Effect of multiple firing and silica deposition on the zirconia–porcelain interfacial bond strength

Abstract

Objectives

To test the hypothesis that multiple firing and silica deposition on the zirconia surface influence the bond strength to porcelain.

Materials and methods

Specimens were cut from yttria-stabilized zirconia blocks and sintered. Half of the specimens (group S) were silica coated (physical vapor deposition (PVD)) via reactive magnetron sputtering before porcelain veneering. The remaining specimens (group N) had no treatment before veneering. The contact angle before and after silica deposition was measured. Porcelain was applied on all specimens and submitted to two (N2 and S2) or three firing cycles (N3 and S3). The resulting porcelain–zirconia blocks were sectioned to obtain bar-shaped specimens with 1 mm² of cross-sectional area. Specimens were attached to a universal testing machine and tested in tension until fracture. Fractured surfaces were examined using optical microscopy. Data were statistically analyzed using two-way ANOVA, Tukey's test (α = 0.05) and Weibull analysis.

Results

Specimens submitted to three firing cycles (N3 and S3) showed higher mean bond strength values than specimens fired twice (N2 and S2). Mean contact angle was lower for specimens with silica layer, but it had no effect on bond strength. Most fractures initiated at porcelain–zirconia interface and propagated through the porcelain.

Significance

The molecular deposition of silica on the zirconia surface had no influence on bond strength to porcelain, while the number of porcelain firing cycles significantly affected the bond strength of the ceramic system, partially accepting the study hypothesis. Yet, the Weibull modulus values of S groups were significantly greater than the m values of N groups.

Introduction

Zirconia-based ceramics is being proved a successful infrastructure material. Studies have reported no fractures of the zirconia infrastructure in short or medium-term periods of clinical observation [1]. The failures of porcelain–zirconia restorations are frequently associated with biological complications [2], such as secondary caries, periodontal support and tooth fracture, which likely are not related to the materials used in fixed prostheses [2] and some structural deficiencies of the material such as marginal discrepancies, weak performance of the veneering ceramics [3] and its limited bond to the zirconia substrate [4]. Chip-off (chipping) and veneer fracture with exposure of the zirconia core ceramic (delaminations) are clinical problems that are currently the subject of comprehensive investigations [3], [5], [6] and remain to be overcome.

Studies showed that residual tensile stresses are created within the porcelain at or near the ceramic–porcelain interface and could contribute to the porcelain fracture in zirconia systems [7]. The stresses are believed to have multifactorial origin, potentially involving the ceramic thermal history, restoration's geometric factors (i.e. infrastructure design and core–veneer thickness ratio), material's elastic properties (ceramics strength) and poor wetting of the core by the veneer [5], [8], [9], [10]. Therefore, these factors should be handled carefully to prevent development of high-magnitude tensile stresses.

The application of several layers of porcelain and, consequently, using multiple firing cycles, may be necessary for the fabrication of all-ceramic restorations, especially when using the standard layering technique to match the esthetics of the natural dentition. The effect of multiple firing cycles on the porcelain–zirconia adhesion remains unclear. Residual stresses can likely accumulate during the heating and cooling firing procedures mainly because of the cooling rate and the coefficient of thermal expansion (CTE) mismatch between core and veneer ceramics [7], [11], [12]. When additional stresses are applied to the restoration, the probability of failure due to fatigue crack propagation might increase [7], [13], [14], [15], [16], explaining the delamination, which is considered a clinical failure mode. In addition, it has been reported on the negative influence of water, alcohol and modeling liquid on faceting Y-TZP grains at the interface with porcelain [17].

In order to improve the porcelain–zirconia mechanical behavior and the bond strength between zirconia and resin cements, Zhang and Kim [18] developed a graded glass–zirconia structure: glass was incorporate to the external surface of the zirconia core. This graded material showed high resistance to intermediate flexural damage, better compatibility with the veneering porcelain and with the resin cements than homogeneous zirconia. This could be particularly interesting to the analysis of the porcelain–zirconia interface, where an improvement of the zirconia surface wettability, by a glass layer deposition, could lead to an increase in the quality of core–veneer bond strength.

A silica thin layer deposited by physical vapor is believed to be a reasonable approach to obtain a similar high-quality graded structure (glass/zirconia) based on the rationale that silicon oxides can be molecular deposited on the zirconia surface as an interlayer [19], improving the wettability at the interface. Additionally, a silica layer could reduce the negative effect of liquids from the slurry on the Y-TZP grains. Thus, assuming that good core–veneer bond strength is a clinical requirement to avoid premature failure of all-ceramic restorations [4], such as veneer delamination, the relationship between sequential number of porcelain firings and the presence of a thin silica layer should be investigated.

The objective of this study was to evaluate the effect of a silica thin layer deposited on the zirconia infrastructure and the number of porcelain firings on the zirconia–porcelain bond strength, testing the hypotheses that the ceramic bond strength increases with the presence of a silica interlayer and with the number of porcelain firing cycles.