Combustion synthesis of TiC–NiAl composite by induction heating

The aim of this work was to develop a new process for the synthesis of TiC and NiAl/TiC composite in which the combustion reaction was ignited using a high frequency induction heater. High density, two-layer TiC–NiAl composites were also produced using this process. Temperature profiles during synthesis were measured with an IR thermometer and a high resolution thermal image camera was used to monitor the reaction process. Phase transformation was investigated using XRD and SEM was used to characterize the microstructure of the synthesized composites. The mechanical properties of the products were evaluated by measuring hardness. The results show that the reaction was complete and that stoichiometric products of NiAl and TiC were produced. The properties of NiAl/TiC composites were found to be functions of composition and processing parameters. The reaction mechanism was analyzed using temperature monitoring, thermodynamic analysis and microstructure investigation.     

Facile synthesis of Ti3SiC2 powder by high energy ball-milling and vacuum pressureless heat-treating process from Ti–TiC–SiC–Al powder mixtures

In this article, Ti/TiC/SiC/Al powder mixtures with molar ratios of 4:1:2:0.2 were high energy ball-milled, compacted, and heated in vacuum with various schedules, in order to reveal the effects of temperature, soaking time, thickness of the compacts, and carbon content on the purity of the sintered compacts. X-ray diffraction and scanning electron microscopy were employed to investigate the phase purity, particle size and morphology of the synthesized samples. It was found that the Ti₃SiC₂ content nearly reached 100 wt.% on the surface layer of the sintered compacts prepared in the temperature range from 1350 °C to 1400 °C for 1 h. Powder containing 91 wt.% Ti₃SiC₂ was successfully synthesized by heating 6 mm green compacts of 4Ti/1TiC/2SiC/0.2Al at 1380 °C for 1 h in vacuum. The excessive carbon content failed to improve the purity of Ti₃SiC₂ powder. TiC phase was the main impurity in the formation process of Ti₃SiC₂.     

Enhanced performance of low-carbon MgO–C refractories with nano-sized ZrO2–Al2O3 composite powder

Low-carbon MgO–C refractories are facing great challenges with severe thermal shock and slag corrosion in service. Here, a new approach, based on the incorporation of nano-sized ZrO₂–Al₂O₃ composite powder, is proposed to enhance the thermal shock resistance and slag resistance of such refractories in this work. The results showed that addition of ZrO₂–Al₂O₃ composite powder was helpful for improving their comprehensive performances. Particularly, the thermal shock resistance of the specimen containing 0.5 wt% composite powder was enhanced significantly which was related to the transformation toughening of zirconia and in-situ formation of more spinel phases in the matrix; also, the slag resistance of the corresponding specimen was significantly improved, which was attributed to the optimization of pore structure and formation of much thicker MgO dense layer.     

In situ synthesis of NbB2–Al2O3 nanocomposite powder by mechanochemical processing

This research is based on the production of NbB₂–Al₂O₃ nanocomposite powder using mechanochemical processing. For this purpose, a mixture of niobium, aluminium and boron oxide powders was subjected to high-energy ball milling. The structural evaluation of powder particles after different milling times was conducted by the X-ray diffractometry (XRD), scanning electron microscopy, and transmission electron microscopy. The results showed that during ball milling the Nb/Al/B₂O₃ reacted with a combustion mode producing NbB₂–Al₂O₃ nanocomposite. The XRD analyses exhibited that the NbB₂–Al₂O₃ nanocomposite was formed after 10?h milling time and increasing milling time up to 30?h had no significant effect other than refining the crystallite size. In the final stage of milling, the crystallite sizes of NbB₂ and Al₂O₃ were estimated to be less than 50?nm.     

Microstructure and mechanical properties of Al2O3/Cu joints brazed with Ag–Cu–Ti+Zn composite fillers

In this study, Al₂O₃ ceramic and Cu bars were brazed with newly designed Ag–Cu–Ti(ABA)+Zn composite fillers. Systematic analysis of the microstructure of the brazed joints indicated that the volatilization of Zn atoms during the brazing process could promote the spreading of liquid brazing fillers on the surface of the Al₂O₃ ceramic, resulting in a uniform dendritic interfacial structure. The typical interfacial structure was an Al₂O₃/TiO/(Cu, Al)₃Ti₃O+Ag(s, s)/Cu interface. Notably, the tensile strength was improved to 20.89 MPa for Al₂O₃/Cu joint brazed with ABA+Zn composite fillers at 900 °C for 20 min, approximately 67.6% higher than the sample brazed without Zn foil. In this case, the fracture model was straight and sharp-angled inside the Al₂O₃ ceramic. In addition, the joint strength decreased with increased brazing temperatures from 900 to 940 °C.     

Mechanism and microstructural evolution of combustion synthesis of ZrB2–Al2O3 composite powders

Mechanism of combustion synthesis (CS) of ZrB₂–Al₂O₃ composite powders was systematically analyzed by a combustion front quenching method (CFQM). The microstructural evolution during the CS process was investigated by field-emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectrometer (EDS). The combustion temperature and wave velocity were measured by the data acquisition system. Moreover, the phase constituents of the final product were examined by X-ray diffraction (XRD). The thermal behaviors of the stoichiometic powders under the thermal exposure were characterized using differential scanning calorimetry (DSC) and thermogravimetric (TG). The results showed that the combustion reaction started from the melting of the B₂O₃ and Al particles, which was followed by the formation of ZrO₂–B₂O₃–Al solution. The ignition temperature of this system was determined to be around 800 °C. B and Al₂O₃ were then precipitated from the solution. As the CS reaction proceeded, Zr and Al₂O₃ were produced by the reaction between ZrO₂ particles and Al and precipitated from the solution. ZrB₂ could then be formed by the direct reaction between Zr and B. Finally, the ZrB₂–Al₂O₃ composite powders were obtained. Furthermore, a model corresponding to the dissolution–precipitation mechanism was proposed.     

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.     

Sintering,mechanical, electrical and oxidation properties of ceramic intermetallic TiC–Ti3Al composites obtained from nano-TiC particles

The paper discusses the development of a new material system for interconnect application in Solid Oxide Fuel Cells (SOFC) based on TiC–Ti₃Al. Nano-sized TiC powders utilized in this research were synthesized using carbon coated TiO₂ precursors from a patented process. The pressureless sintering of TiC–Ti₃Al in a vacuum was applied at temperatures between 1100 °C and 1500 °C and content of Ti₃Al was varied in the range of 10–40 wt%. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used for phase evaluation and sintering behavior. Relative density increased markedly with increasing sintering temperature because of grain growth and formation of the Ti₃AlC₂ secondary phase. Dense products (>95% TD) were prepared from nanosized TiC powders with 10 and 20 wt% Ti₃Al, but with about 8 to 10% porosity for 30 and 40 wt% Ti₃Al. The mechanical properties were determined from Vickers hardness and fracture toughness calculations. Vickers hardness decreased and fracture toughness increased with increasing Ti₃Al content. The electrical conductivity and oxidation behavior of TiC–Ti₃Al composites were investigated to evaluate the feasibility for SOFC interconnect application. The electrical conductivity measurements in the air at 800 °C for 100 h were made using the Kelvin 4-wire method.     

Effects of additives on combustion synthesis of Al2O3–TiB2 ceramic composite

Al₂O₃, SiC and kaolin were employed as additives in combustion synthesizing Al₂O₃–TiB₂ ceramic composite. Effects of the additives on adiabatic temperature, combustion wave velocity, volume change and composite density were studied, and bending strength of the synthesized ceramics was evaluated. By theoretical calculation, the adiabatic temperature of Al–TiO₂–H₃BO₃ system is 2314.85 °C and decreases with increasing the additive addition. With Al₂O₃ addition, the phases presented in the ceramic composite are unchanged, and the phases of SiC and 3Al₂O₃·2SiO₂ emerges when SiC and kaolin are added. The addition of the additives results in a refined TiB₂ particulate size and reduces combustion wave velocity. The highest density is achieved with the addition of kaolin from 10 to 30 wt.% making the volume change from ?4.6 to ?1.2%. The bending strength of the TiB₂–Al₂O₃ composite is improved eight times with the addition of 30 wt.% kaolin.     

Microstructures and enhanced mechanical properties of (Al3Ti+Al2O3)/Al–Si composites with co-continuous network structure prepared by pressure infiltration

The (Al₃Ti + Al₂O₃)/Al–Si composites with three-dimensional co-continuous network structures are fabricated by a pore-forming agent and the pressure infiltration technique. The effect of the Al₃Ti content on the mechanical and wear properties of the developed composites is investigated. The Al₂O₃ (alumina) formation, fracture, and wear mechanisms of the composites are also analyzed. The results demonstrate that the granular Al₂O₃ particles scatter around Al₃Ti phases which are synthesized in-situ during the sintering process. The 20 vol.% (Al₃Ti + Al₂O₃)/Al–Si composites possess the optimal mechanical properties, i.e., compressive and flexural strength of 585 MPa and 489 MPa, respectively, which are 64.8% and 46.0% higher than those of the matrix. The specific wear rate of the composites (16.5 × 10^?14 m³/Nm) is 79% lower than that of the matrix. By further increasing the Al₃Ti content, the network structure is completed, the wear resistance properties are improved, while the mechanical properties are decreased. The enhanced mechanical properties can primarily be attributed to the three-dimensional co-continuous network structure of the Al₃Ti and Al₂O₃ phases, as well as the pinning effect of Al₂O₃ particles.     

Fabrication and mechanical properties of Al2O3–TiC ceramic composites synergistically reinforced with multi-walled carbon nanotubes and graphene nanoplates

In this study, Al₂O₃–TiC composites synergistically reinforced with multi-walled carbon nanotubes (MWCNTs) and graphene nanoplates (GNPs) were prepared via spark plasma sintering (SPS). The effects of the MWCNT and GNP contents on the phase composition, mechanical properties, fracture mode, and toughening mechanism of the composites were systematically investigated. The experimental results indicated that the composite grains became more refined with the addition of MWCNTs and GNPs. The nanocomposites presented high compactness and excellent mechanical properties. The composite with 0.8 wt% MWCNTs and 0.2 wt% GNPs presented the best properties of all analysed specimens, and its relative density, hardness, and fracture toughness were 97.3%, 18.38 ± 0.6 GPa, and 9.40 ± 1.6 MPa m^1/2, respectively. The crack deflection, bridging, branching, and drawing effects of MWCNTs and GNPs were the main toughening mechanisms of Al₂O₃–TiC composites synergistically reinforced with MWCNTs and GNPs.     

Microstructure and mechanical properties of Al2O3 ceramic and TiAl alloy joints brazed with Ag–Cu–Ti filler metal

Al₂O₃ ceramic was reliably joined to TiAl alloy by active brazing using Ag–Cu–Ti filler metal, and the effects of brazing temperature, holding time, and Ti content on the microstructure and mechanical properties of Al₂O₃/TiAl joints were investigated. The typical interfacial microstructure of joints brazed at 880 °C for 10 min was Al₂O₃/Ti₃(Cu,Al)₃O/Ag(s.s)+AlCu₂Ti+Ti(Cu,Al)+Cu(s.s)/AlCu₂Ti+AlCuTi/TiAl alloy. With increasing brazing temperature and time, the thickness of the Ti₃(Cu,Al)₃O reaction layer increased, and the blocky AlCu₂Ti compounds aggregated and grew gradually. The Ti dissolved from the TiAl substrate was sufficient to react with Al₂O₃ ceramic to form a thin Ti₃(Cu,Al)₃O layer when Ag–Cu eutectic alloy was used, but the dissolution of TiAl alloy was inhibited with an increase in Ti content in the brazing filler. Ti and Al dissolved from the TiAl alloy had a strong influence on the microstructural evolution of the Al₂O₃/TiAl joints, and the mechanism is discussed. The maximum shear strength was 94 MPa when the joints were brazed with commercial Ag–Cu–Ti filler metal, while it reached 102 MPa for the joint brazed with Ag–Cu+2 wt% TiH₂ at 880 °C for 10 min. Fractures propagated primarily in the Al₂O₃ substrate and partially along the reaction layer.     

Fabrication of WSi2–Al2O3 and W5Si3–Al2O3 composites by combustion synthesis involving thermite reduction

Formation of WSi₂–Al₂O₃ and W₅Si₃–Al₂O₃ composites was studied by thermite-based combustion synthesis. The addition of two thermite combinations composed of WO₃+2Al and 0.6WO₃+0.6SiO₂+2Al into the W-Si reaction systems facilitated the combustion wave propagating in a self-sustaining manner and contributed to the in situ formation of tungsten silicides along with Al₂O₃. Experimental results showed that the former thermite mixture is more exothermic than the latter, and a decrease in the combustion temperature and flame-front velocity with increasing silicide phase formed in the composite. Depending on the reaction stoichiometry, the combustion wave velocity varied from 9.5 to 3.7 mm/s and temperature from 1650 to 1280 °C. A complete phase conversion and a broad range of the molar ratio of WSi₂/Al₂O₃ from 0.8 to 4.0 were achieved for the production of the WSi₂–Al₂O₃ composites. Due to the lower formation exothermicity, the W₅Si₃–Al₂O₃ composites were produced with a narrower range of W₅Si₃/Al₂O₃ from 0.4 to 2.0, beyond which combustion failed to proceed. Moreover, there exist WSi₂ and unreacted W in the as-synthesized W₅Si₃–Al₂O₃ composites.     

Microstructure and mechanical properties of Al2O3–Ti(C,N)–cBN composites prepared by a novel hot-forging process

Al₂O₃–cBN has received considerable attention in the field of ceramic cutting tools due to its high hardness, high wear resistance, and low cost, but poor interfacial bonding affects the performance of the composite. In this study, a novel hot-forging process was used to prepare high-performance Al₂O₃–cBN composites using Ti(C,N) as a binder. The evolution of the morphology, phase, and microstructure of the hot-forged Al₂O₃–Ti(C,N)–cBN composites was determined, and the mechanical properties were measured. The relative density of the composites increases significantly after hot forging, and the deformation of the composites increases with the hot-forging temperature. The highest performing Al₂O₃–Ti(C,N)–cBN composite was prepared by hot forging at 1600°C and has a hardness of 20 GPa, a bending strength of 647 MPa and a fracture toughness of 5.37 MPa m^1/2, which are superior to those of a directly hot-pressed sintered composite. However, at hot-forging temperatures higher than 1700°C, Al₅O₆N and TiB₂ are formed in the composite. In the composite hot forged at 1800°C, serrated grain boundaries promote the strength and toughness of the composite to 877 MPa and 6.76 MPa m^1/2, respectively. Therefore, the novel hot-forging process is expected to enhance material properties.     

Microstructure and mechanical properties of multi-walled carbon nanotubes containing Al2O3–C refractories with addition of polycarbosilane

Carbon nanotubes (CNTs) are a promising reinforcement for fabricating Al₂O₃–C refractories. However, CNTs are prone to agglomerate or react with antioxidants or reactive gaseous phases such as Al (g), Si (g) and SiO (g), etc. at high temperatures. To overcome the problems above, polycarbosilane (PCS) and multi-walled carbon nanotubes (MWCNTs) were firstly mixed with micro-alumina powder in a liquid medium and then incorporated into Al₂O₃–C refractories. Then the microstructure and mechanical properties of Al₂O₃–C refractories fired in the temperature range from 800 °C to 1400 °C were investigated in this work. The results showed that the MWCNTs were well dispersed in the specimens with addition of PCS in contrast to the specimens without PCS due to the PCS adsorption on the surface of MWCNTs during the mixing process. And the mechanical properties, such as cold modulus of rupture (CMOR), flexural modulus (FM), forces and displacements of Al₂O₃–C refractories with PCS were much higher than those without PCS, which was attributed to more homogeneous dispersion of MWCNTs, more residual MWCNTs as well as different morphologies of ceramic whiskers. Meanwhile, the oxidation resistance of Al₂O₃–C refractories with PCS was improved greatly, which was supposed that the in situ formed SiC_xO_y coating prevented the oxidation of MWCNTs to some extent.     

Al2O3 loss prediction model of selective laser melting Al2O3–Al composite

The element/phase loss is undesirable but existing during selective laser melting (SLM) of materials with volatile element/phase, which not only changes the material composition but also affects the molten pool flow. In the previous researches, the effect of remelting on the element/phase loss was neglected during the SLM process, instead, laser energy density was thought to be uppermost. In fact, the SLM process fabricates the parts in a manner of line by line and layer by layer, i.e., additive character, and the remelting in the overlap zone occurs during the SLM process. In this paper, three different Al₂O₃ loss prediction models of SLM Al₂O₃–Al composite by considering the additive character of SLM and the distribution of the Al₂O₃ associated with the different molten pool driving forces were developed. By comparing with the experimental results and predicted results, it is found that the Al₂O₃ is distributed on both sides of the molten pool under the combined action of the Marangoni flow and the evaporation recoil pressure. This kind of Al₂O₃ distribution enhances the effect of the remelting on the Al₂O₃ loss, i.e., the remelting brings a logarithmic increase in the Al₂O₃ loss rate. This determines the final Al₂O₃ loss rate of the SLMed 3D samples. During this study, although the Al₂O₃ loss rate of the single-track is only 33%, the loss rate of SLMed 3D samples increases significantly to 97% when the hatching space of 60 μm and scanning speed of 200 mm/s are utilized, i.e., almost no Al₂O₃ in the 3D sample. Thus, it is more important to reduce the remelting, i.e., overlap rate for reducing the element/phase loss. This study is a benefit for understanding and reducing the element/phase loss in SLM.     

Microstructure and properties of SiCf/SiC composite joints with CaO–Y2O3–Al2O3–SiO2 interlayer

Using CaO, Y₂O₃, Al₂O₃, and SiO₂ micron-powders as raw materials, CaO–Y₂O₃–Al₂O₃–SiO₂ (CYAS) glass was prepared using water cooling method. The coefficient of thermal expansion (CTE) of CYAS glass was found to be 4.3 × 10^?6/K, which was similar to that of SiC_f/SiC composites. The glass transition temperature of CYAS glass was determined to be 723.1 °C. With the increase of temperature, CYAS glass powder exhibited crystallization and sintering behaviors. Below 1300 °C, yttrium disilicate, mullite and cristobalite crystals gradually precipitated out. However, above 1300 °C, the crystals started diminishing, eventually disappearing after heat treatment at 1400 °C. CYAS glass powder was used to join SiC_f/SiC composites. The results showed that the joint gradually densified as brazing temperature increased, while the phase in the interlayer was consistent with that of glass powder heated at the same temperature. The holding time had little effect on phase composition of the joint, while longer holding time was more beneficial to the elimination of residual bubbles in the interlayer and promoted the infiltration of glass solder into SiC_f/SiC composites. The joint brazed at 1400 °C/30 min was dense and defect-free with the highest shear strength of about 57.1 MPa.     

Development and evaluation of Al2O3–ZrO2 composite processed by digital light 3D printing

Digital Light Processing (DLP) is a promising approach to fabricate delicate ceramic components with high-fidelity structural features. In this work, the alumina and zirconia/alumina ceramic suspensions with low viscosity and high solid loading (40 vol%) were prepared specifically for DLP 3D printing. After debinding and sintering, the final parts were obtained without any defects. The surface morphologies and mechanical properties of alumina (Al₂O₃) and zirconia toughened alumina (ZTA) composites were investigated and the results showed that the final parts exhibited high relative densities and good interlayer combination at the sintering temperature of 1600 °C. Comparing with the Al₂O₃, the ZTA composites exhibited significantly enhanced density (99.4%), bending strength (516.7 MPa) and indentation fracture toughness (7.76 MPa m^1/2).     

Particle occlusion and mechanical properties of Ni–Al2O3 nanocomposites

Polycrystalline alumina, doped with MgO below the solubility limit, was reinforced with sub-micron particles of Ni by infiltration of Ni-nitrate into fired alumina green bodies, followed by reduction and sintering. The Ni particle size and location were monitored both after reduction and after sintering by transmission electron microscopy. Particle occlusion was found to increase with sintering time and temperature, and is correlated with experimentally detected Mg segregation to the Ni–alumina interfaces, resulting in partial depletion of Mg at the alumina grain boundaries and thus their increased mobility. Occlusion of Ni particles reduces the fracture strength and Weibull modulus of the composites, indicating that particle location is a key microstructural parameter for reaching high fracture strengths, and that this can be controlled via grain boundary and interface adsorption.     

Load-dependence of Knoop hardness of Al2O3–TiC composites

Nine samples of Al₂O₃–30 wt.% TiC composites were prepared by hot-pressing the Al₂O₃ powder mixed with TiC particles. The average sizes of the TiC particles used for preparing the nine samples were different with each other. Knoop hardness measurements were conducted on these nine samples, respectively, in the indentation load range from 1.47 to 35.77 N. For each sample, the measured Knoop hardness decreases with the increasing indentation load. The classical Meyer's power law and an empirical equation proposed originally by Bückle were verified to be sufficiently suitable for describing the observed load-dependence of the measured hardness. Analysis based on Meyer's law can not provide any useful information about the cause of the observed ISE while true hardness values, which are load-independent, can be deduced from the Bückle's equation. It was found that the deduced true hardness increases with the average size of TiC particles existing in the sample.