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.     

Oxidation of the intermetallics MoSi2 and TiSi2—A comparison

The oxidation behavior of two MoSi₂ variants, one Mo-rich and one Si-rich, and TiSi₂ was investigated between 1000 and 1400°C in air, oxygen and an 80/20-Ar/O₂ mixture. A protective SiO₂ scale develops on MoSi₂ in all atmospheres in the temperature range investigated. The SiO₂ modification changes around 1300°C from tridymite to cristobalite. This change in SiO₂ modification seems to cause an enhanced formation of SiO₂ and evaporation of MoO₃. The SiO₂ grows at the MoSi₂-scale interface. In air a two-layer scale grows on TiSi₂ between about 1000 and 1200°C with an inner inwards growing fine-grain mixture of SiO₂ + TiO₂ and an outer outward-growing TiO₂ partial layer. TiN formation in the transient oxidation is responsible for the formation of the inner mixed partial layer because in N -free atmospheres a scale of a SiO₂ matrix with some Ti oxide precipitates inside is formed. A one-layer scale structure similar as that in N-free atmosphere is found on TiSi₂ in air at T > 1200°C. In oxygen the TiO₂ precipitates grow as needles mostly oriented perpendicular to the surface. Due to the faster oxygen transport in TiO₂ compared with SiO₂, these TiO₂ needles act as oxygen pipes, causing an enhanced oxidation of TiSi₂ in front of these needles. The SiO₂ scale dissolves about 1–2% TiO₂. This doping causes a mixed oxygenand Si transport with the consequence that the SiO₂ scale on TiSi₂ grows partly by oxygen transport inwards and Si transport outwards. The SiO₂ modification is cristobalite over the entire temperature range investigated.     

Oxidation of SiOC/MoSi2/SiC Composites Prepared by Polymer Pyrolysis

From the pre-ceramic polymer of polymethylsiloxane (PMS) and powders of MoSi₂, SiC and Si, new ceramic composites that consisted primarily of an amorphous SiOC matrix containing dispersed particles of MoSi₂ and SiC were synthesized. The composites displayed superior oxidation resistance at both high and low temperatures by forming SiO₂ on the surface. The thin, amorphous SiO₂ layer that formed initially gradually to crystallized during oxidation between 1000 and 1300°C. An outer highly porous and an inner superficial SiO₂ layer that formed from the initial stage of oxidation between 400 and 500°C protected the composites from pesting.     

MoSi2 oxidation resistance coatings for Mo5Si3/MoSi2 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°C. 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°C. 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.% Mo₅Si₃/MoSi₂ composites with a MoSi₂ coating showed much better oxidation resistance at high temperature.     

Preventing the accelerated low-temperature oxidation of MoSi2 (pesting) by the application of superficial alkali-salt layers

Previous work showed that MoSi₂ diffusion coatings formed by a NaF-activated pack cementation process did not pest. A Na–Al-oxide by-product layer resulting from the NaF activator formed a Na-silicate layer to passivate MoSi₂. Superficial NaF layers were then used to prevent the pesting of MoSi₂ diffusion coating that were otherwise susceptible to pest disintegration. In this study, the use of superficial alkali-salt layers to prevent the accelerated oxidation of bulk MoSi₂ at 500°C is investigated more broadly. The application of Na-halide, KF, LiF, Na₂B₄O₇, or Na-silicate layers prior to oxidation prevented accelerated oxidation and pesting for at least 2000 hr at 500°C in air. The formation of a fast-growing, Na-silicate layer passivates MoSi₂. The MoO₃ that forms during oxidation absorbs sodium by intercolation to form stable Na-molybdate precipitates. Na₂B₄O₇, Na-silicate, LiF, and KF prevented accelerated oxidation at 500°C by a similar mechanism. The application of alkali-halide salts is a simple, effective solution to prevent the accelerated oxidation and pesting of MoSi₂.     

Effect of SiC, Si3N4 and TiB2 additions on the oxidation resistance of TiAl alloys

The oxidation behavior of TiAl alloys containing dispersed particles of (5, 10, 15 wt.%) SiC, (3,5 wt.%) Si₃N₄ or (3, 5, 10 wt.%) TiB₂ was studied between 800 and 1200°C in atmospheric air. The TiAl−(SiC, Si₃N₄) alloys oxidized to TiO₂, Al₂O₃, and SiO₂. The TiAl−TiB₂ alloys oxidized to TiO₂, Al₂O₃, and B₂O₃ which evaporated during oxidation. Improvement in oxidation resistance accompanied by thin, dense scale formation due to the addition of dispersoids originated primarily from the enhanced alumina-forming tendency, improved scale adhesion by oxide grain refinement owing to the beneficial effect of dispersoids, and the incorporation of SiO₂ within the oxide scale in the case of TiAl−(SiC, Si₃N₄) alloys.     

ZrB2-玻璃陶瓷复合材料氧化分析

对ZrB₂-玻璃陶瓷复合材料氧化行为进行热力学分析,对氧化形成的氧化层进行物相分析和显微结构分析。结果表明：在1000°C-1400°C的反应温度范围内,ZrB₂氧化生成ZrO₂,B₂O₃玻璃相,氧化产物ZrO₂与SiO₂反应生成ZrSiO₄,当温度低于1177°C(1450K)时,氧化层主要包括ZrO₂,B₂O₃玻璃相,ZrSiO₄。当氧化温度超过1177°C(1450K)时,B₂O₃玻璃相蒸发,此时SiO₂玻璃相具有良好的流动性,氧化层主要包括ZrO₂,SiO₂玻璃相,ZrSiO₄。氧化过程中的反应产物B₂O₃玻璃相,ZrSiO₄和流动性良好的SiO₂玻璃相,均对氧气向基体的扩散均起到了良好的阻碍作用。     

Microstructure ofin situ MoSi2/SiC nanocomposite coating formed on Mo substrate by displacement reaction

The microstructure of an in-situ Mosi₂/β-SiC nanocomposite coating formed by the solid-state displacement reactions of Si deposited by chemical vapor deposition (CVD) with Mo-carbide layers at 1100°C, which had previously been formed on the surface of a Mo substrate by a CVD process, was investigated. The Mo-carbide layers formed by the simultaneous CVD of Mo and carbon at 900°C for 5 h using a gas mixture of C₂H₄−MoCl₅−Ar consisted of two layers, an inner layer of Mo₂C and an outer layer of MoC. While the monolithic MoSi₂ coating showed a typical colummar microstructure perpendicular to the Mo substrate, the MoSi₂/β-SiC nanocomposite coating formed by the solid-state displacement reactions between the Mo-carbide layers and Si was composed of equiaxed MoSi₂ grains with an average size of 150–500 nm and β-SiC particles with an average size of 80–105 nm. The β-SiC particles exhibited an oblate-spheroidal shape and were located mostly at the grain boundaries of MoSi₂. The volume percentage of β-SiC particles ranged from 18.5 to 29.2% with respect to the carbon concentration in Mo-carbide layers.     

Establishing multilayered coating on Mo–12Si–8.5 B alloy by Si pack cementation and its oxidation behavior at 900°C–1300°C

A multilayered oxidation protection coating consisting of MoSi₂ outer layer, Mo₅Si₃ internal layer, and Mo₅SiB₂/MoB inner layer was developed on the surface of Mo–12Si–8.5B 1.0 wt% ZrB₂ alloy via Si pack cementation. The multilayered coating significantly enhanced the oxidation resistance of the alloy at 900°C, 1100°C, and 1300°C in the air by exhibiting negligible oxidation recession. MoSi₂ outer layer provided admirable oxidation protection for the alloy at high temperatures by forming a thin and protective SiO₂-rich glass scale on its surface. This was supplemented by the Mo₅Si₃ internal layer and Mo₅SiB₂/MoB inner layer that reduced the thermal expansion mismatch between the MoSi₂ outer layer and substrate, and therefore no obvious cracks were found in the MoSi₂ outer layer. More importantly, the Mo₅SiB₂/MoB layer as an in situ barriers of Si interdiffusion ensured the stable existence of MoSi₂ and Mo₅Si₃ layers without obvious thickness change during oxidation at 900°C and 1100°C. Mechanical property test indicated that the formation of the coating layers could not affect the fracture toughness of the alloy.     

Quantitative Electron Microprobe Characterizations of Oxidized ZrB2 Containing MoSi2 Additives

This study reports the oxidation behavior of hot-pressed ZrB₂ ceramics, with 10, 20 and 40 vol.% MoSi₂ additives, exposed to dry air at 1500 °C for up to 10 h. The effect of the amount added MoSi₂ on oxidation resistance was assessed. Quantitative electron microprobe characterizations of the oxidized ZrB₂ with MoSi₂ additives were carried out with electron probe micro analysis. Parabolic oxidation behavior was observed for the three compositions. The oxidation resistance was significantly improved with MoSi₂ additives. Reaction-product phase compositions and phase distribution were thoroughly characterized from the oxidized surface through to the unreactive bulk material. It was found that the oxidation products consisted of nonstoichiometric amorphous SiO₂, stoichiometric crystalline ZrO₂, and MoB. The amounts of these phases in the oxidized reactive region were qualitatively determined.     

Microstructure Evolution of Plasma-Sprayed MoSi<Subscript>2</Subscript> Coating at RT-1200 °C in Air

In this work, the phase composition and microstructure evolution of vacuum plasma-sprayed MoSi₂ coating between room temperature and 1200 °C in air was evaluated and characterized. The results showed that hexagonal MoSi₂ (h-MoSi₂) became the main phase in the deposited coating, which remained even after 50 h oxidation at 500 °C, exhibiting excellent thermal stability. MoO₃ bundles and SiO₂ clusters were generated by consuming tetragonal MoSi₂ (t-MoSi₂) after 1 h, and white powders formed on the coating’s surface after 10-h exposure to air at 500 °C. Most h-MoSi₂ transformed to t-MoSi₂ at 800 °C; moreover, a protective silica layer formed on the coating surface. Similar phenomenon was observed for the coating exposed to 1000 °C where grain growth also occurred. Vacuum heat treatment at 900 °C effectively improved the thermal stability of the MoSi₂ coating. The formation of silica layer alleviated negative effects of structural defects and helped the MoSi₂ coating serve as a protective coating for varied substrates.     

Molybdenum silicide based materials and their properties

Molybdenum disilicide (MoSi₂) is a promising candidate material for high temperature structural applications. It is a high melting point (2030 °C) material with excellent oxidation resistance and a moderate density (6.24 g/cm³). However, low toughness at low temperatures and high creep rates at elevated temperatures have hindered its commercialization in structural applications. Much effort has been invested in MoSi₂ composites as alternatives to pure molybdenum disilicide for oxidizing and aggressive environments. Molybdenum disilicide-based heating elements have been used extensively in high-temperature furnaces. The low electrical resistance of silicides in combination with high thermal stability, electronmigration resistance, and excellent diffusion-barrier characteristics is important for microelectronic applications. Projected applications of MoSi₂-based materials include turbine airfoils, combustion chamber components in oxidizing environments, missile nozzles, molten metal lances, industrial gas burners, diesel engine glow plugs, and materials for glass processing. In this paper, synthesis, fabrication, and properties of the monolithic and composite molybdenum silicides are reviewed.     

Effect of MoSi2/rare earth composite particles on microstructure and mechanical properties of molybdenum

MoSi₂/La₂O₃ and MoSi₂/Y₂O₃ composite particles were prepared by mechanical milling and doped into molybdenum by solid-solid method, respectively. Rods with a diameter of 17 mm were made by pressing and sintering. The effects of different composite particles on microstructures and strength of the as-sintered molybdenum were investigated. Results show that the MoSi₂/La₂O₃ and MoSi₂/Y₂O₃ composite particles transformed to La₂O₃/Mo₅Si₃ and Y₂O₃/Mo₅Si₃ composite particles due to the in situ reaction between Mo and MoSi₂ during sintering process. Mo₅Si₃/La₂O₃ and Mo₅Si₃/Y₂O₃ composite particles can reduce the grain size and improve both strength and toughness of sintered molybdenum significantly. Mo₅Si₃/Y₂O₃ composite particles contribute more to the strength, while the effect of Mo₅Si₃/La₂O₃ on toughness is greater than that of Mo₅Si₃/Y₂O₃.     

A new TiB2 + CrSi2 composite – Densification,characterization and oxidation studies

A new composite of TiB₂ with CrSi₂ has been prepared with excellent oxidation resistance. Dense composite pellets were fabricated by hot pressing of powder mixtures. Microstructural characterization was carried out by XRD and SEM with EDAX. Mechanical and physical properties were evaluated. Extensive oxidation studies were also carried out. A near theoretical density (99.9% TD) was obtained with a small addition of 2.5 wt.% CrSi₂ by hot pressing at 1700 °C under a pressure of 28 MPa for 1 h. The microstructure of the composite revealed three distinct phases, (a) dark grey matrix of TiB₂, (b) black phase – rich in Si and (c) white phase – Cr laden TiB₂. Hardness and fracture toughness were measured as 29 ± 2 GPa and 5.97 ± 0.61 MPa m^1/2, respectively. Crack branching, deflection and bridging mechanisms were responsible for the higher fracture toughness. With increase in CrSi₂ content, density, hardness and fracture toughness values of the composite decreased. Thermo gravimetric studies revealed the start of oxidation of the composite at 600 °C in O₂ atmosphere. Isothermal oxidation of these composites showed better oxidation resistance by formation of a protective oxide layer. TiO₂, Cr₂O₃ and SiO₂ phases were identified on the oxidized surface. Effects of CrSi₂ content, temperature and duration of oxidation on the oxide layer formation are reported. Activation energy of the composite was calculated as ∼110 kJ/mol using Arrhenius equation. Diffusion controlled mechanism of oxidation was observed in all the composites.     

Effect of SI Content on the Oxidation Resistance of Ti3Al1-x Si x C2 (x⩽ 0.25) Solid Solutions at 1000–1400°C in Air

The oxidation behavior of Ti₃Al_1-x Si _x C₂ (x ⩽ 0.25) solid solutions was investigated in flowing air at 1000–1400°C for up to 20 hrs. Similar to Ti₃AlC₂, Ti₃Al_1-x Si _x C₂ (x⩽ 0.15) solid solutions display excellent oxidation resistance at all temperatures because of the formation of the continuous α-Al₂O₃ protective layers. However, Al₂(SiO₄)O formed during oxidation of Ti₃Al_1-x Si _x C₂ (x=0.2 and 0.25) solid solutions at and above 1100°C, which is believed to be responsible for the deterioration of oxidation resistance of Ti₃Al_0.75Si_0.25C₂ at 1300°C. Additionally, Ti₅Si₃ was also found in the oxidized samples. This implies that Ti₅Si₃ precipitated from Ti₃Al_1-x Si _x C₂ solid solutions during oxidation. But it has been proven that Ti₅Si₃ has little effect on the oxidation resistance of the material, which is attributed to an interstitial carbon effect.     

Oxide scale formation and isothermal oxidation behavior of Mo–Si–B intermetallics at 600–1000°C

Initial scale formation in the range 600–1000°C and isothermal oxidation behavior at 1000°C was investigated for Mo–Si–B intermetallics containing 81–88 wt% molybdenum. All compositions exhibited an initial transient oxidation period consisting of a mass gain due to MoO₃ and SiO₂ formation, followed by a rapid mass loss starting at 750°C due to MoO₃ volatilization. After the initial transient oxidation period, oxidation proceeded at a much slower rate. During isothermal oxidation at 1000°C the oxidation rate was found to vary inversely with the ratio of B/Si in the intermetallic, indicating that viscous flow of the scale was an important factor in determining the isothermal oxidation rate at 1000°C.     

The oxidation behaviour of uniaxial hot pressed MoSi2 in air from 400 to 1400 °C

MoSi₂ samples were prepared by hot uniaxial pressing from a 2 μm grain-size powder of commercially available MoSi₂. The oxidation behaviour of MoSi₂ was systematically studied from 400 °C to 1400 °C, which includes the pest-oxidation temperature range. It was observed that the rate and mechanism for oxidation of MoSi₂ change significantly with increasing temperature. Five temperature regimes have to be considered regarding both kinetic results and cross-sections: i) 400 < T < 550 °C; ii) 550 ≤ T < 750 °C; iii) 750 ≤ T < 1000 °C; iv) 1000 ≤ T < 1400 °C; v) T ≥ 1400 °C. In the first range, pesting did not occur in samples that were free of cracks and residual stresses and the oxidation kinetics were governed by surface or phase boundary reactions. Above 550 °C, there was a change in the physical properties of the oxidation products due to the evaporation of MoO₃. The formation of Mo₅Si₃ was observed above 800 °C showing that the thermodynamic previsions were satisfied above this temperature. At higher temperatures (>1000 °C), the oxide scale became very protective and transport in the silica scale (amorphous and β cristobalite) governed the oxidation kinetics. The Mo₅Si₃ phase did not appear anymore at 1400 °C, indicating that another oxidation mechanism has to be proposed.     

Microstructure Characteristics and Oxidation Behavior of Molybdenum Disilicide Coatings Prepared by Vacuum Plasma Spraying

In this study, molybdenum disilicide (MoSi₂) coatings were fabricated by vacuum plasma spraying technology. Their morphology, composition, and microstructure characteristics were intensively investigated. The oxidation behavior of MoSi₂ coatings was also explored. The results show that the MoSi₂ coatings are compact with porosity less than 5%. Their microstructure exhibits typical lamellar character and is mainly composed of tetragonal and hexagonal MoSi₂ phases. A small amount of tetragonal Mo₅Si₃ phase is randomly distributed in the MoSi₂ matrix. A rapid weight gain is found between 300 and 800 °C. The MoSi₂ coatings exhibit excellent oxidation-resistant properties at temperatures between 1300 and 1500 °C, which results from the continuous dense glassy SiO₂ film formed on their surface. A thick layer composed of Mo₅Si₃ is found to be present under the SiO₂ film for the MoSi₂ coatings treated at 1700 °C, suggesting that the phenomenon of continuous oxidation took place.     

低氧分压下NiTi记忆合金氧化行为的研究

本文研究了NiTi形状记忆合金在H₂-H₂O气氛下400-700 °C的氧化行为。合金的氧化过程遵循立方规律,氧化激活能127.52 kJ/mol。氧化显著降低了试样表面的Ni含量。400 °C氧化的试样,其表面形貌与其他试样不同,并且氧化膜较薄,截面结构无法用SEM分析。500 °C、600 °C、700 °C氧化的试样,表面有两种形貌的氧化物,一种是颗粒状氧化物,另一种是晶须状氧化物。截面分析表明,氧化膜分为两层,上层由TiO₂构成,下层由Ni₃Ti构成,两层界面处有孔洞生成。     

High temperature oxidation and microstructure of MoSi2/MoB composite coating for Mo substrate

The two-layer MoSi₂/MoB composite coatings were developed using the halide activated pack cementation (HAPC) method on Mo substrate. Oxidation resistance property and microstructural evolution of the coatings at high temperatures were investigated. During oxidation exposure, the coatings exhibited a good oxidation resistance property. The mass gains of the coated specimens oxidized at 1200 °C for 100 h and at 1300 °C for 80 h were 0.270 and 0.499 mg/cm², respectively. Compared with the monolithic MoSi₂ coatings, the transformation of MoSi₂ phase in the MoSi₂/MoB composite coatings was more sluggish at elevated temperatures. The growth rate constant of the Mo₅Si₃ layer in the composite coatings was two orders of magnitude lower than that of the Mo₅Si₃ layer in the monolithic coatings at 1300 °C. The microstructural degradation of MoSi₂ in the composite coatings at high temperatures was slowed by the introduced MoB layer. The MoB layer in the composite coatings is useful to prolong the service life of MoSi₂ coatings at high temperatures.