Laying It on Thick with
Specialized Coatings
Printed with permission from Moldmaking Technology, May 2003 issue.
Written by Sherry L. Baranek
Phygen coatings offer low friction, high hardness and exceptional wear
resistance to extend tool life.
While conventional coatings are
designed to extend the life of a
tool by making it stronger, often
quite the opposite occurs. High temperature
chemical vapor deposition (CVD) and
thermal diffusion (TD) processes used to
apply hard wear-resistant coatings are able
to change a tool’s critical dimensions and
mechanical properties. Many known low temperature
physical vapor deposition
(PVD) processes typically suffer from
insufficient adhesive and cohesive strength
and might create substantial stresses below
the tool’s working surface, which result in
a reduction of fatigue strength. Excess
wear leads to premature tool repair, and the
tool can often become distorted after that.
According to Dave Bell, president of
Minneapolis, MN–based Phygen Coatings, Inc. — a
manufacturer of specialty coatings — the
beneficial properties of high hardness, low
friction and excellent wear resistance make
carbides and nitrides of transition metals
excellent materials for various thin film
applications. “During the last ten years,
thin, hard coatings deposited by physical
vapor deposition (PVD) processes on
engineering components — such as
forming and cutting tools — have helped to
substantially reduce tool wear and prolong
tool life,” he explains. “The coatings have
permitted more cost-effective machining
processes and helped to fulfill the
increasing demands put on production
processes in terms of performance, reliability
and environmental compatibility.”
Hard coatings’ performance depends on
the type of deposition technology used to
apply the coatings, Bell points out. Many
coatings — such as titanium nitride (TiN) or
chromium nitride (CrN) — if produced by
conventional coating technologies, cannot
meet the improved industry standards.
Specialty Coating
Phygen developed a proprietary physical vapor deposition
coating technology based on principles of plasma acceleration,
which result in a higher plasma density and an intense low-energy
ion bombardment during the coating deposition. The technology
allows deposition of a wide variety of coatings (metals, alloys
and compounds) and produces a Phygen® ST.3 SuperTough™ Chromium
Nitride Coating which exhibits superb hardness and wear resistance
compared to conventional PVD and CVD-based or thermal sprayed
coatings, Bell explains. “Phygen’s technology provides a unique
combination of the coating properties by creating a dense,
non-columnar coating structure having the highest possible
adhesion level.” he says. “The microstructure of the coating
is precisely controlled by proprietary process parameters.
As a result, this coating actually constitutes a single-phase,
stoichiometric, nanocrystalline chromium nitride having a
highly textured, dense structure. The fine-grained structure
and high cohesive strength of these chromium nitride coatings
allow the deposition of extremely tough and hard coatings
— which possess much higher abrasive wear resistance than
provided by conventional PVD processes.”
Additional Features
- Less brittle than conventional hard
coatings and may withstand much
higher mechanical loads than conventional
coatings
- Microhardness that exceeds that of
conventional PVD CrN coatings
- Exceptional abrasive wear resistance
due to the combination of high hardness
and toughness
- Low friction properties (coefficient of
friction less than 0.1) under properly
lubricated conditions and in an oxidizing
type of environment
- Chemically and thermally stable in the
air up to at least 1,550ºF
- Excellent corrosion resistance properties
and chemically inert because of their
dense, noncolumnar microstructure
- Does not change critical dimensions of
properly heat-treated tools or precise
components
- Can be easily (and with no harm to the
substrate) stripped at the end of useful
tool’s life and the tool can then be repaired, refinished and coated again using Phygen’s technology
“Phygen’s coatings have been extensively
investigated during the last few years at
major U.S. National Laboratories and
leading research institutions,” Bell notes.
Figure 1. Rutherford backscattering spectroscopy (RBS)
results confirm 100 percent stoichiometric composition of
Phygen’s SuperTough chromium nitride. |
Figure 2. SEM image made using secondary electrons on a
fractured cross-section of the chromium nitride film
deposited on a silicon wafer. The silicon wafer substrate is
seen to the right of the film. |
The research conducted by Othon Monteiro
at Lawrence Berkeley National Lab
(Berkeley, CA) confirmed stoichiometric
chemical composition of the coatings (see
Figure 1). No presence of any other crystallographic
phases had been found. Scanning
Electron Microscopy (SEM) analysis
performed by scientists at Argonne National
Lab (Argonne, IL) revealed noncolumnar, equiaxially grained microstructure of the coatings (see Figure 2).
John Woodford, and Mohumad al-Zoubi
— both Ph.D.s at Argonne National
Laboratories — recently conducted a study
of Phygen coatings and found that the
coatings exhibited high toughness and wear
resistance. They studied three samples of
hardened H-13 steel flats — each coated
with different process formulations and
compared them to an uncoated flat. The
three CrN-coated flats and one uncoated
flat were subjected to an endurance wear
test in the presence of synthetic diesel fuel.
The tests were conducted in a Cameron-Plint high-frequency rig, which reciprocates
(moves the flats back and forth) under
a fixed 0.25-inch 52100 steel ball carrying
a load of 10 Newtons.
Woodford reports that the friction coefficient
traces for the three different coatings
were then examined. All three coatings
showed a reduction in friction over
uncoated steel. Sample one showed the
least improvement, but was still 20 percent
better. Sample two exhibited the lowest
average friction coefficient with an average
value of 0.09. Phygen’s sample three had
the best consistency in friction coefficient
and had only slightly higher friction coefficient
on average than sample two (see
Figure 3).
Figure 3. Plot of friction coefficient versus sliding distance for reciprocating
tests described in the article. |
Upon examination, the three coated flats
showed no sign of wear, according to
Woodford. The uncoated flat had a distinct
wear track, but optical profilometry
showed it to be partially filled with
material transferred from the ball. Because of this, it was impossible to measure wear rates on the flats.
Success Stories
Out in the field Phygen also has proven
successful. Harvey Bruce, moldmaker and
coatings specialist for Thermotech
(Hopkins, MN) — an engineered plastic
injection molding company for the
automotive, medical, electronics and
communications industry — has been
using the coating for two years. “We were
looking for something that could stand up
to high abrasion,” Bruce explains. “We do
a lot of high-tech parts, and there are
actually minerals and really small strands
of fiberglass in the plastic to give the parts
strength. But what happens is that some
plastics, minerals and fiberglass will attack
and erode the steel. So we needed a coating
that was able to stop that erosion.”
According to Bruce, Phygen worked
really well on the one tool they tried it on. “We then had another opportunity to use
the coating,” he recalls. “We had a instance
where the plastic would just wear the mold
out so quickly, so we tried Phygen. With the previous coatings, we would run
twenty-six hundred shots in eight hours
and would already see that the plastic was
wearing out vents and the detail on the
tool. Then we’d have to repair the mold. It
was terrible. Since we applied the Phygen
coating, it has made more than fifty
thousand shots, which is about forty times
the wear resistance. I pulled the tool off the
shelf and looked at it and there was
absolutely no wear on the tool. It’s very impressive.” Harvey also notes that the
Phygen coating polishes easily and a high
shine is achievable on the tool.
Square D Co. (Lincoln, NE) — a
provider of Thermoset injection and
injection compression molding — also has
met with success using Phygen coatings.
Molding Manager John MacKichan says,
“We mold highly abrasive compounds with
fiber glass content ranging from 9 percent
to 25 percent. Over the years we have tried
a wide variety of surface treatments to
extend tool life, but even with the surface
treatments, tooling wear is still a major
issue.”
In 2000 Square D began testing Phygen
coatings on their high-wear tooling
components. “The Phygen coating seems to
work extremely well — even with our
abrasive materials,” says Chad Nadgwick,
tool room manager. “We have tested
Phygen-coated mold components in the
most critical wear points, such as gate
inserts and the liners on the injection
compression molds with favorable results.
The Phygen coating seems to be pretty
remarkable.”
The company also found that Phygen
coating did not degrade the fine surface
finish they put on the tools and it did not
create any dimensional tolerance issues.
Normally Square D’s high-wear molds
would be pulled for repairs after twelve
months of production, Nadgwick notes, but
the Phygen coating appears to have significantly
extended the available production life
of the tooling. Square D expects success
with the Phygen coating to provide benefits
of cost savings from decreased tool replacement
and part quality enhancements.
Bill Strommen, president of Osseo, MN based
Strommen Engineering — a builder
of close-tolerance plastic injection molds
predominantly for the medical industry —
has realized similar advantages. “We have
found longevity of the tool to be the main
advantage,” Strommen states. “It feels as if
it is going to last forever — it has very
good wear characteristics compared to the
TiN coating. It also has eliminated a lot of
extra costs in repair — the TiN coating
would wear out and the mold would come
back to us for repair work.” Additionally,
Strommen notes that there is no buildup on
the corners like in chrome plating, and the
finish is excellent.
Strommen does recommend caution when
it comes to heat-treating. “Since Phygen is
applied at a 950° temperature, you have to be
careful on which metals you put it,” he
states. “Different tool steels obviously heat
treat at different temperatures and you have
to be equal to 950° or above or else you will
get warpage in your parts when they go
through this treatment. The draw temperature
should be more than 950°.”
Bell and Strommen have been working
together to test Phygen with other substrates.
“We just ran a test on a silicone mold of ours
and it has worked well,” Strommen states.
Bell adds that Phygen coating has exhibited
“excellent” release properties with silicone.
In conclusion, Phygen coatings not only
protect surfaces, they reduce friction and
wear, which results in better mechanical
properties and longer tool life.
|
