Structural stability

Ceramic borides, carbides and nitrides all have very strong chemical bonds that give them high temperature structural stability.^5,7,9

As a result of the extremely strong bond between carbon atoms, they follow the classical definition of brittle ceramics. They come in three general classes: ionic, covalent and interstitial. None of the ionic carbides have engineering uses because of their extreme brittleness.¹⁰ The two mostly diffused covalent carbides with covalent bonds, SiC and B4C, both are valuable for their extreme hardness as well as excellent thermal and chemical stability. The largest class of carbides, those of the interstitial type, includes carbides of the metals Hf, Zr, Ti, and Ta. These materials benefit from strong carbon networks and have some of the highest melting points of known materials (see Fig. 8.1).

Fig. 8.1: A comparison of the melting temperatures of the most refractory members of several classes of materials. Several borides, carbides and nitrides have melting temperatures above 3000°C and belong to the class UHTs.

Carbides posses the cubic close packed NaCl-type structure with a fcc B1 symmetry, where the metal-to-metal bond is relatively weak and the metal-to-carbon bond is strong. The only stable composition is the monocarbide with carbon atom in all octahedral sites as stoichiometry. They are nearly completely soluble among each other, as shown in Fig. 8.2. They are also known to have high strengths at elevated

2400

temperatures.^10,11 From this standpoint, carbides offer a tremendous benefit in many engineering applications. Unfortunately, these materials are hard to fabricate because of their refractoriness, and little has been achieved beyond the laboratory scale.^10,11

Fig. 8.2: mutual solubility of refractory carbides.

Ceramic nitrides have many of the same properties as carbides since they are difficult to fabricate as well, especially in the pure form, due to strong covalent bonding.

Silicon nitride and boron nitride are the primary materials in the family of nitrides to be developed for engineering applications.⁹

Ceramic borides benefit from very strong bonding between boron atoms, although their bonding is not typically as strong as seen in the carbides, thus these materials often have melting points below that of the carbides.^7,10 The unique feature of the electronic nature of the boron bonding in these materials results in having high thermal and electrical conductivities, higher than typically found in carbides and nitrides, as well as low coefficients of thermal expansion, which, combined, give the borides relatively good thermal shock resistance.⁷

Boride ceramics also appear to have improved oxidation resistance over those of carbides and nitrides, the nature of which is discussed in the following section. Table 8.I lists a number of metallic elements that form binary diboride compounds, with the AlB2

structure, shown in Figure 8.3.^12,13

Tab. 8.I: Observed phase stability of transition metal diborides of the AlB2 structure.

Sc Ti V Cr Mn Fe Co Ni Y Zr Nb Mo Tc Ru Rh PD La Hf Ta W Re Os Ir Pt

Stable AlB2 phase Stable at high temp.

Prepared or detected at high temp.

AlB2phase is metastable

Fig. 8.3: A) Atomic projections of the structure showing top down and side view. B) An illustration of metal atom deformation within the structure.¹²

The AlB2 structure contains graphite-like layers of boron separated by hexagonal close-packed (h.c.p.) layers of metal atoms. The diborides are comprised of rigid covalent boron lattices, such that the boron atoms have a trigonal prismatic metal environment with three close boron neighbours. The metal atoms coordinate twelve boron atoms, six metallic atoms in the same layer and two metal atoms in the two adjacent layers (top and bottom).^12,13 The boron nets have very strong covalent bonds that hinder an increase in the direction, though no such hindrance occurs in the direction, giving borides the ability to accommodate a wide variety of metals.¹²

Spear performed calculations on chemical bonding parameters of borides and deduced that M-B bonding is likely the leading contributor to the structural integrity of AlB2

type borides, more so than M-M or B-B bonding. Typically, the less distortion there is to the unit cell, the stronger are the bonds that hold it together. As the degree of bonding increases so does the melting point, modulus and hardness, in the diborides as well as for carbide and nitride ceramics.^7,8,10

Enthalpies of formation for several boride systems were reported by Samsonov and Vinitiskii.¹⁴ From those results it is clear that the stability of diborides decrease in the order HfB2>TiB2>ZrB2>>TaB2>NbB2>VB2, with VB2 the least stable of the diborides, having the lowest energy of formation.

The phase diagram of Zr and Hf carbides and borides are reported in Fig. 8.4.

Fig. 8.4: Phase diagram of a) ZrC, b) HfC, c) ZrB2 and d) HfB2.¹⁵

8.2.2 Structure

Following the previous description of the atomic structure of boride, carbide and nitride ceramics, Table 8.II lists physical crystalline structural differences of a variety of UHTCs along with respective density and melting point.^16-20 Note that density increases with increasing mass of the metal atom. Note also the differences in melting points between materials whereby the carbides typically have the highest melting points, above borides or nitrides of the same metal constituent.

b

c d

a

Tab. 8.II: Crystal strucure, density and melting point of UHTCs.