Ion implantation and ion assisted coating of metals

Abstract

The ion implantation of nitrogen into steels and titanium alloys can markedly improve their wear resistance especially under mild abrasive conditions, but it is significantly less successful in overcoming metal–metal adhesive wear under high loads. Certain metallic species such as Ti, Ta, Nb, Y, and Sn give better performance under such conditions when implanted into steels, but most of these additives are effective only in the presence of carbon or nitrogen. Among them, yttrium is particularly successful in reducing wear and friction after implantation at relatively low fluences (1016 ions cm−2), thus avoiding problems due to surface sputtering and limited retention in work pieces of complex geometry. Yttrium and other oversized species (e.g., Sn) may act by forming a complex with interstitial carbon or nitrogen: this nonspherical defect interacts strongly with mobile dislocations and so strengthens the material. Ion implantation provides a nonequilibrium method for introducing significant quantities of large atoms on substitutional sites. Abrasive wear is lessened by the application of hard, chemically inert coatings such as the nitrides or carbonitrides of titanium, boron or hafnium, but conventional CVD or PVD coatings of these compounds are often lacking in adhesion to the substrate. When deposited at relatively low temperatures, moreover, they may have an undesirable columnar structure and a nodular surface topography. To overcome such problems research is now being carried out on thin coatings of Ti, B, or Hf on steel which are bombarded heavily with energetic ions. This can bring about ion beam mixing at the interface, followed by the conversion of at least part of the coating to a nitride or carbonitride. It is advantageous to support such coatings by implanting the metal substrate first. Ion assisted coatings of TiN or BN appear promising from the standpoint of their microhardness, friction, adhesion and oxidation properties and so they may reduce adhesive wear.