Plasma spray forming of functionally graded materials mould

A new technology of functionally graded materials(FGM) mould fabricated by plasma spraying and arc spraying was developed. According to applied characteristic of plastic mould, the reasonable coatings of FGM were designed and their microstructures were analyzed. At the same time, some key problems were solved including spray mould fabricating, FGM forming and demoulding, etc. The results show that the service performance of the FGM mould is much more excellent than the one composed of the traditional materials, and the life span can also be greatly increased. The technology will have a significant influence on materials development in mould industry.     

Preparation of Al/Si functionally graded materials using ultrasonic separation method

Functionally graded materials （FGM） have been widely used in many industries such as aerospace, energy and electronics. In this experimental study of fabricating FGM, an approach was developed to prepare AI/Si FGM using power ultrasonic separation method. Material sample with continuously changing composition and performance/properties was successfully produced. Results showed that the microstructure of the FGM sample transited, from its top to bottom, from the hypereutectic structure with a large quantity of primary Si gradually to the eutectic, and finally to the hypoeutectic with numerous primary AI dendrites. The distribution of primary Si and microhardness of the FGM sample also presented graded characteristics, resulting that the wear resistance of the FGM sample decreased from top to bottom. Preliminary discussion was made on the mechanism of the formation of AI/Si FGM.     

Mechanical properties of CNT reinforced hybrid functionally graded materials for bioimplants

The hybrid functionally graded materials (FGM) of hydroxyapatite (HA), stainless steel 316L (SS316L) and carbon nanotubes (CNT) were synthesized for biomedical implants. Three different types of FGM were produced by the combination of SS316L and CNT to reinforce HA in discrete layers of FGM. In the first type of FGM, concentration of SS316L was varied from 10% to 40% (mass fraction) with an increment of 10% to reinforce micro HA. In the second type of FGM, 0.5% (mass fraction) functionalized CNT was added by maintaining the rest of composition as that of the first type of FGM. In the third type of FGM, mixture of micro and nano HA (mass ratio1:1) was used, keeping rest of composition similar to the second type of FGM. All types of FGM were subjected to uniaxial compaction and sintered by pressureless sintering technique at similar compaction and sintering parameters. The results show that the densification is enhanced with the addition of CNT and nanocrystalline HA in the FGM. Hardness and fracture toughness increase in both FGM reinforced with CNT, but the increase of the hardness and fracture toughness are more pronounced in FGM with micro and nanocrystalline HA.     

表面无立方相层功能梯度硬质合金的研究进展

综述了目前应用于涂层基体的无立方相层含氮功能梯度硬质合金的研究进展;详细介绍无立方相层的形成热力学基础、梯度结构特征、机理和动力学研究进展以及力学性能和切削性能;重点评述C、N含量以及组分对无立方相层的影响规律;提出获取合金系统真实的热力学相图和动力学数据是今后研究工作的重点。     

Microstructures of functionally graded materials fabricated by centrifugal solid-particle andin-situ methods

Functionally graded materials (FGMs) belong to a relatively new class of inhomogeneous composite materials, in which the composition and/or microstructure undergo a gradual change along some directions. In this review article, the microstructures and composition gradients in Al/SiC, Al/Shirasu (volcanic eruptions commonly found in south Kyushu in Japan), Al/Al₃Ti, Al/Al₃Ni, Al/Al₂Cu FGMs have been investigated. The Al/SiC, Al/Shirasu and Al/Al₃Ti FGMs are fabricated by the centrifugal solid particle method where the distribution particles of SiC, Shirasu and Al₃Ti are solids in the melts. On the other hand, Al/Al₃Ni and Al/Al₂Cu FGMs are fabricated by the centrifugalin-situ method where Al/Al₃Ni and Al/Al₂Cu systems have lower liquidus temperatures than the processing temperatures. The feature of Al/(Al₃Ti−Al₃Ni) hydrid FGM, which is fabricated by a method combining both the centrifugal solid-particle andin-situ methods, is also shown.     

Use of plasma spraying in the manufacture of continuously graded and layered/graded molybdenum disilicide/alumina composites

Plasma spraying was used to produce continuously graded and graded/layered structures of molybdenum disilicide (MoSi₂) and alumina (Al₂O₃). These functionally graded materials (FGMs) were achieved by manipulating the powder hoppers and plasma torch translation via in-house created computer software. The resultant microstructures sprayed uniformly and were crack free. The interface between MoSi₂ and Al₂O₃ was continuous and no evidence of debonding or cracking at the interface was found. The mechanical strength of these sprayed materials was evaluated using C-ring samples (in diametrical compression). Weibull analysis conducted on the C-ring data indicated that the continuously graded samples were slightly stronger and had a significantly narrower strength distribution than the graded/layered samples. Although the average strength values of both types of functionally graded samples were closer to those of monolithic MoSi₂, the fracture energy of the graded samples was significantly larger (∼2–3 times) compared with the monolithic materials. Scanning electron microscopy (SEM) conducted on the fracture surfaces of the FGMs illustrated a wavy and tortuous crack path through the composite cross section of the sample, with extensive crack kinking. This study has two important results. First, we demonstrated the ability to produce such functionally graded composite ceramic microstructures using a conventional plasma spraying process. Second, we quantified the improvements in mechanical performance provided by these FGMs over conventional monolithic materials.     

Plasma spraying of functionally graded yttria stabilized zirconia/NiCoCrAlY coating system using composite powders

Pre-alloyed and plasma spheroidized composite powders were used as the feedstock in the plasma spraying of functionally graded yttria stabilized zirconia (YSZ)/NiCoCrAlY coatings. The ball milling parameters of the composite powders and the plasma spraying parameters for preparing functionally graded materials (FMGs) coatings were optimized to obtain the best performance for the thermal barrier coatings (TBCs). Microstructure, physical, mechanical, and thermal properties of YSZ/NiCoCrAlY FGMs coatings were investigated and compared with those of traditional duplex coatings. Results showed that the advantages of using pre-alloyed composite powders in plasma spraying were to ensure chemical homogeneity and promote uniform density along the graded layers. Microstructure observation showed the gradient distribution of YSZ and NiCoCrAlY phases in the coating, and no clear interface was found between two adjacent different layers. Oxidation occurred during plasma spray and the resultant aluminum oxide combines with YSZ in a wide range of proportions. The bond strength of functionally graded coatings was about twice as high as that of the duplex coatings because of the significant reduction of the residual stresses in the coatings. The thermal cycling resistance of functionally graded coating was much better than that of duplex coating.     

Gradient profile prediction in functionally graded materials processed by electrophoretic deposition

Electrophoretic deposition (EPD) allows the formation of plate-shaped binary functionally graded materials (FGM) by depositing from a powder suspension to which a second suspension is continuously added during the process. The deposition yield is described as a function of time and starting composition of both suspensions, resulting in a model of the EPD process that allows predicting the composition gradient in the green deposit as well as in the sintered material. The model presented enables calculation of the composition gradient in the FGM material from the starting composition of the suspensions, the EPD operating parameters and the powder-specific EPD characteristics. The powder-specific parameters, i.e. the effective charge, the electrophoretic mobility, the specific conductivity of the intermicellar liquid and the specific resistance of the deposit, incorporated in the model were experimentally determined from EPD of the individual homogeneous powder suspensions. The model was verified for the ZrO₂-Al₂O₃ system by the actual preparation and analysis of two different FGM materials. Because of the excellent correlation between the predicted and the measured concentration profiles, the described model allows to precisely engineer and design the composition profile in FGM materials produced by EPD.     

Damage tolerant functionally graded WC-Co/Stainless Steel HVOF coatings

In this paper, effective damage tolerance of a functionally graded coating (FGC) deposited by high velocity oxygen fuel (HVOF) spraying is observed. The thick FGC (≈ 1.2 mm) consists of 6 layers with a stepwise change in composition from 100 vol.% ductile AISI316 stainless steel (bottom layer) to 100 vol.% hard WC-12Co (top layer) deposited onto an AISI316 stainless steel substrate. Damage tolerance is observed via 1) an increase in compliance with depth, and 2) an increase in fracture resistance by containment, arrest and deflection of cracks. A smooth gradation in the composition and hardness through the coating thickness is found by scanning electron microscopy and depth-sensing microindentation, respectively. The in-situ curvature measurement technique reveals that during the deposition of the FGC, compressive stresses exist in the lower, metallic layers owing to peening effect of successive impact, and these gradually evolve to high tensile, in the top layers. Tensile stresses appear to be due to quenching alone; thermal stresses are low because of the gradation. All of this is beneficial for the deposition of a thick coating.The FGC structure shows the ability to reduce cracking with increased compliance in the top layer during static and dynamic normal contact loading, while retaining excellent sliding wear resistance (ball-on-disk tests). Results are discussed in comparison to the behavior and properties of coatings of similar individual compositions and thicknesses, as well as a thick monolithic WC-12Co sprayed coating. Further improvements in the processing are proposed to enhance the adhesion strength and avoid coating delamination under high load contact-fatigue conditions.     

Effect of spray parameters on residual stress build-up of HVOF sprayed aluminium/tool-steel functionally graded coatings

The High Velocity Oxy–Fuel (HVOF) process is one of the most versatile thermal spray technologies and has found use in many industries due to its flexibility and cost effectiveness. It is normally used to protect components against wear, heat and/or corrosion and has the potential to produce functionally graded coatings. Gradual changes of microstructure in functionally graded coatings reduce possible residual stress build-up by gradually attaining property changes between the substrate and the coating. Effectively this is done by depositing layers of coatings of varying composition. The present study investigates the effect of spray parameters on residual stress build-up in aluminium/tool-steel functionally graded coatings (FGC). A simple 3³ factorial design of experiments was employed to establish the effects of spray parameters on residual stress. Residual stress was measured using Clyne's analytical method. Parameters such as flow rate ratio of the oxygen to propylene, flow rate of the compressed air and spray distance were varied during coating deposition. Apart from the thickness of the coated sample, the spray distance showed greater effect on residual stress build-up in the graded coatings compared to flow rate ratio of the oxygen to propylene, and flow rate of the compressed air. Finally a set of values of spray parameters giving the best compromise between low residual stress and high deposited coating thickness were identified. This paper concluded that the optimised set of parameters for FGC resembles the parameters recommended for the deposition of the powder with the lowest melting and boiling point; in this case aluminium.     

Thermal spraying of reactive materials to form wear-resistant composite coatings

The dispersion of more than 20 vol.% submicrometer ceramic particles within a metallic matrix and the deposition of such a cermet to form a thick and tough coating presents problems. Most of the coating techniques have failed in attempting to homogeneously disperse very fine and hard particles in large amounts while avoiding their decomposition or reaction with the metal matrix during the deposition process. A simple and efficient method has been developed for producing ceramic-containing composite coatings. It consists in synthesizing cermet-based materials and in depositing them by a rapid solidification process such as thermal spraying. Boride- and carbide-based materials have been successfully obtained by plasma spraying reactive powders comprising the basic reagents. These materials, with a microstructure of submicrometer ceramic particles dispersed in a metallic matrix, exhibit good wear-resistant properties (abrasion and sliding wear). Finally, reactive core wire arc spraying is suggested as a flexible way to produce coatings containing up to 25 vol.% TiB₂.     

Adhesive/cohesive properties of thermally sprayed functionally graded coatings for polymer matrix composites

High-velocity oxyfuel (HVOF) sprayed polyimide/WC-Co functionally graded (FGM) coatings with flame-sprayed WC-Co topcoats have been investigated as solutions to improve the solid-particle erosion and oxidation resistance of polymer matrix composites (PMCs) in the gas flow path of advanced turbine engines. Porosity, coating thickness, and volume fraction of the WC-Co phase retained in the graded coating architecture were determined using standard metallographic techniques and computer image analysis. The adhesive bond strength of three different types of coatings was evaluated according to ASTM D 4541. Adhesive/cohesive strengths of the FGM coating were measured and compared with those of pure polyimide and polyimide/WC-Co composite coatings and also related to the tensile strength of the uncoated PMC substrate perpendicular to the thickness. The FGM coatings exhibited lower adhesive bond strengths (∼6.2 MPa) than pure polyimide coatings (∼8.4 MPa), and in all cases these values were lower than the tensile strength (∼17.6 MPa) of the reference uncoated PMC substrate. The nature and locus of the failures were characterized according to the percent adhesive and/or cohesive failure, and the interfaces tested and layers involved were analyzed by scanning electron microscopy. The original version of this paper was published as part of the ASM Proceedings, Thermal Spray 2003: Advancing the Science and Applying the Technology, International Thermal Spray Conference (Orlando, FL), 5–8 May, 2003, Basil R. Marple and Christian Moreau, Eds., ASM International, 2003.     

梯度功能材料等离子熔积成形过程的Level-Set方法模拟

利用Level-Set方法与有限体积法(finite volume method,FVM)建立梯度功能材料(functionally graded material,FGM)等离子熔积制造(plasma deposition manufacturing,PDM)过程多相混态场统一模型;研究了在不同热输入功率下熔池形貌、温度场与流场以及溶质分布的变化规律.结果表明,热功率是影响梯度功能材料成分分布和性能的重要工艺参数.并用等离子熔积制造方法制备Al2O3-AISI316梯度功能材料,验证了模拟计算结果的正确性和可靠性.     

Present status of research on design and processing of functionally graded materials

Recent activities in the research of functionally graded materials (FGMs) are reviewed with an emphasis on the fabrication techniques for graded microstructure and functions. The research activities have focused on the processing of graded structures that satisfy the designed functions. Several examples of practical applications of FGMs are described in the present paper to demonstrate the fabrication approaches typical to this kind of material. The remainder of the paper is devoted to summarizing a recently finished national project supported by the Ministry of Education of Japan. Research and developments with respect to FGMs in various fields including physics, chemistry, medical science, and biology are briefly reviewed. This article is based on a presentation made in the symposium “The 3rd KIM-JIM Joint Symposium on Advanced Powder Materials”, held at Korea University, Seoul, Korea, October 26–27, 2001 under auspices of The Korean Institute of Metals and Materials and The Japan Institute of Metals. Present at the Fine Ceramic Center, Atsuta-ku, Nagoya, Japan     

超高压梯度烧结法制备W／Cu功能梯度材料

提出了一种制备具有递变电阻及高熔点差功能梯度材料的新方法－超高压梯度烧结法,并成功制备出了相对密度达到96％的W／Cu梯度功能材料。推导了通电烧结过程中梯度材料内部的温度分布,表明温度场与电流密度及材料的厚度大致成平方的梯度分布模式;考察了不同的烧结助剂对W／Cu梯度材料致密化的影响,发现Ni比V和Zr有更好的致密化效果;观察了W／Cu梯度材料显微结构并对梯度烧结的过程机理进行了初步探讨。     

有裂纹的功能梯度材料板平面问题的随机动态响应(英文)

研究有裂纹的功能梯度材料板平面问题的随机动态断裂,建立平面问题的随机模型。功能梯度材料的物理力学性能沿厚度方向的非均匀随机变化为沿厚度方向的随机场。作用在裂纹面上的动荷载为时间的平稳随机过程,裂纹与功能梯度材料面平行。通过分层法,将随机场的问题转化为多个随机变量的问题进行求解。利用积分变换方法和奇异积分方程技术,得到动态响应的解析形式和裂纹起始扩展的概率计算公式,分析了各参数(裂纹长度、随机场参数和裂纹位置)对动态应力强度因子的影响。结果表明:随着裂纹长度、随机场参数β以及裂纹位置比率 h2/h 的增大,应力强度因子的数学期望和标准方差均相应增大。     

等离子熔射制备梯度功能MOLB式SOFC及其复阻抗分析

采用一种融合功能梯度材料设计思想与快速原型制造等离子熔射与机器人数字化成形技术于一体的新方法,借助自主研发的梯度功能送粉系统,多层连续熔射快速制造立体结构MOLB(mono-block layer built)式固体氧化物燃料电池SOFC(solid oxide fuel cell)三合一电极PEN(positive electrolyte negative)部件.分析了PEN部件的成分、微观组织和阳极、阴极孔隙率的连续梯度变化规律,并利用复阻抗技术对其进行导电性能测试.结果表明,采用该方法能够获得含有成分、组织呈连续梯度变化的功能涂层的SOFC 核心部件PEN;与不含有梯度层的PEN相比,不仅提高电极与电解质间的匹配性,改善电极孔隙率及其分布;而且大幅度减小功能梯度PEN部件界面接触电阻,有助于提高SOFC的电性能.     

Hydroxyapatite coatings for biomedical applications deposited by different thermal spray techniques

Thermally sprayed hydroxyapatite (HAp) coatings are widely used for various biomedical applications due to the fact that HAp is a bioactive, osteoconductive material capable of forming a direct and firm biological fixation with surrounding bone tissue.Bioceramic coatings based on nanoscale HAp suspension and microscale HAp powder were thermally sprayed on Ti plates by high-velocity suspension flame spraying (HVSFS) technique and atmospheric plasma spraying (APS) as well as high velocity oxy fuel spraying (HVOF) technique. HVSFS is a novel thermal spray process developed at IMTCCC, for direct processing of submicron and nano-sized particles dispersed in a liquid feedstock.The deposited coatings were mechanically characterized including surface roughness, micro hardness and coating porosity. The bond strength of the layer composites were analyzed by the pull-off method and compared for the different spray techniques. Phase content and crystallinity of the coatings were evaluated using X-ray diffraction (XRD). The coating composite specimen and initial feedstock were further analysed by scanning electron microscope (SEM) and rheology analysis.     

非对称HA/316L不锈钢梯度生物材料的制备与表征

采用干粉铺叠法和热压工艺制备了非对称HA/316L不锈钢功能梯度生物材料,并测定了其相对密度和抗弯强度,采用X射线衍射仪、扫描电镜、金相显微分析技术等对材料进行了物相和显微组织分析.结果表明:非对称HA-316L不锈钢生物FGM在宏观上呈现明显的梯度分布,微观上则各成分分布连续、均匀,各梯度层之间没有明显的宏观界面,界面结合紧密;随着316L不锈钢含量的增大,材料的相对密度增加,抗弯强度提高,平均抗弯强度达450 MPa左右,体现出FGM的热应力缓和行为;此外,在生物FGM中,HA和316L不锈钢两相在热压过程中发生了不同程度的固溶,表明HA和316L不锈钢能够形成好的结合.