Higher-temperature diffusion

At higher temperatures, diffusion may deposit contaminant on the surface. 

At relatively higher temperatures, diffusion of atoms within the grains, that is, bulk diffusion, becomes more important in metals and ceramics. This is called Herring—Nabarro creep (Figure 10.29b). In this model, the slope of the secondary creep curve is given by ... 

Variants which make additional allowances for micro-mechanical material structure and phase transition are particularly suitable for very high strain rates and temperatures of less than half the melting temperature of the material. For even higher temperatures, diffusion and recovery processes must also be taken into account. 

At low external stresses and low temperatures, the material deforms elastically. At higher temperatures, diffusion creep starts, being stronger at small stresses than dislocation creep because of its lower creep exponent. Because of the lower activation energy for grain boundary diffusion, this mechanism is more important than bulk diffusion at low temperatures. Since the creep exponent is the same in both cases, the two regions are separated by a vertical line. 

In both the stable and the metastable cases, the higher-temperature diffusion of positrons proceeds by the adiabatic mechanism , and the following equation approximately holds for the hopping frequency ... 

The net result of the competition between reaction rate and gas transport rate in Equation 12.2 as a function of temperature is shown schematically in Figure 12.3. At low temperatures the exponential dependence of reaction rate on temperature reduces the reaction rate below the gas transport rate sufficiently that the process is surface reaction rate limited. At higher temperatures diffusion is the dominant rate-limiting step. 

For the examination of the applied metallic or ceramic layer, the test object is heated up from the outside The heat applying takes place impulse-like (4ms) by xenon-flash lamps, which are mounted on a rack The surface temperature arises to approx 150 °C Due to the high temperature gradient the warmth diffuses quickly into the material An incorrect layer, e g. due to a delamiation (layer removal) obstructs the heat transfer, so that a higher temperature can be detected with an infrared camera. A complete test of a blade lasts approximatly 5 minutes. This is also done automatically by the system. In illustration 9, a typical delamination is to be recognized. 

There are two mechanisms of PAN-based carbon fiber oxidation dependent on oxidation temperature ((67,68). At temperatures below 400°C, oxygen diffuses into the fiber and attacks at pores resulting in significantly increased fiber surface area. At higher temperatures impurities catalyze the oxidation reaction. 

---