Introduction

The surfaces that undergo friction wear and are lubricated in the mechanical components are called tribological surfaces. These surfaces have different surface topography depending on the type of applications. Most commonly tribological surfaces are polished and have a clear surface finish when observed with the naked eye, however, the microstructure will have surface irregularities and asperities. The irregularities may vary in different shapes and irregularities might vary in the order of inter-atomic distances. The properties of the surfaces are very important as it influences various tribological properties such as friction, wear, and lubrication. Any precise machining operations cannot guarantee a flattened surface at the molecular levels [1]. The Fig-1 shows the surface morphology with the different wear mechanisms.

Fig-1 Schematic illustration of surface morphology and tribological mechanisms for (a) N, (b) N+F and (c) F+N series [2]

Types of surfaces

The classification of the tribological surfaces into smooth and rough is very difficult as none of the surfaces are molecularly flat. However, based on the performance of the surfaces and the wear at the interface between two tribological interactions it can be classified into smooth surfaces and rough surfaces. Fig-2 shows the schematic illustrations of the smooth and the rough surfaces.

Fig-2 Schematic illustration of smooth and rough surfaces [3]

1. Smooth surface: The tribological surface is said to be smooth when the surface irregularities are very less and exists at the microscopic levels. This surface does not cause the wear at the surface interface upon sliding and the friction at the interface is comparatively less [4]. The smooth surfaces do not cause an enormous impact on the energy efficiency of the mechanical components.
2. Rough surface: The tribological surface is said to be rough when the surface irregularities are very high which leads to wear causing high friction at the interface of the materials upon sliding. The rough surfaces cause a reduction in the efficiency of any mechanical components leading to the wastage of energy.

Surface layers in the solid surface

Apart from the irregularities on the surface present the tribological surfaces also has zones, these zones have been formed due to the various processes. In the Fig-3, we can see the surface layers on the solid surface which are physisorbed, chemisorbed, chemically reactive, heavily deformed, lightly deformed, and base material layers. Upon the base material the formation of the deformed layer is due to the forming process. The materials on the surface might react physically or chemically with the surrounding atmosphere which leads to the formation of the physisorbed and chemisorbed layers. These layers have a very high influence on the surface tribological properties of the surfaces.

Fig-3 Schematic illustration of different surface layers [1]

Characteristics of the surface layers

The individual surface layers on the tribological surface affect friction and wear. The effects of the chemical reactivity by the absorbing layers on the surface which leads to the change in extrinsic properties such as surface tension and surface free energy are important to be noted. Studying the characteristics of this individual layers provides the information about the type of reaction caused by them [5]. Fig-4 shows the schematic representations of the different types of reactions involved in the surface layers.

Fig-4 Schematic illustration of different types of reactions in various surface layers [1]

1. Deformed layer: The deformed layer is formed during the preparation of the material forming process such as lapping, grinding, or polishing. These surface layers are plastically deformed with or without change in temperature and it results in high strained regions. This deformed layer’s formation mainly depends on the amount of energy that was used to form this layer and the nature of the material used.
2. Chemically reactive layer: These layers are formed when the surface material reacts with the atmospheric oxygen to form the oxide layers. Also, there are other elements such as nitrides, sulphites, chlorides, etc which react with the surface materials forming their respective oxide layers. This layer is mainly formed during the machining or the friction process.
3. Physisorbed layer: This layer is formed by the interaction of the material surface with the molecules of oxygen, hydrocarbons from the environment, and the water vapors which get physically adsorbed forming a layer. The molecular interaction is with the weak van der Waals forces and which can be removed very easily when the surface interacts.
4. Chemisorbed layer: In the case of this the Chemisorbed layer is formed due to the interaction of the strong bonding between the molecules such as covalent bonds. This layer needs high energy to be removed which is because the chemical bonds are really stronger compared to the physical bonds which release heat during the removal of these layers.

Research on tribological surfaces

The research on tribological surfaces has been done by various researchers based on its characteristics, classification and the formation etc. The classification of tribological surfaces might vary when the surface is analysed using different techniques involved. In a study GW Stachowiak et.al., studied the classification of tribological surfaces based on a fractal modelling which involves in construction of the mathematical models such as partition iterated function system. This method uses the classification of the model based on method pattern recognition of the obtained images and converging many images to original images [6]. The surface properties are the results of the undulations present on the surfaces such as surface curvatures, these surface curvatures effects the leakage of the mechanical seals friction wear etc. Iman Maleki et.al., have compared the various surface curvatures characterisations methods to choose the best method [7].

Reference

[1] Bhushan, B. and Ko, P.L., 2003. Introduction to tribology. Appl. Mech. Rev., 56(1), pp.B6-B7.

[2] Kikuchi, S. and Komotori, J., 2015. Evaluation of the gas nitriding of fine grained AISI 4135 steel treated with fine particle peening and its effect on the tribological properties. Materials Transactions, 56(4), pp.556-562.

[3] https://www.toppr.com/ask/content/story/amp/factors-affecting-friction-1851/

[4] Israelachvili, J., Giasson, S., Kuhl, T., Drummond, C., Berman, A., Luengo, G., Pan, J.M., Heuberger, M., Ducker, W. and Alcantar, N., 2000. Some fundamental differences in the adhesion and friction of rough versus smooth surfaces. In Tribology Series (Vol. 38, pp. 3-12). Elsevier.

[5] Gatos, H.C. (1968), “Structure of Surfaces and Their Interactions,” in Interdisciplinary Approach to Friction and Wear (P.M. Ku, ed.), SP-181, pp. 7–84, NASA, Washington, DC.

[6] Stachowiak, G.W. and Podsiadlo, P., 2004. Classification of tribological surfaces. Tribology International, 37(2), pp.211-217.

[7] Maleki, I., Wolski, M., Woloszynski, T., Podsiadlo, P. and Stachowiak, G., 2019. A comparison of multiscale surface curvature characterization methods for tribological surfaces. Tribology Online, 14(1), pp.8-17.