2023 has been an eventful year at WZR ceramic solutions – mainly because of the new 3d printer. With the CeraFab S65 (VPP-printer) from Lithoz we got a new AM method in our house, whose potential is used to the fullest for our public funded project Redox3D.

The ceramic material, that is used to achieve the goal of the project is the technical ceramic cerium oxide. Only a few might have heard about this fancy oxide, that’s why it’s not surprising that there are no commercial VPP-suspensions for this material.

The development for this project therefore starts from scratch.

How do we do this?

In this article we would like to give you a short behind the scenes of our engineering-team that develops the suspensions. But before that, we would like to give a short summary of the VPP process.

VPP is an abbreviation for Vat photopolymerization. It includes a term for a container and the chemical process of linking molecules (polymerisation) by light. In practice, this means that a polymer mixture (with particles) is placed in a glass tank that can be irradiated with light from below or above. If a building platform is then placed in this suspension, polymerisation is started by targeted exposure at the desired location (e.g. outline of the component). In this way, the component is built up layer by layer.

But up to the point of building a component, it’s a long road with a lot of tests.

Firstly, the choice of powder is important for the suspension production. As with other additive manufacturing processes, it is also important to pay attention to the particle size in VPP, as this has a major influence on the strength, strength and edge sharpness of the component. The refractive index of the particles also plays an important role in VPP, as it influences the curing depth of the suspension.

We are currently working with a cerium oxide powder that we grind to a particle size of approx. 1-2 µm. As soon as the cerium oxide powder has the right grain size, it can be weighed in together with the photopolymer, the photoinitiator and the co-initiator. As the photoinitiators become reactive on contact with UV light, the mixture must no longer come into contact with light. For this reason, we have also covered the windows in our VPP laboratory with orange-coloured foil. As the proportion of solids in the suspension should be as high as possible, we also use liquefiers to obtain a flowable suspension.

With the right mixing ratio of powder to organic material, the suspension has roughly the consistency of melted chocolate. This is significantly thinner than the pastes that we use for 3D screen printing or material extrusion, but is necessary so that the suspension can be distributed well and evenly in the tank. The device is then filled and the printing process can start.

Firstly, we “print” an adhesive layer over the entire surface of the building platform. The components should adhere to this and can then be easily and non-destructively removed from the platform. Only then is the first layer of the component printed on.

With VPP, the time required to print a component depends on the exposure time and the layer thickness. As cerium oxide, for example, has a high refractive index, the curing depth is low. We therefore work with layer thicknesses of 10 µm. In combination with a long exposure time, printing a component with a height of approx. 20 mm takes over 17 hours, so it is always a good idea to position several components on the platform at the same time. If there are overhangs, care must also be taken to ensure that support structures are introduced. Even with heavy/large components, enough support on the construction platform must be ensured, because nothing is more annoying than seeing the next morning that the long-awaited component is half-finished in the tub because it was too heavy and has fallen off. Our engineering team also had to experience this, as every additive mixture and every component behaves differently.

But it is not only the printing process that requires development. Once the green body has been removed from the build platform and cleaned, the organic material, which accounts for up to 50 % of the total volume, must be burnt out. In order not to damage the component, the debinding curve must be well matched to the corresponding additives. Finding the right combination of as fast as possible and as gentle as necessary is the crucial point here. A TGA/DSC or dilatometer measurement can provide good indications of the temperature ranges in which dwell times need to be incorporated. With our cerium oxide components, for example, debinding currently takes about a week.

The sintering process can only begin once the components have survived the debinding process without cracking. As with other additive manufacturing processes, the sintering temperature depends on the material and the grain size – in the case of our cerium oxide, for example, this is between 1600°C and 1650°C.

Anyone who visited our booth at Formnext 2023 will have already seen the terracotta-coloured components made of CeO₂. The first SEM images of our material have now also been added.

It is often generally said that the lower the porosity, the better the material. After all, it is known that dense ceramics sometimes have the highest strengths, little surface area for chemicals to attack and a high hardness. In one of our current projects, for example, the ceramic must also be gas-tight so that it can be used as a catalyst membrane.

But there are also cases where the porosity needs to be in a certain (higher) range, or generally as high as possible.

But why?

We would like to explain this in more detail in this blog post, starting by highlighting the use of ceramics as filters. Here, the properties of ceramics such as chemical inertness, temperature and wear resistance make them ideal candidates for many different applications in the chemical and pharmaceutical industries, as well as in gas, water and wastewater treatment.

In filtration, a gas or solution flows through the channels of a porous structure, retaining particles whose size exceeds the radius of the pores. The residue flows through the pores of the filter.

Ceramic filters with large pores in the micrometre range function with simple sieving action (macro- and microfiltration); if the pore size is reduced in microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO), other forces such as pressure or chemical potential are required for effective filtration.

Regardless of the filtration principle, separation always occurs through open pores. Their presence is critical to the function of filtration (dense membranes, in which mass transport occurs by ion diffusion through the crystal lattice, are excluded). Closed pores, on the other hand, are dead ends and obviously unfavourable for mass transport.

The opposite is the case with ceramics, which should have the highest possible thermal shock resistance. Whereas with filters the aim is to keep the open pore space as high as possible, for these materials there is a pore optimum – preferably closed.

In the case of ceramics, temperature fluctuations lead to thermal stresses and trigger microcracks that can damage the entire component.

Experimental studies of various dense and porous ceramics confirm that introducing a certain number of pores into the ceramic structure can potentially improve the crack-pore interaction and increase the fracture energy required for crack propagation. This means that by introducing a certain amount of porosity, we can improve the σ/E ratio (strength to elastic modulus) and thus optimize thermal shock resistance.

Another topic that we do not want to ignore in our article is the application in medicine. Here, some ceramics have a very special property: bioactivity.

Bioactivity is the property of materials to develop a direct, adherent and strong bond with the surrounding tissue, which is the key parameter for the development of further connections between implant and bone. Bone implants, for example, are often used to support bone growth as they dissolve in the body. Therefore, for many biomaterials, good biocompatibility and good strength and degradation rates are desirable.

In a porous bioactive ceramic component (implant or scaffold), the size of the pores and the interconnectivity between the pores are the critical factors for bone or living cell ingrowth. The optimal pore size for bone ingrowth has been reported to be 100-400 µm; long or narrow interconnects beyond this point hinder tissue ingrowth and should be avoided.

In summary, porous ceramics are used in many, very different areas. However, the formation of the pores cannot be left to chance. Rather, an attempt is made to create a porous structure whose pores are precisely defined in terms of size, shape and distribution.

AM techniques offer promising possibilities for the production of porous ceramics and the precise control of pore size distribution. In binder jetting, this is provided by controlling the print layer thickness and the ink applied. By varying the print layer thickness or using multiple print heads, membrane supports and membrane layers with different porosities can also be produced in a single manufacturing step. In addition, the use of burnout materials in the powder mixture is an option when a specific pore size is required. The pore size of 3D-printed porous ceramics can vary from one to hundreds of micrometers for fine ceramics and up to several millimeters for coarser structures or, if required, a functionally graded porosity structure, resulting in a solid material with remarkably high thermal shock resistance.

Many advanced AM processes are already used to produce customized bioceramic parts with specific structure and desired porosity, which have both biocompatible and bioactive properties for various applications.

On the printer, we will carry out part of the development work for our project Redox3D, which is publicly funded (Bundesministerium für Wirtschaft und Klimaschutz BMWK).

But what would the development of ceramics be without the accompanying analysis of the material?

In addition to strength testing and characterization using scanning electron microscopy, thermal analysis of the material is also important. For this reason, we have now invested in a second dilatometer to determine the thermal expansion. Especially in terms of time, this purchase is a great advantage for us, because most dilatometer measurements take more than 7 hours, so that usually only one sample can be measured per day. Now we can perform the measurements with both devices at the same time. In addition, with the connection for manual gas control, we can specifically simulate the application environment or simulate sintering under inert gas.

The project is devided into two main topics:

• One is the development of numerical models to evaluate the behaviour of various reactive structures in operation. This part of the project is being carried out by DLR.
• The other is the production of the reactive ceramic component using additive manufacturing. As part of the project, we will test various AM processes at WZR for manufacturing the complex geometry.

In the future, we will keep you updated on the progress of this renewable energy research project here and on our LinkedIn page.

We would like to thank the Bundesministerium für Wirtschaft und Klimaschutz (German Federal Ministry of Economics and Climate Protection) for funding this future-oriented project.

For WZR, the ceramitec in Munich is the most important trade fair. There we meet almost all our customers and establish important contacts with new customers. This year it was finally time again: after 4 years we could present our topics to an international audience. We had chosen a joint booth with Rösler CeramInno and concr3de, an exhibitor team that fitted very well.

We were able to present our entire range of 3D printing processes, the topic of temperature-resistant labeling (“CerTrace®”) and, last but not least, our cold-curing ceramics.

The best feedback for us was that the visitors see us as the experts in ceramic 3D printing. This confirms us in our work and also in the way we report on it in journals and on LinkedIn.

Formnext 2022

In recent years, Formnext has established itself as the most important 3D printing exhibition. This year, WZR exhibited 3D printed parts and material developed by us (powder and ink for the binder jetting process) at the concr3de booth. Also present was a 92 Al2O3 that we developed together with Nabaltec AG and that will be distributed by concr3de from the beginning of 2023. By using the specially developed powder mixture and our particle-containing ink, strengths of over 150 MPa and porosities of less than 7% by volume can be achieved in the binder jetting process. If you have any questions about the binder jetting process, please contact Michael Lüke and Hakimeh Wakily from our team.

At the WASP booth, we showed a 3D printed copper component and a copper paste. Here, the feedback shows us that paste-based 3D material extrusion offers some advantages over paste-based processes, especially for metal 3D printing. If you have any questions on this topic, Niklas Brünker from our team is your contact.

Although we were not an official exhibitor, we were able to meet many customers and partners from the “3D printing scene” again and make important new contacts. For us it was very nice to see that more and more ceramic manufacturers are exhibiting at the show.

What is the task of a ceramic membrane?

Ceramic membranes are part of membrane reactors, the use of a catalyst on both sides is mandatory. Membrane reactors are used to selectively separate gases – mostly low molecular weight gases such as hydrogen or hydrocarbons – to efficiently control chemical reactions.

Die Hauptaufgaben der keramischen Membran in diesem System sind vereinfacht in zwei Bereiche eingeteilt:

• Firstly, they work as a permeation barrier, meaning their structures are gas-tight and thus impermeable to the molecules involved in the reaction.
• But secondly, they are a transport medium for certain ions. For this reason, they are also referred to as ceramic electrolyte membranes.

Basically, a distinction can be made between four different membrane functionalities:

• Oxygen-ion (O2-) conducting membranes.
• proton (H+)-conducting membranes
• so-called MIEC membranes (mixed ionic-electronic conducting membranes) for O2-/e–conducting and H+/e–conducting.

The appropriate membrane type is selected depending on the application: Pure ionic-conducting membranes can be used as electromotive generators (fuel cell) or as electromotive consumers (electrolysis cell), MIEC membranes do not require electrical contact and are recommended for industrial processes. The following figure shows schematically the different types.

Why are ceramics used for this purpose?

Chemical reactions are always equilibrium reactions. This means that the reactants and products of the reaction are in a dynamic equilibrium defined by thermodynamics, the enthalpy of formation or decomposition, and the kinetics of the outward or reverse reaction. Nevertheless, there are equilibria in which the starting materials are preferred because they are energetically more favourable (endothermic reactions). In this case, the kinetics can be “outsmarted” by continuously removing a (side) product from the reaction area, thus preventing the reverse reaction for lack of a reaction partner, which is particularly useful in gas reactions. In membrane reactors, the same trick is used by diffusing the permeable ions through a membrane.

However, the diffusion of permeable ions through the membrane is passive and depends on the partial pressure gradient of the ion species within the membrane. The degree of mobilization of the ions, such as catalytic ionization or diffusion rate across the membrane, determines the delta of this partial pressure gradient and thus the efficiency of the membrane. These partial aspects, as well as the usually strongly endothermic ionization step of the chemical reaction, gain speed with increasing temperature. The most efficient process performances of membrane reactors are observed at operating temperatures between 500 °C and 900 °C. It is now clear why the membrane must be made of a heat-resistant material such as ceramic.

Which ceramics are used?

The development of an efficiently ion-conducting ceramic is basic research at a high level, carried out by universities and major research institutions. The chemical basis of crystals is a structure in which metal atoms face an oxygen sublattice. Different metal oxides are mixed and synthesized in such a way that valence differences lead to the formation of vacancies in the oxygen sublattice. Ion conduction can now take place across these vacancies – for example, O2 ions formed at the membrane, which are introduced into the membrane by activation and passed through the membrane via the vacancy structure. Catalytic recombination then takes place on the backside of the membrane and the recombination molecule is quickly flushed away, keeping the O2 partial pressure gradient in the membrane high.

Oxygen holes in the membrane material are also required for proton conduction, since the protons – according to one model – “ride” through the membrane as hydroxide ions on oxygen ions. O2–conducting and H+–conducting membranes thus have the same structural basis.

About a hundred different material compositions have been described, mostly derived from the perovskite or bixbyite structure. An example of a perovskite variant is the barium zirconate-derived compound BaCe0.2Zr0.7Y0.1O3 (BCZY271), in which the zirconium layer is proportionally replaced by cerium and yttrium.
An example of a bixbyitic variant is lanthanum tungstate La6-xWO12-δ, in which about 1/3 of one of the lanthanum layers is replaced by tungsten. Both ceramics have the necessary thermal and thermochemical stability to withstand the aggressive reaction conditions in the membrane reactor.

How is a ceramic membrane manufactured?

Optimized membranes should be as thin as possible to keep diffusion paths small. Membrane thicknesses of 10-30 µm show efficiency maxima, but must be 100% gas-tight. Such thin membranes are not fracture resistant enough unless supported over the desired large area. A gas-permeable, mechanically stable support is needed to periodically support the membrane.

This is where additive manufacturing processes join the manufacturing process to build the membrane and support structure in as few steps as possible. Particularly in a two-step setup, care must be taken to ensure that the membrane and the support part are made of the same or a very similar material so that less stress builds up during sintering, leading to bending or cracking, but also so that the crystal structure of the membrane is not contaminated or completely altered.

The next figures show membrane support structures made of lanthanum tungstate in the development stage before the catalyst layers are applied. The manufacturing process is 3D screen printing, which can be used to build dense and filigree structures. A flat channel structure is realized using a filigree transverse wall as a support structure. In transmitted light (center), the homogeneity and low layer thickness of the membrane can be seen. The scanning electron microscope visualizes the low thickness (right).

An example of the two-stageed production of membrane and support structure components is the combination of tape casting for the membrane and 3D material extrusion for the support structure.

If you would like to know more about additive manufacturing of ceramic membranes, please contact us.

If you would like to know more about the development of membrane materials, we recommend the homepage of the Institute for Energy and Climate Research (IEK) of the Forschungszentrum Jülich.

¹⁾ Deibert, W., Guillon, O., Ivanova, M. E., Baumann, S., Meulenberg, W. A.; Ion-conducting ceramic membrane reactors for high-temperature applications; Journal of Membrane Science, 543, 2017, 79-97

Everyone has already come across supposedly expensive ceramics in everyday life: Be it the Sunday dinnerware, the ceramic knife or the bathroom sink. If you compare these prices with those of everyday plastic objects – garden chairs, mugs or toilet seats – you will quickly get the impression that ceramic objects are always more expensive than those made of plastic. Is this justified and is it true at all? To get to the bottom of this question, we first need to take a closer look at the two groups of materials.

What makes ceramics so special?

Not all ceramics are the same, and in fact they are not only present in objects we encounter every day.

Because: the areas in which ceramics are used are manifold. As an example, we can consider the following areas in which ceramics are used:

• as building ceramics: e.g. bricks, roof tiles or clinkers
• as refractory ceramics: e.g. furnace linings
• or as technical ceramics for thread guides in the textile industry

In general, ceramics can shine with different positive properties. Technical ceramics in particular often have high hardness, high wear resistance and high corrosion resistance. In particular, the usually high melting temperatures of ceramics make them the material of choice in areas where temperatures can reach up to 1600 °C. Within the ceramic material group, however, opposing properties can also occur when comparing two materials: Silicon carbide, for example, can be used as a resistance heating element in industrial furnaces, while porcelain is used as an electrical insulator for high-voltage lines.

This is, of course, only a small overview of the material, but the (positive) properties can still be clearly seen.

What are plastics and what are they used for?

The word “plastic” in the vernacular usually refers to synthetic plastics, which can be described as artificially produced polymers (linking of smaller molecules).

Polymers can be found in a wide variety of applications and have become an indispensable part of everyday life – the PET bottle for transporting liquids or the housing of the computer mouse with which you probably clicked on this article.

This broad field of application is no coincidence. Plastics can be produced comparatively inexpensively from crude oil and can have a variety of properties depending on the type. There are rigid, flexible or elastic plastics – all with their own application niches.

What influences the price of the components?

1. the raw material price

First and foremost, the raw material of the material to be used in the respective component must be obtained. Raw materials for ceramics are usually extracted in open-pit or underground mining. Depending on the material, it must then be processed further. There are major differences here:

Clay – a mixture of various layered silicates – as used for tiles is probably one of the cheapest ceramic raw materials. This is partly because the material does not have to be “pure” and could, quite casually, be put “straight from the open pit into the kiln.” By contrast, the high-performance ceramic zirconium oxide, which is used for hip joint implants, for example, already costs a thousand times more, since the starting material (usually zirconium) still has to be processed after mining. Even more cost-intensive is, for example, yttrium-barium-copper oxide, which has to be synthesized at great expense, but is then superconducting at -181°C. In this case, the costs are even higher. Here, the costs again amount to about 100 times those of clay. The problem with all these raw materials: Only very few of them can be easily recycled after use.

As already mentioned at the beginning of this article, plastics are mostly obtained from one raw material: Crude oil. Due to the wide range of applications and the great demand for energy, crude oil is nowadays extracted in very large quantities and is comparatively cheap. The hydrocarbons contained in crude oil form the basis for the production of most polymers. In addition, some plastics can be easily recycled, as demonstrated by the return of deposits on beverage bottles.

Even if price differences exist for plastics, they do not vary as much as for ceramics. Even high-performance plastics such as PEEK are not more expensive by a factor of 1000 than conventional plastics such as PET or ABS.

2. The cost of sales

For the production of ceramic components, first and foremost a so-called green compact (ceramic mass brought into shape) must be produced. Green compacts can be produced by various processes such as pressing, extrusion or casting. 3D printing processes such as binder jetting or material extrusion also produce a green compact in a first step. With a few exceptions, this green compact must now undergo a firing process known as sintering. This part of the manufacturing process is comparatively expensive, as high temperatures (approx. 1600°C for aluminum oxide) have to be reached and kept.

While plastic components are also manufactured by pressing, extruding or casting, the sintering process is omitted with a few exceptions. Post-processing is usually not necessary.

Conclusion

As is so often the case, the initial question cannot be answered in a generalized way. Yes, in principle it is obvious: the raw material costs for ceramics are in the vast majority of cases higher than those of high-performance plastics, and the manufacturing costs are also usually higher – especially if the ceramic components are not mass-produced items such as bricks. When making the comparison, however, it is important to know which materials are being considered – and where they will be used. This is because it is important to note that the higher (purchase) costs of ceramics can be accompanied by considerable added value in some areas. Not only can the service life of a machine be extended by replacing plastic elements with ceramics – plastics will probably not be able to replace the porcelain insulator of the high-voltage line easily.

The fracture of a ceramic occurs spontaneously. If one tests metals or plastics in this test arrangement, one sees that the specimens are very clearly deformed before they fail.

If you compare the strength and the force-deformation curve of different ceramics, you can see very clear differences: A ceramic with relatively low strength (approx. 50 MPa) is, for example, cordierite, a technical ceramic which, due to its very low thermal expansion, is often used for applications subject to temperature changes (e.g. catalyst carriers in cars). A ceramic with significantly higher strength is aluminum oxide (approx. 350 MPa), which is used, for example, as a sealing washer or electrical insulator. Another significantly higher strength is achieved with a zirconium oxide ceramic, bending strengths of over 1200 MPa are possible. It is frequently used in dental ceramics (crowns and bridges) or implants.

Especially when it comes to dynamic loading, measuring the bending strength is no longer sufficient. In reality, components are often loaded to a certain extent and then unloaded again many thousands or millions of times. If we take the example of an implant for human hip joints into account, there is an impact load on the joint head with every step. None of these impacts exceeds the strength of the ZrO₂ ceramic, but calculated over a period of e.g. 20 years and a daily number of steps of 4000, the load is over 29 million “impacts”. Here, a new characteristic value must be used, for which crack toughness has proven itself in the field of ceramics.

The destruction of ceramics occurs through cracks in their interior. The starting point are flaws created, for example, by molding, drying or sintering. When ceramics are loaded, these cracks grow. Loads can be chewing movements in dental ceramics, walking with hip implants or temperature changes of ceramics, e.g. in waste incineration plants. Under defined conditions, such loads are simulated in a bending test, in which the force is increased until the ceramic breaks. If only a force is applied that does not completely destroy the ceramic, this is referred to as a subcritical load. On closer inspection, however, it can be seen that such loads cause existing cracks to grow further. This is referred to as subcritical crack growth. There is a threshold value above which the crack length leads to destruction of the ceramic, this value is called the kIC value. Other terms for this are the “critical stress intensity factor” or the “crack toughness”. This value is given by a number that typically ranges from 0.5 to about 20, and the unit is MPa m-0.5. Ceramics that have high crack toughness are ZrO₂ or silicon nitride. Fiber-reinforced ceramics have even higher fracture toughness due to their structure.

There are several options to choose from for testing crack toughness:

A very simple but also very imprecise method to determine the kIC value is to measure the Vickers hardness. This involves pressing a pyramid-shaped diamond syringe into the surface of a specimen. The hardness is derived from the cross-section of the indentation. To obtain a statement about the crack toughness, one can measure the crack lengths in the corners of the indentation and calculate the kIC value via a formula. The following figure shows such a hardness indentation in an Al2O3 ceramic: it is well visible that the cracks at the 4 corners of the indentation are of different lengths and the determination of their correct length is very difficult. Thus, the measurement error is large. Moreover, different formulas are available to convert the crack lengths into the kIC value.

WZR uses a slightly more complex but many times more precise method to determine the fracture toughness. The measured variables are the hardness, the modulus of elasticity, the strength and the strength after a defined damage to the specimen. Especially for this “defined damage of the specimen” there are again several possibilities. A great amount has been described in the literature on this subject as well. In the past, WZR has carried out series of tests to evaluate the fluctuation of the measured values with the various damage methods. Very consistent results at comparatively low cost could be obtained by damaging the specimen with a Vickers indentation before measuring the flexural strength. This method is abbreviated to ISB, which stands for “Indentation/Strength in Bending”.

If we compare the kIC values of different ceramics (see following table), we see that brittle ceramics such as aluminum oxide have a relatively low kIC value, but highly stressable ceramics such as zirconium oxide or silicon nitride have very high kIC values. In principle, the higher the kIC value, the more damage-tolerant the material. So if damage tolerance is an important characteristic value for a component (as in the example of the hip joint), a suitable material can be identified by comparing the kIC values of different ceramics.

Literature data and values in the manufacturers’ data sheets are always based on the respective raw materials, production parameters and sintering conditions. If something changes in these parameters, e.g. an alternative raw material is used or the production is changed from injection molding to axial pressing, this will have an effect on the crack toughness. WZR offers testing of materials as an independent service provider certified according to DIN ISO 9001:2015. In addition to determining measured values, we also offer our customers support in interpreting the measured values.

Finally, the measured values are sometimes outside the expected range using the same raw material or show an unexpected scatter. Here, we are happy to support you in finding the cause in the manufacturing process and help to optimize the processes in order to achieve an improvement in the fracture toughness.

So-called hybrid materials represent this combination of two different types of materials and utilize their advantages while compensating for their disadvantages. However, there are hybrid materials of different types, which are presented in the following.

Components with homogeneous mixing ratio

In the industry, hybrid materials are used in a wide variety of areas. In cutting and chipping tools, the hardness can be increased by a ceramic, while the metal exhibits a tougher behavior. The ceramic is held together by the metal.

The combination of cross-material blends and the geometric freedom of 3D printing introduces in a new generation of hybrid components. For example, in addition to the high hardness of a cutting tool, it is also possible to optimize heat transfer through internal cooling channels.

WZR conducted initial trials using a 316L stainless steel as the matrix and aluminum oxide as the hardness enhancing additive. The material was mixed in different proportions and printed through our WASP 2040 Clay material extrusion printer.

Due to the metallic matrix, tensile bars were printed as test specimens and then sintered.

Initial results showed the following microstructure after sintering:

Microstructure of a hybrid material

Individual aluminum oxide particles can be seen in the sintered, metallic matrix. Nevertheless, isolated pores can be seen, which is why further sintering tests must be carried out.

The tensile test showed a more ceramic-typical behavior: The specimens broke in a brittle fracture at the clamping surfaces.

In principle, an altered deformability behavior is not surprising with the combination of ceramic and metal. On the one hand, the aluminum oxide achieves a higher hardness but on the other hand it impacts the elastic deformability and thus favors a brittle fracture. In future tests, we will therefore resort to test methods for brittle materials, such as the 3-point bending test method.

Multi-material with clear border

In addition to the mixed distribution of both components, materials can also be present in clearly defined areas in the component. Such a multi-material structure makes it possible to print electrical conductors (made of MoSi, for example) in an insulator. A practical example of this can be found here (german version).

Multi-material structure with clear delineation

However, multi-material can also support the process: In order to be able to remove support structures more easily, it is possible to print them from a water-soluble material. In this way, they provide support during the printing of complex structures, but can be easily removed from the component afterwards.

Material with function gradient

The process of additive manufacturing also makes it possible to vary the mixing ratio of the components in a component – an option that injection molding or casting and pressing do not offer with the same precision and accuracy. In this way, components can be produced with a so-called “functional gradient”. This means that the distribution of the different material components is not the same throughout the entire component, but that the ratio of one material to the other increases over a distance. This makes it easier, for example, to absorb thermal stress as the temperature rises.

In order to produce a material with a functional gradient in the material extrusion process, a mixing extruder can be used which mixes component A and B. The mixing extruder can react accordingly in areas where a higher proportion of A is required. In these areas, the mixing extruder can react accordingly and thus increase this proportion. In this way, for example, a component can be produced with a higher hardness at the edges but a high toughness on the inside.

Combination of several multi-material concepts

But not only material extrusion can be used for the production of hybrid components. Binder jetting with particle-filled inks opens up further possibilities for multi-material concepts: local modification of the microstructure. For example, appropriate microstructure reinforcement can be applied to areas where reinforcement of the component is required. With multiple print heads, it is even possible to print more than just one particle-filled ink, thus introducing yet another material into a component.

Metal-organic compounds are also conceivable. In this case, the secondary material is not present in particle form but as a metal-organic compound in the ink, which forms metal oxides during the sintering process and is subsequently finely distributed in the structure.

Now, to return to the initial question:  As the previously listed possible applications show, there are good reasons for selecting a multi-material structure. However, it must be noted that various physical quantities, such as the thermal expansion of the two materials, must be adjusted to each other. In addition, compromises may have to be made with certain material combinations, as can be seen in the example of MEX tension rods.

During the coating process the properties of the powder get improved or changed. The change of the surface properties and / or the functionality of fine particles or powders is important for many different industrial purposes and it extends the possibilities to use coated ceramic powders.

Powder coating is of special importance for the additive manufacturing of ceramics. Very fine powders tend to agglomerate when exposed to air. This worsens the processability e.g. reducing the powders flowability. It is possible to influence properties like the flowability or wettability of the powder by treating the surface of it. This can be useful to customize a powder according to the use case.

There are various methods and processes to coat particle surfaces:

Silanization

The silanization is a common and widely used process for the coating of particles which is based on wet chemical sol-gel technology.

The target flowability and wettability can be customized by surface treatment of the particles with silanes or silane-based compounds.

Silanes are bifunctional compounds consisting of hydrolysable reactive OH groups and stable organofunctional R groups. The attachment to the particle surface is achieved by hydrolyzable groups. Meanwhile, the surface functionality of the powder is determined by the organofunctional R groups. Parameters that often have to be optimized are hydrophobic properties or the achievement of a certain wettability.

Hydrophobicity of the particle surface helps to avoid agglomeration of the powder or powder mixture. This improves the trickle of the powder, which ensures a constant layering during printing.

These properties are used by WZR for…

• …3D printing of ceramic components using the binder jetting process. By using modified powder, an optimal powder layering is achieved, on which the print quality depends and affects the properties of the printed components, e.g. green strength, density and porosity of the final product after sintering.
• …3D printing of ceramic-filled polymer systems according to Vat Photopolymerization  (VPP) process. For this application, the required wettability of ceramic particles in a polymer matrix is of great importance. This ensures optimum compatibility at the interface of the polymer matrix and the ceramic powder used as filler. Coated powder is easier to incorporate into a polymer or resin than uncoated powder. The optimum wetting of the ceramic powder with the polymer matrix can be specifically adjusted by certain organofunctional R groups of a silane. Silanization of a powder as a filler also makes it possible to increase the degree of filling of the powder in the polymer matrix. This increases the ceramic content in a composite material, resulting in increased green strength.

Binder coating

In this process, the particles are coated with a binder. A binder-containing system based on water or solvent serves as the coating solution. Powder coated in this way is also used in the binder jetting process. In this way, a higher green strength of the printed parts can be achieved.

Powder nanoparticle coating

Another application is the coating of a ceramic powder (large particles) with another powder (smaller particles). This gives the coated ceramic powder the properties of the coating powder.

The coarse particles are coated with the coating suspension by spraying. After evaporation of the liquid, the nano particles are attached to the surface of the large particles. This attachment can be achieved by two methods:

• By covalent bonding, via wet chemical sol-gel process. The coating suspension contains nano- and microscale particles dispersed in a sol.
• By direct binding by a binder. For this purpose, a binder-containing dispersion is prepared.

Application at WZR

One process for coating powders, which we dealt with last year, was coating by precipitation of SiO2 onto FeSi powder. Such powder is used, among other things, in magnetic field concentrators. The experiments were accompanied by an observation of the material in a scanning electron microscope, which allowed us to assess the quality and thickness of the coating.

We are currently working internally, but also with cooperation partners, on further exciting powder coatings and look forward to sharing them with you soon.