MATERIALS INFORMATION SERVICE

The Materials Information Service helps those

interested in improving their knowledge of

engineering materials and highlights the national

network of materials expertise.

This Profile is one of a series produced by the

Materials Information Service.

For advice relating to your particular materials

problem, you can contact the MIS at:

The Materials Information Service

The Institute of Materials, Minerals and Mining

Danum House, South Parade

Doncaster DN1 2DY

Tel: 01302 320 486

Fax: 01302 380 900

MIS Profiles are produced by IOM Communications Ltd, a wholly owned subsidiary of the Institute of Materials, Minerals & Mining

Ref: 8/96

Introduction

Surface Engineering means ’modifying the surface’ of a material or component todevelop surface properties which are usefully different from the bulk properties. Thepurpose may be to minimise corrosion, reduce frictional energy losses, reduce wear,act as a diffusion barrier, provide thermal insulation, exclude certain wavelengths ofradiation, electrically insulate or improve the aesthetic appearance of the surface.With the increasing emphasis on cost effective manufacture and performance ofengineering parts, whilst conserving energy and costly or strategic materials, thepossibilities of surface engineering cannot be underestimated.

This Profile deals with those techniques which may reduce the cost of componentproduction, whilst securing improvements in both service performance and life-timesof engineering parts by improving the tribological performance and delaying theonset of surface degradation processes.

Selecting Engineering Materials

The ’in-service’ engineering and environmental demands must be understood beforeselecting the material for a particular engineering component. Three requirementsmust be satisfied:

i) the bulk mechanical properties must be adequate to ensure a satisfactoryoperating life,

ii) it must be feasible to fabricate the components economically in the numbersrequired; and,

iii) the material must be capable of withstanding the expected environmentalconditions during its design lifetime.

When there is a severe service environment to be considered, e.g. with aggressivechemicals or high temperatures, clear limitations are placed on the choice of the bulkmaterial.

For example, gas turbine blade and ring materials require resistance to creepfatigue, high temperature oxidation, erosion, fretting, thermal shock, thermal cyclingand must also be tribologically compatible with the surfaces they contact. Theelevated temperature mechanical properties are of primary importance and the bulk

CORROSION

K T Stevens, A T Poeton & Son Ltd

material is selected on these requirements, the choice being a superalloy.Superalloys do not have adequate corrosion and tribological performance so thesemust be provided by surface engineering.

In other cases, there may not be such an essential requirement for surfaceengineering, but there could be considerable economic benefits from using it.Savings can accrue in production, with cheaper materials, in easier and perhapsfewer production stages, in superior service performance or increased servicelifetimes.

The message is:Before you choose the material and fabrication route, consider the possible benefitsof surface engineering and incorporate it into the design and production plan.

Surface Engineering Processes

There are many ways of treating metal surfaces to enhance the corrosion resistanceand/or their tribological properties. These may be grouped into three broadcategories (Fig 1.).

Fig.1: Categories of Surface Engineering TechniquesModifying the Surface Without Altering the Substrate's ChemicalConstitution

i) By local heatingSome steels and cast irons can be hardened locally using flame, induction, laser orelectron beam techniques. Only the heated surface is transformed to martensite, sothe bulk properties, specifically the toughness, remain unaffected, and componentdistortion is minimised. The processes are very reproducible and amenable toautomation.

ii) By mechanical workingCold working the surface by peening or other processes to produce deformed layersincreases the stored energy and compressive stress, thereby increasing thehardness, fatigue and stress corrosion resistance. These processes are specialisedand only applicable to certain alloys and applications. Surface topography andsurface hardness are altered, both of which are tribologically useful. Shot peening isa sophisticated technique, with highly reproducible properties generated byautomated treatment under computer control and it is used to complement othersurface engineering techniques which impair the fatigue or mechanical performanceof a component.

Altering the Chemistry of the Surface Regions

Thermochemical diffusion treatments mainly introduce interstitial elements, suchas carbon, nitrogen and boron, individually or in combination, into a ferrous metal

surface at elevated temperatures. The processes are not confined to interstitialdiffusion; substitutional elements are used in processes such as chromising andaluminising.

Interstitial element diffusion into steels falls into two groups: Those carried out at lowtemperatures, i.e. within the ferritic range.

Ferritic processes include gas nitriding (typically 525°C), plasma nitriding (400 to600°C) and nitrocarburising processes (approx 500°C). For ferritic nitrocarburisingprocesses, many different media may be employed, salt baths (cyanides or non toxiccyanate mixtures), endothermic/ammonia gas mixtures, and methane orpropane/ammonia/oxygen mixtures. Typically, such processes produce case-depthsof around 250 m on alloy steels, but they can also be applied to a variety of ferrousalloys. On low carbon mild steel they can produce a ’compound layer’, only 10 mthick, which improves both wear and corrosion resistance.

High temperature treatments in the austenitic range.The austenitic treatments include carburising (using pack, salt bath or gaseousmedia) carbo-nitriding and boronising. They are performed at temperatures near900°C and produce much greater case depths (up to several mm) than the ferritictreatments, but with greater surface growth and distortion.

Thermochemical treatments involving diffusion of substitutional elements, chromium(chromising) or aluminium (aluminising) which may be pack, salt bath or vapourprocesses are generally used for elevated temperature service. The substrates areoften nickel-based super-alloys or nickel/chromium gas turbine materials.

Electroplating and thermal diffusion treatments, when used in combination, areincluded in this category.

One process involves the deposition of tin on to ferrous materials followed by adiffusion treatment at 400 to 600°C to form Fe/Sn compounds which resist scuffingand confer some corrosion resistance. Bronze coatings may be developed in asimilar way to add a bearing surface to a steel substrate. Furthermore, treatmentsare available for non-ferrous substrates such as titanium, copper and aluminiumbased alloys to provide anti-scuffing layers and enhance the tribological compatibilitywith the counterface.

Oxide coatings on the surface can produce significant tribological advantages.When oil is present they prevent scuffing, adhesive wear and metal transfer. Onferrous substrates, these chemical conversion layers may be produced by immersionin caustic nitrate solutions. This process is applied to needle or roller bearings, gearsand piston rings. Similar coatings can be developed by thermal exposure at 300 to600°C to produce an oxide film. Steam tempering, or autoclaving, is applied to high-speed steel drills and zirconium alloy components for this purpose.

Anodising treatments for aluminium alloys produce oxide layers which reduceadhesive wear and are significantly harder than the substrate (up to 500Hv). Hardanodising is carried out in an oxidising acid at around 0°C, so that a layer of oxide upto 500 m thick is produced. Surface growth is half of that layer thickness. Thinner

layers, for decorative or corrosion protection purposes, are produced at roomtemperature.

Anodising may be followed by treatments to seal the surface and improve thecorrosion resistance or to lower friction and reduce wear rates by incorporating solidlubricants into the surface. The cellular structure of the layer provides a key andreservoir for low friction polymers.

Anodising can also be applied to magnesium and titanium alloys, but the resultinglayers do not generally match those produced on aluminium alloys in terms ofhardness and thickness.

Treatments incorporate sulphur into the surface of ferrous components.Sulphur, because of its low melting point, and some sulphides because of theircrystal structures, have good lubricating properties. These processes are used foranti-scuffing purposes on cylinder liners, gears, CV joints, heavy duty rear axlecomponents, textile machinery parts, etc. The processing temperature is generallybelow 200°C.

Phosphate coatings absorb oil and grease, thereby assisting ’running-in’ bypreventing adhesive wear and fretting. Phosphating processes are generallyproprietary, based on dilute phosphoric acid solutions of iron, zinc and manganesephosphates. Accelerators are added to shorten the process times to just a fewminutes at approx 40 to 70°C. The simplest phosphate coatings consist of grey orblack crystals of Fe3(P04)2 and some FeP04. Zinc and manganese produce morecomplex layers which absorb lubricant more readily. They are effective in reducinggalling, pick up and scuffing.

Phosphates are used as a pre-treatment for painting on steel surfaces and for somecold working lubrication systems.

Ion Implantation. In this process, atoms of gaseous or metallic elements areionised, accelerated and implanted into the target component. The implantedspecies occupy interstitial sites and distort the lattice. It is a low temperatureprocess, typically 150°C for small items and less for larger components. The depth ofeffect is very shallow, 0.2 m but the surface properties such as wear resistance,friction and oxidation/corrosion resistance are enhanced. This process is used toimprove the performance of plastic forming tools, press tools and surgical implants.

Adding a Layer of Material to the Surface

There are many processes which involve coating with a layer, not necessarilymetallic, to satisfy specific service environments. Provided the coating process iscost effective, it is possible to overcome most tribological and corrosion problems inthis way.

Weld or roll cladding usually involves relatively thick layers (1mm to several cm).Weld cladding can be used to good effect where abrasive wear is a problem, such asdigger teeth, tank tracks and mineral handling equipment. Roll cladding is usuallyassociated with corrosive or mild erosive wear problems, typically those encounteredin the chemical, wood pulp, paper and food process industries. The two processesare to a certain degree complementary. Hard, abrasion resistant materials that areimpractical to fabricate by conventional techniques are best deposited by weldingtechniques, whilst the more corrosion resistant materials are amenable to rollcladding and forming.

Laser Alloying: the power of the laser can be used to alloy a mixture of metal orcermet powders on a component surface. Coating is normally concurrent, with thelaser spot following the spray nozzle, so that the coating is fused into an alloy andmixed with the outer regions of the substrate material.

Thermal Spraying involves heating metal, ceramic or mixtures of metal and ceramicpowders to a semi-molten state and impacting them at high velocities onto thecomponent surface. These line of sight processes include flame, wire, electric arc,plasma arc and detonation gun techniques. In spray fusing, the coating is heated(usually by a torch) after deposition to fuse the material into a dense alloyedstructure diffusion bonded to the substrate.

The process is very versatile, both coating material and application method can betailored to produce specific surface properties. These can range from extremeabrasion resistance with cermets (e.g. WC/Co) and ceramics (e.g. chromium oxide),adhesive wear and corrosion resistance (e.g. Ni/Cr with carbide additions), anti-scuffing (e.g. molybdenum), abradables (e.g. ceramic/graphite coatings for gasturbine stators), thermal barriers (e.g. zirconia) and corrosion resistant coatings (e.g.zinc).

The processes can be automated, with robot manipulation of the gun, rotation of thecomponent being sprayed, and computer control of the spray parameters. For highintegrity coatings hot isostatic pressing (HIP) after coating seals the porosity andimproves adhesion.

Thermochemically formed coatings are formed from a slurry of ceramic particlesin an aqueous chromium-based solution. Through a sequence of applications(spraying, painting or dipping) and heat curing cycles, the composite, which is freefrom porosity, can be built to a thickness over 100 m. The coatings being hard areeffective against low stress abrasion, but tend to be brittle under high loading.

Electroplating. More than 30 metals can be deposited from aqueous solutions.There are many engineering and tribological applications for electroplates despitethe tendency to think that they are used mainly for corrosion resistance, decoration(e.g. gold, rhodium and platinum) or electronic / electrical applications. Both hard andsoft deposits are used, depending on the particular function required.

Hard chromium plates (typically l000Hv and up to 1mm thick) are ideal for resistingabrasive wear, pick-up and corrosion/abrasion. Porous or intentionally crackedchromium deposits are used for oil retention as in automotive cylinder liners,precision bearing sleeves and piston rings. The structure of this chromium plate canbe used as a key for treatment with solid lubricant or polymer layers, which improvecorrosion resistance or reduce friction. Softer (600Hv), crack-free chromium plates(maximum 30 m) are also available.

Nickel and copper deposits are applied mainly as corrosion barriers, often as anundercoat for hard chrome, so that the combination provides both wear andcorrosion protection. Nickel deposits are also available with the addition of adispersion of fine ceramic particles. Such layers provide excellent oil retention andwear properties for cylinder liners in high revving engines, out-performing hardchromium.

Cadmium and zinc coatings (usually around 10 ( m thick) are used to providesacrificial corrosion protection to ferrous materials. Given their position relative toiron in the galvanic table, they continue to protect the substrate even if damaged orworn. There is strong environmental pressure to replace cadmium; zinc/nickelcoatings currently provide the best alternatives.

Soft deposits, such as tin, are used to facilitate ’running in’, prevent fretting andconfer corrosion resistance, silver is used for anti-fretting.

Cobalt is used for high temperature oxidation resistance and electrolyticallydeposited cobalt incorporating chromium carbide has been successfully used in bothdry and lubricated conditions at 800°C. Derivatives of such systems can be used astip coatings on turbine blades.

Electroless plating. Nickel/phosphorous and nickel/boron layers produced byautocatalytic deposition have many useful corrosion and tribo-corrosion applications.Unlike the electrolytic processes, they produce a deposit with completely uniformcoverage.

Nickel/phosphorus deposits are 25 to 50 m thick with a hardness of about 500Hvand ageing at 400°C develops hardnesses in excess of l000Hv. These values arenot retained at high temperatures. Nickel / boron deposits are superior in this respectand excellent results have been obtained with Ni B on glass containermanufacturing moulds. Composite coatings produced by code position of adispersion of fine particles with the metal have useful properties. Electroless nickelcontaining silicon carbide exhibits superior abrasive wear resistance compared to

hard chromium plate in some applications, and I to 5 ( m sized particles of PTFE innickel produces low friction, self-lubricating surfaces. These are finding use in theaerospace, offshore oil and automotive industries. Other composites are producedby the successive deposition of electroless nickel and self-lubricating polymersproviding the combination of hardness and low friction which find applications in theaerospace and food industries. Good mould release properties are one particularadvantage.

Galvanising and bath aluminising are widely used for sacrificial corrosionprotection of steels, for instance in the construction industry and automotiveexhausts. They are both based on submersion in liquid metal (zinc, in the case ofgalvanising), usually with a strip steel product being continuously fed through thebath.

Chemical Vapour Deposition (CVD) involves the dissociation of metal compoundvapours at temperatures in excess of 850¡C to produce thin, adherent, diffusion-bonded, coatings of metal carbides, nitrides, carbo-nitrides and oxides; typically TiN,TiC, Ti(CN) and A1203. The process produces duplex and layered coatings whichprovide diffusion barriers and optimum surface properties. CVD has been in use formore than 20 years on carbide tool indexable inserts and on some metal workingtools.

The high processing temperature limits the choice of substrate e.g. carbides or steelitems of relatively simple shapes which can be re heat treated without excessivedistortion. Plasma assisted chemical vapour deposition (PACVD) permits coatings tobe deposited below 550°C a typical tempering temperature for high speed steel. Thistechnique allows the deposition of ultra-hard Diamond-Like Carbon (DLC), whichcombines a unique combination of wear resistance, low friction and ’kindness’ to thesliding counterface. Such coatings are finding increased applications on tools and inthe medical and automotive industries.

Physical Vapour Deposition (PVD) is important for relatively small, steelcomponents. Processing temperatures are relatively low, up to 400°C, minimisingdistortion and preserving the heat-treated state of the steel. The processes arelimited to thin layers and for heavily loaded contacts the supporting substrate musthave a relatively high hardness.

PVD embraces several technologies which generate nitrides, or carbo-nitrides, oftitanium, zirconium, hafnium or chromium and other metals on components toprovide thin (3 to 5 m), hard (>3000Hv) layers of inert, low friction coefficientcompounds. These ceramic layers enhance the performance of cutting tools andhave considerable potential for many other small components.

Sputter ion plating techniques are used to deposit diamond like carbon (DLC) andsolid lubricants like MoS2~ PTFE and lead onto bearing surfaces, for service invacuum and other demanding applications. Corrosion protection layers andcompounds for high temperature tribological service in gas turbines are alsodeposited by PVD techniques.

Painting to protect a substrate against corrosion and improve its aestheticappearance is probably the most used and familiar surface modification process.There have been considerable advances in paints, application techniques andpre-coating/painting treatments. Surface preparation and corrosion protection

methods such as phosphating bring paints into the range of engineeringcoatings. Figure I shows how paints and organic coatings now form part of thisspectrum. Both corrosion and tribological requirements can be satisfied by painting,dipping or spraying with organic resins and polymeric materials, to which metallic,ceramic or solid lubricant compounds are added. One process, consisting of zincflakes bonded with zinc chromate and a proprietary organic material, providesexcellent surface protection and is widely used in the automotive industry forfasteners, springs, clips, sintered parts and steering gears.

Powder coating is now increasingly used for the dry application of organics andpolymers. The process of air-spraying and electrostatic deposition without the needfor solvents or carriers, and ease of cure, have obvious environmental benefits. Thereduction and re-use of ’overspray’ reduces costs without loss of quality. It is arapidly growing area of surface engineering.

Do I need surface engineering for my part?

Which substrate?The objective is the lowest cost component which will satisfy the engineeringdemands. Substrate selection at the design stage is relatively easy, afterconsideration of the service conditions e.g. temperate, wear and corrosion.Components with more demanding or conflicting duties may necessitate additionalexpenditure on uprating the base material or surface engineering a cheapersubstrate.

Which surface treatment?A step by step process can be followed (Fig 2) to decide whether surfaceengineering is required and, if so, which techniques would be most suitable.Ideally, the choice will be assisted by knowledge of the wear and corrosionperformance of the various options, information is available from many sources. Themost reliable is that which has been obtained under standardised conditions (suchas ASTM), and Tables I, 2 and 3 provide some typical data for abrasive wear,adhesive wear and salt-spray corrosion performance derived from such tests.

Table 1: Comparative Abrasive Wear Factors

Table 2: Comparative Adhesive Wear Factors

Table 3: Comparative Salt Mist Corrosion Performance

Fig.2: Selecting the Surface Engineering Programme 1

The practical issuesAt this stage, the designer will have short listed materials, considered the bestmanufacturing route, decided whether he will need surface engineering, and how thecomponent will be made. The final decision depends upon what is practical,available and economic (Fig 3)

Fig.3: Selecting the Surface Engineering Programme IIThicknessAny surface modification process must be of adequate thickness to withstand thecontact stresses throughout its operating life. Fig 4 indicates the coating thickness

ranges for surface engineering processes, generally surface layers a few micronsthick are more suitable for lower loads and milder corrosive conditions, even onhardened substrates.

Fig.4: Approximate Thickness of Treatment Effect

CostFig 5 indicates relative costs and that coatings should be selected with care to justifythem on economic grounds.

Fig.5: Approximate Relative Costs of some Surface TreatmentsDistortion and dimensional changesThermochemical treatments and other high temperature processes often causedistortions or dimensional changes particularly when dealing with complexgeometries, this should be appreciated and evaluated. Similarly, the condition of thesurface after treatment must be considered, particularly if a further machiningoperation is required.

General considerationsThe component mass, size and numbers to be treated are important considerationsat an early stage in selecting the surface engineering process. Seek advice fromcontractors on the practicalities.

Quality Assurance and Quality ControlThe key properties of a surface engineered product influence its performance. Twoseparate but related quality activities, assurance and control, determine andmeasure these properties. Quality control techniques measure properties likehardness, surface finish and thickness, effective quality assurance defines andmonitors all steps in the treatment process. The user must work closely with hissupplier to define the key properties and identify procedures that will providereproducible quality. In the case of coatings, adhesion to the substrate is a recurringconcern. A destructive test, bending, indenting, thermal shocking, etc., on thecomponent itself is self-defeating. The alternative of sacrificial tab samplesprocessed alongside the components is often used, the search continues for a non-destructive test with thermal wave or acoustic techniques the most promising. Atpresent, the user should challenge the supplier with the following:

'Show me the quality assurance procedures on your process that will provideconfidence that the coating adhesion will always be optimum for my application'

Where to get Advice

General Surface Engineering queriesNASURFHull University ROSEMaterials Information ServiceNational Centre of Tribology

Electro and Electroless PlatingA T Poeton LtdMetal Finishing Association

Welding and similar Surfacing ProcessesTWI

Thermal SprayingA T Poeton LtdTWIThermochemical and DiffusionTreatmentsWolfson Heat Treatment Centre

Painting and Protective TreatmentsPaint Research Association

Loughborough University IPTME

PVD and Ion ImplantationAEA TechnologyTeer CoatingsRSCE Hull University

In addition most coating companies will give advice willingly.

Contact Addresses

NASURFI RAE Road, FarnboroughHants GU14 6XETel; 0345 627873 Fax: 01252 395000e-mail: NASURF@DRA.HMG.GB

The University of HullResearch Centre in Surface EngineeringHull HU67RXTel: 01482 466474 Fax: 01482 466477

Wolfson Heat Treatment CentreAston UniversityAston Triangle, Birmingham B4 7ETTel: 0121 359 3611 Fax 0121 359 8910

TWIAbington HallAbington, Cambridge CBI 6ALTel: 01223 891162 Fax: 01223 3598910

A T Poeton LtdEastern Avenue, Gloucester GL4 3DNTel: 01452 300500 Fax: 01452 300050

The Metal Finishing AssociationFederation House, 10 Vyse StreetBirmingham B18 6LTTel: 0121 237 1122 Fax: 0121 237 1124

The Paint Research AssociationWaldergrave RoadTeddington TWII 8LDTel: 0181 977 4427 Fax: 0181 943 4705

Institute of Polymer Technology andMaterials Engineering

Loughborough University of TechnologyLeicestershire LEII 3TUTel: 01509 223165 Fax: 01509 223949

AEA TechnologyMATNET, Harwell LaboratoryOxfordshire 0X11 ORATel: 01235 434714 Fax: 01235 434136

Teer Coatings Ltd.290 Hartlebury Trading EstateHartlebury, Kidderminster DYIO 4JBTel: 01299 251399 Fax: 01299 250171

National Centre of TribologyRisley, Warrington WA3 6ATTel: 01925 252000 Fax: 01925 252579

Sources of Further Information

There is a huge body of published information on Surface Engineering and it isgrowing all of the time. The following are useful texts and journals, the list is far fromcomprehensive.

Surface Engineering and Heat Treatment Past, Present and FutureEd. P H Morton, Inst of Metals in Association with CEST.Inst Metals 1991. ISBN 0 901716 01 4

Canning Handbook : Surface Finishing Technology23rd edition 1989. ISBN 0 419129 00 6W Canning plc, Great Hampton Street, Birmingham B18 6ASTel: 0121 236 8621

Finishing Directory and Handbook1996 edition. ISBN 0 861089 83 9FMJ International Publications Ltd., Queensway House, 2 Queensway, Redhill,Surrey RH1 1QS. Tel: 01737 768611

Magazines and journals also provide much useful and up to date information. Titlesinclude:

Surface Engineering, published quarterly by the Institute of Materials and theWolfson Institute for Surface Engineering in association with the Surface EngineeringSociety.

Product Finishing, published 12 times a year by the Turret Group plc

Acknowledgements

With thanks to Mike Farrow

MATERIALS INFORMATION SERVICE

The Materials Information Service helps those

interested in improving their knowledge of

engineering materials and highlights the

national network of materials expertise.

This Profile is one of a series produced by the

Materials Information Service.

For advice relating to your particular materials

problem, you can contact the MIS at:

The Materials Information Service

The Institute of Materials, Minerals and Mining

Danum House, South Parade

Doncaster DN1 2DY

Tel: 01302 320 486

Fax: 01302 380 900

MIS Profiles are produced by IOM Communications Ltd, a wholly owned subsidiary of the Institute of Materials, Minerals & Mining

Ref: 8/94

Introduction

Structured polymer composites were initially developed using low viscosity epoxyand polyester thermosetting resins reinforced with continuous filament or staplefibres greater than 10mm in length. The resulting low density products offer highmechanical performance, good insulation and environmental resistance, but theprocess suffers from several limitations. Inherent chemical instability of theimpregnated intermediates (prepregs) results in a limited shelf life even underrefrigeration. Processing rates can be low because of manual lay-up and curingrequirements, and the products can be constrained by matrix brittleness andmoisture sensitivity.

The advantages of a thermoplastic matrix are considerable. When heated,thermoplastics become soft and can be easily moulded without degrading; whencooled they solidify into the finished shape. This heating/cooling cycle can berepeated more than once, hence component manufacture may make use of semi-finished materials of guaranteed quality and indefinite shelf-life. The reclamation ofscrap and recycling are greatly facilitated. Rapid shaping can be achieved using arange of techniques derived from wood and metal working, and the toughness of thethermoplastic matrix translates into products with improved damage tolerance.

Thermoplastic matrix composites (TMC′s), (which here should not be confused withinjection and dough moulding compounds containing chopped fibres generally lessthan 10mm in length), may be split broadly into two categories: Glass MatThermoplastics Composites (GMT′s) and Advanced Thermoplastic Composites(ATC′s). Initially, GMT materials were produced from polypropylene (PP) andreinforced with continuous glass fibre. The level of reinforcement was modest atfirst, with a fibre volume fraction (FVF) below 20% and a random fibre orientation.The material was adopted by the automotive industry, and steady growth has nowestablished a global market in the region of 35,000 tpa for GMT′s. ATC′s resultedfrom the need for tougher composite materials in the aerospace industry. Thesemirror the first generation of thermoset structural composites in having a FVF above50% and highly aligned continuous filament reinforcement. These high performancematerials are of interest in a wide range of automotive and other engineeringapplications. However, because of the unfavourable economic climate in the late1980 s and cutbacks in defence expenditure, uptake of this second categorymaterials has been slow, with global usage in 1993 estimated at around 250 tpa.

CORROSION

Roger Ford, Integrated Materials Technology Ltd

Materials Processing

1) GMTMatrices and FibresAny thermoplastic may be used to produce these materials, but in practice thechoice has been limited to polyvinyl chloride (PVC), PP, polyamide (PA), polyesters(PBT/PET), polycarbonate (PC) and polyphenylene sulphide (PPS) (see Table 1).Of these, low cost PP is by far the most important, accounting for 95%+ of all theproducts that are commercially available. With fibre reinforcement, it can competewith structural materials where temperatures less than about 110°C are experienced.PPS, on the other hand, is a higher priced polymer with thermal and chemicalresistance which makes it acceptable in demanding applications. For example, themaximum sustained use temperature of PPS reinforced with 40% glass fibres isabout 240°C.

E-glass fibre is the most common reinforcement, and can be used in the form ofchopped fibres, random chopped fibre mats or continuous fibre mats.

Table 1: Thermal data for important engineering thermoplastics

Sheet ProductionThere are two basic production routes which depend on the type of fibre used.Chopped fibres are processed in a wet slurry process developed from papermaking technology. Typically the chopped fibre, polymer powder and processingaids are mixed together with water to create a slurry, during which the fibre bundlesfrom filaments. The slurry is then pumped onto a vacuum filter belt where most ofthe water is removed. The resulting non-woven web of intimately mixed fibres andpolymer is then passed through a drier prior to consolidation in a continuous double-belt press. A schematic diagram of the process is shown in Figure 1. Advantages ofthe slurry process are its flexibility to handle highly viscous polymers and high fibreloadings (60-70% by weight). By making use of low cost, high volume paper makingtechnology this approach offers considerable potential for producing cost-competitiveTMC sheet.

Fig 1: Schematic of wet slurry process

Random fibre mats are impregnated with molten polymer by sandwiching theextrudate between two layers of mat which are themselves retained within outer thinsheets of the appropriate polymer prior to entry into a continuous double-beltprocess as shown schematcially in Figure 2.

Fig 2: Schematic of double-belt GMT process

Impregnation takes place in the heating zone of the press with temperature, pressureand residence time key factors controlling the quality of impregnation. This meltimpregnation approach produces consolidated sheet up to about 4mm thick withfibre loadings between 20 and 40% by weight. Below 20% it is difficult to achieve auniform fibre distribution and above 40% problems of fibre anisotropy begin toappear.

Forming ProcessesGMT can be processed by stamping or high speed compression moulding,depending on the nature of the polymer used. A GMT thermoforming line is shownschematically in Figure 3.

Fig 3: Schematic of GMT thermoforming line

Stamping is normally restricted to semi-crystalline polymers which can bepermanently deformed at temperatures between the glass transition and meltingpoints. Stretching of 5-10% is usually involved. Amorphous polymers tend to be toostiff to stamp unless heated to temperatures 50-100oC above their glass transitiontemperatures. Stamping is used for simple parts where there is little change in partthickness relative to the thickness of the blank. It is not suitable for complex partswith deep drawn, ribs or bosses. The process consists of placing a pre-heated blankof near net shape in a lower temperature moulding press. Pressing is rapid withcycle times usually of 15-45 seconds. Secondary movement control is necessary tomaintain constant pressure on the blank throughout the cycle. Moulding pressuresrange from 10-15 MPa for PP to 14-40 MPa for PPS. A reasonable surface finishcan be achieved which improves with higher pressures.

High speed compression or flow moulding can handle amorphous as well assemi-crystalline polymers, since the matrix must be melted before forming. With thisprocess smaller, thicker and precisely weighed blanks are used to ensure completefilling of the mould without flash. Flow moulding allows more complex parts withvarying cross-sectional thickness to be produced. It has advantages for large partswhich would require excessive clamp pressures if injection moulding was attempted.

Extensive flow, from 50-70%, can take place, which may induce anisotropy in thepart. As with stamping, the pre-heated blank is placed in a lower temperature press,but here the pressing speed is adjusted to prevent freezing of the matrix surfacebefore the completion of mould filling. The control of mould temperature is a criticalfactor in determining cycle time and part quality. Cycle times can range from 25-120seconds depending on matrix type, with moulding pressures similar to those used instamping.

2) ATC

Matrices and FibresOriginally, typical matrices were the amorphous resins polyethersulphone (PES) andpolyetherimide (PEI). However, it was soon recognised that solvent resistance wasan important criterion for aerospace applications, and the emphasis of developmentshifted to semi-crystalline polymers such as polyether ether ketone (PEEK) andpolyphenylene sulphide (PPS) (see Table 1). These expensive materials are aimedat aerospace applications. There are also a limited number of pseudo-thermoplastics such as polyamide-imide (PAI) and polyimides. These polymers, incontrast to normal thermoplastics, complete their polymerisation during processing,which includes a post-curing stage. Of the polymers for use at lower temperatures,PA, PBT/PET and PP have received must study. These materials offer aconsiderable span of use temperatures, with prices ranging from commodity pricedPP to more expensive engineering grade PA′s.

The continuous reinforcement used in ATC′s is supplied to the processor in a varietyof forms. Apart from the basic strand, woven, knitted or braided fabrics may be usedfor impregnation. The continuous strand may alternatively be intermingled withfibres composed of the matrix polymer to form a drapable precursor forimpregnation. In all cases however it should be noted that the fibres may requirespecial chemical treatment to their surface (sizing) to optimise the fibre-matrixinterface.

With high temperature polymers, the important fibres are carbon (AS-4, T-300),aramid (Kevlar-409, Twaron) and S- or R-glass, with carbon most commonly found.Conversely, E-glass accounts for the greater part of all the lower temperaturecomposites.

Prepreg Forming ProcessesThe available product forms fall into two groups classified as pre- or post-impregnated. In the former case the fibres are fully impregnated by the matrix in aseparate step prior to part fabrication. With the latter, fibre and matrix are broughtinto close physical proximity without the fibres being fully wetted by the matrix. Fullwetting only takes place during part fabrication.

Pre-impregnation may be achieved by melt or solution techniques. In the caseof melt impregnation the fibre bundle is spread to aid melt penetration prior toentering a crosshead extruder and die which is normally designed to produce a thinflat tape or prepreg. The process, which is shown in Figure 4, is capable of offeringtape widths up to 300mm with thicknesses from 0.125-0.5mm. A typical carbon fibre

prepreg with a thickness of 0.125mm will have a FVF around 60% and a single plyweight of 210g/m2.

Fig 4: Schematic of melt impregnation process

Alternatively, micronised polymer may be used in a fluidised bed process where thepolymer particles are trapped within the fibre bundle by electrostatic charge. Thecoated fibres are then subjected to heat and pressure to complete the impregnationprocess. Because of the high viscosity of most thermoplastics, melt impregnation offabrics has proved difficult and is normally restricted to narrow widths. Particularfeatures of melt impregnated prepregs are that they are stiff and flex like spring steel,but have no tack. These characteristics can be useful in automated processing.

In the case of solution impregnation, which is mainly used with amorphous polymerssuch as PEI, polymer concentration and temperature can be adjusted to givesignificantly lower viscosities than are available via melt impregnation. This givesthe solution route an advantage for fabric impregnation. After passing through abath of the polymer in solution, the material is passed through nip rollers whichcontrol the degree of solution uptake. The impregnated material then passesthrough a solvent evaporation/recovery unit as shown in Figure 5. Although the aimis to achieve total solvent removal, this in practice has proved difficult, with the resultthat care is necessary during fabrication to prevent the release of volatiles which cancause void formation.

Fig 5: Schematic of solution impregnation process

For post-impregnation the matrix polymer may be used in the form of fibres,film or powder. In each case, the ease of processing depends on the degree ofintimate physical contact achieved between the fibres and polymer during prepregformation. With film stacking, which is primarily a laboratory technique, polymer filmis interleaved with the reinforcement (usually fabric) prior to direct part fabrication.Prolonged consolidation at high pressure and temperature is necessary to achieveacceptable impregnation, which is rarely optimum.

Powder impregnation has been used to develop two distinctive forms of prepreg.In the first type, a fluidised bed technique is used to introduce polymer particles intothe fibre bundle which is then passed through a crosshead extruder. Thisencapsulates the fibres and polymer powder within a thin sheath of the samepolymer which prevents polymer loss during handling. The second type, makes useof tacky binders to retain the polymer powder after it has been introduced into thefibre bundle. This product can be draped in complex moulds but care is necessaryduring consolidation because the binder must be removed by volatilisation.

However, the most important route for post-impregnation involved commingling ofmatrix fibres with the reinforcement, which was developed by NASA. Comminglingis accomplished by passing continuous filament reinforcement and matrix yarnsthrough a turbulent jet of compressed air. A binder may be used to stabilise themingled yarn. The attractions of the method are that the FVF can be closelycontrolled and high molecular weight polymers can be used to spin the matrix fibres,whereas lower molecular weight polymers are frequently used for melt impregnation.The importance of the post-impregnated product forms is their ability to acceptdrape, which is a key factor in the fabrication of complex shapes, especially whendeep draws are required. This is particularly the case with commingled yarns whichlend themselves to braiding, knitting and weaving processes. Acceptable levels ofimpregnation can be achieved with the commingled route but relatively long cycletimes are involved.

Fabrication processesThe fabrication process for ATC′s is dictated by the part shape and the form ofprepreg used. Pre-impregnated forms require nominal pressure for only a fewseconds to yield a consolidated product. This makes them particularly suitable forhigh speed automated processing. However, their stiffness and lack of drape havelimited their application to flat or simply contoured parts. On the other hand, theflexibility and drape of post-impregnated forms makes them the preferred materialsfor complex shape fabrication via compression moulding. In practice two routes arefollowed: ′direct from prepreg′ or ′indirect via a semi-finished intermediate′.

(i) Direct FormingThis includes the routes used for producing semi-finished sheet, rod and tube,i.e. tape laying, filament winding and pultrusion. Sheet can be made bymanual lay-up using a platter press but this is only economical fordevelopment quantities. Automation is following two paths: computercontrolled sequential tape laying of the continuous lamination of a stack ofappropriately oriented prepreg plies. Cincinnati-Milacron have developed aTMC tape layer which continuously heats, compacts and cools tape as it is

applied to a panel surface as shown in Figure 6. Large sheets can be laid upwith any required ply orientation sequence at deposition rates up to 45 kg/hr.This scale of operation is of interest to aerospace fabricators, but for volumemarkets continuous multi-ply lamination using a double belt press is thepreferred route. This requires a supply of continuous off-axis prepregs so thatthe reinforcing fibres can be oriented in the desired stress directions within thecomponent. This can be achieved by continuous edge welding and slittingusing equipment under development by Integrated Materials Technology.Here, the projected capacity of a combined off-axis/lamination line producing1220mm wide E-glass/PP sheet is estimated at 230-420 kg/hr depending onsingle ply thickness.

Fig 6: Schematic of thermoplastic tape-layer

(Cincinnati-Milacron type)

Tube formation uses the same basic filament winding equipment developedfor thermoset systems with the addition of a focused heat source. Theprepreg tape or tow is heated just before being wound on the mandrel, whichcan also be heated to aid consolidation. The pressure required forconsolidation is applied by maintaining adequate winding tension and can beaugmented by a following pressure roller. There are several advantages ofATC filament winding which deserve mention. First, non-geodesic windingpatterns are possible because of the rapid solidification of the matrix aftertape deposition. Second, post forming of the wound tube allows the designerto conceive monolithic hollow structures which can include concave surfacesand finally, because no autoclave is required for curing, there is no practicallimit to the size of thermoplastic filament wound structures.

The pultrusion of ATC rod is at an early stage of development. Most use hasbeen made of melt impregnated tow or tape but die design is critical. Gradual

heating is necessary as the prepreg cross section adjusts through a compliantdie which provides a constant pressure for consolidation. Post-impregnatedprepregs are not generally suitable for pultrusion because they requireexcessively long times for melting, shaping and consolidation.

(ii) Indirect FormingPre-consolidated sheet is the main feed stock for the rapid thermoforming ofATC′s. If semi-consolidated feed stock is used, longer cycle times arerequired. With semi-crystalline polymers, cooling rates must be carefullycontrolled to yield the correct morphology which influences physical propertiessuch as toughness and solvent resistance. There are four differentthermoforming techniques: hydro- or rubber, diaphragm, stretch and roll-forming.

In hydro-forming (see Figure 7a) a heavy rubber membrane, pressurised byhydraulic fluid, shapes the preheated blank against the mould face. Withrubber forming, the membrane is replaced by a solid block of rubber. Bothprocesses are suitable for rapid forming of relatively simple parts. Cycle timesof a few minutes can be achieved.

Diaphragm forming is suitable for more complex parts (see Figure 7b). Theykey feature of this process is the placement of the ATC blank between twostretchable diaphragms of superplastic aluminium or polyimide film. Theblank is not edge clamped and may advantageously consist of semi-consolidated sheet. During graduate shaping under heat and pressure inter-laminar slip allows the inextensible continuous filament plies to accommodatecomplex curvature without fibre buckling or breakage. Here cycle times canrange from 20-100 mins.

Conversely, stretch forming intentionally clamps the edges of the blank sothat shaping takes place under tension (see Figure 7c). With impregnatedfabrics or prepregs based on discontinuous aligned fibres, this method allowsrapid forming with minimum fibre buckling. Cycle times are similar to hydro-or rubber-forming.

Finally, roll-forming can be used to produce linear profiles such as hat or Zsections. Here pre-heated consolidated sheet is passed through sets ofshaping and cooling rollers which bend the sheet to the required form. Theprocess is highly productive with rates up to 10 m/min being claimed, but highprecision in sheet thickness is required.

Fig 7: Schematic of indirect forming processes

ApplicationsTMC′s are still at an early stage of their product life-cycle. Only GMT is produced incommodity volume which is almost entirely used for automotive parts. ATC′s, mainlybased on carbon fibre, are used for a limited number of demanding applications inthe aerospace, automotive and sports equipment markets. The combination of GMTand ATC using a common matrix such as PP offers great potential for volumeapplications in construction, material handling, packaging and transportation. Someexamples of active development are listed below.

Aerospace: missile and aircraft stabiliser fins, wing ribs and panels, fuselage walllinings and overhead storage compartments, ducting, fasteners, engine housingsand helicopter fairings.

Automotive: seat frames, battery trays, bumper beams, load floors, front ends, valvecovers, rocker panels and under-engine covers.

Construction: structural profiles, pipes, concrete rebars and lightweight structuraland insulating panels.

Materials Handling: pallets and cargo containers.

Future ProspectsJust as thermoplastics have come to dominate plastic moulding, so TMC′s willbecome the vehicle by which composite materials will establish their long heraldedimportance as serious alternatives to metals in volume engineering applications.The justification for this prediction is based on a simple examination of the facts.

High strength and stiffness and their suitability for distribution in semi-finished formwere key factors that fuelled the growth of metals usage, with ease of post formingand joining also important considerations. The isotropy of the materials aided theprocess because design needs were relatively straightforward. The prolongeddominance of metals has been supported by the remarkable diversity of propertiesthat can be achieved by processing techniques such as alloying, thermally inducedchanges in the microstructure and surface treatment.

When we turn to TMC′s the similarities are striking. Their anisotropy is the only areawhere there is a profound difference. Resistance to the changes in design methodsthat this required has delayed the uptake of composites generally, but with theincreasing adoption of CAD this will be overcome. A lack of ductility, which appliesto ATC′s based on continuous filament reinforcement, could also be cited as aserious processing deficiency, but work on materials with discontinuous alignedfibres is already in progress.

The key to success is, of course, price and this depends on low cost, volumemanufacturing technology. This has never been on the cards for thermoset matrixcomposites where suitable stable semi-finished products do not exist. However, withTMCs the underlying technology is already available for exploitation when adequateinvestment is forthcoming. This awaits the arrival of an economic upturn or,alternatively, the Japanese!

Where to Get Advice

Roger A FordIntegrated Materials Technology Ltd10 Kay Barn MeadowWoolpitNr Bury St EdmundsSuffolk IP30 9TUTel/fax: 01359 241018

Materials Suppliers

GMT — Ahlstrom, BASF, EXXON, GE Plastics, Symalit.ATC-Baycomp, Du Pont, Hoechst, ECI-Fiberite, Statoil, TenCate,Vetrotex, Mitsui Toatsu

Sources of Further Information

‘Mechanical Properties of Reinforced Thermoplastics’Clegg DW & Collyer AA EdsElsevier ASP (1986) ISBN 0853344337

‘Thermoplastic Aromatic Polymer Composites’Cogswell FN, Butterworth/Heinemann (1992) ISBN 0750610867

‘International Encyclopaedia of Composites Vol.5’Lee SM Ed pp 496-530VCH Publishers (1991) ISBN 0895737329

‘Composite Materials Technology’Mallick PK & Newman S Hanser (1990)ISBN 3446156844

‘Design with Reinforced Plastics 5’Pritchard G Ed. Elsevier ASP (1986)ISBN 0853349193

‘Engineering Materials Handbook Vol.1’Reinhart J Ed. Composites ASM International (1987)ISBN 0871702797

Useful Journals

‘Advanced Composites’ Hartcourt BraceJovanovich Publications ISSN 0895-0407

‘Advanced Composites and Materials Engineering’(ACME) supplement to ‘Engineering’ Gillard Welch AssociatesISSN 0013-7782

‘Journal of Thermoplastic Composites’Technomic Publishing Co. ISSN 0892-7057

‘Materials Edge’ Metal Bulletin Journals LtdISSN 0952-5211

‘Reinforced Plastics’ McDonald Publications LtdISSN 0034-3617

‘SAMPE Journal’ Society for the Advancement ofMaterial and Process EngineeringISSN 0091-1062