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 it 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.
Cavitation wear is a process of progressive degradation of a material due to repeated nucleation, growth, and violent collapse of cavities in a liquid flowing near the material. Cavitation fatigue is a specific type of damage mechanism caused by repeated vibration and movement due to contact with flowing liquids, with water being the most common fluid. Cavitation is, in many cases, an undesirable occurrence. In centrifugal pumps, cavitation causes damage to components (erosion of the material), vibrations, noise, and a loss of efficiency.
Perhaps the most important engineering problem caused by cavitation is the material damage that cavitation bubbles can cause when they collapse in the vicinity of a solid surface. Cavitation bubble collapse is a violent process that generates highly localized shock waves and microjets. They force energetic liquid into very small volumes, thereby creating spots of high temperature, and these intense disturbances generate highly localized, transient surface stresses to a solid surface. Signs of erosion will appear as pitting due to the water hammering action of the collapsing vapor bubbles. It has been found that cavitation damage rates increase rapidly with the increase in the volume flow rate.
Softer materials can be damaged even by the short-term occurrence of cavitation, and individual pits can be observed after a single bubble collapse. Therefore harder materials are used for centrifugal pumps. But with the harder materials used in most applications, the cyclic stress due to repeated collapses can cause local surface fatigue failure. Thus cavitation damage to metals usually has the appearance of fatigue failure.
When the cavitation bubbles collapse, they force energetic liquid into very small volumes, thereby creating spots of high temperature and emitting shock waves, the latter of which are a source of the noise. Although the collapse of a small cavity is a relatively low-energy event, highly localized collapses can erode metals, such as steel, over time. The pitting caused by the collapse of cavities produces great to wear on components and can dramatically shorten a propeller or pump’s lifetime.
Cavitation is usually accompanied also by:
- Noise. Typical noise is caused by collapsing cavities, and the level of noise that results from cavitation is a measure of the severity of the cavitation.
- Vibration. Pump vibrations due to cavitation are characteristically low-frequency vibrations, usually found in the 0 to 10 Hz range.
- Reduction in pump efficiency. Decreasing the pump’s efficiency is a more reliable sign of cavitation.
Prevention of Cavitation
Pits can vary from very small to very large, or even they can completely penetrate the thickness of metal. Damage to the structure can be catastrophic, and losses in functional efficiency can be substantial. Methods of dealing with this problem include:
- Increasing the hardness and strength of the metal. However, this may only delay the problem rather than prevent it.
- Increasing the stiffness of the part. This should reduce its amplitude of vibration, thereby increasing its natural vibration frequency. It may be possible to increase wall thickness or add stiffening ribs to change vibration characteristics.
- Increasing the smoothness of the surface. Cavities tend to cluster in certain low-pressure areas. The dispersing of the cavities may be possible to eliminate surface peaks and valleys.
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, abrasion, 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 manufacturing high-temperature applications and other refractories.
- Coated Alloys. Case hardening by surface treatment can be classified further as diffusion treatments 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. During this process, the alloying element concentration is increased at a steel component’s surface. 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.