Effect of graphite addition on mechanical properties of Al2O3 ceramics by directed laser deposition

Abstract

In this article, graphite-reinforced Al2O3 ceramic materials were prepared by directed laser deposition. The effects of graphite addition (3–12 wt.%) on the phase composition, microhardness and fracture toughness of Al2O3 ceramics were studied. The results showed that the process was beneficial for maintaining uniform mixing of graphite and ceramics and thus could avoid the delamination caused by the difference in their densities. When adding 6% and 9% graphite, there were fewer internal defects in the samples, and no obvious new phase was generated after adding more graphite. The microhardness of 6% and 9% graphite-reinforced Al2O3 ceramics reached 18.84 and 18.86 GPa, respectively, and it was close to that of pure Al2O3 ceramics. The microhardness of all graphite-reinforced Al2O3 materials was lower than that of pure Al2O3 ceramics, but the value increased first and then decreased with addition of more graphite. The change in microhardness was similar to the trend of the number of defects, thus, these newly forming defects were an important factor affecting the microhardness. With an increase in graphite addition, the fracture toughness increased gradually. Furthermore, the fracture toughness reached 5.89 MPa m1/2 at 12% graphite addition, which was higher than 4.82 MPa m1/2 of pure Al2O3 ceramics. Adding more graphite was beneficial in enhancing the deflection and pinning of the graphite particles against cracks, resulting in higher fracture toughness. Graphite can improve the fracture toughness of Al2O3 ceramic, prepared by directed laser deposition, and the method can solve the uneven distribution caused by the difference in densities between the two powders. This also provides a new idea for the rapid manufacture of high-toughness ceramic parts at lower cost.