The synthesis of yttria tetragonal zirconia polycrystal powder by hydrothermal method for welding pins used in automotive industry

In this thesis, nano-scale of white and blue-colored Y-TZP powders was synthesized by hydrothermal method. Furthermore, as final product, the manufacturability of white and blue-colored zirconia based welding pins used in nut welding process in automotive industry with high fracture toughness and hardness values was investigated. According to transmission electron microscopy (TEM) results, the white-colored Y-TZP crystals were produced in 5-10 nm grain size range. The bulk samples were prepared from the synthesized powders, and physical and mechanical tests were applied. The highest density value with 94.7% after sintering was reached. With the purpose of production of ZrO2 bodies in higher density, new compositions were prepared by mechanical addition and direct usage from the solution of ZnO. While the density value of bodies was increased by addition of ZnO, it was seen to deteriorate the mechanical properties of ZrO2 ceramics due to grain growth which was formed during the sintering. Therefore, using Y-TZP powders developed without ZnO addition, the alternative high-density bulk sample preparation studies were carried out. For this purpose, the selection of suitable dispersant and additive materials was conducted to prevent the agglomeration of granule powder. Thus, 99.10 % sintered density values were obtained in white-colored ZrO2 samples. At this point, it was seen that 4Zrn-1 and 4Zrn-8 coded samples exhibited the best mechanical properties. Indeed, while the fracture toughness and hardness values for 4Zrn-1 sample were respectively calculated as 15.17 MPa.m1/2 and 8.99 GPa, the 12.24 MPa.m1/2 fracture toughness and 10.04 GPa hardness values were obtained in 4Zrn-8 sample. The production of Y-TZP powders with blue-colored and 5 nm grain size was successfully achieved from ZrO2-Y2O3-Al2O3-CoO system by hydrothermal method in one-step. The blue-colored ZrO2 ceramics with 16.85 MPa.m1/2 fracture toughness and 9.79 GPa hardness values from ZCA-4 coded composition were produced in 98.20% sintering density. Also, the production of ZrO2 ceramic welding pins from ZCA-4 coded composition was performed as final product. Moreover, the industrial application of ZrO2 welding pins, which were developed in the scope of this thesis, were conducted in the production line of a firm operating in automotive industry. It was observed that ZrO2 ceramic welding pin produced from the ZCA-4 coded composition made the nut welding with quite high performance. It was also determined that Y-TZP powders synthesized at nano size via hydrothermal method in this thesis were able to successfully use for many application area that required high wear resistance and mechanical property.