1. Introduction
Functionally graded materials (FGMs) in case of cera-mic-metal composites are materials where the composi-tion varies gradually or stepwise from one component to another one [1–4]. In recent years, it was observed the increasing interest in this type of materials due to their specifi c properties [5–6]. Functionally graded materials have been found to be essential for applications such as sensors, tools, turbine blades, actuators or pipes for trans-port of toxic medium [7–9]. Various fabrication techniques are applied for the preparation of FGMs composites [10]. So far in the Author’s team, research has been conducted on the production of functionally graded ceramic-metal composites from the Al2O3-Ni system by the centrifugal slip
casting method [11–13]. The technique based on a com-bination of classical slip casting in porous plaster moulds and a centrifugal force [12]. Since the microstructure of the obtained composites depends strongly on the slurry cha-racteristics, it is necessary to understand the rheological properties of slips.
This work primarily concerns the development and cha-racterization of a continuous functionally graded Al2O3-Mo
composite by the centrifugal slip casting. Molybdenum was chosen in this study to reinforce the alumina matrix because of its high melting point of 2623 °C and thermal expansion coeffi cient which is very close to the alumina one (αMo = 5.35∙10
–6 1/K, α
Al2O3 = 7.2∙10
–6 1/K).
Therefo-re, the thermal residual stresses arising from the thermal expansion mismatch are expected to be low. In literature, there are works showing the improvement of fracture to-ughness and wear behaviour due to the introduction of molybdenum into the alumina matrix [14–15].
In the present article, alumina and molybdenum powders were taken as the raw materials. Rheological properties of a slur-ry have been measured. The physical properties and microstruc-tures of the sintered composite sample has been analyzed.
2. Experiment
The starting materials were as follows: α-Al2O3 CT3000
LS SG powder with 99.8% purity and an average
partic-Abstract
Al2O3-Mo functionally graded material (FGMs) was fabricated successfully by a centrifugal slip casting method. An aqueous ceramic
slurry containing 50 vol.% of the solid phase and including 10 vol.% of the molybdenum particles was made. Consolidation was per-formed by using a centrifugal slip casting process at 3000 rpm for 1.5 h. The morphology and chemical composition of the produced composite were analysed using a scanning electron microscope equipped with an EDS detector. Moreover, the X-ray diffraction was made. The microstructural observation and the stereological analysis confi rmed that the molybdenum particles are distributed in the material in a gradient way.
Keywords: Al2O3-Mo, Composite, Centrifugal slip casting
FUNKCJONALNY MATERIAŁ GRADIENTOWY Al2O3-Mo
OTRZYMANY PRZEZ ODLEWANIE ODŚRODKOWE MAS LEJNYCH
Funkcjonalny materiał gradientowy z układu Al2O3-Mo zostały wytworzony metodą odlewania odśrodkowego mas lejnych. Wodną
za-wiesinę o zawartość fazy stałej 50% obj. przygotowano na bazie tlenku glinu i proszku molibdenu (10% obj.). Próbki formowano metodą odlewania odśrodkowego; parametry procesu były następujące: v - 3000 obr./min, czas - 1,5 godz. Charakterystyka mikrostruktury oraz składu chemicznego wytworzonego kompozytu została wykonana przy użyciu skaningowego mikroskopu elektronowego z detektorem EDS. Przeprowadzono analizę fazową (XRD) kompozytu. Obserwacje mikrostruktury oraz analiza stereologiczna potwierdziły gradien-towe rozmieszczenie cząstek molibdenu w wytworzonym kompozycie.
le size of 100 nm ± 20 nm, and Mo powder with 99.9% purity and an average particle size of 10.5 μm ± 3.2 μm. The particle size was measured by laser diffraction (Laser Particle Size Analyzer LA-960) conducted in a diluted well--dispersed suspension. The density of Al2O3 and Mo was
3.92 g/cm3 and 10.2 g/cm3, respectively. However, it can be
concluded from microscope observations that the ceramic powder was characterized by a tendency to create agglo-merates while the metallic one showed rather aggregated structure of sub-particles. Fig. 1 shows the microstructure of the used powders.
To prepare the ceramic slurry of appropriate rheological properties, diammonium hydrocitrate (DAC, POCh, Poland) and citric acid (CA, ≥ 99.5% Sigma-Aldrich, Poland) were used as dispersants in an amount of 0.3 wt.% and 0.1 wt.% with respect to a content of the powders, respectively. The selection of dispersants was made basing on previous experimental data (not shown).
The aqueous slurry containing 50 vol.% of the solid pha-se and including 10 vol.% of the molybdenum particles was prepared by adding the alumina and molybdenum powders to the water with the dispersants and milling the mixture in a planetary ball mill PM100 (Retsch) for 60 min with a speed of 300 rpm. Then the slurry has been degassed in a THINKY ARE-250 Mixer and Degassing Machine for 5 min with a speed of 1600 rpm. The equipment allows for the slurry to release bubbles > 1 μm. The slurry was then poured into a gypsum mould of 20 mm in inner diameter. Then, the tubular mould was centrifuged in the radial di-rection with a speed of 3000 rpm for 1.5 hours. After the centrifugation, the sample together with the gypsum mo-uld was removed from the metal momo-uld and was dried in a dryer at 30 °C for 48 hours. The dried sample could be easily removed from the gypsum mould thanks to drying shrinkage. Then, the sample was sintered at 1400 °C for 2 h in H2/N2 atmosphere (N2 of 80 vol.% and balance H2),
a heating rate was 5 °C/min. This procedure gave oppor-tunity to obtain samples in the shape of a hollow cylinder with a concentration gradient of the molybdenum particles.
Several methods were used to characterize properties of the slurry, and microstructure and properties of the fi nal composites.
The rheological properties of the slurry were measured by using a Rheometer (Anton Paar). The viscosity was me-asured as a function of shear rate, when increased from 0.1 s-1 to 260 s-1 and back to 0.1 s-1.
The sintered sample was characterized by X-ray dif-fraction. The XRD study was performed to indentify bulk crystalline phases of the materials. It was conducted using a Rigaku MiniFlex II diffractometer with CuKα radiation (λ= 1.54178 Å). The diffraction pattern was produced at a rate of 1 deg/min in the range 2θ = 20–100°.
Measurements of density, open porosity and micro-structure observations were performed. The density was measured by Archimedes’ method in distilled water. The theoretical density used to determine relative density was calculated from the rule of mixtures. For the microstructu-re observations, a middle part of the sample was chosen. The sample was then prepared by grinding and polishing to eliminate surface fl aws resulting from the cutting process. The microstructure of the sintered sample was observed in a scanning electron microscopy with an EDS detector, HITACHI SEM SU-70. An image analysis equipped with the Micrometer program [16] has been used for measurement of volume fraction of the Mo particles in the cross-section of the composite. The average equivalent diameter was calcu-lated from 500 particles measured in each zone. Moreover, hardness of the composite was measured by the Vickers method on the polished sample surface in three zones un-der a load of 9.8 N. For each measurement, a minimum of 10 indentations was made.
3. Results and discussion
Fig. 2a presents viscosity curves of the Al2O3-Mo slurry.
The slurry is shear the thinning fl uid. It was observed that the slurry has a maximal viscosity equal 35 Pa·s at a mini-mal shear rate of 1.65 s-1. It was found that when the shear
rate increases to 260 s-1 the viscosity decreases to about
0.59 Pa·s. Fig. 2b reveals that the slurry exhibits antithixo-tropic behaviour (rheopexy). It indicates that the molecular structure appears in the slurry what causes the increase of the viscosity. It is caused by the presence of metal particles in the alumina particle suspension and their electrostatic
a) b)
from the outer to inner part of the material. Microscopic ob-servation reveals that the composite consists of three zones with the gradually changed content of Mo particles at the cross-section. This EDS investigation confi rmed different contents of metal particles in each zone (Fig. 5). Note that the maximum volume fraction of the molybdenum particles was observed in the central part of the sample.
The distribution of the particles in the specimen is gra-ded. The Zone I was created as a result of capillary action from the mould. During pouring the slurry into the plaster mould, the liquid was pulled back very quickly before the centrifugal casting process commenced. After the ap-plication of the centrifugal force, molybdenum particles (characterized by higher density than alumina) moved and formed the structure of Zone II. The consolidation process ceased after the deposition of Zone III, mainly composed of alumina.
Fig. 6 shows molybdenum particle size distributions in the three characteristic zones of the composite fabricated by the centrifugal slip casting. The measurements showed that interactions. It was observed that with the minimum shear
rate of 1.65 s-1 a shear stress was 57.4 Pa while for the
maximum shear rate of 260 s-1 a shear stress was 1530 Pa.
The resulting slurry was characterized by high fl uidity. Be-cause of the interaction between the ceramic and metallic particles, no sedimentation of the molybdenum particles was observed before the centrifugation.
The physical properties of the Al2O3-Mo composite
are presented in Table 1. The formation of the compo-site by centrifugal slip casting gave a value of apparent density equal to 4.49 g/cm3 ± 0.03 g/cm3. As a result,
the measured relative density of the sintered composite was 98.76% ± 0.57%. Furthermore, the composite had linear shrinkage of 18.04% ± 1.25% (measured at the he-ight of the sintered sample). The open porosity of the sintered Al2O3-Mo composite was very low and equal to
0.26% ± 0.04%.
The typical microstructure of the Al2O3-Mo composite
produced by the centrifugal slip casting method is shown in Fig. 4. As can be seen, the amount of Mo particles changes
Fig. 3. X-ray diffraction pattern of the Mo-Al2O3 composite sintered
at 1400 °C derived from the slurry with the 50 vol.% content of solid phase.
Fig. 4. Electron micrograph of cross-section of the Al2O3-Mo
hol-low cylinder.
Table 1. Selected physical properties of the Al2O3-Mo composite.
Theoretical density [g/cm3] Apparent density [g/cm3] Relative density [%] Linear shrinkage [%] Open porosity [%] Al2O3-Mo 4.55 4.49 ± 0.03 98.76 ± 0.57 18.04 ± 1.25 0.26 ± 0.04
in Zone I the equivalent diameter was 11.25 μm ± 0.98 μm, while in Zone II it was equal to 13.90 μm ± 1.03 μm. The equivalent average diameter measured for the Mo particles in Zone II is a little bit greater than the diameter for the initial size of Mo particles. This is due to the fact of the presence of agglomerates of Mo particles in this zone. It was obse-rved that the equivalent average diameter for Mo particles in Zone III is equal 7.79 μm ± 0.85 μm. The difference in particle size distribution across the cross-section results from processes affecting the formation of each zone.
The results of hardness measurements in the three zo-nes are presented in the Table 2. Zone III was characterized by the highest hardness due to the lowest metallic phase content, while the lowest hardness values were measured for Zone II with the highest molybdenum particle content.
4. Conclusions
Al2O3-Mo composite were fabricated by the centrifugal
slip casting technique. No additional phases were pre-sent when sintering was carried out at 1400 °C for 2 h in a H2/N2 atmosphere controlled furnace. SEM micrographs
showed a gradient of Mo particles in the obtained compo-site. The maximum content of Mo particles was obtained in the central part of sintered material because of the centri-fugal force action.
Acknowledgement
This work was fi nancially supported by the Faculty of Material Science and Engineering Warsaw University of Technology (statute work).
Fig. 5. EDS elemental analysis of the sintered composite in areas indicated in the inserted SEM image.
Fig. 6. Molybdenum particle size distribution in zones I, II, and III of the Al2O3-Mo composite.
Table 2. Hardness of composite in each zone.
Zone I Zone II Zone III
plates rested on two-parameter elastic foundation, Mech.
Adv. Mat. Struct., 23, (2016), 43–58.
[5] Hirai, T.: Functional gradient materials, Mater. Sci. Tech., 17B, (1996), 293–341.
[6] Suresh, S., Mortensen, A.: Fundamentals of functionally
graded materials, processing and thermomechanical be-havior of graded metals and metal-ceramic composites,
London: IOM Communications Ltd, (1998).
[7] Neubrand, A., Rodel, J.: Graded Materials: An Overview of Novel Concept, Carl. Hanser Verlag, Munchem Z. Metallkd., 88, (1997), 358–369.
[8] Finot, M., Suresh, S., Bull, C., Sampath, S.: Curvature changes during thermal cycling of a compositionally graded Ni-Al2O3 multi-layered material, Mater. Sci. Eng. A, 205,
(1996), 59–71.
Structural Integrity Procedia, 1, (2016), 305–312.
[14] Wei W. Ch., Wang S. Ch., Cheng F. H.: Characterization of Al2O3 composites with fi ne Mo particulates, I.
Microstruc-tural development, Nanostruct. Mater., 10, (1998), 965–981. [15] de Portu, G., Guicciardi, S., Melandri, C., Monteverde, F.:
Wear behavior of Al2O3-Mo and Al2O3-Nb composites, Wear,
262, (2007), 1346–1352.
[16] Michalski, J., Wejrzanowski, T., Pielaszek, R., Konopka, K., Łojkowski, W., Kurzydłowski, K. J.: Application of image analysis for characterization of powders, Mater. Sci.
Po-land., 23, (2005), 79–86.
