Elsevier

Tribology International

Volume 151, November 2020, 106477
Tribology International

On the commensuration of plastic plowing at the microscale

https://doi.org/10.1016/j.triboint.2020.106477Get rights and content

Highlights

  • The transition from indentation to plowing can be divided into two stages: initial depth increase and depth decrease.

  • Depth evolution is partly understood by the conservation of the projected contact area in the surface normal direction.

  • The elastic recovery on the backside of the tip results in a smooth increase of the depth until reaching the maximum depth.

  • In the second stage, the contact area increase due to the front pile-up formation causes a penetration depth decrease.

  • The grain orientation and the relative wear direction determine pile-up evolution and the penetration depth decrease.

Abstract

Improving the wear properties requires an understanding of the mechanisms during the initial stages of wear. This paper focuses on the transition from static indentation to dynamic wear, i.e. the evolution from stationary to sliding contact. By using single micrometer-sized roughness peaks on copper, cementite and austenitic steel, we observe that the conservation of contact area, elastic recovery, and the front pile-up development are the dominant mechanisms. The elastic recovery leads to an additional contact area at the backside of the asperity. The influence of the crystallographic orientation was found to be negligible during the initial increase in wear depth but significant during the later stages. Moreover, the partial wear depth recovery is observed only in ductile materials.

Introduction

Abrasive wear is the material removal due to the relative motion of two bodies [1] and abrasion has been investigated widely by – for instance – quantifying the volumetric loss of engineering structures in contact. For ductile materials at the macroscale, wear modes are classified often into adhesion, abrasion, surface fatigue, and tribochemical reactions. Abrasion can be separated further into plowing and cutting [2]. Some of the first quantitative abrasion models assumed pure cutting in single-track wear motion [3,4]. Subsequently, pile-ups and chips in the front of the wear track as well as on the sides were observed in different proportions based on the experimental conditions [[5], [6], [7], [8]]. In addition, the scratch hardness was introduced as the single parameter to evaluate the material resistance to wear and to model plastic plowing [[9], [10], [11]].

Previous studies have focused on the steady-state region of plowing. Several researchers have developed analytical models for material flow-line field theory for various indenter shapes and orientations [8,[12], [13], [14], [15]]. This theory is based on the orientation of displacement vectors for all the material points in the contact region and the theory predicts plowing and adhesive friction coefficients, pile-up and true contact areas. Johnson [16] introduced a rheological parameter that correlates with the indentation pressure of a blunt indenter. This parameter was recently applied to model pile-ups and sink-ins during wear via the Finite Element Method (FEM) [17,18].

Nonetheless, there is still little systematic examination of the very first stage of wear, i.e. transition from stationary to sliding contact. Goddard and Wilman [6] have shown that the friction coefficient is larger at the start of the wear track until the wear mechanisms reach a stable regime. Additionally, several studies have reported a rapid initial depth increase after the beginning of the lateral movement [[19], [20], [21], [22]]. This depth decreases subsequently to the steady-state regime, which depends on the loading and material configuration and which has much smaller fluctuations (Fig. 1). Kareer [20] has conducted wear experiments with a Berkovich tip in edge forward and face forward orientations to evaluate the scratch hardness of copper at the microscale. The magnitude of the initial depth increase during the transition from indentation to wear correlated with the fraction of the indentation to wear contact area. This initial depth increase is not related to the experiment protocol, i.e. whether or not indentation is followed by intermediate unloading and reloading, and the initial depth increase more pronounced when applying higher normal forces [19]. For viscoelastoplastic polymeric fibers, Cayer-Barrioz et al. [23] observed only a gradual decrease of penetration depth after the initial depth increase in nanoscratch experiments at ultralow normal force. Tayebi [21] has shown the depth evolution during the transition from indentation to wear for fused quartz and the penetration depth did not decrease after reaching maximum value: the behavior of fused quartz is fundamentally different than that of metals. Hence, experimental studies have established the existence of the initial depth increase during the transition from indentation to wear.

The transition from indentation to wear is accompanied by the penetration depth increase also in numerical plowing models. Vedaei-Sabegh [18] studied the influence of the mechanical properties, normal force and friction during sliding wear via finite element method (FEM) simulations. They revealed that the strain hardening capacity, friction coefficient, and the elastic modulus to yield stress ratio affect the pile-up development. Although a clear dependence of the initial depth increase on the normal force is observed in their simulation results, the initial depth increase during the transition from indentation to plowing was not addressed. Chamani and Ayatollahi [22] have calculated the initial depth increase in wear tracks of a Berkovich tip in edge forward orientation and of several conical tips with different apex angles. The transition region is the shortest for the Berkovich tip, and larger cone angles lead to a smaller penetration depth in transition as well as in the steady-state region. Wang et al. [24] calculated nanoscratches in single crystalline copper by a Berkovich tip using a crystal plasticity finite element model (CPFEM). When comparing the numerical and experimental depth curves, they observed the initial depth increase and a subsequent decrease. However, the depth increase at the beginning of the lateral movement was more prominent in the experiments and the origin of the mechanism was not mentioned. Therefore, while the numerical studies have observed the initial depth increase, it was seldom discussed and the evolution mechanisms remain unknown.

The mechanisms that cause the initial depth increase and subsequent depth decrease were discussed in several studies. The initial depth increase is associated with the change in the total force on the sample due to added lateral force [21]. As the indenter tip loses the contact with the back half of the asperity, the change in contact area requires the corresponding depth increase and thus the increase of the frontal contact area to support the evolved total force during wear [20,22]. The gradual depth decrease in wear experiments on viscoelastoplastic polymeric fibers was attributed to a combined effect of viscoplasticity and interface friction [23]. Even though elastic recovery was explicitly investigated for the case of wear in polymers [[25], [26], [27]], its effect on the penetration depth evolution during the transition from indentation to wear was not addressed. A proposed explanation for the subsequent depth decrease is based on the development of the pile-up in front of the indenter tip [22]. This paper investigates the relative contributions of contact area conservation, elastic recovery, and pile-up development to the depth evolution in the transition region.

The dependence of the initial depth increase and decrease upon testing conditions (e.g. material, normal force, tip shape and size), as well as the mechanism of depth decrease, still remain uncertain. The objective of this study is to establish a relationship between the plastic plowing in the transition region and the pile-up formation. We conduct single asperity wear experiments in polycrystalline austenitic steel, copper, and cementite to provide a comparative analysis for multiple materials during the initial stage of wear. Two sphero-conical nanoindenter tips were used to examine the effect of the size of contact area as well as the contact angle. In addition, grain orientation dependent experiments were carried out to discuss the orientation influence on plastic plowing.

Section snippets

Experimental procedure

Austenitic steel (γ-Fe), copper (Cu), and cementite (Fe3C) were chosen to investigate materials with different macroscale mechanical properties (e.g. hardness, elastic modulus). In order to carry out several wear experiments in a single grain as well as perform experiments with longer wear tracks, the as-received γ-Fe was annealed at 1200 °C for roughly one week (see Ref. [19]). After that, crystallographic orientations were determined by electron backscatter diffraction (EBSD), and two large

Overview of transition from indentation to plastic plowing

We demonstrate the typical wear track shape for Fe3C and γ-Fe using the 100 mN wear tracks produced by the 5 μm tip (Fig. 2). The pre-scan, wear track and post-scan were used to determine the elastic and plastic depth. The fractions of elastic and plastic depth in the steady state region are 83.8% and 16.2% for Fe3C and 53.5% and 46.5% for γ-Fe, respectively. Specific values of the penetration depth (hinit, hind, havg) and distance (dinit) are shown for the γ-Fe wear track (Fig. 2b), and these

Discussion

This study aims to describe qualitatively the initial stage of plastic plowing. We observe that the penetration depth curves are overlapping irrespective of lengths in γ-Fe grains (Fig. 4a). Furthermore, the depth curves overlap in the transition region for the [001] and [111] grains (Fig. 4b) but show a significant difference in the steady-state region due to the crystal orientation, i.e. plastic anisotropy. Including a holding segment after the wear segment leads to no change in the

Conclusions

The following conclusions are derived from observations of the initial stage of plastic plowing in single-stroke wear experiments:

  • The transition from indentation to plowing can be divided into two stages: initial depth increase and subsequent depth decrease. The transition stages are partly understood by the conservation of the projected contact area in the surface normal direction: as the normal force is constant, the projected contact area reshapes but remains of equal size.

  • The initial

Statement on originality of the work

Authors confirm that the work is written completely independently, indicating the full name of the source of other authors that were used in the article, and neither the entire work, nor any of its parts have been previously published. The authors confirm that the article has not been submitted to peer review, nor is in the process of peer reviewing, nor has been accepted for publishing in another journal. The authors confirm that the research in their work is original, and that all the data

CRediT authorship contribution statement

Hanna Tsybenko: Investigation, Formal analysis, Writing - original draft, Writing - review & editing. Wenzhen Xia: Investigation, Formal analysis, Writing - review & editing. Gerhard Dehm: Writing - review & editing, Supervision. Steffen Brinckmann: Conceptualization, Methodology, Formal analysis, Writing - review & editing.

Declaration of competing interest

We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgments

The authors thank Prof. Yoshikazu Todaka of Toyohashi University of Technology for providing Fe3C samples. H. Tsybenko acknowledges her Ph.D. fellowship from the International Max Planck Research School for Interface Controlled Materials for Energy Conversion (IMPRS-SurMat).

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