Papers by Author: Sung Hoon Ahn

Abstract: We fabricated a crack-free joining of Ni and Al2O3 using a functionally graded method. Because porosity reduction is important, particle size of the Ni and Al2O3 powders was varied to improve green body density and minimize shrinkage during sintering. As a result, crack-free joining of a Ni–Al2O3 sample with 10 layers was obtained. The ANSYS simulation tool was used to calculate residual stress. The hardness and modulus of each graded layer were measured using an Vickers’ indenter. The experimental values matched the simulation results, showing that this analysis is useful when residual stresses are very difficult to measure experimentally.

Abstract: Joining Si3N4 and Al2O3 using 15 layers has been achieved by a unique approach that introduces SiAlON polytypoids as a functionally graded material (FGM) bonding layer. Previously, the hot press sintering of multilayered FGM with 20 layers, each 500 µm thick, has been achieved successfully. In the present study, the number of layers for FGM was reduced from 20 to 15 to increase optimization. Samples were fabricated by hot pressing at 48 MPa during the temperature ramp to 1650°C and cooling at 2°C/min to minimize residual stresses from sintering. Moreover, a finite element method (FEM) program based on the maximum principal stress theory and the maximum tensile stress theory was applied to design optimized and reduced FGM layers that produced a crack-free joint. The sample had a 3-dimensional cylindrical shape that was transformed to a 2-dimensional axisymmetric mode. By determining the expected thermal stress from the calculated elastic modulus and coefficient of thermal expansion, we were able to predict and prevent damage due to thermal stresses. These analyses are especially useful for FGM samples where it is very difficult to measure the residual stresses experimentally. Finally, oriented Vickers indentation testing was used to qualitatively characterize the strengths of the joint and the various interfaces. The indentation cracks were deflected at the SiAlON layers, implying weak interfaces. In other areas, cracks were not deflected, implying strong interfaces.

Abstract: Cellulose nano whiskers (CNW) are recently known for their tremendous applicability in development of eco-friendly material for various applications. Present attempt was aimed to extract the nano crystals of cellulose from grass of Korea and fabricate the composites with poly (lactic acid) in presence of compatibilizer after modification. Functional group variation, thermal behavior, surface morphology, and crystallinity were monitored by FT-IR, TGA, SEM, and XRD respectively. The water sensitivity measurements were also carried out for study of moisture resistance of composites. It was found that CNW have lower thermal stability than alkali treated long fiber whereas the crystalline nature of composites decreased significantly. The effect of modification of whiskers on the mechanical properties was also discussed in which increase in modulus was observed.

Abstract: Material selection became one of the important activities for engineers in design and manufacturing processes. A selected material at the beginning of design stage affects functionality of the designed part as well as manufacturability and cost of the final product. Unfortunately there are not many accessible material databases that can be properly used for design. In this research, a web-based material database was constructed. In order to assist designers to compare different materials, two-dimensional and three-dimensional graphs for comparison of material properties were provided via the web browser. Using these graphical tools, multi-dimensional comparison was available in more intuitive manner. In addition, this web site is open to the public, and one may add new material properties to the database in order to compare the data with existing materials. To provide the database in a commercial CAD (Computer Aided Design) environment, API (Application Programming Interface) modules were developed using Web services. The web-based material database can be accessible from http://fab.snu.ac.kr/matdb.

Abstract: Cold gas dynamic spray is a relatively new coating process by which coatings can be produced without significant heating during the process. Cold gas dynamic spray is conducted by powder sprayed using supersonic gas jet, and generally called the kinetic spray or cold spray. Its low process temperature can minimize the thermal stress and also reduce the deformation of the substrate. In this study, thick or macro scale deposition was studied while most researches on cold-spray have focused on micro scale coating. Measured material properties of macro scale deposition layer showed that elastic modulus and hardness were lower and electrical resistivity was higher than those of reference substrate material. The main causes of changed material properties were investigated by FE-SEM (Field Emission Scanning Electron Microscope) and EDS (Energy Dispersive X-ray Spectrometer) data. In this result, porous micro structure generated by imperfect plastic deformation might cause decrease in elastic modulus and hardness of the deposition layer by cold spray, and oxidized Al particles increased the electrical resistivity.

Abstract: The rapid prototyping (RP) technology has advanced in various fields such as verification of design, and functional test. Recently, researchers have studied bio-materials to fabricate functional bio-RP parts. In this research, a nano composite deposition system (NCDS) was developed to fabricate three-dimensional functional parts for bio-applications. In the hybrid process, the material removal process by mechanical micro machining and/or the deposition process are combined. NCDS uses biocompatible or biodegradable polymer resin as matrix and various bioceramics to form bio-composite materials. To test drug release rate in vivo environment, two different types of drug delivery system (DDS) were fabricated using the bio-composite materials. 1) Container type DDS used poly(DL-lactide-co-glycolide acid)(50:50) and 5-fluorouracil as the drug composite while polycaprolactone(PCL) served as the container of the drug. 2) Scaffold type DDS formed porous microstructure with poly(DL-lactide-co-glycolide acid)(50:50) and 5-fluorouracil composite. The effect of geometry of the DDS on release rate of drug is under investigation.