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.

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.

This study deals with the influence of spray angle on the deposition of cold-sprayed Al particles. Spray trails were conducted in parallel with finite element simulations of particle deformation and coating build-up as a function of spray angle, powder size, substrate roughness, and surface configuration. Coating cross-sections and splats were examined by SEM; bonding strength and particle adhesion were determined via laser shock adhesion testing. Experimental as well as modeling results show that splats deposited at spray angles less than 60° are highly deformed and poorly adhered. Based on the findings, several conclusions are drawn with regard to the potential use of cold spraying for the repair of aircraft components.

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.

Wear and corrosion of aircraft actuator parts and helicopter gearbox rotating shafts and also wear of seals working against these parts can lead to oil leaks, which require expensive maintenance and increase the risk of failure. The Hardide nanostructured Chemical Vapor Deposition (CVD) Tungsten Carbide coating can help reduce oil leakage from gearboxes and actuators and increase maintenance intervals. The coating protects metal pistons and shafts from abrasion and corrosion, keeping their surface roughness parameters within optimum ranges for longer; this reduces seal wear and makes the whole unit more durable and reliable. The 50-100 microns thick CVD Hardide coating can be applied uniformly on internal and external surfaces and has enhanced fatigue and anti-galling properties. The coating has enhanced wear resistance outperforming Hard Chrome by 14 times and Thermal Spray WC-Co (12%) by 3 times. The fine-grain coating nanostructure wears uniformly so even worn Hardide coating shows no hard micro-grain asperities which are abrasive for seals. The coating is free from porosity and from Cobalt binder and is an excellent barrier against corrosion. As a result the coating keeps the optimal seal-friendly surface finish for longer even in abrasive and corrosive environments. The coating was qualified by Airbus as an environmentally-friendly replacement for Hard Chrome plating.

The recent development of cold spray technology has made possible the deposition of low porosity and oxide-free coatings with good adhesion and with almost no microstructure change of the coated parts. This work focuses on the performance of low pressure cold spray (LPCS) in repairing damaged Al-based thin aircraft skin. The coating quality is investigated through the evaluation of microstructure, microhardness, adhesion strength, surface finish and corrosion resistance of the coatings.

High-velocity oxyfuel (HVOF) spraying is a viable alternative for hard chrome plating because of its excellent coating properties and substantially lower environmental pollution. Initial applications in ball valves, journals, and hydraulic cylinders on earthmoving equipment have proven successful. This paper discusses the advantages and disadvantages of HVOF spraying as an alternative to hard chrome plating for aircraft components and presents some of the early results of laboratory and in-service testing.

Arc spray systems are increasingly used in the overhaul of aircraft engine components and auxiliary power units. The increasing use of arc spray over plasma for metallic coatings has created a demand for new wire approvals. The chemistry is already established as a powder and it is a matter of conversion to a wire and the arc spray process. The increasing popularity of the arc spray process is due to its superior bond strength and microstructure that exceed those of plasma. In one case, there is a two and one-half percent porosity requirement for the arc spray and up to 15% is allowed for plasma. This density approaches HVOF quality requirements. This paper will discuss some historical background of the process, what is approved and then move on to the new materials that are submitted for approval. Microstructures and bond strengths will be presented and some information about a proprietary method to solve a coating problem in the aircraft overhaul industry of long standing. The paper will also discuss new advances in arc spray systems and materials, which makes these systems amenable to replacing plasma sprayed coatings.

Tungsten Carbide (WC) thermal spray coatings have had increased acceptance in commercial aircraft applications driven by the desire to replace chromium electroplate due to environmental and economic considerations [1]. In order to confidently replace chromium electroplate by WC thermal spray coatings, evaluation of wear and fatigue characteristics of the WC thermal spray coatings is necessary. For WC thermal spray coatings to replace chromium electroplate in aircraft applications, the coatings must demonstrate wear and fatigue characteristics as good as or better than those of chrome plating. Previous research in this area has shown that the fatigue life of the WC thermal spray coatings can be improved by inducing compressive stresses in the coating. This paper compares the wear characteristics of several types of WC thermal spray coatings with those of chromium electroplate in sliding wear tests following the "block-on-ring" procedures described in ASTM G77 standard. Wear results are interpreted in terms of coating residual stresses and in terms of X-ray diffraction and Scanning Electron Microscope (SEM) analysis.

This study assesses the viability of using nitrogen instead of helium to cold spray NiCoCrAlTaY coatings onto single-crystal superalloy substrates. The process, though feasible, has a low deposition efficiency, leading to a high level of deformation that affects the microstructure of both the coating and substrate. SEM and TEM analysis revealed metallurgical and mechanical bonding at the interface and grain refinement in the coating. A fine grain structure that developed in the substrate after deposition was also observed possibly caused by dynamic recrystallization during the deposition process. Evidence of element segregation in the substrate, identifiable as zones with a deformed γ/γ’ structure, was found as well.

Many components in helicopter dynamics systems depend on hard, wear resistant coatings for reliable performance. Component replacement times are currently limited by performance of these coatings in many cases. Thermal spray coatings have been evaluated for these applications to replace nickel and chromium electroplate. The effect of coatings on fatigue strength is quantified by a strain limit concept to reduce test requirements during development phase. Full scale bench testing and coupon wear testing were conducted to reveal large improvement in sliding and fretting wear resistance. A fine porosity network in thermal spray coatings, high hardness and high fatigue strength are shown to be the main contributors to improved wear performance. Thermal spray coatings also exhibited enhanced corrosion resistance in salt fog and crevice corrosion conditions. These data permitted implementation on several critical helicopter components.

Tungsten caibide (WC) thermal spray coatings are being used for wear protection on selected components of aircraft. Tungsten carbide coatings are being used on aircraft flap tracks and fan and compressor blade mid-span dampers. However, a larger use of tungsten carbide coatings is being considered for other commercial aircraft applications where it would be used as a replacement for chrome plating. For instance, WC coatings are currently being tested on aircraft landing gear parts. One factor that affects the suitability of WC coatings for these applications is the fatigue life of the coated part. Coatings, whether chrome plating or thermal spray coating, can reduce the fatigue life of the part compared to an uncoated part. This study compares the fatigue life of uncoated 6061 aluminum specimens to the fatigue life of WC thermal sprayed coated 6061 aluminum specimens. The relation between the residual stress level in the coating and the fatigue life of the specimens is also investigated. Fatigue tests were run on cantilever flat beam specimens that were coated on one side. Specimens were cycled in bending so that the coatings experienced tensile fatigue stresses. Residual stress levels for each type of coating were determined using the Modified Layer Removal Method on specimens processed along with the cantilever flat beam specimens. Test results show that the fatigue life of the WC coated specimens is directly related to the level of compressive residual stress in the coating.

Within the Aircraft MRO (Maintenance, Repair and Overhaul) business KLM Royal Dutch Airlines Engineering & Maintenance Division has been involved with Thermal Spraying since the late 60’s. The latest procurement on thermal spraying is the state of the art light weight electric arc spray gun (the EM-14) for the closed-loop electric arc spray system of Praxair TAFA called the CoArc. With this new piece of equipment KLM is up-to-date in the high-tech approach of applying thermal spray coatings on aircraft engines with a electrical twin-wire arc spray system. Close teamwork between an end-user in aircraft industry (KLM) and a thermal spray supplier (Praxair) resulted in a new state of the art arc spray gun. This paper shows the first spray results with the EM-14 gun.

The repair of damaged Ion Vapor Deposition Aluminum coatings on high strength steel aircraft components has generally required the use of brush plating with hazardous materials including cadmium. Inovati has developed a unique Al-Trans (aluminum-transition metal) coating using the Kinetic Metallization process that permits repairs of IVD-Al coatings on high strength steels. Originally the Al-Trans coating formulation was developed for commercial application on telecommunication equipment steel racks as an electrically conductive grounding strip with excellent corrosion resistance. Recent research was completed with NAVAIR to further develop this coating formulation and the Kinetic Metallization process for repair of IVD-Al coatings on aircraft components. This presentation will describe the KM repair process and the tests completed to qualify the repaired coatings. Inovati has recently developed a KM-Mobile Coating System with a handheld Spray Gun for the field repair of corrosion damaged magnesium and aluminum alloy aircraft components.

KLM Royal Dutch Airlines has the oldest Maintenance, Repair and Overhaul (MRO) shop for aircraft in the world (more than 80 years old). The company’s philosophy in all areas, including thermal spray, has always been to pursue the latest innovations and to maintain well-equipped facilities with state-of-the-art processes, equipment and materials for performing repairs. In the area of thermal spray, the latest procurement is a state-of-the-art unique cabinet with a high enthalpy plasma spray process. This new cabinet and new plasma spray process provides up-to-date technology for applying thermal spray coatings on aircraft engines. In this paper, experiences and the latest results obtained using this system to deposit various types of coatings used on aircraft will be reported.

Abradable seals are used in compressors of aircraft and industrial gas turbines to decrease clearance between the stator casing and rotor blade tips and hence to increase compressor efficiency and decrease fuel consumption. The main interest of abradable materials producers has been concentrated on abradable seals for aircraft engines, and special requirements of industrial gas turbine manufacturers have not been met so far. The most significant requirement in industrial gas turbines is durability. This is driven by the need for several times longer periods between overhauls in industrial gas turbines compared to aircraft engines. Westaim Ambeon has developed a new composite powder, Durabrade2413, that meets these requirements. The new abradable seals fabricated by using this powder have been extensively tested over a prolonged period of time. This paper will present the results of an intensive development, evaluation and abradability testing of seal properties. This paper will also show that Durabrade2413 series coating properties can be altered in a broad range by changing spray parameters to tailor the coating to a particular application. The abradable seals are suitable to rub against steel and Ni alloy blades. The abradability results of Durabrade2413 are compared to Durabrade2222 (the Metco 307-NS equivalent), the well known 75Ni25 Graphite abradable that has been on the market for the last 30 years.

The US Navy has adopted High-Pressure Cold Spray (HPCS) as a repair technique for corroded and worn components in their fleet of aircrafts, ships, and submarines. HPCS repairs are not only used for depositing corrosion and wear resistant coatings but is being successfully used for dimensional restoration in metal parts and components. By utilizing HPCS, the Navy ensures the longevity and reliability of critical components, even in harsh environments. Whether safeguarding against corrosion or restoring worn parts, HPCS is playing an increasingly critical role in maintaining operational readiness for the US Navy and other DOD agencies, as well as the commercial industry.

The growth of data generated within thermal spraying is, for many, a daunting business. Yet, this growing resource represents a largely untapped and potentially valuable asset capable of providing “knowledge” rather than just “information”. Many companies already use a range of Web based tools. However, the Web itself is changing and the vision for the future, the “Semantic Web”, is set to revolutionise how business will be done. One important aspect of this Web “future” is that web pages will be greatly enriched and data will have additional information (tags) which help to describe it and more significantly, put the data into a context. This will enable machine readability and the use of query languages to ask direct questions. Following on from ideas introduced at ITSC 2007, a proof of concept demonstrator has been built for thermal spray coatings used in the Maintenance Repair and Overhaul (MRO) of gas turbines. A system has been built which stores and manipulates a range of data including; aircraft deliveries, RSS feeds of aircraft sales, engine types, MRO business details, thermal spray coatings and market dynamics. This paper presents the development of this system and discusses its future potential.

The HVOF sprayed WC-CoCr coatings are widely spread due to their excellent resistance against wear and corrosion. These coatings are one of the most suitable alternatives for hard chromium in many applications. Within the research project, the most suitable hard chromium alternative for hydraulic devices in aircraft is being developed and tested. This application is highly demanding not only on the functional properties of applied coatings but also on the surface quality. Grinding and polishing of the coating are not sufficient, to achieve the necessary surface properties. This study aims to optimize the superfinishing process of HVOF sprayed WC-CoCr coating. The achieved surface quality is primarily measured using profilometry. With optimized surface preparation, the tested parts for aircraft hydraulic parts are treated and tested for leakage of operating fluids and high cyclic lifespan.