Surface Modification of Si3N4 Powders by Coprecipitation of Sintering Aids

Two commercial Si₃N₄ powders were coated with sintering aids by coprecipitation. Lanthanum and yttrium nitrates were used as sintering aid precursors. Electrokinetic sonic amplitude measurements and X-ray photoemission spectroscopy analysis were used to investigate electrokinetic behavior and surface properties, respectively. Coprecipitation produced different effects on the composition of the coating layer depending on the actual features of the starting Si₃N₄ powders. The electrokinetic behavior of aqueous suspensions with coated powders depended strongly on the additives, their solubility, and the rate of oxidation of the coated layer. The coprecipitation conditions had to be carefully controlled to obtain reproducible composition and morphology of the coating layers. Treatments of the starting powder, pH, and washing volumes were optimized to tailor the coating layer and improve the coprecipitation yield.     

Combustion Prevention of Iron Powders by a Novel Coating Method

The effectiveness of a novel coating method in providing fine iron powder particles with a protective barrier against rapid oxidation was systematically studied. Particles were individually coated with an alumina‐based (Al₂O₃) ultra‐thin film using the Atomic Layer Deposition method. The oxidation resistance of the coating layer was found to be greatly dependent on the film thickness. Furthermore, for each film thickness there was a corresponding temperature above which the film drastically lost its protective effectiveness, primarily due to cracking caused by the thermal expansion mismatch between the particle and the alumina film. This problem was largely overcome when, instead of Al₂O₃, the protective film was generated from alternating layers of Al₂O₃/ZnS to ensure that the thermal expansion properties of the resultant film matched that of iron. The technique employed in this study is quite robust and can be adapted for combustion prevention in other types of metal powders.     

Investigation on corrosion and wear resistance of MgO-Al2O3 composite coating prepared by plasma electrolytic oxidation

Ceramic coatings were prepared on 6061 Al alloy in a mixed electrolyte with/without MgO powders at different treatment durations. The results of energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) showed that MgO powder was incorporated into the coatings, and Mg species gradually aggregated into coating inside as prolonging the oxidation time. Scanning electron microscopy (SEM) showed that MgO additive had a certain effect on the microstructures and coating thickness. The corrosion behavior tests evaluated by potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) in 3.5 wt% NaCl solution suggested that at the same treatment time, the addition of MgO powders can improve the corrosion resistance of the coating, and the Mg-rich layer can affect the corrosion resistance of the coating. The tests of mechanical properties showed that the addition of MgO powders improved the stability and hardness of the coating.     

Selection of Suitable Dispersants for Aqueous Suspensions of Zirconia and Titania Powders using Acoustophoresis

Acoustophoresis was used to study the effect of adding various commercially available dispersants onto aqueous suspensions of two zirconia and two titania powders. These powders were characterised for elemental composition by X-ray fluorescence (XRF) spectroscopy and for surface area by BET single point nitrogen adsorption. From the maximum value of the zeta potential, it was possible to select the most promising dispersants. From the shape of the curve the minimum amount of dispersant required to stabilise the powder particles was noted. The iso electric point (i.e.p) of the powders was also identified. Several dispersants can be recommended for the first titania powder, whilst none can be recommended for the second titania as the final zeta potentials on addition of the dispersants were low. The two powders had different surface chemistries which was reflected in a large difference in their i.e.p; the first at pH 7·5 and the second at pH 6·1. This was due to different coatings on the powder surfaces; alumina and an organic material respectively. Removal of this organic coating by calcinatian then enabled the dispersants to fully adsorb. Similarly dispersants for the first zirconia powder could be identified and the i.e.p identified at pH 5·4. However, no dispersants can be recommended for the second zirconia powder as yttria dissolves out of the powder under the naturally occurring weakly acidic conditions. The i.e.p was estimated to be pH _i.e.p 7–7·5.     

氟化物包覆对镁铝合金与水催化反应效率的影响

为了提高镁铝合金与水的反应效率,采用氟化物对镁铝合金粉进行表面包覆,利用扫描电镜、X射线衍射仪和粒度分析仪对合金粉与高温水反应产物进行表征,对比研究了高温下不同比例的氟化物对镁铝合金与水催化反应效率的影响。结果表明,包覆氟化物的镁铝合金与高温水反应产物的粒径减小,分散性明显改善;固相燃烧产物中主要包含Al_2MgO_4、MgO和Al,表明Al未完全反应;合金粉包覆氟化物后铝的反应效率明显提高,其中,包覆质量分数2%氟橡胶和2%有机氟化物的合金粉反应效率高达89.7%,与未包覆样品相比提高了14.6%。     

低水泥浇注料抗爆裂性能的改善

研究了在高铝质低水泥浇注料中加入一定量的有机纤维和金属铝粉对其抗爆裂性的作用。结果表明，加入少量的有机纤维和金属铝粉能有效地改善低水泥浇注料的抗爆裂性而不显著降低其物理性能。同时还探讨了添加金属铝粉、有机发泡剂偶氮酰胺、乳酸铝及有机纤维的作用机理。     

Fibrous Monolithic Ceramics: I, Fabrication, Microstructure, and Indentation Behavior

Monolithic ceramics have been fabricated from coated green fibers to create fibrous microstructures. The fibrous monoliths consist of high aspect ratio polycrystalline regions (cells) of a primary phase regions (cell boundaries) designed to improve fracture resistance. The cells are the remnants of the green fiber which consists of ceramic powder and a polymer binder. The coating applied on the green fiber forms the cell boundaries. Fabrication and microstructure are described for fibrous monoliths in the SiC/graphite, silicon nitride/BN, alumina/alumina–zirconia, alumina/aluminum titanate, alumina/nickel and Ce-TZP/alumina–Ce–zirconia systems. The SiC/graphite fibrous monolith displays noncatastrophic failure in flexure, with shear delamination along the weak graphite layers. Indentations in SiC/graphite cause cells to spall, with crack arrest and extrusion of graphite from the cell boundaries. Crack deflection and spalling of cells are also observed in alumina/alumina–zirconia fibrous monoliths. In the Ce-TZP/alumina system, transformed regions around indentations are significantly modified by the alumina-containing cell boundaries.     

Sintering and Phase Evolution of Electroless-Nickel-Coated Alumina Powder

Alumina-nickel powders have been prepared via electroless-nickel (EN) coating of submicrometer-sized alumina powder. The EN layers contain 5.1 ± 0.4 wt% phosphorus and are very unevenly distributed on the surfaces of the alumina particles. These layers consist of amorphous and microcrystalline phases. At temperatures greaterthan equal to1300°C, the EN layers de-wet from the alumina surfaces to become discrete, round particles in the alumina matrix. The alumina-EN pellets can be sintered to reach ~95% of the theoretical density at 1500°C for 4 h in graphite crucibles. The phases of the sintered samples consist of alumina, nickel, and nickel phosphides. However, in the exterior region of the sample sintered at 1500°C for 4 h, Ni₃Al will also form.     

High strength silica-based ceramics material for investment casting applications: Effects of adding nanosized alumina coatings

A nanosized alumina coating was synthesized on the surface of fused silica particles by electrostatic attraction. The effects of the coated fused silica particles on the cristobalite crystallization behavior, microstructure evolution, and flexural strength of silica-based ceramic cores were investigated. X-ray diffraction (XRD) was used to characterize phase transformations in the specimens, and the results indicated that the formed nanosized alumina coatings could retard cristobalite formation by inducing compressive stress on the fused silica particle surface. A mullite phase was also found due to the reaction of the nanosized alumina coating and the surface of the fused silica when the sintering temperature was increased to 1300 °C. Analysis using scanning electron microscopy equipped with energy dispersive spectrometry (SEM/EDS) suggested that alumina nanoparticles in the coated layer dispersed into a liquid phase and formed a barrier layer to impede the movement of the liquid phase, preventing the pore-filling process and increasing the open porosity of the ceramic specimens. Flexural strengths at room temperature were tested, indicating that increases in the sintering temperature of the specimens without coated fused silica powders had little effect on flexural strength. However, the flexural strength of the specimens with coated fused silica powders increased with increases in sintering temperature. The improvement in flexural strength was related to the reinforcement by sintering necks between particles and the improvement in the strength of the coated fused silica powder.     

Effect on wear resistance of nanoparticles addition to a powder polyester coating through ball milling

The wear properties of a textured polyester powder coating with pyrogenic silica nanoparticles addition were evaluated. Raw powders of a commercial, textured polyester organic coating were mixed with low amounts of SiO₂ nanoparticles (0.5–3 wt%) using ball milling, a simple and economical method. Nanoparticles were mixed into the powder of thermoset organic coating for 10 min in a two-body planetary ball mill. Particle size distribution of the powder was measured to evaluate the milling effect. The coatings were applied and cured in an industrial installation on aluminum substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of the coatings were taken to analyze the homogeneity of the organic coating. Roughness, gloss and color were measured in order to evaluate their appearance. The effect of nanoparticles on abrasive and erosion wear performances was measured. Pin-on-disk wear tests were carried out. Erosion measurements were performed with free fall of sand on the samples, a test based on ASTM D968 standard. The results showed that the milling process provides a good distribution of nanoparticles as no agglomerates were found. The addition of 0.5 wt% silica nanoparticles allows for improvement of the wear resistance of the coatings.     

Synthesis, Characterization, and Processing of Cordierite-Glass Particles Modified by Coating with an Alumina Precursor

The surfaces of cordierite and glass particles were modified by coating them with an alumina precursor using a precipitation process in the presence of urea. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy, X-ray diffraction, electrophoresis, and rheological measurements were used to characterize the coated powders. SEM and transmission electron microscopy morphologies of the coated powders revealed that amorphous and homogeneous coatings have been formed around the particles. The morphology of the coated powders showed a coiled wormlike surface. The coating Al₂O₃ layer dominated the surface properties of the coated glass and cordierite powders. The influence of the coating layer on the processing ability of cordierite-based glass-ceramics substrates by tape casting was studied in aqueous media. It could be concluded that the coating of the powders facilitates the processing and yields green and sintered tapes with denser, more homogeneous microstructures compared with the uncoated powders.     

无机包覆钛白粉研究进展及发展趋势

钛白粉表面无机包覆是钛白粉后处理过程不可或缺的工序之一。剖析钛白粉无机包覆过程机制、构效关系及应用性能提升机制,对设计和制备具有优异应用性能的钛白粉包覆产品具有重要的理论意义和应用价值。基于钛白粉表面单元无机包覆（氧化铝、二氧化硅、二氧化锆、磷酸铝等）,综述了钛白粉表面无机包覆层微观结构调控、包覆层微观结构与应用性能之间的构效关系、应用性能提升机制的研究现状。展望了钛白粉表面无机包覆理论基础研究的发展趋势。     

Investigation of the structural and mechanical properties of micro-/nano-sized Al2O3 and cBN composites prepared by spark plasma sintering

Alumina-cubic boron nitride (cBN) composites were prepared using the spark plasma sintering (SPS) technique. Alpha-alumina powders with particle sizes of ∼15 µm and ∼150 nm were used as the matrix while cBN particles with and without nickel coating were used as reinforcement agents. The amount of both coated and uncoated cBN reinforcements for each type of matrix was varied between 10 to 30 wt%. The powder materials were sintered at a temperature of 1400 °C under a constant uniaxial pressure of 50 MPa. We studied the effect of the size of the starting alumina powder particles, as well as the effect of the nickel coating, on the phase transformation from cBN to hBN (hexagonal boron nitride) and on the thermo-mechanical properties of the composites. In contrast to micro-sized alumina, utilization of nano-sized alumina as the starting powder was observed to have played a pivotal role in preventing the cBN-to-hBN transformation. The composites prepared using nano-sized alumina reinforced with nickel-coated 30 wt% cBN showed the highest relative density of 99% along with the highest Vickers hardness (H_v2) value of 29 GPa. Because the compositions made with micro-sized alumina underwent the phase transformation from cBN to hBN, their relative densification as well as hardness values were relatively low (20.9–22.8 GPa). However, the nickel coating on the cBN reinforcement particles hindered the cBN-to-hBN transformation in the micro-sized alumina matrix, resulting in improved hardness values of up to 24.64 GPa.     

片状玻璃对水性复合锌铝合金涂层耐蚀性的影响

为了改善水性锌铝合金涂层的耐蚀性能,在硅烷钝化液中加入玻璃粉制备了玻璃/水性锌铝涂层,采用中性盐雾试验、电化学阻抗谱、极化曲线等方法研究了玻璃粉用量对 Q235 钢表面该涂层耐蚀性能的影响,利用扫描电镜、能谱、X 射线衍射等技术分析了涂层的形貌、组成以及元素分布状况。结果表明,添加片状玻璃粉可以提高涂层的耐蚀性能。玻璃粉与金属粉以平行叠加的方式排列在锌铝合金涂层中,部分玻璃粉包覆在涂层的外围形成致密的网状结构。当玻璃粉的添加量为 20 g/L时,涂层的耐蚀性能最好,耐中性盐雾试验时间为 3 100 h,是不添加玻璃粉涂层的 5 倍。     

Corrosion‐resistant coating development with potential application in equipment of low‐temperature waste heat recovery

In order to improve corrosion resistance of a condensing heat exchanger, a fluoropolymer, perfluoroalkoxy alkane (PFA), fine powder in 30 wt% aqueous slurry was coated on metal coupons and a coating of high quality finish was obtained. The coating was characterized by microscopy and thickness measurements. The thickness of the coating can be controlled by the number of layers used for the application of wet spray. Hot acid bath corrosion tests showed that the coated coupon possesses the highest adhesion strength and excellent corrosion resistance (1.8–6.7 × 10^?3 mm/year), comparable to super alloys such as Inconel (1.1–26.2 × 10^?3 mm/year) and Hastelloy (0.3–19.8 × 10^?3 mm/year). A heat transfer coefficient analysis showed that across a heat transfer metal tube coated with PFA, the heat transfer resistance of the gas side film is one order of magnitude larger than the resistance from the coated layer. The developed coating could provide an alternative material solution for condensing heat exchangers used in low‐grade waste heat recovery.     

The effect of nano-structured alumina coating on the bond strength of resin-modified glass ionomer cements to zirconia ceramics

The purpose of this study was to evaluate the effect of Y-TZP ceramic surface functionalization with a nano-structured alumina coating on bond strength of the resin modified glass ionomer dental cement. A total of 160 disc-shaped specimens were produced and randomly divided into two groups of 80. Half of the discs in each group received an alumina coating which was fabricated by exploiting the hydrolysis of aluminum nitride (AlN) powder. The shear bond strengths of the resin-modified glass ionomer cement FUJI+ (GC Japan) and the composite resin luting agent RelyX Unicem (3M ESPE, USA) were then studied for the coated and uncoated surfaces The SEM analyses revealed that the application of an alumina coating to the Y-TZP ceramics created a highly retentive surface for bonding. The bond strengths for the coated groups in both cements were significantly higher than the uncoated groups.     

包覆型Al_2O_3粉体制备低温烧成多孔陶瓷膜支撑体

采用22μmα-Al2O3为骨料,0.5μmα-Al2O3为烧结助剂,一是采用简单机械混合得到上述2种氧化铝的混合粉体,二是通过粉体表面修饰的方法,将0.5μmα-Al2O3包覆在22μmα-Al2O3表面得到包覆型氧化铝粉体。采用上述2种原料,通过干压成型法制备出片状多孔支撑体,考察了不同烧成温度下2种粉体路线制备出的多孔支撑体的弯曲强度、平均孔径、孔隙率和纯水通量。结果表明:在获得相同支撑体性能的前提下,以包覆型氧化铝粉体为原料制备出的支撑体的烧成温度大大低于采用简单机械混合后粉体制备支撑体的。在1550℃的烧成温度下,包覆型氧化铝粉体制备的支撑体的机械强度为34.2MPa,孔隙率为34%,平均孔径为2.34μm,纯水通量为205m3/(m2·h·MPa)。     

大比表面积高牢固度堇青石蜂窝涂层的制备

以拟薄水铝石粉为原料,分别采用铝溶胶法、活性Al_2O_3浆料法和混合浆液法,对堇青石蜂窝基体进行涂层处理。通过黏度、SEM、BET和超声振荡等分析测试手段,着重考察了涂层浆液固含量和黏度对蜂窝涂层量、比表面积和牢固度的影响,比较了采用不同方法进行涂层的效果。结果表明,铝溶胶的涂层具有牢固度高的特点,缺点是比表面积小;Al_2O_3浆料的涂层比表面积较大,但牢固度较差。而混合浆液法利用铝溶胶作为Al_2O_3浆料的胶联剂,结合上述两种涂层方法的优势,涂层后的蜂窝既具备大的比表面积(达59m~2·g~(-1)),又保持了较高的涂层牢固度,而且涂层工艺简单可控,有利于工业化应用。     

Structure and mechanical properties of plasma sprayed coatings of titania and alumina

The paper concerns the use of traditional and depth-sensing indentation (DSI) for investigation of deposits produced from powders based on conventional and nano-sized particles by plasma spray technology.

Plasma sprayed coatings of titania and alumina were studied. Polished cross-section of each coating was prepared and matrices of nano-indents with Berkovich tip were applied onto both materials to explore local elastic behavior. Applied load was in the range of mN to create indents with the same size scale as the thickness of splats—the main building units of the coating. The hardness value as well as the load/unload curve for each indent was stored. Titania coating was sprayed from a novel type of nanoscale-size powder agglomerated to particles useful for plasma spraying, whereas fused and crushed conventional powder was utilized for alumina spraying and for titania coating used as reference. The effect of annealing on elastic properties of titania was studied as well. The values of elastic parameters as well as the character of the coating inhomogeneity seem to reflect: (i) the composition of material and the fabrication technique and (ii) microstructural differences between coatings that are partly inherited from the feedstock powders. The results of DSI tests are discussed also in comparison with common technique used for the investigation of plasma coatings hardness—Vickers microhardness measurement.     

Numerical simulation of in-flight particle oxidation during thermal spraying

Metal powders are widely used for thermal spray coating to improve wear, corrosion and temperature resistance of products. The high thermal profiles endured for sprayed particles give rise to oxidation on the surface of metal powders. Metallic oxides are brittle and undermine the performance of coated products. To understand the growth of oxide layers on in-flight metal powders, an oxidation model is implemented into the Lagrangian formula of particle tracking. The numerical simulation is achieved for a 3D combusting gas flow generated by a high velocity oxygen fuel (HVOF) thermal spray gun. The results are able to demonstrate the correlation between in-flight particle oxidation and operation parameters.