During the past
20 years, thousands of pounds of wrought niobium have been sold to the
jewelry industry both here and abroad. It is a small fraction of the metal
used in jewelry manufacturing, but remains a viable choice for the designer.
Niobium features many of the characteristics of precious metals. It is
rare, difficult to refine, and highly resistant to chemical attack. It
is malleable and hypoallergenic - and its price is above the current silver
spot. You might call it a "semiprecious" metal.
Niobium is a lustrous, light gray, ductile, metallic element that resembles
tantalum chemically, platinum in color, and is produced in both pure and
in alloy form. This article will focus on its pure metal form, certified
as ASTM B 392 & 393-99. Number 41 on the periodic table, niobium falls
into the transition metals, along with the more familiar precious metals.
It has a melting point of approximately 2,468°C/4,474°F, and a
density of 8.57 gm/cc. That density is about equal to cartridge brass
(8.5 gm/cc) and a little less than copper (8.9 gm/cc). It sells in bulk,
wrought form for $6 to $12/oz., on average.
Niobium has many properties that make it an excellent candidate for
fabricated parts. Of particular interest to jewelry manufacturers and
designers are niobium's hypoallergenic nature, formability, and the broad
range of anodized colors that can be rendered on its surface. It is considered
hypoallergenic and safe to wear for even those most sensitive to metal
allergies. In the 20 years that I've been in business, there has not been
a documented case of an allergic reaction to this metal due to its presence
in pierced ears or contemporary body piercing.
Working with Niobium
Like the precious metals, niobium is very ductile and extremely slow to
work harden. It cuts easily with a jeweler's saw and can be hand formed,
dapped, chased, and forged with ease. It has moderate density-about a
third that of gold, twice that of titanium, and 10 percent greater than
that of iron. In cold working, such as rolling, drawing, forging, and
swedging, niobium can be reduced as much as 90 percent in cross-sectional
area before annealing. In some processes, annealing is not necessary and
reduction is unlimited.
At room temperatures, a thin adherent oxide film forms on the metal's
surface. This film is transparent and self-healing. It protects the metal
from corrosion and protects the wearer from the metal. Niobium reacts
with atmospheric oxygen and other gases at temperatures as low as 230°C/446°F.
The metal should not be exposed to an atmosphere above 370°C/700°F
for longer than a few minutes to prevent further oxidation.
Niobium's tendency to oxidize also means that annealing requires a high
vacuum (10-4 Torr min.) or an inert atmosphere, vacuum being preferred.
Heat treatment at 1,200°C/2,218°F for one hour provides for complete
recrystalization of material cold worked over 50 percent.
The following is a list of techniques that can be used when working with
niobium, special considerations for the metal, and tips for achieving
the best results:
Drilling. Standard high-speed steel
drills can be used with good results. The peripheral lands of the drill
should be checked often for excessive wear to prevent drilling undersized
holes. Cooling and lubrication will extend bit life.
Machining. Normal machining techniques
can be used with niobium, which behaves very much like soft copper. The
metal has a strong tendency to gall, so special attention must be given
to lubrication and tool design. Although a variety of lubricants can be
applied, one of the most successful problem solvers is a chlorotrifluoroethylene
telomer mixture. This is an expensive lubricant, which is used in very
small quantities and can be recycled easily. (Halocarbon 27 Oil, $76.90/lb,
available from Halocarbon in River Edge, New Jersey.)
Both high-speed steel and carbide tools can be used, but the tendency
to gall is more pronounced with carbide. When turning, metal should be
removed with a shaving action and the chip should be allowed to slide
off the tool surface. Chip build-up can result in pressure, breaking the
cutting edge of the tool.
The recommended minimum surface speed is about 80 feet per minute, as
slower speeds will tear annealed stock. Unannealed metal is preferred
for lathe operations, where cooling and lubrication are of great importance.
Water soluble and vegetable oils are recommended. However, if those do
not work, use the more expensive chlorotrifluoroethylene telomer mixtures.
Thread Cutting. Standard techniques
for thread cutting can be used with sufficient lubrication to eliminate
galling and tearing. It is better to cut the threads on a lathe rather
than with a threading die. When dies or taps are used, they must be kept
free of chips and cleaned frequently. Rolled threads are an excellent
Grinding. Grinding niobium is difficult.
Using silicon carbide wheels is best, but rubber impregnated Cratex-type
wheels also work well. An adequate supply of cooling water is recommended.
Cleaning. Prior to annealing, joining,
and anodizing, the metal must be cleaned to remove all traces of lubricant
and soil. Niobium reacts with common gases as well as contaminants such
as oil, grease, and residues from degreasing. Parts should be degreased
in hydrocarbons or alkaline cleaners and rinsed in distilled or deionized
water. Parts may be stored in water to help protect surfaces from contamination.
Joining. Niobium cannot be soldered
using conventional jewelry equipment. The natural oxides of the metal
protect it and resist the penetration of solders. Heating it just makes
the oxides thicker and more impenetrable. Cold joining with rivets, blind
rivets, miniature nuts and bolts, bezels, and adhesives is common. Standard
fusion findings can be welded using equipment that is currently on the
market. Fusion findings in sterling, gold, titanium, stainless steel,
and nickel all weld well. It should be possible to laser weld in a purified
argon atmosphere, as with titanium.
Niobium can be TIG (Tungsten Inert Gas), EB (Electron Beam), and GTA
(Gas Tungsten Arc) welded with special consideration for argon shielding.
It is essential to completely cover the area of the molten pool and the
heated zone with inert gas to avoid contamination on the weld metal. This
protection must be given to the back of the weld as well as the front,
and remain through the cooling phase. This is best accomplished in a vacuum-assisted,
Blanking and Punching. Dies and punches
made of steels generally used for punching and blanking are satisfactory
for niobium. Simple dies made with the RT Blanking System, which is available
from Rio Grande, work well for small production. In standard die cutting,
an allowance of six percent of the metal thickness for clearance between
the punch and die is recommended. Light oils or similar lubricants should
be used to prevent scoring the dies. It also may be necessary to provide
"strippers" (a mechanical release in the die) to help release
Forming and Stamping. Beryllium copper,
aluminum bronzes, and steel may be used for form stamping tools. The techniques
used for stamping of steel are adequate. Tools should be polished to prevent
galling. Light oils or similar lubricants should be used and strippers
may be required.
Hand forming is a viable option as well. Traditional techniques include
die forming, chasing, repoussé, rolling, forging, fold-forming,
and corrugation. The metal moves so easily that it is possible to use
wooden and plastic tools.
Deep Drawing. Beryllium copper, aluminum
bronzes, and steel may be used for drawing. Single draws, where the depth
of the draw does not exceed the diameter of the blank, can be accomplished
easily. If more than one draw is to be made, the first draw should have
a depth greater than 40 percent of the blank diameter. Intermediate vacuum
annealing may be desirable with multiple draws. Sulfonated tallow and
Johnson's 150 Drawing Wax are suitable lubricants.
Spinning. Niobium can be spun by conventional
techniques using wood formers and steel roller wheels in conjunction with
an adequate lubricant, such as sulfonated tallow or Johnson's 150 Drawing
Wax. Spinning should be performed at room temperature, never hot. Beryllium
copper, Narite, or aluminum bronzes are also satisfactory for tooling.
The metal should be worked in small steps or stages of about 10 percent
with long sweeping strokes, using light pressure. (Niobium is often referred
to as "gooey" and is prone to thinning.) The peripheral speed
of the workpiece should be about 500 feet per minute.
Finishing. Niobium can be finished
by hand and mechanical means. Hand finishing can proceed as with any other
metal. Polishing compounds such as ZAM and Fabuluster work well. A high
polish can be achieved with white diamond. Use soft mops and low pressures,
and let the compound do the work. Surfaces that have been sandblasted,
bead blasted, and scratch-brushed provide interesting contrasts.
Niobium can be mass finished to a high polish in standard vibratory equipment
with the following caveats: It is very important to keep the process clean
and the media specific to niobium. Some experimentation will be necessary
to determine the correct processing times in the equipment being used.
Overprocessing can lead to darkened finishes. This can also result from
using certain finishing liquids, or using liquids at too high a concentration.
(Rio Grande's Super Sunsheen Burnishing Compound is excellent at the recommended
dilution.) Niobium has a strong tendency to gall and should not be burnished
on itself: Always use tumbling media, such as clean stainless steel shot,
and a liquid compound.
Anodizing. The process of anodizing
members of the reactive metals family, including niobium, can be found
in other publications, so it will not be described here in detail. Anodizing
the reactive metals should not be confused with anodizing aluminum. The
names of the processes are the same, but there are no other similarities.
Anodizing the reactive metals is a high-voltage, low-amperage process
performed with non-toxic electrolytes.
Niobium is easier and more predictable to anodize than titanium. It usually
requires no special chemical treatments or electrolytes. Any finish, from
sand and glass bead blasting to highly polished to deeply textured, will
anodize without restrictions. A full range of colors, from 0 to 100+ volts
DC, is available to the designer.
Anodizing produces a transparent oxide film on the surface that generates
interference colors. It is essentially a patina-an oxide of niobium-and
should be treated as such. The coating can be handled like other patinas.
It can be applied to an entire piece and then buffed to remove the high
areas. This is similar to working with patinas on sterling, but with niobium
there is a choice of colors, not just black. Unlike other patinas, niobium
oxides are chemically stable and will not change over time.
The anodic coating can be applied with brushes and other cathodic applicators.
Masking can be applied by screen printing, painting, and resistive tape.
Niobium can also be inlaid into settings or overlaid to protect the colors.
If you are combining niobium with other metals, such as gold or silver,
the anodizing step must be finished before incorporating the niobium into
the piece. Niobium will not anodize if it is submerged in the electrolytic
bath with another metal: The electrical current passes through the other
metal, leaving the niobium untouched. Because heat causes niobium to oxidize,
all soldering on the other metals must be completed before the niobium
is added via a cold joining method, such as rivets.
Currently, there does not appear to be a way to coat these thin films
with a protective finish. The colors depend on the extremely high refractive
index of the transparent oxide, and any coating tends to dull the colors.
Usually this isn't a problem, since many pieces are designed as inexpensive
jewelry for which long-term use is not expected and the color change due
to wear is not material.
Safety. Niobium is non-toxic and there
are no safety issues for working with it. However, breathing the dust
of any metal should be avoided. Fire can be a minor hazard; powder, chips,
and filings of less than 5 microns may autoignite and should not be allowed
to accumulate. Small fires should be allowed to burn themselves out. To
control them, use a Type D dry powder extinguisher or salt. Never apply
water to a fire, as a violent explosion could occur.
The Semiprecious Metal
As you can see, niobium shares many of the characteristics of precious
metals. I have tried in this article to offer a discussion of niobium's
relevant mechanical working characteristics and cast off most of its mystery.
What remains, I hope, is an enthusiasm for the "semiprecious,"
and an interest in further exploring niobium's qualities.