Plasma-Immersion Ion Implantation
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per cm3. Ions must be replenished near the workpiece by either diffusion or ionization since the workpiece (in effect) behaves like an ion pump. Gaseous discharges with thermionic, radio-frequency, or microwave ionization sources have been successfully used. Surface-enhanced materials are obtained through PHI by producing chemical and microstructural changes that lead to altered electrical properties (e.g., semiconductor applications), and lowfriction and superhard surfaces that are wear- and corrosion-resistant. When PHI is limited to gaseous implant species, these unique surface properties are obtained primarily through the formation
of nitrides, oxides, and carbides. When applied to semiconductor applications PHI can be used to form amorphous and electrically doped layers. Plasma-immersion ion implantation can also be combined with plasma-deposition techniques to produce coatings such as diamondlike carbon (DLC) having enhanced properties. This latter variation of PHI can be operated in a high ionenergy regime so as to do ion mixing and to form highly adherent films, and in an ion-beam-assisted-deposition (IBAD)-like ion-energy regime to produce good film morphology and structure. Tribological Applications A primary focus of PHI today is concentrated on the implantation of nitrogen into metals for tribological applications. The focus on nitrogen is not surprising since conventional ion-beam implantation has demonstrated improved wear resistance of metals implanted with nitrogen ions.3 Despite the extensive literature pointing to its benefits, ion-beam implantation has not been accepted to any great extent in the metallurgical surface-treatment industry due not only to the shallowness of the treatment depth but also because of the perceived difficulty in treating three-dimensional objects with a beam of accelerated ions.
Plasma Source Chamber Chamber Filled With Plasma -Positive Ions Strike All Surfaces Simultaneously
Vacuum Figure 1. A plasma sheath surrounds the target in plasma-immersion ion implantation. Therefore ions bombard all surfaces simultaneously without beam aiming or target manipulation.
MRS BULLETIN/AUGUST 1996
Plasma-Immersion Ion Implantation
Implanting ions by PHI offers a solution to this latter problem. A wide range of metals have been treated by PHI nitrogen implantation, yielding results as least as good as those obtained by conventional implantation. Examples of such materials include the following: Ti-6A1-4V,4 Monel K-500,5 AISI-S1 tool steel,67 mild steel,8 austenitic stainless steel,9 aluminum10, and hard chromium plate.11 The implanted concentration profiles match well with those predicted by theory and despite the shallowness of the implantation depth (typically 50-100 nm at 50 keV), significant improvements in wear resistance have been obtained. For example, almost no increase in microhardness could be detected after Pill-treatment of AISI SI tool steel7 but significant increases in wear resistance (from pin-on-disk wear testing) were obtained even at the highest load of 20 N.
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