Molybdenum disulfide also contains more than one type of chemical bond. Looking at
Figure 1, the horizontal bonds are within the basal planes, and vertical bonds are in between the basal
planes. Structurally, molybdenum disulfide functions in a manner intended to keep the basal planes
intact, even when faced with a tangential force. Strong covalent bonds within the planes function to serve
this purpose. There are two types of vertical bonds: the longer sulfur-sulfur bonds which are 3.08
angstroms long, as well as the shorter sulfur-molybdenum bonds which are 1.54 angstroms long. 7 The
longer sulfur-sulfur bonds are connected by weak van der Waals forces and the shorter sulfurmolybdenum
bonds are connected by strong covalent bonds. 8 The configuration actually creates a net
positive charge on the surface of the layers causing an electrostatic repulsion between the layers. Van der
Waals forces are the weakest of all types of chemical bonds. These bond strengths and locations give
molybdenum disulfide its lubricating properties by promoting shear deformation along the basal plane
between the two sulfur layers. This provides a low coefficient of friction. In static conditions values have
been reported at approximately 0.07, while dynamic conditions are as low as 0.03. 9