1. In pistons, silicon is alloyed to aluminum to reduce its rate of expansion.
Most dirt isa mixture of siliconandaluminiumoxidessoif dirtentryoccursthenaluminiumusually
accompaniesthe silicon
Iron,aluminiumandchromium canoftenbe seentogetherinengineoil samplesastheymake upthe
metallurgyof the liners,pistonsandrings.Thisisusuallyseeninconjunctionwithelevatedsiliconas
dirtentrythrough the air intake systemcancause these componentstowearabnormally.
The primaryconstituents ofdirt are minerals containing silicon and aluminum. The ratio of these two elements
varies widely from place to place.
The easiestwayto determine silicon from dirtversus other sources is to look for secondaryelements,particularly
aluminum,which is often presentfrom the mineral alumina.

2. Silicone-based sealants can leach silicon into the oil,while some gear and engine oils contain methyl silicone as
an anti-foam agent,often ranging from 10 to 20 ppm in new oil.
Silicon also can be presentin other forms.For example,new equipmentthathas been cast in sand (e.g., new
engine blocks) often contains trace silica (silicon) embedded in its walls thatsloughs offduring the first few
hundred hours ofoperation.
My vehicle engine oil has a significant level of silicon and a elevated amount of
aluminum, which suggests dirt ingestion. The high levels of silicon could be any number
of things,
 Silicone sealant used as a sealing agent
 Silica from mechanical seals
 Silicon dry-film lubricants/antiseize/
 Some oils use silicon as an antifoaming agent.
 SiliconisalsousedinSilicone greases
Everytime siliconincreases,ironalsoincreases indicatingthatthe ironweariscausedby the
presence of silicon,likelyfromdirtingression.Thisdirectcorrelationbetweensiliconandironis
calleda locksteptrend.
Silicon: Usuallyindicative of abrasive silica from dirtgetting into the engine despite the induction air filter. Can
also come from silicone sealants and gaskets or from glass beads (usuallyonly after overhaul or major engine
maintenance).
Siliconisalsousedasa anti-foamagentinsome oils.
SiliconisalsousedinSilicone greases
In new engines, its presence could indicate liquid silicon material used as sealant during assembly.
It typically washes out after first oil change. Silica (the oxidative form of silicone) appears in
nature associated with alumina in a typical 5 to 1 ratio. Silicon up to approximately 10-15 ppm
may reflect presence of silicone oil based anti-foam additive.
Dirt is predominantly comprised of two minerals, silica and alumina. These two
minerals are typically present in dirt and dust in the approximate ratio 3.4:1, with
silica being the major component and alumina the minor component. Spectrometric
analysis of oil samples that contain high levels of dirt and dust typically reflect this
ratio of silica to alumina in the ppm of silicon and aluminum measured.
In your case, the ratio of silicon to aluminum is 3.5:1, indicating that the aluminum is
not in this instance a wear metal, but simply a minor, trace component of dirt.
Dustin intake air, antifoam additive in motor oil, worn seals containing silicone,residues ofparting agents and
silicone greases (also in oil sampling syringes),worn aluminium alloys (all-aluminium engines).

3. Silicon,byitself,canalsocome fromrecentlyreplaced sealsandgaskets.
Thisis typical of dirtentrythroughthe airinductionsystem: the siliconindicates dirtwhilstthere isa
rise iniron(liners), chrome (rings) andaluminium(pistons).
The third example alsoshowsanincrease insilicon, butitisdue toan internal coolantleak. When
coolingwaterleaksintothe engine,the waterusuallyevaporatesoff.However,the additives(e.g.
anti-freeze) inthe coolingwaterare leftbehindandsiliconcanbe part of the coolantconditioner
make-up(sodiumMetasilicate).Inthiscase the sodiumandcopperreadings rise sharplybutthe
otherreadingsdonot.The sodiumisalsoan additive andthe copperisnota wearmetal buthas
leachedfromthe radiatorcore.The siliconishighbutisa contaminantfromthe coolingsystemand
not abrasive dirt.
The fourthexample showsavery highlevel of silicon butall the otherreadingsremainmore orless
constant.Thisis an example of asilicone-basedsealantorgasketcompoundbeingused. These
compoundsleach intothe oil butdo notdo any harm. If thishighsiliconlevel were due todirtentry
thenit wouldbe expectedthatthe wearreadingswouldincreasedue tothe abrasive nature of the
dirt.It shouldalsobe notedthatdirt isgenerallyamixture of siliconoxide andaluminiumoxideso,
inthe case of dirtentry,the aluminiumlevelshouldrise aswell. A roughrule of thumbisthat for
dirt,the aluminiumtosiliconratiovariesbetweenAl:Si=1:10 - 1:2, dependingonthe component
and the environment.Inthiscase,the siliconisstill acontaminantbutisnotabrasive andis of no
concern.

4. The fifthexample showsa slightincreaseinthe levelof silicon butthisiscausedbyanadditive inthe
oil, polymethyl siloxane,whichisusedtopreventthe oil fromfoaming.Itdoesnotcause the wear
readingstorise and,therefore,doesnotdoanyharm.
The final setof readingsintable one showsanincrease insilicon,iron,chrome andaluminiumand
looksverymuchlike the secondexampleof dirtentrythroughthe airinductionsystem.However,
the aluminiumtosiliconratioisalmost1:1 whichisunusual.Thisisan example of pistontorching.If
an injectorisfaulty,itcan allowfuel tolie ontopof the pistonandburn.The resultinghigh
temperaturescancause the pistontomeltwiththe resultingincrease inaluminium(piston),iron
(liner) andchrome (ring).The rise insiliconisthe resultof silicon carbide beingalloyedwiththe
pistonmaterial inordertoreduce the coefficientof expansionof the aluminium.Inthiscase,the
siliconisa wearelementandisnotidentifiedassuchbecause of the highlevel butbecause of the
ratioof aluminiumtosilicon.
In Table Two, the first set of results again shows a normal set of readings. The second set
shows top end dirt entry in a Detroit diesel two-stroke engine, where the iron, chrome and
silicon have all increased but the aluminium to silicon ratio seems wrong and the tin level is
higher. The reason for this is that the aluminium piston has been covered with a flashing of
tin to facilitate the conduction of heat.
Table Three shows the normal readings and three examples of dirt entry through the air
induction system (higher iron, chrome and aluminium) without the increase of silicon. These
are engines working at Richards Bay, a manganese mine and a chromium mine. Here, the dirt
consists of the minerals that happen to be in the environment where the machine is working
and the silicon won't necessarily rise when dirt entry occurs.

5. Chromium – chemical symbol Cr. In engines,the rings are normallymade ofchromium or are coated with the
metal.On rare occasions,the liners can be chromed and the rings are then made ofcast iron.Shafts, gears and
anti-friction bearings can contain trace amounts ofchromium as an alloying metal with iron to form certain steels
and this can occur in mosttypes of component.Chromium can also be used as a surface hardening coating on
gears.Chromium can be a contaminanttoo,in areas where the metal is being mined itcan show up in the oil as
an indication ofdirt entry; note that dirt does notalways have to consistofsilicon (and aluminium).
Finally, chromium can sometimes be seen as an indication ofan internal coolantleak. Certain types of coolant
conditioners contain sodium chromate and once again,if water from the cooling system leaks into the oil then this
can evaporate off leaving the chromium and sodium behind in the oil. This is usuallyseen in electromotive diesel
applications.
Iron, aluminium and chromium can often be seen together in engine oil samples as theymake up the metallurgy
of the liners,pistons and rings.This is usuallyseen in conjunction with elevated silicon as dirtentry through the
air intake system can cause these components to wear abnormally.
Iron – chemical symbol Fe (if some ofthe chemical symbols do notmake sense itis because theycome from the
Latin name of the element,in this case ferrum).In engines the cylinder liners and the crankshaftare the major
wearing components along with timing gears,shafts and valves.In gearboxes and drivetrain components iron is
the major constituentofthe gears,shafts and anti-friction (rolling element) bearings.
When iron reacts with water (which contains oxygen) and atmospheric oxygen, rustcan form which may indicate
contamination or componentdegradation.Finally,iron can also be a contaminant.Rust,containing iron,can be
formed in cooling systems.Ifan internal coolantleak occurs whereby the coolantcomes into contact with the
lubricating oil then the coolantmay evaporate at working temperature and pressure whilstleaving coolant
additives and contaminants behind in the oil. This will be covered in more detail under sodium.
Silicon – chemical symbol Si.Although silicon is usuallyseen as an indication ofdirt entry it can have many
different sources.Silicon is partofa chemical added to oils to stop them foaming so silicon can be an additive.
Usuallyfound in a concentration of 5 to 10 ppm,so do not be surprised to see silicon in new engine oil samples;
do not panic,this is not due to dirt.
Silicon is found in chemicals used in coolantconditioners so itcan show up,as a contaminant,ifthere is an
internal coolantleak along with sodium.Greases contain silicon (particularlybentonite greases) and assembly
compounds often contain silicone based chemicals thateasilyleach into the oil.The assemblycompounds can
generate very high levels of silicon butthis is not an indication ofa problem.
Oil Crud
Oil tests showsigns of wear by looking for metals and crud, such as...
Chromium from piston rings, cylinder liners and exhaust valves
Nickel from crankshafts,camshafts and valves
Aluminum from pistons and bearings
Copper from bushings and oil coolers

6. Lead and tin from the crankshaft main bearings
Titanium from bearings, valves, struts and connecting rod pins
Silicon, a “contaminant metal” indicating dirt. It’s more common in samples from land vehicles, but it can
appear in a marine engine with a clogged air intake.
Sodium and potassiumfrom the cooling system. A corrosion pinhole can admit antifreeze, diluting the oil. Most
gas and diesel inboards in salt water have closed-loop freshwater systems,but they still have components
through which raw water flows, like heat exchangers and aftercoolers.
Fuel from blow-by. It dilutes oil, triggering damage when moving parts scuff against one another.
Soot from incomplete combustion. It indicates dirty or worn injectors, improper fuel mixture, inadequate air
intake flow, blow-by or an obstructed exhaust system. Diagnosing the problem and correcting it improves
performance and lowers fuel consumption.
Aluminium–chemical symbol Al.The mostcommonsource of aluminiuminenginesisfromthe
piston.Almostwithoutexception,all pistonsare made fromaluminiumortheiralloys.In
transmissions,torqueconvertorsare made of aluminium.Hydraulicpumphousingsandhousingsin
general are oftenmade of aluminium.Thrustwashers,plainbearingsandbushescanalsobe made
of the metal andfoundina varietyof components.
Aluminiumcanalsobe a contaminant.Itisan additive componentof some greasessoif grease is
transferringintoanoil wettedcomponentthenaluminiummaybe present.Commondirt,dust,grit
ishighlyabrasive andcan be verydangeroustoany lubricatedpiece of machinery. Traditionallydirt
isdetectedbythe presence of silicon,however,mostdirtisa mixture of siliconandaluminium
oxidessoif dirtentryoccurs thenaluminiumusuallyaccompaniesthe silicon –usuallyinaratio of
Al:Si of between1:2up to 1:5.
Bearingswithlinersandoverlaysare usedto avoidseizures.
Duringthe revolutionspriortothe formationof a sufficientlythickoil film, the journal contactsthe
bearingsurface.Insuchsituations,the bearingsprovide sufficientlubricationtoavoidscuffing and
seizure.
The majorityof moderncrankshaftjournal bearingshave three differentlayersof metallicmaterials
withdistinctcharacteristicandfunction. Theyare called trimetalbearings.
The rest are called bimetal bearings.Bimetal bearingsare becomingcommoninautomotive
industry.
All crankshaftjournal bearingshave steelbackings,normally of low carbonsteel.
Steel backingisthe thickestlayerinabearing.
The nextlayerbondedtothe steel backingisbearing liner.This isthe layerwhichsupportsbearing
loadand determines itslife.

7. The third layerbondedtothe bearinglineristhe overlay.Thisisprecisicionelectrodepositedlayerof
 Lead,tin,and copper(Lead+ bronze) Alsoreferredtoasleadedbronze.
 Lead andtin or
 Lead andindium
Nickel
A verythinlayerelectrodeposited layerof nickel isusedasabondinglayerbetweenthe linerand
the lead-tin-copperoverlay. The nickel layerisconsideredapart of the overlay,notaseparate layer.
Bearing Liners:
 Leadedbronze
 Aluminium-tin
 Aluminium-Silicon
There are twoclassesof bearinglinersinuse nowadays.
These are leadedbronzes (copper+tin) andaluminiumbasedmaterialssuchas aluminium–tinand
aluminium-silicon.
The lightgray irregularspotsrepresent leadinamatrix of copper-tin(bronze).The materialis
bondedtomildsteel atthe bottom.
In a limitedapplication, Babbittoverlays are centrifugallycastonbronze liners forslow-speeddiesel
engine journal bearings.
Anotherclassof bearingsis single layersolidmetalbearings-namelysolid bronze andsolid
aluminiumbearings. Thesebearingsare notgenerallyusedascrankshaftjournal bearings.However,
solidaluminium isusedinsome-medium-speedandslow-speeddiesel engines.
The most popularcopper-tin-basedleaded bearinglinerincurrentuse has75% copper.
The bulk of modern crankshaftjournal bearingsis mildsteel. Thisisthe strongestof the twoorthree
layersinthe bearings.Itsupportsthe bearinglinerwithorwithoutoverlay.
The linermaterial containssufficientamountsof antifrictionelements,suchasleadandtin.
Lead isthe most valuable antifrictionelement inthe currentmaterialsandispresentasa separate
phase inthe matrix of copper-tininthe leadedbronze(copper+tin) materials.
The linermaterials playthe mostcritical role inthe bearings.Once the linermaterialisdamaged
significantly,the bearingis considered unfitforfurtheruse.Intrimetal bearings,whenthe overlayis
finisheddue towear,the bronze linerwill continue tosupportthe loadandprovide adequate
lubricationintimesof stress.

8. The soft phase of lead(inbronze) andtin (inaluminium) functionassitesforembedmentof dirt
particles.
THE OVERLAY,ISTHE TOP LAYER OF THE BEARINGSURFACE, ISTHE SOFTEST LAYER IN THE BEARING.
Its functionis to provide lubricationtothe journal inthe initial start-upsituations, adjusttoany
misalignmentorout-of-roundness of the journal,andcapture dirtby embedment.The overlay
providessufficientlubricationduringstart-upandshut-downconditionsalso.
The journal makesa comfortable runningenvironmentinthe bearingassemblyduring initial runsby
“beddingin”.Asa resultof this,the wearrate of the overlayishigherinthe beginning.Aslongasthe
overlay ispresent, the phenomenon of seizurewillnotoccur.Once the wear progresses throughto
the overlay,the bearinglinerwill provide adequate lubricationduringstart-upandshut-down
conditions.However,if the oil supplyisseverelycompromisedorcutoff for a few secondsseizure
can take place.
All modern crankshaftjournal bearingmaterialsare mainlycomposedof five elements:
 Copper
 Aluminium
 Lead
 Tin
 Silicon.
These elementsaccountforleadedbronze, aluminium-tin,aluminium–lead,aluminium-silicon
materials.
Indiumisusedas a constituentof the overlays.
Antimony isusedisbabbits.
Silverisa bearingmaterial withgoodtribological properties,butitistooexpensive touse asa
bearinglinerinjournal bearings.
However,itsisusedinspecial applicationsinsome locomotive engines.
An importantcharacteristicof agood bearingmaterial isitsabilitytoconductheat.
Silver,copperandaluminiumare indeedgood conductors of heat.
Silverhasnoaffinityforiron, cobaltand nickel. Therefore,itisexpectedtorunverywell against
steel shafts. Bothcopperandaluminiumpossessacertaindegree of affinityforiron.Therefore,steel
journalscanbond to these metalsinthe absence of antifriction elements like leadandtin, or
lubricatingoil.

9. Aluminiumspontaneously formsan oxide layer,whichisveryinert,inthe presence of airor water
vapour.Thissuppressesthe seizureorthe bonding tendency of aluminium.Besides, siliconparticles
presentinthe aluminium-siliconmaterialskeepthe journalspolishedtoreduce friction.
The lightgray, irregularspotsrepresenttininthe aluminium-coppermatrix.
Belowthe aluminium –tinlayerisa layerof pure aluminiumwhichfunctionsasabondinglayerto
the mildsteel underneath.

10. Aluminium-tin.Widelyusednowadays,these alloyshave excellentmechanical properties.
Furthermore ,those witha comparativelyhightincontent(usually20percent">can be usedwithout
an overlay.
Aluminium-cadmium.These are commonlyfoundinenginesof Americanorigin.Theyhave good
mechanical strength, butpoorsurface properties.Asa consequence alead-tin-copperoverlayis
necessary.
Aluminium-zinc. Chief usersof thisalloyare Volkswagen. Nosteelbackingisprovidedinthis
instance.Surface propertiesare dependentonzinc content. Where thisis low,anoverlayis
required.
babbitsorwhite metal.Thistin-basedalloyWaswidelyusedformanyyears.It has excellent
surface properties, butlacksmechanical strengthathightemperatures.Asaconsequenceitisrarely
usedon modernhigh-outputengines.
Copper-lead. Althoughmuchstrongerandmore resistanttofatigue ,thislacks white metal’s
excellentsurface properties.Asa consequence, athin,softoverlayisused.Usuallyalead-tinalloy,
thisoverlayiselectrodepositedonthe bearingsurface .Whencopper-leadbearingsare employed,
goodfiltrationisof vital importance.