Elsevier

Acta Materialia

Volume 53, Issue 17, October 2005, Pages 4505-4521
Acta Materialia

Nanostructure and properties of TiC/a-C:H composite coatings

https://doi.org/10.1016/j.actamat.2005.05.045Get rights and content

Abstract

TiC/a-C:H nanocomposite coatings, deposited with closed-field unbalanced magnetron sputtering, have been scrutinized with atomic force, scanning and high-resolution transmission electron microscopy, nanoindentation and tribo-tests. These coatings consist of 2–5 nm TiC nanocrystallites embedded in an amorphous hydrocarbon (a-C:H) matrix. A transition from a columnar to a glassy microstructure has been observed in the nanocomposite coatings with increasing substrate bias or carbon content. Microcracks induced by nanoindentation or sliding wear readily propagate through the column boundaries whereas the coatings without a columnar microstructure exhibit substantial toughness. The toughening of the nanocomposite coatings has been achieved effectively on two different scales, namely by restraining the formation of columns on a microscale and by manipulating the nanostructure on a nanoscale. The hardness (H) and elastic modulus (E) of the coatings are found to increase monotonically with increasing substrate bias, whereas the ratio of the hardness to the elastic modulus (H/E) remains approximately constant. In contrast, H/E increases with C content. Ball-on-disc tribo-tests confirm that the nanocomposite coatings possess superior wear resistance and strong self-lubrication effects with a coefficient of friction as low as 0.05 in ambient air and below 0.02 in dry air, under dry sliding against uncoated bearing steel balls. Physical arguments are presented to explain the toughening mechanism and the ultra-low friction.

Introduction

Nanocomposite coatings composed of crystalline/amorphous nanophases mixture have recently attracted increasing interest with respect to fundamental research and industrial applications, because of the possibilities of synthesizing a surface protection layer with an unusual combination of mechanical and tribological properties that are often not attained even in nanocrystalline materials, such as high hardness and toughness, superior wear resistance and low friction. A wear resistant coating must support high loads in sliding or rolling contact without failure by cohesive fracture, or loss of interfacial adhesion. Most frequently, a low friction coefficient is required, which helps to reduce friction losses and to increase load-carrying capability. The latter is evident from the fact that typical coating failures such as cracking, chipping and delamination are primarily caused by the tangential stress that is proportional to the contact load through the friction coefficient. Only recently it has been possible to realize all of these requirements in nanostructured or nanocomposite coatings.

Introducing nanocrystalline ceramic particles into a relatively compliant amorphous matrix generates a high density of interphase interfaces that assist in crack deflection and termination of crack growth [1], [2]. Moreover, other mechanisms like interface diffusion [3] and sliding [4], [5], [6] were also suggested to further improve ductility providing superplasticity in nanocrystalline single-phase ceramics and multiphase structures. These findings could be expanded into the field of hard wear-resistant coatings to introduce ductility and prevent fracture under a high contact load, leading to the supertoughness observed [7]. By controlling the size and volume fraction of nanocrystalline phases, the properties of the nanocomposite coatings can be tailored in a wide range, creating a balance between hardness and elastic modulus to permit a close match to the elastic modulus of substrates. In particular in this way a high toughness can be attained that is crucial for applications under high loading contact and surface fatigue. In addition to these characteristics, amorphous carbon (a-C) or amorphous hydrocarbon (a-C:H) based nanocomposite coatings are expected to exhibit not only excellent wear resistance but also low friction due to the self-lubrication effects of the diamond-like carbon (DLC) matrix, which make them environmentally attractive because liquid lubricants can be omitted. An example of these (a-C:H) based nanocomposite coatings is the TiC/a-C:H system, in which a correlation between coating mechanical properties and metal concentration has been made by Meng et al. [8], [9] and Patscheider et al. [10].

Although the mechanical properties, such as the Young’s modulus and hardness, of TiC/a-C:H nanocomposite coatings have been reported in some detail, only scant information is available on the correlation between the nanostructure, the mechanical properties and the macroscopic tribological characteristics. In this paper, we report on the deposition and characterization of TiC/a-C:H nanocomposite coatings. The tribological behavior of the nanocomposite coatings is scrutinized in conjunction with detailed examinations of the mechanical properties. Cross-sectional and planar TEM observations and energy filtered TEM are employed to characterize the nanostructure and the distribution of the elements in the coatings. The influence of the volume fraction and size distribution of nanocrystallites TiC (nc-TiC) on the coating properties has been examined.

Section snippets

Experimental

Hydrogenated nc-TiC/a-C:H coatings were deposited with closed-field unbalanced magnetron sputtering (CFUBMS) in an argon/acetylene atmosphere in a Hauzer HTC-1000 coating system, which was configured of two Cr targets and two Ti targets opposite to each other. The detailed set-up of the coating system has been documented elsewhere [11]. The Cr targets were used to create an intermediate layer between the nc-TiC/a-C:H coating and the substrate material. The flow rate of acetylene and substrate

Coating chemistry and microstructure

Table 1 summarizes the deposition parameters and the corresponding compositions of the nc-TiC/a-C:H layers excluding hydrogen. The coatings are named in such a way that the numbers before the character ‘V’ indicate the substrate bias in voltage, followed by the flow rate of acetylene gas in standard cubic centimeters per minute (sccm). An increase in substrate bias from floating to 150 V results in a slight monotonic increase of the Ti/C ratio from 0.191 to 0.231 in the compositions, which is

Design of nanocomposite system

In nanocomposite coatings composed of hard nanograins and a compliant matrix, two different designs have been recently put into practice in thin film applications, namely superhard and supertough nanocomposite coatings. The concept of superhard nanocomposite coatings lies in the suppression of dislocation operation by using 3–5 nm small grains and inducing grain incoherence strains with <1 nm thin matrix for grain separation. Examples of these superhard nanocomposite coatings can be found in

Conclusions

The TiC/a-C:H nanocomposite coatings designed for wear resistance and low friction have been deposited with closed-field unbalanced magnetron sputtering. The TiC nanocrystallites are embedded in an amorphous hydrocarbon matrix, and their size and volumetric fraction can be controlled by changing the flow rate of acetylene gas and the substrate bias. The undesired columnar microstructure in the coatings can be restrained by increasing substrate bias or carbon content. Microcracks induced by

Acknowledgments

The authors acknowledge financial support from the Netherlands Institute for Metals Research (NIMR) and the Foundation for Fundamental Research on Matter (FOM-Utrecht). C. Strondl from Hauzer Techno Coating BV, the Netherlands is acknowledged for his help with the deposition of the coatings. Prof. A. Cavaleiro of the Universidade de Coimbra Pinhal de Marrocos, Portugal is thanked for giving support to the EPMA of the composition of the coatings. Drs. J.-D. Kamminga and G.C.A.M. Janssen of Delft

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