Influence of Cr Content on the High-Temperature Oxidation Behavior and Mechanism of Low-Alloy Steels.

The high-temperature oxidation behavior of low-carbon steel (AISI 1015, AISI 8617, AISI 4115) was investigated over the temperature range from 600 to 1000 °C in humid air containing 25% water vapor. Mass gain of oxidation measurement was performed to study the oxidation kinetics. The microstructure, thickness, and composition of the oxide scale formed were investigated via optical microscope (OM), scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS), X-ray diffraction (XRD), and electron probe microanalyzer (EPMA). The oxidation process was performed from 2 to 100 min. As the oxidation time increased, the trend of mass gain per unit area switched from a linear to a parabolic law, regardless of the steel grade used. As the chromium content increased, the duration of time during which the oxidation rate followed a linear relationship decreased. In the low-alloy steel with higher chromium content, the thickness of the mixed oxide layer containing Cr increased and the oxidation rate decreased at all oxidation temperatures.

Strong and Tough Nacre-Inspired Graphene Oxide Composite with Hierarchically Similar Structure.

We report a graphene oxide (GO)-based composite, featuring GO/cross-linking agent (CA) nanoparticles, inspired by a nacre-like hierarchical structure present in nature. The as-prepared GO/CA composite was powdered to nanoscale particles and then mixed with pure GO to be GO/CA/GO (GCG) composite forming hierarchical GO/CA nanoasperities on the GO surface. The strength and toughness of the nacre-inspired GCG composite films were simultaneously improved by adjusting the nanoparticle concentration and hierarchical level of the GO-based films. Compared to pristine GO films and GO/CA composites, which exhibit a low level of hierarchy in their structures, the tensile strength and toughness of the GCG composites with higher hierarchy were enhanced 3.1 and 1.6 times and 47.6 and 10.9 times, respectively. Furthermore, a plausible mechanism of increasing mechanical properties based on nanoscale asperities and homogeneous interactions between GO and CA has been discussed.

Effect of Functional Fillers on Tribological Characteristics of Acrylonitrile Butadiene Rubber after High-Pressure Hydrogen Exposures.

In a high-pressure hydrogen environment, the sealing rubber material is swelled by hydrogen, and the mechanical and tribological properties are reduced, causing various problems in the sealing performance. The focus of this study was the effect of the filler type and content on the tribological characteristics of rubber after exposure to high-pressure hydrogen. Acrylonitrile butadiene rubber specimens were exposed to high-pressure hydrogen at 96.6 MPa, and the change in the amount of wear with time after exposure was observed. The wear test was performed using a pin-on-disc ball tip to measure the amount of wear before and after hydrogen exposure of the materials under fixed revolutions per minute and normal load. Scanning electron microscopy was used to observe the wear track and cross section of the specimen to examine the changes in the wear mechanism after hydrogen exposure and to analyze the wear mechanism for each filler. The results of this study are expected to contribute to the evaluation of the tribological properties of the sealing materials used in hydrogen environments.

Scratch Properties of Clear Coat for Automotive Coating Comprising Molecular Necklace Crosslinkers with Silane Functional Groups for Various Environmental Factors.

Automotive coatings, which comprise multiple layers, i.e., primer, base coating, and clear coat layers, are exposed to various environmental conditions that pose various types of damages to them. In particular, the outer layer of the automotive coating, i.e., the clear coat, is affected significantly by such damages. Therefore, a reliable and durable clear coat must be developed to improve the appearance of automobiles. In this study, a new clear coat based on an acrylic-based clear coat modified using polyrotaxane crosslinkers, which are necklace-shaped supramolecules composed of ring-shaped host molecules, is developed and characterized. The effects of polyrotaxane and silane on the scratch properties and mechanisms of the clear coating are analyzed. It is observed that the critical loads of the clear coat from scratch tests can be improved by adding optimal molecular necklace crosslinkers comprising silane functional groups. The improvement in the scratch properties of the modified acrylic-based clear coat may be attributed to the crosslinking characteristics and dynamic molecular movements of the polyrotaxane. In addition, the effects of environmental factors on the scratch characteristics of the modified acrylic-based clear coat are investigated by addressing the scratch durability of the clear coat.

Foldable and Extremely Scratch-Resistant Hard Coating Materials from Molecular Necklace-like Cross-Linkers.

A flexible hard coating material displaying extreme scratch resistance and foldable flexibility was developed via the design of an organic-inorganic hybrid coating material employing an alkoxysilyl-functionalized polyrotaxane cross-linker (PRX_Si1). PRX_Si1 has a molecular necklace-like structure that can form organic-inorganic cross-linking points and provide large molecular movements. It was postulated that the scratch resistance and flexibility could be simultaneously increased because of the hybrid cross-linking points and dynamic molecular movements. To confirm this hypothesis, the crystalline structure and mechanical properties of the PRX_Si1-based hard coating material were analyzed via transmission electron microscopy, small-angle X-ray diffraction, tensile, pencil hardness, and scratch tests. Finally, the PRX_Si1-based hard coating material could form homogeneously dispersed nanoscale siloxane crystalline domains, and the strain at the break point was 3 times higher than that of a commercial hard coating material, resulting in no defect formation even after 5000 folding test runs. Moreover, the material displayed extremely high pencil hardness (9H) and scratch resistance.

Genetic and clinical characteristics of korean patients with isolated hypoparathyroidism: from the Korean hypopara registry study.

Isolated hypoparathyroidism (IH) shows heterogeneous phenotypes and can be caused by defects in a variety of genes. The goal of our study was to determine the clinical features and to analyze gene mutations in a large cohort of Korean patients with sporadic or familial IH. We recruited 23 patients. They showed a broad range of onset age and various values of biochemical data. Whole exome sequencing was performed on two affected cases and one unaffected individual in a family. All coding exons and exon-intron borders of GCMB, CASR, and prepro-PTH were sequenced using PCR-amplified DNA. In one family who underwent the whole exome sequencing analysis, approximately 300 single nucleotide changes emerged as candidates for genetic alteration. Among them, we identified a functional mutation in exon 2 of GCMB (C106R) in two affected cases. Besides, heterozygous gain-of-function mutations in the CASR gene were found in other subjects; D410E and P221L. We also found one single nucleotide polymorphism (SNP) in the prepro-PTH gene, five SNPs in the CASR gene, and four SNPs in the GCMB gene. The current study represents a variety of biochemical phenotypes in IH patients with the molecular genetic diagnosis of IH.