Silicide Coatings Research Articles

Synthesis of complex concentrated silicide coatings via reactive melt-infiltration: Exploring interfacial phenomena between Si-B melt and MoNbTaW high-entropy alloy

High-entropy silicide (HES) coatings are a promising solution to the problem of poor oxidation resistance of metallic refractory alloys. However, their practical development is still in the early stages. For the first time, this study evaluates the feasibility of synthesizing complex concentrated silicide coatings using a reactive melt infiltration approach. For this purpose, a sessile drop experiment was carried out in which a binary eutectic silicon‑boron alloy (Si8B at.%) was subjected to contact heating with the MoNbTaW refractory high entropy alloy (RHEA) substrate at temperatures up to 1450 °C. After the high-temperature test, the solidified couple was characterized by X-ray diffraction, scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, electron backscatter diffraction systems, and microhardness techniques. The coating had an average thickness of 75 μm and was composed of different layers, including a primary MSi2 layer, a transition MSi2 + M5Si3 silicide layer, and M5Si3 silicides (M = metal). Additionally, borosilicides and boride particles were dispersed throughout the layers. However, future research should prioritize refining process parameters to eliminate porosity and improve the integrity of the coating layer.

Ablation-resistant yttrium-modified high-entropy refractory metal silicide (NbMoTaW)Si2 coating for oxidizing environments up to 2100 °C

Refractory high-entropy alloys (RHEAs) are pivotal in ultra-high temperature applications, such as rocket nozzles, aerospace engines, and leading edges of hypersonic vehicles due to their exceptional mechanical ability to withstand severe thermal environments (in excess of 2000 °C). However, the selection of materials that satisfy the stringent criteria required for effective ablation resistance remains notably restricted. Here, a novel yttrium-modified high-entropy refractory metal silicide (Y-HERMS) coated on a refractory high-entropy NbMoTaW alloy is developed via pack cementation process. The developed Y-HERMS coating with sluggish diffusion effect demonstrates extraordinary ablation resistance, maintaining near-zero damage at sustained temperatures up to 2100 °C for a duration of 180 s, surpassing state-of-the-art high-performance silicide coatings. Such exceptional ultra-high ablation performance is primarily ascribed to the in-situ development of a high viscosity Si-Y-O oxide layer with increased thermal stability and the presence of high-melting Y(Nb0.5Ta0.5)O4 oxides as skeleton structure. Theoretical results elucidate that the Y-HERMS promotes the formation of SiO2, which impedes the diffusion of O into metal silicide layer, synergistically contributing to the superior ablation resistance. These findings highlight the potential of utilizing high-entropy materials with excellent ablation resistance in extreme thermal environments.

Effect of B on hot corrosion behavior of silicide coatings under different corrosive environments at 900 °C

Silicide coatings and B-modified silicide coatings were prepared on Nb-Si based alloys using halide activated pack cementation. The hot corrosion behavior of the coatings exposed to Na2SO4 and Na2SO4 + NaCl mixture salts at 900 °C was investigated. The results show that B-modified silicide coatings possess better hot corrosion resistance than the silicide coatings. The hot corrosion products of silicide coatings show a regional microstructure, including uplift regions of AlNaSiO4 imbedded with Al2O3 phases, and depressed silica regions with NaNbO3 and TiO2 particles. The addition of B to the silicide coatings promotes the formation of the uniform and dense boroaluminosilicate scale with better fluidity during hot corrosion, which impedes the self-sustaining cyclic chlorination/sulfidation reactions, and thus, improves the hot corrosion resistance of the silicide coatings.

High temperature oxidation behavior at 1250 °C: A new multilayer modified silicide coating design strategy on niobium alloys

Silicide coatings have proven to be promising for improving the high-temperature oxidation resistance of niobium alloy. However, the long-term protective property of single silicide coating remains a long-time endeavor due to the deficiency of oxygen-consuming phases, as well as the self-healing ability of the protective layer. Herein, a silicide-based composite coating is constructed on niobium alloy by incorporation of nano-SiC particles for enhancing the high-temperature oxidation resistance. Isothermal oxidation results at 1250 °C for 50 h indicate that NbSi2/Nb2O5-SiO2/SiC multilayer coated sample with a low mass gain of 2.49 mg/cm2 shows an improved oxidation resistance compared with NbSi2 coating (6.49 mg/cm2). The enhanced high-temperature antioxidant performance of NbSi2/Nb2O5-SiO2/SiC multilayer coating is mainly attributed to the formation of the protective SiO2 self-healing film and the high-temperature diffusion behavior of NbSi2/substrate.

Temperature dependence mechanism of high-temperature oxidation of transition metal silicide MoSi2

Transition metal silicides represented by MoSi2 have excellent oxidation resistance and are widely used as high-temperature anti-oxidation coatings in hot end components of power equipment. However, the mechanism of temperature-dependent growth of MoSi2 oxidation products has not been revealed. Therefore, this study investigated the formation characteristics of oxide film and silicide-poor compound on MoSi2 at temperatures of 1000 °C–1550 °C through high-temperature oxidation experiments, combined with microscopic Raman spectroscopy, scanning electron microscope, and x-ray diffraction (XRD) characterizations. The result showed that MoSi2 underwent high-temperature selective oxidation reactions at 1000 °C–1200 °C, forming MoO2 and SiO2 oxide film on the substrate. As the oxidation temperature increased to 1550 °C, after 100 h of oxidation, along with the disappearance of MoO2 and the phase transformation of SiO2, a continuous Mo5Si3 layer with a thickness of approximately 47 μm was formed at the SiO2–MoSi2 interface. Thermodynamics and kinetic calculations further revealed the mechanism of temperature-dependent growth of oxidation products (MoO2 and Mo5Si3) during high-temperature oxidation process of MoSi2. As the temperature increased, the diffusion flux ratio of O and Si decreased, leading to a decrease in oxygen concentration at the interface and promoting the growth of the Mo5Si3 layer. Its thickness is an important indicator for evaluating the oxidation resistance of MoSi2 coatings during service. This study provides experimental and mechanistic insights into the temperature-dependent growth behavior of Mo5Si3 during the high-temperature oxidation of MoSi2 coating, and provides guidance for predicting the service life and improving the oxidation resistance of silicide coatings.

Microstructural development and oxidation resistance of MSi2-type multicomponent silicide coatings on Nb alloys by spark plasma sintering

MSi2-type multicomponent silicide coatings were prepared on Nb alloys by spark plasma sintering using mixed powders of Mo, Ta, W, Zr, Ti and Si. The coatings were composed of thick outer layers and thin inner layers (interdiffusion zone). A two-phase structure consisting of C11b (Mo,Ta,W,Ti)Si2 and C40 (Ti,Ta,Zr)Si2 was observed in the outer layers of all the coatings sintered for holding times ranging from 15 s to 5 min, but the distribution of these elements in each phase gradually became uniform with increasing holding time. The coatings demonstrated excellent protection during oxidation at 1300 °C for 100 h, and a continuous dense oxide scale mainly composed of ZrSiO4 and SiO2 was generated. The alternation of heating and cooling during oxidation caused cracks to appear at the interface between the outer layer and inner layer due to a mismatch of thermal expansion coefficients. However, crack extension was observed only along the inner layer toward the interior due to the multicomponent structure of the outer layer of the coating. The microstructural development and oxidation behavior of the coatings were discussed.

Insights into a novel refractory multi principal element alloy (Mo98W2)85Nb10(TiHf)5 and advancements in oxidation resistance upto 1300 °C through silicide coatings

In the present work, a novel refractory multi-principal element alloy (Mo98W2)85Nb10(TiHf)5 was developed through comprehensive material testing, including high-temperature exposure and oxidation resistance assessments. It was found that after pack siliconizing, the samples could withstand high temperatures up to 1300 °C with a marginal weight change of 0.255 mg/cm2 during four hours holding time. The overall activation energy for the coating process was found to be 123 kJ/mol, which is a consequence of the decreased atomic mobility of silicon and other atoms to overcome sluggish diffusion phenomena. Minor addition of hafnium created a peg effect in the outer silica scale, which improved its oxidation resistance covering coating cracks. The findings of this research provide valuable insights into the potential applications of such alloys with silicide coatings in high-temperature environments, such as aerospace, energy, and high-temperature manufacturing processes.

Ink-Extrusion 3D Printing and Silicide Coating of HfNbTaTiZr Refractory High-Entropy Alloy for Extreme Temperature Applications.

An oxygen-resistant refractory high-entropy alloy is synthesized in microlattice or bulk formby 3D ink-extrusion printing, interdiffusion, and silicide coating. Additive manufacturing of equiatomic HfNbTaTiZr is implemented by extruding inks containing hydride powders, de-binding under H2, and sintering under vacuum. The sequential decomposition of hydride powders (HfH2+NbH+TaH0.5+TiH2+ZrH2) is followed by in situ X-ray diffraction. Upon sintering at 1400°C for 18h, a nearly fully densified, equiatomic HfNbTaTiZr alloy is synthesized; on slow cooling, both α-HCP and β-BCC phases are formed, but on quenching, a metastable single β-BCC phase is obtained. Printed and sintered HfNbTaTiZr alloys with ≈1wt.% O shows excellent mechanical properties at high temperatures. Oxidation resistance is achieved by silicide coating via pack cementation. A small-size lattice-core sandwich is fabricated and tested with high-temperature flames to demonstrate the versatility of this sequential approach (printing, sintering, and siliconizing) for high-temperature, high-stress applications of refractory high-entropy alloys.

Molybdenum and its alloys in advanced engine applications: From material selection to surface optimization

This article explores the potential of molybdenum and its alloys for use in advanced aerospace engines, focusing on their high-temperature capabilities and corrosion resistance. It highlights the importance of enhancing oxidation resistance through methods such as alloying and the application of specialized coatings. The paper provides a detailed overview of high-temperature alloys, especially molybdenum-based alloys, discussing the benefits of various alloying elements and the types of oxidation-resistant coatings like aluminide and silicide coatings that protect against high-temperature degradation. The review of coating technologies includes modern methods like slurry sintering, atmospheric plasma spraying, and chemical vapor deposition, which improve the performance and longevity of these coatings. The paper concludes by identifying future research directions, emphasizing the need to boost the high-temperature oxidation resistance of coatings to meet the evolving demands of aerospace engine technologies.

“Anomalous” diffusion in multi-layer silicide systems

The transition metal silicide coatings have excellent oxidation resistance. In the high-temperature oxidization environment, the “anomalous” diffusion phenomenon of the reverse concentration gradient occurs in the multilayer silicide coating structure, which has a significant impact on the coating degradation. This study is to explore the physical mechanism of “anomalous” diffusion phase transformation in the MoSi2–NbSi2 bilayer silicide coating on the Nb alloy substrate. Through vacuum annealing experiments, combined with micro-Raman spectroscopy and electron probe microanalysis measurements, the diffuisonal phase transformation of multilayer silicide coating in the oxygen-free environment at high temperatures was studied. By decoupling the oxidation and diffusion, the experiments indicate that high-temperature oxidation is not the dominant driving factor for the “anomalous” diffusion of atoms. The thermodynamic analysis reveals that the reduction in the nucleation barrier of the silicide-poor layer due to multicomponent solid solution and the non-uniform distribution of component chemical potential provide the driving force for the “anomalous” diffusion growth. Based on the diffusion kinetic modeling, the simulation of diffusion-controlled phase transformation in multilayer silicide coating was carried out, and the effect of tracer diffusion coefficients on the growth of the silicide-poor phase was analyzed. The research will have guiding significance for the recognition of failure mechanisms of silicide coating systems and performance improvement.

Plasma Spraying of Silicide Coatings to Protect Zirconium Alloys from Oxidation

The work is devoted to an experimental study of the possibilities of applying a protective silicide coating on an alloy based on zirconium (E110) by atmospheric plasma spraying. Coatings based on the binary eutectic Mo5Si3 + MoSi2 were deposited on the surface of E110 alloy sheet. The features of the structure and phase composition of the coatings after deposition and their evolution as a result of isothermal annealing at a temperature of 1300°C are studied. Upon rapid cooling of silicide particles during coating, nonequilibrium phases are formed. As a result of annealing, the phase composition changes, which corresponds to the phase diagram. The kinetics of diffusion interaction between the coating and the base material has been studied. The possibility of successfully protecting a zirconium alloy from oxidation at 1100°C in air by complex deposition of a coating of molybdenum silicides has been shown for the first time. The minimum radius of curvature of the surface of the protected samples was about 1 mm.

On the Plasma Spraying of Silicide Coatings to Protect Zirconium Alloys from Oxidation

On the Plasma Spraying of Silicide Coatings to Protect Zirconium Alloys from Oxidation

Niobium Silicide Diffusion Coatings: The Impact of Cu and of Ti on the Microstructure and High-Temperature Behavior

Abstract Silicide diffusion coatings on niobium offer attractive properties for applications at temperatures above 1,000°C because of the ability to form a protective oxide, silicon dioxide. The combination of a double layer of silicides, NbSi2 and Nb5Si3, provides a gradient of properties to the coating, mitigating the formation and propagation of cracks that otherwise allow oxygen to interact with the substrate. However, a temperature above 1,200°C is required to process double-layer silicide coatings, increasing the challenge of controlling the process and incrementing the cost of the process. The need to enhance silicide coatings without compromising their competitiveness motivates the study of the individual effects of copper and of titanium. This study processed silicide coatings by double pack cementation adding copper or titanium, respectively, to the pack mixture. Three pack mixture compositions were used: with 15 % wt. silicon, with 15 % wt. silicon + 7 % wt. copper, and 15 % wt. silicon + 7 % wt. titanium. Processing was carried out at 1,000°C for 6 hours. The coatings processed with silicon resulted on a single NbSi2 layer, but the pack mixture containing silicon and copper led to thick double-layer coatings of NbSi2 and a continuous layer of Nb5Si3, in spite of the low processing temperature. In contrast, the pack mixture containing silicon and titanium led to thinner coatings composed of a layer of NbSi2 and the discontinuous presence of Nb5Si3. Results suggest that copper and titanium impact the gaseous diffusion during processing and also the vacancy concentration in the ordered structure of NbSi2. Exposure to temperature reveals a change on the oxidation mechanisms after processing with titanium and a more significant mass gain of coatings processed with copper.

Analysis and Kinetics Modeling of the Isothermal Oxidation Behavior of Silicide Coatings

In this paper, an online apparatus was developed for isothermal thermogravimetric measurement of silicide coatings within a wide temperature range (from −180 °C to 2300 °C) based on thermogravimetric analysis. Firstly, the measuring principle and method regarding silicide coatings of this apparatus were studied. Secondly, on the basis of oxidation kinetics analysis, the intrinsic mechanism and kinetic parameters of three stages (oxidation, diffusion, and fall-off) of silicide coatings were studied, and the oxidation kinetics features were also analyzed. In addition, according to mathematical physics methods, a kinetics model of silicide coatings in different stages of oxidation was established, including parameters such as weight change, oxidation rate, oxidation time, etc. Finally, online isothermal experiments from −180 °C to 2300 °C werecarried out and analyzed. The results showed that the kinetic model established in this paper was in good agreement with the oxidation process of silicide coatings. In this paper, a complete kinetics model including different oxidation stages is proposed for the entire oxidation process of a silicide coating, revealing its oxidation mechanism. The research will play a significant role in the study of preparation technology improvement and high-temperature environment application. This paper studied two measuring methods: weight gain and weight loss measuring methods. Also, an experiment was carried out on the silicide coatings to explore the physical oxidation process between −180 °C and 2300 °C. The results proved the perfect consistency of the kinetics model proposed by this paper and the oxidation process of silicide coatings. This paper will play a significant role in the study of preparation technology enhancement and high-temperature environment application. It also provides a theoretical foundation for accelerated aging and life evaluation methods.

Effect of Reaction Temperature on the Molten Salt Electrodeposition of Silicide Coating on Molybdenum Substrate and its Oxidation Behavior

In this study, silicide coatings were successfully prepared on Mo substrates using NaCl-KCl-NaF molten salt as the reaction medium and K2SiF6 as the silicon source. The electrochemical reduction mechanism of Si(IV) was studied by cyclic voltammetry and chronoamperometry technology. The effect of reaction temperature on the phase composition, microstructure, and growth kinetics of the electrodeposited coatings were investigated in detail. The long-term oxidation behaviors of Mo and MoSi2-coated Mo samples at 873 K were also compared. The results revealed that the electrodeposition of silicide coatings was closely related to the reaction temperature. Dense and coherent single-layer MoSi2 coatings could be obtained at 1023 K and 1073 K, while double-layer coatings with inner MoSi2 layer and outer Si layer could be obtained at 1123 K. Under the same electrodeposition time, the overall thickness of the silicide coatings increased with the reaction temperature. Moreover, long-term oxidation tests at 873 K proved that MoSi2 coatings could effectively prevent the Mo substrate from oxidation. The mass gain of the MoSi2-coated Mo sample was only 0.46 wt% after 500 h of oxidation, which could be attributed to the formation of the continuous SiO2 protective layer.

Orthogonal Experimental Optimization of Preparation and Microstructural Properties of a Diffusion Barrier for Tantalum-Based Silicide Coatings.

To solve the problem of silicide coatings on tantalum substrates failing due to elemental diffusion under high-temperature oxidation environments and to find diffusion barrier materials with excellent effects of impeding Si elemental spreading, TaB2 and TaC coatings were prepared on tantalum substrates by the encapsulation and infiltration methods, respectively. Through orthogonal experimental analysis of the raw material powder ratio and pack cementation temperature, the best experimental parameters for the preparation of TaB2 coatings were selected: powder ratio (NaF:B:Al2O3 = 2.5:1:96.5 (wt.%)) and pack cementation temperature (1050 °C). After diffusion treatment at 1200 °C for 2 h, the thickness change rate of the Si diffusion layer prepared using this process was 30.48%, which is lower than that of non-diffusion coating (36.39%). In addition, the physical and tissue morphological changes of TaC and TaB2 coatings after siliconizing treatment and thermal diffusion treatment were compared. The results prove that TaB2 is a more suitable candidate material for the diffusion barrier layer of silicide coatings on tantalum substrates.

On the Possibilities of Increasing the Diffusion Resistance of Protective Silicide Coatings on the Surface of E110 Alloy

On the Possibilities of Increasing the Diffusion Resistance of Protective Silicide Coatings on the Surface of E110 Alloy

(Digital Presentation) Electroless Deposition of Oxidation Resistant High-Temperature Silicide Coatings on Molybdenum and Tzm Alloy Using Molten Salt

Molybdenum is one of the refractory metals which has high melting point, 2620oC, and good mechanical properties. Unfortunately, oxidation resistance of molybdenum is not satisfactory for certain applications which involves high temperatures; therefore, an oxidation resistant coating is necessary. Molybdenum di-silicide (MoSi2) was coated on pure molybdenum and TZM alloy to increase oxidation resistance at high temperatures by electroless molten salt method. A molten salt solution containing NaCl, KCl and NaF was used to apply coating at temperatures above 600oC. Silicon powder and Na2SiF6 were used as silicon source during coating process. Experiments were carried out at different temperatures, durations and salt compositions. Coating thickness, surface roughness, resulting phases and coating behavior of pure molybdenum and TZM alloy were evaluated. X-ray diffraction (XRD), Inductively Coupled Plasma – Mass Spectrometer (ICP-MS) and scanning electron microscope (SEM) were conducted in characterization of the silicide coatings.

Microstructure and Cyclic Oxidation Behavior of Silicide Coatings Prepared by Molten Salt on Nb–Based Alloys

Microstructure and Cyclic Oxidation Behavior of Silicide Coatings Prepared by Molten Salt on Nb–Based Alloys

Microstructure and Cyclic Oxidation Behavior of Silicide Coatings Prepared by Molten Salt on Nb–Based Multiphase Alloys

Microstructure and Cyclic Oxidation Behavior of Silicide Coatings Prepared by Molten Salt on Nb–Based Multiphase Alloys