化学镀纯镍包覆WC-Ni粉体阴离子增强吸附自催化沉积机理研究

The WC-Ni cemented carbide coatings as protective hard coatings of wear-/corrosion- resistant and anti-fatigue applications are being applied for key components in nuclear power plants and mainly prepared by using thermal spraying technology. However, oxidation of WC phase during the thermal spraying can lead to brittle carbon-deficient phases formation and the chemical reaction resultant gaseous byproducts raising porosity of the coatings. In this project, this problem of WC-Ni thermally sprayed coatings is to be solved by using WC-Ni spray powders encapsulated with a dense, uniform and thickness-controlled pure nickel layer, by which the WC phase oxidation is effectively suppressed in turn leading to a low-porosity in the coatings. To fulfil the application of coatings in nuclear power plants with requirement of prohibited elements, a novel electroless plating process of pure nickel is alternatively proposed to encapsulate the WC-Ni spray powders, as eliminating both the Sn sensitization and Pd activation steps commonly employed in well-established Ni-P electroless plating process. Alternatively, WC-Ni powders will be surface modified using anionic surfactant, and the adsorption of Ni ions onto the powders followed by reduction as well as the self-catalysis effect of Ni for subsequent electroless plating will be systematically investigated to develop a batch process method to encapsulate WC-Ni powders for industrial application. The anionic surfactant type and incorporating approaches with either before plating or directly adding into plating bath are to be studied concerning their effects on Ni ions adsorption and nucleation onto WC-Ni powders, while the factors of bath composition and pH value will be also emphasized to analyze their influence on the density and morphology of adsorbed Ni species. Self-catalysis effect of adsorbed Ni on subsequent growth and deposition efficiency of encapsulating layer is then to be explored to understand the whole process of adsorption, reduction and self-catalysis deposition of nickel and to develop a controllable pure nickel electroless plating process. Based on the enhanced adsorption, reduction and self-catalysis deposition mechanism due to anionic surfactant modification of WC-Ni powders, a batch plating process of kilogram scale powders may be innovated to achieve uniform and dense encapsulating layer on the powders, to ultimately enable the wider applications of nuclear grade WC-Ni coatings with combined wear- and corrosion-resistant and anti-fatigue performance.

针对热喷涂制备核级WC-Ni涂层存在的硬质相WC氧化脱碳致脆及气化增加孔隙缺陷问题，本项目采取适于核电涂层应用的无Sn敏化且无Pd催化的纯镍化学镀工艺，揭示阴离子改性WC-Ni表面的Ni离子强吸附还原及自催化沉积规律，发展阴离子浓度调控镀液的批量化沉积方法，在WC-Ni粉体上制备连续均匀、厚度可控的Ni包覆层，实现WC无氧化和低孔隙涂层的喷涂。研究阴离子种类、镀前或镀液引入方式等对WC-Ni粉体表面Ni离子吸附与形核位置的影响，分析不同组分和酸碱度镀液中吸附Ni密度与形貌变化规律；研究吸附Ni自催化对后续Ni包覆层生长模式和沉积效率影响作用，掌握WC表面阴离子吸附Ni还原沉积及持续自催化生长的控制方法，基于阴离子改性增强吸附-还原-自催化一体的纯镍沉积机理，发展出镀液再循环连续镀工艺，获得公斤级批量粉体均匀致密Ni包覆，为耐磨抗蚀抗疲劳核级WC-Ni硬质合金涂层制备创造条件。

项目针对热喷涂制备核级WC-Ni涂层存在的硬质相WC氧化脱碳致脆及气化增加孔隙缺陷问题，采用无Sn敏化、无Pd活化的纯镍化学镀工艺，在WC以及WC-Ni （WC-10Ni和WC-12Ni）硬质合金粉体表面获得均匀、连续、厚度可控的纯镍覆层。揭示WC以及WC-Ni粉体表面直接化学镀纯镍的沉积机制、弄清不同荷电类型表面活性剂以及其它重要工艺影响因素，如镍盐种类、温度、搅拌方式和搅拌速度等，对WC以及WC-Ni粉体表面Ni离子吸附以及还原和自催化沉积的影响并讨论其中机制；利用低温超声直接化学镀镍机制，研制出适合高密度粉体的小批量化学镀分散装置；通过镀镍前后粉体的热分析，结合喷涂涂层的XRD图谱分析脱碳相浓度和抑制脱碳效果；研究粉体表面镀镍对超音速火焰喷涂工艺的最适宜工艺参数影响，如燃油比、转盘速率以及沉积率等，解析粉体表面覆镍对涂层相结构、孔缺陷以及硬度、耐磨耐蚀性的影响和规律。研究结果表明，WC由于其表面过低等电点（I.E.P ≈3.2）而具有强烈吸附阳离子趋势，同时强还原剂使无Pd催化的镍离子还原反应成为可能，促成了WC以及WC-Ni粉体表面直接化学镀镍的实现。相对于阳离子表面活性剂HPB和CTAB，阴离子表面活性SDS及中性表面活性剂TritonX-100能提高粉体表面镍包覆层的均匀性。在超音速喷涂的高温有氧环境中，粉体表面包覆镍层能一定程度抑制WC的氧化脱碳分解。粉体表面包覆镍层存在适宜的厚度范围，过薄的包覆层对抑制WC氧化脱碳的效果不太明显，过厚的包覆层则造成了沉积涂层镍粘结相的不均匀分布，涂层平均硬度下降，摩擦磨损性能有所降低。需要对喷涂粉体镍包覆层厚度和热喷涂工艺参数的匹配调控，以获得高致密度、耐磨减摩的高性能硬质合金热喷涂涂层。