A comparative study of MoSi2 coatings manufactured by atmospheric and vacuum plasma spray processes

In this work, MoSi<;sub>2 coatings were manufactured by atmospheric plasma spraying (APS) and vacuum plasma spraying (VPS) technologies, respectively. Phase composition and microstructure of the coatings were characterized by X-ray diffraction and scanning electron microscopy. Microhardness, void and oxygen content of the coatings were also determined. Oxidation behavior of the coatings at high temperature was examined. The results showed that the surface of VPS-MoSi<;sub>2 coating was dense and homogeneous. However, there were many microcracks formed on the surface of APS-MoSi<;sub>2 coating. The VPS-MoSi<;sub>2 coating also had lower void and oxygen contents, higher Vickers hardness compared with those of APS-MoSi<;sub>2 coating. Besides, oxidation resistance of the VPS-MoSi<;sub>2 coating was better than that of APS-MoSi<;sub>2 at 1500 °C.     

Mechanical and electrical properties of MoSi2–RSiC composites via a combination of phenolic resin infiltration–pyrolysis and MoSi2–Si–Ti alloy-activated melting infiltration composite processes

A three-dimensional interpenetrated network structure composite was designed and prepared via a combination of phenolic resin infiltration-pyrolysis and MoSi<;sub>2–Si–Ti alloy-activated melting infiltration processes to effectively merge the desirable properties of MoSi<;sub>2 and RSiC. Influence of infiltration temperature on the microstructure, mechanical and electrical properties of the composites was examined. Almost dense MoSi<;sub>2–RSiC composites with the designed structure were obtained at 1900°C. Formation of the gradient interface modified the interface combination and enhanced the mechanical and electrical properties of the composite. Flexural strength of the composites reached approximately 114.262?MPa (room temperature) and 128.392?MPa (1400°C), respectively, indicating corresponding increases of 37.08 and 35.69% compared with the RSiC matrix. Volume resistivity decreased to 57.63?mΩ?cm, nearly five orders of magnitude lower than that of RSiC. Influence of the interpenetrated network structure and interface combination on the electrical conductivity behaviour of the composites was discussed via a modified mixture rule.     

Joint between MoSi2 and 316L stainless steel surfaces and its interfacial mechanical properties

In this study, we proposed a new method using the spark plasma sintering technique to join ceramics to alloys. MoSi<;sub>2 and 316L stainless steel were chosen as sample materials and can be welded well with graded interlayers. We found that dense uniformed joints were achieved because of the comparable coefficient of thermal expansion of the interlayers. Furthermore, such a compatibility between the graded interlayers prevented MoSi<;sub>2 with low toughness from the occurrence of microcracks resulted from the residual stresses formed during cooling of the joint.     

Synthesis of MoSi2/WSi2 nanocrystalline powder by mechanical-assistant combustion synthesis method

MoSi<;sub>2/WSi₂ nanocrystalline powder has been successfully synthesized by the mechanical-assistant combustion synthesis method. This method includes a ball-milling process followed by combustion synthesis. The composition and microstructure of the as-milled powder mixture were detected by X-ray diffraction and scanning electron microscopy analyses. Their results show that the Mo(W) solid solution and Si nanocrystals could be obtained during the ball-milling process. Compared with normal powder mixture (Mo + Si + W), it could be easily ignited and high maximum combustion temperature was achieved. It was also confirmed that MoSi<;sub>2/WSi₂ solid solution powder with nanometric structure could be prepared through combustion synthesis method from the mechanical activated powder mixture.     

等离子喷涂及真空热处理过程中MoSi2涂层的相演变

分别采用MoSi2Mo-66.7%Si和Mo-70%Si(摩尔分数)粉末为原料,在45钢上大气等离子喷涂MoSi2涂层,对喷涂前后相以及真空热处理对涂层相和组织的影响进行了研究.结果表明,以MoSi2为原料喷涂后,涂层主要是由MoSi2和Mo5Si3相组成,MoSi2由亚稳定的MoSi2(h)六方结构和稳定的MoSi2(t)四方晶体结构组成.而Mo-66.7%Si和Mo-70%Si两种粉末喷涂后,涂层均由Mo,Si单质粉组成,喷涂过程没有形成硅化物.Mo-66.7%Si涂层在1 050,1 200℃热处理1 h后,涂层中主相是Mo5Si3,次相为MoSi2.Mo-70%Si涂层在800,1 000 ℃热处理30 min后,生成了少量的MoSi2,Mo5Si3和Mo3Si.在1 050 ℃热处理时,随着时间的延长,MoSi2相含量相对增多,当热处理时间为30,45 min时,由于Mo,Si和Fe之间的反应扩散,还生成了FeMoSi和Fe3Si相.     

低压沉积温度对MoSi2涂层微观结构与性能影响

以SiCl₄和H₂为原料,采用低压化学气相沉积（LPCVD）渗硅法在Mo基体表面原位反应制备了MoSi<;SUB>2涂层,研究了沉积温度对MoSi<;SUB>2涂层微观形貌、物相组成、沉积速率、涂层的硬度、涂层与基体结合强度的影响. 研究结果表明：在1100~1200℃下制备的涂层结构致密,由单一MoSi<;SUB>2组成,沉积速率、涂层的硬度以及与基体的结合强度均表现为増加的趋势;当沉积温度高于1200℃,涂层出现开裂现象,由游离Si和MoSi<;SUB>2两相组成,涂层沉积速率、硬度和结合强度均出现下降的趋势. 1100℃以下沉积的主要控制步骤为Si与Mo反应,而1100℃以上Si在涂层中的扩散对沉积过程起控制作用.     

The role of microstructure on pesting during oxidation of MoSi2 and Mo(Si,Al)2 at 773 K

The pesting behavior of MoSi<;sub>2 and Mo(Si,Al)₂ has been examined in air at 773 K to clarify the origin and mechanism of pesting phenomena and the effect of aluminum on pesting phenomena. The initial cracks play a much more important role than the grain boundaries and the initial oxide layer in pesting. Mo and Si oxidize to amorphous Mo-Si-O simultaneously with about a 200% volume expansion. Therefore, large stress appears at the cracktips and induce many new cracks. MoO₃ vaporizes from the Mo-Si-O layer on the external surface and crack surfaces causing the oxides in the initial cracks to become porous. Oxygen has a short-circuit path to enter the sample in the cracks. Therefore, the partial pressure of oxygen is sufficiently high to allow oxidation of Mo in the materials. The platelet-like MoO₃ grows on the external surface and also in the cracks. Finally, the sample distintegrates into powder. Pesting of Mo(Si,Al)₂ occurs in the same way, however, its rate is much lower than that of MoSi<;sub>2. The role of Al is to decrease the initial crack density of the samples from the melt. Other effects of Al might be to decrease the oxygen flux toward the oxide-intermetallic interface and to increase the plasticity of the amorphous oxide being formed in the cracks.     

Synthesis of dense MoSi2 by high-frequency induction heated combustion and its mechanical properties

Dense MoSi<;sub>2 compound was synthesized with the high-frequency induction heated combustion synthesis method in one step from elemental powders of Mo and Si within 2 min. Simultaneous combustion synthesis and densification were carried out under the combined effects of induced current and mechanical pressure. A highly dense MoSi<;sub>2 with a relative density of up to 98% was produced with simultaneous application of 60 MPa pressure and induced current. The percentages of the total shrinkage occurring before and during the synthesis reaction were 16% and 53%, respectively. The average grain size was about 15 μm and a slight amount of Mo₅Si₃ was observed at the boundaries of the MoSi<;sub>2 grains. The fracture toughness and hardness values obtained were 3.5 MPa·m^1/2 and 1050 kg/mm², respectively. These values were similar to those of commercial ones.     

Main recent contributions to SHS from France

Both in situ TRXRD and IR thermography, on the one hand, and different levels of modeling, on the other, have generated a strong progress in the knowledge and control of numerous SHS reactions. The SHS of simple binary materials, such as intermetallics (FeAl, MoSi<;sub>2, NbAl₃, etc.), oxides (e.g. ZrO₂), carbides (e.g. SiC) or nitrides, more complex materials, such as mullite, SiAlONs, MAX phases, composites (SiO₂-Al₂O₃, NiAl-ZrO₂), powders in their more complicated states, such as well controlled microstructures, fully densified intermetallics, smart composites, and hard coatings carried out by GFA researchers greatly contributed to the worldwide competition to harness the potential of SHS. On the occasion of the 40th anniversary of SHS, this paper is giving an overview of the main results obtained by GFA over the last ten years. Dominique Vrel (LIMHP UPR 1311 CNRS-Paris XIII, Villetaneuse)     

Synthesis of MoSi2-TiC nanocomposite powder via mechanical alloying and subsequent annealing

MoSi<;sub>2-30 wt.% TiC nanocomposite powder was successfully synthesized by ball milling and following heat treatment. Effect of milling time and annealing temperature were investigated. The products synthesis and reactions progress were characterized by XRD. Morphology and microstructure of milled powders were monitored by SEM and TEM, respectively. Results showed that the synthesis of this composite begins after 10 h of milling and progresses gradually up to 30 h of milling. MoSi<;sub>2-TiC composite was completely synthesized after annealing of 30 h milled powder at 900 °C. On the basis of Reitveld refinement method, the mean grain size and microstrain of 13.2 nm and 0.44% were obtained, respectively for 30 h milled powder that is in consistent with TEM image. In the spite of grain growth and strain release, this nanocomposite powder maintained its nanostructure after annealing.