Fraunhofer Institute for Mechanics of Materials IWM, MicroTribology Center μTC, Wöhlerstraße 11, 79108, Freiburg, Germany.
Fraunhofer Institute for Material and Beam Technology IWS, Winterbergstraße 28, 01277, Dresden, Germany.
Nat Commun. 2019 Jan 11;10(1):151. doi: 10.1038/s41467-018-08042-8.
Superlubricity of tetrahedral amorphous carbon (ta-C) coatings under boundary lubrication with organic friction modifiers is important for industrial applications, but the underlying mechanisms remain elusive. Here, combined experiments and simulations unveil a universal tribochemical mechanism leading to superlubricity of ta-C/ta-C tribopairs. Pin-on-disc sliding experiments show that ultra- and superlow friction with negligible wear can be achieved by lubrication with unsaturated fatty acids or glycerol, but not with saturated fatty acids and hydrocarbons. Atomistic simulations reveal that, due to the simultaneous presence of two reactive centers (carboxylic group and C=C double bond), unsaturated fatty acids can concurrently chemisorb on both ta-C surfaces and bridge the tribogap. Sliding-induced mechanical strain triggers a cascade of molecular fragmentation reactions releasing passivating hydroxyl, keto, epoxy, hydrogen and olefinic groups. Similarly, glycerol's three hydroxyl groups react simultaneously with both ta-C surfaces, causing the molecule's complete mechano-chemical fragmentation and formation of aromatic passivation layers with superlow friction.
具有四面体非晶碳 (ta-C) 涂层的超滑润性在边界润滑条件下与有机摩擦改性剂一起非常重要，这对工业应用很重要，但潜在的机制仍难以捉摸。在这里，组合实验和模拟揭示了一种普遍的摩擦化学机制，导致 ta-C/ta-C 摩擦副的超滑润性。销盘滑动实验表明，通过不饱和脂肪酸或甘油润滑可以实现超低和超摩擦，且磨损可以忽略不计，但通过饱和脂肪酸和碳氢化合物则不行。原子模拟表明，由于同时存在两个反应中心（羧基和 C=C 双键），不饱和脂肪酸可以同时化学吸附在两个 ta-C 表面并桥接摩擦间隙。滑动引起的机械应变引发一系列分子断裂反应，释放出钝化的羟基、酮、环氧、氢和烯烃基团。类似地，甘油的三个羟基基团同时与两个 ta-C 表面反应，导致分子的完全机械化学断裂，并形成具有超低摩擦的芳烃钝化层。
