2.1       Refractory Materials

The term refractory means resistance to heat.  Refractory metals are a specific group of metals having melting points over 1650°C.  In fact, precious metals also have high melting points but this is not as important as their financial value.  The most important metals in the refractory metal group are zirconium, molybdenum, tungsten, niobium and tantalum.  From a commercial point of view these are the most widely used because of their stability at elevated temperatures.  Applications are limited to non-oxidising conditions where the load bearing requirements are not too severe.

Since metals are of limited use, compounds of the refractory metals have been investigated and developed such as carbides, borides, nitrides and aluminides as well as hybrid materials such as the cermets and metals reinforced with refractories.  In general, these ‘new’ materials are brittle with poor thermal conductivity and high expansion offering poor resistance to thermal shock.  The requirements that have to be met by hard metals or ceramic high temperature materials will vary considerably within large limits dependant upon the intended application.  Creep resistance and stress rupture strength are probably the most important amongst the mechanical requirements and high strength to weight ratio materials will obviously be favoured.

2.2       Transition metal carbides

The carbides of the transition metals in Groups IV - VI have extremely high melting points  (Table 1) and are therefore referred to collectively as the “refractory carbides.” In addition to their stability at high temperatures, these compounds are extremely hard (Table 2), finding industrial use in cutting tools and wear-resistant parts. Their hardness is retained to very high temperatures, and they have low chemical reactivity – they are attacked only by concentrated acid or base in the presence of oxidising agents at room temperature, and retain good corrosion resistance to high temperatures. The refractory carbides are strong, with Young’s modulus values – a measure of elastic deformation resistance – rivalling those of silicon carbide (SiC) at room temperature. In addition, they have good thermal shock resistance and good thermal conductivity, permitting heat to be drawn away from the working surface of the tool. This gives them a benefit over other refractory materials, which do not conduct heat as well. (Table 3).

For high-temperature applications, the carbides are used as pure-material sintered parts or in a composite such as the Co/Mo/W/carbide sintered composite. These materials outperform the standard alloys and so-called “super-alloys” in such applications as rocket nozzles and jet engine parts, where erosion resistance at temperatures in excess of 2500^oC is crucial. TiC_1-x and VC_1-x in particular maintain high strengths up to 1800^oC and therefore can be used as high temperature structural materials, provided that internal and surface flaws, such as stress cracks and pores introduced during fabrication and sintering, are removed. Such defects lead to high room-temperature brittleness; plastic flow relieves internal stresses caused by defects and leads to reduced brittleness at high temperatures. Plastic deformation occurs particularly via a mechanism of dislocation glide along {111} planes.

It must be noted that there is some variation in the literature with respect to the reports of assorted mechanical and thermodynamic properties of the refractory carbides. The transition metal carbides show a range of non-stoichiometries (where stoichiometry is the ratio of cation to anions as specified by the chemical formula) and possibilities for vacancy ordering, so the precise phases being tested for a given property are often unclear. Furthermore, small concentrations of oxygen present as metal oxides are famously difficult to remove – or even detect – and can be expected to affect the properties of the material.

Tungsten carbide (WC) is the most commonly used monocarbide.  Approximately two third of tungsten production in the United States is used to make tungsten carbide for use in cemented carbide manufacture. Fabrication as a “cemented carbide” tools for cutting steel, is essentially by a diffusion process.  Tungsten powder is carburised and the tungsten carbide powder is bonded in a metal matrix, usually cobalt. Cobalt is used because it wets the carbide particles and therefore behaves as a good binder without having significant ability to dissolve the carbide, so that the carbide is left pure in the bound form. However, pure WC-Co cemented carbides tend to weld locally with the steel being cut. TiC, TaC, and NbC are often used in conjunction with WC because TiC locally forms a layer of TiO or TiO₂ which protects the tool from wear, and TaC and NbC which raise the melting temperature and oxidation resistance of the tool.

The melting points of the mixed-metal carbides outperform those of the pure-metal carbides, as well. Samples prepared by vacuum reduction of the mixed oxide powders at 2000^oC, followed by sintering at 2200^oC and 2500^oC in good vacuum were tested for their melting behavior. 8TaC^.ZrC and 4TaC^.HfC had melting points 3890^oC and 3990^oC, respectively, somewhat higher than those of the pure-metal carbides (measured at 3470, 3750, and 3840^oC for Zr, Hf, and Ta carbides, respectively). The higher melting points in the mixed-metal carbides were attributed to composition changes due to the selective evaporation of carbon during melting.

Mixed-metal carbides have been examined for their melting point and hot-hardness behaviour as well. The hardness arc-cast or zone-melted samples of (Ta_0.8Hf_0.2)C_1+x was tested (by indentation using diamond or B₄C tips) and compared with that of the similarly-prepared pure-metal carbides TaC_1-x and HfC_1+x over the temperature range 800 - 2000^oC.  In all cases the hot hardness decreased with increasing temperature. For temperatures up to 1400^oC the hardness order increased as HfC_1+x < (Ta_0.8Hf_0.2)C_1+x < TaC_1-x, but over 1400^oC the mixed carbide began to outperform the tantalum carbide. Hardness varied from 600 kg/mm² to 1500 kg/mm² over the temperature range tested.( Koester, R. D.; Moak, D. P. J. Am. Ceram. Soc. 1967)

2.2.1.   Structures of Transition Metal Carbides

Most of the transition metal mono-carbides form in the B1 (NaCl) or rocksalt crystal structure. A unit cell for this structure is generated from a face centred arrangement of anions (a negatively charged non-metallic ion) with one cation (a positively charged metallic ion) situated at the cube centre and one at the centre of each of the 12 cube edges. Therefore the rocksalt crystal structure may be thought of as two interpenetrating fcc lattices, one made up of cations and the other made up of anions. The co-ordination number for both cations and anions is 6.

These materials contain ions of at least two kinds and therefore defects, such as interstitials or vacancies for each kind may occur.  It is of course unlikely that there would be anion interstitials because the anion is relatively large and would introduce substantial strain on the surrounding ions if forced into an interstitial position.  The atoms exist as charged ions and when defect structures are being considered, conditions of electroneutrality must be considered.  Because of this, defects in ceramics do not occur in isolation but in defects pairs such as the Frenkel defect and the Schottky defect.  The Frenkel defect involves a cation vacancy and a cation interstitial pair, whereas the Frenkel defect involves a cation and anion vacancy pair.  The ratio of cations to anions is not altered by the formation of either the Frenkel or the Schottky defect and if no other defects are present the material is said to be stoichiometric.  Stoichiometry is defined as the state for ionic compound where there is an exact ratio for cations and anions predicted by the chemical formula.  A ceramic compound is nonstoichiometric when there is any deviation from the exact ratio.

The shortest metal -to -metal (M-M) distance is about 30% greater in the rocksalt structure (B1) carbide than in the pure metal for the Group IV and V carbides, but drops to less than 10% greater for the Group VI or VII carbides. At 100% site occupancy, the stoichiometry of the carbide is MC_1.0, though this situation is rarely realised. The concentration and ordering, if any, of the vacancies that result from a non-stoichiometric metal-to-carbon (M-C) ratio have a great effect on the thermodynamic, mechanical, electronic, and magnetic properties of the metal carbides. However, the details of these effects are a matter of some debate in the literature, due to the difficulties inherent in synthesizing pure compounds and in measuring the exact details of the crystal structure of a given sample. The metal carbides share many characteristics with the metals themselves, having (the same slip systems) a plastic deformation like as the fcc metals which, while lowering the high-temperature hardness, protects parts fabricated from the carbides from catastrophic failure in response to stresses.

 2.3.      Group IV - V Carbides

2.3.1.   General Trends

Approximate preparation of the transition metal carbides is straightforward, but ensuring a given stoichiometry and purity against oxygen contamination is famously difficult. Variations in the quantity of vacancies on the carbon (or, less frequently, the metal) lattice, as well as variations in the amount of dissolved oxygen, lead to a wide range of claims regarding even basic thermodynamic, mechanical, and electromagnetic data for the early transition metal carbides. Removal of the Oxy-carbide phases, which can be considered to be solid solutions of MO and MC, depends on the partial pressure of CO over the sample to be purified. At the high-carbon end of the stoichiometric range, Equation 2.1 leads to additional dissolved oxygen in the lattice if CO is removed. At the low-carbon end, Equation 2.2 occurs independently of the CO partial pressure. Thus if excess graphite is present, a high CO partial pressure leads to more nearly stoichiometric carbide as Equation 2.1 is forced left; but the success of this approach in purifying a given metal carbide depends on the stability of the oxycarbide, the annealing temperature and time, and, clearly, the partial pressure of carbon monoxide.

M_xO_y + C (dissolved in MC) = MO (dissolved in MC) + CO _(g)                   eqn.2.1

M_xO_y + M (dissolved in MC) = MO (dissolved in MC)                            eqn. 2.2

Group IV carbides are difficult to purify without melting; heating up to 2000^oC will result in increased oxygen contamination if the vacuum is not better than 10^-6 torr. Moreover, as noted in the previous section, few straightforward chemical methods exist for finding the oxygen level in the Group IV carbides MC_1-x bulk materials; none are reliable. The Group V and VI carbides purify readily at temperatures over 1800^oC.

Slow diffusion of carbon in all of the refractory carbides results in stoichiometry gradients which are difficult to detect in bulk materials but which may compromise the material strength, hardness, and high-temperature behaviour. The lattice parameter and the sharpness of the XRD pattern can give some rough indication of the homogeneity, however.

Group IV metals tend to form in a single cubic phase with a limiting stoichiometry near MC_1.0, but which normally varies from MC_0.5 to MC_0.97 depending on the synthesis conditions. Group V metals form an M₂C phase in addition to the monocarbide. The composition range of the M₂C phase is narrow at room temperature, with decomposition into the cubic phase plus liquid at high temperatures. The V-C system has a cubic phase extending to MC_0.88, while NbC and TaC approach MC_1.0 and melt congruently even at carbon-deficient stoichiometries. The Group VI metals have a more complex M-C phase diagram, forming a number of distinct compositions. The chromium carbides behave uniquely, while the Mo-C and W-C systems have common features, with the MC and M₂C phases stable at high temperatures. (Table 4) The trends in melting points indicate that the Group IV, V, and lower two Group VI metals have strong M-C and M-M bonds, distinct from the Groups IA - IIIB metals, which form acetylenic M-C bonds, and from the Groups VIII - IIB metals, which form unstable carbides, if at all.

Removal of bound carbon causes the lattice parameter to decrease in most of the refractory carbides, but to increase in TiC and ZrC. The behaviour of HfC on decrease of the lattice carbon-to-metal ratio is uncertain due to the variation in this behaviour among reports. For several metal carbides, the variation of lattice parameter with carbon content is linear. Removal of carbon from the lattice also is associated with reductions in hardness, at least for the Group IV carbides. Dissolved oxygen lowers the lattice parameter in Group IV carbides, while it increases it in Group V carbides, and has an uncertain effect for carbides of Group VI metals. The effect of oxygen contamination on mechanical properties is not clearly reported in the literature.

The refractory carbides show high chemical resistance but will react under certain conditions. At high temperatures, the high-carbon compositions form hydrocarbons in the presence of hydrogen. The reactions with oxygen have been indicated above (Equations 2.1, 2.2), and are complex. The carbides will form the nitrides at high temperatures and in the presence of N₂, NH₃, or N₂/H₂ mixtures; however, the cubic carbides and nitrides are completely miscible.

2.3.2     Group IV

The conversion of TiO₂ to TiC occurs via the intermediates Ti₃O₅, Ti₂O₃, and TiO in the temperature range 1000 - 1500^oC. The carbide closest to TiC_1.0 forms at 1600-1700^oC under 1-10 torr of CO. Titanium hydride and carbon form TiC_1.0-x after 1 hour in vacuum at 1200^oC. TiC has also been formed by heating a tungsten wire or carbon filament in an atmosphere of TiCl₄, H₂, and hydrocarbon, or by reaction of CaC₂, TiCl₄, and H₂ at 800^oC (CaC₂ and CaCl₂ are removed by washing with water after the reaction is complete). The last traces of oxygen are difficult to remove, and have a significant effect on the material properties. Later heating may recontaminate even a “pure” sample of titanium carbide if the vacuum is not sufficiently good; a non-protecting, non-adherent TiO₂ (anatase) layer forms at about 450^oC. The Ti-C system has one cubic compound of formula TiC, although other phases have occasionally been claimed. TiC is metallic and gray, and is stable to most concentrated acids or bases. It will dissolve completely in HNO₃ and combinations of HNO₃ with HCl (aqua regia), HF, and H₂SO₄.

The reduction of ZrO₂ proceeds via Zr₂O₃ and ZrO to the carbide between 950-1200 ^oC. It has also been formed using ZrH plus graphite or from ZrCl₄ in the presence of hydrogen and hydrocarbon vapour. Attempts to remove oxygen completely are generally unsuccessful except under melting conditions. “Pure” ZrC heated at temperatures under 1800^oC tends to gather oxygen up to several percent. The Zr-C system contains one cubic compound, ZrC, and the lattice parameter varies with oxygen contamination noticeably at levels of 1000 ppm. ZrC is somewhat more susceptible to acid attack than is TiC and oxidises rapidly above 500^oC.

HfO₂ forms an oxy-carbide of constant composition between 1743 - 2033^oC and under 70-1000 torr of CO, with Hf₂O₃ forming at 1000-1200^oC and the HfC-HfO solid solution between 1300-1800^oC. HfC forms a carbon-deficient lattice between 1800 - 2000^oC, but can be made stoichiometric by repeated heating at 190^oC. HfC has also been formed from HfCl₄ + H₂ + CH₄ in the presence of a hot tungsten wire and from hafnium hydride and carbon. It is one the most difficult carbides to rid of its oxygen, only becoming “pure” when melted or heated at temperatures in excess of 2500^oC in good vacuum. The Hf-C system has one cubic phase HfC, but the composition can range to a low of HfC_0.52. Its melting point increases with increasing carbon content up to a maximum, then HfC forms a eutectic with carbon. There has been no study of lattice parameter variation with either oxygen or carbon content.

2.3.3.    Group V

Heating V₂O₅ or V₂O₃ with carbon for two hours at 1800^oC in 1-10 torr of CO has formed vanadium carbide. V₂O₅ begins reacting with carbon at 435^oC, and the oxygen concentration is relatively easily reduced to below detectable limits by higher-temperature treatments or by the reaction of vanadium metal or hydride with carbon. Loss of vanadium at high temperatures and low carbon content presents a difficulty, but near-saturated VC can be heated to 2000^oC without loss of vanadium. The carbide has been made by treatment of VCl₄ in an atmosphere of hydrogen and methane at 1500 - 2000^oC. The two main phases are cubic VC, available over the range VC_0.78-VC_1.0, and the hexagonal beta-V₂C, available for C/V atom ratios of 0.4-0.5 between approximately 1500 - 2000 ^oC, but presenting a very narrow range of stable compositions at room temperature. Reports of V₅C and V₄C₃ have been discredited. VC will react with dry HCl gas at 750^oC to produce VCl₂, methane, and hydrogen, and has a high rate of oxidation in air, with powdered VC and V₂C being attacked slowly by air even at room temperature. V₂C is soluble in hot 50% HCl solution, leaving a carbon residue, but VC is inert under these conditions; both of the vanadium carbides are attacked by concentrated nitric, sulphuric, and perchloric acids.

Niobium oxide begins to react with carbon at 675^oC, forming NbO₂ and NbC_x below 1200^oC and forming an NbC_xO_y solid solution between 1450 and 1500^oC. Pure NbC is accessible by heating the metal and carbon powders directly, but high temperatures and heating times are required to complete the reaction and drive off oxygen and nitrogen contaminants. The conditions are made less stringent by the presence of an H₂ atmosphere. NbCl₅ heated in the presence of hydrogen and methane forms the pure carbide at 900 -1000^oC. The Nb-C system has the cubic phase NbC and two crystal forms (alpha, room temperature, with a very narrow composition range and beta, existing between 2300-3000^oC and over the C/Nb ratio range 0.4-0.5) of Nb₂C. A third, zeta phase of Nb₂C (C/Nb range 0.5-0.7) has been suggested to exist on the basis of a single weak powder pattern line, but has not been verified. The lattice parameter increases as the C/Nb atom ratio approaches unity, and is increased as well by the presence of oxygen and nitrogen. NbC is inert even to boiling aqua regia but will dissolve in HNO₃/HF mixtures; it is severely corroded in air at temperatures above 1100^oC. Its colour ranges from grey (NbC_0.9) to lavender (NbC_0.99).

The tantalum carbide system is relatively easy to free of oxygen impurities, but due to the slow rate of carbon diffusion it tends to have in-homogeneities in its bulk composition. Evaporation of carbon at temperatures above 2400^oC renders the use of high temperatures to establish a uniform composition problematic. Reaction from the elements in vacuum begins at approximately 1000^oC but is slow to reach completion. Use of hydrogen or methane atmospheres increases the reaction rate but requires a post-synthesis vacuum-annealing step to remove dissolved hydrogen. TaC cannot be made from TaCl₅ in a hydrocarbon/hydrogen atmosphere due to the formation of metallic tantalum, but has been made with varying success by heating Ta wire in methane. Arc melting tends to produce C-deficient, in-homogenous carbides. The Ta-C system has the cubic phase TaC and a hexagonal compound Ta₂C (actually the C6 anti-CdI₂ structure type due to ordering of the carbon atoms) with a transition near 2000^oC. A phase has been claimed at 2000 - 3000^oC over the C/Ta atom ratio 0.70-0.75 but its existence remains a point of debate. The cubic phase persists over a wide temperature and composition range, possibly as low as TaC_0.58 and verified down to at least TaC_0.74. The composition TaC_0.89 is the highest-melting substance known. Very small amounts of carbon (Ta₆₄C) result in a tetragonal distortion to the normally bcc Ta parent lattice. The TaC lattice varies linearly with composition, with the equation C/Ta = -25.641 + 5.9757a. It is grey and metallic in appearance up to about TaC_0.85, then becomes increasingly brown with rising carbon content until the golden TaC_0.99 is reached. It is the most acid-stable of the refractory carbides, dissolving in a nitric/hydrofluoric acid mix, and reacts with pure oxygen above 800^oC. Loss of carbon results from lower-temperature reactions with oxygen in air.

Table 4. Range of Melting Points for Group IV-VI Carbides

2.4.1.   Tantalum

Tantalum is a grey, heavy, and very hard metal. When pure, it is ductile and can be drawn into fine wire, which is used as a filament for evaporating metals such as aluminium. Tantalum is almost completely immune to chemical attack at temperatures below 150oC, and is attacked only by hydrofluoric acid, acidic solutions containing the fluoride ion, and free sulphur trioxide. Alkalis attack it only slowly. At high temperatures, tantalum becomes much more reactive. The element has a melting point exceeded only by tungsten and rhenium. Tantalum is used to make a variety of alloys with desirable properties such as high melting point, high strength, good ductility, etc. Tantalum has a good "guttering" ability at high temperatures, and tantalum oxide films are stable and have good rectifying and dielectric properties

Isolation of tantalum is very complicated. Usually niobium and tantalum are both contained in tantalum bearing minerals. It is difficult to separate niobium and tantalum since they are chemically so similar. Tantalum can be extracted from the ores by fusing the ore first with alkali, and then extracting the resulting mixture into hydrofluoric acid. Recent methodology involves the separation of tantalum from these acid solutions using a liquid-liquid extraction technique. In that process tantalum salts are extracted into the ketone MIBK (methyl isobutyl ketone, 4-methyl pentan-2-one) .The niobium present in tantalum mineral remains in the hydrofluoric solution. After conversion to the oxide, metallic tantalum can be made by reduction with sodium or carbon. Electrolysis of molten fluorides are also used.

Separation of tantalum from niobium requires several complicated steps. Several methods are used to commercially produce the element, including electrolysis of molten potassium fluorotantalate, reduction of potassium fluorotantalate with sodium, or reacting tantalum carbide with tantalum oxide.

Tantalum carbide of the chemical formula TaC, with a theoretical carbon content of 6.23%, is a metallic powder of a dark light-brown colour. The colour in influenced by nitride admixtures and very thin oxide films. Pure crystals isolation from the metal bath have a gold lustre. Preparations described as grey powder are probably Ta₂C.

Tantalum carbide in only slightly soluble in acids .It burns in air with a bright flash. The density of TaC was given by Friederich and Sittig as 13.96 g./cm.³, while Mckenna”s preparations, which was formed in a aluminium bath, had the density 14.48 g/cm.³, which is close to the X-ray density of 14.53 g. /cm.³.

According to Friederich and Sittig the Mohs hardness of TaC is 9 to 10 while Styri and Foster and co-workers give a Brinell hardness of 840 and a Knoop hardness of 840 kp/mm² The micro hardness obtained by Kieffer and Kolbl using a load of 50g was 1,88 kg/ mm.². The same value has been measured by Hinnuber  using a 20g. load. The modulus of elasticity is, according to Koster and Rauscher  29,500 kg./mm² (41.5*106 psi). The tensile strength of vapour-deposited TaC wire at room temperature was determined by Becker and Ewest as 2000-4000 psi.

The carbide Ta₂C melts at 3400°C with decomposition, according to Ellinger . The mono carbide melts, at 4730 –4830°C with decomposition, according to Friederich and Sittig

The specific elastic resistivity of TaC is 200 microhm-cm(µW)  according to Friederich and Sittig , 100 microhm-cm according to Moers ,  and 170 microhm-cm according to Andrews , while recent measurements reported by Schwarzkopf and Sindeband ,  give a values of only 30 microhm-cm . The variation of resistivity  with temperature was measured by Becker and Ewest ,. According to Meissner and co-workers , the mono carbide becomes super conductive between 9.5  Kelvin and 7.3 Kelvin.

Haddan, Goldwater and Morgan  measured the thermionic emission of TaC at 2300°C. The emission was 2.8 times smaller than that of metallic Ta. The values obtained did not indicate any practical possibilities for the use of TaC as emitter. The same authors, as well as Morgan , determined the spectral emissivity of TaC coatings and found the value of 0.67 at 0.655 micron.

2.4.4.   Phase diagram of the system Ta-C

On the basis of microscopic and X –ray investigations and of melting point determinations on tantalum carbide specimens produced by vacuum-sintering, fusion, or surface carburization of tantalum, Ellinger  established a phase diagram of the system tantalum-carbon, the tantalum-rich corner of which is shown in Fig 3. There exists two defined compounds, Ta₂C (3.21%of C),with a melting point at about 3,400 degree Celsius, and TaC (6.23% of C) with a  melting point at about 3,800 degree Celsius . At 0.6% of C, Ta forms a eutectic with Ta₂C at a temperature of about 2800 degree Celsius, Ta₂C being able to dissolve in 0.2% of Ta. An appreciable solubility of carbon in tantalum is not observable. The melting point of TaC of about 3800 degree Celsius is lowered by the absorption of carbon to a second eutectic at 10% of C and 3300degrss Celsius. At 3400 degree Celsius TaC is able to dissolve up to 1.2% of Ta, as had been surmised by Burgers and Basart . While TaC has a cubic face-centred lattice, Ta₂C, first described by Burgers and Basart  has a hexagonal close-packed structure corresponding to a c/a ratio of 1.59.

                                                        Fig 1        

                                       (Phase Diagram of the system Ta-C)

Like W₂C, a₂C occurs, according to Burgers and Basart  in two allotropic modifications, a-Ta₂C and b-Ta₂C. Examinations of the surface of Ta wires, which had been carburised from the gaseous phase under controlled conditions, showed the b-Ta₂C modification. After pulverisation of the wire, however, the X-ray diagram exhibited the lines of a-Ta₂C. Whether b-Ta2c occurs only at the surface of the wire or is transformed into a-Ta₂C on pulverizing remains an open question.

On rapid cooling of burned out TaC wires from above 2530 degree Celsius, a-Ta₂C appears (at variance with W₂C which occurs as b-modification after rapid cooling). Since a weakening of certain lines in X-ray diagram is observed rather than their disappearance during the formation of the two modifications,it is to be assumed that transition is continuous. Ellinger  who studied a number of Ta₂C-bearing preparations, was unable to demonstrate the existence of different modifications.

Table 4. Properties of tantalum monocarbide in the homogeneity range

Tantalum carbide in the form of wires has been proposed for use as wound filaments in incandescent lamps. The low strength of tantalum carbide wires however is prohibitive for general use. For similar applications, protective coatings of tantalum carbide and rhenium or tungsten wires have been suggested. High-sintered tantalum carbide tubes have been used to reach extreme high temperatures as required, e.g., for the determination of the melting point of high-melting hard metals.

Tantalum carbide is of practical importance in the production of cemented multicarbide hard metals. In machining-tool materials, TaC, like TiC, reduces the tendency of welding between steel chips and tool material and thus the so-called cratering, which is due to such welding, and the subsequent separation of the welds.