Mechanochemical finishing: developing a mechanical finishing process to form tribologically superior surfaces

Firth, James Jonathan (2022). Mechanochemical finishing: developing a mechanical finishing process to form tribologically superior surfaces. University of Birmingham. Ph.D.

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Abstract

Both mechanical and chemical finishing process are regularly used to reduce frictional losses and improve the wear resistance of components within mechanical systems like internal combustion engine cylinders and fluid pumps. Coatings often require highly specialised equipment and are usually expensive in comparison to mechanical processes. In many applications, both mechanical finishing and chemical coatings are applied to component surfaces in order to achieve a surface with all the desired properties. This thesis applies tribofilm forming research when using particular nano-additives to mechanical finishing processes in order to mechanochemically generate a tribologically superior surface through a simple, single stage operation.

Cast iron plates were honed against grit paper with dry Cu\(_2\)S microparticles causing a reduction reaction in the particles for sulphur to then react with the iron in the plates. Energy Dispersive Spectroscopy (EDS) revealed a huge increase in the quantity of copper in the cast iron caused by the mechanochemical honing. X-ray Photoelectron Spectroscopy (XPS) did suggest a small quantity of iron sulphides had formed. While the honing process did successfully reduce the copper sulphide, a lot of the sulphur was lost in the process while the copper sintered into the surface. Lubricated friction tests at a variety loads showed a small reduction in friction with the copper honed samples.

In order to better understand mechanisms for tribofilm formation and discover any synergistic behaviours between nano-MoS\(_2\) and ceramic nanospheres, SiO\(_2\) was tested with MoS\(_2\) as lubricant additives in a variety of High Frequency Reciprocating Rig (HFRR) tests. At high load and 50 °C the two nanoparticles on their own could not improve friction or wear but when used together, the SiO\(_2\) improved the conditions for MoS\(_2\) to chemically react with the steel contacts forming a protective tribofilm. This was characterised through a combination of EDS and Hard X-Ray Photoelectron Spectroscopy (HAXPES). Attempts to recreate these results in the TE 77 High Frequency Friction Machine (TE 77) low load adapter were unsuccessful.

Keeping lubricant additives successfully dispersed is a well documented issue. To get round this, Cu\(_2\)S and Al\(_2\)O\(_3\) microparticles were mixed with a base grease whose viscosity was great enough that once dispersed the particles could not settle. The formulated greases were put under very high load (920N) in line contact in the TE 77 to generate a tribofilm style layer that was then tested with point contact wear in a base lubricant. The formulated greases were tested against a commercial grease containing MoS\(_2\). At low loads, only the MoS\(_2\) treated surface produced lower friction than the untreated plate. In 50 °C tests at a higher load, all the treated surfaces slightly reduced friction compared to the untreated plate and the hybrid Cu\(_2\)S, Al\(_2\)O\(_3\) surface had the lowest friction. MoS\(_2\) produced the shallowest scar with a tribofilm similar to that generated with the hybrid lubricant in the previous chapter.

This thesis introduces a proof of concept for two mechanochemical finishing processes as well as developing understanding for the synergistic behaviour of Transition Metal Dichalcogenides (TMDC) and ceramics as nano-additives in lubricants. The potential scope for this research is huge as the finishing processes could be beneficial to any mechanical system where there are sliding metal contacts. A large amount of work is still needed to bring this work from its concept stage to full realisation in a commercial setting.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):