Structural evolution of mechanically alloyed Al-12Si/TiB2/h-BN composite powder coating by atmospheric plasma spraying

In the present study, mechanically alloyed Al-12Si/TiB₂/h-BN composite powder was deposited onto an aluminum substrate by atmospheric plasma spraying. The effect of mechanical alloying (MA) and plasma spray parameters on composite powder and coating structure were investigated. It has been observed that the MA process has a significant effect on the composite powder morphology and reactivity between the selective powders. Results also demonstrate that, at relatively high milling time h-BN decomposes into B and N and forms a solid solution. Also, it has been found that, the relative amount of the in-situ formed AlN through the reaction between h-BN and Al and/or the decomposition of Al-B-N solid solution is independent from the plasma parameters (arc current and secondary gas flow rate). However, spray parameters remarkably affects the coating hardness due to coarsening of Si during the solidification of the coating.     

In-situ plasma spraying: Alumina formation and in-flight particle diagnostic

In-situ plasma spraying (IPS) is a promising process to fabricate composite coatings with in-situ formed thermodynamically stable phases. In the present study, mechanically alloyed Al-12Si and SiO₂ powder was deposited onto an aluminum substrate by atmospheric plasma spraying (APS) to obtain a composite coating consisting of in-situ formed alumina reinforced hypereutectic Al-18Si matrix alloy. The effects of spray parameters (arc current and spray distance) and in-flight particle characteristics (temperature and velocity) on in-situ reaction intensity (alumina and silicon) have been investigated. The results show that, in-situ alumina formation and silicon intensity strongly depend on in-flight particle characteristics, spray distance and substrate temperature.     

In-situ TiB2 and Al2O3 formation by DC plasma spraying

In-situ plasma spraying (IPS) is a promising process to fabricate composite coatings with in-situ formed thermodynamically stable phases. In the present study, mechanically alloyed Al-12Si, B₂O₃ and TiO₂ powder was deposited onto an aluminum substrate using atmospheric plasma spraying (APS). It has been observed that, during the coating process, TiB₂ and Al₂O₃ are in-situ formed through the reaction between starting powders and finely dispersed in hypereutectic Al-Si matrix alloy. Also, obtained results demonstrate that in-situ reaction intensity strongly depends on spray conditions.     

Microstructure and mechanical properties of Al2O3-Cr2O3 composite coatings produced by atmospheric plasma spraying

Al₂O₃, Al₂O₃-Cr₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying. Phase composition of powders and as-sprayed coatings was determined by X-ray diffraction. Electron probe microanalyzer was employed to investigate the polished and fractured surface morphologies of the coatings. Mechanical properties including microhardness, fracture toughness and bending strength were evaluated. The results indicate that the addition of Cr₂O₃ is conducive to the stabilization of α-Al₂O₃. Compared with the pure Al₂O₃ and Cr₂O₃ coatings, Al₂O₃-Cr₂O₃ composite coatings show lower porosities and denser structures. Heterogeneous nucleation of α-Al₂O₃ occurs over the isostructural Cr₂O₃ lamellae and partial solid solution of Al₂O₃ and Cr₂O₃ might be occurring as well. Furthermore, grain refining and solid solution strengthening facilitate the mechanical property enhancement of Al₂O₃-Cr₂O₃ composite coatings.     

Controlling graphite content in plasma sprayed cast iron coatings via in-flight particle diagnostic

Graphite formation and degradation in thermally sprayed cast iron coatings is a technological barrier for achieving superior wear resistant coatings. Therefore, there is a need to understand the in-flight particle behavior of cast iron powder and introduce new approaches to control the graphite content. In this study, it has been demonstrated that the graphite content can be controlled by means of in-flight particle diagnostic. For this purpose, cast iron coatings were plasma sprayed under a variety of spray conditions and characterized by using an optical microscope, X-ray diffractometer and electron probe micro-analyzer. As a result, a significant amount of graphite with respect to a wide range of in-flight particle temperature and velocity was preserved in cast iron coatings.     

纳米粉末对轴向等离子喷涂TiB2-Al2O3复合涂层的影响

采用三阴极轴向送粉等离子喷涂制备TiB2/Al2O3陶瓷复合涂层,其中一种喷涂粉末是自蔓延高温合成的常规微米级TiB2/Al2O3复合粉末,另一种是由TiB2/Al2O3复合粉掺杂质量分数10%的纳米Al2O3粉经喷雾干燥造粒制备的纳微米结构喂料.比较研究两种涂层的微观组织、耐磨性能,探讨纳米粉末对涂层的影响:扫描电镜、X射线衍射分析涂层微观结构和物相组成;转盘磨损试验测试涂层的耐磨性能.结果显示两种涂层表面洛氏硬度相当,纳米掺杂涂层组织致密性、耐磨性明显高于常规粉末涂层,以及TiB2的氧化产物TiO2含量低于常规粉末涂层.     

大气等离子喷涂TiB2-316L不锈钢基复合涂层及其耐磨性能

分别采用高能球磨制备了TiB2含量(质量分数)为10%的316L不锈钢基复合粉,高能球磨与喷雾干燥造粒工艺制备了TiB2含量(质量分数)为40%的316L不锈钢基复合粉,大气等离子喷涂制备相应的TiB2-316L不锈钢基金属陶瓷涂层与316L不锈钢涂层.室温下采用高速环块磨损试验研究TiB2-316L不锈钢基金属陶瓷涂层的磨损特性.采用X射线衍射分析涂层物相,扫描电镜分析喷涂粉末、涂层结构和摩擦副磨损表面形貌.结果表明,大气等离子喷涂两种制粉工艺获得的316L不锈钢基TiB2复合粉能获得较耐磨的316L不锈钢基TiB2复合涂层,耐磨性高于316L不锈钢涂层,且TiB2在复合涂层中增强涂层耐磨性的原因是TiB2颗粒在涂层316L韧性基体中充当强化相,且TiB2在摩擦接触处摩擦氧化形成的氧化产物具有自润滑特性,能减少涂层的磨损量.     

Mechanical properties of the plasma-sprayed Al2O3/ZrSiO4 coatings

The mechanical properties of plasma-sprayed Al₂O₃/ZrSiO₄ coatings were investigated by indentation-based techniques. Two types of feedstock were used to prepare the coatings: spray-dried powders and plasma-spheroidized powders. A 100-kW direct current (d.c.) thermal plasma system was employed. The values obtained were found to exhibit a close relationship with the microstructure of the as-sprayed coatings, which composed of zircon, alumina, amorphous silica and tetragonal zirconia. The coatings produced with plasma-spheroidized powders had higher microhardness, Young's modulus and fracture toughness than that produced with the spray-dried powders. The coatings produced with plasma-spheroidized powders by a 100-kW computerized system at 15 kW of net plasma energy had the best mechanical properties, while those deposited at 19 kW of net plasma energy had the worst properties due to the high density of cracks in the coatings.     

Mechanical synthesis and rapid consolidation of a nanocrystalline 3.3Fe0.6Cr0.3Al0.1-Al2O3 composite by high frequency induction heating

Nanopowders of 3.3Fe_0.6Cr_0.3Al_0.1 and Al₂O₃ were synthesized from Fe₂O₃, Cr, and Al powders by high-energy ball milling. A high density nanocrystalline 3.3Fe_0.6Cr_0.3Al_0.1-Al₂O₃ composite was consolidated by a high frequency induction heated sintering (HFIHS) method within 3 min from mechanically synthesized powders of Al₂O₃ and 3.3Fe_0.6Cr_0.3Al_0.1. The average grain sizes of Al₂O₃ and 3.3Fe_0.6Cr_0.3Al_0.1 were 84 and 32 nm, respectively.     

Mechanical and tribological performance of Al2O3-TiO2 coatings elaborated by flame and plasma spraying

Mechanical properties and wear rates of Al₂O₃-13 wt.% TiO₂ (AT-13) and Al₂O₃-43 wt.% TiO₂ (AT-43) coatings obtained by flame and atmospheric plasma spraying were studied. The feed stock was either ceramic cords or powders. Results show that the wear resistance of AT-13 coatings is higher than that of AT-43 and it seems that the effect of hardness on wear resistance is more important than that of toughness. Additionally, it was established that, according to conditions used to elaborate coatings and the sliding tribological test chosen, spray processes do not seem to have an important effect on the wear resistance of these coatings.     

Microstructure and composition analysis in plasma sprayed coatings of Al2O3/ZrSiO4 mixtures

Plasma spraying of Al₂O₃/ZrSiO₄ was performed using spray dried and plasma spheroidised powder feedstock. The mixtures were sprayed using different spray stand-off distances and plasma power levels. X-Ray diffraction (XRD) was used to characterise the phase composition and scanning electron microscopy (SEM) examined the morphology of the sprayed surface and polished cross-sections. The results showed that the plasma spray process parameters played an important role in the final outcome of microstructures of the coatings. The coatings produced with spheroidised powders displayed a much denser structure than those produced with the spray-dried powders. The phase composition analysis showed the presence of amorphous phases in addition to crystalline alumina, zircon and tetragonal (t) zirconia (ZrO₂). Transmission electron microscopy (TEM) showed that amorphous phases and t-ZrO₂ crystals with particle size 100–200 nm could coexist within a single splat due to the relatively low local cooling rate.     

Artificial neural networks implementation in plasma spray process: Prediction of power parameters and in-flight particle characteristics vs. desired coating structural attributes

Artificial neural networks (ANN) were implemented to predict atmospheric plasma spraying (APS) process parameters to manufacture a coating with the desired structural characteristics.The specific case of predicting power parameters to manufacture grey alumina (Al₂O₃-TiO₂, 13% by wt.) coatings was considered. Deposition yield and porosity were the coating structural characteristics.After having defined, trained and tested ANN, power parameters (arc current intensity, total plasma gas flow, hydrogen content) and resulting in-flight particle characteristics (average temperature and velocity) were computed considering several scenarios. The first one deals at the same time with the two structural characteristics as constraints. The others one deals with one structural characteristic as constraint while the other is fixed at a constant value.     

Synthesis of TiB2 by electric discharge assisted mechanical milling

In this study, synthesis of titanium diboride from elemental powders of Ti and B by electric discharge assisted mechanical milling technique was investigated. This recently developed technique has the following advantages: rapid reaction rate, controlled reaction, direct reaction between Ti and B without adding another element into the system and cost effectiveness. TiB₂ samples were prepared using an in-house built reactor with an ac high voltage transformer, generating impulses within kV/mA range. The structures of reaction products were characterized by X-ray diffractometry, and powder morphologies by SEM. X-ray diffraction studies showed that the milling product was TiB₂ with small fraction of TiB.     

Effect of in-flight particle characteristics on the coating properties of atmospheric plasma-sprayed 8 mol% Y2O3-ZrO2 electrolyte coating studying by artificial neural networks

In-flight particle characteristics (surface temperature and velocity upon impact) are among the most important parameters which influence the coating microstructures and properties in atmospheric plasma spraying (APS) process. The purpose of this paper is to study hydrogen fraction used as secondary plasma forming gas on the in-flight particle surface temperature and by extension on the coating microstructures of atmospheric plasma-sprayed 8 mol% yttria stabilized zirconia electrolyte coatings implementing in particular artificial neural networks (ANN). Then, the predicted in-flight particle characteristics were on the one hand compared to experimental values and on the other hand correlated to some of the coating structural attributes (porosity and gas specific permeability). The predicted results were in good accordance with the experimental data. Results showed that the H₂ flow rate had obvious influence on particle temperature and had almost no significant effect on particle velocity. Increasing the particle temperatures induced dense coating microstructure and improved the gas-tightness performance.     

Thermal shock behavior of nanostructured and conventional Al2O3/13 wt%TiO2 coatings fabricated by plasma spraying

The thermal shock behavior of three kinds of Al₂O₃/13 wt%TiO₂ coatings fabricated by plasma spraying was studied in this paper. One kind of those coatings was derived from conventional fused and crushed feedstock powder available commercially; the other two kinds of coatings were derived from nanostructured agglomerated feedstock powders. These two nano coatings possess moderate pores and pre-existing microcracks, they were composed of fused structure and three-dimensional net or skeleton-like structure. For conventional coatings, the pores and pre-existing cracks were bigger, sharp-point and mostly distributed between splats. Thermal shock tests for the three coatings were performed by water quenching method. Testing result showed the two kinds of nano coatings had much higher thermal shock resistance than the conventional coatings. The improved thermal shock resistance for nano coatings could attribute to their improved microstructure and crack propagation mode. The damage evolution and failure mechanism of coatings was quite different at thermal shock temperature of 650 °C and 850 °C, which was explained by a simple model. Different crack propagating modes in nanostructured and conventional coatings during thermal shock tests were due to their different microstructures in these two kinds coatings. The stress state of coating surfaces during the thermal cycles was also discussed in this paper.     

过渡材料对等离子喷涂Al2O3梯度陶瓷涂层性能影响

针对Al2O3陶瓷涂层结合强度低、孔隙率高的实际,选择NiAl金属间化合物和金属铜粉作为过渡材料,利用等离子喷涂制备Al2O3梯度陶瓷涂层,并对梯度涂层进行组织形貌观察,测试结合强度和孔隙率.结果表明,梯度涂层的组织表现出宏观的不均匀性和微观连续性的分布特征,NiAl和Cu是金属基体与Al2O3涂层之间过渡层的理想材料,可以有效地提高涂层的结合强度,而Cu-Al2O3梯度涂层又比NiAl-Al2O3梯度涂层结合强度高;梯度涂层的孔隙率远低于双涂层的孔隙率,在Cu-Al2O3梯度涂层中随Al2O3含量的增加,涂层的孔隙率降低,而且孔隙率低于NiAl-Al2O3梯度涂层.     

Quantitative analysis of phases formed in the aluminothermic reduction of Ta2O5 by plasma

A novel technique of aluminothermic reduction of tantalum oxide is developed to produce tantalum in form of powder. In this technique, hydrogen plasma is used to trigger the reaction in a plasma reactor. The reacted powders were analyzed by XRD and SEM. Rietveld method was used to quantify the phases present in the product of reaction. The results showed that a tantalum rich phase with a dendritic structure, typical of molten phases is formed. This phase occurred in significant amounts onto the surface and in bulk of the reacted grains.     

Investigation of particle in-flight characteristics during atmospheric plasma spraying of yttria-stabilized ZrO2: Part 1. Experimental

A detailed investigation of the relationship between the parameters of the spray process and the in-flight properties of the particles was carried out using a multivariate statistical approach. A full factorial designed experiment concerning the spray process was performed, the spray gun parameters’ current, argon flow rate, hydrogen flow rate, and powder feed rate being selected to control the process. The particle properties, viz. velocity, temperature, and diameter, were determined using an optical measurement system, DPV 2000. In addition, the standard deviations of, and the correlations between, the measured particle properties were analyzed. The results showed current to have the strongest impact on particle velocity and particle temperature and argon flow rate to be the only parameter with an inverse effect on velocity and temperature.     

等离子喷涂Cu-Al2O3梯度涂层的组织与耐磨性分析

针对纯陶瓷涂层由于结合强度低、孔隙率高、影响涂层耐磨性的实际,用等离子喷涂法制备了Cu-Al2O3梯度涂层,用电子扫描显微镜(SEM)、金相显微镜等手段对涂层进行微观组织和成分分析,用自制销盘式固定磨料磨损试验机,检测了Cu-Al2O3梯度涂层的耐磨料磨损性能.结果表明,采用等离子喷涂法制备的Cu-Al2O3梯度涂层无明显的组织突变和宏观层间界面,涂层的组织表现出宏观不均匀性和微观连续性分布特征;梯度涂层中当Al2O3含量(质量分数,%)达到80%时(GC6),涂层的耐磨性最高,约为基体的3倍,随着Al2O3含量继续增大,纯陶瓷涂层(GC7)的耐磨性有所下降.     

Fabrication and wear characteristics of MoSi2 matrix composite reinforced by WSi2 and La2O3

MoSi₂ matrix composites (RWM) reinforced by the addition of both WSi₂ and La₂O₃ were fabricated by mechanical alloying and self-propagating high-temperature synthesis (SHS) technique. This composite was analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is difficult to synthesize RWM composite by mechanical alloying with Mo–W–Si–La₂O₃ powder mixture, and suitable by self-propagating high-temperature synthesis. The hardness and toughness of MoSi₂ was improved significantly by the addition of both, WSi₂ and La₂O₃ more than by only WSi₂. By adding 0.8 wt.% La₂O₃ and 50 mol.% WSi₂ into the MoSi₂ matrix, this composite has the highest hardness and toughness and exhibits more wear resistance than monolithic MoSi₂ during the sliding wear test under oil lubrication, in this case, the material removal mechanism has been observed to be micro-cutting and micro-fracture.