Chemically vapour deposited titanium carbide coatings on high speed steel cutting tool inserts
A detailed study has been made of titanium carbide (TiC) coatings chemically vapour deposited on powder metallurgy (PM) BT42 grade high speed steel (HSS) indexable cutting tool inserts, in collaboration with Edgar Allen Tools, Sheffield. Initially, the characteristics of a TiC coating deposited under the standard conditions employed by Edgar Allen Tools were determined, both before and after the obligatory HSS substrate heat treatment. This was done using various techniques including x-ray diffraction, scanning electron microscopy, micro-hardness testing, profilometry, optical microscopy and scratch-adhesion testing. Although the characteristics of the TiC coating were found not to be significantly affected by the HSS substrate heat treatment, some of them were different to those presented in the literature for TiC coatings chemically vapour deposited on cemented carbide substrates. A comprehensive survey of the literature revealed that the process parameters employed during the chemical vapour deposition (CVD) of TiC coatings, particularly the CHy/TiCly mole ratio and deposition temperature, have a significant effect on coating characteristics. Consequently, it was decided to investigate the effect of these parameters on the characteristics of the TIC coatings chemically vapour deposited on the PM HSS inserts and, further, their effect on the tool lives exhibited by the coated inserts. The latter were determined from a series of turning tests on 817M40 workpiece material, carried out under conditions of builtup- edge (bue) formation, at cutting speeds in the range 37.5 - 60 m min 1. Both process parameters were found to have a marked effect on the characteristics of the TiC coatings and on the tool lives exhibited by the coated inserts. The reasons for this are discussbd. The TiC coated inserts which displayed the most favourable combination of coating characteristics and tool life were used in two further series of turning tests carried out to investigate tool wear, bue size, tool forces, apparent coefficient of friction and workpiece surface finish. Uncoated inserts were also tested. The application of the TiC coating was found to have led to a marked improvement in the cutting properties of the PM HSS inserts. Its role during cutting was studied from metallographic sections through worn inserts and the reasons for its effect on cutting properties are discussed.