Ion-Beam Mixing
Ion-beam mixing phenomena deal with compositional and structural changes in a two- or multiple-component system under the influence of ion radiation. The effect commonly occurs during sputteríng and results in changes in surface composition during depth profiling anaJysis by SIMS and AES techniques. For example, consider a thin film of A on substrate B bombarded by a beam of inert-gas ions. Typically, the ion range (/?) exceeds the escape depth of the sputtered A atoms. If R does not exceed the thickness of A, then only A atoms sputter. If, after some sputteríng, R now extends into the substrate región, the atomic displacements and interdiffusion that occur within colusión cascades will cause A and B to intermix. The mixing occurs locally at the interface and eventually links with other similarly intermixed zones to créate a continuous ion-beam mixed layer. Now B atoms also enter the stream of sputtered atoms because the combination of continued surface erosión and interfacial broaden-ing, due to ion mixing, has brought them closer to the surface.
Through the use of high-energy ion beams, mixing reactions occur over substantial dimensions. Films can, therefore, be effectively alloyed with sub-strates and layered, but normally immiscible films can be homogenized with the assistance of ion implantation. As an example, consider the multiple-layer structure consisting of alternating Au and Co films. According to the phase diagram, these elements do not dissolve in each other; but they form a uniform metastable solid solution under a flux of 3 x 1015 Xe ions/cm2 at an energy of 300 keV. Colusión cascades, ballistic effects of recoils, and defect migration during room-temperature irradiation all con¬tribute, in a complex way, to the observed mixing.
In suicides, equilibrium as well as metastable phases have been observed after mixing. Experiment has shown that the suicide thickness is both dose-and ion-species-dependent. At the same energy and dose, more mixing occurs the heavier the ion. In metal film systems, extended solubility is virtually always observed, metastable phase formation is a frequent occurrence, and amorphous phases occasionally form at cryogenic temperatures.
Modification of Mechanically Functional Surfaces
By enhancing the ability of surfaces to resist plástic deformation, the benefits of reduced wear, less tendency to surface cracking, and greater dimensional stability are effected. In recent years there has been considerable research on the use of ion implantation to realize these desirable ends. For the case of steel, the implantation of light interstitial ions, such as nitrogen, boron, and carbón, yields considerable improvement in wear and fatigue resistance. The reason is due to the elastic interaction between dislocations and undersized interstitials; this results in their mutual attractíon and the segregation of the atoms to the defects. Long known in metallurgical circles, the interaction occurs even at room temperature and is quite effective in pinning dislocations, thus restrictíng their motion. In addition, iron nitrides, carbides, borides, etc., form when the limited solubilities of the interstitial atoms are locally exceeded; these precipí-tales are also effective barriers to dislocation movement. Since surface damage processes depend on plástic flow of surface layers, the importance of limiting dislocation motion is apparent.
An altérnate approach to improving the wear resistance of surfaces through the deposition of hard coatings was addressed at length. It is instructive to compare this approach with that of ion implantation. Although CVD deposits manage to conformally coat external as well as internal surfaces, ion implantation is limited by geometry to line-of-sight processing. CVD coatings are bonded to the substrate across an interfacial región, which is frequently a source of adhesión difficulty; on the other hand, ion-implantation modified layers are not subject to adhesión problems because no sharp inter-face exists. Since CVD deposition is conducted at elevated temperatures, the substrate is frequently heat-affected and sometimes softened in the process; ion implantation only modifies a very thin surface layer, leaving the remainder of the substrate unaffected. Lastly, thick CVD coatings several microns thick imply less stringent substrate smoothness requirements than for ion-implanta-tion processing. The latter is the only practical method available for modifying precisión surfaces while preserving extreme dimensional tolerances. Clearly, ion implantation is only cost-effective for high-value added components, such as surgical implants or dies.
The issue of the effective surface depth modified by ion implantation is an interesting one. It has been observed that ion implantation effects persist well beyond the shallow depth of the projected ion range. Implanted atoms are frequently observed considerably deeper within the substrate than can be accounted for by the geometry of wear tracks. The cause has been attributed to the generation of fresh dislocation networks that effectively trap and drag atoms deeper below the damaged surface layers. Frictional wear is also accompanied by temperature increases of as much as 600-700 °C at contacting asperities. Migration of mobile impurities is thus encouraged, especially where the dislocation density is high. Such effects provide an unexpected wear protection bonus for ion-implanted surfaces.
A number of industrial applications involving wear reduction by means of ion implantation methods is Usted in Table 13-2. Hardness and resistance to adhesive and abrasive wear are the attributes required of the assorted cutting, mechanical forming, and molding tools. In steel matrices nitrogen is a favored interstitial ion, and implanted cobalt has been explored as a means of modify-ing tungsten carbide tools. Typical doses are well into the 1017/cm2 range and impart two- to fivefold decreases in wear rate with corresponding increases in tool life. Specific examples of ion-implantation modified tools and components.
A totally different application which nevertheless exploits the benefits of enhanced hardness and reduced wear involves metallic surgical implants. Tens of thousands of titanium alloy (Ti-6 wt%-Al-4 wt% V) hip and lcnee replace-ment prostheses have already been ion-implanted with nitrogen resulting in improved tribological properties. In service, the implant moves in contact with a high-molecular-weight polyethylene mating socket, so that wear of this couple is of concern. Apparently, the formation of hard TiOz particles results in abrasión of the unimplanted alloy surface during use. Implantation produces a surface containing hard titanium nitride precipitates that effectively resists such wear. Part of the improvement in properties may be attributable to the enhanced corrosión resistance ion-implanted surfaces exhibit. High defect concentrations promote thickening of air-formed oxide films and enhance chemical homogenization of the underlying metal. The former effect affords an added measure of surface passivation and protection, and the latter helps eliminate localized galvanic corrosión. Lastly, note that there are no practica! alternatives to modifying the surface properties of orthopedic prostheses. Unlike tools whose surfaces can tolérate CVD or PVD coatings, chemical biocompatibility with contac'ting body fluids places severe restrictions on the surface composition of surgical implants.
Ronellys Flores---CRF---libro the materials science of thin films
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