Development of Nano Diamond Polymer Coating on Piston Skirt for Fuel Efficiency

Various polymer-based coatings are applied on piston skirt to reduce friction loss between the piston skirt and cylinder bore which is one of main factors of energy loss in an automotive engine system. These coatings generally consist of polymer binder (PAI) and solid lubricants (graphite or MoS₂) for low friction property. On the other hand, the present study found that PTFE as a solid lubricant and nano diamond as hard particles can be used to improve the low friction and wear resistance simultaneously. In the process of producing coating material, diamond particles pulverized to a nano size tend to agglomerate. To prevent this, silane (silicon coupling agent) treatment was applied. The inorganic functional groups of silane are attached to the nano diamond surface, which keep the diamond particles are apart. In addition, the organic functional group in the silane has property to combine with polymer like PAI or PTFE in the coating solution which improves the bonding energy between nano diamond and the polymer. To find a coating of optimal properties, nano diamond and silane contents as well as the types of silane are controlled. The properties of coating were tested using Ball-on-Plate tribology test machine. As a result, the coating containing 1 wt% of nano diamond and 4 wt% of amino functional silane compared to the nano diamond contents showed the lowest fiction coefficient and wear rate. The optimized nano diamond polymer coating was applied on real piston and tested using single-cylinder rig test machine. The friction force of the nano diamond polymer coating is 21% lower than graphite coating and 7% lower than MoS₂ coating. Furthermore, in contrast to the MoS₂ coating, the nano diamond polymer coating showed no wear mark after the test. Using the real engine test, the nano diamond polymer coating showed 1.8% decrease in friction force measured as FMEP compared to the MoS₂ coating which is on mass production and nano diamond polymer coating showed superior durability according to the full load and full speed engine test.