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.     

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.     

Preparation and Microstructure of a ZrB2–SiC Composite Fabricated by the Spark Plasma Sintering–Reactive Synthesis (SPS–RS) Method

A mixture of Zr, B₄C, and Si powders was adopted to synthesize a ZrB₂–SiC composite using the spark plasma sintering–reactive synthesis (SPS–RS) method. SPS treatments were carried out in the temperature range of 1350°–1500°C under a varying pressure of 20–65 MPa with a 3-min holding time. A dense (∼98.5%) ZrB₂–SiC composite was successfully fabricated at 1450°C for 3 min under 30 MPa. The microstructure of the composite was investigated. The in situ formed ZrB₂ and SiC phases dispersed homogeneously on the whole. The grain size of ZrB₂ and SiC was <5 and 1 μm, respectively. A number of in situ formed ultrafine SiC particles were observed entrapped in the ZrB₂ grains.     

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.     

Microstructural Evolution during Transient Plastic Phase Processing of Titanium Carbide-Titanium Boride Composites

Transient plastic phase processing is a form of reactive hot pressing for fabricating fully dense ceramic-ceramic composites at relative low homologous temperatures. In this study, this technique has been used on two powder mixtures—4:1 Ti/B₄C and 1:1 TiC_0.5/TiB₂, which are equivalent in terms of elemental compositions—to produce fully dense titanium carbide-titanium boride composites. The composites formed in each case are comprised of the same final phases—TiC_x, TiB₂, and Ti₃B₄, in roughly the same volume fractions—but exhibit distinctly different grain morphologies. Ti₃B₄ phase nucleates and grows as platelets for the 4:1 Ti/B₄C starting composition but as equiaxed grains for the 1:1 TiC_0.5/TiB₂ composition. TiB has been identified as an intermediate phase in the "platelet" composition and appears to be important to the development of the Ti₃B₄ platelets. X-ray diffractometry and scanning electron microscopy results indicate that the evolution of the microstructure is governed by the diffusion of boron and carbon, rather than titanium. In addition, the faster diffusion of carbon, relative to boron, is instrumental in the microstructural evolution of the platelet composite. The produced composites possess >99% density and good mechanical properties. The higher strength and toughness of the platelet composite are believed to be due to the platelet morphology of the Ti₃B₄ phase.     

Electrical Resistivities of Monocrystalline and Polycrystalline TiB2

The electrical resistivity of monocrystalline and polycrystalline TiB₂, was measured under an inert atmosphere by a four-point ac impedance technique over the range 298 to 1373 K. The results are expressed in the form ρ-ρ₂₉₈= m(T -298). The following values of ρ₂₉₈ (μω.cm) and m (nω.cm-K^-1) were determined: for polycrystalline TiB₂ (69% dense) 18.2 and 95; for polycrystalline TiB₂ (99% dense) 7.4 and 42; and for monocrystalline TiB₂, 6.6 and 34.9.     

Combustion Synthesis of the Titanium-Aluminum-Boron System

TiB₂ and Al base composite powders, which will offer a weight-saving improvement in stiffness, were produced by combustion synthesis of Ti, Al, and B ternary powder mixtures. Finely dispersed TiB₂ was synthesized by reacting a mixture of Ti, Al, and B in the molar ratio of 1:1:4. The grain size of the TiB₂ formed was <0.5 μm, which was much smaller than that obtained from the reaction of a mixture of Ti, Al, and B in the molar ratio 1:1:2. These results are discussed in light of the reaction propagating velocity and heat removal during the combustion synthesis process.     

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.     

Chemical Vapor Infiltration of TiB2-Matrix Composites

The efficiency of the Hall–Heroult electrolytic reduction of aluminum can be substantially improved by the use of a TiB₂ cathode. The use of TiB₂ components, however, has been hampered by the brittle nature of the material and the grain boundary attack of sintering-aid phases by molten aluminum. In the current work, TiB₂ is toughened through the use of reinforcing fibers, with chemical vapor infiltration used to produce the TiB₂ matrix. In early efforts it was observed that the formation of TiB₂ from chloride precursors at fabrication temperatures below 900–1000°C may have allowed the retention of destructive levels of chlorine. At higher fabrication temperatures (>1000°C), using appropriate infiltration conditions as determined from the use of a process model, TiB₂/THORNEL P-25 fiber composites have been fabricated in 20 h. The improved composite material has been demonstrated to be stable in molten aluminum in short-duration (24 h) tests.     

Synthesis of Dense TiB2-TiN Nanocrystalline Composites through Mechanical and Field Activation

The synthesis of dense nanometric composites of TiN-TiB₂ by mechanical and field activation was investigated. Powder mixtures of Ti, BN, and B were mechanically activated through ball milling. Some powders were milled to reduce crystallite size but to avoid initiating a reaction. In other cases powders were milled and allowed to partially react. All these were subsequently reacted in a spark plasma synthesis (SPS) apparatus. The products were composites with equimolar nitride and boride components with relative densities ranging from 90.1% to 97.2%. Crystallite size analyses using the XRD treatments of Williamson-Hall and Halder-Wagner gave crystallite sizes for the TiN and TiB₂ components in the range 38.5–62.5 and 31.2–58.8 nm, respectively. Vickers microhardness measurements (at 2 N force) on the dense samples gave values ranging from 14.8 to 21.8 GPa and fracture toughness determinations (at 20 N) resulted in values ranging from 3.32 to 6.50 MPa·m^1/2.     

Simultaneous Synthesis and Densification of MoSi2 by Field-Activated Combustion

A method to simultaneously synthesize and consolidate MoSi₂ from powders of Mo and Si was investigated. Combustion synthesis was carried out under the combined effect of an electric field and mechanical pressure. Highly dense molybdenum silicide up to (99.2%) was produced from elemental powders in one step. Minor amounts of Mo₅Si₃ were present at the boundaries of MoSi₂ grains in the interior of samples made from stoichiometric reactants. The addition of 2.5 mol% Si excess, however, resulted in Mo₅Si₃-free, dense MoSi₂ products.     

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.     

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.     

Sintering of in-Situ Synthesized Sic–TiB2 Composites with Improved Fracture Toughness

TiB₂-particle reinforcement is one of the most successful methods for improving the fracture toughness of SiC ceramics.^1–3 Commercially available TiB₂ powders, however, have a large particle size and/or are highly reactive so that they are not favorable as a starting powder. In the present work, TiB₂ particles are formed by an in situ reaction between TiC and boron. The reaction takes place during sintering between 1000° and 1600°C and is accompanied by a large volume expansion. Under optimum conditions, dense composites (> 98% of theoretical) can be obtained by pressureless sintering using B and C as sintering additives. The in situ reaction method enables, for the first time, a complete densification of SiC-particulate composites by pressureless sintering. The fracture toughness of the composites was approximately 30% higher than that of the monolithic SiC ceramic.     

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.     

Homogeneous TiB2 Ceramics Achieved by Electric Current-Assisted Self-Propagating Reaction Sintering

Using spark plasma sintering techniques, homogeneous microstructures of titanium diboride (TiB₂) ceramics were obtained by sintering of boron and titanium powder mixtures. The results show that an additional electric current is essential for achieving a large number of evenly distributed ignition points that ensure that the self-propagating reaction simultaneously takes place within the entire volume. The effects of the electric current, the use of Mg additions, and the heating rates on the resulting TiB₂ ceramic densities and microstructures are discussed.     

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.     

Oxidation Behavior of Titanium Boride at Elevated Temperatures

The oxidation behavior of dense TiB₂ specimens was investigated. Hot-pressed TiB₂ with 2.5 wt% Si₃N₄ as a sintering aid was exposed to air at temperatures between 800° and 1200°C for up to 10 h. The TiB₂ exhibited two distinct oxidation behaviors depending on the temperature. At temperatures below 1000°C, parabolic weight gains were observed as a result of the formation of TiO₂( s ) and B₂O₃( l ) on the surface. The oxidation layer comprised two layers: an inner layer of crystalline TiO₂ and an outer layer mainly composed of B₂O₃. When the oxidation temperatures were higher than 1000°C, gaseous B₂O₃ was formed along with crystalline TiO₂ by the oxidation process. In this case, the surface was covered with large TiO₂ grains imbedded in a highly textured small TiO₂ matrix.     

Processing and Properties of TiB2 with MoSi2 Sinter-additive: A First Report

The densification of non-oxide ceramics like titanium boride (TiB₂) has always been a major challenge. The use of metallic binders to obtain a high density in liquid phase-sintered borides is investigated and reported. However, a non-metallic sintering additive needs to be used to obtain dense borides for high-temperature applications. This contribution, for the first time, reports the sintering, microstructure, and properties of TiB₂ materials densified using a MoSi₂ sinter-additive. The densification experiments were carried out using a hot-pressing and pressureless sintering route. The binderless densification of monolithic TiB₂ to 98% theoretical density with 2–5 μm grain size was achieved by hot pressing at 1800°C for 1 h in vacuum. The addition of 10–20 wt% MoSi₂ enables us to achieve 97%–99%ρ_th in the composites at 1700°C under similar hot-pressing conditions. The densification mechanism is dominated by liquid-phase sintering in the presence of TiSi₂. In the pressureless sintering route, a maximum of 90%ρ_th is achieved after sintering at 1900°C for 2 h in an (Ar+H₂) atmosphere. The hot-pressed TiB₂–10 wt% MoSi₂ composites exhibit high Vickers hardness (∼26–27 GPa) and modest indentation toughness (∼4–5 MPa·m^1/2).     

Microcracking in B4C-TiB2 Composites

Boron carbide/titanium diboride composites with 20 and 40 vol% particulate TiB₂ and various amounts of free carbon were investigated with respect to microcrack toughening. In agreement with previous work, the mere addition of TiB₂ was found to raise the toughness from 2.2 MPa·m^1/2 up to 3.0 and 3.5 MPa·m^1/2, respectively. A further and very significant increase of composite toughness up to 6.0 MPa·m^1/2 was discovered upon the incorporation of free carbon. SEM and TEM observations reveal that this toughening is associated with microcracking at B₄C-TiB₂ phase boundaries. Microcracking is triggered by thin carbon interlayers, which are located at hetero interfaces and supply a weak fracture path.