2018-09-15 Fatigue life of thermal barrier coatings (TBCs) – physically based modeling Sten Johansson, Håkan Brodin, Robert Eriksson, Sören Sjöström, Lars Östergren, Xin-Hai Li Detta är en generell mall för att göra PowerPoint presentationer enligt LiUs grafiska profil. Du skriver in din rubrik, namn osv på sid 1. Börja sedan skriva in din text på sid 2. För att skapa nya sidor, tryck Ctrl+M. Sidan 3 anger placering av bilder och grafik. Titta gärna på ”Baspresentation 2008” för exempel. Den sista bilden är en avslutningsbild som visar LiUs logotype och webadress. Om du vill ha fast datum, eller ändra författarnamn, gå in under Visa, Sidhuvud och Sidfot. Linköpings universitet

Introduction

The components of a TBC system BC 150-300 µm, TC 300-1500 µm top coat (TC), zirconia + 6–8% yttria bond coat (BC), MCrAlY M = Ni or Ni + Co Substrate, Ni-base (Haynes 230, Hastelloy X) light optic micrograph of TBC

plasma spraying, image from Sulzer Metco Manufacturing of TBC Raw material (powder) is fed into a plasma flame. The coatings are built up by droplets, forming a splat-on-splat structure on impact. Plasma spraying can be done in: air (APS) or vacuum (VPS). plasma spraying, image from Sulzer Metco

oxidation / corrosion / fatigue Problems with TBC if combustion temperature is increased, or when new fuel types are used oxidation / corrosion / fatigue Fatigue needs to develop life models for thick TBC needs to include effect of corrosion in life models More severe oxidation Sulphur in fuel might form sulphides Salts might cause rapid oxidation damaged oxide in TBC damaged top coat in TBC

Problem description Reliability of TBCs must improve! stresses the need for more robust life models that takes into account: oxidation, corrosion, BC/TC interface morphology, coating thickness, different types of thermal cycling How to accomplish this? Physically based fracture mechanical models

Experimental work: Surface roughness

interface morphology It influences: grayscale image It influences: adhesion stress distribution in BC/TC interface How to model the surface roughness? binary image surface profile filtered profile model

correlation between Ra and TCF life length Results Correlation between Ra and life length. Interface morphology influences fatigue life correlation between Ra and TCF life length from Fracture mechanical modelling of a plasma sprayed TBC system, Brodin et al.

experimental work: Adhesion of heat treated thermal barrier coatings

Isothermal and cyclic heat treatments isothermal oxidation 1100°C continuous high temperature exposure thermal cycling fatigue 1100°C 1 h in furnace => 10 min cooling to ~100°C burner rig test ~1100°C 75 s heating => 75 s cooling

Microstructural degradation Al depletion of the bond coat is one of the mechanisms that limits life of TBCs β-depletion in heat treated bond coat, a) BRT 300 cyc. b) BRT 1150 cyc. c) isoth. ox. 1 h d) isoth. ox. 23 h

Microstructural degradation formation of bulky oxide cluster in TCF-subjected specimen Bulky cluster of oxides formed in the interface may act as crack initiation sites.

Fracture surfaces from adhesion test By gluing the TBC coated specimens to two rods, the adhesion of the TBC can be tested using a tensile machine 3 18.4 MPa 1 19.7 MPa 2 20.9 MPa Fracture surfaces from adhesion test

Changes in adhesion with heat treatment type and treatment length changes in adhesion with high temperature exposure Cross-sectioned fracture surfaces from adhesion test, 300 cycles TCF Burner rig testing gives lower adhesion, but slower decrease in adhesion per cycle. Isothermal oxidation is beneficial.

Fractography, fracture in top coat Failure occurs between splats in the top coat fractured TC, as-sprayed specimen

fracture in top coat Through-splat cracks are rare. Mostly occur in pre-existing cracks splat-on-splat nucleation fractured TC, as-sprayed specimen fractured TC, as-sprayed specimen

High temperature exposure: grain coarsening a) as-sprayed, b) isothermal oxidation ~300 h, c) burner rig test 1150 cycles, d) thermal cycling fatigue 300 cycles

High temperature exposure: sintering cross-sections showing sintering a) as-sprayed b) isothermally oxidised 300 h fracture surfaces showing in-splat sintering a) as-sprayed b) isothermal oxidation ~300 h c) burner rig, 1150 cycles d) thermal cycling fatigue, 300 cycles

Fracture in BC/TC interface Increase in amount of interface fracture due to fatigue damage. Furnace cycled specimens have cracked oxide layer and cut-through oxide clusters. black fracture: a) as-sprayed, b) 300 cycles of thermal cycling fatigue, c) 300 cycles of burner rig test, d) 1150 cycles of burner rig test

Fracture in BC/TC interface: revealed oxide layer Al2O3 BC Al2O3 BC Cr-rich / spinel Al2O3 TC TCF 300 cycles (Al,Cr)2O3 / (Ni,Co)(Al,Cr)2O4 BC

Fracture in BC/TC interface: revealed oxide layer Al2O3 NiO BC Cr-rich / spinel TC TCF 300 cycles Cut-through oxide clusters Usually cracks during thermal cycling NiO Cr-rich/ spinel

Modelling work: influence of chamfer angle on edge cracking

Basic failure mechanisms Spallation from flat surfaces Spallation from convex surfaces 3. Spallation at sharp edges Substrate

Edge effects: effect of end chamfering TC BC substrate TGO x z TBC chamfer angle Substrate end angle  (2) equivalent ’fatigue evaluation’ stress Surprising conclusion: Practically no improvement until  (1) is made < 60 º

Ongoing experimental work: Oxidation study and corrosion testing

Corrosion rig Corrosion cycle: spraying with Na2SO4+NaCl 1 h in furnace, 650–950 °C cooling in air, reaching a minimum temperature of ~100 °C Currently running corrosion on BC coated specimen, SL30 NiCoCrAlY+Si

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