www.ptcer.pl/mccm
P
AWEŁR
UTKOWSKI1*, P
IOTRK
LIMCZYK2, L
UCYNAJ
AWORSKA2, L
UDOSŁAWS
TOBIERSKI1,
D
ARIUSZZ
IENTARA1, K
AROLINAT
RAN11AGH University of Science and Technology, Faculty of Material Science and Ceramics, al. A. Mickiewicza 30, 30-059
Kraków, Poland
2Institute of Advanced Manufacturing Technology, ul. Wrocławska 37A, 30-011 Kraków, Poland
*e-mail: Pawel.Rutkowski@agh.edu.pl
1. Introduction
Ceramic materials occupy a stable position on the market in terms of the range of species and scope of applications. For example, ceramic cutting tools are frequently used in the machining of diffi cult-to-cut materials. The majority of companies utilize ceramic tools in the form of mechanically fastened indexable inserts.
Ceramic cutting tools are constructed mainly from alumi-na (Al2O3) and silicon nitride (Si3N4, SiAlON) based materials
[1]. Alumina has high hardness, good abrasion resistance and chemical stability even at elevated temperatures. Howe-ver, fracture toughness of those materials is low because the dislocation movement is extremely limited by their ionic and/ or covalent bonds. Brittleness and poor damage tolerance have limited so far their application as advanced engineering materials especially for cutting tools [2] .
Graphene shows very good electrical, thermal and mechanical properties [3, 4]. This phase can be added to ceramic particles to obtain sintered composite tool materials with improved thermal and electrical conductivity as well as
Mechanical properties of pressure sintered
alumina-graphene composites
Streszczenie
The work concerns the alumina-graphene materials sintered by two different pressure methods. The preparation route of the matrix--graphene mixture was discussed in the paper. The prepared compositions with different amount of graphene were hot-pressed or spark plasma sintered. The infl uence on uniaxial pressure during the sintering process on the microstructure was studied by the SEM microstruc-tural observations and ultrasonic measurements. The work presents also the results of measurements of elastic and mechanical properties. Wear tests were made and friction coeffi cients determined by the ball-on-disk method.
Keywords: Alumina, Graphene, Hot pressing, Spark plasma sintering, Mechanical properties
WŁAŚCIWOŚCI MECHANICZNE KOMPOZYTÓW KORUND-GRAFEN SPIEKANYCH CIŚNIENIOWO
Praca dotyczy materiałów korund-grafen spiekanych za pomocą dwóch metod ciśnieniowych. Dyskusji poddano metodę przygotowa-nia mieszaniny złożonej z proszku osnowy i grafenu. Mieszaniny o różnej zawartości grafenu prasowano na gorąco lub konsolidowano w warunkach spiekania wspomaganego polem elektrycznym. Wpływ jednoosiowego prasowania podczas spiekania na mikrostrukturę zbadano, stosując obserwacje mikrosrukturalne SEM i pomiary ultradźwiękowe. Praca zawiera również wyniki pomiarów właściwości sprężystych i mechanicznych. Test odporności na zużycie i pomiary współczynników tarcia wykonano za pomocą metody kula na dysku.
Słowa kluczowe: korund, grafen, prasowanie na gorąco, spiekanie wspomagane polem elektrycznym, właściwości mechaniczne
with better mechanical properties, especially with higher fracture toughness in comparison to currently used compo-sites. Some papers indicate that a small addition of graphene platelets in to the ceramic matrix can signifi cantly improve the fracture toughens due to changing the mechanism of crack propagation [5].
In the case of ceramic materials, graphene particles are used to improve the mechanical properties of such well known materials as alumina, silicon nitride and silicon carbide [6-20].The typical methods of the ceramic-graphene composite manufacturing are hot-pressing and spark plasma sintering [6-20]. As for the case of thermal properties, a higher content of graphene additive can lead to an anisotropy of thermal properties. Effects of uniaxial pressure applied during the hot-pressing process of well packed groups of oriented fl akes lead to an improvement of thermal conductivity in the direction perpendicular to the pressing axis, so in the direction of oriented graphene particles’ planes. This is observed in silicon nitride-graphene composites [20], where the intro-duction of the oriented graphene phase leads additionally to a strong anisotropy of thermal conductivity of the sintered
bodies. A similar situation has been recorded in case of the thermal anisotropy of alumina-graphene composites [16].
In the presented work two grades of graphene platelets and two pressure assisted methods have been used to prepare alumina based composites with a wide composition range. The graphene was incorporated the alumina matrix in quantity up to 10 wt.%. The infl uence of graphene content in the materials, which can modify thermal properties [21] on their mechanical properties, was investigated. The properties such as hardness, fracture toughness, bending strength, wear resistance, and friction coeffi cient are presented in the article.
2. Preparation and examination route
Al2O3-graphene composites were prepared using
com-mercial powders listed in Table 1. The graphene addition to the aluminium oxide matrix was set at a range of 0-10 wt.%.
2.1. Preparation of hot-pressed composites
In the case of hot-pressing, two kinds of mixtures were prepared: (i) alumina (T
aimicron TM-DAR)
with 0-10 wt.% of 8 nm graphene fl akes (Gn(8)), and (ii) alumina (Taimicron TM-DAR) with 0-2 wt.% of 4 nm graphene (Gn(4)). The pow-der mixtures were homogenized for 10 h in propanol, using a rotary-vibratory mill and alumina grinding media. Dried and granulated powders were hot-pressed for 1 h at 1400 °C under 25 MPa in the argon fl ow, using a HP50-7010G hot press (Thermal Technology LLC). A heating rate was 10 °C/min. Sintered bodies with a diameter of 50 mm were obtained.2.2. Preparation of spark plasma sintered
composites
In the case of the SPS method, the alumina powder (A16SG, Almatis) with an addition of 0.3 wt.% of MgO na-nopowder (Inframat) has been used as a starting material. The multilayer graphene nanoparticles (Gn(4); Table 1) were used as a dispersed phase for the alumina ceramic matrix composites.
The mixtures were prepared using a Fritsch Pulverisette 6 planetary mill equipped with a grinding vessel and balls made from ZrO2. The powders were milled for 8 h in isopropanol
with a rotation speed of 200 rpm. The mixtures were dried and sieved through 0.5 mm mesh.
The composites were spark plasma sintered using a HP D5 machine made by FCT-System GmbH in a temperature range from 1250 °C to 1750 °C for 10 min, applying 35 MPa of uniaxial pressure during the whole cycle. The sintered specimens were disk-shaped with dimensions of 20 mm in diameter and ~5 mm in thickness.
2.3. Testing methods
The apparent density of the sintered samples was me-asured by the Archimedes method. The phase composition of the composites was determined quantitatively by using X-ray diffraction and Rietveld refi nement. Raman spectroscopy (Horriba Yvon Jobin LabRAM HR) was used for identifi cation of the graphene phase. FTIR spectra were recorded with a Bruker Vertex 70vt spectrometer. Spectra were collected in the mid infrared regions (4000-100 cm-1) after 256 scans at an
instrument resolution of 4 cm-1. The samples were prepared
using the KBr pellet method. The microstructural features of the composites were studied by using the SEM technique (Nova NanoSEM - FEI). Both cross-section surfaces and fractures were observed.
The anisotropy of longitudinal ultrasonic wave velocity and Young’s modulus were determined for alumina and alumina-graphene composites by the ultrasonic method with the use of 4 MHz ultrasonic heads. Two samples of each material and four series of measurements were used. The standard deviation was calculated. The fl exural strength was determined by the three-point bending test using beam specimens, a support span of 20 mm, and a traverse speed rate of 1 mm/min. Only for the hot-pressed materials, the critical stress intensity factor, KIc, was calculated, basing
on the Evans’ formula, using the results of the three-point bending of notched beams at a distance of 20 mm between supports and a feed of 0.1 mm/min. Additionally for the hot--pressed and SPS’ed materials, the fracture toughness was measured on the way of Vickers indentation at a 1 kgf load. The “ball-on-disk” method was applied to measure the fric-tion coeffi cient and the abrasion wear of materials by using a UMT-2MT tribotester of CETR equipped with an alumina ball of 3.2 mm in diameter, and a load of 10 N.
Table 1. Characteristics of powders used for preparation of alumina-graphene composites.
Powder ID Supplier Characteristics
Submicron alumina Taimicron TM-DAR Taimei CHEMICALS CO., LTD Average particle size 0.1 μm
Submicron alumina grade A16SG Almatis, Germany Particle size 0.3-0.6 μm
Graphene Gn(8) Graphene Loboratories, Inc, USA
Colour black, purity 99,9%
Average fl ake thickness 8 nm (20-30 monolayers) Average particle (lateral) size ~0.5 μm (0.15-3.0 μm)
Specifi c surface area 100 m2∙g-1
Graphene Gn(4) Cheap Tubes, USA
Colour black, purity 99% Average fl ake thickness < 4 nm Average particle (lateral) size 1-2 μm
3. Results and discussion
3.1. Densifi cation
The results of density measurements of the hot pressed and spark plasma sintered alumina-graphene composites are presented in Table 2.
An analysis of the density results shows that the addi-tion of up to 4 wt.% of 8 nm graphene allows us to obtain composites with the relative density higher than 98% by the use of the hot-pressing method. The higher additions result in a decrease in the densifi cation of sintered bodies. The densifi cation measurements of the spark plasma sintered composites showed the densities higher than 97.8% of the-oretical density for up to 10 wt.% of 4 nm graphene.
3.2. Phase structural and microstructural
analysis
Raman spectra of the hot-pressed two-phase polycry-stals shown in Fig. 1 confi rm the presence of undestroyed graphene structures, coinciding with the reference graphene powder. The wave numbers of pure graphene powder are 3243 cm-1, 2727 cm-1, 1578 cm-1 and 1349 cm-1. The same
analysis was made for the spark plasma sintered materials, which also confi rmed the graphene existence in the unde-stroyed form.
The e xamples of SEM images for the hot-pressed ma-terials with the various graphene content are shown in Figs. 2-4. The microscopic observations indicate the orientation of graphene fl akes and groups of fl akes that are perpendicular with respect to the axis of applied pressure during the hot--pressing process. Such a texture is especially well visible in the case of 8 wt.% and 10 wt % of graphene contents in Figs. 3 and 4. The unidirectional orientation of the Gn(8) particles should be the reason of mechanical property anisotropy, what was confi rmed by measurements of longitudinal ultra-sonic wave velocity in different directions of the hot-pressed alumina-graphene Gn(8) composites. The anisotropy of the
ultrasonic wave velocity exceeded 30% for the sintered body containing 10 wt.% of graphene (Fig. 5).
The microstructural observations of the spark plasma sintered materials with the 4 nm graphene particles are presented in Figs. 6-8. Neither the polished surface images nor fractures show the oriented graphene particles. The lar-ge agglomerates of graphene fl akes are visible in the SEM images. Also The graphene orientation in SPS’ed materials was not recorded also on fractures at high graphene particle contents (Fig. 8).
3.3. Mechanical properties
The hardness measurements were performed in two different directions with respect to the hot pressing axis (perpendicular and parallel) because of the high anisotropy confi rmed by the microstructural observations and ultrasonic tests. The results of Vickers and Knoop hardness measure-ments at a load of 1 kgf for the hot pressed alumina-graphene composites are collected in Table 3.
Table 2. Densifi cation of alumina-graphene composites versus manufacturing conditions and graphene content.
Graphene addition and type Pressing method Apparent density [g∙cm-3] Relative density [%] 0 wt.% Gn(8) HP 3.98 100 0.5 wt.% Gn(8) HP 3.95 99.6 1 wt.% Gn(8) HP 3.93 99.5 2 wt.% Gn(8) HP 3.89 99.2 4 wt.% Gn(8) HP 3.79 98.1 6 wt.% Gn(8) HP 3.69 96.9 8 wt.% Gn(8) HP 3.62 96.5 10 wt.% Gn(8) HP 3.52 95.2 0 wt.% Gn(4) SPS 3.90 98.0 0.5 wt.% Gn(4) SPS 3.91 98.5 1 wt.% Gn(4) SPS 3.90 98.7 2 wt.% Gn(4) SPS 3.83 97.7 10 wt.% Gn(4) SPS 3.62 97.8
Fig. 1. Raman spectra of hot-pressed alumina-graphene composites; the content of Gn(8) indicated.
a) b)
Fig. 2. SEM images of hot-pressed composite Al2O3 - 4 wt.% graphene Gn(8): a) polished surface, b) fracture.
a) b)
Fig. 3. SEM images of hot-pressed composite Al2O3 - 8 wt.% graphene Gn(8): a) polished surface, b) fracture.
a) b)
The results show that the hot-pressed composites con-taining up to 2 wt.% of graphene have the hardness in the range of 12-14 GPa and 15-18 GPa for Knoop and Vickers indentation, respectively. The higher additives of graphene particles decreased dramatically the values of hardness in both measurement directions. The difference in hardness values between the measurement directions is about 1 GPa
what is a result of the graphene fl akes orientation caused by the applied uniaxial pressing.
The data concerning the measurements of bending strength and critical stress intensity factor are presented in Table 4. The measurements allowed calculating only the values of elastic modulus, E, for the composites with low concentration of graphene. At the higher contents of gra-phene, the very high anisotropy did not allow to calculate the values of Young’s modulus. The data of Table 4 show that the composites with up to 2 wt.% of graphene have the mechanical properties at the level of pure polycrystalline alumina. Only the data obtained from the bending test indi-cate a slight improvement of fracture toughness in the case of 2 wt.% content of graphene particles. The higher addition of graphene fl akes leads to a strong decrease in mechanical properties what can be connected to the increased porosity, week interface, and agglomerates of graphene. The orienta-tion of graphene particles results in the difference between fracture toughness values measured in different directions by the Vickers indentation method.
In order to understand the negative infl uence of graphene addition on mechanical properties of the studied composites, the crack propagation paths were observed using SEM, and the results are shown in Fig. 9. For the small quantities of
Fig. 5. Anisotropy of ultrasonic wave velocity as a function of gra-phene content in hot-pressed alumina-gragra-phene Gn(8) composites.
a) b)
Fig. 6. SEM images of polished surfaces of spark plasma sintered composiue Al2O3 - 2 wt.% graphene Gn(4): a) general view, b) micro-structural details.
a) b)
Fig. 7. SEM images of polished surfaces of spark plasma sintered composite Al2O3 - 10 wt.% graphene Gn(4): a) general view, b) micro-structural details.
Table 3. Hardness of hot-pressed Al2O3-graphene composites (HK and HV stands for Knoop and Vickers indentation, respectively). Graphene nanoparticle content [wt.%] HK1 Parallel [GPa] HK1 Perpendicular [GPa] HV1 Parallel [GPa] HV1 Perpendicular [GPa] 0 13.2 ± 0.3 14.2 ± 0.2 17.7 ± 1.0 16.7 ± 0.9 0.5 12.8 ± 0.3 14.0 ± 0.3 17.9 ± 0.8 16.9 ± 0.3 1 12.2 ± 0.2 13.2 ± 0.3 16.4 ± 1.3 15.9 ± 0.5 2 11.5 ± 0.3 12.9 ± 0.2 15.2 ± 0.8 14.9 ± 0.6 4 9.4 ± 0.5 9.6 ± 0.3 12.3 ± 1.0 12.4 ± 0.6 6 7.7 ± 0.4 7.4 ± 0.5 9.3 ± 1.1 10.0 ± 0.8 8 4.5 ± 1.2 7.6 ± 0.4 7.0 ± 0.3 8.1 ± 0.7 10 4.4 ± 0.5 6.2 ± 0.4 5.5 ± 0.4 6.1 ± 0.3
Table 4. Mechanical properties of the hot-pressed Al2O3-graphene Gn(8) composites.
Graphene nanoparticle content [wt.%] Bending strength [MPa] KIc at 1kgf [MPa∙m0.5] (Vickers) Parallel KIc at 1kgf [MPa∙m0.5] (Vickers) Perpendicular KIc [MPa∙m0.5] (notched beam bending) E [GPa] 0 526 ± 30 3.8 ± 0.3 3.7 ± 0.2 4.6 ± 0.8 399 ± 7 0.5 533 ± 37 3.7 ± 0.2 3.4 ± 0.2 4.7 ± 0.3 386 ± 8 1 501 ± 71 3.6 ± 0.2 3.5 ± 0.2 5.3 ± 0.4 371 ± 4 2 473 ± 29 3.4 ± 0.2 3.6 ± 0.2 5.4 ± 0.8 ----4 427 ± 22 2.9 ± 0.1 3.6 ± 0.4 4.8 ± 0.1 ----6 328 ± 18 3.0 ± 0.3 3.4 ± 0.3 4.8 ± 0.6 ----8 246 ± 17 2.3 ± 0.1 2.9 ± 0.3 4.2 ± 0.2 ----10 193 ± 5 2.0 ± 0.2 2.6 ± 0.3 3.4 ± 0.2
----graphene, the crack propagates mostly between the alumina grains (Fig. 9b). When the content of graphene increases the amount of crack propagation along the weak graphene/ alumina interface increases, what is a cause of mechanical properties drop. The chipping of the material is visible at the highest graphene content (Fig. 9d).
The mechanical properties of spark plasma sintered com-posites are collected in Table. 5. Likewise to the hot-pressed materials, the elastic modulus, hardness and critical stress intensity factor stay at a high level for less than 2 wt.% of graphene contents. Large quantities such as 10 wt.% make a twice decrease in these properties. For SPS’ed composites there is almost no difference between hardness and fracture toughness measured in different directions. It confi rms a lack of the graphene orientation in the materials. A slight increase
in the fracture toughness is visible only in the case of 0.5 wt.% graphene addition.
The value level of mechanical properties collected in Ta-ble 5 is obtained for materials sintered at least at 1400 °C or 1350 °C. This is confi rmed by the ultrasonic measurements, where the value of elastic modulus becomes steady at the temperatures above 1400 °C (Fig. 10). The maximal density of composites is for the sintering temperature of 1400 °C.
3.4. Wear properties
The wear and friction coeffi cients of the hot-pressed alu-mina-graphene Gn(8) composites, measured by the ball-on--disk method, are presented in Table 6. The sintered bodies with up to 2 wt.% of dispersed Gn(8) particles show good
Table 5. Mechanical properties of spark plasma sintered Al2O3-graphene Gn(4) composites.
Graphene nanoparticle content [% wt.] E [GPa] HV1 [GPa] KIc at 1 kgf [MPa∙m0.5] (Vickers)
Parallel Perpendicular Parallel Perpendicular
0 381 ± 8 --- 18 ± 0.3 --- 5.6 ± 0.2
0.5 377 ± 7 17.0 ± 0.6 16.5 ± 0.5 5.9 ± 0.3 5.8 ± 0.3
1 342 ± 4 --- 16.6 ± 0.9 --- 5.4 ± 0.3
2 307 ± 6 14.8 ± 0.1 14.7 ± 0.5 5.2 ± 0.2 5.1 ± 0.2
a) b)
a) b)
Fig. 9. Crack propagation paths in hot-pressed alumina and alumina graphene composites: a) alumina, b) 2 wt.% Gn(8), c) 6 wt.% Gn(8), and d) 10 wt.% Gn(8).
Fig. 10. Young’s modulus and density versus spark plasma sintering temperature for alumina-graphene composite with 1 wt.% Gn(4).
wear resistance, but a higher friction coeffi cient than that of the alumina reference sample. For the higher quantities of graphene fl akes, the wear increases dramatically as a result
of the higher porosity, existence of graphene clusters and weak alumina/graphene interfaces. On the other hand, the higher concentration of graphene delivered a decrease in
Fig. 8. SEM observation of fracture surface of spark plasma sintered composite Al2O3 - 10 wt.% graphene Gn(4).
friction coeffi cient. The thermal stresses generated during the material manufacturing can be a reason of the incre-ased material wear in case of the spark plasma sintered composite, when compared to the hot-pressed one with the same graphene content. The reversed situation is of the high graphene content. Wear of the SPS originated material is lower than the HP manufactured one, when 10 wt.% of graphene is added. The lower density obtained for the hot pressed materials is responsible for the higher material wear. For the low graphene concentration, the friction coeffi cient of the spark plasma sintered materials is similar to the hot--pressed ones.
4. Conclusions
The maximal graphene content is 2 wt.% which gives a material with good mechanical and wear properties. The higher concentration of graphene leads to a signifi cant de-crease in elastic and mechanical properties.
Only small quantities of graphene can lead to a slight increase in fracture toughness.
The orientation of graphene, perpendicular to the pres-sing axis, is only obtained for hot-pressed materials. No graphene orientation was recorded for spark plasma sintered composites.
The friction coeffi cient of alumina-graphene materials is similar or higher than that of pure alumina in the case of low graphene contents. For higher concentrations of graphene particles, the friction coeffi cient decreases, and higher wear is detected.
Acknowledgement
The study constitutes a part of the project no. GRAF--TECH/NCBR/03/05/2012 “Ceramic-graphene composites for cutting tools and devices parts with unique properties”.
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Otrzymano 3 lutego 2016, zaakceptowano 6 kwietnia 2016. Table 6. Wear properties of alumina-graphene composites.
Graphene addition and type
Pressing method Wear [mm3/(N∙m)] Friction coeffi cient 0 wt.% HP 0.25 ± 0.06 0.55 ± 0.06 0.5 wt.% Gn(8) HP 0.84 ± 0.09 0.72 ± 0.07 1 wt.% Gn(8) HP 0.17 ± 0.04 0.64 ± 0.05 2 wt.% Gn(8) HP 0.58 ± 0.09 0.78 ± 0.09 4 wt.% Gn(8) HP 10.77 ± 4.08 0.47 ± 0.08 6 wt.% Gn(8) HP 24.85 ± 4.71 0.56 ± 0.07 8 wt.% Gn(8) HP 8.24 ± 2.55 0.38 ± 0.07 10 wt.% Gn(8) HP 22.54 ± 4.77 0.38 ± 0.07 0 wt.% SPS 1,15 ± 0,37 0.58 ± 0.02 0.5 wt.% Gn(4) SPS 2,39 ± 0,39 0.64 ± 0.05 1 wt.% Gn(4) SPS 3.71 ± 0.97 0.51 ± 0.03 2 wt.% Gn(4) SPS 5.20 ± 1.01 0.63 ± 0.02
