Surface Damage: Causes and Mechanisms
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
2.2 Common Factors Leading to the Deterioration of Surfaces
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
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.
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.
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.
Relative motion between wear sample and 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.
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
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
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.
In case when hardness of abrasives is significantly higher than surface, abrasive wear varies linearly with the size of abrasive particles.
2.3.3 Erosive Wear
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
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.
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.
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
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.
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.
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
Questions for self-assessment
How can performance of an engineering component be related with service life?
Describe common causes of deterioration in performance of material.
What are the common types of wear experienced by metals?
Explain the mechanisms of adhesive wear.
Describe factors affecting adhesive wear.
How do service conditions affect the adhesive wear?
Explain classical law of adhesive wear.
What is abrasive wear? Describe mechanisms of abrasive wear.
Explain the factors affecting abrasive wear of metals.
How do material properties affect the erosive wear?
Explain mechanism of erosive wear.
Describe the factors affecting erosive wear of metals.
Explain melting wear with the help of schematic diagram.
What is diffusive wear? Explain the mechanism of diffusive wear observed in cutting tool.
Describe method used to measure the wear.
What is significance of surface and subsurface studies of worn-out samples?
How can effect of wear on surface and subsurface of worn-out sample be studied?