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Due to the promising combination of chemical, thermal and mechanical properties, springs made of advanced ceramics have attracted much attention as a replacement for metal springs in highly demanding applications, operating at high temperatures and in harsh environments when hardened metals can no longer be used. A further application was recently proposed by using ceramic springs with metalized surfaces as capacitive force sensors. Prior to any design of an instrument, application-specific static and/or dynamic loading experiments are necessary to investigate the stability of spring properties under the given conditions. These experiments can also be used to determine seldom measured material properties like the shear modulus of ceramics.
Helical Springs with a rectangular cross-section have been machined from straight tubes of alumina (99.99% α-AI203,) and zirconia (Y/Ce-TZP). The sintered density of both materials was above 99% of the theoretical density.
The stress/displacement curves turned out to be extremely linear and the spring constants were not altered, even after more than one million cycles of compression loading at various temperatures from -15°C to +60°C. This means that such springs can be a far more reproducible and reliable source of an elastic response to applied forces than handmade springs from molten quartz wires. The behavior found for in-house fabricated springs contrasts to the behavior found for a commercial ceramic spring, which was produced by injection molding and exhibits a less linear response.
Furthermore, high-temperature displacement behavior of fabricated alumina and zirconia springs was tested under static loading conditions in different atmospheres (air, N2 and H2) at temperatures up to 1000 °C.
Helical springs with a rectangular cross-section have been machined from sintered and grinded hollow cylinders with high geometrical precision and good reproducibility. Such springs made from tetragonal zirconia polycrystal (TZP) ceramic show excellent edge quality because of high fracture toughness and bending strength of the starting material. Hence, springs with desired geometric dimension and tailored spring constant can be manufactured for highly demanding applications at high temperatures and in harsh environments.
Prior to any practical use, application limits of springs under mechanical and thermal load have to be analyzed. Therefore, different displacement experiments were carried out on the helical TZP springs.
- Dynamic displacement tests at various temperatures from -15°C to +60°C using a piezo actor to load/unload springs with frequencies between 1 and 40 Hz: Springs remained undamaged and the spring constants were not altered, even after more than one million cycles of compression loading.
- Long-time displacement measurements under static tensile loading at room temperature with a high-precision interferometer test facility: Significant spring elongation under constant strain was surprisingly proved over a period of many hours already at room temperature.
- Creeping experiments for 48 h under static compression load at different temperatures up to 1000 °C: After cooling down and load removing no permanent length reduction of springs was observed for test temperatures up to 700 °C. However, reshaping of TZP springs by plastic deformation is possible at higher temperatures and opens up additional possibilities for spring design and manufacturing.