ON THE ROLE OF A NIAL2O4 INTERMEDIATE LAYER IN THE SINTERING BEHAVIOROF NI ALPHA-AL2O3/

To investigate the reduction of NiAl2O4 and the sintering of Ni on alp ha-Al2O3, Ni layers have been deposited onto polycrystalline alpha-Al2 O3 substrates. Some samples were oxidized at 700 degrees C and anneale d at 1100 degrees C in N-2/O-2 to convert the nickel layers completely into NiAl2O4. The reduction and sintering behavior of these layers wa s studied by Rutherford backscattering spectrometry, scanning electron microscopy, and X-ray diffraction. Temperature-programmed reduction s howed that the onset temperature for the reduction of bulk NiAl2O4 is about 870 degrees C. Due to the high temperatures required for the red uction of the NiAl2O4/Al2O3 samples, sintering of Wi during this treat ment was inevitable. Our results indicate that there is a critical tra nsition temperature for Ni sintering between 450 and 500 degrees C. Ni ckel layers deposited onto NiAl2O4/Al2O3 samples showed exactly the sa me sintering behavior as Ni layers on bare alpha-Al2O3; thus, a contin uous interfacial layer of NiAl2O4 does not inhibit sintering. However, substantially less sintering was observed for Ni/NiAl2O4/Al2O3 sample s containing Ni particles on top of NiAl2O4 islands. A discontinuous i nterfacial NiAl2O4 layer is apparently required to slow the sintering of nickel on alpha-Al2O3. (C) 1995 Academic Press, Inc.