3.1 Phase composition analysis
The phase composition of powders and corresponding coatings (Al2O3-YAG and Al2O3-Y2O3) were displayed in Fig. 1(a) and (b). As illustrated in Fig. 1(a), a big bulge appeared in the Al2O3-YAG coating, indicated that there was the amorphous phase existed and the content of amorphous phase was about 80%. The crystallization diffraction peaks of α-Al2O3 (rhombohedral) and YAG (cubic) also existed in the Al2O3-YAG coating, which originated from un-melted powders or the recrystallization of the droplets. Notably, there were no γ-Al2O3 (cubic) in as-sprayed Al2O3-YAG coating, while the α-Al2O3 had undergone phase transformation to γ-Al2O3 in Al2O3-Y2O3 coating. It is widely accepted that the lower critical nucleation energy between liquid and solid of γ-Al2O3 than that of α-Al2O3 is the main reason why γ-Al2O3 is preferential forming in the coating[34,35]. However, there had no γ-Al2O3 in Fig. 1(a), so it speculated that the crystallization diffraction peaks of α-Al2O3 might be from the un-melted powders, and the component of α-Al2O3 in the droplets not crystallized or crystallized very little, so the crystallization diffraction peaks of YAG might be also from the un-melted powders. Beyond that, the c-Y2O3 (cubic) also transformed to m-Y2O3 (monoclinic) in Al2O3-Y2O3 coating. Be similar to γ-Al2O3, the m-Y2O3 was also formed. Noticeably, the crystallization diffraction peaks of Al2O3-Y2O3 coating in Fig. 1(b) were broadened, which meant some nanocrystals existed in the coating. The compounds of Al2Y4O9 (YAM)、AlYO3 (YAP) and Al5Y3O12 (YAG) were not found in Fig. 1(b), which indicated that micro-sized α-Al2O3 and c-Y2O3 powders hardly reacted during deposition, so the deposition process of Al2O3/Y2O3 powders in plasma spray was mainly belong to physical changes. Moreover, the existence of α-Al2O3 and c-Y2O3 indicated that there might also had un-melted powders in Al2O3-Y2O3 coating.
3.2 Micro-morphologies of as-sprayed coatings
Fig. 2 showed the micro-morphologies of as-sprayed Al2O3-YAG amorphous coating. The coating was deposited on the 1Cr18Ni9Ti substrate and the thickness was about 300μm. The characteristics of most droplet spreading were obviously observed in the surface morphologies of the coating. Beyond that, there were few particles in the yellow circle in Fig. 2(b), which might be caused by the impact crushing of un-melted granulate powders. Some un-melted areas appeared in the cross-sectional morphologies, which may be explained that the part of powders was insufficiently heated during deposition. Based on this result, the crystallization diffraction peaks of α-Al2O3 and YAG that existed in Fig. 1(a) were supposed to be from un-melted powders. Meanwhile, a few micro-cracks appeared in the cross-sectional morphologies of Al2O3-YAG coating, which may result from the volume shrinkage due to the rapid cooling of the droplets during deposition. Moreover, there had some bright or dark tiny stripes distributed in the cross-sectional morphologies. The result of EDS showed that the bright stripes contained higher Y element content, which denoted the YAG crystalline phase. Similar stripes features have been found in the publicly reported about some amorphous coatings, but there had no crystal diffraction peaks in the XRD pattern of these coatings[36,37]. Hence, these stripes in Fig. 2(d) might be the melted elements in amorphous phase were not uniformly distributed or nanocrystals from recrystallization.
The morphologies of as-sprayed Al2O3-Y2O3 coating were displayed in Fig. 3. According to the principle of backscattered electron imaging, Al2O3 and Y2O3 can be easily distinguished. In the surface morphologies of Al2O3-Y2O3 coating, the distribution of Y2O3 was not uniformly. Moreover, the coating cross-sectional morphologies also had bright and dark stripes, and the bright stripe was Y2O3 while the dark one was Al2O3. Comparing to the Al2O3-YAG coating, the difference was that there were more stripes in the Al2O3-Y2O3 coating and the stripes size was larger, which caused by large particle size of powder. Furthermore, the characteristics of the un-melted region did not be clearly observed in the SEM image of the coating although the powder was presumed not completely fused due to the reservation of α-Al2O3 and c-Y2O3 in the coating.
3.3 The phase distribution of as-sprayed coating
Fig. 4 demonstrated EBSD images of Al2O3-YAG amorphous ceramic coating. Most of the area in the image was black due to the 100nm resolution of EBSD, which indicated the amorphous phase and nanocrystals (no more than 100nm) were not collected. Notably, the bright and dark tiny stripes from Al2O3-YAG amorphous coating in Fig. 2(d) were also not found, which also proved that the tiny stripes may be from the melted elements non-uniformly distributed in amorphous phase or nanocrystals from recrystallization. There only had α-Al2O3 and YAG in the un-melted area and not found γ-Al2O3, so the speculation proposed in section 3.1 that the component of α-Al2O3 not crystallized or crystallized very little was right.
The EBSD images of as-sprayed Al2O3-Y2O3 coating were revealed in Fig. 5. The results of phase detection showed that there were m-Y2O3 and γ-Al2O3 beside the initial phase of α-Al2O3 and c-Y2O3. From Fig. 5(a), the deposition of the powder droplet was mainly in the form of lamellar stacking, and there were many voids between the lamellar structures. Moreover, it can be clearly seen that the edge of the voids was dominated by small grains, so the voids may be nano-crystals or amorphousness. After being sprayed, the amount of α-Al2O3 phase in Al2O3-Y2O3 coating decreased a lot, and most of α-Al2O3 transformed to γ-Al2O3, so the component of α-Al2O3 phase in droplets might mainly be formed to γ-Al2O3. While the residual α-Al2O3 phase in the coating might be dominated from un-melted powders. Of course, the α-Al2O3 phase can also be reserved by increasing the interface temperature between liquid and solid[38,39]. By contrast, there were not so much c-Y2O3 transformed to m-Y2O3, but the m-Y2O3 was dominated by small grains and also mainly appeared in the edge of the voids.
In the analysis of Fig. 1(b), the YAM、YAP and YAG were not found, so the deposition process of Al2O3/Y2O3 powders in plasma spray speculated might belong to physical changes. This phenomenon was mainly attributed to the fact that the mechanically uniformly mixed α-Al2O3 and c-Y2O3 powders were not uniformly distributed in nano-scale or sub-microscale. The distance of different phases between the powder droplets was large so that the internal ions cannot effectively diffused and reacted with each other. Actually, the effectively diffusion and reaction of ions only occurred at the interface of different phase droplets, but the reaction was incomplete due to the insufficient reaction time during deposition and improper size ratio of powder[40]. The crystallization process of droplets was not easy at the edge of the lamellar structures from Fig. 5. The reason was that the micro-powders of Al2O3 and Y2O3 reacted, but the new phases (YAM, YAP or YAG) formed by reaction maybe not easy to crystallize. It was verified that the YAG coating was prepared by plasma spraying was dominated by amorphousness from XRD pattern[41,42]. Thusly, the crystallization resistance of droplets increased as the more sufficient mutual diffusion of Al2O3 and Y2O3 at the nano-scale or sub-microscale. From this point of view, the Al2O3-YAG amorphous ceramic coating can be successfully deposited was ascribed to the fact that the particle size of the feedstock powder was nanometer or sub-micrometer and mixed more uniformly. Therefore, the crystallization resistance of Al2O3/YAG powders droplets was large during deposition. However, the phenomenon that the powder droplets with different phases distributed uniformly were difficult to crystallize was not be explained clearly. The possible reason may be related to the chemical behavior of crystallization. Beyond that, the composition ratio of powders and particle size can be also considered as the key factors for the preparation of amorphous coatings.
3.4 Crystallization chemical process in powder droplets solidification
From the result of XRD and EBSD, the α-Al2O3 was heated and turned into droplet, the droplet transformed as γ-Al2O3 in recrystallization. Many literatures have explained this phenomenon from the viewpoint of nucleation energy, but few reports have considered it from the perspective of crystallization chemistry of melt. Usually, low coordination number means low probability of between ions meeting in the melt during crystallization [43]. Fig. 6 displayed the schematic diagram of the crystal structure of the possible existed phases in this study. The first row in Fig. 6 is the initial phases crystal structure of the powders used in this study. The second row is new -generated phases crystal structure in the coating. The second row is the possible mesophases crystal structure from the chemical reaction of Al2O3 and Y2O3. Table 2 listed the coordination number of Al element or Y element in above-mention crystals. The coordination number of Al element in α-Al2O3 is 3 and that in γ-Al2O3 is 4, so the reason that α-Al2O3 droplets transformed to γ-Al2O3 is that because Al element has higher coordination number and had more chance to form lattice in γ-Al2O3, so the γ-Al2O3 preferentially crystallized. The coordination number of Y element for c-Y2O3 and m-Y2O3 is 6, so the droplets of c-Y2O3 transformed to m-Y2O3 or recrystallized to c-Y2O3. Notably, the density of m-Y2O3 was less than c-Y2O3 in table 2, which meant m-Y2O3 has less stacking density than c-Y2O3, so the m-Y2O3 is probably easier to form. Specially, the coordination number of Al and Y element in YAG was really high, but the Al2O3/YAG powders via plasma spray formed to amorphousness. The coordination number of Al and Y element was also high in YAM and YAP crystals. And the strip-like voids in Fig. 5 were that the new phase like YAM, YAP and YAG reacted by Al2O3 and Y2O3 were not easy to crystallize due to ultra-fast cooling of plasma spraying. Therefore, higher coordination numbers may be more preferable to form amorphousness in a particular situation, which seems conflict to the viewpoint that low coordination number denotes low probability between ions meeting during crystallization.
Table 2
The relevant crystallographic data from some compounds in this study
Compound
|
Crystal system
|
Space group
|
Density, g/cm3
|
Coordination number
|
Ref
|
Al
|
Y
|
α-Al2O3
|
Rhombohedral
|
R-3c
|
4.05
|
3
|
-
|
[44]
|
γ-Al2O3
|
Cubic
|
Fd-3m
|
3.67
|
4
|
-
|
[45]
|
c-Y2O3
|
Cubic
|
Ia-3d
|
5.03
|
-
|
6
|
[46]
|
m-Y2O3
|
Monoclinic
|
C 2/m
|
4.98
|
-
|
6
|
[47]
|
YAG
|
Cubic
|
Ia-3d
|
4.12
|
4, 6
|
8
|
[48]
|
YAP
|
Orthorhombic
|
Pbnm
|
5.35
|
6
|
6
|
[49]
|
YAM
|
Monoclinic
|
P1 21/c1
|
4.52
|
4
|
6,7
|
[50]
|
According to other research[51,52], the phase diagram of the Al2O3 -Y2O3 system was drawn in Fig. 7. The Al2O3 -Y2O3 system has four eutectic point. And when the melt simultaneously crystallizes to YAG and Al2O3, this eutectic point has the lowest melting points (1820℃). It is worth noting that the melting point of melt decreases as the melt contains both Al2O3 and Y2O3, so the crystallization chemical behavior of different phases will hinder each other's crystallization process can be proved. Fig. 8 illustrated the schematic diagram of possible bonding modes of Al and Y with O in droplet. Although the high coordination number ion/atom means high probability of between ions meeting to form ion groups, low coordination number ion/atom also has chance to bond. The crystal structures formed by different bonding methods are not always stable at a given temperature, and those crystals are easy to transform to the most thermodynamically stable structure at that temperature. For example, γ-Al2O3 are preferential crystallization from high temperature to room temperature in APS deposition, but it conflicts to the fact that γ-Al2O3 crystal is not stable in high temperature (≥1000℃) and can transform to α-Al2O3 at high temperature. One possible explanation is that γ-Al2O3 crystal structure can be formed in high temperature, and it is unstable and need time to transform as the more stable structure like α-Al2O3, but it can be reserved due to the insufficient time during the APS deposition. Therefore, the crystal with thermodynamically stable structure is usually not easy to form due to the fact that the formation of different ion groups also need time to transform to the thermodynamically stable structure, so the crystallization process was hindered.
3.5 The formation mechanism of amorphous phase in Al2O3-YAG amorphous ceramic coating
Fig. 9 displayed the formation mechanism of amorphousness in plasma-sprayed Al2O3-YAG amorphous ceramic coating. The amorphous phase formation process of Al2O3-YAG coating can be summarized as follows: the nano-size or sub-micro size scale uniformly distributed multivariate powders with eutectic molar ratio were rapidly heated and fully melted to form a high temperature melt in the plasma plume, then the molten droplets impacted the surface of the substrate/coating at a high speed and quenched, resulted in a steep temperature gradient between the melt and the deposition interface. The melt possessed priority to form ion/atom groups with high coordination number of cations. Meanwhile, the low coordination number of cations in ion/atom groups also formed. Different ion/atom groups can interfere with each other, which hindered the nucleation and growth of crystals, so that the amorphization can be realized due to the fact that droplets were not structurally regulated within a limited time in ultra-fast cooling process of plasma spraying.
Accordingly, there are three requirements for the formation of Al2O3-YAG amorphous ceramic coating: ① A heat source with a high enough temperature can heat the crystal materials to be melt that the internal atoms/ions tend to be disordered. ② Ultra-fast cooling rate which makes the internal atoms/ions of the melt have insufficient time to diffuse into the lattice of crystal and crystallize. ③ The as-sprayable powder should chose multivariate powders with low eutectic point ratio distributed uniformly at nano-scale or sub-micro scale and can be reacted to form the new phase crystal with high coordination numbers of cations.