Modelling of Tape Casting for Ceramic Applications

Functional ceramics find use in many different applications of great interest, e.g. thermal barrier coatings, piezoactuators, capacitors, solid oxide fuel cells and electrolysis cells, membranes, and filters. It is often the case that the performance of a ceramic component can be increased markedly if it is possible to vary the relevant properties (e.g. electrical, electrochemical, or magnetic) in a controlled manner along the extent of the component.

Such composites in which ceramic layers of different composition and/or microstructure are combined provide a new and intriguing dimension to the field of functional ceramics research. Advances in ceramic forming have enabled low cost shaping techniques such as tape casting and extrusion to be used in some of the most challenging technologies.

These advances allow the design of complex components adapted to desired specific properties and applications. However, there is still only very limited insight into the processes determining the final properties of such components. Hence, the aim of the present PhD project is to obtain the required knowledge basis for the optimized processing of multi-material functional ceramics components.

Recent efforts in the domain of ceramic processing are generally focused on the control of the microstructure while the importance of shaping is often underestimated. Improved performance requires the design and shaping of both controlled architectures and microstructures. Novel functionally graded ceramic materials may be formed by multilayers or adjacent grading of different ceramic materials.

Such grading is often desired for optimal performance. An example is when there is a gradient in temperature or chemical environment along the component during operation; in this case the properties of each section of the component should be optimized for the local environment by grading. The grading may be between entirely different ceramic materials or merely a minor compositional alteration within one type of material.

However, there are several challenges to be met for the successful fabrication of such complex structures. Rheological properties play an extremely important role for the co-processing of more than one material. Only by matching the rheological properties of the different pastes, a reproducible and well defined gradient composite will be formed.

Tape casting involves the casting of a slurry onto a flat moving carrier surface. The slurry passes beneath a knife edge (doctor blade) as the carrier surface advances along a supporting table. The solvents evaporate to leave a relatively dense flexible sheet that may be stored on rolls or stripped from the carrier in a continuous process.

Today, multilayers are achieved by laminating layers of different materials on top of each other. The challenge is to be able to tape cast layers of different materials simultaneously both stratified in the horizontal and in the lateral direction. Understanding how to achieve that and perfection of such a technique will open up a large variety of applications.

General challenges with this process is, as mentioned, controlling the rheological properties of the slurries/pastes as they strongly affect the process and the quality of the final product, maintaining uniform composition during the process and controlling/understanding the shrinkage in drying and sintering.

Furthermore, understanding the tape delamination and film cracking of multilayers as well as of interface fracture modes in multilayers is also an important topic that needs to be considered and understood. In the present PhD thesis the focus is on the numerical modelling of the tape casting process of functionally graded ceramic materials for fuel cell applications as well as magnetic refrigeration.

Models to simulate the shaping of monolayer/multilayer and graded materials by tape casting are developed. The emphasis is on analyzing the entry flow of multiple slurries from the reservoir into the doctor-blade region as well as the exit region where a free surface (meniscus) forms. This encompasses a detailed fluid model capable of tracking the material flow/deformation taking the formation of the free surface into account.

In the work it was chosen to focus on developing analytical/numerical flow solvers in both Ansys Fluent and Matlab. Analytical approaches for fluid flow analysis in the tape casting process showed that a relative good agreement could be achieved between the results of the modelling and the experimental data.

The study, furthermore, demonstrated that the aforementioned agreement was increased by improving the steady state model with a quasi-steady state analytical model. In order to control the most important process parameter, tape thickness, the two-doctor blade configuration was also modeled analytically.

The model was developed to control the tape thickness based on the machine configuration and the material constants. Many of the affecting parameters in the process were embedded and they can easily be varied to evaluate their influence. This study showed that using computational fluid dynamics (CFD) the process can be modeled with more details in order to better control the produced tapes.

Very importantly, the free surface of the ceramic as leaving the doctor blade region was modeled. The rheological behavior of the ceramic slurry was also taken into account. The influence of the main process parameters, i.e. the substrate velocity, the initial slurry load, and the doctor blade height, were investigated.

Based on the developed model, one phenomenon inherit to the process called side flow was also modeled. The results showed that to reach a desired uniform tape the side flow factor should be kept as close as to the value of one. The impact of the process parameters were also discussed in details in order to control the side flow, and consequently the tape thickness.

Moreover, a CFD model was developed to simulate multiple flow of the ceramic slurry in tape casting. The simulation was aimed to analyze the production of functionally graded ceramics (FGCs) which are used for magnetic refrigeration applications. Numerical models were developed to track the migration of the particles inside the ceramic slurry.

The results showed the presence of some areas inside the ceramic in which the concentration of the particles is higher compared to other parts, creating the resulting packing structure. And finally a numerical code was developed to simulate the drying process. The results showed that the mass loss due to the evaporation is increasing close to linearly with the drying time corresponding to an almost constant drying rate.

However, the rate starts to decrease after some time in the simulation. This is in good agreement with the real life process where the drying categorized into two stages: (1) constant rate period (CRP), in which the rate of evaporation per unit area of the drying surface is independent of time, (2) falling rate period (FRP), in which the evaporation rate is reduced, as a consequence of low migration of the water from the bottom layers to the top ones due to diffusion (which is highly dependent to the temperature).