Unlubricated sliding frictional behaviour of oxide films on plasma nitrocarburised steel

The sliding frictional behaviour of oxide films formed on low alloy steel by the combined plasma nitrocarburising and post oxidation treatment has been investigated. Unlubricated sliding tests of the as-nitrocarburised and nitrocarburised-oxidised specimens showed that the frictional behaviour depends not only on the oxidation treatment conditions, but also on the counterface material and the testing conditions. When sliding against a steel counterface, all the tested specimens exhibited similar frictional behaviour, which is dominated by the material transfer from the counterface to the sliding surface. On the other hand, when sliding against an alumina slider, all the oxidised specimens exhibited a low friction regime. The duration of this low friction regime depends on the treatment and test conditions, with the low temperature oxidised/nitrocarburised specimen being the most durable. The mechanisms associated with the observed frictional characteristics are discussed in terms of material transfer and the structure and mechanical integrity of the films.