CHAPTER 2 Mechanical Causes of Materials Degradation1.Introduction2. Wear1.Adhesive wear2.Abrasive and erosive wear3.Wear induced by mechanical fatigue of the worn surface4.Melting wear, fretting wear and diffusive wear5.Analytical models of wear6.Wear resistant materials3.Fatigue, fracture and creep1.Mechanisms of fatigue and creep2.Fatigue and creep resistant materials4. Summary
Chapter 2 Mechanical causes of materials degradationIntroduction•Mechanical causes of materials degradation usually involvesmechanical forces.•Most of these forces e.g. the weight force are not destructive but some forces (i.e. oscillating) and stresses (i.e. fatigue, localized high stress-wear) can cause rapid materials degradation and failure.•Wear, fatigue and fracture are discussed.
2.2 Wear•Wear involves contact between 2 solid bodies (i.e. wheel/axle, wheel/road, axle/bearing).•Profilometry studies indicate rough surfaces.•Smoothing-polishing only reduces roughness.•Model of localized contact between rough surfaces is illustrated in Figure 2.1.•Surface roughness scale is 0.1-10
Fig 2.1 Nature of contact between rough surfacesNo perfectly smooth surfaces.
2.2 Wear•True contact area is 1-0.1% of nominal contact area, andcontact stress is 100-1000 times greater than nominal contact stress (load/nominal contact area).•Wear begins with sliding motion, some rolling motion, impact and frictional heat created.•Figure 2.2 shows relations between adhesion, deformation, frictional heating and wear mechanisms.
Figure 2.2 The relationship between wear mechanisms and their physical causes
2.2 Wear•Adhesion leads to adhesive wear and frictional seizure in extreme cases.•Plastic deformation and fracture cause subsurface crack growth.•Hard particles cause abrasive and erosive wear.•Small sliding promotes fretting with debris in contact and different from sliding wear.
2.2 Wear•Frictional or process heating cause melting and diffusive wear.•Wear is only prevented by lack of surface contact (i.e. using a thin film of lubricant of 1 micron thick, magnetic levitation).
2.2.1 Adhesive wear
Fig. 2.4 Contaminant and oxide layer removal on metals by wearing contact to initiate adhesive wear
2.2.1 Adhesive wear•Wear particles can form transfer layers composed of wear debris.•Transfer layers become the layers in contact.•The friction and wear coefficients are lower as in Figure 2.5.•Microstructural affects of non-metals (carbon, interstitial carbides) on steel weaken adhesion relative to steel on steel as in figure 2.6.