Won Seung Cho

Microstructure and Mechanical Properties of Silicon Nitride/h-BN Based Machinable Ceramics

The effects of h-BN content on the microstructure, mechanical properties, and machinability of Si3N4 ceramics were investigated. The relative density of the sintered compact decreased with increasing BN content. The flexural strength also decreased with h-BN content, mainly due to lower Young’s modulus of h-BN compared to Si3N4. With increasing h-BN content, Si3N4/h-BN based ceramic composites revealed enhanced crack resistance (R-curve) behavior. The Vickers indentation crack paths in specimens are sinusoidal due to bridging and pull out of grains during crack propagation. The grain size of ß-Si3N4 slightly decreased with h-BN content. During milling and micro-drilling process, monolithic Si3N4 ceramic could not be machined, due to brittle fracture. However, thrust forces measured for Si3N4/h-BN composites decreased with increasing h-BN content, showing the excellent macro and micro machinabilty.

Effect of Al2O3 Concentration on Density and Structure of (CaO-SiO2)-xAl2O3 Slag

The effect of Al2O3 concentration on the density and structure of CaO-SiO2-Al2O3 slag was investigated at multiple Al2O3 mole percentages and at a fixed CaO/SiO2 ratio of 1. The experiments were conducted in the temperature range of 2154 K to 2423 K (1881 °C to 2150 °C) using the aerodynamic levitation technique. In order to understand the relationship between density and structure, structural analysis of the silicate melts was carried out using Raman spectroscopy. The density of each slag sample investigated in this study decreased linearly with increasing temperature. When the Al2O3 content was less than 15 mole pct, density decreased with increasing Al2O3 content due to the coupling of Si (Al), whereas above 20 mole pct density of the slag increased due to the role of Al3+ ion as a network modifier.

The Effect of Tungsten Content on Nanocrystalline Structure of Ni-W Alloy Electrodeposits

The effects of tungsten content on the nanocrystalline structure of Ni-W alloy electrodeposits were investigated. Direct current electrodeposition was conducted with various W mole fractions in a nickel sulfate solution containing citric acid as a complexing agent. Current efficiency was an almost constant value of 52% up to W mole fraction of 0.58. However, it was sharply decreased owing to severe H2 evolution with increasing cathode overvoltage at a W mole fraction higher than 0.74. The X-ray diffraction peak was decreased and broadened with increasing W content, and an amorphous pattern appeared clearly at 49 wt.% W. It was confirmed that Ni-W alloy deposit was transformed from crystalline to amorphous structure between 41 and 49 wt.% W. This result was also verified through a TEM analysis.

Microstructure and Mechanical Properties of AlN-BN Based Machinable Ceramics

AlN based machinable ceramics with h-BN contents in the range of 5 to 30 vol% were prepared by hot-pressing at 1800°C for 2 h, using 3 wt% Y2O3 as sintering aids. The effects of hBN content on microstructure, mechanical properties, and machinability were investigated. The relative density of sintered compact decreased with increasing h-BN content. The four-point flexural strength also decreased with increasing h-BN content, mainly due to the much lower Youngs modulus of BN compared to AlN. In addition, the residual tensile stress, formed by the thermal expansion coefficient difference between AlN and h-BN, might cause the drop of the strength. The h-BN particles had the desired crack deflection increasing effect. However, the fracture toughness decreased with h-BN content. During end-milling process, feed and thrust forces measured for all kinds of specimens decreased with increasing h-BN particles. From the experimental works, excellent machinability of the composites was investigated, and good surface roughness of specimens is observed after machining process. Also, irrespective of h-BN content, relatively good surfaces with roughness less than 0.5 μm (Ra) could be achieved within short lapping time. Introduction Aluminum nitride (AlN) is an excellent candidate for use as substrates in electronic packaging and electrical, optical material field because of high thermal conductivity, high translucent properties and thermal expansion coefficient close to that of silicon (Si) [1]. It is need to precision machining in order to expand those applications for precision parts. However, high resistance force of cutting and difficulty of machining due to brittle fracture behavior of ceramics makes it a difficulty for manufacture as complicated shapes in various products. Though it is possible to machining by diamond abrasive [2] and cutting by laser tool [3], it has a limitation to manufacture complicated shapes and a problem of price [4]. AlN-BN based ceramics can be machined easily since cleavage plane of h-BN can facilitate crack propagation during machining, thereby decreasing cutting resistance [5,6]. The purpose of this study is to optimize machinability of AlN-BN composite by investigating the effect of h-BN content on microstructure, mechanical properties, and machinability. Experimental Procedure In this experiment, AlN (2.9 μm, Tokuyama) and h-BN (2.6 μm, T.D.K) in range from 0 to 30 vol% was used. As sintering aids, 3 wt% of Y2O3 (<0.5 μm, Shinestu Chemical Co.) were added. The powder mixture was prepared by wet ball milling in ethanol for 72 h in alumina pot. After mixing, Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 873-876 doi:10.4028/www.scientific.net/KEM.264-268.873

Application of Powder Blasting Techniques to Micro-Pattern Making Process for Si3N4-hBN Composites

In this study, micro powder blasting techniques are applied to micro-pattern making process using developed Si3N4-hBN composites. Material properties of the developed machinable ceramics according to the variation of h-BN contents, those are used to give good machinability to the ceramics, are evaluated. And, a series of required experimental works are performed to determine optimum powder blasting conditions for micro-pattern making. The experiments are performed for the prepared samples with no mask, and samples with three different mask patterns. As the results, it can be observed that the machinability of the developed Si3N4-hBN composites increases as the h-BN contents in the composites. Also, from the experimental results, it is possible to determine the optimum blasting conditions for micro-pattern making process with Si3N4-hBN composites.

Machinability Evaluation of Si3N4-hBN Composites for Micro Pattern Making Processes

In this study, machinablilty evaluation of Si3N4-hBN composites is performed for micro pattern making processes. First, R-curve behavior of Si3N4-hBN composites is investigated to understand its machinability. And, micro-powder blasting and micro-drilling processes are performed for the machinability evaluation of the composites. In the micro-powder blasting process, the machining effects on the erosion depth of samples using three different mask patterns are analyzed by varying the nozzle scanning times and compositions of h-BN. Also, in the micro-drilling process, small holes are drilled using micro-drilling machine under variable conditions. Finally, machinablity of Si3N4-hBN composites is investigated based on the obtained data. Introduction Si3N4 composites are very difficult-to-cut materials because of its high strength and hardness. Their machining mechanism can be characterized by cracking and brittle fracture due to relatively high strength and temperature resistance. However, the Si3N4-hBN composites are well known as one of the machinable ceramics, which can be easily machined with hard-alloy cutting tools. Si3N4-hBN composite materials have been used as special refractory nozzles, tubes, break rings for the continuous casting of steel, etc, owing to their high temperature resistance with chemical inertness and thermal shock resistance. Also, recently, development of special purpose parts, such as parts for semiconductor processing, parts and sensors for micro-machine fabrication, etc., has been expanded. Thus, it is essential to develop micro-machining technologies for hard and brittle materials such as glass, ceramics, etc. In this study, machinability of the Si3N4-hBN composites is evaluated for micro-pattern making processes. As a first step, R-curve behaviors of the composites are analyzed to predict the machinability. And, micro-powder blasting and micro-drilling processes are performed for the machinability evaluation of the Si3N4-hBN composites in real situation. In the micro-powder blasting process, the machining effects on the erosion depth of samples with three different mask patterns are analyzed by varying the nozzle scanning times and contents of h-BN. Also, in the micro-drilling process, small holes are drilled using micro-drilling machine under variable conditions. Finally, the machinablity of the Si3N4-hBN composites is investigated based on the obtained data. R-curve behavior analysis for machinability evaluation The Si3N4 powder used in this study is highly pure except the oxygen content of 0.8 wt% and has an average particle size of 0.17 μm, and the BN has a hexagonal crystal phase. The h-BN content in Si3N4 is varied from 0 to 30 vol%. They are hot-pressed at 1800°C for 2 hours in a nitrogen Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 869-872 doi:10.4028/www.scientific.net/KEM.264-268.869

Microstructure and Mechanical Properties of Si3N4/hBN Composites Prepared by Hot-Pressing Using Nitrided Si3N4 Powders

In order to produce Si3N4/hBN composite with low cost, it seems necessary to use nitrided Si3N4 powders since the cost of Si powder is much cheaper than that of Si3N4 powder. The purpose of the present work is to investigate the nitride conditions, and in particular, we focused on the relationship between microstructures and mechanical properties of hot-pressed Si3N4/hBN ceramic composite using nitrided Si3N4 powders. The mixed powders of Si3N4 and hBN were prepared by nitriding Si powders at 1380oC for 24 h, and subsequently sintered by hot-pressing at 1800oC for 2 h in N2 atmosphere. The microstructure and mechanical properties of the Si3N4/hBN composites were investigated. Flexural strength, Young’s modulus, and hardness decreased by the addition of 20 vol% hBN. The addition of BN resulted in a decrease in the modulus as well as an increase in the size of fracture source, both contribute to the observed decrease in mechanical properties. The Si3N4/BN based ceramic composites revealed enhanced crack deflection. The Vickers indentation crack paths in specimens are sinusoidal due to pull-out of grains during crack propagation.

Preparation of Porous Alumina Ceramics by Spark Plasma Sintering

Porous alumina bodies were successfully prepared by spark plasma sintering of alumina powders with different amounts of graphite, and by subsequently burning out the graphite. Highly porous bodies were fabricated by spark plasma sintering at 1000°C for 3 min under a pressure of 30 MPa. The heating rate was 80°C/min, and the pulse pattern (on-off) was 12:2. For example, alumina bodies prepared by the addition of 10 ~ 30 vol% graphite showed high porosity of 50 ~ 57%. Porous alumina bodies prepared by the addition of 10 ~ 30 vol% graphite had a high compressive strength of 200 ± 55 MPa, about 35 times higher than those obtained on samples prepared by pressureless sintering, and about 2.5 times higher than those in samples prepared by hot-pressing. The significant improvement in strength relative to values obtained with conventional sintering was attributed to better sintering resulting from the rapid heating between particles.

Mechanical Properties and Machinability of Si3N4/hBN Composites Prepared by Pressureless Sintering

The effects of hBN content on microstructure, mechanical properties, and machinability of the pressureless-sintered Si3N4 ceramics were investigated. Flexural strength, Young’s modulus, and hardness decreased with increasing h-BN content. The mechanical properties are decreased mainly because of increased porosity of composite, and the much lower Youngs modulus of BN compared to that of Si3N4. Pressureless-sintered Si3N4/hBN composites exhibit strong texture of BN grains oriented with the c-axis parallel to the cold-pressing direction. Cutting resistance of Si3N4 ceramic composites with more than 10 vol% hBN decreased with increasing hBN content, demonstrating a good machinability of the composites. The residual pores can be attributed to improved machinability of pessureless-sintered Si3N4-BN composite.

Fabrication and Thermoelectric Properties of N-Type Bi2Te3 Based Compounds by Spark Plasma Sintering

N-type Bi2Te3 based thermoelectric compound was prepared by spark plasma sintering with a temperature range of 340~460°C and powder size of ~75㎛, 76~150㎛, 151~250㎛. Thermoelectric properties of the compound were measured as a function of the sintering temperature and powder size. With increasing sintering temperature, the electrical resistivity and thermal conductivity of the compound greatly changed because of the increase in relative density. The Seebeck coefficient and electrical resistivity were varied largely with increasing powder size. Therefore, the compound sintered at 460°C, with the powder of ~75㎛, showed a figure of merit of 2.44 x 10-3/K. Also, the bending strength was 75MPa.