Carbon Nanofibers Reinforced Ceramic Matrix Composites

Field: Materials Science (Physical Sciences)

IntroductionModern ceramic materials have, thanks to their crystallographic structure and strong atomic bonds, many excellent properties, such as extremely high hardness, strength, high thermal and chemical stability, high corrosion resistance, and wear resistance.Their weakness is low fracture toughness and crack growth resistance and hence high brittleness and lower reliability.One of the ways how to overcome these drawbacks is preparation of composite materials, where the base ceramic matrix is reinforced by secondary phases in forms of particles/whiskers and in recent years increasingly in a form of fibrous structures.In advanced fine grained ceramics these usually take form of nanofibers and/or nanotubes.Among the most promising candidates are carbon-based filamentous nanomaterials such as carbon nanotubes (CNTs) and also carbon nanofibers (CNFs), which attracted a lot of attention due to their outstanding mechanical properties, excellent thermal performance and useful electrical characteristics (high electrical conductivity).Nowadays, new ceramic/carbon nanotube composites are being developed mostly with two aims: to improve the mechanical properties of the ceramic materials by reinforcing with carbon nanofibers and to develop functionalized ceramics with improved magnetic and electric properties.Studies show that CNTs (both single-wall and multi-wall) should be ideal reinforcing/functionalizing elements for composites due to their small size, low density and good electrical and thermal conductivity.This work focuses on investigations of ceramic matrix composites based on alumina, zirconia and silicon nitride reinforced by carbon nanofibers and nanotubes.The basic characteristics of commercially available nanofibers/nanotubes are studied by various techniques.The chapter then focuses on mechanical properties of reference monolithic and experimental composite materials.The effect of volume fraction of carbon nanofibers on hardness and fracture toughness is illustrated.Further, the possibilities of improving the tribological and wear properties are discussed.The chapter concludes with the section that explores important aspect of functionalization of ceramics composites by improving their electrical properties, namely electrical conductivity. Description of constituentsRecently a lot of experimental work and development has been going on in preparation of ceramic matrix composites with carbon nanofibers as great improvements of material www.intechopen.comNanofibers -Production, Properties and Functional Applications 242 properties are expected.Up-to-now only modest enhancement of properties have been reported in CNT reinforced silicon carbides (Ma et al., 1998) and silicon nitride (Balázsi et al., 2003;Belmonte et al., 2010).Some other works reported improved fracture toughness achieved in alumina matrix systems (Zhan et al., 2003a) but rather often the performance of new composite materials was disappointing due to difficulties in dispersing the CNTs in ceramic microstructures and rather weak bond with them.More success has been achieved in increasing the electrical conductivity in many originally insulating ceramic materials (Zhan et al., 2003b, Balázsi et al., 2006) and in optimizing tribological behaviour of brittle systems (González-Julián et al., 2010;Hvizdoš et al., 2010).This section describes basic characteristics of alumina, zirconia and silicon nitride as materials of ceramic matrices and provides the information about the ongoing effort in combining these materials with carbon nanofibers and nanotubes. Aluminium oxideAluminium oxide (Al 2 O 3 ), commonly referred to as alumina, is the most cost effective and widely used material in the family of engineering ceramics.The raw materials from which this high performance technical grade ceramic is made are readily available and reasonably priced, resulting in good value for the cost in fabricated alumina shapes.With an excellent combination of properties and an attractive price, it is no surprise that fine grain technical grade alumina has a very wide range of applications.Alumina possesses strong ionic interatomic bonding giving rise to its desirable material characteristics.It can exist in several crystalline phases which all revert to the most stable hexagonal alpha phase at elevated temperatures.This is the phase of particular interest for structural applications.Alpha phase alumina is the strongest and stiffest of the oxide ceramics.Among its key properties one can emphasize high hardness and wear resistance, excellent dielectric properties from DC to GHz frequencies, resistance to strong acid and alkali attack at elevated temperatures, good thermal conductivity, excellent size and shape capability, high strength and stiffness.The material is industrially available in purity ranges from 94% (an easily metallizable composition), to 99.5% for the most demanding high temperature applications.Typical uses of alumina include gas laser tubes, wear pads, seal rings, high temperature electrical insulators, high voltage insulators, furnace liner tubes, thread and wire guides, electronic substrates, ballistic armor, abrasion resistant tube and elbow liners, thermometry sensors, laboratory instrument tubes and sample holders, instrumentation parts for thermal property test machines, grinding media, etc. Zirconium oxideFor polycrystalline zirconia ceramics is typical combination of high strength, fracture toughness and chemical stability, which enables its use in extreme conditions (Garvie et al., 1975).It is an extremely refractory material which can be used at temperatures up to 2400°C.It offers chemical and corrosion inertness to temperatures well above the melting point of alumina.It has relatively high density and low thermal conductivity (20% that of alumina).It resists molten metals and has high wear resistance.It is electrically conductive above 600°C and is used in oxygen sensor cells and as the susceptor (heater) in high temperature induction furnaces, where it can be used at over 2000°C in oxidizing atmospheres.Other typical uses include cutting tools for cutting problematic materials such as Kevlar, polymer www.intechopen.com