ATMOSPHERIC PLASMA SPRAY COATINGS OF AL2O3 … · 424 Int. J. Mech. Eng. & Rob. Res. 2014 Yathin Kumar L et al., 2014 B. Plasma Generation and Formation The arc is initiated between

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Int. J. Mech. Eng. & Rob. Res. 2014 Yathin Kumar L et al., 2014

ISSN 2278 – 0149 www.ijmerr.com

Vol. 3, No. 4, October, 2014

© 2014 IJMERR. All Rights Reserved

Research Paper

ATMOSPHERIC PLASMA SPRAY COATINGS OF

AL2O3-YSZ SYSTEM FOR HIGH TEMPERATURE

APPLICATIONS

Yathin Kumar L1*, Rahul S2 and Rakshith M2

*Corresponding Author: Yathin Kumar L � [email protected]

Thermal barrier coatings (TBCs) used in gas-turbine engines afford higher operatingtemperatures, resulting in enhanced efficiencies and performance. Here we demonstrate theuse of the commercial manufacturing method of atmospheric-plasma-spray (APS) to fabricateYttria-stabilized zirconia (YSZ)-based TBCs. Thermal Barrier Coatings (TBCs) of Al2O3 - 7-8%YSZ were deposited by Atmospheric Plasma Spray (APS) method onto stationary flat Nickelsuper alloy substrate. This paper describes the APS technique for creating a bond coat of Al2O3

and a top coat of 7-8% YSZ to resist the thermal shock and for high temperature applications.Phase characterization was done by X-ray diffraction and micro structural characterization byscanning electron microscopy. The porosity and mechanical characterizations were alsodetermined by SPIP software. The observations suggest that the surface morphology plays asignificant role in determining the thermal resistance of the coated surface. These coatings andmany other such coating techniques provides with resistance of heat fluxes and retain theoriginal strength of the material even after subjecting it to high thermal shock.

Keywords: Atmospheric plasma spray, SPIP, Zirconia coating, Microstructure characterization

1 Mechanical Engineering, KSIT , Bangalore, India.2 The National Institute of Engineering, Mysore, India.

INTRODUCTION

The materials with superior performance bothin mechanical strength and oxidationresistance are required to be applied for hightemperature applications. Thermal-barriersystems are an excellent example of multi-layered, multifunctional material assembliesthat have enabled substantial improvementsin the performance and efficiency of gas

turbine engines. The combination of ametallic substrate, an inter metallic (or mixedmetallic and inter metallic) interlayer, and alow conductivity ceramic top coat permits thesystem to operate near, or even above, themelting point of the substrate in a highlyoxidizing combustion environment. Intermetallic coatings have long been a centralrequirement for high temperature operation

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of propulsion and power generation systems.In early applications in aero and land basedturbines (before ceramic thermal barrierswere developed), environmental coatingstypically served a single function. Aluminide-type coatings based on NiAl and NiCoCrAl-family coatings became the standard systemsfor oxidation protection, while diffusionchromides and overlay CoNiCrAl materialswere used to protect against hot corrosion.As turbine operating temperatures haveincreased, fuels have become cleaner,substrate alloys have evolved to refractoryrich nickel-based single crystals, andenvironmental coatings have becomemultifunctional. Thermal barrier coatingsystems on super alloys combine a thermalinsulating zirconia top coat, which has arelatively low thermal conductivity and largethermal expansion coefficient compared withother technological ceramic, with a metallicbond coat that protects the substrate fromboth oxidation ( usually with the developmentof stabilized protective thin layer) and hotcorrosion when operated at high temperature.For ease and economy of fabrication, agraded thermal barrier coatings is comprisedof finite number of layers, each have a certainceramic-bond coat alloy proportions. Zirconiais an excellent refractory owing to its highmelting point (about 2680°C), its lowcoefficient of expansion and thermalconductivity, and its resistance to chemicalaction so it is used as a raw material for TBCsystem. Pure ZrO2 has three polymorphs.Monoclinic zirconia (m-ZrO2) is stable up to1170°C, where it transforms to tetragonalzirconia (t. ZrO2), the later tetragonal zirconiachanged into cubic zirconia (c-ZrO2) at2370°C and finally zirconia melt at 2680°C

(E Ryshkowitch and D W Richardson, 1985).The challenges are increasing for developinginter-metallic bond coatings that will reliablyperform in present and future thermalengineering systems. New experimental andcharacterization tools and computationaldesign approaches are to be explored andthis paper makes an attempt towards futureprospects for discovery of new materials andmaterial combinations for prediction of theirperformance in complex thermo- mechanicalenvironments.

EXPERIMENTAL PROCEDURE

The studies presented in the paper focus ongaining a systematic understanding of theinfluence of layered coatings on micro-structure development in plasma-sprayedcoatings. To do this, a fully quantitativemicrostructure characterization was carriedout for aps sprayed coatings by SEM for eachdeposited layers. SPIP software gives thedetailed theoretical results of the mechanicalparameters of the coatings.

A. Fundamentals of Plasma Spray

Spray torch and in most cases (99%) plasmaspraying is achieved by using plasmatorches. A high intensity arc is operatedbetween a stick type cathode and nozzleshaped water cooled anode. Plasma gas,introduced along the cathode, is heated byarc to plasma temperatures, leaving theanode nozzle as a plasma jet or plasmaflame. Fine powder suspended in a carriergas is injected into the plasma jet where thepowder particles are accelerated and heated.As the molten powder particles impinge withhigh velocities on substrate, they form a moreor less dense coating.

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B. Plasma Generation and Formation

The arc is initiated between the tip of thecathode (typically thoriated tungsten) and thewater cooled anode nozzle. The working gasis introduced either axially or with anadditional swirl component. The latterimproves arc stability in the vicinity of thecathode and rotates the anode arc root whichis desirable for reducing anode erosion. Thegas heated by the arc emanates as a plasmajet from the torch orifice. The gas flow rate issufficiently high to ensure a highly turbulentjet with a visible length of several centimeters.Argon and mixtures of argon with other noble(He) or molecular gases (H2, N2, O2 etc.)are frequently used for plasma spraying. Theaddition of He and in particular of moleculargases results in a drastic increase in theenthalpy of the plasma. The maximumtemperature in the plasma jet is a function ofthe design and of the operating parameters.

C. Materials

In the present study C - 263 Nickel superalloywas used as substrate. All the specimenswere in the shape of a flat plate (×40 mm).The specimen’s surfaces were shot blastedwith alumina grit in the range of 50-80 meshand under a pressure of 40-50psi. Thesurface oxides were removed using methylethyl kethon cleaner, and degreasing wasperformed by trichloro ethylene vapor. Afterwashing they were preheated at 150-200°Fand finally coatings were applied overspecimens. Argon was used as primaryplasma gas whilst hydrogen was thesecondary gas. Amdry 962 trade markNiCrAlY micro-powders, micro-sized Metco204NS-G trade mark YSZ conventionalzirconia powders were used. The nano size

Yttria stabilized zirconia powders wereprepared via chemical co-precipitationprocess, with particle sizes of <200nm, fromplasma spray able agglomerated sphericalmicrometer sized granules (typical size rangein 15-150µm) and coating was appliedthrough plasma spraying. The plasmaspraying was carried out with Metco 3MBatmospheric plasma spraying equipment.The NiCrAlY bond coat with a thickness of150µm was plasm a sprayed on thespecimens. Having applied NiCrAlY coating,the specimens were then plasma sprayedwith as follows: (a) Conventional YSZ coatingwith a thickness of 350µm, (b) Nano-structured YSZ coating with a thickness of350µm. The main compounds in hotcorrosion process were V2O5 and Na2SO4

which were prepared from high puritymaterials mixed in weight ratio of 55wt.%V2O5 and 45wt.% Na2SO4. 30mg of themixture per square centimeter of specimen'ssurface was spread to form an even film ofcorrosive material on coating surface keeping3 mm distance from the edge. The specimenswere then placed in electric furnace in cyclichot corrosion process at 1050°C in 4 hoursduration. They were then inspected and theirperipheral conditions were observed. If therewere any cracks or spallation in coating edgethe test stopped and the time was noted. It isto be mentioned that the test temperature andthe concentration of corrosive materials wereselected according to zirconia phase trans-formation temperature to accelerate the test.Also the mechanical properties of the coatingwere evaluated before and after hot corrosiontests. Optical microscopy (OM), scanningelectronic microscopy (SEM, OxfordCAMSCANMV2300) equipped with energy

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dispersive spectrometer (EDS) and X-raydiffraction (XRD, Philips X’pert) were usedto study the specimens. Nano indentationtests were performed on the cross section ofsample including; substrate, band coat andYSZ coating before and after hot corrosion.In this experiment, the mechanical propertiescomprising the hardness (H) and elasticmodulus (E) were determined by using anano indenter (Micro Materials LTD,wrexham, UK) with Berkovich tip. Theaverage values for ten points wereconsidered as H and E. The parameterswhich were used in nano indentation test are:maximum load = l00mN, loading rate = 5mN/S, unloading rate = 5mN/S and dewell periodat maximum load = 10S.

Table 1: Process Parameters Employedfor APS Coating of YSZ Top Coat

Gas Flow 4.2m3/hr

Flame or Exit Plasma Temp. 5500°C

Particle impact velocity 240 m/s

Spray rate 5kg/hr

Power 30-80kw

SOD 50-60mm

RESULTS AND DISCUSSION

A. XRD

About 95% of all solid materials can bedescribed as crystalline. When X-rays interactwith a crystalline Substance diffraction patternis obtained. Every crystalline substance givesa pattern the same substance always givesthe same pattern and in a mixture ofsubstances each produces its patternindependently of the others. The X-raydiffraction pattern of a pure substance is,therefore, like a fingerprint of the substance.The powder diffraction method is thus ideally

suited for characterization and identificationof polycrystalline phases.

Figure 1: XRD Pattern for C-263 Substrate

Figure 2: SEM Image ShowingSurface of Bare Substrate

Figure 3: XRD Pattern forAPS Deposit of Al2O3

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Figure 4: SEM Image ShowingSurface After Al

2O3 Bond Coat

Figure 5: SEM Image Showing PointDefect in Al

2O3 Bond Coat

XRD results demonstrated the formationof solid solution of Al and Al2O3 phases whichcould enhance the life time of the material.The more broadened diffraction peaksconfirmed the formation of nano-crystallitesince broadening was inversely proportionalto crystallite size. However, broadening wasassociated with vacancies, point defects,lattice distortions and micro-strain developedduring deposition process. Moreover, theconcentrations of oxygen species associatedwith these parameters were directly coupledwith the micro hardness which increased the

life time of the material. XRD data was alsoemployed to estimate the residual stressesdeveloped in Al2O3layer. The up and downshifting of diffraction peaks from their stressfree values indicated the existence of residualstresses. The peak shifting from their stressfree values was due to diffusion of oxidesinterstitially which distorted the lattice,creating point defects and vacancies. Thepeak shifting could also be due to the removalof hydrogen creating vacancies resulted inthe development of defects. It is well knownthat more diffusion could take place at hightemperature which may shift the diffractionpeaks from their corresponding stress freevalues (Figure 6).

Figure 6: XRD Pattern for TypicalAPS Deposit of YSZ

The young’s modulus of Al2O3 layerranged from 176 to 370 GPa depending onthe deposition temperature and the purity ofAl2O3 phase. The young’s modulus of Al2O3

phase (500°C temperature) was found to be344 GPa. In our case, the depositiontemperature was 550°C which was very closeto the above mentioned temperature forwhich the young’s modulus of Al2O3 phasewas 344 GPa. Thus, we multiplied the strain

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with 344 GPa (young’s modulus) in order toestimate the residual stresses developed inAl2O3 phase at 500°C temperature. Thestress was found to be ~ - 0.53 GPa in Al2O3

layer deposited for 5 h treatment time wherethe negative sign indicated the compressivenature of stress.

Figure 7: SEM Image Showing SurfaceAfter APS Deposit of YSZ

Figure 8: SEM Image ShowingSpallation in YSZ Coating

In X-ray diffraction technique, the positionof the peaks indicates the crystal structureof the material while the intensity of the peaksdepends on the material distribution in thestructure (J R Connolly, 2010). All the heat-

treated YSZ+20Al+5Ti APS TBCs are foundto contain one Al-lean t-ZrO2 phase and α-Al2O3, in addition to minor amounts of TiO2

and Y2O3 phases. The t-ZrO2 phase in the1200°C 24 h heat treated TBC shows highertetragonality (1.015), which is the result ofAl-depletion by diffusion and concomitantprecipitation of α-Al2O3. Higher temperatureheat-treatments (1300°C) result in evenhigher tetragonality in t-ZrO2 phase, and areconsistent with the higher concentrations ofprecipitated α-Al2O3. The heat-treatment ofYSZ+20Al+5Ti APS TBC at 1300°C for 72 hresults in about a third of the added Al2O3

remaining in solid solution in t-ZrO2. XRDpeak broadening analysis showed the sameaveragea-Al2O3 precipitate size as the1200°C (24 h) case: 200 nm.

B. Corrosion Rate

The weight losses of after the corrosion testsin a V2O5 and Na2SO4 corrosion environmentat 1050°C as a function of the time are shownin Figure 9.

Figure 9: Weight Loss of the AlloysCorroded at 650°C as a Function of Time

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Under a high temperature oxidative moltensalt environment, the reason for the weightloss of the specimens with time was attributedto a cracking of the protective layer whichled to its spallation from the base metalsurface. Therefore, it was concluded that theweight variations of a specimen would begreatly Under a high temperature oxidativemolten salt environment, the reason for theweight loss of the specimens with time wasattributed to a cracking of the protective layerwhich led to its spallation from the base metalsurface. Therefore, it was concluded that theweight variations of a specimen would begreatly affected by the adhesive strengthbetween a corrosion layer and a base metalby considering the formation, sustenance,and spallation of a corrosion layer and, alsothe stability of a corrosion layer’s growth. Theyoung’s modulus of Al2O3 layer ranged from176 to 370 GPa depending on the depositiontemperature and the purity of Al2O3 phase.The young’s modulus of Al2O3 phase (500°Ctemperature) was found to be 344 GPa. Inour case, the deposition temperature was550°C which was very close to the abovementioned temperature for which the young’smodulus of Al2O3 phase was 344 GPa. Thus,we multiplied the strain with 344 GPa(young’s modulus) in order to estimate theresidual stresses developed in Al2O3 phaseat 500°C temperature. The stress was foundto be ~ - 0.53 GPa in Al2O3 layer depositedfor 5 h treatment time where the negative singindicated the compressive nature of stress.

CONCLUSION

A number of new coating methods have beenemployed and older methods are beingrefined. Many analytical tools are introduced

which have gained importance for the fact ofbeing much precise. The development andevaluation of the coatings have gained muchpriority and has become the basic need thedevelopment and evaluation of coatings areexpensive and there is now a need, morethan ever, to conserve resources, optimizetest programmed and for a greatercoordination dissemination of information. Inthis study, the TBC system was investigatedusing several processes to conclude theinfluence of plasma spray parameters on thecoating performance, from the obtained data,discussion and interpretation of the results,it could be concluded that:

1. The total porosity of the APS coatingdecreases as the hydrogen flow rateincreases. TheSEM micrographs alsoreveal that uncontrolled hydrogen flowrate leads to cracking of the coatings.Achieving control of the microstructureof plasma-sprayed thermal barriercoating (TBC) systems offers anopportunity to tailor coating propertiesto demanding applications.

2. The presence of gaseous atmosphereduring coating was mainly responsiblefor accelerating the powder particles.The higher particle velocity resulted ina worse melting behavior of the powderdue to the shorter dwell time, leadingto a lower coating thickness, namely alower deposition efficiency of thepowder.

3. As the injection angle decreases (or asthe injection distance increases) led toincrease the porosity, decrease thedroplet temperature and decrease thenumber of droplets which impactnormally.

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4. It is worth noting that amorphousmicrocrystalline, and meta-stable phaseare commonly observed in plasmasprayed materials due to the high rate(rapid solidification and rapid cooling)that are inherent in this process.

5. The deviation of the spray parametersfrom the optimum condition led toincrease corrosion rates. Acknowled-gment. This work is financiallysupported by Department of Scienceand Technology - Nano Mission, NewDelhi.

REFERENCES

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3. B D Choules, K Kokini and T A Taylor(2001), “Mater. Sci. Eng. A”, Vol. 299, No.296.

4. E Ryshkowitch and D W Richardson(1985), “Oxide Ceramics, Second Edition,Academic Press Inc.

5. J R Connolly (2010), “IntroductionQuantitative X-Ray Diffraction Methods”.

6. K Kokini and M Case (1997), “ASMETrans. J. Eng. Mater. Technol.,” Vol. 119,No. 148.

7. K Kokini and Y R Takeuchi (1994), “Mater.Sci. Eng. A”, Vol. 189, No. 301.

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11. Y R Takeuchi and K Kokini (1994), “ASMETrans. J. Eng. Gas Turbines Power”, Vol.116, No. 266.

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ATMOSPHERIC PLASMA SPRAY COATINGS OF AL2O3 … · 424 Int. J. Mech. Eng. & Rob. Res. 2014 Yathin Kumar L et al., 2014 B. Plasma Generation and Formation The arc is initiated between