Comparative Investigation of the Microstructure, Hardness, and Tribological Properties of Electroless Ni–B Composite Coatings Reinforced with TiO₂ and ZrO₂

This study focuses on the electroless deposition and characterization of Ni–B composite coatings reinforced with TiO₂ and ZrO₂ ceramic particles. Coatings were deposited on steel substrates using an acidic bath (NiSO₄·6H₂O, DMAB, thiourea, sodium acetate) under controlled conditions (65 ± 3 °C, pH 6, 350 rpm, 45 min) with reinforcement contents of 0 to 15 g/L. The coatings were systematically evaluated in terms of morphology, structure, hardness, and tribological performance. Ni–B exhibited the typical nodular morphology, while the incorporation of TiO₂ and ZrO₂ progressively modified surface features. At moderate concentrations, both reinforcements were uniformly dispersed within the matrix, whereas higher contents led to particle clustering and microstructural heterogeneity. XRD confirmed the amorphous–nanocrystalline nature of Ni–B; TiO₂ acted mainly as an inert phase, while ZrO₂ promoted localized crystallization, resulting in harder and more stable structures. Microhardness increased from ~750 HV for plain Ni–B to ~985 HV (TiO₂) and ~1080 HV (ZrO₂) at 10 g/L, with a slight decline at higher loadings due to agglomeration. Tribological tests demonstrated that both reinforcements improved wear resistance and reduced friction, with TiO₂ yielding more stable friction coefficients and ZrO₂ providing superior resistance to material loss. These results highlight the potential of TiO₂- and ZrO₂-reinforced Ni–B coatings as sustainable alternatives with improved mechanical and tribological functionality for engineering applications.