Aero engine coatings – an overview

Citation

(2005), "Aero engine coatings – an overview", Aircraft Engineering and Aerospace Technology, Vol. 77 No. 5. https://doi.org/10.1108/aeat.2005.12777eaf.002

Aero engine coatings – an overview

Aero engine coatings – an overview

Keywords: Aircraft engines, Coatings technology

Indestructible Paint Ltd, Birmingham, have been a supplier of surface coatings to the aerospace industry for more than 25 years.

Specialising mainly, but not entirely, in coatings for use on the inside and outside components of gas turbine engines and their associated parts the company also produces a number of airframe coatings. It now supplies in excess of 200 diverse products to the aerospace market.

In this feature Brian Norton, Indestructible Paint's Managing Director, discusses the technical requirements for surface coatings for this international market and some of the relevant materials produced by his company.

Some of the toughest demands on surface coatings encountered anywhere are in the harsh environment that exists on the inside and, to some extent, on the external parts of gas turbine aircraft engines. Existing in a very corrosive environment, engine coatings have to resist high temperatures – sometimes above 600°C – and stand up to chemicals such as ester lubricants and hydraulic fluids.

Trends in design that affect the formulation of new surface coatings include an ongoing requirement among engine manufacturers for higher efficiency and lightness. From the point of view of the coating manufacturer, these two areas manifest themselves in higher operating temperatures and the increasing use of magnesium on some external components, respectively. As a result, when new engines are designed, new or improved coatings with increased performance are usually required.

Easier or reduced engine maintenance is another potential area for cost savings. A recently developed “surface coating”, in the form of a special dry film lubricant, is relevant to this and is described later. In addition, there is an increasing need for more environmentally friendly paints and coatings of all types. Although chromate free and low VOC coatings have been a part of the scene for some time, legislative requirements are becoming more stringent and “no VOC” coatings will soon be the norm.

These and other factors, including product cost, have steadily increased pressure on the development facilities of the coatings industry. As a result, manufacturers have had to produce significant numbers of new and innovative coatings during the last decade or so. These include inorganic surface coatings with very high corrosion, chemical and heat resistance that also provide a film with high resistance to physical damage. “Traditional” organic product types, however, are still used in quantity on engines, many having been developed to the limit. No doubt some of these will continue to be used for some time.

Although Indestructible Paint's aerospace products protect and/or decorate a multitude of airframe parts, its main thrust in aircraft coating technology is the provision of materials for jet engines and their associated components. Operating at the high tech end of the aerospace and industrial markets Indestructible Paint's many customers for aero engine coatings include Rolls-Royce, BAC, Pratt & Whitney, Sikorsky, the Royal Air Force and several civil airlines. The aircraft industry is the company's largest outlet but it also provides paint and surface coatings to the general industrial and marine sectors. However, many of the products supplied to these markets are derivatives of the company's aerospace materials.

To meet demand in North America, especially for coatings to protect magnesium, Indestructible Paint Inc., was established at Milford Connecticut in the 1990s and now distributes coatings throughout the USA and Canada. Used in many diverse applications, the company's engine coatings include both organic and inorganic materials. Inorganic coating materials are a necessity for interior parts of engines as organic paints and coatings cannot stand-up to this high temperature environment.

A few examples of components requiring special coatings include turbine blades, helicopter gearbox housings, heat exchangers, engine covers, shafts, jet engine air intakes and thrust nozzles. These products may be made from a variety of materials including steel, aluminium, magnesium, nickel and titanium metals. They also include metal composites, carbon fibre, honeycomb and ceramics.

Some examples of the types of coatings produced by Indestructible Paint to protect these diverse substrates involved are described below.

High performance inorganic coatings

Ipcote products are a range of specialised inorganic aluminium/silicate ceramic materials.

Introduced in 1994, Ipcote 9183 was designed to provide sacrificial corrosion protection and erosion resistance at the increasingly high temperatures required for greater engine efficiency. It currently operates at 500-600°C on engine rotor blades – an environment that makes conventional organic paints simply disappear – but has the capability of providing cathodic protection at service temperatures as high as 700°C (Plate 3).

Plate 3 EGY1-Turbine blades coated in Ipcote 9183

To obtain maximum corrosion protection it is necessary for the surface of Ipcote to be electrically conductive and to this end it contains particulate metal. Surface conductivity is maximised in one of the two ways – the paint film can either be processed after application, at 540°C, or, alternatively, processed to 350°C and then vibro polished. Polishing of Ipcote on turbine blades also has the benefit of improving gas flow through the engine resulting in greater efficiency.

One of only two ranges of this type of coating available anywhere in the world, Ipcote products are employed on Rolls-Royce and Pratt & Whitney jet engines. Other applications include its use on the thrust guide vanes of the Harrier jump jet. It is also used on the turbine blades of the same aircraft's Rolls-Royce Adour engine.

Exhibiting exceptionally high corrosion resistance, Ipcote 9183 has been salt spray tested to 3,000 h (ASTM B117) on mild steel. It is also completely impervious to organic solvents, even at the elevated operating temperatures prevalent in aero engines. With this level of performance the product obviously has uses in many other types of application outside the aerospace industry.

PL134 is another ceramic coating used to protect combustor nozzles on jet engines. These suffer from an effect called “green rot” that occurs in nickel-based alloys at high temperature if not suitably protected. Providing effective protection for this, PL134 is cured initially at 1,020°C and functions at operating temperatures up to and including 850°C. The coating, although extremely hard, is reasonably flexible.

High temperature resistant inorganic marking paints

Less exotic, but nevertheless quite important, are high temperature and chemical resistant engine marking paints. Typical of the many uses for these materials was a requirement by the RAF to distinguish a new series of Rolls-Royce Adour engines from a previous series. The example, supplied by Indestructible Paint for this purpose, was PL150A. PL150A had to be green in colour and permanently resistant to operating temperatures of 500°C. It also had to resist hot phosphate ester lubricants – notorious as very effective paint removers.

Dry film lubricants

PL470. In complete contrast, PL470 is a dry film lubricant specially designed for fast on-wing overhaul/repair work on aircraft engines.

Indestructible Paint has, for many years, provided a range of dry film lubricants for use on aircraft. Many of these are based on graphite or molybdenum disulphide, while others use far more exotic materials. A new product in this range, referred to above, is PL470. Recently developed by Indestructible Paint in conjunction with major aero engine manufacturers, this material enables certain critical maintenance operations to be carried out without removal of engines from the aircraft. This facility results in a significant reduction in maintenance costs.

“Conventional” organic coatings

IP714/715. Designed for engine casings and components, IP714/715 is a more conventional organic coating system formulated to meet the current and future environmental legislation. It now replaces previous engine coatings supplied by Indestructible Paint for engine casings. The system comprises IP714, a high corrosion resistant chromate free primer and IP715, an aluminium based heat-resisting topcoat. Both materials are low VOC 2-pack epoxy based coatings containing no toluene or xylene solvents or chromate pigments. Approved for use on aluminium, magnesium and steel, the system is used by OEMs Pratt & Whitney Canada and Airbus and is awaiting final approval by other American aero engine manufacturers.

Rockhard. Indestructible's Rockhard coatings are a good example of a long established range of conventional materials that have been extensively developed to meet modern requirements over the years. Rockhard coatings are very high corrosion and chemical resistant epoxy phenolic materials originally formulated for the magnesium skins of Westland helicopters and hovercraft used in the marine environment. They are now available as either stoving or two-pack, cold cure, materials (Plate 4).

Plate 4 EGY3-Sikorsky helicopter gearbox castings coated in Rockhard epoxy phenolic

The current range includes a number of high corrosion resistant products including primers, topcoats and various single coat materials – either pigmented or unpigmented – for a multitude of applications. A good example of these applications is their use on magnesium. Used increasingly in aircraft components because of its lightness, magnesium is very reactive and therefore, prone to corrosion. Surface coatings for this metal have to provide excellent corrosion resistance and be as light as possible. For the latter reason they are preferably used as single coat products.

Providing excellent corrosion resistance, Rockhard 985-111-800, used in conjunction with a heavy film anodic coating, provides the necessary protection for the magnesium gearbox housings used in Sikorsky Blackhawk helicopters. An extremely hard coating, Rockhard 985-111-800 is also resistant to humidity and chemicals including solvents, and can easily withstand anodizing or re-chromating processes.

Rockhard coatings are also used to protect heat exchangers, fuel pump diaphragms and many other engine parts as well as number of airframe components.

Attrition compounds

Another interesting product used on various aero engines including that of the Harrier AV8B, is a two-part graphite based attrition compound. This material, supplied in the form of a two-pack dough or filler, is applied to the inside of engine casings to provide a seal between the casing and the turbine blades. After curing it is machined to provide an exact fit. Slight contact between the rotating blades and compound “wears” the compound away to provide a perfect fit.

Future developments in engine coatings are not easy to forecast but one major area, protection of the environment, will affect the development of all future surface coatings. New materials, as they become available, will also change the performance characteristics of new coatings bringing technical, and therefore, economic benefits. Nano technology is a good example of this.

The use of ceramic nano (sub-micron) particles, a relatively new advance in surface coatings has only taken place in recent years but a number of coatings of various types utilising the technology are now available. When used in combination with the organic paint resins common to conventional paints, these tiny particles are invisible and do not affect gloss or form any settlement. They are normally used to reinforce the coating film and provide hardness and wear resistance of a very much higher order than can otherwise be obtained with conventional organic coatings. Often described as semi-organic, these materials represent another advance in coating technology that has great potential for aero engine coatings. More information on this will be available in the near future.

Details available from: Indestructible Paint Co. Ltd. Tel: +44 (0) 121 702 2485.