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

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 mechanism is expected.

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

**Figure 6.** Ashby plot comparing the normalized wear rate *k*<sup>a</sup>

(reproduced from Ashby [71] with permission from Elsevier).

Vickers (*H* in MPa = 10 H<sup>v</sup>

to the hardness *H*, here expressed in MPa rather than

(MPa−1) is defined as the ratio

is

) to the distance slid (m) multiplied by the normal load

). The chart gives an overview of the way in which common engineering materials behave.

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

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

(N). That quantity represents a measure of the propensity of a sliding couple for wear: if *k*<sup>a</sup>

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,

high this would correspond to a rapid or severe wear at a given bearing pressure.

questioned by the scientific community (see Section 3.3).

ceramics, and polymers. In that figure, the wear-rate constant, *k*<sup>a</sup>

of the volume of material removed (m<sup>3</sup>

102 Metallic Glasses - Properties and Processing

the wear rate is strongly correlated to the hardness.

load stresses become apparent, and ultimately driving to the breaking-up of the surface with the formation and detachment of large fragments (i.e. sheet-like particles or spalling process), leaving large pits in the surface. This mechanism can be predominant, for example, in periodically loaded dies and tools, in roller bearings or in pumps that are exposed to cavitation.

**8. Future trends**

loads.

**Author details**

Abdenacer Berradja

**References**

core, and which can conceive a composite pattern.

Address all correspondence to: a.berradja@gmail.com

Raton: CRC Press; 2003. ISBN: 9780824708788

of Applied Mechanics. 1957;**24**:361-364

Polymer Science. 2000;**77**:409

Materialia. 2005;**52**(7):669-673

ISBN: 978-0-444-89235-5

2006;**431**(1):158-165

MTM Department, K.U. Leuven, Leuven, Belgium

All the metallic glass materials described in this chapter are subject to further development. Therefore, the attempt to sketch out all possible developments does not seem appropriate. In the design of materials, however, there is a general tendency to form graduated structures and multiphase models, i.e., materials that exhibit a property gradient from the surface to the

Metallic Glasses for Triboelectrochemistry Systems http://dx.doi.org/10.5772/intechopen.78233 105

With a better understanding of the mechanisms of tribocorrosion, knowledge-based development can lead to new microstructures capable of counteracting specific mechanochemical

[1] Schweiter PA. Metallic Materials: Physical, Mechanical, and Corrosion Properties. Boca

[2] Demetriou M, Launey ME, Garrett G, Schramm JP, Hofmann DC, Johnson WL, Ritchie

[3] Irwin G.Analysis of stresses and strains near the end of a crack traversing a plate. Journal

[4] Liang JZ, Li RKY. Rubber toughening in polypropylene – A review. Journal of Applied

[5] Fan C, Qiao D, Wilson T, Choo H, Liaw PK.As-cast Zr–Ni–Cu–Al–Nb bulk metallic glasses containing nanocrystalline particles with ductility. Materials Science and Engineering: A.

[6] Fu H, Zhang H, Wang H, Zhang Q, Hu Z. Synthesis and mechanical properties of Cu-based bulk metallic glass composites containing in-situ TiC particles. Scripta

[7] Stachowiak GW, Batchelor AW. Engineering tribology. Tribology Series. 1993;**24**:872.

[8] Landolt D, Mischler S. Tribocorrosion of Passive Metals and Coatings. Cambridge UK:

Woodhead Publishing Limited; 2011. ISBN: 978-1-84569-966-6

RO. A damage-tolerant glass. Nature Materials. 2011;**10**(2):123-128

Under tribological/tribocorrosion conditions, the location of maximum shear stress moves towards the near-surface during tangential sliding and micro-cracking occurs at the locations of surface defects. The transition from sliding wear to fatigue wear is evidenced by the initiation and propagation of one or more surface micro-cracks into the bulk material. The analysis of crack initiation in sliding (e.g., fretting) is far more challenging than that of propagation. A wide range of factors affects the number of cycles required for damage to accumulate but they can be conveniently be grouped into three categories [74]: (a) Material factor (grain size, morphology, fracture toughness, flow stress, etc.), (b) Environmental factors (temperature, humidity, etc.), and (c) Mechanical factors (surface tractions, slip amplitude, frictional dissipated energy etc.). A full understanding of the problem will only be gained when all these factors are brought under consideration. However, the range of expertise necessary to master these three areas is considerable. At present, few effects are particularly well understood (*viz.* contact stress field and surface slip amplitude) and approaches are developed accordingly. Two main approaches are available to develop an understanding of crack formation, either a "bulk" or macroscopic approach or a "local" or micromechanics approach. The actual crack initiation process occurring on the scale of a few grains in non-homogenous, anisotropic material riddled with defects, inclusions, and imperfections. There is some discrepancy in the literature as to what fraction of the total fatigue life is consumed by the crack initiation process.

Another challenge under hot working conditions has caught attention in mechanical loaded contacts: an unexpected heat may induce surface or volume expansion and the development of stresses between the surface and the bulk material. Between two loaded contact events, the surface cooling once more generates stresses, and "thermal fatigue" may occur resulting in a network of cracks resembling a mosaic.

Generally, contact fatigue can be mitigated by all usual measures that reduce the susceptibility to cracks, i.e., a high strength to impede crack initiation and a high toughness to retard crack growth (*cfr.* **Figure 1** in Section 2).

The relationship between these two material properties, namely toughness and strength, have been introduced in Section 2 (**Figure 1**). It is shown that, generally, ductile metals exhibit virtually the greatest fracture toughness; however, they may display weak strength. Metallic glasses, on the other hand, often have toughness and strength that lie between brittle ceramics and marginally tough materials. Nevertheless, following the development of very recent bulk metallic glasses, the introduction of newly metal glass composites on the market has led to overcome the early success of metals by acquiring a very high resistance to deformation, and toughness, which imparts them a good resistance to cyclic stresses and solicitations. A strong interface between the glassy matrix and second phase particles is to be guaranteed. Therefore, these glassy composites can substitute metals in application areas not achieved yet so far. Among these novel alloys, there are Pd-based glasses, and ductile-phase-reinforced metallic glass composites [2].
