NIST Standard Reference Database 150

**Last Update to Data Content: **2002

**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

Nearly fully dense polycrystalline TiB_{2} 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 TiB_{2} 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, CrB_{2}, C, Ni, NiB, and Fe. The solubility of TiB_{2} 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 TiB_{2} were determined for a mass fraction of TiB_{2} of at least 98 %, a density of (4.5±0.1) g/cm^{3}, and a mean grain size of (9±1) µm.

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

Purity (mass fraction of TiB

Density : (4.5±0.1) g/cm

Mean grain size : (9±1) µm

TiB_{2}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 ratioc/aranges from 1.066 ± 0.001 at 25 °C to 1.070 ± 0.001 at 1500 °C. Individually, the lattice parameters may be expressed as:

whereTis 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 %.

For a specimen density of 4.29 g/cm

the Norton model of creep, Creep Rate = A (s/s

applied to the collected literature data, yields:

Activation energy {Q}: 426 kJ/mol

Stress exponent {n}: 2.3

Amplitude factor {A}: 4.806x10

The wear behavior of TiB_{2}appears to be complicated by its interaction with oxygen in the atmosphere. Results from a ring on block test of the wear of TiB_{2}for a density of 4.32 g/cm^{3}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 B_{2}O_{3}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.

The values presented here are trend values derived for polycrystalline TiB_{2}specimens with a purity (mass fraction of TiB_{2}) of at least 98 %, a density of (4.5±0.1) g/cm^{3}, 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.

Property [unit] | 20 °C | 500 °C | 1000 °C | 1200 °C | 1500 °C | 2000 °C | u_{r} [%]^{a} |
---|---|---|---|---|---|---|---|

Bulk Modulus [GPa] | 240 | 234 | 228 | 24 | |||

Compressive Strength [GPa] | 1.8 | ? | |||||

Creep Rate^{b} [10^{-9} s^{-1}] |
0.005 | 3.1 | 20 | ||||

Density^{c} [g/cm^{3}] |
4.500 | 4.449 | 4.389 | 4.363 | 4.322 | 4.248 | 0.07 |

Elastic Modulus [GPa] | 565 | 550 | 534 | 5 | |||

Flexural Strength [MPa] | 400 | 429 | 459 | 471 | 489 | 25 | |

Fracture Toughness [MPa m^{1/2}] |
6.2 | 15 | |||||

Friction Coefficient^{d } [] |
0.9 | 0.9 | 0.6 | 15 | |||

Hardness^{e} [GPa] |
25 | 11 | 4.6 | 12 | |||

Lattice Parameter^{f} a [Å] |
3.029 | 3.039 | 3.052 | 3.057 | 3.066 | 3.082 | 0.03 |

Lattice Parameter^{f} c [Å] |
3.229 | 3.244 | 3.262 | 3.269 | 3.281 | 3.303 | 0.04 |

Poisson's Ratio [] | 0.108 | 0.108 | 0.108 | 70 | |||

Shear Modulus [GPa] | 255 | 248 | 241 | 5 | |||

Sound Velocity, longitudinal [km/s] | 11.4 | 11.3 | 11.2 | 5 | |||

Sound Velocity, shear [km/s] | 7.53 | 7.47 | 7.40 | 3 | |||

Specific Heat [J/kg·K] | 617 | 1073 | 1186 | 1228 | 1291 | 1396 | 1.5 |

Thermal Conductivity [W/m·K] | 96 | 81 | 78.1 | 77.8 | 6 | ||

Thermal Diffusivity [cm^{2}/s] |
0.30 | 0.17 | 0.149 | 0.147 | 6 | ||

Thermal Expansion^{g}, a axis [10^{-6}K^{-1}] |
6.4 | 7.0 | 7.7 | 7.9 | 8.3 | 8.9 | 7 |

Thermal Expansion^{g}, c axis [10^{-6}K^{-1}] |
9.2 | 9.8 | 10.4 | 10.6 | 11.0 | 11.6 | 5 |

Thermal Expansion^{h}, average [10^{-6}K^{-1}] |
7.4 | 7.9 | 8.6 | 8.8 | 9.2 | 9.8 | 6 |

Wear Coefficient^{d}[10^{-3}] |
1.7 | 24 | |||||

Weibull Modulus [] | 11^{i} |
? |

a) Estimated combined relative standard uncertainty expressed as a percentage.

b) Flexure creep rate at 100 MPa for density = 4.29 g/cm

c) Single crystal density

d) Density = 4.32 g/cm

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/x

h) Bulk average, for cumulative expansion from 20 °C.

i) Three values have been reported in the literature: 8, 11, and 29.