Mechanical and tribological properties of electrolytic hard chrome and HVOF-sprayed coatings

The microstructure, mechanical properties (Vickers microhardness, fracture toughness, elastic modulus) and tribological behaviour (abrasion and unlubricated sliding wear resistance) of various kinds of electrolitic hard chrome (EHC) coatings and of metallic and cermet HVOF-sprayed coatings (WC-17Co, WC-10Co-4Cr, Co-28Mo-17Cr-3Si) have been studied. EHC coatings are very tough, in fact no cracks could be produced with 10 N Vickers microindentations. However, their superficial morphology and Vickers microhardness are significantly affected by the substrate preparation technique, the deposition process characteristics, and by post-deposition treatments. This causes the coatings to show different behaviors in different wear conditions; in particular, harder coatings are more abrasion-resistant, but undergo a more severe mass loss under unlubricated sliding conditions. HVOF-sprayed cermet coatings are harder but less tough than EHC ones. Therefore, they undergo a comparable or even higher mass loss when subjected to three-body abrasion conditions. However, their two-body sliding resistance definitely overcomes that of EHC coatings, because they form a tough and uniform surface film protecting them from further damage.