Dependence of Tribological Performance and Tribopolymerization on the Surface Binding Strength of Selected Cycloalkane-Carboxylic Acid Additives
Dependence of Tribological Performance and Tribopolymerization on the Surface Binding Strength of Selected Cycloalkane-Carboxylic Acid Additives
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DOI：
10.1007/s11249-020-01329-2
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发表时间：
2020-08
影响因子：
3.2
通讯作者：
Qiang Ma;Qiang Ma;A. Khan;Q. Wang;Yip-Wah-Chung
中科院分区：
文献类型：
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作者：
Qiang Ma;Qiang Ma;A. Khan;Q. Wang;Yip-Wah-Chung
我们最近报道了使用环丙烷甲酸（CPCa）作为一种模型添加剂，它可以在钢摩擦接触中的闪热和应力的综合作用下很容易发生反应，形成摩擦聚合物，并显着改善摩擦学性能。在本文中，我们通过实验和分子动力学模拟展示了 CPCa 的化学结构改性如何影响摩擦聚合物的形成，从而影响摩擦和磨损性能的结果。研究了四种润滑油添加剂，即 CPCa、环丁烷甲酸 (CBCa)、环丙烷-1,1-二甲酸 (CPDCa) 和环丁烷-1,1-二甲酸 (CBDCa)，它们由亚稳环结构和溶解在酯基础油中的一个或两个羧基组成。使用这些添加剂的摩擦和磨损率的顺序为CPDCa < CBDCa < CPCa < CBCa。拉曼光谱分析表明，这些添加剂分子在接触区域形成摩擦聚合物薄膜。分子动力学模拟表明，具有不太稳定的环丙烷环片段的CPCa比CBCa更容易形成。这种碎裂似乎对于随后的摩擦聚合和保护性摩擦膜的形成至关重要。这些模拟进一步证明，在 CPDCa 的情况下具有两个羧基会导致添加剂分子与表面的结合更强，从而增加停留时间，并因此促进机械或热诱导的解离和随后的聚合。最终结果是，在我们的测试条件下，CPDCa 具有最低的摩擦力和可忽略不计的磨损。
We recently reported the use of cyclopropanecarboxylic acid (CPCa) as a model additive that can readily react under the combined effect of flash heating and stress in steel tribocontacts to form tribopolymers, along with marked improvement in tribological performance. In this paper, we present results of how chemical structural modification of CPCa may impact on the formation of tribopolymers and hence friction and wear properties, both by experiments and molecular dynamics simulation. Four lubricant additives, viz., CPCa, cyclobutanecarboxylic acid (CBCa), cyclopropane-1,1-dicarboxylic acid (CPDCa), and cyclobutane-1,1-dicarboxylic acid (CBDCa) consisting of a metastable ring structure and one or two carboxyl groups dissolved in an ester base oil were studied. Friction and wear rate using these additives rank in the order of CPDCa < CBDCa < CPCa < CBCa. Raman spectroscopy analysis reveals that these additive molecules form tribopolymer films at the contact area. Molecular dynamics simulation shows that CPCa with the less stable cyclopropane ring fragments more readily than CBCa. Such fragmentation appears to be essential for subsequent tribopolymerization and formation of protective tribofilms. These simulations further demonstrate that having two carboxyl groups as in the case of CPDCa results in stronger binding of the additive molecules to the surface, thus increasing the residence time and hence facilitating mechanically or thermally induced dissociation and subsequent polymerization. The net result is that CPDCa gives the lowest friction and negligible wear under our testing conditions.