Applied Fundamental Research On Preparation Of Ti(C,N) Based On Carbothermal Reduction-Nitridation Of Nano-TiO₂

Ti(C,N)-based cermets has more advantages than WC cemented carbide in tool materials, such as comprehensive properties, economic cost, and resource utilization. Thus it has been one of key research fields in tool materials. Recent years, study results show its performance maybe greatly improved, if Ti(C,N)-based cermets was prepared or modified with submicron, ultrafine, or nanosized Ti(C,N) powders. However, only commercial submicrometric(0.5-2μm) powders are available nowadays on the market. Therefore, many studies have been focused on preparing ultrafine or nanosized Ti(C,N) powders.Based on analysis of existing preparation methods, a low-cost method was adopted to prepare Ti(C,N) in this study. That is, Ti(C,N) was synthesized by Carbothermal Reduction-Nitridation(CRN) method from mixtures of nanosized anatase and carbon black. For acceleration of CRN reaction and synthesis of nano-Ti(C,N), two new ideas involved in mechanical milling, i.e. mechanical activation and multiple activation, were brought forward and carried out in preparation of Ti(C,N) powders. The mechanical milling processes, CRN reaction sequences, and characterization of powders in the various states were analyzed using X-ray diffraction(XRD), differential scanning calorimetry(DSC), thermogravimetric analyzer(TGA), X-ray photoelectron spectroscopy(XPS), scanning electron microscope(SEM), transmission electron microscope(TEM). To this day, reaction mechanism of TiO₂ CRN was still very unclear, thus it was investigated in this paper. The experiments confirm that TiO₂ CRN processes can be divided into three continuous stages. In the first stage, TiO₂ was reduced to a series of intermediate titanium oxide(Ti_nO_2n-1), and reductive reaction rate is from slow to fast. In the second stage, Ti(C,N,O) was formed by carbonization and nitridation reaction of Ti₂O₃,or Ti(N,O) was formed by nitridation reaction of Ti₃O₅, and its reaction rate is the biggest among three stages. In the third stage, Ti(C,N) was formed by substitution reaction between nonmetal atoms C and N and Ti(C,N,O) or Ti(N,O), and its reaction rate is the smallest one. At the same time, the results also indicate that nitrogen gas content has a great impact on phase evolution sequences during CRN reaction. As N₂ is sufficient in close reaction system, phase evolution sequences are: Anatase→Rutile→Ti_nO_2n-1(n>10)→Ti_nO_2n-1(10≥n≥4)→Ti₃O₅→Ti(N,O)→TiN→Ti(C,N);otherwise its sequences are: Anatase→Rutile→Ti_nO_2n-1(n>10)→Ti_nO_2n-1(10≥n≥4)→Ti₃O₅→{Ti(N,O) Ti₂O₃→Ti(C,O)}→Ti(C,N,O)→Ti(C,N). The reaction sequences are consistent with thermodynamics analysis on TiO₂ CRN reaction processes.Submicron(～0.5μm) Ti(C,N) powders were prepared by CRN method from nanosized starting materials. For gained Ti(C,N) powders with smaller grain size, high-energy ball milling was used as an activation means, and nanosized anatase and carbon black were activated to a certain extent in advance in air. Ti(C,N) particles with average size of below 100 nm have been synthesized via CRN reaction of activated nanosized starting powders in this study. The experimental results show that, if starting powders were activated by mechanical milling prior to CRN reaction, onset formation temperature and holding time of Ti(C,N) decreased from 1300℃to 1150℃and from 4 h to 2 h, respectively. Due to structure change, crystalline refinement, and homogeneous blend in nano-level induced by mechanical force, reaction activation of starting materials was elevated, reaction driving and diffusing force of TiO₂ CRN were improved, and thus Ti(C,N) nucleation ratio increased. However, mechanical milling did not alter phase evolution sequences of TiO₂ CRN reaction. For synthesis of nano-Ti(C,N) at a lower CRN reaction temperature, even room temperature, a new method was put forward in this paper. That is, some Ti powders were mixed into nanosized anatase and carbon black at first, then starting materials were milled in N₂/Ar atmosphere, and nano-Ti(C,N) powders were formed at last. The experimental results suggested that 40 h milling time was so little that synthetic reaction of Ti(C,N) did not complete in the process of mechanical milling. However, the existence of C-N chemical bond in 40 h activated powders also possibly implied that some C atoms have integrated with N atoms at atomic level to a great extent during mechanical milling. The new means lowered further TiO₂ CRN reaction temperature of 200℃and holding time of 1 h, respectively. The synthesized products were Ti(C,N) powders with below 100 nm particle size. It was main causes that heat energy released by Ti-C-N₂ exothermic reaction during milling or subsequent heat treatment promoted TiO₂ CRN endothermic reaction. In addition, it was an important factor that mechanical milling has resulted in decrement of required activation energy for formation of Ti(C,N) solid solution during subsequent heat treatment.For preparing Ti(C,N)-Al₂O₃ composite cermets with higher hardness and better wear resistance, it is necessary for synthesis of Ti(C,N)-Al₂O₃ composite powders with homogeneous blend and as small as possible grain size. In this paper, mixtures of nanosized anatase, micron-sized Al powders, and nanosized carbon black were reaction-milled for 40 h in N₂/Ar atmosphere, then were heat-treated for 1 h at 1100℃. The formed products were nanocrystalline Ti(C,N)-Al₂O₃ composite powders with submicron-sized particles below 0.5μm. The results revealed that some TiO₂ and Al reacted with N₂ or C to produce TiN or TiC and Al₂O₃. At the same time, mechanical force has also induced bond of between some C atoms and N atoms at atomic level. Due to activation effect during mechanical milling, residual starting materials completed reaction at below 800℃in subsequent heat treatment.