Fabrication and thermal properties of a YSZ–NiCr joint with an interlayer of YSZ–NiCr functionally graded material

Joining yittria stabilized zirconia (YSZ) to Ni–20Cr (NiCr) was fabricated using YSZ–NiCr functionally graded materials (FGM) interlayer by hot pressing process. The thermal properties of this YSZ–FGM–NiCr joint were studied by thermal cycling testing and shear testing. Microscopic observations indicated that the graded distribution of the composition and microstructure in the joint eliminate the macroscopic ceramic/metal interface. The joint shows good thermal stability and good oxidation resistance up to 1000 °C. The thermal fatigue cracks initiate and grow in the way of aggregation of microvoids and their connection in the YSZ-rich NiCr–75 vol.%YSZ interlayer during the thermal cycling to 1000 °C. However, these discontinuous cracks in interlayer release the thermal stress and avoid spallation in the joint interface of direct ceramic/metal bonding. No spallation was found in the joint with three interlayers of 2.0 mm theoretical thick each after 30 thermal cycles. The shear strengths were found to be 207.0 and 75.0 MPa in NiCr–50 vol.%YSZ and NiCr–75 vol.%YSZ interlayers respectively for this joint after 30 thermal cycles.