Gelcasting of alumina using the hexamethylenediamine–paraformaldehyde monomer system

A new aqueous alumina gelcasting system using hexamethylenediamine (HMDA) and paraformaldehyde monomers has been studied. The 500 vol% aqueous alumina slurries ‘A’ and ‘B’ containing paraformaldehyde and HMDA, respectively, undergo gelation after thorough mixing of the two due to the polymerization of HMDA and formaldehyde. The gelation time of the slurries cast in a mold is in the range of 7–2.4 min at HMDA to formaldehyde mole ratio in the range of 1.1–1.5. The faster reaction between HMDA and formaldehyde prevents the formaldehyde emission during the processing. The minimum HMDA to formaldehyde mole ratio required for the formation of a mechanically stable gel is 1.1. The compressive strengths and Young's modulus of the wet and dry alumina bodies increased with an increase in HMDA to formaldehyde mole ratio. Though the wet gelcast alumina bodies had low compressive strength (11.2–88.7 kPa) and Young's modulus (0.17–5.9 MPa) the dried ones showed high strength (6–11.7 MPa) and Young's modulus (209–364 MPa). The binder removal by slow heating to a temperature below 500 °C followed by sintering at 1600 °C produced alumina ceramics with ~97% of theoretical density.     

Mechanical properties of sintered La9.33Si2Ge4O26 oxyapatite materials for SOFC electrolytes

Mechanical properties of La_9.33Si₂Ge₄O₂₆ prepared by mechanical alloying and subsequent sintering at 1300–1400 °C for 1 h were evaluated. Hardness and Young's modulus values in the range 7.3–9.6 GPa and 106–135 GPa, respectively, were obtained from nanohardness tests. The fracture toughness values derived from the Palmqvist method varied between 3.5 and 3.9 MPa m^1/2 from classical microindentation test with an indentation load of 9.8 N. Yield stress (σ_y) was determined by inverse analysis from microhardness tests. The maximum value of σ_y (1829 MPa) was obtained for the sample sintered at 1400 °C showing the highest density (5.42 g/cm³).     

A novel polyborosilazane for high-temperature amorphous Si–B–N–C ceramic fibres

Polyborosilazane synthesised from BCl₃, HMeSiCl₂, and Me₃SiNHSiMe₃ is easy to cross-link for dehydrogenation of Si–H and N–H, which limits its practical applications for Si–B–N–C fibres on an industrial scale. Therefore, in this context, MeSiCl₃ was used instead of HMeSiCl₂ to synthesise a novel polyborosilazane with limited cross-linking density to fabricate Si–B–N–C fibres. The polyborosilazane synthesised from BCl₃, MeSiCl₃, and Me₃SiNHSiMe₃ exhibits good melt-processability and 1 km long polyborosilazane fibre can be obtained by melt spinning. Prior to pyrolysis, chemical curing with vapour HSiCl₃ at 80 °C was utilised to make the λ green fibres infusible. The as-cured fibres were subsequently pyrolyzed at 1200 °C in nitrogen atmospheres to provide Si–B–N–C ceramic fibres with ca. 1.5 GPa in tensile strength, ca. 160 GPa in Young's modulus, ca. 12 μm in diameter and keeping amorphous up to 1700 °C, which makes them to be promising reinforcements in ceramic matrix composites for high temperature applications.     

Synthesis and characteristics of fly ash and bottom ash based geopolymers–A comparative study

The research was carried out to develop geopolymers mortars and concrete from fly ash and bottom ash and compare the characteristics deriving from either of these products. The mortars were produced by mixing the ashes with sodium silicate and sodium hydroxide as activator solution. After curing and drying, the bulk density, apparent density and porosity, of geopolymer samples were evaluated. The microstructure, phase composition and thermal behavior of geopolymer samples were characterized by scanning electron microscopy, XRD and TGA-DTA analysis respectively. FTIR analysis revealed higher degree of reaction in bottom ash based geopolymer. Mechanical characterization shows, geopolymer processed from fly ash having a compressive strength 61.4 MPa and Young's modulus of 2.9 GPa, whereas bottom ash geopolymer shows a compressive strength up to 55.2 MPa and Young's modulus of 2.8 GPa. The mechanical characterization depicts that bottom ash geopolymers are almost equally viable as fly ash geopolymer. Thermal conductivity analysis reveals that fly ash geopolymer shows lower thermal conductivity of 0.58 W/mK compared to bottom ash geopolymer 0.85 W/mK.     

Improved mechanical properties of SnO2:F thin film by structural modification

This study reports the improvement in the mechanical properties of SnO₂:F (FTO) thin films through the modification of the structure and surface morphology. The FTO thin films are deposited on glass substrates by the atmospheric pressure chemical vapor deposition method on an industrial production line. Both the average grain size and the surface roughness were progressively increased by increasing the flow rate of metal organic monobutyltin trichloride (MBTC). The hardness and Young's modulus of the FTO films increased from 9.01 GPa to 15.08 GPa, and from 125.24 GPa to 206.93 GPa, respectively, according to the nanoindenter results. Post-heat treatment at 650 °C for 10 min resulted in a further increase in the hardness and Young's modulus, reaching maximum values of ~15.89 GPa and ~235.9 GPa, respectively. The enhancement in mechanical properties can be attributed to the reduced grain boundaries and the improved structural densification.     

In Vitro Dissolution,Microstructural and Mechanical Characterizations of Microplasma‐Sprayed Hydroxyapatite Coating

Hydroxyapatite coating was developed with high degree of crystallinity on SS316L substrate by the microplasma spraying technique. Systematic in vitro study of the coating was conducted after the immersion into the simulated body fluid for 1–14 days. Inductively coupled plasma–atomic emission spectroscopy, X‐ray diffraction, Fourier transformed infrared spectroscopy, and scanning electron microscopy were utilized for physicochemical and microstructural characterizations. Nanoindentation technique employed to evaluate the nanohardness and Young's modulus of the coating at a constant load of 100 mN. Further, the tribological characteristic was also examined by microscratch testing at a ramping normal load of 10–10.6 N.     

Evaluation of microwave processed glass–ceramic coating on nimonic superalloy substrate

MgO–Al₂O₃–TiO₂ based glass–ceramic coatings were formed on nimonic superalloy substrates by microwave and conventional heat treatment processes. The resultant glass–ceramic coatings were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), image analysis, nanohardness and Young's modulus evaluation by depth sensitive indentation (DSI) technique. Nanohardness and Young's modulus values of the microwave heat treated glass–ceramic coatings were improved in comparison to those of the conventionally treated glass–ceramic coatings due to presence of finer sized crystallites in the microwave processed coatings. Slight enhancement in the nanohardness and Young's modulus values with soaking time for the microwave processed coatings were explained in terms of the microstructural refinement and the reinforcement of the parent glass matrix.     

Influence of microstructure and phase composition on the nanoindentation characterization of bioceramic materials based on hydroxyapatite

The aim of the study was to investigate the influence of microstructure and phase composition on the mechanical behaviour of hydroxyapatite (HAp) and biphasic HAp/β-tricalcium phosphate (β-TCP) bioceramic materials using nanoindentation. The formation of β-TCP phase in the HAp ceramic had the predominant influence on the nanomechanical properties of compact ceramics. For investigated microstructures there appear to be a slight decrease in the elastic modulus with increasing load and a higher decrease in hardness, which are in agreement with upper bounds of the results reported in literature. Maximal value of reduced modulus and hardness is yielded with pure HAp, and is measured to be 133.76 GPa for average grain size of 3 μm and 12.18 GPa for average grain size of 140 nm, respectively. The average modulus and hardness results for HAp/β-TCP ceramics with higher (101.61 GPa, 6.76 GPa) and lower grain size (115.72 GPa, 8.76 GPa) show sufficient mechanical properties in order to serve as hard tissue replacement material.     

Investigation of mechanical behavior of CPC/bone specimens by finite element analysis

Calcium phosphate cement (CPC), as an important injectable biomaterial, is extensively used for bone repair in clinical application. If mechanical properties of CPC match well with that of bone tissue, it can create an appropriate mechanical environment for bone repair. In our study, the objective was to investigate the responses of bone tissue to CPC in different series of elastic modulus combinations. Finite element analysis (FEA) was applied to calculate the stress/strain on CPC-bone specimens and to further forecast the potential risky area. The predicted results indicated that CPC materials and bone tissue had different stress distribution patterns under the same loading condition. For CPC material, the Von Mises Stress peak occurred in the bone–cement joint area; while for bone tissue, the risky area was located at the bridge area among trabecular bones. The porous and loose structure of cancellous bone induced a greater Von Mises Stress in bone tissue. Quantitative analysis indicated that stress/strain distribution was directly correlated with the elastic modulus of material. When Young's modulus of bone and CPC was 1 GPa and 6.10 GPa respectively, the optimal stress matching between bone and CPC was achieved. In sum, this work confirmed that FE modeling was the ideal method for predicting fracture behavior of bone–CPC specimen both qualitatively and quantitatively.     

Effect of functional groups (methyl,phenyl) on organic–inorganic hybrid sol–gel silica coatings on surface modified SS 316

Organic–inorganic hybrid sol–gel based silica coatings derived from hydrolysis and condensation of organically modified silane precursors like phenyltrimethoxysilane and methyltriethoxysilane along with tetraethoxysilane were deposited on different surface pre-treated (as-cleaned, plasma-treated, shot-blasted) SS 316 grade stainless steel substrate, using dip coating technique. The coatings were heat treated at 150 °C for 2 h in air. The pre-treated surfaces were characterized using X-ray Photoelectron Spectroscopy and Scanning Electron Microscopy. The water content of the sols was determined by Karl Fischer titration to evaluate the degree of completion of hydrolysis and condensation reactions. Cured coatings were characterized to evaluate thickness, water contact angle, pencil scratch hardness, gloss, and shrinkage in coating thickness. Impact test was carried out on pigmented coatings derived from sols synthesized using the two silane precursors. The corrosion resistance and water durability tests were carried out to compare the coatings derived from using different precursors and different surface pre-treatments. The corrosion tests were carried out for 1 h and 24 h exposure to a 3.5% NaCl solution by electrochemical polarization measurements. It was found that coatings from methyl substituted organically modified alkoxysilane exhibited better hydrophobicity, scratch hardness, impact resistance and barrier properties with respect to corrosion, when compared to those derived from phenyl substituted trialkoxysilane. The difference in performance of coatings was explained on the basis of difference in hydrolysis and condensation rates between the two organically modified silane precursors used for the sol synthesis.     

Nanoscale elastic-plastic deformation and stress distributions of the C plane of sapphire single crystal during nanoindentation

The nanoscale elastic-plastic characteristics of the C plane of sapphire single crystal were studied by ultra-low nanoindentation loads with a Berkovich indenter within the indentation depth less than 60 nm. The smaller the loading rate is, the greater the corresponding critical pop-in loads and the width of pop-in extension become. It is shown that hardness obviously exhibits the indentation size effect (ISE), which is 46.7 ± 15 GPa at the ISE region and is equal to 27.5 ± 2 GPa at the non-ISE region. The indentation modulus of the C plane decreases with increasing the indentation depth and equals 420.6 ± 20 GPa at the steady-state when the indentation depth exceeds 60 nm. Based on the Schmidt law, Hertzian contact theory and crystallography, the possibilities of activation of primary slip systems indented on the C surface and the distributions of critical resolved shear stresses on the slip plane were analyzed.     

Mechanical behavior of La0.8Sr0.2Ga0.8Mg0.2O3 perovskites

This paper examines the important mechanical properties of commercially purchased La_0.8Sr_0.2Ga_0.8Mg_0.2O₃ at room temperature and 800 °C. Sr and Mg-doped lanthanum gallates (LSGM) are strong candidates for use as solid electrolytes in lower temperature solid oxide fuel cells operating at or below 800 °C. The material was found to be phase pure with a Young's modulus value of ∼175 GPa. The four point bending strength of the LSGM samples remained almost constant from 121 ± 35 MPa at room temperature to 126 ± 20 MPa at 800 °C. The fracture toughness, as measured by the single edge V notch beam (SEVNB) method, was 1.22 ± 0.06 MPa√m at room temperature, 1.04 ± 0.09 MPa√m at 700 °C followed by a small increase 1.31 ± 0.16 MPa√m at 800 °C. We also report, for the first time, the static subcritical (or slow) crack-growth (SCG) behavior of natural cracks in LSGM performed in four point bending tests at room temperature. The exponent of a power-law representation in the SCG tests was found to be n = 15, a rather low value showing LSGM to be highly susceptible to room temperature SCG.     

Effect of the addition of polyvinylpyrrolidone as a pore-former on microstructure and mechanical strength of porous alumina ceramics

The effect of the solvent on the properties of porous alumina ceramics was studied when polyvinylpyrrolidone (PVP) was used as an organic pore-former. In particular, porous alumina ceramics were produced by dry-pressing of mixed PVP–alumina powder; the mixing of PVP and alumina powder was achieved via ball milling using water or acetone as solvent, or dry ball milling. Due to the different solubility of PVP in water and acetone, porous alumina ceramics with different pore structures and mechanical properties were obtained. Because of its cylindrical pores being aligned to some extent, the sample prepared using acetone as solvent exhibited the highest bending strength (140.2 MPa) and Young's modulus (57.4 GPa), which were 1.6 times and 3.4 times higher compared to that prepared without PVP. Moreover, the addition of PVP via wet ball milling led to more uniform dispersion of PVP in alumina, hence limiting the grain growth during sintering process and increasing the grain bonding.     

Spark plasma sintering technique for reaction sintering of Al2O3/Ni nanocomposite and its mechanical properties

Al₂O₃/Ni nanocomposites were prepared by spark plasma sintering (SPS) using reaction sintering method and the mechanical properties of the obtained nanocomposites are reported. The starting materials of Al₂O₃–NiO solid solution were synthesized from aluminum sulfate and nickel sulfate. These Al₂O₃–NiO powders were changed into Al₂O₃ and Ni phases during sintering process. The obtained nanocomposites showed high relative densities (>98%). SEM micrographs showed homogeneously dispersed Ni grains in the matrix. The 3-point strength and the fracture toughness of the composites significantly improved from 450 MPa in the monolithic α-Al₂O₃ to 766 MPa in the 10 mol% (2.8 vol.%) Ni nanocomposite and from 3.7 to 5.6 MPa m^1/2 in 13 mol% (3.7 vol.%) Ni nanocomposite. On the other hand, Young's modulus and Vickers hardness of the nanocomposites were mostly same as those of the monolithic α-Al₂O₃.     

Evaluation of mechanical properties and hydrophobicity of room-temperature,moisture-curable polysilazane coatings

Polysilazane coatings have a broad need in real-life applications, which require low processing or working temperature. In this work, five commercially available polysilazanes have been spin-coated on polycarbonate substrates and cured in ambient environment and temperature to obtain transparent, crack-free, and dense films. The degree of crosslinking is found to have a significant impact on the hardness and Young's modulus of the polysilazane films but has a minor influence on the film thickness and hydrophobicity. Among all five polysilazane coatings, the inorganic perhydropolysilazane-based coating exhibits the largest hardness (2.05 ± 0.01 GPa) and Young's modulus (10.76 ± 0.03 GPa) after 7 days of curing, while the polyorganosilazane-derived films exhibit higher hydrophobicity. The molecular structure of polysilazanes plays a key role in mechanical properties and hydrophobicity of the associated films, as well as the adhesion of coatings to substrates, providing an intuitive and reliable way for selecting a suitable polysilazane coating material for a specific application.     

Corrosion resistant low temperature carburized SS 316 as bipolar plate material for PEMFC application

One of the current challenges for application of PEM fuel cell is to find corrosion resistant, electrically conductive, light weight, cost competitive bipolar plate material. Low temperature carburization (LTC) of stainless steels is a novel, patented process by Swagelok Company. This paper addresses the corrosion resistance characteristics of LTC SS 316 for polymer electrolyte membrane fuel cell (PEMFC) bipolar plate applications. Corrosion properties of this material were studied using potentiodynamic and potentiostatic tests in simulated (1 M H₂SO₄ + 2 ppm HF, 0.5 M H₂SO₄, pH: 4.0, and 5% HCl + 5% Na₂SO₄) PEMFC conditions. LTC SS 316 showed excellent corrosion resistance in these conditions compared to SS 316. The mechanism of anodic dissolution and general corrosion of LTC SS 316 was observed to be similar to SS 316 however the extent of LTC SS 316 corrosion was less. LTC SS 316 showed corrosion currents well below 16 μA cm⁻² in anodic and cathodic atmospheres under potentiostatic conditions. The potentiostatic current rapidly falls to ∼4.0 and ∼1.5 μA cm⁻² under anodic and cathodic conditions, respectively. LTC SS 316 was observed to form a thinner oxide layer as compared to SS 316 after 24 h of potentiostatic testing. Moreover LTC SS 316 lowered the interfacial contact resistance by approximately 24% as compared to SS 316 after corrosion testing. Hence this study clearly states the performance advantage of using LTC SS 316 as bipolar plate material as compared to conventional materials.     

Preparation and characterization of hydroxyapatite-forsterite-bioactive glass nanocomposite coatings for biomedical applications

In order to improve biological and mechanical properties of hydroxyapatite, the concept of hydroxyapatite-included nanocomposite coatings was introduced. By judiciously choosing constituent ceramics for composites preparation, the biological and mechanical performance of coatings can be tailored in order to meet various clinical requirements. The aim of this work was fabrication, development and characterization of novel hydroxyapatite-forsterite-bioactive glass nanocomposite coatings. The sol-gel technique was used to prepare hydroxyapatite-forsterite-bioactive glass nanocomposite in order to apply coating on 316L stainless steel (SS) by dip coating technique. The X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) were used to investigate the phase structure, microstructure and morphology of the coating. In order to evaluate the forsterite incorporation influence upon bioactivity, the changes on the surfaces of the prepared composite coatings after the predicted days of contact with simulated body fluid (SBF) were investigated by SEM. Results showed that the suitable calcined temperature for nanocomposite coatings with different amounts of forsterite was 600 °C. At this temperature, the homogenous and crack-free coating could attach to the 316L SS substrate. The crystallite sizes of the prepared coatings were lower than 100 nm. The EDX analysis of hydroxyapatite-forsterite-bioglass, coated 316L SS surface, indicated consisting elements of prepared coatings and the substrate. During immersion in the SBF at pre-determined time intervals, apatite layer was formed and stimulation for apatite formation was increased with increase in forsterite amounts. It seems that hydroxyapatite-forsterite-bioactive glass nanocomposite coatings might be good candidates for biomedical applications.     

Microstructure and mechanical properties of hydroxyapatite obtained by gel-casting process

In the present work, well-shaped HAp green bodies were obtained by the gel-casting process with 50 vol.% slurry. After drying, the microstructure and pore distribution of the green body were investigated. The density, compressive strength and flexural strength of the green body were 1.621 g/cm³, 32.6 ± 3.2 MPa and 13.8 ± 1.0 MPa, respectively. After pressureless sintering at the range of 1100–1300 °C for 2 h, the relative density of the final product ranges from 71.8 to 97.1% th. The maximum value of flexural strength, elastic modulus, hardness and fracture toughness were 84.6 ± 12.6 MPa, 138 ± 7 GPa, 4.45 ± 0.18 GPa and 0.95 ± 0.13 MPa m^1/2, respectively. SEM images show a compact and uniform microstructure; the average grain size was found by using the linear intercept method. XRD and FTIR determined the phase and the radical preserved after sintering.     

Determination of hardness, Young's modulus and fracture toughness of lanthanum tungstates as novel proton conductors

Lanthanum tungstate is a promising material to be used as electrolyte in proton conducting fuel cells, or as a mixed proton-electron conducting membrane for hydrogen separation, and its mechanical properties are crucial for these applications. Lanthanum tungstates with a La/W atomic ratio between 4.8 and 6.0 have been investigated at room temperature at micro/nanoindentation range. Lanthanum tungstates exhibit a strain gradient plasticity at the vicinity of the imprints, which implies that the hardness presents an indentation size effect that was corrected using the Nix and Gao approach. The hardness and Young's modulus have therefore been determined to be 8-9 GPa and 130 ± 15 GPa, respectively. The fracture toughness was estimated to be ∼2 MPa m^1/2 for LWO56 using the Palqmvist equation. Both hardness and Young's modulus did not present a significant dependence with neither the sintering temperature nor the composition. The different imprints were visualized by Atomic Force Microscopy.