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

采用激光喷涂方法制备了由钛和氮化钛组成的功能梯度涂层。喷枪具有用于作为喷涂材料的线材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束熔化钛丝。氩气流产生熔融钛颗粒的细喷雾，其涂覆在铝和低碳钢基底上。通过改变气体中的氩-氮比，形成氮化钛含量与钛的比例不同的后续层。最后一层通过仅使用氮气涂覆有氮化钛。每层的化学成分通过涂层（约1 mm厚）分级;每层的硬度和热膨胀系数通过涂层分级。反向梯度涂层也是通过反向工艺制备的。制备的梯度功能涂层具有以下特点：（1）耐反复热震性能优异。（2）硬质表面涂层提供良好的耐磨性。（3）耐磨性不受涂层的顺序和分度的影响，而取决于摩擦表面的硬度。表面硬度越高，耐磨性越高。（4）在摩擦条件下涂膜与基材之间的粘合强度随着涂层厚度的增加而增加。在基体上镀氮化钛的涂层的结合强度比镀钛的涂层的结合强度高，即如果涂层表面有较厚的氮化钛层，而基体上有较低含量的氮化钛膜，那么所研制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表面。

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.