Influence of microstructure on the mechanical properties of thermally sprayed NiCoCrAlY coatings determined by small punch testing

Jackson, George Anthony (2018) Influence of microstructure on the mechanical properties of thermally sprayed NiCoCrAlY coatings determined by small punch testing. PhD thesis, University of Nottingham.

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Abstract

The development of new thermal barrier coatings (TBCs) capable of increasing the efficiency of gas powered turbines requires an understanding of the time-dependent and time-independent properties of MCrAlY bond coats. High velocity oxy-fuel (HVOF) thermal spraying was used to manufacture free-standing coatings from one commercially available CoNiCrAlY bond coat alloy powder and three experimental NiCoCrAlY powders with potential application as new bond coat alloys. All coatings were subsequently heat treated at 1100 °C for 2 h to simulate a high temperature heat treatment stage used in manufacturing coated components. X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and electron back-scatter diffraction (EBSD) were used to characterise the microstructure of powders and coatings and characterise crack growth and fracture behaviour following mechanical testing.

Microstructural characterisation revealed that the CoNiCrAlY coating exhibited a two-phase microstructure consisting of a FCC γ-Ni matrix with BCC B2 β-NiAl as a secondary phase. After the same heat treatment, all three NiCoCrAlY coatings exhibited a β-NiAl matrix with γ-Ni as a secondary phase. The TCP phase σ-Cr2Co was also observed in the NiCoCrAlY coatings as well as the ordered L12 γ'-Ni3(Al,Ta) phase in two of the NiCoCrAlY coatings. All the coatings exhibited a fine scale microstructure with grain sizes typically in the range 1 - 5 µm. All coatings formed the desired protective Al2O3 scale after an accelerated oxidation test of 96 hours at 1100 °C in air, but the scale thickness in NiCoCrAlY coatings ranged from ≈ 5 to 7.5 µm compared to ≈ 2.5 µm in the CoNiCrAlY.

CALPHAD methods were employed, using ThermoCalc and the TTNi7 database, to model the phase equilibria of each of the MCrAlY alloys as a function of temperature. A comparison was made with the experimental observations for the novel alloys for the first time. Agreement was found to be surprisingly good using this database which was originally designed for compositions used in the manufacture of single crystal turbine blades.

Small punch tensile (SPT) tests were conducted on the CoNiCrAlY coating and two of the NiCoCrAlY coatings between room temperature (RT) and 750 °C in order to evaluate the influence of coating microstructure on their time-independent mechanical properties; specifically, strength, ductility and brittle to ductile transition. The SPT tests demonstrated the ductile to brittle transition temperatures (DBTTs) of the CoNiCrAlY coating and two NiCoCrAlY coatings were 500-700 °C, 600-700 °C and 650-750 °C respectively. One NiCoCrAlY coating exhibited superior yield strength above 650 °C compared to the CoNiCrAlY coating, but both NiCoCrAlY coatings exhibited lower ductility and lower fracture strengths below 600 °C. Increasing the phase fraction of β-NiAl was shown to increase the DBTT and also increase the yield strength above 650 °C, but also caused lower ductility and lower fracture strength below 600 °C. The TCP phase σ-Cr2Co was shown to decrease the ductility of the NiCoCrAlY coatings at ≤ 750 °C because it increased the density of phase boundaries in the coatings. The phase boundaries were found to be the crack nucleation sites for the CoNiCrAlY coating and two NiCoCrAlY coatings during SPT testing.

Small punch creep (SPC) tests were also conducted at 750 and 850 °C to determine the creep properties of the CoNiCrAlY coating and two of the NiCoCrAlY coatings. These tests revealed the NiCoCrAlY coatings exhibited higher creep resistance at 750 and 850 °C compared to the CoNiCrAlY coating. At 750 °C, the stress exponents of the CoNiCrAlY and two NiCoCrAlY coatings were calculated as 7.5, 7.8 and 9.1 respectively. Higher phase fractions of β-NiAl and the addition of γ'-Ni3(Al,Ta) were shown to improve the SPC lifetime of the coatings. The presence of σ-Cr2Co decreased the SPC strain to fracture of the NiCoCrAlY coatings.

At 850 °C large displacements were observed for the CoNiCrAlY coating and one NiCoCrAlY coating over a rage of the experimentally applied stresses, leading to significantly increased strain to fracture and lifetime of the samples. A possible explanation for this phenomenon is the onset of superplasticity.

The results provide important insights into the structure-property relationships of thin, HVOF sprayed bond coat alloys and the quantitative mechanical property data will be useful in the design of new TBC systems for superalloy turbine blades.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: McCartney, D.G.
Sun, W.
Keywords: HVOF, Small Punch Testing, MCrAlY, NiCoCrAlY, DBTT, Creep
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Faculties/Schools: UK Campuses > Faculty of Engineering
Item ID: 48778
Depositing User: Jackson, George
Date Deposited: 13 Jul 2018 04:40
Last Modified: 13 Jul 2018 04:40
URI: http://eprints.nottingham.ac.uk/id/eprint/48778

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