The effect of chromium oxide addition on alumina and alumina-zirconia composites

Alumina is one of the most important advanced ceramic materials that has been frequently used in ceramic matrix materials for many years due to its optical, dielectric, mechanical and thermal properties. It has high-end features such as high elastic modulus, high temperature chemical resistance, abrasion resistance, low density, biocompatibility and corrosion resistance. However, almost all structural ceramics cannot meet all engineering requirements on their own. It is known that the fracture toughness of the new composite material created by adding 10%-20% zirconia by weight to alumina increases significantly compared to alumina. This increase occurs through the transformation toughening mechanism of zirconium dioxide. As the toughness of alumina increases, its hardness values also decrease. Alumina, one of these ceramics, has disadvantages such as relatively low toughness and low bending strength. In this study, the effect of chromium oxide (Cr2O3) addition on the microstructure and mechanical properties of zirconia toughened alumina (Al2O3) composites was examined. In the experimental study, while examining the effect of zirconia on the toughening of alumina at 1550 °C and for different periods of time, the effect of adding low amounts of chromium oxide was compared with composites containing only alumina-zirconia. Al2O3, Al2O3+Wt. 10% Zirconia, Al2O3+Wt. 0.5%Cr2O3, Al2O3+Wt. 1%Cr2O3, Al2O3+Wt. 1.5%Cr2O3, Al2O3+Wt. 10%Zirconia+Wt. 0.5-1.-1.5% Cr2O3 powders were mixed dry in a ball mill at 200 rpm for 4 hours. The prepared powders were compressed in one direction under 170 MPa pressure and sintered in an open atmosphere electric resistance furnace at 1550 °C for 2, 4, 6 hours. Scanning electron microscope (SEM), x-ray analysis (XRD), density and hardness measurement tests were performed on the obtained powders. SEM examination observed that the addition of Cr2O3 prevented the grain size increase and abnormal grain growth of alumina by increasing the sintering temperature. It was observed that chromium was completely dissolved in the alumina matrix, forming a 100% solid solution. It was determined that the fracture toughness values of the composites obtained by adding ZrO2 to the alumina chromium oxide samples increased. The density value of the samples sintered with the addition of chromium increased compared to alumina. With the addition of chromium oxide, the hardness values of the alumina samples increased significantly, while the fracture toughness values decreased slightly. While the hardness values increased significantly with the addition of chromium oxide to the alumina-zirconia composite, the fracture toughness values remained almost the same as the values of pure alumina. Alumina-chromium oxide has been shown to have a relatively equiaxed, fine-grained structure (∼2-5 μm) and the addition of chromium inhibits abnormal grain growth, while chromium dissolved in alumina slightly increases the matrix grain size. Although abnormal grain growth was greater and larger in size in alumina-zirconia composites than in alumina-chromoxide samples, it decreased with the addition of chromoxide and disappeared completely in some samples.