Hardness evolution of electroless nickel–phosphorus deposits with thermal processing

Four samples of electroless nickel–phosphorus (EN) deposits coated on mild steel substrate have been analysed for their hardness changes in relation to the deposit phosphorus contents as well as different heating temperatures at isothermal (100–500 °C for 1 h) and linear heating (to 300–600 °C at 20 °C/min) conditions. It was found that the hardness of the EN samples increased with decreasing phosphorus content at as-deposited condition, and could be enhanced by appropriate heating. The results of Vickers microindentation testing showed that the peak hardness of the EN samples could be achieved after heat-treating at 400–450 °C. This is caused by the formation of intermetallic Ni3P stable phase at this temperature range, acting as a function of precipitation hardening. The Knoop microindentation testing on the cross sections of the samples indicated variations of the hardness across the depth (distance from the sample surface towards the sample/substrate interface), but for most samples the tendencies of change were not clear. Scanning electron microscopy analysis has shown that the lamellar structure present in the cross sections of the as-deposited EN samples tends to disappear, and agglomeration occurs when the heat-treating temperature is increased. The concept of kinetic strength (Ks) is adopted to interpret the kinetic energy (Q) of increased hardening effects using the Vickers hardness data after the isothermal experiments, but the result has not been satisfactory.