Multifunctional nanolaminated PVD coatings in the system Ti–Al–N–C by combination of metastable fcc phases and nanocomposite microstructures
Introduction
Nanocomposite single layer coatings being composed of nanocrystalline hard materials and amorphous phases are subject to intensive R&D efforts towards the design of new multifunctional protective coatings with superior properties and performance since the first reports on superhard composite coatings were published one decade ago [1], [2], [3], [4], [5], [6], [7], [8], [9]. The selection of materials to be combined in such a multiphase structure should refer to their phase relations in the corresponding phase diagram. The resulting microstructure and property profile of a multiphase or nanocomposite coating is clearly determined by the plasma process parameters and by the kinetics of the deposition and growth process [10], [11], [12], [13], [14].
These can be completely different by either fine-tuning the ratio of the volume fraction of the nanocrystalline and of the amorphous phases, or, by fine-tuning the crystallite sizes [15], [16]. Consequently, the properties of nanocomposite coatings are dominated by the grain boundaries and interfaces [17], [18].
Focusing on tribological applications, two main routes can be identified in this field of thin film engineering: (a) the development of hard, tough and lubricious coatings using amorphous carbon as a solid lubricant phase [2], [15], [19], [20], [21], [22], and, (b) the development of superhard coatings applying an amorphous Si–N network as secondary phase [1], [9], [23], [24], [25].
This paper addresses the development of wear-resistant and low friction carbon-based nanocomposite coatings with nanocrystalline hard phases. The combination of a crystalline carbide phase, either thermodynamically stable or metastable, such as WC1−x [20], [26], [27], [28], [29], [30], [31], [32], TiC1−x [2], [15], [33], [34], [35], [36], [37], [38], [39], [40], or TaC1−x [41], together with amorphous carbon in a nanocomposite thin film structure is well-known since the pioneering work done in the 1980s on the metal containing carbon films [19], [42], [43], [44]. Methods applied successfully for the deposition of carbon-based nanocomposite coatings include magnetron-sputtering, either reactively [15], [22], [26], [32] or non-reactively [38], co-sputtering [31], arc deposition [29], plasma assisted CVD [39], as well as non-reactive PVD hybrid deposition techniques [2], [35] or PVD-CVD hybrid deposition techniques using reactive atmospheres [21], [30], [37]. Even if much progress in the development of such coatings was achieved in the past decades on the laboratory scale, the correlation between the kinetics of the deposition and growth, the microstructure and constitution, and the properties and performance of these coatings is not yet fully understood. Therefore, these coatings still are subject to optimisation and scale-up activities. Just a few coatings are available commercially today, for example WC–C coatings are used in combination with (Ti, Al)N coatings for tool applications [45], [46]. Other carbide building elements like Nb and Fe or hard materials like TiB2−x have been reported also to be incorporated in carbon-based nanocomposite coatings [19], [43], [47].
Alternative developments in the field of solid lubricating films deal with the incorporation of soft phases like WS2 in a carbon matrix and address the idea of smart, self-adaptive coatings [48], [49], [50]. An emerging new approach is the design of nanocomposite coatings being composed of ternary or quaternary metastable hard phases such as fcc (Ti, Al)(N, C) or fcc (Ti, Cr) (C, N) and amorphous carbon [51], [52], [53], [54], [55].
Results on the microstructural evolution and on selected properties are shown and discussed both for magnetron sputtered TiC/a-C and (Ti, Al)(N, C)/a-C single layer coatings. These nanocomposite coatings were combined with binary and ternary hard phases like TiN or (Ti, Al)N in advanced nanocomposite multilayer coatings. Only a few reports are found on multilayer coatings of this type of combination of individual layer materials [56], [57]. While in the classical understanding all multilayer coatings that have dimensions at the nano-scale (in example the thickness of individual layers or the grain size of at least one layer material) are designated as nanoscale multilayers or simply as nanocomposite coatings [1], [58], the approach introduced in this paper goes far beyond this definition: innovative multilayer coatings with a special nano-architecture are presented. These multilayers are composed of nanometer thin layers of different carbon-based nanocomposite coatings as described above and will further be referred to as nanolaminated composite coatings. Results on their constitution, microstructure, and properties are shown. For all coatings presented, the interesting issue of scaling-up of the deposition processes to industrial scale is discussed.
Section snippets
Coating deposition
The coatings presented and discussed in this paper were deposited by non-reactive or reactive D.C. magnetron sputtering, either with a laboratory PVD equipment (Leybold Z 550 machine) or with an industrial PVD machine (Hauzer HTC 625).
In the case of TiC/a-C nanocomposite coatings, new ceramic composite targets of various TiC:C molar mixtures were used for non-reactive deposition experiments. These targets were industrially manufactured by hot pressing and sintering from homogeneously mixed
Results and discussion
In this chapter, various carbon-based nanocomposite coatings that were used as individual layered components of advanced multilayer coatings are described first with respect to their microstructure, constitution and properties. Then, different concepts of multilayer thin films with integrated multiphase nanocomposite layers and addressing the development of wear-resistant surfaces are presented.
Conclusions
In this paper, different aspects of the synthesis, deposition methods, growth, microstructural evolution and properties of carbon-based nanocomposite thin films are discussed. Advanced nanoscale multilayer concepts addressing the development of tailored wear-resistant and low friction coatings by integrating such nanocomposite layers are introduced.
For TiC/a-C nanocomposite coatings, a new deposition route for magnetron-sputtering method using ceramic composite targets is suggested. The
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