Tungsten Carbide Coating

Tungsten carbide (WC) hardened coatings are materials of extraordinary interest due to their outstanding properties & great technological applications. The tribological applications require high hardness & high strength, a low friction coefficient, & chemical also thermal stability. WC-Co and WC coatings offer a great opportunity to develop such properties. This kind of coating has relatively high hardness (about 2200HV) and high melting temperature (about 2800C). The WC demonstrated an extremely elevated modulus of elasticity, well above 700 GNm−2, a value overextended by only diamond, & has a high thermal conductivity of 1,2 J (cm.s.K)−1.
It is difficult to get a purified WC phase, as W-C phase diagram shows a narrow range of homogeneity for WC. The further procedures are engaged for tungsten carbide deposition; PVD, CVD, hybrid approach as well as tungsten carbide thermal spray coating methods. The same investigations suggested the obsession of WC coatings on the PVD deposition parameters, for instance, gas arrangement or sputtering time. These outcomes prove that the deposition parameters can forcefully exploit the microstructure, composition, & properties of coatings. The influence of gas parameter flow on tungsten carbide HVOF thermal spray coating was also investigated in the work. It was found that the nanostructured WC-Co coatings show better mechanical and wear properties in comparison to coatings with conventional microstructures. The adherence functioned by TEM revealed that the nanoparticles of WC were implanted in the amorphic matrix. It proposes that the coatings could be a mixture of nanomaterials and amorphic materials. The detailed investigations of microstructure through the WC cross-section coatings were presented in the work.

Tungsten Carbide Coating Process

Thermal spray technologies find a lot of use in all industrial sectors due to the wide range of materials that can be sprayed by means of these technologies and the large range of thickness that can be obtained. The principle of these technologies is melting the coating materials by means of heating in gaseous environment and its projecting with high velocity on the surface that shall be coated. The tungsten carbide spray coating materials are generally in the form of tungsten carbide powder or (less frequently) wire. Thermal spray technologies can be classified according to the energy source utilized to melt the coating material: flame spray and HVOF (High Velocity Oxygen Fuel) technologies use the energy deriving from a combustion, electric arc spray technology uses an electric arc to melt the coating materials while plasma spray technology uses a plasma generated by an electric arc.

The best results in order to get dense and hard WC based coatings have been achieved by HVOF tungsten carbide coating technologies.
These systems utilize the energy obtained by combustion. In the commercial HVOF gun the powder is injected in the flame by a suitable carrier gas (usually nitrogen), it is molten and projected to the substrate surface. High velocity results in very dense coatings due to the ideal condition of splatting on to the surface being coated.

It is possible to identify different systems that can be classified according to:

• the powder feeding that can be radial or axial
• the fuel/oxygen feeding: fuel and comburent can be feeded premixed or separated.

HVOF tungsten carbide coating begins its development in the years 50s when Detonation Gun (HVOF D-gun) has been introduced on the market: the fuel mixture explosion was achieved by spark (4-8 cycles/s), the combustion chamber pressure was high. Further development brought the detonation to become continuous. In these way new system are developed with axial tungsten carbide powder feeding. A third HVOF generation brings systems with radial injection, with liquid fuel and with improved chamber/nozzle design to get always higher speed. Due to the high flame speed it is possible to obtain very dense and very hard coatings: namely with carbides HVOF technology obtained its best results.

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