7/12/16
Andrew Wright
HW #4 PROBLEM 1
The explanation for the strange dependence of applied load on wear rate has to do with
the phase diagram of steel (shown below). At a constant sliding speed and a low applied load,
the wear mechanism in the steel disc is most likely mild sliding. The wear rate for this
mechanism is low, with small, fine particles broken off of the surface. As the applied load is
increased, the wear gets more severe which increases the wear rate and produces larger particles.
The larger particles have larger mass, obviously, which results in a higher wear rate. As the load
is increased further, the wear rate actually decreases. This is because at this increased friction
state, the flash temperature reaches a value that causes a phase transformation in the steel. The
steel disc forms martensite, a very hard and strong phase of iron-based steel, on the surface.
Because martensite is much harder than the previous phase of the steel, the wear rate is reduced.
In conclusion, the flash temperature can have a significant impact on wear characteristics of steel
if it results in phase transformations.
HW #4 Problem 1