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).     

Densification, Sintering Reactions, and Properties of Titanium Diboride With Titanium Disilicide as a Sintering Aid

In the present investigation, we explore the feasibility of using TiSi₂ as a sintering aid to densify titanium diboride (TiB₂) at a lower sintering temperature (<1700°C). The hot-pressing experiments were conducted in the temperature range of 1400°–1650°C for 1 h in an argon atmosphere and TiSi₂ addition to TiB₂ was restricted up to 10 wt%, with an overall objective to densify the materials with a fine microstructure as well as to assess the feasibility of enhancing the mechanical and electrical properties. When all the materials were hot pressed at 1650°C, the hot-pressed TiB₂– X % TiSi₂ ( X =0, 2.5, 5, 10 wt%) composites were found to be densified to more than 98%ρ_th (theoretical density), except monolithic TiB₂ (∼94%ρ_th). An interesting observation is the formation of a Ti₅Si₃ phase and this phase formation is described by thermodynamically feasible sintering reactions. Our experimental results suggest that the optimal TiB₂–5 wt% TiSi₂ composite can exhibit an excellent combination of properties, including a high hardness of 25 GPa, an elastic modulus of 518 GPa, an indentation toughness of ∼6 MPa·m^1/2, a four-point flexural strength of more than 400 MPa, and an electrical resistivity of 10 μΩ·cm.     

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.     

Reactive Hot Pressing of Titanium Nitride–Titanium Diboride Composites at Moderate Pressures and Temperatures

Dense composites in the Ti-B-N system have been produced by reactive hot pressing of titanium and BN powders. The effect of the addition of a small amount of nickel (1–3 wt%) on the reaction kinetics and densification of TiN–TiB₂ (40 vol%) composite has been studied. Composites of ∼99% of theoretical density have been produced at 1600°C under 40 MPa for 30 min with 1% nickel. The hardness and fracture toughness of these composites are 24.5 ± 0.97 GPa and 6.53 ± 0.27 MPa·m^1/2, respectively. The microstructural studies on samples produced at lower temperatures indicate the formation of a transient liquid phase, which enhances the kinetics of the reaction and densification of the composite.     

Simultaneous Spark Plasma Synthesis and Densification of TiC–TiB2 Composites

The simultaneous synthesis and densification of dense TiC–TiB₂ composite has been investigated starting from Ti, B₄C, and C as reactants, and using the spark plasma sintering (SPS) technique. The optimal conditions for complete conversion of the reactants to the composite were determined for different applied DC current levels. A kinetic investigation performed allows us to conclude that solid-state diffusion is the mechanism governing the synthesis process. It is seen that TiC is the first phase formed, while TiB₂ formation occurs afterward. Two intermediate boride phases, i.e. TiB and Ti₃B₄, are also formed but, as the SPS holding time was augmented, they were gradually and completely converted to TiB₂. Moreover, it is found that in order to reach relatively high dense products, an electric current needs to be applied for time intervals longer than those required for obtaining complete conversion. A pure dense product (relative density ∼98%) was obtained when an electric pulsed current of 1100 A and a mechanical pressure of 20 MPa were applied for about 4 min.     

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.     

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.     

Al2O3–Ni Composites with High Strength and Fracture Toughness

Al₂O₃–Ni composites were prepared by the reactive hot pressing of Al and NiO. The composites had a two-phase, interpenetrating microstructure and contained ∼35 vol% Ni. They exhibited an impressively high combination of strength and toughness at room temperature; the four-point bending strength was in excess of 600 MPa with a fracture toughness of more than 12 MPa·m^1/2. Examination of fracture surfaces showed that Ni ligaments underwent ductile deformation during fracture. SEM analysis revealed knife-edged Ni ligaments with a limited amount of debonding around their periphery (i.e., at the Ni–Al₂O₃ interface), indicating a strong Ni–Al₂O₃ bond.     

Multiwalled Carbon Nanotubes–TiB2–Ni Composite: Microstructure and Mechanical Properties

Carbon nanotubes (CNTs)–TiB₂–Ni composites with improved mechanical properties were fabricated by hot pressing at 1600°C/30 MPa/1 h. The effects of CNTs content on the microstructural feature and mechanical properties were investigated. The incorporation of multiwall carbon nanotubes (MWNTs) could significantly improve the mechanical properties of the TiB₂ cermet matrix composites. The main toughening mechanisms include CNTs pulling-out, crack deflexion, bridging, and CNTs rupture.     

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.     

Wear Mechanisms of TiB2 and TiB2–TiSi2 at Fretting Contacts with Steel and WC–6 wt% Co

Unlubricated fretting wear tests on TiB₂ and TiB₂–5 wt% TiSi₂ ceramics against two different mating materials (bearing grade steel and WC–6 wt% Co balls) were performed with a view to understand the counterbody-dependent difference in friction and wear properties. The fretting experiments were conducted systematically by varying load (2–10 N) at an oscillating frequency of 4 Hz and 100 μm linear stroke, for a duration of 100,000 cycles. Adhesion, abrasion, and three-body wear have been observed as mechanisms of material damage for both the TiB₂/steel and TiB₂/WC–Co tribosystems. The third body is predominantly characterized as tribochemical layer for TiB₂/steel and loose wear debris particles for TiB₂/WC–Co tribocouple. An explanation on differences in tribological properties has been provided in reference to the counterbody material as well as microstructure and mechanical properties of flat materials.     

Fabrication and Characterization of Anode-Supported Tubular Solid-Oxide Fuel Cells by Slip Casting and Dip Coating Techniques

High-performance anode-supported tubular solid-oxide fuel cells (SOFCs) have been successfully developed and fabricated using slip casting, dip coating, and impregnation techniques. The effect of a dispersant and solid loading on the viscosity of the NiO/Y₂O₃–ZrO₂ (NiO/YSZ) slurry is investigated in detail. The viscosity of the slurry was found to be minimum when the dispersant content was 0.6 wt% of NiO/YSZ. The effect of sintering temperature on the shrinkage and porosity of the anode tubes, densification of the electrolyte, and performance of the cell at different solid loadings is also investigated. A Ni/YSZ anode-supported tubular cell fabricated from the NiO/YSZ slurry with 65 wt% solid loading and sintered at 1380°C produced a peak power output of ∼491 and ∼376 mW/cm² at 800°C in wet H₂ and CH₄, respectively. With the impregnation of Ce_0.8Gd_0.2O₂ (GDC) nanoparticles, the peak power density increased to ∼1104 and ∼770 mW/cm² at 800°C in wet H₂ and CH₄, respectively. GDC impregnation considerably enhances the electrochemical performance of the cell and significantly reduces the ohmic and polarization resistances of thin solid electrolyte cells.     

Tribological Behavior of Mo5Si3-Particle-Reinforced Silicon Nitride Composites

The tribological behavior of Mo₅Si₃-particle-reinforced silicon nitride (Si₃N₄) composites was investigated by pin-on-plate wear testing under dry conditions. The friction coefficient of the Mo₅Si₃–Si₃N₄ composites and Si₃N₄ essentially decreased slowly with the sliding distance, but showed sudden increase for several times during the wear testing. The average friction coefficient of the Si₃N₄ decreased with the incorporation of submicrometer-sized Mo₅Si₃ particles and also as the content of Mo₅Si₃ particles increased. When the Mo₅Si₃–Si₃N₄ composites were oxidized at 700°C in air, solid-lubricant MoO₃ particles were generated on the surface layer. Oxidized Mo₅Si₃–Si₃N₄ composites showed self-lubricating behavior, and the average friction coefficient and wear rate of the oxidized 2.8 wt% Mo₅Si₃–Si₃N₄ composite were 0.43 and 0.72 × 10⁻⁵ mm³ (N·m)⁻¹, respectively. Both values were ∼30% lower than those for the Si₃N₄ tested in an identical manner.     

In Situ TiC/TiB2 Particulate Locally Reinforced Steel Matrix Composites Fabricated Via the SHS Reaction of Ni–Ti–B4C System

The composites synthesized with three kinds of B₄C particles mainly consist of TiC, TiB₂, and the alloy austenite containing Ni element. Ceramic particulate sizes in the composites synthesized with ∼3.5 and ∼45 μm B₄C particles are larger than that synthesized with ∼140 μm B₄C particle. No pores are found between the reinforcing region and matrix in the composites synthesized with ∼3.5 and ∼45 μm B₄C particles, while some large pores exist in the composites synthesized with ∼140 μm B₄C particle. With the decrease of B₄C particle size, the pores in the composites become fewer and the hardness and wear resistance of the composites increase.     

Influence of MoSi2 Addition on Load-Dependent Fretting Wear Properties of TiB2 Against Cemented Carbide

In an earlier work, it was observed that the use of MoSi₂ (up to 10 wt%) enhanced the densification and mechanical properties of TiB₂. Therefore, the motivation of this study is three-fold: (a) to assess whether a small amount of MoSi₂ addition can enhance wear resistance property, (b) to study whether the MoSi₂ addition will influence the formation of a tribochemical layer, and (c) to correlate the wear resistance with material properties in TiB₂–MoSi₂ materials. In order to address these issues, a series of fretting experiments were conducted systematically by varying load (2, 5, and 10 N) at an oscillating frequency of 4 Hz and a 100 μm linear stroke, for a duration of 100 000 cycles with a cemented carbide (WC—6 wt% Co cermet) ball as a counterbody. The average coefficient of friction of the TiB₂ samples varied within a narrow range (0.50–0.54), without being much affected by either the sintering additive or the load. The wear volume increased with increasing load, while the specific wear rate of all the TiB₂ compositions falls within a mild wear regime (1.1–3.4 × 10⁻⁶ mm³/Nm). Based on the experimental results, it can be said that the addition of MoSi₂ degrades neither the wear resistance properties nor the frictional properties of TiB₂, within the investigated load regime. The microcracking-induced spalling has been found to be the dominant mechanism and, consequently, the wear volume is observed to have a linear dependency on the abrasion parameter. It is noteworthy that the tribo-oxidation as well as the formation of finer wear debris particles occurs to a limited extent.     

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.     

Effect of Iron and Boron Carbide on the Densification and Mechanical Properties of Titanium Diboride Ceramics

The effect of Fe and B₄C on the sintering behavior and mechanical properties of TiB₂ ceramics have been studied. Sintering was performed in an Ar atmosphere at 2000° using attrition-milled TiB₂ powder (mean particle size = 0.8 μm). When a small amount of Fe (0.5 wt%) was added, abnormal grain growth occurred and the sintered density was low. In the case of B₄C added along with 0.5 wt% Fe, however, abnormal grain growth was remarkably suppressed, and the sintered density was increased up to 95% of theoretical. But with excess Fe addition (5 wt%), B₄C grains did not act as a grain growth inhibitor, and B₄C grains were frequently trapped in large TiB₂ grains. The best mechanical properties were obtained for the TiB₂–10 wt% B₄C–0.5 wt% Fe ceramics, which exhibited a three-point bending strength of 400 MPa and a fracture toughness of 5.5 MPa · m^1/2.     

Effect of Y2O3 Additions on the Densification of an Al2O3–TiC Composite

The densification of Al₂O₃–30TiC (in weight percent) composite is investigated as a function of Y₂O₃ additions. It is observed that very small amounts of Y₂O₃ are effective in aiding the densification. The density was observed to pass through a maximum at 0.35 wt% of Y₂O₃. The gas-generating reaction of Al₂O₃ with TiC is likely to be suppressed by the addition of Y₂O₃.     

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.     

Rapid Rate Sintering of Nanocrystalline ZrO2−3 mol% Y2O3

Conventional ramp-and-hold sintering with a wide range of heating rates was conducted on submicrometer and nanocrystalline ZrO₂–3 mol% Y₂O₃ powder compacts. Although rapid heating rates have been reported to produce high density/fine grain size products for many submicrometer and smaller starting powders, the application of this technique to ZrO₂–3 mol% Y₂O₃ produced mixed results. In the case of submicrometer ZrO₂–3 mol% Y₂O₃, neither densification nor grain growth was affected by the heating rate used. In the case of nanocrystalline ZrO₂–3 mol% Y₂O₃, fast heating rates severely retarded densiflcation and had a minimal effect on grain growth. The large adverse effect of fast heating rates on the densification of the nanocrystalline powder was traced to a thermal gradient/differential densification effect. Microstructural evidence suggests that the rate of densification greatly exceeded the rate of heat transfer in this material; consequently, the sample interior was not able to densify before being geometrically constrained by a fully dense shell which formed at the sample exterior. This finding implies that rapid rate sintering will meet severe practical constraints in the manufacture of bulk nanocrystalline ZrO₂–3 mol% Y₂O₃ specimens.