What is Activated Combustion High Velocity Air/Fuel Spraying (AC-HVAF)?

AC-HVAF is a thermal spray technology used for simple and inexpensive application of premium quality coatings, which meet or exceed HVOF standards for hardness, density, bond strength and oxide content, as well as wear, corrosion and fatigue resistance.

The AC-HVAF process uses air and fuel gases, usually propane, propylene or natural gas to spray metal and carbide powders at supersonic velocities. Particle velocities exceed 2300 ft/s, while particle temperatures remain slightly below the melting point of metal and metallic binders, allowing AC-HVAF to operate in a solid-particle spray mode.

Better Coatings Through Research

Unique coating morphology

UniqueCoat Technologies’ AC-HVAF process represents the highest quality thermal spraying technology on the market today. The highest coating quality is the result of unique coating morphology that is radically different from that of HVOF. HVOF coatings are formed from molten particles, which do not bond together properly due to solidification shrinkage and oxide scale. This results in defects in the coating in the form of voids and other imperfections, which reduce coating quality. AC-HVAF coatings do not exhibit such voids and imperfections, with all of the particles tightly bonded together.

 

These SEM pictures reveal a unique void-free morphology of AC-HVAF carbide coatings (top), while HVOF coatings (bottom) formed from molten particles exhibit voids and carbide deterioration.

No thermal deterioration of carbides yields premium wear resistance

Since spray particles are not heated above their melting point, there is no thermal deterioration of carbides or dissolution in the metallic binder. This, in conjunction with better morphology, results in AC-HVAF WC-Co and WC-Co-Cr coatings that exhibit 1.3-4.0 higher resistance to abrasive wear when compared to various HVOF systems during ASTM G65 wear test.

Extremely low oxidation of sprayed powders

The solid particles of AC-HVAF process are far less susceptible to oxidation during application than molten HVOF particles. The oxide content of AC-HVAF coatings is typically below 0.2%, and coating porosity is nearly eliminated (below 0.5%). The AC-HVAF has demonstrated itself to be particularly useful in brazing applications. Braze material is applied to parts before assembly. Solid particle spraying does not deteriorate wetting and flow ability of brazing material – something not achievable with other thermal spray processes. This technique is more reliable than conventional brazing.

High bond strength

High velocity solid particles effectively remove oxides from the substrate upon impact, providing the coatings with exceptional bond strength. This grit-blasting action is so effective that in many cases it completely eliminates the need for surface preparation.

Excellent fatigue resistance

Tungsten carbide-cobalt coatings are alternatives to chrome plating on high-strength steel landing gear components. In independent testing conducted by a US Air Force Research lab the WC-17Co AC-HVAF coatings outperformed all HVOF counterparts in fatigue testing at increased loads and especially when thicker layers were applied, meeting the metallurgical criteria established by General Electric Aircraft Engines for HVOF coatings. Engineers rated the AC-HVAF coating as superior to the HVOF coatings and equal to EHC plating.

 

AC-HVAF coating integrity at 185 ksi and 220 ksi surpasses HVOF, and is rated similar to or better than electroplated hard chrome coatings.

Hard Chrome Replacement

Good coating morphology permits polishing of WC-based coatings to an optical mirror finish. High spray rates, absence of oxygen and little consumption of spare parts make the cost of the AC-HVAF sprayed carbides very competitive to other hardfacing technologies, including hard chrome electroplating, braze cladding and weld overlay. It is the best alternative to hard chrome plating for OEMs that industry currently has to offer.

Thick Coatings are Possible

Because of the lowered residual stresses in the AC-HVAF coatings due to solid particle impact, thicker carbide layers are possible. Carbide coatings are routinely applied to 0.1” (2.5 mm) thick. In special cases coatings as thick as 13 mm can be applied.

Application of 1.5 mm carbide coating to jet mill component.

Consistent Coating Quality

Due to the forgiving nature of the AC-HVAF process the quality of the applied coating remains largely unchanged at shallow spray angles and a wide range of gas parameters. Only the particles with the proper temperature and velocity characteristics form a coating; the particles with poor adhesion are removed by the grit-blasting effect of the AC-HVAF process. In other words, changing the spray parameters will only affect the deposit eficiency, not coating quality. Deposit efficiency varies between 50 and 75% for WC-based powders, and 65 to 85% for metals and alloys.

AC-HVAF Microstructures

AC-HVAF Articles and Laboratory Reports