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 (CVD) of aluminium oxide (Al2O3) coatings on powder metallurgy (PM) BT42 grade high speed steel (HSS) indexable cutting tool inserts. To facilitate this, a laboratory-scale CVD reactor was initially designed and purpose-built. The literature on the effect of process parameters on Al203 coatings chemically vapour deposited on cemented carbide substrates was comprehensively surveyed. With reference to deposition conditions quoted in this literature, a series of trial Al203 coating runs were then performed using uncoated and TiN, TiC and Ti(C1N) precoated PM HSS inserts. Concurrently, modifications to the laboratory-scale CVD reactor were made. Eventually, deposition conditions were established under which a preliminary a Al203 coating could be deposited on the PM HSS inserts and, in the case of the Ti(C1N) precoated inserts only, substantially retained during the obligatory post-coating HSS substrate heat treatment. The effect of the absence/presence and type of precoating on the CVD of AI203 coatings on the PM HSS inserts are discussed. Subsequent coating runs were carried out only on Ti(C1N) precoated inserts. The characteristics of the preliminary AlP, coating on the Ti(C1N) precoated PM HSS inserts were determined, both before and after the obligatory HSS substrate heat treatment, using an established characterisation procedure. This involved the following techniques: X-ray diffraction, Auger electron spectroscopy, optical microscopy, fractography, scanning electron microscopy, microhardness testing, profilometry and scratch-adhesion testing. Although many of the characteristics of the preliminary AI20, coating were found to be inferior to those presented in the literature, they were also established to be essentially unaffected by the post-coating HSS substrate heat treatment. To im prove upon the preliminary Al203 coating, the effect of two of the most important CVD process parameters quoted in the literature; C02JH 2 mole ratio and Alel3 concentration, on the characteristics of Al20, coatings chemically vapour deposited on the Ti(C1N) precoated PM HSS inserts was investigated. Both parameters were found to have a significant effect on Al203 coating characteristics. The reasons for this are discussed in detail.