Authors: Yong Suk Kim, Jong Eun Lee, Suk Ha Kang, Tai Woong Kim
Abstract: Effect of phase transformation and grain-size variation of hot-pressed cobalt on its dry
sliding wear was investigated. The sintered cobalt specimens were heat treated under different
conditions and their tribological characteristics were examined. The sliding wear test was carried
out against glass (83% SiO2) beads at 100N load using a pin-on-disk wear tester. A constant sliding
speed of 0.38m/s and sliding distance of 600m were adapted. Worn surfaces, cross sections, and
wear debris were examined by a scanning electron microscopy (SEM). X-ray diffraction (XRD)
was utilized to identify phases of the specimen and wear debris. The cobalt specimens exhibited low
friction coefficients of around 0.2. Thermal transformation of the cobalt from the hcp ε phase to the
γ (fcc) phase during the wear was detected, which was deduced as a wear mechanism of the sintered
cobalt. Typical wear characteristics of the cobalt including a thin detaching surface layer and fine
wear debris were explained by the transformation. A correlationship between the grain size and the
transformation was also explored.
1109
Authors: Wolfgang Tillmann, Miriam Gathen, Christian Kronholz
Abstract: Due to their better mechanical and physical properties diamond tools have largely
replaced cemented carbide tools for machining of mineral materials like concrete and rocks. The
decomposition tendency of diamond has to be taken into consideration during the manufacturing
process as well as during their employment in machining tools. By using water cooling the diamond
decomposition is reduced, but the contamination of occupied buildings by concrete/rock-watermixture
and the need of water supply units on building sites are unfavourable. However, absence of
water cooling lead to an increased tribological and thermal wear of conventional diamond tools.
Due to the heat development the diamonds in direct contact with mineral materials as well as the
diamonds in deeper layers are deteriorated.
The Institute of Materials Engineering pursues a novel thermal protection shield concept, in which
thermal insulating materials such as Al2O3, ZrO2 or glass in diamond impregnated composite
structures act as heat shield, which protects diamonds in deeper layers against high temperature and
graphitisation. Before the effectiveness of this concept could be investigated suitable composites
have to be manufactured. In this paper the powder metallurgical production processes of diamondalumina-
cobalt-composites with varying alumina and cobalt particle sizes, their microstructures and
porosities are described. In comparison to composites with larger alumina particle sizes it could be
observed that the distribution of alumina particles with particle sizes below 70 ,m in the cobalt
matrix is uniform and the porosity of the composite decrease.
1145
Authors: Yan Sheng Yin, Shou Gang Chen, Xue Ting Chang, Alan Kin Tak Lau
Abstract: Al2O3-TiC composites were fabricated by hot-pressing Cobalt coated powders at low
temperature through liquid phase sintering. Cobalt coating layers effectively hinder the growth of
grain and harmful reaction between Al2O3 and TiC particles, which would improve the mechanical
properties of Al2O3-TiC composites. The hardness decreases with the increase of Co content and the
bending strength was enhanced to 782MPa owing to the grain refinement effect of Co phase. The
fracture toughness, about 9.23 MPa.m1/2, was measured for the composites containing 3wt% Co
sintered at 1550°C. SEM images of fracture surface show that the toughening effect should be
attributed to the conversion of fracture mode caused by the Co phase.
913
Authors: Du Shu Huang, Jin Gang Yu, Rui Min Xiao, Zi Jing Li, Li Da Sun
Abstract: The extraction process of cobalt using microemulsion system saponified with P204 as extracting agent was studied. The influence of initial concentration, the concentration of P204, initial pH, the extracting time and the concentration of cobalt and nickel on extraction coefficient were also studied. Experimental results show that the saponification microemulsion system using P204 as extractant is thermodynamic stable, and it can separate cobalt better. The optimum conditions were achieved as: the initial concentration is 0.05 mol/L, pH is 2; The concentration of P204 is 10%, extraction time is 10 min; The volume ratio of organic phase to water phase is 2:1.
407
Abstract: The effect of titanium and aluminum contents, strain, strain rate and tested temperatures on the mechanical properties and microstructural properties will be investigated in this study. These cobalt base super alloys are to be tested using material testing system (MTS) at strain rates of 10-3, 10-2 and 10-1s-1 and at temperatures of 700°C, 500°C and 25°C respectively. It is found that the flow stress increases with increasing strain rate and Ti and Al contents, but decreases with increasing temperature. Furthermore, the strain rate sensitivity increases with increasing strain rate, but decreases with increasing temperature. The microstructural observations confirm that the mechanical response of the cobalt superalloy specimens is directly related to the effects of the titanium and aluminum contents, strain rate and temperature on the evolution of the microstructure. It can be observed that the strengthening effect in cobalt suprealloy is a result primarily of dislocation multiplication. The dislocation density increases with increasing strain rate, but decreases with increasing temperature.
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