Hard machining, glaze firing and hydrofluoric acid etching: Do these procedures affect the flexural strength of a leucite glass-ceramic?

DENTAL-2545; No. of Pages 10

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Hard machining, glaze firing and hydrofluoric acid etching: Do these procedures affect the flexural strength of a leucite glass–ceramic? Sara Fraga a , Luiz F. Valandro b , Marco A. Bottino c , Liliana G. May b,∗ a b c

Dental Science Post Graduate Program, Federal University of Santa Maria, Santa Maria, RS, Brazil Department of Restorative Dentistry, Federal University of Santa Maria, Santa Maria, RS, Brazil Department of Dental Materials and Prosthodontics, São Paulo State University, São José dos Campos, SP, Brazil

a r t i c l e

i n f o

a b s t r a c t

Available online xxx

Objective. To evaluate the effects of hard machining, glaze firing and hydrofluoric acid etch-

Keywords:

ing on the biaxial flexural strength and roughness of a CAD/CAM leucite glass–ceramic; to

Machining

investigate if ceramic post-machining surface roughness is influenced by the machining

Leucite glass–ceramic

order and by the pair of burs used for it.

CAD/CAM

Methods. A hundred forty four discs were machined by six nominally identical pairs of burs

Hydrofluoric acid etching

and divided into groups (n = 24): (1) machining—M, (2) machining and glaze firing—MG, (3)

Glaze firing

machining and hydrofluoric acid etching—MA, (4) machining, glaze firing and hydrofluoric

Biaxial flexural strength

acid etching—MGA, (5) machining followed by polishing, as a control—MP, (6) machining, polishing and hydrofluoric acid etching—MPA. The roughness after each treatment (Ra and Rz ) was measured. The discs were submitted to a piston-on-three ball flexure test (ISO 6872/2008) and strength data analyzed through Weibull statistics (95% CI). Results. M resulted in lower characteristic strength ( 0 ) (128.2 MPa) than MP (177.2 MPa). The glaze firing reduced 0 (109 MPa), without affecting roughness. Hydrofluoric acid etching increased the roughness without affecting 0 . Spearman’s coefficient (rs ) indicated strong and significant correlation between machining order and roughness (rs Ra = −0.66; rs Rz = −0.73). The ceramic post-machining surface roughness differed significantly according to the pair of burs employed (p < 0.05). Significance. hard machining and glaze firing reduced the leucite ceramic strength, while hydrofluoric acid etching did not affect the strength. Variability in the roughness might be expected after machining, since it was influenced by the machining order and by the bur pairing. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

∗

Corresponding author. Tel.: +55 55 3220 9276; fax: +55 55 3220 9272. E-mail address: [email protected] (L.G. May).

http://dx.doi.org/10.1016/j.dental.2015.04.005 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Fraga S, et al. Hard machining, glaze firing and hydrofluoric acid etching: Do these procedures affect the flexural strength of a leucite glass–ceramic? Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.04.005


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1.

Introduction

Due to the brittle nature of ceramics, their fracture strength is strongly influenced by the presence of defects, which can be considered especially critical when located at the cementation surface of a ceramic crown [1]. A compressive load applied to the occlusal surface of a ceramic single crown induces tensile stresses on the cementation surface, which may contribute to initiate the clinical fracture of a ceramic restoration [2]. The introduction of CAD/CAM (Computer-Aided Design; Computer-Aided Machining) technology in Dentistry has enabled the production of complex restorations using prefabricated ceramic blocks, which are reduced by diamond cutting tools in the dimensions and anatomy informed in a software compatible with the machining system [3]. In spite of reducing processing defects, once the ceramic blocks are produced in a standard process independent of the manual ability of a technician, the machining induces a complex network of events in the ceramic, resulting in radial and lateral cracks, chipping, subsurface damage, residual stresses and even plastic deformation [4–6]. Therefore, knowledge of the effects that machining has on the ceramic strength is essential. Machining can be performed using fully sintered ceramics (hard machining), such as with feldspar-, leucite- and lithium disilicate-based ceramics, or by using partially sintered blocks (soft machining), such as with Y-TZP ceramics. Depending on the type of machining (hard or soft), different effects to the strength of the ceramics may be expected [7]. Wang et al. [8] reported that CAD/CAM soft machining produced surface damage and significantly reduced the strength of zirconia, which may result in unexpected failures at stresses much lower than the ideal strength of the material. After machining and clinical adjustments, CAD/CAM glass–ceramic restorations can undergo a final glaze to promote a glossy and smooth surface. During the glazing process, the ceramic may be submitted to a range of temperatures near the glass transition temperature (Tg ). Residual stress relief has been reported due to thermal treatment, leading to different effects on ceramic strength. Giordano, Cima and Pober [9] reported a reduction in strength after heat treatment, which was attributed to the relief of compressive stresses resulting from polishing. Griggs, Thompson and Anusavice [10] reported that self-glazing did not improve the strength of indented porcelain discs, although having provided flaw modification through a crack-blunting mechanism. According to those authors, a compressive residual stress created during manufacture of the specimens may be removed by annealing the discs during a self-glaze process. In the study of Addison et al. [11], thermal treatment followed by slow cooling did not affect the strength of machined feldspathic ceramic discs. Considering that machining may be able to introduce a thin compressive layer within the ceramic surface, producing a positive effect on the material strength [4], glaze firing may reduce the ceramic fracture strength due to relief of the residual compressive stresses. Regarding ceramic surface conditioning, hydrofluoric acid etching (HF) is an imperative procedure for the success of adhesive cementation of silica-based ceramics. HF creates irregularities in the ceramic surface, increasing the surface

energy, and optimizing the resin bond to the ceramic. However, these surface irregularities could negatively affect the material fracture strength [12]. Currently, according to the current authors’ knowledge, there is no study that assesses the isolated effect of hydrofluoric acid etching using biaxial flexure strength testing of hard machining ceramic discs. Therefore, taking into account the increasing use of CAD/CAM technology in Restorative Dentistry, in which machining may introduce new features to the cementation surface of ceramic restorations, this study assessed the effect of hard machining on the biaxial flexure strength (BFS) and roughness of a leucite glass–ceramic. The impact of procedures preceding cementation to the BFS of machined ceramic discs (the thermal cycle used in glazing and hydrofluoric acid etching) were also investigated. The hypothesis tested was that all procedures (machining, glaze firing and hydrofluoric acid etching) would reduce the ceramic BFS. Another aim of the current study was to investigate whether the ceramic surface roughness would be influenced by the order of machining and by the pair of burs used for the production of the discs. The hypothesis tested was that the post-machining ceramic roughness would be independent of the machining order and of the pair of burs used.

2.

Materials and methods

2.1.

Machining of ceramic discs by CAD/CAM

Two different stone models were produced: a preparation model (Fig. 1a) and a reference model (Fig. 1b). The preparation model was a customized cylindrical crown with an internal preparation (13.5 mm diameter; 1.4 mm depth) and, adjacent to it, two simple cylindrical crowns with flat occlusal surfaces. Three simple crowns formed the reference model. These models were scanned (Scanner CEREC inLab, Sirona Dental Systems Gmbh, Germany) and the three-dimensional images were processed in CAD CEREC inLab 3D software, version 3.85 (Sirona Dental Systems Gmbh, Germany). Using the software interface, a correlation of equality was established between the models, indicating that the restoration to be created for the preparation model should follow the same anatomy of the reference model. Thus, it was possible to machine a disc shaped restoration that had a 13.5 mm diameter and 1.4 mm thickness (Fig. 1c and d) using leucite glass–ceramic fully sintered blocks (14 mm × 14 mm × 18 mm) (IPS Empress CAD C14L, Ivoclar Vivadent AG, Liechtenstein), using CEREC inLab MC XL machine (Sirona Dental Systems Gmbh, Germany). Six nominally identical pairs of diamond burs (A–F), each containing one cylindrical (Cylinder pointed bur 12S, Sirona Dental Systems Gmbh, Germany) and one stepped pattern (Step bur 12S, Sirona Dental Systems Gmbh, Germany) bur, were used for machining 154 ceramic blocks. Each block was used for one disc, with the machining order (from 1st up to 28th) and the pair of burs (A, B, C, D, E or F) recorded for each individual disc. For the first pair of burs used, the cylindrical bur fractured while machining the 29th disc. Therefore, the maximum number of discs obtained from each pair of burs was set at 28, with the exception of the last pair of burs (F) which was used to prepare 13 discs due to the number of




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Fig. 1 – (a) preparation model formed by a customized cylindrical crown with an internal preparation (black arrow) and, adjacent to it, two simple cylindrical crowns with flat occlusal surfaces; (b) reference model, formed by three simple crowns; (c) CEREC inLab 3D® software interface showing the disc-shaped restoration fit to the cavity of the preparation model; (d) disc inside the ceramic block, ready for machining.

remaining ceramic blocks. From the total number of machined discs, nine were employed in a pilot study and one was eliminated due to the bur fracturing while it was bring machined. The remaining 144 discs were randomly divided into six experimental groups (n = 24).

2.2.

Experimental groups

The experimental groups consisted of discs submitted to the following treatments: (1) machining (M), (2) machining and glaze firing (MG), (3) machining and hydrofluoric acid etching (MA), (4) machining, glaze firing and hydrofluoric acid etching (MGA), (5) machining and polishing (MP), (6) machining, polishing and hydrofluoric acid etching (MPA). The glaze firing treatment was performed in a Vita Vacumat 6000 MP furnace (Vita Zahnfabrik, Germany) and followed the glaze firing protocol indicated for IPS Empress CAD® by the manufacturer, without the use of any glaze paste (initial temperature 403 ◦ C; temperature rise 100◦ /min; final glazing temperature 790 ◦ C for 90 s; vacuum initiated at 450 ◦ C and released at 789 ◦ C) [13]. Ten minutes after opening the furnace, the discs were removed and bench cooled. Polishing was performed manually using 400, 600 and 1200 grit silicon carbide paper on the bottom surface of the discs, removing 80 ␮m of the machining surface, in order to remove defects caused by machining. The depths of removal in the center of the discs were controlled by means of a micrometer (210 MAP, Starrett, USA). Etching was applied on the bottom surface of the discs, using 10% hydrofluoric acid (Condac Porcelana, FGM, Brazil) for 60 s. The specimens were then washed in distilled water for 60 s and air dried for 30 s. The final thickness at the center of the discs, as measured by a micrometer (210 MAP, Starrett, USA), was adjusted using 240, 400 and 600 grit silicon carbide paper on the upper surface of the discs to a thickness of 1.32 ± 0.02 mm.

The sprue attachment created during the machining process was located on the lateral circumference of the discs and allowed for the re-orientation of the specimens during the roughness measurements. The sprue was removed for the flexural test using a diamond bur at low speed.

2.3.

Measurement of the surface roughness

The roughness of the bottom surface of every disc was measured after machining (initial roughness) and at the end of each treatment (final roughness), using a contact stylus profilometer (SJ-410, Mitutoyo, Japan). The Ra (average surface roughness; ␮m) and Rz (the arithmetic mean peak-to-valley height; ␮m) values were determined using the average of three measurements, transversal to the machining path. The sampling length was equivalent to five times the cut-off value (c ), as defined according to ISO 4287:1997 [14], using the Ra values recorded at a first reading as a reference. Thereafter, the final roughness of the group MP was evaluated using c = 0.25 mm (tabulated value for 0.02 < Ra ≤ 0.1), resulting in a sampling length of 1.25 mm. The initial and final roughness measurements for the other groups were conducted using c = 0.8 mm (tabulated value for 0.1 < Ra ≤ 2.0), resulting in a sampling length of 4 mm. In addition to the use of a correct cut-off value, a Gaussian filter was employed to differentiate between shape defects and the roughness profile.

2.4.

Analysis of leucite crystals: X-ray diffraction

In order to evaluate the effect of the glaze firing on the leucite crystals, two discs were submitted to X-ray diffraction (D8 Advanced XRD, Bruker AXS GmbH, Germany) before and after ˚ (CuK␣ ), a scan heat treatment, using a length wave of 1.5416 A from 10◦ to 90◦ , a step wise of 0.02◦ and a time of 187 s.



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The average size of leucite crystallites was determined before and after glaze firing for the most intense peaks (0 0 4 and 4 0 0) using the Debye equation (Eq. (1)) [15]. T=

0.9 ˇ cos

(1)

where T is the crystallites average diameter, is the wave˚ is the location of the peak and length of an X-ray (1.5416 A), ˇ is the width of the middle-height of a selected peak, in radians. The real value of ˇ, the free of measurement errors, was calculated using the following equation: 2 ˇ2 = ˇM − ˇS2

(2)

where ˇM is the value of ˇ obtained using ceramic diffraction and ˇs is the value of ˇ obtained for a crystal with a size greater ˚ in this case, silicon. than 1000 A,

2.5.

Piston-on-three ball biaxial flexure test

The fracture strength of the discs was determined using a piston-on-three ball test, according to ISO 6872/2008 [16], using a universal testing machining (DL-1000 Emic, Brazil). The bottom surface of the disc – the experimental surface – was positioned on the top of three steel spheres (2.5 mm in diameter, 120◦ apart and forming a circle of 10 mm diameter), with a load applied at a rate of 1 mm/min, perpendicular to the center of the top surface of the disc, by a circular cylinder steel piston with a 1.4 mm diameter flat tip. An adhesive tape of 25 ␮m was placed between the piston and the disc before loading to fracture. The fracture strength, in MPa, was calculated using Eqs. (3)–(5): −0.2387P (X − Y) = d2 X = (1 + v) ln

B 2 C

Y = (1 + v) 1 + ln

+

(3)

(1 − v) 2

A 2 C

B 2 C

+ (1 − v)

A 2 C

(4)

(5)

where P is the load at fracture (N), d is the disc thickness (mm), v is Poisson’s ratio (0.25), A is the support ball radius (5 mm), B is the radius of the tip of the piston (0.7 mm) and C is the specimen radius (6.75 mm). The number of fragments resulting from each fractured disc was recorded.

2.6.

Scanning electron microscopy analysis (SEM)

To assess the effectiveness of the polishing protocols in removing the machining damage, one machined disc (M group) and one machined and polished disc (MP group) had their cross sectional surface analyzed using a scanning electron microscope (SEM) (JSM 5400, Japan). The discs were embedded in self-curing acrylic resin and cut, perpendicularly to the machined and polished surfaces using a diamond saw at lowspeed (Isomet 1000, Buehler, Lake Buff, USA). The samples had their cross sectional surfaces polished (400, 600 and 1200 grit

silicon carbide paper) and sputter-coated with gold-palladium for SEM. After the flexural tests, some discs were randomly chosen and had their fractured surfaces also evaluated under SEM to identify the region of fracture origin.

2.7.

Statistical analysis

Kruskal–Wallis and post hoc LSD tests (˛ = 0.05) were used to compare the final roughness values (Ra , R) among the experimental groups, since the data presented heterogeneity of variances (p < 0.05 based on the Levene test) and non-normal distribution (p < 0.05 based on the Shapiro–Wilk test). Weibull statistics [17,18] were used for the analysis of the flexural strength, as indicated by the characteristic strength ( 0 ), which is the strength occurring at a probability of failure of 63.2% for a particular test specimen, and Weibull modulus (m), which represents the reliability of the fracture strength. The basic form of the Weibull distribution is shown in the following equation:

m

Pf = 1 − exp −

0

(6)

where Pf is the probability of failure and is the failure stress. The biaxial flexure strength data were ranked in ascending order and the probability of failure for each failure stress (Pf(i) ) was calculated by Eq. (7), where i is the rank order (1, 2, 3,. . .,24) and N is the number of specimens per group. Pfi =

(i − 0.5) N

(7)

A rank regression was performed on the bi-axial flexure strength data to estimate the Weibull parameters ( 0 and m) [16]. The upper and lower limits of the 95% confidence intervals for 0 and m were calculated according to ENV 843-5:1996 [19]. Differences were considered significant when confidence intervals failed to overlap. Spearman correlation coefficients (rs ) were calculated for the fracture strength vs. number of fragments; and for machining order vs. surface roughness. The correlation was considered strong when rs > 0.6 [20]. The Ra and Rz values obtained for each pair of burs were compared using the Kruskal–Wallis test (˛ = 0.05), since the data presented heterogeneity of variances (p < 0.05 based on the Levene test) and non-normal distribution (p < 0.05 based on the Shapiro–Wilk test).

3.

Results

The M group presented higher values of roughness and significantly lower characteristic strength when compared to the MP group (Table 1). The SEM images of the cross sectional area of the machined disc and of the machined and polished disc are illustrated in Fig. 2. Glaze firing did not affect the roughness parameters, but it reduced the characteristic strength, as can be observed when comparing groups M vs. MG and MA vs. MGA (Table 1). X-ray diffraction analysis (Fig. 3) showed changes in the ceramic


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0.61d (0.06) 4.65d (0.47) 169.1D [161.6–176.8] 10.9A [7.3–14.4] Different letters indicate statistically significant differences (p < 0.05 in LSD test for Ra and Rz ; 95% CI overlapping for 0 and m).

MPA MP

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0.04c (0.01) 0.30c (0.06) 177.2D [166.0–189.0] 7.5A [5.0–9.9] 1.75b (0.30) 11.67b (1.74) 110.1C [105.9–114.5] 12.6A [8.4–16.6]

MGA MA

1.67b (0.20) 10.99b (1.32) 123.3A [117.7–129.0] 10.7A [7.1–14.0] 1.29a (0.27) 8.32a (1.74) 109.3B,C [104.6–114.2] 11.1A [7.4–14.6]

MG M

1.37a (0.18) 8.80a (1.22) 128.2A [122.2–134.4] 10.3A [6.8–13.5] Ra (␮m) Rz (␮m) 0 (MPa) [CI = 95%] m [CI = 95%]

Groups Variables

Table 1 – Means and standard deviation of the roughness values (Ra and Rz ) and 95% confidence interval (CI) of characteristic strength ( 0 ) and Weibull modulus (m) for the experimental groups.


Fig. 2 – SEM images of the cross section of: (a) machined disc (M group) showing the defects created by machining (black arrows) and (b) machined and subsequently polished disc (MP group), with a smoother surface.

Fig. 3 – X-ray diffraction patterns before (black line) and after (red line) the glaze firing protocol. The arrow indicates the presence of a wide peak, which is characteristic of an amorphous material. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)



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Fig. 5 – Plots of failure probabilities against biaxial flexure strengths for the experimental groups: (M) machining; (MG) machining and glaze firing; (MA) machining and acid etching; (MGA) machining, glaze firing and acid etching; (MP) machining and polishing; (MPA) machining, polishing and acid etching.

microstructure after glaze firing, indicating the presence of tetragonal leucite in narrow peaks and the presence of a very wide peak, representing amorphous material. After glaze firing, the leucite crystallites size ranged from 54 nm to 51.5 nm for the peak at 0 0 4, and from 35.5 nm to 35.65 nm for the peak at 4 0 0. Hydrofluoric acid etching (MA, MGA and MPA) resulted in rougher surfaces than the corresponding unetched groups (M, MG and MP) (Table 1). When comparing the roughness profiles of groups MP and MPA (Fig. 4), it is evident that hydrofluoric acid introduced new irregularities on the ceramic surface, but it did not affect the 0 (Table 1). The scattering of strength data was similar among groups, since there was no significant difference in Weibull moduli (Table 1). The plots of failure probabilities against biaxial flexure strengths for the experimental groups are illustrated in Fig. 5. SEM analysis of the fractured discs showed that failure appeared to originate from the surface submitted to tensile stress during the flexural test (Fig. 6). The Spearman correlation coefficient demonstrated a significantly (p < 0.01) strong correlation (rs = 0.86) between fracture strength and number of fragments generated. The median of fragments for the weaker groups (MG and MGA) was 3, while for the more resistant groups (MP and MPA) was 5. A negatively strong and significant correlation was found between machining order vs. Ra (rsRa = −0.66) and vs. Rz (rsRz = −0.73), as showed in Fig. 7. The values of initial roughness of the machined discs differed significantly depending on the pair of burs used (Table 2).

Fig. 4 – Roughness profiles of discs submitted to the differing surface treatments: (a) M; (b) MG; (c) MA; (d) MGA; (e) MP; (f) MPA.

4.

Discussion

As can be observed in the SEM images (Fig. 2) and in the roughness profiles (Fig. 4), machining introduced defects on the ceramic surface, which were removed by polishing. As a result, for a same failure probability level, the M group




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Fig. 6 – SEM images of the fracture surface where it is possible to observe the presence of wake-hackles pointing to the origin of the failure in discs of groups MP (a) and MGA (b).

presented a strength that was significantly lower when compared to the MP group (Fig. 5). Therefore, the hypothesis that machining would reduce biaxial flexure strength when compared to polished discs was accepted. Similar results were found for Y-TZP, where the surface damage produced by soft machining reduced the zirconia uniaxial flexure strength [8].

The polishing protocol, which removed 80 ␮m from the machined surface, aimed to isolate the effect of machining on the fracture strength of the ceramics, ensuring that defects from machining were removed. Defects introduced by the CEREC® CAD/CAM system were indicated to be the origin of the failure in ceramics subjected to a uniaxial bending test.

Fig. 7 – Mean Ra and Rz values vs. machining order (1st to 28th). Each mean Ra and Rz point was obtained from the average calculation among the roughness values from A, B, C, D, E and F bur sets. Please cite this article in press as: Fraga S, et al. Hard machining, glaze firing and hydrofluoric acid etching: Do these procedures affect the flexural strength of a leucite glass–ceramic? Dent Mater (2015), http://dx.doi.org/10.1016/j.dental.2015.04.005


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Table 2 – Mean and standard deviation of roughness values (Ra and Rz ) for the six nominally identical pairs of burs (A–F) used in the study. “N” represents the total number of ceramic blocks machined per bur set and “n” represents the number of discs used in the roughness analysis. ␮m

Ra Rz

Pairs of burs A

B

C

D

E

F*

N = 29 n = 23**

N = 28 n = 27***

N = 28 n = 28

N = 28 n = 28

N = 28 n = 28

N = 13 n = 10****

1.46 (0.16)a 9.33 (1.16)a

1.20 (0.25)b 7.71 (1.77)b

1.38 (0.14)a,d 8.89 (0.83)a,c

1.46 (0.28)a 9.31 (1.60)a,c,d

1.31 (0.11)c,d 8.39 (0.75)b,c

1.55 (0.16) 10.23 (0.99)

Different letters indicate statistically significant differences (p < 0.05 in LSD test). F pair was excluded from the bur comparisons. ∗∗ 29 Discs were machined with the pair of bur A. During the machining of the 29th disc, the cylindrical bur fractured and the machining was not completed. The 1st, 2nd, 17th, 20th and 25th discs were excluded from the roughness analysis and used in the pilot study (training the polishing protocol), since they were dislocated during the machining. ∗∗∗ 28 Discs were machined with the pair of bur B. The 13th disc was excluded from the roughness analysis and used in a pilot study (training the polishing protocol) since it was dislocated during the machining. ∗∗∗∗ 13 Discs were machined with the pair of bur F. The 11st, 12nd and 13th disc were employed in the scanning electron microscopy analysis. ∗

Residual surface damage and machining grooves appeared to define flaw size that initiate the failure, which was estimated to be 9–15 ␮m for a glass–ceramic and 15–30 ␮m for feldspathic porcelains [21]. In another study, the machining damage surface was assessed to be 60 ␮m for a feldspathic ceramic [6]. The current authors highlight that, in the studies mentioned above, the machining was performed by a robust CEREC® system, which employed diamond wheels to modify the ceramic and only allowed the preparation of inlay restorations [22]. On the other hand, machining in the present study was performed using the CEREC MC XL® system, which provided a more refined mechanism using a pair of specific diamond burs, possibly resulting in less aggressive damage to the surface when compared to grinding with diamond wheels. Therefore, the current authors suggest that further studies, which use currently available machining systems, are necessary to assess the impact of machining on the mechanical properties of ceramics with different microstructures and subjected to different machining techniques (hard and soft). Additionally, when considering the differences in surface roughness and fracture strength between the M group and MP group, the development of technological innovations in machining, with the creation of new grinding tools and mechanism to provide a better finishing of the cementation surface of a CAD/CAM ceramic restoration, may be an alternative to minimize the impact of the machining process. The hypothesis that glaze firing, without the use of any glaze paste, would significantly reduce the ceramic strength was accepted. The relief of compressive residual stress from machining [4,9], changes in the material microstructure and the development of microcracks and residual stress due the large mismatch in coefficients of thermal expansion between the leucite crystals (˛c ) and the glass matrix (˛m ) [23,24] are some hypotheses that may explain the decrease in the strength of the ceramics. According to Marshall et al. [4], machining would introduce a thin compressive stress layer on the ceramic surface, with a positive effect on the fracture strength. For a conventional feldsphatic ceramic, investigators have demonstrated that heat treatment reduces flexural strength due to the relief of compressive residual stresses originated from polishing

[9]. Therefore, glaze firing could have relieved compressive stresses from machining, thus reducing the biaxial flexure strength of the material. However, the analysis of possible stress relief after glaze firing was not the purpose of the present study. X-ray diffraction analysis indicated an increase of amorphous content in the ceramic surface after glaze firing (Fig. 3). The size of the leucite crystallites for the more intense peaks (0 0 4 and 4 0 0) was not substantially modified by glaze firing, possibly indicating that the amorphous phase originated from the degradation of other particles present in the ceramic. These changes in the microstructure may also have negatively affected the ceramic fracture strength. Residual stresses and the development of microcracks after thermal treatment may also be expected due to the large mismatch in the coefficients of thermal expansion between the leucite crystals (˛c ) and the glass matrix (˛m ), where ˛c > ˛m [23,24] with a possible negative effect on the ceramic strength. The lack of significant differences in the Ra and Rz values between the M and MG groups (Table 1) showed that glaze firing was not enough to change the micro roughness patterns introduced by machining, even when reaching a temperature above the glass transition temperature of IPS Empress CAD (625 ± 20 ◦ C) [25]. The current results are in agreement with one study, which tested a heating treatment with a temperature near the ceramic softening temperature, and observed no effect on the Ra values of feldsphatic ceramic discs obtained from CEREC MC XL® machining [11]. The impact of the glaze firing protocol on the ceramic biaxial flexure strength indicates the importance of investigating the influence of thermal processing on the microstructure and fracture strength of ceramic restorations. Attention should be paid mainly to the protocols of cooling rate and furnace opening, since these steps may not be completely described by the manufacturers. The hypothesis that hydrofluoric acid etching would reduce the ceramic biaxial flexure strength was rejected. Significant differences in the Ra and Rz values among the etched and unetched groups (M vs. MA; MG vs. MGA; MP vs. MPA), in addition to the analyses of the roughness profiles, mainly for the groups MP and MPA (here the acid etching variable




is isolated) (Fig. 4), indicated that the hydrofluoric acid introduced new irregularities on the ceramic surface, without any effects on the characteristic strength. One possible explanation for these findings is that hydrofluoric etching produces surfaces with a uniform distribution of defects, mainly microporosities [26], and may also round off the tips of the microcracks [27], which may contribute to a reduction of stress concentration. On the other hand, Addison et al. [12] reported that hydrofluoric acid etching in different concentrations (5, 10 and 20%) and application times (45, 60 and 180 s) significantly reduced the strength when compared to untreated feldspathic ceramic. Similar results were also found for leucite reinforced glass–ceramic and lithium disilicate when treating with 9% hydrofluoric acid for 2 min [28]. The Weibull modulus (m) is inversely related to the scattering of strength data, indicating a distribution of defects in the ceramic [29]. As the m value did not differ significantly among the groups (Table 1), it may be suggested that the different treatments resulted in similar distributions of flaws. Different from the expectations of the present authors, the MP group showed the lowest value of m (7.5). A possible explanation for this finding is that the polishing was performed manually, without load and speed control, and that some residual damage from machining may have remained at the disc surfaces. The strong direct correlation between fracture strength vs. number of fragments (rs = 0.86) is in agreement with other studies, in which higher values of strength are associated with higher fracture energy and, therefore, a greater number of fragments [30]. The SEM images of the fractured surface of the discs (Fig. 6) showed some typical features of brittle material failures, such as the presence of wake-hackles pointing to the failure origin, indicating that ceramic failures started from the surface subjected to tensile stresses during the test [30]. These findings can be considered one indicator that the biaxial flexure test provided adequate sensitivity to evaluate the ceramic fracture strength. In the present study, a significantly strong negative correlation was found between machining order vs. Ra (rRa = −0.66) and vs. Rz (rRz = −0.73). Observing Fig. 7 is possible to notice that when machining order increase, the roughness decrease. These findings contrast with another study, in which six pair of burs were used in the manufacture of feldsphatic ceramic discs using hard machining in a CEREC inLab MC XL® , and no correlation was found between machining order and surface roughness [11]. The difference in the number of discs produced by each pair of burs, 28 discs in the present study against 14 discs for the latter study, may explain the differences in the results. Since the correlation was negative, it seems that repeated machining with used burs is not detrimental to the roughness of the ceramic. Considering the strong correlation found between machining order and surface roughness, the last pair of burs (F), which produced a small number of discs (13 discs), was excluded from the statistical analysis comparing Ra and Rz values among the different pair of burs. The Kruskal–Wallis test showed that post-machining roughness was significantly altered when different pairs of burs were used (A–E) (Table 2), even though each pair of burs was equal in composition and geometry. Although significant, this difference was small,

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since the largest value of Ra was 1.46 ␮m for pairs A and D, and the lowest value of Ra was 1.20 ␮m for pair B. These results are in agreement with a study by Addison et al. [11], which suggests that differences in the distribution and orientation of the diamond particles may be expected for burs nominally identical in composition and geometry. In their study, bur set influenced roughness and the ceramic strength. Based on the current results, procedures performed prior to the cementation of leucite glass–ceramic CAD/CAM restorations have different impacts on the material strength, since: (1) hard machining introduces defects on the ceramic surface and reduces its biaxial flexure strength; (2) glaze firing reduces the strength of the ceramic and has the potential to change the ceramic microstructure by forming amorphous material; (3) 10% hydrofluoric acid etching for 1 min produces a rougher surface but does not negatively impact the ceramic strength. The current authors suggest that improvements in machining systems should be considered to reduce the impact of hard machining on the ceramic strength and to standardize grinding tools.

Acknowledgements This investigation was supported in part by FIPE ARD 2012 from Federal University of Santa Maria and by FAPERGS/CAPES 009:2011. These authors would like to thank Ivoclar Vivadent for their donation of the some ceramic blocks. The authors are also grateful to Lúcio Dorneles, from the Department of Physics, Federal University of Santa Maria, for his assistance with the X-ray diffraction analysis, and Angela Dullis, from the Department of Statistics, Federal University of Santa Maria, for her assistance with the statistical analysis.

references

[1] Kelly JR, Giordano R, Pober R, Cima MJ. Fracture surface analysis of dental ceramics: clinically failed restorations. Int J Prosthodont 1990;3:430–40. [2] May LG, Kelly JR, Bottino MA, Hill T. Effects of cement thickness and bonding on the failure loads of CAD/CAM ceramic crowns: multi-physics FEA modeling and monotonic testing. Dent Mater 2012;28:e99–109. [3] Giordano R. Materials for chairside CAD/CAM-produced restorations. JADA 2006;137(Suppl. 9):14S–21S. [4] Marshall DB, Evans AG, Khuri Yakub BT, Tien JW, Kino GS. The nature of machining damage in brittle materials. Proc R Soc Lond A 1983;385:461–75. [5] Zhang GM, Satish KG, Ko WK. The mechanics of material removal mechanisms in machining ceramics. In: Technical report TR 94-22rl. Maryland: Institute for Systems Research (NIST); 1994. [6] Sindel J, Petschelt A, Grellner F, Dierken C, Greil P. Evaluation of subsurface damage in CAD/CAM machined dental ceramics. J Mater Sci Mater Med 1998;9:291–5. [7] Denry I. How and when does fabrication damage adversely affect the clinical performance of ceramic restorations. Dent Mater 2013;29:85–96. [8] Wang H, Aboushelib MN, Feilzer AJ. Strength influencing variables on CAD/CAM zirconia frameworks. Dent Mater 2008;24:633–8.



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[9] Giordano R, Cima M, Pober R. Effect of surface finish on the flexural strength of feldspathic and aluminous dental ceramics. Int J Prosthodont 1995;8:311–9. [10] Griggs JA, Thompson JY, Anusavice KJ. Effects of flaw size and auto-glaze treatment on porcelain strength. J Dent Res 1996;75:1414–7. [11] Addison O, Cao X, Sunnar P, Fleming GJP. Machining variability impacts on the strength of a ‘chair-side’ CAD-CAM ceramic. Dent Mater 2012;28:880–7. [12] Addison O, Marquis PM, Fleming GJP. The impact of hydrofluoric acid surface treatments on the performance of a porcelain laminate restorative material. Dent Mater 2007;23:461–8. [13] Ivoclar Vivadent. IPS Empress® CAD: instructions for use labside. Liechtenstein: IPS Empress® ; 2006. [14] ISO 4287. Geometrical product specifications (GPS)—surface texture: profile method, terms, definitions and surface texture parameters. Geneva: International Organization for Standardization; 1997. [15] Cullity BD. Elements of X-ray diffraction. 2nd ed. United States of America: Addison-Wesley Publishing Company; 1978. [16] ISO 6872. Dentistry—ceramic materials. 3rd ed. Geneva: International Organization for Standardization; 2008. [17] Weibull W. A statistical theory of the strength of materials. Swed Inst Eng Res 1939;151:1–45. [18] Weibull W. A statistical distribution of wide applicability. J Appl Mech 1951;18:293–7. [19] ENV 843-5. Advanced technical ceramics—monolithic ceramics—mechanical tests at room temperature—Part 5: Statistical analysis. In: European Standards; 1996.

[20] Crespo AA. Estatística fácil. fifth ed. São Paulo: Saraiva; 1997. [21] Kelly JR, Lüthy H, Gougoulakis A, Pober RL, Mörmann WR. Machining effects on feldspathic porcelain and glass ceramic: fractographic analysis. In: Mörmann WH, editor. International symposium on computer restorations (Switzerland). Berlin: Quintessence Publishing; 1991. p. 253–73. [22] Mörmann W. The evolution of the CEREC system. JADA 2006;137:7S–13S. [23] Mackert JR, Rueggeberg FA, Lockwood PE, Evans AL, Thompson WO. Isothermal anneal effect on microcrack density around leucite particles in dental porcelain. J Dent Res 1994;73:1221–7. [24] Serbena FC, Zanotto ED. Internal residual stresses in glass–ceramics: a review. J Non-Crystalline Solids 2012;358:975–84. [25] Ivoclar Vivadent. IPS Empress® system: scientific documentation. Liechtenstein: IPS Empress® ; 2006. [26] Della Bona A, Anusavice KJ. Microstructure, composition and etching topography of dental ceramics. Int J Prosthodont 2002;15:159–67. [27] Kobayashi K. Crystallized fused SiO2 glass strength improvement by etching. Phys Chem Glasses 1962;3:7–27. [28] Hooshmand T, Parvizi S, Keshvad A. Effect of surface acid etching on the biaxial flexural strength of two hot-pressed glass ceramics. J Prosthodont 2008;17:415–9. [29] Quinn JB, Quinn GD. A practical and systematic review of Weibull statistics for reporting strengths of dental materials. Dent Mater 2010;26:I35–47. [30] Quinn GD. A NIST recommended practice guide. Fractography of ceramics and glasses. Washington, DC: National Institute of Standards and Technology; 2007.


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