3.1. Microstructure of nano Yb2SiO5 feedstocks and micron Yb2SiO5 feedstocks.
Figure 2(a) shows the surface morphology of the n-YbMS feedstocks, and Figure 2(d) shows the surface morphology of the m-YbMS feedstocks. The feedstocks are spherical and smooth, Fig. 1 also shows the flowability of the powders. To observe the internal structure of the feedstocks, the fracture morphology is shown in Figs. 3(a) and 3(b). The microstructure of the n-YbMS feedstocks is denser than that of the m-YbMS feedstocks. Spherical and dense feedstocks with a smooth surface can enhance the flowability of feedstocks and the deposition rate [26, 27]. Moreover, dense feedstocks produce dense coatings with improved wear resistance [26, 28]. On the basis of the heredity of the material tissue, nanostructured feedstocks are denser than conventional feedstocks. Therefore, the coating of the n-YbMS feedstocks is tighter and has better abrasion resistance than that of the m-YbMS feedstocks. Fig. 1(c) is a TEM image of the n-YbMS feedstocks. The grain size is approximately 30–50 nm, showing that the n-YbMS feedstocks are composed of nanostructured agglomerates.
Figure 4 shows the XRD patterns of the n-YbMS and m-YbMS powders. It can be observed that the phase composition is Yb2SiO5 (compared with the X-ray diffraction data of Yb2SiO5 (JCPDS card PDF#40-0386) whether it is n-YbMS powder or m-YbMS powder.
3.2. Microstructure of nanostructured and conventional Yb2SiO5 coatings
The surface morphologies of the nanostructured and conventional Yb2SiO5 coatings are shown in Fig. 5. Some voids and cracks can be observed. The nanostructured Yb2SiO5 coatings have voids and fine cracks,as shown in Fig. 5(a). However, the conventional Yb2SiO5 coatings have more voids and coarse cracks (Fig. 5(b)) than the nanostructured Yb2SiO5 coatings. Therefore, the distribution of cracks in the nanostructured Yb2SiO5 coatings is scattered and narrow, whereas that in the conventional Yb2SiO5 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 Yb2SiO5 coatings are fewer and finer than those in the conventional Yb2SiO5 coatings. However, the conventional Yb2SiO5 coatings have more and coarser cracks, including horizontal and vertical cracks, than the nanostructured Yb2SiO5 coatings. These results are consistent with the analysis illustrated in Fig. 5(a) and 5(b). Fig. 6(c) and 6(d) show the Yb2SiO5 topcoats of the partially enlarged detail of the nanostructured and conventional Yb2SiO5 coatings, respectively. Fig. 6(e) is TEM images of nanostructured Yb2SiO5 coatings, it can be seen that there are nano particles in nanostructured Yb2SiO5 coatings.
The XRD patterns of the nanostructured and conventional Yb2SiO5 coatings are shown in Fig. 7. The analysis of the XRD pattern of the coatings reveals that some changes occurred during the spraying process. The phase composition is Yb2SiO5 and Yb2O3 (compared with the XRD data of Yb2SiO5 (JCPDS card PDF#40-0386, 2/a (15)) and Yb2O3 (JCPDS card PDF#65-3173, la-3 (206))). Fig. 7 shows that Yb2O3 crystal planes (222), (400), (411), (134), (422), (440), (622), (136), (444) and (142) are all reflected, indicating that the volatilization of silicon dioxide in plasma spraying leads to the presence of Yb2O3 in the coating. The strength of Yb2O3 crystal plane (222), (400), (440), (622), (136) and (444) in the nanostructured Yb2SiO5 coatings is stronger than that in the conventional Yb2SiO5 coatings, and the nanostructured Yb2SiO5 coatings also show Yb2O3 crystal planes (134) and (136). These findings can be attributed to the easy melting of the nano particles in the nanostructured Yb2SiO5 coatings during the APS preparation process, but the SiO2 volatilization is more serious than that in the conventional Yb2SiO5 coatings.
3.3 Mechanical properties of nanostructured and conventional Yb2SiO5 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 [25-27, 30-33]. Fig. 8 shows the typical load–displacement curves derived from the nano-indentation of nanostructured and conventional Yb2SiO5 coatings. According to the load–displacement curves in Fig. 8, the maximum penetration depth of the nanostructured Yb2SiO5 coatings is lower than that of the conventional Yb2SiO5 coatings (853.30 nm vs. 1185.47 nm), and the load-displacement curve distribution of the nanostructured Yb2SiO5 coatings is concentrated and uniform compared with that of the conventional Yb2SiO5 coatings. This finding indicates that the regions in the nanostructured Yb2SiO5 coatings can be considered as melted regions, and those in the conventional Yb2SiO5 coatings where the penetration depth reaches the maximum (1185.47 nm) can be considered as unmelted or partially melted regions. This result is consistent with the SEM images.
When the feature size of the material is at the nanometer level, the microstructure characteristics of the material will change significantly. 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 shows that the nanohardness (H) of the nanostructured Yb2SiO5 coatings is not very different from that of the conventional Yb2SiO5 coatings, but the elastic modulus (E) of the nanostructured Yb2SiO5 coatings is higher than that of the conventional Yb2SiO5 coatings (167.37077 ± 16.88070 GPa vs. 153.72856 ± 19.69907 GPa ), indicating that the nanostructure can increase the toughness of the coating [28]. Therefore, to further illustrate the influence of nanostructures on the coatings, statistical analysis should be performed on the measured elastic modulus (E) and nanohardness (H). Weibull distribution can further describe the difference in the microstructure and performance of different coatings[25, 34].
Table 2
The averages of nanohardness (H) and elastic modulus (E) of coatings.
Coatings
|
Er(GPa)
|
H(GPa)
|
nanostructured Yb2SiO5 coatings
|
167.37077 ± 16.88070
|
11.21317 ± 1.52426
|
conventional Yb2SiO5 coatings
|
153.72856 ± 19.69907
|
10.21402 ± 2.27309
|
The Weibull distribution can be expressed as following [25]:
where p is the cumulative probability density function; x is the value of elastic modulus (E) or nanohardness (H); x0 = 63.2%; m is the Weibull modulus; i = 1, 2, 3,…, 22; N = 22; m is the dispersion coefficient. Equation (1) is mathematically deformed to obtain Equation (3):
The value of m is calculated by mathematically fitting equation (3). Fig. 9 is a Weibull diagram of the elastic modulus and nanohardness of the cross-sections of the nanostructured and conventional Yb2SiO5 coatings.
As shown in Fig. 9, the nanostructured Yb2SiO5 coatings present a mono-modal distribution, indicating that the nanostructured Yb2SiO5 coatings are evenly distributed. During the spraying process, the nanostructure caused the particles to melt completely, so the nanostructured coatings presented a mono-modal distribution, whereas the conventional Yb2SiO5 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 coefficient. The area with a low m value is the unmelted area of the conventional Yb2SiO5 coatings, and the low m value reflects the large dispersion and fluctuation of the elastic modulus (E) and nanohardness (H) of the area, also indicating that the conventional Yb2SiO5 coatings have larger holes or wider cracks in the unmelted zone. In Fig. 9(a), m is a single value, indicating that the nanostructured Yb2SiO5 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 Yb2SiO5 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 influence the hardness properties [35]. Therefore, the nanohardness (H) of the nanostructure Yb2SiO5 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, 36-38]. In the previous analysis, the elastic modulus (E) of the nanostructured Yb2SiO5 coatings is higher than the conventional Yb2SiO5 coatings, indicating that nanostructured coatings have fewer pores than the conventional coatings. This result is consistent with the SEM images.
Table 3
Summary of results obtained from Fig. 9.
Set
|
nanostructured Yb2SiO5 coatings
|
conventional Yb2SiO5 coatings
|
mE1
|
11.17
|
14.35
|
R-Square
|
0.94
|
0.96
|
mE2
|
N.A.
|
0.49
|
R-Square
|
N.A.
|
0.55
|
mH1
|
8.50
|
10.32
|
R-Square
|
0.96
|
0.95
|
mH2
|
N.A.
|
2.89
|
R-Square
|
N.A.
|
0.87
|