Helge Prielipp

Strength and fracture toughness of aluminum/alumina composites with interpenetrating networks

The mechanical properties of metal reinforced ceramics, especially Al/Al2O3 composites with interpenetrating networks, are described. Key parameters to tailor the characteristics of these materials are the ligament diameter and volume fraction of ductile reinforcement. Fracture strength and fracture toughness data are given as a function of both variables and are compared with the corresponding values for the porous preforms. A simple model accounts for the influence of metal volume and metal ligament diameter on the plateau toughness of the composites. The increase in fracture strength from the porous preform to the composite is found to be much larger than the gain which can be predicted from the increase in fracture toughness alone. A discussion of fracture strength in these composites therefore must include at least two issues, crack propagation through the matrix as well as crack initiation at metal filled pores.

Ni3AlAl2O3 composites with interpenetrating networks

Homogenous microstructures of Ni3Al/A2O3 composites can be produced by gas pressure infiltration of the intermetallic phase into the porous ceramic. Since the nickel aluminide is single crystalline in the reinforcing ligaments, it exhibits ductile deformation and provides appreciable toughening for the composite. The mechanical property data of this material at room temperature and at 1073 K are comparable.

Ni{sub 3}Al/Al{sub 2}O{sub 3} composites with interpenetrating networks

Homogenous microstructures of Ni3Al/A2O3 composites can be produced by gas pressure infiltration of the intermetallic phase into the porous ceramic. Since the nickel aluminide is single crystalline in the reinforcing ligaments, it exhibits ductile deformation and provides appreciable toughening for the composite. The mechanical property data of this material at room temperature and at 1073 K are comparable.

Mechanical properties of Al/Al2O3 and Cu/Al2O3 composites with interpenetrating networks

The rising fracture resistance with crack length in metal-toughened ceramics due to ductile bridging has been discussed from some selected microstructures and metal-ceramic combinations. An intriguing feature of these composites is the influence of interfacial fracture strength. Strong interfacial bonding leads to high geometrical constraint for the metal and high degree of triaxial tension in the metal ligament, thereby increasing the uniaxial yield strength by a factor of 5--7. This in turn increases the closure stress of the metal ligament, but ultimately limits the total plastic dissipation in the ductile reinforcement. The intent of this paper is to provide some insight on the influence of metal ligament size on both fracture toughness and fracture strength. The materials chosen are Al/Al[sub 2]O[sub 3] and Cu/Al[sub 2]O[sub 3] composites, both prepared by gas-pressure metal-infiltration of porous alumina preforms. SEM observations of fracture surfaces in conjunction with preliminary TEM and PEELS investigations of the metal-ceramic interfaces are used to explain the trends in mechanical property data.

Mechanical properties of AlAl2O3 and CuAl2O3 composites with interpenetrating networks

The rising fracture resistance with crack length in metal-toughened ceramics due to ductile bridging has been discussed from some selected microstructures and metal-ceramic combinations. An intriguing feature of these composites is the influence of interfacial fracture strength. Strong interfacial bonding leads to high geometrical constraint for the metal and high degree of triaxial tension in the metal ligament, thereby increasing the uniaxial yield strength by a factor of 5--7. This in turn increases the closure stress of the metal ligament, but ultimately limits the total plastic dissipation in the ductile reinforcement. The intent of this paper is to provide some insight on the influence of metal ligament size on both fracture toughness and fracture strength. The materials chosen are Al/Al[sub 2]O[sub 3] and Cu/Al[sub 2]O[sub 3] composites, both prepared by gas-pressure metal-infiltration of porous alumina preforms. SEM observations of fracture surfaces in conjunction with preliminary TEM and PEELS investigations of the metal-ceramic interfaces are used to explain the trends in mechanical property data.

Better ceramics through metal modification

The mechanical properties of metal reinforced ceramics, especially Al/Al2O3 and Cu/Al2O3 composites with interpenetrating networks, are described. Key parameters to tailor the characteristics of these materials are ligament diameter and volume fraction of the ductile reinforcement as well as fracture energy of the metal/ceramic interface. Data for fracture strength and fracture toughness are given as a function of all three variables and qualitative descriptions are provided for trends observed in the experiments. Finally, drastic improvements in thermal shock resistance of Al/Al2O3 composites with increasing ligament diameter highlight the opportunities afforded by tailoring the microstructure of metal reinforced ceramics.

Mechanical properties of Al/Al[sub 2]O[sub 3] and Cu/Al[sub 2]O[sub 3] composites with interpenetrating networks

The rising fracture resistance with crack length in metal-toughened ceramics due to ductile bridging has been discussed from some selected microstructures and metal-ceramic combinations. An intriguing feature of these composites is the influence of interfacial fracture strength. Strong interfacial bonding leads to high geometrical constraint for the metal and high degree of triaxial tension in the metal ligament, thereby increasing the uniaxial yield strength by a factor of 5--7. This in turn increases the closure stress of the metal ligament, but ultimately limits the total plastic dissipation in the ductile reinforcement. The intent of this paper is to provide some insight on the influence of metal ligament size on both fracture toughness and fracture strength. The materials chosen are Al/Al[sub 2]O[sub 3] and Cu/Al[sub 2]O[sub 3] composites, both prepared by gas-pressure metal-infiltration of porous alumina preforms. SEM observations of fracture surfaces in conjunction with preliminary TEM and PEELS investigations of the metal-ceramic interfaces are used to explain the trends in mechanical property data.

Multifunctional Ceramics for Thermal Shock Applications

Infiltration of molten metals in a porous ceramic preform represents a viable method to produce multifunctional ceramic matrix composites. In this paper fabrication of these composites as well as mechanical properties prior to and after thermal shock are outlined. Metal infiltrated ceramics are identified as attractive materials for applications requiring both, high strength and thermal shock resistance. Possible mechanisms operating during thermal shock are discussed by comparing energy release rate and R-curve behavior.