Effect of annealing on the structure and hardness of Ni–P–W multilayered alloy coatings produced by pulse plating

Ni–P–W multilayered alloy coatings were deposited on copper foils by pulse plating, using a rotating cylinder electrode. They consisted of layers of high (Ni–5 wt.% P–45 wt.% W) and low (Ni–8 wt.% P–15 wt.% W) W content. The wavelengths studied were 8, 40 and 400 nm and the total thickness of the coatings was approximately 25 μm. The multilayered coatings were annealed in the temperature range between 200 and 800°C and the effect of annealing on their structure was studied by using X-ray diffraction, cross-sectional transmission electron microscopy equipped with selected area electron diffraction facility, and differential thermal analysis. The initially amorphous multilayered coatings crystallise in steps as the annealing temperature increases (first the low-W crystallise layers and then the high-W layers), and the layered structure is lost around 600°C. The main crystalline phases formed during annealing are Ni–W solid solution and Ni3P. The wavelength of the coatings seems to affect the onset temperature of the crystallisation processes. The hardness of the coatings initially increases reaching a maximum in the 500–600oC range, and then decreases as the annealing temperature increases.