The High Temperature Creep Properties of a Thermally Sprayed CoNiCrAlY Coating via Small Punch Creep Testing

Abstract

Thermal barrier coatings (TBC’s) protect superalloy components from excessively high temperatures in gas turbines. TBC’s comprise of a ceramic top coat, a metallic bond coat and a thermally grown oxide (TGO). The creep behaviour of the MCrAlY bond coat, which is sensitive to the composition and the method of deposition, has a significant effect on the life of the TBC. High velocity oxy-fuel (HVOF) thermal spraying is a popular deposition method for MCrAlY bond coats however the creep properties of HVOF MCrAlY coatings are not well documented. The creep behaviour of a HVOF thermally sprayed CoNiCrAlY coating has been determined by small punch creep (SPC) testing. Tests were conducted between an equivalent uniaxial stress range of 37-80 MPa at 750 °C on two different SPC rigs and between 30-49 MPa at 850 °C on a single SPC rig. The measured steady-state creep deformation rates at 750 °C were consistent across the two rigs. A comparison with previous work demonstrated that the creep behaviour of HVOF CoNiCrAlY coatings is not sensitive to the manufacturing variability associated with HVOF thermal spraying. The CoNiCrAlY coating exhibited typical SPC deformation at 750 °C. At 850 °C the CoNiCrAlY coating showed significantly different creep behaviour which could be attributed to the onset of superplasticity.

Some considerations in using the small punch testing for thermally sprayed CoNiCrAlY coatings

“…Soltysiak et al reported a study on using SPT to characterise a thermally sprayed IN625 coating [33]. Chen et al applied SPT to evaluate the ductile-to-brittle transition and creep behaviour of CoNiCrAlY coatings [34][35][36]. These are the limited studies available on using the SPT for coatings, which have demonstrated the importance and suitability of using the SPT for mechanical characterisation of thermally sprayed coatings.…”

Small Punch Testing - The Transition from a Code of Practice to a European Testing Standard

“…A specific European action related to the standard concerns modelling and analysis, which is to be covered in the paper by Abendroth Small Sample Test Technique [37] in this conference. Other European work on SP creep testing which may later be considered for incorporation into the SP creep standard is presented also by other authors namely for very high temperatures (Jackson et al [38], Dymacek et al [39] and Jeffs et al [40]), for ring specimens (Hyde [41]) and for fatigue loading (Lancaster et al [42]) On the other hand the JRC have chosen to show their results [44] using the Wilshire equations [45,46] deriving F/Fmax for Small Punch tests as described in [47] Their results are compared to NIMS uniaxial data in Figure 19 where the difference in the SP and uniaxial results can be eventually related to the k SP in the equation relating SP load and stress given in the Code of Practice, namely…”