A. Pauschitz

Microstructure and mechanical properties of HVOF sprayed nanocrystalline Cr3C2-25(Ni20Cr) coating

The objectives of this work are to deposit nanocrystalline Cr3C2-25(Ni20Cr) powder by thermal spraying and to compare the performance of this coating with that obtained using conventional powder. Towards that purpose, Cr3C2-25(Ni20Cr) powders with nanocrystalline grain size and with conventional grain size were deposited using OSU-SJS high-velocity oxyfuel (HVOF) system. The microstructural features, such as morphology of the coated surface, thickness of the coating, the interface of the coating with the substrate, distribution of various phases, and grain sizes etc, were characterized with the help of optical microscope, scanning electron microscope (SEM), and transmission electron microscope (TEM). The amount of oxide phases and pores were determined by means of image analyzer. The presence of various phases was identified by x-ray diffraction (XRD) technique. Hardness, elastic modulus, and indentation toughness were evaluated employing micro indentation technique. The results indicate the presence of three different zones containing only orthorhombic Cr3C2 phase, FCC NiCr phase, and mixture of Cr3C2 and NiCr phases in, both coatings. The grain sizes in the nanocrystalline coating were in the range of 80 to 100 nm. Nanocrystalline coating exhibits 20% increase in hardness, 40% decrease in surface roughness, and comparable fracture toughness and elastic modulus with respect to conventional coating.

Nanoindentation and AFM studies of PECVD DLC and reactively sputtered Ti containing carbon films

Amorphous carbon film, bdalso known as DLC film, bdis a promising material for tribological application. It is noted that properties relevant to tribological application change significantly depending on the method of preparation of these films. These properties are also altered by the composition of the films. In view of this, bdthe objective of the present work is to compare the nanoindentation and atomic force microscopy (AFM) study of diamond like carbon (DLC) film obtained by plasma enhanced chemical vapour deposition (bdPECVD with the Ti containing amorphous carbon (Ti/a- C : H) film obtained by unbalanced magnetron sputter deposition (UMSD). Towards that purpose, DLC and Ti/a- C : H films are deposited on silicon substrate by PECVD and UMSD processes, respectively. The microstructural features and the mechanical properties of these films are evaluated by scanning electron microscopy (SEM), bdtransmission electron microscopy (TEM), nanoindentation and by AFM. The results show that the PECVD DLC film has a higher elastic modulus, hardness and roughness than the UMSD Ti/a- C : H film. It also has a lower pull off force than Ti containing amorphous carbon film.

Scratch test method: Energy dissipation and crack morphology

Abrasive wear is one of the most dominant wear mechanisms in industrial applications. Abrasive particles groove the metallic surface under various tribological interactions. Different wear mechanisms show a major influence on the abrasive wear behaviour, e.g. depending on the kinematic properties of abrasive particles. The scratch test method can be used to investigate a two-body-abrasion on a lab-scale and is defined as an ideal single tribological contact. Depending on the material behaviour, a certain amount of elastic and plastic deformation occurs during the scratch test. Especially, the characteristics of plastic deformation can give important information about the wear behaviour of the material investigated. The objective of this work is the characterization of selected material properties by the use of a standard scratch tester (Millennium 100, TRIBOtechnic) in combination with special analysis methods, e.g. SEM, confocal white light microscope, and Matlab©. The main focus is laid on the characterization of plastic deformation behaviour correlated with energy dissipation during grooving. Furthermore, a detailed study on the interface properties of multi-phase materials is done. Tests were carried out on single-phase materials with hardness in the range between 270–1100 HV5. The selected single-phase materials can be divided into brass materials, martensitic chromium steels, and complex high-alloyed Fe-based materials. Further investigations were done on multi-phase materials with Ni-based matrix and high content of synthetically added tungsten carbide (WC) with focus on crack initiation and direction of crack propagation. The influence of matrix hardness as well as carbide properties (e.g. shape, content, and orientation) are related to the crack behaviour of the composite material investigated. Results showed a strong correlation between crack morphology and matrix hardness. A soft metallic matrix (450 HV) tends to stop carbide-cracks in the interface, whereas a hard metallic matrix (600 HV) provides less resistance against crack propagation.

Biotribological model systems for emerging nanometer scale technologies

Technological devices such as pressure sensors, gyroscopes and accelerometers get smaller and smaller. This increases the necessity for the fundamental understanding of tribological phenomena at the micro- and nanometer scale. Biological systems excel also at this scale. The thesaurus that nature has developed during the last millions of years of evolution comprises self-cleaning surfaces, systems with friction coefficients smaller than any occurring in man-made systems and organisms that produce macromolecules with ice binding properties. Such systems with well adapted biotribological properties shall serve as inspiration for innovation in micro- and nanotechnology.

Surface analysis of different boundary layers on steel discs formed in a lubricated tribocontact during laboratory test compared with field application

AbstractThe aim of this study was to gain knowledge of tribological layers on a steel disc. This steel disc has been used in a transmission system in the field. ATF-oil (automatic transmission fluid), which contains different surface-adhering additives, serves as a lubricant and as a cooling fluid in the tribological system. Over time, the tribological characteristics of the system changed. Special interest in this study lies in the modifications of the steel surface. The field-used disc was therefore compared with model tested discs. In a disc-on-disc tribometer, all samples are tested for a short period of time to evaluate the actual condition of the system regarding friction behaviour. Analysis and characterisation of surfaces and layer formation were carried out with, among other techniques, 3D topography, SEM / EDX, AFM, XPS, and ToF-SIMS. Results indicate that in some regions a complex composed tribological layer is formed. Differences were detected between the steel discs used in the field and the model discs. This indicates the problem of evaluation of long-term behaviour exclusively by use of short or accelerated model experiments. A combination of carefully selected and sophisticated analytical methods is necessary to trace small changes of the system. FigureAtomic force measurement of field used and model tested “new” friction discs

An analysis of the nanotribological behaviour of Ti-containing hard carbon film

Abstract Diamondlike carbon (DLC) coatings are well known for their self-lubricating properties. Metal-containing DLC films exhibit reduced internal stress, improved adhesion and reduced sensitivity to humidity. Furthermore, Ti-containing DLC coatings are compatible with biological environments. In view of this, the primary aim of the present work was to evaluate and analyse the friction force behaviour of Ti-containing hard carbon films. This work is primarily intended for application to microelectromechanical systems. In order to achieve the above mentioned objective, a hard carbon layer containing 6 at.-%Ti was deposited using an unbalanced magnetron sputter system on a silicon substrate. The deposition rate of the film was monitored by a quartz microbalance. Using a stylus profilometer, the thickness of the film was found to be 1·67 μm. The hardness and the elastic modulus of the film were determined using a nanoindenter. The hardness and the elastic modulus were determined to be 1·7 GPa and 17·7 GPa, respectively. The pull-off force, topography and friction force surface of the film were characterised with the use of an atomic force microscope. The pull-off force and the friction force were found to be 165·9 nN and 1·7 nN, respectively. The surface roughness Ra was found to be 2·65 nm. The friction coefficient was evaluated as a ratio of the friction force and the summation of the pull-off force and the normal force. Using the topography, and lateral force image, the topography induced friction force surface and the adhesion induced friction force surface were evaluated. The influences of applied loads and scan speeds on friction coefficient and various friction force surfaces are described in detail.

High-resolution wear analysis of a ball-on-disc contact using low-activity radioactive isotopes

In this study, an advanced wear volume measuring technique making use of the radioisotope concentration method is applied to a ball-on-disc experiment, which allows highly precise online wear volume measurements and distinguishes among different wear regimes. Furthermore, the adhesive transfer of material from the activated specimen to the counteracting body was investigated with this method; it appears to be a promising technique for evaluating oil-protective capability. The experiments were carried out in a model tribometer following the ASTM D 6425 standard. A polyalphaolefin with zincdithiophosphate was used as lubricant. Two series of reciprocating sliding tests were conducted with different frequencies, 50 and 100 Hz. Results show a frequency-dependent running-in behaviour though the applied load was identical and constant. Using this precise online technique, a more accurate lifetime prediction can be estimated for applications like rolling bearings to which ASTM D 6425 refers.

A comparative AFM study of carbon alloyed Mo-Se-C and W-S-C films for tribological applications

Abstract Transition metal dichalcogenides have a layered structure and are therefore promising self-lubricating films. They can be considered as potential substitutes for carbon based films in various environmental conditions. In this work, a comparative atomic force microscopy study of co-sputtered Mo–Se–C and W–S–C films is performed to evaluate their nanotribological performances. Both films are alloyed with carbon. The microstructural features of these films are characterised using scanning electron microscopy and X-ray diffraction. The hardness and elastic modulus of these films are measured employing nanoindentation. The topography, friction forces and pull off forces of the films are evaluated by means of atomic force microscopy and force spectroscopy. The results show that the roughness parameters of Mo–Se–C films are lower than that of W–S–C films at high carbon content whereas the reverse is true at low carbon content. Adhesion forces of these films based on pull-off force measurements show that Mo–Se–C films have higher pull off forces than W–S–C films. An atomic force microscopy technique is developed to estimate microscopic values of friction coefficients and to characterise the nature of surface changes due to nanotribological experiments. The friction coefficient of Mo–Se–C films is higher than that of W–S–C films at low carbon content and these friction coefficients are comparable at high carbon content.

Tribochemistry of mono molecular additive films on metal surfaces, investigated by XPS and HFRR

In order to see how oxygen and nitrogen containing substances influence the wear relating behaviour of diesel fuels various hydroxychinolines were investigated by means of a high frequency reciprocating rig (HFRR) tribome-ter. Attention was especially given to the connection between the position of the hydroxyl group at the molecule and the resulting influence on the “lubricity” properties of low sulfur diesel fuel. Monomolecular lubricant films were deposited from the liquid phase onto ultra thin copper films sputtered onto silicon wafers. This substrate serves as a model system for bronze materials. In order to compare the results of X-ray photoelectron spectroscopy (also called electron spectroscopy for chemical analysis, ESCA) measurements with tribological experiments performed with the high frequency reciprocating rig method additional substrates made of 100Cr6 steel were used. X-ray photoelectron spectroscopy investigations were performed using a VG ESCALAB Mk III equipped with a special preparation chamber, permitting the transfer of samples from a fluid cell to the analysis chamber under Helium protective gas. The structure of the molecular film is elucidated using angular resolved X-ray photoelectron spectroscopy. Preliminary X-ray photoelectron spectroscopy investigations of ultra thin layers of 8-Hydroxychinoline on copper samples were performed. Angular resolved measurements demonstrated that, without tribological stress, full coverage of the surface with 8-hydroxyquinoline is not possible. Those results however still need corroboration by atomic force microscopy investigations.

Investigation of Stick-Slip Effects in Wet Friction Drive Elements

Up to now, the suitability of uniform tractor transmission oils (UTTO) for truck and building machine drive, brake and hydraulic systems were often investigated in expensive onvehicle or specific laboratory rig tests. Present work describes the layout and adoption of a test rig for application-oriented testing of wet brake system elements. The results gained from the tribometer experiments were compared with on-vehicle tests.