FGM's are considered the first of the fusion materials in the material
technology trend ranging from the conventional "separation type" centered
around separate functions, via the "composite type" aiming at the synergism
of dominant property among component elements, up to the "fusion type" in
which the scope reaches the microscopic level whereby even the distinction
between different types of materials is not clear.
Since intended gradient functions can be selected in these materials by
means of multiple changes in the composition and structure in regard to
thickness and a variety of materials can be selected for combination, new
functions totally different from those now common are available. When one
reflects that this type of heterogeneous material was often conventionally
treated as a defective material, it appears that we are living in a quite
different age.
Although opinions are divided on the background of why such a new materials
concept has appeared, it appears that after all it is largely due to the
inception of such huge projects as the future aerospace and energy projects,
the development of materials by computer-aided means, etc.
The future spacecraft regarded as the immediate objective is a gigantic
project in which various advanced nations such as the United States, the EC
nations, and Japan are engaged in a fierce development competition with each
other, with the development target set at the beginning of the 21st century.
The breakthrough in this gigantic project, after all, solely depends upon
whether or not the advanced technologies in regard to materials have reached
the appropriate level at the present stage. For example, the achievement
of the appropriate temperature for each element of the space shuttle based
upon the estimated speed of Mach 8 (eight times the speed of sound) when it
reenters the atmosphere is as shown in Figure 1. The temperature range
reaching the highest level as a result of aerodynamic heating is at the nose
and at the inlet of the propulsion system, where it reaches a temperature
of almost 1,800°C. Moreover, the shuttle is also exposed to a temperature
as low as -253°C when the liquid hydrogen engine is used as the propulsion
system and it becomes an incredible "fireball flying shuttle with a built-in
ultralow- temperature tank."
Among the various materials presently considered as candidates for fuselage
materials for such spacecraft, the use of various inorganic system composite
materials for high-temperature parts is under consideration by the United
States and other countries, and a competition in this, including FGM's, can
be expected in the future. It is evident that the key to the success of a
space shuttle operating in a harsh environment lies in the propulsion
system, and the material's technology, the importance of the material for
this system, is also great. After all, materials have reached the position
of the latest stage high technology. We recently came across an expression
to this effect also in the United States.
Up to now, our discussion has centered on heat-resistant characteristics for
the aerospace field, which is the immediate area of concern. However, the
concept of functionally gradient materials is not limited to such thermal
and mechanical functions. It is believed that gradient materials will be
oriented to multiple functions in the future, and that the range of
application will also be diversified, as indicated in Table 1. Expectations
are especially high in regard to the polymer and biological areas, in which
the practical use impact will be great.
For FGM's to become widely accepted as industrial materials in the society,
taking into account the environment and characteristics required, the issues
and problem points of these materials must be ascertained and appropriate
countermeasures taken.
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