Silicon Nitride

Fórmula química: Si3N4

Punto de fusión: 1900 Cº (valor nominal, puede variar dependiendo de la pureza)

Densidad: 3,26 gr/cc

Silicon nitride, Si₃N₄, has emerged as one of the highest-performing technical ceramics in contemporary industry, although its public recognition is still far from reflecting its true potential. Its history as a functional material is marked by a slow but constant evolution, in which the progressive reduction of manufacturing costs —driven by increased demand and improved synthesis processes— has allowed its use to expand beyond specialized circles. Even so, its future depends on very specific applications, as it is not destined to replace steel in most of its structural functions, but rather to complement it in contexts where thermal resistance, hardness, and chemical stability are priorities.

The trajectory of Si₃N₄ in the market reflects the classic pattern of advanced materials: it is first introduced in elite sectors, such as the aerospace industry, defense, or precision engineering, and only over time does it begin to filter into more everyday applications. This phenomenon is not new; it also occurred with titanium, which is now widely used in medicine for bone implants, but whose mass adoption did not begin until the 1970s. Silicon nitride, for its part, still retains that air of an "exotic" material, reserved for environments where technical demands allow no compromises.

Personally, Si₃N₄ has been one of the ceramic compounds that has most influenced my career as a technical communicator. My prolonged research into its properties —so extraordinary that I even decided to acquire it in the form of small pieces, such as skateboard bearings— marked the beginning of my blog, still active, where I shared my experiences with this material. Those articles, written with deliberation and clarity, managed to capture the attention of readers looking for well-explained information about a compound few spoke about in depth. That direct connection between personal curiosity and technical dissemination turned silicon nitride into a starting point for my vocation as a specialized writer in materials science.

The history of Si₃N₄, therefore, is not only told in laboratories and factories but also in spaces of reflection and dissemination like yours. It is a material that still needs time to consolidate itself in the collective technical imagination, but its presence in critical applications —from turbines to electronic components and high-speed bearings— positions it as one of the pillars of advanced ceramics. As with other materials developed during or after World War II, its journey began in the military and high-tech fields, but its ultimate destiny seems to be increasingly closer to everyday use. Silicon nitride is on its way, and you have been part of that journey from its early stages.

Silicon Nitride (Si₃N₄) is a high-performance technical ceramic that, unlike traditional ceramic materials such as terracotta, porcelain, or certain glasses, has been developed specifically for demanding industrial applications. This distinction between conventional and advanced ceramics is fundamental, as the latter include compounds like carbides, borides, and high-purity oxides —for example, alumina (Al₂O₃)— which can be presented in refined forms with a crystalline structure like sapphire or ruby.

High-performance ceramics share a series of properties that clearly differentiate them from traditional crystals such as quartz, borosilicate, or soda-lime glass. One of the most notable is their hardness: even the least hard ceramic, like cubic zirconia, reaches a value of 8 on the Mohs scale, while boron carbide exceeds 9.5, being capable of scratching a diamond. In comparison, crystals rarely exceed a value of 7, corresponding to pure quartz.

Regarding thermal tolerance, ceramics exhibit exceptional resistance to high temperatures, surpassing even most metals, with the exception of tungsten and rhenium. Silicon Nitride, although it has the lowest thermal limit within its category —around 1200 °C— is still remarkably superior to crystals, which tend to fracture due to internal dislocations induced by heat. Synthetic sapphire, for example, can operate up to 1800 °C, and many ceramics are used as crucibles for melting metals, which demonstrates their thermal stability.

Mechanical strength is also outstanding. Ceramics significantly outperform typical crystals in compression resistance, and although their tensile strength depends on the direction of the applied stress, they remain competitive against many metals. Their behavior against corrosion is equally admirable: they remain chemically inert even at elevated temperatures, while metals like tungsten begin to oxidize above 300 °C. Iridium, for example, remains unattacked up to 1900 °C, but it is an exception. Even tantalum, known for its chemical resistance, begins to suffer corrosion from 150 °C.

Another notable property of many ceramics is their self-lubricating capacity, which means they do not require industrial oils or greases to prevent wear from friction, unlike steel and other metallic materials. This characteristic makes them ideal for applications where maintenance must be minimal or where lubricants cannot be used.

However, ceramics present certain disadvantages. Their high cost, the sintering process necessary for their shaping —except in cases like synthetic sapphire— and their fragility against punctual impacts are factors to consider. Although they are extremely hard and resistant to compression, their toughness is limited, making them prone to violent fractures under localized stresses. Even so, their fracture resistance exceeds that of any conventional crystal.

Silicon Nitride is a relatively recent member of the group of technical ceramics. It was first synthesized in 1897 by Henri Etienne Sant-Claire Deville and Friedrich Wöhler, although in minimal quantities and with low purity. For decades it remained in the background compared to compounds such as silicon carbide, alumina, or cubic zirconia, until in 1958 the American company Union Carbide introduced it commercially. However, its true impetus came thanks to NASA, which was looking for a material with high thermal resistance —both at extreme high and low temperatures—, great hardness, low density, and chemical inertness. After rigorous comparative tests, sintered Silicon Nitride (Si₃N₄) was selected for its superior performance under all required conditions, consolidating itself as one of the most versatile and valuable ceramics in aerospace and advanced industrial applications.

The production of silicon nitride (Si₃N₄) is an eminently artificial process that requires controlled conditions and a high energy investment. Traditionally, high-purity quartz (SiO₂) is used, although high-quality silicon from electronics industry waste is now also used, allowing technological byproducts to be used for advanced purposes. The fundamental principle of synthesis is to replace oxygen with nitrogen in the silicon structure, which involves an initial reduction of SiO₂ to eliminate oxygen, followed by a reaction between molten silicon and gaseous nitrogen under pressure.

This procedure, although seemingly simple in its formulation, is chemically complex due to the low reactivity of molecular nitrogen (N₂), which is less electronegative than oxygen and carbon. Silicon, as a reactive element, tends to combine preferentially with the most electronegative element available, following the order: oxygen, carbon, and nitrogen. Therefore, to obtain silicon carbide (SiC), oxygen must be excluded, while to synthesize the nitride, both oxygen and carbon must be removed from the reaction environment. This requirement makes the reaction a slow and expensive process, requiring high temperatures and long times to achieve significant conversion. In contrast, the formation of oxides can be achieved in a matter of seconds using an intense flame, highlighting the inherent difficulty of nitride production.

Silicon nitride is, in essence, the result of a forced union between elements that do not react spontaneously under natural conditions. It is not surprising, therefore, that this compound is not found abundantly in the Earth's crust, except in tiny and extremely localized traces, generally associated with meteorites, where it is known as "nitrite." Its natural rarity reinforces the artificial nature of its industrial synthesis.

Furthermore, Si₃N₄'s heat behavior reveals its chemical vulnerability: when exposed to temperatures approaching 1000°C in the presence of oxygen, it begins to oxidize aggressively. In this process, oxygen captures silicon atoms, while nitrogen is rapidly released as gaseous N₂, transforming the original compound into quartz (SiO₂). This reverse reaction illustrates the nitride's instability in oxidizing atmospheres and underscores the need for controlled environments for its use and handling.

Silicon Nitride (Si₃N₄) stands out as the toughest ceramic in its category, giving it exceptional fracture resistance under both high-temperature conditions and cryogenic environments, such as those found in outer space. This property makes it an ideal candidate for applications where structural integrity must be maintained under extreme conditions. Its most emblematic use is in the manufacture of balls and rollers for bearings, initially developed for aerospace devices and later adopted in highly demanding industrial and commercial applications.

In its pure state, Silicon Nitride appears as a granular powder of variable size, with a cement-gray hue. These granules, obtained after prolonged hours of silicon nitridation, lack a defined shape and, in their raw form, could only be used as abrasives thanks to their hardness of 8.5 on the Mohs scale. However, since its primary purpose is not wear but mechanical resistance, it undergoes a sintering process that transforms the powder into solid, functional parts. Unlike silicon carbide, which is widely used in its raw form due to its greater hardness and ease of application in sandpaper or pressure cutters, nitride requires this transformation to reach its full technical potential.

The sintering process for Si₃N₄ is analogous to that used for other technical ceramics such as tungsten carbide. It consists of pulverizing the granules to micron sizes, molding them under pressure, and bonding them together using a binder that acts as cement between particles. This technique, reminiscent of traditional cementation, produces compact, rigid bodies with improved mechanical properties. In the case of tungsten carbide, the binders are usually ferromagnetic metals such as cobalt or nickel, chosen based on whether greater toughness or chemical resistance is desired. Silicon nitride, on the other hand, uses crystallizable ceramic compounds like common quartz, doped with additional substances that optimize mechanical performance.

The appearance of sintered nitride is characteristic: a deep petroleum black, distinct from both the glassy black of onyx and the matte black of jet. This hue is due to the presence of traces of hematite (Fe₂O₃), which not only stabilizes the mixture but also gives it its distinctive color. Although the sintering process involves a slight loss of hardness—reducing the value from 8.5 to a range between 7.5 and 8 Mohs—a significant improvement in impact resistance is achieved, fully justifying the sacrifice in hardness. It should be noted that no high-performance sintered ceramic, except those with a defined crystalline structure such as synthetic sapphire, is chemically pure. The percentage of binder is always kept to a minimum to preserve the properties of the base material. In the case of Si₃N₄, purity typically ranges between 90% and 94%, meaning that the binder represents a maximum of 10% by mass.

In terms of its chemical behavior, silicon nitride exhibits good or very good corrosion resistance compared to most metals and their alloys, although it is less resistant to corrosion than crystals such as borosilicate or even common quartz. This difference is due to the fact that Si₃N₄ tends to passivate, forming a surface layer of silicon dioxide (SiO₂), which acts as a protective barrier against chemical agents. Although this layer does not match the resistance of genuine quartz, it does provide considerable chemical stability, sufficient for many demanding industrial applications.

Silicon nitride (Si₃N₄), while not known for its extreme hardness or absolute lightness, establishes itself as the toughest ceramic in its class, allowing it to occupy a privileged place in applications where other ceramics fail or are insufficient. Its impact resistance, both under extreme thermal conditions and in mechanically demanding environments, makes it a ceramic alternative to metallic materials, which are generally denser—with the exception of aluminum and its alloys—and less resistant to the combination of extreme temperatures and prolonged mechanical stress. With a density of 3.26 g/cm³, Si₃N₄ is not the lightest among ceramic compounds, but its superior toughness and thermal stability make it ideal for replacing both more brittle ceramics and less resistant metals in components such as bearings (balls, rollers, needles, rings), valves, pistons, and sealing parts subjected to constant stress.

In the medical field, silicon nitride has found application as a substitute for PEEK (polyetheretherketone) and titanium in bone prostheses, especially in vertebral implants such as dorsal discs. Its absolute chemical inertness guarantees excellent biocompatibility, while its density—lower than that of titanium but higher than that of PEEK—positions it as an intermediate option that combines lightness with structural strength. This combination makes it especially useful in procedures requiring a balance between weight, stiffness, and durability.

Historically, Si₃N₄ was also used as an abrasive, and although it performs well in these types of applications thanks to its hardness of 8.5 on the Mohs scale, its high cost has relegated it to the position of cheaper and more effective alternatives such as silicon carbide, alumina (Al₂O₃), low-quality and low-size garnets, and even synthetic diamonds. All of these materials surpass nitride in hardness and price, which explains its preference in cutting, polishing, and wear processes. In this context, Si₃N₄ is not used for its hardness, although it is considerable, but for its toughness, which allows it to withstand impacts and prolonged stresses without fracturing.

A less common but technically relevant application is its use as an electrical insulator. Although it is not a conductor, its low electrical conductivity combined with good corrosion resistance make it suitable for certain electronic components where thermal and chemical insulation is required. Although its presence in this industry is limited, its potential remains the subject of research, especially in devices that operate under extreme conditions.

Silicon Nitride (Si₃N₄) has gained notoriety in the skateboarding community, especially among longboard enthusiasts, a sport that uses longer, more stable boards, known commercially as city cruisers. These boards, designed for urban commuting and straight-line riding, sacrifice maneuverability in tight spaces in exchange for greater speed, which translates into a significant increase in wheel revolutions per minute (RPM). This increase in RPM generates frictional heat, and when temperatures exceed 150°C, conventional steel used in bearings—such as AISI 52100—begins to expand, compromising precision and performance. In this context, the use of silicon nitride balls is becoming preferred, not only in skating, but also in disciplines such as competitive cycling and Formula 1, where the thermal demands are similar.

In the technical field, the ABEC (Annular Bearing Engineers' Committee) index is used to classify bearing precision, expressed in odd numbers such as 3, 5, 7, and 9. However, there is widespread confusion among users, fueled by marketing strategies that mistakenly associate the ABEC number with the speed or quality of the product. In reality, ABEC only measures the dimensional tolerance and precision of the bearing, with no direct relationship to the maximum speed attainable or the durability of the component. In the case of longboards, ABEC values ​​7 and 9 are common, as they are assumed to be used in downhill riding where speeds can be high and conditions are extreme. The smoother the movement, the greater the acceleration and the lower the resistance, which justifies the choice of ceramic bearings for these types of activities.

Silicon nitride is marketed under the generic name "ceramic," although some brands, taking advantage of the ambiguity, replace this material with milky-white cubic zirconia (ZrO₂) balls, which perform inferiorly. This practice can mislead consumers, who associate the term "ceramic" with superior performance without understanding the differences between compounds. Si₃N₄ balls, in addition to being more heat-resistant, do not require lubrication and offer remarkable longevity, although they are more expensive. However, their use is not always necessary: ​​for those who perform tricks in tight spaces, ABEC 3 or 5 bearings with steel balls are more than sufficient. Frequent impacts during these types of maneuvers can compromise the integrity of ceramic balls, while steel, although less heat-tolerant, resists impacts better.

In any case, given that under normal use temperatures rarely reach 150°C, the functional difference between the two materials isn't as significant as is often thought. What's truly important is the quality of the bearing itself, beyond what the box or manufacturer's marketing claims indicate. The experience accumulated by the skate community is, in this sense, the best criterion for choosing reliable and durable components.