Some general interest material on tribology from
pathfinder.com
Computer disk drives.
The tribology tribe points proudly to its
crucial role in the $30-billion-a-year data-storage industry. When it
comes to surfaces in motion, this is an especially harrowing arena.
Yet it's through tribological know-how that makers of hard-disk
drives have been able to squash more and more data into less and
less space. The 60% annual growth in storage density has outpaced
even the awesome rise in semiconductor performance.
"Historically, tribology has been the No. 1 technical challenge,
problem, and opportunity in rotating mass storage," remarks Gary
C. Rauch (rhymes with "how"), a vice president in the Recording
Media Group at Seagate Technology in Fremont, Calif. Bharat
Bhushan, a professor of mechanical engineering at Ohio State
University and an authority on data-storage technology, explains the
basic tribological challenge confronting Seagate and its competitors
in the magnetic-data-storage industry. The head that reads and
writes information to and from a hard disk, he says, flies about 50
to 100 nanometers above the disk surface. That's about
one-thousandth the width of a human hair. Meanwhile, the disk
typically spins beneath the head at about ten to 20 meters per
second.
To make such numbers comprehensible, disk-drive makers have
long resorted to airplane analogies. "A 747 flying eight inches above
the ground" was the analogy they used in the 1980s to describe the
relationship of read-write head and spinning disk. Woody Monroy,
head of corporate communications for Seagate, says that in terms
of speed and clearance, it's now the equivalent of an F-16 fighter
plane flying one-62nd of an inch above the ground, counting blades
of grass as it goes, at Mach 813. Yes, that's 813 times the speed of
sound.
Tribology is the limiting and enabling factor for these advances, says
Rauch. The airplane analogy is particularly apropos because the last
thing anybody flying in a plane or using a computer wants is a crash.
There are many reasons computers go down, but one of the most
dreaded is when the head assembly literally crashes into the
spinning disk's surface, tearing up precious data.
It's a tribological triumph that, despite all the hazards, vulnerabilities,
and abuse by users, most storage systems operate fine most of the
time. As in the electric-power industry, the triumph owes a lot to
the use of proper coatings. Hard-disk manufacturers, including
Seagate, Western Digital, and IBM, typically apply two extremely
thin layers over the film of magnetic recording material, which in
turn sits atop the disk's aluminum or glass base.
The first protective layer is at most 20 nanometers thick. It consists
largely of carbon with some hydrogen or nitrogen atoms in an
amorphous arrangement that researchers often refer to as
diamondlike carbon. Over this amorphous carbon layer is a much
thinner, two-nanometer layer of lubricant made of slippery
teflon-like polymers, which produces a banana-peel effect that
reduces head-to-disk friction during starts and stops.
The two-layer arrangement has been working quite well, but the
insatiable demand for more storage capacity brings with it a
maddening engineering challenge. One primary way to increase a
disk's storage capacity is to shrink the "magnetic domains" in which
the ones and zeros of the digital age are stored. It's like using
smaller and smaller type to pack ever more words onto a page. But
as the domains get smaller, it becomes harder for the head to
discern the signals, which can cause blurring and errors.
To prevent such errors, data-storage engineers keep reducing the
space between head and disk, but there's precious little remaining
altitude to play with. "We are getting into the regime where
roughness and smoothness are not much above atomic levels, and
so we will have to design systems where 'intermittent contact' of the
head and disk means contact a good part of the time," says Barry
Schechtman, executive director of the National Storage Industry
Consortium.
One leading-edge tribo-tactic is to fiddle with the molecular
structure of the thin lubrication layer on top of the disk. The idea,
says Rauch, is to keep the loopy, stringy lubricant molecules in
place while making them more flexible. Another leading-edge idea
is to reduce the head-disk spacing by making the wear-resistant
carbon coatings thinner. That sounds straightforward, but it takes a
lot of sophisticated chemistry, analysis, and engineering to get the
needed reliability, uniformity, and manufacturability.
Yet another angle, says Rauch, is to do "laser texturing" on specific
areas on the inside of disks where the heads take off and land.
Although this creates additional roughness on the disk's surface, the
overall area of contact between the head assembly and the disk is
reduced. And that minimizes a crash-inducing phenomenon known
in the industry as "stiction," which occurs when two surfaces come
close together and then stick.
Just which kind of laser texturing does a better job is one of the
arcane technical controversies in data-storage R&D, Schechtman
says. "If we don't find ways of solving these problems, we will fall
off the 60% cumulative annual growth rate in storage density,"
Rauch warns. "And if we don't stay on that curve, then you need to
have more heads and disks" in your disk-drive systems, he adds.
That, in turn, raises costs while increasing the likelihood of crashes,
which is not the way for companies like Seagate to stay
competitive. Says Rauch: "Tribologists won't be without their
work." It's all part of making disk drives, and for that matter the
whole U.S. economy, run smoothly. |
