The metal finishing process of electrolytic hard chrome (EHC) plating has been identified as a source of environmental pollution in most industrialized countries like Australia, Europe and USA. The key driver for the technology replacement is that the EHC plating process uses hexavalent chromium, which is a known carcinogen. Our previous research has identified that cold spray nanostructured tungsten carbide cobalt (WC-Co) coatings can be a suitable alternative to provide a functional coating in wear applications. This work explores at another similar technology- Kinetic Metallization for deposition of WC-Co coatings. In this work, the objective is to characterize the residual stress profile of these WC-Co coatings that are deposited by the latest KM systems. These coating systems are used in critical applications such as landing gear pistons and axle journals, hydraulic rods, engine shaft journals, and numerous other external surfaces that operate under high cyclic loading conditions. As such, the residual stress developed during the KM coating process has a significant influence on the fatigue properties of the components. Thus, knowledge of stresses and their linkage with other properties and production parameters is essential for the quality control of these critical structures.

The low temperature Kinetic Metallization (KM) deposition process is compliant with the recently revised MIL-STD-3021 standard for cold spray materials deposition. KM deposition process and equipment was supported through the US Military via multiple Small Business Innovative Research (SBIR) grants leading to delivery of KM deposition equipment to multiple branches. Coating application areas discussed in this paper include wear resistant tungsten-carbide and chromium-carbide hard-phase coatings. These are used for aircraft engines and landing gear surfaces with firm low temperature deposition requirements. Other areas include aluminum and magnesium dimensional restorations, in particular for Aircraft Mounted Accessory Drive (AMAD) gearboxes for F-18 platforms, and land vehicle housings and casings for the US Marines Corps.

Applications such as landing gears and turbine blades place new demands on near-net coating technologies. Such demands include the replacement of traditional grinding, finishing and grit basting techniques with better, more efficient methods. A method is described for near-net-shape spraying of complex internal and external geometries which eliminates the need for grinding. This is achieved by combining automatic, mass-flow controlled HVOF grit blasting with Nano-HVOF methods. The resulting coating displays an as-sprayed surface roughness of less than 2 μm Ra and a tight control over coating thickness and distribution. By carefully controlling the coating thickness and surface properties, it is possible to hone the required dimension and surface roughness.

This paper describes an inner diameter HVOF spraying technique and associated tests methods. In the experiments, WC-CoCr was applied to an internal diameter of 150 mm and evaluated using a dedicated pin-on-disk wear test and standard axial fatigue tests. The results are presented and discussed along with the potential for further development of the inner diameter HVOF spraying technique.

High production rate and feed powder efficiency are critical in the HVOF hardfacing of aircraft landing gear, turbine, and actuator components traditionally electroplated with carcinogenic hard chromium. Desired improvements are hindered by rapid heat build-up in substrate component and thermal expansion mismatch between the carbide coating and steel, titanium or aluminum substrate. A new, cryogenic nitrogen gas (-195°C) cooling system has been developed which limits the thermal expansion and substrate softening problems, and enables a non-stop, gun-on-target spraying. Fully automated, the operation of the new AP LIN-Cooling System is based on thermal imaging of the entire substrate and multi-zone cooling with novel, cryofluidic nozzles. Thermal logs and images of components processed are saved by the system for quality auditing purposes. This paper presents results of industrial tests of the system during WC-10Co4Cr coating of Boeing 737 landing gear, demonstrating a 50% reduction in spraying time, corresponding reductions in the consumption of powder and HVOF gases, and additional labor savings due to the use of flexible masking, unfeasible with the traditional cooling methods. Analysis of residual stresses, structures and properties of the coating and AISI 4340 steel substrate shows that the cryogenic nitrogen cooling results in high-quality products.

Electroplated hard chromium (EHC) is widely coated onto parts to provide resistance to corrosion, wear and impact. The electroplating process, however, has significant health and environmental impacts. Air emissions during the electroplating process contain hexavalent chromium (Cr+6) - a known carcinogen, furthermore the process is energy intensive and generates hazardous waste. Because of health and environmental issues related to hard chromium plating, there have been several efforts to find alternatives. One of the more efficient technologies among the substitutes is High Velocity Oxy-Fuel (HVOF) thermal spraying. This technology is commercially available today, with a major commercial opportunity in aerospace applications. In this paper, we therefore compare the life cycle environmental footprints of hard chromium and HVOF coatings for aircraft landing gear. Our results indicate that from an environmental perspective, HVOF spraying is generally preferable to EHC plating, with 5-10 times lower human health impacts and 30-50 times lower ecosystem impacts. However, in terms of resource consumption, the processes have similar impact profiles with EHC plating having a potential for lower impact on resources in areas with a significant share of renewable electricity.

Thermal spray coatings applied for aircraft gas turbine maintenance often needs to be stripped and removed. Various methods are used, such as chemicals or a very high pressure water jet. None of them are environmentally friendly and they must be applied with care to prevent substrate damage. Also, these methods can produce a great deal of waste that has to be managed. As regulation gets tougher, a new technology using very high pressure liquid nitrogen could be the future for such treatments. As the worldwide exclusive licensee, over several years Nitrocision LLC has developed and commercialized this technology, which initially was conceived in the late 90s by the Idaho National Laboratory. The NITROJET 6000 can pressurize Liquid Nitrogen up to 4000 bars and to temperatures as low as - 170°C. Various sets of tools can provide single or multiple nozzles on a fixed, or alternatively, a rotary head that provides greater coverage and efficiency. This publication presents case studies of the technology applied to coating removal on aeronautical parts in gas turbines and landing gears.

High velocity oxygen fuel (HVOF) sprayed WC coatings have been found wide applications as substitutions of chrome plating in the airplane industry. The wear of HVOF WC coated landing gear against Al-Ni-Bronze alloy leads to the wear or spalling of WC particles, and finally results in the degradation of sealing property for landing gear system. WC-Co coating with nano-size WC particles is expected to exhibit good wear resistance, and the weight loss of Al-Ni-Bronze alloy can also be reduced, which is beneficial to the sealing property of landing gear system. Three types of coatings are prepared using agglomerated, sintered nano-scaled WC-12Co, conventional WC- 12Co and WC-10Co4C powders, respectively. The wear behaviors of the as-sprayed coatings against Al-Ni- Bronze alloy are investigated at the different loads and linear velocities. The study is expected to provide the basis for the application of HVO/AF nano-scaled WC coating in landing gear.

High velocity oxygen fuel (HVOF) coatings have been in development for chrome replacement for aerospace applications and implemented for many applications over the past couple of decades. Now that these coatings have become common for newly manufactured parts, the need for repair and overhaul coating thicknesses beyond 0.254 mm (0.010 inch) for rework that traditionally would have thick chrome and nickel build-ups with chrome caps has started to develop. This requirement has been identified for high strength steel landing gear applications and will follow most likely in similar aircraft applications such as flap and slat tracks, actuators and other aircraft support structure. This paper will discuss the development and qualification requirements necessary, and related concerns in order to replace these thick repair coating applications with duplex coatings, or possibly with thick single coating applications that will meet the original design criteria for each part.

The Airbus A380 program marked the Goodrich Landing Gear introduction of High Velocity Oxygen Fuel (HVOF) applied tungsten carbide cobalt chrome (WC-Co-Cr) coating as a replacement coating for electrolytic hard chrome. HVOF is a new coating technology when applied to aircraft landing gear so specifications for coating application, finishing, powder and supplier qualification were developed to reflect the unique function of landing gear components. Since the materials, sizes and shapes of landing gear components are dissimilar to other aerospace parts currently HVOF sprayed, the capabilities of each spraying and grinding supplier needed careful assessment. Both suppliers and internal customers required training on the requirements specific to landing gear. This paper will discuss the development of HVOF specifications specific to aircraft landing gear, the methods developed for qualifying HVOF suppliers, and some challenges encountered when introducing HVOF-applied WC-Co-Cr as a hard chrome replacement.

Replacement of Electrolytic Hard Chrome plating (EHC) using High Velocity Oxygen Fuel (HVOF) thermal spray processes to apply dense, wear and corrosion resistant coatings has been identified as a significant goal for the advancement and growth of the thermal spray industry for several years. This paper will provide some details of the development and production implementation of a WC-Co-Cr coating to be used as an alternative to EHC. WC-Co-Cr has been demonstrated in most situations to be superior to the wear performance of EHC with respect to providing functionality in aircraft landing gear hydraulic seal and bearing service. The developed coating is produced via the HVOF process using natural gas as fuel. EHC alternatives using WC-Co-Cr HVOF coatings require significant development prior to implementation into practice. Suitable wear, corrosion and lack of fatigue debit are examples of a few critical properties of coatings for use in landing gear applications. In addition a brief description of the production implementation of a turnkey spray cell for the coating of landing gears will be provided in this paper.

A commercial air carrier has replaced the chrome plating on line-of-sight landing gear installed on several aircraft with HVOF sprayed WC-Co-Cr coatings. Pending the results of ongoing testing and service evaluations, the carrier plans to switch to HVOF coatings on all line-of-site landing gear and possibly other structural and engine components. This paper discusses the infrastructure that will be necessary to build, qualify, and maintain a high-velocity oxyfuel spraying facility that will serve one of the largest fleets of aircraft in the world. Paper includes a German-language abstract.

One candidate alternative to chrome plating and hard anodizing is a tungsten carbide (WC) coating applied by the High Velocity Oxy-Fuel (HVOF) process. HVOF WC coatings are currently being evaluated in many service life tests, including fatigue. The purpose of this paper is to compare the fatigue life of HVOF WC coated specimens with the fatigue life of hard anodized and bare aluminum specimens. This work examines WC thermal spray coatings as candidates for replacement of hard chrome plating and hard anodizing in aircraft and helicopter applications such as landing gear. In fatigue testing, the results showed an expected fatigue deficit for hard anodizing as compared to bare aluminum. Paper includes a German-language abstract.

Environmental constraints on exposure to hexavalent chrome and its compounds has prompted increased utilization of HVOF thermal spray coating technology to implement new overhaul repairs for components that are currently chrome plated. Proposed 1998 EPA and OSHA standards place very tight restrictions on the Personal Exposure Limit (PEL) for hexavalent chrome, which will impact chrome plating operations and most likely attendant grinding operations. Historically, HVOF or D-gun type repairs involved bearing bores, journals on main shafts, mid span supports, dimensional restorations and clearance control applications predominantly on gas turbine engines. Currently, the Naval Aviation Depot Jacksonville is further developing HVOF capabilities for repairing all P-3 aircraft hydraulic component piston rods, landing gear piston struts, and landing gear axle journals with a tungsten carbide 17% cobalt coating in lieu of chrome plating. Once implemented, the repair technology will be transferred to other Naval aircraft components beginning with F14, F/A-18, EA-6B, E-6, and will eventually reduce personnel exposure to chrome and reduce environmental disposal costs for chrome plating waste by two-thirds. This paper provides a case study of the development and implementation of a chrome plating replacement for the P-3 aircraft including fatigue, corrosion, and seal wear testing, as well as HVOF parameter optimization development.

Tungsten carbide thermal spray coatings have been used for more than twenty years in the commercial aircraft industry in applications such as turbine blade and flap-track wear surfaces. Additionally, the evaluation of tungsten carbide (WC) coatings to replace chrome plating in other aircraft applications has been underway for several years. For example, WC coatings applied by the high velocity, oxy-fuel (HVOF) process are being evaluated for use on aircraft landing gear parts. One factor that affects the suitability of WC coatings is the fatigue life of the coated part. This study compares the fatigue life of electrodeposited chrome plated specimens to the fatigue life of WC HVOF-sprayed specimens on aircraft landing gear alloys. Fatigue tests were run on cantilever flat beam specimens coated on one side and subjected to bending fatigue loads. Residual stress levels for the coatings were determined using the Modified Layer Removal Method on rectangular residual stress specimens processed with the flat beam specimens. Also, the Young's modulus and Poisson's ratio of the coating were determined using the Cantilever Beam Bending Method performed on beam specimens that were processed with the fatigue specimens and the residual stress specimens. Results indicate that certain levels of residual stress in the coating can enhance the fatigue life of the parts. The fatigue lives in bending tests of several WC coated specimens are compared with the fatigue life of chrome plated specimens.