Research On Preparation And Performance Of Ni-Al-Cr Coating By High Frequency Induction-Heated Combustion Synthesis

Ni-Al coating has attracted much attention due to its advantages such as low density,high specific strength and good high temperature performance.However,its poor strength and creep resistance at high temperatures and the easy peeling of oxide scale limit its application.The Cr-alloyed NiAl intermetallic compound coating can greatly compensate for its short board with insufficient high-temperature strength and high-temperature oxidation resistance,so it can be used as a promising high-temperature coating material.In this paper,the high-frequency induction heated combustion synthesis technology(HFIHCS)was used to prepare the Ni-Al-Cr intermetallic compound coating in situ on the surface of the 5CrNiMo die steel substrate.The thermodynamic calculations of the Ni-Al-Cr reaction system were carried out,and appropriate experimental process parameters were formulated.Then,the microstructure of the coating was analyzed by SEM and other detection techniques.Finally,the high temperature oxidation and corrosion performances of the received coating were studied through high-temperature oxidation and electrochemical corrosion experiments.The main conclusions are as follows:(1)When the CS reaction occurs in the Ni-Al-Cr system,the free energy change of the products of the Ni-Al binary system is the smallest;the addition of Cr element will reduces the adiabatic temperature of the reaction system,and the maximum Cr content of self-sustained combustion synthesis reaction of Ni-Al-Cr system at T₀=933K is 27.5 at.%;when the Cr content is 15 at.%,the minimum preheating temperature to ensure the continuous reaction is 874 K.(2)The surface of Ni-Al coating is a single Ni Al phase,and there are NiAl,Ni₃Al and annular island-like mixed structures with Ni as the core at the interface;the Ni-Al-Cr coating is mainly composed of NiAl and?-Cr,and a small amount of chrysanthemum-like eutectic structure was also observed in the coating containing 15at.%Cr.The Cr content in the coating increases,the grain size becomes smaller,and the hardness becomes larger;the grain size and surface hardness of the Ni-Al coating are about 15-50?m and 287.13 HV_0.2respectively,while those of the Ni-Al-15Cr coating are about 2-7?m and 616.52 HV_0.2respectively.In this experiment,the Ni-Al-Cr combustion synthesis reaction is controlled by a combination of dissolution-precipitation mechanism and diffusion mechanism;The strengthening of NiAl alloy by Cr element is dominated by fine-grain strengthening,followed by solid solution strengthening and second phase strengthening.(3)When the Ni-Al-Cr coating was oxidized at 900?for 100 h,the kinetic curve of oxidation film conformed to the parabolic law.With the increase of Cr content,the parabolic index decreased,and the high temperature oxidation resistance of the coating became stronger.The oxide film of the Ni-Al coating is mainly composed of dense clustered Al₂O₃in the inner layer and the mixed oxide of Al and Ni in the outer layer with obvious cracks and peeling traces.The oxide film of the Ni-Al-Cr coating presents a dense and flat Al(Cr)₂O₃with coarse-grained NiAl(Cr)₂O₄spinel phases distributed on it.The overall structure is relatively compact and the delamination is not obvious.With the increase of Cr content,the integrity and stability of the oxide increase,the migration and diffusion speed of electrons or ions decrease,and the adhesion between the oxide film and the coating increases.(4)The self-corrosion potentials and self-corrosion current densities of the Ni-Al-Cr coating in acid,alkali,and salt solutions are higher and lower than those of the substrate respectively,which lead to its better corrosion resistance.The corrosion stability of Ni-Al-15Cr coating is the best in salt solution;the corrosion tendency of Ni-Al-6Cr is the least in acid and alkali solutions.After immersing in Na Cl solution for 168 h,the lowest frequency impedance modulus(|Z|_0.01Hz)of Ni-Al-3Cr coating with lower porosity is the largest,which contributes to higher protection performance.