DoD Programs - Alternative Summaries

Hard Chrome Plating Alternatives - Thermal Spray

Hard chrome alternatives summary – thermal sprays  (primary alternatives in blue).

The most commonly used technology for replacing hard chrome plating on military hardware is thermal spray (primarily, but not exclusively, HVOF).

Technology

Thermal spray is the general term for a number of processes in which particles of coating material are heated and sprayed using high speed gas onto the surface to be coated.  It is a fully commercial industrial process that is used for coating everything from bridges and radar towers to aircraft parts and prosthetic knee joints. Since thermal spray coatings are readily sprayed up to 0.020” (500 microns) thick, they are good for rebuilding worn components, which is the primary use for hard chrome plate.

Of the various thermal spray technologies (click here for comparison), the most widely used is plasma spray for thermal barrier coatings and abradables in turbine engines. 

Availability

Most depots have some thermal spray capability, often plasma or flame spray for engine repair.  However, the most widely used technology for chrome replacement is High Velocity Oxy-fuel (HVOF).  HVOF capabilities are in place at several depots

• Ogden ALC – landing gear

• Oklahoma City ALC – gas turbine engines

• Warner Robbins ALC – propeller hubs

• NADEP Jacksonville – engines, landing gear

• NADEP Cherry Point – engines, propeller hubs, helicopter components

• Corpus Christi Army Depot – helicopter components

In addition, Patrick AFB uses arc spray Zn-Al for corrosion protection of ground support equipment.

Most manufacturers are now supplying equipment that uses HVOF coatings in place of hard chrome, and its increasing penetration in the commercial marketplace is expected to drive its use on a broader range of military hardware, including aircraft landing gear and actuators, and hydraulics for vehicles and ships.

There are a large number of companies around the US and the world who supply thermal spray services, and the equipment and supplies (thermal spray powders) are available from a number of international companies.

OEM use of HVOF on military equipment

• F-35 CTOL landing gear (baselined)

• F-35 hydraulic actuators

• F-22 thrust vectoring actuator

• F-22 hydraulic actuators

• C-17 main landing gear post

• X-45 UCAV landing gear

 Depot use of HVOF in repair

• Ogden ALC, landing gear components (pistons, axle journals, pins, etc.) qualified for

  • A-10

  • B-1

  • KC-135

  • C-5

  • F-15

  • F-16

  • Major qualification and implementation program under way

• NADEP Jacksonville, engine components for

  • J52

  • F404

  • TF34

  • P-3 and EA-6B landing gear in flight test

• Oklahoma City ALC, engine components

  • Initial production

• NADEP Cherry Point

  • CH-53 blade damper actuator qualified and approved

  • C-2, C-130 propeller hubs qualified

Advantages

HVOF is a widely available technology that has been in production use for a number of years and is fully aerospace-qualified, with many commercial and OEM specifications.  In general it has a number of advantages over chrome plate:

• No Cr⁶⁺ – The most common chrome alternative powder used in DoD is WC-Co, which contains no Cr.  Even when the spray powder contains Cr metal (WC-CoCr) or chromium carbide ceramic (Cr₃C₂-NiCr), the material is not in hexavalent form.  The HVOF process is not known to emit Cr⁶⁺; the only place that Cr⁶⁺ is known to be formed in production thermal spray processes is in plasma spray of Cr₂O₃, which is used for Anilox print rolls.

• Relatively simple industrial process – Commercially available equipment and materials.

• Able to build up from 0.001” to >0.020” – same range as hard chrome.

• Much better wear resistance than hard chrome – Typically 3 or more times life.  Tends not to scratch or damage on assembly and disassembly.  Avoids striations and scratches on hydraulic rods.

• Better corrosion resistance – Although it looks worse in B117 cabinet corrosion tests, it usually performs much better than hard chrome in service.

• No hydrogen embrittlement

• Faster turnaround time - Turnaround is reduced to hours instead of days because of faster processing and no need for embrittlement relief.

• Cost - Process cost is usually 1.5-2x higher than hard chrome. Life cycle cost is usually much lower that hard chrome because of improved performance.

Limitations of HVOF for DoD applications

The only serious way in which HVOF falls short of hard chrome performance is in its response to high load and high cycle fatigue.  Because the strain-to-failure of most HVOF carbide coatings is only about 0.7%, it will crack above this strain.  The cracks do not run into the underlying material, but they can run along the interface and cause pieces of coating to spall off.

High load:  Under high cyclic load HVOF coatings can spall, with the coating coming off in large flakes from test specimens.  (Note: this has been seen in fatigue testing but not in service.)  This behavior is typically seen in high strength steels (300M, 4340) at loads above about 180ksi in tensile loading, or 20-40ksi higher in bending.  Thin coatings are less susceptible (in fact 0.003” (75mm) coatings show no spalling, but coatings tend to spall at lower loads as the thickness increases.  This is not a problem for most applications of HVOF, since loads are generally well below this value, even for landing gear.  However, it has led to a great deal of concern for its use on the landing gear of carrier-based aircraft, which see high reversed loads on launch and landing.  It has not been found to be an issue for Air Force landing gear cylinders, journals or pins, or for hydraulics, engines, or other components.

High cycles:  After high cyclic loading at low load the coating usually shows signs of cracking in a circumferential pattern around landing gear cylinders.  This cracking pattern can be seen in landing gear that have been in service for several years, but, like the well-known cracking pattern seen in chrome plate, it does not appear to affect sealing or corrosion in service.

HVOF spray gun.

HVOF coating of a landing gear hydraulic cylinder (courtesy Jon Devereaux, NADEP JAX)

Major thermal spray processes.

www.dodcorrosion

exchange.org

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