SULZER TECHNICAL REVIEW 1/2006 74164

HVOF-Sprayed Materials Replace Hard-Chromium Plating

In industrial applications, hard-chromiumplating is increasingly being replaced withHVOF-applied, carbide-containing materials.While these coatings have good wearprotection properties, many hard-chromiumreplacement applications also requirecorrosion resistance. Sulzer Metco Woka, aGerman subsidiary of Sulzer Metco,develops corrosion-resistant coatingmaterials. During the development, a varietyof factors is taken into account, includingthe coating chemistry, the spray process,and the service environment of the coating.

ANDREAS KIRSTENMANFRED OECHSLE

RICHARD MOLL SULZER METCO

Sulzer Metco DiamondJet®

8 SULZER TECHNICAL REVIEW 1/2006

Hard chromium, appliedelectrochemically as plating,

has a long and comprehensivehistory as a surface solution usedto prevent wear and corrosion.Industrial applications include air-craft landing gear, turbine engines,hydraulics, and propeller hubs(Fig. 1). However, human healthand environmental concerns overthe presence of hexavalent chromi-um associated with this processhave caused many industries tolook for alternative solutions. Forapplications with larger surfaces,thermally-spraying of carbide-containing materials using thehigh-velocity oxy-fuel (HVOF)process has proven to be a com-petitive substitute (see box).

Lower Cost—Better PerformanceExperience shows that, in manycases, the right choice of the coat-ing material can lower costs whileimproving performance (Fig. 2). Inlaboratory tests, however, someHVOF-sprayed coatings havedemonstrated insufficient corro-sion resistance. Therefore, the de-velopment of coating materials

that exhibit both wear and corro-sion resistance and can be used inthe HVOF process has high priori-ty. For carbide-containing materi-als, the agglomeration and sinter-manufacturing technique is highlypromising, as many differentmaterial combinations are possibleand because metallic matrices,which influence the applicationsuccess, can be tailored precisely. Cermet-type coatings, where thecarbide constituent is cementedwithin a metallic matrix, are themost suitable carbide-containingmaterials with respect to wear andcorrosion resistance. In such acoating system, the carbide pro-vides wear protection, and themetallic matrix can be formulatedfrom a corrosion-resistant alloy.Tungsten carbide and chromiumcarbide are the most commonlyused carbides for thermal spray.

Specialized Alloys for Difficult ApplicationsWhile tungsten carbide andchromium carbide are highly cor-rosion-resistant, they are not usedas HVOF coating material in pureform but rather in metallic matri-

ces. As in the cemented-carbide in-dustry, for thermal spray powders,single metal matrixes of nickel andcobalt are most often used as well.For applications requiring morespecific properties, specialized al-loys can be used, such as cobalt-chromium, nickel-chromium, andnickel-chromium/ -molybdenum.These metals are added either asprealloyed powders or, for cost-saving reasons, as finely pow-dered elemental metals during thehomogenizing process of the spraypowder. During sintering, they formpseudoalloys with a corrosion re-sistance that is nearly identical tothat of their prealloyed counter-parts. The matrix material, however, re-mains the weak point of corrosion-resistant coatings. In addition, thecoatings must be dense to preventthe worst-possible condition ofelectrolyte penetration to the sub-strate. This penetration causes gal-vanic coupling and results in cor-rosion of the substrate and delam-ination of the coating.

1 Apart from health concerns, legalrequirements also compel the elimi-nation of hard-chromium plating inindustrial use. Sulzer Metco developscoating materials that can replacehard chromium in applications thatrequire high corrosion resistance, likethe coating of aircraft landing gear.

HVOF: An Alternative to Chromium Plating HVOF is a thermal spray technology where a powder is fed into a spray gun. There, afuel—usually ethylene, hydrogen, or kerosene—is burned with oxygen, and the heatedand softened powder is ejected as a spray with the supersonic gases (see STR 1/2004,

p. 4). HVOF is a flexible dry-coating technology withlow environmental impact. HVOF covers many ap-plications with a variety of possible coating materi-als. The use of hard chromium, however, is so preva-lent that no single technology or material can replaceit generally. Among all choices, carbide-containingmaterials are very promising due to their wide rangeof application.

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3 Sulzer Metco Woka in Barchfeld (DE) carried out theresearch. In the corrosion chamber (right), the polarizationresistance of the coatings was measured. Per definition, thepolarization resistance Rρ (kΩ · cm–2) is inversely proportion-al to the corrosion rate of the coatings.

4 The long-term corrosion tests revealed the strong influ-ence of the particle temperature, which is high for lowlambda values, or short stand-off distances. It was foundthat the hotter the flame and, therefore, the hotter thespray particles in the flame, the better the corrosion per-formance of the coating—measured by the high polarizationresistance and high corrosion potential.

ContactSulzer Metco Woka GmbH Andreas KirstenIm Vorwerk 2536456 Barchfeld GermanyPhone +49 (0)36961 861 42Fax +49 (0)36961 861 33andreas.kirsten@sulzer.com

Extensive ResearchSulzer Metco carried out a re-search program to identify themain parameters influencing thehardness and corrosion resistanceof HVOF coating materials byvarying the fuel/oxygen ratiolambda (λ) and the distance be-tween the HVOF spray gun andthe surface. The coatings were sub-jected to standardized hardnessand wear test procedures and theirproperties compared with those ofhard-chromium plating. In almostevery instance, the carbide coat-ings exhibited higher coatinghardness and greater wear resist-ance than galvanic hard-chromi-um plating. To study the corrosion resistanceof HVOF materials, the coatingswere electrochemically tested insaline, acidic, and alkaline aque-ous solutions for two hours andtwenty-four hours immersion time(Fig. 3). The corrosion rate of thecoatings was compared with thatof the uncoated, low-carbon steelsubstrate and with that of galvan-ic hard chromium. HVOF coatings

with matrices of nickel-chromiumalloys demonstrated superior re-sistance in all solutions. Further-more, post-testing examination ofthe interface of the coating to thesubstrate revealed no signs of gal-vanic corrosion.

Award-Winning ResultsCarbide-containing materials cer-tainly rival hard-chromium plat-ing in terms of hardness and wearresistance; tungsten carbide mate-rials also have potentially bettercorrosion resistance than hard-chromium plating. Very gooddurable corrosion resistance in dif-ferent corrosive environments canbe obtained through the choice ofmatrix material and the carefulcontrol of the HVOF sprayprocess. Coatings must be verydense in terms of interparticlebonding to prevent electrolytesfrom penetrating the coating. Thisshields the substrate from attack,which is particularly importantwith less noble substrate materi-als. The HVOF spray parametersshow a strong influence on the cor-rosion behavior of coatings inservice (Fig. 4). Provided that theformation of brittle carbide phasesis avoided, a higher particle tem-perature is helpful. Thanks to thisextensive research—which re-ceived a certificate of merit by tworeputed materials technology as-sociations—Sulzer Metco now of-fers carbide-containing coatingmaterials that can replace hard-chromium plating even in applica-tions where high corrosion resist-ance is required.

2 Coating landing gear—here aSulzer Metco DiamondJet spraying alanding gear strut—is the major useof hard chromium in aircraft. With itsimproved performance, the HVOFprocess lowers the expected overhaulfrequency, thereby reducing the life-time cost of ownership.

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