Thermal properties and Calcium-Magnesium-Alumina-Silicate (CMAS) resistance of LuPO4 as environmental barrier coatings

Journal Pre-proof Thermal Properties and Calcium-Magnesium-Alumina-Silicate (CMAS) Resistance of LuPO4 as Environmental Barrier Coatings Xunxun Hu, Fangfang Xu, Kunwei Li, Yizhou Zhang, Yue Xu, Xinqing Zhao

PII:

S0955-2219(19)30764-2

DOI:

https://doi.org/10.1016/j.jeurceramsoc.2019.11.018

Reference:

JECS 12843

To appear in:

Journal of the European Ceramic Society

Received Date:

7 May 2019

Revised Date:

15 October 2019

Accepted Date:

5 November 2019

Please cite this article as: Hu X, Xu F, Li K, Zhang Y, Xu Y, Zhao X, Thermal Properties and Calcium-Magnesium-Alumina-Silicate (CMAS) Resistance of LuPO4 as Environmental Barrier Coatings, Journal of the European Ceramic Society (2019), doi: https://doi.org/10.1016/j.jeurceramsoc.2019.11.018

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Thermal

Properties

and

Calcium-Magnesium-Alumina-Silicate

(CMAS) Resistance of LuPO4 as Environmental Barrier Coatings

Xunxun Hua, Fangfang Xub, Kunwei Lic, Yizhou Zhangd, Yue Xud,*, Xinqing Zhao a,*

a

School of Materials Science and Engineering, Beihang University, Beijing 100191, China

b

Guobiao (Beijing) Testing＆Certification Co., Ltd, 2 Xinjiekouwai Street, Beijing 100088,

ro of

China China National Institute of Standardization, No.4 Zhichun Road, Beijing 100191, China

d

School of Chemistry, Beihang University, Beijing 100191, China

*

Corresponding authors. E-mail addresses: [email protected];

-p

c

re

[email protected];

Abstract

lP

LuPO4 ceramics have been synthesized by chemical precipitation and calcination approaches. The phase stability and the thermal properties of the ceramics as well as the

na

thermochemical reactions between LuPO4 pellet and calcium–magnesium–alumina–silicate at 1300℃ have been investigated. The results indicated that LuPO4 has a relatively lower thermal conductivity (1.86 W m-1K-1 at 1200 ℃) and a well-matched coefficient of thermal

ur

expansion (5.92 × 10-6K-1) with SiC based ceramic matrix composites. No phase transformation occurs in single phase LuPO4 during the heating from room temperature to

Jo

1300 ℃. Corrosion tests showed that CMAS can dissolve LuPO4 and cause precipitation of crystalline phases such as Lu2Si2O7, CaAl2Si2O8, CaMgSi2O6 ， Ca2Lu8 (SiO4)6O2 and Ca8MgLu (PO4)7. Extending the corrosion duration results in the formation of Ca2Lu8 (SiO4)6O2 apatite and other crystalline phases with a dendritic-like structure in the CMAS layer. CMAS corrosion resistance of LuPO4 is significantly greater than Lu2Si2O7 since less LuPO4 need to be dissolved to form a protective apatite barrier layer. Ca8MgLu (PO4)7 tends to form a continuous reaction layer at the CMAS/LuPO4 interface. This dense 1

and continuous layer can effectively inhibit molten CMAS penetration into LuPO4.

Keywords: LuPO4 ceramics, Environmental barrier coatings, Thermal properties, CMAS

Jo

ur

na

lP

re

-p

ro of

corrosion.

2

1．Introduction Owing to light-weight, good oxidation resistance and excellent high temperature performance, silicon carbide based ceramic matrix composites (SiC-CMCs) are the most promising thermal-part material to replace nickel-based superalloys for the next generation aero-engine [1]. The key barrier for restricting the application of SiC-CMCs is their poor stability in aero-engine working environments [2-3]. Environmental barrier coatings (EBCs)

ro of

could provide an effective protection for SiC-CMCs. The advantages of utilizing EBCS on the surface of SiC-CMCS include enhanced high temperature capability and extended service life [4-5]. The performance of coating materials plays a vital role in the service life of EBCS.

-p

Potential EBC candidates usually have high temperature phase stability, suitable coefficient of

alumina–silicate (CMAS) corrosion.

re

thermal expansion (CTE), favorable resistance to water vapor and calcium–magnesium–

lP

Rare-earth silicates, especially ytterbium and lutetium silicates, are the most promising

na

EBC candidates due to their superior thermal properties [7-9]. RE2Si2O7 (RE=Yb, Lu) have a close CTE match with SiC-CMCS substrate, but its water vapor corrosion resistance is poor

ur

[10-12]. By comparison, RE2SiO5 (RE=Yb, Lu) have lower silica activity than their counterparts, but still suffer from water vapor corrosion due to the presence of silica

Jo

component. Besides, the significantly high CTEs comparing to that of SiC-CMCS substrate makes RE2SiO5 more easier to crack [13,14]. Owing to the silica evaporation during the fabrication process, it is difficult to achieve single phase, dense and crack-free rare earth silicate layers by conventional atmospheric plasma spraying (APS) deposition techniques [15]. Therefore, volatilization of silica is an intractable problem for the use of rare earth silicate as 3

EBCs. CMAS corrosion is another obstacle for the application of rare earth silicate. Generally, CMAS deposited on the surface of rare earth silicate EBCS possess relatively low melting point (lower than 1250℃). The molten CMAS could react with rare earth silicate, forming new crystalline phase with unfavorable properties such as high CTES and low toughness. This could lead to an eventual failure in the use of rare earth silicate as EBCS [16-20].

ro of

In dealing with these problems, Wang et al. [21] investigated the properties related to applications of YPO4 as EBCs. The corresponding results indicated that YPO4 has good water vapor corrosion resistance in quasi-static water vapor environment due to the

-p

absence of Si-O bond. Moreover, YPO4 have high temperature phase stability, well matched

re

CTES with SiC-CMCS substrate and superior corrosion resistance to molten salts. These combined properties endows YPO4 a promising EBC candidate material [21]. As is well

lP

known, the properties of substance are closely associated with their crystal structures. When

na

the materials have the same structure, they would exhibit similar properties. Hence, LuPO4 has a similar crystal structure to YPO4, it might be a material suitable for EBC application.

ur

However, there are few investigations reported on the thermal properties, especially CMAS corrosion behavior of LuPO4.

Jo

In the present study, LuPO4 were synthesized by chemical precipitation and calcination

methods. The thermal diffusivity, CTE, thermal conductivity and phase stability of LuPO4 were characterized. The hot corrosion behavior of LuPO4 exposed to molten CMAS at high temperature for different durations were systematically investigated, and the corresponding mechanisms were discussed. 4

2. Materials and methods The LuPO4 powders were synthesized by chemical co-precipitation method using Lu2O3 (99.99%)，HNO3 (68%＞wt.%>65%), H3PO4 (85 wt.%) and (NH4)2HPO4 (99%) as the raw materials. Lu2O3 and HNO3 were taken in appropriate proportion and heated at 75℃ to obtain Lu (NO3)3 solutions. After that, the obtained Lu(NO3)3 solutions were heated to 115℃. Subsequently, a 2.4 mol/L (NH4)2HPO4 solution was added to it until the solution PH=3.

ro of

Magnetic stirring was carried out during the whole process to ensure that the reaction solution was homogeneous. The resultant particles were collected via centrifugation, washed with ethanol, and dried in air at 60℃. Then, the dried particles was calcined in air at 1550℃ for 4

-p

h in a tube furnace. The chemical reaction equations during the process were as follows:

re

Lu2 O3 + 6HNO3 → 2Lu(NO3 )3 + 3H2 O

(2)

lP

Lu(NO3 )3 + 2(NH4 )2 HPO4 → LuPO4 + 3NH4 NO3 + (NH4 )H2 PO4

(1)

At last, the calcined LuPO4 was ground into superfine powders. The LuPO4 pellet was

na

fabricated by sintering the superfine LuPO4 powder in vacuum through the application of the spark plasma sintering instrument (Sinter 1050 SPS) in a Φ15 mm cylindrical graphite die at

ur

40MPa and 1400℃ for 3 min. The produced pellets were ground by SiC abrasive papers and then polished for 2 min.

Jo

The bulk density (ρ) was measured by Archimedes technique. Phase stability, thermal

diffusivity and CTE of LuPO4 ceramics were determined by a high temperature thermal analysis instrument (STA-449F3, Netzsch, Germany), laser flash testing set (LFA 427, Netzsch, Germany) and a thermal dilatometer (DIL402C, Netzsch, Germany), respectively. The specific heat capacity (Cp) of LuPO4 was calculated via Neumann-Kopp rule [22]. The 5

thermal conductivity (κ) of LuPO4 was calculated according to Eq (3) [23] and further corrected based on Eq (4) [23]: （3）

κ=αρCp k k'

=1-4/3𝜙

(4)

Where the 𝜙 and k' represent thermal diffusivity (α), porosity and fully dense bulk’s thermal conductivities respectively.

ro of

A 33CaO-9MgO-13AlO1.5-45SiO2 (C33M9A133S45) model of CMAS was used in the present study. The melting point of this kind CMAS melt was approximately 1235℃ [24-26]. CaO, MgO, Al2O3, and SiO2 Powders were used as raw material and mixed according to the

-p

stoichiometric ratio of C33M9A133S45. Then the four different oxides were ball-milled for 24h,

re

followed by dried at 100 °C for 6 h. After that, the powder mixture was heated to 1200°C for 12 h to obtain CMAS glass. The resulted CMAS glass was ground to obtain a uniform

lP

granularity distribution. CMAS corrosion studies were performed by dropping the CMAS

na

slurry (CMAS was dispersed in alcohol) on the polished surfaces of LuPO4 pellets with a concentration of about 20mg/cm2. The LuPO4 coated with CMAS samples were heated to

ur

1300 ◦C in an electric furnace (BLMT-1700 ◦C) under air circumstance with a temperature increasing rate of 6°C/min. and, kept at 1300 °C for 5, 25 and 45 h, respectively.

Jo

Phase composition were confirmed by X-ray diffraction (XRD: D/MAX-2500, Rigaku,

Japan, CuKα). The polished cross section microstructures of the CMAS corroded samples were examined by a scanning electron microscopy (SEM, Gemini 300, Carl Zeiss, Germany) with an energy-dispersive spectroscopy (EDS) systems. (Inca, oxford Instruments, X-Max50, UK). The Ca, Si, Lu and P maps were collected and used to evaluate the reaction between 6

CMAS and LuPO4 pellets. 3．Results and discussion 3.1. Thermal properties and phase stability of LuPO4. Fig. 1(a) shows the XRD patterns of the LuPO4 powders and pellet. Both peaks consist well with the standard diffraction patterns of LuPO4, and suggest that the prepared powders and pellets were pure phase. By comparison, it could be found that the LuPO4 pellet has

ro of

stronger diffraction peaks compared to those of LuPO4 powders, indicating that sintering at 1550 °C for 4 h leads to a high degree of crystallization. In general, phase transformations accompanied with the changes in volume and density could play key roles in the formation of

-p

cracks in coating materials. In the present study, a TG-DSC analysis was employed to

re

investigate the phase transformation behavior of the LuPO4 powders during the heating from room temperature (RT) to 1300°C. As shown in Fig. 1 (b), there are no sharp endothermic or

lP

exothermic peak appearing in DSC curves and no sudden mass changes occur in the TG curve

na

(＜1%), suggesting that the LuPO4 powders have superior high temperature stability. The temperature dependence of the thermal diffusivities and thermal conductivities of

ur

LuPO4 ceramics are shown in Fig. 2. From Fig. 2 (a), it can be seen that the heat capacity curves are smooth without any discontinuity, implying there is no phase transformation in the

Jo

heating of the LuPO4 sample up to 1200 °C. The thermal diffusivity decreased monotonously with temperature from RT up to 1200 °C, suggesting that the heat transfer is predominantly by phonon thermal conduction during the tested temperature range. The thermal conductivity of LuPO4 is shown in Fig. 2 (b). This thermal conductivity decreases with increasing temperature from RT to 1200°C, and the values locate in the ranges 7

of 3.82~1.86 W m–1K–1. The high temperature value of 1.86 W m–1K–1 is lower than those of some promising RE-silicate EBC materials, such as Lu2SiO5 [27]. The low thermal conductivity of LuPO4 is favorable in reducing the surface temperature of substrate. Fig.3 shows the time dependence of CTE for LuPO4 ceramics from 100 to 1300 °C. The linear CTE vary from 4.5 to 6.6 10-6/K in this temperature range. This value is well consistent with that of SiC-CMCs substrates (～4.5 to 5.5×10-6·K-1 ) [1,5]. The suitable CTE of LuPO4

the ceramic layers and CMCs substrates. 3.2. The chemical reaction between LuPO4 and CMAS

ro of

make it beneficial in reducing the residual stresses caused by the mismatch of CTE between

-p

Fig. 4 shows the XRD patterns of the surface layer of LuPO4 pellet with CMAS deposits

re

after the heat treatment at 1300 ◦C for 5, 25 and 45 h, respectively. Diffraction peaks indexed to Ca8MgLu (PO4)7, CaAl2Si2O8, CaMgSi2O6，Lu2Si2O7 and Ca2Lu8 (SiO4)6O2 phases can be

lP

observed in the XRD patterns. The presence of Lu2Si2O7, Ca2Lu8 (SiO4)6O2 and Ca8MgLu

na

(PO4)7 phases indicate that a reaction between LuPO4 and CMAS occurs with the formation of these crystals on the surface. It is observed that the expand of corrosion duration from 5 to

ur

45 h cause an increasing diffraction peaks for Ca2Lu8 (SiO4)6O2 and Ca8MgLu (PO4)7 phase and a first increasing and then decreasing peaks for Lu2Si2O7 phase. Detailed discussions on

Jo

the phase evolution will be presented in following sections. Fig. 5A presents the back scattering cross-section images of LuPO4 after CMAS

corrosion at 1300 °C for 5h. An approximate 320μm residual CMAS layer remained on the top of LuPO4. It is found that a lot of acicular compounds (Region 1) interspersed with a few small block-shaped phase (Region 2) are randomly distributed on this layer. EDS results 8

suggests that Region 1 mainly consist of Ca, Al, Si, and O elements, while the small block-shaped phase primarily contains of Ca, Si, Mg and O elements. In conjunction with XRD results, it could be probably identified that the acicular compounds and the small block-shaped phase are CaAl2Si2O8 and CaMgSi2O6, respectively. Between the CMAS glass layer and LuPO4 pellet, a ～7μm thick reaction layer with holes and gaps could be observed, and beneath the layer, LuPO4 bulk keeps structure integrity.

ro of

EDS analysis shows that the LuPO4 pellet (region 5) contains P, Lu and O elements. A high-magnification of the dashed box in the reaction layer is depicted in Fig. 5B, in which the white block compounds (region 3) and gray contrast phase (region4) constitutes the reaction

-p

layer. Table 1 presents the chemical compositions of regions 3-5 in Figure 5B.

In referring

re

to the EDS results, the white block compounds are composed of Lu, Si and O elements, which is confirmed to be Lu2Si2O7 in reference with the XRD result. According to the EDS and

lP

XRD results, the gray contrast phase (region4) formed in the reaction layer could be

na

determined to be Ca8MgLu (PO4)7 phase.

EDS mapping analysis shows that Lu and P present in the CMAS layer, suggesting the

ur

dissolution of LuPO4 in molten CMAS. The significant difference in Ca and Si content on both sides of reaction layer indicate that CMAS infiltration has been arrested by the reaction

Jo

layer to a certain extent. However, Ca2Lu8 (SiO4)6O2 does not observed in the CMAS layer, demonstrating that the concentration of LuO1.5 in molten CAMS is lower than the nucleation concentration of Ca2Lu8 (SiO4)6O2 after 5 h corrosion. Fig. 6 displays the back scattering cross-section images and corresponding EDS elemental maps of LuPO4 reaction with CMAS at 1300 °C for 25 h. Fig.6 B and C show 9

higher magnification of the dashed box in Figure 6A, and the corresponding chemical compositions of regions 1-7 are listed in Table 2. Similar to the result of the corrosion for 5 h, there is still remained a certain amount of CMAS on the LuPO4 surface. But, the residual CMAS layer is thinner (～260 m) than the 5h corrosion sample, indicating that some CMAS have been consumed with the prolongation of corrosion time. At the bottom of this layer, one can identified a locally scattered small amount of acicular CaAl2Si2O8 anorthite (region 5).

ro of

An obvious reaction layer can be observed at the CMAS/LuPO4 interface. From Fig.6 C, one can see that the reaction layer predominantly composed of Lu2Si2O7 phase (region 3) and Ca8MgLu (PO4)7 phase (region 4 and 6). Compared to 5 h corrosion samples, more and larger

-p

Lu2Si2O7 phase formed on the upper edge of the reaction layer. In the meantime, more gray

re

contrast phase formed on the LuPO4 surface. As a consequence, the reaction layer become thicker and more continuous, and the average thickness has increased to～18μm. This

lP

demonstrates that LuPO4 react with the CMAS melt with the increasing of corrosion time.

na

It should be noted that some dendrite-like phases (region 2) randomly distribute in the CMAS glass layer, which is different from 5 h corrosion sample. According to E DS results,

ur

the dendrite-like phase mainly contains Ca, Si, Lu, O and Al. Since the XRD pattern confirms the presence of Ca2Lu8 (SiO4)6O2, CaAl2Si2O8 and CaMgSi2O6 in the corrosion products, thus,

Jo

the dendrite-like phase is presumably a combination of Ca2Lu8 (SiO4)6O2, CaAl2Si2O8 and CaMgSi2O6. Ca2Lu8 (SiO4)6O2 phase, not observable in 5 h corrosion samples, but appeared in the cross section of 25 h corrosion sample, i.e. the dendrite-like phase. This suggests that LuO1.5 is local saturated in residual CMAS of 25 h corrosion samples. Fig. 7 illustrates the back scattering cross-section images and corresponding Lu, Si, P and 10

Ca maps of LuPO4 interaction with CMAS at 1300 °C for 45 h. Fig. 7 B, C and D are higher-magnification SEM images of the dashed box in Fig. 7A. A CMAS layer with a thickness approximately 240 um can still be observed on the upper surface of LuPO4. The thickness is slightly thinner than that of 25 h corrosion samples. In the CMAS layer, obvious acicular compound can hardly be observed, even at the bottom of the CMAS layer. On the contrary, dendritic-like grains become more concentrated and coarser, almost covering the

ro of

entire residual CMAS layer.

At the CMAS/LuPO4 interface, a continuous reaction layer consisting of different contrast compounds, namely gray contrast compounds and dark-gray contrast area, can be

-p

clearly observed. Compared to 25 h corrosion samples, white block Lu2Si2O7 compound has

re

disappeared, replaced by continuous gray contrasted compounds phase (region 5), suggesting that Lu2Si2O7 has been dissolved in the molten CMAS. In the meantime, the dark-gray

lP

contrasted compounds becomes thicker and more continuous. From the EDS elemental maps

na

of the cross-sections, Ca and P is rich in the reaction layer. The chemical composition of the marked regions in the higher-magnification SEM images are reported in Table 3. Results from

ur

XRD/EDS revealed that the reaction layer (region 5 and 6) mainly consists of Ca8MgLu (PO4)7, which is well consistent with the EDS mapping analysis results. In addition, the

Jo

dendrite-like crystalline precipitates (region 2) are ascertained to be a mixture of Ca2Lu8 (SiO4)6O2, CaAl2Si2O8 and CaMgSi2O6. Residual CMAS (region 1, 3 and 4) with different content of Lu dissolutions are immobilized by the interlaced space formed among crystalline. Beneath the reaction layer, the LuPO4 substrate (region 7) still keeps structure integrity, even after 45 h of CMAS corrosion, exhibiting excellent CMAS resistance. 11

A comparison of cross-section SEM images in Fig.5A, 6A and 7A indicates that there is a time-dependent change of reaction layer thickness and residual CMAS thickness. The average thickness calculated from the cross-section images for 5, 25 and 45 h corrosion samples were depicted in Fig. 8. As shown, from 5 to 25 h, the thickness of CMAS layer decrees from～320 to～ 260μm, correspondingly, the thickness of reaction layer increases from ～7 to ～18μ m. On the contrary, it is found that the thickness of CMAS layer has changed very little in the

ro of

later 20 h of corrosion. However, the thickness of reaction layer increase significantly from ～18 to ～35μm. This demonstrates that the CMAS infiltration into LuPO4 can be inhibited by the dense continuous reaction layer and subsequent reaction between LuPO4 and

-p

CMAS melt caused by increasing of corrosion time occurs above the reaction layer.

re

3.3. CMAS Corrosion mechanism of LuPO4 ceramic

Referring to the results mentioned above, the formation of a dense and continuous

lP

reaction layer composed of high melting point Ca8MgLu (PO4)7 phase at the CMAS/LuPO4

na

interface play an important role in preventing molten CMAS infiltration. At the temperature higher than the melting point of CMAS (＞1235℃), CMAS glass on the LuPO4 surface will

ur

melt and react with LuPO4. Once the concentration of Lu and P reaches a supersaturate state, Ca8MgLu (PO4)7 and Lu-Si-O apatite phases can be crystallized and precipitated. As the

Jo

reaction proceeds, a large amount of Ca8MgLu (PO4)7 phases with high melting point are produced at the interface between CMAS and LuPO4 substrate. Accordingly, a dense and continuous reaction layer is generated, and the further infiltration of the molten CMAS could be blocked effectively. The dissolution of LuPO4 consumes CaO and MgO in the molten CMAS, leading to the reduction of CMAS and the precipitation of Ca8MgLu (PO4)7 and 12

Lu-Si-O apatite. It should be noted that the Lu: Ca mole ratio in Ca8MgLu (PO4)7 phase is 0.125, while the ratio in Ca2Lu8 (SiO4)6O2 crystals is 4. The significantly lower stoichiometric ratio of Lu: Ca in Ca8MgLu (PO4)7 than that in Ca2Lu8 (SiO4)6O2 phase means that less LuPO4 are needed to form the former phase. This is why Ca8MgLu (PO4)7 crystallization layer forms before the formation of Ca2Lu8 (SiO4)6O2 phase.

ro of

From the viewpoint of material balance, the dissolved LuPO4 will produce equal amounts of Lu and P. However, the formation of Ca8MgLu (PO4)7 consumes lots of Ca, P and a certain amounts of Mg and Lu. The Lu: P ratio in Ca8MgLu (PO4)7 is less than 1, therefore, there

-p

must be a lot of Si, Lu and Al remained in the molten CMAS above the Ca8MgLu (PO4)7 layer.

re

When the local content of Lu and Si in the CMAS glass accumulate to a certain value, the Lu2Si2O7 byproduct crystallizes and precipitates, as shown in Fig. 5 A and 6 A. On the other

lP

hand, the local excess Al2O3 could be dissolved, which benefits the composition of the molten

na

CMAS convert to an anorthite field for easy crystallization, and result in the formation of CaAl2Si2O8 and CaMgSi2O6 phase [28, 29]. Poerschke et al found that rare earth disilicate can

ur

react with molten CMAS, forming a Ca-RE-Si apatite [30]: 4 RE2Si2O7 +2 CaO (melt)→Ca2RE8 (SiO4)6O2+ 2 SiO2 (melt)

(5).

Jo

The above dissolution/precipitation process continues until the depletion of RE2Si2O7 or

CaO (melt), or the reaction reaches an equilibrium state. Similar phenomenon was also observed in the present study. For example, upon corrosion for 45 h, Lu2Si2O7 dissolved with the consumption of CaO (melt), and release equivalent SiO2 by the consumption of Lu2Si2O7. In this case, the volume of CMAS layer will not change, which seems to be inconsistent with 13

the current experimental results. In fact, part of LuO1.5 produced by dissolution of Lu2Si2O7 will react with molten CMAS to form Ca8MgLu (PO4)7 phase, resulting in significant increase for the thickness of reaction layer from 25 to 45 h (Fig .7A and 8). On the other hand, the dissolution of Lu2Si2O7 generate a large amounts of Ca2Lu8 (SiO4)6O2 phase, leading to the formation of more dendrite-like crystalline phase in the CMAS layer than the 25 h corrosion samples. It is worth noting that lower amount of Lu is needed to form the Ca8MgLu

ro of

(PO4)7 relative to the Ca2Lu8 (SiO4)6O2 (a 32-fold improvement based on Ca: RE), indicating that CMAS resistance of LuPO4 is significantly greater than Lu2Si2O7. 4. Conclusions

-p

In the present work, LuPO4 powders have been synthesized via a chemical precipitation

re

and calcination method. The LuPO4 ceramics exhibit superior phase stability in wide temperature range from RT to 1300 ℃. The thermal conductivity of LuPO4 at 1200°C is –1

, which is relatively lower than that of Lu2SiO5. The average linear CTE

lP

1.86W m –1K

na

between RT to 1300°C is 5.92×10-6K-1, well-matched to the SiC-CMCs substrate. At 1300 ℃, the molten CMAS interacts with LuPO4 to form Ca8MgLu (PO4)7, CaAl2Si2O8, CaMgSi2O6

ur

and Lu2Si2O7 at early stage. With increasing corrosion time Lu2Si2O7 is dissolved in molten CMAS, accompanied by the precipitation of dendritic-like Ca-Lu-Si apatite phase in the

Jo

CMAS layer and the Ca8MgLu (PO4)7 phase in reaction layers, respectively. As a result, the reaction layer consisting of the Ca8MgLu (PO4)7 phase become thicker and denser. In corrosion tests the reaction layer plays an important role in suppressing penetration of molten CMAS. Since less LuPO4 need to be dissolved to form a protective apatite barrier layer, LuPO4 display significantly greater CMAS corrosion resistance than Lu2Si2O7. LuPO4 14

with suitable thermal properties and good CMAS corrosion resistance could be a promising EBC material for the application in aero-engine.

Declaration of Interest Statement: The authors declare that they have no conflict of interest.

Acknowledgements

ro of

This work is financially supported by the National Natural Science Foundation of China (Nos. 51372009 and 51971009).

-p

Reference:

Jo

ur

na

lP

re

[1] N. P. Padture, Advanced structural ceramics in aerospace propulsion, Nat. Mater, 15 (2016) 804-809. [2] K. L. More, P. F. Tortorelli, M. K. Ferber, J. R. Keiser, Observations of Accelerated SiC Recession by Oxidation at High Water-Vapor Pressures, J. Am. Ceram. Soc, 83 (2010) 211-13. [3] E. J. Opila, Oxidation and Volatilization of Silica Formers in Water Vapor, J. Am. Ceram. Soc, 86(2010) 1238-1248. [4] K.N. Lee, Fox, S. Dennis, Bansal, P. Narottam, Rare earth silicate environmental barrier coatings for SiC/SiC composites and Si3N4 ceramics, J. Eur. Ceram. Soc, 25(2005)1705-1715. [5] X. Yue, X. Hu, F. Xu, K. Li, Rare earth silicate environmental barrier coatings: Present status and prospective, Ceram. Int, 43(2017) 5847-5855. [6] K.N. Lee, Current status of environmental barrier coatings for Si-based ceramics, Surf. Coat. Technol, 133(2000)1-7. [7] N. Al Nasiri, N. Patra, D. Horlait, D. D. Jayaseelan, W. E. Lee, Thermal Properties of Rare-Earth Monosilicates for EBC on Si-Based CeramicsComposites, J. Am. Ceram. Soc, 99(2016)589-596. [8] A. J. Fernández-Carrión, M. Allix, A. I. Becerro, Thermal Expansion of Rare‐Earth Pyrosilicates, J. Am. Ceram. Soc, 96(2013) 2298-2305. [9] J. Felsche, The crystal chemistry of the rare-earth silicates, Struct. Bond, 13(1973). 99-197. [10] S. Ueno, T. Ohji, H. T. Lin, Recession behavior of Yb2Si2O7 phase under high speed steam jet at high temperatures, Corros. Sci, 50(2008)178-182. [11] S. Ueno, D. D. Jayaseelan, T. Ohji, Water vapor corrosion behavior of lutetium silicates at high temperature, Ceram. Int, 32(2006) 451-455. 15

Jo

ur

na

lP

re

-p

ro of

[12] S. Ueno, D. D. Jayaseelan, T. Ohji, H. T. Lin, Recession mechanism of Lu-Si-O phase in high speed steam jet environment at high temperatures, Ceram. Int, vol. 32(2006) 775-778. [13] B. T. Richards, M. R. Begley, H. N. G. Wadley, Mechanisms of Ytterbium Monosilicate/Mullite/Silicon Coating Failure During Thermal Cycling in Water Vapor, J. Am. Ceram. Soc, 98(2016) 4066-4075. [14] X. Hu, F. Xu, K. Li, G. Jin, X. Yue, X. Zhao, Water vapor corrosion behavior and failure mechanism of plasma sprayed mullite/Lu2Si2O7-Lu2SiO5 coatings, Ceram. Int, 44(2018) 14177-14185. [15] Z. Tao et al., Influence of phase composition on thermal aging behavior of plasma sprayed ytterbium silicate coatings, Ceram. Int, 44(2018) 17359-17368. [16] V. L. Wiesner, U. K. Vempati, N. P. Bansal, High temperature viscosity of calcium-magnesium-aluminosilicate glass from synthetic sand, Scr. Mater,124(2016)189-192. [17] F. Stolzenburg, P. Kenesei, J. Almer, K. N. Lee, M. T. Johnson, and K. T. Faber, The influence of calcium–magnesium–aluminosilicate deposits on internal stresses in Yb2Si2O7 multilayer environmental barrier coatings, Acta Mater, 105(2016) 189-198. [18] D. L. Poerschke, J. H. Shaw, N. Verma, F. W. Zok, C. G. Levi, Interaction of yttrium disilicate environmental barrier coatings with calcium-magnesium-iron alumino-silicate melts, Acta Mater, 145(2018)451-461. [19] V. L. Wiesner, B. J. Harder, N. P. Bansal, High-Temperature Interactions of Desert Sand CMAS Glass with Yttrium Disilicate Environmental Barrier Coating Material, Ceram. Int, 44(2018)22738-22743. [20] F. Jiang, L. Cheng, Y. Wang, Hot corrosion of RE2SiO5 with different cation substitution under calcium–magnesium–aluminosilicate attack, Ceram. Int, 43. (2017) 9019-9023. [21] Y. Wang, X. Chen, L. Wen, L. Cheng, L. Zhang, Exploration of YPO4 as a potential environmental barrier coating, Ceram. Int, 36(2010)755-759. [22] J. Leitner, P. Chuchvalec, D. Sedmidubský, A. Strejc, and P. Abrman, Estimation of heat capacities of solid mixed oxides, Thermochim. Acta, 395, (2002) 27-46. [23] Schlichting, W. K., Padture, P. N., Klemens, G. P., Thermal conductivity of dense and porous yttria-stabilized zirconia, J. Mater. Sci, 36(2001)3003-3010. [24] M. Bulletin, Environmental degradation of thermal-barrier coatings by molten deposits-Related Articles, Mrs Bull, 37(2012)932-941. [25] S. K, J. Yang, C. G. L, Infiltration-Inhibiting Reaction of Gadolinium Zirconate Thermal Barrier Coatings with CMAS Melts, J. Am. Ceram. Soc, 91(2010)576-583. [26] J. Liu, L. Zhang, Q. Liu, L. Cheng,Y. Wang, Calcium–magnesium–aluminosilicate corrosion behaviors of rare-earth disilicates at 1400°C, J. Eur. Ceram. Soc, 33(2013)3419-3428. [27] Y. Li, Y. Luo, Z. Tian, J. Wang, J. Wang, Theoretical exploration of the abnormal trend in lattice thermal conductivity for monosilicates RE2SiO5 (RE = Dy, Ho, Er, Tm, Yb and Lu), J. Eur. Ceram. Soc, 38(2018) 3539-3546. [28] D. U. Tulyaganov, Phase Equilibrium in the Fluorapatite–Anorthite–Diopside System, J. Am. Ceram. Soc, 83(2010)3141-3146. [29] B. S. Senturk, H. F. Garces, A. L. Ortiz, G. Dwivedi, S. Sampath, N. P. Padture, 16

Jo

ur

na

lP

re

-p

ro of

“CMAS-Resistant Plasma Sprayed Thermal Barrier Coatings Based on Y2O3-Stabilized ZrO2 with Al3+ and Ti4+ Solute Additions, J. Therm. Spray Technol, 23, (2014)708-715. [30] D. L. Poerschke, R. W. Jackson, C. G. Levi, Silicate Deposit Degradation of Engineered Coatings in Gas Turbines: Progress Toward Models and Materials Solutions, Annu. Rev. Mater. Res, 47(2017) 297-330.

17

Figure captions Fig. 1. （a）XRD patterns for the LuPO4 powders and the pellet and (b) The TG-DSC curves of LuPO4 powders calcined from RT to 1300 °C with a temperature increasing rate of 10 °C/min at N2 atmosphere. Fig. 2. (a) Theoretical thermal diffusivity of LuPO4 ceramics as a function of temperature.（b） Thermal conductivities of LuPO4 ceramics compared to the data of Lu2SiO5. Fig. 3. Line expansion rate of LuPO4 ceramics from 100°C to 1300°C. Fig. 4. XRD patterns of LuPO4 reacts with molten CMAS at 1300 ℃ for 5, 25 and 45 h.

ro of

Fig. 5. SEM back-scattered electron cross-section images and corresponding EDS elemental maps of LuPO4 interaction with molten CMAS at 1300 °C for 5 h: (A) low and (B) A high

-p

magnification images of dashed boxes in A.

Fig. 6. SEM back-scattered electron cross-section images and corresponding EDS elemental

re

maps of LuPO4 reaction with CMAS for 25 h at 1300 °C: (A) low and (B) (C) high

lP

magnification views of the corresponding dashed boxes in A.

Fig. 7. SEM back-scattered electron cross-section images and corresponding Lu, Si, P and Ca

na

maps of LuPO4 after 45 h of CMAS corrosion at 1300 °C: (A) low and (B) (C) (D) high magnification images of the corresponding dashed boxes in A.

ur

Fig. 8. The thickness of reaction layer and residual CMAS layer of LuPO4 after CMAS

Jo

corrosion at 1300 ℃ for 5, 25 and 45 h.

18

ro of

-p

re

lP

na

ur

Jo Fig. 1

19

ro of

-p

re

lP

na

ur

Jo Fig. 2

20

ro of

-p

re

lP

na

ur

Jo Fig. 3.

21

ro of

-p

re

lP

na

ur

Jo Fig. 4

22

ro of

-p

re

lP

na

ur

Jo Fig. 5

23

ro of

-p

re

lP

na

ur

Jo Fig. 6

24

ro of

-p

re

lP

na

ur

Jo Fig. 7

25

ro of

-p

re

lP

na

ur

Jo Fig. 8

26

Table captions Table 1. Chemical compositions (at. %) from the marked regions shown in Fig. 5. Table 2. Chemical compositions (at. %) from the marked regions indicated in Fig. 6. Table 3. Chemical compositions (at. %) from the marked regions illustrated in Fig. 7

Table 1. Chemical compositions (at. %) from the marked regions shown in Fig. 5.

Mg

Al

Si

P

1

12.77

0.67

21.11

30.32

-

2

16.84

10.26

2.19

33.56

-

3

2.47

0.00

1.66

38.34

4

34.43

2.46

0.18

-

-

O

-

35.13

-

37.14

0.00

31.03

26.50

19.31

2.46

41.15

38.09

43.58

re

-

-

Jo

ur

na

lP

5

Lu

27

ro of

Ca

-p

Region

17.75

ro of

Ca

Mg

Al

Si

1

12.36

4.26

9.03

2

25.04

2.97

2.91

3

-

-

4

33.37

2.77

5

10.22

1.65

6

32.92 -

Lu

O

35.21

-

2.46

36.68

20.73

-

8.90

39.45

-

30.43

32.19

-

0.38

17.02

4.09

42.37

na

lP

37.38

17.50

36.32

-

0.31

33.99

2.72

-

1.00

18.66

4.25

40.45

-

-

-

18.31

38.09

43.58

ur

-

Jo

7

P

re

Region

-p

Table 2. Chemical compositions (at. %) from the marked regions indicated in Fig. 6.

28

ro of -p re lP

Table 3. Chemical compositions (at. %) from the marked regions illustrated in Fig. 7

Ca

1

11.18

4.90

2

14.23 8.00

Si

P

Lu

O

8.00

30.89

-

3.38

41.66

3.28

5.23

24.81

-

6.15

46.30

4.07

8.95

32.01

-

1.82

45.14

ur

Jo

3

Mg

Al

na

Region

4

8.96

4.09

8.26

31.83

-

2.34

44.51

5

27.70

2.20

-

2.69

15.41

4.69

47.31

6

27.58

2.44

-

-

19.87

2.63

47.78

7

-

-

-

-

18.60

32.02

49.38

29

30

ro of

-p

re

lP

na

ur

Jo