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Organisationseinheit der BAM
A new type of test sample for the determination of lateral resolution in surface analysis is presented. The certified reference material BAM-L002 Nanoscale strip pattern for length calibration and testing of lateral resolution is an embedded cross-section of epitaxially grown layers of AlxGa1-xAs and InxGa1-xAs on GaAs substrate. The surface of the sample provides a flat pattern with strip widths of 0.4-500 nm. The combination of gratings, isolated narrow strips and sharp edges of wide strips offers improved possibilities for the calibration of a length scale, the determination of lateral resolution and the optimization of instrument settings. The feasibility of the reference material for an analysis of lateral resolution is demonstrated for SIMS.
Mechanical and thermal response of a nickel-base superalloy upon grinding with high removal rates
(1997)
Results are reported from a pilot study under the Consultative Committee for Amount of Substance (CCQM) to compare measurements of and resolve any relevant measurement issues in, the amount of thermal SiO2 oxide on (100) and (111) orientation Si wafer substrates in the thickness range 1.5 - 8 nm. As a result of the invitation to participate in this activity, 45 sets of measurements have been made in different laboratories using 10 analytical methods: medium-energy ion scattering spectrometry (MEIS), nuclear reaction analysis (NRA), RBS, elastic backscattering spectrometry (EBS), XPS, SIMS, ellipsometry, grazing-incidence x-ray reflectrometry (GIXRR), neutron reflectometry and transmission electron microscopy (TEM). The measurements are made on separate sets of 10 carefully prepared samples, all of which have been characterised by a combination of ellipsometry and XPS using carefully established reference conditions and reference parameters.
The results have been assessed against the National Physical Laboratory (NPL) data and all show excellent linearity. The remaining data sets correlate with the NPL data with average root-mean-square scatters of 0.15 nm, half being better than 0.1 nm and a few at or better than 0.05 nm. Each set of data allows a relative scaling constant and a zero thickness offset to be determined. Each method has an inherent zero thickness offset between 0 nm and 1 nm and it is these offsets, measured here for the first time, that have caused many problems in the past. There are three basic classes of offset: water and carbonadeous contamination equivalent to ~1 nm as seen by ellipsometry; adsorbed oxygen mainly from water at an equivalent thickness of 0.5 nm as seen by MEIS, NRA, RBS and possibly GIXRR; and no offset as seen by XPS using the Si 2p peaks. Each technique has a different uncertainty for the scaling constant and consistent results have been achieved. X-Ray photoelectron spectroscopy has large uncertainties for the scaling constant but a high precision and, critically, if used correctly, has zero offset. Thus, a combination of XPS and the other methods allows the XPS scaling constant to be determined with low uncertainty, traceable via the other methods. XPS laboratories returning results early were invited to test a new reference procedure. All showed very significant improvements. The reference attenuation lengths thus need scaling by 0.986 ± 0.009 (at an expansion factor of 2) deduced from the data for the other methods. Several other methods have small offsets and, to the extent that these can be shown to be constant or measurable, then these methods will also show low uncertainty. Recommendations are provided for parameters for XPS, MEIS, RBS and NRA to improve their accuracy.
Residual stress depth profiles of a cast nickel-base superalloy were measured by regarding the deflections occurring in plate-shaped specimens while successively removing layers from the machined or treated surface by electropolishing. The results are in good agreement with previous findings showing (i) the influence of grinding parameters on the width of so-called white layers, which correspond to steep gradients of residual tensile stress, and (ii) a broad zone of compressive residual stress in the case of shot peening.
This work focuses on surface changes induced by repeated brake applications and tries to provide explanations, how such material modifications might affect friction and wear properties of automotive disc brakes. Surface films were investigated locally by transmission electron microscopy (TEM) after having prepared thin cross-sections with a focused ion beam instrument (FIB). Since the observed friction layers revealed a nanocrystalline structure, modelling with the method of movable cellular automata (MCA) was performed by assuming an array of linked nanometer-sized particles. In spite of complicated material combinations at the pad surface, two very characteristic features were always observed at both the pad and disc surface, namely a steel constituenteither ferritic (pad) or pearlitic (disc), partly covered with patches of nanocrystalline iron oxide, on a zone of severe plastic deformation with fragmented grain structure. When using an automata size of 10 nm, reasonable values for the mean coefficient of friction (COF) were obtained, namely 0.35 and 0.85 for oxide-on-oxide and metal-on-metal contacts, respectively. Immediately after brake application mass-mixing and bond-breaking was observed within a narrow zone at both surfaces.
The development of two metallizations based on the solid-phase regrowth principle is presented, namely Pd/Sb(Zn) and Pd/Ge(Zn) on moderately doped In0.53Ga0.47As (p=4×1018 cm-3). Contact resistivities of 23×10-7 and 67×10-7 ?cm2, respectively, have been achieved, where both systems exhibit an effective contact reaction depth of zero and a Zn diffusion depth below 50 nm. Exhibiting resistivities equivalent to the lowest values of Au-based systems in this doping range, especially Pd/Sb(Zn) contacts are superior to them concerning metallurgical stability and contact penetration. Both metallizations have been successfully applied for contacting the base layer of InP/In0.53Ga0.47As heterojunction bipolar transistors.
A rather new tool, the focused ion beam (FIB) technique, was used to characterise superficial layers at micro-contact areas of a commercial brake pad. The friction material was a polymer matrix composite (PMC) with approximately 50% metal content (semi-metallic) and the counter part was a cast iron rotor. Though the contact areas were not visible as topographic features, they could be identified with the aid of their increased secondary electron emission during FIB-scanning of the surface after tribological activation. Target preparation of micron-sized cross-sections with the FIB enabled the study of superficial layers at predetermined sites at high magnification.
Depending on the constituent of the pad, one, two or three layers were identified. The three layer structure comprised: (i) a 100 nm thick friction film containing nanocrystalline metal oxides and an amorphous phase which was enriched with sulphur, (ii) a nanocrystalline friction layer of compacted wear debris accommodating surface roughness and (iii) a severely deformed layer if the supporting constituent was a metal particle. Though the majority of loose wear particles was iron oxide, the friction film which adhered tightly to the pad surface contained a large amount of copper and sulphur, whereas zinc was transferred to the cast iron rotor.
Ti/TiN multilayer coatings with multilayer periods in the range 550 nm and a final thickness of 2 µm were deposited on steel substrates by cyclic modulation of nitrogen gas flow into the chamber of a PVD sputtering device. Coating characterization was performed by cross-sectional transmission electron microscopy, glancing-angle X-ray diffraction and instrumental indentation testing. Individual a-titanium and titanium nitride layers were always observed, although for the finer microstructures, the TiN layers were thicker than the Ti layers by a factor three. The plastic hardness of the films increased steadily with decreasing layer spacing, following a HallPetch relationship. Finally, a hardness value of 42 GPa was reached, which is similar to that of a thick TiN monolayer, prepared under the same coating conditions.
The surfaces of a brake pad and rotor were investigated after a run-in period during which a stable coefficient of friction had developed. The Focused Ion Beam Technique (FIB) was used to reveal tribologically induced surface films and for cross-sectional preparation of superficial layers. Additional information was obtained by TEM/EDS of thin lamellae prepared with the FIB and by surface analytical methods (GDOS, XPS and RS). Microscopic contact areas of the pad showed bright contrast in Scanning Ion Microscopy (SIM). This was attributed to severe plastic deformation finally leading to a nanocrystalline microstructure. Metallic particles of the pad, the so-called primary contact areas, were mostly covered with a smooth oxide layer of less than 1 µm thickness. Above this layer a thin (100 nm) partly amorphous film was often observed. The film was not only restricted to the metal particles, but also spread over adjacent regions, suggesting that secondary plateaus had formed. Similar layers and films, although with slightly different composition and structure, were also observed at the surface of the rotor.
The frictional behavior at local contacts in an automotive brake system was analysed on the basis of computer simulation by movable cellular automata method. The boundary conditions of the model were adjusted to experimental observations obtained by TEM. The model proved to be adequate for simulating mechanical mixing and velocity accommodation at the pad-disc interface. Dynamics of particle interaction were visualized by showing rotation angles and velocity vectors. The model provided information on the development of plastic deformation for metal-on-metal contacts and on crack formation at graphite lamellae of cast iron disc. Results are in agreement with conventional friction theories.
The size, surface charge and agglomeration state of nanoparticles under physiological conditions are fundamental parameters to be determined prior to their application in toxicological studies. Although silica-based materials are among the most promising candidates for biomedical applications, more systematic studies concerning the characterisation before performing toxicological studies are necessary. This interest is based on the necessity to elucidate the mechanisms affecting its toxicity. We present here TEM, SAXS and SMPS as a combination of methods allowing an accurate determination of single nanoparticle sizes. For the commercial material, Ludox TM50 single particle sizes around 30 nm were found in solution. DLS measurements of single particles are rather affected by polydispersity and particles concentration but this technique is useful to monitor their agglomeration state. Here, the influence of nanoparticle concentration, ionic strength (IS), pH and bath sonication on the agglomeration behaviour of silica particles in solution has been systematically investigated. Moreover, the colloidal stability of silica particles in the presence of BSA has been investigated showing a correlation between silica and protein concentrations and the formation of agglomerates. Finally, the colloidal stability of silica particles in standard cell culture medium has been tested, concluding the necessity of surface modification in order to preserve silica as primary particles in the presence of serum. The results presented here have major implications on toxicity investigations because silica agglomeration will change the probability and uptake mechanisms and thereby may affect toxicity.
8 wt% Y2O3-ZrO2 spray powder and thermally sprayed as thermal barrier coatings on outer air seals were characterized by Raman microprobe spectroscopy (RMS) and transmission electron microscopy to investigate the lateral distribution, composition, structure and grain size of ZrO2 phases and to separate ZrO2 and Y2O3. RMS of transfer layers on ZrO2-coated shrouds after rub tests against SiC laser-tipped metallic blades showed the formation of metallic oxides MeIMeII2O4 of spinel type. Wear tracks were partly covered by graphite and spinel-type oxides. In some cases, the latter were better detectable after laser enhanced cleaning by burning off the covering carbon layer.
Weld metal microstructure for alloy 6060 aluminum welds, made using the gas-tungsten arc process and alloy 4043 filler metal, has been characterized using optical metallography, EPMA microprobe analysis, SEM/EBSD and STEM/EDX electron microscopy, and single-sensor differential thermal analysis (SS-DTA). In addition, alloy 6060 castings were solidified at variable cooling rates approaching that of welding, to provide a reference for comparison with weld microstructure. It was found that a major change in cast microstructure occurs at cooling rates higher than 27 K/s resulting in a structure similar to that observed in weld metal. Rapid cooling is believed to favor low temperature solidification reactions that normally would be achieved only at higher silicon content. Accordingly, additions of 4043 filler metal that increase the weld metal silicon content have only limited affect on weld solidification range and microstructure. This has direct implications regarding how 4043 filler additions improve weldability and weld quality.
Tribological screening tests (simple, reciprocating ball-on-flat tests) were performed with the objective to identify an appropriate coating for the articulating surfaces of artificial hip joints whose acetabular cups and femoral stems are made from Ti6Al4V alloy, which is appreciated for its light weight, good biocompatibility and elastic properties similar to those of natural bone. Standard coatings like TiN or CrN performed better than more complicated multi-layer systems, though not as good as different types of amorphous carbon coatings, generally referred to as diamond-like carbon or DLC coatings. Among the latter, hydrogenated amorphous carbon (a-C:H) displayed the best properties, especially if the hydrogen content was increased by reducing the bias voltage during PA-CVD-deposition.
The optimised a-C:H coating revealed the most promising wear behaviour under the applied testing conditions, i.e. the increase of linear wear with the number of cycles was close to zero. Regarding the materials examined in this study, correlation of wear with mechanical properties obtained by nano-indentation revealed that high hardness was not an adequate criterion for selecting appropriate coatings. A high ratio of hardness and elastic modulus (H/E) proved to be more important. Microstructural and micro-analytical investigations revealed transformation of TiN and CrN to TiO2 and Cr2O3, respectively, and amorphous carbon was, at least partly, transformed to graphite. Furthermore, incorporation of Al2O3 from the ball was observed at a very fine scale. The wear debris of favourable coatings always formed agglomerates of nano-scale particles. It was shown that commercial nano-particles of Al2O3, Cr2O3 and carbon black are comparable to particles generated by the tribological tests. However, it is uncertain whether they are comparable to those formed during simulator studies or in vivo. Furthermore, the performance of the favourable coating has to be tested in a hip joint simulator before its potential for application in prostheses can be assessed.
Turbine components from conventionally cast nickel-base alloy René 80 show different hot cracking susceptibilities depending on their heat treatment conditions leading to slightly different microstructures. Electron probe micro-analysis, focused ion beam technique and analytical transmission electron microscopy were applied to reveal and identify grain boundary precipitates and the γγ'-microstructure. The distribution of borides along grain boundaries was evaluated statistically by quantitative metallography. The following features could be correlated with an increase of cracking susceptibility: i) Increasing grain size, ii) increasing fraction of grain boundaries with densely spaced borides, iii) lack of secondary γ'-particles in matrix channels between the coarse cuboidal γ'-precipitates. The latter feature seems to be responsible for linking-up of cracked grain boundary precipitates which occurred as an additional cracking mechanism after one heat treatment, whereas decohesion at the boride-matrix-interface in the heat affected zone of laser-drilled holes was observed for both heat treatments.
In the paper the method of discrete modeling (movable cellular automata method) and combined discrete-continuous description of the simulated medium are used to analyze processes occurring in the local contact of the automotive brake system. The characteristic size of the considered region is 1.5 ?m. The following contact situation is simulated: steel fiber coated by an iron oxide film as the brake pad and pearlitic steel also coated by an iron oxide layer as the disc. On the assumption of oxide layer wearing we simulate the iron oxide - iron oxide, iron oxide - metal and metal - metal contacts.
The calculation results for the friction coefficient for various contact situations give quite adequate values. For example, for the oxide - oxide system the calculated coefficient is approximately equal to 0.4, while for the metal - metal contact the obtained value varies from 0.7 to 0.9. Analysis of a set of the obtained results allows concluding that oxide is formed more rapidly than the sliding layer, which in turn makes the friction coefficient value stabilized.
Copper is a major ingredient in friction materials used for automotive braking. The purpose of this study was to find out how copper contributes to good brake performance properties in addition to providing good thermal conductivity. Microstructural investigations of copper chips at the surfaces of brake pads revealed a zone of severe plastic deformation which provides high hardness, but there is also evidence of recrystallized copper nano-particles which are incorporated into friction layers as soft ingredient once detached from the pad surface. Thus copper seems to play a dual role, firstly as reinforcing element of the brake pad providing primary contact sites, and secondly as solid lubricant by contributing to the formation of a layer of granular material providing velocity accommodation between the rotating disc and fixed pad. Confirmation for this hypothesis was obtained by modelling contact sites on the nanometre scale with the method of movable cellular automata. Results show both, the similarity of steel fibres and copper macro-particles in respect to providing primary contact sites, as well as similar sliding behaviours of friction layers containing either copper or graphite as soft inclusions. Furthermore, it is shown that not only material properties, but also the concentration of solid lubricant particles in the friction layers, determine conditions for friction force stabilization and smooth sliding behaviour.
The unique nanostructure formed during severe as well as moderate braking on the surface of brake discs was investigated by conventional and analytical Transmission Electron Microscopy. In both cases nanocrystalline magnetite mixed with carbon nanoinclusions and minor amounts of other pad constituents were identified. On the basis of these observations the friction performance of a single micro-contact was simulated with the method of Movable Cellular Automata. Inspite of a simplified nanostructure which was examined in two dimensions only, the calculated mean coefficient of friction fitted well to the value usually demanded for automotive braking. Furthermore, the model predicts that oxide films without soft nanoinclusions are not capable of providing smooth velocity accommodation at the pad–disc interface and thus lead to unstable friction behaviour.
In the paper, a model typical for contact situations of automotive brakes is established based on the method of movable cellular automata. The processes taking place at local contacts in an automotive brake system are analysed. Based on microscopic and micro-analytical observations, the following contact situations were simulated: (i) a couple of ferritic steel against pearlitic steel, both covered by an oxide layer mixed with graphite nanoparticles and (ii) the same situation but without oxide layers. The results of calculated mean coefficients of friction of the oxide-on-oxide contact correspond well to expected values for a real braking system, whereas steel-on-steel contact are twice as high. This allows one to make some conclusions; for example, oxide formation will take place more quickly than friction layer elimination, and finally this is responsible for the stabilisation of the coefficient of friction.
An attempt was undertaken to obtain a better understanding of the tribological properties of two wear-resistant coatings on tool steel by structural and microchemical analysis of wear particles using a transmission electron microscope. Coatings were deposited by physical vapor deposition and plasma-assisted chemical vapor deposition techniques and tribological properties were derived from reciprocating sliding tests of the coatings against alumina balls. Three types of wear particles were identified by electron diffraction and energy dispersive X-ray spectroscopy: nanocrystalline rutile (TiO2), nanocrystalline graphite and microcrystalline graphite. Low coefficients of friction, of the order of 0.2, were attributed to the formation of solid lubricant films of sub-stoichiometric TiO2-x Magnéli phases and/or graphite.
XTEM and TFXRD investigations of ohmic Ti/Al/Ti/Au/WSiN contacts on AlGaN/GaN HFET layer systems
(2002)
The microstructural features of the high-temperature-stable ohmic contact system Ti/Al/Ti/Au/WSiN on AlGaN/GaN were investigated using analytical transmission electron microscopy and thin film x-ray diffraction. For two typical rapid thermal annealing steps at 750 °C (non-ohmic behaviour) and 850 °C (ohmic behaviour) the intermetallic phases at the metal-semiconductor interface are presented. Increased annealing leads to the transformation of an Al2Au-AlAuTi phase mixture to a mixture of Al2Au-Al3Au8 phases and the formation of Ti-Al-nitride layers at the interfaces. In light of these results the electrical contact properties are discussed.
Cross-sectional transmission electron microscopy, in combination with energy dispersive X-ray spectroscopy and focused beam microdiffraction, was applied to study the solid-state reactions taking place during contact formation of the system Ge(115 nm)/Pd(50 nm)In0.53Ga0.47As. In order to get information about the sequence of the different processes, rapid thermal, annealing experiments in the range 225400 °C were performed. The following features were observed: at 225 °C Pd reacted with the substrate forming the quaternary phase PdxIn0.53Ga0.47As (x 4), and with the Ge-layer forming mainly PdGe and Pd2Ge. Between PdxIn0.53Ga0.47As and In0.53Ga0.47As, a 5 nm thick amorphous Pd-In-Ga-As layer remained, indicating that the first reaction step was solid-state amorphization. After annealing at 350 °C, PdxIn0.53Ga0.47As disappeared and regrowth of In0.53Ga0.47As occured. Finally, at 400 °C, residual Ge from the amorphous top layer diffused to the interface and grew epitaxially on the regrown In0.53Ga0.47As, thus separating the IIIV compound semiconductor from the Pd-Ge reaction products. The interface remained flat, while only about 10 nm of the active In0.53Ga0.47As layer had been modified during the annealing processes.
Transmission electron microscopy study of rapid thermally annealed Pd/Ge contacts on IN0.53Ga0.47As
(1996)
TEM study on the effect of machining on the microstructure of a soft magnetic Ni76Fe17Cu5CrMo2 alloy
(1991)
Low-cycle fatigue experiments under combined axial-torsional loading have been carried out on alloy 800 H tubular specimens at room temperature. In comparison with proportional loading, an extra cyclic hardening effect produced by nonproportional loading was observed. The microstructure study highlights the fact that the dislocation arrangement under proportional loading is significantly different from that under nonproportional loading. Mechanical twinning was found in specimens cycled under axial loading and nonproportional loading. It is suggested that mechanical twinning depends not only on shear stress but also on normal stress on the plane of maximum shearing. The extra hardening can be interpreted in terms of the deformation microstructure. Fatigue cracking was initiated generally at the specimen surfaces along the plane of maximum shearing, but under nonproportional loading cracks were found also in the bulk of the specimens. Transcrystalline crack propagation was observed in the specimens after proportional and nonproportional LCF tests.