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Spray Tips: Thermal spray coatings advantages and disadvantages

February 01, 2018
Source: ASM

Buildup of a thermal spray coating is a chaotic process, because molten particles spread out and deform (splat) as they strike the substrate, at first keying onto asperities, then interlocking with one another. Voids can occur if the growing deposit traps air. Particles overheated in the spray jet can become oxidized. Unmelted particles may simply be embedded in the accumulating deposit.

Many of the defects identified in the diagram are controllable through appropriate equipment and feedstock selection, but by far the greatest influence on coating structure is the actual processing step. Properties of a sprayed coating stemming from the lamellar shape of the splats and residual porosity can be overcome by postdeposition treatments. Porosity can sometimes be a benefit, as in the cases for oil retention on bearing surfaces, for chemically active structures such as batteries, for bone attachment in orthopedic implants, and so on. Rapid particle cooling rates and the lamellar splat shapes are two features that distinguish thermal spray coatings from other coatings. Brittleness, hardness, anisotropic properties, and residual stresses are the result of very rapidly cooled and flattened particles. Properly applied thermal spray coatings have many uses and offer several advantages.

Low Processing Costs. Rapid deposition rates on the order of 1 to 45 kg/h (2 to 100 lb/h) or more can be achieved. Typically, 2 to 7 kg/h (5 to 15 lb/h) is normal practice. Rapid spray rates and high deposit efficiencies result in relatively low processing costs.

A wide range of materials can be deposited as coatings, including metals, metal alloys, oxide and nonoxide ceramics, plastics, cermets, and composite structures comprised of metals, ceramics, and plastics, can be applied using thermal spray. Competitive coating processes do not afford this versatility.

This information is from ASM Handbooks Online, Volume 5A:  Thermal Spray Technology

Subject Classifications

Materials Processing and Treatment | Additive Manufacturing

Materials Processing and Treatment | Thermal Spray Technology

Metals and Alloys

Nonmetallic Engineering Materials | Ceramics

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