Combustion Synthesis Reaction Behaviors And Mechanism Of The Ni-Ti-C/B₄C-BN Systems

iv TiCxNy-based cermets have became the new type materials used in cutting tools due to their relatively high wear resistance, hot hardness, transverse rupture strength (TRS), good thermal conductivity and oxidation resistance. Traditionally, a two-step process was used to fabricate the TiCxNy-based cermets, i.e., produced TiCxNy powder first, mixed them with the binder phase powders in a composition and then hot-pressed or pressureless sintered them. Such a process was usually done in high temperature and pressure, not only requiring equipment with high quality but also greatly increasing cost. Recently, use of combustion synthesis (CS) and subsequent densification technique to fabricate cermets materials is receiving considerable attention since the process offers certain advantages with respect to simplicity and a relatively low energy requirement. However, data on the self-propagating high temperature synthesis of TiCxNy-based cermets are absent. There are two main factors for this absence. Namely, titanium carbonitrides, in general, were produced in a N2 atmosphere via the combustion reaction among Ti, C and N2, and the pressure of N2 showed a great influence on the stoichiometry and microstructure of TiCxNy. Consequently, it was somewhat difficult to control the offered pressure and microstructure of the synthesized cermets. On the other hand, during the processing, metal of carbide, oxides and metal binder phase were usually needed for the densification of the materials. The exposure of these additives in a N2 atmosphere would result in the nitridation of them, leading to an uncompacted microstructure of the cermets. In the present work, hexagonal BN (h-BN) was used as a solid source of nitrogen to synthesize the TiCxNy-based cermets via CS technique. However, even with many advantages of the CS process, the control of the process is difficult mainly because of the high temperature, high reaction rate in the character of CS process and the complexity in the reactants. As a consequence, the understanding of the reaction behaviors and mechanism of the SHS process is essential and meaningful for better control of the process and the obtained materials.Therefore, Ni-Ti-C/B4C-BN systems were selected as the objective of the present study. The study mainly focused on the exploration of the reaction path and the thermodynamic and dynamic conditions for the ceramic formation under different reaction modes (DTA, SHS and TE), as well as the common regularity and individual difference in the reaction mechanism and behaviors. It is expected that these results can provide an optimal technical parameter range and control the microstructure and the properties of the (TiCxNy-TiB2)/Ni cermets. The main results of the present study can be described as follows:[1] Changes in the standard Gibbs free energy (ΔG0) and enthalpy for the possible reactions in both the systems and the adiabatic temperatures of the two systems were calculated. Thermodynamic calculation results indicated that the ceramic phases, TiC, TiN and TiB2, are stable, while NixTiy,NixBy and TiB phases in the products are metastable. With the increase of the C/(C+N) ratio in the reactants, Tad increases, while it decreases with the increase in the Ni contents. For the Ni-Ti-C-BN and Ni-Ti-B4C-BN systems, SHS reaction can't occur unless the Ni content is below 53.31 and 61.7 wt.%, respectively, and these results were verified by the subsequent experiments.[2] The common regularity and individual difference in the reaction synthesized TiCxNy and TiB2 from the Ni-Ti-C/B4C-BN systems are opened out.(i) The common regularity in the DTA, SHS and TE reaction mechanism for the formation of TiCxNy and TiB2 in the Ni-Ti-C-BN system can be concluded as follows: the reaction starts with the formation of TiNx, TiB and Ni3Ti from the reactions between Ti and BN, Ti and Ni, respectively. Subsequently, the formed TiNx reacts with Ni to form the Ni-Ti compounds such as NiTi and Ni3Ti. As the temperature increases further, Ni-Ti liquid appears between NiTi and Ni3Ti, promoting the dissolution of the C, B and N atoms in the Ni-Ti liquid to form the Ni-Ti-B-N-C liquid. Finally, [Ti] reacts with [B], [C] and [N] in the liquid to form the TiCxNy and TiB2 phases, respectively, and precipitate from the liquid.(ii) The common regularity in the DTA, SHS and TE reaction mechanism for the formation of TiCxNy and TiB2 in the Ni-Ti-B4C-BN system can be concluded as follows: firstly, Ni reacts with B4C to form NixBy intermetallic compounds; meanwhile, Ti reacts with BN to form TiNx; subsequently, the formed TiNx reacts with Ni to form the NixTiy compounds. As the temperature increases further, Ni-Ti and Ni-B liquids appear between NiTi and Ni3Ti, Ni2B and Ni4B3, greatly promoting the dissolution of the C, B and N atoms in the liquid to form the Ni-Ti-B-N-C liquid. Finally, [Ti] reacts with [B], [C] and [N] in the liquid to form the TiCxNy and TiB2 phases, respectively, and precipitate from the liquid. (iii) The common regularity and individual difference in the reaction mechanism of the Ni-Ti-C-BN and Ni-Ti-B4C-BN systems are opened out and can be concluded as follows:a) Common regularity: the TiCxNy and TiB2 phases are formed via the dissolution-reaction-precipitation mechanism, i.e., the formation of Ni-Ti or Ni-B liquid promotes the dissolution of the B, N and C atoms in the liquid to form the Ni-Ti-B-N-C liquid; once the thermodynamic conditions for the formation of TiCxNy and TiB2 are meet, they precipitated from the liquid through the reactions among the [Ti], [B], [N] and [C] atoms.b) Particular regularity: for the Ni-Ti-C-BN system, the reaction starts with the formation of TiNx and TiB from the Ti-BN reaction. The SHS reaction could not be ignited until the formation of Ni-Ti liquid. For the Ni-Ti-B4C-BN system, the solid-state reactions between Ni and B4C, Ti and BN occur firstly, yielding to the NixBy compounds and TiNx, respectively. Subsequently, Ni reacts with TiNx to form NixTiy. As the reaction advanced, Ni-B and Ni-Ti liquids form between NiTi and Ni3Ti, Ni2B and Ni4B3, igniting the SHS reaction in the system.[3] It is found that the reaction paths of the Ni-Ti-C/B4C-BN systems are not consistent with those of Ni-Ti-C/B4C systems due to the incorporation of BN. For the Ni-Ti-C/B4C-BN systems, the reaction commences with the formation of TiNx because of the solid-state reaction between Ti and BN, and the Ni-Ti liquid was formed between NiTi and Ni3Ti (melting point is 1118oC). For the Ni-Ti-C/B4C systems, however, the reaction starts with the formation of Ti2Ni due to the reaction between Ni and Ti, and the Ni-Ti liquid was formed between Ti2Ni and Ti, which possesses much lower melting point 942oC).[4] The kinetics regularity of the combustion synthesized TiB2 and TiCxNy from the Ni-Ti-C/B4C-BN systems were opened out.(i) The initiation and the degree of conversion of the SHS reaction in the Ni-Ti-C/B4C-BN systems are mainly dependent on the formation of the Ni-Ti-B-N-C liquid; the particle sizes of TiB2 and TiCxNy mainly depended on the combustion temperature, which is mainly dependent on the rate and amounts of TiB2 and TiCxNy precipitated from the Ni-Ti-B-N-C liquid.a) The proper addition of Ni to the system benefits the formation of the Ni-Ti-B-N-C liquid, promoting the ignition of the reaction, while the increase in the BN and C/B4C particle sizes would decrease the dissolution rate of the B, N and C atoms in the liquid, resulting in the decrease in the concentrations of [B], [C] and [N] in the liquid and leading to the decrease in combustion temperature, which in turn leads to the decrease in the particle sizes of TiB2 and TiCxNy. Therefore, the initiation of the SHS reaction was retarded and the degree of conversion was decresase.b) An increase in the Ti particle size shows a minor influence on the concentrations of [B], [C] and [N] in the liquid and the precipitation rate of TiB2 and TiCxNy, resulting in a weak influence on the combustion temperature and particle sizes of the ceramic phases.(ii) Due to the Ni-B liquid could form easily in the Ni-Ti-B4C-BN system, the formation of Ni-Ti-B-N-C liquid is easier than that in Ni-Ti-B4C-BN system; consequently, the SHS reaction in the Ni-Ti-B4C-BN system could be ignited more easily and occur more completely.[5] The Ni-Ti-B4C-BN system is suitable to prepare the (TiCxNy-TiB2)/Ni cermets rather than the Ni-Ti-C-BN system. A good combustion reaction processing parameters for the preparation of the (TiCxNy-TiB2)/Ni cermets from the Ni-Ti-B4C-BN system is given as follows: Ni contents: 0~60wt.%, C/(C+N) ratio:0.1~0.9, Ni particle size ~45μm, Ti particle size≤75μm, BN particle size≤75μm, B4C particle size≤40μm.[6] The (TiCxNy-TiB2)/Ni cermets with different Ni contents and C/(C+N) ratios were successfully fabricated in one-step through the combustion synthesis and densification process. The synthesized TiCxNy and TiB2 ceramic phases with relatively fine grain sizes distribute homogeneously in the material and the ceramic particles show an excellent interface with the binder phase. The microstructure and the properties of the (TiCxNy-TiB2)/Ni cermets could be readily controlled through the adjustment of the Ni content and the C/(C+N) ratio. With the increase in the TiCxNy-TiB2 content, the wear resistance of the (TiCxNy-TiB2)/Ni cermets shows an first increase and then decrease with the maximum value at Vceramic=81.43vol.% (i.e., WNi=30wt.%). In this work, the abrasive wear behavior of the (TiCxNy-TiB2)/Ni cermets is"plough"abrasion.In a word, the present study provides theoretical foundation for the fabrication of (TiCxNy-TiB2)/Ni cermets using CS and densification technique. Also, through the investigations of the reaction mechanism, behaviors and final products involved in the Ni-Ti-C/B4C-BN systems, the optimum reaction parameters for the control of the microstructure and properties were determined. At the same time, the present work enriched the research field of (TiCxNy-TiB2)/Ni cermets and provided the theoretical basis and a new way for the preparation of the novel TiCxNy-based cermets, which is meaningful for the industrial application of such materials.