The subject of coatings is a very wide one, for there are many

tasks we ask of them. For a time, there seemed to be little

progress in coatings technology, but in the past ten to 15

years there has been a huge increase in its research and

development.

Consequently, there has been an explosion in the number of

coatings available, and their performance in some cases is truly

impressive. They allow us to run our engines hotter, faster, with

increased stresses and lower frictional losses. Without coatings many

modern bespoke racing engines simply won’t work.

This is one area where racing engine manufacturers are leading

series-production vehicle manufacturers to a lower-friction future.

Within a few years, we will see relatively low-cost cars fitted with

coated valvetrain components. Where our goal is performance via

lower friction, theirs is lower fuel consumption and emissions again

via lower friction.

BasicsThere are several reasons why we would consider coating a

component, but most probably fall into one of the following

categories:

• To prevent corrosion

• To reduce wear

• To change the coefficient of friction

• To change a materials’ compatibility problems

• To change other material properties

In terms of changing the corrosion behaviour of metals, we are

generally looking to change the material which is in contact with the

corrosive media. In this sense, we look on corrosion as encompassing

a number of mechanisms, oxidation being the chief among those

that we are looking to minimise. We have all seen the damage that is

done by leaving iron or steel items exposed to air, especially if the air

is damp. Aluminium suffers similarly in that the surface can become

covered in an unattractive oxide.

In many cases, the corrosion is simply that – something we

wish to avoid because it looks unattractive. But it can have serious

implications in terms of weakening the underlying material. Coating

the affected part in a material which is more resistant to the effects

of corrosion can help. It may seem strange, but coating a part in a

material which is more prone to oxidation than the underlying surface

is also helpful, as in galvanising of steel, where zinc is more active and

more easily corroded than the steel. Even when the steel is exposed,

the zinc is preferentially attacked. Plating is a traditional method of

preventing corrosion.

Moreover, corrosion can adversely affect the function of important

parts in the engine. Corrosion also affects polymers and elastomers,

the finished products made from which can be supplied with a

number of surface treatments and coatings.

To change the wear behaviour of a material, we are trying to prevent

one of a small number of wear mechanisms taking place. These are not

necessarily isolated and many occur simultaneously.

Adhesive wear occurs where there is deformation and cold-welding

of materials that slide against one another. The cold-welding at the

tips of the asperities (microscopic peaks on the parts in contact) can

take place when the materials are not moving relative to one another.

When there is relative movement, the bond at the weld may be strong

enough to cause material from the weaker of the two parts to detach,

hence causing wear. Coating one of the pair of materials can mitigate

or prevent this cold-welding and thereby minimise adhesive wear.

Abrasive wear can be simply imagined as the ploughing of a soft or

smooth material by one which is harder and rougher. This can cause

the detachment of particles, which can cause their own problems

in accelerating wear. Wear causes loss of precision in contacts and

therefore higher contact pressures and hence more wear. For these

reasons wear is often a ‘runway problem’.

Further common wear mechanisms are surface fatigue, where sub-

surface fatigue failures can cause particles to become detached from

a surface, and this can happen to materials even in fully lubricated

contacts if the pressures and their amplitudes are sufficiently high.

Fretting is a particular wear mechanism associated with very small

scale relative movements and, in general, it causes oxidation of

contact surfaces, although some large-scale material transfer can often

be observed, especially where identical or similar materials are in

contact.

In all cases, coating can be a solution to these problems.

Friction is generally unwelcome, although that’s not the case in

circumstances such as contact between the valve stem and the collets.

But the race engine designer normally views friction as the enemy.

Coating research and development to reduce friction has certainly

taken most of the research funding in recent years, and we cannot

pretend that the progress has not been rapid and impressive.

In reducing frictional losses we improve engine performance and

reduce heat rejection. The alternative is to maintain performance with

26

Coatings technology has advanced hugely in recent years. Wayne Ward gives a guide to choosing the latest materials

Film review

27

FOCUS : COATINGS

immersion in water, fuels and oils are all

situations in which we place coatings –

often for those very reasons. So we must be

careful that the coating we select can not

only perform its primary function, but also

withstand the rigours of the environment.

The rest of this article will look at the

major components or systems of the engine

and look at how certain coatings may

be successfully used to our advantage.

We’ll begin with the components of the

basic mechanism of most engines, namely

pistons, connecting rods and crankshafts

(see Fig. 1).

PistonsPistons are often coated, as they have

been for many years, and the reasons for

doing so are numerous. Epoxy-polymer

coatings, for example, have been used for

the rapid development of optimised piston

skirt profiles. They are quickly worn into a

certain shape, which is measured and reproduced in the parent metal.

This method has been successfully used to produce piston profiles that

need little subsequent optimisation, and was the subject of a 1982

SAE paper by Tada et al (Experimental Method of Determining Piston

Profile By Use of Composite Materials).

Various polymer coatings are regularly applied to piston skirts to

improve running-in behaviour and reduce friction. These are applied

by methods including spraying and screen-printing.

In the past few years there has been a move to DLC-coated piston

skirts, certainly in Formula One, although the technology is now more

widely available and is being used in both car and motorcycle racing

engines. Where the application of DLC to many other components is

simply a matter of allowing for the thickness of the coating, the design

and manufacture of the piston is fundamentally changed to allow the

successful use of DLC. The machining of the skirt is more involved and

time-consuming so, concerning pistons, the premium to be paid for

DLC coating goes beyond the basic cost of the coating (see Fig. 2).

There are a number of engine builders and now piston

a smaller engine or less fuel, hence the immense interest in these

types of coatings from road vehicle producers. A large number of low-

friction coatings are now available, and the best known of these is the

diamond-like carbon (DLC) family.

And yet friction can be our friend. Coating a bolted joint that’s

subjected to shear loads with a high-friction coating can allow

us to reduce the number or size of the bolts used, and in critical

situations increasing friction at one joint might lead to a much lighter

engine overall. I know of one instance (not in racing) where the

level of friction at one joint dictates the sizing of many of the major

components of the engine and means that the engine is much heavier

than we might expect. There are specific high-friction coatings that can

help in such situations.

We have mentioned materials incompatibility problems in terms

of adhesive wear, but there are also materials that exhibit bulk cold-

welding and complete seizure under even low pressures. Austenitic

stainless steels and titanium alloys are particularly troublesome in this

regard, and coating the affected materials can reduce or eliminate this

problem.

In changing a material’s properties, we are

looking to improve some aspect of engine or

vehicle performance. For example, thermal

barrier coatings might help improve component

life, mitigate turbo-lag and reduce heat

rejection to the coolant or the underbody area.

Operating environmentWe ask coatings to operate in all environments

inside and outside the engine. High

temperatures, thermal cycling, high sliding

velocities, high contact stresses and amplitudes,

▼

Fig. 1 – The piston, rod and crankshaft attract lots of R&D spending in

order to increase reliability and endurance (Courtesy Pankl Racing Systems)

Fig. 2 – DLC-coated pistons are now available outside of Formula One,

albeit at a premium (Courtesy of Bekaert/Sorevi)

FOCUS : COATINGS

manufacturers who have embraced thermal barrier coatings for

pistons (see Fig. 3). By reducing heat transfer to the piston, the

piston might be made lighter or require less cooling in terms of oil

jets. There are theoretical benefits too in terms of fuel conversion

efficiency by keeping the heat of combustion in the combustion

chamber.

There may be disadvantages though regarding volumetric efficiency

if the combination of coating mass and its heat capacity causes the

piston crown to remain hotter than normal during the intake stroke,

causing the incoming charge to be heated. Some involved in engine

development report that engine performance in terms of power

output is decreased due to thermal barrier coatings, especially in

naturally aspirated engines. The coating material and its thickness

must be correctly specified if the aims of the thermal barrier coating

are to be achieved.

The application to pistons has been helped by the process

development in recent times. One coating supplier has reduced the

maximum process temperature to 185º C (365º F) and furthermore

uses a polishing process for this coating to better reflect heat.

Coating pistons to prevent damage from knocking and hot-spots

is not new, and a number of coatings companies as well as piston

manufacturers offer coatings to protect the piston against this damage.

The coatings can range from simple metallic plating processes to

ceramics. Some engines produce best performance on the edge of

knock, or even in a consistently light knocking situation. Knock-

resistant coatings can help with performance as well as preventing

damage. In engines controlled by ECUs which don’t have any form of

active knock control, and which are mapped close to the knock limit,

these coatings can save costly damage.

In the early days of DLC coatings, piston pins were among the

first parts to be widely coated (see below). But early DLC coatings

seemed to cause wear of the pin bore, and pistons were successfully

coated with a ‘composite’ plating consisting of PTFE particles within

Fig. 3 – Two NMCA Pro-Street Camaro pistons from engine builds at the same state of tune.

The damaged piston is uncoated, while the other benefits from a low-friction skirt coating

and a thermal barrier coated crown (Courtesy Swain Tech)

an electroless nickel matrix, preventing this problem until the DLC

technology was more mature.

The ring grooves of pistons are often coated to prevent the rings

sticking in the groove and prevent other general wear problems.

Ring groove problems are most commonly found where the piston

design incorporates a short top ring land. Clearly the amount of heat

reaching the ring grooves is increased in this situation and the higher

temperature means the piston material is closer to its operating limit,

with lower service temperature strength and stiffness. Coatings here

can often mean the difference between success and failure of a given

piston design.

Thermal dispersant coatings have been applied to pistons to prevent

local hot-spots and increase the effectiveness of oil-cooling jets.

Piston ringsWhile plain cast-iron rings remain popular, steel rings with coated

outer diameters have been common for many years. Originally these

were faced with a more wear-resistant metal such as molybdenum,

which was a sprayed coating. After spraying, the correct form was

ground onto the outer diameter of the ring. But the recent trend,

certainly for steel rings, has been to use one of the hard, thin coatings,

normally applied by chemical vapour deposition.

In this process, coatings are deposited from a gas and this allows

complex geometries to be coated. Coatings that have proved popular

for piston rings are titanium nitride (TiN), chromium nitride (CrN) and

DLC, although some suppliers of bore coatings don’t recommend

DLC.

The advantage of these low-friction coatings comes from their

behaviour when the piston is at TDC and BDC. At these points, the

piston velocity is zero and therefore the lubrication regime is no longer

hydrodynamic. The piston ring lubrication therefore relies on there

being some oil trapped in the surface finish of the bore coating. Until

hydrodynamic lubrication is re-established when the piston is moving

again, a low-friction interface is of benefit, as part of the work done

to overcome friction depends on the dry static coefficient of friction

between the ring and liner. Having the top and bottom surfaces of the

ring coated can also help prevent the ring ‘sticking’ in its groove.

Piston pinsPiston pins were among the first pieces of racing engines to be widely

coated, and this has been common now for more than ten years in

many forms of racing. Coatings commonly used on piston pins are

DLC and tungsten carbide/carbon (WC/C) coatings.

Connecting rodsConnecting rods have long been coated on thrust faces, especially at ▼

30

the big end, although the type of coating used here has changed in

recent years. Often designed as pairs of flat surfaces in sliding contact,

thrust bearings have no way of generating pressure in a lubricating

film, although design features which are capable of doing so are easily

incorporated.

It has therefore been common for connecting rods to have a sprayed

metal coating applied to the thrust faces, and this is subsequently

ground to achieve the correct dimensions and surface finish. Recently,

however, these have been supplanted to some extent by CrN coatings

applied to the thrust faces. Providing that the underlying surface

is sufficiently well finished, these CrN coatings don’t require any

subsequent surface finishing operations, although the masking of the

connecting rods is more involved prior to coating.

Some experts report that it is now possible in the case of both steel

and titanium rods to dispense with the interfered small-end bush by

coating the inner diameter of the small end. The benefits of this are a

potentially smaller and lighter rod, and less concern over problems

with the fitting of the bush, which can be serious in the case of

titanium rods.

Oil-shedding coatings are applied to connecting rods (see below)

in order to reduce friction. One company reports that there may be an

advantage in applying a coating that attracts oil to connecting rods.

Both types of coating are polar opposites in terms of function, yet both

are perceived to offer an advantage.

There has been some success reported in drag racing with the

application of thermal dispersant coatings to aluminium connecting

rods – not in terms of increased performance, but in terms of

extended life.

The most exciting developments in connecting rod coatings,

however, are those that might allow us to dispense with the shell

bearing. Many years ago, bearings were ‘metalled’ in place with a soft-

bearing material known as white-metal, the coating being manually

sized with bearing scrapers. But for reasons of economy and accuracy

this method has been replaced by fitting shell bearings.

By automating the process and maintaining stringent controls, it

is again possible to apply soft-metal coatings directly to the bearing

housing. By dispensing with the shell bearing, we may be able to save

weight and money and remove a source of occasional unreliability.

Shell bearings can, if heavily loaded, sometimes fret between the

backing and the rod bore. Moreover, the bolts move closer to the big-

end bore, lowering the applied bending stresses and making a more

compact rod.

This may have benefits for car packaging, allowing the crank axis to

be made marginally lower in the car. But where through-rod positive

oil feed is provided to the small end, the shell bearing is here to

stay. This new method of direct application of bearing materials to

connecting rods and caps is still under evaluation but we aren’t aware

of anybody racing with rods coated in this way yet.

CrankshaftsThe vast majority of crankshafts have no coating, and work perfectly

well without one. There is however an increasing number of parts to

which coatings are applied, although this may not be obvious from

visual inspection.

Beginning with coatings that can be discerned by eye, we find that

some people have experimented with DLC-coated crankshafts. There

are slight benefits in terms of reduced frictional losses from doing

this, but the fact that even Formula One engine manufacturers have

not universally applied DLC probably points to the fact that the gains

are small.

One family of coatings which are widely applied but which aren’t

necessarily visible are ‘oil-shedding’ coatings. In conjunction with

coated connecting rods, the counterweights of the crankshaft are

treated with an oil-shedding coating. The aim is to minimise the

amount of oil on the surfaces in regions of high shear rates in the

engine. The claimed performance increases resulting from the use

of these coatings are truly impressive, with brake horsepower gains

in double figures at high engine speed being reported after dyno

testing.

ValvesValves have been coated in various different materials for many

years, with titanium valves especially requiring coating. Sprayed

metal coatings have been applied to valve stems in the past, with

molybdenum being a favourite. In more recent times, however,

we have seen a change, with TiN becoming much more popular

– sometimes on non-titanium valves as well – as this offers a lower

coefficient of friction than for austenitic steels. A materials expert from

a leading UK research university once warned that we have to balance

the benefits of such coatings with a possible fatigue strength penalty.

In recent times though there seems to have been another shift away

from TiN and towards CrN and DLC. These coatings offer even lower

friction. With all coated valves, any seat lapping must be done with a

‘slave valve’. Lapping with a coated valve can damage the coating and

scrap the valve.

The combustion face of the valve can be coated using a thermal

barrier coating; the reduced conduction through the stem should aid

in fuel conversion efficiency and also better cylinder filling by having

a cooler valve head and seat. How widespread the use of this coating

is in highly optimised bespoke racing engines isn’t clear, although one

valve expert quizzed on this recently was sceptical as to its use in the

highest levels of racing.

Spring retainersValve spring retainers, particularly those made from titanium, are

commonly coated in order to prevent damage being caused by the

edges of the flat ends of the spring. TiN and CrN are common for this

application.

Valve shimsValve shims are often coated where rockers or finger followers

make direct contact with the shim. This is in an effort to reduce the

coefficient of friction at the contact between rocker (or finger) and

shim, mainly to minimise the bending load the valve experiences

owing to the sliding motion of the rocker/finger contact with the shim.

DLC is a common coating here.

31

FOCUS : COATINGS

Cam followersIn overhead cam engines, it is common in racing applications to

coat the cam followers with a low-friction hard coating such as DLC

or WC/C. For lower cost applications, phosphating has been used

with great success to improve the running-in behaviour of ‘bucket’

followers in direct acting systems.

Both finger followers and bucket-type followers (on both OHC

and OHV engines) operate under high contact pressures, and there is

always a desire to see the allowable contact pressures increase. This

allows greater acceleration of the valve in the opening and closing

stages of the lift profile.

But with high contact pressures and sliding

velocities comes the penalty of friction and high

contact temperatures. In the case of low-friction

coatings, the contact temperature (flash temperature)

is kept to a minimum when lubrication is marginal.

Once temperatures become too high, the material is

weakened and parts can fail. In the case of DLC-coated

parts, there is a limit to the temperatures that the

coatings can run at, and this is in the region of 350-

400º C (660-750º F). While we certainly don’t expect

to see the parts getting this hot, the combination of

pressure and sliding leads to local temperatures in the

contact which can reach these levels. At this point, the

coating begins to degrade, leading to higher friction

and therefore higher temperatures.

As we saw in the recent Race Engine Technology

article on pushrod valvetrains (see issue 45), coatings find wide

application on roller valve-lifters too. In this case DLC is the most

popular choice, and the aim here is to reduce friction between the lifter

and its bore (see Fig. 4).

RockersReferring again to the recent RET article on pushrod valvetrains, the

picture of the rollerless rocker concept for NASCAR shows a coated

steel rocker. Again this is a DLC coating.

CamshaftsCoated camshafts have been popular for a number of years in big-budget

racing, and coating both the cams and the followers is said to offer an

advantage in terms of friction over coating only one of them; it is also

likely to offer some advantage in terms of reliability. It is usual to coat the

camshaft using the same coating as the follower (see Fig. 5).

Pneumatic valve return systemsIn engines where the PVRS is of the type with the seal moving up

and down with the valve in a bore, some manufacturers have used

ceramic-coated bores, although these have largely been replaced with

other surface treatment processes recently.

GearsIn an effort to reduce friction in gear trains, some people have

successfully run and raced with gears coated with DLC or WC/C to

reduce friction. A study by a Formula One engine manufacturer in

the 1990s looked at the practicalities of running titanium gears in

the timing drive. One of the final processes used, in addition to other

novel surface treatments, was an early DLC coating.

Ten years ago there were few, if any, processes available whereby

gears could be coated without exceeding the tempering temperature

during coating. Only special carburising steels, developed to have a

higher tempering temperature, were suitable for coating at the time.

Currently there are many different coatings that can be applied at

temperatures which won’t harm a conventional carburising steel.

Tungsten disulphide is finding some application as a coating for ▼

Fig. 4 – A wide variety of cam followers are coated to

increase endurance and to reduce friction. These are

DLC coated (Courtesy of Bekaert/Sorevi)

Fig. 5 – Camshafts are commonly coated to reduce

friction, using the same coating as that on the cam

followers (Courtesy of Bekaert/Sorevi)

Fig. 6 – Gears have been commonly coated in recent

years; these are coated with tungsten disulphide

(Courtesy of Tecvac)

32

FOCUS : COATINGS

to retain as much heat in the

exhaust gas as possible until it

reaches the turbine in order for

it to be as effi cient as possible.

Ceramic coatings in the exhaust

port have been reported in

literature as being successful, and

it might be that these have been

applied in racing in the past. If

however the next version of the

Formula One rules mandates

small-capacity turbocharged

engines, we could see these

coatings applied to cylinder

heads. There are a lot of candidate

materials with low thermal

conductivity, mainly based on

metal oxides. There are also a

number of different spray methods

by which these can be applied.

A secondary benefi t, which all

engine manufacturers could avail

themselves of, is to use a coated exhaust port in order to minimise

heat transfer to water. The subsequent lower cooling requirement

might allow a smaller radiator.

ExhaustsBy logical extension from the coating of the exhaust port, coated

exhausts are widely used for retaining heat within the exhaust gas,

both on naturally aspirated and turbocharged engines. Not only is this

useful in terms of increasing turbocharger effi ciency, the decrease in

radiated heat within the engine bay can have a benefi cial effect on

engine performance, cooling requirements and both mechanical and

electrical reliability. Sprayed ceramic coatings are popular for this

application, as shown in Fig. 8.

gears in motorsport (see Fig. 6).

There are also water-borne ceramic based dry-fi lm lubricants that

fi nd use on gears in everything from heavy industrial use to racing

engines, and everything in between. The gears shown in Fig. 7 aren’t

engine gears, but are coated with an example of this type of coating.

Cylinder headsThere are some applications, mainly for road cars but also in big-

budget racing, where valve seats are not interfered inserts, but sprayed

metal coatings applied directly to the cylinder head. These are

typically bronze alloys although the technology is capable of applying

almost any desired coating.

The combustion chamber could be coated with a thermal barrier

coating, and some suppliers

canvassed for this article market

products that are used for this

purpose. The aim is to keep the

heat within the combustion

chamber, increasing the effi ciency

of the engine while decreasing

the heat rejected to coolant.

The coating has to be carefully

selected, however, as coating

the combustion chamber with

something with high thermal

inertia may lead to lower

volumetric effi ciency due to higher

temperature walls during the

cylinder fi lling process.

For turbocharged applications,

the textbooks show that we need

▼

Fig. 7 – These aren’t racing engine

gears but the same thin, water-borne,

ceramic-based dry-fi lm lubricant is

used widely on racing engine gears

(Courtesy of Techline Coatings)

Fig. 8 – These pipes have been burnished after thermal barrier

coating, leaving them with a polished fi nish (Courtesy of HM Elliott)

DLC COATINGS CERTESS® DCCERTESS® DCx

High-performance coatings and a wealth of experience in the surface engineering sector makes a huge difference to:

Minimize friction losses, decrease wear and improve performance.

Extensive knowledge of surface engineering, interactions and failure-mechanisms allow us to provide solutions to the most demanding applications.

Tel: +1 (207) 985 3232 Fax: +1 (207) 985 4416www.northeastcoating.com

info@northeastcoating.com

Tel: +33 (0)4 77 555 222 Fax: +33 (0)4 77 555 221www.pvd-coatings.com pvd@hef.fr

For all coating related enquiries in the USA contact:

Northeast Coating Technologies

For all coating related enquiries in Europe contact:

34

combination of soft metals which form the bearing. Recently there

have been a number of companies coating the bearing material itself

to provide another running surface. This is said to provide greater

resistance to high loads and is also useful at conditions of marginal

lubrication, especially at start-up. Fig. 9 shows the results of having

coated bearings.

On the back of the bearing shell it is common to find coatings

whose aim is to reduce fretting damage in the block/sump/connecting

rod.

FastenersThere are a number of reasons why we might want to apply a

coating to fasteners. Steel studs, nuts and bolts can easily corrode

and we often see fasteners with a metal plating applied to prevent

unsightly corrosion. Zinc and chromium are most commonly used,

with cadmium not as popular as it once was. Electroplated fasteners

are, if not correctly treated, prone to failure as a result of hydrogen

embrittlement. If you are considering plated fasteners, make sure you

take advice on any de-embrittlement treatments that may be necessary.

Titanium fasteners come with their own problems, owing to the

surface behaviour of the material which is given to smearing and

galling. A range of coating processes are used, from dry lubricants

such as tungsten disulphide through aerosol and dip coatings based in

molybdenum disulphide to silver plating. Some of these processes are

used in conjunction with other surface finish processes.

Fasteners such as exhaust stud nuts which are expected to operate at

high temperatures have special platings applied, and a favourite here

is silver plating. Some commercially available nuts are silver-plated for

such applications.

Of course, we also find the new hard coatings, and Fig. 10 shows

some bespoke fasteners coated with such products.

O-ringsThere are a number of coatings available for O-rings which are aimed

at both reducing friction to allow easier fitting and allow the part to act

better as a seal where movement needs to be accommodated. Many

of the coatings are based on PTFE, although there are others based on

molybdenum disulphide, talc and synthetic waxes.

Heat shieldsThere are times when people feel uneasy about coating exhaust

systems and in these cases, where heat is a problem, its effects

are mitigated by the use of heat-shields that are coated to further

reduce the damaging effects of radiated exhaust heat. Again, ceramic

coatings are popular for this application because of their low thermal

conductivity, and sometimes an additional reflective coating is also

applied. Exhaust heat is a serious consideration for very high-revving

naturally aspirated engines, especially in the case of turbocharged

engines.

Cylinder linersIn the case of loose cylinder liners fitted to four-stroke racing engines,

there are a number of candidate coatings for the cylinder bore. The

most widely used are the thermal or plasma-sprayed metallic coatings

most often based on molybdenum or iron alloys, and the electroless

composite coatings that have particles of silicon-carbide within a

nickel-phosphorous matrix. The sprayed coatings offer the chance

to repair damaged bores and bring worn bores back to original size

without having to resort to oversize pistons.

Chromium bore coatings are not as popular as they once were, but

are still used in some cases at a high level in motor racing.

There are many problems on the outside surfaces of the cylinder

liner – fretting damage, scratching during installation, corrosion, and

cavitation – and aluminium liners suffer more from these problems

than their heavier steel alternatives. Electroless nickel plating and

polymer coatings are commonly used for these applications, although

there are many others that would be equally suitable.

Cylinder blocksThe same bore coatings that are applied to the bore of loose cylinder

liners can also be applied to the bores of linerless cylinder blocks,

although we aren’t aware of anyone using chromium bores in linerless

blocks at present.

One company that offers a specialist plasma-sprayed coating service

says its main market in racing is to coat linerless blocks for high-end

racing applications. The most popular material used is a blend of low

alloyed steel and 30-50% of molybdenum, although metal-matrix

composite coatings and pure ceramics are growing in popularity.

Molybdenum is used in many applications where a sliding motion is

present. The hardness of the plasma-sprayed coatings can be in the

range of 300-1500HV depending on the material being deposited.

Elsewhere on cylinder blocks, oil-shedding coatings are used to try

to minimise frictional losses.

Sumps and windage traysThe same types of oil-shedding coatings as described above are

applied to sumps and windage trays. They aim to facilitate the easy

passage of oil away from the moving components in the engine.

BearingsIn the case of shell bearings, these have been commonly coated for

many years. A steel or bronze backing (the shell) is coated with a

Fig. 9 – These big-end bearings have come from the same engine!

Those on the right are coated while those on the left are uncoated (Courtesy HM Elliott)

35

Metallic sealsThere are a number of metallic seals in use, especially on cylinder

head-to-block joints. For those which resemble metallic O-rings, soft-

metal plating processes are used, and the most common of these for

racing engines is silver. This has a lubricating effect but is also easily

displaced to fill minor surface imperfections.

For gaskets, there are a number of different coatings applied,

including waxes and elastomeric compounds.

ShimsIn order to minimise fretting between castings, prevent movement in

bolted joints and increase the shear load capacity of bolted joints,

steel shims are available. These are cut to the desired form and coated

in an electroless nickel composite with sharp, rough, hard particles

embedded within, and protruding from, the matrix. They are designed

to embed themselves into the surfaces of the main joint components,

basically keying or pinning them together, thereby increasing friction.

The same coating can be applied directly to the surface of one of the

joint components, although the shims are used much more often than

coated joint parts.

Cooling systems There are coatings aimed at improving the performance and efficiency

of the cooling system. These thermal dispersant coatings are applied

to radiators or intercoolers with the aim of making the temperature

distribution more even. One high-end racing application tested on a

pressurised water system for which results were revealed showed a

9º C (16º F) reduction in water system temperature. While welcome,

the increased efficiency allowed the radiator to be partially blanked

and therefore less cooling air flow was required. So we can see that

this coating can give an aerodynamic advantage to the vehicle. An

example is shown in Fig. 11.

Future developments and economic considerationsMany of those questioned for this article seemed to see the drivers

of automotive engine coating development as one or more of the

following:

• Lower friction

• Greater reliability

• Greater load capacity

• Lower part cost

The first three reasons are easy to understand – we want more power,

we want the engine to last longer and we want to load components

more heavily rather than using larger components. These aims we have

in common with engineers developing series-production engines.

We can’t hide from the fact that DLC coatings have been a

revelation and have brought great benefits, and it is no surprise that

they continue to attract research and development funding. New

coatings with lower friction and greater wear resistance are being

developed. The load capacity of coatings is being increased, and

even DLC can be improved upon by applying it over other coatings. I

have seen independent test results showing the benefits of combining

different coatings in order to improve on the capabilities of them both.

One test project increased the seizure load while lowering the friction

of a certain coating, simply by applying it over the top of another

coating.

Some companies we spoke to offer multi-layer coatings that consist

of two, three or more – sometimes many more – layers to build a

better coating for increased adhesion, toughness or wear resistance.

Multi-layer surface treatments, often called ‘duplex’ treatments,

which combine a coating with an underlying surface treatment, give

the coating some useful support and are becoming commercially

available. These are particularly useful for surface treatment of

titanium.

The impetus behind these developments comes in part from the

aircraft industry, where an imperative to continually lower the mass of

an aircraft means substituting steel with titanium, which attracts a lot

FOCUS : COATINGS

▼

Fig. 10 – These bespoke racing fasteners are

coated with titanium nitride (TiN). Such coatings

are commonly used to prevent galling of titanium

fasteners (Courtesy of T&K Precision)

Fig. 11 – The efficiency of a radiator or cooler can be improved by thermal dispersants

applied to their surfaces. This can lead to lower fluid temperatures, or lower cooling

flow requirements (Courtesy of HM Elliott)

36

and he has DLC-coated parts in there, financed from his own pocket.

Applied intelligently to the right parts, coatings of many kinds, not

only DLC, will save money by increasing component life.

Disadvantages of hard coatingsThe application of coatings such as TiN, CrN and DLC doesn’t

come without penalty, and we have to balance the advantages with

the disadvantages. We also have to be careful to design parts in

conjunction with advice from the coating supplier.

For example, if we design a part that is not stiff enough to support

a hard coating properly, it will flex too much and the coating may

crack and flake. The resulting hard debris can cause accelerated wear

in adjacent components or in bearings and so on. In this example, the

uncoated part may have been perfectly suitable for its intended use,

but in the quest to decrease friction we might give ourselves another

problem.

We also need to consider the effect of some of these coatings on

fatigue. The fatigue limit of some materials is degraded due to being

coated. If you have a reasonable factor of safety against fatigue failure

of the coated region, then coating may not cause any problem, but

if your uncoated parts are marginal in terms of fatigue, then coating

them could cause problems.

ConclusionsAn ever-increasing number of specially developed coatings

have become available in recent years and they have brought

us genuine steps forward in developing lighter, more powerful

engines. Developments from Formula One have passed down to

other racing series and will appear in series-production vehicles

in a surprisingly short time. Coatings which are seen by some as

expensive and unnecessary are, in reality, likely to save money if

applied intelligently. If they were genuinely an expensive sideshow,

motor manufacturers would not be looking to use them in large-scale

production.

This is one area where, in recent times, motor racing has taken the

lead and shown the benefits of coatings to the road-vehicle makers,

eventually to the benefit of us all. As gasoline and diesel prices climb

ever higher, in years to come these ‘expensive’ coatings might save us

all some money.

Credits:This article would not have been possible without the kind help

and advice of the following: Dr Andy Bloyce of Materials and

Surface Engineering, Mike Cope of MC Materials, Mark Boghe of

Bekaert/Sorevi, Ian Arnold of Arrow Precision, Chad Elliott of HM

Elliott, Tim Forster of T&K Precision, Peter Ernst of Sulzer Metco, Ian

Haggan of Tecvac, Christoph Wachmann of Pankl Racing Systems,

Val Liebermann of Systec, Jack McInnis of Dart Machinery, Dr Andy

McCabe of Zircotec, Desiree Driesenaar of Hauzer Techno Coating

(NL), Chris Gorvin of Anochrome Group, Sven Schreiner of ESK

Ceramics, Yuri Zhuk of Hardide Coatings, Nikos Douvras of Omega

Pistons, Leonard Warren of Techline Coatings and Richard Tucker of

Swain Tech.

of research and development. There are other processes whereby the

surface of steels is improved by diffusion processes before adding a

low-friction high hardness coating.

One problem with many ‘new coatings’ such as TiN and DLC is

that only thin layers are possible owing to the build-up of large tensile

stresses within the coating. Thick coatings with lower tensile stresses

are being developed, with one company going as far to say it has

coatings that are stressed in compression.

Furthermore, DLC now consists of a family of coatings, some of

which are ‘doped’ with metals such as tungsten, tantalum and niobium

or metalloids including silicon and boron in order to tailor properties

to specific applications or requirements, with greater hardness, lower

residual stresses, more wear resistance and greater temperature

capabilities being mentioned.

DLC is not only being ‘modified’: one company told us it has ‘very

promising results’ from creating a ‘composite’ coating with a metal-

doped CrN coating aimed at improving frictional behaviour and wear

resistance.

The opposite approach is being taken in other applications, with

very hard particulates being used in a metallic matrix. While this

seems similar in principle to something like Nikasil, both the coating

process and the materials involved are very different: the coating

method is chemical vapour deposition with the matrix being based on

tungsten. The coating shows great promise, with high hardness and

wear resistance coupled with toughness and the ability to produce

thick coatings. It is being used in ‘performance engine’ applications,

but for reasons of confidentiality we can’t reveal which parts are

coated.

Developments have seen the application temperatures of many

coatings fall in recent years, so that they can be applied to different

materials. The example of carburising steels is an obvious one as they

can now be coated without fear of losing the mechanical properties

of the core material. It has often been the case that lower application

temperatures have produced coatings with lower adhesion and

increasing the adhesion of coatings applied at low temperature is an

active area of research.

The coatings industry is not a charity, however, so coating parts to

achieve these aims costs us money. Coatings, particularly DLC, have

been criticised as being the preserve of the rich teams. It’s here that

we need to stand back from the obvious costs and look for the not-so-

obvious benefits.

When we look at high-quality cutting tools, they are often coated

and made from carbides and ceramics rather than traditional steel.

They are certainly much more expensive than the steel tools they

replace and which are widely available, so why does industry buy

them? Because it is cheaper to do so, providing of course, that we

intend to tap more than one hole or mill more than one short slot.

They are much more durable, and so it is often the case with racing

parts.

Valvetrain components may be expensive when coated relative to

uncoated parts, but it is not unrealistic to expect them to last five times

longer. DLC is often maligned as being expensive and the preserve

of the high-budget racer. I have a friend with a tuned ‘monkey bike’

FOCUS : COATINGS

▼

Polymer Dynamics, Inc. 11211 Neeshaw Drive, Houston, TX 77065Tel: +1-888-765-9396 Website: www.polydyn.com

The #1 Choice of Champions Since 1979, PolyDyn Performance Coatings have been protecting and enhancing vital engine parts with our unique and innovative engine coatings. Our range of polymer coatings protect from heat saturation, reduce friction – and in most cases, strengthen the component, making it more efficient. From the back roads of La Carrera to the winners circle at Daytona, be it two wheels or four, PolyDyn Performance Coatings have stood proudly behind Champions in every form of motorsport. Are you on the PolyDyn team?

38

FOCUS : COATINGS

EXAMPLES OF COATING COMPANIES

BELGIUMBekaert Sorevi +32 9 338 5910 www.bekaert.com

FRANCEHEF Group +33 47 75 55 222 www.hef.fr

GERMANYAxyntec +49 8217 4999141 www.axyntec.deCapricorn Group +49 2161 477760 www.capricorngroup.deESK Ceramics +49 8315 6180 www.esk.comGramm Technik +49 7152 50090 www.gramm-technik.deSystec SVS Vacuum Coatings GmbH +49 9353 79030 www.systec-vacuum.com

THE NETHERLANDSHauzer Techno Coatings +31 77 355 9777 www.hauzer.nl

SWITZERLANDSulzer Metco +41 56 6188181 www.sulzer.com

UKAnochrome Group +44 (0)1902 567567 x250 www.anotec.co.ukBodycote +44 (0)1695 721361 www.bodycote.co.ukCamcoat Performance Coatings +44 (0)1925 445 688 www.camcoat.comELTRO (GB) LTD +44 (0)1252 523 000 www.eltro.co.ukHardide +44 (0)1869 353 830 www.hardide.comLangcourt +44 (0)1934 612226 www.langcourt.comMaterials & Surface Engineering Ltd +44 (0)7958 077234 www.mat-surf-eng.comMetal Improvement Company +44 (0)1635 279621 www.metalimprovement.comOerlikon Balzers +44 (0)1908 377 277 www.oerlikon.comPoeton +44 (0)1452 300 500 www.poeton.co.ukTecvac +44 (0)1954 233 700 www.tecvac.comTeer Coatings +44 (0)8702 203910 www.teercoatings.co.ukWS2 Coatings +44 (0)1430 861222 www.ws2.co.ukZircotec +44 (0)1235 434 320 www.zircotec.org.uk

USAAnatech (Extremeion) +1 704 489 14889 www.extremeion.comApplied Diamond Coatings +1 203 605 4408 www.applieddiamondcoatings.comCalico Coatings +1 888 236 6079 www.calicocoatings.comCentral Connecticut Coatings +1 860 528 8281 www.centralctcoatings.comDart Machinery +1 248 362 1188 www.dartheads.comElectronic Chrome & Grinding Inc +1 562 946 6671 www.ecgrinding.comELTRO Services Inc +1 248 628 9790 www.eltroservices.comHigh Performance Coating +1 800 432 3379 www.hpcoatings.comHM Elliott +1 704 663 8226 www.hmelliottcoatings.comIon Bond +1 973 586 4700 www.ionbond.comJet Hot Coatings +1 800 432 3379 www.jet-hot.comMorgan Advanced Ceramics +1 610 366 7100 www.diamonex.comMicro Surface Corporation +1 800 248 4221 www.microsurfacecorp.comnCoat Inc +1 336 447 2028 www.ncoat.comNIC Industries +1 541 826 1922 www.nicindustries.comNortheast Coating Technologies +1 207 985 3232 www.northeastcoating.comPanacea Powder Coating +1 260 728 4222 www.panaceapowder.comPolyDyn Performance Coatings +1 888 765 9396 www.polydyn.comRobbjack (Crystallume) +1 866 783 9700 www.robbjack.comSermatech +1 713 849 9474 www.sermatech.comSub-One Technology +1 925 924 1020 www.sub-one.comSwain Tech Coatings +1 585 889 2786 www.swaintech.comTech Line Coatings +1 972 775 6130 www.techlinecoatings.comUS Chrome Corp +1 203 378 9622 www.uschrome.com

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