Denominación

2.3.1 Introduction to Ultra-High Pressure Fabrication

The materials that are usually fabricated under extremes of ultra-high pressure (UHP) are sintered masses of diamond and cubic boron nitride (cBN) for the production of superhard, industrial cutting tool materials. Pressures of a similar magnitude to those used in diamond or cBN synthesis are necessary to maintain their thermodynamic stability at high sintering temperatures, so preventing reversion to soft, graphitic phases.

In most commercial UHP apparatus the diamond or cubic BN powders used to prepare the superhard composites are encapsulated in a deformable, high melting point material (usually tantalum). This is immediately surrounded by a solid 'pressure- transmitting' medium inside a high pressure capsule that is uniaxially compressed

during the fabrication process. Materials which are typically used as 'pressure-

transmitting' media are sodium chloride [79], boron nitride (hexagonal phase), and pyrophyllite [80-82] (an alumina silicate), selected according to the criteria discussed in

reference [83]. However, the use of solid 'pressure-transmitting' media prevents the

attainment o f truly hydrostatic pressures inside the high pressure capsule and also provides a major source of difficulties in the calibration of effective fabrication pressures. Pressure calibration is additionally complicated by any structural changes in the 'pressure-transmitting' media, such as volume expansion or phase transitions which may occur at the extreme pressure and temperature conditions.

Temperature measurement inside the capsule is inhibited by the practical difficulties o f passing thermocouples into the high pressure region and then protecting

them against mechanical failure. Another problem encountered in temperature

measurement is defining the influence of UHP on the thermocouple e.m.f. [84].

In general, progress in the determination of exact processing conditions has been constrained by the many factors involved. High pressure equipment is usually calibrated using the pressure-induced electrical phase transitions of bismuth to provide reference points [85], sample capsule temperatures being estimated from the input heating power.

2.3.2 Cubic Boron Nitride

Cubic BN (cBN) was first synthesised from hexagonal BN (hBN) in 1957 [86] following speculation concerning an analogy between the structural forms of carbon and boron nitride. Hexagonal BN (hBN), a soft refractory and lubricant, has a similar crystal structure to graphite while cubic BN has a diamond-like structure. The analogy between carbon and boron nitride was carried further in 1963 [87] by the discovery of a hexagonal BN phase with the 'wurtzite' crystal structure common to meteoritic

diamonds [88]. In the cubic BN crystal lattice, boron and nitrogen atoms are

tetrahedrally co-ordinated from the intersection of two separate, face-centred-cubic sublattices for boron and nitrogen atoms. This crystal arrangement is also shared by zincblende (3-zinc sulphide). The relationship between the crystal structures of BN is illustrated in figure 2.7 (below).

( i l l ) a) [0001) (c-axis) (0001] (c-axis) b)

Figure 2.7 : Crystal structures o f boron nitride: (a) cubic BN, (b) hexagonal BN, (c) wurtzitic BN

Tile cubic phase of BN, second only to diamond in hardness, has an excellent combination of physical and chemical properties that make it eminently suitable as a 'superabrasive' or cutting tool material. In such applications, cBN has advantages over diamond, being more resistant to chemical reaction in ferritic environments. The early experiments [86] showed that cBN is a good electrical insulator, is not attacked by the more common acids, scratches and is scratched by diamond and is only slowly oxidised in air, at about 2000°C. Other properties are given in the following table.

(i) Properties of Cubic BN

Table 2.1: Physical Properties o f Cubic Boron Nitride

PROPERTY CUBIC BN DIAMOND REFERENCE

Lattice Parameter _{3.6157 + 0.001 A} [89]

Crystal Space Group _F43m Fd3m [86]

Density _{3.487± 0.003 g/cm3 3.515 ± g/cm3} [89,86]

Microhardness (Vickers) 60 - 75 GPa 120 GPa [90]

Linear Thermal Expansion x lO ^K "1 200 K: 0.50 400 K: 1.80 600 K: 3.23 800 K: 4.70 1000 K: 5.96 1200 K: 6.45 200 K: 0.45 400 K: 1.79 600 K: 3.17 800 K: 3.81 1000 K: 4.38 1200 K: 4.93 [91]

Thermal Conductivity 1300 W/m K (theoretical) _{200 - 900 W/m K} 2000 W/m K _{[93, 94]}[92]

Young's Modulus 890 GPa [95]

Poisson's Ratio _{0.138 [95]}

Compressive Strength 450 MPa [96]

Stability in Air oxidation begins in air at

2.1 x 10 Pa and 950 K 6.67 Pa and 1000 K

[97]

apart from surface oxidation, stable in air at 1300 K for 30 mins.

(ii) The Boron Nitride Phase Diagram

The boron nitride phase diagram was first determined by Bundy and Wentorf [87] from their studies of direct, non-catalysed transitions of hBN to denser phases under ultra-high pressures. A direct phase transition to the cubic BN phase involves thermal disruption of the hBN atomic lattice and the reformation of bonds under cBN- stable conditions. Wakatsuki et al. [96] showed that less extreme conditions are needed for this phase transition if the starting powder has a poorly defined crystal structure which lowers the high transformation energy barrier.

The boron nitride phase relationships can be determined in studies of 'direct' transitions since the presence of catalysts causes a displacement of the phase equilibrium boundary line. In a patent describing the uncatalysed conversion of hBN to cBN, Sirota and Mazurenko [98] claimed that the phase boundary between hBN and cBN as given by Bundy and Wentorf should be shifted towards higher temperatures. This finding met agreement with investigations of BN phase transitions by Corrigan and Bundy [99] who used both static pressures and shock-compression methods and also the results of Corrigan [93] in the conversion of pyrolytic BN to cBN. The phase diagram determined by Corrigan and Bundy is given in fig.2.8. In this diagram, the hBN-cBN equilibrium line is parallel to the graphite-diamond equilibrium boundary, a feature that is disputed by Solozhenko [100, 101]. Taking experimental data on the thermodynamic characteristics of cBN, Solozhenko calculated the position of the hBN-cBN equilibrium line and concluded that it should intersect the temperature axis at 1570 K rather than the pressure axis at 1.3 GPa. Solozhenko suggests that cubic BN is the stable phase under low pressures and is wrongly represented as a metastable state.

The determination of a thermodynamically stable region for the wurtzitic BN phase is still unclear. Wurtzitic BN has been obtained under both static pressures [102] and by shock compression [99] in a diffusionless ('martensitic') transformation from hBN, but it has been concluded that this phase is probably thermodynamically unstable

[100,102], Tani et al. [102] reported that wBN transforms to hBN between 1200 and

P re ssu re (kbar)

Figure 2.8 :Boron Nitride Phase Diagram

(Hi) Catalytic Synthesis o f Cubic BN

The initial synthesis of cBN was by direct conversion from hBN under simultaneous conditions of 85000 atmospheres and 1800°C [86]. However, it was later discovered that the high activation energy barrier for the phase transformation could be reduced with the aid of a catalyst such as the alkali, alkaline earth metals and their nitrides [103,104], The phase transformation then occurs by chemical dissolution of the hBN followed by precipitation of BN in the cubic form. Commercial cBN production takes place by catalytic synthesis and has stimulated wide-ranging research to determine a lower pressure limit for cBN synthesis. The main considerations involved are the minimum temperature at which a solvent can dissolve hBN and the cubic-hexagonal phase boundary. Some of the more unusual catalysts that have been effective in cBN synthesis include water, urea, ammonium nitrate and boric acid [105,106].

2.3.3 Fabrication of Cubic Boron Nitride Composites

The fabrication processes for cBN aggregates can be divided into two groups: (i) spontaneous sintering of cBN formed during synthesis from hBN and (ii) sintering powdered cBN (starting phase), usually with a binder phase.

Polycrystalline cBN fabricated from direct, uncatalysed conversion of hBN has been reported to have extremely high wear resistance [98], very high Vickers' microhardnesses (60-70 GPa) [90] and thermal conductivities as high as 9 W/cm°C [93] that approach the theoretical prediction of 13 W/cm°C for a single crystal [92], The excellent physical properties are attributed to the absence of any other phases that may degrade the material. However, this fabrication process generally requires pressures above 60 kbar and temperatures exceeding 1800°C whereas less extreme conditions can be used to produce a cBN material which has secondary, binder phases. Such materials can be fabricated by spontaneous sintering during the catalysed conversion of hBN or by embedding cBN in a metal or resin matrix.

Polycrystalline cBN can be fabricated with properties that can be modified for specific applications by the judicious choice of catalysts for cBN synthesis. Examples include the fabrication of a light-transparent material [107], also of high hardness (57- 64 GPa) obtained using catalysts formed from lithium or alkaline earth metals that were reported to achieve this result by uniformly diffusing into the initial hBN powder. A cBN material of fairly high thermal conductivity (6.5 W/cm°C) and extreme hardness (60-65 GPa) resulted from hBN conversion catalysed by an alkaline earth metal hydroxide [108]. Aluminium nitride is also reported to be a good catalyst, allowing the synthesis of cBN together with formation of a strongly bonded material [109]. Another cBN material of high hardness (51 GPa) has been produced using an ammonium nitrate catalyst, to promote hBN transformation with seed crystals of cBN (to promote direct bonding between cBN grains) [110].

The physical properties of these composites and the conditions required for their fabrication are summarised in table 2.2.

T a b l e 2 .2 : F a b r ic a tio n c o n d itio n s a n d p ro p e rtie s o f c a ta ly tic a lly - s y n th e s is e d c B N c o m p o s ite s

Fabrication Conditions Catalyst Properties Reference