Nanocomposites for top-end Functional Applications: the CINN-LECAST experience
This lecture presents the accumulated experience in this field during the last 10 years at both CINN and LECAST, with special emphasis on “what we don’t know” and “questions to which we still seek answers”.
Currently ceramics and composites are not suitable for the industrial development of many technological innovations due to the impossibility of combining in the same material high mechanical performance with critical functional material properties. Nanocomposites entirely made of ceramic and metallic nanoscale particles or nanoscale phases, is a term that denotes a broad, new class of engineered materials where unique and otherwise unattainable properties can be revealed. The industrial applications of nanocomposites rely on the successful consolidation of these materials preserving their nanostructures. Traditional processing techniques have strong limitations of not being able to retain the nanoscale grain size if conventional nanoparticles are used as starting raw materials.
Scientific approaches to design and develop new advanced multifunctional materials in CINN and LECAST (STANKIN)
Bulk Superhard Nanocomposites
Previous experimental results of hardness of single phase nanostructured metals or metallic superlattices clearly indicate that hardness increases with decreasing grain size (below 100 nm) up to 5-7 times following a d-1/2 dependence known as Hall-Petch effect. The origin of superhardness in these composites is attributed to the following factors: i) the suppression of dislocations due to the small crystal size of nanoparticles; ii) the supermodulus effect in the nanocrystal core due to the compressive stress of the non-crystalline shell; iii) a strong interaction in the interface between different components. In this sense, the optimization of microstructural parameters of nanocomposites is a crucial subject, which has been treated in many studies.
In order to optimize the hardness of ceramic/metal nanocomposites, it would be necessary to increase the fraction of the hard phase through increasing the ratio r0/R (being R the radius of the matrix grain). It can be done by using ceramic matrices with very small grains. However, the sintering of such composites is not easy, considering that the growth rate of ceramic nanoparticles is very high, inducing exaggerated grain growth.
In order to overcome this roadblock, in STANKIN Megagrant project we use the singular properties of nanocrystalline metals embedded in dense rigid matrices opening a new avenue to prepare superhard materials suitable to be used for metallurgical applications where diamond-based materials do not work. The enhancement of the fraction of monodisperse nanoparticles in harder matrices can produce new superhard cermets (H>40 GPa) at reasonably low cost.
High Creep Resistant Nanocomposites
The cohesive strength and diffusion at interfaces often control engineering properties of structural materials such as hardness, yield strength, fracture toughness, creep and creep fracture, and fatigue behaviours both at low and elevated temperatures. The interface and grain boundary effects begin to dominate properties as the microstructure is reduced to the nanoscale. Therefore, the basic premise is that novel functionalities for mechanical engineering applications (hardness/strength/ductility/creep) designed into advanced nano and innovative composites by employing a suitable nanoscale or other fine scale architecture to control the properties. In this part of STANKIN Megagrant project, we seek to address the design of nanostructured and innovative nanocomposites not by just refining the polycrystalline microstructural scale, but rather by exploring how the electrical current assisted sintering allow as controlling the structure of grain boundaries of developed nanoceramics and ceramic-metal nanocomposite materials to achieve unusual mechanical and possibly other properties. By this way we try to fulfil the requirements of advance materials working in extreme conditions. In the case of nanoceramics, poor toughness results from the lack of dislocations and their mobility. To alleviate some of these problems, it was proposed up to now to reduce grain size. But the problem arises when these developed materials have to work at high temperatures under extreme conditions. Grain size reduction enhances grain-boundary sliding and grain-boundary diffusion related creep phenomena thereby inducing ductility at low temperatures. In the nanoscale regime, we envisage that grain-boundary sliding and creep phenomena dominate and control their mechanical properties. In our project we try to resolve this important issue by developing nanostructured nanoceramics with a bimodal grain structure in order to take profit of the nanostructure of some phases both as functional nanoparticles and as grain boundary modificators, by introducing residual stresses and by changing the nanochemistry of grain boundaries and as a consequence grain boundary diffusion rate at high temperatures.
Taking into account the nature of the pore and grain scattering, the conditions to prepare transparent ceramics resumed: i) we have to use nanometer sized powders; ii) the quality of the green body must be excellent free of defects, bubbles or impurities; iii) sintering by electrical current assisted sintering should eliminate porosity (<0.1%) with a pore size smaller than 10 nm; iv) the resulting grain size should be smaller than 1 micron and/or some texture with the c-axis perpendicular to the surface of the sample should be induced.
In order to attain the requisites, a series of specific steps carried out. First of all, high purity nanometer sized powders are required and have to be conditioned. They must be homogenised and subjected carefully to a forming process before sintering to avoid ambient contamination and processing defects, such as bubbles or large packing defects. The obtained green body must be thoughtfully calcined inside the mould to remove any organic residual. The subsequent sintering process by electrical current assisted sintering should densify the material to theoretical density, but keeping the grain size small. This later process was done by doping the starting powders with some other oxide like CeO2, that help to increase the green density and hinder the grain growth and/or change the mass transfer mechanism depending on the atmosphere (changing from Ce4+ to Ce4+).
Low thermal expansion nanocomposites
In our LECAST STANKIN laboratory we studied the phase equilibrium diagram L-A-S containing the compositions with negative CTE revealing the most desirably phases for the fabrication of the composite according to the chemical compatibility. This diagram reveal the temperature range in order to obtain dense bodies after sintering of the designed composites. And this is a key issue in this project as LAS phases with negative thermal expansion coefficient have relatively low melting points compared with silicon carbides and nitrides. Furthermore, the invariant points in the LAS system are relatively close to the pure LAS phase compositions. The temperatures of these points are with around 1000 ºC quite low. This is why the electrical current assisted sintering can be the only way to obtain dense LAS/SiC(for example) composites with tailored thermal expansion coefficient as this method can lower the sintering temperature which is a very convenient point in these systems avoiding the formation of a melted phase (glasses).
Once the most suitable compositions be chosen and the nanocomposite was be designed, it necessary to synthesize the NTE materials (LAS or cordierite). The nanocomposites was made from mixtures of these materials with second phases of nanometric size (nSiC, nSi4N3, CNF…). Especially the addition of CNF would help to improve the requirements of electrical conductivity. CNF also help to get better thermal diffusivity if the amount and dispersion of this component in the matrix of the nanocomposite is under control.
Once dense nanocomposites are obtained, they was be characterized from the physicochemical and structural point of view. It is mandatory to get to a final compromise between the CTE values of the composites and their mechanical properties in order to obtain a suitable material for the functional final application.