Research Areas

Advanced Ceramic Composites

Advanced Ceramic Composites

Ceramic Matrix Composites (CMC) with fibre reinforcement or other composite materials as layered structures with stiff and weak components provide an exemplary way to increase the fracture toughness of engineering ceramics, while other superior properties of ceramic structures are retained. The behaviour of these composites is strongly dependent on the components used. A smart combination of reinforcement, interface and matrix materials leads to sophisticated composites achieving highest performances especially in severe environmental conditions. This enables the designing engineer to adjust the composite properties specifically to various application requirements and mechanical load conditions. The major scopes of ceramic composites can be divided into three areas: Advanced Ceramic Composites for biological applications (e. g. bioceramics), Advanced Ceramic Composites for room and high temperature applications and Advanced Ceramic Composites for applications in extremely corrosive environments. Beside oxidation and corrosion resistance, the mechanical properties of the composites as e.g. fracture toughness and damage tolerance are of major interest. Thus, the mechanisms being responsible for the damage tolerant behaviour have to be adjusted to achieve crack deflection and high energy dissipation in general. A focus lies in the investigation and interpretation of mechanical behaviour of ceramic fibre reinforced ceramic matrix composites (CMC). In all variations, whether short- or long-fibre reinforced, the interface or the matrix has to meet this challenge. In consequence, different CMC strategies have been followed, i.e. Weak Interface Composites (WIC) or Weak Matrix Composites (WMC). The WMC-concept is characterised by a fibre dominant behaviour, while the interface is of minor importance. The opposite case is followed by the WIC-concept which is governed by a weak interface taking also into account that the mechanical properties of the fibres (reinforcements) generally exceed the matrix properties. However, it has to be pointed out that these CMC strategies represent idealised models and the optimized performance is reached somewhere in between these concepts meeting the requirements of different applications. Research activities in this area are given below.

CMC 

Room and high temperature behaviour of CMC

Due to their superior specific strength and toughness, non-oxide ceramic composites (CMC) can be considered as materials of choice for highly demanding thermostructural lightweight applications as e. g. components for the re-entry phase of space vehicles. Here, we investigate comprehensively the mechanical behaviour of CMC as a function of processing and thereby induced microstructural properties. One main research focus is the understanding of the different behaviour of oxide and nonoxide composites. The the detailed investigations include evaluation of strength, creep and fatigue as well as environmental effect on the overall performance of CMC. The following equipment is available also for service:
· Tensile, compressive, bending, shear tests up to 1600°C in different atmospheres
· Cyclic, quasistatic, and static testing mode
· Fracture mechanics
· Micromechanical testing
· Contactless measurement of local strain up to 1600°C

CMC 

Modelling of mechanical behaviour of advanced ceramic composites

The development of new mechanical models for ceramic matrix composites is one main interest in our investigations. Several modelling approaches are adapted to describe experimentally observed complex laminate behaviour at ambient and high temperature conditions. Linear elastic properties of embedded single plies are derived by applying inverse laminate theory and used for prediction of the elastic composite performance. The nonlinear composite behaviour is simulated by phenomenological models based on continuum damage mechanics. Focusing on matrix dominated stiffness degradation, the progression of damage is predicted by using damage potential and scalar damage variables. Ply-level failure models are also developed to predict local brittle failure within one or more plies and the resulting stiffness degradation as well as the ultimate composite failure. The model approaches are used for suitable designs of ceramic composite materials and structural parts. The hierarchical, highly heterogeneous and anisotropic structure of CMCs requires non-classical approaches of analysis and numerical calculation. On the basis of the finite element code MARC and by implementation of our recently developed material models we are able to cover several characteristics of the material behaviour, such as stiffness degradation, inelastic behaviour, brittle, and non-brittle failure, respectively. Linear and non-linear structural calculations as well as analysis of coupled structural-thermal problems are our principal expertise in numerical simulation of ceramic matrix composites. Also, the properties of layered structures with stiff and weak layers of different geometrical extension are calculated using finite element models in order to explain the experimentally measured properties. The processing as well as the choice of components follows a bionic approach and results in composites containing high volume fraction of ceramic and low content of polymeric interfaces. The processed composites show impressive combination of stiffness with enhanced toughness and high strength which is described experimentally, analytically and numerically.