Fabrication and properties of CNT-, RGO- reinforced ceramic composites

학위논문 (박사)-- 서울대학교 대학원 : 재료공학부, 2016. 2. 홍성현.Ceramic materials has been extensively used in a various applications such as cutting tools, abrasive tools, gas turbine, constructions and dental applications owing to their outstanding mechanical properties and chemical, thermal reliability. However, the brittleness in nature of ceramic materials, friction problems in tribological applications due to the insulating feature and very low thermal conductivities are still an obstacle to limit their wide applications as high-performance ceramic parts or some functional applications. To overcome this problem, several attempt have been devoted to develop ceramic composite such as composite material, microstructure and composition optimization. In this research, ceramic matrix composite was intensively studied and carbon nanotube, graphene nanoplatelets were introduced as an additive materials due to high mechanical properties (hardness, toughness, elastic modulus) and electrical, thermal conductivities. Two main topics will be discussed to investigate reinforcing mechanism and enhance their physical properties. 1) Fabrication and characterization of carbon nanotube reinforced ceramic, reduced graphene oxide reinforced ceramics and 2) Comparisons on the role of additive material (CNT, graphene) First, fully dense yttria-stabilized zirconia (YSZ) ceramics reinforced with single wall carbon nanotubes (SWCNTs) were fabricated by spark plasma sintering (SPS), and their electrical and mechanical properties were investigated. Dimethylformamide (DMF) was used as a solvent and tip-sonicator was employed to disperse SWCNTs homogeneously throughout the matrix and reduce the damage on the SWCNTs during mixing. The microstructure of the composite ceramics indicated that undamaged SWCNT bundles were well distributed throughout the matrix with intimate contact with ZrO2 grains without interlayer or amorphous carbon layer. The electrical resistivity of ZrO2 ceramics drastically decreased with SWCNT addition and it reached 0.3cm at 1.0 wt%. The SWCNT addition to ZrO2 ceramics increased the fracture toughness from 4.4 to 5.2 MPa1/2 at 1.0 wt%. The nanotube pull-out and crack bridging contributed to the improved fracture toughness. The frictional behavior was not affected, but the wear resistance of ZrO2 ceramics was significantly improved by SWCNT addition. Additionally, fully dense yttria-stabilized zirconia (YSZ) ceramics reinforced with reduced graphene oxide (RGO) were fabricated by spark plasma sintering (SPS), and their electrical, thermal, and mechanical properties were investigated. Graphene oxide (GO) was exfoliated by a short sonification in dimethylformamide (DMF)/water solution and uniformly mixed with ZrO2 powders. The microstructure of the composites showed that undamaged RGO sheets were homogeneously distributed throughout matrix grains. The electrical resistivity of YSZ composites drastically decreased with the addition of RGO, and it reached 0.0081 cm at 4.1 vol. %. However, the thermal diffusivity increased only 12% with RGO addition. The hardness decreased slightly with RGO addition, whereas the fracture toughness significantly increased from 4.4 to 5.9 MPa1/2. The RGO pull-out and crack bridging contributed to the improved fracture toughness. Second, we fabricated CNT and RGO (Reduced graphene oxide) added alumina ceramic composite and compared their electrical, thermal and mechanical properties. The composite was prepared by SPS which is very similar method as mentioned before. Each specimen was consolidated above ~98%. However, RGO added composite exhibits relatively low density compared with that of CNT added composite. Elastic modulus and hardness were decreased in both CNT and RGO composite, and rapid decrease was confirmed in CNT added composite due to the low relative density. Fracture toughness of the both composite was slightly increased and the increasing in RGO composite was more striking because of high surface area of RGO and 2-dimentional wrapping effect. It is believed that the toughening mechanisms were very similar with CNT-YSZ and RGO-YSZ composite. The electrical conductivity of CNT and RGO composite was rapidly increased at a certain concentration. In point of view for electrical percolation, CNT has advantages in very small amount of additions, and it was also confirmed by simple simulations. Thermal conductivity was drastically decreased in both CNT and RGO composite. The residual carbon defects located intra-grains shorten the phonon mean-free path in CNT, RGO-ceramic composite.Chapter 1. General background 1 1.1. Ceramic matrix composites for toughening 1 1.2. CNT and Graphene reinforced composite 4 1.2.1. Fabrication and properties of carbon nanotube 4 1.2.2. Fabrication and properties of graphene 5 1.2.3. Basic concept of CNT and Graphene reinforced composite 6 1.3. Spark plasma sintering (SPS) 8 Chapter 2. Synthesis of zirconia ceramic composite reinforced with carbon nanotube and reduced graphene oxide 19 2.1. Microstructure and mechanical properties of single wall carbon nanotube reinforced yttria stabilized zirconia ceramics 20 2.1.1. Introduction 20 2.1.2. Experimental 21 2.1.3. Results and discussion 24 2.1.4. Conclusions 32 Reference 44 2.2. Fabrication and properties of reduced graphene oxide reinforced yttria-stabilized zirconia composite ceramics 49 2.2.1. Introduction 49 2.2.2. Experimental 51 2.2.3. Results and discussion 53 2.2.4. Conclusions 59 References 69 Chapter 3. Synthesis and properties of alumina ceramic composite reinforced with carbon nanotube and graphene nano-platelets 75 3.1. Introduction 76 3.2. Experimental 77 3.3. Results and discussion 80 3.4. Conclusions 91 References 108 Abstract (Korean) 116Docto