Abstract

In this work, we focus on the advantages of soft (ultra high molecular weight polyethylene, UHMWPE) and hard (diamond like carbon, DLC) composite film over either soft or hard film alone for tribological performance. Thin hard tetrahedral amorphous carbon, ta-C (DLC) is deposited onto Si substrate. Micron scale thin soft UHMWPE is coated onto Si and Si/DLC films by simple dip coating method. In order to get higher tribological performances, perfluoropolyether (PFPE) is coated onto Si/UHMWPE and Si/DLC/UHMWPE. The friction and wear tests are carried out using a custom-built high speed tribometer at a constant speed of 500rpm. A 4mm diameter silicon nitride ball under a normal load of 4g is used as counterface. The nanoscratch tests show that the load carrying capacity of Si/DLC/UHMWPE film is superior to that of Si/UHMWPE. The coefficients of friction of UHMWPE film on Si and Si/DLC are 0.18 and 0.13, respectively. After applying PFPE on UHMWPE, the coefficients of friction drop to 0.06 and 0.07 for Si/UHMWPE and Si/DLC/UHMWPE respectively. Hard DLC film can provide high load carrying capacity, high hardness, high penetration resistance and low contact area for the soft polymer film. The wear durability of Si/DLC/UHMWPE is approximately five times higher than that of Si/UHMWPE. Overcoating of PFPE further extends wear durability by approximately five times for Si/UHMWPE and three times for Si/DLC/UHMWPE (experiments stopped). This study shows that the combination of soft and hard composite film is better than either soft or hard film alone for tribological properties.

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