There is an increasing demand for a cheap alternative to electrolytic hard chrome coatings in corrosive environments. The market needs environmentally friendly long-lasting tungsten carbide coatings or inexpensive stainless steel coatings.
Kermetico HVAF technology and equipment provide a way for hard chrome replacement with impermeable, hard and ductile coatings that are inexpensive and easy to apply. Numerous researchers have pointed out that HVAF coatings work several times longer than electrolytic hard chrome does having the same or lower cost.
How do we thermal spray coatings as cheap as hard chrome plating?
- We spray thin gas impermeable coatings with roughness low enough to avoid grinding.
- We spray it fast.
- We have the systems to spray it onto outer and internal surfaces.
- We have developed the technology to do it without failures.
Features of Kermetico HVAF Coatings for Hard Chrome Replacement
The Kermetico High Velocity Air Fuel (HVAF) process has been shown to be very competitive as an environmentally friendly alternative to electrolytic hard chromium (EHC).
Kermetico HVAF carbide coatings are superior to HVOF and EHC rivals in corrosion protection, wear resistance and production cost. The high velocity of the in-flight particles (faster than 1,000 m/s | 3,280 ft./sec.) in our HVAF process enables the production of very dense coatings with high bond strength.
Moreover, the low combustion spraying temperature (1,960-2,010°C | 3,560-3,650°F depending on fuel gas) and gentle particle heating lead to minimal feedstock phase transformation and almost nonexistent elemental depletion/decomposition of the in-flight particle.
Furthermore, the replacement of pure oxygen in the HVOF process by using air in the HVAF process significantly reduces the oxide content in the coatings, which is desirable for high-performance coatings.
The Main Features of HVAF and Typical HVOF Tungsten Carbide Coatings
| WC-10Co-4Cr Coating Features | A Typical HVOF | Kermetico HVAF Modes | ||
| Economy | Balanced | Ultra | ||
| Deposition efficiency, % | 40-55 | 60-70 | 48-58 | 36-42 |
| Hardness, HV300 | 1,050-1,250 | 1,050-1,250 | 1,250-1,350 | 1,450-1,600+ |
| Young’s Modulus E, GPa | 300 | 400 | 450 | 450 |
| Apparent metallographic porosity | <0.8 | <0.8 | <0.5 | <0.1 |
| Bond strength, MPa (PSI) | 80+ (12,000 +) | |||
| Range of as-sprayed roughness, µm (µ in.) | 3.5-4.5(150-180) | 1-3.2 (40-130) | ||
A Case of a Flash Carbide Coating of Titanium Parts for Hard Chrome Replacement
Thermal spray applications available with Kermetico HVAF technology cannot be provided well with traditional thermal spray processes. We develop new hard chrome replacement applications frequently and would like to share one of these cases with you.
Background:
A manufacturer had a mission-critical titanium part in a new product which contained two OD seal surfaces. The use condition subjected the part to fully reversing hydraulic pressure cycles of 5 KSI (with occasional spikes to 8 KSI).
Note: An NDA covers this application and our relationship with this customer, so, for this reason, details are intentionally vague. This report has been reviewed, edited, and approved by the customer for our use.
Customer Initial Requirements:
- The seal surfaces should not experience excessive wear over the service life of the part.
- The seals themselves should not be negatively affected due to the reciprocating motion while in contact with the part.
- Application of the coating should not affect the substrate material properties. In particular, it should not cause heating which would result in the formation of beta phase within the Ti crystalline structure.
- The coating should not be cost prohibitive.
Initial Customer Testing:
Customer performed testing which simulated operational conditions over a period of several months.
Preliminary Customer Results:
Parts exhibited longitudinal cracks through the coating which formed a stress concentration resulting in crack propagation through the titanium alloy substrate. Once cracks progressed through the substrate, this created a passage between two isolated fluid circuits, rendering the equipment unfit for continued use. Fatigue life was on the order of 2e5 cycles – far below the expected infinite fatigue life. These failures prompted the customer to specify an additional performance requirement.
Additional Customer Requirements:
Application of the coating should not cause a fatigue debit.
Kermetico Recommendation for the coating thickness:
After discussions with the customer, Kermetico recommended reducing the layer thickness from 250 microns (.010”) to 65-80 microns (0.0025”-0.0035”).
Customer Development Efforts:
In addition to Kermetico’s recommendation, the customer pursued the addition of a shot-peening process prior to the application of the WC-Co-Cr coating as well as modifications to the machining and finishing processes.
Kermetico Responsiveness:
The customer ordered some variants in preparation to test the effect of modifying dimensional and process parameters. Knowing the criticality of the parts, we completed the turnaround time of our processing in a matter of days.
Customer Development Testing:
Fatigue testing under worst-case loading conditions, as well as seal endurance testing, was performed over the course of roughly a 12-month period.
Customer Hard Chrome Replacement Results:
Fatigue testing exhibited a fatigue life more than 10e7 cycles. As this number of cycles is more than the published endurance limit for the substrate material at the stress ratio experienced by the parts, testing was halted with the expectation of infinite fatigue life. One of these two designs was adopted for production and subsequently placed into service. As of this writing, no field failures using the updated part design have been reported.
Other Hard Chrome Plating Alternatives
Numerous studies of Kermetico HVAF sprayed Fe-based coatings have shown their high corrosion resistance in different environments such as acid, alkaline, and chloride solutions. High-quality microstructures with low oxide content, high retention of the powder chemistry and low porosity have been reported and make this family of coatings suitable as a low-cost hard chrome replacement in many applications.
Notes on Kermetico HVAF Equipment to Replace Hard Chrome Plating
The traditional thermal spray approach is to melt and atomize the feedstock, propel it to the surface of the target part whereupon contact ‘splat cooling’ builds up a coating.
The Kermetico HVAF process operates a little differently.
We heat the feedstock material to near its liquid phase temperature without exceeding it.
Then we accelerate the particles to an optimized high velocity, and when the particles impact the substrate, there is a rapid conversion of kinetic to thermal energy that allows for plastic deformation of the particle and a bond that we cannot accurately measure.
In the ASTM 633C bond test, the only result we get is broken glue, even with 0.040“ (1 mm) of WCCo 88/12.
There are two factors here that need to be understood.
The first is coating quality: regardless of the HVAF gun you choose and the size of the part, you will get a good coating.
The second – you need to choose HVAF gun carefully to get the best efficiency from its power.
Using a large gun (AK7) on a small part, you will have a long cooling time between passes and consequently a job cost that was higher than it needed to be.
Using a compact gun (AK5) for a large part, you will have a low ‘spot’ temperature, and this will give you a low DE (deposit efficiency) making the job more costly than necessary.
You could choose an AK7 for 1,400 mm parts or an AK6 for 50 mm parts or an AK5 for <25 mm parts or our ID gun (AK ID) for ID surfaces.
The Surface of a Polished Rod with a Kermetico HVAF Tungsten Carbide Coating
Hard Chrome Replacement with Kermetico HVAF Coatings
Blast and Spray with Kermetico HVAF Equipment
Usually, we deposit hard chromium replacement coatings using robotic blast and spray operations.
We blast a surface with a Kermetico HVAF gun (it is extremely fast and uniform).
Then we spray with the same gun after switching the powder feed hose and perhaps changing the nozzle
It is much faster, more accurate and needs much less grit than manual blasting.
Hot blasting also provides very even surface preparation and induces less stress into the base metal.
Kermetico HVAF and HVOF Thermal Spray Coating Equipment
Kermetico designs and manufactures three families of HVAF thermal spray equipment.
- Convertible HVOF and HVAF equipment:
- Multi-purpose HVAF AK systems, with a range of spray guns:
- AK7 – a high-power gun for the thermal spray of large parts
- AK6 – a gun to spray carbides onto a balanced mix of parts – recommended for hard chrome replacement
- AK5 – a compact gun to spray smaller parts, thin walls and complex surfaces
- AK-ID – an 80 mm (3.15”) and larger internal diameter spray gun – recommended as a chrome plating alternative for internal surfaces
- AK-IDR – a rotating ID gun to spray 4” (100 mm) and larger bores
- AK-HH – a handheld gun for on-site coating
- Specialized HVAF S equipment:
- Additional equipment:
- An HVAF grit feeder for the grit blast preparation of parts to reduce coating costs
- A Gas Permeability Tester for an instant check of a coating’s through porosity
Kermetico Inc. – Material Science Excellence, HVAF and HVOF Equipment Manufacturing and Thermal Spray Coating Services Since 2006
We create equipment that helps material scientists, engineers and business managers achieve their goals.
We have installed more than 60 Kermetico HVAF & HVOF 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.
We proudly design and produce our HVAF& HVOF thermal spray equipment in California and install it all over the world.
You can visit our R&D center in Benicia to meet our designers and see our HVAF and HVOF equipment in action or visit the factory of our partner in China.
We also service our U.S. customers with HVAF and HVOF thermal spray coatings – ourselves and through the network of our partners.
