Page 355 - Book Hosokawa Nanoparticle Technology Handbook

6.2 MECHANICAL PROPERTIES FUNDAMENTALS
Hardness test is effective for the evaluation of nanoparticles, for example, by hot pressing, hot isosta-
mechanical properties on coatings and printed cir- tic pressing, sinter forging and spark plasma sintering
cuits. It can be applied on the characterization on (SPS). The creep and superplasticity are also related to
nanomaterials, too. those stress assisted densification processes [5].
Creep and superplasticity behavior of nanomateri-
als have been characterized in creep tests [6] and
References
superplasticity tests at elevated temperatures.
[1] W.C. Oliver, G.M. Pharr: J. Mater. Res., 7, 1564 (1992).
6.2.3.1 Creep test
`[2] M. Sakai, S. Shimizu: Ceramic Trans., 133, 105 (2002).
In a creep test a constant load is applied to a tensile
[3] M. Sakai, S. Shimizu: J. Mater. Res., 14, 1471 (1999).
specimen maintained at a constant temperature. Strain
[4] M. Sakai: J. Mater. Res., 14, 3630 (1999).
is then measured over a period of time t. A typical
[5] S. Sakaguchi, N. Murayama, Y. Kodama and F. Wakai: creep curve is shown in Fig. 6.2.11, and the slope of
.
J. Mater. Sci. Lett., 10, 282 (1991). the curve is the strain rate . The creep curve is usu-
[6] JIS R 1602: 1995, Testing methods for elastic modulus ally divided into three regions. Primary creep is a
of fine ceramics. period of decreasing creep rate. The strain rate even-
[7] K. Yamanaka, H. Cho and Y. Tsukahara: Appl. Phys. tually reaches a minimum and becomes almost con-
Lett., 76, 2797 (2000). stant. This is known as secondary creep or steady
creep. The “creep strain rate” is typically the rate in
[8] JIS K 7244: 1998, Plastics – Determination of dynamic
the secondary stage, or the minimum creep rate.
mechanical properties.
Tertiary creep occurs when there is a reduction in
[9] JIS R 1642: 2002, The test method for internal friction
cross sectional area due to necking or effective reduc-
test of fine ceramics at elevated temperatures.
tion in area due to internal cavity formation.
[10] JIS Z 2244: 2003, Vickers hardness test – test method. Japanese Industrial Standards for testing creep have
[11] JIS R 1610: 2003, Test methods for hardness of fine been established for bulk materials in tension, com-
ceramics. pression and bending.
JIS Z 2271: Method of creep and creep rupture for
6.2.3 Creep/superplasticity
metallic materials.
To use nanomaterials in structural applications, design JIS R 1631: Method for tensile creep of fine
engineers must have knowledge of the mechanical ceramics.
response of the material. Deformation is the change in JIS R 1612: Method for bending creep of high-
shape due to an applied force. While elastic deformation
is reversible, plastic deformation is not reversible. Creep performance ceramics.
refers to the process in which a material continues to JIS R 2658: Testing method for compressive creep
strain with time upon application of stress. As the grain of refractory bricks.
size in metals and ceramics moves into the nanoscale,
an increasingly large proportion of atoms in the solids Similar testing methods can be applied to nanocrys-
are found on the grain boundaries. In those nanocrys- talline materials as well. When the available specimen
talline materials, the diffusional creep occurs at an inter-
mediate temperature range (0.5 T T 0.4T , T :
m
m
m
Melting temperature), because the strain rate is
enhanced by grain boundary diffusion much more than
that of the conventional microcrystalline materials Fracture
[1–3]. Creep is a property that is related to the mechan-
ical strength of nanocrystalline materials. Primary creep
On the other hand, superplasticity refers to the ability
of polycrystalline solids to exhibit exceptionally large Strain
elongations in tension at elevated temperatures [4]. It is
a property commonly found in many metals, alloys, Tertiary creep
intermetallics and ceramics when the grain size is very
small: less than several micrometers for metals and less Secondary creep
than one micrometer for ceramics. Further grain
refinement leads to high-strain rate superplasticity or
low-temperature superplasticity. Such behavior is of
interest because the ability to achieve large strains Time t
makes superplastic forming an attractive option for the
manufacture of complex shaped components. Figure 6.2.11
Nanocrystalline materials are often fabricated from Typical creep curve.

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