What stress components drive mechanochemistry? A study of ZDDP tribofilm formation
What stress components drive mechanochemistry? A study of ZDDP tribofilm formation
复制标题
是什么应力成分驱动了机械力化学？ZDDP摩擦膜形成的研究
DOI：
10.1039/d2fd00123c
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发表时间：
2022-07-27
影响因子：
3.4
通讯作者：
Carpick, Robert W.
中科院分区：
文献类型：
--
作者：
Fang, Lu;Korres, Spyridon;Carpick, Robert W.
二烷基二硫代磷酸锌（ZDDP）是发动机油中使用最广泛的抗磨添加剂，在过去几十年中已被广泛研究，以帮助了解其有效性的来源。玻璃状磷酸盐基摩擦膜，约100纳米厚，通常形成在含ZDDP的油中滑动的表面上，这有助于防止或减少磨损。最近的研究表明，施加的剪切应力和压缩应力的组合驱动机械化学反应，促进摩擦膜生长，并且通过增加温度进一步加速生长。虽然最近的工作已经表明，压缩应力单独是不足以形成摩擦膜，剪切应力和压缩应力的个别影响没有完全理解。在这里，剪切和压缩应力分别研究使用不同比例的高粘度，高牵引力的流体进行测试。这允许当被限制在加载的滑动界面处时流体中的面积平均压缩应力和剪切应力在驱动摩擦膜生长的同时被独立地控制，这是我们称为应力控制的机械化学反应器的系统。来自二次ZDDP的摩擦膜使用碳化钨/碳化钨球-盘接触在全弹性流体动力润滑（EHL）状态下使用微型牵引机（MTM）产生，这意味着避免了固体-固体接触。的MTM配备了一个间隔层成像（SLIM）的能力，允许在其生长过程中的摩擦膜厚度的原位测量。由高粘度流体产生的良好分离的滑动表面证实了摩擦膜形成不需要固-固接触。在这些全流体膜弹流润滑条件下，剪切应力和温度促进摩擦膜的增长，根据应力增强的热激活。相反，在恒定的剪切应力和温度下，压缩应力具有相反的效果，抑制摩擦膜的生长。使用扩展的Eyring模型剪切和静水压力影响的反应动力学，活化能为0.54 +/- 0.04 eV的发现，与以前的研究ZDDP一致。发现剪切应力的活化体积为0.18 +/- 0.06 nm（3），而压缩应力组分的活化体积要小得多，为0.010 +/- 0.004 nm（3）。这不仅证实了先前的工作支持剪切应力驱动摩擦膜生长，但演示和量化压缩应力如何抑制生长，与涉及键断裂反应的摩擦膜生长中的限速步骤一致。这些研究结果的影响进行了讨论。
Zinc dialkyldithiophosphate (ZDDP), the most widely used antiwear additive in engine oils, has been extensively studied over the last few decades to help understand the origin of its effectiveness. Glassy phosphate-based tribofilms, approximately 100 nm thick, are often formed on surfaces sliding in ZDDP-containing oils, which help to prevent or reduce wear. Recent studies reveal that a combination of applied shear and compressive stresses drive mechanochemical reactions that promote tribofilm growth, and that growth is further accelerated by increased temperature. While recent work has shown that compressive stress alone is insufficient to form tribofilms, the individual effects of the shear stress and compressive stress are not fully understood. Here, shear and compressive stresses are studied separately by using different ratios of high-viscosity, high-traction fluids for testing. This allows the areal mean compressive and shear stresses in the fluid when confined at a loaded sliding interface, to be independently controlled while driving tribofilm growth, which is a system we refer to as a stress-controlled mechanochemical reactor. Tribofilms derived from a secondary ZDDP were generated using a tungsten carbide/tungsten carbide ball-on-disk contact in the full elastohydrodynamic lubrication (EHL) regime using a mini-traction machine (MTM), meaning that solid-solid contact is avoided. The MTM was equipped with a spacer layer imaging (SLIM) capability, permitting in situ measurement of the tribofilm thickness during its growth. The well-separated sliding surfaces generated by the high-viscosity fluids confirm that solid-solid contact is not required for tribofilm formation. Under these full fluid film EHL conditions, shear stress and temperature promote tribofilm growth in accordance with stress-augmented thermal activation. In contrast, under constant shear stress and temperature, compressive stress has the opposite effect, inhibiting tribofilm growth. Using the extended Eyring model for shear- and hydrostatic pressure-affected reaction kinetics, an activation energy of 0.54 +/- 0.04 eV is found, consistent with prior studies of ZDDPs. The activation volume for shear stress is found to be 0.18 +/- 0.06 nm(3), while that for the compressive stress component is much smaller, at 0.010 +/- 0.004 nm(3). This not only confirms prior work supporting that shear stress drives tribofilm growth, but demonstrates and quantifies how compressive stress inhibits growth, consistent with the rate-limiting step in tribofilm growth involving a bond-breaking reaction. Implications of these findings are discussed.