WC-based hardmetal coatings with Co-free matrices: microstructure and properties

Cobalt it is considered a critical raw material to many industrialized economies, due to its strategic importance coupled to supply risks, and is health hazardous, having been classified as a Cat. 1B human carcinogen within the European CLP regulation. Therefore, this work aims to replace Co-based hardmetal coatings by testing High Velocity Oxy-Fuel (HVOF) sprayed WC-18%(Ni-16Mo-15Cr-5Fe-2Co), WC-15%(Fe-30Ni-28Cr-7Mo-1Cu) and WC-15%(Fe-20Cr-6Al) compositions (all values in wt.%). The microstructure, phase composition, microhardness, sliding and abrasive wear resistance, and electrochemical corrosion behaviour of these coatings in a 3.5% (w/v) NaCl aqueous solution are compared to those of HVOF-sprayed WC-10%Co-4%Cr benchmarks.

WC-NiMoCrFeCo and WC-FeNiCrMoCu compositions coatings with porosity <1% and comparable hardness values as the WC-CoCr references. Mild ball-on-disc wear rates <10^-7 mm³/(N∙m) testify to their suitability for sliding conditions. Wear mechanisms involve micro-scale plastic flow with minimal brittle fracture. The Ni-based matrix also provided better corrosion resistance than the WC-CoCr references. On the other hand, in high-stress abrasion, wear rates of ≈10^-3 mm³/(N∙m) are marginally higher than for the WC-CoCr benchmarks.

The WC-FeCrAl composition, on the other hand, suffered from slightly higher wear rates under all test conditions, because of greater brittleness, and from higher corrosion current density values, due to selective matrix dissolution.