Papers by Keyword: MoSi₂

Abstract: Molybdenum di silicide (MoSi2) has lower mismatch of coefficient of thermal expansion (CTE) with Hastelloy X as compare to silicon carbide. So, MoSi₂ coatings can be a potential candidate to protect Hastelloy X against high temperature sulfuric acid. Herein, MoSi₂ thin films were deposited via electron beam physical vapor deposition (EBPVD) method. Corrosion behavior of MoSi₂ coated HX and bare HX was analyzed. Corrosion tests of bare Hastelloy X and coated samples was performed at different temperatures (60°C and 120°C) in 98% sulfuric acid using the weight-loss method. The morphology of pre and post corrosion test was also analyzed by using optical microscopy. Bare Hastelloy X showed a slight weight gain at 60°C and a weight loss at 120°C. The MoSi₂ coated Hastelloy X showed slight weight gain at 60°C and a weight loss at 120 °C. The weight gain is the result of oxides formation whereas, weight loss may be attributed to corrosion due to H+ ions. These findings were also accompanied by optical microscopy as corroded and very rough patches were evident for bare HX, whereas, for MoSi₂ coated samples, the coating was still intact.

Abstract: Glow plasma deposition (GPD) and halide activate pack cementation (HAPC) methods were combined to prepare B-modified MoSi₂ coatings on C103 alloy. The pure Mo layer was firstly deposited on the substrate through GPD process, and then Si and B were co-deposited into the Mo layer through HAPC process to form the B-modified MoSi₂ coating. Experimental results revealed that the Mo layer exhibited metallurgical bonding with the substrate. Two typical coatings were produced through the above combined method. One consisted of the MoSi₂ outer layer, the MoSi₂+MoB two-phase middle layer and the Mo₅Si₃ inner layer, the other consisted of an outer deposits layer composed of MoSi₂ matrix and MoB dispersions, and an inner diffusion layer composed of NbSi₂, NbB₂ and Nb₅Si₃ phases. The latter coating was oxidized at 1250 °C, and it has been found that the protective scale can form on the coating surface, which indicates the good oxidation resistance of such B-modified MoSi₂ coating prepared through the combined method.

Abstract: 800x600 Intermetallics are compounds of two metals or of metal(s) and semimetal(s). Their structures are usually different from those of the constituents. Some intermetallics are interesting functional materials, others have attracted attention as high-temperature structural materials. We remind the reader of some fundamentals of solid-state diffusion and to the major techniques for tracer diffusion measurements, interdiffusion studies and the growth kinetics of layers in solid diffusion couples. Starting from self-diffusion, which is the most basic diffusion phenomenon in any solid, the paper covers the main features of diffusion in binary intermetallics from the systems Cu-Zn, Ni-Al, Fe-Al, Mg-Al, Ni-Ge, Ni-Ga, Fe-Si, Ti-Al, Ni-Mn, Mo-Si, Co-Nb and Ni-Nb.. We illustrate the influence of phase transitions on diffusion and point out some common features of diffusion in intermetallics. We discuss in detail diffusion in silicides of iron, molybdenum and of silicides of refractory metals. We also consider aluminides of iron, nickel, and titanium and in the aluminium-magnesium system. We consider diffusion in intermetallics of the cobalt-niobium and nickel-niobium system and in in the Nb-Sn and V-Ga systems. We finish with some remarks about grain boundary diffusion in intermetallics. Normal 0 21 false false false UK X-NONE X-NONE MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Calibri","sans-serif";}

Abstract: Non-planar dislocation dissociations and cores play a fundamental role in mechanical properties of many materials. In this contribution, we concentrate on the complex structures arising from a large number of possible metastable stacking faults in disilicides with the C11_b structure, specifically MoSi₂. First, gamma-surfaces for the planes with high atomic density in this intermetallic are presented. Employing these results, possible configurations of dislocation dissociations are discussed and related to the deformation behavior.

Abstract: In the present paper the influence of the addition of MoSi₂ particles on the microstructure and pitting corrosion behaviour of laser cladding Co based alloy coatings deposited on 304 stainless steel substrate has been reported. The coating microstructure was investigated by SEM, OM, XRD and EPMA etc.. And the pitting corrosion resistance of coating was evaluated in the 3.5% NaCl solution. The results showed that: (1) The microstructure is fined by increasing MoSi₂ percentage. And the coating microstructure evolved from dendrites and interdendritic eutectics to various faceted dendrites with the bamboo leaf, flower, or butterfly morphology, when the MoSi₂ content is from 0~20% to 30~40%; (2) the (E_pit-E_prot) of Co based alloy/MoSi₂ composite coating was lower than that of Co based alloy, and which presented higher self-repairing capability. The pitting potential E_pit of Co +(0~20wt.%) MoSi₂ cladding coatings is higher than that of stainless steel, the pitting corrosion resistance is enhanced; When more MoSi₂ (30wt.%) was added, the pitting corrosion resistance decreases due to microstructure inhomogeneity and exiting of inclusion.

Abstract: The single and synergism effect of SiC and ZrO₂ nanoparticles on low temperature oxidation resistance of MoSi₂ were investigated by thermo-gravimetric analysis (TGA), phase and micrograph analysis of surface film. The results show that at the oxidation temperature of 600°C, the addition of 10%ZrO₂ results in obvious oxidation mass gain of MoSi_2, and discontinuous protective film forming on the surfaces, which makes “pesting” phenomenon still occur. The addition of 10%SiC accelerates the oxidation of MoSi₂, but compact SiO₂ protective film formed on the surface after some time of oxidation, avoiding the occurrence of large scope of “pesting” phenomenon. The synergism of 10%ZrO₂ and 10%SiC promotes the formation of compact, even silicate glass film on the surface of MoSi₂, as a result, significantly improves the low temperature oxidation resistance of MoSi₂.

Abstract: An approach for fabricating functionally graded specimens of Co-based alloy and molybdenum disilicide (MoSi₂) compositions via Selective Laser Cladding (SLD) is presented. The focus aimed at using the functionally graded material (FGM) concept to gradually grade powdered compositions of MoSi₂ within a base material of Co-based alloy. A high power CO₂ laser was used to process the material compositions to a high density with gradual but discrete changes between layered compositions. The graded specimens initially consisted of 100% Co-based alloy with subsequent layers containing increased volume compositions of MoSi₂ (0-80wt.%). Specimens were examined for porosity, phase composition and microstructure. It was found that crack sensitivity of layer can be reduced by adding Co-based alloy into the pure MoSi₂ powder.

Abstract: In this work, the influences of the particle size and morphology of raw materials on the formation of MoSi2 by self-propagating high-temperature synthesis (SHS) were investigated. A series of Si powders with different particle sizes and Mo powders with different morphologies were obtained by grinding for 1, 5 and 10 hours, respectively. X-ray diffraction pattern characterization (XRD) and scanning electron microscopy (SEM) were used to characterize the samples. It was found that, the phase compositions and morphologies of the combustion products depended on the particle size and morphology of the raw materials. The particle sizes of Si powders decreased with increasing the grinding time, and a secondary phase of Mo5Si3 was detected in the obtained MoSi2 powders when the smallest particle size of Si powders was about 1μm. While, the particle sizes of Mo powders increased with increasing the grinding times, and the obtained MoSi2 showed massive flaky structures, which were similar to the morphologies of Mo particles.

Abstract: On the principle of alloying modification of the same crystal structure compound, MoSi₂-AlCr₂ quasibinary system alloys were prepared by mechanical activation and hot pressed sintering.The phase composition and crystal structure evolution were investigated by X-ray diffractometer(XRD).The results showed that the main phase composition of alloys were MoSi₂ and Mo(Si,Al)₂,the crystal structure was gradually transformed from tetragonal C11_b MoSi₂ to hexagonal C40 Mo(Si,Al)₂. (Mo,Cr)(Si,Al)₂ phase has not found in alloys.

Abstract: Wear behaviours of La₂O₃-MoSi₂ composite against SiC under different loads at 1000°C and 0.126m/s sliding speed in air were investigated by using an XP-5 type high temperature friction and wear tester. The worn surfaces and phase of the sample were observed by scanning election microscopy (SEM) and X-ray diffraction (XRD), respectively. Results showed that friction coefficient and wear rate of La₂O₃-MoSi₂ composite and SiC decreased with the increase of loads. The wear mechanisms of La₂O₃-MoSi₂ composite are oxidation, adhesion abrasion and fatigue pitting.