Nanocomposite TiC/a-C coatings: structure and properties
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Nanocomposite TiC/a-C coatings: structure and properties Jeff Th.M. De Hosson 1, Yutao Pei 1, Damiano Galvan 1 and Albano Cavaleiro 2 1
Dept. of Applied Physics, Materials Science Centre and the Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands. 2 Departamento de Engenharia Mecanica, FCTUC - Universidade de Coimbra Pinhal de Marrocos, 3030 Coimbra, Portugal ABSTRACT This contribution deals with fundamental and applied concepts in nano-structured coatings, in particular focusing on the characterization with high-resolution (transmission) electron microscopy. Both unbalanced and balanced magnetron sputtering systems were used to deposit nc-TiC/a-C nanocomposite coatings with hydrogenated or hydrogen-free amorphous carbon (a-C) matrix, respectively. The contents of Ti and C in the coatings have been varied over a wide range (7~45 at.% Ti) by changing the flow rate of acetylene gas or the locations of substrates relative to the center of C/TiC targets. Different levels of bias voltage and deposition pressure were used to control the nanostructure. The nanocomposite coatings exhibit hardness of 5~35 GPa, hardness/E-modulus ratio up to 0.15, wear rate of 4.8×10-17 m³/Nm, coefficient of friction of 0.04 under dry sliding and strong self-lubrication effects. The nanostructure and elemental distribution in the coatings have been characterized with cross-sectional and planar high-resolution transmission electron microscopy (HRTEM) and energy filtered TEM. The influences of the volume fraction and size distribution of nanocrystallites TiC (nc-TiC) on the coating properties were examined. INTRODUCTION Nanocomposite coatings consisting of TiC nanoparticles embedded in an amorphous carbon (a-C) matrix are able to combine high fracture toughness and wear resistance with low friction coefficient. Coatings currently employed for such applications are based on a hydrogenated a-C matrix. However, if a hydrogen-free a-C matrix could be made, improvement in thermal stability of the nanocomposite coating is anticipated, together with a decrease in the coefficient of friction (CoF) of sliding in a humid environment. By controlling the size and volume fraction of nanocrystalline phases in the a-C matrix and consequently the separation width of the amorphous matrix among the nanocrystallites, the properties of the nanocomposite coatings can be tailored, e.g. to make a balance between hardness and elastic modulus to permit close match to the elastic modulus of substrates [1], and particularly to obtain high toughness that is crucial for applications under high loading contact and surface fatigue [2, 3, 4]. In addition to these characteristics, amorphous carbon based nanocomposite coatings exhibit not only excellent wear resistance but also low friction due to self-lubrication effects, which make them attractive for environmental reasons because liquid lubricants can be omitted. This paper focuses on the deposition and characterization of TiC/a-C nanocomposite coating
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