Study on Magnetic Properties of Vacuum Sintered Metal Ceramics

Research on Magnetic Properties of Vacuum-sintered Metal-ceramics in Materials Science Zhou Shu-Zhu 1 Zhuo Hai-Yu 2 Hu Mao-Zhong 2 (1. Hunan Metallurgical Vocational and Technical College, Zhuzhou 412000, Hunan, China; 2. Zhuzhou Cemented Carbide Group Co., Ltd., Zhuzhou, Hunan 412000, China) , lattice constant and its cobalt magnetic and coercive force relations. The experimental results show that the cobalt magnetic and coercive force of the cermet alloy increase with the increase of the total content of carbon, nitrogen and oxygen in the alloy. The cobalt magnetic and coercive force of the cermet have a strong positive correlation. Cobalt magnetic energy is used to characterize the changes in the total amount of carbon, nitrogen, and oxygen in the cermet alloy; the coercive force cannot be used as a criterion for evaluating the size of the cermet hard particles. When the total content of carbon, nitrogen, and oxygen is low, the cermet alloy is not magnetic.

2.1 Preparation of metal ceramic alloys 3.1 Fundamental theory of magnetic properties of metal ceramics 19i Zhou Shu-Zi, Nan JingfeiMet. The Powder Metallurgy Research Institute of Central South University is engaged in the development of cemented carbides, ceramics, and new forming agents during the Ph.D. growth period: Research ww.cnki.net 1 In the production of cemented carbide, cobalt magnetic and coercive force are two important physical performance indicators. Cobalt magnetization is the weight percent of the alloy's saturation magnetization that is converted into the cobalt content of the alloy. The magnetic properties of cemented carbide come from the phase composition, quantity, and distribution of the binder phase in the alloy. Magnetic properties are commonly used as a nondestructive testing method to characterize the composition, microstructure, and mechanical properties of cemented carbide. The carbon content of the alloy, while the coercivity as a measure of the grain size of its hard phase. In recent years, cermets have been extensively studied as cutting tool materials for their excellent anti-oxidation, anti-crater wear and high-temperature red hardness properties, and people also want to use magnetic properties to characterize their structure and properties. However, the raw material composition of the cermet alloy is complicated, in particular, carbon, nitrogen, and oxygen therein undergo complex reactions in the sintering process to form a complex structure, and the influence factors of the magnetic properties thereof are also complicated. There have been few reports on research in this area. In this paper, different components of the cermet mixture compaction, sintering in vacuum; using X-ray diffraction, cobalt magnetometer, coercivity and other analytical means of the series of alloys of magnetic properties and composition of the alloy, cobalt crystal phase The relationship between the lattice constants was preliminary studied.

2 Test method The main raw material powder of cermet is commercially available. First, the raw material powder is formulated into a series of mixtures in different proportions. The approximate composition is Ti*WC*15% (Ni+Co). The prepared mixed powder was ball milled and broken with an alloy ball mill barrel, and the mixture was evenly mixed. The ball material ratio was 51 and the ball milling medium was anhydrous alcohol. The ball milling time was 72-96 hours. 2% of a water-soluble polymer was added as a binder, and the mixed slurry was dried and sieved.

The binder-added mix was pressed into bars. The H2 degumming and vacuum sintering of the test strips were completed at a time using a multi-atmosphere sintering furnace produced in Germany. The sintering temperature was 1460° C. and the holding time was 1 hour.

2.2 Magnetic properties of the test strips The sintered test strips were grit blasted and the cobalt magnets and coerciveness of the alloy strips were measured using a 6502 cobalt magnetometer produced by French SETARAM Corporation and a 60 coercivity gauge (stock 5). magnetic force. The cobalt magnetization obtained by the cobalt magnetometer is the saturating magnetization of the cermet that is automatically converted to the weight percentage of the cobalt content of the alloy.

2.3 Chemical composition and phase analysis of metal-ceramic alloys Each of the cermet test bars was crushed in a cemented carbide mortar and pestle, and sieved through an 80-mesh screen. The smashed alloy powders were chemically analyzed to determine their total carbon contents. , oxygen, nitrogen.

3 Results and analysis Table 1 Effects of doping atoms on the magnetic properties of nickel and cobalt alloys. Matrix Doping According to modern physics, the magnetic properties of an atom consist of three parts: the orbital magnetic moment of an electron; the spin magnetic moment of an electron; 3 The magnetic moment of the nucleus (about 1/2000 of the magnetic moment of the electron). The magnetic moment of an electron is the vector sum of the orbital magnetic moment and the spin magnetic moment. As long as one of the electron shells is not filled, the sum of the electronic magnetic moments of the shell is not zero, and the atom must show magnetic moments to the outside. Therefore, when atoms with different electron shell structures form different substances, Showing different magnetic properties. When they condense into crystals, their magnetic properties are derived from 3d electrons because the outer electron orbit is fluctuating by the direction of the lattice periodic field and cannot generate a joint magnetic moment (ie, the orbital torch does not contribute to the total magnetic moment). Shell electrons do not have full spin magnetic moments. However, the key to the material's ferromagnetism is not the size of the magnetic moments of the atoms that make up the material itself, but rather the interactions between the atoms after forming the condensed state. The magnetic properties of a material depend on the interaction between the atomic structure and the atom, which in turn depends on the crystal structure and microstructure of the material. Starting from the analysis of the electronic structure of a material, the mechanism of the generation of various physical properties centered on magnetic properties will be explained. The filling of the outermost layers of the electrons that make up some of the elements of the cermet (ie, the outside determines the shape of the band, and the number of valence electrons determines the extent to which these bands are filled. The 3d electrons in the transition metals Co and Ni are non-stationary. Because of the electron exchange between the s-shell and the d-shell, the ferromagnetism is not only determined by the 3d-shell but also by the 4s-shell electrons.The d-band of Ni splits into spin-up. The difference between the number of electrons per atom spin up and down is called the Bohr magnetic moment, and the two spins downward, and each branch can hold 5 electrons, and the spin up branch is full. Spin-down subbands were only filled with 4.4 holes, leaving 0.6 holes remaining, so on average there were 0.6 spin-unpaired electrons per atom, so the saturation magnetic moment of pure Ni is 0.6H/atom. The saturation magnetic moment is 1.7H/atom.

The change of the magnetic moments of cobalt and nickel caused by the transition metal doping atoms is caused by the following two reasons: 1 the magnetic moment of the doping atoms and the difference in the cobalt and nickel bonding phases; 2 the binding phases around the doping atoms The magnetic moment has changed. The size of the change is determined by the nuclear charge difference between the dopant atoms and the cobalt nickel matrix atoms. From the alloy composition to predict its magnetic properties, cobalt, nickel metal atoms are replaced, the alloy is a mixture of atoms with different potentials, mainly changes in the d state, due to the d-state energy of the doped atoms and cobalt, nickel matrix metal atoms d Different energy states change the energy of the d-band of the substrate. The magnetic properties of some transition metal doping atoms dissolved in Co, Ni, and their alloys are shown in Table 1. From the ferromagnetic brewing curve (see), it can be seen that the magnetic properties of some alloys formed between transition metals vary with composition. . The relationship between the Curie temperature and the concentration of cobalt and nickel alloys d*/dc and the relationship between the magnetic moment and the doping atomic concentration dH/dc are almost the same. When different components make up a cermet alloy, different structures are formed as the composition changes. The magnetic properties of alloys also vary. If a paramagnetic or diamagnetic metal is dissolved in the cobalt-nickel binder phase to form a substitutional solid solution, the saturation magnetization decreases and decreases with the increase in the solute atom concentration, and the higher the solute atom value, the more drastically the reduction occurs. - Electrons enter the unfilled 3d-shell of nickel, resulting in a decrease in the number of Bohr magnets.

3.2 The effect of chemical composition on the magnetic properties of cermets The magnetic properties of cermets come from their magnetic binder metals, whereas the enamels of the cermets are mainly transition metal nickels, and some have a certain proportion of metal cobalt. Cobalt and nickel can form an infinitely soluble solid solution. Tungsten, titanium, niobium and other atoms in the hard phase undergo dissolution and high temperature diffusion in liquid-phase sintering. After sintering and cooling of the alloy, tungsten, titanium, niobium and other atoms in the cobalt-nickel solid solution A certain amount of cobalt and nickel atoms are replaced to form a substitutional solid solution. Light elements such as carbon, nitrogen, and oxygen enter the Cobalt-nickel solid solution gold saturation moment and the average outer layer 3+4 § electron number of the Slat Hshing interstitial spaces to form interstitial solid solution e solute atomic size n. The affinity between the atomic radius and negative charge of the elements in the metal ceramic raw material and the total content of carbon and nitrogen in the total alloy of crucible and the magnetic properties of the metal ceramic (electronegativity), electron concentration and crystal structure and other factors on the solid solubility There are obvious rules of influence. The properties of a solid solution are related to its composition and structure. Specifically, it is related to the atomic size of the solute, the valence of the solute, and the filling of electrons in the energy band. When the cobalt-nickel binder phase contains a small amount of impurities, the conduction electrons of the binder phase are disturbed by the local potential caused by the impurities, and localized changes occur in the distribution of electrons around the impurities. When the Coulomb force interaction between the electrons at the impurity site is particularly large, the local magnetic moment-conduction electrons accompanying the impurity atoms are strongly coupled with the local magnetic moment, and the magnetic properties are abnormal. Table 2 shows the atomic radii and electronegativity of some possible elements in the cermet raw material, and the amount of each element atom in the cobalt-nickel solid solution is different. The number, shape and distribution of these non-magnetic elemental atoms in the cobalt-nickel solid solution and the number and distribution of cobalt-nickel solid solution alloys determine the magnetic properties of the cermet material.

Table 3 shows the contents of carbon, nitrogen, and oxygen before and after the sintering of the various components of the cermet. It can be seen that comparing the atomic radius and the electronegativity in Table 2, carbon is shown. The total carbon and nitrogen oxide content of the total a (wt4) alloy of Yenhe is related to the magnetic properties of the cermet to form interstitial solid solutions. The carbon, nitrogen and oxygen atoms dissolved in the cobalt and nickel binder phase should be very small. The contents of carbon, nitrogen and oxygen have a great influence on the magnetic properties of cermets. From the above, it can be seen that as the contents of carbon, nitrogen, and oxygen increase, the degree of saturation of metal carbides and carbonitrides increases, and the number thereof increases. . The number of atoms of transition elements such as titanium, tungsten, tantalum, and niobium dissolved in the cobalt-nickel binder phase to form a substitutional solid solution decreases, and the cobalt magnetic and coercive forces of the cermet increase at the same time. From Table 3, it can be seen that due to the difference in the manufacturing process of the cermet mixture, the oxygen content of the mixture is relatively different, and part of it is oxygen adsorption. This part of oxygen generally evaporates during the sintering process of the cermet. During the sintering process of cermet, carbon reacts with carbon to produce carbon monoxide. A portion of the carbon in the mixture is lost by oxygen, so that the carbon content in the cermet alloy is reduced to a different extent than the carbon content in the mixture. As the content of carbon and nitrogen in the alloy decreases, the degree of saturation of the metal carbide and the metal nitride decreases, and the metal element dissolved in the cobalt-nickel binder phase increases, and the non-ferromagnetic element is added to the bond metal alloy. The increase of the Curie point and the saturation magnetization of carbon 9, nitrogen and oxygen 2 oxygen and titanium, tungsten, niobium, etc. Joining elements hing reduce cochlear bean coercive force at the same time t drop: low. Comparison of t-i gold e3 and bookmark5 cermet backscattering scanning electron microscopy Photograph 4 Magnetic properties of cermet after vacuum sintering Alloy numbering alloy 4 It can be seen that nitrogen has a greater influence on the magnetic properties, and the increase of nitrogen makes the cermet cobalt magnetic And coercivity increases faster. The difference between the sum is due to the influence of the oxygen content in the alloy, and it can be seen that as the total carbon, nitrogen, and oxygen contents of the alloy increase, the cobalt magnetism and magnetic force of the cermet increase. Comparing the atomic radii and electronegativity of carbon, nitrogen, and oxygen in Table 2 and the curves in the graph, it can be considered that the effects of carbon, nitrogen, and oxygen on the magnetic properties of cermets are that oxygen is greater than nitrogen, and nitrogen is greater than carbon. At the same time, a small amount of carbon, oxygen, and nitrogen enter the cobalt-nickel bonding phase to form interstitial solid solution, so that the Curie point and saturation magnetization of the alloy are increased, and the coercivity increases with the increase of the doping atom concentration. From the magnetic properties in Table 4 and Table 4, it can be seen that there is a strong positive correlation between the cobalt magnetic and coercive force of the cermet.

When the total amount of carbon-nitrogen-oxygen of the cermet alloy is at a relatively high level, the transition metal atoms dissolved in the cobalt-nickel solid solution are few, and the cobalt magnetic and coercive force of the alloy are also high; at this time, there will be a certain amount of Carbon and nitrogen are dissolved in the binder phase solid solution, and atoms of non-metal elements such as carbon, nitrogen, oxygen, etc. can be formed as interstitial solid solution in the solute. The increase of nitrogen content can promote the increase of the amount of solid solution of molybdenum in the binder phase. Conducive to the emergence of ferromagnetic state. Such as alloy 1. As the total amount of carbon-nitrogen-oxygen of the cermet alloy decreases, the transition metal atoms dissolved in the cobalt-nickel solid solution gradually increase, when the total amount of the cermet carbon and nitrogen is at a relatively low level, tungsten, molybdenum, The solubility of transition element atoms such as neodymium, ytterbium, titanium, etc. in the binder phase reaches a high value. At this time, the cobalt magnetic and coercive force of the alloy is close to zero, such as alloy 2. Backscattered SEM for 1 and 2 cermets. photo. From the electron micrographs, the microstructure of the cermet is much more complicated than that of the tungsten-cobalt hard alloy. Due to the formation of the ring structure, the determination of the particle size between the hard phase and the cobalt-nickel binding phase is also difficult. Therefore, unlike tungsten cobalt carbides, it is difficult to use coercivity to accurately reflect the grain size of the cermet hard phase.

4 Conclusion The oxygen content of the metal ceramic mixture is relatively high, in which the oxygen content is about 1.5%. After vacuum sintering, the total carbon of the corresponding alloy will decrease by 1.5%, and the oxygen content in the alloy is 0.1%~0.2%. The vacuum sintering metal The cobalt magnetic and magnetic forces of the ceramic alloy increase with the increase of the total content of carbon, nitrogen, and oxygen in the alloy. When the total content of carbon, nitrogen, and oxygen is low, the cermet alloy is not magnetic. The effect of carbon, nitrogen, and oxygen on the magnetic properties of cermet alloys is that nitrogen is greater than carbon.

The cobalt magnetic and coercive forces of the cermet have a strong positive correlation; the magnetic properties can characterize the changes in the total amount of carbon, nitrogen, and oxygen in the cermet alloy. The coercive force cannot be used as a criterion for evaluating the size of the cermet hard particles.

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