Wear in tribological contacts results in generation of wear particles of various sizes and shapes and these particles impact the performance of the mechanical devices. These particles may create additional mechanical damage or act as catalysts and adversely affect the lubricating properties of lubricants. The size of the generated wear particles determines the severity of the impact.
The size of particles also determines the evolution of friction coefficient during initial stage of wear – running-in. In mixed lubrication regime the change of roughness profile is closely related to the wear particle generation. In turn, the evolution of surface roughness determines the friction coefficient. Therefore, the knowledge of the wear particles size evolution allows to predict the evolution of the friction coefficient.
In Surface Technology and Tribology group of University of Twente we developed a model to predict the size of wear particles formed in lubricated contacts and it was applied to calculate the evolution of the friction coefficient during running-in. Typical evolution of friction coefficient with wear during running-in is shown in Fig. 1.
As shown in the Fig. 1, the typical friction process starts with high friction coefficient (Initial) and after initial phase stabilizes at a lower – (Run-In) level. The theoretical model was validated using experimental data by comparison of initial and run-in values of coefficient of friction. The results are shown in Fig. 2.
Prediction of the initial friction coefficient by the model is in a reasonable agreement with the experimental data for both the initial and run-in friction coefficient. It can be noticed that for the sliding speeds of 0.03 and 0.05 m/s, the model underestimates the friction. This is ascribed to the roughening of the surface due to abrasive wear of particles re-entering the contact, an effect that is not included in the model.
The largest change in the mean coefficient of friction due to running-in is achieved in the mixed lubrication part of the friction (Stribeck) curve (0.03–0.05 m/s). Obviously, the effect of running-in was absent in the EHL regime, 0.2–0.4 m/s. In boundary lubrication the effect of running-in was less pronounced. After all, the fraction of the contact separated by the lubricant is already very small and will not change much by wear.
The model was also validated by the measurement of wear particles size using Dynamic Light Scattering and a good agreement was observed. Developed model can be used to optimize the design of the mechanical components. As discussed in the original work, the hardness of the material can be chosen to obtained a minimum friction coefficient after running-in.
For further details see “Generation of wear particles and running-in in mixed lubricated sliding contacts”, Aydar Akchurina, Rob Bosman, Piet M. Lugt, Tribology International 110 (2017) 201–208, http://doi.org/10.1016/j.triboint.2017.02.019.
 “Generation of wear particles and running-in in mixed lubricated sliding contacts”, Aydar Akchurina, Rob Bosman, Piet M. Lugt, Tribology International 110 (2017) 201–208, http://doi.org/10.1016/j.triboint.2017.02.019.