Cobalt-coated Al <sub> 2 </ Sub> The O <sub> 3 </ Sub>-tic Composite Mechanical Properties And Strengthening And Toughening Mechanisms

As an important structural material, Al₂O₃-TiC (AT for short) composites have been widely used owing to the high hardness, good chemical stability and wear resistance. However, its facture toughness still needs improving in view of its rupture and spalling during application.Coating surfaces of Al₂O₃ and TiC powders with Co by electroless method is believed to be an effective way to prevent powders from amalgamating and growing, avoiding heterogeneous distribution of low content additive when using conventional ball-milling method, to achieve synergistic toughening by interfacial modification.In this paper, with Co coated Al₂O₃ and TiC powders as staring powders, sintered composites (ATC for short) was obtained by hot-pressing the mixture of powder. The main contents are as follows:Hot pressing were carried out in the range of 1500℃ to 1700℃ with different Co content (1.5⁵vol%). For comparison, AT composites was prepared by hot pressing at 1700℃ .The results of mechanical properties of the samples with different Co content sintered at different temperature were investigated and compared. The optimum comprehensive mechanical properties with 4vol% Co are considered to be HRA=92.7, σ=782MPa, K_IC=8.71. In comparison with AT composites, the fracture toughness and fracture strength are improved by 59% and 40%, respectively. The processing parameters of ATC composites with 4vol% Co were further optimized to be 1650℃/30min/30MPaThe microstructure of ATC4 composites was investigated using XRD, EPMA, SEM, TEM and HRTEM. The results indicate that the composites are composed of α-Al₂O₃, TiC and Co. The TiC particles are well dispersed in the Al₂O₃ matrix, partly homogeneously distributed along the interfaces between TiC and Al₂O₃. The intergranular Co particles can effectively pin dislocations and refine the matrix grain size. The fracture mode is the mixture of intergranular and transgranular fracture, occasionally with some grains pulling out HRTEM studies revealed that the interfacial-bonding mode between of Co and Al₂O₃ is semi-coherent The atom arrangement of Co tends to bond with Al₂O₃ crystal plane according to preferred orientation.The thermal residual stress toughening and ductile metal phase toughening from Co particle makes little contribution to toughness. The intergranular Co particles in ATC4 composites can either refine the matrix grain size or strengthen the grain boundary. The fractal analysis of indentation cracks indicated that the fracture toughness is positively proportional to the fractal dimension, with ATC4 possessing higher value. The improvement of toughness mainly resultsfrom the indirect contribution of Co phase: the occurrence of large ratio TiC particle and weakened interfacial bonding make the crack bridging and deflection more effectively.The thermal shock resistance behaviors of AT and ATC4 composites were investigated by quenching method and indentation-quench technique, respectively. The results show that thermal shock resistance of ATC composites are better than that of AT. Both composites are, however, sensitive to cyclic thermal shock, and the resistance to cyclic thermal shock of both composites is close after cyclic thermal shock in large numbers, which should be due to the heavy oxidation of TiC particles after cyclic thermal shock at 400 °C.The thermal shock resistance parameters (R> R\ R"'\ Rs) indicate that the high ratio of fracture toughness to fracture strength of ATC composite makes it possess better thermal shock resistance in comparison with AT. Thermal property changed a little after adding a few Co, but the fracture toughness of ATC4 composites was improved greatly, which effectively increase the resistance to crack propagation.