Microstructure and mechanical properties of MoSi2 coatings produced by laser processing

The feasibility of depositing MoSi2 on steel or graphite by using a laser beam has been studied. The main advantage of this technique is to give a dense material in the form of fine layers, which bonds well with the substrate. Coatings have been manufactured on steel substrate in order to improve resistance to high temperature and on graphite to obtain a 'massive' MoSi2. This compound can be formed from two distinct powders of Mo and Si on steel or graphite substrate, provided that supplied energy is sufficient to start the reaction between the initial products. Coatings have also been manufactured directly from a MoSi2 powder. In both cases, layers are dense but with a high density of cracks due to thermal stress induced by phase transformation and thermal gradients during coating manufacturing. On steel substrate, a high dilution rate is observed. Two routes of processing have been used to remedy to this problem: by preheating the steel substrate to 450 degrees C, to reduce the thermal gradients, and by using a graphite substrate to avoid dilution of the substrate at high temperatures. Unfortunately, these attempts appear insufficient to obtain a deposit of sound MoSi2, free of cracks. Tensile stresses appearing during solidification can lead to cracking of the deposit if they locally exceed the rupture strength of the material, The average microhardness of these coatings is approximate to 1000 HV0.3, four times larger than that of the steel substrate. (C) 1998 Elsevier Science S.A. All rights reserved.