ABSTRACT The effects of Ti particle loading in a plating bath on the morphology and hardness of Ni-Ti composite coatings via an electrochemical co-deposition coating were investigated. The Ti-reinforced Ni-matrix composite coatings were co-deposited on Austenitic Stainless Steel (316 L) substrates using a Ni ion electrolytic solution stably suspended with -20 ?m max. diameter Ti particles and heat-treated under vacuum atmosphere for six hours at 950 °C prior to oxidation testing. During heat treatment, coating morphological development was found to occur by the exothermic reaction Ni +Ti = NiTi. Further, the oxidation behavior of nickel matrix/ titanium particle composite coatings in the air at (700-900°C) for 50 hrs. was studied. The prepared coatings were characterized by X-ray diffraction (XRD), optical microscope, scanning electron microscopy (SEM), Vickers hardness, and atomic absorption. For all samples, weight gain kinetics was obtained from furnace exposure and the composition and morphology of the oxidation products were examined using XRD analysis and optical microscopy. An outer TiO2 layer was formed on the composite coating surface. At test temperatures 700, 800, and 900 °C, the coatings benefit from a small grain size that enhances Ti diffusion to the surface to form the protective TiO2 layer.
CONCLUSIONS
1. The Ni-Ti composite coating was successfully produced through the electrochemical co-deposition route. The composite coating produced a gray, rough metallic surface with thickness ranging up to 115 µ m and Ti content up to 35%.
2. Heat treatment at 950 oC for 3 hrs. under vacuum atmosphere results in diffusion-reaction between the Ti particles and nickel matrix to produce a range of Ni-Ti Intermetallic compounds like NiTi and Ni3Ti phases.
3. Over the temperatures range between 700-900 oC, the parabolic rate constant (kP) varies from a (kP = 6.696x10-7 (mg2 /cm4 )/s.) at 700 oC to a value of (kP = 1.813x10-6 (mg2 /cm4 )/s.) at 900 oC.
4. Titanium dioxide is the major phase that exists in the outer layer under oxidation conditions.