6 research outputs found
Combination of biological mechanisms for a concept study of a fracture-tolerant bio-inspired ceramic composite material
The biological materials nacre and wood are
renowned for their impressive combination of toughness and
strength. The key mechanisms of these highly complex
structures are crack deflection at weak interfaces, crack
bridging, functional gradients and reinforcing elements.
These principles were applied to a more fracture-tolerant
model material which combined porous stiff ceramic layers,
manufactured by freeze casting, infiltrated and bonded by a
polymer phase reinforced with fabric layers. In the hybrid
composites, crack deflection occurred at the ceramic–fabric
interface and the intact fabric layers served as crack-bridging
elements. Fabric-reinforced epoxy layers stabilized the
fracture behaviour and delayed catastrophic failure of the
material. The influence of the different components was
analysed by varying the ceramic, fabric and interface
properties. More ductile fabrics lead to larger strain to failure
and more crack bridging but reduced the composite
strength and stiffness after initial cracking. Higher elastic
mismatch between the components improved crack deflection
and bridging but resulted in deterred load transfer and a
lower strength. The stiffness and strength of the ceramic
layers influenced the elastic properties of the laminar composite
and the initial crack resistance. Flaw tolerance was
increased with polymer infiltration. We show with our
hybrid ceramic–fabric composite as a bio-inspired concept
study how fracture toughness, work of fracture and tolerance
for cracking can be tailored when the contributing factors,
i.e. the ceramic, the fabric and their interface, are modified
Influence of fiber orientation and matrix processing on the tensile and creep performance of Nextel 610 reinforced polymer derived ceramic matrix composites
The tensile and creep performance of two Nextels 610(N610) reinforced polymer derived ceramic
composites (UMOX™, OXIPOL) is studied up to 1200C. Independent of the fiber orientation(45 or
0/90deg) all samples exhibit a segment where the strain rate was constant. The creep performance in
45deg is matrix dominated and shows a more pronounced primary creep regime, due to changes within
the matrix. The following creep regime with a constant strain rate might be attributed to viscous flow of
the SiOC within the matrix based on activation energy(283kJ/mol) and stress exponent (0.6). In0/90 deg
orientation the creep and tensile performance is independent of oxidation, but directly influenced by
grain structure of the fiber. The coarser and non-uniform microstructure of the fibers in UMOX™
decreases the stationary creep rates and changes the diffusiona lcreep mechanism. The possibility to
modify the microstructure of the fiber during the manufacturing process might be used to adjust e.g. the
strength and creep stability of these materials related to the desired applications
Development and validation of oxide/oxide CMC combustors within the HiPOC program
In the framework of the High Performance Oxide Ceramics program (HiPOC), three different oxide/oxide ceramic matrix composite (CMC) materials are studied for a combustion chamber application in continuation of the work reported in Gerendas et al. [1]. A variation in the micro-structural design of the three CMC materials in terms of different fiber architecture and matrix processing are considered in a first work stream. By modification of the matrix and the fiber-matrix interface as well as the application of an environmental barrier coating (EBC), the high temperature stability is enhanced. Furthermore, design concepts for the attachment of the CMC component to the metal structure of the engine are finalized in a second work stream. Issues like sealing of cooling leakage paths, allowance for the different thermal expansion and the mechanical fixation are addressed. An interim standard of the mechanical attachment scheme is studied on a shaker table. Also the friction coefficient between the metallic and ceramic components is analyzed in order to set the proper tightening torque. The manufacturing of the CMC combustor is improved in several iterations in order to achieve a high quality material with optimized fiber architecture. Afterwards, two CMC materials are selected for the combustion testing and the finalized design of the metallic and CMC components is manufactured. A fit check is performed prior to EBC application and laser drilling of the effusion holes in order to evaluate the impact of the manufacturing tolerances on the function of the sealing and attachment scheme and to correct small issues at this stage. First results from the validation testing in a high-pressure tubular combustion rig up to a Technology Readiness Level 4 (TRL4) are reported.</jats:p