Merit, Drawbacks and Applications of HVOF Spraying
The most prominent advantage of HVOF spraying is the ability to spray high-quality tungsten carbide coatings with properties of density, high bonding strength and low oxide content. It is used in aviation & space, pumps & compressor shafts, valves, sealing surfaces, pulp rollers and petrochemical industries where wear and corrosion resistance is required. Compared to plasma spray coating, HVOF has higher bonding strength, lower oxide content, better abrasion resistance and higher deposition rate. The coating can be thicker due to the lower internal stress of the coating. Moreover, HVOF spray only has half of control parameters than plasma spray, and it thus causes a simpler process and better repeatability of coating quality.
These merits expand thermal spray applications. For example, the use of HVOF coating instead of hard chrome plating has been a great success. This eliminates the extremely difficult problems of dispose of plating waste. Also it solves the thick coating (above 0.5 mm) plating difficulties.
The improvement of coating quality brought by HOVF also make the application area of thermal spray coating to a new level. For instance, HVOF Sprayed Inconel coatings have successfully solved some of the stress corrosion problems. Besides, the use of HVOF coatings to replace welding overlays has an optimal progress in terms of corrosion resistance demands.
Due to the low temperature of HVOF, it is generally not suitable for the spraying of ceramic materials. However, some spray guns (e.g. Top Gun, HV-2000) have a high heating capacity and can accordingly create very dense ceramic coating.
Another drawbacks of HVOF are the higher cost and bigger noise. In order to heat the particles adequately at a very high speed, it requires very high thermal power, which consume very large quantity of fuel and oxygen. A considerable proportion of heat lose because of the cooling water. For example, when using JP-5000, the cooling water takes away 80kW, accounting for 30% of total combustion heat. The extremely high thermal powder also make the work piece to be strongly heated, which requires a strong cooling substrate. An alternative way is to use a very high relative speed between gun and work piece, which further increases the cost.
