Multiscale microshaping of functionally graded polymer-derived SiCN ceramics

Polymer-derived ceramics (PDCs) exhibit excellent properties and are compatible with many shaping techniques due to the liquid character of the preceramic polymers (PCPs). Furthermore, their properties are adjustable by modifying the precursor composition and controlling the processing conditions. In this thesis, poly(methylvinylsilazane) (PMVSz) is chosen as the precursor basis and varied amounts of divinylbenzene (DVB) are added to increase the carbon content, which significantly increases the electrical conductivity of the silicon carbonitride (SiCN) obtained after pyrolysis. Based on molding and pressureless processing, samples are manufactured with varied homogeneous composition and functional properties as well as with engineered property contrasts, so-called functionally graded ceramics.Starting with shaping aspects, several mold materials, mold fabrication techniques, and the casting and thermal processing are discussed in the first part. The liquid precursor formulation is directly pipetted into the molds, either CNC-machined PTFE for bulk samples or anisotropically etched Si for microsized parts. Thermally cured bulk green bodies (GBs) are removed from the PTFE molds before pyrolysis whereas the monolithic and resilient character of Si allows for an in-mold pyrolysis which eliminates the need for mold release of fragile GBs. While the Si molds define the parts' general shape, sacrificial 3D microstructures are optionally added onto the mold bottom by two-photon polymerization (2PP). After pyrolysis, the PDC parts are obtained loose in the mold with sub-µm resolution features replicated as imprints.The second part focuses on the processing conditions and the tuning of the PDCs' bulk properties. Thanks to a thermal initiator, the PMVSz-based precursors are cured without pressurization within 30 min at 200°C. The pore-free GBs, filled with 0-60 wt.% DVB, are converted to SiCN with varied carbon (C) concentration by pyrolysis at 800-1400°C under Ar atmosphere. Dense and defect-free PDCs are obtained whose free carbon content is adjusted in the range 15-25 wt.%. The influence of the pyrolysis temperature on the ceramics' microstructure is analyzed and correlated to the electrical conductivity, tunable across 10 orders of magnitude up to 0.4 S/cm. Very high characteristic flexural strength of up to 1.65 GPa is observed and the SiCNs' cytocompatibility demonstrated.Thirdly, PCPs with and without DVB-filling are combined by consecutive casting to fabricate functionally graded SiCN parts. The dissimilar precursors are joined in vertical or lateral arrangement in dedicated PTFE molds, typically with an intermediate thermal curing step. Monolithic and defect-free property-contrast PDC parts are obtained after pyrolysis which exhibit seamless transitions and no signs of delamination at the interface. The casting order and extent of intermediate curing are varied, showing that composition gradients are adjustable from sub-µm to mm-range transition lengths. Significant contrasts in C-concentration and consequently electrical conductivity are observed within monolithic samples entirely consisting of SiCN. Finally, a cm-scale plate with two seamlessly integrated contrast regions is designed and fabricated. The all-SiCN parts have an insulating matrix and two conductive feedthroughs. An LED connected to the areas with higher C-concentration is powered through the ceramic and lit up.

Influence of carbon enrichment on electrical conductivity and processing of polycarbosilane derived ceramic for MEMS applications

Jürgen Brugger, Jonas Grossenbacher, Rosario Maurizio Gullo, Markus Niederberger

An analysis about the effect of carbon enrichment of allylhydridopolycarbosilane SMP10® with divinylbenzene (DVB) as a promising material for electrical conductive micro-electrical mechanical systems (MEMS) is presented. The liquid precursors can be micropipetted and cured in polytetrafluoroethylene (PTFE) molds to produce 14 mm diameter disc shaped samples. The effect of pyrolysis temperature and carbon content on the electrical conductivity is discussed. The addition of 28.7 wt.% of DVB was found to be the optimum amount. Carbon was preserved in the microstructure during pyrolysis and the ceramic yield increased from 77.5 to 80.5 wt.%. The electrical conductivity increased from 10^−6 to 1 S/cm depending on the annealing temperature. Furthermore, the ceramic samples obtained with this composition were found to be in many cases crack free or with minimal cracks in contrast with the behavior of pure SMP10. Using the same process we demonstrate that also microsized ceramic samples can be produced.

Elsevier Sci Ltd2014

Mechanical properties and microstructure of a high carbon steel

Iva Tkalcec Vâju

This work is focussed on a high carbon tool steel used for the production of files. The mechanical properties of the files depend on a hardening thermal treatment that produces a martensitic structure. Thus, the motivation for this work is technological: we want to understand the effects of two types of thermal treatment on the file's mechanical properties. The first is a traditional thermal treatment in a cyanide bath (CN), while the other is performed in a conveyor belt furnace (BF). The files resulting from BF treatment have sometimes a poorer performance. In order to improve the quality of BF files, we study the relationship between the mechanical properties and the microstructure resulting from a particular thermal treatment. Transmission electron microscopy does not reveal a change in the microstructure produced by different thermal treatments, yet it does permit visualization of the complex material microstructure, consisting of a martensitic matrix with embedded carbides. On the other hand, thermoelectric power measurements allow a differentiation between CN and BF files in a fast and non-destructive manner: the measurements performed on the whole files show that the thermopower of BF files is lower. This result is attributed to a higher concentration of carbon soluted in the martensitic matrix. The yield stress and compression strength determined by compression testing, as well as the hardness measured by nanoindentation, are higher for the bulk of BF files, emphasizing the importance of soluted carbon in hardening the martensite. The mobility of point defects and dislocations, and their interaction, are studied by mechanical spectroscopy. The martensitic structure is characterized by a rich internal friction spectrum comprising several overlapping relaxation and material transformation peaks. The internal friction measured at room temperature is related to an athermal background due to the interaction between soluted carbon and dislocations. This background is higher for BF files. The internal friction spectrum indicates that, upon heating above 375 K, the material structure irreversibly changes due to tempering processes. Tempering effects on the material are studied combining several experimental techniques. A first stage of tempering, observed by differential scanning calorimetry, starts around 375 K and is related to carbon precipitation in the form of transition carbides. At this temperature, the thermopower and Young's modulus increase, and the internal friction and hardness decrease, implying that all these quantities are sensitive to soluted carbon. The hardness continuously decreases with tempering and no precipitation hardening is observed. A second stage of tempering is observed around 525 K, leading to cementite precipitation. X-ray diffraction allows quantitative determination of the concentration of carbon in solid solution by measuring the tetragonality of the martensite lattice. The evolution of carbon content during tempering is compared to the evolution of internal friction and thermoelectric power revealing a strong correlation between these quantities. Internal friction measured at room temperature is directly proportional to the carbon content. The measurement of the concentration of carbon by X-ray diffraction is used to determine the variation of carbon content in the file profile. Thermopower measurements indicate that BF files have a lower concentration of carbon in the teeth than in the bulk, while the opposite is found for CN files. This is confirmed by X-ray diffraction. Moreover, diffraction performed on the very surface of the teeth reveals a lower carbon concentration in BF than in CN files teeth, which is probably the origin of their lesser performance. It can be concluded that the concentration of carbon in solid solution in martensite is responsible for the steel's hardness and therefore for its resistance to wear.

EPFL2004

Mechanical properties and microstructure of a high carbon steel

Iva Tkalcec Vâju

EPFL2004