Xiaobao Fan

Generation of pulse-modulated induction thermal plasma at atmospheric pressure

The radio frequency induction thermal plasma of sufficiently high electric power for materials processing has been successfully generated with a pulsemodulated operating condition. A solid-state amplifier, which supplies the electric power with a nominal frequency of 1 MHz, was employed for the pulsing plasma generation. The Ar–H2 plasma was generated at a high power level of 17 kW at atmospheric pressure. Typically, the plasma remained stable until the pulse duty factor went down to 30%, when the period of the high power level was 5 ms and the low power level was about 6 kW.

Phase formation in molybdenum disilicide powders during in-flight induction plasma treatment

The in-flight modification of MoSi{sub 2} powders has been carried out by using an Ar{endash}H{sub 2} induction plasma. Reactor pressure, powder feed rate, and plate power level were taken as the experimental parameters to alter the thermal history of the injected powder particles. Metastable hexagonal structure of {beta}{endash}MoSi{sub 2} is the major phase observed in the Ar{endash}H{sub 2} induction of plasma-treated molybdenum disilicide powders, while the stable phase of tetragonal structure of {alpha}{endash}MoSi{sub 2} usually retains no less than 30 wt.{percent}. Depending on the experimental condition and the deviation from stoichiometry in raw materials, low silicides, Mo{sub 5}Si{sub 3} and Mo{sub 3}Si, and free Si were observed. {copyright} {ital 1997 Materials Research Society.}

Calculated C–MoSi2 and B–Mo5Si3 pseudo-binary phase diagrams for the use in advanced materials processing

Abstract The phase diagrams were calculated for the pseudo-binary systems, C–MoSi2, and B–Mo5Si3, in which the composites have significant potentials to function in high-temperature environments. The calculations have been executed with the aid of the software ChemSage and thermodynamic data derived based on the solution database of Scientific Group Thermodata Europe (SGTE). The constructed phase diagrams are discussed with respect to some published experimental information for the fabrication of molybdenum silicide materials and provide a guideline for the experimental work of advanced materials processing, such as second-phase reinforcement for MoSi2 and other low silicides in the Mo–Si system.

Critical free energy for nucleation from the congruent melt of MoSi2

Abstract As a potential candidate for high-temperature applications, the intermetallic compound molybdenum disilicide, MoSi 2 , has been the focus of many recent investigations. Plasma spraying has been demonstrated as a viable technique for producing dense monolithic and composite forms of MoSi 2 . It has been reported that in the plasma spraying process, in which the melting of powders and the rapid solidification are involved, the metastable hexagonal structure of β-MoSi 2 is the major phase observed in the as-sprayed molybdenum disilicide deposit and/or coating. This study examines the formation of β-MoSi 2 in terms of the classical nucleation theory. It is concluded that the critical free energy for nucleation of β-MoSi 2 is lower than that of the stable phase of α-MoSi 2 on solidification and thereby β-MoSi 2 will be the prevailing phase formed by the homogeneous nucleation mechanism from liquid droplets of a certain undercooling, resulting from the rapid solidification rate.

Heat and mass transfer during in-flight nitridation of molybdenum disilicide powder in an induction plasma reactor

Abstract The present study reveals some of the important parameters which control the in-flight nitridation of molybdenum disilicide (MoSi 2 ) powders when carried out in an induction thermal plasma reactor. Initially, gradients of temperature, velocity and concentration were evaluated, using an enthalpy probe system, for the plasma flow without injection of MoSi 2 powders. Radial profiles were then measured at the torch exit to examine the mass and energy transfer mechanisms occurring under different nitridation conditions. These measurements were performed using an induction plasma torch connected to a 50 kW radio-frequency (r.f.) power supply, the torch being attached to a water cooled cylindrical reactor. The process operating conditions studied were plasma plate power, chamber pressure, sheath gas composition, composition and flow rate of quench gas. The effect of last named parameter on the nitridation of the powders was found to be the most important parameter in the nitridation process. The results show that there is an optimum flow rate value for each type of quench gas and the temperature and concentration mapping demonstrates that the combination of high temperatures and high concentrations of N 2 are necessary to reach maximum nitridation levels in MoSi 2 .

Compositional modification of boron carbide induced by induction plasma treatment

Abstract The in-flight compositional modification of boron carbide powder with a chemical composition of B5.25C and an average grain size of 22.3 μm was carried out in an Ar–H2 RF induction plasma. Reactor pressure, powder feed rate and plate power level were taken as the experimental parameters to alter the thermal history of the injected powder particles. The powder was heated to be partially melted and evaporated during the plasma treatment. The carbon content in plasma-treated powders was decreased to 16.2–16.8 wt.% from that of original one, 17.0 wt.%. The formation of free boron, which agreed with the thermodynamic prediction, and the decrease of cell volume of non-stoichiometric boron carbide BxC were shown in plasma-treated powders. The very fine particles coagulated from a vapor phase consisted of carbon and boron carbide.

Mo5Si3-B and MoSi2 deposits fabricated by radio frequency induction plasma spraying

Induction plasma-spray processing was used to produce free-standing parts of Mo5Si3-B composite and MoSi2 materials. The oxidation resistance, up to 1210 °C, of the Mo5Si3-B composite was compared with MoSi2, which is known to be resistant to high-temperature oxidation. The deposits were oxidized isothermally in air at atmospheric pressure. The structural performance of these materials under high-temperature oxidation conditions was found to depend on the boron content in the specimens. In particular, the composite containing 2 wt.% boron exhibited excellent resistance to oxidation, as indicated by the specimen mass change, which was found to be near zero after the 24 h oxidation test.

Parametric Study on Nitridation and Carburization of MoSi2 Powders in an Induction Plasma

An investigation of in-flight nitridation and carburization of MoSi2powders has been carried out using the induction plasma process. Response surface methodology (RSM) with the Box–Behnken experimental design was utilized in this investigation, in order to study the influence of some induction plasma processing parameters on the efficiency of nitridation and carburization reactions performed on inflight treated MoSi2powders. The parameters examined included the level of inductive power, reactor pressure, and the quantity of reaction gases supplied. Under the optimal process conditions achieved during this investigation, about 2.8 wt.% of nitrogen (in the case of nitridation) and about 8.0 wt.% of carbon (in the case of carburization) were able to be incorporated into the MoSi2powder particles.

In-flight nitriding of the MoSi2 powders in an Ar–N2 induction plasma

The in-flight nitridation of MoSi 2 powders has been carried out by using an Ar-N 2 induction plasma, with and without tail gas quenching. The effects of plate power level, reactor pressure and plasma gas composition on the nitridation of MoSi 2 powder particles were investigated. The average nitrogen content measured in the in-flight plasma treated MoSi 2 powders was approximately 2.8 wt.%. The distribution of nitrogen inside the nitrided MoSi 2 powder particles was determined by scanning the cross-section of mounted plasma nitrided MoSi 2 powder particles by means of Auger electron spectroscopy (AES). The nitridation mechanism for the MoSi 2 powders in the induction plasma process is briefly discussed.