Surface Treatment and Coating of PM Components

Abstract

The range of surface engineering technologies applicable to PM components is reviewed and the importance of a clear design concept capable of optimizing substrate and coating properties to provide the desired overall performance is emphasised. Fully dense components can be processed without difficulty using established technology. An example is given of ASP 23 cold forming dies, which have shown a 25 fold increase in life following a hardening, TiN/TiC chemical vapour deposition coating, and rehardening treatment that eliminates pickup on the tool and also improves die lubricity. However, problems have been encountered in the treatment of porous components. Retention of process fluid following salt bath treatment or of quenching media can accelerate corrosion and is likely to degrade any subsequent layer. The variation in density also makes it difficult to produce consistent cases and quenching rates and hence consistent performance. These problems can be overcome by steam treatment to seal porosity or by the modern gaseous and plasma nitrocarburising processes which can successfully produce cases on the internal surfaces of pores. The latest generation of coatings is being produced by duplex processing, e.g. the consolidation by hipping or by laser or electron beam melting of coatings deposited by thermal spraying. PM/0570