Kenji Matsunaga

Structure and properties of Si-Ti-C-O fibre-bonded ceramic material

Si-Ti-C-O fibre-bonded ceramic material was synthesized from pre-oxidized Si-Ti-C-O fibre with an oxide layer 400–600 nm thick, by hot-pressing at 2023 K under 50–70 MPa. The interstices in the Si-Ti-C-O fibre-bonded ceramic material were packed with an oxide material which existed on the surface of the pre-oxidized Si-Ti-C-O fibre, and the oxide material formed a small amount of the matrix phase (⩽10 vol%). At the fibre-matrix interface, aligned turbostratic carbon, which was oriented around the fibre, was formed during hot-pressing. The existence of the interfacial carbon layer indicated the Si-Ti-C-O fibre-bonded ceramic material to have a fibrous fracture pattern with high fracture energy. The Si-Ti-C-O fibre-bonded ceramic material showed excellent durability even at 1773 K in air, because a protective oxide layer is formed on the surface at a high temperature (above 1273 K) in air. Moreover, the Si-Ti-C-O fibre-bonded ceramic material almost maintained its initial strength in the bending and tensile tests, even at 1773 K in air.

Temperature-dependence mechanism of tensile strength of Si-Ti-C-0 Fiber-Aluminum matrix composites

The mechanism for the temperature dependence of the tensile strength of unidirectional hybrid type Si-Ti-C-O (Tyranno) fiber-reinforced aluminum matrix composite, in which SiC-particles are dispersed in the matrix, is discussed, focusing on the temperature dependencies of the stress concentration arising from broken fibers and critical length and their influences on the composite strength, by means of a shear-lag analysis and a Monte Carlo simulation. The main results are summarized as follows. The softening of the matrix at high temperatures raises the composite strength from the point of decrease in stress concentration, but on the other hand, it also reduces strength from the point of increase in critical length, which reduces the stress-carrying capacity of broken fibers over a long distance. The reason why the measured strength of composite decreased with increasing temperature could be attributed to the predominacy of the latter effect over the former one. The results of the simulation indicated that the hybridization of the composites improved room-temperature and high-temperature strengths through the strengthening of the matrix.

A new type of fiber-bonded-ceramic material synthesized from pre-oxidized Si-Ti-C-O fiber

Two types of fiber-bonded-ceramic material (FBC2123 or FBC1873) were synthesized from preoxidized Si-Ti-C-O fibers with oxide layers of 150 to 500 nm in thickness at 2123 K or 1873 K under 50 to 70 MPa. The interstices in both types of the materials were packed by an oxide material, which had existed on the surface of the pre-oxidized Si-Ti-C-O fiber. So, the dense, fiber-bonded-ceramic materials with small amount of the oxide matrix were obtained. During hot-pressing, carbon in excess of the non-stoichiometric ratio was released from the fiber and formed an interfacial layer on the surface of the fiber, beneath the pre-existing oxide material. Both FBC2123 and FBC1873 showed fibrous fracture patterns with high fracture energies at temperatures up to 1573 K and 1773 K, respectively. FBC2123 exhibited some plasticity in air at a temperature of 1673 K or over, due to the existence of amorphous silica in the matrix, and then a reduction in bending strength was observed at 1773 K in air. On the other hand, FBC1873 maintained its initial bending strength up to 1773 K in air, which is attributed to reduced crystallization of Si-Ti-C-O fiber and to the formation of cristobalite in the matrix.

Observation and analysis of fracture process of fibre, matrix and interface in UD Si-Ti-C-O fibre-bonded composite

The detailed observation of fracture process of unidirectional Si-Ti-C-O fibre-bonded ceramic composites at room temperature revealed the fracture behaviour as follows: first, matrix crack initiated at the strain level of about 0.2%. Then, matrix cracks accumulated with an increasing strain. In these processes the interfacial debonding was suppressed by the thermal compressive residual stress of the matrix. The slope of the stress-strain curve scarcely decreased from the initial one because of the high fibre volume fraction (around 0.9) and of the suppression of debonding. The breakage of fibres, followed by large scale interfacial debonding, occurred just below the ultimate load. Then, overall fracture of the composite occurred, accompanied by a large number of fibre breakage. A simulation of the fracture process was carried out using the modified shear lag analysis combined with the Mote Carlo method. The characteristics of the fracture behavior observed in experiments could be simulated fairly well by this method.

Notched strength of satin-woven Si-Ti-C-O fiber-bonded ceramic composite

The fracture behavior of Si-Ti-C-O fiber-bonded ceramic composite produced by hot-pressing oxidized 8 harness-satin-woven Si-Ti-C-O fibers was investigated by using unnotched and double edge notched tensile test specimens with different width (8 and 40 mm). The main results are summarized as follows. (i) The tensile strength of unnotched specimens for 8 mm width was higher than that for 40 mm width. Such a width-dependence of the unnotched strength could be described fairly well from the viewpoint of effective volume by application of the experimentally estimated Weibulls shape parameter. (ii) The applicability of the fracture toughness criterion (fracture arises when the stress intensity factor reaches the critical value) and net section stress criterion (fracture arises when the strength of the ligament reaches the unnotched strength) to the present composite was examined. The fracture strength of a notched specimen for 8 mm width was described by the net stress criterion. On the other hand, the strength for 40 mm width obeyed the net stress criterion for a small notch length but it shifted toward the fracture toughness criterion for large one. The shift of the fracture criterion from net strength- to fracture toughness-criterion arose around at the relative notch length 0.2 (notch length 8 mm), corresponding to periodical spacing of fiber strands (8 harness). (iii) The fiber pull-out length (0.4 mm on an average) was nearly the same as the half length of the fiber strand whose deformation is not constricted by the other strands in the satin-weave. (iv) The present fiber-bonded ceramic composite is insensitive to notch under the condition where the width of specimen is narrow and the notch length is smaller than 8 mm. This composite could be therefore applicable to industrial objects safely when the objects are designed as to satisfy the notch-insensitive condition.

Cryogenic properties of Si-Ti-C-O fibre-bonded ceramic using satin weave

Mechanical and thermophysical characteristics of Si-Ti-C-O fibre-bonded ceramic produced by hot-pressing the laminated material of oxidized satin-woven Si-Ti-C-O fibre have been investigated at room and cryogenic temperatures. The fibre element (diameter: 8 μm, fibre volume fraction: 85 ± 1%) constructing the Si-Ti-C-O fibre-bonded ceramic showed a close-packed structure of the oxidized Si-Ti-C-O fibre mainly composed of fine SiC crystals, amorphous SiO2-based phase and turbostratic carbon. The Si-Ti-C-O fibre-bonded ceramic with lightweight (density: 2.45 × 103kg/m3) and low porosity (<1 vol%) showed a markedly higher fracture energy (notched, cross-plied specimen: approximately 10kJ/m2) and lower thermal conductivity (1/10 the value of stainless steel). The reason why the fibre- bonded ceramic showed such a low thermal conductivity in spite of very high thermal conductivity of a pure SiC and carbon could be attributed to the complicated microstructure of Si-Ti-C-O fibre-bonded ceramics.

Fabrications and Mechanical Properties of Sintered SiC Fiber-Bonded Ceramics.

A sintered SiC fiber-bonded ceramic was synthesized by hot-pressing the plied sheets of an amorphous Si-Al-C-O fiber. Here we describe the microscopic structure and mechanical properties of the two-directional sintered SiC fiber-bonded ceramic synthesized from three kinds of starting fiber with different concentration of excess carbon and oxygen and different fiber diameter. The desirable sintered SiC fiber-bonded ceramic has been found to show a perfectly close-packed structure of the hexagonal columnar fibers with a very thin interfacial carbon layer. Furthermore, the interior of the fiber element was composed of sintered β-SiC crystal without an obvious second phase at the grain boundary and its triple points. Its mechanical properties at high temperatures are closely relate to the microscopic structure. The strength and the fracture behavior are strongly dominated by the uniformity of the interfacial carbon layer and a densified structure of the fiber element. A reduction in the excess carbon and oxygen included in the starting fiber resulted in the improved mechanical strength of the fiber-bonded-ceramic. Furthermore, it was found the highest four-point bending strength (-500MPa) could be obtained by the use of a thinner starting fiber (8μm) with low concentration of carbon and oxygen. It was concluded from these facts that the improvement in mechanical strength was strongly related to the formation of the uniform interfacial carbon layer and the most densified structure of the fiber element.