processing effects on porosity-property correlations in plasma sprayed yttria-stabilized zirconia...

Processing effects on porosity-property correlations in plasmasprayed yttria-stabilized zirconia coatings

Anand Kulkarni *, A. Vaidya, A. Goland, S. Sampath, H. Herman

Department of Materials Science and Engineering, Center for Thermal Spray Research, State University of New York, Old Engineering, Room 314,

Stony Brook, NY 11794-2275, USA

Received 11 November 2002; received in revised form 4 April 2003

Materials and Engineering A359 (2003) 100�/111

www.elsevier.com/locate/msea

Abstract

For plasma sprayed thermal barrier coatings (TBCs), control of thermal conductivity is critical since low thermal conductivity

depends not only on the intrinsic property of the yttria-stabilized zirconia (YSZ) TBC, but also on the morphology of pores and

cracks introduced during spray process. They are closely linked to process methodology as well as to chemistry, structure and

morphology of the ceramic feed materials. This paper addresses the influence of feedstock characteristics on particle state in the

plasma and the resultant coating properties. In addition, substrate temperature, angle-of-impact and thermal cycling effects on

porosity (quantity and morphology) and its resultant influence on thermal conductivity and elastic modulus of plasma sprayed YSZ

TBCs. The results show increased porosity with particle size, due to an increase in the degree of particle fragmentation and unmelted

particles, leading to lower thermal conductivity and modulus. Furthermore, higher substrate temperatures and low particle velocity

lead to lower porosity and improved inter-splat contact and, thus, enhanced coating properties. Sintering during thermal cycling

reduces porosity and increases thermal conductivity and modulus.

# 2003 Elsevier B.V. All rights reserved.

Keywords: Thermal barrier coating; Plasma; Yttria-stabilized zirconia; Porosity; Thermal conductivity; Elastic modulus

1. Introduction

There is an extensive effort to incorporate prime-

reliant ceramic thermal barrier coatings (TBCs) into

advanced gas turbine and diesel engine components.

This is because of the increased thermodynamic effi-

ciency due to the higher operating temperatures and

reduced cooling air requirements allowed by these

coatings as well as prolonged substrate lifetimes due to

lower metal surface temperatures. TBCs based on 6�/8

mol% yttria-stabilized zirconia (YSZ) have been widely

used, owing to their low thermal conductivity, chemical

and thermal stability, relatively high coefficient of

thermal expansion (relative to the metallic substrate)

and high resistance to spallation under thermal fatigue

(toughening) [1�/4]. The development and acceptance of

TBCs are also closely linked to processing technology:

plasma spray and electron beam-physical vapor deposi-

tion (EB-PVD). In the EB-PVD process, vapors are

produced by heating the source material with an

electron beam, and the evaporated atoms condense on

the substrate. Crystal nuclei form on favored sites,

growing laterally and in thickness to form individual

columns, which provide in-plane compliance [5]. For the

case of plasma spraying, feedstock powder is melted and

accelerated to high velocities, impinging upon the

substrate, and rapidly solidifying to form a ‘‘splat’’ (a

flattened particle). The deposit develops by successive

impingement and inter-bonding among the splats. The

deposit microstructure is strongly dependent on proces-

sing conditions, spray parameters and feedstock materi-

als. The splats are separated by interlamellar pores

resulting from rapid solidification of the lamellae, very

fine voids formed by incomplete inter-splat contact or

around unmelted particles, and cracks due to thermal

stresses and tensile quenching relaxation stress [6�/8].

The pores and cracks interfere with the direct flow of

heat (thermal barrier) resulting in lowered thermal

* Corresponding author. Tel.: �/1-631-632-4511; fax: �/1-631-632-

8440.

E-mail address: [email protected] (A. Kulkarni).

0921-5093/03/$ - see front matter # 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0921-5093(03)00342-3

mailto:[email protected]

conductivity. The cracks also increase the overall

compliance of the coating and, hence, enhance the

thermal shock resistance [9�/11]. Due to thermal cycling

conditions encountered by TBCs in service, the influence

of thermal cycling on the thermal protection and

spallation resistance has been studied in great detail

[12�/15]. In this paper, we examine processing-micro-

structure-property correlations in plasma sprayed YSZ

coatings. Studies involving the influence of feedstock

particle size, particle conditions (particle temperature,

velocity and flux), deposition conditions (substrate

temperature and angle-of-impact) are related to micro-

structure development and properties. The experimental

results are presented as follows: particle�/plasma inter-

actions to show the inflight behavior of feedstock in the

plasma flow; correlations of particle state to splat

morphologies on impact with the substrate; splat�/splat

interactions leading to the microstructure development;

correlation of microstructure to porosity and properties

of the coatings. Results are also given of microstructure

and property alterations that occur upon thermal

cycling. Such studies will allow tailoring of coating

microstructure to meet future design needs of prime

reliant TBCs.

2. Experimental

In order to examine particle behavior in the plasma

flow, in-flight diagnostic studies were carried out on

feedstock powders with different manufacturing meth-

ods and shaped morphologies. Three powders were

chosen, made by different manufacturing methods and

two different particle size distributions. Table 1 lists the

characteristics of powders that were studied.In order to explore the effects of particle size

distribution, three different particle sizes (classified as

‘Fine’, ‘Medium’ and ‘Coarse’ were sieved from the as-

received plasma densified powder and compared to the

as-received/ensemble powder. Two of the powders were

chosen for the diagnostic study*/‘Fine’ and ‘Medium’

size distributions: Table 2.

2.1. Processing parameters

Initially, a study was carried on the effects of feed-

stock particle size on the properties of YSZ coatings.

Coatings were deposited using a Sulzer Metco 3 MB

plasma gun at a 100 mm standoff distance. The spray

parameters are listed in Table 3. Splats were collected on

polished stainless steel substrates. Freestanding deposits

were also obtained for characterization. Some of the

coatings were subjected to ten cycles of heating at

1150 8C for 30 min, followed by cooling in air for 15

min to observe resultant deposit properties on thermal

cycling.

The second part of the study was an investigation of

the effects of particle velocity and substrate temperature

on coating properties. The particle velocity was changed

by varying the plasma conditions during spraying (e.g.

using nozzles of different internal diameter, 6 and 8 mm,

for high and low velocities, respectively). The particle

conditions used are shown in Table 4. In-flight particle

diagnostics were performed using the Tecnar DPVTM

system to monitor particle temperature and velocity, the

results of which are shown in Table 5. Previous work

[16,17] has shown the substrate temperature to have a

dramatic effect on splat morphology, thus influencing

the adhesion and bonding between splats and, hence, the

physical and mechanical properties of the coatings. With

this in mind, the splats and coatings were also produced

at two substrate temperatures, designated ‘‘low’’ and

‘‘high’’. By controlling the preheating and air-cooling

during spraying, it was possible to vary substrate

temperatures from 100 8C (Cold) to 300�/350 8C (Hot).

Lastly, a study was carried out on the influence of

spray angle on the microstructure. The splats were

collected and coatings were sprayed at two different

spray angles, normal and 458 to the substrate. The spray

process parameters are given in Table 6. Coatings were

sprayed using a fused-and-crushed powder at very high

power to develop dense microstructure [18]. The influ-

ence of particle flux on the microstructure was also

investigated by changing the traverse speed of the

plasma gun during spraying; 70 and 30 m s�1 for

standard and slow (high particle flux) traverse, respec-

Table 1

Feedstock characteristics

Powder type Processing Manufacturer Mean particle size-d50

(mm)

S.D.

(mm)

Surface area (m2

g�1)a

Fused and crushed (F&C) Cast material, subsequently

crushed

MSM ZY7 45 22 0.08

Agglomerated and sintered

(A&S)

Spray dried and sintered Sulzer-Metco AE

7216

45 28 0.4

Plasma densified (HOSP) Plasma processed/densified Sulzer-Metco

AE7593

61 33 0.48

a Characterized using BET surface area technique at NIST, Gaithersburg, MD.

A. Kulkarni et al. / Materials and Engineering A359 (2003) 100�/111 101

tively. Deposition with high particle flux produce

thermal stresses to generate vertically cracked structure

in the coatings [19,20], the preferential orientation of

these cracks can be observed on a macroscale with

respect to spray angle.

2.2. Deposit characteristics

Splats were observed under SEM, since splat

morphologies and interactions between them control

deposit integrity and properties. To quantify the surface

profile and dimensionality of these splats, they wereobserved under a ZygoTM surface interferometer. Free-

standing YSZ deposits were evaluated for porosity

content and thermal conductivity. Surface-connected

porosity in as-sprayed and thermal-cycled specimens

was measured by mercury intrusion porosimetry (MIP)

using a Quantachrome Autoscan 33 porosimeter. Ther-

mal conductivity measurements were carried out on

disk-shaped specimens, coated with carbon, using aHolometrix laser flash thermal properties instrument.

Depth-sensitive indentation studies were carried out

with a Nanotest 600 instrument with a 1/16-in. WC-

Co spherical indenter with a maximum load of 10 N.

Phase and microstructural characterization were also

carried out. Microstructural changes upon thermal

cycling/sintering were monitored through thermal ex-

pansion measurements on a Netzsch 402CTM Dilat-ometer at a heating rate of 10 K min�1 up to 1400 8C.

3. Results and discussions

3.1. Particle�/plasma interactions

The results from diagnostic studies, summarized in

Fig. 1, show that the particle size and shape of the

feedstock powder play key roles in determining the

velocity and temperature of the particles in the plasma

flow. The Fig. 1 indicates that the coarser powder

(‘‘medium’’) travel slower through the plume as com-

pared with the finer distribution. This can be under-

stood from the fact that the momentum transfer fromthe gas to the particle is strongly dependent on the mass

of the injected particle. Thus, the coarser (heavier)

particles attain a lower velocity as compared with the

Table 2

Particle size distributions for sieved plasma densified powder

Powder size Manufacturer Sulzer-Metco Mean particle Size-d50 (mm) S.D. (mm) Surface area (m2 g�1)a

Fine AE7592 35 18 0.91

Medium AE7591 54 21 0.44

Coarse AE7590 103 24 0.32

a Characterized using BET surface area technique at NIST, Gaithersburg, MD.

Table 3

Processing parameters for particle size effects

Plasma gun Sulzer-Metco 3 MB

Current 650 A

Voltage 66�/68 V

Primary gas 40 SLM Ar

Secondary gas 8 SLM H2

Carrier gas 3000 SCCM Ar

Feed rate 20�/40 g min�1

Standoff distance 100 mm

Table 4

Processing parameters for particle velocity and substrate temperature

effects

Plasma gun PT F4 torch

Current 600 A

Voltage 66�/68 V



Secondary gas 30 SLM He




Table 5

In-flight diagnostics

Low velocity High velocity

Temperature (8C) 33469/200 32279/200

Velocity (m s�1) 1809/30 2359/30

Table 6

Processing parameters for angle-of-impact studies

Plasma gun PT F4 torch

Current 650 A

Voltage 66�/68 V






A. Kulkarni et al. / Materials and Engineering A359 (2003) 100�/111102

finer ones. On the other hand, the average surface

temperature recorded for the coarser particles is higher

than that recorded for the finer particles. This difference

arises due to the different trajectories of the injected

particles and the longer residency time. Fig. 2 represents

a cross-sectional snapshot of the plasma plume showing

the particle size distribution within the flow at two

different distances from the nozzle, 60 and 120 mm. The

center of plasma plume corresponds to coordinates (0,

0) and the particles are injected from the top-above the

coordinate (0, 10). As can be seen from the two contour

maps, the larger particles travel closer to the center of

the flow and the finer ones are on the periphery. The

coarser particles, which have a larger momentum than

the finer ones, penetrate the plasma plume to a greater

extent. The coarser particles also travel much slower

than the fines and hence spend a longer time in the

plume. This results in a trajectory closer to the high

temperature zones of the plasma. Hence, the coarser

particles attain a higher surface temperature for similar

injection parameters. For the particle size distribution

chosen in this study, the particles were in a range

suitable for plasma spraying and not so coarse as to

cause the particles to ‘fall’ through the flow. The other

factor, which is significant with regards to heat and

momentum transfer, is the particle shape. The powders

with particles of different shape*/dense spherical (ag-glomerated and sintered) and angular (fused and

crushed) result in different particle velocities and

temperatures.

3.2. Droplet�/substrate interactions

It is clear that droplet impact, spreading and splat

solidification largely determine microstructure develop-

ment. The intrinsic properties of the individual splatsand the intricate correlations among them are affected

by the conditions of the impacting particles, such as size,

temperature and velocity. Substrate condition (or that

of previously solidified particles) affect wetting, thermal

contact resistance, roughness/splat profile, chemical

interactions, etc. The governing interrelations of droplet

impact and solidification history to process parameters

and substrate condition are examined here. The resultsdemonstrate the role of particle energy and substrate

temperature in determining the nature of splat spreading

and solidification.

Fig. 1. Diagnostic data for different powders shows differences in particle�/plasma interactions measured at the standoff distance.


Fig. 3 shows the microstructure of a single YSZ splat.

Fig. 3A shows a disk-shaped splat, collected on a

polished stainless steel substrate at 100 8C. ‘‘Mud

cracks’’ due to stress relief of YSZ are also seen togetherwith the core and rim regions. Fig. 3B shows the

droplet�/substrate interaction. Columnar grain struc-

ture, having an average grain diameter of 50 nm, is

observed in regions where the particle droplet first

comes in contact with the substrate. The columns

grow in a direction perpendicular to the substrate, since

heat is extracted through the substrate and the solid/

liquid interface moves vertically. Also seen is themicrocrack inside the splat.

3.3. Splat�/splat interactions/coating buildup

Fig. 4 shows a fractured cross-section of a plasma

sprayed YSZ coating showing layers of splats, along

with other features such as interlamellar pores, cracks

and globular pores. Fig. 4B shows columnar epitaxy

preserved in a splat�/splat interaction along with glob-

ular pores from gas and interlamellar pores due to

improper localized adhesion. The interlamellar pores

result from poor wetting/adhesion between the splats as

they accumulate to form the coating. Although they

provide significant reduction in through-thickness ther-

mal conductivity, these poorly bonded regions can result

in delaminations, causing premature failure during

thermal cycling. The microcracks or ‘‘mud-cracks ’’

present within splats result from the release of ‘‘quench-

ing’’ stresses during deposition. These stresses arise dueto constrained shrinkage of a solidifying splat and

generally have a large tensile magnitude for materials

such as YSZ [8]. In the case of most ceramics, the large

quenching stresses cannot be retained by the splat

during deposition and cooling and result in micro-

cracking (Fig. 3A). These micro-cracks can provide a

small degree of in-plane compliance. The globular pores

Fig. 2. This figure represents a cross-sectional snapshot of the plasma

plume showing the particle size distribution within the flow at two

different distances from the gun (A) 60 and (B) 120 mm.

Fig. 3. Microstructure development of a single splat, (A) shows single splat morphology collected on a polished substrate and (B) shows columnar

structure along with microcracks (M).


result from lack of filling around the undulations of

splats and debris. In general, globular pores areuncontrollable and undesirable since they provide sites

for failure initiation. Another aspect in the TBC

microstructure is the existence of the large vertical

macro-cracks , which form due to the relief of thermal

stresses associated with the mismatch between thermal

expansion coefficients of the substrate and the coating.

When properly controlled, these cracks can provide the

desired in-plane compliance, such as those in EB-PVDdeposits.

3.4. Effect of feedstock characteristics

Fig. 5 shows the different splat morphologies ob-

served for the three different particle sizes. It is evident

that the morphology of the splats changes from a disk-

like shape to fragmented shape with increasing particle

size. These shapes are highly representative of the vast

number of splats for each size distribution. This

difference in splat morphology manifests itself in coating

microstructure and properties.These differences in microstructure show themselves

as wide variations in the porosity levels in the coatings.

Fig. 6 shows particle size effects on porosity and thermal

conductivity. The scatter in porosity values is 9/5% and

that in thermal conductivity is 9/3%. The porosity

increases with increased particle size. This can be

attributed to two factors: reduced melting efficiency of

coarser particles in the plasma plume compared with

fine particles and, secondly, due to the increased

fragmentation of splats resulting in poor adhesion and

porosity. The thermal conductivity obeys the inverse

relationship, decreasing with increasing particle size.

The modulus results, plotted versus porosity, are shown

along with the microstructures in Fig. 7. Vast differences

are observed, showing fragmented splats leading to poor

splat�/splat contact and formation of pores. It is evident

that the coating prepared from fine powder shows well-

adhered splats, while the unmelted and poorly adhered

particles can be seen in the case of the coating prepared

from coarse powder. In-plane and out-of-plane modulus

measurements were carried out to show the anisotropy

in these coatings. The modulus, both in through-

thickness and in-plane direction, decreases with increas-

ing particle size, obeying the same trend as the thermal

conductivity. However, the well-adhered splats along

with the interlamellar pores and intrasplat cracks

generate greater anisotropy in the case of coating

made from fine powder particles. This anisotropy

reduces with increasing particle size owing to the

microstructure developed from the unmelted particles.

Fig. 4. Cross-section micrograph of a plasma sprayed YSZ coating, (A) shows splats along with other features with interlamellar pores, cracks and

globular pores and (B) shows splat�/splat interface along with globular pores and interlamellar pore (improper adhesion).

Fig. 5. Particle size effects on splat morphologies observed, collected on polished stainless steel substrate at 100 8C.


3.5. Particle velocity and substrate temperature effects

Fig. 8 shows the splat morphologies observed under

an interferometer for the low and high particle velo-

cities: 180 and 235 m s�1, respectively. The splats

appear to be fragmented for both the conditions, the

extent of fragmentation appearing greater for the high

velocity case. The surface topography indicates a

disturbed/rough surface profile for the high velocity

condition vis-a-vis the low velocity condition. As

anticipated, the dimensionality of the splats show the

splats to be thinner for the higher velocity case. The

splat morphologies observed for the different substrate

temperatures were similar to those observed in the

previous study [16], confirming its role in enhancing

contact area and wetting between the splat and the

substrate.

Fig. 9 shows the significantly different microstructural

features observed for these particle conditions. The

coating sprayed under low velocity condition shows

well-adhered splats, compared with the high velocity

case. Certain regions of unmelts are also observed in the

high velocity condition, owing to shorter flight times in

the plasma. Also, the thickness of the splats is different;

thicker for the low velocity condition than for the high

velocity conditions, thus fewer interfaces are observed in

the former case. MIP porosity shows an increase in

surface-connected porosity with increasing velocity,

arising from the interlamellar and globular pores due

to improper adhesion and unmelted particles in the

microstructure (Fig. 9). As a consequence of the higher

velocity (at almost similar particle temperatures (Table

5) and a reduced residence time of the particle in the

plasma), it can be concluded that the microstructure of

the coating is influenced to a greater degree by particle

velocities than it is owing to a change in substrate

temperature.

The thermo-mechanical properties observed for the

two different velocity conditions at two substrate

temperatures are shown in Fig. 10. As indicated earlier,

the scatter in thermal conductivity is 9/3% and that for

elastic modulus values is 9/1%. A lower porosity is

Fig. 6. Influence of feedstock particle size on porosity and thermal conductivity of YSZ coatings.

Fig. 7. Microstructural variations explaining anisotropy in coatings with respect to elastic modulus.


observed for the high substrate temperature, leading to

increased thermal conductivity and elastic modulii,

consistent with prior studies [16]. Also, the increase inporosity combined with the large number of splat

interfaces (as a result of thinner splats for the higher

velocity case) decrease thermal conductivity and elastic

modulus.

3.6. Effect of particle flux and angle-of-impact

In this section are discussed the splat morphologies

and properties with reference to the particle flux effects

and the angle-of-impact of impinging particle. Fig. 11

shows the splat morphologies and dimensions observed

for two (45 and 908) impinging angles. The thickness of

the splats is nominally 1.29/0.3 mm for both cases. It is

observed that the splats are disk-shaped for normalincidence and elongated for the 458 impact angle. This is

expected to influence microstructure development,

which is shown in Fig. 12. Fused-and-crushed powder

was selected, given its ability to produce dense coatings

as observed in previous studies [18]. The microstructures

show segmented crack networks due to the biaxial stress

developed during deposition at high power and particle

flux [20]. Earlier work on the effect of spray angle ongray alumina coatings showed the tendency for cracks to

orient preferentially with the spray direction [19]. This

surprising effect is seen for YSZ coatings (tougher than

alumina), Fig. 12, showing cracks tilted by 70�/808 to the

substrate plane. In order to determine the effect of spray

angle and flux on physical properties, a study was

carried out on thermal conductivity and modulus as a

function of two spray angles.

The observed thermal conductivity and elastic moduli

values are shown in Fig. 13. The effect of particle flux is

controlled by traverse speed of the plasma gun during

spraying; 70 and 30 m s�1 for standard (low particle

flux) condition and slow (high particle flux) condition,

respectively. The variation in surface connected porosity

was not significant for each case owing to large crack

network contributing to porosity. However, the proper-

ties will depend on the intrinsic structure of the coating.

The values of thermal conductivity and elastic modulus

are higher for coatings sprayed by slow traverse speed

because of higher particle flux impinging on the

substrate leading to lower porosity coatings. The pre-

ferential crack orientation for the angle-of-impact

studies show lower thermal conductivity for coatings

sprayed at 458 impact angle as opposed to coatings

sprayed at 908 impact angle.

An investigation of microstructural changes observed

upon heating and thermal cycling would improve our

current understanding of the sintering-related failure

mechanisms in these coatings. The change in micro-

structure reflects changes in specimen dimensions, which

are given in the thermal expansion curve in Fig. 14.

Fig. 8. Surface topography of splats observed under interferometry.


Fig. 9. This figure shows the fractured cross-sections observed for different particle conditions, the table shows the variation in surface-connected

Fig. 10. This figure shows the experimental thermal conductivity and elastic modulus of the YSZ coatings for the varying velocity and temperature

conditions.


Curves (1) and (2) are for measurements in the through-

thickness and the in-plane direction, respectively. It is

seen that the dimensional changes are different for both

cases as reflected in the coefficient of thermal expansion

values. It can be inferred that the dimensional change is

larger for the through-thickness direction than the in-

Fig. 11. The figure shows single splat morphologies for different angle-of-impacts.

Fig. 12. The figure shows effect of angle-of-impact on coating microstructures (cracks seen preferentially oriented).

Fig. 13. The figure shows the effect of particle flux and angle-of-impact on the coating properties.


plane direction, because of the larger dimensions of

interlamellar pores compared with the intrasplat cracks.

Another interesting result is the behavior of the coating

in the temperature range of 450�/800 8C, which likely is

due to the monoclinic-to-tetragonal transformation [21].

Further shrinkage above 1100 8C is due to sintering.The change in microstructure reflects itself as changes

in porosity and thermal conductivity, the results of

which are shown in Fig. 15. This change in thermal

conductivity can be attributed to sintering upon heating

[22]. The change is greater for the lower porosity

coatings (fine particle size) as compared with high

porosity coatings (large particle size) because of differ-

ences of the mean opening dimensions of pores [23].

4. Summary and conclusions

There is a significant influence of processing condi-

tions on the properties of plasma sprayed YSZ coatings.

Feedstock particle size, particle condition in the plasma

(particle temperature, velocity and particle flux) and

deposition conditions (substrate temperature and angle-of-impact) have been identified as critical parameters

that influence the microstructure and properties. Results

show that the particle state in the plasma is strongly

influenced by feedstock size and shape. This in turn

dictates microstructure development to a large extent.

The particle size and substrate temperature studies show

that from among the different microstructural features,

the major factors influencing coating properties areporosity and interlamellar contact. While wide variation

in microstructures and resultant properties are observed,

depending on processing conditions, the vertical macro-

Fig. 14. The figure shows thermal expansion curves observed for YSZ coatings heated to 1400 8C.

Fig. 15. The figure shows porosity thermal conductivity relationships

observed in YSZ coatings.


cracks seem to provide an ideal combination of lower

thermal conductivity with high strain tolerance for

TBCs in service.

Sintering during thermal cycling results in a decreasein porosity leading to an increase in thermal conductiv-

ity and a loss of the strain tolerant behavior. The level of

sintering depends on the initial deposit density realized

from various powder morphologies and particle sizes.

Quantitative analysis on the interlamellar and globu-

lar pores and intrasplat cracks are being carried out

using multiple small-angle neutron scattering for a

better insight into the porosity-property relationships.This analysis is also being used to develop a predictive

capability of the microstructural property correlations

[18].

Acknowledgements

This research work was sponsored by the MRSEC

program of National Science Foundation under award

DMR-0080021.

References

[1] R.A. Miller, Surf. Coat. Technol. 30 (1987) 1.

[2] W.J. Brindley, R.A. Miller, Adv. Mater. Proc. 8 (1989) 29.

[3] W. Mannsmann, H.W. Grunling, J. Phys. IV 3 (1993) 903.

[4] R.L. Jones, in: K.H. Stern (Ed.), Metallurgical and Protective

Coatings, Chapman and Hall, London, 1996, pp. 194�/235.

[5] O. Unal, T.E. Mitchell, A. Heuer, J. Am. Ceram. Soc. 77 (4)

(1994) 984.

[6] H. Herman, Sci. Am. 259 (3) (1988) 112.

[7] R. McPherson, Thin Solid Films 83 (1981) 297.

[8] J. Matejicek, S. Sampath, P.C. Brand, H.J. Prask, Acta Mater. 47

(2) (1999) 607.

[9] R.H.J. Hannink, P.M. Helly, B.C. Muddle, J. Am. Ceram. Soc. 83

(3) (2000) 461.

[10] M.V. Swain, L.F. Johnson, R. Syed, D.P.H. Hasselman, J. Mater.

Sci. Lett. 5 (1986) 799.

[11] R. McPherson, Thin Solid Films 112 (1984) 89.

[12] J. Gutleber, S. Usmani, S. Sampath, in: C.C. Berndt (Ed.),

Thermal Spray: A United Forum for Scientific and Technological

Advances, ASM International, Metals Park, OH, 1997, pp. 285�/

289.

[13] G. McDonald, R.C. Hendricks, Thin Solid Films 73 (1980) 491.

[14] B.C. Wu, E. Chang, S.F. Chang, C.H. Chao, Thin Solid Films 172

(1989) 185.

[15] C.C. Berndt, H. Herman, Thin Solid Films 108 (1983) 427.

[16] S. Sampath, X. Jiang, Mater. Sci. Eng. A304�/306 (2001) 144.

[17] S. Sampath, X. Jiang, J. Matejicek, A.C. Leger, A. Vardelle, Mat.

Sci. Eng. A272 (1999) 181.

[18] A. Kulkarni, Z. Wang, T. Nakamura, S. Sampath, A. Goland, H.

Herman, A. Allen, J. Ilvasky, G. Long, J. Frahm, R. Strinbrech,

Comprehensive microstructural characterization and predictive

property modeling of plasma-sprayed zirconia coatings, Acta

Mater., 51 (2003) 2457.

[19] J. Ilavsky, A.J. Allen, G.G. Long, S. Krueger, C.C. Berndt, H.

Herman, J. Am. Ceram. Soc. 80 (3) (1997) 733.

[20] P. Bengtsson, T. Ericsson, J. Wigren, J. Therm. Spray Technol. 7

(2) (1998) 340.

[21] Q. Liu, S. An, W. Qui, Solid State Ionics 121 (1999) 61.

[22] J. Ilavsky, G.G. Long, A.J. Allen, C.C. Berndt, Mater. Sci. Eng.

A272 (1999) 215.

[23] A. Kulkarni, A. Goland, S. Sampath, H. Herman, A. Allen, Small

angle neutron scattering studies to investigate particle size effects

in plasma sprayed zirconia coatings, unpublished work.


processing effects on porosity-property correlations in plasma sprayed yttria-stabilized zirconia...

Documents