用于瓦楞辊防护的WC-12Co涂层组织和性能研究

采用团聚烧结法制备了一种含有纳米粒子的新型瓦楞辊专用WC-12Co喷涂粉末,并使用超音速火焰喷涂工艺（HVOF）制备了该粉末的两种涂层。测试了涂层的结合强度、显微硬度、气孔率、开裂韧性和单道次沉积厚度。并利用XRD对喷涂粉末及涂层进行了相结构分析,用扫描电子显微镜和金相显微镜对喷涂粉末、涂层的组织结构进行了观察,并与进...     

Synthesis of nanostructured WC-12 pct Co coating using mechanical milling and high velocity oxygen fuel thermal spraying

A nanostructured WC-12 pct Co coating was synthesized using mechanical milling and high velocity oxygen fuel (HVOF) thermal spraying. The variation of powder characteristics with milling time and the performance of the coatings were investigated using scanning electron microscope (SEM), X-ray, transmission electron microscope (TEM), thermogravimetric analyzer (TGA), and microhardness measurements. There is no evidence that indicates the presence of an amorphous phase in the sintered WC-12 pct Co powder, and the binder phase in this powder is still crystalline Co. Mechanical milling of up to 20 hours did not lead to the formation of an amorphous phase in the sintered WC-12 pct Co powder. During the initial stages of the milling, the brittle carbide particles were first fractured into fragments and then embedded into the binder phase. This process gradually formed polycrystal nanocomposite powders of the Co binder phase and W carbide particles. The conventional cold welding and fracturing processes primarily occurred among the Co binder powders and polycrystal composite powders. The nanostructured WC-12 pct Co coatings, synthesized in the present study, consist of an amorphous matrix and carbides with an average particle diameter of 35 nm. The coating possesses an average microhardness of 1135 HV and higher resistance to indentation fracture than that of its conventional counterpart.     

Synthesis of nanostructured Cr3C2-25(Ni20Cr) coatings

On the basis of the nanocrystalline Cr₃C₂-25 (Ni20Cr) feedstock powders produced by mechanical milling, a nanostructured coating has been synthesized using high velocity oxygen fuel (HVOF) thermal spraying. The properties of the nanostructured coating were compared to those of the conventional coating of the same composition using scanning electron microscope (SEM), transmission electron microscope (TEM), and microhardness tests. The nanostructured Cr₃C₂-25 (Ni20Cr) coating synthesized in this study had an average carbide particle size of 24 nm. Discontinuous elongated amorphous phases were observed in the nanostructured coating. The conventional Cr₃C₂-25 (Ni20Cr) coatings produced using blended elemental powders exhibited an inhomogeneous microstructure. The observed homogeneity of the nanostructured coating is attributed, in part, to the microstructural improvement of the starting powder. The nanostructured Cr₃C₂-25 (Ni20Cr) coating yielded an average microhardness value of 1020 DPH₃₀₀, which corresponds to a 20 pct increase in microhardness over that of the conventional coating. The nanostructured Cr₃C₂-25 (Ni20Cr) coating also exhibited a higher apparent fracture toughness relative to that of the conventional coating. The apparent mechanical property improvements in the nanostructured coating were thought to result from the uniformity of the microstructure and the high performance associated with a nanostructured structure. In addition, the mechanism that is present during the milling of a system containing nondeformable particles is discussed in light of the TEM observations.     

Cold spray deposition of nanocrystalline aluminum alloys

Aluminum 5083 powder was mechanically milled under liquid nitrogen to achieve a nanocrystalline grain size in the range of 20 to 30 nm. The powder was subsequently sprayed using a nozzle designed with a validated numerical model for cold spray technology. The resulting coatings were evaluated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), micro- and nanoindentation. The TEM analysis shows that the nanocrystalline grain structure of the cryomilled feedstock powder was retained after the cold spray process. A significant increase in hardness from 104 to 261±8 (HV_300g) was observed when comparing the nanocrystalline coating with cast, cold-worked, Al 5083. The ability to use cold spray to produce nanocrystalline large deposits was also demonstrated in this work. An erratum to this article is available at .     

一种新型WC-12Co粉末喷涂工艺优化与涂层性能研究

采用JP-8000型超音速火焰(HVOF)喷涂设备,在低碳钢基体上采用新型WC-12Co粉末和不同喷涂工艺参数制备了5种涂层,测试了涂层的结合强度、显微硬度、气孔率、开裂韧性和单道次沉积厚度.并利用XRD对喷涂粉末及涂层进行了相结构分析,用扫描电子显微镜和金相显微镜对喷涂粉末、涂层的组织结构进行了观察.结果表明:该粉末...     

Synthesis and nanoindentation study of high-velocity oxygen fuel thermal-sprayed nanocrystalline and near-nanocrystalline Ni coatings

In the present work, the nanoindentation technique was used to study the behavior of nanocrystalline Ni coatings. Two different types of Ni coatings were synthesized. One of the coatings was prepared with a commercial-grade Ni powder (as received, near-nanocrystalline), and the second coating was sprayed with the same powder, after having been mechanically milled in liquid nitrogen for 15 hours (nanocrystalline). Identical high-velocity oxygen fuel (HVOF) spray parameters were used for both types of coatings. The oxide-phase content in each coating was analyzed. The microstructure and properties of the milled powders and as-sprayed coatings were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nanoindentation. The average grain size of the as-received powder was 140±52 nm, and that of the as milled powders was 15.7±5.1 nm. The near-nanocrystalline coating microstructure was composed of grains with an average grain size of 280±39 nm, and the nanocrystalline coating was composed of nanocrystalline grains with an average grain size of 92±41 nm. The nanoindentation technique was applied to characterize the coating hardness under different penetration depths. The indentation size effect (or ISE) has been observed and correlated to the microstructure of the coatings. The results show that the assumption of geometrically necessary dislocations was valid for this study. A critical indentation depth was identified for measuring the intrinsic properties of the constituent material of the coating (≲500 nm).     

Microstructure and thermal cycling behavior of nanostructured yttria partially stabilized zirconia (YSZ) thermal barrier coatings

Nanostructured yttria partially stabilized zirconia(YSZ) coatings were prepared by atmospheric plasma spraying(APS) using the conglomeration made by zirconia nanoparticle as the raw materials.The measurement methods,which consisted of scanning electron microscopy(SEM),transmission electron microscopy(TEM) and thermal cycling behavior,were used to character the morphology,composition and thermal oxidation behavior of the powder and the coatings.From the results,it was shown that the YSZ coating was the laminar structure,and the elements distribution in the bond and top coat were well-proportioned.The YSZ coatings were composed of fine grains with size ranging from 30 to 110 nm.The laminar layers with columnar grains were surrounded with unmelted parts of the nanostructured powder and some equiaxed grains.In the as-sprayed nanostructured zirconia coatings,there existed pores that were less than 1 μm.The cracks were observed on some of the crystal border.The cyclic oxidation experiment showed that the nanostructured coating had longer thermal cycling lifetime to exhibit the promising thermal cyclic oxidation resistance.The failure of the nanostructured TBC was similar to the failure of conventional APS TBC.     

Synthesis of nanosized WC/MgO powders by high energy ball milling and analysis of reaction thermodynamics

Abstract

In the present work, a powder mixture of pure WO₃, graphite and Mg with a definite atomic ratio was milled at room temperature using a high energy ball mill method, and ball milled powders were analysed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results indicated that after ball milling for a period of time, an oxidation–reduction reaction was successfully achieved among the Mg, graphite and WO₃ powders to obtain MgO and WC. The extension of the ball milling led to the refinement of the powders. After ball milling 50 h, nanocrystalline WC grains (25 nm) were embedded into the fine matrix of MgO and formed fine nanocomposite MgO/WC powders (～100 nm in diameter). The experimental results and thermodynamic analysis showed that the formation of nanocomposite MgO/WC was a mechanically induced self-propagating reaction, and very short milling time was needed to complete the reaction.     

Characterisation and sintering studies of mechanically milled nano tungsten powder

Abstract

Elemental tungsten powder was mechanically milled by planetary mill for 100 h. Particles were thinned down to nanometre scale. The shape of the milled powders was flat cylindrical with average diameter and length 12˙5 and 46˙5 nm respectively. The corresponding crystallite size obtained by X-ray diffraction (XRD) was 26˙96 nm. The results obtained by XRD and small angle X-ray scattering were well supported by transmission electron microscopy and high resolution transmission electron microscopy results. The maximum shrinkage of the compact has been observed at ～1500 K, which has been used as a guideline for sintering experiments. The powders sintered at 1773 K have resulted in 96% relative density.     

喷涂参数对内送粉等离子喷枪制备YSZ涂层微观结构和性能的影响

为了进一步提高大气等离子喷涂YSZ热障涂层性能并降低其成产成本,开发了内送粉等离子喷枪.本文采用Spray Watch 2i在线监测系统测量了YSZ粉末粒子温度和飞行速度,采用实验室手段表征了涂层的微观结构、结合强度、弯曲性能和抗热震性能.研究结果表明,内送粉方式下喷涂距离对涂层微观结构和性能的影响规律与外送粉相似,而电流和主气流量表现出了独特的作用效果：随电流的增大,涂层综合性能先增加后降低,高电流下制备的涂层结合强度较低;在30～50 L/min范围内,增大主气流量可显著提高涂层结合强度.与外送粉最优参数制备YSZ涂层相比,内送粉时的功率消耗大幅降低,但粒子熔化效果明显增强,相同送粉速率下粉末沉积效率略有增加,涂层热循环寿命有所提高.     

Structural evolution in mechanically alloyed Al-Fe powders

The structural evolution in mechanically alloyed binary aluminum-iron powder mixtures containing 1, 4, 7.3, 10.7, and 25 at. pct Fe was investigated using X-ray diffraction (XRD) and electron microscopic techniques. The constitution (number and identity of phases present), microstructure (crystal size, particle size), and transformation behavior of the powders on annealing were studied. The solid solubility of Fe in Al has been extended up to at least 4.5 at. pct, which is close to that observed using rapid solidification (RS) (4.4 at. pct), compared with the equilibrium value of 0.025 at. pct Fe at room temperature. Nanometer-sized grains were observed in as-milled crystalline powders in all compositions. Increasing the ball-to-powder weight ratio (BPR) resulted in a faster rate of decrease of crystal size. A fully amorphous phase was obtained in the Al-25 at. pct Fe composition, and a mixed amorphous phase plus solid solution of Fe in Al was developed in the Al-10.7 at. pct Fe alloy, agreeing well with the predictions made using the semiempirical Miedema model. Heat treatment of the mechanically alloyed powders containing the supersaturated solid solution or the amorphous phase resulted in the formation of the Al₃Fe intermetallic in all but the Al-25 at. pct Fe powders. In the Al-25 at. pct Fe powder, formation of nanocrystalline Al₅Fe₂ was observed directly by milling. Electron microscope studies of the shock-consolidated mechanically alloyed Al-10.7 and 25 at. pct Fe powders indicated that nanometer-sized grains were retained after compaction.     

Near-nanostructured WC-18 pct Co coatings with low amounts of non-WC carbide phase: Part I. Synthesis and characterization

Near-nanostructured WC-18 pct Co coatings, with low amounts of non-WC carbide phases, have been synthesized using high velocity oxygen fuel (HVOF) thermal spraying under spraying conditions of varying fuel chemistry, fuel-oxygen ratio, and powder particle size. The results show that the temperature the particles experience during spraying depends on the preceding parameters. Compared to available published results on WC-Co system coatings, nanostructured WC-18 pct Co coatings, synthesized in these experiments, contain very low amounts of non-WC carbide phase (less than 10 pct vol). This is comparable to that of the conventional WC-12 pct Co coating, prepared in the present study for comparison purposes. Regardless of whether the binder phase in the agglomerated feedstock powder particles melt or not, the WC particles do not appear to experience significant growth as a result of the spraying. The size of WC particles remains in the 200 to 500 nm range, consistent with that present in the feedstock powder. The as-received near-nanostructured WC-18 pct Co feedstock powder exhibits morphological characteristics that lead to low amounts of non-WC carbide phases in the coatings. The microstructure and phase constitution of the coatings depend on particle size of the feedstock powder and flame characteristics of the fuels during spraying. A higher particle temperature causes more decomposition of the WC phase but reduces porosity in the coatings, this occurs with higher flame temperature and smaller particle sizes. Propylene fuel produces less decomposition of the WC phase despite the higher flame temperature and, thus, provides the best combination of dense coating with low amount of non-WC phase.     

Formation and Characterization of Plasma Sprayed Photocatalytic TiO2- ZnO Nano- Compounded Coatings

Two kinds of TiO2-ZnO nano-compounded powders aggregated by spray-drying process and evaporation method were used to deposit photocatalytic coatings by atmospheric plasma spraying technique. The phase compositions, morphologies of the agglomerated TiO2-ZnO powders and the as-sprayed coating were characterized using X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) respectively. Furthermore, roughness measurements were carried out on their surfaces of the plasma sprayed TiO2-ZnO nano-comPounded coatings. Compared with the TiO2ZnO nano-compounded coating deposited from the spray-dried powder, it was found that the TiO2-ZnO nano-compounded coating deposited from the evaporated powder possesses higher anatase phase. It is ascribed to the existing of partially melted or non-melted microstructure in the TiO2-ZnO coating deposited from the evaporated powder. The partially-meltedor non-melted microstructure was retained from the starting agglomerated powder. This microstructure is beneficial to improve the photocatalytic properties of plasma sprayed TiO2-ZnO nano-compounded coatings.     

Microstructural changes in a mechanically alloyed Al-6.2Zn-2.5Mg-1.7Cu alloy (7010) with and without particulate SiC reinforcement

Elemental powders of Al, Zn, Mg, and Cu (corresponding to the composition of 7010 aluminium alloy) were milled in a high-energy attritor with and without additions of SiC particulates. The microstructural changes taking place in the milled powders (which eventually lead to mechanical alloying) are found to be retarded by SiC additions. High-resolution techniques such as electron probe microanalysis (EPMA) and transmission electron microscopy/energy-dispersive X-ray analysis (TEM/EDX) revealed the presence of localized solute-rich regions long after the diffraction line from these solutes had ceased to appear in the X-ray diffractograms. Zinc appears to be more difficult to be mechanically alloyed into aluminum than either Cu or Mg in spite of its comparatively larger diffusivity in aluminum.     

柱状晶对等离子YSZ涂层性能的影响

本文选用微米级YSZ粉末和纳米级YSZ粉末喂料,采用等离子喷涂技术制备了两种YSZ涂层,并通过扫描电镜(SEM)对涂层的微观组织和结构进行了研究。结果表明:微米级YSZ粉末制备涂层内部柱状晶的尺寸较短,长度在1～3μm之间;纳米级YSZ粉末制备涂层内部柱状晶的尺寸较长,长度在8～12μm之间;这种组织结构特征的差异是导致YSZ涂层结合强度和热震性能不同的主要原因。     

Optimisation of mechanical milling process for production of AA 7075/(SiC or TiB2) composite powders

Abstract

The present work concerns the processing of composite powders based on 7075 aluminium alloy by mechanical milling. A premixed powder (Alumix 431D, Ecka Granules, Germany) was used as the matrix material, and two different ceramic reinforcements (SiC and TiB₂) were chosen as reinforcements. The main objective was to evaluate the effect of the content and addition method of the process control agent as well as the content and type of reinforcement on the microstructural and morphological evolutions of the powder particles during milling process and the as milled properties of the processed materials. Results showed that regardless of the starting composition, alloying took place through three stages, in which deformation, cold welding and fracturing of powder particles were the main mechanisms involved respectively. The mechanically milled composite powders showed a fine and homogenous distribution of reinforcement particles. A higher content of reinforcement resulted in a lower crystalline size for the milled powders (～18 nm for composite powders containing 15 vol.-% ceramic particles).     

Effect of separate and combined milling of Cu and TiB2 powders on the electrical and mechanical properties of Cu–TiB2 composites

The present work compares the properties of the Cu–TiB₂ composites prepared by varying the mechanical milling conditions. The Cu–TiB₂ composites were processed using Cu–TiB₂ powders combined milling, a powder mixture consisting of separately milled Cu & TiB₂ and a powder mixture prepared by the combination of separate and combined milling. The hardness and flexural strength of the combined milled powders were found to be maximum, despite of their lower sintered density. The separately milled powders achieved excellent electrical properties combined with moderate hardness and flexural strength. The properties of composites processed using the combination of separate and combined milling laid in between the two conditions of combined and separate milling.     

Correlation of microstructure and wear resistance of Al2O3-TiO2 coatings plasma sprayed with nanopowders

Correlation of microstructure and wear resistance of Al₂O₃-TiO₂ coatings plasma sprayed with nanopowders was investigated in this study. Four kinds of nanostructured Al₂O₃-13 wt pct TiO₂ coatings were fabricated by varying plasma-spraying parameters and were compared with an Al₂O₃-13 wt pct TiO₂ coating fabricated with conventional powders. The nanostructured coatings showed a bimodal microstructure composed of fully melted regions of γ-Al₂O₃ and partially melted regions, while the conventional coating mostly consisted of fully melted γ-Al₂O₃, together with some TiO₂-rich regions and unmelted Al₂O₃ powders. The wear test results revealed that the wear resistance of the nanostructured coatings was 3 or 4 times better than that of the conventional coating, because the preferential delamination seriously occurred along TiO₂-rich regions in the conventional coating. In the nanostructured coatings, TiO₂ was homogeneously dispersed inside splats and around, thereby leading to higher splat bonding strength and to better wear resistance over the conventional coating.     

Strain Energy During Mechanical Milling: Part II. Experimental

The strain energy stored in mechanically milled 5083?Al powders was investigated using two experimental approaches: thermal and microstructural analysis. The experimental results show that mechanically milled 5083?Al powders store strain energy on the order of a few tens of joules per gram. These experimental results are consistent with the calculated strain energy stored in mechanically milled powders. The experimentally measured strain energy stored in powders increases with an increase in attritor diameter, impeller??s rotational frequency, and ball-to-powder mass ratio; however, it decreases with an increase in ball diameter. These trends were in good agreement with the calculated strain energy stored in powders as a function of the corresponding processing parameters.     

内送粉等离子喷涂枪的研究与应用现状

内送粉等离子喷涂枪将难熔粉末送入喷枪内部的高温区,增强了粉末的熔化效果,可制备出高质量的涂层,同时获得较高的粉末沉积效率。本文总结了等离子喷涂枪内、外送粉方式的特点,综述了国内外典型内送粉等离子喷涂枪的研究与应用现状。