CORROSION STUDIES ON SURFACE TREATED TIALN AND ALCRN COATINGS IN 3.5 NACL

摘要

In recent years,developments in the use of thin hard coatings for cutting tool and metal forming applications have focused on multi-component alternatives to traditional TiN coatings,such as TiCN,TiAIN,CrN,AlCrN and AlCrON.These coatings are becoming established in many engineering applications where enhanced wear resistance and reduced friction is required.Typical method for deposition of these coatings on an industrial scale are by various physical vapour deposition(PVD)technologies,particularly cathodic arc evaporation,electron beam evaporation and magnetron sputtering and consequently,a feature common to all these techniques is the generation of coating defects,such as voids,pin-holes and macro-particles which can result in reduced corrosion resistance,when subject to corrosive environments.Consequently,there is a need to develop techniques for modifying the surface morphology to produce dense,defect-free coatings,thus improving the corrosion resistance.Post deposition treatments,such as various blasting brushing and polishing techniques,commonly used for de-burring,polishing and genera! cleaning,have shown to improve the surface texture of PVD coatings,which can influence the resultant corrosion behaviour of the coatings.However,these studies have been limited to TiN coatings.In the current study,a series of post deposition treatments,to include drag polishing(DP),wet and dry blasting(WB and DB respectively)and nylon abrasive filament(NAF)brushing and were employed on TiAIN and AlCrN coatings deposited on 1020 carbon steel substrates using cathodic arc evaporation PVD process.Electrochemical corrosion studies were conducted in saline(3.5% NaCl)solutions using potentiodynamic scanning.Ecorr and Icorr values were determined using Tafel Extrapolation.M orphological characterization of the surface of the coatings was conducted using optical and SEM imaging,prior to and after corrosion testing,to determine mechanisms of corrosion.The results of this study showed a slight improvement in the corrosion resistance for the surface treated TiAIN and AlCrN coatings.Corrosion resistance was limited by the presence of defects such as pin holes,voids and macro-particles which controlled the corrosion behaviour of the coating system at lower anodic potentials,while at higher potentials,corrosion behaviour was controlled by the development of large pitted regions,coating spallation and significant underlying substrate attack.In general,improved corrosion resistance was observed for the gentler,lower impact surface treatments,such as wet blasting(WB)compared with the higher impact treatments,such as dry blasting(DB),which are more likely to increase the amount of defects which could act as corrosion initiation sites.