High temperature coatings based on β-NiAl

Ni-based superalloys have been the workhorse materials in the high temperature alloy industry. Further increases in operating temperatures of these alloys have been limited due to low melting temperatures of constituent phases. A new alloy system has been proposed based on the Mo-Ni-Al system, which appears to be a promising alternative alloy. However, the intrinsic oxidation resistance of these alloys is relatively poor compared to current Ni-based superalloys. A two-step coating process based on electroplating and pack cementation was used to synthesize β-NiAl coatings on top of the Mo-Ni-Al alloys. These coatings were seen to be stable when exposed to expected operating conditions, and substantially improved the oxidation resistance of the Mo-Ni-Al alloys. Grain size of β-NiAl has been shown to be important during oxidation. Platinum group metals and Hf have been identified as effective alloying additions. However, less expensive additions need to be identified. Additions of 2 nd phase TiB2 particles were found to decrease the initial grain size of β-NiAl, and limit grain growth after long term exposure at 1300°C