| Zirconia ceramic material possesses very excellent chemical and physical properties. It has good physical and mechanical properties at room temperature, and also because of its high temperature resistance, good chemical resistance, it usually served in harsh environments and load conditions, so it is a very promising high-temperature ceramics. However, its intrinsic brittleness of ceramic and poor tribological properties, greatly limit it’s the wider applications. To achieve high mechanical performances, much effort has been devoted to develop zirconia ceramic-based composites, in whch Zr O2 matrix is toughened with high strength fibers, whiskers, hard particles and carbon nanotubes(CNTs).Graphene nanosheets(GNSs) are composed of a few graphene layers held together through weak van der Waals force, and they display compatible properties similar to that of monolayer graphene(e.g. tensile strength 130 GPa and Young’s modulus 0.5-1 TPa), and GNSs are expected to be used as nanofillers in ceramics to produce composites with improved fracture toughness. Therefore, fabrication of the GNS reinforced 5 mol.% yttria stabilized tetragonal zirconia polycrystals(TZP) composites using spark plasma spray sintering(SPS) was proposed in this research, and the microstructure characteristics and the mechanical properties such as elastic modulus, hardness and fracture toughness of sintered composites were analyzed.Ultrasonic dispersion with the addition of surfactant was applied to disperse GNS/TZP mixture powders(the contents of the added GNSs in the composite powders were selected as 0wt.%, 0.5wt.%, 0.75 wt.% And 1.0wt.%), The resulting powders were consolidated by SPS.X-ray diffraction and Raman spectroscopy were used for analysis phase constituents of the sintered composites. The XRD results showed no tetragonal to monoclinic phase transformation occurred during SPS consolidation. Raman spectra depicted the presence of D, G and 2D peaks in all composites, which confirms the retention of GNSs after SPS consolidation. By means of field emission scanning electron microscopy(SEM), the microstructure characteristics of the sintered composites were observed, and GNSs distribute uniformly in the composite matrix to hinder significantly the grain growth.Regarding the mechanical properties of the sintered composites, the measured hardness and elastic modulus of the 0.5wt.%GNS/TCP composite show improvements of ~12% and ~18%, respectively, as compared to monolithic Zr O2. This is due to grain size refinement, low porosity and high elastic modulus of the GNSs. Further increase in GNS content(0.75 and 1.0wt.%) leads to a slight decrease in hardness and almost same elastic modulus of the composites owing to the increased porosity. Most importantly, all composites exhibit improved fracture toughness, it reaches a maximum value(~5.6 MPa m0.5) and increases by up to ~36% at 0.5% weight fraction, strongly indicating the added GNSs exhibit significant toughening effect on TZP even at very low concentration. The main toughening mechanisms such as GNS pullout, crack bridging, crack deflection and crack branching are responsible for the improved fracture toughness.In terms of scratching tests, though the sintered TZP sample exhibits lower coefficient of friction than that of sintered composites, the damage mechanism of the TZP sample is found to be a brittle fracture, and the residual scratch marks of the sintered composites showed more smoother, especially for 0.5wt.% GNS/TZP composite. Careful SEM observations illustrates that the adhesive wear is the predominant damage mechanism, which mainly resulted from the improved fracture toughness. |
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