Prof. Valery Mitin
Call for a better lubricant
About one fifth of all energy produced annually is lost to friction. More than a half of this loss comes from industry and transportation. To fight this we need lubricants that can significantly decrease friction and wear, are easy to produce and apply. Currently, most popular are liquid lubricants such as oils and greases, but lubricants can also be solid, gaseous and plastic. While liquid lubricants do a good job in mild conditions, they fail under high load and in low and high temperatures. They also need constant re-supply, which in some cases, e.g. in vacuum, can be proven difficult. These conditions are all around us — in cars, airplanes, manufacturing, wind and solar energy systems, etc. As we push for more efficient technologies, conditions become more complex and extreme, and we find ourselves in need for new versatile lubricants. This is a call for better lubricants — and the answer is solid lubricants.
There are many types of solid lubricants. They could be organic (polytetrafluoroethylene, polyamides) or non-organic (boron nitride, metal chlorides, layered (molybdenum disulfide, graphite), homogenous (tin, copper, silver), or even composite with nanoparticles embedded into matrix (copper-carbon, silver-carbon). Unique and tailored solid lubricant can be found for each existing friction problem — selection of solid lubricant depends on operating conditions of friction pair. And, while each solid lubricant excels under specific conditions — e.g. a sharp decrease in the coefficient of friction (up to 0.01) was achieved with molybdenum disulfide coating when heated in vacuum to 700 °C — they also have limiting factors for their application. For example, the limiting temperature of graphite performance in air is determined by its oxidation temperature. It also exhibits antifriction properties that can be explained by the weakening of bonds in parallel to the direction of sliding surface, but it can become abrasive if particles are oriented perpendicularly or obliquely to the sliding surface.
Production of solid lubricants
Currently most solid lubricants are produced in the form of coatings applied either by chemical or physical vapor deposition (CVD or PVD). The choice of a coating method depends on the substrate material and its composition, design and requirements. Among these requirements are good adhesion of the coating, wear rate and friction coefficient of solid lubricant, counterpart type, friction type, corrosion resistance, temperature resistance and load capacity. Based on the above requirements and taking into account the development of a new solid lubricants for extreme conditions, a high-speed ion-plasma magnetron sputtering or HsIPMS (see Future of PVD) is preferred production method, as it has two natural advantages:
Ecological impact. It is a substitute for environmentally harmful electrochemical and CVD coating methods. It provides higher quality of coatings and wider range of sprayed materials, compounds and composites that cannot be obtained by chemical methods. HsIPMS technology is using environmentally friendly process of physical sputtering of materials by ion bombardment that results in no special liquid drains, gaseous emissions, transportation and storage of toxic reagents, and equipment operators are not exposed to chemical influences and radio-electronic radiation.
Easy-to-control structure. Nanoscale structure of the coating and the size of nanocrystals can be determined by specific modes of sputtering and tailored design of sputtering coatings.
Among other pros of HsIPMS technology are: versatility, i.e. it is possible to deposit metallic, ceramic, semiconductor and even magnetic coatings; wide operating temperature range from 20 °C up to 1500 °C; high quality and uniformity of coatings; high precision in coatings thickness from sub-micron up to hundreds of microns; capability to coat outer and inner surfaces, and cavities in bushings, sleeves, pipes, rings, nets; and last, but not least — high productivity and low cost of coating process. Our technology allows to effectively deposit traditional solid lubricants such as graphite, molybdenum disulfide, molybdenum diselenide, copper, silver, niobium diselenide, etc. But it also enables development of new solid lubricants on basis of hard coatings (TiN, ZrN, CrN, MoN) in combination with nanostructured materials with a set of properties necessary for the operation of tribological pairs in extreme conditions.
We designed new type of solid lubricant that can be made with HsIPMS by joint sputtering of metal and carbon from mosaic targets. Metal should be selected from ones that do not chemically interact with carbon: nickel, cobalt, copper, silver, etc. The carbon content can vary in wide range from 5 at.% up to 90%, depending on required properties of the coating. Homogeneous introduction of carbon on the atomic level into metal structure creates composite coating and reduces coefficient of friction in dry conditions up to 0.06. Such coating can be enhanced by alternation of pure metallic layers with composite Me-C layers, nanostructured not only in composition, but also in thickness. Such enhancement provides increased strength, wear and corrosion resistance. Such solid lubricant coatings, where the metal is not the main component and does not interact with carbon, can be used as low-temperature solid lubricant operating both in air and in an inert atmosphere. They are suitable both for sliding and rolling bearings and for friction pairs, including various types of gears. In addition to reducing the coefficient of friction, these solid lubricants can provide backlash-free couplings without sticking and slipping. Slip-free engagement significantly reduces the noise level in gearboxes and friction pairs and can significantly increase accuracy of movement in mechanisms of coordinate positioning systems.
Body of mosaic targets (shown above) can also be made out of metal that chemically interacts with carbon (tungsten, tantalum, molybdenum, niobium, chromium). This type of solid lubricant coatings can be used for friction pairs operating at temperatures higher than 500°С and high contact loads in atmospheric conditions, vacuum, inert atmosphere and at cryogenic temperatures of liquid gases (oxygen, nitrogen, hydrogen, helium). In this case, carbides of refractory metals are formed during sputtering and multilayer structure Me/MeC/Me/... gives these materials increased strength, microhardness up to 50-70 GPa, increased wear resistance, corrosion resistance and heat resistance in vacuum or inert atmosphere, and in case of chromium carbide even in pure oxygen atmosphere. Metal content can vary from 5 to 85 at.%. Coatings with excess metal content exhibit dry friction coefficient lower than 0.25 and show low sticking at high temperatures to non-ferrous metals (>400 °C) and stainless steels and high-alloyed alloys (>1000 °C). Coatings of this type can also be widely used to increase the durability and lifetime of metalworking and cutting tools, especially in high-speed machining of high-alloy, chromium-nickel-containing steels and alloys, as well as in the processing of viscous materials.
Solid lubricants are nothing new — ore molybdenite was known to early Greeks and graphite has been used as lubricant since middle ages. But true potential of solid lubricants is yet to be explored. High-speed ion-plasma magnetron sputtering with its combination of materials and nanoscale design capabilities is the perfect tool to unlock their potential and lead the way to energy lossless future.