Papers by Author: Monika Kašiarová

Abstract: The scratch resistance of the silicon nitride with the addition of 1 and 7 wt% of graphene multiplatelets prepared by hot press sintering has been studied. The scratch resistance behaviour of Si₃N₄-GNP composites were investigated using a Rockwell indenter for normal applied loads ranging from 1-150 N. Si₃N₄-7-wt%GNP composite behaved differently during the scratch test depending on the normal applied load. The coefficient of friction changed dramatically at higher load and extensive crack propagation resulting in the chipping is observed.

Abstract: 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.

Abstract: 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.

Abstract: The decrease of the mechanical properties – hardness and reduced elastic modulus after corrosion in white wine was measured. Under static corrosion conditions no significant decrease was observed up to 8 hours of corrosion. Dynamic corrosion conditions cause detrimental decrease of properties (one order of magnitude) compare to the results of static corrosion test. This is due to the removal of the harder outer layer of the enamel during polishing. To obtain a relevant data concerning corrosion test, natural surface of a tooth should be investigated and tested.

Abstract: 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.

Abstract: The paper deals with the determination of the characteristic strength and as well as the Weibull moduli and of the Si3N4+SiC micro/nanocomposite determined by the four-point bending test and the contact test using opposite spheres both applied to specimens of different size, respectively. Material failure in the bending and contact modes is caused by the presence of processing defects as fracture origins, and by the formation of cone cracks, respectively, where a stable growth of the cone cracks initiated during contact loading is assumed to be a reason of , . A microstructural analysis of the processing flaws, and a mathematical analysis of the propagation of the cone cracks regarding different dimensions of a specimen are also presented.

Abstract: The present work studies the corrosion of three most widely used types of structural ceramics – silicon nitride, solid state sintered alumina and liquid phase sintered alumina – in 3 % aqueous solutions of sodium chloride at temperatures up to 290 °C and pressures up to 7 MPa. The corrosion of silicon nitride was controlled by attack of Si3N4 matrix grains, while yttrium oxynitride amorphous grain boundary phase was corrosion resistant. Corrosion of Si3N4 in reference media -distilled water - at 290 °C was characteristic by formation of passivation layer, which hindered further dissolution of silicon nitride matrix. The presence of sodium chloride resulted in formation of discontinuous layer of corrosion products, resulting in more severe corrosion than in distilled water. The corrosion of liquid phase sintered alumina was mainly attributed to congruent dissolution of SiO2 and CaO from grain-boundary amorphous film, which was accelerated at higher temperature, and accompanied by precipitation of siliceous phases from oversaturated solution at 200 °C. Pure polycrystalline alumina corroded by loss of alumina grains, which did not dissolve in the corrosion media. The corrosion impaired significantly the fracture strength of silicon nitride, creating new, corrosion related defects at the surface, while the influence of corrosion on fracture strength of polycrystalline aluminas was negligible.

Abstract: The paper deals with the determination of the characteristic strength and the Weibull modulus m of Si3N4 and SiC ceramic materials using conventional four-point bending and unconventional contact tests between opposite rollers and opposite spheres. Ceramographic and fractographic methods were used for the characterization of strength degrading defects represented by processing flaws and by cracks of different types arising during the loading. The processing flaws influenced the Weibull parameters mainly in the bending mode, and the strength and its scatter in contact modes was influenced by lateral, median and contact end cracks, originated during the contact test using rollers, and by cone cracks originated during the contact test using spheres.

Abstract: Bending and contact strength of a carbon derived in-situ reinforced Si3N4-SiC micro/nanocomposite have been investigated. Four-point bending strength was measured using specimens with different effective volume and Weibull statistical analysis has been used for characterization of the strength values in the form of characteristic strength and Weibull modulus. The characteristic strength values of the investigated composite were σ0 = 675 MPa and σ0 = 832 MPa with the Weibull modulus of 6,4 and 8,6 for the specimens with effective volume 15,9 mm3 and 1,28 mm3 respectively. Contact strength of the investigated material was carried out using sphere on sphere and roller on roller methods and the obtained results are σ0S = 1997 MPa, σ0R = 1242 MPa, and mS = 17,1 mR = 6. Fracture origins in the specimens with effective volume of 15.9 mm3 tested in four-point bending were surface and volume located technological defects with dimensions from 10 μm to 180 μm, mainly in the form of clusters of pores and large SiC grains. Fracture mirror sizes were measured and the mirror constant was calculated. Fracture origins in the specimens with effective volume of 1.28 mm3 tested in four-point bending were surface (subsurface) defects with the same type as for the specimens with a higher effective volume. The fracture during the contact strength test „roller/roller“ has been initiated under the surface of the specimens and during the contact strength test „sphere/sphere“ by creation and growth of the cone cracks to critical size.

Abstract: 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.