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Nanoindentation of Pressure Quenched Fullerenes and Zirconium Metal from a Diamond Anvil Cell
Nanoindentation of Pressure Quenched Fullerenes and
Zirconium Metal from a Diamond Anvil Cell

Shane A. Catledge, Philemon T. Spencer, Jeremy R. Patterson, and Yogesh K. Vohra
Department of Physics, University of Alabama at Birmingham (UAB)
Birmingham, AL 35294-1170

ABSTRACT


The sample size employed in high pressure diamond anvil cells is limited to a diameter of
typically 25 to 150 microns. While this size is often sufficient for diagnostics using synchrotron
x-ray diffraction and Raman scattering, ex-situ measurements of mechanical properties using
conventional microhardness indentation techniques is not feasible. For some materials, the high
pressure phase(s) can be quenched to ambient pressure allowing further characterization by other
techniques. We make use of the very small probe volume allowed by nanoindentation to
investigate the pressure-quenched structures of both C
70
fullerene and zirconium. For the case of
C
70
, we show that the amorphous phase established above 35 GPa can be quenched to ambient,
and that it shows a largely elastic indentation loading behavior with a hardness of 30 GPa. We
establish that this hard carbon phase contains a mixture of sp
2
- and sp
3
-bonded carbon and that it
can be produced from C
70
fullerene by application of pressure at room temperature. With regard
to zirconium metal, we confirm the irreversible transformation from the ambient hexagonal-
close-packed phase to the simple hexagonal
-phase (AlB
2
structure) and document an 80%
increase in hardness that may be attributed to the presence of covalent bonding based on sd
2
-
hybridized orbitals forming graphite-like nets in the (0001) plane of the AlB
2
structure.

INTRODUCTION


Due to the advent of the diamond anvil cell (DAC), the physical properties of materials
can be investigated under static pressures of millions of atmospheres and temperatures of several
thousand Kelvin. Investigation of materials over such a large pressure and temperature range has
resulted in the identification of new structures and has provided a fundamental understanding of
the physical transformations that occur under extreme conditions [1]. Characterization of
materials using DACs is typically performed in situ (under high pressure and/or temperature) via
the use of x-ray diffraction, Raman scattering, or electrical transport measurements. In some
cases, the high pressure and/or high temperature phase of interest can be quenched to ambient
conditions, allowing further ex situ characterization. Of particular interest is investigation of
mechanical properties, such as hardness, of the quenched phase [2]. However, this type of
measurement has been impractical due to the very small sample volumes required in DAC
devices in which the sample chamber is typically only 25-150
µm in diameter, depending on the
maximum pressure needed. We have overcome this difficulty by taking advantage of the small
probe volume available from a depth-sensing nanoindenter. In this way, we have obtained
hardness measurements from pressure quenched samples of C
70
fullerene and zirconium metal
compressed in a DAC at room temperature. Both of these materials exhibit irreversible structural
transformations upon compression, leading to a dramatic increase in hardness of the pressure
quenched phase.
Mat. Res. Soc. Symp. Vol. 649 © 2001 Materials Research Society
Q7.24.1
The fullerenes C
60
and C
70
can form a wide variety of polymeric and
disordered/amorphous carbon phases upon application of high pressure and high temperature due
to intermolecular interactions between the large cage-like molecules. Most of these phases can
be retained at ambient conditions after synthesis for further investigation. While most of the
polymeric phases have Vickers hardness of about 1 GPa, much of the interest and study has
been devoted to formation of superhard amorphous carbon phases with hardness as high as
around 70 GPa (about 70% of the hardness of bulk crystalline diamond) [3]. Most of the earlier
high pressure/high temperature synthesis of C
60
and C
70
was performed using a large volume
press limited to below 12 GPa, followed by bulk hardness measurements of the quenched
samples. However, the extended pressure and temperature range offered by the DAC (P to 400
GPa and T to 4000 K) combined with the small probe volume hardness measurements obtainable
with nanoindentation techniques offer new opportunities in characterization of pressure and
temperature synthesized phases. The mechanical properties of the pressure quenched amorphous
phase produced by compression at room temperature need to be characterized for comparison
with other superhard amorphous phases [4] produced by the high pressure/high temperature
techniques.

Another unique opportunity for mechanical property studies of pressure quenched
materials involves the group IV transition metals Ti and Zr which transform to the
-phase upon
compression and can be retained at ambient conditions. The
-phase has been the focus of
intense theoretical and experimental work due to its complex morphology, its interesting kinetics
of formation, and its effects on physical properties such as ductility and superconductivity [5].
The increase in microhardness with the formation of the
-phase during temperature quenching
in Group IV transition metal alloys has been well documented [5]. The pressure-induced
transformation in zirconium from the hexagonal close-packed structure (hcp)
-phase to simple
hexagonal
-phase usually starts at approximately 4 GPa at room temperature. The equilibrium
- transformation in Zr is around 2 GPa in the presence of shear stresses. The -phase has the
AlB
2
structure with a c/a ratio of 0.612. The (0001) planes form a sequence of the type
(ABAB) with trigonal coordination in the B-plane, resulting in a graphite-like net structure.
Formation of the
-phase leads to an increase in hardness and a consequent loss of ductility. This
has previously been experimentally verified for bulk samples of
-titanium in which the
hardness was greater than the
-phase by a factor of 1.8 at room temperature [5]. We report a
similar increase in hardness exhibited by pressure-quenched zirconium metal that has undergone
a phase transformation to the simple hexagonal
-phase in a diamond anvil cell at room
temperature.

EXPERIMENT


All high pressure DAC experiments employed diamonds with culet size of 600
µm and
the sample was placed in a chamber of 150
µm in diameter drilled in a spring steel gasket. C
70

fullerene of sample purity 99% was purchased from Alfa AESAR. The C
70
sample was annealed
at 400
°C in an argon atmosphere for 2 h in order to minimize the amount of solvent present in
the sample before experiments. Zirconium metal foil (annealed) of 0.25 mm thickness and 99.8%
purity was purchased from Alfa AESAR. Energy dispersive x-ray diffraction spectra were
recorded at the superconducting wiggler beam line X-17C at National Synchrotron Light Source,
Brookhaven National Laboratory. The x-ray diffraction experiments were performed with a
Q7.24.2 Fig. 1. Energy dispersive x-ray diffraction
spectra of C
70
in a diamond anvil cell upon
increasing and decreasing pressure. The
product of Energy and interplaner spacing
ED = 81.799
± 0.005 keV Å. The peak
marked g is from the surrounding steel
gasket.
microcollimated beam using a focusing mirror to produce a spot size of 10
µmx11µm. Micro-
Raman spectra of the pressure-quenched C
70
sample were obtained with 514.5 nm excitation
from an argon ion laser. Nanoindentation studies were carried out using a Materials Testing
System-Nanoinstruments XP system with continuous stiffness measurement and atomic force
microscope (AFM) attachment. The Berkovich diamond tip used for indentation had a nominal
radius of 50 nm.

RESULTS


Figure 1 shows energy dispersive x-ray diffraction patterns for the C
70
sample in a DAC
at a pressure of 4 GPa, 17 GPa, 27 GPa, 43 GPa, and at ambient conditions after release of
pressure. The starting C
70
sample at ambient pressure shows a mixture of face centered cubic
phase (a=14.95
± 0.04 Å) and hexagonal close packed phase (a=10.67 ± 0.03 Å, c = 16.75 ± 0.04
Å). Upon increasing pressure, a general broadening of the diffraction peaks is observed above 12
GPa with the eventual loss of the diffraction peaks above 35 GPa. At the highest recorded
pressure of 43 GPa, no residual crystalline phase could be detected. The amor