Image Credits: maximmmmum/shutterstock.com
How two surfaces interact with each other is very important for many applications, and these interactions at the interface can define whether a defined system is efficient or not. Tribocontact is the interaction at these interfaces between two tribological surfaces—i.e. two surfaces which are in relative motion to each other. Tribology and the interaction of these tribological surfaces are vitally important in lubricant and oil systems, such as those found in combustion engines and industrial machinery. In this article, we look at how the topography of a surface, and in particular surface roughness, plays a major role at different tribological interfaces.
There are many levels to surface roughness. The most obvious to the observer is the bulk surface roughness that can be felt by simply running your hand over the surface. There is also a micro-scale roughness, a nanoscale roughness and an Angstrom-scale roughness (atomic level roughness), which is present even if the surface feels smooth to the touch. Another fact of surface roughness is that the smaller the roughness dimensions, the higher the relative surface area, which in turn leads to a greater degree of contact between two surfaces at the nanoscale interface. How these surfaces interact with varying degrees of roughness—which is also dependent on the materials in question and whether any lubrication is present—affects both the contact efficiency and the properties (and efficiency) of the whole tribological system. One of the key properties of nanoscale roughness is that the effects are seen in the bulk system.
Friction and Lubrication
In addition to the surface roughness itself, the degree of friction and lubrication between two tribological surfaces play a role in the properties of the tribological system. However, whilst this article is on surface topography, it should be noted that friction and lubrication at an interface are defined by the nanoscale topography of a surface and its interactions with other surfaces and/or mediums. In general, the greater the nanoscale contact between two surfaces, the greater the friction between two bare solid surfaces. This changes when a lubricant or a liquid/solid that displays lubricating properties is used between two surfaces. When a lubricating medium is used, the lubrication forces offset the friction generated between the two surfaces and enables the topographical geometries to pass over each other smoothly.
Adhesion is another type of interaction that can happen between two surfaces. Because all materials are made up of molecules—which in turn contain specific functional groups—there are always going to be other molecules/materials that are attracted or repelled by another material. Some adhesion may be physically obvious, whereas others may only be a weak interaction at the molecular level. Coatings can sometimes help with generating higher adhesive forces between surfaces, and in general, adhesion forces between two surfaces can take the form of weak van der Waals forces, to stronger intermolecular forces such as hydrogen bonding, and ionically charged attraction forces. Adhesion is not necessarily governed by surface roughness or geometry; rather, it is more the chemical composition of the surface which is important.
Effects on the Whole Tribological System
The various effects, forces, and interactions detailed above determine how effective a tribological system is. Given that many tribological systems play important roles in machinery and engine systems, how the topography of the components and the lubricants interplay within the system are important for reducing wear and ensuring longevity in the system.
Because the surface interactions define the efficiency of the mechanical system, if the surface topography of the components are changed, or the lubricating medium is altered, then it can have a negative effect on the whole system. This is why there are different types of wear. On one hand, if the mechanical (solid) components become worn, the topography of the surface changes at the nanoscale level and the effectiveness is lowered—this can be due to higher degrees of surface roughness causing greater degrees of friction, or sharp geometries at the nanoscale which cause higher energy (and more localized) impacts to other components and cause further wear.
The other type of wear stems from a lack of inadequate lubrication. Many lubricants are designed so that they ‘fill the gaps’ so to speak between different surfaces. If there is degradation within the lubricant, the wrong lubricant, or a lubricant mix-up, then the ability for the lubricant to fill the solid gaps and provide a means of smooth movement between the surfaces can be lost. This can then lead to wear of the mechanical system, which can then come back full circle and contaminate the lubricant with particulate matter, and this can cause a vicious cycle where the wear is exponentially increased—especially when unwanted solid particles interact with the solid surfaces of the system.
Sources and Further Reading
- “Diesel Engine System Design”- Xin Q., Woodhead Publishing in Mechanical Engineering, 2011
- “Effect of Surface Topography on Contact Fatigue in Mixed Lubrication”- Epstein D. et al, Tribology Transactions, 2003, DOI: 10.1080/10402000308982657
- “How Surface Topography Relates to Materials’ Properties”- Assender H. et al, Science, 2002