Microstructure and high-temperature friction and wear behavior of WC-(W,Cr)2C-Ni coating prepared by high velocity oxy-fuel spraying

WC-(W,Cr)₂C-Ni coating was prepared by high velocity oxy-fuel spraying (HVOF). The microstructure and phase composition of the as-sprayed coating and that after oxidation at high temperature were analyzed by means of scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The oxidation behavior of as-sprayed coating and starting powders was evaluated by thermogravimetry. Dry sliding friction and wear behavior of the WC-(W,Cr)₂C-Ni coating sliding against Si₃N₄ ball at different temperatures (room temperature 20 °C and elevated temperature of 700 °C and 800 °C) was evaluated using an oscillating friction and wear tester. Besides, the microhardness and fracture toughness of the coating was also measured. Results show that sintering agglomerated WC-20 wt.%Cr-7 wt.%Ni powder is an effective method to prepare agglomerated and sintered WC-(W,Cr)₂C-Ni composite powder. The excellent oxidation resistance of WC-(W,Cr)₂C-Ni coating is mainly resulted from a double-decker shell-core microstructure formed in the coating. The composition of the outer shell is (W,Cr)₂C phase and that of the inner shell is Cr₃C₂. During high-temperature friction and wear test, well remained hard WC phase in the WC-(W,Cr)₂C-Ni coating can guarantee its good mechanical properties and wear resistance, and newly generated nano NiWO₄, CrWO₄ and Cr₂WO₆ particles can further improve these properties significantly.     

Experimental and simulation studies on grain growth in TiC and WC-based cermets during liquid phase sintering

The grain growth behaviors of TiC and WC particles in TiC-Ni, TiC-Mo₂C-Ni, WC-Co and WC-VC-Co alloys during liquid phase sintering were investigated for different Ni or Co contents and compared with the results of Monte Carlo simulations. In the experimental study, TiC-Ni and WC-Co alloys had a maximum grain size at a certain liquid volume fraction, while the grain size in TiC-Mo₂C-Ni and WC-VC-Co alloys increased monotonically with an increasing liquid volume fraction. These results mean that the grain growth of these alloys cannot be explained by the conventional mechanisms for Ostwald ripening, namely diffusion or reaction controlled processes. Monte Carlo simulations with different energy relationships between solidliquid interfaces predicted the effect of the liquid volume fraction on grain size similar to the experimental results. The contiguous boundaries between solid (carbide) particles appear to influence the grain growth behavior in TiC- and WC-based alloys during liquid phase sintering.     

VC, Cr3C2 doped ultrafine WC-Co cemented carbides prepared by spark plasma sintering

The influence of the addition of 0.3, 0.5 and 0.7 wt.% VC on the density, microstructure and mechanical properties of WC-Cr₃C₂-11 wt.% Co with 0, 0.2, 0.4 and 0.6 wt.% Cr₃C₂ hard metals prepared by spark plasma sintering (SPS) at a temperature of 1200 °C (5 min, 40 MPa) was investigated. Microstructure analysis revealed that the WC grain size in the sintered hard metals was strongly influenced by the VC and Cr₃C₂ content. With the addition of inhibitors and the increased amount of Cr₃C₂, the density is reduced, and on the contrary, the addition of VC as an inhibitor contributes to promoting the densification. The combined addition of Cr₃C₂ and VC could strongly reduce the WC grain growth to about 350 nm. Observation suggests that the fracture of WC-Co cemented carbide is brittle and intergranular. The amount of added VC/Cr₃C₂ should be controlled in a certain range. Samples with an appropriate proportion of VC/Cr₃C₂ added exhibit higher hardness which can be up to 1938 HV₃₀. Toughness, too, can reach 16.34 MPa m^1/2.     

Dispersed hard particle strengthening of metals

A review is presented of the various methods utilized to strengthen metals and alloys for high temperature service by a suitable dispersion of finely divided hard particles. Alloys such as SAP, produced by powder metallurgy, and other sintered aluminum powders, molybdenum containing small quantities of refractory oxides and magnesium containing a dispersed intermetallic, are considered. Internal oxidation and other methods of achieving a hard particle dispersion are discussed. New data on the high temperature strength and stability of the alloy systems Cu-Al₂O₃, Ni-Al₂O₃ and Be-Be₂C are shown. The theoretical aspects of the hardening and strengthening mechanism are presented and consideration is given to the problems associated with the powder metallurgy of the various processes.     

Microstructural and mechanical evaluation of Al-TiB2 nanostructured composite fabricated by mechanical alloying

Production of bulk Al-TiB₂ nanocomposite from mechanically alloyed powder was studied. Al-20 wt.% TiB₂ metal matrix nanocomposite powder was obtained by mechanical alloying (MA) of pure Ti, B and Al powder mixture. A double step process was used to prevent the formation of undesirable phases like Al₃Ti intermetallic compound, which has been described in our previous papers. The resultant powder was consolidated by spark plasma sintering (SPS) followed up by hot extrusion. The structural characteristics of powder particles and sintered samples were studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Hardness measurements were conducted on the cross section of powder particles and sintered sample and the tensile behavior of extruded samples was evaluated. The results showed that the prepared Al-20 wt.% TiB₂ nanocomposite has good thermal stability against grain growth and particle coarsening. Extruded Al-20 wt.% TiB₂ showed a hardness value of 180 VHN and yield and tensile strength of 480 and 540 MPa, respectively, which are much higher than those reported for similar composites made by other processes.     

Influence of feedstock powder characteristics and spray processes on microstructure and properties of WC-(W,Cr)2C-Ni hardmetal coatings

The composition WC-(W,Cr)₂C-Ni (commercial designations WC-‘CrC’-Ni, WC-Cr₃C₂-Ni and WC-NiCr) is unique among the WC-based materials used for the preparation of thermally sprayed hardmetal coatings. These coatings show a significantly higher oxidation resistance and high-temperature sliding wear resistance than WC-Co and WC-CoCr coatings do. Unlike WC-Co and Cr₃C₂-NiCr, WC-(W,Cr)₂C-Ni is not a simple binary hard phase-binder metal composite as it is composed of two hard phases: WC and (W,Cr)₂C. Surprisingly this composition has been poorly investigated in the past.In this paper coating microstructures and properties obtained from five commercial feedstock powders of different origins using two different liquid-fuelled high velocity oxy-fuel (HVOF) systems (K2 and JP-5000) were investigated. Additional experiments were performed with one powder using atmospheric and vacuum plasma spraying (APS and VPS, respectively). The microstructures and phase compositions of the powders and the coatings were studied. Focus was on the appearance, composition and distribution of the (W,Cr)₂C phase which might form or might change its Cr/W ratio during the spray process. The composition of the (W,Cr)₂C phase was estimated from the lattice parameters. Hardness HV0.3 was measured for all coatings. The density, Young's modulus and abrasion wear resistance of HVOF-sprayed coatings were studied.     

Effect of cobalt replacement by nickel on functionally graded cemented carbonitrides

Functionally graded cemented carbonitrides (FGCCs) are applied in cutting tools industry. Indexable inserts made from mentioned alloys have superior cutting performance and tool life thanks the formation of a surface modified layer with enhanced properties as well as crater wear resistance. Cemented carbonitrides are made of hard carbide/nitride/carbonitride particles that have been embedded in a metallic binder. Excellent wetting ability of tungsten carbide with cobalt has made this metal the first choice as binder. However, cobalt has high cost and environmental pollution impacts. Substitution of cobalt with other metals has always been figured out. Some other metals that have been used as binder are iron, nickel and manganese. In addition to lower cost, nickel has higher corrosion resistance than cobalt. In the present work, gradual substitution of cobalt with nickel in WC/2.5TiC_0.7N_0.3/2.5TiC/0.2VC/8Co (numbers indicate wt.%) cemented carbonitride has been studied. This is supposed to make some alterations in mechanical and magnetic properties as well as formation of Cubic Free Layer. As sintered samples with different binder characteristics were prepared by powder metallurgy techniques and use of Hot Isostatic Pressure (HIP) and vacuum furnaces. Magnetic saturation, coercive force, Vickers hardness, and transverse rupture strength tests were carried out together with optical and scanning electron microscopy. Computational thermodynamics was applied to explain changes in magnetic properties of investigated alloys as well as plotting phase diagrams. Complete displacement of cobalt by nickel made an increase in transverse rupture strength (TRS) of final alloy by 37% with less than 6% decrease in Vickers hardness and doubled the thickness of Cubic Free Layer.     

Correlation of microstructure with high-temperature hardness of (TiC,TiN)/Ti–6Al–4V surface composites fabricated by high-energy electron-beam irradiation

Correlation of microstructure with high-temperature hardness of (TiC,TiN)/Ti–6Al–4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. TiC, TiN and TiC+TiN powder mixtures containing 50% CaF₂ flux were deposited on the surface of a Ti–6Al–4V alloy substrate, and irradiated by high-energy electron beam to form 1-mm-thick, defect-free surface composite layers. The surface composite layers contained a large amount (over 30 vol.%) of precipitates such as TiC, TiN, (Ti_xAl_1−x)N and Ti(C_xN_1−x) in the martensitic or N-rich acicular α-Ti matrix. This microstructural modification including the formation of hard precipitates and hardened matrices in the surface composite layers improved hardness and high-temperature hardness two to four times greater than that of the substrate. In particular, the surface composite fabricated with TiN powders had the highest hardness because of the highest volume fraction of TiN and (Ti_xAl_1−x)N distributed in the hardened N-rich acicular α-Ti matrix. These findings suggest that the (TiC,TiN)/Ti–6Al–4V surface composites can be used for structural materials requiring excellent thermal resistance.     

Microstructure and Properties of HVOF-Sprayed WC-(W,Cr)<Subscript>2</Subscript>C-Ni Coatings

The composition WC-(W,Cr)₂C-Ni is one of the standard compositions used for the preparation of thermally sprayed coatings by high velocity oxy-fuel (HVOF) spraying. Surprisingly, this composition has been poorly investigated in the past. Frequent use of commercial designations WC-‘CrC’-Ni, WC-Cr₃C₂-Ni, and WC-NiCr indicates the insufficient knowledge about the phase compositions of these powders and coatings. The properties of these coatings differ significantly from those of WC-Co and WC-CoCr coatings. In this paper, the results of different series of experiments conducted on HVOF-sprayed WC-(W,Cr)₂C-Ni coatings are compiled and their specific benefits pointed out. The focus of this study is on the analysis of the microstructures and phase compositions of the feedstock powders and coatings. Unlike WC-Co and Cr₃C₂-NiCr, WC-(W,Cr)₂C-Ni is not a simple binary hard phase—binder metal composite. The phase (W,Cr)₂C with unknown physical and mechanical properties appears as a second hard phase, which is inhomogeneously distributed in the feedstock powders and coatings. As examples of coating properties, the oxidation resistance and dry sliding wear properties are compared with those of WC-10%Co-4%Cr coatings.     

Effect of Atmospheric Plasma Spraying Power on Microstructure and Properties of WC-(W,Cr)2C-Ni Coatings

WC-(W,Cr)₂C-Ni coatings were prepared by atmospheric plasma spraying (APS) with different spraying powers. The effect of spraying power on microstructure, phase composition, hardness, fracture toughness, and oscillating dry friction and wear behaviors of the coatings were studied. Simultaneously, the microstructure and properties of the as-sprayed coatings were compared with those of WC-17Co coating prepared under the optimal spraying power. It was found that spraying power had significant effect on the molten degree of feedstock powder and phase composition as well as microstructure and properties of WC-(W,Cr)₂C-Ni coatings. WC-(W,Cr)₂C-Ni coating deposited at a moderate spraying power of 22.5?kW had the highest fracture toughness and the best wear resistance. WC-17Co coating obtained under the moderate spraying power had poor fracture toughness and wear resistance. Moreover, the four kinds of coatings were all dominated by subsurface cracking and removal of materials when sliding against Si₃N₄ ball under unlubricated conditions.     

Effect of h-BN addition on the properties of nanostructured Al2O3-TiB2-TiN based coatings developed by combined SHS and laser surface alloying

A hard coating was obtained on AISI1025 steel substrate by the action of a high power laser beam on a powder mixture of Al, TiO₂ and h-BN pre-placed on the substrate surface. The precursor powder mixture underwent self-propagating high-temperature synthesis (SHS) at the high temperatures induced by the incident laser. The products of SHS were subsequently laser alloyed onto the substrate, whereby, a hard, nanostructured coating was formed comprising of Al₂O₃, TiB₂ and TiN. Excess h-BN in the precursor resulted in the presence of free h-BN in the coating. Microhardness and coefficient of friction (with WC-Co as counterbody) of the coating were found to reduce with increase in h-BN content in the precursor. It was possible to develop a coating with a property combination of high hardness, low wear rate and low friction coefficient.     

AlN粉体表面抗水解涂层的制备及其对AlN陶瓷热导率的影响

提升AlN陶瓷粉体的抗水解性能对于其储存和成型加工至关重要。使用一种抗水解涂层作为阻止水分与AlN表面接触的屏障,以提升AlN粉体的抗水解性能。采用化学沉淀工艺在AlN粉体表面制备均匀、全包覆的非晶Y₂O₃涂层。利用TEM、XPS和Zeta电位测试详细研究了包覆层的有效性和完整性。通过测试室温下水基AlN悬浮液的pH-时间曲线以研究AlN粉体的水解性能。结果表明,经包覆处理的AlN粉体能够在水中保持稳定至48 h,这说明Y₂O₃表面包覆处理可以有效钝化AlN粉体,从而避免了其水解反应的发生。此外,与球磨工艺引入烧结助剂相比,化学沉淀工艺有利于提升AlN陶瓷的热导率。     

Effect of medium temperature on the dispersion of WC/ZrO2/VC composite powders

This article proposed a novel method to disperse WC/ZrO₂/VC composite powders so as to attain a perfectly uniform suspension. Besides using conventional dispersing means such as adding dispersant (PEG, polyethylene glycol), mechanical stirring, ultrasonic vibration and ball milling, the temperature adjustment of dispersing-medium distilled water had also been employed. The agglomerating and dispersing mechanisms were analyzed by means of TEM observation of WC/ZrO₂/VC composite powders dispersed under five different temperatures, with the results showing that the most uniform dispersion was obtained under the temperature of 100 °C based on the criterion for conglomeration number per unit. The dispersed WC/ZrO₂/VC composite powders were dried and consequently sintered by hot-press sintering in nitrogen atmosphere at 1580 °C with pressure of 30 MPa. The testing results of mechanical properties such as relative density, hardness, bending strength and fracture toughness show that the optimal properties are obtained by using the WC/ZrO₂/VC composite powders dispersed under 100 °C. The surface crack morphologies of sintered samples are investigated and the results show that crack extended in a more tortuous path for the sample sintered from well-dispersed composite powders.     

Sintering characteristics and microstructure of WC-Co-VC/Cr<Subscript>3</Subscript>C<Subscript>2</Subscript> ultrafine cemented carbides

The sintering characteristics, microstructure, and mechanical properties of ultrafine WC-12%Co-0.2%VC/0.5%Cr₃C₂ cemented carbides were investigated. Dilatometric and differential thermal analyses (DTA) indicate that the compacts start to shrink at 600°C, the shrinkage rate peak is at 1190°C, and the liquid formation temperature is lower than the W-C-Co eutectic temperature (1330°C). Microstructure analysis results show that the cemented carbides with fine and homogeneous microstructure were obtained when sintered at 1430°C. Continuous and discontinuous grain growth was suppressed due to the synergistic action of VC/Cr₃C₂. The transverse rupture strength (TRS) of the samples reaches 4286 MPa, with the hardness HRA 92.1. The fine and homogeneous microstructure, alloy strengthening, and different phase constitutions of binder in the cemented carbides result in high hardness and TRS. Continuous and discontinuous grain growth was observed in the cemented carbide sintered at 1450°C, which results in significant decreases of hardness and TRS. It indicates that VC/Cr₃C₂ additions in the cemented carbides can only suppress the grain growth at a certain temperature.     

Spraying TiN by a Combined laser and low-pressure plasma spray system

The formation of a TiN-Ti composite coating by thermal spraying of titanium powder with laser processing of the subsequent coating in a low-pressure N₂ atmosphere was examined. A low-pressure plasma spray system was used in combination with a CO₂ laser. First, the coating was plasma sprayed onto a mild steel substrate using a N₂ plasma jet and titanium powder in a controlled low-pressure N2 atmosphere. The coating was then irradiated with a CO₂ laser beam in a N₂ atmosphere, and the coating was heated with a N₂ plasma jet. The amount of TiN formed in the coating was characterized by X-ray diffraction analysis. The influence of plasma spraying conditions such as plasma power, flow of plasma operating gases, chamber pressure, and laser irradiating conditions on the formation of TiN was investigated. The effect of TiN formation in the titanium coating on Vickers hardness of the coatings was examined. It was evident that coating hardness increased with an increase in TiN content in the coating and that a TiN-Ti composite coating with a hardness of more than 1200 H V can be obtained with the use of laser irradiation processing.     

Laser surface hardening of austenitic stainless steel

For the purpose of studying the possibilities of increasing the wear resistance, keeping a high level of corrosion strength, austenitic stainless steel specimens mainly containing 19.2%Cr and 9.4%Ni were two-step surface alloyed using added materials (AMs) with hard particles of carbides (WC), nitrides (TiN), and borides (TiB₂). The simultaneous melting of AM and surface layer was performed by a CO₂ continuous wave laser on a numerically controlled X-Y table. On these specimens, the microstructural characteristics, microhardness, and depth of the molten zone were determined, which allowed definition of the AM with the best hardening effect. The research continued by two-step laser surface alloying of the same base material with different effective AM quantities. The specimens were processed by continuous wave laser radiation, by multiple-pass with 35% overlap. The alloyed layers were described by light optical microscopy, x-ray diffractometry, flash spectrometry, and hardness measurement. The conditions to obtain compact surface layers with 2.5 to 3 times higher hardness than the base material were determined.     

超细钨铜合金粉末在金刚石工具中的应用研究

采用超细钨铜合金粉末和单质钨、铜粉末热压烧结成两组金刚石胎体块,利用硬度计,排水法,万能实验机,扫描电镜和锯切实验等测试分析手段分别分析测量了胎体的硬度、致密度、抗弯强度、断口形貌和锯切性能。实验结果表明:在高钨基胎体配方中添加超细钨铜合金粉末比添加单质钨、铜粉末可以显著提高胎体的硬度10 HRB左右,改善胎体合金使之均匀化,在本实验配方中,干切硬花岗岩时,高钨基胎体配方中添加超细钨铜合金粉末比添加单质钨、铜粉末切割平稳,形成胎体和金刚石有效的磨损匹配,可以显著提高工具的使用寿命30%左右。烧结温度范围内,添加合金粉的胎体抗弯强度均比添加单质钨、铜粉末低100 MPa左右,这主要是使用的超细钨铜粉末的氧含量较高所致。     

TiN含量对Ti(C,N)/NiCr金属陶瓷微观结构和力学性能的影响

采用粉末冶金真空烧结方法制备了Ti（C,N）／NiCr金属陶瓷．研究了TiN含量对Ti（C,N）／NiCr金属陶瓷微观结构与力学性能的影响、结果表明,TiN的加入既改变了金属陶瓷硬质相颗粒的尺寸,使其变小,也改变了硬质相颗粒的形貌,使其由圆形变为多边形;随TiN含量的增加,金属陶瓷的抗弯强度均出现先增加后降低的规律,但在较低的烧结温度下,抗弯强度在TiN含量为4％时达到最大值,而在较高的烧结温度下,抗弯强度在TiN含量为6％时达到最大值;硬度在TiN含量〈10％时变化不明显,TiN含量〉10％时硬度急剧下降;抗弯断口以穿晶解理为主要的断裂模式．     

Preparation and properties of sintered molybdenum doped with La2O3/MoSi2

Sintered Mo with the addition of La₂O₃/MoSi₂ was prepared via the process of solid–solid doping + powder metallurgy. X-ray diffraction experiment, hardness test, three-point bending test and high-temperature tensile test were carried out to characterize the samples. The XRD pattern of a typical sample shows that the sintered Mo was mainly composed of Mo, La₂O₃ and Mo₅Si₃. Mo₅Si₃ was probably formed through the reaction between MoSi₂ and the Mo matrix. Densities and fracture toughnesses of both doped Mo and pure Mo were measured and contrasted. Sintered Mo with the addition of 0.2 wt% La₂O₃/MoSi₂ has the highest toughness, while more addition of La₂O₃/MoSi₂ has smaller effect on improving toughness or even embrittles Mo. The results of three-point bending test and high-temperature tensile test show that the bending strength and high-temperature tensile strength of doped Mo are both higher than those of pure Mo. The formation of Mo₅Si₃ improves the high-temperature strength. The La₂O₃/Mo₅Si₃ dispersed in the Mo matrix refined the grains, and thus strengthened the Mo matrix by dispersion strengthening and grain refinement.     

Fabrication of Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript> ceramic material by mechanical alloying

Ti₃AlC₂ has the properties of ceramics and metals. These excellent properties indicate that Ti₃AlC₂ is a very promising material to extensive applications. Ti₃AlC₂ ceramic material was prepared by mechanical alloying. The effects of milling time and sintering temperature on the fracture, microstructure and mechanical properties of Ti₃AlC₂ ceramic material were analyzed by laser particle analyzer, X-ray diffraction, and scanning electron microscopy. The experimental results showed that Ti₃AlC₂ had the best comprehensive properties after the composite powder was milled for 3 h and sintered at 1630°C for 2 h. The relative density, bending strength, and hardness of the sample reached 92.23%, 345.2 MPa, and HRA 34.1, respectively. The fracture surface indicated that the fracture of the material belonged to ductile rapture.