Nanocoatings - Making Lightweight Engine Components A Reality for the Aerospace Industry by MERI

Topics Covered

Background
Introduction
A Very Promising Family of Materials
Providing Cover
PVD Technologies Making New Coatings Possible
Conclusion

Background

MERI (formerly MRI) has been a centre for research excellence in materials since 1990. This excellence was recognised in 2001 when the Research Assessment Exercise (RAE) awarded a 5 rating to the research carried out at MRI. This makes MERI the highest rated department of its type in the new university sector and rated alongside the materials departments of universities such as Liverpool and Queen Mary's.

MERI's research and consultancy activities are supported by a large advanced equipment base ranging from the latest electron microscopes to high performance computing hardware.

Introduction

The aerospace industry is of course notorious for its drive to reduce weight - in an on-going bid to reduce fuel consumption and, by extension, operating costs. Further, pressure for air travel to be 'more green' has also come to bear.

Almost since man first took to the skies, much work has been done to reduce airframe weight, and today's civil aircraft are rich with composite materials and light-weight alloys. Most recently though, attention has turned to reducing the weight of aerospace engines, and the components therein, and much development work has been conducted into the use of lightweight materials to replace the traditional steels and nickel-base alloys used in engines.

A Very Promising Family of Materials

A very promising family of materials are the so-called Titanium Aluminides (TiAl), and in particular gamma TiAl. These materials, also known as intermatillic compounds, boast great strength and are light weight. For example aluminide base alloys offer superior high temperature performance with low weight and non-burn, and gamma aluminide turbine blades based on Ti47Al2Cr2Ni have been proven to be as strong as nickel based alloys, up to 760°C - but at only half the weight. This will of course allow for smaller engines, reduced fuel consumption, less hazardous emissions and will therefore be better for the environment.

These new materials are ticking several boxes in not only the aerospace industry but also the automotive and industrial sectors. However, not all boxes are being ticked. Aluminides are difficult to process as they have limited heat treatability and low ductility at room temperature. Moreover, wear and erosion resistance are further compromised when the operating temperature exceeds 650°C, limiting the use of these materials in harsh environments. Under these conditions the material surface rapidly fails due to the combination of intensive oxidation and mechanical wear.

Providing Cover

Seeking solutions to the above problems, and endeavouring to harness the true potential of gamma TiAL, is the Innovatial project (a play on 'innovative and TiAl), which started in May 2005 and is supported by the European Commission through the Sixth Framework Programme for Research and Development.

Innovatial involves 24 high-profile European partners - including Fiat Research Centre, German Aerospace, Siemens, General Electric, Hauzer and IonBond - and is focussed on developing innovative processes and materials to allow ultra-performance nanostructured coatings to be applied to gamma TiAl using Physical Vapour Deposition (PVD).

PVD is carried out under vacuum conditions and is essentially a three-step process of transferring material at the atomic level.

  • Evaporation - the donor material is thermally evaporated or sputtered by energetic ion bombardment to vaporise (sputter) its surface atoms.
  • Transfer - of the vaporised atoms.
  • Deposition - the build up of a surface coating by condensation where, for some processes, reactions take place between donor and recipient atoms (and, in some instances, reactive gases are also introduced into the mix).

Heavily involved in Innovatial, and a world-leader in its field, is the Nanotechnology Centre for Physical Vapour Deposition Research (NTCPVD), which is part of the Materials and Engineering Research Institute (MERI) at Sheffield Hallam University.

The NTCPVD has a very successful history in the research and development of high-performance functional coatings for applications in extreme environments, and the centre has pioneered the deposition of several application-specific nanoscale multilayer (superlattice) coatings.

PVD Technologies Making New Coatings Possible

These new coatings were possible thanks to PVD technologies, such as magnetron sputtering, cathodic arc evaporation, and hybrid technologies such as low pressure plasma nitriding combined with PVD. Also, the NTCPVD has taken the lead in ground-breaking High Power Impulse Magnetron Sputtering (HIPIMS) PVD technology for adhesion-enhancing substrate pre-treatment - which many consider to be the most significant breakthrough in PVD in the last 30 years.

One of four plasma sources within the High Power Impulse Magnetron Sputtering (HIPIMS) chamber. The plasma sources produce the coating material flux in ionised form by sputtering. Magnetic and electric fields then draw the ionised material(s) towards the components to be coated, which are rotated within the chamber so that they can receive from each of the donors.

Coating families developed by the centre in Sheffield include TiAlYCrN, CrAlYN/CrN, CrN/NbN, TiAlN/VN, TiAlCN/VCN and Me/C, which offer greatly reduced friction, increased wear and corrosion resistance and good protection against high temperature oxidation. Further, most have already transferred to industrial applications such as cutting tools used for dry high-speed machining, parts for textile machines, metal forging and glass moulds protected with the coatings developed at Sheffield - all are available on the market, and have been for a number of years.

Turbine blades made from gamma TiAl with a nanoscale multilayer structure coating (4 microns thick) of CrAlYN/CrN, deposited using the HIPIMS PVD process.

Further, Dr. A. P Ehiasarian, Senior Research Fellow of Sheffield Hallam University, not only invented and up-scaled the HIPIMS technology for surface pre-treatment but also allowed the centre to patent and license the technology to a number of world-leading PVD system manufacturers, as well as market a number of coating recipes for a variety of commercial applications.

The HIPIMS technology is also used to produce extremely dense PVD coatings, which is a key requirement when it comes to protection of aerospace or automotive engine parts against environmental attack.

A cross section Transmission Electron Microscope (XTEM) image of the nanoscale multilayer PVD coating.

Conclusion

The Innovatial project, and theNTCPVD's activities, will provide a substantial increase in the durability and performance of coated TiAl components, and the aerospace applications envisaged for these components will push the material to its very limits.

At Sheffield, the NTCPVD is equipped with unique world-class industrial and laboratory scale PVD systems and a large variety of advanced systems for plasma diagnostics which allows both fundamental and applied research to be carried out.

Professor Papken Hovsepian by the HIPIMS PVD chamber at Sheffield Hallam University.

The centre collaborates with large number of companies in Europe, USA and the Far East, as well as leading research organisations such as the Lawrence Berkeley National Laboratory, the Centre for Microanalysis of Materials, University of Illinois and the Fraunhofer Institute.

Source: MERI - Materials and Engineering Research Institute

For more information on this source please visit MERI - Materials and Engineering Research Institute.

Date Added: Oct 12, 2008 | Updated: Jun 11, 2013
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