Thermal Spray: Current Status and Future Trends

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Current Status and Future Trends Herbert Herman, Sanjay Sampath, and Robert McCune

Introduction Thermal spray is a continuous, directed, melt-spray process in which particles (e.g., 1–50 m in diameter) of virtually any material are melted and accelerated to high velocities, through either a combustion flame or a dc or rf nontransferred thermal-plasma arc. The molten or semimolten droplets impinge on a substrate and rapidly solidify to form a thin “splat.” The deposit is built up by successive impingement and interbonding among the splats. The splats accumulate into a wellbonded deposit, generally 10 m thick. In the case of ceramic deposits, it is necessary to melt the particles using either a combustion flame or a thermal-plasma arc. The deposit microstructure, and thus its properties, aside from being dependent on the spray material, rely on the processing parameters, which are numerous and complex. Nonthermal, high-velocity methods have entered the lexicon of spray coatings: for example, “cold spray,” in which nominally ductile metals are supersonically propelled and impacted onto a substrate, resulting in deposit buildup. A schematic overview of the thermalspray process is shown in Figure 1. Numerous publications on this subject are available, some of which are listed in References 1–7. The spray materials range from low- to high-melting-point metals and alloys, ceramics, and polymers. The torch can be based on a combustion process or on an electric arc, the choice of which will generally be determined by the type of spray material used and the specific application. The features of each type of torch and its range of applications are described in the next section. The chemistry of the coatings is determined by the composition of the feedstock. Generally, minimal chemical interactions

MRS BULLETIN/JULY 2000

occur at the interfacial layer during the deposition process. The integrity and quality of a coating deposition are measured by three fundamental factors: oxide content, porosity, and bond strength. Different variables of these measures apply to the different types of thermal-spray processes. One measure may displace the criticality of another, depending on the type of process used and the desired end result.

Types of Thermal-Spray Processes Combustion flame spraying uses compressed air or oxygen mixed with a fuel (e.g., acetylene, propylene, propane, or hydrogen). Generally (with the exception to be noted shortly), the process yields low-performance coatings, and it is not employed when high-density, wellbonded coatings are required. The reasons for these deficiencies have to do with the low flame velocity of about 50 m/s and the low temperature achieved within the combustion flame. In recent years, high-velocity oxy-fuel (HVOF) spraying, a novel variation of combustion spraying, has had a dramatic influence on the field of thermal spray. This technique is based on special torch designs in which a compressed flame undergoes free expansion upon exiting the torch nozzle, thereby experiencing high gas acc

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