STUDY AND ELABORATION OF TERNARY CHROMIUM BASED COMPOUNDS (CR, O, N) DEPOSITED BY VACUUM-ARC EVAPORATION

Ternary chromium based films (Cr, O, N) have been deposited using a co nventional are coating system. The effect of N-2 and O-2 partial press ures, p(O2), p(N2), on the formation of the Cr-O-N compounds and on th e structural properties (texture, grain size, residual stress) has bee n studied. The mechanical properties (microhardness, adhesion and wear resistance) have also been investigated. Two kinds of coatings are ob tained. For low oxygen flow rates (Q(O2)/Q(N2) < 0.44) only the chromi um nitride phases can be detected by XRD analysis. The addition of oxy gen leads to a better crystallization state of the coatings. Though th e compressive stress increases from about-1.5 GPa (Q(O2)/Q(N2) = 0) to -3 GPa (Q(O2)/Q(N2) = 0.35) the mechanical properties do not decrease . The microhardness is about 25 GPa and the critical load between 45 a nd 50 N. For Q(O2)/Q(N2) greater than or equal to 0.44 the chromium ni tride phases can no longer be detected, and only Cr2O3 as well as a no n-identified phase (whose diffraction peak is located at Theta = 38.5 degrees) are observed. The microhardness is largely improved (45 GPa f or Q(O2)/Q(N2) = 0.44), but in contrast the adhesion and the wear resi stance are drastically reduced. For Q(O2)/Q(N2) less than or equal to 0.4, the oxygen partial pressure is low (p(O2)/p(N2) = 1% for Q(O2)/Q( N2) = 0.4). The major part of the introduced oxygen molecules are phys isorbed on the walls of the deposition chamber due to the gettering ef fect of chromium. Therefore, few reactive oxygen species impinge on th e growing film. On the contrary, when Q(O2)/Q(N2) > 0.4 all the oxygen molecules cannot be absorbed, the oxygen partial pressure increases, and this leads to a poisoning of the chromium cathode provoking simult aneously a decrease of the evaporation rate. In this case, a lot of re active oxygen species impinge on the growing films promoting the growt h of the chromium oxide. (C) 1997 Elsevier Science S.A.