等离子喷涂熔滴快速撞击基体非稳态凝固行为研究进展

涂层成形过程中的缺陷含量、残余应力、沉积效率、组织结构及力学性能等指标均会随着工艺参数与基体预处理状态的不同而发生显著变化,因而需要从更加微观的角度深入理解等离子喷涂涂层的微观构筑过程,即单个熔滴的铺展凝固现象。本研究分别从熔滴凝固的类型与机理、不同因素对熔滴凝固过程的影响及凝固斑点形态的定量表征方法 3个方面详细综述了等离子喷涂熔滴撞击基体快速凝固过程的研究现状。结果表明,熔滴的铺展形态主要可以分为5类,包括圆盘型、破碎型、放射型、花瓣型及气泡型,影响铺展过程的因素主要包括熔滴特性(速度、温度、粒径、材料属性、熔化状态等)与基体状态(表面粗糙形貌、表面化学状态、吸附物及冷凝物、界面润湿性及接触热阻等)2大类,综合采用一系列参数对熔滴铺展几何形态进行表征,可实现熔滴沉积质量的定量评价。

Status Quo of Unstable Solidification Process after of Plasma Sprayed Droplets Impinging to Substrate

As is known, phases, defects, residual stress, deposition efficiency and mechanical properties of plasma sprayed coatings depend dramatically on the fabrication parameters and substrate pretreatment state. In other words, slight changes might lead to distinguished differences between coatings. Therefore, it is of great importance to perform researches on the formation process of plasma sprayed coatings microscopically. Basically, plasma sprayed coatings are constituted of enormous molten or semi-molten droplet, which are heated and accelerated by plasma torch. Hence, the flattening and solidification behavior of single droplet after impacting on the substrate are the most elementary process of the formation of coatings. The factors influencing the flattening and solidification behavior can be divided into two categories, namely, droplet properties: velocity, temperature, size, material properties, molten status, etc.; substrate status: roughness, preheated temperature, surface chemistry status, deposition of adsorbates and condensates, wetting properties and interfacial thermal contact resistance between droplet and substrate, etc.. With different droplet properties and substrate conditions, the droplets might spread into five different types, i.e., disk-like, fragmented, finger-like, flower-like and bubble-like. Each type represents specific deposition mechanism and quality. Therefore, series of parameters can be used to quantificationally characterize the morphologies of splats, such as perimeter, area, volume, depth, circularity, eccentricity, solidity, flattening degree and so on.