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Article

In Situ Deposition of Amorphous Al2O3-GAP Ceramic Coating with Excellent Microstructure Stability and Uniformity via Atmospheric Plasma Spraying

by
Linya Qiang
1,2,
Xiaozhen Zhang
1,*,
Yizhaotong Ai
2,
Yin Zhuang
2,
Jing Sheng
2,
Jinxing Ni
2 and
Kai Yang
2,*
1
School of Materials Science and Engineering, Jingdezhen Ceramic University, Jingdezhen 333403, China
2
Key Laboratory of Inorganic Coating Materials CAS, Shanghai Institute of Ceramic, Chinese Academy of Sciences, Shanghai 201899, China
*
Authors to whom correspondence should be addressed.
Coatings 2022, 12(2), 119; https://doi.org/10.3390/coatings12020119
Submission received: 13 December 2021 / Revised: 16 January 2022 / Accepted: 20 January 2022 / Published: 21 January 2022
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Abstract

:
A novel Al2O3-GdAlO3 (GAP) amorphous ceramic coating was in situ prepared via atmospheric plasma spraying (APS). The Al2O3/Gd2O3 sprayable powders possessed excellent sphericity and fluidity after heat treatment at 1173 K without solid-phase reaction, which indicated that the Al2O3-GAP coating deposition process was significantly simplified. The Al2O3-GAP amorphous coating showed high glass transition temperature (1155.1 K), initial crystallization temperature (1179.2 K), local activation energy (847.6 kJ/mol) and nucleation resistance (88.3). Compared with almost all amorphous materials, the Al2O3-GAP amorphous coating possessed greater crystallization activation energy, which was conducive to its high-temperature microstructure stability. Furthermore, the hardness and elastic modulus of Al2O3-GAP coating fluctuated with a tiny range when increasing the nanoindentation depth from 500 nm to 2000 nm, which exhibited excellent uniformity of microstructure and mechanical performance of the coating. Therefore, the results showed that the Al2O3-GAP amorphous coating had a better potential for large-scale engineering application under harsh service conditions.

Graphical Abstract

1. Introduction

The friction and wear of materials under high PV (P: contact pressure, V: friction velocity, PV ≥ 15 MPa·m·s−1) are often accompanied by high temperature, oxygen enrichment and thermal shock in a wide temperature range [1,2]. Ceramic coatings have to bear large contact pressure, high friction velocity, great friction heat (maximum frictional contact temperature is close to or even exceeds 1000 °C), serious oxidation and strong thermal shock under severe conditions with high PV values [3,4,5]. Al2O3 coatings are considered to be the typical representative of wear-resistant oxide ceramic coatings [6,7]. In the past, investigations on Al2O3 coating mainly included optimization of spraying, post-treatment process and variation of coating composition, such as increasing the substrate deposition temperature [8,9,10], dry-ice blasting [10], laser remelting [11,12], nanostructure [13,14] and adding elemental phases (Al [11], graphene [12], carbon nanotubes [13], etc.) or compounds (ZrO2 [14], YSZ [15], TiO2 [16], etc.). Dong S. J. et al. used dry-ice blasting technology to increase the bonding strength of plasma sprayed Al2O3 coating by 30% (the value exceeds 60 MPa) and the porosity was reduced from 9.3% to 6.8%. The internal compressive stress of the coating was nearly doubled, which was beneficial to suppress crack propagation and improvement of fracture toughness [10]. Plasma sprayed Al2O3 coating surface for laser remelting can increase the content of α-Al2O3 phase in the coating and obtain a coating with fine structure, low porosity and high hardness [11,12]. The nanoscale Al2O3 coating with fine-grained strengthening and toughening effect exhibited higher hardness and toughness, better anti-sliding and anti-erosion wear properties [13,14]. However, the common problem is that it is difficult to effectively improve the high-temperature mechanical properties, thermal conduction and wear resistance of the coating simultaneously. Some new methods in our previous work have been applied to solve the above-mentioned problem [17,18,19]. The “self-healing” and partial coherent interface strengthening and toughening mechanisms of Al2O3 coating microcracks filled with in situ generated nanocrystals were discovered and clarified. Moreover, this method for self-toughening Al2O3 coatings with in situ regulation of the content of α-Al2O3 nanocrystals via stress-induced phase transformation was put forward [20]. In order to suppress the thermal conductivity of Al2O3 coating from showing negative temperature coefficient characteristics and to meet the requirements of more severe wear conditions, the Al2O3-Cr2O3 composite coatings with positive temperature coefficient characteristics were designed and prepared [18]. Heterogeneous nucleation and partial solid solution strengthened the phase interface, refined the crystalline grains and improved the interlayer bonding of the coating. Meanwhile, it was taken into account thermal conduction, mechanical properties and anti-wear of the coating over the full temperature range [19].
Furthermore, to inhibit the reduction in the coating strengthening effect caused by the long-term high-temperature creep, based on the deep eutectic solidification mechanism, the Al2O3-Y3Al5O12 (YAG) amorphous ceramic coating was deposited via plasma spraying with the great under-cooling in our previous investigations [21]. Compared with other thirty kinds of amorphous materials (such as Fe-based alloy, Zr-based alloy, polystyrene and ceramic, etc.), the Al2O3-YAG amorphous ceramic coating shows excellent high-temperature microstructure stability [22]. Additionally, the Al2O3-YAG coating possesses good plastic toughness, thermal conductivity and crack arrest characteristics, exhibiting superior wear resistance to Al2O3 coatings and Al2O3-Cr2O3 composite coatings under high PV value conditions [23]; however, there still exist some problems in the whole preparation process of Al2O3-YAG amorphous coatings:
  • High-temperature calcination for solid-phase reaction (direct heat treatment at 1673~1873 K or step-by-step heating cycle calcination at 1173~1773 K), roughened the surface of the spray powder and resulted in the agglomeration effect of powder, which brought some negative effects on the powder sphericity and fluidity [24]. The step-by-step heating cycle calcination process was very cumbersome, which was difficult to control powder quality and reliability. Moreover, the time and cost were greatly increased;
  • The plasma spheroidization process of the powder needed to be carried out to remove the unfavorable factors caused by high-temperature solid-phase reaction;
  • Al2O3/YAG powders obtained by high-temperature solid-phase reaction may lead to gun blockage in the long-term plasma spraying process, which would be adverse to the uniformity of coating preparation.
The fully densified Al2O3-GAP ceramics with a eutectic composition prepared by hot-pressing methods described flexural strengths of 556 MPa at room temperature and 515 MPa at 1000 °C [25]. The fracture toughness of the Al2O3-GAP bulk eutectic ceramic system ranges from 5 to 6 MPa m1/2 from room temperature to 1600 °C. Al2O3-GAP eutectic ceramics have good high-temperature strength and toughness [26,27]. In this current work, a novel Al2O3-GdAlO3 (GAP) amorphous ceramic coating was in situ deposited via APS. Being different from the preparation of Al2O3-YAG amorphous ceramic coating, no high-temperature solid-phase reaction would occur in the sprayable powders for the deposition of Al2O3-GAP coating, and thus the original phases in feedstocks can be retained, namely, α-Al2O3 and c-Gd2O3. The sprayed Al2O3/Gd2O3 granulated powders had outstanding sphericity and fluidity, which would effectively simplify the whole coating preparation process. Simultaneously, the consistency and reliability of the coating deposition process were greatly improved, which could significantly reduce the time and economic cost of coating deposition. Exceptional mechanical performance, high-temperature microstructure stability and coating uniformity can be achieved for the as-sprayed Al2O3-GAP coating; therefore, the Al2O3-GAP amorphous ceramic coating had the better potential for large-scale engineering applications under harsh service conditions, such as Turbine pump dynamic seal of a liquid rocket engine, high-speed and over-loading bearing, ultra-high temperature molten salt energy storage system, etc.

2. Experimental Procedure

2.1. Feedstock Preparation and Coating Deposition

Commercially available sub-micro Al2O3 and Gd2O3 powders (Kouting, China) were used as feedstocks. The median particle size (D50) of Al2O3 and Gd2O3 powders was 0.486 μm and 0.730 μm, respectively. The main phases of the two powders were α-Al2O3 and c-Gd2O3, respectively. The homogeneous Al2O3/Gd2O3 composite powders were prepared by the spray granulation method (PLG-5, China) at the eutectic ratio of Al2O3:Gd2O3 = 77:23 (mol.%). The Al2O3 and Gd2O3 powders were put into the ball milling tank, which contained zirconia grinding balls (at the weight ratio of balls: feedstocks: deionized water is equal to 2:1:1). Then the feedstocks were milled with dispersant (Arabic gum, 0.2 wt.%) and binder (polyvinyl alcohol, 0.8 wt.%) for 6 h to obtain a homogeneously mixed suspension. The conventional spray drying process was carried out for the preparation of Al2O3/Gd2O3 sprayable powders that contain nano/sub-micro scale structures. The main controlled operating parameters included the rotation speed of atomizer (16,800 rpm), the air temperature at the entry (220 °C) and the exit (110 °C), respectively. The median particle size (D50) of the granulated powder obtained was 15.2 μm. The differential scanning calorimetry (DSC) method was used to determine the heat treatment temperature of granulated Al2O3/Gd2O3 powders and evaluate the thermal stability of as-sprayed Al2O3-GAP coating. The Al2O3/Gd2O3 granulated powders were sintered at 1173 K for 2 h so as to remove the organic additives and improve the density of granules. The density of sintered Al2O3/Gd2O3 granulated powders was 0.92 g/cm3. The phase composition of the sprayable powder did not change after heat treatment. The heat-treated powders were further sieved to cater to the sprayable demands. The Multicoat APS system equipped with an F4 MB-XL plasma gun (Sulzer Metco AG, Wohlen, Switzerland) was employed to deposit coating. The rectangular stainless steel with the dimension of 30 mm × 15 mm × 1.25 mm and the round graphite disk with the dimension of Φ 30 mm × 3 mm were used as substrates. Before spraying, the substrates were grit-blasted with 20# white corundum, and then ultrasonically cleaned in alcohol and dried with compressed air. NiCr powder was used to prepare a bondcoat prior to spraying ceramic coating. The density of as-sprayed Al2O3-GAP coating was 2.732 g/cm3. The plasma spray parameters of Al2O3-GAP amorphous coatings are listed in Table 1. For performance comparison, Al2O3 and Al2O3-YAG coatings were also fabricated according to our previous works [18,24].

2.2. Microstructure and Property Characterization

The phase compositions of the as-deposited and annealed Al2O3-GAP coatings were identified by X-ray diffraction (XRD) using a Rigaku D/Max2550 Diffractometer with nickel-filtered CuKα radiation. The particle size of granulated Al2O3/Gd2O3 powders was analyzed by a laser particle size instrument (MASTERSIZER-300, UK). Transmission electron microscopy (TEM) images were taken on JEM 2100F to characterize the microstructures of the coating. Electron backscattered diffraction (EBSD) technology was carried out on Magellan 400 to detect the phase structure of the coating samples and the cross-sectional morphologies of the coating were also characterized by Magellan 400. The crystallization process of the Al2O3-GAP amorphous coating was evaluated by a Netzsh 404C high-temperature DSC tester. The microhardness and elastic modulus of the Al2O3-GAP coating were measured on the cross-section of the coating after being submerged and polished. The measuring instrument was the G-200 nano-indenter produced by Agilent Technologies with Berkovich indenter.

3. Results and Discussion

3.1. Structural Characterization

Figure 1a,b represent the morphology of the original granulated powders and corresponding element distribution, indicating that the powders have good uniformity and dispersibility. A solid-state reaction would occur in granulated powders in the range of 1460~1620 K, as observed in Figure 1c. According to the DSC curve, the sintering temperature of the powder was chosen to be 1173 K, which ensured the improvement effects on density and strength of powders and without solid-state reaction being occurred. The morphology of the heat-treated powders and corresponding element distribution are shown in Figure 1d,e. After heat treatment, powder morphology was not changed, which still possessed good sphericity and fluidity. Figure 1f shows the XRD patterns of the original and heat-treated powders. The phase composition of the heat-treated powder remains unchanged (namely, α-Al2O3 and c-Gd2O3). The angle of repose, Carr index and Hausner ratio of Al2O3/YAG and Al2O3/Gd2O3 sprayable powders are shown in Table 2. The fluidity of the Al2O3/Gd2O3 powder is obviously better than that of the Al2O3/YAG powder in our previous work [28], which is beneficial to the subsequent plasma spraying process.
There were obvious amorphous scattering peaks in the XRD pattern of the as-sprayed coating, as shown in Figure 2a, indicating that a large amount of amorphous phase was formed in the coating. In addition, few weak diffraction peaks of α-Al2O3 and GAP crystalline phases appeared. According to XRD analysis, the content of the amorphous phase of Al2O3-GAP amorphous coating was estimated to be 91.8 wt.%, which means that the as-sprayed coating was mainly composed of the amorphous phase. A small number of α-Al2O3 and GAP crystalline grains was distributed in the amorphous matrix. Figure 2b,c show the cross-sectional morphology of as-sprayed Al2O3-GAP amorphous coating deposited on a stainless-steel substrate. Prior to depositing the ceramic topcoat, NiCr bond-coat was prepared. The thickness of NiCr bond-coat and Al2O3-GAP topcoat was about 50~60 μm and 250~270 μm, respectively. The results showed that there was no obvious two-phase feature in the SEM morphology of the Al2O3-GAP coating, further indicating that the coating matrix mainly consisted of an amorphous phase and the uniformity of coating was good. The black zoom and white zoom were analyzed by EDS in Figure 2c. It presents the enlarged view of the selected area in Figure 2b. The element line scan along the yellow line (marked in Figure 2c as shown in Figure 2d). It can be judged that the black zoom corresponds to the α-Al2O3 phase, and the white zoom corresponds to the GAP phase. On the whole, these phases were well dispersed in the gray amorphous matrix without obvious segregation. The element distribution between the white phase on the left and the black phase on the right was less fluctuating. Consequently, the uniformity of the element distribution in the amorphous matrix demonstrates good consistency of the coating structure. In accordance with the image analysis (IA) method, the porosity of Al2O3-GAP coating was 3.71 ± 0.20%. In contrast, the porosity of the plasma-sprayed Al2O3-YAG coatings was 8.54 ± 1.57% in our previous work [21]; therefore, the Al2O3-GAP coating had fewer microcracks and higher density than that of the Al2O3-YAG coating. The electron backscatter diffractometer (EBSD) and field emission transmission electron microscope (FETEM) were applied to further characterize the coating structure, as shown in Figure 2e,f. The distribution of α-Al2O3 and GAP crystalline grains could be evaluated by EBSD analysis. The content of α-Al2O3 phase marked with yellow was 1.51%. The content of the GAP phase marked with green was 1.73%. Figure 2f shows the TEM image of the as-sprayed Al2O3-GAP coating. The coating matrix was chiefly made up of the zone without boundary feature mark with “C”. The corresponding selected area electron diffraction (SAED) pattern (shown in Figure 2f) revealed a diffuse halo, which denoted the obvious amorphous phase feature. There were two kinds of crystalline grains with different morphologies and obvious boundary characteristics in the amorphous matrix, which were labeled with A and B, respectively. The EDS analysis showed that zone A with surface wrinkled and zone B with a darker color were α-Al2O3 and GAP, respectively. A few α-Al2O3 and GAP crystalline grains were distributed in an amorphous phase matrix. Combined with the above-mentioned analysis, the content of the amorphous phase of Al2O3-GAP coating was 96.76%, which was consistent with the calculated results from the XRD analysis (as shown in Figure 2a). According to EBSD and TEM analyses, the crystalline size of α-Al2O3 was larger than that of GAP, and the grain number of α-Al2O3 was less than that of GAP. This phenomenon may be ascribed to the fact that one large α-Al2O3 grain is actually originated from many small nuclei; therefore, the difficulty of α-Al2O3 to nucleate is misleading since the larger grain may come from many small nuclei. On the other hand, the small and many grains of GAP may not form larger ones due to the ineffective Ostwald ripening process [29]. In this case, the diffusivity/mobility of small grains is low, thus preventing the large grain forms.

3.2. Thermal Stability Analysis

From the DSC curve shown in Figure 3a, the glass transition temperature (Tg), initial crystallization temperature (Tc1), supercooled liquid region (ΔT = Tc1–Tg), crystallization peak temperature (Tp), local activation energy (Ec) and nucleation resistance (Eg/RT) of Al2O3-GAP amorphous coating can be calculated, as shown in Table 3. In this paper, the glass transformation temperature and initial crystallization temperature of Al2O3-GAP coating were 1155.1 K and 1179.2 K, respectively. The characteristic temperature (Tg and Tc) values of the Al2O3-GAP and the Al2O3-YAG [22] amorphous coatings were very close. Ec can be calculated by the Kissinger equation, which is the most common method to analyze the crystallization behavior [28,29]. The onset activation energy calculated of the Al2O3-GAP amorphous coating was 847.6 kJ/mol, which was greater than that of Al2O3-YAG amorphous coating. Figure 3b compares the initial crystallization activation energy obtained by the Kissinger method for some typical amorphous materials [22,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48]. Conspicuously, the crystallization activation energy of the Al2O3-GAP amorphous coating reported in this work was higher than that of almost all amorphous materials, which indicated that it was extremely difficult to overcome the energy barrier to precipitate crystalline phase in the Al2O3-GAP amorphous coating. Meanwhile, it can be seen that the initial crystallization temperature of Al2O3-YAG coating is slightly higher than that of Al2O3-GAP coating. This may be due to the fact that the nanocrystals present in the Al2O3-GAP coating can serve as the crystalline core of the amorphous coating during the crystallization process (as shown in Figure 4a,b). Furthermore, the grain size was further analyzed by EBSD technology. Figure 4c,d) describe that the GAP grain size range (0.25~0.85 μm) in the as-sprayed Al2O3-GAP coating is indeed smaller than the YAG grain size range (0.25~1.35 μm) in the as-sprayed Al2O3-YAG coating. So, the crystalline phase of the former begins to precipitate at a lower temperature, resulting in a small decrease in the initial crystallization temperature. In addition, Ec/RTg can also be suggested to evaluate the standard for nucleation resistance of amorphous materials and characterize the degree of difficulty of supercooled liquid crystallization. Greater Ec/RTg (88.3) of the Al2O3-GAP amorphous coating indicates higher nucleation resistance, which would show a stronger anti-crystallization tendency and better thermal stability of amorphous materials. In our previous investigations [22], the Al2O3-YAG amorphous coatings possessed greater crystallization kinetic parameters and better high-temperature microstructure stability than thirty kinds of amorphous materials. Particularly, the Al2O3-GAP amorphous coating has greater Ec and Ec/RT than that of the Al2O3-YAG amorphous coating, which indicates the greater efforts needed to overcome the energy barrier during crystallization, directly leading to outstanding microstructure stability of the former.

3.3. Microstructure and Mechanical Property Uniformity

During coating deposition, some defects such as microcracks, pores and un-melted particles are unavoidable. The presence of these defects degrades material homogeneousness and influences corresponding mechanical properties. The nanoindentation tests were carried out to evaluate structure uniformity and mechanical performance of Al2O3, Al2O3-YAG and Al2O3-GAP coatings. To avoid the influence of indentation size effect caused by micro-defects and residual stress on mechanical performance testing, high load conditions should be selected. Ten load-displacement curves were obtained with the nanoindentation process for every kind of coating. It can be seen from the load-displacement curves that the maximum displacement of Al2O3 coating was about 1890~4263 nm. However, the maximum displacement of Al2O3-YAG and Al2O3-GAP amorphous coatings were 2034~2393 nm and 2125~2351 nm, respectively, shown in Figure 5a–c. The order of maximum displacement range variation of three kinds of coatings follows the sequence of Al2O3 > Al2O3-YAG > Al2O3-GAP.
Meanwhile, the hardness- and modulus-displacement curves are exhibited in Figure 5d–i. The measurement values of the corresponding mechanical property were selected at a depth of 500~2000 nm for statistical analysis. Ten indentation tests were made in each sample and the results were presented in Table 4. The hardness of Al2O3, Al2O3-YAG and Al2O3-GAP coatings were 5.22 ± 1.87, 9.46 ± 0.63 and 9.25 ± 0.57 GPa, respectively. The corresponding modulus were 158.64 ± 41.89, 149.71 ± 8.98 and 140.77 ± 7.10 GPa, respectively. It is noteworthy to point out that the hardness of Al2O3 coating was significantly smaller than that of Al2O3-YAG and Al2O3-GAP coatings; the corresponding elastic modulus was higher than that of Al2O3-YAG and Al2O3-GAP coatings. With regard to the hardness and elastic modulus of Al2O3-YAG and Al2O3-GAP coatings, the former was slightly higher than the latter. Compared with Al2O3-YAG and Al2O3-GAP coatings, the values of Al2O3 coating in the ten nanoindentation tests were quite different. That was ascribed to the cause that Al2O3 coating may produce additional defects during non-uniform plastic deformation as the indentation depth increased, namely free volume defects [49]. The existence of these defects greatly reduced the uniformity of the Al2O3 coating and affected its micromechanical properties. Relatively speaking, the Al2O3-GAP coating had fewer defects than Al2O3-YAG coating, which was beneficial to its microstructure stability. Furthermore, the ratio of standard deviation to average (denoted as Δε) was used to estimate numerical fluctuation degree. The Δε of hardness for Al2O3, Al2O3-YAG and Al2O3-GAP coatings were 35.8%, 6.7% and 6.2%, respectively, and that of corresponding modulus values were 26.4%, 6.0% and 5.0%. The Δε of Al2O3-GAP coating was less than that of Al2O3 and Al2O3-YAG coating, indicating that the mechanical performance fluctuation range of Al2O3-GAP coating was the smallest. The load–displacement, hardness–displacement and modulus–displacement curves of Al2O3-GAP coating basically coincided, indicating that the coating possessed outstanding structural and property uniformity.

4. Conclusions

In summary, this work exhibits the results of an experimental study of the phase structure, microstructure stability and mechanical properties of a novel amorphous Al2O3-GAP ceramic coating produced by APS. Notably, the sprayed powder did not need to undergo high-temperature solid-phase reaction and plasma spheroidization, and then the resulting Al2O3/Gd2O3 granulated powders possessed excellent sphericity and fluidity. So, the in situ Al2O3-GAP coating deposition process was effectively simplified, which would be conducive to the consistency and reliability improvement of coating preparation process. The amorphous phase was dominant in as-sprayed Al2O3-GAP coating. The Al2O3-GAP amorphous coating possessed excellent microstructure stability, which was attributed to its greater crystallization activation energy, compared with almost all amorphous materials. Ten nanoindentation experiment results showed that the load, hardness and modulus–displacement curves of Al2O3-GAP coating basically coincided. Moreover, the Δε of hardness and modulus for Al2O3-GAP coating were smaller than Al2O3 and Al2O3-YAG coating. In contrast to Al2O3 and Al2O3-YAG coatings, Al2O3-GAP coating possessed better microstructure uniformity and performance consistency. The Al2O3-GAP coating retains superior high microstructure stability and structure/performance uniformity, which will provide a fundamental contribution to the engineering applications of the coating under harsh service conditions.

Author Contributions

K.Y. conceived the overall projects; L.Q. designed/performed the experiments, data analysis and wrote the manuscript; X.Z. helped with characterization and discussion; Y.A. helped with the manuscript; Y.Z., J.S. and J.N. deposited the Al2O3-GAP coating. All authors participated in discussions and comments on the paper. All authors have read and agreed to the published version of the manuscript.

Funding

The study is jointly supported by Sub-project of Key Basic Research Projects of Basic Strengthening Program (172-04), National Nature Science Foundation of China (51772311) and Youth Innovation Promotion Association, Chinese Academy of Sciences (2016230). We are grateful to Inorganic Materials Analysis and Testing Center of Shanghai Institute of Ceramics for providing the support for this work.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) SEM image of granulated powders (the lower-left corner shows the particle size distribution of granulated powders); (b) the morphology of the single granule and related element distribution; (c) DSC curve of Al2O3/Gd2O3 granulated powders at the heating rate of 30 K/min; (d) SEM image of granulated powders sintered at 1173 K for 2 h; (e) the morphology of the single granule sintered at 1173 K for 2 h and related element distribution; (f) XRD patterns of Al2O3/Gd2O3 granulated powder.
Figure 1. (a) SEM image of granulated powders (the lower-left corner shows the particle size distribution of granulated powders); (b) the morphology of the single granule and related element distribution; (c) DSC curve of Al2O3/Gd2O3 granulated powders at the heating rate of 30 K/min; (d) SEM image of granulated powders sintered at 1173 K for 2 h; (e) the morphology of the single granule sintered at 1173 K for 2 h and related element distribution; (f) XRD patterns of Al2O3/Gd2O3 granulated powder.
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Figure 2. (a) XRD pattern of as-sprayed Al2O3-GAP coating; (b,c) cross-sectional SEM images of Al2O3-GAP coating; (d) the zoomed morphology image of the Al2O3-GAP coating and line scan from image (c); (e) electron backscattered diffraction (EBSD) images of the Al2O3-GAP amorphous coating; (f) TEM image of as-sprayed Al2O3-GAP coating and selected area electron diffraction (SAED) pattern corresponding to zone C.
Figure 2. (a) XRD pattern of as-sprayed Al2O3-GAP coating; (b,c) cross-sectional SEM images of Al2O3-GAP coating; (d) the zoomed morphology image of the Al2O3-GAP coating and line scan from image (c); (e) electron backscattered diffraction (EBSD) images of the Al2O3-GAP amorphous coating; (f) TEM image of as-sprayed Al2O3-GAP coating and selected area electron diffraction (SAED) pattern corresponding to zone C.
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Figure 3. (a) DSC curve of the Al2O3-GAP amorphous coating at the heating rate of 20 K/min; (b) initial crystallization activation energy Ec(x) obtained by the Kissinger method for some typical amorphous materials reported [22,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48].
Figure 3. (a) DSC curve of the Al2O3-GAP amorphous coating at the heating rate of 20 K/min; (b) initial crystallization activation energy Ec(x) obtained by the Kissinger method for some typical amorphous materials reported [22,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48].
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Figure 4. (a,b) HRTEM images of as-sprayed Al2O3-GAP amorphous coatings after heat treatment at 1173 K and 1223 K for 10 min; (c,d) grain size distribution of different crystalline phases in Al2O3-GAP and Al2O3-YAG amorphous coating.
Figure 4. (a,b) HRTEM images of as-sprayed Al2O3-GAP amorphous coatings after heat treatment at 1173 K and 1223 K for 10 min; (c,d) grain size distribution of different crystalline phases in Al2O3-GAP and Al2O3-YAG amorphous coating.
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Figure 5. The load–displacement (a–c), hardness–displacement (d–f) and modulus–displacement (g–i) curves of the Al2O3, Al2O3-YAG and Al2O3-GAP coatings.
Figure 5. The load–displacement (a–c), hardness–displacement (d–f) and modulus–displacement (g–i) curves of the Al2O3, Al2O3-YAG and Al2O3-GAP coatings.
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Table
Table 1. Plasma spray parameters of Al2O3-GAP coatings.
Table 1. Plasma spray parameters of Al2O3-GAP coatings.
ParametersNiCr BondcoatAl2O3-GAP Topcoat
Arc current, A600650
Primary plasma gas (Ar), slpm5749
Secondary plasma gas (H2), slpm7.5–8.08.5–9.0
Carrier gas (Ar), slpm3.54.0
Powder feed rate, g/min1030
Power, kW40–4545–50
Stand-off distance, mm120110
Table
Table 2. Angle of repose, Carr index and Hausner ratio of Al2O3/YAG and Al2O3/Gd2O3 sprayable powders.
Table 2. Angle of repose, Carr index and Hausner ratio of Al2O3/YAG and Al2O3/Gd2O3 sprayable powders.
Angle of ReposeCarr Index *Hausner Ratio *
Al2O3/YAG powders 24]46.5°26%1.35
Al2O3/Gd2O3 powders30.0°8%1.09
* When the Carr index is less than 10%, the powder has excellent fluidity; when it is greater than or equal to 38%, the powder fluidity is extremely poor. When the Hausner Ratio is 1.0–1.1, the powder has excellent fluidity; when it is greater than 1.6, the powder fluidity is extremely poor.
Table
Table 3. A series of evaluation parameters of thermal stability for Al2O3-YAG and Al2O3-GAP amorphous materials.
Table 3. A series of evaluation parameters of thermal stability for Al2O3-YAG and Al2O3-GAP amorphous materials.
Amorphous CoatingsTg (K)Tc1 (K)ΔT (K)Tp1 (K)Ec (kJ/mol)Ec/RTgβ (K/min)
Al2O3-YAG [22]1166.01198.429.31215.5807.883.320
Al2O3-GAP1155.11179.217.41215.0847.688.320
Table
Table 4. The nanoindentation hardness and elastic modulus of several coatings for different nanoindentation depth.
Table 4. The nanoindentation hardness and elastic modulus of several coatings for different nanoindentation depth.
Displacement
(nm)		Al2O3
Hardness (GPa)		Al2O3 Modulus (GPa)Al2O3-YAG Hardness (GPa)Al2O3-YAG Modulus (GPa)Al2O3-GAP Hardness (GPa)Al2O3-GAP Modulus (GPa)
500–10005.16 ± 1.35157.24 ± 41.909.85 ± 0.68154.78 ± 9.909.54 ± 0.68144.36 ± 6.78
1000–15005.22 ± 2.38157.04 ± 47.029.24 ± 0.64146.66 ± 8.779.02 ± 0.69138.74 ± 8.39
1500–20005.61 ± 2.63161.84 ± 41.308.80 ± 0.66141.29 ± 8.178.90 ± 0.70134.97 ± 8.09
500–20005.22 ± 1.87158.64 ± 41.899.46 ± 0.63149.71 ± 8.989.25 ± 0.57140.77 ± 7.10
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Qiang, L.; Zhang, X.; Ai, Y.; Zhuang, Y.; Sheng, J.; Ni, J.; Yang, K. In Situ Deposition of Amorphous Al2O3-GAP Ceramic Coating with Excellent Microstructure Stability and Uniformity via Atmospheric Plasma Spraying. Coatings 2022, 12, 119. https://doi.org/10.3390/coatings12020119

AMA Style

Qiang L, Zhang X, Ai Y, Zhuang Y, Sheng J, Ni J, Yang K. In Situ Deposition of Amorphous Al2O3-GAP Ceramic Coating with Excellent Microstructure Stability and Uniformity via Atmospheric Plasma Spraying. Coatings. 2022; 12(2):119. https://doi.org/10.3390/coatings12020119

Chicago/Turabian Style

Qiang, Linya, Xiaozhen Zhang, Yizhaotong Ai, Yin Zhuang, Jing Sheng, Jinxing Ni, and Kai Yang. 2022. "In Situ Deposition of Amorphous Al2O3-GAP Ceramic Coating with Excellent Microstructure Stability and Uniformity via Atmospheric Plasma Spraying" Coatings 12, no. 2: 119. https://doi.org/10.3390/coatings12020119

APA Style

Qiang, L., Zhang, X., Ai, Y., Zhuang, Y., Sheng, J., Ni, J., & Yang, K. (2022). In Situ Deposition of Amorphous Al2O3-GAP Ceramic Coating with Excellent Microstructure Stability and Uniformity via Atmospheric Plasma Spraying. Coatings, 12(2), 119. https://doi.org/10.3390/coatings12020119

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