Combinations of coating and heat treating processes: establishing a system for combined processes and examples

High resistance of metals against wear, fatigue and corrosion can be achieved by several different treatments, like thermal, mechanical, thermochemical and coating processes. Combining successful single processes into one treatment can result in an even higher resistance of materials against complex loads, e.g. superimposed wear, fatigue and corrosion, because of the addition of the single process advantages. For a substantial choice of technical and economical promising combinations, a classification for combined processes was set up. The single processes were divided into five groups: thermal, mechanical, thermochemical, ion implantation and coating processes. A 5X5 matrix out of these five groups was set up, which contains a large number of possible combined processes. This matrix holds for steels as well as for non-ferrous alloys. The potential of different combined processes was analyzed theoretically and experimentally. Several combined processes for steels and non-ferrous metals were reviewed, popular combined processes for steels are thermochemical treatment and coating as well as coating and thermal treatment. Examples like carburizing and CVD, nitriding and PVD, CVD and quench hardening, and CVD and induction hardening will be presented. The combined process CVD and quench hardening illustrates the principle of combined processes: The high hardness of the thin CVD-coating is supported by the high strength of the quench hardened steel substrate. Examples for non-ferrous alloys are plasma nitriding and precipitation hardening of aluminum alloys and also nitriding and CVD of titanium alloys. These examples will highlight the great potential of combined processes. (C) 1998 Elsevier Science S.A. All rights reserved.