Couches minces dures de carbone amorphe pour des applications biomédicales
Abstract (summary)
The goal of this project was to develop and characterize DLC (Diamond-Like Carbon) films on stainless steel 316L substrate with the aim of creating new components to be used for hip implants. The short time goals are to improve the tribological and electrochemical properties of DLC deposited by PECVD (Plasma Enhanced Chemical Vapor Deposition). Based on the literature, DLC is known to have high hardness and wear resistance, a smooth surface, a good biocompatibility, and to be chemically inert. The hypothesis is then made that DLC can greatly enhance the surface properties of the stainless steel 316L substrate for biomedical applications.
Etching, followed by nitriding of a minimum of three hours was revealed essential for the adherence of the film. Adding an interface, especially silicon, has been a major factor affecting the properties of the samples. The adhesion of the films was measured with the critical loads, which ranged from 6 N to 9.5 N.
Tribological measurements were made using a 10 mm reciprocal ball-on-flat wear machine using loads of 10 N and 22.5 N in dry and Ringer's solution for 30 minutes. In dry conditions, DLC obtained friction coefficients of 0.05, 0.09 and 0.18 with ceramic, stainless steel and polyethylene balls respectively. The wear coefficients were measured at 5x.10-8 mm3N -1m-1 or less during these tests. In most cases, the wear created changes on the films but didn't reach the substrate. In Ringer's solution, the friction coefficient of DLC was 0.1 and 0.15 against ceramic and polyethylene. All surfaces tested showed great damages after the wear. Nevertheless, electrochemical analysis clearly demonstrates the superiority of DLC compared to the substrate.
The silicon nitrided interface greatly improved the adhesion by a factor of 40%, while it also greatly enhanced the tribological and electrochemical properties of the films. In Ringer's solution, the lowest friction coefficient was obtained at 0.09 and some samples barely showed damage even on the scanning electron microscope. Since the wear did not affect the DLC film, the open circuit potential showed no variations during the tests and stayed stable at –0.05 V. Moreover, the impedance measurements in the range of giga Ohm-cm2 did not change after the wear. For these DLC films, the pitting potential reached 1.6 V, the passivation current 10-5 A/cm2, and the corrosion current 10-11 A/cm 2. Some of these samples are now undergoing further researches in the hope that one day, they will make hip implants last for a lifetime.