Wear failure analysis

Date：2021-07-30 18:20:29 Views：3251

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Introduction to wear failure analysis items    

Wear is a phenomenon of continuous damage caused by the gradual separation of wear debris on the surface when the object moves relatively under the action of friction. Wear failure refers to the failure of mechanical parts to reach the original design efficiency due to the occurrence of wear phenomenon, that is, they can not reach the original design level. Wear failure is a process of gradual development and gradual change, ranging from a few hours to a few years. Wear, fracture and corrosion are called three forms of metal failure analysis, and their harm is very amazing.

The types of wear failure are: 

① Adhesive wear failure;

When the two friction surfaces slide relative to each other, the material will be transferred from one surface to the other after cold welding, and become a surface protrusion, which promotes the further wear of the friction surface, which is called adhesive wear.

Main factors affecting adhesive wear: similar friction pair materials are easier to adhere than dissimilar materials. Surface treatment (such as heat treatment, spray plating, chemical treatment, etc.) can reduce adhesive wear. Brittle materials have higher anti adhesion ability than plastic materials. The smaller the material surface roughness value, the stronger the anti adhesion ability. Control the temperature of friction surface The lubricant used can reduce adhesive wear, etc.

② Abrasive wear failure;

The hard particles or protrusions entering the friction surface from the outside plough out many grooves on the surface of softer materials, resulting in a wear phenomenon caused by the migration of materials, which is called abrasive wear.

The main factors affecting this wear are: in most cases, the higher the hardness of the material, the better the wear resistance; The wear amount increases with the increase of the average size of wear particles, and the wear amount increases with the increase of the hardness of wear particles.

③ Corrosion wear failure;

In the process of friction, the chemical or electrochemical reaction between the friction surface and the surrounding medium causes the loss of surface materials, which is called corrosive wear.

Main factors affecting corrosion wear: properties of corrosive media (such as acid, alkali and salt), properties of oxide film on the surface of parts, ambient temperature and humidity, etc.

④ Deformation and wear failure;

After elastic deformation, the object will naturally wear. But some wear and tear is very small and difficult to find, but it does happen. Even if it is invisible to the naked eye, it actually occurs. Some, of course, are very obvious. Therefore, there must be wear and tear.

⑤ Surface fatigue wear failure;

When the two materials move relative (rolling or sliding), the contact area is subjected to the repeated action of cyclic stress. When the cyclic stress exceeds the material contact fatigue strength, fatigue cracks are formed on the contact surface or somewhere under the surface, resulting in the local falling off of the surface layer, which is called fatigue wear.

The main factors affecting fatigue wear: the higher the surface hardness of parts, the less the risk of fatigue cracks; Reducing surface roughness can improve the fatigue life of parts; High viscosity lubricating oil can improve the ability of anti fatigue wear and is conducive to improving fatigue life.

⑥ Impact wear failure;

When the fluid containing flowing particles (solid, liquid or gas) strikes the material surface, a wear phenomenon is called erosion wear.

The main factors affecting erosion wear are the impact velocity and angle of flowing particles.

⑦ Micro vibration wear failure, etc.

Due to small amplitude vibration between mutually compressed metal surfaces, oxidation wear particles are generated on the contact surface, which is difficult to be removed from the contact part, and fretting wear will occur.

The main factors affecting fretting wear: the wear of similar materials is much more serious than that of dissimilar materials.

Wear failure analysis and influencing factors

Material properties. The greater the compatibility of matching materials, the greater the adhesion tendency and the greater the adhesion wear.

Material microstructure. The ferrite structure is soft. Under the same other conditions, the more ferrite content in the steel, the worse the wear resistance.

Load and sliding speed. The results show that there is a critical pressure for all kinds of materials. When the surface pressure of the friction pair reaches this critical value, the adhesive wear will increase sharply until it bites.

Surface temperature. The heat generated by the friction process increases the surface temperature and produces a large temperature gradient along the depth direction in the contact surface. The increase of temperature will affect the properties of the friction pairs, the properties of the surface film and the properties of the lubricant. The temperature gradient will cause thermal stress on the contact surface, which will affect the adhesive wear.

Environmental atmosphere and surface film. The environmental atmosphere mainly affects the adhesive wear by affecting the tribochemical reaction. All kinds of surface films have a certain anti adhesive wear effect. The oil additives, wear-resistant additives in the lubricating oil, the chemical reaction film formed by the adsorption film, the extreme pressure additive, and the surface film produced by other methods, such as sulfide, phosphide and oxide, can significantly improve the adhesive wear resistance.

lubricant. Lubrication is one of the important ways to reduce wear.

Methods to prevent various wear and failure of parts

1. Lubrication. Filling the two friction surfaces with lubricating oil can greatly reduce the friction coefficient, reduce the friction resistance and reduce the mechanical wear. Therefore, it is very important for the operation of the machine to have lubricating oil, correctly select lubricating materials, reasonably formulate lubrication system and strengthen lubrication management, which has a great impact on the service life of the machine.

2. Surface finish quality. After machining, it is impossible for the friction surface of the machine to obtain the ideal geometry. It is always necessary to leave the tool mark of the cutting tool or the grinding mark of the grinding wheel to form a concave convex unevenness. Generally, the surface is rough and begins to wear quickly.

When the unevenness is roughly eliminated after grinding for a certain time, the wear will slow down. Therefore, the requirements of surface machining accuracy should be selected according to the characteristics of parts, and do not blindly pursue too high machining quality. Experiments show that too smooth surface does not necessarily have good wear resistance, because at this time, the lubricating oil can not form a uniform oil film, and the two contact surfaces are easy to bond, which will deteriorate the wear resistance.

3. Materials. The wear resistance of a material mainly depends on its hardness and toughness. The hardness of a material depends on the resistance of the metal to its surface deformation. However, too high hardness is easy to increase brittleness and cause the peeling of abrasive particles on the material surface. The toughness of the material can prevent the generation of abrasive particles and improve its wear resistance.

In addition, increasing the chemical stability of materials can also reduce corrosive wear. Increasing the porosity of the material itself can accumulate lubricant, so as to reduce mechanical wear and improve the wear resistance of parts. Different materials have different mechanical properties. The mechanical properties of the same material can be improved by different heat treatment methods. Therefore, reasonable selection of materials and heat treatment methods is of great significance to reduce mechanical wear.

4. Quality of installation and maintenance. The correctness of installed parts has a great impact on the service life of the machine. For example, incorrect tightening of the connecting screws between the bearing cover and the bearing seat, misalignment of the two joint surfaces, uneven mating surfaces and improper adjustment of bearing clearance can cause incorrect distribution of unit load on the surface or additional load, so as to accelerate its wear.

Wear failure analysis test procedure

1. Field investigation and macro analysis 2 3. Measurement of wear failure Check lubrication and lubricant quality 4 Inspection of friction pair material 5 Conduct necessary simulation test 6 Determine the wear mechanism, analyze the failure causes, and put forward improvement measures.