Study On Composite Titanium Nitride Permeation Ceramic Layers And Plasma Characteristic By Double Glow Plasma Surface Alloying Process

Titanium nitride (TiN) coatings have outstanding properties, such as high hardness, low friction coefficient, exceptional wear resistance, high electrical conductivity and fine gold yellow in color, so they have been extensively used for machine, material, chemical engineering, electron, metallurgy, navigation, spaceflight and decoration etc. At present, the main methods of forming TiN coatings are chemical vapor deposition(CVD) and physical vapor deposition(PVD). CVD needs a set of equipment to get TiCl4 gas. The gas has corrosion and will cause serious environmental pollution. Chlorine ion can enter into substrate at the high temperature and cause inter-crystalline corrosion in the substrate and increase the brittleness of TiN coating. Therefore, these all have limited the application of conventional CVD technique to a certain extent. TiN coatings formed by PVD have poor thickness and film quality and mechanical properties and bonding strength. As we know, the bonding strength between film and substrate is a very important parameter in coating applications. It decides the film's mechanical and physical properties. These prevent PVDTiN coating from being widely applied. A interlayer firstly formed by a surface treatment technique under the PVD TiN coating is a measured to improve the film quality and bonding strength. However, these processes are difficult to be widely applied due to complex processing, high cost and expensive equipments. Thus it is very important to find a new way to get TiN multilayer of thick film and excellent bonding strength.To solve these problems, the paper successfully forms TiN/Ti permeation multilayer on carbon steel using a double glow-discharge plasma process(DGP). The TiN/Ti permeation multilayer can be formed at one time by DGP and it has deeper film and better bonding strength. Compared with the general CVD and PVD techniques, the DGP can obtain the permeation diffuse layers with required thickness and excellent bonding strength, and the method also possesses a series of properties, such as save precious metal materials, save energy, good composition controllable and friendly to environment.The general idea of forming TiN/Ti permeation multilayer is:in a double glow plasma surface alloying equipment, the Ti wires are sputtered in the forms of Ti ion and Ti atom by using multiple hollow cathode effect, glow discharge sputtering phenomenon and point discharge of Ti wire among auxiliary cathode. Firstly Ti solid solution diffusion layer is formed on the surface of carbon steel. After Ti permeation is heated for a certain time, N2 is put in and certain proportion is maintained between Ar and N2. Then N2 is also ionized to N atoms and ions under glow discharge plasma, a part of which could participate in cathode surface sputtering of Ti wires and some will combine with Ti atoms on the surface of sample to form TiN deposition layer, other parts are absorbed by the surface of sample and diffuse into the sample to form TiN precipitated phase. When the synthesizing is to the end, the structure of TiN deposition layer +TiN precipitated phase+Ti,N solid solution diffusion layer on the substrate is completely formed. The thickness of this kind of permeation multilayer is above 10μm and the composition are gradually distributed. It has metallurgical bonding with substrate because of phase relations of formingTiN deposition layer on Ti solid solution diffusion. The bonding strength is so high that there is not any spallation phenomenon. The internal stress and thermal expansion coefficient are low due to forming the transition layer of Ti diffusion in advance. The TiN/Ti permeation multilayer can enhance load-bearing capacity to the coating and increase adhesion strength-resistance and corrosion-resisting properties.A complete study of TiN/Ti permeation multilayer via Ti-N alloying layer by DGP technique has not, to our knowledge, been made up to now. The glow discharge plasma characteristic and tribological behaviors and corrosion properties of TiN/Ti permeation multilayer have been studied. The main results have been obtained as follows:1.The composition and structure of TiN/Ti permeation multilayer have been studied. The surface appearances of TiN/Ti permeation multilayer are uniform and compact celluar structure. The permeation multilayer is a duplex layer, composed of surface deposition layer and diffusing layer, that is, the structure of TiN deposition layer+TiN precipitated phase+Ti,N solid solution diffusion layer+substrate. The structure in which the chemical components and hardness change gradiently can enhance load-bearing capacity to the coating and ensure the durability of the coating. The test results show that the thickness of permeation multilayer is above 10μm and can been controlled by the surface-treatment temperature and time. The diffraction apex of TiN(200) is the preferred orientation. Other nitride phasescan be not found.It also demonstrates that the surface TiN layer has a single crystal phase structure.2.The microhardness and scratch tests show that TiN/Ti permeation multilayers enhance the surface strength of substrate. The average values of hardness of TiN/Ti permeation multilayer is 2200 HV0.1. The hardness gradually decreases from the surface to the inner. Bonding strength of DGPTiN and PVDTiN is evaluated by the scratch test. The sound signal fluctuations of DGPTiN do not generate during the loading process, but those of PVDTiN generate under 25N. TiN/Ti permeation multilayer shows the better bonding strength due to its good matching of strength-toughness.3.The tribological properties and mechanism of the TiN permeation multilayer have been studied. Under unlubricated condition, the friction coefficient of DGPTiN nearly does not have any changes and its curve is very smooth which mean fricition coefficient always maintained at 0.06, however the friction coefficient of PVDTiN and PVDTiB2 has large fluctuations and their mean friction coefficient is 0.4 and 0.79, respectively. The friction coefficient of substrate is about 0.26.According to the surface appearance after unlubricated friction of DGPTiN the grinding crack is smooth and shallow. Ihis shows that its friction-abrasion property is improved largely. From the grinding crack appearance of PVDTiN, a great deal of spallation appear around the grinding crack can be seen and furrow phenomenon is severe. The reason is that the excellent wear resistance of DGPTiN comes from its strong enough load-bearing capacity and bonding strengthUnder lubricating condition, with loading incressing the friction coefficient of DGPTiN is almost invariable. It indicates that it has low friction coefficient and high wear resistance.4.Electrochemistry tests show that the corrosion resistance of DGPTiN/Ti permeation multilayer is 10 times,17 times and 143 times as that of PVDTiN, stainless steel and substrate respectively in 10%H2SO4 solution. In 3.5%NaCl solution the corrosion resistance of DGPTiN/Ti permeation multilayer is almost 1.24 times,8.17 times and 30.03 times more than that of PVDTiN, stainless steel and substrate, respectively. In 5%HCl solution its corrosion resistance is about 1.2 times and 2.69 times as that of PVDTiN and substrate. Therefore the corrosion resistance of DGPTiN permeating samples is excellent in acidic solution and in salt water.5.The test of glow discharge strength shows that glow discharge of the non-equal potential and equal potential is uniform and stable. With the increase of the gas pressure and voltage, the strength of glow discharge enchances.The glow discharge characteristic of equal potential (working voltage 400-500V,pressure 30-40Pa) is similar to that of non-equal potential (working voltage 400-500V, source voltage 800-900V, pressure 40-50Pa).6.The electron temperature of glow discharge plasma has been studied. The results show that the main processing parameters of workpiece voltage and working pressure are effective to the electron temperature. The electron temperature increases with workpiece voltage. While the electron temperature changes little with source electrode voltage. When working pressure increases, the electron temperature reduces. Thus gas pressure is usually controlled about 35Pa. The electron temperature reduces initially, then increases and declines finally with voltage increasing when working gas pressure is constant. The electron temperature and power of wire source electrode is higher than those of plane. It is indicated that source sputtering quantities are extremely high in wire source electrode of equal potential.7.The electron density of equal potential glow discharge test results show that the electron density is about 1021 m-3 in a range of discharge voltage 500-1000V, gas pressure 30-100Pa and electron temperature 102-1025eV. The discharge belongs to abnormal glow discharge and can produce a large number of free electron on the cathode because of high electron density. At the same time, closed assistant cathode can not only inhibit free electron move towards boundary but also enhance the effect of thermal retardation and electron density. So in that way the discharge characteristic of equal potential hollow cathode effects has been formed.The electron density of wire source electrode reduces with voltage increasing, but that of plane source electrode increases in equal potential plasma. The electron density of wire source electrode reduces with gas pressure increasing, however that of plane source electrode increases initially, and then reduces when gas pressure charges. The electron density of wire source electrode is up to 9.75×1021m-3 at pressure 30-50Pa, voltage low 500V which almost agree to optimal process of forming TiN permeation multilayer.