TC6 Programme

Special session TC6 – Ceramics, 10.7.2020

Information

Chairmen: J. Dusza, P. Hvizdoš

Summary: The school will provide information on research and development of advanced structural and functional ceramics and ceramic matrix composites. It will be focused on showing and discussion of the recent trends in design, processing, fabrication, characterization and application of novel ceramic-based materials, on new time and energy saving sintering techniques, on new structures, on advanced characterisation methods.

The topics include:

· Novel ceramics and ceramic matrix composites: preparation, microstructure, properties;

· Mechanical characterisation of ceramics and CMCs at macro-, micro- and nano-levels;

· Ultrahigh temperature ceramics (UHTC) and their structural integrity;

· High entropy ceramics: preparation, challenges and perspectives;

· Fracture mechanics of brittle materials, tribology and wear of hard materials.

Timetable (Time zone is CEST) - ROOM: LINK

9.009.45: Micro/Nano-mechanical testing of advanced ceramics

J. Dusza

9.45 – 10.30: Tribology of ceramic materials – scratch, friction, and wear properties

P. Hvizdoš

10.30 – 11.00: Break

11.00 – 11.45: Mechanical and tribological properties of TiB2-SiC and TiB2-SiC-GNPs ceramic composites

A. Kovalčíková

11.45 – 12.30: Development of boron carbide/graphene platelets ceramics prepared by different processing technologies

R. Sedlák

12.30 – 14.00: Lunch break

14.00 – 14.45: Finite element modeling of cohesive and adhesive cracking in the hard coating/softer substrate system during nanoindentation and scratch testing

F. Lofaj

14.45 – 15.30: Deformation and fracture of high-entropy carbide grains during various micro/nanomechanical testing

T. Csanádi

15:30 – 16:00: Final test

Prof. Ján Dusza: Micro/Nano-mechanical testing of advanced ceramics

The deformation and fracture characteristics of differently oriented WC grains/crystals in WC – Co, Si3N4 grains/crystals in reaction bonded Si3N4 system and ZrB2 grains/crystals in ZrB2 polycrystal were investigated. Depth-sensing nano-indentation and scratch tests of grains and micro-compression tests of micropillars and microcantilevers prepared by focused ion beam from oriented facets of grains and grain boundaries were studied. Electron backscatter diffraction (EBSD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were performed to determine the grain orientation and to study the surface morphology and the resulting deformation and damage mechanisms around the indents and in micropillars. The hardness and scratch resistance of the differently orientated grains showed significant angle dependence from the basal towards the prismatic directions. A strong influence of the grains orientation on compressive yield stress and rupture stress values was found during the micropillar test, too. The active slip systems for individual ceramics have been recognized. The different properties of the basal and prismatic planes were found to be connected with the different deformation mechanisms – slip and dislocation activities. The bending strength of the individual gains are controlled with nano-size defects.

Dr. Tamás Csanádi: Deformation and fracture of high-entropy carbide grains during various micro/nanomechanical testing

Deformation and fracture behaviour of high-entropy carbide grains of different orientation were investigated by nanoindentation, micropillar compression and micro-cantilever bending tests. The (Hf-Ta-Zr-Nb)C sample was prepared by spark plasma sintering and crystallographic orientation of grains was determined by electron backscatter diffraction (EBSD). Micropillar and micro-cantilevers were milled out from grains of specific orientations, based on EBSD maps, by focused ion beam (FIB) technique. Micro/nanomechanical tests were carried out on an Agilent G200 NanoIndenter using different diamond tips such as Berkovich tip for indentation, flat punch for micropillar compression and spherical for micro-cantilever bending. Nanoindentation and micropillar compression were directly compared to the constituent mono/binary carbides. Deformation around indents, compressed pillars and cantilevers and their fracture surface were studied by scanning electron microscopy. Nanoindentation tests exhibited a significantly enhanced hardness (36.1±1.6 GPa,) compared to the hardest monocarbide (HfC, 31.5±1.3 GPa) and the binary (Hf-Ta)C (32.9±1.8 GPa). Micropillar compression test of {001} oriented grains revealed that (Hf-Ta-Zr-Nb)C had a significantly enhanced yield and failure strength compared to the corresponding base monocarbides, while maintaining a similar ductility to the least brittle monocarbide (TaC) during the operation of {110}〈11 ̅0〉 slip systems. It was found that the deformation behaviour of cantilevers depends mostly on their defect structure (i.e. existence of pores/inclusions) and grain orientation ({001} and {101}) has only slight influence on it. Based on the load-displacement curves, plastic behaviour was inferred in about the half of the micro-cantilevers tested, which were broken mostly at their fixed end and characterized as ‘defect free’ based on SEM images. The corresponding fracture strength of grains was measured to be ~11 GPa. The other ‘defected’ beams, which were broken further from the fixed end, exhibited a brittle linear fracture with visible granular shape pores on the SEM images. Based on the dimension and location of pores, the amplified stress values were determined at the vicinity of pores. The real fracture strength of micro-cantilevers, including both ‘defected’ and ‘defect free’ ones, were calculated to 12.59 ± 3.2 GPa. According to the analysis of EBSD and SEM images, fracture surface was found to the (001) type plane for both types of micro-cantilevers.

Dr. Alexandra Kovalčíková: Mechanical and tribological properties of TiB2-SiC and TiB2-SiC-GNPs ceramic composites

TiB2-SiC and TiB2-SiC-graphene nanoplatelets (GNPs) composites were prepared using field-assisted sintering technology at 2100°C in argon atmosphere, and the influence of the SiC and different GNPs addition on microstructure development, mechanical and tribological properties has been investigated. Instrumented hardness, bending strength, chevron-notched fracture toughness and ball-on-flat tribological tests were used for the testing and characterization of the composites. The increasing GNPs addition to TiB2-20 % SiC from 1 to 10 wt. % with two types of platelets resulted in significantly improved densification, with almost 100 % TD, but in decreased hardness from 24 to 14 GPa, and decreased elastic modulus from 370 to 236 GPa. According to the results for optimal strength and fracture toughness, the amount of GNPs additive should be 2 wt. % for both type of additives. The highest strength was measured for the system TiB2-20SiC-2GNP with the value of 729 MPa and the highest fracture toughness for the system TiB2-20SiC-10GNP with values 6.2 MPa.m1/2. Fractography revealed no fracture origin in the form of processing flaws such as pores or clusters after bending strength test and toughening mechanisms on fracture lines and surface mainly in the form of crack bridging and crack branching by GNPs were observed. The very low coefficient of friction, below 0.5, at GNPs content above 5 wt. % and the very low specific wear rate 10-7 mm3/N.m at 5N and 10-6 mm3/N.m at 50 N are probably thanks to the formation of zones of tribochemical layer (SiO2 -TiO2-C based tribofilm) on the worn surfaces. Type of the graphene platelets did not show a significant influence on the wear behaviour.

Dr. Richard Sedlák: Development of boron carbide/graphene platelets ceramics prepared by different processing technologies

Boron carbide/graphene platelet (B4C/GPLs) composites have been prepared with the addition of different weight percent and various types of GPLs by hot-press processing technology (HP), conventional Spark Plasma Sintering (SPS) and application of the novel and superfast processing method Flash Sintering (FS), which reduces sintering time only to 24 seconds. The effect of processing technology and influence of the GPLs addition on microstructure development, fracture toughness, electrical conductivity and tribological properties was investigated. The microstructure was studied by SEM, TEM, HRTEM, XRD and Raman spectroscopy. SEVNB method was used for fracture toughness and four-point Van der Pauw method for electrical conductivity measurement. Almost fully dense B4C/GPLs composites have been prepared with lower wt.% of GPLs additives with relatively homogeneously distributed platelets in the matrix. With increasing amount of GPLs additives, the fracture toughness increased due to the activated toughening mechanisms in the form of crack deflection, crack bridging, crack branching and graphene sheet pull-out. The highest fracture toughness of 4.48 MPa.m1/2 was achieved at 10 wt.% of GPLs addition, which was ~50 % higher than the KIC value of reference material. A significant improvement of electrical conductivity around two orders of magnitude was noticed. The friction and wear behaviour of B4C/GPLs composites have been investigated using the ball-on-flat technique with SiC ball under dry sliding conditions at room temperature. The coefficient of friction for composites was similar, however, the wear rate significantly decreased ~77 % in the case of B4C+10 wt.% GPLs when compared to reference material at a load of 5 N, and ~60 % at a load of 50 N. For revealing and observation of the wear damages under the worn surfaces, focused ion beam (FIB) technique was used for the preparation of the cross-section of wear tracks.

Prof. František Lofaj: Finite element modeling of cohesive and adhesive cracking in the hard coating/softer substrate system during nanoindentation and scratch testing

Finite element modelling (FEM) combined an Extended FEM (XFEM) and Cohesive Zone Model (CZM) methods were applied to study the formation of cohesive and adhesive cracks during indentation and scratch testing in the W-C:H coatings deposited on steel substrate, respectively. The distributions of von Mises stresses, principal stresses and equivalent plastic deformation in the coating and in the substrate during the whole loading cycle were analyzed within conventional FEM. The formation of cohesive cracks during later stages of nanoindentation at large penetration depths were modeled based on extended FEM and verified by focused ion beam (FIB) technique with scanning electron microscopy. These observations showed formation of a system of circular cohesive microcracks in the coating around the indent in the sink-in zone. CZM suggested also formation of adhesive microcracks at the coating/substrate interface and coating delamination depending on the maximum applied load. The results of modeling of nanoindentation were applied to the modelling of cohesive cracks during scratch testing. Principal agreement between modelling and experimental observations was achieved at the initial stage of scratching. The combination of advanced FEM with nanoindentation or with scratch testing can be used to determine the strength and fracture toughness of thin hard coatings on softer substrates.

Dr. Pavol Hvizdoš: Tribology of ceramic materials – scratch, friction, and wear properties

Principles of friction, contact damage, scratch and wear behaviour. The motivation of tribological testing. Frequently used geometries, methods, wear mechanisms and regimes, damage mechanisms. Evaluation of the tribological testing - coefficient of friction, wear rate, construction of wear maps. Specifics of wear of ceramics and ceramic matrix composites. Connection to microstructure, topography, mechanical properties. Examples of tribology of oxides, carbides, nitrides, composites with metals, with carbon based nanobjects (graphene, nanotubes). perspectives for designing and development of materials for tribological applications.