Fracture behavior of dental porcelain modified by zirconia-parylene thin-film

Objective: To evaluate the basic fatigue parameters of a dental porcelain modified by deposition of a YSZ-parylene thin-film, and compare the data to that of an unmodified control (porcelain, sandblasted). Methods: 200 bars (2mm x 2mm x 15mm) were cut from ProCAD (Ivoclar-Vivadent) blocks. Specimens were wet-polished through 1200-grit SiC abrasive. One surface of each bar was sandblasted with 50µm Al₂O₃ abrasive (50psi). Half of the specimens were further modified through deposition of 5 layers of 1.25µm yttria-stabilized zirconia (YSZ) + 0.75µm parylene thin-film on the sandblasted surface. Depositions were performed using an RF magnetron sputter system (working pressure of 15mT, 150ºC, 30:1 Ar/O₂ gas ratio) for YSZ layers and a vapor deposition process for the parylene layers. Specimens were tested at different stressing rates: 5.0, 0.1, and 0.01 MPa/s (n=25/group) in DI water (37ºC), and inert strength was determined in air (25ºC, 70MPa/s). All strength measurements were carried out by three-point bending in a servo-electric test system (MTS-Evolution). Results: The table displays the average flexural strength and standard deviation of both tested groups.

Stressing Rates MPa/s	Uncoated	Coated
70 (inert)	126.3 (15.0)	151.7 (23.6)
5	105.7 (10.6)	143.7 (21.9)
0.1	99 (7.9)	120.9 (13.9)
0.01	89.5 (9.1)	106.7 (13.6)

The fatigue parameters n and lnB were calculated by linear regression of dynamic fatigue data. For the uncoated group, n=37 and lnB=18MPa²s, and n=20 and lnB=12MPa²s for the coated group. Weibull analysis was also performed showing that the characteristic parameter (s_o) was 133 and 161MPa for the uncoated and coated group respectively. Conclusion: Although the multilayer film promoted an increase in the strength of porcelain, it was susceptible to subcritical crack growth. Ideally, in terms of long-term performance and applicability not only a high initial strength is important for a material but also its ability to resist strength degradation.