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.     

Development and Characterization of Y2O3-Stabilized ZrO2 (Y-TZP) Composites with TiB2, TiN, TiC, and TiC0.5N0.5

Up to 50 vol% of TiB₂, TiC_0.5N_0.5, TiN, or TiC was added to Y₂O₃-stabilized tetragonal ZrO₂ polycrystals (Y-TZP) and hot pressed under vacuum. The influence of the type of secondary phase on the microstructure and mechanical properties was studied, as a function of the hot-pressing temperature. The influence of the secondary-phase content on the mechanical properties was studied by varying the TiB₂ content up to 50 vol%. Fully dense Y-TZP-based composites with very high toughness (up to 10 MPa·m^1/2), excellent bending strength (up to 1237 MPa), and increased hardness, with respect to ZrO₂ (Vickers hardness up to 1450 kg/mm²), were obtained.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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).     

Mechanical Properties of Hot-Pressed TiB2-ZrO2 Composites

The use of monoclinic ZrO₂ as an additive improves the mechanical properties of TiB₂-based composites without the use of stabilizers. In particular, TiB₂-30% ZrO₂ compacts exhibited a transverse rupture strength of 800 MN/m², few pores, and a K_{I c} of 5 MPa·m^1/2. The high strength and toughness are thought to result mainly from the presence of partially stabilized tetragonal ZrO₂ and from solid solution of (TiZr)B₂ formed in sintering.     

Densification and Mechanical Properties of Titanium Diboride with Silicon Nitride as a Sintering Aid

Titanium diboride (TiB₂) was hot-pressed at a temperature of 1800°C, and silicon nitride (Si₃N₄) was added as a sintering aid. The amount of Si₃N₄ that was added had a significant influence on the sinterability and mechanical properties of the TiB₂. When a small amount (2.5 wt%) of Si₃N₄ was added, the Si₃N₄ reacted with titania (TiO₂) that was present on the surface of the TiB₂ powder to form titanium nitride (TiN), boron nitride (BN), and amorphous silica (SiO₂). The elimination of TiO₂ suppressed the grain growth effectively, which led to an improvement in the densification of TiB₂. The formation of SiO₂ also was deemed beneficial for densification. The mechanical properties-especially, the flexural strength-were enhanced remarkably through these improvements in the sinterability and microstructure. On the other hand, when a large amount (greaterthan equal to5 wt%) of Si₃N₄ was added, the mechanical properties were not improved much, presumably because of the extensive formation of a glassy Si-Ti-O-N phase at the grain boundaries.     

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.     

Preferential Grain Orientation in Hot Pressed TiB2

Preferential grain growth is reported for hot pressed titanium diboride (TiB₂) prepared at 1800°C and 50 MPa. Orientation imaging microscopy and X-ray diffraction revealed that the grains in the material were preferentially orientated with the [001] direction parallel to the mechanical field. The experimental findings are discussed, with emphasis on the anisotropic properties of TiB₂.     

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.     

Process-Structure-Reflectance Correlations for TiB2 Films Prepared by Chemical Vapor Deposition

The process-structure-reflectance interrelationships for TiB₂ films prepared by CVD were determined using statistically designed experiments. A hot wall CVD reactor employing graphite substrates and the TiCl₄+ BCl₃+ H₂ reagent system were used at pressures of 2.7 and 6.7 kPa. Single-phase polycrystalline TiB₂ films were obtained. An increasing percentage of the grains were oriented with their (001) planes parallel to the substrate as the deposition temperature was increased and as the BCl₃:TiCl₄ ratio decreased. Grain size increased from ∼0.5 to 3 µm as the deposition temperature was increased from 900° to 1100°C and as the coating rate was decreased from 0.6 to 0.1 µm/min. Fine-grained, smooth, highly reflective films were obtained at low deposition temperatures and high BCl₃:TiCl₄ ratios.     

Polytypes, Grain Growth, and Fracture Toughness of Metal Boride Particulate SiC Composites

In order to understand the relation between microstructure and toughening behavior in SiC materials, NbB₂, TaB₂, TiB₂, and ZrB₂ particulate SiC composites were fabricated with pressureless sintering. In the composites, 3(cubic)-SiC powder was used as starting material for the matrix. The p-SiC powder transformed to a(noncubic) phase during sintering. The transformation, the behavior of which was influenced by the existence of metal boride particles, was accompanied by normal or exaggerated grain growth. The metal boride particles suppressed large-scale exaggerated grain growth of SiC, and it had a tendency to simulate grain growth with a high aspect ratio of the SiC grains. Increase in the fracture toughness of the composites was observed when the grain size and the aspect ratio of the SiC grains increased together. The toughening behavior is discussed based on a grain bridging mechanism.     

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.     

The Effect of Surface Oxides During Hot Pressing of TiB2

Hot pressing of TiB₂ has been investigated with particular emphasis on the evolution of secondary phases originating from the initial surface oxide layer on the TiB₂ powders. Carbothermal reduction of the surface oxides during sintering was also investigated by adding carbon to the TiB₂ powder. TiO_{1− x} C _x was shown to be the main secondary phase in hot-pressed TiB₂, and carbon was shown to strongly influence on sintering process and the amount, composition and distribution of the secondary phase TiO_{1− x} C _x . The formation of TiO_{1− x} C _x is discussed in relation to volatile boron oxide, which reacts with the graphite die to produce CO gas, which further may cause transport of carbon into TiB₂ during sintering before pore closure. Finally it was demonstrated that the density could be controlled by addition of carbon to the TiB₂ powder.