Nikasil and Alusil

8/12/2019 Nikasil and Alusil

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Nikasil®

and Alusil A primer on some of the more common specialty coatingsand cylinder materials found in small displacement engines

BY JOHN GOODMAN

This issue of Engine Professional isdedicated to specialty engines and niche

marketing opportunities. Part of thatmarket involves small displacement

engines of all types and configurations.

Past experience with these small bore

engines has demonstrated a need tounderstand materials and coating in a

way quite different from the run-of-the-

mill gasoline engine. Motorcycle, watersport, snow mobile, off-road, stationary

power plant and other specialty engines

offer a machine shop many high profit

opportunities. What has generally keptshops from exploiting these niche markets

is a lack of knowledge on how to

approach it, a clear understanding of

what is required by way of equipmentand an understanding of materials and

coatings used in the manufacture of these

engines.We will attempt to give the reader a

primer on some of the more common

coatings and cylinder materials that can

be found in these specialty engines as wellas many automotive engine applications.

BackgroundAny shop considering a cylinder repairthat has been coated with Nikasil®

should be aware of a few materialproperties and some of the confusionsurrounding them. It will not be thepurpose of this article to cover many

manufacturing details but developingapplication and machining do’s and don’tswill be fundamental.

Let’s begin by covering what Nikasil®is. Nikasil® or NiCom® (1967 Mahle

trademark) is electrodeposited oleophilic

nickel silicium carbide coating. Simplyput, it is a nickel plated silica carbidecoating often applied to aluminumcylinder bore IDs but can be used for

many applications where tight tolerancesand superior wear surfaces are indicated.The oleophilic feature of Nikasil® gives ita natural tendency to absorb oil, which in

turn helps the oil retention of the coating.Although there are several proprietaryformulations of nickel plated silicacarbide, application parameters remainthe same for all of them.

What Nikasil® and other similarcoatings are not remains the center of confusion. Alusil’s like Lokasil®,Mercasil® and other high silica substratesare not coatings at all but ratheraluminum metal alloys. These alloys havealso been referred to as hypereutecticaluminum due to the percent of silicaparticles contained in the alloy. It is asubtle distinction but Alusil alloys do notcontain silica carbide like nickel platingdoes. Instead of silica carbide, Alusilemploys glass-like silica particles similarto sand. Beside silica particles, other hard

particle formations are built into the alloythrough smelting. These other hardparticles can be regulated in size andshape depending upon perceived need.

Nikasil® is also not to be confusedwith plasma spray cast iron or carboncoatings. Spray cast iron has been used onaluminum cylinder bore ID’s as a cost-effective and efficient wear surface. Butspray cast iron is much softer thanNikasil® and subject to greater wear. Aquick glance at Nikasil® and spray castiron does not reveal many differences

FEATURE


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because both have a grey look to thesurface so further investigation isnecessary to determine what coating youare working with. Carbon coatings aremuch easier to identify as they are muchdarker (almost black). Generally, carboncoatings are not found in cylinder boreID’s but have been used successfully inthat application.

Physical Properties

Nikasil®

As has been mentioned, many variationsof Nikasil® like Elnisil®, Kanisil®,Electrosil® and SCEM or SuzukiComposite Electro-chemical Material areavailable from a variety of manufacturersbut generally, these nickel silicon carbidecomposites use about 14% silicon carbidewith the rest in nickel and other additivesas binders. This would be similar toconventional vitrified abrasive materialwhere ceramic binders are used to holdabrasive crystals in place. Figures 1 and 2are representations of one type of nickelsilicon carbide composite provided byElectrosil Performance CylinderTechnology in Melbourne, Australia.

By drawing a comparison to vitrifiedabrasives, one might conclude thatNikasil® is an abrasive and by somedefinitions, it could be. The reason it isnot stems largely from low crystal densityand the nickel binder covering all but themicroscopic peaks of those crystals. Thesesilicon carbide micro peaks are oftensmaller than oil molecules which residebetween crystals and allow pistons andrings to slide easily along the cylindersurface. I suspect this is one of the reasonsNikasil® exhibits greatly reduced friction

for sliding seal components such aspistons and rings.

AlusilAlthough not a coating, some hypereutec-tic aluminum alloys suggest by name thatthey possess Nikasil® properties.Lokasil®, Mercosil® and Silumal® arebut three such alloys and often confusedwith nickel silicon carbide composites.For a more detailed look at this alloy, Iwould refer you to the April-June 2008issue, pages 20-26 of Engine Professionalmagazine and read Tim Meara’s article.Tim is Sunnen Products Senior HoningTechnician and explains in detail all dis-creet features of Alusil. It is vital thatshops do not mix coatings and alloystogether when determining best course of action to take with a repair or machiningoperation. I often hear machinists speakabout preparing a hypereutectic alloy asthough it were Nikasil®. This can lead todisastrous results and ultimately blamethe alloy or coating for a failure. So let’stake a closer look at these hypereutecticalloys often referred to as Alusil.

KB Pistons describes hypereutecticaluminum alloy like this: Silicon additionsto aluminum are very similar to addingsugar to iced tea. Silicon can be addedand dissolved into aluminum so it toobecomes inseparable from the aluminum.If these additions continue, the aluminumwill eventually become saturated withsilicon. Silicon added above thissaturation point will precipitate out in theform of hard, primary silicon particlessimilar to the excess sugar in the iced tea.

This point of saturation in aluminumis known as eutectic and occurs when thesilicon level reaches 12%. Aluminum with

silicon levels below 12% are known ashypoeutectic (the silicon is dissolved intothe aluminum matrix). Aluminum withsilicon levels above 12% are known ashypereutectic (aluminum with 16%silicon has 12% dissolved silicon and 4%shows up as primary silicon crystals).).

A hypereutectic aluminum-siliconalloy produced by a powder metallurgytechnique comprises 12 to 50% by weightof silicon, 1.0 to 5.0% by weight of copper and 0.01 to 0.05% by weight of phosphorus, the content of calcium (Ca)as an impurity being controlled to be0.03% by weight or less. Hypereutecticaluminum-silicon alloy exhibits excellentmechanical strength and is therefore usedfor manufacturing pistons and otherengine components.

A hypereutectic aluminum-siliconcasting alloy used in cylinder blocks formarine engines is composed by weight of 16% to 19% of silicon, 0.4% to 0.7%magnesium, up to 0.37% copper and thebalance in aluminum. With the statedsilicon content, this alloy has goodfluidity and the precipitated siliconcrystals provide excellent wear resistance.In addition, the alloy has a narrowsolidification range of less than 150° F.,thereby providing the alloy with excellentcastability. The copper content ismaintained at a minimum so that thealloy has improved resistance to saltwater corrosion.

These expressed features of Alusilhave gained popularity as a structuralmaterial and less so as a sliding seal wearsurface. But many engine manufacturersof all sizes and types use Alusil as a wearsurface too and should be treatedaccordingly. (continued on next page)

Figure 2:Photomicrographof Electrosil platedcylinder bore

Figure 1



Machining and PreparationMost of the confusion surroundingNikasil® and Alusil exists because theyare both used as cylinder ID surfaces forexactly the same application. Even theirname suggests they are the same so it iseasy to mix them up and treat themidentically when machining. So how dowe machinists and assemblers treat thesetwo distinctly different processes?

Let’s start with Nikasil®. Nickelsilicon carbide composites are applied as athin coating of about .002”-.006” thick.We are told that anything thicker than

.006” and the coating may lose some of its elasticity and become brittle. So,depending on how much thickness youhave to work with, careful honing is aviable process to restore a factory surfacefinish. If you cannot determine coatingthickness, assume it is thin and treat it asthought it were no more than .002” thick.Due to the thin nature of this coating,boring the cylinder to the next oversizewill remove the coating entirely. Underthese circumstances, the block orcylinders should be sent to a facility that

specializes in re-coating with nickel siliconcarbide.

Mahle recommends honing no morethan .001” from Nikasil® coated boresand only to restore cylindricity to anywear areas. If wear is greater than .001”,it is advisable to have the coating stripped(in many cases this is a simple electrochemical process and does not requireboring) and fresh nickel silicon carbideapplied. Check with the supplier of coating material for correct honing stonesfor this process. Please note that piston towall clearance for Nikasil® coated linersand hypereutectic pistons can be as closeas .0008” so careful attention to detail isadvised.

Alusil is a different material and howthis substrate is honed leaves little roomfor error. So as not to be redundant, Iwould refer you once again to TimMeara’s article on honing Alusil.

One final note on machining Nikasil®coated cylinders; if decking has to bedone, this may but not always require are-plate of the bore ID. Many cylindercoaters bring Nikasil® over and aroundliner edges. This is done to tie the coating

at both ends of an edge and reducepotential separation or flaking. Enginemanufacturers can do this because theymask off coating just inside the headgasket sealing ring area of the cylinder.Decking will eliminate this edge featureand may leave the coating unsecured.

Pistons, Rings and CoatingsYears ago, GM used a hypereutectic alloyin their Vega four cylinder engine. Thatengine had its own problems butrebuilders were left in the dark as to howrebuilding should take place. Given therelatively high tech nature of materialsand machining vital to restoring wearsurfaces in these cylinders, rebuildersexperienced failures when employing triedand true methods used for cast ironblocks. Even when correct surfacegeneration processes were employed,failures continued. The fix then was tosleeve with cast iron liners and installstandard aluminum pistons and gardenvariety cast rings.

So what was missing in this exercise?What eventually became clear was thatpiston skirts had to be coated and ring

NIKASIL® AND ALUSIL

BY JOHN GOODMAN

Original Equipment

Direct Injected CoatedPiston for GeneralMotors 2009 2.0LEcotec Turbo Engine.


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faces should be barrel shaped if Alusilcylinder bores are to be retained as a wearsurface. When this news finally hit therebuilding industry, rebuilders already hada workable fix by sleeving with cast ironliners. But that fix is not always possible

with modern engines using Alusil as asliding seal or wear surface.

Early in the process, piston coating foruse in aluminum cylinder bores becameknow as “tining”. As it turns out,“tining” piston skirts was indeed a thinlayer of plated tin used as a wear or scuff barrier between aluminum pistons andaluminum cylinders. “Tining” thenbecame a euphemism for several pistoncoatings that acted as wear barriers. Somemanufacturers used a variety of coatingsthat included nickel–tungsten (Ni–W)plating, electroless Ni plating, Ni–Pcoatings with ceramic particles such asboron nitride (BN), SiC, or Si3N4, as wellas titanium nitride physical vapordeposition (PVD) coating, diamond-likecarbon (DLC) coating, spray cast iron andhard anodizing. Not all of these coatings

proved reliable against scuffing Alusilcylinder bores and were eventuallyreplaced by more robust piston coatingswe see in use today.1

Due to the abrasive nature of Alusiland the affinity aluminum has to itself;

this coated barrier greatly reduces seizureof component parts. A stock aluminumpiston would simply not survive even in awell prepared Alusil cylinder bore.Further, barrel shaped ring faces wereused to glide across correctly honed andetched silicon particles without dislodgingthem. Beveled ring faces utilize a scrapingaction which may be detrimental to anAlusil wear surface. Exercise cautionwhen selecting a suitable ring pack for usein Alusil bores.

Further, many different ring materialsare used for Nikasil® and Alusil but someof the better ones are Gas Nitrided, IonNitrided or titanium coated tool steel.There appears to be some consensus thatusing a barrel shaped ring for eitherNikasil® or Alusil cylinder bores worksbest. However, always go with the coating

or cylinder alloy manufacturer’srecommendation for a suitable ring pack.

CautionsSome of the early factory Nikasil® coatedengines suffered piston seizures. The

culprit turned out to be use of high sulfurgasoline sold mostly in Europe. Evenmoderate use of this gasoline began thedestructive process that only got worsewith time. We won’t go into the chemicalreasons why nickel breakdown occurs inthe presence of sulfur but it is a very realproblem nonetheless. Some engines withNikasil® coated liners have been runsuccessfully on LPG due to its extremelylow sulfur content. The caveat here is toquestion your customer about fuel to beused especially if a nickel silicon carbidecoating is what you are working with.

Additionally, both Nikasil® and Alusilhave been used simultaneously by severalengine manufacturers. Porsche, Mercedes-Benz and BMW offered both in differentengine families within the same modelyear. In some cases, Alusil may be the



NIKASIL® AND ALUSIL

BY JOHN GOODMAN

parent cylinder material that is coated withNikasil® or some other wear surface. Be sure tocorrectly identify the material or cylinder coating

you are working with before starting any job.

ApplicationsThis list is far from complete but should give youan idea of how many engines use or have usedNikasil® and/or Alusil for cylinder bores:

• Chevrolet LT5 used in the Corvette ZR-1• Chevrolet Vega 2.3 and 2.0 Cosworth• Citroen Visa twin• Citroen GS birotor wankel• BMW M52 inline six• BMW M60 V8• BMW R80GS• Ferrari F50 V12• Ford RS200• Honda RS125R• Honda XR650R• Jaguar AJ-V8 check series and VIN numbers• KTM LC4• Lambretta TS1• Lotus Esprit Turbo 2.2• Maserati Biturbo 2.0 V6• Moto Guzzi 850 T3• NSU Ro80• Porsche 912• Porsche 911 1973 on

(excluding 1975-1978 911S)• Suzuki RGV250• Yamaha R1 1000cc

There are many more engines out there thanthose listed above and more will be using thesecoatings and alloys in the future. They can besuccessfully and profitably renewed using toolsand equipment you already own. Only correctidentification of materials and careful attentionto machining are necessary to do these jobssuccessfully.

1 Yucong Wang & Simon C. Tung (January,2000). Scuffing and wear behavior of aluminum piston skirt coatings against aluminum cylinder

bore. GM Powertrain, General Motors, Saginaw,MI 48605-5073, USA; GM R&D Center,General Motors, Warren, MI 48090, USA.

Nikasil and Alusil

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