Friction is the phenomenon of mutual obstruction between the surfaces of relatively moving objects. Friction can be divided into the following categories:
(1) Internal friction: The phenomenon of the obstruction relative motion between molecules that occur within a substance.
(2) External friction: The phenomenon of mutual obstruction that occurs between the surfaces of relatively moving objects.
(3) Static friction: The friction only in relative motion.
(4) Kinetic friction: The friction in relative motion.
(5) Sliding friction: The form of motion between the surfaces of objects is relative sliding friction.
(6) Rolling friction: The form of motion between the surfaces of objects is relative rolling friction.
Wear is the loss or transfer of material on the surface of the object due to friction, which can affect the efficiency of the machine, reduce the reliability of the workpiece, and causing the machine to be scrapped in advance.
Mechanical wear can be divided into three stages:
The running-in stage – the new parts are usually in this stage at the beginning of use, with a high wear rate.
The stable wear stage – belongs to the normal working stage of parts, and the wear rate is stable and low.
The severe wear stage – the stage that parts will be scrapped, and the wear rate will increase sharply.
The three stages of wear are shown below:
According to the classification of the wear mechanism, it can be generally summed up as:
1. Abrasive wear, or wear, is the abrasion caused by the outer surface of the free hard particles or hard contour peaks.
2. Fatigue wear, also known as pitting, is the wear caused by the fatigue of material caused by repeated deformation of the micro-volume of the friction surface material under the action of alternating friction.
3. Adhesive wear, also known as gluing, occurs when the profile peak of the frictional surface “cold welds” at various points of interaction due to transient temperature rises and pressures. During relative motion, the material migrates from one surface to the other, and then forms adhesive wear.
4. Erosion wears, the entrainment of hard substances or particles in the fluid, the wear caused by the impact force of the fluid on the friction surface;
5. Corrosion wear, when the frictional surface material causes corrosion under the chemical or electrochemical action of the environment, the wear caused by the relative motion of the friction pair is corrosion wear.
6. Fretting wear refers to the composite wear formed by the above-mentioned wear mechanisms when the friction pair moves slightly, and the micro-motion can be understood as the relative motion that is insufficient to separate the abrasive particles from the friction pair.
The friction of mechanical components is universal and unavoidable. Friction causes heat, temperature rise and energy loss leading to loss and transfer of material at the contact surface (that is to causing contact surface wear). Wear causes the shape and size of the parts to be damaged, which reduces the efficiency and reliability of the machine until it loses its original performance.