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Novel silazane modified phenol formaldehyde (SPF) resins were synthesized by reacting varying amounts of 1, 3, 5-trimethyl-1', 3', 5'-trivinylcyclotrisilazane (CTS) with phenol formaldehyde (PF) resin. For the selection of an appropriate pyrolysis condition, polymer-to-ceramic conversion of SPF was carried out at 1450 °C and 1650 °C under argon and nitrogen atmosphere. The pyrolysis conditions significantly affected the evolution of ceramic phases, excess carbon and morphology which were thoroughly investigated through XRD, Raman and FESEM techniques. This study establishes silazane modified phenol formaldehyde as a new class of preceramic polymer for high-temperature applications. The study also reveals that nitrogen atmosphere is a more suitable gas atmosphere than argon atmosphere for preparation of desired C/Si3N4/SiC ceramics with higher ceramic yield.

Polymer derived Pyrocarbon (PyC) interphase coating with varying thickness were deposited onto the surface of carbon fiber reinforcement by dip coating technique. The graphitic layered structure of PyC interphase was characterized by X-ray diffraction analysis (XRD) and Raman spectroscopy. Two-dimensional carbon fiber reinforced SiBOC matrix (2D C/PyC/SiBOC) composites were fabricated by Polymer Infiltration and Pyrolysis (PIP) process. Influence of the interphase thickness on the flexural strength of C/SiBOC composites was investigated using three-point bending test. The Extended Pressure Scanning Electron microscope (EPSEM) was used to evaluate the PyC coating thickness. The results indicated that the flexural strength of composites initially increased with increasing PyC coating thickness and reached a peak value of 163 ± 2 MPa at a coating thickness of 0.5 μm. Further increase in the coating thickness led to decrease in the flexural strength of the composite. Oxidative stability of C/PyC/SiBOC composites were investigated and the oxidation rates were compared with that of C/C and C/SiBOC. The results indicated that PyC interphase can improve the oxidation resistance of C/SiBOC composites.

Cf/PyC/SiC-Si3N4 composites are fabricated using silazane modified phenol formaldehyde (SPF) as preceramic matrix resin, pyrocarbon (PyC) as interphase and 2D carbon fabric as reinforcement. The PyC interphase was deposited via chemical vapor infiltration (CVI) technique on the carbon fabric reinforcement and densified with matrix via polymer impregnation and pyrolysis (PIP) process. The changes in the SPF composition significantly affects the mechanical properties as shown by the changes observed in stress-strain behavior, flexural strength and flexural modulus. Cf/PyC/SiC-Si3N4 composites exhibited a gradual increase in flexural strength from 50 ± 6 MPa to 92 ± 5 MPa with change in SiC-Si3N4 composition. The fractography of Cf/PyC/SiC-Si3N4 composites were closely evaluated using optical microscope and scanning electron microscope techniques. The fracture behavior of the Cf/PyC/SiC-Si3N4 composites highlights fiber pull-out or partial delamination phenomenon depending on the SiC-Si3N4 composition. This study establishes silazane modified phenol formaldehyde as a potential preceramic matrix resin for the fabrication of Cf/PyC/SiC-Si3N4 composites to achieve improved mechanical properties for thermo-structural applications.

Cf/SiBOC was fabricated from 2D carbon fabric as reinforcement and slurry-containing boron-modified phenol formaldehyde (BPF) resin with silicon as matrix resin using reaction-bonded silicon carbide method. The processing involves synthesis of (BPF) resin by reacting various amount of boric acid with phenol formaldehyde resin, polymer to ceramic transformation at 1450°C under argon atmosphere, with and without silicon, thermal transformation of the polymer matrix composite into a ceramic matrix composite and evaluation of isothermal oxidation for ceramics and its composites at 1000, 1250 and 1500°C. The ceramic studies, confirmed the formation of B4C, SiC and SiB4 (SiBOC) mixed phase and the role of boron as a catalyst for graphitisation of free carbon present in the ceramic. Oxidation of Cf/SiBOC composite at various temperatures leads to the formation of borosilicate glass which heals the cracks, hindering the inwards diffusion of oxygen.

Ceramic matrix composites (CMCs) are potential thermo-structural materials for use in space applications. Fiber/matrix (F/M) interface plays a key role in determining the mechanical properties of CMCs. Present study focuses on the optimization of F/M volume ratio and the influence of Pyrocarbon (PyC) interphase coating on the mechanical properties of CMCs derived from precursor route. CMCs are fabricated using phenol formaldehyde (PF) resin and boron modified PF (BPF) resin as precursor slurries, 2D carbon fabric (Toray, T300 3K, 8H, satin weave) as reinforcement and PyC as interphase. The deposition of PyC interphase was done by chemical vapor infiltration on the carbon fabric followed by densification of the matrix using reaction bonded silicon carbide method. In CMCs prepared from PF resin, without interphase the flexural strength improves from 25 ± 3.9 MPa (fiber content-40) to 63 ± 9.9 MPa (fiber content-60) on increasing the fiber vol%. In the second part of the investigation, the effect of PyC interphase was studied using CMCs prepared from BPF resin with fiber volume ratio of 60 %. The CMCs with PyC interphase shows an improvement in flexural strength (102 ± 11.5 MPa) compared to that of CMCs prepared without interphase (38 ± 4.4 MPa). The fractography of CMCs with and without interphase was closely evaluated under a scanning electron microscope. CMCs without interphase show no fiber pull-out, indicating the strong fiber-matrix bonding. While CMCs with interphase show fiber pull-out phenomenon and hence fails in a ductile manner.