The influence of speed on metallic wear

Abstract

The rate of wear of brass on hardened steel has been measured on a pin and ring apparatus over a very wide range of speeds (10$^{-2}$ to 10$^3$ cm/s). As speed increases the rate of wear first decreases to a minimum and then begins to increase again. Radioactive tracer experiments show that over the whole range of speeds the mechanism of wear remains the same; the wear is of the `severe' type in which metallic debris is produced and a film of transferred brass is built up on the steel surface. The decrease in the rate of wear with speed at low speeds is shown to be due to a reduction in the number and in the size of brass fragments transferred to the steel. At high speeds, the number of fragments transferred continues to decrease and the observed increase in the rate of wear arises solely from an increase in the size of the fragments. This increase is due to softening of the brass by frictional heating and can be prevented by cooling the surfaces. A crossed-cylinders friction apparatus has been used to determine the cause of the observed variations in the rate of wear with speed. It is found that transfer of brass to the steel occurs when a local region of the brass surface has been deformed a critical number of times. This critical number increases with speed and in consequence the frequency of transfer is less at high speeds. Metallographic sections through brass surfaces show that an extensive pattern of strain develops beneath the surface and the rate of development is again less at high speeds. The fragments of brass which are transferred to the steel are platelets, and their thickness shows that they originate from the region of greatest strain near to the surface. It is suggested that transfer occurs when the brass within this region has been weakened sufficiently by successive deformations. Transfer is not therefore equally probable at each local encounter between surfaces; it depends upon the previous history of the surfaces and is less frequent at high speeds because more encounters are then required to develop the critical state of strain.