Coating contribution

There are immense economic and political pressures on the aerospace industry to reduce operational costs and improve environmental performance. These are embodied in the targets set by the Clean Sky initiative of The Advisory Council for Aeronautics Research in Europe (ACARE) to reduce CO2 emissions by 50% and nitrogen oxide (NOX) by 80% by 2020. Simeon Collins, sales director at metal improvement specialist the Wallwork Group explains how advanced coatings can contribute to this goal.

Wallwork is probably best known as a heat treatment company. Our sites in Manchester and Birmingham provide traditional vacuum, sealed quench and salt heat treatments for aerospace components. We still undertake bespoke development work in this area, but for the most part these are mature technologies where the capability of the process is well established.

This is not the case with advanced coatings provided by Wallwork group member Tecvac in Cambridge. Here there are numerous technical initiatives underway to extend and improve aerospace component performance by the alteration of surface characteristics and application of micro thin films.

Aerospace objectives – operational cost reduction

Around half the cost of long haul aircraft operation is due to the engine and this in turn is attributable to fuel burn, maintenance and ownership costs. These factors are related. Fuel burn is a function of engine efficiency, commonly expressed as specific fuel consumption.

This will vary with load and over time as the engine ages. Preventative maintenance will maintain engine efficiency, but there is a loss or revenue when planes are not in use so operators need to maximise flight hours in order to amortise the cost of the engine if they are to achieve optimal ownership cost. This latter point applies whether the operator owns its engines outright or uses a leasing or power by the hour type of arrangement.

Weight reduction by material substitution is one route that the industry has taken. The use of composites and light advanced aluminium structural components is advancing and will make a major contribution. Bombardier's CSeries commercial aircraft is a good example of this.

The role of titanium in weight reduction

Titanium is increasingly used as a substitute for steel and other dense metals – in bearings and landing gear struts for example – because of its outstanding strength to weight ratio, fatigue strength and corrosion resistance. Over the lifetime of an aircraft the weight reduction can contribute to significant fuel savings.

However, the use of titanium has certain disadvantages. The material has very poor tribological characteristics with a poor coefficient of friction and a propensity for metal to metal adhesion or galling due to low hardness. Without surface treatment this can lead to 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. This is insufficient, however, in circumstances where components are subject to frequent heavy loading as the treatment will have a limited life due to the instability of the substrate.
Tecvac's answer to this problem is a duplex coating where the substrate is first hardened by the production of a hardened nitride diffusion zone extending to 15-20µm beneath a lubricious PVD applied surface coating. Both hardening and final coating are achieved in a single process.

Our team in Cambridge has developed cutting edge variants of the duplex coating technique with the involvement of Sheffield University. Some of these are proprietary to clients such as Airbus and NMB-Minebea however, the established core process, Nitron-O is universally available and capable of meeting a multitude of aerospace industry needs. Tecvac's R&D development department also works collaboratively with clients on bespoke approaches to the surface treatment of titanium and other metals to meet specific technical goals.

Engine Enhancement

PVD applied coatings have many applications within the engine where improvement of surface characteristics of components will extend intervals between services. One of the best examples of this is in the critical compressor stage of the engine where evidence suggests that there can be a significant and sustained fuel saving and CO2 reduction.

High temperature, pressure and the erosive forces of airflow and contaminants in the airstream all shorten the life of compressor aerofoils. Nitron Flight is a family of multi-layer coatings based on titanium, chromium, chromium/aluminium or carbon/metal carbide that can overcome these problems. The coating may be applied in from six to eight layers depending on the process, making a surface that complies closely with the polished substrate. This creates a low stress surface coating that is more resilient to delamination than a single layer coating of the same thickness.

The chromium-based coating has been extensively proven with a major international maintenance overhaul and repair contractor on compressor blades. This has been shown to improve fuel consumption by around $48,000 per year (based on rates prevailing in 2010) and reduce CO2 emissions by around 290 tonnes. Significantly it has been shown to maintain the blade's optimum smoothness and aerofoil profile resisting the erosive effects of harsh contaminants in the airstream. Fuel savings are sustained longer and the interval required between overhaul is further extended to improve time on the wing and engine ownership cost.

Nitron flight coating systems are applicable to other engine components such as bearings and high temperature engine seals. Generally they will prolong the life of components such as actuators and landing gear.

Research and Development The economic and environmental drivers for change are unremitting. To answer future needs Tecvac employs around 10% of the workforce on R&D to create new and optimise existing coating processes. The factory produces PVD coating equipment for supply to universities, research establishments and manufacturers throughout the world and this knowledge and experience feeds into the development of coating techniques.
Research is backed by in-house laboratory facilities that are the equal to those of leading universities. As a production unit, the site can also transfer research projects into a commercial production environment to ensure they scale up and results can be produced consistently.

Wallwork
www.wallworkht.com