Fracture toughness and toughening mechanisms in a (ZrB2–SiC) composite reinforced with boron nitride nanotubes and boron nitride nanoplatelets

Abstract

A ZrB 2-SiC composite reinforced with the mixture of boron nitride nanotubes (BNNTs) and boron nitride nanoplatelets (BNNPs) was prepared by spark plasma sintering. The results show that the fracture toughness of a (ZrB 2-SiC) composite increases by up to $24.4% at 1.0% (BNNT-BNNP) weight fraction. The synergetic toughening mechanisms, including BNNT and BNNP pullout, crack bridging by BNNP, crack branching and crack deflection, and bending of multilayered BNNPs, are observed to contribute to the improved toughness.

Spherical indentation with multiple partial unloading for assessing the mechanical properties of ZrB2–SiC composites

“…As listed in Table 1, the computed indentation yield strength is $ 920 MPa for ZrB 2 -SiC composite, $ 890 MPa for 0.5 wt% (BNNT-BNNP)/(ZrB 2 -SiC) composite, and $ 870 MPa for 1.0 wt% (BNNT-BNNP)/(ZrB 2 -SiC) composite, strongly indicating that the added mixture of BNNTs-BNNPs slightly decreases the indentation yield strength of the sintered ZrB 2 -SiC composites. As reported in our previous research [12], the fracture toughness of a ZrB 2 -SiC composite increases by up to $ 24.4% (from 3.7370.11 to 4.6470.36 MPa m 1/2 ) at 1.0% (BNNT-BNNP) weight fraction, and the improved fracture toughness is likely due to activated toughening mechanisms such as BNNT and BNNP pullout, crack bridging by BNNP, crack branching and crack deflection. Hence, it is clear that the added mixture of BNNTs-BNNPs is expected to significantly improve the toughness of ZrB 2 -SiC composites without comprising its mechanical strength.…”

“…As discussed in our previous research [12], high elastic modulus of $ 750-1200 GPa [24] and tensile strength of 424 GPa [25] associated with BNNTs is expected to favor the enhancement of elastic modulus and hardness of (ZrB 2 -SiC) composite. By contrast, BNNPs with low hardness and low elastic modulus ( $ 33.89 GPa in the c direction and $ 85.95 GPa in the a direction) [26] would decrease the hardness and the elastic modulus of the ZrB 2 -SiC composites.…”

“…Such a dislocation configuration leads to an assumption that the Orowan-type strengthening effect might be operative [28], and the resistance to plastic deformation associated with Orowanloop-like configuration increases with the added content of BNNTs-BNNPs, leading to an increase in strain-hardening exponent. On the other hand, it is worth to note that the volume fraction of BNNPs is $ 60% in the mixture of BNNTs-BNNPs [12]. Most importantly, our previous research [12] revealed that multilayered BNNPs with low hardness and low elastic modulus undergo such deformation that the bending angle approaches $ 901 without any fracture due to its inherent softness.…”

“…By contrast with carbonaceous materials, boron nitride nanotubes (BNNTs), a structural analog of CNTs, not only possess excellent mechanical properties comparable to those of CNTs, but also exhibit higher starting oxidation temperature of BNNTs ($ 900 1C) [11], which is believed to guarantee less degradation of high-temperature performances of the BNNT/(ZrB 2 -SiC) composite. Also, the choice of using soft and ductile BN platelets seems to be an attractive option to toughen brittle ceramics [6,12]. More recently, the fracture toughness of ZrB 2 -SiC composite reinforced with a mixture of boron nitride nanotubes (BNNTs) and boron nitride nanoplatelets (BNNPs) has been reported to increase by up to 24.4% (from 3.73 7 0.11 to 4.64 7 0.36 MPa m 1/2 ) at 1.0% (BNNT-BNNP) weight fraction in our previous research [12].…”

“…Also, the choice of using soft and ductile BN platelets seems to be an attractive option to toughen brittle ceramics [6,12]. More recently, the fracture toughness of ZrB 2 -SiC composite reinforced with a mixture of boron nitride nanotubes (BNNTs) and boron nitride nanoplatelets (BNNPs) has been reported to increase by up to 24.4% (from 3.73 7 0.11 to 4.64 7 0.36 MPa m 1/2 ) at 1.0% (BNNT-BNNP) weight fraction in our previous research [12].…”