OPTIMUM FILM THICKNESS OF THIN METALLIC COATINGS ON SILICON SUBSTRATES FOR LOW-LOAD SLIDING APPLICATIONS

The frictional behaviour of thin metallic films on silicon substrates sliding against 52100 steel balls-is presented. The motivation of this work is to identify an optimum film thickness that will result in low friction under relatively low loads for various metallic films. Dry s liding friction experiments on silicon substrates with soft metallic c oatings (silver, copper, tin and zinc) of various thickness (1-2000 nm ) were conducted using a reciprocating pin-on-flat type apparatus unde r a controlled environment. A thermal vapour deposition technique was used to produce pure and smooth coatings. The morphology of the films was examined using an atomic force microscope, a non-contact optical p rofilometer and a scanning electron microscope. Following the sliding tests, the sliding tracks were examined by various surface characteriz ation techniques and tools. The results indicate that the frictional c haracteristics of silicon are improved by coating the surface with a t hin metallic film, and furthermore, an optimum film thickness can be i dentified for silver, copper and zinc coatings. In most cases ploughin g marks could be found on the film which suggests that plastic deforma tion of the film is the dominant mode by which frictional energy dissi pation occurred. Based on this observation, the frictional behaviour o f thin metallic coatings under low loads is discussed and friction coe fficients are correlated with an energy based friction model. Copyrigh t (C) 1996 Elsevier Science Ltd