On the Role of a Nial 2O 4 Intermediate Layer in the Sintering Behavior of Ni/α-Al 2O 3

Abstract

To investigate the reduction of NiAl 2O 4 and the sintering of Ni on α-Al 2O 3, Ni layers have been deposited onto polycrystalline α-Al 2O 3 substrates. Some samples were oxidized at 700°C and annealed at 1100°C in N 2/O 2 to convert the nickel layers completely into NiAl 2O 4. The reduction and sintering behavior of these layers was studied by Rutherford backscattering spectrometry, scanning electron microscopy, and X-ray diffraction. Temperature-programmed reduction showed that the onset temperature for the reduction of bulk NiAl 2O 4 is about 870°C. Due to the high temperatures required for the reduction of the NiAl 2O 4/Al 2O 3 samples, sintering of Ni during this treatment was inevitable. Our results indicate that there is a critical transition temperature for Ni sintering between 450 and 500°C. Nickel layers deposited onto NiAl 2O 4/Al 2O 3 samples showed exactly the same sintering behavior as Ni layers on bare α-Al 2O 3; thus, a continuous interfacial layer of NiAl 2O 4 does not inhibit sintering. However, substantially less sintering was observed for Ni/NiAl 2O 4/Al 2O 3 samples containing Ni particles on top of NiAl 2O 4 islands. A discontinuous interfacial NiAl 2O 4 layer is apparently required to slow the sintering of nickel on α-Al 2O 3.