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.     

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.     

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).     

Thermal and Electric Properties in Hot-Pressed ZrB2–MoSi2–SiC Composites

The thermal and electrical properties of MoSi₂ and/or SiC-containing ZrB₂-based composites and the effects of MoSi₂ and SiC contents were examined in hot-pressed ZrB₂–MoSi₂–SiC composites. The thermal conductivity and electrical conductivity of the ZrB₂–MoSi₂–SiC composites were measured at room temperature by a nanoflash technique and a current–voltage method, respectively. The results indicate that the thermal and electrical conductivities of ZrB₂–MoSi₂–SiC composites are dependent on the amount of MoSi₂ and SiC. The thermal conductivities observed for all of the compositions were more than 75 W·(m·K)⁻¹. A maximum conductivity of 97.55 W·(m·K)⁻¹ was measured for the 20 vol% MoSi₂-30 vol% SiC-containing ZrB₂ composite. On the other hand, the electrical conductivities observed for all of the compositions were in the range from 4.07 × 10–8.11 × 10 Ω⁻¹·cm⁻¹.     

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.     

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.     

Elastic Properties and Microcracking Behavior of Particulate Titanium Diboride–Silicon Carbide Composites

The spontaneous microcracking of particulate TiB₂–SiC composites is studied as a function of TiB₂ volume fraction. The degree of microcracking was examined by measuring elastic properties from room temperature to 1300°C. The results showed that only one composition contains microcracks. All other compositions did not microcrack regardless of TiB₂ volume fraction. This was attributed to the difference in the sintering aids. In particular, the Al₂O₃ sintering aid needed in these compositions had reacted with SiO₂ to form an amorphous grain boundary phase that allowed residual stresses to relax by viscous flow at moderate to high temperatures. The existence of this amorphous grain boundary phase was directly observed by transmission electron microscopy.     

Narrowband Microwave Dielectric Resonance and Negative Permittivity Behavior in Hydrogen-Fired Al2O3–CuO Composites

The frequency-dependent microwave complex permittivity of composites based on an Al₂O₃–CuO system is investigated at room temperature. The composites are formed by solution infusion of copper precursors into a porous Al₂O₃ matrix, followed by thermal decomposition to copper oxides and localized formation of copper aluminate compounds, and finally, H₂ firing. The material exhibits a strong amplitude, with relatively narrowband dielectric resonance in the microwave frequency regime at intermediate levels of mass gain, which is reminiscent of a plasmon resonance. Large mass gains cause negative permittivity behavior below 6 GHz, consistent with a collisional Drude model of a semiconducting oxide phase.     

Homogeneous Fabrication of Al2O3–ZrO2–SiC Whisker Composite by Surface-Induced Coating

Al₂O₃–ZrO₂–SiC whisker composites were prepared by surface-induced coating of the precursor for the ZrO₂ phase on the kinetically stable colloid particles of Al₂O₃ and SiC whisker. The fabricated composites were characterized by a uniform spatial distribution of ZrO₂ and SiC whisker phases throughout the Al₂O₃ matrix. The fracture toughness values of the Al₂O₃–15 vol% ZrO₂–20 vol% SiC whisker composites (∼12 MPa.m^1/2) are substantially greater than those of comparable Al₂O₃–SiC whisker composites, indicating that both the toughening resulting from the process zone mechanism and that caused by the reinforced SiC whiskers work simultaneously in hot-pressed composites.     

Novel Fabrication Route to Al2O3–TiN Nanocomposites Via Spark Plasma Sintering

A novel method for the preparation of Al₂O₃–TiN nanocomposites was developed. A mixture of TiO₂, AlN, and Ti powder was used as the starting material to synthesize the Al₂O₃–TiN nanocomposite under 60 MPa at 1400°C for 6 min using spark plasma sintering. X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy were used for detailed microstructural analysis. Dense (up to 99%) nanostructured Al₂O₃–TiN composites were successfully fabricated, the average grain size being less than 400 nm. The fracture toughness ( K _{I C} ) and bending strength (σ_b) of the nanostructured Al₂O₃–TiN composites reached 4.22±0.20 MPa·m^1/2 and 746±28 MPa, respectively.     

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.     

Modeling of Thermal Spalling During Electrical Discharge Machining of Titanium Diboride

Erosion in electrical discharge machining has been described as occurring by melting and flushing the liquid formed. Recently, however, thermal spalling was reported as the mechanism for machining refractory materials with low thermal conductivity and high thermal expansion. The process is described, here, by a model based on a ceramic surface exposed to a constant circular heating source which supplies a constant flux over the pulse duration. The calculations were based on TiB₂ mechanical properties along a and c directions. Theoretical predictions were verified by machining hexagonal TiB₂. Large flakes of TiB₂ with sizes close to grain size and maximum thickness close to the predicted values were collected, together with spherical particles of Cu and Zn eroded from cutting wire. The cutting surfaces consist of cleavage planes sometimes contaminated with Cu, Zn, and impurities from the dielectric fluid.     

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.     

Phase Relations in the System NaF-CaF2-NaAlSiO4-CaAl2Si2O8

The phase diagram of the pseudoternary reciprocal system NaF–CaF₂–NaAlSiO₄–CaAl₂Si₂O₈ is reported in this paper. The phase relations in the system have been investigated by differential thermal analysis, quenching melts, X-ray diffractometry, and optical and electron microscopies. The stable diagonal CaF₂–NaAlSiO₄ divides the system in two pseudoternary systems. The solidus temperatures in the two subsystems NaF–CaF₂–NaAlSiO₄ and CaF₂–NaAlSiO₄–CaAl₂Si₂O₈ are 805°± 2°C and 1095°± 4°C, respectively. An extensive region of liquid–liquid immiscibility is evident in the NaF–CaF₂–NaAlSiO₄ subsystem. The compositions of the two liquids fall outside the compositional surface NaF–CaF₂–NaAlSiO₄–CaAl₂Si₂O₈, but only a small deviation from the ternary behavior is observed.     

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.     

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.     

Fabrication and Microstructures of MoSi2 Reinforced–Si3N4 Matrix Composites

Details of the fabrication and microstructures of hot-pressed MoSi₂ reinforced–Si₃N₄ matrix composites were investigated as a function of MoSi₂ phase size and volume fraction, and amount of MgO densification aid. No reactions were observed between MoSi₂ and Si₃N₄ at the fabrication temperature of 1750°C. Composite microstructures varied from particle–matrix to cermet morphologies with increasing MoSi₂ phase content. The MgO densification aid was present only in the Si₃N₄ phase. An amorphous glassy phase was observed at the MoSi₂–Si₃N₄ phase boundaries, the extent of which decreased with decreased MgO level. No general microcracking was observed in the MoSi₂–Si₃N₄ composites, despite the presence of a substantial thermal expansion mismatch between the MoSi₂ and Si₃N₄ phases. The critical MoSi₂ particle diameter for microcracking was calculated to be 3 μm. MoSi₂ particles as large as 20 μm resulted in no composite microcracking; this indicated that significant stress relief occurred in these composites, probably because of plastic deformation of the MoSi₂ phase.     

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.     

Oxidation of ZrB2–SiC: Influence of SiC Content on Solid and Liquid Oxide Phase Formation

The effect of SiC concentration on the liquid and solid oxide phases formed during oxidation of ZrB₂–SiC composites is investigated. Oxide-scale features called convection cells are formed from liquid and solid oxide reaction products upon oxidation of the ZrB₂–SiC composites. These convection cells form in the outermost borosilicate oxide film of the oxide scale formed on the ZrB₂–SiC during oxidation at high temperatures (≥1500°C). In this study, three ZrB₂–SiC composites with different amounts of SiC were tested at 1550°C for various durations of time to study the effect of the SiC concentration particularly on the formation of the convection cell features. A calculated ternary phase diagram of a ZrO₂–SiO₂–B₂O₃ (BSZ) system was used for interpretation of the results. The convection cells formed during oxidation were fewer and less uniformly distributed for composites with a higher SiC concentration. This is because the convection cells are formed from ZrO₂ precipitates from a BSZ oxide liquid that forms upon oxidation of the composite at 1550°C. Higher SiC-containing composites will have less dissolved ZrO₂ because they have less B₂O₃, which results in a smaller amount of precipitated ZrO₂ and consequently fewer convection cells.     

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.