Boric acid is a weak acid and has been used as a lubrication additive because of its special structure. In this study, we report that boric acid could achieve a robust superlubricity (μ < 0.01) as an additive in polyethylene glycol (PEG) aqueous solution at the Si₃N₄/SiO₂ interfaces. The superlow and steady friction coefficient of approximately 0.004–0.006 could be achieved with boric acid under neutral conditions (pH of approximately 6.4), which is different from the acidic conditions leading to superlubricity. The influence of various factors, including boric acid concentration, sliding speed, applied load, PEG molecular weight, and the volume of lubricant on the superlubricity, were investigated. The results reveal that the PEG aqueous solution with the boric acid additive could achieve superlubricity under a wide range of conditions. The surface composition analysis shows that the synergy effect between boric acid and PEG provides sufficient H⁺ ions to realize the running-in process. Moreover, a composite tribochemical film composed of silica and ammonia-containing compounds were formed on the ball surface, contributing to the superlubricity. The film thickness calculation shows that superlubricity was achieved in a mixed lubrication region, and therefore, the superlubricity state was dominated by both the composite tribochemical film formed via the tribochemical reaction on the contact surfaces and the hydrodynamic lubricating film between the contact surfaces. Such a liquid superlubricity achieved under neutral conditions is of importance for both scientific understanding and engineering applications.

中文翻译：

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硼酸添加剂在聚乙二醇水溶液中的液体超润滑性

硼酸是一种弱酸，由于其特殊的结构已被用作润滑添加剂。在这项研究中，我们报告硼酸可以在Si ₃ N ₄ / SiO ₂处作为聚乙二醇（PEG）水溶液中的添加剂而具有强大的超润滑性（μ<0.01）接口。硼酸在中性条件下（pH约为6.4）可以达到大约0.004-0.006的超低和稳定摩擦系数，这与导致超润滑的酸性条件不同。研究了硼酸浓度，滑动速度，施加载荷，PEG分子量和润滑剂量等因素对超润滑性的影响。结果表明，含硼酸添加剂的PEG水溶液在很宽的条件下都能获得超润滑性。表面组成分析表明，硼酸与PEG的协同作用提供了足够的H ⁺离子来实现磨合过程。此外，在球表面形成了由二氧化硅和含氨化合物组成的复合摩擦化学膜，有助于超润滑性。膜厚的计算表明，在混合润滑区域中实现了超润滑，因此，超润滑状态主要由在接触表面上通过摩擦化学反应形成的复合摩擦化学膜和在接触表面之间的流体动力润滑膜所决定。在中性条件下获得的这种液体超润滑性对于科学理解和工程应用都是重要的。