Papers by Keyword: MoSi₂

Abstract: In order to improve wear resistance of carbon steel, laser cladding experiments were carried out using a 3kW continuous wave CO2 laser. The diameter of the laser beam was 3-5mm, the scanning velocity was 3-10mm/s, and the laser output power was 1.0-1.3kW. The experimental results showed that MoSi2/SiCP composites coating could be in-situ synthesized from mixture powders of molybdenum, silicon and SiC by laser cladding. A good metallurgical bond between the coating and the substrate could be achieved. The microstructures of the coating were mainly composed of MoSi2, SiC and FeSiMo phases. The average microhardness of the coating was about HV0.21300, about 6.0 times larger than that of steel substrate.

Abstract: Fe-SiAlON-MoSi2 composites were prepared using FeSi75, α-Al2O3, and MoSi2 powders as starting materials at 1400 oC, 1500 oC, 1600 oC and 1700 oC, respectively, for 3h in a flowing nitrogen under the pressure of 0.9MPa. The results showed that the main phases of Fe-SiAlON-MoSi2 composite were β-SiAlON with deferent Z-values, MoSi2, and Fe3Si. Z-value of the obtained β-SiAlON depanded on α-Al2O3 content and the sintering temperature. The morphology of SiAlON phase changed from fibre-like of low Z-value to hexagonal prism-like of high Z-value.

Abstract: MoSi2 was prepared by SHS, and then pressed under 300 MPa at room temperature and sintered at 1600 °C for 1 h in a vacuum furnace. The tribological properties of MoSi2 against Al2O3 in the temperature range from 700°C to 1100 °C were investigated. Microphotographs and phases of the worn surface of MoSi2 were observed by SEM and XRD. Results showed that MoSi2 has well friction and wear properties below 900 °C. When temperature rises from 900 °C to 1000 °C, wear rate of MoSi2 is raised by 20.8% which is attribute to the change of wear mechanism. The main wear mechanisms of MoSi2 are adhesion and oxidation at high temperatures. When over 900 °C, because of ductile - brittle transition characteristic of this material, plastic deformation and fracture are also found on the worn surface of MoSi2. This leads to the high wear rate of MoSi2.

Abstract: Transformation of the microstructure of molybdenum disilicide heating unit in nitrogen atmosphere at 1700°C was investigated by XRD, SEM and EDS. The results showed that MoSi2 was transformed to polyhedral Si3N4 crystals groups distributed across in the matrix with irregular geometry Mo5Si3 and Mo3Si crystals, and the Si3N4 crystal groups were dense areas, while the Mo5Si3 and Mo3Si groups were loose areas after being heated in nitrogen atmosphere at 1700°C for 3h. The thickness of conversion zone of MoSi2 matrix was about 30μm and the loose Mo5Si3 and Mo3Si areas can damage the dense structure of MoSi2 matrix.

Abstract: SiC whisker reinforced (Mo,W)Si2 composite powder has been successfully synthesized by a novel process, named as chemical oven self-propagating high temperature synthesis (COSHS). The mixtures of Si and Ti powders were ignited as chemical oven. XRD result shows that the combustion product is mainly composed of (Mo,W)Si2 solid solution and SiC phases. SEM photo and EDS result show that SiC whisker is formed during this process. The as-prepared SiCW/(Mo,W)Si2 composite powder has been pressureless sintered. The microstructure and mechanical properties of the composite were investigated. Relative densities of the monolithic material and composite are 91.2% and 92.2%, respectively. The composite containing SiC whisker and (Mo,W)Si2 solid solution has higher Vickers hardness than monolithic MoSi2. Especially the room-temperature flexural strength of the composite is higher than that of monolithic MoSi2, from 135.5MPa for MoSi2 to 235.6MPa for composites with 10 vol.% WSi2 and 15 vol.% SiC, increased by 73.9%. The morphology of fractured surface of composite reveals the mechanism to improve flexural strength of MoSi2. The results of this work show that in situ SiCW/(Mo,W)Si2 composite powder prepared by COSHS technique could be successfully sintered via pressureless sintering process and significant improvement of room temperature flexural strength could be achieved. It could be a cost-effective process for industry in future applications.

Abstract: MoSi2 is a potential high temperature structural materials and shows excellent oxidation resistance at high temperatures. To explore its oxidation behaviour and mechanism, MoSi2 single crystals were prepared and investigated in the present work. The experimental results showed that different from its polycrystalline, MoSi2 single crystal showed good oxidation resistance at both 773 and 1473K. Near parabolic law of oxidation kinetics was followed for all the investigated surfaces of MoSi2 single crystal. The close-packed (110) surface of MoSi2 single crystal showed the best oxidation resistance at both temperatures. After exposure at 773K, the molybdenum oxide formed on the (110) surface of MoSi2 single crystal was found to be Mo4O11 instead of MoO3 formed on the other surfaces. Morphology observation showed a columnar growth of Mo4O11 on the (110) surface. No difference was found on the oxide formed for different surfaces of MoSi2 single crystal at 1473K.

Abstract: In this study, the high temperature oxidation behavior of polycrystalline MoSi2 in a low-pressure atmosphere was investigated. Polycrystalline MoSi2 was produced by the spark plasma sintering process. Oxidation tests were carried out at 1500°C at either 10Torr or 760Torr in an Ar-20%O2 atmosphere. For both conditions, the weight change peaked at the initial oxidation stage, and then their weights gradually increased with increasing oxidation time. The sample weight became heavier in the ambient pressure than in the low-pressure, but the evaporation oxidation was not significant in the low-pressure condition. After the low-pressure oxidation tests, the formation of Mo5Si3 in the MoSi2 substrate was identified. The oxidation resistance of MoSi2 at 1500°C is discussed based on the obtained results.

Abstract: MoSi2-SiC precursor powder has been synthesized via the mechanical alloy method with the elements Mo, Si and C powder. The SiC/MoSi2 composites with different SiC volume fraction have been prepared by reactive hot-pressing the precursor powder at 1350 °C. There is a significant increase in the fracture toughness due to addition of SiC reinforcement. However, the intermediary ternary phase exists in this system—namely, Mo5Si3C, Nowotny phase, which has negative effect on the fracture toughness, and can be eliminated through high temperature treatment at 1600 °C for 2 hrs.

Abstract: MoSi2 is one of the few intermetallics to have potential for further systems. However, the use of MoSi2 has been hindered due to the brittle nature of the material at low temperatures, inadequate creep resistance at high temperatures, accelerated (pest) oxidation at temperatures between 450~ 550°C. In this investigation Mo(Al,Si2)/Ti3SiC2 composites has been prepared by reaction hot-pressing from Mo, Si, SiC, Ti, Al powder mixture under different temperatures. XRD results show that the main products are Mo(Si,Al)2 and Ti3SiC2. Part of TiC and SiC also appeared at low treating temperature. With the treating temperature increasing SiC disappeared. No evidence show lattice change of Mo(Si,Al)2. It must be the results of sufficient Al added. The electrical conductivity properties were also investigated. Samples treating under different temperatures showed different changes. Samples under high treating temperature showed a near linear change ranging from 27~800°C and Samples under low treating temperature showed a nonlinear change.

Abstract: The tribochemical effects of MoSi2 powder in the ball milling process have been studied by X-ray diffraction (XRD) and QM-4H milling machine. It has been found that the intensity of diffraction peak of MoSi2 powder is continuously decreased and the width increased with the increase of milling time. Specifically, the crystallite size decreased largely in the early stage of milling. Contrary to the above, the microstrain and the effective temperature factor increased considerably. After ninety hours of milling, the crystallite size had little changes. The relations among the tribochemical effect factors are as follows: the microstrain and the effective temperature factor increase with the decrease of the crystallite size. The microstrain increases with the increase of the effective temperature factor.