Ceramics to Overcome Brittleness

As discussed previously, ceramic matrix composites were originally developed to overcome the brittleness of monolithic ceramics. Thermal shock, impact and creep resistance can also be improved, making CMCs premium replacement choices for some technical ceramics. Industrial applications such as in automotive gas turbines or advanced cutting tools are already taking advantage of such characteristics. 

Ceramic-matrix composites are utilised to overcome the inherent brittleness of ceramics. The reinforcement consists of fibres or particles. The materials used include silicon carbide and alumina. The toughening comes about because the fibres or particles deflect or bridge cracks in the matrix. 

Monolithic oxide ceramics are problematic as stmctural materials due to their inherent brittleness. Therefore many efforts have been made during the last two decades to overcome the problem of brittleness by reinforcing oxide matrices. To achieve damage-tolerant and favorable failure behavior reinforcing components such as Zr02 particles (e.g. [1]), whiskers... 

Ceramic matrix composites (CMCs), in which carbon or ceramic fibers are embedded in a ceramic matrix, have been designed to overcome the intrinsic brittleness of monolithic ceramics with a view toward structural uses at extremely high service temperatures. The most commonly used are carbon (C/C) and SiC matrix composites (C/SiC and SiC/SiC). Ceramic matrix composites with a silica based glass or glass-ceramic matrices have also been studied [12] [53-56]. 

Developing appropriate ceramic—matrix composites [henceforth CMCs] for possible aero-engine applications started more than a decade ago. CMCs continue to be developed for improved toughness, to overcome the inherently brittle nature of most of the monolithic ceramics. Many experiments have shown that long-term loading of CMCs (for thousands of hours) produce improved high-temperature properties in monolithic ceramics by causing the dispersion of ceramic whiskers or... 

Too great a load chosen for deep penetration to overcome or minimize the sample surface factors can produce erratic hardness results as brittle ceramics crack locally around the indent and energy is expended on crack propagation. Deliberate overloading has now been shown to be a nondestructive microscopic way of determining important properties of ceramic systems, and as such occupies the whole of Chapter 5. However, when a polycrystalline ceramic is not obviously cracked and the indented area extends over many grains, then other microstructural features become dominant. For example, equation (1.2) is found to relate hardness to porosity in sintered materials. ... 

Ceramics have low fracture toughness because dislocations are difficult to move in ceramics especially at room temperature. It is, therefore, hypothesized that a frontal process zone (FPZ) ahead of a crack tip is composed of many nano-cracks rather than dislocations as in metals. To overcome the inherent brittleness of ceramics, a new microstructural design concept must be developed. The design concept of nanocomposites is a new, and significantly improved strengths are achieved with moderate enhancement in fracture toughness. The typical microstructure of nanocomposites consists of second-phase nano-size particles dispersed within the matrix grains. Thermal expansion mismatch between the matrix and second-phase particles improves several mechanical properties of nanocomposites. 

CNTs have received an enormous degree of attention in recent years, due to the remarkable physical and mechanical properties of individual perfect CNTs, which are considered to be one of the most promising new reinforcements for structural composites. The researchers have particularly focused on CNTs as toughening elements to overcome the intrinsic brittleness of the ceramic or glass material. Although there are now a number of studies published in the literature, these inorganic systems have received much less attention than CNT/pol5uner matrix composites. 

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