Nanoindentation study of nickel manganite ceramics obtained by a complex polymerization method

The chemical synthesis of nickel manganite powder was performed by a complex polymerization method (CPM). The obtained fine nanoscaled powders were uniaxially pressed and sintered at different temperatures: 1000–1200 °C for 2 h, and different atmospheres: air and oxygen. The highest density was obtained for the sample sintered at 1200 °C in oxygen atmosphere. The energy for direct band gap transition (Eg) calculated from the Tauc plot decreases from 1.51 to 1.40 eV with the increase of the sintering temperature. Indentation experiments were carried out using a three-sided pyramidal (Berkovich) diamond tip, and Young's modulus of elasticity and hardness of NTC (negative temperature coefficient) ceramics at various indentation depths were calculated. The highest hardness (0.754 GPa) and elastic modulus (16.888 GPa) are exhibited by the ceramics sintered at highest temperature in oxygen atmosphere.     

Microstructure and integrity of leucite ceramic derived from potassium-based geopolymer precursor

In this paper, leucite ceramics with different shapes and fine integrity were directly developed from geopolymer precursors by the optimized heat treatment procedures. Phase compositions and microstructure, coupled with the mechanical properties, were systematically investigated. The effect of heating rates on the integrity of ceramics was also discussed. After high-temperature treatment at 800 °C for 120 min, the geopolymer was converted into a leucite ceramic. The relative density of the sample treated at 1000 °C for 120 min was 93% and the size of leucite grain was approximately 1–4 μm. TEM results showed that the leucite grain was composed of lamellar and needle-shaped twins, which resulted from the displacive phase transformation as it went from cubic to tetragonal symmetry when it was cooled down. Samples with plate and bullet shapes maintained their fine integrity by the optimization of the heating procedures. Meanwhile, the flexural strength, Young's modulus, fracture toughness and Vickers hardness of the leucite ceramic that resulted was 70.0 MPa, 65.0 GPa, 1.3 MPa m^1/2 and 7.39 GPa, respectively.     

Mechanical characterization of a polysiloxane-derived SiOC glass

Silicon oxycarbide glass with the composition Si_1.0O_1.6C_0.8 was synthesized from a commercial polysiloxane by polymer pyrolysis. Dense SiOC samples were obtained by cross linking of the polysiloxane followed by warm pressing to form cylindrical samples and subsequent pyrolysis of the shaped polymer at 1100 °C in Ar. Hardness (H), Young's modulus (E) and Poisson's ratio (ν) of the as-prepared SiOC glass were evaluated from indentation studies and from acoustic microscopy. Indentation studies showed that E depends on the applied load and amounts to 90 GPa for low load and to 180 GPa for high load. Average values of 6.4 and 101 GPa were obtained for H and E, respectively, by the Vickers indentation method. Acoustic microscopy analysis yielded values of 96 GPa and 0.11 for E and ν, respectively. Compared to vitreous silica, the Young's modulus of the SiOC glass is about 1.3–1.5 times higher. To the knowledge of the present authors, the measured Poisson's ratio (ν = 0.11) is the lowest reported so far for glasses and polycrystalline ceramics.     

A study of the microstructure and mechanical properties of SiC coatings on spherical particles

We have investigated the effect of the microstructure on the mechanical properties of three nearly stoichiometric SiC coatings (SiC, SiC + C and SiC + Si coating), which were coated onto spherical particles as simulated nuclear fuel particles by fluidized-bed chemical vapour deposition (FBCVD). The mechanical properties of the SiC coatings were studied using micro- and nano-indentation. The microstructure was characterised using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). TEM was also used to elucidate the deformation behaviour under the indentation. The FBCVD SiC coatings studied exhibited a higher hardness than conventional CVD SiC coatings, and SiC coating gave the highest hardness among the three coatings. TEM confirmed that the presence of pores affect the Young's modulus of SiC coatings. The high hardness was attributed to the high density of dislocations and their interactions. The initiation and propagation of micro cracks under the confined shear stress was found to be responsible for the mechanism of plastic deformation. Based on this hardness-related plastic deformation mechanism, the variation of hardness in the three types of SiC coating was due to different grain morphologies.     

Columnar and DVC-structured thermal barrier coatings deposited by suspension plasma spray: high-temperature stability and their corrosion resistance to the molten salt

High-temperature stability of SPS YSZ coatings with the columnar and deep vertically cracked (DVC) structures and their corrosion resistance to 56 wt% V₂O₅+44 wt% Na₂SO₄ molten salt mixture were investigated. Both the columnar and DVC-structured YSZ coatings were sintered at 1000 °C, but a significant increase in porosity in combination with significant reductions in Vickers’ hardness and Young's modulus were observed at the temperatures from 1200 °C to 1400 °C. The DVC-structured YSZ coating exhibited superior corrosion resistance against the molten salt mixture attack to the columnar-structured one due to its higher density behaving as a sealing protective top layer at 950 °C.     

Mechanical behavior and ultrasonic non-destructive characterization of elastic properties of cordierite-based ceramics

The structural and morphological evolutions of cordierite-based ceramics produced from stevensite/andalusite mixture sintered from 1150 to 1350 °C were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical behavior was investigated by three-point bending and Brazilian tests. The elastic properties were evaluated using ultrasonic non-destructive testing (NDT). XRD results revealed that the main crystalline phase formed at 1300 and 1350 °C was cordierite with traces of mullite. A linear-elastic behavior followed by brittle fracture was observed in three-point bending test with the presence of multiple discontinuities. Flexural and diametral compression strength values of cordierite sintered at 1300 °C were 39.4±4 and 21.8±2 MPa, respectively. The elastic properties such as Young's modulus (38.7–45.1 GPa), shear modulus (17.90–19.42 GPa) and Poisson ratio (0.08–1.6) of cordierite-based ceramics produced at 1350 and 1300 °C were also determined.     

SiC ceramic micropatterns from polycarbosilanes

Micropatterned SiC ceramics were fabricated from polycarbosilanes applying a softlithographic replication technique. A polydimethylsiloxane mould replicated from a photolithographic microstructured silicon wafer was used as master structure. The polydimethylsiloxane mould was coated with a solution containing a mixture of two different polycarbosilanes in n-octane. After treatment at 200–400 °C the cross-linked polycarbosilane films were debonded and pyrolysed at 900 °C in nitrogen and subsequently crystallised at temperatures up to 1500 °C in argon. The cross-linking and thermal degradation behaviour of the polycarbosilanes was investigated by Fourier-transform infrared spectroscopy, differential scanning calorimetry and thermogravimetric analysis. X-ray diffractrometry showed the expected development of a nanocrystalline β-SiC (3 nm) as the main phase with increasing temperature. However, traces of α-SiO₂ derived from the polycarbosilane precursors were also detected by X-ray analysis. Removal of the α-SiO₂ dioxide with hydrofluoric acid in the pyrolysed samples and subsequent increased the crystallite size to 7 nm. The Young's modulus determined by nanoindentation was increased from 3 GPa after cross-linking to 110 GPa after crystallisation. Scanning electron microscopy revealed, that the initial micropatterns were fully retained in the pyrolysed and crystallised SiC ceramics. The micropatterned cross-linked and crystallised β-SiC based substrates exhibited light scattering characteristics, which qualify them as promising candidates for diffractive optical elements in microoptical applications.     

Preparation and properties of porous alumina ceramics with oriented cylindrical pores produced by an extrusion method

Porous alumina ceramics with unidirectionally-oriented pores were prepared by extrusion. Carbon fibers of 14 μm diameter and 600 μm length to be used as the pore-forming agent were kneaded with alumina, binder and dispersing agent. The resulting paste was extruded, dried at 110 °C, degreased at 1000 °C and fired at 1600 °C for 2 h. SEM showed a microstructure of dispersed highly oriented pores in a dense alumina matrix. The pore area in the cross section was 25.3% with about 1700 pores/mm². The pore size distribution of the fired body measured by Hg porosimetry showed a sharp peak corresponding to the diameter of the burnt-out carbon fibers. The resulting porous alumina ceramics with 38% total porosity showed a fracture strength of 171 MPa and a Young's modulus of 132 GPa. This strength is significantly higher than the reported value for other porous alumina ceramics even though the present pore size is much larger.     

Temperature dependent hardness and strength properties of TiB2 with TiSi2 sinter-aid

From the perspective of high temperature structural applications, it is important to evaluate temperature dependent mechanical properties of titanium diboride (TiB₂) ceramics. The present study reports the effect of TiSi₂ content (up to 10 wt.%) and temperature on hardness and strength of TiB₂. The hardness properties were measured from room temperature (RT)—900 °C in vacuum; the four-point flexural strength properties were evaluated at selected temperatures in air up to 1000 °C. An attempt has been made to discuss the difference in hardness and strength properties with sinter-aid amount and microstructure. Our experimental results clearly indicated that the addition of 2.5 wt.% TiSi₂ to TiB₂ resulted almost full densification at a lower hot pressing temperature of 1650 °C without compromising on the high temperature strength and hardness properties. The hot pressed TiB₂–2.5 wt.% TiSi₂ ceramic could retain moderate strength of more than 400 MPa and hardness of 9 GPa at 1000 °C and 900 °C, respectively.     

Elastic behaviour of zirconium titanate bulk material at room and high temperature

Zirconium titanate (ZrTiO₄) is a well known compound in the field of electroceramics, however, its potential for structural applications has never been analysed. Moreover, it is compatible with zirconia, thus, zirconium titanate–zirconia composites might have potential for structural applications in oxidizing atmospheres. Nevertheless, there are currently no data about elastic properties of zirconium titanate materials in the literature. In view of the importance of these properties for the structural integrity of components subjected to high temperature and mechanical strains, an attempt was done in this work to determine the elastic properties of ZrTiO₄, both at room and high temperature. Young's modulus (161 ± 4 GPa), shear modulus (61 ± 1 GPa) and Poisson's ratio (0.32 ± 0.01) values at room temperature have been estimated for a fully dense single phase ZrTiO₄ material from experimental data of sintered single phase ZrTiO₄ materials with different porosities (6–19%). Values for room temperature Young's modulus are in agreement with those obtained by nanoindentation. Young's modulus up to 1400 °C shows an unusual dependence on temperature with no significant variation up to 500 °C an extremely low decrease from 500 to 1000 °C (≈0.02–0.03% every 100 °C) followed by a larger decrease that can be attributed to grain boundary sliding up to 1400 °C.     

Thermomechanical properties of Y-substituted SrTiO3 used as re-oxidation stable anode substrate material

Re-oxidation robustness is important to warrant a reliable operation of anode-supported solid oxide fuel cell systems. The current work concentrates on the mechanical properties of re-oxidation stable Y-substituted SrTiO₃ ceramic for the use as anode substrate material. Room temperature micro-indentation yielded Young's modulus and hardness of 160 and 7 GPa, respectively, whereas the temperature-dependent modulus was measured with a resonance-based method up to ∼950 °C. The effective Young's modulus as a function of porosity was measured at room temperature and compared with fracture strength data. The fracture toughness was assessed using a combination of pre-indentation cracks and bending test. Creep rates were measured at 800 and 900 °C in a 3-point bending configuration. Post-test fractographic analysis performed using stereo, confocal and scanning electron microscopy, revealed important information on fracture origins and critical defects in the material. A methodology to assess the mechanical properties of porous materials is suggested.     

Microstructure and mechanical properties of tantalum carbide ceramics: Effects of Si3N4 as sintering aid

Stoichiometric Tantalum carbide (TaC) ceramics were prepared by reaction spark plasma sintering using 0.333–2.50 mol% Si₃N₄ as sintering aid. Effects of the Si₃N₄ addition on densification, microstructure and mechanical properties of the TaC ceramics were investigated. Si₃N₄ reacted with TaC and tantalum oxides such as Ta₂O₅ to form a small concentration of tantalum silicides, SiC and SiO₂, with significant decrease in oxygen content in the consolidated TaC ceramics. Dense TaC ceramics having relative densities >97% could be obtained at 0.667% Si₃N₄ addition and above. Average grain size in the consolidated TaC ceramics decreased from 11 µm at 0.333 mol% Si₃N₄ to 4 µm at 2.50 mol% Si₃N₄ addition. The Young's modulus, Vickers hardness and flexural strength at room temperature of the TaC ceramic with 2.50 mol% Si₃N₄ addition was 508 GPa, 15.5 GPa and 605 MPa, respectively. A slight decrease in bending strength was observed at 1200 °C due to oxidation of the samples.     

Investigation of the tribological and biomechanical properties of CrAlTiN and CrN/NbN coatings on SST 304

This study examines the influence of thin layer coatings of CrAlTiN and CrN/NbN, deposited via physical vapor, on the biocompatibility, mechanical, tribological, and corrosion properties of stainless steel 304. The microstructure and morphology of the thin CrAlTiN and CrN/NbN layers were characterized by scanning electron microscopy (SEM), EDX, and X-ray diffraction. The pin on disc wear test was performed on bare and metal-nitride coated SST 304 under a 15 N load at 60 rpm and showed that the wear rates of the thin CrAlTiN and CrN/NbN film coatings were lower than the bare substrate wear ratio. The coefficients of friction (COFs) attained were 0.64, 0.5, and 0.55 for the bare substrate, CrN/NbN coating, and CrAlTiN coating, respectively. Nano indentation tests were also performed on CrAlTiN-coated and CrN/NbN-coated SST 304. The nanohardnesses and Young's moduli of the coated substrates were 28 GPa and 390 GPa (CrN/NbN-coated) and 33 GPa and 450 GPa (CrA1TiN-coated), respectively. For comparison, the nanohardness and Young's modulus of the uncoated substrate were 4.8 GPa and 185 GPa, respectively. Corrosion tests were conducted, and the behaviors of the bare and metal nitride-deposited substrates were studied in CaCl₂ for seven days. The corrosion Tafel test results showed that the metal-nitride coatings offer proper corrosion resistance and can protect the substrate against penetration of CaCl₂ electrolyte. The CrN/NbN-coated substrates showed better corrosion resistance compared to the CrAlTiN-coated ones. In evaluating the biocompatibility of the CrAlTiN and CrN/NbN coatings, the human cell line MDA-MB-231 was found to attach and proliferate well on the surfaces of the two coatings.     

Mechanical and electrical properties of micron-thick nitrogen-doped tetrahedral amorphous carbon coatings

The effect of nitrogen doping on the mechanical and electrical performance of single-layer tetrahedral amorphous carbon (ta-C:N) coatings of up to 1 μm in thickness was investigated using a custom-made filtered cathode vacuum arc (FCVA). The results obtained revealed that the hardness and electrical resistance of the coatings decreased from 65 ± 4.8 GPa (3 kΩ/square) to 25 ± 2.4 GPa (10 Ω/square) with increasing nitrogen gas ratio, which indicates that nitrogen doping occurs through substitution in the sp² phase. Subsequent AES analysis showed that the N/C ratio in the ta-C:N thick-film coatings ranged from 0.03 to 0.29 and increased with the nitrogen flow rate. Variation in the G-peak positions and I(D)/I(G) ratio exhibit a similar trend. It is concluded from these results that micron-thick ta-C:N films have the potential to be used in a wide range of functional coating applications in electronics.     

Mechanical behaviour of MgO–C refractory bricks evaluated by stress–strain curves

In this work, the mechanical behaviour of a set of resin- and pitch-bonded MgO–C refractories containing metallic additives was evaluated in laboratory tests at high temperature in a non-oxidant atmosphere. Commercial bricks were used for this evaluation, and a comprehensive characterization of the as-received materials was performed using several techniques (mineralogical analysis by X-ray diffraction, density and porosity measurements, differential thermal and thermogravimetric analyses and microstructural analysis by reflected light microscopy coupled with cathodoluminiscence accessory and scanning electron microscopy). Stress–strain curves in compression were obtained at room temperature, 600, 1000 and 1400 °C under flowing N₂ gas. An Instron 8501 servo-hydraulic machine was used with a capacitive extensometer suitable for axial strain measurements at high temperatures. A constant displacement of 0.1 mm/min was applied until specimen failure. Several parameters were calculated from the stress–strain curves: failure stress, failure strain, yield stress and secant Young's modulus. Moreover, a comprehensive characterization of the tested specimens was carried out. The analysis of the mechanical behaviour has been based on previous research and the results have been interpreted in terms of the thermal evolution of the brick's microstructure. The resin-based refractory exhibited the higher values of mechanical strength and Young's modulus in the entire range of testing temperatures. Up to 1000 °C, the mechanical behaviour was controlled by the type of binder and the changes in porosity whereas at 1400 °C, the main differences between the responses of resin- and pitch-based refractories were mainly caused by the metallic additive reactions.     

Preparation and characterization of novel bioactive dicalcium silicate ceramics

The beta- and gamma-dicalcium silicate (β- and γ-Ca₂SiO₄) ceramics were prepared by sintering β-Ca₂SiO₄ greens at 1100, 1300, and 1450 °C, respectively, after compacting with cold isostatic pressure. The phase transition from β- to γ-phase of polymorphic ceramics occurred at 1100–1300 °C. Bending strength and Vickers hardness of β-Ca₂SiO₄ ceramic sintered at 1100 °C were only 25.6 ± 3.8 MPa and 0.41 ± 0.05 GPa. In contrast, the mechanical properties of the γ-Ca₂SiO₄ were improved remarkably when the ceramics were sintered at 1450 °C, corresponding to bending strength, 97.1 ± 6.7 MPa; Vickers hardness, 4.34 ± 0.35 GPa, respectively. The ceramics were soaked in the simulated body fluid (SBF) for various periods were characterized by SEM, XRD, FTIR, and EDS analysis, and the results indicated that the carbonated hydroxyapatite (CHA) was formed on the surface of the ceramics within 3 days. In addition, cell attachment assay showed that the ceramics supported the mesenchymal stem cells adhesion and spreading, and the cells established close contacts with the ceramics after 1 day of culture. These findings indicate that the γ-Ca₂SiO₄ ceramic possesses good bioactivity, biocompatibility and mechanical properties, and might be a promising bone implant material.     

Densification,microstructure, elastic and mechanical properties of reactive hot-pressed ZrB2–ZrC–Zr cermets

In this study, cermets composed of zirconium diboride and zirconium carbide with intergranular zirconium were sintered by reactive hot-pressing. Relative density exceeding 97% was obtained for the sintered cermets having four distinct compositions varying in concentration of excess Zr. Their densification behaviour was examined by monitoring displacement during sintering. The microstructure was characterized by scanning electron microscopy and X-ray diffraction, and the elastic and mechanical properties were evaluated at room temperature. The effects of Zr concentration on the densification and mechanical properties were assessed. The ZrB₂ and ZrC micron-grains coarsened with increasing amount of Zr starting material. In addition, the cermets exhibited high flexural strength (546–890 MPa) and fracture toughness (6.63–10.24 MPa m^1/2), which simultaneously increased with increasing Zr concentration. However, the elastic moduli and hardness (11–18 GPa) decreased with increasing Zr. The shear modulus and Young's modulus were in the range of 150–190 GPa and 360–440 GPa, respectively.     

Sintering behaviour and translucency of dense Eu2O3 ceramics

Eu₂O₃ ceramics have been obtained at sintering temperatures of between 1000 °C and 1550 °C. X-ray diffraction and scanning electron microscopy, in combination with dilatometry experiments, allowed understanding the sintering behaviour. Moderate grain growth followed an efficient densification process between 1400 °C and 1550 °C, which yielded high-density ceramics with an average grain size of 4 μm. The ceramics had Young modulus of 125 GPa, in agreement with the previously published data. The dense Eu₂O₃ ceramics were translucent (35.1% transmittance at 800 nm of 0.8 mm thick discs), showing in addition a slightly pink colour. We propose that the combination of high density and an average grain size of 4 μm is responsible for this novel functionality.     

Microstructure,mechanical, thermal,and oxidation properties of a Zr2[Al(Si)]4C5–SiC composite prepared by in situ reaction/hot-pressing

The microstructure, mechanical and thermal properties, as well as oxidation behavior, of in situ hot-pressed Zr₂[Al(Si)]₄C₅–30 vol.% SiC composite have been characterized. The microstructure is composed of elongated Zr₂[Al(Si)]₄C₅ grains and embedded SiC particles. The composite shows superior hardness (Vickers hardness of 16.4 GPa), stiffness (Young's modulus of 386 GPa), strength (bending strength of 353 MPa), and toughness (fracture toughness of 6.62 MPa m^1/2) compared to a monolithic Zr₂[Al(Si)]₄C₅ ceramic. Stiffness is maintained up to 1600 °C (323 GPa) due to clean grain boundaries with no glassy phase. The composite also exhibits higher specific heat capacity and thermal conductivity as well as better oxidation resistance compared to Zr₂[Al(Si)]₄C₅.