**8. Future trends**

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.

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

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

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

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

process.

network of cracks resembling a mosaic.

104 Metallic Glasses - Properties and Processing

crack growth (*cfr.* **Figure 1** in Section 2).

glass composites [2].

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 core, and which can conceive a composite pattern.

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