Abstract:Thermal spraying has been developed to a well-established coating technology to provide materials surfaces with protective and functional coatings with different properties and performances which are required by service conditions of engineering parts where the substrate do not pocess. As one of the most important surface engineering technologies, the most distinct feature of thermal spraying is that the coating application developments are primarily based on microstructure defect control technologies, and the coatings can be sprayed with different levels of porosity. Moreover, the processing ability to deposit coatings with different microstructure of porosity level from more than 50% to near zero of dense one which looks like wrought bulk makes thermal spray coatings able to fulfill different service requirements from abradable, thermal barrier, wear-resistant to corrosion-resistant. Based on over 100 year research and development, several spray technologies such as plasma spraying, arc spraying, high velocity oxy-fuel spraying, conventional flame spraying have been established for wide applications in different industrial fields. Typical fields include aeronautics and aerospace, transportation, petroleum and chemicals, power and energy, metallurgy and steel manufacturing, textile and paper industries, machinery manufacturing etc. Thermal spraying has become indispensable to above industrial fields to prolong service lifetime of engineering parts and improve their properties as well as cost performance. The most important applications of thermal spraying are to provide engineering parts with excellent anti- wear, anti-corrosion, high temperature protections in the form of materials protective coatings. Among wide applications, thermal spraying is most popularly employed to deposit protective coatings against wear. HVOF WC-Co coatings can provide materials with excellent abrasive wear resistance. However, excellent wear-resistant potential of coating materials themselves is not utilized yet under wear conditions of dynamic loading such as erosion, cavitation and fatigue wear, and high stress wear conditions. It should be understood that as-sprayed coating cannot be directly used as materials protection against corrosion, since pores are inevitably present in coating. Thus, sealing treatment using proper organic or inorganic sealants must be applied to thermal spray coatings which are used in a corrosive environment. It has been reported that high temperature protections by thermal spray coatings for aero gas turbine and industrial gas turbine account about 60% of thermal spray market share in Northern American and Europe. Therefore, market potential in such field is expected to expand thermal spray market in China gradually with increasing application of gas turbine. The challenges facing further improvement of wear resistance of thermal spraying coatings include development of high performance anti-wear new coating materials and self-lubricating materials in a necessary temperature range, and also processing technology to create the coating with sufficiently bonded lamellae based on spray powder design. Moreover, it is also effective to use the coating in the conditions without significant spalling of lamellae by establishing the relationship between coating microstructure and service conditions. The challenge for anti-corrosion is how to deposit a dense coating that aggressive gas or liquid phase does not penetrate. Moreover, emerging new thermal spray technologies including cold spraying, plasma spraying physical vapor deposition, and liquid feedstocks thermal spraying have been developed in last two decades and are expected to explore new applications such as cold spraying in additive manufacturing and remanufacturing along with corresponding powders manufacturing technology. The great demand for high performance functional coatings such as electrical conduction, catalyst, bioactivity, insulation, anti-plasma eching in new energy, medical, semiconductors etc will lead to further development of new applications fields of thermal spraying. In this review paper, the applications and research progresses of thermal spraying will be summarized. The typical features of thermal spray coatings dominating coating application performances will be discussed. Moreover, typical challenges for future development of thermal spray technology from powders and processing technologies to fundamental researches will be put forwarded. It would be expected to provide engineers involving in thermal spray coatings with a reasonable understanding to coating features and current problems and future challenges.