Fatigue behavior in water of Y-TZP zirconia ceramics after abrasion with 30 μm silica-coated alumina particles
Introduction
Due to its excellent mechanical properties (i.e. bending strength and toughness) Yttria stabilized tetragonal zirconia polycrystal (Y-TZP) has become an attractive high-toughness core material for fixed dental restorations [1], [2], [3], [4], [5], [6]. The innovation of zirconia ceramics lies in its stress-induced toughening mechanism, which impedes crack propagation [7], [8], [9]. During stress application, the tetragonal crystallites (t) transform into their monoclinic (m) form at the crack tip. The concomitant 3–5% expansion in volume [10] leads to the development of internal compressive stresses that oppose crack propagation and thus increase the material's fracture toughness (KIc) and strength. Transformation can be induced by any externally applied stress such as generated by grinding, upon impact or during outright fracture. Grinding Y-TZP and sandblasting will create compressive stresses in the surface due to the t → m phase transformation [11], [12]. The magnitude of the increase in strength depends on the volume % of transformed ZrO2 and the depth of the compressive layer underneath the surface (i.e. the “transformed zone depth”) [11], [12]. Still, forceful grinding will introduce deep surface flaws, which extend beyond the surface compressive layer. These will act as stress concentrators and curtail the strength of the workpiece [11], [13], [14], [15].
Sandblasting of zirconia ceramic with 50 or 110 μm alumina particles improves adhesion to resin-based cements due to increased surface roughness [16], [17]. Also, high adhesive bond strengths were demonstrated when sandblasting with 30 μm silica-coated alumina particles at ca. 2.8 bar in combination with phosphate monomer resin-based cements [18], [19], [20], [21], [22]. Concerns have been raised regarding the surface damages after sandblasting with alumina particles of 50–120 μm at pressures of 2.8–4 bar [13], [23], [24], [25]. Kosmač [13] reported a decline in the reliability (from Weibull m = 11–7.5) of zirconia specimens after sandblasting with 110 μm alumina particles. The deterioration was in direct relation with the damages affecting the ceramic surface. The authors warned that these impact flaws might develop into severe stress intensifiers when exposed to a wet environment under cyclic loading and thus facilitate crack nucleation, even at lower levels of applied stress. Along the same lines, Zhang et al. [23], [25] reported a 30% decrease in strength during contact fatigue testing after sandblasting mirror polished surfaces of Y-TZP with 50 μm alumina particles at 2.8 bar. The decline was explained by the formation of microcracks (>4 μm) in the specimens which under sustained loading would grow and promote radial crack initiation [24].
Clinically, the typical surfaces of zirconia frameworks for dental restoration are not mirror polished but utilized as machined by the CAD–CAM devices. They thus harbor a variety of initial surface flaws, which may be further affected by sandblasting. On CAD–CAM machined Lava™ zirconia specimens with Ra = 0.21 μm, Curtis et al. [26] found no difference in biaxial strength and reliability when tested in water after sandblasting with 25, 50 or 110 μm Al2O3. In contrast Wang et al. [27] demonstrated an increase in dry static flexural strength and reliability after sandblasting a CAD–CAM machined Y-TZP (Cercon®) (Ra = 1.9 μm) ceramic with 50 μm alumina particles. These seemingly contradictory findings may actually be related to a variety of factors such as the nature of the defects induced by machining and processing, the t–m phase distribution in the surface and the residual stress state prior to testing.
Concerns have been also expressed regarding the degradation of zirconia ceramic in the oral environment due to the exposure to water and mechanical stress over prolonged periods [28], [29], [30], [31], [32]. Still, Y-TZP is currently the strongest ceramic in clinical use [33], [34], [35]. Further, its sandblasting with 30 μm silica-coated alumina particles has been recommended to improve the adhesion to resin-based cements [18], [20], [21], [22]. It is unclear whether or not a “less aggressive” sandblasting with 30 μm silica-coated alumina particles does affect the fatigue resistance of CAD–CAM machined Y-TZP ceramics. In this context, stress vs. number of cycles diagrams (S–N- or Wöhler curves) will allow the experimentalist to define a stress range (i.e. the “fatigue limit”) below which failure will not occur on any realistic time scale.
The objectives of the present study therefore were: (1) To draw S–N diagrams and determine the fatigue limits of four Y-TZP zirconia materials “as received” and “sandblasted” with 30 μm silica-coated alumina particles. Further to determine median Kaplan–Meier survival stresses under cyclic fatigue loading. (2) To characterize the type of flaws affecting fatigue failure using fractographic analysis. (3) To determine the proportions of tetragonal vs. monoclinic phases “as received” and “sandblasted” using X-ray diffraction.
Section snippets
Materials and methods
The zirconia materials used in this study are summarized in Table 1. Bars 40 mm × 3 mm × 5 mm with beveled edges were provided “as received” by the manufacturers for each Y-TZP material. Prior to testing, each bar was subjected to four firing cycles (930 °C, 900 °C, 890 °C, 880 °C) corresponding to the firing programs for “liner”, “dentin 1”, “dentin 2” and “glaze” of a zirconia veneering ceramic (Zirox®, Wieland) to duplicate the effect of veneering ceramic application.
Results
The materials characteristics regarding microstructure, density, roughness and Weibull parameters are summarized below.
Discussion
The essential issue of this project was to determine whether the fatigue limit of CAD–CAM machined Y-TZP ceramics was negatively affected by sandblasting with 30 μm silica-coated alumina particles. The results clearly demonstrate that this did not occur. On the contrary, the “soft” (30 μm at 2.5 bar) sandblasting procedure augmented the fatigue limit by 15–31%. Further, for all 4 Y-TZPs, the failure stress at the median probability of survival was also raised. The fractographic analysis provided
Acknowledgements
The authors are grateful to the following companies, Wieland, KaVo and 3M Espe for providing the zirconia bars. Many thanks are expressed to Mr. Paul Goering from 3M Espe for providing the CoJet Sand, Mr. Radovan Cerny (University of Geneva, Department of Crystallography) for the Rietveld analysis, Mr. Robert Schnagl (3M Espe) for the microstructure images as well as Mr. George Quinn (Univ. of Baltimore, Maryland) for his comments and fruitful discussions.
This study was supported in part by a
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