Evolution of structural, mechanical and tribological properties of Ni-P/MWCNT coatings as a function of annealing temperature

The structural, mechanical and tribological properties of Ni-P/MWCNT coatings annealed at various temperatures (350-500 °C) were investigated using XRD, SEM, nanoindentation and tribometer to determine the optimal annealing temperature for their enhanced tribological properties. The results showed that the annealed coatings comprised a hard Ni3P phase, and consequently presented a higher hardness (from 7.0 ± 0.3 to 8.2 ± 1.4 GPa) than the as-plated sample (6.0 ± 0.9 GPa). With the annealing temperature increasing from 350 °C to 500 °C, the crystallinity of coating was enhanced with larger crystal grains of Ni and Ni3P, which led to a decline in hardness (from 8.2 to 7.0 GPa) due to the Hall-Petch effect. Owing to the lubrication effect of H3PO4 arising from the tribochemical reaction of Ni3P with ambient environment, the annealed samples exhibited lower friction coefficients (0.71 ~ 0.86) compared to the as-plated coating (0.87). A combination of low surface roughness and the reduction of oxides on wear track contributed to the lowest friction coefficient of Ni-P/MWCNT annealed at 400 °C. However, the decomposition of amorphous carbon in MWCNT over 380 °C produced less dense coatings (for annealing temperatures 400-500 °C), and their incompact structure led to a higher wear rate (2.9-3.0 × 10-5 mm3/Nm) compared to the as-plated sample (2.4 × 10-5 mm3/Nm). In contrast, Ni-P/MWCNT coating annealed at 350 °C (<380 °C) exhibited a better wear resistance (4.3 × 10-6 mm3/Nm). Thus, 350 °C was found to be the optimal annealing temperature to lower the friction coefficient and enhance the wear resistance of Ni-P/MWCNT coatings.