Coated machine elements — fiction or reality?

In the last decade tremendous progress has been made in the field of hard coatings. The majority of research work refers to improvement of mechanical and tribological properties of coatings, deposited on ‘hard’ substrates, i.e. ceramics and tool steels, and used for cutting and forming tools. However, requirements in the case of machine elements are quite different from those for tools. In addition to a hard, wear resistant surface with good frictional characteristics, a tough, fracture resistant core is necessary. In contrast to tool steels, hardened and tempered low-alloy steels have high fracture toughness. On the other hand, high hardness and internal compressive stresses of the case formed by proper thermochemical treatment can lead to increased load-carrying capacity of the steel substrate as well as to improved tribological properties of coated parts. Therefore, the combination of thermochemical treatment and hard-coating, known as a duplex treatment, would allow function sharing between the core material, the hardened case and wear resistant surface, which is of special interest for applications involving machine elements subjected to complex stresses. This paper shows that by proper selection and control of substrate treatment and coating deposition, thin hard coatings could also be successfully used in the case of machine elements subjected to complex stresses.