Fabrication And Mechanical Performance Of ZrO₂-NiCr Functionally Graded Materials

Thermal stress of ZrO₂-NiCr functionally graded materials(FGM) after sintering has been calculated and optimized for stress relaxation in this paper. Based on the optimized compositional profile, ZrO₂-NiCr functionally graded materials (FGM) were developed by powder metallurgy. Their microstructure and mechanical properties were investigated by means of X-ray diffractometry (XRD), scanning electron microscopy (SEM) and other mechanical test.FGM as well as the uniform material samples corresponding to each graded layers were fabricated by the following steps: firstly, the raw powder mixture was mixed by ball-milled for 24 hours with liquid medium; then, prepare the green-bodies by cold pressing; finally, sinter in vacuum at 1250℃with pressure of 5¹0MPa for 1 hour.The results showed that ZrO₂-NiCr FGM samples exhibit the predicted distribution of graded layers. The middle graded layers of the ZrO₂-NiCr FGM compose of the nickel chrome alloy and zirconia components. Chrome solved in nickel to form a solid solution. The zirconia component exists mainly in the form of tetragonal t-phase with a little m-phase.The density of ZrO₂-NiCr composite decreases with the increase of ZrO₂ volume fraction, and the relative density is higher than 94%. The Vickers hardness ascends with the increase of ZrO₂ volume fraction and climbs quickly in the ceramic rich layers. Young's modulus and the bending strength change almost in the opposite way of that of the porosity. The bending strength of Nickel chrome alloy reached 560 MPa, which was much higher than that of pure nickel and accorded with the expected strength.Fractographs of ZrO₂-NiCr FGM sample reveal that in the metal rich layers the main failure mechanism is plastic fracture. With the increase of ZrO₂ volume fraction, the plasticity decreases, and there is a mixture fractograph of plasticity and brittleness when the volume ratio of metal and ceramic components is near to 0.5. The fracture face shows brittleness in the ceramic rich layers. Since ZrO₂-NiCr functionally graded material is heterogeneous in macro-structure and continuous in microstructure, the crack propagation path is deflected, which is good for increasing...