DEVELOPMENT OF FUNCTIONALLY GRADIENT COATINGS FOR WEAR-RESISTANCE BY LASER SPRAYING

采用激光喷涂的方法制备了钛和氮化钛梯度功能涂层。喷枪具有作为喷涂材料的金属丝的中心出口和用于辅助气体的两个同轴环形喷嘴。用高能量密度的二氧化碳激光熔化了一根钛丝。一股氩气流产生了熔化的钛颗粒的精细喷雾，它覆盖了铝和低碳钢的衬底。通过改变气体中的Ar/N比，形成了不同氮化钛含量与钛含量比的后续层。最后一层仅用氮气包覆一层氮化钛。每一层的化学成分通过涂层(约1 mm厚)分级；每一层的硬度和热膨胀系数通过涂层分级。此外，还采用反向工艺制备了反向梯度涂层。所制备的功能梯度涂层具有以下特点：(1)优异的耐热震性。(2)硬面涂层具有良好的耐磨性。(3)耐磨性不受涂层顺序和分级程度的影响，而取决于摩擦表面的硬度。表面硬度越高，耐磨性越好。(4)涂层与基材在摩擦条件下的结合强度随涂层厚度的增加而增加。氮化钛涂层的结合强度比氮化钛涂层强，也就是说，如果涂层表面有较厚的氮化钛涂层和较低的氮化钛含量，所开发的成分梯度激光喷涂涂层可以提供良好的耐磨性和抗热震性。

Functionally gradient coatings, which consists of titanium and titanium nitride, have been developed by a laser spraying method. The spray gun has a center outlet for a wire as a spraying material and two coaxial ring-nozzles for assist gas. A titanium wire was melted by a high-energy-density carbon dioxide laser beam. An argon stream produced a fine spray of melted titanium particles which coated an aluminum and a mild steel substrates. The subsequent layers having different ratio of titanium nitride content to titanium were formed by changing the argon-to-nitrogen ratio in the gas. The last layer was coated with titanium nitraide by using nitrogen only. The chemical components of each layer were graded through the coatings (about 1 mm thick); the hardness and the thermal expansion coefficient of each layer are graded through the coatings. The reversal gradient coatings were also produced by the reverse process. The functionally gradient coatings produced have the following characteristics: (1)Excellent resistance for repeated thermal shock. (2)The hard surface coatings provide a good wear resistance. (3)The wear resistance is not influenced by the order of coatings and the degree of graduation but depends on the hardness of the rubbing surface. The higher hardness of the surface, the higher wear resistance. (4)The adhesion strength between the coated film and the substrate material under rubbing conditions increases with increase in the coating thickness. The adhesion strength in case where the substrate is coated with titanium nitride is stronger than that in case where the substrate is coated with titanium.That is, the developed compositionally gradient laser-sprayed coatings can provide a good wear resistance and thermal shock resistance surface if they have a relatively thick layer of titanium nitride at the surface and a low titanium nitride content film on the substrate.