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).     

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.     

前驱体碳化粉末反应火焰喷涂制备TiC增强金属复合涂层

采用前驱体碳化复合技术制备了Ti-Fe-C和Ti-Ni-C两种体系的反应热喷涂复合粉末,通过氧乙炔火焰喷涂原位合成并沉积了TiC增强Fe基和Ni基复合涂层.利用XRD、SEM和EDS研究了复合粉末、涂层的相组成和组织结构,考察了TiC/Fe、TiC/Ni复合涂层的硬度和耐磨性.结果表明: 复合粉末在喷涂过程中反应充分,可分别生成以Fe和Ni为粘结相的TiC增强涂层;两种涂层都是由TiC颗粒均匀分布的复合强化片层和TiC聚集片层叠加而成,TiC/Fe复合涂层的片层较薄,而TiC/Ni涂层中TiC的聚集片层较少;TiC/Fe涂层的硬度高于TiC/Ni涂层,两者的耐磨性能分别约为Ni60涂层的11倍和6倍.     

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.     

Ni–ZrO2 Composite Coating by Electro-Co-Deposition

In the present investigation Ni–ZrO₂ metal matrix composite coatings were prepared on steel substrate using watt’s type solution through electro-co-deposition process with different weight percentages of zirconia powder dispersed in the bath. In the coating, nickel is present with faceted appearance along with ZrO₂. The microhardness and wear resistance of the coatings increase with increasing weight percentage of particles content in the coating. The hardness of the resultant coatings was found to be 325 VHN for pure Ni coating whereas 401VHN for Ni–ZrO₂ (15 g/l ZrO₂) coating depending on the particle volume in the Ni matrix. The results also showed that the wear resistance of the composite coatings was improved as compared to unreinforced Ni deposited material. Strengthening of the coating was attributed to the ZrO₂ dispersion and partially favorable texture.     

Synthesis and Properties of Pulse Electrodeposited Ni-CeO2 Nanocomposite

Ni-CeO₂ nanocomposite coatings were pulse electrodeposited from the Watt??s electrolyte containing different concentrations of nanosized CeO₂ particles (10, 20, 30, 40, and 50?g/L). The microhardness, thermal stability, and corrosion resistance of these coatings were evaluated and compared with those of pure nanocrystalline Ni deposited under the same conditions. The results show that the Ni-CeO₂ nanocomposite coating, synthesized from the electrolyte containing 30?g/L CeO₂, has significantly higher hardness, thermal stability, and corrosion resistance than those of the pure nanocrystalline Ni and other Ni-CeO₂ nanocomposite coatings.     

Comparison in the Oxidation and Corrosion Behavior of Aluminum and Alumina-Reinforced Ni/Ni-Co Alloy Coatings

In this study, a comparison in the oxidation and corrosion behavior of Ni/Ni-Co aluminum and alumina-reinforced electrodeposited composites has been made. The developed coatings were characterized for the morphology, structure, microhardness, oxidation, and corrosion resistance. It was found that the incorporation of Al particles in NiCo matrix is higher (9 wt pct) compared to Ni matrix (1 wt pct). In the case of aluminum oxide particles, about 5 and 7 wt pct had been obtained in Ni and NiCo matrices respectively. The difference in the surface morphology was observed with respect to metallic (Al) and inert ceramic (Al₂O₃) particle incorporation. X-ray diffraction studies showed the presence of predominant Ni (200) reflection in the coatings. Also, peaks corresponding to Al and Al₂O₃ particles were present. The Ni/NiCo-Al coatings exhibited higher microhardness values at 1273 K (1000 °C) compared to alumina-reinforced coatings, indicating better thermal stability of the former coatings. The NiAl coating showed one and two orders of magnitude improved oxidation resistance compared to NiCoAl and Ni/NiCo-Al₂O₃ coatings, respectively. It was observed that the Ni-Al composite coating exhibited poor corrosion resistance in 3.5 pct NaCl solution compared to the other coatings studied.     

超音速火焰喷涂技术制备的双峰WC–CoCr涂层磨粒磨损特性

采用超音速火焰喷涂(high velocity oxy-fuel,HVOF)工艺分别制备了双峰结构和常规结构的WC–CoCr复合涂层。比较了不同结构WC–CoCr涂层的组织结构、显微硬度和断裂韧性;在涂层磨粒磨损实验的基础上,探讨了双峰结构WC–CoCr涂层的磨损机理。结果表明:与常规结构的WC–CoCr复合涂层相比,在由含质量分数30%超细WC粉末制备的双峰结构涂层中,WC在黏结相中溶解最多,断裂韧性最低;由含质量分数50%超细WC粉末制备的双峰结构涂层最致密,显微硬度与断裂韧性最高,耐磨粒磨损性能最优良。     

超细及纳米WC-Co复合粉的低成本短流程制备及应用

综述了在超细及纳米WC-Co粉末的制备研究方面国内外的相关研究进展,系统介绍了北京工业大学硬质合金研究组在研究开发超细及纳米WC-Co复合粉的原位反应合成技术、超细及纳米复合粉在金属陶瓷防护涂层及烧结硬质合金块体材料中的应用等方面开展的系列工作和取得的研究结果。以合成的WC-Co复合粉为原料,制备的超细结构硬质合金涂层、超细晶及纳米晶硬质合金块体材料均具有优良的综合性能。     

Grain growth behavior of cryomilled INCONEL 625 powder during isothermal heat treatment

Nanocrystalline INCONEL 625 powders were fabricated via cryomilling (mechanical alloying under a liquid nitrogen environment), and their grain growth behavior during isothermal heat treatment was investigated in detail. The grain size after milling for 8 hours was approximately 22 nm, measured by transmission electron microscopy (TEM) observations and X-ray diffraction (XRD). Along with this refined structure, the NiO and Cr₂O₃ oxide particles were distributed in the cryomilled material with average size of 3 nm. Following heat treatment at 800 °C, correspond to T/T _m = 0.65, for 4 hours, the grain size was approximately 240 nm, which represents an improved grain stability compared to that of conventional INCONEL 625 and cryomilled pure Ni. The improved grain stability of cryomilled INCONEL 625 is originated from a particle pinning effect by the oxide particles in addition to solute drag. The grain stability of the cryomilled powders at 900 °C was better than that at lower temperatures. This behavior was attributed to the formation of two types of secondary particles that precipitated at this temperature, which were identified as spherical NbC carbides and cylindrical-shaped Ni₃Nb intermetallic precipitates. These precipitates promote grain growth resistance at this particular temperature via a grain-boundary pinning effect. Contribution of 30 pct Nb solute atoms in alloy on the forming precipitates on grain boundary, the grain growth will be restricted to approximately 200 nm, on the basis of a Zener mechanism. This calculation is in qualitative agreement with the experimental results. The observation that precipitation kinetics were accelerated over those of conventional INCONEL 625 was rationalized on the basis of the shortened diffusion paths and more nucleation sites available in the nanocrystalline materials.     

低温球磨法制备块体纳米晶Al-Zn-Mg-Cu系铝合金

采用低温球磨法制备了纳米晶Al—Zn—Mg—Cu系铝合金粉末，对纳米晶粉末进行真空热压，获得纳米晶铝合金块体。采用显微分析方法研究了纳米晶粉末和块体材料的微观组织结构：试验结果表明，纳米晶铝合金粉末具有良好的热稳定性，热压块体的平均晶粒尺寸在100nm以下。低温球磨所得铝合金粉末晶粒尺寸为48nm；真空退火试验表明其具有良好的热稳定性；热压块体的平均晶粒尺寸在100nm以下。     

Development of CeO2 nanorods reinforced electrodeposited nickel nanocomposite coating and its tribological and corrosion resistance properties

A simple electrodeposition technique was used to prepare Ni-CeO_2 nanorods composite coating(Ni-CeO_2 NRs) using Watt's nickel plating bath containing CeO_2 nanorods(NRs) as the reinforcement phase under optimized process conditions. The X-ray diffraction analysis(XRD) was used for the structural analysis of Ni-CeO_2 NRs composite coatings and their average crystalline size is ～22 nm for pure Ni and ～18 nm,respectively. The crystalline structure is fcc for the Ni-CeO_2 nanocomposite coatings. The surface morphology of the electrodeposited Ni-CeO_2 NRs composite coatings was analyzed by scanning electron microscopy(SEM). Microhardness of pure Ni and Ni-CeO_2 NRs composite coatings are found to be 253 HV and 824 HV, respectively. The inclusion of CeO_2 NRs increases the microhardness of Ni-CeO_2 NRs composite coatings. The corrosion resistance behavior of Ni-CeO_2 NRs composite coating was evaluated by Tafel polarization and AC impedance methods. It is revealed that CeO_2 NRs reinforced Ni matrix shows higher microhardness and corrosion resistance than existing reported electrodeposited pure Ni and CeO_2 nanoparticles reinforced Ni coatings.     

Synthesis of nanocrystalline Zn-22 Pct Al using cryomilling

In the present investigation, the synthesis of nanocrystalline Zn-22 pct Al by ball milling was studied. The microstructural evolution during cryomilling and subsequent annealing was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Observations made during the cryomilling of the alloy reveal three findings. First, minimum average grain sizes of about 33 nm for the Al phase and 41 nm for the Zn phase are reached as cryomilling time increases to 16 hours. Second, the morphology of the powders changes from spherical (as-sprayed) to flaky (milled 8 hours) and then back to spherical again (milled 16 hours). Third, the microstructure transforms from two-phase coarse structure (0.8 μm, as-sprayed) to bimodal structure (milled 8 hours) and then to a uniform fine-grained structure (milled 16 hours). The minimum grain size characterized by the peak broadening of the XRD patterns is much larger than that reported in previous work on Al and Zn but agrees well with those predicted from the approximate linear relationship between the minimum grain size and the critical equilibrium distance between two edge dislocations in a pileup. The mechanism of grain size refinement is discussed at three different levels: macroscopic level (individual powders), mesoscopic level (individual small fragments), and microscopic level (individual grains). The excellent thermal stability of the milled powders during subsequent annealing has been attributed to three factors: the nature of the eutectoid structure, grain-boundary pining by dispersions, and microporosity in the particles.     

Preparation and tribological performance of electrodeposited Ni-TiB2-Dy2O3 composite coatings

TiB2 and Dy2O3 were used as codeposited particles in the preparation of Ni-TiB2-Dy2O3 composite coatings to improve its per-formance. Ni-TiB2-Dy2O3 composite coatings were prepared by electrodeposition method with a nickel cetyltrimethylanunonium bromide and hexadecylpyridinium bromide solution containing TiB2 and Dy2O3 particles. The content of codeposited TiB2 and Dy2O3 in the compos-ite coatings was controlled by adding TiB2 and Dy2O3 particles of different concentrations into the solution, respectively. The effects of TiB2 and Dy2O3 content on microhardness, wear mass loss and friction coefficients of composite coatings were investigated. The composite coat-ings were characterized by X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES) and scanning electron microscopy (SEM) techniques. Ni-TiB2-Dy2O3 composite coatings showed higher microhardness, lower wear mass loss and friction coefficient compared with those of the pure Ni coating and Ni-TiB2 composite coatings. The wear mass loss of Ni-TiB2-Dy2O3 composite coatings was 9 and 1.57 times lower than that of the pure Ni coating and Ni-TiB2 composite coatings, respectively. The friction coefficient of pure Ni coating, Ni-TiB2 and Ni-TiB2-Dy2O3 composite coatings were 0.723, 0.815 and 0.619, respectively. Ni-TiB2-Dy2O3 composite coat-ings displayed the least friction coefficient among the three coatings. DY2O3 particles in composite coatings might serve as a solid lubricant between contact surfaces to decrease the friction coefficient and abate the wear of the composite coatings. The loading-bearing capacity and the wear-reducing effect of the Dy2O3 particles were closely related to the content of Dy2O3 particles in the composite coatings.     

Preparation of tungsten base composite powder by electroless nickel plating

Abstract

Fine tungsten powder with an average size of 8 μm was coated by electroless nickel plating with hydrazine and sodium hypophosphite reducing agents to obtain Ni and Ni–P coatings, respectively. The influence of process parameters such as temperature, pH and time of electroless plating was investigated. As coated composite powders were characterised by energy dispersive spectrometer analysis and scanning electron microscopy. It was found that, high homogeneity Ni/Ni–P coatings are deposited around the tungsten particles. Also it was shown that deposited mass on the powders increases as the temperature and pH of bath increase, but with different deposition rates depending on coating type. Furthermore, other results indicate that at higher pH values, the P content in the Ni–P coating decreases, leading less impurity in the final composite powders.     

Fabrication and Characterization of Ni-SiC Nanocomposite Coatings on Al Substrates by Ball Impact Deposition Method

Micron-size Ni and SiC powder mixtures were used to prepare Ni-SiC nanocomposite coatings on an Al substrate by employing a high-energy ball milling technique. Ni:SiC weight ratio was varied over a wide range to explore the effect of the charge composition on the microstructure, composition, microhardness, and wear properties of the depositions. It was observed that the composition of the produced coating was correlated to the charge composition in a complex manner, which suggests that deposition rates for Ni and SiC particles significantly vary depending on the charge composition; SiC deposition rate was higher than that of Ni when Ni:SiC weight ratio was greater than 3:1. Diffusion of Al from the substrate into the Ni matrix provided evidence for the metallurgical bonding at the interface. Both microstructural and mechanical properties of the produced coatings were found to be crucially dependent on the charge composition. By increasing the SiC content in the charge from about 5 to 33 wt pct, the mechanical properties enhanced due to the dispersion strengthening effect of the incorporated SiC particles in the coatings and the crystallite size of the Ni matrix decreasing to the nanometer range. However, a further increase resulted in the formation of a coating with a poor degree of compaction. It was found that the composite coating with about 15 vol pct SiC, produced from the charge with Ni:SiC weight ratio of 2:1, showed a microhardness as high as 830 HV_0.05 along with excellent wear resistance. Despite the current sample size limitations for applying high-energy ball milling, the present findings demonstrate that the adopted technique holds good prospect for the synthesis of nanostructured metal matrix composite coatings with enhanced and tunable properties.

     

Grain growth of nanocrystalline Ni powders prepared by cryomilling

Grain growth of nanocrystalline Ni powders with an average grain size of ∼22 nm prepared by cryogenic mechanical milling (or cryomilling) was investigated by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). A dispersion of NiO and Ni₃N particles with a size less than 5 nm was formed in the cryomilled powders. The Ni₃N particles decomposed at 773 K. It was found that at 0.56 homologous temperature (T/T _M ), Ni grains were retained at ∼150 nm even after long annealing times (e.g., 4 hours). For 0.45 to 0.62 T/T _M , the time exponent n deduced from D ^1/n −D ₀ ^1/n =kt was 0.16 to 0.32, tending toward 0.5 as T/T _M increased. The activation energy for grain growth in the Ni sample was determined to be 113 kJ/mol, which is close to the activation energy for grain boundary self-diffusion in polycrystalline Ni. The observed high grain size stability was attributed primarily to a grain boundary pinning mechanism arising from the NiO particles as well as impurity segregation.     

Preparation and performance of electrodeposited Ni-TiB₂-Sm₂O₃ composite coatings

Sm2O3 and TiB2 were used as codeposited particles in electrodeposition Ni-TiB2-Sm2O3 composite coatings to improve its performance. Ni-TiB2-Sm2O3 composite coatings were electrodeposited in the nickel sulfate,hexadecylpyridinium bromide and cetyltrimethylammonium bromide solution containing TiB2 and Sm2O3 particles. The content of codeposited Sm2O3 in the composite coating was controlled by changing the concentrations of Sm2O3 particles in the solution. The composite coatings were characterized with X-ray diffraction(XRD) and inductively coupled plasma-atomic emission spectrometer(ICP-AES) . The effects of Sm2O3 content on microhardness,wear weight loss and friction coefficient of composite coatings were investigated,respectively. The microhardness of the Ni-TiB2-Sm2O3 composite coatings was 19.35%,16.58%,2.03% higher than that of the Ni coating,Ni-Sm2O3 and Ni-TiB2 composite coatings,respectively. The wear weight loss of the Ni-TiB2-Sm2O3 composite coatings was 7,2.33,1.22 times lower than that of the Ni coating,Ni-Sm2O3 and Ni-TiB2 composite coatings,respectively. The friction coefficient of the Ni coating,Ni-Sm2O3,Ni-TiB2 and Ni-TiB2-Sm2O3 composite coatings were 0.712,0.649,0.850 and 0.788,respectively. The loading-bearing capacity and the wear-reducing effect of the Sm2O3 particles were closely related to the content of Sm2O3 particles in the composite coatings.     

用电弧等离子体制备超细AlN粉末的研究

介绍了利用直流电弧等离子体制备超细AlN粉末的方法。实验结果表明,在N_2-Ar气氛中制备的超细粉末是Al和AlN混合物,而在N_2-NH_3气氛中能制备出高纯度的超细AlN粉末。X射线衍射分析证明,在适当分压比的N_2-NH_3混合气体中所制出的超细AlN粉末不存在Al(即其氮化率达到100％),所得AlN粒子的最大几率直径为5.29nm,几何平均直径为21.0nm。本文最后对电弧等离子体的作用,AlN超细颗粒的生成机理及影响超细粉末中AlN含量的主要因素作了分析。     

Ti-Y2O3 Composites with Nanocrystalline and Microcrystalline Matrix

Mechanical milling of a Ti-2 pct Y₂O₃ powders mixture led to the synthesizing of a composite powder with a nanocrystalline Ti matrix having a mean crystallite size of 19 nm. Both the nanocomposite powder prepared through milling and the initial mixture of powders were consolidated by hot pressing under the pressure of 7.7 GPa at the temperature of 1273 K (1000 °C). The transmission electron microscopy (TEM) investigations of the bulk sample produced from milled powder revealed that Y₂O₃ equiaxial particles of less than 30 nm in size are distributed uniformly in the Ti matrix with a grain size in the wide range from 50 nm to 200 nm. The microhardness of the produced nanocrystalline material is 655 HV0.2, and it significantly exceeds the hardness of the microcrystalline material (the consolidated initial mixture of powders), which is equal to 273 HV0.2. This finding confirms that reducing the grain size to the nanometric level can have a beneficial influence on the hardness of titanium alloys. Dispersion hardening also contributes to the hardness increase.