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Emerald Group Publishing Limited
Copyright © 2004, Emerald Group Publishing Limited
Electron beam-based PVD technologies offer new choices to aerospace engineers
Electron beam-based PVD technologies offer new choices to aerospace engineers
Keywords: Aerospace, Engineering, Coatings
Aerospace engineers are under more pressure than ever before to reduce costs. This is expressed in demands for longer maintenance cycles, increased fuel economy, lower aircraft weight and, of course, improved performance.
A unique set of advanced coating technologies is making its own special contribution here, both at the core of gas turbine technology and for moving flying parts within control systems and other powered devices.
These coating technologies are based upon electron beam evaporation and physical vapour deposition (PVD) in a vacuum chamber, mainly developed by Cambridge company Tecvac Limited, a member of the Wallwork Group. They allow thin film techniques, originally developed for electronics, to radically extend the life of metal components, whether they are the advanced nickel-based alloys for turbine blades, high grade steel alloys or titanium alloys. While aluminium is still mainly specified for airframes, in many cases performance can still be improved using these and allied technologies in places where it is used for moving parts.
PVD allows very thin, high integrity, high adhesion coatings of very hard or lubricious materials to be applied to metal surfaces. These coatings, between 1 and 30 µm (or more), extend from super-hard titanium nitride (TiN) and chromium nitride (CrN) – some three times as hard as high quality chrome plate – to highly lubricious (and hard) diamond-like coatings with very low coefficients of friction. The metal-based nitrides, including TiN, CrN, titanium carbonitride (TiCN) and titanium aluminium nitride (TiAlN), can be applied by PVD to faithfully replicate even the smoothest mirror finish, at the same time providing very low levels of friction.
Physical vapour deposition technology is at present straightforward and well understood. Tecvac developed a method using electron beams to vaporise high purity metals, such as titanium, chromium and aluminium, in a vacuum chamber, and then combine them with other elements, typically nitrogen, to produce ceramic films. These compounds are then deposited on the workpiece under the direction of highly focused electrostatic fields at a typical temperatures between 300 and 450°C. This relatively high process temperature tends to restrict the PVD process to steels, titanium alloys and nickel alloys. One notable advantage of electron beam evaporation is that it enables perfect replication of surface finish and avoids the surface defect problems often associated with other coating methods.
Most of the nitride-based coatings produce dramatic increases in surface hardness to radically extend wear resistance (Table I).
|Hard coatings||Maximum operatingtemperatures (8C)||Typicalthickness(mm)||Hardness(HV)||Substrate|
|Titanium nitride||400||3||2,500||Steels – high alloys, titanium|
|Titanium carbonitride||380||3||3,000||Steels – high alloys, titanium|
|Chromium nitride||700||3||2,000||Steels – high alloys, titanium|
|Titanium aluminium nitride||700||3||3,000||Steels – high alloys, titanium|
|Diamolith (DLC)||200-300||3-5||4,500||Aluminium, steels – high alloys, titanium|
Many PVD coated parts have already clocked up impressive service histories. Flying parts maker APPH of Runcorn saw a radical increase in the life of hydraulic cylinders, which enabled service life to be doubled for an elevator damper.
APPH, part of BBA Aviation, was asked to design a special elevator damper unit to damp out transient 2-8 Hz oscillations in flight. The design solution provided a unit weighing less than 1.8 kg using a piston moving in a hydraulic cylinder, which had a maximum stroke of 150 mm to allow damping to operate across the full envelope of elevator movement.
To ensure maximum service life, the steel piston drive rod was ground, hard chrome- plated, then polished to minimise wear where the rod passed through the composite seals at each end of the cylinder. While this design met the requirements, subsequent analysis revealed that actual wear was concentrated in very small, but very variable locations on any part of the rod. Each wear area typically occupied a band less than 1 mm in width (less than 0.6 per cent of the rod stroke).
Despite the hard chrome plating, with a typical hardness of 1,000 HV, the wear concentrated in these very limited areas eventually resulted in the loss of seal efficiency and fluid leakage to give a limited estimated service life of around 150 million cycles. While this performance was completely acceptable, the replacement of the unit, its components or the hydraulic fluid created undesirable routine maintenance costs, so APPH decided to investigate methods of extending service life.
Following an investigation of hard coating, APPH ultimately specified 3 mm Tecvac TiN hard coating on top of the hard chrome applied by APPH's in-house plating shop. The TiN coating was applied after polishing by APPH and faithfully replicated the mirror finish in a single coating process, which did not require any subsequent finishing before assembly.
Tests carried out by APPH indicated a considerable gain in wear resistance, which extended the effective life before significant fluid loss to more than 300 million cycles. Most of this gain is due to the increased hardness of the rod surface (TiN 2,000 HV, hard chrome 1,000 HV).
Perhaps the greatest contribution made by these super-hard coatings is in desert conditions, where dust and grit are the enemies of precision engineering. When quantities of fine, hard silica particles are ingested in high speed engines, wear becomes rapid and, left unchecked, terminal, even when using the most resistant nickel-based alloys.
Tecvac has developed a range of multi-layer coatings for compressor blades, at present regularly specified by engine makers for arduous conditions, that has shown impressive gains in the performance by reducing in-service erosion on both nickel-based and titanium-based alloys. These gains are produced both by low friction final surface layer of TiN or CrN, which reduces micro-impact effects, and by the hardness of the surface, which is typically between 2,500 and 3,000 HV much harder than uncoated nickel alloys or titanium alloys.
The low friction properties of TiN have also been exploited by leading suppliers to the engine builders, to prevent wear and improve efficiency. Much of this application work has been in engine bearings, where various coatings have been used, both to provide wear resistant surfaces and “sacrificial” surfaces to protect critical elements.
One key advantage of PVD coatings is that they can be applied effectively to titanium alloys. While titanium has always had clear advantages in strength to weight ratios, engine and airframe makers have not been able to exploit its full potential because titanium/titanium mating surfaces can gall or bind against each other to produce rapid wear. Coatings of titanium nitride overcome this difficulty and allow titanium alloy gears, brake parts, clutch mechanisms and other mating parts to fully benefit from the higher strength/weight ratio achieved by the use of titanium rather than aluminium or steel.
This is not just an advantage for aerospace. It also gives the automotive world a whole new set of lightweight design options – fuel economy on the ground is likely to be just as important as it is in the air.
Vacuum coating technology has more recently been applied to diamond-like coatings. Tecvac currently offers a set of coatings that apply a “diamond-like” coating to provide very high levels of lubricity with a typical friction coefficient of 0.1 (typical steel/steel 0.45) which confers many advantages to all types of moving metal parts. The coating techniques for these materials, though still relying upon vacuum technology, operate at temperatures some 1008 lower than current PVD processes. This enables many aluminium alloys to be coated, and, with hardness up to 4,500 HV will confer many benefits in terms of wear resistance and lubricity.
Of course, applications of all these technologies depend on the choice of substrate. Generally steel, nickel-based alloys and titanium are easy to coat, since they are stable at the process temperatures of PVD technology. While many engineering alloys can be coated by PVD, sometimes other alloys require lower temperature hardening techniques. Here, Tecvac has also pioneered another process, again borrowed from the world of transistors and chip manufacture-ion implantation. Nitrogen ions are implanted in metal surfaces at relatively low temperatures, typically 200°C or below. This allows aluminium and other elements within aluminium alloys to be converted to the nitride form to produce great gains in hardness and wear resistance.
Development is continuing apace in the world of PVD and ion implantation. The range of elements and compounds that can be applied is growing all the time, and the processes are all environmentally friendly. Aerospace engineers can at present specify surfaces to meet whatever in-service requirements in terms of hardness, lubricity or wear resistance. These surfaces can be designed to provide a barrier to wear, or to enable sacrificial processes to operate and protect critical areas.
One thing is for sure; these new technologies of PVD, electron beam evaporation, nitride thin film coatings, diamond-like coatings and ion implantation will give aerospace design engineers new opportunities to enhance performance, reduce maintenance costs and generate fuel economies.
Details available from: Wallwork Heat Treatment/Tecvac Ltd