Elsevier

Wear

Volumes 444–445, 15 March 2020, 203162
Wear

Efficacy of surface microtexturing in enhancing the tribological performance of polymeric surfaces under starved lubricated conditions

https://doi.org/10.1016/j.wear.2019.203162Get rights and content

Highlights

  • Effect of microtexturing on static and sliding COF.

  • Role of mechanical properties on texturing’ tribological effectiveness.

  • Role of core void volume in indicating wear intensity for texturing.

  • Failure of microtexturing due to fluid pressure build-up in the dimples.

  • Dependence of surface microtexturing performance on lubrication regime.

Abstract

The efficacy of microtexturing on the tribological performance of Aromatic Thermosetting CoPolyester (ATSP) and Ultra High Molecular Weight Polyethylene (UHMWPE) were investigated under oil-lubricated starved conditions. Nominal contact pressures of 5.02 and 10.04 MPa were applied over a range of sliding velocities, 0.008–2.278 m/s. Static and kinetic coefficients of friction (COF) for untextured and textured surfaces were obtained and compared. Reduction in the average kinetic COF up to 29% for microtextured ATSP and 25% for microtextured UHMWPE were obtained. Static COF did not exhibit a notable change as a result of microtexturing. The core void volume was identified as a reliable indicator of the wear intensity of the dimples and consequently lubricant retention capacity for texturing. While both microtextured polymers illustrated reduction in the kinetic COF, only ATSP showed mechanical integrity for applications that experience high contact load under starved conditions. Microtextured UHMWPE experienced mechanical failure due to fluid pressure build up in the dimples under high load tests.

Introduction

Surface texturing has been successfully utilized to enhance the performance of different components in various applications [1]. Cutting tools are among industrial applications which benefitted from surface texturing [[2], [3], [4]]. The focus of the present study is on the application of surface texturing to improve the tribological performance of parallel sliding contacts, such as thrust bearings [5,6]. To describe the experimental conditions of the tests conducted in the present study, it is essential to differentiate between lubrication regime and lubrication condition of the tests. To clarify, there are studies that interchangeably use the terms mixed and starved to describe a lubrication regime/condition [[7], [8], [9]]. Other studies attempted to define starved lubrication [9], or impose certain constraints, such as limited supply of the lubricant on the contact to describe starved lubrication [10]. In this work, we assign specific definitions for each term.

Supply rate of the lubricant to the contact could be categorized into three readily distinguishable conditions namely dry, starved, and fully-flooded or submerged conditions. When the lubricant is provided at a limited supply rate in the form of spraying, capillary effect, or other minimal supply rate, the term starved condition could be appropriately used. Every lubrication regime could transpire at each of these lubrication supply rates from hydrodynamic regime at a dry contact which is prevalent in air bearings [11,12] to the mixed or even hydrodynamic regime under starved conditions [[13], [14], [15]]. There are several studies [[16], [17], [18], [19], [20], [21]] which have embraced this terminology that seems crucial for more accurate description of a contact lubrication history.

Implementing parametric studies using sliding velocity and obtaining its effect on the COF as a tool for detecting lubrication regimes have been used by many authors [14,15,22,23]. The present study utilizes COF changes with velocity as an indicator of the lubrication regime.

One could expect a shift in the Stribeck curve towards the boundary regime for a textured surface because of lower bearing area, thus higher contact pressure. A leveling effect in the COF behavior in the Stribeck curve was mentioned to signify the optimum texturing parameters [24]. The effect of texturing area density and the side of the tribocontact which is textured, whether the polymer side or the metallic side, was studied for UHMWPE-stainless steel tribopairs. Constant reduction of the COF with increasing velocity for water-lubricated textured UHWMPE was observed for a contact pressure of 0.32 MPa [20], and an optimum area density of 30% was suggested. The effect of elastic modulus on the tribological performance of textured surfaces was discussed in other studies [20,25]. Pressurizing the cavities is essential for generating higher lifts. However excessive pressurizing could damage the structural integrity of the texturing if the mechanical properties such as yield strength are not integrated into the texturing design carefully. Numerical studies focus on the role of texture spatial distribution [26], and texture shapes [27] for journal bearings, and suggested enhanced load capacity via partial texturing, compared to full texturing in the hydrodynamic regime. Other studies show the effective reduction of slip-stick behavior through texturing under starved lubrication conditions [28]. Recent review papers highlight the key role of texturing in enhancing tribological performance of piston-ring and seals [29], as well as machining processes [30] in starved and flooded conditions. In addition, significant improvement in anti-scuffing performance of textured surfaces [31], as well as implementing texturing to reduce adhesive wear [32] under starved lubrication conditions were shown.

Concentration of stress along the perimeter of the textures, usually around sharp corners of the texture features, has been shown numerically [20,33,34]. The negative effect of such stress concentration on friction and wear of textured surfaces were discussed [35] and its elimination is one of the criteria implemented in texture optimization [36]. Hence, the significance of the dimple shapes. Recent studies under starved lubrication on the influence of dimple shape on tribofilm formation, revealed that the tribofilm could outweigh the texturing tribological effects [37]. In this work we also study the effect of plastic deformation of the dimples on effective wear and friction reduction.

Trapping of wear debris by microtextures such as microdimples, retaining lubricant as microreservoirs for liquid lubricants, and generating microhydrodynamic lift were identified as key mechanisms which contribute to enhanced tribological performance of textured surfaces [7,38,39]. Understanding the effect of geometrical specifications of the texturing, such as area density of the dimples, on changes in wear and friction of the sliding components has been studied [38,[40], [41], [42]]. The texture distribution was found to have a strong influence on the wear characteristics of a textured surface [41], while the area density was identified as the most influential parameter on the load carrying capacity, and friction behavior for polymers. The efficiency of texturing also depends on the operating conditions of a bearing contact [43].

The present study investigates the efficacy of textured surfaces under starved lubrication conditions and compares their performance with plain (untextured) surfaces. The focus of this work is on the role of sliding velocity, normal load, polymer material, and type of contact, i.e., kinetic or static. The main objective is to obtain further insight to the texturing of polymers under starved lubrication, which directly contributes to better engineering design and optimization of systems operating under such conditions.

Section snippets

Materials and fabrication of microtextures on polymers

ATSP is an advanced polymer with excellent wear performance under different tribological conditions, including high temperature, starved and dry, high pressure, high speed and abrasive conditions [[44], [45], [46], [47], [48]]. UHMWPE has shown promising performance in various tribological applications, including textured surfaces for bio implants as well as industrial applications [20,24,49,50]. The mechanical properties of ATSP and UHMWPE are given in Table 1. For the pin-on-disk experiments,

Effect of sliding velocity on textured ATSP

The COF values for both untextured and textured ATSP surfaces were obtained under 5.02 MPa nominal contact pressure at a wide range of sliding velocities. Fig. 3 (a) depicts typical data obtained for a single run conducted on the ATSP textured pin under 5.02 MPa contact pressure. The run-in period followed by the incrementally decreasing velocity section is shown. The experiment is concluded by a final reciprocating motion to obtain the static COF. Fig. 3 (b) depicts the obtained average static

Conclusion

Static and kinetic COFs of two different polymers, namely ATSP and UHMWPE were measured under starved lubricated conditions. The effects of texturing, sliding speed and contact pressure were investigated. The evolution of surface topography as a result of harsh testing conditions was investigated and the wear mechanisms were analyzed. In summary, the main findings are:

  • Under starved lubricated conditions with a high viscosity lubricant, the efficiency of texturing is most pronounced in the mixed

CRediT authorship contribution statement

Reza Gheisari: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing. Pixiang Lan: Conceptualization, Methodology, Visualization, Writing - original draft, Writing - review & editing. Andreas A. Polycarpou: Conceptualization, Data curation, Funding acquisition, Project administration, Resources, Software, Supervision, Writing - original draft, Writing - review & editing.

Acknowledgement

Partial funding of the present study was provided from the Meinhard H. Kotzebue ‘14 endowment in the J. Mike Walker ‘66 Department of Mechanical Engineering at Texas A&M University. The authors also acknowledge the use of the Texas A&M Materials Characterization Facility (MCF).

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