In situ X-ray diffraction has been used to investigate the stability of expanded austenite during annealing in vacuum for the austenitic stainless steel 316Ti, the super-austenitic stainless steel 904L, and the duplex steel 318LN. Expanded austenite has been formed using plasma immersion ion nitriding before. Time-of-flight secondary ion mass spectrometry before and after annealing yielded complementary information regarding nitrogen depth profiles and CrN precipitation using cluster analysis. The decay of expanded austenite during annealing was found to be thermally activated with an activation energy of 1.8 ± 0.3 eV, starting within five minutes at 550 °C and taking more than two hours below 450 °C. The decay occurs simultaneously throughout the whole nitrogen-containing zone—and not at the surface as during nitriding. Nitrogen diffusion occurring in parallel slightly complicates the data analysis. Further transmission electron microscopy investigations are necessary to understand the microstructure after annealing in vacuum. The limit for operating hard and wear-resistant expanded austenite layers at elevated temperatures of up to 350 °C is given, however, by nitrogen diffusion and not the decay into CrN.

Plasma nitrocarburizing of an austenitic stainless steel AISI 316L was performed in an N-2-H-2 atmosphere (50 % H-2 and 50 % N-2) with a carbon-fiber reinforced carbon (CFC) active screen. The experimental concept of a plasma-discharged CFC active screen in conjunction with a substrate holder at floating potential ensures that no charged particles are present on the sample surface and no sputtering occurs. As a result, carbon contaminations are formed on the surface which were studied in detail with a combination of scanning electron microscopy (SEM) and secondary ion mass spectrometry (SIMS). The results indicate that a layer of about 25-50 nm of carbon is formed. At the same time, a strong segregation of Mn, Ni and Cr is observed below the coating within the steel substrate. Both effects progress with treatment time and there exists a clear correlation between the formation of the carbon layer and the segregation effect.

In situ X-ray diffraction (XRD) was used to compare nitrogen low-energy ion implantation (LEII) into austenitic stainless steel 316Ti and super austenitic stainless steel 904L. While the diffusion and layer growth were very similar, as derived from the decreasing intensity of the substrate reflection, strong variations in the observed lattice expansion—as a function of orientation, the steel alloy, and nitriding temperature—were observed. Nevertheless, a similar resulting nitrogen content was measured using time-of-flight secondary ion mass spectrometry (ToF-SIMS). Furthermore, for some conditions, the formation of a double layer with two distinct lattice expansions was observed, especially for steel 904L. Regarding the stability of expanded austenite, 316Ti had already decayed in CrN during nitriding at 500 °C, while no such effect was observed for 904L. Thus, the alloy composition has a strong influence only on the lattice expansion and the stability of expanded austenite—but not the diffusion and nitrogen content.

Expanded austenite - austenitic stainless steel containing a colossal amount of nitrogen in solid solution - is still some kind of conundrum with the nitrogen atom transport from the surface into the bulk being enhanced for higher nitrogen content. At the same time, atomic ordering due to an enhanced affinity between chromium and nitrogen is observed.Here, nitrogen transport experiments are performed at different temperatures using 14N and 15N as isotopic tracers. The depth distribution of each of these isotopes as well as the total nitrogen content is investigated with time-of-flight secondary ion mass spectrometry (ToF-SIMS) and glow discharge optical emission spectroscopy (GDOES), respectively. In parallel, phase information is obtained using in situ X-ray diffraction (XRD) during nitriding (for time-resolved data) and during sputter depth profiling (for depth-resolved data).During nitriding, no delayed transport of nitrogen, i.e. trapping, is observed for the expanded austenite. However, as soon as CrN precipitates are formed, nitrogen trapping is a dominant factor with an almost perfect correlation between precipitation and trapping. Cluster ion analysis in SIMS confirms this effect with the abundance of Cr2+ ions significantly increased compared to FeCr+ ions in the presence of precipitation. As nitrogen transport and CrN formation occur in parallel at higher temperatures, further experiments to separate the CrN formation at higher temperatures from the nitrogen transport at lower temperatures have been performed.

Expanded austenite formed by inserting nitrogen into austenitic stainless steel is a puzzling phase since its discovery about 30 years ago: variable nitrogen concentration correlated with a variable lattice constant together with an unusual depth profile normally indicating a compound layer instead of a diffusion layer. While expanded austenite has been described for nitriding experiment durations as low as 5 min and >48 h, the initial phase transition has not been investigated in detail. Here, very low ion current density experiments prolonging the initial phase together with in-situ X-ray diffraction (XRD) experiments during nitriding are presented. Additionally, the formed layers are investigated by depth-resolved in-situ XRD measurements during successive removal of the surface layer. It is shown that two phases, a low expansion phase and a high expansion phase, are formed sequentially over time with the growth rate of the latter phase being much higher, thus obliterating any trace of the initial phase. It is surmised that the driving force is a threshold in the nitrogen concentration.

Plasma nitriding is a well‐established technique to improve hardness and tribological properties of austenitic stainless steel. It is proved that it is also possible to preserve the corrosion resistance after nitriding by controlling the process parameters such as time and temperature, obtaining the so‐called S phase. Herein, the corrosion behavior is evaluated for nitrided layers produced by three different plasma treatments: DC plasma nitriding, plasma immersion ion implantation (PIII), and low‐energy ion implantation (LEII), using different parameters in order to obtain the S‐phase in thickness from 1.2 to about 6 μm without nitride precipitation. The microstructure and chemical composition of the nitrided layers is characterized by means of X‐ray diffraction, secondary‐ion mass spectroscopy, and scanning electron microscopy–focused ion beam. The corrosion behavior is evaluated by means of the cyclic polarization tests in NaCl solution. The morphology of the corrosion attack is studied by optical microscopy and SEM‐FIB, revealing a change from crevice to pitting after the nitriding process. The inclusions are observed to be corrosion initiation sites. Due to this, the thickest nitrided layers (with high N concentration) show better corrosion behavior than the thinner ones.

In situ x-ray diffraction (XRD) during ion implantation or thin film deposition is a powerful method to follow the time evolution of diffusion and phase transition processes in thin films, even as the depth resolution is still dominated by the information depth of the x rays. However, in the case of sputter etching with energetic ions at moderate temperatures, where no diffusion or phase transformation processes are active, this limitation is no longer of concern. Here, thin surface layers which are removed by sputtering can be identified with a depth resolution of 25 nm or better—while information from the substrate—despite overlayers of several micrometers—is accessible. However, considerable mathematical operations are necessary to convert the time series of diffractograms measured by XRD into a depth series. In this paper, a method is highlighted describing which depth-resolved properties of thin films can be accessed using such in situ measurements during ion beam sputtering in the model system austenitic stainless steel + nitrogen: (i) the influence of concentration gradients on the peak shape and peak width for conventional XRD scans in Bragg–Brentano geometry is determined; (ii) correlations between the local nitrogen concentration and the local lattice expansion can be established; and (iii) the evolution of the scattering intensity with depth becomes accessible, thus depth-resolved information on defect densities or grain size (normal to the surface) can be extracted without resorting to transmission or scanning electron microscopy.

Expanded austenite γN formed after nitrogen insertion into austenitic stainless steel and CoCr alloys is known as a hard and very wear resistant phase. Nevertheless, no single composition and lattice expansion can describe this phase with nitrogen in solid solution. Using in situ X-ray diffraction (XRD) during ion beam sputtering of expanded austenite allows a detailed depth-dependent phase analysis, correlated with the nitrogen depth profiles obtained by time-of-flight secondary ion mass spectrometry (ToF-SIMS) or glow discharge optical emission spectroscopy (GDOES). Additionally, in-plane XRD measurements at selected depths were performed for strain analysis. Surprisingly, an anomalous peak splitting for the (200) expanded peak was observed for some samples during nitriding and sputter etching, indicating a layered structure only for {200} oriented grains. The strain analysis as a function of depth and orientation of scattering vector (parallel/perpendicular to the surface) is inconclusive.

The present investigation reports the modification of Ti substrates by a plasma technique to enhance their physio-chemical properties as biocompatible substrates for the deposition of artificial membranes. For that purpose, nitrogen ions are implanted into Ti substrate using the plasma immersion ion implantation & deposition (PIII&D) technique in a capacitively coupled radio frequency plasma. The plasma was characterized using optical emission spectroscopy, together with radio frequency compensated Langmuir probe, while the ion current towards the substrate was measured during the implantation process using an opto-electronic device. X-ray photoelectron spectroscopy (XPS) was used for chemical analysis of the surface, confirming the presence of δ-TiN. The penetration depth of the nitrogen ions into the Ti substrate was measured using secondary ions mass spectroscopy (SIMS) while the morphological changes were observed using atomic force microscopy (AFM). A calorimetric assay was used to prove that the TiN samples maintain the biocompatibility of the untreated Ti surface with its native oxide layer. The ion implantation increases the load bearing ability of Ti surface by the formation of α-Ti(N) and δ-TiN phases on the sub-surface of Ti, and maintains the bio compatibility of Ti surface. After the plasma treatment a thin layer of chitosan (CH) was deposited in order to provide a moisturizing matrix for the artificial membrane of 1,2-dipalmitoyl-sn-3- phosphor glycerocholine (DPPC). The CH and subsequently the DPPC were deposited on the plasma deposited TiN substrate by using physical vapor deposition. The formation of artificial membranes was confirmed by AFM, measuring the topography at different temperatures and performing force curves.

While plasma immersion ion implantation (PIII) has shown a remarkable success story in its development, recently it appears to be stagnating. This review tries to shed some light on the dichotomy between the industrial success of plasma doping and the superficial dearth of industrial applications for functional coatings or biomedical implants. It is surmised that the use of high voltage pulses between 5 and 25 kV leads to excessive heat deposition, reducing the effective deposition rate, and large gradients in sputtering across the substrate surface, which is more detrimental than those in comparable recent methods, e.g. high power impulse magnetron sputtering (HiPIMS). However, the peculiarities of PIII can still be used rewardingly when focusing more on model experiments to understand specific fundamentals of ion surface interactions. The first example concentrates on dynamic measurements of secondary electron emission coefficients measured while the surface is actually changing. Another example shows how a combination of dedicated experiments can help to elucidate information on the formation of expanded austenite formed by nitrogen insertion into austenitic stainless steel. Nevertheless, for that case there are still open questions requiring further work.

Though X-ray diffraction (XRD) is a well-established technique—yet still continuously evolving—for characterization of thin films and diffusion layers, there are two inherent limitations due to the underlying physical principles leading to the diffraction: (i) all data are a weighted integral across the information depth and (ii) X-ray amorphous or nanocrystalline phases are very difficult to be detected. The purpose of this Perspective is to show how a straightforward combination of in situ XRD with depth profiling using a laboratory experiment in combination with a low-energy broad-beam ion source may allow us to overcome these limitations for functional surfaces. Whereas time-resolved investigations of diffusion and phase formation during ion implantation using reactive species are well established, using ion beam sputtering with nonreactive species for depth profiling of the phase composition during the measurements is a rather recent development. While employing analysis of differential spectra leads to a depth resolution below the information depth, ongoing in situ sputtering will lead to information obtained from layers deeper than the information depth. Keeping track of the total diffraction intensity permits the identification of phases without reflections in the XRD measurements, i.e., they appear to be amorphous, and their depth distribution. Selected examples drawn mainly from investigations of the nitriding kinetics of austenitic stainless steel will be used to demonstrate the potential and possible limitations of this method.

Plasma immersion ion implantation (PIII) is characterized by applying short high voltage pulses to a substrate immersed in a low pressure plasma discharge. Compared to a more conventional experimental setup for glow discharge optical emission spectroscopy (GDOES) where sputtered atoms are detected, the PIII experiment is characterized by higher ion energies of the primary ions, lower pressures with a larger mean free path, shorter pulses and a higher relative energy density deposited by the secondary electrons emitted from the surface. Using time resolved optical emission spectroscopy (OES) during PIII high voltage pulses, detailed insights into the interactions of sputtered atoms, secondary electrons and the low pressure plasma discharge are possible. Thus, a direct excitation of the sputtered particles by the secondary electrons is highly unlikely due to the small cross sections. However, a transient increased optical emission was observed with the maximum intensity about 20-80 mu s after the end of the high voltage pulse. This delayed excitation of Ar I, Ar II and Me I lines scale with the secondary electron emission coefficient. Nevertheless, transient plasma excitations including long-living metastable states by secondary electrons could be probed in this setup.

The study of biocompatibility of an artificial membrane on a suitable substrate is an important aspect for the construction of a biosensor. A good candidate for this kind of substrate is titanium nitride (TiN), which is harder and more corrosion resistant than titanium. With this motivation, we synthesize coatings of TiN on titanium substrate using a plasma immersion ion implantation (PIII) technique, which allows us to optimize substrate hardness and biocompatibility. Here we fabricate and analyze the system, starting from pure titanium, treated with the PIII technique to form a biocompatible coating composed of TiO2 and TiN. Our TiN coatings where characterized by Vickers micro-hardness test, Atomic Force Microscopy (AFM), X-ray Photo-electron Spectroscopy (XPS) and Secondary Ions Mass Spectroscopy (SIMS). After TiN coating, we deposit a chitosan (CH) layer, which acts as hydrated layer to support the phospholipid membrane (DPPC). Phospholipids were deposited on the Chitosan/TiN coating by Physical Vapor Deposition (PVD). The phospholipid bilayer on our Chitosan/TiN coating was studied with AFM. By heating the sample, we were able to observe phase transitions of the lipid bilayer. The formation of artificial membranes was studied by AFM, measuring the topography and performing force curves. A calorimetric assay was used to demonstrate that the TiN samples maintain the biocompatibility of the un-treated titanium with its native oxide film. Supported by FONDECYT grant 1170261 (HB), 1141105 (UGV), Postdoctoral Fellowship FONDECYT 3160803 (MJR) and 3160179 (PS), FONDECYT INICIACION 11160664 (TPC), CONICYT-PIA ACT 1108 and 1409, Conicyt Master and PhD fellowships (MC).

A passive thermal probe has been used to detect dynamic changes in the secondary electron emission (SEE). Oxidized and nitrided materials have been studied during argon ion sputtering in a plasma immersion ion implantation process. Identical measurements have been performed for the metallic state with high voltage pulses accelerating nitrogen ions towards the surface, supposedly forming a nitride layer. Energy flux data were combined with scanning electron microscopy images of the surface to obtain information about the actual surface composition as well as trends and changes during the process. Within the measurements, a direct comparison of the SEE within both employed ion species (argon and nitrogen) is possible while an absolute quantification is still open. Additionally, the nominal composition of the investigated oxide and nitride layers does not always correspond to stoichiometric compounds. Nevertheless, the oxides showed a remarkably higher SEE compared to the pure metals, while an indistinct behavior was observed for the nitrides: some higher, some lower than the clean metal surfaces. For the aluminum alloy AlMg3 a complex time dependent evolution was observed with consecutive oxidation/sputtering cycles leading to a very rough surface with a diminished oxide layer, leading to an almost black surface of the metal and non-reproducible changes in the SEE. The presented method is a versatile technique for measuring dynamic changes of the surface for materials commonly used in PVD processes with a time resolution of about 1 min, e.g. magnetron sputtering or HiPIMS, where changes in the target or electrode composition are occurring but cannot be measured directly.

Expanded austenite is a very hard and wear resistant phase observed after nitrogen insertion into austenitic stainless steels and CoCr alloys and while maintaining their excellent corrosion resistance. Plasma nitriding, plasma immersion ion implantation (PIII), and low energy ion implantation (LEII), among others, are employed to form this phase in the temperature region of 350°C–450°C. The nitrogen supply rate does influence the transport with a transition from a supply limited to a diffusion limited mode is observed. This saturation shifts to higher current densities for higher process temperatures. In saturation, activation energies of 0.8 eV for the nitrogen diffusion is obtained for both classes of alloys. At elevated temperatures, the expanded phase starts to decay into CrN/Cr2N and a Cr-depleted matrix. Initially, a competition between forming these precipitates and long-range diffusion for reduction of concentration gradients is present. Without additional nitrogen insertion from the surface, the gradual decay of expanded austenite leads to very unusual nitrogen depth profiles. The phase transition is initiated from the surface, while the continuing nitrogen redistribution from the expanded phase towards the substrate dominates at the interface. For additional nitrogen insertion from the surface, a much more complex picture emerges. Whereas austenitic steels allow fast nitrogen diffusion with accelerating layer growth for longer times with faster transport through the formed Cr-free FeNi network, no additional nitrogen uptake occurs for CoCr alloys as the Cr-depleted Co matrix does not permit nitrogen transport. Thus, the layer thickness at lower temperatures can be larger than at higher temperatures in this case.

A method to obtain depth-resolved structural information from hydrogen-free amorphous carbon films of high roughness, high thickness and grown on unpolished substrates was developed. The characterization was based on combining secondary ion mass spectrometry (SIMS) sputtering for craters at defined depth with X-ray photoelectron spectroscopy (XPS) and visible Raman spectroscopy analysis. After determination of the etching rate for the carbon film, areas with a defined end position corresponding to different known depths of the films were obtained. The SIMS etching process used did not significantly affect the structure of the amorphous carbon layer. XPS measurements of the crater bottoms provided information about the composition of the films. Visible Raman spectroscopy measurements inside of the craters were correlated with the XPS results taking into account the penetration depths of both techniques, and models aimed at predicting the sp2/sp3 ratio from Raman measurements were evaluated.

TiO2:N is known for its photoactivity upon illumination with visible light. Using filtered arc with energetic particle fluxes, deposition near room temperature on sensitive substrates, e.g. polymers should be possible. However, addition of nitrogen gas flux during deposition results in very small nitrogen contents. Incorporation of nitrogen up to 5–7.5at.% for either cathodic arc deposition or plasma based ion implantation and deposition leads to a reduction of the band gap down to 2.7eV before the films become semimetallic. However, only deposition at a temperature of 200°C allows avoiding the early formation of defects within the band gap. The nitrogen content was determined using secondary ion mass spectroscopy (SIMS) and calibrated with nitrogen implanted TiO2 samples using conventional beamline implantation. The results show that the nitrogen/oxygen flow ratio in two completely different deposition systems is a reliable indicator of the physical properties.