Matrix cracking in ceramic-matrix composites with unbonded frictional interface has been studied using fracture mechanics theory. The critical stress for extension of a fiber-bridged crack has been analyzed using the stressintensity approach. The analysis uses a new shear-lag formulation of the crack-closure traction applied by the bridging fibers based on the assumption of a constant sliding friction stress over the sliding length of the fibermatrix interface. The new formulation satisfies two required limiting conditions: (a) when the stress in the bridging fiber approaches the far-field applied stress, the crack-opening displacement approaches a steady-state upper limit that is in agreement with the previous formulations; and (b) in the limit of zero crack opening, the stress in the bridging fiber approaches the far-field fiber stress, This lower limit of the bridging stress is distinctly different from the previous formulations. For all other conditions, the closure traction is a function of the far-field applied stress in addition to the local crack-opening displacement, the interfacial sliding friction stress, and the material properties. Numerical calculations using the stress-intensity approach indicate that the critical stress for crack extension decreases with increasing crack length and approaches a constant steady-state value for large cracks. The steadystate matrix-cracking stress agrees with a steady-state energy balance analysis applied to the continuum model, but it is slightly less than the matrix-cracking stress predicted by such theories of steady-state cracking as that of Aveston, Cooper, and Kelly. The origin of this difference and a method for reconciliation of the two theoretical approaches are discussed.

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The effects of cobalt content and heat treatment temperature on hardness and wear resistance of nickel-cobalt-phosphorus (Ni-Co-P) are systematically investigated on the electroless alloy deposits composing of cobalt with contents up to 45%. The increase of the cobalt content leads to improved stabilization of grain size and crystallographic structure upon thermal annealing. A moderate annealing temperature of 300°C applied on Ni-45Co-2.6P deposits grants optimum hardness of 959 Hv and relatively low wear loss of 8.85 mg/1000 cycles in abrasive mode, as assessed by Taber abraser wear test. An exclusion of cobalt, however, shows benefit in wear resistance enhancement under the ball-on-disc wear test, where a combined abrasive and adhesive wear is induced in this class of nickel-based alloys.

Interfacial sliding friction stress (q) was assessed using both pushout and pullout tests on Sic-borosilicate glass composite specimens. Single-filament composite specimens were fabricated by heating to 950°C in argon borosilicate glass rods with fine-diameter (250-pm) capillary in which SIC filaments were inserted. The composite specimens prepared in this manner showed only frictional bonding. The maximum frictional sliding loads for pushout and the initial frictional sliding loads in pullout were measured as functions of the embedded length of the filament in the glass rods. The nonlinear variations of the frictional loads were analyzed using shear-lag models that include corrections for the effects of Poisson expansion or contraction on the sliding friction stress. It is shown that under identical conditions of composite fabrication the two tests give nearly identical properties for the interfaces. Pushout tests on hotpressed bulk composite specimens, however, showed both chemical bonding and a higher sliding friction stress relative to the single-filament capillary specimens. The presence of compressive residual stress on the filaments was independently confirmed by evidence of stress-induced birefringence. [