Microstructure and Properties of Atmospheric Plasma Sprayed (Al,Cr)2O3\u2013TiO2 Coatings from Blends

Maximilian Grimm,Lutz-Michael Berger,Susan Conze,Thomas Lampke,Rico Drehmann

doi:10.1007/s11666-021-01289-6

Abstract

Coatings prepared from chromia-rich (Al,Cr)2O3 solid solution (ss) feedstock powders are intended to improve the properties of Cr2O3 coatings, but are rarely studied so far. In this work, the processability of a commercial (Al,Cr)2O3 solid solution (ss) powder containing 78 wt.% Cr2O3 by atmospheric plasma spraying (APS), the corresponding coating microstructures and properties were investigated. Possible further improvements were expected by blending with 2, 23 and 54 wt.% TiOx powder. For comparison, plain Cr2O3 and TiOx coatings were studied as well. The microstructures were analyzed using SEM, EDS and XRD measurements. Hardness (HV0.3) was measured, as well as the dry unidirectional sliding wear resistance and the abrasion wear resistance (ASTM G65). Moreover, the corrosion and electrical insulating properties were measured. The (Al,Cr)2O3 ss showed only a small change of the composition, and the formation of γ-Al2O3, as found for alumina-rich (Al,Cr)2O3 ss powders, was avoided. Compared to the plain chromia coating, some improvements of the processability and coating properties for the ss (Al,Cr)2O3 coating were found. The most balanced coating performance was achieved by blending the ss (Al,Cr)2O3 with 2 wt.% TiOx, as this coating showed both a high sliding and abrasion wear resistance, in combination with a high corrosion resistance.

Highlights

Sprayed Cr2O3 coatings are widely used due to their excellent wear and corrosion properties (Ref [1-3])
The AC75 powder particles show a largely spherical morphology consisting of a large number of small grains, typical for powders produced by agglomeration and sintering
The (Al,Cr)2O3 solid solution coatings already possess some advantages over plain chromium oxide coatings, such as a higher deposition rate or a significantly improved corrosion resistance

Summary

Introduction

Sprayed Cr2O3 coatings are widely used due to their excellent wear and corrosion properties (Ref [1-3]). Cr2O3 does not show any phase transformation during spraying, but has a very high melting temperature of over 2300 °C and shows low deposition efficiencies of about 40%, maximum 60% in the case of APS (Ref 2). Bolelli et al (Ref 1) indicate that Cr2O3 volatilizes at high temperature in both reducing and oxidizing environments, and in both cases, this occurs well below its melting point. A reduction in the reducing part of the plasma can be responsible for the change of color from green to black due to a small oxygen deficiency in Cr2O3 (Ref 5) or, in more severe cases, for formation of metallic chromium (Ref 1).

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