Petr Glogar

Analysis of carbon fibers and carbon composites by asymmetric X-ray diffraction technique

Abstract The effect of annealing on the microstructure of three sets of carbon fibers and their composites with a phenol-formaldehyde matrix was investigated by X-ray diffraction. An asymmetric diffraction geometry with parallel beam optics was used to get more information in comparison with conventional diffraction experiments. It was found that the microstructure of the samples is improved after annealing up to 2800°C in terms of the following parameters: the narrowing of the orientation distribution function of ( 00l ) planes by a factor between 2 and 3 was observed, and this effect of the annealing ordering is more pronounced for the composites than for the fibers alone. The degree of graphitization increases by annealing, which is reflected in the decrease of the interplanar distance of ( 002 ) planes from about 3.51 A to 3.40 A, without any significant difference between the fibers and composites. The line broadening due to microstrains and small size effects is also strongly reduced by the annealing but the asymmetric shape of 100 and 110 reflections suggests that the layer stacking is still turbostratic.

Carbonization behaviour of some polyimide resins reinforced with carbon fibers

Abstract It was shown that the low weight loss makes the polyimide resins based on acetyl derivatives of aromatic diamines a promising candidate for carbon–carbon composites. The weight loss of this polyimide resin can reach about 30% in the composite, which is 1.5 times lower than that of the phenol-formaldehyde resin. It is suggested that the lower weight loss of the matrix in carbon fiber reinforced polyimide is due to strong fiber–matrix interaction, which can result from cross-linking between the fiber surface and the matrix. On the other hand due to this strong fiber–matrix interaction carbon–carbon composites can undergo brittle failure with crack propagation normal to the fibers.

Resonant frequency study of tensile and shear elasticity moduli of carbon fibre reinforced composites (CFRC)

Abstract The dynamic elastic properties are important characteristics of composite materials. They control the vibrational behaviour of composite structures and are also an ideal tool for monitoring of the development of CFRCs’ mechanical properties during their processing (heat treatment, densification). The present studies have been performed to explore relations between the dynamic tensile and shear moduli and some structural features (viz., fibre fraction, fibre type, porosity, weave pattern of woven reinforcement) of various unidirectional or bi-directional fibre reinforced carbon/carbon composites, made out of PAN- or pitch-based fibres as reinforcements and phenolic resin or coal tar pitch as matrix precursors. The dynamic tensile and in-plane shear moduli were determined from resonant frequencies of a beam with free ends. The longitudinal dynamic Young’s modulus of unidirectional CFRC composites – besides its dependence on the original fibre modulus and fibre volume contents – also reflects changes induced in matrix and fibres by heat treatment. The in-plane shear modulus does not depend on the fibre type but there exists its distinct tendency to increase with increasing fibre fraction. For bi-directionally reinforced composites, the longitudinal tensile modulus is more sensitive to the fabric weave pattern than to the fibre type. Tensile modulus of diagonally cut specimens and in-plane shear modulus of longitudinally cut ones are mutually correlated and, therefore, simultaneously controlled by densification steps and graphitisation heat treatment.

Carbon/carbon composites based on a polyimide matrix with coal tar pitch

Abstract Blending of coal tar pitch with a polyimide precursor based on acetyl derivatives of aromatic diamines during its synthesis leads to a homogeneous, highly thermostable matrix for carbon fibre reinforced composites. If the weight content of the pitch in the polyimide matrix does not exceed 40%, the mechanical properties (flexural strength, shear modulus and fracture toughness) of these composites are comparable to those of similar composites based on a pure polyimide matrix. Carbonisation and graphitisation of the composites with a properly blended matrix precursor leads to carbon fibre reinforced carbon composites with lower open porosity and higher density, elastic modulus and flexural strength than those of composites based on a pure polyimide matrix.

Strength, elasticity and failure of composites with pyrolyzed matrices based on polymethylsiloxane resins with optimized ratio of D and T components

Two mixtures of T and D siloxane monomeric components labelled as TxDy (molecular ratio x:y equal 3:1 or 4:1) were chosen as matrix precursors for manufacturing Nextel720 reinforced unidirectional composites which, after pyrolysis at 1000 or 1100°C, revealed good endurance in an oxidizing environment up to 1500°C. Vickers hardness of the heat treated (1000–1500°C) samples of pyrolyzed matrices T3D1 and T4D1 are mutually similar (1100–1400 HV0.2) and reach their maximum between 1200–1300°C. Flexural strength of the pyrolyzed composites is 150–170 MPa and 170–250 MPa for T3D1 and T4D1, respectively. After annealing 3 h in air at 1200–1300°C, the strength slightly decreases but similar treatment at 1500°C yields strengths exceeding those of the pyrolyzed material. Shear modulus of the pyrolyzed T4D1 composite is roughly twice that of the T3D1 one (15 GPa vs. 8 GPa) and both increase sharply to 22–25 GPa after annealing at 1500°C, which manifests substantial improvement of the matrix properties. Fracture toughness of the composites, as measured by chevron notch test at RT, 550°C, and 1100°C, yields 4–5 MPa.m−1/2 for T3D1 and 3–4 MPa.m−1/2 for T4D1. For both composite types, the fracture toughness drops by 1 MPa.m−1/2 when measured at 550°C, which can be attributed to suppression of fibre pull-out due to stress state changes caused by the coefficient of thermal expansion (CTE) mismatch. Fracture surfaces generated during flexural tests of the annealed samples reveal decreasing occurrence of pullout towards the highest annealing temperature.

Studies on the influence of secondary heat treatment on structure and properties of unidirectional carbon/carbon and carbon/furfural-siloxane composites

Abstract Unidirectional carbon/carbon composites have been prepared using high strength and high modulus carbon fibres as reinforcements and phenolic, polyfurfuryl alcohol and furfuryl-siloxane resins as matrix precursors. The composites were pyrolyzed to 1000 °C and subsequently heat treated to 1450 and 2800 °C in the presence of argon. Changes in volume and density were measured and were found to be dependent on reinforcement and matrix precursors. Porosity and pore size distribution measured by mercury porosimetry suggests that during secondary heat treatment pore and crack closing takes up if the fibre/matrix bonding is strong and the opposite phenomenon takes place otherwise. This has been substantiated by optical and SEM microscopy. The porosity and pore size distribution influences the flexural strength of the composites while the dynamic modulus has been found to be dependent on the graphitic contents of the composite. The secondary heat treatment enhances the oxiresistivity of the composites though porosity is still a determining factor. Presence of silicon compounds in the matrix enhances the oxidation resistance and in the composites with this matrix system is found to be more effective at the fibre/matrix interface.

Fracture Properties of Basalt Fibre Composites with Cured or Pyrolysed Matrix

Composite materials based on polysiloxane matrix reinforced by basalt fibres were prepared in laboratories of the IRSM ASCR. The composite samples were pyrolysed at 400 ÷ 750 °C after moulding and curing at 250 °C. Measurement of several mechanical characteristics (flexural strength, fracture toughness, impact strength, and measurement of elasticity) demonstrates a favourable influence of pyrolysis in comparison with the cured-only composite material. Fracture toughness was measured by chevron-notch technique and fracture surfaces were investigated using a scanning electron microscopy.