CARBIDES

Up until now, we have discussed the use of elemental carbon in metallurgy, which, except in applications like crucibles or refractory parts, is considered more a fuel than a structural material. In its pure state, carbon does not possess the necessary mechanical properties to perform structural functions in metallic alloys, and its direct use is limited. However, this perception changes radically when carbon forms compounds with other elements, especially with transition metals, giving rise to what are known as carbides, whose relevance in metallurgy is difficult to overestimate. 

The definition of a carbide encompasses any compound in which carbon (C) is bonded to an element whose electronegativity is lower than its own—2.2 on the Pauling scale—regardless of the type of bond established. This ability to form carbides with a wide variety of elements makes carbon a primary structuring agent. In the metallurgical field, metallic carbides stand out for their chemical stability, extreme hardness, wear resistance, and exceptional thermal behavior, which makes them indispensable in demanding industrial applications.

A paradigmatic example of the importance of carbides is steel, an alloy of iron (Fe) and carbon that constitutes the true core of modern metallurgy. All equilibrium phases, crystalline transformations, and mechanical properties of steel are intrinsically linked to the formation—or absence—of cementite (Fe₃C), iron carbide, as well as the amount of uncombined graphite present in the matrix. The presence, distribution, and proportion of these compounds determine not only the hardness and toughness of steel, but also its response to heat treatments and its behavior under mechanical stresses.

To illustrate the magnitude of steel study, it is enough to point out that while titanium (Ti) might occupy a few dozen academic monographs, steel has generated hundreds of scientific papers, doctoral theses, and technical treatises that meticulously analyze the variations between its multiple grades. Each of these grades is distinguished by factors such as the percentage of cementite, the amount of pure graphite by mass, secondary alloying elements, and applied heat treatments. This wealth of information has made steel not just another branch of metallurgy, but its true trunk, the starting point from which other disciplines branch out.

However, carbides are not exclusive to iron. They also form in alloys of different base metals, such as titanium, vanadium (V), tungsten (W), molybdenum (Mo), and others, each with particular properties and specific applications. Although they share certain structural features—such as the formation of compact crystalline lattices and strong bonds with carbon—each carbide responds differently to thermal, mechanical, and chemical processes, allowing for their adaptation to very specific uses, from cutting tools to aerospace components.

Over time, you will learn that the study of carbides not only reveals the versatility of carbon as a chemical element, but also offers a privileged window into the very heart of metallurgy. Each carbide, each crystalline structure, each equilibrium phase is a piece of the puzzle that makes up a deep understanding of materials, and understanding them is to understand how science transforms matter into utility.

Before continuing, I would like to explain how a carbide forms. This way, I believe, you will more easily understand why it resembles, in some sense, other non-carbon-containinginorganic compounds, such as borides, nitrides, and widely used oxides.

To begin, you must think of a carbide as a substitute for a common oxide, in which the oxygen atoms have been replaced by carbon atoms. How is this possible?

If you have previously read about why metals oxidize so easily, you will know by now that the vast majority of metals opt to form bonds with more electronegative elements rather than combining with themselves. That is, when we go to a mine, it is normal to find metals in their mineral form, with the vast majority of these minerals being, in fact, oxides. But why?

The abundant presence of oxygen and the fact that it is more electronegative and forms chemically very stable compounds (you can't oxidize an oxide, of course) encourage the formation of combinations of this element with other more reactive elements that readily donate their electrons, with greater or lesser speed depending on their "nobility." Thus, as you will have read, gold does not naturally mix with oxygen, while alkali metals practically "explode" upon mere contact with water; they oxidize rapidly. The fact that we find so many oxide minerals and not carbides is precisely due to three factors that favor oxygen over carbon in these compounds.

The first is electronegativity. While carbon is quite electronegative (2.2), oxygen is even more so (3.44), such that if a metal is going to donate an electron, oxygen will always take it first. The second factor is that oxygen is much more abundant, by mass, than carbon, at least on the Planet's surface. Although this data may not be as important when producing a carbide, it helps to explain why there is such a difference in abundance between oxides and carbides. The third reason, no less important, is that oxides are always more chemically stable than carbides. In fact, some carbides like calcium carbide (CaC₂) cannot come into contact with water, as they would react. On the other hand, calcium oxide (CaO), although also reactive, is more stable in this sense. In any case, the main reason remains the first: if the metal has to "choose" between forming carbides or oxides, it will always choose the latter, since oxygen will prevail over carbon (do not forget that carbon itself is oxidizable by oxygen, forming carbon monoxide CO or carbon dioxide CO₂).

It is for this reason that the vast majority of carbides are artificial and are generated in environments where the amount of carbon is so massive compared to oxygen that saturation occurs in two steps: oxygen will form CO₂ instead of oxidizing the metal, while the remaining free carbon, due to excess mass, will combine with the metal. This is why when producing a carbide, the amount of accessible carbon must be enormous, otherwise the quantity of carbide would be very small or of poor quality. Although simply summarized, this is the method used to extract iron from its minerals. Another important clarification is that to form carbides, it is not necessary to supply the metal in its pure form (it can be added as a mineral, provided it is refined), nor in the case of carbon (common coke is used in most cases).

As I said before, not all carbides are chemically stable. Furthermore, not all metals form carbides.

Although many authors cite tin and lead as the only metals "completely incapable" of forming carbides "under any conditions," the truth is that the list, although long, must focus on the elements found in groups 4, 5, and 6 of the Periodic Table, or what is the same, in order of increasing atomic number: Titanium, Vanadium, Chromium, Zirconium, Niobium, Molybdenum, Hafnium, Tantalum, and Tungsten. These metals are the kings among metallic carbides (some are so famous, like tungsten carbide, that sometimes when ordered, the buyer refers to it simply as "Carbide" without specifying "of Tungsten"). Iron forms its own carbide, cementite, but it is thermodynamically unstable and can only exist within a steel matrix. Furthermore, the amount of carbon that iron is capable of dissolving (beyond its crystalline structure) never reaches 3% dissolved carbon by mass, at least at normal pressures.

It is important to distinguish between carbides that can be isolated in massive quantities, such as those of metals from groups 4, 5, and 6, and those formed, among others, by iron, manganese, and cobalt, which only dissolve small amounts of carbon when in a liquid state. Metals like copper, zinc, silver, gold, tin, et cetera, are incapable of forming carbides under normal conditions.

In contrast to this, the metalloids silicon and germanium (from the carbon family) form carbides with carbon with relative ease. Silicon carbide, in particular, is the second (or first, depending on the context) most used and/or famous carbide, after tungsten carbide.

Other metals, such as the alkaline earth calcium or the post-transition aluminum, form carbides, but they are chemically very reactive and dissolve easily in water.

The "best" carbides from an engineer's perspective, seeking the best materials, are, as I have said, those formed by the metals of groups 4, 5, and 6, in addition to cementite (iron carbide with formula Fe₃C) and silicon carbide with formula SiC. Note that I have not mentioned the formulas of each metallic carbide. The reason is that, in fact, there is more than one subtype per element (e.g., WC for monotungsten carbide and W₂C for semitungsten carbide), although one formula always prevails over the rest.

Traditional metallic carbides are also known as Cermets, an Anglo-Saxon word that blends the terms Cer (from "ceramic") and Mets (from "metals"), alluding to the characteristics of these compounds, which are considered similar to those of high-end ceramics themselves, such as alumina and cubic zirconia, and transition metals. In other words, metallic carbides, or Cermets, share characteristics with both ceramics and metals, but do not fall into either category, instead forming one of their own.

Cermets are the most popular in the discipline of metallurgy as they are widely used in the manufacture of special steels with extraordinary performance, such as the so-called HSS (High Speed Steel). Most applications of these compounds exploit their unique mechanical properties, which we will now examine.

Although each has a specific name and formula, they generally behave so similarly that it is sometimes difficult to distinguish between them.

The attributes they all share are: extremely high hardness (most have a Mohs hardness of 9 to 9.5), high corrosion resistance, and above all, an extremely high melting temperature.

Temperature resistance is a point of emphasis among scholars because it applies not only to the ability of these compounds to operate in extreme conditions where other materials would fail mechanically or chemically.

Mechanically, because heat weakens the bonds, for example, of a typical mild steel from 120ºC or even less (without considering the problem of corrosion, crackling, and rapid wear). A quality Cermet is capable of operating up to 600ºC or more and does not lose rigidity in the process.

This is because the carbon-metal bonds are very strong, even if they are difficult to form.

Manufacturing carbides is relatively easy compared to other similar compounds like nitrides or borides, due to the strong affinity that the element carbon has for some transition metals. The main advantage is that to manufacture them, it is not necessary to expose the metal in its pure form to carbon, as we saw before, but rather the oxide (or even more complex minerals, but the simple oxide is preferable) can be used. This detail is of paramount importance when considering cost. For example, when trying to obtain metallic titanium (pure), many steps are required to refine the oxide, and from there to "detach" the oxygen atoms from the metal without contaminating it with carbon. The same happens with vanadium, molybdenum, et cetera; it is a problem that all present. However, in the case of titanium, it is curious, since unlike most other transition metals that easily form Cermets, it does absorb nitrogen, so the fusion must take place in a controlled atmosphere. In any case, the method consists of combining fuming carbon with the purified oxide of the metal with which we want to form the compound. This procedure applies to all transition metals that form carbides, and also to silicon (a metalloid).

To obtain a carbide of high purity, high purity carbon and the oxide of the metal in question are used (it is possible to use pure metal, but it is rare as it does not compensate). The carbon must be present in large quantities, as it not only has to reduce the oxide (separate the oxygen atoms by bonding with them, forming CO and CO₂) but also combine (be absorbed) by the metal/metalloid with which it seeks to combine.

The result of the "carburization" of the oxide yields high-purity crystals, normally dark in color (usually due to the presence of traces of iron) and with a density similar to that of the pure metal. These are then processed to reduce their size (if we are talking about high-precision parts, a nanometer-sized grain powder is required) which will then be added to steel (in the case of High Speed Steels) or shaped through the sintering process (to manufacture Cermets). A famous example of the latter is tungsten carbide, well explained in this book.

The Cermet itself is a powder, or a sandy substance depending on the "grain size." The grain size, as its name suggests, measures the dimensions of a typical particle. The smaller they are, the more mobile they become, to the point that if sufficiently minute, they act like a pseudo-liquid. The grain size is typified as Mesh Grain Size and is expressed in approximations of less than a millimeter. This is important to know because sometimes the same Cermet is used as more or less large grains, depending on the desired result. Normally, fine grain size translates into greater robustness, while superfine is used for the manufacture of parts demanding high precision. An example is that the tungsten carbide grains used in sandpaper are considerably larger than those used for the manufacture of bearing balls or high-end jewelry rings.