Liquid‐Film‐Assisted Formation of Alumina/Niobium Interfaces

Abstract

Alumina has been joined at 1400°C using niobium-based interlayers. Two different joining approaches were compared: solid-state diffusion bonding using a niobium foil as an interlayer, and liquid-film assisted bonding using a multilayer copper/niobium/copper interlayer. In both cases, a 127-µm thick niobium foil was used; ≈1.4-µm or ≈3-µm thick copper films flanked the niobium. Room-temperature four-point bend tests showed that the introduction of a copper film had a significant beneficial effect on the average strength and the strength distribution. Experiments using sapphire substrates indicated that during bonding the initially continuous copper film evolved into isolated copper-rich droplets/particles at the sapphire/interlayer interface, and extensive regions of direct bonding between sapphire and niobium. Film breakup appeared to initiate at either niobium grain boundary ridges, or at asperities or irregularities on the niobium surface that caused localized contact with the sapphire.

Transient-Liquid-Phase and Liquid-Film-Assisted Joining of Ceramics

“…The sample preparation procedures in our current work are generally similar to those in our prior work, and detailed descriptions are available in the literature [26][27][28][29][30][31][32][34][35][36][37][38][39]. The summary provided here emphasizes those procedures or materials that differ from those used previously.…”

“…The liquid film provides a high diffusivity pathway for niobium that allows the formation of extensive alumina-niobium contact (an area fraction >ª90%) along the alumina-interlayer interface [26,29,39,43], as illustrated in Figure 3.…”

“…When properly implemented the average fracture strength of the bonded assemblies is ª90% of the average fracture strength of the alumina that was bonded, and the standard deviations in strength for the joined and reference assemblies are comparable [29,39,43]. It 13 is important to note that even at elevated temperatures, above the melting point of the dispersed phase (ª1080°C), the joints retain a significant fraction of their room temperature strength; loss of strength reflects in large part the strength loss of the alumina due to softening of an intergranular glassy film [29], and partly the drop in the yield strength of the copper particles.…”

“…The energetics of the process lead to a breakup of the continuous film (described as dewetting) and the formation of discrete copper-rich droplets along the alumina-interlayer interface [26,29,39,43] that enhance interfacial toughness by undergoing ductile tearing when the failure propagates along the interface [44]. When properly implemented the average fracture strength of the bonded assemblies is ª90% of the average fracture strength of the alumina that was bonded, and the standard deviations in strength for the joined and reference assemblies are comparable [29,39,43].…”

“…Multilayer interlayers based on the copper-niobium system have been used to implement LFAJ, and the use of Cu/Nb/Cu interlayers for joining alumina has been studied extensively [26,29,39,43,44]. A thin copper-rich film has a desirably low melting temperature (relative to niobium).…”