Research on Denitrification Behavior and Strengthening-toughening Mechanism of Cobalt-free Ti(C,N)-based Cermets with High Nitrogen Content

Author:Zhou Wei

Supervisor:zheng yong


Degree Year:2018





In order to solve the denitrification problems of nitrogen-containing compounds in Ti(C,N)-based cermets,which makes it difficult to obtain full densification during vacuum sintering,the denitrification behavior of cobalt-free Ti(C,N)-based cermets with high nitrogen content,as well as the effects of sintering parameters,chemical composition and preparation methods on denitrification rate,microstructure and mechanical properties had been systematically investigated by X-ray diffraction(XRD),thermal analysis(DTA),scanning electron microscopy(SEM),transmission electron microscopy(TEM),and energy dispersive spectrometer(EDS)in this dissertation.The denitrification and strengthening-toughening mechanisms were both obtained and the comprehensive properties of Ti(C,N)-based cermets were improved greatly.In the first part of this dissertation,the influence factors on the stability of Ti(C1-x,Nx)solid solution and the effects of its composition on the denitrification rate and microstructure of the cermet were studied.The results were obtained as follows:the stability of Ti(C1-x,Nx)solid solution was closely related to its composition and sintering atmosphere,and the final phase composition of the sintered body depended on the values of N/C ratio in Ti(C1-x,Nx)and nitrogen partial pressure in sintering atmosphere.The denitrification rate of Ti(C1-x,Nx)solid solution increased with the increasing of the sintering temperature and decreased with the increasing of N/C ratio in Ti(C1-x,Nx).The denitrification rate of Ti(C,N)-based cermets using different Ti(C1-x,Nx)solid solutions as the nitrogen-containing hard phases increased with the decreasing of N/C ratio.However,the increase of denitrification rate within a certain range did not significantly reduce the density of the cermets.The microstructure of Ti(C,N)-based cermets using Ti(C0.6,N0.4)as the nitrogen-containing hard phases distributed more evenly,and the rims around the surface of hard core phases were more complete with moderate thickness.The typical denitrification process of Ti(C,N)-based cermets with high nitrogen content during sintering and the influence of main sintering parameters on its denitrification behavior were studied.The results show that the denitrification rate at the heating stage was obviously higher than that at the holding stage during the whole sintering process.The denitrification rate was the highest at the heating stage from 1240℃to 1400℃but the lowest at the cooling stage.The denitrification processes of the cermets were mainly controlled by three mechanisms:oxidation,decomposition and dissolution of the nitrogen-containing compounds.The Ti(C,N)-based cermets,which were sintered at 1450℃for 1 h with the heating rate of 3℃/min during 1000℃-1240℃and 2℃/min during 1240℃-1450℃,showed a relatively lower denitrification rate and better microstructure.However,when the heating rate was too fast or the final sintering temperature was too low,the cermet could not be completely densified and the rims became incomplete,which would lead to a significant increase of denitrification rate.The effects of main chemical composition on the microstructure,mechanical properties and denitrification behavior of Ti(C,N)-based cermets with nitrogen content were studied.The main conclusions were obtained as follows:when the Ni content increased from 15wt.%to 45wt.%,the denitrification rate of the cermet increased gradually,the transverse rupture strength(TRS)increased firstly and then decreased and the hardness decreased gradually but the fracture toughness increased gradually.As the Mo content increased from 6wt.%to 18wt.%,the size of hard phase particles was gradually refined,the denitrification rate of cermets was gradually reduced,the TRS and hardness increased firstly and then decreased,while the fracture toughness decreased gradually.The WC contents had an obvious effect on the microstructure of Ti(C,N)-based cermets with high nitrogen content.With the WC content increasing from 0wt.%to9wt.%,the proportion of the black hard core ceramic phase and the thickness of the rims around the black cores decreased gradually while the number of“white core-grey rim”particles and the denitrification rate of the cermet increased slightly.The TRS and hardness of the cermets decreased while the fracture toughness increased gradually.Ti(C,N)-based cermets with high nitrogen content prepared by in situ carbothermal reduction,using MoO3 as the molybdenum source and adding with a proper amount of carbon,has been investigated.The results show that the rapid densification process of the cermets prepared by in-situ carbothermal reduction method proceeded earlier and the volume shrinkage was larger than that of traditional cermets.The carbothermal reduction reactions with gas formation were all completed before the appearance of liquid phase,and the final sintered body reached a relative density of 99.85%and no visible holes remained in the microstructure.The hard ceramic hard phases included both large particles and a large number of small particles.The interfaces among hard cores,inner rims and metal binder phase all showed coherent relationship.The coherent interfaces would improve the interfacial bonding strength of the cermet and resulted in significantly improvement of the comprehensive mechanical properties of the material.The hardness,tranverse rupture strength and fracture toughness of Ti(C,N)-based cermets with the composition of Ti(C0.6,N0.4)-21.0MoO3-35.0Ni-6.2C(wt.%)prepared by in-situ carbothermal reduction were 88.0±0.1HRA,2461±62MPa and 22.3±0.4MPa·m1/2,respectively.The relationship between the non-metallic vacancies in rims caused by denitrification of Ti(C,N)-based cermets during sintering and strengthening-toughening mechanism of the cermets were studied.The results show that the interfacial electron density difference between the(-11-1)plane of rim phase and(110)plane of binder phase decreased monotonically when the Z value,which represents the vacancy concentration of nonmetallic atoms in the rims,decreased linearly from 1.0 to 0.5 but decreased exponentially when the Z value further decreased from 0.5 to 0.25.When the Z value was between 1.0 and 0.25,the crystal structure of the rims remained the same as NaCl-type crystal structure.If the Z value was between 0.64 and 0.25,the interfacial electron density difference between the(-11-1)plane of rim phase and(110)plane of binder phase could be decreased to lower than 10%,so that the electron density on the interface kept continuous and the bonding strength of the interface would be increased obviously,and resulted in great improvement of strength and toughness of the nitrogen-containing cermets.Finally,the microstructure and mechanical properties of Ti(C,N)-based cermets with dual grain structure fabricated by in situ carbothermal reduction of MoO3 and subsequent liquid sintering,using Ti(C1-x,Nx)solid solutions with different particle sizes and N/C ratios as the ceramic phases had been studied.It was found that there were both coarse and fine hard ceramic particles in the microstructure of the cermets with dual grain structure.The coarse ceramic grains still appeared to be the typical“black core-white rim”structure,whereas the fine grains included“white core-gray rim”structure and“black core-rimless”structure.With the increase of the addition of coarse Ti(C0.6,N0.4),the number of hard phase particles with“black core-white rim”structure increased gradually and the number of hard phase particles with“white core-gray rim”structure increased firstly and then decreased.The transverse rupture strength and fracture toughness of the cermets also increased firstly and then decreased while the hardness decreased gradually.When the weight ratio of coarse Ti(C0.6,N0.4)solid solution to fine grain Ti(C0.3,N0.7)solid solution was 50%:50%,the obtained Ti(C,N)-based cermets with dual grain structure showed a better comprehensive mechanical properties having a Rockwell hardness of 92.0±0.2HRA,a transverse rupture strength of 2332±45MPa and a fracture toughness of 11.2±0.4MPa·m1/2,respectively.