To use functional ceramics successfully, sufficient mechanical reliability must be guaranteed in addition to a clearly defined functional profile. Therefore, knowledge of mechanical properties such as hardness, toughness and strength is essential for the design.
The sample material investigated in this thesis is a commercial "Low Temperature Cofired Ceramic". LTCCs are used in special applications as electrical circuit boards due to their outstanding dielectric and thermophysical properties. Conductor tracks are printed on ceramic foils, stacked and sintered at temperatures between 900° and 1000° Celsius.
A key topic of this thesis is the analysis of the mechanical strength and failure of LTCCs. Empirical evidence shows that the strength of almost all ceramic materials can be described in many load scenarios based on a defect model. In this model, structural components such as pores, microcracks and foreign phases are interpreted as defects that can act as fracture origins. The statistical nature of such defect distributions ultimately also leads to distributions of measured strength values of a sample of nominally similar specimens. The Weibull distribution has proven itself for the mathematical description of strengths. Within the range of validity, this allows extrapolation to other load situations or to other sample geometries, which is why the strengths of “Weibull materials” exhibit a volume effect. Statistically, it can be shown that samples of different sizes have different strengths. In order to test this volume effect or Weibull behavior, the disc-shaped LTCC specimens were initially tested using two different, established test methods with different test volumes: the ring-on-ring test and the ball-on-three-balls test. In order to test the applicability of the Weibull volume effect with extremely small volumes under load, a “Hertzian contact damage test” was developed, which leads to very local, short-range tensile stress fields. The specimen is pressed symmetrically between two spheres with increasing force until ring cracks are induced by the tensile stresses outside the contact surface. These tests were evaluated using a finite element method (FEM) simulation.
It could be shown that the failure of the studied LTCC follows Weibull’s statistical approach when tested by the ring-on-ring and the ball-on-three-balls tests. However, the failure in a highly localized stress field (such as in the Hertzian contact damage test) presented in this work, shows significant deviations from the previously determined statistics.
Furthermore, the influence of surface finishes on the strength of ceramics was discussed. After an “industrial” lapping process, the strength of the material was examined using the ring-on-ring- and ball-on-three-balls test.