Characterization of magnetron sputtered Cr–B and Cr–B–C thin films for electrical contact applications

• B segregates to the CrB2 − x grain boundaries despite a B/Cr atomic ratio of ≤ 1.5.
• A two phase amorphous structure forms at C contents above 17 at.%.
• The alloying of C improves adhesion and reduces the coefficient of friction.

We have deposited Cr–B and Cr–B–C thin films by co-sputtering from chromium boride and carbon targets. The binary Cr–B films consist of nanocrystalline and substoichiometric CrB2 − x grains (B/Cr atomic ratio ≤ 1.5) with a (101)-texture, where B segregates to the grain boundaries forming a B-rich tissue phase. A hardness of 25 GPa is measured for these films. They have a low wear resistance, attributed to a (101)-texture and limited adhesion. As a consequence, wear debris in the CrB2 − x wear track from delaminated film and steel-to-steel contact between the exposed substrate and the counter surface result in a high friction (0.52–0.78 against stainless steel) making the Cr–B films unsuitable as sliding electric contacts. Cr–B–C films, on the other hand, form a two phase amorphous structure at > 17 at.% C consisting of an amorphous Cr-rich phase containing both B and C and an amorphous matrix phase containing mainly B and C. The addition of C improves the adhesion and tribological properties and a coefficient of friction of 0.12 is obtained at 38 at.% C. The improved tribological properties are explained by the formation of the matrix phase, which acts as a solid lubricant forming a graphite-like tribofilm during ball-on-disc test. However, the formation of an amorphous structure is not beneficial for the electrical contact resistance, which increases from 0.5 Ω for the Cr–B film to 1.5 and 2.3 Ω for the Cr–B–C films containing 17 and 26 at.% C, respectively. Finally, the importance of a chemical analysis of the chromium boride sputtering target composition is discussed.