d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) e272–e282
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Effect of heat treatment and in vitro aging on the microstructure and mechanical properties of cold isostatic-pressed zirconia ceramics for dental restorations Anna Vatali a , Eleana Kontonasaki a , Panagiotis Kavouras b,d , Nikolaos Kantiranis c , Lambrini Papadopoulou c , Konstantinos K.M. Paraskevopoulos d , Petros Koidis a,∗ a
Department of Fixed Prosthesis and Implant Prosthodontics, School of Dentistry, Aristotle University of Thessaloniki, Greece b Technological Educational Institute of Thessaloniki, Department of Applied Sciences, Sindos, Greece c Geology Department, Aristotle University of Thessaloniki, Greece d Physics Department, Aristotle University of Thessaloniki (A.U.TH.), Greece
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The temperature variations during the veneering firing cycles of a zirconia dental
Received 14 April 2013
ceramic can negatively affect its mechanical properties. A possible synergistic effect of both
Received in revised form 8 July 2013
heat-treatment and aging while exposed to the oral environment could result to catastrophic
Accepted 21 May 2014
failure. The aim of the present study was to investigate the effect of heat treatment followed during veneering and in vitro aging on the mechanical and microstructural properties of zirconia dental ceramics.
Keywords:
Methods. Three specimens from each of two zirconia blocks (Ivoclar IPS e.max ZirCAD (IV) and
Y-TZP zirconia ceramics
Wieland ZENO Zr (WI)) were cut by CAD/CAM technology, fully sintered and polished. Each
Nano-hardness
one was cut in four equal parts. One part was used as control (C), one was heat-treated (H),
Elastic constant
one was aged (A) (134 ◦ C, 2 bar, 10 h) and one was heat-treated and subsequently aged (HA).
Heat-treatment
The mechanical properties (nano-hardness (H) and elastic modulus (E*)) were investigated
Aging
by nano-indentation tests while the surface characterization was carried out with XRD, FTIR
Monoclinic zirconia
and SEM.
XRD
Results. Different treatments on IV and WI samples resulted in a reduction of both H and E*
FTIR
values, however the differences were not statistically significant (p > 0.05). The combination
SEM
of treatments imposes an overall effect (p < 0.001), enhancing the influence on both H and E* values. This reduction in mechanical properties was followed by an increase of monoclinic content. Greater variations in both H and E* values were recorded for WI samples. Significance. The clinical performance of zirconia dental ceramics may be affected during firing and aging resulting in increased probability of failure. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author at: Department of Fixed Prosthesis and Implant Prosthodontics, Aristotle University of Thessaloniki, University Campus, Dentistry Building, GR 54124, Thessaloniki, Greece. Tel.: +30 2310 999659; fax: +30 2310 999676. E-mail address: [email protected] (P. Koidis). http://dx.doi.org/10.1016/j.dental.2014.05.017 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) e272–e282
1.
Introduction
Y-TZP zirconia ceramics have attracted the interest of dental technology in manufacturing core materials for fixed all-ceramic restorations due to their excellent mechanical properties (i.e. bending strength and toughness) [1,2]. The high strength of Y-TZP zirconia ceramics is attributed to a toughening mechanism related to the transformation of the tetragonal phase (t), to the monoclinic (m) phase (natural equilibrium) [3]. This t→m transformation may be triggered by an applied stress and is associated with a net 4 vol% expansion due to the larger volume occupied by the monoclinic phase compared to the tetragonal one [4]. In case of a pre-existing crack the volume expansion associated to the t→m transformation, enables the closure of the crack and diminishes its propagation. This crack propagation prevention minimizes the risk of catastrophic failure of the material due to fracture and is called the “transformation toughening mechanism” [5]. The main drawback of Y-TZP zirconia ceramics is their sensitivity to low temperature degradation (LTD) – aging that leads to exaggerated t→m transformation and a degradation that starts on the surface and propagates into the depth of the material, diminishing its mechanical properties [6–8]. This is of particular interest for dental zirconia ceramics, as their degradation in the oral environment due to the exposure to oral fluids and mechanical stress over prolonged periods of time cannot be overlooked. Although minimally exposed to the oral environment, the margins of zirconia restorations allow a continuous contact of zirconia core with saliva or other fluids that may start and progressively lead to the degradation of the material. Furthermore, there is growing popularity of monolithic zirconia restorations where much larger areas of zirconia are in contact with the oral environment.The majority of the models proposed to explain the spontaneous t→m transformation taking place during LTD-aging, are either based on the formation of zirconium hydroxides (Zr-OH) [9,10] or yttrium hydroxides (Y(OH)3) or Y(O)OH) [11] due to the diffusion of water through the material, promoting phase transition with local stress concentration or variation of the yttrium/zirconium ratio. According to the most recent proposed model [12] oxygen anions are responsible for the transformation nucleation and therefore for the LTD. Due to volume changes the transformed grains cause microcracks and the material becomes degraded. Currently, accelerated tests at intermediate temperatures (100–300 ◦ C) are the only basis for the estimation of the transformation rate and, hence, of the product lifetime [13], although a lot of controversy exists about the validity of the extrapolated predictions [14]. According to Chevalier [13] aging of zirconia with 1 h of autoclave treatment at 134 ◦ C and 2 bar pressure results in a significant t→m transformation that has theoretically the same effect as 3–4 years in vivo, while the ISO standard imposes a maximum of 25 wt% of monoclinic zirconia to be present after an accelerated aging test conducted for 5 h at 134 ◦ C and 2 bar [15]. However Lughi and Sergo [14] state that these accelerating in vitro tests provide only a rough estimate and any extrapolation could lead to a significant error in estimating body temperature lifetimes.
e273
To achieve highly aesthetic zirconia restorations, zirconia core is veneered with a feldspathic porcelain coating and is subjected to firing at high temperatures (750–900 ◦ C) followed by subsequent cooling. This process takes place at least once, but usually it takes two to five firing cycles in order to obtain an aesthetically acceptable restoration [16,17]. The fracture strength [16], microhardness [17] and flexural strength [17] are significantly reduced after veneering firing cycles. According to Oilo et al. [17] this effect is predominantly observed after the first firing cycle while subsequent cycles do not cause a further deterioration of properties. The reduced mechanical properties of zirconia ceramics after heat-treatment have been attributed to residual compressive stresses due to milling and various processing steps – such as grinding and sandblasting – that are released during the heat treatment, as well as to the t→m transformation and the alteration of the grain size that take place during the firing cycles of the veneering process [17]. This degradation of mechanical properties can further reduce the strength of the material after exposure to the oral environment, so a possible synergistic effect of both heat-treatment and aging could result to a probably irreversible premature failure. The aim of the present work was to investigate the effect of heat treatment followed during veneering and the in vitro aging on critical mechanical and microstructural properties at the nano-scale of cold isostatic-pressed zirconia ceramics for dental restorations. The research hypotheses investigated were: 1. the heat treatment does not affect the nano-hardness (H) and the elastic (E*) constant of the zirconia ceramic cores; 2. the in vitro aging does not affect the nano-hardness (H) and the elastic (E*) constant of the zirconia ceramic cores; 3. the combination of heat-treatment and subsequent in vitro aging does not affect the nano-hardness (H) and the elastic (E*) constant of the zirconia ceramic cores.
2.
Materials and methods
Three bar-shaped specimens (28 mm × 4 mm × 2 mm) milled from two zirconia blocks [Ivoclar IPS e.max ZirCAD (LOT: PX0075), and Wieland ZENO Zr (LOT:20090728-07)] by the CAD/CAM technology were sintered to full density, polished with diamond pastes of 3 and 1 m under running water until a mirror-like surface was achieved, and cut with a diamond bur under water cooling into four equal parts each (7 mm × 4 mm × 2 mm). One part was used as control (designated as C), one was heat treated (designated as H), one was aged (designated as A) [steam 121 ◦ C/2 bar/10 h, Kavo autoclave sterilizer 2100 (KavoDental, Biberach/Riss, Germany)] and the last was heat-treated and subsequently aged (designated as HA). The applied heat-treatment corresponded to four firing cycles of the veneering porcelain as shown in Table 1 according to manufacturers’ instructions.
2.1.
Nano-mechanical properties evaluation
The H and E* values were assessed by means of a Hysitron Ubi1 TriboLab modular nano-indentation instrument (Tribolab,
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Table 1 – The applied firing cycles for each zirconia ceramic core. Firing cycle
B (standby temperature) S (pre-heating temperature time) T (heating rate) T (final temperature) H (holding time at T) V1 (vacuum in) V2 (vacuum out)
1st shoulder
1st dentin
Glazing
IV
WI
IV
WI
IV
WI
IV
WI
403 ◦ C 4 50 ◦ C/min 800 ◦ C 1 450 ◦ C 799 ◦ C
575 8 45 980 1 575 980
403 ◦ C 4 50 ◦ C/min 750 ◦ C 1 450 ◦ C 749 ◦ C
575 9 45 900 2 575 900
403 ◦ C 4 50 ◦ C/min 750 ◦ C 1 450 ◦ C 749 ◦ C
575 8 45 890 1 575 890
403 ◦ C 6 60 ◦ C/min 725 ◦ C 1 450 ◦ C 724 ◦ C
575 5 45 880 1 – –
Hysitron Incorporated, Minneapolis). A pyramidal diamond Berkovich indenter with a total included angle of 142.3◦ , and a tip with a radius of curvature of approximately 120 nm, has been utilized. In all cases, a trapezoidal loading–unloading profile was used, with 5 s loading and unloading segments, including a 2 s holding segment. The H and E* values were calculated from the experimental unload–displacement curves using the Oliver and Pharr model [18]. Nano-indentations in all samples were performed with constant load of 4 mN. This load was selected after tests with varying loads performed on the untreated samples, based on the aspect that the produced indentation should have a contact depth higher than one half of the tip’s radius of curvature. This is necessary, in order to have proper onset of plasticity. The contact depth, in all indentation tests, was kept between 80 and 110 nm. The eight different groups of materials were produced in triplicates and each sample was tested in two different locations, where ten indentations were made in every location, thus making a total of 20 indentation prints in every sample and a total of 60 in each different material. Every experimental point in Fig. 2a and b is the mean value of 60 indentations. In order to reach equilibrium with the thermal mass inside the cabinet, all samples were placed into the acoustic and thermal enclosure of the nano-indenter apparatus for several hours prior to testing. Before performing each array of indents, the stability of the nano-indentation instrument was checked by measuring a fused-quartz reference sample. In all cases, the measured values of H and E* of the fused-quartz reference sample were in good agreement with the values reported by the manufacturer, i.e. 9.25 GPa and 69.6 GPa, respectively.
2.2.
2nd dentin
Statistical analysis
Descriptive statistics were calculated by means of min, max, median, mean and standard deviation. The assumption of normality was tested with the Shapiro–Wilk test and it was not rejected. Levene’s test for equality of variances was used to test the homogeneity of variances assumption which also was not rejected. One-way ANOVA was performed to investigate statistically significant differences between groups of specimens, while pair wise comparisons were conducted with the Bonferroni multiple comparison tests for the adjustment of the Type I error. Two independent samples t test were performed to investigate statistically significant differences between the two zirconia ceramics. The analysis was performed with the SPSS 15.0 software and the statistical significance was set for p < 0.05 [19].
2.3.
Surface characterization
X-ray diffraction (XRD) surface analysis was carried out for the determination of the amount of m-ZrO2 transformation using a Philips diffractometer (PW1710; Philips, Eindhoven and Almelo, The Netherlands) with Ni-filtered CuK␣ radiation. The counting parameters of the XRD study were: step size: 0.05◦ 2Â, start angle: 5◦ , end angle: 53◦ and scan speed: 1, 2◦ 2Â/min. The monoclinic phase fraction Xm was calculated according to the Garvie and Nicholson [20] method. Fourier Transform Infrared Spectroscopy (FTIR) was used to qualitatively evaluate the m-ZrO2 transformation. The FTIR reflectance spectra of the specimens were obtained in the spectral area of MIR and FIR (4000–120 cm−1 ), using a Brüker FTIR extended spectrometer, model IFS113v with a resolution of 2 cm−1 . Scanning electron microscopy-Energy dispersive spectroscopy (SEM-EDS) analysis was performed on the surfaces of carbon-coated specimens (J.S.M. 840A; JEOL, Tokyo, Japan) in order to observe any differences concerning surface morphology or elemental composition owed to the various treatments applied. SEM microphotographs of arbitrarily selected specimens were obtained and backscattered microphotographs were collected. Chemical analyses were performed with 20 kV accelerating voltage and 0.4 mA probe current. Pure Co was used as an optimizing element.
3.
Results
3.1.
Nano-mechanical properties evaluation
The results of nano-mechanical tests are listed in Table 2 and presented in the diagrams of Fig. 1. Different treatments on IV and WI samples produce similar trends of the variation of the H value. More specifically, when thermal treatment and aging are applied separately the H value is reduced by a factor of approximately 7.5%. When the above treatments are applied in combination and consecutively (first thermal treatment followed by aging) the H value is reduced by a factor of approximately 20% for IV and 25% for WI samples. From the above it is clear that the combination of thermal treatment and aging on the same sample impose a cumulative effect, enhancing the influence on the H value by a factor of 3.0 up to 3.5. Although H value is reduced after ageing and thermal treatment, these differences are not statistically significant (Table 3). However, thermally treated and consequently aged specimens presented statistically significant lower H compared to control and separately applied aging and heat-treatment (Table 3).
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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) e272–e282
Table 2 – Descriptive statistics of the H and E* values. Sample Control Heat-treatment Aging Heat-treatment + aging
IV H (GPa)
WI H (GPa)
IV E* (GPa)
WI E* (GPa)
15 ± 0.06 13.7 ± 0.5 13.8 ± 0.08 11.9 ± 0.83
14.7 ± 0.81 14 ± 0.49 13.3 ± 0.7 10.7 ± 0.47
223.5 ± 4.8 214.4 ± 9.5 217.9 ± 6.6 209.6 ± 5.2
237.7.0 ± 14.8 214.1 ± 3.8 215.3 ± 2.9 198.7 ± 7.1
Similarly to H, different treatments on IV and WI samples produce similar trends of the variation of the E* value but those of the WI sample are greater. When thermal treatment and aging are applied separately the E* value is reduced by a factor of approximately 4% for the IV sample and 10% for the WI sample. When the above treatments are applied in combination, the E* value is reduced by a factor of approximately 7% for IV and 16% for WI samples. Again, a cumulative effect is observed for the combination of thermal treatment and aging (Table 4). Comparing the two zirconia ceramics, although a more pronounce reduction of both H and E* was recorded for WI zirconia, statistically significant differences were not recorded in any group (Nano-hardness H: Control: t = 0.618, df = 2.019 (unequal variances), p = 0.599, Heat treated: t = −0.81,
a
18
16
Hardness (GPa)
Control
Heat-treatment
Aging
14
Heat-treatment + aging
12
10 IV
WI
8
b
250 Control
Elastic constant (GPa)
240
230 Aging Heat-treatment Heat-treatment + aging
220
210
200
190 IV
df = 4 (equal variances), p = 0.463, Aged: t = 1.253, df = 2.053 (unequal variances), p = 0.334, Heat treated and aged: t = 2.152, df = 4 (equal variances), p = 0.098, Elastic constant E*: Control: t = −1.585, df = 4 (equal variances), p = 0.188, Heat treated: t = 0.051, df = 4 (equal variances), p = 0.962, Aged: t = 0.659, df = 4 (equal variances), p = 0.570, Heat treated and aged: t = 2.124, df = 4 (equal variances), p = 0.101).
3.2.
Surface characterization
XRD surface analysis was performed on all specimens from each group and all XRD patterns were almost identical. Representative XRD patterns from each zirconia ceramic group are presented in Fig. 2. Monoclinic zirconia was not detected on the XRD pattern of the control specimens of either ceramic. However, the tetragonal phase was dominant with percentages around 94–95 wt%. Y2 O3 and HfO2 were detected in small amounts on both ceramics. An average increase of 3–4 wt% of the m-ZrO2 phase and an equivalent reduction of the t-ZrO2 phase was recorded by XRD on both ceramics when heat treatment and aging were applied separately to the starting samples. The greatest variation of zirconia fractions (6.5–9 wt%) was recorded when the above treatments were applied in combination. FTIR analysis was performed on all specimens from each group. As all the spectra were almost identical a representative FTIR spectrum from each zirconia ceramic is presented in Fig. 3. The FTIR spectra of the control specimens present 2 strong broad bands at about 140 cm−1 and about 500 cm−1 assigned to the t-phase and a weak band at about 330 cm−1 assigned to the m-phase of zirconia crystal structure. The FTIR spectra of the specimens after heat treatment, aging, heat treatment and subsequent aging present new distinct peaks which – in comparison with the reference spectrum of monoclinic ZrO2 – suggest a prominent existence of the monoclinic phase in the material. In particular the above spectra present new better defined bands at 585, 510, 443, 350, 258 cm−1 attributed to the m-phase [21]. SEM-EDS analyses revealed similar morphological features, with surfaces become dull after aging, and similar elemental composition for both ceramics. However analyses of three areas on each specimen, presented indicatively for IV ceramics in Fig. 4, revealed that the % weight percent of yttrium was reduced about 25% after aging compared to the control specimens.
WI
180
Fig. 1 – Graphical representation of (a) nano-hardness estimated means (95%CI) ± SE and (b) elastic constant estimated means (95%CI) ± SE for both zirconia cores.
4.
Discussion
According to the results presented previously, the first and second null hypotheses are accepted, while the third that the
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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) e272–e282
Fig. 2 – Representative XRD patterns of (a) IV ceramics, (b) WI ceramics.
combination of heat-treatment and subsequent in vitro aging does not affect the nano-hardness and the elastic constant of the zirconia ceramic cores, was rejected. Different treatments on both ceramic core materials resulted in decrease of
both nano-hardness and elastic constant. Despite the small size of the sample used, when statistical analysis with a linear mixed model based on a split-plot design considering the repeated measurements in each specimen were further
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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) e272–e282
Table 3 – Bonferroni multiple comparison tests of mean H values, between control and treated specimens after One way ANOVA (IV: F(3,8) = 20.383, p < 0.001, WI: F(3,8) = 22.456, p < 0.001). (I) Treatment (J) Treatment
Mean difference (I − J)
Standard error (SE)
p-value
95%CI Lower limit
Upper limit
IV-C
IV-H IV-A IV-HA
1.313 1.153 3.071*
0.397 0.397 0.397
0.064 0.119
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Effect of heat treatment and in vitro aging on the microstructure and mechanical properties of cold isostatic-pressed zirconia ceramics for dental restorations Anna Vatali a , Eleana Kontonasaki a , Panagiotis Kavouras b,d , Nikolaos Kantiranis c , Lambrini Papadopoulou c , Konstantinos K.M. Paraskevopoulos d , Petros Koidis a,∗ a
Department of Fixed Prosthesis and Implant Prosthodontics, School of Dentistry, Aristotle University of Thessaloniki, Greece b Technological Educational Institute of Thessaloniki, Department of Applied Sciences, Sindos, Greece c Geology Department, Aristotle University of Thessaloniki, Greece d Physics Department, Aristotle University of Thessaloniki (A.U.TH.), Greece
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The temperature variations during the veneering firing cycles of a zirconia dental
Received 14 April 2013
ceramic can negatively affect its mechanical properties. A possible synergistic effect of both
Received in revised form 8 July 2013
heat-treatment and aging while exposed to the oral environment could result to catastrophic
Accepted 21 May 2014
failure. The aim of the present study was to investigate the effect of heat treatment followed during veneering and in vitro aging on the mechanical and microstructural properties of zirconia dental ceramics.
Keywords:
Methods. Three specimens from each of two zirconia blocks (Ivoclar IPS e.max ZirCAD (IV) and
Y-TZP zirconia ceramics
Wieland ZENO Zr (WI)) were cut by CAD/CAM technology, fully sintered and polished. Each
Nano-hardness
one was cut in four equal parts. One part was used as control (C), one was heat-treated (H),
Elastic constant
one was aged (A) (134 ◦ C, 2 bar, 10 h) and one was heat-treated and subsequently aged (HA).
Heat-treatment
The mechanical properties (nano-hardness (H) and elastic modulus (E*)) were investigated
Aging
by nano-indentation tests while the surface characterization was carried out with XRD, FTIR
Monoclinic zirconia
and SEM.
XRD
Results. Different treatments on IV and WI samples resulted in a reduction of both H and E*
FTIR
values, however the differences were not statistically significant (p > 0.05). The combination
SEM
of treatments imposes an overall effect (p < 0.001), enhancing the influence on both H and E* values. This reduction in mechanical properties was followed by an increase of monoclinic content. Greater variations in both H and E* values were recorded for WI samples. Significance. The clinical performance of zirconia dental ceramics may be affected during firing and aging resulting in increased probability of failure. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author at: Department of Fixed Prosthesis and Implant Prosthodontics, Aristotle University of Thessaloniki, University Campus, Dentistry Building, GR 54124, Thessaloniki, Greece. Tel.: +30 2310 999659; fax: +30 2310 999676. E-mail address: [email protected] (P. Koidis). http://dx.doi.org/10.1016/j.dental.2014.05.017 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) e272–e282
1.
Introduction
Y-TZP zirconia ceramics have attracted the interest of dental technology in manufacturing core materials for fixed all-ceramic restorations due to their excellent mechanical properties (i.e. bending strength and toughness) [1,2]. The high strength of Y-TZP zirconia ceramics is attributed to a toughening mechanism related to the transformation of the tetragonal phase (t), to the monoclinic (m) phase (natural equilibrium) [3]. This t→m transformation may be triggered by an applied stress and is associated with a net 4 vol% expansion due to the larger volume occupied by the monoclinic phase compared to the tetragonal one [4]. In case of a pre-existing crack the volume expansion associated to the t→m transformation, enables the closure of the crack and diminishes its propagation. This crack propagation prevention minimizes the risk of catastrophic failure of the material due to fracture and is called the “transformation toughening mechanism” [5]. The main drawback of Y-TZP zirconia ceramics is their sensitivity to low temperature degradation (LTD) – aging that leads to exaggerated t→m transformation and a degradation that starts on the surface and propagates into the depth of the material, diminishing its mechanical properties [6–8]. This is of particular interest for dental zirconia ceramics, as their degradation in the oral environment due to the exposure to oral fluids and mechanical stress over prolonged periods of time cannot be overlooked. Although minimally exposed to the oral environment, the margins of zirconia restorations allow a continuous contact of zirconia core with saliva or other fluids that may start and progressively lead to the degradation of the material. Furthermore, there is growing popularity of monolithic zirconia restorations where much larger areas of zirconia are in contact with the oral environment.The majority of the models proposed to explain the spontaneous t→m transformation taking place during LTD-aging, are either based on the formation of zirconium hydroxides (Zr-OH) [9,10] or yttrium hydroxides (Y(OH)3) or Y(O)OH) [11] due to the diffusion of water through the material, promoting phase transition with local stress concentration or variation of the yttrium/zirconium ratio. According to the most recent proposed model [12] oxygen anions are responsible for the transformation nucleation and therefore for the LTD. Due to volume changes the transformed grains cause microcracks and the material becomes degraded. Currently, accelerated tests at intermediate temperatures (100–300 ◦ C) are the only basis for the estimation of the transformation rate and, hence, of the product lifetime [13], although a lot of controversy exists about the validity of the extrapolated predictions [14]. According to Chevalier [13] aging of zirconia with 1 h of autoclave treatment at 134 ◦ C and 2 bar pressure results in a significant t→m transformation that has theoretically the same effect as 3–4 years in vivo, while the ISO standard imposes a maximum of 25 wt% of monoclinic zirconia to be present after an accelerated aging test conducted for 5 h at 134 ◦ C and 2 bar [15]. However Lughi and Sergo [14] state that these accelerating in vitro tests provide only a rough estimate and any extrapolation could lead to a significant error in estimating body temperature lifetimes.
e273
To achieve highly aesthetic zirconia restorations, zirconia core is veneered with a feldspathic porcelain coating and is subjected to firing at high temperatures (750–900 ◦ C) followed by subsequent cooling. This process takes place at least once, but usually it takes two to five firing cycles in order to obtain an aesthetically acceptable restoration [16,17]. The fracture strength [16], microhardness [17] and flexural strength [17] are significantly reduced after veneering firing cycles. According to Oilo et al. [17] this effect is predominantly observed after the first firing cycle while subsequent cycles do not cause a further deterioration of properties. The reduced mechanical properties of zirconia ceramics after heat-treatment have been attributed to residual compressive stresses due to milling and various processing steps – such as grinding and sandblasting – that are released during the heat treatment, as well as to the t→m transformation and the alteration of the grain size that take place during the firing cycles of the veneering process [17]. This degradation of mechanical properties can further reduce the strength of the material after exposure to the oral environment, so a possible synergistic effect of both heat-treatment and aging could result to a probably irreversible premature failure. The aim of the present work was to investigate the effect of heat treatment followed during veneering and the in vitro aging on critical mechanical and microstructural properties at the nano-scale of cold isostatic-pressed zirconia ceramics for dental restorations. The research hypotheses investigated were: 1. the heat treatment does not affect the nano-hardness (H) and the elastic (E*) constant of the zirconia ceramic cores; 2. the in vitro aging does not affect the nano-hardness (H) and the elastic (E*) constant of the zirconia ceramic cores; 3. the combination of heat-treatment and subsequent in vitro aging does not affect the nano-hardness (H) and the elastic (E*) constant of the zirconia ceramic cores.
2.
Materials and methods
Three bar-shaped specimens (28 mm × 4 mm × 2 mm) milled from two zirconia blocks [Ivoclar IPS e.max ZirCAD (LOT: PX0075), and Wieland ZENO Zr (LOT:20090728-07)] by the CAD/CAM technology were sintered to full density, polished with diamond pastes of 3 and 1 m under running water until a mirror-like surface was achieved, and cut with a diamond bur under water cooling into four equal parts each (7 mm × 4 mm × 2 mm). One part was used as control (designated as C), one was heat treated (designated as H), one was aged (designated as A) [steam 121 ◦ C/2 bar/10 h, Kavo autoclave sterilizer 2100 (KavoDental, Biberach/Riss, Germany)] and the last was heat-treated and subsequently aged (designated as HA). The applied heat-treatment corresponded to four firing cycles of the veneering porcelain as shown in Table 1 according to manufacturers’ instructions.
2.1.
Nano-mechanical properties evaluation
The H and E* values were assessed by means of a Hysitron Ubi1 TriboLab modular nano-indentation instrument (Tribolab,
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d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) e272–e282
Table 1 – The applied firing cycles for each zirconia ceramic core. Firing cycle
B (standby temperature) S (pre-heating temperature time) T (heating rate) T (final temperature) H (holding time at T) V1 (vacuum in) V2 (vacuum out)
1st shoulder
1st dentin
Glazing
IV
WI
IV
WI
IV
WI
IV
WI
403 ◦ C 4 50 ◦ C/min 800 ◦ C 1 450 ◦ C 799 ◦ C
575 8 45 980 1 575 980
403 ◦ C 4 50 ◦ C/min 750 ◦ C 1 450 ◦ C 749 ◦ C
575 9 45 900 2 575 900
403 ◦ C 4 50 ◦ C/min 750 ◦ C 1 450 ◦ C 749 ◦ C
575 8 45 890 1 575 890
403 ◦ C 6 60 ◦ C/min 725 ◦ C 1 450 ◦ C 724 ◦ C
575 5 45 880 1 – –
Hysitron Incorporated, Minneapolis). A pyramidal diamond Berkovich indenter with a total included angle of 142.3◦ , and a tip with a radius of curvature of approximately 120 nm, has been utilized. In all cases, a trapezoidal loading–unloading profile was used, with 5 s loading and unloading segments, including a 2 s holding segment. The H and E* values were calculated from the experimental unload–displacement curves using the Oliver and Pharr model [18]. Nano-indentations in all samples were performed with constant load of 4 mN. This load was selected after tests with varying loads performed on the untreated samples, based on the aspect that the produced indentation should have a contact depth higher than one half of the tip’s radius of curvature. This is necessary, in order to have proper onset of plasticity. The contact depth, in all indentation tests, was kept between 80 and 110 nm. The eight different groups of materials were produced in triplicates and each sample was tested in two different locations, where ten indentations were made in every location, thus making a total of 20 indentation prints in every sample and a total of 60 in each different material. Every experimental point in Fig. 2a and b is the mean value of 60 indentations. In order to reach equilibrium with the thermal mass inside the cabinet, all samples were placed into the acoustic and thermal enclosure of the nano-indenter apparatus for several hours prior to testing. Before performing each array of indents, the stability of the nano-indentation instrument was checked by measuring a fused-quartz reference sample. In all cases, the measured values of H and E* of the fused-quartz reference sample were in good agreement with the values reported by the manufacturer, i.e. 9.25 GPa and 69.6 GPa, respectively.
2.2.
2nd dentin
Statistical analysis
Descriptive statistics were calculated by means of min, max, median, mean and standard deviation. The assumption of normality was tested with the Shapiro–Wilk test and it was not rejected. Levene’s test for equality of variances was used to test the homogeneity of variances assumption which also was not rejected. One-way ANOVA was performed to investigate statistically significant differences between groups of specimens, while pair wise comparisons were conducted with the Bonferroni multiple comparison tests for the adjustment of the Type I error. Two independent samples t test were performed to investigate statistically significant differences between the two zirconia ceramics. The analysis was performed with the SPSS 15.0 software and the statistical significance was set for p < 0.05 [19].
2.3.
Surface characterization
X-ray diffraction (XRD) surface analysis was carried out for the determination of the amount of m-ZrO2 transformation using a Philips diffractometer (PW1710; Philips, Eindhoven and Almelo, The Netherlands) with Ni-filtered CuK␣ radiation. The counting parameters of the XRD study were: step size: 0.05◦ 2Â, start angle: 5◦ , end angle: 53◦ and scan speed: 1, 2◦ 2Â/min. The monoclinic phase fraction Xm was calculated according to the Garvie and Nicholson [20] method. Fourier Transform Infrared Spectroscopy (FTIR) was used to qualitatively evaluate the m-ZrO2 transformation. The FTIR reflectance spectra of the specimens were obtained in the spectral area of MIR and FIR (4000–120 cm−1 ), using a Brüker FTIR extended spectrometer, model IFS113v with a resolution of 2 cm−1 . Scanning electron microscopy-Energy dispersive spectroscopy (SEM-EDS) analysis was performed on the surfaces of carbon-coated specimens (J.S.M. 840A; JEOL, Tokyo, Japan) in order to observe any differences concerning surface morphology or elemental composition owed to the various treatments applied. SEM microphotographs of arbitrarily selected specimens were obtained and backscattered microphotographs were collected. Chemical analyses were performed with 20 kV accelerating voltage and 0.4 mA probe current. Pure Co was used as an optimizing element.
3.
Results
3.1.
Nano-mechanical properties evaluation
The results of nano-mechanical tests are listed in Table 2 and presented in the diagrams of Fig. 1. Different treatments on IV and WI samples produce similar trends of the variation of the H value. More specifically, when thermal treatment and aging are applied separately the H value is reduced by a factor of approximately 7.5%. When the above treatments are applied in combination and consecutively (first thermal treatment followed by aging) the H value is reduced by a factor of approximately 20% for IV and 25% for WI samples. From the above it is clear that the combination of thermal treatment and aging on the same sample impose a cumulative effect, enhancing the influence on the H value by a factor of 3.0 up to 3.5. Although H value is reduced after ageing and thermal treatment, these differences are not statistically significant (Table 3). However, thermally treated and consequently aged specimens presented statistically significant lower H compared to control and separately applied aging and heat-treatment (Table 3).
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Table 2 – Descriptive statistics of the H and E* values. Sample Control Heat-treatment Aging Heat-treatment + aging
IV H (GPa)
WI H (GPa)
IV E* (GPa)
WI E* (GPa)
15 ± 0.06 13.7 ± 0.5 13.8 ± 0.08 11.9 ± 0.83
14.7 ± 0.81 14 ± 0.49 13.3 ± 0.7 10.7 ± 0.47
223.5 ± 4.8 214.4 ± 9.5 217.9 ± 6.6 209.6 ± 5.2
237.7.0 ± 14.8 214.1 ± 3.8 215.3 ± 2.9 198.7 ± 7.1
Similarly to H, different treatments on IV and WI samples produce similar trends of the variation of the E* value but those of the WI sample are greater. When thermal treatment and aging are applied separately the E* value is reduced by a factor of approximately 4% for the IV sample and 10% for the WI sample. When the above treatments are applied in combination, the E* value is reduced by a factor of approximately 7% for IV and 16% for WI samples. Again, a cumulative effect is observed for the combination of thermal treatment and aging (Table 4). Comparing the two zirconia ceramics, although a more pronounce reduction of both H and E* was recorded for WI zirconia, statistically significant differences were not recorded in any group (Nano-hardness H: Control: t = 0.618, df = 2.019 (unequal variances), p = 0.599, Heat treated: t = −0.81,
a
18
16
Hardness (GPa)
Control
Heat-treatment
Aging
14
Heat-treatment + aging
12
10 IV
WI
8
b
250 Control
Elastic constant (GPa)
240
230 Aging Heat-treatment Heat-treatment + aging
220
210
200
190 IV
df = 4 (equal variances), p = 0.463, Aged: t = 1.253, df = 2.053 (unequal variances), p = 0.334, Heat treated and aged: t = 2.152, df = 4 (equal variances), p = 0.098, Elastic constant E*: Control: t = −1.585, df = 4 (equal variances), p = 0.188, Heat treated: t = 0.051, df = 4 (equal variances), p = 0.962, Aged: t = 0.659, df = 4 (equal variances), p = 0.570, Heat treated and aged: t = 2.124, df = 4 (equal variances), p = 0.101).
3.2.
Surface characterization
XRD surface analysis was performed on all specimens from each group and all XRD patterns were almost identical. Representative XRD patterns from each zirconia ceramic group are presented in Fig. 2. Monoclinic zirconia was not detected on the XRD pattern of the control specimens of either ceramic. However, the tetragonal phase was dominant with percentages around 94–95 wt%. Y2 O3 and HfO2 were detected in small amounts on both ceramics. An average increase of 3–4 wt% of the m-ZrO2 phase and an equivalent reduction of the t-ZrO2 phase was recorded by XRD on both ceramics when heat treatment and aging were applied separately to the starting samples. The greatest variation of zirconia fractions (6.5–9 wt%) was recorded when the above treatments were applied in combination. FTIR analysis was performed on all specimens from each group. As all the spectra were almost identical a representative FTIR spectrum from each zirconia ceramic is presented in Fig. 3. The FTIR spectra of the control specimens present 2 strong broad bands at about 140 cm−1 and about 500 cm−1 assigned to the t-phase and a weak band at about 330 cm−1 assigned to the m-phase of zirconia crystal structure. The FTIR spectra of the specimens after heat treatment, aging, heat treatment and subsequent aging present new distinct peaks which – in comparison with the reference spectrum of monoclinic ZrO2 – suggest a prominent existence of the monoclinic phase in the material. In particular the above spectra present new better defined bands at 585, 510, 443, 350, 258 cm−1 attributed to the m-phase [21]. SEM-EDS analyses revealed similar morphological features, with surfaces become dull after aging, and similar elemental composition for both ceramics. However analyses of three areas on each specimen, presented indicatively for IV ceramics in Fig. 4, revealed that the % weight percent of yttrium was reduced about 25% after aging compared to the control specimens.
WI
180
Fig. 1 – Graphical representation of (a) nano-hardness estimated means (95%CI) ± SE and (b) elastic constant estimated means (95%CI) ± SE for both zirconia cores.
4.
Discussion
According to the results presented previously, the first and second null hypotheses are accepted, while the third that the
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Fig. 2 – Representative XRD patterns of (a) IV ceramics, (b) WI ceramics.
combination of heat-treatment and subsequent in vitro aging does not affect the nano-hardness and the elastic constant of the zirconia ceramic cores, was rejected. Different treatments on both ceramic core materials resulted in decrease of
both nano-hardness and elastic constant. Despite the small size of the sample used, when statistical analysis with a linear mixed model based on a split-plot design considering the repeated measurements in each specimen were further
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Table 3 – Bonferroni multiple comparison tests of mean H values, between control and treated specimens after One way ANOVA (IV: F(3,8) = 20.383, p < 0.001, WI: F(3,8) = 22.456, p < 0.001). (I) Treatment (J) Treatment
Mean difference (I − J)
Standard error (SE)
p-value
95%CI Lower limit
Upper limit
IV-C
IV-H IV-A IV-HA
1.313 1.153 3.071*
0.397 0.397 0.397
0.064 0.119
