Reddyhoff Tom, Ewen James P, Deshpande Pushkar, Frogley Mark D, Welch Mark D, Montgomery Wren
Department of Mechanical Engineering, Imperial College London, Exhibition Road, South Kensington, London SW7 2AZ, United Kingdom.
Soft Condensed Matter Group, Diamond Light Source, Diamond House, Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom.
ACS Appl Mater Interfaces. 2021 Feb 24;13(7):9239-9251. doi: 10.1021/acsami.0c21918. Epub 2021 Feb 10.
Simple -alcohols, such as 1-dodecanol, show anomalous film-forming and friction behaviors under elastohydrodynamic lubrication (EHL) conditions, as found inside bearings and gears. Using tribometer, diamond anvil cell (DAC), and differential scanning calorimetry (DSC) experiments, we show that liquid 1-dodecanol undergoes a pressure-induced solidification when entrained into EHL contacts. Different solid polymorphs are formed inside the contact depending on the temperature and pressure conditions. Surprisingly, at a moderate temperature and pressure, 1-dodecanol forms a polymorph that exhibits robust macroscale superlubricity. The DAC and DSC experiments show that superlubricity is facilitated by the formation of lamellar, hydrogen-bonded structures of hexagonally close-packed molecules, which promote interlayer sliding. This novel superlubricity mechanism is similar to that proposed for the two-dimensional materials commonly employed as solid lubricants, but it also enables the practical advantages of liquid lubricants to be maintained. When the pressure is increased, 1-dodecanol undergoes a polymorphic transformation into a phase that gives a higher friction. The DAC and DSC experiments indicate that the high-friction polymorph is an orthorhombic crystal. The polymorphic transformation pressure coincides with the onset of a dimple formation in the EHL films, revealing that the anomalous film shapes are caused by the formation of rigid orthorhombic crystals inside the contact. This is the first demonstration of a macroscale superlubricity in an EHL contact lubricated by a nonaqueous liquid that arises from bulk effects rather than tribochemical transformations at the surfaces. Since the superlubricity observed here results from phase transformations, it is continuously self-replenishing and is insensitive to surface chemistry and topology. This discovery creates the possibility of implementing superlubricity in a wide range of machine components, which would result in enormous improvements in efficiency and durability.
简单醇类，如1-十二醇，在弹性流体动力润滑（EHL）条件下表现出反常的成膜和摩擦行为，在轴承和齿轮内部就存在这种情况。通过摩擦计、金刚石对顶砧（DAC）和差示扫描量热法（DSC）实验，我们发现液态1-十二醇在进入EHL接触时会发生压力诱导固化。根据温度和压力条件的不同，接触内部会形成不同的固体多晶型物。令人惊讶的是，在适度的温度和压力下，1-十二醇会形成一种表现出强大宏观超润滑性的多晶型物。DAC和DSC实验表明，六方密堆积分子形成的层状氢键结构促进了层间滑动，从而实现了超润滑性。这种新颖的超润滑机制与通常用作固体润滑剂的二维材料所提出的机制相似，但它也能保持液体润滑剂的实际优势。当压力增加时，1-十二醇会发生多晶型转变，变为摩擦系数更高的相。DAC和DSC实验表明，高摩擦多晶型物是正交晶体。多晶型转变压力与EHL膜中凹坑形成的起始点一致，这表明反常的膜形状是由接触内部刚性正交晶体的形成引起的。这是首次在由非水液体润滑的EHL接触中证明宏观超润滑性是由体效应而非表面摩擦化学反应产生的。由于这里观察到的超润滑性是由相变引起的，它是持续自我补充的，并且对表面化学和拓扑结构不敏感。这一发现为在广泛的机器部件中实现超润滑性创造了可能性，这将带来效率和耐久性的巨大提升。
