Wear phenomena at the nanoscale are essential for applications involving miniaturized specimens. Furthermore, stochastic nano-events affect in general tribological processes, eventually also at the macroscale. Hence, it is of fundamental importance to perform nanotests with materials-such as steel-which are widely used also at the macroscale. In this paper, we present the analysis of tribotests performed with self-mated 100Cr6 steel (AISI 52100) at the submicron scale by means of an atomic force microscope. To this aim, steel particles with micrometer size were glued to the cantilever as "colloidal particles". The microscope was employed for wear generation, for the imaging of scars and colloidal particles, and for the determination of wear volumes of both specimens. The analysis is focused on wear volume and its dependence on normal force and total sliding distance. Nanotests are compared with previously presented macrotests, also performed with self-mated steel. Nanotests exhibit, compared with macrotests, a significantly larger scattering and poor repeatability. Especially the analysis of these features reveals that, with small forces (<= 10 mu N) and surfaces (<= 2 mu m(2)), the random number of asperities inside the contact surface plays a crucial role, by far more decisive than the normal force or the sliding distance. Moreover, in several cases, only few asperities (<10) are involved in the wear process. Such low numbers lead to a breakdown in the applicability of tribological laws (e.g., Archard's law) based on statistical methods and on average variables.

EDITORIAL article Front. Mech. Eng., 21 February 2022 | https://doi.org/10.3389/fmech.2022.853934

Friction at the microscale during reciprocal sliding tribotests was studied for the first time with self-mated steel (100Cr6/AISI 52100) taking advantage of an atomic force microscope (AFM). To this aim, microsized steel particles were glued to the AFM-cantilever and employed as colloidal tips to perform tribotests on a steel disc. The torsion of the cantilever, which correlates with the friction force, was measured during the tests. Due to the irregular shape of the test particles and their wear, it is not possible to calibrate the torsional response of the cantilever and absolute quantification of the friction force cannot be achieved. Nevertheless, the model system used in the presented measurements is more representative of the tribology of real mechanical tribo-elements than already studied systems, in which for example only one tribopartner is worn. Few tests with the same load did not yield any wear and show that the load and adhesion contributions to friction stay constant when the shape of the test particle does not change. Most of the presented tribotests engendered wear. For those tests, the increase of friction during the tribotests was detected and was attributed to the emerging plowing contribution. Furthermore, analysis of both torsion and local slope experienced by the cantilever during the tests gives information on the creation of wear particles and their influence on friction.

Single asperity nanowear phenomena are fundamental for understanding basic tribological mechanisms. Yet, they are studied mostly through theoretical and simulation works. Few experiments were conducted in the past decades, usually with materials which are commonly used in micro- and nanotechnology, but not for macroscopic components with relevance in tribology. In the present work, we show for the first time tribotests performed with self-mated 100Cr6 steel, a very widespread material at the macroscale, taking advantage of an AFM, employed as a tribometer for the tribotests as well as for the inspection of wear of both tribopartners. Emphasis is put on the morphology of the scars, on wear particles, and on wear of the “colloidal” particles glued on the AFM cantilever. Measurements demonstrate the possibility of characterizing single asperity events leading to very small wear (scars with isolated, down to 1-nm-deep scratches). We highlight several phenomena, for example, transfer of wear particles and their negative contribution to wear volume, which are elementary key constituents of tribological processes. Such phenomena, probably occurring also at the macroscale, can be detected, identified, and characterized with high spatial and time resolution only at the nanoscale, thus giving insight into conditions and causes of their emergence.

Quantitative bioimaging of Quantum Dots (QDs) uptake in single cells by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a challenging task due to the high sensitivity and high spatial resolution required, and to the lack of matrix-matched reference materials. In this work, high spatially resolved quantitative bioimaging of CdSe/ZnS QDs uptake in single HT22 mouse hippocampal neuronal cells and in single HeLa human cervical carcinoma cells is novelty investigated combining: (a) the use of a ns-LA-ICP-Sector Field (SF)MS unit with mono-elemental fast and sensitive single pulse response for 114Cd+; and (b) the spatially resolved analysis of dried pL-droplets from a solution with a known concentration of these QDs to obtain a response factor that allows quantification of elemental bioimages. Single cells and dried pL-droplets are morphologically characterized by Atomic Force Microscopy (AFM) to determine their volume and thickness distribution. Moreover, operating conditions (e.g. spot size, energy per laser pulse, etc.) are optimized to completely ablate the cells and pL droplets at high spatial resolution. Constant operating conditions for the analysis of the single cells and calibrating samples is employed to reduce potential fractionation effects related to mass load effects in the ICP. A number concentration of CdSe/ZnS QDs between 3.5 104 and 48 104 is estimated to be uptaken by several selected single HT22 and HeLa cells, after being incubated in the presence of a QDs suspension added to a standard cell culture medium. Mono-elemental bioimaging at subcellular resolution seems to show a higher number concentration of the CdSe/ZnS QDs in the cytosol around the cell nucleus.

When compliant samples such as polymer films are scanned with an atomic force microscope (AFM) in contact mode, a periodic ripple pattern can be induced on the sample. In the present paper, friction and mechanical properties of such ripple structures on films of polystyrene (PS) and poly-n-(butyl methacrylate) (PnBMA) are investigated. Force volume measurements allow a quantitative analysis of the elastic moduli with nanometer resolution, showing a contrast in mechanical response between bundles and troughs. Additionally, analysis of the lateral cantilever deflection when scanning on pre-machined ripples shows a clear correlation between friction and the sample topography. Those results support the theory of crack propagation and the formation of voids as a mechanism responsible for the formation of ripples. This paper also shows the limits of the presented measuring methods for soft, compliant, and small structures. Special care must be taken to ensure that the analysis is not affected by artefacts.

Considerable scattering of experimental wear results affects seriously the evaluation of repeatability and reproducibility of tribological measurements and hampers detecting, studying, and verifying tribological laws. An accurate characterization of the statistics of results, the detection of the influence of operating parameters, and the verification of equations describing tribological phenomena can be achieved only through the analysis of large datasets with wide variation of parameters. Taking advantage of more than 400 experiments performed with the same material pairing (100Cr6) under the same conditions (unlubricated, ball-on-disc configuration, reciprocating sliding) on four different tribometers, the repeatability and reproducibility of volumetric wear measurements has been evaluated. The effect of the tribometer used for the acquisition, of five operating parameters (frequency, number of cycles, stroke, velocity, and normal force), and of the friction coefficient could be characterised using Welch's unequal variances t-test. The proportionality between volumetric wear and the product of sliding distance and normal force (sFN) is described in Archard's law through the proportionality factor k, commonly known as wear coefficient. By testing Archard's law over seven orders of magnitude of sFN, it was found that this law is not universally valid and that, under certain conditions, the wear coefficient depends on sFN itself. An alternative equation for the dependence of the volumetric wear on sFN is presented. The limits of the validity of Archard's law were investigated by varying the normal force and the sphere radius, using among others experiments with a reused, worn ball.

Customized piezoresistive cantilever microprobes with a deflection range of 120 ​μm and silicon tips of 100 ​μm height were operated in a Cypher AFM showing their functionality for measuring topography together with viscoelastic properties of thin films. For drop-in mounting in the AFM a holder was developed comprising the piezoresistive microprobe and its voltage-supply and signal-conditioning electronics. With the probe tip in contact to a glass sample we found a vertical resolution of 2.8 ​nm in a bandwidth of 1 ​kHz, which is close to the theoretical limit of 3.0 ​nm ​at a deflection of 2.5 ​μm. This resolution could be verified in topographic images of a scratch of approximately 300 ​nm in depth. Force-volume images with lithographically patterned photoresist (AZ 5214E) of approximately 300 ​nm thickness on silicon revealed contrast of the resist-covered and bare regions in topography, stiffness and adhesion. With contact-resonance imaging using the Dual AC Resonance Tracking (DART) method, patterned AZ 5214E photoresist of approximately 50 ​nm thickness could be distinguished from the bare silicon in topography, contact stiffness (indicated by contact resonance frequency shift) and adhesion (indicated by phase shift). Finally, a droplet of lubricant (Lupranol VP 9209) on glass could be detected by force volume imaging revealing a thickness of approximately 90 ​nm of the liquid layer with a sharp lateral limitation, which was clearly detected. We conclude that the piezoresistive silicon microprobe is a promising tool for emerging tasks of industrial surface metrology on manufacturing machines, including micro-finish of work pieces and elasticity, thickness, adhesion, etc. of thin solid or liquid deposits on top.

Force–distance curves have been recorded on thin films of nine different lubricants to extend the results of a previous work of one of the authors. The lubricants wet the AFM tip, which causes a capillary force. This depends on the shape of the tip, as well as on properties of the lubricants such as surface tension, contact angle, and viscosity, which have been additionally measured with other methods. Thus, their influence on the shape of the curves could be analyzed. The main features of force–distance curves on different lubricants have been characterized and the underlying phenomena could be explained. Results contribute to a better understanding of fundamental mechanisms influencing lubrication and hence friction and wear at the micro- and nanoscale.

Raw data, scripts and figures used for the article "Calibrating a high-speed contact-resonance profilometer", published in Journal of Sensors and Sensor Systems on 07 Jul 2020. The data/scripts can be opened/executed by the software "Matlab"

Wear test results exhibit often large scattering and hence poor repeatability, reproducibility, and reliability. The study of spreading of tribological experimental results requires so called big datasets, allowing to understand the origin of scattering and to characterize statistical distributions. In various studies, wear coefficients were found to have lognormal distributions. Therefore, it was supposed that the wear coefficient is affected by two normally distributed variables, which combine through a product, namely the number of contacting asperities and the size of the produced wear particles. In the present study, taking advantage of large sets of measurements collected in the last decades in the Federal Institute for Materials Research and Testing, we demonstrate that a lognormal distribution may arise from a non-constant wear coefficient, too, i.e., when a system does not follow Archard's law.

The influence of relative humidity on friction and wear is subject of several studies in the last decades. A comprehensive understanding of physical and chemical phenomena affecting the tribology is hampered by the lack of reproducible experimental results, by the large number of variables, and by several difficulties in the detection of tribochemical processes and products. In the present work, we analyze the wear coefficient and the wear volumes of 686 unlubricated tests performed on different oscillating tribometers with 100Cr6 balls on 100Cr6 planes at different relative humidity. Aim of this work is to assess the repeatability and reproducibility of data, to determine the dependence of the wear coefficient on the relative humidity, to understand the underlying physicochemical phenomena and to build three dimensional maps of the wear coefficient as a function of both humidity and the product of normal force and sliding distance.

Raw data and figures of the article "Application of contact-resonance AFM methods to polymer samples", published in Beilstein Journal of Nanotechnology on 12 Nov 2020 The raw data can be opened with the software "Igor Pro"

The accurate determination of wear volumes is a prerequisite for the study of numerous tribological phenomena. Wear volumes can be measured with different techniques (profilometry, confocal microscopy, white light interferometry, atomic force microscopy) or else be calculated starting from some quantities (usually the width and the planimetric wear) measured from the wear scar. Advantages and drawbacks of the mentioned measuring techniques are shown by means of wear scars and calottes resulting from ball-on-plane tests with 100Cr6 specimens. When measuring wear volumes, white light interferometry results to be one of the most suitable techniques, since it offers high accuracy and is not as time consuming as atomic force microscopy. When wear volumes are calculated, errors result mainly from two sources: (1) the arbitrary choice of one or few line profiles for the determination of the width and of the planimetric wear, and (2) approximations in the calculation, which are even necessary when values of the wear volumes of the single tribological partners, i.e., ball and plane, and not only the total volume, are of interest. The effect of both the statistical distribution of values of the width and of the planimetric wear and the propagation of errors due to approximations on the accuracy in the determination of wear volumes is characterized and elucidated by examples. It is found that errors due to approximations are negligible when compared to errors due to the arbitrary choice of one line profile.

A European EMPIR project, which aims to use large-scale, 5 mm × 200 µm × 50 µm (L×W×H), piezoresistive microprobes for contact resonance applications, a well-established measurement mode of atomic force microscopes (AFMs), is being funded. As the probes used in this project are much larger in size than typical AFM probes, however, some of the simplifications and assumptions made for AFM probes are not applicable. This study presents a guide on how to systematically create a model that replicates the dynamic behavior of microprobes. The model includes variables such as air damping, nonlinear sensitivities, and frequency dependencies. The finished model is then verified by analyzing a series of measurements.

Contact-resonance AFM (CR-AFM) has been used in recent years for the measurement of mechanical properties of rather stiff materials, such as ceramics or metals, but also of some polymers. Compared with other techniques providing information on the mechanical properties of a sample, notably force-distance curves, CR-AFM has a much shorter acquisition time. This compensates in part the incomplete theoretical understanding of the underlying physical phenomena and of factors influencing the measurements. A commonly used method to analyze CR data requires the determination of the relative position of the tip, the calculation of the normalized contact stiffness, and the use of a calibration sample for the calculation of the elastic modulus of the sample. In the present paper, we propose an alternative procedure, based on approximations of the equations describing the system, which allows one to determine the elastic modulus of the sample as a parameter of the fit of the CR frequency as a function of the load. After showing that CR modes including scanning under continuous contact wear and damage the sample and/or alter the surface roughness, the results of point CR measurements on bulk and thin films are presented. Though Young's moduli of bulk polystyrene and poly(methyl methacrylate) could be determined through the presented analysis, it is concluded that CR measurements are not appropriate for polymer samples. Major drawbacks are the bad resolution for moduli lower than ca. 10 GPa and the lack of a comprehensive physical model accounting for many factors affecting the dynamic response of a cantilever in contact with a sample.

The analytical potential of a nanosecond laser ablation inductively coupled plasma mass spectrometer system, equipped with an ultra-fast wash-out ablation chamber, is critically investigated for fast and highly spatially resolved (∼μm) qualitative elemental distribution within single cells.

The determination of mechanical properties via force–distance curves outlined in the previous chapter, most of all through force–volume measurements, is rather complex and time-consuming. For example, for the determination of the elastic modulus of PnBMA as a function of temperature shown in Sect. 3.14 , several thousand curves had to be acquired and analysed. This requires much more time than the measurement and the analysis with, e.g., DMA. On the other hand, AFM is able to determine the mechanical properties of samples, which cannot be analysed with other techniques. The fundamental difference between AFM and other techniques such as DMA or also nanoindentation is that an AFM enables to determine the local properties of the sample, whereas other techniques give information only about the bulk properties. Hence, an AFM can be employed to study inhomogeneous samples, i.e. samples consisting of two or more phases with one or more interfaces. There are two wide categories of inhomogeneous samples: thin films and blends. Blends are the object of Chap. 5 . In this chapter, thin polymer films and a particular category of them, i.e. polymer brushes, are handled.

This Springer Laboratory volume is a practical guide for scientists and students dealing with the measurement of mechanical properties of polymers at the nanoscale through AFM force-distance curves. I

The atomic force microscope (AFM) is increasingly employed not only to acquire topography images of samples but also to measure force–distance curves. Such curves, beyond playing a major role in the theoretical study of surface interactions, are meanwhile a fundamental tool in surface science, nanotechnology, biology and many other fields of research. Force–distance curves find their application in the study of numerous material properties, such as mechanical properties, surface charge densities, adhesion and Hamaker constants. One of the most important applications of AFM force–distance curves is the study of mechanical properties of polymers. Compared to other instruments, the AFM has in this case two major advantages. First of all, elastic moduli of samples can be measured with high resolution from some GPa down to some MPa, which is the range of the elastic moduli of common polymers. Second, force–distance curves can be acquired in an array over the sample. This is a fundamental tool for the characterization of the lateral variation of sample properties and hence for the study of confined polymers and polymer blends. The first part of this book is divided in two chapters dealing with the theoretical and practical aspects of force–distance curves. Theoretical aspects, handled in this chapter, are focused on mechanical properties of polymers.