Mechanochemical Activation of Anthracene [4+4] Cycloadducts.

Controlled formation and breaking of weak chemical bonds is a versatile method for modifying the properties of materials. Anthracene [4+4] cycloadducts are a prime example that can be formed by light and opened by external forces. We address the theoretical description of mechanochemistry of these cycloadducts, where the standard constraint geometry simulates forces approach fails due to the lack of consideration of temperature. Explicit inclusion of external forces reveals the corresponding transition barriers that are clearly dominated by rupture of the [4+4] inter-anthracene bonds. Other bonds come into play at extremely large forces only, which cannot be expected to be reached under ambient conditions. The theoretical results are in line with the experimental rheology of [4+4]-linked anthracene polymers, which indicates reversible re-formation of [4+4] cycloaddition bonds with ultraviolet light after mechanochemical bond breaking due to applied shear stress.

Effects of Microstructure Modification by Friction Surfacing on Wear Behavior of Al Alloys with Different Si Contents.

In this work, Al alloys with 6.6%, 10.4%, and 14.6% Si were deposited as thick coatings by Friction Surfacing (FS), resulting in grain refinement and spheroidization of needle-shaped eutectic Si phase. Lubricated sliding wear tests were performed on a pin-on-disc tribometer using Al-Si alloys in as-cast and FS processed states as pins and 42CrMo4 steel discs. The chemical composition of the worn surfaces was analyzed by X-ray photoelectron spectroscopy (XPS). The wear mechanisms were studied by scanning electron microscopy (SEM) and focused ion beam (FIB), and the wear was evaluated by measuring the weight loss of the samples. For the hypoeutectic alloys, spheroidization of the Si phase particles in particular leads to a significant improvement in wear resistance. The needle-shaped Si phase in as-cast state fractures during the wear test and small fragments easily detach from the surface. The spherical Si phase particles in the FS state also break away from the surface, but to a smaller extent. No reduction in wear due to FS was observed for the hypereutectic alloy. Here, large bulky primary Si phase particles are already present in the as-cast state and do not change significantly during FS, providing high wear resistance in both material states. This study highlights the mechanisms and limitations of improved wear resistance of Si-rich Al alloys deposited as thick coatings by Friction Surfacing.