Experimental Results on Lamellar-Type Solid Lubricants in Enhancing Minimum Quantity Lubrication Machining

Abstract

This paper studies the effect of various lamellar-type solid lubricants (graphite and hBN) that can be mixed into a lubricant to potentially improve the machinability of minimum quantity lubrication (MQL) machining. To examine this, the solid lubricants are classified into particles and platelets based on their aspect ratios as well as their respective sizes. In particular, the particles are classified into microparticles and nanoparticles based on their dimensions (average radius), while the platelets were classified, based on their average thickness, into two types: the “microplatelets” if the thickness is typically up to few tens of microns and the “nanoplatelets” if the thickness is well below a tenth of a micron (even down to few nanometers). Our previous work has shown that the mixture of an extremely small amount (about 0.1 wt. %) of the graphitic nanoplatelets and vegetable oil immensely enhanced the machinability of MQL machining. In this paper, many lubricants, each mixed with a particular variety of nano- or micro-platelets or one type of nanoparticles, were studied to reveal the effect of each solid lubricant on MQL machining. Prior to the MQL machining experiment, the tribological test was conducted to show that the nanoplatelets are overall more effective than the microplatelets and nanoparticles in minimizing wear despite of no significant difference in friction compared to pure vegetable oil. Consequently, the MQL ball-milling experiment was conducted with AISI 1045 steel yielding a similar trend. Surprisingly, the oil mixtures with the microplatelets increased flank wear, even compared to the pure oil lubricant when the tools with the smooth surface were used. Thus, the nanoscale thickness of these platelets is a critical requirement for the solid lubricants in enhancing the MQL machining process. However, maintaining the nanoscale thickness is not critical with the tools with the rough surfaces in enhancing the MQL process. Therefore, it is concluded that finding an optimum solid lubricant depends on not only the characteristics (material as well as morphology) of solid lubricants but also the characteristic of tool surface.