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Dry film lubricants have been used for years in nuclear power plants in the United States due to their high performance, extreme temperature, and pressure resistance. This article is intended to explain what these lubricants are, how they work, and why engineers in these industries continue to depend on them for lubrication and anti-seize applications.
Dry film lubricant coatings
Lubricants, as most people understand them, generally come in liquid form – oils and greases. They work through hydrodynamic lubrication. Through the addition of a liquid layer between two solid moving parts, these lubricants smooth and cushion the bumps between the two surfaces. The lubricants flow between the surfaces, ideally in two layers moving in opposite directions. These layers slide past one another, reducing friction between the two parts. The quality that makes these substances effective lubricants is called lubricity.
What is a dry lubricant, and how does it provide lubrication in the absence of hydrodynamic properties? A dry lubricant is any material that can reduce friction between two surfaces moving against each other, despite being in the solid – rather than liquid – phase. In this case, the lubricity is attributable to the substances’ lamellar structure. Lamellae are fine layers between surfaces. Dry films or powders form lamellae between the two parts to which they are applied, orienting themselves along the surfaces in the direction of motion. A remarkable fact about these lamellae is that, even between two heavily loaded structures, under extreme pressure or extreme temperatures, they are still able to prevent contact between the moving parts – even when applied in relatively thin layers.
The most common forms of dry film lubricant coatings are molybdenum disulfide, graphite dry film lubricants, hexagonal boron nitride, and tungsten disulfide. Each of these has properties that distinguish it from the others and is used in very different applications. Hexagonal boron nitride, also known as white graphite, is used in space vehicles, for example. Graphite and molybdenum disulfide have numerous uses, which will be expanded on below. Tungsten disulfide is very similar to molybdenum disulfide but has a much higher cost and are harder to come by.
Molybdenum disulfide dry film lubricant
Molybdenum disulfide (MoS2) is a silvery black solid that occurs naturally in molybdenite ore. It has several qualities that make it a perfect industrial lubricant. It has a very high in lubricity – with a coefficient of friction averaging around less than 0.1. It is robust and stable at temperatures ranging up to 350°C. It is impervious to oxygen, which makes it ideal for oxidizing environments. MoS2 is also an indirect bandgap semiconductor, which makes it useful in situations where conductive lubricants are required.
MoS2 is used in a wide variety of applications and can be marketed in several forms. It can be sold as a powder or film on its own but is most often used as an additive in other products, such as graphite, oils, and greases. It can be applied in engines – particularly the two-stroke ones found in motorbikes, CV, and universal joints. There are also various uses in the nuclear power industries, and in ballistics. Huron Industries markets a special preparation of MoS2 in isopropanol, a lubricant designed for applications where control of impurities is required. It is intended for use on threaded fasteners and other anti-seize applications of closely-fitted parts. It is certified to military specification MIL-L-24478 and comes in two forms: a premixed 8-ounce bottle with an applicator brush and a two-part kit that includes 1500 grams of MoS2 and 1000ml of isopropanol.
Choosing a high-temperature lubricant
For nuclear energy, aeronautics industries, and a variety of military functions, one of the most important criteria for lubricants is that they remain stable and functional at high temperatures. There are several high-temperature lubricants available, but how do you choose the correct one for your application?
When selecting a lubricant to use at high temperatures, users generally consider three critical factors: viscosity, thermal degradation, and oxidation. Viscosity is the magnitude of internal friction within a fluid, expressed colloquially as a substance’s thickness or stickiness. For a lubricant to be effective, it needs to maintain its viscosity at high temperatures. Many gels and greases tend to thin out when overheated, reducing their utility. Thermal degradation refers to a substance’s propensity to break down at the molecular level due to increasing temperatures. Oxidation is the breakdown or denaturing of a material due to contact with oxygen. For a lubricant to be favorable for use at high temperatures, it needs to retain its viscosity and molecular structure at high temperatures, while also being relatively impervious to oxidation.
MoS2 has proven to be extremely resistant to high temperatures and to be virtually unaffected by oxygen. This is why it has been so popular for uses under extreme conditions. The quality of viscosity does not apply to MoS2, since it is a solid. However, it can be included as an additive in grease-based lubricants. The viscosity of which will depend on other factors. When assessing a lubricant for viscosity, you need to look at what is known as the dropping point. This is the temperature at which a grease starts to lose its viscosity. Of the five main types of grease thickeners used in the manufacture of lubricants (lithium complex, sodium complex, PTFE, calcium complex, and aluminum complex), only one of them is suitable for use at temperatures of 200 to 300°C: lithium complex. In cases where the working environment rises towards 400°C, there are no grease-based lubricants that can withstand the heat. The solution is to use solid lubricants or composites.
The higher your operating temperature, the fewer options you have when it comes to lubricants, making the choice much more straightforward. At temperatures ranging above 300°C, the only lubricants that offer sufficient resilience against thermal degradation and oxidation are MoS2 and a few liquid metals and liquid oxides. Of these, MoS2 remains the most reliable and cost-effective.
An excellent high temperature lubricant for nuclear applications is Huron Industries’ NEOLUBE® No. 1260. This is intended for critical service applications under intense heat (up to 635°C) and pressure (2300 psi). It exhibits extremely high lubricity under all conditions and is suitable for containment and/or secondary side in nuclear applications. It has high chemical purity, excellent thermal stability, low halogen content and excellent radiation resistance.
Oxidation – the lubricant’s nemesis
Oxidation is the most common reaction of oil and grease-based lubricants, especially when used under extreme conditions. It results in a variety of problems such as viscosity increase or decrease, sludge formation, sedimentation, depletion of active additives, rust and corrosion, and more. When formulating lubricants, chemists need to pay particular attention to the control of oxidation.
Every lubricant is formulated to include antioxidants. These are often sacrificial, meaning they oxidize so that the rest of the lubricant does not. The critical thing for chemists to calculate is which of these antioxidants should be included in what proportions. This becomes unnecessary with dry film lubricants, however, because oxidation is less of a consideration. The four most common compounds used as dry solid lubricants are all relatively impervious to oxidation. This means that lubricants maintain their nature and function even under extreme conditions.
Tribology: the key to proper lubricant selection
Tribology is the study of friction and wear – and their prevention. In the specific context, tribology is the science of interacting surfaces in relative motion fixating on the design of bearing, moving parts, and lubricants formulated to protect them. This science underlies the design of all machines, their parts, and the creation of lubricants to prevent wear and ensure proper safe functioning.
There was a time when the formulation and selection of lubricants was a trial-and-error process. This is no longer possible in a high-performance environment where machines must run better, longer, and at higher temperatures. A more scientific approach is required, involving the study of a machine’s tribological system and the resulting lubrication needs.
A machine’s tribological system consists of its type, speed of motion, operating temperature, and environment, and load. Once tribology experts have defined these parameters, they can select the most suitable lubricant.
– Type of motion: there are two basic types of motion in any tribological system: sliding and rolling. Some systems may also include a combination of the two. This is an important consideration for tribology engineers as some lubricants may work well for rolling but may not work well for sliding or vice versa.
– Speed: The speed of the system is generally defined in one of three general categories: fast, moderate and slow. Once the speed of the contact between the parts is determined, an appropriate lubricant can be selected.
– Temperature: All lubricants have specific temperature ranges within which they can work optimally. Many lubricants work in a kind of broad middle range, while others are best applied in very hot or very cold temperatures. An ether base oil thickened with PTFE, for example, maintains optimal function at 220°C, while a dry film application composed of MoS2 can work at temperatures higher than 300°C. Knowing the average optimal temperature of the system makes it relatively easy to select a lubricant.
– Load: Is the load light or heavy? The answer will determine how much frictional torque the system creates, which will, in turn, dictate the lubrication requirements. Heavy loads may require additives to prevent pitting and wear, while low loads would demand the minimization of fluid friction.
– Operating environment: Here, engineers consider the external factors acting upon the system. Is it operated underwater? In a vacuum? Or is it a critical service application such as in a nuclear powerplant? In all of these cases, special considerations would have to be added to the choice of lubricant. For instance, if water is a factor, then anti-corrosion preparations must be considered.
Conductive lubricants
In some tribological systems, an additional factor needs to be taken into consideration over the movement of the parts. There may be instances where there is surface-to-surface contact – particularly metal-to-metal contact, where a continuous electrical path is required – perhaps a switch or commutator. In these cases, the lubricant between the parts needs to have the capacity to conduct electricity to keep the circuit closed and ensure the proper function of the system.
Conductive lubricants can also work in a protective capacity in systems where static discharge poses a hazard. They are often applied to ball bearings in several applications, where they act as a ground and allow static discharges to pass through the bearings instead of building up and arcing. Silicon, graphite and MoS2 are commonly used to add this property to lubricant greases.
Lubricants for nuclear reactor systems
Nuclear reactor systems present extremely rigorous tribological demands. They operate under high temperatures, with combinations of rolling and sliding motions at high speed and heavy loads. The lubricants employed in these applications need to be suitably robust. The properties of graphite and MoS2 make them ideal solutions to these needs. They are rugged and durable, maintaining form and function at high temperatures, resist oxidation, and provide excellent lubrication through their lamellar structures.
Huron Industries is a woman-owned and woman-led industrial sealant and lubricant supplier for the nuclear industry. Our NEOLUBE® products and our MoS2/ isopropanol kits are products of extremely high lubricity and powerful resistance to extreme temperatures and pressures, and are well suited to use in nuclear applications.
For more information on our range of dry film lubricants and other solutions for nuclear, aeronautic and military applications, contact us.