High temperature oxidation and erosion–oxidation behaviour of HVOF sprayed Ni–20Cr,WC–20Cr–7Ni and Cr 3 C 2 –Ni–20Cr coatings

The oxidation and erosion–oxidation (E–O) behaviour of high velocity oxy fuel sprayed Ni–20Cr alloy as well as WC and Cr₃C₂ cermet coatings on a steel substrate were studied. The E–O tests were carried out in a rig with specimen assemblies that were rotated through a fluidized bed of erodent particles in the temperature range 500–850°C and with erodent impact velocities of 2·5– 19·5 m s⁻¹. Alumina powder (∼200 μm) was used as the erodent. The oxidation behaviour of the coatings was determined gravimetrically. The Ni–20Cr and Cr₃C₂–25(Ni–20Cr) coatings showed significantly higher oxidation resistance than the WC–20Cr–7Ni coating, especially at temperatures above 800°C. The E–O resistance of the coatings was determined by wastage, as a function of temperature. The three coatings did not show any significant change in E–O at temperatures up to 500–600°C. At higher temperatures, wastage increased with temperature, reached a maximum at 700°C and then decreased with further increase in temperature. Different E–O regimes were identified. The specimen surfaces were examined in a scanning electron microscope and their roughness determined. E–O maps have been drawn to define conditions under which the coatings undergo low, moderate and severe wastage.