In general, wear is mechanically induced surface damage that results in the progressive removal of material due to relative motion between that surface and a contacting substance or substances. A contacting substance may consist of another surface, a fluid, or hard, abrasive particles contained in some form of fluid or suspension, such as a lubricant. As is with friction, the presence of wear can be either good or bad. Productive, controlled wear can be found in processes like machining, cutting, grinding, and polishing. However, in most technological applications, the occurrence of wear is highly undesirable and is an enormously expensive problem since it leads to the deterioration or failure of components. In terms of safety, it is often not as serious (or as sudden) as a fracture, and this is because the wear is usually anticipated.
Certain material characteristics such as hardness, carbide type, and volume percent can have a decided impact on the wear resistance of a material in a given application. Wear, like corrosion, has multiple types and subtypes that are predictable to some extent and are rather difficult to test and evaluate in the lab or service reliably.
Erosion wear is a process of progressive removal of material from a target surface due to repeated impacts of solid particles. The particles suspended in the flow of solid-liquid mixture erode the wetted passes limiting the service life of the equipment used for the slurry transportation system. Each particle cuts or fractures a tiny amount of material (known as wear chips) from the surface. If this is repeated over a long period, a significant amount of material loss can result.
Erosive wear is common in pumps, impellers, fans, steam lines, and nozzles inside sharp bends in tubes and pipes. Therefore, it is a widely encountered mechanism in industry and power engineering. Due to the nature of the conveying process, piping systems are prone to wear when abrasive particles have to be transported.
Erosion wear is caused by the kinetic energy transferred to the target surface by impinging solid particles. The rate of erosive wear is dependent upon many factors. The material characteristics of the particles, such as their shape, hardness, impact velocity, and impingement angle, are primary factors, along with the properties of the surface being eroded. A material loss of target material is higher for the higher kinetic energy of an impinging particle. So impact velocity largely affects the erosion wear of target material. The impingement angle is one of the most important factors and is widely recognized in the literature. Sharp curves or bends tend to produce more erosion than gentle curves.
Erosion wear can be classified into three categories:
- Solid particle erosion. Solid particle erosion is the loss of material volume from a target material due to continuous impingement of solid particles in the flowing fluid.
- Liquid impact erosion. The continuous striking of liquid jet on the material surface causes liquid impact erosion.
- Cavitations erosion. The vapor or gas in a liquid forms cavities or bubbles that cause wear.
In general, erosive wear resistance may be improved by increasing the hardness of the surface, by proper materials, and by proper design of the product. Some specific steps that can be taken to change flow conditions include: reducing fluid velocity, eliminating turbulence at misalignments, and avoiding sharp bends.
Erosion – Corrosion
Erosion can also occur in combination with other forms of degradation, such as corrosion. This is referred to as erosion-corrosion. Erosion corrosion is a material degradation process due to the combined effect of corrosion and wear. Nearly all flowing or turbulent corrosive media can cause erosion corrosion. The mechanism can be described as follows:
- mechanical erosion of the material, or protective (or passive) oxide layer on its surface,
- enhanced corrosion of the material if the corrosion rate of the material depends on the thickness of the oxide layer.
Wear is a mechanical material degradation process occurring on rubbing or impacting surfaces, while corrosion involves chemical or electrochemical reactions of the material. Corrosion may accelerate wear, and wear may accelerate corrosion.
Surface Hardness and Wear Resistance
Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness. If the hardness of the material is higher than that of the abrasive material, less wear rate will occur.
Case hardening or surface hardening is the process in which the hardness of an object’s surface (case) is enhanced while the inner core of the object remains elastic and tough. After this process enhances surface hardness, wear resistance, and fatigue life. This is accomplished by several processes, such as a carburizing or nitriding process by which a component is exposed to a carbonaceous or nitrogenous atmosphere at elevated temperatures. As was written, two main material characteristics are influenced:
- Hardness and wear resistance is significantly enhanced. In materials science, hardness is the ability to withstand surface indentation (localized plastic deformation) and scratching. Hardness is probably the most poorly defined material property because it may indicate resistance to scratching, resistance to abrasion, resistance to an indentation, or even resistance to shaping or localized plastic deformation. Hardness is important from an engineering standpoint because resistance to wear by either friction or erosion by steam, oil, and water generally increases with hardness.
- Toughness is not negatively influenced, and toughness is the ability of a material to absorb energy and plastically deform without fracturing. One definition of toughness (for high-strain rate, fracture toughness) is that it is a property that is indicative of a material’s resistance to fracture when a crack (or other stress-concentrating defects) is present.
The case-hardening process involves infusing additional carbon or nitrogen into the surface layer for iron or steel with low carbon content, which has poor to no hardenability. Case hardening is useful in parts such as a cam or ring gear that must have a very hard surface to resist wear and a tough interior to resist the impact that occurs during operation. Further, the surface hardening of steel has an advantage over hardening (that is, hardening the metal uniformly throughout the piece) because less expensive low-carbon and medium-carbon steels can be surface hardened without the problems of distortion and cracking associated with the through hardening of thick sections. An atomic diffusion from the gaseous phase introduces a carbon- or nitrogen-rich outer surface layer (or case). The case is normally 1 mm deep and harder than the material’s inner core.
Typical Wear-resistant Materials
In general, wear is mechanically induced surface damage that results in the progressive removal of material due to relative motion between that surface and a contacting substance or substances. Therefore, there is perfect wear-resistant material; in every case, it depends strongly on many variables (e.g., materials combination, contact pressure, environment, temperature). The hardness of the material is correlated to the wear resistance of the material. If the hardness of the material is less than that of the hardness of the abrasive material, then the wear rate is high. The hardness of material plays a major role in wear resistance. Some materials exhibit special wear characteristics:
- Ni3Al – Alloy. Nickel aluminide is an intermetallic alloy of nickel and aluminium with properties similar to ceramic and metal. Nickel aluminide is unique because it has very high thermal conductivity combined with high strength at high temperatures. These properties, combined with its high strength and low density, make it ideal for special applications like coating blades in gas turbines and jet engines. Composite materials with Ni3Al-based alloys as a matrix hardened by, e.g., TiC, ZrO2, WC, SiC, and graphene, are advanced materials. In 2005, the most abrasion-resistant material was reportedly created by embedding diamonds in a matrix of nickel aluminide.
- Tungsten Carbide. Impact wear is of the highest importance in mining and mineral processing. Mining and mineral processing demand wear-resistant machines and components because the energies and masses of interacting bodies are significant. For this purpose, materials with the highest wear resistance must be used. For example, tungsten carbide is used extensively in mining in top hammer rock drill bits, downhole hammers, roller-cutters, long wall plow chisels, long wall shearer picks, raise-boring reamers, and tunnel boring machines.
- Silicon Carbide. Silicon carbide is an exceedingly hard, synthetically produced crystalline compound of silicon and carbon, and its chemical formula is SiC. Silicon carbide has a Mohs hardness rating of 9, approaching that of a diamond. In addition to hardness, silicon carbide crystals have fracture characteristics that make them extremely useful in grinding wheels. Its high thermal conductivity, high-temperature strength, low thermal expansion, and resistance to a chemical reaction make silicon carbide valuable in the manufacture of high-temperature applications and other refractories.
- Coated Alloys. Case hardening by surface treatment can be further classified as diffusion or localized heating treatments. Diffusion methods introduce alloying elements that enter the surface by diffusion as solid-solution or hardenability agents that assist martensite formation during subsequent quenching. In this process, the concentration of alloying element is increased at the surface of a steel component. Diffusion methods include:
- Carburizing is a case hardening process in which the surface carbon concentration of a ferrous alloy (usually low-carbon steel) is increased by diffusion from the surrounding environment. Carburizing produces hard, highly wear-resistant surface (medium case depths) of product with an excellent capacity for contact load, good bending fatigue strength, and good resistance to seizure.
- Nitriding is a case hardening process in which the surface nitrogen concentration of a ferrous is increased by diffusion from the surrounding environment to create a case-hardened surface. Nitriding produces hard, highly wear-resistant surface (shallow case depths) of product with a fair capacity for contact load, good bending fatigue strength, and excellent resistance to seizure.
- Boriding, also called boronizing, is a thermochemical diffusion process similar to nitrocarburizing in which boron atoms diffuse into the substrate to produce hard and wear-resistant surface layers. The process requires a high treatment temperature (1073-1323 K) and long duration (1-12 h) and can be applied to a wide range of materials such as steels, cast iron, cermets, and non-ferrous alloys.
- Titanium-carbon and Titanium-nitride Hardening. Titanium nitride (an extremely hard ceramic material) or titanium carbide coatings can be used in the tools made of this kind of steel through a physical vapor deposition process to improve the performance and life span of the tool. TiN has a Vickers hardness of 1800–2100 and a metallic gold color.
- Case Hardened Steels. Case hardening based on martensitic transformation is usually performed to enhance the wear resistance of steels. Martensitic transformation hardening is one of the most common hardening methods, primarily used for steels (i.e., carbon steels and stainless steels).
- Flame hardening. Flame hardening is a surface hardening technique that uses a single torch with a specially designed head to heat the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough. It is a similar technique to induction hardening. The carbon content of 0.3–0.6 wt% C is needed for this type of hardening.
- Induction hardening. Induction hardening is a surface hardening technique that uses induction coils to provide a very rapid means of heating the metal, which is then cooled rapidly, generally using water. This creates a “case” of martensite on the surface. The carbon content of 0.3–0.6 wt% C is needed for this type of hardening.
- Laser hardening. Laser hardening is a surface hardening technique that uses a laser beam to provide a very rapid means of heating the metal, which is then cooled rapidly (generally by self-quenching). This creates a “case” of martensite on the surface, while the inner core of the object remains elastic and tough.
Some common materials:
Ductile cast iron. Ductile iron, also known as nodular iron or spheroidal graphite iron, is very similar to gray iron in composition, but during solidification, the graphite nucleates as spherical particles (nodules) in ductile iron rather than as flakes. Typical applications for this material include valves, pump bodies, crankshafts, gears, and other automotive and machine components because of its good machinability, fatigue strength, and higher modulus of elasticity (compared to gray iron), and in heavy-duty gears because of its high yield strength and wear resistance.
- Aluminium Bronze. The aluminium bronzes are a family of copper-based alloys that combine mechanical and chemical properties unmatched by any other alloy series. They contain about 5 to 12% of aluminium. Aluminium bronze has been recognized for various applications requiring resistance to mechanical wear. Its wear resistance is based on the transfer from the softer metal (aluminium bronze) to the harder metal (steel), forming a thin layer of softer metal on the harder metal.