www.elsevier.com/locate/surfcoat

Surface & Coatings Technolo

Thermal barrier coatings deposited by laser CVD

Takashi Goto*

Institute for Materials Research, Tohoku University, 2-1-1 Katahira Aoba-ku Sendai 980-8577, Japan

Available online 2 December 2004

Abstract

The ceramic coating is a key issue for thermal barrier coatings (TBCs) in gas turbine technology. Atmospheric plasma spray (APS) and

electron-beam physical vapor deposition (EBPVD) are commonly employed in practical applications. An alternative route, chemical vapor

deposition (CVD), is a candidate process for TBCs. Although CVD has been understood to be too slow to obtain thick coatings, a laser CVD

process has achieved an extremely high deposition rate (660 Am/h) for yttria-stabilized zirconia (YSZ) coatings. The high deposition rates

have caused a large number of nano-pores in grains leading to a significantly smaller thermal conductivity of 0.7 W/mK. This paper briefly

reviews the process for TBCs, and introduces laser CVD for thick oxide coatings.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Laser; Photolaser CVD; Zirconium oxide; Yttrium oxide; Thermal barrier

1. Introduction

The gas turbine blade is mainly constructed from a Ni-

base super alloy substrate, an intermediate bond-coat layer

and ceramic (usually yttria-stabilized zirconia, YSZ)

thermal barrier coatings (TBC). Due to strong demand of

high-temperature operation of gas turbines, an advanced

TBC is essential to improve the performance of gas

turbines. The gas inlet temperature used to be about 900

8C at 1960s, but now the TBC system has enabled one to

raise the gas inlet temperature to more than 1500 8C.Further increase in the operation temperature could be

strongly dependent on the improvement of coating process

and nano-structure of TBCs [1].

Since the ceramic material for TBCs should have low

thermal conductivity, high mechanical properties and

thermal shock resistance, an engineering ceramic of YSZ

has been widely utilized in practical applications. However,

a slightly smaller thermal coefficient of YSZ than that of Ni-

base super alloy substrates often causes significant thermal

stresses and cracks in TBCs yielding a disastrous peeling off

from the substrate. The thermal stress could be relaxed by

0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2004.10.084

* Tel.: +81 22 215 21105; fax: +81 22 215 2091.

E-mail address: goto@imr.tohoku.ac.jp.

controlling microstructures of YSZ using various coating

processes. This paper focuses on the recent deposition

process of YSZ, particularly laser chemical vapor deposition

(CVD).

2. Deposition process of TBCs

Since the thickness of TBCs should be more than a few

100 Am to sustain severe temperature gradient of few

100K along TBCs, high deposition-rate processes such as

atmospheric plasma spray (APS) and electron-beam

physical vapor deposition (EBPVD) have been practically

employed. In the APS process, YSZ powders are melted

by a plasma torch in an atmospheric pressure, and then the

melted YSZ particles are rapidly solidified on the substrate

surface forming thick coatings. Although the APS has

several advantages such as relatively simple equipment

configuration, availability for complicate substrates and a

low cost, this process causes particularly flat lamella

(splat) microstructure containing a large number of voids

and defects [2]. Such microstructure leads to favorably

high thermal insulation; however, cracks often exist along

the substrate resulting peeling off over a wide substrate

area.

gy 198 (2005) 367–371

Fig. 1. Temperature dependence of deposition rates for YSZ by thermal

CVD [10–12].

T. Goto / Surface & Coatings Technology 198 (2005) 367–371368

In the EBPVD process, an YSZ sintered body will be

evaporated by a high power electron beam and its vapor

will condense on substrates. The advantageous columnar

structure will be easily grown vertically from substrates [1].

Since the gaps between columns are effective for the

relaxation of thermal expansion mismatch between sub-

strate and YSZ coating, EB-PVD YSZ has excellent

durability for severe thermal cycles. EBPVD would be also

feasible to contain a large number of nano-pores in YSZ

coatings due to high super-saturation being effective to

decrease thermal conductivity. On the other hand, the gaps

between each column would be disadvantageous for the

oxidation of layers underneath the top coating. Al2O3-

related phases may be formed on the bond-coat layer that is

often called as thermally grown oxide (TGO). Whole TBC

would peel off from the substrate with increasing the TGO

layer. The high cost of high power electron beam and a

large-scale vacuum chamber could be also drawbacks of the

EBPVD process.

CVD can be a candidate for TBCs because of the

superior conformal coverage and microstructure controll-

ability [3]. CVD is often categorized according to the

precursor material and source energy for the deposition

reactions [4]. Conventional CVD could be called as

thermal CVD where thermal energy such as ohmic or

radio-frequency (RF) induction heating are used. Metal-

organic CVD (MOCVD) is one of thermal CVDs, where

MO precursors are utilized owing to high vapor pressures

at low temperatures without forming harmful by-products.

Many papers have been published on the CVD of YSZ

films due to their attractive properties for oxide ion

conductors and buffer layers for Y–Ba–Cu–O films on Si

wafers. In the past study, the deposition rates ranged

from 0.5 to 20 Am/h with the thickness from 0.2 to 2.5

Am. At first, halide precursors of ZrCl4 and YCl3 have

been employed as precursors, and then changed to MO

precursors such as acetylacetonate (acac), tetrametyl-

heptanedionate (thd) and dipivaloylmethanato (dpm)

compounds. A metal-alkoxide, Zr(O–C4H8)4, is also a

candidate due to its high vapor pressures and low cost.

Plasma-enhanced CVD (PECVD) can be employed to

decrease deposition temperatures because the plasma

would significantly enhance the reactivity of source

materials.

The deposition rates of CVD are usually small.

Because of this reason, the CVD has been mainly

applied to thin film coating deposition. However, the

deposition rates of CVD can be increased by optimizing

CVD parameters as reported for non-oxides, typically

Si3N4 and SiC [5,6]. On the other hand, the increase in

deposition rates for oxides is not easy mainly due to the

homogeneous powder formation in a gas phase [7]. Such

difficulty may be overcome by choosing appropriate

precursors and geometrical configuration of CVD cham-

bers. There have been several research groups trying to

apply CVD YSZ for TBCs. Wahl et al. [8] achieved a

high deposition rate of 50 Am/h having a well-developed

columnar microstructure by using Zr(thd)4 and Y(thd)3 as

precursors and a hot-wall type CVD chamber. The crystal

structure was mainly cubic phase, and Y2O3 content

ranged from 1.4 to 19 mol%. They reported that a

deposition rate of more than 100 Am/h may be possible,

but the films might have low adherence to substrates

losing the specific columnar microstructure. We have

constructed a cold-wall type CVD chamber using

Zr(dpm)4 and Y(dpm)3 precursors. A high deposition

rate of 108 Am/h has been obtained with a well-

developed columnar structure having (200) oriented

tetragonal YSZ films [9]. Fig. 1 demonstrates the

deposition rates as a function of temperature reported in

literatures, where several results on high deposition rates

are selected. Generally, there is a trend that the deposition

rates in CVD increase with increasing temperature, and

become almost temperature independent or sometimes

slightly decrease at higher temperatures. The rate con-

trolling step in thermal CVD could be changed from

reaction kinetic to diffusion with increasing temperature,

and therefore, the diffusion control region may be

selected to obtain high deposition rates. In the reaction

kinetic region, the nucleation will initiated at the bottom

of deposit, often called as kinks, and then the grain

growth would proceed upward resulting fully dense

materials. On the other hand, in the diffusion limited

region, the nucleation may occur at the highest gas

concentration region, i.e., the top of deposits, and then

the grains will grow downward forming voids (nano-

pores) at grain boundaries [13]. We have reported that the

diffusion control region could be favorable to prepare

YSZ coatings at high deposition rates containing nano-

pores having a low thermal conductivity less than 1 W/

mK [3]. PECVD has been also applied to TBCs.

Preauchat et al. [14] have prepared YSZ by PECVD at

Table 1

Literature data of laser CVD

Deposit Deposition

rate (Am/h)

Power

(W)

Spot Size

in diameter

(Am)

Laser

density

(W/m2)

Ref.

Al 1.1�104 1.1 45 6.9�108 [19,20

AlxOy 2.2�105 0.002 5 1.0�108 [21]

AlxOy 1.6�105 0.0013 3 1.8�108 [22]

T. Goto / Surface & Coatings Technology 198 (2005) 367–371 369

high deposition rates of 100 to 250 Am/h. They obtained

65- to 200-Am-thick coatings at 1173 K, 106 Pa and

micro-wave power of 1700 W using ZrCl4 and Y(thd)3precursors. The coatings were (200) oriented non-trans-

formable tetragonal (tV) phase with excellent thermal

shock resistance and low thermal conductivity (1.6 to

1.7 W/mK).

B 4.0�106 0.2 20 6.4�108 [23]

C 1.3�107 12 10 1.5�1011 [24]

C 3.6�108 6 25 1.2�1010 [25]

Diamond 9.1�105 0.15 10 1.9�109 [25]

Ge 1.3�105 0.2 20 6.4�108 [25]

Si 1.8�106 0.2 20 6.4�108 [25]

Si 3.6�103 0.28 13 2.1�109 [26,27

SiC 4.5�105 0.2 20 6.4�108 [23,28

SixNy 2.7�106 0.2 20 6.4�108 [23]

Ti 5.0�103 10 50 5.1�109 [29]

TiB2 7.2�103 200 5000 1.0�108 [29]

TiB2 1.4�103 80 9000 1.3�106 [31,33

TiC 432 50 800 10.0�108 [34]

TiNxCy 3.6�104 10 50 5.1�109 [32]

W 5.8�105 3 10 3.8�1010 [24]

WC 6.3�105 12 10 1.5�1011 [24]

ZrO2 660 200 20,000 6.4�105 Our

study

3. A high speed deposition process: laser CVD

In spite of the low deposition rates of usual CVD,

many efforts have been conducted to prepare large-scaled

materials such as SiC nose-cone for space vehicles or

MgO domes for missile radars [15]. However, no paper

has been published on the high speed deposition of oxide

coatings on complicated substrates such as turbine blades.

As mentioned above, we have prepared YSZ films at a

high deposition rate of 108 Am/h by conventional thermal

CVD. In that process, the substrate should be limited to

relatively simple shape due to the difficulty of uniform

heating of the substrate and supplying precursor vapors.

More recently, we have developed a CVD system using

laser to heat the substrate and to enhance the reactivity of

source gases [16]. Fig. 2 demonstrates an YSZ coating

deposited by laser CVD on a test piece for evaluating

thermal shock resistance of TBC. Even for a rather

complicated shaped substrate, the whole surface has been

uniformly coated by broadening the laser beam to about

20 mm in diameter. More complicated and large-scaled

substrates could be coated by scanning the laser beam

and by moving the substrate.

In the past, many papers on laser CVD have been

published, mainly for semiconductor devise applications

[17]. Laser CVD can be generally categorized into pyrolytic

laser CVD and photolytic laser CVD. In the pyrolytic CVD,

laser can be a heat source, focusing and scanning at a specific

position of a substrate. Micro-scale coils, dots and lines were

prepared by pyrolytic laser CVD. In photolytic laser CVD,

laser may excite source gases promoting photochemical

reactions. Several reviews on laser CVD are available in the

literatures [18]. Literature data of laser CVD for wide-ranged

Fig. 2. Appearance of an YSZ coating left: coated, right: un-coated.

Fig. 3. Relationship between volume deposition rate and laser density in

laser CVD.

]

]

]

]

laser powers and spot sizes are presented in Table 1. In

conventional thermal CVD, the deposition rate cannot be so

enhanced due to the premature powder formation around the

substrate. The laser CVD is advantageous to prevent the

premature powder formation because the reaction site is

usually restricted to a small area at the substrate surface by a

focused laser beam. Extremely high deposition rates up to

3.6�108 Am/h was reported for carbon at the laser spot size

of 25 Am. Fig. 3 demonstrates the relationship between the

volume deposition rate (spot area�deposition rate in thick-

ness) and laser density for the data shown in Table 1. Since

usual laser CVDs employ focused laser beams, the volume

deposition rates are significantly small even at a high laser

Fig. 6. Effects of substrate pre-heating temperature and laser power on

deposition rates.Fig. 4. Cross-section of an YSZ coating prepared at a deposition rate of

230 Am/h.

T. Goto / Surface & Coatings Technology 198 (2005) 367–371370

power density as encircled in Fig. 3. High volume deposition

rates of non-oxides such as TiB2 [31,33] and TiC [30] have

been reported; however, their total thicknesses are rather

small ranging from a few to several tens of micrometers.

Many kinds of oxides have been also prepared [35], and the

deposition rate of TiO2 reached to 120 to 1200 Am/h.

However, the volume deposition rates of oxides are

significantly small ranging inside the circle in Fig. 3 due to

focused laser beams around 100 to 200 Am in size.

No laser CVD in the past has accomplished oxide coating

several 100 Am thick for large-scaled substrates around a few

cm. Fig. 4 depicts YSZ coating prepared at a deposition rate

of 230 Am/h by our laser CVD at a laser power of 200 W,

substrate pre-heating temperature of 750 8C and Zr-precursor

flux of 1.2�10�6 mol/s. A well-developed columnar micro-

structure with a (200) orientation was observed. The

deposition rate was further increased with increasing the

Fig. 5. Cross-section of an YSZ coating prepared at a deposition rate of

660 Am/h.

precursor flux rate. Fig. 5 represents a cross-section of an

YSZ coating prepared at a deposition rate of 660 Am/h at a

Zr-precursor flux of 2.0�10�6 mol/s. So-called cone

structure, typically observed at a very high deposition rate

in CVD, was observed. Fig. 6 shows the effects of substrate

pre-heating temperature and laser power on deposition rates

at a Zr-precursor flux of 1.2�10�6 mol/s. The deposition rate

increased significantly at the laser power above 70 W,

accompanying strong plasma light emission around the

substrate [36]. The deposition efficiency in the laser CVD

has reached to more than 80%, whereas it could be less than a

few % in conventional thermal CVD. This suggests that the

precursor gases have been particularly excited by laser. A

Fig. 7. Nano-structure of an YSZ coating prepared at a deposition rate of

450 Am/h.

T. Goto / Surface & Coatings Technology 198 (2005) 367–371 371

plasma diagnosis revealed that electrons and ions have been

produced in the plasma zone. Fig. 7 represents the nano-

structure of a YSZ coating prepared at a deposition rate of

450 Am/h. At the deposition rate below 50 Am/h, each grain

in the columnar structure was almost single crystal forming

nano-pores along the grain boundary. The columns became

poly-crystal at the deposition rates over 150 Am/h. A large

number of nano-pores were observed not only at the grain

boundary but also around the coating/substrate interface. In

the case of EBPVD YSZ coatings, nano-pores have aligned

along the sub-column boundary, where each column of

EBPVD has a feather-like microstructure [37]. Columnar

grains can be easily sintered at high-temperature heat

treatment, leading the increase in thermal conductivity. On

the other hand, the YSZ coatings by laser CVD have a low

thermal conductivity of 0.7 W/mK, about a quarter of bulk

YSZ body, due to a large number of nano-pores. This value is

almost the same as that of EBPVD YSZ coatings; however,

the nano-pores in the laser CVD YSZ are relatively stable

without significant increase in thermal conductivity after

heat-treatment for more than 20 h at high temperatures

around 1000 to 1100 8C.

4. Summary

Although APS and EBPVD have been employed in the

industry, an alternative coating process is expected to

improve the thermal and mechanical properties of TBCs.

CVD could be a candidate for TBCs due to non light-of-sight

nature, having excellent conformal coverage and micro-

structure controllability. The low deposition rates in CVD

may be overcome by selecting CVD parameters. Conven-

tional thermal CVD and PECVD have attained at a high

deposition rate of about 200 Am/h. The laser CVD is

significantly effective to further increase the deposition rate

up to 660 Am/h being comparable to that of APS and EBPVD.

The laser CVD is also able to obtain other oxides such as

Al2O3 and TiO2 coatings at deposition rates around 1 to 2

mm/h. The thick coatings more than several 100 Am at high

speeds by laser CVD may find many applications in

industries for corrosion resistant and abrasive coatings.

Acknowledgment

This work was performed as a part of Nano-Coating

Project sponsored by New Energy and Industrial Technology

Development Organization (NEDO), Japan.

References

[1] D.R. Clarke, C.G. Levi, Annu. Rev. Mater. Res. 33 (2003) 383.

[2] P.A. Kammer, Handbook of Thin Film Technology, IOP Pub, 1995,

p. A4.1:1.

[3] J.R.V. Garcia, T. Goto, Sci. Technol. Adv. Mater. 4 (2003) 397.

[4] C.F. Powell, Vapor Deposition, John Wiley, 1966, p. 249.

[5] T. Hirai, K. Niihara, T. Goto, J. Mater. Sci. 12 (1977) 631.

[6] T. Hirai, T. Goto, T. Kaji, Yogyo Kyokaishi 91 (1983) 502.

[7] W.A. Bryant, J. Mater. Sci. 12 (1977) 1285.

[8] G. Wahl, W. Nemetz, M. Giannozzi, S. Rushworth, D. Baxter, N.

Archer, F. Cernuschi, N. Boyle, Trans. ASME 123 (2001) 520.

[9] R. Tu, T. Kimura, T. Goto, Mater. Trans. 43 (2002) 2354.

[10] Y. Akiyama, T. Sato, N. Imaishi, J. Cryst. Growth 147 (1995) 130.

[11] N. Bourhila, F. Felten, J.P. Senateur, F. Schuster, R. Madar, A.

Abrutis, in: M.D. Allendorf, C. Bernard (Eds.), Proc. 14th Intern.

Conf. and EUROCVD-11, Proc., Electrochem. Soc., vol. 97-25, 1997,

p. 417.

[12] M. Pulver, W. Nemetz, G. Wahl, Surf. Coat. Technol. 125 (2000) 400.

[13] W.R. Holman, F.J. Heugel, J. Vac. Sci. Technol. 11 (1974) 701.

[14] B. Preauchat, S. Drawin, Surf. Coat. Technol. 142–144 (2001) 835.

[15] T. Hirai, T. Goto, Kogyo Zairyo 32 (1984) 1.

[16] T. Kimura, T. Goto, Mater. Trans. 44 (2003) 421.

[17] D. Bauerle, Laser Processing and Chemistry, Springer, 2000, p. 337.

[18] C. Duty, D. Jean, W.J. Lackey, Int. Mater. Rev. 46 (2003) 271.

[19] D. Tonneau, J. Bouree, Y. Pauleau, Appl. Surf. Sci. 86 (1995) 488.

[20] T. Baum, C. Larson, R. Jackson, Appl. Phys. Lett. 55 (1989) 1264.

[21] M. Wanke, O. Lehmank, K. Muller, Q. Wen, M. Suke, Science 275

(1997) 1284.

[22] Lehman, M. Stuke, Mater. Lett. 21 (1994) 131.

[23] F. Wallenberger, Ceram. Int. 23 (1997) 119.

[24] K. Williams, J. Maxwell, K. Larsson, M. Boman, IEEE (1999) 232.

[25] J. Maxwell, M. Boman, K. Williams, K. Larsson, N. Jaikumar, G.

Saiprasanna, Proc. Conf. Micromachining and Microfabrication

Process Technology V, 1999, Proc.-SPIE, vol. 3874, SPIE-Interna-

tional Society for Optical Engineering, Bellingham, p. 227.

[26] H. Westberg, M. Boman, J. Appl. Phys. 73 (1993) 7864.

[27] S. Park, S. Lee, Jpn. J. Appl. Phys. 29 (1990) L129.

[28] H. Chang, L. Lee, R. Hwang, C. Yeh, M. Lin, J. Lou, T. Hseu, T. Wu,

Y. Chen, C. Tang, Mater. Chem. Phys. 44 (1996) 59.

[29] J. Maxwell, R. Krishnan, S. Haridas, Solid Freeform Fabrication

Symp. Proc., 1997, p. 497.

[30] V. Hopfe, A. Techel, A. Baier, J. Scharsig, Appl. Surf. Sci. 54 (1992)

78.

[31] J. Elders, P. Quis, B. Rooswijk, J. van Voorst, J. Nieuwkoop, Surf.

Coat. Technol. 45 (1991) 105.

[32] J. Maxwell, J. Shat, T. Webster, J. Mock, Solid Freeform Fabrication

Symp. Proc., 1998, p. 575.

[33] J. Elders, J. van Voorst, Appl. Surf. Sci. 69 (1993) 267.

[34] Zergioti, A. Hatziapostolou, E. Hontsopoulos, A. Zervaki, G.

Haidemenopoulos, Thin Solid Films 271 (1995) 96.

[35] J. Mazumder, A. Kar, The Theory and Application of Laser Chemical

Vapor Deposition, Plenum, 1995, p. 16.

[36] H. Miyazaki, T. Kimura, T. Goto, Jpn. J. Appl. Phys. 42 (2003) L316.

[37] T.J. Lu, C.G. Levi, H.N.G. Wadley, A.G. Evans, J. Am. Ceram. Soc.

84 (2001) 2937.