Synthesis and properties of MoAlB composites reinforced with SiC particles

Abstract: Novel MoAlB composites reinforced with 5–15 vol% SiC have been firstly prepared and characterized in the present study. The SiC reinforcement is stable with MoAlB at a sintering temperature of 1200 °C in Ar. The 5 vol% SiC/MoAlB composite exhibited improved mechanical properties and enhanced oxidation resistance. A flexural strength of 380 MPa and a Vickers hardness of 12.7 GPa were achieved and increased by 24% and 51%, respectively, as compared with those for MoAlB, indicating the enhanced strengthening effect of SiC. Cyclic oxidation tests at 1200 and 1300 °C for 10 h in air showed that the 5 vol% SiC/MoAlB composite has better oxidation resistance than MoAlB due to the formation of a dense and continuous scale composed of Al₂O₃ and SiO₂, which prevents the oxygen inward diffusion and the evaporation of oxides. We expect that the general strategy of second phase reinforcing for materials will help to widen the applications of MoAlB composites.

Keywords: SiC/MoAlB composite; mechanical properties; oxidation; microstructure

1 Introduction

MoAlB is an attractive ternary boride in MAB phase family (where M is a transition metal, A is aluminium or zinc, and B is boron ^[1]) not only due to its ease of fabrication and densification at relatively lower temperatures (1050–1200 ℃) but also to its excellent high temperature oxidation resistance as compared with the vast majority of binary transition metal borides. The binary transition metal borides generally require high temperatures above 1700 ℃ to achieve full density, but have poor oxidation resistance at temperatures above 1000 ℃ ^[2]. 

MoAlB, as the most studied MAB phase, has been widely investigated from theoretical simulations ^[3–5] to experimental studies, and exhibits attractive mechanical properties, high temperature properties, radiation tolerance, and ablation resistance ^[6–12]. So far, MoAlB as the only one in the MAB phase family has been demonstrated with better oxidation resistance than other Al-containing MAB phases (such as Fe₂AlB₂ and Cr₂AlB₂) at temperatures below 1300 ℃ due to the formation of a dense and continuous α-Al₂O₃ layer ^[13–16]. The oxidation resistance of MoAlB is even better than that of Ti₂AlC and Cr₂AlC MAX phases at 1100 ℃ ^[17–19]. For example, the oxidation rate constant K_c of MoAlB is about 7.1 × 10⁻²³ m³/s at 1100 ℃ ^[8], lower than 1.0 × 10⁻²¹–1.8 × 10⁻²¹ m³/s for Ti₂AlC ^[17]. However, increasing oxidation temperature leads to the rapid mass gains of MoAlB. To further improve its oxidation resistance, the replacement of some Al by Si in MoAlB to form a Mo(Al,Si)B solid solution has been designed because the dissolved Si can be oxidized to SiO₂ which retards the inner diffusion of O upon oxidation. The Mo(Al,Si)B solid solution exhibited better oxidation resistance than its natural counterpart ^[19]. In the temperature range of 1200–1400 ℃, the mass gain and the scale thickness of Mo(Al,Si)B are lower than those of MoAlB ^[19]. The above results confirm that Si can effectively improve the oxidation resistance of MoAlB. However, the solubility of Si in MoAlB is limited, with the maximum solubility of ~0.91 at% Si ^[20], causing the difficult study of the influence of Si content on the oxidation behavior of MoAlB.

Another facile route to enhance the oxidation resistance of MoAlB is the incorporation of ceramic particles. Among ceramic particles, SiC is preferred to reinforce MoAlB due to the following two reasons. First, SiC has high hardness and excellent oxidation resistance. Second, SiC has been proved to be a good reinforcement in MAX composites. The incorporation of SiC in Ti₃SiC₂ matrix is one of the simplest yet effective approaches to prepare composites with greatly improved mechanical properties and high temperature oxidation resistance ^[21–26]. Up to now, to the best of our knowledge, there are no reports on the synthesis and characterization of SiC/MoAlB composites. 

In the present study, the main purpose is to prepare SiC/MoAlB composites for the first time. The mechanical properties and oxidation resistance of composites were investigated. The phase composition and microstructure of the prepared composites were characterized. 

2 Experimental 

2. 1 Material preparation 

To prepare MoAlB samples, Mo (-300 mesh, 99.5% purity, General Research Institute for Nonferrous Metals GRINM, China), Al (-300 mesh, 99.5% purity, Beijing Reagent Company, China), and B (-300 mesh, 99% purity, GRINM, China) powders with a molar ratio of Mo:Al:B = 1:1.3:1 were mixed for 10 h, and then pressureless-sintered at 1200 ℃ for 1 h in Ar atmosphere. The sintered samples were pulverized, and then sieved with a 200 meshed sieve to obtain MoAlB powders.

To prepare SiC/MoAlB composites, powders of SiC (an average size of ~28 m, > 99% purity, GRINM, China) and MoAlB were mixed for 10 h, and then hot pressed at 1200 ℃ under 25 MPa for 1 h in Ar atmosphere. The volume contents of SiC are 5, 10, and 15 vol%.

2. 2 Mechanical property measurement

The prepared SiC/MoAlB samples were cut into bars with different sizes by a wire electrical discharge machine. The bars were polished to 1 μm by diamond paste, cleaned with ethanol, and then dried in an oven at 50 ℃ for 4 h. The different-sized bars were used for mechanical properties and oxidation tests. 

3 mm × 4 mm × 36 mm bars were used to measure the flexural strength by the three-point bending test in a WDW-100E compression machine (China). The span size and crosshead speed were 30 mm and 0.5 mm/min, respectively. 

The Vickers hardness test was performed in a TH700 hardness tester under loads of 1–20 kg with a dwelling time of 15 s. Six measurements in different areas were performed for each sample to obtain an average value.

2. 3 Oxidation test 

3 mm × 4 mm × 10 mm bars were used to investigate the cyclic oxidation behavior in a TSK-5-14 high temperature tube furnace at 1200 and 1300 ℃ for 10 h in air. The weight gain was measured at the interval of 2 h. The weight change of the samples was measured using an analytical balance with an accuracy of 0.0001 g, and converted into a specific weight change per surface area. The oxidized samples were used to the following microstructure and phase characterization. 

2. 4 Characterization

The phase compositions of mixtures, hot-pressed samples before and after oxidation were analyzed by X-ray diffraction (XRD) analysis using an Ultima IV diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 0.154 nm) operated at 40 kV and 40 mA. The morphologies of mixture powders, polished and fractured surfaces of hot-pressed samples, and oxide scale were characterized with a ZEISS EVO 18 scanning electron microscope (SEM; Carl Zeiss SMT, Germany) equipped with an energy dispersive spectrometer (EDS) system. 

For transmission electron microscopy (TEM) examination, focused ion beam (FIB) cross-section was prepared with a dual-beam FIB/SEM (FEI Scios, USA) by using a Ga⁺ ion source operating at 30 kV. A thin slice representing a cross-sectional cut perpendicular to the SiC/MoAlB boundary was analyzed by an FEI Talos F200X (FEI, USA) TEM with an operating voltage of 200 kV. 

3 Results 

3. 1 Preparation and characterization of SiC/MoAlB composites

MoAlB composites reinforced with 5, 10, and 15 vol% SiC were prepared by hot pressing at 1200 ℃ under 25 MPa for 1 h in Ar atmosphere. Figure 1 presents the XRD results for the prepared composites. The composites are primarily composed of MoAlB and SiC, with a small amount of Al3Mo. No new phases were found in the XRD patterns, which probably indicated that no reaction happened between SiC and MoAlB at the sintering temperature of 1200 ℃. The back-scattered SEM micrograph (Fig. 2(a)) depicts the distribution of SiC particles (black color) with irregular shapes in the 10 vol% SiC/MoAlB composite. SiC particles with an average size of ~28 μm were homogeneously distributed in MoAlB matrix (with sizes of less than 40 μm). Al₃Mo (light grey color in Fig. 2(a)) and Al₂O₃ (small black particles in Fig. 2(b)) as impurity phases appeared, which should be from the initial MoAlB powders. No Al₂O₃ peaks were detected in the XRD patterns (Fig. 1), possibly due to their amount below the detection limit of XRD. An enlarged SEM image, taken from the dashed rectangle area in Fig. 2(a), clearly shows the boundary areas between SiC and MoAlB particles (Fig. 2(b)). Neither new formed phases nor reaction layers could be detected in the boundary areas, confirming the thermal stability of SiC with MoAlB at 1200 ℃. 

Fig. 1 XRD patterns of (a) 5 vol%, (b) 10 vol%, and (c) 15 vol% SiC/MoAlB composites.

Fig. 2 Back-scattered SEM micrographs of the polished surface of 10 vol% SiC/MoAlB: (a) a low magnification image and (b) a high magnification image taken from (a). 

TEM analysis was performed to further examine the microstructure of the 5 vol% SiC/MoAlB composite. Figure 3(a) shows the TEM analysis of a representative SiC/MoAlB phase boundary. No reaction zones are observed in the phase boundary. A high resolution TEM (HRTEM) image presents the lattice fringes of SiC and MoAlB (Fig. 3(b)). The measured interplanar spacing is ~0.1822 nm corresponding to the (102) plane of SiC and 0.704 nm corresponding to the (020) plane of MoAlB. The HRTEM image reveals that the interface is clean and continuous, without new phases or amorphous phases at the interface, further confirming the stability of SiC with MoAlB at the sintering temperature. 

Fig. 3 (a) TEM image of SiC/MoAlB and (b) HRTEM image of the zone marked with a dashed rectangle in (a).

3. 2 Mechanical properties 

The measured mechanical properties as a function of reinforcing phase content are presented in Fig. 4. A flexural strength of 380 MPa and a Vickers hardness of 12.7 GPa have been successfully achieved in the 5 vol% SiC/MoAlB composite and increased by 24% and 51%, respectively, as compared with 306 MPa and 8.4 GPa for MoAlB. However, both the strength and the hardness decrease with the increase of SiC content (Figs. 4(a) and 4(b), respectively). The deterioration of the mechanical properties should be caused by the decreased relative density. Under the hot pressing conditions of 1200 ℃ with 25 MPa, the 5 vol% SiC/MoAlB composite has a relative density of ~92.6%, close to 93% for MoAlB material, whereas the 15 vol% SiC/MoAlB composite has a low relative density of 83.8%. Further increasing hot pressing temperature to 1250 ℃ contributes little to the increase in the density of SiC/MoAlB composites. 

Fig. 4 (a) Flexural strength and (b) Vickers hardness of SiC/MoAlB composites, together with those of MoAlB for comparison.

The fracture surface demonstrates that the fracture mode is mainly transgranular (Fig. 5(a)). The cracks are deflected in the MoAlB grains due to their nanolaminated structure. The nanolaminated structure of MoAlB can prolong the crack propagation paths, consuming more energy as the cracks propagate in the grains, and thus endowing MoAlB with good damage tolerance and thermal shock resistance ^[27]. Figure 5(b) further illustrates a crack propagation path emitting from an indentation corner. It can be found that the hard SiC particles play roles of “bridging”, “pinning”, and “deflecting” cracks in the crack propagation resistance. 

Fig. 5 SEM micrographs of crack propagation paths in the 5 vol% SiC/MoAlB composite: (a) a back-scattered electron image of the fractured surface and (b) a second electron image of a crack emanating from the indentation corner. 

The above results confirm that the incorporation of 5 vol% SiC reinforcement in MoAlB is preferred and its strengthening effect is obvious. Therefore, the following study mainly focused on the 5 vol% SiC/MoAlB composite. 

3. 3 Oxidation behavior 

Figure 6(a) depicts the oxide scale thickness as a function of time after cyclic oxidation at 1200 and 1300 ℃ for 10 h in air. The scale thicknesses of SiC/MoAlB are lower than those of MoAlB, i.e., 2.0 vs. 2.7 μm after oxidation at 1200 ℃, and 3.9 vs. 5 μm after oxidation at 1300 ℃. This result indicates that the scale growth rate on the SiC/MoAlB samples is slower than that on the MoAlB samples. The cross-sectional back-scattered SEM micrographs present the continuous oxide scales after oxidation at 1200 ℃ (Fig. 6(b)) and 1300 ℃ (Fig. 6(c)). The formed scales are thin and dense, and mainly composed of Al₂O₃. 

Fig. 6 (a) Scale thickness as a function of oxidation time for 5 vol% SiC/MoAlB composite. Cross-sectional back-scattered SEM micrographs of the composite after oxidation at (b) 1200 ℃ and (c) 1300 ℃ for 10 h in air. 

The XRD result demonstrates that Al₂O₃ is the main oxide phase after cyclic oxidation at 1200 ℃ (Fig. 7(a)). Diffraction peaks with strong intensities belonging to MoAlB are detected, further confirming the thinner scale. No SiC peaks are detected, possibly due to that some SiC are oxidized and the residual SiC with content exceeds the detection limit of XRD. A broaden peak with low intensity locating at the diffraction angle of ~20° may be contributed by the amorphous SiO2 (Fig. 7(a)). The morphologies of oxides on the composite are illustrated in Figs. 7(b)–7(d). On the oxidized surface, SiO₂ particles are larger, while Al₂O₃ particles are smaller and irregular (Figs. 7(b) and 7(c)). The compositions of the two phases are identified by EDS (Fig. 7(d)). The surfaces of SiO₂ particles are smooth, suggesting the viscous phase formed at above 1200 ℃. Small pores formed in the Al₂O₃ scale are irregular, whereas these formed in the SiO₂ particles are round. The appearance of round pores confirms the evaporation of gas phases from the viscous SiO₂ particles upon oxidation at high temperatures. It should be noted that the pores became much round in the SiO₂ particles after oxidation at 1300 ℃ (Fig. 7(c)). This feature suggests that the higher the temperature, the lower the viscosity of the formed SiO₂ glass phase. The formation of the viscous SiO₂ glass phase at high temperatures is beneficial to sealing the channels for the inward diffusion of O and the outward diffusion of elements from the matrix, contributing to the improvement of oxidation resistance especially at the early stage of oxidation. 

Fig. 7 (a) XRD pattern of 5 vol% SiC/MoAlB composite after oxidation at 1200 ℃ for 10 h. SEM micrographs of oxides on the composite after oxidation at (b) 1200 ℃ and (c) 1300 ℃ for 10 h in air. (d) EDS mapping for (c). 

4 Discussion 

Fully dense MoAlB bulk samples are not easy to obtain under hot pressing conditions, possibly due to the existence of small pores caused by the evaporation of MoO₃ or B₂O₃ formed with the absorbed O at sintering temperatures. For example, a relative density of 94% is achieved for the MoAlB samples by hot pressing of MoB and Al powders at 1200 ℃ with 39 MPa for 5.8 h in vacuum ^[16]. In the present study, a relative density of about 93% for MoAlB was prepared by hot pressing of a mixture of Mo, Al, and B at 1200 ℃ with 25 MPa for 1 h in Ar. No full density obtained in the SiC/MoAlB composites should not only be ascribed to the above reason, but also to the absence of sintering reactions between SiC and MoAlB phases. This has been confirmed by the SEM and TEM examinations (Figs. 2 and 3, respectively). The densification of composites without sintering reactions has always been a major challenge. For example, relative densities of 94.4% and 95% are obtained, respectively, in monolithic CrB₂ and 5 wt% MoSi₂/CrB₂ composite after hot pressing at 1400 ℃ under 35 MPa for 2 h ^[28]. To further increase the density of SiC/MoAlB composites, hot isostatic pressing or spark plasma sintering can be considered. It is reasonable to believe if the density of SiC/MoAlB composites was improved, the properties would be further enhanced. 

Upon high temperature oxidation, the following oxidation reactions occur: 

2MoAlB(s) + 6O₂(g) = Al₂O₃(s) + B₂O₃(g) + 2MoO₃(g) (1) 
SiC(s) + 2O₂(g) = SiO₂(s) +CO₂(g) (2)

For Reaction (1), the sequence of phase formation at temperatures below 1400 ℃ is Al₂O₃ > B₂O₃ > MoO₃^[29]. Before a dense and continuous Al₂O₃ scale forms, B₂O₃ and MoO₃ evaporate, thus causing the weight loss upon oxidation at temperatures above 1200 ℃^[15]. Once the dense and continuous Al₂O₃ scale forms, it acts as a barrier to prevent the inward diffusion of O and the outward diffusion of B and Mo, decreasing the evaporation rate of MoO₃ and B₂O₃. Therefore, the weight gain is predominant in the MoAlB material during oxidation at high temperatures. 

For the oxidation of SiC/MoAlB composites, both Reactions (1) and (2) occur. Reaction (2) induced a fluid glassy phase of SiO₂ at above 1200 ℃, similar to the oxidation reaction in a SiC/Ti₃Si(Al)C₂ composite ^[22]. The fluid phase fills some pores and accelerates the formation of dense and continuous oxide scale, decreasing the oxidation rate and leading to the thinner oxide scale on the SiC/MoAlB as compared to the pure MoAlB material.

A cross-sectional back-scattered SEM micrograph presents the microstructure of scale after oxidation at 1300 ℃ for 10 h in air. It should be noted that the formed SiO₂ is just over a SiC particle (Fig. 8(a)). The two phases are identified by EDS mapping, as shown in Fig. 8(b). This feature suggests that the SiC particles on the surface are gradually in situ oxidized to SiO₂. The small and round pore can be found in the SiO₂ particle, which should be resulted from the evaporation of CO₂ according to Reaction (2) at 1300 ℃. It is reasonable to believe that fine SiC particles in the MoAlB will further improve the oxidation resistance of composites due to the fact that the fine reinforcement leads to the rapid formation of SiO₂ which seals microcracks and channels in the Al₂O₃ scale to retard the evaporation of B₂O₃ and MoO₃ in the initial oxidation stage. 

Fig. 8 (a) Cross-sectional back-scattered SEM micrograph and (b) corresponding EDS mapping of 5 vol% SiC/MoAlB composite after oxidation at 1300 ℃ for 10 h in air.

Based on the above results, a proposed oxidation resistance mechanism for SiC/MoAlB is presented in Fig. 9. Before the formation of a continuous Al₂O₃ and SiO₂ scale, the evaporation of MoO₃, B₂O₃, and CO₂ at above 1200 ℃ induces the weight loss at early oxidation stage. However, the fluid glass of SiO₂ formed at above 1200 ℃ can seal phase boundaries and fill small pores for retarding the inward diffusion of oxygen. Hence, the oxidation resistance of SiC/MoAlB is better than that of MoAlB even at early oxidation stage. Once the continuous and dense scale forms, the inward diffusion rate of oxygen is retarded, and a low oxygen partial pressure under the scale generates. As a result, the oxidation of Mo and B followed by evaporation of MoO₃ and B₂O₃ is suppressed, leading to weight gain. In addition, the fluid glass of SiO₂ will fill microcracks and small pores in the scale, and further increases the bonding strength between the oxide scale and the matrix. The above analysis explains the reason why SiC/MoAlB has thinner oxide scales than MoAlB after oxidation at 1200 and 1300 ℃.

Fig. 9 Schematic of the oxidation mechanism of SiC/MoAlB as exposed to air. 

5 Conclusions 

5–15 vol% SiC/MoAlB composites were prepared by hot pressing of SiC and MoAlB at 1200 ℃ under 25 MPa for 1 h in Ar atmosphere. Such hot pressing conditions led to ~92.6% theoretical density in the 5 vol% SiC/MoAlB composite, close to 93% for MoAlB. Increasing SiC content in composites induces the decrease in density. SiC and MoAlB are chemically stable without any reaction products at the sintering temperature of 1200 ℃. The 5 vol% SiC/MoAlB composite is stronger and harder than MoAlB. A flexural strength of 380 MPa and a Vickers hardness of 12.7 GPa were achieved in the 5 vol% SiC/MoAlB composite and increased by 24% and 51%, respectively, as compared with those for MoAlB. In addition, the incorporation of SiC also improved the oxidation resistance of MoAlB due to the formation of a dense and continuous scale composed of Al₂O₃ and SiO₂, which prevents the oxygen inward diffusion and the evaporation of oxides. 

Reference: Omitted