Combustion synthesis of TiC–TiB2 composites

An experimental study on formation of TiC–TiB₂ in situ composites with a broad range of compositions was conducted by self-propagating high-temperature synthesis (SHS) using the reactant compacts from different combinations of Ti, B₄C, C, and B powders. Direct reaction of Ti with B₄C at stoichiometry of Ti:B₄C = 3:1 yields a TiB₂-rich composite with TiC:TiB₂ = 1:2. Formation of the products containing 20, 33.3, and 50 mol% of TiB₂ was achieved by the Ti–B₄C–C reactants. In addition, the test specimen composed of Ti, B₄C, and B was employed for the synthesis of a composite with 80 mol% TiB₂. Among three different types of the powder compacts, the boron-containing sample was characterized by the fastest combustion wave and the highest reaction temperature. The lowest combustion temperature and wave velocity were observed in the Ti–B₄C compact. When fine Ni particles were added to the Ti–B₄C reactant, it was found that the propagation rate of the reaction front was increased and the densification of the end product was enhanced significantly. This was attributed to formation of the Ti–Ni eutectic liquid during the reaction. As a result, the relative density of a TiC + 2TiB₂ composite increases from 30 to 86% with the Ni content from 0 to 20 mol%. Based upon the XRD analysis, small amounts of TiNi₃ and TiB were detected in the Ni-reinforced TiC–TiB₂ composites.     

The influence of powder characteristics on mechanical properties of Al2O3–TiC–Co ceramic materials prepared by Co‐coated Al2O3/TiC powders

Ultrafine Al₂O₃–TiC–Co (ATC) ceramic is prepared in order to improve the bending strength and fracture toughness of ceramic materials. The ultrafine Co‐coated Al₂O₃ and TiC powders have been synthesized by electroless plating at room temperature, and the composite powders were sintered by hot‐pressing to compact ATC samples. The average bending strength, average hardness and average fracture toughness values of ATC ceramic with different particle sizes and Co contents were investigated. The toughening mechanism of the ultrafine ATC ceramic was studied by transmission electron microscopy (TEM) and ceramic performance testing methods. The results show that the relative density, bending strength and fracture toughness values increase remarkably with the increase of Co content. The ultrafine grain of original powders is beneficial to improve the relative density, strength and toughness values of ATC ceramic. The Co phase hinders the growth of ATC ceramic grains during sintering. The Co phase forms a three‐dimensional mesh skeleton structure during sintering, improving the fracture toughness and strength of the composite ceramic.     

Mechanical properties and microstructure of TiB2–TiC composite ceramic cutting tool material

TiB₂–TiC composite ceramic cutting tool material was prepared by sintering during hot-pressing in vacuum. The effects of nano-scale Ni and Mo additives and sintering heating rate on mechanical properties and grain characteristics were investigated. TiB₂ and TiC grains exhibited prismatic and equiaxed shapes respectively. The diameter and aspect ratio of prismatic TiB₂ grains were influenced by nano-scale Ni/Mo additives. A higher heating rate could cause a higher aspect ratio of prismatic TiB₂ grains. The good mechanical properties of TN1((TiB₂–TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min) were ascribed to a relatively fine and homogenous microstructure. And a brittle B₄MoTi solid solution phase and wider distribution of grain size induced the lower flexural strength of TNM2((TiB₂–TiC)/(Ni,Mo) composite ceramic sintered at heating rate of 100 °C/min), but the higher aspect ratio of TiB₂ grains could prevent cracks from propagating and ameliorated the fracture toughness. The optimum resultant mechanical properties were obtained by (TiB₂–TiC)/Ni composite ceramic sintered at a heating rate of 50 °C/min.     

Combustion synthesis of FeAl−Al2O3 composites with TiB2 and TiC additions via metallothermic reduction of Fe2O3 and TiO2

Combustion synthesis involving metallothermic reduction of Fe₂O₃ and TiO₂ was conducted in the mode of self-propagating high-temperature synthesis (SHS) to fabricate FeAl-based composites with dual ceramic phases, TiB₂/Al₂O₃ and TiC/Al₂O₃. The reactant mixture included thermite reagents of 0.6Fe₂O₃+0.6TiO₂+2Al, and elemental Fe, Al, boron, and carbon powders. The formation of xFeAl−0.6TiB₂−Al₂O₃ composites with x=2.0−3.6 and yFeAl−0.6TiC−Al₂O₃ composites with y=1.8−2.75 was studied. The increase of FeAl causes a decrease in the reaction exothermicity, thus resulting in the existence of flammability limits of x=3.6 and y=2.75 for the SHS reactions. Based on combustion wave kinetics, the activation energies of E_a=97.1 and 101.1 kJ/mol are deduced for the metallothermic SHS reactions. XRD analyses confirm in situ formation of FeAl/TiB₂/Al₂O₃ and FeAl/TiC/Al₂O₃ composites. SEM micrographs exhibit that FeAl is formed with a dense polycrystalline structure, and the ceramic phases, TiB₂, TiC, and Al₂O₃, are micro-sized discrete particles. The synthesized FeAl−TiB₂−Al₂O₃ and FeAl−TiC−Al₂O₃ composites exhibit the hardness ranging from 12.8 to 16.6 GPa and fracture toughness from 7.93 to 9.84 MPa·m^1/2.     

Synthesis and characterization of Al2O3–TiC nano-composite by spark plasma sintering

Al₂O₃–10TiC composite was synthesized by high energy ball milling followed by spark plasma sintering (SPS) process. Microstructure of the sintered composite samples reveals homogeneous distribution of the TiC particles in Al₂O₃ matrix. Effect of sintering temperature on the microstructure and mechanical properties was studied. The sample sintered at 1500 °C shows a measured density of 99.97% of their theoretical density and hardness of 1892 Hv with very high scratch resistance. These results demonstrate that powder metallurgy combined with spark plasma sintering is a suitable method for the production of Al₂O₃–10TiC composites.     

Using coated ceramic particles to increase wear resistance in high-speed steels

The use of chemical-vapor-deposition (CVD)-coated ceramic particle reinforcements in metal-matrix composites allows the control of reactivity at the particle/matrix interface. Wear-resistant, high-speed, steel-based composites containing uncoatedAl₂O₃ uncoated TiC, and CVD-coated A1₂O₃ were liquid-phase sintered and characterized using pin-on-disk wear testing. TiC or TiN CVD coating of Al₂O₃ resulted in a porosity decrease at the particle/matrix interface in addition to better ceramic/metal cohesion due to improved wettability. Lower wear rates were obtained with the composites containing TiC-or TiN-coated Al₂O₃.     

Densification, microstructure, and fracture behavior of TiC/Si3N4 composites by spark plasma sintering

TiC/Si₃N₄ composites were prepared using the β-Si₃N₄ powder synthesized by self-propagating high-temperature synthesis (SHS) and 35 wt.% TiC by spark plasma sintering. Y₂O₃ and Al₂O₃ were added as sintering additives. The almost full sintered density and the highest fracture toughness (8.48 MPa·m^½) values of Si₃N₄-based ceramics could be achieved at 1550°C. No interfacial interactions were noticeable between TiC and Si₃N₄. The toughening mechanisms in TiC/Si₃N₄ composites were attributed to crack deflection, microcrack toughening, and crack impedance by the periodic compressive stress in the Si₃N₄ matrix. However, increasing microcracks easily led to excessive connection of microcracks, which would not be beneficial to the strength.     

Effects of ultrafine refractory carbides on microstructure and mechanical properties of hot-pressing sintered TiB2-ZrC cermet composites at different temperatures

TiB₂-ZrC cermet composites were sintered at different temperatures by hot pressing in vacuum. Effects of ultrafine refractory carbides and sintering temperatures on microstructures and mechanical properties were evaluated. The relationships between mechanical properties and microstructure are discussed. A typical black core-grey rim structure was observed; the grain size of TiB₂ remarkably decreased when ultrafine refractory carbides were added into the matrix. The incorporation of vanadium carbide (VC) and niobium carbide (NbC) clearly improved the flexural strength and hardness. This can be attributed to the fine uniform grain size and solid solution strengthening mechanism. It was also observed that the introduction of VC and tantalum carbide (TaC) enhanced the indentation fracture resistance of composites. This can be attributed to the favorable solubility of VC in TiB₂-ZrC system and the pinning effect of precipitated TaC. In this study, the typical core-rim structure, good solubility of VC, pinning effect of TaC, and crack deflection and branching improved the mechanical properties of TiB₂-ZrC cermet composites.     

Oxidation behavior of micro-sized Al2O3–TiC–Co composites prepared from cobalt-coated powders

The oxidation behavior of hot-pressed Al₂O₃–TiC–Co composites prepared from cobalt-coated powders has been studied in air in the temperature range from 200 °C to 1000 °C for 25 h. The oxidation resistance of Al₂O₃–TiC–Co composites increases with the increase of sintering temperature at 800 °C and 1000 °C. The oxidation surfaces were studied by XRD and SEM. The oxidation kinetics of Al₂O₃–TiC–Co composites follows a rate that is faster than the parabolic-rate law at 800 °C and 1000 °C. The mechanism of oxidation has been analyzed using thermodynamic and kinetic considerations.     

Effect of sintering process on the microstructures and properties of in situ TiB2–TiC reinforced steel matrix composites produced by spark plasma sintering

Spark plasma sintering (SPS) is a new technique to rapidly produce metal matrix composites (MMCs), but there is little work on the production of TiB₂–TiC reinforced steel matrix composites by SPS. In this work, in situ TiB₂–TiC particulates reinforced steel matrix composites have been successfully produced using cheap ferrotitanium and boron carbide powders by SPS technique. The effect of sintering process on the densification, hardness and phase evolution of the composite is investigated. The results show that when the composite is sintered at 1050 °C for 5 min, the maximum densification and hardness are 99.2% and 83.8 HRA, respectively. The phase evolution of the composite during sintering indicates that the in situ TiB₂–TiC reinforcements are formed by a hybrid formation mechanism containing solid–solid diffusion reaction and solid–liquid solution-precipitation reaction. The microstructure investigation reveals that fine TiB₂–TiC particulates with a size of ～2 μm are homogeneously distributed in the steel matrix. The TiB₂–TiC/Fe composites possess excellent wear resistance under the condition of dry sliding with heavy loads.     

A novel Al–Ti–B alloy for grain refining Al–Si foundry alloys

Al–Ti–B refiners with excess-Ti (Ti:B > 2.2) perform adequately for wrought aluminium alloys but they are not as efficient in the case of foundry alloys. Silicon, which is abundant in the latter, forms silicides with Ti and severely impairs the potency of TiB₂ and Al₃Ti particles. Hence, Al–Ti–B alloys with excess-B (Ti:B < 2.2) and binary Al–B alloys are favored to grain refine hypoeutectic Al–Si alloys. These grain refiners rely on the insoluble (Al,Ti)B₂ or AlB₂ particles for grain refinement, and thus do not enjoy the growth restriction provided by solute Ti. It would be very attractive to produce excess-B Al–Ti–B alloys which additionally contain Al₃Ti particles to maximize their grain refining efficiency for aluminium foundry alloys. A powder metallurgy process was employed to produce an experimental Al–3Ti–3B grain refiner which contains both the insoluble AlB₂ and the soluble Al₃Ti particles. Inoculation of a hypoeutectic Al–Si foundry alloy with this grain refiner has produced a fine equiaxed grain structure across the entire section of the test sample which was more or less retained for holding times up to 15 min.     

Effect of Al2O3 content on the synthesized products by the laser ignited self-propagating high-temperature synthesis reaction of Al2O3–Ti–C system

The effect of Al₂O₃ content on ignition temperature and combustion temperature, the phase composition, the density of the products and the grain size of TiC was investigated by self-propagating high-temperature synthesis reaction of Al₂O₃–Ti–C system. The results show ignition temperature increases and combustion temperature decreases with the increasing of Al₂O₃ content; the density of the products varies with Al₂O₃ content, TiC and Al₂O₃ are the two stable phases after SHS, TiC particle size decreases with the increasing of Al₂O₃ content, furthermore, the fracture type of the sintered specimens is a nearly completely intergranular mode.     

High-temperature compressive properties of TiC–TiB_2/Cu composites prepared by self-propagating high-temperature synthesis

TiC–TiB2 /Cu composites were prepared by self-propagating high-temperature synthesis with pseudo hot isostatic pressing using Ti, B4 C, and Cu powders. The compressive deformation of the composites at high temperature was investigated. It is found that the maximum compressive strength decreases with the increase of temperature and Cu content. The deformation of the composites includes the steps of elastic, stable rheology, and inaction. The maximum strain is in the range of 5 %–10 %. Before fracture, TiC–TiB2 /40Cu becomes drum-shaped at 1123 K; however, TiC–TiB2 /20Cu only has a brittle fracture along the axial direction of 45°. The results show that the compressive strength of TiC–TiB2 /Cu decreases from 823 to 1223 K. However, the maximum compressive strength of TiC–TiB2 /20Cu reaches 1850 MPa at 823 K, which predicts that this series of composites could be applied to high-temperature compressive materials.     

Study of structure formation in TiC–TiB2–MexOy ceramics fabricated by SHS and densification

Bulk ceramic materials based on TiC–TiB₂–Me_xO_y systems have been produced by means of pressure-assisted self-propagating high-temperature synthesis (SHS). The selection of the SHS systems was based on thermodynamic analyses in order to obtain regimes of synthesis with formation of liquid phase allowing a proper densification. The liquid phase formed in the products consisted of either melted oxides or TiC–TiB₂ eutectic depending on the combustion temperature. The mechanisms of structure formation and the effect of grain growth inhibition due to the metal oxide particles have been evaluated. The microstructural investigations have confirmed the role of the metal oxides as promoter of the final products densification and of the grain refinement in the SHS ceramic materials. A core–shell model is proposed to explain the structure formation of the SHS composites through a chemical interaction involving two stages. The good agreement between calculated and experimental results confirmed the validity of the core–shell mechanism.     

Electrospark coatings produced by ceramic nanostructured SHS electrode materials

The paper considers the possibility to expand the number of materials used for electrospark alloying. Ceramic nanostructured TiB₂–TiC–Al₂O₃–ZrO₂-based materials produced by means of SHS extrusion are used as electrodes. The regularities of the alloyed layer formation with electrospark alloying (ESA) are studied. Three different modes of anode erosion and cathode gain are observed for single pulse energies ranging from 0.05 to 2.5 J. The microstructure of the coatings is studied. It is shown that, in the case of ESA by ceramic SHS electrodes, nanosized crystallites are formed. The microhardness of the alloyed layer is 1250 HV. The tribological tests of the protective coatings demonstrate a high wear resistance (10^–5 mm³/N/m) and low friction coefficient (0.2).     

Preparation,mechanical properties and microstructure of TiB2 based ceramic cutting tool material toughened by TiC whisker

TiC whiskers were synthesized by carbothermal reduction process. By using these whiskers as the toughening phase, a novel TiB₂ based ceramic cutting tool material was prepared. Due to that the thermal expansion coefficient of TiC is close to that of TiB₂, the addition of no more than 30 vol% TiC whisker not only has little adverse effect on the density and flexure strength of the composite, but also can refine the grains, reduce the defects and improve the grain strength. As a result, both the fracture toughness and flexure strength of the TiB₂ based ceramic composite can be significantly improved. Appropriate sintering temperature and holding time can reduce defects, improve the strength of grains and grain boundaries and enhance the toughening effect of TiC whiskers. Experimental results showed that when the whisker content was 30 vol%, the sintering temperature was 1700 °C and the holding time was 30 min, the flexure strength, fracture toughness and Vickers hardness of the TiB₂ based ceramic cutting tool material was 860 MPa, 7.9 MPa·m^1/2 and 22.6GPa, respectively.     

Proposal of self-healing coatings for nuclear fusion applications

Avoiding cracks in ceramic coatings is one of the most important problems to be solved for the thermally sprayed tritium permeation barriers in fusion reactor. In this paper, a self-healing composite coating composed of TiC + mixture (TiC/Al₂O₃) + Al₂O₃ was developed to address this problem. The coating was deposited on certain martensitic steel by plasma spraying. The morphology and phase of the coating were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) while the porosity was analyzed by using Image Pro software. The thermal shock resistance test and residual stress measurement of the coating were also performed. In the experiment, NiAl + TiC + mixture (TiC/Al₂O₃) + Al₂O₃ and mixture (TiC/Al₂O₃) + Al₂O₃ films were also fabricated and studied respectively. The results showed that the TiC + mixture (TiC/Al₂O₃) + Al₂O₃ coating exhibited the best mechanical integrity and self-healing ability among the three samples with the porosity decreased by 90% after heat-treatment under normal atmosphere. The oxidation/expansion of TiC in the coating played an important role in the sealing of pores. This self-healing coating made by thermal spraying is proposed as a good candidate for tritium permeation barrier in fusion reactors.     

Fabrication of dense Al2O3-FeAl composites by using solid-state sintering

Highly densified alumina-iron aluminide (Al₂O₃-FeAl) composites consisting of ubiquitous elements were fabricated by using pulse current sintering technique under a certain uni-axial pressure. The solid-state sintering without melting FeAl was the highlight in this study. The mechanical properties of the Al₂O₃-FeAl composites were much greater than previously reported ones fabricated by reaction sintering technique. The poor wettability of FeAl against Al₂O₃ strongly influenced the mechanical properties and made it difficult to be highly densified Al₂O₃-FeAl composites by liquid phase sintering especially when volume fraction of FeAl to Al₂O₃-FeAl was high (>30.5 vol%). However, highly densified Al₂O₃-FeAl composites were obtained by solid-state sintering with control of Al₂O₃ grain size and sintering temperatures. It was concluded that highly controlled powder metallurgy made it possible to fabricate dense ceramic-metal (intermetallic) composites from the combination of materials having poor wettability.     

Al–Ti–C–Sr master alloy—A melt inoculant for simultaneous grain refinement and modification of hypoeutectic Al–Si alloys

Present article is focused on the microstructural features of Al–Ti–C–Sr master alloy, an inoculant for simultaneous grain refinement and modification of hypoeutectic Al–Si alloys. This master alloy is basically a metal matrix composite consisting of TiC and Al₄Sr phases formed in situ in the Al-matrix. TiC particles initiate the refinement of primary α-Al through heterogeneous nucleation in molten hypoeutectic Al–Si alloy, while Al₄Sr phase dissolves in molten Al–7Si alloy enriching the melt with Sr, which eventually leads to modification of eutectic silicon during solidification of the Al–7Si alloy casting. Thus present master alloy serves in both ways, as a grain refiner and a modifier for hypoeutectic Al–Si alloys.     

Al–Ti–B grain refiners via powder metallurgy processing of Al/K2TiF6/KBF4 powder blends

The response to thermal exposure of ball-milled Al/K₂TiF₆/KBF₄ powder blends was investigated to explore the potential of PM processing for the manufacture of Al–Ti–B alloys. K₂TiF₆ starts to be reduced by aluminium as early as 220 °C when ball-milled Al/K₂TiF₆/KBF₄ powder blends are heated. The reaction of KBF₄ with aluminium follows soon after. The Ti and B thus produced are both solutionized in aluminium before precipitating out as Al₃Ti and TiB₂. All these reactions take place below the melting point of aluminium. The ball-milled Al/K₂TiF₆/KBF₄ powder blends heat treated at approximately 525 °C can be compacted to produce Al–Ti–B pellets with in situ formed Al₃Ti and TiB₂ particles. These pellets are shown to be adequate grain refiners for aluminium alloys.