Abstract: The effect of seeding on the microstructure and mechanical properties of hot-pressed silicon nitride was investigated by introducing large, elongated b-Si3N4 particles into an α-Si3N4 matrix. Densification and fracture toughness were followed in the presence of Y2O3 as sintering aid. The concentration of seeds varied from 0 to 5 wt%. The maximum fracture toughness was 8.0 MPam1/2 for a material with 3 wt% seeds, hot pressed at 1750oC, for 4h.
Abstract: This paper deals with densification and a®b phase transformation of Si3N4, with a
constant b-Si3N4 seeds concentration regarding sintering temperature and different additive types. The seeds of b-Si3N4 were obtained by sintering of silicon nitride in a-form and additive mixture consisting of Y2O3 and SiO2. Two different Si3N4/additive mixtures (Y2O3 + Al2O3 and CeO2) containing constant amount of seeds were prepared and tested. Characterization of sintered samples involved phase analysis by X−ray diffraction and density measurements. The results indicated that in the presence of yttria-alumina mixture both the phase transformation and the densification were enhanced as compared to samples containing only CeO2 as a sintering additive. The reasons for observed behavior are discussed in detail.
Abstract: The process of filament winding is particularly suited to the manufacturing of composite pipes. This paper first describes the overall process and its advantages for composite structures. Particular attention is devoted to bi-directional pipes [+j, -j]n. The static behaviour exhibits damage and plastic phenomena depending on the loading and stacking sequence. In the main part of the paper we focus our presentation on [+55, -55]n. The failure mode under tensile-internal pressure loads of bi-directional pipes is described, as well as their fatigue behaviour with the effect of several parameters (such as frequency) on the lifetime and the kinetics of damage. In particular, this paper shows that the effect of frequency is related to two phenomena: temperature and fatigue-creep coupling, which have opposing action on the lifetime.
Abstract: A BN interphase has been deposited by infiltration of a Hi-Nicalon fibre tow in a Low
Pressure Chemical Vapour Deposition reactor using a boron nitride molecular precursor: the tris(dimethylamino)borane (TDMAB), an halogen-free precursor. This precursor prevents fibre and CVD apparatus from chemical damage. Then SiC/BN/SiC 1D mini-composites were produced by classical CVD of SiC from hydrogen and methyltrichlorosilane mixture. The interphase composition was characterised using FT-IR and XPS analyses and the presence of carbon in the BN has be related to surface pollution and not from carbon bounded to the coating. The tensile properties of the mini-composites were tested with unload-reload cycles and have shown very good mechanical properties corresponding to a high interfacial shear stress. The observations of the interphase using TEM reveal that it is made of an anisotropic turbostratic BN. The fibre/matrix debounding, which occurs during mechanical loading, was shown to be located within the BN interphase.
Abstract: Due to favorable chemical and mechanical properties of yttrium silicate coatings (low Young’s modulus, low thermal expansion coefficient, low evaporation rate and oxygen permeability, good erosion resistance), this material is a promising complement to SiC coatings for protecting C/C-Si-SiC composites. For the preparation of silicate powders, the Pechini method was used. As a coating method, electrophoretic deposition from stable suspensions based on isopropanol was chosen. Under controlled deposition voltage and duration, coatings of various thicknesses were deposited. The deposited layers were characterized by SEM and EDX analysis. The protectiveness of these coatings was tested by isothermal thermogravimetry.
Abstract: The protectiveness of mullite layers electrophoretically deposited on C/C-Si-SiC
composites, against isothermal oxidation in air in the temperature range from 1200 to 1550 °C, was investigated by means of thermogravimetry (TG). The experimental results are interpreted with the help of a phenomenological model. At lower temperatures or short oxidation times the overall oxidation kinetics is determined by transport processes in the EPD mullite layer, which leads to a linear growth law. At higher temperatures or longer times of oxidation the oxidation rate is controlled by solid-state diffusion processes in the growing silica layer, which leads to a parabolic
growth law. Comparison of experimental parabolic and linear rate constants with calculated ones suggests, in the framework of the model, the conclusion that carbon monoxide (CO) diffusion in the oxide layers is the rate determining step for the overall oxidation of the C/C-Si-SiC substrates.
Abstract: The change in elastomer tensile moduli, as formulated in the Gaussian statistical theory of rubber elasticity, with deformation, is considered both experimentally and theoretically. Gum elastomers of different structures and corresponding materials filled with carbon black, as reinforcing filler, are investigated experimentally. For all materials considered, the same scaling pattern with negative and low slope for small deformations, and positive and higher slope for large deformations is obtained, indicating two distinct mechanisms of elastic response. Most pronounced is the similarity of small deformation responses for filled materials. Considering the modulus as an elastic energy density gradient dependent on structure changes with deformation, and interpreting the changes for small deformations in terms of conformational energy change, the fractal dimension of a new type is formulated. It describes the decrease in elastomer network connectivity with deformations, which is discussed in terms of conformon dynamics. Possibilities of application of Faynman's path integral method and statistical method of random walk to the lattice are considered for the conformon, as well.
Abstract: Wear of ultra-high molecular weight polyethylene (UHMWPE) and its composites is one of the main obstacles that limit the longevity of total joint replacements. Compression molded UHMWPE/quartz composites with organosiloxane as a cross-linking agent for UHMWPE matrix, were tested in nanoindentation and nanowear. The nanomechanical properties of the composite were examined in the light of nanoindentation experiments performed with a diamond tip of nominal radius of curvature of about 150 nm under conditions of various contact loads. Results from nanowear tests show that, in addition to the nanohardness and elastic modulus, the crosslinking procedure has the most pronounced effect on the tribological properties and at 0.5 phr organosiloxane, composites reache their maximum nanowear resistance. These findings are in agreement with the results of conventional mechanical and wear tests performed on these materials.
Abstract: This paper deals with the control of the adhesion strength in metal-elastomer joints by
fillers and the use of rubber blends to produce advanced high performance adhesive systems. The curing behavior of rubber compounds and dynamic mechanical properties of crosslinked rubber composites were considered in this technologically applicable study. The effects of two types of reinforcing filler, used to increase the adhesion strength between steel and rubber matrix based on acrylonitrile-butadiene rubber (NBR) and chlorosulfonated polyethylene rubber (CSM) was investigated. Precipitated silica (with the average size of primary particle 15 nm) and diatomaceous earth (with the average size of primary particle 28 µm) were used for both rubber and its blend (NBR/CSM). The filler loading range was from 0 to 35 phr. The determination of curing characteristics was estimated by Monsanto Rheometer. The crosslink density of the rubber composites was determined by swelling measurement. Dynamic mechanical behavior was measured by mechanical spectroscopy (in a single cantilever bending mode).
Abstract: The possibility of applying optical fibers as sensors for investigation of real time low
energy impact damage in laminar thermoplastic composite materials has been studied. For that purpose intensity based optical fibers were embedded in composite material specimens. Kevlar 129 (DuPont’s registered trade-mark for poly (p-phenylene terephthalamide)) woven fabric was used as reinforcement. Impact toughness testing by the Charpy impact pendulum was conducted in order to investigate low energy impacts. Transient intensity of optical signal during the impact, were compared with material crack initiation energy and crack propagation energy. Following this approach, development of damage in material was monitored. Obtained results show that intensity based optical fibers could be used as detectors for material damage appearance, and also, for level evaluation of its degradation caused by low energy impacts.