High Temperature Oxidation Behavior And Self–healing Effect Of Si C_p Or MoSi_2p Reinforced ZrO₂–Al₂O₃ Ceramics

Polycrystalline ZrO₂（Y2O3）–Al₂O₃ ceramics（ZrO₂–Al₂O₃） have attracted great attention due to its high hardness, excellent room-temperature strength, fracture toughness and other excellent performance such as high wear resistance and chemical corrosion resistance, and has been widely used in harsh environments and load-bearing conditions. However, ZrO₂–Al₂O₃ ceramic is still quite sensitive to flaws due to the nature of brittleness, and its mechanical properties drastically degrade at high temperatures. One of effective approaches applied to improve the mechanical properties of oxide ceramics is to strengthen and toughen these oxide ceramics by incorporating different reinforcements of particles. This present work aims to improve the mechanical properties of ZrO₂（3mol.%Y2O3）–20wt.%Al₂O₃（ZrO₂–Al₂O₃） ceramics and oxidation resistance at elevated temperatures by dispersing Si C or Mo Si2 particles into the oxide matrix, due to high elastic modulus, high strength and crack healing ability of Si C or Mo Si2 particles at high temperatures. The fracture toughness of ceramic composites could be improved by the crack self-healing effect of second phases to further enhance the lifetime and reliability.Si C or Mo Si2 particles with various volume fractions were added into the ZrO₂–Al₂O₃ powder, and then the mixture was subsequently hot-pressed with different sintering temperatures, respectively, to form dense ZrO₂–Al₂O₃–Si C and ZrO₂–Al₂O₃–Mo Si2 ceramics. The microstructure, mechanical properties, oxidation resistance and crack healing ability of ZrO₂–Al₂O₃–Si C and ZrO₂–Al₂O₃–Mo Si2 composites are invesitigated to obtain a better understanding on the influences of self-healing particles and other technological paraneters, and to further reveal the mechanisms due to oxidation and self-healing effects at elevated temperatures.The addition of Si C or Mo Si2 particles refines microstructure of the oxide matrix, and the relative densities of different ZrO₂–Al₂O₃–Si C and ZrO₂–Al₂O₃–Mo Si2 ceramic composites are over 98%. The room temperature flexural strength, high temperature flexural strength at 1000 oC, Vickers hardness and fracture toughness for ZrO₂–Al₂O₃–15vol.%Si C ceramics hot pressed at 1600 oC are 1044 MPa, 518 MPa, 16.3GPa and 14.1MPa·m1/2, respectively. However, for ZrO₂–Al₂O₃–15vol.%Mo Si2 ceramics hot pressed at 1600 oC, the room temperature flexural strength, high temperature flexural strength at 1000 oC, Vickers hardness and fracture toughness are 1097 MPa, 589 MPa, 16.5GPa and 13.4MPa·m1/2, respectively. Room temperature mechanical properties of different ZrO₂–Al₂O₃–Si C and ZrO₂–Al₂O₃–Mo Si2 composites are related to their relative densities, phase constituents and grain sizes of different phases. However, the enhancement of high-temperature strength is due to the dispersion strengthening effect of the reinforcements with high elastic modulus and high strength. Under high temperature environments, surface cracks can be healed by the oxidized products of Si C or Mo Si2 particles, and phase boundaries were toughened by Si C or Mo Si2 particles at elevated temperatures, all above this improve high temperature flexural strength of ceramic composites. The fracture modes of different ZrO₂–Al₂O₃–Si C and ZrO₂–Al₂O₃–Mo Si2 composites convert from the inter-granular fracture to a mixed fracture mode of both trans-granular fracture and intergranular.The oxidation behavior of ZrO₂–Al₂O₃–Si C and ZrO₂–Al₂O₃–Mo Si2 ceramics composites was investigated by a variety of isostatic oxidation experiments at elevated temperatures. For ZrO₂–Al₂O₃–10vol.%Si C ceramics, a passive oxidation process of Si C occurs. The oxidized product of Y2Si2O7 is observed during heat treatment at 800 oC. However, after heat treatment at 1100 oC, an oxidized product of Si O2 is generated first, and then transforms gradually to Y2Si2O7. For ZrO₂–Al₂O₃–Mo Si2 composites, Zr Si O4 and a glassy phase of Si–O are generated at the initiation stage, and then both Si O2 and a mullite phase of 3Al₂O₃·2Si O2 are formed at higher temperature. After prolonging the oxidation time over 10 h at 1100 oC, the oxidation process becomes slow due to the coverage of oxidized products on ceramic surfaces.Cracking-healing ability and strength recovery of different ZrO₂–Al₂O₃–Si C and ZrO₂–Al₂O₃–Mo Si2 composites have been investigated by tailoring heat treatment temperature, heat treatment time, particle content and crack size. Heat treatment in air enhances distinctly the flexural strength of ZrO₂–Al₂O₃-Si C and ZrO₂–Al₂O₃–Mo Si2 composites. Crack self-healing mechanisms associated with crack closure and rebonding of the crack walls are due to the formation of Si2O2 glassy phase by various oxidation reactions of Si C or Mo Si2 particles in the oxide matrix. A complete crack-healing and strength recovery of ZrO₂–Al₂O₃–10vol.%Si C ceramics can be realized by optional heat treatments at 800 oC for 30h（1023MPa）, 1000 oC for 10h（1052MPa） or 1100 oC for 5h（964MPa）, respdectively. Clearly, increasing the heat treatment temperature will shorten the crack self-healing time. Appropriate heat treatment condition for a complete crack self-healing and strength recovery at 1100 oC for 3h for ZrO₂–Al₂O₃–10vol.%Mo Si2（984MPa）. When the volume fraction of Si C particles in ZrO₂–Al₂O₃ ceramic matrix is 15 vol.%, a complete crack self-healing and strength recovery（1003MPa） could be attained by heat treatment at 1000 oC for a shorter crack-healing time as compared with the ZrO₂–Al₂O₃–10vol.%Si C composite. Increasing the size of self-healing particles could shorten the healing time of cracks, and Si C particles with a size of 300 nm will reduce a crack-healing time of 2h as contrasted with the fine reinforcement particle with a size of 100 nm, and the crack-healed samples with the 300 nm Si C particles have a strength recovery up to 986 MPa. The maximum crack sizes in ZrO₂–Al₂O₃–Si C and ZrO₂–Al₂O₃–Mo Si2 composites that can be completely healed depend on both the crack length and width. For ZrO₂–Al₂O₃–10vol.%Si C ceramics, the maximum crack size which can be completely healed is with a crack length of 150μm and a width of 0.32μm, and the strength can be recovered to a value of 978 MPa under a heat treatment condition of 1000 oC for 10 h. However, for ZrO₂–Al₂O₃–10vol.%Mo Si2 ceramics, the maximum crack size which can be completely healed is with a crack length of 120μm and a width of 0.25μm, and the strength can be recovered to a value of 984 MPa under a heat treatment condition of 1100 oC for 3h.