Key Engineering Materials Vols. 321-323

Abstract: In this report, the indentation method of measurement for residual stress in structural ceramics is discussed. The residual stresses in Al2O3, Si3N4 and ZrO2 ceramics were evaluated with the indentation method. The value obtained by the indentation method was examined as a function of the microstructure, using Al2O3 ceramics differing in microstructure (grain size and aspect ratio). The residual stress values in Al2O3 and Si3N4 obtained by the indentation method at 98 N agreed well with the values obtained by the X-ray method. The residual stress value in ZrO2 obtained by the indentation method was close to the value obtained by the X-ray method, when the indentation load was 294 N. For estimating the residual stress in ZrO2 with the indentation method, the influence of the phase transformation induced by the indentation is added to the original residual stress, when the indentation is small. The value obtained by indentation method differed with the aspect ratio of the grains in Al2O3. It was thought that the origin of the variation was the difference in crack propagation resistance in different materials

Abstract: Bi-Te-Sb compound material was developed by spark plasma sintering process. The electrical, thermal, thermoelectric and mechanical properties also obtained by the experiments. This paper discusses performance and strength of the thermoelectric cooler made of Bi-Te-Sb compound alloy. The effect of the current has been investigated. The coefficient of performance (COP) is the primary factor to evaluate the performance of the coolers. Using the FEMLAB software package finite element (FE) analysis was carried out with properties of Bi-Te-Sb compound alloy from experiments. The temperature distributions, the stress distributions and the COP in the thermoelectric cooler were obtained.

Abstract: Effects of TiAlN coating conditions on the cutting characteristics during machining high hardened tool steel using coated ball end mill were studied in this research. KP4 steel and STD 11 steel were used as workpieces. Minimum quantity lubrication (MQL) was utilized in the cutting. Scanning electron microscope, optical tool, and a tool dynamometer were used to measure coating thickness and progress of tool wear. Results showed that the cutting distance using TiAlN coated ball end mills was longer than that using WC-Co ball end mill by 2.3~5.7 times for KP4 steels and about 2.5~4.3 times for STD11 heat treated steels. The multi layer TiAlN coated ball end mill showed better performance of machining of hard steels than single layered coated tools.

Abstract: An effect of anisotropy on the thermoelectric properties of Bi1Sb3Te6 added with Au alloys prepared by a mechanical alloying process has been studied. The conduction properties including electrical conductivity and thermal conductivity were increased with Au content. The electrical conductivity and the power factor of the perpendicular direction to the pressing direction were larger than those of the parallel direction to the pressing direction. The intensity of (1 1 0) perpendicular plane was larger than that of the parallel plane. It was suggested that the increase of intensity of the (1 1 0) plane would contribute to improve the thermoelectric performance. Although the power factor and thermal conductivity revealed the anisotropic behavior with direction, the Z value showed almost the equal value regardless of direction. From these results, it appeared that the Z value of the Bi1Sb3Te6 added with Au alloy prepared by powder metallurgy process was almost isotropic.

Abstract: A study has been made on the effects of antimony (Sb) and strontium (Sr) additions on the microstructure of Mg-Al-Si alloys. Results showed that the additions of Sb and Sr can modify the morphology of Mg2Si particles from Chinese script shape to refined polygonal shape. Tensile strength and creep resistance were improved and tensile elongation was also increased in the modified alloy with Sb and Sr. The addition of Sr was more effective than Sb modification of AS52 alloy for refining microstructure and thus improving properties.

Abstract: The effect of Al content on the castability, tensile and creep properties of Mg-xAl-Zn alloy was investigated. The molten Mg-(3~11)Al-Zn alloys of 700 °C were poured into the mould designed to estimate castability. Tensile and creep tests were performed for the alloys. It has been shown that creep resistance decreased while tensile strength improved with increasing Al amount. Microstructure study revealed that morphology of Mg17Al12 is main factor determining the tensile and creep properties of as cast Mg-xAl-Zn alloys. The final purpose of this study is to develop the heat resistant Mg alloys to apply transportation systems such as automobile. Further study on effect of heat treatment and additional alloying elements on the microstructure and mechanical properties of Mg-xAl alloys is going to be performed.

Abstract: In this study, we performed fracture tests on thermally aged DLC thin films by using micro-indentation and nano-indentation and investigated the deformation and fracture characteristics of these films. The aged high hydrogen films have better characteristics such as thermal and mechanical stability than low hydrogen films.

Abstract: The residual stresses in a polymeric thin film deposited on Si wafer induced during cooling from a cure temperature down to room temperature are investigated. The laser scanning method and the boundary element method (BEM) are employed to investigate the residual stresses. A 3 μm thick polyimide film is deposited on a relatively thick Si wafer. The normal stress across thickness of the thin film is estimated from wafer curvature measurements to be 20MPa . The boundary element method is employed to investigate the whole stresses in the film. The numerical result for the normal stress across thickness of the film, σ xx , shows good agreement with the experimental result obtained by using the laser scanning method. The singular stress is observed near the interface corner. Such residual stresses are large enough to initiate interface delamination to relieve the residual stresses.

Abstract: This study is conducted in order to estimate the structural characteristics for inlet port of air breathing engine under flight condition at 12 ~ 15 km height and Mach Number 2 ~ 4. Inlet port of air breathing engine is heated up from 307 oC to 427 oC, so that is reached plastic zone partially. The material properties at high temperature such as young’s modulus, thermal expansion coefficients and thermal conductivity are applied to the analysis. The transient temperature and stress analysis are performed by the finite element method using nonlinear commercial code ABAQUS. The analysis is performed for the two materials, Titanium alloy and AISI4130 Steel and two FEM element type, shell and solid, to select preliminary design model for flight condition. Thermal boundary conditions are applied at inlet port until 5 s, and these conditions are maintained until 150 s. The results of the analysis recommend tendency of thermal stress and temperature contour. This study is expected useful to select the structural material and to determine shape of air inlet port of air breathing engine which satisfied the structural design safety.

Abstract: A new type of thermotropic main-chain liquid crystalline polyurethanes containing biphenyl units was synthesized by polyaddition reaction of diisocyanates such as 2,6-tolylene diisocyanate, 2,5-tolylene diisocyanate, 2,4-tolylene diisocyanate, and 1,4-phenylene diisocyanate, with 4,4′-Bis(11-hydroxyundeyloxy)biphenyl (BP11). The structure of the monomer and the corresponding polymers were confirmed FT-IR and 1H NMR spectroscopic methods. BP11 exhibited a smectic type mesophase, however, nematic phase was found for all synthesized liquid crystalline polyurethanes except for 1,4-phenylene diisocyanate/BP11 based polyurethane. For example, polyurethane 2,5-TDI/BP11 exhibited monotropic liquid crystallinity in the temperature ranges from 173 to 156 °C on the cooling stage.