Synthesis and Properties of Crystalline Carbon Nitride Composite Superhard Coatings
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Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208, [email protected] ABSTRACT Multilayer carbon nitride/TiN coatings were synthesized using a dual-cathode dc magnetron sputtering system. Fully crystalline films can be obtained when the thickness of the carbon nitride component is on the order of one nm. There appears to be a strong correlation between the occurrence of strong TiN(1 11) texture and the hardness of these multilayer coatings, as controlled by target powers, nitrogen partial pressure and substrate bias. These coatings can be synthesized at near room temperatures, with smooth surface morphology and hardness in the range of 45-55 GPa. INTRODUCTION Liu and Cohen [1, 2] predicted that P-C 3N,, which has the same structure as 3-Si 3N4, possesses mechanical properties similar to those of diamond. This has precipitated much excitement in the materials community. While there is debate among theorists on whether or not this and other carbon-based materials have bulk moduli comparable to diamond [3 - 5], the search for ultrahard materials as wear protective coatings continues. There are several reports describing the successful synthesis of some carbon-nitrogen crystalline phases. All involve the use of highly non-equilibrium techniques such as laser ablation [6-8], rf sputtering [9] and arc-plasmas [10]. None reported nanoindentation hardness
and modulus data. One recent paper reported the synthesis of fullerene-like carbon nitride thin films with hardness up to 60 GPa [11], although the standard analysis [12] indicates hardness of at most 25 GPa. Most investigations succeeded in synthesizing amorphous carbon nitride coatings. Several studies centered around the analysis of carbon and nitrogen Is core level peaks [13,14] of amorphous carbon nitride films made by plasma techniques. These analyses suggest that a significant fraction (up to 40%) of the carbon atoms exhibits the sp3 state characteristic of P-C3N,. However, because of the unavoidable incorporation of hydrogen and oxygen into these films, it is this author's opinion that such XPS analysis is not very meaningful. Even if this analysis were correct, the low hardness (10-25 GPa) of amorphous carbon nitride coatings [14-17] indicates that having the local atomic arrangement of p-C3N, is not a sufficient condition for achieving high hardness. Earlier success in synthesizing some crystalline carbon-nitrogen phases using highly nonequilibrium techniques suggests that p-C 3N, may be metastable. One possible approach to seed the growth of O-C3N4 is substrate stabilization. For example, in the work on polycrystalline nitride superlattices such as TiN/NbN and TiN/CrN, it was observed that the cubic TiN layers force NbN and CrN to adopt the same cubic structure under growth conditions where the formation of hexagonal phases of NbN and CrN should have been favored [18,19]. It turns out that the TiN(1 11) surface has hexagonal symmetry, same as P-C3N,(0001). Since the TiN(111) unit vector is 0.30 nm, one unit vector (=0.64
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