放电等离子烧结制备W/Cu功能梯度材料的磨损性能和显微硬度

C. Nicolicescu, V. Nicoară, F. Popa, T. Marinca
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The wear behavior is influenced by the composition of the layers and by ball testing material (100Cr6 and alumina). Introduction Copper alloys are frequently used in applications that require high electrical and thermal conductivity. In some applications that require strength and wear resistance it is necessary to alloy copper with others metals. Alloys based on copper and tungsten attract the attention due to the combination of properties such as low thermal expansion coefficient, high melting point, high strength and wear resistance conferred by tungsten with a high electrical and thermal characteristics conferred by copper [1-6]. The researches in the field of W/Cu alloys are focused on the controlling the microstructure by optimizing the composition or processing techniques [7-11]. Due to their insolubility and high differences between densities and melting points it’s very difficult to produce W/Cu composite. There are different methods to produce W/Cu composite/nanocomposite namely: copper infiltration and liquid phase sintering [10, 12] which are considered classical methods, respectively new methods as mechanical alloying (MA), mechano-chemical processes (MCP) [13], mechanothermochemical processing (MTP) [14], the thermo-mechanical method [15], wet-chemical methods [16] and spray drying [17]. Functional graded materials (FGM) based on W/Cu represent a new category of materials consisting in two or more layers, in which the microstructure and the composition vary from the Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 183 top layer to the bottom layer and vice versa. This class of materials presents some advantages comparative to the single layer materials, namely: the properties are different in each layer, residual and thermal stresses are reduced and the fracture strength is optimized [18-20]. The main fields of applications of W/Cu FGM are: electrical contacts, plasma facing materials, heat sink materials, etc. [21] In recent decades, Spark Plasma Sintering (SPS) became a popular sintering method which is widely used in fabrication process of different materials as: ceramics, cermets, metals, hard materials, composite materials and FGMs [22–31]. The SPS process is different by conventional sintering in that, the cold pre-pressed powder is sintered by discharge impact, spark plasma, Joule heat and intrinsic field effects produced by additional strong pulse currents through the powders. The SPS is assisted by a vertical uniaxial load in order to accelerate the process. The main advantages of the SPS process are: rapid heating (over 200 C/min), short dwell times, lower temperatures and fine microstructures [32, 33]. The present work is focused on the elaboration of three layers W/Cu FGM by SPS process. The composition of the layers consisting in W100-xCux, where x is 25, 30 and 40 %weight. The influence of the SPS temperature and layers composition on the wear behavior and microhardness were studied. Experimental work Raw materials Tungsten nanopowders (Fig. 1a) obtained by mechanical milling for 35 hours with particle size about 50 nm (according to Dynamic Laser Scattering analysis) and copper micrometric powders (type SE Pometon) with particle size around 10μm (Fig. 1b) were used as raw materials in order to prepare the mixtures. Fig. 1. SEM images of the starting powders: a) tungsten nanopowders; b) copper powders Tungsten nanopowders are agglomerated and have irregular shape (Fig. 1a) comparative with copper powders (Fig. 1b) which have dendritic shape, corresponding to electrolytic process. Three types of mixtures with the following concentrations (% weight): 75W/25Cu, 70W/30Cu and 60W/40Cu were subjected to mechanical milling process for 20 hours in order to obtain composite powder. For mechanical milling (MM) process planetary ball mill Pulverisette 4 made by Fritsch was used. The milling parameters were: material of vials: stainless steel, vials volume: 250 ml, materials of balls: stainless steels, balls diameter: Φ=10 mm, balls to powder ratio: 2/1, Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 184 rotational speed of the main disk: 400 rpm, rotational speeds of the vials: -800 rpm, milling medium: argon, milling type: dry. The SEM images and element distribution maps obtained by EDX analysis show that inside of particles of all mixtures, the component elements are homogenous dispersed (Fig. 2). Fig. 2. SEM images and EDX analysis of the mixtures used for the research: a) 75W/Cu; b) 70W/Cu; c) 60W/Cu Elaboration of functional graded materials The mixtures obtained after MM process was used in order to obtain three layered functional graded materials (FGM) by spark plasma sintering (SPS) according to the flow chart presented in (Fig. 3). Fig. 3. Experimental flow chart A thin graphite paper was put into the inner part of the die for lubrication and other two papers were put on the bottom and upper of the part in order to prevent the powder to stick on the graphite punches. The samples were sintered at four temperatures (950, 850, 750 and 650 C) without dwell time and a pressure equal to 20 MPa using a SPS homemade system from Technical University of Cluj-Napoca. The heating rate was about 300 C/min. After SPS the density of the samples was measured by the Archimedes’ method. The samples were cut and polished in order to study the microstructural aspects and microhardness. The optical microscopy was made using an NIKON microscope with NIS ELEMENTS image software. SEM characterisation was performed using a JEOL JSM5600LV microscope equipped with EDX spectrometer (Oxford Instruments, INCA 200 software). The micro hardness was measured using a Namicon tester with a load of 9.8 Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 185 N and a dwell time of 15 seconds. The investigation of wear behavior was performed using a CSM Instruments tribometer TRB 01-2541 and a Taylor Hobson Surtronic 25+ profilometer. The parameters for wear testing were: type: ball on disk; load – 2N; testing method linear; amplitude 6mm; speed 10cm/s; distance 60 m; ball material – 100Cr6 and alumina; temperature 25 C. Results and discussion The relative density function of the SPS temperature is plotted in (Fig. 4). Fig. 4. Relative density function of the SPS temperature The highest value of the density is attained at 950 C and it is observed that it decrease with the decreasing of the sintering temperature. Fig. 5 presents optical microscopy images of the samples (not etched) at the four sintering temperatures. The microstructures of the samples are homogenous and present structural gradient. The dark grey colour corresponds to the layer 1 with 75%W and so on. The thickness of the middle layer increases with the decreasing of the SPS temperature. The lowest value of the thickness (1216.85 μm) was attained for the sample sintered at highest temperature 950 C. Higher density lead to lower thickness as expected. Fig. 5. Optical microscopy aspects of the three layered samples (75X): a) 950 C; b) 850 C; c) 750 C; d) 650 C The EDX line distribution on the three layers, (Fig. 6) shows the distribution of W and copper function the scanning distance. It is observed that the content of W decrease from left (first layer – 75%W) to the right (third layer 60%W). In the same time a slight increase in Cu is recorded, according the layer composition change. In (Fig. 7) are presented the interfaces of the three layers. Analysing (Fig. 6, 7) it is obvious that the graded structure was attained. The interfaces seems to assure continuity from one layer to another as it can be seen in (Fig. 7). The composition Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 186 differences between the layers are small, probably due to the powder superficial mix at the interface. In (Fig. 8) is plotted the evolution of the microhardness. Fig. 6. SEM image (left) and EDX line distribution of W and Cu elements of the three layered W/Cu FGM obtained by SPS at 950 C Fig. 7. SEM images of the interfaces of W/Cu FGM obtained by SPS at 950 C: a) between layer 1 and layer 2; b) between layer 2 and layer 3. Fig. 8. Evolution of microhardness as a function of sintering temperature The measurements of the microhardness were performed in all the three layers, even if only the layer 1 and 3 are important from this point of view, because only these are in contact with other materials. The bonding between W and copper particles influences the microhardness of the layers. The highest value of the microhardness was attained in the layer 1 (75W/Cu) sintered at 950 C and it decreases with the decreasing of the W content. Powder Metallurgy and Advanced Ma","PeriodicalId":20390,"journal":{"name":"Powder Metallurgy and Advanced Materials","volume":null,"pages":null},"PeriodicalIF":0.0000,"publicationDate":"2018-11-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Wear behavior and microhardness of some W/Cu functionally graded materials obtained by spark plasma sintering\",\"authors\":\"C. Nicolicescu, V. Nicoară, F. Popa, T. Marinca\",\"doi\":\"10.21741/9781945291999-21\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"This paper is focused on the elaboration of some W/Cu functionally graded materials (FGM) by spark plasma sintering (SPS) process, as well as on their characterization, from the wear behavior and microhardness point of view function of composition and sintering temperature. The raw materials used for the research were W/Cu mechanically alloyed powders for 20 hours, which were subjected to consolidation in three layers of compositions W100-xCux, where x is 25, 30 and 40 % wt. by SPS. The evolution of tribological parameters and microhardness function of the chemical composition and SPS temperature were investigated. Microhardness is influenced by the SPS temperature and composition of the layers namely, the highest value was attained for the sample sintered at 950 C and layer 1 which consists in W75Cu25. The wear behavior is influenced by the composition of the layers and by ball testing material (100Cr6 and alumina). Introduction Copper alloys are frequently used in applications that require high electrical and thermal conductivity. In some applications that require strength and wear resistance it is necessary to alloy copper with others metals. Alloys based on copper and tungsten attract the attention due to the combination of properties such as low thermal expansion coefficient, high melting point, high strength and wear resistance conferred by tungsten with a high electrical and thermal characteristics conferred by copper [1-6]. The researches in the field of W/Cu alloys are focused on the controlling the microstructure by optimizing the composition or processing techniques [7-11]. Due to their insolubility and high differences between densities and melting points it’s very difficult to produce W/Cu composite. There are different methods to produce W/Cu composite/nanocomposite namely: copper infiltration and liquid phase sintering [10, 12] which are considered classical methods, respectively new methods as mechanical alloying (MA), mechano-chemical processes (MCP) [13], mechanothermochemical processing (MTP) [14], the thermo-mechanical method [15], wet-chemical methods [16] and spray drying [17]. Functional graded materials (FGM) based on W/Cu represent a new category of materials consisting in two or more layers, in which the microstructure and the composition vary from the Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 183 top layer to the bottom layer and vice versa. This class of materials presents some advantages comparative to the single layer materials, namely: the properties are different in each layer, residual and thermal stresses are reduced and the fracture strength is optimized [18-20]. The main fields of applications of W/Cu FGM are: electrical contacts, plasma facing materials, heat sink materials, etc. [21] In recent decades, Spark Plasma Sintering (SPS) became a popular sintering method which is widely used in fabrication process of different materials as: ceramics, cermets, metals, hard materials, composite materials and FGMs [22–31]. The SPS process is different by conventional sintering in that, the cold pre-pressed powder is sintered by discharge impact, spark plasma, Joule heat and intrinsic field effects produced by additional strong pulse currents through the powders. The SPS is assisted by a vertical uniaxial load in order to accelerate the process. The main advantages of the SPS process are: rapid heating (over 200 C/min), short dwell times, lower temperatures and fine microstructures [32, 33]. The present work is focused on the elaboration of three layers W/Cu FGM by SPS process. The composition of the layers consisting in W100-xCux, where x is 25, 30 and 40 %weight. The influence of the SPS temperature and layers composition on the wear behavior and microhardness were studied. Experimental work Raw materials Tungsten nanopowders (Fig. 1a) obtained by mechanical milling for 35 hours with particle size about 50 nm (according to Dynamic Laser Scattering analysis) and copper micrometric powders (type SE Pometon) with particle size around 10μm (Fig. 1b) were used as raw materials in order to prepare the mixtures. Fig. 1. SEM images of the starting powders: a) tungsten nanopowders; b) copper powders Tungsten nanopowders are agglomerated and have irregular shape (Fig. 1a) comparative with copper powders (Fig. 1b) which have dendritic shape, corresponding to electrolytic process. Three types of mixtures with the following concentrations (% weight): 75W/25Cu, 70W/30Cu and 60W/40Cu were subjected to mechanical milling process for 20 hours in order to obtain composite powder. For mechanical milling (MM) process planetary ball mill Pulverisette 4 made by Fritsch was used. The milling parameters were: material of vials: stainless steel, vials volume: 250 ml, materials of balls: stainless steels, balls diameter: Φ=10 mm, balls to powder ratio: 2/1, Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 184 rotational speed of the main disk: 400 rpm, rotational speeds of the vials: -800 rpm, milling medium: argon, milling type: dry. The SEM images and element distribution maps obtained by EDX analysis show that inside of particles of all mixtures, the component elements are homogenous dispersed (Fig. 2). Fig. 2. SEM images and EDX analysis of the mixtures used for the research: a) 75W/Cu; b) 70W/Cu; c) 60W/Cu Elaboration of functional graded materials The mixtures obtained after MM process was used in order to obtain three layered functional graded materials (FGM) by spark plasma sintering (SPS) according to the flow chart presented in (Fig. 3). Fig. 3. Experimental flow chart A thin graphite paper was put into the inner part of the die for lubrication and other two papers were put on the bottom and upper of the part in order to prevent the powder to stick on the graphite punches. The samples were sintered at four temperatures (950, 850, 750 and 650 C) without dwell time and a pressure equal to 20 MPa using a SPS homemade system from Technical University of Cluj-Napoca. The heating rate was about 300 C/min. After SPS the density of the samples was measured by the Archimedes’ method. The samples were cut and polished in order to study the microstructural aspects and microhardness. The optical microscopy was made using an NIKON microscope with NIS ELEMENTS image software. SEM characterisation was performed using a JEOL JSM5600LV microscope equipped with EDX spectrometer (Oxford Instruments, INCA 200 software). The micro hardness was measured using a Namicon tester with a load of 9.8 Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 185 N and a dwell time of 15 seconds. The investigation of wear behavior was performed using a CSM Instruments tribometer TRB 01-2541 and a Taylor Hobson Surtronic 25+ profilometer. The parameters for wear testing were: type: ball on disk; load – 2N; testing method linear; amplitude 6mm; speed 10cm/s; distance 60 m; ball material – 100Cr6 and alumina; temperature 25 C. Results and discussion The relative density function of the SPS temperature is plotted in (Fig. 4). Fig. 4. Relative density function of the SPS temperature The highest value of the density is attained at 950 C and it is observed that it decrease with the decreasing of the sintering temperature. Fig. 5 presents optical microscopy images of the samples (not etched) at the four sintering temperatures. The microstructures of the samples are homogenous and present structural gradient. The dark grey colour corresponds to the layer 1 with 75%W and so on. The thickness of the middle layer increases with the decreasing of the SPS temperature. The lowest value of the thickness (1216.85 μm) was attained for the sample sintered at highest temperature 950 C. Higher density lead to lower thickness as expected. Fig. 5. Optical microscopy aspects of the three layered samples (75X): a) 950 C; b) 850 C; c) 750 C; d) 650 C The EDX line distribution on the three layers, (Fig. 6) shows the distribution of W and copper function the scanning distance. It is observed that the content of W decrease from left (first layer – 75%W) to the right (third layer 60%W). In the same time a slight increase in Cu is recorded, according the layer composition change. In (Fig. 7) are presented the interfaces of the three layers. Analysing (Fig. 6, 7) it is obvious that the graded structure was attained. The interfaces seems to assure continuity from one layer to another as it can be seen in (Fig. 7). The composition Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 186 differences between the layers are small, probably due to the powder superficial mix at the interface. In (Fig. 8) is plotted the evolution of the microhardness. Fig. 6. SEM image (left) and EDX line distribution of W and Cu elements of the three layered W/Cu FGM obtained by SPS at 950 C Fig. 7. SEM images of the interfaces of W/Cu FGM obtained by SPS at 950 C: a) between layer 1 and layer 2; b) between layer 2 and layer 3. Fig. 8. Evolution of microhardness as a function of sintering temperature The measurements of the microhardness were performed in all the three layers, even if only the layer 1 and 3 are important from this point of view, because only these are in contact with other materials. The bonding between W and copper particles influences the microhardness of the layers. The highest value of the microhardness was attained in the layer 1 (75W/Cu) sintered at 950 C and it decreases with the decreasing of the W content. 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引用次数: 0

摘要

铣削参数为:小瓶材料:不锈钢,小瓶体积:250 ml,球材料:不锈钢,球直径:Φ=10 mm,球粉比:2/1,粉末冶金与先进材料- RoPM&AM 2017材料研究论坛LLC材料研究进展8 (2018)182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 184主盘转速:400 rpm,小瓶转速:-800 rpm,铣削介质:氩气,铣削类型:干燥。SEM图像和EDX分析得到的元素分布图表明,在所有混合物的颗粒内部,组成元素都是均匀分散的(图2)。用于研究的混合物的SEM图像和EDX分析:a) 75W/Cu;b) 70 w /铜;c) 60W/Cu功能梯度材料的细化利用MM工艺后得到的混合物,通过火花等离子烧结(SPS)得到三层功能梯度材料(FGM),流程图如图3所示。在模具内部放一张薄石墨纸润滑,在模具底部和上部放两张薄石墨纸防止粉末粘在石墨冲头上。采用克卢日-纳波卡工业大学自制的SPS烧结系统,在950、850、750和650℃4种温度下无保温时间和20 MPa的压力下进行烧结。升温速率约为300℃/min。SPS后,用阿基米德方法测量样品的密度。对样品进行了切割和抛光,以研究其显微组织和显微硬度。光学显微镜采用尼康显微镜,使用NIS ELEMENTS成像软件。SEM表征采用JEOL JSM5600LV显微镜,配备EDX光谱仪(Oxford Instruments, INCA 200软件)。显微硬度采用负载为9.8的Namicon测试仪进行测量。粉末冶金与先进材料- RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 185 N,停留时间为15秒。使用CSM Instruments的TRB 01-2541摩擦计和Taylor Hobson Surtronic 25+轮廓仪进行磨损行为调查。磨损试验参数为:类型:球盘;负载- 2N;测试方法线性;振幅6毫米;速度10厘米/秒;距离60米;球料——100Cr6和氧化铝;SPS温度的相对密度函数绘制在(图4)中。SPS温度的相对密度函数,密度在950℃时达到最大值,随烧结温度的降低而减小。图5给出了四种烧结温度下样品(未蚀刻)的光学显微镜图像。样品的显微组织均匀且呈结构梯度。深灰色对应于图层1的75%W,以此类推。随着SPS温度的降低,中间层厚度增大。在最高温度950℃下烧结的样品厚度最小,为1216.85 μm,密度越大,厚度越小。图5所示。光学显微镜方面的三个层状样品(75X): a) 950℃;b) 850 C;c) 750℃;三层上的EDX线分布(图6)显示了W和铜随扫描距离的分布。W的含量由左(第一层- 75%W)向右(第三层- 60%W)递减。同时,根据层成分的变化,Cu的含量略有增加。图7中给出了三层的界面。分析(图6、7)可以明显地看出,得到了梯度结构。界面似乎确保了从一层到另一层的连续性,如图7所示。粉末冶金与先进材料- RoPM&AM 2017材料研究论坛LLC材料研究进展8 (2018)182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 186层之间的差异很小,可能是由于界面处的粉末表面混合。在(图8)中绘制了显微硬度的演变。图6所示。图7.在950℃时SPS获得的三层W/Cu FGM的SEM图像(左)和W、Cu元素的EDX线分布。950℃时SPS获得的W/Cu FGM界面SEM图像:a)第一层和第二层之间;B)在第2层和第3层之间。图8所示。显微硬度随烧结温度的变化。显微硬度的测量在所有三层中进行,即使从这个角度来看,只有第1层和第3层是重要的,因为只有这两层与其他材料接触。 W和铜颗粒之间的结合影响层的显微硬度。显微硬度在950℃烧结的第一层(75W/Cu)达到最大值,随W含量的降低而降低。粉末冶金与高级硕士
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Wear behavior and microhardness of some W/Cu functionally graded materials obtained by spark plasma sintering
This paper is focused on the elaboration of some W/Cu functionally graded materials (FGM) by spark plasma sintering (SPS) process, as well as on their characterization, from the wear behavior and microhardness point of view function of composition and sintering temperature. The raw materials used for the research were W/Cu mechanically alloyed powders for 20 hours, which were subjected to consolidation in three layers of compositions W100-xCux, where x is 25, 30 and 40 % wt. by SPS. The evolution of tribological parameters and microhardness function of the chemical composition and SPS temperature were investigated. Microhardness is influenced by the SPS temperature and composition of the layers namely, the highest value was attained for the sample sintered at 950 C and layer 1 which consists in W75Cu25. The wear behavior is influenced by the composition of the layers and by ball testing material (100Cr6 and alumina). Introduction Copper alloys are frequently used in applications that require high electrical and thermal conductivity. In some applications that require strength and wear resistance it is necessary to alloy copper with others metals. Alloys based on copper and tungsten attract the attention due to the combination of properties such as low thermal expansion coefficient, high melting point, high strength and wear resistance conferred by tungsten with a high electrical and thermal characteristics conferred by copper [1-6]. The researches in the field of W/Cu alloys are focused on the controlling the microstructure by optimizing the composition or processing techniques [7-11]. Due to their insolubility and high differences between densities and melting points it’s very difficult to produce W/Cu composite. There are different methods to produce W/Cu composite/nanocomposite namely: copper infiltration and liquid phase sintering [10, 12] which are considered classical methods, respectively new methods as mechanical alloying (MA), mechano-chemical processes (MCP) [13], mechanothermochemical processing (MTP) [14], the thermo-mechanical method [15], wet-chemical methods [16] and spray drying [17]. Functional graded materials (FGM) based on W/Cu represent a new category of materials consisting in two or more layers, in which the microstructure and the composition vary from the Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 183 top layer to the bottom layer and vice versa. This class of materials presents some advantages comparative to the single layer materials, namely: the properties are different in each layer, residual and thermal stresses are reduced and the fracture strength is optimized [18-20]. The main fields of applications of W/Cu FGM are: electrical contacts, plasma facing materials, heat sink materials, etc. [21] In recent decades, Spark Plasma Sintering (SPS) became a popular sintering method which is widely used in fabrication process of different materials as: ceramics, cermets, metals, hard materials, composite materials and FGMs [22–31]. The SPS process is different by conventional sintering in that, the cold pre-pressed powder is sintered by discharge impact, spark plasma, Joule heat and intrinsic field effects produced by additional strong pulse currents through the powders. The SPS is assisted by a vertical uniaxial load in order to accelerate the process. The main advantages of the SPS process are: rapid heating (over 200 C/min), short dwell times, lower temperatures and fine microstructures [32, 33]. The present work is focused on the elaboration of three layers W/Cu FGM by SPS process. The composition of the layers consisting in W100-xCux, where x is 25, 30 and 40 %weight. The influence of the SPS temperature and layers composition on the wear behavior and microhardness were studied. Experimental work Raw materials Tungsten nanopowders (Fig. 1a) obtained by mechanical milling for 35 hours with particle size about 50 nm (according to Dynamic Laser Scattering analysis) and copper micrometric powders (type SE Pometon) with particle size around 10μm (Fig. 1b) were used as raw materials in order to prepare the mixtures. Fig. 1. SEM images of the starting powders: a) tungsten nanopowders; b) copper powders Tungsten nanopowders are agglomerated and have irregular shape (Fig. 1a) comparative with copper powders (Fig. 1b) which have dendritic shape, corresponding to electrolytic process. Three types of mixtures with the following concentrations (% weight): 75W/25Cu, 70W/30Cu and 60W/40Cu were subjected to mechanical milling process for 20 hours in order to obtain composite powder. For mechanical milling (MM) process planetary ball mill Pulverisette 4 made by Fritsch was used. The milling parameters were: material of vials: stainless steel, vials volume: 250 ml, materials of balls: stainless steels, balls diameter: Φ=10 mm, balls to powder ratio: 2/1, Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 184 rotational speed of the main disk: 400 rpm, rotational speeds of the vials: -800 rpm, milling medium: argon, milling type: dry. The SEM images and element distribution maps obtained by EDX analysis show that inside of particles of all mixtures, the component elements are homogenous dispersed (Fig. 2). Fig. 2. SEM images and EDX analysis of the mixtures used for the research: a) 75W/Cu; b) 70W/Cu; c) 60W/Cu Elaboration of functional graded materials The mixtures obtained after MM process was used in order to obtain three layered functional graded materials (FGM) by spark plasma sintering (SPS) according to the flow chart presented in (Fig. 3). Fig. 3. Experimental flow chart A thin graphite paper was put into the inner part of the die for lubrication and other two papers were put on the bottom and upper of the part in order to prevent the powder to stick on the graphite punches. The samples were sintered at four temperatures (950, 850, 750 and 650 C) without dwell time and a pressure equal to 20 MPa using a SPS homemade system from Technical University of Cluj-Napoca. The heating rate was about 300 C/min. After SPS the density of the samples was measured by the Archimedes’ method. The samples were cut and polished in order to study the microstructural aspects and microhardness. The optical microscopy was made using an NIKON microscope with NIS ELEMENTS image software. SEM characterisation was performed using a JEOL JSM5600LV microscope equipped with EDX spectrometer (Oxford Instruments, INCA 200 software). The micro hardness was measured using a Namicon tester with a load of 9.8 Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 185 N and a dwell time of 15 seconds. The investigation of wear behavior was performed using a CSM Instruments tribometer TRB 01-2541 and a Taylor Hobson Surtronic 25+ profilometer. The parameters for wear testing were: type: ball on disk; load – 2N; testing method linear; amplitude 6mm; speed 10cm/s; distance 60 m; ball material – 100Cr6 and alumina; temperature 25 C. Results and discussion The relative density function of the SPS temperature is plotted in (Fig. 4). Fig. 4. Relative density function of the SPS temperature The highest value of the density is attained at 950 C and it is observed that it decrease with the decreasing of the sintering temperature. Fig. 5 presents optical microscopy images of the samples (not etched) at the four sintering temperatures. The microstructures of the samples are homogenous and present structural gradient. The dark grey colour corresponds to the layer 1 with 75%W and so on. The thickness of the middle layer increases with the decreasing of the SPS temperature. The lowest value of the thickness (1216.85 μm) was attained for the sample sintered at highest temperature 950 C. Higher density lead to lower thickness as expected. Fig. 5. Optical microscopy aspects of the three layered samples (75X): a) 950 C; b) 850 C; c) 750 C; d) 650 C The EDX line distribution on the three layers, (Fig. 6) shows the distribution of W and copper function the scanning distance. It is observed that the content of W decrease from left (first layer – 75%W) to the right (third layer 60%W). In the same time a slight increase in Cu is recorded, according the layer composition change. In (Fig. 7) are presented the interfaces of the three layers. Analysing (Fig. 6, 7) it is obvious that the graded structure was attained. The interfaces seems to assure continuity from one layer to another as it can be seen in (Fig. 7). The composition Powder Metallurgy and Advanced Materials – RoPM&AM 2017 Materials Research Forum LLC Materials Research Proceedings 8 (2018) 182-191 doi: http://dx.doi.org/10.21741/9781945291999-21 186 differences between the layers are small, probably due to the powder superficial mix at the interface. In (Fig. 8) is plotted the evolution of the microhardness. Fig. 6. SEM image (left) and EDX line distribution of W and Cu elements of the three layered W/Cu FGM obtained by SPS at 950 C Fig. 7. SEM images of the interfaces of W/Cu FGM obtained by SPS at 950 C: a) between layer 1 and layer 2; b) between layer 2 and layer 3. Fig. 8. Evolution of microhardness as a function of sintering temperature The measurements of the microhardness were performed in all the three layers, even if only the layer 1 and 3 are important from this point of view, because only these are in contact with other materials. The bonding between W and copper particles influences the microhardness of the layers. The highest value of the microhardness was attained in the layer 1 (75W/Cu) sintered at 950 C and it decreases with the decreasing of the W content. Powder Metallurgy and Advanced Ma
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