Composite coatings constitute a new class of materials, which are mostly used in mechanical and tribological applications as well as because of anti-corrosion properties. Laser surface alloying of 35NCD16 bainitic steel and iron substrates with silicon powder was studied. It is principally the nature of substrate, which determines the thickness of heat affected zone (HAZ) and melted zone (MZ). The MZ was found to be homogeneous in composition and concentration in alloying elements and mainly constituted of rapid solidified structure. Depending on carbon and Si contents, residual stresses and phase transformations, the superficial zone could be cracked. The cracking phenomenon is observed in the MZ of iron substrate whenever the Si deposit thickness and when more than 20 mum of Si powder is alloyed with steel substrate. Hardness and cracking depend on residual stress level. The result is a superficial hardness increase up to a maximum and then a decrease with apparition and generalization of cracks associated to reduction of stress level.