Suspension and Solution Precursor Plasma and HVOF Spray: A Review

Abstract Thermal spray, being a cost- and time-efficient process, is used extensively in industrial and engineering sections for mass production of desired coating structures, allowing to deposit a wide range of materials on various substrates. Conventionally, powder feedstocks are used in plasma and high-velocity oxy-fuel (HVOF) thermal spray that has limitations such as limited feedstock particle size (10-100 µm), clogging and limited options for coating materials. Liquid feedstocks, in the form of suspensions or precursor solutions could potentially resolve these issues by allowing nano- and submicron particles to be deposited, where unlike dry feedstock, the liquid medium helps in reducing the friction and avoiding the clogging. Also, liquid feedstocks, especially precursor solutions, provide the opportunity to deposit a wide range of coating materials with better control over coating microstructure, material composition and stoichiometry by varying the properties of the feedstock. Despite benefits, liquid feedstock has its own complexities, such as complex feedstock preparation, thermo-physical reactions during interaction with the energy source and gases. Therefore, it becomes essential to understand how different suspension and solution precursor feedstock properties affect the coating microstructures and properties. This review paper covers a detailed discussion on the role of different process parameters such as feedstock properties, injection methods, different torches and surface properties, affecting the coating quality and performance and related recent developments and challenges are discussed. This would be beneficial in optimizing the spray parameters to obtain coatings with desired microstructures. The later part of the review focuses on the economic aspect of the suspension/solution precursor-based plasma and HVOF spray methods and their various applications.