Relationship between structure and mechanical properties in hard Al-Si-Cu-N films prepared by magnetron sputtering

This article reports on properties of Al-Si-Cu-N films with 5-9 at.% Si and 2-12 at.% Cu magnetron sputtered from a composed (Al, Si)-Cu target. The films were deposited under the following conditions: Magnetron discharge current I_d = 1 A, negative substrate bias U_s = - 100 V, substrate ion current density i_s = 0.6 and 0.9 mA/cm², substrate temperature T_s = 300 and 500 °C, substrate-to-target distance d_s-t = 60 mm, partial pressure of nitrogen p_N(2) ranging from 0.05 to 0.17 Pa and a constant total pressure p_T = p_Ar+p_N(2) = 0.5 Pa. It was demonstrated that (1) the structure of Al-Si-Cu-N films can be controlled by energy delivered to the film during its deposition; (2) the Al-Si-Cu-N films can form a superhard material with hardness H≥40 GPa; (3) superhard films with the same microhardness can exhibit different structures characterized either with a low-intensity h-AlN(0 0 2) reflection or an X-ray amorphous structure, i.e. can be composed of nc-AlN_x grains of different size; (4) the system Al-Si-Cu-N easily forms a disordered structure characterized with X-ray amorphous diffraction pattern; (5) the Al-Si-Cu-N film with a maximum microhardness of 59 GPa exhibits (i) an X-ray amorphous structure; (ii) contains 44 at.% Al, 42 at.% N, 7 at.% Cu and 6 at.% Si and (iii) is substoichiometric, i.e. x = N/(Al+Si) = 0.84 < 1 and (6) the Al-Si-Cu-N films with nitrogen content N≥44 at.% are electrically insulating. The demonstration of a possibility to form the X-ray amorphous superhard films is of key scientific importance. It makes possible to start investigation of the grain size dependent phenomena in nanostructured films prepared by two-step process based on nanocrystallization from amorphous phase using a post-deposition annealing.