Claus Rebholz

Structure, mechanical and tribological properties of Ti–B–N and Ti–Al–B–N multiphase thin films produced by electron-beam evaporation

Titanium-based multiphase ceramic coatings were deposited by electron-beam plasma-assisted physical vapor deposition, evaporating mixtures of Ti, TiB2, and (Ti0.6Al0.4)B2.19N1.47 material in Ar or Ar/N2 plasmas. All exhibited dense microstructures, however varying amounts of droplets or spits incorporated in the coatings produced could be observed for the different evaporation materials depending on their fabrication route. Results on the chemical composition of the coatings, obtained from glow discharge optical emission spectroscopy and Auger electron spectroscopy, showed no preferential evaporation of any element from the different evaporation source materials used, resulting in very similar compositions between evaporant and coating. Hardness values of up to 40 GPa were found for Ti–B–N based coatings containing both TiB2 and c-BN phases. Lower hardness values of around 30 GPa were observed for coatings deposited within the quaternary Ti–Al–B–N system, due to the presence of h-BN. Two micron thick Ti–A...

In situ observation of rapid reactions in nanoscale Ni–Al multilayer foils using synchrotron radiation

The observation of rapid reactions in nanoscale multilayers present challenges that require sophisticated analysis methods. We present high-resolution in situ x-ray diffraction analysis of reactions in nanoscale foils of Ni0.9V0.1–Al using the Mythen II solid-state microstrip detector system at the Material Science beamline of the Swiss Light Source Synchrotron at Paul Scherrer Institute in Villigen, Switzerland. The results reveal the temperature evolution corresponding to the rapid formation of NiAl intermetallic phase, vanadium segregation and formation of stresses during cooling, determined at high temporal (0.125 ms) and angular (0.004°) resolution over a full angular range of 120°.

Investigations on the self propagating reactions of nickel and aluminum multilayered foils

The self-propagating reaction of nickel-aluminum thin film multilayers with a bilayer thickness of ∼43 nm was investigated using high-speed optical camera and infrared thermometry. The results indicate a two-stage reaction with two different characteristic temperatures. Following ignition, the flame front propagates near the reverse peritectic transformation temperature of Ni2Al3 into NiAl and liquid at 1406 K. The reaction continues with the growth of NiAl until the melting temperature of 1911 K is reached. The reaction mechanism and kinetics are discussed.

Plasma-based surface engineering processes for wear and corrosion protection

Vacuum plasma‐based coating and treatment methods have considerable potential for reducing wear and corrosion. However, they have not been widely applied to engineering components other than tools such as those used in metal cutting. This is due to several factors, not least of which are cost and an unsuitability for cheaper substrate materials. Here we discuss six plasma‐based processing routes that can widen the applicability of these processes and ensure greater market penetration. These methods are: (i) duplex plasma nitriding plus physical vapor deposition (PVD) coating, (ii) low temperature plasma diffusion treatment, (iii) postcoat plasma nitriding, (iv) layered diamondlike carbon/metal carbide PVD/chemical vapor deposition coating, (v) thick metal/metal nitride PVD coating, and (vi) duplex electroless nickel plus PVD coating. The characteristics of these methods are discussed, especially with regard to their beneficial influence on wear or corrosion.

Room temperature synthesis and high temperature frictional study of silver vanadate nanorods

We report the room temperature (RT) synthesis of silver vanadate nanorods (consisting of mainly beta-AgV O(3)) by a simple wet chemical route and their frictional study at high temperatures (HT). The sudden mixing of ammonium vanadate with silver nitrate solution under constant magnetic stirring resulted in a pale yellow coloured precipitate. Structural/microstructural characterization of the precipitate through x-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed the high yield and homogeneous formation of silver vanadate nanorods. The length of the nanorods was 20-40 microm and the thickness 100-600 nm. The pH variation with respect to time was thoroughly studied to understand the formation mechanism of the silver vanadate nanorods. This synthesis process neither demands HT, surfactants nor long reaction time. The silver vanadate nanomaterial showed good lubrication behaviour at HT (700 degrees C) and the friction coefficient was between 0.2 and 0.3. HT-XRD revealed that AgV O(3) completely transformed into silver vanadium oxide (Ag(2)V(4)O(11)) and silver with an increase in temperature from RT to 700 degrees C.

Microstructural and morphological effects on the tribological properties of electron enhanced magnetron sputtered hard coatings

X‐ray diffraction measurements and scanning electron microscopy studies on close to stoichiometric titanium carbonitride [TiCxN(1−x)] and carbon containing tungsten coatings produced in an electron enhanced unbalanced magnetron system have been undertaken. These coatings are deposited as interlayers to improve adhesion and load support for metal diamondlike carbon (DLC) multilayers. In this study, however, only the interlayer without the top coating (metal DLC) was deposited to study and optimize this layer. The films were deposited on stainless steel substrates with an argon background pressure in the range of 0.1 Pa, introducing acetylene while sputtering tungsten and a mixture of acetylene and nitrogen while sputtering titanium. The chemical composition of the film was analyzed by a glow discharge optical spectrometer. The peak positions and integral breadth and shape parameters were determined by x‐ray diffraction. Microhardness, scratch adhesion, and the tribological performance under sliding wear ha...

Novel CeO2-based Screen-Printed Potentiometric Electrodes for pH monitoring

Nuclear waste repositories are being installed in deep excavated rock formations in some places in Europe to isolate and store radioactive waste. In France, the Callovo-Oxfordian formation (COx) is a possible candidate for nuclear waste storage. This work investigates the applicability of CeO(2)-based oxides (CeO(2), Ce(0.8)Sm(0.2)O(2) and Ce(0.8)Zr(0.2)O(2)) for monitoring the pH of the COx pore water (T=25°C). The study is limited to the pH range between 5.5 and 13.2, which includes the pH values that have been encountered or are anticipated in the COx formation during its evolution as radioactive waste repository due mainly to alkalinisation, an increase in salinity, and a decrease in redox potential. Screen-printing was done to assemble electrodes and rapidly generate data sets. The electrochemical behavior of CeO(2)-based screen-printed electrodes (CeO(2)-based SPEs) was determined by cyclic voltammetry and electrochemical impedance spectroscopy. The use of the electrodes for pH sensing was then evaluated by potentiometric measurements. The feasibility of measuring pH with CeO(2)-based SPEs was first tested in NH(4)Cl/NH(3) buffer solutions, leading to electrode calibration over the widest range of pH, from around neutral to basic pH. Experiments were then conducted in NaHCO(3)/Na(2)CO(3) buffer samples similar to conditions prevailing in the COx formation. Ce(0.8)Zr(0.2)O(2) SPEs exhibit a near-Nernstian behavior (sensitivity -(51±2)mV/pH) in the pH range of 5.5-13.2 at 25°C. Electrode response was slightly affected by the direction of the pH change. Electrode reliability was clearly demonstrated for pH monitoring. Probes based on the same components, but more durably designed, could be considered for pH measurements in radioactive waste repositories.

Synthesis and characterization of Cr–B–N coatings deposited by reactive arc evaporation

Nanocomposite Cr–B–N coatings were deposited from CrB 0.2 compound targets by reactive arc evaporation using an Ar/N 2 discharge at 500 °C and −20 V substrate bias. Elastic recoil detection (ERDA), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and selected-area electron diffraction (SAED) were used to study the effect of the N 2 partial pressure on composition and microstructure of the coatings. Cross-sectional scanning electron microscopy (SEM) showed that the coating morphology changes from a glassy to a columnar structure with increasing N 2 partial pressure, which coincides with the transition from an amorphous to a crystalline growth mode. The saturation of N content in the coating confirms the formation of a thermodynamically stable CrN–BN dual-phase structure at higher N 2 fractions, exhibiting a maximum in hardness of approximately 29 GPa.

Growth and characterization of ceria thin films and Ce-doped γ-Al2O3 nanowires using sol–gel techniques

γ-Al(2)O(3) is a well known catalyst support. The addition of Ce to γ-Al(2)O(3) is known to beneficially retard the phase transformation of γ-Al(2)O(3) to α-Al(2)O(3) and stabilize the γ-pore structure. In this work, Ce-doped γ-Al(2)O(3) nanowires have been prepared by a novel method employing an anodic aluminium oxide (AAO) template in a 0.01 M cerium nitrate solution, assisted by urea hydrolysis. Calcination at 500 °C for 6 h resulted in the crystallization of the Ce-doped AlOOH gel to form Ce-doped γ-Al(2)O(3) nanowires. Ce(3+) ions within the nanowires were present at a concentration of < 1 at.%. On the template surface, a nanocrystalline CeO(2) thin film was deposited with a cubic fluorite structure and a crystallite size of 6-7 nm. Characterization of the nanowires and thin films was performed using scanning electron microscopy, transmission electron microscopy, electron energy loss spectroscopy, x-ray photoelectron spectroscopy and x-ray diffraction. The nanowire formation mechanism and urea hydrolysis kinetics are discussed in terms of the pH evolution during the reaction. The Ce-doped γ-Al(2)O(3) nanowires are likely to find useful applications in catalysis and this novel method can be exploited further for doping alumina nanowires with other rare earth elements.

Fabrication of nanorods by metal evaporation inside the pores of ultra-thin porous alumina templates

Porous alumina has attracted a great deal of attention as a template material for the growth of nanowires and nanodots. Typically, the pores have a high aspect ratio, which forbid the use of evaporation techniques for filling them, due to a pore closure effect. For this reason electrochemical methods are mainly used. However, there are materials, such as Al, which are very difficult to deposit electrochemically. In this work, the fabrication of Al nanorods by electron gun evaporation into low aspect-ratio pores of ultra-thin porous alumina templates is described. The thicknesses of the templates are in the range from 50 to 70 nm, while their pores have diameters from 20 to 40 nm, i.e. their diameter:height aspect ratios are very low, from 1:1.5 to 1:3. These properties make it possible to completely fill the pores with evaporation techniques. This method can be generalized to any target and substrate material.