Optimization of the interfaces in Nicalon-fibre-reinforced-AIPO-matrix composite materials

The optimization of fibre/matrix interfaces in Nicalon-fibre-reinforced aluminium-phosphatematrix composite materials is addressed. First, the structure and chemical composition of the fibre/matrix interfaces were characterized for the as-fabricated composite materials and for the same materials after high-temperature exposures simulating the conditions of their intended application. Transmission electron microscopy (TEM) showed considerable Si diffusion from the fibres into the matrix accompanied by the formation of an interfacial diffusion/reaction zone in the process of heat treatment at 816 °C and higher temperatures. A BN-fibre coating did not prevent Si diffusion. Next, the interfacial bond strength was measured for the uncoated and some of the coated interfaces. The measurements showed a much lower bond strength in the carbon and carbon/BN coated interfaces than in the uncoated and BN-coated interfaces. Finite-element modelling was used to evaluate the interfacial bond strength which would result in the highest strength of the composite material. This optimal bond strength was found to be characterized by a critical energy-release rate close to 50 J m^–2. Further increase in the interfacial bond strength above 50 Jm^–2 resulted in brittle failure of the composite materials. Finally, the potential fibre coatings which were stable and not reactive with the fibres and the matrix at elevated temperatures were identified for the projected service temperatures of 816 and 1093 °C.     

Effect of heat treatment in air on the thermal properties of SiC fibre-reinforced composite. Part 1: a barium osumilite (BMAS) matrix glass ceramic composite

The thermal properties have been studied on a glass ceramic composite comprised of a barium osumilite (BMAS) matrix reinforced with SiC (Tyranno) fibres which has been subjected to a heat treatment in air in the range of 700–1,200 °C. Microstructural studies were carried out especially on of the interface between fibre and matrix. The presence of a carbon thin layer in the interface is a typical observation in SiC fibre-reinforced glass ceramic matrix composite systems. The microstructural evaluation and thermal properties showed a degradation of interfacial layer occurred at low heat treatment temperatures, (700–800 °C) this was attributed to the fact that, at those heat treatment temperatures the carbon rich layer formed during processing was oxidised away leaving voids between fibre and matrix, which were linked by isolated silicon-rich bridges. After heat treatment at higher temperatures of 1,000–1,200 °C, the thermal properties were retained or even enhanced by leaving a thick interfacial layer.     

Interface characterization and fracture of calcium aluminosilicate glass-ceramic reinforced with nicalon fibres

An investigation of the structure and properties of a calcium aluminosilicate glass-ceramic reinforced with Nicalon fibres is described. Microstructural analysis of the interface showed that during manufacture of the composite a reaction zone rich in carbon formed between the Nicalon fibre and the anorthite matrix. Tensile strengths were approximately 330 MPa for unidirectional material and around 210 MPa for a (0°/90°)_3s. composite, little more than half that predicted by the mixtures rule. Flexural strengths were, however, higher than tensile strengths, by a factor 1.5–2.5 depending on lay-up. Studies carried out on specimens heat treated in air for 24 h at temperatures in the range 600–1200 °C showed a progressive change of interface microstructure in the outermost regions of the specimens due to oxidation of the carbon-rich layer; at 1000 °C and above the carbon had disappeared to leave voids and silica-rich bridges between fibre and matrix. These changes affected the strength of the interfacial bond, as measured by an micro-indentation technique, and also the degree of fibre pull-out produced in mechanical tests. Thus as-received material exhibited appreciable pull-out whilst heattreated samples were characterized by brittle behaviour in the outer (oxidized) regions. Nevertheless, the composites whilst in the unstressed condition appeared to survive these short-term exposures to oxidizing environments. An interfacial shear stress of around 5 MPa was calculated by applying the Aveston, Cooper and Kelly theory to crack spacings measured in our room-temperature deformation experiments, a value which agreed well with the 3–5 MPa obtained by the micro-indentation method.     

Synthesis of nickel ferrite-dispersed carbon composites by pressure pyrolysis of organometallic polymer

Nickel ferrite-dispersed carbon could be synthesized by pressure pyrolysis of divinylbenzene (DVB)-vinylferrocene (VF)-nickelocene (Cp₂Ni) polymer in the presence of water under 125 MPa and at temperatures below 700°C. By heat treatment at 550°C with water, nickel ferrite particles could be dispersed finely in the carbon matrix, although a small amount of nickel-iron carbide also began to form above 600°C. The morphologies of the carbon particles formed were observed to be polyhedral, coalescing spherulitic and spherulitic. When 30 wt% H₂O, spherulitic carbons a few micrometres in diameter were prepared, in which nickel ferrite particles from 10–30 nm were dispersed in the carbon matrix. The saturation magnetization of carbon composites formed from DVB-3.0 mol% Cp₂Ni-6.0 mol% VF and 20 wt% H₂O at 550°C was about 30 e.m.u.g^–1 and increased with pyrolysis temperature. The coercive force of the carbon composite was 120 Oe and was affected by the amount of added water using pressure pyrolysis. Thermomagnetic measurement shows that the Curie temperature of nickel ferrite-dispersed carbon was about 580 °C.     

Synthesis of boron-dispersed carbon by pressure pyrolysis of organoborane copolymer

Boron-dispersed carbon was synthesized by pressure pyrolysis of divinylbenzene-tris(allyl)-borane and styrene-tris(allyl)borane at 125 MPa below 650° C. Amorphous boron dispersed in a carbon matrix was oxidized easily to yield boric acid by heat treatment under air at 300° C. The BK image of the product showed that boron was dispersed uniformly in a carbon matrix. Boron-dispersed carbon had the morphology of coalescing spherulite and polyhedra depending upon the concentration of boron in the parent copolymer. The grain size of carbon polyhedra decreased from 2.0m to 0.2m with an increase in the boron concentration from 1.3 to 5.7 wt%. The presence of 0.5 wt% boron in a carbon matrix enhanced the graphitization at 4.0 GPa and 1200° C, decreasing the lattice spacing with an increase in the crystallite size. The crystallite sizes were comparable to each other after heat treatment at 1100° C and 4.0 GPa when the specimen contained boron from 0.5 to 2.5 wt%. The lattice constant (c ₀) and crystallite size (L _c) of boron-dispersed carbon containing 2.5 wt% boron were 677.0 pm and 30 nm, respectively, after heat treatment at 1200° C and 4.0 GPa.     

Thermal stability and oxidation resistance of novel carbon-silicon alloy fibres

The effect of thermal treatment on the properties and structure of carbon-silicon alloy fibres produced from a novel silicon-containing carbon precursor is reported. The precursor, containing about 22 wt% Si, was melt spun into fibres and then oxidatively stabilized under different conditions to render the fibres infusible. The fibres were pyrolysed and heat treated to 1600 °C in inert atmosphere. The extent of stabilization was found to be critical to the development of mechanical strength of the fibres which varied with heat treatment temperature, showing a maximum at 1200 °C when the strength was 1.2–1.4 GPa. Moduli were low because of the lack of orientation of the carbon layer planes along the fibre axis. The maximum strength and the thermal stability at high temperatures is considerably reduced if the fibres are excessively oxidized at the stabilization stage. Optimally stabilized fibres show a drop in strength at 1300 °C but this stabilizes at about 600 MPa over the range 1300–1600 °C. These strengths are remarkably good considering the low modulus which is due to the quite high failure strains. The fibres can show excellent resistance to oxidation if given an initial short exposure to oxygen at high temperature. This is considered to be due to an imperceptible layer of silica.     

Thermophysical properties of ceramic matrix composites produced by polymer pyrolysis

A unidirectional composite and a series of bidirectionally reinforced composites were fabricated using carbon fibre reinforcement in a silicon carbide matrix, which was produced by the pyrolysis of a polymer precursor. The thermal expansion over the temperature range 20–1000 °C has been measured and the thermal diffusivity measured over the temperature range 200–1200 °C. Thermal diffusivity data was converted to conductivity data using measured density and literature specific heat data. Metallographic examination has been carried out on the composites and the results are discussed in terms of the observed microstructural features.     

Raman study of SiC fibres made from polycarbosilane

We have examined the evolution of Raman spectra of SiC fibres through structural and compositional transformations caused by heat treatment. The SiC fibre was made from polycarbosilane. Raman spectra of the SiC fibre indicate that it consists of (i) amorphous or microcrystalline SiC, (ii) carbon microcrystals, and (iii) silicon oxide. The amount of microcrystalline carbon in the fibre increases with heat treatment temperature up to 1400° C, and it decreases abruptly in those fibres heat treated above 1500° C. The tensile strength of the fibre drops virtually to zero after the heat treatment at 1500° C. Carbon microcrystals are precipitated from the Si-C random network with excess carbon, and they are distributed uniformly in the fibre. These carbon particles suppress the growth of SiC crystals. It is shown that the carbon microcrystals play an important role in maintaining the high mechanical strength of the SiC fibre.     

Fabrication,structure and properties of quasi-carbon fibres

Partially carbonized fibres, termed quasi-carbon fibres (QCF), with good thermal stability and acceptable mechanical properties were developed from a polyacrylonitrile-based precursor. Heat treatment temperature (HTT), in the 400–950 °C range, played a major role in determining the thermal, mechanical and electrical properties of the QCF. The thermal stability of the QCF was increased by increasing the HTT. An appreciable amount of the graphite-like structure in QCF began to develop at ca. 650 °C. The Young modulus magnitudes of QCF scaled almost linearly with the pyrolysis temperature. In contrast, the QCF exhibited a decreasing trend in both tensile strength and failure strain up to a HTT of 650 °C, above which both the tensile strength and failure strain of the QCF increased with the HTT. Electrical resistivity values of the QCF covered a very wide range from 10⁷to 10^–2 cm. QCF showed semiconducting behaviours with activation energies falling between 0.690 and 0.0052 eV when the pyrolysis temperature was in the 400–850 °C range.     

Rate and temperature effects on crack blunting mechanisms in pure and modified epoxies

The failure mechanisms of several epoxy polymers (including pure, rubber- and particulatemodified, as well as rubber/particulate hybrid epoxies) were investigated over a wide range of strain rates (10^–6 to 10² sec^–1) and temperatures (–80 to 60° C). A substantial variation in fracture toughness, G_Ic, with rate was observed at both very high and very low strain rates. Under impact testing conditions, G_Ic for both pure and rubber-modified epoxies displayed peaks at about 23 and –80° C which appeared to correlate with the corresponding size of the crack tip plastic zone. In order to explain these rate and temperature-dependent G_Ic results, two separate crack blunting mechanisms were proposed: thermal blunting due to crack tip adiabatic heating and plastic blunting associated with shear yield/flow processes. Thermal blunting was found to occur in the pure- and rubber-modified epoxies under all impact testing conditions and temperatures above 0° C. For temperatures below –20° C under impact conditions, the fracture toughness is dependent on viscoelastic loss processes and not thermal blunting. Plastic blunting was predominant at very slow strain rates less than 10^–2 sec^–1 for the pure- and rubber-modified epoxies and at impact strain rates for the fibre and hybrid epoxies. Microstructural studies of fracture surfaces provided some essential support for the two proposed crack blunting mechanisms.     

The environmental response of hybrid composites

Hybrid composite specimens containing a total of 60 or 75 vol % of unidirectional fibre were prepared from HT S-carbon fibre and E-glass fibre, HT S-carbon fibre and Kevlar 49 fibre, and E-glass fibre and Kevlar 49 fibre with a standard anhydride cured epoxide resin. The specimens were divided into four groups and subjected to the following environments: (A) room temperature and humidity; (B) soaked in water for 300 h at 95° C and then oven dried at 60° C to a constant weight; (C) thermally cycled 100 times between –196 and 95° C; (D) cycled 35 times between –196 and water at 95° C. The flexural properties of the samples were measured at room temperature after exposure. The modulus of the hybrid materials was not significantly affected by any of the treatments, although thermal cycling with or without water caused a large decrease in the modulus of all Kevlar fibre/resin and to a lesser extent all glass fibre specimens. The flexural strength of the unexposed carbon fibre/glass fibre and glass fibre/Kevlar fibre hybrids showed a positive deviation from the rule of mixtures behaviour at low volume loadings of the lower extension fibre. Wet thermal cycling or soaking in water caused a substantial reduction in the flexural strength of glass fibre/Kevlar fibre specimens. The interlaminar shear strength of all three fibre combinations was not affected by dry thermal cycling, but the effects of soaking in water and especially thermal cycling with water exposure were significant and irreversible.     

Hydration of tricalcium germanate

The hydration characteristics of tricalcium germanate were examined. Kinetics of tricalcium germanate hydration, and hydration product morphologies and compositions were determined at 5 °C intervals between 5 and 70 °C. Complete hydration was rapidly achieved, X-ray diffraction indicated that tricalcium germanate reacted completely within 2 h at all the temperatures investigated. However, a constant value for an Arrhenius activation energy could not be determined. At hydration temperatures above 45 °C a small heat peak, which preceded the main calorimetric peak, appeared in the calorimetric curves. Calcium hydroxide was rarely observed by SEM for hydration at temperatures below 45 °C. Dense regions of calcium hydroxide were readily observed for hydration above this temperature. In contrast to the hydration of tricalcium silicate, an induction period was not observed nor was an extended period of diffusionally controlled reaction. Calcium germanate hydrate was fibrous with the fibre thickness exhibiting a temperature dependence. Hydration product compositions were determined by thermal gravimetry. Expressing the composition of calcium germanate hydrate as (CaO)_3–x GeO₂·nH₂O, the value of 3–x decreases from 1.68 to 1.59 with increasing temperature from 5–70 °C. The values obtained forn varied inconsistently between 2.4 and 3.3. The unit cell of the calcium germanate hydrate was determined to be monoclinic. Cell parameters werea=1.851,b=1.147,c=0.531 nm and =98.10 °.     

Effects of boron addition on some properties of hard-type carbons

The boron-containing hard-type (HT) carbons were prepared by heating the raw coke compacts with 1.6 wt% boron at temperatures ranging from 1000 to 2800° C. Some physical and mechanical properties of boron-doped HT carbons have been measured and compared with those for boron-free materials. It was confirmed that the boron enters the HT carbon at a relatively low temperature of 1400° C and enhances the densification process of compacts during heat-treatment above 1800° C. The addition of boron caused increases in Young's modulus and thermal conductivity, and decreases in hardness and electrical conductivity of HT carbons. The effects are discussed, and compared with those for graphitizable carbons.     

Interface characterization by TEM,AES and SIMS in tough SiC (ex-PCS) fibre-SiC (CVI) matrix composites with a BN interphase

The fibre-matrix interfacial zone formed during the isothermal/isobaric chemical vapour infiltration processing of SiC fibres (ex-polycarbosilane)/boron nitride/SiC matrix composites has been analysed by TEM/electron energy loss spectroscopy, Auger electron spectroscopy, and secondary ion mass spectroscopy. In the composites, the boron nitride interphase (deposited from BF₃-NH₃) is made of turbostratic boron nitride, almost stoichiometric but containing some oxygen (less than 5 at %). The boron nitride layer stacks are randomly orientated except in the very vicinity of the fibre surface where they lie almost parallel to the substrate. The long chemical vapour infiltration treatment at 1000 °C used to infiltrate the SiC matrix acts as an annealing treatment for the metastable ex-polycarbosilane fibres which gives rise to the growth of an SiO₂/carbon amorphous double layer at the boron nitride/fibre interface. Deflection of microcracks arising from the failure of the brittle SiC-matrix occurs at the boron nitride/SiO₂ interface considered to be the weaker link in the matrix/boron nitride interphase/SiO₂/carbon/fibre sequence. It is suggested that the combination of the boron nitride layered interphase and SiO₂/carbon fibre decomposition products might play an important role in determining the propagation path of microcracks in the fibre/matrix interfacial zones and could be responsible, at least to some extent, for the non-brittle behaviour of such composites.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

Development of pitch-based carbon–copper composites

Carbon–copper composites with varying copper to carbon ratio of 0.66–1.5 (by weight) were developed from coal-tar-pitch-derived green coke (as such or modified with natural graphite) as carbon source and electrolytic grade copper powder at different heat treatment temperatures (HTTs) of 1000–1400 °C. The physical, mechanical, and electrical properties differ depending upon the HTT and also on copper to carbon ratio (Cu/C). The composites prepared at HTT of 1100 °C having Cu/C ratio of 0.66 and 0.9 exhibited a high bending strength of 150 and 140 MPa, bulk density of 2.63 and 2.81 gm/cm³, electrical resistivity of 1.6 and 0.96 m Ω cm and shore hardness of 88 and 84, respectively, in spite of well-known inadequate wettability between copper and carbon. Increasing the temperature from 1100 °C for processing of the composites deteriorated the properties mainly due to the loss of copper through melting above 1100 °C as revealed by X-ray, scanning electron microscopy, thermal analysis and EDAX studies.     

Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon)

The oxygen free Si–C fibre (Hi-Nicalon) consists of -SiC nanocrystals (5nm) and stacked carbon layers of 2–3nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si–C–O fibre (Nicalon NL202), the Si–C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200–1400°C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400°C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600°C could be caused by SiC coarsening or superficial degradation. Si–C fibres have a good oxidation resistance up to 1400°C, due to the formation of a protective silica layer.     

Electrically conductive PANi multifilaments spun by a wet-spinning process

Electrically conductive polyaniline (PANi) filaments were successfully spun from a spinning solution prepared from the PANi protonated with 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPSA) in dichloroacedic acid (DCA) as a solvent by a wet-spinning process. The conductivity of the fibre is in a range of 145 (±35) Scm^–1 to 1440 (±300) Scm^–1, which depends on the orientation of polymer chains. The fibre has a Young's modulus about 3.2 GPa, and a tensile strength about 0.23 GPa. Thermal analysis by TGA and DSC show that the fibre has five major weight losses at around 100 °C, 165 °C, 215 °C, 315 °C and 465 °C which are associated with the removal of moisture, residual solvent, the decompositions of the AMPSA, and the degradation of the PANi, respectively. The AMPSA in doped PANi performs two-stage thermal decompositions. The conductivity of the fibre was adversely affected by the thermal ageing due to the evaporation of the residual solvent at the temperatures lower than 100 °C and the decompositions of the dopant AMPSA at the temperatures above 100°C. The temperature dependent conductivity of both aged and unaged fibres is thermally activated at the temperatures between 15 K and 295 K. A negative temperature coefficient was observed in the temperature range of 240 K to 270 K for the unaged fibres. This disappeared when the fibres were thermally aged at 100 °C for 24 hours in vacuum. These results indicate that the residual solvent trapped inside the fibre enhances the electrical conductivity of the fibres, and possibly affects the negative temperature coefficient at the temperatures around 260 K.     

Dynamic mechanical analysis of a two-dimensional carbon-carbon composite

A two-dimensional carbon-carbon composite material consisting of stacked carbon cloths densified with a carbon matrix has been characterized with respect to its dynamic mechanical properties. Unprotected specimens and some protected against oxidizing conditions were investigated over the temperature range of subambient (–100°C) to 500°C. The reproducibility of the calculated storage modulus as well as the loss factor was within 5% at any one temperature over the entire temperature range. Results concerning protected and unprotected specimens were compared analytically and the difference of the relative plots above 400°C reflects the bad corrosion resistance of the unprotected samples under oxidizing conditions, where the carbon matrix material is more sensitive to oxidation than the fibre. The transition regions which appeared on the loss factor plots are strongly connected with the relevant secondary transition regions of the antioxidation protected material used. This technique, which has been demonstrated to be non-destructive for the sample analysed, proved that no differences exist in the dynamic mechanical properties of the specimens with respect to fibre orientation (warp or weft direction). Details of the experimental technique and assumptions made are also presented.     

Carbon fibre-reinforced silicon nitride composite

The processing of silicon nitride reinforced with carbon fibre was studied. The problems of physical and chemical incompatibility between carbon fibre and the silicon nitride matrix were solved by addition of a small amount of zirconia to the matrix and by low-temperature hot-pressing. The composite material possesses a much higher toughness than hot-pressed silicon nitride. Its work of fracture increased from 19.3 J m^–2 for unreinforced Si₃N₄, to 4770 J m^–2; its fracture toughness,K _lc , increased from 3.7 MN m^–3/2 for unreinforced material, to 15.6 MN m^–3/2. The strength remains about the same as unreinforced Si₃N₄ and the thermal expansion coefficient is only 2.51×10^–6 ° C^–1 (RT to 1000° C). It is anticipated that this composite may be promising because of its mechanical and good thermal shock-resistance properties.