Two ways to add a thick layer
Vapor-deposited films are usually measured in micrometers. Many wear and corrosion problems call for much more material, often a few tenths of a millimeter up to several millimeters. Two families of processes handle that: thermal spray, which throws molten or softened particles at a surface, and cladding, which fuses a layer of different material onto the part. Both are often thought of as alternatives or complements to vapor routes such as thin film deposition by vapor.
How thermal spray works
Thermal spray feeds powder, wire or rod into a hot gas stream or electric arc. The feedstock melts, or softens, and the stream propels the droplets toward the part. On impact, each droplet flattens into a thin disk called a splat and freezes quickly. Thousands of overlapping splats build up a layered coating. Because the layers are made of discrete splats, the structure is lamellar, with small pores and oxide films between layers.
Several variants exist, differing in how the heat and speed are generated.
- Flame spray. A fuel gas flame melts the feedstock. It is simple and low cost, with comparatively low particle speed and more porous coatings.
- Electric arc wire spray. An arc between two wires melts them and compressed air blows the droplets onto the surface. It is productive and commonly used for large areas and corrosion protection.
- Plasma spray. A plasma jet reaches very high temperatures and can melt ceramics with high melting points. It is the usual route for ceramic layers such as thermal barrier coatings.
- High-velocity oxy-fuel (HVOF). Combustion gases push particles at very high speed, which tends to give dense coatings with good bond strength. It is frequently used for carbide-in-metal coatings that resist abrasive wear.
- Cold spray. Particles are accelerated well below their melting point and bond by impact. It limits oxidation and thermal effects on the part.
How cladding works
Cladding, also called overlay or hardfacing, deposits a layer of one material on another by melting. A welding arc, a laser or another heat source melts the added material and a thin region of the part, and the two fuse as they solidify. Laser cladding uses a focused beam with powder or wire and allows precise control of heat input and layer shape. Because the interface is metallurgically bonded, the layer is typically dense and strongly attached. Cladding is common for rebuilding worn parts and for applying hard or corrosion-resistant alloys.
The trade-off is heat. Melting the surface mixes some substrate into the clad layer, which is called dilution, and heats a region of the part below it. That heat-affected zone can distort the part or change its properties. Good control keeps dilution and distortion small.
Comparing with vapor-deposited films
| Aspect | Thermal spray | Cladding | Vapor deposition |
|---|---|---|---|
| Typical thickness | Tenths of a millimeter and up | Often around a millimeter or more | Mostly micrometers |
| Bonding | Mainly mechanical, some chemical | Metallurgical fusion | Chemical and mechanical, depends on preparation |
| Density | Some porosity, varies by method | Typically dense | Typically dense |
| Heat on part | Low to moderate | Higher, local melting | Varies from moderate to high |
| Surface finish | Rough, usually finished by grinding | Rough, usually machined | Smooth, follows the part |
| Coverage of shapes | Line of sight spray | Accessible surfaces | Depends on process |
These are general tendencies, not guarantees, and each process has exceptions.
Where each makes sense
Spray and cladding are often chosen for:
- Large or heavy parts, such as rolls, shafts, pump components and valve parts, where vacuum chambers are impractical.
- Repair and rebuilding, restoring dimensions on worn or machined-too-far parts.
- Heavy wear or corrosion duty, where a thick layer is needed to last through long service. Wear-resistant carbide coatings built on a binder phase are one such example, and tungsten carbide in a cobalt binder is the best-known case of cemented carbide.
- High-temperature protection, as in sprayed ceramic layers on hot engine parts.
Vapor-deposited films are more suitable for:
- Precision tools and components, where sharp edges and tight tolerances must be kept.
- Thin, smooth, hard surfaces, such as on cutting tools, molds and sliding parts.
- Special functions, like optical or electronic layers and crystalline diamond films.
Preparation and finishing
All coatings need clean, prepared surfaces. For thermal spray, grit blasting is the standard preparation, since roughness helps the splats lock on. Skipping it risks poor coating adhesion. After spraying, the surface is rough, and parts often need grinding or lapping to final dimensions. Hard sprayed coatings are commonly finished with diamond grinding wheels, because conventional abrasives struggle with very hard carbides.
Practical limits
Sprayed coatings are not perfectly dense and can allow corrosive fluids to reach the substrate through connected pores, so sealing steps are sometimes added. Residual stress in thick layers can cause cracking or delamination, particularly with brittle ceramics. Cladding can distort parts. Neither process matches vapor deposition for thin, smooth, precisely controlled layers. Choosing between them usually comes down to how much material is needed, how much heat the part can accept, how accurate the final dimensions must be, and what the coating must withstand in service.