Structure, thermal stability and mechanical properties of electroless Ni–P–W alloy coatings during cycle test

The electroless Ni–P–W coatings were deposited on the mild steel in the alkaline solutions. The compositions of Ni–P–W and Ni–P coatings were evaluated by electron probe microanalysis. The effect of thermal cycle at 400 and 450 °C on the crystal of the electroless deposit structure was studied by X-ray diffraction (XRD). The thermal stability of electroless Ni–P and Ni–P–W coatings with the identical phosphorus content was analyzed by differential scanning calorimetry. With similar precipitated Ni3P in both heat-treated Ni–P and Ni–P–W films, the tungsten addition into the nickel–phosphorus based coatings effectively increases microhardness and thermal stability. After eight times of thermal cycling test at 400 °C, the XRD analysis shows that Ni–P films have completely crystallized and the hardness is degraded to a lower value. On the contrary, the Ni–P–W films have not completely crystallized and thus a high value of hardness can be retained without appreciable degradation.