- Email: [email protected]
X2-Yb2SiO5 coatings using laser chemical vapor deposition
Ceramics International xxx (xxxx) xxx–xxx
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Self-oriented growth of β-Yb2Si2O7 and X1/X2-Yb2SiO5 coatings using laser chemical vapor deposition Akihiko Itoa,∗, Masato Sekiyamab, Tomohiro Haraa, Takashi Gotob a b
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan
ARTICLE INFO
ABSTRACT
Keywords: Films Surfaces Silicate Environmental barrier coating
We demonstrated the self-oriented growth of ytterbium silicate films at high deposition rates using laser chemical vapor deposition (laser CVD). β-Yb2Si2O7, X1-Yb2SiO5, and X2-Yb2SiO5 phases preferred (001)β and (110)β, (100)X1 and (021)X2, and (221)X2 orientations, respectively. The self-oriented growth was associated with crystallographic features, namely the edge-sharing Yb–O polyhedral networks in each phase. Deposition rates of the β-Yb2Si2O7 and X1/X2-Yb2SiO5 films ranged 114–423 and 353–943 μm h−1, respectively. Post annealing implied that β-Yb2Si2O7 and X2-Yb2SiO5 were the stable phases at elevated temperatures and the obtained films maintained their cone-like and columnar structures after annealing at 1673 K in air.
1. Introduction SiC- and Al2O3-fiber-reinforced ceramic matrix composites (CMC) are promising light-weight structural materials for high-temperature and harsh environments [1]. Among them, SiC-CMC could be an alternative to Ni-based superalloys used in the gas turbine engines of airplanes. Recession of SiC occurs in high-temperature atmosphere containing water vapor which limits the operating temperature of SiCCMC [1]. Therefore, the development of an environmental barrier coating (EBC) for CMC components is in strong demanded to ensure reliable and durable operation of SiC-CMC at high-temperatures [2,3]. Ytterbium silicates, typically β-Yb2Si2O7, X1-Yb2SiO5 and X2Yb2SiO5, are candidate materials for EBC because these compounds possess excellent chemical stability [3], low volatilization rate against high-temperature water vapor [4], low thermal conductivity [5] and a compatible coefficient of thermal expansion with the underlying SiCCMC [6]. To date, ytterbium silicate coatings have been prepared using spraying [7,8] and electron-beam physical vapor deposition (EB-PVD) [9,10]. Chemical vapor deposition (CVD) is a versatile technique to produce conformal protective coatings with controlled orientation and microstructure [11]. CVD of crystalline rare-earth silicate coatings, including ytterbium silicates, has rarely been explored because amorphous sili-
∗
cate would likely be formed from a precursor vapor. Introducing laser irradiation into the CVD process can enhance the chemical reaction between precursor vapor and the surface of the film, resulting in the CVD of crystalline Y2Si2O7 and Y2SiO5 at a high deposition rate (several hundred micrometers per hour) [12,13], which is comparable the rates reported using EB-PVD [10]. However, Y2Si2O7 has many polymorphs of α, β, δ, γ and y phases [12,13], and volume changes due to phase transitions at elevated temperatures cause delamination of coatings. Because no polymorph has been reported on Yb2Si2O7 at elevated temperatures owing to a small ionic radius of Yb ion, high-speed deposition of ytterbium silicates with self-oriented growth via laser CVD would be alternative technique for EBC. In the present study, we demonstrated the self-oriented growth of βYb2Si2O7 and X1/X2-Yb2SiO5 films using laser CVD at a high deposition rate. The high-temperature stability of the ytterbium silicate films was also studied via post annealing. 2. Experimental procedure Ytterbium silicate films were prepared using the CVD apparatus, which was previously reported [14], and the deposition conditions are listed in Table 1. Polycrystalline AlN plate was selected as a substrate because AlN is accessible material and has a similar thermal expansion
Corresponding author. E-mail address: [email protected] (A. Ito).
https://doi.org/10.1016/j.ceramint.2019.12.217 Received 28 September 2019; Received in revised form 18 December 2019; Accepted 25 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Akihiko Ito, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.217
Ceramics International xxx (xxxx) xxx–xxx
A. Ito, et al.
Table 1 Deposition conditions. Yb source and vaporization temperature Si source and vaporization temperature Carrier gas and flow rate Reactant gas and flow rate Total chamber pressure Substrates Laser source and power Deposition time
Yb(dpm)3 at 473–523 K TEOS at 313–353 K Ar at 100 sccm O2 at 100 sccm 0.8 kPa Polycrystalline AlN plate (dimensions: 8 mm × 8 mm × 1 mm) Diode laser (λ = 808 nm) at 68–168 W 180–300 s
coefficient to SiC-CMC. Ytterbium tris-dipivaloylmethanate (Yb(dpm)3) and tetraorthosilicate (TEOS) were used as precursors and the Yb molar fraction in the vapor (MYb) was changed from 0 to 100 mol%Yb by changing the vaporization temperatures of each precursor. For the laser CVD process, a diode laser irradiated the substrate through a quartz window in the chamber. The deposition temperature increased from 1023 K to 1373 K as the laser power increased from 88 to 168 W. For the thermal CVD process, the substrate could be heated up to 1473 K on a hot stage installed with an electric heater without laser irradiation. The phase composition of the coatings was investigated using X-ray diffraction (XRD; Rigaku Ultima VI and Bruker D2 Phaser). The microstructure was observed using a scanning electron microscope (SEM; Hitachi H-3100 and JEOL JCM-6000) and a transmission electron microscope (TEM; Topcon EM-002B). The crystal structure was schematically illustrated using the VESTA software package [15].
Fig. 1. XRD patterns of ytterbium silicate films prepared on polycrystalline AlN substrate at various MYb and PL: (a) 16 mol%Yb and 128 W, (b) 55 mol%Yb and 128 W, (c) 76 mol%Yb and 168 W, and (d) 85 mol%Yb and 168 W.
3. Results and discussion 3.1. Phase composition and self-orientated growth of ytterbium silicate coatings
structures. The crystal structure of X1-Yb2SiO5 is described as alternating layers of YbO7 and YbO7–SiO4 planes along a-axis direction (Fig. 2(c)). On the other hand, the crystal structure of X2-Yb2SiO5 contains edge-sharing YbO6–YbO7 zig-zag chains, and one side is on the (221)X2 plane (Fig. 2(d)). These crystallographic features might result in each preferred oriented growth. Fig. 3 summarizes the effects of MYb and PL on phase composition and the preferred orientation of the ytterbium silicate films prepared using laser CVD. As the MYb increased, the phase composition of the films changed: amorphous SiO2, β-Yb2Si2O7, X1-Yb2SiO5, X1/X2Yb2SiO5, bixbyite Yb2O3. A minor amount of X-Yb2Si2O7 co-existed in the β-Yb2Si2O7 films. As the PL increased, the orientation of β-Yb2Si2O7 was changed from (001) to (110) plane at MYb = 15–50 mol%Yb. This change could be associated with the microstructural evolution of the films from granular to columnar, as discussed in the next section, because β-Yb2Si2O7 film with a columnar structure prepared via EB-PVD also favored a (110) orientation [10]. At MYb = 50–70 mol%Yb, primary phase was changed from X1-Yb2SiO5 to β-Yb2Si2O7 as the PL increased because TEOS precursor was much activated at high PL, resulting in the formation of Sirich phase. No significant changes on phase composition and orientation plane of X2-Yb2SiO5 was observed at MYb = 70–90 mol%Yb. Although ytterbium silicate films were also prepared using thermal CVD for comparison, all the thermal CVD films were amorphous at deposition temperatures from 923 to 1273 K.
Fig. 1 shows XRD patterns of the ytterbium silicate films at MYb = 0–33 mol%Yb and 66–100 mol%Yb, respectively. Amorphous SiO2 films were prepared without Yb(dpm)3 supplication (MYb = 0 mol %Yb) independently of PL, whereas Yb2O3 films prepared without TEOS supplication (MYb = 100 mol%Yb) had a bixbyite structure with a significant (100) orientation independent of PL. β-Yb2Si2O7 films were prepared at MYb = 15–53 mol%Yb and PL = 128–168 W (Fig. 1(a) and 1(b)), while the obtained films were amorphous at 88–128 W. Orientation of the β-Yb2Si2O7 films changed from (001) to (110) to (100) plane as MYb was increased from 15 to 55 mol%Yb. The crystal structure of β-Yb2Si2O7 (space group: C2/m) is described as alternating layers of YbO6 and SiO4 planes along c-axis direction [17], and this crystallographic feature might prefer (001) and (110) oriented growth (Fig. 2(a) and 2(b)). A minor amount of XYb2Si2O7 co-existed in the β-Yb2Si2O7 films at MYb = 33–47 mol%Yb. X-Yb2Si2O7 (P412121), which is isostructural with X-Lu2Si2O7, has been previously reported as a high-pressure phase of Yb2Si2O7 [16]. Almost single-phase X1-Yb2SiO5 films were obtained at MYb = 65–66 mol%, and the X1-Yb2SiO5 films exhibited a significant (100) orientation (Fig. 1(c)). A mixture of X1- and X2-Yb2SiO5 (X1/X2Yb2SiO5) films were obtained at MYb = 65–85 mol%Yb. The X1 and X2 phases had (021) and (221) orientations, respectively, as pictured in Fig. 1(d). Yb2SiO5 has two polymorphs of X1 (P21/c) and X2 (C2/c)
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Fig. 2. Schematic illustrations for self-oriented growth of (a) (001)-oriented β-Yb2Si2O7, (b) (110)-oriented β-Yb2Si2O7, (c) (100)-oriented X1-Yb2SiO5, and (d) (221)oriented X2-Yb2SiO5.
columnar grains (Fig. 5). A selected-area electron diffraction pattern can be indexed with X1-Yb2SiO5 revealing a [010] zone axis confirming the (100)-oriented growth of the X1-Yb2SiO5 film (inset in Fig. 5). 3.3. Deposition rate of ytterbium silicate coatings Deposition rates of β-Yb2Si2O7 films using laser CVD ranged from 114 to 423 μm h−1 (circles in Fig. 6), while the rates for the X1/X2Yb2SiO5 films were 353–943 μm h−1 (squares in Fig. 6). These deposition rates are comparable to those reported for EB-PVD of (110)oriented β-Yb2Si2O7 film (168 μm h−1) [10]. Deposition rates of the ytterbium silicate films using thermal CVD are also plotted in the same figure for comparison. The deposition rates increased from 1 to 93 μm h−1 as the deposition temperature rose from 923 to 1273 K (reversed triangles in Fig. 6). The corresponding apparent activation energy was 102 kJ mol−1, suggesting that the thermal CVD process was limited by the chemical reaction on the surface [18]. Conversely, deposition rates of the β/X-Yb2Si2O7 and X1/X2Yb2SiO5 films prepared by laser CVD were independent of PL indicating an apparent activation energy of almost zero. The surface reaction was activated by laser irradiation, and thus the rate-limiting step changed from the chemical reaction rate to the mass transport rate in the gas phase [18].
Fig. 3. Effects of MYb and PL on phase composition of the ytterbium silicate films. 227 and 215 denote for Yb2Si2O7 and Yb2SiO5, respectively. A dot mark indicates the coexistence of X-Yb2Si2O7 in β-Yb2Si2O7.
3.2. Microstructure of ytterbium silicate coatings Amorphous SiO2 films had a dense structure in cross section with a smooth surface, namely glassy morphology. (001)- and (110)-oriented β-Yb2Si2O7 films had a cone-like structure composed of granular and slightly faceted fine grains as shown in Fig. 4(a) and 4(b), respectively. A similar one-like structure was observed in the (100)-oriented X1Yb2SiO5 film (Fig. 4(c) and 4(d)) but composed of elongated fine grains. Fig. 4(e) and 4(f) show a (221)-oriented X2-Yb2SiO5 film with a fine columnar grain with pyramidal caps. The cross-sectional TEM observations for the (100)-oriented X1Yb2SiO5 film revealed that the X1-Yb2SiO5 film consisted bundles of thin
3.4. Post annealing of ytterbium silicate coatings Yttrium silicate films were post annealed at 1673 K in air for 3.6 ks. Amorphous SiO2 films prepared without Yb supplication (MYb = 0 mol %Yb) were crystallized into α-cristobalite SiO2 (“α” area in Fig. 7). Formation of α-cristbalite SiO2 was also found in the β-Yb2Si2O7 films prepared at MYb = 10–20 mol%Yb after annealing, suggesting the coexistence of an amorphous SiO2 in the as-deposited β-Yb2Si2O7 films (“β+α” area in Fig. 7). β-Yb2Si2O7 was stable at 1673 K, while X-Yb2Si2O7 transformed into
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Fig. 4. Cross-sectional (a, c, e) and surface (b, d, f) SEM images of ytterbium silicate films prepared various MYb and PL: (a, b) (110)-oriented βYb2Si2O7 film at 55 mol%Yb and 128 W, (c, d) (100)-oriented X1-Yb2SiO5 film at 76 mol%Yb and 168 W, and (e, f) a mixture of (021)-oriented X1Yb2SiO5 and (221)-oriented X2-Yb2SiO5 film at 85 mol%Yb and 168 W.
Fig. 6. Arrhenius plots for the deposition of ytterbium silicate films using thermal and laser CVDs.
Fig. 5. Cross-sectional bright-field TEM image of the (100)-oriented X1Yb2SiO5 film. Inset shows the selected-area electron diffraction pattern.
with the results reported in literature. Amorphous ytterbium silicate films prepared at low PL of 88–108 W were crystallized into β-Yb2Si2O7 for MYb = 15–45 mol%Yb and X2Yb2SiO5 for MYb = 55–65 mol%Yb. Yb2O3 films maintained the bixbyite structure after annealing. Fig. 8 shows cross-sectional and surface SEM images of the ytterbium silicate films, which were presented in Fig. 4, after post annealing. For the Yb2Si2O7 films, grain growth was observed in the cone-like structure in cross section (Fig. 8(a)) and gaps between aggregates became wider (Fig. 8(b)). Although fine grains of the Yb2SiO5 films became rounded (Fig. 8(d), 8(f)), the films maintained the cone-like and columnar structures in cross section (Fig. 8(c), 8(e)). Post annealing results indicated that β-Yb2Si2O7 and X2-Yb2SiO5
β-Yb2Si2O7 during annealing (“β-227” area in Fig. 7). X-Yb2Si2O7 has been reported as a high-pressure phase and β-Yb2Si2O7 is the only stable phase of Yb2Si2O7 thermodynamically, therefore X-Yb2Si2O7 could be formed kinetically during the CVD process. X1-Yb2SiO5 transformed into X2-Yb2SiO5 during annealing (“X2/β” and “X2-215” areas in Fig. 7). Although the X1 and X2 phases are generally known as low- and high-temperature phases of rare-earth silicates, respectively, the X2 phase is much more stable than X1 phase for small rare-earth ions (Dy–Lu) including Yb [19]. Wang et al. reported that sol–gel-delivered X1-Yb2SiO5 powder transformed into X2Yb2SiO5 at 1273 K [19]. Kolitsch et al. reported that X1-Yb2SiO5 bulk bodies synthesized by solid-state reaction transformed into X2-Yb2SiO5 at 1423 K [21]. The phase transformations observed in this study agree
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Yb2Si2O7 was related to the alternating layers of YbO6 and SiO4 planes on (001) plane, while (100) orientation of X1-Yb2SiO5 related to edgesharing YbO7 plane on (100) plane. The (221) orientation of X2Yb2SiO5 could be associated with the edge-sharing YbO6–YbO7 chains. Deposition rates of β/X-Yb2Si2O7 and X1/X2-Yb2SiO5 films were 114–981 and 160–943 μm h−1, respectively. After post annealing at 1673 K in air, X-Yb2Si2O7 and X1-Yb2SiO5 were transformed into βYb2Si2O7 and X2-Yb2SiO5, respectively, implying that β-Yb2Si2O7 and X2-Yb2SiO5 were stable phases at elevated temperatures. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was partly supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Structural Materials for Innovation” (Funding agency: JST), Japan. This study was also supported in part by
Fig. 7. Phase composition of the ytterbium silicate films prepared after annealed at 1673 K in air. 227, 215, α, and B denotes Yb2Si2O7, Yb2SiO5, αcristobalite SiO2, and bixbyite Yb2O3, respectively.
Fig. 8. Cross-sectional (a, c, e) and surface (b, d, f) SEM images of the ytterbium silicate films after annealing at 1673 K in air for 3.6 ks. The films before annealing were (a, b) (110)-oriented βYb2Si2O7, (c, d) (100)-oriented X1-Yb2SiO5, and (e, f) a mixture of (021)-oriented X1-Yb2SiO5 and (221)-oriented X2-Yb2SiO5, which are presented in Fig. 4.
were stable phases at 1673 K for CVD films. As a result, the Yb2SiO5 coating would exhibit higher durability as a top coat of barrier coatings at elevated temperatures.
JSPS KAKENHI Grant Number JP17H03426, Japan. X-ray diffraction measurement (Ultima IV) was partly carried out at Instrumental Analysis Center, Yokohama National University, Japan.
4. Conclusions
References
We demonstrated the first CVD of crystalline ytterbium silicate films with self-oriented growth at high deposition rates. The phase composition of the films changed from amorphous SiO2 to β-Yb2Si2O7 to X1Yb2SiO5 to X2-Yb2SiO5 to bixbyite Yb2O3 as MYb increased. β-Yb2Si2O7 films showed (001)β, (110)β and (100)β orientations with changes in MYb, whereas X1-Yb2SiO5 and X2-Yb2SiO5 films preferred (100)X1 and (021)X1, and (221)X2 orientations, respectively. The preferred orientation of the ytterbium silicate films was associated with the crystallographic features of each phase. The (110) and (001) orientations of β-
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Self-oriented growth of β-Yb2Si2O7 and X1/X2-Yb2SiO5 coatings using laser chemical vapor deposition Akihiko Itoa,∗, Masato Sekiyamab, Tomohiro Haraa, Takashi Gotob a b
Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya-ku, Yokohama, 240-8501, Japan Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, 980-8577, Japan
ARTICLE INFO
ABSTRACT
Keywords: Films Surfaces Silicate Environmental barrier coating
We demonstrated the self-oriented growth of ytterbium silicate films at high deposition rates using laser chemical vapor deposition (laser CVD). β-Yb2Si2O7, X1-Yb2SiO5, and X2-Yb2SiO5 phases preferred (001)β and (110)β, (100)X1 and (021)X2, and (221)X2 orientations, respectively. The self-oriented growth was associated with crystallographic features, namely the edge-sharing Yb–O polyhedral networks in each phase. Deposition rates of the β-Yb2Si2O7 and X1/X2-Yb2SiO5 films ranged 114–423 and 353–943 μm h−1, respectively. Post annealing implied that β-Yb2Si2O7 and X2-Yb2SiO5 were the stable phases at elevated temperatures and the obtained films maintained their cone-like and columnar structures after annealing at 1673 K in air.
1. Introduction SiC- and Al2O3-fiber-reinforced ceramic matrix composites (CMC) are promising light-weight structural materials for high-temperature and harsh environments [1]. Among them, SiC-CMC could be an alternative to Ni-based superalloys used in the gas turbine engines of airplanes. Recession of SiC occurs in high-temperature atmosphere containing water vapor which limits the operating temperature of SiCCMC [1]. Therefore, the development of an environmental barrier coating (EBC) for CMC components is in strong demanded to ensure reliable and durable operation of SiC-CMC at high-temperatures [2,3]. Ytterbium silicates, typically β-Yb2Si2O7, X1-Yb2SiO5 and X2Yb2SiO5, are candidate materials for EBC because these compounds possess excellent chemical stability [3], low volatilization rate against high-temperature water vapor [4], low thermal conductivity [5] and a compatible coefficient of thermal expansion with the underlying SiCCMC [6]. To date, ytterbium silicate coatings have been prepared using spraying [7,8] and electron-beam physical vapor deposition (EB-PVD) [9,10]. Chemical vapor deposition (CVD) is a versatile technique to produce conformal protective coatings with controlled orientation and microstructure [11]. CVD of crystalline rare-earth silicate coatings, including ytterbium silicates, has rarely been explored because amorphous sili-
∗
cate would likely be formed from a precursor vapor. Introducing laser irradiation into the CVD process can enhance the chemical reaction between precursor vapor and the surface of the film, resulting in the CVD of crystalline Y2Si2O7 and Y2SiO5 at a high deposition rate (several hundred micrometers per hour) [12,13], which is comparable the rates reported using EB-PVD [10]. However, Y2Si2O7 has many polymorphs of α, β, δ, γ and y phases [12,13], and volume changes due to phase transitions at elevated temperatures cause delamination of coatings. Because no polymorph has been reported on Yb2Si2O7 at elevated temperatures owing to a small ionic radius of Yb ion, high-speed deposition of ytterbium silicates with self-oriented growth via laser CVD would be alternative technique for EBC. In the present study, we demonstrated the self-oriented growth of βYb2Si2O7 and X1/X2-Yb2SiO5 films using laser CVD at a high deposition rate. The high-temperature stability of the ytterbium silicate films was also studied via post annealing. 2. Experimental procedure Ytterbium silicate films were prepared using the CVD apparatus, which was previously reported [14], and the deposition conditions are listed in Table 1. Polycrystalline AlN plate was selected as a substrate because AlN is accessible material and has a similar thermal expansion
Corresponding author. E-mail address: [email protected] (A. Ito).
https://doi.org/10.1016/j.ceramint.2019.12.217 Received 28 September 2019; Received in revised form 18 December 2019; Accepted 25 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Akihiko Ito, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.217
Ceramics International xxx (xxxx) xxx–xxx
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Table 1 Deposition conditions. Yb source and vaporization temperature Si source and vaporization temperature Carrier gas and flow rate Reactant gas and flow rate Total chamber pressure Substrates Laser source and power Deposition time
Yb(dpm)3 at 473–523 K TEOS at 313–353 K Ar at 100 sccm O2 at 100 sccm 0.8 kPa Polycrystalline AlN plate (dimensions: 8 mm × 8 mm × 1 mm) Diode laser (λ = 808 nm) at 68–168 W 180–300 s
coefficient to SiC-CMC. Ytterbium tris-dipivaloylmethanate (Yb(dpm)3) and tetraorthosilicate (TEOS) were used as precursors and the Yb molar fraction in the vapor (MYb) was changed from 0 to 100 mol%Yb by changing the vaporization temperatures of each precursor. For the laser CVD process, a diode laser irradiated the substrate through a quartz window in the chamber. The deposition temperature increased from 1023 K to 1373 K as the laser power increased from 88 to 168 W. For the thermal CVD process, the substrate could be heated up to 1473 K on a hot stage installed with an electric heater without laser irradiation. The phase composition of the coatings was investigated using X-ray diffraction (XRD; Rigaku Ultima VI and Bruker D2 Phaser). The microstructure was observed using a scanning electron microscope (SEM; Hitachi H-3100 and JEOL JCM-6000) and a transmission electron microscope (TEM; Topcon EM-002B). The crystal structure was schematically illustrated using the VESTA software package [15].
Fig. 1. XRD patterns of ytterbium silicate films prepared on polycrystalline AlN substrate at various MYb and PL: (a) 16 mol%Yb and 128 W, (b) 55 mol%Yb and 128 W, (c) 76 mol%Yb and 168 W, and (d) 85 mol%Yb and 168 W.
3. Results and discussion 3.1. Phase composition and self-orientated growth of ytterbium silicate coatings
structures. The crystal structure of X1-Yb2SiO5 is described as alternating layers of YbO7 and YbO7–SiO4 planes along a-axis direction (Fig. 2(c)). On the other hand, the crystal structure of X2-Yb2SiO5 contains edge-sharing YbO6–YbO7 zig-zag chains, and one side is on the (221)X2 plane (Fig. 2(d)). These crystallographic features might result in each preferred oriented growth. Fig. 3 summarizes the effects of MYb and PL on phase composition and the preferred orientation of the ytterbium silicate films prepared using laser CVD. As the MYb increased, the phase composition of the films changed: amorphous SiO2, β-Yb2Si2O7, X1-Yb2SiO5, X1/X2Yb2SiO5, bixbyite Yb2O3. A minor amount of X-Yb2Si2O7 co-existed in the β-Yb2Si2O7 films. As the PL increased, the orientation of β-Yb2Si2O7 was changed from (001) to (110) plane at MYb = 15–50 mol%Yb. This change could be associated with the microstructural evolution of the films from granular to columnar, as discussed in the next section, because β-Yb2Si2O7 film with a columnar structure prepared via EB-PVD also favored a (110) orientation [10]. At MYb = 50–70 mol%Yb, primary phase was changed from X1-Yb2SiO5 to β-Yb2Si2O7 as the PL increased because TEOS precursor was much activated at high PL, resulting in the formation of Sirich phase. No significant changes on phase composition and orientation plane of X2-Yb2SiO5 was observed at MYb = 70–90 mol%Yb. Although ytterbium silicate films were also prepared using thermal CVD for comparison, all the thermal CVD films were amorphous at deposition temperatures from 923 to 1273 K.
Fig. 1 shows XRD patterns of the ytterbium silicate films at MYb = 0–33 mol%Yb and 66–100 mol%Yb, respectively. Amorphous SiO2 films were prepared without Yb(dpm)3 supplication (MYb = 0 mol %Yb) independently of PL, whereas Yb2O3 films prepared without TEOS supplication (MYb = 100 mol%Yb) had a bixbyite structure with a significant (100) orientation independent of PL. β-Yb2Si2O7 films were prepared at MYb = 15–53 mol%Yb and PL = 128–168 W (Fig. 1(a) and 1(b)), while the obtained films were amorphous at 88–128 W. Orientation of the β-Yb2Si2O7 films changed from (001) to (110) to (100) plane as MYb was increased from 15 to 55 mol%Yb. The crystal structure of β-Yb2Si2O7 (space group: C2/m) is described as alternating layers of YbO6 and SiO4 planes along c-axis direction [17], and this crystallographic feature might prefer (001) and (110) oriented growth (Fig. 2(a) and 2(b)). A minor amount of XYb2Si2O7 co-existed in the β-Yb2Si2O7 films at MYb = 33–47 mol%Yb. X-Yb2Si2O7 (P412121), which is isostructural with X-Lu2Si2O7, has been previously reported as a high-pressure phase of Yb2Si2O7 [16]. Almost single-phase X1-Yb2SiO5 films were obtained at MYb = 65–66 mol%, and the X1-Yb2SiO5 films exhibited a significant (100) orientation (Fig. 1(c)). A mixture of X1- and X2-Yb2SiO5 (X1/X2Yb2SiO5) films were obtained at MYb = 65–85 mol%Yb. The X1 and X2 phases had (021) and (221) orientations, respectively, as pictured in Fig. 1(d). Yb2SiO5 has two polymorphs of X1 (P21/c) and X2 (C2/c)
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Fig. 2. Schematic illustrations for self-oriented growth of (a) (001)-oriented β-Yb2Si2O7, (b) (110)-oriented β-Yb2Si2O7, (c) (100)-oriented X1-Yb2SiO5, and (d) (221)oriented X2-Yb2SiO5.
columnar grains (Fig. 5). A selected-area electron diffraction pattern can be indexed with X1-Yb2SiO5 revealing a [010] zone axis confirming the (100)-oriented growth of the X1-Yb2SiO5 film (inset in Fig. 5). 3.3. Deposition rate of ytterbium silicate coatings Deposition rates of β-Yb2Si2O7 films using laser CVD ranged from 114 to 423 μm h−1 (circles in Fig. 6), while the rates for the X1/X2Yb2SiO5 films were 353–943 μm h−1 (squares in Fig. 6). These deposition rates are comparable to those reported for EB-PVD of (110)oriented β-Yb2Si2O7 film (168 μm h−1) [10]. Deposition rates of the ytterbium silicate films using thermal CVD are also plotted in the same figure for comparison. The deposition rates increased from 1 to 93 μm h−1 as the deposition temperature rose from 923 to 1273 K (reversed triangles in Fig. 6). The corresponding apparent activation energy was 102 kJ mol−1, suggesting that the thermal CVD process was limited by the chemical reaction on the surface [18]. Conversely, deposition rates of the β/X-Yb2Si2O7 and X1/X2Yb2SiO5 films prepared by laser CVD were independent of PL indicating an apparent activation energy of almost zero. The surface reaction was activated by laser irradiation, and thus the rate-limiting step changed from the chemical reaction rate to the mass transport rate in the gas phase [18].
Fig. 3. Effects of MYb and PL on phase composition of the ytterbium silicate films. 227 and 215 denote for Yb2Si2O7 and Yb2SiO5, respectively. A dot mark indicates the coexistence of X-Yb2Si2O7 in β-Yb2Si2O7.
3.2. Microstructure of ytterbium silicate coatings Amorphous SiO2 films had a dense structure in cross section with a smooth surface, namely glassy morphology. (001)- and (110)-oriented β-Yb2Si2O7 films had a cone-like structure composed of granular and slightly faceted fine grains as shown in Fig. 4(a) and 4(b), respectively. A similar one-like structure was observed in the (100)-oriented X1Yb2SiO5 film (Fig. 4(c) and 4(d)) but composed of elongated fine grains. Fig. 4(e) and 4(f) show a (221)-oriented X2-Yb2SiO5 film with a fine columnar grain with pyramidal caps. The cross-sectional TEM observations for the (100)-oriented X1Yb2SiO5 film revealed that the X1-Yb2SiO5 film consisted bundles of thin
3.4. Post annealing of ytterbium silicate coatings Yttrium silicate films were post annealed at 1673 K in air for 3.6 ks. Amorphous SiO2 films prepared without Yb supplication (MYb = 0 mol %Yb) were crystallized into α-cristobalite SiO2 (“α” area in Fig. 7). Formation of α-cristbalite SiO2 was also found in the β-Yb2Si2O7 films prepared at MYb = 10–20 mol%Yb after annealing, suggesting the coexistence of an amorphous SiO2 in the as-deposited β-Yb2Si2O7 films (“β+α” area in Fig. 7). β-Yb2Si2O7 was stable at 1673 K, while X-Yb2Si2O7 transformed into
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Fig. 4. Cross-sectional (a, c, e) and surface (b, d, f) SEM images of ytterbium silicate films prepared various MYb and PL: (a, b) (110)-oriented βYb2Si2O7 film at 55 mol%Yb and 128 W, (c, d) (100)-oriented X1-Yb2SiO5 film at 76 mol%Yb and 168 W, and (e, f) a mixture of (021)-oriented X1Yb2SiO5 and (221)-oriented X2-Yb2SiO5 film at 85 mol%Yb and 168 W.
Fig. 6. Arrhenius plots for the deposition of ytterbium silicate films using thermal and laser CVDs.
Fig. 5. Cross-sectional bright-field TEM image of the (100)-oriented X1Yb2SiO5 film. Inset shows the selected-area electron diffraction pattern.
with the results reported in literature. Amorphous ytterbium silicate films prepared at low PL of 88–108 W were crystallized into β-Yb2Si2O7 for MYb = 15–45 mol%Yb and X2Yb2SiO5 for MYb = 55–65 mol%Yb. Yb2O3 films maintained the bixbyite structure after annealing. Fig. 8 shows cross-sectional and surface SEM images of the ytterbium silicate films, which were presented in Fig. 4, after post annealing. For the Yb2Si2O7 films, grain growth was observed in the cone-like structure in cross section (Fig. 8(a)) and gaps between aggregates became wider (Fig. 8(b)). Although fine grains of the Yb2SiO5 films became rounded (Fig. 8(d), 8(f)), the films maintained the cone-like and columnar structures in cross section (Fig. 8(c), 8(e)). Post annealing results indicated that β-Yb2Si2O7 and X2-Yb2SiO5
β-Yb2Si2O7 during annealing (“β-227” area in Fig. 7). X-Yb2Si2O7 has been reported as a high-pressure phase and β-Yb2Si2O7 is the only stable phase of Yb2Si2O7 thermodynamically, therefore X-Yb2Si2O7 could be formed kinetically during the CVD process. X1-Yb2SiO5 transformed into X2-Yb2SiO5 during annealing (“X2/β” and “X2-215” areas in Fig. 7). Although the X1 and X2 phases are generally known as low- and high-temperature phases of rare-earth silicates, respectively, the X2 phase is much more stable than X1 phase for small rare-earth ions (Dy–Lu) including Yb [19]. Wang et al. reported that sol–gel-delivered X1-Yb2SiO5 powder transformed into X2Yb2SiO5 at 1273 K [19]. Kolitsch et al. reported that X1-Yb2SiO5 bulk bodies synthesized by solid-state reaction transformed into X2-Yb2SiO5 at 1423 K [21]. The phase transformations observed in this study agree
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Yb2Si2O7 was related to the alternating layers of YbO6 and SiO4 planes on (001) plane, while (100) orientation of X1-Yb2SiO5 related to edgesharing YbO7 plane on (100) plane. The (221) orientation of X2Yb2SiO5 could be associated with the edge-sharing YbO6–YbO7 chains. Deposition rates of β/X-Yb2Si2O7 and X1/X2-Yb2SiO5 films were 114–981 and 160–943 μm h−1, respectively. After post annealing at 1673 K in air, X-Yb2Si2O7 and X1-Yb2SiO5 were transformed into βYb2Si2O7 and X2-Yb2SiO5, respectively, implying that β-Yb2Si2O7 and X2-Yb2SiO5 were stable phases at elevated temperatures. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was partly supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), “Structural Materials for Innovation” (Funding agency: JST), Japan. This study was also supported in part by
Fig. 7. Phase composition of the ytterbium silicate films prepared after annealed at 1673 K in air. 227, 215, α, and B denotes Yb2Si2O7, Yb2SiO5, αcristobalite SiO2, and bixbyite Yb2O3, respectively.
Fig. 8. Cross-sectional (a, c, e) and surface (b, d, f) SEM images of the ytterbium silicate films after annealing at 1673 K in air for 3.6 ks. The films before annealing were (a, b) (110)-oriented βYb2Si2O7, (c, d) (100)-oriented X1-Yb2SiO5, and (e, f) a mixture of (021)-oriented X1-Yb2SiO5 and (221)-oriented X2-Yb2SiO5, which are presented in Fig. 4.
were stable phases at 1673 K for CVD films. As a result, the Yb2SiO5 coating would exhibit higher durability as a top coat of barrier coatings at elevated temperatures.
JSPS KAKENHI Grant Number JP17H03426, Japan. X-ray diffraction measurement (Ultima IV) was partly carried out at Instrumental Analysis Center, Yokohama National University, Japan.
4. Conclusions
References
We demonstrated the first CVD of crystalline ytterbium silicate films with self-oriented growth at high deposition rates. The phase composition of the films changed from amorphous SiO2 to β-Yb2Si2O7 to X1Yb2SiO5 to X2-Yb2SiO5 to bixbyite Yb2O3 as MYb increased. β-Yb2Si2O7 films showed (001)β, (110)β and (100)β orientations with changes in MYb, whereas X1-Yb2SiO5 and X2-Yb2SiO5 films preferred (100)X1 and (021)X1, and (221)X2 orientations, respectively. The preferred orientation of the ytterbium silicate films was associated with the crystallographic features of each phase. The (110) and (001) orientations of β-
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