Among various types of continuous fibre reinforced ceramic matrix composites (CMCs), SiC fiber (SiC_f)-reinforced SiC matrix composites (SiC_f/SiC) with superior mechanical and thermal properties have obtained increasing attention in industrial applications like aerospace, nuclear and braking system over past few decades^[1,2,3]. However, major issues like sensitivity to structural defects and brittle behavior in external load require effective solution for optimizing the performance of SiC_f/SiC in application^[4]. Nanotubes-reinforced hierarchical composites introduce a practicable way to handle the issues due to their significant improvement in toughness and strength^[5]. Nanotubes such as carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) present better mechanical properties by enhancing brittle microstructural parts relatively away from fiber bundles, monolithic composites are efficiently toughened and strengthened owning to ductile behavior like sliding, pull-out and bridging of these micro-scale counterparts of fibers^[6,7]. Apart from CNT and BNNT, SiC_nw and SiC whisker with high tensile modulus and strength also acquire good performance as hierarchical reinforcement in both C_f/SiC and SiC_f/SiC composites owing to their restriction effect on crack propagation in and into fiber bundles^[8].

However, research on the mechanism of toughening and strengthening of SiC_nw is still not rich enough to elaborate the effect on mechanical behavior of hierarchical composites under external load. According to previous study^[9], the mechanical improvements of SiC_nw reinforced C_f/SiC composites were mainly attributed to crack deflection and bridging of SiC nanowires as well as their pull-out and sliding effect in the matrix. In our work, acoustic emission (AE) was introduced to monitor the damage evolution in real time,Vickers Hardness Impresses was employed to evaluate the propagation of obvious transverse cracks. As the mechanical behavior and fracture mechanism of SiC_f/SiC composites modified with SiC_nw is still unclear, this study is expected to show a better understanding of SiC nanowire as a mechanical reinforcement.

SiC_nw were in situ grown on SiC fiber (KD-II, with 500 nm thick BN interphase deposition in advance) surface after 10 h Ni²⁺ solution treatment by chemical vapor infiltration (CVI). H₂ (30 sccm) and methyl trichlorosilane (MTS, 100 sccm) were utilized as reductant as well as source gas. The growth of SiC_nw was carried out under a pressure of 2 kPa at 1000 ℃ for 1 h. To achieve the optimum of SiC_nw/matrix interfacial bonding, BN interphase was deposited on SiC_nw surface via chemical vapor deposition (CVD) method at 800 ℃ using BCl₃ (10 sccm) and NH₃ (20 sccm).The composites were densified using precursor impregnation and pyrolysis (PIP) method. Finally, original SiC_f/SiC composites, as-grown and BN-coated SiC_nw modified SiC_f/SiC hierarchical composites were fabricated. These composites were marked as SiC_f/SiC, SiC_f/SiC-SiC_nw and SiC_f/SiC-SiC_nw/BN, respectively.

The bulk density and open porosity were evaluated by the Archimedes principle. Three-point bending test was performed on a CRIMS-DDl20 universal testing machine with a span of 30 mm at a cross head speed of 0.1 mm/min. Composites were cut into bar specimens with the dimension of 2.0 mm×4.0 mm×40 mm for bending test. AE technique was employed to detect in situ information of damage process in composites during the three-point bending test. AE activity, containing the information of damage events, was real-timely recorded. AE detection was conducted by using a two-channel MISTRAS acquisition system (Physical Acoustics Corporation) with a sampling rate of 2 MHz. Two nano-30 (Physical Acoustics Corporation) sensors were mounted on the surface of specimens to record AE signals. Two pre-amplifiers were also adopted and set with the pre-amplification of 40 dB and band-pass filtering of 20-1200 kHz. To avoid possible noise recording, the threshold was set at 45 dB (as shown in Fig. 1). Hitachi SU8220 field-emission scanning electron microscope (SEM) were used to characterize the morphology and microstructure of as-grown and BN-coated BNNTs. The fracture morphology of fibers in composites were also investigated via SEM. The Vickers Hardness Impresses was applied to induce the crack formation of SiC_nw/SiC matrix by a Vickers hardness tester (HVS-5) with a load of 500 g for 10 s, and the crack propagation was observed by SEM.

The morphologies of fibers covered by SiC_nw before matrix infiltration are shown in Fig. 2. As seen in Fig. 2(a-c), the microscale network composed of SiC_nw filled the gap between fiber bundles which surfaces are barely exposed. The length of nanowires as secondary reinforcement is approximately 10-20 μm and the diameter is about 60-100 nm. Macroscopic properties like density and porosity for each group are listed in Table 1. Compared to original SiC_f/SiC, the density of composite SiC_f/SiC-SiC_nw increases from 1.98 g/cm³ to 2.08 g/cm³, and the open porosity decreases from 17.64% to 11.58%. Combined with Fig. 1(d, e) demonstrating the pores in fiber bundles, it indicates that the incorporation of SiC_nw densifies the monolithic composite by suppressing the formation of large-sized micro-pores. ZHAO et al.^[10] studied similar structure and concluded that this network was preferred due to less possibilities to cause undesirable clogging in the matrix infiltration process and simultaneous enough strength for enhancing microdomains near or between fiber bundles.

The aim of deposition of BN interphase on SiC_nw is to render composites more optimized mechanical strength, since the direct contact between SiC nanowires and matrix leads to an undesirable strong bond^[11]. Moreover, the influence of weak-bonded interface between nanowires and matrix on damage evolution requires investigation in detail.

Mechanical properties of these composites are evaluated and gathered in Table 2. It can be implied that SiC_nw as secondary reinforcement indeed produce positive influence on mechanical behavior. Flexural strength of SiC_f/SiC-SiC_nw and SiC_f/SiC-SiC_nw/BN is 15.7% and 42.1% higher than that of original SiC_f/SiC, respectively, which may be attributed to the strengthening effect of nanowires in and between fiber bundles, as shown in Fig. 2. This impact is further intensified by deposition of BN interphase on the surface of nanowires. The fracture morphologies of each group are shown in Fig. 3 and Fig. 4. As discussed above, direct contact of nanowires with matrix causes a strong interfacial bond. It can be confirmed by the observation in Fig. 3(a) that few pull-outs of nanowires are found. Instead of tending to be pulled out, nanowires prefer to breaking. On the contrary, obviously much more pull-outs are inspected in composites SiC_f/SiC-SiC_nw/BN, as illustrated in Fig. 3(b). Furthermore, a local part of Fig. 3(b) is magnified in Fig. 3(c), indicating that nanowires are easier to be pulled out with assistance of weaker interfacial bonding. Fig. 4 demonstrates different fracture morphologies of fibers in three types of composites. It can be concluded that fiber/matrix interfacial bonding is exceedingly enhanced after introducing SiC_nw, for the SiC_f/SiC-SiC_nw shows apparent brittle fracture mode in Fig. 4(a). However, above mentioned effect is effectively alleviated by introducing BN interphase, phenomenon of long pull-outs of fibers happens again in SiC_f/SiC-SiC_nw/BN, indicating better mechanical performance for material after the formation of obvious transverse cracks^[12].

It is noticeable that the strain at maximum load for composites SiC_f/SiC-SiC_nw is relatively bigger than that for original SiC_f/SiC, a possible explanation could be illustrated as following. In general, the failure strain of composites is mainly decided by fiber/matrix bond and stress status in fibers. The SiC_nw/matrix strong bond may help load transfer among fibers, and improve their ability to bear load. Therefore, load concentration of fibers is reduced and break of fibers is delayed, causing bigger failure strain in composites SiC_f/SiC-SiC_nw. However, stronger evidence requires more profound study.

As inspiration of this research, the restriction effect of nanowires on the formation and evolution of matrix cracks was observed and significantly benefits fracture behavior of materials owing to an increase of cracks propagation path and extra energy dissipation^[13]. Li et al.^[14] studied the modification effect of nanowires on composites and found their contribution to the absorption of breaking energy by rubbing against matrix after formation of obvious transverse cracks, which could transform otherwise mechanical catastrophic failure to ductile behavior. Such results may offer assistance to the discussion about AE diagrams in the following section.

To get further understanding of the impact of SiC_nw on damage evolution of composites, AE measurement was performed in the three-point bending tests. The AE events with corresponding energy and numbers was detected and recorded, reflecting damage evolution such as generation and propagation of cracks. On basis of AE energy information, normalized cumulative AE energy versus stress curve are plotted to clarify damage threshold stress. Normalization is determined by the total energy of all AE events as denominator for every cumulative AE energy in specific stress. The slope of curve represents damage process rate^[15]. As shown in Fig. 5, each type of composites shows different AE activities as external stress increases. For simplified analysis, the point highlighted by solid arrows where AE energy starts to be non-zero is denoted by $\sigma_{min}$ (first AE stress), indicating the stress where the first AE signal is detected or the onset of microcrack generation. The other point highlighted by dotted arrows where the slope acquires a considerable increase is denoted by $\sigma_{onset}$ (onset stress), representing the happening of severe damage inside composites like the formation of large transverse cracks and subsequent propagation^[16,17]. All above mentioned values of three groups are listed in Table 2, it can be inferred from Fig. 5(b) that the introduction of SiC_nw delays the presence of $\sigma_{min}$ as well as $\sigma_{onset}$ $\sigma_{min}$ and $\sigma_{onset}$ of SiC_f/SiC-SiC_nw are 12.5% and 33.6% higher than that of original SiC_f/SiC, respectively. Besides, the deposition of BN interphase further enhances the hysteresis, for the values of SiC_f/SiC-SiC_nw/BN shift to higher level compared with SiC_f/SiC by 34.5% and 47.4%, respectively.

The effect of SiC_nw can also be confirmed by stress- strain curves in Fig. 5. As is well-known, with external stress increasing, the traditional fracture mode of composites could be illustrated as following^[18]: initial microcracks generate, microcracks propagate and merge into obvious transverse crack, fiber/matrix interfacial debonding, final fiber pull-out and breakage. The formation of transverse cracks makes main contribution to the transformation of stress-strain curve from linear part to non-linear part^[15], namely, proportional limit as the knee point in this curve determines the stress where damage in composites starts to be severe and irreversible. By comparison, the proportional limit of SiC_f/SiC-SiC_nw and SiC_f/SiC-SiC_nw/BN are 20.9% and 61.4% higher than that of original SiC_f/SiC, respectively, which roughly agrees with the differences of $\sigma_{onset}$ values among these three types of composites. Therefore, AE results throw some light on detecting the important signal for identifying when composites bear noticeable interior damage. However, the stress recorded by AE is lower than proportional limit without exception, which may result from high sensitivity of AE devices, energy will be in record once high-energy event occurs, while one transverse crack is not enough for deviation from linear region in stress-strain curve.

Above analyses hint that early damage evolution is delayed, i.e., the required stress for the formation of microcracks or transverse cracks is lifted by introducing SiC nanowires, and the restriction effect is more notable with the assistance of BN interphase. For confirming the assumption, the scattering diagram of individual AE events over time is demonstrated in Fig. 7. As is shown, the first detected AE activity is delayed in composites SiC_f/SiC-SiC_nw and SiC_f/SiC-SiC_nw/BN as anticipated, yet the obvious increase of AE activity representing the formation of apparent transverse crack does not show the same trend by comparison. With the aim of explaining the contradiction, Vickers Hardness test were performed to obtain information about crack propagation in composites. As shown in Fig. 8, after tests of same parameters, crack deflection and bridging are clearly observed in composites SiC_f/SiC, inferring that the cumulative fracture energy has been consumed and the seriously damaged matrix is brittle. By comparison, there are much less cracks caused by the indentation in composites SiC_f/SiC-SiC_nw and SiC_f/SiC-SiC_nw/BN, the latter even obtain few cracks after the test, indicating that the fracture energy is far from enough for the formation of obvious cracks and subsequent propagation.

Considering observation given above, the inconsistence of previous assumption with AE scattering diagrams might be explained as following. In composites SiC_f/SiC and SiC_f/SiC-SiC_nw/BN, cracks tend to deflect along the weak-bonded BN interphase between fiber/matrix or nanowire/matrix, while in composites SiC_f/SiC-SiC_nw, cracks are more likely to be suppressed owing to the strong bonding between nanowires/matrix. In composite SiC_f/SiC, the propagation of cracks in brittle matrix is considerably fast, resulting in earlier presence of both the first recorded AE events and high-energy AE events, as confirmed in Fig. 7(a). On the other hand, as shown in Fig. 8(b, c), SiC_nw can effectively decrease the size of cracks by bridging between cracks, causing the improvement of $\sigma_{min}$ and $\sigma_{onset}$ in Fig. 5(b). In composites SiC_f/SiC-SiC_nw, nanowires tend to break as external stress increases, which brings forward the evident increase of AE activities. By contrast, due to the weak BN interphase in favor of sliding and pull-outs of nanowires, there are much more AE events recorded in composites SiC_f/SiC-SiC_nw/BN. The presence of rapid increase of AE activities is postponed, and the high-energy period lasts for a shorter time. It is also noteworthy that AE events gain an obvious boost after the hysteresis at early stage of damage evolution, which may result from the merging of microcracks or the breaking of short nanowires.

SiC nanowires were introduced in composite SiC_f/SiC to realize a hierarchical structure. The modification effect of nanowires on microstructure and mechanical properties was investigated. Compared to original SiC_f/SiC, the density and porosity of SiC_f/SiC-SiC_nw and SiC_f/SiC-SiC_nw/BN are both positively optimized to a higher level. SiC nanowires wrapped by BN interphase significantly improve the fractural strength and proportional limit. AE results show that the proportional limit may correlates with onset stress of composites. With the assistance of Vickers Hardness Impresses, it is found that the reinforcing effect of SiC_nw may be attributed to delaying the presence of microcracks and bridging cracks. In addition, the resultant hindering effect of cracks of SiC_nw could not be fully optimized when SiC_nw are not coated with BN, although the brittle matrix is effectively enhanced. Thus, the strength of SiC_f/SiC-SiC_nw is comparable to original composites SiC_f/SiC.

The supproting materials related to this article refer to https://doi.org/10.15541/jim20210045.