AbstractSurface topographic features of plasma nitrided En40B steel have been investigated in the present work using the multitrace prolifometric technique recently developed for three-dimensional characterisation of engineering surfaces. It is shown that, during plasma nitriding, cones of various morphologies tend to form on the surface, such that the finished nitrided surface is densely populated with cones of varying sizes. The formation of cones on the surface during nitriding is affected not only by parameters such as temperature, time, and gas pressure, but also by the original surface topography. In conjunction with metallographic examination of well prepared microsections and metallurgical results obtained by others, the mechanisms associated with surface topographic changes during plasma nitriding are discussed. The potential influence of plasma nitrided surface topography on tribology under highly stressed contact conditions is briefly discussed in terms of various calculated functional parameters for each surface investigated.