Recent Developments in the Research of Splat Formation Process in Thermal Spraying

Hindawi Publishing Corporation Journal of Materials Volume 2013, Article ID 260758, 14 pages http://dx.doi.org/10.1155/2013/260758 Review Article Recent Developments in the Research of Splat Formation Process in Thermal Spraying Kun Yang, Min Liu, Kesong Zhou, and Changguang Deng Department of New Materials, Guangdong General Research Institute of Industrial Technology (Guangzhou Research Institute of Non-Ferrous Metals), No. 363 Changxing Road, Tianhe District, Guangzhou 510651, China Correspondence should be addressed to Kun Yang; Received 12 November 2012; Accepted 7 December 2012 Academic Editor: Iwan Kityk Copyright © 2013 Kun Yang et al. is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ermal spraying is a well-established surface modi�cation technology which has been widely used in industrial applications. As the coating properties were mainly determined by the �attening nature of each splat, much attention has been increasingly paid to the study on the splat formation process of thermal sprayed particles. is paper is concerned with the development in the research of the splat formation process of the individual splat deposited by thermal spraying during the past few decades, including the experimental and numerical simulations up to today; some classical splashing models were also reviewed. As a simulation of the actual thermal spray process, the development of the �attening behavior of free falling droplet has been mentioned as well. �n the basis of the current investigation, some recommendations for the future work have been advised. 1. Motivation to Study Splat Formation Process “God made the bulk; the surface was invented by the devil” [1]. is comment indicates that the surfaces of parts are the location for many phenomena between the material and its environment, where all physical and chemical interactions and exchanges take place [2]. As most of the material damage and failure start from the surface of the material, beside the so-called nanotechnology, surface modi�cation technology has been quickly developed. is method is a hopeful way to realize the sustainable human society in future, as it enables both high performance and recyclability of the material, through modifying the surface of a material by bringing physical, chemical, or biological characteristics different from the ones originally found on the surface of a material, which have been attracting a great deal of attention from various industries, as they present a way to get an entirely different material performance from the surface of materials merely through surface engineering techniques [3]. In general, thermal spraying is such a typical process that can provide thick coating ranging from 20 𝜇𝜇m to several millimeters on the substrate. is method has been developed since the early 1910 when Dr. Schoop introduced the technique with a �ame as the heat source and molten lead for feedstock materials [4], while both electrical (plasma or arc) or chemical means (combustion �ame) were usually used as the source of energy for thermal spraying nowadays. Using this technology, thick coatings can be deposited on the substrate over a large area at a high deposition rate as compared to other coating processes in a very short time; in addition, almost all kinds of materials feed in powder or wire form can be used as the feedstock, including metals, alloys, ceramics, plastics, cermets, and composites. Accordingly, it has been increasingly applied in various �elds. Including mechanics, aeronautics, aerospace, chemistry and oil, electronic, military, automotive, medical, marine, and mining, and their development has continuously increased over the last decade [5–11], for example, typically used as thermal barrier coating, TBCs, in power plants; this method has also been widely used to protect the substrate or remanufacturing of the surface damaged products. However, the controllability or reliability of the process has not been established yet until today. Hence, the question of how to improve the thermal-spray technology is quite worthwhile for paying more attention to it. e coatings fabricated by thermal spraying are always built up on the roughen substrate by the impingement of the molten or semimolten particles, which �atten a�er impact 2 and solidify quickly, forming a layer. At the beginning of the coating buildup, particles impact directly onto the substrate. e phenomena occurring at this stage determine the adhesion of the coating to the substrate. Following this, the next layer is deposited on the top of the �rst one, until a coating of the desired thickness is obtained [12]. As the individual splat is unit cell for the entire coating buildup, the �attening and solidi�cation of the individual particle on the substrate surface is the fundamental process for the coating fabrication. Coating microstructure and corresponding properties, such as porosity and adhesion strength, depend strongly on the �attening nature of each splat [13]. erefore, it is necessary to make clear of the splat formation mechanism of the thermal sprayed particles to establish the controlling way for the coating fabrication. Consequently, splat characteristics can be recognized by measuring the in-�ght condition and investigating on the �nal splat shape. e desired splat formation can be obtained by changing the spraying parameters and splat-substrate materials combinations and contact nature. By selecting the optimum conditions, thermal spray process can be controlled effectively. In general, the deposition process of the sprayed particles can be divided into three steps as illustrated in Figure 1. Firstly, the feedstock material was accelerated and heated by the �ame prior to impact onto the substrate, then the molten or half-molten particle with high velocity impacts onto the substrate; following this, the molten particle spreading on the substrate surface was driven by the dynamic impact pressure and inertial of the particle; �nally, it solidi�ed on the substrate surface with various shapes. Although the feedstock powder size used in the spraying is always ranging from several tens to several hundreds micrometers, which might not be indenti�ed with the human eyes, the splat �attening and solidi�cation process should be determined by the joint effects of many factor which will be introduced in Section 2. On the other hand, due to the complex shape of the semimolten particles [14] on rough substrate [15, 16], most of the common studies are focused on a smoothed �at surface. According to controlling the spraying parameters and splat-substrate natures, many types of splats can be obtained. e investigation is always focused on two typical types of splats as shown in Figure 2. Splash splat with a center splat surrounded by a ring of fragments or numerous sp (...truncated)