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The high-alloy carbon steel used in mining tools of Wear mechanisms
Archard’s equation describes the easiest forms of wear, see Equation 1. The theory of Archard says that the volume loss will be a linear function of rubbing distance at constant wear speed and the volumetric wear rate will be proportional to the applied load.
Presence of water:
The presence of water in a tribology system contributes to both a corrosive effect and a lubricating effect. It may also act like a coolant or a means of flushing wear debris away from the system. The lubricating effect depends on the wear mechanism. For abrasion with small particles, water lubricates effectively, but as the particle size increases, the lubrication becomes less effective. The presence of water often reduces the severity of damage caused by the abrasive particles resulting in a less disrupted surface. The lubricating effect increases with smaller and/or more round abrasives [9]. Studies have shown that the corrosion process between metal and fluid generally is slower than the wear process. Thus, the speed of material removal by wear is faster than allowing the material to corrode.
Adhesion wear:
Adhesion wear occurs when two solid surfaces are in sliding or in rolling contact with each other. At local spots, so-called asperities, atomic contact between the surfaces are formed.
Because of sliding movements, shear deformation between the contact spots will appear forming a plastic zone on the surface. Continued sliding will cause shear failure. Adhesion wear is schematically presented in Figure 5.
Adhesion wear is characterized by its flaky surface forming when the surface of a specimen is sliding against another surface and starts to wear, seen in Figure 6.
Sliding abrasion：
Abrasive wear is when particles or sharp irregularities on the surface scratches a softer surface and makes a volume loss or plastic deformation of the specimen. Two-body and three-body abrasion wear are two different forms of sliding abrasion. In two-body abrasion no wear-debris is present and the mating surfaces wear by direct contact. In three-body abrasion, wear-debris between the mating surfaces play an important role, forming a number of rolling contacts between the two mating surfaces. This is schematically presented in Figure 7.
Abrasion wear causes different scratching mechanisms on the material; micro-ploughing, microcracking, micro-fatigue or micro-cutting. Which mechanism that affects the material most depends on how the material is being exposed to abrasive wear. In a real situation it is often a combination of different active mechanisms that wears the material. Generally, the combination is very specific for each application.
Micro-cutting is the basic mechanism of abrasive wear. The micro-cutting mechanism changes the material either by plastic deformation or by removal of material. Micro-ploughing, microcracking and micro-fatigue are different forms of micro-cutting. Micro-ploughing is when the abrading medium cuts in the specimen and there are no volume loss, only plastic deformation. Micro-cracking is a micro-cutting mechanism that removes material from a brittle specimen.
Micro-fatigue is when the abrading medium?s continuous sliding on the specimen and occasionally causes deformation. The three different mechanisms of micro-cutting abrasion are presented in Figure 8.
At lower loads micro-ploughing-micro-cutting is the dominating damage mechanism, but with increased load micro-cracking-micro-cutting is the damage mechanism. Micro-cutting is more common among ductile materials, whilst cracking is more common among brittle materials.
Impact abrasion: Particle erosion：
When a surface is being hit by solid particles, affecting the surface by plastic deformation or removal of material it is called particle erosion, see Figure 9. As Hutchings defined erosion: “An abrasive wear process in which the repeated impact of small particles entrained in a moving fluid against a surface result in the removal of material from that surface”.
The erosion rate is highly dependent on the properties of the particles and the target, as well as the impact angle and impact velocity. [19]The erosion rate increases with increased impact angle and impact velocity. The mechanism of erosion depends on the rate of hardness on target to particle. As for two- and three-body wear, a ductile target results in micro-cutting-microploughing and a brittle target results in micro-cutting-micro-cracking.
Metal Corrosion：
Corrosion is a chemical reaction between a metal and the surrounding environment. All metals are susceptible to corrosion. The speed of which a metal corrodes depends on the alloy and environmental factors. The driving force for a metal to corrode is reaching the most stable phase. Corrosion can be described in a corrosion cell, where metal and hydrogen based fluid interacts. The metal will react with the fluid and emit electrons to the fluid. The hydrogen in the fluid will bond with the electrons; this is called the cathode reaction. The metal ion will react with the oxygen and oxidized and new material is formed on the surface, with other mechanical properties than the original one as shown in Figure 10.
The corrosion rate depends on the pH-value. For example, in Figure 11, a Pourbaix diagram is presented. From the Pourbaix diagram the most stable phase according to pH-value and potential can be read.
2019-07-12 10:10
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