J Mater Sci: Mater Med DOI 10.1007/s10856-014-5217-1
Survival-rate analysis of surface treated dental zirconia (Y-TZP) ceramics Cedomir Oblak • Ivan Verdenik • Michael V. Swain Tomaz Kosmac
•
Received: 31 December 2013 / Accepted: 11 April 2014 Ó Springer Science+Business Media New York 2014
Abstract The role of surface preparation, hydrothermal ageing exposure and subsequent cyclic fatigue testing on the biaxial strength of a dental Y-TZP material are investigated. The initial strength and survival rate of a dental Y-TZP ceramic material to fatigue testing was found to be highly dependent upon surface preparation more so than exposure to various hydrothermal exposure conditions. The results suggest that the monoclinic phase generated by either surface damage (especially sandblasting) and to a lesser extent hydrothermal exposure does appear to mitigate strength and fatigue degradation. The results are discussed in terms of the size of defects generated following various surface treatments and the role of cyclic fatigue induced crack growth. A critical ratio is established between the monotonic strength and fatigue stress survival. C. Oblak Department of Prosthodontics, Medical Faculty, University of Ljubljana, Ljubljana, Slovenia I. Verdenik University Medical Center, Ljubljana, Slovenia M. V. Swain Biomaterials, Faculty of Dentistry, University of Sydney, Sydney, Australia M. V. Swain Biomaterials, Faculty of Dentistry, Otago University, Dunedin, New Zealand M. V. Swain Prosthehetic Department, University of Freiburg, Freiburg, Germany T. Kosmac (&) Engineering Ceramics Department, Jozef Stefan Institute, Ljubljana, Slovenia e-mail: [email protected]
From the specimens that failed and exhibited reduced strength after cycling a plot of averaged crack growth rate versus max cyclic stress intensity factor was established which closely matched existing results for Y-TZP ceramics.
1 Introduction In restorative dentistry, the replacement of metal-based fixed partial dentures (FPDs) with all-ceramic crowns and bridges has become increasingly popular because of improved aesthetics and biocompatibility. Zirconia containing 3 mol % yttria (3Y-TZP) has become the material of choice for multi-unit posterior bridges and implant supported crowns because of its superior strength, fracture toughness and damage tolerance, compared to other dental ceramics [1–5]. Y-TZP-based all-ceramic FPDs are produced by CAD/ CAM milling of biscuit-sintered Y-TZP blanks, which are then sintered to full density. Before porcelain veneering, a final adjustment by dental grinding is usually required, while sandblasting is commonly used to improve the adhesion between the luting cement and Y-TZP framework [6, 7]. The surfaces of Y-TZP ceramics ground and/or sandblasted core will be partially transformed to the monoclinic (m) phase as well as damaged. In earlier studies we have shown, that dry grinding and airborne particle abrasion result in heavily damaged and partial plastically deformed surfaces [8, 9]. Grinding-induced surface flaws are likely to become strength-determining because their length commonly exceeds the depth of grinding-induced surface compressive layer [9, 10]. In contrast, substantial strength improvement was reported with sandblasting indicating that impact-induced flaws are of comparable size
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to pre-existing flaws of these materials [9, 11]. A recent study by Scherrer et al. [12] has shown that sandblasting with 30 lm alumina particles at 2.5 bar, not only enhances the monotonic strength but also improves the fatigue limits and survival probabilities of the dental Y-TZP ceramics and was therefore recommended as a last clinical step before cementation. Under clinical conditions, dental restorations are exposed to cyclic mechanical and thermal loadings in the chemically aggressive environment of the oral cavity, where they are expected to be in service for at least 7–10 years. The strength tends to diminish steadily with time, from stress corrosion and fatigue as well as other mechanisms, e.g. mechanical surface treatment and/or enhanced low temperature degradation (LTD), i.e. ageing of Y-TZP ceramics. The latter occurs when exposed to moist environments at slightly elevated temperatures over long periods, during which the surface of Y-TZP transforms spontaneously from the tetragonal (t) to monoclinic (m) structure [13]. The transformation initiates from isolated surface grains and gradually proceeds into the bulk [14]. Due to a considerable volume expansion associated with the t ? m transformation, this process is accompanied by surface compressive stresses and extensive microcracking, which may lead to strength degradation and may result in spontaneous fracture [13–15]. Currently, clinically reported fractures originating in the zirconia framework are relatively scarce [4, 16, 17]. This may partially arise from the protection provided by the veneering porcelain on the Y-TZP to LTD degradation. However, with the introduction of porcelain-free fully anatomic FPDs in clinical practice, of Y-TZP sintered at higher temperatures, this situation may drastically change, as these FPDs will be directly exposed to the moist environment of the oral cavity. This in vitro study of a densely sintered 3Y-TZP material was designed to evaluate the combined effect of mechanical surface treatment, notably dental grinding and sandblasting, and accelerated ageing on the survival rate with fatigue testing in an artificial saliva solution, with groups tested in air serving as controls.
2 Materials and methods A biomedical-grade zirconia powder (TZ-3YB-E, Tosoh, Tokyo, Japan), containing 3 mol % Y2O3 and 0.25 wt % of Al2O3 was used. Disc-shaped specimens (15.5 ± 0.03 mm in diameter and 1.5 ± 0.03 mm thick) were fabricated by uniaxial dry pressing (150 MPa) and pressureless sintering at 1,520 °C for 2 h. The density of the sintered specimens exceeded 99 % with the mean grain size of 0.40 lm and consisted of nearly 100 % t ? c zirconia (from XRD
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analysis). The exact amount of the cubic zirconia was not determined. Considering the composition and the sintering temperature the Scott’s phase diagram suggests that about 27 % should be untransformable cubic with the reminder transformable tetragonal phase [18]. Sintered discs, (320 in total) were randomly divided into groups of 10 and subjected to different surface treatments and/or ageing conditions, as indicated in Fig. 1. Samples were ground with a coarse-grit (150 lm) diamond burr (ISO-No. 806 314 110 534 018, Dica, Dendia, Austria) mounted on a high-speed hand piece for the dry surface grinding, to simulate clinical and dental laboratory procedures. Sandblasted samples were subjected to abrasion with 110 lm Al2O3 particles at 4 bar, 25 mm from the nozzle (5 mm in diameter) (Duostar Z2, Bego, Bremen Germany). Half of sandblasted samples were annealed (1 h at 920 °C) eliminating the impact-induced surface compressive stresses by re-transforming the m ? t zirconia on the surface [9]. Accelerated aging was carried out by autoclaving at 134 °C for 2 and 24 h in an artificial saliva solution prepared according to Arvidson and Johansson [19]. Based on the recently published results by Keuper et al. [20], who investigated the LTD of 3Y-TZP at human body temperature in water over a longer period of time, it can be anticipated that autoclaving for a few hours at the sterilization temperature will result in a transformed layer thickness equivalent to several years of storage at body temperature. Before and after each surface treatment and/or ageing, the samples were phase analyzed using XRD (CuKa radiation). The relative amount of transformed m-zirconia on the specimens’ surfaces was determined according to the method of Garvie and Nicholson [21]. Biaxial, flexural strength (punch-on-3 balls, ISO 6872) was measured at a loading rate of 1 mm/min, using a universal testing machine (Model 4301, Instron Corp., Canton, USA). Tests were performed both in air and in artificial saliva; in the latter case, the specimens were stored for 24 h in artificial saliva at 37 °C prior to strength testing. The surface-treated specimens were fractured with the surface-treated side under tension. The failure load was recorded and biaxial flexural strength calculated following Wachtman et al. [22]. For biaxial fatigue testing, the specimens were subjected to a sinusoidal cyclic loading ranging from 50 to 850 N at a frequency of 15 Hz exerted by a flat punch (1.6 mm diameter). The lower and upper loads corresponded to a stress of 36 and 620 MPa, respectively. For each specimen, the number of cycles to failure was registered. A fatigue cycle limit of 106 was set and the specimens that survived the mechanical fatigue testing were subsequently monotonically loaded to fracture. The tests were performed in air and in artificial saliva using a servo-hydraulic testing system (Fast Track 8871, Instron, High Wycombe, UK). This
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Fig. 1 Experimental design of the study
evaluation procedure was chosen because it allows us to explore the effects of all clinically relevant mechanical and/or hydrothermal surface treatments in a relevant environment using a limited number of specimens.
2.1 Cyclic fatigue crack growth Cyclic fatigue crack growth for the fatigued specimens was estimated as follows. From the monotonic strength test results sizes of the critical flaws at failure were determined using the expression (c ¼ ðr=YKc Þ2 ) where Y is the crack shape function 1.025, r is the strength and Kc is the fracpffiffiffiffi ture toughness, taken as 4 MPa m. This value is slightly lower than K1c but as the monotonic tests were conducted in air at modest loading rates some subcritical crack growth would have occurred. A previous study by Chevalier et al. [23] evaluating a similar grain size 3Y-TZP used 5 mm/ min in a silicone oil environment to eliminate sub-critical pffiffiffiffi crack growth to determine K1c. At a K value of 4 MPa m these authors found for a well-established crack that a crack growth rate of 10 lm/s occurred, which is not
dissimilar to the extent of the initial crack size calculated for the monotonic strength specimens. The fatigue results were then ranked from the fewest cycles to fracture to the highest resultant strength for the samples that survived. From the ranked monotonic strength results the stress intensity factor at the max cyclic stress (620 MPa) was determined as well as the critical flaw size for failure at this stress with the same conditions as mentioned above. The difference between the initial flaw and critical flaw size was considered the extent by which the crack grew during the fatigue cycling if the specimen fractured. If the samples survived the 106 cycles then upon subsequent monotonic loading to fracture the critical flaw size could be estimated and any growth during the cyclic loading determined. As the slope of the fatigue crack growth rate versus K is very steep for zirconia ceramics the lifetime or number of cycles to failure is dominated by the lowest crack growth rate. As such the averaged cyclic crack extension (dc/dN) was plotted as a function of Kmax at the onset of testing. Such an analysis was conducted for all specimens that either fractured during cyclic stressing or showed strength reduction.
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2.2 Statistical analysis The data analysis was performed using SPSS 16 software (Statistical Package for the Social Sciences, SPSS., IL, USA). Analysis of variance (ANOVA) was used to determine whether exist significant differences in biaxial flexural strength between different surface treatment and/or ageing. When the overall F-test showed a significant difference (meaning that the strength is statistically significant different in differently surface treated/and or ageing samples), the multiple pairwise comparisons were used to determine which mean values differed from another with a significance level of P \ 0.05. To account for increased error due to multiple comparison, Bonfferoni correction was applied. The survival rate of all the tested groups (i.e. specimens subjected to different mechanical surface treatment and/or ageing under hydrothermal conditions in artificial saliva) was assessed using the Cox survival analysis whereby the (logarithm of) number of cycles survived was considered as a dependent variable with survival status serving as censoring variable.
3 Results XRD analysis of surface treated specimens revealed detectable monoclinic peaks with a marked preference for the M(111) orientations and a substantial T(111) peak broadening accompanied by a reversed intensity of the tetragonal (200) (002) peaks. As shown in Table 1, the highest amount, about 14–15 %, of the monoclinic phase was found after sandblasting. A considerably lower amount, \5 %, of the m phase was obtained after grinding, and almost no monoclinic zirconia was observed on the surface of the sandblasted specimens that were subsequently annealed, indicating that during heating up to 920 °C the reverse m ? t transformation occurred. It is worth mentioning, however, that after annealing the T(111) peak intensity and sharpness were re-established, but the reversed intensity of the tetragonal (002) (200) peaks was preserved. The exposure to artificial saliva for 24 h at 37 °C resulted in a detectable amount of m-zirconia in the surface of the as-sintered specimens; a still higher amount of t-zirconia transformed to m in these samples with accelerated ageing at 137 °C for 2 h, while after ageing for 24 h at this temperature the amount of m-zirconia exceeded 25 %. If we assume that the increase in the monoclinic content upon ageing was a measure of the phase instability of the Y-TZP ceramics under hydrothermal conditions, the highest instability was observed with pristine, i.e., as-sintered samples, and the lowest with sandblasted samples that were subsequently annealed above the m ? t temperature.
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The results of the bi-axial flexural strength measurements in air and in artificial saliva obtained by monotonic loading are presented in Fig. 2. The mean strength and standard deviation (SD) of the as sintered samples at RT in air and in artificial saliva stored at 37 °C was 1012 ± 92, and 886 ± 108 MPa, respectively. After ageing in artificial saliva at 134 °C the strength actually improved and was only slightly lower than the control group, with statistically significant difference (P \ 0.05) found only between the control group (air) and the samples tested in artificial saliva (after 24 h at 37 °C prior to testing). When the samples were ground with diamond burs, the strength values obtained by testing in air (804 ± 106 MPa) and when tested in artificial saliva (either before or after accelerated ageing in an autoclave) were significantly lower (P \ 0.05) compared to all other groups tested in the study. It is worth mentioning here, that the strength reduction in ground specimens was largest after exposure to the wet environment at 37 °C before testing (746 ± 72 MPa) and lowest after short-term autoclaving. In contrast to grinding, sandblasting increased the strength in air (1,289 ± 49 MPa) and, more importantly, there was no strength reduction when tested in artificial saliva either before or after short- or long-term accelerated ageing at 134 °C. As a consequence, there were statistically significant differences between the groups of sandblasted samples and control (as sintered) or ground samples (P \ 0.05). Sandblasted samples upon annealing at 920 °C had strengths comparable to the as sintered specimens although the scatter in strengths did increase with artificial saliva exposure, especially after autoclave accelerated ageing. The results of fatigue testing are summarized in Table 2. The survival rate of the controls in air was 60, and 50 % in the artificial saliva solution. After grinding, the survival rate dropped down to only 20 % in air and to 10 % in the artificial saliva solution, whereas none of the sandblasted samples failed during the fatigue testing, neither in air nor in artificial saliva. The survival rate of the sandblasted and annealed samples was 90 % in air, higher than that of the controls, but this dropped down to 60 % in artificial saliva. It is worth noting, that all failures in air occurred during initial (first) 10,000–20,000 cycles and even earlier when tested in artificial saliva. After that, no further failures were registered. In contrast, a steady decrease in the survival rate with the number of loading cycles was observed after accelerated ageing in artificial saliva at 134 °C prior to fatigue testing. The strength of the surviving specimens after fatigue testing corresponded well to the mean flexural strength of the particular group before fatigue testing, with a much higher standard deviation, though. The survival rate analysis after mechanical fatigue testing for various surface treatments and (pre)testing are presented in Fig. 3. In relation to the as sintered control
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Fig. 2 Mean biaxial flexural strength values and SD after monotonic loading for assintered and surface-treated Y-TZP ceramics: in air, in artificial saliva, after accelerated ageing in artificial saliva for 2 h at 134 °C and after accelerated ageing in artificial saliva for 24 h at 134 °C
Xm before ageing (%)
Material
Xm after ageing (%) 37 °C/24 h
As sintered
\1
Sandblasted
14.5 ± 2.1
134 °C/2 h
134 °C/24 h
1.5 ± 0.2
5.1 ± 0.4
25.6 ± 4.1
14.0 ± 1.6
14.5 ± 1.4
23.7 ± 2.4
Sandblasted ? annealed 920 °C
1.5 ± 0.5
2.5 ± 0.5
2.9 ± 1.2
9.4 ± 2.6
Ground
4.3 ± 0.8
6.3 ± 1.5
7.2 ± 2.6
24.8 ± 2.1
1600 Air
Art. Saliva 134C 2h
Art. Saliva 37C 24h
Art. Saliva 134C 24h
1400
Mean biaxial flexural strength (MPa)
Table 1 Relative amounts of Xm (monoclinic phase) on the surface of as-sintered and surface treated biomedicalgrade Y-TZP ceramics before and after ageing in artificial saliva
1200
1000
800
600
400
200
0 As sintered
group, any mechanical surface treatment resulted in a substantial difference in the survival rate whereas the influence of (pre)testing conditions used in this work was not found to be equally important. In order to determine whether the noted differences are statistically significant we used Cox proportional hazard regression. According to results listed in Table 3, the effects of mechanical surface treatment are indeed statistically significant, while the effects of accelerated ageing are not. Thus, survival rates of sandblasted samples were significantly higher in comparison to as sintered or ground ceramics tested in air or in artificial saliva. Lower failure rates were found for sandblasted samples compared with as sintered samples (HR: 0.1; 95 % CI: 0.03–0.3), and also for sandblasted samples subsequently annealed at 920 °C compared with as sintered samples (HR: 0.23; 95 % CI: 0.07–0.8). No difference (P [ 0.05) was observed between sandblasted specimens and specimens subsequently annealed at 920 °C. Higher but statistically insignificant
Ground
Sandblasted
Sandblasted + annealed
failure rates were found for all specimens aged in artificial saliva. In an attempt to determine whether there was a critical strength for the onset of fatigue susceptibility for all the specimens all the monotonic strength results were combined, ranked and the probability of failure (Pf = i/n ? 1, where i is the rank of the test result and n is the number of specimens, 120) versus strength was plotted as shown in Fig. 4. The tacit assumption here was that the monotonic strength of the specimens, irrespective as to whether there were residual stresses present or not, provides a basis for the distribution of the ‘‘effective’’ flaws of the tested materials. As such any reduction of strength of the materials as a consequence of the exposure to cyclic fatigue loading provides an indication as to the susceptibility of the original ‘‘effective’’ flaws to have grown during fatigue. Also shown in Fig. 4 are the strengths of the specimens that survived cyclic fatigue testing for 1 million cycles to 620 MPa. These strengths were similarly ranked because
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J Mater Sci: Mater Med Table 2 Fatigue behavior of as-sintered and surface treated Y-TZP ceramics in air and in artificial saliva As sintered (control group)
Dry ground (150 mm)
Sandblasted
Sandblasted ? annealed
Survival ratea (%)
6 (60 %)
2 (20 %)
10 (100 %)
9 (90 %)
Survival strength ? SD (MPa)
1,016 (105)
926 (2)
1,266 (107)
1,037 (50)
Survival ratea (%)
5 (50 %)
1 (10 %)
10 (100 %)
6 (60 %)
Survival strength ? SD (MPa)
886 (98)
940
976 (339)
981 (167)
6 (60 %)
1 (10 %)
8 (80 %)
9 (90 %)
1,038 (68)
816
1,125 (193)
979 (165)
Survival ratea (%)
5 (50 %)
1 (10 %)
8 (80 %)
6 (60 %)
Survival strength ? SD (MPa)
877 (188)
722
1,159 (177)
1,004 (91)
Air
Art. Saliva, 37 °C 24 h
Art. Saliva, 134 °C 2 h Survival ratea (%) Survival strength ? SD (MPa) Art. Saliva, 134 °C 24 h
6
After 10 cycles (50–850 N) at a frequency of 10 Hz
100%
80%
Survival rate
100%
sandblasted
80%
sandblasted+900ºC
as sintered (control)
60%
40%
Survival rate
a
60% artificial saliva (2h, 134ºC)
40%
air (control) artificial saliva (24h, 134ºC)
20%
artificial saliva (24h, 37ºC)
20% ground
0%
0% 10
1
10
2
10
3
10
4
10
5
10
6
number of cycles
10 1
10 2
10 3
10 4
10 5
10 6
number of cycles
Fig. 3 The Cox survival curves after mechanical fatigue testing: left various surface treatments (each line includes together specimens treated in air and aged in artificial saliva), right various (pre)testing conditions (each group includes together as sintered and surface treated specimens)
they were assumed to have the same probability of failure as the initial monotonically loaded specimens. The estimated dc/dN versus K at the max cyclic stress for the samples that fractured during cyclic loading and displayed subsequent monotonic strength reduction is shown in Fig. 5 for all specimens. The form of the results following various treatments show very similar trends with already reported threshold Kmax for crack growth occurring pffiffiffiffi *2–2.5 MPa m, or approximately K1c/2 [23].
4 Discussion The present results clearly show that current dental clinical and laboratory procedures do influence the strength, fatigue survival outcomes and LTD t ? m susceptibility of
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Y-TZP. Grinding appears to have the most deleterious effect on the strength and fatigue failure rate while sandblasting has a substantial strengthening response. The role of accelerated ageing (autoclave exposure) was less pronounced. The LTD induced t ? m transformation, the extent of which varied considerably between different surface conditions, has only a modest (not significant) strengthening effect. The XRD results show major differences between the sandblasted and ground conditions. The latter, although generally acknowledged to be a more severe deformation procedure was less effective in generating the t ? m transformation (Table 1). This difference has also been seen by a number of other authors and debate still continues as to the interpretation of the t/c peak broadening at lower 2 theta and also for the reversal of the 200–002 t
J Mater Sci: Mater Med Table 3 Results of the Cox survival comparisons and hazard rate ratio with 95 % confidence interval
B
SE
Stat. sign. P
-2.381
0.657
\0.001
0.092 (0.025–0.335)
1.223
0.380
0.001
3.397 (1.613–7.151)
-1.457
0.599
0.015
0.233 (0.072–0.753)
\0.001
As sintered (control) Sandblasted Ground Sandblasted ? 920 °C
HR (95 % CI)
Air (control)
0.193
Artificial saliva (24 h, 37 °C)
0.636
0.368
0.084
1.889 (0.919–3.883)
Artificial saliva (2 h, 134 °C)
-0.030
0.389
0.938
0.970 (0.452–2.081)
0.562
0.597
0.346
1.754 (0.545–5.648)
Artificial saliva (24 h, 134 °C)
Fig. 4 A plot of the fracture strength of the monotonic strength results for all Y-TZP specimens versus the probability of failure. Also included are the results of the monotonic strengths for those specimens that survived cyclic loading. Three regions are identified: (i) samples with initial strengths between 620 and 890 MPa that failed during 1 million cycles at 620 MPa, (ii) samples between 890 and 1,000 MPa initial strength that displayed strength reduction, and (iii) samples with initial strengths above 1,000 MPa that did not show strength degradation
peak intensities observed. Some have focused on the broadening and often clear peaks at lower angles following grinding [24, 25] whereas others have suggested that strain induced ferroelastic domain switching has occurred with the reversal of the t peak intensities [26, 27]. Upon annealing the sandblasted specimens at 920 °C there was almost complete m ? t phase reversal and sharpening of the t/c peak but the t reversal at 35 degrees was still present. The influence of autoclave exposure to induce accelerated LTD was most pronounced for the as sintered and least influential for the sandblasted and subsequently annealed specimens. For all other samples there is a substantial increase in m phase after 24 h exposure. For sandblasted and annealed specimens greater resistance to LTD has also been reported by Whalen et al. [28] who have suggested this is related to grain size reduction due to
Fig. 5 Plot of the averaged fatigue crack growth rate (lm/cycle) during stressing at 620 MPa for the specimens that failed and for those that exhibited monotonic strength degradation after surviving 1 million cycles. Previous observations by Chevalier et al. and others indicate that the threshold value for cyclic fatigue is *0.45 K1c suggesting that the Y-TZP material had a fracture toughness of 5.5 MPa m1/2
recrystallization which would have the effect of increasing the stability of the t phase. In addition, as demonstrated by Munoz-Tabares et al. [27] the mechanically induced transformation is accompanied by considerable changes of the internal stresses and reduction of the near surface grain size that hinder the hydrothermally induced transformation. This is evident from slower degradation rate of sandblasted samples. At present we do not know for sure why the ground specimens transformed at a higher rate than the sandblasted ones; it is speculated that this is because during rough grinding cracks have been formed, that opened paths for water penetration, but this explanation is yet to be proven. The monotonic strength results before and after accelerated LTD show no correlations with the m contact measured by XRD. The only clear evidence for the presence of the m phase was for the sandblasted specimens when compared with the subsequent annealed materials. Here nearly a 300 MPa increase in strength is observed which is almost equivalent to the compressive stress generated by the t ? m transformation (r ffi EVf DV) where E is the modulus, Vf the t ? m volume fraction
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transformed and DV is the volume dilatation associated with the t ? m phase change [15]. The effectiveness of this compressive stress will be dependent upon the ratio of the transformed depth to that of the flaws generated [10, 23]. With autoclaving LTD t ? m it may have been anticipated, from the above expression, that this would increase the strength because after 24 h as there is almost a doubling of the m phase. Only for the as sintered and ground specimens when comparison is made between the 24 h/37 °C versus the 24 h autoclave results is there almost a 100 MPa increase in strength. The calculated hazard rate ratio (probability of failing with regard to intact controls) is 10.9 (95 % CI: 3–40) times lower for sandblasted samples, 4.3 (95 % CI: 1.3–13.9) times lower for sandblasted and annealed samples and 3.4 (95 % CI: 1.6–7.2) times higher for ground samples. With regard to testing in air, the hazard is almost twice as high when tested in wet environment, which is in line with the results of Chevalier et al. [23], Morena et al. [29] and Studard et al. [30] who reported on a significantly shorter lifetime for Y-TZP specimens under cyclic loading in water than in air. The additional effect of accelerated ageing, in contrast, is statistically insignificant. The superposition of the combined monotonic strength data plotted versus probability of fracture in Fig. 4 shows a clear monotonic trend. The assumption here was that as the Y-TZP material was the same the various treatments resulted in different effective defect sizes that were the same for the samples monotonically and cyclically loaded. It is clear from Table 2 that the sandblasted specimens dominated the upper portion of this curve and ground surfaces the lower portion. The results for the retained strength following cyclic fatigue also shown in Fig. 4 identify three critical regions. At stresses above 1,000 MPa the strength probability plot is almost identical to that of the monotonic strength indicating that no extension of the defect population occurred during cyclic stressing. However for specimens, which initially would have had strengths between 890 and 1,000 MPa, there was a gradual change of the slope from the original distribution indicating that the size of the largest defects in these materials had increased during cyclic loading resulting in the observed lower strength. For specimens that initially had a monotonic strength of 890 MPa or less sufficient crack extension occurred during cyclic stressing to extend the defect above a critical size such that at 620 MPa the crack tip stress intensity factor exceeded the K1c. An alternate approach to view the above data is shown in Fig. 6 in which the results presented in Fig. 4 are reploted as the probability of survival (Ps = 1 - Pf) versus the cyclic fatigue stress divided by the monotonic strength. Superimposed upon this normalized strength distribution curve, as horizontal lines, are the results for various surface treatments. The width of these lines represents the range of
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strength results for two standard deviations and the position was determined by the mean strength of the specific treatment as listed in Table 2. The choice of 2 SDs was chosen as only 2 % of the specimens would fall outside this range. Also shown, as the vertical line, is the normalized strength (890 MPa) below which fracture was seen to occur during cyclic loading at 620 MPa as shown in Fig. 4. If the horizontal line representing the range of strengths for the various treatments crossed the vertical line then the number of specimens likely to fail during fatigue testing may be estimated from the known normal distribution function. The plotted results suggest that most of the ground samples would have failed during fatigue testing as observed whereas the sandblasted samples would have survived. The results also show a good correlation with the Cox survival data shown in Fig. 3. That is the further to the right the data for a specific surface treatment is in Fig. 6 of the vertical line the fewer the cycles resulting in fracture and in a greater failure rate in the Cox survival plots. The results shown in Fig. 4 clearly indicate that cyclic induced sub-critical fatigue crack growth occurs and compliment the results estimated for the crack growth rates shown in Fig. 5. The calculated averaged dc/dN results from the fatigue tests shown in Fig. 5 do agree reasonably well with the results of direct cyclic crack extension studies by Chevalier et al. [23]. The present analysis is based upon simple assumptions namely that the flaw size distributions for the monotonic strength tests are similar to the fatigue results. Invariably there will be slight differences however the reasonable overlap between the differently treated surfaces for the two conditions (as sintered and ground) along with the comparable outcomes to that by Chevalier et al. [23] and Studart et al. [30] does generate some support for the approach, particularly in that cyclic fatigue crack growth becomes important at Kmax values approaching half the K1c value of the material. From the results shown in Fig. 4 there is a critical strength identified (1,000 MPa) above which there is no indication of strength reduction as a consequence of cyclic fatigue. That is for these specimens the effective flaw size is such that the stress intensity factor at the maximum cyclic stress (620 MPa) was less than Kth for cyclic fatigue crack growth. From the literature [23, 30] this threshold value is *0.45 K1c. For specimens that had monotonic strength results between 890 and 1,000 MPa there was a clear indication that subcritical crack growth during the 1 million cycles occurred with the extent of the degradation being greater as the monotonic strengths approached 890 MPa. This increase in defect size upon cyclic loading was also identified in the fatigue crack growth results shown in Fig. 5. Chevalier et al. [23] identified two regimes of the cyclic fatigue crack growth rate response, in the range close to the threshold there is a very steep response but above this the slope is significantly reduced.
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1
Probablity of Survival versus Normalised Stress Strength distribution Critical stress 890 MPa
Probability of Survival
Fig. 6 A plot of the probability of survival of the initial (ranked) monotonic strength data for all Y-TZP specimens normalized by the maximum fatigue stress (620 MPa). The vertical broken line is the critical initial monotonic strength (890 MPa) below which fracture occurred during fatigue cycling. The short horizontal lines represent 2 standard deviations from the mean for the different specimens. If these horizontal lines cross the vertical line then fracture during fatigue testing is anticipated
0.8 AS Dry ArtSaliva 37C
0.6
AS Art Saliva 2/134C 0.4
AS Art Saliva 24 134C Ground Dry
0.2
Ground Art Salica 37C Ground Art Saliva 2h 134C
0 0.00
0.20
0.40
0.60
0.80
1.00
1.20
Sand Blasted 34C 24hr
Fatigue Stress/Strength Sand Blasted annealed Dry
5 Conclusions The present investigation has shown that a Y-TZP dental ceramic undergoes strength degradation upon surface damage and cyclic fatigue loading. The extent of the strength degradation was greatest for ground specimens while sandblasting increased the strength. Autoclave exposure to enhance low temperature tetragonal to monoclinic transformation was also found to be surface treatment dependent with as-sintered samples having a faster rate than ground samples and sandblasted samples having the slowest rate of transformation. It was also found that LTD had a minimal effect on the strength and fatigue reliability outcomes. Hazard and probability of fracture analysis of the results showed that the initial monotonic strength provided a very good basis to identify whether failure would occur during subsequent cyclic stressing. The number of cycles to failure and the retained strength after survival of cyclic loading enabled an averaged cyclic crack growth rate versus cyclic stress intensity factor to be determined which agreed well with results from more sophisticated methods available in the literature. The approach adopted should have relevance for predicting the lifetime of more complex dental crowns and bridges subjected to cyclic stressing.
3.
4.
5. 6. 7.
8.
9.
10.
11.
12.
13.
References 14. 1. Rekow D, Thompson VP. Engineering long term clinical success of advanced ceramic prostheses. J Mater Sci Mater Med. 2007;18:47–56. 2. Sailer I, Pjetursson BE, Zwahlen M, Ha¨mmerle CHF. A systematic review of the survival and complication rates of allceramic reconstructions after an observation period of at least
15. 16.
3 years. Part II: fixed dental prostheses. Clin Oral Impl Res. 2007;18:86–96. Pjetursson BE, Sailer I, Zwahlen M, Ha¨mmerle CHF. A systematic review of the survival and complication rates of all-ceramic reconstructions after an observation period of at least 3 years. Part I: single crowns. Clin Oral Impl Res. 2007;18:73–85. Raigrodski AJ, Hillstead MB, Meng GK, Chung KH. Survival and complications of zirconia-based fixed dental prostheses: a systematic review. J Prosthet Dent. 2012;107:170–7. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater. 2008;24:299–307. Kern M. Bond strength of luting cements to zirconium oxide ceramics. Int J Prosthodont. 2000;13:30–5. Re D, Augusti D, Sailer I, Spreafico D, Cerutti A. The effect of surface treatment on the adhesion of resin cements to Y-TZP. Eur J Esthet Dent. 2008;3:186–96. Kosmacˇ T, Oblak Cˇ, Jevnikar P, Funduk N, Marion L. The effect of grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater. 1999;15:426–33. Kosmacˇ T, Oblak Cˇ, Jevnikar P, Funduk N, Marion L. Strength and reliability of surface treated Y-TZP dental ceramics. J Biomed Mater Res. 2000;53:304–13. Xu H, Jahanmir S, Ives LK. Effect of grinding on strength of tetragonal zirconia and zirconia-toughened alumina. Mach Sci Technol. 1997;1:49–66. Sato H, Yamada K, Pezzotti G, Nawa M, Ban S. Mechanical properties of dental zirconia ceramics changed with sandblasting and heat treatment. Dent Mater J. 2008;27:408–14. Scherrer SS, Cattani-Lorente M, Vittecoq E, Mestral F, Griggs JA, Wiskott HW. Fatigue behavior in water of Y-TZP zirconia ceramics after abrasion with 30 lm silica-coated alumina particles. Dent Mater. 2011;27:28–42. Kobayashi K, Kuwajima H, Masaki T. Phase change and mechanical properties of ZrO2-Y2O3 solid state electrolyte after ageing. Solid State Ion. 1981;3:489–95. Chevalier J, Gremillard L, Deville S. Low temperature degradation and implications for biomedical implants. Annu Rev Mater Res. 2007;37:1–32. Swain MV. Impact of oral fluids on dental ceramics: what is the clinical relevance? Dent Mater. 2014;30:33–42. Raigrodski AJ, Yu A, Chiche GJ, Hochstedler JL, Mancl LA, Mohamed SE. Clinical efficacy of veneered zirconium dioxide-
123
J Mater Sci: Mater Med
17.
18. 19. 20.
21. 22. 23.
24.
based posterior partial fixed dental prostheses: five-year results. J Prosthet Dent. 2012;108:214–22. Pela´ez J, Cogolludo PG, Serrano B, Lozano JF, Sua´rez MJ. A prospective evaluation of zirconia posterior fixed dental prostheses: three-year clinical results. J Prosthet Dent. 2012;107:373–9. Scott HG. Phase relationships in the zirconia-yttria system. J Mat Sci. 1975;10:1527–35. Arvidson K, Johansson EG. Galvanic current between dental alloys in vitro. Scand J Dent Res. 1985;93:467–73. Keuper M, Berthold C, Nickel KG. Long-time aging in 3 mol.% yttria-stabilized tetragonal zirconia polycrystals at human body temperature. Acta Biomater. 2014;10:951–9. Garvie RC, Nicholson PS. Phase analysis in zirconia systems. J Am Ceram Soc. 1972;55:303–5. Wachtman JB, Capps W, Mandel J. Biaxial flexure tests of ceramic substrates. J Mater. 1972;7:188–94. Chevalier J, Olagnon C, Fantozzi G. Subcritical crack propagation in 3Y-TZP ceramics: static and cyclic fatigue. J Am Ceram Soc. 1999;82:3129–38. Kitano Y, Mori Y, Ishitani A, and Masaki T, A study of rhombohedral phase in Y2O3-partially stabilized zirconia. In:
123
25.
26.
27.
28.
29.
30.
Materials Research Society symposia proceedings, vol. 78. 1987. p. 17–24 Denry IL, Holloway JA. Microstructural and crystallographic surface changes after grinding zirconia-based dental ceramics. J Biomed Mater Res B. 2006;76:440–8. Virkar AV, Matsumoto RLK. Ferroelastic domain switching as a toughening mechanism in tetragonal zirconia. J Am Ceram Soc. 1986;69:C224–6. Munoz-Tabares JA, Jimenez-Pique E, Reyes-Gasga J, Anglada M. Microstructural changes in ground 3Y-TZP and their effect on mechanical properties. Acta Mater. 2011;59:6670–83. Whalen PJ, Reidinger F, Antrim RF. Prevention of low-temperature surface transformation by surface recrystallization in yttriadoped tetragonal zirconia. J Am Ceram Soc. 1989;72:319–21. Morena M, Beaudreau GM, Lockwood PE, Evans AL. Fatigue of dental ceramics in a simulated oral environment. J Dent Res. 1986;65:993–7. Studart AR, Filser F, Kochler P, Gauckler LJ. Fatigue of zirconia under cyclic loading in water and its implications for the design of dental bridges. Dent Mater. 2007;23:106–14.
Survival-rate analysis of surface treated dental zirconia (Y-TZP) ceramics Cedomir Oblak • Ivan Verdenik • Michael V. Swain Tomaz Kosmac
•
Received: 31 December 2013 / Accepted: 11 April 2014 Ó Springer Science+Business Media New York 2014
Abstract The role of surface preparation, hydrothermal ageing exposure and subsequent cyclic fatigue testing on the biaxial strength of a dental Y-TZP material are investigated. The initial strength and survival rate of a dental Y-TZP ceramic material to fatigue testing was found to be highly dependent upon surface preparation more so than exposure to various hydrothermal exposure conditions. The results suggest that the monoclinic phase generated by either surface damage (especially sandblasting) and to a lesser extent hydrothermal exposure does appear to mitigate strength and fatigue degradation. The results are discussed in terms of the size of defects generated following various surface treatments and the role of cyclic fatigue induced crack growth. A critical ratio is established between the monotonic strength and fatigue stress survival. C. Oblak Department of Prosthodontics, Medical Faculty, University of Ljubljana, Ljubljana, Slovenia I. Verdenik University Medical Center, Ljubljana, Slovenia M. V. Swain Biomaterials, Faculty of Dentistry, University of Sydney, Sydney, Australia M. V. Swain Biomaterials, Faculty of Dentistry, Otago University, Dunedin, New Zealand M. V. Swain Prosthehetic Department, University of Freiburg, Freiburg, Germany T. Kosmac (&) Engineering Ceramics Department, Jozef Stefan Institute, Ljubljana, Slovenia e-mail: [email protected]
From the specimens that failed and exhibited reduced strength after cycling a plot of averaged crack growth rate versus max cyclic stress intensity factor was established which closely matched existing results for Y-TZP ceramics.
1 Introduction In restorative dentistry, the replacement of metal-based fixed partial dentures (FPDs) with all-ceramic crowns and bridges has become increasingly popular because of improved aesthetics and biocompatibility. Zirconia containing 3 mol % yttria (3Y-TZP) has become the material of choice for multi-unit posterior bridges and implant supported crowns because of its superior strength, fracture toughness and damage tolerance, compared to other dental ceramics [1–5]. Y-TZP-based all-ceramic FPDs are produced by CAD/ CAM milling of biscuit-sintered Y-TZP blanks, which are then sintered to full density. Before porcelain veneering, a final adjustment by dental grinding is usually required, while sandblasting is commonly used to improve the adhesion between the luting cement and Y-TZP framework [6, 7]. The surfaces of Y-TZP ceramics ground and/or sandblasted core will be partially transformed to the monoclinic (m) phase as well as damaged. In earlier studies we have shown, that dry grinding and airborne particle abrasion result in heavily damaged and partial plastically deformed surfaces [8, 9]. Grinding-induced surface flaws are likely to become strength-determining because their length commonly exceeds the depth of grinding-induced surface compressive layer [9, 10]. In contrast, substantial strength improvement was reported with sandblasting indicating that impact-induced flaws are of comparable size
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to pre-existing flaws of these materials [9, 11]. A recent study by Scherrer et al. [12] has shown that sandblasting with 30 lm alumina particles at 2.5 bar, not only enhances the monotonic strength but also improves the fatigue limits and survival probabilities of the dental Y-TZP ceramics and was therefore recommended as a last clinical step before cementation. Under clinical conditions, dental restorations are exposed to cyclic mechanical and thermal loadings in the chemically aggressive environment of the oral cavity, where they are expected to be in service for at least 7–10 years. The strength tends to diminish steadily with time, from stress corrosion and fatigue as well as other mechanisms, e.g. mechanical surface treatment and/or enhanced low temperature degradation (LTD), i.e. ageing of Y-TZP ceramics. The latter occurs when exposed to moist environments at slightly elevated temperatures over long periods, during which the surface of Y-TZP transforms spontaneously from the tetragonal (t) to monoclinic (m) structure [13]. The transformation initiates from isolated surface grains and gradually proceeds into the bulk [14]. Due to a considerable volume expansion associated with the t ? m transformation, this process is accompanied by surface compressive stresses and extensive microcracking, which may lead to strength degradation and may result in spontaneous fracture [13–15]. Currently, clinically reported fractures originating in the zirconia framework are relatively scarce [4, 16, 17]. This may partially arise from the protection provided by the veneering porcelain on the Y-TZP to LTD degradation. However, with the introduction of porcelain-free fully anatomic FPDs in clinical practice, of Y-TZP sintered at higher temperatures, this situation may drastically change, as these FPDs will be directly exposed to the moist environment of the oral cavity. This in vitro study of a densely sintered 3Y-TZP material was designed to evaluate the combined effect of mechanical surface treatment, notably dental grinding and sandblasting, and accelerated ageing on the survival rate with fatigue testing in an artificial saliva solution, with groups tested in air serving as controls.
2 Materials and methods A biomedical-grade zirconia powder (TZ-3YB-E, Tosoh, Tokyo, Japan), containing 3 mol % Y2O3 and 0.25 wt % of Al2O3 was used. Disc-shaped specimens (15.5 ± 0.03 mm in diameter and 1.5 ± 0.03 mm thick) were fabricated by uniaxial dry pressing (150 MPa) and pressureless sintering at 1,520 °C for 2 h. The density of the sintered specimens exceeded 99 % with the mean grain size of 0.40 lm and consisted of nearly 100 % t ? c zirconia (from XRD
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analysis). The exact amount of the cubic zirconia was not determined. Considering the composition and the sintering temperature the Scott’s phase diagram suggests that about 27 % should be untransformable cubic with the reminder transformable tetragonal phase [18]. Sintered discs, (320 in total) were randomly divided into groups of 10 and subjected to different surface treatments and/or ageing conditions, as indicated in Fig. 1. Samples were ground with a coarse-grit (150 lm) diamond burr (ISO-No. 806 314 110 534 018, Dica, Dendia, Austria) mounted on a high-speed hand piece for the dry surface grinding, to simulate clinical and dental laboratory procedures. Sandblasted samples were subjected to abrasion with 110 lm Al2O3 particles at 4 bar, 25 mm from the nozzle (5 mm in diameter) (Duostar Z2, Bego, Bremen Germany). Half of sandblasted samples were annealed (1 h at 920 °C) eliminating the impact-induced surface compressive stresses by re-transforming the m ? t zirconia on the surface [9]. Accelerated aging was carried out by autoclaving at 134 °C for 2 and 24 h in an artificial saliva solution prepared according to Arvidson and Johansson [19]. Based on the recently published results by Keuper et al. [20], who investigated the LTD of 3Y-TZP at human body temperature in water over a longer period of time, it can be anticipated that autoclaving for a few hours at the sterilization temperature will result in a transformed layer thickness equivalent to several years of storage at body temperature. Before and after each surface treatment and/or ageing, the samples were phase analyzed using XRD (CuKa radiation). The relative amount of transformed m-zirconia on the specimens’ surfaces was determined according to the method of Garvie and Nicholson [21]. Biaxial, flexural strength (punch-on-3 balls, ISO 6872) was measured at a loading rate of 1 mm/min, using a universal testing machine (Model 4301, Instron Corp., Canton, USA). Tests were performed both in air and in artificial saliva; in the latter case, the specimens were stored for 24 h in artificial saliva at 37 °C prior to strength testing. The surface-treated specimens were fractured with the surface-treated side under tension. The failure load was recorded and biaxial flexural strength calculated following Wachtman et al. [22]. For biaxial fatigue testing, the specimens were subjected to a sinusoidal cyclic loading ranging from 50 to 850 N at a frequency of 15 Hz exerted by a flat punch (1.6 mm diameter). The lower and upper loads corresponded to a stress of 36 and 620 MPa, respectively. For each specimen, the number of cycles to failure was registered. A fatigue cycle limit of 106 was set and the specimens that survived the mechanical fatigue testing were subsequently monotonically loaded to fracture. The tests were performed in air and in artificial saliva using a servo-hydraulic testing system (Fast Track 8871, Instron, High Wycombe, UK). This
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Fig. 1 Experimental design of the study
evaluation procedure was chosen because it allows us to explore the effects of all clinically relevant mechanical and/or hydrothermal surface treatments in a relevant environment using a limited number of specimens.
2.1 Cyclic fatigue crack growth Cyclic fatigue crack growth for the fatigued specimens was estimated as follows. From the monotonic strength test results sizes of the critical flaws at failure were determined using the expression (c ¼ ðr=YKc Þ2 ) where Y is the crack shape function 1.025, r is the strength and Kc is the fracpffiffiffiffi ture toughness, taken as 4 MPa m. This value is slightly lower than K1c but as the monotonic tests were conducted in air at modest loading rates some subcritical crack growth would have occurred. A previous study by Chevalier et al. [23] evaluating a similar grain size 3Y-TZP used 5 mm/ min in a silicone oil environment to eliminate sub-critical pffiffiffiffi crack growth to determine K1c. At a K value of 4 MPa m these authors found for a well-established crack that a crack growth rate of 10 lm/s occurred, which is not
dissimilar to the extent of the initial crack size calculated for the monotonic strength specimens. The fatigue results were then ranked from the fewest cycles to fracture to the highest resultant strength for the samples that survived. From the ranked monotonic strength results the stress intensity factor at the max cyclic stress (620 MPa) was determined as well as the critical flaw size for failure at this stress with the same conditions as mentioned above. The difference between the initial flaw and critical flaw size was considered the extent by which the crack grew during the fatigue cycling if the specimen fractured. If the samples survived the 106 cycles then upon subsequent monotonic loading to fracture the critical flaw size could be estimated and any growth during the cyclic loading determined. As the slope of the fatigue crack growth rate versus K is very steep for zirconia ceramics the lifetime or number of cycles to failure is dominated by the lowest crack growth rate. As such the averaged cyclic crack extension (dc/dN) was plotted as a function of Kmax at the onset of testing. Such an analysis was conducted for all specimens that either fractured during cyclic stressing or showed strength reduction.
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2.2 Statistical analysis The data analysis was performed using SPSS 16 software (Statistical Package for the Social Sciences, SPSS., IL, USA). Analysis of variance (ANOVA) was used to determine whether exist significant differences in biaxial flexural strength between different surface treatment and/or ageing. When the overall F-test showed a significant difference (meaning that the strength is statistically significant different in differently surface treated/and or ageing samples), the multiple pairwise comparisons were used to determine which mean values differed from another with a significance level of P \ 0.05. To account for increased error due to multiple comparison, Bonfferoni correction was applied. The survival rate of all the tested groups (i.e. specimens subjected to different mechanical surface treatment and/or ageing under hydrothermal conditions in artificial saliva) was assessed using the Cox survival analysis whereby the (logarithm of) number of cycles survived was considered as a dependent variable with survival status serving as censoring variable.
3 Results XRD analysis of surface treated specimens revealed detectable monoclinic peaks with a marked preference for the M(111) orientations and a substantial T(111) peak broadening accompanied by a reversed intensity of the tetragonal (200) (002) peaks. As shown in Table 1, the highest amount, about 14–15 %, of the monoclinic phase was found after sandblasting. A considerably lower amount, \5 %, of the m phase was obtained after grinding, and almost no monoclinic zirconia was observed on the surface of the sandblasted specimens that were subsequently annealed, indicating that during heating up to 920 °C the reverse m ? t transformation occurred. It is worth mentioning, however, that after annealing the T(111) peak intensity and sharpness were re-established, but the reversed intensity of the tetragonal (002) (200) peaks was preserved. The exposure to artificial saliva for 24 h at 37 °C resulted in a detectable amount of m-zirconia in the surface of the as-sintered specimens; a still higher amount of t-zirconia transformed to m in these samples with accelerated ageing at 137 °C for 2 h, while after ageing for 24 h at this temperature the amount of m-zirconia exceeded 25 %. If we assume that the increase in the monoclinic content upon ageing was a measure of the phase instability of the Y-TZP ceramics under hydrothermal conditions, the highest instability was observed with pristine, i.e., as-sintered samples, and the lowest with sandblasted samples that were subsequently annealed above the m ? t temperature.
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The results of the bi-axial flexural strength measurements in air and in artificial saliva obtained by monotonic loading are presented in Fig. 2. The mean strength and standard deviation (SD) of the as sintered samples at RT in air and in artificial saliva stored at 37 °C was 1012 ± 92, and 886 ± 108 MPa, respectively. After ageing in artificial saliva at 134 °C the strength actually improved and was only slightly lower than the control group, with statistically significant difference (P \ 0.05) found only between the control group (air) and the samples tested in artificial saliva (after 24 h at 37 °C prior to testing). When the samples were ground with diamond burs, the strength values obtained by testing in air (804 ± 106 MPa) and when tested in artificial saliva (either before or after accelerated ageing in an autoclave) were significantly lower (P \ 0.05) compared to all other groups tested in the study. It is worth mentioning here, that the strength reduction in ground specimens was largest after exposure to the wet environment at 37 °C before testing (746 ± 72 MPa) and lowest after short-term autoclaving. In contrast to grinding, sandblasting increased the strength in air (1,289 ± 49 MPa) and, more importantly, there was no strength reduction when tested in artificial saliva either before or after short- or long-term accelerated ageing at 134 °C. As a consequence, there were statistically significant differences between the groups of sandblasted samples and control (as sintered) or ground samples (P \ 0.05). Sandblasted samples upon annealing at 920 °C had strengths comparable to the as sintered specimens although the scatter in strengths did increase with artificial saliva exposure, especially after autoclave accelerated ageing. The results of fatigue testing are summarized in Table 2. The survival rate of the controls in air was 60, and 50 % in the artificial saliva solution. After grinding, the survival rate dropped down to only 20 % in air and to 10 % in the artificial saliva solution, whereas none of the sandblasted samples failed during the fatigue testing, neither in air nor in artificial saliva. The survival rate of the sandblasted and annealed samples was 90 % in air, higher than that of the controls, but this dropped down to 60 % in artificial saliva. It is worth noting, that all failures in air occurred during initial (first) 10,000–20,000 cycles and even earlier when tested in artificial saliva. After that, no further failures were registered. In contrast, a steady decrease in the survival rate with the number of loading cycles was observed after accelerated ageing in artificial saliva at 134 °C prior to fatigue testing. The strength of the surviving specimens after fatigue testing corresponded well to the mean flexural strength of the particular group before fatigue testing, with a much higher standard deviation, though. The survival rate analysis after mechanical fatigue testing for various surface treatments and (pre)testing are presented in Fig. 3. In relation to the as sintered control
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Fig. 2 Mean biaxial flexural strength values and SD after monotonic loading for assintered and surface-treated Y-TZP ceramics: in air, in artificial saliva, after accelerated ageing in artificial saliva for 2 h at 134 °C and after accelerated ageing in artificial saliva for 24 h at 134 °C
Xm before ageing (%)
Material
Xm after ageing (%) 37 °C/24 h
As sintered
\1
Sandblasted
14.5 ± 2.1
134 °C/2 h
134 °C/24 h
1.5 ± 0.2
5.1 ± 0.4
25.6 ± 4.1
14.0 ± 1.6
14.5 ± 1.4
23.7 ± 2.4
Sandblasted ? annealed 920 °C
1.5 ± 0.5
2.5 ± 0.5
2.9 ± 1.2
9.4 ± 2.6
Ground
4.3 ± 0.8
6.3 ± 1.5
7.2 ± 2.6
24.8 ± 2.1
1600 Air
Art. Saliva 134C 2h
Art. Saliva 37C 24h
Art. Saliva 134C 24h
1400
Mean biaxial flexural strength (MPa)
Table 1 Relative amounts of Xm (monoclinic phase) on the surface of as-sintered and surface treated biomedicalgrade Y-TZP ceramics before and after ageing in artificial saliva
1200
1000
800
600
400
200
0 As sintered
group, any mechanical surface treatment resulted in a substantial difference in the survival rate whereas the influence of (pre)testing conditions used in this work was not found to be equally important. In order to determine whether the noted differences are statistically significant we used Cox proportional hazard regression. According to results listed in Table 3, the effects of mechanical surface treatment are indeed statistically significant, while the effects of accelerated ageing are not. Thus, survival rates of sandblasted samples were significantly higher in comparison to as sintered or ground ceramics tested in air or in artificial saliva. Lower failure rates were found for sandblasted samples compared with as sintered samples (HR: 0.1; 95 % CI: 0.03–0.3), and also for sandblasted samples subsequently annealed at 920 °C compared with as sintered samples (HR: 0.23; 95 % CI: 0.07–0.8). No difference (P [ 0.05) was observed between sandblasted specimens and specimens subsequently annealed at 920 °C. Higher but statistically insignificant
Ground
Sandblasted
Sandblasted + annealed
failure rates were found for all specimens aged in artificial saliva. In an attempt to determine whether there was a critical strength for the onset of fatigue susceptibility for all the specimens all the monotonic strength results were combined, ranked and the probability of failure (Pf = i/n ? 1, where i is the rank of the test result and n is the number of specimens, 120) versus strength was plotted as shown in Fig. 4. The tacit assumption here was that the monotonic strength of the specimens, irrespective as to whether there were residual stresses present or not, provides a basis for the distribution of the ‘‘effective’’ flaws of the tested materials. As such any reduction of strength of the materials as a consequence of the exposure to cyclic fatigue loading provides an indication as to the susceptibility of the original ‘‘effective’’ flaws to have grown during fatigue. Also shown in Fig. 4 are the strengths of the specimens that survived cyclic fatigue testing for 1 million cycles to 620 MPa. These strengths were similarly ranked because
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J Mater Sci: Mater Med Table 2 Fatigue behavior of as-sintered and surface treated Y-TZP ceramics in air and in artificial saliva As sintered (control group)
Dry ground (150 mm)
Sandblasted
Sandblasted ? annealed
Survival ratea (%)
6 (60 %)
2 (20 %)
10 (100 %)
9 (90 %)
Survival strength ? SD (MPa)
1,016 (105)
926 (2)
1,266 (107)
1,037 (50)
Survival ratea (%)
5 (50 %)
1 (10 %)
10 (100 %)
6 (60 %)
Survival strength ? SD (MPa)
886 (98)
940
976 (339)
981 (167)
6 (60 %)
1 (10 %)
8 (80 %)
9 (90 %)
1,038 (68)
816
1,125 (193)
979 (165)
Survival ratea (%)
5 (50 %)
1 (10 %)
8 (80 %)
6 (60 %)
Survival strength ? SD (MPa)
877 (188)
722
1,159 (177)
1,004 (91)
Air
Art. Saliva, 37 °C 24 h
Art. Saliva, 134 °C 2 h Survival ratea (%) Survival strength ? SD (MPa) Art. Saliva, 134 °C 24 h
6
After 10 cycles (50–850 N) at a frequency of 10 Hz
100%
80%
Survival rate
100%
sandblasted
80%
sandblasted+900ºC
as sintered (control)
60%
40%
Survival rate
a
60% artificial saliva (2h, 134ºC)
40%
air (control) artificial saliva (24h, 134ºC)
20%
artificial saliva (24h, 37ºC)
20% ground
0%
0% 10
1
10
2
10
3
10
4
10
5
10
6
number of cycles
10 1
10 2
10 3
10 4
10 5
10 6
number of cycles
Fig. 3 The Cox survival curves after mechanical fatigue testing: left various surface treatments (each line includes together specimens treated in air and aged in artificial saliva), right various (pre)testing conditions (each group includes together as sintered and surface treated specimens)
they were assumed to have the same probability of failure as the initial monotonically loaded specimens. The estimated dc/dN versus K at the max cyclic stress for the samples that fractured during cyclic loading and displayed subsequent monotonic strength reduction is shown in Fig. 5 for all specimens. The form of the results following various treatments show very similar trends with already reported threshold Kmax for crack growth occurring pffiffiffiffi *2–2.5 MPa m, or approximately K1c/2 [23].
4 Discussion The present results clearly show that current dental clinical and laboratory procedures do influence the strength, fatigue survival outcomes and LTD t ? m susceptibility of
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Y-TZP. Grinding appears to have the most deleterious effect on the strength and fatigue failure rate while sandblasting has a substantial strengthening response. The role of accelerated ageing (autoclave exposure) was less pronounced. The LTD induced t ? m transformation, the extent of which varied considerably between different surface conditions, has only a modest (not significant) strengthening effect. The XRD results show major differences between the sandblasted and ground conditions. The latter, although generally acknowledged to be a more severe deformation procedure was less effective in generating the t ? m transformation (Table 1). This difference has also been seen by a number of other authors and debate still continues as to the interpretation of the t/c peak broadening at lower 2 theta and also for the reversal of the 200–002 t
J Mater Sci: Mater Med Table 3 Results of the Cox survival comparisons and hazard rate ratio with 95 % confidence interval
B
SE
Stat. sign. P
-2.381
0.657
\0.001
0.092 (0.025–0.335)
1.223
0.380
0.001
3.397 (1.613–7.151)
-1.457
0.599
0.015
0.233 (0.072–0.753)
\0.001
As sintered (control) Sandblasted Ground Sandblasted ? 920 °C
HR (95 % CI)
Air (control)
0.193
Artificial saliva (24 h, 37 °C)
0.636
0.368
0.084
1.889 (0.919–3.883)
Artificial saliva (2 h, 134 °C)
-0.030
0.389
0.938
0.970 (0.452–2.081)
0.562
0.597
0.346
1.754 (0.545–5.648)
Artificial saliva (24 h, 134 °C)
Fig. 4 A plot of the fracture strength of the monotonic strength results for all Y-TZP specimens versus the probability of failure. Also included are the results of the monotonic strengths for those specimens that survived cyclic loading. Three regions are identified: (i) samples with initial strengths between 620 and 890 MPa that failed during 1 million cycles at 620 MPa, (ii) samples between 890 and 1,000 MPa initial strength that displayed strength reduction, and (iii) samples with initial strengths above 1,000 MPa that did not show strength degradation
peak intensities observed. Some have focused on the broadening and often clear peaks at lower angles following grinding [24, 25] whereas others have suggested that strain induced ferroelastic domain switching has occurred with the reversal of the t peak intensities [26, 27]. Upon annealing the sandblasted specimens at 920 °C there was almost complete m ? t phase reversal and sharpening of the t/c peak but the t reversal at 35 degrees was still present. The influence of autoclave exposure to induce accelerated LTD was most pronounced for the as sintered and least influential for the sandblasted and subsequently annealed specimens. For all other samples there is a substantial increase in m phase after 24 h exposure. For sandblasted and annealed specimens greater resistance to LTD has also been reported by Whalen et al. [28] who have suggested this is related to grain size reduction due to
Fig. 5 Plot of the averaged fatigue crack growth rate (lm/cycle) during stressing at 620 MPa for the specimens that failed and for those that exhibited monotonic strength degradation after surviving 1 million cycles. Previous observations by Chevalier et al. and others indicate that the threshold value for cyclic fatigue is *0.45 K1c suggesting that the Y-TZP material had a fracture toughness of 5.5 MPa m1/2
recrystallization which would have the effect of increasing the stability of the t phase. In addition, as demonstrated by Munoz-Tabares et al. [27] the mechanically induced transformation is accompanied by considerable changes of the internal stresses and reduction of the near surface grain size that hinder the hydrothermally induced transformation. This is evident from slower degradation rate of sandblasted samples. At present we do not know for sure why the ground specimens transformed at a higher rate than the sandblasted ones; it is speculated that this is because during rough grinding cracks have been formed, that opened paths for water penetration, but this explanation is yet to be proven. The monotonic strength results before and after accelerated LTD show no correlations with the m contact measured by XRD. The only clear evidence for the presence of the m phase was for the sandblasted specimens when compared with the subsequent annealed materials. Here nearly a 300 MPa increase in strength is observed which is almost equivalent to the compressive stress generated by the t ? m transformation (r ffi EVf DV) where E is the modulus, Vf the t ? m volume fraction
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transformed and DV is the volume dilatation associated with the t ? m phase change [15]. The effectiveness of this compressive stress will be dependent upon the ratio of the transformed depth to that of the flaws generated [10, 23]. With autoclaving LTD t ? m it may have been anticipated, from the above expression, that this would increase the strength because after 24 h as there is almost a doubling of the m phase. Only for the as sintered and ground specimens when comparison is made between the 24 h/37 °C versus the 24 h autoclave results is there almost a 100 MPa increase in strength. The calculated hazard rate ratio (probability of failing with regard to intact controls) is 10.9 (95 % CI: 3–40) times lower for sandblasted samples, 4.3 (95 % CI: 1.3–13.9) times lower for sandblasted and annealed samples and 3.4 (95 % CI: 1.6–7.2) times higher for ground samples. With regard to testing in air, the hazard is almost twice as high when tested in wet environment, which is in line with the results of Chevalier et al. [23], Morena et al. [29] and Studard et al. [30] who reported on a significantly shorter lifetime for Y-TZP specimens under cyclic loading in water than in air. The additional effect of accelerated ageing, in contrast, is statistically insignificant. The superposition of the combined monotonic strength data plotted versus probability of fracture in Fig. 4 shows a clear monotonic trend. The assumption here was that as the Y-TZP material was the same the various treatments resulted in different effective defect sizes that were the same for the samples monotonically and cyclically loaded. It is clear from Table 2 that the sandblasted specimens dominated the upper portion of this curve and ground surfaces the lower portion. The results for the retained strength following cyclic fatigue also shown in Fig. 4 identify three critical regions. At stresses above 1,000 MPa the strength probability plot is almost identical to that of the monotonic strength indicating that no extension of the defect population occurred during cyclic stressing. However for specimens, which initially would have had strengths between 890 and 1,000 MPa, there was a gradual change of the slope from the original distribution indicating that the size of the largest defects in these materials had increased during cyclic loading resulting in the observed lower strength. For specimens that initially had a monotonic strength of 890 MPa or less sufficient crack extension occurred during cyclic stressing to extend the defect above a critical size such that at 620 MPa the crack tip stress intensity factor exceeded the K1c. An alternate approach to view the above data is shown in Fig. 6 in which the results presented in Fig. 4 are reploted as the probability of survival (Ps = 1 - Pf) versus the cyclic fatigue stress divided by the monotonic strength. Superimposed upon this normalized strength distribution curve, as horizontal lines, are the results for various surface treatments. The width of these lines represents the range of
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strength results for two standard deviations and the position was determined by the mean strength of the specific treatment as listed in Table 2. The choice of 2 SDs was chosen as only 2 % of the specimens would fall outside this range. Also shown, as the vertical line, is the normalized strength (890 MPa) below which fracture was seen to occur during cyclic loading at 620 MPa as shown in Fig. 4. If the horizontal line representing the range of strengths for the various treatments crossed the vertical line then the number of specimens likely to fail during fatigue testing may be estimated from the known normal distribution function. The plotted results suggest that most of the ground samples would have failed during fatigue testing as observed whereas the sandblasted samples would have survived. The results also show a good correlation with the Cox survival data shown in Fig. 3. That is the further to the right the data for a specific surface treatment is in Fig. 6 of the vertical line the fewer the cycles resulting in fracture and in a greater failure rate in the Cox survival plots. The results shown in Fig. 4 clearly indicate that cyclic induced sub-critical fatigue crack growth occurs and compliment the results estimated for the crack growth rates shown in Fig. 5. The calculated averaged dc/dN results from the fatigue tests shown in Fig. 5 do agree reasonably well with the results of direct cyclic crack extension studies by Chevalier et al. [23]. The present analysis is based upon simple assumptions namely that the flaw size distributions for the monotonic strength tests are similar to the fatigue results. Invariably there will be slight differences however the reasonable overlap between the differently treated surfaces for the two conditions (as sintered and ground) along with the comparable outcomes to that by Chevalier et al. [23] and Studart et al. [30] does generate some support for the approach, particularly in that cyclic fatigue crack growth becomes important at Kmax values approaching half the K1c value of the material. From the results shown in Fig. 4 there is a critical strength identified (1,000 MPa) above which there is no indication of strength reduction as a consequence of cyclic fatigue. That is for these specimens the effective flaw size is such that the stress intensity factor at the maximum cyclic stress (620 MPa) was less than Kth for cyclic fatigue crack growth. From the literature [23, 30] this threshold value is *0.45 K1c. For specimens that had monotonic strength results between 890 and 1,000 MPa there was a clear indication that subcritical crack growth during the 1 million cycles occurred with the extent of the degradation being greater as the monotonic strengths approached 890 MPa. This increase in defect size upon cyclic loading was also identified in the fatigue crack growth results shown in Fig. 5. Chevalier et al. [23] identified two regimes of the cyclic fatigue crack growth rate response, in the range close to the threshold there is a very steep response but above this the slope is significantly reduced.
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Probablity of Survival versus Normalised Stress Strength distribution Critical stress 890 MPa
Probability of Survival
Fig. 6 A plot of the probability of survival of the initial (ranked) monotonic strength data for all Y-TZP specimens normalized by the maximum fatigue stress (620 MPa). The vertical broken line is the critical initial monotonic strength (890 MPa) below which fracture occurred during fatigue cycling. The short horizontal lines represent 2 standard deviations from the mean for the different specimens. If these horizontal lines cross the vertical line then fracture during fatigue testing is anticipated
0.8 AS Dry ArtSaliva 37C
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AS Art Saliva 2/134C 0.4
AS Art Saliva 24 134C Ground Dry
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Ground Art Salica 37C Ground Art Saliva 2h 134C
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5 Conclusions The present investigation has shown that a Y-TZP dental ceramic undergoes strength degradation upon surface damage and cyclic fatigue loading. The extent of the strength degradation was greatest for ground specimens while sandblasting increased the strength. Autoclave exposure to enhance low temperature tetragonal to monoclinic transformation was also found to be surface treatment dependent with as-sintered samples having a faster rate than ground samples and sandblasted samples having the slowest rate of transformation. It was also found that LTD had a minimal effect on the strength and fatigue reliability outcomes. Hazard and probability of fracture analysis of the results showed that the initial monotonic strength provided a very good basis to identify whether failure would occur during subsequent cyclic stressing. The number of cycles to failure and the retained strength after survival of cyclic loading enabled an averaged cyclic crack growth rate versus cyclic stress intensity factor to be determined which agreed well with results from more sophisticated methods available in the literature. The approach adopted should have relevance for predicting the lifetime of more complex dental crowns and bridges subjected to cyclic stressing.
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5. 6. 7.
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References 14. 1. Rekow D, Thompson VP. Engineering long term clinical success of advanced ceramic prostheses. J Mater Sci Mater Med. 2007;18:47–56. 2. Sailer I, Pjetursson BE, Zwahlen M, Ha¨mmerle CHF. A systematic review of the survival and complication rates of allceramic reconstructions after an observation period of at least
15. 16.
3 years. Part II: fixed dental prostheses. Clin Oral Impl Res. 2007;18:86–96. Pjetursson BE, Sailer I, Zwahlen M, Ha¨mmerle CHF. A systematic review of the survival and complication rates of all-ceramic reconstructions after an observation period of at least 3 years. Part I: single crowns. Clin Oral Impl Res. 2007;18:73–85. Raigrodski AJ, Hillstead MB, Meng GK, Chung KH. Survival and complications of zirconia-based fixed dental prostheses: a systematic review. J Prosthet Dent. 2012;107:170–7. Denry I, Kelly JR. State of the art of zirconia for dental applications. Dent Mater. 2008;24:299–307. Kern M. Bond strength of luting cements to zirconium oxide ceramics. Int J Prosthodont. 2000;13:30–5. Re D, Augusti D, Sailer I, Spreafico D, Cerutti A. The effect of surface treatment on the adhesion of resin cements to Y-TZP. Eur J Esthet Dent. 2008;3:186–96. Kosmacˇ T, Oblak Cˇ, Jevnikar P, Funduk N, Marion L. The effect of grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater. 1999;15:426–33. Kosmacˇ T, Oblak Cˇ, Jevnikar P, Funduk N, Marion L. Strength and reliability of surface treated Y-TZP dental ceramics. J Biomed Mater Res. 2000;53:304–13. Xu H, Jahanmir S, Ives LK. Effect of grinding on strength of tetragonal zirconia and zirconia-toughened alumina. Mach Sci Technol. 1997;1:49–66. Sato H, Yamada K, Pezzotti G, Nawa M, Ban S. Mechanical properties of dental zirconia ceramics changed with sandblasting and heat treatment. Dent Mater J. 2008;27:408–14. Scherrer SS, Cattani-Lorente M, Vittecoq E, Mestral F, Griggs JA, Wiskott HW. Fatigue behavior in water of Y-TZP zirconia ceramics after abrasion with 30 lm silica-coated alumina particles. Dent Mater. 2011;27:28–42. Kobayashi K, Kuwajima H, Masaki T. Phase change and mechanical properties of ZrO2-Y2O3 solid state electrolyte after ageing. Solid State Ion. 1981;3:489–95. Chevalier J, Gremillard L, Deville S. Low temperature degradation and implications for biomedical implants. Annu Rev Mater Res. 2007;37:1–32. Swain MV. Impact of oral fluids on dental ceramics: what is the clinical relevance? Dent Mater. 2014;30:33–42. Raigrodski AJ, Yu A, Chiche GJ, Hochstedler JL, Mancl LA, Mohamed SE. Clinical efficacy of veneered zirconium dioxide-
123
J Mater Sci: Mater Med
17.
18. 19. 20.
21. 22. 23.
24.
based posterior partial fixed dental prostheses: five-year results. J Prosthet Dent. 2012;108:214–22. Pela´ez J, Cogolludo PG, Serrano B, Lozano JF, Sua´rez MJ. A prospective evaluation of zirconia posterior fixed dental prostheses: three-year clinical results. J Prosthet Dent. 2012;107:373–9. Scott HG. Phase relationships in the zirconia-yttria system. J Mat Sci. 1975;10:1527–35. Arvidson K, Johansson EG. Galvanic current between dental alloys in vitro. Scand J Dent Res. 1985;93:467–73. Keuper M, Berthold C, Nickel KG. Long-time aging in 3 mol.% yttria-stabilized tetragonal zirconia polycrystals at human body temperature. Acta Biomater. 2014;10:951–9. Garvie RC, Nicholson PS. Phase analysis in zirconia systems. J Am Ceram Soc. 1972;55:303–5. Wachtman JB, Capps W, Mandel J. Biaxial flexure tests of ceramic substrates. J Mater. 1972;7:188–94. Chevalier J, Olagnon C, Fantozzi G. Subcritical crack propagation in 3Y-TZP ceramics: static and cyclic fatigue. J Am Ceram Soc. 1999;82:3129–38. Kitano Y, Mori Y, Ishitani A, and Masaki T, A study of rhombohedral phase in Y2O3-partially stabilized zirconia. In:
123
25.
26.
27.
28.
29.
30.
Materials Research Society symposia proceedings, vol. 78. 1987. p. 17–24 Denry IL, Holloway JA. Microstructural and crystallographic surface changes after grinding zirconia-based dental ceramics. J Biomed Mater Res B. 2006;76:440–8. Virkar AV, Matsumoto RLK. Ferroelastic domain switching as a toughening mechanism in tetragonal zirconia. J Am Ceram Soc. 1986;69:C224–6. Munoz-Tabares JA, Jimenez-Pique E, Reyes-Gasga J, Anglada M. Microstructural changes in ground 3Y-TZP and their effect on mechanical properties. Acta Mater. 2011;59:6670–83. Whalen PJ, Reidinger F, Antrim RF. Prevention of low-temperature surface transformation by surface recrystallization in yttriadoped tetragonal zirconia. J Am Ceram Soc. 1989;72:319–21. Morena M, Beaudreau GM, Lockwood PE, Evans AL. Fatigue of dental ceramics in a simulated oral environment. J Dent Res. 1986;65:993–7. Studart AR, Filser F, Kochler P, Gauckler LJ. Fatigue of zirconia under cyclic loading in water and its implications for the design of dental bridges. Dent Mater. 2007;23:106–14.
