Characterization of interface debonding in a ceramic-ceramic fibre composite using the indentation method and acoustic emission

Abstract

The indentation method was first used by Marshall [1] to estimate the frictional stress at the fibre-matrix interface in an SiC(Nicalon)/glass-ceramic composite. In the case of frictional sliding at the interface, the effects of the Poisson's ratio of the fibre and of residual thermal stresses were discussed [2, 3] and it was shown that these effects could best be analysed by using an instrumented indentation set-up giving the loaddisplacement curves by loading and unloading. An alternate interface behaviour is the perfect bonded case where debonding occurs when critical conditions are fulfilled. In this case, during fibre loading, there is no sliding at the interface and all displacements are fully elastic. For a given critical fibre load, debonding between fibre and matrix occurs, and then frictional sliding can take place. This type of behaviour under indentation was studied by Mandell et al. [4] for glass and graphite fibres in epoxy. The authors concluded that the critical parameter was the interracial strength (stress condition). Another approach to debonding, based on energy considerations, was previously proposed by Outwater and Murphy [5]; Wells [6] measured the debond stress (critical fibre stress for debonding) in tension of steel wires of varying diameter embedded in epoxy and clearly showed that the debond stress is more accurately predicted by the energy condition. In this letter we describe debonding experiments by indentation on a ceramic-ceramic composite and compare the observed debond stress with those predicted by the two criteria for debonding (stress condition and energy condition). To do so, the Nicalon fibres are of considerable interest because they exhibit a very large scatter in diameters (say, from 6 to 30 #m) so the effect of fibre radius can be checked for the two conditions. The indentation apparatus is a standard inverted optical microscope equipped with a microhardness accessory. The displacement of the Vickers indentor is obtained by turning the microscope focus knob. The microhardness accessory permits load measurement optically by observing the displacement of a spiderline in front of a scale in the ocular lens. Acoustic emission (AE) is used to determine the moment of debonding. The AE resonant transducer is fixed on the opposite side of the specimen (Fig. 1); the signal delivered is amplified and processed by a ring-down counter whose value is erased each 0.1 sec. The analogue counting value is converted to frequency and injected, after adequate amplification, into a loudspeaker. Thus an AE event leads to an audible note