Phone: +1 (707) 745-3862


HVAF systems and technology from Kermetico.
High velocity air-fuel process description.
Thermal spray coating services in Bay Area, California.

HVAF – High Velocity Air Fuel process description

Kermetico High-Velocity Air-Fuel Spraying (Kermetico HVAF) is a process for deposition of high-quality coatings of metals, alloys, cemented carbides, and borides onto metallic surfaces.

The coating material in the form of pre-alloyed or composite powder is fed into a Kermetico HVAF spray gun. The gun combusts compressed air and fuel gas (propane, propylene, natural gas, etc.) generating a high-velocity jet of combustion products. In the jet, the spray powder is heated and accelerated and, when impacting a workpiece (substrate), forms a coating.

Since the combustion temperature of air-fuel mixtures is relatively low, it is possible to gradually and precisely heat the spray powder to the desired temperature, usually a few degrees below the material's melting point, performing deposition of coatings in a "solid particle" spray mode.
The Kermetico HVAF gun accelerates the powder to a very high velocity, in the range  from 800 to over 1200 meters/sec., even for such heavy particles as tungsten carbide. This is essential for producing dense and well-bonded coatings.

Precise temperature control of the spray material near the melting point and high impact velocities are the distinguishing characteristics of the Kermetico HVAF coating deposition process.

Kermetico HVAF Systems

Kermetico Inc. manufactures an HVAF system that is capable of operating several different AK-HVAF guns with equivalent power ranging from 20 to over 200 kW for the deposition of cemented carbide and metal coatings, including guns specifically designed or modified for internal diameter spraying. The coatings created by our Kermetico HVAF system exhibit high density and hardness, combined with excellent ductility. This high quality comes with the additional benefit of lower coating cost due to the efficient use of the Kermetico systems’ combustion energy which results in high spray rates and deposits per pass four to eight times higher than HVOF.

Oxidant and cooling gas: compressed air;
Optional: oxygen injection in combustion air (for selected applications);
Fuel: propane, propylene, propane-butane, natural gas, MAPP-gas.

The quality and efficiency of the Kermetico systems family is the result of the relatively low combustion temperature of the air-fuel mixture combined with the axial injection of the feedstock through the long combustion chamber, where the low gas velocity provides sufficient time for the powder particles gradual heating. The materials’ acceleration occurs in the nozzle that has a large diameter. The large diameter of the nozzle is needed to pass through the volume of nitrogen present in the combustion air. It also assures that the nozzle walls do not interact with the particles, allowing the use of a nozzle that is as long as needed for feedstock acceleration, typically from 800 to over 1,000 m/s. The Ceramic Catalytic Insert is used in the combustion chamber to provide a wide range of stable combustion parameters.

A diagram of the Kermetico HVAF configuration

Coating Quality: The combustion temperature in air-fuel mixtures is typically 1000°C lower than in oxygen-fuel combustion. This lower temperature is ideal for the gradual heating of the feedstock particles of metals and cemented carbides to or slightly above the metals’ melting temperature. The initial oxygen content in the combustion gas mixture is 5-fold lower in the HVAF process compared to any HVOF process. Both factors prevent the oxidation of metals and the decomposition of carbides and their dissolution into the metal binder. These factors allow the retention of the original ductility of the powder feedstock material in the applied coating, even when the hardness of the cemented carbide coating exceeds 1700 HV300.

Heating Efficiency:
 Heating of the spray particles in the combustion chamber occurs at high pressure, where heat transfer is the most efficient. The size of the chamber provides a long residence time for the spray particles to heat. Thus the energy transfer efficiency is increased noticeably compared to other high-velocity spraying methods, resulting in much higher spray rates. Heating efficiency is a primary reason for the reduced cost of the Kermetico HVAF applied coatings.

Acceleration Efficiency:
 The large diameter of the nozzle eliminates its length limitations, known to other guns. The spray particles can be accelerated up to the gas velocity.

Uniformity of Coating Quality:
 The diameter of the spray particle jet is several times smaller than the nozzle diameter. The influence of the nozzle walls on the spray particle velocity is negligible, providing even acceleration of the particles and improved consistency of the coating structure and quality.

Kermetico HVAF in a Family of Thermal Spray Processes

The thermal spray industry celebrated its 100-year anniversary in 2006. Many great processes and materials have been developed and have found applications in aerospace, power generation, automotive, pulp and paper machines, steelworks, oil drilling and refinery, medical devices, electronics, and numerous other industries worldwide. We are proud to be part of this family!
Among the well-known and established thermal spray processes are flame spraying, electric arc spraying, plasma spraying in air (APS) and vacuum (LPPS), high velocity oxy-fuel spraying (HVOF) and detonation spraying.
The principle characteristics of each process are spray particle temperature and velocity, as achieved during coating application. They are frequently used to compare the processes. While this comparison is rather vague and schematic - particles of different materials and of different sizes are heated and accelerated very differently even using the same process, different spraying guns exist within the same process, etc. - it is useful to understand the technological windows in thermal spray, as well as the trends in equipment and technology development.

The chart on the below demonstrates the technological windows of major thermal spray processes through the relationship between the typical spray particle temperature and velocity utilized in each process. The chart indicates that the majority of the processes require melting (often, substantial overheating) of the spray materials. To the contrary, the newest HVOF processes, and the Kermetico HVAF process relies heavily on the high velocity of the spray particles while targeting their lowest temperature.

Kermetico HVAF, HVOF, plasma, detonation, cold spray particle temperature vs. velocity.


How Low is "Low Oxygen Content" in the Coating?

Here is an example of chemical analysis data for oxygen content in the Ni-45Cr-1Ti coatings, applied by different thermal spray processes. In each coating, oxygen is present in the form of an oxide scale and as dissolved oxygen in the metal matrix. Due to high chromium content, the chosen material is sensitive to oxidation in a spray jet, thus, ideal for comparison purposes. All data are in wt.%.

Electric Arc »
stock: 0.02%
coating: 7.0%

Plasma »
stock: 0.05%
coating: 3.8%


stock: 0.06%
coating: 1.9%


Kermetico HVAF »
stock: 0.06%
coating: 0.28%

Kermetico HVAF

Kermetico HVAF Equipment

Kermetico is not just an HVAF gun developer. We produce complete high velocity air fuel systems. Since our incorporation in 2006, we have installed more than 60 HVAF systems in the USA, Europe, Japan, and China. Some of the systems are at work in Universities and National labs, but most of them are used in  production thermal spray shops.

Our systems can be equipped with different guns; they are robust and technologically efficient. And they are not expensive. If you spray tungsten carbide for just 3 hours per day, our system pays back your investment in only three months.
Here you can watch videos of our HVAF systems working.

We proudly design and produce our HVAF thermal spray equipment in California and install it all over the world. You can visit our R&D center in Benicia or our distributors` showroom in Beijing, China to meet our designer and see our HVAF equipment in action. We provide thermal spray coating services for customers around the San Francisco Bay Area in California, USA.

Phone: +1 (707) 745-3862 • Contact: • © 2006-2016 Kermetico, Inc.