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Tribology | 3. Introduction of Wear and Wear Mechanism

Introduction of Wear

Undesirable removal of material from operating solid surface is known as wear. There are two definitions :

(1) Zero wear : Removal of material which causes polishing of material surfaces may be known as “Zero wear”. It may increase performance. It is for betterment, so it is not undesirable.

Zero wear is basically a polishing process in which the asperities of the contacting surfaces are gradually worn off until a very fine, smooth surface develops. Generally, “polishing-in” wear is desirable for better life of tribo-pair. Fig. 1 shows polished surface of helical gear which occurs due to slow loss of metal at a rate that will have a little affect on the satisfactory performance within the life of the gears.

(2) Measurable wear : Removal of material from surface that increases vibration; noise or surface roughness may be treated an “Measurable wear”. Often we measure wear in volume/mass reduction. Undesirable removal of material occurs in measurable wear.

Measurable wear refers to a loss of material which must be counted to estimate the life of tribo-pair. The extent of measurable wear depends on the lubrication regime, the nature of the load, the surface hardness and roughness, and on the contaminants in the lubricating oil. A typical example of measurable wear in helical gear is shown in Fig. 2 which is typically known as pitting wear.

Pitting :

Pitting is a surface fatigue failure which occurs due to repeated loading of tooth surface and the contact stress exceeding the surface fatigue strength of the material. Material in the fatigue region gets removed and a pit is formed. The pit itself will cause stress concentration and soon the pitting spreads to adjacent region till the whole surface is covered with pits. Subsequently, higher impact load resulting from pitting may cause fracture of already weakened tooth. Sometimes impurities in materials provide nucleus for crack generation as shown in Fig. 3 (c). Fig. 3 (d) shows merger of generated cracks, which finally detaches from the surface as shown in Fig. 3 (e). Such formation of pits (removal of material) comes under measurable wear.

Removal of material from operating solid surfaces by solid particles depends upon Load, Velocity, Environment, and Materials. Removal of material from operating solid surface by Fluid (liquid/gas) depends upon Velocity, pressure, Environment and material.

As wear increases power losses increases, oil consumption increases, rate of component replacement also increases. Ultimately, it reduces efficiency of the system. Therefore, as far as possible wear should be minimized.

Wear Mechanisms :

Wear can be classified based on the ways that the frictional junctions are broken, that is, elastic displacement, plastic displacement, cutting, destruction of surface films and destruction of bulk material. There are many types of wear mechanisms, but we shall discuss about common wear mechanisms, which are:

• Abrasive Wear : polishing, scouring, scratching, grinding, gouging.

• Adhesive Wear : galling, scuffing, scoring.

• Cavitation (interaction with fluid).

• Corrosive Wear (Chemical nature).

• Erosive Wear.

• Fatigue : delamination.

• Fretting Wear.

1. Adhesive Wear

Adhesive wear is very common in metals. It is heavily dependent on the mutual affinity between the materials. Let us take example of steel and indium [Fig. 4 (a)]. When steel pin under load is pushed [Fig. 4 (b)] in indium block, and subsequently retracted [Fig. 4 (c)], a thin layer of indium transferred on the steel pin. Similar behavior is observed by pushing brass metal in indium metal. This behavior demonstrates the loss of indium material, which occurs due to high value of adhesive force between steel and indium. If steel pin is subjected to normal load as well as tangential load [Fig. 4 (d)] then severe wear of indium material occurs. By introducing a thin layer of lubricant at the interface of indium and metal, the severe wear can be reduced to mild wear. Shear strength of lubricant layer is much smaller than shear strength of indium metal, therefore weak interface between steel and indium occurs which can be sheared easily and wear rate reduces to mild value.

Scoring wear, a severe form of adhesive wear, occurs due to tearing out of small particles that weld together as a result of overheating (due to high contact pressure and/or high sliding velocity) of the tooth mesh zone, permitting metal to metal contact shown in Fig. 5. After welding, sliding forces tear the metal from the surface producing a minute cavity in one surface and a projection on the other. The wear initiates microscopically, however, it progresses rapidly. Scoring is sometimes referred to as galling, seizing or scuffing.

Steps leading to Adhesive Wear :

Deformation of contacting asperities Fig. 6 (a).
Removal (abrasion) of protective oxide surface film.
Formation of adhesive junctions Fig. 6 (b).
Failure of junction by pulling out large lumps and transfer of materials Fig. 6 (c).

2. Abrasive Wear

Abrasive wear, sometimes called cutting wear, occurs when hard particles slide and roll under pressure, across the tooth surface. Hard particle sources are: dirt in the housing, sand or scale from castings, metal wear particles, and particles introduced into housing when filling with lube oil. Scratching is a form of abrasive wear, characterized by short scratch-like lines in the direction of sliding. This type of damage is usually light and can be stopped by removing the contaminants that caused it. Fig. 7 shows abrasive wear of a hardened gear.

Following are few well-known reasons of abrasive wear mechanisms :

– Micro-cutting : sharp particle or hard asperity cuts the softer surface. Cut material is removed as wear debris.

– Micro-fracture : generally occurs in brittle, e.g. ceramic material. Fracture of the worn surface occurs due to merging of a number of smaller cracks.

– Micro fatigue : When a ductile material is abraded by a blunt particle/asperity, the worn surface is repeatedly loaded and unloaded, and failure occurs due to fatigue.

– Removal of material grains : Happens in materials (i.e. ceramics) having relatively week grain boundaries.

Basic modes of abrasive wear are classified as two body abrasion and three body abrasion

Two – Body Abrasion :

This wear mechanism happens between two interacting asperities in physical contact, and one of it is harder than other. Normal load causes penetration of harder asperities into softer surface thus producing plastic deformations. To slide, the material is displaced/removed from the softer surface by combined action of micro ploughing and micro-cutting.

Three Body Abrasion :

Three body abrasion is material removed from softer surface by hard loose particles(Fig. 8), which are free to roll as well as slide over the surface, since they are not held rigidly. The hard particles may be generated locally by oxidation or wear from components of tribological system. Iron oxides wear debris produced during adhesive wear cause further damage due to abrasion. Due to rolling action, abrasive wear constant is lower compared to 2-Body abrasion.

Wear rate is lesser in three body abrasion than two body abrasion.

3. Corrosive Wear

• Chemical reaction + Mechanical action = Corrosive wear

The fundamental cause of Corrosive wear is a chemical reaction between the material and a corroding medium which can be either a chemical reagent, reactive lubricant or even air.

Stages of corrosive wear :

• Sliding surfaces chemically interact with environment (humid/industrial vapor/acid)

• A reaction product (like oxide, chlorides, copper sulphide)

• Wearing away of reaction product film

Fig. 9 Passivation of corrosion. Continuous corrosion.

The most corrosion films passivate (Fig. 9) or cease to grow beyond a certain thickness. This is favourable as corrosion process stops its own. But most corrosion films are brittle & porous, and mechanical sliding wears away the film. The formation and subsequent loss of sacrificial (Fig. 9) or short life-time corrosion films is the most common form of corrosive wear.

Sliding surfaces may wear by chemically reacting with the partner surface or the environment, or both. The oxide layers resulting from reactions with the environment are typically 10 microns thick, and they may have a protective role unless the thickness tends to grow during the cyclic contact process. If the oxide layer grows, it becomes liable to break in brittle fracture, producing wear particles. Hard, broken-off oxide particles may then profoundly affect subsequent wear life as abrasive agents. If soft, ductile debris results, it may form a protective layer on the surface.

4. Fretting Wear

Fretting Wear coined in 1927 by Tomlinson. It refers to small amplitude(1 to 300 μm), with high frequency oscillatory movement mainly originated by vibration. This generally occurs in mechanical assemblies (press fit parts, rivet / bolt joints, strands of wire ropes, rolling element bearings), in which relative sliding on micron level is allowed. It is very difficult to eliminate such movements and the result is fretting. Fretting wear and fretting fatigue are present in almost all machinery and are the cause of total failure of some otherwise robust components.

Fretting occurs wherever short amplitude reciprocating sliding between contacting surfaces (Fig. 10) is sustained for a large number of cycles. The centre (Fig. 10) of the contact may remain stationary while the edges reciprocate with an amplitude of the order of 1 micron to cause fretting damage. One of the characteristic features of fretting is that the produced wear debris is often retained within the contact due to small amplitude sliding. The accumulating wear debris gradually separates both surfaces(Fig.11) and, in some cases, may contribute to the acceleration of the wear process by abrasion. The process of fretting wear can be further accelerated by temperature. Reciprocating movements as short as 0.1 micron in amplitude can cause failure of the component when the sliding is maintained for one million cycles or more.