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Vol. 9. Issue 1.
Pages 282-290 (January - February 2020)
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Vol. 9. Issue 1.
Pages 282-290 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.10.056
Open Access
Investigation of mechanical properties and oxidation resistance of CVD TiB2 ceramic coating on molybdenum
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Xiaoxiao Huang, Shuchen Sun
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sunsc@smm.neu.edu.cn

Corresponding author.
, Ganfeng Tu
School of Metallurgy, Northeastern University, Shenyang 110819, China
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Table 1. Deposition condition of TiB2 coating by CVD.
Abstract

TiB2 anti-oxidation ceramic coatings were successfully synthesised on a Mo substrate by using chemical vapor deposition (CVD). A scratch test was performed to measure the adhesion strength of the TiB2 ceramic coatings to the Mo substrate. The results indicated good adhesion between the two materials; the adhesion strength was 125mN. Furthermore, high-temperature oxidation tests indicated that the CVD TiB2 ceramic coating protected the Mo substrate against oxidation up to 900℃. Isothermal oxidation experiments revealed that the Mo substrate covered with a 13-μm-thick TiB2 ceramic coating was protected against oxidation for a duration of 5h. The oxidation kinetics followed a parabolic law for the first 2.5h and a linear law in the next 2.5h.

Keywords:
Titanium diboride
Coating
Chemical vapor deposition
Oxidation
Full Text
1Introduction

Mo is a useful metal with many excellent physical and chemical properties, such as corrosion resistance, good electrical and thermal conductivities, and a high melting point (2622°C) [1,2]. Therefore, Mo and Mo alloys are widely used in the aerospace, metallurgy, nuclear, and energy industries, as well as in lighting technology, high-temperature furnace construction, sputtering targets, and high-performance electronics [3–5]. Nevertheless, the applicability of Mo is limited owing to its extremely poor resistance to oxidation at high temperatures. Mo is easily oxidised in air even at 427°C [5].

To improve the oxidation resistance of Mo, various methods have been employed, such as anti-oxidation coating and the addition of anti-oxidation elements (Si, B, etc.). However, the addition of B and Si is limited by the low solubility and poor workability [6]. The oxidation-resistance coating method is widely used for Mo and Mo-based alloys. In previous studies, two major types of coating materials were used for obtaining anti-oxidation protective layers on the surface of Mo: 1) MoSi2[5] and MoSi2-based composites, e.g., MoSi2–SiO2[2], (Mo, W)Si2–Si3N4[7,8], MoSi2+ X SiC [9], and Mo-Si-B [10–17], and 2) metals (alloys), e.g., Ir [18], W [19–21], and Ni-20Cr [4]. Some ceramic materials also exhibit excellent properties, such as high-temperature oxidation resistance, a high melting point, and high hardness. Titanium diboride (TiB2), which is a candidate material for ultrahigh-temperature ceramics, is attracting increasing attention. It has superior properties, such as a high melting point (approximately 3100°C [22]), high hardness (maximum 45GPa [23]), good electrical and thermal conductivities, and chemical inertness [24]. Additionally, as a very important anti-oxidation ceramic material, TiB2 can resist oxidisation in air up to 1100°C [25]. Thus, TiB2 has been utilised for a protective coating for various applications. TiB2 as a protective coating has been used for cemented carbide cutting tools [26], moulds, and other tools [27,28]. However, there has been little research on the TiB2 ceramic as an anti-oxidation coating on the surface of Mo. In using a ceramic anti-oxidation coating, the major problem is poor adhesion between the ceramic coating and the metal substrate. However, the adhesion strength can be improved either via substrate surface treatment [29] or by increasing the substrate temperature [30]. Therefore, the TiB2 ceramic can be considered as a protective layer to improve the oxidation resistance of Mo metal.

In discussions regarding the oxidation resistance of TiB2 coatings for Mo, Mo-B-Si coatings are also mentioned owing to their similar composition and oxidation performance. Mo-B-Si coatings have exhibited excellent high-temperature oxidation resistance for Mo and Mo alloys [11,12,14,15,31,32], particularly in the temperature range of 1200–1700°C [11,12,14]. However, the oxidation resistance of Mo-B-Si coatings is poor at temperatures below 1000°C [31–34]. At temperature of 650–750°C, pest oxidation of the coatings occurs [31,34]. Conversely, the TiB2 ceramic exhibits good and reliable oxidation resistance below 1000°C. Thus, it is of interest to examine the mechanical and oxidative properties of TiB2 ceramic coatings on Mo substrates.

In our previous work [35], a high quality TiB2 ceramic coating was successfully synthesized on metal Mo by using a chemical vapor deposition (CVD) method, and the microstructure and morphology of the coatings on the Mo substrate have been examined. The objective of this study was to systematically investigate the effectiveness of a TiB2 ceramic coating for improving the oxidation resistance of Mo. Firstly, the adhesion strength and microhardness of the coating were measured. Furthermore, the oxidation behaviour of the TiB2 ceramic coatings on the Mo substrate was investigated. Oxidation tests were performed on the coatings from room temperature to 900°C, which is the temperature required for Mo moulds in our future work.

2Experimental method2.1Materials

The experimental materials are from our previous work. The TiB2 ceramic coatings were deposited on a sheet of metallic Mo using a gas mixture of H2 +TiCl4 + BCl3 under 1000°C by CVD. The overall chemical reaction is as follows.

Within this work, the TiB2 ceramic coating was deposited on Mo substrate using a conventional vertical CVD reactor. The experimental equipment was self-made in this study. The schematic design of the experimental CVD system is illustrated in Fig. 1. The graphite deposition chamber was heated with a graphite resistance heating element. The temperature was measured by a platinum-rhodium thermocouple and with a temperature controller (CKW-3100, BCHY, Beijing, China) to ± 5°C. The TiCl4 was heated to boiling point of 135°C in a resistance furnace. TiCl4, BCl3 and H2 were introduced into the bottom of the deposition chamber via stainless steel tubing, respectively. The TiCl4 gas path was heated by conventional heating tapes to o ensure that no condensation of the reagents occurred within the lines.

Fig. 1.

Schematic design of the CVD system.

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It is well known that the adhesion between ceramic coatings and metal substrates is poor. Many factors affect the adhesion strength. Among the most important factors are the surface characteristics of the substrate. A roughened surface can improve the adhesiveness of ceramic coatings [36,37]. Hence, the surfaces of the Mo substrates were roughened by 600 grit silicon carbide sandpaper to improve the adhesiveness of the TiB2 ceramic coatings to the substrates. Then, the Mo substrates were ultrasonically cleaned with acetone for 30min. Finally, the substrates were suspended in the centre of the deposition chamber by a Mo wire. A. J. Caputo et al. [38] reported that some properties of CVD TiB2 depend on the deposition temperature. They found that TiB2 coatings deposited at ≥900°C exhibited very low erosion rates and that the coating hardness increased with the deposition temperature. Hence, we used a high deposition temperature (1000°C) to obtain good coating properties. More detailed deposition conditions are presented in Table 1. The appearance and cross-section images of the materials we used in this work are shown in Fig. 2[35].

Table 1.

Deposition condition of TiB2 coating by CVD.

substrate  molybdenum 
Deposition temperature  1000 [°C] 
Deposition time  2 [h] 
Deposition pressure  40–50 [kPa] 
Pressure of H2  0.15 [MPa] 
Flow rate of H2  635 [cm3/min] 
Temperature of TiCl4  135 [°C] 
Flow rate of TiCl4  130 [cm3/min] 
Flow rate of BCl3  195 [cm3/min] 
Fig. 2.

Appearance and cross-section images of experimental materials.

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2.2Mechanical-property test of TiB2 ceramic coatings on Mo substrate

Adhesion measurements were performed according to a standard test method for evaluating the adhesion strength between a vapor deposition film and a substrate (JB/T 8554, People’s Republic of China Machinery Industry Standard (PRCMIS)). The scratch method was employed for testing the adhesion of the TiB2 ceramic coatings to the Mo substrates. The adhesion strength was evaluated using a nanoscratch tester (TriboIndenter, Hysitron, Minneapolis, USA) with a diamond cube-corner stylus under a continuously increasing load. The load ranged from 0 to 250mN, with a loading rate of 5mN/s. Furthermore, a microhardness tester (FM-700, Future-tech, Kawasaki, Japan) was used to evaluate the microhardness of the TiB2 ceramic coating.

2.3Oxidation test

To study the oxidation process of the specimens, oxidation experiments were performed in a tube furnace with a temperature accuracy of ±5°C. These experiments were conducted in dry air at 300, 450, 600, 750, and 900°C. Additionally, isothermal-oxidation tests were performed at 900°C for 1, 2, 3, 4, and 5h. Thermal analysis, thermogravimetry (TG), and differential scanning calorimetry (DSC) were performed to investigate the oxidation mechanisms. The tests were performed using a Netzsch STA449F3 thermoanalytical system (TG measurement error was 10−4 mg, temperature measurement error was <1°C) at a constant heating rate of 10°C/min in flowing air at a flow rate of 150mL/min in the temperature range of 20–900°C. The duration of the isothermal was 5h at 900°C. For comparison, thermogravimetric tests of uncoated Mo were also performed under the same oxidation conditions.

2.4Characterisations of coating before and after oxidation

The phases of the coatings formed on the Mo substrate were examined using X-ray diffraction (XRD, X’ Pert Pro, PANalytical B.V., Almelo, Netherlands) at 40mA and 40kV with Cu Kα radiation (λ=1.541Å, step size=0.0334°, integration time of 19.69s/step, scan rate of 0.216° s−1 from 5° to 90°). Scanning electron microscopy (SEM, Ultra Plus, Zeiss, Heidenheim, Germany) and energy-dispersive X-ray spectroscopy (EDS, X-Max 50, Oxford Instruments, Oxford, UK) were performed to investigate the microstructure, morphology, and composition of the coatings. XRD was also used to determine the phase composition of the oxidation products on the surface of the specimens after the oxidation experiments. The oxidation products were imaged and analysed using SEM, and their compositions were analysed using EDS.

3Results and discussion3.1Mechanical properties of TiB2 coatings

According to a standard method for testing the adhesion strength between a vapor deposition film and a substrate (JB/T 8554 from PRCMIS), scratch adhesion tests were performed by applying continuous loading from 0 to 250mN with a 5-mN/s increase rate. The adhesive strength was assessed according to the critical load, which was identified by an abrupt change in the coefficient of friction. To reduce the influence of the surface roughness in the evaluation of the coefficient of friction, the surface roughness must be <0.32μm according to the test standard JB/T 8554. Hence, the coating was polished by 2000 grit silicon carbide sandpaper before the scratch tests. The surface roughness of the coating was measured using a three-dimensional laser scanning microscope (LEXT OLS4100, Olympus, Tokyo, Japan). The results of the roughness test indicated that after polishing, the average surface roughness was 0.26, which is 0.32 lower than the value specified by the test standard JB/T 8554. Fig. 3 shows the coefficient of friction plot for the scratch testing of the TiB2 ceramic coating on a pure Mo substrate. Two turning points are observed in the coefficient of friction plot: the lower critical load and upper critical load, corresponding to the critical loads for failure of the coating. Before the lower critical load was reached, the coefficient of friction increased slowly and fluctuated slightly with the increasing load. At this stage, the coating exhibited good adhesion to the substrate and was polished by the indenter. Then, we observed an abrupt increase in the coefficient of friction over a short period of time, indicating that the coating began to crack and peel off. Therefore, the lower critical load was defined as the adhesion strength of the coating to the substrate. As the load continued to increase, an upper critical load was observed. Subsequently, the coating was removed from the substrate. Hence, the friction coefficient increased slowly with the increasing load. According to the results, as shown in Fig. 3, the adhesion strength of the TiB2 ceramic coatings to the Mo substrate was approximately 125mN. Q. L. Lv and J. Gao [39] reported that good adhesion of a Hf metal coating to a Mo substrate was achieved using ion-beam sputtering; the adhesion strength was approximately 120mN. The adhesion of the as-prepared CVD TiB2 coatings to the Mo substrate in the present study was as good as that of the previously reported Hf metal coating. Additionally, the TiB2 coating exhibited high microhardness relatively to MoSi2 coating on Mo substrate. The mean average microhardness value of the coating we obtained was 28GPa and the maximum microhardness value was 30GPa. But the MoSi2 coating microhardness only have a mean value of 13GPa [40]. Generally, the hardness values for TiB2 films can vary between 25 and 36GPa [27]. In our work, hardness of as-preparation TiB2 coatings on Mo substrate is in accordance with the consistently range of TiB2 hardness value [23,41–43].

Fig. 3.

Coefficient of friction plots for the scratch testing of the TiB2 coating on the Mo substrate.

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3.2TG and DSC of TiB2 coated on Mo

Fig. 4 presents the weight-gain data obtained during continuous oxidation of Mo covered with the TiB2 coating. For comparison, the data for a pure Mo specimen are also shown. The possible oxidation reactions are as follows.

Fig. 4.

TG weight-gain data under continuous oxidation for the Mo substrate covered with the TiB2 coating and the pure Mo specimen in air up to 900°C with a heating rate of 10°C/min.

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The weight gain of the pure Mo and the Mo covered with the TiB2 coating started at 550 and 720°C, respectively, in agreement with previously reported data for the oxidation of TiB2[44]. The rate of weight gain for the pure Mo specimen increased up to ˜790°C; subsequently, the weight-gain rate remained high up to 850°C, and then rapid weight loss occurred. These results can be explained by the fact that Mo is very easily oxidised at low temperatures in air, and its oxide (MoO3) is easily volatilised according to reaction (5) [45]. However, the Mo covered with the TiB2 coating exhibited entirely different weight-gain trends. Above 720°C, the sample exhibited very slow weight gain up to the final temperature. This indicates that the TiB2 functioned as a protective layer on the surface of the pure Mo metal and improved the oxidation resistance of the Mo.

The thermal-analysis results for the TiB2 coated on Mo in air are presented in Fig. 5. The heating rate was 10°C/min in the heating segment up to 900°C, and the duration of the isothermal oxidation was 5h at 900°C. According to the TG curve in Fig. 5, a very small weight gain (0.03%) occurred in the initial oxidation period up to 120°C. Although this weight gain was negligible, it indicates the start of the oxidation, as supported by the DSC curve. The oxidation reaction was followed by reaction (2). Subsequently, the weight did not change until the temperature reached 400°C, owing to the formation of low-viscosity B2O3 glass that functioned as an O barrier. When the oxidation temperature increased to 450°C, the vaporisation of B2O3 (liquid) (see reaction (3)) occurred more vigorously, resulting in weight loss. When the temperature increased to 720°C, slight weight gain occurred, because a large amount of B2O3 fluid covered the surface of the TiB2 coating, preventing its further oxidation. Above this temperature, rapid weight gain occurred up to 900°C because of the rapid vaporisation consumption of the B2O3 glass, causing part of the TiB2 to be exposed in air, which led to further oxidation. Thereafter, isothermal oxidation occurred for 5h. The weight-change rate of the specimen was dependent on the oxidation time. For the first 2.5h, the weight-gain rate decreased with the oxidation time, and the oxidation kinetics of the specimen essentially followed a parabolic law. Then, the weight-gain rate increased, and the oxidation kinetics followed a linear law.

Fig. 5.

TG and DSC curves for the oxidation of TiB2 coated on Mo in air, with the heating segment at a heating rate of 10°C/min up to 900°C and isothermal oxidation at 900°C up to 5h.

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3.3Phase and microstructure of oxide scales

To further examine the oxidation process of the TiB2 ceramic coatings formed on Mo, samples prepared under identical conditions were oxidised at 300, 450, 600, 750, and 900°C for 5h. XRD was employed to identify the phases of the oxide scales, and their morphology was examined using SEM. The results are shown in Figs. 6 and 7, respectively. When the samples were exposed to air at temperatures of 300 and 450°C for 5h, only the peaks of TiB2 were detected, as shown in Fig. 6. However, as shown in Fig. 5, the TG and DSC curves indicated that oxidation (reaction (2)) occurred at these two temperatures. It is considered that the phases of the reaction products were undetected because of their small amount. Indeed, the weight gain was only 0.03%, according to the foregoing TG data. At 600°C, the TiO2 peaks were observed, but the B2O3 peaks were absent. Additionally, the B2O3 peaks were absent at 750 and 900°C, as shown in Fig. 6. This is because the B2O3 evaporated with the increasing temperature, as confirmed by the EDS mapping in Fig. 7. With the increasing temperature, the B content gradually decreased, and the O content increased. When the temperature increased to 900°C, the TiB2 peaks completely disappeared, and only TiO2 peaks remained. After the total consumption of TiB2 (T=900°C and 5-h oxidation time), important changes were observed, as shown in Fig. 7(e): high-crystallinity TiO2 (crystallites) was formed on the surface. The XRD patterns (Fig. 6) and EDS mapping (Fig. 7(e)) indicated that Mo was present on the surface of the coating, suggesting that the TiB2 coating (13μm thick) lost its ability to protect the Mo substrate after 5h of oxidation at 900°C in air.

Fig. 6.

XRD patterns of Mo coated with TiB2 oxidised in air at different temperatures for 5h.

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Fig. 7.

SEM images and EDS mapping of Mo coated with TiB2 oxidised in air at 300–900°C for 5h.

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Furthermore, isothermal oxidation studies for the Mo substrate coated with the TiB2 ceramic coating were performed in air at 900°C for 1, 2, 3, 4, and 5h. Fig. 8 presents the XRD patterns of Mo coated with TiB2 oxidised in air at 900°C for different durations. As shown, TiO2 and B2O3 peaks were detected, as well as TiB2 peaks, after 1h of oxidation. Thus, both TiO2 and B2O3 were formed on the surface after 1h of oxidation. The intensity of the B2O3 peak was reduced after 2h, and the B2O3 peak disappeared after 3h. As described previously, the volatilisation of B2O3 increased with time, causing the disappearance of B2O3. After 2h of oxidation, only the TiB2 peak remained. After 5h of oxidation, the TiB2 peak disappeared, and a Mo peak was observed. Hence, when the sample was exposed to air for 5h at 900°C, the only phase on the surface was TiO2.

Fig. 8.

XRD patterns of Mo coated with TiB2 oxidised in air at 900°C for different durations.

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Fig. 9 shows SEM images of the specimens after oxidation at 900°C for various durations. As indicated by the SEM images of the as-prepared TiB2 coating surface in our previous work [35], the coating typically exhibited a rod-like particle microstructure with a small gap. As shown in Fig. 9(a), a liquid phase filled the small gap. According to the results of the foregoing analysis, the liquid phase was B2O3. This is consistent with the EDS mapping results in Fig. 9(a). Comparing Fig. 9(a)–(c) reveals that the B2O3 content decreased with the exposure time. Fig. 9(d) and (e) indicate that crystalline TiO2 was formed on the surface of the Mo substrate, supporting the foregoing XRD results and TG data.

Fig. 9.

SEM images and EDS mapping of Mo coated with TiB2 oxidised in air at 900°C for different durations.

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Fig. 10 presents cross-sectional SEM images and element line scanning EDS analysis results for the oxidation of the TiB2 coating at 900°C for different durations. Oxide scales were formed after oxidation for 1, 2, 3, 4, and 5h at 900°C. As shown in Fig. 10(a)–(e), the TiB2 coatings were generally oxidised with the increasing oxidation time, and there were obvious boundaries between the oxide layer and the unoxidised layer (TiB2). For the five aforementioned oxidation durations, the thicknesses of the oxide scales were approximately 3, 6, 9, 10, and 13μm, respectively. These results indicate that the oxidation rate was higher in the initial oxidation period than in the later stage, in accordance with the TG results shown in Fig. 5. The results of element line scanning EDS analysis indicated that the oxide scales mainly comprised O and Ti. The B content was lower in the oxide scales than in the TiB2 coatings. This suggests that the B2O3 evaporated during the oxidation at 900°C. The jagged concentration line profiles of B, Ti, and O for the oxide scales resulted from pores in the oxide scales. The pores were due to the evaporation of B2O3 formed during the oxidation of the dense TiB2 coating. Conversely, the smooth concentration line profiles indicate that the TiB2 coatings were dense.

Fig. 10.

Cross-sectional SEM images and element line scanning EDS analyses of the TiB2 coating after oxidation at 900°C for different durations in air.

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3.4Oxidation mechanism

The foregoing results indicate that the CVD TiB2 ceramic coating formed on the surface of the Mo metal protected the Mo substrate against oxidation up to 900°C in air. B2O3 was formed in the process of oxidation, and the protection against the oxidation of the coating depended on the formation of B2O3. The oxidation process of the TiB2 coating started at a low temperature (100°C), as indicated by the TG data (Fig. 5). At this temperature, the surface of the coating with the rod-like TiB2 phase was surrounded by low-viscosity B2O3 glass resulting from the slight oxidation of the TiB2 coating (see Figs. 8(b) and 9 (a)). The B2O3, as a protective layer, prevented the further access of O ions. Thus, the formation of the B2O3 layer prevented the further oxidation of the TiB2 coating. However, the volatilisation of B2O3 became noticeable above 720°C, resulting in rapid oxidation of the TiB2 coating. A high oxidation rate was detected up to 900°C through TG (Fig. 5). Isothermal oxidation studies (at 900°C) revealed that the oxidation rate decreased with the oxidation time, owing to the formation of a new B2O3 layer. However, the oxidation kinetics of the sample exhibited different trends at isothermal temperature 900°C duration of 5h. For the first 2.5h, the oxidation kinetics of the sample followed a parabolic law. For the next 2.5h, the oxidation kinetics essentially followed a linear law. After oxidation for 5h at 900°C, XRD data (Fig. 8) and SEM (Fig. 9) images indicated the complete consumption of TiB2 and the volatilisation of B2O3, and only a porous and non-protective TiO2 crystal was observed on the Mo substrate.

4Conclusions

To improve the oxidation resistance of Mo metal, a TiB2 coating was synthesised on the surface of pure metallic Mo using CVD. According to the experimental results, the following conclusions are drawn.

  • (1)

    The TiB2 ceramic coating, as an anti-oxidation protective layer on the Mo metal, was successfully obtained on the pure Mo substrate via CVD. The as-deposited TiB2 ceramic coating exhibited good adhesion to the Mo substrate and a high microhardness. These good mechanical properties of the coating were due to the roughened surface of the substrate and the high deposition temperature.

  • (2)

    The weight gain of only 0.03% indicates that the TiB2 coating exhibited high oxidation resistance during oxidation below 720°C. The TiB2 coatings also exhibited good oxidation resistance in the range of 720–900°C; the weight gain increased to 0.4% but was far smaller than the 2.3% weight gain of pure Mo. In this temperature range, the oxidation resistance depended on the coating-layer thickness and oxidation time. At 900°C, the 13-μm-thick TiB2 layer protected the Mo substrate against oxidation for up to 5h.

  • (3)

    The oxidation mechanism of the TiB2 coating was examined from room temperature to 900°C. The oxidation process of the TiB2 coating started at a low temperature (100°C). The oxidation resistance of the TiB2 coating depended on the state of the reaction product B2O3. Below 720°C, B2O3 functioned as a low-viscosity diffusion barrier that covered the surface of the coating, preventing further ingress of O ions and thereby preventing further oxidation of the TiB2. At 720–900°C, the B2O3 glass was consumed by vaporisation, which slightly reduced the protective ability.

  • (4)

    The oxidation kinetics of the sample exhibited different trends during oxidation for 5h at 900°C. For the first 2.5h, the oxidation kinetics essentially followed a parabolic law. For the next 2.5h, they followed a linear law.

Conflict of interest statement

The authors declared that they have no conflicts of interest to this work.

Acknowledgments

The authors thank Prof. Puqi Jia (Lanzhou University) for technically reviewing the manuscript.

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