[Paper Review] The Relation Between Wear and Irreversible Entropy Generation in the Dry Sliding of Metals

Hisham A. Abdel-Aal|arXiv (Cornell University)|Aug 2, 2010

Mechanical stress and fatigue analysis22 references3 citations

TL;DR

This paper proposes a thermodynamic model linking wear in dry sliding metals to irreversible entropy generation within the material asperity zone (MAZ), treating the MAZ as a Carnot-like heat engine. It finds that wear rates correlate with entropy generation and flow, with opposite trends in mass wear rate versus specific wear rate across copper and titanium, while a residual entropy ratio predicts wear behavior: excess entropy transport increases wear, while entropy generation reduces it due to system re-equilibration.

ABSTRACT

We examine the relationship between wear and the generation of entropy in dry sliding of metals. It is postulated that wear is related to irreversible entropy generation within the MAZ. We present a model, that treats the MAZ as a heat engine in the Carnot sense. The model assumes that the MAZ is a heat engine that transports heat from a high temperature reservoir, represented by the asperity contact layer, to a low temperature reservoir, represented by the sub-contact layer. Consequently an entropy generation source that represents the irreversibilities within the MAZ is defined and a study of the entropy generation is attempted. Wear data, published elsewhere, of two materials, Oxygen Free High conductivity Copper, and Commercially Pure Titanium are analyzed using the developed model. It is found that wear for both materials is correlated to entropy generation, and to the entropy flow within the MAZ. Interestingly, moreover, in view of the contrasting wear trends of the test materials, the relationship of the mass wear rate and the specific wear rate of each material with respect to entropy generation are totally opposite to each other. A common feature between the behavior of the two materials, was found in wear behavior with respect to a so called Ratio of Residual Entropy. This ratio determines wether the system is over or under supplied with entropy. It is found that when the capacity to transport entropy exceeds the entropy supply the mass wear rate increases, and when the entropy transport capacity of the system is exceeded and entropy generation takes place the mass wear rate decreases. This is attributed to the re-establishment of equilibrium within the system.

Motivation & Objective

• To investigate the thermodynamic basis of wear in dry sliding metals using entropy generation principles.
• To model the material asperity zone (MAZ) as a heat engine to analyze entropy transport and irreversibility.
• To correlate wear behavior with entropy flow and generation, particularly through a residual entropy ratio.
• To explain contrasting wear trends in oxygen-free high-conductivity copper and commercially pure titanium.
• To identify conditions under which entropy transport capacity or generation dominates wear response.

Proposed method

• Model the MAZ as a Carnot heat engine transferring heat from the asperity contact layer (hot reservoir) to the sub-contact layer (cold reservoir).
• Define an entropy generation source term to represent irreversibilities within the MAZ.
• Use published experimental wear data for oxygen-free high-conductivity copper and commercially pure titanium to test the model.
• Introduce a 'ratio of residual entropy' to quantify whether the system is over- or under-supplied with entropy transport capacity.
• Analyze mass wear rate and specific wear rate as functions of entropy generation and entropy flow.
• Apply thermodynamic equilibrium principles to interpret wear rate variations based on entropy dynamics.

Experimental results

Research questions

• RQ1 How is wear in dry sliding metals related to irreversible entropy generation within the material asperity zone?
• RQ2 Can the MAZ be modeled as a thermodynamic heat engine to explain wear behavior?
• RQ3 Why do mass wear rate and specific wear rate show opposite trends with respect to entropy generation in different materials?
• RQ4 What role does the residual entropy ratio play in determining wear rate trends?
• RQ5 How does system re-equilibration due to entropy generation affect wear rates?

Key findings

• Wear for both oxygen-free high-conductivity copper and commercially pure titanium correlates with entropy generation and entropy flow within the MAZ.
• The mass wear rate increases when entropy transport capacity exceeds supply, indicating system overcapacity.
• The mass wear rate decreases when entropy generation exceeds transport capacity, suggesting system re-equilibration reduces wear.
• The specific wear rate shows an opposite trend to the mass wear rate with respect to entropy generation, indicating distinct scaling behaviors.
• A common behavior across both materials is that wear is minimized when the system is balanced in terms of entropy supply and transport capacity, as defined by the residual entropy ratio.
• The residual entropy ratio serves as a predictive indicator: over-supply of entropy transport increases wear, while entropy generation reduces it due to thermodynamic equilibrium restoration.