The layer of oxides and impurities third body has a fundamental tribological importance, in fact it contributes to reducing friction. Another fact of fundamental importance regarding oxides is that if you could clean and smooth the surface in order to obtain a pure "metal surface ", what we would observe is the union of the two surfaces in contact In fact, in the absence of thin layers of contaminants, the atoms of the metal in question, are not able to distinguish one body from another, thus going to form a single body if put in contact.
The contact between surfaces, in reality, is a contact between roughness and the origin of the phenomenon of friction, and therefore of the dissipation of energy, is due precisely to the deformations that such bumps undergo due to the load and of relative movement. Plastic, elastic or rupture deformations can be observed:. The energy that is dissipated during the phenomenon is transformed into heat, thus increasing the temperature of the surfaces in contact. The increase in temperature also depends on the relative speed and the roughness of the material, it can be so high as to even lead to the fusion of the materials involved.
The involvement in temperature friction phenomena is fundamental in many aspects of application, an example can be found in the case of brakes. If the temperature rises too much the risk is that of an excessive reduction of the friction coefficient and consequently a sharp reduction in the effectiveness of the brakes. If at this point the two surfaces are sliding between them, a resistance to shear stress t is observed, given by the presence of adhesive bonds, which were created precisely because of the plastic deformations, and therefore the frictional force will be given by.
At this point, since the coefficient of friction is the ratio between the intensity of the frictional force and that of the applied load, it is possible to state that. In the case of lubricants, in fact, we use a substrate of material with low cutting stress t , placed on a very hard material. It should be noted that the force acting between two solids in contact will not only have normal components, as implied so far, but will also have tangential components.
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This further complicates the description of the interactions between roughness, because due to this tangential component plastic deformation comes with a lower load than when ignoring this component. A more realistic description then of the area of each single junction that is created is given by. To obtain even more realistic considerations, the phenomenon of the third body should also be considered, ie the presence of foreign materials, such as moisture, oxides or lubricants, between the two solids in contact.
A coefficient c is then introduced which is able to correlate the shear strength t of the pure "material" and that of the third body t t. Keeping in mind what has just been said, it is possible to correct the friction coefficient formula as follows:. Also for the elastic bodies the tangential force depends on the coefficient c seen above, and it will be. The simplest and most immediate method for evaluating the friction coefficient of two surfaces is the use of an inclined plane on which a block of material is made to slide.
This allows us to state that the coefficient of friction can be calculated very easily, by means of the tangent of the angle in which the block begins to slip.
History of Tribology
In fact we have. Then from the inclined plane we moved on to more sophisticated systems, which allow us to consider all the possible environmental conditions in which the measurement is made, such as the cross-roller machine or the pin and disk machine.
Today there are digital machines such as the "Friction Tester " which allows, by means of a software support, to insert all the desired variables. Another widely used process is the ring compression test. A flat ring of the material to be studied is plastically deformed by means of a press, if the deformation is an expansion in both the inner and the inner circle, then there will be low or zero friction coefficients.
Otherwise for a deformation that expands only in the inner circle there will be increasing friction coefficients.
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To reduce friction between surfaces and keep wear under control, materials called lubricants are used . Unlike what you might think, these are not just oils or fats, but any fluid material that is characterized by viscosity, such as air and water. Of course, some lubricants are more suitable than others, depending on the type of use they are intended for: air and water, for example, are readily available, but the former can only be used under limited load and speed conditions, while the second can contribute to the wear of materials.
What we try to achieve by means of these materials is a perfect fluid lubrication, or a lubrication such that it is possible to avoid direct contact between the surfaces in question, inserting a lubricant film between them. To do this there are two possibilities, depending on the type of application, the costs to address and the level of "perfection" of the lubrication desired to be achieved, there is a choice between:. The viscosity is the equivalent of friction in fluids, it describes, in fact, the ability of fluids to resist the forces that cause a change in shape.
Thanks to Newton's studies, a deeper understanding of the phenomenon has been achieved. He, in fact, introduced the concept of laminar flow : "a flow in which the velocity changes from layer to layer". From what has been said, it is possible to state that the force F , necessary to cause a rolling motion in a fluid contained between two plates, is proportional to the area of the two surfaces and to the speed gradient:. To conclude we want to underline that it is possible to divide the fluids into two types according to their viscosity:.
Modeling and Analytical Methods in Tribology
Temperature and pressure are two fundamental factors to evaluate when choosing a lubricant instead of another. Consider the effects of temperature initially. There are three main causes of temperature variation that can affect the behavior of the lubricant:. In order to classify the various lubricants according to their viscosity behavior as a function of temperature, in the viscosity index V. These assigned the best lubricant then available, namely the oil of Pennsylvania, the viscosity index , and at the worst, the American oil of the Gulf Coast, the value 0.
In the case of oils with V. We can therefore say, in conclusion, that an increase in temperature leads to a decrease in the viscosity of the oil. It is also useful to keep in mind that, in the same way, an increase in pressure implies an increase in viscosity. To evaluate the effects of pressure on viscosity, the following equation is used. To determine the viscosity of a fluid, viscosimeters are used which can be divided into 3 main categories:.
The first two types of viscometers are mainly used for Newtonian fluids, while the third is very versatile. The wear is the progressive involuntary removal of material from a surface in relative motion with another or with a fluid. We can distinguish two different types of wear: moderate wear and severe wear. The first case concerns low loads and smooth surfaces, while the second concerns significantly higher loads and compatible and rough surfaces, in which the wear processes are much more violent.
Wear plays a fundamental role in tribological studies, since it causes changes in the shape of the components used in the construction of machinery for example. These worn parts must be replaced and this entails both a problem of an economic nature, due to the cost of replacement, and a functional problem, since if these components are not replaced in time, more serious damage could occur to the machine in its complex. This phenomenon, however, has not only negative sides, indeed, it is often used to reduce the roughness of some materials, eliminating the asperities.
Erroneously we tend to imagine wear in a direct correlation with friction, in reality these two phenomena can not be easily connected. There may be conditions such that low friction can result in significant wear and vice versa. In order for this phenomenon to occur, certain implementation times "are required, which may change depending on some variables, such as load, speed, lubrication and environmental conditions, and there are different wear mechanisms, which may occur simultaneously or even combined with each other:. As known, the contact between two surfaces occurs through the interaction between asperities .
If a shearing force is applied in the contact area, it may be possible to detach a small part of the weaker material, due to its adhesion to the harder surface. What is described is precisely the mechanism of the adhesive wear represented in the figure. This type of wear is very problematic, since it involves high wear speeds, but at the same time it is possible to reduce adhesion by increasing surface roughness and hardness of the surfaces involved, or by inserting layers of contaminants such as oxygen, oxides, water or oils. In conclusion, the behavior of the adhesive wear volume can be described by means of three main laws.
An important aspect of wear is emission of wear particles into the environment which increasingly threatens human health and ecology. The first researcher who investigated this topic was Ernest Rabinowicz . The abrasive wear consists of the cutting effort of hard surfaces that act on softer surfaces and can be caused either by the roughness that as tips cut off the material against which they rub two-body abrasive wear , or from particles of hard material that interpose between two surfaces in relative motion three-body abrasive wear.
At application levels, the two-body wear is easily eliminated by means of an adequate surface finish, while the three-body wear can bring serious problems and must therefore be removed as much as possible by means of suitable filters, even before of a weighted machine design.
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