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6.3 Erosion wear
Erosion wear occurs when a liquid or gas flow acts on the material surface.
Depending on the type of influencing media, substances distinguish between hydroerosive, gas-erosive, electroerosive types of wear, as well as cavitation and ablation.
A characteristic feature of the manifestation of erosive wear is the formation of irregularities and coarsening of the surface as a result of the removal of local microvolumes of the material under the influence of a medium flow.
The intensity of erosive wear depends on the flow rate, angle of attack, mechanical properties and concentration of the acting particles, aggressiveness of the medium, physical-mechanical and chemical characteristics of the surface and near-surface layers of the material.
Hydroerosive wear is wear resulting from the action of a fluid flow on a solid body. The destruction of a solid body is caused by friction and impacts of a continuous flow on the surface. Friction causes loosening and washing out of microvolumes of the material. The wear rate is low in this case. The dynamic (impact) impact of a flow or jet is capable, depending on the properties of the material, of pulling out volumes of material or groups of grains with an unfavorable orientation in relation to the acting forces. In ductile materials with the ability to harden, microplastic deformations of individual sections first accumulate, and when their ability to harden is exhausted, they are destroyed and removed. The liquid penetrating into the microcracks formed during impacts pushes the side walls apart like a wedge.
Hydroerosive wear is prone to sharp edges of spools of hydraulic units, valves of shut-off and control devices of hydraulic systems, etc.
cavitation wear . When a solid body is exposed to a liquid flow containing gas bubbles that collapse near the surface, hydroerosive wear turns into cavitation wear.
This type of wear occurs in a stream moving at high speed, with its narrowing and the presence of obstacles to its movement. As a result, the pressure in the stream may drop to a pressure corresponding to the vaporization pressure at a given temperature. There is a break in the flow and a violation of its continuity. The resulting void is filled with steam and gases released from the liquid. Bubbles with sizes on the order of tenths of a millimeter move along with the flow and enter high pressure zones. Water vapor condenses, gases dissolve, and liquid particles rush into the formed microvoids with acceleration. There is a restoration of the continuity of the flow, accompanied by a blow. Shock waves, acting on the surface, lead to the separation of microvolumes of the material. Cavitation wear is typical for machine parts operating in liquid media. Microdefects, called cavities , form on the friction surface . The frequency of formation of caverns H is characterized by the Strouhal criterion:
, (6.3)
where H is the number of caverns that occur per second; d - characteristic body size; V is the fluid flow rate.
During cavitation wear, an increase in surface roughness occurs, similar to chemical corrosion.
Propellers, blades of centrifugal and propeller pumps, blades of hydraulic turbines are subject to this type of wear.
It is possible to reduce cavitation wear by constructive solutions, designing the hydromechanical system in such a way that the pressure at all points of the flow does not fall below the vaporization pressure. Another way to combat cavitation wear is to introduce substances into the water that form or promote the formation of emulsions that lower the surface tension of the liquid medium. Resistance to cavitation wear is also increased by alloying steels with nickel, chromium or by using special processing methods: hardening with high-frequency heating, carburizing, surfacing with hard materials.
Gas erosion wear is a type of erosion wear as a result of the impact of a gas flow on the surface of a solid body. The mechanisms of gas-erosion and hydro-erosion wear are similar.
Gas erosion wear, for example, occurs on the piston rings of aircraft engines as a result of gas breakthrough. In this case, the least strong particles of the alloy, as a result of local heating of the working surface of the ring, are torn off and carried away by the gas flow. The stronger structural components of the alloy, being isolated, are also destroyed in the future. Characteristic oblong shells with branches are formed on the friction surface.
ablation wear is observed , during which chemical transformations and destruction of the surface layers of the product material occur. External elements of space objects are subject to ablative wear when they pass through dense layers of the atmosphere, heads of plasma torches, nozzles of jet engines, etc.
Electroerosive wear occurs when electrical discharges act on friction surfaces. This type of wear is typical for electrical contacts, current collectors, collectors of electrical machines and is the result of several types of wear: mechanical and electrical.
Electrical wear depends on the mechanism of current flow through the contact and the material of the contact elements and is mainly due to the transfer of ions of the material of one element to another, the fritting of oxide films, leading to an increase in the molecular forces of adhesion between pure metals and subsequent microseizure and tearing; sparking and arcing, leading to the release of large thermal energy in the gap between the contacts and evaporation or splashing of metal into the contact gap with a sharp deterioration in the smoothness of the surface, which in turn increases the mechanical wear of the friction pair.
Mechanical wear in electrical contacts occurs by mechanisms similar to friction pairs operating with light loads. Reducing the magnitude of electroerosive wear of electrical contacts is achieved by using special lubricant compositions.