High temperature oxidation and erosion–oxidation behaviour of HVOF sprayed Ni–20Cr, WC–20Cr–7Ni and Cr3C2–Ni–20Cr coatings

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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.