Effect of the annealing on the microstructure of HVOF deposited coatings

Purpose: The objective of this paper is to present how process of the coatings annealing deposited by high velocity oxy-fuel (HVOF) method influence on the microstructure changes. The differences in the microstructure and microhardness after different variants of the annealing in the comparison to the HVOF deposited coats were presented. Design/methodology/approach: Two different coatings: WC-Co and Cr3C2-NiCr deposited on the AK9 substrate by HVOF method were investigated. The coats were annealing at the nitrogen in the conditions as follows: a) T = 550°C, t = 5.5 h, b) T = 500°C, t = 24 h. After, the samples were subjected by using optical (MO) and scanning electron microscopy (SEM). Also the microhardness was determined by Vickers method, the applied load was 200 gram. Findings: The microstructure of the WC-Co and Cr3C2-NiCr coatings was build from the equiaxial grains distributed relatively uniform. Also characteristic was large number of discontinuous, voids and pores, especially in the WC-Co coat. After annealing, both WC-Co and Cr3C2-NiCr coating, the microstructure was more homogenous. It was observed reduction of the pore and voids amount. The microhardness after annealing was almost at the same level as after HVOF deposition. Practical implications: The performed investigations could be useful in the industrial practice and give the information about working WC-Co and Cr3C2-NiCr coats at the elevated temperatures. Originality/value: The HVOF deposited and successive annealed WC-Co and Cr3C22-NiCr coats have more uniform microstructure which could contribute to the improvements of some properties, for example wear resistance.