Journal of Wuhan University of Technology-Mater. Sci. Ed. www.jwutms.net Feb.2016

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DOI 10.1007/s11595-016-1319-6

A Review: Structural Oxide Coatings by Laser Chemical Vapor Deposition

Takashi GOTO

(Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan) Abstract: Yttria-stabilized zirconia and -alumina films were prepared by laser chemical vapor deposition at deposition rates of several hundred micrometers per hour. Moreover, the structural oxide coatings by laser chemical vapor deposition are reviewed. The laser can significantly accelerate the chemical reaction and grain growth in CVD, yielding high deposition rates. The films contain large amounts of nanopores that act as thermal insulation and are thus promising as coating materials for gas turbine blades of Ni-based superalloys and WC-Co cutting tools. Key words: laser CVD; yttria-stabilized zirconia; alumina; plasma; nano-pores

1 Introduction CVD (chemical vapor deposition) is a versatile technique to provide highly pure and dense films, hence oxide thin films by CVD have widely been studied for electronic and optical application. CVD has also been applied to prepare non-oxides films, such as SiC and Si 3N 4. Since various precursors and good substrate (mainly graphite) are available for preparing nonoxides, thick films or even bulky forms of non-oxide materials can be prepared by CVD[1,2]. Although there are many useful applications of thick oxide films such as thermal barrier coating (TBC) and protective coating for cutting tools, thick or bulky oxide coating cannot be realized by conventional CVD. The precursors of oxide films generally react easily with oxidant gas (usually O2 or CO2) resulting in premature reactions in a gas phase. The oxide films are reactive with substrate material at high temperature, thus the temperature of CVD oxide films cannot be increased like non-oxide films. Therefore, the deposition rate of CVD for oxide films are low, commonly about a few micrometers per hour. Auxiliary energy sources, such as lasers, can increase the deposition rate and decrease the deposition temperature of CVD. Laser CVD (LCVD) is known ©Wuhan University of Technology and SpringerVerlag Berlin Heidelberg 2016 (Received: Oct. 20, 2015; Accepted: Nov. 24, 2015) Takashi GOTO: Prof.; Ph D; E-mail: [email protected]

to significantly increase the deposition rate locally on an area having the diameter of the laser beam size[3]. It has been believed that LCVD cannot prepare films on a wide area and complicated shaped substrates because the diameter of the deposition area by LCVD is too small, about a few millimeters at most. On the other hand, we have found that LCVD is able to prepare oxides thick films on wide-area substrate at significantly high deposition rate by using continuousmode high power laser. This paper demonstrates the preparation of oxide thick films for structural use, thick YSZ coating as TBC on turbine blade and -Al2O3 coating for cutting tools.

2 Experimental Fig.1 shows a schematic diagram of the LCVD apparatus. A cold wall CVD chamber made of stainless steel was used. For YSZ films, Zr(dpm) 4 (dpm: dipivaloylmethanate) and Y(dpm) 3, and for Al 2O 3 films, Al(acac) 3 (acac: acetylacetonate) were used. The source vapors were carried by Ar gas into the CVD chamber. O2 gas was separately introduced with a double tube nozzle and mixed with the precursor vapors above the substrate. The distance between the nozzle and the substrate was 25 mm. Total pressure was kept at 0.93 kPa. Al2O3 and AlN substrates (15 × 15 × 2 mm) were mainly used for preparing YSZ and Al2O3 films, respectively, and were pre-heated up to 1 023 K on a hot stage. Practical Ni-base super alloy

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Vol.31 No.1 Takashi GOTO: A Review: Structural Oxide Coatings by Laser Chemical...

gas turbine blade was also coated with YSZ film, and WC-Co cutting tool was coated with -Al 2O 3 film. A Nd:YAG laser (wavelength: 1 063 nm; maximum power: 250 W) irradiated the substrates with a spot size that covered the entire substrate surface (approximately 20 mm in diameter). The substrate temperatures under the laser irradiation were measured using a pyrometer (IR-FBIH-SP, Chino) with an InGaAs photo detector (=1 550 nm). An optical filter was used to cut off the incident laser light. The experimental procedure was described elsewhere in detail[4].

Surface and cross-sectional microstructures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Crystal structure was determined by X-ray diffraction (XRD). A Langmuir probe was inserted in the CVD chamber to measure the space charge current in plasma formed around the substrate.

3 Results and discussion 3.1 YSZ coating In CVD, the deposition rate would be determined by the deposition temperature, total pressure in the CVD chamber and partial pressure of each precursor. In LCVD, however, the laser power also affects the deposition rate. Laser can dissociate gases forming ions and electrons as it is well known in laser ablation called plasma plume. The plasma formation can be identified in LCVD, and the plasma can significantly affect the enhancement of deposition rate in the present LCVD. Figs.2 (a) and 2(b) show the effects of the laser power on the deposition rate and the space charge current of the plasma, respectively[5]. The laser power threshold to form plasma for preparing the YSZ film was 70 W. A significant increase in the deposition rate can be observed over this threshold, which depends on the wavelength of the laser and the precursors. Plasma

formation occurs only above the substrate (deposition zone), and the substrate temperature is increased by the heat radiation from the plasma. The substrate temperature was uniform in a laser irradiated area about 20 mm in diameter[6]. Fig.3 shows the deposition rates of YSZ films by LCVD[7] and conventional CVD[8-12]. The deposition rate of LCVD was several to ten times greater than those of conventional CVD[13].

Journal of Wuhan University of Technology-Mater. Sci. Ed. www.jwutms.net Feb.2016

Fig.4 depicts a typical cross-sectional microstructure of YSZ film prepared by LCVD [7]. The YSZ film prepared at 230 µm/h (Fig.4(a)) exhibited vertically elongated grains. Such columnar microstructures relax the thermal expansion mismatch between the YSZ film and the metal substrate. The YSZ film prepared at higher deposition rate of 660 µm/h (Fig.4(b)) shows a cone-like structure. This microstructure is commonly observed in CVD material prepared at high-deposition rate. Fig.5 shows nanopores in the YSZ film of Fig.4 (a). A large number of nano-pores about a few nanometers in diameter are contained in grains Fig.5(a). The columnar grains also contain nano-pores a few tens nanometers in diameter (Fig.5(b)). These nanopores are similar to that in YSZ films prepared by EBPVD (electron-beam physical vapor deposition)[14]. It is known that these nano-pores are effective to decrease the thermal conductivity of YSZ film. Fig.6 shows the YSZ coating on a Ni-based super alloy turbine blade. By rotating and moving the turbine blade substrate, the entire surface of the blade was uniformly coated by a 300 µm thick YSZ film taking about four hours. Although APS (atmospheric plasma spray) and EBPVD have been practically utilized for TBC, present LCVD can be applied for TBC of YSZ film on gas turbine blades.

3.2 -Al2O3 coating WC-based composites, typically WC-Co, are widely employed in cutting tools because of

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their excellent wear resistance, high hardness, and ductility [15]. WC-based cutting tools are generally coated by a TiCN film as intermediate and an Al2O3 film as topcoat by CVD using halide precursors (halide CVD). Metal-organic CVD (MOCVD) using metal-organic compound as a precursor has also been employed to prepare -Al 2O 3 coating. However, -Al 2O 3 coating cannot be prepared at temperature lower than about 1 300 K by conventional CVD.

Fig.7 shows the effects of deposition temperature and laser power on the crystal structure of the Al2O3 film. g-Al 2O 3 formed at 980 K, a-g mixed phases formed at 1 100 K, and single-phase a-Al2O3 film was obtained above 1 170 K[16]. The deposition temperature

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Vol.31 No.1 Takashi GOTO: A Review: Structural Oxide Coatings by Laser Chemical...

of the a-Al2O3 film by LCVD was approximately 200 K lower than that of conventional MOCVD. Fig.8 shows the typical surface microstructure of an a-Al2O3 film prepared by LCVD. Since the (001) plane is flat with higher mechanical properties than other planes, (001) oriented Al 2O 3 film is required in practical applications. Hexagonal facets were observed on the surface, implying significant (001) orientation. LCVD can prepare highly (001) oriented a-Al2O3 films. Fig.9 shows the effect of the deposition temperature on the deposition rate of the Al2O3 film. Almost no deposition occurred below 900 K, a significantly high deposition rate of g-Al2O3 film of approximately 600 µm/h was obtained around 950 K, and the deposition rate of a-Al2O3 film decreased to approximately 200 µm/h at higher than 1 050 K. The temperature dependence of the deposition rate of the Al2O3 film by LCVD is compared with that by conventional halide[17,18] and MOCVD[19-21] in Fig. 10. The highest deposition rates of a-Al2O3 and g-Al2O3 films by LCVD were 252 and 570 µm/h, respectively. These values were several 100 times higher than that by conventional thermal CVD.

Fig.11 presents a-Al 2O 3 coating on practical WC-Co cutting tool. Since WC-Co would be readily oxidized in O 2 atmosphere, a-Al 2O 3 coating using Al(acac) 3 and O 2 gas has caused oxidation of WCCo substrate and a-Al2O3 film was easily delaminated from the substrate. By LCVD, a-Al2O3 was deposited on WC-Co without O2 gas. Although the deposition rate was slightly decreased, oxygen in Al(acac)3 (acac : C5H8O2) was enough for depositing a-Al2O3 by LCVD. Although conventional halide CVD is generally employed to coat a-Al2O3 film on WC-Co cutting tools, the halide CVD needs high temperature, and brittle η-phase (typically W3Co3C) often forms at the interface between the coating and substrate. LCVD can be a candidate process to coat a-Al 2O 3 coating at lower temperature without forming the brittle phase.

Fig.12 shows TEM images of the a-Al2O3 film. As in the YSZ film, the nanopores were uniformly distributed. In general, the CVD process has two distinct temperature-dependent rate-controlling steps; chemical reactions are limited at low temperatures and diffusion (mass transport) is limited at high temperatures. The activation energy of the deposition rate is higher than about about 400 kJ/mol for the chemical reaction-limited step and lower than about about 200 kJ/mol for the diffusion-limited step. In this

Journal of Wuhan University of Technology-Mater. Sci. Ed. www.jwutms.net Feb.2016

study, the activation energy of both YSZ and a-Al2O3 films was significantly low (a few tens to almost zero kJ/mol) suggesting diffusion-limited process. Fig.13 shows the diagrams of films prepared at the chemical reaction- (Fig.13(a)) and diffusion-limited steps (Fig.13(b))[22]. In the chemical reaction-limited step, the film becomes solid (no pores) because grains nucleate at kinks and steps (i e, bottom of the film).On the other hand, in the diffusion-limited process, nucleation begins at the top of the film and grain growth proceeds downward forming nanopores inside the film. By the proposed LCVD, the deposition rate is significantly high and the rate-controlling step is diffusion; thus, nanopores form in the film. These nanopores act as thermal insulation in TBC for gas turbines and top coatings for cutting tools.

4 Conclusions CVD has wide-ranged applications because of good conformal coverage; the backside or even inside of porous substrates can be coated. However, the deposition rate of common CVD is low, and cannot be used to make thick oxide coatings. The laser can significantly enhance the deposition rate of CVD, and thus, LCVD can be used to fabricate thick coating. LCVD can be applied not only to highperformance structural thick oxide coatings but also to functional thick oxide coatings such as in antiplasma Y2O3 coatings[23], photocatalytic TiO2 coatings[24], Licompounds for thin film battery[25,26], and ferroelectric Ba–Ti–O films[27]. LCVD can be expected for a new route of thick coating.

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