TEM characterization and reaction mechanism of composite coating fabricated by plasma spraying Nb–SiC composite powder

Niobium carbide composite coatings with Nb₂C, NbC, Nb₃Si as the main phases were prepared in situ on the surface of TC4 titanium alloy by plasma spraying Nb–SiC composite powder. The microstructure of the coating was characterized in detail by TEM, and the reaction mechanism of Nb–SiC was revealed. Sub-micron and nano-scale NbC grains dispersed in Nb₃Si region, nano-Nb/Nb₃Si cellular eutectic region, and equiaxed Nb₂C nanograins region were formed in the coating. The research results show that Nb and SiC reacted firstly to form cubic NbC and Nb₃Si phases during the plasma spraying process. Then, NbC with a higher melting point took the lead in crystallization during the cooling process of the coating, forming sub-micron and nano-scale NbC granular fine grains. Nb₃Si with a lower melting point crystallized around the sub-micron and nano-scale NbC granular fine grains in the subsequent cooling process. In the plasma spraying process, the molten droplets formed Nb/Nb₃Si cellular eutectic structure under large temperature gradient and extremely fast cooling rate. The remaining Nb in the raw material powder formed a diffusion couple with NbC to generate fine and dispersed nano-equiaxed Nb₂C with cubic structure. The present investigation provides a reference for the reaction synthesis of advanced nanocomposites using Nb–SiC system.     

Nanostructured ceramic composite coating prepared by reactive plasma spraying micro-sized Al–Fe2O3 composite powders

Nanostructured ceramic matrix composite coating was prepared in-situ by reactive plasma spraying micro-sized Al-Fe₂O₃ composite powders. The microstructure, toughness and Vickers hardness of these coatings were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and mechanical tests. The results indicated that the coating exhibited nanostructures which consisted of FeAl₂O₄, Al₂O₃, Fe (or Fe solid solution) and a little FeAl. The composite coating showed significantly higher toughness and wear resistance than the conventional Al₂O₃ coating.     

Ablation resistance of HfC–SiC coating prepared by supersonic atmospheric plasma spraying for SiC-coated C/C composites

In order to improve the ablation properties of carbon/carbon composites, HfC–SiC coating was deposited on the surface of SiC-coated C/C composites by supersonic atmospheric plasma spraying. The morphology and microstructure of HfC–SiC coating were characterized by SEM and XRD. The ablation resistance test was carried out by oxyacetylene torch. The results show that the structure of coating is dense and the as-prepared HfC–SiC coating can protect the C/C composites against ablation. After ablation for 30 s, the linear ablation rate and mass ablation rate of the coating are −0.44 μm/s and 0.18 mg/s, respectively. In the ablation center region, a Hf–Si–O compound oxide layer is generated on the surface of HfC–SiC coating, which is conducive to protecting the C/C composites from ablation. With the ablation time increasing to 60 s, the linear ablation rate and mass ablation rate are changed to −0.38 μm/s and 0.26 mg/s, respectively. Meanwhile, the thickness of the outer Hf–Si–O compound layer also increases.     

Effect of temperature on the structure and mechanical properties of SiC–TiB2 composite ceramics by solid-phase spark plasma sintering

SiC composite ceramics have good mechanical properties. In this study, the effect of temperature on the microstructure and mechanical properties of SiC–TiB₂ composite ceramics by solid-phase spark plasma sintering (SPS) was investigated. SiC–TiB₂ composite ceramics were prepared by SPS method with graphite powder as sintering additive and kept at 1700 °C, 1750 °C, 1800 °C and 50 MPa for 10min.The experimental results show that the proper TiB₂ addition can obviously increase the mechanical properties of SiC–TiB₂ composite ceramics. Higher sintering temperature results in the aggregation and growth of second-phase TiB₂ grains, which decreases the mechanical properties of SiC–TiB₂ composite ceramics. Good mechanical properties were obtained at 1750 °C, with a density of 97.3%, Vickers hardness of 26.68 GPa, bending strength of 380 MPa and fracture toughness of 5.16 MPa m^1/2.     

Dynamic response of the TiB2–SiC/B4C composite ceramic under dynamic loading

TiB₂–SiC/B₄C composite ceramics have application as lightweight armor ceramic material, and therefore, it is particularly important to research their dynamic mechanical properties and fracture behavior under high strain rates. In the present research, the dynamic compression strength and Hugoniot elastic limit (HEL) of these composite ceramics were determined by split Hopkinson pressure bar technology and plate impact equipment, respectively. The experimental investigation results illustrated that the composite ceramic exhibited outstanding dynamic compression strength (2750 MPa) and displayed a remarkable strain-rate strengthening effect. The HEL was calculated as 18.49 GPa, and the commonly used P–μ form of the Hugoniot curve was derived from the calculated data of the shock velocity and particle velocity. The dynamic fracture morphologies of the TiB₂–SiC/B₄C composite ceramic and monolithic B₄C ceramic (correlation data) at macro and micro scales were observed and analyzed in detail. The dynamic fracture mechanisms were summarized in-depth into the following three points. First, the strengthening mechanism offered by the in situ second phases could significantly improve the comprehensive property. Second, the mixed-inter/transgranular fracture mode was conducive to energy absorption. Third, the anchoring effect and thermal expansion coefficient mismatch contributed to the energy dissipation and fracture toughness improvement. This work provides the foundation for the application and development of lightweight armor ceramic materials.     

Microstructures and mechanical properties of B4C–SiC and B4C–SiC–TiB2 ceramic composites fabricated by hot pressing

Boron carbide (B₄C) ceramic composites with excellent mechanical properties were fabricated by hot-pressing using B₄C, silicon carbide (SiC), titanium boride (TiB₂), and magnesium aluminum silicate (MAS) as raw materials. The influences of SiC and TiB₂ content on the microstructural evolution and mechanical properties of the composites were systematically investigated. The mechanism by which MAS promotes the sintering process of composites was also investigated. MAS exists in composites in the form of amorphous phase. It can effectively remove the oxide layer from the surface of ceramic particles during the high temperature sintering process. The typical values of relative density, hardness, bending strength, and fracture toughness of B₄C–SiC–TiB₂ composites are 99.6%, 32.61 GPa, 434 MPa, and 6.20 MPa m^1/2, respectively. Based on the microstructure observations and finite element modeling, the operative toughening mechanism is mainly attributed to the crack deflection along the grain boundary, which results from the residual stress field generated by the thermal expansion mismatch between B₄C and TiB₂ phase.     

Multidimensional nano-graphite constructed TiB2–SiC–B4C composite ceramics with the integration of mechanical and electromagnetic properties

The goal of this study is to create structure-functional integrated ceramic matrix composites with high structural strength and electromagnetic absorbing properties. The multidimensional nano-graphite (1-Dimensional rod-like nano-graphite, 0-Dimensional dispersive nano-graphite, and 2-Dimensional lamellar nano-graphite) were employed to construct TiB₂–SiC–B₄C composites via high-energy ball milling, vacuum filtration, and reactive SPS sintering. The microstructure of multidimensional nano-graphite was investigated using XRD and HRTEM and determined to be a crystal-amorphous coexisting. Furthermore, solid solution reaction and interfacial evolution are confirmed as the primary influence on the microstructure of TiB₂–SiC–B₄C composite. A significant improvement occurs on the flexural strength (647.6 MPa) and bending toughness (5.1 MPa m^1/2). Meanwhile, the multi-dimensional nano-graphite gives the TiB₂–SiC–B₄C composite the loss ability of electromagnetic waves, and the matching thickness of the 10 vol% sample is 2.4 mm and the absorption range is 10.4–11.3 GHz.     

Microstructure and properties of ZrB2–SiC composites prepared by spark plasma sintering using TaSi2 as sintering additive

ZrB₂–SiC composites were fabricated by spark plasma sintering (SPS) using TaSi₂ as sintering additive. The volume content of SiC was in a range of 10–30% and that of TaSi₂ was 10–20% in the initial compositions. The composites could be densified at 1600 °C and the core–shell structure with the core being ZrB₂ and the shell containing both Ta and Zr as (Zr,Ta)B₂ appeared in the samples. When the sintering temperature was increased up to 1800 °C, only (Zr,Ta)B₂ and SiC phases could be detected in the samples and the core–shell structure disappeared. Generally, the composites with core–shell structure and fine-grained microstructure showed the higher electrical conductivity and Vickers hardness. The completely solid soluted composites with coarse-grained microstructure had the higher thermal conductivity and Young's modulus.     

Mechanical properties and microstructure evolution of pressureless-sintered B4C–SiC ceramic composite with CeO2 additive

Boron carbide ceramic composites (B₄C)-silicon carbide (SiC) with the cerium oxide (CeO₂) additive, which was varied from 0 wt% to 9 wt%, were prepared by pressureless sintering at 2150 °C for 60 min. The effect of CeO₂ additive content on the microstructure and mechanical properties of the B₄C–SiC ceramic composites was investigated in detail. In-situ synthesised cerium hexaboride (CeB₆) was identified in the B₄C–SiC ceramic composites. B-rich transition zones (such as B_38.22C₆, B_51.02C_1.82) were formed between the B₄C and CeB₆ grains, which introduced local lattice distortion to increase the sintering driving force. The thermal conductivity coefficient of CeB₆ was higher than that of B₄C, which benefited the delivery of heat quantity and helped achieve a highly dense and uniform sintered body. When the CeO₂ additive was excessively increased (more than 5 wt%), the CeB₆ grains had a large grain size and exhibited an increase in the amount of generated carbon monoxide (CO) gas, which led to an increase in the porosity of the B₄C–SiC ceramic composites and decrease in the mechanical properties. The optimum values of the relative density, Vickers hardness, flexural strength, and fracture toughness of the B₄C–SiC ceramic composite with 5 wt% CeO₂ additive were 96.42%, 32.21 GPa, 380 MPa, and 4.32 MPa m^1/2, respectively.     

High-performance B4C–TiB2–SiC composites with tuneable properties fabricated by reactive hot pressing

In this work, we systematically studied the effects of powder characteristics (B₄C, TiC and Si powders) on the existential form of toughening phases (SiC and TiB₂) as well as the overall microstructure and properties of B₄C–TiB₂–SiC composites fabricated by reactive hot pressing. The particle size of the TiC powder plays a largely determining role in the development of novel toughening phases, the TiB₂–SiC composite structure, that are formed in the B₄C matrix, while the Si particle size affects the agglomerate level of the SiC phase. The TiB₂–SiC composite structure and SiC agglomerates enhance the fracture toughness, but decrease the flexural strength. Both the microstructure and mechanical properties of B₄C–TiB₂–SiC composites can be effectively tuned by regulating the combinations of the particle sizes of the starting powders. The B₄C–TiB₂–SiC composites demonstrate flexural strength, fracture toughness and Vickers hardness in the respective range of 567–632 MPa, 5.11–6.38 MPa m^1/2, and 34.8–35.6 GPa.     

Long-sized rods of Al2O3–SiC–TiB2 ceramic composite material obtained by SHS-extrusion: Microstructure,X-ray analysis and properties

Long-sized rods of Al₂O₃–SiC–TiB₂ ceramic composite material were obtained by SHS-extrusion. The material was synthesized by self-propagating high-temperature synthesis (SHS) followed by high-temperature shear deformation. Ceramic samples app up to 465 mm in length and 5 mm in diameter were obtained. According to the results of XRD and SEM the obtained rods have a composite structure. The matrix is Al₂O₃ with distributed titanium diboride and silicon carbide particles. A uniform phase distribution was observed along the entire length of the rod. The microhardness of the matrix was 25–26 GPa, that of the dispersion-strengthening phases - 32–34 GPa. Heat resistance tests showed that during heat treatment at T = 1000 °C for 21 h, the sample specific weight gain and its real rate were 8.3 g/m² and 1 g/(m²∙h), respectively. The density, hardness and electrical resistivity of the samples obtained in this work were 3.27 g/cm³, 19.5 GPa, 3.1∙10⁻⁵ Ohm∙m, respectively.     

Structure and properties of nanostructured Fe(AlCr)2O4–Cr–(AlCr)2O3–Fe composite coating prepared by plasma spraying

In-situ nanostructured Fe(AlCr)₂O₄-based composite coating (FACr_52.5 coating) was prepared by reactive plasma spraying with micro-sized Al–Fe₂O₃–Cr₂O₃ powders. The microstructure, toughness and Vickers hardness, and adhesive strength of the coating were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and mechanical tests. The results indicated that the interlamellar spacing of the FACr_52.5 coating is only 1 μm. The coating exhibited nanostructured microstructure. The in-situ Cr (20 nm) and Fe (50–200 nm) particles were uniformly distributed in an Fe(AlCr)₂O₄ matrix, while the grain size of the Fe(AlCr)₂O₄ matrix is about 60 nm. The FACr_52.5 composite nano-coating exhibited much higher hardness, better wear resistance, stronger adhesive strength and toughness as compared to those of the composite nano-coating sprayed with Fe₂O₃–Al powders. Excellent mechanical properties of the FACr_52.5 coating were attributed to the uniform distribution of the in-situ nano-sized Cr particles in the coating matrix.     

Synthesis of ZrB2–SiC–ZrC nanocomposite by spark plasma in ZrSiO4/B2O3/C/Mg system

A novel reduction process by spark plasma is proposed for the synthesis of homogeneous intermixed ZrB₂–SiC–ZrC nanostructure powder using ZrSiO₄, B₂O_3, C and Mg as starting materials. In the present research, the reaction mechanisms were investigated by the displacement–temperature–time (DTT), the displacement rate–temperature and the displacement rate–time diagrams which were obtained during spark plasma cycles. The synthesis process of the composite was completed after 15 min at temperature of 400 °C. The total shrinkage of 5.5 mm was observed due to the rearrangement and reaction phenomenon. The main shrinkage occurs within a period of less than 1 min at 400 °C similar to the self propagating high temperature synthesis. Finally using Rietveld method, the mean crystallites sizes of about 26, 25 and 22 nm were calculated for ZrB₂, SiC and ZrC phases, respectively.     

Crucible-free fabrication of ZrB2–SiC composite ceramics with ultra-fine eutectic microstructure by laser surface zone-melting: The selection of lasers

Here we report the crucible-free fabrication of ZrB₂–SiC composite ceramics by laser surface zone-melting (LSZM, a rapid solidification method) using Nd: YAG laser, ytterbium-doped fiber laser, and CO₂ laser, respectively. Based on the comparative investigation of the thickness and the microstructure of the laser processed zone, this work highlights that the CO₂ laser is the most suitable laser resource for the fabrication of highly dense and large-scale ZrB₂–SiC composite ceramics with the eutectic colonies consisting of maze-shaped eutectic microstructure by LSZM. The high temperature gradient up to ～4 × 10⁵ K/m using CO₂ laser extremely reduces the phase size down to submicron-scale, which contributes to the high relative density (～99%), Vickers hardness (24.18 GPa) and fracture toughness (7.4 MPa m^1/2) of the ZrB₂–SiC composite ceramics. This work demonstrates for the first time that LSZM is a promising method for the rapid fabrication of dense ZrB₂ based ultra-high temperature bulk ceramics.     

Preparation and characterization of ultrafine ZrB2–SiC composite powders by a combined sol–gel and microwave boro/carbothermal reduction method

A combined sol–gel and microwave boro/carbothermal reduction technique was investigated and used to synthesize ultrafine ZrB₂–SiC composite powders from raw starting materials of zirconium oxychloride, boric acid, tetraethoxysilane and glucose. The effects of reaction temperature, molar ratios of n(B)/n(Zr) and n(C)/n(Zr+Si) on the synthesis of ultrafine ZrB₂–SiC composite powders were studied. The results showed that the optimum molar ratios of n(B)/n(Zr) and n(C)/n(Zr+Si) for the preparation of phase pure ultrafine ZrB₂–SiC composite powders were 2.5 and 8.0, respectively, and the firing temperature required was 1300 °C. This temperature was 200 °C lower than that require by using the conventional boro/carbothermal reduction method. Microstructures and phase morphologies of as-prepared ultrafine ZrB₂–SiC composite powders were examined by field emission-scanning electron microscopy (FE-SEM) and transmission electron microscope (TEM), showing that SiC grains were formed evenly among the ZrB₂ grains, and the grain sizes of ZrB₂ in the samples prepared at 1300 °C for 3 h were about 1–2 μm. The average crystalline sizes of these two phases in the as-prepared samples were calculated by using the Scherrer equation as about 58 and 27 nm, respectively.     

Microstructure and mechanical behaviour of transient liquid phase spark plasma sintered B4C–SiC–TiB2 composites from a B4C–TiSi2 system

Almost fully-dense B₄C–SiC–TiB₂ composites with a high combination of strength and toughness were prepared through in situ reactive spark plasma sintering using B₄C and TiSi₂ as raw materials. The densification, microstructure, mechanical properties, reaction, and toughening mechanisms were explored. TiSi₂ was confirmed as a reactive sintering additive to promote densification via transient liquid-phase sintering. Specifically, Si formed via the reaction between B₄C and TiSi₂ that served as a transient component contributed to densification when it melted and then reacted with C to yield more SiC. Toughening mechanisms, including crack deflection, branching and bridging, could be observed due to the residual stresses induced by the thermoelastic mismatches. Particularly, the introduced SiC–TiB₂ agglomerates composed of interlocked SiC and TiB₂ played a critical role in improving toughness. Accordingly, the B₄C–SiC–TiB₂ composite created with B₄C-16 wt% TiSi₂ achieved excellent mechanical performance, containing a Vickers hardness of 33.5 GPa, a flexural strength of 608.7 MPa and a fracture toughness of 6.43 MPa m^1/2.     

The preparation of ZrO2–ZrC ceramic microspheres by internal gelation process with sucrose as a carbon source

ZrO₂–ZrC (ZrCO) ceramic microspheres were fabricated by internal gelation and carbothermal reduction. The effect of various carbon sources on the stability of the broth was investigated, the carbon sources which have little effect on the stability of the broth were chosen as the carbon sources for carbothermal reduction, and the component distribution and microstructure of the sintered ZrCO microspheres were analyzed. The results indicate that the broth was stable at room temperature when sucrose was used as the carbon source, and crack-free ZrC–ZrO₂ ceramic microspheres with good sphericity and uniform distribution of ZrC and ZrO₂ could be successfully prepared. Moreover, the ZrC–ZrO₂ ceramic microspheres fabricated have no obvious pores and free carbon.     

Comprehensive study on corrosion protection properties of Al2O3, Cr2O3 and Al2O3–Cr2O3 ceramic coatings deposited by plasma spraying on carbon steel

In this study, individual Al₂O₃ and Cr₂O₃ coatings and Cr₂O₃-25, 50, 75 wt% Al₂O₃ composite coatings were applied on carbon steel by atmospheric plasma spraying method. Corrosion experiments were performed on as-sprayed and epoxy resin sealed coatings including potentiodynamic polarization, electrochemical impedance spectroscopy and long-term immersion in 3.5 wt% NaCl solution. Phase composition and microstructure of the coatings were investigated by x-ray diffraction, optical microscopy and scanning electron microscopy, before and after the corrosion experiment. The results showed that the Cr₂O₃ coating exhibited the best corrosion resistance, due to the densest microstructure and highest adhesion strength. The Cr₂O₃-25 wt% Al₂O₃ coating had the highest interconnected porosities and thus had the least corrosion resistance compared to other coatings. In general, the as-sprayed coatings induced a maximum increase of 3.93 times the polarization resistance (R_p) in the polarization experiment and a 3.5 times increase in the charge transfer resistance (R_ct) in the EIS experiment, which was not significant. Stresses caused by increased volume of corrosion products in the coating-substrate interface resulted in the spallation of Cr₂O₃-25, 50 wt% Al₂O₃ coatings from the substrate over long-term of immersion. The adhesion strength of the coatings was a determining criterion for the long-term durability of the coatings. The sealing treatment resulted in a significant increase in R_p and R_ct.     

Synthesis of a (Ti,Mo)B2 coating by electro-codeposition in NaCl–KCl–AlCl3–MoO3 melt containing TiB2 nanoparticles

This paper demonstrates the availability of electro-codeposition (i.e., the simultaneous occurrence of electrophoretic deposition of nanoparticles and electrochemical deposition of metal ions) in molten salts without the assistance of stirring of the bath. In molten NaCl–KCl–AlCl₃–MoO₃ system containing TiB₂ nanoparticles at 710°C, the electro-codeposition of TiB₂ nanoparticles and Mo(VI) ions has been achieved, and a (Ti, Mo)B₂ coating has been prepared.     

Improvement of the properties of AZ91D magnesium alloy by treatment with a molten AlCl3–NaCl salt to form an Mg–Al intermetallic surface layer

A process for surface modification of AZ91D magnesium alloy has been carried out in AlCl₃–NaCl molten salts melt at 573 K for 7 h. The process was used to form a diffusion alloying layer on the surface of the sample to improve corrosion resistance. The diffusion alloying layer was mainly composed of Mg–Al intermetallic compounds. The continued immersion test in 5% NaCl solutions displayed that molten salt diffusion-treated specimen had better corrosion resistance compared to the untreated AZ91D specimen. The polarization test indicated that the Mg–Al intermetallic compounds of the layer were an effective corrosion barrier to decrease the corrosion rate for AZ91D magnesium alloy when exposed to 3.5% NaCl solutions. The microhardness values of the Mg–Al intermetallic compounds were higher than those of the substrate.