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Titanium Diboride ( TiB₂ )

Database Reference: NIST Structural Ceramics Database, SRD Database Number 30

Journal Reference: "Material Properties of Titanium Diboride," R. G. Munro, Journal of Research of the National Institute of Standards and Technology, Vol. 105, pp. 709-720 (2000).

Other materials: Property Data Summaries

Material Summary:

Nearly fully dense polycrystalline TiB₂ can be produced by a variety of processing methods, including sintering, hot pressing, hot isostatic pressing, microwave sintering, and dynamic compaction. The relatively strong covalent bonding of the constituents, however, results in low selfdiffusion rates. Consequently, given also a high melting point of (3225 ± 20) °C, pressureless sintering of TiB₂ requires a relatively high sintering temperature, on the order of 2000 °C. Unfortunately, grain growth is also accelerated by the higher temperature, and the anisotropy of the hexagonal grain structure results in deleterious internal stresses and the onset of spontaneous microcracking during cooling. Grain growth can be limited and densification enhanced by the use of sintering aids such as Cr, CrB₂, C, Ni, NiB, and Fe. The solubility of TiB₂ in liquid Ni and Fe appears to be especially useful in this regard. In such cases, the mass fraction of the sintering aid in the specimen may range from 1 % to 10 %, while the sintering temperature may be reduced to the range of 1700 °C to 1800 °C for sintering times on the order of 1 h. Successful hot pressing with Ni additives can be achieved with a hot pressing temperature as low as 1425 °C with a sintering time of 2 h to 8 h.

The data presented here were derived from reported values for a narrowly defined material specification. Using trend analysis, property relations, and interpolation methods, the selfconsistent trend values for the properties of polycrystalline TiB₂ were determined for a mass fraction of TiB₂ of at least 98 %, a density of (4.5±0.1) g/cm³, and a mean grain size of (9±1) µm.

Property Summary:

For references to the source data and detailed discussions of the properties, please refer to the Journal Reference.

Special Notes on Properties:

Data Status: Validated
Except where noted otherwise:
           Purity (mass fraction of TiB₂) : at least 98 %
           Density : (4.5±0.1) g/cm³
           Mean grain size : (9±1) µm
Crystallography:

TiB₂ crystallizes with hexagonal symmetry, space group P6/mmm, and has one formula unit per unit cell. The lattice parameters (a,c) have a slight quadratic dependence on the temperature which accounts for the linear temperature dependence of the coefficient of thermal expansion. The ratio c/a ranges from 1.066 ± 0.001 at 25 °C to 1.070 ± 0.001 at 1500 °C. Individually, the lattice parameters may be expressed as:

            a/Å = 3.0236 + 1.73x10^-5 (T/K) + 3.76x10^-9 (T/K)²
            c/Å = 3.2204 + 2.73x10^-5 (T/K) + 3.95x10^-9 (T/K)²

where T is in the range from 293 K to 2000 K. The relative standard uncertainties, u_r, when using these expressions are estimated to be u_r(a) = 0.03 % and u_r(c) = 0.04 %.

Creep Characteristics (flexure at 100 MPa and T > 1500 °C):
           For a specimen density of 4.29 g/cm³ and a mean grain size of 18 µm,
           the Norton model of creep, Creep Rate = A (s/s_o)ⁿ exp[-Q/RT],
           applied to the collected literature data, yields:
                Activation energy {Q}: 426 kJ/mol
                Stress exponent {n}: 2.3
                Amplitude factor {A}: 4.806x10^-4 s^-1
Tribology Characteristics (dry TiB₂ on TiB₂):

The wear behavior of TiB₂ appears to be complicated by its interaction with oxygen in the atmosphere. Results from a ring on block test of the wear of TiB₂ for a density of 4.32 g/cm³ and a grain size of 2 µm showed that for temperature less than 600 °C, the amount of material removed during the test increased with increasing sliding distance, but decreased with increasing temperature. For temperature greater than 600 °C, the specimens gained mass with the amount of mass gain increasing with increasing sliding distance. The decrease of mass loss and the occurence of mass gain appear to be the result of the formation of B₂O₃ in the wear track of the specimens. The coefficient of friction appears to have a power law dependence on the ratio of the sliding speed v_slide and the contact stress P_load. For v_slide/P_load = 0.2 m s^-1 MPa^-1, the coefficient of friction may be taken to be 0.8 ± 0.1 for temperature less than or equal to 400 °C and 0.4 ± 0.1 for temperature in the range 800 °C to 1000 °C.

Property Table

The values presented here are trend values derived for polycrystalline TiB₂ specimens with a purity (mass fraction of TiB₂) of at least 98 %, a density of (4.5±0.1) g/cm³, and a mean grain size of (9±1) µm. Estimated combined relative standard uncertainties of the property values are listed in the last column. For example, a value of 3.0 with u_r = 5 % is equivalent to 3.0 +/- 0.15. A question mark, (?), for u_r means the uncertainty could not be determined with the available data.

           ===== Notes for Table Entries =====
           a) Estimated combined relative standard uncertainty expressed as a percentage.
           b) Flexure creep rate at 100 MPa for density = 4.29 g/cm³, grain size = 18 µm.
           c) Single crystal density
           d) Density = 4.32 g/cm³, grain size = 2 µm, v_slide/P_load = 0.2 m s^-1 MPa^-1.
           e) Vickers indentation, load = 5 N.
           f) Single crystal, hexagonal unit cell.
           g) Single crystal, for cumulative expansion from 293 K (20 °C).
               CTE = (1/x₂₉₃)(x-x₂₉₃)/(T/K - 293), x = a or c.
           h) Bulk average, for cumulative expansion from 20 °C.
           i) Three values have been reported in the literature: 8, 11, and 29.