Ceramic matrix composites CMCs properties

The program also addressed the need to develop lough ceramic-matrix composites (CMCs) with much greater resistance to brittle fracture. Early in the program, researchers round that the chemical structure that imparts superior thermal and mechanical properties to ceramics also results in negative altribuies. panicularly of brittleness, which easily can lead 10 catastrophic failure. 

The applications of glass/glass-ceramic matrix composites (CMC) can be divided into two specific categories aerospace applications and non-aerospace applications. In aerospace applications, performance is the prime consideration, while in non-aerospace applications cost-effectiveness is paramount. The characteristic properties of materials for aerospace applications should be... 

Fiber reinforced ceramic matrix composites (CMCs) are under active consideration for large, complex high temperature structural components in aerospace and automotive applications. The Blackglas resin system (a low cost polymer-derived ceramic [PDC] technology) was combined with the Nextel 312 ceramic fiber (with a boron nitride interface layer) to produce a sihcon oxycarbide CMC system that was extensively characterized for mechanical, thermal, and electronic properties and oxidation, creep mpture, and fatigue. A gas turbine tailcone was fabricated and showed excellent performance in a 1500-hour engine test. 

Depending on the method, the SiC composites vary in their properties as well as in their manufacturing costs. The advantages of the MI processes are a short manufacturing time and the use of low-cost raw materials. The unique thermal and mechanical properties of these MI ceramic matrix composites (CMCs) has opened up a wide field of new applications beyond aerospace. The data relating to these different materials are listed in Table 4.4. 

The criteria for designing fibers for use in ceramic matrix composites (CMCs) are different from those for designing fibers for use in poiymer or metal matrix composites. The key properties are thermal stability and mechanical properties at high temperatures [43]. As a consequence, relatively coarse microstructures are obtained at elevated temperatures, corresponding to somewhat lower failure strengths (-2 GPa), but high thermal stability and creep resistance are preferable to ultrafine microstructures. 

Composites usually consist of a reinforcing material embedded in various matrices (binder). The elfective method to increase the strength and to improve the overall properties of composites is to incorporate dispersed phases into the matrix which can be an either polymer or engineering materials such as ceramics or metals. Hence, metal matrix composites (MMCs), ceramic matrix composites (CMCs) and polymer matrix composites (PMCs) are obtained. Besides, hybrid composites, metal/ceramic/polymer composites and carbon matrix composites can also be obtained. MMC and CMC composites are developed to withstand high temperature applications. MMCs are also used in heat dissipation/electronic transmission applications due to the conductive nature of metals (electrically and thermally). 

There is ever increasing interest in Ceramic Matrix Composites (CMCs) due to the fact that mechanical properties are relatively constant with temperatures up to the maximum use temperature [1]. This is shown in the interest of CMCs for extended high temperature use where superalloys are usually considered [2,3]. 

There is ever increasing interest in Ceramic Matrix Composites (CMCs) due to the fact that mechanical properties are relatively constant with temperatures up to the maximum use temperature [1]. This is shown in the interest of CMCs for extended high temperature use where siqteralloys are usually considered [2,3]. As characterization of this class of material proceeds to enable such applications, key constituent properties need to be looked into in both the as-received state as well as after relevant exposure. The purpose of this paper is look at the interfacial shear stress of a known CMC system in bodi cases just mentioned and to additionally recommend a new sample design to make this property easier to measure. 

Table18.7. Properties ofselecteri ceramic matrix composites (CMCs)...

In ceramic matrix composites (CMC), the fibres mainly serve to increase the fracture toughness. Because one important property of the fibres is their small defect size, it is possible to use the same material for fibre and matrix. The advantage of this is that the elastic properties of fibre and matrix are identical, avoiding the formation of stress concentrations, and that the coefficient of thermal expansion is also the same, so no residual stresses are generated during cooling. Chemical reactions do not occur as well. Possible fibre materials are mainly ceramics. 

Elevated temperature applications require materials that can maintain good mechanical properties such as strength and hardness. Ceramics have good mechanical properties at high temperature and, thus, appear to be good candidates for elevated temperature applications. However, due to their brittle nature, monolithic ceramics are unsuitable for many applications where reliability is a critical issue. In the last few years, a new class of ceramic materials has been developed and studied. It is understood that two brittle materials can show non-brittle behavior if they are properly mixed. Fiber-reinforced ceramic matrix composites (CMCs) exhibit pseudo-plastic behavior at room temperature, as well as in an elevated temperature environment. Since the fiber and the matrix are made of ceramic material, creep behavior and hazardous emissions are reduced considerably. 

Ceramic matrix composites (CMCs) can be thought of as an improved form of carbon matrix composite in which the carbon matrix is replaced with ceramics that are stronger and much more resistant to oxidation. CMCs employ a variety of reinforcements including continuous fibers, discontinuous fibers, whiskers, and particles. Continuous fibers provide the best properties. There are many different types of CMCs, and they are at various stages of development. 

SiCf/SiC ceramic matrix. composites (CMC) are considered as structural materials in next generation fission nuclear reactors. However, thermal conductivity of SiC is reduced on the one hand at the highest temperatures, but also under irradiation. Titanium carbide, because of its peculiar thermal properties is an attractive material to be used as a matrix in a CMC to enhance the thermal conductivity of CMC under irradiation and at high temperature. 

Ceramic Matrix Composites (CMCs) 11 in the broad category of technical ceramics [I]. Unlike monolithic ceramics where surface and sub-surface flaws are known to be clearly detrimental from a tensile and durability [2] point of view, the effect of defects in CMCs is not as clear. The range of porosity (key defect) foimd in oxide/oxide is 25% [3], melt infiltrated nonoxide CMCs is 2% [3], polymer infiltrations pyrolysis ntm-oxide CMCs is 5% [3] and chemical vapor infiltrated non-oxide CMCs is 12% [4]. The properties vary widely between and wifliin these overall CMC classes. The above percent porosity for these classes of CMCs covers the conventional expectations from fabrication and does not consider local variations or unexpected processing concerns. Within all these systems, there is a range of durability behavior seen (both fatigue and creep). 

A ceramic matrix composite or CMC is composed of two or more solids, the matrix of which consists of a ceramic material or carbon. The crystalline, ceramic matrix is moulded and/or densified at a temperature of at least 1000 K. To the matrix one ormore solid inorganic substances are added, e.g. in the form of particles or fibres in order to alter the (thermo) mechanical properties of the pure matrix. In the composite s microstructure these additives can still be distinguished by their chemicalcomposition or geometry even after they have undergone a temperature treatment of at least 1000 K. 

FIGURE 12.11 Improvements of the mechanical properties of three-dimensional reinforced CMCs by hybrid infiltration routes (a) R.T. flexural stress-strain plots for a three-dimensional carbon fiber reinforced composite before and after cycles of infiltration (comparison between eight cycles with zirconium propoxide and fonr cycles pins a last infiltration with aluminum-silicon ester (b) plot of the mechanical strength as a fnnction of the final open porosity for composites and matrix of equivalent porosity, before and after infiltration (Reprinted from Colomban, R and Wey, M., Sol-gel control of the matrix net-shape sintering in 3D reinforced ceramic matrix composites, J. Eur. Ceram. Soc., 17, 1475, 1997. With permission from Elsevier) (c) R.T. tensile behavior (d) comparison of the R.T. mechanical strength after thermal treatments at various temperatures. (Reprinted from Colomban, R, Tailoring of the nano/microstructure of heterogeneous ceramics by sol-gel routes, Ceram. Trans., 95, 243, 1998. With permission from The American Ceramic Society.)... 

FIGURE 12.12 Examples of FGM CMCs (left) the combination of NLM Nicalon SiC fiber reinforced zirconia (in white) and mullite (in black) matrices (right) a Nasicon matrix reinforced with mullite (Nextel) and SiC (NLMTM) fibers (see the sketch). (Reprinted from Colomban, R, Process for fabricating a ceramic matrix composite incorporating woven fibers and materials with different compositions and properties in the same composite, Mater. TechnoL, 10, 89, 1995. With permission.)... 

In some applications the lack of toughness of ceramics or CMCs prohibits their use. In cases where enhanced stiffness, wear resistance, or elevated temperature capabilities greater than those provided by metals are necessary, metal matrix composites (MMCs) offer a reasonable compromise between ceramics or CMCs and metals. Typically, MMCs have discrete ceramic particulate or fiber reinforcement contained within a metal matrix. In comparison to CMCs, MMCs tend to be more workable and more easily formed, less brittle, and more flaw tolerant. These gains come primarily at the expense of a loss of high-temperature mechanical properties and chemical stability offered by CMCs. These materials thus offer an intermediate set of properties between metals and ceramics, though somewhat closer to metals than ceramics or CMCs. Nonetheless, like ceramic matrix composites, they involve physical mixtures of different materials that are exposed to elevated temperature processes, and therefore evoke similar thermodyamic considerations for reinforcement stability. 

R. Weiss, Caibon Fibre Reinforced CMCs Manufacture, Properties, Oxidation Protection, High Temperature Ceramic Matrix Composites ( jAs. W. Krenkel, R. Naslain, H. Schneider), WILEY-VCH, Weinheim, Germany, (2001), p. 440-456. 

This chapter will describe the processing and properties of an oxide fiber reinforced ceramic matrix composite with a silicon oxycarbide matrix based on a PDC technology, introduced by AlliedSignal (now Honeywell International) under the trademark of Blackglas ceramic. The oxide fiber in this CMC system is the Nextel 312 fiber (3M, Inc.) that has been treated to form a boron nitride surface coating. The information that follows was primarily developed from Low Cost Ceramic Matrix Composites (LC ) program funded by DARPA from 1991-1997. 

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... 

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