Chemically Vapour Deposited Alumina Coatings on High Speed Steel Cutting Tools

Abstract

A detailed study has been made of the atmospheric pressure chemical vapour deposition\n(CVD) of aluminium oxide (Al2O3) coatings on powder metallurgy (PM) BT42 grade high\nspeed steel (HSS) indexable cutting tool inserts. To facilitate this, a laboratory-scale CVD\nreactor was initially designed and purpose-built.\nThe literature on the effect of process parameters on Al203 coatings chemically vapour\ndeposited on cemented carbide substrates was comprehensively surveyed. With reference to\ndeposition conditions quoted in this literature, a series of trial Al203 coating runs were then\nperformed using uncoated and TiN, TiC and Ti(C1N) precoated PM HSS inserts. Concurrently,\nmodifications to the laboratory-scale CVD reactor were made. Eventually, deposition\nconditions were established under which a preliminary a Al203 coating could be deposited\non the PM HSS inserts and, in the case of the Ti(C1N) precoated inserts only, substantially\nretained during the obligatory post-coating HSS substrate heat treatment. The effect of the\nabsence/presence and type of precoating on the CVD of AI203 coatings on the PM HSS inserts\nare discussed. Subsequent coating runs were carried out only on Ti(C1N) precoated inserts.\nThe characteristics of the preliminary AlP, coating on the Ti(C1N) precoated PM HSS inserts\nwere determined, both before and after the obligatory HSS substrate heat treatment, using an\nestablished characterisation procedure. This involved the following techniques: X-ray\ndiffraction, Auger electron spectroscopy, optical microscopy, fractography, scanning electron\nmicroscopy, microhardness testing, profilometry and scratch-adhesion testing. Although\nmany of the characteristics of the preliminary AI20, coating were found to be inferior to those\npresented in the literature, they were also established to be essentially unaffected by the\npost-coating HSS substrate heat treatment.\nTo im prove upon the preliminary Al203 coating, the effect of two of the most important CVD\nprocess parameters quoted in the literature; C02JH 2 mole ratio and Alel3 concentration, on\nthe characteristics of Al20, coatings chemically vapour deposited on the Ti(C1N) precoated\nPM HSS inserts was investigated. Both parameters were found to have a significant effect\non Al203 coating characteristics. The reasons for this are discussed in detail.