Preparation And Corrosion Resistance Of Ti₃AlC₂ MAX Phase Coatings

Ti₃AlC₂ MAX phase materials combine the excellent properties of metals and ceramics,such as machinability,high temperature oxidation resistance,corrosion resistance and radiation resistance,and have broad application prospects in the fields of aerospace and nuclear energy.However,the synthesis of MAX phase is mainly based on bulk materials,and there are difficulties in the preparation process such as high temperature,high pressure and long time,which greatly limits the application of Ti₃AlC₂ materials.At present,coating method can effectively reduce the synthesis temperature of Ti₃AlC₂ MAX phase,and has the advantages of simple process and low cost,which shows great potential for industrial application.In this paper,the rapid preparation of Ti₃AlC₂ MAX phase coatings was achieved at low temperature based on a two-step method of magnetic filtered cathodic vacuum arc（FCVA）and solid-state reaction,and the influences of C₂H₂ flow rate,annealing temperature and annealing time on the microstructure and corrosion resistance of the coatings were systematically investigated.The main work and conclusions are as follows:（1）The effect of C₂H₂ flow rate on Ti₃AlC₂ coatings:The as-deposited coatings with different C₂H₂ flow rate showed smooth,dense and crack-free microstructures.The coatings all showed good adhesion to the substrate,and exhibited amorphous structures.As the C₂H₂ flow rate increased from 9 to 18 sccm,the C content in the coating gradually increased,while the Ti and Al contents gradually decreased and the atomic ratio of Ti and Al in the as-deposited coatings were both 1:1,which was consistent with the Ti Al target.After annealing at 800?C for 1 h,the atomic ratio of Ti and Al in the annealed coating gradually increased from 1:1 to 2:1 with the increase of C₂H₂ flow rate.At low C₂H₂ flow rate（9 sccm）,almost no Ti₃AlC₂ phase was formed in the annealed coating.When the C₂H₂ flow rate was higher than 9 sccm,the annealed coatings mainly exhibited Ti₃AlC₂ phases,the texture of which transformed from（104）to（105）planes.In addition,the hardness,elastic modulus and H/E value of the annealed coating increased gradually with the increase of C₂H₂ flow rate,while the corrosion resistance first increased and then decreased.When C₂H₂ flow rate was 12 sccm,the annealed coating exhibited high hardness（～18.5 GPa）,high elastic modulus（～225GPa）,high H/E（0.081）,high corrosion potential（?0.325 V）,low corrosion current density(5.63?10^-9 A/cm²)and high low-frequency impedance value（4.5?10⁶Ωcm²）.（2）The effect of annealing temperature on Ti₃AlC₂ coatings:As the annealing temperature increased from 600 to 1000?C,the content of Al element in the coating gradually decreased,and the atomic ratio of Ti and Al increased from 1:1 to 3:1.When the annealing temperature was 600?C,the coating contained Ti₃AlC₂ phase with poor crystallinity and a small amount of Ti_xAl_y phase.With the annealing temperature was700?C,the coating showed a structure dominated by Ti₃AlC₂ phase.As the annealing temperature continued to increase to 1000?C,the Ti₃AlC₂ phase in the coating gradually decreased,while the Ti C phase gradually increased.In addition,the increase of the hard Ti C phase caused by the reduction of Al element due to the increase of annealing temperature led to the increase of the hardness and elastic modulus of the coating.However,the corrosion resistance of the coating first increased and then decreased with the increase of annealing temperature,and the coating exhibited the best corrosion resistance when the annealing temperature was 800?C.（3）The effect of annealing time on Ti₃AlC₂ coatings:With the annealing time increased from 1 h to 24 h,the surface roughness of the coating increased,and the content of Ti₃AlC₂ phase in the coating gradually decreased,while the content of Ti C phase gradually increased.In addition,the hardness and elastic modulus of the coating gradually increased with the increase of annealing time,while the corrosion resistance increased first and then decreased.The coating with annealing time of 5 h exhibited better corrosion resistance than the coating with the annealing time of 1 h because of its higher corrosion potential（?0.305 V）,lower corrosion current density(2.98?10^-9A/cm²),higher capacitive arc radius and higher low-frequency impedance value（4.0?10⁶Ωcm²）.