A Nanoscratch Method for Assessing Wear of Metal Carbide-Metal Nacreous Nanostructures
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A Nanoscratch Method for Assessing Wear of Metal Carbide-Metal Nacreous Nanostructures Rachel W. Obbard and Todd S. Gross University of New Hampshire Materials Science & Engineering Program Kingsbury Hall Durham, NH 03824-3591 ABSTRACT The wear resistance of metal carbide-metal nacreous nanostructures is compared to that of a solid metal carbide hard coating. A reciprocating nanoscratch method has been developed which allows the assessment of wear resistance of the film independent of substrate type or filmsubstrate adhesion. A series of nominally 1 micron thick TiC-based multilayer films on Si substrates were produced using RF magnetron sputtering. A scratch protocol consisting of 70 reciprocal scratches each 9.5 µm in length was applied. Data reduction techniques were used to remove the effects of sample tilt and drift. The resulting normal displacement as a function of scratch cycles is used to compare the wear of different films on Si substrates. Specific wear characteristics related to film morphology are examined. This investigation showed that this nanoscratch technique holds promise as a thin film wear characterization technique. INTRODUCTION Thin hard metal carbide and metal nitride films are widely used for cutting tools and bearing applications. Hardness, fracture toughness and wear are important mechanical properties of such films. Nanostructures imitating natural nacre are of interest because they possess unique properties of hardness and fracture toughness. Nacreous nanostructures are characterized by a laminar arrangement of hard and ductile layers, similar to the alternating layers of CaCO3 and proteins that comprises oyster shells. Synthetic nacre is typically made with carbide or nitride hard layers and ductile metal interlayers. [1] Thin film hardness is assessed through the nanoindentation technique. Other researchers have shown that thin carbide or nitride nacreous nanostructures with metal interlayers have hardesses greater than the volume-weighted mean of the hardness of the two components. He, Wang, W.Z. Li and H.D. Li prepared TiN/Pt multilayer films with very thin layers (1-8 nm). They found that found the hardness of these films was greater than the volume weighted mean of the hardness of the two components and that certain nitride/metal combinations resulted in a higher hardness than that of pure TiN. This phenomenon was found to be related to two competing factors: the individual layer thickness and the number of interfaces between layers. [1] In separate work, Wang et al found that by adjusting the periodicity, of TiC/Mo multilayer films between 2 and 14 nm, nanoindentation hardness larger than either the rule-of-mixtures value or the hardness of TiC could be achieved. [2,3] Hardness, however, is not the only hallmark of a useful coating. It must also hold up under wear and be forgiving of imperfections in the substrate produced during manufacturing or introduced during use. P8.22.1
The toughness of thin films remains difficult to assess. Scale and orientation factors make conventiona
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