3D Assessment of Surface Integrity and Performance

In many cases failure mechanisms initiate and propagate from the surface, including failure under corrosion, fatigue and wear. Critical to this is the surface finish (SF) and the surface integrity (SI). While surface finish has received much attention, surface integrity, a term used to describe the localised sub-surface region that differs from the bulk (residual stresses, plastic deformation, chemical changes, hardness, etc) has received much less attention. Traditionally people have used simple cross sections to examine the surface microstructure.In this project we will apply a suite of state-of-the-art methods to characterise as fully as possible the local microstructure in 3D across a range of scales. These include serial sectioning using a focused ion beam (FIB), mechanical sectioning and X-ray tomography. In the latter X-rays are used to obtain a 3D picture without mechanically sectioning the sample. Critical to the former methods are the means of removing material quickly and efficiently without introducing damage. Emerging methods to remove the damaged layer will be developed such that we can obtain EBSD, texture, chemical mapping, residual stress and insights into plastic deformation near-surface. This will lead to one of the best surface integrity assessment facilities in the world to support industry. In addition we will develop micromechanical methods to assess mechanical properties and corrosion and wear performance. In this way we will relate surface integrity to surface durability. This is critical if we are to predict and engineer surface performance. In addition to developing these metrology tools we will apply them to a set of industrial case studies including corrosion of stainless steel for the energy sector, the performance of thermal barrier coatings for the turbine engine sector, the wear performances of WC-Co coatings and nanostructured coatings. Further case studies will be identified by our industrial steering group.

Society Ensuring that unexpected failures rarely happen is key to the well-being of society. Critical to this is a knowledge of the stresses and environmental loadings placed on materials, but of equal importance is the definition of acceptable levels of surface finish and integrity. NPL takes a leading role in the UK towards the standardised quantification of surface finish and the definition of surface integrity standards. These standards need to be based on well founded evidence. However at present due to the lack of surface microstructural characterisation methods, very little is known about how best to characterise surface integrity nor its effect of surface performance and durability. This project will have an immediate impact in this area. Economy In the short term we will maintain very close contact to the related industrial sectors (e.g. metal producers, surface finishers, aerospace, automotive, oil and gas, nuclear, etc) by reporting our work to 4 Industrial Advisory Groups (IAGs), namely Surface Technology, Oil and Gas, Power Industries and High Temperature Degradation. Each involves around 10 industrial members thus providing an immediate and direct pathway to impact to a significant community. In addition, members of the IAGs will be identified to attend our 3 Industrial Advisory Group steering workshops. Our initial industrial case studies have been chosen to include wear, oxidation, thermal fatigue and corrosion failure mechanisms and will include deformed, residual stresses and coated systems in order to help provide a surer foundation for surface engineering. We will hold an Industrial Advisory Group meeting to determine the second batch of case studies to ensure appropriate studies of maximal impact to our industrial partners. Further we have asked for funding to hold a workshop at month 30 to support technology transfer. Another pathway to impact is to publish an NPL Good Practice Guide . Beyond the length of the grant, the establishment of a comprehensive Facility at NPL for the characterisation of surface integrity/durability relationships will provide an on-going resource aimed specifically at supporting industry in terms of surface engineering. Further it is envisaged that the industrial case studies will have industrial partners. This will provide a focused short term pathway to impact. These outcomes of these case studies will be reported to the IAGs and made available on the NPL website as downloaded information sheets. It is expected that a number of these mini-proof of principle studies will spawn third party studies, Technology Strategy Board projects, EU projects etc. Knowledge As stressed above, at present there is a knowledge gap regarding the influence of structural integrity on many aspects of durability (Table 1). This metrology project will, in a very simple way, develop tools that will provide the data necessary to plug the gap. This will lead to a better knowledge of how SI affects various failure mechanisms. Furthermore, it will provide the bases for the development and the validation of finite element models, structural integrity codes and material performance models. People The most direct way in which this project will lead to improved skills will be by developing a PDRP placed in NPL with all the skills necessary to support the structural integrity function at NPL and to deliver knowledge from the dual beam FIB/microtest facility. Further they will have a high level of appreciation and access to complementary techniques that expand the length scale, provide residual stress and non destructive imaging capabilities of the near-surface RoI.