Effect of nanocrystalline binder on the microstructure and mechanical properties of ultrafine Ti(CN) cermets

The influence of highly deformed nanocrystalline binder ingredient on the overall microstructure and mechanical properties of ultrafine grade Ti(C_0.5N_0.5) cermet was investigated. Nanocrystalline Ni and Ni-aluminides viz. NiAl and Ni₃Al, synthesized by mechanical milling/alloying, were blended to the cermet powder prior to sintering. The mechanically milled nano-Ni contained a mixture of f.c.c. and hexagonal Ni phases. The cermet with nanocrystalline Ni showed a considerably improved microstructure over cermets with commercial coarse grade Ni and exhibits a high Vickers hardness of 16.1 GPa, along with a good fracture toughness value of 9 MPa m^1/2. However, no significant change in the mechanical properties could be detected in the case of nanocrystalline NiAl and Ni₃Al binder addition. The enhanced properties of the cermets containing nano-Ni were attributed to the finer particle size of the hard phase after sintering along with improved particle size distribution caused by rapid dissolution, which prevents excessive coalescence of the hard particles. On the other hand, the presence of high porosity in the cermets containing nano-NiAl and -Ni₃Al apparently nullified any such enhancement.     

WC与Mo2C的添加对Ti(C,N)基金属陶瓷显微组织和耐腐蚀性能的影响

目的 研究Ti(C,N)-WC-Mo2C-Ni-Co金属陶瓷的成分、显微组织和耐腐蚀性能之间的关系,以期提高金属陶瓷材料的耐腐蚀性能.方法 采用粉末冶金方法 在金属陶瓷中添加不同比例的WC与Mo2C,并对金属陶瓷显微组织、力学性能及在酸性和碱性溶液中的腐蚀行为进行系统研究.结果 随着WC与Mo2C比例的降低,金属陶瓷组织中黑芯-灰环硬质相增多,白芯-灰环硬质相减少,环形相的厚度增加,材料的硬度先升高后下降,断裂韧性先下降后升高.当w(WC)/w(Mo2C)为0︰3时,金属陶瓷的硬度、断裂韧性和抗弯强度分别为90.5HRA,12.12 MPa·m1/2和2030 MPa.金属陶瓷在硫酸溶液中,较低的Mo2C添加量可以改善金属陶瓷的耐腐蚀性能.当w(WC)/w(Mo2C)为2︰1时,材料在硫酸溶液中有较好的耐腐蚀性能.在KOH溶液中,Mo2C添加量的提高会使材料在KOH溶液中的耐腐蚀性能降低.结论 对于设计和制备应用于不同腐蚀环境下的金属陶瓷材料有较好的指导意义,可以有效拓宽金属陶瓷的应用领域.     

Properties of porous FeAlO y /FeAl x ceramic matrix composite influenced by mechanical activation of FeAl powder

Porous ceramic matrix composites FeAlO _y /FeAl _x with incorporated metal inclusions (cermets) were synthesized by pressureless method, which includes hydrothermal treatment of mechanically alloyed FeAl powder followed by calcination. Their main structural, textural and mechanical features are described. Variation of FeAl powder alloying time results in non-monotonous changes of the porosity and mechanical strength. Details of the cermet microstructure and its relation to the mechanical properties are discussed.     

Carbon nanotube/ultrafine grade Ti(C,N) based cermets composite

Abstract

Effects of CNT addition on the microstructure and mechanical properties of Ti(C,N)–WC–Mo–Ni cermets were studied. Ti(C,N) based cermets present the typical dark core/grey rim and white core/grey rim grains. The amount of the typical dark core/grey rim grains decreases with the increase in CNT addition. For 1 wt-% CNT addition, the value of TRS is 1174·5 MPa, which is about 1·45 times than that of cermet without CNT addition whose strength is 807·3 MPa. The cermet with 5·0 wt-% CNTs increased the fracture toughness from 10·2 to 15·9 MPa m^1/2. The hardness does not change initially, decreases with further increase in the addition of CNTs.     

Processing and characterisation of cermet/hardmetal laminates with strong interfaces

Cemented carbides and cermets are potential materials for high speed machining tools. However, cemented carbides are not chemically stable at high temperature and cermets present poor fracture toughness. Novel cermet/hardmetal multilayer systems show a huge potential for this intended application. It would be possible to achieve the right balance of the required thermomechanical properties using cermet as temperature protective outer layers and hardmetal as reinforcement layers. In this work, preliminary results on the microstructural and mechanical characterisation of a multilayer TiC_xN_1−x–Co/WC–Co composite densified by hot pressing are presented, with special attention to the properties of the interface. Microstructural observations revealed the existence of strong bonding interfaces between cermet and hardmetal layers due to chemical interaction during the sintering process. As a consequence, owing to the different coefficient of thermal expansion between cermet and hardmetal, a tensile and compressive biaxial residual stress of σ_res,Cermet ≈ +260 ± 50 MPa and σ_res,WC–Co ≈ −350 ± 70 MPa was estimated in the corresponding layers. Microindentation cracks introduced in the cermet layers (the less toughness material) and propagated transversely to the layers were arrested at the interface, showing the combined effect of toughness and compressive stresses on crack shielding.     

Effect of Carbon Addition on Microstructure and Properties of WC-Co Cemented Carbides

Based on a unique method to synthesize WC-Co composite powder by insitu reactions of metal oxides and carbon, the effects of the carbon addition in the initial powders on the phase constitution, microstructure and mechanical properties of the cemented carbides were investigated. It is found that with a suitable carbon addition the pure phase constitution can be obtained in the sintered bulk from the composite powder. The mechanical properties of the cemented carbides depend on the phase constitution and the WC grain structure. To obtain the excellent properties of the WC-Co bulk, it is important to obtain the pure phase constitution from the appropriate carbon addition in the initial powders and a suitable grain size.     

硬质相粒度对金属陶瓷断裂韧性的影响

用压痕法测定了具有不同粒度硬质相的Ti(C,N)基和WC基金属陶瓷的断裂韧性,结果发现,Ti(C,N)基金属陶瓷的断裂韧性随硬质相粒度的增大而减小,而WC基金属陶瓷的断裂韧性随硬质相粒度的增大而增大。产生上述现象的主要原因与硬质相的晶体结构有关:在室温条件下,面心立方结构的Ti(C,N)晶体中可能存在｛110｝<11 0>和｛111｝<11 0>两个滑移族(含18个独立滑移系);裂纹从一个Ti(C,N)颗粒扩展至另一个Ti(C,N)颗粒时很容易形成取向有利。当Ti(C,N)颗粒较粗时,极易发生穿晶断裂,并且裂纹连续穿晶扩展时亦不会发生显著的偏转或分叉,Ti(C,N)呈现较强的脆性断裂特征。而密排六方结构的WC晶体只有｛101 0｝<112 3>一个滑移族(含4个独立的滑移系);由于取向不利,裂纹难以连续穿晶扩展,且随WC粒度的增大,其对裂纹的偏转和分叉作用增强,从而导致断裂面表面积增大而增韧。     

碳化钨/钴热喷涂粉末和涂层的研究进展

热喷涂传统碳化钨/钴金属陶瓷作为耐磨涂层已得到广泛的应用。近几年来,纳米结构涂层的热喷涂研究成为新的发展趋势。本文总结了传统和纳米结构 WC/Co热喷涂粉末的制备方法及其性质对涂层性能的影响因素,分析了涂层的微观结构和脱碳机理,简述了热喷涂纳米结构及纳米结构 微米结构 WC/Co涂层的研究进展,并指出了其发展方向。     

搅拌球磨制备亚微米晶粒Ti(C,N)基金属陶瓷

用搅拌球磨方法制备了亚微米TiC-TiN-WC-Mo-Ni-C金属陶瓷复合粉，并烧结成亚微米晶粒Ti(C,N）基金属陶瓷；研究了原始粉末粒度，磨球大小，球磨时间对复合粉粒度的影响，研究了球磨过程中氧和铁元素对粉末的污染情况；并对烧结合金的组织，性能进行了分析，表明亚微米晶粒Ti(C,N）金属陶瓷的性能优良。     

Use of Ti in hard metal alloys – Part II: mechanical properties

WC‐Co hard metal is a material of high hardness, high compressive strength and wear resistance while maintaining good toughness and thermal stability. Samples of nanosized WC powders with 10 wt% Co, WC‐10 wt% Ti, WC‐9 wt% Ti‐1 wt% Co were cold pressed at 200 MPa and sintered at 1500°C during 1 hour under vacuum of 10^–2 mbar. The characterization of the sintered materials was performed by the measurements of densification, HV30 hardness, fracture toughness and compression strength. The results showed that it is possible to process a hard metal through a Powder Metallurgical conventional route from nanoscaled WC grains, using Ti (or a Ti‐Co mixture) as a binder phase, with good mechanical properties.     

Microstructure of Nanostructured WC-Co Coating Prepared by Thermal Spraying

Nanostructured WC-Co coatings were produced with Atmospheric Plasma Spraying (APS) and Low Pressure Plasma Spraying (LPPS). Microstructure characteristics and phase compositions of the coatings were studied by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD) respectively. The microstructure compositions were analyzed with EDS. During APS deposition the existence of oxygen causes the decarburization of the coating, and the coating was composed of melted area and the WC/12Co powder. Besides WC phase, there was W2C phase. However in the coating deposited by LPPS the content of oxygen was so low that there were a limit degradation of the WC/12Co powder. The coatings were made up of compact block and loosen porosity area. There were large quantities of nanostructure WC grains and a small quantity of microstructure WC grains presented in the coating. Besides WC phase, a little W2C and WC1-x or Co6W6C phases occured. Consideration of the characteristics of the highly porous, spherical-shell morphology of nano-WC/12Co power, heterogeneous melting and localized superheating of the power were two main factors which caused the microstructure and phase composition of nano-WC-Co coatings.     

Decarburization mechanisms of WC–Co during thermal spraying: Insights from controlled carbon loss and microstructure characterization

Decarburization behavior of WC–Co particles in terms of transformation of WC to W₂C and W, and formation of η and γ phases and microstructure evolution during plasma spraying have been systematically investigated in this study. The extent of the carbon loss of WC was tailored by either altering cooling conditions of substrate/pre-coating or spraying the particles into the media with different temperatures. It is revealed that loss of carbon of WC was alleviated by protection of Co during the coating formation stage. W₂C exhibits epitaxial growth on the WC substrates in perpendicular direction and forms a nearly complete shell around the WC particles. η phase was formed as a result of decarburization and diffusion of associated phases and is located around WC–Co splats with its crystals being in cross shape. The γ phase in rod-like shape with a size of 10–20 nm embeds within the binder Co and is clearly well separated from WC grains. Further decarburization-induced W was detected mainly in Co binder, being apart entirely from WC grains. The main advantage of Co for preventing decarburization in WC–Co particles is not associated with oxidation, but instead the diffusion-controlled carbon loss. These findings would facilitate fabrication of the WC-based cermet coatings with excellent mechanical properties in particular wear resistance for extreme wear applications.     

Observation of Co nanoparticle dispersions in WC nanograins in WC-Co cermets consolidated from chemically synthesized powders

High resolution, and analytical electron microscopy has been used to analyze morphological features in consolidated specimens of nanostructured tungsten carbide-cobalt cermet materials prepared from chemically synthesized nanostructured WC-Co powder. The investigation includes studying materials produced by consolidation (i) above, and (ii) below the eutectic temperature by thermo-mechanical working. In the case of liquidphase sintered material, the study includes examining the influence of a vanadium carbide additive on morphological features, as well as its effect in inhibiting grain growth. Electron microscope examination of all consolidated materials reveals a dispersion of nanoprecipitates within the WC nanograins of the WC-Co cermet. Micro-diffraction and analytical studies show that these nanoprecipitates are f.c.c. cobalt. This novel discovery is consistent with the concept that the nanoprecipitates nucleate from cobalt retained within the WC nanograins, which is a consequence of the intimate intermixing of tungsten and cobalt in the original chemical synthesis process.     

纳米晶复合碳化钨-钴粉末的工业化制备技术

超细晶粒的ＷＣ－Ｃｏ硬质合金具有高硬度、高强度的性能,纳米晶ＷＣ－Ｃｏ复合粉末的合成是制备该合金的首要前提工作,中国正努力保持和强化“钨钴液相复合－喷雾干燥－流态化连续还原碳化技术”在国际上的前沿优势。该技术将偏钨酸铵和水合硝酸钴以分子级水平混合,经喷雾干燥制备出ＣｏＷＯ４／ＷＯ３复合氧化物前驱体粉末,然后将前驱体粉末置于流态化反应炉中,在Ｈ２／ＣＨ４／ＣＯ２／Ｎ２气氛下经过连续的还原、碳化、调碳     

Cr对超音速火焰喷涂WC-Co涂层抗中性盐雾腐蚀性能的影响

研究了超音速火焰喷涂(HVAF)WC-Co和WC-CoCr涂层的抗化学腐蚀性能。采用XPS对超音速火焰喷涂涂层的表面和内部进行了分析,在中性NaCl溶液中对涂层进行了电化学分析。结果表明,WC-Co中的粘结相Co主要呈金属单质,而WC-CoCr涂层中粘结相分别呈单质Co和Cr2O3。电化学极化测试表明,WC-Co涂层处理明显提高了基体的腐蚀电位,而添加Cr后则进一步提高了腐蚀电位。400 h中性盐雾腐蚀结果表明,WC-CoCr涂层的抗化学腐蚀性能优于WC-Co涂层的。腐蚀后的SEM表明,腐蚀介质先腐蚀涂层中的粘结相,当腐蚀介质扩散到基体中则优先腐蚀基体。添加少量的Cr在粘结相中形成了Cr2O3陶瓷钝化相,有利提高HVAF WC-Co涂层的抗腐蚀性能。     

Effect of WC nanoparticle size on the sintering temperature,density, and microhardness of WC-8 wt % Co alloys

Using X-ray diffraction, scanning electron microscopy, and density measurements, we have studied the effect of WC particle size (20 to 150 nm) on the optimal sintering temperature of the WC-8 wt % Co alloy and the effect of sintering temperature (800–1600°C) on its phase composition, density, and microhardness. The results indicate that, during sintering of the starting powder mixture, the first to form is the ternary carbide phase Co₆W₆C. At sintering temperatures of 1100°C and above, this phase reacts with carbon to form Co₃W₃C. Sintering above 1000°C leads to the formation of a cubic solid solution of tungsten carbide in cobalt, β-Co〈WC〉, along with the ternary carbide phases. The density and microhardness of the alloy have been measured as functions of sintering temperature. The use of WC nanopowder has been shown to reduce the optimal sintering temperature of the WC-Co alloy by about 100°C.     

Influence of Ni Interlayers on the Mechanical Properties of Ti6Al4V/(WC-Co) Friction Welds

Ni interlayers were introduced prior to dissimilar friction welding of Ti6Al4V base material to three cemented carbide substrates. The fracture strength of Ti6Al4V/(WC-6 wt% Co) welds were poor and were markedly improved when 20-µm thick Ni interlayers were introduced prior to dissimilar friction welding. These results were only produced when the (WC-6 wt% Co) cermet was electroplated prior to friction welding. When the Ti6Al4V alloy was electroplated prior to friction welding, fractured WC particles and cracking were observed in the (WC-Co) carbide substrate. The fracture strengths of Ti6Al4V/(WC-11 wt% Co) and Ti6Al4V/(WC-24 wt% Co) welds were not improved when 20-µm thick Ni interlayers were introduced prior to friction welding. During mechanical testing, the Ni layer retained at the dissimilar joint interface created a region of weakness.     

Effect of different binders and secondary carbides on NbC cermets

Due to the rapidly increasing price of tungsten carbide and the significant health risks associated with the wear products of WC-Co (Co₃O₄ and Wo₃), an alternative is required. Niobium carbide (NbC) is well suited as a cutting tool due to its high melting point and low solubility in iron. Compared to pure NbC, a complete substitution of WC to NbC-Co resulted in an increased toughness and strength. As alternative binders, nickel and iron-based binders were subsequently investigated. Although iron-based cermets would be an economical, low-cost alternative to NbC-Ni cermets, they showed a higher coefficient of friction and wear rate. So far, NbC-Ni cermets best met the requirements of high hardness and toughness. Various secondary carbides such as VC, Mo₂C, TiC, but also WC were added to further improve the hardness. Elemental analyses of NbC-Ni-MeC cermets (Me = metal) showed that the binder is a face-centered cubic solid solution, while the NbC phase is a solid solution of the type (Nb, Me)C.

     

Synthesis and processing of nanostructured WC-Co materials

In this study a novel approach, termed the integrated mechanical and thermal activation (IMTA) process, was used to synthesize nanostructured WC-Co powder. As a result of the integration of mechanical and thermal activation, nanostructured WC-Co powder was synthesized below 1000°C, starting from WO₃, CoO and graphite powder mixtures. Furthermore, consolidation of the nanostructured WC-Co powder via high velocity oxy-fuel (HVOF) thermal spraying and solid state sintering was investigated. The results demonstrated the feasibility of converting the nanostructured WC-Co powder to coatings and bulk components, the properties of which are either comparable to or better than that of the conventional coarse-grained counterparts.     

Design of composite porous cermets synthesized by hydrothermal treatment of CrAl powder followed by calcination

The microstructure of the porous Cr–Al metal–oxide cermet was studied by means of XRD, SEM, EDX as well as IR and Raman spectroscopy. This cermet was synthesized by mechanical alloying of Cr–Al powders in an AGO-2 planetary ball mill followed by hydrothermal treatment in a special stainless steel die and calcination in air. As a result, a highly porous monolith comprised of metal-like particles randomly distributed in the oxide matrix (Cr₂O₃ and Al₂O₃) was formed. Two types of the composite cores were found in cermets. The first one consisted of chromium phase containing nanoparticles sized from 50 to 140 nm and Al-enriched phase at the interfaces. The second one consisted of new chromium oxide phases with hexagonal Cr₂N-like and fcc CrN-like structures probably with Cr₂O and CrO stoichiometry. These new phases were stabilized within aggregates of the nanocomposite particles containing inclusions of alumina. The relations between different preparation stages and the cermet microstructure are discussed.