Due to pollution, chemical heat treatment, electroplating, and the effects of smoothing agents, an extremely thin outer film is formed on the surface of the metal, such as oxide film, sulfide film, phosphating film, chloride film, confinement film, cadmium film, aluminum film, etc., which gives the surface different properties from the substrate. If the outer film is within a certain thickness, the actual contact area is still scattered on the substrate material instead of the outer film, and the shear strength of the outer film can be lower than that of the substrate material; On the other hand, due to the presence of the outer film, adhesion is less likely to occur, resulting in a decrease in friction force and coefficient of friction.
The thickness of the outer film also has a significant impact on the friction coefficient. If the outer film is too thin, the film is easily crushed and presents direct contact with the substrate material; If the outer film is too thick, on the one hand, it increases the practical contact area due to the softness of the film, and on the other hand, the micro peaks on the two pairs of surfaces also have a more prominent plowing effect on the outer film. It can be seen that the outer film has a better thickness that is worth seeking.
The friction coefficient of metal friction pairs varies with the properties of the paired data. Generally speaking, friction pairs of the same metal or metals with greater mutual solubility are prone to adhesion, resulting in a higher friction coefficient; On the contrary, the friction coefficient is smaller. Different structures of data have different friction characteristics. Graphite, due to its stable layered structure and low interlayer separation force, is prone to sliding, resulting in a lower friction coefficient; For example, the friction pair paired with diamond is less prone to adhesion due to its high hardness and small practical contact area, and its friction coefficient is also relatively small.
The influence of the temperature of the surrounding medium on the friction coefficient is mainly caused by changes in the properties of the surface data. Bowden et al.'s experiments have shown that the friction coefficient of many metals (such as molybdenum, tungsten, Qin, etc.) and their compounds shows small values when the temperature of the surrounding medium is 700-800 ℃. This phenomenon is due to the initial temperature rise causing a decrease in shear strength, and further temperature rise causing a sharp drop in the yield point, resulting in a significant increase in the practical contact area. However, during high polymer friction pairs or pressure processing, the friction coefficient will exhibit a maximum value with changes in temperature.
As can be seen from the above, the influence of temperature on friction coefficient is variable, and the relationship between temperature and friction coefficient becomes very complex due to the influence of detailed operating conditions, data characteristics, changes in oxide film, and other factors.
Under normal conditions, the sliding speed will cause surface heating and temperature rise, thereby altering the properties of the surface. Therefore, the friction coefficient will inevitably change accordingly. When the relative sliding speed of the friction pair on the surface exceeds 50m/s, a large amount of frictional heat will be generated in contact with the surface. Due to the short duration of continuous contact at the contact point, a large amount of frictional heat generated in an instant cannot diffuse into the interior of the substrate in time. Therefore, frictional heat is concentrated in the surface layer, resulting in a higher temperature and a condensed layer. The condensed metal liquid plays a smoothing role, causing the friction coefficient to decrease with increasing speed. For example, when copper slides at a speed of 135m/s, its friction coefficient is 0.055; At 350m/s, it drops to 0.035. But the friction coefficient of some materials (such as graphite) is simply not affected by sliding speed, because the mechanical properties of such materials can remain unchanged over a wide temperature range.
Regarding border friction, within the low speed range below 0.0035m/s, which is the transition from static friction to dynamic friction, as the speed increases, the friction coefficient of the adsorption film gradually decreases and tends to a constant value, while the friction coefficient of the reaction film also gradually increases and tends to a constant value.
Under normal conditions, the friction coefficient of metal friction pairs decreases with increasing load and then tends to stabilize. This phenomenon can be explained by the adhesion theory. When the load is very small, the two pairs of surfaces are in an elastic contact state. At this time, the practical contact area is proportional to the power of 2/3 of the load, while according to the adhesion theory, the friction force is proportional to the practical contact area, so the friction coefficient is inversely proportional to the power of 1/3 of the load; When the load is large, the two pairs of surfaces are in an elastic-plastic contact state, and the actual contact area is proportional to the power of 2/3 to 1 of the load. Therefore, the friction coefficient decreases slowly and tends to stabilize with the increase of the load; When the load is so large that the two dual surfaces are in plastic contact, the friction coefficient is completely independent of the load.
The magnitude of the static friction coefficient is also related to the duration of static contact between the two pairs of surfaces under load. Under normal conditions, the longer the duration of static contact, the greater the static friction coefficient. This is due to the effect of the load, which causes plastic deformation at the contact point. With the extension of the static contact time, the practical contact area will increase, and the micro peaks will be more deeply embedded, causing more trouble.
In plastic contact conditions, the influence of surface roughness on the practical contact area is minimal, so the friction coefficient is almost unaffected by surface roughness. Regarding dry friction pairs with elastic or elastic-plastic contact, when the surface roughness value is small, the mechanical action is also smaller, while the molecular force effect is greater; The opposite is also true. It can be seen that the friction coefficient will have a minimum value as the surface roughness changes.
The influence of the above factors on the friction coefficient is not isolated, but interconnected and mutually influential.