Wear behaviour of low-, medium-, and high-phosphorus electroless nickel–phosphorus (ENP) coatings deposited on DMR249A naval-grade steel

Electroless-Nickel-Phosphorus (ENP) coatings with three phosphorus categories, low-P (LP, ∼3–5 wt.%), medium-P (MP, ∼6–9 wt. %), and high-P (HP, ≥10 wt. %), were deposited at three thickness levels (∼8, ∼20, and ∼33 µm) on DMR249A naval-grade high-strength low-alloy (HSLA) steel. A single-factor, three-level experimental design (phosphorus class x coating thickness, nine coated conditions + uncoated reference n = 5 hardness indentation per condition) was used to systematically map coating microstructure onto tribological response. Wear behaviour was evaluated using a ball-on-disk tribometer (ASTM G99–05) at 10 N, 0.5 m/s, and 500 m sliding distance against a WC counter-ball (ø10 mm, 2000 HV). LP coatings achieved the highest as-deposited hardness (up to 790 ± 27 HV0.2 for LP-3 (30.3 µm), best-three averages, a 264% increase over the uncoated substrate, 217 ± 5 HV0.2) and the best overall tribological performance (LP-3 recorded zero measurable mass-loss over 500 m at a CoF of 0.525, a 100% reduction in specific wear-rate related bare DMR249A (60.96 × 10 −15 mm 3 /Nm), while HP coatings showed the highest wear rates and lowest hardness (up to 431.5 × 10 −15 mm 3 /Nm and ∼337 HV0.2 for HP-1), but offered the best corrosion resistance (electrochemical corrosion rate as low as 0.0033 mm/yr for HP-3 versus 0.080 mm/yr for bare steel). AFM confirmed LP deposits are the roughest (R q ≈ 14.99 nm) and MP the smoothest (R q ≈ 2.14 nm). SEM, XRD, EDAX, and FTIR were combined to develop a mechanistic interpretation of the observed wear trends. The results provide a practical, quantitatively benchmark framework for selecting ENP coating class and thickness for Indian naval platform maintenance and new-build applications.