Metal fiber
Metal fibers are defined as fibers composed of metals, plastic-coated metals or metal-coated plastics. Single component metallic fibers are fine-drawn metal filaments. These fibers are mostly used as reinforcing elements in composite materials.
For this, the fiber should
have high modulus and tenacity, better thermal expansion co-efficient,
resistance to solvents and comparatively lower density. Based on this, the composition or the type of metal or the structure of the fiber is usually
determined. The filaments are generally fabricated by one of the following techniques:
Those are
(a) wire-drawing technique,
(b) melt-forming technique,
(c) Powder metallurgical process and
(d) vapor deposition method.
Wire-drawing
The wire-drawing method is very well
known and common. However, the production cost, wire breakage, and
non-uniformity of the filament cross-section are very high. By this technique
fine filaments can be manufactured by means of the following principle:
(I) Encasing the sheath or core
wire with a matrix. The matrix may be plastic, glass or ceramic.
(ii) The matrix embedded wires
are heated to a temperature, to soften or melt the metalcore. Simultaneously
the sheath material is also softened.
(iii) Drawing the wire at high
temperature.
Melt-formatting technique
Molten metals can be
ordinarily extruded and pulled as per the principle of melt spinning. Because
of low viscosity at high temperature, as soon as the molten filament comes out
of the spinnerets, it breaks into separate droplets. However, it can be extruded
and processed by a modified technique. The principle of the technique is as follows:
(1) Melting of the metal in
a furnace.
(i) Solidification of the
molten metal’s immediately after extrusion by means of a chill block or chill
wheel.
Instantaneous solidification
takes place by surface contact of the molten metal with the chilled surface.
The chill roll melt spinning technology is very common for the production of
metal fibers.
Powder metallurgical method
By this method, metal powders are mixed with binders and extruded at High temperature. The extruded material is further processed for compactness.
Powder metallurgical method
By this method, metal powders are mixed with binders and extruded at High temperature. The extruded material is further processed for compactness.
Vapour deposition method
Boron fibers are prepared by
a different technique usually referred to as a vapor deposition method.
According to this method a heated core in filament form e.g., a tungsten wire
is continuously drawn through a sealed deposition chamber.
A vaporized reactant is
introduced in the chamber, which is decomposed and deposited on the core to
produce a composite filament. This is generally done by pyrolysis of boron
halide on a heated tungsten wire at 1500°C. The heating techniques for a
uniform boron wire was developed and done by radiofrequency or high-frequency
heating.
However, with the invention
of carbon and graphite fiber, the deposition is generally done by the chemical
vapor deposition method. The filaments thus formed is chemically treated by or
thermally post-treated to remove surface defects.
The chemical post-treatment
consists of an etching by some chemicals and the thermal post-treatment are done to
remove residual stresses. Some of the metal fibers commonly used for-composites
are Steel, bronze, alumina, boron, tungsten, copper-tin alloy,
Nickel-aluminum alloy.
