The frictional properties of ZDDP tribofilms at low entrainment speeds in boundary lubrication conditions have been studied in both rolling/sliding and pure sliding contacts. It has been found that the boundary friction coefficients of these tribofilms depend on the alkyl structure of the ZDDPs. For primary ZDDPs, those with linear alkyl chains give lower friction those with branched alkyl chain ZDDPs, and a cyclohexylmethyl-based ZDDP gives markedly higher friction than non-cyclic ones. Depending on alkyl structure, boundary friction coefficient in rolling-sliding conditions can range from 0.09 to 0.14. These differences persist over long duration tests lasting up to 120 h. For secondary ZDDPs, boundary friction appears to depend less strongly on alkyl structure and in rolling-sliding conditions stabilises at ca 0.115 for the three ZDDPs studied. Experiments in which the ZDDP-containing lubricant is changed after tribofilm formation by a different ZDDP solution or a base oil indicate that the characteristic friction of the initial ZDDP tribofilm is lost almost as soon as rubbing commences in the new lubricant. The boundary friction rapidly stabilises at the characteristic boundary friction of the replacement ZDDP, or in the case of base oil, a value of ca 0.115 which is believed to represent the shear strength of the bare polyphosphate surface. The single exception is when a solution containing a cyclohexylethyl-based ZDDP is replaced by base oil, where the boundary friction coefficient remains at the high value characteristic of this ZDDP despite the fact that rubbing in base oil removes about 20 nm of the tribofilm. XPS analysis of the residual tribofilm reveals that this originates from presence of a considerable proportion of C-O bonds at the exposed tribofilm surface, indicating that not all of the alkoxy groups are lost from the polyphosphate during tribofilm formation. Very slow speed rubbing tests at low temperature show that the ZDDP solutions give boundary friction values that vary with alkyl group structure in a similar fashion to rolling-sliding MTM tests. These variations in friction occur immediately on rubbing, before any measurable tribofilm can develop. This study suggest that ZDDPs control boundary friction by adsorbing on rubbing steel or tribofilm surfaces in a fashion similar to organic friction modifiers. However it is believed that, for primary ZDDPs, residual alkoxy groups still chemically bonded to the phosphorus atoms of newly-formed polyphosphate/phosphate tribofilm may also contribute to boundary friction. This understanding will contribute to the design of low friction, fuel efficient crankcase engine oils.

Graphical Abstract

中文翻译：

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ZDDP摩擦膜的边界摩擦

在滚动/滑动和纯滑动接触中，都研究了ZDDP摩擦膜在低夹带速度下在边界润滑条件下的摩擦性能。已经发现，这些摩擦膜的边界摩擦系数取决于ZDDP的烷基结构。对于初级ZDDP，具有直链烷基链的ZDDP的摩擦较小，而具有支链烷基链的ZDDP的摩擦较小，而基于环己基甲基的ZDDP的摩擦明显高于非环状的ZDDP。根据烷基的结构，在滚动滑动条件下的边界摩擦系数可以在0.09至0.14的范围内。这些差异在长达120小时的长时间测试中仍然存在。对于次级ZDDP，边界摩擦似乎不太依赖于烷基结构，并且在滚动滑动条件下稳定在约研究的三个ZDDP为0.115。在通过不同的ZDDP溶液或基础油形成摩擦膜后更换含ZDDP润滑剂的实验表明，几乎在新润滑剂开始摩擦后，初始ZDDP摩擦膜的特性摩擦就消失了。边界摩擦迅速稳定在替代ZDDP的特征边界摩擦上，或者在基础油的情况下，约为0.115被认为代表了裸露的多磷酸盐表面的剪切强度。唯一的例外是当将含环己基乙基ZDDP的溶液替换为基础油时，尽管在基础油中摩擦会去除约20 nm的摩擦膜，但边界摩擦系数仍保持该ZDDP的高值特性。对残留的摩擦膜的XPS分析表明，这是由于暴露的摩擦膜表面存在相当比例的CO键所致，这表明在摩擦膜形成过程中，并非所有烷氧基都从多磷酸盐中丢失。低温下非常慢速的摩擦测试表明，ZDDP解决方案提供的边界摩擦值随烷基结构的变化而变化，其滚动滑动MTM测试的方式与此类似。这些摩擦力变化会在摩擦发​​生时立即发生，然后才能形成可测量的摩擦膜。这项研究表明，ZDDP以类似于有机摩擦改性剂的方式通过吸附在摩擦钢或摩擦膜表面上来控制边界摩擦。然而，据信，对于伯ZDDP，残留的烷氧基仍化学键合到新形成的聚磷酸盐/磷酸盐摩擦膜的磷原子上，也可能有助于边界摩擦。这种理解将有助于设计低摩擦，省油的曲轴箱发动机油。仍旧化学键合到新形成的聚磷酸盐/磷酸盐摩擦膜的磷原子上的残留烷氧基也可能有助于边界摩擦。这种理解将有助于设计低摩擦，省油的曲轴箱发动机油。仍旧化学键合到新形成的聚磷酸盐/磷酸盐摩擦膜的磷原子上的残留烷氧基也可能有助于边界摩擦。这种理解将有助于设计低摩擦，省油的曲轴箱发动机油。

图形概要