The Microstructures And Properties Of Nitrided Layers Formed On Aluminum Substrates By Nitrogen Thermal Plasma

Plasma surface treatment is a new rapid developing technology for materials surfacemodification. By changing chemical compositions and microstructures of materials surfacethe materials performance can be improved significantly. Aluminum nitride (AlN) andother reinforcement phases will be formed in surface layers on aluminum and its alloysubstrates treated by using the plasma contained nitrogen. Thus their wear resistance willbe improved.In this paper, the thermal plasma contained nitrogen was obtained using the plasmaarc equipment. The nitrided layers reinforced by AlN were formed on6082aluminumalloy and1060pure aluminum substrates and the surface nitriding was carried out atatmospheric pressure. The experiment results showed that the nitrided layers were formedwidely and smoothly with uniform microstructure distribution when plasma current is140A, nitrogen and argon gas flow rates were40L/h and25L/h respectively, and theshielding argon gas flow rate was400L/h. The nitrided layers were consisted of aluminumnitride and aluminum solid solution phases. According to the microstructure characteristics,the nitrided layers could be divided into three regions. They were the thin transition regionat the bottom, the AlN dendrite region in the lower part and the thicker lamellar structuralregion in the middle and upper part. Among them, the lamellar structural region wascomposed of thin AlN lamellae and thick aluminum solid solution lamellae in which thetwo phases alternatively distributed. The different microstructures in three regions of thenitrided layers were depended on the AlN growth condition and the mass transfer ofaluminum melt by capillary attraction. According to the dynamic surface nitriding processin which aluminum and its alloy substrates were nitrided by the thermal plasma containednitrogen, a forming mechanism of the nitrided layer was put forward under theexperimental condition.Based on above aluminum substrate nitriding directly by the thermal plasmacontained nitrogen, the nitriding experiments of pure aluminum substrate assisted bytitanium were carried out as well. Under the conditions of sending and pre-coated titanium powders, the nitriding experiments were done by changing processing parameters such asarc currents and nitrogen proportions in arc atmosphere. The microstructures of the nitridedlayers were examined and their properties were studied. The e xperimental results showedthat in the nitrided layers prepared by two kinds of nitriding processes assisted by titanium,the Al3Ti and TiN strengthening phases appeared besides the AlN phase. Among them, theneedle-like Al3Ti phase was blue in the microscope, and the TiN phase was yellow anddistributed among Al3Ti needles. But AlN looked grey. The nano-indentation analyses ofdifferent regions in the nitrided layers showed that the average elastic modulus and theaverage hardness of the solid aluminum solution were77.546Gpa and0.418Gpa. But theywere138.721Gpa and2.649Gpa in the blue Al3Ti intensive distribution region, and192.283Gpa、9.960Gpa in the TiN intensive distribution region respectivelyThe friction wear experiment results of the nitrided layers showed that the wearresistance of the nitrided layer prepared by sending titanium powders was better when thearc current was120A, nitrogen ratio in arc atmosphere was60%. Its average frictioncoefficient was0.2155and the wear weight loss was4.3mg. Much better wear resistance ofthe nitrided layer prepared by pre-coated with titanium powders was achieved when the arccurrent was140A, nitrogen ratio in arc atmosphere was20%. Its average frictioncoefficient was0.2005and the wear weight loss was2.1mg. While in the sameexperimental condition, the friction coefficient of pure aluminum substrate was0.3189andthe wear weight loss was15.6mg. Thus for the nitrided layer formed by sending titaniumpowders, the friction coefficient was decreased to32%and the wear resistance was3.6times higher compared to the aluminum substrate. Moreover, for the nitrided layer formedby pre-coated with titanium powders, the friction coefficient was decreased to37%and thewear resistance was7.4times higher compared to the aluminum substrate. The nitridingprocess assisted by titanium made the harder Al3Ti and TiN phases in addition to AlNformed in the nitrided layers and their wear resistance was obviously improved.