Microstructure and hardness of reactively r.f. magnetron sputtered Cr-V-O thin films in dependence on composition and substrate bias
Introduction
New transition metal oxide-based thin films have recently gained widespread interest with regard to their large potential for offering advanced functionalities through specific combinations of mechanical, electrical, optical, thermal or chemical properties [1], [2], [3]. While this type of thin film material is already intensively investigated in fields such as optics, photonics or superconductivity [4] the potential use of either multi-phase composite or single-phase solid solution mixed oxides in mechanical engineering is currently developing. The most relevant material in this context is aluminum oxide, either in α-phase (corundum) or γ-phase. Alumina thin films and coatings have been in industrial use for a long time [5], [6]. Recent research focuses on physical vapor deposition (PVD) synthesis and development of advanced solid solution oxide thin films such as (Al,Cr)2O3 and related nitrogen containing materials [3], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. To realize such materials, an important fact is utilized: both α-Al2O3 (corundum) and Cr2O3 are isostructural and offer a solid solubility over a wide range of compositions of the quasi-binary phase diagram at high temperatures [18]. Cr2O3 crystallizes only in the corundum structure and is known as a hard material with high thermal and chemical stability and can be obtained in PVD already at temperatures below 500 °C [19], [20].
Therefore, Cr2O3 is an appropriate base material for the deposition of advanced mixed oxide thin films in corundum structure. A challenging task is the identification of suitable binary oxides that would form solid solution thin films with Cr2O3 in corundum structure. At this point, one has to state that several relevant systems have been examined by means of phase diagram calculations, among them Al-Cr-O [18], Cr-V-O [18], [21], Cr-Mn-O [22] or more complex systems like Al-Cr-Si-O [23] and Al-Cr-Fe-O [24]. These publications describe equilibrium thermodynamics, while the modeling of metastable phases (resulting from non-equilibrium PVD) is a relatively new topic in materials science. Furthermore, only few theoretical simulations on such new materials, for example by ab-initio calculations, are available on very specific systems (see for example Wallin et al. [25]). Thus, it makes sense to roughly estimate potential appropriate partner compounds by comparing their structural characteristics. This approach led to the conclusion that vanadium sesquioxide, V2O3, is a promising candidate: the ionic radii of V3 + and Cr3 + ions differ only very little (rV3 + = 64 pm and rCr3 + = 61.5 pm [26]) and V2O3 is isostructural to Cr2O3. This correlation between radii and structure is well investigated in materials science [27].
Consequently, the Cr-V-O system (and in more detail, its Cr2O3-V2O3 section) can be of particular interest for the development of new oxide thin film materials. Cr2O3 is a very well-investigated thin film material which can be deposited with a variety of PVD techniques like dc magnetron sputtering [28], [29], [30], r.f. magnetron sputtering [19], [20], [31], [32], r.f. diode sputtering [33] and arc ion plating [34], [35]. Cr2O3 is known as one of the hardest oxides [36] which makes it interesting especially for wear resistant coatings. V2O3 itself is a material that has attracted a lot of interest due to various specific features: it belongs to the so-called Magnéli phases, for example described by VnO2n-1 (3 ≤ n ≤ 9), which have been recognized for example as potential high performance tribological materials [37], [38], [39]. In addition, V2O3 exhibits a metal-insulator transition (MIT) at 168 K, which makes the material very interesting for optical and electronic applications [40], [41]. This MIT is accompanied by a structure transition from monoclinic to corundum structure. However, the V2O3 phase occurs only in a small range of compositions in the V-O phase diagram [42], which makes the PVD deposition of this phase rather difficult.
The thermodynamic modeling of the pseudo-binary section Cr2O3-V2O3 shows a wide miscibility gap below 1200 K [18]. Above this temperature both phases are fully miscible. Thus, it should be possible to obtain such solid solution structured mixed oxide phases by PVD at low or medium deposition temperatures.
In this work, we describe the influence of elemental composition and ion bombardment (induced by substrate bias variation) on microstructure and mechanical properties, i.e. indentation hardness and reduced Young’s modulus, of reactively r.f. magnetron-sputtered Cr-V-O thin films.
Section snippets
Experimental
The deposition of the Cr-V-O thin films was done with a Leybold Z550 PVD coating machine using a segmented chromium–vanadium (Cr-V) target. As demonstrated for other material combinations in previous works [14], [43], the realization of such an experimental combinatorial approach enables the deposition of Cr-V oxide thin films with a large variation of composition, i.e. from Cr-rich to V-rich, in one deposition process (see Fig. 1). The segmented target was made of two metal pieces, cut from
Cr-V-O thin films deposited without substrate bias
In this section, the Cr-V-O thin film deposition is described when no substrate bias was applied (i.e. zero volt substrate bias, substrates grounded). This enables the examination of the impact of elemental composition on microstructure formation with regard to the experimental combinatorial approach.
The deposition rate was determined for all Cr-V-O thin films. Thin films which were placed below the Cr part of the segmented Cr-V target exhibit values of 22 nm/min to 27.2 nm/min, resulting in
Summary and Conclusions
Cr-V-O thin films with different elemental compositions and microstructures were deposited with a combinatorial experimental approach. By varying the substrate bias voltage in lower regimes (between 0 V and − 100 V), resulting in moderate ion bombardment conditions during thin film deposition, the composition, microstructure and mechanical properties of these films could be influenced.
All Cr-rich films are nanocrystalline in corundum structure with an Me/O ratio of about 2/3. All films with high V
Acknowledgements
The authors thank K. Erbes, B. Rabsch, S. Schweiger and S. Zils for their kind support and advice. This work was partially carried out with support of the Karlsruhe Nano Micro Facility (KNMF, www.knmf.kit.edu), a Helmholtz research infrastructure at Karlsruhe Institute of Technology (KIT, www.kit.edu).
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