Designing compounds for friction reduction: from software development to application in conjunction with experiments

The design of novel, environmentally friendly materials to reduce friction and wear is crucial to save energy and reduce CO2 emissions. To improve the effectiveness of such materials, a clear understanding of the interactions between lubricant additives and the surface at the atomistic level is paramount. To investigate the complex additive-surface interactions by means of ab initio calculations, this thesis introduces Xsorb, a user-friendly Python-based program for identifying the adsorption energy and geometry of complex molecules on crystalline (reconstructed) surfaces. Working in conjunction with a DFT code, it generates multiple adsorption configurations via symmetry operations, identifies the most representative ones through a fast pre-optimization, and finds the potential energy surface (PES) global minimum with a full structural optimization. Two test-cases employing 1-hexene adsorption on iron and diamond surfaces showed Xsorb's effectiveness in reducing computational workload. The program was then applied to the design of a new lubrication system, in a combined computational and experimental work with the University of Coimbra. Experiments demonstrated that chemical modifications in lubricant additives and substrates provide enhanced friction reduction through synergistic effects. The simultaneous inclusion of N-containing 2-(Dimethylamino)ethyl methacrylate and silicon dopants respectively in lauryl-methacrylate additives and diamond-like carbon coatings resulted in a significant reduction of friction, with the formation of a tribofilm. Ab initio calculations of molecular adsorption and a charge density analysis uncovered the key role of the N-Si interaction in enabling the chemisorption of the additive molecules, which constitutes the initial stage for the tribofilm formation. These results open the way to high throughput approaches for discovering the optimal additive/substrate modifications to achieve lower friction and wear.