Experimental and finite element study of residual thermal stresses in veneered Y-TZP structures

Abstract

The main complications of zirconia-based laminated systems are chipping and delamination of veneering porcelain, which has been found to be directly associated with the development of residual thermal stresses in the porcelain layer. This study investigates the effects of cooling rate and specimen geometry on the residual stress states in porcelain-veneered zirconia structures. Bilayers of three different shapes (bars, semi-cylindrical shells, and arch-cubic structures) with 1.5 mm and 0.7 mm thickness of dentin porcelain and zirconia framework, respectively, were subjected to two cooling protocols: slow cooling (SC) at 32 °C/min and extremely-slow cooling (XSC) at 2 °C/min. The residual thermal stresses were determined using the Vickers indentation method and validated by finite element analysis. The residual stress profiles were similar among geometries in the same cooling protocol. XSC groups presented significantly higher tensile stresses (p = 0.000), especially for curved interfaces. XSC is a time-consuming process that showed no beneficial effect regarding residual stresses compared to the manufacturer recommended slow cooling rate.

Do thermal treatments affect the mechanical behavior of porcelain-veneered zirconia? A systematic review and meta-analysis

“…The authors also observed that the fast cooling (furnace opening at 800˚C -aboveT g ) led to high levels of transient tensions, which were associated to internal microcracks and, consequently, early failures. Moreover, previous studies have observed distinct residual stresses in curved areas of PVZ crowns, [41][42][43] especially when fast cooling was applied. 43 Thus, it is plausible that the difference in residual stress distributions due to different geometries and thicknesses led to the contradictory meta-analyses results regarding flexural strength and critical load to failure.…”

“…Finite element analysis (FEA) studies reported that cooling down restorations from 50˚C below T g is enough to decrease residual stresses and avoid thermal gradients; 42 and that an overextended cooling protocol (closed furnace until 25˚C at 2˚C/min), besides time consuming, added more tensile and compressive residual stresses throughout the porcelain layer, compared to a more reasonable slow cooling regimen (closed furnace until 450˚C). 41 Our two meta-analyses comparing fast cooling from above T g versus slow cooling, for flexural strength (Fig. 2) and critical load to failure (Fig.…”

“…40 Nonetheless, one should note that the authors compared fast cooling with an overextended slow cooling regimen (down to room temperature, taking around 600 min). An aforementioned FEA study 41 showed that overextended slow cooling below T g is deleterious even for flat specimens.…”

“…The majority of studies evaluated cooling rate (10 studies), 13,[22][23][24][36][37][38][39][40][41] two evaluated number of veneer firings, 26,27 one evaluated self-glaze, 25 and one investigated alternative firings. 28 The included studies used 9 different commercial brands of veneer porcelains, presenting a range of physical characteristics, such as elastic modulus (E), coefficient of thermal expansion (CTE) and glass transition temperature (T g ).…”

Using glass-graded zirconia to increase delamination growth resistance in porcelain/zirconia dental structures

“…In general, when cracks in the porcelain veneer reach the veneer/core interface, they tend to graze along the interface before deflecting back into the porcelain veneer rather than penetrate the stiffer and tougher zirconia core [4,7]. This type of failure is motivated by the low interfacial fracture energy G C of PFZ [8,9] as well as presence of deleterious residual tensile stresses, formed during the veneer firing or glaze firing process due to mismatch in thermal expansion coefficients between veneer and core and low thermal diffusivities characterizing both porcelain and zirconia [10–16]. The weak delamination resistance of the P/Z interface is generally attributed to the chemical inertness of the zirconia phase.…”