Oxidation protection of graphite materials by single-phase ultra-high temperature boride modified monolayer Si-SiC coating

To improve the oxidation resistance of Si-SiC coating, single-phase ultra-high temperature boride (ZrB₂ or TaB₂) modified Si-SiC coating was designed and established on graphite substrates by combination of dipping and reactive infiltration process. ZrB₂ or TaB₂ phase was introduced in Si-SiC coating by directly mixing raw materials and phenol formaldehyde resin in the slurry, and then the ZrB₂-SiC-Si and TaB₂-SiC-Si coatings were fabricated on the graphite samples by dipping-curing, pyrolysis, and siliconizing. The crystalline phases and microstructure of the as-obtained multiphase coatings were investigated by X-ray diffraction analysis and scanning electron microscopy. The interrupted oxidation tests from room-temperature to 1500?°C were conducted to assess the anti-oxidation property of the prepared coatings. After 1200?h of oxidation at 1500?°C in air (30 times thermal cycles), the mass losses of the graphite substrates coated with ZrB₂-SiC-Si and TaB₂-SiC-Si coatings were 0.086% and 0.537%, respectively, and the high-temperature stability of the modified coatings was greatly improved compared to the Si-SiC coating. The excellent anti-oxidation performances of the compound coatings were attributed to the compact structure of the coatings and the formation of compound oxide layers covering on the surfaces. The compound Zr-Si-O and Ta-Si-O films possessed low oxygen diffusion rate and appropriate viscosity, which can provide appreciable oxidation protection for the internal coatings, thus obtaining the excellent oxidation and spallation resistance property.     

Preparation of TaB2-SiC oxidation protective coating for carbon materials by liquid phase sintering

To control the microstructure and amounts of TaB₂ phase in the TaB₂-SiC coating, a novel liquid phase sintering method was developed on the basis of in-situ reaction method to prepare the TaB₂-SiC coating, which includes synthesis of TaB₂ powders and further preparation of TaB₂-SiC coating. With Ta₂O₅, B₂O₃ and C employed as raw materials, hexagonal TaB₂ powders were prepared by carbothermal reduction method at 1500?°C, whose mean particle size is 491?nm. The TaB₂, SiC, C powders, and the low melting point phases Si and silica sol were used to prepare the TaB₂-SiC coating by liquid phase sintering at 2373?K. The thickness of the coating is about 350?µm. Compared with the SiC coating, the weight loss of the samples modified by TaB₂ decreased from 17.7% to 11.8%, and the average weight loss rate of the fastest weightloss zone reduced from ?6?×?10^?3 mg?cm^?2 s^?1 to ?5?×?10^?3 mg?cm^?2 s^?1. During oxidation, the Ta-oxides would gradually dissolve in the silicate glass to form Ta-Si-O glass ceramics with dendritic structure, which significantly improved the toughness and stability of the glass layer. The Ta-Si-O glass ceramics possesses the ability of sealing and arresting the microcracks, which can enhance the oxidation protective ability of the coating.     

Synthesis of ultra‐fine TaB2 nano powders by liquid phase method

Ultra‐fine TaB₂ powders were synthesized by a liquid phase method using tantalum ethoxide, boric acid and sucrose as the sources of tantalum, boron, and carbon. The TaB₂ precursor powders is a Ta–B–C–O network system, which were heat‐treated at lower temperature (1500°C) in normal argon atmosphere to obtain the TaB₂ powders. XRD confirms the presence of only hexagonal TaB₂, while EDS and XPS spectrums confirm the composition and element chemical states of TaB₂. The TEM images show a platelet shape of the TaB₂ powder and the monocrystal SAED pattern confirms the presence of hexagonal TaB₂. Particle size distribution curves show that particle size of the TaB₂ powders distributes in the range of 30‐160 nm, whose mean particle size is 106 nm.     

Synthesis,densification, and microstructure of TaC‐TaB2‐SiC ceramics

The usual way to prepare TaC‐TaB₂ ceramics by adding B₄C to TaC leads to formation of residual C, which degrades samples’ mechanical properties. To eliminate the residual C, we suggest incorporating Si together with B₄C into TaC ceramics, resulting in new ultrahigh‐temperature ceramics (TaC‐TaB₂‐SiC). Dense ceramics (>99%) with SiC volume fraction ranging from 15.86% to 41.04% were fabricated by reactive spark plasma sintering at 1900°C for 5 minutes. The formation of SiO₂‐based transient liquid phase was evidenced by the “film” in intermediate products, which can promote densification. The fine‐grained microstructure in final products was found to be associated with the in situ formed SiC, which impeded TaC and TaB₂ grains from coarsening by the pinning effect. Besides, ultrafine TaB₂ grains (~100 nm) produced during the reaction and then rearranged in liquid also contributed to grain refinement. Compared to TaC‐TaB₂(‐C) ceramics prepared from TaC and B₄C, the acquired composites exhibit better mechanical properties, due to their fine‐grained microstructures and the elimination of residual C.     

Low temperature synthesis of TaB2 nanorods by molten‐salt assisted borothermal reduction

TaB₂ powders were synthesized by a molten‐salt assisted borothermal reduction method at 900°C‐1000°C in flowing argon using Ta₂O₅ and amorphous B as starting materials. The results indicated that the presence of liquid phase, such as B₂O₃ and NaCl/KCl, accelerated the mass transfer of reactant species and resulted in the complete finish of the reaction at low temperatures. The obtained TaB₂ powders exhibited a flow‐like shape assembled from nanorods grow along [001] direction or c‐axis. The morphology of the synthesized TaB₂ powders could be tailored by the amount of B₂O₃ or NaCl/KCl.     

Low temperature synthesis of pure phase TaB2 powders and its oxidation protection modification behaviors for Si-based ceramic coating in dynamic oxidation environments

To reveal the generation mechanisms of the Ta-Si-O glass ceramics layer in dynamic oxidation environments, a 40?wt% TaB₂-SiC coating was prepared by liquid phase sintering method. To obtain pure phase TaB₂ powders at lower temperature (1500?°C), excessive B₂O₃ powders were added in raw materials to eliminate the TaC byproduct phase. The hexagonal pure phase TaB₂ powders own average particle size of about 386?nm. During the TGA dynamic oxidation tests, after the modification of 40?wt% TaB₂, the initial weight loss temperature of the sample delayed by about 48%, while the weight loss percentage and rate in fastest weight loss zone decreased by about 61% and 53%, respectively. During oxidation, the generated Ta-oxides were peeled and carried away by the formed fluid SiO₂ glass layer to form “Ta-oxides halation” at first, which results the dissolution of Ta-oxides in the SiO₂ glass, thus forming the Ta-Si-O glass ceramics with dendritic structure. With the spread of the SiO₂ glass layer and growth of the Ta-Si-O dendrite, the Ta-Si-O glass ceramics gradually cover on the surface of the SiO₂ glass layer, forming the structure of Ta-Si-O/SiO₂ double glass layer that is capable of sealing and arresting of microcracks.     

Synthesis of TaB2 powders by borothermal reduction

Borothermal reduction processes of Ta₂O₅ with boron under vacuum were investigated. Ta₂O₅ reacted with boron to form various borides (TaB₂, Ta₃B₄, and TaB), depending on the boron/Ta₂O₅ molar ratio and temperature. In order to prepare pure TaB₂ powders, two routes were developed. The first route was one‐step heat treatment at 1550°C. With boron/Ta₂O₅ molar ratio of 9.0, pure TaB₂ powders with strong agglomeration were synthesized by the first route, and the particle size and oxygen content were 0.7 μm and 0.9 wt%, respectively. The second route consisted of two‐step heat treatment at 800°C and 1550°C plus intermediate water washing. With lower boron/Ta₂O₅ molar ratio of 8.2, pure TaB₂ powders with less agglomeration and more uniform distribution were synthesized by the second route, and the particle size and oxygen content were 0.8 μm and 0.8 wt%, respectively. Moreover, the particle size similarity of TaB₂ powders by the two routes suggested that byproduct boron oxides, which were previously reported as the most important factor in promoting the coarsening of ZrB₂ powders by borothermal reduction, did not lead to the significant coarsening of TaB₂ powders.     

Influence of the ZrB2 content on the anti-oxidation ability of ZrB2-SiC coatings in aerobic environments with broad temperature range

ZrB₂-SiC coatings with different ZrB₂ contents were prepared by liquid phase sintering. The oxidation processes of coatings were explained according to TG results of ZrB₂-SiC coatings and powders tested from 298 K to 1773 K. Results show that, increasing ZrB₂ content made the weight of the samples changed from weight-loss of 10.04% to weight-gain of 0.14%, while the fastest weight-loss regions were narrowed, whose inflection points reduced from 1310℃ to 1050℃. Increasing ZrB₂ content made the relative oxygen permeability of the ZrB₂-SiC/SiC coatings reduced from 40%–60% to -10%-5%. Increasing ZrB₂ content enhanced high-temperature stability of coatings, making final weight of samples changed from weight-loss of 0.16% to weight-gain of 0.11% after oxidation at 1773 K for 200 h. The peeling and dispersion of Zr-oxides formed Zr-B-Si-O compound glass layer, presenting enhanced stability, dispersion strengthening and pinning effect of Zr-oxides, which were responsible for the excellent anti-oxidation protective effect of coatings in a broad temperature region.     

Oxidation of TaB2-SiC coatings prepared by spark plasma sintering and effect of pre-oxidation treatments

To suppress the oxidation of TaB₂-SiC coatings, the effects of pre-oxidation temperature on the oxygen hindering properties of TaB₂-SiC coatings were investigated to prepare TaB₂-SiC coatings with enhanced oxidation behavior. The addition of 40 wt% TaB₂ made the oxygen permeability of the coating decrease by 62.16%. However, excessive TaB₂ weakened the oxygen hindering ability of the coating due to the large ion complex ability of Ta⁵⁺. The pre-oxidation temperature at 1500 °C led to a homogeneous dispersion of Ta-oxide nanocrystal particles in the Ta-B-Si-O complex-phase glass layer. In contrast with the untreated samples, the active factor and inert factor values of the TaB₂-SiC coating after pre-oxidation treatment at 1500 °C decreased by 43.12% and 17.33%, respectively, which improved the dynamic stability of the coating during oxidation.     

Comparison of plasma sprayed NbB2-NbC coatings obtained by ex-situ and in-situ approaches

NbB₂-NbC composite coatings were fabricated on the surface of TC4 by plasma spraying NbB₂-NbC and Nb-B₄C composite powders. The microstructure and properties of the as-prepared coatings were investigated, and the reaction mechanism of the Nb-B₄C composite powder in the plasma jet and the formation mechanism of the NbB₂-NbC coatings were revealed. During the plasma spraying process, NbB₂-NbC composite powder underwent melting-depositing and no phase transformation occurred. The formation mechanism of the coating by plasma spraying Nb-B₄C composite powder was solid phase diffusion-reaction-melting-deposition, and NbB₂ and NbC were formed in situ by the solid phase diffusion reaction between Nb and B₄C in the plasma jet. The coating obtained by reactive plasma spraying Nb-B₄C composite powder has obvious lamellar structure, low porosity, high density, higher microhardness, good toughness, and better wear resistance compared with the coating prepared by NbB₂-NbC composite powder, which is attributed to the exothermic reaction between Nb and B₄C.     

Reducing the self-healing temperature of Ti2AlC MAX phase coating by substituting Al with Sn

In an effort to overcome the property degradation of Ti₂AlC MAX phase coating used in harsh environments, we fabricated a solid solution Ti₂(Al_0.6Sn_0.4)C coating with amount of Ti₅Sn₃ (20 wt.%) by a combined technique composing of magnetron sputtering and post-heat treatment. The cracks induced by Vickers indentation on coating surface were self-healed at 700 ℃, which is the lowest self-healing temperature among the Al-based MAX phase coatings till now. The structural evolution and kinetic diffusion revealed that the formation of SnO₂ is the key factor to achieve the crack self-healing at such a low temperature for Al-based MAX phase coatings. Additionally, the self-healed Ti₂(Al_0.6Sn_0.4)C coating exhibited better oxidation resistance compared to the unhealed one at 800 ℃. The results provide a novel and facile strategy to develop the protective MAX phase coatings with high performance at high temperature by partially substituting Al with Sn.     

Preparation and characterization of antistatic coatings with modified BaTiO3 powders as conductive fillers

The antistatic coatings were prepared by using rare earths modified BaTiO₃ powders as conductive fillers instead of the traditional metal additives. The composition and preparation technology of the antistatic coatings were researched. The effects of thinner, curing agent, dispersant, and conductive fillers on surface resistance and performances of the antistatic coatings were studied. The determined composition of the antistatic coatings was that of epoxy resin 10.00?g with curing agent 13%, BaTiO₃ powders 5%, dispersant 2%, and thinner 4?ml?g^?1. FTIR and SEM analyses illustrated that the dispersant agent and ultrasonic can make modified BaTiO₃ powders in the coatings to disperse completely. The parameters of antistatic coatings were as following: surface resistance is 1.18?×?10¹⁰?Ω, dry time is 6.0?h, solid content is 94.0%, stiffness is 0.419, viscosity is 255?s, adhesive force is 3, and flexibility is 2. The antistatic coatings were prepared by using rare earths modified BaTiO₃ powders will be prospective candidates for reducing static electricity.     

Influence of Al content and post-annealing on synthesis and mechanical properties of plasma sprayed Cr–Al–C composite coatings

Due to ultra-high temperature and short reaction time, it was very challenging to produce high purity MAX phase by plasma spraying. In this study, Cr–Al-graphite agglomerated powders with different Al additions (x = 0.2–1.5) was used to prepare Cr–Al–C composite coatings by atmospheric plasma spraying followed with annealing. Results showed that the as-sprayed coatings displayed typical lamellar structure, mainly composed of Cr–C binary carbides (Cr₇C₃ and Cr₂₃C₆) and residual Al. After annealing at 700 °C, the newly formed Cr₂AlC phase increased significantly in the coatings. The higher addition of Al, the more Cr₂AlC phase formed after annealing. The enhanced atomic diffusion, sufficient Al source and existence of (Cr, Al)C_x contributed to the formation of Cr₂AlC under annealing. Annealing treatment improved the hardness of the coating, but with the increase of Cr₂AlC phase content, the hardness decreased slightly. The Al content and post-annealing had a synergistic effect on the formation of Cr₂AlC phase in the sprayed coatings. This provided an effective route to control the Cr₂AlC content in sprayed Cr–Al–C composite coatings.     

Ablation behaviors of ZrC-TiC coatings prepared by vacuum plasma spray: Above 2000 °C

ZrC-TiC coatings were fabricated by vacuum plasma spray and their ablation resistance were evaluated and compared with ZrC-SiC coating by a plasma flame with a heat flux of 4.02 MW/m². The microstructure and phase compositions of the as-sprayed and ablated coatings were characterized and the function of TiC addition on the ablation resistance was investigated. The results showed that the ablation resistance of the ZrC-TiC coating was much better than that of the ZrC-SiC coating under the present ablation conditions. The decrease of surface temperatures with the increasing of TiC content were observed. (Zr, Ti)O₂ eutectic phase in liquid state was observed. The low vapor and decomposition pressures of TiO₂, combined with the formation of (Zr, Ti)O₂ liquid contributed to the excellent ablation resistance of the ZrC-TiC coating. This work affirmed that TiC could be an ideal addition to improve the ablation resistance of the ZrC coating in harsh environment above 2000 °C.     

Study on microstructure evolution and reaction mechanism of in-flight Ti–Si–C agglomerates during reactive plasma spraying using in situ water quenching

An in situ water quenching method was used to explore the microstructure evolution and reaction mechanism of Ti–Si–C agglomerates during reactive thermal spraying. The quenched powders exhibit a melting process from outside to inside and small particles to large ones, forming complete droplets with increasing spray distance. The formation of the liquid phase is the basis of the reaction; once the liquid phase is formed, reactions begin to form new phases (TiC and Ti₅Si₃). The melting point of the new phase and the temperature of the droplet determine the formation mechanism and morphology of the new phase in the coating. As the melting point is higher than the droplet temperature, the new phase, TiC, grows to a large submicron size in flight. When the melting point is lower than the droplet temperature, like Ti₅Si₃, it dissolves into the liquid phase and re-precipitates in nanometer size at impact. Moreover, the droplet surface absorbed and dissolved O element from the atmosphere, and thus Ti₃O coexisted with Ti₅Si₃ in lamellae of coatings.     

High-temperature oxidation resistance of spent MoSi2 modified ZrB2–SiC–MoSi2 coatings prepared by spark plasma sintering

The spent MoSi₂ modified ZrB₂–SiC–MoSi₂ coatings were prepared on carbon matrixes by spark plasma sintering. A continuous metallurgical bonding was formed at the interface between the coating and matrix, and no obvious defects such as pores and cracks were observed inside. The effects of spent MoSi₂ content and trace doping in the spent powder on the oxidation behavior of the coatings in air at 1700 °C were investigated. During the active oxidation stage, the spent MoSi₂ promoted the densification of the coating and enhanced the structural oxygen barrier properties. With the increase of service time, during the inert oxidation stage, doping an appropriate amount of spent MoSi₂ helped to increase the fluidity of the rich-SiO₂ protective layer so that the Zr oxides fully dispersed in the generated Zr–B–Si–O–Al multiphase glass layer, which could impede the penetration of oxygen and enhance the oxidation protection efficiency. However, excessive spent MoSi₂ exacerbated the volatilization of gas by-products, forming pores and cracks in the glass layer and rising the oxidation loss. When the content of spent MoSi₂ was 20 vol%, the glass layer is dense and uniform, with few defects and the best oxygen resistance property. Moreover, compared with commercial powders, spent MoSi₂ contained Al₂O₃ and SiO₂. Al₂O₃ had an excellent modification effect, while SiO₂ glass can promote liquid phase sintering and seal the defects in the coatings. By adding spent MoSi₂, the modified ZrB₂–SiC–MoSi₂ composite coatings could inhibit the formation of defects and improve the dynamic stability of the coatings effectively.     

Microstructural formation and evolution of near-eutectic Al2O3–ZrO2 powders prepared by a rapid cooling technique: An exploration on coupled eutectic growth range

Regulation of eutectic compositions is a difficult challenge due to the limited formation range of this microstructure. Al₂O₃–ZrO₂ powders were successfully prepared through a modified high-temperature melt-assisted air atomization technique. Micron-sized raw powders of α-Al₂O₃ and m-ZrO₂ were used to regulate the temperature of system and the composition of final products. The high-temperature melt of 3500 K (3227 °C) was obtained based on the exothermic reaction between Al and O₂, which was subsequently atomized and quenched to form composite powders. The phase composition and microstructure of the as-prepared powders were characterized by X-Ray diffraction and scanning electron microscopy. Eutectic colonies with nanolamellae (62–79 nm) were observed in the as-prepared Al₂O₃-(28–58 mol%) ZrO₂ powders, with growth rates ranging from 1.9 to 2.8 mm s⁻¹. Importantly, the powders with ZrO₂ contents of 38 mol% and 42 mol% showed complete eutectic microstructures without primary dendrites, which indicated an extended coupled growth range deviating by ∼4.9 mol% from the eutectic composition (C_E). The microstructural formation mechanism was discussed based on solidification behavior analysis. Furthermore, the evolution of the microstructure and phase in Al₂O₃-42 mol% ZrO₂ powders was investigated. This work aims to develop a novel strategy for preparing oxide powders with eutectic structures and provide a reference for the composition regulation of eutectics.     

Phase and structure formation mechanisms of SHS synthesized composite coatings

High performance composite coatings were synthesized by self-propagation high-temperature synthesis followed by gravitational-separation process based on thermite reaction. The phase, structure and composition of generated composite coatings were investigated, and formation mechanism was studied by thermodynamic analysis. Results showed that phase composition of Al-Fe₂O₃ reaction system consisted of Al₂O₃, Fe and FeAl₂O₄. In Al-Fe₂O₃/Al-Cr₂O₃ composite reaction system, Fe-Cr alloy was formed and FeAl₂O₄ phase disappeared, which could improve the corrosion resistance of composite coatings. Furthermore, the addition of SiO₂ in SHS reaction favored the formation of low-melting point phase Al₂O₃·SiO₂, which filled into voids of Al₂O₃ dendrites and reduced the porosity of composite coatings, thus improving their strength and densification level. Moreover, the generated transition structure in different reaction systems could buffer the residual stress to promote the binding between the composite coating and steel pipe.     

Effect of different nucleating agent ratios on the crystallization and properties of MAS glass ceramics

The effects of different kinds of nucleating agents on crystallization, microstructure and performances of Magnesium Aluminosilicate (MgO-Al₂O₃-SiO₂, MAS) glass-ceramics which were fabricated by melting method in this study. Also, this paper systematically investigated the mechanism of glass stability, crystallization kinetics and element distribution of MAS glass-ceramics. Herein, we used three kinds of nucleating agents, which was TiO₂, ZrO₂ and composite nucleating agent (TiO₂/ZrO₂). The results showed after the doping of nucleating agent, the content of α-cordierite was increased, the stability and crystallization kinetics of glass was changed, the precipitated crystal phase was finer and more compact. Wherein, the sample with composite nucleating agents (TiO₂, ZrO₂) has the best performance due to the highest contents of α - cordierite, uniform distribution of elements without agglomeration in the crystal phase and the most compact structure, whose Vickers hardness and bending strength can reach 9.70 GPa and 312 MPa, respectively.     

Synthesis of well‐dispersed β‐Si3N4 with equiaxed structures using carbothermal reduction–nitridation method

Well‐dispersed β‐Si₃N₄ powders with a novel equiaxed structure and eminent crystal integrity were prepared by carbothermal reduction–nitridation (CRN) strategy with the assistance of CaF₂ additive. The growth mechanism of Si₃N₄ particles in the CRN process was elucidated. It is proposed that the liquid phase formed by SiO₂ and CaF₂ additive is crucial to the formation of equiaxed β‐Si₃N₄, and with an appropriate content of CaF₂, Si₃N₄ powders with pure β phase, superior dispersity and crystal integrity can be obtained.