Understanding Tribofilm Microstructure by Means of Atomic Force Microscopy

Using Tosca 400 to determine tribofilm 3D surface structure and morphology for understanding the sliding effect of different materials on ZDDP tribofilm.

1 Introduction

Engine technology for vehicles is constantly developing to minimize fuel consumption. One central issue is that certain engine parts have metal to metal contact, leading to extensive friction and wear, lowering the engine efficiency and increasing the fuel consumption. By reducing the friction, the energy loss can be reduced by up to 30 %. 

A widely used approach to reduce friction and prevent wear is to use lubricants with additives such as zinc dialkyldithiophosphate (ZDDP).  ZDDP breaks down under wear conditions to form a thin layer (100 to 200 nm) of amorphous polyphosphate, which protects the metal components by maintaining a socalled tribofilm, a physical barrier that is softer and more flexible than the substrates. During engine operation, the ZDDP tribofilm is apparently worn away and reformed, working as sacrificial layer to reduce wear on the metal parts.  Investigation of the surface topography of ZDDP films would provide insight into their tribological effects.

In this report, we use the atomic force microscope (AFM) from Anton Paar, Tosca 400, to scan the surface of ZDDP tribofilms on stainless steel with two different counter-sliding parts made of Al and CuSn. The AFM enables high-resolution surface topography images to be generated with a sharp probe. It provides not only spatial but also vertical high resolution and offers a real 3D surface morphology, which can be used to visualize the tribofilm surface structure and morphology. Understanding the effects of ZDDP tribofilms should lead to design of better formulations, and ultimately, to more efficient energy use.

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