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Plasma spraycoatingprocesses

B. J. GillR. C. Tucker, Jr

Plasma spray deposition is one of the most important technologies available forproducing the high-performance surfaces required by modern industry. Over thepast 25 years, there have been significant advances in the understanding of plasmaphysics and in the development of spraying equipment and techniques. This hasenabled a range of materials including metals, alloys, ceramics, and cermets to beplasma sprayed on to a great variety of substrate types and geometries. Duringthis period, the uniquely aggressive environment within the gas turbine engine hasprovided not only some of the greatest challenges to plasma spraying technology,but also some of its most successful applications. In this paper, the nature ofplasma and plasma spray devices are discussed and factors affecting coatingquality are considered. Practical aspects of plasma spraying are considered andfinally the application of plasma spray coating processes to the protection ofhigh-temperature gas turbine components is discussed using as examples turbineblade overlay coating, coatings for hot gas path seals, and ceramic thermalbarrier systems. MST/282

© 1986 Union Carbide Corporation. Dr Gill is with Union Carbide UK Ltd,Coatings Service Division, Swindon, Wiltshire. Dr Tucker is with MaterialsDevelopment, Union Carbide Corp., Indianapolis, Ind., USA.

Plasma spray principles

Plasma spray processes utilize the energy contained in athermally ionized gas to melt partially and propel finepowder particles on to a surface such that they adhere andagglomerate to produce coatings.1 Plasma itself consists ofgaseous ions, free electrons, and neutral atoms.

The functions of a plasma spray torch are to generateand sustain a captive high-temperature region so thatpowder particles introduced into that region can be heatedand accelerated on to a workpiece. High temperature isachieved by concentrating the power of an electric arc intoan extremely small volume,2 while acceleration is achievedby appropriate design of the spraying nozzle.

Plasma torch design

The elements of a plasma spraying torch are shown inFig. 1. In such a torch, a gas, normally argon or nitrogen,is caused to flow around a tungsten cathode and throughthe annular space between the cathode and a contouredcopper anode. While the gas is flowing, a high-frequencyelectrical discharge is used to initiate a direct current arcbetween the electrodes which is carried by the ionized gasplasma. This produces a small region of extremely hightemperature into which powder particles can be intro-duced. Arc current and voltage in a plasma torch vary withelectrode design, gas flow, and composition while powerconsumption is normally in the range 5-80 kW. Localplasma temperatures in spraying torches are normally inthe range 10000-15000°C, but can in certain devices reach30000°C.

The plasma temperature and gas velocity profilesthrough the anode are governed by electrode geometry, gasdensity, mass flow rate, and current-voltage conditions.Arc core temperature depends on the degree to which thearc can be constricted inside the torch. Arc constriction isachieved by reducing the anode bore diameter and byutilizing phenomena known as thermal and magnetohydrodynamic pinch.

While high local temperatures cause gas expansion, thegas velocity through the torch anode is controlled bygeometry. Most conventional plasma spray torches use

subsonic gas flow conditions, but the' incorporation ofconverging/diverging nozzle geometry in the anode canenable supersonic gas flow rates to be achieved.3,4 Thevelocity achieved by powder particles in the plasma streamdepends on the mass flow rate of the plasma and thedistance over which particles are carried. Powder tempera-ture is a function of plasma temperature, plasma composi-tion, and transit time in the plasma stream. Powdervelocity and temperature are also functions of particle size,composition, density, heat capacity, conductivity,emissivity, and several other variables. 5 It follows,therefore, that the point of entry of powder into a plasmastream is of great importance in the design of a plasmaspraying torch.

Ideally, the powder should be introduced uniformly andupstream of the anode to allow the best distribution in theplasma stream and the longest transit time. Many torchmanufacturers have not been able to overcome powderadhesion to the throat of the anode and the resultantanode blockage and overheating. As a result, powderentry is normally arranged either where the nozzle divergesor just beyond the exit (as shown in Fig. 1). There is nodoubt that powder introduction and metering will continueto receive attention from torch designers. In the meantime,with powder velocities for conventional torches in therange 120-350 m s-1, and with values of 400-550 m S-1

claimed for certain high-velocity torches,4 it is widelyaccepted that powder velocity is one of the major factorsaffecting the final quality of a plasma sprayed coating.

Transferred arc surfacing torches are outside the scope ofthis paper and will not be discussed. However, theycontribute significantly where substrate heating is required,for example, to fuse a deposit in situ. Heating is achievedby making the substrate anodic rather than the torchnozzle.6

Plasma and materials interaction

Plasma sprayed coatings are built up particle by particlewith extremely high cooling rates of 106-108 K s -1 beingcommonplace because of the differences in thermal mass ofthe particles and most substrates. With local plasmatemperatures of 15000°C it is clear that given sufficientdwell time, the temperatures in most plasma torches are

Materials Science and Technology March 1986 Vol. 2 207

208 Gill and Tucker Plasma spray coating processes

PLASMAANODE FLAME

GAS

1 Schematic of plasma torch

high enough to melt and vaporize any· material. Powderheating in a plasma stream is largely a result of therecombination of electrons and ions on the surfaces of theparticles themselves.

The final temperature attained by powder particles is,therefore, a function of dwell time in the plasma stream,catalytic surface activity, emissivity, heat capacity, thermalconductivity, and shape.7 Spraying parameters requirecareful development to avoid overheating of the powderparticles which may cause vaporization. Metals tend toheat up more rapidly than most oxides and, thus, indeveloping spraying parameters it is necessary to takeaccount of this to achieve dense and well bonded deposits.

So far, the interaction of plasma and particles has beenconsidered in terms of the physical effects of heating andacceleration. Chemical reaction between plasma gas andparticles is generally assumed not to occur because of theinert nature of argon and helium, the gases used mostfrequently in plasma spraying. If however, a high-velocitystream of hot, fine metallic particles is projected throughthe air on to a substrate it is found that air is inspiratedinto the gas stream and that oxygen in particular reactswith the surfaces of the powder particles while they are inflight from torch to substrate. On arrival at the substrate,the powder particles are not only cool, but they have alsobeen oxidized. Although they may impact and deform, theyare prevented from achieving good density and bonding bythe presence of surface oxide films. Air-sprayed depositsare characterized by their high volume fractions of includedoxide stringers, low as-sprayed density, and relatively lowbond and cohesive strengths. However, it is essential torealize that for many industrial purposes, such deposits are

/truly cost effective as coatings. Oxides are obviously lesssensitive to the effects of oxidation than are metals.

The quality of metallic coatings with respect to oxidelevel, density, and cohesive strength can be improved inseveral ways, but notably by reducing the effects ofoxidation during spraying. Coatings can be improved, at aprice, by protecting the powder stream in flight from torchto substrate. A.number of devices have been designed fortorch effluent protection, ranging from inert gas shieldingto spraying inside chambers under a low pressure of inertgas. The advantages and disadvantages of some of thesedevices in practical plasma spraying are discussed below.

Equipment for plasma spraying

The equipment required for plasma spraying in practicedepends on the purpose for which the deposited coating isintended. Thus, it can be in a form as simple as a plasmatorch with power and gas controls together with a powder

Materials Science and Technology March 1986 Vol. 2

dispenser for spraying in air or as complex as a lowpressure spraying chamber with automated roboticmanipulation of torch and component. Similarly, theancillary equipment can be in the simple form of a gritblast cabinet to prepare surfaces for coating or it caninclude a~tomated sputtering or transferred arc cleaning,post-coatIng vacuum heat treatment, and automatedsurface finishing techniques.

Returning to basic principles, the requirements of aprotective coating whether it be applied by plasmaspraying or by any other method are:

(i) it must resist the effects of its operating environment(ii) it must be chemically and physically compatible

with its substrate(iii) it must adhere to its substrate throughout its

working life(iv) it must be a consistent performer(v) it must be capable of being applied uniformly to

complex components to meet quality standards(vi) it must represent a cost effective solution to an

engineering problem.

The requirements of the coating clearly impose a numberof conditions on the plasma spray operation and it isequally evident that process control is critical.lt is'appropriate to consider the several steps in plasma coating.

SURFACE PREPARATIONTo ensure that a plasma sprayed deposit adheres to itssubstrate, the first requirement is that the substrate is cleanand ready to accept it. This means not only that all oxidescales and other foreign matter must be removed, but alsothat oils and machining lubricants must be eliminated.Components should therefore be degreased chemically.Virtually all plasma sprayed coatings require a roughenedsubstrate to increase the surface area and providemechanical interlocking for the sprayed particles.

The options to clean by reverse sputtering or transferredarc are available in chamber spraying, but the mostcommon method of surface preparation is by abrasiveblasting. This removes surface scales and roughens thesurface. The type of grit and blast pressure used isdetermined by the nature of the substrate material; for softsubstrates, chilled steel is used frequently. For hardersubstrates, an alumina or silicon carbide grit is used toachieve a -sharp peaked topography and a roughnessaverage of 4-5 Jlm (150x 10-6 in centreline average) ormore. Control of grit blasting parameters is essential toachieve consistently high bond strengths. Overblasting, forexample, can reduce mechanical interlocking by roundingoff sharp peaks and can also embed surface scales. Thisleads to reduced bond strength and corrosion under thecoating.

Having prepared the workpiece surfaces for coating it isessential to carry out the coating operation as soon aspossible to prevent recontamination.

MASKINGMany coating operations are carried out in the final stagesof component manufacture and therefore attention must bepaid not only to the areas where coating is required, butalso to the areas which must be kept free of coating. Inmost cases, prevention of coating deposition by masking ispreferable to removal by grinding. For low-velocity, longstand-off, plasma torches there are many types of tape andpaint which are satisfactory. However, where particlesimpact at higher speeds, heavier duty masking is necessary.Aluminium or steel adhesive back foil, glass fibre re-inforced tape, or sheet steel masking are used success-fully. Good masking design can have a significant effecton the cost and efficiency of a spraying operation.

Coating equipment

There are many types of plasma spray equipment availablein the market-place. Most of these enable the purchaser tostart with a basic system for spraying in air and then tobuild up to a fully automated operation in a controlledenvironment. Similarly, there are companies that operatesophisticated equipment and proprietary processes on aservice basis, freeing component manufacturers from theneed to absorb another technology. A detailed technicalreview of the plasma equipment available on the market oroperated by service companies is beyond the scope of thispaper, but certain broad comments can be made.

In general, power supplies to the plasma torch shouldbe ripple-free and ammeters and voltmeters should be cali-brated regularly. Gas control can be achieved with criticalflow orifice columns or with rotating flowmeters, but inboth cases the control device must be capable of measur-ing accurately and delivering constant flow.

The requirements of monitoring and control applyequally to the supply of powder to the plasma torch and,to achieve consistently high quality coatings, powder of thecorrect type must be delivered at a constant rate. Anumber of different types of powder dispenser are availablein the market-place based on spiral or aspirated flow, or onfluidized-bed principles, but, irrespective of type, calibra-tion and control are essential.

A discussion of powder feed criteria would be incompletewithout considering the nature of the powder itself. Mostof the powder used for plasma spraying is between 5 and60 J.lm in diameter and, to achieve uniform heating andacceleration of a single component powder, a narrow sizedistribution is preferred. It is found that the cost of particlesizing is recovered by improved deposition efficiency andcoating quality. In general terms, finer powders areaccelerated and heated more rapidly in the plasma stream,but correspondingly they lose momentum more rapidlywhen sprayed at greater stand-off distances. Finer particlesalso generally give denser deposits, but with higher internalstresses and oxide levels. They also tend to cause moretorch operating problems than do coarse particles becauseof fusion and torch blockage.

Clearly, control of powder is essential, not only duringmanufacture, but also during use. In particular, the powdershould be kept clean and dry to avoid dispenser and torchblockage problems.

Auxiliary equipment and processlimitations

In a line-of-sight process such as plasma spraying, themetallurgical structure of the coating varies with the angleof deposition. Coatings having the highest density andbond strength are achieved with 90° spraying incidence andthe extent to which quality changes with spray angle isrelated to the type of plasma torch used. For example, inlow-velocity torches, a greater sensitivity to spray angle-may be observed. 8

The need to achieve a spray angle of as near to 90° aspossible and to maintain torch-to-workpiece distanceduring spraying can cause problems in coating complexcomponents. In these cases, components may have to beset up several times to coat various surfaces or alternativelyan automated torch and part manipulation machine can beused.

In spraying the surfaces of complex components, it hasbeen found that the use of hand held torches leads tostand-off variations, poor thermal control, non-uniform

Gill and Tucker Plasma spray coating processes 209

coating thickness and quality, and variations in depositionrate. This has led to the development of a number of torchand part handling devices varying in complexity from thosebased on the lathe to those incorporating computercontrolled part and torch movement.

The hardware and software required for part manipula-tion are obtainable in the market-place, at a price, andprovide highly reproducible coating quality provided thatthe basic controls over torch parameters and powder feedare exercised. One of the points most frequentlyoverlooked in plasma spraying is that the plasma torchitself occupies a volume that may physically prevent thepowder stream being delivered at the most suitable angleand stand-off to give optimum coating quality. As anexample, some gas turbine blades are much smaller thanmany plasma torches and therefore stand-off and angle areless than ideal. This can mean that coating quality can bedifferent on different parts of a small turbine blade. Theproblem can be overcome by improved torch and parthandling, by inert gas shrouding to give effluent protection,or by the use of low-pressure chamber spraying9 to reducethe effects of stand-off.

A further limitation related to the physical size of torchesis the ability to apply coatings to internal diameters.Equipment manufacturers1o,11 have addressed this andthere are torches which will fit physically inside diameters~ 45 mm and apply coatings at 90° spraying incidence.Small torches in general require careful assembly andmaintenance and also require very precise control of powerand gas supplies.

Considerable effort by equipment manufacturers, servicecompanies, and part handling systems manufacturers isdevoted to developing processes which will allow all partsof modern engineering components to be coated underoptimum conditions. The demands for coatings on high-temperature gas turbine components is a major drivingforce for service companies and equipment manufacturers.

The application of plasma spray processes to theprotection of high-temperature gas turbine components isconsidered below and the various processes are compared.

Protection of gas turbine components

The gas turbine environment is one of the most aggressivein modern engineering. Materials in the hottest parts ofengines commonly operate at temperatures above theirmelting points, under creep and fatigue conditions, and inenvironments which are erosive and corrosive.

To achieve the goals of greater power and efficiency withlonger lifetimes and lower costs, gas turbine materialsselected primarily for their high-temperature strength mustbe protected by coatings. It is essential to recognise thatany coating applied to a gas turbine component must becost effective and fit-for-purpose. Thus, the pursuit of themetallographically perfect deposit is not a commercialproposition unless engine performance indicates thatnothing less will satisfy and the particular engine builderindicates that the price for perfection is acceptable.

TURBINE BLADE OVERLAY COATINGThe protection of the gas washed surfaces of gas turbineblades against oxidation and hot corrosion has attractedgreat interest in terms of the alloys developed for thispurpose and the competition between coatings processes.Turbine blade overlay coating requires the uniformdeposition of dense, oxide-free alloy, which is diffusionbonded to the base material. Coatings are usually between75 and 125 J-lmthick. The materials used as coatings aremetallic and formulated largely for their oxidation



a

ca CoNiCrAIY (LCO 22); b CoCrAIY (LCO 29); c high-Cr CoNiCrAIY (LCO 37); d NiCrSi

2 Coatings deposited by argon shrouded plasma spraying

resistance. They are often members of the MCrAIY family(M = Co, Ni, Co-Ni, or Fe). There are however a numberof coating materials that include other oxide formers suchas silicon, which are designed specifically for resistance toindustrial and marine environments. There are more thanfifty MCrAIY coating compositions available, a largenumber of which are patent protected. Many of thesecoatings have been deposited on turbine blades byspraying, evaporation, and other processes. Selectedcompositions of typical MCrAIY materials are given inTable 1.

The challenges presented in gas turbine blade coatingprocesses are:

(i) to apply thin, uniform, dense, clean, and wellbonded coatings

(ii) to achieve the above on part geometries whichinvolve small radii and re-entrant angles

(iii) to achieve the above without affecting blade coolingpassages

(iv) to achieve the above with a surface finish that isaerodynamically acceptable

Table 1 Nominal compositions of some MCrAIY coat-ing alloys, wt-%

Desig nation * Co Ni Cr AI Y Mo

UCAR LCO 5 70 19 10 0·5UCAR LCO 7 63 23 13 0·6UCAR LCO 22 39 32 21 7·5 0·5UCAR LCO 29 73 18 8 0·5UCAR LCO 37 44 23 30 3 0·5UCAR LN 11 23 48 17 12 0·3UCAR LN 21 22 49 21 7·5 0·5UCAR LN 34 0·5 67 20 11 0·5 0·5

* UCAR is a trademark of the Union Carbide Corp.


(v) to achieve the above consistently on large numbersof blades in a cost effective manner.

Metallurgical quality requirements usually preclude the useof air spraying for turbine blade overlays, because usingthat process the bond strength is too low and porosity andoxide levels are too high. Thus, the process used forturbine blades requires a form of torch effluent protection.

The principal plasma spray process in the UK forcoating turbine blades in production quantities is the argonshrouded torch process,12 which is offered by UnionCarbide on a service basis using proprietary technology.Torch effluent protection, which results in clean oxide-freedeposits, is achieved by surrounding the powder streamwith a local shield of argon gas. The torch used is ofpatented design13 and the total service offered includessurface preparation, plasma spray coating, vacuum heattreatment, surface finishing, and peening, together with fullinspection, quality control, documentation, and technicalspecialist support. The microstructure of CoNiCrAIY andCoCrAIY coatings produced by the argon shroudedprocess is shown in Fig. 2a and b, respectively. Themicrostructure of a high-chromium CoNiCrAIY and asilicon-containing coating for industrial turbine bladesdeposited by the same process is shown in Fig. 2c and d,respectively. Argon shrouded plasma spraying operated byUnion Carbide UK is a proven industrial and commercialprocess with more than 30000 HPI military turbine bladesprocessed in the last two years.

In addition to current blade coatings, the argonshrouded spray process is being used by Union Carbide tospray metallic and ceramic coatings for marine andindustrial turbines on components ranging from blades andvanes to combustor cans and flare heads.12,14

Apart from the clear advantage of having componentscoated as a service by a firm having the necessary technical

expertise, the technical advantages of the argon shroudedtorch process are:

(i) the equipment is simple to operate and requires arelatively low capital outlay

(ii) all component· handling and processing is carriedout under normal clean workshop conditions

(iii) components can be processed in large numberbatches

(iv) torch movement is programmed to give consistentlyaccurate and uniform deposition

(v) spraying is carried out in a localized Inertatmosphere provided by an inert gas shield on thetorch front to give clean, dense coatings

(vi) metals and ceramics can be sprayed on to blades,vanes, and combustor components as overlays andthermal barriers

(vii) over 95% product yield is obtained in productionwith this coating process.

As with all processes, there are certain limitations andthese are:

(i) relatively short stand-off distances require pro-grammed torch movement to coat complexcomponents

(ii) deposition on to substrates at high temperaturecannot normally be carried out, because the processoperates under normal atmospheric conditions.

The alternative plasma spray processes for overlay gasturbine blades and vanes are the low pressure or vacuumchamber processes.9 These processes also provide torcheffluent protection and among their advantages are:

(i) components can be preheated without oxidation tominimize internal stress effects and allow low-stressthick deposits to be applied

(ii) the plasma jet becomes elongated allowing longerstand -off distances to be used minimizing the effectsof component and torch geometry.

Among the disadvantages of chamber processes are:

(i) the equipment is available to purchase, but thecapital outlay for large vacuum chamber sprayingequipment is extremely high

(ii) vacuum is not a normal working environmentI5 andcomponents must be handled through vacuuminterlocks, and there may be shape or sizelimitations

(iii) powder particles enter the pumping systems andcause poor environment control; thus, maintenancecosts are reportedly very high

(iv) chambers must be kept scrupulously clean toachieve high quality coatings.

Examples of the high-quality MCrAIY deposits obtainableusing chamber spraying are shown in sales literature (seee.g. Refs. 9 and 11).

The argon shroud process and the chamber sprayprocesses are each used to coat turbine blades and othercomponents. However, at the time of writing, the argonshrouded plasma spray process is making the greatestimpact in the blade overlay coatings field, because of itsgreat versatility and consistent quality. Conversely, thechamber processes have some advantages when extremelythick deposits (3 mm) are required, e.g. on hot gas pathseals.

COATING OF HOT GAS PATH SEALSClearance control between turbine blade tips is a majorfactor in achieving turbine efficiency. Hot gas path sealsare required to resist high-temperature oxidation and to be


able to tolerate blade tip rubbing without damaging theturbine blade seal fins. Materials of the MCrAIY type usedfor overlay blade coating are used in thicknesses of up to3 mm on hot gas path seal segments for both military andcivil gas turbines. The current challenge in coating hot gaspath seals is basically to apply thick MCrAIY coatings ofhigh density, low oxide content, and low porosity ascheaply as possible. There is also great interest in thedevelopment of plasma sprayed ceramicI6 and bondedceramic seal systems. 1 7

For metallic coatings, plasma spraying in air is usuallyruled out, because of the high oxide content in the deposits,The main competitive plasma processes are argonshrouded plasma spraying and chamber spraying.

The main advantage of chamber processes for thickcoatings is that substrates can be heated without oxidationbefore coating and therefore thick low-stress deposits canbe applied to a hot ( '" 900DC) alloy in two steps or often inone. Deposition on to a hot substrate under vacuum orlow-pressure conditions enables a good bond with thesubstrate to be developed in situ. The main disadvantage,however, is that it is not straightforward to raise a largebatch of seal segments to temperature in a chamber andmanipulate them in front of the torch. This can mean thatthey have to be processed in small numbers or evensingly.

The benefits of the argon shrouded plasma spray processare the same as those for turbine blades, but with thecapability of part handling in large batches under normalatmosphere conditions. Coating quality is excellent, beingwell bonded to the base material after heat treatment andalso having a flat profile which minimizes the waste as theseal segments are later machined. The disadvantage isthat the coating is applied to a metallurgically coldsubstrate and a thick deposit has to be build up in layerswith intermediate vacuum heat treatments. While thisensures a high degree of densification, special attention isrequired to minimize part handling.

Both the argon shrouded and chamber spray processesare in current production for the coating of seal segmentsfor a variety of US, UK, and European military and civilengines.

THERMAL BARRIERSFor many years there has been interest in reducing the rateof heat flow through cooled turbine blades to eliminate theneed for cooling air from the compressor.18 Thermalbarriers based on ceramic insulating layers bonded withoxidation resistant alloys have been tested in manyconfigurations, in many applications, and with differingdegrees of success.

On non-rotating components such as combustor canflare heads, fuel vaporizers, and deflector plates, MCrAIYbonded ceramic systems have shown great benefits. Inthese applications, even air sprayed nickel aluminide andMCrAIY bondcoats with high oxide contents combinedwith magnesia stabilized zirconia insulating top coats haveshown great improvements in practice. The challenges incoating combustor cans are those of coating internaldiameters at non-optimum spraying angles on a wide rangeof component sizes. Component size may preclude bondcoat heat treatment.

Thermal barriers have failed in service by loss of ceramicas a result of erosion or impact damage, by spallationunder thermal cycling, and by bond coat oxidation.

For the highest duty applications, there has been animprovement in MCrAIY bond coats from NiCrAIY toCoNiCrAIY and a progressive change from magnesia-to yttria-stabilized zirconias. The objectives here are toimprove bond coat oxidation resIstance and to unitebond coats with ceramics of improved thermal shock



3 MCrAIY bonded yttria-zirconia thermal barriersystem

resistance.19 Typical MCrAIY bonded thermal barriersystems are given in Table 2. There are alternative systemsapplied by chamber spraying.

As yet, the application of bonded ceramic thermalbarriers to rotating turbine blades is far from routine,although there is intense activity in. this field. MCrAIY-bonded yttria-stabilized zirconia patches have been appliedsuccessfully, using the argon shrouded spFay system, to thegas washed surfaces of a number of guide vanes andturbine blades. The microstructure of such a system isshown in Fig. 3. Activity in thermal barrier development isintense and many types and configurations of thermalbarrier are available from both the argon shrouded and thechamber spray processes.

During development work with the argon shroudedsystem, it was shown that high-quality MCrAIY coatingscould be applied to certain types of pack aluminizing andthat these layers in turn could be used as thermal barrierbond coats. An example-of the structure of such coatings isshown in Fig. 4. This capability may extend the applicationof MCrAIY-bonded thermal barriers because, for manyapplications, pack aluminizing is the industry standard forturbine blades.

Conclusions

The application of technically successful and. commerciallycost effective coatings for gas turbine components has-presented some of the greatest challenges in the develop-ment of plasma spray processes. As turbines are required to

4 MCrAIY bonded thermal barrier sprayed over packaluminizing using Ar shrouded spraying process

produce more of the world's power from fuels of decreasingpurity this trend must continue. The following can be saidconcerning future developments.

1. Argon shrouded and chamber plasma spray processeswill both be required to deposit better materials at lowercost and with as few auxilliary process steps as possible.

2. Thickness profiling around aerofoils will be requiredto maintain turbine throat size and component efficiencies.

3. Coatings will have to be deposited with aerodynami-cally acceptable surface finishes.

4. Plasma sprayed coatings will have to be made frommaterials which can be deposited to be engine cyclecompatible in mechanical and corrosion terms.

5. Plasma spray processes will be used to fabricatecomplex gas turbine components from metallic andceramic materials.

6. The gas turbine will continue to challenge plasmaspray processes to produce appropriate surfaces.

While an objective view of a number of plasma sprayprocesses is presented, the authors of this paper are morefamiliar with argon shrouded· plasma torch technology.Technical information on competitive processes taken frompublished literature has not necessarily been confirmed bythe authors.

Acknowledgment

The assistance of Mr G. J. Higgs of Union Carbide UKLtd with metallography is gratefully acknowledged.

Table 2 Nominal compositions of some MCrAIY bonded thermal barrier systems, wt-%

Bond coat Ceramic

Designation Co Ni Cr AI Y Zr02 MgO Y203

UCAR LTB 4 23 48 17 12 0·3 75 25UCAR LTB 5 22 48 20 9 0'5 75 25UCAR LTB {) 23 48 17 12 0·3 88 12UCAR LTB 7 22 48 20 9 0·5 88 12UCAR LTB 8 39 32 21 7·5 0'5 75 25UCAR LTB 12 '-,,39 32 21 7·5 0·5 93 6-5UCAR LTB 13 39 32 21 7·5 0·5 93 6-5

* Bond coat thickness is usually 181lm (0'007 in) and ceramic thickness is usually 250-300 Ilm (0-01 in-0'012 in)_ LTB 13 is processed to produce acontrolled microcrack density. In this table the development of thermal barrier systems is shown; not all the coatings are available commercially.



References

1. R. M.GAGE:US Patent 2806124, Union Carbide Corporation.2. R. M. GAGE,O. H. NESTOR,and D. M. YENNI:US Patent

3016447, Union Carbide Corporation.3. E. MUEHLBERGER:US Patent 3914573.4. 'Metco 7M Plasma Spray Process', Metco Bulletin 205,

Metco, Westbury, NY.5. M.VARDELLE,R. McPHERSON,and P. FAUCHAIS:in Proc. 9th Int.

Conf. on 'Thermal spraying', The Hague, May 1980,NederlandsInstituut voor Lastetechnik, 155.

6. D. R. MARANTZand s. J. RICHARDSON:Proc. 9th Int. Conf. on'Thermal spraying', The Hague, May 1980, Union Carbide,207. .

7. N. EL-KADAH,1. MCKELLIGET,and J. SZEKELY:Me tall. Trans.,1984, l5D, 59.

8. R. F. SMARTand J. A. CATHERALL:in 'Plasma spraying';1972, London, Mills and Boon Technical Library.

9. P. C. WOLFand F. N. LONGO:in Proc. 9th fnt~Co~-on-'Thermal spraying', The Hague, May 1980,Nederlands Instituutvoor Lastetechnik, 187.

10. 'Metco Type 11MB plasma gun', Metco Bulletin 258, Metco,Westbury, NY.

fT. 'pla-sin-a-Inslae spray gun type SM-Fl', Sales Literature,Plasma Technik AG, Wohlen, Switzerland, 1984.

12. B.J.GILL:in'Thermal spraying', 164;1983,Dusseldorf, DeutscherVerlag fUr Schweisstechnik.

13. 1. E.JACKSON:US Patent 3470347, Union Carbide Corporation.14. R.C.TUCKER,Jr,T.A.TAYLOR,and M.H.WEATHERLY:in Proc. 3rd

Conf. on 'Gas turbine materials in the marine environment',Bath, Sept. 1976, US Naval Ship Engineering Centre and UKShips Department, paper 2.

15. P. C. WOLF:in 'Thermal spraying', 95; 1983, Dusseldorf,Deutscher Verlag fur Schweisstechnik.

16. G. T. SHIEMBOB:'Development of a plasma sprayed ceramic gaspath seal for high pressure turbine applications', CR 135183,NASA, 1977.

17. A. SICKINGER:in 'Thermal spraying', 140; 1983, Dusseldorf,Deutscher Verlag fur Schweisstechnik.

18. D.S.DUVALLand D.L.RUCKLE:in Proc. Conf. on 'Gas turbines',London, Apr. 1982, Institution of Mechanical Engineers,82-GT-322.

19. T. A. TAYLOR,M. O. PRICE,and R. C. TUCKER,Jr: in Proc. 84thAnnual Meeting of the American Ceramic Society, Cincinnati,Ohio, May 1982, American Ceramic Society.

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months to predict the full creep strain by RWEvans and BWIlshIreand creep life characteristics ofcomplex alloys for times up to 10 yearsand more.

Whether readers support or opposethe innovative concepts presented, itwill be impossible to ignore the impactof this major contribution to theliterature available to scientists andengineers concerned with theoreticaland practical problems in the field ofcreep and creep fracture.
