Microstructure transformation of Cr-Al-BN coating on low carbon steel prepared by ball milling method

Cr-Al-BN powder was successfully applied as a coating agent on low carbon steel substrate by mechanical alloying method. The addition of BN as a doping material is considered as a feasible option because of excellent thermal and chemical stability presented by BN. In this paper, the compositions of Cr-Al-BN are varied Cr49.5-Al49.5-BN, Cr48.5-Al48.5-BN3, and Cr47.5-Al47.5-BN5. Each powder was milled by shaker mill for 2h in a stainless steel chamber with stainless steel balls to powder ratio 10:1. Subsequently, each powder compositions were mechanically alloyed onto substrate surface in air for 2h. The 2h heat treatment at temperature 800°C was conducted to each coated sample in vacuum furnace. In order to achieve the characteristics of phase composition and microstructure of the coating before and after heat treatment, XRD and Optical Microscope were performed, while the automatic microhardness tester was carried out to get the hardness of coating layer. The oxidation behaviour of coated substrates was also studied by heating treatment at 800°C for 8 cycles where each cycle is 20 h. The results show that the ball milling induces the formation of homogeneous Cr-Al-BN coating structure with a thickness of about 62.5 μm The optimum coating hardness and oxidation resistance was achieved by Cr47.5-Al47.5-BN5 coating composition. The addition of higher concentration ofBN increases the tendency onthe formation of intermediate phase of Cr2B and Cr2N. The detailed result of this study is shown in this paper.