Characterization of magnetron sputtered Cr–B and Cr–B–C thin films for electrical contact applications
Introduction
Electrical contacts are important devices in our electrified society. A good material for sliding electrical contacts requires a combination of properties such as a low friction and wear rate combined with a high corrosion resistance and a low electrical contact resistance. Today, Ag and Au are typically used as contact materials, but a disadvantage of these materials is their high friction and wear rates combined with a high cost of materials. A potential alternative to the noble metals is transition metal borides, which exhibit many of the properties required for a good contact material. An interesting feature of the borides is the potential to form lubricating boric acid surface layers. Hitherto, however, few studies have been carried out to evaluate the use of borides in contact applications. A recent study on magnetron sputtered NbB2 films showed a low resistivity, coefficient of friction, and wear rate and thus a potential for sliding contact applications [1]. A problem, however, was that the films exhibited a high electrical contact resistance. This was attributed to a high hardness (> 40 GPa), which obstructs the penetration of the surface oxide. Several studies of the Ti–B–C system can be found in the literature mainly focusing on protective coatings and hence mechanical and tribological properties [2], [3], [4], [5], [6], [7], [8]. Lauridsen et al. have studied electrical contact properties of Ti–B–C coatings [9]. However, the B content was ≤ 17 at.% and therefore no TiB2 phase existed in the coatings. It is well known from the Ti–B–C system, that the hardness of the boride films can be reduced by alloying with carbon. This is explained by the formation of a nanocomposite structure with improved tribological properties compared to the binary Ti–B films [2], [3], [4]. A similar structure is also formed in magnetron sputtered Nb-B–C films, but a problem in this system is crack formation and subsequent film fragmentation and delamination resulting in poor tribological properties during friction tests [10].
An alternative to Nb–B–C could be magnetron sputtered films of the Cr–B–C system. CrB2 is known to exhibit excellent corrosion properties, exceeding those of TiB2 and ZrB2 [11], [12], as well as good mechanical and electrical properties. Furthermore, it is known that the stability of diborides with the AlB2 structure decreases when going from group 4 to group 6 of the transition metals. This is attributed to the filling of anti-bonding states as the valence electron concentration exceeds four, i.e. the group number for the transition metal exceeds four [13], [14]. According to this model, slightly weaker bonds are expected in a MeB2 phase constituting Cr from group 6 compared to Nb from group 5. A lower hardness, and thus possibly a lower electrical contact resistance, is therefore expected for the Cr–B films in comparison to the previously studied Nb–B films. The alloying of carbon into the boride could also have beneficial effects on the friction and wear properties as seen for Ti–B–C [2], [3].
The purpose of this study is to make an in-depth study of the microstructure and chemical bonding in Cr–B and Cr–B–C thin films as well as to bench mark them to Nb–B–C films in terms of mechanical and electrical contact properties. Magnetron sputtering was used to deposit Cr–B and Cr–B–C films with different C contents. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) have been used to characterize the film microstructure.
Section snippets
Experimental
Cr–B–C films were deposited using non-reactive DC-magnetron sputtering from circular 50 mm CrB2 and C (claimed purity 99.999%) targets in an ultra-high vacuum chamber (base pressure of ≤ 10− 6 Pa). Two different Cr–B targets were used in this study. The targets had a claimed B/Cr ratio of 2 but ERDA analysis showed a B/Cr ratio of only 1.5 corresponding to about 60 at.% B (see Table 1 for details). The magnetrons were directed towards a rotating substrate holder at a distance of 15 cm. The Ar-plasma
ERDA analysis of targets and films
Initial analysis of the magnetron sputtered films suggested a substoichiometric composition with B/Cr ration of < 2. XPS analysis of the sputtering targets used in this study indicated a B/Cr ratio of < 2 also for the targets, although they had a claimed B/Cr ratio of 2 by the suppliers. A more detailed analysis of the two targets was therefore carried out using a combination of ERDA and XPS. The analyses are summarized in Table 1. As can be seen, both targets exhibited a significantly lower B
Discussion
Studies on magnetron sputtered transition metal boride films have reported on deviations in the stoichiometry of magnetron sputtered boride films compared to the claimed target composition [31], [32], [33]. Our films have a typical measured B/Cr ratio of 1.5, which is significantly lower than the specified ratio of a CrB2 target. However, ERDA and XPS analysis of two commercial “CrB2” targets from two independent manufacturers show a significant deviation in claimed and analyzed composition. We
Conclusions
Sub-stoichiometric CrB2 − x films with a B/Cr ratio of 1.5 have been deposited by magnetron sputtering from a Cr–B target with a B/Cr ratio of 1.5. The films have a columnar structure with (101)-textured CrB2 − x grains (5–10 nm wide). STEM and XPS reveal that B segregates to the CrB2 − x grain boundaries forming a B-rich tissue phase. The (101)-texture results in a rather low hardness of 25 GPa and low wear resistance. A poor adhesion together with the low wear resistance result in a high friction
Acknowledgments
Kristian Nygren and Fang Mao are acknowledged for assistance with electrical contact resistance measurements and the short ball-on-disc measurements, respectively. The work was financially supported by Vinnova (Swedish Governmental Agency for Innovation Systems) through the VINN Excellence Centre FunMat. J. L. and U. J. also acknowledge the Swedish Foundation of Strategic Research through the Synergy Grant FUNCASE. U. J. acknowledges the Swedish Research Council (VR). The Knut and Alice
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