Adhesive Wear Analyzed
The effects of surface wear reaches across many industries including production, manufacturing, engineering, transportation, roads, and machinery, to name only a few. Given its wide reaching tentacles, it can be surprising to realize that surface wear is still not completely understood.
Surface wear is the gradual removal or damage of material from solid surfaces through motion and contact with other substances or surfaces. The relative size of the removed or damaged material ranges from the large and obvious to the microscopic and invisible to the human eye. These wear particles removed from the surface can create long lasting circumstances, both in economical and medical terms. Consequently, research is coming to the forefront on a regular basis in order to attempt to minimize any possible negative effects of surface wear across industries. Researchers at the Ecole Polytechnique Federale de Lausanne are involved in this research, as described in the article “Researchers Simulate the Process of Adhesive Wear,” published March 8, 2019 in Nature Communications, a scientific journal published by the Nature Publishing Group.
While surface wear is created through various methods, one of the most common causes is adhesive wear. Adhesive wear occurs between two surfaces as they rub against each other, causing friction, which removes particles from one of the surfaces, typically the surface that is less wear-resistant. The resulting debris of removed particles consequently sticks or adheres to the opposing surface.
Adhesive wear depends on several factors such as material properties, presence of chemicals, velocity, and load. Outcomes of adhesive wear vary, with the changing roughness of the surfaces leading the way. When two surfaces first come into contact, they touch at only a few rough points, which is where the process of any wear and friction originates. Gradually, the surface texture changes as material is transferred, creating more areas of roughness and increased friction. Understanding the process more clearly would provide invaluable information regarding innovations to reduce the wearing process and subsequently decrease the required energy input, and pollution output.
This is where the researchers at Ecole Polytechnique Federale de Lausanne have focussed their attention. To date they have completed three studies regarding adhesive wear. The 2016 study incorporated digital simulations in order to illustrate the processes involved in producing minute particles of debris through adhesive wear. Next, the 2017 study delved deeper, determining the possibly of accurately predicting the level of debris and its general appearance. The study demonstrated that these predictions are possible to accurately accomplish.
This all leads to the most recent research into the evolution in change of the rough surfaces over time. Digital simulations (molecular dynamic simulations) are employed for this process In order to overcome experimental limitations. The digital simulations tracked surface changes due to adhesive wear on two-dimensional objects. The final simulation outcomes agree with the outcomes determined during past experiments. Any contact between surfaces results in increasing wear and accumulation of particles. These particles, as they adhere to the opposing surface, increase the roughness of that surface, thus increasing the friction and rate of production of subsequent debris. The process continues growing the accumulation over time, increasing damage, required energy input as well as pollution output.
Credit for the video: Emergence of self-affine surfaces during adhesive wear, Enrico Milanese et al, .
Molecular dynamic simulations proved to be superior to experimental methods in this case. Experiments are limited due to the difficulty of continually monitoring changes in the contacting surfaces. Attempting to do so becomes time and cost consuming, often producing inaccurate results. Building a digital simulation to replicate the adhesion wear process allows for accurate and continuous monitoring of the most minute of changes over time.
Continued research is required to determine the exact effects of the adhesion process on a variety of materials within a wide range of circumstances, such as the use of different materials, differing velocities, differing loads, differing temperatures, etc. Compiling such information would give insight into the general process, direction into how to predict the type and amount of debris accumulated, and how to best stop, or at least minimize the wear and tear caused by the changes.
Due to the limitation of physical experiments in this process, future digital simulations would prove to be invaluable in the process. Future simulations could look at three-dimensional models in order to accurately replicate real world scenarios.
 Enrico Milanese, Tobias Brink, Ramin Aghababaei & Jean-François Molinari, Nature Communicationsvolume 10, Article number: 1116 (2019), https://doi.org/10.1038/s41467-019-09127-8.
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