Although the driver was originally regulatory, growth in the use of engineered hard chrome (EHC) alternatives has accelerated as original equipment manufacturers (OEMs) and their customers have recognized that alternatives can provide better wear and corrosion performance with faster turnaround in maintenance, repair and overhaul (MRO). Many companies have found that high-velocity oxygen-fuel (HVOF) technology reduces their cost of ownership, even though the process itself is more expensive.

U.S. Environmental Protection Agency regulations for hexavalent chrome (Cr 6+) air emissions (regulated under the Clean Air Act) have become increasingly stringent over the last few years . In addition, chrome plating sludge, used maskant, and any other material containing Cr (whether Cr, Cr3+or Cr6+) must be disposed of as toxic waste. The liability risks associated with air emissions, waste disposal and worker health are a major concern for any organization engaged in aerospace manufacturing or overhaul using engineered hard chrome (EHC).

Although the driver was originally regulatory, growth in the use of EHC alternatives has accelerated as original equipment manufacturers (OEMs) and their customers have recognized that alternatives can provide better wear and corrosion performance with faster turnaround in maintenance, repair and overhaul (MRO). Many companies have found that high-velocity oxygen-fuel (HVOF) technology reduces their cost of ownership, even though the process itself is more expensive.

Figure 1. A schematic of a double-ended aircraft hydraulic actuator showing coatings and damage mechanisms.

Typical Applications and Requirements

EHC is used for two primary purposes in the aerospace industry: increasing wear resistance and rebuilding worn components.

For wear resistance, a plating thickness of 0.003 to 0.005 in. (75-125 microns) is typical, although there are some applications in bearings and internal diameters that use nodular thin dense chrome (TDC) with a thickness of 0.0003 to 0.0006 in. (7.5-15 microns).

When rebuilding worn components to print specifications (often where EHC had not originally been used), thicknesses up to 0.015 in. are usually allowed. Sulfamate nickel (Ni) capped with EHC is used for larger buildups.

Figure 1 shows a typical aircraft actuator, which is subject to a variety of wear and damage mechanisms. Surfaces are plated thicker than required and then ground back to the final dimensions and finish. Most aircraft applications are on cylindrical surfaces such as this that are subject to wear, although some are more complex, including landing gear pistons, journals for wheel and engine bearings, hydraulic actuator rods, and flap and slat tracks (a non-cylindrical surface). Additionally, thin EHC and TDC are frequently applied to the internal diameters (IDs) of hydraulic actuator and landing gear outer cylinders and some bearing races. Any alternative must not only match or exceed the performance of EHC, but it also must be compatible with typical substrate materials, and it must fit with manufacturing and overhaul processes. Requirements for these processes include:
  • It must be usable on shot-peened high-strength steel, which cannot exceed 375°F (190°C). There are some structural Al alloys that are even more heat-sensitive.
  • It must be possible to build up to a thickness of at least 0.015 in. without excessive stress (especially tensile stress, which tends to cause a fatigue debit).
  • Its wear resistance should be comparable with or better than EHC.
  • Its surface finish should be suitable for use in fluid-wetted wear applications (ideally a smooth bearing surface that traps oil).
  • It should be suitable for use in an OEM or MRO plant, or be widely available from aerospace-qualified vendors.
  • It must be suitable for MRO, including:
    - able to be deposited at final spec or to be ground or superfinished to the required size and finish,
    - trippable for subsequent repair and overhaul, and
    - suitable for non-destructive inspection (NDI), especially fluorescent dye penetrant inspection (FPI) for substrate cracks.

Figure 2. A schematic of an HVOF gun.

Alternatives for Exterior Surfaces

An evaluation of various options for EHC replacement carried out in the 1990s concluded that HVOF thermal spray was the best available option for use by aerospace OEMs, military depots and commercial aerospace maintenance, repair and overhaul (MRO) shops.1The Hard Chrome Alternatives Team (HCAT) was formed to generate all the performance data required for validating and qualifying HVOF coatings such as Tribaloy, WC-Co and WC-CoCr, including laboratory testing, full scale rig testing, and flight testing.2HCAT is funded by an international partnership between the US Department of Defense (DoD) (primarily Strategic Environmental Research and Development Program [SERDP]/Environmental Security Technology Certification Program [ESTCP]), the Canadian Department of National Defense (DND) and Industry Canada.

Figure 3. HVOF is sprayed on a nose landing gear piston. The HVOF gun (left) moves up and down as piston rotates.

The HVOF process is entirely different from chrome plating (see Figures 2 and 3). HVOF coatings are deposited with a supersonic oxy-hydrogen or oxy-kerosene torch through which alloy or cermet powder is sprayed, so that it softens or melts, hits the surface and spreads out to form a dense, well-adhered coating. Spraying is done in much the same manner as spray painting, spinning the component on its axis while moving the gun back and forth along its length (or, for non-cylindrical components such as flap tracks, moving the gun back and forth over the area to be coated). This makes it possible to spray the component uniformly without the typical chrome plate dog-bones at the ends. At the same time, however, the painting process means that complex parts are difficult to spray, and while shallow internal diameters can be sprayed, deep IDs cannot.

Unlike EHC, where the sole material that can be deposited is Cr, HVOF is a technology that can deposit hundreds of different alloys and cermets. Although this feature makes it very flexible, it is also far more complicated than simply using hard chrome. The most commonly used alternative is HVOF WC-10Co4Cr (although the Air Force uses WC-Co, and Cr3C2-NiCr is used for some applications). When sprayed, this material takes the form of hard carbide particles embedded in an alloy matrix. (Sintered WC-Co is the material used to make highly wear-resistant lathe cutter inserts, drills and other tools.)

Figure 4. Leakage comparison of coated hydraulic rods, EHC vs. HVOF ground and superfinished.

HVOF coatings are superior to EHC in many respects:
  • Although the material contains Cr, HVOF does not produce Cr6+. Non-Cr alternatives such as WC-Co can be used to avoid Cr-containing materials entirely.
  • With a hardness of 1,000-1,500 Vickers hardness number (VHN), WC-CoCr is far more resistant to wear and damage than EHC (800-1,000 VHN).
  • When properly superfinished, the leakage rate of HVOF-coated hydraulic rods is well below that of chrome plated rods (see Figure 4). Note that superfinishing is critical to avoid creating an excellent rasp that will destroy seals and bushings.
  • The corrosion resistance of HVOF coatings is superior to EHC in service.
  • HVOF coated components cause little or no fatigue debit, in contrast with the large debit from EHC.
  • The HVOF process is non-embrittling, and environmental embrittlement appears to be less severe than with EHC. Hydrogen baking is not required.
  • A typical landing gear cylinder can be HVOF sprayed in 1-2 hours, while EHC typically requires 24 hours for plating and a further 23 hours for hydrogen baking (embrittlement relief). HVOF thus reduces overhaul turnaround time, which is especially important for airlines.
      The only performance shortcoming of HVOF is that it can spall at high stress (180 ksi or above) or at high fatigue cycles. The only aircraft application for which this is an issue is the landing gear on carrier fighters, which sees high stresses on both launch and landing. Even there, however, no serious issues exist for thin (0.003 to 0.005 in.) OEM coatings.

    • HVOF is now being specified for all new landing gear designs (e.g. Airbus 380, Boeing 787, Boeing 767-400, F-35 Joint Strike Fighter) and on many hydraulic rods and some flap tracks. Since most of the world’s commercial landing gear components are made in Canada, several new spray shops have been opened in Canada to meet the growing demand.

      Although HVOF is the predominant EHC alternative for externals, it is not the only one. While it is an efficient way of coating large components, it is relatively inefficient for small items, and it is especially unsuitable for small components such as brackets that need to be coated all over to provide wear protection in some areas and corrosion protection in others. Such components can be coated simply in a plating bath, but they are complicated to spray. For these types of components, electroless Ni-P (EN) is becoming the chrome alternative of choice. It has the advantage of being a widely available aerospace-qualified material able to coat complex shapes, through holes, blind holes and grooves. It is not as hard as chrome, but its wear resistance can be comparable, although its abrasion resistance would be expected to be lower.

      Because electroless Ni is autocatalytic, it is very sensitive to surface contamination. Surfaces must be clean and properly activated to ensure good adhesion and prevent holes in the coating. Fatigue is also an issue, since the coating becomes more tensile (and the fatigue debit increases) as the plating solution ages. Specifications and quality control measures should take this weakness into account.

      Care must be taken when using EN on aerospace components. For most high-strength substrates, the EN must be used as-deposited because the heat treatment needed for hardening it (typically 350°C or 660°F) is above the permissible limit.

      The correct choice of EN phosphorus content is always an issue, since it depends on the application and whether or not corrosion is a concern. Low-P Ni has higher hardness as-deposited, but high-P Ni has higher hardness when heat treated. When EN is used as-deposited, high-P Ni has better corrosion resistance than low-P. When EN is to be used on high-strength steels that cannot be heat treated, low-P EN will usually provide better abrasion and wear but worse corrosion resistance. For steels that are able to withstand the heat treatment, high-P Ni is usually preferred.

      A number of EN composites are now on the market that incorporate silicon carbide (SiC) or diamond particles for hardness or PTFE for lubricity (or even both). Hard particles allow EN to achieve the hardness of EHC without heat treating, but coating uniformity is difficult to maintain in production, especially for complex shapes.

      Alternatives for Interior Surfaces

      The major limitation of HVOF is that it cannot be used for most internal diameters. IDs can be coated up to a depth-to-diameter ratio of about 2:1 by angling the gun in from outside.3Deeper holes can only be sprayed by inserting the gun and its flame into the hole, which limits the method to holes larger than about 11 in. in diameter (landing gear outer cylinders as large as those used on the A380), or 6 in. with one new gun on the market from Northwest Mettech. It is possible instead to use plasma spray, which can be done on internals as small as 1.5 in., although the coating quality is inferior to HVOF.4For any thermal spray, blind holes present a difficult problem because spraying creates a dust storm inside the hole that entrains unmelted powder into the coating, degrading coating quality.

      Because of the limitations of thermal spray, the most common alternative for IDs is electroless Ni-P. Because internals generally experience less severe wear than pistons, EN is often a good choice. It is even possible that some of the EN-polytetrafluoroethylene (PTFE) composites may be an option for internals, although they are usually significantly softer than standard EN. However, at this point there does not appear to be any consensus among aerospace users on how well these materials work for internals.

      There are other electro and electroless plates, but availability is an issue because most are sole source and are not aerospace-qualified. When the Air Force Research Lab (AFRL) carried out screening tests of a number of Ni-based plates, initial results pointed toward NiPlate 700, a SiC EN composite, as a potential candidate,5but detailed testing and validation have not been done. With most EN composites, it is difficult to ensure uniformity over complex shapes, an issue that any qualification testing would need to address. AFRL is now evaluating non-Ni plates for ID coating as an alternative because of uncertainty over the long term regulation of Ni, and some manufacturers have begun to test electroplates such as Ni-W-B.

      NanoPlate, a nanophase Co-P electroplate from Integran in Toronto, Ontario, Canada, was developed under DoD SERDP/ESTCP funding specifically as a chrome alternative for IDs. The process is unusual in that it uses pulse electroplating to deposit Co-3%P as a nanomaterial, which is corrosion resistant and considerably harder and more wear resistant than standard DC-plated Co. Since this coating is still under final development and validation, it is not yet ready for aerospace use. If validation is successful, this material might, in principle, be an option to replace both TDC and EHC on IDs, and could, of course, also be an alternative for externals.

      Note that the need for rebuild means that typical thin physical vapor deposition (PVD) coatings (such as nitrides and diamondlike carbon) are not suitable for most alternative applications, even though they are some of the smoothest, hardest and most wear-resistant materials. In principle, a very hard PVD coating could be a “lifetime finish” OEM chrome alternative, just as it is on consumer plumbing and door hardware. At this point, however, there is no aerospace-qualified PVD material widely available for this type of application.

      Requirements and Specifications

      From all the testing that has been done, it is clear that we cannot just strike out EHC from drawings and substitute HVOF, EN, or any other material. The specifications for material chemistry, heat treat and surface finish will vary depending on the alternative chosen and the application. This is especially true for HVOF, where the largest amount of data is available.

      For example, in EHC plating, labor is the largest cost, whereas for HVOF materials cost predominates. While chrome plating often is applied to an entire component to avoid masking, only the relevant areas should be HVOF coated, since coating non-functional areas is expensive and wasteful.

      Additionally, EHC coatings can be as thin as 0.0003 in., but HVOF cannot be continuous at less than about 0.002 in.

      While EHC is usually ground with an alumina wheel, HVOF carbides require a diamond wheel, which will load up when grinding metals. Methods have had to be developed to grind both HVOF and the surrounding steel with the same wheel to minimize equipment set-ups.

      Finally, EHC coated hydraulic rods are usually specified with an 8 or 16 µ-in. finish. Rough HVOF coatings make excellent cutting tools, however, typically requiring a 4 µ-in. superfinish for seal performance significantly superior to EHC.

      There are a number of commercial specifications for hard chrome alternatives,6as well as extensive performance data, reports and briefings on HVOF.7 Despite the large volume of available data, the requirements for designing with these technologies are unfamiliar to most design engineers. As aerospace companies have begun to replace EHC with HVOF and electroless Ni, experts in the field have developed HVOF guidelines for designers, and will probably need to do so for other chrome alternatives as well.

      Future Directions

      Spalling remains the primary performance drawback of HVOF. Efforts are under way to resolve this problem through different equipment, coating materials and powder formulations, including nanophase powders. Lower cost is likely to come from the qualification of new guns such as high-energy plasma spray and hybrid guns, which appear to provide coatings with similar performance while avoiding the use of hydrogen fuel.

      Thermal spray is not a good method for coating IDs, leaving the industry with different technologies for external and internal areas. Gun sizes are being reduced to accommodate smaller and smaller IDs, but the problem still remains that any internal coating by thermal spray generates non-adhered overspray powder and excess heat, which requires careful attention to gun efficiency and the use of gas jets. The alternative approach is to develop and qualify a plating method with sufficient performance. Any plating that meets the performance of EHC could be a contender for both ID and OD chrome replacement, simply by avoiding multiple coating technologies and vendors, even if it does not match the performance of HVOF.

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