1. Surface properties

Due to pollution, chemical heat treatment, electroplating and lubricants, a layer of extremely thin surface film (such as oxide film, vulcanization film, phosphating film, chlorination film, prohibition film, cadmium film, aluminum film, etc.) is formed on the metal surface, which makes the surface layer have different properties from the matrix.If the surface film is within a certain thickness, the actual contact area still depends on the substrate material rather than the surface film, and the shear strength of the surface film can be lower than that of the substrate material.On the other hand, due to the existence of the surface film, it is difficult to adhere, so the friction force and friction factor can be reduced.

The surface film thickness also has great influence on the friction factor.If the surface film is too thin, the film is easy to be broken and there is direct contact with the matrix material.If the surface film is too thick, on the one hand, the actual contact area will be increased because of the soft film; on the other hand, the furrow effect of the micro-peaks on the surface film is also prominent.It can be seen that the surface film has an optimal thickness worth seeking.

2. Material properties

The friction factors of metal friction pairs vary with the properties of the matched materials.Generally speaking, the same metal or metal friction pair with greater mutual solubility, easy to adhere, its friction factor is larger;On the contrary, the friction factor is small.Materials with different structures have different frictional properties.For example, graphite has a stable lamellar structure and the interlamellar binding force is small, easy to slide, so the friction factor is small;For another example, the friction pair paired with diamond is not easy to stick due to its high hardness and small actual contact area, and its friction factor is also small.

3. The temperature

The influence of the temperature of the surrounding medium on the friction factor is mainly caused by the changes in the properties of the surface material. Bowden et al. showed that the friction factor of many metals (such as molybdenum, tungsten, chins, etc.) and their compounds is minimized when the temperature of the surrounding medium is between 700 ℃ and 800 ℃.This phenomenon is caused by the decrease of shear strength caused by initial temperature rise and the sharp decrease of yield point caused by further temperature rise, resulting in the increase of the actual contact area.However, the maximum value of the friction factor will appear with the change of temperature during the high polymer friction pair or pressure machining.

It can be seen from the above that the influence of temperature on friction factor is variable, and the relationship between temperature and friction factor becomes very complicated due to the influence of specific working conditions, material characteristics, oxidation film changes and other factors.

4. Relative movement speed

In general, sliding speed will cause surface heating and temperature rise, thus changing the surface properties, so the friction factor will change accordingly.

Figure 1 shows the results of the experiment by klagelski et al.For the friction pair in the general elastic-plastic contact state, the friction factor passes a maximum value with the increase of sliding speed, as shown in curves 2 and 3, and with the increase of the normal load between the dual surfaces, the position of the maximum value moves to the origin of coordinates.When the load pole is small, the curve has only the ascending part.When the load is extremely large, the curve has only the descending part, as shown in figure 1 and 4.

When the relative sliding speed of the dual surface of the friction pair exceeds 50m/s, a large amount of friction heat is generated on the contact surface.Due to the contact point of continuous contact time is short, a large number of frictional heat generated at the moment of no internal diffusion to the substrate, so the friction heat on the surface, the surface temperature is higher in melting layer, molten metal fluid lubrication effect, make the friction coefficient decreased with increasing speed, such as copper when sliding rate of 135 m/s, its friction coefficient is 0.055;At 350m/s, it drops to 0.035.However, the friction factor of some materials, such as graphite, is hardly affected by the sliding speed, because the mechanical properties of such materials can remain constant over a wide range of temperatures.

For boundary friction, in the low speed range with the speed below 0.0035m/s, that is, the transition from static friction to dynamic friction, with the acceleration of the speed, the friction factor of the adsorbed film gradually decreases and tends to a constant value, and the friction factor of the reaction film also gradually increases and tends to a constant value.

FIG. 1 influence of sliding speed

1 -- very small load 2, 3 -- medium load 4 -- very large load

5. The load

In general, the friction factor of metal friction pair decreases with the increase of load and then tends to be stable, which can be explained by the theory of adhesion.When the load is very small, the two dual surfaces are in elastic contact state, then the actual contact area is proportional to the 2/3 power of the load, and according to the adhesion theory, the friction force is proportional to the actual contact area, so the friction factor is inversely proportional to the 1/3 power of the load;When the load is large, the two dual surfaces are in an elastoplastic contact state. The actual contact area is proportional to the 2/3 ~ 1 square of the load. Therefore, the friction factor decreases slowly and tends to be stable with the increase of the load.When the load is large enough that the two dual surfaces are in plastic contact state, the friction factor is basically independent of the load.

The static friction factor is also related to the duration of the stationary contact between the two pairs of surfaces under load.In general, the longer the duration of static contact, the greater the static friction factor.This is due to the action of load, so that the contact place plastic deformation, with the extension of static contact time, the actual contact area will be increased, micro peaks embedded in each other.More deeply caused.

6. Surface roughness

In the case of plastic contact, because the surface roughness has little effect on the actual contact area, it can be considered that the friction factor is almost unaffected by the surface roughness.For the elastic or elastic-plastic contact dry friction pair, when the surface roughness value is very small, the mechanical effect is smaller, but the molecular force is larger;And vice versa.It can be seen that the friction factor will have a minimum value with the variation of surface roughness.

The influence of the above factors on the friction factor is not isolated, but interrelated, which should be noted in the analysis.