There is an increasing demand to replace electrolytic hard chrome coatings on hydraulic rods’ surfaces with environmentally friendly long-lasting WC-CoCr or less expensive stainless steel coatings. Kermetico HVAF technology and equipment provide a way to protect hydraulic rod surfaces with impermeable, hard and ductile layers.
Numerous researchers have pointed out that HVAF coatings work several times longer than electrolytic hard chrome and even HVOF WCCoCr.
Kermetico Convertible systems work in HVOF or HVAF mode, providing an even wider technological choice for hydraulic rod coatings.
The Features of Kermetico HVAF Coatings
Our High Velocity Air Fuel (HVAF) technology has been shown to be very competitive for hydraulic rods protection.
Kermetico HVAF carbide coatings are superior to conventional HVOF counterparts regarding wear resistance, corrosion protection and production cost. The high velocity of the in-flight particles (higher than 1,000 m/s) in our HVAF process enables the production of very dense coatings.
Moreover, the low combustion spraying temperature (1,960-2,010 °C | 3,560-3,650°F) and gentle particle heating lead to minimal feedstock phase transformation and almost nonexistent elemental depletion/decomposition of the in-flight particles.
Furthermore, the replacement of pure oxygen necessary for HVOF with air in our HVAF process significantly reduces the oxide content of the coatings, which is desirable for high-performance coatings.
The Main Features of HVOF and Kermetico HVAF Tungsten Carbide Coatings
| WC-10Co-4Cr Coating Feature | 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) | ||
Numerous studies on 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.
To maximize seal life, superfinishing with an oscillating stone, tape or abrasive film is required.
According to ARP 5935, recommended surface finish parameters may include:
- Ra 4 μinch or better
- Rz 40 μinch max (average peak-to-valley)
- Rp 8 μinch max (mean line to highest peak)
- Rmr 70 to 95% @ Co = 5% and C1 = 0.25Rz
A Case of Hard Chrome Replacement for Hydraulic Rods used on a Sea Dock
Thermal spray applications that can be done with Kermetico HVAF technology cannot be accomplished well with traditional thermal spray processes. The one we have capitalized on in our shop is the replacement of hard chrome plating on hydraulic rods.
Traditional coatings and hydraulics have never worked long enough. Even with a sealer the porosity eventually allows the hydraulic fluid under pressure to penetrate the coating. Once the pressurized fluid hits the substrate, it spreads out under the coating and lifts it off.
About five and a half years ago we coated about 30 hydraulic rods used at a local refinery on their dock cranes. They had been getting only six months out of the chrome plated rods. We coated them with HVAF WCCoCr 86-10-4 and the first ones we sprayed have been in service for over five years.
A report of a visual inspection about six months ago was, “They still pretty much look like they did when we first put them in.”
Kermetico HVAF Coating of a Hydraulic Rod
Notes on Kermetico HVAF Equipment
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.
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 the HVAF gun carefully to get the most economical result 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 Ak5 gun 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 a 1,400 mm (55″) part or an AK6 for a 50 mm (2″) part or an AK5 for <25 mm (1″) part or our ID gun (AK ID) for ID surfaces.
Blast and Spray Rods with Kermetico High Velocity Equipment
Usually, we deposit rod coatings using robotic blast and spray operations.
We blast a surface with the Kermetico HVAF gun (it is extremely fast and uniform) and spray with the same gun after switching the powder feed hose and perhaps changing the nozzle.
HVAF hot blasting is much faster, more accurate and consumes much less grit than the conventional compressed air blasting.
It also provides very even surface preparation and induces less stress into the base metal.
Kermetico HVAF 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 variety of spray guns:
- AK7 – a high-power gun for the thermal spray of large parts
- AK6 – a carbide gun to spray a balanced mix of parts in a job shop – recommended for hydraulic rods
- AK5 – a compact gun to spray smaller parts, thin walls and complex surfaces – recommended for smaller rods
- AK-ID – an 80 mm (3.15”) and larger internal diameter spray gun
- 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 coating through porosity
Kermetico, Inc. – HVAF and HVOF Material Science Excellence, Equipment Manufacturing and Thermal Spray Coating Since 2006
We create equipment that helps material scientists, engineers and business managers achieve their goals.
We have installed more than 60 Kermetico HVAF and 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 thermal spray equipment in California and install it all over the world.
You can visit our R&D center in Benicia, CA to meet our designers and see our HVAF and HVOF equipment in action.
We also deposit HVAF and HVOF thermal spray coatings for customers in North America.
