Patent Application
20110262295
The present invention relates to composite materials, and more particularly, a method for fabricating hard particle-dispersed metal matrix-bonded (cemented) composites. The hard particles can be selected from carbides such as titanium carbide, nitrides, borides, silicides, oxides and/or diamond.
Hardmetals include a class of composite materials that are specifically designed to exhibit superior properties such as hardness (resistance to deformation), toughness (resistance to fracture), and wear resistance. Examples of hardmetals include cermets or sintered or cemented carbides such as cobalt cemented tungsten carbide (WC/Co). Cemented carbides, or metal matrix composites, generally comprise ceramic or carbide grains or particles (e.g., WC) as the aggregate bonded with binder metal particles (e.g., Co) as the matrix. Certain compositions of cemented carbide have been documented in the technical literature. For example, a comprehensive compilation of cemented carbide compositions is published in Brookes' World Dictionary and Handbook of Hardmetals, sixth edition, International Carbide Data, United Kingdom (1996).
Cemented carbides, such as WC/Co, exhibit desirable properties including hardness, wear resistance and fracture toughness suitable for broad applications such as cutting tools for cutting metals, stones, and other hard materials, mining tools for cutting coals and various ores and rocks, and drilling tools for oil and other drilling applications. Other applications include, but not limited to, protective coatings, wear parts, wire-drawing dies, knives, machine tools, drill bits, and armor. The cemented carbide is generally formed by first dispersing hard, refractory particles of carbides (e.g., WC) in a binder metal matrix (e.g., Co). Then the resulting mixture is cold pressed and sintered at low pressure (in vacuum) or sintered at high pressure (e.g., hot isostatic pressing (HIP)) for preparing a bulk composite. The mixture can also be thermally sprayed or welded onto the surface of a bulk metallic substrate for preparing functionally graded coating (e.g., cladding or hardfacing).
During this sintering process, the binder metal enters the liquid state and the carbide particles remain in the solid state. As a result of this process, the binder metal embeds or cements the carbide particles and then solidifies to yield the metal matrix composite with its distinct physical properties. The hard particles primarily contribute to the hard and refractory properties of the resulting cemented carbide. The naturally ductile metal serves to offset the characteristic brittle behavior of the carbide ceramic, thus enhancing toughness and durability. The physical properties can be changed by grain size, hard particle content, metal content, and degree of bonding between the hard particles and the metal matrix.
The hardmetal composite material of choice in nearly all applications is currently cobalt-cemented tungsten carbide (WC/Co), which is known and preferred for its high hardness, superior wear resistance and good fracture toughness. Recent improvements in WC/Co-based materials have been realized by the addition of diamond particles into the powder starting materials prior to pressure sintering. The addition of diamond particles yield C(diamond)/WC/Co composites.
C(diamond)/WC/Co composites exhibit excellent physical properties. During the fabrication process, it is known that diamond exhibits a tendency to undesirably transform into graphite at low pressure and high temperature. Diamond/WC/Co composites, thus, require extremely high sintering pressures to prevent the diamond particles from transforming into graphite as the raw materials consolidate into the final composite. In addition to high sintering pressure, the raw materials, tungsten and cobalt, are difficult to acquire since they must be imported from abroad and their availability is subject to foreign nations. These factors greatly affect the overall cost and complexity in making composites from such materials.
It is further known that WC/Co and diamond/WC/Co composites are especially susceptible to corrosion and corrosive wear. In sour-gas well drilling, for example, drill bits made from these composites experience disproportionate corrosive wear, which severely limits their useful life. As a result, there is an increase in “trip-time” associated with replacing the worn bit, which is both time-consuming and expensive. In geothermal wells, submersible pump bearings made from such composites are also exposed to severe corrosive wear which limits bearing lifetime, and hence increases overall operating costs.
Accordingly, there is a need for a method for making a hard particle-dispersed composite material that confers advantages over conventional cemented carbide composites. There is a further need to provide a method for making a hard particle-dispersed composite material that is super-hard, lightweight and corrosion resistant at more economical processing pressures and temperatures. There is a further need to provide a method for the making a hard particle-dispersed composite material that is more cost effective and simpler to fabricate for various applications.
The present invention relates generally to a method for the making a hard particle-dispersed composite material, and preferably a titanium carbide- and/or a carbon in diamond phase-dispersed composite material. The composite material produced by the method of the present invention exhibits desirable properties including high hardness, superior wear resistance, good fracture toughness and excellent corrosion resistance, while being simpler and more cost efficient to fabricate than prior art composite materials of similar properties. The method of the present invention have been found to afford considerable flexibility in tailoring the properties of the resulting composite material suitable to meet a range of performance requirements for different applications. The method of the present invention utilizes existing materials and commercially available equipment, and reduces the time and cost needed for production.
The composite materials of the present invention are fabricated from a mixture of metal or ductile particles and hard particles including metal carbide such as titanium carbide, and/or diamond particles. The composite materials of the present invention can be produced by different methods including, but not limited to, pressure assisted sintering, or by thermal spraying or weld overlaying, or by pressure assisted extrusion, but examples for the present invention as provided below describe the method using pressure-assisted sintering. Prior to pressure-assisted sintering, the mixture can be pressed together at room temperature. The mixture undergoes pressure-assisted sintering under elevated pressures and at elevated temperatures for a predetermined holding time. While maintaining the sintering pressure, the resulting composite is thereafter cooled.
The resulting composite can be heat treated at high temperatures and under low pressure in the presence of an inert gas such as argon. The hard particles can also be selected from carbides, borides, nitrides, silicides, oxides, and/or carbon in diamond phase, (“C(diamond)” or “C(d)”). A preferred composition of the present invention includes a combination of carbon (C) and titanium (Ti) to yield TiC/Ti, and C(d)/Ti and/or C(d)/TiC/Ti. The metal particles can be selected from titanium, aluminum, beryllium, and alloys thereof. Preferably, the metal is titanium and titanium alloys.
In one aspect of the present invention, there is provided a method of making a hard particle-dispersed metal matrix-bonded composite, which comprises the steps of:
mixing hard particles and ductile metal particles to yield a mixture thereof; and
sintering the mixture under a pressure of less than 2.0 GPa and at a temperature of less than 1200° C. for a sufficient time to yield the composite.
In another aspect of the present invention, there is provided a composite produced by the method above.
The following drawings are illustrative of embodiments of the present invention and are not intended to limit the invention as encompassed by the claims forming part of the application.
The present invention directed generally to a method for the making a hard particle-dispersed metal matrix-bonded (cemented) composite material. The composite material produced by the method of the present invention exhibits desirable properties including high hardness, superior wear resistance, good fracture toughness and corrosion resistance, while being simpler and more cost efficient to fabricate than prior art composite materials of similar properties. The methods of the present invention have been found to afford considerable flexibility in tailoring the properties of the resulting composite material suitable to meet a range of performance requirements for different applications. The method of the present invention utilizes existing materials and commercially available equipment, and reduces the time, temperature requirements, and cost needed for production.
In accordance with one embodiment of the present invention, there is provided a method for making the composite materials of the present invention. A mixture of metal particles and hard particles such as metal carbide particles and/or diamond particles are prepared. The components of the mixture may be homogenous and uniformly dispersed or functionally graded. Optionally, the mixture can be pressed together at room temperature to yield a preform having increased apparent density prior to forming the final composite.
The mixture then undergoes pressure-assisted sintering under elevated pressures and at elevated temperatures for a predetermined holding time. While maintaining the sintering pressure, the resulting composite is cooled to ambient temperature. The mixture of metal particles and hard particles such as diamond particles and/or metal carbide particles (e.g., titanium carbide) can be graded over varying volume percent ratios from one region to another to yield a functionally graded composite material.
In accordance with a preferred embodiment of the present invention, the metal particles are selected from titanium, aluminum, beryllium, and alloys thereof. Preferably, the metal particles are selected from titanium and alloys thereof. The average particle or grain sizes of the metal particles are at least 10 μm, and preferably from about 10 μm to 50 μm.
In accordance with a preferred embodiment of the present invention, the hard particles are selected from carbides, borides, nitrides, silicides, oxides, and/or diamond. Preferably the hard particles are selected from diamond, metal carbides such as titanium carbide, and combinations thereof. The average particle or grain sizes of the hard particles selected carbides, borides, nitrides, silicides, and oxides are at least 50 nm, and preferably from about 1 to 2 μm. The diamond particles (C(d)) can be selected from monocrystalline diamond grains, polycrystalline diamond grains, and combinations thereof. Preferably, the diamond particles are selected from polycrystalline diamond grains. The average particle or grain sizes of the diamond particles are at least 5 μm, and preferably from about 50 μm to 500 μm.
The mixture is pressed together under pressure of about 0.5 GPa and at about room temperature (i.e., 18° C. to 28° C.) to produce a compact, green body or preform. The compact undergoes pressure-assisted sintering under pressures of about less than 2.0 GPa, and at temperatures, of about less than 1200° C. for a predetermined holding time of about 1 to 15 minutes. While maintaining the sintering pressure, the resulting composite is thereafter cooled to about room temperature.
The term “homogenous and uniformly dispersed” is intended to refer to a characterization of the composite material of the present invention in which the composition and structure is substantially the same throughout the composite as homogenous mixtures.
The term “functionally graded” is intended to refer to a characterization of the composite material of the present invention in which the composition and structure vary gradually over volume, resulting in corresponding changes in properties of the material. Generally, the location, volume fraction and compositional gradient of the individual material components (i.e., metal matrix and hard particles) can be varied within the mixture in preparation for pressure-assisted sintering to yield the composite material of the present invention as a functionally graded material. For example, two or more components are blended during forming and the ratio is continuously varied over a specified volume from 100% of component 1 through to 100% of component 2 (or variation thereof).
Various approaches as known in the art can be used to fabricate the present composite materials into the form of functionally graded materials. Examples of such approaches include, but are not limited to, bulk (particulate processing), controlled-blend processing (impeller dry blend processing), controlled-segregation processing, preform processing, layer processing and melt processing. Such processing techniques for producing functionally graded materials are generally known to one skilled in the art.
The starting powders are produced by any known suitable processes including, but not limited to, inert gas atomization (metal powders), carbothermic methods (carbide powders) and the like.
Examples of methods for preparing the starting material before pressure-assisted sintering include ball mill mixing, thermal spraying, plasma spraying, flame spraying and the like. Thermal spray technology includes both plasma spraying and flame spraying. In plasma spraying, an aggregated titanium carbide/titanium-base powder is fed at a controlled rate into a plasma stream, where the titanium-base particles melt and wet the un-melted titanium carbide particles to form semi-solid or “mushy” particles, which then impact on the substrate to form a relatively dense coating. The as-sprayed coating consists of a uniform dispersion of hard titanium particles in a titanium-base matrix phase. Some dissolution of the titanium carbide particles in the liquid titanium occurs, depending on the degree of superheat of the melted particles.
The coating builds up by the superposition of “splat-quenched” mushy particles and has a characteristic micro-laminated structure. To mitigate decarburization of the titanium carbide particles during plasma spraying in ambient air, various strategies can be used, such as inert-gas shrouding or reducing the enthalpy of the plasma. The latter approach is simplest to implement. Typical operating parameters are given in Table 1 for air plasma spray systems (APS).
In flame spraying, in its most advanced form called high velocity oxy-fuel (HVOF) spraying, the processing methodology is similar. However, because of the much reduced enthalpy of a combustion flame, it is easier to avoid overheating the aggregated feed powder, so that decarburization of the titanium carbide component is minimized. Another positive feature of HVOF spraying is the high velocity of the gas stream with its entrained particles. Upon impact with the substrate, a high density coating with minimal decarburization is formed. Typical operating parameters are given in Table 2.
Referring to
The hard particles can also be selected from other carbides including WC, SiC, Cr3C2, and B4C, borides including WB4, TiB2, AlB12, and HfB2, nitrides including BN (hard cubic phase), Si3N4, TiN, and ZrN, silicides including B4Si, Ti5Si3, and TiSi, TiSi2, and oxides including BeO, Al2O3, SiO2, and TiO2, and combinations thereof. The starting materials (e.g., titanium carbide, diamond and titanium) are produced by any known suitable processes including, but not limited to, inert-gas atomization, carbothermic methods, and the like. The combination of the components of the mixture can be compositionally graded in varying volume ratios of each to yield a functionally graded composite.
In one preferred embodiment of the present invention, the volume fractions of the constituent phases of hard particles (diamond and titanium carbide) and metal particles (titanium), respectively, are given as follows.
Examples of vol. % of TiC/Ti mixtures in top layer (hardfacing layer): 90%, 75%, 70%, 60%, 50%;
Examples of vol. % of C(diamond)/Ti mixtures in top layer (hardfacing layer): 75%, 70%, 60%, 50%, 40%, 30%;
Grain sizes of diamond particles: 500 μm (600/400), 50 μm (60/40), 5 μm (5/3);
Examples of mixtures of diamond particles: (1)[500 μm (600/400), 75 wt. %+50 μm (60/40), 25 wt. %]; (2)[50 μm (60/40), 75 wt. %+5 μm (7/5), 25 wt. %]; (3)[500 μm (600/400), 68 wt. %+50 μm (60/40), 23 wt. %+5 μm (5/3), 9 wt. %];
Grain sizes of TiC particles: 2 μm or 50 nm (80/50); and
Grain sizes of Ti (or alloy) particles: 50 μm (60/40), or 10 μm.
In step 3, a homogenous or functionally-graded green body that is relatively solid, but weak and not machinable due to fragility is prepared. The mixture, which can be a homogenous mixture or a compositionally graded mixture, is cold pressed or compressed together under pressure, preferably less than 0.5 GPa, and at about ambient or room temperature. The cold pressed mixture results in a uniformly dense mass or preform, or a green body, which is a weakly bound and fragile solid that is not machinable. In step 4, the preform is subjected to pressure-assisted sintering under elevated pressures and temperatures for a sufficient time. The elevated pressure is preferably less than 2.0 GPa, and more preferably from about 0.2 GPa to 2.0 GPa. The elevated temperature is preferably less than 1200° C., and more preferably from about 700° C. to 1200° C. The temperature is typically about 700° C. for titanium, beryllium and alloys thereof, and about 500° for aluminum and alloys thereof. The time for the pressure-sintering step depending on the temperature, can be from about 1 to 15 minutes.
The diamond particles are incorporated into the present composite material to provide enhanced hardness characteristics for especially demanding applications such as, for example, rock drill bits or slide bearing surfaces. The interface of the composite material can be compositionally graded to further enhance resistance against wear and thermal and elastic misfit stresses, thus minimizing coating/substrate delamination or spallation during use. In this manner, the pressure, temperature, time and concentration of the components can be adjusted to achieve variable properties and performance. Increasing the volume fraction of the hard particles enhances the hardness of the composite material, while increasing the volume fraction of metal particles enhances the crack or fracture resistance of the composite material.
In reference to
Referring to
Referring to
The diamond particles are dispersed in the titanium metal (a binder) primarily on a top surface of the region 11 where superhardness is needed. The concentration of the diamond particles decreases toward bottom of region 11. The concentration of titanium carbide particles in the graded diamond/titanium carbide/titanium middle region 12 is greater under region 11 where higher hardness is needed, and the concentration of titanium carbide is maximal at the top of graded titanium carbide/titanium substrate region 13 where high hardness is needed. The concentration of the titanium carbide particles in region 13 decreases to zero toward its bottom where high impact strength and resistance to cracking are needed.
In one embodiment of the present invention, region 11 is composed of diamond in an amount of 50 volume percent and titanium in an amount of 50 volume percent, each based on the total volume of region 11. Region 12 is composed of diamond in an amount of 25 volume percent, titanium carbide in an amount of 25 volume percent, and titanium in an amount of 50 volume percent, each based on the total volume of region 12. Region 13 is composed of titanium carbide in an amount of 50 volume percent, and titanium in an amount of 50 volume percent, each based on the total volume of the hard-faced portion of region 13, and titanium in an amount of 100 volume percent, based on the total volume of the ductile bottom portion of region 13. The diamond particles can be uniformly dispersed throughout the regions 12 and 13, if needed.
Referring to
As previously indicated for
Vickers hardness measurements have shown that for comparable volume fractions of carbide phase, TiC/Ti by this method has higher hardness than known industrial composites based on WC/Co, Cr. Toughness and impact strength of TiC/Ti are also greater at the higher hardness.
The container unit 40 is composed of clay material surrounding the reaction cell 42, and further includes a deformable ring 39 extending therearound. The container unit 40 and the deformable ring are each disposed between the anvils 34. The reaction cell 42 includes a graphite heater 41 extending around a sample 44. The high pressure unit 35 retains and squeezes the container unit 40 with the reaction cell 42 between the anvils 34. The anvils 34 are supported via the steel rings 36.
The loading force is applied to the anvils 34 from ram 46 and frame 20 through the bearing discs 24. The bearing discs 24 are supported via the steel rings 26. The reaction cell 42 is adapted to hold the sample 44, which is in the form of a green body (homogenous or functionally graded mixture of powders) used to make the composite material of the present invention. The cylinder 54 and ram 46 generates the necessary force on the high pressure unit 35 to compress the container unit 40. The container unit 40 comprises a clay-sand mixture or a suitable electrically non-conductive material, and the reaction cell 42.
The power supply 27 includes copper cables 32 connected to the graphite heater 41, a shunt 30, and voltage and current meters 28 and 31. An electrical current is supplied to the reaction cell 42 via a power supply (PS) 27 to generate the heat energy needed to raise the temperature of the reaction cell 42. Insulating layers 22 are provided between the frame 20 and the cylinder 54 and ram 46 of the hydraulic unit 38 for electrical insulation. The electronic control unit 62 includes a processor 63, a timer 48, a pressure gage 50, and electrical motors 64 for driving the pumps 58 and 60. It will be understood that the control devices and electrical components are suitably arranged as known in the art to accurately provide the proper control and programming of the pressure and temperature over time needed to yield the composite materials of the present invention.
The design of the high pressure and high temperature apparatus 19 is similar to the HPHT apparatus as taught in Voronov (U.S. Pat. No. 6,942,729), the content of which is incorporated herein by reference, but is simplified for moderate (lower) pressure and temperature ranges that are used in the present invention. The design of the apparatus described in Voronov is more complicated, since it was invented to generate very (extremely) high static pressure and temperature in relatively large volume. When such a high pressure and high temperature are not needed the Voronov apparatus can be simplified and adjusted for moderate ranges as shown in system 19. The last modification is very economical and technically viable for industrial applications.
The hardness and density properties of titanium carbide/titanium composites of the present invention are listed below in Table 3.
~9½
~7½
A comparison of the density and hardness properties of different hard particle-dispersed composites are provided in Table 4 below.
A comparison of sliding coefficients of diamond-hardfaced composites and carbide-hardfaced composites is shown in Table 5 below.
A comparison of the wear resistance of various materials is shown in Table 6 below.
Test pieces of diamond-hardfaced carbide/metal composites (or carbide-hardfaced carbide/metal composites) and experimental bearing components can be fabricated by the following multi-step process:
This process can be used to make experimental components of the new composite bearings for submersible pumps in geothermal wells and other applications.
A step by step protocol used to make the present TiC/Ti composite material is provided below.
A step by step protocol used to make the present functionally graded (FG) TiC/Ti composite material is provided below.
A step by step protocol used to make the present Diamond/TiC/Ti composite material in a functionally (compositionally) graded form is provided below.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
This invention was made with government support under U.S. Department of Energy Contract No. DE-FG02-08ER85139 effective date Jun. 30, 2008-03/29/09. The government has certain rights in the invention.
