Dynamic Consolidation of Combustion-Synthesized Alumina-Titanium Diboride Composite Ceramics

A mixture of TiO₂, B2O₃, and Al powders was reacted exo-thermically to form a low-density Al₂O₃–TiB₂ foamed product. After the cessation of the combustion synthesis reaction, this hot foam was densified by the pressure waves generated by the detonation of an explosive charge. The delay time between the initiation of the combustion synthesis and detonation of the explosive, the thickness of the explosive charge, and the strength of sample confinement were found to influence the effectiveness of the consolidation. Analysis of the combustion synthesis reaction temperature histories and microscopy of the reacted product sponge are used to develop additional information about this reaction/consolidation process. These results, and the microstructural properties of the combustion-synthesized and dynamically consolidated products, are discussed.     

Simultaneous Synthesis and Densification of Titanium Nitride/ Titanium Diboride Composites by High Nitrogen Pressure Combustion

Composites of TiN/TiB₂ were synthesized by a combustion process of BN, Ti in a nitrogen atmosphere. The effect of the BN/Ti ratio and the nitrogen gas pressure on the synthesis of these composites was investigated. Dense TiN/TiB₂ composites with relatively high hardness and toughness were fabricated by combustion synthesis from Ti and BN under a nitrogen pressure of 4.0 MPa. The Vickers microhardness of the products obtained from reactants with a BN/Ti mole ratio of 0.11 increased with an increase in nitrogen pressure and had a maximum value of ∼25 GPa. Fracture toughness, K _IC, of the products increased from 3.1 to 5.9 MPa·m^1/2 as the BN/Ti ratio increased from 0.11 to 0.20. However, products formed under nitrogen pressures higher than 6.0 MPa exhibited circumferential macrocracks due to thermal shock.     

Solid-State Diffusion Bonding of Carbon–Carbon Composites with Borides and Carbides

Solid-state diffusion bonding of carbon–carbon (C─C) composites by using boride and carbide interlayers has been investigated. The interlayer materials used in this study were single-phase borides (TiB₂ or ZrB₂), eutectic mixtures of borides and carbides (ZrB₂+ ZrC or TiB₂+ B₄C), and mixtures of TiB₂+ SiC + B₄C produced in situ by chemical reactions between B₄C, Ti, and Si or between TiC, Si, and B. The double-notch shear strengths of the joints produced by solid-state reaction sintering of B₄C + Ti + Si interlayers were much higher than those of joints produced with other interlayers. The maximum strength was achieved for C─C specimens bonded at 2000°C with a 2:1:1 mole ratio of Ti, Si, and B₄C powders. The reaction products identified in the interlayers, after joining, were TiB₂, SiC, and TiC. The joint shear strength increased with the test temperature, from 8.99 MPa at room temperature to an average value of 14.51 MPa at 2000°C.     

Production of TiB2–TiN Composites by Combustion Synthesis and Their Properties

The self-propagating high-temperature synthesis (SHS) process has been applied to formation of composites consisting of TiB₂ and TiN ceramics synthesized simultaneously. Ti, B, and BN powders were used as raw materials. The SHS reaction was initiated by a tungsten heating coil. XRD experiments confirmed that the reaction was complete, and that only TiB₂ and TiN phases were detected. Microstructural observations revealed that both TiN and TiB₂ crystal grains had small sizes of less than 1 μm in the composites with high TiN content. Inhibition of grain growth can be attributed to the pinning effect of TiN grains. Excellent corrosion resistance was obtained for HCl reagent.     

Mechanochemical Synthesis and Pressureless Sintering of TiB2–AlN Composites

TiB₂-AlN composites have been fabricated by the pressureless sintering of a mechanochemically processed Ti, Al, and BN powder mixture. TiB₂-AlN powder was obtained from the mixture of Ti, Al, and BN, which had a composition corresponding to 45.7 wt% TiB₂-54.3 wt% AlN, after mechanochemical processing for longer than 24 h. X-ray diffraction and transmission electron microscopy analysis showed that the powder subjected to mechanochemical processing for 60 h consisted of crystallites less than 300 nm in size with a disordered crystal structure. TiB₂-AlN composites with 95% relative density, a flexural strength of 172 MPa, a fracture toughness of 4.6 MPa·m^1/2, a hardness of 12.0 GPa, and an electrical resistivity of 1488 μΩ·cm were obtained by pressureless sintering at 1700°C for 2 h of the powder subjected to mechanochemical processing for 60 h.     

Combustion Synthesis/Dynamic Densification of a TiB2-SiC Composite

The Ti + 2B exothermic chemical reaction was used in combination with a high-velocity forging step to produce dense TiB₂-(20 vol%)SiC composites. Densities in excess of 96 % of the theoretical were achieved for both SiC particulate and fiber additions. X-ray diffractometry revealed the products of the reaction to be TiB₂ and SiC. The microstructures are composed of spheroidal TiB₂ phase, a highly contiguous SiC binder phase, and an apparent eutectic between TiB₂ and SiC located at regions of preexisting SiC additions. These microstructural features suggest that SiC underwent a peritectic phase transformation. Thermodynamic analysis predicts that at least 41 vol% SiC addition is needed to prevent the loss of the starting morphologies by the peritectic reaction.     

Ablation-Resistance of Combustion Synthesized TiB2–Cu Cermet

TiB₂–Cu ceramic–metal composites were prepared by combustion synthesis of elemental titanium, boron, and copper powders. The synthesized product consisted of two phases: TiB₂ and copper. The addition of copper improved the strength and fracture toughness, thermal expansion coefficient, and thermal conductivity of TiB₂. Thermal shock and ablation resistances of TiB₂–Cu composites were studied using a plasma torch arc heater. Monolithic TiB₂ failed catastrophically when the plasma arc flow reached the specimen surface. However, no cracks were found on the ablation surface of the TiB₂–Cu ceramic–metal composites. The fractional mass loss was 4.09% for a TiB₂–40Cu composite, which was close to the traditional W/Cu alloys. Volatilization of metal binder and mechanical erosion of TiB₂ were observed to be the major ablation mechanisms. An ablation process model is proposed for the TiB₂–Cu composites.     

Effect of SiC on Interfacial Reaction and Sintering Mechanism of TiB2

Compacts of TiB₂ with densities approaching 100% are difficult to obtain using pressureless sintering. The addition of SiC was very effective in improving the sinterability of TiB₂. The oxygen content of the raw TiB₂ powder used in this research was 1.5 wt%. X-ray photoelectron spectroscopy showed that the powder surface consisted mainly of TiO₂ and B₂O₃. Using vacuum sintering at 1700°C under 13–0.013 Pa, TiB₂ samples containing 2.5 wt% SiC achieved 96% of their theoretical density, and a density of 99% was achieved by HIPing. TEM observations revealed that SiC reacts to form an amorphous phase. TEM-EELS analysis indicated that the amorphous phase includes Si, O, and Ti, and X-ray diffraction showed the reaction to be TiO₂+ SiC → SiO₂+ TiC. Therefore, the improved sinterability of TiB₂ resulted from the SiO₂ liquid phase that was formed during sintering when the raw TiB₂ powder had 1.5 wt% oxygen.     

Acid Leaching of SHS Produced Magnesium Oxide/Titanium Diboride

The stoichiometric self-propagating high-temperature synthesis (SHS) thermite reaction involving magnesium (Mg), titanium dioxide (TiO₂), and boron oxide (B₂O₃) forms MgO and titanium diboride (TiB₂) as final products. Selective acid leaching is used to remove the MgO leaving TiB₂ powder. This study investigates the acid leaching of SHS-produced MgO/TiB₂ powders and a stoichiometric mixture of commercially obtained MgO and TiB₂ powders. Leaching was conducted at pH levels of 4.0, 2.5, and 1.0 by the introduction of concentrated aliquots of HNO₃. This method maintains a minimum pH target throughout the leaching process, thereby sustaining a dynamic concentration to remove the oxide. The optimal leaching conditions were determined to be at 90°C at a minimum pH target of 2.5 for the SHS-produced product. At these conditions, conversion percentages of 83%–84% of MgO were measured with only trace amounts of TiB₂ measured in the solution (<100 μg/L). Conversion percentages for each leaching condition and dissolution mass of solid MgO and TiB₂ at each pH are also reported. Results from powder X-ray diffraction confirm the removal of MgO and minimal dissolution of TiB₂, and indicate the formation of unidentified compounds. Inductively coupled plasma mass spectrometry (ICP) was used to analyze the ionic composition and extent of leaching. Scanning electron microscopy was used to observe the particle morphology of the leached powders.     

Solid Solubility Effect on Properties of Titanium Diboride

The effect of solute additions on the lattice constants of (Ti,Me)B₂ alloys is reported. The data are based on the literature and on additional experimental work. Variation of lattice constants, calculated from crystallochemical principles, agrees satisfactorily with measured values for dilute solid solutions. Increased solute concentration causes positive deviation for (Ti,Cr)B₂, (Ti,Zr)B₂, and (Ti,Al)B₂ because lattice dilation is affected by both the valence of the solute and the size of the metallic radii. It is postulated that lattice anisotropy and elastic strain energy increase in the solid solutions and that they determine the grain-boundary cohesion in polycrystalline TiB₂. Knowledge of the lattice dilation properties of TiB₂ solid solutions indicates how the mechanical behavior of diboride ceramics could be improved.     

Effect of Ni and Co Additives on Phase Decomposition in TiB2–WB2 Solid Solutions Formed by Induction Field Activated Combustion Synthesis

Solid solutions of TiB₂–WB₂ were densified and annealed simultaneously to cause the decomposition into the phases (Ti,W)B₂ and (W,Ti)B₂. Ni and Co were added to solid solutions formed by induction field activated combustion synthesis. The presence of these metals as additives markedly enhanced the kinetics of the subsequent decomposition process. With these additives, decomposition to the two phases occurred within minutes (360 s) in contrast to hours when the solutions did not include the additives. The phases resulting from decomposition, (Ti,W)B₂ and (W,Ti)B₂, were identified by X-ray to have the hexagonal AlB₂ and W₂B₅ structures, respectively. The precipitated phase, (W,Ti)B₂, occurred as elongated grains with aspect ratios of as high as about 10 in samples containing Ni as the additive.     

Low-Temperature Densification of TiN–TiB2 Composites Through Reactive Hot Pressing with Excess Ti Additions

Reactive hot pressing of Ti and BN powder mixtures is used to produce dense TiN _x –TiB₂ composites. The effect of excess Ti along with a small addition, ∼1 wt% Ni, on the reaction and densification of the composite was investigated. A composite of ∼99.9% relative density (RD) was produced at 1200°C at 40 MPa for 30 min with 1 wt% Ni, whereas composites produced without Ni are porous and contain residual reactants. The microstructural studies on composite samples with excess Ti produced at short durations indicate the presence of a transient (Ni–Ti) phase from which Ti is finally removed to form substoichiometric TiN _x . The hardness of the dense TiN _x –TiB₂ composite is ∼22 GPa. The densification mechanism in this system is contrasted with the role of nonstoichiometry in the Zr–B₄C system.     

Titanium Diboride–Tungsten Diboride Solid Solutions Formed by Induction-Field-Activated Combustion Synthesis

Solid solutions of titanium diboride–tungsten diboride (TiB₂–WB₂) were synthesized by induction-field-activated combustion synthesis (IFACS) using elemental reactants. In sharp contrast to conventional methods, solid solutions could be formed by the IFACS method within a very short time, ∼2 min. Solutions with compositions ranging from 40–60 mol% WB₂ were synthesized with a stoichiometric ratio (Ti + W)/B =½; however, samples with excess boron were also made to counter the loss of boron by evaporation. The dependence of the lattice constants of the resulting solid solutions on composition was determined. The "a" parameter decreased only slightly with an increase in the WB₂ content, whereas the "c" parameter exhibited a significant decrease over the range 40–60 mol% WB₂. Solid-solution powders formed by the IFACS method were subsequently sintered in a spark plasma sintering (SPS) apparatus. After 10 min at 1800°C, the samples densified to relative density 86%. XRD analysis showed the presence of only the solid-solution phase.     

Self-Propagating High-Temperature Synthesis Flammable Range and Dominant Parameters for Synthesizing Several Ceramics and Intermetallic Compounds under Heat-Loss Condition

Extensive comparisons have been conducted between experimental and theoretical results for the nonadiabatic self-propagating high-temperature synthesis combustion characteristics of many solid-solid systems subjected to volumetric heat loss. The nonadiabatic flame propagation theory-which describes the premixed mode of bulk flame propagation supported by the nonpremixed reaction of dispersed nonmetal (or higher-melting-point metal) particles in the liquid metal, with finite-rate reaction at the particle surface and temperature-sensitive Arrhenius-type condensed-phase mass diffusivity-is used to compare with experimental results with heat loss. Systems examined are ceramics (TiC, TiB₂, and ZrB₂) and intermetallic compounds (NiAl, TiCo, and TiNi). By using a consistent set of physicochemical parameters for these systems, satisfactory quantitative agreement is demonstrated for the flammable range (defined in terms of the mixture ratio, degree of dilution, particle size, and/or compact diameter.     

Mechanism of AlN Formation through the Carbothermal Reduction of Al2O3 in a Flowing N2 Atmosphere

Mixtures of Al₂O₃, and C powders in a molar ratio of 1:3 were reduced in a N₂ atmosphere. The effects of time, temperature, particle size of both reactants, and N₂ flow rate were investigated. Kinetics show that the rate-limiting process is the combustion of carbon, which leads to the reaction mechanism through the following basic steps:
Al₂O₃= 2Al + 3/2 O₂
3C + 3/2 O₂→ 3CO (slow, with or without intermediate formation of CO₂)
2Al + N₂= 2AlN (fast)
All of the experimental results have been discussed on the basis of this model, which allows one to predict the best conditions for preparing AlN powders.     

Microstructure and Mechanical Properties of In Situ Produced TiC/TiB2/MoSi2 Composites

A novel microstructure of in situ produced TiC/TiB₂/MoSi₂ composite and its mechanical properties were investigated. The results indicate that TiC/TiB₂/MoSi₂ composites can be fabricated by reactive hot pressing the mixed powders of MoSi₂, B₄C, and Ti. A novel microstructure consisting of hollow particles of TiC and TiB₂ grains in an MoSi₂ matrix was obtained. Grains of in situ produced TiC and TiB₂ were much finer, from 100 to 400 nm. During the fracture process, hollow particles relieved crack tip stress, encouraging crack branching and changing the original direction of the main crack. The highest bending strength of this composite achieved was 480 MPa, twice that of monolithic MoSi₂, and the greatest fracture toughness of the composite reached 5.2 MPa·m^1/2.     

Microstructural Properties of Combustion-Synthesized and Dynamically Consolidated Titanium Boride and Titanium Carbide

Full-density TiB₂ and TiC have been fabricated by combustion synthesis reactions followed by dynamic compaction of the still hot, porous ceramic body. The relationship between the morphologies and purities of the precursor powders used and the ceramic product structures is presented. Intergrain bonding and residual porosity of the dynamically consolidated products are found to depend strongly on the impurity levels of the precursor powders. Analysis of the TiC indicates that density and microhardness increase as a function of the C/Ti ratio, with maximum values at the ratio of 1.0.     

SHS Combustion Characteristics of Several Ceramics and Intermetallic Compounds

Extensive comparisons have been conducted between experimental and theoretical results for the SHS combustion characteristics of a number of solid-solid systems. The heterogeneous flame propagation theory describes a premixed mode of bulk flame propagation supported by the nonpremixed reaction of dispersed nonmetal (or higher melting point metal) particles in the liquid metal, with finite-rate reaction at the particle surface and temperature-sensitive Arrhenius-type condensed-phase mass diffusivity. Systems examined are those of borides (TiB₂, ZrB₂, and HfB₂) and intermetallic compounds (NiAl, TiCo, and TiNi). By using a consistent set of physico-chemical parameters for these systems, satisfactorily quantitative agreement is demonstrated for the effects of mixture ratio, degree of dilution, and particle size on the burning velocity. Experimental flammability limits are also predicted by the theory.     

Factors Affecting the Porosity and Bending Strength of Ti(CN)-TiB2 Materials

This paper describes the effects that the particle size of Ti(CN) and TiB₂ powder, oxygen content of the TiB₂ powder, and Co impurity content in the starting raw materials have on the porosity and bending strength ofTi(CN)-30% TiB₂ materials obtained by ordinary sintering.     

High-pressure Self-Combustion Sintering for Ceramics

The high-pressure self-combustion sintering process can be used to synthesize and simultaneously sinter ceramics in a very short time by taking advantage of exothermic reactions under high pressure. A dense TiB₂ compact was fabricated in a few seconds by electric ignition on one end of a powdered mixture of Ti and B under 3 GPa.