Study and elaboration of ternary chromium based compounds (Cr, O, N) deposited by vacuum arc evaporation

Abstract

Ternary chromium based films (Cr, O, N) have been deposited using a conventional arc coating system. The effect of N 2 and O 2 partial pressures, p O2, P N2 on the formation of the Cr-O-N compounds and on the structural properties (texture, grain size, residual stress) has been studied. The mechanical properties (microhardness, adhesion and wear resistance) have also been investigated. Two kinds of coatings are obtained. For low oxygen flow rates ( Q O2/ Q N2 < 0.44) only the chromium nitride phases can be detected by XRD analysis. The addition of oxygen leads to a better crystallization state of the coatings. Though the 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 and 50 N. For Q O2/ Q N2 ≥ 0.44 the chromium nitride phases can no longer be detected, and only Cr 2O 3 as well as a non-identified phase (whose diffraction peak is located at Θ = 38.5 °) are observed. The microhardness is largely improved (45 GPa for Q O2/ Q N2 = 0.44), but in contrast the adhesion and the wear resistance are drastically reduced. For Q O2/ Q N2 ≤ 0.44, the oxygen partial pressure is low ( p O2/ p N2 = 1% for Q O2/ Q N2 = 0.44). The major part of the introduced oxygen molecules are physisorbed on the walls of the deposition chamber due to the gettering effect of chromium. Therefore, few reactive oxygen species impinge on the 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 simultaneously a decrease of the evaporation rate. In this case, a lot of reactive oxygen species impinge on the growing films promoting the growth of the chromium oxide.