Effects of aging procedures on the topographic surface, structural stability, and mechanical strength of a ZrO₂-based dental ceramic

Abstract

Objective

To determine the effects of different aging methods on the degradation and flexural strength of yttria-stabilized tetragonal zirconia (Y-TZP)

Methods

Sixty disc-shaped specimens (∅, 12 mm; thickness, 1.6 mm) of zirconia (Vita InCeram 2000 YZ Cubes, VITA Zahnfabrik) were prepared (ISO 6872) and randomly divided into five groups, according to the aging procedures (n = 10): (C) control; (M) mechanical cycling (15,000,000 cycles/3.8 Hz/200 N); (T) thermal cycling (6,000 cycles/5–55 °C/30 s); (TM) thermomechanical cycling (1,200,000 cycles/3.8 Hz/200 N with temperature range from 5 °C to 55 °C for 60 s each); (AUT) 12 h in autoclave at 134 °C/2 bars; and (STO) storage in distilled water (37 °C/400 days). After the aging procedures, the monoclinic phase percentages were evaluated by X-ray diffraction (XRD), and topographic surface analysis was performed by 3D profilometry. The specimens were then subjected to biaxial flexure testing (1 mm/min, load 100 kgf, in water). The biaxial flexural strength data (MPa) were analyzed by 1-way ANOVA and Tukey's test (α = 0.05). The data for monoclinic phase percentage and profilometry (Ra) were analyzed by Kruskal–Wallis and Dunn's tests.

Results

ANOVA revealed that flexural strength was affected by the aging procedures (p = 0.002). The M (781.6 MPa) and TM (771.3 MPa) groups presented lower values of flexural strength than did C (955 MPa), AUT (955.8 MPa), T (960.8 MPa) and STO (910.4 MPa). The monoclinic phase percentage was significantly higher only for STO (12.22%) and AUT (29.97%) when compared with that of the control group (Kruskal–Wallis test, p = 0.004). In addition, the surface roughnesses were similar among the groups (p = 0.165).

Significance

Water storage for 400 days and autoclave aging procedures induced higher phase transformation from tetragonal to monoclinic; however, they did not affect the flexural strength of Y-TZP ceramic, which decreased only after mechanical and thermomechanical cycling.

Introduction

The quest for esthetics in dentistry has led to the search for metal-free restorative materials, such as ceramics [1]. The use of stabilized zirconium oxide (ZrO₂) as an esthetic material in ceramic restorations has emerged as meeting the needs of most resistant prostheses. Zirconium oxide is a polymorphic crystal found in three crystallographic forms: monoclinic, tetragonal, and cubic. The addition of 3% mol of yttrium oxide to zirconium oxide improves its molecular stability, allowing for the production of zirconia stabilized by yttria (Y-TZP). Thus, zirconium oxide is kept in its tetragonal phase at room temperature [2] and has improved mechanical properties [3], [4], [5], [6].

Improvement of the mechanical behavior of Y-TZP is attributed to the following: when a fracture in this material is initiated, the grains of tetragonal-phase zirconia are transformed in monoclinic phase at the crack tip. In this process an increase of grains volume caused by the phase transformation induces to compressive stress difficulting the fracture propagation [3], [4], [5], [6], [7].

However, problems with the degradation of zirconia containing yttria as a stabilizer have been reported in femoral prostheses [8]. Tetragonal-to-monoclinic transformation was observed on the ceramic surfaces maintained at a temperature of 100–500 °C [9], [10] in contact with water [11] or body fluids at 37 °C, and this is called low-temperature degradation (LTD). Degradation of Y-TZP is a process dependent on the time of exposure [8], and the tetragonal-to-monoclinic transformation is accompanied by a 4% volume increase of zirconia grains under the surface in contact with moisture [12]. The pressure generated in the grains around the monoclinic zirconia results in stress in this area, inducing phase transformation in the surrounding grains. Monoclinic grain detachment occurs, creating a roughened surface [12] with microcracks [13]. The uncontrolled transformation at the surface is accompanied by decreased mechanical performance [14], [15].

Studies on the degradation of dental zirconia have been conducted to determine whether degradation has an impact on the survival of this material [15], [16]. It is known that stress can affect the tetragonal-to-monoclinic transformation. When tetragonal zirconia undergoes tension, the stress is minimized through the increase in its dimensions when it becomes monoclinic. This expansion reduces the initially applied tractional tension [7]. This stress can be caused by laboratory tests, with cyclic loading in water, which significantly reduces the mechanical properties of the material and can then affect their long-term performance [17]. In contrast, according to Borchers et al. [17], the decreased load-support capacity after thermal and mechanical cycling and after long-term storage in water at 37 °C did not appear to be caused by degradation of zirconia, perhaps because the transformation zone did not extend to a sufficient depth; thus, additional studies on this topic are needed.

Y-TZP has been the material of choice for infrastructure in metal-free restorations. Thus, better understanding is needed of aging mechanisms and their relationship with Y-TZP strength and microstructure. The effects of zirconia aging, specifically in the oral environment, and its relation to phase transformation from tetragonal to monoclinic have not yet been elucidated.

Since zirconia restorations are exposed to wet environments with cyclic loading in the oral cavity, an evaluation of this material under similar conditions is essential to its successful use in dentistry [16]. Therefore, the aim of this study was to evaluate the influence of aging procedures on the topographic surface, structural stability, and flexural strength of a ZrO₂-based dental ceramic. The hypotheses investigated were that: (1) the aging procedures would decrease the flexural strength of the ceramic, (2) the monoclinic phase percentage would be higher for all experimental groups, and (3) the aging procedures would increase the roughness values for ZrO₂ ceramic.