Overview of Polymer tribology

Introduction

The tribological characteristics of polymers is different from those of metals and ceramic materials because of their distinct chemical composition, physical properties, and surface attributes. Polymers exhibit low surface free energy and possess viscoelastic properties which significantly affect tribological responses, particularly causing adhesion and mechanical frictional interactions. Furthermore, because polymers can be easily modifiable both physically and chemically, it is easy to prepare materials that have different properties. This means that polymers are good for controlling their interaction and wear out when they slide against each other.

Friction of polymers

Friction in polymers involves both adhesion and deformation components. Adhesion is affected by the cutting or plowing action of harder particles or asperities on the surface, leading to two-body or three-body abrasion depending on whether these cutting points are embedded in the counterface or loose within the contact zone. Deformation during friction arises from the elastic, plastic, or viscoelastic deformation of the sliding surfaces’ asperities upon contact. This deformation dissipates mechanical energy, influenced by various factors such as strain type, sliding conditions, mechanical properties, and environmental factors. The tribological properties of polymers are further influenced by factors including the structure and positioning of macromolecules on the surface, degree of crystallinity, polymer type, composition, chain orientation, and molecular weight distribution. Polymers, being viscoelastic, generate heat during movement due to mechanical energy conversion, which occurs during plastic deformation, hysteresis, dispersion, and viscous flow. Friction mechanisms in polymers primarily involve adhesion across smooth surfaces, accompanied by deformation of the polymer surface layers. These observations hold true for amorphous, rubber, and semicrystalline polymers.

Figure-1 Two-term model of friction and wear processes.

Wear of Polymers

The primary wear mechanism in materials is adhesion which is a critical component of friction. Adhesive wear involves the creation, growth, and breaking of adhesive bonds, leading to material transfer between surfaces. Wear typically results from a combination of various mechanisms, leading to the unwanted loss of solid material from a surface due to mechanical interactions. Wear is commonly quantified using the specific wear rate, determined by measuring the volumetric loss of a sample under applied force and sliding distance. However, traditional methods like ASTM G 65-85 may yield unreliable results due to the dependency of debris generation on relative surface speed and the generation of heat during testing. The test temperature directly influences polymer wear, with different polymers exhibiting varied responses to the same relative velocity due to differences in specific heat capacity and behavior relative to their glass transition temperature. Consequently, comparing wear results between polymers using mass loss measurements is challenging, and there is currently no universally accepted test method for polymer wear assessment. Moreover, the absence of apparent mass loss in many polymers, particularly softer materials, renders some ASTM protocols unsuitable for accurate assessment.

Important properties

The macroscopic properties of polymers are intricately linked to their micro-nano structure and molecular-level interactions. For instance, the addition of carbon black as a filler in polymer blends demonstrates this connection. At a specific concentration of carbon black, known as the percolation threshold, there is a sharp decline in both electrical resistance and static friction. This phenomenon occurs because the carbon black particles form a continuous network, reducing electrical resistivity and creating a surface with a smaller contact area during friction testing, primarily composed of carbon black particles. The study of micro- and nano-tribology serves as a crucial link between fundamental science and engineering, enhancing our comprehension of interfacial behaviors across different length scales. While tribology is not a novel field, developing specific approaches for polymer-based materials (PBMs) is essential to understand their unique properties and disseminate knowledge about PBMs’ tribological characteristics.

Figure-2 Tribological performance of polymers, composites and nanocomposites

Future of Polymer tribology

Enhancing the understanding and improving the tribological properties of polymers and PBMs, such as composites and blends, can be achieved by identifying links between friction factors and wear mechanisms, as well as by modifying surface structures at micro- and nano-levels. Exploring the mechanical and tribological behavior of PBMs opens new possibilities for their applications in response to evolving challenges in science and technology. Key polymer characteristics like viscoelasticity, brittleness, and structural changes resulting from processing modifications play vital roles in establishing functional relationships with tribological behavior. Additionally, computer simulations offer insights into polymer tribology by modelling structural changes and their effects on tribological properties, potentially complementing, or replacing experimental approaches. Understanding the relationships between surface tension and tribological properties, including friction, scratch resistance, and wear, provides valuable insights for optimizing desired material properties. Incorporating polymer tribology into university curricula and fostering further research in this field are essential for advancing knowledge and addressing emerging needs in materials science and engineering.

Reference

[1] Brostow, W., Kovačevic, V., Vrsaljko, D. and Whitworth, J., 2010. Tribology of polymers and polymer-based composites. Journal of Materials Education, 32(5), p.273.

[2] https://www.atspinnovations.com/blog/the-whys-of-polymer-coatings-for-tribological-applications

[3] https://www.lehigh.edu/~intribos/polymers.html