Tribology - Materials,Surfaces & Interfaces

Adsorbed water is present on nearly all surfaces exposed to the ambient and can greatly influence tribological properties such as adhesion, friction and wear. This paper reviews how the effects of the surface chemistry, such as oxidised or terminated by different functional groups, greatly influences the water adsorption isotherm thickness as well as the structure of adsorbed water. The thickness and activity of the adsorbed water greatly affect the wear between different material surfaces. Sufficient adsorbed water could induce a detrimental effect such as galvanic corrosion between dissimilar metals or a beneficial effect strengthening soda lime glass.

The gap between academic science/engineering and industry concerning problem solving competencies is addressed in this paper by proposing the New Asian Case Method, an approach inspired by, but substantially different from, the trademark of professional education of the Harvard Business School. The paper puts forward the idea that knowledge in tribology shall be arranged in cases with practical examples that allow students and researchers to learn tribology on real world problems, and obtain a toolbox they subsequently use in their work as tribologists in academia or industry. Micro- and nanotribology are accentuated as high potential role model fields for successful interdisciplinary, inherently application oriented approaches. The New Asian Case Method increases quality approaches in tribology, by structuring the ways of thinking and addressing problems. Establishing the New Asian Case Method in tribology shall set the basis for an international standard in the field.

There are a number of different mechanisms by which tooth wear occurs, including tooth–tooth contact, temperature changes and the chemical environment inside the mouth; these different mechanisms can occur simultaneously. ‘Attrition’, ‘abrasion’, ‘abfraction’ and ‘erosion’ are the main terms used to describe tooth wear. Abrasion occurs in the presence of abrasive particles from various sources. Damage by abrasion occurs on enamel or dental porcelain surfaces when particles become trapped in the occlusal zone, a mechanism called three-body wear abrasion. In the current study, three commercial dental powder–liquid porcelains (used as restoratives in posterior teeth) and one amalgam restorative were tested using an in vitro method with a Plint TE66 microwear tester. Wear conditions were 0·25 N of load applied, 0·1 m s⁻¹ of ball speed and 7·98 m of sliding distance at room temperature. Mixtures of distilled water and artificial saliva were used as liquid suspension, each containing 22 vol.-% silicon carbide (SiC F1200), with an average size of 4 μm. Lost volume and wear coefficient of the restoratives were determined. Scanning electron microscopy images were obtained, the wear patterns (scars) on the specimens were analysed and the wear mechanisms (including brittle fractures for the porcelains, and ploughing, cutting and plastic deformation for the amalgam) were identified and discussed. Wear topography was obtained from atomic force microscopy images.

A complex approach based on atomic force microscopy (AFM) is developed to establish influence of nanoscale layer thickness on its elastic, adhesive and frictional properties of polymeric coatings for microelectromechanical systems. Thermoheating element was applied to perform AFM measurements with thermal effects in the temperature range from 20 to 120°C. Friction coefficients at high velocities of sliding and dependences of friction coefficient on the temperature of heated films at low velocities of sliding are defined. This study concludes that the Young's modulus of ultrathin polymeric films on silicon substrate is reduced when thickness or temperature is increased.

Nanoscratch testing, as an important technique for the assessment of the mechanical failure behaviour and adhesion strength of ceramic coatings and a simulation tool of single asperity contact in tribological experiments, is increasingly becoming an established nanomechanical characterisation method. This paper reviews recent work in nanoscratch testing in different engineering applications including thin ceramic films, automotive organic coatings, chemical–mechanical polishing and biomaterials. In the main part of the paper, nanoscratch results from experiments performed using NanoTest systems fitted with tangential force sensors and spherical indenters as scratch probes are presented and discussed. The types of nanoscratch tests described include constant load nanoscratches, ramped load nanoscratch tests and multipass repetitive unidirectional constant load nanoscratch tests (nanowear). The results are discussed in terms of critical load sensitivity to intrinsic and extrinsic factors, impact of scan speed and loading rate, influence of probe radius and geometry, estimation of tip contact pressure, influence of surface roughness and film stress and thickness, and finally role of ploughing on friction evolution.

The minimisation of friction and adhesion during sliding contacts is crucial for the industrial fabrication of many micro/nanodevices (e.g. MEMS/NEMS), as well as in nanotechnological processes, e.g. in nanoimprint lithography where a silicon mould is used to fabricate polymeric nanostructures by imprinting. We have conducted intensive research on the contact between the mould and PMMA polymeric resist film via advanced modelling and computer simulations. The properties of the contacting surfaces have been identified with the atomic force microscope and nanoindentation, as well as wettability tester applied for the identification of the surface free energy. A model of contact has been elaborated and adequate original software was used to calculate the frictional and adhesive forces in particular at the silicon mould/polymeric resist interface.

Access to the tribological contact behaviour of individual small particles is crucial with respect to many applications in particle technology. In this paper, we present a simple nanoindentation based approach to study the rolling friction of micrometre sized spherical particles. The results are compared to sliding friction tests, which are carried out by a nanoindentation based colloid probe technique. The potential of the approach is evaluated for borosilicate glass spheres featuring nominal radii of 2·5 and 10 μm in contact with silicon surfaces. The roughness of the latter is modified by a plasma etching process. Significant differences of sliding and rolling friction with respect to roughness are observed even though the variation of root mean square roughness only ranges from 0·3 to 2·7 nm respectively.