phase composition and its changes during annealing of plasma-sprayed ysz

Ž .Surface and Coatings Technology 127 2000 120]129

Phase composition and its changes during annealing ofplasma-sprayed YSZq

Jan Ilavsky a,b,U, Judith K. Stalicka

aNIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, MD 20899, USAbUni ersity of Maryland, College Park, MD 20742, USA

Received 25 May 1999; accepted in revised form 16 February 2000

Abstract

Ž .The phase composition of plasma-sprayed yttria stabilized zirconia YSZ , ZrO with 8% by mass Y O , was studied using2 2 3neutron and X-ray diffraction. Comparison shows that neutron diffraction is superior for analysis of the phase composition aswell as for the analysis of the yttria content of the tetragonal phase. The presence of large amounts of the cubic phase is probablyoften neglected or underestimated in standard XRD analysis due to scattering-related limitations and the inherent difficulty ofthe analysis. The importance of this fact needs to be addressed in future studies. The amount of monoclinic, tetragonal, and cubicphases was determined using neutron Rietveld refinement for feedstock powders, as-sprayed deposits and for samples annealedfor 1 h at temperatures of 1100, 1200, 1300, and 14008C. The two studied feedstock materials were manufactured by differentproduction methods, and contained various amounts of the monoclinic phase depending on manufacturing method. While thetetragonal phase dominated, both feedstock powders also contained significant amounts of the cubic phase. The as-sprayeddeposits were composed of mostly tetragonal phase, with only traces of the monoclinic phase; the amount of the cubic phase was

Žreduced with respect to the feedstock. For one of the materials the cubic phase content remained significant approx. 25% by. Ž .mass , and for the other the cubic phase content was significantly lower approx. 6% by mass . The cubic phase content after

annealing at 14008C was in both cases similar } approximately 40% by mass. There was no significant change in monoclinicphase content observed in this experiment. The yttria fraction within the tetragonal and cubic phases was followed and changesare discussed. Q 2000 Elsevier Science S.A. All rights reserved.

Keywords: Phase transitions; X-Ray diffraction; Sintering; Zirconium oxide; Yttrium oxide; Neutron scattering

q ŽU .Certain trade names and company products are identified in order to adequately specify the experimental procedure. In no case does suchidentification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the productsare necessarily the best for the purpose.

U Corresponding author. Tel.: q1-630-252-0866; fax: q1-630-252-0862.Ž .E-mail address: [email protected] J. Ilavsky

0257-8972r00r$ - see front matter Q 2000 Elsevier Science S.A. All rights reserved.Ž .PII: S 0 2 5 7 - 8 9 7 2 0 0 0 0 5 6 2 - 4

( )J. Ila¨sky, J.K. Stalick r Surface and Coatings Technology 127 2000 120]129 121

1. Introduction

Ž .Yttria stabilized zirconia YSZ ceramic deposits areŽ .often applied as thermal barrier coatings TBCs for

thermal insulation in the hot areas of gas turbinecomponents. Operating temperatures of the aircraft

w xengine components currently reach over 13008C 1 .The major goal of application of TBCs is an increase inthe fuel efficiency by an increase in the operatingtemperatures. A second, but not secondary, purpose is

w xto increase the thermal cycling lifetime 2,3 .Ž .Zirconia ZrO at ambient pressures occurs in three2

polymorphic forms: monoclinic at room temperature,tetragonal at higher temperatures, and cubic at yet

Ž .higher temperatures. Yttria Y O is added to zirconia2 3

to stabilize the intermediate temperature tetragonalŽphase so-called partially stabilized zirconia, approxi-

mately 8% by mass Y O equivalent to 8.7 mol%2 3. ŽYO or the high-temperature cubic phase fully1.5

.stabilized zirconia, with higher contents of Y O at2 3room temperature.

Since the ZrO ]Y O ceramics have been used ex-2 2 3

tensively in various applications, this material has at-tracted the interest of many researchers. A number ofextensive studies investigating the phase relationshipsin this system have recently been published; for a

w xreview see Yoshimura 4 . As mentioned in this review,there are still open questions about the exact boun-daries of the phase diagram. However, the genericshape of the phase diagram does not change and there-

w xfore we have selected one 5 for further discussions inw xthis work. Modifications proposed by Yoshimura 4

w xand Jayaram et al. 6 and others do not alter theoverall conclusions.

Experience from TBC applications shows that tightcontrol of the coating phase composition is imperativefor optimal coating lifetime and survival. Largeramounts of the monoclinic phase in the deposits areconsidered undesirable, as these transform on heatinginto the tetragonal phase. Since this transformation isaccompanied by a large volume change, thermal cyclinggenerates stresses in the deposits leading to prematurefailure. The role of the cubic phase is, however, un-clear. Coatings made of fully stabilized zirconia haveshorter lifetimes and generally inferior properties com-pared to partially stabilized zirconia coatings.

As is shown in this paper, the phase analysis of YSZmay be a challenge. Whereas the monoclinic phase iseasily identified and quantified relative to the tetrago-nal phase in the X-ray pattern, determination of thetetragonalrcubic phases is much more difficult. Asprevious work has indicated, the cubic phase content is

w xoften underestimated or even ignored 7 .

w xFig. 1. Phase diagram of YSZ 5 .

1.1. Phase diagram

w xAccording to the phase diagram 4]6 of ZrO ]Y O2 2 3Ž .Fig. 1 the room temperature phase of ZrO contain-2

Ž .ing 8% by mass Y O 8.7 mol% YO is tetragonal.2 3 1.5There is a two-phase region at temperatures betweenapproximately 6008C and 20008C where the phase com-position is a mixture of cubic and tetragonal phases. Athigher temperatures the stable phase is cubic.

Plasma spraying, as a rapid solidification process,often results in the formation of metastable phases in

w xthe deposits 8 . Thus if the YSZ deposit is in ametastable state } and therefore the phase composi-tion is equivalent to that for some higher temperature

} the material composition should be a mixture ofthe tetragonal and cubic phases rather than only thetetragonal phase which is predicted at room tempera-ture.

1.2. Comparison of neutrons and X-rays

The diffraction of both neutrons and X-rays is de-w xscribed by Bragg’s law 9 . There are, however, impor-

tant differences in the interaction of neutrons andX-rays with matter. Neutrons interact with the nuclei,whereas X-rays interact with the electrons surrounding

Žthe nuclei. The strength of the X-ray interaction de-.scribed as an atomic form factor depends on the

number of electrons in the atom, whereas the strengthŽof neutron interaction with atomic nuclei atomic

( )J. Ila¨sky, J.K. Stalick r Surface and Coatings Technology 127 2000 120]129122

.cross-section cannot be described in simple terms. It iscommon for atoms with similar weights and even dif-ferent isotopes of the same element to have signifi-cantly different neutron atomic cross-sections. For thesystem being studied, all component elements havesimilar neutron scattering cross-sections. Thus the oxy-gen atoms contribute more to the diffracted intensitieswith neutron diffraction than with X-ray diffraction,and in fact dominate the diffraction pattern owing totheir greater relative abundance.

Another difference between the X-ray and neutroninteraction is the angular dependence of the diffractedintensity. The geometric spread of electrons in an atomcauses the scattering form factor to decrease withscattering angle for X-rays, whereas for neutrons theatomic cross-section remains at a constant value. Ac-cordingly, the neutron powder patterns have relativelystronger diffraction peaks at large scattering angles.

Since neutrons penetrate most materials with mini-mal attenuation, the volume sampled is much larger

Žthan in X-ray diffraction. Surface effects such as sur-.face roughness and surface texture are therefore usu-

ally negligible for neutron diffraction.

1.3. Use of Riet eld analysis

w xRietveld refinement 10 is a technique in which theentire powder pattern is fit based upon a model that

Žincludes known crystallographic parameters unit cell,.atomic positions, etc. , as well as pattern line shape,

background, and phase composition. The modelparameters are then adjusted via least-squares refine-ment. This technique can be used with either X-ray orneutron radiation, but has proven most successful withneutron powder diffraction owing to the symmetricpeak shape, scattering at higher diffraction angle, andgreater sensitivity to light atoms that are obtained withneutron radiation. These factors are of particular im-portance in the phase analysis of zirconia, as the

diffraction peaks of the cubic phase are not clearlyseparated from those of the tetragonal phase.

1.4. Purpose of this study

This study uses neutron powder diffraction to studyboth the phase composition of the YSZ deposits andchanges of this phase composition occurring duringsimulated in-service conditions. These in-service condi-tions were obtained by annealing at temperatures of11008C, 12008C, 13008C, and 14008C for 1 h prior todata collection at room temperature.

2. Experimental

Deposits made of SylvaniaU SX233 and AmdryU 142Ž .feedstock powders for details see Table 1 were manu-

factured by Plasma TechnikU PT F4 system. The spraynozzle diameter was 8 mm, the powder injector diame-ter was 1.8 mm, and the current was 500 A at 68 V. The

Ž y1 .primary gas was argon 40 l min , the secondary gasŽ y1 .was hydrogen 10 l min , and the carrier gas was

Ž y1 . y1argon 3 l min . The feed rate was 26 g min andthe spray distance was 90 mm. Deposits of thicknessapproximately 5 mm were sprayed on a steel substrateŽ .25 mm=50 mm=2.5 mm , which was grid blastedprior to spraying and then covered by an Al layerdeposited by wire arc spraying. The Al layer was dis-solved after spraying using HCl to obtain freestandingdeposits.

Ž .Smaller samples approx. 20 mm=3 mm=5 mmwere cut using a diamond saw. Annealing for 1 h inambient atmosphere was done in the furnace at varyingtemperatures. Samples were inserted into a cool fur-nace, and the furnace was heated at approximately6008Crh to the selected temperature. After 1 h thesamples were pulled out directly from the hot furnaceand left to cool down outside of the furnace. This

Table 1Parameters of the feedstock material referred to in this work

a bManufacturerrmaterial Chemistry Size range CommentŽ .d , d mm10 90

Alloy Metals, Inc. ZrO , 8% by mass Y O 41, 113 Fused and crushed2 2 3UAmdry 142

cUSylvania SX233 ZrO , 8% by mass Y O 26, 96 HOSP , equivalent2 2 3Uto Metco 204

a The chemical composition was supplied by manufacturer of the feedstock material. It was obtained for each material by chemical wetmethod.

bSize range is given as d and d , which represent the particle size below which lie 10% and 90% of the size distribution. Measured by10 90MicrotrackU.

c The HOSP techniqueU is a proprietary technique used for powder processing. It is similar to plasma spheroidization.


provided a fast cooling rate needed to prevent changesin the phase composition during cooling.

Neutron diffraction data were obtained at room tem-perature using the National Institute of Standards andTechnology 32-detector high-resolution powderdiffractometer. Data were collected over the range

˚Ž .5]1658 2u with a neutron wavelength of 1.5396 1 A.The scan range was 108, with a step size of 0.058, so thateach data point was collected in two adjacent detectors.

The data were then processed into a single-detectorsimulation and refined using the GSAS suite of pro-

w xgrams for structural analysis 11 . The overall back-ground was fit using Chebychev polynomials. The unitcell parameters, atomic coordinates, thermal parame-ters, and powder line shape were refined independentlyfor the monoclinic, tetragonal, and cubic phases; forthe plasma-sprayed coatings, the small amount ofmonoclinic phase precluded structural refinement ofthis phase and these parameters were assumed to bethe same as determined for the feedstock powder. Thephase fraction of each of the three phases was alsorefined.

The calculation of yttria content within the cubic andtetragonal phases was based on the changes of lattice

w xparameters as described by Scott 5 . These data havew xbeen reinterpreted by Howard 12 to give the relation-

ships:

Ž .mol% YO s ay5.1159 r0.0015471.5

for the cubic phase, where a is the cubic latticeparameter, and

Ž .mol% YO s 1.0225ycra r0.0013111.5

for the tetragonal phase, where the ratio cra of thetetragonal lattice parameters is calculated for an F-centered lattice. However, the total yttria content cal-culated using these expressions has been found to begreater than the nominal composition in several stud-

Ž .ied materials to be reported elsewhere , most probablydue to unrecognized cubic phase content in the pre-

w xsumed phase-pure tetragonal materials 7 . Since thecubic phase typically has a higher yttria content thanthe tetragonal phase, the scaling factor for the tetra-gonal phase was empirically adjusted to obtain resultswith the overall level of yttria content equal to thenominal composition of a wide range of samples, sothat

Ž .mol% YO s 1.0225ycra r0.00161.5

Using this scaling the Amdry feedstock appears to below in overall yttria content.

Fig. 2. Comparison of XRD and neutron spectra with fit fromŽ . Ž .Rietveld method on neutron data . Peaks for tetragonal T and

Ž . Žcubic phase C are indexed; peaks for monoclinic phase discernable.ones only are marked M.

3. Results

Figs. 2 and 3 show a comparison of the neutrondiffraction data, Rietveld fit on these data, and XRDdata for the SX feedstock powder. To allow for simplecomparison, the scattered intensities of the diffractionline at 508 2u were scaled to the same height. Fig. 3shows in detail the difference between the XRD andneutron patterns. The split in the neutron data of thegroup of peaks at approximately 748 2u documents the

Žpresence of the cubic phase the middle peak is fromthe cubic phase; the peaks on the sides are from the

.tetragonal phase . The XRD data do not exhibit thisclear peak definition. No attempt was made to analyzefurther the XRD data owing to the inherent difficultiesdescribed previously.

The neutron diffraction data around the 748 2u range

Ž .Fig. 3. Detail of Fig. 2: angles 65]808 2u. Peaks for tetragonal TŽ . Žand cubic phase C are indexed; peaks for monoclinic phase dis-.cernable ones only are marked M.

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Table 2aResults for SX 233 deposits

Annealing % by mass % by mass % by mass Tetragonal Tetragonal Mol% Cubic phase Mol% Mol%conditions monoclinic tetragonal cubic phase lattice phase lattice YO in lattice YO in YO1.5 1.5 1.5

cphase phase phase parameter a parameter c tetragonal parameter cubic totalb˚ ˚ ˚w x w x w x Ž .content content content A A phase A phase "1.0

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž ."1 "3 "3 "0.0001 "0.0002 "0.2 "0.0005 "1.0

Powder 25 49 26 3.6106 5.1647 6.9 5.1371 14 8AS 2 74 24 3.6116 5.1629 7.3 5.1365 13 911008C 2 72 26 3.6098 5.1685 6.3 5.1369 14 812008C 2 73 25 3.6094 5.1705 6.0 5.1381 14 813008C 3 71 26 3.6086 5.1700 5.9 5.1365 13 814008C 3 60 37 3.6074 5.1743 5.2 5.1364 13 8

a Results are from Rietveld analysis of neutron diffraction data. Included in each column are estimated standard uncertainties.b w x Ž .Recent work has suggested that the values calculated using the data from publication 5 may be somewhat high. Therefore an experimental correction was used see text . The relative values

remain unaffected. Uncertainty for feedstock powder is "1.0.c Total YO content was calculated assuming 3 mol% YO in the monoclinic phase. Nominal composition is 8.7 mol% YO .1.5 1.5 1.5

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Table 3aResults for Amdry 142 deposits

Annealing % by mass % by mass % by mass Tetragonal Tetragonal Mol% Cubic phase Mol% Mol%conditions monoclinic tetragonal cubic phase lattice phase lattice YO in lattice YO in YO1.5 1.5 1.5

cphase phase phase parameter a parameter c tetragonal parameter cubic totalb˚ ˚ ˚w x w x w x Ž .content content content A A phase A phase "1.0

Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž ."1 "3 "3 "0.0001 "0.0002 "0.2 "0.0005 "1.0

Powder 1 59 40 3.6084 5.1696 5.9 5.1269 7 6dAS 1 93 6 3.6162 5.1599 8.5 5.1153 ] 8d11008C 1 93 6 3.6138 5.1664 7.3 5.1219 ] 7

12008C 1 81 18 3.6138 5.1672 7.2 5.1266 7 713008C 1 73 26 3.6126 5.1681 6.8 5.1285 8 714008C 1 58 41 3.6091 5.1745 5.4 5.1310 10 7

a Results are from Rietveld analysis of neutron diffraction data. Included in each column are estimated standard uncertainties.b w x Ž .Recent work has suggested that the values calculated using the data from publication 5 may be somewhat high. Therefore an experimental correction was used see text . The relative values

remain unaffected. Uncertainty for feedstock powder is "1.0.c Total YO content was calculated assuming 3 mol% YO in the monoclinic phase. Nominal composition is 8.7 mol% YO .1.5 1.5 1.5dCould not be resolved because of small amount of this phase.


are presented in Fig. 4 for the SX feedstock powder,the as-sprayed deposit, and deposits annealed at vari-ous temperatures. This allows direct visual comparisonof the diffraction patterns. Similar results were ob-tained for the Amdry powder and deposits, but visualcomparisons are more difficult owing to the differencesin yttria content between the various phases of the SX

Ž .and Amdry materials see below . The phase composi-tions and calculated yttria content obtained from theneutron Rietveld refinements are given in Tables 2 and3; the changes in phase composition as a function ofannealing temperature are also presented in Fig. 5.

4. Discussion

4.1. Use of XRD ¨s. neutrons

The difficulty with recognizing the cubic phase inthese materials, particularly when present in small

w xquantities, was discussed by Argyriou and Howard 7 .The XRD technique has limitations discussed in thebeginning of this paper, which lessen the sensitivity ofthe results compared with the neutron analysis. De-tailed analysis of the phase composition of these mate-rials may, therefore, require the use of neutrondiffraction. This is especially true for specific purposessuch as measurements of standards, experimental sam-ples, and other cases when the precise tetragonalrcubicphase composition is needed.

4.2. Scattering comparison

Fig. 4 documents the magnitude of the changesobserved upon annealing at increasingly higher temper-

atures. It can be seen that the intensity of the cubicw xphase 400 reflection increases with annealing relative

to the intensities of the tetragonal phase reflections.Furthermore, it also shows changes in the peak shapesand positions, most notably for the tetragonal phase.Numerical analysis of the peak shapes shows a slightincrease in the peak width for the tetragonal phase asthe samples are annealed. Although this material is

w xfine grained 13 , the increase in peak width with an-nealing is inconsistent with fine grain-size peakbroadening. Our analysis suggests that this peakbroadening is related to an increase in chemical inho-mogeneity, more specifically to formation of areas oftetragonal phase with varying yttria content. This mat-ter is under further evaluation and will be reported inmore detail in future work.

4.3. Feedstock

The Amdry 142 feedstock contained a minimalamount of monoclinic phase and a relatively largeamount of cubic phase. The SX 233 feedstock was quitedifferent, with cubic and monoclinic phase contentsimilar, each approximately 25% by mass. Such differ-ences are probably related to the powder manufactur-ing method as well as the apparently low yttria contentof the Amdry feedstock. The Amdry powder, manufac-tured by the melt-and-crush method, is probably morehomogeneous than the SX powder, manufactured bythe HOSP method. The HOSP method, a proprietarymethod similar to plasma spheroidization, is a rapidsolidification process and the time the material has tohomogenize in the flame is short. Therefore it is possi-ble that there are chemical inhomogeneities even withinone feedstock grain. Areas with low yttria content are

Fig. 4. Comparison of scattered intensity in the 72]768 2u range for SX feedstock powder, as-sprayed deposit, and annealed deposits. Note thew x w x w xincrease in the cubic 400 peak intensity and the shift of the tetragonal 004 to lower 2u and 220 to higher 2u with increasing annealing

temperature, owing to an increase in cubic phase content and a decrease in yttria content of the tetragonal phase.


Ž . Ž .Fig. 5. Phase composition of the deposits depending on the annealing temperature. a Amdry 142 deposits; b SX 233 deposits. Annealing time1 h. Estimated standard uncertainties are approximately "3 for the tetragonal and cubic phases and approximately "1 for the monoclinic phase.

then monoclinic. In addition, the differences in chemi-cal composition between different grains may be higherin the SX than in the Amdry feedstock.

4.4. As-sprayed deposits

Both as-sprayed deposits had a relatively small con-Ž .tent of monoclinic phase -2% by mass . This is

important especially for the SX powder, where theplasma spraying resulted in a significant reduction ofmonoclinic phase contained in the feedstock.

There were, however, differences in cubic and tetra-gonal phase content. The Amdry as sprayed deposits

Ž .contained a small amount of cubic phase 6% by mass ,while this phase represented approximately 24% by

mass of the SX as sprayed deposits. The reason for thisdifference is currently unclear. The lack of homogene-ity in the SX material is an unlikely cause, since thedecrease in the monoclinic phase content after plasmadeposition suggests chemical homogenization duringplasma deposition. It seems probable that the lowercubic phase content of the Amdry coatings is the resultof the low overall yttria content, which favors forma-tion of the tetragonal phase.

However, another possible explanation is a differ-ence in the thermal history of the deposits. Thermalspraying results in deposits with metastable composi-tion and is sensitive to spray parameters. While thesamples were sprayed with as similar spray parameters

Žas possible same torch power, same spray distance,


.etc. , the difference in feedstock powder propertiescould not be avoided. This variation in feedstockproperties is reflected in the different microstructures

Ž .of the formed deposits porosity and pore characterw x14,15 . Therefore the deposits also differ in theirproperties such as thermal conductivity. Thermal con-

Ž .ductivity heat loss into the substrate significantly in-fluences the thermal history of the deposits and cancause different phase composition in the as sprayedstate. A similar effect, caused by changing the spraydistance, has already been documented by the authorsw x16 for the SX material.

4.5. Annealing

Annealing of the deposits resulted in changes lead-ing to similar phase compositions at higher annealingtemperatures: approximately 60% by mass of tetra-gonal phase and approximately 40% by mass of cubic

Ž .phase with traces of monoclinic phase . This composi-tion agrees with values estimated from the phase dia-gram for 14008C. The changes for the Amdry samplesare more pronounced since the starting phase composi-

Ž .tion as sprayed is further from the 14008C phasecomposition than is the case for the SX material.

4.6. Yttria content ¨ariations

ŽThe yttria fraction in the tetragonal phase Tables 2.and 3 decreased with increased annealing temperature

for both the Amdry and SX materials. This is alsow xdocumented by the shift of the tetragonal 004 reflec-

w xtion to lower 2u and the 220 reflection to higher 2uwith increased annealing temperature as shown in Fig.4. Such loss of yttria is also in agreement with thephase diagram. As the annealing temperature in-creases, the thermodynamically stable yttria content inthe tetragonal phase decreases.

The yttria content in the cubic phase did not exhibitthe same trend for both materials. This is most likelydue to the differing fraction of cubic phase within thedeposits as well as the total yttria content. For theAmdry samples, where the amount of cubic phase isinitially small and increases significantly with annealingat higher temperatures, we observed initial formationof cubic phase with relatively low yttria content. As theannealing temperature is increased and the yttria con-tent of the tetragonal phase decreases, both the amountof cubic phase and the yttria content of this phaseincrease. For the SX samples, which have a relativelyconstant but larger cubic phase fraction, we observedlittle variation in the yttria content of the cubic phase.In both materials this behavior is consistent with atransfer of yttria from the tetragonal to the cubic phaseupon annealing.

5. Conclusions

This experiment documents important processes inthe plasma sprayed YSZ deposits for industrial prac-tice. Engineering applications require long-term stabil-ity of the phase composition of these deposits and ofthe microstructure in general. Our results show that at

Ž .high temperatures close to 14008C the phase compo-sition and the tetragonal phase yttria content maychange significantly and rapidly, and that even at con-siderably lower temperatures some compositionalchanges are evident. The loss of yttria from the tetra-gonal phase is especially important, since it may leadeventually to formation of the monoclinic phase oncooling, which may be detrimental to deposit lifetime.The present results, however, do not prove thisdestabilization and subsequent monoclinic phase for-mation, as in our current experiments the monoclinicphase content did not vary appreciably. However, this

w xshould occur at longer aging times. Miller et al. 17reported a significant increase in the monoclinic phasecontent with annealing for 100 h at 14008C based onXRD analysis; however, the initial monoclinic phase

Žcontent of the material studied was much higher ;.10% by mass than in the present study. They also

noted a decrease in yttria content of the tetragonalphase on annealing at higher temperatures along withan increase in cubic phase content, in qualitative agree-ment with our results.

The current experiment showed that the yttria lostby the tetragonal phase goes into the cubic phase. Nodecrease of total yttria content or formation of free

Ž .yttria was observed Tables 2 and 3 . Unlike in previousstudies, the use of the Rietveld refinement techniqueand neutron diffraction data permit the determinationof the cubic lattice parameter a and thus the yttriacontent of the cubic phase along with a more reliableestimate of the cubic phase fraction. It is possible thatthe amount of cubic phase which can be formed islimited by the amount of yttria available. Further inves-tigation on changes at longer annealing times is inprogress.

For industrial practice this work presents potentiallyimportant conclusions about the limitations of XRDfor phase analysis on these materials. Relying solely onXRD phase analysis may result in neglecting or under-estimating the cubic phase content. The temperaturedependence of this phase shows that its behavior maybe an important marker for the rates of phase transfor-mation and chemical element redistribution. Neglect-ing the cubic phase itself in the deposits may not beimmediately detrimental from the practical point ofview. However, by being able to follow the cubic phasevariations we may be able to study the dynamics of thephase changes in the deposits } and these changes


may be extremely important for the optimization ofYSZ deposits for their lifetime at higher temperatures.

Acknowledgements

This research was supported in part by the MRSECprogram of NSF under award DMR 96-32570 at theNSF Center for Thermal Spray Research at the StateUniversity of New York at Stony Brook.

References

w x Ž .1 Z. Mutasim, W. Brentnall, J. Thermal Spray Technol. 6 1997105]108.

w x2 C.C. Berndt, H. Herman, 10th International Thermal SprayingConference, Essen, Deutscher Verlag fur Schweißtechnik¨DVS-Verlag GmbH, Dusseldorf, 1983, pp. 175]178.¨

w x Ž .3 S. Alperine, L. Lelait, J. Eng. Gas Turbines Power 116 1994258]265.

w x Ž .4 M. Yoshimura, Ceram. Bull. 67 1988 1950]1955.w x Ž .5 H.G. Scott, J. Mater. Sci. 10 1975 1527]1535.w x6 V. Jayaram, M. De Graef, C.G. Levi, Acta Metall. Mater. 42

Ž .1994 1829]1846.w x Ž .7 D.N. Argyriou, C.J. Howard, J. Appl. Cryst. 28 1995 206]208.

w x Ž .8 J. Ilavsky, J. Forman, P. Chraska, J. Mater. Sci. Lett. 11 1992573]574.

w x9 C. Kittle, Introduction to Solid State Physics, 3rd ed, JohnWiley & Sons, Inc, New York, 1967.

w x Ž .10 H.M. Rietveld, J. Appl. Crystllogr. 2 1969 65]71.w x11 A.C. Larson, R.B. Von Dreele, Rep. LAUR 86]748, Los Ala-

mos Laboratory, 1994.w x12 C.J. Howard, private communication, Australian Nuclear Sci-

ence and Technology Organization, NSW, Australia, 1998.w x Ž .13 T. Chraska, A. King, J. Mater. Sci. Lett. 18 1999 1517]1519.w x14 J. Ilavsky, G.G. Long, A.J. Allen, C.C. Berndt, H. Herman, in:

Ž .C.C. Berndt Ed. , 1st United Thermal Spray Conference }

Thermal Spray: A United Forum for Scientific and Technologi-cal Advances, Indianapolis, Indiana, USA, ASM International,Materials Park, OH 44073-0002, USA, 1997, pp. 697]702.

w x15 J. Ilavsky, G.G. Long, A.J. Allen, H. Herman, C.C. Berndt, in:Ž .C.C. Berndt Ed. , 9th National Thermal Spray Conference }

Thermal Spray: Practical Solutions for Engineering Problems,Cincinnati, Ohio, USA, ASM International, Materials Park,OH 44073-0002, USA, 1996, pp. 725]728.

w x Ž .16 J. Ilavsky, J.K. Stalick, in: C.C. Berndt Ed. , 1st United Ther-mal Spray Conference } Thermal Spray: A United Forum forScientific and Technological Advances, Indianapolis, Indiana,USA, ASM International, Materials Park, OH 44073-0002,USA, 1997, pp. 691]695.

w x Ž .17 R.A. Miller, J.L. Smialek, Adv Ceramics 3 1981 241.

phase composition and its changes during annealing of plasma-sprayed ysz

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