This paper describes a method for detecting stress in the surface of magnetic materials, such as steels, based on the measurement of magnetic Barkhausen emissions. The paper presents a simple practical linear calibration curve that can be used to determine stress, and also provides a theoretical explanation of the reason for such a relationship. The influence of elastic tensile and compressive stresses of various magnitudes on the magnetic Barkhausen emissions was studied, with the objective of developing a technique for quantitative measurements of surface stress in machined steels. The peak amplitude of the Barkhausen emissions was found to correlate with both residual and applied stress, showing a clear rising trend for transition from compressive to tensile stress. The relationship between stress dependence of the maximum differential anhysteretic susceptibility and Barkhausen peak amplitude was observed. Plots of reciprocal values 1 max and 1 V MBN max against stress showed a linear relationship providing a convenient method for detecting stress levels in near-surface regions from Barkhausen measurements.
We present the result of a systematic study of the tribological properties of industrial Polytetrafluorethylene (PTFE)-based coatings carried out with an atomic force microscope. A new characterization protocol allowed the reliable and quantitative assessment of the friction coefficient and adhesion forces at the sub-micrometer scale even for highly corrugated industrial samples. We have studied and compared PTFE coatings charged with different additives in dry and humid environment. The influence of additives and humidity on the friction coefficient and on adhesion forces has been investigated using standard silicon nitride tips as sliders in the low-load regime.
Atomic Force Microscopy (AFM) is a suitable tool to perform tribological characterization of materials down to the nanometer scale. An important aspect in nanofriction measurements of corrugated samples is the local tilt of the surface, which affects the lateral force maps acquired with the AFM. This is one of the most important problems of state-of-the-art nanotribology, making difficult a reliable and quantitative characterization of real corrugated surfaces. A correction of topographic spurious contributions to lateral force maps is thus needed for corrugated samples. In this paper we present a general approach to the topographic correction of AFM lateral force maps and we apply it in the case of multi-asperity adhesive contact. We describe a complete protocol for the quantitative characterization of the frictional properties of corrugated systems in the presence of surface adhesion using the AFM.
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