WO1999029465A1

WO1999029465A1 - Microwave brazing process and brazing composition for tsp diamond

Info

Publication number: WO1999029465A1
Application number: PCT/US1998/026154
Authority: WO
Inventors: Robert Paul Radtke
Original assignee: Robert Paul Radtke
Priority date: 1997-12-10
Filing date: 1998-12-08
Publication date: 1999-06-17
Also published as: AU1811699A

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"

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

compositions suitable for use in manufacturing drill bit cutters having improved

resistance to stress failure when drilling hard formations.

BACKGROUND OF THE INVENTION

1. Drilling hard rock and dealing with high well bore temperature gradients have been persistent problems in the drilling industry. At

this time, drill bits using conventional 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 rock, where drill bit cutter temperatures can reach

900 °C. However, TSP diamond cutters have relatively low wear rates at

temperatures which reach 1200°C. The current state-of-the-art TSP diamond

cutter attachment procedure is to furnace heat or induction braze thermally stable polycrystalline diamond (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 TiCuSil braze alloy, all components are

heated slowly to 800° to 900 °C and melted to form discontinuous two-phase

micro structures. 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 direct resistance, induction, and furnace heating methods. However, because of

the stresses developed in the diamond layer due 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 700 °C) due to internal stresses developed within the PDC diamond

by the expansion of a cobalt binder. SP diamond does not contain a cobalt binder.

This limitation 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 the exposure above the bit face needed to drill at high rates of

penetration. Also needed is a 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 800 °C, the PDC cutter fails altogether. Commercial

grade diamond disks will graphitize and lose their 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 have uneconomical yields due to 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. The development of a new

brazing technique and new braze filler metals is required to produce high

attachment shear strength diamond cutters.

Detailed Description of the Inventon 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 also

provides new microwave braze filler metals for use with that microwave brazing

technique. A typical microwave system includes a 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 microwave device is tuned to its

resonant frequency and the device to be brazed placed in the cavity. 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 interlayer or filler metal (i.e., TiCuSil) 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.

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 950 °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 catalyzing metals used to grow diamond crystals from carbon to form synthetic diamond grains,

including, but not limited to iron, is 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 catalyzing metals is

a good absorber of microwaves (proportional to a high dielectric constant),

whereas tungsten carbide with about 6 to about 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 filler metal will

depend on the 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 filler metal compositions

with melting temperatures near the temperature stability limit of TSP diamond can

be used.

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 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.

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 material 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. Specifically, because the presence of catalytic metals, such as iron metals,

silicon carbide and other dielectric materials, in TSP diamond creates a high

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 MISSING UPON FILING.

when employing other than microwave heating methods, such induction heating

commonly used to braze PDC diamond to tungsten carbide.

By virtue of the invention's microwave brazing process, including the

process for controlling diamond dielectric content, 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

shear and impact strength in order to attain the highest possible cutter 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 ofa 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 (hereinafter "TiCuSil"). The TiCuSil

braze filler compositions are compositions that contain a silver(Ag)-copper(Cu)

eutectic plus specified titanium(Ti) contents. The industry standard TiCuSil 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

(TiC) regions near the TSP diamond interface. However, current TSP brazing

methods using the industry standard TiCuSil 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 (MW) 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 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 °s 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 world 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. The coating thickness is selected so that when combined with a

copper-silver eutectic braze filler 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 TiCuSil braze alloy mentioned above.

Other braze filler metals which can substituted for the CuSil eutectic, include

other compositions of CuSil, copper palladium alloys, gold and gold alloys.

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 I 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 TiCuSil 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 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 new the 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 a 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 at least 200 ┬░C higher than the

substrate material.

3. 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.

4. A cutter according to Claim 1 in which the diamond material

includes at least one metal dopant, and in which the amount of dopant varies from

locations relatively near the surface of a diamond crystal to locations relatively

near the interior of the diamond crystal.

5. A cutter according to Claim 3 wherein the TSP diamond

crystals have geometry-specific iron profiles.

6. A cutter according to Claim 3 wherein the braze material

includes from 10 weight percent to 15 weight percent titanium.

7. A cutter according to Claim 6 wherein the braze material

further includes a silver-copper eutectic composition.

8. A cutter according to claim 3 including a substantially

continuous layer of titanium carbide on the TSP diamond surfaces.

9. A cutter according to Claim 8 wherein said titanium carbide

layer does not exceed a thickness of about one to about five microns.