Dy:YAG Phosphor Coating Using the Solution PrecursorPlasma Spray Process

Dianying Chen,w,z Eric H. Jordan,y Michael W. Renfro,y and Maurice Gellz

zDepartment of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University ofConnecticut, Storrs, Connecticut 06269

yDepartment of Mechanical Engineering, University of Connecticut, Storrs, Connecticut 06269

Dy:YAG phosphor coatings were deposited using the solutionprecursor plasma spray process. The phase composition, micro-structure, and photoluminescent properties of the as-depositedcoatings were investigated. X-ray diffraction analysis confirmedthat the coating is mainly composed of the YAG phase with asmall amount of an intermediate YAP phase. Scanning electronmicroscopy micrograph revealed that the as-sprayed coating hascolumnar structures with a thickness of B60 lm. The measure-ment of photoluminescent properties indicated that the phosphorcoating exhibits two predominant emission regions: a blue region(470–500 nm) and a yellow region (560–600 nm), which are as-signed to

4F9/2–

6H15/2 and

4F9/2–

6H13/2 transitions, respectively.

I. Introduction

YTTRIUM aluminum garnet (Y3Al5O12, YAG) materials havebeen widely used as scintillators (YAG:Ce31),1 solid-state

lasers (YAG:Nd31)2 as well as phosphors.3,4 Dysprosium-dopedYAG (Dy31:YAG) is one of the thermographic phosphors thatcan be used to measure surface temperature by applying a thincoating of phosphors to the substrate.5–7 The Dy:YAG phos-phor exhibits temperature sensitivity in the range of 295–1973 Kand is therefore well suited for use in hostile high-temperatureenvironments such as gas turbine combustors.6–9

Phosphor coatings have been deposited by a variety of tech-niques such as chemical vapor deposition,10 spray pyrolysis,11

pulsed laser deposition,12 etc. One disadvantage of these tech-niques is their low deposition rate. Recently, a solution precur-sor plasma spray (SPPS) process has been developed for thedeposition of highly durable thermal barrier coatings,13–15 denseand hard coatings,16,17 bioactive coatings,18 porous coatings19 aswell as Eu:Y2O3 phosphor coatings.

20 In the SPPS process, liq-uid-precursor solutions are injected directly into the plasma jet.The atomized droplets undergo a series of physical and chemicalreactions before deposition on the substrate as a coating. TheSPPS method for the deposition of ceramic coatings offers sev-eral advantages such as high-rate deposition, better control overthe chemistry of the deposit, and deposition of nanostructuredcoatings. These advantages and the potential to deposit a widerange of ceramics make the SPPS method technologicallyattractive.

In this research, a Dy31:YAG phosphor coating was depos-ited using the SPPS process. The phase composition, micro-structure, and photoluminescent properties of the as-depositedcoatings were investigated.

II. Experimental Procedures

(1) Precursor Preparation

Aluminum nitrate (Al(NO3)3 � 9H2O, 497%, Alfa Aesar, WardHill, MA), yttrium nitrate (Y(NO3)3 � 6H2O, 499.9%, AlfaAesar), and dysprosium nitrate (Dy(NO3)3 � 5H2O, 499.9%,Alfa Aesar) were used as the starting materials. The abovechemicals were dissolved in deionized water based on the chem-ical formula of Y2.85Dy0.15Al5O12 (5 at.% Dy relative to yttriumions) and then stirred for 2 h. To study the phase evolution ofthe solution precursor, the solution was dried on a hot plate atB1001C and then the dried precursor powders were heated to8001 and 9001C at a heating rate of 101C/min, and held for 3 h.

(2) Plasma Spray Deposition

The Dy:YAG coatings were deposited using the direct currentplasma torch (Metco 9MB, Sulzer Metco, Westbury, NY),which was attached to a six-axis robotic arm. Argon and hy-drogen were used as the primary and the secondary plasmagases, respectively. An atomizing nozzle attached to theplasma torch was used to inject solution precursor mist intothe plasma jet. Nitrogen was used as the solution precursor at-omizing gas. The coatings were deposited on grit-blasted (Al2O3

grit of #30 mesh size) superalloy substrates (25 mm diameter,3 mm thickness). The coating deposition parameters used in thisresearch are presented in Table I.

(3) Characterization

Both differential thermal analysis (DTA) and thermal gravimet-ric analysis experiments were performed simultaneously on theas-dried Dy:YAG precursor powders using an SDT-Q6000 ther-mal analyser (TA Inc., New Castle, DE). Thermal analysis wasperformed in flowing air by heating the sample from room tem-perature to 10001C at a rate of 101C/min. A12O3 powder wasused as a reference material.

The crystalline phase composition of all the samples was de-termined using X-ray diffraction (XRD). (CuKa radiation;D5005, Bruker AXS, Karlsruhe, Germany). The XRD patternswere collected in a 2y range from 201 to 801 at a scanning rate of21/min. The photoluminescent properties of the Dy:YAG coat-ing were investigated using a Perkin–Elmer LS50B Fluorometer(Perkin-Elmer Instruments, CT). An environmental scanningelectron microscope (ESEM 2020, Philips Electron Optics,Eindhoven, the Netherlands) and a Jeol JSM-6335F field emis-sion scanning electron microscope (FESEM, Tokyo, Japan)were used to characterize the coating microstructure.

III. Results and Discussion

(1) Precursor Crystallization Behavior

The XRD patterns of the Dy:YAG precursor powders heated inthe lab furnace at 8001 and 9001C are displayed in Fig. 1. At

S. Allison—contributing editor

This work is supported by National Science Foundation under Grant No. CTS-0553623.

wAuthor to whom correspondence should be addressed. e-mail: chendy@ims.uconn.edu

Manuscript No. 25142. Received August 22, 2008; approved October 17, 2008.

Journal

J. Am. Ceram. Soc., 92 [1] 268–271 (2009)

DOI: 10.1111/j.1551-2916.2008.02846.x

r 2008 The American Ceramic Society

268

8001C, the powders are still amorphous. When the temperatureis increased to 9001C, a pure YAG crystalline phase begins toform and no intermediate phases like YAM (Y4Al2O9) and YAP(YAlO3) can be detected.

The weight loss of the precursor as a function of temperaturewas measured using a TG-DTA instrument for the as-synthe-sized powders in air. Typical TG-DTA curves for the crystalli-zation of a Dy:YAG precursor obtained at a heating rate of101C/min in air are shown in Fig. 2. The sample weight de-creases with increasing temperature continuously from roomtemperature to B5001C, and the total weight loss is about 72wt%. There are three strong endothermic peaks in the DTAcurve at 1571, 2101, and 4771C. The peak at 1571C can be as-cribed to water evaporation. The peaks at 2101 and 4771C aredue to the decomposition of the precursor, because an abruptweight loss occurs in this temperature range. The exothermicDTA peak at 9181C can be ascribed to the crystallization ofYAG from the amorphous phase, which is confirmed by theXRD analysis.

(2) Phase Composition and Microstructure of As-SprayedCoating

Figure 3 shows the XRD pattern of the as-sprayed coating. Thecoating is mainly composed of a YAG phase with a smallamount of an intermediate YAP phase. There are no crystal-line Al2O3 or Y2O3 phases in the as-sprayed coating.

When the precursor powders are heat treated, the solutionprecursor will undergo solvent vaporization, precursor decom-position, and pure YAG phase formation, as confirmed byXRD and TG-DTA analyses (Figs. 1 and 2). The precursordroplets in the plasma jet are assumed to undergo similar phys-ical and chemical changes as was observed in the laboratoryexperiments performed under near-equilibrium conditions.

However, the presence of a small amount of the YAP phasein the as-sprayed coating indicates that some of the droplets inthe plasma jet did not undergo the same process as that underequilibrium conditions. Because the droplets in the plasma jetunderwent a different thermal history, the droplets injected intothe cooler area of plasma jet may undergo the decompositionprocess and form a metastable YAP phase; however, the energyrequired for the complete transformation from the YAP to theYAG phase is not enough. Therefore, the YAP phase remains inthe as-sprayed coating.

Figure 4(a) shows the representative surface morphology ofthe as-sprayed Dy:YAG coating. The coating is composed ofultrafine splats (0.5–2 mm) and dense fine spheres. These splatsand spherical particles indicate that melting and solidificationtake place during SPPS Dy:YAG coating formation. A typicalpolished cross section of the SPPS Dy:YAG coating is shown inFig. 4(b). It can be seen that the coating is composed of colum-nar structures with a thickness of B60 mm. Such columnarstructures, which could enhance the coating strain tolerance atan elevated temperature and thus prevent spallation, are desir-able for SPPS Dy:YAG thermographic phosphor coatings usedat a high temperature.

(3) Photoluminescent Properties of As-Sprayed Coating

Figures 5(a) and (b) show the excitation and emission spectra ofthe SPPS Dy:YAG phosphor coating, respectively. The excita-tion spectra of the phosphor coating monitored at 488 nm showfour strong bands with peaks at 326, 352, 366, and 386 nm,

10

G:YAG

GGG

GGGG

GG

GG

G

GGG

(b)

(a)

2 theta

20 30 40 50 60 70 80

Fig. 1. X-ray diffraction of YAG powders calcined at various temper-atures: (a) 8001C and (b) 9001C, 3 h.

00

20

40

60

80

100

–200

–150

–100

–50

0

157°C477°C

918°C

Hea

t flo

w (

mW

)

Wei

ght p

erce

nt (

%)

200 400 600 800 1000

Temperature (°C)

210°C

Fig. 2. Typical thermogravimetry-differential thermal analysis curvesof dried YAG precursor powders at a heating rate of 101C/min.

100

200

400

600

800

1000

G O

Inte

nsity

(C

PS)

G

G

GO

GG

O

G

G

G G

O

G

GG

GGG

2 θ (°)20 30 40 50 60 70

O: YAP (YAlO3)

G: YAG (Y3Al5O12)

Fig. 3. X-ray diffraction pattern of the as-sprayed Dy:YAG coating(G:YAG; O:YAP).

Table I. SPPS Conditions for Spraying of Dy:YAG Coatings

Parameters Value

Power 40–50 kWAtomizing gas N2

Liquid flow rate 10–30 mL/minSpraying distance 40–70 mmTraverse speed 1000 mm/sSubstrate SuperalloyNo. of pass deposited 40

SPPS, solution precursor plasma spray.

January 2009 Communications of the American Ceramic Society 269

which correspond to the transition from the ground state 6H15/2

to the excited states 6P3/2;4I11/2,

4M15/2,6P7/2;

4P3/2,6P3/2,5/2; and

4I13/2,4F7/2,

4K17/2, and4M19/2,21/2, respectively (Fig. 5(a)). The

emission spectrum of the phosphor coating under the excitationof 353 nm UV light (lex5 353 nm) consists of two predominantemission regions: a blue region and a yellow region (Fig. 5(b)).Blue emission between 470 and 500 nm is assigned to the tran-sition of Dy31 ion from the 4F9/2 excited state to the 6H15/2

ground state. Yellow emission is observed between 560 and 600nm, corresponding to a 4F9/2 to

6H13/2 transition. The emissionintensity of blue emission is stronger than that of yellow emis-sion. Heyes et al.5 reported that the ratio of the emission inten-sities of the two color distinct spectral lines is temperaturedependent and has been successfully used to monitor the coatedsurface temperature. The columnar structure Dy:YAG phos-phor coatings made using the SPPS process could find potentialapplications for temperature measurement of the coated com-ponent in the hot section of a gas turbine.

Although a small amount of an intermediate YAP phase ispresent in the as-sprayed coating, it will not affect the coatingphotoluminescent properties; the excitation and emission spec-tra obtained in the present experiment are very similar to that ofpure YAG phase phosphors doped with Dy31.3

IV. Conclusion

Columnar structure Dy:YAG phosphor coatings were depositedusing the SPPS process. The coating is mainly composed of aYAG phase with a small amount of an intermediate YAP phase.The phosphor coating exhibits two color emission regions: a

blue region (470–500 nm) and a yellow region (560–600 nm),which are assigned to 4F9/2–

6H15/2 and4F9/2–

6H13/2 transitions,respectively. Columnar structures that can enhance the coatingstrain tolerance at an elevated temperature and thus preventspallation are desirable for SPPS Dy:YAG thermographic phos-phor coatings used at a high temperature.

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(a)

(b)

Fig. 4. Microstructure of a solution precursor plasma spray Dy: YAGcoating: (a) surface morphology; (b) polished cross section.

250

0

20000

40000

60000

80000

100000

(a)

386

366352

Inte

nsity

Wavelength (nm)

326

450

0

20000

40000

60000

80000

100000

(b)

Inte

nsity

Wavelength (nm)

300 350 400 450

500 550 600 650

λem=488 nm

λex=353 nm

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