L. Pranevicius

High current density nitrogen implantation of an austenitic stainless steel

Abstract Surface modification of AISI 304L austenitic stainless steel under high flux and low energy nitrogen ion implantation has been carried out at moderate temperatures in the range 270–400 °C with 1.2 keV ions and current densities between 0.5 and 1 mA/cm2. The influence of temperature, current density and fluence on the nitrogen transport and the microstructure of the nitrided layer have been investigated. The nitrogen depth profiles have been determined by nuclear reaction analysis, the microstructure of the modified layers has been analysed by X-ray diffraction and transmission electron microscopy. It is shown that the processing temperature and the ion flux have a major influence on both the profile shapes and the unusually deep penetration depth of nitrogen. The results suggest that above a critical temperature the nitrogen transport increases rapidly giving rise to a flat depth profile. The formation of the well known expanded austenite γN has been observed and its structure identified to a f.c.c. lattice containing a high density of stacking faults induced by the high level of internal stresses. The X-ray photoelectron spectroscopy study of the Cr2p3/2, Fe2p3/2 and N1s binding states demonstrates clearly the preferential bonding of chromium with nitrogen. The atomic transport of nitrogen and the profile shapes are discussed in relation with both the specific role of Cr and additional processes such as the formation of surface vacancies and adatoms.

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.

Ion beam nitriding of single and polycrystalline austenitic stainless steel

Polycrystalline and single crystalline [orientations (001) and (011)] AISI 316L austenitic stainless steel was implanted at 400 °C with 1.2 keV nitrogen ions using a high current density of 0.5mAcm−2. The nitrogen distribution profiles were determined using nuclear reaction analysis (NRA). The structure of nitrided polycrystalline stainless steel samples was analyzed using glancing incidence and symmetric x-ray diffraction (XRD) while the structure of the nitrided single crystalline stainless steel samples was analyzed using x-ray diffraction mapping of the reciprocal space. For identical treatment conditions, it is observed that the nitrogen penetration depth is larger for the polycrystalline samples than for the single crystalline ones. The nitrogen penetration depth depends on the orientation, the ⟨001⟩ being more preferential for nitrogen diffusion than ⟨011⟩. In both type of samples, XRD analysis shows the presence of the phase usually called “expanded” austenite or γN phase. The lattice expansion de...

Flux effect on the ion-beam nitriding of austenitic stainless-steel AISI 304L

The effect of flux and Ar pretreatment during ion-beam nitriding of austenitic stainless steel is investigated. The ion energy and temperature were 1.2keV and 400°C, respectively, the ion current densities were 0.5, 0.67, and 0.83mAcm−2. The nitrogen distribution profiles were measured using nuclear reaction analysis. The obtained nitrogen distribution profiles were analyzed by the means of the nitrided layer thickness evolution due to sputtering and diffusion and the model of trapping–detrapping. Both approaches could fit well the experimental results, however, different diffusion coefficients have to be assumed for each current density. In addition, the diffusion coefficients are higher for higher current densities. On the other hand, it is shown that the pretreatment with Ar-ion beam at nitriding temperatures produces only a thermal effect without any other influence on the following nitrogen diffusion. The results are discussed in relation with surface and temperature effects and atomic transport mech...

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.

A phenomenological study of the initial stages of film growth

The kinetics of island nucleation, coalescence and growth during deposition of Au atoms on amorphous carbon are studied experimentally and the obtained results are analysed using rate equations. Rutherford backscattering spectroscopy (RBS), transmission electron microscopy (TEM), grazing-incidence small angle scattering (GISAXS) and atomic force microscopy (AFM) techniques are used to measure the quantity of deposited Au, density, size and average height of the islands as a function of time. Parameters used in rate equations are deduced from a quantitative comparison between calculated and experimental evolution of the above dependencies. Suggested rate equations take into account the adsorption rate on a bare substrate as well as on existing islands, the mobility of adatoms, possible redistribution of atoms between islands and bare substrate and coalescence of islands. The rate equations are used to study the kinetics of the coverage of a carbon substrate by gold atoms and the kinetics of the island density. It is shown that experimental and calculated dependencies are in agreement if the mobility of islands is included.

Development of the microstructure of sputter deposited gold on amorphous carbon

Abstract The deposition of sputtered gold atoms on the substrate of amorphous carbon was investigated using transmission electron microscope, atomic force microscope and grazing-incidence small-angle scattering techniques. It was found from Rutherford back-scattering that, in the range of deposited gold 10 15 –1.5 × 10 16 cm −2 , the condensation rate is constant and does not depend on the fractional coverage which exponentially increases versus time. It it shown that the majority of the gold atoms arriving onto the substrate are collected by the growing clusters. The experimentally measured average height of gold clusters only slightly increases with the amount of deposited gold and is always larger than the radius of clusters, indicating a quite cylindrical shape.

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.

Simulation of silicon dry etching through a mask in low pressure fluorine-based plasma

The ion beam assisted etching of silicon through a mask in a low pressure fluorocarbon plasma is considered. The two-dimensional profiles of etched grooves are calculated using a proposed model involving a function of mask size, the fluxes of incident chemically active and non-active species from the plasma and bombarding ions. The model also includes the processes of adsorption, heterogeneous reactions, desorption, physical sputtering, activation of surface atoms and stochastic mixing between monolayers. Special attention is given to the etching anisotropy, lateral etching and elemental composition at the surface of a groove. It is shown, that formation of an inhibiting film on the sidewall of groove increases the etching anisotropy, however, the process of stochastic mixing leads to the formation of the altered layer in the near surface region. The thickness of altered layer and elemental composition at different surface regions of etched groove is considered.