JORNADAS SAM/ CONAMET/ SIMPOSIO MATERIA 2003 06-50 600 PLASMA NITRIDING AND PAPVD HARD COATING: A CRITICAL OVERVIEW OF DUPLEX COATING PROCESSING Junia Cristina Avelar-Batista a , Cristina Godoy b , Rafael D. Mancosu b , Janaína Morais b , A. Matthews c a Tecvac Ltd., Buckingway Business Park, Swavesey, Cambridge, CB4 5UG, United Kingdom. juniacri@yahoo.co.uk b Department of Metallurgical and Materials Engineering, Universidade Federal de Minas Gerais, Rua Espírito Santo 35, Belo Horizonte-MG, 30160 030,Brazil. gcgodoy@ufmg.br c Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, United Kingdom. A.Matthews@sheffield.ac.uk 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 the coating/substrate interface can be strongly affected by the presence of a compound layer, 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 work was to produce well-adherent PAPVD duplex coatings on AISI H13 steel substrates for tribological applications. In the first stage, 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 sputtering mechanism. The same adhesion strength was measured for duplex TiN coatings with nitrided cases consisting of mono-phase ε compound layer + diffusion zone and for duplex TiN coatings with nitrided cases consisting of a diffusion zone only. In the second stage, non-duplex and duplex (Ti,Al)N and Cr-N coatings were prepared and characterised by X-ray diffraction (XRD), glow discharge optical emission spectroscopy (GDOES), surface profilometry, nanoindentation and Knoop hardness measurements, scratch tests and scanning electron microscopy (SEM). The corrosion response of duplex (Ti,Al)N and Cr-N coatings and that of the uncoated substrate was also investigated using potentiodynamic tests. The corrosion results indicated that both duplex coatings improved the corrosion resistance of a hardened AISI H13 steel substrate, with the duplex Cr-N coating exhibiting the best corrosion response. The wear resistance of duplex and non-duplex (Ti,Al)N, Cr-N and TiN coatings was evaluated by micro-abrasive wear tests. Impact tests were also carried out to investigate their resistance to dynamic loading. Duplex (Ti,Al)N coatings exhibited the best wear resistance in micro-abrasive wear tests. The results obtained in such tests suggested the occurrence of abrasive wear by plastic deformation, with the hardest coatings displaying the lowest wear rates. The highest wear rates were recorded for duplex and non-duplex Cr-N coatings. However, a duplex Cr-N coating exhibited the best performance in impact tests using a tungsten carbide ball. The duplex Cr-N coating displayed the lowest elastic modulus among all tested coatings, which could be attributed to a small presence of an α-Cr metallic phase. This coated system also showed the smallest difference between the substrate and coating elastic moduli. For the duplex-coated systems, compressive subsurface stresses due to a nitriding treatment in conjunction to compressive stresses in the film (produced by the ion plating technique) probably lowered the magnitude of the tensile stresses in the affected zone. The better impact resistance obtained for duplex coatings in comparison to their non-duplex counterparts seems to be supported by such statement. The ambivalent behaviours exhibited by duplex (Ti,Al)N and Cr-N coatings in different tribological tests reveal the important role of selecting the most adequate coating for a given tribological application. Keywords: PAPVD coatings, plasma nitriding, load-bearing capacity, elasticity property mismatch, micro- abrasive wear testing, impact testing. 1. INTRODUCTION 1.1. PAPVD duplex coating development Failure of a hard coating/soft substrate system under many tribological conditions is rarely promoted by conventional wear. Instead, the tribological failure is often caused by debonding of the coating from the substrate (adhesive failure), fracture of the coating (cohesive failure) or even by subsurface fracture (substrate failure). The effects of coating thickness and substrate properties on the initiation and development of the plastic zone, and also on the load-bearing capacity of a coating/substrate system, are extremely important aspects which should be considered when analysing the fracture behaviour of these systems. Duplex treatments consisting of a plasma nitrided