HP-HVOF θ-GUN System
WHITCO JAPAN
＜ Back Ground ＞ The HVOF （ H igh V elocity O xy F uel ） spray process produces very dense coatings with the least decomposition but has not much been applied in actual case because of it's high running cost though many applications have been studied in various fields. Some technical problems remain unsolved, such as high residual stress in coatings and low tensile strength of coatings. They make it hard to build thick coatings and cause cracks and bonding failures at twist, vibration and other external stress. The θ -Gun spray system has been developed to solve these technical problems and cut the spray cost down, finding more applications of high quality coatings in various industries.
＜ Problems with Conventional HVOF ＞
Most of conventional HVOF spray guns have a nozzle at the jet flame outlet. Powders are fed into the nozzle, melted and sprayed. This mechanism has the following technical problems causing high coating cost.
The internal powder feed process restricts the particle size distribution of spray powders within a very narrow range. Fine powders, being out of range, are over melted and clogged at the inside of nozzle, and therefore, the spray system has often to shut down it's operation even on a substrate during jobs.
The lower limit of particle distribution depends on the material properties and is in a range, 10 ～ 15 μ . Powders, finer than the range, are much produced in actual powder manufacturing and not easily separated from those with acceptable size.
After all, only about one fourth of manufactured powders meet the specified size distribution, and accordingly the material costs rise very high.
The powder feeding, more than the proper volume, also makes a trouble with nozzle clogging. The possible max feed rate is very low, and the energy generated at the spray gun is not efficiently used.
Even in normal operation a nozzle and related parts are worn by fed powders in a short period resulting in high running cost.
Particles at deposition are tend to be in either molten or half molten status as the energy adjusting range is narrow, and powders to be used are specified in a narrow range. The residual stress in coatings is tensile with molten particles and compressive with half molten particles. In either case the stress is very high making limitation of coating thickness, and the coatings with high residual stress easily crack by external force.
＜θ -Gun Characteristics ＞
The θ -Gun shown in (Fig.1) has the following characteristics offering high quality coatings at low cost.
Powders are fed into the jet flame at the outlet. As the θ -Gun has no nozzle which brings most of troubles, low cost powders with wide size distribution containing much finer size can be used and fed at large quantity meeting the energy of flame, and consequently, coating cost is very much cut down.
An air tunnel located at around the jet flame keeps the flame concentration and assists powders to be well melted and sufficiently accelerated.
The properly blended fine powders being easily melted can control the status of particles at deposition. The mixture of molten particles and half molten particles can build nearly zero residual stress in coatings.
As no consumable parts are required and no troubles with spitting caused by not specified powders are expected, the long term consistent operation is possible at maintenance free.
＜ Properties of θ -Gun Coatings ＞
The θ -Gun Coatings have much different characteristics compared with conventional HVOF coatings.
Low Coating Oxidation (Coatings by Super-sonic Reduced Flame)
The θ -Gun can be consistently operated at fuel rich, generating reduced flame. With the supply of oxygen and fuel in a balance that keeps the jet flame burning even at outside of the gun outlet, the supersonic reduced high energy jet flame, collimated by air jet stream, can be obtained. Powders injected into the flame are melted or softened, half melted, and accelerated. These particles are exhausted into the atmosphere at a little away from a substrate. As the dwell time while particles get oxygen in the air is so short that even molten particles are least oxidized. One of the ways to get the least oxidation in spraying in the air is to put particles into low heat energy and accelerate them at very fast speed, as with HVOF process, but there still remain some problems with such as extremely high compressive residual stress in coatings and as low deposit efficiency. In the θ -Gun spray process enough high heat energy and high accelerating energy are, at the same time, given to particles and therefor tough coatings with the least residual stress and less oxidation are obtained. Though it is not easy to know the degree of coating oxidation quantitatively, the view of coating cross section shows oxidized phase to much extent. The comparison of similar coatings makes the evaluation of oxidation easier. a cross section of θ -Gun coating and that of conventional HP/HVOF coating, respectively. Oxides are observed around each particle on either cross section and it means that molten or soften particles get oxygen in the flame or the air during the flight and the oxidation gets into particles from the surface. Oxygen content in the reduced flame of θ -Gun is very low compared with the other spray processes. The θ -Gun has a great advantage in spraying a material, such as chromium carbide, which needs high heat energy to get quality coatings with the least oxidation.
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