DIN Wear Classification

Figure 35: Tribology System Characterization Parameters.

Table 8: Types of Wear present in different tribological systems [28].

1.17.1 Abrasive wear

If one of the system bodies is harder and rough enough or if hard abrasive particles are detached from one of the materials, this type of wear is the one that dominates in the tribological system. The abrasive wear happens when the hard abrasive particles present in one body or cloister into one of the bodies, start cutting or sliding the other body particles

that is removed from the softer body doesn’t fall but displaces and ploughs leaving furrows.

This relationship between wear resistance and hardness of the particles can be shown in the following figure:

Figure 36: Relationship between abrasive wear and hardness.

As it usually happens in hard materials as metals, the table bellows shows three different trends to understand the abrasive wear. When the metal is pure and does not undergoes plastic deformation, thus no effect of work hardening is present, the result of abrasive wear can be represented by a positive slope and if the material is harder the wear will be greater. If there is a heat threatening the slope of the relationship will decrease after a wear value or when the heat treatment is present, meaning that the hotter the material is less abrasive wear will happen, this could be because rounding of the edges that produces the heat treatment in the cloistered particles. Finally, it is shown that if the material undergoes plastic deformation and therefore is a hard material in which can be present work hardening, the wear will remain constant when the work harden will still increase until the material will eventually enter into the rupture zone and the fracture will take place.

The abrasive wear has a dense theory background and starts by the assumption that the hard negative angle particle is sharp enough to move pieces from the other body from one side to the other. This particle can be modeled as a conic geometric figure and will dig furrows into the other body, such as shown in Figure 37.

Figure 37: Geometrical assumptions for abrasive wear demonstration [9].

Following the proper analysis for the correct analytical and geometrical assumptions one can demonstrate that the wear coefficient (-) is given by:

- =2 ∗ /01 3

( ∗ 42

Equation 4: Wear Coefficient in abrasive wear [24].

where k2 can be understood as the proportion of all the abrasive events produced by the appearances of different hardened particles. This wear coefficient is only valid for the conic shape seen in Figure 37, but it can vary depending on the shape that is wanted to analyze [24].

As the conditions in which abrasive wear dominates the tribological system, the preventive measures to prevent it are widely known, as it only happens when the harder and rougher material enters in contact with the softer material. One preventing measure is to reduce the sharpness, this means to decrease the roughness in the harder material to decrease the sharpness of the hard particles that are responsible for the furrows in the softer material [29]

1.17.2 Adhesive Wear

Adhesive wear happens when the materials that are able to adhere together through a solid phase of welding asperities. Posteriorly, the junction breaks resulting in material loss of one or both bodies. As in previous case, the study of metals materials is of interest to this subject and some general conclusions have been find, i.e. [25]:

- When there is the adhesive wear between a harder metal and a softer metal, the softer metal tends to loss more particles than the harder metal after the break down [25].

- Adhesive wear seems to happen more for similar metals than for those of diverse metals [25].

- Adhesive wear seems to happen more for materials that have mutual solubility at the temperature contact [25].

In the void space the metals tend to join into one due to Van der Waal’s attraction forces and the bond strength will be higher with the time and the pressure. Therefore, this adhesive wear will not take place, in the open environment are present different things that prevent material bounding to happen i.e. air, surface contaminants and oxides [25].

Figure 38: Adhesive wear [25].

By studying the adhesive interaction between two bodies, assuming an isothermal process (thus a constant temperature) and constant mechanical properties, a conclusion can be made:

the wear rate is not dependent of the sliding velocity, nor the apparent area of contact and is because the true area of contact depends on the load and wear rate depends on the true area of contact [29].

Many wear processes start as adhesive wear, as seen before if there is a detached particle in an interaction between two bodies, it can act as a lubrication or as a hard particle that cloisters into one of the materials giving the start for abrasive wear or pass for air which contains oxides and will produce hard and abrasive particles into one of the surfaces. This effect can be seen when two surfaces are subjected to high vibration frequencies between each other, there will be particles detachment and it will lead to the transformation between adhesive wear to abrasive wear or to wear due to fatigue [29].

1.17.3 Fatigue Wear

Fatigue Wear can happen in macroscopic and microscopic scale, the difference between them is that at macroscopic scale its referred to a machine or component of a machine, in microscopic scale is referred to an individual asperity contact. A typical failure mode for fatigue wear is when wholes of removed material are present in the junction, sometimes, these wholes grows bigger and bigger and corresponds exactly to the maximum shear stress of the structure or the piece of structure, causing the structure to arise to elastic limit and thus causing elastic deformation and finally rupture in the material. As usually, fatigue is represented and determined with the number of cycles that are needed to achieve failure with the accumulative stress [25].

Depending in the failure condition that the material reach, there can be several conclusions:

-The environment determines how the amount of stress required for crack nucleation and the rate of crack propagation.

-The presence of surface flaws can determine the direct surface crack to become as important as the interior crack.

-If surfaces are subjected to high tangential stresses, the position of maximum shear stress arises to the surface and will lead to fracture.

-The precious conclusion will arise if there is bad lubrication or surface roughness, good lubrication and smooth surfaces will be supported if fatigue wear leads to a stress failure in interior fatigue.

-A lubricant can accelerate crack propagation if it enters into a gap and develops high fluid pressures in the opening and closing cavity.

In the following figures there can be evidenced two different types of fatigue wear, the first on was due to elastic deformation and the second one is due to crack propagation in the surface. As seen before for the adhesive wear, in Figure 39 is shown that some particles are separated from the surface, this phenomenon can induce another type of wear such as abrasive wear and further failure of the structure [24].

Figure 39: Fatigue wear by elastic deformation [30].

Figure 40: Fatigue wear by crack propagation in the surface [30].

1.17.4 Corrosive Wear

This type of wear happens by the interaction of the surface with the environment, antagonistic with the other fatigue effects that can be explained with the stress interactions and deformation properties of the interaction surfaces, it is due to a chemical reaction with the oxide that is present in the ambient or with the external substances that can be present in a tribological system, as is known, oxide particles are very hard and rough. This reaction produces asperities in the surface and crack formation, relating this type of wear with the others, such as abrasion and fatigue (crack nucleation), this is the most difficult wear case to be explain by mathematical models but with some assumptions an approximation (it is always an approximation, in all types of wear) of the wear coefficient can be made [25].

The following assumptions must be taken into account if there is wanted a mathematical analysis of the corrosive wear:

-Surface rub is due to asperity contacts as in adhesive wear.

-A reaction environment produces a slow and growing protective thin layer upon the surfaces that are interacting.

- This protective layer will remain protective and undamaged until it reaches a thickness determined by 5 and then it will be removed by rubbing.

-If there is oxidation in the surfaces, it will be removed by rubbing.

-The growth of the protective film will be directly proportional to the thermal process.

Under the previous assumption, the corrosive wear is protective for the surfaces and is present to protect against severe metallic wear. These assumptions are made having knowledge that the minimum changes in the environment reactivity or in the temperature may cause intolerable changes in the thickness of the corrosive layer [25].