Mechanical characterisation of CMSX-3 and CMSX-4 with PT and PTAL Coatings under thermo-mechanical fatigue loading conditions

Background/Overview
The approach for the assessment of fatigue damage from cyclic variation of stress, strain and temperature as experienced in the hot section of a gas turbine uses the range of stress or strain and the maximum temperature attained in the cycle against which to predict the damage from fatigue curves. This results in a fatigue life based on an elastic prediction which is then assessed against the engine life requirements. Recent methods have proposed the generic use of analyses with plasticity and creep included to determine the relaxed stress and strain state with a revised fatigue damage assessment approach.

In the case of hot section components, the stress or strain may vary in-phase with the temperature, or there may be a phase difference which at its extreme would be completely out-of-phase with the temperature. This case occurs in a hotspot where the maximum temperature condition generates a significant compressive stress.

In other types of cycle where transient effects occur due to the boundary conditions of the external hot gas temperature and the internal cooling temperature combined with the mechanical loading due to rotational and gas loading forces, the peak stress or strain may develop on the rise or fall to the peak temperature condition and would correspond to a much lower temperature than the peak.

Such components do have the added benefit of thermal barrier coatings, but to date, little understanding is currently available that truly captures how the mechanical behaviour differentiates across the two materials.


Project Aims
The aim of this project is to investigate thermo-mechanical fatigue damage in high temperature coated systems through a thorough review of existing data available on single crystal materials, to develop a TMF lifing model that can represent the true service behaviour observed including the influence of creep and oxidation that occurs at high stresses and temperatures, to validate this model with targeted TMF tests and to investigate the fracture behaviour of the TMF experiments in comparison with predicted stress fields from finite element models.

The work could investigate how damage is accumulated in a range of TMF cycles, predicting the first cycle and stabilised cycle responses under these conditions. The incremental accumulation of damage around the cycle may generate an understanding of the driving behaviours in TMF. In particular, a holistic knowledge of the damage evolution experienced in a counter-clockwise -135 degree cycle is of great interest. Research could also incorporate the use of PD crack monitoring and Digital Image Correlation (DIC) for advanced characterisation of the TMF behaviour.

The materials of choice would be the nickel based single crystal superalloys CMSX-3 and CMSX-4 with Pt and PtAl coating systems.

TMF test data from Rolls-Royce would be referred to in addition to that available from open literature.

Outline Plan
1) Familiarisation with previous work, literature, and fatigue data on TMF in single crystals, with particular emphasis on coated materials.
2) Develop TMF testing capability and propose validation tests.
3) Perform validation tests including fractographic studies.
4) Demonstrate application to an engine component.

The CDT will produce 50 graduates with doctoral level knowledge and research skills focussed on the development and manufacture of functional industrial coatings. Key impact areas are:

Knowledge
- The development of new products and processes to address real scientific challenges existing in industry and to transfer this knowledge into partnering companies. The CDT will enable rapid knowledge transfer between academia and industry due to the co-created projects and co-supervision.
- The creation of knowledge sharing network for partner companies created by the environment of the CDT.
- On average 2-3 publications per RE. Publications in high impact factor journals. The scientific scope of the CDT comprises a mixture of interdisciplinary areas and as such a breadth of knowledge can be generated through the CDT. Examples would include Photovoltaic coatings - Journal of Materials Chemistry A (IF 8.867) and Anti-corrosion Coatings - Corrosion Science (IF 5.245), Progress in Organic Coatings (IF 2.903)
- REs will disseminate knowledge at leading conferences e.g. Materials Research Society (MRS), Meetings of the Electrochemical Society, and through trade associations and Institutes representing the coatings sector.
- A bespoke training package on the formulation, function, use, degradation and end of life that will embed the latest research and will be available to industry partners for employees to attend as CPD and for other PGRs demonstrating added value from the CDT environment.

Wealth Creation
- Value added products and processes created through the CDT will generate benefits for Industrial partners and supply chains helping to build a productive nation.
- Employment of graduates into industry will transfer their knowledge and skills into businesses enabling innovation within these companies.
- Swansea University will support potential spin out companies where appropriate through its dedicated EU funded commercialisation project, Agor IP.

Environment and society
- Functionalised surfaces can potentially improve human health through anti-microbial surfaces for health care infrastructure and treatment of water using photocatalytic coatings.
- Functionalised energy generation coatings will contribute towards national strategies regarding clean and secure energy.
- Responsible research and innovation is an overarching theme of the CDT with materials sustainability, ethics, energy and end of life considered throughout the development of new coatings and processes. Thus, REs will be trained to approach all future problems with this mind set.
- Outreach is a critical element of the training programme (for example, a module delivered by the Ri on public engagement) and our REs will have skills that enable the dissemination of their knowledge to wide audiences thus generating interest in science and engineering and the benefits that investments can bring.

People
- Highly employable doctoral gradates with a holistic knowledge of functional coatings manufacture who can make an immediate impact in industry or academia.
- The REs will have transferable skills that are pertinent across multiple sectors.
- The CDT will develop ethically aware engineers with sustainability embed throughout their training
- The promotion of equality, diversity and inclusivity within our cohorts through CDT and University wide initiatives.
- The development of alumni networks to grow new opportunities for our CDT and provide REs with mentors.