WO2000034001A1

WO2000034001A1 - Microwave brazing process and brazing composition for tsp diamond

Info

Publication number: WO2000034001A1
Application number: PCT/US1999/025395
Authority: WO
Inventors: Robert Paul Radtke
Original assignee: Robert Paul Radtke
Priority date: 1998-12-08
Filing date: 1999-10-28
Publication date: 2000-06-15
Also published as: AU1456100A

Abstract

A drill bit cutter is disclosed. The cutter comprises a thermally stable polycrystalline ('TSP') diamond, a tungsten carbide substrate, and a braze filler metal composition containing relatively large amounts of titanium. A method of brazing the TSP diamond to the substrate by microwave heating is also disclosed.

Description

S P E C I F I C A T I O N

TITLE: "MICROWAVE BRAZING PROCESS AND BRAZING COMPOSITION FOR TSP DIAMOND"

RELATED APPLICATION

This is a continuation-in-part of PCT application no. PCT/US98/26154 filed on December 8, 1998. FIELD OF THE INVENTION

The present invention relates to the fabrication of drill bit cutters. More particularly, the invention relates to a new microwave brazing process and brazing composition suitable for use in manufacturing drill bit cutters having improved resistance to stress failure and wear when drilling hard formations. BACKGROUND OF THE INVENTION

1. Drilling hard and abrasive rock and dealing with high well bore temperature gradients have been persistent problems in the drilling industry. At this time, drill bits using conventional polycrystalline diamond composite ("PDC") cutters are unable to sustain a sufficiently low wear rate and resulting bit life at the cutter high temperatures associated with drilling in hard and abrasive rock, where drill bit cutter temperatures can reach 900 °C. However, thermally stable product diamond cutter have relatively low wear rates at temperatures which reach 1200°C. The current state-of-the art diamond cutter attachment procedure is to furnace heat, resistance heat or induction braze thermally stable polycrystalline ("TSP") diamond to 6% cobalt-bonded, fine grain, tungsten carbide substrates by means of a suitable braze filler metal composition. For example, when using a commercial titanium-copper-silver braze filler metal, all components are heated slowly to 800°C to 900°C and the metal alloy is melted to form a two-phase microstructure that includes a discontinuous titanium carbide ("TiC") layer on the diamond surface and a copper silver eutectic layer between the TiC layer and the tungsten carbide substrate. Higher temperature brazes (i.e. over 1200°C) cannot be used with these heating methods without TSP diamond damage. Average shear strength levels of 138 Mpa to 207 Mpa (20,000 to 35,000 psi) have been achieved using conventional direct resistance, induction, and furnace heating methods. However, because of the stresses developed in the diamond layer due to the difference in thermal expansion coefficients between diamond and tungsten carbide, the diamond layer can crack on cool-down. This is called the bi-metal effect. However, drill bits using conventional PDC cutters fail as they reach higher cutter temperatures (e.g., above about 750 °C) due to internal stresses developed within the PDC diamond by the expansion of a cobalt binder and due to chemical degradation by cobalt on the diamond structure. Greater high temperature performance is possible with the TSP diamond which does not contain a cobalt binder. This limitation of the PDC cutter significantly increases the cost of drilling in hard rock with roller cone drill bits which have a much lower rate of penetration. What is therefore needed is the ability to braze TSP diamond to a tungsten carbide support material which provides a required rigid substrate in addition to the ability to provide the exposure above the bit face needed to drill at high rates of penetration. Also needed is a TSP diamond cutter with a stronger braze joint leading to an improved shear strength at higher temperatures.

The major problem in developing these improved cutters is that PDC conventional cutters cannot be used when cutter temperatures exceed about 350°C due to higher wear rates. At about 750 °C, the PDC cutter fails altogether. TSP diamond will graphitize and lose strength and wear resistance if they are heated to more than 1200°C. Additionally, those current techniques to braze TSP diamond cutters to tungsten carbide which produce the highest shear and fatigue strength result in high residual thermal stress and frequent TSP diamond fracture during brazing. Those limitations have hindered the brazing of diamond disks since it is difficult to heat all of the cutter components to the required brazing temperature without damaging the diamond component. As a result, TSP-to-tungsten carbide brazed cutters are not commonly used commercially as abrasive cutting materials. The development of a new brazing technique and new braze filler metals is required to produce high attachment strength diamond cutters. DETAILED DESCRIPTION OF THE INVENTION The present invention provides a unique microwave brazing technique for fabricating TSP diamond to tungsten carbide cutters for drag bits. The microwave brazing technique produces the attachment strength necessary to enable the design of a petroleum drag bit with the cutter exposure necessary to consistently achieve economical rates of penetration. The present invention provides a tailored TSP diamond composition which enhances the ability to microwave braze diamond to tungsten carbide with controllable residual thermal stress in the diamond layer. The present invention also provides new microwave braze filler metals for use with the microwave brazing technique.

Microwave systems can employ magnetron, transwave tube ("TWT") and klystron power sources and operate with constant or variable frequency. A typical microwave system includes a magnetron generator, a waveguide, an applicator and a control system. The generator produces the microwaves which are transported by the waveguide to the applicator (typically in the form of a cavity) where they are manipulated for the desired purpose. Microwave systems can be single or multimode. Generally, the device to be brazed is placed in the cavity, and the microwave device is tuned to its resonant frequency. Generated microwaves of the desired frequency are then focused preferentially onto the target material. The general principles of microwave operation are well known and are not discussed.

Initially, it was sought to use the unique selective heating property of microwaves to develop required brazing temperatures in the braze filler metal without over-heating the diamond cutter component. It is widely known that diamond is a very poor absorber of microwaves, whereas the support substrate absorber and the proposed braze interlay er or braze filler metal were moderate to good absorbers. Thus, it was intended to preferentially heat the braze filler metal and, thereby, reduce the bi-metal effect, described more fully below. For example, in one experiment, a thin diamond disk was stacked coaxially on top of a thick tungsten carbide (WC) cylinder with a very thin braze foil sandwiched between them. Typically, each of the three elements in the stack had about the same diameter; around 13.5 mm. The foil was 75 microns thick. The WC cylinder was about 7.5 mm in height and the diamond disk was about 3.0 mm in height. When joined by microwave brazing, that assembly comprised a diamond cutter. The diamond braze joint was achieved in an argon atmosphere inside a rectangular waveguide cavity powered by a one kilowatt Magnetron power source. A three stub tuner was located near the entrance to the cavity for impedance matching. One of the stubs was specially instrumented for automatic control with the aid of a feedback circuit that sensed the reflected power. A movable plunger under automatic control was provided for maintaining the resonant frequency of the loaded cavity at 2.45 Ghz, the operating frequency of the Magnetron.

Contrary to the prior art teaching that diamond is not a lossy dielectric (i.e., that it does not absorb electromagnetic energy and convert it to heat), the microwave brazing process heated the diamond much faster than the tungsten carbide and the metal foil. After about 1.5 minutes of heating, the temperature of the diamond reached about 640 °C and the diamond started to dimly glow red, while the tungsten carbide cylinder and the foil remained dark (i.e., cool). The temperature of the diamond then climbed steadily to about 840 °C during the next minute and the light emitted from it grew brighter, while changing to orange, then yellow, then white. The tungsten carbide and the foil remained dark during this period. The heat of the diamond layer caused the braze foil to melt, initially in the center. This center first heating is a volumetric heating characteristic of microwave energy on a lossy material.

Subsequent testing demonstrated that the TSP diamond material was a better absorber of microwave energy than the tungsten carbide. When both materials were assembled in a microwave chamber and heated, the TSP diamond easily reached white heat while the tungsten carbide remained black. That finding was completely contrary to the prior art teaching that TSP diamond is a poor absorber of microwave energy. Moreover, when the TSP diamond temperature was maintained at white heat (approximately 850 °C) for 60 minutes, the heat conducted from the TSP diamond to the tungsten carbide only raised the tungsten carbide to red heat.

Chemical analysis of the TSP diamond revealed a concentration of iron metals, including metallic iron. It is now understood that the heating of the TSP diamond is affected by chemistries of non-carbon elements, such as nucleating and catalyzing metals. These metals are used to grow diamond crystals from carbon to form synthetic diamond grains, including, but not limited to iron. In addition, for certain types of TSP diamond where metals are acid leached from the material, small amounts of residual metal is also causing, the TSP diamond to be preferentially heated by the microwave energy.

Based on that discovery, a new method has been developed for the manufacture of TSP diamond to tungsten carbide braze joints. The preferred heating method for this invention is microwave heating. While brazing TSP diamond to tungsten carbide, the tungsten carbide temperature can be minimized to control differential thermal expansion (i.e., bi-metal effect) with the lower thermally expanding/contracting TSP diamond. The present microwave brazing invention takes advantage of the fact that diamond containing non-carbon elements, such as nucleating and catalyzing metals, can be a superior absorber of microwaves (proportional to high dielectric constant), whereas tungsten carbide with about 6 to 18 wt. % cobalt or any other sufficiently rigid refractory metal is a moderate absorber (lower dielectric constant) and the proposed braze filler metals, described below, are good absorbers (high dielectric constant). The temperature of the braze filler metal will depend on the microwave energy absorbed and the conduction (and radiation) occurring between the TSP diamond and base material.

The ability of the present microwave brazing invention to preferentially heat TSP diamond has several important consequences. This brazing method permits the use of higher temperature braze filler metals. The temperature of the braze interlayer material will depend on the energy absorbed and the conduction (and radiation) occurring between the TSP and support substrate as they are being heated. Braze line shear strength typically increases with melting temperature. Microwave energy will preferentially heat the TSP diamond and filler metal, with substantially less absorptivity by the tungsten carbide. Braze filler metal compositions which melt between 900°C to nearly 1200°C can be used without exceeding the inherent temperature stability limit of TSP diamond. Under controlled conditions, it is possible to exceed 1200°C, but only for short times (i.e., less than about 5 minutes). Therefore, high strength braze filler metal compositions with melting temperatures near the temperature stability limit of TSP diamond can be used. These braze filler metals can be in the form of a metal foil, paste, coating or other similar form. Another important aspect of the present invention is related to the fact that diamond has a lower coefficient of thermal expansion (as low as 1.0 x 10^"6 cm/cm/ °C) than tungsten carbide (as high as 4-6 x 10^"6 cm/cm/ °C), depending on cobalt content. TSP diamond and tungsten carbide are dissimilar materials because of the difference in the thermal expansion coefficients between TSP diamond and tungsten carbide. The coefficient of expansion of TSP diamond is less than that of tungsten carbide. Therefore, TSP diamond contraction is less than tungsten carbide while both parts are cooling from the same brazing temperature.

Differential contraction of these two materials upon cooling from the brazing temperature can cause excessive stresses (e.g., compressive/bending loads) to develop in the TSP diamond. This is termed the bi-metal effect. If the stress exceeds the strength of the TSP diamond, cracking and fractures occur. In fact, TSP diamond cracking has occurred when using conventional heating methods such as resistance, induction, electron beam, and furnace brazing. Lower process yields are a result of both TSP diamond fracture and structural degradation when the brazing temperature exceeds a thermal stability limit of 1200°C. The degradation mechanism is a diamond-to-graphite crystal transformation called graphitization. Thus, it is desirable to have the TSP diamond at a higher temperature than the tungsten carbide so that the diamond contraction will more closely match that of the tungsten carbide. With control of the temperature differentials, the residual thermal stress remaining in the TSP diamond can also be controlled.

In order to control the temperature difference between the TSP and tungsten carbide, the present invention takes advantage of one of the unique characteristics of microwave heating relative to conventional heating. That is, unlike materials heated conventionally, materials heated with microwaves manifest higher temperatures in their interior than at their surface. This is due primarily to the volumetric deposition of microwave energy in the heated material. The temperature gradient across the material is due to the fact that even though microwave energy is deposited on the surface of the materials as well as its interior, the heat produced is radiated away from the surface faster than it can be replaced by thermal conduction from the interior. The surface of the material therefore remains cooler than the interior.

The present invention provides a mechanism for controlling and flattening the temperature gradient across the TSP diamond and therefore, the deleterious effects, such as TSP diamond cracking on cool down, due to the different coefficient of expansion between the TSP diamond and the tungsten carbide. Further, the stress field within the cutter material can also be controlled. Specifically, because the presence of nucleating catalytic metals, such as iron metals, residual cobalt, silicon and other similar dielectric materials, in TSP diamond creates a higher dielectric, it is possible to fabricate TSP diamonds with geometry-specific iron profiles. For example, a TSP diamond can be fabricated having an iron content that is higher near the outside of the diamond than it is near the inside of the diamond (i.e., less of the dielectric can be added to the center portion while manufacturing the diamond). Thus, the relative temperature of the TSP can be fabricated having an iron content that is higher near the outside of the diamond than it is near the inside of the diamond (i.e., less of the dielectric can be added to the center portion while manufacturing the diamond). Thus, the relative temperature of the TSP can be precisely controlled by varying the average amount of dielectric iron metal or other suitable element while manufacturing the TSP diamond. One form of TSP diamond is manufactured by forming a composite diamond and cobalt. The cobalt is subsequently acid leached from the diamond structure resulting in less than five volume percent interconnected porosity. Therefore, suitable dielectric metals can be added to the porosity of certain TSP diamond materials.

As should be apparent, the dielectric iron metal content can be varied axially, radially or perpendicularly, or in any desired profile. Careful control of the variation in iron/dielectric content correspondingly controls the TSP diamond heating characteristics. Thus, appropriate variation in the iron content as a function of location in the diamond can be used, for example, to flatten the temperature gradient across the diamond during cutter fabrication. In turn, flattening the temperature gradient reduces stresses in the TSP diamond caused by the bi-metal effect and also contributes to minimizing stresses in the diamond layer after brazing. Likewise, controlled variation in chemistry may be used with other diamond or diamond like materials, including PDC diamond and cubic boron carbide. For example, for PDC materials, both the iron metal content and the cobalt binder constituent can be varied in a controlled way. Additionally, the carbide backing on the PDC can have a compositional variation which controls the temperature gradients. The concept of varied chemistry can also be desirable when employing other than microwave heating methods, such as induction heating commonly used to braze PDC diamond to tungsten carbide.

In addition to varying the amount of dielectric metal, the shape of the TSP diamond may also be varied to control the relative temperature of the TSP diamond during the microwave brazing process. As previously noted, the surface of the TSP diamond remains cooler than the interior when the TSP diamond is subject to microwave heating. Therefore, the shape of the TSP diamond can be altered to maximize the surface area to volume ratio of the TSP diamond.

A variety of different shapes may be utilized. For example, the TSP diamond has a thickness and a surface that defines an outer edge. The surface area to volume ratio can be increased by varying the thickness across the surface of the TSP diamond. Specifically, the thickness is varied such that it is greatest at the outer edge. Preferably, the thickness varies from being greatest at the outer edge to being substantially equal to zero at a region on the surface at a distance from the outer edge. In particular, the region is approximately located in the middle of the surface. The size of the region is limited to the extent that the structural integrity of the TSP diamond is compromised and to the extent that it effects the temperature gradient within the TSP diamond.

By virtue of the invention's microwave brazing process, including the process for controlling diamond dielectric content and shape, the temperature difference between the TSP and tungsten carbide can be precisely controlled and the two materials can be made to expand and contract at the same rate, notwithstanding, or perhaps because of, the different coefficients of thermal expansion. Thus, the two materials may be brazed with minimal thermal stress developed within either material. Similarly, the temperature difference can be controlled such that the tungsten carbide expands and contracts at a controlled greater rate, thus imposing a sometimes desirable compressive stress on the TSP diamond. Other material pairs can be used which exhibit the compatible physical characteristics of TSP diamond and tungsten carbide; for example, aluminum oxide doped with silicon carbide when brazed to titanium.

As should be evident, the present microwave brazing invention permits processing and material variables to be correlated to provide improved cutter wear shear and impact strength in order to attain the highest possible cutter wear and impact strength with shear strength greater than 35,000 psi. As previously noted, the prior art TSP diamond-to- tungsten carbide brazing techniques provide shear strengths ranging from an average of 138 to 207 Mpa (20,000 to 35,000 psi) which are not adequate for hard rock drilling applications. Strength levels of less than 35,000 psi are adequate for drilling softer formations.

The ability to microwave braze TSP to tungsten carbide by microwave heating the TSP diamond according to the present invention has also made possible heretofore unknown improvements in the braze filler metal composition used to facilitate creation of a cutter joint. The improvements in braze filler metal compositions described herein are also part of this invention.

There are many filler metal types which have been tested for brazing TSP diamond to tungsten carbide. In each case, the braze metal is shaped and placed between the TSP diamond and tungsten carbide base materials. For example, the braze metal may be in the form of a foil or screen shaped in accordance with the shape of the TSP diamond and tungsten carbide support. Tungsten carbide is wetted by many braze metal compositions. However, the wetting of TSP diamond requires that the braze filler metal alloy contain a reactive metal to form intermediate compounds with the TSP diamond. The most successful braze alloy compositions are alloys which contain a metal which reacts with carbon, including chromium, titanium, tantalum, molybdenum, hafnium and zirconium. Braze filler metal compositions which are able to set the TSP diamond brazing surface do so because of a diffusion process initiated by an active element (e.g., chromium or titanium) in the melt, whereby the melt creates a reaction product with the normally non-wettable TSP diamond surface before that surface is wetted and brazed.

The state-of-the-art TSP diamond cutter attachment procedure is to braze TSP diamond to 6% cobalt-bonded, fine-grain, tungsten carbide substrates with a titanium- copper-silver braze filler metal. The titanium-copper-silver braze filler compositions are compositions that contain a silver("Ag")-copper("Cu") eutectic plus specified titanium("Ti") contents. The industry standard titanium-copper-silver braze filler metal composition is (4.5Ti-26.7Cu-68.8Ag) which has a liquidous melting temperature of 850 °C. When heated to 850 °C, the braze filler metal flows and wets the TSP diamond to form a microstructure with dispersed titanium carbide regions near the TSP diamond interface. However, current TSP brazing methods using the industry standard titanium- copper-silver alloy result in an undesirable discontinuous layer of TiC adjacent to the TSP diamond surface. In other words, the braze metal does not completely wet the surface of the TSP diamond. Maximum strength properties are not realized unless a thin continuous layer of reaction product forms on the TSP surface (i.e., unless wetting is complete).

Thus, in addition to a new microwave brazing process, the present invention provides new braze filler metal compositions that may be used in conjunction with, or separately from, that process. The new braze metal filler compositions is a copper-silver eutectic composition containing about 10% to 15% by weight of Ti. Examples are (10.OTi-25.4Cu-64.6Ag) and (15.OTi-24.OCu-61.OAg.), which have liquidous melting temperatures of 960°C and 985°C, respectively. The novel braze filler material can be supplied in a metal foil, paste or other conventional form. The TSP diamond to tungsten carbide brazing temperature is typically 20 °C to 50 °C above the liquidous melting temperature. Therefore, the maximum brazing temperature for the higher Ti content alloy is 1,135 °C. Controlled variation of the Ti content according to the present invention permits selection of the optimum temperature to gain sufficient wetting of the diamond surface without excess alloying with the reactive metal. Excessive temperature over the braze filler metal melting point can result in the molten alloy being pushed out of the joint, a phenomenon known as braze starvation. Because the above braze temperature is within the 1200 °C temperature stability limit for TSP diamond, thermal damage to the TSP diamond is avoided. Compositions in excess of 15 weight % Ti can be used but may cause TSP diamond damage when brazing temperatures approach 1200°C. Generally, above 1200°C, all diamond materials change crystal structure to graphite and the desired wear resistance characteristics of natural and synthetic diamond materials are destroyed. However, if increasing degrees of wear rate are tolerable, it is possible to exceed 1200°C for a limited period of time before the TSP diamond structure converts completely to a graphitic structure.

During the microwave brazing temperature cycle, the TSP diamond temperature is to be greater than the WC temperature, particularly while the assembly is cooling from the braze temperature or braze filler metal liquidous temperature (whichever is less) to a temperature approximately 200 °C below the braze filler metal solidus temperature. Using finite element modeling (FEM), it was determined that the preferred ΔT should be 150° to 200 °C, which results in a minimum residual thermal stress in the TSP diamond. This FEM analysis was performed for a specific sample geometry and set of material properties. They were a 13.7 mm diameter and 3.5 mm and 8 mm length for TSP and 13 wt.% cobalt bonded tungsten carbide, respectively and 0.05 mm copper-silver eutectic braze filler metal foil containing 10 wt.% Ti. For this combination, the preferred ΔT for minimum residual thermal stress is 200 °C. Other material and geometric combinations will result in somewhat different preferred ΔT.

A minimum thermal stress in the TSP diamond is desired in order to achieve the optimum, drilling (or cutting) performance. High residual thermal stresses in the TSP would result in diminished resistance to impact stress developed in the drilling application, and reduced wear resistance in the drilling application. In other words, the TSP will crack and chip away due to high thermal stresses induced by brazing techniques which use heating techniques which do not provide the preferred ΔT between the TSP and tungsten carbide.

Also preferred is for the titanium to be introduced as a coating on the TSP diamond in combination with a suitable braze filler metal, such as copper silver eutectic composition (72 Cu-28 Ag). As a result, a higher strength braze will be formed. An advantage of this braze filler metal combination is that the braze foil can have a hollow center. Thus, as the TSP diamond is preferentially heated in the center, the braze metal will not melt until the brazing temperature has been reached throughout the TSP diamond interface with the braze filler metal layer.

The preferential heating of the TSP diamond center by microwave energy is termed a "volumetric heating effect." Center heating causes the full round braze filler metal to melt first in the center. In turn, center melting causes the higher bulk temperature (when compared to the tungsten carbide) of the TSP diamond to be reduced because the braze filler metal provides a thermally conductive path for heat loss from the TSP diamond to the tungsten carbide. In practice, this results in an increased tungsten carbide temperature, and a drop in an overall cutter assembly temperature so that the molten braze solidifies. As a result, more power to reheat the cutter assembly is required in order to achieve again a higher temperature in the TSP diamond and a uniform melt of the braze filler metal. On the other hand, the hollow braze filler metal foil allows for a desirable uninterrupted braze temperature-time cycle that exhibits an improved control of the desirable temperature difference between the TSP diamond and tungsten carbide. The present invention is not limited by the hollow center shape of the braze foil. Other configurations may be utilized so that the braze filler metal melts uniformly.

In addition, the Ti coating thickness is selected so that when combined with a copper-silver eutectic braze filer metal composition the reductive titanium content is between 1 to 15 wt.%Ti. Less than 1% is not adequate and 10 wt.%Ti is preferred. The coating may be applied to the TSP diamond by various coating techniques such as sputtering, plasma spray, and vapor deposition. Sputtering is preferred because it provides the highest density and is more economical. Other reactive metals can be substituted for titanium (Ti) including chromium (cr.) vanadium (V), molybdenum (Mo), and tungsten (W). A reactive metal is defined as one which will react with TSP diamond to form a carbide. According to the invention, the ideal microstructure and wetting will be developed by combinations of, for example, 10 to 15 weight % Ti and the Ag-Cu eutectic composition, plus variable braze temperature and time. No practicable combination of brazing temperature and time can develop the desired microstructure with the industry standard titanium-copper-silver braze alloy mentioned above. Other braze filler metals which can be substituted for the CuAg eutectic, include other compositions of CuAg, copper palladium alloys, gold, gold alloys and other similar compositions. However, these braze filler metal compositions also do not result in the desirable continuous TiC layer of the present invention.

According to a preferred embodiment, the desired microstructure includes a continuous layer of TiC on the TSP diamond surface that does not exceed a thickness of about 1 to about 5 microns. Greater thickness will increase the brittleness of the braze and, thereby, limit the ultimate shear and impact strength of the TSP diamond to tungsten carbide attachment. As noted, the prior art commercial titanium-silver-copper braze composition containing 4.5% Ti creates a discontinuous layer of TiC, resulting in lower shear strength attachments. The reason the new compositions develop the desired microstructure is that the use of braze filler metal compositions with, for example, 10 to 15% Ti, create a higher Ti composition gradient when the braze filler metal is heated and becomes molten. A higher Ti content braze alloy creates a composition gradient that increases the Ti rate of diffusion from the molten braze filler metal to the TSP diamond surface. Thus, the desired TiC reaction progresses at a faster rate to form the desired continuous layer of TiC microstructure. The more ductile composition containing greater proportions of Ag and Cu remains in the majority of the braze, thus providing desired ductility. Too great a proportion of TiC results in undesirable embrittlement of the braze. According to the present invention, the new braze filler metal compositions may be melted using the new microwave brazing process. Microwave energy may be preferentially applied to the dielectric-bearing TSP diamond, whereby subsequent heat conduction causes the braze filler metal to melt, or microwave energy may be preferentially applied directly to the new braze filler metals. It is evident that alternatives, modifications and variations of the invention will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and broad scope of the disclosure.

Claims

CLAIMS:

1. A method of joining thermally stable polycrystalline ("TSP") diamond material to tungsten carbide substrate comprising the steps of providing a braze metal filler composition comprising titanium, copper and silver at the location where the TSP diamond and substrate are to be joined, and heating the TSP diamond, braze metal filler and substrate by the application of microwaves.

2. A method according to Claim 1 including the step of heating the TSP diamond material to a temperature higher than the substrate material when using a titanium-copper-silver braze filler metal.

3. A method according to Claim 2 including the step of heating the TSP diamond material to a temperature of about 150°C to about 200 °C higher than the substrate material when using the titanium-copper-silver braze filler metal.

4. A method according to Claim 1 including the step of providing the TSP diamond having a thickness and a surface defining an outer edge, the thickness varies across the surface and comprises a maximum thickness at the outer edge.

5. A drill bit cutter comprising, in combination, a thermally stable polycrystalline ("TSP") diamond material, a tungsten carbide substrate, and a braze filler metal composition joining the diamond material to the substrate.

6. A cutter according to Claim 5 in which the diamond material includes at least one non-carbon element, and in which the amount of non-carbon element varies from locations relatively near the surface of a diamond crystal to locations relatively near the interior of the diamond crystal.

7. A cutter according to Claim 6 wherein the TSP diamond crystals have geometry-specific iron profiles.

8. A cutter according to Claim 5 wherein the TSP diamond has a thickness and a surface defining an outer edge, the thickness varies across the surface and comprises a maximum thickness at the outer edge.

9. A cutter according to Claim 8 wherein the thickness is substantially equal to zero at a location on the surface at a distance from the outer edge. 10. A cutter according to Claim 5 wherein the braze material includes from

10 weight percent to 15 weight percent titanium.

11. A cutter according to Claim 10 wherein the braze material includes a silver-copper eutectic composition.

12. A cutter according to Claim 5 including a substantially continuous layer of titanium carbide on the TSP diamond surfaces.

13. A cutter according to Claim 12 wherein said titanium carbide layer does not exceed a thickness of about one to about five microns.

14. A cutter according to Claim 12 wherein said cutter further comprises said braze filler metal having a hollow center.