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Possibility of High Pressure Transformation during
Nanoindentation of SiC
Maneesh Mishra, Izabela Szlufarska, Materials Science
Program
High-pressure transformation to rocksalt
structure has been proposed as a mechanism that
underlies a ductile wear observed during
nanomachining of SiC. However, in contrast to
other brittle materials (e.g., Si), no such
transformation has been directly observed either
during machining or during nanoindentation of
SiC. We have performed large-scale molecular
dynamics simulations of nanoindentation with
spherical indenters of various sizes and surface
roughness to determine whether SiC can undergo a
nanoindentation-induced transformation from
zincblende to rocksalt structure. Load
displacement curve and stress distribution for a
tip with radius of curvature R=10 nm
indentation are shown in figure 1.
Figure 1.(a) Load vs. displacement (P vs.
h) curve for R=10 nm nanoindentation.(b)
Distribution of hydrostatic pressure under the
indenter during nanoindentation of SiC at indent
depth of 18 Å averaged locally over 3.4 Å
spherical volume.
The calculations of possible states of stresses
under the indenter have been combined with a
thermodynamic analysis to estimate the effects
of dislocation density, shear stresses, and
temperature on the phase transformation pressure
in SiC. Dislocation and shear stress lower the
transformation pressure by a maximum of 20 GPa
and 10 GPa, respectively. Temperature (which can
be as high as 1400K) increases the
transformation pressure, but the change is
smaller than 3 GPa. Due to local fluctuations
pressure in small volumes in the deformed region
can reach 60 GPa (see figure 1(a)), but for the
transformation to take place in a small volume,
the pressure required for a transformation also
increases due to interface energy. The results
are summarized in figure 2(a).
Figure 2.(a) Summary of how dislocations,
shear stresses, temperature, and granularity of
stress affect the transformation pressure. The
maximum pressure under the indenter (light grey)
has been plotted along with the pressure
required for transformation to occur (dark
grey). (b) Maximum local stresses under the
indenter as a function of indenter load for R
=10 nm tip. Circles and triangles correspond
to the shear and the hydrostatic stresses,
respectively. Dashed lines are the required
hydrostatic stress for pressure transformation
(blue) and critical resolved shear stress for
dislocation nucleation on the
 (111)
system (red).
Our analysis indicates that a nanoindentation-induced
HPPT is not likely in SiC, which is consistent
with the fact that no such transformation has
been observed directly in experimental and
modeling studies on SiC. On the other hand, the
shear stresses under the indenter tip are high
enough to cause a slip on the {111} family of
planes (figure 2(b)). Dislocation plasticity is
thus the most likely mechanism for ductile
deformation during nanoindentation and
nanomachining of SiC.
A more detailed discussion can be found in Ref.
[1]
[1] Mishra M. and Szlufarska I. Possibility of
high-pressure transformation during
nanoindentation of SiC. Acta Mater (2009),
doi:10.1016/jactamat.2009.08.041 |
