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Thermal and Mechanical Behaviors of High Temperature Coatings
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
##### Abstract [en]

With superior oxidation and corrosion resistance, metallic coatings (i.e. diffusion coatings and MCrAlX coatings) are widely used to protect hot components made of superalloys in turbine engines. Two issues are critically important for the coating at high temperatures: thermal property related to oxidation/corrosion behavior and microstructure stability, and mechanical properties (e.g. creep and fatigue). The aim of this project is to develop better understanding of the thermal and mechanical behaviors of metallic coatings on superalloys and to improve the accuracy of prediction of their lifetime by thermodynamic modeling. The present work includes an investigation on the oxidation behavior of MCrAlX coating with a new lifetimeprediction model and a study on the influence of diffusion coatings on creep and fatigue behaviors of the superalloy IN792.

Experiments on isothermal and thermal cycling oxidation were designed to investigate the oxidation behavior of a HVOF CoNiCrAlYSi coating on superalloy IN792. It is found that the oxidation behaviors of the coating are related to its thermodynamic property. A diffusion model has been established using the homogenization models in the DICTRA software and taking into consideration of the influence of surface oxidation, coating-substrate interdiffusion and diffusion blocking effect caused by internal voids and oxides. The simulation results show an improved accuracy of lifetime prediction by introducing the diffusion blocking effect.

Microstructural evolution during creep process at high temperatures was studied in different diffusion coatings (NiAl and PtAl). It is found that the inward diffusion of aluminum controls the thickening rate of the diffusion coatings. The developed coatings displayed two types of mechanical behavior - being easily plasticized or cracked - dependent on temperature and type of coating, and therefore could be considered as non-load carrying material during creep test. The influence of cracking of PtAl coating on the high-cycle fatigue (HCF) behavior of the IN792 was also investigated. The results show that precracking of the coating prior to the fatigue test has little influence on the fatigue limit of specimens with thin coating (50 μm) but lowers the fatigue limit of specimens with thick coating (70 μm). The through-coating crack has enough mobility to penetrate into the substrate and causes fatigue failure only when the driving force for crack propagation is increased above a critical value due to a higher applied stress or a larger crack length (thicker coating).

##### Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1569
##### National Category
Engineering and Technology Materials Engineering
##### Identifiers
Local ID: LIU-TEK-LIC-2013:3ISBN: 978-91-7519-708-1 (print)OAI: oai:DiVA.org:liu-89500DiVA: diva2:608163
##### Note

Som minor changes has been done in the electronic version compared to the printed version such as acknowledgement among others.

Available from: 2013-03-14 Created: 2013-02-26 Last updated: 2013-03-15Bibliographically approved
##### List of papers
1. Modeling of the microstructural evolution and lifetime prediction of MCrAlX coatings on Nickel based superalloys in high temperature oxidation
Open this publication in new window or tab >>Modeling of the microstructural evolution and lifetime prediction of MCrAlX coatings on Nickel based superalloys in high temperature oxidation
2013 (English)In: Surface and Coatings Technology, Vol. 232, no 15, 204-215 p.Article in journal (Refereed) Published
##### Abstract [en]

At high temperature, MCrAlX coatings are deposited onto the surface of superalloy to provide oxidation and corrosion protection by forming a thermalgrowing oxide scale. In this project, the oxidation behavior of an HVOF CoNiCrAlYSi coating on IN792 was studied in both isothermal oxidation (900, 1000 and 1100 °C) and thermal cycling (1100-100 °C). The microstructural evolution in the MCrAlX coatings after oxidation was investigated. It was found that Al-rich β phase is gradually consumed due to two effects: surface oxidation and coating-substrate interdiffusion. Some voids and oxides along the coating-substrate interface, or inside the coating, were considered to play a role in blocking the diffusion of alloying elements. Based on the microstructural observation, an oxidation-diffusion model was developed by using Matlab and DICTRA software to predict the lifetime of MCrAlX coatings. Several effects influencing the microstructural evolution were included: surface oxidation, coating-substrate interdiffusion and diffusion blocking. The results showed good agreement between the experimental composition profiles and modeling ones. Furthermore, by choosing suitable diffusion blocking factors, the accuracy of coating life prediction could be improved.

Elsevier, 2013
##### Keyword
MCrAlX coating; HVOF; Life prediction; Interdiffusion; Oxidation; Diffusion blocking
##### National Category
Engineering and Technology Materials Engineering
##### Identifiers
urn:nbn:se:liu:diva-90013 (URN)10.1016/j.surfcoat.2013.05.008 (DOI)000327691300027 ()
Available from: 2013-03-14 Created: 2013-03-14 Last updated: 2014-10-08Bibliographically approved
2. Analysis on Microstructural Evolution of PtAl Diffusion Coating on Ni-based Superalloy Influenced by Creep Process
Open this publication in new window or tab >>Analysis on Microstructural Evolution of PtAl Diffusion Coating on Ni-based Superalloy Influenced by Creep Process
2012 (English)Conference paper, Oral presentation only (Other academic)
##### Abstract [en]

The microstructural evolution of PtAl diffusion coating on a nickel-base superalloy IN792 was studied to assess the influence of high temperature (850 °C and 950 °C) and static creep loading conditions. The chemical composition analysis by EDS reveals that local enrichment of Cr, W and Mo is responsible for the formation of σ, Cr-α and μ/R particles in the different characteristic zones in the PtAl coating. Being taken as an approach to describe the coating’s microstructure evolution, a so-called Effective Coating Thickness (ECT) is defined and was measured for different creep conditions. It is found that the value of ECT is controlled by the inward diffusion of Al and Pt, being demonstrated by the existence of a linear correlation between ECT and $\sqrt{D^A^l x {t}}{(D^A^l:}$ diffusion coefficient of Al in substrate; t: diffusion time). Simulated ECT development of the PtAl diffusion coating agrees well with the experimental results at 850 °C and 950 °C.

##### Keyword
Creep, PtAl diffusion coating, IN792, microstructural evolution
##### National Category
Engineering and Technology Materials Engineering
##### Identifiers
urn:nbn:se:liu:diva-84300 (URN)
##### Conference
The 4th Conference on the Integrity of High Temperature Welds & 9th International Conference on Creep and Fatigue, September 25-27 2012, London, UK
Available from: 2012-10-04 Created: 2012-10-04 Last updated: 2013-03-14
3. Creep Fracture Mechanism of Polycrystalline Ni-based Superalloy with Diffusion Coatings
Open this publication in new window or tab >>Creep Fracture Mechanism of Polycrystalline Ni-based Superalloy with Diffusion Coatings
2013 (English)Conference paper, Published paper (Other academic)
##### Abstract [en]

Diffusion coatings are widely used to increase oxidation and corrosion resistance of hot superalloy components for gas turbines. The aim of this study is to investigate the effect of coatings (NiAl and PtAl) on the creep fracture mechanism of samples with a substrate of IN792. The samples have been creep tested at two temperatures (850 °C and 950 °C) and different applied tensile stresses, until failure between 205 and 21000 hrs. The observation of cross-sections by SEM shows that the microstructural evolution in the coating is dependent on the diffusion of alloying elements in the sample. Furthermore the time and temperature induced growth of the coating is found to be controlled only by inward diffusion of Al. Grain-boundary cracking is the basic fracture mode in the substrate in all samples irrespective if the crack is initiated from coating or substrate. The analysis of microstructure shows that the diffusion coatings display two types of mechanical behavior - being easily plasticized or cracked - dependent on temperature and type of coating, and therefore can be considered as non-load carrying regions. After recalculating the creep stress to exclude the final effective coating thickness from the total sample thickness, the coated samples showed similar creep rupture behavior as the uncoated samples in the Larson-Miller diagram.

##### Keyword
Creep; Ni-based superalloy; Diffusion coating; Crack
##### National Category
Engineering and Technology Materials Engineering
##### Identifiers
urn:nbn:se:liu:diva-89497 (URN)
##### Conference
13th International Conference on Fracture ICF13, June 16-21, 2013, Beijing, China
Available from: 2013-02-26 Created: 2013-02-26 Last updated: 2013-03-14Bibliographically approved
4. Influence of Precracked Diffusion Coating of Pt-Modified Aluminide on HCF Fracture Mechanism of IN 792 Nickel-Based Superalloy
Open this publication in new window or tab >>Influence of Precracked Diffusion Coating of Pt-Modified Aluminide on HCF Fracture Mechanism of IN 792 Nickel-Based Superalloy
2012 (English)In: Applied Mechanics and Materials, ISSN 1660-9336, E-ISSN 1662-7482, Vol. 148-149, 24-29 p.Article in journal (Refereed) Published
##### Abstract [en]

High-cycle-fatigue (HCF) fracture mechanism of nickel-based superalloy IN 792 coated with Pt-modified aluminide outward-diffusion coating is studied with focus on the influence of coating cracks. It is found that cracking of the diffusion coating prior to HCF tests has little influence on the fatigue limit of specimens with thin coating (50 μm) but lowers the fatigue limit of specimens with thick coating (70 μm). By fractographic analysis, three types of fractural modes are established according to their crack initiations: internal, external and mixed. While external fractural mode is related to the propagation of existing cracks in the coating, internal facture initiates often at Ti-Ta-W-rich carbides and/or topological-close-packed (TCP) phases and grainboundaries in the superalloy. Increasing the thickness of diffusion coating or the amplitude stress promotes the fractural mode transition from internal/mixed to external. The influence of precracking of coatings on the HCF fracture mechanism can be qualitatively explained by its influence on the stress intensity factor.

##### Place, publisher, year, edition, pages
Trans Tech Publications Inc., 2012
##### Keyword
Diffusion coating, Nickel-based superalloy, HCF, Fracture mechanism
##### National Category
Engineering and Technology Materials Engineering
##### Identifiers
urn:nbn:se:liu:diva-72633 (URN)10.4028/www.scientific.net/AMM.148-149.24 (DOI)
##### Conference
ICMEME2011, International Conference on Mechanical Engineeering in Dalian, China, October 18-20, 2011
Available from: 2011-12-01 Created: 2011-12-01 Last updated: 2017-12-08Bibliographically approved

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