THERMAL BARRIER COATINGS MATERIAL SELECTION, METHOD OF .
Int. J. Mech. Eng. & Rob. Res. 2014Vishnu Sankar, 2014ISSN 2278 – 0149 www.ijmerr.comVol. 3, No. 2, April, 2014 2014 IJMERR. All Rights ReservedReview ArticleTHERMAL BARRIER COATINGS MATERIALSELECTION, METHOD OF PREPARATION ANDAPPLICATIONS - REVIEWVishnu Sankar1**Corresponding Author: Vishnu Sankar, vishnusankar369@gmail.comThe desire to reach higher efficiencies, lower specific fuel consumption and reduced emissionsin modern engines has become the primary focus of engine researches and manufacturersover the past three decades. Ceramic coating is a solution to such problems as they providegood thermal barrier properties for designers. In the design of adiabatic engines, reducing incylinder heat rejection requires very special thermal barrier coatings on the engine combustionchamber. Partial Thermal barrier coating (TBC) on the top surface of the piston is considered asa solution for reduction of unburned Hydrocarbon (HC) emission produced by incompletecombustion with respect to crevice volume when engines start. The TBC on the top pistonsurface decreases the thermal conductivity and increases the unburned charge oxidation, sothat the metallic substrates will be exposed to lower peak temperature thereby reducing thethermal stress in engine components. Also thermal barrier coatings on other elements ofcombustion chamber of internal combustion engine offer advantages including fuel efficiency,multifuel capacity and high power density. Therefore, thermal barrier coating (TBC) technologyis successfully applied to the internal combustion engines, in particular to the combustion chamber[1].Keywords:Thermal Barrier Coatings, Yttria Stabilized Zirconia, Electrostatic SprayAssisted Vapour Deposition, Hot corrosion, High Velocity Oxygen Fuel process.INTRODUCTIONMetallic coatings were introduced to sustainthese high temperatures. The trend for themost efficient gas turbines is to exploit morerecent advances in material and coolingtechnology by going to engine operatingcycles which employ a large fraction of themaximum turbine inlet temperature capabilityfor the entire operating cycle (A Parlak andV Ayhan, 2007).GeneralHeat engines are based on consideringvarious factors such as durability, performance and efficiency with the objective ofminimizing the life cycle cost. For example,the turbine inlet temperature of a gas turbinehaving advanced air cooling and improvedcomponent materials is about 1500 C.1M.Tech., Thermal Engineering [Department of Mechanical Engineering] Toc H Institute of Science and Technology, Arakkunnam,Ernakulam - 682 313.510
Int. J. Mech. Eng. & Rob. Res. 2014Vishnu Sankar, 2014Thermal Barrier Coatings, as the namesuggests are coatings which provide a barrierto the flow of heat. Thermal Barrier Coatings(TBC) performs the important function ofinsulating components such as gas turbineand aero engine parts operating at elevatedtemperatures. Thermal barrier coatings(TBC) are layer systems deposited onthermally highly loaded metallic components,as for instance in gas turbines. TBC’s arecharacterized by their low thermal conductivity, the coating bearing a large temperaturegradient when exposed to heat flow. The mostcommonly used TBC material is YttriumStabilized Zirconia (YSZ), which exhibitsresistance to thermal shock and thermalfatigue up to 1150 C. YSZ is generallydeposited by plasma spraying and electronbeam physical vapour deposition (EBPVD)processes. It can also be deposited by HVOFspraying for applications such as blade tipwear prevention, where the wear resistantproperties of this material can also be used.The use of the TBC raises the processtemperature and thus increases theefficiency.First, a thermally protective TBC layer witha low thermal conductivity is required as theouter layer to maximize the thermal dropacross the thickness of the coating. This layeris also called the top coat. This coating islikely to have a thermal expansion coefficientthat differs from the component to which it isapplied. This layer should therefore have ahigh in-plane compliance to accommodatethe thermal expansion mismatch between theTBC and the underlying nickel super alloycomponent. In addition, it must be able toretain this property and its low thermalconductivity during prolonged environmentalexposure. A porous, columnar, 100-200 µmthick, yttria stabilized zirconia (YSZ) layer iscurrently preferred for this function.Second, an oxidation and hot corrosionresistant layer is required to protect theunderlying turbine blade from environmentaldegradation. This layer is required to remainrelatively stress free and stable during longterm exposure and remain adherent to thesubstrate to avoid premature failure of theTBC system. It is important that it also providean adherent surface for the TBC top coat.Normally, the thin ( 1 µm), protectivealuminium rich oxide which is thermally grownupon the bond coat is utilized for this purpose(M Cerit et al., 2011.)Structure of TBCModern TBC’s are required to not only limitheat transfer through the coating but to alsoprotect engine components from oxidationand hot corrosion. No single coating composition appears able to satisfy these multifunctional requirements. As a result, a coatingsystem is being used (Bose S, 2007).Researchers have led to a preferred coatingsystem consisting of three separate layersto achieve long term effectiveness in the hightemperature, oxidative and corrosive useenvironment for which they are intended tofunction.Since the aluminium content of modernnickel based super alloy is not typically highenough to form a fully protective aluminascale, an aluminium rich layer (bond coat) isapplied onto which the thermally grown oxidemay get deposited. A 100µm thick layer ofeither a low sulphur platinum aluminide orMCrAlY (where M is Ni or Co) is utilized forthis purpose. Either low pressure plasmaspray (LPPS) or pack cementations are usedto apply the bond coat.511
Int. J. Mech. Eng. & Rob. Res. 2014Vishnu Sankar, 2014 High melting point: The coating shouldbe having high melting point so that itcan withstand high operatingtemperatures without melting away.Figure 1: Layer Structure of ThermalBarrier Coating on a Turbine Blade Low thermal conductivity: The coatingshould have very low thermalconductivity so that it produces aconsiderable drop in temperatureacross the coating. Low density: The coating materialshould be having low density and weightin order to reduce the payload.The thermally generated oxide layer is required to protect the substrate from oxidationand hot corrosion. The choice of base material (Co or Ni) is dependent on the primarycorrosion mechanism, but as engine temperatures increase, the trend is towardsCoNiCrAlY compositions. Cr and Al arepresent in the MCrAlY composition becausethey form highly tenacious protective oxidescales, whilst Y promotes formation of thesestable oxides. MCrAlY coatings may beapplied by a number of processes including: High thermal shock resistance [4]. Resistance to oxidation and chemicalenvironment: The coating materialshould protect the underlying metalfrom oxidation and corrosion. High surface emissivity: The coatingshould have high emissivity so that amajor portion of the incident heat isemitted away. Resistance to mechanical erosion: Thecoating should provide resistancetowards the mechanical erosion causeddue to the various particles present inthe exhaust gas coming from thecombustion chamber. Physical vapour deposition (PVD) [8]. Low pressure (LPPS), vacuum plasma(VPS) or air plasma spraying (APS). High velocity oxy-fuel (HVOF) spraying.PVD and VPS offer high quality in termsof minimal oxidation of the coating during thedeposition process, but are the mostexpensive. Approvals have been granted forthe use of APS and HVOF coatings on certaincomponents with significant cost savings. Itis common for MCrAlY coatings to bedeposited onto components pre-coated withAl, PtAl or Cr, which have been produced byvapour deposition techniques or diffusionprocesses. High coefficient of thermal expansion:The coating material should have acoefficient of thermal expansion whichis higher than that of the substrate sothat it will not crack or fail when it issubjected to high temperatures.Design Options for TBCSome of the innumerable design options withregard to TBC are given below:Fuel FlexibilityCorrosion resistance [5]Alternative fuelsNo derating for heavy fuelsCharacteristics of TBCThere are certain characteristics that a goodThermal Barrier Coating should satisfy. Theyare listed as follows:512
Int. J. Mech. Eng. & Rob. Res. 2014Vishnu Sankar, 2014Availability and ReliabilityCorrosion / Erosion resistanceLower metal temperatureLower transient thermal stressthermal conductivity, k and its relatively highthermal coefficient of expansion (comparedto many other ceramics). This reduces thethermal expansion mismatch with the highthermal expansion coefficient metals to whichit is applied. It also has good erosionresistance which is important because of theentrainment of high velocity particles in theengine gases. The low thermal conductivityof bulk YSZ results from the low intrinsicthermal conductivity of zirconia and phononscattering defects introduced by the additionof yttria. These defects are introducedbecause yttria addition requires the creationof O²? vacancies to maintain the electricalneutrality of the ionic lattice. Since both theyttrium solutes and the O²– vacancies areeffective phonon scattering sites the thermalconductivity is decreased as the yttria contentis increased. YSZ has a room temperature,grain size dependent, thermal conductivity of2.2-2.6 W/mK in the densest form. Addingporosity further reduces k and can improvethe in-plane compliance (Parlak A, et al.,2003; Chan S, H, 2001; Srinivasan K K,2010; Uzun A, 1999; Taymaz I, 2005; ChanS H and Khor K A, 2000; Buyukkaya E et al.,2006; Barbezat G, 2006).EfficiencyReduce coolant flowIncrease the turbine inlet temperatureCapital costEasily cast super alloySimplified coolingThe options stressed above depend on theapplication of TBC. For aircraft turbines,emphasis has been placed on efficiency,durability and capital cost. For example,calculations have shown that the applicationof 1 mm oxide coating to the first two stagesof an aircraft gas turbine can reduce coolingair consumption by 6.1% yielding a net thrustspecific fuel consumption improvement of1.3%. Alternatively, metal temperature andtransient thermal stresses can be reducedsignificantly with more than a four-foldimprovement in blade life.For stationary gas turbines and diesel engines, emphasis has been placed on fuelflexibility and durability. In some cases,ceramics are more corrosion resistant thanpotential metallic coated materials, thuspermitting firing with minimally processedfuels. They also result in lower metaltemperatures; improve creep and thermalfatigue resistance of the substrate metal.METHODS TO PRODUCE TBCAs stated earlier the Thermal Barrier Coatingscan be produced in industries by the followingmethods:(i) Air Plasma Spray (APS)(ii) Electron Beam Physical VapourDeposition (EBPVD)(iii) High Velocity Oxygen Fuel (HVOF)(iv) Electrostatic Spray Assisted VapourDeposition (ESAVD) and(v) Direct Vapour Deposition (DVD)For small aircraft and land vehicles on ICengines, efficiency improvement has beenemphasized.Material SelectionYttria stabilized zirconia has become thepreferred TBC layer material for gas turbineengine applications because of its lowThe EBPVD and APS processes are widelyused in industries whereas HVOF, ESAVD513
Int. J. Mech. Eng. & Rob. Res. 2014Vishnu Sankar, 2014and DVD processes are less frequently used.Two processes are explained below.Figure 2: Schematic of ESAVD ProcessElectrostatic Spray AssistedVapour Deposition (ESAVD)PrincipleESAVD is the process of producing coatingon a heated substrate by spraying chemicalprecursors through an electric field. It is anon- line-of-sight-process. The electric fieldhelps to direct the chemicals on to thesubstrate and initiate the chemical reaction.This leads to the formation of an adherentcoating with the correct chemical and physicalcharacteristics, together with the desiredmicrostructure.The unique spray reaction zone is a distinct environment that enables the chemicalvapour deposition to occur unhindered. Thismeans that coatings normally applied in dedicated reactors using moderate to high vacuums, and hence expensive vacuum systems,can now be applied in open atmosphere. Theresulting deposition equipment is simple toconstruct and can be maintained easily withminimum equipment downtime, requiring arelatively low capital investment.ProcessThe ESAVD process involves the sprayingof atomized, charged droplets containingcarefully formulated mixtures of coatingprecursor material through an electric field,in an otherwise ambient environment,towards a mildly heated substrate. Carefulcontrol of process conditions in the sprayreaction zone (i.e. zone between spraynozzle and substrate) allows the appropriatechemical reactions to occur. These includeevaporation/decomposition of aerosoldroplets and formation of intermediatereactants that undergo chemical reactions inthe vicinity of the heated surface of thesubstrate. This leads to the formation of anadherent coating with the correct chemicaland physical characteristics, together with thedesired microstructure. ESAVD enables theuse of a simple aerosol type precursordelivery system to be combined with thevapour phase based coating of ChemicalVapour Deposition. ESAVD is a non-line-ofsight process, and therefore able to coatcomplex geometries, the electric field helpsto ensure that a very high proportion of theprecursor ends up on the substrate viaelectrostatic attraction. (Boehman A L, 1997).Features The equipment is simple to constructand can be maintained easily with minimum equipment downtime, requiring arelatively low capital investment. This coatings which were normally applied in dedicated reactors using moderate to high vacuums, and hence expensive vacuum systems, can be applied in open atmosphere. The use of electrostatics also ensuresthat ESAVD is a ‘non-line-of-sight’ process and can therefore be used to coateither flat surfaces or complex 3D geometries, e.g. hip implants, engine components and curved windscreens. Produces coatings with variable thickness at variable deposition rates depending on the conditions.514
Int. J. Mech. Eng. & Rob. Res. 2014Vishnu Sankar, 2014 Uniformity and microstructure can beprecisely controlled to produce veryhigh standard coatings, whether involving a dense coating onto a porous substrate or a porous coating onto a densesubstrate.mixing chamber and subjected to combustion in the combustion chamber. This combustion product is made to flow through anozzle and directed on to the substrate toform the coating.Process The electric field directs the precursorto the substrate, thereby minimizinglosses to the surroundings, unlike otherChemical Vapour Deposition process.When ESAVD is optimized over 90%of the precursor will end up on the substrate.In High Velocity Oxygen Fuel process, oxygen and a suitable fuel (acetylene, propylene,propane or hydrogen) is fed into a gun whereit undergoes combustion to produce a highpressure flame. Ceramic powder is also fedinto it axially. This melts the powder, which isthen passed through a nozzle to increase it'svelocity. This process produces dense strongcoatings.High Velocity Oxygen - Fuel (HVOF)PrincipleThe High Velocity Oxygen Fuel (HVOF) process is a subset of flame spray process.There are two distinct differences betweenconventional flame spray and HVOF. HVOFutilizes confined combustion and an extendednozzle to heat and accelerate the powderedcoating material. Typical HVOF devices operate at hypersonic gas velocities, greaterthan MACH 5. The extreme velocities provide kinetic energy which help produce coatings that are very dense and very well adhered in the as-sprayed condition.FeaturesHVOF is commonly used to produce verywear resistant coatings such as cermets (ceramic and metal mixes) like tungsten-carbidecobalt. Coatings of this type have wear resistance similar to sintered carbide materials. Since HVOF produces very dense coatings (porosity levels less than 0.5%), it canbe used to produce very good corrosion resistant coatings from materials such asInconel, Stellite, stainless steel and ceramics.ApplicationsHVOF coatings can be incorporated into thedesign of complex components such as hightech medical devices used for performingcomplex surgeries, to simple componentssuch as bolts used in agricultural combines.Combines process incredible amounts ofcrops during harvesting. The wear createdby the crop going through the machine canbe extreme and in those cases the bolts thatare used to attach critical internal components are coated with carbide materials togreatly extend the life of the bolt. The improved bolt assures the component that thebolt is holding remains attached.Figure 3: Cross Section of an HVOF GunEquipmentThe HVOF gun has three different inlets forthe fuel gas, ceramic powder and for oxygen. The three components are mixed in the515
Int. J. Mech. Eng. & Rob. Res. 2014Vishnu Sankar, 2014APPLICATIONS OF TBCCONCLUSIONThermal barrier coating is actually a ceramiccoating, which is having a layer structure. Itnot only reduces thermal fatigue but also protects the underlying metal from oxidation andcorrosion. It helps to increase the operatingtemperature and also improves the engineperformance. The life of the coated part isincreased to a great extent.Direct vapour deposition is mainly used forproducing coatings on complex surfaces. Itis capable of producing coatings on internalsurfaces of machine parts which cannot beattained by other methods.Turbine Blade ApplicationsThermal barrier coatings have been extensively used to protect the internal surfacesof the combustion chambers in aircraft gasturbines (Ekrem Buyukkaya et al., 2006). Dueto their low absorptivity and low thermal conductivity, such coatings yield a substantial reduction in metal temperature.The currently used coating material (yttriastabilized zirconia) is capable of providingconsiderable protection for the existing engines, but in future, more powerful engineswill be developed and there is a need for abetter coating material (Chan S H and KhorK A, 2000).In turbines, the demands placed on TBCare far more stringent than in the combustor.The high convective heat fluxes encounteredin the turbine results in large thermo mechanical stress in ceramic coatings under bothtransient and steady state conditions. Theaverage and high metal temperature on thehot spots of the turbine range from 50 to100 C higher than in the combustor. Thisplaces great demand on the environmentalresistance of the bond coat. At these highsurface temperatures, plasma sprayed coatings are subjected to process such as sintering and phase changes.Various methods like plasma spray technology, electron beam physical vapour deposition etc. have significantly improved the reliability of TBC turbines, diesel engines andother heat engines. Processing improvementin the control and development of TBC arerequired. Further study on the mechanismscontrolling coating adherence and degradation in clean and dirty environments, the effects of coating composition and structure oncoating properties and correlation of modelsof engine tests are necessary to obtain thermal barrier coating that have even better tolerance to high temperature and thermomechanical stresses.Earlier, thermal barrier coatings for turbineblade applications involved calcia and magnesia stabilized Zirconia and nichrome (Ni20Cr) bond coats (all compositions are inweight percent). The first major advance wasthe identification of ZrO2-12Y2O3 / Ni-16Cr6Al-0.6Y system (Felicia M Pitek and CarlosG Levi, 2007. This new generation TBC offered superior durability compared to the oxide coat mentioned above. The bond coatoperating temperatur
Thermal Barrier Coatings, as the name suggests are coatings which provide a barrier to the flow of heat. Thermal Barrier Coatings (TBC) performs the important function of insulating components such as gas turbine and aero engine parts operating at elevated temperatures. Thermal barrier coatings (TBC) are layer systems deposited on
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