The expanding role of coatings

Aviation is a highly competitive business with operators seeking to maximise their return on their investment by achieving greater time on-wing and reductions in both fuel cost and workshop bills. This in turn leads to the use of lighter, stronger, more resilient materials and with them a new set of manufacturing challenges and in-service performance requirements.
Overlaying these technical and economic considerations is a set of pressing environmental concerns. Fuel cost is not the driving issue that it was, but it would be unwise to assume that the current oversupply is anything but temporary.

Notwithstanding this, the urgent need to reduce CO2 emissions still remains. There are also environmental concerns about the hazardous wastes arising from some traditional hardening and finishing processes.

Which process provides the best technical, commercial and environmental solution depends on many factors. As a supplier of both heat treatments and a wide spectrum of hard coatings, Wallwork provide access to multiple surface treatments including new and hybrid technologies. These are provided in factories that have extensive research and development facilities, quality assurance laboratory back-up and independent accreditations from Nadcap, aerospace industry primes, and other national and international bodies.

Cutting composites

Carbon fibre reinforced plastic (CFRP) composites have revolutionised the manufacture of aerostructures, but cutting this material or stacks combining CFRP with aluminium or titanium is problematic. Tooling can wear quickly, requiring frequent downtime for changes and so undermining productivity. The heat generated can cause tooling to smear the resins, rapid wear can create ragged cuts and in some cases result in delamination to undermine quality and safe performance.

The answer lies in diamond-coated carbide tooling. Though we have an extensive capability in all forms of carbon coating from highly lubricious graphite to resilient diamond-like coating (DLC), we chose to partner with a world leader in this field, CemeCon, for whom we are the only official UK service provider. This allows us to offer a complete family of industrial diamond coatings for tools used in the machining of standard composites and stacks with aluminium and titanium within the structure.

Diamond-coated tooling has alternate layers of crystalline and nano-crystalline carbon within a micro thin layer of smooth even coating that conforms closely to the profile of the tool. These coatings have good surface adhesion, outstanding hardness and good thermal conductivity. Staying sharper for longer means fewer tool changes, longer runs and higher productivity in addition to cleaner cutting, higher yields and improved quality.

Prolonging engine efficiency

Maintaining aerofoil geometry is crucial to sustained engine performance. In operation blades are subject to both erosive forces of airflow and contamination. This leads to surface pitting and a loss of material in some areas of the blade and the accumulation of encrustation in others.

Nitron Flight, a family of multi-layer coatings based on titanium, chromium, chromium/aluminium or carbon/metal carbide, overcomes these problems. The coating may be applied in multilayer form making a surface that complies closely with the polished substrate. This multi-layering creates a low stress surface coating that is more resilient to delamination than a single layer coating of the same thickness.

The chromium-based variation of this coating has been extensively proven with a major international maintenance overhaul and repair contractor on compressor blades in long term side-by-side engine trials on the same aircraft.

Trials have shown that the surface coating maintains the blades optimum smoothness and aerofoil profile, so resisting the erosive effects of harsh contaminants in the airstream. Optimum fuel consumption is sustained longer and the interval required between overhaul is further extended to improve time on the wing and return on engine ownership. As untreated blades eroded they were more prone to encrustation that was difficult to remove. Surfaces on treated blades were also subject to some scale build-up, but were more easily descaled because the surface coating prevented firm adhesion.

The benefits of titanium

Widely used as an alternative to steel and other dense metals, titanium weighs less, has greater strength, good fatigue resistance and natural resistance to corrosion. The material unfortunately also has a very poor coefficient of friction due to low hardness. Without surface treatment this can cause adhesion and galling when used in load bearing parts such as landing gear bearings, creating a need for more frequent maintenance attention.

In lightly loaded situations this problem can be addressed by the application by physical vapour deposition (PVD) of titanium or chromium nitride to aid lubricity. This is insufficient, however, in situations where components are subject to frequent heavy loading. Here the single layer treatment will have a limited life due to the instability of the substrate, a situation known as the choc-ice effect.

To answer this problem a duplex coating is required. The substrate is first hardened by the diffusion of nitrogen into the surface of the titanium to create deep hardened cases. On top of this a lubricious PVD surface coating is applied. Both hardening and final coating are achieved in a single process cycle.

The R&D department at Wallwork Cambridge work closely with aerospace majors, leading universities and research organisations to both develop and validate coatings. The department is equipped to a standard equal to that found in the material science departments of the universities themselves.

Research work undertaken is varied and includes, evaluation of the surface topography and wear rate of coated surfaces, establishing the wear rates of different coatings, failure analysis, measurement of coating thickness on components with complex geometry and the measurement of the depth of hardened layers in substrates. This research leads to the development of better coatings, improved production techniques and provides key data to pre-qualify new processes before trials in the field.

The factory extension at Cambridge will provide more space to expand current operations and develop new processes. While Cambridge remains the centre of excellence in coating, the company has also invested heavily in facilities in Manchester and Birmingham to continue to extend services in heat treatment, vacuum brazing and heat resistant castings.

www.wallworkht.com