Study On TiN Layers Prepared By Nitrogen Arc Melting Technique

With the development of scientific technology and domestic economy, all industries have an increasing demand for materials properties, particularly the surface properties of materials. Materials surface modification technology developed continuously. Surface melting technology could change the chemical composition, microstructure and morphology, and reinforce the hardness and wear resistance by preparing the surface layers on the surface of metals, which improves material surface properties effectively and increases productivity. It has become an important manufacturing technology to improve the quality and life of products.Titanium and titanium alloys having excellent properties (high specific strength and corrosion resistance) are widely used in many areas such as aerospace, military, chemical engineering, biomedical engineering, etc. However the application of these metals in engineering is restricted because of high friction coefficient and poor wear resistance. Therefore, raising the wear resistance of Ti and its alloys has become a research focus for materials researcher at home and abroad. Owing to the superior mechanical properties, excellent corrosion and wear resistance, TiN coatings are often prepared to modify the surface properties of titanium and titanium alloys.A number of techniques have been reported to prepare the TiN coatings. Among nitriding techniques, chemical vapor deposition (CVD) and physical vapor deposition (PVD) methods are the most commonly used. In both processes, nitride coatings are deposited on the substrate surfaces in the gaseous environment. As an interaction between the substrate and the deposit is not requisite for coating growth in either CVD or PVD, the nitride coatings often suffer from lace of adherence. Most of vapor deposition technologies need vacuum chambers with high voltage power supply and special equipments with complicated operations and high cost. In addition, owing to the limit of deposition rate, the nitride coatings are relative thin. Surface treatment by laser nitriding is one of the up-to-date methods for improving the wear and corrosion behavior of metal alloys. During laser nitriding of titanium,titanium substrate is directly involved in the reaction of layer formation, which results in an excellent adhesion of the layer to the substrate. However, the expensive laser equipment makes this process inherently cost much. Nitrogen arc melting technique is a new method for in-situ preparing the nitrided layers on titanium and titanium alloy substrates at atmospheric pressure. During nitrogen arc melting process, a thick TiN layer forms on the titanium and titanium alloy substrate, which is directly involved in the reaction between titanium and nitrogen from arc atmosphere. Its advantages are that there is no need for complex and expensive equipment, and the operation is simple and flexible. In addition, the nitrided layers are metallurgically bonded to the substrates.In this research, nitrogen arc melting technique is presented, which allows for in-situ preparing TiN layers of titanium and its alloy at atmospheric pressure, and the microstructrue, hardness and wear properties of the layers were analyzed systematically. At same time, the growth mechanism of TiN reinforced phases, the factors and rules influencing microstructure and properties are investigated.In the nitrogen arc melting technique for preparing the nitrided layer on the surface of titanium substrate, the nitrogen gas issuing from the ceramic nozzle effectively avoids the formation of oxides and also provides nitrogen for the formation of TiN. The formation process of TiN prepared by nitrogen arc melting may be described as follow. After the arc is struck between the tungsten cathode and the titanium substrate by a high frequency ignition, the titanium substrate surface is heated and molten to form the molten pool and the nitrogen is ionized to ions and atoms. The active nitrogen ions (N~+ or N~-) and atoms (N) in the arc atmosphere absorb on the surface of liquid titanium, overcome the surface energy and diffuse into the molten pool, and react with molten Ti at high temperature. TiN dendrites are then formed in molten pool during cooling.The nitrided layers have a golden color, which obtained by arc melting technique on titanium and its alloys, and have a good metallurgical bonding with substrates. The nitrided layers consisted mainly of TiN phase and small amounts of TiN0.3 phase dispersed in the TiN dendrites. From the microstructure of cross-section of the layers, the TiN is mainly dendrites and the content of the TiN dentrites gradually decreased towards substrates. The nitrided layers have a good adhesion with substrate because it is directly involved in the reaction of layer formation. The TEM examination indicats that the gray phase with stripe distributed between the TiN phases is TiN0.3 phase. As increasing the arc current, and decreasing the arc traveling speed and the nitrogen to argon ratios in mixture gas, the amount of TiN increased. These are all attributed to the increasing of arc heat input. The hard TiN phase has an obvious effect on the crack tendency. Decreasing arc current and increasing arc traveling speed favor reducing the nitrided layer crack. Decreasing nitrogen to argon ratio in the mixture gas and reducing the proportion of TiN can restrain the crack formation in nitrided layers. There are less TiN and moreα-Ti in the nitrided layer parepared on titanium alloy substrate. Compareing with TiN phase, theα-Ti phase has plastic and can inhibit formation and expansion of crack in the nitrided layers. Therefore, there is smaller crack tendency in the nitriede layers prepared on titanium alloy than that prepared on titanium substrate.In this study, it is found that under the same nitrogen arc discharge parameters, the size and amount of TiN dentrite in nitriding layer prepared on titanium alloy substrate is smaller than that prepared on titanium substrate, and the amount ofα-Ti phase is more than that in titanium nitrided layer. This may be due to the elements of Al and V in titanium alloy substrate having little reaction with N to form dentrides, and the amount of Ti is fewer. In addition, the element of Al promotes the formation ofα-Ti. The result of element map distribution has proved this point that the element Al and V did not react with N to formate dendrite, but as Ti solid solution distributing between the dendrites. The study also find that some small needle-like martensiteα′-Ti occurred in the heat-affected zone on titanium alloy substrate, and less martensite also occurred in the heat-affected zone on titanium substrate. This may be due to the element of V in titanium alloy substrate inhibits theβphase transiting toαphase, and theβphase directly transit to martensite during fast cooling.The cooling rate is 10-2～10-3 K/s from the research of the secondary dendrite spacing of the nitrided layer, so it is the rapid cooling process. Under the non-equilibrium condition of nitrogen arc melting, because of the compositive function of directionality of thermal conduction and latent heat emission in the course of the fabrication of the TiN in molten pool, the temperature distribution in the molten pool has a certain direction. This resulted in TiN newborn nucleation along the heat conduction directions. The macroscopical growth mode of TiN crystal is that, the TiN nuclei formation along the direction of thermal conduction to formation the stem of dendrite in the molten pool. The crystallization latent heat released during stem crystallization process has the minimal effects on the dendrite arm at the direction perpendicular to the stem. Therefore, the secondary dendrite arm grows in the sticks of the stem along the direction perpendicular to the stem. From the microcosmic perspective, the TiN crystal grows by spiral dislocation lateral growth mode at the location of small undercooling in the molten pool. With increasing of undercooling, the growth mode changes to the alternating of continuous growth and lateral growth.The nitrided layer has a high microhardness of about 2600 HV, and the hardness decreases with the increase of layer depth. The arc current, arc traveling speed and nitrogen to argon flow ratios have obvious effects on both the hardness and the wear properties of the nitrided layers. Increasing the arc current, decreasing the arc traveling speed and increasing nitrogen to argon flow ratio favor increasing the amount of TiN phase, the hardness and wear resistance of the nitrided layers.According to the results of abrasive wear test, main abrasive mechanism of the nitrided layer is pull-out, short and shallow grooves developed on the worn surface of nitrided layers. The worn surfaces of titanium and titanium alloy substrate have deep groove cutting and floc tear, so it is typical abrasive wear.In this thesis, the electrospark deposition (ESD) technology is put forward to deposit TiN hard reactive coatings on the surface of 45 and stainless steel substrates with pure titanium electrode and nitrogen as the reacting and shielding atmosphere. In the ESD thermal process, a pulsed micro-arc discharge comes into being between revolving titanium electrode and substrate. The high temperature generated in the discharging micro-zone by the spark results in partially melting of the tip of the electrode and superficial layer of substrate material. The liquid droplets form in the tip of electrode are accelerated by nitrogen ion flow and impinge on the molten pool, and the metal transfer takes place from the electrode into the molten pool on the substrate surface. At the same time, nitrogen in the arc atmosphere is partly ionized by pulsed micro-arc discharge. The active N~+, N– and N absorb on the surface of liquid metal, overcome the surface energy and diffuse into the molten pool, and then combine with molten Ti to form TiN. The TiN precipitates to form a TiN coating during molten pool cooling.The phase composition and interface behavior of deposition coatings and the formation mechanism of TiN deposition coatings are analyzed in this thesis. The TiN coatings have a golden color, and their surfaces consist of countless deposition spots which overlap remelting with remarkable splash appearance. There is a transition zone existing between the TiN coating and substrate. The element transition happens in the transition zone, in which there is a metallurgical bonding between deposition coating and substrate.The microhardness test of deposition specimens shows that the deposition coatings can remarkably enhance the superficial hardness of the steel substrates. The hardness of deposition coatings is up to 1515HV and 877HV for 45 steel and stainless steel respectively, which are 5 and 3 times higher than that of substrates. The hardness decreases with the increase of layer depth.because of decreasing the amount of TiN. Experimental results of wear and abrasion show that the wear resistance of the deposited sample is improved remarkably. The abrasive surface morphologies of deposited samples are smooth and flat, without obvious grooves and chip accumulation. The deposition coating has better dry slip wear resistance than the substrates.Nitrogen arc melting technique can be used to prepare surface TiN layers with appropriate thickness, which metallurgically bond to substrates. It is easy for application because of the cheap equipment, simple technology and low cost. In one word, it is a new and convenient technique to prepare TiN surface layers and has a potential application in future. The two different and complementary processing techniques given by the research will provide the more important theoretical basis and a new way for the research of TiN layers.