Carbon Fiber Reinforced Silicon Nitride Research Articles

Carbon fiber/Si3N4 composites with SiC nanofiber interphase for enhanced microwave absorption properties

Abstract In this paper, SiC nanofibers (SiCNFs) as multifunctional interphase were prepared on carbon fibers via catalysis chemical vapor deposition (CCVD) in order to improve the microwave absorption for carbon fiber reinforced silicon nitride (CFs/Si3N4) composites. Experimental results indicated that the chemical compatibility between CFs and Si3N4 matrix under high temperature could be effectively improved by incorporating SiCNFs interphase. Additionally, the dielectric responses and microwave absorbing performance of CFs/Si3N4 composites are significantly influenced by the mass fraction of SiCNFs on CFs. An optimal reflection loss (RL) of −14 dB and 96% absorbing bandwidth (BW) of X-band could be achieved in the CFs/Si3N4 composites with 8 wt% of SiCNFs. Furthermore, experimental points of permittivity in Cole-Cole plane are inclined to distribute on distinguishing circular arcs, corresponding to three types of electric polarization modes. It is concluded that the hopping relaxation of electrons between SiCNFs plays more pronounced contribution to the superior microwave attenuation capability of CFs/Si3N4 composites with SiCNFs interphase than the relaxation loss of free electrons inside short CFs.

Impact Strength of Continuous‐Carbon‐Fiber‐Reinforced Silicon Nitride Measured by Using the Split Hopkinson Pressure Bar

Three‐point impact bending tests, using the split Hopkinson pressure bar method, were performed to evaluate the fracture resistance of monolithic silicon nitride (SN) and carbon‐fiber‐reinforced silicon nitride (CFRSN) ceramics. By applying ramped incident‐stress waves in the split Hopkinson pressure bar apparatus, relatively smooth stress‐time curves could be recorded without using any artificial filtering process. The maximum load in the load‐deflection curve of the CFRSN material increased, in comparison to its static value, when impact testing was applied. Such behavior was substantially different from that of the monolithic SN material, for which the maximum load values from impact and static testing were almost the same. The time dependence of strength in the CFRSN ceramic was then investigated by using relaxation tests, and the impact strength behavior could be explained by these results. Also, the shear strength was significantly dependent on the deformation rate, whereas the tensile strength was almost independent of it. The experimental results were compared with the numerical predictions of the stress distribution that were obtained by using finite‐element analysis.

Static and Impact Shear Strength Estimation of a Continuous Carbon Fiber-Reinforced Silicon Nitride Using Double Notched Specimens

Finite element analysis was performed to investigate the shear stress distribution in double notched isotropic and orthotropic specimens under compressive loading. The shear stress distribution between the notches in the orthotropic material was found to be relatively uniform except at the notch roots where a distinct stress concentration was predicted at both for the isotropic and the orthotropic materials. In brittle orthotropic materials, microcracks initiate in the adjacent regions to the notch roots and may relax stress concentration. Static and dynamic tests were performed on double notched specimens of continuous carbon fiber-reinforced silicon nitride ceramics. The shear strength in the fiber direction is much lower on the perpendicular plane than on the parallel plane to the hot pressing direction. The strain rate dependencies of the shear strength were clarified through the present method. It is found that the shear strength evaluated by this method was much lower than that evaluated by conventional tests, such as the short beam or the compression tests.

衝撃 カーボン長繊維強化型窒化ケイ素の衝撃三点曲げ試験

Bending strength of a continuous carbon fiber-reinforced silicon nitride was experimentally investigated at high rate of strain by a split Hopkinson pressure bar method. Ramped incident waves were applied to the specimen to obtain a smooth load-deflection curve without high frequency oscillations. Acoustic emission during impact deformation was successfully detected as well as quasi-static deformation and the evidence of micro-cracking in the relatively early stage of high velocity deformation was confirmed. The dependences of strength and fracture morphology on fiber orientation and specimen size were clarified and their rate dependences were also identified. Tensile strength was found to be almost independent of deformation rate, while shear strength along fiber direction was significantly increased with rise of strain rate. Fiber bridging over a fracture surface affected shear strength and provided a reasonable explanation of experimental results.

衝撃荷重下におけるカーボン長繊維強化型窒素ケイ素の圧縮強度

The compressive strength of continuous carbon fiber-reinforced silicon nitride ceramics was investigated experimentally at low and high rates of strain. The orientation dependence of the strength and fracture characteristics was well rationalized in terms of shear deformation pararell and/or perpendicular to the fiber direction. A significant effect of deformation rate on the strength was clarified in the case of shearing delamination, while there was no effect on the strength in the case of splitting fracture. The specimen loaded along the fiber direction was fractured by splitting at the stress well below the tensile fracture stress. Microstructural inhomogenity and microcracks induced during the fabrication process play an important role on the strength and fracture morphology of the material.

Changes in Fracture Surface Area and Actual Fracture Energy of a Carbon Fiber-Reinforced Silicon Nitride Composite during Failure

The change in total surface area, namely fracture surface area, of a carbon fiber-reinforced silicon nitride composite (unnotched specimens) was measured by the BET adsorption technique during loading/unloading tests in three-point bending. The actual fracture energy of unnotched specimens, which had been defined as the ratio of the work done during one cycle of loading/unloading to the fracture surface area, was compared with that of notched specimens reported previously. Changes in fracture surface area and actual fracture energy corresponded to the transition of fracture mechanism during failure, and obviously reflected the difference in the damage process of the specimens with and without notch. The actual fracture energy had a physical meanings close to the effective fracture energy ordinary reported when the main fracture mechanism was the macroscopic crack extension. It was considered that the evaluation of fracture surface area and actual fracture energy was an effective method to investigate fracture behavior of fiber-reinforced ceramic composites.

Fiber pullout processes and mechanisms of a carbon fiber reinforced silicon nitride ceramic composite

Details of fiber pullout processes and mechanisms are addressed for a carbon fiber reinforced silicon nitride ceramic composite. The energy for fiber pullout, γpo, is experimentally measured by the work-of-fracture technique using test specimens with a circumferential notch. A precise determination of the distribution of pullout fibers combined with the experimental results of the pullout energy provides an important insight into the micromechanical processes and mechanisms of the composite fracture. Various energy dissipation processes associated with the composite fracture, the frictional interface shear stress, the Weibull modulus of the reinforcing fibers, and composite designs for improved toughening are discussed.

The Evaluation of the Actual Fracture Energy of the Carbon Fiber Reinforced Silicon Nitride Composite by the Kr-BET Adsorption Technique

The fracture surface area of the chevron-notched carbon fiber reinforced silicon nitride composite was measured by the Kr-BET adsorption technique. The energy-absorbing mechanism of the composite was discussed on the basis of the actual fracture energy. The average actual fracture energy of the composite is nearly the same as the fracture energies of polycrystalline ceramics. This fact suggests that the large energy-absorbing capacity of the composite depends on how the fracture surface area can increase. Furthermore, application of the loading/unloading technique reveals that the actual fracture energy of the composite increases to a maximum at an initial stage of the failure, and then decreases to a constant value at a final stage. This indicates that the energy-absorbing mechanism changes from the formation of the fracture surface to the pullout of fibers.

Izod Impact Test for Measuring the Fracture Energy of Fiber-Reinforced Ceramics

(1) Izod impact test was proposed to evaluate the fracture energy of mode I failure of fiber-reinforced ceramics.(2) By selecting the notch geometry of a specimen and the loading condition, the fracture of carbon fiber-reinforced silicon nitride resulted from mode I crack.(3) The fracture energy of carbon fiber-reinforced silicon nitride measured by Izod impact test was about 62kJ/m2 which 20 times as large as that of monolithic silicon nitride.

通液溝付き砥石によるセラミックスとその複合材料のクリープフィード研削

Electroplated and vitrified bonded diamond grinding wheels with unique structure were newly developed which have slots/segments and cooling holes in the wheel working surface for creep feed grinding of ceramic materials. Silicon nitride was ground with 64% of the grinding power required with a conventional wheel. Wear flats of diamond grits on the leading edge and trailing edge of the cutting segments were observed. Residual stresses of the ground surface of ceramics were measured by the X-ray diffraction method. Aluminum reinforced alumina was slightly easier to grind than alumina, but its surface finish was 150% as rough as that of alumina. Carbon fiber reinforced silicon nitride was ground with 44% of the grinding power required for silicon nitride, in the case when the grinding direction was normal to the fibers, but its surface roughness increased by 130% compared with that for silicon nitride. When the grinding direction was longitudinal, the grinding power was 10%, but the surface roughness increased by a factor of four. Specific energy for grinding ceramic composites with the newly developed wheel does not increase with increasing wheel depth of cut up to 8 mm.