ICMCTF2012 Session G1-1: Innovations in Surface Coatings and Treatments

In aero engines, coatings are facing severe attack under multiple loading conditions. Sand erosion, e.g., can cause great damage to turbine hardware in the compressor, while hot corrosion and oxidation are of concern in the hotter parts of the engines. Today, coatings are widely applied to protect high pressure turbine airfoils, however, their use in the compressor and the low pressure turbine is scarce yet.

The paper will review recent developments in the field of erosion protection of aerospace alloys such as titanium and nickel alloys indicating that coatings can substantially improve the component lifetimes under erosion attack. Moreover, examples of coatings for intermetallic titanium aluminide alloys will be addressed. These alloys are the latest aerospace materials brought into service by General Electric in their GEnx for stage 6 and 7 of the low pressure turbine. Yet unprotected today, future application of titanium aluminides at even hotter temperatures will require oxidation and potentially heat protection. Therefore, considerable research efforts are underway to develop coating systems including thermal barrier coatings which will be highlighted in this paper as well.

Recent Iceland Volcanic action (known as Eyjafjallajokul volcano) posed a serious threat to jet engine reliability over European sky. Any solid particles sucked into jet engine system are likely to hit the compressor blade at high velocity with various angles. Erosion of the compressor blade leads to change of blade shape and consequence is loss of aero dynamical integrity and engine efficiency. Erosion resistant coatings such as TiN, TiAlN applied by PVD processes can be used to improve the life time of compressor blade. Kobe Steel is developing a new AIP cathode (SFC) which can reduce the residual stress of the coating substantially, make it possible to deposit a thick coating.

In the present investigation solid particle erosion resistance of several nitride coatings including TiAlN deposited by SFC with different thicknesses has been evaluated by using the sand blast test equipment. The sand blast tests have been done by using comparatively large size solid particles as an erodent. To understand the effect of the film thickness to the large particle erosion resistance, TiAlN films with several 10um thicknesses have been tested by using ave.diameter 190um-silica sand as the erodent with approximately 100m/s air velocity on the test surface and 90deg impact angle. As a result, doubling the film thickness improved the sand blast erosion resistance of the coating by more than 10 times.

Hardide is a new family of nano-structured CVD Tungsten Carbide/Tungsten coatings used to increase the life of critical parts and tools operating in abrasive, erosive and chemically aggressive environments.

The Hardide coatings consist of Tungsten Carbide nano-particles dispersed in metal Tungsten matrix. This structure gives a combination of ultra-hardness with excellent toughness, crack and impact resistance. From extensive laboratory and field testing it was found that the combination of sufficient hardness with enhanced toughness achieves the optimum protection against both wear and erosion in most applications. The coating’s ultra-hardness inhibits the micro-cutting mechanisms of wear and erosion, while its toughness, ductility, residual compressive stresses and homogeneous micro-structure prevent fatigue micro-cracking/chipping and platelet mechanisms of erosion.

Hardide coatings are typically 50 microns thick – exceptionally thick among hard CVD coatings – tough and ductile to withstand 3000 micro-strain deformations without damage; this deformation will crack or chip most other thick hard coatings.

The company developed a low-temperature CVD technology to deposit the coatings at around 500^oC. This enables the coating of a wide range of materials: stainless steel, tool steels stable at 500^oC, Ni-, Cu-, and Co-based alloys, Titanium. The coating has a strong metallurgical adhesion to these substrates, with the bond strength typically exceeding 70 MPa.

The gas-phase CVD process enables the uniform coating of external and internal surfaces and complex shapes, such as valves, pump cylinders ID and extrusion dies.

Pore-free CVD coatings resist acids and aggressive media.

Among other hard coatings Hardide fills the gap between thin film PVD and CVD coatings, and much thicker rough and non-uniform thermal spray coatings. Compared to thin PVD/CVD coatings, 50-microns thick Hardide has higher load-bearing capacity and is much more durable in abrasive and erosive applications such as oil drilling tools. Unlike thermal spray coatings, Hardide gas-phase CVD coatings can be uniformly applied to internal surfaces and complex shapes.

Hardide is an attractive replacement for Hard Chrome plating, which is under pressure from the US OSHA and the EU REACH regulations, and is especially suitable for coating complex shapes and internal surfaces.

In recent years, electrophoretic deposition (EPD) process has been envisaged as one of the convenient, low temperature and cost effective solution-based techniques that produce carbon nanotube (CNT) thin films on virtually any substrate. Some of the crucial application fields of electrophoretically deposited carbon nanotube films are micro-electronics and microelectromechanical systems (MEMS) technologies where silicon substrates are used predominantly. However, the research and fabrication trend of EPD of carbon nanotubes has been, so far, focused mostly on conductive/metal substrates such as stainless steel, aluminum, nickel, titanium and ITO (indium tin oxide)-coated glass plates. Published reports on carbon nanotube coatings deposited by EPD on silicon substrates are relatively few and thus it offers an interesting thin film research subject to explore. In this study, EPD has been performed extensively to obtain appreciable deposits of carbon nanotubes on silicon substrates with various surface coatings. The process resulted in CNT film thickness up to ~15 µm on metal-coated silicon samples from aqueous suspensions. In addition, successful attempts in selective deposition and characterization of CNT thin films by the subsequent EPD experiments on patterned metals atop insulating layers like silicon dioxide and silicon nitride show compatibility of this process with conventional silicon processes. Interesting phenomenon of agglomeration of carbon nanotubes and subsequent degradation of the CNT dispersed medium during the EPD process has also been observed. The deposited nanotubes exhibited preferential deposition and adhesion only on the metal surfaces even when the DC voltage was supplied to the silicon substrate which was electrically isolated from the metal layer. The observed deposition and adhesion of CNT films on the conducting surfaces is attributed to both electrophoretic mobility of the charged CNTs in the suspension and the hydrophilic interaction on the target surface. Deposition of the nanotubes was confirmed by scanning electron microscopy and Raman spectroscopy. Thickness of the deposited film showed a trend of linear relationship to the electric field strength and the deposition duration.The results present great potential of CNT films for micro-electronics and MEMS applications.

Military aircraft must operate in significantly more demanding environments and are often required to continue in service much longer than commercial aircraft. Readiness and life-cycle costs associated with maintenance are critical issues associated with weapons systems. This presentation will provide information on the implementation of three different inorganic coatings technologies that are having a major impact on performance and cost reduction: (1) HVOF thermal spray coatings, (2) Cathodic arc PVD coatings, and (3) Cold spray coatings.

The Department of Defense conducted extensive studies to qualify high-velocity oxygen-fuel (HVOF) WC/Co or WC/CoCr thermal spray coatings as a technologically superior, cost-effective alternative to electrolytic hard chromium (EHC) plating which is widely used in manufacturing and repair of aircraft components. EHC plating uses chemicals containing hexavalent chromium, a known carcinogen. Because of the extensive use of EHC on aircraft, separate efforts were undertaken to qualify HVOF coatings on different categories of components including landing gear and engine components. Results of materials and flight testing to qualify the HVOF coatings will be presented. The Air Force is now implementing HVOF coatings on most of its landing gear and they are designed for the newest aircraft, the Joint Strike Fighter.

During aircraft operation, gas turbine engines are continuously exposed to erosive media, such as sand and dirt suspended in the air, that is extremely damaging to the compressor section of the engine, leading to reduced performance and increased fuel consumption. Cathodic arc PVD coatings consisting of a multi-layer ceramic metal matrix , developed by MDS Coatings Technologies Corp., have been implemented on compressor airfoils in the U.S. Marine Corps H-53 and CH-46 helicopters. These coatings have accrued over 1 million operational hours in desert environments, increasing engine reliability and lowering costs.

Research efforts in cold spray (CS) have shown it to be a promising technology to impart surface protection to Mg and other alloy components on helicopters and fixed-wing aircraft. Applications have been developed by ARL, implemented into production, and have been incorporated into such weapon systems as the B1 bomber and the UH-60 Blackhawk helicopter. For the latter application, CS is in the process of qualification by Sikorsky Aircraft Co. for use on the UH-60 to reclaim Mg components. The CS repair has been shown to have superior performance, can be incorporated into production, and has been modified for field repair, making it a feasible method for recovering components, thereby reducing cost.