Microstructural Effects on Fracture Strength of YSZ Thin-film Modified Constructs

Objective: Recent reports have shown improvements in fracture strengths of dental ceramics of up to 32% when coated with YSZ films. By combining substrate bias during sputter deposition, unique microstructures consisting of lateral defects and increased film stress can be achieved.  These attributes are thought to enhance fracture strength of a construct by way of crack-deflection and resistance to crack expansion. Here, we propose a model for the strengthening effect based on compressive film forces acting on a critical surface flaw.

Method: YSZ films with varying microstructures were deposited onto porcelain (ProCAD Ivoclar-Vivadent) substrates (n=12) that were wet-polished through 1200-grit using SiC abrasive, then air-abraded on one surface (50µm, Al₂O₃, 275kPa). Depositions were performed via r.f. magnetron sputtering (5mT, 25ºC , 30:1 Ar/O₂ gas ratio) with varying deposition times (film thickness) and bias powers. Flexural and biaxial fracture strength tests were performed in de-ionized water at 37ºC and fractures surfaces characterized by optical microscopy and scanning electron microscopy (SEM).

Result: Film stress behaviors are found to vary with changes in film thickness and substrate bias as a result of changes in microstructure. A model will be presented to describe the contribution of compressive forces as a function of film thickness on overall fracture strength. Mechanical testing data showed an increase in fracture strength of up to 30% over unmodified specimens.  

Conclusion: Data suggest that interrupted microstructures exhibit increased film stresses resulting in a larger magnitude of compressive forces acting on the substrate surface. The control of film microstructure and stress are expected to result in improved fracture strengths of dental constructs in the future.  This work is supported through NIH-NIDCR grant 2RO1DE013511-10.