Name: Claudio Ferraro

Supervisor: Prof Eduardo Saiz

Sponsor: FP7

          The progress of a wide range of strategic fields from aerospace, construction, transportation or medicine depends on our ability to develop new light weight composites with outstanding mechanical properties. In this respect, natural materials such as bone, silk or nacre offer clear examples of how enhanced mechanical performance can be reached in low density materials through structural manipulation. Much attention has been paid to the idea of applying the design principles observed in nature to the fabrication of synthetic composites. One of the common design features in natural composites seems to be the use of layered and brick-and-mortar structures in which stiff layers or bricks are joined by thin soft interlayers. An interesting possibility is the replication of these designs using technical ceramics as the stiff phase and metallic alloys as the mortar. However, the challenge still remains on how to effectively translate natural principles to synthetic metal-ceramic materials and how to develop effective fabrication technologies to implement them in practical dimensions.

           In this project we have used a combination of freeze-casting and reactive presureless infiltration to fabricate layered alumina/Al₄Mg composites. By using ceramic particles with different aspect ratios it is possible to manipulate the layer thickness at the microscopic scale between few microns up to 50 µm and to control the structure of the ceramic layer (grain size and porosity). The resulting composites are lightweight, strong and tough. We have characterized their mechanical response as a function of their architecture. The composites can exhibit high flexural strengths and initiation fracture toughness (up to 800 MPa and 16 MPa.m^1/2). More interestingly, these materials exhibit several toughening mechanisms that result in a unique R-curve behaviour with fracture resistance values that can reach up 100 MPa.m^1/2. Crack propagation has been investigated with different techniques (optical microscope, in-situ SEM and x-ray tomography) to analyse the key toughening mechanisms. The observations have served to identify the role of phenomena such as crack propagation or bridging and to propose strategies to enhance their contribution. We also investigated the best approaches towards the characterization of the mechanical response of this kind of bio-inspired materials.