Dae-Soon Lim

Synergistic effects of aramid pulp and potassium titanate whiskers in the automotive friction material

Friction and wear of automotive brake friction materials were investigated using a pad-on-disk type friction tester to study the synergistic effects of aramid pulp (fibrillated organic fibers) and potassium titanate whiskers (fine ceramic whiskers). The friction materials were based on a simple formulation with five ingredients (phenolic resin, aramid pulp, potassium titanate whiskers, molybdenum disulfide, and barite). Friction stability, wear rate, and morphology of sliding surfaces were carefully examined to investigate the effect of the two different fibrous ingredients in the friction material on various friction properties. The results showed that both aramid pulp and potassium titanate whiskers played a crucial role for the friction characteristics by maintaining the durable friction film on the rubbing surface. The friction film provided improved friction stability and wear resistance. Microscopic observations of the friction material exhibited that the fine ceramic whiskers adhered to the fibrillated aramid fibers, providing heat resistance and strength to the friction film at the rubbing interface. The beneficial synergistic effect from the two fibrous ingredients, however, was significantly diminished when only one of them was employed.

Temperature effects on the tribological behavior of alumina reinforced with unidirectionally oriented SiC whiskers

In the present study, the influence of temperature on the tribological behavior of alumina reinforced with SiC whisker addition up to 20 vol.% is presented. Wear tests were performed at selected temperatures with alumina composites prepared by a modified tape casting method. Above a 673 K wear test temperature, the wear and frictional characteristics behaved differently compared with tribological behavior at 403 K. The results of this study indicate that as the temperature increased, the reinforcement effect of the unidirectionally oriented SiC whisker increased and the grain size effect was decreased. The effect of whisker orientation on friction and wear characteristics at the higher temperature was not dependent on test temperatures. The highest wear rate was obtained with a SiC whisker oriented parallel to the tape casting direction at all tested temperatures. The friction and wear mechanism of alumina composites reinforced with SiC at a high temperature is discussed based on SEM observations of worn surfaces and energy dispersion X-ray analysis.

Tribological behavior of alumina reinforced with unidirectionally oriented SiC whiskers

Silicon carbide whisker reinforced alumina ceramic composites were subjected to tribological tests by using ball-on-reciprocating flat geometry at 403 K in order to study wear progress under mild wear regime. Alumina composites were prepared by a modified tape casting followed by lamination, binder removal and hot pressing in order to align the whiskers in tape casting direction. Wear coefficients on three directions were measured; parallel and normal to the tape casting direction on the tape casting surface and normal to lamination direction on surface normal to the tape casting direction. For comparison, samples were prepared by hot pressing the powder mixture. The highest wear rate was obtained in the direction parallel to the tape casting direction and the lowest in the direction normal to lamination direction. The results of this study also indicated that matrix grain size and roughness due to whisker orientation were responsible for variations of wear rate and friction coefficient.

Effect of microstructural orientation on erosion behavior of self-reinforced silicon nitride

Erosion tests were conducted on silicon nitride containing uniaxially oriented large elongated grains using silicon carbide angular particles at impact angles from 45 to 90°. Two different impact directions were selected to investigate the effect of elongated grain orientation on erosion behavior of self-reinforced silicon nitride. Erosion weight loss after impact of SiC particles was measured and the erosion behavior depending on impact angle and grain orientation were studied. Higher erosion rate was obtained in the normal direction with respect to the aligned large grains in all tested impact angles. The erosion models based on grain pullout and lateral cracking were proposed to explain the observed anisotropic tendency in erosion behavior of self reinforced silicon nitride at the low impact angle.

Fabrication and Characteristics of Yttria-stabilized Zirconia (7.5 wt% Y2O3-ZrO2) Coating Deposited via Suspension Plasma Spray

Yttria-stabilized zirconia (YSZ) coatings are fabricated via suspension plasma spray (SPS) for thermal barrier applications. Three different suspension sets are prepared by using a planetary mill as well as ball mill in order to examine the effect of starting suspension on the phase evolution and the microstructure of SPS prepared coatings. In the case of planetary-milled commercial YSZ powder, a deposited thick coating turns out to have a dense, vertically-cracked microstructure. In addition, a dense YSZ coating with fully developed phase can be obtained via suspension plasma spray with suspension from planetary-milled mixture of Y2O3 and ZrO2.

Thermal Durability of 4YSZ Thermal Barrier Coating Deposited by Electron Beam PVD

Abstract 4 mol% Yttria-stabilized zirconia (4YSZ) coatings with 200 µm thick are fabricated by Electron BeamPhysical Vapor Deposition (EB-PVD) for thermal barrier coating (TBC). 150 µm of NiCrAlY based bond coat is pre-pared by conventional APS (Air Plasma Spray) method on the NiCrCoAl alloy substrate before deposition of top coat-ing. 4 mol% YSZ top coating shows typical tetragonal phase and columnar structure due to vapor phase depositionprocess. The adhesion strength of coating is measured about 40 MPa. There is no delamination or cracking of coatingsafter thermal cyclic fatigue and shock test at 850 o C.Keywords: Electron Beam Physical Vapor Deposition(EB-PVD), Yttria Stabilized Zirconia(YSZ), Thermal Barrier Coat-ing(TBC), Thermal Cyclic Fatigue ······························································································································· ································································································· 1. 서론 열차폐 코팅(TBC, Thermal Barrier Coating)기술은 발전용 가스터빈, 항공용 터빈엔진부품 등에 적용되어 높은 온도와 압력에 노출되는 금속 합금소재 부품의 내구성 및열, 에너지 효율 향상을 목표로 발전되어 왔다[1-4]. 일반적으로 열차폐 코팅은 고온의 가스환경에 직접 노출되는세라믹 소재의 탑코팅(Top coating)과 침식이나 산화저항성을 위한 금속합금소재의 본드코팅(Bond coating)으로 구성된다. 탑코팅에 사용되는 소재는 지르코니아 (ZrO