Coefficient of Friction of CAD/CAM Restorative Materials Against Zirconia Antagonists

Objectives: To describe time-dependent wear behavior by evaluating the coefficient of friction of CAD/CAM and direct restorative materials opposed to a zirconia antagonist in a pin-on-disc setup.
Methods: Flat specimen discs (n=3; d=35mm) were prepared from two direct composite resins (Grandio SO, VOCO/ Filtek Supreme XTE, 3M), four CAD-CAM based composite and hybride-ceramic blocs (Grandio blocs, VOCO/ Tetric CAD/ Lava Ultimate, 3M/ VITA Enamic, VITA), one lithium disilicate ceramic CAD-CAM material (IPS e.max CAD, Ivoclar Vivadent), and bovine enamel. Specimens were used in segments, luted onto holders and levelled plane parallel using a cup grinder up to D15 roughness. After storage (28d, aqua dest., 37°C), testing was performed in a pin-on-disc setup according to EN 1071-13 (50,000cycles, 120rpm, F=31,3N) in 60ml mucine based saliva (Saliva Orthana, AS Pharma, Salisbury, UK) opposed to a zirconia- pin (Y-TZP, Frialit, d=3mm). Mean vertical substance loss, volume loss and coefficient of friction (initial/steady-state) were analysed using a 3D optical non-contact profilometer (CT100, CyberTechnologies, Eching-Dietersheim, Germany) equipped with a confocal white-light sensor CHR-600 (CyberTechnologies (z-res.=0.02µm)).
Results: Mean values for the coefficient of friction varied between 0.165 (Tetric CAD) and 0.490 (IPS e.max CAD). The lithium disilicate ceramic showed the significantly greatest wear loss while hybride or composite materials performed comparable to enamel or amalgam. Vertical substance loss thereby correlated (r^2=0.995) with volume loss. A principal correlation of wear rates and surface hardness can be postulated. The zirconia pin did not experience any degradation.
Conclusions: All tested materials showed considerable wear facets against the zirconia antagonist. The combination of two hard restorative materials seems to be deleterious in terms of material degradation. Hybride or composite materials showed wear resistance in the range of enamel and amalgam.