Thermal spraying
The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame).
Coating materials available for thermal spraying include metals, alloys, ceramics, plastics and composites.
Coating quality is usually assessed by measuring its porosity, oxide content, macro and micro-hardness, bond strength and surface roughness.
A spark is used to ignite the gas mixture, and the resulting detonation heats and accelerates the powder to supersonic velocity through the barrel.
The high kinetic energy of the hot powder particles on impact with the substrate results in a buildup of a very dense and strong coating.
The coating adheres through a mechanical bond resulting from the deformation of the base substrate wrapping around the sprayed particles after the high speed impact.
There are a large number of technological parameters that influence the interaction of the particles with the plasma jet and the substrate and therefore the deposit properties.
These parameters include feedstock type, plasma gas composition and flow rate, energy input, torch offset distance, substrate cooling, etc.
This technique with variation may also be used to create porous structures, suitable for bone ingrowth, as a coating for medical implants.
Plasma jet generation: Plasma-forming medium: Spraying environment: Another variation consists of having a liquid feedstock instead of a solid powder for melting, this technique is known as Solution precursor plasma spray Vacuum plasma spraying (VPS) is a technology for etching and surface modification to create porous layers with high reproducibility and for cleaning and surface engineering of plastics, rubbers and natural fibers as well as for replacing CFCs for cleaning metal components.
Techniques like X-ray photoelectron spectroscopy and scanning electron microscopy are used for surface analysis to identify the processes required and to judge their effects.
As a simple indication of surface energy, and hence adhesion or wettability, often a water droplet contact angle test is used.
PTWA can be used to apply a coating to the wear surface of engine or transmission components to replace a bushing or bearing.
The resultant hot gas at a pressure close to 1 MPa emanates through a converging–diverging nozzle and travels through a straight section.
Since the maximum flame temperature is relatively close to the melting point of most spray materials, HVAF results in a more uniform, ductile coating.
The method was originally developed in the Soviet Union – while experimenting with the erosion of the target substrate, which was exposed to a two-phase high-velocity flow of fine powder in a wind tunnel, scientists observed accidental rapid formation of coatings.
[1] In cold spraying, particles are accelerated to very high speeds by the carrier gas forced through a converging–diverging de Laval type nozzle.
Upon impact, solid particles with sufficient kinetic energy deform plastically and bond mechanically to the substrate to form a coating.
The critical velocity needed to form bonding depends on the material's properties, powder size and temperature.
[7] Soft metals such as Cu and Al are best suited for cold spraying, but coating of other materials (W, Ta, Ti, MCrAlY, WC–Co, etc.)
It is possible to accelerate powder particles to much higher velocity using a processing gas having high speed of sound (helium instead of nitrogen).
On the other hand, lower temperatures of warm spraying reduce melting and chemical reactions of the feed powder, as compared to HVOF.
These advantages are especially important for such coating materials as Ti, plastics, and metallic glasses, which rapidly oxidize or deteriorate at high temperatures.
Thermal spray application is not compatible with the substrate if the area to which it is applied is complex or blocked by other bodies.
Ideally, equipment should be operated automatically in enclosures specially designed to extract fumes, reduce noise levels, and prevent direct viewing of the spraying head.
There are occasions when the type of components being treated, or their low production levels, require manual equipment operation.
Under these conditions, a number of hazards peculiar to thermal spraying are experienced in addition to those commonly encountered in production or processing industries.
[10] The atomization of molten materials produces a large amount of dust and fumes made up of very fine particles (ca.
[13] Proper extraction facilities are vital not only for personal safety, but to minimize entrapment of re-frozen particles in the sprayed coatings.
[13] Certain materials offer specific known hazards:[10] Combustion spraying guns use oxygen and fuel gases.
