wwceramics. org/ACT Properties of Sic Matrix Composite However, there still exists a great deal of matrix micro- Experimental Procedure cracks in the C/Py C/SiC composite. The microcracks in SiC matrix are regarded as one of the obstacles to the Preparation Process widespread use of C/SiC an inward diffuse through the matrix microcracks, Two kinds of fibers, Hi-Nicalon SiC fiber from fibers at 700oC, which led to the minimum strength carbon fiber from Japan Toray Tokyo, Japan)consist- retained ratio at the range of room temperature to 1250C. Therefore, it is necessary to reduce the ma- diameter of 14 and 7 um per-filament were used to pre trix microcracks of C/PyC/SiC composite for long-time pare fiber preforms using four-step three-dimensional wide temperature range. techniques by Nanjing Fiberglass Research and Design It is indicated that the trs can be controlled Institute, People's Republic of China. The major prop through the appropriate choice of the fiber-matrix erties of the raw materials are summarized in Table IIn pair. The TRS in C/PyC/SiC composite has been cal order to understand the effect of hybrid fibers on the ulated and measured in the previous work. compare mechanical properties of the composite, two types of with the C/PyC/SiC composite, the SiC/Py C/Sic preforms were braided. The first type contained both cause the Sic fiber has nearly the same CTE as the togethe arbon ibers, in which two iber tows were put composite yielded a negligible tRS (close to zero value) fiber bundles for braiding, and the sec- SiC matrix. The properties of fiber, interphase, ond one only contained carbon fiber. The volume frac- will occur or not when the compos under the preforms. PyC interphase and SiC matrix were depos- ited by CVI process. PyC interphase was deposited on Based on the above considerations, SiC matrix the surfaces of fibers by decomposition of C3H at composite reinforced by both SiC and carbon hybrid 900C for 144h at reduced pressure of 5kPa in fibers([SiC-CJ/Py C/SiC)using the PyC as the inter- CVI reactor, obtaining a thickness of 150 nm.One of phase may have a lower residual thermal stress and less the preforms only containing carbon fiber with the Pyc matrix cracks than C/PyC/SiC composite. Moreover, interphase was heat treated at 1800.C in argon for 1h heat treatment of the PyC interphase in C/Py C/SiC to increase the crystalline degree of PyC interphase. Sic composite may improve the crystalline degree of inter matrix was prepared at 1000C at a reduced pressure of phase, and the composite named as C/Pyc/SiC may using also have lower TRS and less matrix cracks than C/PyC/ with a molar ratio of 10 between H2 and MTS, which SiC composite. The major objectives of this work are to was carried by bubbling hydrogen in gas phase and have a knowledge on the microstructure and mechanical argon as the dilute gas to slow down the chemical re- action rate during deposition. For each CVI cycle, the oxidation behaviors at 700%C in air, which were com- deposition time was 80h. After CVI SiC for 6 cycles, pared with those of C/Py C/SiC and C/PyCHT/SiC the as-received composites were machined and polished ino3mm×4mm×40 mm samples. Consequently Table L. Properties of Raw Materials.11-16 Tensile Fracture Diameter Poisson's Modulus CTE strength energy onstituent ratio(v) (GPa) (X 10/K (GPa) T ( /m) Hi-Nicalon SiC fiber 2.74 14 4.6 20 T300 carbon fiber 0.3 1.12 8.6 0.23 Sic matrix 3.21 350However, there still exists a great deal of matrix micro￾cracks in the C/PyC/SiC composite. The microcracks in SiC matrix are regarded as one of the obstacles to the widespread use of C/SiC composite. Oxidizing species can inward diffuse through the matrix microcracks, leading to the oxidation of PyC interphase and carbon fibers at 7001C, which led to the minimum strength retained ratio at the range of room temperature to 12501C.3–5 Therefore, it is necessary to reduce the ma￾trix microcracks of C/PyC/SiC composite for long-time use in a wide temperature range. It is indicated that the TRS can be controlled through the appropriate choice of the fiber–matrix pair.6 The TRS in C/PyC/SiC composite has been cal￾culated and measured in the previous work.7 Compared with the C/PyC/SiC composite, the SiC/PyC/SiC composite yielded a negligible TRS (close to zero value) because the SiC fiber has nearly the same CTE as the SiC matrix.8 The properties of fiber, interphase, and matrix decide whether the debonding of interphase will occur or not when the composite is under the loading. Based on the above considerations, SiC matrix composite reinforced by both SiC and carbon hybrid fibers ([SiC–C]/PyC/SiC) using the PyC as the inter￾phase may have a lower residual thermal stress and less matrix cracks than C/PyC/SiC composite. Moreover, heat treatment of the PyC interphase in C/PyC/SiC composite may improve the crystalline degree of inter￾phase, and the composite named as C/PyCHT/SiC may also have lower TRS and less matrix cracks than C/PyC/ SiC composite. The major objectives of this work are to have a knowledge on the microstructure and mechanical properties of the (SiC–C)/PyC/SiC composite and its oxidation behaviors at 7001C in air, which were com￾pared with those of C/PyC/SiC and C/PyCHT/SiC composites. Experimental Procedure Preparation Process Two kinds of fibers, Hi-Nicalon SiC fiber from Japan Nippon Carbon (Takauchi, Japan) and T300 carbon fiber from Japan Toray (Tokyo, Japan) consist￾ing of bundles of 500 and 1000 filaments with the diameter of 14 and 7 mm per-filament were used to pre￾pare fiber preforms using four-step three-dimensional techniques by Nanjing Fiberglass Research and Design Institute, People’s Republic of China. The major prop￾erties of the raw materials are summarized in Table I. In order to understand the effect of hybrid fibers on the mechanical properties of the composite, two types of preforms were braided. The first type contained both SiC and carbon fibers, in which two fiber tows were put together as one fiber bundles for braiding, and the sec￾ond one only contained carbon fiber. The volume frac￾tion of the fibers was approximately 40 vol% in both preforms. PyC interphase and SiC matrix were depos￾ited by CVI process. PyC interphase was deposited on the surfaces of fibers by decomposition of C3H6 at 9001C for 144 h at reduced pressure of 5 kPa in a CVI reactor, obtaining a thickness of 150 nm. One of the preforms only containing carbon fiber with the PyC interphase was heat treated at 18001C in argon for 1 h to increase the crystalline degree of PyC interphase. SiC matrix was prepared at 10001C at a reduced pressure of 5 kPa by using methyltrichlorosilane (MTS, CH3SiCl3) with a molar ratio of 10 between H2 and MTS, which was carried by bubbling hydrogen in gas phase and argon as the dilute gas to slow down the chemical re￾action rate during deposition. For each CVI cycle, the deposition time was 80 h. After CVI SiC for 6 cycles, the as-received composites were machined and polished into 3 mm 4 mm 40 mm samples. Consequently, Table I. Properties of Raw Materials7,11–16 Constituent Density (g/cm3 ) Diameter (lm) Poisson’s ratio (n) Modulus (GPa) CTE ( 106 /K) Tensile strength (GPa) Fracture energy C (J/m2 ) Hi-Nicalon SiC fiber 2.74 14 0.2 270 4.6 2.8 20 T300 carbon fiber 1.76 7 0.3 230 1.12 3.1 8.6 PyC 1.76 — 0.23 35 5.57 — 2–6 PyCHT 1.76 — 0.23 35 5.57 — 0–2 SiC matrix 3.21 — 0.21 350 4.6 — 6 www.ceramics.org/ACT Properties of SiC Matrix Composite 309