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

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 solid-

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 underly-

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,

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 regard-

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

In general no equation can fully describe the adhesive wear process but the most widely used

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.

the lubricity of a wide variety of solid oxides at high temperature could be explained.

state attraction is therefore a feasible concept to avoid adhesive wear.

ing previously welded material.

100 Metallic Glasses - Properties and Processing

is the Archard equation (Eq. (1), see Section 3.3).

less of low friction.

expected.

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 unit on Mohs scale of mineral hardness.

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

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

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

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, is commonly used for abrasive wear, although this is derived from a completely different set of material removal mechanisms. However, the validity of the Archard wear criterion is still questioned by the scientific community (see Section 3.3).

The hardness of a metal is more likely structure dependent, so that the hardness of an annealed metal may be lower than that of a work-hardened metal. The state of a metal in fully workhardened condition corresponds to the state, which a metal surface might achieve after repeated sliding over it had occurred. Steels, for example, when hardened by quenching may be less prone to abrasive wear than soft-annealed. The application of this concept is, however, limited especially against mineral abrasives: metallic materials cannot reach the required hardness. Ceramics, on the other hand, are sufficiently hard but owing to their lack of toughness (**Figure 1**, see Section 2) and ductility they cannot claim to be eligible candidates as basic materials in many specialized fields. In addition, they are prone to micro-cracking during abrasive loading, which reduces their wear resistance. To counteract this issue, metallic materials are reinforced with hard phases. The tough/hard metallic matrix provides high ductility and strength, while the hard phases prevent indentation and grooving of the surface [72]. They are most effective if they are harder than the abrasive, and larger than the groove width. The hardness of precipitatedhard phases is raised by hard phase forming alloying elements. Their size usually decreases with the temperature of precipitation, i.e., primary, eutectic and secondary carbides. Tool steels and white cast irons belong to this group. Another group comprises metal matrix composites, e.g., Al-, Fe- or Co-based with a mixed hard phases, such as carbides (fused tungsten carbide,

sufficient to back up the hard phases and to minimize the indentation depth of the abrasive.

Fatigue (delamination) wear is defined as, "the removal of particles detached by fatigue arising from cyclic stress variations". The delamination or fatigue theory of wear was proposed by Suh [73], as an attempt to explain weaknesses in the Archard theory of adhesive wear [26].

Repeated cycles of contact are not necessary in adhesive and abrasive wear for the generation of wear particles. There are other cases of wear where a critical number of repeated contacts are essential for the generation of wear particles. Wear generated after such contact cycles is called "fatigue wear". When the number of contact cycles is high, the high-cycle fatigue mechanism is expected to be the wear mechanism. When it is low, the low-cycle fatigue

To better guide the choice of materials in the field, where surface interactions interferes with fatigue wear, it is necessary to understand the mechanisms and processes that govern the wear by contact fatigue with or without sliding. Detailed explanations can be found elsewhere [73, 74]. Nevertheless, in the following, a summary of this material is recalled concisely. This form of contact fatigue-induced wear is often observed in systems where cyclic contact stresses (e.g., loaded tools) take place, but in most cases during sliding or rolling contacts.

In loaded mechanical parts, the contact surface undergoes compression stresses and shear stresses are developed beneath the surface. The repeated loading and unloading cycles to which the materials are exposed may induce the formation of surface and/or sub-surface nano- and microcracks, at critical zones where, for example, imperfections, inclusions or second phases are located. This eventually will result in the growth of fatigue cracks as further

**7.4. Materials oriented approach to act in opposition to fatigue wear**

), cubic boron nitride or diamond. The matrix hardness needs to be

Metallic Glasses for Triboelectrochemistry Systems http://dx.doi.org/10.5772/intechopen.78233 103

TiC), oxides (Al<sup>2</sup>

mechanism is expected.

O3 , ZrO<sup>2</sup>

According to the Archard equation, a timely way to avoid abrasion is to increase the surface hardness of the component [70]. However, it should be pointed out that for a number of metallic glass composites and bulk glassy alloys, the wear rate may deviate and even do not follow the Archard equation [26]. Only a good combination of the hardness and the toughness taken together can allow the metallic glass to be wear resistant. A convenient way of bringing out this choice is by a series of figures or charts, in which one parameter of interest is plotted against another. The Ashby chart [71] plotted in **Figure 6** compares the normalized wear rate and the hardness for most of the common engineering materials including metals, technical ceramics, and polymers. In that figure, the wear-rate constant, *k*<sup>a</sup> (MPa−1) is defined as the ratio of the volume of material removed (m<sup>3</sup> ) to the distance slid (m) multiplied by the normal load (N). That quantity represents a measure of the propensity of a sliding couple for wear: if *k*<sup>a</sup> is high this would correspond to a rapid or severe wear at a given bearing pressure.

The wear rate of metals are markedly hardness dependent, however, technical ceramics show nearly the lowest wear rate and the largest hardness over metals, polymers, and elastomers. Note how certain engineering materials lie roughly on a diagonal (dotted lines). Interestingly, the wear rate is strongly correlated to the hardness.

**Figure 6.** Ashby plot comparing the normalized wear rate *k*<sup>a</sup> to the hardness *H*, here expressed in MPa rather than Vickers (*H* in MPa = 10 H<sup>v</sup> ). The chart gives an overview of the way in which common engineering materials behave. (reproduced from Ashby [71] with permission from Elsevier).

The hardness of a metal is more likely structure dependent, so that the hardness of an annealed metal may be lower than that of a work-hardened metal. The state of a metal in fully workhardened condition corresponds to the state, which a metal surface might achieve after repeated sliding over it had occurred. Steels, for example, when hardened by quenching may be less prone to abrasive wear than soft-annealed. The application of this concept is, however, limited especially against mineral abrasives: metallic materials cannot reach the required hardness. Ceramics, on the other hand, are sufficiently hard but owing to their lack of toughness (**Figure 1**, see Section 2) and ductility they cannot claim to be eligible candidates as basic materials in many specialized fields. In addition, they are prone to micro-cracking during abrasive loading, which reduces their wear resistance. To counteract this issue, metallic materials are reinforced with hard phases. The tough/hard metallic matrix provides high ductility and strength, while the hard phases prevent indentation and grooving of the surface [72]. They are most effective if they are harder than the abrasive, and larger than the groove width. The hardness of precipitatedhard phases is raised by hard phase forming alloying elements. Their size usually decreases with the temperature of precipitation, i.e., primary, eutectic and secondary carbides. Tool steels and white cast irons belong to this group. Another group comprises metal matrix composites, e.g., Al-, Fe- or Co-based with a mixed hard phases, such as carbides (fused tungsten carbide, TiC), oxides (Al<sup>2</sup> O3 , ZrO<sup>2</sup> ), cubic boron nitride or diamond. The matrix hardness needs to be sufficient to back up the hard phases and to minimize the indentation depth of the abrasive.
