Seung Ho Kim

Effects of Different Solid Lubricants on Mechanical and Tribological Properties of Al2O3/ZrO2 Nanocomposites

Solid lubricants were suited for the applications of high temperature parts. Recently, researches for self-lubricating ceramic matrix composites using solid lubricants, such as MoS2, WS2, h-BN, and graphite etc., became active. This work is to use the spark plasma sintering (SPS) technique in order to make self-lubricating Al2O3/15wt% ZrO2 nanocomposites with higher mechanical properties. To optimize the self-lubricating Al2O3/15wt% ZrO2 nanocomposite systems were added with different solid lubricants (CaF2, BaF2, MoS2, WS2, h-BN, and graphite). The coefficient of friction of CaF2, h-BN and graphite added composites in steady-state at room temperature are below 0.3.

Pulsed Electric Current Sintering of the Al2O3 - 15wt% ZrO2 Nanocomposites with 3wt% of Different Solid Lubricants

High temperature low friction materials are sought for use in engines in order to reduce energy consumption of the machines. Due to the high service temperatures solid lubricating materials are necessary. This study is designed to find the optimal processing conditions for preparing these materials by pulsed electric current sintering. In this study, the Al2O3 - 15wt% ZrO2 (AZ) nanocomposite was modified with 3 wt% of self-lubricating component (CaF2, BaF2, MoS2, WS2, h-BN, or graphite). After the preparation of the alumina-zirconia powder mixture solid lubricant powder was added. Powders were then mixed in ethanol for 24 h, dried in a rotary evaporator, and in oven at 80°C for 24 h. The particle size distribution of the powders was established with the laser method. Powders were compacted by using pulsed electric current sintering technique at 1300 °C with 50 MPa for 5 min in vacuum. The structure of the materials was studied with XRD and SEM. Density of the compacts was measured with the Archimedes method and their hardness was evaluated by applying HV1 hardness with the instrumented indentation techniques. Their mechanical behavior was further studied with the instrumented scratch testing.

Effects of Fabricated Method on the Coefficient of Friction of Al2O3-15 wt% ZrO2-3 wt% Solid Lubricant Composites

The coefficient of friction was very important factor for the applications of high temperature parts. In vehicles, the coefficient of friction was decreased due to lubricants as like engine oil etc. Lubricant such as oils is difficult to apply at high temperature. To apply high temperature parts, lubricants were demanded for high temperature stability. This work is to use the pulsed electric current sintering (PECS) technique and the atmospheric plasma spraying (APS) method in order to make self-lubricating Al2O3-15wt% ZrO2-solid lubricant composites. We focused on the coefficient of friction for the fabrication method of self-lubricating Al2O3-15wt% ZrO2-solid lubricant composites. We compared with the coefficient of friction of PECSed and APSed composites. The surface roughness of PECSed Al2O3-15wt% ZrO2-solid lubricant composites were 0.06 ~ 0.31 μm of Ra and 10.16 ~ 33.12 μm of Ry. In the case of APSed Al2O3-15wt% ZrO2-solid lubricant composites, as-coated samples were 6.56 ~ 11.42 μm of Ra and 59.68 ~ 81.79 μm of Ry, and polished samples were 1.12 ~ 3.70 μm of Ra and 11.66 ~ 32.22 μm of Ry. The coefficient of friction of PECSed and APSed Al2O3-15wt% ZrO2-solid lubricant composites were 0.19 ~ 0.49 and 0.41 ~ 0.61, respectively.

A Study on Mechanical and Tribological Properties of Hot Pressed Al2O3/ZrO2/h-BN/TiO2 Composites

Oxide ceramics such as alumina and zirconia are industrially utilized as cutting tools, a variety of bearings, biomaterials, and thermal and corrosion-resistant coatings due to their high hardness, chemical inertness, high melting point, and ability to retain mechanical strength at elevated temperatures. In this research, the effect of other ceramic additives (TiO2) and h-BN within alumina(α-Al2O3) and yttria-stabilized tetragonal (Y-TZP) composite was studied with respect to the mechanical and tribological properties. The lowest coefficient of frction of 0.45 was observed for the ZTA ceramic composite with hBN-TiO2. The highest hardness, fracture toughness and flexural strength were obtained as 15.7GPa, 5.2MPam-1/2, 712MPa, respectively.

Effect of Reinforcement on the Tribological Behavior of CuFe-Based Composites

Two types of CuFe matrix composites with different reinforcements: silica and alumina particulates were developed using powder metallurgy. The mechanical properties were determined from Brinell hardness and flexural strength. The dry sliding friction and wear performance of the composites were investigated by the friction test. The results indicated that mechanical properties of alumina were superior to those of silica. The friction tests of the composites showed that the alumina reinforcement particle provides better braking performance. A wear track examination of composites showed that same abrasive wear. Our results indicated that composites with alumina reinforcement particles of high compatibility are to be preferred for braking performance.

Tribological Behaviors of Dense Gelcasting Nanocomposites

Gelcasting/pressure less sintered Al2O3/SiC nanocomposites has a low sinterability. Also, mechanical and wear properties of these nanocomposites was degraded. Wear mechanism of low sinterability gelcasting nanocomposites was dominated by fracture mode of surface during wear tests. In this study, gelcasting processing and followed plressureless (PL) and hot isostatic pressing (HIP) sintering was attempted to fabricate dense Al2O3/SiC nanocomposites. Wear behaviors of high densities gelcasting nanocomposites were investigated under the identical wear test condition. The comparative specimen was used to hot pressed nanocomposites. Wear rates of dense gelcasting nanocomposites were related to closely initial friction coefficient.

High Strength and Toughness Al2O3 composite materials for the Improvement of Ceramic Channel in SRL Hot Dipping System

The ceramic channel is very important in SRL hot dipping system. High strength and fracture toughness of ceramic channel materials can improve the quality, productivity and economic feasibility of zinc plated steel. The purpose of this research was to find out the most suitable conditions of the ceramic channel that have best fracture strength and fracture toughness. The hot pressed composite materials was carried out by hot pressing Al2O3 with different content of ZrO2. The composite contained from 0-20 wt.% ZrO2. Hot pressed composite materials were observed for mechanical properties (density, hardness, fracture toughness and flexural strength) and microstructure.

Boron Nitride Doped Transparent Polycrystalline Silicon Nitride Ceramics

The addition of h- BN to a polycrystalline Si3N4 was to increase the fracture toughness and other mechanical properties such as flexural strength and hardness of the material. The hot pressed samples were prepared from the mixture of α-Si3N4, AlN, MgO and h-BN. The composite contained from 0 to 2 wt.% BN powder with sintering aids (9% AlN + 3% MgO). The transparency, mechanical properties and microstructure of hot pressed polycrystalline Si3N4-BN composite materials were investigated by UV/VIS spectrophotometer, scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques. The transparency decreased with increasing the content of h-BN into Si3N4.