Development Of New B₄C Composite Ceramic Nozzle And Study On Its Erosion Mechanisms

Pure B₄C is a kind of ceramic material, difficult to be densified and always with low bending strength and toughness. In order to promote its densification rate, high sintering temperature is demanded. In this paper, two new B₄C matrix composite nozzles were prepared by the in-situ and hot-pressing technology. Phase analysis, mechanical properties, microstructures and erosion properties of the composites were analyzed and studied systematically.The design objectives of B₄C matrix composites and selecting principles for toughening phases were proposed. TiC, Al₂O₃ and Mo were chosen as additives, and the maximum content of TiC phase in B₄C matrix was calculated. The mechanism for in-situ and hot-pressing method of B₄C matrix composites was discussed. It was indicated that TiB₂ was formed in composite nozzles by the reaction between TiC and B₄C by chemical compatibility analysis. The conclusion was confirmed by XRD technology.B₄C/TiB₂/Mo (BMT) and B₄C/TiB₂/Al₂O₃ (BAT) composites were prepared successfully by in-situ and hot-pressing technology. B₄C, Mo and TiB₂ could exist stably in BMT composites; B₄C, Al₂O₃ and TiB₂ could exist stably in BAT composites. The optimal comprehensive mechancal properties, including bending strength, hardness and fracture toughness, of two composites were abtained. The optimal comprehensive mechanical properties of BMT composites were 525 MPa, 24.34 GPa and 4.27 MPa.m^1/2, respectively; those of BAT composites were 420 MPa, 26.27 GPa and 3.95 MPa.m^1/2.The effect of TiC content on mechanical properties of BMT and BAT composites was analyzed. The optimal content of TiC was 10 wt.%. The effect of sintering temperature and holding time on BMT and BAT composites were discussed, and the optimal preparing parameters was 1950℃, 60 min and 35 MPa.The erosion behaviors of BMT and BAT composite ceramic nozzles were studied. The erosion resistance of BMT21 composite ceramic nozzle was the lowest in BMT composite ceramic nozzles, and erosion rate of BMT21 composite ceramic nozzle was 0.81×10^-3 mm³/g. that of BMT13 composite ceramic nozzle was the highest, and erosion rate of BMT21 composite ceramic nozzle was 2.12×10^-3 mm³/g. The erosion rate of BMT composite ceramic nozzles ranged from the lowest to the highest was as follows: BMT21 -3 mm³/g. Erosion rate of BMT13 composite ceramic nozzle was the highest, and erosion rate of BMT21 composite ceramic nozzle was 1.1×10^-3 mm³/g. The erosion rate of BAT composite ceramic nozzles ranged from the lowest to the highest was as follows: BAT21 4C matrix composite nozzles on erosion rate was analyzed. It was shown that erosion rate decreased with the increasing of hardness and fracture toughness. The effect of ratio between impacting particle hardness versus nozzle hardness on erosion rate was studied. Erosion rate of B₄C matrix composite nozzles impacted by Al₂O₃ particle was lower than that impacted by SiC owing to samller hardness of Al₂O₃ impacting particle.The velocity distribution of impacting particle in different entry angle nozzles was simulated using Fluent software. Impacting particle was accelerated in nozzle, and impacting particle velocity changed with the changing of entry angle of nozzle. Particle velocity at exit of nozzle increased with the increasing of nozzle entry angle as nozzle entry angle was lower than 15°, and that of nozzle decreased with the increasing of nozzle entry angle as nozzle entry angle ranged from 15°to 45°, and that of nozzle hardly changed with the increasing of nozzle entry angle as nozzle entry angle ranged from 45°to 90°.Effect of particle velocity and impacting angle on weight loss was simulated using Abaqus software. It was shown that weight loss increased with the increasing of particle velocity and impacting angle due to the increasing of impacting stress, and weight loss reached the maximum value as impacting angle up to 90°.Erosion mechanisms at entry, exit and middle of nozzle were different. The erosion mechanism at entry of BMT composite nozzle was brittle fracture, and the erosion mechanism at exit was fatigue fracture, and that at middle was micro-cutting. The erosion mechanism at entry of BAT composite nozzle was fatigue fracture, and the erosion mechanisms at exit and middle were micro-cutting.The erosion resistance of BAT composite nozzle was better than that of BMT composite nozzle, because the mechanical properties of former higher than that of latter, furthermore, the stress imposing at different zone of BAT composite nozzle lower than that of BMT composite nozzle.