Structural complexity improves stability: The pressure and temperature-dependent behaviour of lithium greases

Abstract Greases are characterised by a complex structure in which an oil is transformed into a gel-like state by a thickener, i.e. often metal soaps. Depending on the requirements for industrial application, different thickeners are used to adapt the microstructure of the grease. The crystal structure of the thickener should be stable over a wide pressure and temperature range to guarantee reliable operation of the lubricants. In this work, small- and wide angle x-ray measurements on two different greases are discussed. The crystal structures of the thickeners depend non-linearly on pressure and temperature. We show that in the complex Li grease containing Li salts with mono- and dicarboxylic acids as thickeners, the gel-like structure of the grease is stabilized by the structural complexity in contrast to a simple Li grease containing a Li-12-hydoxystearate thickener only.