Oxide ceramic matrix composites (O-CMCs) are gaining attention as a serious substitute for metals in several applications. They offer lower density, comparable strength and high oxidation resistance, enabling efficiency improvements originally in the aerospace sector as well as more recently in automotive or commercial aviation. However, the production process is time-consuming and limited to simple geometries. Fused deposition modeling (FDM) as a category of additive manufacturing (AM) is a promising rapid processing technique offering the production of complex shapes, and little waste.
In this project, we investigated a CMC material based on zirconia-toughened alumina (ZTA) matrix and NEXTEL 610 Al2O3 fibers. The ceramic material was mixed with a thermoplastic binder in a high shear mixing process and filaments with a diameter of 1.75 mm were extruded using a capillary rheometer. Up to 30 vol.% fiber content could be added without technical problems. However, above 20 vol.% fiber content, the surface of the filaments became very rough.
In the first step, ceramic filaments were used to optimize the post-printing process like solvent and thermal debinding steps as well as final sintering. Interestingly, by adding fibers into the ceramic filament recipe torque during mixing and the extrusion pressure were decreasing. We can explain this by the orientation of the fiber during the processing. Therefore, the fibers get aligned in the direction of the shear force. After sintering at 1200°C a relative density of 55 % could be achieved for all samples. Using four-point bending test the mechanical strength could be almost doubled, and strength above 100 MPa was calculated by the Maximum likelihood method for the ceramic matrix composites with 10 and 15 vol.% of fibers. Unfortunately, filaments with 15 vol.% were not printable with commercial direct extruder head and therefore pure ZTA and CMCs with 10 vol.% of short fibers were selected to continue the study. Printing parameters were investigated to achieve dense 0° and 0°/90° fiber reinforced ceramic structures. The printing of 90° fiber orientation resulted in a significantly higher porosity based on printing defects. The 4-point bending experiments showed that printed structures with 0° fiber orientation resulted in a slightly lower strength (90 MPa) whereas the 0°/90° printed sample resulted in a strength of 45 MPa, as expected. Surprisingly, the mechanical strength of the porous ZTA without fiber resulted in a significantly lower strength (> 20 MPa). This could be explained by the delamination of the printed layers during solvent debinding.