Table of contents
Wear is defined as the removal of material from the surface of bodies in contact subject to relative motion. Wear affects the surfaces of contact members of machines causing a progressive decay of the functional characteristics. This phenomenon, however, has not only negative sides, on the contrary, it is often used to reduce the roughness of some materials by eliminating their asperities. Wrongly we tend to imagine wear in a direct correlation with friction, in reality these two phenomena are not connected in a simple way. There can be conditions where low friction can lead to significant wear and vice versa. For this phenomenon to occur, certain “lead times” are required, which can change depending on a number of variables, such as load, speed, lubrication and environmental conditions.
The breakdown of a machine can be traced back to three main causes:
- the breakage of machine elements, due to material defects or to the fact that the system is subjected to loads greater than those of the project;
- obsolescence, i.e. ageing due to the appearance on the market of more evolved machines capable of performing the same function in a more convenient way from the point of view of both speed of execution and energy saving;
- wear and tear, inherent in the operation of the machine itself, causing a progressive decline in functionality, not always in proportion to the passage of time. Usually, in fact, wear phenomena show a higher growth rate as the overall level of wear increases.
Today, wear is the subject of theoretical and experimental studies, since the tolerances allowed for proper operation of machine elements are very small compared to the past. Wear is manifested by:
- an increase of the clearances in the couplings with consequent inaccuracies in the movement and increased noise;
- a possible appearance of impact phenomena and consequent vibrations causing dynamic overloads and fatigue stresses;
- uneven distribution of pressures;
- operating inaccuracies;
- possible increase in the rate of wear itself due to the increase in the actions exchanged between bodies in contact and the abrasion of wear particles created in the contact zone.
Types of wear and tear
Wear is classified into four categories, for each of which the nature of the phenomena that cause it is now clarified, albeit qualitatively. The laws that govern it are always of a statistical nature and therefore interpretable in terms of “survival time” under certain operating conditions.
Adhesive wear is the most frequent and dangerous, because while the other forms of wear can be practically eliminated with appropriate interventions, adhesive wear can be reduced, but it is always present. When two surfaces creep or are simply in contact with each other, countless micro-welds are created between the tips of the surface asperities due to the roughness of the contacting bodies.
During the relative sliding motion between the bodies, these microjoints tear. Upon tearing, minute fragments of one solid remain adhered to the other, sometimes in the form of a solid solution. In the continuation of the phenomenon such fragments can also be returned to the original surface or roll forming free particles. In conclusion, the behavior of the adhesive wear volume can be described by means of three main laws:
- Law 1. The mass involved in the wear is proportional to the distance traveled in the rubbing between the surfaces.
- Law 2. The mass involved in wear is proportional to the applied load.
- Law 3. The mass involved in wear is inversely proportional to the hardness of the less hard material.
Abrasive wear consists of the shear stress of hard surfaces acting on softer surfaces, as the roughness of the former scratches the latter like a tool. The material removed usually forms loose fragments. Abrasive wear is very destructive because it can occur by: scratching, tearing, grinding or erosion.
Every material is chemically attacked by the environment; often the product of the attack is an oxide which, once formed, protects the material from further attack. Corrosive wear is chemical because in the sliding of two surfaces the oxide layer, which is less resistant, is removed and allows the renewal of the chemical attack.
In a corrosive environment, mechanical and chemical action can enhance each other’s effects: the surface layers, chemically protective but easily removable, are continuously removed and immediately reform, thus triggering a mechanism of wear that can sometimes be very rapid.
Wear due to surface fatigue
Surface fatigue wear is the typical phenomenon that occurs between surfaces with a relative rolling motion prevailing over the sliding motion, such as in rolling bearings or in gear wheels.
In the rhythmic cycle of load application, cracks are caused under the surfaces, typically at the depth where the stress reaches the maximum value, which proceed isolating fragments of material that come off. Often this type of wear goes unnoticed when other types of wear are present, as it is generally of lesser magnitude than adhesive, abrasive or corrosive wear. In some cases, however, it is considerable: for example, in the case of roads subject to vehicle traffic.
Rubbing or fretting wear
Fretting wear occurs in systems subject to more or less intense vibrations, which cause relative movements between the surfaces in contact of the order of a nanometer. These microscopic relative movements cause both adhesive wear, caused by the displacement itself, and abrasive wear, caused by the particles produced in the adhesive phase, which become trapped between the surfaces. This type of wear can be accelerated by the presence of corrosive substances and increased temperature.
Erosion wear occurs when free particles, which can be either solid or liquid, strike a surface, causing abrasion. The mechanisms involved are varied and depend on parameters such as the angle of impact, particle size, impact velocity and the material of which the particles are made.
Criteria for reducing wear
Lubrication has beneficial effects on adhesive, abrasive and corrosive wear. The beneficial effects of lubrication are reflected in the effective contact area which, as it decreases, results in a smaller wear coefficient k and, therefore, a smaller volume removed. As far as corrosive wear is concerned, the lubricant protects the surfaces from the corrosive action of chemical agents.
Choice of contact metals
Microsalts are made of real metal alloys. Therefore the contact metals must be as little miscible as possible to hinder the formation of micro-welds and thus the effects of adhesive wear. In particular, a benefit is already achieved when the atomic radii of the contact metals differ by more than 7%.
The practice of surface treatment is to make surfaces “less metallic”, i.e. to metallize them. Protection consists of surface treatments such as carburizing, nitriding or phosphatizing that produce anti-soldering material. These processes are advantageous not so much for the surface hardening, but precisely for the metallization that hinders the formation of metal alloys and therefore micro-welding.
Lapping of surfaces
Lapping the surfaces and avoiding the presence of hard and free particles reduces abrasive wear.