Electroless Ni–P Composite Coatings: The Effect of Heat Treatment on the Microhardness of Substrate and Coating

Several types of hard particles, SiC, Al sub 2 O sub 3 and B, were co-deposited by electroless within a Ni-(8.22 wt.%)P matrix, on 6063-T6 Al substrate. Investigation of the Ni-P deposit structure by XRD and DSC revealed an amorphous phase with a crystallization temperature approx350 deg C. The heat treatment applied to achieve a maximum hardness of the coating caused a significant decrease of the 6063 Al microhardness due to coarsening of Mg sub 2 Si precipitates and recrystallization of the Al matrix. Thus, for this substrate-coating sytsem, heat treatments > 250 deg C are not recommended because the structural stability of the substrate, and consequently, its properties are negatively affected. Substrate: 6063-T6 Al.