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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.