ELECTROLESS Ni-P-Ti BASED NANOCOMPOSITE COATINGS
Infrastructures in various industries, such as oil and gas, chemical, automotive etc., are subjected to aggressive erosive and corrosive environments. A potential candidate for the protection of such infrastructures is electroless Ni-P coating owing to its high hardness and good corrosion resistance. Nevertheless, Ni-P has low toughness, which tends to readily crack and fracture under load. To toughen Ni-P coating, superelastic NiTi particles are an attractive addition. The high price of the NiTi particles, however, restricts their application. Ti particles have much lower price (5-10 times lower) than NiTi particles, which can be employed as an alternative to the NiTi particles. Therefore, to reduce cost, Ti particles were utilized as an addition in the present study. Ni-P-NiTi composite coatings with different Ti concentrations were successfully prepared on low carbon steel substrates by co-plating Ni-P and Ti nanoparticles followed by annealing of Ni-P-Ti coatings. To systematically investigate the effects of the formation of superelastic NiTi phase after annealing on the mechanical, wear, and corrosion properties of the composite coatings, a series of tests were conducted such as scratch, bend, tensile, nanoindentation, single particle erosion, corrosion, and erosion-corrosion tests. The formation of NiTi phase after annealing was substantiated by slow scan XRD, point and line scan EDS. The superelastic effect of NiTi particles was also verified by nanoindentation tests. Compared to Ni-P-Ti coatings annealed at 700°C and 800°C for 5 hours, Ni-P-Ti coatings annealed at 650°C for 2 hours exhibited higher Vickers hardness, scratch toughness, scratch resistance due to the formation of high amount of superelastic NiTi phase. Different toughening mechanisms such as crack deflection, crack bridging, transformation toughening induced by superelastic NiTi particles were observed and identified in the aforementioned tests. In conclusion, the formation of superelastic NiTi phase not only improved the toughness, scratch, indentation, erosion resistance but also considerably enhanced the corrosion and erosion-corrosion resistance of Ni-P coating. As a result, the annealed Ni-P-Ti coatings (i.e., Ni-P-NiTi coatings) may be employed to protect low carbon steel components or in applications where the high cracking resistance, high wear, or impact resistance, and/or high corrosion resistance are required.