Laser alloying using carbide forming elements (Ti, Nb, and V) was carried out to form an MCFe composite coating on a carbon steel substrate for improving wear resistance. The microstructure of laser alloyed zone was studied by OM, SEM/EDS, STEM, and XRD to reveal the effect of additional elements and laser alloying conditions on the distribution of carbides. The laser alloyed zone was found to consist of MC carbide and iron matrix. Hardness and wear properties of the laser alloyed zone were improved as compared with the laser transformation hardened sample. Dispersion of carbide within the laser alloyed zone is shown to be effective in decreasing the friction coefficient and improving wear resistance.Keywords: laser surface alloying, microstructure, metal matrix composite, carbide, steel
IntroductionMetal matrix composites (MMC) that use ceramic particles for reinforcement have good toughness and wear resistance when applied to wear resistant materials such as tools and die [1]. MMCs are generally produced by a powder metallurgy or casting processes. In the powder metallurgy process, it is difficult to produce large and complex shaped components. In contrast, complex shapes can be produced using the casting process, however, it requires a lot of energy to melt a high melting temperature matrix such as iron. However, it is efficient if MMC coatings are locally formed on only the required area of low cost materials by using a laser surface modification process. Recently, a number of researchers have investigated MMC coating using laser cladding or laser surface alloying [2][3][4][5][6].Laser surface alloying is one route of producing an MMC coating on a substrate. This process consists of melting the surface of a substrate with additional materials, mixing these components, followed by rapid solidification of the mixture [7][8][9]. Advantages of this process are as follows: low thermal strain, fine microstructure, and flexibility of the choice of substrates and additional materials. MMC coatings can be produced by injection of hard particles into the molten matrix [10][11] or in-situ synthesis of hard particles in the molten pool [12][13].In this study, laser surface alloying using graphite and pure carbide forming elements such as titanium, niobium, and vanadium was carried out to form MC-Fe composite coating on carbon steel in order to improve wear resistance. Carbon steel is a typical low cost industrial material, and MC carbides such as TiC, NbC, VC, have high hardness and are chemically stable in a ferrous matrix, hence each carbide is effective for improving wear resistance [14].The issue of MMC coating by the laser surface alloying process is to control the distribution of reinforcements and to enhance the uniformity of the coating layer. In the laser
