Rare-earth modified amorphous carbon films: Effects of erbium and gadolinium on the structural evolution and mechanical properties

Modifying amorphous carbon (a-C) with rare-earth elements is a highly auspicious concept to synthetize functional films with unique characteristics. Among the rare earth elements, Er and Gd demonstrate abundant physicochemical properties and, hence, are of remarkable interest for the element modification of a-C films. Therefore, Er-containing a-C:Er and Gd-containing a-C:Gd films are prepared in a reactive-free magnetron sputtering process. The a-C:Er and a-C:Gd films have an amount of up to 5 at.-% Er and 4.8 at.-% Gd. High-resolution x-ray photoelectron spectroscopy analyses show the formation of Er-C and Gd-C components, which rise proportionally with increasing amount of the rare-earth element. The addition of Er and Gd lowers the sp(3) content of C bonds. At the highest concentrations of the respective rare-earth elements, the a-C:Er and a-C:Gd films exhibit a reduced sp3 content of 8%. The number and size of sp(2)-carbon clusters in the amorphous network are enhanced with increasing amount of Er and Gd which is evaluated by Raman scattering measurements. X-ray diffraction analyses reveal Er and Gd carbide phases, indicating the formation of a nano composite structure consisting of carbidic nanocrystallites and an a-C network. In nanoindentation tests, the non modified a-C demonstrates a hardness of (21.7 +/- 1.6) GPa and an elastic modulus of (232 +/- 10) GPa. With increasing Er and Gd contents, the hardness linearly decreases to (16.7 +/- 0.9) GPa and (14.8 +/- 0.9) GPa, respectively. An analogous behavior is also identified for the elastic modulus. The reduced hardness and elastic modulus are attributed to the lower sp(3) content and the larger number and size of the sp(2)-hybridized carbon atoms. Additionally, the adhesion was slightly improved by the addition of Er and Gd in comparison to non modified a-C.