Evaluation of the ductile-to-brittle transition temperature in a thermally sprayed CoNiCrAlY coating by small punch multi-step loading tests

Abstract

High velocity oxy-fuel thermal spraying was used to prepare free-standing CoNiCrAlY (Co-31.7%Ni-20.8%Cr-8.1%Al-0.5%Y, all in wt%) coatings of an approximate thickness of 0.5 mm. Small punch tests under multi-step loading conditions were performed between room temperature and 600 C on these samples to evaluate the ductile-to-brittle transition temperature. The microstructure of the coatings was characterised using a scanning electron microscope with energydispersive X-ray analysis. A two-phase structure consisting of fcc g-Ni and bcc b-NiAl was found to exist. The displacements obtained from small punch multi-step loading tests at each load increment were relatively small and similar at temperatures below 500 C but a significant increase in displacement was noted at 600 C. Fractographic investigation showed that the main fracture mode was dominated by extensive g matrix tearing at elevated temperatures. A distinct stress and strain behaviour was found at 600 C, indicating that the ductile-to-brittle transition temperature of this CoNiCrAlY coating occurred between 500 C and 600 C.

Mechanics of the small punch test: a review and qualification of additive manufacturing materials

“…Sensitivity to location of sample extraction from the original body with respect to depth from the surface has also been shown [153]. The SPT has also been employed to test thin coatings, such as in [154,155] and [156], which successfully determined the DBTTs of two bond coats utilized for thermal barrier coatings for the first time with the SPT. Additionally, the DBTT calculated via SP has been shown to be sensitive to evolutions in microstructure such as those seen when post-processing AM materials [24].…”

Fabrication and microstrain evolution of Al-TiB2 composite coating by cold spray deposition

“…One method that appears to have the potential to meet the above requirements is the cold gas dynamic spraying or, more simply, cold spray. Unlike plasma spray and high velocity oxy-fuel thermal spray, [23][24][25][26][27][28] which use high temperature jets, cold spray involves the acceleration and impact of solid particles onto a substrate to form a coating at moderate to low temperatures. The bulk of the particles do not undergo any melting in cold spray; however, the temperature at the interfacial zone can rise significantly.…”