Design of Surface Texture for the Enhancement of Tribological Performance

Surface texturing is a method of surface modification that fabricates micro patterns on the contacting surfaces to improve the tribological performance in sliding, lubricated system. It is found that the geometric design of textures has a significant influence on the performance of textured surfaces. Some important geometric parameters, such as the area ratio and the depth-over-diameter ratio, have been identified for textures in the form of circular dimples. The current study aims to improve the friction reduction effect of surface texturing by using textures with novel designs. Some new factors considered in the design of textures are: internal structure and geometric shape. A new type of texturing is also proposed for the application in piston rings to reduce engine friction. In the research on the dimple with internal structures, both experimental and numerical studies are conducted to compare the performance of dimples with three internal structural shapes: rectangle, oblique triangle and isosceles triangle. The numerical model uses Jakobsson-Floberg-Olsson (JFO) cavitation theory to predict the load-carrying capacity of textures. It is found that the value of cavitation pressure plays an important role in the implementation of the JFO theory. Therefore, the analysis on the selection of cavitation pressure is performed for the steady state lubrication. In the work of texture shape optimization, a numerical approach based on the sequential quadratic programming (SQP) algorithm is used to determine the optimum texture shape for different operating conditions, including unidirectional and bidirectional sliding and rotating. The optimization result for rotating is further verified by experiments. For the application of surface texturing in piston rings, a new design of lasered oil pockets is proposed to improve the lubrication at the piston ring/cylinder liner interface. Its friction reduction effect is evaluated with both bench tests of piston ring prototypes and motorized engine tests of production piston rings

Multilevel Diversity Coding with Secure Regeneration: Separate Coding Achieves the MBR Point

The problem of multilevel diversity coding with secure regeneration (MDC-SR) is considered, which includes the problems of multilevel diversity coding with regeneration (MDC-R) and secure regenerating code (SRC) as special cases. Two outer bounds are established, showing that separate coding of different messages using the respective SRCs can achieve the minimum-bandwidth-regeneration (MBR) point of the achievable normalized storage-capacity repair-bandwidth tradeoff regions for the general MDC-SR problem. The core of the new converse results is an exchange lemma, which can be established using Han's subset inequality