In order to provide the mechanical data needed to design a ceramic component for a combustion engine, mechanical testing of a silicon nitride material and a sialon material will be performed in various conditions: static and cyclic loading; high temperatures; diesel exhaust gas environment; with special attention paid to the specific influence of the aggressive atmosphere.
The fracture behaviour will be examined under 2 approaches assuming that the failure is dictated by a subcritical crack growth law, and by a creep law, the subcritical crack growth being a consequence of a local creep.
These approaches will be developed with a view to their application in finite elements programmes for the lifetime prediction of ceramic components. In parallel, the physicochemical mechanisms responsible for the degradation of the mechanical properties of ceramics in the aggressive environment will be identified.
This programme gives important results on:
tensile and bending creep;
influence of the environment (air or exhaust gases and temperature) on corrosion and subcritical crack growth (static or cyclic);
relation between microcracks (natural flaws) and macrocracks (test specimens);
testing of a postprocessor to a 3-dimensional finite element programme which gives failure probability of components;
development of specific numerical posttreatments to give a failure probability of components after damage (creep and crack growth in static and cyclic loading).
In order to provide the mechanical data needed to design a ceramic component for a combustion engine, mechanical testing of sintered silicon nitride (SSN) materials (with glassy and recrystallized intergranular phases) has been performed under various conditions (static and cyclic loading, high temperatures). Special attention has been payed to the specific influence of the aggressive atmosphere of diesel exhaust gases. This work has necessitated the development of specific testing devices and models to describe the material behaviour.
SSN with recrystallized intergranular phases exhibits good mechanical properties (strength, oxidation, creep) up to 1100 C (900 C for glassy intergranular phases SSN). No significant effect of frequency on lifetime has been found or any effect of exhaust gas on high temperature mechanical properties. Creep behaviour, however, is very sensitive to loading direction, with a greater creep rate in tension than in compression.
Postprocessors to finite element codes have been developed to predict lifetimes of ceramic components for a given failure probability under the testing conditions of this programme.
IN ORDER TO PROVIDE THE MECHANICAL DATA NEEDED TO DESIGN A CERAMIC COMPONENT FOR A COMBUSTION ENGINE, MECHANICAL TESTING OF A SILICON NITRIDE MATERIAL AND A SIALON MATERIAL WILL BE PERFORMED IN VARIOUS CONDITIONS - STATIC AD CYCLIC LOADING, HIGH TEMPERATURES, DIESEL EXHAUST GAS ENVIRONMENT - WITH A SPECIAL ATTENTION PAID TO THE SPECIFIC INFLUENCE OF THE AGGRESSIVE ATMOSPHERE.
THE FRACTURE BEHAVIOUR WILL BE EXAMINED UNDER TWO APPROACHES ASSUMING THAT THE FAILURE IS DICTATED:
1.- BY A SUBCRITICAL CRACK GROWTH LAW
2.- BY A CREEP LAW, THE SUBCRITICAL CRACK GROWTH BEING A CONSEQUENCE OF A LOCAL CREEP.
THESE APPROACHES WILL BE DEVELOPED IN VIEW OF THEIR APPLICATION IN FINITE-ELEMENTS PROGRAMMES FOR THE LIFETIME PREDICTION OF CERAMIC COMPONENTS.IN PARALLEL, THE PHYSICO-CHEMICAL MECHANISMS RESPONSIBLE FOR THE DEGRADATION OF THE MECHANICAL PROPERTIES OF CERAMICS IN THE AGGRESSIVE ENVIRONMENT WILL BE IDENTIFIED.
Funding SchemeCSC - Cost-sharing contracts
79108 Freiburg (In Breisgau)