On the atomic structure of the Nb/Al₂O₃ interface and the growth of Al₂O₃ particles

Abstract

The growth mechanism for small precipitates of A12O3 formed by internal oxidation in the Nb-Al2O3 interface is studied in detail. The observations show that the Nb (001)/Al2O3 (00.1) interface is almost atomically flat and that there are no interface compounds. We suggest that the final layer on the A12O3 side of this interface consists of oxygen atoms. The effects of image forces on misfit dislocations are found to result in a standoff distance between dislocation cores and the interface, in good agreement with the recent theory. The implications of this for the strength of metal-ceramic bonding are discussed.

Effects of processing conditions and ambient environment on the microstructure and fracture strength of copper/niobium/copper interlayer joints for alumina

“…After such orientation adjustments, the close-packed planes of niobium and alumina are not exactly parallel indicating that an ≈3° tilt of the niobium close-packed planes with respect to the oxygen close-packed planes in alumina is energetically more favorable than perfect alignment. [74,78,83,91,84] Samples diffusion-bonded with orientation relationship A [80] revealed an interface with some similarities as well as notable differences to those observed in the internal oxidation studies [92,96] . Niobium (3,3.5 at% aluminum alloy) was internally oxidized † Although one study [92] on the internal oxidation of niobium-aluminum alloy states an orientation relationship equivalent to C as the lowest energy relationship, other studies refer to the results of this internal oxidation study as orientation relationship A [80,93] .…”

“…No additional diffraction spots for such a NbO x phase were present in these studies [74,83] which were presumably bonded under similar processing conditions as used by Morozumi et al [77] . Additionally, no reaction layer has been observed at niobium/alumina interfaces produced by internal oxidation of niobium-aluminum alloys [92,96] , which were processed at higher partial oxygen pressures (≈10 -7 atm) and lower temperatures (1450-1500°C) compared with those (total vacuum pressure ≈10 -8 atm, 1600-1800°C) used by Morozumi et al [77] .…”

“…Although Morozumi et al [77] report some convincing evidence supporting the formation of a NbO x reaction layer at the niobium/sapphire interface, no other studies [19,20,73,74,78,80,83,84,[91][92][93][94][95][96] on these interfaces have observed such a layer. The hardness values reported by Morozumi et al [77] are comparable to those measured by Elssner et al [97] for niobium containing a few atomic percent oxygen.…”

“…Many of the studies [74,78,83,91,84] focused on orientation relationship C (see Table 2.2) . Other studies [80] examined an orientation relationship (A in Table 2.2) that was more fundamental to the system as it was observed for alumina particles in a niobium matrix formed by the internal oxidation of a niobium-aluminum alloy [92,96] . Orientation relationship A may be obtained by a 90° rotation of the niobium crystal about its [110] axis from orientation C and implies that the close-packed planes of the sapphire and niobium are still parallel † .…”

“…by exposure to a partial oxygen pressure of ( 5 10 [92,96] , and the diffusion-bonded interfaces were processed under similar conditions (1500±50°C for 1 h) in high vacuum [80] . The alumina precipitates observed in internal oxidation did not exactly duplicate the corundum structure of sapphire used in diffusion bonding, but were a closely related four-layer stacking sequence variant of alphaalumina [92] .…”