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1 DIAMOND-SILICON CARBIDE COMPOSITE AND METHOD
This application is a continuation-in-part of U.S. patent application Ser. No. 11/110,252 filed on Apr. 19, 2005, now U.S. Pat. No. 7,060,641, which is a divisional of U.S. patent application Ser. No. 10/448,672 filed on May 30, 2003, now issued as U.S. Pat. No. 6,939,506, which issued on Sep. 6, 2005, all hereby incorporated by reference.
STATEMENT REGARDING FEDERAL RIGHTS
This invention was made with government support under Contract No. W-7405 -ENG-36 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
The present invention relates generally to composite materials and more particularly to diamond-silicon carbide composites with high fracture toughness and to a method for preparing such a composite.
BACKGROUND OF THE INVENTION
In view of their exceptionally high hardness, excellent wear resistance, and thennal stability, diamond-silicon carbide (SiC) composites have been used in various industrial applications such as machining, grinding, drilling, and mining. Diamond-SiC composites have been prepared by a variety of methods that include chemical vapor deposition, highpressure high temperature (HPHT) liquid phase sintering, and low vacuum liquid phase infiltration.
Most available diamond-SiC composites are composed of microcrystalline diamond held together by microcrystalline SiC. Despite their extraordinary hardness and wear resistance, these composites have relatively low fracture toughness (<6 MPa~m1/2), which limits their potential applications.
Fracture toughness of diamond-SiC composites has been improved by incorporating nanocrystalline diamond into the composites. It is believed that the nanocrystalline diamond and SiC hinder dislocation growth and microcrack propagation in the composite better than microcrystalline diamond and SiC do, which improves fracture toughness. Such a composite has been reported by E. A. Ekimov, A. G. Gavrilliuk, B. Palosz, S. Gierlotka, P. Dluzewski, E. Tatianin, Yu. Kluev, A. M. Naletov, and A. Presz in “High-Pressure, High-Temperature Synthesis of SiC-Diamond Nanocrystalline Ceramics,” Applied Physics Letters, vol. 77, no. 7, pp. 954-956). The composite was prepared by the liquid silicon infiltration of nanocrystalline diamond powder under HPHT conditions HPHT (7.7 GPa, 1700-2300 K). The composite displayed high fracture toughness (10 MPa~m1/2) but was not unifonnly dense. It was only partially densified. The infiltration depth was only 1-2 millimeters (mm) because the pores closed very quickly during infiltration due to the “self-stop process”; as silicon infiltrates through the pores, it reacts rapidly with diamond to fonn a silicon carbide phase that seals the pores and prevents further infiltration. Alternative methods that are not limited by the self-stop process may be required to overcome problems relating to the self-stop process in order to provide unifonnly dense composites with high fracture
toughness. Such a method would also minimize grapl1itization of nanocrystalline diamond, which has also been a problem in the past.
Unifonnly dense, diamond-SiC composites having high fracture toughness remain desirable.
In accordance with the objects and purposes of the present invention, as embodied and broadly described herein, the present invention includes a process for preparing a umformly dense diamond-silicon carbide composite. The method involves consolidating a powder mixture of diamond and amorphous silicon at a pressure and temperature sufficient to produce a uniformly dense diamond-silicon carbide composite having a Vickers hardness of at least 35 GPa, a Knoop hardness of at least 30 GPa, and a fracture toughness of at least 8 MPa~ml/2.
The invention also includes a unifonnly dense diarnondsilicon carbide composite. The composite is prepared by a process that includes consolidating a powder mixture of diamond and amorphous silicon at a pressure and temperature sufficient to produce a unifonnly dense diamond-silicon carbide composite having a Vickers hardness of at least 35 GPa, a Knoop hardness of at least 30 GPa, and a fracture toughness of at least 8 MPa~m1/2.
The invention also includes a unifonnly dense diarnondsilicon carbide composite having a Vickers hardness of at least 35 GPa, a Knoop hardness of at least 30 GPa, and a fracture toughness of at least 8 MPa~m1/ 2.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and fonn a part of the specification, illustrate the embodiment (s) of the present invention and, together with the description, serve to explain the principles of the invention. In the draw1ngs:
FIG. 1 shows a scamiing electron micrograph of ballmilled diamond and silicon powder;
FIG. 2 shows an x-ray diffraction pattem of the ball-milled powder of FIG. 1;
FIG. 3 shows a Raman spectrum of the ball-milled powder of FIG. 1;
FIG. 4a-g shows x-ray diffraction patterns for composites prepared using the ball-milled powder of FIG. 1;
FIG. 5a-g shows Raman spectra of the composites having the x-ray diffraction pattems of FIG. 4a-g, respectively; and
FIG. 6 shows a scamiing electron micrograph of the invention diamond-silicon composite of EXAMPLE F.
The invention includes diamond-SiC composites having high hardness, high fracture toughness, and high thennal stability. The invention also includes a method for preparing the composites. According to the invention, a homogeneous powder mixture of diamond and amorphous silicon are subjected to a pres sure in a pres sure range and a temperature in a temperature range sufiicient to consolidate the powder into a unifonnly dense diamond-silicon carbide composite.
The diamond portion of the powder mixture may be microcrystalline diamond, or a combination of microcrystalline diamond and nanocrystalline diamond. It is believed that in order to produce a diamond-silicon carbide composite with high hardness, a high percentage of microcrystalline diamond is preferable. In order to impart high fracture toughness to the
composite, nanocrystalline diamond may also be included in the powder mixture. The amount of microcrystalline diamond and nanocrystalline diamond in the powder mixture may be adjusted in order to provide a composite with high hardness and high fracture toughness. For producing an embodiment composite, the amount of microcrystalline diamond is from about 60 percent to 100 percent, and the amount of nanocrystalline diamond is from about zero percent to about 40 percent.
The powder mixture of diamond and amorphous silicon may be prepared using a variety of techniques. In each of these techniques, the amorphous silicon is homogeneously distributed around the diamond particles.
In an embodiment, diamond powder and crystalline silicon powder are ball milled until the silicon becomes amorphous. The ball-milled powder is then sintered to form a composite. The ball milling may be performed by a liigh-energy ball milling technique, or by using a cryogenic milling technique at low temperatures of from about 77 degrees Kelvin and higher.
In another embodiment, diamond powder and silicon powder are jet-milled until the silicon becomes amorphous.
In yet another embodiment, the powder mixture of diamond and amorphous silicon is produced by a thermal technique such as, thermal spray, high temperature plasma synthesis, and the like.
The amorphous silicon component of the pre-consolidated powder mixture can be produced from crystalline silicon as the milling technique or spraying technique is applied. Alternatively, amorphous silicon is a commercially available material, and can be purchased and then combined with diamond powder to form a mixture. It should be understood that amorphous silicon is critical for the preparation of composites of this invention because amorphous silicon reacts with the surface carbon from the diamond powder to form nano structured silicon carbide, which is believed to bond the diamond grains together. It is believed that this bonding between the grains enhances the fracture toughness greatly. The reaction between amorphous silicon and the surface carbon of the diamond also consumes surface defects of the diamond. This also is believed to contribute to the high hardness of the product diamond-silicon carbide composite. Diamond powder generally has defects, and the defects are generally concentrated on the surface of diamond grains, so the reaction between the amorphous silicon and the diamond removes these surface defects as the composite is being formed.
The powder mixture of diamond and amorphous silicon is then consolidated to fonn a uniformly dense diamond-silicon composite. A variety of techniques may be used to consolidate the powder.
In an embodiment, the diamond and amorphous silicon powder mixture is consolidated by a reactive sintering technique known in the art as the HPHT (high pressure high temperature) technique to form the diamond-silicon carbide composite. A pressure range useful with this technique for preparing composites of the invention is a range from about 1 GPa to about 10 GPa, preferably from about 3 GPa to about 8 GPa, and more preferably from about 5 GPa to about 7 GPa.
In the HPHT technique, a press is used to consolidate the powder. Presses useful for the HPHT technique include, but are not limited to, multi-anvil presses, toroidal anvil presses, piston cylinder presses, and the like. It should be understood that any press capable of consolidating the powder could be used.
In the HPHT technique for powder consolidation, a temperature in a temperature range useful for sintering is from about 800 Kelvin to about 2600 Kelvin, preferably from about
1200 Kelvin to about 2400 Kelvin, more preferably from about 1400 Kelvin to about 2200 Kelvin, and even more preferably from about 1600 Kelvin to about 2000 Kelvin.
In another embodiment, the powder mixture is consolidated by a technique known in the art as the HIP (hot isostatic pressing) technique. The HIP technique involves exposing the powder mixture to a vacuum in a confined volume, subjecting the powder to an elevated pressure and an elevated temperature.
In another embodiment, the powder mixture of diamond and amorphous silicon is consolidated by a spraying technique known in the art as “plasma spraying” or “thennal spraying” or “spray coating”. In this technique, the powder is applied to a substrate by a high velocity, high temperature spraying application that results in a uniformly dense coating on the substrate.
A variety of techniques were used to analyze pre-consolidated powder and composites prepared from the powder. The morphology and microstructure of the powder was examined using a field emission SEM LEO 1530 apparatus (LEO ELECTRON SPECTROSCOPY, LTD., Cambridge, UK), which has the ability to minimize the charging effect and take high-resolution pictures under low working voltage.
The phase composition of the powder and composites was analyzed by powder x-ray diffraction and Raman scattering spectroscopy. X-ray diffractograms were obtained using a SCINTAG XDS 2000 with a CuKa source. The step was 0.02° and the expose time was 1 second/step during the measurement. Raman spectra were obtained using the incident wavelength of 785 mn. The scattered light passed through a KAISER optical spectrometer (Ann Arbor, Mich.) equipped with a notch filter, holographic gratings, and a charged-coupled device (CCD) detector. The spectral resolution was 4 cm_1.
The bulk and shear modulus of the composites were measured on an ultrasonic interferometer (AUSTRIALIAN SCIENTIFIC INSTRUMENTS). To obtain the elastic modulus of each composite, the velocities of the longitudinal and shear waves were measured using the ultrasonic interferometer. At least five successive sample echoes were clearly visible. Overlap and interference of these sample echoes with those from the WC buffer rod enable the determination of the travel time at high frequencies (20-60 MHz) with a precision of 2><10‘6 seconds (see reference 20).
The densities of composites were measured on AT261 DELTA RANGE (METTLER TOLEDO, Columbus, Ohio) by using the Archimedes method.
The Vickers microindentation hardness was measured on a MICRO4 micron hardness tester (BUEHLER LTD). The load applied to the indenter was 9.8 Newtons and holding time was 15 seconds. Twelve indentations were made to obtain the average hardness value for each composite. The fracture toughness was measured with a larger applied load (490 Newtons) using MACROVICKERS 51 14 (BUEHLER LTD).
Pre-consolidated powder of the invention was sintered using a tungsten carbide, toroidal high-pressure apparatus (SUPERHARD MATERIALS INSTITUTE, Kiev, Ukraine, references 18 and 19) equipped with a hydraulic press (ROCKLAND RESEARCH, NY). The pressure was calibrated by detecting phase transitions in bismuth (Bi) and lead-tellurium (PbTe) alloy. The temperature of the cell was calibratedusing a W3% Re/ W25% Re thermocouple by measuring the temperature near the center of the cell as a function of dissipated electric power. The calibration curves, pressure versus load, and temperature versus power were reproducible, and the estimated maximum deviation of temperature