Boundary-lubricated wear of cast irons to simulate automotive piston ring wear rates

Abstract Boundary-lubricated pin-on-ring tests have been performed with cast iron materials to produce specific wear rates between 10 −13 and 10 −10 mm 3 mm −1 N −1 , which is the range observed in practically operating machine elements such as automotive piston rings on cylinder liners. The tests were also done to study the wear mechanisms responsible, particularly the relative importance of high cycle metal fatigue and chemical reaction film wear. Two types of cast iron used as piston rings (a grey and a carbidic iron) were used as pins and a cylinder bore material was used as the ring. Specific loads were varied from 20 to 400 MPa and the sliding speed was 0.4 m s −1 . Boundary lubrication was achieved with a light medicinal paraffin oil and with the paraffin to which was added 1% by weight of zinc dialkyldithiophosphate (ZDDP). No attempt was made to reproduce engine gaseous atmospheres or temperatures. There was visual evidence for both direct metallic and chemical reaction film wear. However, changes in wear with operating conditions did not correlate with mechanical parameters such as plasticity index and surface shear strength, but with chemical film formation changes. There were differences between the wear resistances of the pin materials. When paraffin alone was the lubricant, wear occurred by the removal of oxide films: these were more protective on the grey than on the carbidic iron. Addition of ZDDP changed the nature of the film for both materials. An insulating film developed on the grey iron which reduced the wear rate. This film did not build up on the carbidic iron, the wear of which remained relatively high. It is suggested that the matrix carbides in the carbidic iron scraped away the film as it formed.