1. The Base Coat
The coating which carries the aluminum, chromium or other metals
deposited in a high vacuum process
Abstract
High vacuum metalizing is the method to efficiently apply a thin metal surface on almost
any substrate. In order to give this metal surface a specific texture – usually high gloss- a
lacquer based coating is applied to the substrate with the main purpose that this “Base
coat” becomes the actual substrate for the metal.
Base coats as I write about here are typically found in applications like these:
Lighting equipment, especially automotive headlamps and rearlamps, sputter chromium
as replacement for electroplating, high gloss metal on toys and bottle caps.
This article is about this base coat, describes in a “nutshell” what metalizing base coat is
all about and goes into specifics with those problems in metalizing like:
Rainbow colored aluminum which is called iridescence, cracking chromium, that what
makes gloss, adhesion or loss of adhesion.
Some basic Physics and Chemistry
Leveling or how to make a rough substrate glossy
Here we are not talking about sanding and polishing; that would be a way to make a solid
metal piece glossy. This is about making a high gloss surface by coating it with lacquer
or varnish. Actually, in the past there often was some kind of sanding prior to base
coating, but this was done because those base coats in the past did not have the leveling
capability which today’s base coats posses.
How does this leveling process work? Here is a model which I use to explain it:
Assumed there is a metal surface with a roughness of 10µ m, means distance between the
average peak and valley of the surface texture is 10µ m. Apply a wet coating of 30µm to
this surface and let it level out. The liquid covers the roughness entirely and the surface
has a gloss with the appearance of a tranquil lake in the moonshine. But unfortunately,
after the paint has dried, the gloss is gone; it’s not as rough as the initial bare metal
surface, but definitely not glossy.
Ok, 30µm was not enough; 50µ m of coating is applied, that should take care of the
roughness.
It will not. What happened?
Our model coating may consist of 50% solids and 50% solvent; so, after all the solvent
has evaporated out of the 30µm initial wet layer, a 15µm thick layer of pure resin, -not
liquid anymore-, would remain and this should still bury all the peaks and valleys,
shouldn’t it? It does, as long as the resin is not yet cured.

2. Now we assume that this model resin would shrink during the curing process by 50%
(exaggerated by today’s standard, but this is just a model), means from low molecular
resin to hard and cross-linked macromolecule. The shrinking happens at 50% above the
peaks and at the same rate above the valleys with the result that the original roughness is
just reduced by 50%. Off course, applying 50µm of the same model coating would not
make any difference in smoothing out the initial roughness.
So, how to finally make gloss with our model coating on the rough metal surface?
The answer is multiple layers and if possible sanding in between layers to reduce the
height of the still protruding peaks. This is exactly what was done in the past when horse
drawn carriages received that high gloss black paint finish and it was still so in the early
years of automotive body coating.

3. Leveling
capability of a
coating with low
shrinkage
With these above mentioned model coatings in mind it becomes obvious that to obtain a
high gloss base coat one has to be in control of these parameters:
• Initial substrate roughness as low as possible
• Coating thickness of solvent free resin must cover initial roughness
• The resin remains liquid as long as possible prior to curing
• Low chemistry shrink of the resin in the phase from gel to fully cured
• Low solvent shrink; means little or no solvent in the resin during the phase from
gel to fully cured
• Low temperature shrink; means curing at low temperature
I will try to explain effect of the above mentioned parameters with examples of real
world coatings
Nitrocellulose based
Was the coating used on early mass produced automotive bodies. Has low solid content
(≈ 20%) and still lots of solvent left in gel; thus, overall high shrinkage and low leveling
capability. With these coating materials multiple layers had to be applied to achieve an
acceptable gloss level.
Linseed oil paint, air dry; as used to paint wood or metal with brush.
Slowly drying (oxygen cross-linking) oil. Little or no solvent left in gel, no temperature
shrink, and low chemical shrink. Has good leveling capability. One coat provides good
gloss, but dries very slowly and is thus rather unsuitable for an industrial process.

4. High temperature bake enamels, e.g. Polyester/Acrylic/Melamine
Has considerable shrinkage due to chemistry and high temperature processing, but still
leveling capability is typically much better than with Nitrocellulose, especially when
formulated as “high solid”.
Two part Urethane or Epoxy
Very low shrinkage due to chemistry and processing at low temperature, typically
provide excellent leveling performance.
Radiation curing –UV and EBC
Moderate chemical shrinkage, usually not as low as Epoxy or Urethane; however,
provides very good leveling due to low processing temperature and low or no solvent
content.
Powder coating
Chemical shrinkage can be low to moderate pending the type of resin. High temperature
processing is responsible for some shrinkage which is offset by total absence of solvent;
typically good leveling performance.
The leveling capability of a coating is one of its prime attributes and it is high demand for
many applications. Modern coatings typically have a much higher leveling capability
than coatings used in the past which for example is not only the reason for the very high
gloss of today’s car body coatings but also for a more effective coating process compared
to what was done in the old days.
It is obvious that the leveling capability of a metalizing base coat is of utmost importance
since it is the high gloss of surface where the metal is being deposited which is
responsible for the mirror-like appearance.
Is it possible to measure the leveling capability of a coating and in this way distinguish
between lesser or better materials when developing a new product? To a degree, yes.
I once did it quite successfully with the aid of a resolution test picture similar to this one.
It was mirrored over a (standard) sheet metal sample plate which
was coated and metalized with the test material. The higher the
resolution is, the more leveling capability has the coating.
For documenting and comparing results, the mirrored test picture
can be photographed.

5. Hardness or the mechanical modulus of a base coat
This is a critical aspect of a base coat’s properties. Why?
In vacuum metalizing the metal is deposited out of the vapor phase; this is why the
process is called PVD which means Physical Vapor Deposition. Rather independent of
how the part of evaporation is achieved, the metal vapor condenses everywhere within
the vacuum chamber as long as the condensing area is positioned in “line of sight” to the
evaporation source. (The “line of sight” thing is a key element of the PVD process in
high vacuum; the number of particles –be it molecules or atoms- in the gas space is
greatly reduced compared to ambient pressure which is why the metal atoms can “fly”
directly from the evaporation source to the condensing area which actually makes the
metalizing process possible in the first place.)
Pending on the type of metal which is evaporated/deposited different mechanical
properties of the deposited metal layers will be obtained – independent from the inherited
mechanical properties of the metal.
Example Chromium
The metal atoms condense out of the high energy vapor phase and arrange themselves to
a solid structure which once fully developed eventually occupies a smaller volume than
when the layer starts to build up, the layer wants to shrink. If the layer of chromium can
shrink because the modulus (hardness) of the substrate –the base coat- is low enough, the
basecoat will crack. This is visible in the form of many micro cracks or even large ones
popping nastily into the eye. Thus only if the base coat is hard enough, an acceptable
metalizing with chromium can be obtained; and not only must the basecoat’s modulus be
sufficient at ambient temperature but over the entire range where the metalized part shall
be used, otherwise the cracks will appear once the basecoat softens. (Details of the
change in modulus are explained below). The chromium layer has tensile stress.
Example Aluminum
The metal atoms condense out of the vapor phase and form a solid structure. But due to
the aluminum’s affinity to oxygen and here especially water something else also happens.
The remaining gas at high vacuum level is consisting mostly of water (vapor). This water
is adherent on the vacuum chamber walls, all the internal chamber devices and off course
is also in and on the substrate surface. Now, if an aluminum atom and a water molecule
hit each other during the condensation process, an aluminum oxide/hydroxide-compound
is being created and that occupies a larger space than an aluminum atom alone. The result
is that the final metal deposit layer wants to be larger than the one initially formed, the
layer wants to expand. If the metal layer is allowed to expand, it deforms the substrate’s
surface to wavy structures. These “waves” typically have a wavelengths in the range of
λ/4, the wavelengths of visible light and such a deformed surface suddenly appears in all
the rainbow colors due to light interference. In metalizing technology this effect is called
iridescence. Thus, only if the substrate’s –usually the base coat – modulus is high enough
to withstand the metal layer’s desire to expand, a successful metal deposition can be

6. obtained and same as in the case of chromium the modulus needs to be high enough over
the entire temperature range in which the metalized part shall be used.
The aluminum layer has compression stress.
Iridescence occurs when the aluminum layer warps the surface of the basecoat because of
too much aluminum-oxide/hydroxide in the metal layer and/or a basecoat with too low a
modulus.
If iridescence happens only at elevated temperature,
then the basecoat softens too much at this temperature to
still withstand the metal layer’s tendency to expand.
The warped base coat and thus the rainbow colored surface
remain irreversible as long as the metal layer is present.
If one would remove the metal, e.g. with acid, and reheat
the coating above its glass transition temperature, the
coating surface pulls itself flat again. After metalizing
there is no more iridescence. More about the mechanical properties vs. temperature in the
next chapter.
The modulus of a coating
When developing a basecoat for a specific application it helps tremendously if one has
the capability to measure the mechanical properties of the cured coating over its entire
temperature range in which the metalizing eventually shall perform.
One will see how pending on the formulation the modulus changes with temperature, e.g.
glass transition points and its scale become visible. This glass transition temperature is
often the point where the metalizing starts to fail, be it cracks or iridescence.
Here are some graphs of e-modulus and elongation under stress vs. temperature
(these are e-modulus/elongation charts of actual coating formulations measured at a time when PCs were not available
everywhere, thus the old style graph created on paper by hand)

7. graph #1,2,3 is a coating formulation with
increasing amount of a softer resin type. The result
is a decrease of the glass transition temperature
and the consequence that the iridescence
resistance falls from 140°C to 120°C and
eventually to 100°C.
graph 4 is a formulation where the Tg drop due to
the selected co-resins is so little that iridescence
barely occurs even above Tg.
graph 5 is a radiation cured base coat; it shows
that the modulus over the observed range is
virtually unaffected by temperature. Infect, a base
coat like this one is unknown for iridescence.
The actual stress level in the metal which would cause cracks or iridescence is unknown
and probably very difficult to measure; but in case of iridescence with aluminum it
became known that the level has to be somewhere in the range of about 100 N/mm²,
because a basecoat with a modulus above 100 rarely fails. Here it has to be mentioned
that the actual obtained modulus data depend very much on the applied measurement
method (I measured elongation vs. applied load on free film), also that there are different
moduli such as elastic and shear and I remember that I obtained results composed of both
elasticity and shear modulus. But in this case it was not important to measure the absolute
level but to have a tool to explain why certain formulations produce coatings which fail at
specific temperatures when others do not. Also it has to be mentioned that the metalizing
process itself has an effect on the tendency to iridescence as well. Substrates with higher
humidity content produce more iridescence than those with dryer substrates. Also if one
would pump down deep and long enough to remove virtually all water out of chamber
and substrate, then little or no aluminum - oxide/hydroxide would be formed with the
result of less compression stress in the aluminum. This however would be rather
unpractical in actual metalizing operation because it can take forever.
Iridescence with aluminum as well as cracking with chromium or stainless steel used to
be more of a problem than it is today. The reason is not only that the issues of vacuum
metalizing and the mechanics of base coats are understood more and more, but it is

8. especially due to UV curing technology which today is widely applied for coatings of
vacuum metalizing. See the mechanics of the UV cured coating in graph #5
Why do I explain this complex issue of coating modulus, cracks and iridescence when the
UV coatings seem to have solved all the problems?
The basic principle of the coating’s modulus and stress in the metal applies to the UV
coatings as well and one very easily can formulate UV (or general radiation curing)
coatings with all the mechanical problems. Infect, if there is a problem with a bad coating
batch, insufficient curing or out of control temperature, iridescence or cracking suddenly
may occur even with UV coatings and then it helps to understand what the factors are that
contribute to the problem. Also, for a variety of reasons UV coatings may not be
available or suitable for the specific application and then all the shortcomings of
“conventional” coating materials are there to deal with.
Other effects and/or problems with metalizing base coats
Cloudy or hazy appearance of the metalizing
The haze may have its origin in the metalizing process itself, which is possible, but
usually the root cause of this problem lies in the base coat. If the chosen components for
the base coat are not completely compatible with each other to form a crystal clear matrix
over the entire applied thickness range, then the metal layer on this coating will have a
hazy appearance. The chosen resins itself may be the cause for this “incompatibility”,
incompatible solvents, solvents which are trapped and did not evaporate prior to curing,
inappropriate curing conditions, contaminations in the coating material, additives which
are part of the formulation for a specific purpose but unfortunately as a side effect create
haziness.
A simple test to check for haziness in the base coat is to create a free film or apply and
cure the coating on clear glass. If there is the slightest haziness visible within thin or
thicker sections of the otherwise clear coating, expect to see a hazy metal applied to this
coating.
No adhesion of the metal to the base coat, the metal washes of in flakes when
exposed to humid environment
Typically base coats provide good adhesion to the metal. Some resins in the coating’s
formulation may not promote adhesion, but this case is rather rare. The culprit for poor
metal adhesion is most likely found in additives which are supposed to improve the
coatings surface and/or the coatings capability to wet out the substrate. Not all these
additives come with this negative side effect, but a lot of them do. To make things more
confusing, the metalizing process itself also has an effect on adhesion, primarily that

9. what is done during a plasma treatment. For example: A plasma etching step can offset
the otherwise negative side effect of certain flow additives in the base coat; thus this base
coat works for the one who has the plasma process available but it would not work for the
one who does not have it. Also, the plasma process may be out of control and then
suddenly strange failures occur which have a root cause in the base coat, but were
covered up most of the time by the plasma process, be it intentionally or unintentionally.
Discolored metal, yellow or brownish with aluminum
This can be trapped solvent in the base coat. Since the metal is evaporated/condensed in
high vacuum, trapped solvent would outgas from the base coat with the result that the
metal is deposited on this outgassing surface at too high a pressure level and this metal
would not have its typical color anymore. But there are other process conditions which
can make discolored metal as well.
Conclusion
This was basically a description of the key elements in a metalizing base coat. It may be
helpful for selecting basecoats as they are offered by a few suppliers of such special
coating material. It will be helpful for the one who has to troubleshoot problems coming
up in the entire metalizing process where it may guide to a root cause in the coating.
Definitely will the knowledge of details as described above enable the developer of the
coating material reach the goal faster and probably with a better product. Understanding
the base coat also improves the efficient development of an entire metalizing process, or
of a new product which could be a component in an automotive headlamp that is exposed
to very high temperature and made of a material previously never used. Another
application where the base coat plays a major role is using sputter chromium or sputter
stainless as replacement for electroplating which has its environmental concerns.
Of course, there are more aspects to the base coat than I covered in this article such as:
- Solvents; much, little, none, water
- Application technique; spray, flood, spin, etc
- Adhesion to substrate and metal
- Barrier function between substrate and metal
- and others
These usually apply to most other types of coating material as well and in this article I
wanted to focus on those aspects which specifically categorize the base coat. I hope that
the paper was understandable to the reader and that it can provide help to the one who has
to deal in real life with this rather exotic material.