Effect of Oxygen Segregation at Grain Boundaries on Deformation of B, C-Doped Silicon Carbides at Elevated Temperatures

The effect of oxygen segregation at grain boundaries on the deformation of 1 wt% boron (B)- and carbon (C)-doped β-silicon carbide (B, C-doped SiC) was investigated by compression testing at 2073 K. We studied the deformation of sinter-forged B, C-doped SiC (sinter-forged SiC), which contained the minimum amount (0.07 wt%) of oxygen as an impurity, and that of hot isostatically pressed B, C-doped SiC (HIPed SiC), which contained 1 wt% oxygen. Oxygen was detected at grain boundary in HIPed SiC by energy-dispersive X-ray spectroscopy, but it was not detected in sinter-forged SiC. The strain rate of sinter-forged SiC was one order of magnitude lower than that of HIPed SiC at the same grain size. The grain growth rate of sinter-forged SiC was lower than that of HIPed SiC also. These results suggest that the oxygen segregation at grain boundaries, together with boron segregation, promoted the grain-boundary diffusion in B, C-doped SiC. But, the oxygen segregation without boron was less effective in promoting deformation than the boron segregation without oxygen.     

Fabrication and characterization of arc melted Si/B co-doped boron carbide

Boron carbide undergoes stress-induced amorphization when subjected to large non-hydrostatic stresses that exceed its elastic limit. This has been proposed as the source for the abrupt loss of shear strength in boron carbide which limits its engineering applications. Si/B co-doping was suggested as one of the means to suppress stress-induced amorphization but this has not been experimentally verified. Here, by utilizing arc melting, we prepared Si/B co-doped boron carbide with increased Si content as compared to conventional methods. Through Raman analysis in conjunction with indention and elemental analyses based on SEM and STEM (ζ-factor microanalysis), it is suggested that Si/B co-doping is a promising avenue for suppressing stress-induced amorphization. A comprehensive characterization of microstructure, chemistry, and structural change of boron carbide as a result of Si/B co-doping was elucidated.     

The grain-boundary resistance of CeO2 ceramics: A combined microscopy-spectroscopy-simulation study of a dilute solution

Weakly acceptor-doped ceria ceramics were characterized structurally and compositionally with advanced transmission electron microscopy (TEM) techniques and electrically with electrochemical impedance spectroscopy (EIS). The grain boundaries studied with TEM were found to be free of second phases. The impedance spectra, acquired in the range 703 ≤ T/K ≤ 893 in air, showed several arcs that were analyzed in terms of bulk, grain-boundary, and electrode responses. We ascribed the grain-boundary resistance to the presence of space-charge layers. Continuum-level simulations were used to calculate charge-carrier distributions (of acceptor cations, oxygen vacancies, and electrons) in these space-charge layers. The acceptor cations were assumed to be mobile at high (sintering) temperatures but immobile at the temperatures of the EIS measurements. Space-charge formation was assumed to be driven by the segregation of oxygen vacancies to the grain-boundary core. Comparisons of data from the simulations and from the EIS measurements yielded space-charge potentials and the segregation energy of vacancies to the grain-boundary core. The space-charge potentials from the simulations are compared with values obtained by applying the standard, analytical (Mott–Schottky and Gouy–Chapman) expressions. The importance of modelling space-charge layers from the thermodynamic level is demonstrated.     

Absorption Effects in STEM Microanalysis of Ceramic Oxides

A scanning transmission electron microscope (STEM) equipped with an X-ray energy dispersive spectrometer (EDS) was used for quantitative X-ray tnicroanalysis of an MgO-10 mol% NiO ceramic. Using the Cliff-Lorimer standardless ratio technique for quantitative X-ray microanalysis, absorption of the Mg characteristic X rays, particularly by oxygen in the specimen, was found to be a critical problem and corrections were required. This problem is characteristic of low atomic number oxide ceramics. However, preferential absorption of characteristic X rays by carbon contamination buildup during X-ray analysis was negligible. A grain-boundary composition profile, corrected for absorption, showed no segregation of the Ni to the grain boundary, in agreement with current segregation theory.     

Study of chemical bonds in carbon and boron materials by EPMA

This paper describes the study of chemical bonds in carbon and boron materials (single-crystalline graphite, natural diamond, boron carbide and pure boron) by electron probe microanalysis. BK and CK X-ray emission spectra of the investigated materials are presented and discussed.     

Solid-Sample 11B Nuclear Magnetic Resonance and X-ray Photoelectron Spectroscopy Study of Boron-Doped β-Silicon Carbide

Solid-sample magic angle spinning (MAS) nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS), in conjunction with scanning electron microscopy (SEM), were used to investigate the fate of boron used as a sintering aid for silicon carbide. The results of the NMR studies indicated that the boron penetrated the silicon carbide grain boundaries during sintering, and was incorporated in a tetrahedral form in the bulk, regardless of the gas used during the process. The NMR spectrum of a sample sintered under nitrogen indicated the formation of a trigonal form of boron as well. XPS identified this trigonal boron as boron nitride; however, no boron was detected by XPS in any form on the fracture surface of the silicon carbide sintered under argon, even though the NMR results confirmed the presence of tetrahedral boron in the bulk sample. The SEM results indicated that the fracture process for these materials was predominantly intergranular. This suggested that the boron in the silicon carbide sintered under argon penetrated the grains and left the grain boundaries depleted of boron.     

Ab Initio Calculations of Pristine and Doped Zirconia Σ5 (310)/[001] Tilt Grain Boundaries

The structure of the cubic-ZrO₂ symmetrical tilt Σ5 (310)/[001] grain boundary is examined using density functional theory within the local density and pseudopotential approximations. Several pristine stoichiometric grain-boundary structures are investigated and compared with Z-contrast scanning transmission electron microscopy and electron energy loss spectroscopy results. The lowest-energy grain-boundary structure is found to agree well with the experimental data. When Y³⁺ is substituted for Zr⁴⁺ at various sites in the lowest-energy grain-boundary structure, the calculations indicate that Y³⁺ segregation to the grain boundary is energetically preferred to bulk doping, in agreement with experimental results.     

Detection of Boron Segregation to Grain Boundaries in Silicon Carbide by Spatially Resolved Electron Energy-Loss Spectroscopy

Boron segregation to grain boundaries in SiC was directly observed for the first time by using spatially resolved electron energy-loss spectroscopy methods. The hot-pressed, fully dense material was doped with 0.3 wt% of boron and was free of other additives, except for 2 wt% of free carbon. The detection of boron was achieved in the difference spectra at all the grain boundaries that were examined. Its interfacial excess was in the range of 15–29 atoms/nm², or approximately one monolayer. Concurrently, silicon depletion occurred at these boundaries, although to a lesser extent (−13.5 atoms/nm² on average), which indicated that boron mainly replaces silicon and bonds with carbon at the grain boundary. These findings validate the dual role of boron at the grain boundary for promoting densification via improved grain-boundary diffusivity while maintaining a covalent grain boundary without an oxide phase.     

Experimental observations of amorphization in multiple generations of boron carbide

Boron carbide is an excellent armor material due to its light weight and ultrahigh hardness. However, high-rate mechanical behavior can be degraded by stress-induced amorphization. In this paper, we review the progressive advances in the understanding of amorphization in three successive generations of boron carbide: stoichiometric (undoped), B-rich, and B/Si codoped boron carbides. For each generation of boron carbide, the crystal structure and microstructure are first discussed. Then, we outline the experimental observations of amorphization made by Raman spectroscopy and transmission electron microscopy. The susceptibility of amorphization in each generation of boron carbide will be compared and the fundamental mechanisms that explain the reduction in amorphization for B-rich and B/Si codoped boron carbides elucidated. Comments on future research directions to further broaden and deepen the understanding of stress-induced amorphization of boron carbide are also provided.     

Zirconia Nanostructures: Novel Facile Surfactant‐Free Preparation and Characterization

Zirconia nanostructures have been prepared via a facile precipitation route using Zirconium (IV) oxynitrate hydrate and tetraethylenepentamine (TEPA). Here, TEPA was used as novel precipitator to fabricate zirconia nanostructures. The influence of reaction time, dosage of TEPA, and solvent was also examined to control the shape and particle size. Results of this work indicate that these reaction parameters have important impact on the control of shape and grain size of the zirconia. To characterize the as‐synthesized nanostructures, techniques such as X‐ray diffraction (XRD), ultraviolet–visible (UV–vis) spectroscopy, energy dispersive X‐ray microanalysis (EDX), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared (FT‐IR) spectroscopy, and photoluminescence (PL) spectroscopy were applied. In addition, the formation mechanism of zirconia nanostructures was discussed.     

Role of carbon nanotubes in the ballistic properties of boron carbide/carbon nanotube/ultrahigh molecular weight polyethylene composite armor

Composite armor panels with boron carbide ceramic as striking face, backed with carbon nanotube-ultra high molecular weight polyethylene (CNT-UHMWPE), have been fabricated as lightweight body armors. The armor panels were tested against actual ammunitions through self-loading rifle (SLR) and AK-47. Slip lines were observed in boron carbide with high energy bullets. CNT-UHMWPE composite backing resulted in a low back face signature (BFS) due to the formation of a strong bond between CNT and UHMWPE and high energy dissipation through this composite. The role of CNT in the ballistic protection of this composite armor has been explained with the help of necessary characterization by electron microscopy, X-ray radiography, Fourier transformed infrared spectroscopy and differential scanning calorimetry.     

Effect of boron carbide nanoparticles on the thermal stability of carbon/phenolic composites

In this work, the use of nano‐boron carbide as a nanomodifier of phenolic matrix was envisioned. Particularly, nano‐boron carbide/phenolic‐based nanocomposites were produced and investigated. The obtained nanostructured matrices were also used to produce carbon fiber‐based bulk molding compounds (BMC). The thermal stability of nanocomposites and BMCs was investigated by thermogravimetric analysis both in nitrogen and in air atmospheres. The good dispersion and distribution of the nanosized particles in the matrix was confirmed by transmission electron microscopy while the post‐burning appearance of the BMCs was investigated by visual inspection and scanning electron microscopy. The experimental data highlighted the remarkable effects of nano‐boron carbide on the thermal stability and oxidation resistance of the carbon fiber‐based BMCs. Moreover, the boron oxide produced by the conversion of boron carbide allowed a substantial improvement of the dimensional stability of the BMC which also exhibited considerable residual structural integrity after burning. POLYM. COMPOS., 38:1819–1827, 2017. © 2015 Society of Plastics Engineers     

Strengthening boron carbide by doping Si into grain boundaries

Grain boundaries, ubiquitous in real materials, play an important role in the mechanical properties of ceramics. Using boron carbide as a typical superhard but brittle material under hypervelocity impact, we report atomistic reactive molecular dynamics simulations using the ReaxFF reactive force field fitted to quantum mechanics to examine grain-boundary engineering strategies aimed at improving the mechanical properties. In particular, we examine the dynamical mechanical response of two grain-boundary models with or without doped Si as a function of finite shear deformation. Our simulations show that doping Si into the grain boundary significantly increases the shear strength and stress threshold for amorphization and failure for both grain-boundary structures. These results provide validation of our suggestions that Si doping provides a promising approach to mitigate amorphous band formation and failure in superhard boron carbide.     

Microstructural Characterization of Commercial Hot-Pressed Boron Carbide Ceramics

Two commercial hot-pressed boron carbide ceramics were investigated by transmission electron microscopy. Atomic-scale observations suggest that the grain boundaries of the two materials are free of grain-boundary films. Two triple-junction phases were found and characterized to be rhombohedral Fe₂B₁₀₃ and orthorhombic Ti₃B₄. In addition, intra-granular precipitates, AlN, Mo₂(C, B) and graphite, were identified and found to have coherent relationships with the boron carbide matrix. Micron-scale inclusions were also observed and most of them were determined to be graphite. The formation mechanisms of the secondary phases and their possible influence on mechanical properties are also discussed.     

Analysis of Grain-Boundary Impurities and Fluoride Additives in Hot-Pressed Oxides by Auger Electron Spectroscopy

Auger electron spectroscopy was used to characterize grain-boundary chemistry in MgO, Al₂O₃, and MgAl₂O₄ (spinel) hot-pressed with LiF or NaF. The presence of additives (F, Na) at the grain boundaries was confirmed; observation of grain-boundary impurities in MgO extends previous studies.     

On the phase and grain boundaries in dual phase carbide/boride ceramics from micro to atomic level

The phase and grain boundary characteristics of recently developed fine-grained dual-phase high-entropy (Ti-Zr-Nb-Hf-Ta)C/(Ti-Zr-Nb-Hf-Ta)B₂ was investigated throughout all the accessible length scales using scanning electron microscopy (SEM), aberration-corrected scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The system exhibits relatively homogeneous grain size distribution where the average size is approximately 0.97 µm, with chemical composition (Ti_0.14 Zr_0.2 Nb_0.2 Hf_0.2 Ta_0.26)C + (Ti_0.38 Zr_0.18 Nb_0.22 Hf₀.₁₁₅ Ta_0.105)B₂. SEM analyses revealed no micro-crack formation and second – phase segregation at the boundaries or micro-pores at the triple – points. Investigation down to the sub-nanometer scale revealed that the phase and grain boundaries were typically clean and sharp with an indistinct 1 – 1.5 nm thin gradient of metallic elements at boride/boride and carbide/carbide interfaces. The sharp phase and grain boundaries do exhibit elemental enrichment from a trace amount of Fe being incorporated in interstitial positions of carbide and boride grains locally at boride/carbide boundaries or are present in boride and carbide grains in the form of continuous thin layer at boride/boride and carbide/carbide interfaces with the probably origin from starting powders.     

Microstructure and Electrical Properties of Sodium-Diffused and Potassium-Diffused SrTiO3 Barrier-Layer Capacitors Exhibiting Varistor Behavior

In sodium-diffused strontium titanate internal-boundary-layer capacitors exhibiting good varistor properties, high-resolution transmission electron microscopy and scanning transmission electron microscopy revealed sodium segregation at grain boundaries in the absence of intergranular phases. The segregation layer is narrow (≅10 nm), unlike much broader diffused boundary layers which have also been observed. The nonohmic behavior in these and in potassium-diffused specimens suggests that segregated acceptor alkali ions act as grain-boundary electron traps leading to varistor electrical barriers.     

High-temperature oxidation of BN-coated sylramic SiC fibers in dry and wet atmosphere

The oxidation behavior of SiC Sylramic fibers coated with chemically vapor deposited Si-doped boron nitride (BN) was investigated at temperatures between 800 and 1200°C in dry and wet O₂ atmospheres. Thermogravimetric analysis was used to study the oxidation kinetics of the fiber and the influence of the BN layer and the environment. The morphology and composition of the thermally grown oxide scale was determined posttest by scanning electron microscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma optical emission spectrometry. This study gives new insights into the synergistic effects of BN and water vapor on the oxidation behavior SiC Sylramic fibers. The vulnerability of the BN fiber interphase and the behavior of the fiber under conditions relevant to high-temperature turbine applications are discussed.     

Effect of Amount of Boron Doping on Compression Deformation of Fine-Grained Silicon Carbide at Elevated Temperature

The effect of the amount of boron doping in the range of 0 to 1.0 wt% on the high-temperature deformation of fine-grained β-silicon carbide (SiC) was investigated by compression testing. Flow stress at the same grain size increased as the amount of boron doping decreased. The stress exponent increased from 1.3 to 3.4 as the amount of boron doping decreased. The strain rates of undoped SiC were ∼2 orders of magnitude lower than those of 1.0-wt%-boron-doped SiC of the same grain size. The apparent activation energies of SiC doped with 1.0 wt% boron and of undoped SiC were 771 ± 12 and 884 ± 80 kJ/mol, respectively. These results suggest that the actual contribution of grain-boundary diffusion to the accommodation process of grain-boundary sliding decreased as the amount of boron doping decreased. Consequently, the apparent contribution of the dislocation glide increased.     

Auger Analysis of Hot-Pressed and Sintered Silicon Carbide

Intergranular and transgranular chemistries of hot-pressed and sintered silicon carbides were investigated by Auger electron spectroscopy. Results indicated major differences in grain-boundary compositions between the two. Hot-pressed silicon carbide displayed a complex intergranular chemistry. Sintered silicon carbide displayed grain facets that were free of impurities and additives. The observed intergranular chemistries for both silicon carbides are discussed in terms of their relation to the processing methods.