Abstract

A Study on the Wear Properties of Nanocrystalline Diamond Films


by Im Jong Hwan
Advisor : Prof. Kang Chan Hyoung
Course for Energy Technology
(Department of Advanced Material Engineering)
Graduate School of Knowledge-based
Technology and Energy
Korea Polytechnic University

Nano-crystalline diamond (NCD) films were deposited on SiC balls by microwave plasma chemical deposition (MPCVD) method to be applied in a ball-on-disk type tribometer. Deposition parameters were the microwave power of 1.2 kW, the working pressure of 100 Torr, the Ar/CH4 gas ratio of 200/2 sccm, the substrate temperature of 600 ℃, and the deposition times of 1 ~ 6 h. Bare SiC and ZrO2 balls were prepared for comparison as test balls. The FE-SEM observation of the cross section of the samples machined by FIB revealed that complete continuous films formed after 4 h of deposition. NCD films were identified by X-ray diffraction and Raman spectroscopy. Tribology tests were performed in air with pairs of three different balls and mirror polished steel SKH51 disk. The wear tracks on balls and disks were examined by optical microscope and alpha step profiler. Under the load of 3 N, the friction coefficients of steel against SiC, ZrO2, and NCD coated balls were between 0.4 and 0.8. After a few thousands sliding laps corresponding to less than 100 m, the friction coefficient of NCD coated balls dropped from 0.45 to below 0.1 and maintained thereafter. Under a higher load of 10 N or 20 N with a long sliding distance of 2 km, SiC and ZrO2 balls showed the similar friction coefficient as above. Under the same conditions, however, NCD coated balls exhibited the friction coefficient less than 0.1 from the start maintaining steadily or with some fluctuation as sliding distance increased. It is believed that the carbon debris formed during initial sliding play a role as lubricant and resulted in a low friction coefficient. The wear tracks on the balls and the disks were observed to be wider and deeper in the case of NCD coated balls than SiC and ZrO2 balls. In conclusion, the new NCD coated balls can be applied in the tribometer with low load and short sliding conditions and a longer life is expected than the conventional balls.