Vacuum Coating: A Clean Technology

It is only natural that a clean and environmentally friendly technology such as Low Temperature Arc Vapor Deposition (LTAVD) was developed in Boulder, Colorado. Here many people ride bicycles to work and school. A clear running creek winds through the town, and the Rocky Mountains provide a perfect backdrop for this growing city.

In 1981, Vapor Technologies of Boulder, a division of Masco Corp., developed LTAVD along with other thin film coating techniques. The present company was incorporated in 1988, and the first major commercialization of the processes and equipment started in 1989. Since that time, Vapor Technologies has continued to thrive and grow.

The LTAVD process is the technical centerpiece of its business. The process deposits a variety of functional and decorative metallic coatings on virtually any substrate from metals to plastic. It is used for automotive decorative styling and functional components, medical devices and instruments, EMI/RFI/ESD shielding on plastic electronic housings, functional coatings for aerospace applications and decorative coatings on household hardware, jewelry and plastics. Obviously, the parts can vary widely in shape, size and configuration.

LTAVD is a physical vapor deposition process that employs a high-current, low-voltage electric arc to evaporate essentially any electrically conductive material. The material to be deposited is fashioned as a cylindrical source on which the electric arc is ignited. The deposition process is operated in a highly controlled vacuum to deposit adherent, dense, thin-film coatings.

LTAVD has a number of advantages over other physical vapor deposition techniques, as well as benefits not found in chemical vapor deposition. LTAVD coatings are highly adherent as well as wear and corrosion resistant. Also, LTAVD can coat at room temperature or higher and does not add significant thermal energy (heat) to the substrate. Heat can be detrimental to parts, causing loss of temper, deformation or a change in crystal structure.

The process is highly productive; deposition occurs over a 360 degree field rather than a 180 degree field. Thus, parts being coated are always in the coating plasma, a significant advantage for film quality and deposition rate.

Electrodes can be configured in any shape necessary to "fit" the parts being coated. When a planetary fixture is not feasible, the electrode can be configured to coat the substrate without moving the substrate, such as a pipe coated on the inside and outside with a single electrode. However, both the substrate and electrode can be moved if necessary. Electrodes can be made from solid sintered, cold-pressed powder ingot of any metal, alloy or carbon class materials.

Because there is little waste heat and low thermal radiation, the process has the ability to deposit materials with high melting points onto substrates with low melting points. So, high-melting-point metals like tungsten (3410C) can be applied to plastic films without using a heat sink and without damaging the substrate. The energies of the metallic ions ejected from the electrode are high (60 to 100 electron-volts/ion). The ionization percentage is excellent, greater than 90 pct, which allows for good mechanical adhesion even if the substrate temperature is ambient.

When substrates are heated, there is good mechanical adhesion, depending on the substrate. Elevated temperatures do not interfere with the process, so the temperature of the substrate is totally controllable.

Because the arc travels on the electrode surface, its speed is controllable. The area to be coated can be fine tuned from a narrow area to a full 360 degrees of coverage. Coating uniformity is provided by the uniformity of the arc's traverse across the surface of the electrode, with a spent electrode appearing as if it has been uniformly eroded or milled.

Coating rates and uniformity are excellent. Deposition rates are rapid. Film thickness varies from 300 angstroms to a few mils, depending on what is specified.

Deposition rates are inversely proportional to distance. With high melting point, high-density metals and ceramics, deposition rates are lower. The erosion rate for nickel is more than one gram per min per electrode. A typical uniformity specification is plus or minus five pct for thin-film processes. Large areas or volumes can be coated uniformly because of the high rate of deposition, capacity for large target sizes and the large vacuum chamber.

Alloy Coatings. Dissimilar materials can be alloyed using LTAVD. Materials not commercially available can be deposited using the process. This requires synthesis of a target, then deposition using LTAVD. This is useful for compounds such as titanium/aluminum and others that have different melting and evaporation points.

The purity of the deposited materials is the same as the starting materials. Alloys preserve their composition through the coating process. Ceramic materials can be synthesized by evaporating a metal or alloy in the presence of a low-pressure reactive gas, resulting in materials such as titanium carbonitride (TiCN) and titanium oxide. Pure carbon can also be evaporated.

Environmental Issues. The LTAVD process is safe for the environment and workers. No hazardous or toxic materials are produced. The process produces no VOCs. The only effluent generated is vaporized oil from the vacuum pumps; however most of this is captured in a cold trap that condenses the effluent and causes the oil to separate out.

Vapor Technologies has 14 batch coating service chambers at its facility. At the time of this writing, Vapor Technologies had four units under fabrication for proprietary customers.

LTAVD systems typically accommodate one target to produce three to four coatings. The new VT3500 can accommodate more than one target. This means users do not have to dedicate a system to one target material. The targets (material sources) can be interchanged quickly and easily.

Vapor Tech performs contract coating, custom equipment design and fabrication as well as contract research and development. The company will help users develop the precise process to fit their needs, giving full consideration to the functional and decorative requirements of each application.

Vapor Technologies' quality initiative is to be ISO 9000 certified within the next 12 to 13 months. The well-equipped materials laboratory includes a scanning electron microscope with elemental analysis, x-ray diffractometer, spectroradio-meter to measure transmittance and reflectivity, atomic absorption spectroscope and equipment for measuring the density, surface profile and microhardness, stress, sheet and point conductivity and coating adhesion, among other capabilities.

Applications. Plastic aerodynamic forms have been metallized. Plastic automobile grilles, bumpers, buttons, knobs and handles have been coated with scratch-resistant, reflective materials such as gold, chromium, brass, copper, titanium. The coatings give plastic the cosmetic metal appearance consumers prefer as well as providing for weight and cost reductions.

A wide array of highly reflective coatings can be deposited on the plastics used in headlamp housings. Single layer reflective coatings are corrosion and scratch resistance. Highly reflective coatings with smooth basecoats and protective topcoats can also be produced.

LTAVD has also been used to coat substrates that cannot be heated more than a few hundred degrees, such as epoxy metal composites, and substrates with complex surface geometries. The coatings include a variety of metals and reacted metal nitrides, carbides and oxides. These coatings solve problems caused by corrosive and oxidative hot gases, acids, caustics, organics and molten materials.

Areas of ongoing research at Vapor Technologies include diamond coatings using a patented process called Compact Linear Filtered Arc and overall improvement of LTAVD coatings.

Vapor Technologies' customers prefer to keep their names confidential for competitive reasons, albeit, applications of LTAVD are abundant throughout the door lock, plumbing and window hardware industries.

Coatings are applied to locksets using LTAVD. The finish provides not only the specularity and cosmetic appeal the company is looking for, but the corrosion resistance its customers demand. Coated locksets have been salt spray tested for more than 1,200 hrs, without any sign of discoloration. The BHMA standard is 96 hrs. The locksets also withstood UV and scratch tests with "equal vigor." This has allowed the lock company to provide a lifetime anti-tarnis warranty on its locksets' finish in addition to its standard mechanical components.

LTAVD is used to apply a brass color coating over brass faucets and other plumbing accessories. Plumbing hardware coatings must withstand more than 90 cleaning products from store-bought brands to specially made custom-cleaning solutions. The faucets are scratch resistant. The finish has allowed the customers to provide a lifetime warranty against corrosion, pitting and discoloration.

Vapor Technologies' coating applications are as diverse as the imaginations of its customers. With coatings as thin as one-thousandth the thickness of a human hair, the properties and performance of a substrate may be entirely changed. The coating processes can alleviate material limitations as the critical path in product improvement and development.