Download Hard Materials with Functionally Designed Mesostructure

Structure of Materials
Microstructure
The microstructure of hard materials has a strong
impact of the performance of the material. There are
many parameters which describe the microstructure
of hard materials such as
Grain Size and Size Distribution
Grain Shape
Spacial Distribution
Volume Fraction
Figure 1. Micrographs of (a) polycrystalline diamond and (b)
tungsten-carbide/cobalt.
Hard Materials with Functionally Designed
Mesostructure
David P. Harding, Zhigang Zak Fang,
Department of Metallurgical Engineering, University of Utah
J. Daniel Belnap, MegaDiamond, Smith International
Beginning with the Voigt and Ruess models the
theoretical flexural strength of the composites have
been derived for both plane-stress conditions
(dotted lines in figure 8a) and plane-strain
conditions (dotted lines in figure 8b).
Mesostructure vs. Properties
Like
microstructology,
mesostructology
is
important to understand material properties and how
to
optimize
those
properties.
Unlike
microstructology, the study of mesostructure is in
its infancy. There is much to learn about how FDM
enhances
the
performance
of
materials.
Experimental testing, analytical modeling, and
numerical modeling are all being used to study the
mechanisms involved.
The material we are currently investigating is shown
in Figure 2a.
Double cemented tungsten carbide, with a
hierarchical structure, has more degrees of freedom
and can be designed to perform above the standard
WC-Co combination of wear resistance and fracture
toughness, as seen by the top two lines in figure
4.[1]
Two-dimensional numeric modeling was conducted
using OOF, objection oriented finite element
analysis. The residual stresses due to both cool
down and pressure release after sintering is mapped
in figure 9a. A stress map after axial loading is
presented in figure 9b. The estimated flexural
strengths are presented in figure 8. It was
unexpected that the plane strain model would
produce more accurate results.
Figure 6. Wear resistance data.
Wear Resistance
The wear resistance data is shown in Figure 6. It is
significant that the wear resistance does not begin to
drop off until a substantial amount of WC-Co is
added into the PCD. This allows for flexibility in
designing the mesostructure to improve fracture
toughness without sacrificing wear.
Figure 2. Micrographs of FDM PCD/WC-Co composites with
(a) a granular mesostructure and (b) a honeycomb
mesostructure. Images of their corresponding 3-D computed
tomography are shown in figures 2c and 2d. They were
generated using a microCT machine.
For example, the FDM material in figure 2a has 60
v/o PCD granules of size 200-275 µm regularly
spaced in a tungsten carbide matrix to minimize
contiguity.
Figure 9. Analytical and numerical analysis predictions of the
flexural strengths using (a) plane-stress and (b) plane-strain
assumptions.
2-D Numerical Model
Figure 3. (a) Conventional WC-Co with 27 w/o cobalt. (b)
Double cemented tungsten carbide (DC Carbide) with WC
granules (6 w/o cobalt) embedded in a cobalt matrix, totaling
27 w/o cobalt overall.
Figure 8. Stress maps generated by OOF showing the (a)
residual stresses and (b)stresses after loading. Image (a)
shows compression in blue and tension in red. Image (b) has a
thermal color scheme with hotter colors indicating greater
stress.
3-D Numerical Modeling
Mesostructural characteristics can not be controlled
by thermo-mechanical methods. The mesostructure
must be created by powder processing methods.
FDM materials have a two tier substructure. The
materials have a mesostructure composed of two or
more distinctly different phases with a manipulated
arrangement. Each of these phases, in turn, have
their own microstructures. Both the microstructures
as well as the mesostructure can be engineered to
obtain functional properties.
Analytical Model
Many important mechanical and physical properties, including hardness, fracture toughness, and
wear resistance, are functions of the microstructure and mesostructure of materials. Although the
dependence of properties on microstructure has been well studied over the past century, there has
been little research on the effect of the mesostructure and how to obtain materials with hierarchical
structure to maximize properties. This poster presents some recent work on improving the fracture
toughness of hard and superhard materials without sacrificing their wear resistance by using
functionally designed mesostructures (FDM). Materials with FDM have mesostructures designed to
optimize overall functionality. Results of several material systems show that the combination of
wear resistance and fracture toughness can be improved while the hardness vs. fracture toughness
correlation remains unchanged. Two dimensional and three dimensional models are being
developed to help explain and quantify the impact of the functionally designed hierarchy on
functional properties. This knowledge is necessary to intelligently design the mesostructure for
optimal performance.
For example, the PCD in figure 1a has ~2µm round Case Studies
grains of diamond with small pools of cobalt evenly
distributed throughout the material. The Double Cemented Tungsten Carbide.
tungsten-carbide cobalt in figure 1b has 6 µm
angular grains evenly embedded in a cobalt matrix. The microstructure of WC-Co, shown in figure 3a,
can be be modified several ways such as varied
The microstructural parameters of a material are cobalt fraction or WC grain size. The microstructure
typically set by heat treating, mechanical will determine the fracture toughness and wear
processing, or thermo-mechanical processing.
resistance of the material, but not independently. All
conventional WC-Co has a wear resistance-fracture
toughness relationship described by the lowest line
Mesostructure
in figure 4. All conventional WC-Co materials will
The mesostructure of a material, if it has one, also have combinations which fall along this line.
has a strong impact on material performance. Many
mesostructural parameters are analogous to
microstructural parameters, though they typically
two orders of magnitude larger in size.
Hierarchal Structure
Modeling
Figure 4. Wear resistance vs. fracture toughness for
conventional and DC tungsten carbide.
Honeycomb PCD
Field tests of PCD/WC-Co enhanced composite
inserts with a Honeycomb mesostructure (shown in
figure 2b) have shown that the functionally
designed material has a higher performance and a
longer service life than conventional PCD enhanced
Materials with hierarchal substructures have inserts. This is due to the higher impact resistance
broader performance ranges than materials with of the FDM material, as shown in figure 5.[2]
only engineered microstructures. Materials with
functionally designed mesostructures may be
designed outperform the equivalent material
without a hierarchal structure.
Some overall substructural parameters, such as
mean free path in the tungsten carbide, are
influenced by both the microstructure and the
mesostructure.
Figure 7. Flexural strength data.
In order to eliminate errors due to assuming plane
stress or plane strain conditions, three-dimensional
modeling of the FDM material is being conducted
using ANSYS software. A sketch of the 3-D
mesostructure is shown in figure 10. We have
begun to use the model predict flexural strength.
Once the model is fully implemented we plan to use
that model to predict the contact fatigue toughness.
Flexural Strength
Flexural strength is an important property of hard
materials. It is related to other intrinsic properties
like hardness, strength and toughness, but it is much
easier to measure. The results of the flexural
strength testing are shown in Figure 7. Contrary to
intuition, the flexural strengths of the composites
are less than the flexural strengths of either
component. Modeling has helped explain the
underlying mechanisms which cause this.
Figure 10. Early 3-D model of a granule mesostructure.
Figure 5. Comparison of chipping on a (a) functionally
designed PCD/WC-Co enhanced insert and a (b) conventional
PCD enhanced insert. Notice how the chip has been contained
to a small area by the WC-Co cell boundaries in the FDM
material.