| The Ni-W-P alloy has outstanding physical and mechanical properties such as high hardness, high wear resistance, high heat stability, and excellent corrosion resistance, which makes it suitable for many engineering applications. The surface of the Ni-W-P alloy provides a sense of beauty and cleanness, but their silver color will give people the cold sense of alienation. With the improvement of people’s living standards, civil products such as constructions, household appliancs, cars and kitchen supplies are also expanding demand for Ni-W-P alloy products. The need of colored Ni-W-P alloys is also rising in the decorative and artistic development. The coloring technology of Ni-W-P alloy meets this requirement. The paper proproses two coloring methods. Coatings of uniform color, excellent wear resistance and corrosion resistance can be obtained. The detailed research work is listed as follows:1. Thermal coloration of the Ni-W-P alloy and study on the component, properties of colored coatings. A novel process for thermal coloration of Ni-W-P alloy deposits was developed in this study. After several experiments the best coloring process was determined. Ni-W-P alloys were dipped in the H2O2for30min and annealed for1h. Uniform organe, blue, and yellow color films formed on the surfaces of Ni-W-P deposits by heating them at350,410and450℃, respectively. Then their morphology and content were studied in detail by AEM and XPS. The adhesion, corrosin resistance, hardness, wear resistance and oxidation resistance were also tested. Uncolored Ni-W-P alloy did not have obvious raised grain. The average size of the crystallites or grains of the colored Ni-W-P alloys increased and got finer with increase in coloring temperature. The colored films were composed of Ni2O3, NiO, WO3and NiWO4. After coloring, the adhesion, corrosion resistance, hardness, wear resistance and oxidation resistance were greatly enhanced.2. The oxidation mechanism of the Ni-W-P alloy deposit and coloring mechanism of colored coatings. During thermal coloration of Ni-W-P alloy deposits at350-450℃, the colored films were composed mainly of NiO and Ni2O3since the content of tungsten and phosphorus was very low at the begininng of the oxidation process. Then stronger oxygen diffusin into the substrate along grain boundaries took place, leading to the internal oxidation of tungsten. When the thickness of NiO and Ni2O3film inceased, the oxide/alloy interface shifted internally. As comparison of the Gibbs free energies for the formation of NiO, NiWO4, WO2and WO3shows that the absolute value for the Gibbs free energy for the formation of tungsten (nickel) oxides is higher than that for NiO formation with the highest value shown for NiWO4formation, being5to10times higher than that for NiO. This leads to the effect that W can crack NiO and form NiWO4. The rate of oxidation process deponded on the positive, negative ions passing through the oxide film, these colors arised from both the strong absorption of compositions in the visible region and interference of interface, the color change could be ascribed to the increase in the film thickness.3. The study of chemical coloring process of Ni-W-P alloy deposits. A novel process for chemical coloring of Ni-W-P alloy deposits was developed in this study. Uniform blue, yellow and violet color films formed on the surfaces of Ni-W-P deposits by immersing them in an aqueous solution containing (NH4)6Mo7O24-4H2O, NaH2PO2-H2O, and MnSO4-2H2O with different immersing time. Mechanism study of coloring process showed that these colors arised from both the strong absorption of compositions in the visible region and interference of interface. Properties study of colored films showed that they had excellent corrosion resistance and high thermal stability. |
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