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The present study is a systematic investigation into the effect of three different types of surfactants (anionic: SDBS, cationic: CTAB, and non-ionic: PVP) on the behaviour of electroless Ni-P baths containing TiC nanoparticles, and on the formation of Ni-P-TiC composite coatings. The effect of surfactants can be observed in the interfacial phenomena, i.e. wetting and electrokinetic behaviour during the formation of the coating. The wetting properties of the coating bath were investigated by the sessile drop method, which revealed the wetting improvement on all three substrates (W302 steel, TiC, Ni-P coating substrates) in the case of the anionic and cationic surfactant. The Zeta potential measurement showed a stability decrease in Ni-P bath compared to the distilled water medium, which led to the agglomeration of the TiC particles. TiC agglomerate size below 100 nm was achieved by all three types of surfactant (1 g/L) in distilled water; however, only larger agglomerates were measured in the Ni-P bath. Finally, the Ni-P-TiC composite coatings on steel substrate were created. The investigation of the cross-section of the coatings revealed that, the highest amount of TiC (0.53 wt.% and 0.52 wt.%) was incorporated in the Ni-P matrix with the use of 0.1 g/L CTAB and 0.1 g/L PVP respectively, although these particles formed larger agglomerates.

Electroless Ni-P coatings were obtained on steel substrates using different bath compositions, which lead to different phosphorous contents of the coatings. In this paper the effect of the P-content in the Ni-P coatings was experimentally studied on the thickness of the coating, on its nano-micro-structure and on its micro-hardness. The as-received samples were nano-crystalline (mostly amorphous according to XRD) and their micro-hardness was found to decrease with increasing the P-content. Upon annealing at 400 °C a new Ni3P phase was formed and the nano-cystalline Ni-rich grains coarsened to micro-grains. In this annealed state the micro-hardness was found to increase with increasing the P-content. A complex model was built to explain the experimental results. It was supposed that the as-received Ni-P coating contains almost pure Ni nano-grains surrounded by segregated P atoms; as the grain grows, its surface is covered more and more by the P atoms. When the grain is fully covered by the P atoms, further grain growth is inhibited and the coating can grow further only due to nucleation of a new grain. Thus, the size of the grains was found inversely proportional to the P-content of the Ni-P alloy. The need for a larger number of nucleation events with decreasing grain size explains why the coating has a smaller thickness for smaller grain size, i.e. higher P-content. The inverse Hall-Petch rule was found for the grain size dependence of micro-hardness of the as deposited samples due to the grain boundary sliding of relatively hard Ni-rich nano-crystals along the soft P atoms (higher P-content lead to lower micro-hardness through smaller grain size). After annealing the micro-hardness was found to increase with the volumetric phase fraction of the harder Ni3P phase within a relatively soft Ni matrix, i.e. it was found to increase with the P-content of the Ni-P coating. The extrapolated value for the micro-hardness of the Ni3P phase is found about 757 ± 20 HV 0.01 .

In this study electroless Ni-P coatings were deposited on W302 steel substrates. The effects of bath pH and heat treatment at 400?C were investigated on the surface morphology, phase structure, phosphorus content, thickness and microhardness of the coatings. It was observed that both the phosphorus content and coating thickness are dependent on the bath pH. In an acidic/neutral bath, low and medium phosphorus coatings with thickness of 13.9-19.8 ?m were synthesized, while in an alkaline bath, high phosphorus, 4.8-5.8 ?m-thick coatings were formed. Coatings containing medium or high P seemed to be amorphous, while low P coatings had microcrystalline structures. Hardness was also dependent on the composition of the coating. After heat treatment, the structure of the coatings transformed into crystalline Ni with the precipitation of Ni3P phases, which resulted further increases in hardness.