POSTER

1
Enhanced Mechanical Strength of Single Crystal
PMN-PT Achieved Through Surface Preparation
Methods D.J. Quesnel, J. Harker and N. Di
Mechanical Engineering University of
Rochester
Strength Measurements
Summary
336 MPa
The enhanced transduction properties of next
generation piezoelectric single crystals require
enhanced mechanical properties to realize their
full potential. Typical bend strengths at
fracture for commercially available crystals have
been in the neighborhood of 50-60 MPa. These
bend strengths are the result of subsurface
damage in the form of flaws, cracks, or other
defects caused by surface preparation and do not
represent the intrinsic strength of the crystal.
Because these crystals have relatively low
elastic modulus and can plastically deform by
phase transformation, the depth of the damage
caused by sawing operations is relatively large,
approaching 200 microns. When these damaged
layers are removed, the crystals are much
stronger showing apparent bending strengths in
excess of 330 MPa, a significant improvement.
In this work, the polishing protocol needed to
remove the subsurface damage and achieve the
significantly improved strengths is outlined.
Subsurface damage associated with saw cutting and
subsurface damage associated with polishing is
characterized. Results of mechanical tests are
given to show that the crystals can be bent to
the point of visible curvature without fracture
if the surfaces have been made free of defects
that lead to premature fracture. Strength levels
are more than sufficient to achieve the potential
of next generation piezocrystals in high-strain
transduction applications. This work supported
by ONR code 332, Dr. A. K. Vasudevan, Scientific
Officer. Collaboration with Dr. Lynn Ewar (NUWC
Newport), Dr. Pengdi Han (HC Materials), and Dr.
Jun Luo (TRS) is gratefully acknowledged.
The round roller set up above is a miniature
version of the ASTM Standard set up. The
undersized samples have been prepared by
polishing rather than grinding so do not strictly
comply with ASTM specifications. Results obtained
with this set up are comparable to the results
obtained by the D roller set up.
318 MPa
Polishing to Achieve High Strength
Samples are polished with the rule of threes.
Starting with 15 micron diamond abrasives,
successively finer abrasives each are used to
take off three times the depth corresponding to
the particle size of the previous abrasive. For
example, 6 micron diamond is used to take off 45
micron of materials that contains the damage from
the 15 micron abrasive. This is continued with
finer abrasive to 1 micron, and subsequently, a
fine polish removing only a fraction of a micron
is applied with 0.05 micron colloidal silica.
The structures left behind from the 1 micron
abrasive are discussed in a separate poster. A
total of 200 micron is taken off by all steps to
remove the damage believed to be present from the
sawing operations.
Experiment Set-ups
Observations when at High Stresses
Photos inset in the figures at right are taken
from movies of the samples during the tests. It
is clear that both samples show visible bending
(curvature) just before breaking, a surprising
result. The images in ambient light show that
the sample becomes increasingly transparent with
increasing load. This starts from the surfaces
(both tension and compression) and spreads inward
as the load increases. The crossed-polar images
show photoelastic fringes which become so
numerous above 80 MPa that individual fringes can
no longer be resolved. There is a general
increase in brightness as the transmittance of
the sample increases at higher stresses. This
also appears to start at both tension and
compression surfaces and migrate inward with
increasing load.
Summary
300 MPa is now the benchmark strength for PMN-PT
We are ready to test samples prepared in the
manufacturing environment to assess their
strength against this benchmark.
We are working on better understanding the
elastic response so that we can determine an
apparent Youngs modulus and more accurate strain
values
Acknowledgements The Rochester Piezogroup
In collaboration with Prof. David Quesnel
Professor Dr. Lynn Ewart at NUWC Newport,
RI Na (Dina) Di Ph.D. Student Dr.
Pengdi Han at H.C. Materials, Urbana, IL John
(Jace) Harker Ph.D. Student Dr. Jun Luo
at TRS, State College, Pa Stephen R. Robinson
Technical Associate This work is supported by
The Office of Naval Research, Code 332 under the
direction of Dr. A. K. Vasudevan, Scientific
Officer.
For further discussion, please contact David J.
Quesnel Department of Mechanical
Engineering Rochester, NY 14627-0132 dque_at_me.roche
ster.edu [phone] Presentation 920-93
5 AM, Wednesday, May 16, 2007 Session IV.5
The D roller set up has the advantage of being
able to use very small samples and still allow
accurate placement of the loading rollers.
Samples can be placed on flat platen compression
fixtures. This set up also allows unrestricted
observation of the sample during the test as
shown in the images at right.