A composite (CMC) with a ceramic matrix such as SiC reinforced by ceramic fibers. A weak interface lets fibers pull out, giving a material that does not fail suddenly.
Japanese name: セラミックス基複合材料
Ceramic matrix composite appears in 1 practice question across 1 topic, including Composite materials.
Field: Composite materials
Ceramics have excellent heat resistance, but their greatest weakness is brittleness: they fail all at once from a small crack. A CMC is reinforced with ceramic fibers so that it does not fail immediately even when the matrix cracks. Whereas composites with polymer or metal matrices add fibers mainly to raise strength or stiffness, the main aim in CMCs is to change how they fail and so raise toughness. Fibers bridge matrix cracks and absorb energy as they debond at the interface and pull out, so the stress–strain curve continues smoothly instead of breaking off.
CMCs include SiC/SiC, with silicon carbide fibers and matrix; C/SiC, reinforced with carbon fibers; C/C, with carbon fibers and matrix; and oxide/oxide, with alumina-based fibers and oxide matrices. SiC fibers were made possible by a method announced in 1975 by Seishi Yajima and coworkers at Tohoku University, in which an organosilicon polymer is spun and fired, and they were industrialized in Japan. C/C withstands very high temperatures but oxidizes easily and is used with surface protection. Matrices are produced by chemical vapor infiltration (CVI) from gases, polymer infiltration and pyrolysis (PIP), and melt infiltration of molten silicon.
When a CMC is loaded, many fine cracks begin to form in the matrix at a certain stress (the proportional limit), and beyond that the fibers carry most of the load. Once cracks form, oxygen and water vapor enter through them to the fibers and interfaces and accelerate degradation, so designs often keep CMCs below the proportional limit. In addition, SiC forms a protective SiO₂ film in high-temperature oxygen, but in environments rich in water vapor, such as combustion gases, the SiO₂ volatilizes and recedes. The surface is therefore covered with an environmental barrier coating (EBC).
SiC/SiC CMCs are about a third of the density of Ni-based superalloys and withstand higher temperatures, so their use in the hot sections of jet engines is advancing. Since the mid-2010s, CMCs have begun to be used in commercial aircraft engines for parts such as the shrouds that surround the turbine. Because less air is needed to cool the parts, fuel efficiency improves. The challenges are that manufacturing is slow and expensive and that internal damage is hard to inspect, calling for design and inspection approaches different from those for metal parts. For brakes, C/SiC discs are used in high-performance cars.
Answer: C. To make the interface suitably weak so that cracks are deflected and energy is absorbed by fiber debonding and pull-out
The ceramic matrix is brittle, so under load cracks first form in the matrix. If the interface is too strong, these cracks run straight through the fibers and the material fractures all at once, just like a monolithic ceramic. A layered, easily sliding coating such as BN or pyrolytic carbon makes the interface suitably weak, so cracks are deflected along the interface, and the fibers bridge the crack while absorbing considerable energy through debonding and pull-out. The result is a fracture mode that does not fail suddenly, with a large increase in apparent toughness. Bonding the interface as strongly as possible is the approach for improving load transfer in resin matrices, and it is counterproductive in CMCs. The coating is not meant to lower the sintering temperature.
Ordinary ceramics fail all at once when a crack forms. In CMCs, fibers bridge cracks and absorb energy as they pull out, so the material does not fail suddenly when cracks form and can tolerate a certain amount of damage.
Because if the interface is too strong, a matrix crack runs straight through the fibers and the material fails like a monolithic ceramic. A coating such as BN makes the interface suitably weak, deflecting the crack and letting the fibers pull out.
It depends on the type and environment, but SiC/SiC aims at use around 1200–1300 °C and is said to be usable at higher temperatures than Ni-based superalloys. C/C withstands temperatures above 2000 °C in the absence of oxygen.
Sintering CFRP (carbon-fiber-reinforced plastic) Critical fiber length Delamination GFRP (glass-fiber-reinforced plastic) Metal matrix composite Quasi-isotropic laminate Silane coupling agent