Phase and Stress Analysis of a New Glass/Zirconia/Glass Restorative Material

Objective: Recent studies demonstrate that a graded glass/zirconia/glass (GZG) structure exhibits improved flexural damage resistance relative to monolithic zirconia. The objective of this study is to elucidate weather the strengthening effect is due to a surface compressive stress developed from the glass infiltration process and/or zirconia tetragonal-to-monoclinic phase transformation.

Method: GZG plates (12x12x1.5 mm) and crowns (anatomically-correct, average thick 1.5 mm) were prepared by infiltrating heat-treated monolithic zirconia plates and crowns (1350°C for 1h) with an in-house prepared glass at 1450°C for 2 h. Specimens were sectioned across the graded layers; microstructures were revealed by SEM. Using micro-level beam size X-ray, with a step size of 50μm, a series of diffraction patterns from the outer surface glass layer, through graded glass/zirconia zone to bulk zirconia were obtained. The Sin²Ψ technique was employed to analyze the residual stresses at the surface of graded layers and the interior zirconia core.

Result: There is no detectable monoclinic zirconia phase present in GZG plates and crowns. The main diffraction peak at 2θ = 28.1º indexed by tetragonal phase increased first and remained constant in intensity while the micro X-ray irradiated consecutively from the outer glass layer, through the graded glass/zirconia zone to the bulk zirconia.  Micro x-ray analysis revealed that the glass-infiltration depth in these zirconias was ~150–200µm, consistent with SEM observations. Residual stresses at the graded glass/zirconia surface and at the center of the zirconia core were approximately 25MPa (tension) and -33MPa (compression), respectively.

Conclusion: Neither tetragonal-to-monoclinic phase transformation nor significant residual stresses have been found in GZG plates and crowns. Our results suggest that the strengthening effect from surface grading is not due to the surface compressive residual stress; it may be due to the elastic modulus gradients at the surface. Supported by NIH/NIDCR-R01DE017925 and NSF/CMMI-0758530.