Wire-drawing lubricant and method of use

A process for drawing wire employing a lubricant composed of perfluorocarbon compounds having the general formula C.sub.n F.sub.2n+2. Such fully fluorinated carbon compounds exhibit a very high degree of thermal and chemical stability, due to the strength of the carbon-fluorine bond. Further, because the compounds are fully fluorinated, and therefore do not contain chlorine and bromine, they have zero ozone depletion potential (ODP). Further, because the PFCs are photochemically non-reactive in the atmosphere, they are not precursors to photochemical smog and are exempt from the federal volatile organic compound (VOC) definition.

FIELD OF THE INVENTION 
The present application relates to a process for drawing refractory and 
reactive metal wire, and more particularly tantalum fine wire. 
BACKGROUND OF THE INVENTION 
Wire drawing is one of the most difficult of the metal-forming operations. 
Wire is produced by reducing the cross-section of metal rod through a 
series of reduction dies until the desired final geometry is obtained. 
Wire has been produced from all of the common metals, including steel, 
copper, aluminum, gold, silver, etc., as well as from the refractory and 
reactive metals, including tantalum, niobium, molybdenum, tungsten, 
titanium, zirconium, etc. Because of the severe sliding contact between 
the wire and the die, lubricants are used in all wire drawing operations 
to reduce friction between the die and the wire, to flush the die to 
prevent the buildup of fines and dirt on the die surface, to reduce wear 
and galling between the die and the wire, to remove heat generated during 
plastic deformation, and to protect the surface characteristics of the 
finished wire. 
The lubricants used today to draw the common metals are a complex blend of 
various esters, soaps, and other extreme-pressure lubricants. Oil- or 
polyglycol-based lubricants are often used in the form of emulsions in 
water at concentrations on the order of 10%, sometimes with additives to 
give the emulsions the necessary detergency to keep both the dies and wire 
clean. Ease of cleaning is a fundamental parameter in the selection of 
wire-drawing lubricants. In the state-of-the-art, these classes of 
lubricants have been found to be inadequate in the production of 
refractory and reactive metal wire. 
Various chlorinated oils have been used over phosphate precoats, as well as 
mixtures of various graphite and molybdenum disulfide lubricants, with 
limited success to draw refractory and reactive metal wire. More recently, 
chlorotrifluoroethylene (CTFE)-based oils have become the lubricant of 
choice in the production of refractory and reactive metal wire, generally 
in a viscosity range of 20 to 150 centistokes. While CTFE lubricants are 
now used almost exclusively in the production of electronic-grade tantalum 
wire, they present a number of serious operating limitations. Because of 
the poor heat transfer characteristics of the CTFE lubricants, drawing 
speeds must be very slow, generally in the range of 100 to 300 FPM. 
Typical wire-drawing speeds for the common metals are in the range of 5000 
to 20,000 FPM. As a result, drawing costs for refractory and reactive 
metals are very high by comparison. In addition, the CTFE lubricants are 
only marginally effective in reducing wear and galling between the wire 
and the die and in flushing the wear products away from the die entrance. 
These problems are very evident in the short die life (&lt;20 pounds per set) 
obtained when using carbide dies to draw tantalum wire and in continuing 
problems with surface roughness and dimensional control (including both 
diameter and roundness). All of these limitations associated with CTFE 
lubricants make refractory and reactive metal wire drawing an inherently 
high-cost process that results in a marginal quality product. 
A more serious limitation of the CTFE lubricants is found when attempting 
to remove them from the surface of the finished wire. The removal of these 
lubricants is typically accomplished using solvents, typically 
1,1,1-trichloroethane. With the increasing restrictions placed on solvent 
use because of flammability, toxicology, ozone depletion, and global 
warming, it is almost completely impossible to remove the CTFE lubricants 
from wire products. A number of hot, aqueous degreasing systems, with and 
without ultrasonics, have been used to attempt to remove these lubricants 
with limited success. As a result, CTFE lubricant residues on 
electronic-grade wire surfaces continue to be a cause of component 
failure. 
Accordingly, it is the object of this invention to provide an improved 
process of drawing refractory and reactive metal wire, avoiding the 
foregoing problems. 
A further object of the invention is to use in a conventional wire-drawing 
process a nonflammable and nontoxic lubricant. 
It is another object of the invention to use in a conventional wire-drawing 
process a lubricant having zero ozone depletion potential (ODP). 
It is a still further object of the invention to use in a conventional 
wire-drawing process a lubricant that is photochemically nonreactive in 
the atmosphere, is not a precursor to photochemical smog, and is exempt 
from the United States Environmental Protection Agency's volatile organic 
compound (VOC) definition. 
SUMMARY OF THE INVENTION 
The foregoing objects are achieved in a process for drawing wire using a 
conventional wire-drawing machine, including the use of perfluorocarbon 
fluids as lubricants while drawing refractory and reactive metal wire 
through the dies. 
Perfluorocarbon fluids originally were developed for use as heat-transfer 
fluids. They are currently used in heat-transfer, refilteration, and 
cleaning applications. The present process employs a lubricant composed of 
perfluorocarbon compounds (PFCs) selected from noncyclic perfluoroalkanes 
having the general formula C.sub.n F.sub.2n+2 and perfluoroamines, either 
alone or in combination. Such fully fluorinated carbon compounds exhibit a 
very high degree of thermal and chemical stability due to the strength of 
the carbon-fluorine bond. PFCs are also characterized by extremely low 
surface tension, low viscosity, and high fluid density. They are clear, 
odorless, colorless fluids with boiling points from approximately 
30.degree. C. to approximately 300.degree. C. 
Importantly, because PFCs are fully fluorinated, and therefore do not 
contain chlorine or bromine, they have zero ozone depletion potential 
(ODP). They are nonflammable and nontoxic Further, because the PFCs are 
photochemically nonreactive in the atmosphere, they are not precursors to 
photochemical smog and are exempt from the federal volatile organic 
compound (VOC) definition. In addition, they cost significantly less than 
the chlorotrifluoroethylene oils currently in use. Accordingly, PFCs are 
now found to be the preferred lubricants in high-speed fine wire drawing 
of refractory and reactive metals. 
In the wire drawing process, the perfluorocarbon fluids of the present 
invention have greatly extended the ranges of the major wire drawing 
variable available to the process engineer. While using the CTFE 
lubricants, the reduction per die was limited to approximately 15%. The 
use of PFC lubricants allows reductions as large as 26% per die. This will 
allow the next generation of wire drawing equipment to be much more 
productive. In addition, operating speeds can be increased by more than 10 
fold, greatly reducing the number of wire drawing machines required at a 
given production level. The CTFE lubricants were limited to approximately 
200 FPM while the PFC lubricants have been used at speeds of over 2,000 
FPM with no signs of having reached an upper limit. In addition, die wear 
is minimized to the point that wire can be drawn without annealing from 
0.103" (2.5 mm) to a final diameter of 0.005" (0.127 mm). 
All grades of the perfluorocarbon fluids evaluated to date have been used 
to produce high-quality tantalum wire. PFC fluids ranging from 
perfluoroalkanes, such as 3M's PF-5050 (perfluoropentane (C.sub.5 
F.sub.12)) having a boiling point of only 30.degree. C. and a viscosity of 
0.4 centistokes, to perfluoroamines, such as 3M's FC-70 (a blend of 
perfluorotripropylamine (C.sub.3 NF.sub.9) and perfluorotributylamine 
(C.sub.4 NF.sub.11)) (C.sub.15 F.sub.33 N) having a boiling point of 
215.degree. C. and a viscosity of 14 centistokes have all been used to 
produce high quality wire at high drawing speeds. 3M Company's FC-40 has 
been extensively evaluated because of its combination of low price and 
high boiling point (155.degree. C.). This fluid has a viscosity of only 2 
centistokes and a vapor pressure at room temperature of 3 torr. All of the 
data suggest that there are many other PFC fluids that are good 
metalworking lubricants. 
The fact that lubricating characteristics are not dependent upon PFC fluid 
viscosity is unique to this class of fluids and is not yet understood in 
terms of current metalworking lubrication theory. In fact, the use of a 
wire-drawing lubricant having a viscosity of less than 1 centistoke is 
contrary to most lubrication theories. 
A variety of metal wire-drawing tasks can be enhanced through the above 
process. But particular benefits are realized in the context making fine 
tantalum wire to be used as anode lead wires in tantalum electrolytic 
capacitors. The tantalum wire (typically 5 mils to 20 mils (0.127 mm to 
0.508 mm in diameter) is buttwelded to a porous, sintered powder anode, or 
is embedded therein prior to sintering and bonded thereto in sintering. 
Minimizing leakage of the capacitor using such an anode depends in part on 
the cleanliness of the lead wire, which is directly affected by lubricant 
selection. 
Significant reduction in wire DC leakage has been achieved with wires 
produced in accordance with the present invention. The leakage current is 
directly related to the surface topography of the wire, as well as the 
amount of lubricant that remains trapped in the cracks and crevices on the 
surface of the wire. DC leakage currents can be reduced by producing a 
smoother wire surface and eliminating residual lubricant from the wire 
surface. The DC leakage is measured by anodizing a length of wire to 
completely cover the surface with a tantalum oxide dielectric film. This 
anodized wire is placed in on electrolyte and a DC voltage is applied to 
the tantalum lead itself. The DC current "leaking" through the dielectric 
film is measured at a fixed voltage. This leakage current is a measure of 
the integrity of the dielectric film. The dielectric film integrity itself 
is a measure of the overall surface roughness and cleanliness of the wire 
surface. By producing a smooth surface free from residual lubricants, 
improved dielectric films are produced, thus improving the DC leakage 
characteristics of the wire.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The practice of the invention according to preferred embodiments thereof is 
indicated by the following non-limiting examples: 
EXAMPLE 1 
169.5 lbs (77.1 kg) of 0.0098" (0.0249 cm) half-hard temper tantalum wire 
was drawn through a Heinrich wire-drawing machine (MODEL # 21W21) using 
FC-40 perfluorocarbon fluid (3M Company) as the lubricant. Wire speed 
ranged from 200 ft/min (61 m/min) to 1386 ft/min (424.5 m/min). The 
average roundness measured using a laser micrometer at the beginning of 
each of the coils of wire was 16 millionths of an inch (40.6 .mu.m) with 
the average roundness at the end of each coil averaging 18 millionths of 
an inch (45.7 .mu.m). An average of 42.4 lbs of wire was produced per set 
of dies. 
EXAMPLE 2 
70.2 lbs (31.9 kg) of 0.0079" (0.0201 cm) extra-hard temper tantalum wire 
was drawn through a Heinrich wire-drawing machine, as in Example 1, using 
3M's FC40 perfluorocarbon fluid as the lubricant. Wire speed ranged from 
500 ft/min (152.4 m/min) to 1000 ft/min (304.8 m/min). The average 
roundness at the beginning of each of the coils of wire was 11 millionths 
of an inch (27.9 .mu.m) with the average roundness at the end of each coil 
averaging 11 millionths of an inch (27.3 .mu.m). An average of 35.1 lbs of 
wire was produced per set of dies. 
EXAMPLE 3 
231.8 lbs. (105.4 kg) of 00079" (0.0201 cm) hard temper tantalum wire was 
drawn through a Heinrich wire-drawing machine, as in Example 1, using 3M's 
PC-40 perfluorocarbon fluid as the lubricant. Wire speed ranged from 800 
ft/min (243.8 m/min) to 1480 ft/min (451.1 m/min). The average roundness 
at the beginning of each of the coils of wire was 12 millionths of an inch 
(30.5 .mu.m) with the average roundness at the end of each coil averaging 
16 millionths of an inch (40.6 .mu.m). An average of 46.4 lbs of wire was 
produced per set of dies. 
EXAMPLE 4 
49.4 lbs (22.5 kg) of 0.0075" (0.0191 cm) hard temper tantalum wire was 
drawn through a Heinrich wire-drawing machine, as in Example 1, using 3M's 
FC-40 perfluorocarbon fluid as the lubricant. Wire speed ranged from 1480 
ft/min (451.1 m/min) to 1600 ft/min (487.7 m/min). The average roundness 
at the beginning of each of the coils of wire was 15 millionths of an inch 
(38.1 .mu.m) with the average roundness at the end of each coil averaging 
17 millionths of an inch (43.2 .mu.m). An average of 24.7 lbs of wire was 
produced per set of dies. 
EXAMPLE 5 
71.6 lbs (32.6 kg) of 0.091" (0.0231 cm) annealed temper tantalum wire was 
drawn through a Heinrich wire-drawing machine, as in Example 1, using 3M'6 
FC-40 perfluorocarbon fluid as the lubricant. Wire speed was 1200 ft/min 
(365.8 m/min). The average roundness at the beginning and the end of each 
of the coils of wire was 20 millionths of an inch (50.8 .mu.m). An average 
of 71.6 lbs of wire was produced per set of dies. 
EXAMPLE 6 
In addition to the normal dimensional, visual, and mechanical property 
evaluation performed on the wire as it is produced, the wire drawn using 
the perfluorocarbon lubricants was evaluated using scanning electron 
microscopy (SEM). 
Scanning electron micrographs taken at 300X and 1000X of capacitor-grade 
tantalum wire drawn using FC-40 at 200 ft/min (61 m/min), 500 ft/min 
(152.4 m/min), and 1000 ft/min (304.8 m/min) are shown in FIGS. 1-3, 
respectively. The 300X pictures show that wire surface quality actually 
improves with increasing drawing speed. Overall, the frequency and depths 
of the cracks and crevices on the surface of the wire drawn using 
perfluorocarbon fluid lubricant diminish with increasing wire-drawing 
speed. 
EXAMPLE 7 
The surface of a capacitor grade tantalum wire drawn using a CTFE lubricant 
at 200 ft/min (61 m/min) is shown in FIG. 4 at 1000X. This picture shows 
the typical structure seen on wire drawn using a conventional 
chlorotrifluoroethylene lubricant. As can be seen, this wire shows a great 
deal of surface damage, particularly in the form of relatively thin 
platelets of material torn from the surface of the wire. This appears to 
be the mechanism by which most of the "fines" observed in the fine 
wire-drawing process are generated. The fact that fines are not observed 
in wire drawn using the perfluorocarbon fluid lubricant indicates that 
surface damage due to this flaking caused by galling and seizing (as a 
result of lubricant breakdown) has been eliminated. 
EXAMPLE 8 
In order to evaluate the overall degree of cleanliness of the as-drawn wire 
produced using a perfluorocarbon lubricant, samples were submitted to 
micro-FTIR infrared analysis. The reference spectrum of the 3M FC-40 
lubricant is shown in FIG. 8. The spectrum of the methylene chloride 
extract from a sample of TPX 501G wire drawn using the perfluorocarbon 
lubricant, together with the reference spectrum of the FC-40, are shown in 
FIG. 9. It is important to note that essentially no lubricant residue of 
any kind is found on the wire, and that whatever residue that is present 
is definitely not FC-40. The overall absorbence values can be compared to 
the data shown in FIG. 10, which shows the FTIR spectrum of the extract 
removed from a sample of TPX 501G after cleaning in an ultrasonic strand 
cleaning system used to remove CTFE lubricants. Total absorbence values on 
the order of 0.1 absorbence units are typical of wire cleaned in the unit. 
In general, these absorbency values represent less than one monolayer of 
residual lubricant on the surface of the wire. The perfluorocarbon wire as 
drawn has less than 20% of this amount of surface contamination and is 
truly an electronically clean material. 
FIG. 11 shows the as-cleaned spectrum superimposed on the reference spectra 
of CTFE oil and an ester-based rod-rolling oil used in earlier stages of 
the wire production process. These two materials account for essentially 
100% of the residue found on the surface of our uncleaned capacitor-grade 
wire. No indication of any residual FC-40 was found. As a result of this 
analysis, it appears that wire drawn using the perfluorocarbon lubricant 
can be used as drawn. Subsequent ultrasonic cleaning will only serve to 
contaminate the surface of the wire. 
EXAMPLE 10 
In order to further verify this finding experimentally, samples of both 
0.0079" (0.0201 cm) and 0.0098" (0.0249 cm) diameter wire were submitted 
for as-received leakage tests. The DC leakage is measured by anodizing a 
length of wire to completely cover the surface with a tantalum oxide 
dielectric film. This anodized wire is placed in an electrolyte and a DC 
voltage is applied to the tantalum lead itself. The DC current "leaking" 
through the dielectric film is measured at a fixed voltage. This leakage 
current is a measure of the integrity of the dielectric film. The 
dielectric film integrity itself is a measure of the overall surface 
roughness and cleanliness of the wire surface. By producing a smooth 
surface free from residual lubricants, improved dielectric files are 
produced; thus improving DC leakage characteristics of the wire. These 
data are shown in FIG. 12 and indicate that the as-received leakage values 
for as-drawn wire fall in the range of 1 to 3 .mu.amps/cm.sup.3. They 
certainly compare favorably with recent production and compare very 
favorably with the specification maximum of 10 .mu.amps/cm.sup.3 commonly 
seen in the industry. In actual production trials employing the 3M 
Company's FC-40 perfluorocarbon fluid, the most significant advantages 
observed include a greater than five-fold increase in die life, a greater 
than ten-fold increase in wire-drawing speed, "electronically clean" 
as-drawn wire, and a five-fold reduction in lubricant cost. In addition, a 
major reduction in the amount of submicron tantalum fine particle debris 
has been observed. While using the CTFE lubricants, the filters on the 
wire-drawing machines are changed at the end of every production shift. 
When using PFC fluids, these filters are changed every one to two months. 
It will now be apparent to those skilled in the art that other 
embodiments, improvements, details, and uses can be made consistent with 
the letter and spirit of the foregoing disclosure and within the scope of 
this patent, which is limited only by the following claims, construed in 
accordance with the patent law, including the doctrine of equivalents.