NDE of ceramic components manufactured by additive technologies
Additive Manufacturing (AM) adds in the first instance a new shaping method to the portfolio of established shaping methods of ceramics. New, unseen, shapes can be created directly from the computer model, potentially after numerical optimization (functional design). Sensory or actuator functions can be added by printing functional layers.
The additive shaping or process will not result in successful components without considering the technology chain from raw materials and preprocessing to post processing and testing. Non-destructive evaluation (NDE) shall be applied ideally already to prefinished components for optimizing process steps and reducing waste. Additive methods offer new challenges (e.g. internal stresses, imperfect interfaces between layers) and new opportunities for NDE methods.
These new opportunities are opened up by investigating thinner layers of materials directly in-operando of the AM shaping process. Ceramics structures, often difficult to investigate, will become better accessible.
Fraunhofer IKTS qualifies new optical methods for in-operando use during additive manufacturing. Recent examples of work will be presented.
Independent from the specifics of the application, a cost efficient manufacturing of solid oxide fuel cells (SOFC), its electrolyte membranes and other stack components, leading to reliable long-life stacks is the key for the commercial viability of this fuel cell technology.
Tensile and shear stresses are most critical for ceramic components and especially for thin electrolyte membranes as used in SOFC cells. Although stack developers try to reduce tensile stresses acting on the electrolyte by either matching CTE of interconnects and electrolytes or by putting SOFC cells under some pressure – at least during transient operation of SOFC stacks ceramic cells will experience some tensile stresses. Electrolytes are required to have a high Weibull characteristic fracture strength. Practical experiences in stack manufacturing have shown that statistical fracture strength data generated by tests of electrolyte samples give limited information on electrolyte or cell quality. In addition, the cutting process of SOFC electrolytes has a major influence on crack initiation.
Typically, any single crack in one the 30 to 80 cells in series connection will lead to a premature stack failure drastically reducing stack service life. Thus, for statistical reasons only 100% defect free SOFC cells must be assembled in stacks. This underlines the need for an automated inspection. So far, only manual processes of visual or mechanical electrolyte inspection are established. Fraunhofer IKTS has qualified the method of optical coherence tomography for an automated high throughput inspection. Alternatives like laser speckle photometry and acoustical methods are still under investigation.