Oxidation Resistance Of MoSi2 Research Articles

Structure, Mechanical Properties, and Oxidation Resistance of MoSi2, MoSiB, and MoSiB/SiBC Coatings

Single-layer MoSi2, MoSiB, and multilayer MoSiB/SiBC coatings are fabricated by magnetron sputtering. Coating structures are investigated using X-ray diffraction, a scanning electron microscopy, and glow-discharge optical emission spectroscopy. Mechanical properties of coatings are determined by nanoindentation. The thermal stability of coatings is studied in a temperature range of 600–1200°C and oxidation resistance is studied upon heating to 1500°C. It is established that single-layer MoSiB coatings possess a hardness of 27 GPa, elasticity modulus of 390 GPa, and elastic recovery of 48%. They can also resist oxidation up to 1500°C inclusively, which is caused by the formation of the SiO2-based protective film on their surface. The MoSi2 coatings can have hardness comparable to the hardness of MoSiB coatings, but they are somewhat worse than them in regards to oxidation resistance. Multilayer MoSiB/SiBC coatings have hardness 23–27 GPa and oxidation resistance restricted by 1500°C, but they herewith have higher elastoplastic properties when compared with MoSiB.

Structure, mechanical properties and oxidation resistance of MoSi2, MoSiB and MoSiB/SiBC coatings

Magnetron sputtering was used to obtain single-layer MoSi2, MoSiB and multilayer MoSiB/SiBC coatings. Coating structures were studied using X-ray phase analysis, scanning electron microscopy and glow discharge optical emission spectroscopy. Mechanical properties of the coatings were determined by nanoindentation. The coatings were tested for oxidation resistance and thermal stability at temperatures between 600 °С and 1200 °С. It was found that single-layer MoSiB coatings have a hardness of 27 GPa, elasticity modulus of 390 GPa and elastic recovery of 48 % and exhibit short-term oxidation resistance up to 1500 °С inclusive due to a SiO2-based protective film formed on their surfaces. MoSi2coatings have hardness comparable to that of MoSiB but slightly lower oxidation resistance. Multilayer MoSiB/SiBC coatings feature 23–27 GPa hardness and oxidation resistance limited to 1500 °С, but at the same time they have higher elastoplastic properties as compared to MoSiB.

High temperature oxidation behavior of ZrB2-SiC added MoSi2 ceramics

In this paper, MoSi2, MoSi2-20 vol% (ZrB2-20 vol% SiC) and MoSi2-40 vol% (ZrB2-20 vol% SiC) ceramics were prepared using pressureless sintering. The oxidation behaviors of these MoSi2-(ZrB2-SiC) ceramics were investigated at 1600 °C for different soaking time of 60, 180 and 300 min, respectively. The oxidation behaviors of the MoSi2-(ZrB2-SiC) ceramics were studied through weight change test, oxide layer thickness measurement, and microstructure analysis. Further investigation of the oxidation behaviors of the MoSi2-(ZrB2-SiC) ceramics was conducted at a higher temperature of 1800 °C for 10 min. The microstructure evolution of the ceramics was also analyzed. It was finally found that the oxidation resistance of MoSi2 was improved by adding ZrB2-SiC additives, and the MoSi2-20 vol% (ZrB2-20 vol% SiC) ceramic exhibited the optimal oxidation resistance behavior at elevated temperatures. From this study, it is believe that it can give some fundamental understanding and promote the engineering application of MoSi2-based ceramics at high temperatures.

Improvement of MoSi2 oxidation resistance via boron addition: Fabrication of MoB/MoSi2 composite by mechanical alloying and subsequent reactive sintering

The purpose of this study is to determine the influence of boron addition on the oxidation resistance of MoSi2. Powder mixtures of Mo–Si with stoichiometric ratio of 1:2 and 30Mo–47Si–23B (at.%) were used, respectively to produce MoSi2 and MoB/MoSi2. First powder mixtures were separately milled for 20h in an attritor ball mill. After degassing of as-mechanically alloyed powders at 450°C, the powders were pressed into cylindrical samples under a pressure of 600MPa. The final diameter of samples were 15mm. Compacted samples were sintered in tube resistance furnace using Ar controlled atmosphere. 30Mo–47Si–23B alloy with a MoB/MoSi2 two-phase structure and monolithic MoSi2 were obtained by sintering at 1300°C for 3h and 1100°C for 7h, respectively. The sintered alloys were subjected to high temperature continuous oxidation test at 1000°C. Microstructure of sintered alloys and as-mechanically alloyed powders, phase composition of sintered and oxidized samples were evaluated by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The results showed that in 30Mo–47Si–23B alloy no pest disintegration is observed. The presence of boron acted as an effective barrier against oxygen diffusion during oxidation test of MoB/MoSi2 composite. Excellent oxidation resistance of MoB/MoSi2 composite was achieved through homogeneously distributed and fine grained microstructure and also very high relative density.

Effects of Nb-Alloying on High-Temperature Oxidation of MoSi2

The e ects of alloying with 0.5, 1.0, and 2.0 at.% Nb on oxidation resistance of MoSi2 are investigated at temperature range of 1400 ◦C 1700 ◦C. Rapid formation of a stable protective layer of silica resulted in a parabolic oxidation rate. The oxide layer thickness and the sample weight increased with increasing oxidation time. Impurities accelerated oxidation rate of MoSi2 only slightly, however, did not a ect the rate controlling mechanism for oxidation. There was no correlation between oxidation rate and the amount of impurity. The values of activation energies for oxidation of pure MoSi2 and Nb-alloyed samples were similar to activation energy for di usion of O2 through silica. Di usion of O2 through the oxide scale remained the rate controlling mechanism with or without Nb impurity.

Oxidation resistance of Mo(Si1−xAlx)2 nanocrystalline films and characterization of their oxide scales by electrochemical impedance spectroscopy

To explore the influence of Al alloying on the oxidation resistance of MoSi2, four Mo(Si1−xAlx)2 nanocrystalline films were fabricated on Ti–6A1–4V substrates and their cyclic oxidation behavior was characterized.

Mechanically Activated Combustion Synthesis of Molybdenum Silicides and Borosilicides

ABSTRACTMolybdenum silicides and borosilicides are promising structural materials for advanced power plants. A major challenge, however, is to simultaneously achieve high oxidation resistance and acceptable mechanical properties at high temperatures. For example, molybdenum disilicide (MoSi2) has excellent oxidation resistance and poor mechanical properties, while Mo-rich silicides such as Mo5Si3 (called T1) have much better mechanical properties but poor oxidation resistance. One approach is based on the fabrication of MoSi2−T1 composites that combine high oxidation resistance of MoSi2 and good mechanical properties of T1. Another approach involves the addition of boron to Mo-rich silicides for improving their oxidation resistance through the formation of a borosilicate surface layer. In particular, Mo5SiB2 (called T2) phase and alloys based on this phase are promising materials.In the present paper, MoSi2−T1 composites and materials based on T2 phase are obtained by mechanically activated self-propagating high-temperature synthesis (MASHS). To obtain denser products, the so-called SHS compaction (quasi-isostatic pressing of hot combustion products) has been employed. Thermal analysis has shown that SHS compaction significantly improves the oxidation resistance. Self-sustained combustion of Mo/Si/B mixtures for the formation of T2 phase becomes possible if the composition is designed for adding a more exothermic reaction of MoB formation. These mixtures exhibit spin combustion. Oxidation resistance of the obtained multi-phase Mo−Si−B materials is independent on the concentration of Mo phase in the products. The “chemical oven” technique has been used to obtain a single Mo5SiB2 phase and an alloy consisting of α-Mo, Mo5SiB2, and Mo3Si phases.

Effect of SiC and ZrO<sub>2</sub> on Low Temperature Oxidation Resistance of MoSi<sub>2 </sub>Nanocomposites

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 MoSi2, 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.

Molybdenum-based oxidation-resistant MoSi2–Al2O3 and WSi2–Al2O3 coatings

Various studies and comparative tests are conducted on oxidation-resistant unmodified and modified MoSi2–Al2O3 and WSi2–Al2O3 silicide coatings in air at 1800 and 1900°C. Metallographic, x-ray diffraction, electron microprobe, and chemical analyses show that the introduction of alumina into molybdenum and tungsten disilicides improves their hardness and produces fine-grained silicides with high corrosion resistance. The composition with fine alumina particles (O < 0.1 μm) with the total alumina concentration from 3 to 15 wt.% in the coating is most favorable for improving the oxidation resistance of disilicides. It is shown that the high oxidation resistance of MoSi2–Al2O3 and WSi2–Al2O3 coatings is due to the presence of alumina in disilicides, which contributes to the formation of an external high-temperature oxide layer. This layer consists of α-Al2O3 and a mullite sublayer in which Al2O3 crystals are introduced into SiO2.

Phase constitutions of Mo [sbnd]Si [sbnd]N anti-oxidation multi-layer coatings on C [sbnd]C composites by fused slurry

To improve the oxidation resistance of Mo Si fused slurry coating fabricated in vacuum, Mo Si N multi-layer coatings were synthesized on C/C composites in nitrogen atmosphere by fused slurry using same Mo and Si element powders. The phase compositions and microstructures were characterized by X-ray diffractometry (XRD), optical microscopy (OM), scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS). The results indicate that the Mo Si N coating contains SiC inner layer and MoSi 2/Si main layer, which was similar with Mo Si coating. Additionally, a thin outer layer with nano-filiform morphology has been found on the coating surface, which consists of SiC, Si 3N 4, AlN, Al 2O 3 and sialon phase. Oxidation experiments show that the Mo Si N multi-layer coating exhibits excellent oxidation resistance at 1400 °C and anti-oxidizing potential ability at 1450 °C.

High Temperature Oxidation Behavior of MoSi&lt;sub&gt;2&lt;/sub&gt; under Low Pressure Atmosphere

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.

Effect of microstructure on oxidation resistance of MoSi 2 fabricated by spark plasma sintering

In order to clarify the effect of microstructure on oxidation resistance of MoSi 2, structure and oxidation resistance of sintered MoSi 2 were studied. Sintered MoSi 2 was fabricated by a spark plasma sintering method from the two kinds of starting material, MoSi 2 powder and mixed powders of elemental Mo and Si. Dense MoSi 2 containing few SiO 2 inclusions was fabricated by applying in situ synthesis using mixed powders of Mo and Si. Oxidation behavior in the accelerated oxidation temperature region of MoSi 2 showed that accelerated oxidation was remarkably suppressed.

Synthesis and oxidation resistance of MoSi2-SiC composites

Synthesis of fully dense MoSi 2 -SiC composites without SiO 2 -inclusions was performed, and oxidation behavior of the composites was studied. For the fabrication of MoSi 2 and MoSi 2 -SiC composites, the mixed powders consisting of elemental Mo, Si and C were sintered at 1673 or 1873 K with a spark plasma sintering method. Namely, in-situ synthesis of MoSi 2 and SiC and sintering of the compounds were simultaneously performed. As the result, fully dense MoSi 2 and MoSi 2 composites which little contain SiO 2 inclusions were fabricated. Oxidation tests of the sintered bodies were carried out in air at 773 K in the accelerated oxidation region and 1773 K in the passive oxidation region. In the composites, occurrence of simultaneous oxidation of Mo and Si in the accelerated oxidation region was remarkably suppressed. In oxidation at 1773 K, the composites show outstanding oxidation resistance by a formation of a protective SiO 2 layer.

Ternary MoSi2 Compounds for High Temperature Structural Applications

AbstractThe microstructure and phase composition of MoSi2 modified with Al, B, Ge, Hf, Nb, and Re have been investigated. B and Hf substitutions for Si and Mo, respectively, exhibited very low solubilities in MoSi2. Al and Nb substitutions for Si and Mo, respectively, changed the crystal structure from tetragonal C11b to hexagonal C40. Phase boundaries and solubility limits were determined for Al and Nb substitutions. Ge and Re substitutions for Si and Mo, respectively, exhibited complete solubility and maintained the tetragonal Cllb crystal structure. The mechanical properties evaluation as determined by four-point flexural testing indicate a ductile-to-brittle transition temperature (DBTT) of 1250-1350°C for all of the modifications evaluated. Isothermal oxidation testing at 1400°C indicates no significant debit in oxidation resistance of MoSi2 that can be attributed to alloying, however, alloys containing higher concentrations of the ternary elements may exhibit reduced oxidation resistances.

Oxidation of molybdenum silicide thin film temperature sensors

The oxidation characteristics of molybdenum silicide (MoSi x ) thin films used in resistance-type temperature sensor applications were investigated. In the present study, the electrical resistance variation of sensor films as a function of temperature was measured as an indication of sensor oxidation characteristics. The oxidation resistance of MoSi x films was observed to depend significantly on their silicon content. For comparatively molybdenum-rich MoSi x (as against stoichiometric MoSi 2) sensor films, poor oxidation resistance was observed. Sensors of this category showed linear positive, i.e. metallic, temperature coefficient of resistance (TCR) characteristics up to about 700°C, followed by sudden disarray of the electrical resistance due to intense oxidation. At the other extremity, relatively silicon-rich MoSi x sensor films, e.g. containing 92 at.% Si, exhibited non-linear and negative TCR characteristics, although good oxidation resistance could be obtained by the formation of a protective SiO 2 layer. In contrast, a near stoichiometric MoSi 2 sensor film showed relatively good oxidation resistance as sell as positive TCR values of (3.1–3.9) × 10 −3 K −1 up to 1450°C for at least five test cycles. This result indicates that a self-passivating SiO 2 layer forms effectively on the top surface of MoSi 2 sensors as expected, thus giving the desired sensor performance.

Prediction of the High Temperature Oxidative Life of Intermetallics

A method is presented to predict the oxidative life of intermetallics. The method is demonstrated by predicting the lifetimes of several aluminides undergoing cyclic oxidation at 1200°C. For NiAl and NiAI-Zr alloys, the lifetimes were predicted at several other temperatures as well. Using a critical surface recession failure criterion, it is shown that several aluminides (e.g., NiAl and FeAl) have long-term oxidation resistance (∼ 10,000 hours) to approximately 1150°C. Aluminides containing reactive elements (e.g., NiAI-Zr alloys) have long-term oxidation resistance to approximately 1 200°C. The method is also applied to predict the oxidative lifetime of MoSi2 at temperatures of 1200°-1400°C which shows that the oxidation resistance of MoSi2 is significantly better than that for the aluminides. For the same failure criterion, it is shown that MoSi2 can exhibit long-term oxidation resistance (∼ 10,000 hours) at temperatures in excess of 1400°C. Use of the method to predict the maximum use temperature is also demonstrated for NiAl and NiAI-Zr alloys.

The oxidation resistance of MoSi2 composites

Recent work at the Naval Air Development Center demonstrates that the oxidation resistance of MoSi2 is not significantly reduced by XD™ compositing. The MoSi2 + SiC material exhibited an oxidation resistance equal to or better than that of base MoSi2. This excellent oxidation resistance, in conjunction with the enhanced high-temperature strength provided by XD particles, makes these materials highly attractive for use as future gas turbine engine materials. The problem of low room-temperature ductility and fracture toughness will be approached by incorporating other types of reinforcement phases.