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The effect of nickel silicide on the friction properties of Cu-based wet sintered friction materials was investigated. The microstructure and mechanical properties of nickel silicide added Cu-based wet sintered friction materials were examined. Nickel silicide was prepared by mechanically alloying Ni and Si powder. Three types of Cu-based wet sintered friction materials with different nickel silicide contents (0 wt%, 5 wt%, 10 wt%) were fabricated by the powder metallurgy route. The phase transformation of Ni₂Si to Ni₃₁Si₁₂ was observed in nickel silicide after a sintering process. The friction coefficient and wear rate decreased with increasing nickel silicide content. The wear scar changed from ploughing to micro-cutting with increasing nickel silicide content. As a result, it is suggested that nickel silicide can be used as a friction modifier when added to Cu-based wet sintered friction materials. (Received June 17, 2021; Accepted November 22, 2021)

keyword : wet sintered friction material, nickel silicide, hardness, friction coefficient, wear rate

The aim of this study was to investigate the effect of precipitates and cold working on the reheat cracking of STS 347 austenitic stainless steel. Three different heat treatments to form NbC and M₂₃C₆ precipitates were applied to the STS 347 austenitic stainless steel and 10% pre-strain was additionally introduced to simulate cold working. The NbC and M₂₃C₆ precipitates were examined by SEM and XRD analysis, and creep rupture test was conducted under various temperature and stress conditions. The specimen having intergranular NbC precipitate prior to cold working had enhanced intergranular creep resistance and the tendency to reheat cracking decreased at temperature conditions between 650℃ and 750℃, compared to the specimens processed by solution treatment or M₂₃C₆ formation treatment. This means that for STS 347 that has not been subjected to stabilization treatment, the cold working strain limits required by ASME’s boiler and pressure vessel code should be lowered, or if stress relief treatment is impractical, the stabilization heat treatment should be performed prior to cold working. Although the precipitation of M₂₃C₆ is not the major cause of reheat cracking, it should still be considered as a factor that increase the sensitivity of reheat cracking because M₂₃C₆ forms mostly at the grain boundary and grows rapidly, adversely affects creep ductility. (Received September 26, 2021; Accepted October 29, 2021)

keyword : reheat cracking, STS 347, austenitic steel, precipitate, cold working

The study investigated the effect of repeated repair welding on the mechanical properties and pitting corrosion resistance of 25% Cr super duplex stainless steels (UNS S32750). The test coupons were prepared by gas tungsten arc welding (GTAW) with ER2594 filler metal and repaired 3-times at the same weld locations, where the remaining previous welds were 2-3 mm in depth. The number of repairs provided various reheating to the original welds (as-welded condition) and to the previously repaired welds. The effect of repair welding reheating on the microstructure, ferrite content, hardness, impact toughness and pitting corrosion was investigated. The pitting corrosion occurred in the root region of the first repair weld (R1). Secondary phases such as austenite (γ₂) and sigma (σ) were observed in the reheated regions with a consumption of ferrite content, where impact toughness and pitting corrosion resistance were significantly reduced. The vicinity of the pitting corrosion region was analyzed, and showed that sigma (σ) was precipitated in the secondary austenite (γ₂) and at the boundary between ferrite and austenite. The fraction of the sigma phase was determined by electron back scatter diffraction (EBSD) to be 0.3% in the vicinity of the pitting corrosion region. This study also discusses the applicability of 3-times repair welding in the offshore industry. (Received October 18, 2021; Accepted November 4, 2021)

keyword : super duplex stainless steel, pitting corrosion, repair welding, GTAW, sigma phase

Neodymium is the most used of rare earth elements. Its demand is rapidly increasing as the raw material for Nd-Fe-B permanent magnets. In the present study, the preparation of neodymium metal by molten salt electrowinning was investigated, using the support salt of LiF:NdF₃:BaF₂ = 7.5:67.4:25.1 (in wt%, FLiNdBa). It focused particularly on the effects of the electrowinning temperature, the cathodic current density, and the feeding rate of the raw material (Nd₂O₃) on the current efficiency. The current efficiency decreased almost linearly with the electrowinning temperature regardless of the cathodic current density in the temperature range (1030~1130 ℃). Considering current efficiency and workability, the most suitable electrowinning temperature was 1040~1050 ℃. The current efficiency increased linearly with the cathodic current density in the range of 1~4 A/cm² irrespective of the electrowinning temperature. The effect of the feeding rate of the raw material (Nd₂O₃) on the current efficiency showed the same tendency as that of the cathodic current density on the current efficiency. The current efficiency of 73.6% and the metal purity of 99.8% were achieved in this study when the cathodic current density was 3.6 A/cm² and the electrowinning temperature was 1045℃. (Received July 21, 2021; Accepted October 26, 2021)

keyword : molten salt electrowinning, neodymium, neodymium oxide, neodymium fluoride, current efficiency, current density

Spent electroplating solutions contain a small amount of Pd(II). This Pd(II) can be concentrated by cementation with zinc metal powder. In order to recover pure palladium, a hydrometallurgical process consisting of leaching followed by solvent extraction was developed in this work. First, suitable extractants for the selective extraction of Pd(II) over Zn(II) were selected from synthetic sulfuric acid solutions. Among cationic, solvating and basic extractants employed in this work, Cyanex 301, LIX 63 and Aliquat 336 showed selectivity for Pd(II) over Zn(II) from 5 M sulfuric acid solutions. Stripping results for the loaded phases of these three extractants showed that HCl was effective at stripping Pd(II) from the loaded LIX 63 and Aliquat 336, while a mixture of inorganic acid and NaClO as an oxidizing agent was necessary to strip Pd(II) from the loaded Cyanex 301. The cemented Pd was completely dissolved using 5 M sulfuric acid solution at 25℃, and the concentration of Pd(II) and Zn(II) in the real leaching solution was 1.926 and 0.074 g/L, respectively. Application of the optimum conditions for the selective extraction of Pd(II) over Zn(II) to the real sulfuric acid leaching solution of cemented Pd indicated that only LIX 63 and Cyanex 301 could separate Pd(II) from Zn(II). The stripping efficiency of Pd(II) by a mixture of 5 M HCl and 3% NaClO was much higher from the loaded LIX 63 than from the loaded Cyanex 301. Considering the extraction and stripping characteristics of Pd(II), LIX 63 was selected as an optimum extractant for the separation of Pd(II) from the real sulfuric acid leaching solution of cemented Pd. (Received October 6, 2021; Accepted November 2, 2021)

keyword : palladium, spent electroplating solution, sulfuric acid leaching, solvent extraction, commercial extractant

A lead-free piezoelectric ceramic, (1-x)Ba(Zr_0.2Ti_0.8)O_3-x(Ba_0.7Ca_0.3)TiO₃ (hereafter (1-x)BZT-xBCT), was investigated for functional device applications. Lead-free piezoelectric materials with high piezoelectric charge coefficients and voltage coefficients are indispensable for functional industrial applications. Piezoelectric ceramic specimens tend to shrink in size during the sintering process without melting. This study focused on this phenomenon as a unique characteristic of piezoelectric ceramic materials, and derived the materials’ activation energies from this phenomenon, employing the Arrhenius relationship. Then the estimated activation energy was employed to determine the relationship between the piezoelectric properties and crystalline properties, while varying sintering temperature and chemical stoichiometric composition. Piezoelectric properties can change depending on the sintering temperature and process conditions. Crystalline properties and piezoelectric properties can be influenced by the sintering temperature and holding times. In this research, sintering temperature dependent structural and dielectric properties were investigated. (1-x)BZT-xBCT ceramics showed the highest piezoelectric coefficient of 470 pC/N. Sintering temperature dependent shrinkage rate and activation energy were estimated and calculated for the various sintering processes. In this research, two different activation energies for the shrinkage process were estimated. (Received March 11, 2021; Accepted October 28, 2021)

keyword : piezoelectric material, (1-x)Ba(Zr_0.2Ti_0.8)O_3-x(Ba_0.7Ca_0.3)TiO₃, shrinkage mechanism, activation energy

Conductive atomic force microscopy (C-AFM) is one of the most commonly used characterization techniques for piezoelectric one-dimensional nanomaterials. However, a comprehensive understanding of the effects of certain scan parameters on the C-AFM signals remains elusive. In the present work, the dependency of C-AFM signals on the normal force, scan speed, and Z scanner feedback gain was studied in conjunction with lateral force microscopy (LFM) signals. As the normal force increased, the C-AFM and the LFM signals increased for the following two possible reasons. When larger normal force was applied, ZnO nanorods were more effectively deflected, intensifying the piezoelectric effect. Additionally, the triboelectric effect was enhanced via the increased force of friction between the AFM tip and the ZnO nanorods. When the scan speed increased to 0.5 Hz, the LFM signals and the C-AFM signals increased owing to the enhanced degree of deflection in the ZnO nanorods. However, when exceeding 0.5 Hz, both the LFM signals and the C-AFM signals started to decrease because the AFM tip did not come into contact with the short ZnO nanorods at a high scan speed. Finally, with an increase in the feedback gain to 0.5, both the LFM signals and the CAFM signals increased. However, when the feedback gain exceeded 1.0, the Z scanner feedback loop was too sensitive to deflect the ZnO nanorod, considerably reducing the total LFM signals. In contrast, the total CAFM signal showed only a moderate decrease. (Received September 7 2021; Accepted November 4, 2021)

keyword : ZnO nanorods, conductive AFM, lateral force microscopy, scan parameters, piezoelectric, triboelectric

ZnSe quantum-dots were synthesized in an aqueous system and UV irradiation was introduced as a post-treatment process. The synthesis temperature was varied from 25℃ to 90 ℃, and the surface of the nano powder was stabilized using different ligands such as glutathione (GSH), 3-mercaptopropionic acid (MPA) and thioglycolic acid (TGA). Then, the effect of UV irradiation on the structure and the absorption/ emission properties of the nano powder were investigated. As the UV light was irradiated, the PL intensity of the quantum dots increased. UV irradiation for 16 h increased the PL intensity of the synthesized quantum dots at room temperature to the level of those at 90 ℃. Particle aggregation was not found with UV irradiation of at least 24 h. XRD patterns confirmed that the UV irradiation released sulfur from the GSH, inducing the formation of the ZnSe/ZnS composite-typed nano powders. The luminescence characteristics of the GSH or TGA-added quantum dots were enhanced by UV irradiation regardless of temperature, while MPA-added quantum dots were not significantly affected by UV irradiation. The post-treatment process with UV irradiation showed the possibility that even ZnSe quantum dots, which are synthesized at low temperature and have low luminescence intensity, can have luminescence characteristics comparable to those of hightemperature synthesis. (Received September 13, 2021; Accepted October 25, 2021)

keyword : ZnSe, nanoparticle, UV irradiation, post-treatment process, PL intensity