The Base Coat The coating which carries the aluminum, chromium or other metals deposited in a high vacuum processAbstractHigh vacuum metalizing is the method to efficiently apply a thin metal surface on almostany substrate. In order to give this metal surface a specific texture – usually high gloss- alacquer based coating is applied to the substrate with the main purpose that this “Basecoat” 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 chromiumas 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 isall about and goes into specifics with those problems in metalizing like:Rainbow colored aluminum which is called iridescence, cracking chromium, that whatmakes gloss, adhesion or loss of adhesion.Some basic Physics and ChemistryLeveling or how to make a rough substrate glossyHere we are not talking about sanding and polishing; that would be a way to make a solidmetal piece glossy. This is about making a high gloss surface by coating it with lacqueror varnish. Actually, in the past there often was some kind of sanding prior to basecoating, but this was done because those base coats in the past did not have the levelingcapability 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 theaverage peak and valley of the surface texture is 10µ m. Apply a wet coating of 30µm tothis surface and let it level out. The liquid covers the roughness entirely and the surfacehas 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 metalsurface, but definitely not glossy.Ok, 30µm was not enough; 50µ m of coating is applied, that should take care of theroughness.It will not. What happened?Our model coating may consist of 50% solids and 50% solvent; so, after all the solventhas evaporated out of the 30µm initial wet layer, a 15µm thick layer of pure resin, -notliquid 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.
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 molecularresin to hard and cross-linked macromolecule. The shrinking happens at 50% above thepeaks and at the same rate above the valleys with the result that the original roughness isjust reduced by 50%. Off course, applying 50µm of the same model coating would notmake 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 theheight of the still protruding peaks. This is exactly what was done in the past when horsedrawn carriages received that high gloss black paint finish and it was still so in the earlyyears of automotive body coating.
Leveling capability of a coating with low shrinkageWith these above mentioned model coatings in mind it becomes obvious that to obtain ahigh 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 temperatureI will try to explain effect of the above mentioned parameters with examples of realworld coatingsNitrocellulose basedWas 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 levelingcapability. With these coating materials multiple layers had to be applied to achieve anacceptable 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 temperatureshrink, and low chemical shrink. Has good leveling capability. One coat provides goodgloss, but dries very slowly and is thus rather unsuitable for an industrial process.
High temperature bake enamels, e.g. Polyester/Acrylic/MelamineHas considerable shrinkage due to chemistry and high temperature processing, but stillleveling capability is typically much better than with Nitrocellulose, especially whenformulated as “high solid”.Two part Urethane or EpoxyVery low shrinkage due to chemistry and processing at low temperature, typicallyprovide excellent leveling performance.Radiation curing –UV and EBCModerate chemical shrinkage, usually not as low as Epoxy or Urethane; however,provides very good leveling due to low processing temperature and low or no solventcontent.Powder coatingChemical shrinkage can be low to moderate pending the type of resin. High temperatureprocessing 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 formany applications. Modern coatings typically have a much higher leveling capabilitythan coatings used in the past which for example is not only the reason for the very highgloss of today’s car body coatings but also for a more effective coating process comparedto what was done in the old days.It is obvious that the leveling capability of a metalizing base coat is of utmost importancesince it is the high gloss of surface where the metal is being deposited which isresponsible for the mirror-like appearance.Is it possible to measure the leveling capability of a coating and in this way distinguishbetween 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 whichwas coated and metalized with the test material. The higher theresolution is, the more leveling capability has the coating.For documenting and comparing results, the mirrored test picturecan be photographed.
Hardness or the mechanical modulus of a base coatThis 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 theprocess is called PVD which means Physical Vapor Deposition. Rather independent ofhow the part of evaporation is achieved, the metal vapor condenses everywhere withinthe vacuum chamber as long as the condensing area is positioned in “line of sight” to theevaporation source. (The “line of sight” thing is a key element of the PVD process inhigh vacuum; the number of particles –be it molecules or atoms- in the gas space isgreatly 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 themetalizing process possible in the first place.)Pending on the type of metal which is evaporated/deposited different mechanicalproperties of the deposited metal layers will be obtained – independent from the inheritedmechanical properties of the metal.Example ChromiumThe metal atoms condense out of the high energy vapor phase and arrange themselves toa solid structure which once fully developed eventually occupies a smaller volume thanwhen the layer starts to build up, the layer wants to shrink. If the layer of chromium canshrink because the modulus (hardness) of the substrate –the base coat- is low enough, thebasecoat will crack. This is visible in the form of many micro cracks or even large onespopping nastily into the eye. Thus only if the base coat is hard enough, an acceptablemetalizing with chromium can be obtained; and not only must the basecoat’s modulus besufficient at ambient temperature but over the entire range where the metalized part shallbe used, otherwise the cracks will appear once the basecoat softens. (Details of thechange in modulus are explained below). The chromium layer has tensile stress.Example AluminumThe metal atoms condense out of the vapor phase and form a solid structure. But due tothe 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 wateris adherent on the vacuum chamber walls, all the internal chamber devices and off courseis also in and on the substrate surface. Now, if an aluminum atom and a water moleculehit each other during the condensation process, an aluminum oxide/hydroxide-compoundis being created and that occupies a larger space than an aluminum atom alone. The resultis that the final metal deposit layer wants to be larger than the one initially formed, thelayer wants to expand. If the metal layer is allowed to expand, it deforms the substrate’ssurface to wavy structures. These “waves” typically have a wavelengths in the range ofλ/4, the wavelengths of visible light and such a deformed surface suddenly appears in allthe rainbow colors due to light interference. In metalizing technology this effect is callediridescence. Thus, only if the substrate’s –usually the base coat – modulus is high enoughto withstand the metal layer’s desire to expand, a successful metal deposition can be
obtained and same as in the case of chromium the modulus needs to be high enough overthe 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 oftoo much aluminum-oxide/hydroxide in the metal layer and/or a basecoat with too low amodulus.If iridescence happens only at elevated temperature,then the basecoat softens too much at this temperature tostill withstand the metal layer’s tendency to expand.The warped base coat and thus the rainbow colored surfaceremain irreversible as long as the metal layer is present.If one would remove the metal, e.g. with acid, and reheatthe coating above its glass transition temperature, thecoating surface pulls itself flat again. After metalizingthere is no more iridescence. More about the mechanical properties vs. temperature in thenext chapter.The modulus of a coatingWhen developing a basecoat for a specific application it helps tremendously if one hasthe capability to measure the mechanical properties of the cured coating over its entiretemperature 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 isoften 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 availableeverywhere, thus the old style graph created on paper by hand)
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 unknownand probably very difficult to measure; but in case of iridescence with aluminum itbecame 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 mentionedthat the actual obtained modulus data depend very much on the applied measurementmethod (I measured elongation vs. applied load on free film), also that there are differentmoduli such as elastic and shear and I remember that I obtained results composed of bothelasticity and shear modulus. But in this case it was not important to measure the absolutelevel but to have a tool to explain why certain formulations produce coatings which fail atspecific temperatures when others do not. Also it has to be mentioned that the metalizingprocess itself has an effect on the tendency to iridescence as well. Substrates with higherhumidity content produce more iridescence than those with dryer substrates. Also if onewould pump down deep and long enough to remove virtually all water out of chamberand substrate, then little or no aluminum - oxide/hydroxide would be formed with theresult of less compression stress in the aluminum. This however would be ratherunpractical in actual metalizing operation because it can take forever.Iridescence with aluminum as well as cracking with chromium or stainless steel used tobe more of a problem than it is today. The reason is not only that the issues of vacuummetalizing and the mechanics of base coats are understood more and more, but it is
especially due to UV curing technology which today is widely applied for coatings ofvacuum metalizing. See the mechanics of the UV cured coating in graph #5Why do I explain this complex issue of coating modulus, cracks and iridescence when theUV coatings seem to have solved all the problems?The basic principle of the coating’s modulus and stress in the metal applies to the UVcoatings 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 coatingbatch, insufficient curing or out of control temperature, iridescence or cracking suddenlymay occur even with UV coatings and then it helps to understand what the factors are thatcontribute to the problem. Also, for a variety of reasons UV coatings may not beavailable 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 coatsCloudy or hazy appearance of the metalizingThe haze may have its origin in the metalizing process itself, which is possible, butusually the root cause of this problem lies in the base coat. If the chosen components forthe base coat are not completely compatible with each other to form a crystal clear matrixover the entire applied thickness range, then the metal layer on this coating will have ahazy 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 whichare part of the formulation for a specific purpose but unfortunately as a side effect createhaziness.A simple test to check for haziness in the base coat is to create a free film or apply andcure the coating on clear glass. If there is the slightest haziness visible within thin orthicker sections of the otherwise clear coating, expect to see a hazy metal applied to thiscoating.No adhesion of the metal to the base coat, the metal washes of in flakes whenexposed to humid environmentTypically base coats provide good adhesion to the metal. Some resins in the coating’sformulation may not promote adhesion, but this case is rather rare. The culprit for poormetal adhesion is most likely found in additives which are supposed to improve thecoatings surface and/or the coatings capability to wet out the substrate. Not all theseadditives come with this negative side effect, but a lot of them do. To make things moreconfusing, the metalizing process itself also has an effect on adhesion, primarily that
what is done during a plasma treatment. For example: A plasma etching step can offsetthe otherwise negative side effect of certain flow additives in the base coat; thus this basecoat works for the one who has the plasma process available but it would not work for theone who does not have it. Also, the plasma process may be out of control and thensuddenly strange failures occur which have a root cause in the base coat, but werecovered up most of the time by the plasma process, be it intentionally or unintentionally.Discolored metal, yellow or brownish with aluminumThis can be trapped solvent in the base coat. Since the metal is evaporated/condensed inhigh vacuum, trapped solvent would outgas from the base coat with the result that themetal is deposited on this outgassing surface at too high a pressure level and this metalwould not have its typical color anymore. But there are other process conditions whichcan make discolored metal as well.ConclusionThis was basically a description of the key elements in a metalizing base coat. It may behelpful for selecting basecoats as they are offered by a few suppliers of such specialcoating material. It will be helpful for the one who has to troubleshoot problems comingup 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 thecoating material reach the goal faster and probably with a better product. Understandingthe base coat also improves the efficient development of an entire metalizing process, orof a new product which could be a component in an automotive headlamp that is exposedto very high temperature and made of a material previously never used. Anotherapplication where the base coat plays a major role is using sputter chromium or sputterstainless 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 othersThese usually apply to most other types of coating material as well and in this article Iwanted to focus on those aspects which specifically categorize the base coat. I hope thatthe paper was understandable to the reader and that it can provide help to the one who hasto deal in real life with this rather exotic material.