Mass-transport driven by surface instabilities in metals under reactive plasma/ion beam treatment at moderate temperature

Abstract

This paper presents a generalized approach to the mechanisms of oxidation, hydrogenation and nitriding of metals under ion irradiation with reactive particles at elevated temperatures. Experimental results on the plasma oxidation of bilayered Y/Zr lms, the plasma hydrogenation of Mg lms and the ion beam (1.2 keV N + 2 ) nitriding of stainless steel are presented and discussed. We make special emphasis on the analysis of surface e¬ects and their role in the initiation of mixing of bilayered lms, the ingress of reactive species in the bulk and the restructuring of the surface layers. It is suggested that primary processes driving reactive atoms from the surface into the bulk are surface instabilities induced by thermal and ballistic surface atom relocations under reactive adsorption and ion irradiation, respectively. The di¬usion of adatoms and vacancies, at temperature when they become mobile, provide the means to relax the surface energy. It is recognized that the stabilizing e¬ect of surface adatom di¬usion is signi cant at temperatures below 300-350 o C. As the temperature increases, the role of surface adatom di¬usion decreases and processes in the bulk become dominant. The atoms of subsurface monolayers occupy energetically favorable sites on the surface, and result in reduced surface energy.