The effect of DLC wear particles on friction under lubrication

Almost sliding parts in machine elements are immersed in lubricant to prevent wear, to reduce friction coefficient and to eliminate wear particles from sliding interface between two faces. Recently, automobile industries are required to use less lubricant for environment friendly, high normal load and low speed at the contact or sliding part which generates less amount of oil film thickness between faces and it causes severe friction condition. In this situation, carbonaceous coating such as Diamond-Like Carbon (DLC) is one of the attractive candidates to generate low friction under boundary lubrication because of its low friction property at dry. However, wear particles coming from DLC is believed to be a kind of hard slurry that scratches surfaces as abrasive and it shortens lifetime of DLC. Generally, these wear particles are collected by filters that 0.1 μm pore size is the smallest in commercial, therefore, nm scale wear particles are eliminated from the discussion. So, it is unclear that nm scale particles have affection onto tribological properties. In this study, we carried out friction test between the side surface of DLC coated cylinder against DLC coated disk under lubricant at boundary lubrication condition. The friction test was conducted 2000 cycles, then lubricant was replaced to new lubricant, friction test was continued at same contact point after the sequence. We carried out these procedures to reach to 12000 cycles, then wear particles were collected from lubricant by using electric field to gather nm scale particles. The lubricant was kerosene at room temperature. The wear particles were gathered near plus voltage electrode, then these particles stayed onto glass plate, then the lubricant was dried out. Finally, we conducted scanning electron microscope (SEM) observation and laser microscope observation to the particles.