Mechanical properties and microstructure of vacuum plasma sprayed Cr3C2 – 25(Ni20Cr) coatings

Abstract

This paper analyzes vacuum plasma spray VPS - Cr₃C₂ - 25(Ni20Cr) coatings. Commercial powder marked Sulzer Metco Woka 7205 is used. The powder is deposited with a plasma gun F4 at a distance of 340 mm from the substrate. The main objective of the study was to eliminate, at the reduced pressure of inert gas Ar, the degradation of primary Cr₃C₂ carbide into Cr₂₃C₆ carbide which significantly reduces the microhardness and mechanical properties of the coating. The coating is deposited with a thickness of 100 - 120 µm on a steel substrate. The microhardness of the coating was tested by HV_0.3. The microhardness values were in the range of 1248 - 1342 HV_0.3. The bond strength of the coating was tested by tension. It was found that the bond strength between the substrate and the coating has a value of 89 MPa. The microstructure of the coating was tested by the light microscopy technique. The structure of the coating consists of an NiCr alloy base with a dominant primary Cr₃C₂ carbide phase. In addition to the Cr₃C₂ phase,  the Cr₇C₃ phase is also present. The coating etching was done with the reagent 1HNO₃ : 4HCl : 4H₂O that primarily  dissolves nickel to enable the distribution of the carbide phase to be clearly seen in the coating. Etching the coating with this reagent  revealed the presence of the largely undegraded primary Cr₃C₂ carbide phase   which provides high hardness values to the coating.