SYNTHESIS AND PHASE DEVELOPMENT IN THE Cr-Al-N SYSTEM

摘要

The ternary nitride system Cr-Al-N has been investigated by sintering different powder compositions. The powder compositions belong to four groups, AlN- + Cr-powder (5 compositions between 20-90 molar percent A1N), Al- + Cr_2N-powder (5 compositions between 15-80 molar percent Cr_2N). AlN-+ Cr_2N-powder (50- and 90 molar percent Cr_2N) and Al- + Cr-powder. The powders were dry mixed and pressed into pellets by uniaxial pressing followed by cold isostatic pressing (C1P). Sintering took place in a graphite lined reaction bonding furnace under nitrogen atmosphere at three different temperatures, 1350 deg C, 1500 deg C and 1800 deg C and in an alumina tube furnace in order to avoid access to carbon. Holding limes were varied, from 2 hours up to 72 hours. The phase development was evaluated by thermal analysis and XRD. CrAlN was formed at 1350 deg C but decomposed at higher temperatures. Both pure Al and Cr-powder were prone to react with carbon in the graphite furnace. Thermal analysis showed a sublimation of Cr_2N at temperatures around 1050 deg C and nitridation of pure Al-powder between 680-750 deg C and of pure Cr-powder between 610-1080 deg C. Samples with pure Al-powder showed a very large expansion due to melting of aluminium in combination with nitridation. AlN was found to be more stable than Cr_2N at higher temperatures and longer holding limes. The mixtures of AM Cr-powder produced an intermediate Al-Cr-phase.