CC BY
30
UDK 621.762
M. Ahmadi Daryakenari, A. Jafarzade, A. F. Dresvyannikov,
E. V. Petrova
SPARK PLASMA SINTERING OF ALUMINA: MICROHARDNESS AND TEMPERATURE DISTRIBUTION MODEL AS A FUNCTION OF POWDER PREPARATION METHOD
AND SINTERING MODE
Key words: spark plasma sintering, electrochemical synthesis, supercritical fluid, alumina, microhardness.
The spark plasma sintering (SPS) of two types of f-Al2O3 powders (600 nm, 50 nm) was investigated. The evolution of microhardness was studied after sintering of the specimens at temperatures ranging from 1300 to 1650 0C. The micro-hardness of specimens decreased slightly from 17.3 to 14.5 GPa for former powder (600 nm). The microhardness of the specimens increased for latter powder (50 nm) when sintering temperature rose from 1300 to 1400 0C because of increase in specimen density and then decreased in the range of1400-1650 0C and also there was grain growth. A finite element model for simulating the coupling of thermal, electrical behavior of the SPS process was developed using the commercially available ANSYS software. It allowed to obtain precise temperature distribution and investigate micro-hardness at different position of specimens.
Ключевые слова: искровое плазменное спекание, электрохимический синтез, сверхкритические флюиды, алюминия оксид,
микротвердость.
В условиях искрового плазменного спекания исследовали поведение образцов оксида алюминия, полученных электрохимическим и сверхкритическим флюидным методами. Микротвердость объемных образцов, полученных путем спекания в диапазоне температур 1300-1650оС, определяли по Виккерсу. Для описания SPS - процесса предложена математическая модель, основанная на использовании конечных элементов, которая применена для расчета значений микротвердости объемного образца в зависимости от распределения поля температур.
1. Introduction
Nowdays, Spark Plasma Sintering (SPS) is a modern technique that has been utilized for the densifica-tion of different kinds of materials. The SPS process is a pressure-assisted pulsed current sintering process where densification is highly promoted at lower temperatures coMPared to usual processes. This process generally leads to highly dense materials with control of grain structure. Spark plasma sintering (SPS) makes possible to sinter powders with next to-theoretical values of density and little grain growth (<1 ^m). This technique can work at heating rates of hundreds degrees per minute, getting high temperatures in a short time. In particular, SPS was used for the densification of Al2O3 specimens [1-5].
Alumina (Al2O3) as a multifunction compound has been found a wide range of applications in electronics, biomedicine, chemistry, optics, and production of materials with high refractory properties [6-8].
Besides SPS powder of alumina has mechanical properties that makes it differ from other materials sintered by various methods. D. Pravarthana et al. [9, 10] found that SPS alumina ceramic showed microhardness which was better than the alumina materials obtained by other sintering methods.
Recently, there are several efforts were taken to analyze the temperature distributions in SPS process. Zavaliangos et al. [11] applied the commercial package ABAQUS and performed a coupled electrical - thermal analysis of the process, paying special attention to the effect of contact resistances on the temperature gradient.
In this research, effect of different temperature of sintering on microhardness and temperature distribution in specimen were studied by ANSYS software. The Vickers indentation test is a general method used to characterize the microhardness of materials. These experiments are
easy to perform; need a small quantity of material, are commonly non-destructive and can be performed repeatedly.
2. Finite element modelling
For thermal-electric coupling complications, the electrical potential and temperature distributions are ran by the following system of simultaneous partial differential equations [12]:
V.J = 0 (1)
dT
vf +pcp & = qe (2)
Where J = aE is the current density, a is the electrical conductivity, and E is the electric field intensity defined as E = — Vcfs. where <(> is the electric potential. Also, f = —ÀVT is the heat flux density, T is the temperature, and A is the thermal conductivity, p is the density, cP is the specific heat capacity, and qe is the heat generated by the flowing current per unit volume and per unit time. Consistent with the Joule's law, qe=J.E = JE.
Substituting J = aE, f =-ÀVT, and qe = JE into Equations (1) and (2), the governing equations of the coupled system can be written as:
V. (-O-V0) = 0 (3)
dT
V.(-m)+pcp—= JE (4)
dt
A schematic representation of the SPS apparatus used in the simulation is shown in Fig 1.
Fig. 1 - Schematic of the spark plasma sintering (SPS) apparatus
The thermal and electrical properties of the graphite die, spacers and punches were all assumed to be spatially uniform and consequently, treated as an isotopic material in the model. In addition, the sintering samples, which were composed of alumina, were also modeled as an iso-topic and fully dense solid. It should be noted that any changes to the listed properties occurring during the densi-fication process were not considered in the model in this paper. Hence, this model only simulates their thermal and electrical behaviors at the final stage of SPS sintering (fully dense powder).
The initial and boundary conditions are:
- the initial temperature of the entire system was
- the process took place in vacuum so heat losses by conduction or convection through the gas were neglected. However, all of the lateral surfaces had heat losses by radiation towards the chamber walls, which are held at room temperature;
- the temperature of the two extreme upper and lower spacer surfaces was also fixed at (boundary condition);
- the current flux can be considered as constant current ( ).
Thermal and electrical contact resistances were also neglected.
3. Experimental procedure
3.1 Preparation of specimens
Two types of y-Al2O3 powders are used. The first powder has an average particle size of 0.6 ^m which is synthesized by supercritical method (99.99%, RIKOM Co, Saint Petersburg) and the second powder has an average particle size 50 nm which is synthesized by electrochemical method [13].
The powder was sintered by SPS (Thermal Technology LLA., USA). A graphitic sheet was placed between the punches and the powder, and between the die and the powder for trouble-free removal. Sintering was performed in vacuum (residual cell pressure <0.03 torr.). An optical pyrometer was focused on a small hole at the surface of the die to measure the temperature.
For all sintering, heating rate of 200 0C min-1 was used from room temperature to the desired temperature. The cooling rate was fixed at 100 0C min-1 for all samples. The uniaxial pressure was released during cooling for all
samples. Samples were sintered in the temperature interval; 1300°-1650 °C by step 50 0C.
The holding time at dwelling temperature was set to 5 min. Also samples were prepared at applied pressure of 60 MPa and temperature of pressure application of 20 0C. Fig.2 and Fig.3 present the cycles of the sintering temperature and pressure depending on sintering time.
Fig. 2 - Pressure cycles during SPS sintering
Fig. 3 - Temperature cycles during SPS sintering
3.2 Characterization
After sintering, the microhardness of the materials was studied. Before the microhardness measurements, the specimens were carefully polished by standard diamond polishing techniques, which allows us to use a diamond particle size of 1 ^m by Buehler machine. The microhardness (HV) at room temperature was evaluated by the Vickers indentation technique at a load of 4/903 N and time of 10s according to Jean-Marc Schneider et al [14]. Temperature distribution in the specimens in order to characterize microhardness at different positions of specimens was obtained by ANSYS 12 software. The phase composition was identified by X-ray diffractometry (XRD) using Co Ka radiation (D2 Phaser, Bruker, Germany). The microstructure of the surface was observed using a scanning electron microscope (SEM) (Evex Mini- SEM, USA).
4. Result and discussion 4.1 Phase composition microstructure
and
The XRD patterns of the initial powders are shown in Fig. 4 and 5.
Fig. 4 - XRD pattern of AI2O3 powder obtained by supercritical method
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Fig. 5 - XRD pattern of Al2O3 powder obtained by electrochemical method
In specimens obtained by supercritical method , the diffraction peaks correspond the □ phase of Al2O3. There are diffraction peaks of □, n and 5 phases for the specimens of aluminium oxide obtained by electrochemical method. The XRD patterns of specimens with SPS different condition and different powders are shown in Fig 6, 7.
Fig. 6 - XRD patterns of specimens which sintered at different temperatures (1250, 1300 and 1350 0C) with the powder synthesized by supercritical method
In this specimens, all of phases of initial powders are transformed to a phase.
The SEM image of the fracture surface of the specimen which was sintered at 1300 0C, 60 MPa, 5 min with the powder synthesized by electrochemical method, is shown in Fig 8.
Fig. 7 - XRD patterns of specimens which sintered at different temperatures (1250, 1300 and 13500C) with the powder synthesized by electrochemical method
Fig. 8 - SEM micrograph of Al2O3 specimen which sintered at 1300 0C, 60 MPa, 5 min with the powder synthesized by electrochemical method
Here the grain size is in the range of 1 to 2 ^m. In addition, there is a few voids. Therefore, it is necessary to increase temperature in order to destroy voids and increase mechanical properties. Fig 9 presents the fracture surface of the specimen which sintered at 1300 0C, 60 MPa, 5 min with the powder synthesized by supercritical method. Here the grain size is the range of 2 to 3 ^m and there are no voids.
Fig. 9 - SEM micrograph of Al2O3 specimen which sintered at 1300 0C, 60 MPa, 5 min with the powder synthesized by supercritical method
4.2 Effect of preparation method and sintering condition on microhardness of bulk specimens
Fig. 10 shows the effect of sintering temperature on the Vickers hardness (HV) of different powders.
Fig. 10 - Effect of sintering temperature on microhard-ness
In supercritical method, the microhardness of the specimen firstly increased in the range of 1200-1300 0C because of increasing density of specimen and then decreased in the range of 1300-1650 0C and in this range the density of specimens is almost equal to theoretical value and then the Vickers hardness of specimens was slightly decreased from 17.3 to 14.5 GPa with increasing sintering temperature from 1300 to 1650 C0 because of increasing of grain size.
The higher the sintering temperature is, the bigger the size of grains is. The less the interface of grains is, and the less the resistance of dislocation motion is and so Mi-crohardness specimens was decreased from 1300 to 1650 C0.
In electrochemical method, the microhardness of the specimen firstly increased in the range of 1300-1400°C because of increasing density of specimen and then decreased in the range of 1400-1650 0C because of grain growth. There are two main factors that affect the micro-hardness of the specimen, which are, density of specimen and the grain size.
4.3 Effect of temperature distribution on microhardness
Contour plots of steady-state temperature distribution in the region of alumina specimens for sintering temperatures 14000C, 15000C and 16000C are shown in Fig 11.
The temperature difference curves along radius of specimen are shown in Fig 12.
At the sintering temperature of 1400 0C, temperature was increased from 1672 K (edge of specimen) to 1737 K (center of specimen). Also at the sintering of 1500 0C and 1600 0C, temperature was increased from 1779 and 1889K (edge of specimens) to 1861 and 1992 K (center of specimen) respectively. The microhardness difference curve along radius of specimen is shown in Fig 13.
Fig. 11 - Steady-state temperature distribution in the region of the specimen for sintering temperature a) 14000C, 6) 15000C, b) 16000C
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Fig. 12 - Temperature difference curve along radios of specimen for sintering temperature a) 14000C, 6) 15000C, b) 16000C
At the sintering of temperature 1400 0C for supercritical and electrochemical powder, microhardness was respectively decreased from 1584 and 1649 kg -force/mm2 (9 cm from center) to 1427 and 1513 (1cm of center) kg- force/mm2 because of grain growth. The same behavior can be seen at the sintering of temperature 1500 0C and 1600 0C.
5. Conclusion
The spark plasma sintering of two types of y-Al2O3 powders (600 nm, 50 nm) obtained by various methods, were investigated. In the first case, Vickers microhardness of the samples slightly decreased from 17.3 to 14.5 GPa. In the latter case, Vickers microhardness of the SPS samples firstly increased then decreased with the increase of sintering temperature reaching the maximum microhardness, at which Vickers microhardness is 19.7 GPa. Temperature distribution in the specimens was studied using ANSYS 12 software. At certain sintering temperature, temperature from edge of
specimen to its center increases and temperature in center of specimen is greater by dozen of degrees than the sintering temperature. Microhardness from edge of the specimen to the center of the specimen decreases because of increasing temperature that generated the bigger size of grain and decreased microhardness.
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© M. Ahmadi Daryakenari - PHD student of the Department of Analytical Chemistry, certification and quality management KNRTU, dariakenari@gmail.com; A. Jafarzade - PHD student of KNRTU-KAI; A. F. Dresvyannikov - Doctor of chemical science, Professor of the Department of Analytical Chemistry, certification and quality management KNRTU, alfedr@kstu.ru; E. V. Petrova -candidate of chemical science, docent of the Department of Analytical Chemistry, certification and quality management KNRTU.
© M. Ahmadi Daryakenari - асп. каф. аналитической химии, сертификации и менеджмента качества КНИТУ, dariakenari@gmail.com; A. Jafarzade - асп. КНИТУ-КАИ; А. Ф. Дресвянников - д-р хим. наук, проф. каф. аналитической химии, сертификации и менеджмента качества КНИТУ, alfedr@kstu.ru; Е. В. Петрова - канд. хим. наук, доц. той же кафедры.
