Yong-Bum Choi

Titanium matrix composites (TMCs) have garnered substantial attention from researchers owing to their outstanding properties. Nonetheless, the strength and ductility of TMCs hardly co-exist and often show a trade-off between each other. In this study, we employ an ultra-thin graphite powder sheet as the carbon source and employ Ti/C composites with varying carbon contents, prepared via a layer-stacked laminated sintering method, to ensure a comprehensive in-situ reaction and uniform reinforcement distribution. With increasing carbon content, noticeable alterations occur in the size, concentration, and morphology of the titanium carbide (TiC) particles. The increase of TiC particle content is found to boost the ultimate tensile strength of the composite. However, this improvement comes at the expense of reduced elongation. Notably, as the carbon content reaches 1.81 wt%, the yield strength and ultimate tensile strength of the composites soar to 354.4 MPa and 575.4 MPa, respectively. These values represent a remarkable increase of 75.4% and 65.0% compared to pure titanium, while maintaining an acceptable elongation of 6.45%. This study unveils a promising approach for significantly enhancing the mechanical properties of titanium alloys.

In this study, polyvinyl alcohol (PVA) - polytetrafluoroethylene (PTFE) - vapor grown carbon fiber (VGCF) sheets are fabricated as a new paper-like thermal interface material (TIM), which is a potential substitute for traditional TIMs. Two types of PVA-PTFE-VGCF sheets were fabricated by using 120nm and 300nm PTFE particles. The microstructure shows that the VGCFs were arranged in random directions inside the sheet and interconnected via the aggregate of PVA. 300nm PTFE particles were well distributed within the sheet, while 120nm PTFE particles aggregated partially and formed pores nearby. The fabricated sheets have a low thickness of 31.2um and 28.2um, and lightweight properties with a density of 0.79 × 106 g·m-3 and 0.91 × 106 g·m-3, respectively. With the addition of 300nm PTFE particles, the fabricated sheet has higher thermal conductivities of 9.81 W·m-1·k-1 in the in-plane direction and 2.11 W·m-1·k-1 in the through-plane direction. In the high temperature and humidity test, the thermal conductivities of the fabricated sheet were increased due to the rearrangement of PVA and PTFE particles.

The application of ultrasonic to molten metal has many effects, such as improving its wettability and fracturing the oxide layer. However, a few research studies have been done to apply ultrasound to the Hot-Dip Galvanizing plating method. This study focuses on improving the interfacial bonding strength and removing the oxide layer using ultrasonic waves. In this study, a high-tensile steel plate will be plated with ultrasonic waves, and the plated steel will be examined by microstructure observation and the mechanical properties of the interfacial bonding structure between SUS310S and Zn-Al alloy. It was confirmed that ultrasound waves affect the removal of impurities and pores between the SUS310S steel plate and the interface of the Zn-0.13 wt.% Al alloy during Hot-Dip galvanizing process. Furthermore, to observe the effect of ultrasonic waves on the removal of oxides, a kind of impurity, from the surface of SUS310S, an oxide layer having a thickness of 2 µm was deposited on the surface of SUS310S by Physical Vapor Deposition (PVD). When ultrasonic waves were applied to the oxide-coated SUS 310S surface for 60 seconds, a part of the oxide layer was fractured by the cavitation effect of ultrasonic.

Titanium matrix composites (TMCs) with TiC reinforcements were fabricated by an in-situ method that evolves pure titanium foils (thick: 100 μm) and graphite powder sheets by spark plasma sintering. 20 μm thick graphite powder sheets with PVA (polyvinyl alcohol) were fabricated as carbon resources. The effects of different sintering temperatures and heating time on microstructural features, interface, and properties of the composites were investigated. The structural and microstructural analyses were performed by EPMA, FE-SEM, and EDS. The XRD patterns taken from the cross-section of the prepared composites revealed the composites are composed of TiC_x and hexagonal close-packed (HCP) α-Ti. Homogeneous rod-like TiC_x particles reinforced TMCs were evaluated by tensile property. The tensile properties of the rod-like TiC_x-reinforced TMC show that the tensile strength (UTS) is 479 Mpa, which is 81.4% higher than pure titanium. The formation mechanism and enhancement mechanism of rod-like TiC_x particles are also discussed.

The conventional manufacturing process of fiber-reinforced metal matrix composites via liquid infiltration processes, preform manufacturing using inorganic binders is essential. However, the procedure involves binder sintering, which requires high energy and long operating times. A new fabrication process without preform manufacturing is proposed to fabricate short carbon fiber (SCF)-reinforced aluminum matrix composites using a low-pressure infiltration method. To improve the wettability between fiber and matrix, fibers were plated copper using an electroless plating process. The low-pressure infiltration method with preformless succeeded in manufacturing a composite with a volume fraction of about 30% of carbon fibers.The fiber orientation of the composite material manufactured without preform and the fiber orientation of the composite material manufactured using an inorganic binder was almost the same. The manufactured composites with preformless have high hardness and high thermal conductivity.

A new fabrication process without preform manufacturing has been developed for carbon short fiber (CSF) reinforced various aluminum matrix composites. And their mechanical and thermal properties were evaluated. Electroless Ni plating was conducted on the CSF for improving wettability between the carbon fiber (CF) and aluminum. It was confirmed that pores in Ni plated CSF/Al and Al alloy matrix composites prepared by applied pressure, 0.8 MPa, had some imperfect infiltration regions between the CF/CF and CF/matrix in all composites. However, pores size in the region between the CF/CF and CF/matrix to use the A336 matrix was about 1 µm. This size is smaller than that of other aluminum-based composites. Vickers hardness of Ni plated CSF/A1070, A356 alloy, and A336 alloy composites were higher as compared to matrix. However, the A1070 pure aluminum matrix composite had the highest hardness improvement. The Ultimate tensile strength of the A1070 and A356 aluminum matrix composite was increased due to carbon fiber compared to only aluminum, but the Ultimate tensile strength of the A336 aluminum matrix composite was rather lowered due to the highest content of Si precipitate and large size of Al₃Ni compounds. The Thermal Conductivity of Ni plated CSF/A1070 composite has the highest value (167.1 W·m⁻¹·K⁻¹) as compared to composites.

Conventional manufacturing process of fiber reinforced metal matrix composites via liquid infiltration processes, preform manufacturing using inorganic binders is essential. However, the procedure involves binder sintering, which requires high energy and long operating times. A new fabrication process without preform manufacturing is proposed to fabricate carbon short fiber (CSF)-reinforced aluminum matrix composites using a low-pressure infiltration method. To improve the wettability and avoid interfacial reactions in CSF/Al matrix composites, the fibers were plated with copper using electroless plating process. Various volume fractions of CSFs were used to determine optimum fiber content which would produce versatile mechanical and thermal properties. Effect of CSFs content on properties such as Vickers hardness and thermal conductivity was studied. Cu-plated CSFs showed good bonding with the Al matrix and CSFs were randomly dispersed inside the composites, with CSF content of up to 29.1 vol.%, through the new manufacturing process. It showed better fiber distribution than the composite fabricated perform with SiO₂ binder, which was determined by comparing the relative frequency distribution of CSFs in composites. Vickers hardness of the composites showed an obvious improvement over that of the Al matrix, and the hardness increased as the CSF content increased. The Cu-plated CSF (14.3 vol.%) reinforced Al matrix composite exhibited the highest thermal conductivity (184.1 W·m^-1·K^-1). However, the thermal conductivity decreased as CSF content increased to 29.1 vol.% due to the defects in composite.

Polyvinyl alcohol (PVA)-based carbon nanofiber (CNF) sheets are fabricated as an innovative thermal interface material (TIM), which is a potential substitute for traditional TIMs. Five types of PVA-based CNF sheets were fabricated at different mass ratios of PVA:vapor-grown carbon fiber (VGCF) (1:0.100, 1:0.070, 1:0.050, 1:0.030, 1:0.025). The thickness of the PVA-based CNF sheets was 30–50 µm, which was controlled by the amount of VGCF. The microstructure of the CNF sheets indicated that VGCFs were arranged in random directions inside the sheet, and PVA was formed as a membrane between two VGCFs. However, many pores were found to exist between the VGCFs. The porosity of the PVA-based CNF sheets decreased from 25 to 13% upon decreasing the mass ratio of VGCF from 43.38 to 16.13%. The density and Shore hardness of all CNF sheets were 1.03–1.15 × 10⁶ g m⁻³ and 82.4–85.0 HS, respectively. The highest thermal conductivity, measured as the mass ratio of PVA:VGCF, was achieved at 1:0.05, with the in-plane thermal conductivity of the fabricated sheet being 14.3 W m⁻¹ k⁻¹.