Chrome and nickel electroplating have been used over decades for corrosion and wear protection and for sealing surfaces. In hard chrome plating, a relatively thick layer of chromium is deposited directly on the base metal to provide a surface with wear resistance, a low coefficient of friction, high hardness, and moderate corrosion resistance, or has been used to build up surfaces that have been eroded by use. The multi-step process typically consists of pretreatment, alkaline cleaning, acid dipping, chromic acid anodizing, and chromium electroplating, depending on the material to which the chromium is deposited. Hexavalent chromium plating baths are the most widely used baths to deposit chromium on metal. These baths contain chromic acid, sulfuric acid, and water. The chromic acid provides the source of the hexavalent chromium that reacts to form chromium deposits on the metal. Unfortunately, this is also the primary source of ambient exposure to humans and the environment. For more information on HexChrome, visit DoD’s Chemical and Material Risk Management Program.
The process of nickel electroplating requires cleaning and degreasing in alkaline water-based cleaners, and then acid dip pickling before actual electroplating occurs. Fumes generated during this and subsequent finishing processes all contribute toward nickel exposures.
Cold Spray (CS) deposition of protective coatings is an environmentally friendly and field-deployable process, that is capable of replacing electroplating processes. Originally developed in the 1990’s, this additive manufacturing technique utilizes a high-pressure gas stream to carry powder particles through a converging-diverging de Laval nozzle where they are accelerated to supersonic velocities before impacting on a solid substrate, see Figure 1. The solid state process has no mechanism by which the powders will break down into more harmful versions of the elements, eliminating known exposures.
Powder-material and a carrier gas are the only system consumables. The powder can be a metal, polymer, or cermet, and the gas is generally nitrogen but air and helium can also be used. CS has successfully coated substrates, such as titanium, steel alloys, aluminum alloys, magnesium alloys, Ni-super alloys and even ceramics with coatings such as stainless steel, aluminum, titanium, tantalum, brass, zinc and even elemental chrome or nickel, as well as with mixtures of metals such as tantalum-tungsten. The dense nature of the coatings and the superior bond at the interface with the substrate are unique. CS can form a metallurgical bond which can be stronger than the base material.
Particles deposited by CS are physically merged into the substrate through impact, without added heat or melting the substrate remains relatively cool – below any transformation temperatures, stress relief temperatures, or even the melting point of many sensitive materials like tin or polymers. As a result, the granular structures of both coating material and substrate are not modified. Carbide powders are not degraded because of the low temperature and nano-powders can be incorporated without concern about grain growth. Powders do not undergo a phase transformation during CS as with thermal spray processes. Field-deployable CS units are commercially available, need only electrical power and an air compressor or nitrogen gas and can be used in theater.
SERDP-funded efforts by Mr. Victor Champagne at the Army Research Laboratory and his team have advanced the state-of-the-art for CS by providing a better understanding of the particle/substrate interaction and bonding mechanisms, and the compaction and consolidation of powders (Project Web Page). This has resulted in novel CS coating powders/material formulations that can be used by both DoD and industry to replace Cr and Ni electroplating. A “Materials by Design” approach was undertaken through modeling/simulation, innovative nozzle design, synthesis and development of CS powders and process parameters that produced protective coatings with properties comparable to those of Cr and Ni plating for production and/or field relevant performance characteristics.
The objectives of this work were:
Often CS powders are comprised of dense spherical powder particles of the material to be sprayed. With this typical powder morphology, the extremely hard particles needed for hard surfaces are not conducive to cold spray due to the impact deformation limitation and subsequent inability to adhere. This deformation is the key to the cold spray process. The solution combines a very hard phase such as a carbide with a soft phase such as nickel, cobalt, etc. such that the soft phase deforms and retains the hard phases to a sufficient level to give the deposit a high bulk hardness. The objective was to develop cold sprayable powders that resulted in deposits like electroplated chrome in hardness and durability.
Finite element modeling was used to predict how powder modifications affected the resulting cold spray deposit as demonstrated in Figure 2. These mathematical simulations pointed to the powder compositions most likely to yield the desired deposit. Over 160 different spray trials were performed on different powder formulations. From these trials, solutions for nickel replacement in the 400 HV hardness range as well as chrome replacements in the >700 HV hardness range have been identified. In addition, it has been noted from several wear test data points that even the lower hardness CS deposits show improvements over chrome in some wear situations. Some of the powders that have been developed, along with their deposit hardness and cross section micrographs are given by Figure 3. The wear suppressing ability of these developed cold spray coatings are compared with conventional hard coatings in Figure 4.
Power transmitting shafts experience wear at bearing contacts and the wear resistant, hard, surfaces of shafts are an example of where CS wear deposits can be applied. In order to demonstrate CS hard surface repair, simulated helicopter engine turboshafts were cold sprayed with three of the selected powders for the evaluation of repair potential. The shafts were rotated with a lathe mechanism and sprayed in a longitudinal direction, yielding a spiral coating on the shaft. Three CS deposits were evaluated. WC-17Co spray dried and sintered powder with a sub-micron carbide grain size, A blend of a dense spherical chrome carbide powder with –nickel powder and a fine-grained WC-12Co spray dried powder that was subsequently granulated with 18% fine Ni were cold sprayed on simulated shafts. The shafts were evaluated by first grinding them to achieve the desired finish, then a ball on surface wear test was employed. Qualitative results show that all of the shafts performed well, and the special design WC-12Co +18% Ni Spray dried+ Sinter/Granulated + Sinter powder worked well in wear in comparison to chrome and was superior to the chrome plating in the wear test. The hardness is lower than the typical hardness of chrome at approximately 700 HV but performance was better. Hardness of alternative CS WC-Co coatings can be as much as 1,300 V.
A similar hard surface repair was done on a section of damaged submarine periscope with CrC-NiCr powder. Figure 5 shows the result.
Repair of a damaged and corroded helicopter rotor shaft was also demonstrated. In this case the area of damage was machined until the corrosion was removed. The area was locally masked and cold sprayed nickel was applied. Unlike conventional repair methods, there were no limitation on thickness and hydrogen bake-out is not required. Figure 6 shows the shaft configuration.
This effort to produce acceptable cold sprayed hard coatings that can replace environmentally unacceptable electroplated surfaces has been demonstrated. New coatings have shown equivalence or better than conventional coatings in both hardness and wear. Successful repair of aircraft and submarine components were accomplished. Military depots are installing cold spray systems and the results of this work will allow these depots to utilize powders and operating procedures that assure successful repairs.