Altering Material Surfaces to Prolong Service Life
A versatile ion implantation facility opens at SwRl
by Geoffrey Dearnaley, Ph.D.
Material surfaces we encounter in everyday life are important, because in many cases surface properties determine how particular components, tools, or devices such as medical implants will perform and how long they will last. For instance, TeflonŽ, a durable, non-stick coating for cookware, proved highly unsuitable as a joint replacement material.
Wear, fatigue failure, corrosion, fretting, and oxidation all begin at the surface and can rapidly lead to stress concentration, fracture, increased friction, seizures, and other problems caused by the formation of wear debris and corrosion products. Engines, power plants, bridges, and pipelines all suffer such degradation, but we are never so conscious of the penalties of material failure until it occurs in our own bodies, as it can in a replacement hip or knee joint.
Southwest Research Institute (SwRI) has invested in a versatile facility in which the surfaces of all kinds of materials can be modified using energetic ion beams. This ion implantation facility will allow the development of advanced processes to overcome surface-related problems for client organizations. Already, we have been able to demonstrate dramatic reductions in wear in orthopedic materials, to lower friction in ceramics, and to formulate ideas that could lead to novel electronic device structures.
How Surfaces Are Engineered
Many surface treatments and coating procedures have been developed over the years to combat failure mechanisms such as wear and corrosion. The proper design, use, and integration of surface properties with those of the component's bulk material make up the relatively modern approach called surface engineering. The day when a coating was applied after the event to cope with unexpected wear or corrosion is over. Quality management, sound medical practice, and competition in the marketplace demand that material properties and surface treatments or coatings of a device or tool be well-designed and tailored to meet product requirements from the start.
There are other factors to consider as well. Some traditional coating methods, such as electroplating with cadmium or chromium, are now viewed with disfavor because of the toxic waste or effluent produced. This also applies to traditional methods of hardening steel in baths of cyanide as a means of diffusing carbon and nitrogen into the surface. Aluminum alloy is often given a protective coating by anodic oxidation, but such oxides contain many pores, and the preferred method for sealing these is by immersion in vats of boiling chromate solution, which also produces hazardous wastes. Environmental factors provide a strong driving force for the development of clean, controllable metal treatment processes that will not degrade the properties of the substrate material.
Methods of coating or hardening metals often involve the use of high temperatures in reactive chemical vapors, or require the assistance of electrical energy within a plasma. However, temperatures of between 800 and 1,500 degrees F often are not compatible with optimum substrate behavior, and it can be difficult to scale up these thermal treatments for large tools or components, which cannot be subjected to the risk of thermal distortion. For these reasons, there are few options for the surface protection of machined parts larger than six feet long or 10 square feet in area.
There exists a real need for a new, versatile, and clean low temperature method for the surface treatment of all kinds of materials. Ion beams, used sometimes in conjunction with coatings, can provide an exciting range of possibilities for surface engineering. These methods, pioneered by the author 20 years ago in Britain, led to development of a large-scale ion implantation chamber in which tools and components weighing up to three tons have been successfully surface treated.
Surface Modification by Ion Beams
Ion beams, accelerated electrically in vacuum to very high velocities, were first used to embed, or implant, selected species such as boron atoms into the surfaces of silicon wafers in the manufacture of microelectronic chips. This process became a key technology in the semiconductor industry, and today is applied a dozen times or more in the manufacture of each microcircuit.
In addition, ion implantation provides a versatile means to modify nonsemiconductor materials to impart properties such as hardness or corrosion resistance. A great deal of research has been carried out around the world on these and other property changes in metals, ceramics, and polymers. The technology has matured to the point where it is timely to apply it to the broad range of materials-related activities at SwRI.
It is easy to combine ion implantation with a vacuum coating process, such as thermal evaporation of a solid from an electron-beam heated hearth. The two fluxes of particles overlap at the specimen, or workpiece, surface to build up, layer by layer, a coating with a composition that incorporates both the vapor deposit and particles constituting the ion beam. Such a process is called ion beam-assisted deposition (IBAD).
The special properties of an IBAD coating derive from the fact that the ions have energies that are typically one hundred thousand times those caused by thermal motion, and a thousand times those occurring in an electrically stimulated plasma. The impacts of these ions break interatomic bonds near the coating-substrate interface. If there is a favorable interfacial chemistry, good adhesion can be achieved through new, strong interatomic bonds between coating and workpiece. Sometimes an intermediate bond coat can be used to impart adhesive bonding to both substances, effectively gluing them together.
The transfer of forward momentum in atomic collisions during ionic bombardment serves to compress the deposited coating to its full bulk density : voids and vacancies are filled up. For this reason, IBAD has been most widely used in the deposition of stable and reproducible optical coatings and filters with the predictable refractive index of the bulk material. Most manufacturers of high-grade optical instruments now use IBAD for precise control of light transmitted through the equipment.
A Case History
Industry employs many tools for the injection molding of plastics that often contain mineral fillers, glass-fiber reinforcing materials, and abrasive pigments such as titanium dioxide. These widely used composites produce severe wear on the hardened tool steel molds or extrusion dies, screws, and nozzles. The simple process of ion implantation of nitrogen, however, can strengthen the surfaces of such tools by the formation of a fine dispersion of second-phase nitrides, such as chromium. This ceramic and metal (cermet) composite layer can double the surface hardness and increase the life of the tool by a factor of four to five, for example, in the case of molds used to make automobile intake manifolds.
When the circumstances are favorable, the results can be even more dramatic. For example, in a case history from Great Britain, a $30,000 steel tool used to mold polyester formers for the coils of an electric motor became severely worn, a common occurrence. Normally, it would have been refurbished by the application of chromium followed by remachining, an expensive operation. Instead, nitrogen ion implantation was applied, extending the tool's life 22-fold to produce a reported 680,000 moldings without problems. The nitrogen implantation treatment represented about 10 percent of the tool's cost.
This example represents a particularly successful application of ion implantation in Britain, one that has also spread to Germany. A lack of appropriate equipment, however, has prevented exploitation of this technology in the United States. The large facility under construction at SwRl will give U.S. clients the opportunity to evaluate ion implantation in a variety of tools and components.
The emphasis during the first phase of the process development program at SwRI will be on an ion beam-assisted method for depositing coatings of an amorphous form of one of the more versatile elements, carbon. The result is known as diamond-like carbon.
Because it is black and amorphous (non-crystalline), diamond-like carbon (DLC) is not strictly diamond-like, but it has been given the name because it is extremely hard (harder than tungsten carbide) due to the three-dimensional carbon-carbon bonds within it. Its hardness varies according to the ratio of hydrogen atoms to carbon atoms, which can be as much as 50 percent, depending on how the DLC is made.
The usual starting material for DLC is a hydrocarbon, such as methane, which can be dissociated in a plasma containing argon to deposit carbon coatings. This chemical vapor deposition procedure requires high temperatures. An alternative method that yields low hydrogen content is pulsed laser ablation of a graphite target at high power densities. It is difficult, however, to reproduce either process to satisfy full-scale industrial applications.
The technology to be used and further developed at the SwRI facility allows the coating of items with a surface area of several square feet. It also allows low processing temperatures, and therefore can be applied even to polymers.
The process is a simple one. A fluid precursor, generally a hydrocarbon or silicone, is volatilized under vacuum conditions within an electrically heated vessel. A stream of boiled-off precursor molecules is then directed towards the workpiece through a nozzle. In this ion beam-assisted process, bombardment with either nitrogen or argon ions at levels of a thousand electron volts of energy can disrupt most of the carbon-hydrogen bonds. In fact, each ion liberates up to 100 hydrogen atoms that then migrate to the surface and are removed by the vacuum pumps. This is a highly efficient process, with a low arrival ratio of ions to carbon-coating atoms.
The friction coefficient under dry sliding conditions against a steel ball can be as low as 0.02 (up to a contact pressure of one GPa), and the coatings produced from a poly-dimethylsiloxane precursor maintain this low value even at high values of relative humidity. Other forms of DLC, deposited from plasmas, do not behave in this way and have friction coefficients as high as 0.3 in humid air. There is evidence that amorphous ion-assisted DLC provides low friction and thus acts as a solid lubricant under high vacuum conditions.
These dense, chemically inert coatings of amorphous DLC, free from grain boundaries or the columnar microstructure characteristic of many other deposited coatings, are expected to have excellent corrosion resistance. If a siloxane precursor is used, the coatings are a form of nano-composite, consisting of nanometer-sized regions in which carbon is bonded to hydrogen and silicon is preferentially bonded to oxygen. These regions are knitted together by strong silicon-carbon bonds, providing a structure that is more resistant to corrosion than conventional DLC.
The hydrogen content of ion beam assisted DLC is relatively low -- around 12 percent by atom. This hydrogen : carbon ratio has a dramatic influence on electrical conductivity, and resistivities may change by eight orders of magnitude (i.e., from one ohm cm to a hundred million ohm cm). These properties, combined with the feasibility of doping the material (it is a semiconductor, the analog of amorphous hydrogenated silicon), open up several interesting possibilities for electronic applications. For instance, it could be used to coat silicon crystals to form what is called a hetero-junction, which consists of two different semiconductor materials in combination. Properties of hetero-junctions differ from those of conventional silicon diode junctions.
SwRI's Ion Implantation Facility
When fully instrumented, the new ion beam facility at SwRI will be the most versatile such installation in the world. Ion guns and other components will be added in stages to enable all the ion beam processes discussed above to be carried cut, including ion implantation of metallic and gaseous ions, IBAD, ion beam mixing, and implantation of various forms of DLC. Combined with intense ultraviolet irradiation, these capabilities can provide yet another means of surface modification. A four-pocket electron beam-heated hearth will enable a variety of coating materials to be evaporated for ion beam-assisted deposition of metals, oxides, and nitrides. Precursors for DLC will be vaporized from a resistively heated reservoir.
The ion beam processes will be conducted in a stainless steel vacuum chamber six feet long and four feet in diameter -- the largest such device in the United States dedicated to ion beam surface modification. This will allow trials to be made on large, full-size tools and components used by the Institute's clients. Once these trials have proved successful, it will be possible to work with a client to design an industrial plant able to apply the process economically for volume production. Although scaling up ion beam treatments is technically a straightforward matter, and one that brings major benefits in unit cost, the technology has been held back in North America by the lack of any facilities comparable in size to those that exist in Europe.
Future plans include installation of a robotic manipulator inside the vacuum chamber, so specimens can be exposed controllably to sequential treatments using a variety of ion beams or coating fluxes. This novel application of robotics could be extended to other low-pressure or vacuum processes.
A versatile and powerful facility for the modification and coating of metals, ceramics, polymers, and composites has potential for use in a variety of ways. In the immediate future, however, it is necessary to focus on selected applications for which there is a demonstrable need.
The orthopedic application of DLC to combat wear of polyethylene components in replacement hip and knee joints is an area where there is wide consensus that surface coatings need careful evaluation. Research in this area, supported by SwRI internal research funds as well as outside sponsors, is discussed in the accompanying sidebar [link to sidebar]. A second timely application is to reduce dependence on chromate baths for the surface treatment of aluminum alloys. An ion-assisted process would overcome the pollution and industrial discharge problems associated with toxic hexavalent chromium. A third important application is the use of nitrogen ion implantation to extend the lives of tools used in injection molding and extrusion of filled polymers. Also, oxidation resistant coatings could increase the life of costly turbine blades. Finally, the electrical properties of DLC need to be explored to assess its commercial potential for use in fabricating simple electronic devices.
Successful demonstrations and field trials of ion beam-modified materials will lead to the need for large-scale, dedicated equipment to be operated in-house by client organizations. The Institute is in a position to provide designs for such equipment, or to team with a manufacturer to supply it to the client's specifications. Such developments are a necessary accompaniment to the research and development occurring in this field.
Published in the Spring 1994 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Joe Fohn.