Effect of Surface and Heat Treatments on the Biaxial Flexural Strength and Phase Transformation of a Y-TZP Ceramic Renata Garcia Fonsecaa / Filipe de Oliveira Abi-Rachedb / Filipe Samuel Correia Pereira da Silvac / Bruno Alexandre Pacheco de Castro Henriquesd / Ligia Antunes Pereira Pinellie Purpose: To evaluate the effect of grinding and airborne-particle abrasion on the biaxial flexural strength (BFS) and phase transformation of a Y-TZP ceramic, and examine whether sintering the veneering porcelain renders the previous heat treatment recommended by the manufacturer unnecessary. Materials and Methods: Lava zirconia specimens (N = 108) were obtained with the following dimensions: 14.0 mm diameter × 1.3 mm thickness (n = 36) and 14.0 mm × 1.6 mm (n = 72). The thicker specimens were ground with diamond burs under irrigation and received (heat-treated groups) or not (non-heat–treated groups) a heat treatment (1000ºC for 30 min) prior to the four firing cycles applied to simulate the sintering of the veneering porcelain. All specimens were air abraded as follows (n = 12): 1) 30-μm silica-modified Al2O3 particles (Rocatec Soft); 2) 110-μm silica-modified Al2O3 particles (Rocatec Plus); and 3) 120-μm Al2O3 particles, followed by Rocatec Plus. Three specimens of each group were analyzed by x-ray diffraction (XRD) to assess the monoclinic phase content (%). The BFS test was performed in a mechanical testing machine (Instron 8874). Data (MPa) were analyzed by two-way ANOVA (grinding × airborne-particle abrasion and heat treatment × airborne-particle abrasion) and Tukey’s post-hoc test (α = 0.05). The strength reliability was analyzed using the Weibull distribution. Results: Grinding significantly decreased the BFS of the non-heat–treated groups (p < 0.01). Within the ground groups, the previous heat treatment did not influence the BFS (p > 0.05). Air abrasion only influenced the BFS of the ground/heat-treated groups (p < 0.01). For the non-heat–treated groups, the grinding did not decrease the Weibull modulus (m), but it did decrease the characteristic strength (σ0). For Rocatec Soft and 120-μm Al2O3 particles + Rocatec Plus, the heat-treated groups presented lower m and higher σ0 than the ground/non-heat–treated groups. The independent variables did not seem to influence phase transformation. Air-abraded surfaces presented higher monoclinic zirconia content than the as-sintered and ground surfaces, which exhibited similar content. Conclusion: Even under irrigation, grinding compromised the Y-TZP ceramic strength. The sintering of the veneering porcelain rendered the previous heat treatment recommended by the manufacturer unnecessary. Airborneparticle abrasion influenced the strength of heat-treated zirconia. Keywords: zirconia, airborne-particle abrasion, grinding, heat treatment, biaxial flexural strength. J Adhes Dent 2014; 16: 451–458. doi: 10.3290/j.jad.a32663

Submitted for publication: 29.01.13; accepted for publication: 17.06.14

Y

-TZP ceramic frameworks for fixed dental and implant-supported prostheses have been extensively used due to their high fracture toughness25,29 and flexural strength.12,17,25 However, these frameworks

may undergo grinding with diamond burs1,11,15 and/ or airborne-particle abrasion.12,26,30 Both grinding (performed on the outer surface of the zirconia framework when there is not enough space for the veneering porce-

a

Associate Professor, Department of Dental Materials and Prosthodontics, Araraquara Dental School, UNESP – Univ Estadual Paulista, Araraquara, São Paulo, Brazil. Idea, wrote manuscript.

d

PhD Student, Department of Mechanical Engineering, School of Engineering, Minho University, Guimarães, Portugal. Performed the biaxial flexural strength test and Weibull analysis.

b

Adjunct Professor, Department of Dental Materials and Prosthodontics, Araraquara Dental School, UNESP – Univ Estadual Paulista, Araraquara, São Paulo, Brazil. Adjusted the method, performed the experiments, contributed to discussion, proofread the manuscript.

e

Assistant Professor, Department of Dental Materials and Prosthodontics, Araraquara Dental School, UNESP – Univ Estadual Paulista, Araraquara, São Paulo, Brazil. Adjusted the method and supervised its development.

c

Associate Professor, Department of Mechanical Engineering, School of Engineering, Minho University, Guimarães, Portugal. Experimental design, statistical analysis.

Correspondence: Profa. Dra. Renata Garcia Fonseca, Departamento de Materiais Odontológicos e Prótese, Faculdade de Odontologia de Araraquara – UNESP, Rua Humaitá, no. 1680, Araraquara, São Paulo, Brazil 14801-903. Tel: +55-16-3301-6426, Fax: +55-16-3301-6406. e-mail: [email protected]

Vol 16, No 5, 2014

451

Fonseca et al

sion techniques on the biaxial flexural strength (BFS) and phase transformation of a commercially available Y-TZP ceramic, as well as to investigate whether sintering of the veneering porcelain renders the heat treatment recommended by the manufacturer unnecessary in terms of BFS and phase transformation. The null hypothesis was that grinding, airborne-particle abrasion, and the heat treatment do not influence the two dependent variables.

SINTERED Y-TZP DISKS

non-ground (as-sintered) n = 36

ground n = 72

no heat treatment n = 36

heat treatment n = 36

veneering porcelain firing cycles

Rocatec Soft n = 12

Fig 1

Rocatec Plus n = 12

120 μm Al2O3 + Rocatec n = 12

Experimental procedures for preparation of Y-TZP disks.

lain and/or on its intaglio surface to improve the fitting of the restoration11) and airborne-particle abrasion (one of the most effective methods performed on the inner surface of the zirconia framework to improve bonding with luting cements3,5,6,31) can be performed according to different protocols, which may determine the severity of these procedures.11,21 Consequently, the behavior of the surface flaws created on the zirconia by grinding and airborne-particle abrasion is unpredictable, resulting in an increase in its strength by the transformation toughening phenomenon11,15,19 or a decrease as a consequence of crack initiation and propagation towards the bulk of the material.1,12,16 When these procedures are severe (coarse diamond burs and particles), a decrease in strength of the zirconia occurs due to: a) an excessive transformation from tetragonal to monoclinic phase26 or b) the reverse phase transformation (when the local temperature exceeds the monoclinic to tetragonal transformation temperature) with the ensuing release of the compressive stresses.8,11,19 In order to reduce the damage resulting from grinding and airborne-particle abrasion, heat treatments performed prior to firing the veneering porcelain are recommended by some Y-TZP dental ceramic manufacturers; however, its efficacy has not been sufficiently proven in the literature.23,26 Moreover, the specific thermal cycles applied during the sintering of the veneering porcelain can in fact perform the function of the heat treatment proposed by the manufacturer. Thus, the purpose of this in vitro study was to evaluate the influence of grinding and airborne-particle abra452

MATERIALS AND METHODS One hundred eight disk-shaped zirconia specimens were obtained from pre-sintered Lava Y-TZP blocks (3M ESPE; Seefeld, Germany), which were cut in a diamond saw (Isomet 1000, Buehler; Lake Bluff, IL, USA) under wet conditions. The specimens were washed in tap water to remove the smear resulting from cutting and were then finished manually using a ceramic polisher (Exa Cerapol 0361HP, Edenta; Au, SG, Switzerland) in a slow-speed handpiece with water cooling. All specimens were sintered according to the manufacturer’s protocol in a special oven (Lava Furnace 200, Dekema Dental-Keramiköfen; Freilassing, Germany). The final dimensions of the specimens were as follows: 14.0 mm diameter × 1.3 mm thickness (n = 36) and 14.0 mm diameter × 1.6 mm thickness (n = 72). The thicker specimens were positioned in a matrix with a circular central hole (14.0 mm diameter  ×  1.3 mm thickness) and the top surface (0.3 mm excess of zirconia) was ground with 125-μm-grit diamond burs (ZR6881, Komet-Brasseler; Lemgo, Germany) in a high-speed handpiece under constant irrigation until achieving the final thickness of 1.3 mm, which was confirmed with a digital caliper at three different points. The ground specimens received (heat-treated groups) or did not receive (non-heat– treated groups) heat treatment (1000ºC for 30 min) in a porcelain oven (Aluminipress, EDG Equipamentos e Controles; São Carlos, SP, Brazil) as similarly recommended by the Lava manufacturer (1000ºC for 20 min). Next, all zirconia disks were subjected to four firing cycles (“wash”: 950ºC, “dentin 1”: 910ºC, “dentin 2”: 900ºC, and “glaze”: 900ºC) in the Aluminipress porcelain oven as recommended by the manufacturer of VM9 zirconia veneering porcelain (Vita Zahnfabrik; Bad Säckingen, Germany). This step was performed in order to simulate the effect of the veneering porcelain application. Afterwards, the tensile surface of all specimens was subjected to one of the following airborne-particle abrasion techniques (n = 12): 1) 30-μm silica-modified Al2O3 particles (Rocatec Soft); 2) 110-μm silica-modified Al2O3 particles (Rocatec Plus); and 3) 120-μm Al2O3 particles, followed by Rocatec Plus (Fig 1). Airborne-particle abrasion was performed for 20 s with an air-abrasion unit (Basic Classic, Renfert; Hilzingen, Germany). The specimens were air abraded at a pressure of 0.28 MPa and a perpendicular distance of 10 mm from the zirconia surface,2 standardized by a special holder. In the ground specimens, this air abrasion was performed on the surface opposite to that of the grinding technique. The Journal of Adhesive Dentistry

Fonseca et al

Table 1 Two-way ANOVA (grinding × airborne-particle abrasion) Source of variation

SS

df

MS

Grinding

56638.0

1

56638.0

11.4 < 0.01

Abrasion

22634.6

2

11317.3

2.3 > 0.05

9233.8

2

4616.9

.9 > 0.05

327225.0

66

4958.0

76501116.0

72

Grinding × abrasion Residual Total

F

p

Before determining the biaxial flexural strength, three specimens of each group were subjected to x-ray diffraction (XRD) analysis to assess the effect of grinding, heat treatment, and airborne-particle abrasion on the phase transformation of zirconia. This analysis was performed with an x-ray powder diffractometer (D8 Advance; Bruker Axs; Cheshire, UK) with graphite-monochromated Cu Kα radiation (λ = 1.5406 Å, generator set at 40 kV and 40 mA). Scans were performed from 10 degrees to 100 degrees (2θ) at a step size of 0.02 degrees with a measuring time of 6 s per step. The monoclinic phase fraction (%) was calculated using the Garvie-Nicholson method.10 The biaxial flexural strength test (ISO 6872 standard13) was conducted at room temperature in a mechanical testing machine (Instron 8874; Canton, MA, USA). For the test, the zirconia disks were placed over three stainless steel balls (3.0 mm diameter) 120 degrees apart on a support circle (11.0 mm diameter).24 A flat punch (1.5 mm diameter × 1.0 mm length) running at a crosshead speed of 0.5 mm/min directed a uniaxial compressive load to the as-sintered and ground surfaces, while the air-abraded surfaces were subjected to the tensile force until failure. The force (N) required for the failure of each specimen was recorded and the biaxial flexural strength values (MPa) were calculated using the equations recommended by the ISO 6872 standard.13 The biaxial flexural strength data (MPa) were analyzed using two-way ANOVA (grinding  ×  airborne-particle abrasion and heat treatment  ×  airborne-particle abrasion), followed by Tukey’s Honestly Significant Difference (HSD) post-hoc test at a significance level of α = 0.05. The software used was SPSS 19.0 (SPSS; Chicago, IL, USA). The Weibull analysis of the biaxial flexural strength values determined both the Weibull modulus (m) and the characteristic strength (σ0) of each group. The Weibull distribution is given by the following formula: P = 1 – exp [(– σ/σ0)m] where P is the probability of failure, σ is the biaxial flexural strength, σ0 is the characteristic strength at the fracture probability of 63.2%, and m is the Weibull modulus. Vol 16, No 5, 2014

Table 2 Mean (SD) biaxial flexural strength values in MPa of the non-heat–treated groups Rocatec Soft

Rocatec Plus

120-μm Al2O3 Total + Rocatec Plus

Nonground

1076.0 (53.8)

1072.8 (68.9)

1019.3 (83.2)

1056.0 (72.6)a

Ground

990.5 (55.3)

1020.4 (90.4)

988.9 (63.0)

999.9 (70.6)b

Total

1033.2 (68.9)A

1046.6 (83.0)A

1004.1 (73.8)A

Different uppercase superscript letters indicate significant differences among airborne-particle abrasion techniques (p < 0.05). Different lowercase superscript letters indicate significant differences between nonground and ground conditions (p < 0.05).

Table 3 Two-way ANOVA (heat treatment × airborneparticle abrasion) Source of variation

SS

df

MS

F

p

Heat treatment

7588.3

1

7588.3

1.0

> 0.05

Abrasion

33971.9

2

16986.0

2.3

> 0.05

Heat treatment × abrasion

87796.2

2

43898.1

6.0

< 0.01

482003.2

66

7303.1

74087408.0

72

Residual Total

RESULTS The results from the two-way ANOVA (grinding  ×  airborne-particle abrasion) (Table 1) revealed that only grinding affected the biaxial flexural strength (p < 0.01). Table 2 shows the mean biaxial flexural strength values (MPa), standard deviations, and statistical analysis results of the non-heat–treated groups (ground or not). Regardless of performing the grinding or not, there were no significant differences among the airborne-particle abrasion techniques. Grinding significantly decreased the BFS of the non-heat–treated groups (p < 0.01). The results of the two-way ANOVA (heat treatment × airborne-particle abrasion) (Table 3) revealed that only the interaction between these independent variables was significant (p < 0.01). Table 4 shows the mean biaxial flexural strength values (MPa), standard deviations, and statistical analysis results of the ground groups (heat treated or not) according to Tukey’s HSD test. Within the non-heat–treated groups, there was no significant difference among the airborne-particle abrasion techniques. However, for the heat-treated groups, the abrasion with 453

Fonseca et al

Table 4

Mean (SD) biaxial flexural strength values (MPa) of the ground groups Rocatec Soft

Rocatec Plus

120-μm Al2O3 + Rocatec Plus

No heat treatment

990.4 (55.3)AB

1020.4 (90.4)AB

988.9 (63.0)AB

Heat treatment

1025.7 (112.1)AB

949.0 (91.0)B

1086.7 (88.3)A

Different uppercase superscript letters indicate significant differences among airborne-particle abrasion techniques (p < 0.05).

Table 5

Mean (± SD) and coefficient of variation (CV, %) of biaxial flexural strength (σ, MPa) values.

Experimental group

n

σ (SD)

CV (%)

m

95% CI (m)

σ0 (MPa)

95% CI (σ0)

Non-ground Rocatec Soft

12

1076.0 (53.8)

5.0

24.0

21.5 – 26.5

1100

1093 – 1106

Non-ground Rocatec Plus

12

1072.8 (68.9)

6.4

18.2

15.8 – 20.6

1103

1094 – 1112

Non-ground 120-μm Al2O3 + Rocatec Plus

12

1019.3 (83.2)

9.3

14.6

12.9 – 16.2

1056

1045 – 1067

Ground Rocatec Soft

12

990.5 (55.3)

5.6

21.3

17.5 – 25.0

1014

1003 – 1025

Ground Rocatec Plus

12

1020.4 (90.4)

8.9

13.1

11.9 – 14.4

1061

1053 – 1068

Ground 120 μm Al2O3 + Rocatec Plus

12

988.9 (63.0)

6.4

18.8

17.1 – 20.5

1017

1010 – 1023

Ground/ heat-treated Rocatec Soft

12

1025.7 (112.1)

10.9

9.6

7.0 – 12.2

1073

1047 – 1100

Ground/ heat-treated Rocatec Plus

12

949.0 (91.0)

9.6

12.3

9.4 – 15.1

988

963 – 1011

Ground/ heat-treated 120 μm Al2O3 + Rocatec Plus

12

1086.7 (88.3)

8.1

14.7

12.1 – 17.3

1125

1106 – 1143

Weibull’s modulus (m), Weibull’s characteristic strength (σ0, MPa), and corresponding confidence intervals (CI) at 95% confidence levels.

Table 6 Relative amount (%) of monoclinic zirconia on the different conditions Non-ground

Ground No heating

Heating

454

As-sintered Rocatec Soft

2.5 17.5

As-sintered Rocatec Plus

1.7 17.5

As-sintered 120-μm Al2O3 + Rocatec Plus

2.2 17.0

Grinding + no heating Rocatec Soft

2.0 22.9

Grinding + no heating Rocatec Plus

2.2 17.9

Grinding + no heating 120-μm Al2O3 + Rocatec Plus

0.9 20.0

Grinding + heating Rocatec Soft

0.5 23.3

Grinding + heating Rocatec Plus

3.4 17.0

Grinding + heating 120-μm Al2O3 + Rocatec Plus

2.1 17.0

120 μm followed by Rocatec Plus promoted higher BFS than Rocatec Plus alone (p < 0.01). Regardless of the airborne-particle abrasion techniques, the heat treatment did not influence the BFS; that is, the firing cycles used to simulate the sintering of the veneering porcelain made the previous heat treatment unnecessary. The results of the Weibull statistical analysis are presented in Table 5 and Fig 2. For the non-heat–treated groups, grinding did not decrease the Weibull modulus (m), although it did decrease the characteristic strength (σ0). For Rocatec Soft and 120-μm Al2O3 particles + Rocatec Plus, the heattreated groups presented a lower Weibull modulus and higher characteristic strength than did the the ground/ non-heat–treated groups. Table 6 shows the relative amount (%) of the monoclinic phase and Fig 3 presents the representative XRD pattern of zirconia surface after each experimental treatment. The independent variables grinding, heat treatment, and airborne-particle abrasion do not seem to influence the zirconia phase transformation. Airborne-particle abrasion provided higher percentages of monoclinic zirconia than the as-sintered and ground surfaces, which exhibited similar percentages of monoclinic phase content. The Journal of Adhesive Dentistry

Fonseca et al

95

Probability of fracture, Pf (%)

90 80 70 63.2% 60 50 40 30 20 10 Non-ground Ground Ground/Heat-treated

5 2 1 600

800

1000

1200

1400

1600

Fracture stress (MPa)

a

95

Probability of fracture, Pf (%)

90 80 70 63.2% 60 50 40 30 20 10 Non-ground Ground Ground/Heat-treated

5 2 1 600

800

1000

1200

1400

1600

Fracture stress (MPa)

b

95

Probability of fracture, Pf (%)

90

Fig 2 Weibull plots of fracture data for the airborne-particle abrasion techniques. a: Rocatec Soft; b: Rocatec Plus; c: 120-μm Al2O3 + Rocatec Plus.

Vol 16, No 5, 2014

80 70 63.2% 60 50 40 30 20 10 5 2 1 600

c

Non-ground Ground Ground/Heat-treated 800

1000

1200

1400

1600

Fracture stress (MPa)

455

Fonseca et al t 500

t

t t

Relative Intensity (a.u.)

3000

m

250

t

0 2000

27

32

37

t – ZrO2 (tetragonal) t

1000

m – ZrO2 (monoclinic) tt

t

t

m

t

tt

t

tt

as-sintered air-abraded (Rocatec Soft) air-abraded (Rocatec Plus) air-abraded (120 μm Al2O3 particle + Rocatec Soft) ground/non heat-treated ground/heat-treated

0 40

60

80

100

2θ (degree)

DISCUSSION The results of this study found no evidence to support the null hypothesis, since both grinding (p < 0.01) and the interaction between heat treatment and airborneparticle abrasion (p < 0.01) affected the biaxial flexural strength. The mean BFS values (MPa) of all groups and the monoclinic phase content (%) of the as-sintered surface obtained in the present study are in agreement with the results reported by Borchers et al.4 These authors evaluated the Lava zirconia and found BFS mean values varying from 942 to 1054 MPa and a monoclinic phase content of 2% for the non-treated group. Kosmac et al19 reported a transformation zone depth of 0.3 μm when zirconia was abraded with 110-μm Al2O3 particles, while Sato et al26 found a value lower than 10.0 μm for the abrasion with 70-μm Al2O3 particles or 125-μm SiC powder. These values are 4300 or 130 times thinner than the thickness (1.3 mm) of the specimens used in the present study. Based on this finding, we can assume that in our study, the non-ground/non-heat–treated/non-abraded surface can be considered an as-sintered one in terms of the phase transformation. In the non-heat–treated groups, ground or not, the airborne-particle abrasion techniques resulted in statistically similar BFS and provided very similar percentages of tetragonal to monoclinic phase transformation (17.5%, 17.5%, and 17.0% in the non-ground groups and 22.9%, 17.9%, and 20.0% in the ground/non-heat–treated groups), which were considerably higher than the opposite surfaces (2.5%, 1.7% and 2.2% for the as-sintered surface and 2.0%, 2.2%, and 0.9% for the ground/nonheat–treated surface). This statistical similarity of BFS and the slight variation of monoclinic phase percentage found for the three airborne-particle abrasion techniques are surprising in view of the variability of the particle size and the number of steps 456

Fig 3 Representative XRD pattern obtained from zirconia surface after each experimental treatment.

involved in this procedure. Since the particles of Rocatec Soft (30-μm silica-modified alumina ones) are smaller than the other ones used in this study (110 μm /120 μm), it would be expected that the former would provide lower monoclinic content than the latter. Fonseca et al9 reported a lower monoclinic phase content after air abrasion with Cojet Sand (30-μm silica-modified alumina particles) than with Rocatec Plus (110 μm) and 120 μm/Rocatec Plus, which exhibited no significant difference between them. Among the studies9,22,26-28 that evaluated the effect of the particle used for air abrasion on phase transformation, only that by Yamaguchi et al32 is in accordance with the results of the current study, since they also observed no significant difference between 30-μm and 110-μm silicamodified alumina particles. However, those authors also did not find an explanation for this phenomenon. Regarding the number of steps involved in air abrasion, for the Rocatec Soft (30 μm) and Rocatec Plus (110 μm) groups, this procedure was performed once, while for the 120 μm/ Rocatec Plus groups (120 μm + 110 μm), it was performed twice. However, Curtis et al7 also found no significant difference in BFS when the Lava zirconia was abraded with 25-, 50- and 110-μm Al2O3 particles, reporting that the airborneparticle abraded groups exhibited higher percentages of phase transformation when compared with the as-sintered group. On the other hand, in contrast to the present study, those authors7 observed that by increasing the average alumina particle size (25, 50, and 110 μm), the amount of transformed phase (17%, 22%, and 28%, respectively) also increased. In the study by Sato et al,26 there was also no significant BFS difference between abrasion with 70-μm Al2O3 particles and 125-μm SiC powder, and the percent monoclinic zirconia content after abrasion with 125-μm SiC powder (approx. 10%) was higher than that obtained with 70-μm Al2O3 particles (approx. 4%). The present study and those previously cited7,26 indicate that, regardless of the differences observed in the monoclinic percentage values and in the behavior of zirconia The Journal of Adhesive Dentistry

Fonseca et al

regarding phase transformation, the particle size used in airborne-particle abrasion did not influence the BFS of this material. This result may be explained by the fact that the airborne-particle abrasion did not produce enough compressive stress to increase the strength and also by the fact that the microcracks resulting from the airborne-particle abrasion did not penetrate deep enough into the bulk of the zirconia to significantly affect its strength. Borchers et al4 estimated that a crack length of 11 μm is required to decrease the strength of the zirconia. Sato et al26 reported a transformation zone depth less than 10 μm after abrasion with 70-μm Al2O3 particles or 125-μm SiC powder, while Zhang et al33 observed damage 4 μm below the surface after airborne-particle abrasion with 50-μm Al2O3 particles. In the study by Kim et al,16 abrasion with this particle size (50 μm) and grinding with 120-grit (approx. 162 μm) diamond disks created subsurface cracks 2 to 4 μm below the surface, which propagated laterally. Although in the present study the transformation zone depth was not determined, the response of the zirconia concerning its biaxial strength under the three airborne-particle abrasion techniques suggests that the crack depth resulting from this procedure was not sufficient to affect the BFS. However, this finding does not indicate that the airborne-particle abrasion can be indiscriminately performed without compromising the zirconia strength. This may be true for the immediate BFS, but the long-term flexural strength of zirconia must be taken into consideration, since mechanical stress12 and moisture18 may accelerate the propagation of microcracks, which could affect the strength of the zirconia. In this study, it was observed that grinding significantly decreased the BFS of the non-heat–treated groups and provided percentages of monoclinic zirconia similar to and lower than those observed on the as-sintered and airborneparticle abraded surfaces, respectively. Kosmac et al19,20 also reported that grinding decreased the BFS of a Y-TZP zirconia and that the airborne-particle abraded groups (110μm Al2O3 particles) promoted higher percentages of monoclinic zirconia (13.9% to 15.2%) than those observed after grinding with 150-μm–grit diamond burs (3.4% to 4.2%). According to these authors,19 due to the high stresses developed during grinding, surface cracks must have formed, which lowered the strength of the material. Işeri et al14 commented that surface flaws act as stress concentrators if their length largely exceeds the depth of the surface compressive layer induced by grinding. The decrease not only in the BFS but also in the characteristic strength (σ0) of the non-heat–treated groups observed in the present study after grinding is possibly the result of the diamond bur friction on the zirconia surface, creating the microcracks reported by Kosmac et al19 which propagate towards the bulk of the material. Regarding the Weibull modulus (m), although it did not change statistically for Rocatec Soft and Rocatec Plus after grinding, there was a tendency to decrease, indicating that this procedure tends to reduce the reliability of the material, as observed in some related studies.19,20,30 Concerning the phase transformation, Kosmac et al19 believe that the lower percentages of monoclinic zirconia found after grinding vs airborne-particle abrasion may be due to the reverse transformation (monoclinic to Vol 16, No 5, 2014

tetragonal) initiated by the locally high temperatures developed during grinding. The reverse phase transformation releases the compressive stresses, also contributing to decreasing BFS of the zirconia.19 When comparing their results with those obtained by Kosmac et al,19 Guazzato et al11 stated that unlike the study of Kosmac et al,19 they observed no reverse phase transformation, possibly due to their use of a less aggressive grinding configuration (developing comparatively lower temperatures) than that adopted by Kosmac et al.19 As observed in the study by Guazzato et al11 and based on the same argument presented by these authors, there was also no reverse phase change observed after grinding in the present study. Therefore, in this study, the significant BFS decrease observed after grinding may not be related to the release of the compressive stresses, but rather to the damage caused by extensive erosive wear on the zirconia surface. In the present study, heat treatment (1000ºC for 30  min) was performed before sintering the veneering porcelain as an attempt to revert the possible damage caused by grinding, which was confirmed by the significant BFS decrease. Wang et al30 observed that the sintering procedure might have a “healing” effect on the surface damage caused by the grinding procedure. Kim et al16 reported that the sintering process partially healed microcracks and eliminated voids and flaws. In the present study, for the three airborne-particle abrasion techniques, no significant differences were observed between the ground and ground/heat-treated groups, that is, the heat treatment performed before the sintering of the veneering porcelain did not influence the BFS and also appears not to have changed the percentages of phase transformation on either the air-abraded (approx. 19.1%) or the ground (approx. 2.0%) surfaces. This behavior can probably be explained by the fact that the specific thermal cycles applied during the sintering of the veneering porcelain can in fact perform the function of the heat treatment. On the other hand, we did not find an explanation for how the ground/ heat-treated groups (except for the group abraded with Rocatec Plus) presented higher characteristic strength (σ0) and lower reliability than the ground/non-heat–treated groups. Another effect of heat treatment was reported by Guazzato et al,11 who observed that this treatment (first cycle 930ºC, second 910ºC), when performed on the ground (91 μm) or sandblasted (110 μm) specimens, decreased the flexural strength and the monoclinic phase content to negligible percentages. According to these authors,11 heat treatment is accompanied by the reverse mŠt phase transformation, which releases the compressive stresses, thus affecting the strength. Sato et al26 noted that heat treatment performed after sandblasting (500°C to 1200ºC for 5 min) caused a decrease in BFS and in monoclinic zirconia content, which achieved 1% after 1200ºC. Although Doi et al8 also observed the reverse phase transformation (based on the negligible amount of monoclinic phase) after heat treatment (1000ºC for 10 min) of the sandblasted specimens, assuming that the compressive stresses were released, this procedure did not influence flexural strength, as noted in the present study. The reason for the innocuous behavior of BFS and 457

Fonseca et al

phase transformation, as well as for the decrease in the reliability of the ground zirconia observed in this study regarding heat treatment, is not yet known. The fractography analysis might possibly provide complementary information to better understand the behavior of the ground groups submitted or not to the heat treatment as recommended by the manufacturer after grinding. Additional studies for alternative procedures that effectively accomplish this purpose must be conducted. Moreover, studies that evaluate how the zirconia behaves under these same treatments when submitted to thermomechanical aging and moisture, simulating the oral environment over time, are urgently required.

CONCLUSIONS 1. The three airborne-particle abrasion techniques did not influence the biaxial flexural strength of the nonheat–treated groups. However, all of them initiated phase transformation, resulting in an increase in monoclinic phase percentages. 2. Grinding decreased the biaxial flexural strength without promoting phase transformation. 3. Regarding the biaxial flexural strength and phase transformation, the sintering of the veneering porcelain made the heat treatment recommended by the manufacturer after grinding unnecessary.

REFERENCES 1.

Aboushelib MN, Wang H. Effect of surface treatment on flexural strength of zirconia bars. J Prosthet Dent 2010;104:98-104. 2. Attia A, Lehmann F, Kern M. Influence of surface conditioning and cleaning methods on resin bonding to zirconia ceramic. Dent Mater 2011;27:207-213. 3. Blatz MB, Chiche G, Holst S, Sadan A. Influence of surface treatment and simulated aging on bond strengths of luting agents to zirconia. Quintessence Int 2007;38:745-753. 4. Borchers L, Stiesch M, Bach FW, Buhl JC, Hübsch C, Kellner T, Kohorst P, Jendras M. Influence of hydrothermal and mechanical conditions on the strength of zirconia. Acta Biomater 2010;6:4547-4552. 5. Casucci A, Mazzitelli C, Monticelli F, Toledano M, Osorio R, Osorio E, Papacchini F, Ferrari M. Morphological analysis of three zirconium oxide ceramics: Effect of surface treatments. Dent Mater 2010;26:751-760. 6. Cavalcanti AN, Foxton RM, Watson TF, Oliveira MT, Giannini M, Marchi GM. Bond strength of resin cements to a zirconia ceramic with different surface treatments. Oper Dent 2009;34:280-287. 7. Curtis AR, Wright AJ, Fleming GJ. The influence of surface modification techniques on the performance of a Y-TZP dental ceramic. J Dent 2006;34:195-206. 8. Doi M, Yoshida K, Atsuta M, Takahashi S. Influence of pre-treatments on flexural strength of zirconia and debonding crack-initiation strength of veneered zirconia. J Adhes Dent 2011;13:79-84. 9. Garcia Fonseca R, de Oliveira Abi-Rached F, Dos Santos Nunes Reis JM, Rambaldi E, Baldissara P. Effect of particle size on the flexural strength and phase transformation of an airborne-particle abraded yttria-stabilized tetragonal zirconia polycrystal ceramic. J Prosthet Dent 2013;110:510-514. 10. Garvie RC, Nicholson PS. Phase analysis in zirconia systems. J Am Ceram Soc 1972;55:303-305. 11. Guazzato M, Quach L, Albakry M, Swain MV. Influence of surface and heat treatments on the flexural strength of Y-TZP dental ceramic. J Dent 2005;33:9-18. 12. Guess PC, Zhang Y, Kim JW, Rekow ED, Thompson VP. Damage and reliability of Y-TZP after cementation surface treatment. J Dent Res 2010;89:592-596.

458

13. International Organization for Standardization. ISO 6872:2008(E): Dentistry – Ceramic materials. Berlin, Germany: ISO, 2008. Available at http://www.iso.org/iso/ch/iso/prods-services/isostore/store.html 14. Işeri U, Özkurt Z, Kazazoğlu E, Küçükoğlu D. Influence of grinding procedures on the flexural strength of zirconia ceramics. Braz Dent J 2010;21:528-532. 15. Karakoca S, Yilmaz H. Influence of surface treatments on surface roughness, phase transformation, and biaxial flexural strength of Y-TZP ceramics. J Biomed Mater Res Part B Appl Biomater 2009;91:930-937. 16. Kim JW, Covel NS, Guess PC, Rekow ED, Zhang Y. Concerns of hydrothermal degradation in CAD/CAM zirconia. J Dent Res 2010;89:91-95. 17. Kitayama S, Nikaido T, Takahashi R, Zhu L, Ikeda M, Foxton RM, Sadr A, Tagami J. Effect of primer treatment on bonding of resin cements to zirconia ceramic. Dent Mater 2010;26:426-432. 18. Kohorst P, Borchers L, Strempel J, Stiesch M, Hassel T, Bach FW, Hubsch C. Low-temperature degradation of different zirconia ceramics for dental applications. Acta Biomater 2012;8:1213-1220. 19. Kosmac T, Oblak C, Jevnikar P, Funduk N, Marion L. The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic. Dent Mater 1999;15:426-433. 20. Kosmac T, Oblak C, Jevnikar P, Funduk N, Marion L. Strength and reliability of surface treated Y-TZP dental ceramics. J Biomed Mater Res 2000;53:304-313. 21. Mochales C, Maerten A, Rach A, Cloetens P, Mueller WD, Zaslansky P, Fleck C. Monoclinic phase transformations of zirconia-based dental prostheses, induced by clinically practised surface manipulations. Acta Biomater 2011;7:2994-3002. 22. Monaco C, Tucci A, Esposito L, Scotti R. Microstructural changes produced by abrading Y-TZP in presintered and sintered conditions. J Dent 2013;41:121-126. 23. Oilo M, Gjerdet NR, Tvinnereim HM. The firing procedure influences properties of a zirconia core ceramic. Dent Mater 2008;24:471-475. 24. Pittayachawan P, McDonald A, Petrie A, Knowles JC. The biaxial flexural strength and fatigue property of Lava Y-TZP dental ceramic. Dent Mater 2007;23:1018-1029. 25. Qeblawi DM, Muñoz CA, Brewer JD, Monaco EA Jr. The effect of zirconia surface treatment on flexural strength and shear bond strength to a resin cement. J Prosthet Dent 2010;103:210-220. 26. 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;27:408-414. 27. Souza RO, Valandro LF, Melo RM, Machado JP, Bottino MA, Ozcan M. Air-particle abrasion on zirconia ceramic using different protocols: effects on biaxial flexural strength after cyclic loading, phase transformation and surface topography. J Mech Beh Biomed Mater 2013;26:155-163. 28. Turp V, Sen D, Tuncelli B, Goller G, Özcan M. Evaluation of air-particle abrasion of Y-TZP with different particles using microstructural analysis. Aust Dent J 2013;58:183-191. 29. Uo M, Sjögren G, Sundh A, Goto M, Watari F, Bergman M. Effect of surface condition of dental zirconia ceramic (Denzir) on bonding. Dent Mater J 2006;25:626-631. 30. Wang H, Aboushelib MN, Feilzer AJ. Strength influencing variables on CAD/CAM zirconia frameworks. Dent Mater 2008;24:633-638. 31. Wolfart M, Lehmann F, Wolfart S, Kern M. Durability of the resin bond strength to zirconia ceramic after using different surface conditioning methods. Dent Mater 2007;23:45-50. 32. Yamaguchi H, Ino S, Hamano N, Okada S, Teranaka T. Examination of bond strength and mechanical properties of Y-TZP zirconia ceramics with different surface modifications. Dent Mater J 2012;31:472-480. 33. Zhang Y, Lawn BR, Rekow D, Thompson P. Effect of sandblasting on the long-term performance of dental ceramics. J Biomed Mater Res B Appl Biomater 2004;71:381-386.

Clinical relevance: The damage caused to zirconia ceramic by diamond burs during the coping adjustments is not minimized by heat treatment. Concerning the zirconia mechanical behavior, since the heat treatment is not performed after grinding, any air-abrasion technique could be employed, and therefore its selection could be based on the capability of providing bonding.

The Journal of Adhesive Dentistry

Effect of surface and heat treatments on the biaxial flexural strength and phase transformation of a Y-TZP ceramic.

To evaluate the effect of grinding and airborne-particle abrasion on the biaxial flexural strength (BFS) and phase transformation of a Y-TZP ceramic, ...
186KB Sizes 0 Downloads 10 Views