Wear properties and thermal stability of new PVD coating compositions based on titanium with combined metallic, metalloid and non-metallic alloying additions

Titanium nitride and chromium nitride PVD ceramic coatings are now used widely in manufacturing industry to protect cutting and forming tools against wear. Both coatings possess certain technical advantages and disadvantages, which lend themselves to specific (and different) applications. For example, TiN behaves well in sliding wear against steel and against mild abrasion; CrN often performs well in impact wear and when some measure of corrosion protection is also needed (eg. in polymer injection moulding). However, with new requirements for dry, or minimally-lubricated, machining (where contact temperatures are high) and for machining and forming of non-ferrous alloys (where tribochemical interactions can be severe), more sophisticated (and preferably ‘adaptive’) coating systems are needed. Alloyed coatings such as TiAlN and CrAlN are now used commercially, where the aluminium additions provide improved oxidation resistance by promoting the formation of a protective alumina film in service. Such coatings can also, with careful selection of composition and/or processing route, exhibit ‘nanocomposite’ structures with two- or multi-phase compositions that are claimed to improve both hardness and toughness. For extreme hardness and high temperature stability, there has for many years been an interest in boron based coatings (eg. cubic boron nitride, boron carbide and transition-metal borides such as TiB2 and, more recently, CrB2). However, such films tend to be brittle and exhibit poor adhesion to many substrate materials. On the other hand, the addition of metalloid elements such as boron or silicon to TiN coatings has shown that ‘pseudo-binary’ nanocomposite structures (eg. TiN/TiB2, TiN/BN, TiN/Si3N4) can be generated, with exciting combinations of high hardness, improved toughness, chemical inertness and – particularly in the case of TiSiN – impressive thermal stability, to temperatures in excess of 1200°C. This project aims to explore the combined addition of metallic (eg. Cr, Al) and metalloid (eg. Si, B) elements to the Ti-N metal/non-metal binary system, with a view to developing new, adaptive, nanostructured coatings to satisfy future industrial requirements for the machining and forming of non-ferrous alloys and composites.