Cold Spray for Chrome Plating Replacement
U.S. Army Research Lab and research partners investigate cold spray as an alternative to electrolytic chrome and nickel plating.
Hard chrome plating is a technique that has been in commercial production for over 60 years, and is a critical process associated with manufacturing and maintenance activities on a wide variety of military aircraft, vehicle and munitions applications, including landing gear, hydraulic actuators, propeller hubs, helicopter rotor heads, shafts, splines and gun barrels.
The U.S. Department of Defense (DoD) relies on hard chrome for dimensional restoration, as well as a wear-resistant, corrosion-inhibiting surface. The current process to apply hard chrome utilizes chromium in the hexavalent state, a known carcinogen.
Hard chrome itself is not inherently toxic, and provides ideal wear resistance and surface protection. Cold spray (CS) technology can be used to replace hard chrome with alternative surface processes, or to produce a hard chrome layer without using Cr6.
CS will enable the DoD to comply with environmental regulations (i.e. AERTA PP-2-02-04) and executive orders (i.e. 13148) to eliminate chromic acid used in the current process, a hazardous substance containing Cr6, while adhering to the DoD Strategic Sustainability Performance Plan. The EO requires the usage reduction of Cr6 by 50 percent; the major issue for hard chrome plating is Cr6 mist, for which the OSHA PEL is 5 µgm-3 8-hour time-weighted average (TWA). The EPA Cr6+ emission limit is 6 µgm-3 for new sources.
The method used for hard chrome plating at various depots is standard hard chrome, rather than high efficiency chrome plating or trivalent chrome, which is currently only used for decorative chrome. For DoD, the most serious problem is the requirements established by OSHA—requiring washing and showering, changing clothes, etc.—which can effectively reduce the work day from eight hours to seven. OSHA also requires that Cr6 be used only in regulated areas of the shop, and carries out wipe tests for Cr6 in all other areas such as offices, break rooms, etc.
Cold Spray Process
Cold spray is a solid-state materials deposition process where metallic, polymeric and/or combinations of metallic and non-metallic particles are consolidated to form a coating or a near-net shaped part by means of ballistic impingement upon a suitable substrate. The particles utilized are in the form of commercially available powders—typically ranging in diameter from 5 to 100 µm—that are accelerated from 300 to 2,500 m/sec. by injection into a high-velocity stream of gas. The high velocity gas stream is generated via the expansion of a pressurized, preheated gas through a converging-diverging de Laval nozzle. The pressurized gas is expanded to supersonic velocity, with an accompanying decrease in pressure and temperature. The particles—initially carried by a separate gas stream—are injected into the nozzle either prior to the throat of the nozzle (upstream injection) or downstream of the throat (downstream injection). The particles are subsequently accelerated by the main nozzle gas flow and impacted onto a substrate after exiting the nozzle. Upon impact, the solid particles deform and create a bond with the substrate. As the process continues, particles continue to impact the substrate and form metallurgical, as well as mechanical bonds with the substrate and subsequent consolidated material, resulting in a uniform deposit with little porosity and high bond strength.
If the critical impact velocity of the accelerating particles is attained upon impact, the solid particles deform and create a bond with the substrate. Adequate velocity is necessary for optimal particle consolidation and coating density, and several important CS process parameters, including gas conditions, particle characteristics and nozzle geometry, affect the particle velocity.
It has been well established that impacting particles must exceed the critical velocity to deposit; otherwise, they may rebound off the substrate. The magnitude of the critical velocity can be estimated by using empirical relationships, which generally depend on particle material characteristics such as density, ultimate strength, yield and melting point, as well as the particle temperature. Process development is an important aspect of CS.
Advantages of Cold Spray
The uniqueness of cold spray lies in its ability to produce a coating, provide dimensional restoration or even produce a near-net shaped part at temperatures well below the melting point of the powders being applied, thereby avoiding or minimizing many deleterious high-temperature reactions, which are indicative of thermal spray processes. It is this characteristic of cold spray that makes it attractive as a method for coatings or for dimensional restoration, while retaining the unique material properties of the feed stock powders. The CS process can deposit coatings with deposition rates exceeding 20 lbs. per hour, and can lay down a coating of 0.030 inches thick in a matter of seconds; electroplating would take over 30 hours to produce the same thickness. In addition, CS provides the opportunity to apply multiple materials using the same process, providing the option of buildup repair and hard surfacing from one operation.
CS is suitable for a wide variety of common military parts and substrates, to include internal and external diameters, surfaces and complex shapes in an economically feasible manner. It is versatile in that it can be adapted to high-volume production or can be used as a portable process for local/spot repair and operate in unpredictable field conditions only requiring an air compressor and electric power.
Chrome replacement is an ongoing effort in many industries, as the cost of chrome plating processes gets higher due to increased regulatory compliance costs. Many of these efforts have found that a single solution will not be sufficient to address all applications in which chrome is currently used. In addition, the cost, environmental friendliness, speed and quality of deposits produced through the cold spray process provide benefits over other competing technologies with respect to chrome replacement.
The efforts at ARL and UTRC have included the use of feed stock powders containing a hard phase and a soft phase to produce coatings with a combination of acceptable hardness, adhesion and wear and corrosion resistance. Generally, hard particles such as spherical 1200 HV dense metallic particles will not deposit alone due to the high hardness, which limits the ability of the particle to undergo extreme plastic deformation during the CS process, a necessity to form a dense, adherent deposit with minimal porosity. The deformation of the impacting particles is the key to a superior coating.
One solution is to combine a very hard phase—such as a carbide or hard metallic phase—with a soft phase, such as nickel or cobalt, so that the soft phase deforms and retains the hard phases to a sufficient level to provide the deposit a high bulk hardness. There are several conventional and non-conventional methods being investigated and/or developed for creating cold spray powders in this fashion. These include mechanical blending of conventional powders, use of conventional agglomerated cermet powders, granulation cladding of hard particles with fine metallic powder, electroplating and electroless plating of hard particles, high energy milling and combinations of these processes. In each case, the key is to properly select the hard phase and soft phase for maximum compatibility and to understand the need, if any, for heat treatments to diffusion bond the two phases such that they can survive the CS process.
Chrome plating is typically used in wear due to its relatively high hardness, and tends to perform relatively well in wear situations ranging from abrasive to adhesive wear and fretting. There are several chrome plating specifications that range in hardness requirement from 650 HV to 800 HV. Typical hard chrome plating, however, is closer to 850-900 HV. This is a relatively high hardness, and provides the greatest challenge for CS, so this hardness range has been the focus. It is important to note, however, the specific performance of chrome under certain conditions. Wear can be measured in many ways, with the best methods typically matching the actual operating conditions of the component.
Typical applications might include:
- Plated shaft rotating against a bronze bearing in oil.
- Plated piston axially sliding against a glass filled polymer bearing and a shaft seal in an oil film environment.
- A surface in light contact with Inconel or steel, which experiences an oscillatory motion at room to elevated temperature due to equipment vibration.
- The surface of a part against which a nut or washer rubs during assembly or disassembly.
Each of these potential applications has a different set of counter-faces and types of motion, and generally are evaluated by using full component level testing of sub-element testing closely matching the conditions of operation. The wear life also includes other important aspects that lead to failure at lower times that might be predicted by a typical test. The most overlooked and critical piece that ultimately causes large field failures is service contamination. This is because chrome plating is not particularly wear resistant in an abrasive field. With that in mind and reviewing the list of applications, we can compare these by types of potential wear:
- Sliding wear against bronze to third body contamination as well as embedded contaminants in the bronze counter-face.
- Abrasion from glass fibers in bearing and environmental contaminations between seals and bearing scratching the piston and leading to advanced seal wear and leakage.
- Metal-to-metal oscillating wear with Inconel or steel resulting in fretting debris formation, acting as a third body abrasive, as well as high-temperature, metal-to-metal contact and fretting.
- High load and high stress metal contact with potential for metal-to-metal adhesive wear and galling.
When wear is tested in a laboratory coupon test, each of these wear types can be isolated and materials can be ranked against each other. This has the benefit of understanding the specific pros and cons for a material solution, but can confuse the adoption of a solution. It is important to find a variety of materials that have beneficial performance in several types of tests, and then to conduct an experiment more closely matching the application. One case in point is performing ASTM G133 ball on flat reciprocating wear tests with 0.25-inch ball bearings and a plated specimen. When running chrome plate against an M-50 ball bearing in a clean, dry environment, the wear is extremely low. Simply substituting an aluminum oxide ball more closely represents the material combination that might be present if an engineering system were contaminated with sand and/or similar hard particle debris. This completely changes the situation producing tremendous amounts of wear. Running a typical HVOF cermet coating with a hardness close to chrome will produce a slightly higher wear performance against an M-50 ball, but a dramatically lower wear performance against the alumina ball.
After evaluating the microstructure of the dozens of CS coatings produced from feed stock powders by the methods previously described, the best candidates having low porosity levels and no internal stress cracks or spallation were chosen for wear testing.
Hardness indents were performed resulting in a hardness of 1362 HV200 and 904 HV200 for samples identified as CS-16-209-10 and CS-16-209-5, respectively. Both coatings exceed the typical 850 HV200 threshold chosen for chrome plate alternatives. These were wear tested resulting in favorable results using a simple ASTM G133 test and the results are shown in Figure 2, while Figure 3 shows the representative microstructure of a CrC-NiCr CS coating produced from a powder from HC Stark, (Amperit 587-074 -325+15µm).
It is clear from the wear results that the CS version of WC-Co and Cr3C2-(Ni 20Cr) fit well with comparable high quality HVOF coatings. The variations observed between the different carbide coatings are directly linked to the ratio of carbide to metal in the powder. If plotted by hardness, the trends would be similar. The outlier is chrome plating, which performs well when coupled to a material like steel, but performed very poorly when coupled to other materials like Al2O3.
The properties of chrome plating vary significantly and have been widely used in a variety of engineering applications due to the relative simplicity and longtime use of the process. In fact, the specific properties of chrome plating may not be the critical enabler for all applications, and understanding this is key to finding appropriate solutions to its replacement.
For instance, a chrome plated surface in a wear application completely submerged in an oil bath may need the wear properties of chrome but not the corrosion resistance. Likewise, in an overhaul situation where a press fit surface is slightly oversized due to some type of damage unrelated to operation, and chrome plating is used to build the surface back to dimension, the plating is being used simply to fill space and therefore does not require the hardness or likely the corrosion resistance of chrome. In a final case, chrome may have been used in a wear application (such as abrasive wear) where the mode or wear was not especially favorable to chrome, but due to the engineering familiarity with the plating, it was used.
Because of the variety of favorable properties for chrome plating it is important to understand a wide variety of engineering properties for a substitute material.
The primary goal of ARL is to compile several potential solutions and, more generally, to develop a physics-based understanding around materials that can produce high-hardness, high-quality CS deposits.
In this way, given an application and the general properties required from that application, one of several options are available, and in unique applications where very specific benefits may accrue from the use of an alternate hard phase or soft phase chemistry, the tools are available to identify the method of combining those materials in such a way that will be successful for CS.
Victor Champagne is with the U.S. Army Research Laboratory at the Aberdeen Proving Ground, Maryland.
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Originally published in the August 2017 issue.
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