Molybdenum disulfide (MoS2) is used as a dry lubricant to reduce friction and wear. It can be placed in powder form between two sliding surfaces to prevent wear, seizure and sticking of the surfaces. MoS2 occurs as a mineral ore, Molybdenite and Jordisite. To be used as a lubricant, MoS2 has to be extracted and purified from abrasive particles and moisture.
Liquid lubricants are used in applications operating in normal conditions, but under certain operating conditions, dry lubricants like MoS2 are used. MoS2 is a preferred lubricant in high-temperature, high-vacuum, high-radiation, extreme load and corrosive conditions. In such conditions, the conventional lubricants can fail and become ineffective. Molybdenum disulfide powder can be directly applied to the friction surface by rubbing or as pastes, coatings, additives and films.
MoS2 is classified as a Transition Metal Dichalcogenide (TMD), which has a structure of type MX2. M is a transition metal, like Molybdenum, and X is a chalcogen, like Sulphur. MoS2 exhibits a low coefficient of friction due to its layered crystal structure.
MoS2 has a layered lattice structure, and within one layer, a plane of Molybdenum atoms is sandwiched between two planes of Sulphur atoms. This forms the S-Mo-S structure bonded with covalent bonds. The individual layers are connected together by weak Van der Waals forces.
The layers can easily slide against each other when a shear force is applied, resulting in a low coefficient of friction. The layers align themselves parallel to the sliding direction. The layers with the covalent bonds provide excellent load-carrying capacity perpendicular to the sliding direction. This mechanism is used in lubrication.
MoS2 is an expensive lubricant, but it can be prepared by synthesis. The synthetic MoS2 does not have inferior properties to the natural MoS2 and is available at half the cost.
Synthesis of MoS2
1. Reacting Molybdenum trioxide (MoO3) and Sodium carbonate (Na2CO2) with Sulphur in its melt.
The mixture of Molybdenum trioxide, Sodium carbonate and Sulphur is heated in a porcelain crucible and covered with a lid. It is heated to a temperature of 700 °C for an hour. The sintered mass is leached several times with water heated to a temperature of 60 °C. This helps to remove the impurities not consumed in the reaction. The product is filtered and dried at 90 °C in a vacuum-dry oven. The resulting product is MoS2.
2. Reacting Calcium molybdate (CaMoO4) and Sodium carbonate (Na2CO3) with Sulphur in a melt.
The mixture of Calcium molybdate, Sodium carbonate and Sulphur is heated in a porcelain crucible to a temperature of 700 °C for 1 hour. The resulting product is treated with water and then a 5% solution of hydrochloric acid, and again with water. The resulting Molybdenum disulfide contains 60% Mo and 40% S.
3. Reacting Molybdenum trioxide (MoO3) with Sulphur vapour
The reaction takes place at a velocity at a temperature higher than 300 °C. The components are synthesised in a vertical stainless steel tube with a porous partition. A layer of powdered Molybdenum trioxide is placed on the partition. Sulphur vapours are passed through the partition. The upper part of the tube is filled with Ar gas. The reaction occurs at 700 °C and produces MoS2. Instead of Molybdenum trioxide, pure Molybdenum can also be placed in the porous partition.
Applications of MoS2
- MoS2 in powder form can be applied by smearing or compacting, or impacting the metal surface to be lubricated. The metal surface should be cleaned before applying. To improve the adhesion between MoS2 and the metal surface, the metal surface can be heated. The excess layers of MoS2 are difficult to remove mechanically and can be removed using hydrochloric acid.
- When MoS2 is rubbed on the sliding surface, the irregularities get filled in, and this increases the total contact area. This allows the bearing to operate at a higher capacity. A thin layer of MoS2 ensures good functioning of the sliding surface up to a specific pressure.
- MoS2 is used as a lubricant in the fields of aviation, technology, automotive, construction, in cutting tools and for drilling and boring refractory material, food and chemical industry, in textile machines and more.
- MoS2 can be used as a lubricant for moving parts of an engine. This promotes a low coefficient of friction and provides greater cooling and power.
- When MoS2 is used as a lubricant, it leads to a reduction in noise when the bearings function. The force required to twist in threaded and screw connections is reduced by 50%. The service life of the machinery increases by 50%.