1. Introduction
Ceramic is a type of material with unique characteristics, such as high-temperature oxidation resistance, high strength, and elastic stiffness, but it has inherent brittleness and low machinability [
1,
2,
3,
4]. A special group of materials in the ceramic family are the MAX-phase ceramics, which have a hexagonal structure and combined metal-ceramic properties. MAX-phase materials have broad application prospects owing to their excellent properties. However, compared with conventional ceramics, the hardness and high-temperature oxidation resistance of MAX-phase materials are lower, which significantly limits their application in the engineering field. Therefore, it is necessary to improve their mechanical properties and high-temperature stability. Ti
3SiC
2 is a MAX-phase compound with a layered structure that is a promising candidate for high-temperature applications [
5,
6]. In addition to its simple machinability, this material has excellent properties, such as electrical conductivity, thermal conductivity, and thermal shock resistance [
7,
8]. As a typical MAX-phase material, Ti
3SiC
2 is a promising structural ceramic for high-temperature applications such as heating elements in high-temperature furnaces and fuel-combustion components in automobiles and aircraft engines [
9,
10].
Notably, the oxidation resistance of Ti
3SiC
2 is crucial and has been investigated extensively, whether in the application of high-temperature structural ceramics or connection materials for solid oxide fuel cells. The preferential oxidation behavior of Ti
3AlC
2 is different from that of Ti
3SiC
2, which has a continuous Al
2O
3 layer [
11,
12,
13]. The antioxidation capacities of Ti
3SiC
2 require further improvement for its effective application.
Reinforcement phases, including TiC, SiC, c-BN, TiB
2, and ZrO
2, have been used to improve the mechanical properties and oxidation resistance of Ti
3SiC
2 [
14,
15,
16,
17]. Li et al. [
18] prepared dense SiC/Ti
3Si(Al)C
2 composites using an in situ hot-pressing sintering method and reported that the oxide layers formed at 1200 and 1300 °C were divided into outer, middle and inner layers. To obtain high-purity Ti
3SiC
2, Xu et al. [
19] demonstrated that the incorporation of a small amount of Al was beneficial for improving the purity of Ti
3SiC
2. Moreover, the addition of Al was advantageous for improving the oxidation resistance of the composites [
20,
21]. Some researchers believe that the optical and electrical properties of alumina at high temperature have crucial application value and prospects for fusion technology [
22,
23,
24].
Thus, the dense Al2O3 layer formed during the oxidation of Ti3AlC2 is the design inspiration for this study. Moreover, considering the lack of an effective anti-oxidation protective layer in the Ti3SiC2 oxidation process, thermally stable Al2O3 is selected as a reinforcement phase in this study to change the oxidation resistance of Ti3SiC2. A Ti3SiC2/Al2O3 composite is synthesized in situ using the hot-pressing sintering method, and the high-temperature oxidation resistance of the composite is reported. Therefore, this study aims to present a detailed investigation of the high-temperature oxidation resistance of Ti3SiC2/Al2O3.
2. Experimental Procedure
The volume capacity of 30%, 40% and 50% Al2O3 were added and the powders of Ti:Si:TiC:Al in the molar ratio of 1:1.2:2:0.3 were used to synthesize Ti3SiC2/Al2O3 composites. In situ synthesis of Ti3SiC2 and Al2O3 was designed, namely TSC70 (Ti3SiC2/30 vol.% Al2O3), TSC60 (Ti3SiC2/40 vol.% Al2O3), TSC50(Ti3SiC2/50 vol.% Al2O3). TiC (99.9% purity, average particle size 1 μm, Shanghai ST-Nano Technology Co., Ltd., Shanghai, China), Ti (99.9% purity, average particle size 1–3 μm, Shanghai ST-Nano Technology Co., Ltd., Shanghai, China), Al (99.9% purity, average particle size 50 nm, Shanghai ST-Nano Technology Co., Ltd., Shanghai, China), Si (99.9% purity, average particle size 1 μm, Shanghai ST-Nano Technology Co., Ltd., Shanghai, China) and Al2O3 (99.9% purity, average particle size 30 nm, Shanghai ST-Nano Technology Co., Ltd., Shanghai, China) were used as raw materials. The original powders were mixed into ethanol by ball-milling for 4 h. Then the slurry was dried in a drying oven at 40 °C for 6 h and then sieved under 100 mesh. The Ti3SiC2/Al2O3 composites were in situ fabricated by vacuum hot-press sintering (VVPgr-80-2200, Shanghai, China) at 1450 °C with an applied pressure of 30 MPa for 1.5 h (the vacuum degree was 6.71 × 10−3 MPa).
For the oxidation experiments, rectangular blocks of size 5 mm × 4 mm × 4 mm were cut using a cylindrical SiC blade. The surface was polished with SiC paper. Thereafter, the samples were ultrasonically cleaned with ethanol to remove surface impurities. Oxidation tests was carried out in an alumina tube furnace at 1400 °C and the samples were exposed for up to 20 h. After the alumina tube furnace was heated to the test temperature, the block Ti3SiC2/Al2O3 composites to be tested were placed in the furnace. Using an electronic balance of accuracy 1 × 10−7 kg, the difference in weight gain was calculated.
X-ray diffraction (XRD) (D8 ADVANCE, Bruker, Saarbrucken, Germany) was used to confirm the phases of the samples before and after oxidation. The microstructure of the oxidized samples was observed by scanning electron microscopy (SEM) (FEI QUANTA FEG 250, Hillsboro, OR, USA) with energy dispersive X-ray spectrum (EDS).
3. Results and Discussion
The phase components of the Ti
3SiC
2/Al
2O
3 composite were characterized by X-ray diffraction (XRD).
Figure 1 shows the XRD patterns of TSC70, TSC60 and TSC50 before oxidation. As shown in
Figure 1, Ti
3SiC
2 and Al
2O
3 did not generate Ti–Al compounds, indicating that the composite degree between Ti
3SiC
2 and Al
2O
3 was appropriate. This plays an important role in the subsequent investigation of the high-temperature oxidation resistance of the Ti
3SiC
2/Al
2O
3 composite. Notably, several TiC peaks were observed in TSC70, but none were detected in TSC50 and TSC60. Under the same sintering conditions, the peak intensities of Al
2O
3 increased with increasing Al
2O
3 content, corresponding with the change in the volumetric fraction added during synthesis. This phenomenon indicates that the selection of the raw material was successful. Sun et al. [
25] reported that the oxidation rate of Ti
3SiC
2 is slower than that of TiC, and TiC is detrimental to the oxidation resistance of Ti
3SiC
2. Therefore, in the subsequent experiments, two sets of samples without TiC were selected for the oxidation test.
Figure 2 and
Figure 3 show the XRD patterns of the TSC50 and TSC60 samples, respectively, oxidized at 1400 °C. After the oxidation of TSC50 for up to 4 h, XRD showed that the TiO
2 content was relatively low. The presence of matrix Ti
3SiC
2 and Al
2O
3 was caused by the short oxidation time, and a dense and continuous oxide layer was not formed. Thus, the exposure of the matrix to the surface was accompanied by a small amount of TiO
2. In contrast to TSC50, the oxidation products of TSC60 with less Al
2O
3 were different after 4 h of oxidation. The intensity of the Ti
3SiC
2 peaks in TSC60 were significantly reduced, and the oxidized surface was mainly composed of TiO
2, Al
2O
3, and newly generated Al
2TiO
5, indicating that a continuous and thin oxide layer was formed on the sample surface.
As shown in
Figure 4, the scanning electron microscopy (SEM) results show the surface morphology of the oxide layer after the oxidation of TSC50 and TSC60 for 4 h. Ti
3SiC
2 and Al
2O
3 remained on the surface of the TSC50 oxide layer. The morphology of the grains was massive and layered, and the pores were clearly observed. After oxidation, some grains were aggregated, which indicates oxide layer growth. The newly generated TiO
2 was connected to Ti
3SiC
2 with an evident layered structure, indicating its tendency to encapsulate Ti
3SiC
2. Conversely, after the oxidation of TSC60, the grain morphology on the oxide layer surface did not exhibit a lamellar structure. Although the grain size on the oxide layer surface of TSC60 was larger than that of TSC50, the grain morphology of TSC50 was more regular with a more distinct orientation. Stomata were observed on the surface of TSC50, and these pores may serve as channels for oxygen diffusion into the matrix. Compared with the sparsely oxidized surface of TSC50, TSC60 exhibited a state of mutual fusion and airtightness among the grains. Moreover, the fusion boundary of TSC60 could be clearly observed. Trace pores and cracks were observed on the oxidized surface of TSC60. The cause of the cracks may be the short oxidation time and the incomplete growth of the oxide layer. The difference in the oxidized surface morphology may be owing to the variation in the Al
2O
3 contents, which causes different degrees of oxidation between oxygen and Ti
3SiC
2 during the oxidation process.
Figure 5 and
Figure 6 show the surface morphology and energy dispersive X-ray spectroscopy (EDS) results of TSC50 and TSC60 after 4h of oxidation. The oxidized surface formed a well-shaped crystal. EDS analysis indicated that the dense massive crystals on the oxidized surface of TSC50 (
Figure 5) mainly contained Ti, Al, and O. Based on the types of elements and grain structures observed, the main components observed at points 1 and 2 in
Figure 5 were identified as Al
2O
3 and TiO
2. Moreover, Si was not detected, indicating that SiO
2 was not present on the oxidized surface. Furthermore, a minor difference in the contents of Ti and Al was observed. Additionally,
Figure 6 shows two types of crystal morphologies, in which the content of Al (the flat grain identified by point 1) was much higher than that of Ti. Based on the XRD results, the oxidized surface of TSC60 contained a small amount of Al
2TiO
5. Aluminium titanate has a plate-titanite-type crystal morphology with typical plate-like and blade-like crystals. Therefore, the grain indicated at point 1 in
Figure 6 is proposed to be Al
2TiO
5. Simultaneously, the grain indicated at point 2 had a higher content of elemental Ti and appeared columnar, which is consistent with the crystal structure of rutile. Therefore, the EDS results were in good agreement with the XRD results.
Figure 7 shows the change in the weight gain per unit area of the two composites over time at a temperature of 1400 °C. During the oxidation period of 0–4 h, the weight gain per unit area decreased with the increasing Al
2O
3 content in the composites. However, when the oxidation time exceeded 4 h, the weight gain per unit area increased with the increasing Al
2O
3 content. With increasing time, the weight gain of TSC60 stabilized, indicating that TSC60 had transformed after 4 h of oxidation, thus reducing the degree of the subsequent oxidation processes. After 20 h of oxidation, the weight gain of TSC50 was 42.253 × 10
−3 kg/m
2, whereas that of TSC60 was 29.411 × 10
−3 kg/m
2, which is approximately 70% of the former. According to the XRD and EDS results, a mixed layer of Al
2TiO
5 and TiO
2 formed on the surface of TSC60 after 4 h of oxidation. Therefore, it is proposed that the presence of a mixed layer effectively reduces the rate of the subsequent oxidation of the composites. As a material with good thermal shock resistance and excellent high-temperature stability, Al
2TiO
5 played a significant role in reducing the oxidation rate of Ti
3SiC
2/Al
2O
3 in this study.
As discussed, pores and cracks were present on the oxidized surface of Ti
3SiC
2/Al
2O
3, providing a diffusion channel for oxygen. Therefore, the oxidation of Ti
3SiC
2/Al
2O
3 was a diffusion-controlled process. Moreover, the oxidation of Ti
3SiC
2 was caused by the outward diffusion of Ti, Si and carbon, and the inward diffusion of oxygen. Although the oxidation of Ti
3SiC
2/Al
2O
3 was a diffusion-controlled process, the addition of Al
2O
3 changed the composition of the oxide layer. The cross-section of the oxide layer after 20 h of oxidation is shown in
Figure 8. The thickness of the oxide layer of TSC60 was approximately 253 μm. Moreover, the oxide layer exhibited a silicone-free outer layer of approximately 40 μm. Based on the EDS results, the outer layers were clearly composed of Al
2TiO
5 and TiO
2. The presence of Si in the composite layer was detected, indicating that Si did not undergo external diffusion when it reached the composite layer, as shown in
Figure 9. Thus, SiO
2 formed by silicon diffusion, and Al
2TiO
5 and Al
2O
3 compounded to form a glass phase, which increased the compactness of the composite layer and prevented the diffusion of Si and Ti [
26]. As shown in
Figure 9, a large amount of TiO
2 was present in the Al-deficient layer. Because the oxygen pressure in the outer layer was higher than that in the inner layer, Si gave priority to SiO gas generation. As the diffusion process progressed, SiO gas became a SiO
2 barrier that encapsulated TiO
2, resulting in a condition of Si enrichment; therefore, only a small amount of Al was detected [
27]. In contrast, TSC50 did not have dense layers but it also had an Al-deficient layer of approximately 18 μm (
Figure 10). At elevated temperatures and extended periods, such as 1400 °C and 20 h, the oxidation rate was high. Notably, this temperature is similar to the melting point of Si; thus, Si was more reactive. The Si content may be one of the reasons for the difference between the TSC60 and TSC50 oxide layers. Owing to its excellent high-temperature stability, Al
2O
3 did not decompose at this oxidation temperature; however, it reacted with other oxides to protect the matrix.
Based on the above analysis, it is evident that Al
2O
3 improves the high-temperature oxidation resistance of Ti
3SiC
2. Compared with the studies reported by Zhang et al. [
28] and Gao et al. [
29], the oxide layer of Ti
3SiC
2/Al
2O
3 formed in this study was thinner. Moreover, the Al
2O
3 content was one of the factors influencing the oxidation resistance of the composite. Increasing the Al
2O
3 content did not always yield positive results. Thus, it is proposed that an optimal range of Al
2O
3 content exists, in which the high-temperature oxidation resistance of Ti
3SiC
2/Al
2O
3 is improved. This aspect will be investigated further in future research studies.