Joining of alumina via copper/niobium/copper interlayers

Abstract

Alumina has been joined at 1150°C and 1400°C using multilayer copper/niobium/copper interlayers. Four-point bend strengths are sensitive to processing temperature, bonding pressure, and furnace environment (ambient oxygen partial pressure). Under optimum conditions, joints with reproducibly high room temperature strengths (≈240 ± 20 MPa) can be produced; most failures occur within the ceramic. Joints made with sapphire show that during bonding an initially continuous copper film undergoes a morphological instability, resulting in the formation of isolated copper-rich droplets/particles at the sapphire/interlayer interface, and extensive regions of direct bonding between sapphire and niobium. For optimized alumina bonds, bend tests at 800°C-1100°C indicate significant strength is retained; even at the highest test temperature, ceramic failure is observed. Postbonding anneals at 1000°C in vacuum or in gettered argon were used to assess joint stability and to probe the effect of ambient oxygen partial pressure on joint characteristics. Annealing in vacuum for up to 200 h causes no significant decrease in room temperature bend strength or change in fracture path. With increasing anneal time in a lower oxygen partial pressure environment, the fracture strength decreases only slightly, but the fracture path shifts from the ceramic to the interface. Cu/Nb/Cu Interlayers R. A. Marks et al. Joining of Alumina via

Fracture and Fatigue Behavior at Ambient and Elevated Temperatures of Alumina Bonded with Copper/Niobium/Copper Interlayers

“…Although discrete copper-rich particles persist along the interface and melt at ~1083°C, the high area fraction of niobium/alumina contact allows joints to retain reasonably high strength levels up to 1300°C, well above the melting point of copper, with the majority of failures occurring away from the joint in the alumina. 5,6 While strength is an important property governing the suitability of joints for structural applications, fracture and cyclic fatigue properties (i.e., flaw tolerance) must also be assessed to predict the reliability and lifetime of joints. To our knowledge, no previous studies have investigated the fracture toughness and fatigue-crack growth properties at or near PTLP bonded joints.…”

Transient-liquid-phase and liquid-film-assisted joining of ceramics

“…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).…”

“…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.…”

“…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].…”

“…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.…”