Bexxon Global

HVAF Thermal Spray Equipment and Wear Resistant Coatings for Mud Motor Rotors

Wear resistance of the mud rotor surface is one of the key factors of an oil well directional drilling cost. 

Kermetico HVAF technology and equipment provide a way to protect mud motor rotors with gas-tight, 1,350-1600+ HV300 hard and ductile tungsten carbide coatings.  Other mud motor components, such as flow restrictors and drive shafts, also benefit from our coatings. And we produce  Convertible HVAF + HVOF equipment for companies that still need to spray conventional HVOF coatings for conservative clients.

HVAF coating schlumberger pump rotor 861004
tungsten carbide mud pump rotors

The Kermetico HVAF System Spraying a WCCoCr Coating onto a Mud Motor Rotor

Numerous researchers have found that Kermetico HVAF WCCoCr coatings provide wear resistance several times longer than HVOF or electrolytic hard chrome.

Let’s find out why.

The Wear of Mud Rotors

Downhole drilling motors, or positive displacement motors (PDM), consist of a helically shaped metallic rotor rotating within a molded stationary elastomer lined stator.

The heart of the mud motor is the rotor-stator pair. The circulation of the rotor transforms the pressure of hydraulic energy of drilling mud into the mechanical energy causing the motion of the drill.

Mud-rotors have been plagued with excess wear, corrosion, erosion and coating delamination resulting in small drilling times. Most mud rotors had historically been chrome plated or nickel plated, but frequent coating failures have forced the industry search for other solutions.

Drilling mud is contaminated with chlorides that permeate the microcracks and pores of traditional chrome plating and corrode the underlying steel.

The appropriate coating should increase drilling times, reduce wear, increase corrosion resistance and minimize repair times.

The coating must be capable of sustaining horizontal and vertical flexing and needs to be impact resistant.

For performance in the harsh conditions the coating should:

  • have very low residual stresses
  • be as ductile as possible
  • have very high cohesive and adhesive strength


Kermetico HVAF Coating Features

Kermetico High Velocity Air Fuel (HVAF) has been shown to be very competitive for mud rotors’ protection.

HVAF carbide coatings ARE superior to HVOF rivals regarding both wear protection and production cost. The high velocity of the in-flight particles (greater than 1,000 m/s) in our HVAF process enables the production of very dense coatings with high bond strength.

Moreover, the small combustion spraying temperature (1,960-2,010°C | 3,560-3650°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 particles.

Furthermore, the replacement of HVOF using pure oxygen with air in the HVAF process significantly reduces the oxide content in the coatings, which is desirable for high-performance coatings.

A Comparison of Main Features of HVAF and HVOF Tungsten Carbide Coatings for Mud Rotors


The Performance of Ceramic Sealants

1,600+ HV300 is still lower than 2,000 HV some manufacturers advertise for their ceramic sealants. Well, we are not familiar with any real-life proof of a sealant’s advantages.

We have heard from field operators that ceramic sealants don’t really help in the long-term as they wear off fast.

And we believe it, because otherwise, one doesn’t need an HVOF coating there; he can just apply this sealant as a coating. Sealing might help to close the porosity and permeability of a poor WCCoCr coating, but it can’t be a key factor.

Our HVAF tungsten carbide coatings are impermeable to gas or water and contain the same or less amount of brittle WC phases as the original powder, which gives them much higher toughness and hardness than any HVOF specimens.

These factors result in a longer service life.

HVAF vs. HVOF for Mud Motor Rotors Coating

For testing of WC-10Co-4Cr HVAF coatings in comparison to the best HVOF counterparts, we would refer to the late 2011 Schlumberger UK data, several works performed by Hunan University (Changshu, Hunan, China) and the data from the Central Power Research Institute (CPRI, Bangalore, India).

The improvement of our HVAF coatings versus the best HVOF coatings of similar chemistry:
abrasive wear: 35-50%
slurry erosion (jet) at 90° attack angle:
3-4-fold (Schlumberger and Hunan, different powders manufacturers, different shops)

cavitation resistance:
5-fold for a 1,320 HV300 hardness HVAF coating,
10-fold for a 1,450 HV300 HVAF coating
15-fold for a 1,550 HV300 HVAF coating (CPRI India).

Note that Hunan was using local-made agglomerated-sintered powder for HVAF while comparing it to sintered-crushed Amperit used with a Diamond Jet HVOF. And CPRI was using agglomerated-sintered Amperit for HVAF while comparing it to fine-carbide Durmat with the JP5000. T

diamond belt grinding pump rotor
pump rotor polishing

HVAF Equipment Mud Motor Rotors

High Quality Tungsten Carbide