Cr3C2 assisted ultrafast high-temperature sintering of TiC

The strong covalent bonding of TiC renders its densification through conventional sintering difficult. Here, we propose a method involving liquid-phase-assisted ultrafast high-temperature sintering (UHS) for obtaining nearly full density of TiC ceramics by the addition of Cr₃C₂. The samples were heated at a rate of 600 °C/min to 2200 °C and held at this temperature for 1 min. The effects of sintering parameters and the Cr₃C₂ content on the relative density and microstructure of the sintered samples were investigated. The main causes of rapid densification were particle rearrangement associated with the Cr₃C₂ liquid phase, dissolution-reprecipitation associated with the solid solution, and the weak evaporation of Cr formed during UHS. In particular, the addition of Cr₃C₂ helped increase the hardness and elastic modulus of TiC significantly. This paper presents an effective and extensible method involving UHS for rapidly obtaining dense ceramics.     

Synthesis of Ti2(InxAl1-x)C (x = 0–1) solid solutions with high-purity and their properties

Herein, high-purity Ti₂(In_xAl_1-x)C (x = 0–1) solid solutions were successfully synthesized. The crystal structure and actual composition of solid solutions were confirmed using XRD, SEM, and TEM analyses, and their formation mechanism was revealed by thermal analysis. On the In-rich side (x ≥ 0.5), primary Ti₂InC first formed and then acted as a crystalline seed for the subsequent solid solutions, resulting in a cluster-like morphology. The lattice constants of Ti₂(In_xAl_1-x)C were found to well follow Vegard’s law. The examined properties of Ti₂(In_xAl_1-x)C also greatly depended on their A-site compositions. Ti₂AlC exhibited the highest hardness and elastic moduli, while the best corrosion resistance was achieved at Ti₂InC, and all Ti₂(In_xAl_1-x)C displayed active dissolution in 0.5 M HCl solution. Thus, adjusting the In/Al ratio at A-site can yield a desired set of performances, which provides a good example for regulating the performance of MAX phases via A-site solid solution strategy.     

Identification and characterization of autoreactive T cell responses to bullous pemphigoid antigen 2 in patients and healthy controls

Antibodies against the extracellular domain of bullous pemphigoid antigen 2 (BPAG2) are thought to play a key role in the pathogenesis of bullous pemphigoid (BP), the most frequent autoimmune bullous disease of the skin. Autoreactive T cell responses to BPAG2 were investigated in 16 BP patients and 24 healthy controls by coculture of PBMC with two recombinant BPAG2 proteins (extracellular domain of BPAG2). Primary in vitro T cell responses to BPAG2 were observed in 10/12 BP patients expressing the BP-associated HLA-DQB1*0301 allele and 8/10 DQB1*0301 positive healthy individuals. DQB1*0301 also restricted three autoreactive T cell lines from two BP patients and a healthy donor. In contrast, PBMC from 14 normal patients carrying HLA class II alleles other than DQB1*0301 were not stimulated by BPAG2. Autoreactive BPAG2-specific CD4(+) T cell lines and clones from five BP patients produced both Th1 and Th2 cytokines, whereas three autoreactive T cell lines from three DQB1*0301 positive normal patients produced exclusively IFN-gamma. The absence of BPAG2-specific Th2 cells in healthy individuals strongly suggests that autoreactive Th2 responses to BPAG2 are restricted to BP patients and may thus be critical in the pathogenesis of BP.     

Thermal stability of superhard Ti-B-N coatings

Ternary transition-metal boron nitride Ti-B-N offers outstanding hardness and thermal stability, which are increasingly required for wear resistant applications, as the protective coatings are subjected to high temperature, causing thermal fatigue. Ti-B-N coatings with chemical compositions close to the quasibinary TiN-TiB₂ tie line and boron contents below ∼ 18 at.% contain a crystalline supersaturated NaCl structure phase, where B substitutes for N. Annealing above the deposition temperature causes precipitation of TiB₂, which influence dislocation mobility and hence the hardness of TiB_0.40N_0.83 remains at a very high level of ∼ 43 GPa with annealing temperature T_a up to 900 °C. Growth of Ti-B-N coatings with B contents above ∼ 18 at.% results in the formation of nm sized TiN and TiB₂ crystallites embedded in a high volume fraction of disordered boundary layer. The compaction of this disordered phase during annealing results in a hardness increase of TiB_0.80N_0.83 coatings from the as-deposited value of ∼ 37 GPa to ∼ 42 GPa at T_a = 800 °C. Excess B during growth of TiB_2.4 coatings causes the formation of bundles of ∼ 5 nm wide TiB₂ subcolumns encapsulated in a B-rich tissue phase. This nanocolumnar structure is thermally stable up to temperatures of ∼ 900 °C, and consequently the hardness remains at the very high level of ~ 48 GPa, as nucleation and growth of dislocations is inhibited by the nm sized columns. Furthermore, the high cohesive strength of the B-rich tissue phase prevents grain boundary sliding.     

Factors affecting the perception of whole-body vibration of occupational drivers: an analysis of posture and manual materials handling and musculoskeletal disorders

Due to the high cost of conducting field measurements, questionnaires are usually preferred for the assessment of physical workloads and musculoskeletal disorders (MSDs). This study compares the physical workloads of whole-body vibration (WBV) and awkward postures by direct field measurements and self-reported data of 45 occupational drivers. Manual materials handling (MMH) and MSDs were also investigated to analyse their effect on drivers' perception. Although the measured values for WBV exposure were very similarly distributed among the drivers, the subjects' perception differed significantly. Concerning posture, subjects seemed to estimate much better when the difference in exposure was significantly large. The percentage of measured awkward trunk and head inclination were significantly higher for WBV-overestimating subjects than non-overestimators; 77 and 80% vs. 36 and 33%. Health complaints in terms of thoracic spine, cervical spine and shoulder–arm were also significantly more reported by WBV-overestimating subjects (42, 67, 50% vs. 0, 25, 13%, respectively). Although more MMH was reported by WBV-overestimating subjects, there was no statistical significance in this study.     

Sheet metal forming and springback simulation by means of a new reduced integration solid-shell finite element technology

The paper deals with the validation of a recently proposed hexahedral solid-shell finite element in the field of sheet metal forming. Working with one integration point in the shell plane and an arbitrary number of integration points in thickness direction, highly non-linear stress states over the sheet thickness can be incorporated in an efficient way. In order to avoid volumetric locking and Poisson thickness locking at the level of integration points the enhanced assumed strain (EAS) concept with only one EAS degree-of-freedom is implemented. A key point of the formulation is the construction of the hourglass stabilization by means of different Taylor expansions. This leads to the advantage that the sensitivity with respect to mesh distortion is noticeably reduced. The hourglass stabilization includes the assumed natural strain (ANS) concept and a kind of B-Bar method. So transverse shear locking and volumetric locking are eliminated.The finite element formulation incorporates a finite strain material model for plastic anisotropy as well as non-linear (Armstrong–Frederick type) kinematic and isotropic hardening. In this context the plastic anisotropy can be modeled by representing the yield surface and the plastic flow rule as functions of so-called structural tensors. The integration of the evolution equations is performed by means of an exponential map exploiting the spectral decomposition. The element formulation and material model have been implemented into the commercial code ABAQUS/Standard by means of the UEL interface for user-defined elements. Using an implicit time integration scheme numerical results for classical deep drawing simulations as well as springback predictions are presented in comparison to experimental measurements.     

Effects of Sulfur Additive EP in Ester Coolants during Friction with CBN

A study was undertaken to investigate the effects of the EP additives during tribological tests using a CBN tool against steel. Ester oil with and without sulfur additive were used as lubricants in a tribometer. Tribochemical interactions between the S additive and steel have been investigated under boundary lubrication conditions by SEM and EDX analysis. The relative abundance of different elements on the surface of the CBN tools, which are present in the workpiece material such as Fe (iron) and Cr (chromium), suggests that adhesion occurred when the ester oil without sulfur additive was tested. Tribochemical interactions between the additive and the steel could be observed when using the ester oil containing the sulfur additive. These interactions contribute to the formation of a uniform layer on the CBN tool. This layer is composed by S (sulfur), Fe, and O (oxygen). The presence of these elements indicate that FeO (iron oxide) and FeS (iron sulfide) were formed.     

Densification, Microstructure, and Wear Property of In Situ Titanium Nitride-Reinforced Titanium Silicide Matrix Composites Prepared by a Novel Selective Laser Melting Process

This work presents the densification behavior, microstructural features, microhardness, and wear property of in situ TiN/Ti₅Si₃ composite parts prepared by a novel Selective Laser Melting (SLM) process. The occurrence of balling phenomenon at a low laser energy density combined with a high scan speed and the formation of thermal cracks at an excessive laser energy input generally decreased densification rate. The in situ-formed TiN reinforcing phase experienced a successive morphological change: an irregular polyangular shape—a refined near-round shape—a coarsened dendritic shape, as the applied laser energy density increased. The variations in liquid-solid wettability and intensity of Marangoni convection within laser molten pool accounted for the different growth mechanisms of TiN reinforcement. The TiN/Ti₅Si₃ composite parts prepared under the optimal SLM conditions had a near-full 97.7 pct theoretical density and a uniform microhardness distribution with a significantly increased average value of 1358.0HV_0.3. The dry sliding wear tests revealed that a considerably low friction coefficient of 0.19 without any apparent fluctuation and a reduced wear rate of 6.84 × 10⁻⁵mm³/Nm were achieved. The enhanced wear resistance was attributed to the formation of adherent strain-hardened tribolayer covered on the worn surface.     

On the microstructure and properties of 100Cr6 steel processed in the semi-solid state

After semi-solid treatment and rapid quenching, bearing steel 100Cr6 exhibits a martensitic structure with a large amount of retained austenite along the grain boundaries. The toughness values are significantly lower than after conventional hardening, even when the alloys are post-processed by an additional heat treatment step. Slow cooling from the freezing range results in a reduced amount of retained austenite and improved toughness properties. The low impact toughness values of the rapidly cooled conditions are associated with the appearance of intergranular fracture, while the slowly cooled condition fails in a transgranular manner. The structural development and the mechanical behaviour are explained by severe segregation of the main alloying elements during solidification, but also by sulphur enrichment in the remaining liquid phase. Chain-like precipitates of MnS are formed along the grain boundaries, similar to the well-known ‘burning’ phenomenon of low-alloyed steels.     

Modelling the effects of various contents of fillers on the relaxation rate of elastomers

This paper describes numerical investigations conducted on filled rubbers. Some experimental data on stress relaxation tests carried out on various filled elastomers evidenced that increasing the content of rigid particles such as fillers or changes in the crystallization index, result in higher values of the stress relaxation rate. This result seems to indicate that higher rigid particle contents promote macroscopic viscoelastic strain. To better understand this effect, finite element analyses were carried out on axi-symmetric unit cells as well as unit cell for numerical homogenization with periodic computations.