Plasma- and vacuum-plasma-sprayed Cr 3C 2 composite coatings

Abstract

Plasma- and detonation-sprayed chromium carbide-Nichrome coatings have long been used for applications requiring superior wear resistance at temperatures up to 820°C. The coatings are typically sprayed from mechanical blends of powder containing from 17 to 35 wt.% Nichrome. These coatings are susceptible to non-uniformity of microstructure because of segregation of the blended powders. Oxide formation occurs in both ambient atmosphere plasma and detonation-applied coatings. A new Cr 3C 2 Nichrome composite powder was developed for application by the plasma and vacuum plasma processes. The developed material consists of 50 wt.% Cr 3C 2 clad with 50 wt.% 80-20 Nichrome. Unlike powder blends, each Cr 3C 2 powder particle is clad with an essentially continuous layer of Nichrome. The developed material is sized −270 mesh + 5 μm. Coatings of the composite Cr 3C 2 material were sprayed using the conventional non-transferred arc plasma and the low pressure low oxygen vacuum plasma processes. These coatings were compared with coatings sprayed from a commercially available blend of 75 wt.% Cr 3C 2 25wt.% Nichrome. Unlike the blend, the microstructure of both composite coatings showed Cr 3C 2 to be present as discrete second-phase particles embedded in the Nichrome matrix. The vacuum-plasma-sprayed composite coating showed no visible oxide. Macrohardness (Rockwell C hardness) and microhardness (diamond pyramid hardness for a load of 300 gf) readings of the conventionally sprayed coatings were 50 HRC and 600 HDP 300 respectively. The hardness values for the vacuum-plasma-sprayed composite were 60 HRC and 860 HDP 300. The abrasive slurry wear resistance of the conventionally sprayed composite was three times that of the blend, while the wear resistance of the vacuum-plasma-sprayed composite was four times that of the standard blend coatings. The air-plasma-sprayed composite Cr 3C 2 Nichrome coatings are expected to exhibit performance characteristics comparable with similar detonation-applied coatings. The vacuum-plasma-sprayed composite coatings combine superior wear resistance with low oxide and are recommended for severe high temperature wear environments.