Wear resistance, variation with

Figure 9o Variation of wear resistance 1/K of PET specimens with Vickers hardness Hy o...

Although microeffects and macroeffects have their own characteristics, they often act closely together. This is demonstrated by the example of AI2O3, in which variation of the crystalhte size and controlled phase transition by means of heat treatment have a major effect on the wear resistance and compressive strength of the material. Interactions between die active component and the support material are discussed in detail in Section 5.4. Chapter 14 deals with the influence of the reactor type on the choice of catalyst. 

In both PWRs and BWRs, corrosion of the primary circuit materials is an essential factor in the buildup of contamination layers on the surfaces of the pipes and the components. The materials used in BWRs which are in contact with the reactor water and, therefore, are potential sources of radionuclides are mainly stainless steels wear-resistant hardfacing alloys such as Stellite are also present in most of the plants. Zircaloy as the material of fuel rod claddings, spacers and fuel assembly casks need not be considered in this context, because of the extremely small release of activated constituents from this material. Due to differences in temperature and environment, the mechanisms of the corrosion process and the resulting metal release rates, which contribute to the input of corrosion products into the region of the reactor core, may show differences in different regions of the plant. Thus, corrosion of materials in the water-steam cycle exhibiting H2O phase transformations and considerable temperature differences will proceed differently than in the recirculation lines and the reactor water cleanup system, which are in contact with liquid water exclusively and show comparatively small variations in operating temperature. 

In this section, the friction and wear of PTFE-based composites with different nano-scaled fillers are explicitly discussed. The friction coefficients of PTFE-based composites with different nanoscaled fillers differ with each other because of the dissimilar physical and chemical properties of different types of nanofiUers. However, despite the different nanofiller type and content, the variation of friction coefficient between PTFE-based composites and pure PTFE is evident under different experimental conditions. On the one hand, this is caused by the very low friction coefficient of pure PTFE so that a further decrease in friction coefficient becomes a formidable issue. On the other hand, due to the material nature of the nanofillers—for instance the lubrication property of nano-EG significantly lowers the friction coefficient of PTFE/nano-EG composites while friction coefficient of PTFE/nanoserpentine composites barely changes, which is greatly related to the material nature of the nanofillers. Conversely, a dramatic reduction in wear rate is observed in all PTFE-based composites. It is believed that the strong interfacial interaction, high shear strength, enhanced load capacity, and extra lubrication effect of PTFE-based composites with nanoscaled fillers are responsible for the improvement of wear resistance. However, the specific enhancement mechanism remains unsolved. 

Once the particle sizes are diminished down to the nanoscale (< 100 nm), the wear performance of these nanocomposites differs significantly from that of micron particle-filled systems. Polymers filled with nanoparticles are recently under discussion because of some excellent properties they have shown under various testing conditions. Some results were achieved in various studies, suggesting that this method is also promising for new processing routes of wear resistant materials. For instance, Xue et al. found that various kinds of SiC particles, i.e., nano, micron and whisker, could reduce the friction and wear when incorporated into a PEEK matrix at a constant filler content, e.g., 10 wt.% ( 4 vol.%). However, nanoparticles resulted in the most effective reduction. Nanoparticles were observed to be of help to the formation of a thin, uniform, and tenacious transfer film, which led to this improvement. The variation of Zr02 nanoparticles from 10 to 100 nm was conducted by Wang et al. The results showed a similar trend as most of the micron particles, i.e., the smaller the particles, the better was the wear resistance of the composites. 

The variation in the wear rates, which were calculated from the slope of Figure 7 in the unit of g/m, are presented in Figure 10 as a function of test temperature. The addition of Mg to A1 matrix resulted with an improvement of the wear resistance of the composites. Thus, the high strength composite exhibited high resistance to abrasion at ambient and elevated temperature. 

The variation of relative abrasive wear resistance (RAWR) as a function of temperature for ductile iron is presented in Fig. 6.14a [74], It is clear that the abrasive wear resistance shows a maximum at around 323-373 K. The RAWR is 20% lower at higher temperature than at ambient temperature. The high value of the RAWR at around 323-373 K is attributed to dynamic strain ageing. In contrast, the decrease of RAWR with increase of temperature is related to loss of strength and ductility of the ductile iron at high temperature. 

Constant product quality requires an even feed rate, homogeneous bulk density of the material to be treated, uniform densification, and reproducible maximum pressure. This statement is true for all pressure agglomeration methods. However, while these conditions can be met relatively easily in die and roller presses with proper feed preparation and specific equipment parameters, it is rather difficult to achieve in extrusion. The reason for this is that densification and maximum pressure depend on the resistance to flow in the die channel or holes. Small variations in feed homogeneity or frictional properties can yield major differences in equipment performance and product quality. Wear or buildup in the extrusion die are among the most important parameters influencing the back-pressure which, in turn, is responsible for the amount of densification prior to extrusion. 

Alternatively, squeeze type seals operate by distorting under compressive load, and the hardness specified for such an application must be sufficient to ensure adequate retention of the sealing pressure. The squeeze resistance can be enhaneed by incorporating one or more plies of fabrics in the rubber section rather than increasing the hardness of the compound as this might adversely affeet other properties. Dimensional variations due to contact with fluids can be adjusted to achieve a small positive swell which can maintain seal efficiency, by compensating for wear and compression set. The choice of eompound will depend on the effects of the fluids with which the seal is in contact, the operating temperature, and mechanical conditions such as pressure, relative veloeity and abrasion. 

A variation of the cathode test with vibration (which imitates the movement of the metal) is combined with lab electrolysis. In Thomstad s variation [92], both electrochemical and abrasion resistance are taken into account. This variant of testing is very complex. The test sample under negative potential is in the alumina crucible with aluminium and the bath with 8 % alumina. The crucible is in the lab furnace, the temperature is increased up to 960 °C, the anode is from tin oxide, and the rotor is from bormi nitride. The time of testing is 6 h. The registered parameters are voltage and the CO/CO2 concentratiOTi. After the testing, a visual inspection is carried out, but the wear forecast is made based on the volume of CO/CO2 output. 

---