Formation Mechanism of Al2O3-YAG Amorphous Ceramic Coating Deposited via Atmospheric Plasma Spraying

This study mainly investigated the formation mechanism of Al 2 O 3 -YAG(Al 5 Y 3 O 12 ) amorphous coating prepared by atmospheric plasma spraying. Nano and micro-sized powders with low eutectic point ratio were selected as raw materials for comparison. XRD, SEM and EBSD were used to analyze the phase composition, morphologies, phase distribution and structure of the coating. The crystal structures of the possible existed phases were studied to analyze the crystallization chemistry of powder droplets. It is concluded that the composition ratio of powders and particle size should be also considered as the key factors for the preparation of amorphous coatings besides the high enthalpy and ultra-fast cooling rate of thermal spray technology. The as-sprayable powder chose multiple components with low eutectic point ratio distributed uniformly at nano-scale or sub-micro scale, and can reacted to form the new phase crystal with high coordination numbers of cations.


Introduction
Amorphous materials have the characteristic of long-range disordered structure and short-range ordered clusters, and possess scienti c and engineering interest for decades [1,2]. As one of the numerous amorphous materials, inorganic amorphous coatings have been widely used in mechanical, electrical, magnetic, wear-resistant and anti-corrosion elds [3][4][5]. The common inorganic amorphous coatings are mainly amorphous alloys [6][7][8][9], such as Fe-based, Ni-based and Co-based amorphous alloys. However, in the early research of amorphous alloy, because the cooling speed of molten metal powder was very fast, the amorphous alloys were mostly formed in the shape of thin strip or lament, and their application scope was greatly limited due to the small size [2,5]. Therefore, the amorphous formation mechanism of amorphous alloys had been deeply studied to prepare the bulk amorphousness. And it is generally believed that as long as the cooling rate of the melt is fast enough, the melt of all the substances can form amorphousness [10,11]. Nevertheless, not every preparation method can achieve the su cient cooling rate in the actual preparation of amorphous alloys. According to the preparation results of multiple series of amorphous alloys, Inoue [12] proposed the three requirements for the fabrication of amorphous alloys: The alloy must be a multi-element system composed of more than three elements; The atomic size ratio of the main constituent elements in the alloy is greater than 12%; The three main constituent elements in the alloy have a negative mixing enthalpy. The summary of this law provides empirical guidance for the preparation of amorphous alloys. However, the glass transition temperature of amorphous alloys is generally not high (300 ~ 600℃)[8, [13][14][15][16], and the initial crystallization activation energy of them is also low (No more than 600 kJ/mol) [17][18][19][20]. Therefore, the stability of these amorphous alloys was poor under harsh service conditions, which changed the microstructure of the coating and further affected the performance of the coating.
Recently, some manufactured amorphous ceramic coatings have been deposited. Noticeably, these amorphous ceramic coatings have high crystallization temperature [21][22][23][24], which make them have excellent microstructure stability in a wide temperature range for the application under harsh services conditions. In our previous work, the Al 2 O 3 -YAG amorphous ceramic coating was deposited via atmospheric plasma spraying (APS) used the Al 2 O 3 /YAG powders. The glass transition temperature of the Al 2 O 3 -YAG amorphous coating was 905.5℃ at 5 ℃/min, and the initial crystallization activation energy of that was 807.8 kJ/mol by Kissinger method, so Al 2 O 3 -YAG amorphous ceramic coating had better high temperature microstructure stability than other 30 kinds of amorphous materials [21].
Moreover, this coating possessed greater crack propagation resistance and lower wear rate under severe wear conditions with high PV values (P: contact pressure; V: friction velocity) than conventional crystalline oxide ceramic coatings (such as Al 2 O 3 coating, Al 2 O 3 -Cr 2 O 3 coating) [25,26]. The crack propagation resistance was due to the existence of amorphous phase, which increased the crack growth tolerance of the coating and the ability of plastic deformation. Therefore, as-sprayed Al 2 O 3 -YAG amorphous ceramic coating with high content of amorphous phase is expected to be used under thermalmechanical coupling harsh conditions with high PV value. However, the formation mechanism of amorphous ceramic coating is still ambiguous. In order to better guide the application of Al 2 O 3 -YAG amorphous ceramic coating, the formation mechanism of Al 2 O 3 -YAG amorphous ceramic coating need to be further studied.
Previous studies suggested that the formation mechanism of amorphous ceramic coatings was due to the thermal spraying with high enthalpy and ultra-fast cooling rate [26,27]. However, this conclusion was slightly inadequate. It has been publicly reported that the XRD patterns of single Al 2 O 3 coating and Y 2 O 3 coating prepared by various plasma spraying processes were a mainly crystalline state, some coatings contain nanocrystals and have no obvious characteristics of amorphous phase [28,29]. Therefore, it is not enough to form the amorphous ceramic coating only with high enthalpy and ultra-fast cooling rate under the current technology. According to the results of reported studies, it is concluded that the ceramic coatings containing the amorphous phase successfully prepared were belong to the multicomponent system like Al 2 O 3 -ZrO 2 Al 2 O 3 -YSZ Al 2 O 3 -ZrO 2 -SiO 2 and TeO 2 -Bi 2 O 3 -V 2 O 5 -Na 2 O -TiO 2 [22][23][24][30][31][32]. Especially, the ratio of multi-component in sprayable powders was at low eutectic point, then the amorphous ceramic coating can be successfully prepared. Accordingly,the powders with the low eutectic point ratio combined with thermal spraying technology can be used to fabricate high melting point amorphous ceramic coating. In this study, the Al 2 O 3 -YAG amorphous ceramic coating was prepared by plasma spraying, and the main purpose was to study the formation mechanism of the amorphous phase in Al 2 O 3 -YAG coating to supply the guidance for the coating application. were used as feedstocks. Before spraying, the powders were mixed uniformly with the low eutectic molar ratio (Al 2 O 3 :Y 2 O 3 were equal to 82:18) to spray granulation. And the granulation powders were further heat-treated to obtain Al 2 O 3 /YAG powders for the preparation of Al 2 O 3 -YAG amorphous ceramic coating, the preparation process can refer to our previous work [33]. The stainless steels (1Cr18Ni9Ti) with the dimension of 30mm×15mm×1.25mm were used as substrate. Prior to spray, the substrates were sandblasted to increase the surface roughness and then cleaned by ultrasonic. The NiCr powders were deposited as bond coating. The Al 2 O 3 /YAG powders were prepared for Al 2 O 3 -YAG amorphous coating.
The powders were deposited by the Multicoat atmospheric plasma spraying system equipped with F4MB-XL gun (Sulzer Metco AG, Wohlen, Switzerland). The spraying parameters are listed in Table 1.

Coating characterization
The phase composition of powders and coatings were characterized by X-ray diffraction (XRD, D/Max-2550, Rigaku, Tokyo, Japan) with the 2θ range of 10° 80° and the scanning speed was 2 °/ min. The morphologies of coatings were observed by scanning electron microscope (SEM, TM3000, HITACHI, Tokyo, Japan) with Energy disperse spectroscopy (EDS). Electron backscattered diffraction (EBSD) technology was carried out to analyze the phase structure in the coatings preparation. Prior to EBSD testing, ion beam polishing was necessary for coating samples. To better analyze the crystallization chemistry behavior of powder droplets in the deposition process, the software Diamond 3.2 was used to draw the crystal structure of the phase involved in this study.

Phase composition analysis
The phase composition of powders and corresponding coatings (Al 2 O 3 -YAG and Al 2 O 3 -Y 2 O 3 ) were displayed in Fig. 1(a) and (b). As illustrated in Fig. 1(a), a big bulge appeared in the Al 2 O 3 -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-Al 2 O 3 (rhombohedral) and YAG (cubic) also existed in the Al 2 O 3 -YAG coating, which originated from un-melted powders or the recrystallization of the droplets.
Notably, there were no γ-Al  Fig. 2(b), which might be caused by the impact crushing of unmelted granulate powders. Some un-melted areas appeared in the cross-sectional morphologies, which may be explained that the part of powders was insu ciently heated during deposition. Based on this result, the crystallization diffraction peaks of α-Al 2 O 3 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 Al 2 O 3 -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 crosssectional 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.  In the analysis of Fig. 1(b), the YAM YAP and YAG were not found, so the deposition process of 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 insu cient 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 Al 2 O 3 and Y 2 O 3 reacted, but the new phases (YAM, YAP or YAG) formed by reaction maybe not easy to crystallize. It was veri ed 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 su cient mutual diffusion of Al 2 O 3 and Y 2 O 3 at the nano-scale or sub-microscale. From this point of view, the Al 2 O 3 -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 Al 2 O 3 /YAG powders droplets was large during deposition. However, the phenomenon that the powder droplets with different phases distributed uniformly were di cult 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.

Crystallization chemical process in powder droplets solidi cation
From the result of XRD and EBSD, the α-Al 2 O 3 was heated and turned into droplet, the droplet transformed as γ-Al 2 O 3 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 rst 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 Al 2 O 3 and Y 2 O 3 . Table 2 Fig. 5 were that the new phase like YAM, YAP and YAG reacted by Al 2 O 3 and Y 2 O 3 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 con ict 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 According to other research [51,52], the phase diagram of the Al 2 O 3 -Y 2 O 3 system was drawn in Fig. 7. The 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-Al 2 O 3 are preferential crystallization from high temperature to room temperature in APS deposition, but it con icts to the fact that γ-Al 2 O 3 crystal is not stable in high temperature (≥1000℃) and can transform to α-Al 2 O 3 at high temperature. One possible explanation is that γ-Al 2 O 3 crystal structure can be formed in high temperature, and it is unstable and need time to transform as the more stable structure like α-Al 2 O 3 , but it can be reserved due to the insu cient 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 Al 2 O 3 -YAG amorphous ceramic coating Fig. 9 displayed the formation mechanism of amorphousness in plasma-sprayed Al 2 O 3 -YAG amorphous ceramic coating. The amorphous phase formation process of Al 2 O 3 -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 ultrafast cooling process of plasma spraying.
Accordingly, there are three requirements for the formation of Al 2 O 3 -YAG amorphous ceramic coating: 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. Ultra-fast cooling rate which makes the internal atoms/ions of the melt have insu cient time to diffuse into the lattice of crystal and crystallize. 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.

Conclusion
The formation mechanism of amorphous phase in plasma-sprayed Al 2 O 3 -YAG amorphous ceramic coating prepared by Al 2 O 3 /YAG powder with low eutectic point was mainly investigated, and the Al 2 O 3 /Y 2 O 3 micro-powder as a contrast to prepare crystalline coating. Different to other research, this study revealed the effect of melt crystallization chemistry on crystallization process, and proved that the selection of powder also has an important effect on the preparation of amorphous ceramic coating. In summary, there have three requirements to prepare Al 2 O 3 -YAG amorphous ceramic coating in preparation technology and as-sprayed powder. The preparation technology requires high enthalpy and ultra-fast cooling rate. And the as-sprayable powder should choose multivariate powders with low eutectic point ratio distributed uniformly at nano-scale or sub-micro scale and could react to form the new phase crystal with high coordination numbers of cations. The schematic diagram of the crystal structure of the possible existed phases in this study The formation mechanism of amorphous phases in plasma-sprayed Al2O3-YAG amorphous ceramic coating