Tooling Up a Tough Thin Film

Article From: Products Finishing, , from Products Finishing

Posted on: 9/1/2002

These thin-film coatings applied using PVD and CVD processes are tougher and longer lasting than your average tool coating…

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When you think of thin film coatings, you aren't likely to envision industrial equipment such as drills bits, fasteners, stampings and other parts and pieces that would go into heavy-duty applications. When you think of these types of parts, you are more apt to think of heavy-duty, thick coatings, something tough and strong. Not so at IonBond Inc. (Madison Heights, MI). It provides customers all of these benefits in a thin film. Strong, wear-resistant coatings are applied using state-of-the-art cathodic arc physical vapor deposition (PVD), enhanced arc PVD, unbalanced magnetron sputtering, chemical vapor deposition (CVD) and plasma assisted chemical vapor deposition.

"We have seven coating centers throughout the United States," explained David Johns. "Each center has practical and technical expertise in thin-film coating, surface conditioning and industrial applications. The technical and engineering staffs at each facility support the Corporate Technology Group in Madison Heights, which develops new coatings and applications and provides failure analysis and analytical services."

This can be seen in the QSFV (quality, speed, flexibility, value) program within IonBond. The company has invested in its own fleet of trucks to help with pickup/delivery and JIT requirements of its customers. Also, IonBond has invested in a customized system for monitoring the business. "We worked with a firm to customize the software to fit our needs," commented Rajiv Ahuja, president and CEO.

This specialized management tool is called "MAJIC." MAJIC is designed to track a customer product and record processing history, while monitoring a whole range of operational parameters, like product turnaround times and capacity utilization. Structured around the ISO 9000 quality system, MAJIC tracks a tool from the time it enters a facility to the moment it is delivered. "Managing several service centers from a central location can be difficult. We needed a way to maintain the data in a centralized location so that everyone could access it anytime. Any of our employees in the country can call in and check the status of a job," said Mr. Ahuja.

Although it has been challenging to meet the needs of customers, IonBond has found that educating customers is its greatest challenge. "Our most significant challenge has been customer education and awareness," stated Mr. Ahuja. "These coatings (CVD, PVD) have been around for 30 years; however, they are only now seeing significant activity in the market. People do not realize what these coatings can do for their tools, plumbing fixtures, drill bits and more." Even though the coating costs more initially, it is more cost-effective because it lasts longer.

Determining which deposition process to use depends on the end use of the product. PVD is used for close tolerance tooling, such as milling tools, round shank tools, trim tooling, plastic molds, punches and brazed carbide tooling. PVD uses low temperature, line-of-sight operation. Parts rotate within the chamber, and it can be used on most metals and requires no post-heating.

CVD is used on loose tolerance tooling. The process involves high temperatures and a gaseous environment. With CVD, everything is coated, and masking is almost impossible. Often heat treating is required after CVD, and there are material restrictions.

"Vacuum coatings offer more flexibility. I see large potential for growth, particularly in the decorative business," stated Mr. Ahuja. "But it depends on how fast we educate designers and engineers and push down prices. We also have to continue to develop new coatings, such as diamond-like coatings that are good for cutting tools and other components. We have to remind engineers to design products with the coating in mind. That way, they will have a more effective product than if they simply adapted a coating to product."

Bernex® PaCVD is a new process for depositing amorphous coatings consisting of diamond-like carbon (ADLC). The coatings are deposited by a plasma activated CVD process. The patented process enables the deposition of smooth, adherent coatings at temperatures below 200C. This opens a new field in the coating of steels in hardened and tempered conditions and aluminum and titanium alloys, since the mechanical properties of the base material are not affected.

Here is a brief tutorial on the different deposition processes a summarized from IonBond's website, www.ionbond.com.

Physical Vapor Deposition.

PVC includes a number of vacuum-coating processes during which material is physically removed from a source by evaporation or sputtering and then transported through a vacuum using the energy of the vapor particles and finally condensed as a film on the substrate. All PVD processes can be separated into three stages: 1) Emission from a vapor source; 2) Vapor transport in a vacuum; and 3) Condensation on the substrate.

The IonBond PVD process uses a vacuum arc to evaporate material from a cathode to produce a stream of highly active and excited coating material. The coating units consist of a vacuum chamber and a number of arc evaporation sources that are arranged in the chamber walls. Each source is about 63 mm in diameter, although some are larger.

A number of metals, alloys and electrically conductive, semi-conductive and insulating compounds can be deposited. PVD is used almost exclusively to deposit films 0.03-5mm thick. The use of PVD hard coatings, particularly titanium nitride (TiN), on cutting tools and other wear components has grown because it has a hardness of greater than Rc 80, increasing the wear rate. The TiN coating also provides chemical resistance, because it is a stable coating. It prevents chip welding in cutting tools because of the anti-galling properties of the coating. TiN also has a lower coefficient of friction than hard chromium.

The most commonly used PVD processes are ion-assisted processes: vacuum arc evaporation, electron beam evaporation and high-rate magnetron sputtering. These can be deposited using reactive and non-reactive methods.

Reactive PVD

During reactive PVD, carbides, carbonitrides, nitrides, oxides and other compound types are deposited by introducing a reactive gas such as nitrogen or oxygen into the physical vapor stream. Reactions between the gas and vapor occur at the substrate surface, in transit or at the source surface, as well as on the chamber walls and other surfaces. The primary reaction site is the substrate.

Plasma Enhancement

Plasma enhancement increases the number of ions, electrons, fragmented molecules and excited neutrons in the gas or vapor. To use plasma enhancement, inert and/or reactive gases must be present at pressures that can maintain a gas discharge. When used with a negative substrate bias, the increased ion bombardment generally results in improved film properties, since the atoms, molecules and molecular fragments react more readily with the growing film.

The electrode attached to the auxiliary discharge supply is the plasma enhancement device. High frequency voltage applied to this electrode is another way to create a gas discharge. Another way to generate plasma enhancement is to accelerate electrons from a thermionic filament through the transport region to the auxiliary anode.

The voltage needed is about 100-1000v at pressures of 10 to 30e -5 torr. Once started, the gas discharge clamps the voltage at a level determined by the electrode geometry, surface composition and gas pressure and composition.

Enhanced Arc

This system is a patented deposition system that uses high strength magnetic fields in direct line-of-sight configuration to suppress macro-particle emission. This results in the enhancement of plasma ionization, energy and density and the near elimination of macroparticles in the deposited film. High ionization assists in the substrate conditioning and promotes adhesion, particularly when depositing amorphous diamond.

Ion Bombardment

Ion bombardment produces films with extremely high adherence and good physical properties. In most ion-assisted PVD processes, ions gain energy because of a negative bias voltage applied to the substrates. For nonconductive substrates or coatings, a high frequency bias voltage is applied and positive ions are accelerated toward the substrates during the negative phase. Electrons neutralize substrate charging during the positive phase of each cycle. In most PVD processes, ion bombardment is supplied by argon or reactive gas ions from a gas discharge.

Vacuum Arc Deposition

During vacuum arc evaporation, metal evaporates from a cathode source as a result of intense localized heating by arc spots that move randomly across the cathode surface. The vapor source remains solid while the major portion of the metal vapor is ionized.

Because this type of PVD is highly localized, cathodes can be used in any position, allowing for greater flexibility in designing systems with multiple sources for coating large parts or large quantities of parts. Also, the metal vapor is used for heating the substrate and surface preparation as well as coating. Substrates are heated using metal-ion bombardment and operating one or more arc sources while applying a voltage to the substrates. This heats the parts and conditions their surfaces to increase film adhesion.

Reactive deposition occurs while operating arc sources in the presence of a reactive gas using substrate bias voltages -75 to -400v. This produces dense stoichiometric films with high adhesion.

Electron Beam Reactive Deposition

The current electron beam industrial processes are thermionically enhanced triode-ion plating, hollow cathode discharge and coaxial. In each process, an electron beam heats metal, evaporating it from a molten pool. The substrates collect the vapor from bottom-mounted sources. Substrates are usually rotated to increase film thickness. The reactive gas pressure is closely controlled to achieve good film stoichiometry.

Thermionically enhanced. Once the chamber is pumped down, substrates are heated to at least 570F using argon ion bombardment. An argon ion discharge is used to sputter clean the substrate surface. The thermionic electrons accelerated from the hot filament enhance the strength of this discharge. During coating, a bias of several hundred volts is applied to the substrate, and the reactive gases are introduced into the chamber. The electron beam is a high-voltage, low-current unit the produces little ionization.

Hollow cathode discharge. This process is similar to thermionically enhanced triode ion plating. The hollow cathode discharge gun produced a high-current electron beam that strikes the metal source at about 100 eV. About 10-20% of the metal vapor is ionized as it passes through the electron flux.

Coaxial electron beam. This reactive deposition process differs from the others. The electron source contains magnetically enhanced, thermionically supported discharge that emits a high electron current along with ionized gas. The substrates are heated using electron bombardment. The positive side of the discharge supply is connected to the substrate. When the substrate temperature reaches 570F, electrons from the source are accelerated to the auxiliary anode and then sputter cleaned with argon ions. During cleaning the substrates are biased negatively.

High Rate Reactive Magnetron Sputtering

During this process, metal vapor is sputtered from cathode sources by an intense inert gas discharge that is confined to a region within four inches of the cathode source. The vaporization rate per unit area is relatively low, but large single sources and multiple sources can be used. The arrangement of the components and gas inlets maximize the amount of molecules that will react on the substrate before reaching the cathode. It is necessary to balance the flow rate of reactive gas with the metal vaporization rate to achieve film stoichiometry while maintaining a clean cathode surface. Argon ion bombardment is often used to sputter clean the substrate and provide additional heat.

Chemical Vapor Deposition

This is a different process from vacuum evaporation, ion plating or sputtering. It is a heat-activated process that relies on the reaction of gaseous chemical compounds with heated substrates. The main reactive vapor can be chloride, bromide, iodide or fluoride, or a metal. Generally, deposition of a pure metal from the halide occurs either by hydrogen reduction at a specific temperature or by direct pyrolytic decomposition at a higher temperature.

Whenever carbide, nitride or borides are desired, the metal halide vapor is accompanied by an additional reactive species such as methane, nitrogen or boron trichloride. The addition of reactive species lowers the free energy of the reaction products so that the compound is formed in preference to the pure metal. Manipulation of the reactor pressure, temperature and/or reactant composition can also influence the deposition products.

Advantages

CVD is a versatile process that can be used on a wide configuration of substrates. It produces high density, high purity and high strength coatings. The strength is dependent on crystal structure and size, purity, density and internal stress. CVD deposits are usually more ductile because of their purity and have comparable or higher mechanical strength than wrought material. However, not all materials exhibit high strength. Pure tantalum and pure columbium have low yield strengths and high ductility in the vapor deposited condition.

Coatings Deposited

A number of pure metals, carbides, nitrides, borides, silicides and oxides of metals can be deposited using CVD. Coating thickness ranges from 0.0002-0.050 inch. Co-deposition of TiC and TiN to form titanium carbonitride using the Bernex Moderate Temperature CVD process is expanding in use. Chromium deposition on steel substrates, resulting in carbides of chromium and iron, (CrFe)7C3, provides excellent resistance to corrosion and cold welding.

Applications

The greatest application for CVD is the hard coating of tools fabricated from high-speed steels, stainless steels and cemented carbides. Both TiC and TiN enhance the life of carbide cutting tools and high-speed punches and dies. The process is also used to coat tungsten, platinum and rhenium used in electronic hardware, as well as integrated circuits for microelectronic application and solar cell fabrication.

The Process

Important aspects of the process include gas pressure; temperature and velocity; reactant composition; material composition; substrate cleanliness and temperature; storage flow and recovery systems; scrubber systems for the by-products; and the composition and construction of the vessel.

Substrates must be free of grease and oil. IonBond uses aqueous cleaners, sodium hydroxide and ultrasonic cleaning. The substrates must tolerate intense heat, since temperatures in the chamber vary from 400-3600F. Small pieces are much easier to heat up than large pieces. Small pieces may have to be racked or rotated. Because the surface temperature is critical, and cracks, crevices and recesses frequently maintain a higher temperature, deposits will be easily formed in these areas. However, the reactant gases are limited in their ability to penetrate crevices. This tends to offset the effect of temperature on the deposition rate.

Gas systems

CVD systems require a gas feed system so that the active material can be injected into the deposition chamber and exhaust products removed.

Reaction temperature

Temperature must be maintained during the process to prevent excess coating buildup on tanks as well as on parts. When decomposing the metal compound, the substrate is held at a substantially higher temperature than any other part of the system. Because of this, the reaction chamber may be more of a high temperature problem than any other part of the system. Most reactions occur in an anhydrous and anaerobic environment, often at sub-atmospheric pressures.

Temperatures during deposition range from 1500-2200F. Temperature and pressure depend on what properties are wanted in the deposit. The deposition rate can vary from 0.07 mm/min at low temperatures to 25 mm/min at higher temperatures. Thick uniform coatings with refined grain structure are also the result of deposition temperature. Low temperature processing is usually desirable, although a tradeoff with deposition rate is often made. Fewer CVD reactions occur at temperatures below 1500F; however, exposing the substrate to electrical plasma in the gas phase during coating can lower the temperature (plasma enhanced).

Reaction pressure

Deposition at sub-atmospheric pressure improves the uniformity, grain size and composition of most CVD films. Special pumping systems are required because the gases are hot and often corrosive. One way around this is to trap the gases and cool them before they reach the vacuum pumps. If pressures greater than 50 torr are used, water jet exhausters or watering pumps can be used. Inexpensive polyvinyl chloride exhausters can be used if gases are cooled before entering the equipment. Toxic by-products should be neutralized or eliminated.

"The coating processes take a lot of time," explained Babu Mamidipally, engineer. "From a customer service standpoint it is often difficult to explain the time involved, since the customer wants the part as soon as possible. But these parts are not just sitting around the shop. Most customers have learned, however, that the process is better for their tools and equipment. It is just that they don't turn it over to us until they are 'at the end of their rope.'"

IonBond is a pioneer in the thin film coatings industry. More than 20 years ago the company made high performing low-cost TiN coating a reality. It designed and built the first commercial cathodic arc PVD systems. The patented enhanced arc source is another IonBond technical advancement. The enhanced arc reduces the macro particles in cathodic arc coatings, providing a smoother, defect-free thin film and allows deposition of innovative materials like TETRABOND® tetrahedral amorphous carbon.Over the years, the company's technical developments have expanded the scope of PVD and CVD coating applications. It invented the recoating concept that is used to extend the life of perishable tooling and pioneered the use of PVD coatings on medical devices. When it became apparent that the company's TiN did not meet all customer needs, it developed Cathodic Arc TiAIN, AITiN, TiCN, CrN and ZrN coatings. And it continues to meet its customers' needs by developing new coatings and application technologies.

 



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