SURFACE MODIFICATION AND COATING TECHNOLOGY (MT60139)

A Term Paper on

Corrosion assisted surface degradation

commonly encountered in auto-engines

Submitted By

Gaurav Potnis (14MT60R32)

Prashant Agarwal (14MT60R47)

AnupDalal (14MT60R52)

Introduction

Corrosion

Corrosion is the gradual destruction of materials (usually metals) by chemical reaction with

their environment. In the most common use of the word, this means

electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the

formation of iron oxides, is a well-known example of electrochemical corrosion. This type of

damage typically produces oxide(s) or salt(s) of the original metal. Corrosion can also occur

in materials other than metals, such as ceramics or polymers, although in this context, the

term degradation is more common. Corrosion degrades the useful properties of materials and

structures including strength, appearance and permeability to liquids and gases.Many

structural alloys corrode merely from exposure to moisture in air, but the process can be

strongly affected by exposure to certain substances. Corrosion can be concentrated locally to

form a pit or crack, or it can extend across a wide area more or less uniformly corroding the

surface. Because corrosion is a diffusion-controlled process, it occurs on exposed surfaces.

As a result, methods to reduce the activity of the exposed surface, such

as passivation and chromate conversion, can increase a material's corrosion resistance.

However, some corrosion mechanisms are less visible and less predictable.

Types of corrosion

Basic Engine Parts The core of the engine is the cylinder, with the piston moving up and down inside the

cylinder.

Spark plug

The spark plug supplies the spark that ignites the air/fuel mixture so that combustion can

occur. The spark must happen at just the right moment for things to work properly.

Valves

The intake and exhaust valves open at the proper time to let in air and fuel and to let out

exhaust. Note that both valves are closed during compression and combustion so that the

combustion chamber is sealed.

Piston

A piston is a cylindrical piece of metal that moves up and down inside the cylinder.

Piston rings

Piston rings provide a sliding seal between the outer edge of the piston and the inner edge of

the cylinder.

Connecting rod

The connecting rod connects the piston to the crankshaft. It can rotate at both ends so that its

angle can change as the piston moves and the crankshaft rotates.

Crankshaft

The crankshaft turns the piston's up and down motion into circular motion just like a crank on

a jack-in-the-box does.

Sump

The sump surrounds the crankshaft. It contains some amount of oil, which collects in the

bottom of the sump (the oil pan).

Corrosion in Automotive engine parts

Case -1 : Corrosion in Aircraft engine

All internal engine components that are manufactured from any steel alloy are susceptible to

it and can prematurely fail from it.

The areas of the engine most commonly subject to the detrimental effects of corrosion are the

cylinder barrel surfaces, camshaft lobes, tappet faces, and gear teeth. Problems in these parts

of the engine can cause varying safety issues and can be very expensive to repair.

Excessive corrosion on a cylinder barrel surface will cause premature wear to itself, the

piston rings riding on its surface and the piston that those rings are installed on. This in turn

will cause high oil consumption and low compression readings. The only fix is to remove the

affected cylinders and overhaul or replace them as necessary.

Corrosion on a cam lobe or tappet face will eventually cause a spalling condition on both

surfaces. This results in extreme wear of those surfaces. The operational results of this

condition are; low power output, possible engine roughness at high R.P.M. settings and metal

in the oil screens or filters. To repair this condition it is necessary to remove the engine from

service, completely disassemble it and to repair or replace all affected components before the

engine can be reassembled and used again.

If corrosion on the mating surface of a gear tooth is significant enough to cause spalling of

that surface, the resultant wear will cause the actual profile of the gear tooth to change. If this

happens and the gear is not replaced before the change in profile becomes dramatic,

catastrophic engine failure will result. If the aircraft is still in one piece after that,

replacement rather than repair, of the engine is often necessary.

It should be stressed that the corrosion itself does not actually cause the part failures, but that

the corrosion is the catalyst that starts a long chain of events over several hundred hours of

operation that eventually leads to the conditions mentioned.

As an engine sits idle, not being used, it is subject to day to day changes in ambient

temperature and humidity. Internal engine components, as well as the oil in the engine itself,

are subject to moisture accumulation caused by natural condensation from these changes. If

an engine is not run for an extended period of time, the oil film that was left on any individual

component within the engine at the last shutdown deteriorates to a point that allows the

accumulated moisture to make contact with the surface the oil was on. Corrosion of that

surface will result. At the same time, accumulated moisture in the engine oil reacts with

deposits in the oil that have developed from normal engine operation. This reaction can cause

the engine oil itself to become acidic and corrosion on the part that the oil is trying to protect

will result.

The only true protection we have against any of these corrosive elements is keeping the

engine oil moisture and acid free and physically on the internal parts of the engine. This

allows the oil to prevent corrosion from occurring.

Flying the aircraft and engine long enough to develop an oil temperature of at least 160

degrees fahrenheit for 30 to 40 minutes will remove almost all corrosive elements that build

up in the oil from its non-use and exposure to the atmosphere. This process also provides a

fresh renewed protecting film of oil, that contains little or no moisture or acids, on all internal

parts of the engine at shut down. This film of oil does not last forever and if enough time

elapses between engine runs, the internal parts of the engine will become subject to corrosion

all over again. The only way to prevent this is to fly the aircraft, as prescribed, fairly often.

This will repeatedly burn the moisture and acids out of the oil, as well as providing a

continuous renewed film of protecting oil.

Another consideration of the ability of the oil to protect against corrosion is the amount of

combustion by products and engine produced deposits that are present in the oil. As

previously mentioned, running the engine will remove most of the harmful contaminants

present in the oil. However, it will not remove all of them. The longer an oil is run in the

engine, the more contaminants it has a chance to absorb. The more contaminants present in

the oil increases the likelihood of a reaction with those contaminants that can cause corrosion.

Consequently, changing your oil frequently will help reduce the possibility of corrosion as

well.

As a general rule, changing your oil at least every 50 hours of operation and flying your

aircraft at least once a week will go a long way in preventing internal corrosion and its

associated maintenance headaches. If your aircraft is based in an extremely corrosive

environment (i.e. high humidity, near the seashore, etc.) or in extremely favorable anti-

corrosive area (stable temperatures, low humidity, desert like conditions, etc.) these intervals

can be adjusted accordingly but as a rule of thumb they are a pretty good guideline for

preventing excessive internal engine corrosion and the unexpected maintenance costs caused

by it.

Case 2wet liners in maritime dieselengines

This study investigates the causes ofcracking occurred in one of the water cooling channels

(top cooling groove) of wet cylinder liners of maritimediesel engines used in naval frigates

(warships). Two damaged cylinder liners were tested, as shown inFig. 1, and identified as A

and B.Cylinder liners in diesel engines are subjected to low cycle fatigue (LCF), mainly due

to starts and stopsof the engine. In the wet type designs, LCF is aggravated by corrosion and,

hence, stress corrosion andcorrosion fatigue may occur in high stress concentration areas,

such as in grooves and assembly notches.Although no complete failures were observed in the

liners, the extent of cracking and the corrosiondamage was sufficiently great so as not to

allow an economical repair to be carried out in the upper part ofthe liners. Therefore, all the

liners were scrapped and replaced by new ones. Cylinder liners in diesel engines are

subjected to low cycle fatigue (LCF), mainly due to starts and stopsof the engine. In the wet

type designs, LCF is aggravated by corrosion and, hence, stress corrosion andcorrosion

fatigue may occur in high stress concentration areas, such as in grooves and

assemblynotches.Although no complete failures were observed in the liners, the extent of

cracking and the corrosiondamage was sufficiently great so as not to allow an economical

repair to be carried out in the upper part ofthe liners. Therefore, all the liners were scrapped

and replaced by new ones.

Fig. 2.Macro showing a frontal view of one of the cracks found in liner A

Fig. 3 Macro of the lateral view of the crack in the depth direction

Fig 4.Micrographs showing the microstructure of the grey cast iron along the thickness of the liner:

(a) along the crack surfaces,showing a rosette type distribution of graphite. Unetched, 50; (b) pearlitic

matrix on the inside wall revealing the larger size of thelamellas.Nitaletch, 1000; (c) pearlite matrix

on the outside wall.Small dimensions of graphite lamellae.Unetched.

Case 3 Cavitation erosion of wet sleeve liners of heavy-duty diesel engine

Cavitation erosion can occur if the pressure of liquid is reduced sufficiently tocause

formation of vapour bubbles (cavities). In the case of heavy-duty dieselengines, the following

is happening: The fuel detonates in the combustion chamber. This causes the wet-sleeve liner

wall to flex outward slightly, due to the pressureinside the liner. When the liner returns to its

original shape, the water cannot followquickly enough. As a result, microscopically small

vapour bubbles are created on thewall of the liner. When the wet-sleeve liner stops moving,

the vapour bubblescollapse violently. Frequently, the impact is so hard that it breaks off a

microscopic flake of iron from the liner wall . If it is allowed to proceed, the wet-sleeve

linermay be completely penetrated. The pitting can penetrate the liner wall untilperforations

go all the way through to the combustion chamber. Cavitation erosion ofthe outside of

cylinder liners is more rapid than progressive wear of the insideSurface by piston ring action.

It is difficult to estimate the periods of time that cancavitation occur.In some cases, can be

identified within the first several hours of use.Sometimes it takes 10001200 hours or more.

Thus cavitation erosion of liners oftenseriously reduces the reliability and life of many types

of diesel engines.

The widespread use of electrodeposited coatings of cadmium, chromium,copper and also

diffusion aluminising, chromizing, and impregnation of grey cast ironwith carbide-forming

elements, has been used in order to protect wet-sleeve linersfrom cavitation erosion.

Fig. 4 Cavitation erosion with removal of

further material layers in wet-sleeveliners

was observed after 1500 working hours

Fig 5. Cavitation erosion with removal of

further material layers in wet-sleeveliners was

observed after 1500 working hours

Case -4 -Corrosion of ductile iron exhaust brake housing in heavy diesel

Engines

Exhaust brakes, typically made of ductile iron, are a means of slowing down vehicles by

generating negative torque in theengine when closing off the exhaust path from the engine. In

certain operating conditions, the exhaust brakes malfunctionedafter a short period of time due

to extensive corrosion product formation.The general oxidation of ductile cast irons in air

results in the formation of Fe2O3 at the gas/oxide interface, then Fe3O4and, finally, FeO.

Many cast irons include silicon among the alloying elements, where the products of its

oxidation are typicallycohesive and cause a buildup on the surface. It has been reported

previously that for low Si cast irons (

large in volume, which form by the migration of ferrous ions from the underlying metal to the

surfaceand reacting with the environment. In the case of spheroidal graphite cast irons, the

resistanceto corrosion decreased as the size of the spheroids decreased. Some reaction

variables for the burning of exhaust valves were studied. In their study, corroding medium

was defined as a combination of CO2, CO, H2O, SO2, and hydrocarbons present in the

gasphase. Additionally, unburned oxygen and nitrogen were mentioned among the

constituents of the corroding medium. Existenceof an ash deposit was observed on the

surface that was a mixture of oxides, halides, sulfates, and phosphates. They alsomentioned

that there was a possibility of temperature rising in the system when the gas reactions were

catalyzed by theformation of scales.

Fig. 6. Image of the failed brake cut to show different parts.

Exhaust brake (ductile iron 65-45-12) failure caused by a large volume of corrosion product

was examined and it wasconcluded that, according to the characterization results, the failure

was a result of higher than normal operating temperatures,which significantly affects the

oxidation rate. Cast irons with higher silicon content, such as ASTM 518 or nickel corrosion

resistant cast irons such as ASTM 436 and ASTM 439, are recommended for this application.

Case -5 -Investigation of failure in main fuel pump of an aeroengine

The components in an aeroengine always operate under most arduous conditions and the

application demands working of the components in tandem. Failure in any of the components

leads to catastrophe with resultant loss of the aircraft and also, loss of valuable lives.

Aeroengine being a safety critical system, is normally manufactured, serviced and maintained

to the highest standards. Yet, aircraft accidents do take place due to engine failures. Failures

in aeroengines are more likely the result of a chain of events rather than due to a single cause.

Investigation of aeroengine failures is, therefore, complex and challenging.

The components of aeroengines are subjected to cyclic loading during their operational

usage. Because of this, the components experience fatigue leading to initiation and

propagation of cracks at highly stressed areas. Statistics show that fatigue is the predominant

mode of failure in aircraft components and it accounts for nearly 60% of the total

failures and . If one considers the failures in engine components alone, the percentage of

fatigue failure is still higher.

Generally, the components of aeroengines are designed for safe life with the minimum

probability of fatigue cracking during the life span. In spite of this, fatigue failures of the

components are common. Fatigue cracks generally initiate at locations of high stress

concentrations. Study shows that the stress concentrators are invariably related to defects of

various types, introduced mostly inadvertently, at various stages of the life cycle of the

components, starting from manufacturing to service retirement , and . Stress concentration

also arises because of corrosion during service due to interaction of the components with the

operating environment. In these cases, the failure generally does not take place due to loss of

substantial amount of material by corrosion. Instead, the stress concentration arising from the

corrosion facilitates premature fatigue crack initiation in the component and its eventual

failure by fatigue fracture. In this paper, investigation of a failure in the main fuel pump of an

aeroengine has been reported. The failure resulted in loss of power in the engine, leading to

an accident. Through systematic investigation and analysis, the primary cause of failure in the

MFP and the sequence of events that led to the accident were established.

Fig. 7 Schematic of (a) MFP in assembly, and (b) components of MFP.

The raw material wire and the unused springs from the batch of production as the fractured

ones were evaluated. Examination revealed corrosion on the surfaces of the raw material as

well as the springs. Compositional analysis showed presence of chlorine in the corrosion

products.

Laboratory investigation showed that all the fractures in spring No. 3 took place by fatigue

mechanism. It was also established that the premature fatigue fracture of the spring was

promoted due to corrosion on the spring surface. Fatigue failures due to premature crack

initiation associated with corrosion pits are commonly observed in Al-alloys and low alloy

steels used in aqueous and chloride bearing environments .For example, a decrease in the

number of cycles to failure for a given stress level was reported in AA6061 alloy wires

subjected to pre-corrosion prior to fatigue testing . Similarly, crack initiation at corrosion pits,

and reduction in fatigue life was reported in a low carbon low alloy (CrNiMo) steel at high cycle regime.

Formation of corrosion pits on the surface is a common phenomenon in low alloy steels used

in chemical environment. Localized corrosion pits are known to be potential origins of

fatigue cracks. Finite element analysis was carried out to estimate the stress concentrator

factor arising from such pits. Findings of an experimental study on Mn-steel wires used in

coalmine show that the corrosion fatigue cracks initiate at surface pits and the corrosion

fatigue life is shortest in acidic solutions compared to caustic environment.

In the sequence of failure, the spring No. 3 had fractured by fatigue leading to a situation

wherein the parts of the spring were either misaligned or telescoped in. This had resulted in

impact load on the plunger at the bottom edge leading to generation of an overload crack of

length 22 mm. This crack had then propagated further by fatigue over a length of about

20 mm. Followed by which, the crack had again propagated by overload and got arrested at

the top inner surface of the plunger. The crack edge at the top surface had then propagated by

fatigue following roughly a circular path leading to dislodgement of material from a sector of

the side wall of the plunger. Following this, the dislodged fragment had fallen in the annular

space between the rotor and the pump body casing. The churning of this fragment during

operation of the pump had damaged the outer conical surface of the rotor and the inner

surface of the pump body casing over a circular band.

Therefore, the primary reason for the failure of plunger No. 3 of the MFP was the fatigue

fracture of the spring. The fatigue fracture of the spring was promoted due to corrosion.The

failure of the springs 4, 6 and 7 was also by fatigue because of the same reason. Evidences

suggest that the failure in spring 3 was the first among the four spring failures. Also, failures

in the four springs were independent of each other and had occurred at different times.

The corrosion pits on the springs of the MFP were not developed during usage. Instead, it

was carried forward from the raw material stage. It was established that there was

development of corrosion pits on the surface of the raw material wires during storage.

Therefore, the primary reason for the spring failure was the improper storage of the raw

material for a long period of time without proper surface protection.

c

Fig. 8. Fractured springs of the MFP; the locations

of the springs in the corresponding plungers

indicated.

Fig 9. Fractures in spring No. 3 showing relative

damages on the faces of the broken parts.

Fig. 9 .Scanning electron fractographs of one of the mating surfaces of fracture 2 of spring No. 3: (a)

fracture surface oriented approximately 45 with the wire axis (arrow), (b) crack arrest marks or beach

marks, (c) fatigue crack origin region, (d) dimple rupture, (e) fatigue striations, and (f) corrosion pits

at the fatigue crack origin (encircled).

Fig. 10. (a) Scanning electron images of the surface of the broken spring No. 3 showing corrosion

pits, and (b) optical micrographs of the longitudinal section of the spring wire showing corrosion on

the surface (encircled).

Recent advances in mitigation of corrosion assisted degradation

Steel coating application for engine block bores by Plasma Transferred

Wire Arc spraying process

Thermal spraying development for automotive industry grows since the end of the nineteen

nineties. In the case of engine blocks, bores were before plated by a chemical process or

liners were inserted in cylinders to ensure the corrosion resistent performances. In fact,

aluminum has poor wear properties. Nowadays, a lot of research is conducted on the

reduction of weight by liner replacement. The main solution is the coating application

directly on the aluminum cylinder bore. In this way, weight is reduced and design can be

improved to furthermore develop performance and efficiency. Different processes are

employed such as plasma, HVOF or electrical arc. One of them is the Plasma Transferred

Wire Arc spraying (PTWA) consisting of a plasma jet formed by a transferred arc between a

cathode and a wire. Its tip is melted and blown by a gas blast toward the bore surface.

Diffusion aluminide coatings

Pack cementation method. Diffusion coatings are typically deposited during the diffusion

aluminising process, or in somecases, during complex Al-Cr, Al-Si, Pt-Al, Ti-Al processes.

During powder processes, the coated parts are placed in special containers, which are then

covered with specific powder mixture, containing neutral filler such as Al2O3, the aluminium

powder or alloy and the chemical activator. Subsequently, the sealed container is located in

the furnace, where the chemical activator produces the transporting vapour source at a

definite process temperature ranging from 700 to 1050C. The diffusion aluminising process

performed in this way usually lasts for up to twenty hours and requires strict powder mixture

protection from oxidation. To obtain thickness or Al concentration diversified coatings, a

number of types of the powder process are employed. The said types might be divided into

high, moderate and low activation powders. During the aluminide coating deposition process

at high temperature, about 1050C, low aluminium content NiAl phases are created (low

activityprocess), whereas at about 700C, NiAl phases containing more aluminium are

formed .

The microstructure of platinum modified aluminide

coating

CVD aluminizing process

The chemical vapour depositionprocess is the effect of evolution of the aforementioned

methodsapplied for diffusion aluminide coating deposition. CVDprocess involves placing the

turbine blades in the retort, whichis fed with gas AlCl3+H2 atmosphere created in the

outerreactor. Gas AlCl3 is formed as a result of processing of HCl inthe heated generator

containing aluminium. Subsequently,AlCl3+H2 gases are preheated and fed into the retort

when theyreach about 1000oC. The CVD retort holding its charge istypically heated to the

processing temperature in the bell typefurnace, soaking pit or elevator furnace. The reaction

gasesleaving the retort are processed by the special gas neutralisingsystem. This method of

aluminising enables both outerand inner surfaces of the turbine blades to be

coatedsimultaneously, particularly cooling channels, which might poseproblems in other

methods of coating. Apart from that,this technology provides the means for regulating the

coolingrate, which is crucial on account of hot working parametersof several foundry

superalloys.

Thermal Barrier Coatings

The temperature of inlet gases may be increased thanks to the use of thermal barrier coatings

on the combustion chamber parts, vanes and blades. The application of barrier coatings

caused a decrease in the temperature of the employed superalloys by about 170C in

comparison with the temperature on the surface of the ceramic coating. What is more, the

TBC shave decreased the amount of the necessary cooling air, while retaining constant

temperature of exhaust gases, as well as increasing significantly the durability of the parts

and theirthermal deformation resistance. The idea of thermal barrier coating application is

presented in fig. 8.The thermal barrier coatings are composed of the outer ceramic zone,

usually consisting of ZrO2 x Y2O3 and the bond coat, containing MCrAlY (M = Ni, Co, Fe).

The insignificant value of thermal conductivity, characterising ceramic materials,triggers the

temperature reduction in the interlayer above the bond coat, which is responsible for the rise

in the oxidation and hot corrosion resistance. The most common methods for obtaining

thermal barrier coatings are APS (Air Plasma Spray), LPPS (Low Pressure Plasma Spray) or

EB-PVD (Electron Beam Physical Vapour Deposition). The thermal spraying method is

generally applied for producing TBCs on combustion chamber parts and vanes, whereas the

blades are normally covered employing EBPVD technology. The microstructure of sample

plasma sprayed thermal barrier coating, made of ZrO2x8Y2O3 forming about 300 mthick

ceramic zone and the MCrAlY bond coat, which is about100 m thick, is shown in fig. 12

The yttria stabilised zirconia coating needs to possess a specific, strictly controlled porosity

and micro cracks determining its cyclic temperature change resistance Fig. demonstrates

sample applications of plasma sprayed thermal barrier coatings on the aircraft engine

combustion chamber and on the air cooled vane.

Fig. 11 Sample applications of plasma

sprayed TBCs on the aircraft engine

combustion chamber and on the air cooled

gasturbine vane

Fig 12. The outline of thermal sprayed TBC

microstructure

Erosion Corrosion resistant coating

During aircraft engine operation, compressor blades are vulnerable to various types of failure,

resulting from the specific, frequently aggressive factors. Mechanical damage resultant from

the impact of particles of ingested foreign matter, e.g. from the airstrips, is one of the most

common problems. All scratches and indentations etc. constitute accidental damage and, as

such, are extremely difficult to prevent. As well as that, the compressor blades are also

subject to the erosive corrosive impact of inflow dusty air. Their damage generally takes the

form of pinholes appearing chiefly on the leading edge and on the pressure said of the blade

The erosion dents, particularly these placed on the leading edge of the compressor blades,

initiate corrosion by retaining humidity. The injured coherence of the material in the

corrosive

environment forms the structural foundation initiating fatigue fracture, which poses a threat

to one of the crucial elements of the compressor blades. The erosive damage of the aircraft

engine parts may be prevented by applying protective coatings. The earliest of these, varnish

coatings, demonstrated extremely low efficiency in terms of erosion and corrosion protection,

being particularly susceptible to destruction consequential to their exposure to the stream of

dusty air. The fastest degradation occurred on the leading edges of the blades. The prolonged

flow of the stream of dusty air could accelerate notably the erosion process in the blade

material, while the corrosion processes are determined by the electrochemical and chemical

interaction of the material and the surrounding environment, e.g. coastal environment. The

enumerated destructive phenomena occur mainly during operation, sometimes varying in

intensity.

Fig. 13 Fracture morphology of Ti/TiN and

Cr/CrN multilayercoatings deposited by Arc-

PVD method on compressor bladesmade of

martensitic stainless steel. View of coated

compressorblades after engine test

Fig. 14 Set of compressor blades coated

with Cr/CrN multilayer

coating deposited by Arc-PVD method

CORROSION MITIGATION BY ADVANCED CYLINDER COATINGS

Along with the optimization of other engine components, the cylinder running surface often

gets into the focus of engine designers. Honing structures are being optimized on existing,

proven cylinder surfaces such as cast iron blocks and sleeves, hypereutectic aluminum alloys

(Alusil), and on various galvanic coatings (e.g. Nikasil). In parallel, the direct coating of

cylinder running surfaces with the application of thermal spray processes has become more

and more important. Applying a coating directly to the cylinder surfaces in aluminum engine

blocks can eliminate the need for cast iron sleeves. This can significantly reduce the weight

of the engine block, leads to improved heat transfer from the combustion chamber into the

water jacket and can if required - give corrosion protection of the running surface.

Apart from the atmospheric plasma spray (APS) technology, which has been applied in mass

production over the past ten years, other thermal spray processes found its way into niche

applications in various market segments. The twin wire arc spray (TWAS) and the plasma

transferred wire arc (PTWA) processes are the most important of these alternative thermal

spray processes used for cylinder surface coating applications. The capability of the APS

process for mass production has been amply proven; it has been used to coat more than 2.5

million passenger car cylinder bores over the past 10 years and the developments in the

various markets have continuously been reported. Recent developments in this area include

the industrialization of the process and obtaining the series production readiness in various

additional market segments (including recreational and commercial vehicles, power

generation, marine propulsion, and rail), as well as the provision of suitable coating materials

to cope with the different challenges in these market segments.

Preparation and Surface Activation

Before the coating is applied, the bore diameter has to be oversized to accommodate the

coating. The oversize is determined by the target coating thickness after final machining

(honing). This pre-machining can be carried out by means of line boring, single point drilling,

honing, or other processes. An arithmetic roughness average Ra < 4m is required after pre-

machining. Thereafter the part has to be thoroughly cleaned to remove any oil and grease and

residues from the machining process from the surface. This is typically done in a dedicated

washing machine. The specified surface tension after cleaning and drying is about 36mN/m

in the area where the coating will be applied. The cleanliness can easily be checked on the

part using a test fluid of defined surface tension [31]. A clean surface is necessary to

guarantee the adhesion of the coating. The next step in the coating process is the so-called

activation of the surface. This step consists of a roughening of the substrate surface to

increase the surface area and produce a structure which facilitates the mechanical interlocking

of the coating with the substrate. Since the adhesion of the coating is not due to a

metallurgical bonding with the substrate, as would be the case for a weld overlay coating, the

adhesion of the coating is dependent on this roughening. The activation (roughening) of the

surface can be accomplished by a number. of methods. Contrary to widespread perception,

APS coatings can be applied to all different kinds of activated surfaces, not only grit blasted

surfaces. Activated surfaces generated with the most commonly used activation processes and

an APS coating deposited onto these surfaces are shown in Figure 2. As can be seen from

previous experiences, not every surface activation process is well-suited for every substrate

material and engine block design. In case of cast iron surfaces, only grit blasting with

corundum (Al2O3) is applied today.

Grit blasting can be applied to all different substrate materials. For very high hardness

materials, the grit could be SiC. In contrast to the high pressure waterjet activation process,

grit blast activation does not enlarge any casting pores that may be present on the cylinder

surface. This will allow a larger acceptable pore size in the casting if grit blast activation is

used, which will increase the casting yield and therefore reduce the costs.

APS COATING MATERIALS AND PROCESSES

The use of powder based materials to build up the coating allows great flexibility in the

choice of the material. Requirements for corrosion resistance, low friction, and low

oil consumption can be met with appropriate material compositions, which can be varied and

adjusted over a wide range. As can be seen in Table 2, the range of materials starts

with a low alloyed carbon steel (XPT512), which has been used to coat more than 2 million

cylinder bores. The column HV0.3 shows typical hardness values of the materials,

measured on the coating. The addition of a ceramic such as Al2O3/ ZrO2 generally

does not increase the hardness, which is deliberate, but it will increase the scuffing and wear

resistance. If carbides are blended with the alloyed powder the hardness could increase

significantly. However, the size of the carbides has to be chosen carefully, in order not to be

abrasive to the piston rings. Such carbides are typically in the size range of 2m or

smaller. For racing applications, molybdenum-containing materials are the most common, as

molybdenum acts like a solid lubricant and can improve the scuffing resistance. In case of

high abrasive wear on the cylinder surface, the materials can be blended with ceramics; this

class of materials is called metal matrix composites (MMC). An example of such a material is

F2056, the composition and hardness of which are shown in Table 2. If there is a need for

corrosion resistant cylinder surfaces, the basis of the MMC may be a 14Cr-2Mo steel

(XPT627); when blended with ceramics, it is called F2071. This material has become the

standard material in diesel engines with high EGR rates.In some applications, it has been

found that 14% chromium is not sufficient to guarantee the corrosion resistance. In this

case, a superferritic matrix with 28Cr-4Mo (F4375) is used, which, when blended with

ceramic, is designated F2220. Should there be a requirement for coatings with higher

hardness, materials with a high content of hard phases are available. These include F5122 and

F2186, both of which contain chromium carbide. In some cases, even pure oxide

ceramics are applied, as can be seen in Table 2. As all of these materials are powder based,

there is almost no limitation if it comes to mixing and blending powders to create new

customized materials. For example, various carbides can be added to corrosion-resistant

matrices in order to produce a coating which is both hard and corrosion resistant. The powder

based approach also makes it easy toblend in solid lubricants, such as MoS2, WS2, or ZnO.

In principle, all powders suited for plasma spraying can be processed. A typical coating

thickness after final machining (honing) is in the range of 120 - 130m for passenger and

high performance car applications. For heavy-duty diesel engines, the target coating thickness

is around 150m. Including the honing allowance, the coating thickness in the as

sprayedcondition is around 230 - 280m.

Final Machining of the Coating (Honing)

The performance requirements for a honed cylinder surface are high, and will further increase

as the injection and combustion pressures increase, and even lower friction coefficients,

greater corrosion resistance, and lower wear rates are targeted. Recent results indicate that the

surface, apart from having the suitable chemical composition, should exhibit small and

homogeneously distributed open pores in order to take the full benefit of a plasma coated

cylinder surface. The final machining of the plasma coating is a very important step which is

carried out by honing with diamond ledges. The final surface has a so-called mirror finish,

without a plateau honing pattern. Over the past few years, the recommended specification for

the honed, plasma coated surfaces has been refined, based on engine tests with measurements

of reduced lube oil consumption (LOC) and fuel-oil consumption (FOC).

From the evidence presented in this paper, it can be seen that the APS cylinder bore coating

solution is both versatile and robust, even in automotive mass production. It has emerged,

especially over the past few years, as a leading-edge choice of Original Equipment

Manufacturers (OEMs) for both new and re-manufactured cylinder blocks and liners. As a

proven and robust product capable of withstanding the rigors corrosion at high speed and

highly automated industrial production processes, this technology has become a preferred

solution for many engine professionals. A broad base of materials extends the solution

capability of this system into all types of reciprocating engines, no matter what the

application or design parameters of the engine are. The optimization of the cylinder running

surface coated with a porous APS coating with an adjusted ring package and piston geometry

led to an improvement of the LOC in a high performance engine of BRP Powertrain of 35%

compared to the current engine in the market that uses a cast iron sleeve. The changes to the

piston rings and the piston geometry were specifically made to take full advantage of the

mirror finished, porous APS cylinder running surface coating.

CERMAKROME SALT SPRAY REPORT

An Independent test was performed comparing TLines Cermakrome to another major

manufacturers similar product. Both coatings were applied using the same procedures. In

the past, salt Spray tests for metallic Ceramic coatings have been terminated at 5000 hrs.

Cermakrome has passed this test at 5000 hrs several times. In this case the company

interested in the results decided to run the test until failure. The photos on the right show

each coating at 1000, 2000, 4000 and 6524 hrs.

Salt Spray testing was done to ASTM B 117 standards. This consists of a continuous spray of

salt water at a 5% concentration. The water is heated to 100f. The test coupons are cold rolled

steel and have had a line scratched through the coating to bare metal. The coated test coupons

are evaluated periodically to see if any rust is penetrating the coating, developing in the

scratched area or is creeping in from the exposed edges of the test coupons. Red rust is the

culprit and is evident in the photos as a brown/red stain. The white areas on the coupons are

areas of the surface that oxidized. This is typical for an aluminum filled coatings such as

these and is not an area of concern. Red rust is the problem, as any significant amount

indicates a failure of the coating to protect the substrate.

PRODUCT DESCRIPTION:

MCX Cermakrome is a metallic ceramic coating capable of providing extremely high

levels of corrosion and chemical protection in very thin films. When applied to exhaust

systems MCX will withstand substrate temperatures of over 1300f. In addition direct flame

will not cause delamination, as long as substrate temperatures do not exceed this temperature.

MCX will handle environmental temperatures of up to 1600f. Due to its unique ceramic

nature, the coating also functions as a very effective thermal barrier, with reduced thermal

radiation characteristics. Requires a 500F cure, with maximum hardness achieved with a

650F cure. After curing the coating requiresburnishing to be sealed. This burnishing

process can produce a very high luster, near chrome appearance. The coating cures out to a

very hard surface with excellent adhesion. The water based solvent system provides for a

bake/cure cycle that is not as hazardous as other thermally cured resin systems. MCX can

be applied to all metals except Magnesium.

References:

1. Ioannis Gravalos, Dimitrios Kateris, anagiotis Xyradakis, Theodoros Gialamas, CAVITATION

EROSION OF WET-SLEEVE LINERS: CASE STUDY Technological Educational Institute of

Larissa, Faculty of Agricultural Technology, Department of Agricultural Machinery &

Irrigation.

2. engines Farzad Mohammadi Akram Alfantazi, Corrosion of ductile iron exhaust brake

housing in heavy diesel , Department of Materials Engineering, The University of British

Columbia, Vancouver, BC, Canada

3. M. Hetmaczyk co-operating with L. Swadba, B. Mendala*, Advanced materials and protective

coatings in aero-engines application, Department of Materials Science and Metallurgy, Silesian University of Technology.