Effect of the bias voltage on the structural and tribo-mechanical properties of Ag-containing amorphous carbon films

The modification of amorphous carbon films by a mixture of Ag atoms is a promising approach to reduce the residual stresses in the coating and to improve its adhesion to the substrate. Besides the Ag concentration, the bias voltage has a crucial impact on the properties of carbon-based films. Therefore, the effect of the bias voltage on the structural and tribo-mechanical properties of hydrogen free a-C:Ag is investigated. The a-C:Ag films are sputtered from graphite targets with varying number of Ag pellets by setting the bias voltage to - 100, - 150, and - 200 V. A non-modified a-C and two a-C:Ag film systems with different Ag content are synthetized to obtain a comprehensive understanding about the influence of the bias voltage on the properties of the a-C:Ag films. A high bias voltage leads to a reduction in the amount of Ag within the a-C:Ag films, since impinging ions remove Ag atoms during the film growth. Additionally, XRD analyses show the formation of large Ag nanocrystallites with rising bias voltage. In Raman scattering studies, an Ag-induced graphitization of the a-C films is identified. The graphitization is less pronounced at low Ag concentrations and high bias voltages. The residual stresses increase with rising bias voltage and decreasing Ag content, which also favor greater values of hardness and elastic modulus. While a high bias voltage results in a poor adhesion strength for the a-C films, a good adhesion behavior is observed for the a-C:Ag films. It is ascribed to lower stresses in the a-C:Ag films as compared to that in a-C. The friction behavior of the a-C:Ag films is not influenced by the bias voltage, since the coefficients of friction vary from 0.26 to 0.32 against a steel counterpart in tribometer tests. An agglomeration of Ag particles in the tribological contact is observed for all a-C:Ag films which contributes to the slightly higher friction when compared to non-modified a-C films. On the whole, it is demonstrated that the tribo-mechanical properties of a-C:Ag are not only affected by the Ag content, but also by the applied bias voltage.