Li Chang-Jiu, Luo Xiao-Tao, Dong Xin-Yuan, Zhang Li, Li Cheng-Xin
State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi'an Jiaotong University, Xi'an, 710049 Shaanxi Province People's Republic of China.
J Therm Spray Technol. 2022;31(1-2):5-27. doi: 10.1007/s11666-022-01327-x. Epub 2022 Jan 28.
Although thermal spray metallic coatings have been widely used for materials protection from wear, corrosion and oxidation, its porous feature limits the full utilization of materials potential. Moreover, the oxidation inherent to thermal spraying in the ambient atmosphere is detrimental to interlamellar bonding formation, which further degrades the performance of thermal spray metal coatings. How to tape out the full potential of spray materials in the form of the coating is a still great challenge to thermal spray coating technology. Facing such challenge, recent efforts have been made to deposit dense metallic coatings with sufficiently bonded lamellae by oxide-free molten droplets through atmospheric plasma spraying. In this paper, the strategies for depositing bulk-like metal coatings will be reviewed. The formation of the bulk-like coating through post-spray treatments is briefly reviewed including post-spray heat treatment and laser remelting following the brief introduction to the features of thermal spray metallic coatings. The effect of the substrate preheating temperature on the splat formation and subsequently the adhesion formation was examined to reveal the dominant limitation of resultant oxide scale. Then, the role of the deposition temperature on the formation of bulk-like metal deposits with neglecting the effect of oxidation during spraying by vacuum plasma spraying practices is shortly presented. The recent progress on the new strategies to develop spread-fusing bonding mechanism and in-situ in-flight deoxidizing mechanism through developing ultra-hot metallic droplets will be introduced. The thermodynamics and kinetics requirements for the in-situ in-flight deoxidizing through deoxidizer elements adding to alloy spray powders for achieving oxide-free molten droplets in the ambient atmosphere are examined. The conditions to develop the spread-fusing mechanism during the spreading of impacting molten metal droplet for metallurgical bonding are presented. It is obvious from this review paper that the realization of two mechanisms depends on both the spray materials design and heating control of in-flight particles. Through the generation of ultra-hot droplets by plasma spraying to achieve oxide-free molten droplets, strategically it will be possible to deposit bulk-like dense metallic coating through spread-fusing of splat surfaces with limited post-spray oxidation. Such strategies will tape out the full potential of coating materials and open up the new application fields for plasma spraying.
尽管热喷涂金属涂层已被广泛用于材料的耐磨、耐腐蚀和抗氧化保护，但其多孔特性限制了材料潜力的充分发挥。此外，在大气环境中热喷涂固有的氧化作用不利于层间结合的形成，这进一步降低了热喷涂金属涂层的性能。如何以涂层的形式充分发挥喷涂材料的潜力，仍然是热喷涂涂层技术面临的巨大挑战。面对这一挑战，最近人们通过大气等离子喷涂，利用无氧化物的熔滴来沉积具有充分结合薄片的致密金属涂层。本文将综述制备块状金属涂层的策略。在简要介绍热喷涂金属涂层的特点之后，将简要回顾通过喷涂后处理（包括喷涂后热处理和激光重熔）形成块状涂层的过程。研究了基体预热温度对熔斑形成以及随后附着力形成的影响，以揭示所得氧化皮的主要限制因素。然后，简要介绍了通过真空等离子喷涂实践，在忽略喷涂过程中氧化影响的情况下，沉积温度对块状金属沉积物形成的作用。将介绍通过开发超热金属液滴来发展铺展-熔合结合机制和原位飞行脱氧机制的新策略的最新进展。研究了通过向合金喷涂粉末中添加脱氧剂元素以在大气环境中实现无氧化物熔滴的原位飞行脱氧的热力学和动力学要求。给出了在冲击熔滴铺展过程中形成冶金结合的铺展-熔合机制的条件。从这篇综述文章中可以明显看出，这两种机制的实现都依赖于喷涂材料设计和飞行中颗粒的加热控制。通过等离子喷涂产生超热液滴以实现无氧化物熔滴，从策略上讲，通过熔斑表面的铺展-熔合并结合有限的喷涂后氧化，有可能沉积块状致密金属涂层。这些策略将充分发挥涂层材料的潜力，并为等离子喷涂开辟新的应用领域。
