Coated machine elements - fiction or reality?

In the last decade tremendous progress has been made in the field of hard c oatings. 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. Howeve r, requirements in the case of machine elements are quite different from th ose for tools. In addition to a hard., wear resistant surface with good fri ctional characteristics, a tough. fracture resistant core is necessary. In contrast to tool steels, hardened and tempered low-alloy steels have high f racture toughness. On the other hand, high hardness and internal compressiv e stresses of the case formed by proper thermochemical treatment can lead t o increased load-carrying capacity of the steel substrate as well as to imp roved tribological properties of coated parts. Therefore, the combination o f thermochemical treatment and hard-coating, known as a duplex treatment, w ould allow function sharing between the core material, the hardened case an d wear resistant surface, which is of special interest for applications inv olving machine elements subjected to complex stresses. This paper shows tha t by proper selection and control of substrate treatment and coating deposi tion, thin hard coatings could also be successfully used in the case of mac hine elements subjected to complex stresses. (C) 2001 Elsevier Science B.V. All rights reserved.