Reinforcement of Hydroxyapatite Bioceramic by Addition of Ti3SiC2

Ti₃SiC₂/HAp composites with different Ti₃SiC₂ volume fractions were fabricated by spark plasma sintering (SPS) at 1200°C. The effects of Ti₃SiC₂ addition on the mechanical properties and microstructures of the composites were investigated. The bending strength and fracture toughness of the composites increased with increasing of Ti₃SiC₂ content, whereas the Vickers hardness decreased. The bending strength and fracture toughness reached 252±10 MPa and 3.9±0.1 MPa·m^1/2, respectively, with the addition of 50 vol% Ti₃SiC₂. The increases in the mechanical properties were attributed to the matrix strengthening and interactions between cracks and the Ti₃SiC₂ platelets.     

Fabrication and Microstructure Characterization of Ti3SiC2 Synthesized from Ti/Si/2TiC Powders Using the Pulse Discharge Sintering (PDS) Technique

Ti/Si/2TiC powders were prepared using a mixture method (M) and a mechanical alloying (MA) method to fabricate Ti₃SiC₂ at 1200°–1400°C using a pulse discharge sintering (PDS) technique. The results showed that the Ti₃SiC₂ samples with <5 wt% TiC could be rapidly synthesized from the M powders; however, the TiC content was always >18 wt% in the MA samples. Further sintering of the M powder showed that the purity of Ti₃SiC₂ could be improved to >97 wt% at 1250°–1300°C, which is ∼200°–300°C lower than that of sintered Ti/Si/C and Ti/SiC/C powders using the hot isostatic pressing (HIPing) technique. The microstructure of Ti₃SiC₂ also could be controlled using three types of powders, i.e., fine, coarse, or duplex-grained, within the sintering temperature range. In comparison with Ti/Si/C and Ti/SiC/C mixture powders, it has been suggested that high-purity Ti₃SiC₂ could be rapidly synthesized by sintering the Ti/Si/TiC powder mixture at relatively lower temperature using the PDS technique.     

Cyclic Fatigue-Crack Growth and Fracture Properties in Ti3SiC2 Ceramics at Elevated Temperatures

The cyclic fatigue and fracture toughness behavior of reactive hot-pressed Ti₃SiC₂ ceramics was examined at temperatures from ambient to 1200°C with the objective of characterizing the high-temperature mechanisms controlling crack growth. Comparisons were made of two monolithic Ti₃SiC₂ materials with fine- (3–10 μm) and coarse-grained (70–300 μm) microstructures. Results indicate that fracture toughness values, derived from rising resistance-curve behavior, were significantly higher in the coarser-grained microstructure at both low and high temperatures; comparative behavior was seen under cyclic fatigue loading. In each microstructure, Δ K _th fatigue thresholds were found to be essentially unchanged between 25° and 1100°C; however, there was a sharp decrease in Δ K _th at 1200°C (above the plastic-to-brittle transition temperature), where significant high-temperature deformation and damage are first apparent. The substantially higher cyclic-crack growth resistance of the coarse-grained Ti₃SiC₂ microstructure was associated with extensive crack bridging behind the crack tip and a consequent tortuous crack path. The crack-tip shielding was found to result from both the bridging of entire grains and from deformation kinking and bridging of microlamellae within grains, the latter forming by delamination along the basal planes.     

Effect of Vacuum Annealing on the Phase Stability of Ti3SiC2

The effect of vacuum annealing on the thermal stability and phase transition of Ti₃SiC₂ has been investigated by X-ray diffraction (XRD), neutron diffraction, synchrotron radiation diffraction, and secondary ion mass spectroscopy (SIMS). In the presence of vacuum or a controlled atmosphere of low oxygen partial pressure, Ti₃SiC₂ undergoes a surface dissociation to form nonstoichiometric TiC and/or Ti₅Si₃C _x that commences at ∼1200°C and becomes very pronounced at ≥1500°C. Composition depth profiling at the near surface of vacuum-annealed Ti₃SiC₂ by XRD and SIMS revealed a distinct gradation in the phase distribution of TiC and Ti₅Si₃C _x with depth.     

Processing and Mechanical Properties of Ti3SiC2: II, Effect of Grain Size and Deformation Temperature

In this article, the second part of a two-part study, we report on the mechanical behavior of Ti₃SiC₂. In particular, we have evaluated the mechanical response of fine-grained (3–5 μm) Ti₃SiC₂ in simple compression and flexure tests, and we have compared the results with those of coarse-grained (100–200 μm) Ti₃SiC₂. These tests have been conducted in the 25°–1300°C temperature range. At ambient temperature, the fine- and coarse-grained microstructures exhibit excellent damage-tolerant properties. In both cases, failure is brittle up to ∼1200°C. At 1300°C, both microstructures exhibit plastic deformation (>20%) in flexure and compression. The fine-grained material exhibits higher strength compared with the coarse-grained material at all temperatures. Although the coarse-grained material is not susceptible to thermal shock (up to 1400°C), the fine-grained material thermally shocks gradually between 750° and 1000°C. The results presented herein provide evidence for two important aspects of the mechanical behavior of Ti₃SiC₂: (i) inelastic deformation entails basal slip and damage formation in the form of voids, grain-boundary cracks, kinking, and delamination of individual grains, and (ii) the initiation of damage does not result in catastrophic failure, because Ti₃SiC₂ can confine the spatial extent of the damage.     

Mechanical-Alloying-Assisted Synthesis of Ti3SiC2 Powder

Mechanical alloying (MA) has been used to synthesize Ti₃SiC₂ powder from the elemental Ti, Si, and C powders. The MA formation conditions of Ti₃SiC₂ were strongly affected by the ball size for the conditions used. MA using large balls (20.6 mm in diameter) enhanced the formation of Ti₃SiC₂, probably via an MA-triggered combustion reaction, but the Ti₃SiC₂ phase was not synthesized only by the MA process using small balls (12.7 mm in diameter). Fine powders containing 95.8 vol% Ti₃SiC₂ can be obtained by annealing the mechanically alloyed powder at relatively low temperatures.     

Thermal Expansion of Polycrystalline Ti3SiC2 in the 25°–1400°C Temperature Range

We measured the volume thermal expansion of Ti₃SiC₂ from 25° to 1400°C using high-temperature X-ray diffraction using a resistive heated cell. A piece of molybdenum foil with a 250 μm hole contained the sample material (Ti₃SiC₂+Pt). Thermal expansion of the polycrystalline sample was measured under a constant argon flow to prevent oxidation of Ti₃SiC₂ and the molybdenum heater. From the lattice parameters of platinum (internal standard), we calculated the temperature by using thermal expansion data published in the literature. The molar volume change of Ti₃SiC₂ as a function of temperature in °C is given by: V _M (cm³/mol)=43.20 (2)+9.0 (5) × 10⁻⁴ T +1.8(4) × 10⁻⁷ T ². The temperature variation of the volumetric thermal expansion coefficient is given by: α_v (°C⁻¹)=2.095 (1) × 10⁻⁵+7.700 (1) × 10⁻⁹ T . Furthermore, the results indicate that the thermal expansion anisotropy of Ti₃SiC₂ is quite mild in accordance with previous work.     

Synthesis of Ti3SiC2 Powders: Reaction Mechanism

Titanium silicon carbide (Ti₃SiC₂) and Ti₃SiC₂-based composite powders were synthesized by isothermal treatment in an inert atmosphere as a function of initial compositions (mixtures). A high content of TiC was obtained in the final product when the initial mixtures contained free carbon. The use of TiC as a reagent was unsuccessful in obtaining Ti₃SiC₂. High Ti₃SiC₂ conversion was found for the initial mixtures containing SiC as the main source for silicon and carbon. An initial mixture with a large excess of silicon, 3Ti/1.5SiC/0.5C, was needed to obtain high-purity Ti₃SiC₂. A reaction mechanism, where Ti₃SiC₂ nucleates on Ti₅Si₃C crystals and grows by long-range diffusion of Ti and C, is proposed. The reaction mechanism was proposed to be based on silicon loss during the formation of Ti₃SiC₂.     

Self-Propagating High-Temperature Synthesis of Ti3SiC2: Study of the Reaction Mechanisms by Time-Resolved X-Ray Diffraction and Infrared Thermography

Ti₃SiC₂ is synthesized by self-propagating high-temperature synthesis (SHS) of elemental titanium, silicon, and graphite powders. The reaction paths and structure evolution are studied in situ during the SHS of the 3Ti+Si+2C mixture by time-resolved X-ray diffraction coupled with infrared thermography. The proposed reaction mechanism suggests that Ti₃SiC₂ might be formed from Ti–Si liquid phase and solid TiC _x . Finally, the effect of the powders starting composition on the Ti₃SiC₂ synthesis is studied. For the investigated initial mixtures, TiC _x is always formed as a major impurity together with the Ti₃SiC₂ phase.     

Intermediate Phases in Ti3SiC2 Synthesis from Ti/SiC/C Mixtures Studied by Time-Resolved Neutron Diffraction

The reactive sintering of 3Ti/SiC/C to form the layered ternary carbide Ti₃SiC₂ was studied in situ by time-resolved neutron powder diffraction. A number of intermediate processes occur during the synthesis beginning with the α-β transition in Ti. Concurrent with the α-β transition, two intermediate phases, TiC _x and Ti₅Si₃C _x ( x ≤ 1), form. These phases account for almost the entire sample in the range 1500–1600°C beyond which they react with each other and a small amount of free C to form the product phase Ti₃SiC₂.     

Interdiffusion Between Ti3SiC2–Ti3GeC2 and Ti2AlC–Nb2AlC Diffusion Couples

In this work, we report on the interdiffusion of Ge and Si in Ti₃SiC₂ and Ti₃GeC₂, as well as that of Nb and Ti in Ti₂AlC and Nb₂AlC. The interdiffusion coefficient, D _int, measured by analyzing the diffusion profiles of Si and Ge obtained when Ti₃SiC₂–Ti₃GeC₂ diffusion couples are annealed in the 1473–1773 K temperature range at the Matano interface composition (≈Ti₃Ge_0.5Si_0.5C₂), was found to be given by
D _int increased with increasing Ge composition. At the highest temperatures, diffusion was halted after a short time, apparently by the formation of a diffusion barrier of TiC. Similarly, the interdiffusion of Ti and Nb in Ti₂AlC–Nb₂AlC couples was measured in the 1723–1873 K temperature range. The D _int for the Matano interface composition, viz. ≈(Ti_0.5,Nb_0.5)₂AlC, was found to be given by
At 1773 K, the diffusivity of the transition metal atoms was ≈7 times smaller than those of the Si and Ge atoms, suggesting that the former are better bound in the structure than the latter.     

In Situ Processing and High-Temperature Properties of Ti3Si(Al)C2/SiC Composites

Ti₃SiC₂ has many salient properties including low density, high strength and modulus, damage tolerance at room temperature, good machinablity, and being resistant to thermal shock and oxidation below 1100°C. However, the low hardness and poor oxidation resistance above 1100°C limit the application of this material. The poor oxidation resistance at temperatures above 1100°C was because of the absence of protective layer in the scale and the presence of TiC impurity phase. TiC impurity could be eliminated by adding a small amount of Al to form Ti₃Si(Al)C₂ solid solutions. Although the high-temperature oxidation resistance was significantly improved for the Ti₃Si(Al)C₂ solid solutions, the strength at high temperatures was lost. One important way to enhance the high-temperature strength is to incorporate hard ceramic particles like SiC. In this article, we describe the in situ synthesis and simultaneous densification of Ti₃Si(Al)C₂/SiC composites using Ti, Si, Al, and graphite powders as the initial materials. The effect of SiC content on high-temperature mechanical properties and oxidation resistance were investigated. The mechanisms for the improved high-temperature properties are discussed.     

Processing and Mechanical Properties of Ti3SiC2: I, Reaction Path and Microstructure Evolution

In this article, the first part of a two-part study, we report the reaction path and microstructure evolution during the reactive hot isostatic pressing of Ti₃SiC₂, starting with titanium, SiC, and graphite powders. A series of interrupted hot isostatic press runs have been conducted as a function of temperature (1200°–1600°C) and time (0–24 h). Based on X-ray diffractometry and scanning electron microscopy, at 1200°C, the intermediate phases are TiC _x and Ti₅Si₃C _x . Fully dense, essentially single-phase samples are fabricated in the 1450°–1700°C temperature range. The time-temperature processing envelope for fabricating microstructures with small (3–5 μm), large (∼200 μm), and duplex grains, in which large (100–200 μm) Ti₃SiC₂ grains are embedded in a much finer matrix, is delineated. The microstructure evolution is, to a large extent, determined by (i) the presence of unreacted phases, mainly TiC _x , which inhibits grain growth; (ii) a large anisotropy in growth rates along the c and a directions (at 1450°C, growth normal to the basal planes is about an order of magnitude smaller than that parallel to these planes; at 1600°C, the ratio is 4); and (iii) the impingement of grains. Ti₃SiC₂ is thermally stable under vacuum and argon atmosphere at temperatures as high as 1600°C for as long as 24 h. The influence of grain size on the mechanical properties is discussed in the second part of this study.     

Preparation and Properties of Ternary Ti3AlC2 and its Composites from Ti–Al–C Powder Mixtures With Ceramic Particulates

A layered ternary carbide phase, Ti₃AlC₂, was synthesized by hot pressing from the starting materials of Ti, aluminum, and activated carbon at 1400°C for 2 h. Its composites were also fabricated through addition of micro-sized SiC and partially stabilized zirconia particulates to the pulverized Ti₃AlC₂ powders. The polycrystalline Ti₃AlC₂ ceramic obtained has a flexural strength of 172 MPa and a fracture toughness of 4.6 MPa·m^1/2, respectively. This compound is relatively soft (Vikers hardness of 2.7 GPa) and exhibits good electrical conductivity with an electrical resistivity of 8.2 μΩ·m. Both the Ti₃AlC₂/SiC and Ti₃AlC₂/ZrO₂ composites are superior to the monolithic Ti₃AlC₂ ceramic in strength, fracture toughness, and micro-hardness.     

Machinable Ti3SiC2/Hydroxyapatite Bioceramic Composites by Spark Plasma Sintering

In this paper, we report a machinable Ti₃SiC₂/hydroxyapatite (HAp) composite prepared by spark plasma sintering. The experimental results of a drilling test demonstrated that the composites exhibit excellent machinability when the Ti₃SiC₂ content is higher than 20 vol%, which can be attributed to the improvement in the mechanical and machinable properties of the composites by addition of Ti₃SiC₂ phase, which possessess unique mechanical and machinable properties and energy-absorbing mechanisms. The superior mechanical and machinable properties of Ti₃SiC₂/HAp composites suggest that the composite system could be attractive for practical applications of novel biomaterials.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel on Ti3AlC2 Substrate

A MgAl₂O₄ (MA) spinel layer was synthesized on Ti₃AlC₂ substrate through the molten salt synthesis (MSS) method. The Ti₃AlC₂ substrate was immersed in MgCl₂·6H₂O powders and treated at 800°, 850°, and 900°C for 4 h in air. A continuous and 10-μm-thick MgAl₂O₄ layer was obtained at 900°C, by which the surface hardness of Ti₃AlC₂ can be effectively improved. The combined scanning electron microscopy observations and crystal morphology simulation further revealed that the as-formed MgAl₂O₄ presents tetragonal bipyramids morphology with (400)-orientation.     

Tribological Properties of Polycrystalline Ti3SiC2 and Al2O3-Reinforced Ti3SiC2 Composites

Tribological properties of Ti₃SiC₂ and Al₂O₃-reinforced Ti₃SiC₂ composites (10 and 20 vol% Al₂O₃) were investigated by using an AISI-52100 bearing steel ball dryly sliding on a linear reciprocating athletic specimen. The friction coefficients were found varying only in a range of 0.1 under the applied loads (2.5, 5, and 10 N), and the wear rates of the composites decreased with increasing Al₂O₃ content. The enhanced wear resistance is mainly attributed to the hard Al₂O₃ particles nail the surrounding soft matrix and decentrale the shear stresses under the sliding ball to reduce the wear losses.     

Hf3AlN: A Novel Layered Ternary Ceramic with Excellent Damage Tolerance

In this work, bulk Hf₃AlN ceramic was synthesized by an in situ reaction/hot pressing method using Hf and AlN as initial materials. The reaction path during the synthesis process was investigated. Hf₃AlN was found to form via the reaction of Hf and AlN above 1000°C. Furthermore, physical and mechanical properties of Hf₃AlN, such as electrical conductivity, flexural strength, and elastic moduli were also characterized. Similar to typical layered ternary ceramics Ti₃SiC₂ and Ti₃AlC₂, Hf₃AlN possesses metallic conductivity and excellent damage tolerance, which is also the first one of this type that has ever been reported to crystallize in an orthorhombic structure. It is believed that a typical layered crystal structure and weak interlayer bondings contribute to the damage tolerance of Hf₃AlN. Moreover, the stiffness of Hf₃AlN can sustain a temperature as high as 1450°C, being 250°C higher than that of Ti₃AlC₂, which renders it a promising high-temperature structural material.     

Phase Relations in the Pseudobinary System Na2TiSi4O11-Na2Ti2Si2O9

The quenching technique was used to study subliquidus and subsolidus phase relations in the pseudobinary system Na₂ Ti₂Si₂ O₁₁-Na₂ Ti₂ Si₂ O₉. Both narsarukite (Na₂TiSi₄O₁₁) and lorenzenite (Na₂Ti₂Si₂O₉) melt incongruently. Narsarsukite melts at 911°±°C to SiO₂+liquid, with the liquidus at 1016°C. Lorenzenite melts at 910°±5°C to Na₂ Ti₆ O₁₃+liquid; Na₂ Ti₆ O₁₃ reacts with liquid to form TiO₂ and is thus consumed by 985°±5°C. The liquidus occurs at 1252°C.     

Effect of Composition on Properties of Alumina/Titanium Silicon Carbide Composites

In the present study, the room-temperature properties of Al₂O₃-Ti₃SiC₂ composites with different Ti₃SiC₂ contents are determined. The composites are prepared by attrition milling Al₂O₃ and Ti₃SiC₂ mixture powders followed by spark plasma sintering (SPS) under vacuum. From a closer examination of the dependencies of the electrical conductivity on compositions in this system, we determined the percolation threshold at which an interconnected network of electrically conductive phase arises. Since the hardness of Ti₃SiC₂ is lower than that of Al₂O₃, the Vickers hardness decreased with the increasing of Ti₃SiC₂ content while the fracture toughness and the strength increased. The maximum strength (673 MPa) and the maximum toughness (9.3 MPa·m^1/2) were reached in the pure Ti₃SiC₂ material.