Effect of In Situ Grown BNNTs and Preparation Temperature on Mechanical Behavior of SiC/SiC Minicomposites

a Powder Metallurgy Research Institute, Central South University, Changsha, Hunan 410083, China b State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China c Hunan Province Key Laboratory of New Specialty Fibers and Composite Material, Central South University, Changsha, Hunan 410083, China d Science and Technology on Thermostructural Composite Materials Laboratory, Northwestern Polytechnical University, Xi¢an, Shanxi 710072, China e School of Aeronautics and Astronautics, Central South University, Changsha, Hunan 410083, China f Institute of Structures and Design, German Aerospace Center Stuttgart, Pfaffenwaldring 38-40, 70569 Stuttgart, Germany g State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha, Hunan 410083, China


Introduction
Continuous silicon carbide fiber reinforced silicon carbide ceramic matrix composites (SiC/SiC CMCs) have the advantages of low density, high temperature resistance, oxidation resistance, high specific strength, and high specific modulus, and have become an important candidate material for the hot-section components of next generation high-performance aero-engine [[1]- [4]]. The introduction of fiber and the design of fiber/matrix interface can stimulate the energy consumption mechanism such as interfacial debonding and crack deflection and endow the material with non-brittle fracture characteristics [ [5]]. However, on the one hand, the matrix between fibers, between fiber bundles and between layers is brittle ceramics and difficult to be strengthened and toughened by the energy consumption mechanism in micron scale; on the other hand, the crack initiation threshold in the matrix is low, and the crack propagation cannot be hindered, which ultimately limits the mechanical properties of the material. Therefore, as the weak region in the material, the micro region of matrix needs to be strengthened and toughened in a finer scale to improve its properties. Boron nitride nanotubes (BNNTs) have excellent mechanical, chemical and thermal stability.
The oxidation resistance temperature is up to 900 o C. The introduction of BNNTs into the material, as the second reinforcement in addition to the microfiber, can improve the mechanical properties of the matrix by strengthening and toughening the micro area matrix in the nano scale, and finally realize the optimization of the overall properties of the material [[6]- [8]].
Many researchers performed experimental and theoretical investigations on the mechanical behavior of ceramic or CMCs with BNNTs. Bansal et al. [ [9], [10]] firstly introduced BNNTs into ceramic materials to prepare BNNTs reinforced barium calcium aluminosilicate (BACS) glass. Experimental results showed that the room temperature bending strength and fracture toughness of the glass increased by 90% and 35% respectively after adding 4 wt% BNNTs, which verified the strengthening and toughening effect of BNNTs. Based on the experimental results, Bansal et al [ [9], [10]] analyzed that the debonding and pull-out mechanism of BNNTs is similar to the strengthening and toughening mechanism of fiber or whisker, which may be the reason for the improvement of glass strength and toughness. Huang

Materials and experimental procedures
To develop the in situ BNNTs on the surface of SiC fiber, mixed powder of B powder, MgO powder and Fe2O3 powder with mass ratio of 2:1:1 was prepared. The precursor powder with uniform dispersion and smaller particle size was obtained by planetary ball mill milling for 100 h, and then spread evenly to the porcelain boat. The SiC fiber bundles with a PyC interface layer were wound on the porcelain boat covered with a layer of precursor powder, and then annealed in a tubular furnace at approximately 1300 o C, as shown in Fig. 1. In the early stage, Ar was used as the protective atmosphere. When the temperature was raised to approximately 1300 o C, NH3 was used as the nitrogen source, and the annealing time was 120 min. After annealing, it is cooled down to room temperature. For comparison, the same SiC fiber bundles were wound on the porcelain boat without precursor powder and annealed accordingly. After the sample was taken out, it was found that the surface of fiber bundles with precursor powder in the porcelain boat became white, while the surface of fiber bundles without precursor powder had no obvious change. Then, the matrix of the two treated fiber bundles was densified by PIP process. Polycarbosilane (PCS) was used as raw material, and the weight gain was less than 1% after three PIP processes. Table 1 listed the general properties of SiC/SiC mincomposites with and without BNNTs.

Micromechanical constitutive model
Nonlinear behavior of mini-CMCs under tensile loading is mainly attributed to multiple micro damage mechanisms in the fiber, the matrix, and the interface between the fiber and the matrix [[16]- [18]]. Tensile nonlinear behavior of mini-CMCs can be divided into three main stages based on the internal damage state, including: • Stage I, the linear-elastic region.
• Stage II, the nonlinear region due to micro damages.
• Stage III, the secondary linear and final fracture region due to gradual fiber fracture.
In this section, a micromechanical constitutive relationship of the three stages mentioned above is developed and related with micro damage state inside of mini-CMCs.

Stage
In Stage I, there is no damages occurred in mini-CMCs, and the linear-elastic stress-strain relationship is, where  is the applied stress, and Ec is the composite's elastic modulus.

Stage II
When multiple micro damage mechanisms occur in the mini-CMCs, the nonlinear stress-strain relationship is, where Vf and Vm are the volume of the fiber and the matrix, Ef is the modulus of the fiber, τi is the shear stress at the interface, rf is the fiber radius, ld and lc are the length of the debonding and the space between the cracking in the matrix, f and c are the axial thermal expansional coefficient of the fiber and the composite, and T is the temperature difference between testing and fabricated temperature. Curtin [[19]] developed a stochastic model to analysis matrix stochastic cracking inside of CMCs, and the relationship between matrix crack spacing and applied stress can be determined by Eq. (3). Gao et al. [ [20]] developed a fracture mechanical approach to determine the interface debonding length when matrix crack propagates to the interface, and the relationship between the interface debonding length and the applied stress can be determined by Eq. (4).
where lsat is the saturation length of matrix cracking, m is the stress carried by the matrix, R is the characteristic stress for cracking in the matrix, and ζd is the debonding energy at the interface.
During damage Stage II, a new damage parameter of interface debonding ratio is,

Stage III
Upon approaching saturation of the cracking in the matrix, fibers gradually fracture inside of the composite. Considering fiber's failure, the stress-strain relationship at the where Φ is the intact fiber stress. The Global Load Sharing (GLS) criterion is adopted to determine the stress distribution between the intact and failure fibers [ [21]] where <L> is the average fiber pullout length, P is the fiber's failure probability.
where σfc is the fiber characteristic strength, and mf is the fiber Weibull modulus.

Results and discussions
In this section, experimental monotonic and compliance tensile behavior of

Original and fracture surface observation of SiC/SiC minicomposites with and without BNNTs under SEM
Using ball millingannealing process, the in situ BNNTs were grown on the surface of SiC fibers. Fig. 9 shows the microstructure of the surface of SiC fibers with BNNTs under SEM. It can be found that the BNNTs were successfully grown on the surface of SiC fibers, and the grown was compact and evenly covered on the fibers' surface.

Experimental comparisons
Under tensile loading, nonlinear behavior appears for the SiC/SiC minicomposite with and without BNNTs. In the section 3, a micromechanical constitutive relationship is developed considering different damage mechanisms occurred in the composites under tensile loading. The nonlinear stress-strain curves of SiC/SiC minicomposite with and without BNNTs can be predicted using the developed constitutive models. The evolution of broken fiber fraction versus applied strain can also be predicted.

Conclusions
In this paper, the SiC/SiC minicomposites with and without BNNTs were fabricated by using the PIP method. Effect of in situ grown BNNTs on monotonic tensile and compliance tensile behavior was analyzed.

Conflict of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.