S. Mändl

Nitriding of austenitic stainless steels using plasma immersion ion implantation

Abstract Austenitic stainless steels show excellent corrosion resistance, in combination with a rather low hardness and high wear, which is lost during conventional hardening processes at elevated temperatures above 450 °C, like plasma nitriding. Here, we present improved hardness and wear behaviour of austenitic stainless steel (X6CrNiMoTil7.12.2 — AISI 316Ti) implanted with nitrogen plasma immersion ion implantation (PIII) while retaining the corrosion properties. Compared to untreated stainless steel, the hardness can be increased up to a factor 4, depending on the amount of implanted nitrogen. The wear behaviour was improved by 1–2 orders of magnitude. A layer of γ N ′—expanded austenite—of more than 10 μm was formed in a few hours, as determined with GDOS. Only a very small fraction of chromium nitride, which is not detrimental to the corrosion resistance, was observed with XRD for the samples implanted at 380 °C.

Formation of TiN, TiC and TiCN by metal plasma immersion ion implantation and deposition

Abstract Titanium nitride, titanium carbide and titanium carbonitride are well known compounds displaying a rather high hardness and melting point, thus enabling their use as hard coatings. With a titanium cathode and both nitrogen and methane gas, a series of compound TiCxNy can be prepared with the final points TiN and TiC using metal plasma immersion ion implantation and deposition. The respective properties of the films are investigated by X-ray diffraction and Rutherford backscattering spectroscopy. With increasing pulse voltage, a decreasing carbon content and growth rate is found for the TiC series, while no such effect is observed for corresponding TiN films.

Thin Film Deposition Using Energetic Ions

One important recent trend in deposition technology is the continuous expansion of available processes towards higher ion assistance with the subsequent beneficial effects to film properties. Nowadays, a multitude of processes, including laser ablation and deposition, vacuum arc deposition, ion assisted deposition, high power impulse magnetron sputtering and plasma immersion ion implantation, are available. However, there are obstacles to overcome in all technologies, including line-of-sight processes, particle contaminations and low growth rates, which lead to ongoing process refinements and development of new methods. Concerning the deposited thin films, control of energetic ion bombardment leads to improved adhesion, reduced substrate temperatures, control of intrinsic stress within the films as well as adjustment of surface texture, phase formation and nanotopography. This review illustrates recent trends for both areas; plasma process and solid state surface processes.

Nitrogen diffusivity in expanded austenite

Abstract Expanded austenite is a metastable phase formed after nitrogen insertion into austenitic stainless steel, at elevated temperatures between 350 and 400 °C with nitrogen contents between 5 and 30 at.%, thus leading to a lattice expansion of up to 13%. The layer thickness can vary up to a factor of ten for different alloys at identical process conditions. The main part of the nitrogen depth profiles can be described by an complementary error function. However, close to the end of the profile, a rather sharp decrease of the nitrogen content is found. Calculating the concentration dependent nitrogen diffusivity, it can be shown that this kink corresponds to steep increase of the diffusivity by nearly a factor of ten. A systematic study of different steel grades is presented, trying to correlate the diffusivities with the alloying content and to predict the ‘suitability’ of austenitic stainless steel for nitriding.

Measured and calculated dose distribution for 2D plasma immersion ion implantation

Nitrogen was implanted into four cylinders (height 37 mm and diameter 4, 6, 8, and 12 mm) covered with aluminum foil using plasma immersion ion implantation at 30 kV. The homogeneity of the implantation, i.e. the spatial variation of the implanted dose, was determined for selected points on the top and the side walls of the cylinders with sputter depth profiling using Auger electron spectroscopy. Calculations approximating the dynamic sheath evolution with discrete sheath positions were performed to obtain the variation of the implanted dose and angle of incidence. Good agreement is found between the calculations and the experimental data. The ion dose exhibits a maximum on the top and decreases on the side walls. A second maximum is formed on the wall of the cylinder. The total dose variation is of the order of 50%.

Development of a new technique for high-energy secondary-electron measurements in plasma immersion ion implantation

A scintillation technique was applied to directly monitor high-energy secondary electrons during the process of plasma immersion ion implantation (PIII). The spectral peak of the scintillation appeared at the wavelength of nm, and the temporal behaviour of the secondary electrons was measured with a response time as short as several microseconds. A maximum absolute secondary-electron flux for a target peak voltage of 25 kV reached at a measuring point, which showed good agreement with a calculated value. The kinetic energy of the secondary electrons was also measured by penetration depth into an aluminium layer coated on the scintillator surface, revealing acceleration of the secondary electrons up to an energy corresponding to the sheath voltage.

Influence of Microstructure on Nitriding Properties of Stainless Steel

Very hard and wear-resistant layers are formed after energetic nitrogen insertion into stainless steel. Here, a systematic investigation of the influence of the microstructure is presented. Nitrogen implantation was performed in austenitic, martensitic, and ferritic steels with the samples investigated with respect to formation of expanded phase, nitrogen depth distribution, hardness, and wear. Microstructure strongly affects the diffusion in austenite and has negligible effect in the case of martensitic/ferritic transformations

Corrosion protection of titanium by deposition of niobium thin films

Abstract Titanium is a promising material for medical implants, replacing bones and teeth. However, at pH values below 2, which occur in the dental environment, the corrosion resistance is compromised. The deposition of niobium layers onto titanium is a possibility to increase the corrosion resistance, as measured in 5 N HCl solution.

Corrosion Properties of Supersaturated Magnesium Alloy Systems

The range of applications for magnesium alloys is still limited due to their relatively poor corrosion behavior. In recent years, various new magnesium alloys were developed, some of them with improved corrosion properties, thus opening new fields of application. However, the number of alloying elements for the use in conventional cast processes is limited due to their interaction with liquid magnesium, other alloying elements or large differences in the melting temperatures. The possibilities for grain refinement by post-processing are also restricted. PVD techniques can help to produce supersaturated precipitation free and microcrystalline magnesium layers. Using ion beam and magnetron sputtering, binary or ternary Mg-Al, Mg-Ti and Mg-Sn alloy systems as well as standard alloys (AM50, AZ91 and AE42) were deposited on silicon and on magnesium substrates. The effect of the microstructure on the corrosion properties was studied by comparing as cast material and PVD coatings using potentiodynamic polarization, linear polarization resistance, and electrochemical impedance techniques.

Phase formation and diffusion after nitrogen PIII in molybdenum

Abstract Using nitrogen plasma immersion ion implantation into Mo, the nitride phase formation and nitrogen diffusion behavior were investigated in the temperature range from 330 to 580 °C. In all cases, the formation of a new phase (beside the Mo substrate) was observed with X-ray diffraction. Good agreement of this phase with powder diffraction files of cubic Mo 2 N was found, albeit the exclusion of a slightly distorted tetragonal modification was not unequivocal due to the low signal intensity. Elastic recoil detection analysis indicated a surface nitrogen concentration of 35–40 at.%. The thickness of this surface layer was always larger than the projected range, as calculated with TRIM, and it increased with longer process time or higher temperature. In addition to this diffusion process, the formation of a deep tail of nitrogen into the bulk Mo was observed, starting near a temperature of approximately 500 °C.