Effect of YSZ thin film coating thickness on the strength of a ceramic substrate

Although ceramics are used for many different biomedical applications they are brittle materials that can be compromised by surface defects when under stress. The objective of this study was to evaluate the effect of surface modification with an yttria-stabilized zirconia (YSZ) thin film coating on the strength of a machinable dental ceramic. Fifty bars (2 mm x 2 mm x 15 mm) were cut from ProCAD (Ivoclar-Vivadent) blocks. Specimens were wet-polished through 1200-grit SiC abrasive. One surface of each bar was sandblasted with 50 μm Al₂O₃ abrasive (0.34 MPa). Specimens were further modified through the deposition of a sputtered YSZ thin film on the sandblasted surface. Different thin film thicknesses were evaluated: 1, 3, 5, and 7 μm. Depositions were performed using a radio frequency magnetron sputter system (working pressure of 15 mT, 150°C, 30:1 Ar/O₂ gas ratio). Flexural strength measurements were carried out by three-point bending (span = 10 mm) in a servo-electric material testing system in DI water (37°C). The results showed that the strength of porcelain significantly increased with the deposition of a 3-μm YSZ thick coating. A nonlinear relationship was observed between film thickness and strength. Strengthening of porcelain is shown through the application of a sputtered YSZ thin film. It is presumed that the strengthening mechanism is due to modification of surface flaws and/or surface residual stress by the applied thin film.