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Fig.5.11. Influence of bulk temperature on the coefficient of friction.


Lecture number 6.
GENERAL CLASSIFICATION OF THE WEAR PROCESS.
6.1 Classification of types of wear
The elementary act of wear is localized in a small volume of the surface layer of the material of the friction part. This volume is separated from the friction surface and enters the friction zone in the form of wear particles. The nature of the impact on the friction surface determines the wear mechanism. Therefore, the kinetic patterns of wear are determined by the combined influence of many mechanical, physical and chemical factors.
Depending on the type and nature of frictional interaction, the following types of wear are distinguished:
- abrasive;
- hydroabrasive;
- gas-abrasive;
-erosive;
- hydroerosive;
- gas erosion;
- cavitation;
- electroerosive;
- adhesive;
- fatigue;
-corrosion-mechanical;
-oxidative;
- fretting, fretting-corrosion.
The experience of studying the types of wear of friction units shows that under specific operating conditions, as a rule, several types of wear are simultaneously observed. However, it is always possible to single out the most characteristic type, which makes a prevailing contribution to the wear intensity of the tribocoupling as a whole.
6.2 Abrasion
Abrasive wear refers to the most intense types of mechanical destruction of the surface, resulting from the cutting or scratching action of solid particles on it, which are in a free or fixed state.
A necessary condition for the manifestation of abrasive wear is the lower hardness of the surface layer of the part compared to the abrasive. The most common abrasive is ordinary quartz sand, which is found in soils, soils and dust and is the main agent that causes wear of parts. The abrasive particles can be scale, ore, coal, structural components of one of the mating bodies, wear products.
The main sign of the manifestation of abrasive wear of the friction surface is the presence on it of well-distinguished scratches, small scratches and depressions of various lengths, oriented in the direction of motion. The formation of scratches and depressions on the friction surface has a dual origin. Risks and depressions are formed as a result of the separation of material in the form of micro-chips, when the material is sufficiently hard and low-plasticity, or it is a mark formed by the displacement of the material into the dumps, when micro-cutting does not occur due to its high plasticity. Displacement by an abrasive particle of material into dumps is the first stage of surface destruction. When subsequent abrasive particles move near the previously formed dumps on their side surfaces, repeated deformation and reorientation of the material of the dumps towards the risk or its separation from the wear surface occurs.
Numerous studies have established that the resistance of metals to abrasive wear of metals depends on the type of crystal lattice and the strength of interatomic bonds in the lattice. MM. Khrushchev, as applied to commercially pure metals and steels in the annealed state, obtained a dependence for estimating the amount of abrasive wear, V:
, (6.1)

where N - normal load; S - friction path; H is the hardness of the abraded material; a - the size of the abrasive grain; c is a coefficient depending on the abrasive properties of the abrasive surface, the conditions of interaction, the contacting bodies, the kinematics of movement and fastening of the abrasive, the sharpness of the edges of the abrasive.

Abrasive wear is observed not only for metals and alloys, but also for non-metallic materials, such as carbon graphite, ceramic, polymer and composite. Polymeric materials, whose cohesive strength is significantly less than most of the tribological materials in contact with them, are especially subject to abrasive wear. Polymeric materials, like metals and alloys, tend to increase the wear intensity with an increase in the size of abrasive particles.
Parts of agricultural, road-building, mining, transport machines and transport devices, components of metallurgical and crushing and grinding equipment, metal-cutting machines, etc. are most exposed to abrasive wear.
In real operating conditions of equipment with abrasive wear, various schemes of abrasive action are implemented. Depending on the type of friction, there are:

• 
  abrasive wear during sliding friction;
  
• 
  rolling friction;
  
• 
  impact of material with abrasive.
  

The interaction of the material with the abrasive during the movement of parts and tools in the flow of abrasive particles carried by air or liquid stands out in particular.
Hydroabrasive and gas-abrasive wear occur as a result of the action of moving solid particles entrained in a gas or liquid flow. Wear in these cases depends on the particle impact velocity and the angle of inclination of the velocity vector to the surface of the wear part - the angle of attack . The amount of wear is also affected by the concentration of abrasive particles in the flow, their shape, the mechanical characteristics of the particles and wear materials, as well as the temperature of the medium and its acidity.
The wear intensity J _G for these types of abrasive wear is related to the particle velocity v by the dependence:
, (6.2)

where K is a coefficient depending on the angle of attack and the properties of the wear and abrasive material; m - exponent, depending on the type of wear material. At a speed of <100 m/s, the value of m for steel St3 is 2.3; for hardened steel 45-2.5; for white cast iron - 2.8; for basalt - 2.9.

The dependence of wear resistance on the angle of attack in most cases is expressed by a curve with a minimum corresponding to the critical angle of attack. The angle of attack for brittle non-metallic materials is close to 90 ⁰ , and for metal alloys its value decreases as their plasticity increases and is 30 ⁰ -40 ^{0 for mild steel} , 50 ⁰ -70 ^{0 for hardened steel} .
Hydroabrasive and gas-abrasive wear is observed in the impellers of hydraulic turbines, gas turbine blades, pipes of water economizers and steam boilers, blades of smoke exhausters, pipes and pumps of dredgers, bearings of propeller shafts of ships, etc.