ASPERITY LEVEL CHARACTERIZATION OF ABRASIVE WEAR USING ATOMIC FORCE MICROSCOPY
Figure 1 - AFM set-up. (Online version in colour.)
Using an atomic force microscope, a nanoscale wear characterization method has been applied to a commercial steel substrate AISI 52100, a common bearing material. Two wear mechanisms were observed by the presented method: atom attrition and elastoplastic ploughing. It is shown that not only friction can be used to classify the difference between these two mechanisms, but also the ‘degree of wear’. Archard's Law of adhesion shows good conformity to experimental data at the nanoscale for the elastoplastic ploughing mechanism. However, there is a distinct discontinuity between the two identified mechanisms of wear and their relation to the load and the removed volume. The length-scale effect of the material's hardness property plays an integral role in the relationship between the ‘degree of wear’ and load. The transition between wear mechanisms is hardness-dependent, as below a load threshold limited plastic deformation in the form of pile up is exhibited. It is revealed that the presented method can be used as a rapid wear characterization technique, but additional work is necessary to project individual asperity interaction observations to macroscale contacts.
In this study, wear mechanisms of mechanical regimes are to be considered, of which there are many subdefined categories [2]. Surface failures due to sliding wear mechanisms that lead to plastic deformation [3], cracking and material removal are the focus of this study rather than fatigue. The ability to interpret between mechanisms for single asperity interactions is made using the difference in the volume of material displaced and that of the pile-up material as well as frictional characteristics.
Figure 2 - LEO 1530-VP SEM image of a new Adama NM-TC probe. (Online version in colour.)
Characterization of wear from a contact level using conventional tribometers, such as pin-on-disc, requires excessively prolonged experimental and measurement phases to form wear maps [4]. Wear on an individual asperity interaction basis has rarely been experimentally explored; with many of the original studies focusing on microscale, rather than on the nanoscale of asperities [4–6]. To characterize wear at the nano-length scale, one approach is to use an atomic force microscope (AFM) with an appropriately selected cantilever and tip. The use of such an instrument forms the basis of the methodology in the current study.
It is important to consider material physical properties at a nano-length scale as they are shown to be different to that of a bulk material. Nix & Goa [7] conducted an analysis on the indentation of crystalline materials to develop a bulk to nanoscale hardness connection. Bhushan & Nosonovsky [8] further developed this work and applied it to wear coefficients. The limitation of such relationships as indicated by Bushan & Nosonovsky is the lack of ability to measure the characteristic length scale, which enables the interpretation of nanoscale properties to the bulk material characteristic.
Figure 3 - Microstructure of AISI 52100 surface etched with 2% Nital using an optical microscope.
When observing contact mechanics at an asperity scale, adhesion is present between the two contiguous surfaces. Depending on the magnitude of the adhesion force of the contacting bodies, plastic deformation of soft materials may occur. Therefore, evaluation of the adhesion significance is imperative as part of an asperity level wear investigation, which can be conducted using Johnson & Greenwood's [9] adhesion map.
Many studies have conducted AFM wear mapping or characterization upon laboratory-scale single crystal silicon or using coating substrates such as diamond-like-coatings (DLC) [10–14]. Therefore, applying similar AFM wear characterization techniques to an engineering steel with grain boundaries will extend the applicability of the methodology to a wider range of commonly used materials. Furthermore, these studies limited their scope of application of AFM scratching to interfacial phenomena of micro-electromechanical systems, and nanomachining [11–14].
Figure 4 - Average of nine nanoindentation repetitions at each load 7−35 mN. (a) Load–penetration graphs for nanohardness test and (b) hardness–load relationship. (Online version in colour.)
Hokkirigawa & Kato [5] experimentally explored different modes of abrasive wear using a micro pin-on-disc apparatus, with in situ scanning electron microscope (SEM) imaging. Wear mode frictional characteristics were observed, and three distinct responses were recorded. While the work shows a novelty in its in situ SEM imaging of wear generation at a microscale, it has limited applicability to real-world scenarios, where the wear particles are typically entrained into the contact normally exacerbating wear.
Celano et al. [14] conducted an AFM experiment using a diamond tip sliding in contact with silicon, silicon–germanium and germanium for the purpose of controlled material removal. Two contact regimes were reported with different wear rates. A load threshold was identified as the transition between ‘sliding’ wear and ‘sliding–ploughing’ wear; the latter included plastic deformation of the contacting area.
Figure 5 - (a) AFM force–displacement curve for pull-off force determination. (b) AFM continuous reciprocating scratch plotted on adhesion map [9]. (Online version in colour.)
As described, AFM nanomachining or nano-scratching techniques in the literature have typically concentrated on silicon [14] and DLC coatings [8,10]. In this study, a scratch methodology using AFM has been developed to enable wear characterization of commercial multi-grain metallic substrates at nanoscale, which has not been reported hitherto, with the future intention of linking together with multi-scale wear phenomena. It is imperative to consistently induce wear through asperity level interaction for use as a building block for a multi-scale approach to wear modelling. Friction characterization of types of nanoscale wear is made possible through the use of AFM in lateral force mode (LFM). The effect of the length scale on the material properties is shown to play an integral role in the relationship between the degree of wear and load. The experimental data conform well to Archard's Law of adhesion at a nano-level for metallic substrates, where plastic deformation is dominant in the wear mechanism. The presented method will enable rapid and efficient wear characterization expandable across different scales of length.
Click on the link for the full print article:
Asperity level characterization of abrasive wear using atomic force microscopy - PMC (nih.gov)
Published June 9. 2021 in the Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences.