Analysis of friction coefficient evolutions on coated systems under sliding conditions

This paper analyses friction coefficient evolution of dry lubricant coatings subjected to unidirectional sliding. Dry lubricant coatings have been investigated for maintaining low friction under sliding conditions, and have been used for this purpose in mechanical components. Experimental characterisation of coating durability can be achieved by measuring the friction coefficient as a function of the number of turns, or, alternatively, the sliding distance. In this paper, literature on abrasive wear tests of various low friction coatings was reviewed, and friction coefficient evolutions of these coatings were analysed with appropriate mathematical forms. The friction coefficient evolutions can be classified into three distinct types: exponential, linear, and logarithmic. In an exponential evolution type, friction coefficient growth rate with respect to sliding distance is expressed as a power-law function of the friction coefficient itself. It was identified that two parameters determine the friction coefficient evolution: the damage rate constant and the damage exponent. The damage exponent was found to be close to unity, suggesting an exponential relation between damage and sliding distance. In a linear evolution type, friction coefficient growth rate is constant, expressed with the damage rate constant. In a logarithmic evolution type, friction coefficient growth rate is expressed as a power-law function of sliding distance. Two parameters, the damage rate constant and the damage exponent, were also identified. The damage exponent was found to be close to minus unity. This characterisation of friction coefficient evolutions with the damage rate constant and the damage exponent helps to understand friction behaviours of coated systems.