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An Investigation Of TiN Coatings Prepared By Electrospark Deposition Under Nitrogen Atmosphere

Posted on:2008-07-09Degree:MasterType:Thesis
Country:ChinaCandidate:J H NiuFull Text:PDF
GTID:2121360212496965Subject:Materials Processing Engineering
Abstract/Summary:PDF Full Text Request
The malfunction of mechanical products is frequently brought about by the individual part failure which is due to the corrosion, friction and abrasion, fatigue cracks and so forth on partial surfaces. If surface engineering technology is applied to prolong the surface failure time of the sensitive parts, the total performance of the products will be improved, especially for those required to work reliably in the high speed, high temperature, high stress and heavy load service condition. This puts forward a new challenge to processing technologies, promotes the development of surface engineering technology, and calls for the extensive application of surface engineering technology in manufacturing industries.Titanium nitride (TiN) has beautiful golden color. And its melting point is up to 2950℃and Vickers-hardness is about 20GPa. Also it has low friction coefficient, chemical stability and superior corrosion resistance. It can be prepared on the surface of tools, dies and cutting tools to improve their wear resistance and properties of components of aircraft, rockets and other aviation and spaceflight devices. There are two aspects of TiN application, firstly as cermets together with other metal carbides and binding metals such as Ni, Co etc. secondly as wear and corrosion resistance coatings on the material surface.There are lots of methods of preparing the TiN surface layers/films which are divided into the physical and chemical methods. The physical methods, such as physical vapor deposition (PVD) have the advantages of simple technology, saving materials, freedom from pollution, dense layers/films and strong bonding to the substrates. But most of them need high vacuum and high voltage power supply and require special equipments with complicated operations and higher cost. For the chemical methods, although the equipments are generally simpler and less expensive but the working temperature is higher and that results in coarse grains in substrates. Because of large deformation and lower properties of components, the choice of substrate materials is limited. Beside, spraying, ion implantation and general nitriding techniques and laser nitriding methods are also utilized to prepare TiN surface layers/films.The electrospark deposition (ESD) technology is used broadly in practice. The main researches at home and abroad focus on forming new hard phases on substrates through metallurgical reaction by using hard alloy electrodes. The electrode is melted to mix with partly melted substrate to further diffuse and form the surface coatings. The physical diffusion reaction happens between solid and solid or solid and liquid. Moreover, the research on chemical reaction is relatively less for preparing hard reactive coatings, which have different compositions and phases from the electrodes.In this thesis, the ESD technology was put forward to deposit TiN hard reactive coatings on the surface of 45 and stainless steel substrates with TA2 electrode and nitrogen gas as the reacting and shielding atmosphere. In the ESD thermal process, a pulsed micro-arc discharge comes into being between revolving electrode and substrate, both of which are melted to form molten pool, around which a high-temperature and high stress discharge micro-area is formed. The tip of electrode melts into small liquid droplets which are accelerated by N ion flow and impinge on the substrate surface. Meanwhile, N2 issuing from applicator gun mouth is partly ionized by pulsed micro-arc discharge, and the high active N+, N–, N and unionized N2 absorb on the surface of liquid metal and diffuse into molten pool through conquering the superficial energy of solid-liquid interface, and then combine with molten Ti to form TiN in high-temperature molten pool. the deposition coatings containing TiN after cooling is prepared.The thickness of coatings is an important criterion to evaluate the technologic properties of preparing TiN coatings on 45 steel and stainless steel substrates by ESD. The processing parameters of output voltage, capacitance, frequency and specific deposition time affect crucially the structures and performance of deposition coatings. Generally speaking, the discharge energy is inadvisably too strong. And the middle energy parameters with high voltage matching middle capacitance are quite a perfect choice; With the increase of ESD frequency, the thickness of TiN coatings also increases and the dense and even deposition coatings could be prepared through choosing specific deposition time reasonably.During ESD process, the deposition could be achieved when the materials from electrode (anode) are transited to the substrate (cathode). According to the characteristics of ESD for TiN coatings on the surface of 45 and stainless steels, the mass transition rules from TA2 electrode and substrates are studied, and the effect of deposition parameters (including power, voltage and electrode diameter ) on the mass transition coefficient (MTC) is investigated as well. With the increase of capacitance, the mass lose of TA2 electrode increases gradually; With the increase of output voltage, the mass lose of TA2 electrode appreciably increases. When the diameter of TA2 electrode is big, whose mass lose is also large. At the beginning of deposition the MTC increases gradually, and then gradually decreases; The capacitance, output voltage and diameter of electrode all affect MTC.The forming mechanism of TiN deposition coatings, single spot multi-pulse deposition, interface behavior of deposition coatings and phase composition are analyzed in this thesis. The most remarkable characteristics of single spot multi-pulse deposition are splash appearance as the gush of liquid metal and splash. The single deposition spot is mostly composed of substrate elements and countless deposition spots randomly overlap with each other to make up the deposition coatings. The cross-section between TiN deposition coating and the substrate can be clearly divided into three areas: deposition coating, transition zone and substrate. The microstructures of deposition coatings are dense and fine, only showing the micro-crystal structures adjacent to the interface. The element transition happens among three areas and the content of each element doesn't break but transits smoothly, which shows there is a metallurgical bonding between deposition coating and substrate. The main phases of ESD coatings on 45 and stainless steels are composed of TiN and substrate phase. TiN is the new phase formed by electrode material and shielding gas, which is main component of deposition coatings. The deposition coatings aren't the mechanical mixture of electrode and substrate materials, but the reactive products deposited on the substrate surface between electrode materials and the shielding gas.The microhardness test of deposition specimens shows 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 and stainless steels respectively, which are 5 and 3 times higher than that of steel substrates. With the increase of the distance from the top surface, the microhardness presents obvious down trend, which is separately declined to 552HV and 343HV on the interface. Abrasion experiments show the wear behavior of ESD TiN coatings under non-lubricant condition, and it is compared with substrates under the same condition. The wear resistance of the deposited sample is improved remarkably, which is 4 times higher than that of the substrates. The abrasive surface morphologies of deposited samples are smooth and flat, without obvious furrows and chip accumulation. The deposition coating has an excellent dry slip wear resistance than the substrates.
Keywords/Search Tags:electrospark deposition (ESD), titanium nitride, reactive deposition coating, dry slip wear
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