Fracture surface energetics

Fracture Energetics and Surface Energetics of Polymer Wear... 

Since the first review on the effect of surface energetics on polymer friction and wear published in 197, (0 many new works have appeared. Some of these papers(2- ) are on fracture mechanics. In this paper we shall review our current knowledge about both fracture energetics and surface energetics of polymer wear. First, we discuss wear mechanisms and then emphasize these two aspects related to each wear mechanism. 

In comparison with metals, most conventional polymers are low in wear resistance. For wear control, we need to understand various wear mechanisms for each polymer system (V). As discussed in a previous paper, for adhesive wear, surface energetics can determine the extent of surface wear. Thus, a low surface energy is preferred to minimize the surface attrition. In addition, a harder polymer is desired to lower the wear rate. For abrasive wear, fracture energetics become important a harder and tougher material should be more wear resistant. 

An electron emitted by either EE mechanism mentioned above can strike a surface and stimulate the emission of an ion. In surface physics, electron-stimulated desorption (ESD) is a widely studied phenomenon that has recently been extended to organic adsorbates (28). The energetics of this mechanism would demand that electrons strike the surface with a minimum energy of several eV, but this is conceivable considering the charging of the fracture surfaces that frequently occurs and the EE energy distributions observed. If this latter mechanism is correct, the flux of EE measured-i-the flux that... 

In the few cases where thermal stimulation of the fracture surface has been performed,01 4,39) EE glow curves similar to those obtained following irradiation with X-rays or energetic electrons were observed. In the case of alkali halides, the temperature of the crystals at fracture had to be above a minimum temperature in order to produce such EE glow curves. The peaks, when they were observed, could be correlated with the activation of known defect chemistry (e.g., and Vp-center annihilation).(ii)... 

Fig. 5.23 Grain boundaries in SrTiOa (mpe = 9-5 10 cm" ) recorded by high-resolution electron microscopy a) atomically shaup, low-energy grain boundary (symmetrical E3 (111), without amorphous interfacial layer), b) triple point with sunorphous intergranular phase (thickness 1 nm) c) faceted grain boundary with amorphous interfaciaJ layer (thickness sa 1 nm). Fracture surface (d) with energetically favourable, step-shaped surfaces. From Refs. [139,140].

A key point to be made in the present context is that such cohesive surface models can be used to describe the energetics of a number of different dissipative processes related to fracture as shown in fig. 12.8. The claim is that each of these different mechanisms is amenable to a treatment in which the interfacial normal tractions can be derived from a nonlinear interplanar potential according... 

Abrasive Wear. Abrasive wear(18) is common for brittle, ductile and elastomeric polymers. Abrasion is the wear by displacement of materials from surfaces in relative motion caused by the presence of hard protruberanees or by the presence of hard particles either between the surfaces or embedded in one of them. As a result, microploughing, microshearing or microcutting can occur. Thus, fracture energetics and contact mechanics are involved in analyzing the wear results. We shall discuss briefly the wear rate with respect to different types of polymers. 

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