Titanium carbide
Titanium carbide (TiC) is a binary refractory ceramic material composed of titanium and carbon, characterized by its exceptional hardness, high melting point, and superior wear resistance, making it a key component in advanced engineering applications.[1][2] With the chemical formula TiC, it often exhibits non-stoichiometry as TiCx (where x ranges from 0.48 to 0.98 due to carbon vacancies), and adopts a face-centered cubic (FCC) crystal structure akin to the NaCl type, with space group Fm-3m and lattice parameter a ≈ 0.4327 nm.[3][2] TiC appears as a grey crystalline powder or solid, with a density of 4.9 g/cm³, melting point of 3140 °C, and boiling point of 4820 °C; it is insoluble in water but soluble in aqua regia and nitric acid.[1][4] Its mechanical properties include a Vickers hardness of 28–35 GPa, Young's modulus of 410–510 GPa, and tensile strength up to 258 MPa, while thermal and electrical conductivities reach 21 W/m·K and 10–20 × 103 S/cm, respectively, contributing to its classification as a transition metal carbide with combined ionic, covalent, and metallic bonding.[2][5] Chemically stable and resistant to oxidation in air up to 450 °C, TiC also demonstrates superconductivity at 1.1 K and good chemical inertness.[4][6] TiC is synthesized via methods such as carbothermal reduction of TiO2 with carbon at 1700–2300 °C, chemical vapor deposition (CVD), self-propagating high-temperature synthesis (SHS), and mechanical alloying, allowing control over particle size and morphology for nanoscale applications.[3][2] These processes enable the production of ultrafine powders or coatings, often integrated into cermets or composites. Notable applications leverage its properties for cutting tools and inserts in machining, wear-resistant coatings on drill bits and engine components, reinforcement in metal matrix composites (e.g., with Cu, Ni, or Al) for enhanced strength in aerospace and automotive sectors, as well as in electronics for diffusion barriers, heat sinks, and electromagnetic shielding materials.[2][3] Emerging uses include catalysis, energy storage, and microwave absorption due to its tunable nanostructures.[3]Chemical and Structural Characteristics
Composition and Nomenclature
Titanium carbide is represented by the chemical formula TiC, indicating a stoichiometric 1:1 ratio of titanium atoms to carbon atoms.[7] This composition reflects the compound's basic structure as a binary interstitial carbide, where carbon atoms occupy octahedral voids in a titanium lattice.[7] The molar mass of TiC is 59.878 g/mol, derived from the standard atomic weights of titanium (47.867 g/mol) and carbon (12.011 g/mol).[7] This value is consistent across authoritative chemical databases and underscores the compound's lightweight nature relative to other transition metal carbides.[8] In nomenclature, the IUPAC name for TiC is titanium carbide, with the alternative designation titanium(IV) carbide emphasizing the +4 oxidation state of titanium.[9] It is commonly abbreviated as TiC and referred to as titanium monocarbide to distinguish it from other titanium-carbon phases, such as the sesquicarbide Ti₂C.[9] These naming conventions align with systematic inorganic chemistry standards for metal carbides.[4] Titanium carbide frequently exhibits non-stoichiometric compositions, expressed as \ce{TiC_{1-x}}, where x (typically 0.01 to 0.5) denotes vacancies in the carbon sublattice.[10] These deviations from ideal stoichiometry arise during synthesis and significantly impact properties, including enhanced ductility and altered electronic characteristics due to the increased vacancy concentration.[11] Such variability allows tailoring of TiC for specific applications while maintaining its core refractory nature.[10]Crystal Structure
Titanium carbide (TiC) adopts a face-centered cubic (FCC) crystal structure, classified as the halite or rock salt type, with the space group Fm\bar{3}m (No. 225).[12] In this arrangement, titanium atoms form the FCC lattice, while carbon atoms occupy all octahedral interstitial sites, resulting in each titanium atom being octahedrally coordinated to six carbon atoms and vice versa.[13] The lattice parameter a is approximately 4.327 Å at room temperature for near-stoichiometric TiC, though it varies slightly with composition due to non-stoichiometry.[14] The bonding in TiC combines strong covalent interactions between Ti and C atoms with metallic bonding among neighboring Ti atoms, contributing to its unique combination of ceramic hardness and metallic conductivity.[15] This hybrid nature arises from the directional charge density in Ti-C bonds and delocalized d-electrons facilitating Ti-Ti interactions, as revealed by cluster model analyses.[15] TiC is typically non-stoichiometric, with a homogeneity range from TiC_{0.75} to TiC_{0.95}, primarily due to carbon vacancies that act as constitutional defects.[13] These vacancies are predominantly located in the carbon sublattice and can be randomly distributed in most compositions, though ordered arrangements may emerge at lower carbon contents, influencing lattice stability and electronic properties.[16] Structurally, TiC resembles the rock salt (NaCl) configuration, where the anion and cation sublattices are interpenetrating FCC arrays, but it exhibits metallic characteristics absent in ionic NaCl, such as high electrical conductivity stemming from partially filled d-bands.[17]Physical and Mechanical Properties
Thermal and Electrical Properties
Titanium carbide (TiC) possesses remarkable thermal stability, characterized by a high melting point of 3140 °C and a boiling point of 4820 °C, making it suitable for extreme high-temperature applications.[1] Its thermal conductivity ranges from approximately 20 to 30 W/(m·K) at room temperature, though this value decreases with rising temperature due to enhanced phonon scattering in its lattice.[18] The coefficient of thermal expansion is about 7.5–8.5 × 10^{-6} /K, which reflects the material's ability to withstand thermal stresses without significant dimensional changes.[5] Additionally, the specific heat capacity lies in the range of 30–40 J/(mol·K), indicating moderate energy absorption per unit mass under heating conditions.[2] Electrically, titanium carbide behaves as a metallic conductor, with an electrical resistivity of approximately 68–120 μΩ·cm at room temperature, a property influenced by its rock-salt crystal structure that facilitates electron mobility.[18][19] This conductivity level supports its use in applications requiring both thermal resilience and electrical performance. In terms of oxidation resistance, TiC begins to oxidize in air above approximately 600–800 °C, forming a protective titanium dioxide (TiO₂) layer that slows further degradation, though prolonged exposure above 900 °C leads to progressive scale formation.[20][21]Hardness and Elastic Properties
Titanium carbide exhibits a black-gray appearance in its crystalline powder or solid form. Its theoretical density is 4.93 g/cm³.[5] The material demonstrates exceptional hardness, ranking 9–9.5 on the Mohs scale, with Vickers hardness values ranging from approximately 2,800 to 3,200 HV and Knoop hardness from 2,500 to 3,000 kg/mm².[22][5][23] In terms of elastic properties, titanium carbide possesses a Young's modulus of approximately 400–450 GPa, a shear modulus of about 188 GPa, and a Poisson's ratio between 0.19 and 0.25.[2][24] Despite its high stiffness, titanium carbide is brittle, with a low fracture toughness of approximately 3–4 MPa·m^{1/2}, rendering it susceptible to cleavage fracture under tensile loading.[18] This brittleness limits its tensile strength to around 250–350 MPa, while compressive strength reaches 3,000–4,000 MPa.[5]| Property | Value/Range | Measurement Type |
|---|---|---|
| Density | 4.93 g/cm³ | Theoretical |
| Mohs Hardness | 9–9.5 | Scratch resistance |
| Vickers Hardness | 2,800–3,200 HV | Indentation (load-dependent) |
| Knoop Hardness | 2,500–3,000 kg/mm² | Microindentation |
| Young's Modulus | 400–450 GPa | Uniaxial tension |
| Shear Modulus | 188 GPa | Torsional deformation |
| Poisson's Ratio | 0.19–0.25 | Lateral strain ratio |
| Fracture Toughness | 3–4 MPa·m^{1/2} | Critical stress intensity |
| Compressive Strength | 3,000–4,000 MPa | Uniaxial compression |
| Tensile Strength | 250–350 MPa | Uniaxial tension |