TRIBOLOGICAL PROPERTIES OF THIN SILVER FILMS SPUTTER-DEPOSITED ON SUPERALLOY SUBSTRATES

Thin silver films have been deposited on superalloy (alacrite) substrates with an average roughness of 0.2-0.3 mu m by radio frequency magnetron sputtering of a silver target in argon atmosphere. The silver films, of 0.4-20 mu m thickness, exhibited a crystallographic structure preferentially oriented in the(lll) direction, with grain sizes of 150 nm. The compressive residual stresses in these films were as low as -0.05 GPa. Argon-free silver films deposited on silicon substrates at the floating potential possessed an electrical resistivity similar to the bulk-resistivity value. The tribological tests were carried out with an alumina ball-on-disk apparatus operating under loads of 9.8, 2.9 and 1 N at room temperature and 500 degrees C with a sliding velocity of 0.02 m s(-1) for 5000 to 90 000 cycles, i.e. for an average sliding distance varying from 385 m to 7 km. After an initial stage of about 1000 cycles, the friction coefficient of silver-coated alacrite disks was essentially independent of the number of cycles. The average friction coefficient values were in the range 0.14 to 0.42, depending upon film thickness and test temperature. After 5000 cycles under loads of 2.9 and 1 N, the bottom of the wear tracks exhibited a periodic sequence of bumps and hollows. The periodicity of these undulations was dependent on the load and test temperature. The formation of these undulations probably resulted from the adhesive friction mechanism of silver films. Under a load of 9.8 N, the bottom of the wear tracks was flat and the wear of silver films was caused by a ploughing mechanism. The variation of the average friction coefficient of silver-coated alacrite disks with film thickness at room temperature was interpreted on the basis of a mechanism of friction proposed for soft metal films deposited on steel substrates when sliding against hard antagonists.