Myeong Woo Cho

Wear Characteristics of Magnetorheological Fluid under Boundary Lubrication

This experimental investigation presents wear characteristics of magnetorheoloigcal (MR) fluid under boundary lubrication contact condition. Three different specimens; aluminum, copper and steel pins are prepared and wear test is performed under consideration of several operational factors such as normal load, sliding distance, and sliding speed by using pin-on-disk test equipment. Wear rate and friction coefficients of the MR fluid are experimentally evaluated with respect to the sliding ranges. Microscopic surface and roughness changes of the worn surface of pin specimens are also analyzed by using the scanning electron microscope (SEM). In addition, Energy Dispersive X-ray Spectroscopy (EDS) analysis is conducted and chemical changes are investigated.

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.

Polishing characteristics of optical glass using PMMA-coated carbonyl-iron-based magnetorheological fluid

Soft magnetic carbonyl iron (CI) particles for magnetorheological (MR) polishing encounter corrosion problems as a result of their oxidation, leading to unpredictable polishing results. To overcome this issue, CI particles have been coated with either polymer or inorganic materials for improved MR polishing. In this study, CI particles were coated with poly(methyl methacrylate) to achieve improved MR polishing and anti-corrosion protection. In addition to an analysis of their rheological properties, a series of MR polishing experiments were performed to investigate the material removal rate and surface roughness for BK7 optical glass by changing experimental parameters, such as the wheel rotating speed and magnetic field intensity. A very fine surface roughness (Ra = 0.86 nm for PMMA coated CI/Ra = 0.92 nm for pristine CI) was obtained at a wheel speed of 1256 mm s−1 and a magnetic field intensity of 15.92 kA m−1.

Tribological Characteristics in Modified Magneto-Rheological Fluid

In this study, Magneto-rheological (MR) fluid was modified by mixing and adding the additives. The researches on MR fluid have been active progress all around the worlds, especially for the application to many devices on the various fields. For the success of application, tribological characterisitcals are important to be satisfied. There are many progress mainly aimed to the methods to improve the stability and performance of MR fluid in application. In this study, the experiments on friction and wear with pin-on-disk tester as well as SRV tester are conducted in different operation and magnetic. From the results, tribological performance of modified MR fluid is analyzed.

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

Prediction of Surface Roughness in High Speed Milling Process Using the Artificial Neural Networks

The objective of this research was to apply the artificial neural network algorithm to predict the surface roughness in high speed milling operation. Tool length, feed rate, spindle speed, cutting path interval and run-out were used as five input neurons; and artificial neural networks model based on back-propagation algorithm was developed to predict the output neuron-surface roughness. A series of experiments was performed, and the results were estimated. The experimental results showed that the applied artificial neural network surface roughness prediction gave good accuracy in predicting the surface roughness under a variety of combinations of cutting conditions.

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

Magnetorheological properties and polishing characteristics of silica-coated carbonyl iron magnetorheological fluid:

While magnetorheological fluids can be used for ultra-precise polishing, for example, of advanced optical components, oxidation of metallic particles in water-based magnetorheological fluids causes irregular polishing behavior. In this study, carbonyl iron microspheres were initially coated with silica to prevent oxidation and were used to polish BK7 glass. In addition, their rheological and sedimentation characterizations were investigated. Material removal and surface roughness were analyzed to investigate the surface quality and optimal experimental conditions of polishing wheel speed and magnetic field intensity. The maximum material removal was 0.95 µm at 95.52 kA/m magnetic field intensity and 1854 mm/s wheel speed. A very fine surface roughness of 0.87 nm was achieved using the silica-coated magnetorheological fluid at 47.76 kA/m magnetic field intensity and 1854 mm/s wheel speed.

A Study on Tool Deflection Trend Using Real Captured Images in Micro Endmilling Process

Recently, ultra-precision micro patterns and shapes have been widely used in optical field. Various methods which are based on semi-conductor fabrication methods are nowadays used in fabrication of micro shapes and patterns, but micro endmilling technology has lately attracted considerable attention because of various available materials, flexibility of process and high-productivity. For the precision micro endmilling process, analysis of micro cutting error is mandatory. In general, tool deflection is a major factor which causes cutting error and limits realization of the high-precision cutting process. Specially, in micro endmilling process, micro tool deflection generates very serious problems compared to macro tool deflection. In this paper, it is performed to observe the real tool deflection shapes in micro endmilling process, so the trend of micro tool deflection was analyzed using real captured images in this study. To get the real images of micro tool deflection, micro slot cutting processes were executed under various cutting conditions using micro endmill and the real images of tool deflection were obtained during cutting process by high-speed camera. Finally, the extent of tool deflection was calculated by the deflection angle according to cutting conditions and two trends (the point of first tool contact and the cutting stage) of micro tool deflection were analyzed.