Promising duplex coatings for tribological applications at elevated temperatures

Abstract

It has already been reported that duplex coatings, consisting of plasma nitriding of steel substrates and subsequent deposition of hard coatings by PAPVD (plasma-assisted physical vapour deposition), can improve both wear and contact fatigue resistance and also the load support capability of steel substrates. However, the adhesion at coating/substrate interface can be strongly affected by the presence of a compoundlayer, which can be produced during the plasma nitriding step. This compound layer can destabilise during coating deposition; its destabilisation would lead duplex coatings to exhibit poor adhesion. The aim of this thesis was to produce well adherent PAPVD duplex coatings on AISI H13 steel substrates for tribological applications at elevatedtemperatures. In the first stage of the project, duplex TiN coatings have been used to investigate the problem of compound layer destabilisation. An intermediate treatment consisting of cooling down the samples in vacuum and carrying out an Ar plasma-etching step has been developed. This treatment allows the production of duplex coatings with good adhesion strength by successfully avoiding compound layer destabilisation. It also elucidates a systematic approach to produce duplex coatings which are compound-layer-free, by removing this iron nitride layer through an Ar sputter mechanism. The same adhesion strength was measured for duplex TiN coatings with nitrided cases consisting of mono-phase a compound layer + diffusion zone and forduplex TiN coatings with nitrided cases consisting of a diffusion zone only. Furthermore, a duplex TiN coating with a mono-phase c compound layer 4- diffusion zone outperformed duplex TiN coatings with diffusion zones in impact tests, suggesting that hard, dense mono-phase compound layers are able to act as a proper interface between harder, more brittle PVD coatings and relatively softer, more ductile diffusionlayers of plasma nitrided substrates. In the second stage, single-layered and duplex Cr-N coatings were prepared and characterised by XRD, GDOES, LOM, SEM, Knoop microhardness measurements and scratch testing. Their wear resistance (and also of single-layered and duplex TiN coatings) was evaluated by micro-abrasive wear tests. Impact tests were also carried out to investigate their resistance to dynamic loading. Results pointed out that the duplex treatment improved the performance of single-layered PAPVD coatings with respect to micro-abrasive wear and impact impact resistance. This was mainly achieved by increasing the load support-for the thin, hard PAPVD coatings and also preventing substrate elastic and plastic defotmation. Therefore, duplex coatings are expected to be successfully used on tribolagical applications that require both wear and dynamic impact resistances. Duplex (Ti,AI)N coatings had the highest wear resistance to micro-abrasion, followed by single-layered(Ti,Al)N coatings whereas duplex (Ti,Al)N and duplex Cr-N coatings had the best performances in the impact tests. This work reveals that both duplex (Ti,A1)N and Cr-N coatings are promising duplex coatings for tribological applications at elevated temperatures. In particular, these coatings have a great potentiality to improve the lifetime of AISI H13 shear knives, which are used to cut steel ingots during a continuous ingot cast process. Nevertheless, further optimisations are still required in order to introduce these coatings on industrial field tests. For instance, film thickness needs to be optimised for the duplex Cr-N coatings in order to yield a satisfactory resistance to abrasive wear