Paul D. Jero

Issues in the control of fiber/matrix interfaces in ceramic composites

Abstract It is argued that virtually all ceramic-matrix composite parts will be at least locally microcracked in service, such that engineered, oxidation-resistant interfaces are a pervasive requirement. In addition, there is a host of complex parameters which affect interfacial properties. This work presents theoretical and experimental work examining the role of interface topography. Fiber push-out tests and an analysis of the test which includes an interface roughness contribution are described. Techniques for characterizing fiber/interface topography are described and results presented. Finally, interface degradation associated with fiber sliding is described.

Interfacial Shear Behavior of Two Titanium-Based SCS-6 Model Composites

Single fiber push-out and push-back tests combined with acoustic response monitoring were used to examine the interfacial behavior in two titanium alloy-SiC fiber composites. Distinctly different behaviors were observed in the two systems. The differences were attributed to the formation of a substantial interfacial reaction layer in one of the composites which changed the interfacial chemistry and the resulting debond topography. The reaction layer caused an increase in the interfacial bond strength and in the roughness of the debonded interface. The latter resulted in substantially increased sliding friction. Although both composite interfaces exhibited some roughness, only one showed a seating drop during fiber push-back. This is related to the fact that the reaction layer which formed in one of the composites was severely degraded during fiber pushout. Although this interface was still rough, the roughness correspondence between fiber and matrix was destroyed during sliding, such that seating was no longer possible.

Fiber-matrix interface -- information from experiments via simulation

&This study explores a novel procedure for obtaining quantitative information on the mechanical properties of the fiber-matrix interface in composite materials. The method, based on lattice discretization of a medium, simulates actual experiments in detail, including fiber breakage, matrix yield and/or cracking, and interface failure. The paper concentrates on two experiments performed commonly, the so-called fragmentation test for metal matrix, and the pushout/ pullout test for metal as well as ceramic matrix composites. Based on the documented capability of the method to simulate actual experimental data, reliable values of (homogenized) interface properties can be obtained. In addition, the simulations provide further understanding of the mechanisms involved during the relevant testing. Although this study presents results from basic problems, the method is general enough to include effects of residual stress, of high temperature environment, and of dynamic crack propagation, as well as threedimensional details of the interface failure process. The potential exists for simulating nondestructive wave-based techniques aimed at evaluating interface properties.

Determination of Fiber/Matrix Interface Mechanical Properties in Brittle-Matrix Composites

It has been evident for some time that the mechanical properties of the fiber/matrix interface play an important role in determining the mechanical behavior of ceramic composites (for reviews, see [1], [2], and [3[). Recently there has been a growing interest in the role of the fiber/matrix interface in intermetallic matrix composites. While ceramic and intermetallic composites are certainly very different materials, understanding the behavior of one will provide insight into the other. Furthermore, the basic issues regarding the determination of interface properties are the same. The accuracy of micromechanics models of any composite system is dependent upon the accuracy of all the constituent and interface properties. It is far preferable to measure actual materials constants rather than test-specific quantities. The tests described here are intended to measure the interfacial shear strength (or mode II toughness) and the interfacial tensile strength. The objective of this work is to briefly outline a few of the approaches which are being evaluated for and applied to ceramic composites, and which may be of interest to investigators working in intermetallic composites.

Ultrasonic Analysis of Interfacial Debond in Ceramic Matrix Composites

Ceramic materials have properties which make them desirable candidates for elevated temperature applications. Most monolithic ceramics, however, have an inherently low fracture toughness which limits their usefulness for critical applications. Therefore, recent research has been aimed at developing ceramic matrix composites (CMC’s) and engineering the microstructure of ceramics to enhance the material’s toughness.