Microstructure and Nanomechanical Property of Plasma-Sprayed Nanostructured Yb 2 SiO 5 Environmental Barrier Coatings

In this study, nanostructured Yb 2 SiO 5 coatings were prepared by atmospheric plasma spraying (APS) using nanostructured Yb 2 SiO 5 feedstocks. Conventional Yb 2 SiO 5 coatings were selected for comparison. The microstructure and nanomechanical property of the nanostructured and conventional Yb 2 SiO 5 coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and nanoindentation. Results indicate that the surface of the nanostructured Yb 2 SiO 5 coatings is uniform and denser than the conventional Yb 2 SiO 5 coatings. In addition, Weibull distribution analysis shows that the molten state of the nanostructured Yb 2 SiO 5 coatings present a mono-modal distribution, whereas the conventional Yb 2 SiO 5 coatings show a bi-modal distribution, i.e. molten and unmelted zones. The nanostructured Yb 2 SiO 5 coatings have a higher elastic modulus than the conventional Yb 2 SiO 5 coatings (167.37077 ± 16.88070 GPa versus 153.72856 ± 19.69907 GPa), reecting their high density.


Introduction
To improve the thermal e ciency, thrust-to-weight ratio and inlet temperature of a gas turbine, improvements on thermal barrier coatings have allowed the industry to increase the gas inlet temperatures up to 1500°C [1][2][3]. Nevertheless, 1500°C exceeds the melting point of Ni-based super-alloys, and new substrate materials should be developed. Non-oxide silicon-based ceramics, such as SiC and Si 3 N 4 , show potential applications in high-performance gas turbine engines due to their low density, superior fracture toughness, high strength and reliability in high-temperature environments [4]. However, SiC-based materials are susceptible to corrosion and performance degradation in high-temperature environments, resulting in disaster damage for gas turbine engine [5][6][7][8][9]. Therefore, environmental barrier coatings (EBCs) were developed to protect the SiC substrate and improve the durability of structural components in high-temperature environments [4,10,11].
To date, the research on environmental barrier coating systems has focused on Si bond coat, mullite coat and rare-earth silicate topcoat. Ideal EBC materials should have a high melting point, low thermal conductivity, low density, low oxygen diffusion, a favorable chemical stability, thermodynamic compatibility with the substrate and a coe cient of thermal expansion (CTE) near the substrate [12][13][14][15][16]. Rare-earth silicates present potential applications for EBC candidate materials owing to their good oxidation resistance, favorable chemical stability, high melting point and appropriate CTE at elevated temperature. Among these rare-earth silicates, Yb 2 SiO 5 and Yb 2 Si 2 O 7 are widely investigated as EBC topcoats. Most researchers investigating EBCs have utilised the atmospheric plasma spraying (APS) [17][18][19][20], electron beam physical vapor deposition (EB-PVD), and plasma spraying-physical vapor deposition (PS-PVD) methods to fabricate EBCs. APS has great advantages, such as simple preparation process and high productivity, and has become the main preparation method.
Recent studies demonstrated that Yb 2 SiO 5 is a promising EBC material candidate [4,10,17]. Yb 2 SiO 5 is a suitable EBC material as a protective layer because of its excellent high-temperature chemical, corrosion Page 3/12 resistance and low CTE. With the development of nano materials science and technology, nano materials have shown unique properties due to some unique effects and consequently become popular in the research and preparation of nanostructured materials [21]. Previous research has established the superior performance of nanostructured coatings over that of conventional structural coatings [22][23][24]. However, the nanomechanical characterization of nanostructured Yb 2 SiO 5 coatings has not been su ciently investigated yet.
In this paper, nanostructured and conventional Yb 2 SiO 5 coatings were fabricated by APS. The microstructure and phase composition of nanostructured and conventional Yb 2 SiO 5 coatings were analysed, and their micromechanical properties were investigated by nanoindentation and discussed in detail via tting the experimental data, including elastic modulus and nanohardness, with probability density function of Weibull distribution.

Characterization
The microstructure and phase composition of the n-YbMS and m-YbMS feedstocks and the corresponding as-sprayed coatings were characterised by scanning electron microscopy (SEM, JSM-7610FPLUS), transmission electron microscopy (TEM, FEI Tecnai G2 F20) and X-ray diffraction (XRD, Ultima IV, RIGAKU, Japan) using Cu Kα radiation with a scanning rate of 4 °/min at Bragger angle (2θ) between 10 ° and 80 °.

Nano-indentation test
The   Some voids and cracks can be observed. The nanostructured Yb 2 SiO 5 coatings have voids and ne cracks,as shown in Fig. 5(a). However, the conventional Yb 2 SiO 5 coatings have more voids and coarse cracks ( Fig. 5(b)) than the nanostructured Yb 2 SiO 5 coatings. Therefore, the distribution of cracks in the nanostructured Yb 2 SiO 5 coatings is scattered and narrow, whereas that in the conventional Yb 2 SiO 5 coatings is dense and wide. To observe the internal structure of the coatings, the fracture morphology is shown in Fig. 6(a). Fig. 6(a) shows that the cracks and voids in the nanostructured Yb 2 SiO 5 coatings are fewer and ner than those in the conventional Yb 2 SiO 5 coatings. However, the conventional Yb 2 SiO 5 coatings have more and coarser cracks, including horizontal and vertical cracks, than the nanostructured Yb 2 SiO 5 coatings. These results are consistent with the analysis illustrated in Fig. 5

Mechanical properties of nanostructured and conventional Yb 2 SiO 5 coatings.
Mechanical properties are vital to the evaluation of the durability and reliability of coatings [29].
Micromechanical properties, such as elastic modulus and nanohardness are among the important mechanical properties for coatings [25][26][27][30][31][32][33]. Fig. 8 shows the typical load-displacement curves derived from the nano-indentation of nanostructured and conventional Yb 2 SiO 5 coatings. According to the load-displacement curves in Fig. 8 When the feature size of the material is at the nanometer level, the microstructure characteristics of the material will change signi cantly. The size of the nano-featured structure of the material decreases, and the proportion of each interface (e.g., grain boundary, phase boundary, etc.) in the material increases, resulting in the change of the mechanical properties of the material. The elastic modulus (E) and nanohardness (H) of coatings can be obtained from each unloading curve, and the date is shown in Table 2. The analysis in Table 2   The value of m is calculated by mathematically tting equation (3). Fig. 9 is a Weibull diagram of the elastic modulus and nanohardness of the cross-sections of the nanostructured and conventional Yb 2 SiO 5 coatings.
As shown in Fig. 9, the nanostructured Yb 2 SiO 5 coatings present a mono-modal distribution, indicating that the nanostructured Yb 2 SiO 5 coatings are evenly distributed. During the spraying process, the nanostructure caused the particles to melt completely, so the nanostructured coatings presented a monomodal distribution, whereas the conventional Yb 2 SiO 5 coatings showed a bi-modal distribution, that is, the coating appeared both molten and unmelted states during the spraying process. The bi-modal distribution led to uneven coating distribution and increased defects. In Fig. 9(b), m represents the Weibull coe cient. The area with a low m value is the unmelted area of the conventional Yb 2 SiO 5 coatings, and the low m value re ects the large dispersion and uctuation of the elastic modulus (E) and nanohardness (H) of the area, also indicating that the conventional Yb 2 SiO 5 coatings have larger holes or wider cracks in the unmelted zone. In Fig. 9(a), m is a single value, indicating that the nanostructured Yb 2 SiO 5 coatings have a single distribution. The nanostructured feedstock have nano-scale grains, and the melting is highly uniform under the same spraying process. Therefore, the nanostructured Yb 2 SiO 5 coatings present a single melting area, and the single distribution makes the coating uniform and dense. The nanostructure feedstocks are composed of raw materials with a nanometre grain size, and the grain size does not in uence the hardness properties [35]. Therefore, the nanohardness (H) of the nanostructure Yb 2 SiO 5 coatings is not very different from that of the conventional coatings. However, the elastic modulus (E) is more sensitive to voids, and the fewer the voids are, the higher the elastic modulus (E) is [25,[36][37][38]. In the previous analysis, the elastic modulus (E) of the nanostructured Yb 2 SiO 5 coatings is higher than the conventional Yb 2 SiO 5 coatings, indicating that nanostructured coatings have fewer pores than the conventional coatings. This result is consistent with the SEM images.   SEM images of (a) n-YbMS powder, (b) partial enlarged detail of (a), (c) TEM images of n-YbMS, (d) m-YbMS powder, (e) partial enlarged detail of (d).   Surface morphologies SEM images of (a) nanostructured Yb2SiO5 coatings ,(b) conventional Yb2SiO5 coatings.

Figure 7
XRD patterns of nanostructured and conventional Yb2SiO5 coatings.

Figure 9
Weibull plots of elastic modulus and nanohardness on cross-section for (a) nanostructured Yb2SiO5 and (b) conventional Yb2SiO5 coatings.