Molybdenum Disulfide
Molybdenum disulfide is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is MoS2.
The compound is classified as a transition metal dichalcogenide. It is a silvery black solid that occurs as the mineral molybdenite, the principal ore for molybdenum. MoS2 is relatively unreactive. It is unaffected by dilute acids and oxygen. In appearance and feel, molybdenum disulfide is similar to graphite. It is widely used as a dry lubricant because of its low friction and robustness. Bulk MoS2 is a diamagnetic, indirect bandgap semiconductor similar to silicon, with a bandgap of 1.23 eV.
Mechanical Properties
MoS2 excels as a lubricating material due to its layered structure and low coefficient of friction. Interlayer sliding dissipates energy when a shear stress is applied to the material. Extensive work has been performed to characterize the coefficient of friction and shear strength of MoS2 in various atmospheres. The shear strength of MoS2 increases as the coefficient of friction increases. This property is called super lubricity. At ambient conditions, the coefficient of friction for MoS2 was determined to be 0.150, with corresponding estimated shear strength of 56.0 MPa. Direct methods of measuring the shear strength indicate that the value is closer to 25.3 MPa.
The wear resistance of MoS2 in lubricating applications can be increased by doping MoS2 with chromium. Micro indentation experiments on Nano pillars of Cr-doped MoS2 found that the yield strength increased from an average of 821 MPa for pure MoS2 (0 at. % Cr) to 1017 MPa for 50 at. % Cr. The increase in yield strength is accompanied by a change in the failure mode of the material. While the pure MoS2 Nano pillar fails through a plastic bending mechanism, brittle fracture modes become apparent as the material is loaded with increasing amounts of dopant.
The widely used method of micromechanical exfoliation has been carefully studied in MoS2 to understand the mechanism of delamination in few-layer to multi-layer flakes. The exact mechanism of cleavage was found to be layer dependent. Flakes thinner than 5 layers, undergo homogenous bending and rippling while flakes around 10 layers thick delaminated through interlayer sliding. Flakes with more than 20 layers exhibited a kinking mechanism during micromechanical cleavage. The cleavage of these flakes was also determined to be reversible due to the nature of van der Waals bonding.
In recent years, MoS2 has been utilized in flexible electronic applications, promoting more investigation into the elastic properties of this material. Nanoscopic bending tests using AFM cantilever tips were performed on micromechanically exfoliated MoS2 flakes that were deposited on a holey substrate. The yield strength of monolayer flakes was 270 GPa, while the thicker flakes were also stiffer, with a yield strength of 330 GPa. Molecular dynamic simulations found the in-plane yield strength of MoS2 to be 229 GPa, which matches the experimental results within error.
Bertolazzi and coworkers also characterized the failure modes of the suspended monolayer flakes. The strain at failure ranges from 6 to 11%. The average yield strength of monolayer MoS2 is 23 GPa, which is close to the theoretical fracture strength for defect-free MoS2.
The band structure of MoS2 is sensitive to strain.
Applications: Lubricants
Due to weak van der Waals interactions between the sheets of sulfide atoms, MoS2 has a low coefficient of friction. MoS2 in particle sizes in the range of 1–100 µm is a common dry lubricant. Few alternatives exist that confer high lubricity and stability at up to 350 °C in oxidizing environments. Sliding friction tests of MoS2 using a pin on disc tester at low loads (0.1–2 N) give friction coefficient values of <0.1.
MoS2 is often a component of blends and composites that require low friction. For example, it is added to graphite to improve sticking. A variety of oils and greases are used, because they retain their lubricity even in cases of almost complete oil loss, thus finding a use in critical applications such as aircraft engines. When added to plastics, MoS2 forms a composite with improved strength as well as reduced friction. Polymers filled with MoS2 include nylon (trade name Nylatron), Teflon and Vespel. Self-lubricating composite coatings for high-temperature applications consist of molybdenum disulfide and titanium nitride, using chemical vapor deposition.
Examples of applications of MoS2-based lubricants include two-stroke engines (such as motorcycle engines), bicycle coaster brakes, automotive CV and universal joints, ski waxes and bullets.
Other layered inorganic materials exhibit lubricating properties (collectively known as solid lubricants (or dry lubricants)) include graphite, which requires volatile additives and hexagonal boron nitride.