MoSi2 Layer Research Articles

Anti-oxidation properties and phase composition evolution of the MoSi2-HfO2 duplex bond coat in air and steam/oxygen atmosphere at 1450 °C

Environmental Barrier Coatings (EBCs) can prevent SiC-based ceramic matrix composites from corrosion at high temperatures. However, the low melting point of conventional Si bond layers limits the operating temperature of EBCs to 1350 °C. Herein, a MoSi2-HfO2 duplex bond coat was designed and showed robust adhesion on SiC substrate after corrosion at 1450 °C for 100 h in steam/oxygen atmosphere. A mixed oxide layer was observed to in-situ form at the interface between the MoSi2 and HfO2 layers, containing SiO2, Hf-Si-O glass and HfSiO4. This oxide layer was proven to effectively inhibit further oxidation/corrosion of the coating and the substrate. The phase composition evolution and anti-oxidation mechanism were systematically analyzed.

Characterization of microstructure and oxidation behavior of Al modified MoSi2 coating on Nb-Si based alloy

The aim of this work is to investigate the effects of Al on the microstructure formation and oxidation behavior of MoSi2 coating on Nb-Si based alloy by slurry sintering. The results indicate that the addition of a small amount of Al has little effect on the formation of MoSi2 coating. The phase constituent and structure of the inter-diffusion zone (IDZ) at the interface change significantly with the increase of Al content. The Al2O3 formed during the coating preparation process can hinder the diffusion of Si and Fe, thus suppressing formation of Mo5Si3 and Nb4Fe3Si5 layers in IDZ of the coatings with higher Al, but instead forming Fe-rich (Ti,Nb)5Si4 layer and (Nb,X)5Si3 layer. The higher amount of Al2O3 will increase the sintering resistance between the Mo and Si, resulting in a high porosity of the MoSi2 coating. By comparison, the 0.1Al coating exhibits the best oxidation resistance at 1250 °C. The preferential oxidation of Al promotes the formation of SiO2, which is conducive to the rapid filling of pores in the lower part of MoSi2 coating, forming a dense mixed layer of MoSi2+Mo5Si3+SiO2. However, adding a higher content of Al reduces the oxidation resistance of the coating.

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.

Enhancing oxidation resistance of Mo metal substrate by sputtering an MoSi2(N) interlayer as diffusion barrier of MoSi2(Si) surface coating

Owing to the diffusion of Si elements towards the metal substrate and the “pesting” behavior caused by volatilization of MoO3, magnetron-sputtered MoSi2 coating is highly susceptible to failure and unable to protect the metal substrate from oxidation. In this work, therefore, an MoSi2(N) interlayer was sputtered on Mo substrates as the diffusion barrier, followed by deposition of Si-doped MoSi2 (i.e., MoSi2(Si)) layer to suppress the “pesting” behavior. The as-deposited coatings were annealed subsequently at 1000 °C in an Ar atmosphere to obtain a stable structure. To evaluate the oxidation resistance, the annealed coating was heated in a muffle furnace in air at 700 °C, 1000 °C and 1200 °C for different durations. The coating structure, surface morphology, cross section and chemical composition were examined by X-ray diffraction, field emission scanning electron microscopy, and energy dispersive spectroscopy. The results showed the MoSi2(N) interlayer was amorphous and kept the most stable thermal stability at Ar/N2 flow rate of 20/20 sccm. It effectively blocked the downward diffusion of Si. Free state Si elements only diffused towards the coating surface. “Pesting” pores in the MoSi2(Si) layer were repaired by the growth of SiO2 layer via the preferential oxidation of Si. The mechanism for the enhanced oxidation resistance was explained through a model.

Oxidation behaviour of a MoSiBTiC alloy coated by a (Si+B) co-deposition pack cementation method

A MoSiB based coating was applied to the 1st-generation MoSiBTiC alloy using pack cementation process. After conditioning at 1300 °C for 10 h in air, the coating developed into four layers: an oxide layer, a MoSi2 layer with Mo5Si3 and MoB precipitates, a Mo5Si3 layer, and a MoB+Mo5SiB2 (T2) layer. However, visible holes often formed in the coating due to defects, and the oxidation proceeded to the substrate during subsequent cyclic oxidation. In a controlled test on a MoSiBTiC alloy with lower TiC volume fraction, an intact coating formed after the conditioning at 1300 °C and provided an effective oxidation resistance.

Захист елементів двигунів транспортних засобів від окислення: технологічний аспект

Multi-component composite coatings based on refractory compounds with a low speed of diffuse interaction, are concerned in the article in order to increase the service life. A study was conducted to increase the heat resistance of working blades, in which a diffusion borosilicide coating was formed on their surface; on the surface of coating a layer of molybdenum disilicide with a thickness of 120 μm was applied using the slip method. Experiments showed that when the thickness (over 120 microns) of the layer of disilicide suspension was increased, its detachment from the borosilicate surface of the working blade was observed. It was found that the thickness of the MoSi2 layer should not exceed 120 microns. Based on the results of the test of borosilicinated blades with a layer of MoSi2, the combined coating at a temperature of 1,6000C works for 450 hours, which is one and a half times longer than the service life of samples with only a borosilicide coating. The slip layer made of MoSi2 is found to increase the service life of the borosilicide coating. A metallographic analysis of the samples was carried out, which proved that the multi-layered coating is a composition based on molybdenum disilicide, reinforced with refractory oxides. An X-ray phase analysis of the protective surface layer of the slip coating was carried out, which showed that after high-temperature treatment, in addition to molybdenum silicides, it contains a number of the following oxides: Zr SiO4, HfSiO4, YAlO3, Y4Al2 O 3. According to the study, organic compounds of one- and two-component additives, introduced into the saturating mixture, ensure the formation of a borosilicide coating on refractory metals with a certain arrangement of boride and silicide layers and an adjustable structure of the silicide layer. Therefore, in aggregate, these factors contribute to increasing the service life of the protective diffuse coating.

Formation and oxidation resistance of MoSi2 coating for Nb-Si based alloy prepared by slurry sintering method

The high-temperature oxidation resistant MoSi2 intermetallic coatings have been successfully prepared on the Nb-Si based alloy by slurry sintering method. The heating rate before the sintering temperature is reached has an obvious influence on the density and interface structure of the coatings. The coating prepared at a heating rate of 10 °C/min is the densest, with a porosity about 18.4 area%, and has a good adhesion strength of 22.5 MPa. After oxidation at 1250 ℃ for 10 h, the coating exhibits multi-layer structure consisted of an SiO2 scale, an MoSi2 layer, an inner oxidation layer and an (Nb,X)5Si3 transitional layer from the outer to the inner.

Surface modification of a Mo-substrate to form an oxidation-resistant MoSi2 layer using a Si-saturated tin bath

Molybdenum disilicide (MoSi2) is one of the most heat and oxidation resistant materials used in high-temperature applications. In this work, the feasibility of forming an oxidation-resistant MoSi2 coating using a low-temperature Sn-Si bath by the hot dipping method was evaluated. It was found that a smooth and dense MoSi2 layer can be successfully synthesized on the surface of a Mo-substrate at 700 °C to 1000 °C. Using this method, MoSi2 can be synthesized more rapidly than using conventional methods and at much low operating temperatures. The diffusion through the product layer controlled the growth of the formed MoSi2 layer, and the growth rate was fastest at 800 °C. This was due to the replacement by the larger tin atoms that led to the expansion of the lattice, allowing the faster diffusion of silicon through the silicide layer towards the unreacted Mo-substrate. Oxidation resistance test at 1150 °C for 2 h confirmed that the MoSi2 layer formed by the present method protects the Mo-substrate from rapid oxidation at elevated temperatures.

Prevention of SiC‐fiber decomposition via integration of a buffer layer in ZrB2‐based ultra‐high temperature ceramics

AbstractA ZrB2‐based ceramic, containing short Hi‐Nicalon SiC fibers, was fabricated with a Mo‐impermeable buffer layer sandwiched between bulk and the outermost oxidation resistant ZrB2–MoSi2 layer, in order to prevent inward Mo diffusion and associated fiber degradation reactions. This additional layer consisted of ZrB2 doped with either Si3N4 or with the polymer‐derived ceramics (PDCs) SiCN and SiHfBCN. Scanning electron microscopy imaging and elemental mapping via energy‐dispersive X‐ray spectroscopy showed that this tailored sample geometry provides an effective diffusion barrier to prevent the SiC fibers from deterioration due to reactions with Mo or Mo‐compounds. In contrast, the structure of the SiC fibers in a reference sample without buffer layer is strongly degraded by MoSi2 diffusion into the fiber core. The comparison of the three buffer‐layer systems showed a moderate alteration of the fiber structure in the case of Si3N4 addition, whereas in the PDC‐doped samples hardly any structural change within the fibers was observed. A stepwise reaction mechanism is deduced, based on the continuous progression of a reaction zone that propagates toward the ZrB2–MoSi2 top layer. The progression of such a reaction zone as a consequence of the different eutectic melts forming in the different layers, that is, first in the SiC‐fiber‐containing bulk, then in the buffer layer itself, and finally in the top layer at high temperature, allows for an effective separation of the ZrB2–MoSi2 top layer from the SiC fibers.Subsequent oxidation at 1500°C and 1650°C for 15 min did not affect the efficiency of all three buffer layers, since no structural changes regarding buffer layer and fibers were observed, as compared to the non‐oxidized samples.

Formation and hot corrosion behavior of MoSi2/NbSi2 composite coating on Nb-based alloys by combined electrodeposition and HAPC processes

MoSi2/NbSi2 composite coating was prepared on Nb-based alloys through electrodeposition and halide activated pack cementation (HAPC) process. Preferred electrodeposition process, the effect of electrolyte with different concentrations of MoO42- on the electrodeposition behavior and coating formation were investigated. Besides, the microstructures of the composite coatings, and the hot corrosion behavior of the MoSi2/NbSi2 composite coated Nb-based alloys in the mixture of Na2SO4/NaCl (3:1 wt%) salt at 900 ℃ were studied. The results had shown that the co-deposited Ni-Mo layer was low oxygen, crack-free, and denser. Correspondingly 55.87 at% O and many cracks were detected in the Mo layer. With the increase of MoO42− concentrations in the electrolyte, the Mo content of the Ni-Mo layer increased and the cellular structures also became prominent. The high Mo content of the Ni-Mo layer was silicified to form a composite coating with thick MoSi2 outer layer. During the hot corrosion process, the composite coating exhibited stronger resistance. A SiO2 scale formed in the MoSi2 outer layer blocked the diffusion of oxygen so it could isolate the NbSi2 layer from the corrosion salt. The SiO2 scale got denser as the Mo content increased, which further improved the corrosion resistance of the composite coating. After the MoSi2 layer was exhausted, the NbSi2 sub-layer turned into the source of silicon dioxide and the second protective layer of the substrate. The satisfactory hot corrosion resistance of the MoSi2/NbSi2 composite coatings was attributed to the compactness and liquidity of the SiO2 scale.

Effect of different fillers on the microstructural evolution and high temperature oxidation resistance of Mo-Si-B coatings prepared by pack cementation

Mo-Si-B coatings were prepared on molybdenum by a pack cementation process. The effects of Al2O3, SiO2, and SiC fillers on the microstructural evolution and high temperature oxidation kinetic of Mo-Si-B coatings were studied. All the three Mo-Si-B coatings were composed of a MoSi2 layer and a MoB transitional layer. The thickness of the transitional layers showed no discernable differences in different coatings, but the coating prepared with the SiC filler showed the thickest MoSi2 outer layer owing to the additional silicon source supplied by the SiC pack agent. After oxidation at 1250 °C for 100 h, the oxidized coatings exhibited a three-layer structure consisting of an outer SiO2 film, a middle Mo5Si3 layer and an inner MoSi2 layer. The coating prepared with the SiO2 filler showed the smallest weight gain and consumption of the MoSi2 layer, which may be due to the residual SiO2 pack agent favoring the rapid formation of a dense SiO2 film.

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

AbstractA 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.

Formation of MoSi2 and Si/MoSi2 coatings on TZM (Mo–0.5Ti–0.1Zr–0.02C) alloy by hot dip silicon-plating method

MoSi2 and Si–MoSi2 coatings were deposited on TZM alloy by using a hot dip silicon-plating method. The composite coatings mainly consist of MoSi2 and Si with a small amount of C/SiO2/ZrSi2. The composite coatings have fine MoSi2 grain size and higher surface silicon concentration. The diffusion layer mainly consists of MoSi2 layer and Mo5Si3C layer under the deposition time is 10 min. The diffusion layer is divided into three layers when the deposition time ranges from 15 to 25 min, which consists of Si–MoSi2 layer, MoSi2 layer and Mo5Si3 interface layer. The gradient structure can reduce the stress mutation between coating and the substrate, and further reduce the possibility of crack propagation. SEM and CLSM results show that a large amount of MoSi2 grains are covered by a silicon layer when the deposition time is longer than 10 min, which results in a very low coating surface roughness. And the lowest values of Sa and Sq are 0.292 and 0.391 μm, respectively. Meanwhile, about 65.375%–79.898% of the coating surfaces are covered by silicon when the deposition time is 20–25 min.

Oxidation resistance and diffusion behavior of MoSi2/WSi2/(Nb,X)Si2 compound coating at 1400 °C

MoSi2/WSi2/(Nb,X)Si2 compound coating showed outstanding oxidation resistance at 1400 °C. After oxidation for 100 h, the specimen kept good integrality and the parabolic rate constant of the scale growth was about 1.497 µm2 h−1. NbSi2 layer could improve the bonding of MoSi2/WSi2 layers to the substrate, postpone their degradation into porous sub-silicide layer and provide adequate Si for oxidation. The intense interdiffusion between MoSi2 and WSi2 layers would result in formation of pores in WSi2 layer and then weaken its barrier effect. In addition, a small amount of alloying elements Ti, Cr and Al from the substrate could diffuse through the MoSi2/WSi2 layers along the grain boundaries during oxidation. As a result, there are TiO2 formed in the scale. After oxidation for 100 h, a gradient compound layer of (Mo,W)Si2/(W,X)Si2/(Ti,Nb)5Si4 formed eventually.

Preparation and 1500 °C oxidation behavior of crack-free bentonite doped MoSi2 protective coating on molybdenum

Spark Plasma Sintering technique was applied to fabricate MoSi2 and MoSi2-bentonite coatings on molybdenum. The phase composition, morphologies and anti-oxidation properties of the coatings at 1500 °C were investigated. Penetrated cracks generated in MoSi2 coating after SPS while the composite coating was crack-free because of the reduction of CTE mismatch. Cracks existed in the silica layer of MoSi2 coating after oxidation while the silica layer of bentonite-containing coating showed dense and crack-free morphologies. Compared with MoSi2, the composite coating possessed thinner Mo5Si3 diffusion layer and silica layer, exhibiting less loss of substrate and coating with improved oxidation resistance.

The enhanced structure stability and the reinforcement mechanism of 0.1Al2O3-ZrO2 reinforced MoSi2 composite coating

The 0.1Al2O3-ZrO2 reinforced MoSi2 composite coating was prepared on pure Mo substrate to improve its high-temperature oxidation resistance, of which the MoSi2 coating was prepared by a modified pack siliconizing process and subsequently the 0.1Al2O3-ZrO2 layer was generated on the as-prepared MoSi2 layer by a non-hydrolytic sol-gel method. The phase composition and microstructure of the as-prepared composite coating before and after oxidation exposure were investigated by XRD, SEM and EDS. Different from the loose structure of the single MoSi2 coating after oxidation exposure, the 0.1Al2O3-ZrO2 reinforced MoSi2 composite coating exhibited a relatively compact internal structure with less through-thickness cracks. The introduction of 0.1Al2O3-ZrO2 layer effectively prevented the cracking and spalling of the glassy SiO2 scale formed on the surface of MoSi2 during the oxidation, and then the SiO2 scale could be well stabilized on the coating surface. The protective SiO2 glassy scale embedded with stable ZrO2 and ZrSiO4 phase was formed, contributing to the improvement in the structure stability of the composite coating. It was proposed that ZrO2, ZrSiO4 and SiO2 developed a special glass-ceramic skeleton structure, in which ZrO2 and ZrSiO4 played the role of “Zr nail” to the SiO2 scale. In addition, the reinforcement mechanism of 0.1Al2O3-ZrO2 layer to the MoSi2 coating was also discussed.

Microstructure and properties of Mo-based double-layer MoSi2 thick coating by a new two-step method

A double-layer MoSi2 thick coating was prepared on Titanium-Zirconium-Molybdenum (TZM) alloy through a new two-step method, including molybdenum slurry vacuum sintering and halide-activated pack cementation. The coating was formed by a porous MoSi2 outer layer and a dense MoSi2 inner layer. The ball-milled fine Mo powder can effectively improve the coating density and the high temperature oxidation resistance. The oxidation protection time of the coatings prepared by coarse and fine Mo powders were 1 h and 6 h at 1700 °C, respectively. After oxidation, the coating exhibited multilayer structure consisted of SiO2 oxide layer, porous MoSi2 layer, dense MoSi2 layer and Mo5Si3 layer from the outer to the inner. With oxidation proceeding, the Mo5Si3 layer thickened, while the MoSi2 layer thinned and eventually transformed into Mo5Si3 completely, leading to coating failure because of the depletion of silicon.

Influence of different coating structures on the oxidation resistance of MoSi2 coatings

Two different structures of MoSi2 coatings were prepared on Niobium based alloys by using a two step process. The as-deposited type(a) MoSi2 coating structure consists of a MoSi2 layer on the surface and a NbSi2 layer underneath, while the type(b) MoSi2 coating consists of an outer MoSi2 layer and an inner unsiliconized Mo layer. The oxidation behaviors of the two different types MoSi2 coatings were examined at 1200 °C for 100 h in air, and the mass gains of type(a) and type(b) MoSi2 coated specimens were 0.64 mg/cm2 and 0.59 mg/cm2 respectively. The excellent oxidation resistance of both type(a) and type(b) MoSi2 coated samples at 1200 °C was due to the formation of a dense and continuous SiO2 scale during oxidation. As the CTE mismatch between the outer MoSi2 coating and the inner layer, cracks distributed within both type(a) and type(b) MoSi2 coating structures.

Structural design and ablation performance of ZrB2/MoSi2 laminated coating for SiC coated carbon/carbon composites

A protective coating alternated with ZrB2 and MoSi2 laminated layers was designed and prepared on carbon/carbon (C/C) composites with SiC inner layer by supersonic atmosphere plasma spraying. After ablated at a heat flux of 2.4 MW/m2 for 30s, ZrB2/MoSi2 laminated coating was in good condition with a linear growth rate and mass gain rate of 1.67 μm/s and 0.44 mg/s, respectively. From the central region to the border region, the calculated residual thermal stress of ZrB2/MoSi2 laminated coating decreased at first and then increased rapidly, illustrating the size change of the generated laminated cracks. The alternate design of ZrB2 layers for erosion and MoSi2 layers for oxidation resulted in the laminated stress distribution and improved ablation resistance.

Electrodeposition of Mo/Re duplex layer and preparation of MoSi2/ReSi2/NbSi2 compound coating on Nb-Ti-Si based alloy

MoSi2/ReSi2/NbSi2 compound coating has been successfully prepared on the Nb-Ti-Si based alloy through electrodeposition and halide activated pack cementation (HAPC) process. Optimization of electrodeposition process, the microstructural evolution and oxidation resistance of the compound coating were investigated. Re is beneficial to the electrodeposition of Mo and then improves the integrity and adherence of MoSi2 layer. The porous structure and limited thickness expansion of MoSi2 layer should be attributed to the oxides in the electrodeposited molybdenum layer. Remarkably, the substrate could be effectively protected by the compound coating at 1150 °C due to the formation of a dense scale.