Modification Of C/C-AlSi For High-frequency Pulse Ablation

High power density internal combustion engine piston,pantograph slide,electromagnetic gun armature and other components are often subjected to reciprocating impact of high temperature heat source,the occurrence of ablation will significantly shorten its service life,affecting its work stability.C/C-Al composites are one of the main alternative materials which can be applied in the above environment.Matrix modification is the main method to improve its microstructure and properties.However,the effects of different matrix modification methods on the microstructure and properties of C/C-Al materials are uncertain,and the ablation damage behavior of C/C-Al composites in the high-frequency reciprocal impact environment of heat sources is also unclear.This work focuses on the above problems and investigates the effects of carbon matrix and alloy matrix modification on the microstructure,physical properties,and resistance to high-frequency reciprocal pulse ablation of C/C-Al composites.Resin derived carbon matrix C/C-AlSi and pyrolytic carbon matrix C/C-AlSi composites were prepared by method of resin impregnation carbonization,thermal gradient chemical vapor infiltration and pressure infiltration.The alloy matrix of the resin derived carbon matrix C/C-AlSi was modified by adding Al-Ti-C,La-Ce mixed rare-earth,and Al50Cu intermediate alloy.The mechanical properties,thermophysical properties,and electrical conductivity of C/C-AlSi were tested,and its resistance to high-frequency reciprocal pulse ablation was evaluated,and its ablation behavior was studied in different single loading durations,different combustion temperatures,and accompanying particle erosion environments.The morphology and phase of the material before and after the test were characterized and analyzed by optical microscope,scanning electron microscope and X-ray diffractometer.The results show that:（1）The distribution of different carbon matrixes in C/C-AlSi varies.The resin derived carbon is mainly present between the carbon fiber bundles with uneven distribution.The pyrolytic carbon is sheathed and wrapped around the individual carbon fibers,with a more uniform distribution.The compressive strengths of the composites with the two carbon matrixes do not differ significantly.But C/C-AlSi with resin derived carbon exhibits brittle fracture and C/C-AlSi with pyrolytic carbon exhibits pseudoplastic fracture.The coefficient of thermal expansion of C/C-AlSi with resin derived carbon is lower than that of C/C-AlSi with pyrolytic carbon.The thermal and electrical conductivity of C/C-AlSi with resin derived carbon is poor due to the low graphitization of the resin carbon matrix.In the high frequency pulse ablation test,the poor texture uniformity and thermal conductivity resulted in many defects in the resin carbon C/C-AlSi matrix,resulting in a higher ablation rate than the pyrolytic carbon C/C-AlSi.（2）The Al and Si phases in the C/C-AlSi alloy matrix were modified by adding 4 wt.%of Al-Ti-C and 0.3 wt.%of La-Ce rare earth,and the compressive strength was increased up to 278MPa by adding the Al₂Cu reinforced phase formed by Cu in the alloy matrix.The strengthening of the alloy matrix causes a weakening of the C/C constraint on it during thermal expansion,resulting in an increase in its coefficient of thermal expansion.The thermal and electrical conductivity of C/C-AlSi decreases due to the refinement of alloy organization and the generation of second phase,the increase in the number of phase interfaces and the decrease in the average free range of electrons.The results of high-frequency pulse ablation tests showed that the refinement and densification of the alloy matrix could improve the ablation resistance of C/C-AlSi,with the most obvious effect of adding 0.3 wt.%rare earths,resulting in an 86%reduction in the linear ablation rate.However,the ablation rate will rise again as the addition amount increases（3）With the increase of single impact duration,the material surface temperature rises sharply and the dominant mechanism of ablation changes from thermal stress damage and mechanical spalling to melting,volatilization and thermochemical ablation,and the mass ablation rate and linear ablation rate increase exponentially,at most from 0.17 mg/s and 3μm/s to 6.2mg/s and 23.25μm/s.The ablation rate changes in stages as the plasma jet temperature increases,when the plasma jet temperature changes below 1500℃,the surface temperature is low,only a small part of the alloy matrix softening and deformation and carbon skeleton oxidation can be observed,the mass ablation rate and linear ablation rate change from 0.07 mg/s and 0.14μm/s to0.09 mg/s and 0.24μm/s.When the plasma jet temperature rises above 1500℃,the surface temperature rises,the thermal stress causes the alloy matrix to tear,the carbon skeleton to crack,the alloy matrix and carbon skeleton to mechanically spall,and the mass ablation rate and linear ablation rate increase exponentially.In the ablation-particle erosion environment,the high temperature plasma jet impact causes high temperature softening of the alloy matrix,oxidation of the carbon skeleton,and degradation of the mechanical properties,and the alloy matrix and carbon skeleton are severely damaged under particle erosion,resulting in an increase in mass ablation rate and linear ablation rate to 1.922 mg/s and 13.04μm/s.