Thermal induced deflection of a porcelain-zirconia bilayer: Influence of cooling rate.

OBJECTIVE: To determine the thermal expansion of a porcelain (VM9) and tetragonal zirconia (Y-TZP) as well as the deflection upon re-heating and cooling of a bilayer fabricated from these two materials after slow and rapid cooling during initial fabrication. METHODS: The coefficient of thermal expansion (CTE) of bulk porcelain and Y-TZP as well as bilayer beam deflection was measured with a novel non-contact optical dilatometer. The influence of cooling rate during initial fabrication of the porcelain-zirconia bilayer and the bulk porcelain during subsequent heating and cooling is investigated. Specimens were heated to 900°C in the dilatometer, well in excess of the glass transition temperature (Tg) and softening temperature (Ts) of the porcelain. RESULTS: The thermal expansion of the porcelain above Tg exhibits a threefold increase in CTE over that observed below Tg. Observations of the bilayer deflection reflect the difference in the CTE of the component materials and enable Tg and Ts temperatures for the porcelain to be estimated. Initial cooling rate of the porcelain and porcelain-YTZP bilayer was found to have a profound influence on the subsequent response to slow reheating and cooling as well as the resultant residual deflection. SIGNIFICANCE: The estimation of the residual stress and potential for chipping of porcelain-zirconia dental restorative systems should not be based solely on thermal expansion data measured below Tg.

Influence of the bonder on the adhesion of porcelain to machined titanium as determined by the strain energy release rate.

OBJECTIVE: To determine the adhesion at the titanium-porcelain interface using a fracture mechanics approach, and to investigate the bonding mechanism using SEM. METHODS: Specimens of five different titanium-porcelain and one base metal-porcelain bonding systems were prepared for a four-point bending interfacial delaminating test on a universal testing machine. The pre-cracked specimen was subjected to load and the strain energy release rate (G, J/m(2)) was calculated from the critical load to induce stable crack extension in each system. The interface for the various materials was investigated in an SEM and compared. RESULTS: The titanium-porcelain with Gold Bonder showed the highest G-value (72.39+/-13.21J/m(2)) among the groups whilst titanium-porcelain with cross-cut-bur preparation showed the lowest (5.78+/-1.39J/m(2)). The former was significantly higher than that of Wiron 99 (base-metal, BEGO, Germany) porcelain (40.01+/-6.67J/m(2)), a clinically accepted bonding system for many years. The G-values of porcelain fused to titanium-Rocatec, titanium-sandblasted and/or titanium-GC-Bonder were 10.81+/-1.49, 12.64+/-3.01 and 35.74+/-5.20J/m(2), respectively. SEM images of the interface fracture crack path for the different bonders enabled the mechanisms responsible for the differences in strain energy release rates to be appreciated. CONCLUSION: The strain energy release rate (G) of titanium-porcelain with a Gold Bonder interface layer was highest among the five different systems.

ADM guidance-ceramics: Fatigue principles and testing.

BACKGROUND: Clinical failure of dental ceramics is usually reported as partial fracture of the restoration (chipping) or as catastrophic fracture of the whole structure. In contrast to metals, ceramics are linear-elastic, brittle materials exhibiting extremely low damage tolerance to failure. Well documented clinical and lab reports have shown this fracture event often occurs at loads far below their fracture strength due to intrinsic fatigue degradation via slow crack growth or cyclic fatigue mechanisms. The presence and development of surface flaws have a dominant role in damage accumulation and lifetime reduction of ceramic structures. AIMS: This ADM guidance document aims to summarize the aspects related to fatigue degradation of dental ceramics, reviewing the concepts of fatigue testing and furthermore aims to provide practical guidance to young scientists entering into fatigue related research. The description of fatigue strength is always accompanied by a clear understanding of the underlying fracture mechanisms.