Investigation Of Reaction Behavior And Mechanism Of Thermal Explosion From Ni-Ti-C System Under The Participation Of Air

TiC particulate possesses many desirable properties, such as high hardness, low density, high melting temperature, high modulus, and high corrosion resistance. But, it is an expensive and energy-wasting process to fabricate Ti matix ceramic by traditional way. Additioning low melting metal to Solid-Solid reaction, it can increase contacting areas of reactants and provide an easier way to mass transfer, and the reaction can undergo easier. It is showed that wettability between melting nickel and TiC ceramics so good that interface of nickel and ceramic particals is firm. Addition of Ni not only increased the reaction rate, but also improved wear resistance property.In recent years, a much higher demand has been requested for the research and production on new materials, and thus a large amount of research is needed for TiC composite and its processing.Combustion synthesis has many attractive advantages, such as high purity of products, low processing cost, and energy and time efficiency, no high-temperature furnace process, non-polluting traits, etc. Since 1960s, combustion synthesis has become a very important technique for material synthesis and processing. Two modes are included in combustion synthesis, plane wave propagating (PWP) and thermal explosion (TE), both of which have been widely applied in the material fields. PWP is the mode that the perform is ignited from one end, while TE is the one that the whole perform is heated to the ignition temperature and then thermal explosion occurs through the whole body, ending in an instant.Compared with traditional processing techniques, combustion synthesis has several merits as follows:(1) Simple processing equipments and machines, short processing period and high production rate. Moreover, it has a high reaction speed. For example, TE needs only several seconds to complete, and the PWP combustion wave can transmit at a speed of 250mm/s.(2) Short synthesis production cycle and the great reduction of the cost.(3) Because combustion synthesis can use for Hear Net Shape (NNS) technique, consumption of reactant is decreased greatly.(4) The reaction temperature of combustion synthesis can reach 4000K, and the heating rate of the exothermic reaction is up to 10⁴-10⁶K·s^-1. In SHS reaction, the temperature gradient is up to 10⁵-10⁶K·m^-1. The combustion synthesis has a high temperature in the synthesis reaction, so it can synthesize high purity products. At the same time, such a high heating and cooling rate can obtain the products with the non-equilibrium structures.In the study, the reaction path in thermal explosion (TE) reaction mechanism of the Ni-Ti-C system under air atmosphere, as well as the TE reaction behavior was studied. Moreover, influences of content of Ni, particle size of Ni,Ti and C to TE behavior was studied primarily .The major research efforts of the present study are as follows:1. In the DTA apparatus under air atmosphere, the exothermic reaction path of the Ni–Ti–C system proceed in such a way that Ni initially reacted with O2 to form NiO and Ti reacted with N₂ to form TiN_0.3, and then Ti reacted with O2, simultaneously, C reacted with O₂. Ti reacted with N₂ and O₂ untill Ti exhausted. All of these reactions accompany with high heat release, and until 1200℃, there is not TiC in the product. 2. TE reaction mechanism under air atmosphere can be described as"chemical oven", The reaction path is as follows: initially, particles in the surface of the sample which can contact with air directly react with element of air. Ni reacted with O₂ to form NiO and Ti reacted with N₂ to form TiN_0.3, and then Ti reacted O₂, meanwhile C reacted with O2,All of these reactions accompany with high heat release, so temperature of the whole sample rose very quickly ,and the above reaction took turns at a very short time. These heats can promote Ni to react with Ti to form Ti₂Ni in the entire sample. The appearance of Ni–Ti eutectic liquid phase between Ti2Ni and Ti were followed, which enabled the mutual diffusion of Ni, C and Ti in solution to form Ni–Ti–C liquid more easily and quickly at a low temperature; finally, TiC was precipitated out of the liquid.3. Compared with the reaction in the vacuum, the TE reaction in the Ni–Ti–C system under air atmosphere is easier to occur. The products mainly consist a large number of TiC. However with the increase of Ni content, the ignition temperature exhibits no markable change, its range is from 519℃to 575℃, while Ni contents is from 10wt.% to 50wt.%. However, when Ni content rose to 60wt. %, the ignition temperature rose to 644℃. Along with the increase of Ni content, the combustion temperature decrease. With the decrease of Ni,Ti and C particle size, the ignition temperature decreases firstily and then , but the combustion temperature increases. Furthermore, particle size of Ni and Ti effect the ignition temperature of TE reaction in air remarkably.