Design and preparation of an ultra-high temperature ceramic by in-situ introduction of Zr2[Al(Si)]4C5 into ZrB2-SiC: Investigation on the mechanical properties and oxidation behavior

Novel ZrB2-matrix composites were designed and prepared by in-situ introducing SiC and Zr2[Al(Si)]4C5 simultaneously for the first time. The obtained composites were dense and showed good mechanical properties, especially the strength and toughness, 706 MPa and 7.33 MPa·m1/2, respectively, coupled with high hardness of 21.3 GPa, and stiffness of 452 GPa. SiC and Zr2[Al(Si)]4C5 constituted a reinforcing system with synergistic effects including grain refinement, grain pull-out as well as crack branching, bridging, and deflection. Besides, the oxidation results of the composites showed that the oxidation kinetics followed the parabolic law at 1600 °C, and the oxidation rate constants increased with the increase of Zr2[Al(Si)]4C5 content. The formation and evolution model of the oxidation structure was also investigated, and the oxide scale of the composite exhibited a three-layer structure.


Introduction 
Because of the extremely high melting points, and excellent physical and chemical properties, borides and carbides of IVB and VB transition metals, such as ZrC, ZrB 2 , HfC, HfB 2 , and TaC, are attractive representatives of ultra-high temperature ceramics (UHTCs) [1][2][3]. This group of materials can be adapted to extreme 2 Experimental

2 Characterization
The apparent porosity and volume density of the specimens were determined by Archimedes method. X-ray diffraction (XRD, Rigaku Max-2200, Tokyo, Japan) was used for phase analysis. The chemical states of elements were characterized by the X-ray photoelectron spectroscope (XPS, Nexsa, Thermo Fisher, Waltham, USA). The XPS data were analyzed by the surface chemical analysis software (Thermo Avantage) and NIST XPS database. The surface and fracture morphologies were analyzed by the scanning electron microscope (SEM, JEOL JSM-5900, Tokyo, Japan). And, the energy spectrometer equipped in SEM, i.e., energy dispersive spectroscopy (EDS), was used to conduct the elemental analysis and phase composition.

3 Mechanical property test
The specimens with the required dimensions were cut by the electric spark and then were polished. The processed samples were used for the tests of strength, toughness, Vickers hardness, and Young's modulus. Ten specimens with dimensions of 3 mm×4 mm×36 mm and ten specimens with dimensions of 2 mm×4 mm×22 mm were used for strength and toughness tests, respectively.
The strength and toughness were tested in the universal testing machine (SUNS UTM2203, Shenzhen, China), and the test method and process were the same with that in our reported work [13]. The bending strength was measured using a three-point bending method with a span of 30 mm and a cross head speed of 0.5 mm/min. The fracture toughness was determined using the single-edge notched beams (SENB) method with a span of 16 mm and a cross head speed of 0.05 mm/min. The notch with a depth of 2 mm and width ≤ 0.26 mm was made by a dicing saw. Vickers hardness of the polished samples with sizes of 8 mm×8 mm was tested by a hardness tester (Huayin 200HV-5, Laizhou, China) under the load of 1 kg for 10 s, and twenty data for each sample were collected to obtain the results. The modulus (sample size of 10 mm×10 mm) was evaluated by ultrasonic material test system (RITEClab UMS-100, Chelles, France).

4 Oxidation resistance test
The high-temperature oxidation test was carried out in a tube furnace (Kejing GSL-1700X, Hefei, China). When the temperature in the furnace reached and was stable at the target temperature (1600 ℃), the polished samples with a size of 3 mm×4 mm×8 mm were placed in the middle of the furnace immediately. After being held for some time (0.5, 1, 2, 3, and 5 h), the oxidized samples were taken out directly. The oxidation weight gain was measured using an electronic balance (Mettler Toledo ME204E, Shanghai, China) with an accuracy of ±0.1 mg. The phase compositions and chemical states of the oxidized scales were characterized using XRD and XPS. The microstructure of the oxide scale was analyzed using SEM, and the chemical analysis was carried out by the EDS.

2 Phase composition
With the Zr 2 [Al(Si)] 4 C 5 content increasing, its relative intensity of the peaks increased, while those of ZrB 2 and SiC decreased. From the backscattered electron images shown in Fig. 3, the three expected phases with  Table 2. The measured data showed that all three composites were dense, and the composites exhibited a high relative density of ˃ 99% and a low apparent porosity of < 0.05%. In addition, due to the lower density of Zr 2 [Al(Si)] 4 C 5 (4.50 g/cm 3 ) [28] than that of  ZrB 2 -SiC (molar ratio of 2:1, 5.37 g/cm 3 [34]), the density of the composite decreased with the Zr 2 [Al(Si)] 4 C 5 content increasing. As is known, the use of lightweight materials is one of the primary considerations in the design and manufacture of aerospace vehicles, which is beneficial for weight reduction and speed increase. Thus, novel ZrB 2 -SiC-Zr 2 [Al(Si)] 4 C 5 ceramic with low density is an attractive candidate for the application to the high-temperature environment.

3 Mechanical properties and microstructure
The measured mechanical properties of ZrB 2 -SiC-Zr 2 [Al(Si)] 4 C 5 are listed in Table 2 [34,35]. It can be concluded that the composites exhibit better strength and toughness than those of ZrB 2 -SiC prepared under the same condition. So, the incorporation of Zr 2 [Al(Si)] 4 C 5 is beneficial for the further improvement of strength and toughness. As can be observed in Fig. 3, the grain sizes of ZrB 2 decreased with the Zr 2 [Al(Si)] 4 C 5 content increasing, which showed the grain refining effect. From the fracture surfaces shown in Fig. 4, the grain refining effect could also be observed. And, there were no pores in the grains or at the grain boundaries, and the composites were dense, which was in good agreement with the density measurement. Besides, the fracture mode exhibited a mixture of intergranular and transgranular mode, and the rough fracture surfaces showed visible grain pull-out. The above fracture characteristics were beneficial for the improvement of strength and toughness. However, the high content of Zr 2 [Al(Si)] 4 C 5 in ZS30Z led to the decrease of strength and toughness. Due to the thermo-elastic mismatch between ZrB 2 and Zr 2 [Al(Si)] 4 C 5 , residual stress arose at the phase interfaces [12]. The coefficients of thermal expansion of ZrB 2 and Zr 2 [Al(Si)] 4 C 5 are 6.8×10 -6 and 8.1×10 -6 , respectively [13,28]. So, according to Hsueh's formula, the radial matrix stress was tensile [12,13]. The high content of Zr 2 [Al(Si)] 4 C 5 in ZS30Z could make the tensile stress increase, resulting in the decreased strength and toughness. Besides, the grain refinement was not evident enough in ZS30Z, and the aggregation of some particles could be observed. These could be the reasons for the decrease of strength and toughness [12][13][14].
The crack propagations of the composites on the polished surfaces are shown in Fig. 5. The crack deflection around the particles could be observed. Besides, the crack branching and bridging could also be observed on the crack path. These obvious characteristics in the composites could improve the strength, especially the toughness. For the specimen with low Zr 2 [Al(Si)] 4 C 5 content, i.e., ZS10Z, the crack was relatively straight, which should be similar to that in ZrB 2 -SiC. In ZS20Z, the crack was obviously circuitous, while that in ZS30Z seemed to be relatively straight again. This shows that the strength and toughness are related to the crack deflection degree, which can extend the crack propagation path, consume the fracture energy, and therefore improve the strength and toughness [12,13]. The crack is easy to propagate  along the load direction during the fracture, so the angle between the directions of the crack and the load may have a direct relationship with the strength and toughness. To further investigate this, the deflection angles between the directions of the fracture crack and the load of all the broken bars were counted. The optical microscope photographs of the fracture crack are shown as representatives in Fig. 6. According to the results of statistical calculation, ZS20Z showed the maximum deflection angle between the directions of the crack (arrows) and the load (dotted lines). The deflection angle of the crack in ZS20Z was ~10°±4°, while those in ZS10Z and ZS30Z were ~3°±3° and ~6°±3°, respectively. This was consistent with the change of strength and toughness. In addition, from the Vickers indentation on the polished surface of ZS20Z (Fig. 5(d)), the crack deflection, bridging, and branching on the crack propagation path and the grain pull-out near the indentation edge could be observed. In summary, these data provided good support for the improved strength and toughness of the prepared materials, especially for ZS20Z.
Additionally, due to the lower Vickers hardness and Young's modulus of Zr 2 [Al(Si)] 4 C 5 than those of ZrB 2 -SiC, the hardness and modulus of the composites should theoretically decrease with the increase of the Zr 2 [Al(Si)] 4 C 5 content according to the mixing rule, which were consistent with the results. The measured hardness and modulus of the composites decreased slowly from ~21.3 and 452 GPa for ZS10Z to ~16.8 and 419 GPa for ZS30Z, respectively.

Oxidation kinetics
The oxidation kinetics was investigated through the relationship curves of the weight gain/surface area (Δw/s) and the oxidation time (t). The corresponding curves of the composites are plotted in Fig. 7(a). All the composites exhibited similar curves, and it indicated that the composites had the same oxidation kinetics in this test condition. From the curves, it could be observed that the Δw/s increased significantly first and then relatively slowed down with t increasing. This was possibly corresponding to two oxidation stages. At the initial oxidation, the oxidation products formed rapidly with a certain thickness, which constituted a protective layer on the surface to prevent further oxidation. After that, the oxidation entered the passivation stage. From the curves, it also clearly showed that the Δw/s increased with the Zr 2 [Al(Si)] 4 C 5 content increasing at the same t. For example, the Δw/s increased from 0.268 kg·m -2 for ZS10Z to 0.534 kg·m -2 for ZS30Z after oxidation at 1600 ℃ for 5 h. The results meant that the oxidation resistance decreased with the Zr 2 [Al(Si)] 4 C 5 content increasing. The relationship curves of (Δw/s) 2 versus t and the corresponding linear fitting curves (dotted line) adjusted are plotted in Fig. 7(b). R 2 ≥ 0.97 is also given. The results indicated that the oxidation kinetics of the composites followed a parabolic law [33] at 1600 ℃. And, the calculated slopes of the linear curves are the oxidation rate constant (k 1 ) of the composites, which are shown in Table 3. The increase of Zr 2 [Al(Si)] 4 C 5 content led to the increase of k 1 , which increased from 0.432×10 -5 kg 2 ·m -4 ·s -1 for ZS10Z to 1.653×10 -5 kg 2 ·m -4 ·s -1 for ZS30Z. Ignoring the volatilization of oxidation products, the results confirm the decreased oxidation resistance of the composites with the Zr 2 [Al(Si)] 4 C 5 content increasing.
Besides k 1 , the oxidation kinetics can also be investigated from the relationship curves between the thickness of the oxidation layer (δ) and t [37], which are plotted in Fig. 8(a). It showed similarity to the shape and trend of Δw/s versus t. The δ 2 displayed in Fig. 8(b) also showed a linear relationship versus t. Based on the results, δ was indispensable in the investigation of the oxidation kinetics for the composites, and the oxidation kinetic curves also followed the parabolic law here. It is notable that the oxidation kinetics may be reflected better by the relationship between δ and t because of the evaporation and volatilization of gaseous oxidation products. According to the data in Fig. 8(b), the calculated rate constant, i.e., k 2 (listed in Table 3), of ZS10Z was ~15.43 μm 2 ·min -2 ,   which was ~59% and ~27% of that of ZS20Z (~26.28 μm 2 ·min -2 ) and ZS30Z (~57.99 μm 2 ·min -2 ), respectively. Accordingly, the measured δ of ZS30Z was ~980 μm after oxidation for 5 h, which was also obviously thicker than that of ZS10Z (~516 μm). Above all, the composites started to decrease their oxidation resistance with the Zr 2 [Al(Si)] 4 C 5 content increasing. Nevertheless, compared with ZrB 2 -SiC-ZrC, the composites still seemed to show improved oxidation resistance [25]. respectively. It is noticeable that aluminosilicate has a complicated phase composition with the chemical formula of xAl 2 O 3 ·ySiO 2 . Additionally, the product formed by the oxidation of B element, i.e., B 2 O 3 , has a high vapor pressure and cannot exist stably under this oxidation condition [38,39]. Therefore, according to the phase composition of the oxide surface and previous works [33,38,[40][41][42], some main reactions involved in the oxidation of the composites were described as follows:

Oxidation surfaces and products
After oxidation under the same condition, with the Zr 2 [Al(Si)] 4 C 5 content increasing, the composition of aluminosilicate changed, and the formation content of aluminosilicate increased. For the same sample, with further oxidation, the formed Al 2 O 3 could be consumed to produce aluminosilicate. However, the aluminosilicate content in the oxide surface seemed to decrease. This could be due to the formation of SiO 2 glass phase covering the surface [41].
To further investigate the chemical state and confirm the composition of the oxide scale, the XPS spectra of representative ZS20Z after oxidation at 1600 ℃ for 2 h is shown in Fig. 10. The XPS results confirmed the presence of Zr, Si, Al, and O in the oxide products. The high-resolution spectra calibrated by the C 1s signal of 284.8 eV were presented as well as the corresponding fitting curves. As shown in Fig. 10(a), the two peaks at ~182.0 eV (Zr 3d 5/2 ) and ~184.4 eV (Zr 3d 3/2 ) corresponded to the Zr-O bond, and the weak peaks detected at ~183.4 and ~185.7 eV represented the presence of Zr-O-Si bond. This indicated that the oxidation products containing Zr were mainly composed of ZrO 2 and a small amount of ZrSiO 4 . The formation of ZrSiO 4 was also confirmed in the spectra of the Si 2p band shown in Fig. 10(b), in which the peak at ~102.8 eV belonging to the bond of Zr-O-Si was detected. Besides, the peaks at ~102.2 and ~103 eV were assigned to the bonds of Al-O-Si and Si-O from aluminosilicate and SiO 2 glass phase, respectively. In the spectra of the Al 2p band, i.e., Fig. 10(c), two peaks at ~74.4 and ~74.6 eV corresponding to the bonds of Al-O-Si and Al-O could be detected and identified as  aluminosilicate and trace Al 2 O 3 , respectively. Therefore, the XPS results were consistent with the composition analysis and the oxidation reactions mentioned above.
The surface morphologies of the composites after oxidation are presented in Fig. 11. From Figs. 11(a) and 11(b), the surface of oxidized ZS10Z at 1600 ℃ for 1 h consisted of a continuous oxide surface, which was relatively flat and dense. This could be ascribed to the abundant SiO 2 glass phase produced during the oxidation. The formation and distribution of oxide products were further investigated by the EDS mapping results shown in Figs. 11(g)-11(i). It is notable that ZrO 2 exhibited two different morphologies, i.e., the dendritic and granular grains. The dendritic morphology of ZrO 2 was also found in Ref. [43]. For the oxide surface of ZS20Z shown in Figs. 11(c) and 11(d), it became rough, and the holes caused by the volatilization of gaseous products, e.g., B 2 O 3 and CO, could be observed. The increasing content of Zr 2 [Al(Si)] 4 C 5 led to a decrease content of SiC. So, the content of formed SiO 2 also decreased, while the content of formed Al 2 O 3 increased, and more aluminosilicate was produced by Reaction (5). Additionally, SiO 2 on the surface could also volatilize gradually by the following reaction [38,40]: Thus, the filling effect of the SiO 2 glass phase among these products was diminished. Despite that, the produced SiO 2 seemed to fill in most of the holes during the oxidation of ZS20Z, as shown in Fig. 11(d). However, with further increasing the content of Zr 2 [Al(Si)] 4 C 5 , the limited SiO 2 seemed to not fill the holes, and the deep holes could be observed clearly in the oxide scale of ZS30Z as shown in Figs. 11(e) and 11(f), resulting in a discontinuous and porous oxide surface and improved diffusion of oxygen through these holes. The cross-section morphologies of the composites after oxidation are presented in Fig. 12,   Fig. 11 Surface morphologies of (a, b) ZS10Z, (c, d) ZS20Z, and (e, f) ZS30Z oxidized at 1600 ℃ for 1 h, and (g-i) the EDS maps of (b).

Fig. 12
Cross-sectional morphologies of (a) ZS10Z, (b) ZS20Z, and (c) ZS30Z oxidized at 1600 ℃ for 1 h. and the residual holes could also be clearly observed in the oxide surface of ZS30Z (Fig. 12(c)). And, with the increase of Zr 2 [Al(Si)] 4 C 5 content, the thicknesses of the oxidation layers increased. Therefore, based on the analysis results, the protection capability of the oxide scale for preventing the further oxidation of the substrate decreased with the increase of Zr 2 [Al(Si)] 4 C 5 content, which was in agreement with the analysis results of the oxidation kinetics.

Oxidation mechanism
The SEM morphologies and corresponding EDS results of the cross-sections of representative ZS20Z oxidized at 1600 ℃ for 1 h are shown in Fig. 13. As presented in Fig. 13(a), the oxide layer on the substrate displayed a three-layer structure, in which the oxide layer interfaces were clearly shown in Fig. 14. It could be observed that the outer layer and the inner layer were dense, while the middle layer seemed to be loose and porous. The EDS mapping results showed that all the elements had different distributions in different oxide layers. The oxygen content shown in Fig. 13(d) was reduced from the outer oxide layer to the middle layer and then to the inner layer. This was corresponding to the three-layer structure of the oxide scale in Fig. 13(a). Zr element seemed more accumulated in the middle and inner layer, while more Al element existed in the outer layer. Si element was depleted from the middle layer. Although the detection signal of C was weak due to the detection limit of the instrument, it could still be observed that there was C residue in the middle and inner oxide layer, of which the signal intensity was equivalent to that recorded in the substrate.
Based on the obtained results, it could be inferred that ZrO 2 , SiO 2 , and Al 2 O 3 were formed and covered the substrate immediately by Reaction (4). And then, aluminosilicate was produced by the reaction of SiO 2 and Al 2 O 3 , i.e., Reaction (5). Thus, the four main phases formed a primary outer layer on the surface in the initial oxidation stage. With the continuously inward diffusion of O 2 , the substrate covered by the oxide scale was further oxidized. Zr 2 [Al(Si)] 4 C 5 seemed to be oxidized first in the inner layer according to the EDS mappings. Zr and Al were oxidized to ZrO 2 and   Al 2 O 3 , respectively. Si could be oxidized to SiO by active oxidation [38,44], and residual C was formed due to the relatively low oxygen concentration, which was caused by the inward diffusion of O 2 through the oxide surface at high temperatures. Therefore, the main oxidation in the inner oxide layer could be described as follows: Zr 2 [Al 3.56 Si 0.44 ]C 5 + 4.89O 2 →2ZrO 2 + 1.78Al 2 O 3 + 0.44SiO↑ + 5C (8) Thus, the inner layer mainly consisted of ZrO 2 , Al 2 O 3 , C, and unreacted ZrB 2 and SiC. With further oxidation, unreacted ZrB 2 and SiC were oxidized to form a Si-depleted layer through the following reactions [38,41,43]: SiC + 0.5O 2 → SiO↑ + C The middle oxide layer mainly consisted of ZrO 2 , Al 2 O 3 , and C covered the newly-formed inner oxide layer. However, the formation of little SiO 2 could be observed in the middle layer, which was due to that SiO diffused outward could be further oxidized [42]: SiO + 0.5O 2 → SiO 2 (11) Additionally, Al 2 O 3 could move toward the outer layer together with SiO 2 by forming aluminosilicate with lower viscosity [45]. Thus, a porous middle scale was formed, which provided an easy path for the inward diffusion of O 2 . During the transfer of Al 2 O 3 and SiO 2 toward the outer layer, SiO 2 could also react with ZrO 2 to form ZrSiO 4 . Besides the reaction consumption, a part of SiO 2 on the surface could volatilize through Reaction (7). Additionally, coarsening ZrO 2 grains could be seen in the outer layer, which could be due to the formed aluminosilicate with lower viscosity promoting the mobility and growth of ZrO 2 particles [45]. Ultimately, an oxide scale with a three-layer structure was formed on the substrate, which was displayed in Figs. 13(a) and 14. Based on the above analysis results, its oxidation sketch is also illustrated in Fig. 15.

Conclusions
Novel ZrB 2 -SiC-xZr 2 [Al(Si)] 4 C 5 UHTCs with relative densities ˃ 99% were successfully obtained via reactive hot-pressing at 1850 ℃ in Ar. With the introduction of Zr 2 [Al(Si)] 4 C 5 into ZrB 2 -SiC, the composites showed good mechanical properties, especially improved strength and toughness, e.g., 706 MPa and 7.33 MPa·m 1/2 , respectively, coupled with Vickers hardness of 21.3 GPa, and Young's modulus of 452 GPa. In-situ formed SiC and Zr 2 [Al(Si)] 4 C 5 constituted an effective binary reinforcing system with different scales and morphologies, which showed synergistic strengthening and toughening effects including grain refinement, grain pull-out, crack branching, bridging, and deflection. Additionally, the oxidation behavior of the composites at 1600 ℃ was investigated. According to the relationship between the oxidation weight gain or the thickness of oxide scale and the time, it showed that the oxidation kinetics of all the prepared composites followed the parabolic law, and the composites showed degraded oxidation resistance with the increase of Zr 2 [Al(Si)] 4 C 5 content. The formation and evolution model of the oxide scale was also given, and the oxide scale exhibited a three-layer structure, i.e., a dense outer layer, a porous middle layer, and a dense inner layer. The obtained results in this paper can provide a basis for the preparation and properties of UHTCs.