MATERIALS AND MANUFACTURING DIRECTORATE AIR FORCE RESEARCH LABORATORY AIR FORCE MATERIEL COMMAND WRIGHT-PATTERSON AIR FORCE BASE OH

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1 AFRL-ML-WP-TR ADVANCED THERMAL BARRIER COATING DR. RABI S. BHATTACHARYA UNIVERSAL ENERGY SYSTEMS, INC DAYTON-XENIA ROAD DAYTON, OH APRIL 2000 FINAL REPORT FOR 05/03/ /03/2000 THIS IS A SMALL BUSINESS INNOVATION RESEARCH (SBm) PHASE I REPORT. APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED MATERIALS AND MANUFACTURING DIRECTORATE AIR FORCE RESEARCH LABORATORY AIR FORCE MATERIEL COMMAND WRIGHT-PATTERSON AIR FORCE BASE OH DTZG w".e7 IäSPICMOD

2 NOTICE WHEN GOVERNMENT DRAWINGS, SPECIFICATIONS, OR OTHER DATA ARE USED FOR ANY PURPOSE OTHER THAN IN CONNECTION WITH A DEFINITELY GOVERNMENT- RELATED PROCUREMENT, THE UNITED STATES GOVERNMENT INCURS NO RESPONSIBILITY OR ANY OBLIGATION WHATSOEVER. THE FACT THAT THE GOVERNMENT MAY HAVE FORMULATED OR IN ANY WAY SUPPLIED THE SAID DRAWINGS, SPECIFICATIONS, OR OTHER DATA, IS NOT TO BE REGARDED BY IMPLICATION OR OTHERWISE IN ANY MANNER CONSTRUED, AS LICENSING THE HOLDER OR ANY OTHER PERSON OR CORPORATION, OR AS CONVEYING ANY RIGHTS OR PERMISSION TO MANUFACTURE, USE, OR SELL ANY PATENTED INVENTION THAT MAY IN ANY WAY BE RELATED THERETO. THIS REPORT IS RELEASABLE TO THE NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). AT NTIS, IT WILL BE AVAILABLE TO THE GENERAL PUBLIC, INCLUDING FOREIGN NATIONS. THIS TECHNICAL REPORT HAS BEEN REVIEWED AND IS APPROVED FOR PUBLICATION. ROLLIE E. DUTTON Metals Develpment & Materials Prcessing Branch Metals, Ceramics & NDE Divisin WLft Uvtt/Vl KAJHERINE A. STEVENS, Chief Metals Develpment & Materials Prcessing Branch Metals, Ceramics & NDE Divisin GERALD L^ETRÄK, Asst Chief Metals, Qe^fnics & NDE Divisin Materials & Manufacturing Directrate IF YOUR ADDRESS HAS CHANGED, IF YOU WISH TO BE REMOVED FROM OUR MAILING LIST, OR IF THE ADDRESSEE IS NO LONGER EMPLOYED BY YOUR ORGANIZATION, PLEASE NOTIFY, AFRL/MLLM, WRIGHT-PATTERSON AFB OH TO HELP US MAINTAIN A CURRENT MAILING LIST. COPIES OF THIS REPORT SHOULD NOT BE RETURNED UNLESS RETURN IS REQUIRED BY SECURITY CONSIDERATIONS, CONTRACTUAL OBLIGATIONS, OR NOTICE ON A SPECIFIC DOCUMENT.

3 REPORT DOCUMENTATION PAGE Frm Apprved OMBN Public reprting burden fr this cllectin f infrmatin is estimated t average 1 bur per respnse, including the time fr reviewing instructins, searching existing data surces, gathering and maintaining the data needed, and cmpleting and reviewing tbe cllectin f infrmatin. Send cmments regarding this burden estimate r any ther aspect f this cllectin f infrmatin, including suggestins fr reducing this burden, t Washingtn Headquarters Services, Directrate fr Infrmatin Operatins and Reprts, 1215 Jeffersn Davis Highway, Suite 1204, Arlingtn, VA , and t the Office f Management and Budget, Paperwrk Reductin Prject ( ), Washingtn, DC AGENCY USE ONLY {Leave blank) 2. REPORT DATE APRIL TITLE AND SUBTITLE ADVANCED THERMAL BARRIER COATING 6. AUTHOR(S) DR. RABI S. BHATTACHARYA 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) UNIVERSAL ENERGY SYSTEMS, INC DAYTON-XENIA ROAD DAYTON, OH REPORT TYPE AND DATES COVERED FINAL REPORT FOR 05/03/ /03/ FUNDING NUMBERS C F C-5206 PE PR 3005 TA ML WU PERFORMING ORGANIZATION REPORT NUMBER 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) MATERIALS AND MANUFACTURING DIRECTORATE AIR FORCE RESEARCH LABORATORY AIR FORCE MATERIEL COMMAND WRIGHT-PATTERSON AFB, OH POC: ROLLIE E. DUTTON. AFRL/MLLM SUPPLEMENTARY NOTES 10. SPONSORING/MONITORING AGENCY REPORT NUMBER AFRL-ML-WP-TR THIS IS A SMALL BUSINESS INNOVATION RESEARCH (SBIR) PHASE I REPORT. 12a. DISTRIBUTION AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE APPROVE FOR PUBLIC RELEASE, DISTRIBUTION UNLIMITED. 13. ABSTRACT (Maximum 200 wrds) This reprt was develped under SBIR cntract fr tpic AF The primary research bjective f this wrk was t develp a nvel thermal barrier cating (TBC) system by depsiting a high quality alpha alumina sublayer n the metallic bnd-cat, prir t the depsitin f the yttria stabilized zircnia (YSZ) cating. A patented filtered cathdic arc depsitin equipment was used t depsit an alpha alumina layer n a superally that was plasma-spray cated with a NiCCrAlY bnd-cat. The filtered arc cating prcess allws drplet-free depsitin i catings. Als, this prcess prvides high inizatin (>90%) cmpared with the standard physical vapr depsitin prcesses (<20%), thereby prviding excellent bnding between the cating and the substrate. Using this apprach, micrn thick alpha alumina catings were depsited n metallic substrate at 925 C. Oxidatin tests reveal that further imprvements are necessary fr the system t perfrm better than state-f-the-art TBCs. The perfrmance was impaired by micrstructural instability and surface rumpling f the bnd-cat, which caused failure f the alumina layer at a number f lcatins. 14. SUBJECT TERMS SBIR Reprt, Thermal Barrier Cating, Alumina Cating, Alpha Alumina, Filtered Cathdic Arc, Physical Vapr Depsitin, Yttria Stabilized Zircnia, Turbine Engine, TBC, Oxidatin. 15. NUMBER OF PAGES 16. PRICE CODE SECURITY CLASSIFICATION OF REPORT 18. SECURITY CLASSIFICATION OF THIS PAGE 19. SECURITY CLASSIFICATION OF ABSTRACT 20. LIMITATION OF ABSTRACT UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED SAR Standard Frm 298 (Rev. 2-89) (EG) Prescribed by ANSI Std Designed using Perfrm Pr, WHSIDI0R, Oct 94

4 PREFACE This technical reprt has been prepared as part f the requirement f the Phase I SBIR Cntract N. F C-5206 with the Air Frce Research Labratry/MLLM, Wright-Pattersn Air Frce Base, Ohi. The reprt cvers wrk cnducted during the perid 3 May 1999 thrugh 3 February 2000, and cnstitutes the final reprt under this cntract. The Air Frce Prject Engineers were Dr. Rllie E. Duttn and Dr. Edmund Mre.

5 ACKNOWLEDGEMENT The authr wishes t acknwledge the supprt f Dr. Bhaskar S. Majumdar, New Mexic Institute f Technlgy, Department f Materials and Metallurgical Engineering, as a subcntractr. He was the Principal Investigatr f this prject prir t his departure fr his new psitin. He has perfrmed mst f the xidatin tests and part f the characterizatins in this prject, and als helped in the preparatin f this reprt. Mr. Bris Brdkin and Dr. A.K. Rai f UES have perfrmed cating depsitin and characterizatin, respectively. in

6 1.0 INTRODUCTION Thermal Barrier Catings (TBCs) are used in the ht sectin f aircraft engine turbines t increase turbine efficiency and t extend the life f metal cmpnents. 1.1 ADVANTAGES OF THERMAL BARRIER COATINGS AND THEIR CURRENT LIMITATIONS The advantages f thermal barrier catings (TBCs) in ht-sectin turbine cmpnents are best illustrated by Figure 1. Uncated t Metal Temperature TBC Failuri Risk t TBC Cated ability T Clant Flw -> Figure 1. Advantages and risks assciated with TBCs as cmpared t uncated metal under expsure t the same temperature envirnment. The main advantages f the TBC are: (i) At cnstant clant flw, they reduce the base metal temperature, thereby increasing the life fthat part. Advanced turbine blades in aerspace engines perate very clse t their melting temperature, and it is estimated that a 15 C reductin in metal temperature can prduce a tw-fld increase in life [1-3]. (ii) At existing metal temperature, the temperature at the TBC surface can be increased by as much as 65 C, leading t a substantial gain in fuel efficiency. It is estimated that this temperature advantage is equivalent t tw generatins f new superally develpment [1-3]. These advantages cnstitute immense cst savings t the Air Frce and the Navy. Unfrtunately, the abve benefits f TBCs have nt been realized in practice. This is largely because the perfrmance f even the advanced TBCs is nt cnsidered sufficiently reliable and predictable [4] (i.e., the risk factr illustrated in Figure 1). In additin, the life f these TBCs need significant imprvement. In military engines that emply state-f-the-art single crystal blades such as thse fabricated frm PW1480 r Rene N5, the maximum temperatures are in the neighbrhd f 1150 C. The typical life f the mst advanced TBC fr a 23 C C temperature cycle ranges between 200 and 500 cycles, and in additin there is the prblem f reliability. Turbine blades in cmmercial aircraft experience lwer temperatures. Cst cnsideratins d nt favr the use f single crystal r directinally slidified blades in mst instances. Als, while chemistry cntrl is strictly adhered t, the levels f allwable impurities in the blade may be larger, since the perating cnditins are less severe. At the same time, hwever, the significantly higher risks assciated 1

7 with TBC failure, and the requirement that it prvides much lnger life than in military engines, stipulates that TBC perfrmance will be even mre critically examined in cmmercial engines. Thus, there is a need t develp TBC systems that have significantly higher reliability and durability than current state-f-the-art systems. 1.2 MECHANISMS AND MECHANICS OF FAILURE OF TBCs We illustrate the fur essential cmpnents f a TBC system in Figure 2. The base superally substrate is nrmally vacuum plasma spray (VPS) cated with a bnd-cat such as MCrAlY (where M is either Ni, C, r bth) r a cmplex Pt-based aluminide f typical thickness Jm. Upn this layer is depsited a 6-8 wt.% yttria stabilized zircnia (YSZ) layer f apprximately 100 Jm thickness. The YSZ is typically applied either by a plasma spray prcess r by electrn beam physical vapr depsitin (EB PVD). The plasma spray depsited YSZ cntains many pres and defects that cnstitute crack nucleatin sites, and failure in this system typically initiates in the YSZ clse t the metal substrate. The life f plasma sprayed YSZ is generally much lwer than YSZ depsited by EB PVD, and hence was nt cnsidered in this wrk. The advantages f EB PVD depsited YSZ are its superir quality, and a clumnar mrphlgy that leads t significant increase in cating cmpliance and hence lwer YSZ stresses. TGO Superally Substrate Figure 2. Sketch illustrating the primary cmpnents f a traditinal thermal barrier cating. Upn expsure t air at high temperatures, an xide layer frms at the interface between the bnd-cat and the YSZ [1,5]. This is called the thermally grwn xide (TGO) layer, whse grwth is aided by the clumnar mrphlgy f the YSZ that allws easy penetratin f xygen. While the TGO is largely cmpsed f alpha alumina, which is the mst stable phase, it can als cntain substantial amunts f mixed xides, such as spinels f the type Ni (Cr,Al) 2 0 4, r Cr 2 0 3, etc. [6-8]. The TGO is a nn-equilibrium regin, and grws in the presence f xygen that is cntinuusly fed by the pen channel YSZ layer. The typical thickness f TGO after thermal expsure is reprted t be abut 5 Jm [9-11]. The failure mechanism f the TBC is intimately cnnected with the frmatin and grwth f the TGO. During the initial stages f xidatin, blister type spts can be bserved n the TBC surface by thermal wave imaging, r even ptically if the blisters are quite large. At a later stage, these blisters either cmbine, r a single ne grws t such a pint that a piece f the YSZ layer detaches frm the blade. The exact mechanics depend n the system under cnsideratin, and Figure 3 prvides a plausible picture f the prgressin f damage. Micrcracks initially frm in the TGO and ften tend t prpagate alng the bnd-cat/tgo interface [3,4,9,12]. Cntinued xidatin and pening f these cracks initiate bulges that manifest as surface irregularities n the YSZ uter surface. At this pint, tw scenaris can perate: (i) either the interface crack kinks int the YSZ ceramic, leading t its cracking and separatin (Figure 3c), r (ii) there is buckling induced delaminatin f the YSZ, because f cmpressive stresses in

8 the TGO and the YSZ layer (Figure 3d). The frmer case des nt require the frmatin f a buckle, but is driven by a greater kink instability with increasing Mde II mixity as the delaminatin size grws. In the latter case, the bending stresses assciated with buckle frmatin ultimately leads t crack kinking int the YSZ, and this is believed t be the primary spallatin mechanism. Micr-Cracks (a) Crack Kinking int the YSZ Micr-Cracks (b) Crack Kinking int the YSZ Final Prpagatin t Create Spall (c) Buckling f Cating (d) Figure 3. Plausible damage prgressin in traditinal thermal barrier catings. Micrcracks initially nucleate either in the TGO (a), r at the TGO/bnd-cat interface (b). These grw and calesce t frm larger interface cracks. These cracks can then either kink int the TBC by an instability mechanism (c), r it can buckle when the crack length becmes larger (d). The latter als finally gives rise t a kinked crack. The current understanding is that spallatin is largely a result f TBC buckling and crack kinking. Stress analysis n Figure 2 indicates that the TGO layer is under very high in-plane cmpressive residual stress at rm temperature; a value f 3 GPa is typical [11]. This is because f the significant thermal expansin mismatch between the Ni-base ally (~12xlO" 6 /C) and the alumina (~8xlO" 6 /C). If the YSZ layer was nt present, and at mdest levels f fracture tughness f the TGO/bnd-cat interface, this level f cmpressive stress in cnjunctin with a micr defect can be sufficient t a buckling induced delaminatin failure f the TGO. The presence f the relatively thick YSZ alng with its assciated stiffness prevents this frm happening, unless the crack length becmes large, as will be illustrated presently. Cnsequently, the initiatin f the micrcrack requires an explanatin as t hw an ut-fplane nrmal stress is generated either within r at the interface f the TGO. One pstulated surce fr the micrcrack frmatin is hetergeneus vlumetric change assciated with xidatin f the NiCrAlY bnd-cat (Figure 4a). This scenari requires vlumetric expansin that has yet t be quantified. A secnd and mre quantifiable mechanism is the presence f

9 ripples in the TGO (Figure 4b). At the start f ur Phase I prgram, we had assumed it t be caused either by an initially rugh bnd-cat surface r an uneven xidatin f the bnd-cat. The results f the current prgram suggest that instabilities in the bnd-cat micrstructures, and thermal expansin difference between the bnd-cat and the superally substrate, likely play an even larger rle. Thse will be amplified in a later sectin. Figure 5 illustrates results frm a finite element methd (FEM) analysis with a sinusidal alumina/ysz interface [12]. The YSZ layer is n the right, and the TGO is n the left side f the figure. The cntur regin marked A, at the bnd-cat/tgo interface, has an ut-f-plane stress (i.e., directed t the right f the figure) f apprximately 450 MPa. Thus, fairly large ut-f-plane stresses can be generated by interface waviness, enugh t nucleate delaminatin cracks at the interface. Crack nucleatin (a) unifrm rate f xidatin, with bulge frmatin Generatin f tensile stress due t waviness f interface (b) Figure 4. Mechanisms fr crack nucleatin in the TGO r at the interface, (a) Hetergeneus xidatin can create a bulge and give rise t nrmal stresses, thereby nucleating cracks that can then prceed twards the bulge, (b) Nn-planar bnd-cat depsitin r nn-unifrm xidatin can create surface ripples, which cause generatin f ut-f-plane tensile stresses. Figure 5. Cntur plt frm ur FEM analysis with a wavy TGO structure. The YSZ layer is n the right, and the bnd-cat is n the left. Significant ut-f-plane tensile stresses (i.e., alng the hrizntal axis in the figure) are generated in the small island A (bright) at the bnd-cat/tgo interface. In the case f the diffusin aluminide Pt-Al bnd-cat system, the ripple effect is less evident. Rather, large thermal ridges have been bserved, ccupying as much as 10 t 20% f the bnd-cat surface. In this case, cracking f the TGO is bserved t ccur at these ridges [13]. While stress cncentratins d ccur in such grves, sme related xidatin studies f aluminide catings suggest that segregatin effects in the grain bundaries can als play a dminant rle in enhancing such TGO damage.

10 The mechanics f buckling induced YSZ spallatin has als been prbed [12]. In the absence f any interface crack, the critical stress fr buckling f the YSZ is apprximately 9 GPa, whereas the actual in-plane cmpressive stress in the YSZ is nly abut 150 MPa. Clearly, an interface crack, r a crack in the TGO, has t be present fr buckling induced spallatin f the YSZ t ccur. When an interface crack, r a crack in the TGO layer is already present, the critical stress (a c ) in the YSZ layer t cause buckling induced spallatin is apprximately given by: a c = (Ec/12).(7ih/a) 2, where a is the radius f the delaminatin crack, h is the cating thickness, and Ec is an effective mdulus. Assuming a YSZ thickness f mm, the critical delaminatin crack radius t cause buckling induced spallatin is apprximately 1.7 mm, which is a little mre than ten times the YSZ thickness. Thus, the interface crack has t grw by a substantial amunt befre buckling, s that methds that prevent delaminatin crack grwth will be beneficial t TBC life. In this calculatin, it is assumed that Ec at RT is 28 GPa, and the cmputed residual stress in the YSZ is 140 MPa. The fracture analyses shw that buckling will be delayed by a smaller delaminatin crack length, lwer cmpressive stress in the YSZ, greater tughness f the YSZ, and a greater thickness f the YSZ. While an increased thickness f the YSZ has the undesirable effect f increasing the delaminatin crack driving frce (which is prprtinal t YSZ thickness), it turns ut that the cmpressive energy in the very thin TGO is actually 5 t 10 times larger than in the YSZ, because f the significantly larger stress in the TGO cmpared with the YSZ. In ther wrds, it is the TGO characteristics that cntrl delaminatin behavir. The stress analyses als indicates that it is extremely imprtant t prevent the frmatin and initial grwth f micrcracks in the TGO (r at its interface with either the bnd-cat r YSZ). Larger thickness f the TGO aids in micrcrack damage, because f the additinal defect sites invlved (a vlume effect), as well as the high cmpressive energy that is available t drive the grwth f incipient cracks in the TGO (energy ~a 2 t/e, where t is TGO thickness, and a is apprximately 3-4 GPa). Typical TGO thickness after thermal cycling is abut 5 um and larger. Clearly, xidatin has t be drastically reduced. 2.0 PHASE I APPROACH UES, Inc.'s Phase I apprach is illustrated in Figure 6. The mst imprtant cmpnent f the research was t develp a TBC system that cmprised an alpha alumina sublayer, depsited by a nvel filtered cathdic arc depsitin system. The standard yttrium stabilized zircnia (YSZ) layer wuld be depsited n this sublayer by the electrn-beam physical vapr depsitin (EB PVD) technique. The ratinale was that the high quality and fully stable alpha alumina layer wuld prevent xidatin f the metallic substrate, which, as explained earlier, is the primary means by which TBC life is degraded. It is als imprtant t nte that discussins with the industry (bth TBC manufacturers as well as engine cmpanies) had shwn that PVD was verwhelmingly favred ver CVD techniques, largely because the system culd easily be integrated with EB PVD systems that are already in use fr depsiting YSZ. The alumina layer was planned t be essentially defect-free, withut the presence f mixed xides that nrmally exist in thermally grwn TGOs. The ratinale was that this high quality alumina sublayer wuld be highly resistant t the inward diffusin f xygen. Yttrium (Y) dping wuld be cnsidered because it has been shwn t significantly reduce grain-bundary xygen diffusin in alumina, the primary rute fr inward xygen diffusin.

11 Thermal barrier YSZ, depsited by EB PVD n the alumina sublayer High Quality Alpha Alumina Sublayer MCrAlY «Superally Substrate Figure 6. The apprach f UES fr the TBC system. It cnsists f a MCrAlY (mre apprpriately NiCCrAlY) layer, fllwed by a Jm alumina sublayer. The alumina is depsited by the filtered cathdic arc technique. The thermal barrier YSZ layer (-100 Jm thick) is depsited by EB PVD. The technical wrk relied n a patented filtered cathdic arc depsitin prcess [14,15] t depsit the alpha alumina sublayer at elevated temperatures (see Figure 7). This prcess prvides high plasma inizatin (>90%) cmpared t standard PVD prcesses (<20%). The multiply charged ins prvide high reactivity f the plasma, thereby allwing very gd bnding t the substrate. With high current densities (~ 4-5 A/cm 2 ) pssible in the arc prcess, the percentage f neutral mlecules can be reduced t less than 0.1%. The filtered arc surce allws depsitin f drplet-free catings by deflecting the plasma flw alng the curvilinear magnetic lines f frce twards the substrate, while the drplets, having straight trajectries, are captured n the baffles. This advanced filter design prvides practically drplet-free cating n large areas, ranging frm abut 250 mm in width t heights n the rder f 300 mm t 2 m r mre. The patented auxiliary ande assembly facilitates the generatin f a unifrm, high density plasma in which the part is immersed. Cnsequently, the prcess is much less sensitive t line-f-sight issues, as is the prblem with standard sputtering and EB PVD techniques. Als, the high degree f inizatin, cmbined with the plasma blanketing effect, permits depsitin f extremely unifrm catings withut rughness r ridges. The vacuum arc cathde is als a theretically unlimited electrn emitter, thus prviding an efficient surce f high-density electrn current. In this mde, it facilitates the generatin f a unifrm, highly cnductive inized gas even withut the metal plasma. This is utilized fr in-cleaning the substrate befre cating depsitin. 2.1 THE ALUMINA SUBLAYER ARCHITECTURE A few pints are wrthy f mentin regarding the alpha alumina sublayer: (i) It is nt clear whether a pm thickness f the alumina layer will be sufficient t drastically reduce xidatin f the superally, since a typical TGO thickness in Pt-Al based bnd-cat system is abut 5 Jm. Sme diffusin data fr Al and O ins in alpha alumina are relevant here. Studies using radi-active tracers [16] indicate that the lattice diffusin in undped alpha alumina is abut 6 rders f magnitude smaller than grain bundary diffusin. Therefre, ne anticipates that grain bundaries wuld be the primary path by which xygen is carried thrugh the alumina t the bnd-cat.

12 AUXILIARY ANODE WITH HEATER CATHODE IGNITER -ANODE DIRECT CATHODIC ARC SOURCE (INTERCHANGEABLE WITH MAGNETRON SOURCE) FILTERED CATHODIC ARC SOURCE LARGE-AREA FILTERED ARC SOURCE ASSEMBLY Figure 7. Sketch f the filtered cathdic arc depsitin equipment at UES, Inc. shwing the arrangements f the direct and filtered arc surces, auxiliary ande assembly, and substrate hlder. The effective zne f depsitin is apprximately 15 inches in diameter and 16 inches high. The rtating carusal has 12 independent munts and there are 4 layers. One can als use the diffusin data t calculate the xidatin parablic rate cnstant. This gives a parablic rate cnstant f abut 10" 18 cm 2 /sec at 1100 C, whereas the measured parablic rate cnstant fr alumina scale grwth n a nickel base ally substrate is abut 10" 13 cm 2 /sec at the same temperature. This large difference suggests additinal shrt circuit paths, mixed transitin metal xide frmatin, and defects in the alumina layer, during xidatin at 1100 C. The significant implicatin here is that if indeed a high quality alpha alumina layer can be depsited n the superally, then it may be pssible t reduce the parablic rate cnstant by as much as ne t tw rders f magnitude. Given that a limited degree f xidatin f the metal will still ccur, it becmes imprtant t prbe the mechanism f xidatin. There can be bth an inward diffusin f xygen thrugh the cathdically cated alumina layer, as well as an utward diffusin f Al thrugh that layer. These mechanisms wuld have imprtant effects n delaminatin stresses and n the mrphlgy f the metal-xide interface. (ii) The grain bundary diffusivity in reference [16] was bserved t be as much as three rders f magnitude lwer fr an alpha alumina dped with 300 ppm Y as cmpared with

13 the undped alumina. The Y 3+ ins at the grain bundary were believed t have caused this reduced diffusivity. In reference [17], 6 wt.% Y reduced the sintering capability f Ce0 2 by a factr f hundred, nce again shwing the large effect f Y 3+ ins in reducing grain bundary diffusin. In references [18,19], the presence f Y in a superally was fund t give rise t well adhering, flat scale f alumina under cnditins where xidatin f thse allys withut Y additins frmed cnvluted, nn-adhering scales. The wrinkled scales were attributed t lateral grwth f xide resulting frm xide frmatin at grain bundaries by reactin with xygen mving alng the bundaries, with Al ins migrating thrugh the bulk f the A grains. This effect was reduced when Al bulk diffusin was reduced by additin f Y. In reference [20], Y 3+ was shwn t be an is-electric dnr in alpha alumina in spite f the same charge as Al +, and this behavir was attributed t the large size f the Y 3+ in. This dnr effect was cnsidered t be respnsible fr the reductin in sintering rate f A dped with Y [21,22]. In the prpsed wrk, we intended t take advantage f this effect, by intrducing minute levels f Y during cathdic arc depsitin f the alumina. It is imprtant t nte here that althugh Y has been added in the past t the bnd-cat fr xide adhesin, there have been very limited attempts at intrducing Y during alumina frmatin by a CVD prcess. The heavy element slws dwn the cating prcess, and has generally nt been very successful. Cnsequently, the use f the cathdic arc depsitin prcess ffers significant advantage in reducing xygen penetratin thrugh alumina. This apprach may be cnsidered as nvel in the TBC develpment, with patent implicatins. (iii) Anther imprtant issue is the xidatin f the metal underneath the alumina layer, since there will always be sme diffusin f O thrugh the alumina. The issue is whether the metal substrate needs t have high levels f Al (-50 atmic percent) t fster A grwth, in preference t nickel xides, chrmium xides, r spinels. These latter xides are fast grwing, and their vlumetric changes can have significant damaging influence n the integrity f the alumina cating. On the ther hand, t high a level f Al in the superally r bnd-cat renders them brittle. Althugh a detailed xidatin study has nt been perfrmed, we can perhaps use thermdynamic data t shed sme insight here. The Ellingham diagram shws that the partial pressure f xygen that is in equilibrium with the alumina at 1100 C is nly abut 10" 32 atmspheres. At this lw partial pressure, nly aluminum xide can nucleate and grw, and nt any transitin metal xide. The nly ther elements that will cmpete fr the xygen will be Y, Hf, etc. Past literature indicates that selective frmatin f such xides belw the uter xide layer is extremely beneficial t xide adhesin, since they tend t frm pegs that hld the alumina t the metal [23]. Als, such pegs are barriers t the grwth f delaminatin cracks. Therefre, as lng as there is a reasnable supply f Al, (a) it is unlikely that mixed xides will nucleate and grw belw the alumina cating, and (b) Y r Hf in the metal will be f great benefit t xide/metal adhesin. In ther wrds, t high an Al level is nt needed in the substrate belw the alpha alumina layer. 3.0 EXPERIMENTAL PROCEDURE The base material selected fr the investigatin was a MAR M-247 superally that was plasma cated at Chrmally, New Yrk, with um thick NiCCrAlY cating. The nminal cmpsitin f the NiCCrAlY was stated as: C 19-25%, Cr %, Al 11-13%, Y %, Bal. Ni, all in wt.%. The cated cupns received Chrmally's standard heat treatment, invlving 4 hrs. at 1100 C in vacuum. The purpse f the heat treatment was presumably t stabilize the tw-phase NiCCrAlY micrstructure. Hwever, as will be shwn later, this treatment did little t reduce the extent f bndcat micrstructure changes that ccurred at temperatures f 1100 C and abve. 8

14 The test cupns were f apprximate size 20 mm x 12 mm x 3 mm. They were metallgraphically plished n the NiCCrAlY side up t 1 Jm diamnd. These were then washed and cleaned, prir t being cated at UES with alumina using the filtered cathdic arc depsitin system. The 1100 grade aluminum targets were used fr the cathdic arc depsitin. The depsitins were cnducted at high temperatures, in rder t btain the stable alpha-alumina phase. Because the existing filtered cathdic arc unit had capability nly up t 450 C, a special tubular furnace was cnstructed with capability up t 1000 C. The sample was munted inside the furnace, which was held hrizntal t allw the impingement f Al and 0 2 ins. This necessarily limited the cating t nly ne face f the sample. Hwever, this limitatin was cnsidered sufficient fr the Phase I prgram, since the real gal was t establish the feasibility f the highly stable alpha alumina film. It is imprtant t nte that the 1000 C capability is unique t any physical vapr depsitin system, since the maximum capability f mst systems is nly abut 850 C. Prir t depsitin f the alumina cating, the substrate was arc cleaned at the desired temperature. Next, a mixture f xygen and argn gas (apprximately in the rati 80:20) was intrduced int the chamber. The carrier argn gas was needed t minimize xidatin f the Al cathde, and thereby minimize instability f the arc. The substrate was RF biased, since prir research suggested that RF biasing was cnducive t bth imprved cating/substrate adhesin, as well as the frmatin f high temperature phases. A ttal f apprximately 25 cating runs were cnducted, in rder t establish rbust cnditins fr the depsitin f the highly stable alpha alumina phase. Cating thickness was limited t Jm. This value was based n tw cnsideratins: (i) having a lw enugh thickness t reduce spalling f the alumina layer, since a smaller vlume wuld necessarily pssess less elastic energy might drive an interface crack, and (ii) having sme level f thickness, t reduce xygen penetratin int the substrate and thereby reduce xide grwth. Hwever, the thickness f [Jm was just an initial estimate, and an ptimum thickness wuld have t be btained in a pssible Phase II prgram. In additin t cating the superally samples, small pieces f Si wafers were als cated. These latter samples were used t btain a quick assessment f the alumina crystal structure using transmissin electrn micrscpy (TEM). Hwever, the final evaluatin f the alumina crystal structure was always cnducted n the NiCCrAlY-cated superally substrates. Tw variatins f the cating runs als were perfrmed. In ne case, a dilute dispersin f fine alpha alumina in distilled water (alumina particle size ranged between 50 nm and 300 nm) was spin cated n the substrates [24]. The ratinale was that these alpha alumina particles wuld act as nucleatin seeds, in that they wuld help t reduce the temperature at which alpha alumina cating culd be btained by the cathdic arc depsitin prcess. It was felt that any lwering f the temperature fr alpha alumina depsitin wuld make the prcess much mre technlgically attractive. It is relevant t nte here that diamnd seeding had been fund t be cnducive t the frmatin f diamnd film n SiC particles/whiskers [25]. In anther variatin, a high vacuum PVD prcess was utilized t depsit alumina that was dped with Fe. This methd was nce again attempted in rder t reduce the temperature fr alpha alumina depsitin. The apprach was cnsidered because manufacturers f alpha alumina fibers take advantage f this dping t reduce the temperature fr the frmatin f alpha alumina. Other than these tw variatins, the bulk f the 25 cating runs invlved the depsitin f alumina using the filtered cathdic arc system. One set f alumina cated samples was further cated with YSZ using the EB PVD system at Chrmally, New Yrk.

15 Fllwing alumina depsitin, the cupns were characterized in terms f their micrstructure, crystal structure, and xidatin behavir. The crystal structure f the alumina cating was determined using the glancing angle X-ray diffractin technique. This technique is fairly sensitive t thin films, since the incident X-ray beam is maintained at a cnstant value f nly 3-5 degrees, s as t btain maximum cunts frm the film cmpared t the substrate. The sample is als spun rapidly abut an axis perpendicular t the film surface, and nly 2-theta is varied during the X-ray scan. The results shw that this technique is a simple and effective methd fr determining the crystal structure f thin alumina films. The crystal structure f the alumina cating was als cnfirmed using TEM. The prcedure invlved cutting a thin strip f the cupn parallel t the alumina face, and then mechanically grinding dwn frm the metal side t apprximately 0.1 mm thickness. Samples were then jet-electrplished in a slutin f 20 ml perchlric acid and 180 ml glacial acetic acid at zer degree centigrade; the vltage was apprximately 12 V and the current density was 1-2 ma/mm 2. The TEM samples were further thinned using an in-milling apparatus, and then bserved n a Hitachi TEM. In additin t the abve tw techniques, we als experimented with the Raman scattering technique t determine the crystal structure f the alumina. The advantages f this technique are twfld: (i) it has a spatial reslutin f 1 urn, s that nn-unifrmity in crystal structure culd be detected, and (ii) it can be used t determine the stress in the alumina film. Preliminary experiments indicated that the technique was nly partially successful, with the Raman peaks ften being masked by flurescence assciated with Cr in the subsurface regin. Hence, the methd was discntinued, althugh it may be used in the future fr detecting stresses in the YSZ and alumina layers. The alumina-cated cupns were sectined and metallgraphically plished, and then examined using a scanning electrn micrscpe (SEM). This prcedure was cnducted fr the as-cated samples, as well as fr samples that were expsed in air r vacuum at temperatures f 1100 C and abve. Mechanical characterizatin f the alumina catings was cnducted using a scratch tester. In this technique, a spherical diamnd indenter is dragged n the cating at increasing indenter lads. The frictinal resistance and acustic emissin (AE) signals are simultaneusly mnitred. The critical lad (L c ) crrespnding t the nset f significant AE signals is nted. Hwever, this by itself is nt sufficient t establish the nset f delaminatin. Frm the lad L c, the lcatin f the scratch at which delaminatin ccurs is estimated. Next, the scratch is bserved using ptical and SEM methds. A separate estimate is then made regarding the lcatin at which delaminatin may have ccurred. Based n a number f such bservatins, bth frm the AE signals and ptical/sem phtgraphs, ne can btain a reasnable estimate f the lad at which cating failure ccurs. It is als imprtant t nte that there are tw distinct failure mdes. One is the delaminatin f the cating, and the ther is transverse cracking f the cating. The latter may r may nt be accmpanied with delaminatin failure. Indeed, the alumina-seeded samples suggest that delaminatin failure did nt ccur even fr fairly high lads. The alumina cated samples were xidized in air, under tw different cnditins: (i) 1150 C fr 100 hurs, and (ii) 1135 C fr 100 hurs. A cmpanin sample withut the alumina layer was als xidized at 1135 C fr 100 hurs, in rder t estimate the effect f the alumina cating n the xidatin behavir. The weights f the cupns were recrded befre and after the xidatin treatment. Hwever, because the alumina was cated n nly ne face f the cupns, the results f the weight lss/gain measurements were nt very cnclusive. 4.0 RESULTS The crss-sectin f the NiCCrAlY layer is illustrated in Figure 8, and it shws a tw-phase micrstructure, cmpsed f light and dark phases. The light phase is a Cr-rich/Al-lean phase with the apprximate cmpsitin: Ni-34.3, C-28.6, Cr-27.7, Al-9.4, Y-0.01, all in at.%, as determined by the micrprbe technique. This phase is the fee gamma phase (and may cntain the Ll 2 gamma-prime 10

16 phase). The darker phase is rich in Al, and has the apprximate atmic cmpsitin: Ni-44.2, C-14.4, Cr-8.1, Al-33.3, and Y 0.04, all in atmic percent. This is the rdered beta (mre apprpriately B2) phase, and is the phase that prvides the maximum xidatin resistance at 1000 C and abve. The initial grain size was abut 3 micrns fr each phase, but as will be shwn later, the grain size increased t apprximately 25 micrns fllwing expsure at 1135 C fr 100 hurs. Figure 8. Micrstructure thrugh a sectin f the NiCCrAlY bndcat. The whiter phase is Cr rich (the disrdered fee gamma phase), whereas the darker grains are Al-rich (the B2 phase). The grains have a smewhat clumnar mrphlgy. Figure 9a shws the crss sectin f sample 119 that was cated with alumina at 950 C. It is difficult t determine any micrstructure f the alumina in the SEM image. Figures 9b and 9c shw back scattered (BSD) images f a different sample, cated als at 950 C using a separate heater system. The defect structure at the NiCCrAlY-superally interface is clearly visible in Figure 9b, as is the relatively fine structure f the tw-phase ally. Figure 9c shws a relatively smth interface between the cating and the substrate, with the alumina cating appearing darker because f the lwer atmic number f the cnstituents cmpared t the bndcat. The EDAX analysis f the cating shwed that the rati f Al t O peak height was nearly identical t that fr a plycrystalline alpha-alumina ceramic. Micrprbe measurement f the alumina layer n a Si substrate shwed stichimetric cmpsitin. Thus, based n current results and thse reprted in the literature, the alumina stichimetry appears t be fairly easily btainable. It is the alpha phase that pses the mst difficult challenge. Auger analysis was cnducted n a sample that was cated at 950 C. It indicated significant carbn cntaminatin. It is likely that this carbn riginated frm hydrcarbn vaprs in the vacuum chamber, althugh pssible degassing frm the heater system cannt be ruled ut. The presence f carbn is detrimental t xide adhesin, and effrts are under way t reduce this cntaminatin t a minimum. 11

17 NM (a) MarM247 #:,-: - NiGÖGrÄlY (b) NiCCrAlY (c) Figure 9. (a) As-cated sample, with the alumina layer appearing white. The darker regin at the bttm is NiCCrAlY. Cating thickness is apprximately 0.5 micrns, (b) A lw magnificatin micrgraph f a separate cated specimen, where the cating is at the bttm. The NiCCrAlY micrstructure is visible, and the pres define the riginal bundary between the EB PVD depsited NiCCrAlY and the MarM247 superally substrate, (c) Higher magnificatin back scattered micrgraph f Figure 9b. The darker thin band running hrizntally in the middle f the picture is the alumina layer. 12

18 Figure 10a shws glancing angle X-ray diffractin data f the 950 C cated sample, illustrated in Figure 9a. Figure 10b is a glancing angle X-ray frm sme alpha alumina pwder. Figure 10c shws X-ray diffractin data fr the NiCCrAlY, withut the alumina cating. The imprtant peaks t cnsider in Figure 10a (alumina cated sample) are the nes at apprximately 3.49, 2.56, 1.74, and 1.60 Angstrm d-spacings. All these peaks crrespnd t alpha alumina, and are absent fr the base NiCCrAlY material. Figure lod shws glancing angle X-ray fr a sample that was cated at 850 C. In this case the alpha alumina peaks are missing. Tgether they indicate that alpha alumina can be btained if depsitin is cnducted at 950 C and higher. T further cnfirm the detectability f alpha alumina, the sample f Figure lod (cated at 850 C) was heat-treated in vacuum fr 4 hurs at 1150 C, t transfrm the alumina t the alpha alumina phase. Except fr sme cracks alng the edges, the alumina cating was essentially intact. Figure loe shws the glancing angle X-ray fr this sample. The characteristic alpha alumina peaks can be bserved nce again fr this sample. The same peaks were bserved n samples xidized in air fr 100 hurs. Thus these cnsistencies established the viability f using the simple glancing angle X-ray technique fr detecting the frmatin f alpha alumina. Althugh nt shwn here, alpha alumina was als btained at a substrate temperature f 925 C. Alpha alumina was nt btained at lwer substrate temperatures. Between 750 C and 850 C, nly gamma alumina was btained. At rm temperature, the alumina phase was amrphus. Figure 11 shws the selected area TEM diffractin pattern fr sample 209 that was cated with alumina at 950 C. The rings with fine spts crrespnd t the cating, whereas the bigger spts crrespnd t the NiCCrAlY. The d-spacings crrespnding t the rings were btained as 2.54, 2.17, 1.73, , and 1.05 Angstrms. These agree very well with the d-spacings fr alpha alumina, namely 2.552, 2.165, 1.74, 1.52, 1.24, and 1.05 Angstrms. Thus, the TEM bservatins cnfirm the X-ray data, in that alpha alumina is btainable if the depsitin is cnducted at 950 C. Figure 12 shws the Raman spectra fr sample 119, and that fr an alpha-alumina pwder (calibrating sample). The backgrund slpe fr sample 119 is caused by flurescence assciated with Cr in the sub-surface regin. Nevertheless, the tw small peaks (marked A and B) agree very well with thse fr the alpha-alumina standard. In sme f the ther samples, fluresecence appeared t cmpletely mask all the alumina peaks, and hence this prcedure was discntinued after initial trials. Figure 13 shws the micrstructure f a sample that was cated at 950 C, and then heat treated in a vacuum fr 4 hurs at 1150 C. The micrgraph shws that the cating was perfectly intact. Hwever, there was sme carsening f the NiCCrAlY micrstructure, as well as precipitatin/carsening f a finer phase in the disrdered gamma phase. Figure 14a shws the crss-sectinal micrgraph f sample number 121 that was cated at 950 C, then expsed in air at 1150 C fr 100 hurs. This is a fairly severe test f any cating withut an uter zircnia layer. EDAX analysis shwed that the dark uter layer at the bttm f the micrgraph (apprximate thickness Jm) was pure alumina. Since the riginal alumina cating was nly 0.5 pm, it is clear that additinal xidatin had ccurred during the high temperature expsure. Hwever, the riginal alumina is nt distinguishable, and it is pssible that the depsited layer may have delaminated ver the 100 hurs expsure. Figure 14b shws the micrstructure f sample (number 119) after a separate xidatin run (1135 C fr 100 hurs). In this case, nly a very thin xide layer is bserved (-0.3 Jm), althugh it is nt clear if this layer is a remnant f the depsited alpha alumina, r whether that alumina layer spalled and was replaced by a spalling fresh xide layer. The sample weight lss was apprximately grams. This may be cmpared with a cmpanin NiCCrAlY cated sample (withut any alumina), fr which the weight lss under the same cnditins was grams. 13

19 <0 in -4-» >C3 O -^ S.S. I«C3 D t: s C OH. LSSO'Z K)6 'Z flflz zzurz BUium VBiidiv'9l70S'e t3 <U H _! * S H.3. g 60 3.S a 8 JO OH I b u «< 1 T3 U CO CO O >H 5? T3 u O u u J3 O : -1= CN e ' S ca ^ E X x x s OH OH 00 a. cn g O cj E 1 cd 60 tn r~ «/ N u CO *H ASM 3 (SO 14

20 00 CO CN c a* a a. "3 'a, > 00.5 ÖD LH -a T3 -** O OH u as JS C VI H s J3 0 a, «S _u "5b e.3 Ö s O 3 60 (s;nn3) Äjisuajuj 15

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22 3 00 VO -H- ON c u i/i u Q. CÜ JÖ "3 u cd u c«- O s s 6 VO U in 00 ft ft ISC u 00 ^H - v 1 * ' «</-> -** tu 43 E H 3 (S CO «4-1 O c in I- -a 2 >< (SO.5 u A Ü I (sjun3) j( isn3;ni 3 17

23 c CO u & w w «.0 H B en cd u D, sj -C &, O J5 a u in 03 s 3 I s si «E B 3 e5 M 2 t 2 ^_> CO '71 & c«ca & ^ «j _u 2 "H, C 5.2 ics (s un3) Äjisuajni 3 E 18

24 Figure 11. Selected area diffractin pattern f the alumina cating fr sample 209, which was cated at 950 C. The rings with the fine spts crrespnd t the alumina layer, whereas the larger spts crrespnd t the underlying NiCCrAlY layer. The d-spacing fr the ring patterns cnfirm that the cating was alpha alumina i i i i 1.2 1(T HO 4 N &8000 v> C ai « \ Aipha-alumina pwder NiCCrAIY+Alumina, Sample 119B J_ III! -I I I L. Raman Shift Figure 12. Raman spectra fr sample 119 (cathdically cated with alumina at 950 C) and an alphaalumina pwder. The tw peaks, marked A and B, d agree quite well with thse f the alpha alumina pwder. 19

25 Figure 13. Micrstructure f sample 119 that cated with alumina at 950 C, and then heat treated in vacuum at 1150 C fr 4 hurs. The alumina cating is perfectly intact. 20

26 (a) L> SEI EM KV I«- 15 mm 0RG- X 20.0 K PHOTO (jm ( i H113B 1135C/100hrs Aluminum Oxide (b) Figure 14. (a) Crss-sectinal BSD image f sample 121, after expsure at 1150 C fr 4 hurs. This sample had been cated with alumina at apprximately 950 C. The alumina layer appears as a darker hrizntal band twards the bttm f the picture, (b) Sample 119 after a separate expsure at 1135 C fr 100 hurs. The tp whitish band is alumina, based n EDAX analysis. Hwever, its thickness is less than the riginal 0.5 micrn thickness. 21

27 One f the prblems faced in the prgram was significant micrstructural changes in the NiCCrAlY bnd-cat, accmpanied with surface rumpling. The frmer is illustrated in Figure 15a, and the latter in Figures 15b. The last figure clearly shws hw surface rumpling has led t delaminatin f the cating. As indicated earlier, surface rumpling is extremely deleterius t cating integrity, and it appears that temperatures abve 1100 C are unsuitable fr the NiCCrAlY bndcat. In a recent paper [26], the mismatch between NiCCrAlY and a single-crystal CMSX-4 ally was estimated as apprximately 0.3% at 1090 C. This is a significant strain level, and was indicated t be a majr surce f surface rippling f bnd-cats n superally substrates [26]. The xidatin behavir was als studied fr specimens that were nt cated with alumina. In this case, the xidatin respnse was erratic. In ne case, n xide was bserved, suggesting that the xide may have spalled. In the secnd case, an xide f apprximately Jm was bserved (see Figure 16), which was almst tw times that fr the alumina cated samples. The uter prtin f the xide cntains Ni, Cr, and Al, suggesting the presence f the respective xides, as well as spinels. The inner part f the xide was simply alumina, and may have ccurred because the lcal xygen partial pressure drps significantly when a sizable xide thickness is built up. Under very lw partial pressure (~10" 3 atmsphere), alumina and rare earth xides wuld frm in preference t xides f Cr, Ni, r C. The bserved scale cnstituents are cnsistent with this descriptin. Hwever, if the layer spalled, then the entire xidatin prcess wuld repeat. Thermal cycling was cnducted between 200 C and 1177 C in air, bth fr an alumina cated sample (number 180) and fr an alumina+ysz cated sample (number 181-ZRO). The hld time at 1177 C was apprximately 45 minutes, and the ttal cycle-time was apprximately 65 minutes. Thermal cycling was cnducted fr nly 25 cycles. Sample 180 shwed many lcatins where the alumina had delaminated frm the substrate, whereas sample 181-ZRO indicated that the cating was intact. Thus, as indicated earlier, thermal expsure f just the alumina cated material is much mre severe than ne with a YSZ tp-cat. The YSZ cated sample was sectined fr metallgraphic examinatin. Unfrtunately, we bserved delaminatin f the cating frm the substrate n either side f the cut plane. Detailed examinatin f the fracture path is still under investigatin. A cntrl platinum-aluminide+ysz cated sample was als thermally cycled during this investigatin. In this case, the sample was ptted in an epxy befre it was sectined, and it did nt exhibit the extent f delaminatin that was bserved in ur alumina+ysz cated sample. Hwever, the plished sectin did indicate a number f lcatins where there was small-scale delaminatin between the TGO and the Pt-Al bnd-cat. Figures 17a and 17b are dark-field micrgraph and diffractin pattern fr sample 139Y that was seeded with alpha-alumina particles, and then cated with alumina using the cathdic depsitin technique. The cating was applied at 850 C. Recall that in unseeded samples, this temperature resulted in the frmatin f gamma alumina rather than alpha alumina. Mst f the rings in Figure 17b, particularly the strng nes, crrespnd t that f alpha alumina. Dark field images at different diffractin cnditins indicated that the alpha alumina phase cvered almst the entire regin f the sample; i.e., the surface cverage was much larger than the initial cverage f alpha-alumina pwder determined by high magnificatin SEM (see Figure 17c). While these results d nt fully cnfirm that alpha alumina seeding may be beneficial t alpha-alumina frmatin at a lwer temperature, the results are encuraging. Future effrts will attempt t cnfirm if this methdlgy f seeding is indeed beneficial in lwering the temperature fr alpha alumina depsitin. 22

28 Pres mark riginal bundary (a) S*Bs****^*w^*^ NiCCrAlY MarM247 (t>) Figure 15. (a) Lw magnificatin micrgraph f sample number 119 expsed in air at 1135 C fr 100 hurs. Nte the large grains (cmpare with Figure 8), and reactins with the superally (the large grains belw the riginal pre bundary), indicating majr micrstructural changes f the NiCCrAlY bndcat. (b) Surface rumpling f the bndcat fr a sample expsed in air at 1150 C fr 100 hurs. Nte hw the xide has spalled. 23

29 i EHT= OOMITI i dized ;:;*S^3pfK'v;:.-;-.-..'f ";,. ^Ä^^-'^'^^^^^^^^^^^-^^SCkt^.t...; ;>j } Oxide NiCCrAlY Figure 16. Oxide layer f a cntrl sample (withut alumina cating), xidized in air at 1135 C fr 100 hurs. The ttal xide thickness is apprximately 4 micrns. The uter bright spts n the xide cntain significant amunts f Ni and Cr, as determined by ED AX analysis. The xide clser t the NiCCrAly cntains nly Al and O, suggesting essentially pure alumina. 24

30 (b) Figure 17. (a) Dark field and (b) diffractin micrgraphs fr sample 139Y. This sample was initially seeded with fine alpha alumina particles, then cathdically cated with alumina at 850 C, which was lwer than the standard temperature f 950 C. Mst f the rings crrespnd t alpha alumina, suggesting that the alumina seeds may have facilitated frmatin f alpha alumina at a lwer temperature. 25