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Carbon coating toughens up parts

June 6, 2012
Diamondlike carbon can improve the surface characteristics of parts and pipes.

Authored by:
Kent E. Coulter
Michael Miller
Christopher Rincon
Ronghua Wei
Southwest Research Institute (SwRI)
San Antonio, Tex.
Edited by Stephen J. Mraz
[email protected]

Southwest Research Institute

This article is based on an item that appeared in Technology

Mechanical components usually fail from the outside in, with failures starting on the surface eventually affecting the entire component. Examples include wear (abrasion, galling, and erosion), corrosion (including pitting, crevice, and galvanic), and fatigue. One approach engineers take to avoid these problems while increasing machine reliability, loads, speeds and productivity, is to develop new materials. Unfortunately, this can be costly and time consuming, with no guarantee of success.

Another approach is to alter the material’s surface proprieties. Heat treatments, for example, have been used for centuries to increase part strength or restore ductility after another metalworking process. Heat treatment and other surface-engineering techniques, including coatings, are cost effective at improving some properties while retaining core qualities of the base material such as toughness. And one of the most-effective coatings is diamond.

Diamond’s downside
Diamond, the world’s hardest material, also has the highest resistance to nearly all types of wear and corrosion, as well as a low coefficient of friction. And diamond coatings have been deposited on a variety of materials. But diamond coatings aren’t suited for every component.

There are two primary problems that hold back broad applications of diamond coatings. First, deposition temperatures — greater than 1,300° F — are too high for most engineering materials, including steels. Second, diamond coatings are rough, and polishing them is difficult and costly.

DLC: The next best thing to diamonds
To overcome these problems, researchers have developed an alternative — diamondlike carbon (DLC). It is carbon based but, unlike diamond, has an amorphous structure. This makes DLC coatings smooth and lets them protect rather than abrade components and surfaces they contact. Still, it has a comparatively high hardness of 15 to 50 gigapascals (GPa). (For comparison, diamond has a hardness of 100 GPa and fully hardened tool steels come in at 6 to 9 GPa). Similar to diamond, DLC’s coefficient of friction is low, To overcome these problems, researchers have developed an alternative — diamondlike carbon (DLC). It is carbon based but, unlike diamond, has an amorphous structure. This makes DLC coatings smooth and lets them protect rather than abrade components and surfaces they contact. Still, it has a comparatively high hardness of 15 to 50 gigapascals (GPa). (For comparison, diamond has a hardness of 100 GPa and fully hardened tool steels come in at 6 to 9 GPa).

Similar to diamond, DLC’s coefficient of friction is low, and the coating resists corrosion. Corrosion resistance stems from three sources: carbon is relatively inert; the coatings are pinhole-free and watertight; and DLC coatings typically resist and suppress electrons moving on top of the coating.

DLC coatings can be deposited at room temperature, making it suitable for steels, aluminum alloys, and even polymers. In addition, DLC can be “doped” with different elements for various purposes. For example, adding silver or titanium oxide gives it antibacterial properties, adding gold increases its oxidation resistance, and putting in fluorine decreases its surface energy for easier deicing and self-cleaning.

Plasma-immersion ion deposition
DLC coatings are applied using physical-vapor deposition (PVD) or plasma-enhanced chemical-vapor deposition (PECVD). In PVD, carbon from a solid source, such as a graphite target, is knocked off or sputtered using accelerated argon ions. This loose carbon collects on and coats the part. In contrast, PECVD uses carbon from gaseous sources, such as acetylene or methane.

In general, DLC coatings made using PVD are hydrogen- free and harder than PECVD coatings. PVD coatings usually retain more stress, which can lead to the coatings delaminating. Therefore, an interfacial layer is added to increase adhesion, or a thinner DLC coating is applied to avoid film spallation.

The biggest limiting factor for PVD is that it is a lineof- sight process. This makes it difficult to uniformly coat three-dimensional components without complicated manipulation.

PECVD, on the other hand, is inherently nonline of sight. It can be used to consistently coat even the most complicated components without moving them because the carbon comes from a gas or gas mixture, which reaches everywhere in the deposition chamber.

Another approach
Researchers have been developing a new type of DLC coating process, plasma-immersion ion deposition (PIID), an advanced version of PECVD. PIID’s main advantage is its use of pulsed-glow discharges to deposit the coating. It can coat several components simultaneously in a larger chamber, which lowers costs. PIID also uses lower deposition temperatures and short voltage pulses that reduce arcing.

For PIID, parts are pl aced in a vacuum chamber, which gets pumped down to a pressure of about 10-6 torr. A gas gets fed into the chamber, raising the pressure to 10 to 15 millitorr. Researchers then apply a pulsed train of negative voltage to the parts, generating a pulsed-glow discharge plasma in the chamber. Because parts are pulsed, the voltage draws carbon ions to the parts’ surfaces.

In a refinement on the technique, researchers first feed argon into the chamber, letting ion-sputter cleaning remove contaminants on the parts’ surfaces. Next, without breaking the partial vacuum or stopping the discharge, the argon flow is slowly reduced to zero while a carbon-rich gas such as acetylene is pumped in, at which time carbon starts depositing onto the parts. Cleaning the parts first ensures strong bonds with the coating. For ferrous parts, however, researchers found that adding a layer of silicon, silicon carbide, or silicon nitride using various siliconcontaining gases before laying down the DLC coating makes for strong bonds between coating and parts.

Coating internal surfaces
Even though there are many ways to coat external surfaces, it’s much more difficult coating internal ones, such as the inner walls of pipes and cylinders. But coatings on internal cylindrical surfaces improve many parts, such as aircraft landing gear, hydraulic cylinders, engine cylinder liners, military-gun barrels, and pipes for transporting petroleum and other liquids. SwRI has developed patented and patent-pending hollow-cathode-discharge (HCD) techniques for applying DLC coatings deep inside tubular products.

In typical PIID on a short tube, applying negative voltage creates a plasma primarily inside the tube. This is the HCD process and it involves high currents at low voltages because electrons generated inside the tube undergo many collisions before they move out of the tube. Because the plasma is generated in the bore of the pipe, the pipe does not need to be straight. Researchers have used HCD to coat curved steel pipes up to 7-ft long and 6 in. in diameter. The coating is quite dense and adheres well. So far, researchers have also put DLC coatings on various materials, including steels, alloys of aluminum, titanium, and magnesium, various ceramics, glass, and polymers such as polycarbonate and Teflon.

For longer pipes, a vacuum chamber is not commercially viable. However, the pipe itself can serve as the vacuum chamber by using proper vacuum and electrical isolation connections. A pulsed voltage is then applied to the pipe, and a DLC coating gets deposited on the inner surface. The insides of pipes up to 40-ft long have been coated with DLC.

Pipes can be too small or thin for standard HCD. For example, HCD cannot create a plasma in a tube greater than 2-ft long with a diameter of less than 1 in. But using a patented SwRI technique of adding a magnetic field to HCD lets a plasma form inside the tube. With this approach, technicians have DLC coated the insides of 10-ft tubes with outer diameters of less than three-quarters of an inch.

The intense plasma generated in the HCD leads to high deposition rates, which are useful for speeding up the coating process.

Using the same principle, SwRI researchers developed a technique in which a meshed metal cage encloses parts for high-rate deposition. With this technique, several components can be coated with much thicker coatings (10 to 20 m). Additionally, short tubes can be coated on the inside and outside simultaneously, due to the high plasma density inside the cage.

DLC coating performance
The hardness and smoothness of DLC coatings let them resist sliding wear, making them well suited to parts in reciprocating pumps and motors. These attributes also led to a successful military application when SwRI researchers developed a version of PIID that could coat the glass windshields of military Humvees with DLC to help them resist abrasion.

In tests, uncoated and DLC-coated glass samples were sprayed with small alumina balls. The results showed a several-fold improvement of erosion resistance with a DLC coating only a few nanometers thick. Based on these results, hundreds of windshields were coated and delivered to the U. S. Marine Corps for use in the field. The final deposition process was later transferred to a commercial company, and SwRI continues to develop other transparent coatings.

In some applications, DLC coatings are being evaluated as replacements for toxic chromium plating, which should reduce environmental problems.

DLC is an established material and can be placed on a wide variety of industrial parts, even those with complex geometries. DLC coatings can help meet a broad demand for better surfaces with more wear and corrosion resistance for markets such as oil and gas delivery, power generation, and automotive engines and drivetrains.

© 2012 Penton Media, Inc.

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