Investigation on the chemical reactions affecting the sinterability and oxide content of Cu–Cr composites during the solid state sintering process

Cu–Cr alloys produced by powder metallurgy are widely used as contact materials in vacuum interrupters. The efficiency of these devices depends on material characteristics, in particular porosity and oxygen content. This paper focusses on densification and oxygen evolution in Cu and Cu–25 wt%Cr compacts during sintering. Chemical reactions and their relationships with sintering are studied by dilatometric and thermogravimetric analyses coupled with mass spectrometry. High density Cu compacts swell during sintering, due to the gas released in closed pores by Cu oxide reduction reactions. The addition of Cr hinders the sintering kinetics by a mechanical effect but prevents swelling phenomena by trapping the released oxygen in solid chromium oxide. The furnace atmosphere has a major effect as its reducing degree determines the chemical reactions. Under neutral atmosphere such as secondary vacuum, copper oxide reduction occurs at high temperature, when densification has already produced close pores. In Cu compacts, this leads to large swelling while in Cu–25 wt%Cr compacts the oxidation of Cr traps the oxygen in CR2O3 interfacial nodules. A reducing atmosphere such as diluted hydrogen flux reduces the Cu oxide at lower temperature, so that part of the released oxygen is evacuated out of the compact. This leads to lower swelling in Cu compact and to a reduced amount of chromium oxide in lower oxygen Cu–25 wt%Cr compacts, which results to better bonding between Cu and Cr particles.