碳化钛粒径对碳氮化钛基金属陶瓷微观结构和力学性能的影响

用粉末冶金真空烧结法制备了超细晶粒碳氮化钛[Ti(C,N)]基金属陶瓷.研究了原始粉末粒径对Ti(C,N)基金属陶瓷微观结构和力学性能的影响.结果表明:在化学成分相同的条件下,晶粒细化使材料的Vickers硬度和抗弯强度上升,但断裂韧性有所下降.在超细晶粒Ti(C,N)基金属陶瓷微观组织中出现了一种新型的白芯/灰壳结构和一种特殊化合物(Ni2Mo2.5W1.3)Cx.初步研究表明:由于原始粉末粒径微小,促进了扩散反应因而生成了这种芯/壳结构.芯/壳结构有利于提高材料的抗弯强度和断裂韧性.(Ni2Mo2.5W1.3)Cx有利于提高材料的Vickers硬度,但是降低了Ti(C,N)基金属陶瓷的抗弯强度和断裂韧性.     

Microstructure evolution and characteristic of Ti(C,N)-based cermets prepared by in situ carbothermal reduction in TiO2

Ti(C,N)-based cermets were prepared by in situ carbothermal reduction in TiO₂ and subsequent liquid phase sintering under vacuum. The prepared cermets were examined using XRD, SEM, TEM, and EDX. During solid-state sintering, fine TiC particles were formed through the carbothermal reduction in TiO₂. A great number of (Ti,W,Mo)C complete solid solutions containing more W and Mo subsequently formed through the counter diffusion of the fine TiC and carbides. The majority of the coarse TiN particles in the raw powders remained undissolved. During liquid phase sintering, Ti-based carbonitride complex solid solutions with less W or Mo precipitated on the coarse TiN particles and fine (Ti,W,Mo)C particles, resulting in black core/gray rim structures and white core/gray rim structures, respectively. Moreover, small amounts of Ti-based carbonitride complex solid solutions precipitated directly from the liquid binder phase in some areas enriched in W and Mo during the cooling stage after sintering, resulting in coreless grains. Ultimately, after being sintered at 1400°C for 1 hour, the present cermets were characterized with white core/gray rim grains, black core/gray rim grains and a few gray grains. In addition, the interfaces between the black core/gray rim grains and binder phase were atomically smooth, exhibiting a orientation relationship with a perfect coherency state.     

Ti(C,N)基金属陶瓷中陶瓷相芯/壳组织的观察与分析

用ＸＲＤ，ＳＥＭ，ＴＥＭ和ＨＲＥＭ观察、分析和方法研究了Ｔｉ（Ｃ，Ｎ）基金属陶瓷中陶瓷相的芯、壳组织。结果表明，在ＳＥＭ，ＴＦＭ观察中，芯、壳之间存在相界面，而ＨＲＥＭ观察显示陶瓷相的芯／壳组织具有连续相同的点阵结构。     

Effect of WC content on the microstructures and corrosion behavior of Ti(C,N)-based cermets

The influence of WC content on the microstructure and corrosion behavior of Ti(C, N)-based cermets in 2 mol/L nitric acid solution was studied in this paper. There exists typical core/rim structure in the cermets. The cores appear black or white, and the rim is divided into white inner rim and grey outer rim. The undissolved Ti(C, N) particles normally appear as black cores, while the white core, inner rim and outer rim are (Ti, W, Mo) (C, N) solid solution formed at different sintering stages. The inner rim and white core appear brighter atomic contrast than the outer rim and black core, which is attributed to their higher W and Mo content. The thickness of the inner rim increases with WC addition, but the grain size of core/rim phase becomes finer. Meanwhile, the amount of white cores increases and that of black cores decreases. WC is more easily oxidized and dissolved in the nitric acid solution, compared with Ti(C, N). Therefore, the degradation of inner rim phase and the white core becomes more considerable with the increase of WC content. Consequently, the corrosion rate of cermets increases and the corrosion resistance of Ti(C, N)-based cermets is deteriorated with the increase of WC content.     

Effects of TiC/TiN addition on the microstructure and mechanical properties of ultra-fine grade Ti (C, N)–Ni cermets

Two series of Ti (C, N)-based cermets, one with TiC addition and the other with TiN addition, were fabricated by conventional powder metallurgy technique. The initial powder particle size of the main hard phase components (Ti (C, N), TiC and TiN) was nano/submicron-sized, in order to achieve an ultra-fine grade final microstructure. The TiC and TiN addition can improve the mechanical properties of Ti (C, N)-based cermets to some degree. Ultra-fine grade Ti (C, N)-based cermets present a typical core/rim (black core and grayish rim) as well as a new kind of bright core and grayish rim structure. The average metallic constituent of this bright core is determined to be 62 at% Ti, 25 at% Mo, and 13 at% W by SEM–EDX. The bright core structure is believed to be formed during the solid state sintering stage, as extremely small Ti (C, N)/TiC/TiN particles are completely consumed by surrounding large WC and Mo₂C particles. Low carbon activity in the binder phase will result in the formation (Ni₂Mo₂W)C_x intermetallic phase, and the presence of this phase plays a very important role in determining the mechanical properties of TiN addition cermets.     

Morphology evolution of TiC-based cermets via different sintering schedules

Solid state sintering, liquid phase and cooling stages play different roles in determining the final morphology and composition of cermets, especially the well-known core-rim structure. In this work, TiC-(5–25 wt%)WC-11Mo₂C-18(Ni-Co) cermets were prepared and sintered by different sintering schedules. Morphology evolution and rim phase composition during sintering from 1250 °C to 1600 °C were investigated. Effects of sintering stages on the final morphology of cermets were also studied. It was shown that submicron (Ti, W, Mo)C grains tend to precipitate in binder during the cooling for cermets with high WC content. After the formation of outer rims during liquid sintering stage, interface reaction began to take effect between the rims and core. Coreless (Ti_0.76, W_0.13, Mo_0.11)C ceramic grains would be formed under high temperature (1600 °C) for TiC cermets with 25% WC. Long time sintering at solid state favored the formation of black core-thick inner rim and bright core-grey rim phases, while cooling near the melting point could result in submicron bright particles. This study provided not only a better view of the formation of rim-core structure but also an easier way to control the final morphology of cermets via reasonable changing the sintering cycle.     

Evolution of microstructure and interfacial characteristics of complete solid-solution Ti(C,N)-based cermets fabricated by mechanical activation and subsequent in situ carbothermal reduction

Complete solid-solution Ti(C,N)-based cermet, with no typical core-rim structure, was synthesized through mechanical activation and subsequent in situ carbothermal reduction method. XRD, SEM, TEM, and C/N analysis were used to investigate the microstructure, phase transformation, and the interfacial characteristics of the present cermets. During solid-state sintering, the (Ti,Mo)C/(Ti,Mo)(C,N) phases formed through the transformation of Mo-based solid solution which generated by mechanical activation. Then, the formed (Ti,Mo)C/(Ti,Mo)(C,N) continuously dissolved into the nickel-based binder above 1100 °C. It was found that in the subsequent stage of liquid sintering, the mechanical activation and also the presence of extremely fine TiC/Ti(C,N) particles accelerated the Mo diffusion into the hard phase, resulting in a large quantity of (Ti,Mo)(C,N) solid solutions formed in the nickel-based binder. Finally, complete (Ti,Mo)(C,N) solid-solution phase was obtained via dissolution and re-precipitation. The higher toughness and transverse rupture strength (TRS) of the synthesized new cermet, as compared with traditional cermets, were mainly caused by the increased crack deflection and transgranular fracture of the novel cermets. Moreover, the interface among the Ni-based binder phase and complete solid solution hard phase exhibited a semi-coherency state with high-density dislocations, which also significantly improved the TRS and toughness of the synthesized cermets.     

Influence of molybdenum addition on the microstructure and mechanical properties of TiC-based cermets with nano-TiN modification

Effect of Mo addition on the microstructure and mechanical properties of TiC–TiN(nm)–WC–Co–Ni–C system cermets was studied in the work. Specimens were fabricated by conventional powder metallurgy techniques. The microstructure was investigated using transmission electron microscope (TEM) and the scanning electron microscope (SEM). Chemical compositions of different phases such as ceramic phase with core/rim structure [the core being TiC and rim being (Ti,W,Mo)(C,N)] and metallic phase were analyzed quantitatively by EDX. Mechanical properties such as flexural strength, fracture toughness and hardness were also measured. Results show that flexural strength and fracture toughness have a trend to decline with increasing Mo addition, but the change of hardness is not apparent with the increase of Mo addition. Results also reveal that finer microstructure and thicker rim phase will be obtained with the increase of Mo addition. The optimal addition of Mo can be estimated to be 4 wt.% with respect to TiC–10TiN(nm)–15WC–5Co–Mo–5Ni–1C system cermets. Fracture micrographs show that main failure mode of the cermets is a mixed one, i.e., trans-granular and inter-granular fractures both exist.     

Microstructure and mechanical properties of Ti(C,N)-based cermets fabricated by in situ carbothermal reduction of TiO2 and subsequent liquid phase sintering

Ti(C,N)-based cermets were prepared by in situ carbothermal reduction of TiO₂ and subsequent liquid phase sintering in one single process in vacuum. The densification behavior, phase transformation, and microstructure evolution of the cermets were investigated by DSC, XRD, SEM, and EDX. The results showed that the carbothermal reduction of TiO₂ was completed below 1250 °C, and Ti(C,N)-based cermets with refined grains were obtained after sintered at 1400 °C for 1 h by this method. The hard phase of the cermets mainly exhibited white core/gray rim structure, in great contrast to the typical black core/gray rim structure of hard phase in traditional cermets. Ti(C,N)-based cermets prepared by this novel method showed excellent mechanical properties with a transverse rupture strength of 2516±55 MPa, a Rockwell hardness of 88.6±0.1 HRA, and a fracture toughness of 18.4±0.7 MPa m^1/2, respectively.     

Grain Growth in Ti(C,N)‐Based Cermets During Liquid‐Phase Sintering

Growth behavior of Ti‐based carbonitride ceramic grains in two high‐Mo Ti(C,N)‐based cermets with Ni and Ni–20Cr (wt%) binders was investigated during liquid‐phase sintering under vacuum at 1410°C, using DSC, XRD, SEM, AEM, and EDS. Grain growth occurred primarily through two‐dimensional nucleation and lateral growth. Most significantly, the grain growth kinetics followed the cubic law, which was controlled by the diffusion of dissolved mass through liquid Ni‐based binder phase. However, when Ni–20Cr (wt%) was used as metallic binder, the inner rim of ceramic grains with the typical core‐rim structure was seldom complete, and there were often some fine Ni‐rich and Mo‐rich speckles in their core. In Ni‐rich and Mo‐rich speckles, there were two kinds of microstructure: one consisted of Ni‐based superlattice phase, and the other consisted of Ti‐based carbonitride ceramic phase and unknown phases. The three‐dimensional thermodynamic equilibrium shape of Ti‐based carbonitride ceramic grains evolved from a {111}‐faceted and round ‐edged octahedron to a {111}‐faceted and sharp‐edged octahedron. In addition, the grain growth rate increased, which was mainly attributed to that the decrease of solid/liquid transformation temperature of Ni‐based binder phase led to the increase of the diffusion rate of dissolved mass through liquid Ni‐based binder phase.     

Effect of short carbon fibre concentration on microstructure and mechanical properties of TiCN-based cermets

Short carbon fibre (C_f) reinforced TiCN-based cermets (C_f/TiCN composites) were produced by powder metallurgy method with pressureless sintering technology. The phase evolution, microstructure and fracture morphology of C_f/TiCN composites were investigated. The results showed that TiC, TiN, WC, Cr₃C₂ and Mo phases disappeared gradually and diffused into core and rim phases by dissolution–reprecipitation process, finally formed new hard TiCN core phases and complex compound (Cr, W, Mo, Ti)(CN) rim phases, with the sintering temperature increasing. The added C_f did not change the ‘core–rim’ microstructure but improved the mechanical properties of TiCN-based cermets. The C_f/TiCN composite containing 3?wt-% C_f achieved the best comprehensive mechanical properties, with fracture toughness and bending strength increasing by about 14.4% and 30.8%, respectively, when compared with the composite without C_f. Toughening and strengthening mechanisms of C_f/TiCN composite were concluded as crack deflection and branch, as well as the pull-out, fracture and bridging of carbon fibres.     

Fabrication of Ti(C,N)‐based cermets by in situ carbothermal reduction of MoO3 and subsequent liquid sintering

Ti(C,N)‐based cermets were fabricated by in situ carbothermal reduction of MoO₃ and subsequent liquid sintering in a single heating process. The densification behavior, phase formation, and microstructure evolution of the cermets were characterized by DSC, XRD, SEM, and TEM. The results showed that near‐fully dense Ti(C,N)‐based cermets with fine carbonitride grains could be obtained by the above‐mentioned method. The carbonitride grains of the cermets still exhibited typical core/rim structures and evenly distributed in the binder phase, but the rim phase was more complete and thinner compared with traditional cermets. In addition, the interfaces between the ceramic phase and binder phase of the cermets were atomically smooth, having the orientation relationship of ()_R//(110)_B with a perfect coherency state. The prepared Ti(C,N)‐based cermets produced with MoO₃ showed excellent comprehensive mechanical properties having a transverse rupture strength of 2461±62 MPa, a Rockwell hardness of 88.0±0.1 HRA, and a fracture toughness of 22.3±0.4 MPa·m^1/2, respectively.     

Ti(C,N)-based cermets containing uniformly dispersed ultrafine rimless grains: Effect of VC additions on the microstructure and mechanical properties

Difference in lattice parameters and physical properties of core-rim structure leads to mismatch at the core-rim interfaces. In this respect, VC was adopted to shrink the lattice of rims and intended to reduce the lattice misfit. Small amount of VC (0.8 wt%) effectively decreased the grain size from 0.77 μm to 0.57 μm, with an increased lattice parameter of the core-rim phase. Higher VC content accelerated the dissolution and precipitation process, resulting in the formation of thicker rims and smaller lattice parameters of core-rim structure. The same lattice parameters of core-rim phases were achieved with 4.8 wt% of VC addition. A distortion zone with a thickness of 3–5 atomic layers was confirmed, which located at the boundary of binder and gray rim. VC induced the formation of spherical rimless grains, which were uniformly dispersed in the binder and usually attached to the coarse core-rim grains. Black ultrafine rimless grains exhibited larger lattice parameters than Ti(C,N) cores due to the increased C/N ratios. Ti(C,N)-based cermets with uniformly dispersed rimless grains were achieved, which showed effectively improved the bending strength of cermets, with in-situ formed dimples inside the binders during the fracture.     

Microstructure evolution and performance improvement of TiN‐based cermets with carbon addition

In this work, TiN‐based cermets with excellent performance and uniform microstructure were successfully manufactured by conventional vacuum sintering with 0‐5 wt% carbon addition at 1500°C. Influence of carbon addition on the microstructure and mechanical properties of cermets was investigated by scanning electron microscope, transmission electron microscope, X‐Ray diffraction, electron probe microanalysis, and mechanical tests. The results showed that small amount of carbon helped to improve significantly the wettability between TiN and Ni/Co, leading to well‐distributed structure and perfect core‐rim phases. As the carbon content increased from 0 to 5 wt%, mechanical properties of cermets increased initially, displayed a maximum and then decreased. For the experimental conditions considered, the cermets with 3 wt% carbon addition revealed best mechanical properties. The relative density, the transverse rupture strength, fracture toughness, and Rockwell hardness of the cermets were 99.78%, 1836 MPa, 14.7 MPa m^1/2, and 88, respectively.     

Fabrication and properties of Ti(C,N) based cermets reinforced by nano-CBN particles

Titanium carbontride (Ti(C,N)) based cermets with and without nano-cubic boron nitride (CBN) particles were prepared by microwave sintering in argon and nitrogen environment, respectively. Two kinds of core–rim microstructure, black core–grey rim and white core–grey rim, are shown in the cermets by scanning electron microscopy (SEM) in combination with energy dispersive X-ray analysis (EDX). It is found that, for the cermet with 1.5% nano-CBN particles sintered at 1500 °C for 30 min in argon, its transverse rupture strength (TRS) and hardness are improved to about 25.9% and 1.4%, respectively. The SEM analysis shows that the inhibition effect of nano-CBN particles on the dissolution of Ti(C,N) is weakened with the increase of content of nano-CBN particles. Moreover, for the cermet sintered in argon reinforced by 1.5% nano-CBN particles, more fine black core–grey rims are found in the microstructure compared to the others. For the material sintered in nitrogen, its microstructure accompanied with many white core–grey rims in number and big black core and thin outer rim in size, results in high hardness and low TRS.     

Ti(C,N)-based cermets with fine grains and uniformly dispersed binders: Effect of the Ni–Co based binders

Metallic binder is a key factor affecting the microstructure and mechanical properties of Ti(C,N)-based cermets. To optimize the overall performances, cermets with various weight ratios of Ni/(Co + Ni) ranging from 0 to 1 were fabricated by gas pressure sintering. Microstructure, phase formation, interface structure and related mechanical properties of the sintered cermets were investigated. With the increase of the Ni/(Co + Ni) ratios, the black cores became smaller and grains of Ti(C,N) dispersed uniformly. Compared to the pure Ni or Co, Ni–Co binders accelerated the formation of rim phases, and avoided the nonuniform dispersed binder pools. When the ratio was 0.5, the cermets showed fine grains, uniformly dispersed binders and small lattice misfit of the core-rim interface, exhibiting the optimal mechanical properties, i.e. satisfactory Vickers hardness of 1670 (HV30) Kgf/mm², bending strength of 1970 MPa and Fracture toughness of 8.94 MPa m^0.5. This work sheds light on constructing the relationship between the microstructure, mechanical performance of Ti(C,N)-based cermets and the Ni/Co-based binders.     

Strengthened Ti(C,N)-based cermets using high-energy ball-milled NiTiC binders: Microstructure and mechanical properties

High-energy ball-milled NiTiC powders were used for preparing Ti(C, N)-based cermets. Effect of NiTiC content on the morphology, composition, interface structure and mechanical properties of cermets were investigated. NiTiC binders promoted the formation of inner rims on Ti(C, N) cores and hindered their coalescence, leading to well-distributed microstructure. Binder had little effect on the composition of rims, but greatly affected the interface structure of core-rim and rim-binder. Complete inner rim could decrease the lattice mismatch between outer rim and core, forming highly coherent interface. With increasing the Ti-C in Ni, the rim-binder boundaries evolved from semi-coherent to coherent interface, due to the decreased lattice mismatch. Small difference in Vickers hardness of cermets was found, with the values ranging from 1622 to 1684 N/mm². Bending strength of cermets increased from 1330 to 2073?MPa, with the Ti-C content from 0 to 20?wt%. Further increasing the Ti-C could lead to thick rims, resulting in decreased strength and toughness. This work showed us a way to strengthen the Ti(C, N)-based cermets via modifying the interface structure.     

Microstructure and mechanical properties of TiC–TiN–Zr–WC–Ni–Co cermets

Cermet cutting tools are widely used for semi-finishing and finishing work on steel and cast iron. However, their brittleness is still an unavoidable limitation for their utilizations. Zirconium was added to improve the fracture toughness of Ti(C, N) based cermets. The microstructure and the fracture surfaces of cermets were studied by using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The experimental results reveal that Zr dissolved and formed solid solutions during the sintering process. The amount of grains with typical core/rim structure decreases and that of coreless grains increases with increasing Zr addition. Moreover, the fracture toughness is improved clearly due to the increased amount of the coreless grains, the spinodal decomposition in cermets, as well as the crack deflection and crack branching mechanisms. Additionally, hardness and relative density were also measured, respectively.     

Experimental and thermodynamic investigation of gradient zone formation for Ti(C,N)-based cermets sintered in nitrogen atmosphere

The influence of N₂ atmosphere on the microstructure of gradient zone in Ti(C,N)-Mo₂C-Ni cermet was systematically investigated by the coupling analysis of experimental characterization and thermodynamic calculation. Under the guidance of calculated carbon window, the composition of Ti(C,N)-Mo₂C-Ni cermet was designed, and the cermet was produced via liquid-phase sintering at 1450 °C for 2 h under N₂ pressure of 20, 200, 400 and 600 mbar. The microstructure and element distribution of cermet were analyzed by using Scanning Electron Microscopy (SEM) equipped with Energy Dispersive X-ray spectroscopy (EDX). A homogeneous microstructure was obtained for cermet sintered in 20-mbar nitrogen atmosphere, whereas the thickness of gradient layer increased with nitrogen pressure. EDX mapping demonstrate that Mo and Ti are enriched in gradient zone, while Ni is lacking and partially segregated near the surface. The diffusion of elements in cermet is caused by the different nitrogen activity between surface and interior. The carbonitride grains show typical black core and gray rim structure in the bulk of cermets, while it present light-gray core and gray rim in the surface gradient layer. In addition, the Vickers microhardness measurement was performed for the gradient zone of cermets, and the hardness increased for cermets sintered in higher nitrogen pressures, which exhibit slower grain growth phenomena.     

Processing and Properties of Ti(C,N)–WC-Based Materials

Two Ti(C,N)–WC powder mixtures, one containing 0.88 wt% Co and the other 6.2 wt% Ni + 2.9 wt% Co, were fabricated by different routes: pressureless and gas-pressure sintering in argon and nitrogen, and hot-pressing under vacuum. The microstructures of all the sintered samples consisted of grains with a core/rim structure, the core being Ti(C,N) and the rim (Ti,W)(C,N). An inner rim also was present at the core/rim interface. The more highly doped materials also had an intergranular Ni-Co-Ti-W binder phase. The compositions and cell parameters of the hard phases, as well as of the binder, were analyzed. The nitrogen partial pressure in the sintering furnace was the main factor that influenced grain growth and phase composition. In fact, sintering under argon enhanced grain growth and was accompanied by a lower tungsten content in the rim. The influence of the microstructure on some mechanical properties (hardness, flexural strength, toughness, and Young's modulus) also was investigated. Flexural-strength values up to 1550 MPa at room temperature and 1200 MPa at 800°C, and fracture-toughness values up to 8 MPa·m^1/2 were measured, depending on the starting composition and processing conditions.