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Structural Ceramics:

• Remain hard and resist deformation at high temperatures where metals soften or melt, e.g. metal cutting, metal filters, brake pads and rotors
• Resist cavitation and ablative wear e.g. nozzles for water cutting, ink jet printing, and rocket engines
• Are inert e.g. bioceramics such as hip joint, knees, teeth, and bone compatibility

Structural ceramic materials are important because they are light weight, harder than metals, withstand higher temperatures and are actually stronger. When they do fracture, most high strength ceramics suffer catastrophic failure, which means there is no yield stress.

Some of the challenges include processing uniform, high green density parts and sintering to high density. Usually high density and fine grain sizes are best because this gives higher hardness and better uniformity and strength. In certain ceramics we want controlled porosity e.g. refractories and filters.

The key to obtain high strength in structural ceramics is to increase the toughness which prevents fracture. Fracture always occurs due to high stresses at flaws or cracks. When flaws are carefully controlled, ceramics can achieve strengths 10 to 100 times higher than metals, typically at 1/3 the weight.

The toughness of ceramics is increased with tricks like using high aspect ratio particles (whisker or needles) or fibers to give mechanisms such as pull out, crack tortuousity, and crack bridging. Another toughening option is to add zirconia for transformation toughening or metals to allow local deformation to absorb crack tip energy. Another approach is to increase the toughness with directional void space or preferential debonding directions to allow for crack propagation without catastrophic failure (one example is in fibrous monolith composites). The strength is usually lower for this last approach, however the delayed fracture allows warning of failure, i.e. there is a yield stress.

Some important properties for various structural ceramics include: wear resistance, strength, hardness, fracture toughness, corrosion resistance, biocompatibility, and thermal shock resistance. Structural ceramics find applications in automotive, aerospace, printing, textile, metal cutting, and many other industries.

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