*2.3.3 Mixed lubrication*

The transition between the boundary lubrication and the hydrodynamic regimes is a gray zone known as mixed lubrication. In this regime, the both mechanisms such as boundary lubrication and hydrodynamic lubrication may be in operational condition. There might be possible for the higher solid contacts, however, a minimum portion of the bearing surface leftover through partial hydrodynamic film. The hard solid interaction between the new metal surfaces can cause to a wear debris formation, adhesion of particle with counterpart, metal transfer from bulk to counterpart and eventual seizure. But, in the case of liquid condition, the chemically formed films protect the surfaces from adhesion during the sliding experiment. This mixed regime is called as thin film lubrication, partial fluid and quasi-hydrodynamic lubrication [21].

### **2.4 Wear**
