The influence of overlay coatings on high cycle fatigue of an advanced tantalum carbide strengthened eutectic composite

High cycle fatigue of coated and uncoated NiTaC 3-116A2, a directionally solidified tantalum carbide strengthened eutectic, has been examined over a range of temperatures. In the uncoated condition, the fatigue strength of the alloy increased with temperature from room temperature to 600 °C, but was slightly lower at 900 °C than at 600 °C, which is consistent with the variation in 0.2 pct yield strength with temperature. At room temperature and 600 °C, the influence of several plasma sprayed coatings in reducing the fatigue life was found to be sensitive to the coating composition and alternating stress level. A Ni-20Cr coating resisted fatigue damage on the basis of its ductility and therefore performed better at high cyclic stress levels than at low cyclic stress levels. By contrast, Ni-20Cr-10Al-2Hf-0.1C. and Ni-20Co-20Cr-10Al-2Hf-0.1C coatings were stronger and so exhibited high fatigue strengths at long lives but, because of their limited ductility, cracked readily when tested at high stress levels. At 900 °C the coatings had little effect on fatigue life. On the basis of the present results and the metallographic observations of early crack growth through the coatings, it is concluded that the selection of a mechanically compatible coating requires a knowledge of the fatigue cycle. To ensure a minimum effect on fatigue lives for the anticipated stress cycle, it is necessary to match the coating and substrate mechanical properties as closely as possible.