喷涂粒子在等离子体射流中的加热历程及熔化状态研究

The Study of Heat Process and Molten State of Sprayed Particles in Plasma Jet

本文通过对水中收集的等离子喷涂熔融粒子进行粒度分析及形貌观察,研究了喷涂粒子在普通大气等离子喷涂(APS)及超音速大气等离子喷涂(SAPS)射流中的加热历程及熔化状态。实验观察到氧化钇部分稳定的纳米颗粒团聚造粒的氧化锆(Y-PSZ)球形粉末颗粒在APS及SAPS射流中传热传质过程中的熔融与"细化"现象有明显不同:在SAPS射流中,尺寸较大(粒径在几十微米)的颗粒大部分形成了十分细小的(小于5μm)颗粒;相对而言,APS中的熔融粒子仍以十几微米以上较大粒径的半熔融粒子为主。进一步研究表明,大尺寸粒子在等离子射流中先从表层熔化,逐渐向颗粒内部扩展,形成柱晶熔区,属典型的梯度熔化模式,最终芯部存留大量细小未熔一次粒子,即所谓半熔融状态;相应的SAPS中大量粒径小于5μm的小颗粒在超音速等离子体射流中整体迅速熔化(即所谓等温熔化模式)。对熔融或半熔融粒子撞击基体后的扁平化过程进行分析表明:在APS射流中扁平粒子的直径一般在100μm以上,且周围飞溅较多,导致典型的层状多孔的粗大组织结构,层间出现较多平行横向裂纹;而SAPS大量细小熔滴撞击基体后,冷却更快,多呈无明显飞溅的直径约10～30μm圆盘状,形成大量细密柱晶交错堆垛而成的、无明显分层的特殊结构。对收集粒子进行X-射线衍射相分析表明,APS及SAPS两种工艺下的熔融粒子均未发生明显相变,相组成主要为室温下非转变四方氧化锆(t’-ZrO2),说明喷涂粒子在高温等离子射流的加热历程中,其晶体结构和化学成分未发生明显变化。

In this work, the particles were collected in the water and the heat process and molten state of particles in plasma jet were studied. The results revealed that the YSZ particles were refined in the APS and SAPS plasma jet, however, this phenomenon was more significant in the latter, leading to the formation of numerous fine particles with size less than 5 μ m. In addition, three molten states were found for the particles in the SAPS plasma jet, they are 1 ） un-melted state; 2） partially melted state, which corresponded to the gradient melting mode; 3 ） fully molten state for the fine particles, which corresponded to the isothermal melting mode. For the APS process, the diameter of splating particles after the in-flight particle impinged onto the substrate was more than 100 IJ m and the splash formed around the splat, however, the splash was not found around the small splat （about diameter of 10-30 ~ m）formed by the fine molten particle in the SAPS plasma jet, as a result, the microstructure of SAPS-coating was much finer than that of APS-coating. Meanwhile, the original and collected particles were composed of t＇-ZrOz, which suggested the compositions （especially the Y203） of powders in the APS/SAPS plasma jets were not changed.