| Titanium and its alloys are widely used in the fields of aerospace, military, chemical, biomedical engineering and others because of their high specific strength and excellent corrosion resistance. However, the friction coefficient of titanium and its alloys is high and the wear resistance is low compared with steels. All of these shortcomings impede them being widely used. Therefore, surface hardening of titanium and its alloys and improving their wear resistance become a significant subject.TiN has excellent mechanical properties and strong anti-friction and corrosion resistance. The nitrided layers can be prepared on the surface of titanium and its alloys to improve their surface properties. There are a lot of research reports about the nitriding techniques. Among them physical vapor deposition (PVD) and chemical vapor deposition (CVD) are the most common methods. The nitrided layers are deposited to the substrate in the gas by the two methods. And the bonding between the layers and the substrate is weak because no reaction takes place between them. These vapor deposition techniques require high vacuum and high voltage power supply. The price of the equipment is high and the operation is complicated. Besides, because of the restrictions of the deposition rate, the vapor deposition nitrided layer is generally thinner. Laser nitriding is a new emerging nitriding technique to improve the wear and corrosion resistance of the material surface. In the process of the laser nitriding of the titanium, the titanium substrate is directly involved in the reaction, which leads to better bonding. But the generation of the laser mostly needs the ultra-high vacuum and the high pressure. The requirement to the equipment is high and the price is expensive with highly consumed energy, high cost and complex procedure. All of these reasons make it difficult to be widely used in the industry production.The nitrogen plasma flame nitriding technique can overcome the weakness of the preparation methods mentioned above. It's a new technology of preparing in situ nitrided layers on the substrates of titanium and its alloys. The advantages of the technology are simple procedure, easily operated, low-cost and able to be applied to the workplace flexibly.This experiment was done in the atmosphere. In the process, a plasma spray gun and the transformed TIG torch were used to produce the nitriding plasma flame which heated the surfaces of the specimens of the titanium and alloys directly in order to keep them at high temperatures (700℃a nd above, solid-state nitriding). And the shielding gas was the pure N2 or N2+Ar mixture. N2 plays the roles not only to protect the specimens from oxidation but to provide the nitrogen source for nitriding and TiN forming. The heat from the nitrogen plasma flame heats the surface of the substrate on one hand and atomizes and ionizes N2 in the arc on the other hand. These high active nitrogen atoms (N) and ions (N+ or N-) in the arc collide and absorb on the surface of the substrate with the flame-flow. They overcome the surface energy, diffuse into the internal substrate and react with titanium at high temperature. Thus the nitrided layer forms in the process of cooling.During the nitriding experiment, different morphologies and properties of the nitrided layers were prepared. First, surface macro-morphologies and the sectional microsturctures of the nitrided layers were checked and measured by metallographic microscope, SEM and EDS. Then the thickness was measured and the defects were analyzed. Finally, the micro-hardness and wear resistance of both substrate and the nitrided layers were tested and analyzed.The surfaces of nitrided layers which were formed by nitrogen plasma flame on heated TA2 and TC4 showed golden yellow and metallurgically bonded with the substrate. The XRD analysis showed that the nitrided layer was consisted mainly of TiN, less TiN0.3 and fine plateletα-Ti solid solution. The substrate microstructures were coarse and typicalα-Ti solid solution. With the increase of the temperature or the nitriding time the thickness and the nitride contents in the nitrided layers were increased gradually. TiN and theα-Ti dendrites in substrates became coarse grains. Because of high temperature or long nitriding time, the heat input from nitrogen plasma flame to the substrate was high and resulted in lower cooling. The amount of nitrogen absorbing and diffusing into the substrate was high to bring about the adequate nitriding reaction. So the thickness of the nitrided layer and the content of nitride increased respectively, which made the grains grew obviously.Crack defects occured at the interface between the nitrided layer and the substrate because of the high hardness and the brittleness of TiN. And the internal stress was not relived in time, which would cause the formation of cracks in the nitride layer. Another defect was the oxidation accompanied with high-temperature nitriding, which declined the properties of nitrided layers and became the crack initiation site and propagation approach. Serious oxidation resulted in the peeling-off phenomenon. From nitriding with the plasma spray gun to the modified TIG torch nitriding, the velocity of plasma flame and the distance between orifice and the specimen were reduced. The lower degree of oxidationa led to a good effect of nitriding.The test of micro-hardness showed that the nitrided layers could be hardened over substrate. The maximum 1434HV which was about more than 5 times of the substrate hardness, occurred in the upper nitriding limit of 1150℃by 4min. The wear test results showed that the weight losses and the friction coefficient were obviously lower than those of the substrate, which conformed that the nitrided layers had better wear resistance. There were a deep cutting groove and fatigue tearing morphorlogies on the worn surface. The wear mechanism was mainly adhesive and tearing. However, the worn surface of the nitrided layer was smooth without obvious furrows and accumulated worn debris. Its mechanism was mainly of fatigue peeling wear.The change of the hardness of the nitrided layer was the same as the wear resistance. As the increase of the nitriding temperature and longer nitriding time, both the micro-hardness and wear resistance of the nitrided layers were increased because the high temperature and long nitriding time brought about bigger temperature gradient, higher heat input in the nitrided layer. More ionized nitrogen in the arc led to more absorbed nitrogen. It diffused into the substrate and made more hardened phase of TiN in the nitrided layer.According to the experiment of nitrided layers by nitrogen plasma flame, the better processing parameters were nitriding temperature from 1000℃to 1150℃together with nitriding time from 1min to 4 min under the nitrogen plus argon mixture. In that case, the nitrided layers could be obtained with moderate thickness, metallurgical bonding to the substrate, homogeneous and dense microstructures. And also they had high hardness and good wear resistance. The advantages of the technology include short duration of nitriding, easy operation, simple facilities, low cost and especially it can be done in the atmosphere. Thus it enlarges the application scope of titanium and its alloys. These advantages make it a new kind of technology and nitriding method applicable to surface modification for preparation of wear-resistant and corrosion-resistant surface with low cost and high performance. It is looked forward to potential application.
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