Deposition and structural properties of two-component metal coatings for tribological applications

Advanced vacuum deposition processes are gaining increasing importance in material manufacturing. This paper describes the deposition of immiscible metal-metal systems by magnetron sputtering. Two basic deposition modes, codeposition from a multicomponent target and sequential deposition from two separate targets, are discussed. We show that it is possible to obtain a film structure of soft metal particles homogeneously dispersed in a tough metal matrix. In practical, e.g. tribological, applications, the tough matrix takes high mechanical loads, while the soft particles act as a solid lubricant. The dependence of the film structure on the process parameters and him composition is discussed with respect to the different deposition modes. A thermodynamic model, which describes the roughness evolution and phase distribution of a system consisting of a layer-forming matrix and an island-forming inclusion component, is presented for the sequential deposition mode. The results are compared with quantitative atomic force microscopy (AFM) measurements of the surface roughness. For the codeposition mode, the phase distribution in the film can be described by a dynamic model of structural evolution based on the far-from-equilibrium process of diffusion-limited aggregation (DLA). The complex morphological properties of the soft phase are qualitatively well matched with the structures obtained from the model. In conclusion, we discuss a practical application of metallic two-component films as coatings for plain bearings in high performance diesel engines. These bearings are always oil lubricated during the operation of the engine to prevent metal-metal contact. Nevertheless, they are exposed to severe pressures and pressure gradients. Therefore the coating has to resist extremely high and time-dependent mechanical loads. The wear properties of aluminum-tin films, which are currently manufactured by high rate post magnetron sputtering in an industrial process, are compared with conventional bearing designs and are found to be superior to all alternative surface- modified designs.