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

Abstract

Interfacial fracture toughness and cyclic fatigue-crack growth properties of joints made from 99.5% pure alumina partial transient-liquid phase bonded using copper/niobium/copper interlayers have been investigated at both room and elevated temperatures, and assessed in terms of interfacial chemistry and microstructure. The mean interfacial fracture toughness, G c , was found to decrease from 39 to 21 J/m 2 as temperature was raised from 25° to 1000°C, with failure primarily at the alumina/niobium interfaces. At room temperature, cyclic fatigue-crack propagation occurred at both the niobium/alumina interface and in the alumina adjacent to the interface, with the fatigue threshold, ∆G TH , ranging from 20 -30 J/m 2 ; the higher threshold values in that range resulted from a predominantly near-interfacial (alumina) crack path. During both fracture and fatigue failure, residual copper at the interface deformed and remained adhered to both sides of the fracture surface, acting as a ductile second phase, while separation of the niobium/alumina interface appeared relatively brittle in both cases. The observed fracture and fatigue behavior is considered in terms of the respective roles of the presence of ductile copper regions at the interface which provide toughening, extrinsic toughening due to grain bridging during crack propagation in the alumina, and the relative crack propagation resistance of each crack path, including the effects of segregation at the interfaces found by Auger spectroscopy.

Effects of impurities on alumina–niobium interfacial microstructures

“…In joints processed with 99.5%-pure alumina, examination of interfacial fracture surfaces was possible and revealed the presence of adherent islands of a silicide phase and tearing of niobium in bonded regions where the niobium was in contact with the finer-grain-size alumina regions [18,20,24]. This indicated that the small amount of glassy silicate phase present in the material may play a role in alumina-niobium contact formation and does modify the alumina-niobium interfacial microstructure.…”

“…Copper particles persist at the interface after bonding due to the low solubility of copper in niobium and a low rate of copper-niobium interdiffusion. These ductile particles affect the fracture strength of the joined assembly in a manner that depends specifically on the morphology and volume fraction of the copper [18,20,23,24]. The inset in Figure 1 shows two of these copper particles at increased magnification.…”

“…Considerable prior research has been conducted on the mechanical properties and interfacial characterization of the alumina-niobium system [2,[9][10][11][12][13][14][15][16][17]. In the case of alumina-niobium interfaces, niobium silicide formation has been observed in previous studies [1,4,18]. In the present study, polycrystalline alumina substrates of two distinct purities and single-crystal alumina (sapphire) substrates were used to fabricate alumina-niobium interfaces by liquid-film-assisted joining (LFAJ).…”

“…Fractography and EDS data obtained from fracture surfaces [18,20] interface and then deviated either along the Nb-Nb 5 Si 3 interface or through the silicide particle.…”

Mechanisms of microstructure development at metallic-interlayer/ceramic interfaces during liquid-film-assisted bonding

“…The microstructure of these interlayer/alumina interfaces contained continuous and discrete second phase precipitates that resulted from a reaction between copper, alumina, and additional oxygen that was either dissolved in the copper or the niobium (this differs from the microstructure that developed in assemblies processed in a lower † p(O 2 ) environment [7][8][9]69 and is to be discussed later). The continuous precipitate was usually found at niobium grain boundaries and fracture often occurred through these precipitates.…”