The effect of erosional wear on strength and residual stress during shaping of ceramic multilayer capacitors

Abstract

Shaping of ceramic multilayer capacitors (CMCs) is a prerequisite for the application of end terminations. This shaping is frequently done by erosional wear in a tumbling process. In this study the tumbling time, tumbling speed and loaded volume of the tumbling bottle were varied to determine the effect on the shape parameters. A power law relationship is observed between the roughness, corner radius and weight vs. time while the overall level depends sensitively on tumbling speed and loaded volume. The tumbling process, however, also affects strength. In the first stage of the tumbling process severe damage is introduced on the surface of the CMC which decreases the strength considerably. Compressive residual stress and crack length reduction, both induced during the tumbling operation, are responsible for an important strength increment in a later stage of the process. The optimum process parameters therefore should be chosen considering shape and strength. Small differences in the loaded volume of the tumbling bottle appear to affect the final strength and shaping considerably. The residual stress introduced during the tumbling process was determined by the sin 2ψ method and indentation techniques. Good agreement between both methods can be obtained only after taking into account all stress components in the sin 2ψ method. After sintering a compressive residual stress of 170 MPa is present in the CMCs. For the tumbled CMCs the amount of residual stress increases up to about 250 MPa. Experiments with dummy CMCs, that is identically processed but without metal electrodes, show that the residual stress after sintering is an overall stress effect in which the electrodes have an important contribution. The stress due to tumbling is highly concentrated on the surface, and is not affected by the electrodes.