Page 36 - Mechanics Analysis Composite Materials

Chapter 1. Introduciion 21

The next step in the development of composite materials that can be treated as
matrix materials reinforced with fibers rather than fibers bonded with matrix (which
is the case for polymeric composites) is associated with ceramic matrix composites
possessing very high thermal resistance. The stiffnesses of the fibers which are
usually metal (steel, tungsten, molybdenum, niobium), carbon, boron, and ceramic
(Sic, A1203) and the ceramic matrices (oxides, carbides, nitrides, borides, and
silicides) are not very different, and the fibers do not carry the main fraction of the
load in ceramic composites. The function of the fibers is to provide strength and
mainly toughness (resistance to cracks) of the composite because non-reinforced
ceramics is very brittle. Ceramic composites can operate under very high
temperatures depending on the melting temperature of the matrix that varies from
1200°C to 3500°C. Naturally, the higher is this temperature the more complicated
is the manufacturing process. The main shortcoming of ceramic composites is
associated with a low ultimate tensile elongation of the ceramic matrix resulting in
cracks appearing in the matrix under relatively low tensile stress applied to the
material.
An outstanding combination of high mechanical characteristics and temperature
resistance is demonstrated by carbon-carbon composites in which both components
- fibers and matrix are made from one and the same material but with different
structure. Carbon matrix is formed as a result of carbonization of an organic resin
(phenolic and furfural resin or pitch) with which carbon fibers are impregnated, or
of chemical vapor deposition of pyrolytic carbon from a hydrocarbon gas. In an
inert atmosphere or in a vacuum, carbon-carbon composites can withstand very
high temperatures (more than 3000°C). Moreover, their strength increases under
heating up to 2200°C while modulus degrades under temperatures more than
1400°C. However in an oxygen atmosphere, they oxidize and sublime at relatively
low temperatures (about 600°C). To use carbon-carbon composite parts in an
oxidizing atmosphere, they must have protective coatings made usually from silicon
carbide. Manufacturing of carbon-carbon parts is a very energy and time
consuming process. To convert initial carbon-phenolic composite into carbon-
carbon, it should pass thermal treatment at 250°C for 150 h, carbonization at about
800°C for about 100 h and several cycles of densification (one-stage pyrolysis results
in high porosity of the material) each including impregnation with resin, curing, and
carbonization. To refine material structure and to provide oxidation resistance, its
further high-temperature graphitization at 2700°C and coating (at 1650°C) can be
required.
Vapor deposition of pyrolytic carbon is also a time consuming process performed
at 900-1200°C under pressure 150-2000 kPa.

I.2.3. Processing
Composite materials do not exist apart from composite structures and are formed
while the structure is fabricated. Though a number of methods has been developed
by now to manufacture composite structures, two basic processes during which
material microstructure and macrostructure are formed are common for all of them.