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Introduction
Powder technology (P / T) processing has become an advanced
important techniques, which many materials were produced by these
processes .
The most important reason to use ( P / T) that many materials are
difficult to melt and cast, as for example ceramic materials, hard metals,
cermets or refractory metals, Another reason for using powder technology
may be that it is economically attractive to make mass production
complex shaped structural parts by powder compaction and sintering, For
other products, such as magnets or multilayer electronic devices, there are
no practical alternative manufacturing process to that of powder
technology production processes.
The subject of ceramics covers a wide range of materials One
recent attempts have been made to divide it into two parts:1) traditional ceramics.
2) Advanced ceramics.
Traditional ceramics still represent a major part of the ceramics
industry. Interest in recent years has focused on advanced ceramics that,
with minor exception , have been developed with in that last 30 years or
so. Advanced ceramics include ceramics for electrical, magnetic,
electronic,
Thermal and optical applications, and ceramics for structural
applications at ambient as well as elevated temperatures. The
development in ceramic technology led to extend this to include other
binary and complex compounds of metals such as oxides, carbides,
nitrides, borides and silicides.
Ceramics are materials that have unique, high performance properties
and characteristics, The traditional ceramic materials are made up of
mainly inorganic nonmetallic compositions which have been formed by
the action of heat on raw material, while advanced ceramic materials are
made by complex thermal, chemical and mechanical routes. Some of the
unique properties of ceramics include :- dimensional stability ouer a wide
temperature range, high temperature resistance ( refractoriness), thermal
shock resistance, extreme wear resistance, electrical insulation, chemical
inertness, corrosion resistance at low and high temperatures, high
mechanical strength, hardness properties.
Many properties or combinations of properties which are not
achievable with other classes of materials give ceramics enormous
technical potential. The main obstacles that prevent the wider use of
ceramics include insufficient reliability, reproducibility, and high cost.
The physical basis of the processing steps is well established.
However, the chemical reactions which occur during the high –
temperature processing frequently influence the densification process and
microstructure development of ceramics in an unpredictable way.
Therefore, an ability to understand and control the chemical processes
that occur during ceramic processing are necessary to advance and open
up new uses for technical ceramics.
The typical application for ceramics include the automotive,
aerospace, power generation, thermocouple protection tubes and
insulaters, semiconductor equipment components furnace tubes, wear
components for the glass and metals industries, labware and process
crucibles.
Mechanical Processes
powder preparation by mechanical disintegration widely employed
in P / M. This is however a time – taking process rendering low yields.
There are six methods of mechanical comminution of metals and alloys,
as indicated above.
1) Machining
This method is employed to produce filings, turnings. Scratchings,
chips, etc. which are subsequently pulverized by crushing and milling.
Since relatively coarse and bulky powders entirely free from fine particles
are obtained by this method, it is particularly in suitable a very few
special cases such as the production of magnesium powders for
pyrotechnic applications (where the explosivenss and malliabilty of the
powder would prohibit the use of other methods, beryllium powders,
silver solders and dental alloys. The powder particles produced are of
irregular shape. This method is highly expensive and therefore has a
limited application.
This is especially employed where cost is not excessive in relation
to the cost of the metals themselves or where the choice of the method is
considered auccessity as in the case of Mg.
2) crushing
This method is mostly used for dis integration oxides
( subsequently reduced to metal powders) and brittle materials, Any type
of crushing equipment such as a tamps; hammers, jaw crushers or
gyratory crushes may be employed for crushing brittle materials.
Various ferrous and non – ferrous alloys, can be heat treated in
order to obtain a sufficiently brittle material which can be easily crushed
into powder from.
Some metals particularly titanium, zirconium, Vanadium, niobium
and tantalum when heated to moderate temperature in hydrogen
atmosphere are converted to brittle hydrides. The powder produced by
this method are of angular shape which are subsequently comminuted by
milling to attain the required fineness of the powder for processing by (P /
M) technique.