Mechanical Properties of DLC/Ti-O Bilayer Films

Abstract

Bilayer diamondlike carbon (DLC) on titanium oxide (DLC/Ti-O) films have important potential application for artificial heart valves, provided that the films are sufficiently hard and wear resistant and maintain their integrity during prolonged use. We have investigated these film characteristics for DLC/Ti-O samples formed on test substrates of titanium alloy (Ti6Al4V) and silicon (for microstructure study). The results indicate substantial differences in the bilayer film performance depending on the precise film deposition parameters. In this paper, the Ti-O films were first deposited at substrates with different O2/Ar ratios by magnetron sputtering. The Ti-O films with different electrical resistances were obtained, and the Ti-O film electrical resistance decreased with the O2/Ar ratio. Then, the DLC film was fabricated on the Ti-O films with different electrical resistances using the dc magnetic filtered cathodic vacuum arc deposition (MFCVAD) system. The properties of the DLC on titanium oxide films with different electrical resistances were studied. The DLC sp3/sp2 ratio (diamond-bonded fraction), bilayer microhardness, adhesion strength, and wear resistance were all seen to increase with decreasing Ti-O film electrical resistance, in other words, increase with decreasing O2/ Ar ratio used in the Ti-O film sputter deposition process. The C+ ion deposited at a lower resistivity Ti-O film has higher ion deposition energy than that deposited at a higher resistivity or an insulating Ti-O film during the DLC film deposition on Ti-O films with the same -80-V dc bias voltage by MFCVAD. The higher ion energy, in turn, leads to a higher sp3/sp2 ratio. Therefore, the sp3/sp2 ratio, bilayer microhardness, adhesion strength, and wear resistance of the DLC deposited at a lower resistivity Ti-O film are higher than those of a DLC deposited at a higher resistivity Ti-O film. All in all, we attribute this to the effect of the O2/Ar ratio on the Ti-O film resistivity and, in turn, on the C+ ion deposition energy.