Process Developments Towards Producing Well Adherent Duplex PAPVD Coatings

Abstract

Duplex treatments consisting of a plasma nitrided layer followed by a PAPVD hard coating have gained attention in recent years. However, obtaining a satisfactory adhesion strength between the PAPVD hard coatings and the nitrided layers is a key issue in duplex coating processing, especially if plasma nitriding and coating deposition processes are to be carried out sequentially in a PAPVD plant. Inadequate process control will lead to the formation of a soft 'black layer' below the coating by thermochemical decomposition of the compound layer, which adversely affects the load bearing capacity of the composite and, consequently, the adhesion between the hard coating and the nitrided subsurface. In the present paper an intermediate treatment which allows the production of duplex TiN coatings with good adhesion is described, even if a compound layer is formed during plasma nitriding. This intermediate treatment also provides a way to produce duplex coatings in which the nitrided subsurface is compound layer free. Moreover, scratch and impact test results indicate that a monophase ϵ compound layer does not have any detrimental effects on the adhesion and impact resistance of duplex TiN coatings. Similar levels of adhesion strength were recorded for duplex TiN coatings with nitrided layers consisting of either a monophase ϵ compound layer + diffusion zone or a diffusion zone only; however, in impact tests, it was found that the former outperformed the latter. Such results indicate that it is feasible to produce well adherent duplex coatings with monophase compound layers if process parameters are carefully controlled in order to avoid compound layer destabilisation.