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.
Highlights
The thermal spray process has evolved with time, and based on the thermal energy source, it can be divided into plasma spray, flame/combustion spray and electric arc
This review focuses on plasma spray and highvelocity oxy fuel (HVOF) spray
Thermal energy in plasma spray is higher than the high-velocity oxy-fuel (HVOF) spray, which helps in the sufficient melting of feedstock powder
Summary
The thermal spray process has evolved with time, and based on the thermal energy source, it can be divided into plasma spray, flame/combustion spray and electric arc. The use of liquid medium allows to spray particles even with a smaller size (\10 lm) and resolve the nozzle clogging issue by exhibiting better flowability than the dry powder (Ref 3). In solution precursor thermal spray, when the solution precursor feedstock droplets are introduced into the system, the evaporation of the solvent, followed by precipitation of precursors due to the high enthalpy of plasma/flame takes place After melting, these precipitates travel toward the substrate. The coating morphology and properties depend on several parameters involved in spraying, such as preparation and insertion of the suspension or solution precursor, suspension medium or solvent and its viscosity, the interaction of liquid feedstock with the energy source, etc.
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