The viscosity of motor oil gets lower in order to decrease resistance at region of the fluid lubrication. On the other hands, this causes increasing of friction at the boundary lubrication region. For this reason, it is required to use a friction modifier to reduce the boundary friction. MoDTC is especially used as friction modifier. Although the decomposition mechanism of MoDTC has been presumed, it is considered only in the case of metal, not hard coatings. So, it is required to understand the effects of hard coatings to friction in motor oils. We tried to clarify the surface and tribofilm changes depending on the kinds of hard coatings by SEM-EDX, TEM and so on. As a result of experiment of hard coatings against metal, we clarified the process as follows when friction of CrN reduces. In the early stage of friction, Fe of the opposite metal transfers to CrN. Mo2S2O2 intermediate product which is formed by MoDTC decomposition is formed on CrN divided into Fe oxide and Mo sulfide. At this time, crystal orientation between Fe oxide and CrN is matching because the lattice constant of Fe oxide is twice that of CrN. These results show that the crystal structure of hard coatings is important for formation of stable interface. And this newly knowledge is necessary to achieve low friction for systems using hard coatings.