Physical Vapor Deposition and Vacuum Metalizing
Industry, under the pressure of the market and regulatory burdens, is forced to improve in order to provide competitively priced, high-quality products demanded by their customers. Companies must find methods that improve quality and reduce cost within regulatory constraints to be successful.
An example of the results of these achievements in process technology is the replacement of expensive processes, difficult-to-manage materials and production wastes with modern surface treatments on automotive lighting reflective surfaces. Automotive lighting has evolved to provide highly sophisticated illumination coupled with low power consumption, which improves driver safety. The demands to achieve this has driven the surface-coatings technology currently employed to manage the high-intensity output from these systems and at the same time eliminate process waste and undesirable materials.
Plastic reflectors for forward beam and rear brake automotive lighting require a highly reflective surface that will withstand nasty environmental conditions. Injection-molded polycarbonate, ABS and polycarbonate-ABS blends among others are used for these assemblies. Glass-filled thermoset bulk molding compounds have also been used.
These reflective surfaces are created using the physical vapor deposition (PVD) process. To achieve the high optical reflectance required, aluminum is deposited in a thin film approximately 1,000 angstroms thick. This thickness is only 4 micro inches! (Note that “half a tenth or 0.00005 is 50 microinches.)
In order to obtain a smooth reflective surface, a lacquer undercoat had been used to “reflow” the molded surface of the plastic components. This was common practice a few years ago, but the large amount of volatile organic compounds (VOCs) generated by the reflow process required expensive catalytic treatment of the flashed-off organic vapors. Advances in mold-cavity polishing treatments are now able to create surfaces that allow direct coating without a lacquer undercoat.
Two PVD processes are in common use: thermal evaporation and sputtering. Both of these processes provide a high flux of incident atoms that will become the coating. They require leak-tight high-vacuum systems operating in the 10-5 to 10-4 Torr pressure levels.
In thermal evaporation, small aluminum canes are loaded into tungsten filaments. The canes typically weigh about 500 milligrams. Multiple tungsten sources are used, heated by a high-current AC source. Up to 40 filament assemblies are mounted in the center of a planetary carousel (Fig. 1). The parts to be coated are racked in the planet positions. The planets spin as the carousel mainframe is rotated, allowing aluminum to be deposited on all the molded parts.
Suitable masking prevents aluminum vapor from coating unwanted areas. In time, the aluminum condenses and builds up on the masking material. This causes unwanted outgassing, which will increase the cycle time. To correct this, the tooling is dipped in a caustic solution to periodically strip the condensed aluminum.
In the evaporation process, the filaments are first preheated to melt the aluminum canes. The aluminum cane is sized to wet the coiled tungsten filament without running out the bottom of the filament. The power output is increased to ramp up the temperature of the aluminum to cause vaporization. This flashing process takes only a minute or two and is under the control of the machine’s programmable logic controller (PLC).
To protect the fragile aluminum film, a lacquer top coat can be applied, but this has VOC concerns. To eliminate the VOCs, top coating is done in the vacuum chamber by bleeding in an organo-metallic compound rich in silicon, typically hexamethyl-di-siloxane (HMDSO). In this plasma-enhanced chemical vapor deposition (PECVD) process, plasma is created by applying a high AC voltage to electrodes within the chamber.
The plasma reacts with the HMDSO to deposit a film on the parts that is stoichiometrically close to silicon dioxide (SiO2). This top-coat film is quality checked by dropping some dilute sodium-hydroxide solution on the part. A poor top coat will allow the aluminum to be consumed rapidly, showing the black plastic underneath. With a decent top coat, there will be no reaction after 30 minutes.
In sputter deposition, planar magnetron cathodes are used as a source of aluminum. The magnetron consists of a target plate of aluminum positioned over a magnet assembly, which forms a race-track-like closed magnetic flux on the cathode target surface (Fig. 3). Free electrons drift away from the negatively charged cathode and interact with neutral argon atoms to form argon-ion/electron pairs. The magnetic flux increases the ion density, which drastically increases the sputter rate as compared to a cathode with no magnetic enhancement.
Argon ions are attracted to the cathode and dislodge the target atoms, which condense on the molded parts. Argon sputter gas is used because it is inert to aluminum. Water-cooling channels within the cathode cool the target plate. The process vacuum level is in the 10-3 Torr range. After many coating cycles, the target plate becomes eroded and must be replaced. Internal shielding and part tooling must be caustically cleaned as in thermal evaporation. After coating, a plasma top coat using HMDS is applied similar to that used in thermal evaporation.
The vacuum system is the precursor process that allows PVD to take place. Creating the vacuum achieves two requirements for the process:
- It reduces the undesirable components of the atmosphere to provide a contaminant-free environment for the film to be deposited
- It reduces the pressure to the point where the vaporized atoms have a long mean free path so nucleation of the vapor does not occur before the atoms deposit onto the desired surfaces.
To get the required cycle time, large positive-displacement blowers along with mechanical vacuum pumps are used to rough down the vacuum chambers (Fig. 4). Pumping down to the high 10-4 Torr range is provided by large diffusion pumps and auto cascade cryotraps running at -135°C to rapidly pump residual water vapor. Typical load-to-load cycle times are in the area of 15-20 minutes, depending on the process used.
Automotive lighting has advanced a long way since the first acetylene gas lamp hung on a vehicle early in the history of automobiles. Today, we enjoy the performance and safety afforded by highly engineered lighting systems that depend on the technology of vacuum-process environments to create conditions compatible with exotic surface coatings. The development is relentless, and newer technology awaits introduction into the market. It will be exciting to witness what comes next.
For more information: Contact Christopher Estkowski, VP of engineering, Metallurgical High Vacuum Corp. 6708 124th Ave. Fennville, Mich., 49408; tel: 269-543-4291; e-mail: Chris@methivac.com; web: www.methivac.com
Products Finishing; Donald M. Mattox, Technical Director from Society of Vacuum Coaters
Posted on: 6/1/1999; From http://www.pfonline.com/articles/vacuum-deposition-processes
Vacuum Metallizing Plastic Parts; W. Bialojan, M. Geisler; Leybold AG, Hahau, Germany
Applications of Vacuum Coating
by Donald M. Mattox, Management Plus, Inc.; From https://www.svc.org/AboutSVC/Applications-of-Vacuum-Coating.cfm
Transparent SiO2 Barrier Coatings: Conversion and Production Status
E. Finson and J. Felts, Airco Coating Technology, Concord, CA; From http://ns3inc.com/PublishedPapers/TransparentBarrier.pdf
Vacuum Deposition: Definition in Wikipedia: https://en.wikipedia.org/wiki/Vacuum_deposition
History of Automotive Headlamps-From Acetylene to LEDS
Headlamp: Definition in Wikipedia; https://en.wikipedia.org/wiki/Headlamp
Physical vapor deposition (PVD) has applications in a variety of industries on many types of materials using vacuum-assisted processes. Introduction of reactive gases like oxygen, acetylene and nitrogen into the vacuum chamber can enhance the bond between the coating and the substrate and tailor the structural, physical and tribological properties of the film. This provides for application on cutting tools to create surfaces that improve machining conditions by increasing tool surface hardness, lubricate tool surfaces, and increase abrasion and wear resistance. The results are faster speeds and feeds, less galling, improved surface finish and less downtime.
Some common industrial coatings that use the PVD application process are titanium nitride, chromium nitride, zirconium nitride, chromium nitride/chromium carbide, titanium carbonitride, diamond-like carbon and decorative PVD films deposited on common materials such as steel, zinc, carbide and plastic. The injection-molding industry is using PVD films on injection-molding tools to enhance lubricity, increase hardness and improve resistance to corrosion. The films have the following advantages over traditional coatings:
- They are highly controlled for uniform thickness.
- There are no post-processing requirements after coating.
- Coatings can be more chemically inert.
- Removing the coating is possible without damaging the underlying surface of the steels.
An additional benefit of PVD coatings is they do not demonstrate the tendency of developing microcracks that electroless and electrolytic-type coatings typically will. It is interesting to note that coating advancements have paved the way for additional coating improvements. As mentioned, plastic part surfaces are now being molded that require no post-molding primer or surface conditioning, in part due to coating improvements on the molding cavities that allow better filling and release of parts and on the cutting tools used to produce them.
Headlamps are critical to driver and passenger safety. Drivers need to illuminate the road and encounter oncoming vehicles and must avoid being distracted by headlight glare. The journey of development to achieve the level of lighting technology we enjoy today is interesting. Coatings are a key element in this development.
The state-of-the-art headlight in the late 1800s was an acetylene lamp originally created for mining with a polished metal reflector and adapted by Prest-O-Light, which also integrated a convenient switch to ignite the lamps from the cockpit. Prest-O-lights were common around 1904. Electricity broke into the industry in 1898 on the Columbia electric car. At the time, electric headlamp technology was limited by the harsh environment and the tungsten filament life, but Peerless had electric headlamps in common use in 1908.
To demonstrate how fast innovations drive development, by 1912 Cadillac, under GM, had created an electrical ignition and lighting system. This was the first integrated automotive electrical system. Corning innovated reflector technology and beam-concentration methods using lenses and polished stamped metal inserts on glass. By the 1920s, the electric Corning Cornaphore Headlamp provided illumination to distances of several hundred feet. The Corning Cornaphore headlight with a metal reflector and glass lens was the foundation of modern automotive lighting.
Sealed-beam technology followed as did government regulatory mandates in the U.S. that placed controls on the sealed-beam design. Lighting design, which evolved as legislation lobbied for by manufacturers pushed new technology forward, became more favorable to innovation. Sealed-beam all-glass reflectors had surface coatings to improve the reflective surface and enhance brightness and were shaped to focus the beam through the lens.
Government regulations did not allow composite headlights to be placed on vehicles until 1983. The removal of this barrier provided consumers the convenience to replace only the bulb of the assembly in the lighting system and removed design restrictions that freed engineers to innovate on beam control, brightness and glare reduction. Early electric headlight bulbs used a tungsten filament in a vacuum that created a residue in the bulb and consumed power.
Halogen bulbs provided the technology to conserve power and enhance brightness and are the common bulb type in use today. Halogen uses a group of gases in an inert-gas filler to burn bright while conserving energy. Another recent innovation is the HID bulb, which uses two tungsten electrodes to charge the gases inside vaporizing metal salts in the bulb to produce a plasma that emits an intense light at temperatures around 2000°F.
The current developing technology is the light emitting diode (LED). The Lexus LS was the first vehicle to incorporate LED beams beginning in 2008. With the variety of lighting technologies and styling variations, reflector and lens shapes were developed from safer, less-expensive and more design-friendly materials like engineered plastics, which enhance design flexibility for style and beam-control shaping to manage the illumination from these modern bulb designs.
Coatings have evolved with the technology to provide the high surface reflectivity, surface quality and durability required for these new designs. Newer designs require uniform metal deposition over intricate details and exotic shapes of the engineered reflectors and lenses using materials with low surface activity. Today, we enjoy automotive lighting that provides extended vision for drivers and is minimally distracting for oncoming traffic, creating safer conditions for drivers and passengers alike.