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.     

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.     

Development of WC–ZrO2 Nanocomposites by Spark Plasma Sintering

In the present work, we report the processing of ultrahard tungsten carbide (WC) nanocomposites with 6 wt% zirconia additions. The densification is conducted by the spark plasma sintering (SPS) technique in a vacuum. Fully dense materials are obtained after SPS at 1300°C for 5 min. The sinterability and mechanical properties of the WC–6 wt% ZrO₂ materials are compared with the conventional WC–6 wt% Co materials. Because of the high heating rate, lower sintering temperature, and short holding time involved in SPS, extremely fine zirconia particles (∼100 nm) and submicrometer WC grains are retained in the WC–ZrO₂ nanostructured composites. Independent of the processing route (SPS or pressureless sintering in a vacuum), superior hardness (21–24 GPa) is obtained with the newly developed WC–ZrO₂ materials compared with that of the WC–Co materials (15–17 GPa). This extremely high hardness of the novel WC–ZrO₂ composites is expected to lead to significantly higher abrasive-wear resistance.     

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.     

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.     

Effect of Oxygen Contamination on Densification of TiB2

In order to identify the mechanisms behind the adverse effect of oxygen contamination on the densification of TiB₂ ceramics, densification and grain growth behavior during pressureless sintering (1700° to 2050°C) were examined as a function of oxygen content of the powder and compared with those observed during hot-pressing (1400° to 1700°C). Marked difference in the grain size-density relations was observed between the two processing methods. Specific roles of oxygen in various kinetic processes were analyzed and the likely oxygen-bearing species were also identified.     

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.     

Mechanical Behavior of MoSi2 Reinforced–Si3N4Matrix Composites

The mechanical behavior of MoSi₂ reinforced–Si₃N₄ matrix composites was investigated as a function of MoSi₂ phase content, MoSi₂ phase size, and amount of MgO densification aid for the Si₃N₄ phase. Coarse-phase MoSi₂-Si₃N₄ composites exhibited higher room-temperature fracture toughness than fine-phase composites, reaching values >8 MP·am^1/2. Composite fracture toughness levels increased at elevated temperature. Fine-phase composites were stronger and more creep resistant than coarse phase composites. Room-temperature strengths >1000 MPa and impression creep rates of ∼10⁻⁸ s⁻¹ at 1200°C were observed. Increased MgO levels generally were deleterious to MoSi₂-Si₃N₄ mechanical properties. Internal stresses due to MoSi₂ and Si₃N₄ thermal expansion coefficient mismatch appeared to contribute to fracture toughening in MoSi₂-Si₃N₄ composites.     

Pressureless Sintering of High-Density ZrB2–SiC Ceramic Composites

Ultra-high-temperature ceramic composites of ZrB₂ 20 wt%SiC were pressureless sintered under an argon atmosphere. The starting ZrB₂ powder was synthesized via the sol–gel method with a small crystallite size and a large specific surface area. Dry-pressed compacts using 4 wt% Mo as a sintering aid can be pressureless sintered to ∼97.7% theoretical density at 2250°C for 2 h. Vickers hardness and fracture toughness of the sintered ceramic composites were 14.82±0.25 GPa and 5.39±0.13 MPa·m^1/2, respectively. In addition to the good sinterability of the ZrB₂ powders, X-ray diffraction and scanning electron microscopy results showed that Mo formed a solid solution with ZrB₂, which was believed to be beneficial for the densification process.     

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.     

Effect of Hot-Pressing Temperature on Densification and Mechanical Properties of Titanium Diboride with Silicon Nitride as a Sintering Aid

The effect of hot-pressing temperature on the densification behavior and mechanical properties of titanium diboride (TiB₂) was investigated. TiB₂ specimens were hot-pressed for 1 h at temperatures in the range of 1500°–1800°C, with an addition of 2.5 wt% of silicon nitride (Si₃N₄) as a sintering aid. The density increased markedly at temperatures in the range of 1500°–1600°C and remained constant thereafter. The formation of a eutectic liquid at 1550°C was attributed to the steep increase in density. The hot-pressing temperature also improved the mechanical properties, such as the flexural strength, Vickers hardness, and fracture toughness of the specimens. Similar to the density, the mechanical properties improved remarkably at ∼1550°C, so that optimum properties were obtainable at temperatures as low as 1600°C.     

Reaction Processing of Al2O3 Composites Containing Iron and Iron Aluminides

Different Fe-Al₂O₃ and FeAl-Al₂O₃ composites with metallic contents up to 30 vol% have been fabricated via reaction processing of Al₂O₃, Fe, and Al mixtures. Low Al contents (<∼10 vol%) within the starting mixture lead to composites consisting of Fe embedded in an Al₂O₃ matrix, whereas aluminide-containing Al₂O₃ composites result from powder mixtures with higher Al contents. In both cases, densification up to 98% TD can be achieved by pressureless sintering in inert atmosphere at moderate temperatures (1450°-1500°C). The proposed reaction sintering mechanism includes the reduction of native oxide layers on the surface of the Fe particles by Al and, in the case of mixtures with high Al contents, aluminide formation followed by sintering of the composites. Density and bending strengths of the reaction-sintered composites depend strongly on the Al content of the starting mixture. In the case of samples containing elemental Fe, crack path observations indicate the potential for an increase of fracture toughness, even at room temperature, by crack bridging of the ductile Fe inclusions.     

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.     

Properties of a Pressureless-Sintered ZrB2–MoSi2 Ceramic Composite

A ZrB₂-based composite was fully densified by pressureless sintering at 1850°C with addition of 20 vol% MoSi₂. The microstructure was very fine, with mean dimensions of ZrB₂ grains around 2.5 μm. The four-point flexural strength in air was in excess of 500 MPa up to 1500°C.     

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.     

Fabrication and High-Temperature Phase Stability of Mo(Al,Si)2—MoSi2 Intermetallics

A two-step processing technique was used to make dense, homogeneous intermetallics in the Mo(Al,Si)₂—MoSi₂ system. A variation of self-propagating high-temperature synthesis was used, in which starting pellets were nucleated at room temperature to make intermetallic powders from metallic precursors, followed by uniaxial hot pressing at 1600°—1800°C to achieve densification. The samples were held at the hot-pressing temperatures for several hours; therefore, this study also provided qualitative phase-stability information. The solubility limit of Al for Si in MoSi₂ was <5% at 1800°C. Samples that had 10% Al substituted for Si yielded approximately equal amounts of MoSi₂ and Mo(Al,Si)₂.     

Pressureless Sintering of Dense Si3N4 and Si3N4/SiC Composites with Nitrate Additives

The effect of aluminum and yttrium nitrate additives on the densification of monolithic Si₃N₄ and a Si₃N₄/SiC composite by pressureless sintering was compared with that of oxide additives. The surfaces of Si₃N₄ particles milled with aluminum and yttrium nitrates, which were added as methanol solutions, were coated with a different layer containing Al and Y from that of Si₃N₄ particles milled with oxide additives. Monolithic Si₃N₄ could be sintered to 94% of theoretical density (TD) at 1500°C with nitrate additives. The sintering temperature was about 100°C lower than the case with oxide additives. After pressureless sintering at 1750°C for 2 h in N₂, the bulk density of a Si₃N₄/20 wt% SiC composite reached 95% TD with nitrate additives.     

Sintering of Nanophase γ-Al2O3 Powder

The sintering of ultrafine γ-Al₂O₃ powder (particle size ∼10–20 nm) prepared by an inert gas condensation technique was investigated in air at a constant heating rate of 10°C/min. Qualitatively, the kinetics followed those of transition aluminas prepared by other methods. Measurable shrinkage commenced at ∼ 1000°C and showed a region of rapid sintering between ∼1125° and 1175°C followed by a transition to a much reduced sintering rate at higher temperatures. Starting from an initial density of ∼0.60 relative to the theoretical value, the powder compact reached a relative density of 0.82 after sintering to 1350°C. Compared to compacts prepared from the as-received powder, dispersion of the powder in water prior to compaction produced a drastic change in the microstructural evolution and a significant reduction in the densification rate during sintering. The incorporation of a step involving the rapid heating of the loose powder to ∼1300°C prior to compaction (which resulted in the transformation to α-Al₂O₃) provided a method for significantly increasing the density during sintering.     

Pressureless Sintering of SiC-Whisker-Reinforced Al2O3 Composites: I, Effect of Matrix Powder Surface Area

Pressureless sintering of SiC-whisker-reinforced Al₂O₃ composites was investigated. In Part I of the study, the effect of the matrix (Al₂O₃) powder surface area on densification behavior and microstructure development is reported. Compacts prepared with higher surface area Al₂O₃ powder showed enhanced densification at lower whisker concentrations (5 and 15 vol%). Samples with 15 vol% whiskers could be pressureless sintered to ∼97% relative density with zero open porosity and ∼1.6-μm matrix average grain intercept size.     

Improvement of Mechanical Properties and Corrosion Resistance of Porous β-SiAlON Ceramics by Low Y2O3 Additions

Addition of Y₂O₃ as a sintering additive to porous β-SiAlON (Si_{6− z} Al _z O _z N_{8− z} , z = 0.5) ceramics has been investigated for improved mechanical properties. Porous SiAlON ceramics with 0.05–0.15 wt% (500–1500 wppm) Y₂O₃ were fabricated by pressureless sintering at temperatures of 1700°, 1800°, and 1850°C. The densification, microstructure, and mechanical properties were compared with those of Y₂O₃-free ceramics of the same chemical composition. Although this level of Y₂O₃ addition did not change the phase formation and grain size, the grain bonding appeared to be promoted, and the densification to be enhanced. There was a significant increase in the flexural strength of the SiAlON ceramics relative to the Y₂O₃-free counterpart. After exposure in 1 M hydrochloric acid solution at 70°C for 120 h, no remarkable weight loss and degradation of the mechanical properties (flexural and compression strength) was observed, which was attributed to the limited grain boundary phase, and with the minor Y₂O₃ addition the supposed formation of Y-α-SiAlON.