||Late 20th-Century Technology: Layer by Layer to the Perfect Blend of Metals
At least since the Bronze Age, people have mixed metals together to obtain
properties like hardness or ductility that are not present in the base materials
themselves. Now scientists are improving on the alloying process by combining
materials in many fine layers only a few atoms thick.
This atomic engineering holds the promise of designer materials, ones whose
properties are built in by blending elements that could not be combined in
other ways. Scientists say the new materials represent a new state of matter
that was previously unavailable.
Troy W. Barbee Jr., a senior scientist at the Lawrence Livermore National
Laboratory in Livermore, Calif., has produced a variety of multilayers,
initially for use in X-ray optics, but lately for their physical properties
rather than optical characteristics.
Some of the multilayer materials are deceptive in appearance. A thin piece
of shiny reddish brown metal looks like copper. But it does not bend easily, as
a thin sheet of copper would. Instead it is resilient, snapping back in place
after a bending force is removed.
The sheet is 110 microns thick, or about one-and-one-half times the diameter
of a human hair. But it is composed of 5,500 individual layers: 2,750 layers of
copper and 2,750 layers of an alloy of copper and zirconium. Although the
zirconium is only 3.1 percent of the combined material -- and is deposited in
layers one-eighth as thick as the copper-only layers -- its presence radically
alters the mechanical properties of copper.
Mr. Barbee said the fineness of the layers contributes to the strength of the
material, which he sees as a substitute for copper-beryllium alloys now used
in springs and tools. Many beryllium compounds are toxic, and cutting back their
use would reduce risks to workers.
Metals form crystalline structures when they cool from the liquid state to
solid, and defects in the crystal structure lead to weakness that prevents
materials from reaching their maximum theoretical strength.
Most materials have theoretical strengths far greater than their real
strength because of the problems of grain structure. Thus, the strength is
usually found only in microscopic single-crystal whiskers. Some multilayers,
though, have realized strength as much as 70 percent of the theoretical maximum.
Because the thin layers present in multilayers are deposited as gases with
high energy levels, they do not develop a conventional grain structure. As a
result, they are stronger and less likely to fail under stress. For example, a
multilayer composed of copper and Monel (itself an alloy of nickel and copper)
exhibits tensile strength more than 10 times that of basic copper. The strongest
materials are the ones with the thinnest layers, which have the most uniform
Multilayers may have important applications where great strength or
resistance to wear or corrosion is needed, said William D. Nix, a professor at
Stanford University and a member of the National Materials Advisory Board.
"They may be used in unusual coatings and protective layers," he said.
Other applications may come in integrated circuits, he said, which is fitting
since many of the techniques to form multilayers were developed for fabrication
of computer chips. "Multilayers may be used in integrated circuits to form very
strong interconnections," Professor Nix said. Problems with connections between
elements on chips have grown as they have become thinner to pack in more
components in the same physical space.
Because materials with different properties can be combined in multilayers,
some of the drawbacks of existing materials can be eliminated. For instance,
most very hard materials are also brittle. With a multilayer, very hard
materials can be combined with those that are very tough to produce something
that is both hard and tough.
According to Mr. Barbee, most materials will stick together in multilayers,
even if they are hard to combine by other means. So far, he said, 75 of the 92
naturally occurring elements have been sandwiched with at least one other
element in different experiments.
Some examples included combining tungsten and tungsten carbide to form a very
hard material that will withstand very high temperatures. A multilayer of
stainless steel and Monel produces a strong material that is highly resistant to
Researchers at Livermore are using a technique called sputtering to turn
solid materials into gases with temperatures of 80,000 to 90,000 degrees
centigrade. Sputtering, which is widely used in the semiconductor industry to
deposit thin films on computer chips, involves firing charged ions at the
material to be deposited on the substrate. The impact of the ions blasts atoms
from the surface, and they are attracted to material to be layered by electrical charge.
High quality silicon wafers are mounted on a device resembling an upside down
Lazy Susan and are rotated between vacuum chambers containing different
materials. As the wafers revolve from chamber to chamber, the alternating layers
are built up on the silicon substrate.
The 5,500 separate layers in the copper-copper zirconium sample took 13 hours
to form several months ago. But improvements in equipment are increasing
deposition rates. A 120 micron thickness now requires only three hours using
equipment now coming on line, Mr. Barbee said. A material 1 millimeter (1,000
microns) thick, containing 200,000 separate layers, takes almost 30 hours to
form. "It's like running between buildings in a rainstorm," Mr. Barbee said.
"The faster you go, the thinner the layer."
Because of the extraordinary physical properties of the multilayer
materials, they pose serious fabrication problems. Superhard, supertough
materials are likely to tear up steel drills and bits if they are machined by
So one of the next research goals is to go from flat plates to
three-dimensional shapes, like cylinders, that can be used with little or no
machining. This effort ties in with work being done at Carnegie-Mellon
University in Pittsburgh to combine masking techniques and technology known as
stereolithography with molten metal sprays to form components that need little
or no machining.
Stereolithography uses lasers guided by a computer-aided-design system to
form plastic prototype parts in a vat of photosensitive liquid polymer. If the
plastic part can be successfully coated with sprayed metal, a hard part can be
formed that is close to final specifications. This is called near-net-shape
Mr. Barbee thinks stereolithography and spraying techniques can be applied to
produce multilayer components rapidly with designer properties and useful
---The New York Times, December 1, 1991, Sunday, Late Edition - Final
Section 3; Page 9; Column 1.