Zoltán Lenčéš

In-situ carbon content adjustment in polysilazane derived amorphous SiCN bulk ceramics

Abstract The present paper is concerned with the in-situ carbon content adjustment in amorphous bulk silicon carbonitride (SiCN) ceramic matrices prepared by the polymer to ceramic transformation of cross-linked and compacted poly(hydridomethyl)silazane powders. Heat treatment in inert (Ar) or reactive atmosphere (ammonia, or mixed Ar/NH 3 with different volume ratio of ammonia) was used for carbon content adjustment. Isothermal annealing steps in Ar and/or mixed atmospheres at various intermediate temperatures were also included into the pyrolysis schedule (i) to adjust the final carbon content, (ii) to control outgassing of low molecular reaction products like methane or hydrogen from the matrix during polysilazane decomposition and thus (iii) to avoid cracking of the pressed polymer powders. Optimal annealing temperature for carbon content adjustment was found to be in the range between 500 and 550°C. Increasing NH 3 contents from 10 to 50xa0vol% in the pyrolysis atmosphere as well as enhanced transient annealing temperature and time promote carbon reduction. In contrast intermediate isothermal annealing in Ar at 500 up to 600°C results in pronounced formation of Si–C bonds and in increased carbon contents after the final pyrolysis process. Depending on the pyrolysis conditions, flawless bulk specimens with carbon contents ranging from 0·3 up to 16·2xa0wt% were obtained. Different possible chemical reactions are considered to explain the generation of the particular Si(C)N compositions found. ©

Multifunctional Si3N4/(β-SiAlON+TiN) layered composites

Abstract Layered multifunctional ceramic composites on the base of Si 3 N 4 and TiN have been prepared by tape casting. The reaction conditions for in situ preparation of β-SiAlON + TiN composite were optimised and dense Si 3 N 4 /(β-SiAlON + TiN) layered materials were prepared by hot pressing. The bending strength and fracture toughness of layered materials measured in the direction perpendicular to the layer alignment were remarkably higher (1184 MPa and 9.75 MPa m 1/2 ) in comparison to the “monolithic” β-SiAlON + TiN composite (647 MPa and 4.71 MPa m 1/2 ). High anisotropy was achieved for the electrical resistance of the layered materials in parallel (6.10 −2 Ω cm) and perpendicular (5×10 11 Ω cm) direction to the layer alignment.

Layered Si3N4 composites with enhanced room temperature properties

Four silicon nitride multilayer composites were prepared by hot pressing. Internal stress distribution along the layer boundary dictates the anisotropy of mechanical properties of the composite. The room-temperature bending strength and fracture toughness of all layered composites was higher than the bending strength of related monoliths. Layered ceramic materials exhibited higher tolerance to flaws in comparison to the monolithic ceramic.

THERMAL ANALYSIS STUDY OF POLYMER-TO-CERAMIC CONVERSION OF ORGANOSILICON PRECURSORS

The organosilicon precursors attract significant attention as substances, which upon heating in inert or reactive atmosphere convert directly to oxide or non-oxide ceramics, like nitrides, carbides, carbonitrides, boroncarbonitrides, oxycarbides, alons, etc. In characterisation, and in study of conversion of these polymers to ceramics thermal analysis plays an important role. The degree of cross-linking of the polymer vital for achievement of high ceramic yield is estimated with the use of thermal mechanical analysis (TMA). Decomposition of polymers and their conversion to ceramics is studied by the combination of differential thermal analysis (DTA), differential scanning calorimetry (DSC) thermogravimetry(TG), and mass spectrometry (MS). The use of these methods in study of the polymer-to-ceramic conversion is illustrated by case studies of a commercially available poly(allyl)carbosilane as the precursor of SiC, and a poly(hydridomethyl)silazane as the precursor of SiCN.

Carbon reduction reaction in the Y2O3–SiO2 glass system at high temperature

Abstract In order to assess the role of carbon with respect to the grain boundary chemistry of Si3N4-based ceramics model experiments were performed. Y2O3–SiO2 glass systems with various amount of carbon (from 1 to 30 wt.%) were prepared by high-temperature treatment in a graphite furnace. High carbon activity of the furnace atmosphere was observed. EDX analysis proved the formation of SiC by the carbothermal reduction of SiO2 either in the melt or in the solid state. The melting temperature of the Y2O3–SiO2 system is strongly dependent on the amount of reduced SiO2. XRD analysis of the products documented the presence of Y2Si2O7, Y2SiO5 and Y2O3 crystalline phases in that order with an increasing amount of free C in the starting mixture. The reduction of Y2O3 was not confirmed.

In situ generated homogeneous and functionally graded ceramic materials derived from polysilazane

The pyrolysis of cross-linked poly(hydridomethyl)silazane pellets via transient isothermal ammonia gas treatment yields amorphous layered ceramic bulk Si/N-Si/N/C-materials with graded carbon content. By this process the C content in the material can be adjusted with high accuracy in the range between 0 and 14 wt.%. The influence of (1) temperature of reactive ammonia treatment (2) time of reactive isothermal ammonia treatment (3) isothermal holding time under inert atmosphere (Ar) before application of ammonia (4) degree of cross-linking of the polycarbosilazane (5) porosity of green compact (6) volume ratio of NH3 in the reactive atmosphere was examined. The temperature of 525 °C and the reactive atmosphere containing 10 vol.% NH3 were found to be optimum for carbon content adjustment. Higher ammonia contents did not allow suitable control of the process, while higher temperature of transient heat treatment caused crack formation in the specimen due to excessive pressure of gaseous reaction products. High degree of cross-linking as well as the annealing of cross-linked green bodies at transient temperature in inert atmosphere decrease the efficiency of the reactive treatment and increase the amount of C in the pre-ceramic continuous random network (CRN). Next to graded materials, samples with bulk homogeneous carbon distribution were generated. However, these require more sophisticated heating schedules and combination of reactive treatment with pre-annealing in inert gas.

Microstructurally Induced Internal Stresses in the Silicon Nitride Layered Composites

A design of composites with layered structure seems to be a way of preparation of ceramic materials with decreased sensitivity to the defects1,2 Recently, this approach was applied also for silicon nitride based composites3–7. The decreased flaw sensitivity of these materials is probably reached by presence of residual stresses which are conserved in these materials after cooling. These stresses can diminish the quantity of the stresses concentrated on the largest defect and so decrease the sensitivity of layered materials to the flaw size6,7. The residual stresses are a consequence of different material constants of individual layers (thermal expansion coefficients and Young’s modulus)6. An increase of the bending strength of layered materials was achieved, however the size of the flaws was the same as in the relative monoliths3–7. Additionally, the stress status of the layer can be modified by the layer thickness, green density of the layer, sinterability of the used powder and used sintering method7. Layered ceramic materials pay for the increased quality by loosing of isotropy of their mechanical properties.

Fracture Characterization of Silicon Nitride Based Layered Composites

Silicon nitride based structural ceramics are a family of advanced materials that exhibit a combination of high hardness, high strength, good corrosion and erosion behaviour, high elastic modulus and dimensional stability. Major application of these ceramics includes wear components, cutting tools and parts of engines (turbochargers, bearings, etc.). Their wide application is, however, still limited mainly due to their brittleness, low flaw tolerance and low reliability1,2. In recent years nitride based ceramics have been very intensively investigated all over the world with the aim to improve their mechanical properties and make them suitable for structural applications. The main ways of improving the room temperature mechanical properties of silicon nitride based ceramics can be summarized as follows: n n nimproving the strength level and reducing the strength values scatter, i.e., enhancing the reliability by reduction of the critical defect size (improved properties of powders, clean room manufacturing, etc.) — the flaw diminution approach3,4; n n npromoting the localized bridging behind the crack tip (in the form of frictional and mechanical interlocking, or pull out) by which the flaw tolerance of the material can be improved — the flaw tolerance approach5−7; n n nimproving the strength values by incorporating into the matrix the nano-sized, second-phase particles with different expansion coefficients — the nano-particle dispersion strengthening8; n n nimproving the structural reliability by designing novel laminar composites with a promoted crack deflection at the interlayer boundaries and utilizing the compressive residual stresses arisen during cooling down from the sintering temperature because of the differences in the thermal expansions between the layers which have different compositions — the laminar structure approach9−12.

Thermodynamic and Dielectric Properties of MgSiN2 Ceramics

Reaction bonded MgSiN2 (RBMSN) was prepared by direct nitridation of a Si/Mg2Si/Mg/Si3N4 powder compact in a temperature range of 1350-1550°C. The oxygen content of MgSiN2 was in the range of 0.4 – 0.6 wt%. A thermal stability examination showed that MgSiN2 is stable up to 1400°C at 0.1 MPa N2 pressure. The activation energy of decomposition calculated from the temperature dependence of weight loss is H = 383 kJ⋅mol-1. The time dependence and nitrogen pressure dependence of MgSiN2 decomposition was also investigated at constant temperature. MgSiN2 is stable at 1560°C in 0.6 MPa nitrogen atmosphere. Using these experimental data together with the heat capacity published in a literature the Gibbs free energy of formation of MgSiN2 was calculated in a temperature range 300-2500 K. Dense MgSiN2 ceramics or MgSiN2/Si3N4 composites with fluorine-based additives were prepared by hot pressing. The composite materials had a 4-point bending strength of 427 MPa and Vickers hardness (HV1) of 20.8 GPa, respectively. The indentation fracture toughness was 5.3 MPa.m1/2, due to the presence of elongated β-Si3N4 grains. The dielectric constant of dense reaction bonded MgSiN2 at 100 kHz was 9.5-10, while that of MgSiN2/Si3N4 composite in a wide range 50 – 6000, depending on composition and heat treatment.

Hardness Limits of SiC and Si3N4 Ceramic Materials

Nano- and macro-hardness of SiC and Si3N4 based ceramic materials prepared by liquid phase sintering were evaluated. The applied loads were 3.5 mN and 9.81 N, respectively. The measurements showed that the nano-hardness of both ceramics is substantially higher compared to the macro-hardness. The influence of solid solutions and grain boundary composition on the hardness of SiC-based ceramics was studied. The macro-hardness is strongly dependent on the grain boundary composition while the nano-hardness was nearly the same for all tested samples with different Re2O3-AlN additives. In the case of Si3N4 based ceramics the SiC nano-inclusions content was varied. As a source of SiC nanoinclusions and grain boundary phase modifierSiNC polymer precursor has been used. Nano- as well as micro-hardness increased with increasing SiC content. Present paper deals with the explanation of both results.