E. Santner

Advances in tribology: the materials point of view

The application of advanced materials in various areas of contemporary technology can lead to improvements in the function, quality and performance of engineering components and systems. In this paper, an overview of the developments in high performance materials, both organic and inorganic based, is given. This includes thin hard coatings because of their increasing importance in tribological improvements. For these types of materials the requirements for tribo-engineering applications are analysed. Research results from BAM concerning ceramics and ceramic composites, polymers and polymer composites as well as hard coatings illustrate the friction and wear behaviour of these materials and their potential for tribo-engineering applications.

Influence of the testing parameters on the tribological behaviour of self-mated PVD-coatings

Abstract Friction and wear of self-mated PVD-coated tribo-couples were investigated using different testing parameters. By means of pin-on-disc geometry, tribological tests with coatings like TiN, CrN and (Ti,Al)N were performed. Fundamental investigations on these sliding pairs show that the tribological behaviour is strongly influenced by the testing conditions. Depending on the ambient medium, relative humidity, sliding distance or velocity etc., very different friction and wear coefficients can be observed before the coating of at least one tribo-partner is worn out. The variation of a single test parameter can lead to a fundamental change of the functional behaviour of the whole tribo-system. While TiN- and CrN-coatings reveal very good results at high relative humidities, (Ti,Al)N-coatings show its best performance at dry environments or higher ambient temperatures. After the tests, the worn surfaces and wear debris were investigated by means of microanalysis, e.g. Auger electron spectroscopy (AES), small-spot electron spectroscopy for chemical analysis (ESCA) and scanning electron microscopy (SEM). Small-spot-ESCA gives valid information about the chemical composition of the surface before and after tribological stressing. The results of microanalytic investigations show that tribochemical reactions are the dominating wear phenomena. The extent of these tribochemical reactions and the chemical composition of the wear debris depends on the ambient conditions and the testing parameters. The tribological behaviour of TiN/TiN sliding pairs gets worse when TiO 2 is built as a result of tribochemical reactions. On the other side, the formation of Cr 2 O 3 and CrO 3 during tribological stressing do not lead to a significant change of friction and wear of CrN/CrN sliding pairs. For (Ti,Al)N/(Ti,Al)N sliding pairs tribo-oxidation is the dominating wear mechanism. The wear debris consist of titanium and aluminium oxide, but the chemical structure is not clear yet.

The influence of roughness on friction: Part I: The influence of a single step

Part I of this paper series presents experiments whose goal is to reveal a correlation between roughness and friction force for contact areas on the micrometer scale. An elastic deformation model was developed to describe this correlation. This model concentrates on friction processes which are dominated by elastic deformation. The experiments were performed on defined model tribosystems with a tribometer of our own design equipped with an atomic force microscope (AFM) and an optical microscope. The samples were silicon wafers whose roughness was created by lithography and the counter bodies were balls of different materials. In contrast to part II the roughness of the samples consists only of a single step. In part II the elastic deformation model will be tested on roughness with two and more steps inside the contact area. Finally, the model will be validated on a surface which has a statistical roughness resulting from various grinding processes.

Comprehensive tribological characterization of thin TiN-based coatings

Innumerable papers have been published so far describing tribological investigations of thin hard coatings based on TiN. Analysis of the presented results demonstrates a large dispersion of measured friction and wear numbers, whereas TiN-coated pieces and tools have proved their benefits in a broad area of application. Therefore an attempt was made to clarify the influences on friction and wear test results by varying the coating process, the tribological stresses due to sliding, fretting and rolling motion and by changing the surrounding medium. The results reveal that machining of substrate surfaces and type of tribological stresses due to sliding, fretting and rolling have an important influence. The formation of reaction layers is dominating the tribological behaviour in most cases.

The influence of roughness on friction: Part II. The influence of multiple steps

Aim of the present work is to derive a correlation between roughness and friction force. This part II of a paper series tests the elastic deformation model presented in part I on more general tribosystems. Where in part I the influence of a single artificially generated step on friction force was investigated, part II considers the influence of two and more structures inside the contact area on friction force. This artificial roughness was created by lithography on silicon wafers, as regular grooves and ridges. The distances between the steps of the grooves ranged from 100 to 0.1 μm. Finally, the correlation between roughness and friction force was validated on surfaces with statistical roughness created by various grinding processes.

Quantitative wear analysis using atomic force microscopy

A new method is presented to measure the volume of locally restricted topographical changes on surfaces due to wear with ultra-high precision. For this purpose a so-called stand-alone atomic force microscope (AFM) is used in combination with a special sample holder that allows to observe the same section of the sample surface before and after tribological stressing in a conventional tribometer. Exceeding the resolution limits of standard profilometry by far, the AFM can be used for quantifying smallest wear events and gives new insights in the details of the tribological process. As examples, measurements of hardness indentations on a Si wafer, as well as wear events due to sliding motion on an AuNi coating and filled polymers for electrical contact systems are shown. In addition, some practical aspects such as avoiding measurement errors and possible applications are discussed in detail.

Topographic Changes on the Surfaces of PVD Coatings in Humid Air: an AFM/LFM Study

Tribological investigations of self-mated physical vapour deposition (PVD) coatings have shown in part very low friction in humid air and a strong dependence of the tribological behaviour on the relative humidity during the test. This behaviour can be interpreted in terms of the growth of surface layers, which act as solid lubricants and reduce friction and wear considerably. The use of surface sensitive analytical methods like SEM, XPS and AES for getting more information about this kind of surface layer, however, can have an adverse effect on these layers because of the necessary vacuum conditions. Therefore, atomic force microscopy (AFM) and lateral force microscopy (LFM) were used to study the topographical changes of two different PVD coatings under the influence of humidity. The aim of this work was to verify and to explain the well-examined macroscopic behaviour of two different engineering coatings on a microscopic scale. Because of the spatial resolution of the LFM technique it is possible to discern species with a different frictional behaviour on the same surface. This allows the beginning of (tribo)chemical reactions to be detected locally in contrast to conventional integrating tribological testing. Depending on the chemical nature of the PVD coatings and the stock holding period in certain environments, topographical changes can be detected by means of AFM/LFM. Under humid conditions small hillocks (droplets) begin to grow on TiN surfaces, the number and size of these hillocks depending on the duration of stock holding. On CrN surfaces this hillock formation does not occur even after extended storage for several weeks in humid air.

Utility and limitations of tribosimulation for quality control and material preselection

Abstract The potential and limitations of tribosimulations for preselection of materials or coatings for technical application and for tribological failure analysis are the objectives of this article. The results of relatively simple pin-on-disc tests correlate reasonably well with the real cutting performance of coated tools, so that less expensive procedures for coating optimization can be proposed. A short-run pin-on-disc test proved to be secure and self-sufficient way to decide between well functioning and failing polymer compound sliders in a tumble drier. However, the greatest effort of analytical methods could not explain the true causes for the differing tribological behaviour of identical compounds which belong only to different production batches.