Advertisement

Surface Damage: Causes and Mechanisms

  • Dheerendra Kumar Dwivedi
Chapter

Abstract

The loss of material from the functional surfaces is caused by various mechanisms. Fundamental mechanisms of wear, namely adhesive wear, abrasive wear, erosive wear, surface fatigue wear, and corrosive wear, have been described. Additionally, properties of the surface material important to control a specific type of wear have been explained. Methods commonly used for measuring the wear loss have been presented.

2.1 Material Properties and Its Effect on Performance of Components

The performance of engineering component during service is predominantly determined by the surface characteristics because their failure is mostly triggered from the surface. Degradation at the surface of the component owing to any physical, chemical, and biological phenomenon/activities leads to loss of materials (wear), corrosion, and fracture. These degradations do not just lead to the loss of materials from the surface but also deteriorate the surface finishing by developing severe undulations at the surface through the formation of oxides and other compound based on the interactions with the surrounding environment. The undesirable features of surface degradation can degrade the performance of engineering components below the critical level, thereby forcing the requirement of repairing or replacing the engineering component with new ones (Fig. 2.1).
Fig. 2.1

Performance versus time relationship for any engineering component

Surface engineers are, therefore, focusing on the development of technologies for reducing surface degradation rate so as to increase the service time by improving surface qualities, to enhance the performance and life of components.

2.2 Common Factors Leading to the Deterioration of Surfaces

The deterioration of functional surfaces begins as soon as they are put in use owing to the interaction of surfaces with heat, radiation, mechanical stresses, ambient and exhaust gases, water and other liquids, biological items, and their wastes during the service. On the basis of fundamental mechanism, various factors causing the deterioration of functional surfaces (due to interactions during service) can be classified or grouped in three categories as shown schematically in Fig. 2.2. This kind of classification of the surface degradation mechanisms is primarily for developing strategy to control the surface damage. Surface damage is caused because of the following three reasons: (a) Physical phenomenon is primarily caused by heat, mechanical loads, and radiation, (b) chemical phenomenon is caused due to chemical reactions of surface with water, atmospheric gases, chemicals, and waste items, and (c) biological phenomenon is caused due to interactions of surfaces with biological items and their wastes.
Fig. 2.2

Schematic diagram showing the classification of the causes of surface degradation

The development of strategy (for surface modification using suitable approach) to control the surface degradation is generally found to be easier when damage occurs due to a single mechanism (such as wear, corrosion) rather than due to simultaneous operations of two or more mechanisms, such as corrosive wear, corrosion fracture, failure of metal. Human body is a typical example of such cases where physical, chemical, and biological phenomena act together.

For controlling the surface degradation under abrasive wear conditions, the surface modification can be designed to have requisite hardness while that for solid particle erosion can be done by developing engineered surfaces having suitable combination of both hardness and toughness. However, abrasion and erosion occurring either in the corrosive environment or at high temperature make the job of surface engineers more complex primarily because of the need of contradicting design property requirement and entirely different nature of operating mechanisms which cause surface damage.

2.3 Types of Wear and Mechanisms and Classical Governing Laws

Wear is one of the most commonly encountered surface degradation mechanism in which gradual and progressive loss of material from the functional surface takes place. This uncontrolled and non-uniform loss of material from the surfaces results in surface undulation and increases surface roughness which eventually leads to change in the size and shape of component to such an extent that it promotes other surface degradation mechanisms, weakens the component and makes it prone to mechanical failure under static or dynamic service loads, and leads to improper movement or malfunction of other mating components or the entire system.

Wear of material from the functional surfaces of engineering components primarily occurs through common mechanisms such as adhesion, abrasion, erosion, corrosion, surface fatigue, fretting. Adhesion, abrasion, and corrosion together contribute to more than 80% of material loss from the engineering components in general. However, the percentage contribution of each wear mechanism varies appreciably with the change of sector such as transport, mining, machinery. In the following sections, the basic principle and factors affecting material loss based on the above wear mechanisms are presented.

2.3.1 Adhesive Wear

Adhesive wear takes place when the interacting surfaces develop a relative motion with respect to each other and are in direct metal-to-metal contact through asperities (peaks and troughs) present at the mating surfaces (Fig. 2.3). These asperities are always present on the mating surfaces irrespective of the manufacturing process used for making the components in question. Even polishing and finishing can just reduce the extent of peaks and valleys present on the surface but cannot eliminate them. In fact, all real contacts occur through asperities at the interacting surfaces; therefore, the actual area of contacts (shaded zones in Fig. 2.3) is just about 0.1–1% of the nominal area of contact, thereby leading to the development of true stress at real contact areas which is approximately 100–1000 times of the nominal stress owing to either the dead weight of interacting member or the external loads. Under such high stress conditions, asperities (peaks and troughs) become plastically deformed and develop direct metal-to-metal contact between the interacting surfaces. This deformation and establishment of direct metallic contact between the mating surfaces result in the formation of cold bond or joints owing to the following three phenomena (Fig. 2.4).
Fig. 2.3

Schematic of contacts at interface in adhesion

Fig. 2.4

Block diagram showing mechanism of adhesive wear

  1. (a)

    Mechanical interlocking: The interaction of asperities at the mating surfaces forms mechanical joints owing to the interlocking of surface irregularities due to deformation. Moreover, the strength of such cold weld bond at the interface is primarily governed by the work hardening of surface layers owing to deformation, thus making the surface layers harder than the subsurface layers and core materials.

     
  2. (b)

    Electron exchange: The development of metallic bond at the interfaces through the exchange of free electrons from one side to other (high energy to low energy side) happens very rapidly when contacts are established just for a fraction of second such as in the case of sliding.

     
  3. (c)

    Diffusion under high temperature at the interface for longer duration leads to the diffusion of alloying elements present in the mating components. Diffusion begins from the areas where direct metal-to-metal contacts exist, and this leads to the development of diffusion at the interfaces.

     

Bonds formed at the interface do not affect the surfaces of interacting components until the relative movement takes place. For relative movement to happen between the mating surfaces, these bonds must be broken. The fracture of interfacial bond preceding the relative motion can occur from any of the three regions: (a) just at the bond interface, (b) subsurface layer zone of mating one component, or (c) other component whichever is weak. In general, the interface bonds are found stronger than subsurface layers due to the work hardening effect of plastic deformation of the surface layers. Therefore, a small lump of metal is detached generally from the subsurface layers of weaker component of the two interacting members. These particles initially tend to roll and plow at the interacting surfaces due to relative motion. Continued removal of material from the functional surfaces helps these particles to grow. Eventually, these are pushed out from the interface in the form of mostly black oxidized wear debris. The presence of this work hardened piece of metal at the interface can act as the third body for abrasion to take place and increase loss of materials from the surfaces.

The resistance to breaking bonds developed at the interfaces primarily determines the force (friction force) required to have the relative movement. All factors encouraging the formation of strong interfacial bonds between the matting components would lead to increase in the wear and friction effects. There are numerous factors and some of them are: the presence of asperities, load (influencing the true stress at contact area), contact duration, surrounding (atmospheric gases or vacuum), counter surface, metallic intimacy, complexity of microstructure, hardness of mating components, extent of work hardening owing to deformation of near-surface layers, etc.

These factors can be grouped under two headings: (a) service or sliding conditions such as load, relative speed (affecting interface contact duration), interface medium, or air (if any) affecting the metallic intimacy, and (b) material-related parameters such as surface roughness, hardness, ductility, work hardening tendency, complexity of structure, presence of nonmetallic components (such as ceramics, polymers).

In general, increase in normal load (so true stress), surface roughness (so depth and height of troughs and peaks, respectively), contact duration at the interface (at low relative velocity between mating parts), absence of active or oxidative environment, vacuum, inert environment, or absence of lubricant, and high temperature increase the interfacial bonding tendency which in turn enhances the wear and frictional losses.

Sliding of mating component in the ambient conditions causes oxidation of interacting surface layers, thereby reducing the metallic intimacy between them, which in turn decreases the tendency of interfacial bond formation that leads to the reduction in friction coefficient and wear. However, the relative motions occurring between the mating components in vacuum or inert atmosphere do not develop any such protective oxide layers at the surfaces and thus cause direct metal-to-metal contact as well as develop seizure-like conditions.

The interacting surfaces of similar metals having low hardness and high ductility as well as low work hardening tendency further increase the wear and friction effects. Therefore, dissimilar mating metal systems with high hardness and low stacking fault energy help in reducing the wear- and friction-related problems.

Mating components composed of complex microstructures having nonmetal components (like ceramic, carbides, nitrides, and graphite) imbedded in metal matrix (ferrite, pearlite, aluminum, nickel) discourage the interfacial bonding through free electron transfer. As free electron transfer between metal (aluminum, iron, cobalt, etc.) and nonmetal system (silicon, graphite, and ceramic particles) does not occur under normal conditions unless meting takes place. Hence, the reinforcement of hard nonmetallic constituents in metal matrix (as in case of MMCs) generally reduces the friction coefficient and increases the wear resistance.

Mild and severe wear

Under severe sliding conditions of normal load and relative velocity, the generation of excessive frictional heat causes thermal softening and even partial or complete melting of surface layers, which can lead to the situation of (a) 100% direct metal-to-metal contact and seizure-like conditions owing to the thermal softening of surface layers, (b) metallic failure of surface layers due to the inability to take up the service loads, and (c) melting and deposition of partially or melted surface layers on to the counter surface and the generation of wear debris in the form of big metallic particles indicating the occurrence of metallic wear. Metallic wear becomes 10–100 times greater than normal mild oxidative. Tribological components are generally designed to work under mild oxidative wear conditions; however, accidental situations like failure of lubricating, overloading, and overspeeding can lead to metallic wear conditions which will eventually lead to failure of the system.

All the metal systems having high thermal stability and the hard and coherent surface layers offer good resistance to the occurrence of metallic wear. The conditions leading to the occurrence of transition from mild oxidative to severe metallic wear are called transition load or transition velocity.

Factors affecting the adhesive wear

There are many test material, counter surface and sliding test-related parameter, which affect degree of metallic intimacy and mechanism of material loss from the contacting surfaces. The most of sliding adhesive wear factors include normal load, sliding speed, type of relative movement (unidirectional, reciprocating), counter surface (hardness, metal, crystal structure, temperature), environment affecting extent of interfacial contact (ambient air, steam, liquid metal, etc.), nature of contact (pin-on-disc, pin-on-flat, ball-on-flat, ball-on-ring) determining the contact stresses at the interface and material properties (hardness, oxidation tendency, thermal stability, resistance to softening, type of oxide, metallurgical structure, crystal structure), work hardening tendency (work hardening exponent, work hardening capacity, stacking fault energy) affect the wear behavior.

Normal load : Increase of load during sliding results in higher metallic intimacy under adhesive wear condition which in turn in general increases the wear rate. The rate of change in wear rate due to increase in normal load is determined by operational wear mechanism for a given set of conditions (Fig. 2.5). Increase in wear rate as function of normal load is found to be low and linear under mild oxidative wear condition. Under mild wear conditions,  wear surface remains coherent and does not experience large-scale plastic deformation. Increase in normal load above a critical value called transition load unstabilizes the wear surfaces causing large-scale plastic deformation and metallic failure. Moreover, the critical load at which change in the wear mechanism from mild oxidative wear to severe metallic wear takes place is influenced by many sliding wear conditions namely sliding speed, counter surface temperature, and its metal (crystal structure, microstructure). In general, high sliding speed, high counter surface temperature, and same crystal structure decrease the transition load. The change in magnitude of wear rate owing to transition from mild to severe wear may be 10–100 folds or even more. Additionally, wear debris changes from dark black oxide particles of fine size (few tens of micrometers) to bright metallic particles of large size (few thousands of micrometers).
Fig. 2.5

Effect of normal load on wear rate at different sliding speeds and counter surface temperature conditions

Sliding speed: The relative speed between the material of component and counter surface is called sliding speed. Sliding speed affects frictional heat generation and localization of heat generated. Rise in temperature due to frictional heating causes (a) oxidation of interacting surfaces, (b) thermal softening of near sliding surface layers, and (c) complete/partial melting of element at the interface. However, minor temperature rise (about 10–30 °C) occurring under mild wear conditions, i.e., low normal load and low sliding speed, primarily causes oxidation of interacting surfaces. These oxides reduce the metallic intimacy between the mating surfaces which in turn decreases adhesive wear. Therefore, under low-load sliding conditions, initially an increase in sliding speed reduces wear rate. Increase in sliding speed above a certain limit increases in heat localization at mating surfaces to such an extent that thermal softening dominates over oxidation (Fig. 2.6). Increased thermal softening of the sliding surfaces increases the metal-to-metal contact which may lead to (a) increase in wear rate with increase in sliding speed and (b) metallic failure of the softened metal results in 10–100 folds or even more increase in wear rate causing severe metallic wear. The sliding speed at which transition from mild oxidative wear to severe wear takes place is called transition speed.
Fig. 2.6

Effect of sliding speed on wear rate at different normal loads

Relative motion between wear sample and counter surface

Relative motion between wear test specimen (pin, ball) and counter surface (disc, flat, ring) can be either unidirectional or reciprocating type. Effect on relative motion on the wear rate depends on the way it causes the presence/absence of wear debris at sliding interface (Fig. 2.7). Wear debris mostly comprises oxides and deformed and work hardened metallic particles. Debris present at the sliding interface between mating components can act as a third body to cause three-body abrasive wear. In case of unidirectional motion like pin-on-disk and ball-on-ring configurations, wear debris generally gets removed from the counter surface due to centrifugal force, and therefore, it is usually not found at the sliding interface. Chances for the presence of wear debris at the sliding interface under reciprocating relative motion condition are found more than unidirectional motion. Hence, unidirectional motion usually causes lower wear rate than reciprocating motion.
Fig. 2.7

Relative motion between wear specimen and counter surface a unidirectional and b reciprocating motion

Counter surface

The metal system against which component is made to slide is called counter surface. The composition, crystal structure, hardness, and temperature of the counter surface affect the metallic intimacy at mating interface in two ways (a) deformation of surface irregularities and (b) oxidation of interacting surfaces which in turn influence wear rate of the specimen. Similarity of composition and crystal structure of counter surface and wear specimen results in more metallic intimacy. During the sliding increased metal-to-metal contact causes higher wear rate. Therefore, counter surface (even for engineering application at the design stage) is generally selected of different compositions and crystal structures from component in consideration. Temperature of the counter surface affects both oxidation tendency and thermal softening. Slight increase in temperature of counter surface increases the oxidation of interacting surfaces which in turn lowers the wear rate due to reduced metal-to-metal contact. However, significant increase in temperature of the counter surface may lead to thermal softening and even partial melting of the softer metal. Softening of metal increases the metallic intimacy, and therefore, it experiences higher wear rate. Hardness of the counter surface also affects the metal-to-metal contact. High hardness of counter surface generally decreases the wear rate due to somewhat reduced metallic intimacy owing to limited interfacial yielding. Therefore, counter surface and mating wear component of matching hardness cause high wear rate.

Environment

The tribological components under adhesive wear conditions are required to work in different environments like ambient air, steam, gases, and liquid metal. Influence of environment on wear rate depends on how far it affects the metal-to-metal contact due to formation of oxides, nitrides, and any other metallic components at the sliding interface. The formation of hard, coherent, and stable oxides and compounds reduces the metallic intimacy between sliding components which in turn decreases the wear rate while the formation of porous, brittle, and weak layers of oxides and compounds increases the wear rate in two ways (a) formation of oxides and their removal under sliding conditions and (b) presence of impurities at sliding interface may act as third body to increase the possibility of three-body abrasive wear.

Classical model on adhesive wear

According to the simplified model developed by Archard, the volume of material (V) removed from the surface under adhesive wear conditions is directly proportional to the product of cross section (A) of the interfacial bond developed because of the plastic deformation and length of groove (D depends on sliding distance) that has formed due to the relative motion on the surface (Fig. 2.8). The cross-sectional area of the interfacial bond is primarily determined by the extent to which plastic deformation occurs in the subsurface region. The extent of deformation depends upon the applied normal load W (in N) and the flow stress (H/3). The flow stress is found to be approximately one-third of the hardness H (Pa) of material.
Fig. 2.8

Schematic of groove formation during adhesive wear for determining wear rate

$${\text{Wear}}\;{\text{volume}}\;V{:}\;(k \times D \times W)/\left( {H/3} \right) = (3 \times k \times D \times W)/H$$
where k is the proportionality constant called wear coefficient, which is a dimensionless number, W is load (N), D is distance (m), V is wear volume (m3) and flow stress (Hardness/3 in Pa). Wear coefficient is found to vary from 0.001 to 10−12 depending upon the interfacial contacts as dictated by oxidation or lubrication condition. The above equation suggests that the adhesive wear rate (V/D in g/m) of the interacting surfaces increases proportionally with the increase in normal load and decreases with the increase in hardness.

2.3.2 Abrasive Wear

The interaction of functional surfaces of components with hard particles like rocks, sand, dust causes one of the most significant forms of the wear called abrasive wear which in a simplified form is supposed to occur in a way that is similar to that of microcutting by the number of randomly oriented fine cutting edge of hard particles. Based on the appearance of wear surface, the abrasion of engineering components takes place through two ways, namely plowing and cutting. Technically, the mechanisms generally held responsible for material loss from the surfaces under abrasive wear conditions include plastic deformation followed by work hardening and then fatigue, grain detachment, and brittle fracture. The common abrasive wear mechanism is depicted in Fig. 2.8a–c.

Mechanisms of abrasive wear

Depending upon the hardness, toughness, and size of the hard abrasive particles as well as the above properties of abrading surfaces, it will be decided whether plowing or cutting will take place. Hard and stable abrasive particle (under given load condition) with favorably oriented sharp cutting edges removes the material by cutting action, whereas the blunted abrasive particles (without sharp cutting edges) result in plowing by displacing the material in sides and producing a wedge-shape groove. However, the formation of wedge-shape groove does not necessarily remove the material but continuous deformation work hardens it and is subsequently removed by fatigue (by coalescence of cracks) due to the repeated action of abrasive particles. In an effort to reduce wear by increasing hardness backfires after certain limit (of increased hardness) owing to the loss of toughness and increased brittleness (Fig. 2.9). Metals with limited or nil ductility or brittle metals with the action of abrasive particles develop the subsurface cracks which in turn lead to the loss of material by brittle fracture. Similarly, poor bonding of microconstituents in a material system such as in composite materials (having SiO2, SiC, WC, Al2O3, etc., in aluminum matrix) leads to the debonding of hard particles from matrix and detachment of grains from materials through abrasive action. Sometimes, matrix holding the hard particles in position is abraded from all around by abrasive action, thereby leading to the loss of support to hard microconstituents which in turn results in grain detachment in the form of pullouts.
Fig. 2.9

Schematic of abrasive wear mechanisms a plowing and cutting, b brittle fracture and grain detachment, and c fatigue

Factors affecting abrasive wear

The volume of material lost from the surface through abrasion primarily depends on the depth of penetration and relative movement of abrasive particles with respect to the indented surface. The depth of indentation is governed by many materials, service load, and abrasive particles related to parameters. In general, the increase in hardness, reduction in applied load, and size of abrasive particle result in decreasing the depth of penetration which in turn lowers the abrasive wear.

Moreover, apart from shape, the mechanical properties of abrasive particles, that is, hardness and toughness predominantly affect the abrasive wear, as these two properties determine the stability of abrasive particles in terms of size and shape under abrasive wear conditions. Hard abrasives generally get fractured and so provide sharp and fresh cutting edges for rapid abrasive wear to take place but the size gets reduced, whereas tough abrasives get blunted with the rounded-off edges and corners which in turn leads to reduced abrasive action (Fig. 2.10). So, there is a critical toughness or hardness combination resulting in maximum abrasive wear due to the balance between formation of sharp cutting edges and survival of grit size.
Fig. 2.10

Schematics showing the ways by which abrasive grits are affected during wear

In case when hardness of abrasives is significantly higher than surface, abrasive wear varies linearly with the size of abrasive particles.

A ratio of the hardness of surface material and abrasive (1.2) is one of the crucial parameters affecting the abrasive wear rate (Fig. 2.11). Abrasives having the above ratio lower than 0.8 result in rapid wear. If this ratio is higher than 1.2 then it results in negligible wear. Designer always makes the effort to reduce gap in the above two harnesses for reducing abrasive wear.
Fig. 2.11

Effect of hardness ratio of substrate and abrasive on abrasive wear rate

Moreover, the hardness of surface during abrasion is found to be different from that of the bulk material owing to difference in the rate of straining or loading. Material under abrasive conditions is strained at much higher rate (just like metal cutting by shearing) than normal hardness tests; therefore, the hardness of severely deformed material is considered to be a more useful parameter in relation to the wear than normal hardness (Fig. 2.12).
Fig. 2.12

Schematics showing the effect of hardness on abrasive wear rate of a under deformed material, and b severely deformed material

2.3.3 Erosive Wear

The erosion of solid surfaces can occur due to high impingement of fine particles (in solid state) or droplets moving at high speed. The mechanism of material removal owing to erosion can vary appreciably depending upon the speed, angle of impingement, size and shape of particles, phase of particle, that is, liquid or solid, and mechanical properties of the surface in consideration (Fig. 2.13). These parameters dictate the mechanisms of material loss from the surface such as shearing and cutting (like abrasion), plastic deformation, brittle fracture and fatigue, super-plastic flow or melting. According to the prevailing erosion mechanisms, the hardness, toughness, fracture toughness, and fatigue resistance of material play a predominant role in determining the life of components.
Fig. 2.13

Schematic of the erosion of surface and important variables

In erosive wear, the kinetic energy of particle at the time of impact and surface characteristics play a major role in controlling the operational mechanism of material loss. The kinetic energy of the particles during the time of impact is found to be the function of mass of particle (so size) and its velocity. In general, an increase of kinetic energy of impacting particle increases rate of erosion.

Effect of angle

Fine solid particles (about 20 μm) moving at a high speed (about 100 m/s) impinge at a very low angle at the surface remove material by abrasion like mechanisms which involve plowing, shearing, and cutting, etc. These mechanisms can also be termed as a ductile mode of erosive wear. Shearing and cutting will occur only if some indentation is caused by the impinged particles because of the kinetic energy during the time of impact. A reduction in angle of impact reduces this energy, and so the related indentation is also decreased which in turn lowers the erosion (Fig. 2.14). Particles impinging at a high angle produce high kinetic energy during the time of impact (maximum at 90°), which results in the loss of material from the surfaces of tough metals through the combination of plastic deformation and fatigue, whereas in case of high strength and low ductility metals loss of material takes place through brittle fracture. For brittle metals, the maximum impact energy of particles at 90° causes the highest erosion rate and the minimum erosion is observed for those impacting at a low angle. For ductile metals, maximum erosive wear is observed at about 30° of the angle of impingement.
Fig. 2.14

Schematic showing the effect of angle incidence on a indentation and b wear rate during erosion

Particle size

Atomic-scale size particles (ions and atoms) moving at very high speed cause surface damage by removing the atoms from the lattice, while in the case of large-size particles (more than 50 μm), melting and super-plastic flow predominantly determine the erosive wear rate. At a moderate speed, the small-sized particles (about 10 μm) having low kinetic energy at the time of impact result in the deformation and work hardening of thin subsurface layer, which in turn makes it crack-sensitive and prone to removal, while the large-size particles make larger volume of subsurface layer deform up to a greater depth that is sensitive to cracking and erosion. Therefore, fine particles encourage the ductile mode (owing to fact that the limited volume of subsurface layer is subjected to lesser plastic deformation), while the large particles promote the brittle mode of erosion. High kinetic energy of large particles moving at a very high speed generates excessive heat at the time of impact which results in the melting of a thin surface layer. Part of this molten layer is lost (with particles or splashing sidewise) from the surface, and the rest of it re-solidifies on the surface.

Substrate materials

Material properties—hardness, toughness, and ductility—predominately affect the operational erosion mechanism which in turn affects the erosion rate (Fig. 2.15). Hard, high strength, and low ductility of materials resist indentation which in turn affect the removal of material by erosion. The particles impinging at low angle to the surface cause erosion through plowing and cutting mechanism, where the first indentation is a prerequisite so any phenomenon or modification (such as alloying, overlaying, work hardening) increasing the surface hardness would reduce the wear by erosion. Erosion by solid particle impinging at high angle is predominantly caused by brittle mode (through crack nucleation and growth mechanism). Any mechanism or factor enhancing the fracture toughness or toughness of surface layer (in case of high ductility and low yield strength metals) would increase resistance of the brittle fracture and high angle erosion, as the increasing hardness beyond a certain level would only deteriorate the erosion resistance. Therefore, increase in toughness with reasonable level of hardness lowers erosion by particles impacting at high angle.
Fig. 2.15

Schematic erosive wear mechanisms: a cutting, b plowing, and c brittle fracture

Particle shape

Particle with sharp edges and corners helps in erosion by cutting and shearing, while spherical particle lowers the erosion wear. Erosion by particles in a liquid state is found to be very sensitive to particle velocity, and erosion is found to be a proportional to the fifth power of velocity.

Melting wear

The development of abnormal and extreme sliding conditions such as excessive normal load, sliding speed, and failure of  lubrication leads to the generation of frictional heat at sliding interface. The rise in temperature results in the melting of any of the mating parts which cause melting of thin surface layers (up to few micrometers depth). Increase in the surface temperature affects the wear and friction behavior in two ways: (a) thermal softening and (b) interaction of metallic surface with the surrounding atmospheric gases. The influence of both on the wear and friction characteristics with rise in surface temperature is found to be opposite. Thermal softening with limits (before melting) facilitates the plastic deformation of surface asperities, so as to increase the metal-to-metal contact area which in turn tends to increase both friction coefficient and wear. However, the interaction of surface layer with surrounding gases (oxygen, nitrogen, etc.) at high temperature results in the formation of oxides and nitrides in accordance with the affinity of surface metal with gases present. The formation of oxides layer at the surface generally performs two functions: (a) reducing the direct metal-to-metal contact between the interacting surfaces, thereby reducing the friction coefficient and wear rate by adhesion and (b) developing the hard protective mechanical mixed surface layers which is generally harder than subsurface layer (Fig. 2.16).
Fig. 2.16

Schematic of the surface erosion by melting wear

The mild increase in interfacial temperature under designed or service low load, sliding speed, and lubrication condition results in mild oxidative wear conditions, whereas the abrupt rise in interfacial temperature (beyond certain critical level) due to accidental increase in load, speed, and lubrication failure leads to localized melting of near-surface layer up to the depth of few micrometers. The presence of thin films of molten layer at the sliding interface decreases the forces required for maintaining the relative motion (owing to poor/negligible interface bonding), which in turn decreases the friction coefficient. However, the peeling of molten metal layer from the interface under the influence of normal load leads to rapid increase in the rate of material loss from the interacting surfaces (especially one having low melting point and resistance to thermal softening). The loss of material from the functional surfaces takes place in the form of big metallic wear particles. This increase in wear rate under melting wear conditions can be 10–100 folds or even greater than that of mild oxidative wear conditions.

2.3.4 Corrosion Wear

The loss of material from the functional surface owing to interaction (in form of chemical reaction) between surface and fluid present around it is called corrosion wear. The nature of reaction product on the surface in the form of oxide, nitrides, etc., determines the rate of material loss (wear) from the surface of metals. A coherent, adherent, and non-porous reaction product in the form of oxides (e.g., aluminum oxide, chromium oxide) results in significantly low rate of wear because these oxides do not allow the access of oxygen in the air to react with the metal below the oxide layer, thus preventing or reducing further oxidation or corrosion. However, the formation of porous, loosely held, and non-coherent reaction products (e.g., iron oxide in the form of rust) formed by the interaction of metal with corrosive fluid results in high rate of material loss from the functional surfaces through corrosion, as these products of reaction allow the access of corrosive media to the metal surface apart from getting dislodged easily from the surface (Fig. 2.17). Any activity such as the interaction of affected metal surface (having corrosion reaction product) with other components, bodies, abrasives, cavitation, fluids, etc., that can remove the reaction products formed on the functional surface will expose the fresh metal surface to corrosive media for further chemical reaction or attack on the surface, which in turn will eventually increase the material loss from the surface by corrosion. For example, the adhesive or abrasive wear in the corrosive environment will have synergic effect of material loss from the functional surfaces by causing material loss through both adhesion or abrasion and corrosion. Wear rate under such condition is found to be many times greater than the combined wear by both the mechanisms, individually. Metallic components working in the corrosion environments like marine, mining, petrochemical environments are found sensitive to such kind of synergic effect of wear and corrosion.
Fig. 2.17

Photograph of corroded surface of FSW joints of Al alloys in 5% NaCl solution

2.3.5 Diffusive Wear

High normal load and sliding speed conditions lead to direct and intimate metal-to-metal contact between the interacting surfaces (like in seizure conditions) which results in the atomic movement (diffusion) of alloying elements from one region or body or surface to another depending upon the presence of driving force in the form of concentration gradient of alloying elements or energy level. However, there are few requisites for the occurrence of any influential diffusion of alloying element from one body or region to another, and these requisites are (a) perfect metallic intimacy, that is, the absence of impurities or oxides layers between the interacting surfaces, (b) high temperature which is enough to have reasonably good diffusion coefficient, and (c) contact duration between interacting surfaces must be long enough for diffusion. The relative movement between mating components under abnormal or normal load and sliding speed can develop such conditions at the interface leading to the diffusion of alloying elements. The diffusion of alloying elements can create areas or zones in the interacting surfaces having unfavorable enrichment or depletion of alloying elements which in turn can deteriorate their mechanical, corrosive, and tribological properties. Wear of metal cutting tool during machining is a typical example of diffusion wear.

It is well known that during metal cutting, peak temperature is generated near the cutting edge (at some distance away from the cutting edge) on the rake face of the cutting tool against which chips sliding under high pressure create a direct metal-to-metal contact (Fig. 2.18a). The presence of high temperature and metal-to-metal contact between the rake face of tool and underside surface of chip facilitates diffusion, especially in the absence of proper cooling. Diffusion wear significantly reduces the life of tools that are made of common materials like high speed steel, tungsten carbide-cobalt, polycrystalline diamond primarily during the machining of carbon steel under the unfavorable machining conditions (cutting speed beyond recommended or specified level and use of excessively high feed rate and depth of cut) lead to high heat (interface temperature) generation and pressure at the cutting edge to facilitate diffusion of alloying elements from the tool surface and subsurface layer to sliding metal chips. The loss of alloying elements (W, Cr, C, etc.) from the rake/flank face of high-speed steel tool and the carbon from the surface of polycrystalline diamond tool during machining of steel reduces high temperature stability and resistance to thermal softening (Fig. 2.18b). Softening of surface and near-surface layers of cutting tool causes the loss of material from the face of cutting tool by adhesion, as cut metal chips slid over the face of the tool under pressure.
Fig. 2.18

Schematic of diffusive wear of a tool during metal cutting and b diffusion of elements from cutting tool

2.4 Techniques to Evaluate Damage of Wear Surfaces

Uncontrolled and progressive loss of material from the surfaces of mating components having sliding or rolling contacts influences the properties of surface and subsurface layers with respect to the roughness, composition, microstructure, mechanical, and corrosion properties. However, the extent of influence of the above characteristics depends on wear mechanism which is responsible for surface damage. For example, the adhesive and surface fatigue wear results in changes in the form of weight loss (dimensional changes), roughening of surface, work hardening of surface and subsurface layer, severe straining and microstructural modification of near-surface layers, formation of mechanical mixed layer (having oxides of element of both sliding components). The quantification related to the wear surface damage is mainly carried out to establish (a) the length of time necessary for the occurrence of changes in the sliding components in terms of dimensional variation and surface roughness that can lead to failure, (b) the steps need to be taken at the design stage of component for reducing the rate of surface degradation so as to enhance the useful service time, and (c) the development of strategy for reclamation or refurbishing of worn-out component.

The characterization of each type of features of surface damage due to wear needs separate and specialized approach. The most common methods of studying the wear consist of the examination of sliding material before and after the test; any difference in material is attributed to wear. The detection of wear generally uses one or the other techniques of weighing, mechanical gauging, and examination of surface and subsurface features and wear debris.

2.4.1 Material Loss

Amount of the material lost from the wear surface is quantified using the three common approaches that are (a) weight loss, (b) height or depth measurement, and (c) optical method. The last method is employed when the first two methods cannot be applied in a situation.

2.4.2 Weighing

This is the simplest way of detecting the wear in which specimen is weighed before and after running test by using sensitive weighing balance (accurate > 0.1 mg), and weight loss is calculated to get wear rate.

2.4.3 Dimensional Measurement

In this method, the wear is measured through the decrease in dimensions using mechanical (dial gauge) or electrical system based on the linear variable displacement transducer (LVDT) principle. This has resolution limit of about 10−5 m.

2.4.4 Optical Method

There are a number of methods for measuring wear using the optical technique. One way is to make small microhardness indentation on a surface and to study how its size is reduced during sliding. The horizontal limit of resolution of this method is about 10−5 m.

2.4.5 Hardness of Surface and Subsurface Layers

Vickers microhardness test is generally carried out using low load (say 100 g) for obtaining hardness profile from the surface to subsurface regions. It indicates the depth up to which the deformation has taken place during wear.

2.4.6 Chemical Composition and Phase Analysis

Spectroscopy and X-ray diffraction analysis are commonly performed to establish the properties with respect to the chemical composition and phases present on the surface generated (e.g., mechanical mixed layer) during service. The presence of elements and their compounds in the form of oxides, nitrides, and chlorides from both interacting components confirms the formation of mechanical mixed layer. The presence of surface compound further confirms the existence of particular kind of environments during service. For example, the formation of Fe2O3 and Fe3O4 indicates the occurrence of corrosion of iron in the presence of oxygen in moist environment. This information can also be used in the failure analysis for establishing the root causes of failure which in turn can be used to avoid failure from that particular mechanism. The formation of hard, strong, and coherent protective mechanical mixed surface layer is considered to be good from the point of view of wear and friction resistance.

2.4.7 Surface Roughness

The roughness of surface subjected to wear is very dynamic characteristics and provides enough information for drawing inference regarding the mode of wear and operating wear mechanisms. However, surfaces subjected to different wear conditions like adhesion, abrasion, erosion, cavitation, corrosion exhibit significant variation with respect to the surface morphology. Typical surface morphologies and their features produced under different wear conditions are shown in Fig. 2.19a–d. Adhesive wear refers to the presence of oxides, and the scoring marks in the direction of sliding besides the presence of cracks observed at high magnification on the wear surface suggest the occurrence of mild oxidation wear, whereas bright shining surface with heavy plastic deformation of grooves and ridges along with the development of large metallic debris suggests the occurrence of severe metallic wear. Severe metallic wear becomes 10–100 times greater than mild oxidative wear. Abrasive wear shows deep and wide abrasive and scratch marks on the surface which suggest the occurrence of high rate of material removal. Erosion refers to cavitation, slurry, and solid particle erosion of the subjected surface that commonly reveals the presence of surface, pits, cracks, and craters. The depth of craters determines the wear rate by these processes.
Fig. 2.19

SEM micrographs showing typical surface features of samples subjected to a abrasive wear, b adhesive wear, c slurry erosion, and d cavitation erosion

Questions for self-assessment

  1. 1.

    How can performance of an engineering component be related with service life?

     
  2. 2.

    Describe common causes of deterioration in performance of material.

     
  3. 3.

    What are the common types of wear experienced by metals?

     
  4. 4.

    Explain the mechanisms of adhesive wear.

     
  5. 5.

    Describe factors affecting adhesive wear.

     
  6. 6.

    How do service conditions affect the adhesive wear?

     
  7. 7.

    Explain classical law of adhesive wear.

     
  8. 8.

    What is abrasive wear? Describe mechanisms of abrasive wear.

     
  9. 9.

    Explain the factors affecting abrasive wear of metals.

     
  10. 10.

    How do material properties affect the erosive wear?

     
  11. 11.

    Explain mechanism of erosive wear.

     
  12. 12.

    Describe the factors affecting erosive wear of metals.

     
  13. 13.

    Explain melting wear with the help of schematic diagram.

     
  14. 14.

    What is diffusive wear? Explain the mechanism of diffusive wear observed in cutting tool.

     
  15. 15.

    Describe method used to measure the wear.

     
  16. 16.

    What is significance of surface and subsurface studies of worn-out samples?

     
  17. 17.

    How can effect of wear on surface and subsurface of worn-out sample be studied?

     

Copyright information

© Springer (India) Pvt. Ltd., part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Mechanical and Industrial EngineeringIndian Institute of Technology RoorkeeRoorkeeIndia

Personalised recommendations