Ablation Mechanism Of ZrB₂-based Composites Irradiated By Intense Laser

The doctoral dissertation focused on the ablation mechanism of ZrB2-basedcomposites irradiated by intense laser. Both ZrB2/SiC and ZrB2/Cu compositeswere successfully prepared by spark plasma sintering technology. Some criticalphysical properties were characterized respectively. The abltion experiments werecarried out by intense near-infared laser. The themo-mechanical response onmaterials under different loading condition was researched, and the ablationmechanisms of ZrB2-based composited were revealed.ZrB2/SiC composites exhibit high reflectivity and good thermo-physical properties,but the fracture toughness was only5.56MPa/m1/2. ZrB2/SiC composite burst apartimmediately when the laser with energy of500w irradiated on it. The materialssuffered more serious mechanical damage when the laser energy was higher. TheZrB2/SiC composite exhibits extremely low mechanical laser resistance, because ofits low fracture toughness and rapid local heating of laser. However, there is noother thermal ablation phenomenon occour when the composite irradiated for20sby the laser with energy of500w. High thermal conductivity can homogenized th etemperature, thus the local ablation can be effectively avoided. When the energy oflaser is more than1000w, the composites were ablated apparently because ofoxidation. SiO2could only be stable when the laser of1000w*5s irradiated on thematerials. The evaporation of SiO2resulted in rare oxidation resistance ofcomposites, which could also deterioration the ablation zone because of activeoxidation of SiC.The melting appeared when ZrB2/SiC irradiated by laser for more than20s under1000w. The low thermal conductivity of ZrO2obstructed the inner thermaltransferring, which caused the melting on the surface. The denudation occouredwhen the composite was irradiated by laser of1500w*5s. The pores caused byactive oxidation of SiC resulted in the denudation, since the local mechanicalproperties and conductivities degenerated.The plastic Cu with high reflectivity and thermal properties was selected asaddition to improve ablation resistance of ZrB2-based composite. Somemicrocracks appeared on the ZrB2/Cuccomposite when it irradiated by laser of1000w*10s, which revealed much higher mechanical ablation resistance than ZrB2/SiC. It resulted from that the crack expansion was effectively blocked sincethe fracture toughness was high and the temperature gradient was relieved by highreflectivity. However, the ZrB2/Cucbegan to melt after irradiation by laser of1000w*10s, which meant the threshold of melting was lower than that of ZrB2/SiC.The isolated ZrB2, which caused by priority evaporation of continuous Cu, was tendto melting since the thermal transferring was poor.The microstructure adjustment of ZrB2/Cu was carried out by prolonging sinteringtime and unloading the sintering stress. ZrB2became more connectivity and Cuexhibit as dispersed phase. The microcracks did not appear in ZrB2/Cupuntil itirradiated by laser of1000w*30s. Moreover, the melting on the surface couldobserved in the sample of1000w*30s, which revealed that the threshold of meltingwas twice than that of ZrB2/Cup. The transpiration cooling of Cu was observed inZrB2/Cup, which was helpful to improve the laser ablation, since more Cu coulddissipate energy by state transform.