For

Vacuum-based technologies have always bad the potential of replac- ing electrodeposited coatin@ for

another process that can build thick coatings). A cathodic arc plasma deposition system bas been devel- oped that can achieve decorative as well as functional (corrosion resistance and wear/abrasion resistance) coatings. This commer- cially available, computer-com- manded and PLC-controlled system provides a bigb degree of versatility and capability, and can coat a quantity of large-area, three- dimensional objects. Vacuum Plating Technology, San Jose, CA,

presented during AESF's '96 technical conference,

recent health and environmental concerns with hexavalent chromium, however, there are several govem- mental programs charged with evaluating various technology alternatives to hexavalent chromium.

As reported in numerous artic1es.l vacuum-based technologies have always had the potential of replacing

carried out by a number of research groups around the world, and some novel and striking advances have been reported. Researchers at Vacuum Plating Technology Cop. have developed commercial applications

for decorative coatings. in vacuum arc technology

ompanied by a deeper

The following is a short overview f some fundamentals as compared to

tional electrodeposition

readers. For comparison, a simplified diagram of a cathodic arc plasma process with the basic components is shown in Fig. 2. These components

e core of the cathodic arc ess are the arc spots that, when

ignited by a mechanical striker, erode the consumable cathode. Metal ions, electrons, microdroplets and neutral particles are emitted from solid cathodes by small and intense arc spots that rapidly traverse the surface, either randomly or under the influence of magnetic fields. A high percentage of particles eroded from the cathode consists of ions. The average kinetic energies of the ions are in the 1CL100 eV range. The phenomenon of vacuum arc ion source has been studied by many investlgators and has formed the basis for some well- established technology? Similarities to electroplating include:

~

~

~

A cloud of plasma is formed through evaporation of the arc

Nicholas P. tiberto. P.E. Powder coating Consultants DIvIsion of Ninan. Inc. 1529 Laurel Avenue Bridgeport. CT 06604 203/366-7244 * FAX 203/367-5245

Editor’s Note: This new monthly column will address, in “questiorr- and-answer” format, issues relating to the powder coating industry, such as system design, coating selection, features, etc. Please e-mail (aesfi journal@worldnet.att.net) or send your questions to “Powder Coating Forum,” 12644 Research Pkwy., Orlando, FL 32826-3298.

Question What are the main functional differences of epoxies, urethanes, TGIC polyesters and hybrids?

Answer There 6an be many subtle differences among all these coatings, and many features can be formulated into each coating. The general functional differences, however, are as follows:

Epoxies have excellent chemical resistance but have no UV (weathering) resistance. Urethanes have great weathering characteristics, but do not have good flexibility or impact resistance. TGIC polyesters have good weathering characteristics but reduced chemical resistance.

* Hybrids is a term that describes a mixture of resin technologies th can have all sorts of properties One of the most common hy rids is an epoxy-polyester that as most of the properties o / ure epoxy coatings, as well as some of the added UV resistance of polyester coatings.

There are many more features and properties in each of these coatings that should be discussed with a powder coating materials supplier. Selecting the right powder coating for your process requires you to list and discuss with a competent supplier the

i I

by your customer. Powder ppliers have a great amount ‘ty in formulating these

roperties into a coating our expectations, as tomer’s. Some

ance criteria. It

08, CMS 565-09, CMS 565-lo)?

Answer I suggest that you contact the Powder Coating Institute in Alexandria, VA, at 703/684-1770. They may have an answer for you, because they keep track of powder coatings that meet military and commercial standards. Their membership directory includes names of powder coating formulators that may have approved materials for your specific use.

Questlon Are there any products that can be used as a “filler” to hide imperfec- tions or to fill seams in cold-rolled steel prior to powder coating? We

are currently using a polyester TGIC powder coating with a cure schedule of 400 “F for 10 min.

Answer Standard fillers will not work under powder coatings, because they will outgas during cure of the powder. In addition, because these materials are nonconductive, they will not attract the lectrostatically charged powder c ating sprayed onto the surface. I do not know of any metal-based fillers available on the market that will give you what you want. Some fillers have metal in them, but they still have resins and hardeners that might cause outgassing, so should be thoroughly tested before using. You might consider a colored caulk that can be applied after the powder coating has been cured to fill unsightly areas. Welding and grinding the defected area has proven to be the most appropri- ate method of correcting metal defects prior to powder coating. If the metal being used is not a heavy enough gage to handle this, then the part will probably have to be scrapped. P ~ S F

,.&

the Columnist ick Liberto is president of Powder ating Consultants, a division of

Nihyn, lnc., 1529 Laurel Ave., Bridgeport, CT 06604. He has more than I7 years’ experience in the powder coating industry, holds a BS in mechanical engineering, and is a registered Professional Engineer in the State of Connecticut. He is a memher of The Powder Coating Institute, the Chemical Canters Association and the Association of Finishing Processes division of SME. He is the editor for and contributing author to the award-winning Powder Coating: The Complete Finisher’s Handbook, and has written numerous articles and columns on powder coating.

March 1997 15

g l-Pl01mng cell

spots when the substrate in the chamber, which is immersed in the plasma, is similar to the substrate immersed in the chemical solution.

* Cathode power supply, usually in DC mode, is characterized by low voltage (about 20 V) and high current (typically 35-500 A). An anode is a conductive device around the cathode, or the chamber wall acting as an anode. The typical range of substrate bias potential is 0-300 volts refer-

deposition in the past included a wide choice of both substrates and coat- ings. The disadvantages, however, far outweighed the advantages of aqueous deposition, which were: Lower cost8, thicker coatings, coating complex shapes, control and modifi-

I

e structure of surfaces and

Fig 2-Cathode arc plasma PVD principle

interfaces. Surface films of a variety of elements, compounds and alloys can be deposited.

Multilayer structure is receiving increasing attention. With multiple cathodic arc sources, some with different metal species, variation of pulse length of the metal plasma pulse, and phasing of the high-voltage bias pulse with respect to the plasma pulse-all of these parameters can be tailored throughout the duration of the surface processing operation to fabricate a wide range of surface structures. By adding a reactive gas flow to the deposition and implanta- tion processes, the variety of films that can form is greatly expanded. In addition to metallic films, for ex- ample, films of compounds, such as ceramic oxides and carbides can be formed.

Cathodic arc plasma technology is, therefore, very versatile. The bonding transition zone between a substrate and film can be tailored to provide a good match with mechanical proper- ties with strong adhesion and low residual stress.

Control & Modification Of Deposit Properties The ionization efficiency for vacuum arc plasma (-70 percent) is signifi- cantly higher than other PVD pro- cesses, such as sputtering and evaporation (2-8 percent). High ionization efficiency and high ion energies involved result in coatings

anced adhesion and density. , these characteristics allow

su bias voltage, gas pressure an tenstlcs are fairly constant for external currents ranging from 50 to 100+ amps. These features contribute to control panel design simplicity as an industrial tool. As a result, a computer-commanded and PLC- controlled unit was integrated into the cathodic arc system.

The benefits of ion bombardment of growing films are well established. They include increased surface mobility of adatoms, formation of dense film structures, good adhesion, desorption of weakly bound species, etc. The energy delivered by ion bombardment allows good films to be grown at low substrate temperatures. The effect of ion bom- bardment is similar to that of elevated substrate temperature in CVD, and evaporation or sputter deposition is required in order to achieve adhesion.

Deposition from solutions in which the metallic ions are combined with other ions or ligands as complex ions involves more complicated mecba- nisms. In the plating process, the chemistry of the electrolyte is a time- and space-dependent, complex function. The current potential changes in relation to the resistance that builds up over the cathodic surface. Advanced integrated sensors map the conductivity of the plating solution, and computerized intelligent inference techniques study the cbemis- try of the plating solution. Other factors, such as electrical contact, volume, temperature, shape and tank size are collected as system influences.

the resistance or conductivity and

ng rate. Vacuum arc charac-

With all the above data collected,

17

me back side of the coming chamber, vacuum pump ~ y ~ r e m nnd power supply vlysrem

linear behavior of the system can be determined. Staff chemists are required to continually monitor the system behavior, and/or a commer- cially available computerized plating controller can sense the plating system's reaction to the influx current, responding with a pre- programmed complex form of voltage and current to eliminate the effects of the shielding layer.4

Coating C Shapes In sharp co conventlonal ion beam processing methods, this PVD technology is not strictly a Iine-of- sight method. In conventional vacuum

on or sputtering techniques, ple, the straight, rigid

tdmanipulated in order to s the entire surface. This is

conductor surface-

y diminished, if not oved. In the current

I8

system, a planetary rotation fixturing system is sufficient to achieve the desired uniformity.

For fixed-arc current and deposition chamber pressure, the front- sidehack-side thickness ratio depends on the distance between the cathode and the substrate. As reported: at a deposition chamber pressure of 10" Pa, the thickness of

Plan View - CAF

ig 3-A eommercral-wed corhodrc ( I T ~ plasma (CAP) syblem

TiN coatings on SUI-^^ ~:es facing away from a cathode (back side) is nor- mally 10 percent of that registered on surfaces facing the cathode (front side). For a deposition chamber pressure of 6 Pa, the ratio of front- side to back-side coating thicknesses approaches unity.

Thicker Coatings Deposition rates for cathodic arc deposition technology vary widely- from nanometers per minute to tens- of-micrometers per minute. The deposition rate required will depend on such factors as the desired film morphology, the substrate tempera- ture limitations. and whether the

'al being deposited I F a metal or

PLATING &SURFACE FINISHING

to the uniformity of the coatings, Typical deposition rates observed for chromium-as high as a w h i n with good adhesion-are achievable under industrial production conditions,

The deposition sate for vacuum arc plasma is directly related to the arc current supply. The inherent feature of macroparticles-small droplets of micrometer size-becomes volatile, however, when the arc current increases. Macroparticles are pro- duced at the cathode spots along with the plasma, and the contamination of the films by macroparticles of sizes up to a few pm represents a major obstacle to the broad application of vacuum arc plasma deposition. Many attempts have been made to separate the plasma and macroparticles, among which filtering of the plasma by magnetic fields is the most common."

The underlying idea of filtenng is to guide the vacuum arc plasma from the cathode spots to the substrate by a magnetic field. The macroparticles cannot reach the substrate because they move along nearly straight trajectones, and their inertia is lost to the duct walls. l l u s magnetic duct design will add to the complexity of the system, as well as a d in reducing the deposition rate.

The degree of contamination by macroparticles allowed depends on

r decorative percentage of

oes not seem to onsumers. In deposition of e macroparticles are integrated into the coating in

such a manner that it is difficult to distinguish them from coating material that arrived as ions, because the ions amve at the substrate with high energy and dense paclang.

The team at Vacuum Plating Technology Corp. uses a propnetary metal shield device to simply block the macroparticles. The deposition rate, therefore, is also reduced. In summary, this problem can he solved through a compromise between deposition rate, degree of macro- particle contamination allowed, and complexity of the system design.

PVD chromium as a r hexavalent chronuum

m large part on the difference the cost of cathodic arc s. electrodeposition. Vacuum efficient means of generating sma. Cathodic arc PVD

s involve large pieces of ment and are designed to apply lic coatings to large substrate . Usually it is a hatch process

as short as 30 min. uipment is shown

unfortunately, is

tion has increased significan f Duct Bias on Transport of could continue to increase w probability of tighter EP

Vacuum Arc Plasmas Throu

produced. In conclusion, the cathodic arc

plasma deposition process offers g potentlal as a means of modifying t surfaces of components facilitating the depositlon of decorative and functional coating with unique properties. Pasf

References I J . Lindsay, “Special Conference

Comes to Grips with Hexavalent Chromium,” Pk 19 (Feb. 1995).

t

e cost of electrodeposi-

2. Handbook for Deposition Technologies for Films and Coatings. R. Bunshah, Ed., Noyes Publica- tions (1994).

3. S. Anders, A. Anders & I. Brown, “Vacuum Arc Ion Sources: Some Vacuum Arc Basics and Recent Re- sults,”Rev. Sci. Instrum. 65,4 (April 1994).

4. Company brochure, Environmental Plating Tech- nology, Inc.

5 . J.E. Daalder, “Cathode Spots and Vacuum Arcs,” Phys.

r

Flee Deloili: Circle 108 on postpoid reodel Service cord.

19

230 p.m.-Break

245 p.m.--l(epair of Metal Coatings Using Environmentally Compliant Rrush.plating Solutions K.R. Eddwin d C.J.E. Smith. Sliuciu~al Morerialr Centre, Defence Reseorch Agency (DRA), Fernborough, HompThire, UK

3 1 5 pm-Improving Test Methods for Hydrogen Embrittlement John GminXur, Afochrm North America, Inc., Long Beach. CA

Practical Applications B. Bodger, Soulhwest Aeroservice, Inc., Tulsa, OK

2 3 0 p.m.-Environmental Eeonomies in tho Aerospace Industry Dorin G. Chiidrrs, CHMM, Bell Helicopler Turron rnc., FI. worth, ix

3 p.m.--RemovsI of Nickel-based Braze Alloys from Gas Turbine Compressor Stators Roben Genereou. Meld Finishing Technologier, rnc.. ~ ~ ~ ~ . ~ i i i ~ , CT

3 3 0 p.m.-Rreak

1-1:30 p.m.-Buses depapoll for UniledAiriines Tour

k30-330 p.m.-Tour of United Airlines Je t Engine Shop & Surface Tachnology Group No~Ticketsforthisplanttaurarc525. TicketrWILL NOTbeavailableonrite.(ifyoudon'twnnt tomiss this impanant tour, your registration and payment mu81 bo received by Monday. March 17.)

3 3 0 p.m-Boord buses for ielnrn 10 hold or dd ivsq lo airport mminol

20 PLATING B SURFACE FINISHING