Interface powders

The primary reason to use lubricants is to reduce friction and wear between two interacting surfaces. Hydrocarbon oils have the proper friction properties to meet these requirements but their low viscosity may cause them to be forced out of the contact region between interfaces. Powders of low MW PIEE may be added to liquid lubricants to provide reserve lubrication in case the liquid phase is forced out. Low-MW PTFE that is used this way is sometimes called an extreme pressure or boundary additive. The type of PTFE used in lubricants may be from either suspension or dispersion polymerization but the small particle size of dispersion-type PTFE is usually preferred to help maintain dispersion in the oil. Many Journal articles and patents have been published that report the performance of lubricating oils with and without the addition of PTFE. For example, Rico et al. [44] provided the results of an extreme pressure wear study of steel balls (Shell four-ball test) with several mineral oils containing four different percentages (1-10%) of PTFE. 

Microcrystals of SrS04 of 30 A diameter have a solubility product at 25°C which is 6.4 times that for large crystals. Calculate the surface tension of the SrS04-H20 interface. Equating surface tension and surface energy, calculate the increase in heat of solution of this SrS04 powder in joules per mole. 

Initially in ceramic powder processing, particle surfaces are created tliat increase tlie surface energy of tlie system. During shape fomiing, surface/interface energy and interiiarticle forces are controlled witli surface active additives. 

The terminal R groups can be aromatic or aliphatic. Typically, they are derivatives of monohydric phenoHc compounds including phenol and alkylated phenols, eg, /-butylphenol. In iaterfacial polymerization, bisphenol A and a monofunctional terminator are dissolved in aqueous caustic. Methylene chloride containing a phase-transfer catalyst is added. The two-phase system is stirred and phosgene is added. The bisphenol A salt reacts with the phosgene at the interface of the two solutions and the polymer "grows" into the methylene chloride. The sodium chloride by-product enters the aqueous phase. Chain length is controlled by the amount of monohydric terminator. The methylene chloride—polymer solution is separated from the aqueous brine-laden by-products. The facile separation of a pure polymer solution is the key to the interfacial process. The methylene chloride solvent is removed, and the polymer is isolated in the form of pellets, powder, or slurries. 

Typical pressure and temperature histories computed are shown in Figs. 6.6 and 6.7. In Figs. 6.6, the pressure is shown as a function of position within the powder compact at various times. For the baratol explosive loading shown, an initial wave, whose pressure is 1.8 GPa, is shown moving slowly from right to left. Upon reflection from the rear interface with the copper, the pressure jumps to a much higher value and then quickly reverberates to a peak pressure of about 11.4 GPa. The shorter reverberation time reflects the higher wavespeed and the major reduction in thickness in the compressed powder. 

Fig. 6.7. The predicted, one-dimensional, mean-bulk temperatures versus location at various times are shown for a typical powder compact subjected to the same loading as in Fig. 6.5. It should be observed that the early, low pressure causes the largest increase in temperature due to the crush-up of the powder to densities approaching solid density. The "spike in the temperature shown on the profiles at the interfaces of the powder and copper is an artifact due to numerical instabilities (after Graham [87G03]).

It is particularly significant that no evidence is found for localized melting at particle interfaces in the inorganic materials studied. Apparently, effects commonly observed in dynamic compaction of low shock viscosity metals are not obtained in the less viscous materials of the present study. To successfully predict the occurrence of localized melting, it appears necessary to develop a more realistic physical model of energy localization in shock-compressed powders. 

In all cases, broad diffuse reflections are observed in the high interface distance range of X-ray powder diffraction patterns. The presence of such diffuse reflection is related to a high-order distortion in the crystal structure. The intensity of the diffuse reflections drops, the closer the valencies of the cations contained in the compound are. Such compounds characterizing by similar type of crystal structure also have approximately the same type of IR absorption spectra [261]. Compounds with rock-salt-type structures with disordered ion distributions display a practically continuous absorption in the range of 900-400 cm 1 (see Fig. 44, curves 1 - 4). However, the transition into a tetragonal phase or cubic modification, characterized by the entry of the ions into certain positions in the compound, generates discrete bands in the IR absorption spectra (see Fig. 44, curves 5 - 8). 

---