Investigation of metal distribution and carbide crystallite formation in metal-doped carbon films (a-C:Me, Me = Ti, V, Zr, W) with low metal content

Abstract

Metal-doped amorphous carbon films (a-C:Me) were deposited at room temperature by magnetron sputtering using a metal (Me = Ti, V, Zr, W) and a graphite target. The metal distribution and the temperature-induced carbide crystallite formation were analyzed by X-ray diffraction (XRD), electron microscopy (TEM, STEM) and X-ray absorption spectroscopy (EXAFS, XANES), focusing on low metal concentrations between 6.5 and 9.5%. In as-deposited samples, the metal atoms are atomically distributed in the carbon matrix without significant formation of carbide particles. With annealing to 900 K the local atomic environment around the metal atoms becomes similar to the carbide. The carbide crystallites grow with annealing up to 1300 K, their size is dependent on the metal type: V > Ti > Zr≈W. W₂C and WC_1 − x crystallites were identified for W-doped films, whereas the monocarbides are formed for the other metals. It is demonstrated, that EXAFS and high resolution electron microscopy are required to get a correct picture of the structure of the analyzed a-C:W films.

Introduction

Metal-containing carbon films (a-C:Me, a-C:H:Me) were intensively studied in the last years and have great importance for application as hard and wear resistant coatings. Most frequently, the carbide forming metals Ti and W are used as dopants [1], [2], [3], [4], [5], [6], [7]. Improved tribological and mechanical properties compared to pure carbon films are achieved by a nanocomposite structure with nanometer-sized carbide particles in an amorphous matrix of (hydrogenated) carbon. Such coatings are also investigated in respect to improved optical and electrical properties [8], [9].

Our interest in a-C:Me films is motivated by research on the chemical sputtering process of carbon by hydrogen impact [10]. This is of great importance for future fusion devices like ITER, where carbon – together with Be and W – is suggested as plasma-facing material (PFM) [11]. The reaction of hydrogen species with carbon-based PFM leads to its degradation and to formation of undesired hydrocarbon layers, depositing in the reactor vessel [12]. If radioactive tritium is used – together with deuterium the fuel for fusion – this leads to an accumulating radioactive inventory, which is of high safety relevance. Doping of graphite with carbide-forming transition metals (Ti, V, Zr, W) is a possible way to decrease its reactivity against hydrogen species [10], [13], [14]. The presence of metals influences the erosion mechanism, and they accumulate at the surface as a result of preferential sputtering of carbon [15], [16]. For a systematic investigation of the effect of doping, erosion experiments have been performed with metal-doped amorphous carbon films (a-C:Me), produced by dual source magnetron sputter deposition. Their reactivity against hydrogen is determined by the kind of metal and its concentration [17], [18], but depends also on the nano-structure of these layers [19]. To study particularly the effect of the a-C:Me nanostructure on the erosion process, the films were deposited at room temperature (RT) and annealed after deposition to induce structural changes.

Therefore, our deposition conditions are in contrast to most publications dealing with characterization of a-C:Me films optimized for tribological applications. For those, the formation of a stable carbide phase in the carbon matrix already during deposition is desired, which requires high adatom mobility. This can be achieved either by increased substrate temperature or by a high energy of the impinging species (e.g. by using ion-beams [20], CVD/PVD hybrid deposition [21], [22], plasmas with higher ionization [23], [24], and laser ablation [25]). A higher metal content is also beneficial for carbide phase formation [9], [26]. Also a sample bias is generally applied to increase the energy deposited in the growing film during deposition.

In contrast, this paper gives a detailed structural analysis of a-C:Me (Me = Ti, V, Zr, W) films with low metal content (1–19%) and the films were deposited at RT by non-reactive magnetron sputtering without bias. We focus on samples with 6.5–9.5% metal content and describe the metal distribution after film deposition and the temperature-induced carbide cluster formation by annealing up to 1300 K. The following methods were used: X-ray diffraction (XRD), (scanning) transmission electron microscopy (TEM, STEM), and X-ray absorption fine structure spectroscopy (XAFS) in the extended energy (EXAFS) and near edge (XANES) region. The influence on the carbon structure was studied by Raman spectroscopy and XRD, and the results have been published in a separate paper [27]. A catalytic effect on the sp² clustering in the carbon phase during film deposition has been observed, which is dependent on the type of metal (W < V<Ti≈Zr).