Under the wear understand the gradual destruction of the friction surfaces of various pairs. There are many types of wear. They are due to various reasons. But all of them are united by one thing - particles are separated from the main material. This leads to disruption of the mechanisms, and in other cases can cause them to break. Gaps in the joints increase, landing starts to hit as a result of the formation of significant backlash. This article discusses the main types of wear, their characteristics and general classification are given.
Features of abrasive wear
An abrasive is a finely dispersed material of natural or artificial origin, which has significant hardness sufficient to scratch other, less hard, materials.
The type of wear of surfaces in which the destruction of the structure and integrity of the surface layer is observed when interacting with solid microparticles is called abrasive. It should be canceled that for this kind of destruction, the friction speed must be very significant (several meters per second). Although during prolonged operation, destruction occurs at lower speeds and clamping forces.
Abrasive substances can be played by both fixed objects (solid phases of steels and alloys), and mobile foreign particles that have fallen into the contact zone of rubbing surfaces (sand, dust, and others).
The following factors affect the amount of abrasive wear and its intensity:
- nature of origin of abrasive particles;
- operating environment of mechanisms (degree of aggressiveness);
- material properties of friction pairs;
- shock loads;
- temperature indicators and many others.
Abrasive wear by solid particles (grains)
This type of mechanical wear occurs when abrasive grains come in contact with metal or other material. The hardness index of such particles significantly exceeds the value of the hardness index of the metal itself. This leads to deformation of the materials of friction pairs, the appearance of fatigue stresses, and surface abrasion.
If the mechanism operates under conditions of frequent alternating loads, the effect of the harmful effects of the abrasive is enhanced. In this case, the abrasive particle leaves not only risks, but also dents on the metal surface.
With an increase in the abrasive fraction, abrasive wear also increases. Particles of abrasives are very hard, but at the same time brittle. Therefore, large bodies can grind into smaller ones.
Features of oxidative wear
This type of wear occurs when a loose oxide film appears on the surface of the rubbing parts, which is quickly removed from the surface as a result of friction. Most engineering materials are prone to oxidation in air at elevated temperatures. Therefore, mechanisms that operate without lubrication and without a cooling system are subject to this type of wear of parts.
The higher the rate of destruction of the oxide film and the higher the rate of its formation, the more intense the wear of surfaces.
This type of wear is characteristic of articulated and bolted joints, various suspension mechanisms, and indeed for all nodes operating without lubrication.
With an increase in the friction speed, the temperature of the rubbing surfaces increases. This leads to the intensification of destructive processes. A similar effect is exerted by an increase in shock loads.
Wear under the influence of plastic deformation
This type of wear of machine parts is characteristic of highly loaded units. Its essence is to change the geometric shapes of the product under the action of significant loads.
It is most characteristic of key and splined joints, as well as threads, pins and so on.
Similar deformations can occur in gear joints. Moreover, they do not have to be fast. The key factor here is the load.
Often, such deformations appear on railway rails and rolling stock wheels. To prevent the need for timely prevention and examination of structural elements.
Chipping wear
The presented classification of types of wear will not be complete if we neglect the so-called wear resulting from chipping. Its essence is as follows. Under severe (possibly even extreme) operating conditions, the surface layers of rubbing parts undergo structural and phase transformations. The reasons in different cases are elevated temperatures, heating and cooling conditions, high pressure and others. The properties of the obtained layers are significantly different from the properties of the starting material. As a rule, these phases are fragile and under load are destroyed.
So, on steel and cast iron in the process of friction without lubrication characteristic white stripes are formed. These areas cannot be etched even with a solution of nitric or hydrofluoric acid in alcohol. Specialists in the field of metallurgy call this formation a white layer. It has a fairly high Rockwell hardness and is very fragile. One laboratory performed phase and structural analysis of the white layer. It turned out that it is a mechanical mixture of martensite and cementite. It also contains a small amount of ferrite. The latter is very small in it and it cannot reduce hardness.
The formation (synthesis) of this substance is accompanied by the occurrence of harmful internal tensile and compressive forces. When the vectors of internal stresses coincide with the external loads on the part, minor cracks form on its surface in the region of the white layer. These microcracks are concentrators and stress accumulators, which leads to brittle destruction of the product as a whole.
Fetting Corrosion Wear
This process occurs on surfaces that are in close contact with each other. The reason is the fluctuation. It should be noted that the materials of the bodies of the friction pair can be very different (metal-metal or non-metal-metal).
This phenomenon occurs even with minimal movements of bodies (of the order of 0.025 micrometers).
As a result of vibrations on the surfaces there are foci of corrosion that grow and lead to the destruction of the surface layer.
Wear through vibration cavitation
This type of wear occurs during the operation of products in a liquid medium. Although it can also occur when a part of a machine or a mechanism of a jet of liquid enters. The physics of the process is as follows. The fluid pressure at the phase interface (between the fluid and the solid) drops, which leads to the appearance of the so-called cavitation bubbles. The intensity of such wear depends on the air content in the liquid and on external pressure.
The catalyst may be sound vibration. Especially harmful in this case are fluctuations in the ultrasonic spectrum. Very often, such a harmful phenomenon occurs in the rubbing parts of internal combustion engines. Research results indicate that wear as a result of sound cavitation occurs three, or even four times faster than with friction.
Thermal cracking wear
This problem is typical for the wheels of railway cars and locomotives. During the movement of the train, the driver often has to slow down. This causes the wheels to slip and heat up. When speeding up, the rubbing surface cools quite quickly. Such thermal cycling leads to the formation of many cracks on the surface of the wheel. This significantly accelerates the wear of the product. Currently, special alloy steels are used for the production of railway wheels. But before they used steel of ordinary quality. Old wheels are used today on many trains, so this problem is still relevant.
Methods of dealing with thermal cracks
The most effective measure to combat thermal cracks will be to provide intensive cooling. Special oils and greases can be used for this. In the case of railway wheels, this measure, for obvious reasons, is not suitable. In this case, you can play on the chemical composition of the material and choose a steel grade that is more favorable from this point of view. Certain grades of alloy steels have a low coefficient of expansion. And this property can be advantageously used.
Some features of erosive wear
Considering the types of friction and wear, one should not lose sight of the so-called erosion wear. In simple terms, this is the destruction of surfaces under the influence of the environment.
In engineering, this concept refers to the destruction of the surfaces of machine parts and machine components under the influence of environmental factors. Such influence factors include air and liquid flows, steam or various gases. The cause of wear is, as before, friction. Only in this case, the surface is affected not by abrasive particles, but by gas or liquid molecules.
During this process, microcracks occur. Molecules of liquid and vapor under high pressure penetrate into them and contribute to the destruction of all surface layers of products.
The liquid or vapor may also contain abrasive particles in suspension. In this case, such a mixture will cause abrasive erosion and wear.
Fatigue wear and its characteristics
Types of wear and geometry violations are very diverse. A lot of problems for structural engineers and mechanical engineers are caused by fatigue chipping of surfaces of parts. This "ailment" is very insidious. The phenomenon of fatigue chipping occurs in parts that operate for a long time under alternating loads. This is a characteristic “disease” of gear joints.
This type of wear is accompanied by the initiation of cracks on the surface and their penetration deep into the product. On a small surface area there is a whole network of such microcracks. Under the influence of pressure and temperature, small scattered pieces of metal exfoliate from the main body and fall off. An important role in this process is played by lubricant (oil), which penetrates microcracks and contributes to the destruction.