Fabrication and mechanical properties of self-toughening ZrB2–SiC composites from in-situ reaction

Self-toughening ZrB2–SiC based composites are fabricated by in-situ reactive hot pressing. The effect of sintering additive content on the microstructure and mechanical properties of the composites is investigated. Microstructure observation found that the in-situ reactive hot pressing could promote the anisotropic growth of ZrB2 grains and the formation of interlocking microstructure. Such microstructure could improve the mechanical properties, especially, for the fracture toughness. The improved mechanical properties could be attributed to the self-toughening structure related to the ZrB2 platelets and the formed interlocking microstructure, which could trigger various toughening mechanisms such as grain pull-out, crack bridging, crack deflection, and crack branching, providing the main contribution to the high fracture toughness.


Introduction
*Corresponding author. E-mail: jjsha@dlut.edu.cn In recent decades, ultrahigh temperature ceramics (UHTCs) are attracting more and more attention with the development of space and industrial applications. As a special class of highly refractory materials, UHTCs have been accepted for potential use as thermal protection structures in next-generation space vehicles, as well as in critical ground-based applications such as refractory linings and cutting tools [1,2]. Zirconium diboride (ZrB 2 ), a member of UHTCs, is the most studied candidate because of its unique combination of relatively low density, superior strength, and high hardness [3][4][5]. Especially, by adding SiC to ZrB 2 , the sintering ability, of MB 2 (transition metal diboride) based composites in UHTC field [1]. Motivated by the results of Si 3 N 4 ceramics [13,14], a unique self-reinforced interlocking and textured microstructures are preferred, which could not only prevent the formation of flaw clusters, but also increase the opportunity for crack bridging and deflection, thereby resulting in both improved strength and fracture toughness [15,16]. Accordingly, ZrB 2 -SiC based composites with a unique interlocking microstructure have been densified by pressureless sintering [16]. The ZrB 2 grains exhibit plate-like morphologies when being sintered above 2100 ℃. However, the high sintering temperature resulting in large grain size may decrease the mechanical properties of composites to some extent, especially the fracture strength. Some other researchers have shown that the in-situ reactive hot pressing process can be used as an alternative method to fabricate materials with some novel microstructures under relative low sintering temperature [17][18][19]. Using elementals Zr, B, Mo, and Si as raw materials, ZrB 2 -MoSi 2 composites with platelet ZrB 2 grains were fabricated by reactive hot pressing at 1800 ℃ [20]. The fracture toughness of the composites was improved owing to the partially textured structure with in-situ platelet ZrB 2 grains. Therefore, the in-situ reaction is thought to have potential advantages for the fabrication of ZrB 2 -SiC based composites with tailored microstructure to improve the mechanical properties.
In the present work, ZrB 2 -SiC based composites were fabricated from in-situ reactive hot pressing. The anisotropic growth of ZrB 2 grain was promoted and a unique self-toughened interlocking microstructure was formed. The X-ray diffractometer (XRD) and fieldemission scanning electron microscope (FE-SEM) with an energy dispersive spectrometer (EDS) were applied to analyze the phase composition and microstructure. Besides, the flexural strength and fracture toughness were measured and the relationship between the mechanical properties and the microstructure was studied.
As we know, it is quite difficult to obtain highly dense ZrB 2 -SiC ceramics below 1700 ℃ because of the covalent bonding characters of ZrB 2 and SiC. The excessive ZrSi 2 used as the sintering additive could reduce the process temperature of ZrB 2 -SiC based composites [21]. The improved sintering ability could be attributed to the ductile deformation of ZrSi 2 particles and liquid phase formation on the grain boundaries at high temperature [22]. Furthermore, it was found that the formation of liquid phase was beneficial for the formation of platelet-shaped ZrB 2 grains [23]. Therefore, different content of excessive ZrSi 2 was also used to investigate its influence on the microstructure features and mechanical properties of the final composites.
The powder mixtures were ball-milled at 250 rpm for 12 h using WC balls and ethanol as the milling media. The mass ratio among powders, balls, and ethanol was 1:10:1. The ball-milled powder mixtures were dried in a rotary evaporator under vacuum condition, and then charged into the graphite die coated with a After a fine grinding, the contaminated surface layer was removed. The bulk density of the samples was measured via Archimedes method. The theoretical density of the composites was calculated based on the composition shown in Table 1 according to the rule of mixtures. The consisting phase of the samples was determined by XRD. The Vickers hardness was measured under a load of 5 kg for 10 s on a polished surface. Flexural strength was tested in three-point bending on 2 mm × 2 mm × 25 mm bars using a 18 mm span and a crosshead speed of 0.5 mm/min. Fracture toughness was measured by means of the single edge notched-beam test on 2 mm × 4 mm × 25 mm bars using a 18 mm span and a crosshead speed of 0.05 mm/min. The depth and width of the notch were 1.5 and 0.3 mm, respectively. The surface morphology and fracture surface after mechanical test were observed by FE-SEM. Grain sizes were estimated from the scanning electron microscopy (SEM) micrographs of the fracture surfaces. Figure 1 shows the XRD spectra of the milled powder mixtures and the fabricated RZSZ-0 sample. In the milled powder mixtures, the peaks of ZrSi 2 and B 4 C were primarily identified. In addition, trace ZrO 2 and WC were detected simultaneously as shown in Fig. 1(a). The formation of trace ZrO 2 should be attributed to the surface oxidation of the ZrSi 2 powders, which may be occurred in the starting powder and handling after the ball-milling. The impurity of WC came from the WC balls which interacted with the B 4 C during high-energy ball-milling because B 4 C is a typical hard material in nature [24].

1 XRD analysis
In the case of the RZSZ-0 composite, ZrB 2 and SiC were identified as the dominant crystalline phases and no obvious peaks from the starting materials were detected. The result indicated that Reaction (1) completely finished after the in-situ reactive hot pressing. Trace ZrO 2 phase was still present in the final composites. Furthermore, WSi 2 was identified as a new crystalline phase besides the products of Reaction (1). The formation of WSi 2 could be associated with the reaction between WC and ZrSi 2 as indicated in our previous work [25]. However, it has been outlined that the W-compounds are helpful for the improvement of the oxidation resistance of ZrB 2 and ZrB 2 -SiC composites [26]. Figure 2 shows the XRD spectra of the fabricated composites with different ZrSi 2 contents. On the whole, the main consisting phase of the four composites presented no significant difference. The primary crystalline phases were still ZrB 2 and SiC, and trace WSi 2 and ZrO 2 were also observed. In addition, some weak peaks of ZrSi 2 were detected in the final composites attributed to the excessive ZrSi 2 introduced as the sintering additive. Among these four composites, the intensity of WSi 2 peak increased with increasing quantity of ZrSi 2 . Furthermore, ZrC as a new phase was detected by XRD analysis when the ZrSi 2 content was above 20 vol%.
As mentioned above, ZrSi 2 would react with WC during the preparation process. The WSi 2 and ZrC were the products as shown in Reaction (2). If B 4 C was present, ZrB 2 would be also probably formed via Reaction (3) [27].
(3) Figure 3 shows the Gibbs free energy of reactions as a function of temperature. From the results of thermodynamic calculations, Reactions (1-3) are feasible at all temperatures. However, the Gibbs free energy of Reaction (1) is much lower than that of Reactions (2) and (3), resulting in the limited B 4 C to react with ZrC. Therefore, the absence of ZrC was observed in the XRD spectra of RZSZ-10 and RZSZ-15, while the ZrC appeared and the WSi 2 intensity increased in XRD spectra of RZSZ-20 and RZSZ-25. Figure 4 shows the relative density of the ZrB 2 -SiC based composites fabricated by the in-situ reactive hot pressing. Without the excessive ZrSi 2 as the sintering additive, the final composite RZSZ-0 presented a low relative density of 91.8% due to the covalent bonding characters of ZrB 2 and SiC. In the case of RZSZ-10 and RZSZ-15, the relative density was 94.9% and 98.5%, respectively, indicating ZrSi 2 could effectively improve the sintering ability of composites during the in-situ reactive hot pressing process. Further increasing the ZrSi 2 content, the relative density was 97.6% for  RZSZ-20 and 96.4% for RZSZ-25, respectively, which slightly decreased in comparison to RZSZ-15 as shown in Fig. 4. Similar result was also indicated in Ref. [28]. In the ZrB 2 -ZrSi 2 -SiC ternary system, it was found that the high ZrSi 2 content would be accompanied by an increase of porosity due to the interaction of ZrSi 2 with components of the ceramics [28]. Figure 5 shows the back-scattered electron (BSE) images on the polished surface, which were used to investigate the typical microstructural features of the in-situ reactive hot pressed composites. It can be observed that different phases with different colors dispersed homogeneously in the composites. The main phases were the grey and black phases identified as ZrB 2 and SiC, respectively. From the low magnification images of RZSZ-15 and RZSZ-20 shown in Figs. 5(a) and 5(b), a larger amount of elongated ZrB 2 grains were observed as the arrows indicated. In addition, some elongated ZrB 2 grains interconnected with each other and generated the interlocking microstructures as shown in Fig. 5(c). From the high magnification image of RZSZ-20, it also could be found that the grain size distribution of the ZrB 2 was non-uniform. During the sintering process, ductile deformation or even local melting of ZrSi 2 particles would occur as Reaction (1) was an exothermic reaction (ΔH < 0). The elongated ZrB 2 with multimodal grain distribution would be much easier to slide and interlock under applied pressure thereby promoting the formation of interlocking microstructure. The interlocking microstructures were considered to improve the damage tolerance of the composites. Some insular elongate ZrB 2 grains were found in the dark phase which could be produced by the liquid ZrSi 2 phase. This eutectic-like microstructure also suggested that local melting occurred during the in-situ reaction process. The EDS analysis was performed on these samples. EDS analysis in zone A in Fig. 5(a) revealed that the white phase with irregular shape was mainly consisting of 59.53 at% Si, 21.38 at% W, 11.42 at% C, and 7.67 at% Zr (Fig. 5(e)). It indicated the white phase could be the coexistence of WSi 2 and ZrC, which were the products of Reaction (2). Furthermore, EDS analysis in zone B in Fig. 5(a) revealed that the dark phase was mainly consisting of 58.91 at% C, 34.51 at% Si, and 6.58 at% Zr (Fig. 5(f)), indicating zone B was the SiC-rich region. The small amount of element Zr might come from the ZrSi 2 and ZrC phases. Owing to the ductile deformation of ZrSi 2 at high temperature, some ZrC particles could be carried by ZrSi 2 to the SiC-rich region under the applied pressure during the reactive hot pressing. Figures 5(a)-5(d) show that the quantity of excessive ZrSi 2 had a significant influence on the grain size evolution of ZrB 2 . For RZSZ-15 and RZSZ-20, these elongated ZrB 2 grains presented a length of 0.5-2 μm approximately with a submicron width. While, ZrB 2 grain coarsening was observed and some large ZrB 2 grains appeared as shown in zone C in Fig. 5(d). It was concluded that increasing the quantity of excessive ZrSi 2 would result in grain growth of ZrB 2 . The grain coarsening could be attributed to the high atom diffusion ability along the grain boundaries due to the occurrence of more liquid phase. However, significant anisotropy growth of ZrB 2 grains occurred in all the fabricated composites. Figure 6 shows the SEM image of the fracture surface of RZSZ-15. It could be seen that the anisotropic growth of ZrB 2 grains was evident and nearly all the ZrB 2 grains developed into the platelet morphology. Moreover, a large number of nano-sized SiC particles were formed among the ZrB 2 grains. The grain size of SiC has a strong influence on the mechanical properties of ZrB 2 -SiC composites. Fine grain size of SiC would result in high mechanical properties, while huge SiC grains in the composites acted as the critical flaws for the failure of ZrB 2 -SiC composites [29].

2 Densification and microstructure
It is known that ZrB 2 has a hexagonal crystal structure which could be promoted to present the anisotropic grain growth similar as Si 3 N 4 . The fastest growing planes of ZrB 2 are the {010}, {110} families. The low-activation energy diffusion path is along <210> and <110> directions, that lead to the ZrB 2 grains tending to grow along c-axis perpendicular to the basal plane [30]. It was also indicated that the formation of ZrB 2 platelets was strongly related to the liquid phase and plastic flow mechanism [20,31]. In the present work, the liquid phases might be from two sources. One was the oxide impurities such as SiO 2 and B 2 O 3 appeared on the surface of the raw powders. The other was the melt of ZrSi 2 at the local region due to the exothermic Reaction (1). Under the applied pressure, the Zr and B atoms in the liquid phase would easily transport to the low surface energy planes of ZrB 2 grains, which favored in the anisotropic growth of ZrB 2 grains resulting to the platelet morphology. Figure 7 shows the mechanical properties of the composites. It indicated that the quantity of excessive ZrSi 2 had great influence on the mechanical properties of the ZrB 2 -SiC based composites. With an increase of the ZrSi 2 content, the Vickers hardness, flexural strength, and fracture toughness of the composites first increased and then decreased. The relationship of the mechanical properties and the ZrSi 2 content is similar to that of the relative densities. However, the RZSZ-20 sample presented the highest Vickers hardness, flexural strength, and fracture toughness, which was 17.1± 0.7 GPa, 655±21 MPa, and 6.08±0.17 MPa·m 1/2 , respectively. Furthermore, comparing with RZSZ-20, the Vickers hardness and the flexural strength of RZSZ-25 decreased more obviously, while the fracture toughness of RZSZ-25 decreased slightly. The typical fracture surfaces and the path of crack propagation of the composites after mechanical tests were observed by SEM as shown in Fig. 8. It can be seen that the fracture surfaces of all the samples are rugged. For the RZSZ-0 composite without excessive ZrSi 2 , several obvious pores were found on the fracture surface ( Fig. 8(a)), in agreement with the relative density data given in Fig. 4. In addition, the formed SiC grains were submicron sized. However, the nano-sized SiC grains were formed in the composites with excessive ZrSi 2 . This indicated that the excessive ZrSi 2 could refine the SiC grains formed during the in-situ reactive hot pressing. Therefore, in the RZSZ-0, the porous microstructure and the coarse SiC grains might be responsible for the lowest mechanical properties. When excessive ZrSi 2 was introduced, only some tiny pores could be observed on the fracture surface of RZSZ-10 ( Fig. 8(b)). Typical mixture mode of intergranular and transgranular fracture of ZrB 2 platelets presented as the arrows indicated. For the composite RZSZ-15, an obvious self-toughening microstructure of ZrB 2 platelets was observed (Fig. 8 (c)). Both intergranular and transgranular fractures were evidenced on the fracture surface, where the larger ZrB 2 platelets fractured mainly in a transgranular mode which might be caused by the local stress concentration, whereas the smaller ZrB 2 platelets fractured predominantly in the intergranular manner. Particularly, on the fracture surface of RZSZ-20 ( Fig. 8(d)), the unique interlocking microstructure formed by amounts of ZrB 2 platelets with multimodal grain sizes could be observed. Benefiting from the unique interlocking microstructure, the mechanical properties of RZSZ-20 did not deteriorate with its decreased density. On the contrary, this composite possessed the highest mechanical properties. Figure 8(e) shows the SEM images of RZSZ-25. Significant coarsening of ZrB 2 platelets appeared on the fracture surface. Commonly, the coarse grains in ceramics easily become the defect source due to the stress concentration and therefore reduce the flexural strength of the ZrB 2 -SiC based composites. Due to the coarsening of ZrB 2 platelets, the fracture of RZSZ-25 presented in the transgranular mode. Furthermore, some secondary cracks propagated across the nanosized SiC-rich region or along the interface of ZrB 2 and SiC grains as shown in Fig. 8(e). The secondary cracks caused by crack branching were shown to be beneficial to the enhancement of fracture toughness of ceramic materials [32]. Therefore, the fracture toughness of RZSZ-25 was as high as 5.96±0.23 MPa·m 1/2 . The improved fracture toughness might be attributed to the self-toughened interlocking microstructure. The in-situ formed interlocking microstructure of ZrB 2 platelets provided definite self-toughening effect on the ZrB 2 -SiC based composites. To achieve a deeper understanding of the toughening mechanism, the path of crack propagation on the polished surface of RZSZ-20 was further inspected by SEM as shown in Fig. 8(f). Benefiting from the effective self-toughening microstructure of the ZrB 2 platelets and the nano-sized SiC, it is easy to trigger various toughening mechanisms such as grain pull-out, crack bridging, crack deflection, and crack branching, resulting in the tortuous path and provided additional fracture energy dissipation, thereby resulting in an enhanced fracture toughness.

3 Mechanical properties and toughening mechanisms
The self-toughening mechanism is schematically illustrated in Fig. 9. On the crack propagation path, when the initial cracks encountered the inter-locked ZrB 2 platelets, they would be stopped by the interlocking structure. With continuously increasing the applied force, the fracture energy would be high enough to break the inter-locked ZrB 2 platelets. The collapse of inter-locked structure could significantly reduce the stress intensity on the crack tips.
Simultaneously, the cracks with reduced stress intensity would hardly break the ZrB 2 platelets and be preferred to propagate along the grain interface with lower bonding strength. The ZrB 2 platelets pull-out was promoted and crack bridging occurred, which gave rise to the longer crack propagation path. When the crack-tip furtherly met the other ZrB 2 platelets, the stress intensity was reduced or even blocked. Furthermore, the crack branching caused by SiC rich region was also observed. For ZrB 2 -SiC composites, because of the mismatch of the coefficient of thermal expansion of ZrB 2 and SiC, tension residual stress would be generated in a tangential direction to the ZrB 2 matrix around the SiC grains during the cooling down [33]. Hence, the cracks propagated to the SiC-rich region resulting in the crack branching.

Conclusions
Self-toughening ZrB 2 -SiC based composites were fabricated via in-situ reactive hot pressing using ZrSi 2 , B 4 C, and C as the raw materials. The effect of ZrSi 2 content on the microstructure and mechanical properties of the composites was investigated. The following results could be drawn from the current work: (1) In-situ reactive hot pressing could promote the anisotropic growth of ZrB 2 grains resulting in the ZrB 2 platelets, and the nano-sized SiC grains were also formed in the fabricated composites.
(2) The anisotropic growth of ZrB 2 grains led to a unique interlocking microstructure when the excessive ZrSi 2 was used. The interlocking microstructure was more obvious in the case of composites with excessive ZrSi 2 higher than 20 vol%. Such microstructure could improve the mechanical properties, especially, for the fracture toughness. (3) The ZrB 2 platelets and the formed interlocking microstructure played an important role on the self-toughening mechanism of composites, which were activated by various toughening mechanisms such as grain pull-out, crack bridging, crack deflection, and crack branching, providing the main contribution to the high fracture toughness.