Thermal spraying is a process by which metals, alloys, or ceramic coatings are applied to a surface in their molten or semi-molten state. Hot thermal spray particles are atomized, accelerated toward the work by a stream of jet air.
As particles strike the working surface they cool and build up, particle by particle, fusing into a cast-like structure described as highly cohesive, lamella platelets. Most thermal spray surfaces are finished before service, though sometimes rough, as-sprayed surfaces may be acceptable or even beneficial. Values typically range from 100 to 400 micro inch. And, depending on material, can be improved to less than 10 micro inch. Densities vary with working temperature and particle impingement velocity.
What types of materials can be applied? Most metals, ceramic spray oxides, cermets, tungsten carbide or other binary, and organic plastics.
Adhesion is chiefly mechanical. Consequently, surface preparation by roughening, such as grit blast, is necessary for best adhesion. Though tensile strength can be sometimes significantly improved with the higher temperature processes, depending on material, through micro-welding and diffusion.
Applications continue to grow with this technology. Food processing, packaging, molding, plastics, paper and chemical processing, are just some of the relevant applications. (Many materials are regarded non-objectionable with FDA.)
So, what was formerly developed for rebuilding worn parts or those with machining errors is now extending into many fields of surface engineering. Characteristics such as wear resistance such as fretting, abrasion, or erosion, and corrosion resistance or non stick properties are now seen.
Common methods of thermal spray application include HVOF (high velocity oxygen flame), which is similar to the combustion powder thermal spray process (LVOF), though with increased density, stronger bonds and lower residual tensile stress. Plasma spray and vacuum arc spray are also popular.
Ideally, look for processes for low heat transfer to minimize risk in surface warping or distortion, and greater kinetic energy systems to ensure best particle to particle cohesive bonding.
Though hard facing relates to thermal spray, it’s really more about depositing ‘filler’ materials onto a metal surface to obtain alternative dimensional or surface characteristics. Normally, the improvements made relate to wear resistance, where bond strengths are metallurgical instead of mechanical. But these "overlays" of material may also contribute to corrosion resistance or low friction.
All of these thermal spray processes can create long-lasting, cost-effective solutions to surface performance. Which is why you need to also consider thickness, surface finish, and whether your critical surfaces are within ‘line-of-sight’.
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