الفهرس 
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Abstract
The objective of this work is to improve the high temperature strength of the Ceramic Steel brazed joints. To achieve this objective. hot pressed 3i3N4 (HPSN) s.as used as a base ceramic material and either low CTE steel of X34iCo(KoVar)or heat resisting steel of XIOCrAII8 was used as parent base metal Commercial AgCulnti active brazing alloy and high temperature brazing of either pure copper o CuPd alloy or both was used to braze HPSN to steel The high temperature brazing materials were used Lo widen the temperature range of the brazed joint applications. [he active brazing alloy was used to pre-metallize’ the HPSN thus forming a dense inIer1ciaI layer along the HPSN surface. this protects the joint from liberated nitrogen as well as wets many brazing materials. Moreover. the metallization process produce another layer rich in Ti and Cu. In the actual brazing process, the pre-metallization layer is dissolved in the braze metal without reducing the high temperature resistance of the brazing A special pars of this work had been devoted to study the macro-defects accompanying with the brazing process. M cru-defects had been classified into two groups according to their origin geometrical and process i)pcs. The causes of these defects were studied and solutions had been introduced Lo prevent them effects of different parameters on the joint strength were studied. These include: thickness of brazing material, brazing time, brazing temperature. influence of the pee metalizing process, cooling rate. holding time as 88U’C, brazing under load, and pre metallizing of XIOCrAII8 with either active brazing or CuPd alloy, The effect of exposure to either ageing or thermal cycles on the joint strength was also investigated The mechanical properties of the joints were evaluated using 4-point bending test, not only at room temperate, but aIso, at elevated temperatures of 4O0C and 6OtJC, The microstructure of the brazing bye: was determined by Scanning Electron Microscope (SEM) and the chemical compositions were evaluated by the Energy Dispersive x-ray Spectroscopy (EDS). In additions. the froactrography of the brazed onls was studied Using both stereo-microscope and SEM to evaluate the weak points in the joint The fractography and the joint microstructure arc related lo the results the bending Test Moreover, the bending results had proved that the pre-metallizing process improves the strength of the HPSn/Kovar joint. Cooper proved to be able to absorb the residual stresses at both HPSn/Kovar and HPSN’X 10CM j I g joints. However, insignificant difference in strength bad been noticed between them in spite of the bet that XIOCrAII8 has higher CTE than Kovar Moreover, the joints performed with Cu had higher strength than those joints performed with CuPd. Thu is due to the higher ductility of the Cu compared to that of the CuPd alloy.