Peter F. Ruggiero* Engelhard CorporationIselin, New Jersey

For more than 70 years, hardchromium coatings have been

“the gold standard” to impart resist-ance to wear, impact, and corrosionto components in aviation, trans-portation, industrial, and consumerproducts. However, in recent* Member of ASM International

years, several performance limita-tions associated with hard chromiumhave driven the engineering commu-nity in both the military and civilianaerospace sectors, as well as many in-dustrial sectors, to seek better coatingmaterials, as well as more cost-effective methods to apply them.

Among the most promising alter-natives are high velocity oxygen fuel(HVOF) tungsten carbide coatings.Extensive performance testing andan increasing number of successful

commercial applications are provingthe performance advantages of HVOF tungsten carbide coatings fora variety of aviation engine and air-frame components. These includeaircraft landing gear, hydraulic actu-ators, jet engine bearings and bearinghousings, turbine shafts, and evensome dynamic components such ashelicopter drive trains and propellerassemblies. Able to provide improvedwear, impact and fatigue resistance,and equal or better corrosion resist-

 TECHSPOTLIGHTHighvelocity oxygenfuel (HVOF) tungstencarbide coatingsare amongthemost promisingalternatives tohardchromiumcoatings.

 HVOF tungsten carbide coatings are applied to a wide range of aircraft landing gear components.

Tungsten carbide coatings replace chromium

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ance, these new coatings are replacingconventional hard chromium coatingsin many demanding applications.

In addition to the performance ad-vantages that the HVOF tungsten car-

 bide coatings can provide in the faceof punishing operating conditions,the HVOF coating process is easier tocarry out than conventional elec-trolytic chrome plating baths. In fact,a variety of previously publishedtechnology assessments (many of them joint projects between the pri-vate and military sectors) have con-cluded that HVOF thermal spray of tungsten carbide is a viable tech-nology for hard chrome replacement.To date, extensive lab and prototypetesting, and increasingly far-reachingcommercial experience, have demon-strated comparable or superior per-formance to chromium plating in

terms of wear, corrosion, andheat resistance; application fea-sibility; and overall lifecyclecosts and process economics.

A better deposition methodHVOF coatings are de-

posited by thermal spray.During this process, fuel gasand oxygen are pre-mixed, andfed at high pressure into a com-

 bustion chamber, where they burn to produce a hot, high-pressure gas stream. Tungstencarbide powder particles areinjected directly into this com-

 bustion region of the gununder automatic control. As the high-velocity gas stream (containing semi-molten powder particles) is directedtoward the substrate, the impact anddeposition of the particles on the sur-face creates a dense, uniform coating,typically with less than 1% porosity,an oxide content of less than 1%, anda bond strength in excess of 80 MPa(10 ksi). The HVOF coating processis carried out in a spray booth, with arobotic apparatus.

The HVOF deposition process isfaster than conventional chromeplating. Typical tungsten carbide coat-ings are applied in an hour or two,versus 24 hours or more required forhard chrome plating. Furthermore,unlike hard chrome coatings, HVOFtungsten carbide coatings do not un-dergo hydrogen embrittlement, so

the time and cost required for embrit-tlement-relief treatment is eliminated.

All of these factors work in tandemto reduce the frequency of repairs andfacilitate faster turnaround duringoverhaul and maintenance activities.The result is reduced out-of-servicetime, and generally reduced overalllifecycle costs associated with criticalindustrial engine and aviation parts.

As shown in the table, the macro-hardness of HVOF tungsten carbidecoatings is greater than 70 Rc (Rock-well scale), compared with typicalvalues of 60 to 70 Rc for hard chromeplating. Microhardness values for thetungsten carbide coating are on theorder of 1050 DPH 300 (DiamondPyramid Hardness), compared withvalues of 750 to 850 DPH that are typ-ical of hard chrome plate.

As mentioned above, greater bondstrength and lower porosity can alsocontribute to the formation of durablecoatings. As summarized in the table,HVOF tungsten carbide coatingshave also demonstrated considerablygreater corrosion resistance than hardchrome coatings (720 hours, com-pared with 55 hours, using ASTMB117 testing protocols), and have en-dured higher surface temperaturelimits (1025°F, as opposed to 750°F).

Gaining in commercial acceptanceHVOF tungsten carbide coatings

are already protecting many landinggear and airframe components. Forexample, Engelhard applies HVOFtungsten carbide coatings on a widearray of components for both militaryand commercial aircraft, includinglanding gear and actuators.

As the U.S. Dept. of Defense, theU.S. Air Force, the U.S. Navy, and agrowing list of private-sector jet en-gine and airplane manufacturers con-tinue to recognize the ability of HVOFtungsten carbide coatings to impartsuperior resistance to abrasion andwear, fatigue failure, and corrosiveattack, and to help operators sidestepthe costs associated with using elec-trolytic chrome plating processes, theuse of these advanced coatings is ex-pected to take off.

For more information: Peter Ruggiero is business manager for aviation services atEngelhard Corp., 101 Wood Avenue,Iselin, NJ 08830; tel: 732/205-5000; e-mail:peter.ruggiero@Engelhard.com; Web site:www.Engelhard.com/surfacetech.

Coating characteristics and performance resultsHVOF tungsten Hard chromium

Property carbide coating plate

Macrohardness, HRC >70 60 – 70Microhardness, DPH 300 >1050 750 – 850Bond strength, MPa (psi) >80 (10,000)* ~41 (6,000)Porosity <1% Inherently crackedCoating thickness, mm (in.) >0.08 (0.003) < 0.13 (0.005)Surface finish, Ra <4 <4Corrosion test (ASTM B117), hours 720 55Surface temperature limits, °C (°F) 550 (1025) 400 (750)

*Results exceed strength limit of epoxy needed for tensile test.

The left image shows a hard chromium coating; the right image shows the much smoothertungsten carbide coating applied by HVOF.

 HVOF tungsten carbide has a bonding strength tothe substrate of 80 MPa (10 ksi).

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