Ni-P-B4C复合镀层秉承化学镀Ni-P合金镀层良好的耐蚀性、高硬度和高耐磨性，又由于B4C颗粒的弥散强化作用，将在机械零件耐磨表面有很好的应用前景。本文通过大量试验，确定了化学镀Ni-P-B4C复合镀层的最佳镀液配方和工艺参数。用金相显微镜和X-射线衍射分析(XRD)研究了Ni-P-B4C复合镀层的微观形貌和组织结构。采用动电位极化曲线的方法，研究了Ni-P-B4C复合镀层的耐蚀性。采用氧化增重的方法研究了Ni-P-B4C复合镀层的高温抗氧化性能。并研究测试了Ni-P-B4C复合镀层的硬度和干摩擦磨损性能。在上述实验的基础上，研发出Ni-P-B4C梯度镀层，并对Ni-P-B4C梯度镀层的性能进行了初步探讨。得出如下结论： (1) 化学镀Ni-P-B4C复合镀层的最佳镀液配方和工艺参数如下：硫酸镍25～30 g/L、次亚磷酸钠25～30 g/L、柠檬酸钠10～20 g/L、无水乙酸钠10～20 g/L、添加剂A适量、添加剂B适量、B4C 5～25 g/L、pH 3.1～5.6、施镀温度85～87℃、搅拌速度150～300 rpm、施镀时间2～4 h。实验表明，以上述镀液配方和工艺参数获得的Ni-P-B4C复合镀层与基体结合力良好，B4C颗粒弥散均匀地分布在Ni-P合金基质中；镀速每小时可达到37μm。 (2) Ni-P-B4C复合镀层在酸、碱、盐三种腐蚀介质中的耐蚀性从强到弱排序为：10%NaOH＞3.5%NaCl＞10%HCl。进一步研究表明，Ni-P-B4C复合镀层的耐蚀性与镍磷比基本无关，而随B4C颗粒含量、热处理温度及添加剂浓度的变化而变化。 (3) Ni-P-B4C复合镀层在抗高温氧化测试开始阶段，增重较多，但是随着时间的延长，Ni-P-B4C复合镀层单位面积的增重不明显；且随着氧化温度的升高，Ni-P-B4C复合镀层氧化增重呈直线增加；而随着B4C颗粒含量的升高，Ni-P-B4C复合镀层氧化增重呈直线减小。 (4) Ni-P-B4C复合镀层的硬度随着B4C颗粒含量和热处理温度的升高而升高。复合了硬质相的Ni-P基化学镀Ni-P-B4C复合镀层，随着B4C颗粒含量的增加，摩擦系数减小，而磨损却增加。经热处理后，Ni-P-B4C复合镀层的摩擦系数和磨损协同减小。 (5) 与Ni-P-B4C均匀复合镀层相比，Ni-P-B4C梯度镀层具有更优良的耐蚀性和干摩擦磨损性能。

Electroless Ni-P alloy coatings, well known of their excellent wear and corrosion resistance and high hardness, have many applications in industries. Codeposition another metallic or non-metallic elements or abrasive/lubricative particles or combination of them in binary Ni-P matrix can greatly enhance their properties. In this paper, electroless Ni-P-B4C composite coatings were prepared on low carbon steel (Q235) sheets by adopting optimum plating process. Metallurgical microscope and X-ray diffraction (XRD) were used to determine surface morphology and microstructure of the coatings. Corrosion resistance characterization of the coatings was enabled by means of potentiodynamic polarization curves. The oxidation resistance of the coatings was showed by their weight gains in a muffle furnace. Hardness and dry friction and wear properties of the coatings were also tested. On the ground of former experiments, electroless Ni-P-B4C gradient coatings were studied elementarily. The main results gained are as follows: (1) The perferable solution compositions and plating conditions for electroless Ni-P-B4C composite coatings are as follows: nickel sulfate 25~30 g/L, sodium hypophosphite 25~30 g/L, sodium citrate 10~20 g/L, sodium acetate anhydrous 10~20 g/L, suitable amount of additive agent A, suitable amount of additive agent B, B4C 5~25 g/L, pH 4.1~4.6, Temperature 85~87 ℃, Stirring rate, 150~300 rpm, Plating time 2~4 h. The results show that the adhesion between Ni-P-B4C composite coating and substrate is qualified and a relatively uniform concentration of particles can be seen along the cross section of electroless Ni-P-B4C composite coatings. The deposition rate is up to 37μm per-hour. (2) The order of corrosion resistance of Ni-P-B4C composite coatings during their exposure to acid, alkali and salt aqueous solution from strong to weak is as follows: 10%NaOH> 3.5%NaCl> 10%HCl. Further studies show that, corrosion resistance of Ni-P-B4C composite coatings is independent of nickel-phosphorus molar ratio, while varies with the contents of B4C particles, heat treatment and additive concentrations. (3) The weight gains of Ni-P-B4C composite coatings are high during the initial stage of the high temperature oxidation test, but decrease with time going on. And the weight gains of Ni-P-B4C composite coatings increase linearly with the oxidation temperature, while the weight gains of Ni-P-B4C composite coatings decrease linearly with the increasing of contents of B4C particles. (4) The hardness of Ni-P-B4C composite coatings increases with both the increasing of contents of B4C particles and heat treatment temperature. With the increasing of contents of B4C particles, the friction coefficient of Ni-P-B4C composite coatings decreases while the wear rate increases. After heat treatment, both the friction coefficient and wear rate of Ni-P-B4C composite coatings reduce. (5) Compared with Ni-P-B4C uniform composite coatings, Ni-P-B4C gradient coatings have higher corrosion resistance and dry friction and wear properties.