Effects of interfacial residual stress on mechanical behavior of SiCf/SiC composites

Layer-structured interphase, existing between reinforcing fiber and ceramics matrix, is an indispensable constituent for fiber-reinforced ceramic composites due to its determinant role in the mechanical behavior of the composites. However, the interphase may suffer high residual stress because of the mismatch of thermal expansion coefficients in the constituents, and this can exert significant influence on the mechanical behavior of the composites. Here, the residual stress in the boron nitride (BN) interphase of continuous SiC fiber-reinforced SiC composites was measured using a micro-Raman spectrometer. The effects of the residual stress on the mechanical behavior of the composites were investigated by correlating the residual stress with the mechanical properties of the composites. The results indicate that the residual stress increases from 26.5 to 82.6 MPa in tension as the fabrication temperature of the composites rises from 1500 to 1650 °C. Moreover, the increasing tensile residual stress leads to significant variation of tensile strain, tensile strength, and fiber/matrix debonding mode of the composites. The sublayer slipping of the interphase caused by the residual stress should be responsible for the transformation of the mechanical behavior. This work can offer important guidance for residual stress adjustment in fiber-reinforced ceramic composites.

Microstructures and Phases of Ytterbium Silicate Coatings Prepared by Plasma Spray-Physical Vapor Deposition

Ytterbium silicate coatings were deposited on SiCf/SiC ceramics matrix composite (CMC) substrates by plasma spray-physical vapor deposition (PS-PVD), and the microstructures and phase constituents of the coatings were studied. The results show that the Yb2SiO5 coating prepared with high power and low pressure (65 kW/2 mbar) had quasi-columnar structure, mainly deposited from the vapor phase, whereas the coating prepared with low power and high pressure (40 kW/10 mbar) had a typical layered structure, mainly deposited from the liquid phase. The deposition efficiency of parameter A (~2 μm/min) was also significantly lower than that of parameter B (~20 μm/min). After annealing at 1300 °C for 20 h, the coating prepared by 65 kW/2 mbar was mainly composed of ytterbium disilicate phase (77.2 wt %). The coating also contained some silicon-rich phase. The coating prepared by 40 kW/10 mbar basically consisted of ytterbium monosilicate (63.6 wt %). In addition, a small amount of silicon-rich phase and ytterbium-rich phase were also present in the coating. Accompanied with calculation results by the FactSage software, the cause of deviations in phase compositions was analyzed.

Study of the Nanocomposite cBN/TiC-SWCNTs by Field Actived Sparck Plasma Sintering Process

Cubic boron nitrid (cBN) bonded TiC and alloyed with single walled carbon nanotubes (SWCNTs or NC) ceramics matrix nanocomposites (CMNCs) tools were manufacturated by a field actived sparck plasma sintering processus (FASPS). The effects of cBN-TiC ratio, carbon nanotubes and optimisation of the sintering process on the microstructure, densification in addition mechanical and vibronic properties of NC-cBN-TiC nanocomposites were studied. The results showed that for the nanocomposite cBN-TiC vol. ratio of 8:2 with 0.1 wt% NC, it was found that microhardness incresses significantly with addition of carbon nanotubes exhibited the highest microhardness and fracture toughness. After sintering of the samples at 1800 °C, 10 mn, 75 MPa of cBN–TiC1-x, x=0.8 with and without addition of 0.1 wt% NC were characterized using field emission scanning electron microscopy (FESEM) and X-ray diffraction. The samples exhibited a dense polycrystalline structure. From the resonant Raman scattering we can locate the vibration frequency of the transformation cBN to hexagonal boron nitrid (hBN) and formation of secondary hard phase TiB2to consolid the (CMNCs) tools. The final product is hBN-TiC-TiB2-NC.The best product contained cBNx-TiC1-x (x=0.8)-0.1 wt % NC which was sintered at 1800 °C, 75 MPa for 10 mn. The Vickers hardness of cBN-TiC1-x (x=0.8) incresses with NC incorporation in the matrix The indentation fracture toughness was calculated to be 12.30 MPa m1/2 for cBNx-TiC1-x (x=0.8 -0.1 wt % NC ceramics matrix nanocomposite (CMNCs) tools with excellent wear resistant will be confirmed. The wear of cBN-TiC of the composites tools have shown that this is predominantly a chemical process involving the interaction of the tool with its environment and is restricted by the formation of protective layers on the exposed faces of the tool by the addition of carbon nanotubes (NC). The wear features of tools used in fine cutting tests under identical conditions will be compared and the results will be interpreted in terms of the existing models for the wear of cBN -based nanomaterials by the effects of the additives in the modified tools

Extraction of Silica Nanoparticles from Rice Husk Ash (RHA) and Study of Its Application in Making Composites

Nanotechnology is the most emerging field in the area of different scientific research. Various methods of synthesis of nanoparticles are available. The wet chemical synthesis method is applicable in the extraction of silica nanoparticles from Rice Husk Ash (RHA). Rice husk is a form of waste product from the rice milling industries which is produced in an abundance amount in and around the country. Rice husk which is generally left on the field as a waste contains 60% silica content and can be economically viable raw materials for the extraction of silica [1-2]. Initially, Rice husk was burnt to obtain its ash. Then, a simple bottom-up approach, the sol-gel method was applied, and fine powder silica in the nanoscopic range was extracted. After extraction of it, internal arrangement and average particle size were recognized by XRD while molecular components and structure present in silica were identified by FTIR. The obtained silica was then used in making of ceramics matrix nanocomposites (particularly silica-sand cement block), and its mechanical properties were identified by compressive strength test using Instron testing machine which was found to be increased in comparing with the compressive strength of ceramics composite (Sand cement block) prepared in the same ratio and same size of mold as that of ceramics matrix nanocomposites. The ceramics matrix nanocomposites acquiring higher mechanical strength than Ceramics composite occurred due to the incorporation of Silica nanoparticles.