Synthesis of (Ti,W, Mo,V)(C,N) nanocomposite powder from novel precursors

(Ti, W, Mo, V)(C, N) nanocomposite powders with globular-like particle of ∼10–100 nm were synthesized by a novel method, namely carbothermal reduction–nitridation (CRN) of complex oxide–carbon mixture, which was made initially from salt solution containing titanium, tungsten, molybdenum, vanadium and carbon elements by air drying and subsequent calcining at 300 °C for 0.5 h. Phase composition of reaction products was discussed by X-ray diffraction (XRD), and microstructure of the calcined powders and final products was studied by scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. The results show that the synthesizing temperature of (Ti, W, Mo, V)(C, N) powders was reduced greatly by the novel precursor method. Thus, the preparation of (Ti, 15W, 5Mo, 0.2V)(C, N) is at only 1200 °C for 2 h. The lowering of synthesizing temperature is mainly due to the homogeneous chemical composition of the complex oxide–carbon mixture and its unusual honeycombed structure.     

Influence factors and microstructure evolution during preparation of nanocrystalline (Ti, W, Mo, V)(C, N)–Ni composite powders

Nanocrystalline (Ti, W, Mo, V)(C, N)–Ni composite powders with crystalline size of about 35 nm were synthesized at 1300 °C from oxides by a simple and cost-effective route which combines traditional low-energy milling plus carbothermal reduction–nitridation techniques. Influence of main technological parameters was investigated by X-ray diffraction, and microstructure of the milled powders and reaction products was studied by scanning electron microscopy. The results show that the phase evolution of TiO₂ follows TiO₂ → Ti₃O₅ → Ti(C, N), and (Ti, W, Mo, V)(C, N)–Ni composite powders with higher nitrogen content and smaller crystalline size can be produced by introducing high nitrogen pressure. By contrast with high nitrogen pressure, high synthesizing temperature and long isothermal time can contribute to dissolution of W, Mo and V atoms into Ti(C, N). In addition, synthesizing temperature has a significant effect on the microstructure evolution of (Ti, W, Mo, V)(C, N)–Ni composite powders.     

Effect of secondary carbides addition on the microstructure and mechanical properties of (Ti, W, Mo, V)(C, N)-based cermets

(Ti, W, Mo, V)(C, N)-based cermets were prepared by mixing Mo₂C, WC and TaC with ultrafine (Ti, W, Mo, V)(C, N) powders, and then processed via a conventional P/M technique. The effect of Mo₂C, WC and TaC on the microstructure and mechanical properties of (Ti, W, Mo, V)(C, N)-8 wt.% Ni-7 wt.% Co systems was investigated. The Mo₂C content was varied from 0 to 10 wt.% and additive WC or TaC was added at a level of 5 wt.% with Mo₂C addition. The results show that the densification of (Ti, W, Mo, V)(C, N)-8 wt.% Ni-7 wt.% Co cermets was improved significantly by the addition of Mo₂C. With the increase of Mo₂C content, there is a coarsening tendency in the microstructure of (Ti, 20W, 15Mo, 0.2V)(C, N)-8Ni-7Co system, but the refinement for (Ti, 15W, 5Mo, 0.2V)(C, N)-8Ni-7Co. TaC addition decreases the density of (Ti, 15W, 5Mo, 0.2V)(C, N)-10Mo₂C-8Ni-7Co cermet and thus weakens its bending strength. (Ti, 15W, 5Mo, 0.2V)(C, N)-10Mo₂C-5WC-8Ni-7Co cermet has optimal mechanical properties: bending strength of 1999 MPa, hardness (H_v) of 1677 MPa and toughness of 9.95 MPa m^1/2 respectively by adding WC, which is due to its ultrafine and weak core/rim structure.     

Effect of (Ti,W, Mo,V)(C,N) powder size on microstructure and properties of (Ti,W, Mo,V)(C,N)-based cermets

Three kinds of (Ti, 15 W, 5Mo, 0.2 V)(C, N) powders with different particle size were prepared from a mixture of oxides and carbon powders by carbothermal reduction-nitridation method. (Ti, W, Mo, V)(C, N)-based cermets were obtained by mixing Co, Ni, WC, MoC₂ and (Ti, 15 W, 5Mo, 0.2 V)(C, N) powders, and then processed via a conventional P/M technique. The influence of (Ti, 15 W, 5Mo, 0.2 V)(C, N) particle size on the microstructure and mechanical properties of (Ti, 15 W, 5Mo, 0.2 V)(C, N)-5WC-10MoC₂-7Co-8Ni cermets has been studied. When the particle size of (Ti, W, Mo, V)(C, N) is 0.2–0.5 μm, (Ti, 15 W, 5Mo, 0.2 V)(C, N)-based cermets can be characterized by weak core/rim structure. With the particle size of (Ti, W, Mo, V)(C, N) increasing to 1–1.5 μm, the microstructure of (Ti, 15 W, 5Mo, 0.2 V)(C, N)-based cermets develops into composite structure that consists of typical core/rim and no core/rim. Accordingly, typical core/rim structure is obtained in the case of the particle size of 3–5 μm. With the coarsening of raw (Ti, 15 W, 5Mo, 0.2 V)(C, N) powders, the fracture toughness of (Ti, 15 W, 5Mo, 0.2 V)(C, N) cermets is greatly improved, but the hardness continues to decline. (Ti, 15 W, 5Mo, 0.2 V)(C, N) cermets with composite structure have higher bend strength of 2165 MPa.     

Ultrafine (Ti, M)(C, N)-based cermets with optimal mechanical properties

A tough and strong cermet with the composition (Ti,20M,0.2V)(C,N)-16M-20Ni/Co (M = W,Mo) was prepared by mixing WC and Mo₂C with ultrafine (Ti,M)(C,N) powders, and then, processed via a conventional P/M technique. It has an ultrafine and distinct core/rim structure, resulting in excellent mechanical properties: bending strength of 2210 MPa, HV hardness of 14.7 GPa and toughness of 10.1 MPa m^1/2. The small concentration gradient in core/rim composition and the disappearance of inner rims benefit the reduction of the stress concentration at the core/rim interface in (Ti,M)(C,N)-M_xC cermets, and thus improve their toughness. In addition, ultrafine microstrucure improves mainly their bending strength and hardness.     

Synthesis of ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders by low-energy milling and subsequent carbothermal reduction–nitridation reaction

Ultrafine (Ti, W, Mo, V)(C, N)–Ni composite powders with globular-like particles of 50–300 nm were synthesized at static nitrogen pressure from oxides by a simple and cost-effective route which combines traditional low-energy milling plus carbothermal reduction–nitridation (CRN) techniques. Reaction path of the (Ti, W, Mo, V)(C, N)–Ni system was discussed by X-ray diffraction (XRD) and thermogravimetry–differential scanning calorimetry (TG–DSC), and microstructure of the milled powders and final products was studied by scanning electron microscopy (SEM) and transmission electron microscope (TEM), respectively. The results show that CRN reaction has been enhanced by nano-TiO₂ and nano-carbon powders. Thus, the preparation of (Ti, 15W, 5Mo, 0.2V)(C, N)–20Ni is at only 1300 °C for 1 h. During synthesizing reaction, Ni solid solution phase forms at about 700 °C and reduction–carbonization of WO₂ and MoO₂ occurs below 900 °C. The reactions of TiO₂ → Ti₃O₅, Ti₃O₅ → Ti(C, O) and Ti(C, O) → Ti(C, N) take place at about 930 °C, 1203 °C and 1244 °C, respectively.     

Fabrication of W-20wt.%Ti alloy by pressureless sintering at low temperature

A powder metallurgical technology of low temperature and pressureless is used to fabricate a W-20wt.%Ti alloy using milled TiH₂ powders and micro-sized W powders. The microstructure of the milled TiH₂ powders and the bulk W–Ti alloy were studied. It is indicated that TiH₂ nanoparticles with the size of 8 to 15 nm were obtained after milling for 48 h and the decomposition temperature decreased from 520.2 °C to 395.5 °C. The W-20wt.%Ti alloy prepared at 1200 °C for 80 min had a relative density of 97.8% which was composed of α-Ti, W and β(W/Ti) solid solution. A preparation mechanism of the W–Ti alloy is also proposed based on the experimental results.     

Synthesis of (Ti,W, Mo,V)(C,N) powders by carbothermal reduction–nitridation with NH4HCO3 addition

(Ti, W, Mo, V)(C, N) powders were synthesized by carbothermal reduction–nitridation in an open system. Effect of NH₄HCO₃ addition on the phase composition and microstructure of the synthesized powders were investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that NH₄HCO₃ addition plays a facilitating role in preparation of high-purity (Ti, W, Mo, V)(C, N) powders. With the temperature increasing, the residual carbon content decreases in the products but the nitrogen content is more stable. It is found that the morphologies of the synthesized powders exhibit high-porosity structure when NH₄HCO₃ additive is added.     

Effect of partial substitution of Cr for Ni on densification behavior, microstructure evolution and mechanical properties of Ti(C,N)-Ni-based cermets

Four cermets of composition TiC-10TiN-16Mo-6.5WC-0.8C-0.6Cr₃C₂-(32 − x)Ni-xCr (x = 0, 3.2, 6.4 and 9.6 wt%) were prepared, to investigate the effect of the partial substitution of Cr for Ni on densification behavior, microstructure evolution and mechanical properties of Ti(C,N)-Ni-based cermets. The partial substitution of Cr for Ni decreased full densification temperature, and the higher the content of Cr additive was, the lower full densification temperature was. The partial substitution of Cr for Ni had no significant effect of the formation of Mo₂C and Ti(C,N) and the dissolution of WC, and however, it had a significant effect on the dissolution of Mo₂C. Cr in Ni-based binder phase diffused into undissolved Mo₂C to form (Mo,Cr)₂C above 1000 °C at 6.4-9.6 wt% Cr additive, and a small amount of (Mo,Cr)₂C did not dissolve after sintering at 1410 °C for 1 h at 9.6 wt% Cr additive. In the final microstructure, Cr content in Ni-based binder phase increased with increasing the content of Cr additive, and however, regardless of the content of Cr additive, coarse Ti(C,N) grains generally consisted of black core, white inner rim and grey outer rim, and fine Ti(C,N) grains generally consisted of white core and grey rim. The partial substitution of Cr for Ni increased hardness and decreased transverse rupture strength (TRS). Ni-based binder phase became hard with increasing the content of Cr additive, therefore resulting in the increase of hardness and the decrease of TRS. TRS was fairly low at 9.6 wt% Cr additive, which was mainly attributed hardening of Ni-based binder phase and undissolved (Mo,Cr)₂C.     

Synthesis of WC nanopowders from novel precursors

Nano-WC powders with granular particle of ~ 20-80 nm were synthesized by a new precursor method, namely carbothermal reduction-carburization of amorphous WO₃-C mixture, which was made initially from salt solution containing tungsten and carbon elements by air drying and subsequent calcining at 400 °C for 1 h. The reaction products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that the synthesizing temperature of WC powders was reduced greatly by the novel precursor method. Thus, the preparation of the single-phase nano-WC powders is at only 1000 °C for 2 h. The lowering of synthesizing temperature is mainly due to the homogeneous chemical composition of the amorphous oxide-carbon mixture.     

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).     

纳米TiN复合Ti(C,N)基金属陶瓷烧结过程中的相演变

本文采用XRD与SEM对纳米复合Ti(C,N)基金属陶瓷烧结过程中的相演变进行了研究,结果发现:对于缺碳体系,Mo在800℃即可以夺取TiC中的C,生成Mo2C;1 000℃时开始生成Ni2(Mo,W,Ti)4C相,其含量在1 250℃时达到最大,随着温度的进一步升高而部分分解并进入Ti(C,N)晶格,生成非化学计量的(Ti,Mo,W)(C,N),导致Ti(C,N)的相对含量升高,晶格常数减少。纳米TiN与亚微米TiN相比并没有显示出更高的烧结活性。当添加适量的C时,纳米TiN复合的Ti(C,N)基金属陶瓷与同一体系的亚微米金属陶瓷烧结过程中的相变规律基本相同。     

(Ti,W,Ta)(C,N)_p/Ti(C,N)基金属陶瓷的组织与性能

采用自制的多元复式碳氮化物陶瓷粉末 ((Ti,W,Ta) (C,N) p)制备 (Ti,W,Ta) (C,N) p/Ti(C,N)基金属陶瓷。研究了 (Ti,W,Ta) (C,N) p 粉末的组织结构特征及其加入对金属陶瓷的组织及性能的影响。结果表明 ,多元复式碳氮化物粉末的晶格常数与元素的固溶度有很好的对应关系 ,调整粉末中元素的固溶度可控制粉末的晶格常数 ,进而控制材料的性能。 Ti(C,N)基金属陶瓷中 (Ti,W,Ta) (C,N) p 粉末的加入 ,有利于重金属元素 W和 Ta向粘结相中扩散 ,从而降低了硬质相在粘结相中的溶解度 ,阻碍了晶粒长大。(Ti,W,Ta) (C,N) p/Ti(C,N)基金属陶瓷各项性能指标优于 Ti(C,N)基金属陶瓷和国外对应的金属陶瓷牌号 CT5 2 5的产品。强化机制主要表现为细晶强化与固溶强化。     

Effect of pre-alloyed raw materials on the microstructure of a (Ti,W)(C,N)–Co cermet

Four alloys manufactured from different combinations of powders (TiC + TiN + WC; Ti(C,N) + WC; (Ti,W)C + TiN and (Ti,W)(C,N)) were studied using X-ray diffractometry, optical microscopy (OM), scanning electron microscopy (SEM) and analytical electron microscopy (AEM). The alloy manufactured from binary powders had a smaller grain size and a more inhomogeneous microstructure than the other alloys. The alloys manufactured with WC contained an inner rim around Ti(C,N) cores, as well as W-rich cores. Thermodynamic calculations suggest that these are formed during solid-state sintering at 900°C in a low nitrogen activity. The outer rim had a composition that is in good agreement with calculations of the equilibrium during liquid phase sintering at 1450°C.     

Stability of (Ti, M)C (M = Nb, V, Mo and W) carbide in steels using first-principles calculations

The lattice parameters, formation energies and bulk moduli of (Ti, M)C and M(C, Va) with the B1 crystal structure have been investigated using first-principles calculations, where M = Nb, V, Mo and W. The replacement at 0 K of Ti by Mo or W in the TiC lattice is found to be energetically unfavorable with respect to the formation energy. However, it decreases the misfit strain between the carbide and ferrite matrix, a factor which is of critical importance during the early stages of precipitation, thus favoring the substitution of Ti by Mo, as is observed in practice. The effect of Mo in enhancing the coarsening resistance of (Ti, Mo)C precipitates is discussed in terms of its role in the nucleation process, but followed by a more passive contribution during coarsening itself. The role of tungsten has been predicted to have a similar effect to molybdenum on the nucleation and coarsening process. Analysis of precipitates in Ti-, Ti-Mo- and Ti-W-bearing steels shows results consistent with the calculations.     

Investigation of metal distribution and carbide crystallite formation in metal-doped carbon films (a-C:Me, Me = Ti, V, Zr, W) with low metal content

Metal-doped amorphous carbon films (a-C:Me) were deposited at room temperature by magnetron sputtering using a metal (Me = Ti, V, Zr, W) and a graphite target. The metal distribution and the temperature-induced carbide crystallite formation were analyzed by X-ray diffraction (XRD), electron microscopy (TEM, STEM) and X-ray absorption spectroscopy (EXAFS, XANES), focusing on low metal concentrations between 6.5 and 9.5%. In as-deposited samples, the metal atoms are atomically distributed in the carbon matrix without significant formation of carbide particles. With annealing to 900 K the local atomic environment around the metal atoms becomes similar to the carbide. The carbide crystallites grow with annealing up to 1300 K, their size is dependent on the metal type: V > Ti > Zr≈W. W₂C and WC_1 − x crystallites were identified for W-doped films, whereas the monocarbides are formed for the other metals. It is demonstrated, that EXAFS and high resolution electron microscopy are required to get a correct picture of the structure of the analyzed a-C:W films.     

Sinter and hot isostatic pressing (HIP) of multi-wall carbon nanotubes (MWCNTs) reinforced ZTA nanocomposite: Microstructure and fracture toughness

This work describes the microstructure and fracture toughness of zirconia toughened alumina (ZTA) nanocomposite in which multi-wall carbon nanotubes (MWCNTs) and nanosized ZrO₂ particles were used as reinforcement. The ZTA nanocomposites with additions of 0, 0.005, and 0.01 wt.% MWCNTs and 2 wt.% nanosized ZrO₂ particles were pressureless sintered in an anti-oxidant sagger with graphite powder bed at 1520 °C during 1 h in air and then HIPed at 1475 °C in argon atmosphere 1 h at a pressure of 150 MPa. Relative densities ranging 94–98% were reached. In HIPed composites the hardness and fracture toughness values were increased up to ∼17% and ∼37%, respectively, compared to the “as sintered” composites free of carbon nanotubes. A combined fracture mode, crack deflection, pull-outs of a small amount of carbon nanotubes, and bridging effect were the mechanisms leading to the improvement in fracture toughness.     

Combustion synthesis and development of Ti-O-C aluminium composites

Aluminium matrix composite reinforced with Ti compounds was successfully fabricated by SHS combustion synthesis and squeeze casting course. Prepared samples from mixture containing Ti, C and Al₂O₃ fibres were heated in microwave reactor to ignite synthesis and produce porous preform for subsequent infiltrating with liquid metal. Studies showed that synthesizing temperature has been remarkably increased by applying higher magnetron power and addition of graphite. Synthesis of specimens prepared from preliminary ball milled Ti and C powders proceeded at the highest propagation wave velocity. Microwave heating of metal Ti powder in the stream of CO₂ resulted in formation of corrugated precipitates composed of titanium oxide with carbon inclusions TiO(C) and Ti₂O₃. The produced preforms were impregnated by squeeze casting with the aluminium alloy AlSi7Mg. Proper interface with slight reduction of Ti oxide between the reinforcement and the matrix was developed. Subsequently, the samples were annealed at 500 and 1000 °C. Annealing at the lower temperature induced creation of Ti₃O₂(C) and Al₂O₃. This process was continued at 1000 °C, and additionally some Ti(Al_0,8Si_0,2)₃ pellets appeared in the matrix. With prolonged annealing, oxygen was completely removed from Ti compound and oval grains of Ti(C) were created, enveloped with Al₂O₃. In the matrix, larger and numerous Ti₃AlSi₅ pellets were formed. Hardness examination showed that the best strengthening effect was achieved after annealing at 1000 °C.     

Vacuum sintering of WC-8 wt.% Co hardmetals from WC powders with different dispersity

The effect of sintering temperature and particle size of tungsten carbide WC on phase composition, density and microstructure of hardmetals WC-8 wt.% Co has been studied using X-ray diffraction, scanning electron microscopy and density measurements. The sintering temperature has been varied in the range from 800 to 1600 °C. The coarse-grained WC powder with an average particle size of 6 μm, submicrocrystalline WC powder with an average particle size of 150 nm and two nanocrystalline WC powders with average sizes of particles 60 and 20 nm produced by a plasma-chemical synthesis and high-energy ball milling, respectively, have been used for synthesis of hardmetals. It is established that ternary Co₆W₆C carbide phase is the first to form as a result of sintering of the starting powder mixture. At sintering temperature of 1100-1300 °C, this phase reacts with carbon to form Co₃W₃C phase. A cubic solid solution of tungsten carbide in cobalt, β-Co(WC), is formed along with ternary carbide phases at sintering temperature above 1000 °C. Dependences of density and microhardness of sintering hardmetals on sintering temperature are found. The use of nanocrystalline WC powders is shown to reduce the optimal sintering temperature of the WC-Co hardmetals by about 100 °C.     

Comparative investigation of Al- and Cr-doped TiSiCN coatings

The aim of this work was a comparative investigation of the structure and properties of Al- and Cr-doped TiSiCN coatings deposited by magnetron sputtering of composite TiAlSiCN and TiCrSiCN targets produced by self-propagating high-temperature synthesis method. Based on X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy data, the Al- and Cr-doped TiSiCN coatings possessed nanocomposite structures (Ti,Al)(C,N)/a-(Si,C) and (Ti,Cr)(C,N)/a-SiC_xN_y/a-C with cubic crystallites embedded in an amorphous matrix. To evaluate the thermal stability and oxidation resistance, the coatings were annealed either in vacuum at 1000, 1100, 1200, and 1300 °C or in air at 1000 °C for 1 h. The results obtained show that the hardness of the Al-doped TiSiCN coatings increased from 41 to 46 GPa, reaching maximum at 1000 °C, and then slightly decreased to 38 GPa at 1300 °C. The Cr-doped TiSiCN coatings demonstrated high thermal stability up to 1100 °C with hardness above 34 GPa. Although both Al- and Cr-doped TiSiCN coatings possessed improved oxidation resistance up to 1000 °C, the TiAlSiCN coatings were more oxidation resistant than their TiCrSiCN counterparts. The TiCrSiCN coatings showed better tribological characteristics both at 25 and 700 °C and superior cutting performance compared with the TiAlSiCN coatings.