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.