氛围气压强对热喷涂颗粒沉积行为及涂层性能影响机理研究

Most of the material damage and failure start from the surface of the material. With the purpose to protect the substrate, thermal spraying is employed to deposite other materials on its surface. This thchnology has been widely applied in industry in recent years. During thermal spraying process, the deposition behavior of individual particle on substrate surface is the fundamental process for the coating fabrication. That is to say, coating microstructure and properties depend strongly on the flattening nature of each splat. Therefore, it is necessary to make clear of the splat formation mechanism of the thermal sprayed particles to establish the controlling way for coating fabrication. Consequently,the interactions between plasma jet and spraying powder, along with the flattening and solidification behavior on the substrate surface, can be traced through measuring the in-fight condition and observing the microstructure of the final depositions. Low-pressure plasma-spraying (LPPS) is one of the typical thermal spraying processes in which the ambient pressure in deposition chamber can be controlled. It has been attracting wide attention because coating particles can be accelerated to much higher speeds without oxidizing the metal, and reaction of the coating materials with plasma operating gases during spraying can be controlled. This technology can develop higher-performance dense coating. Although this technology has been widely used, only few attentions have been paid to the effect of ambient pressure on splat formation and coating fabrication process. Therefore, a detail study of this aspect is needed. With the purpose to control the thermal spray process more effectively and predict the coating properties by optimizing the patricle deposition behavior, this study is devoted to investigating the effect of ambient pressure on the splat formation process of thermal sprayed particles, and try to clarify the role of individual splat formation on the coating performance.

材料的损坏和失效大多是从表面开始的，利用热喷涂技术在材料表面涂覆一层其它材料可实现对基体材料的保护，这一技术近年来已在工业领域得到广泛应用。热喷涂过程中，单个熔融颗粒在基材表面的沉积是涂层的基本构成要素，对涂层的微观组织及性能有着重要影响。因此，通过对熔融颗粒飞行状态的监测及沉积物形态的观察分析可追溯熔滴与热喷涂焰流的交互作用及在基材表面铺展及凝固，从而优化热喷涂过程。低压等离子喷涂技术可降低熔融颗粒飞行过程中氧化程度，加快颗粒飞行速度，利用该技术可得到性能更加优良的致密涂层。然而，关于喷涂腔内氛围气处于低压条件时颗粒沉积行为以及对涂层性能的影响机理尚无系统的研究。为实现通过对单个颗粒沉积行为的控制来优化控制热喷涂过程及预测涂层性能的目的，本研究着眼于研究氛围气压强对单个热喷涂颗粒沉积行为的影响机理，并尝试阐明单个颗粒沉积行为对涂层性能的影响。

热喷涂过程中，单个粉末颗粒在基体或已沉积涂层表面的碰撞、铺展及凝固是形成涂层的基本构成要素。本研究着眼于研究氛围气压强对单个热喷涂颗粒在基体表面沉积行为的影响机理，并尝试阐明单个颗粒沉积行为对涂层性能的影响。. 采用Fluent软件对不同氛围气压强环境下等离子射流的温度速度分布及常压下粉末颗粒在等离子射流中的加热加速特性进行了模拟；在不同氛围气压强环境下采用等离子喷涂向抛光基体上喷涂Ni及NiCoCrAlYTa粉末，在基体上收集一定数量的颗粒沉积物及涂层；另外，在不同预热温度基体上也进行了颗粒沉积物收集和涂层制备。利用光学显微镜、扫描电镜与聚焦离子束技术等对单个颗粒沉积物的形态进行观察分析，对涂层的微观组织、氧含量、结合强度、抗高温氧化性能等进行了系统的表征。. 研究结果表明，随着氛围气压强的降低，在基体表面吸附质吸附/脱附、熔滴与基体间的润湿性能、热传导以及熔滴底部的初期凝固层等综合作用下，热喷涂颗粒沉积物形貌逐渐由呈典型的溅射状向盘状转变。与之对应，低氛围气压强下制备的涂层组织致密，涂层的结合强度、氧含量、截面维氏硬度、抗高温氧化等性能较常压下所制备涂层均有明显改善，涂层性能随氛围气压强变化的趋势与单个熔滴沉积物形貌转变趋势吻合良好。另外，基体预热温度与氛围气压强在喷涂粉末颗粒沉积形貌转变方面有着相似的效果，但预热过程中基体表面的脱附更完全且表面发生纳米尺度上的变化，对于润湿和热传导的效果更为明显，颗粒沉积物由溅射向盘状的转化更为明显。. 基于氛围气压强/基体预热温度对单个热喷涂粉末颗粒沉积行为及涂层性能的影响研究表明，单个热喷涂颗粒作为涂层组成的基本要素，对涂层性能有着重要的影响。通过对热喷涂单个熔滴沉积行为的调控，可为实现有效热喷涂过程优化及涂层性能预测奠定理论基础。