A mechanical view of wear – The third body approach
March, 14 2025
A mechanical view of wear – The third body approach
Table of Contents
Surface roughness is an inherent property of any material, and it is established when two surfaces come in contact, it was assumed that these asperities were rounded than spiky. Further, the clean surfaces adhere to each other increasing adhesion hence artificial screening is performed on the industrial surfaces to avoid friction. Key challenges in this domain include predicting particle detachment during contact (relevant to both material science and mechanical engineering), understanding the mechanical conditions lead to screen destruction, and investigating the kinetics of screen regeneration. The behavior of detached particles is influenced by mechanical factors like velocity fields, while their composition and transformations fall under materials science.
Any contact consists of two primary surfaces i.e. first body and an intermediate layer known as the third body. The third body can be defined in two ways: by its material composition, which differs from the first bodies, or by its role in accommodating velocity differences between the first bodies. Contacts are categorized as either “full,” where the space between first bodies is completely filled by the third body (e.g., in elastohydrodynamic lubrication), or “empty,” where the third body acts as struts to separate the first bodies, common in the initial rubbing of hard materials. Third bodies are introduced into the contact either tangentially (via the motion of the first bodies) or normally (through wear of the first bodies). Normal feeding involves wear, whereas tangential feeding helps prevent it. Lubricants, whether solid or liquid, are typically tangentially fed, while wear debris, such as from rubbing plastics, is fed normally. In multi-pass systems, both feeding methods can occur simultaneously, as newly formed debris mix with previously deposited particles, forming traces that are recirculated after being destroyed and reprocessed.
Figure-1 Schematic showing variables which may influence wear when using the particle approach [2].
To achieve efficiency comparable to lubrication theory, the Third Body Approach (TBA) must predict load and friction for specific operating conditions and materials. For the simplest cases, such as full contacts, this requires:
Understanding the rheological behaviour of the material acting as the third body is essential. This is straightforward when the third body is a material available in sufficient quantities, like a solid lubricant, as it can be studied directly. However, challenges arise when the third body is produced in minute amounts, such as from normal feed during the rubbing of plastics. In such cases, third bodies are extremely thin (less than 1 µm thick) and small in area, making them difficult to analyze with existing microrheometric techniques. Additionally, the behaviour of these third bodies cannot be directly inferred from the properties of the original material (first body), as compacted wear particles exhibit different characteristics. This complicates the understanding of third-body rheology and its relationship with the materials involved.
To fully understand the behaviour of third-body contacts, detailed information is needed about both longitudinal boundary conditions (entry and exit) and transverse boundary conditions (interfaces between the first body and third body). Unfortunately, very little data exists on these topics. Studies of two-body contacts indicate no clear relationship between tangential and normal stresses, and the situation is even more complex in three-body contacts. Observations suggest that particles in three-body interactions may adhere, roll, or slip on the counter face, but the exact boundary kinematics remain uncertain and are not yet well-defined.
Developing a comprehensive theory of thin film mechanics that accounts for all possible rheological conditions is essential but challenging. This difficulty arises primarily from the high aspect ratio of the contact, where the film thickness is extremely small compared to the other contact dimensions. To address this, either generalized equations or advanced numerical discretization methods are needed. These approaches would provide an integrated understanding of contact behaviour, which is critical for making significant advancements in this field.
[1] Godet, M., 1984. The third-body approach: a mechanical view of wear. Wear, 100(1-3), pp.437-452.
I am a postgraduate researcher at the University of Leeds. I have completed my master's degree in the Erasmus Tribos program at the University of Leeds, University of Ljubljana, and University of Coimbra and my bachelor's degree in Mechanical Engineering from VTU in NMIT, India. I am an editor and social networking manager at TriboNet. I have a YouTube channel called Tribo Geek where I upload videos on travel, research life, and topics for master's and PhD students.
