Comparative study of the fabrication of ultrafine Ti（C,N）-based cermets by spark plasma sintering and conventional vacuum sintering

Spark plasma sintering (SPS) and conventional vacuum sintering (VS) were employed to fabricate ultrafine Ti(C,N)-based cermets. The shrinkage behavior, microstructure, and porosity and mechanical properties of the samples fabricated by SPS were compared with those of the samples sintered by VS using optical microscopy, scanning electron microscopy, universal testing machine, and rockwell tester. The results are as follows: (1) The shrinkage process occurred mainly in the range of 1000-1300℃ during the VS process, and only a 0.2% linear shrinkage ratio appeared below 800℃；during the SPS process, a 60% dimensional change occurred below 800℃ as a result of pressure action. (2) By utilizing the SPS technique, it is difficult for obtaining fully dense Ti(C,N)-based cermets. Due to the much existence of pores and uncombined carbon, the mechanical properties of the sintered samples by SPS are inferior to sintered ones by VS. (3) grain size of the samples sintered by SPS is still below 0.5μm, but not by VS; because of low sintering temperature, there are no typical core/rim structures formed in the sintered samples by SPS1; the main microstrures of the sintered samples by SPS2 are a white core/grey shell sstructure, whereas by VS show a typical black core.grey shell structure.     

Effect of sintering on microstructures and properties of sub-micron Ti （C, N）-based cermets

The influence of different sintering processes, including vacuum sintering, vacuum sintering followed by HIP and sintering-HIP, on the microstructure and properties of sub-micron Ti(C,N) cermets with various binder contents was studied. Image analysis based on back-scattered electrons image observations was used to determine the morphologic and structural characteristics. Transverse rupture strength(TRS), hardness, fracture toughness were measured and TRS data were treated by Weibull statistics further. It is shown that a very significant improvement in TRS can be obtained by HIP or sintering-HIP treatment for the alloys with lower and middle binder content at the controlled cooling rate, but the effect is not obvious for the alloys with higher binder content. HIP is also helpful for improving the hardness of sub-micron Ti(C,N) cermets, however, but can lower the fracture toughness. The varia-tion of these properties was interpreted in terms of the difference in morphologic and structural characteristics.     

Effects of molybdenum on the microstructure and mechanical properties of Ti（C,N）-based cermets with low Ni

The effects of Mo on the microstructure and mechanical properties of Ti（C,N）-based cermets with low Ni have been studied systematically. Different contents of Mo （4-12 wt.%） were added into Tl（C,N）-based cermets. Specimens were fabricated by conventional powder metallurgy and vacuum sintered at temperatures of 1440, 1450, and 1460℃ individually. The microstructure and fracture morphology were investigated by scanning electron microscope, and the mechanical properties such as transverse strength and hardness were measured. The results show that the microstructure is uniform and the thickness of rim phase is moderate when the content of Mo is 8 wt.%; the mechanical properties of the specimens sintered at 1450℃ are better than those sintered at 1440 and 1460℃. The integrated properties of transverse strength and hardness are the best when the content of Mo is 8 wt.% and the sintering temperature is 1450℃.     

Effect of RE element Y on properties of Ti(C, N)-based cermet

The effect of RE element Y on the microstructure and properties of Ti(C, N)-based cermet has been investigated. Yttrium can refine the particles of carbide phase because it can purify the interface between binder phase and carbide phase, and increase the thickness of rim phase slightly. The effect is in the most evidence when the mass fraction of Y is 0.8% and when the transverse rupture strength and the hardness of Ti(C, N)-based cermet are maximal.     

Characterization of phase transformation and microstructure of nano hard phase Ti（C,N）-based cermet by spark plasma sintering

By means of optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM), the process of densification, the characterization of phase transformation and the microstructure for spark plasma sintering (SPS) nano hard phase Ti(C, N)-based cermet were investigated. It is found that the spark plasma sintering (SPS) enables the nano hard phase Ti(C,N)-based cermet to densify rapidly, however, the full densification of the sintered samples can not be obtained. The rate of phase transformation is significantly quick.When being sintered at 1 200℃ for 8 min, Mo2C is completely dissolved, and TiN dissolves into TiC entirely and disappears. Above 1200℃, Ti(C,N) begins to decompose and the atoms of C and N separate from Ti(C,N) resul-ting in the generation of N2 and the graphite. Due to the denitrification and the graphitization, the density and the hardness of sintered samples are rather low. The distribution of grain size of the sample sintered at 1350℃ covers a wide range of 90-500 nm, and most of the grain size are about 200 nm. The hard phase is not of typical core-rim structure. Oxides on the surface of particles can not be fully removed and present in sample as titanium oxide TiO2.Graphite exists in band-like shape.     

Growth of brass nanofilms sputtered on organic substrate

The growth of brass nanofilms sputtered on acrylics substrate was studied through experimental investigation of the effect of sputtering voltage, target-to-substrate distance, chamber pressure and sputtering time on the content, growth rate and surface morphology of brass nanofilms. The results show that compared with original brass target, Cu content in brass nanofilms changes by no more than 6.23%(mass fraction). High sputtering voltage and short target-to-substrate distance help to improve brass nanofilm deposition rate. There exists an optimal chamber pressure where deposition rate of nanofilm reaches the maximum. The key factor affecting surface morphology is the kinetic energy of sputtering particles. Low sputtering voltage, large target-to-substrate distance and low chamber pressure are very important for the formation of the high-quality brass nanofilms. The brass films prepared under the conditions of sputtering voltage 1.6 kV, target-to-substrate distance 2.5 cm， chamber pressure 10 Pa and sputtering time 20 rain, possess following characteristics: smooth and uniform surface, thickness of 41 nm and Cu content of 71.0% (mass fraction).     

Evolution of Ti （C, N）-based cermet microstructures

Two series of Ti(C, N)-based cermet materials originating from the same chemical composition but with dif-ferent grain size distribution and sintered to different stages of the sintering cycle have been studied using SEM, TEM,EDX, and XRD. Much of the surrounding structure is formed during solid state sintering. During the solid state sintering, at first, the Mo and W rich (Ti, Mo, W)C inner rim is formed by the interaction among TiC, WC, and Mo2C; then the Mo and W lean (Ti, Mo, W)(C, N)outer rim is formed. During the liquid phase sintering, the outer rim of coarse grains grows rapidly throw a solution-reprecipitation process; aLso coarse grains grow by particle coalescence. The interface between coarse grain outer rim and binder is flat (crystal surface).     

Densification of Ni-NiFe204 cermets for aluminum electrolysis

The density of cermet inert anodes in aluminum electrolysis is of great importance. Ni-NiFe2O4 cermets were studied with respect to their densification affected by ball milling time, particle size of raw powders, contents of metallic phase, sintering atmosphere and temperature. The results show that, prolonging ball milling time will increase the density with the optimum value of 150 min; cermets containing 0 - 15 % Ni(mass fraction) have high relative density ranging from 94 % to 96%, but with Ni content increasing, the density slightly decreases; weak reductive atmosphere is favorable to densification; the relative density increases from 80.38% to 96.85% with the sintering temperature increasing from 1 100℃ to 1 300℃ while it decreases at 1 400℃, which may be due to crystal grain coarsening. So the sintering temperature of Ni-NiFe2O4 cerrnets in current work should be controlled at 1 300℃, where the relative density is 96.85 %.     

Synthesis of Ti3SiC2 by spark plasma sintering(SPS)of elemental powders

Ti3SiC2 materials have been fabricated by spark plasma sintering of the elemental powders with the addition of Al.At the heating rate of 80℃/min and under the pressure of 30MPa,the ideal synthesis temperature of Ti3SiC2 is in the range of 1150-1250℃.The addition of Al is in favor of the formation of Ti3SiC2.The synthesized compound has the molecular of Ti3Si0.8Al0.2C2 and lattice parameters of α=0.3069nm,c=1.7670nm.Its grain is plane-shape with a size of about 50μm in the elongated dimension.The prepared material has Vickers hardness of 3.5-5.5GPa(at 1N and 15s) and is as readily machinable as graphite‘s.     

Warm compaction powder metallurgy of Cu

A series of experiments were carried out using different admixed lubricant contents, different compaction pressures and temperatures in order to study the warm compaction of copper powder. Results show that too much admixed lubricant will lead to the squeeze out of the lubricant from the compact during the warm compaction processing of Cu powder. Results also show that blisters can be found in sintered samples that contain lubricant less than 0.15%(mass fraction). Optimal warm compaction parameters for producing high density powder metallurgy copper material are obtained. Compacts with green density of 8.6 g/cm^3 and a sintered density of 8.83 g/cm3 can be produced by warm compacting the Cu powder, which contains 0.2% admixed lubricant, and is compacted at 145℃ with a pressure of 700 MPa.     

Effect of VC addition on sinterability and microstructure of ultrafine Ti(C, N)-based cermets in spark plasma sintering

The effect of vanadium carbide (VC) addition on the sinterability and the microstructure of ultrafine Ti(C, N)-based cermets consolidated through spark plasma sintering (SPS) was systematically investigated using optical microscope, scanning electron microscope (SEM) with X-ray energy dispersive spectrometer (EDS), X-ray diffractometer (XRD) and transmission electron microscope (TEM). Our results reveal that the addition of VC increases the porosity of sintering body and depresses the sinterability of Ti(C, N)-based cermets. It is also found that the VC addition has a significant influence on the microstructure of ultrafine Ti(C, N)-based cermets, which inhibits the dissolution of titanium-containing compounds and the formation of inner rim phase and outer rim phase, thus preventing from grain growth. Owing to the depressed dissolution and precipitation, nitrogen liberation is mitigated, therefore resulting in less amount of graphite phase in the samples. In substance, VC changes the solubility of metallic elements in the binder, which makes more elements of Mo and W to be reserved in the binder and thus greatly decreases the content of titanium dissolved into the binder. The re-building solubility rule determines the development of phases and microstructure.     

High-quality Ti(C,N)-based cermets via solid-state nitrogen-pressure sintering: Influence of the sintering atmosphere

Owing to the denitrification behavior of Ti(C,N)-based cermets during the sintering process, nitrogen atmosphere sintering is essential for the fabrication of high-quality products. Disclosing the roles of the sintering atmosphere on the microstructure and properties of cermet is of significance. Herein, Ti(C,N)-based cermets were fabricated under six types of sintering atmosphere. Effects of sintering atmosphere on the microstructure, coercive force (Hc), specific saturation magnetization (Ms (%)), and mechanical properties of cermets were investigated. More rimless Ti(C,N) grains with increased fraction of black cores were formed with increasing the nitrogen partial pressure. Three types of gradient structure of cermets were confirmed caused by the different nitrogen activity at the liquid sintering state. Vacuum sintering induced the formation of thicker rims on smaller Ti(C,N) cores at the surface due to the low nitrogen potential, while fcc solid solutions rich in N was formed in nitrogen atmosphere. Vacuum sintered cermets showed the lowest Hc and Ms. (%) of 3.66 KA/m and 2.13%, owing to the denitrification process. High-quality Ti(C,N)-based cermets were achieved via the solid-state nitrogen atmosphere sintering, benefiting from the optimized core-rim structure, i.e. moderate thickness of rims and certain amount of black Ti(C,N) cores.     

氮含量对Ti(C,N)基金属陶瓷显微组织与磨粒磨损行为的影响

采用真空烧结方法制备了4种氮含量的Ti(C,N)基金属陶瓷,测试了试样的抗弯强度、硬度、断裂韧性等力学性能,用扫描电镜、能谱仪等研究了N含量对其显微组织及磨粒磨损行为的影响。结果表明:在一定范围内随N含量的增加,硬质相芯部逐渐细化且分布均匀,环形相厚度变薄、体积分数减小;磨粒磨损形貌中犁沟所占的比例减少,微观脆性断裂形成的凹坑增加,耐磨粒磨损性能逐渐提高。随N含量增加,Ti(C,N)基金属陶瓷的抗弯强度呈现出先增加后减小的趋势,断裂韧度逐渐递减,硬度变化不大。当N含量为3.6%时(文中含量均为质量分数),Ti(C,N)基金属陶瓷综合力学性能最佳,其抗弯强度为1 873 MPa,硬度为89.9 HRA,断裂韧度为20.7 MPa.m1/2。     

Thermodynamics of TiC- and Ti(C,N)-based cermet processing prior to liquid phase sintering stage

The thermodynamics of the various events occurring during heating to liquid phase sintering temperatures of powder compacts of TiC- and Ti(C,N)-based cermets has been investigated. The results show that via a series of dissolution, solute repartitioning and mixed rim formation reactions, Mo₂C can not only help keep the systems free of oxides and residual oxygen but also moderate the Ti content in the Ni binder phase during the heating stage, thus prevent the formation of intermetallic compound and ensure good wetting in the subsequent liquid phase sintering stage.     

Effect of submicron Ti(C,N) on the microstructure and the mechanical properties of Ti(C,N)-based cermets

There is an increased industry demand for Ti(C,N)-based cermets with improved material properties. One of the parameters which are supposed to influence these properties is the mean particle size of the Ti(C,N) powder used. In this study the effects of a newly developed submicron Ti(C,N) powder grade on the properties of Ti(C,N)-based cermets, including hardness, toughness and microstructure were investigated. The cermets showed only small differences with respect to outgassing upon sintering (investigated by MS-EGA) as well as shrinkage (dilatometry). Cermet formulations with submicron Ti(C,N) could be sintered under identical conditions as with fine Ti(C,N), yielding completely dense bodies of A00 porosity. From SEM and XRD investigations it was found that submicron Ti(C,N) powders cause accelerated diffusion and homogenisation of the microstructure leading to a substantially increased amount of outer rim phase, a higher amount of inverse grains and substantially finer and less Ti(C,N) cores. Upon using submicron Ti(C,N), hardness (HV10) is increased and in one grade the fracture toughness (Palmqvist–Shetty) is increased as well.     

纳米TiN改性Ti(C,N)基金属陶瓷的抗热震性能

为研究不同粘结相对纳米TiN改性Ti(C,N)基金属陶瓷抗热震性能的影响,真空烧结制备了粘结相质量分数分别为20%Ni、10%Co-10%Ni和20%Co的Ti(C,N)基金属陶瓷。采用扫描电镜研究了材料的组织,并测量了力学性能。采用压痕-急冷法主要研究了其抗热震性能,结果表明,随着温度的升高和热循环次数的增加,裂纹增长,粘结相为Ni的金属陶瓷的抗热震性优于粘结相为Co的金属陶瓷。显微照片表明,裂纹主要沿硬质相/粘结相界面及粘结相扩展。     

Effect of carbon content and cooling mode on the microstructure and properties of Ti(C,N)-based cermets

Ti(C,N)-based cermets were prepared by vacuum liquid sintering. The effects of carbon content as well as cooling mode on the microstructure, magnetic and mechanical properties of the cermets were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and vibrating sample magnetometer (VSM). Hardness and transverse rupture strength (TRS) were also measured. The grains of Ti(C,N)-based cermets became finer and solid solubility of titanium, molybdenum, tungsten in binder phases decreased with increasing carbon content. The thickness of the rim phases increased when the cermet was annealed at 1360 °C for 30 min during cooling, which resulted in the decrease of the hardness and the transverse rupture strength (TRS). On the other hand, the magnetic saturation of Ti(C,N)-based cermets increased with increasing carbon content, which was due to the decrease of the solid solution of alloy elements in binder phases.     

Ti(C,N)基金属陶瓷抗弯强度的研究

研究了提高Ti(C,N)基金属陶瓷抗弯强度的方法。采用改变粘结相成分、进行低压烧结及快冷处理来制备Ti(C,N)基金属陶瓷。试验发现,粘结相成分对材料的强度有很大的影响,提高Ni/Ni+Co的比例可以提高材料的强度,当然,在实际应用中还要考虑对其它性能的综合影响;低压烧结和快冷处理都可以有效的提高Ti(C,N)基金属陶陶的抗弯强度。     

Effect of WC addition and cooling rate on microstructure,magnetic and mechanical properties of Ti(C0.6,N0.4)-WC-Mo-Ni cermets

Ti(C_0.6,N_0.4)-8Mo-xWC-25Ni (x = 0, 3, 6 and 9 wt%) cermets were synthesized under different cooling rates by vacuum sintering. The influence of WC addition and cooling rate on microstructure, magnetic and mechanical properties of the as-prepared Ti(C,N)-based cermets was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and physical property measurement system (PPMS). The results revealed that the grain size of the Ti(C,N)-based cermets became finer with WC addition. Furthermore, room-temperature saturation magnetization (M_s), remanence (M_r) and Curie temperature (T_c) of the Ti(C,N)-based cermets initially decreased with increasing WC content, followed by a gradual increase. Cermets bacame paramagnetic at x = 6 under the cooling rate of 2 °C/min, x = 6 and 9 under the cooling rate 35 °C/min, respectively. The decrease in magnetic properties could be ascribed to the enhanced solid solubility of alloy elements in Ni-based binder phase. Moreover, the hardness and transverse rupture strength (TRS) of the Ti(C,N)-based cermets initially increased and followed by a gradual decrease, whereas the fracture toughness initially decreased followed by an increase with increasing WC content. At the same value of x, the Ti(C,N)-based cermets exhibited better magnetic and mechanical properties at the cooling rate of 35 °C/min than that at the cooling rate of 2 °C/min, which could be attributed to the grain refinement strengthening and solid-solution strengthening of the binder phase.