**7. Material properties, which influence surface interactions in a triboelectrochemical contact**

has open up the opportunity for alloy design and innovations in new bulk metallic glass (BMG) materials to withstand deformation and flexibility that cannot be achieved by traditional metals, or casual BMGs making them attractive for various tribological systems (e.g.,

The ultimate resolution combining selection and material design is a consensus between economical parameters and technical qualification. Generally, this requires three essential steps,

**2.** The choice of materials assigned for the function of interest, including, inter alia, mechani-

This third step in the choice of a material emphasizes the problem of economic compromise. It is thus brought to compare investment costs (resistant but expensive material) and operating costs (costs of failures, replacements, and stops they may cause). The relative weight of these two types of cost has slowly changed. The current trend is often to prefer high but predictable investment costs and to minimize operating costs when these are too difficult to predict. In addition, in a competitive economy, short-term cash optimization is increasingly being replaced by optimizing long-term profitability. In turn, this may well favor investment, especially in metallic glass alloys whose resistance to mechanical and chemical constraints

In what follows, we will particularly focus on the second step above, namely the behavioral of BMG materials with regard to their wear and corrosion testing, and in particular to their

Actually, despite the cost for developing new BMGs of desirable types and compositions, the examination of their appealing properties remains a persistent issue. Although, wear and corrosion are an important topic, they have never received the attention they deserve. Many tribological and electrochemical aspects of BMGs have not been characterized yet or are not well assumed, and the actual deformation and failure mechanisms are not fully understood. Albeit, localized corrosion, fatigue, wear, and fracture all have been reported in almost every BMG corrosion or mechanical study published to date (see next chapter). Though, theoretically speaking, this fact is unexpected for an ideally homogeneous material. Therefore, an overview of the material-oriented approach to resist the mutual and opposite interactions between the main actors of tribocorrosion involving both mechanical (friction, wear), and

**6. Towards the needs in tribological and electrochemical testing to meet the requirements of effective usage of metallic glass materials**

journal bearings), and mechanical engineering applications.

**1.** Identification of the requirements (e.g., design considerations);

namely:

cal and chemical risk factors;

94 Metallic Glasses - Properties and Processing

(i.e., to tribocorrosion) is optimal.

mutual coupling effect (tribocorrosion).

electrochemical (corrosion) phenomena is given below.

**3.** The election of the least expensive material.

Fundamentally, tribocorrosion depends on the dominating deterioration mechanism of interacting surfaces in chemical environment and under relative motion conditions [10, 55]: *viz.* wear, corrosion, and their mutual interaction (synergism).

Modern research has established a consensus on four main forms of wear, namely, chemical wear (i.e., corrosion and corrosive wear), adhesive wear, abrasive wear or surface fatigue wear [56]. Each process of wear obeys its own laws and, to confuse things, repeatedly one of the modes of wear acts in such a way as to affect the others, hence of the complexity of wear. Typically, there is a combination of wear mechanisms in a mechanochemical dynamic contact. In that respect, the classification of wear mechanisms remains a matter of debate among the scientific community of researchers and authors. Albeit, the terminology used by Burwell in 1957 [56] to describe wear is simple and rational, that of seeking out the primary cause of each form of wear. To avoid any further issue regarding the nomenclature in this field study, all wear mechanisms should be referenced to the ASM standards [57, 58].

Other forms of wear can be found in the literature depending upon the contact configuration (e.g. unidirectional and reciprocal sliding and/or rolling, rolling with slip, etc.). Wear in these contact geometries is reported to as erosion wear, fretting wear, sliding or rolling wear, impact or slurry wear, etc. This is one of the approaches that judges wear by the consequences of the conditions of a tribological contact *vis-a-vis* its environment (e.g., reactivity of tribosurfaces with the environment, the contact system configuration, etc.). Such wear descriptions are all-technical and do not represent wear mechanisms in a scientific manner.

According to the recent critical review on the quantification of wear made by Meng and Ludema [55], several models have been proposed to explain various phenomena of wear. The authors have clearly enumerated 182 equations with 625 variables for explaining wear processes. This clearly shows that wear is not a material property, but rather a material system response. Wear can change drastically even as a result of a relatively small change in dynamical, environmental or material parameters forming the tribosystem. Indeed, wear rates change promptly (10−15 up to 10−1 mm<sup>3</sup> .N−1.m−1) depending upon the conditions in which the materials are exposed to (tribological system, corrosive medium, loading contact parameters, etc.) and the choice of these materials [26, 59–61]. The combination of these two main factors, namely the operating conditions and the choice of materials, are the primary keys for monitoring the wear of materials (modes and rates) exposed to normal working conditions. Optimal solutions have been recommended as a means of meeting these requirements, are the wear maps that predict both modes and rates of wear of materials [8, 62]. A wear map or chart can be considered as one of the best descriptions of tribological/tribocorrosion conditions and as useful strategy in the design of mechanical systems (tribosystems) and for the selection of materials to be used in a wide range of operating conditions.

The wear volume loss measured during or after the end of an operating tribological test provides useful information in characterizing wear. Generally, there are three typical types of

Therefore, there are several aspects of corrosion: the material, the environment, and the material properties. Considerable information is available in textbooks [9, 45, 65]. This general definition of corrosion includes the physicochemical (oxidation/reduction) reaction processes taking place at the surface of widely varying material classes (metallic, ceramic and organic), such as localized corrosion cells in some Fe-based metallic glass ribbons used in waste water treatment, the degradation of dental amalgams or metallic restorations by galvanic corrosion, polymers by ultraviolet radiation, and the chemical attack of refractory bricks during

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The severity of a corrosive medium depends on a variety of parameters. Nevertheless, the following four main parameters can generally be selected, namely the pH of the medium, the presence of chlorides (and other alkyl halides), the oxidizing power, and the temperature. The corrosion resistance properties can then be characterized directly by the limits of use of the materials, which can be expressed, for example, in terms of maximum temperature in-service or maximum concentration of use. In real service and under normal conditions, the understanding and control of corrosion are based on the electrochemical interpretation of corrosion phenomena and the consideration of the relative ranking scales of materials in order to select, by successive approaches,

Since these materials may interact in a complex way with the environment that they are used for, the assessment of the reactivity of their surfaces with respect to their environment, and the evolution of that reactivity with time greatly accounts for their corrosion performance. Usually, certain metallic materials gain their resistance from the formation of a thin, yet dense and adhesive oxide layer, which protects the underlying substrate from further oxidation (a passive layer). Thus, the potential at which this layer is growth and the rate at which it is built (passivity) and rebuilt after being mechanically or chemically damaged (repassivation) are

On a theoretical level, passivity concerns, a priori, most metals (and alloys). However, only a small number of those metals actually allows a systematic usage of this property. Conversely, some metals resist to corrosion without any appealing to passivity. Some of them are chemically inert, or not very reactive; this is the case for example for noble metals, such as gold, platinum and iridium, which are not prone to oxidation due to their high standard electrode potential, and are hardly used in technical applications because of their excessive price and often poor mechanical properties, or even copper and nickel in non-oxidizing acids. Other metals are roughly covered with a protective layer of corrosion products; this is the case of steels exposed to atmospheric weather, copper alloys in natural waters, lead in sulfuric acid, or even, occasionally, the most ordinary steel in the presence

In total, metals and engineering alloys, including metallic glasses, actually used for their passivity are limited to Fe, Co, Nb, and Ta, stainless steels, alloys of Al, of Ni, of Ti, and of Zr, etc. It should be noted that some metals are less passivable than others are; especially this is the case of Fe, and Co, which do not reveal any sufficient passivation or oxidation. Not all oxide layers that form on metals are protective. If the oxide does not form a continuous layer on the surface of the metal, it will not be able to reduce the amount of oxygen reaching the metal

the materials best suited to each application of interest.

, or H<sup>2</sup> S.

decisive for its corrosion resistance.

of acidic gas medium, CO<sup>2</sup>

steel-making.

**Figure 5.** Three typical wear curves in repeated sliding contacts.

wear volume curves as shown in **Figure 5**. Type I shows a constant rate of wear throughout the process (likely one ideally mechanism monitoring wear). Type II shows the transition from an initially high wear rate to a steady-state low wear rate. This type of wear behavior is often observed in metals and metallic alloys [63]. Type III shows a fatal transition taking place from an initial low wear rate up to a high wear rate (e.g. fatigue wear by fracture mode). This is frequently encountered when using ceramic materials [64]. The total number of sliding contacts, before any catastrophic wear can occur, reflects the period for which the crack initiation takes place, and this latter depends on a number of material and system parameters, mainly surface roughness, material properties, and sliding conditions.

When considering the tribocontact system as a whole, including the entire design configuration and components, and specific for a given application either in laboratory or in the field, it is not possible to increase the strength of the solid material *vis-a-vis* one of the wear mechanisms in general. Nevertheless, the selection of suitable materials should always be established based on a more knowledge and a better understanding of the conditions in which the solid material is subjected to (e.g., surface interactions in aggressive medium, and loadings).

In the following sections, an overview of the desired material properties and trends towards how wear and corrosion resistance can be achieved is given.

#### **7.1. Materials oriented approach to act in opposition to corrosion and corrosive wear**

#### *7.1.1. Corrosion*

Corrosion may be defined as "…an irreversible reaction of a material with the environment, which usually (but not always) results in a degradation of the material or its properties…" Therefore, there are several aspects of corrosion: the material, the environment, and the material properties. Considerable information is available in textbooks [9, 45, 65]. This general definition of corrosion includes the physicochemical (oxidation/reduction) reaction processes taking place at the surface of widely varying material classes (metallic, ceramic and organic), such as localized corrosion cells in some Fe-based metallic glass ribbons used in waste water treatment, the degradation of dental amalgams or metallic restorations by galvanic corrosion, polymers by ultraviolet radiation, and the chemical attack of refractory bricks during steel-making.

The severity of a corrosive medium depends on a variety of parameters. Nevertheless, the following four main parameters can generally be selected, namely the pH of the medium, the presence of chlorides (and other alkyl halides), the oxidizing power, and the temperature. The corrosion resistance properties can then be characterized directly by the limits of use of the materials, which can be expressed, for example, in terms of maximum temperature in-service or maximum concentration of use. In real service and under normal conditions, the understanding and control of corrosion are based on the electrochemical interpretation of corrosion phenomena and the consideration of the relative ranking scales of materials in order to select, by successive approaches, the materials best suited to each application of interest.

Since these materials may interact in a complex way with the environment that they are used for, the assessment of the reactivity of their surfaces with respect to their environment, and the evolution of that reactivity with time greatly accounts for their corrosion performance. Usually, certain metallic materials gain their resistance from the formation of a thin, yet dense and adhesive oxide layer, which protects the underlying substrate from further oxidation (a passive layer). Thus, the potential at which this layer is growth and the rate at which it is built (passivity) and rebuilt after being mechanically or chemically damaged (repassivation) are decisive for its corrosion resistance.

wear volume curves as shown in **Figure 5**. Type I shows a constant rate of wear throughout the process (likely one ideally mechanism monitoring wear). Type II shows the transition from an initially high wear rate to a steady-state low wear rate. This type of wear behavior is often observed in metals and metallic alloys [63]. Type III shows a fatal transition taking place from an initial low wear rate up to a high wear rate (e.g. fatigue wear by fracture mode). This is frequently encountered when using ceramic materials [64]. The total number of sliding contacts, before any catastrophic wear can occur, reflects the period for which the crack initiation takes place, and this latter depends on a number of material and system parameters,

When considering the tribocontact system as a whole, including the entire design configuration and components, and specific for a given application either in laboratory or in the field, it is not possible to increase the strength of the solid material *vis-a-vis* one of the wear mechanisms in general. Nevertheless, the selection of suitable materials should always be established based on a more knowledge and a better understanding of the conditions in which the solid material is subjected to (e.g., surface interactions in aggressive medium, and loadings). In the following sections, an overview of the desired material properties and trends towards

**7.1. Materials oriented approach to act in opposition to corrosion and corrosive wear**

Corrosion may be defined as "…an irreversible reaction of a material with the environment, which usually (but not always) results in a degradation of the material or its properties…"

mainly surface roughness, material properties, and sliding conditions.

**Figure 5.** Three typical wear curves in repeated sliding contacts.

96 Metallic Glasses - Properties and Processing

how wear and corrosion resistance can be achieved is given.

*7.1.1. Corrosion*

On a theoretical level, passivity concerns, a priori, most metals (and alloys). However, only a small number of those metals actually allows a systematic usage of this property. Conversely, some metals resist to corrosion without any appealing to passivity. Some of them are chemically inert, or not very reactive; this is the case for example for noble metals, such as gold, platinum and iridium, which are not prone to oxidation due to their high standard electrode potential, and are hardly used in technical applications because of their excessive price and often poor mechanical properties, or even copper and nickel in non-oxidizing acids. Other metals are roughly covered with a protective layer of corrosion products; this is the case of steels exposed to atmospheric weather, copper alloys in natural waters, lead in sulfuric acid, or even, occasionally, the most ordinary steel in the presence of acidic gas medium, CO<sup>2</sup> , or H<sup>2</sup> S.

In total, metals and engineering alloys, including metallic glasses, actually used for their passivity are limited to Fe, Co, Nb, and Ta, stainless steels, alloys of Al, of Ni, of Ti, and of Zr, etc. It should be noted that some metals are less passivable than others are; especially this is the case of Fe, and Co, which do not reveal any sufficient passivation or oxidation. Not all oxide layers that form on metals are protective. If the oxide does not form a continuous layer on the surface of the metal, it will not be able to reduce the amount of oxygen reaching the metal surface, and thereby increasing the brittleness of the layer leading to further corrosion (*cfr.* Section 3.5).

takes place, which often results in a growth of the real contact area as the load increases [32]. This, in turn, leads to increasing the area for which the distribution of bonding and attraction forces may occur by forming an interfacial junction area along the contact, thus resulting in a strong adherence and promoting surface welding at a solid-state (*viz.* adhesion). The removal of the load breaks off most of the junctions, as a result of elastic spring-back. At this point, the chemical, plastic deformation and wear become clear in light of the formation of a series of

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The presence of adsorbed films containing water and other molecules derived from the air serves measurably to reduce the surface interaction of contacting materials. The effect of grease films, if present, however, is even more marked, and reduces, often by one or more orders of magnitude, the severity of surface interaction, and thus friction too. Clean and smooth contacting metallic surfaces are more prone to promote higher attraction than contaminated surfaces (e.g., oxides, corrosion products) and thereby increasing both the adhesion and the static frictional strength of interfaces (i.e. a certain minimum tangential force is

Most non-metals essentially have the same chemical composition at the surfaces as they do within the interior. A handful of metals and alloys will form surface oxide films in air, however, and in other environments they are likely to do, for other films (e.g., nitrides, sulfides,

Under sliding conditions, the first stage involves the removal of the thin surface film (e.g. oxide layer) covering the surface of the material through a mechanical wear process. As the oxide layer is degraded after the first few sliding cycles, the distance between the two surfaces becomes in the order of the interatomic distance of the metal lattice, and metallic bonds are thus established (*viz.* direct adhesive bonding contact) between the two interacting surfaces (e.g., counter-body and underlying bare surfaces), thereby leading the contact surfaces to starting adherence by forming "microwelds" or "cold welds". When the adhesive bonding strength resists the relative sliding motion, large plastic deformation caused by dislocation is produced in the contact zone under the effect of compression and tangential shearing. The generation of slips along slip planes in the contact zone entails the formation of flake-like shear tongues and/ or wedge-like shape, and are followed by a crack initiation and propagation in the combined fracture mode of tensile and shear in the contact zone area. Currently, the real actual contact area is made up of all areas of asperities welded to the surfaces and upon tangential sliding/ shearing, the crack reached in the contact interface causes a mass separation of the asperities in the underlying softer material rather than in the interface, and therefore a wear particle from the

The adhesive wear process is responsible for the initial accumulation of wear debris between the contact surfaces. Additional sliding cycles bring near-surface plastic deformation, new bonds often prevail and fracture occurs at some distance to the surface, additional wear and

Under tribocorrosion conditions, the surface roughness, as well as the oxide film growth or chemical corrosion layers adsorbed on top of the surfaces, normally hinder the direct metal

grain-sized microwelds, microcracks, and material transfer.

bulk material is formed and eventually transferred to other surface.

the potential formation of new oxide [7, 8, 10–13, 16, 32].

required to produce motion).

and chlorides).

### *7.1.2. Corrosive wear*

A more obvious mechanism of triboelectrochemistry is the periodic exposure of fresh bare surfaces when sliding friction between surfaces occurs in corrosive liquids or gases. This results in reaction products mainly driven by chemical and electrochemical interactions. The surfaces of the materials are quickly covered by a scale of the reaction product, the oxide in the case of metals and metallic alloys, acting as a protective barrier layer. The thinner the scale, the faster the reaction, and the weaker the protectiveness.

In the case where these reaction products strongly adhere to the surface and behave as the bulk material, the wear mechanism should be almost the same as that of the bulk material. Otherwise, as observed in many cases in practice, the reaction products behave rather differently compared to that of the bulk material. The resulting wear is therefore very different from that of the bulk material, and it is thus controlled by the scales of reaction products (i.e. tribochemical reactions). In corrosive media, the tribochemical reaction at the contact interface is accelerated by the friction processes (*viz.* elastoplastic deformation, heating, micro-fracture, and successive removal of these scales of reaction products). In the case of metals, friction can cause extensive plastic deformation of a subsurface layer in the material; whereas in the case of ceramics, microfracture predominantly occurs. These strains lead to structural defects (microcracks, grain boundaries, vacancies, dislocations, etc.), thereby accelerating the diffusion of reagents through the protective scale. This results in the acceleration of the chemical reactions, which leads to a material removal from the contact interface. The resultant wear is called "corrosive wear". In air, oxygen prevails as a corrosive medium, and tribochemical wear of metals in air is usually called "oxidational wear". More detail about this type of wear can be found elsewhere [29]. The material removal rate in corrosive wear is governed by the balance between the relative growth rate and the removal rate, which determines the wear rate of the reaction layers.

It is worthwhile to note that in the absence of chemical reactions, the sliding surfaces experience mechanical wear, while in the case of the absence of sliding friction, the surfaces experience corrosion degradation.

### **7.2. Materials oriented approach to act in opposition to adhesive wear**

Adhesive wear is the most common form of wear that exists when one solid surface material is slid over the surface of another (e.g. tribological/tribocorrosion contacts) or is pressed against it (e.g. loaded surfaces under bending conditions, fretting mode II, etc.). The removal of material takes the form of small particles which are usually transferred to the other surface, but which may come off in loose form [27].

In loaded or pressed tools, the tendency of contacting surfaces to adhere arises from the attraction forces, which exist, between the surface atoms of the two materials in intimate contact. In a presumed elastic contact, elastoplastic deformation of the asperities in contact takes place, which often results in a growth of the real contact area as the load increases [32]. This, in turn, leads to increasing the area for which the distribution of bonding and attraction forces may occur by forming an interfacial junction area along the contact, thus resulting in a strong adherence and promoting surface welding at a solid-state (*viz.* adhesion). The removal of the load breaks off most of the junctions, as a result of elastic spring-back. At this point, the chemical, plastic deformation and wear become clear in light of the formation of a series of grain-sized microwelds, microcracks, and material transfer.

surface, and thereby increasing the brittleness of the layer leading to further corrosion (*cfr.*

A more obvious mechanism of triboelectrochemistry is the periodic exposure of fresh bare surfaces when sliding friction between surfaces occurs in corrosive liquids or gases. This results in reaction products mainly driven by chemical and electrochemical interactions. The surfaces of the materials are quickly covered by a scale of the reaction product, the oxide in the case of metals and metallic alloys, acting as a protective barrier layer. The thinner the

In the case where these reaction products strongly adhere to the surface and behave as the bulk material, the wear mechanism should be almost the same as that of the bulk material. Otherwise, as observed in many cases in practice, the reaction products behave rather differently compared to that of the bulk material. The resulting wear is therefore very different from that of the bulk material, and it is thus controlled by the scales of reaction products (i.e. tribochemical reactions). In corrosive media, the tribochemical reaction at the contact interface is accelerated by the friction processes (*viz.* elastoplastic deformation, heating, micro-fracture, and successive removal of these scales of reaction products). In the case of metals, friction can cause extensive plastic deformation of a subsurface layer in the material; whereas in the case of ceramics, microfracture predominantly occurs. These strains lead to structural defects (microcracks, grain boundaries, vacancies, dislocations, etc.), thereby accelerating the diffusion of reagents through the protective scale. This results in the acceleration of the chemical reactions, which leads to a material removal from the contact interface. The resultant wear is called "corrosive wear". In air, oxygen prevails as a corrosive medium, and tribochemical wear of metals in air is usually called "oxidational wear". More detail about this type of wear can be found elsewhere [29]. The material removal rate in corrosive wear is governed by the balance between the relative growth rate and the removal rate, which determines the wear

It is worthwhile to note that in the absence of chemical reactions, the sliding surfaces experience mechanical wear, while in the case of the absence of sliding friction, the surfaces experi-

Adhesive wear is the most common form of wear that exists when one solid surface material is slid over the surface of another (e.g. tribological/tribocorrosion contacts) or is pressed against it (e.g. loaded surfaces under bending conditions, fretting mode II, etc.). The removal of material takes the form of small particles which are usually transferred to the other surface,

In loaded or pressed tools, the tendency of contacting surfaces to adhere arises from the attraction forces, which exist, between the surface atoms of the two materials in intimate contact. In a presumed elastic contact, elastoplastic deformation of the asperities in contact

**7.2. Materials oriented approach to act in opposition to adhesive wear**

scale, the faster the reaction, and the weaker the protectiveness.

Section 3.5).

*7.1.2. Corrosive wear*

98 Metallic Glasses - Properties and Processing

rate of the reaction layers.

ence corrosion degradation.

but which may come off in loose form [27].

The presence of adsorbed films containing water and other molecules derived from the air serves measurably to reduce the surface interaction of contacting materials. The effect of grease films, if present, however, is even more marked, and reduces, often by one or more orders of magnitude, the severity of surface interaction, and thus friction too. Clean and smooth contacting metallic surfaces are more prone to promote higher attraction than contaminated surfaces (e.g., oxides, corrosion products) and thereby increasing both the adhesion and the static frictional strength of interfaces (i.e. a certain minimum tangential force is required to produce motion).

Most non-metals essentially have the same chemical composition at the surfaces as they do within the interior. A handful of metals and alloys will form surface oxide films in air, however, and in other environments they are likely to do, for other films (e.g., nitrides, sulfides, and chlorides).

Under sliding conditions, the first stage involves the removal of the thin surface film (e.g. oxide layer) covering the surface of the material through a mechanical wear process. As the oxide layer is degraded after the first few sliding cycles, the distance between the two surfaces becomes in the order of the interatomic distance of the metal lattice, and metallic bonds are thus established (*viz.* direct adhesive bonding contact) between the two interacting surfaces (e.g., counter-body and underlying bare surfaces), thereby leading the contact surfaces to starting adherence by forming "microwelds" or "cold welds". When the adhesive bonding strength resists the relative sliding motion, large plastic deformation caused by dislocation is produced in the contact zone under the effect of compression and tangential shearing. The generation of slips along slip planes in the contact zone entails the formation of flake-like shear tongues and/ or wedge-like shape, and are followed by a crack initiation and propagation in the combined fracture mode of tensile and shear in the contact zone area. Currently, the real actual contact area is made up of all areas of asperities welded to the surfaces and upon tangential sliding/ shearing, the crack reached in the contact interface causes a mass separation of the asperities in the underlying softer material rather than in the interface, and therefore a wear particle from the bulk material is formed and eventually transferred to other surface.

The adhesive wear process is responsible for the initial accumulation of wear debris between the contact surfaces. Additional sliding cycles bring near-surface plastic deformation, new bonds often prevail and fracture occurs at some distance to the surface, additional wear and the potential formation of new oxide [7, 8, 10–13, 16, 32].

Under tribocorrosion conditions, the surface roughness, as well as the oxide film growth or chemical corrosion layers adsorbed on top of the surfaces, normally hinder the direct metal contact. However, the plastic deformation of the surface can result in the flattening of the roughness as well as the cleavage of the oxide layers. Preventing plastic deformation or solidstate attraction is therefore a feasible concept to avoid adhesive wear.

After mechanical processing, they slightly protrude from the surrounding matrix and impede

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Abrasive wear is defined as wear by displacement of material caused by hard particles or hard protuberances, which results in a significant plastic deformation of the softer surface material. Actually, this is the form, which occurs when a rough hard surface, or a soft surface containing hard particles, slides on a softer surface, and ploughs a series of grooves in it. The material from the grooves is displaced in the form of wear particles (debris) generally loose ones. When these wear particles are attached to one of the surfaces in contact, the phenomenon is called as two-body abrasion. Otherwise, it is known as three-body abrasion. The severity of abrasive wear depends on size and angularity of abrasive wear particles and the ratio between hardness of metal and the abrasive particles too [68, 69]. To be effective, either the hard particles or the harder surface must be 1.3 times harder than the softer surface material undergoing abrasion, which Hutchings [68], and Ludema [69] note is the difference of one

In a tribocorrosion process, abrasion usually occurs when interlocking of inclined or curved asperity contacts at the contact interface takes place or when harder particles are introduced into the tribosystem between the interacting solid surfaces at the contact interface (e.g., through erosion (abrasive particles) or fretting (trapped in contact), etc.). These sharp and hard particles or asperities are then pressed onto the softer surface (usually being the investigated material), causing a plastic flow of the surface. During sliding-corrosion, the softer surface undergoes ploughing, entailing the formation of scratches, and abrasive grooves, which

It is likely that diverse modes of action contribute to the mechanisms by which abrasive wear occurs and proposed models include micro-cutting, micro-chipping, and micro-fatigue (e.g., due to cyclic loading-sliding). Other models have also been emphasized. Hutchings [68] quotes three common models for the occurrence of abrasive wear *via* plastic deformation,

Note that the contact geometry in a tribological/tribocorrosion system remains, inter alia, a parameter either favoring or discouraging the manifestation of abrasion phenomena when a hard surface slides on another softer one or remained in intimate contact with it. In fretting or unidirectional reciprocating sliding conditions, for example, the use of a sphere-on-flat configuration can promote both trapping and ejection of the abrasive particles brought into contact between the two slid protagonist surfaces, whereas, in a flat-on-flat contact geometry, such abrasive particles may remain regularly trapped in the contact zone, driving various possible mechanisms as plastic flow, welding, scratches, grooves, etc. It is therefore recommended that the project designer or engineer consider the incidence that the choice of the

The abrasive wear rate is defined in the same way as for the adhesive wear. Indeed, the Archard equation (Eq. (1), see Section 3.3), which was originally formulated to model adhesive wear,

**7.3. Materials oriented approach to act in opposition to abrasion wear**

metallic contact.

unit on Mohs scale of mineral hardness.

leads to a significant material removal (e.g. as volume).

which can be categorized as cutting, ploughing and wedge-forming.

geometrical contact system may have on the wear of the contacting materials.

The role of the environment on the adhesive wear is of a great importance in tribological contacts. Under dry friction conditions, for example, often part of the energy is dissipated in the form of thermal heat, which boosts the surface reactivity and results in an increase of the temperature of the interacting surfaces. This, in turn, affects both the oxidation and the friction of the two surfaces in contact and entails material transformations (e.g. phase transformation, depletion zone, hardening, welding process, material transfer, etc.). Little part of the frictional work is done to overcome surface roughness, but most of the work being done in inducing shear displacement of the junction interface as well as the removal of bulk underlying previously welded material.

One of the interesting approaches used to explain the friction behavior of metallic oxides just below and above their melting temperature is the polarizability approach, initially introduced by Vesselin et al. [66], and later extended by Prakash et al. [67]. Thus, based on that approach, the lubricity of a wide variety of solid oxides at high temperature could be explained.

A correlation may exist between the dissipated energy due friction and the wear rate. Often a low friction induces a low dissipated energy, which may result in a low wear rate. However, this is not a universal law, and there are numerous examples showing high wear rate regardless of low friction.

Despite its omnipresence in almost all modes of degradation by sliding wear, adhesive wear is not necessarily preventable form of wear, nor is it the most dangerous since adhesive wear rates are usually fairly low. The mechanism behind this type of failure wear process is quite expected.

In general no equation can fully describe the adhesive wear process but the most widely used is the Archard equation (Eq. (1), see Section 3.3).

Adhesive wear can be addressed effectively by deliberate and intelligent choice of materials. Solid materials with high hardness or having undergone strain or work hardening (e.g. metals) lead to a reduction of plasticity of the surface. Alternatively, metals or alloys with a hexagonal or body centered cubic (bcc) crystal lattice are thus preferred to those with a face centered cubic (fcc) structure, since these show a high number of slip systems making them very ductile. Solid materials with covalent bonds are less prone to adhesion than those with metallic bonding. Adhesion, thus, can be effectively reduced by ceramic layers that can be produced by coating technologies or surface treatments. Nitriding of steels, for example, can lead to the formation of a nitride layer that reduces adhesion. If these layers, however, are damaged or removed by superimposed abrasive or cyclic loading, the underlying material become unprotected, and may favor adhesion. This often happens during tribological and tribocorrosion processes.

In case of metallic glasses, surface treatment or coating with material composites can be very useful to improving their hardness, and thus reducing their plastic deformation. A fine dispersion of hard phases into metal glass matrix for instance can effectively reduce adhesion. After mechanical processing, they slightly protrude from the surrounding matrix and impede metallic contact.
