Adhesive Wear Equation and Hardening Processes - Assignment Example

Archard’s Adhesive Wear Equation and Hardening Processes Wear can be defined as damage of a surface that occurs when two bodies in relative motion come in contact causing materials to be detached from one or both bodies. Interactions that produces wear can be both physical and chemical (Watson et al 76). The model of adhesive wear advanced by Archard can be illustrated by the figures below. http://www.sciencedirect.com/science/article/pii/S0257897207008201 The equation of Archard’s adhesive wear is based on some assumptions that are related to the area of contacts and the materials of bodies in contact. The bodies in contact cause impact on each other according to texture, softness and hardness of materials making them. The distance over which the two bodies are in contact determines the amount of wear that results. Also, damage of surfaces in contact varies with variance in pressure and relative speed. Assumptions made are: Contact of the bodies involved happen only at face tips Higher impact is experienced by the body made of softer material as compared to the body made of a harder material and that such impact can cause plastic deformation to the softer body Load is equal to the product of area and coefficient of hardness of the softer body. N=A.H It is assumed that contact area is given by πa2 and, therefore, supports load at the area given is equal to πa2 H In distance 2a fragment of volume 2πa3/3 is formed. Deriving equation for the volume of wear per unit slid distance Wear volume, which is denoted by δQ produced in unit distance is given by: δQ = (2πa3/3)/2a = πa2/3 Total load at the point of contact is given by nπa2 H So Q = N/3H According to the equation, laws below can be stated: 1. Volume worn out varies proportionally with the distance covered by the two bodies in relative motion and lying on each other. 2. Wear volume varies directly according to the total load exerted at the surface of contact by the two bodies in contact. 3. Change in wear volume is realised when there is a change in the area of the bodies in contact (Watson et al 74-76). Inconsistencies are witnessed when the above equation is applied to calculate amount of material of the surface lost in the process of contact between two bodies lying on each other and in relative motion. For this reason, it gives unreliable result because according to conclusions made using the equation, many factors that affect wearing are not put into consideration. Laws stating that: 1. Volume worn out varies proportionally with the distance covered by the two bodies in relative motion and lying on each other; the law is not all inclusive of factors for wearing. It is true but only holds in selected circumstances. This is because the material of the bodies will determine how much wearing will take place. When a very heavy body is placed on a softer body, wearing is possible even without a movement. As the two bodies wear each other, they have a possibility of causing smooth services which are not as destructive as would be in the initial motion. This will means that constants that are not supposed to change in the equation, change. 2. Wear volume varies directly according to the total load exerted at the surface of contact by the two bodies, this law is applicable for lighter loads, it does not hold for very heavy loads for they cause massive destruction to each other when they are in contact and moving in relative direction. Eminent damage is experienced on the soft body. This is especially if a body made of softer material is lighter than the other body made of harder substance and both are in contact. In this case, damage is not realised on one body but total destruction is witnessed on the second body. A case like this, with un-matched loads differences between the two bodies in contact, is known as critical loads. In a case like this, pressure exerted can be denoted by the formula ~ H/3 indicating that other factors like texture plays a part in wearing. 3. Change in wear volume is realised when there is a change in the area of the bodies in contact, this law is possibly verifiable using the equation above, and wearing is influenced by roughness and smoothness of those areas in contact. Pressure rise at the face of contact steps up wear rate. Surfaces hardening I. Maintaining original chemical composition 1. Flame hardening This method entails heating and immediate cooling. In this process, a very hot flame is conveyed to the surface of steel component causing its quick heating. The water used for cooling the steel is conveyed through the same path of acetylene or propane. Cooling follows heating in very quick succession. Due to the length of heating, heat conduction is varied between the centre core and the surface. This explains why the core is never hardened. At the end of the process, the core has the same chemical composition as it was in the beginning while the surface has a column of carbon. The process does not change the composition of the steel materials that make these devices. The steel component is hardened to reduce the rate of wearing when the metal bars are in contact in a relative motion. In the process, the surface of the steel metal component is made resistant to the impact of another metal which would acts upon it. Wearing is reduced by a high degree in order to make components more durable. Materials that have been hardened using this method have the same chemical properties as the original metal. http://www.ibeda.de/en_US/flame_hardening.php 2. Induction hardening In induction process, a component to be hardened is fixed in one position to avoid any movement. Heat for required is produced by an electromagnetic induction coils. The component is surrounded by an inductor block through which a high-frequency current of about 2000 Hz, passes. This raises the temperature of the surface layer to above its upper critical in a few seconds. The surface is then quenched by pressure jets of water which pass through holes made in the inductor block. The carbon atoms in the carbon dioxide in the atmosphere are used. In induction hardening, the material is hardened using high voltage power which is used to produce magnetic flux which consequently produces heat energy required in the process. A metal hardened by induction has its services coated with carbon material from the environment but the core of the component is never chemically changed. http://quality-on.blogspot.co.uk/2011/02/work-coil-design-used-in-induction.html II. Changing chemical composition 1. Case hardening In this process an additional carbon is introduced into the surface of steel to make its surface hard and hence resistant to wear. First, a component is mixed with carbon compound and is heated. Heating process takes a long time because it targets to raise the temperature to very high degrees. This is because breaking the compound of carbon into its constituent elements and to have them react with steel material requires super heat. At the end, a deep coat, rich in carbon, is formed on the surface of the steel component. During this process, carbon compound in any state of matter can be used. The nature of the component determines the choice of state of matter of the carbon compound used. Case hardening is double-step: 1. Carburizing It involves baking steel components which has traces of carbon in a carbon compounds aimed at increasing the level of carbon content in such a component. This is because the more carbon there is in a steel component the harder to wear it is. The carbon is obtained from a hardening compound that is super heated to cause it to react with the steel material. When reaction has taken place, however, the core of the steel metal does not react with carbon compound and therefore does not harden. 2. Heat treatment is the second method that involves hardening the metal used as well as refining its core. This heat used targeting the core of the steel metallic component. At the end, a metal whose core is softer than the surface but very tough is produced. This process changes the chemical composition of the material used because carbon atoms are added to the steel. The figures above indicates the nature of steel component during addition of carbon, after addition and its nature after quenching. 2. Nitriding In nitriding, steel hardening is achieved through addition of nitrogen atoms in a component. A steel alloy supper heated in presence of a nitride or nitrogen oxides and ammonia gas. The alloy is made of little traces of other metals, for example, molybdenum and steel. A hard surface of nitride is formed at the end of the reaction. The process involves heating of the components in ammonia gas at a temperature between 500 and 600 °C for over forty hours. At this temperature the ammonia gas breaks down and the atomic nitrogen is readily absorbed into the surface of the steel. The process is done only to manufactured devices. No subsequent grinding is possible since the hardened case is only a few micrometers thick. However, this is processes does not affect the surface of the metallic material treated. Due to the temperature ranges used, the material is left intact without distortion since the heat is not enough to change the state of the alloy. Work Cited Watson, Matthew; Byington, Carl; Edwards, Douglas and Amin, Sanket; Dynamic Modeling and Wear-Based Remaining Useful Life Prediction of High Power Clutch Systems, 2005, Web. Dec. 20th 2012 Read More