Controlled microstructure and mechanical properties of Al2O3-based nanocarbon composites fabricated by electrostatic assembly method
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This work reports on the microstructure-controlled formation of interconnected carbon-layered Al2O3 ceramics using carbon nanoparticles (CNP)-alumina (Al2O3) composite particles. The Al2O3 micro-particles used in this study were obtained by granulation of nano-sized Al2O3 nanoparticles with an average diameter of 150 nm. Then, CNP-Al2O3 composite was fabricated using an electrostatic assembly method using the granulated Al2O3 and CNP. The decoration of CNP on the surface of granulated Al2O3 was investigated as a function of primary particle size and coverage percentage using a fixed amount of CNP. Notably, an interconnected layer of carbon particles at the interface of Al2O3 that resemble the grain boundaries was obtained. The mechanical properties of the samples obtained with different particle size and CNP coverage on Al2O3 particles were also investigated which presented the possibility to control the mechanical properties through microstructural design of composite ceramic materials.
KeywordsNanocomposite Electrostatic adsorption Carbon microsphere Alumina mechanical property
Scanning electron microscope
It is well known that alumina (Al2O3) possesses good properties such as high hardness, excellent wear resistance, and high chemical stability. On the other hand, the drawbacks of alumina are its poor fracture toughness, low strength at elevated temperature as well as poor thermal shock resistance . This has prompted intense research in alumina-based nanocomposite development at micro- and nano-scales. Functional ceramic composites with well-dispersed nano-size particles in the ceramic matrix are reported to improve not only mechanical properties such as failure strength, fracture toughness, fatigue, and wear resistance but also the electrical, magnetic, thermal, and optical properties [2, 3, 4, 5, 6, 7]. In order to improve and control the mechanical properties of ceramics, microstructural porosity [8, 9], incorporation of additive fillers , and heat-treatment profiles [11, 12] have been used and reported. This shows that by controlling the microstructure of Al2O3, the desired mechanical properties of Al2O3 ceramics could be obtained. However, most reported work merely used the simple method of Al2O3 powders mixing prior to sintering which is insufficient to obtain good control on the microstructure and design of Al2O3 ceramics resulting in poor controllability of its mechanical properties. In the formation of nanocomposite by a conventional mixing method, it remains a huge challenge to obtain a homogenous decoration of nano-sized additive particles onto a designated primary particle due to the additive particles agglomeration. The uneven distribution caused by the agglomeration would then lead to adverse effects on the microstructural design as well as the properties of a ceramic composite. Therefore, a novel method via bottom-up assembly using an electrostatic adsorption method was used in this study to demonstrate the feasibility to obtain a good microstructural control and design that consequently allow controlled desired properties to be introduced into Al2O3 ceramics such as optical, electrical, and mechanical properties. As one of the possible additives for Al2O3 ceramic composites, various shapes of nano-sized carbon materials such as fiber (carbon nanotube (CNT), nanofiber) and plate-like (graphene) as well as particle have been developed. This enables the application of carbon-based materials as an additive for materials fabrication which has been reported recently.
In the development of carbon-based alumina composite, Kumari et al. reported the enhancement of thermal conductivity of carbon nanotube (CNT)-alumina composite up from 60 to 318% compared to pure alumina by changing the weight percentage of CNT addition and sintering temperature . Besides that, owing to the exceptional tribological properties of carbon-based composite materials for applications such as power generation, transportation, and manufacturing, many researchers have focused their interest into the development of carbon-based composites [13, 14]. Ceramics with carbon reinforced surfaces have been reported to exhibit improved wear resistance and a reduced friction coefficient. Despite controversial reports on mechanical strength enhancement using carbon nanofiber (CNF) on alumina and zirconia, most authors have reported improvement in the mechanical properties. A recent study of CNT on the creep property of alumina drew an opposing conclusion as it is reported that depending on the addition amount of CNT, the creep strength could be either strengthened or weakened due to an impediment of grain boundary sliding or promotion of grain boundary diffusion or sliding, respectively . Meanwhile, Crepo et al. reported that graphene oxide-reinforced alumina composite exhibits better creep resistance than CNF-reinforced alumina . Also, due to the excellent lubricating properties of graphite, carbon-based materials are a good candidate for solid lubricant application. During dry friction, carbon-based composites are reported to generate a lubricating film from the exfoliation of carbon and its incorporation with the ceramic debris over the affected contact area . However, most of the reported work involves the usage of sole mixing by either ultrasonic mixing of suspensions or a conventional mechanical milling, and no work has been demonstrated on the controlled decoration of carbon materials on ceramic leading to the formation of microstructure-controlled carbon-based ceramics. Therefore, in this study, CNP-Al2O3 composites were fabricated using electrostatic adsorption assembly which offers more controllability in its composite assembly and design. The Al2O3 micro-particles used in this work were obtained using control granulation of nano-sized Al2O3 particles. Then, the granulated Al2O3 micro-particles obtained were used for the formation of carbon CNP-Al2O3 composite. The study was conducted systematically by varying the amount of carbon nanospheres from 0.3, 0.6, and 1.0 vol% (volume percent) and the average size of alumina particles used. The mechanical properties of carbon-based composite samples were then characterized and compared with a monolithic alumina sample using a three-point bending and indentation test. The inter-correlation between the microstructure obtained and mechanical properties is also discussed and elucidated.
The hardness properties of the composite sample were further evaluated using indentation. The Rockwell indenter used consisted of a diamond (Ei = 1050 GPa, υ = 0.20) with a nominal radius of curvature, R = 200 embedded in a conical tip with an apex angle of 120°. The indenter was set in an Instron type tester (Sanwa Instruments) and was driven in at a crosshead speed of 0.05 mm/s to a fixed depth (20 μm). The load obtained during indentation was measured with a load cell (TCLZ-100KA, Tokyo Gakko), and the indent depth was measured with a non-contact electrostatic displacement meter (VE-222, Ono Sokki).
Results and Discussion
The loading of monolithic Al2O3 demonstrated a correlation with the P-h curve similar to the quadratic Eq. 2 while CNP-Al2O3 composite fabricated using Al2O3 particle with the size of 37 and 62 μm demonstrated a linear with deviated curve from the monolithic Al2O3 loading curve, respectively. This indicates the presence of CNP within the microstructure (at the grain boundary interface) which resulted in local deformation along the grain boundaries. As for CNP-Al2O3 composite fabricated using Al2O3 with the particle size 98 μm, the high density of CNP at the grain boundaries resulted in discontinuity of P-h hysteresis curve and demonstrated the lowest hardness due to occurrence of grain boundary slip or surface microfracture.
In this work, a feasible controlled formation of CNP-Al2O3 composite by an electrostatic adsorption method is demonstrated. The Al2O3 micro-particles used were obtained by granulation of nano-sized (150 nm) Al2O3 particles which enabled better compaction and sinter-ability. In the formation of composite ceramics, parameters involving the amount of CNP (0.3, 0.6, 1.0 vol%) and primary granulated Al2O3 micro-particle sizes (37, 62, 92 μm) were investigated. It is demonstrated that by controlling the amount of CNP additives and Al2O3 micro-particle size, different surface coverage could be obtained leading to controlled microstructure formation with different mechanical properties. Using the homogenous CNP-Al2O3 composite, a continuous interconnected carbon layer was obtained along the grain boundaries of Al2O3. A dense and compact Al2O3 matrix was also observed due to the good sintering of Al2O3 nanoparticles. From the results of a 3-point bending and indentation test, the control of mechanical properties was demonstrated by adjusting the coverage of CNP on Al2O3. The change in elastic modulus was either due to the inhibition of effective sintering or the slipping of the carbon layer generated at the Al2O3 interface. From this study, we have demonstrated the feasibility of ceramics microstructural design with an interconnected interface using CNP-Al2O3 composite. This method of microstructural design will open up greater possibilities and potential for materials design through bottom-up assembly to induce the desired properties for a wide range of applications.
Prof. Hiroyuki Muto and Dr. Wai Kian Tan would like to acknowledge Cross Ministerial Strategic Innovation Promotion Program (SIP) and Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research JP18H01706 for funding of this research work.
HM and WKT designed the study and contributed to the manuscript writing. NH, AY, GK, and AM provided technical and scientific insight and contributed to the editing of the manuscript. All authors read and approved the final manuscript.
Cross Ministerial Strategic Innovation Promotion Program (SIP), Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research JP18H01706 and Toyohashi University of Technology Research Support Fund.
The authors declare that they have no competing interests.
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