MoSi_2 oxidation resistance coatings for Mo_5Si_3/MoSi_2 composites

In order to improve the oxidation resistance properties of 30 at.% Mo5Si3/MoSi2 composite at high temperature in air, a molybdenum disilicide coating was prepared on its surface by a molten salt technology. XRD and SEM analysis showed that only tetragonal MoSi2 phase existed in the coating after being siliconized for 5 h at 900℃. The oxidation film formed on the uncoated sample was not dense, so that oxygen diffused easily through it. The volatilization of MoO3 resulted in the oxidation film separating from the substrate. The MoSi2 coating was proved to be an effective method to prevent 30 at.% Mo5Si3/MoSi2 composites from being oxidized at 1200℃. A dense glassy SiO2 film was formed on the MoSi2 coating surface, which acted as a barrier layer for the diffusion of oxygen atoms to the substrate. The 30at.% MosSi3/MoSi2 composites with a MoSi2 coating showed much better oxidation resistance at high temperature.

Low-temperature oxidation behavior of MoSi_2 powders

The oxidation behavior of molybdenum disilicide （MoSi2） powders at 400, 500, and 600℃ for 12 h in air were investigated by using X-ray diffraction （XRD） and transmission electron microscopic （TEM） techniques. Significant changes were observed in volume, mass, and color. Especially at 500℃, the volume expansion was found to be as high as 7-8 times, the color changed from black to yellow-white, and the mass gain was about 169.34% after 8 h, with SiO2 and MoO3 as main reaction products. The gains in volume and mass were less at 400 and 600℃ compared with those at 500℃, probably due to the less reaction rate at 400℃ and the formation of silica glass scale at 600℃, which would protect the matrix and restrain the diffusion of oxygen and molybdenum. Thus, the accelerated oxidation behavior of MoSi2 powder appeared at 500℃ and the volume expansion was the sign of accelerated oxidation.

Rapid synthesis of MoSi_2-Si_3N_4 nanocomposite via reaction milling of Si and Mo powder mixture

The nanocomposite of MoSi2-SiaN4 （molybdenum disilicide-silicon nitride） was synthesized by reaction milling of the Mo and Si powder mixture. Changing the processing parameters led to the formation of different products such as a- and B-MoSi2, SiaN4, Mo2N, and M05Si3 at various milling times. A thermodynamic appraisal showed that the milling of Moa2Siss powder mixture was associated with highly exothermic mechanically induced self-sustaining reaction （MSR） between Mo and Si. The MSR took place around 5 h of milling led to the formation of a-MoSi2 and the reaction between Si and N2 to produce Si3N4 under a nitrogen pressure of 1 MPa. By increasing the nitrogen pressure to 5 MPa, more heat is released, resulting in the dissociation of Si3N4 and the transformation of a-MoSi2 to β-MoSi2. Heat treatment was also performed on the milled samples and led to the formation of Mo2N and the transformation of a-MoSi2 to β-MoSi2 at the milling times of 10 and 40 h, respectively.

Orthogonal experiment investigation on wear resistance of MoSi2

The influence of lubricant conditions, wear loads, and counterparts on thefriction and wear characteristics of an MoSi_2 material was investigated by means of orthogonalanalysis. Orthogonal experiment results show that the order of influencing degree of the factors islubricant condition > load > counterpart. MoSi_2 has excellent friction-wear comprehensiveproperties against higher-hardness counterparts on the higher load and 20 oil lubricant conditions.

Cyclic Oxidation Behaviors of MoSi_2 with Different Relative Density

The influence of different relative density on the cyclic oxidation behaviors of MoSi2 at 1 273 K were studied. ＂Pesting＂ was not found in all MoSi2 materials after being oxidized for 480 h. All samples exhibited continuous mass gain during the oxidation process. The mass gains of MoSi2 with the lowest relative density （78.6%） and the highest relative density （94.8%） are increased by 8.15 mg·cm^-2 and 3.48 mg·cm^-2, respectively. The surface of the material with lower relative density formed a loose, porous and discontinuous oxidation scale, which accelerated oxygen diffusion and aggravated the oxidation process. However, a dense scale in the material with higher relative density is formed, which acts a diffusion barrier to the oxygen atoms penetrating into the matrix. The high temperature oxidation resistance of MoSi2 can be improved by increasing its relative density.

Dry friction and wear characteristics of MoSi_2 against alloy steels

The dry friction and wear characteristics of three kinds of friction couplesunder different loads, MoSi_2/45 tempered steel, MoSi_2/45 quenched steel, and MoSi_2/CrWMn steel,were investigated by using a friction and wear tester. SEM and X-ray diffraction were employed toanalyze the microphotograph of the worn surface and the phase of worn pieces in order to reveal thewear mechanisms of MoSi_2 material. The results show that MoSi_2/CrWMn steel friction pair has gooddry friction and wear properties under the load of 80 N, where the friction coefficient is 0.255 andthe wear rate of MoSi_2 is only 14.72 mg centre dot km^(-1). But under the load of 150 N, it isMoSi_2/45 tempered steel friction pair that has good tribological properties, where the frictioncoefficient is 0.278 and the wear rate of MoSi_2 is only 10.6 mg centre dot km^(-1). The main wearmechanism of MoSi_2 under low loads is brittle fracture. With the increase of load, the main wearmechanism of MoSi_2 against 45 quenched steel or CrWMn steel is adhesive wear. However, the wearmechanism of MoSi_2 against 45 tempered steel is changed from oxidation-fatigue wear to adhesivewear.

Dry friction and wear properties of intermetallics MoSi_2

The dry friction and wear properties of intermetallics MoSi 2 against 45 steel under different loads were investigated with M 2 type friction and wear tester. Scanning electric microscope (SEM) equipment with microprobe was employed to analyze the morphology of the friction surface. Results show that the dry friction and wear properties are deeply affected by load. The wear rate of MoSi 2 at the load of 80?N is the maximum which is 36.1?μg/m. On the condition of the load of 150?N, MoSi 2 material has the better friction and wear properties: friction coefficient is 0.28 and wear rate is 10.6?μg/m. With the load increasing, the main friction mechanisms change from microslip and plastic deformation to adhesive effect, and the main wear mechanisms change from plough groove wear and oxidation fatigue wear to adhesive wear.

Progress in Recrystallized SiC and Its Composites

Recrystallized silicon carbide（ RSi C）,a high purity Si C material sintered by the process of evaporation-condensation without any additives,is one of the most important structural materials in the fields of high temperatures. However,its low density and porous structure caused by the sintering mechanism in the absence of shrinkage,restrict its wide applications in engineering.This paper reviews the research progress and related technologies on the preparation of high-density RSi C and its composites. RSi C with relative high density up to 2. 75g·cm- 3can be obtained by a combination of pretreatment to Si C raw materials such as reshaping,modification and particle size distribution,and appropriate forming method. Post treatments such as cyclic pyrolysis and impregnation- recrystallization,and slurry impregnation- recrystallization are needed for the further density increase of RSi C（ 2. 99 g·cm- 3）. In addition,high performance RSi C- Mo Si2 and RSi C- Al composites obtained by melt infiltration are also reviewed.