Nanoscale elastic-plastic deformation and mechanical properties of 3C-SiC thin film using nanoindentation

The elastic-plastic deformation of 3C-SiC thin film was investigated by a nanoindenter equipped with the Berkovich tip. Transition from pure elastic to elastic-plastic deformation was evidenced at an approximate load of 0.35 mN, when loading the sample at several peak loads ranging from 0.5 to 5 mN. The indentation size effect observed in 3C-SiC and was analyzed by Nix-Gao model. In purely elastic region, the Oliver- Pharr hardness values were 44 ± 2 GPa. In contrast, indentation size effects were evidenced in 3C-SiC specimen and the average value of Oliver-Pharr hardness in the indentation size effect region was 36 ± 2 GPa. Furthermore, depth independent or intrinsic hardness extracted from Nix-Gao was estimated as H_o = 26 ± 1 GPa which was also validated by proportional specimen resistance model, ie, H₁ = 28 ± 1 and H₂ = 28.5 ± 0.1 GPa. Besides, energy principle was utilized to extract Sakai Hardness as 104 GPa, which is combined elastic and elastic-plastic response. Moreover, based on energy principle, another property, ie, work of indentation was also determined to be 20 nJ/μm³. Similarly, elastic modulus had almost depicted stabilized value of 325 ± 8 GPa in pure elastic and elastic-plastic regions. In addition, plastic zone size was also estimated in elastic-plastic region using Johnson cavity model at pop-in and higher loads. Based on the first pop-in load at 0.35 mN, the distributions of shear and principal stresses were evaluated on various slip planes to elaborate the deformation behavior. Increase in loading rate from 100 to 400 μN/s increased critical pop-in load from 0.35 to 0.64 mN. This increase in critical pop-in load with increasing loading rate and values of maximum contact pressure indicates that no phase assisted transformation will occur at pop-in load. Based on theoretically calculated maximum tensile and cleavage strengths, it was affirmed that the elastic-plastic deformation occurred due to pop-in formation rather than tensile stresses. Moreover, it was also concluded on basis of Hertzian contact theory and Schmidt law that the highest possibility of slippage in 3C-SiC was along the {111} glide plane.     

Nanoindentation of WC–Co hardmetals

WC–Co cemented carbide has been investigated using instrumented indentation with maximum applied loads from 0.1 to 10 mN. The hardness and indentation modulus of individual phases and the influence of crystallographic orientation of WC on the hardness and indentation modulus have been studied. The hardness of the Co binder was approximately 10 GPa and that of WC grains up to 50 GPa with relatively large scatter under the indentation load of 1 mN. Investigation of the role of crystallographic orientation of WC grains on hardness at 10 mN load revealed average values of H_ITbasal = 40.4 GPa (E_ITbasal = 674 GPa) and H_ITprismatic = 32.8 GPa (E_Itprismatic = 542 GPa), respectively. The scatter in the measured values at low indentation loads is caused by the effects of surface and sub-surface characteristics (residual stress, damaged region) and at higher loads by “mix-phase” volume below the indenter.     

Mechanisms of ductile mode machining for AlON ceramics

This paper aims to reveal the mechanisms of ductile mode machining for AlON ceramics. The removal characteristics during machining were studied through ultra-precision grinding experiments. The machined surface consists of fractured and smooth areas, which were generated by brittle and ductile removal, respectively, of the individual AlON grains. The material removal mode has a determining effect on the surface/subsurface quality. The proportion of the fractured areas on the ground surface decreased gradually with a decrease in the depth of cut. The crystal indices of the grains most prone to brittle removal on the workpiece surface were determined using micro-area X-ray diffraction (μXRD) analysis performed using a beam with 50 μm diameter. The results showed that the ductile removal of the {111} planes is critical for the ductile mode machining of AlON. Nanoindentation tests based on electron back-scattered diffraction (EBSD) indicated that AlON shows strong anisotropy in its mechanical properties and machinability. The (111) plane has the highest hardness and lowest fracture toughness, at 22.91 GPa and 1.8 MPa m^1/2, respectively. The material removal mechanism during the grinding of AlON was discussed in detail. The minimum and maximum d_c(hkl) values must be known for classifying whether the removal mode of the workpiece is brittle or ductile. A damage-free surface could be obtained during ductile mode grinding by controlling h_max to be less than d_c(111). The subsurface deformation mechanism during ductile mode grinding was analysed. An amorphous layer was observed close to the ground surface. Further, dense dislocations with no particular orientation were present beneath this amorphous layer. As the crystal structure became clearer with an increase in the depth, the plastic deformation shifted to stacking faults parallel to the {111} planes.     

Titanium diboride–silicon carbide–boron carbide ceramics with super‐high hardness and strength

Triplex particulate composites composed of boride and carbide ceramics were found to have high strength, hardness, and fracture toughness values. Two compositions consisting of 70:15:15 and 1:1:1 volume ratios of TiB_2, SiC, and B₄C were produced from commercially available powders by hot‐pressing. The 70:15:15 ceramic exhibited a strength of ~1.3 GPa, while the 1:1:1 ceramic had a strength of ~0.9 GPa. These strengths are comparable to super‐strong Y₂O₃‐PSZ and β‐SiAlON based composites. The Vickers’ hardness values of these ceramics were ~32 GPa for indent loads of 9.8 N. Hardness increased as indentation load decreased. The 1:1:1 ceramic had a hardness of ~53 GPa at an indentation load of 0.49 N, higher than values reported for so‐called “super‐hard” ceramics, and comparable to c‐BN.     

The Effect of Composition on the Optical Properties and Hardness of Transparent Al‐rich MgO·nAl2O3 Spinel Ceramics

The study investigates the transmittance and hardness of Al‐rich spinel ceramics (MgO·nAl₂O₃, 1 ≤ n ≤ 2.5) prepared by reaction air sintering (up to closed porosity) of different ratios of fine and coarse‐grained commercial Al₂O₃ and MgO raw powders completed by subsequent hot isostatic pressing (HiP). Different compositions give rise to a wide range of presintering temperatures. With starting compositions 1 ≤ n ≤ 1.5, presintering results in a formation of single‐phase spinel, in which the excess of Al is solved. With higher Al contents (n > 1.5), however, a biphasic ceramic of stoichiometric MgAl₂O₄ and residual alumina is formed first. This excess alumina is incorporated into the spinel lattice during the final HiP at a temperature of 1750°C. Single‐phase, highly transparent spinel is obtained by increasing the Al‐content up to n = 2.5, which gives about 85% in‐line transmittance in the visible range of light and about 63% at a UV wavelength of 200 nm. Whereas the optical properties can be improved, the hardness (HV1) slightly decreases with increasing Al content. Depending on the raw powders, the hardness of samples prepared by finer powders tend to higher values enabled by the development of a bimodal microstructure with a finer grain fraction (≤2 μm) between coarser grains (≤156 μm). In contrast, samples made of coarser powders need higher sintering temperatures and exhibit, then, a monomodal microstructure of very large grains (≤622 μm) only.     

Influence of nitrogen vacancy defects incorporation on densification behaviour of spark plasma sintered non-stoichiometric TiN1−x

Highly nitrogen deficient non-stoichiometric TiN_1?x powders within nitrogen vacancy defects (0.3<1?x<0.5) were prepared through mechanical alloying and consolidated through spark plasma sintering. Increasing nitrogen vacancy defects promoted densification behaviour of TiN_1?x. Nitrogen vacancies accelerated material transport and diffusion during sintering. The altered strong covalent bonding nature of TiN_1?x was believed to enhance the sinterability. TiN_1?x (1?x?=?0.32) ceramic reached relative density of 98.8%, Vickers hardness of 17.0 GPa and grain size of 200–300 nm after sintering at 1000°C, 40 MPa and 10 min.     

Coherency strain induced hardening in bulk polycrystalline ceramics: A case study with MgO-based “ceramic alloys”

The role of coherency strain at the matrix/precipitate interface toward hardening of bulk polycrystalline “ceramic alloys” has been established here. Formation of “near ideal” bulk polycrystalline ceramic microstructure characterized by the presence of uniformly distributed coherent “ultra-fine” MgCr₂O₄ particles (size: ~25 nm) within matrix (MgO) grains was achieved via solid-state precipitation during aging treatment of bulk supersaturated MgO–Cr₂O₃ solid solutions (formed during pressureless sintering in air, followed by fast cooling). The as-aged MgO–MgCr₂O₄ “ceramic alloys” exhibited hardness increment by ~73% over that of phase pure bulk MgO upon aging for just 10 hours at 1000°C in air. Evidences toward the presence of significant coherency strains across the MgO/MgCr₂O₄ coherent interfaces were obtained with transmission electron microscopy. Analysis based on hardening mechanisms and comparisons with MgO–MgFe₂O₄ system, having lesser hardening due to lower misfit strain at MgO/MgFe₂O₄ coherent interfaces (despite greater content of second-phase particles), confirm the dominant role of coherency strains toward hardness enhancement in “ceramic alloys.”     

Microstructure and indentation damage resistance of ZrB2‐20 vol.%SiC ipo‐eutectic composites

Zirconium diboride with 20 vol.% silicon carbide bulk composites were fabricated using directionally solidification (DS) and also by spark plasma sintering (SPS) of crushed DS ingots. During the DS the cooling front aligned the c‐axis of ZrB₂ grains and its Lotgering factor of f_(00l) was high as 0.98. The Vickers hardness was anisotropic and it was high as 17.6 GPa along the c‐axis and 15.3 GPa when measured in an orthogonal direction. On both surfaces, even when using 100 N indentation load, no cracks were observed, suggesting a very high resistance to crack propagation. Such anomalous behavior was attributed to the hierarchical structure of DS sample where the ZrB₂ phase was under strong compression and the SiC phase was in tension. In the SPSed sample, the microstructure was isotropic respect to the direction of applied pressure. Indentation cracks appeared around the indent corners but not emanated from the diagonals, confirming high damage resistance.     

Fabrication of ZrB2 ceramics by reactive hot pressing of ZrB and B

Zirconium diboride (ZrB₂) ceramics were prepared by reactive hot pressing of ZrB+B powder mixture. Formation of a transient liquid due to eutectic reaction of ZrB₂+Zr→L_eu(ZrB2+Zr) at 1661°C following peritectic decomposition of 2ZrB=ZrB₂+Zr at 1250°C during heating up of the ZrB+B mixture facilitated densification. The liquid phase was subsequently eliminated via reaction of B with Zr in the eutectic liquid L_eu(ZrB2+Zr) to result in a dense ZrB₂ ceramic. Full density was reached after reactive hot pressing at 1900°C under 30 MPa for 1 h. The ZrB₂ ceramic had a refined microstructure consisting of grains of <1.5 μm in size and relatively good Vickers hardness (21 ± 2 GPa) and flexural strength (595 ± 63 MPa).     

Synthesis of Al2O3/SiO2 nano‐nano composite ceramics under high pressure and its inverse Hall–Petch behavior

We report the synthesis of alumina/stishovite nano‐nano composite ceramics through a pressure‐induced dissociation in Al₂SiO₅ at a pressure of 15.6 GPa and temperatures of 1300°C‐1900°C. Stishovite is a high‐pressure polymorph of silica and the hardest known oxide at ambient conditions. The grain size of the composites increases with synthesis temperature from ~15 to ~750 nm. The composite is harder than alumina and the hardness increases with reducing grain size down to ~80 nm following a Hall–Petch relation. The maximum hardness with grain size of 81 nm is 23 ± 1 GPa. A softening with reducing grain size was observed below this grain size down to ~15 nm, which is known as inverse Hall–Petch behavior. The grain size dependence of the hardness might be explained by a composite model with a softer grain‐boundary phase.     

Evolution of microstructure,mechanical, and optical properties of Y2O3-MgO nanocomposites fabricated by high pressure spark plasma sintering

A high-pressure spark plasma sintering (SPS) process was applied for consolidating Y₂O₃–MgO nanocomposites. This approach enabled to fabricate a fully dense infrared (IR) transparent nanocomposites, which possess an average grain size of ∼70 nm and high hardness, at a relatively low sintering temperature of 1130 °C under a high pressure of 300 MPa. The light transmittance was improved with increasing pressure and reached to the maximum transmittance of 64.5% at a wavelength of 0.2–1.6 μm owing to the fine-grained microstructure. The Vickers hardness exhibited 16.6 ± 0.7 GPa for the grain size of 74 nm, which is significantly higher than that of the sub-micro grains obtained at a conventional sintering pressure of 70 MPa (11.9 ± 0.8 GPa). The hardness rigorously followed the Hall–Petch relationship, that is, it is enhanced with a reduction of the grain size. Successful fabrication of the high-performance Y₂O₃–MgO nanocomposites indicates that the nanopowder processing followed by the high-pressure sintering process can be applied for fabricating fully dense fine-grained nanocomposites with excellent optical and mechanical properties.     

Nano–Nano Composite Powders of Lanthanum Zirconate and Yttria‐Stabilized Zirconia by Spray Pyrolysis

Powders composing of La₂Zr₂O₇ (LZ) and (Zr_0.8Y_0.2)O_1.9 (10YSZ) phases (volume ratio = 1:1) were synthesized by using a sol‐spray pyrolysis method. The effects of annealing temperature on the grain size and lattice parameter of the LZ–10YSZ powders were investigated. XRD results showed that the grain size of LZ and 10YSZ phases gradually grew from 10 to 95 nm and from 5 to 65 nm as the annealing temperature elevated from 900°C to 1200°C. The relative decreasing percentage of grain size comparing to that of the single‐phase LZ and 10YSZ powders were in the range 9%–36% and 37%–86%. The activation energy for grain growth of LZ and 10YSZ phases in the composite powders were 225 ± 12 and 382 ± 17 kJ/mol, which were 20% and 183% higher than that of the single‐phase counterparts. Obvious lattice contraction and lattice expansion for LZ and 10YSZ phases were observed at temperatures below 1100°C, respectively. SEM results revealed that LZ and 10YSZ phases were homogeneously distributed in the sintered bulk. The TEM results suggested that the grain growth was affected by the interaction on nanometer length scales of grain boundaries between LZ and 10YSZ phases in the composite.     

Flash Sintering of dense alumina ceramic discs with high hardness

MgO-doped-Al₂O₃ ceramic discs were fabricated by flash sintering (FS) and pressureless sintering (PS). The results showed that MgO-doped Al₂O₃ exhibited typical characteristics of flash sintering under an electric field in excess 2500 V/cm. Compared with the PS- fabricated specimen, the flash sintered specimens exhibited sub-micron grains (≤760 nm) and homogeneous microstructures. The relative density of the ?ash sintered MgO-doped Al₂O₃ ceramics increased with current density, reaching 99.91 % when the current density increased to 7 mA/mm². The FS-fabricated sample exhibited higher hardness (21.02 GPa) and fracture toughness (3.46 MPa m^1/2) than PS-fabricated sample.     

Microstructure evolution of a W‐doped ZrB2 ceramic upon high‐temperature oxidation

This basic research deals with the microstructure evolution of a W‐doped ZrB₂ ceramic, as‐sintered and upon oxidation at 1650°C. Transmission electron microscopy enabled to disclose microstructural features occurred during oxidation never observed before. In the pristine material, (Zr,W)B₂ solid solutions surround the original ZrB₂ nuclei, whereas refractory W‐compounds at triple junctions and clean grain boundaries are distinctive of this ceramic. After oxidation, the microstructure is typified by intragranular nanostructures, in which nanosized W inclusions remained trapped within ZrO₂ grains, or decorate their surfaces. The understanding of the oxidation reactions occurring in the system as a function of the oxygen partial pressure was fundamental to conclude that W‐based compounds do not notably suppress or retard the oxidation of ZrB₂ ceramics.     

Microstructure and micromechanical properties of GaV4S8 ceramics prepared by single-step solid state synthesis

Despite the current focus on the electronic properties of GaV₄S₈ lacunar spinel, the microstructural and mechanical characterization of this material is scarce in the literature. In this work, we propose an effective GaV₄S₈ ceramics production method and provide a detailed microstructural and micromechanical characterization. Light microscopy, scanning electron microscopy and X-ray diffraction are used to describe the microstructure of the ceramic targets that can be used for thin film deposition. V₂O₃ was found to be the main impurity in ceramic targets and its content is discussed with respect to the sintering atmosphere control. Nanoindentation and microcantilever bending were employed to provide estimates of the indentation modulus, hardness and fracture stress of individual grains. The values of these parameters have been determined as E_r = 130 ± 2 GPa, H = 8.9 ± 0.2 GPa and σ_c = 600 ± 57 MPa, respectively.     

Grain size dependence,mechanical properties and surface nanoeutectic modification of Al2O3-ZrO2 ceramic

The present work aims to provide fundamental insights into the grain size dependence and mechanical behavior of hot-pressed Al₂O₃-ZrO₂ ceramic at its eutectic composition, and further to explore the hardening effect of laser-induced surface nanoeutectic layer. The underlying correlations between densification behavior, grain size distribution and mechanical properties were elucidated. Sintering at 1550 °C promotes the densification without extensive grain growth, and in this case the sample exhibits a critical density of 99.3 %. The average grain size is tailored into a range of 0.6–0.9 μm, and the measured flexural strength and toughness reach 1100 MPa and 11 MPa·m^1/2, respectively. The metastable t-ZrO₂ grains indeed play a pivotal role in energy dissipation at the crack tip through crack deflection and branching. In addition, the mechanical behavior is reasonably explained through constructing a multilevel toughening mechanism map associated with grain size distribution of ZrO₂. Particularly, surface nanocrystallized Al₂O₃-ZrO₂ eutectic layer with a thickness of 1000 μm free of pores and cracks is achieved by a rapid laser melting process. The outmost laser-modified nanoeutectic layer exhibits a fine cellular structure with an interphase spacing of only 105 nm and a hardness of as high as 26.1 GPa, which provides a promising potential in enhancing significantly the hardness and wear resistance for applications as sliding ceramic components.     

Thermal,Structural, and Enhanced Photoluminescence Properties of Eu3+‐doped Transparent Willemite Glass–Ceramic Nanocomposites

The precursor glass in the ZnO–Al₂O₃–B₂O₃–SiO₂ (ZABS) system doped with Eu₂O₃ was prepared by the melt‐quench technique. The transparent willemite, Zn₂SiO₄ (ZS) glass–ceramic nanocomposites were derived from this precursor glass by a controlled crystallization process. The formation of willemite crystal phase, size, and morphology with increase in heat‐treatment time was examined by X‐ray diffraction (XRD) and field‐emission scanning electron microscopy (FESEM) techniques. The average calculated crystallite size obtained from XRD is found to be in the range 18–70 nm whereas the grain size observed in FESEM is 50–250 nm. The refractive index value is decreased with increase in heat‐treatment time which is caused by the partial replacement of ZnO₄ units of ZS nanocrystals by AlO₄ units due to generation of vacancies. Fourier transform infrared (FTIR) reflection spectroscopy was used to evaluate its structural evolution. Vickers hardness study indicates marked improvement of hardness in the resultant glass‐ceramics compared with its precursor glass. The photoluminescence spectra of Eu³⁺ ions exhibit emission transitions of ⁵D₀→⁷F_j (j = 0, 1, 2, 3, and 4) and its excitation spectra show an intense absorption band at 395 nm. These spectra reveal that the luminescence performance of the glass–ceramic nanocomposites is enhanced up to 17‐fold with the process of heat treatment. This enhancement is caused by partitioning of Eu³⁺ ions into glassy phase instead of into the willemite crystals with progress of heat treatment. Such luminescent glass–ceramic nanocomposites are expected to find potential applications in solid‐state red lasers, phosphors, and optical display systems.     

Giant electromechanical strain response in lead‐free SrTiO3‐doped (Bi0.5Na0.5TiO3–BaTiO3)–LiNbO3 piezoelectric ceramics

Lead‐free 0.985[(0.94?x)Bi_0.5Na_0.5TiO₃–0.06BaTiO₃–xSrTiO₃]–0.015LiNbO₃ [(BNT–BT–xST)–LN, x=0‐0.05] piezoelectric ceramics were prepared using a conventional solid‐state reaction method. It was found that the long‐range ferroelectric order in the unmodified (BNT–BT)–LN ceramic was disrupted and transformed into the ergodic relaxor phase with the ST substitution, which was well demonstrated by the dramatic decrease in remnant polarization (P_r), coercive field (E_c), negative strain (S_neg) and piezoelectric coefficient (d₃₃). However, the degradation of the ferroelectric and piezoelectric properties was accompanied by a significant increase in the usable strain response. The critical composition (BNT–BT–0.03ST)–LN exhibited a maximum unipolar strain of ~0.44% and corresponding normalized strain, S_max/E_max of ~880 pm/V under a moderate field of 50 kV/cm at room temperature. This giant strain was associated with the coexistence of the ferroelectric and ergodic relaxor phases, which should be mainly attributed to the reversible electric‐field‐induced transition between the ergodic relaxor and ferroelectric phases. Furthermore, the large field‐induced strain showed relatively good temperature stability; the S_max/E_max was as high as ~490 pm/V even at 120°C. These findings indicated that the (BNT–BT–xST)–LN system would be a suitable environmental‐friendly candidate for actuator applications.     

Spectroscopic analysis and catalytic application of biopolymer capped silver nanoparticle,an effective antimicrobial agent

Microbial reduction of silver ion (conc. 1 mM AgNO₃) was performed by Alkaliphilus oremlandii strain ohILAs in an alkaline pH 10. The synthesized silver nanoparticle was stabilized by poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) biopolymer which was also synthesized by the microbial culture of Alkaliphilus oremlandii strain ohILAs at pH8. The particle size and shape of the silver nanoparticles was studied by dynamic light scattering and under a transmission electron microscope and it was found that the particle size of polymer stabilized colloidal silver was comparatively lower (22–43nm) than that for the unstabilized one (63–93 nm). The stabilization of nanoparticles in polymer dispersed medium after around 60 days was confirmed from analysis of UV‐visible spectroscopy and scanning electron microscopy. The crystalline peaks as recorded with X‐ray's diffraction were observed at 2θ values of 38° and 43°, indicating the fcc crystalline structure of the silver nanoparticle. The antimicrobial activity of silver nanoparticles on gram‐negative bacteria strain (Escherichia coli XL1B) and gram‐positive strain (Lysinibacillus fusiformis) showed better performance by the solution of polymer stabilized nanoparticle than that for the non polymer stabilized one. The reduction of nitro group in p‐nitrophenol to p‐aminophenol was observed from the analysis of UV‐Visible spectroscopy in which, the shifting of absorption peak at 400 to 295 nm and the simultaneous regeneration of light brown color (λ_max 410 nm) of silver nanoparticles confirmed the catalytic activity of silver nanomaterials. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41495.     

Fabrication of New Porous Metal-Bonded Grinding Wheels by Hip Method and Machining Electronic Ceramics

Grinding wheels with different abrasive grains and different bonding materials were fabricated using hot isostatic press (HIP) sintering. Poly-crystal diamond powder of #1000 mesh size, single-crystal diamond powder of #1000 mesh size, and synthetic single-crystal diamond abrasive grains of #325 mesh size were used as abrasive grains. Cast-iron, and two different particle sizes of iron powders were used for the bonding material. The grinding capacity of these grinding wheels as well as conventional grinding wheels was evaluated by constant-pressure-grinding method to grind Al₂O₃-TiC component ceramics, which are typical electronic ceramics used for magnetic memory devices. The hardness of the bonding materials, the adhesion strength between abrasive diamond grains and the bonding materials, and the porosity of sintered body strongly relate to the grinding capacity. The porous bonded grinding wheels showed higher grinding capacity than the conventional wheels. The HIP method enables to fabricate excellent porous metal-bonded grinding wheels.