Influence of the microstructure on the mechanical and tribological behavior of TiC/a-C nanocomposite coatings D. Martínez-Martínez ⁎, C. López-Cartes, A. Fernández, J.C. Sánchez-López Instituto de Ciencia de Materiales de Sevilla, CSIC-Universidad de Sevilla, Avda. Américo Vespucio 49, 41092-Sevilla, Spain abstract article info Article history: Received 10 January 2008 Received in revised form 13 June 2008 Accepted 22 September 2008 Available online 1 October 2008 Keywords: Titanium carbide Raman Hardness Wear Sputtering Transmission electron microscopy Scanning electron microscopy Tribological properties The performance of protective thin films is clearly influenced by their microstructure. The objective of this work is to study the influence of the structure of TiC/a-C nanocomposite coatings with a-C contents ranging from ~0% to 100% on their mechanical and tribological properties measured by ultramicroindentation and pin-on-disks tests at ambient air, respectively. The microstructure evolves from a polycrystalline columnar structure consisting of TiC crystals to an amorphous and dense TiC/a-C nanocomposite structure when the amount of a-C is increased. The former samples show high hardness, moderate friction and high wear rates, while the latter ones show a decrease in hardness but an improvement in tribological performance. No apparent direct correlation is found between hardness and wear rate, which is controlled by the friction coefficient. These results are compared to the literature and explained according to the different film microstructures and chemical bonding nature. The film stress has also been measured at the macro and micro levels by the curvature and Williamson–Hall methods respectively. Other mechanical properties of the coating such as resilience and toughness were evaluated by estimating the H 3 /E⁎ 2 and H/E⁎ ratios and the percentage of elastic work (W e ). None of these parameters showed a tendency that could explain the observed tribological results, indicating that for self-lubricant nanocomposite systems this correlation is not so simple and that the assembly of different factors must be taken into account. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Current research is directed towards designing multifunctional materials that accomplish multiple performance objectives in a single system or advanced materials exhibiting improved or outstanding performance. Examples of both possibilities can be found in the field of protective coatings, combining hardness with other suitable properties as low-friction, thermal and wear-resistance (multifunc- tional) or superhard nanocomposite coatings (advanced). In fact, hardness has traditionally been the most demanded property for protective coatings. Normally, the intrinsic hardness of a material (associated with chemical bond strength) is not achievable, because there are various deformation mechanisms other than breaking chemical bonds that need less energy (dislocation, cracks, etc.). As a result, proper control of the structure of a coating is essential to improve its properties. One typical example of this is the well-known Hall–Petch effect [1,2] for polycrystalline structures, which holds that the hardness is inversely proportional to the crystal size. This effect is attributed to two main reasons. First, creating cracks is more difficult if the crystal size is small. Second, deformation of crystals progresses ideally through crystal planes. When a dislocation arrives at a grain boundary, its advance is hindered because the crystals are randomly oriented. Accordingly, increasing the number of boundaries (i.e., reducing the crystal size) results in hardness improvement. However, the hardness reduces again when the crystal size is reduced below a certain limit (the so-called reverse Hall–Petch effect) due to the appearance of another deformation mechanism, grain boundary sliding, which has been both theoretically [3] and experimentally [4] confirmed. Nanocomposites are structures conceptually similar to polycrystals but which include a second phase among crystals. As a result, they are formed by nano-sized crystals (typically, based on hard phases as carbides or nitrides) embedded in a second phase. The nature of the components, crystal size, and the amount of each phase determines the final properties of the material. According to Musil and Vlcek [5], nanocomposites can be classified depending on the nature of the second phase. If it is hard, grain boundary sliding is blocked, which hinders the plasticity. These nanocomposites were suggested by Veprek and Reiprich [6] and show super- and ultra-hardness, and good toughness (resistance against crack formation). The same can be said for another ultra-hard structure, the superlattices proposed by Koehler [7]. However, according to Zhang et al. [8], a second “class” of toughness can be defined, the fracture toughness, which accounts for the resistance of a material against the crack propagation. In this sense, the behavior of these materials is poor, since they show brittle failure at high loads. The second kind of nanocomposites consists of those formed by crystals of a hard phase embedded in a soft phase. These structures show also good values for hardness, since the crystal size is Thin Solid Films 517 (2009) 1662–1671 ⁎ Corresponding author. Tel.: +34 954 48 95 79; fax: +34 954 46 06 65. E-mail address: dmartinez@icmse.csic.es (D. Martínez-Martínez). 0040-6090/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.09.091 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf