B.J. Dalgleish

New approaches to joining ceramics for high-temperature applications

Abstract Micro-designed multilayer interlayers have been used to join both oxide and non-oxide ceramics. The approach allows the formation of ceramic-ceramic joints with high melting point metals at temperatures that are typically several hundred degrees lower than those required for more conventional joining methods. The new joining approach employs a thin transient liquid phase (TLP) layer to allow joining by a brazing-like process. Several distinct interlayers have been used to join alumina ceramics successfully; work using Ni-based interlayers has demonstrated the potentially beneficial impact of “reactive” metal additions to the TLP. The method has also been applied to the joining of silicon-based ceramics, and has led to the fabrication of joints with reproducibly high strengths.

Mechanics and mechanisms of crack growth at or near ceramic-metal interfaces : interface engineering strategies for promoting toughness

Abstract The strength and toughness of ceramic-metal joints is often controlled by the propagation path selected by stress-induced cracks. Against a background of recent linear elastic mechanics studies, experimental results from fracture tests on ceramic/metal/ceramic sandwich geometries are described which determine both the selection of crack path and the corresponding crack extension rates. It is found that crack path selection is controlled by the path of low microstructural resistance and the driving force directionality, which itself is a function of the far-field loading and the elastic compliance mismatch across the ceramic-metal interface. However, there are instances where the compliance mismatch takes the crack off the weak microstructural path, or where cracking occurs at, or near, both interfaces (crack jumping). Such cracking configurations can be tortuous and high toughness joints result. This paper discusses the potential for predicting and engineering, interfaces with enhanced toughness.

Ceramic joining III bonding of alumina via Cu/Nb/Cu interlayers

A method of ceramic-ceramic joining that exploits a multilayer interlayer designed to form a thin, potentially transient layer of liquid phase has been used to join alumina to alumina. Microdesigned multilayer Cu/Nb interlayers were used to achieve bonding at 1150 °C. Flexure strengths of as-bonded samples ranged from 119 to 255 MPa, with an average of ≈ 181 MPa. The ability to form ‘strong’ ceramic/metal interfaces is also indicated by instances of ceramic failure. Microstructural and chemical characteristics of fracture surfaces were evaluated using SEM, EDS and microprobe. The impact of post-bonding anneals of 10 h duration at 1000 °C in gettered argon on room-temperature joint strength was assessed. High strengths (198 to 238 MPa) were obtained. The retention of strength following annealing in low oxygen partial pressure argon differs from the behaviour previously observed in Cu/Pt bonded alumina. Effects of the anneal on interfacial microstructure were determined, and an explanation for this difference in behaviour is proposed.

Fatigue crack growth and stress redistribution at interfaces

The role of the interface in redistributing stress around cracks in multilayered ceramic/metal composites is investigated. The emphasis is on the different effects of interfacial debonding or of plastic slip in the metal phase adjacent to strongly bonded interfaces. The experiments are conducted on alumina/aluminum multilayered composites. Monotonic loading precracked test pieces causes plastic shear deformation within the aluminum layer at the tip of the notch without debonding. However, interfacial debonding can be induced by cyclic loading, in accordance with a classical fatigue mechanism. Measurements of the stress around the crack demonstrate that debonding is much more effective than slip at reducing the stress ahead of the crack.

Ceramic joining II partial transient liquid-phase bonding of alumina via Cu/Ni/Cu multilayer interlayers

Multilayer Cu/Ni/Cu interlayers that form a thin layer of a Cu-rich transient liquid phase have been used to join alumina to alumina at 1150 °C. The method and bonding conditions yield an assembly bonded by a Ni-rich (>94 at% Ni) interlayer at a temperature substantially lower than those normally required for solid-state diffusion bonding with pure Ni interlayers. Flexure strengths of as-bonded beams ranged from 61 to 267 MPa with an average of 160 MPa and a standard deviation of ±63 MPa. The highest flexure strengths were observed in samples where failure occurred in the ceramic. Post-bonding anneals of 10 h duration in air and gettered-argon at 1000 °C decreased the average room temperature strength to 138 and 74 MPa, respectively. In as-processed and annealed samples, varying degrees of interfacial spinel formation are indicated. Spinel formation may contribute to the scatter in as-processed samples, and the decrease in strength values resulting from annealing.

Interfaces between alumina and platinum: Structure, bonding and fracture resistance

Abstract Various diffusion bonded interfaces have been produced between Pt and either sapphire or polycrystalline Al 2 O 3 . The structure, bonding and fracture resistance of these interfaces have been examined, as well as the influence of intervening silicate phases, both amorphous and crystalline. It is found that the intrinsic fracture resistance Γ i of the Pt/sapphire interface is high, especially when Pt has an epitaxial orientation with respect to the sapphire. The fracture resistance is substantially degraded by amorphous silicate phases at the interface, even when discontinous, but is not adversely influenced by crystalline silicates. By contrast, the silicate phase enhances the diffusion bonding and facilitates formation of void-free bonds. This duality ih the role of silicates is a major feature of this article.

Cyclic Fatigue-Crack Propagation in a Silicon-Carbide Whisker Reinforced Alumina Composite: Role of Load Ratio

The characteristics of subcritical crack growth by cyclic fatigue have been examined in a silicon carbide whisker-reinforced alumina composite, with specific reference to the role of load ratio (ratio of minimum to maximum applied stress intensity, R=Kmin/Kmax); results are compared with similar subcritical crack-growth data obtained under constant load conditions (static fatigue). Using compact-tension samples cycled at ambient temperatures, cyclic fatigue-crack growth has been measured over six orders of magnitude from ∼10−11–10−5 m cycle−1 at load ratios ranging from 0.05–0.5. Growth rates (da/dN) display an approximate Paris power-law dependence on the applied stress-intensity range (ΔK), with an exponent varying between 33 and 50. Growth-rate behaviour is found to be strongly dependent upon load ratio; the fatigue threshold, ΔKTH, for example, is found to be increased by over 80% at R=0.05 compared to R=0.5. These results are rationalized in terms of a far greater dependency of growth rates on Kmax(da/dN ∞ Kmax30) compared to ΔK(da/dN ∞ ΔK5), in contrast to fatigue behaviour in metallic materials where generally the reverse is true. Micromechanisms of crack advance underlying such behaviour are discussed in terms of timedependent crack bridging involving either matrix grains or unbroken whiskers.

Microstructural and Chemical Components of Ceramic-Metal Interfacial Fracture Energies

Failure of ceramic-metal interfaces induced by residual or applied stress is often brittle in nature although plastic strain in one or more bonding layers may add to the fracture energy for decohesion. Thus, the fracture toughness depends on chemical bonding across the interface, the plasticity and flow stress of the metal as well as other factors, arising from local internal stresses and the microstructure of the ceramic-metal couple, that cause crack tip branching, deflection, bridging, blunting or shielding. Electron microscopy and DCB testing of metal-glass systems provide insights into the relative importance of factors that determine the decohesion resistance.

On the strength and toughness of structural ceramics bonded to metals

The structural integrity of ceramic/metal joints is primarily determined by the strength and toughness of the interface formed between the ceramic and metal components. A combination of theoretical and experimental studies of the mechanics and mechanisms of crack growth at or near such interfaces in sandwich geometries demonstrates a strong dependence of strength and toughness on the propagation path selected by stress-induced cracks. As a result, there is potential for predicting, and engineering, stronger and tougher ceramic/metal interfaces.