Miroslav Hnatko

SiC/Si3N4 nano/micro-composite — processing, RT and HT mechanical properties

Two SiC/Si3N4 nano/micro composites were prepared from a starting mixture of crystalline α-Si3N4, amorphous SiNC, Y2O3 and/or Al2O3. The composite material for room temperature (RT) application has high strength of 1200 MPa, Weibull modulus of 19 and moderate fracture toughness of 7 MPa m1/2. The composite for high temperature (HT) application, without Al2O3 has RT strength of 710 MPa and is able to keep 60% of its RT strength up to 1300°C. The creep resistance of composite material is approx. 1 order higher compared to relative monolith up to 1400°C.

Low-cost preparation of Si3N4-SiC micro/nano composites by in-situ carbothermal reduction of silica in silicon nitride matrix

Abstract SiC/Si 3 N 4 nano/micro composites were prepared from a mixture of α-Si 3 N 4 , amorphous carbon (carbon black), and Y 2 O 3 by carbothermal reduction of SiO 2 present at the surface of Si 3 N 4 matrix grains, or added deliberately to the starting mixture. A special heating regime allowed the outgassing of CO(g) (a product of carbothermal reduction) and reduced the residual porosity to less than 2%. The inter- and intragranular SiC inclusions containing the amorphous oxygen-rich layer at the interface between SiC and the Si 3 N 4 grains were present, originating from the reaction of free carbon with the silica melt. The reaction consumes silica in the grain boundary phase. The change of the grain boundary chemistry influences both room and high temperature properties of the nanocomposite.

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.

The Formation of Two Types of SiC Inclusions in Si3N4/SiC Nanocomposites

Detailed study on the formation of SiC nanoinclusions within the SiC/Si3N4 nanocomposite is presented. Inclusions located within the Si3N4 micrograins are of two different origins. The first category consists of inclusions with the oxygen rich interface between SiC and host Si3N4 grain. The second category of inclusions consists of SiC nanograins without this interface layer. The present study proved that oxygen rich SiC inclusions are created by reduction of SiO2 in the presence of carbon during the liquid phase sintering of the composite.

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.

Low Cost Si3N4/SiC Nanocomposites, Processing, RT and HT Properties

Silicon nitride - silicon carbide nanocomposite has been prepared by an in-situ method that utilizes formation of SiC nanograins by C+ SiO2 carbothermal reduction during the sintering process. The developed C/SiO2 derived nanocomposite consists of a silicon nitride matrix with an average Si3N4 matrix grain diameter of approximately 200 nm with inter- and intra- granular SiC inclusions with sizes of approximately 150 nm and 40 nm, respectively. The mean value of room temperature 4-point bending strength is 670 MPa with the Weibull modulus of 7.5 and indentation fracture toughness of 7.4 MPa.m1/2. The creep behaviour was investigated in bending at temperatures from 1200°C to 1450°C, under stresses ranking from 50 to 150 MPa in air. A significantly enhanced creep resistance was achieved by the incorporation of SiC nanoparticles into the matrix. The inserts machined from this composite have three times longer life time compared to those available on the market.

Local Mechanical Properties of Highly Porous Si3N4 for Trabecular Bone Replacement

The study deals with the development of highly porous undegradable ceramics based on silicon nitride as potential replacement of trabecular bone. These materials were produced using replication method with polyurethane foams as pore-forming agents to achieve similar porous structure to trabecular bone. Prepared porous ceramics had a bimodal pore structure with macro-pores larger than 200 μm and micro-pores smaller than 1 μm in diameter, which are necessary for tissue ingrowths, cell adhesion, adsorption of biological metabolites and nutrition delivery in organism. The microstructure and local mechanical properties (Young’s modulus and Yield strength) were evaluated and compared with human trabecular bone. Results showed that studied porous materials have satisfactory porosity and pore sizes for trabecular bone replacement. Young’s modulus of bone was 12.6 ± 2.23 GPa and porous silicon nitride samples ranged from 10.9 ± 3.38 GPa to 12.9 ± 1.13 GPa. The values of Yield strength of trabecular bone was determined as 493 ± 30.7 MPa and the values of porous samples varied from 250 ± 19.3 MPa to 558 ± 36.5 MPa. Young’s modulus and Yield strength increase with increasing of the pre-sintering temperature and multiple infiltrations.

Mechanical Properties of Porous Si3N4 Ceramics

Mechanical properties of porous silicon nitride prepared by two different processing routes have been studied. Depth sensing methods was used to measure the hardness and elastic modulus of experimental materials. The results were compared with the hardness and elastic modulus of trabecular bone in order to find out porous ceramics with properties close to that of trabecular bone. Material prepared by infiltration of polyurethane sponge exhibited properties close to the properties of bone and it is the potential material for further investigation in the bioapplication field.

Determination of Local Mechanical Properties of Si3N4 Based Foams

Local mechanical properties, particularly the hardness and Youngs modulus of highly porous silicon nitride based foams were studied in this work. Silicon nitride foams were prepared using polyurethane foam replication method to obtain appropriate cellular structure suitable for bio-application. Two types of the polyurethane foams were used (with average pore size 0.48 mm and 0.62 mm). Some of these samples were prepared by single or multiple infiltrations. The effects of structures, temperature of calcination, volume fraction of Si3N4 powder and number of the infiltrations on the local mechanical properties were investigated. The Youngs modulus of studied samples range from 12 to 46 GPa at the macroscopic scale measured by resonant frequency technique and from 10 to 28 GPa at the microscopic scale measured by instrumented indentation. Results showed increase of the hardness and Youngs modulus with increasing of the calcination temperature, with increasing of the number of infiltrations and also with increasing of volume fraction of Si3N4 powder in suspension. The results obtained from nanoindentation carry out lower values in comparison with the values measured by resonant frequency technique.

Influence of Calcium Addition on the Chemical Durability of the Model Alumino-Silicate Glasses in Aqueous Solutions

The corrosion resistance of liquid phase sintered (LPS) alumina ceramics in aqueous environments strongly depends on composition and chemistry of grain boundary glass formed during sintering. The chemical durability of model alumino-silicate glasses with various contents of CaO in aqueous solutions was therefore evaluated. Prepared glasses were corroded under hydrothermal conditions in deionized water under static conditions. The examination of surface morphology of corroded specimens after the contact with deionized water, together with the analysis of corrosion solution provided information on mechanism of dissolution of grain boundary glasses in LPS aluminas and confirmed that dissolution process is hindered due to saturation of solution with respect to leached elements. The initial dissolution rates for studied glasses were determined. The results are applicable for optimization and enhancement of corrosion resistance of LPS alumina under hydrothermal conditions.