L.L. Pranevicius

On the mechanism of ion nitriding of an austenitic stainless steel

Abstract The mechanism of nitrogen transport has been investigated in an austenitic stainless steel (AISI 304) under high flux and low energy nitrogen ion beam irradiation at moderate temperatures in the range 270–550°C. The profiles of the distribution of nitrogen have been analyzed with nuclear reaction analysis (NRA) and glow discharge optical spectroscopy (GDOS), and the surface roughness with scanning AFM. The modeling is based on the study of the stochastic mixing of atoms ‘ballistically’ displaced by incident ions. The development of surface roughness and the formation of an altered layer highly enriched by nitrogen are analyzed, and it is concluded that the transport of nitrogen into the bulk results from a flux of matrix atoms driven by mobile vacancies at temperatures above 350°C. This behavior is consistent with an altered layer ‘growth’ that is controlled by the ion-beam-induced displacements of surface atoms.

On the mechanism of synthesis of PbTiO3 thin films by thermal annealing of Pb/Ti layers in air at atmospheric pressure

Abstract Single phase PbTiO3 thin films were synthesized by thermal treatment at temperatures 350–700 °C of Pb/Ti magnetron sputtered layered structures at atmospheric pressure in air. The film stoichiometry was accurately controlled by deposition of individual layers with the required thickness. Scanning electron microscopy views showed that initially smooth surfaces had porous and granular structure after thermal annealing. Surface morphology depends on the substrate material and the cooling rate. X-Ray diffraction patterns revealed the nanometric microstructure of synthesized films. The size of crystallites in oxidized films depends on the annealing temperature and does not depend on the annealing time. The atomic mixing and oxidation kinetics are discussed and applied to PbTiO3. The emphasis is made on the analysis of surface atom relocation and restructuring processes under reactive adsorption at elevated temperatures and their role in the formation of surface instabilities and initiation of transport of oxygen from the surface into the bulk directed to stabilize these instabilities. The experimental results support that the formation of PbTiO3 films homogeneous in structure and composition during the thermal oxidation is a result of intensive mixing of Pb/Ti layers with continuous supply of oxygen though the surface. Under non-equilibrium conditions on the surface, the oxygen atoms are driven into grain boundaries of crystallites, result in high compressive stress inducing fragmentation of grains and their oxidation by oxygen diffusion through boundaries into the nanograins. The oxidation kinetics and the size of crystallites is governed by the mass-transport processes.

Mass-transport driven by surface instabilities under high-flux, low-energy nitrogen ion irradiation at elevated temperatures

Flux effects in ion nitrided AISI 304 stainless steel have been investigated in an attempt to understand the mechanism of nitrogen transport. It is concluded that an interaction between a highly activated surface layer, the internal interfaces and the bulk is critical. Under conditions of non-equilibrium present on the surface, the nitrogen atoms are driven into the grain boundaries and highly compressive stress is formed. The stress relaxation processes initiate plastic flow of atoms in the grains and a corresponding flow of nitrogen atoms.

Nitriding of an austenitic stainless steel in plasma torch at atmospheric pressure

Experiments with a nitrogen torch at atmospheric pressure have been performed in order to identify the role of surface processes in the mechanism of nitrogen transport during nitriding of stainless steel AISI 304. Unusually thick (∼175 μm) layers of supersaturated N solid-solution f.c.c. phase have been obtained for 10 min at 450 °C. Samples treated at 550 °C have a radically different structure. The scanning electron microscopy (SEM) surface and cross-sectional micrographs reveal that surface topography is an indicator of the degree of modification occurring in the nitrided layer. Surface vacancies generated by surface instabilities move deeply into the bulk at elevated temperatures and form a highly defective layer with pores and microcracks. The transport of nitrogen in austenitic stainless steel is driven by the fluxes of matrix atoms directed to stabilize surface instabilities. Nitrogen depth profiles simulated on the basis of a model with a surface-atom relocation process and activation energy of 1.15 eV, and including balanced fluxes of atoms in the bulk for relaxation of surface energy, are in qualitative agreement with experimental results.

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

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 films, the plasma hydrogenation of Mg films and the ion beam (1.2 keV N2+) nitriding of stainless steel are presented and discussed. We make special emphasis on the analysis of surface effects and their role in the initiation of mixing of bilayered films, 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 diffusion of adatoms and vacancies, at temperature when they become mobile, provide the means to relax the surface energy. It is recognized that the stabilizing effect of surface adatom diffusion is significant at temperatures below 300–350°C. As the temperature increases, the role of surface adatom diffusion 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.

Role of Grain Boundaries in the Mechanism of Plasma Hydrogenation of Nanocrystalline MgAl Films

Nanocrystalline aluminum hydrides (alanates) are potential hydrogen storage materials for PEM fuel cell applications. One of candidates is magnesium alanate, Mg(AlH4)2, which contains 9.3 wt. % of hydrogen. In the present work, the effects of Ti catalyst in improving the kinetics of hydrogen uptake and release are investigated. The 2–5 μm thick MgAl films have been hydrogenated employing plasma immersion ion implantation technique as a function of Ti‐content. Nanocrystalline MgAl films were prepared by magnetron sputter deposition in vacuum. Titanium atoms were incorporated simultaneously into the growing film. Morphological and structural properties were studied by scanning electron and atomic force microcopies and X‐ray diffraction technique. It is shown that the microstructure of the hydrided/dehydrided MgAl film is highly defected and demonstrates dispersed/amorphous cluster‐like structure. Ti atoms in MgAl film kinetically enhance the dehydrogenation of magnesium alanate film. For Ti‐doped MgAl film ...

High-flux, low-energy implantation effects on the composition of altered layers

Abstract A simplified model to study high-flux, low-energy ion irradiation of materials is presented, enabling us to establish links between processing parameters, such as the ion energy, ion flux and fluence of irradiation, and the concentration of incident ions accommodated in the altered layer. The altered layer is considered as a ‘black box’ with input and output parameters, such as sputtering rate, flux and fluence of incident atoms and supply rate of atoms from the bulk due to continuous movement of boundaries of the altered layer. Analysis of the rate equations is carried out and the role of the sputtering and flux effects on the kinetics of accommodation of incident atoms is studied. The concentration of incidents as irradiation proceeds increases and saturates at a level defined by the balance between supply and sputtering rates. At steady state, the incorporation of incident atoms does not change the ratio of surface concentrations for matrix atoms.