Ceramics failure

The locus of failure in all PMDA-ODA/MgO cases is consistently a mixed mode type leaving ceramic on the polyimide failure surface, and leaving polyimide on the ceramic failure surface. APS application prior to polyimide coating does not change the failure locus or the peel strength behavior of these interfaces. 

S. S. Scherrer, J. B. Quinn, G. D. Quinn, FL W. A. Wiscott, Fractographic Ceramic Failure Analysis Using the Replica Technique Two Case Studies, Dental Materials, 23 (11), (2006) 1397-1404. 

From the preceding discussion, it should be obvious that ceramic failure in any practical application implies the existence of a flaw that degrades the strength below the theoretical value and a stress sufficiently high to propagate that flaw. The intent of the failure analysis is, usually, to determine the nature of the flaw and, if possible, to ascertain how it was introduced. It is also desirable to determine the magnitude of the stress and how the stress was generated. 

Figure 7.44 Several potential ceramic failure mechanisms of Y-PSZ thermal barrier coatings. (A) Spalling due to stresses introduced by thermal cycling (B) Destabilization ofzirconia due to cubic-monoclinic phase transformation (C) Erosion and...

Overdischarge reaction. From the very first charge the cell has a surplus of sodium in the anode compartment so that for an overdischarge tolerance sodium is available to maintain current flow at a lower voltage, as indicated in Figure 10.5. This reaction is equal to the cell failure reaction but runs without a ceramic failure. 

This example relies on the identification of an element at the failure interface that can be used as a marker for a particular component in the formulation. More usual is the need to resort to high resolution spectroscopy. The data of O Fig. 10.8 relates to a radiation-cured adhesive being developed for the bonding of components to ceramic substrates for surface mount technology. Once again water reduced the adhesion below an acceptable level and XPS and ToF-SIMS analysis of the failure surfaces provides an indication of the cause of the failure. The adhesive has two major components an aromatic adhesive part and an aliphatic reactive diluent. Inspection of the high resolution Cls spectra from the adhesive side of the failure shows the expected Jt —> Jt shake-up satellite diagnostic of the aromatic component of the adhesive (Watts and Taylor 1995). The ceramic failure surface shows a small amount of carbon... 

Difficulties with the Na—S system arise ia part from the ceramic nature of the alumiaa separator the specific P-alumiaa is expeasive to prepare and the material is brittie and quite fragile. Separator failure is the leading cause of early cell failure. Cell failure may also be related to performance problems caused by polarization at the sodium/soHd electrolyte iaterface. Lastiy, seal leakage can be a determiaant of cycle life. In spite of these problems, however, the safety and rehabiUty of the Na—S system has progressed to the poiat where pilot plant production of these batteries is anticipated for EV and aerospace apphcations. 

Ceramic ball beatings are also sometimes effective ki operation with water which would result ki rapid failure with steel beatings. This capabiUty may result from a thin hydrodynamic film formed from very small hydrated Si N wear particles and the water (44). 

The overriding concern with regard to the mechanical performance of ceramics is their brittieness and hence sensitivity to flaws. There is usually htfle or no warning that failure is imminent because deformation strain prior to failure is usually less than 0.1%. As a result, a primary thmst of stmctural ceramics research has been the development of tougher ceramics. Ceramics now exist that have toughness values of 20 and strengths that... 

Many distribution functions can be apphed to strength data of ceramics but the function that has been most widely apphed is the WeibuU function, which is based on the concept of failure at the weakest link in a body under simple tension. A normal distribution is inappropriate for ceramic strengths because extreme values of the flaw distribution, not the central tendency of the flaw distribution, determine the strength. One implication of WeibuU statistics is that large bodies are weaker than small bodies because the number of flaws a body contains is proportional to its volume. 

V. D. Frnchette, Failure Analysis of Brittle Materials, Advances in Ceramics, Vol. 28, The American Ceramic Society, Inc., Westervike, Ohio, 1990. 

A partial answer to the first question has been provided by a theoretical treatment (1,2) that examines the conditions under which a matrix crack will deflect along the iaterface betweea the matrix and the reinforcement. This fracture—mechanics analysis links the condition for crack deflection to both the relative fracture resistance of the iaterface and the bridge and to the relative elastic mismatch between the reinforcement and the matrix. The calculations iadicate that, for any elastic mismatch, iaterface failure will occur whea the fracture resistance of the bridge is at least four times greater than that of the iaterface. For specific degrees of elastic mismatch, this coaditioa can be a conservative lower estimate. This condition provides a guide for iaterfacial desiga of ceramic matrix composites. 

The toughness induced in ceramic matrices reinforced with the various types of reinforcements, that is, particles, platelets, whiskers, or fibers, derives from two phenomena crack deflection and crack-tip shielding. These phenomena usually operate in synergism in composite systems to give the resultant toughness and noncatastrophic mode of failure. 

Fig. 14. Failure mechanisms for continuous fiber reinforced ceramic matrices (58).

These requirements severely limit our choice of creep-resistant materials. For example, ceramics, with their high softening temperatures and low densities, are ruled out for aero-engines because they are far too brittle (they are under evaluation for use in land-based turbines, where the risks and consequences of sudden failure are less severe - see below). Cermets offer no great advantage because their metallic matrices soften at much too low a temperature. The materials which best fill present needs are the nickel-based super-alloys. 

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