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

Thin silver films have been deposited on superalloy (alacrite) substra tes with an average roughness of 0.2-0.3 mu m by radio frequency magne tron sputtering of a silver target in argon atmosphere. The silver fil ms, of 0.4-20 mu m thickness, exhibited a crystallographic structure p referentially 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 th e floating potential possessed an electrical resistivity similar to th e bulk-resistivity value. The tribological tests were carried out with an alumina ball-on-disk apparatus operating under loads of 9.8, 2.9 a nd 1 N at room temperature and 500 degrees C with a sliding velocity o f 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 1 000 cycles, the friction coefficient of silver-coated alacrite disks w as essentially independent of the number of cycles. The average fricti on coefficient values were in the range 0.14 to 0.42, depending upon f ilm thickness and test temperature. After 5000 cycles under loads of 2 .9 and 1 N, the bottom of the wear tracks exhibited a periodic sequenc e of bumps and hollows. The periodicity of these undulations was depen dent on the load and test temperature. The formation of these undulati ons probably resulted from the adhesive friction mechanism of silver f ilms. Under a load of 9.8 N, the bottom of the wear tracks was flat an d the wear of silver films was caused by a ploughing mechanism. The va riation of the average friction coefficient of silver-coated alacrite disks with film thickness at room temperature was interpreted on the b asis of a mechanism of friction proposed for soft metal films deposite d on steel substrates when sliding against hard antagonists.