Wear and corrosion behaviour of thermally sprayed cermet coatings

In order to protect machining parts against wear and corrosion, they are coated by cermet coatings. The coatings consist of WC or Cr3C2 particles in a metal binder, which can be a pure metal or a mixture consisting of Ni, Cr, Co. The examined coatings were produced by high velocity oxy-fuel (HVOF) spraying and were investigated with regard to erosion and corrosion resistance. The combined erosion corrosion tests were carried out at ambient temperature in 0.1 M NaOH and 0.1 M H2SO4 solutions containing sand. Information about the corrosion resistance was gained from electrochemical polarization measurements and salt spray test (NaCl solution). The results of combined wear and corrosion tests showed that the corrosion properties of the sprayed coatings strongly affect the materials loss rate under wear corrosion conditions. Coatings with a less corrosion resistant matrix present enhanced erosion, also. The erosion mechanism of the carbide coatings seams to be controlled by the skeletal network of the carbides. For comparison, flame and plasma sprayed hard Cr2O3 coatings were examined. Due to the low electrical conductivity the corrosion rate of these coatings was very low. Under erosion conditions the hard Cr2O3 coatings exhibited a high erosion rate and the erosion mechanism seems to be that of the brittle erosion due to grain-by-grain removal of oxide grains during impact.