Traditional thermal spray processes are based on this principle: heat fast – spray fast, with the idea being that the faster process is completed, the less oxidation will occur and the coating we will get will be better. Sometimes it works, however, fast heating requires higher gas temperatures, which may cause quite the opposite results: material evaporation, higher oxidation levels and coating material degradation.
While Kermetico are developing HVAF equipment, they continuously keep to one principle: heat slow, spray fast. A large diameter combustion chamber and a comparatively low gas combustion temperature allows the delivery of powder materials axially, directly to the combustion zone, where they are heated gradually and evenly.
The guns have a large diameter, flow optimized nozzle and narrow powder jet and these factors minimize any nozzle wall influence on particle velocity. Multiple gun designs and configurations let us spray materials with different melting points without overheating them.
Depositing the same feedstock using different technologies results in different coating properties. The Kermetico HVAF process lets you spray well-known materials with more desirable properties: higher hardness, ductility and deposition efficiency, with lower porosity and cost.
Coating Properties for a Protective Coating
- Hardness.
- Ductility.
- Gas permeability.
- Fracture toughness.
- Corrosion resistance in the given environment.
- Tribological properties in contact with a given material.
- Electrical conductivity.
- Bond strength.
- Coefficient of thermal expansion.
- Residual stress.
- Porosity.
- Spray rate.
- Deposition efficiency.
- Cost of the feedstock spray material.
- Cost of the applied coating.
HVAF Coating Materials Include:
- Aluminium.
- Amorphous steel.
- Brass.
- Bronze.
- Chromium carbide.
- Copper.
- Hastelloy coatings.
- Non-stick coatings.
- Stainless steel.
- Tin.
- Tungsten carbide.
- Zinc.
The Bond Strength of HVAF Coatings
Due to the high-velocity impact of the spray particles, Kermetico HVAF coatings provide high bond strength to metallic substrates and exhibit excellent cohesion strength.
Some Kermetico HVAF metallic coatings can withstand impacts by a hammer or welding through the coating without cracking or delamination.
Resistance to High Temperature Corrosion
The absence of oxide scales and the high density of Kermetico HVAF coatings results in their superior performance in high temperature corrosion environments.
In oxidizing and sulphurizing environments HVAF coatings of alloy 625 and alloy 671 substantially outperformed their counterparts sprayed with electric arc or HVOF coatings.
Weight Gain of Alloy 671 Coatings Sprayed with an Electric Arc, HVOF, and HVAF, after Testing in N2-1%H2S-1%HCl Gas at 400°C for 1,440 Hours.
The beneficial factor was that HVAF metal coatings efficiently sintered and formed diffusion zones with a substrate at elevated temperatures, afterward performing as a “solid metal.”
Such sintering and inter-diffusion are restricted in other thermal spray coatings because oxide scales are efficient barriers to diffusion.
Weight Gain of HVAF Alloy 671 Coatings and Stock Materials after Testing in Air at 700°C for 1,000 Hours.
Fracture Toughness of HVAF Coatings
Kermetico HVAF coatings reveal excellent crack resistance (fracture toughness). One of the methods for measuring the fracture toughness coefficient, K1C, for brittle materials involves indentation with a Vickers pyramid and measuring of induced crack length. Since K1C ~ (a/c)3/2, when a>c (a is the diagonal of the indentation, c is the length of the induced crack), the ratio “a/c” can be used as the crack resistance factor. In our tests, the crack resistance factor was found to be between 3 and 4 for different HVOF coatings of WC-based materials. Corresponding Kermetico HVAF coatings revealed values between 5 and 7, indicating substantially improved crack resistance.
In addition to improved coating performance, this fact has very practical applications in the creation of new markets for HVAF thermal spray coatings. In particular, improved coating crack resistance results in the possibility of applying very thick layers of carbides, using carbide coatings to combat cavitation, applying coatings on parts working in conditions of alternating stresses, impact, etc.
Spraying Ductile WC Coatings with a Hardness over 1,600 ±6% HV300, and no Cracks
Most HVOF companies will tell you that a non-cracking WC coating 1,600 HV300 with a deviation of 100 HV is impossible in real life. And they will be telling you the truth. It is impossible with the oxygen-kerosene process, where the combustion temperature of 3200 °C prohibits the spraying of fine particles.
The Kermetico HVAF process works within a combustion temperature range of 1,860-2,100°C with a particle velocity higher than 1,000 m/s, which allows spraying fine WC-metal particles while keeping all the feedstock WC phases intact.
Spraying Low-Melting Point Metal Coatings: Al, Cu, Sn, Zn and Alloys
Kermetico HVAF SL System Spraying a Tin Coating
The twin wire arc and flame spray processes apply aluminum, zinc or copper with high spray rates, also with high porosity and high oxide levels so that the coating is inexpensive but oxidized.
The cold spray process does the opposite and sprays a coating without oxidation and with low porosity, but its low spray rate and high equipment and gas costs make the quality coating quite expensive.
Kermetico AK HVAF equipment provides an option to spray low melting point metal coatings at a 10kg (22 lbs.) per hour spray rate, without oxidation, and with an applied cost comparable to twin wire arc spray.
For more information on the equipment or consumables for HVAF Coating Material Selection, call us on 07 3823 1004, or email us using our contact form.
