Dong Liu - Academia.edu (original) (raw)

Papers by Dong Liu

The impact of seeding of the diamond growth on the microstructural properties of GaN-on-diamond w... more The impact of seeding of the diamond growth on the microstructural properties of GaN-on-diamond wafers was studied using in situ focused ion beam cross-sectioning and scanning electron microscopy imaging. Microstructural studies revealed that the seeding conditions are a critical parameter to obtain an optimal material, allowing the manufacture of GaN-on-diamond wafers with no microscopic defects and with structural stability under thermal annealing at 825 ⁰C. The use of the right seeding conditions also results in homogeneous thermal properties across four inch GaN-on-diamond wafers, which is of critical importance for their use for ultra-high power microwave electronic devices.

The mechanical and thermo-mechanical integrity of GaN-on-diamond wafers used for ultra-high power... more The mechanical and thermo-mechanical integrity of GaN-on-diamond wafers used for ultra-high power microwave electronic devices was studied using a micro-pillar based in situ mechanical testing approach combined with an optical investigation of the stress and heat transfer across interfaces. We find the GaN/diamond interface to be thermo-mechanically stable, illustrating the potential for this new material for reliable GaN electronic devices.

Neutron diffraction and synchrotron X-Ray diffraction and imaging have been applied to study, in ... more Neutron diffraction and synchrotron X-Ray diffraction and imaging have been applied to study, in situ, the mechanical response to tensile and bending loading of polygranular Gilsocarbon nuclear grade near-isotropic graphite (grade IM1-24). Digital image correlation of X-ray radiographs and digital volume correlation of tomographs allow measurement of bulk elastic moduli and examination of the heterogeneity of deformation in the microstructure. Both the neutron and X-ray studies show the application of tensile strain reduces the bulk elastic modulus. A permanent set is observed to develop with applied tensile strain. The elastic strains within the graphite crystals were measured by diffraction; a cross-correlation analysis method has been applied for greater speed, robustness and improved precision in the measurement of the change in basal plane separation distance. In compression, a linear relation is observed between the elastic strains in the graphite crystals and the applied strain. In tension, this relationship is non-linear. The results are discussed with respect to the distribution of elastic and inelastic strain within the graphite microstructure. It is deduced that the significant residual elastic strains in the as- manufactured graphite are relaxed by microcracking as tensile strain is applied.

In the present work, the microstructure and mechanical properties of Gilsocarbon graphite have be... more In the present work, the microstructure and mechanical properties of Gilsocarbon graphite have been characterized over a range of length-scales. Optical imaging, combined with 3D X-ray computed tomography and 3D high-resolution tomography based on focus ion beam milling has been adopted for microstructural characterization. A range of small-scale mechanical testing approaches are applied including an in situ micro-cantilever technique based in a Dualbeam workstation. It was found that pores ranging in size from nanometers to tens of micrometers in diameter are present which modify the deformation and fracture characteristics of the material. This multi-scale mechanical testing approach revealed the significant change of mechanical properties, for example flexural strength, of this graphite over the length-scale from a micrometer to tens of centimeters. Such differences emphasize why input parameters to numerical models have to be undertaken at the appropriate length-scale to allow predictions of the deformation, fracture and the stochastic features of the strength of the graphite with the required confidence. Finally, the results from a multi-scale model demonstrated that these data derived from the micro-scale tests can be extrapolated, with high confidence, to large components with realistic dimensions.

It has been demonstrated that the initiation and growth of localised, heterogeneously-distributed... more It has been demonstrated that the initiation and growth of localised, heterogeneously-distributed process zones is associated with the non-linear stress-strain response of graphites used to moderate UK gas-cooled civil nuclear reactors. These graphites, such as Gilsocarbon graphite, have heterogeneous complex polygranular microstructures which contain pores and flaws arising from their fabrication. The macroscopic properties of such nuclear graphites are dictated by their microstructure. Due to the presence of pores and
aggregates, the lattice strain is not expected to change 1:1 with the externally bulk strain applied to macroscale specimen. Deformation of the material containing pores and flaws causes localisation of strains and, hence, initiation of inelastic damage. The length-scale at which the localised damage develops during
loading can be characterised by the lattice strain in a bulk volume of material. Therefore, in situ neutron diffraction on a Gilsocarbon graphite bend geometry test specimen has been undertaken at the ENGIN-X, ISIS facility. It was found that lattice strain changes linearly with applied bulk strain but with reduced magnitude. The results are discussed with respect to the evolution of characteristic process zones, as deformation is increased, and the associated of microcracking.

The evolution of the deformation and fracture of a Gilsocarbon nuclear grade graphite has been in... more The evolution of the deformation and fracture of a Gilsocarbon nuclear grade graphite has been investigated in situ at three length-scales (nano-, micro- and macro-scale): (i) Neutron diffraction was used to quantify the lattice spacing changes with externally applied bulk strain. It was found that the lattice strain changes linearly with applied bulk strain but the magnitude is approximately one order of magnitude less. This relationship deviated from linearity when the applied deformation exceeded about two-thirds of the peak strain. (ii) Micro-scale tests that are capable of isolating a selected volume of material, typically 2 μm x 2 μm x10 μm, were used to observe the propagation of damage at this length-scale. These results are typical of the damage and micro-scale fracture that occurs within an individual process zone. (iii) In situ tests under a scanning electron microscope traced the fracture characteristics of macro-cracking. The results obtained from these approaches are discussed with respect to the characteristics of the elastic and inelastic strain accumulation within individual process zones and the formation of macro-cracks in this quasi-brittle nuclear graphite.

The effect of stress state on the flexural strength of a quasi-brittle material is explored using... more The effect of stress state on the flexural strength of a quasi-brittle material is explored using reactor core
Gilsocarbon graphite. The test geometry adopted has a ‘five-point’ bending configuration, i.e. a cruciformshaped
specimen that creates a tensile biaxial stress on the surface. This allows the effect of changing the
biaxial ratio on the load-displacement and fracture characteristics to be considered. An acoustic emission
(AE) technique has been applied to monitor and identify the occurrence of acoustic events and their locations
in specimens loaded either monotonically to failure or via several progressively increasing load-unloading
cycles. It was found that the fracture path changes with biaxial ratio. AE events occurred from low load in
all loading modes, and increased progressively with the increase of applied load. The total number of AE
events and the sum of the cascade energy from the AE measurements were similar for specimens fractured
at the same load under a particular loading condition.

Procedia Materials Science, 2014

A set of Ni-based single crystal superalloy aero-blade sections with electron beam physical vapou... more A set of Ni-based single crystal superalloy aero-blade sections with electron beam physical vapour deposited thermal barrier coating (EBPVD-TBC) were used to investigate the role of substrate curvature on the microstructure and the residual stresses in the coating system. The residual stresses generated in the thermal barrier coating (TBC) and the thermally grown oxide (TGO) have been evaluated non-destructively by Raman spectroscopy (RS) and photo-stimulated luminescence piezo-spectroscopy (PLPS), respectively. In addition, residual stresses were measured in free-standing individual columns extracted from the TBCs for further understanding of the large Raman shift towards the tension side in the ceramic layer. For the TGO, three areas with varied substrate curvature were mapped by PLPS. These residual stresses are analysed as a function of the thermal exposure and the substrate curvature. Discussion focuses on the implications on how to correctly evaluate the residual stresses in EBPVD-TBCs based on Raman shifts and subsequently the role of substrate curvature on the degradation of the microstructure and evolution of residual stresses in this type of TBCs.

A platinum-aluminide environmental coating has been subjected to simulated operating thermal cycl... more A platinum-aluminide environmental coating has been subjected to simulated operating thermal cycles normally encountered in land-based gas turbines. The oxidation was undertaken in a mixture of gas with 6 vol.% steam. The specimens were subject to long term cycles (1000 hrs) at temperatures 850°C, 900°C, 950°C and 1050°C for up to ten cycles. Growth of the thermally grown oxides with thermal exposure is explored, and the fracture and delamination modes of the protective oxide layer were characterised by scanning electron imaging combined with focus ion beam milling. Two representative failure modes of the thermally grown oxide are proposed and discussed.

range of small scale, from centimetre scale to micrometre scale, mechanical testing techniques ha... more range of small scale, from centimetre scale to micrometre scale, mechanical testing techniques have been used to measure the properties of nuclear reactor core graphite, including Pile Grade A (PGA) graphite and Gilsocarbon graphite. These testing methods include four-point bending (centimetre scale), diametral compression (millimetre scale), micro-scale cantilever bending (micrometre scale) and nano-indentation (micrometre scale). These methods provide both a measure of mechanical properties including elastic modulus and fracture strength and detailed information concerning the deformation and fracture
mechanisms. For each test, an example using a particular specimen geometry is given and discussed with respect to the particular mechanical property evaluated and compared with macro-scale data. Nano-indentation was carried out as a conventional approach to validate and assist the understanding of the mechanical properties obtained via other small scale tests. The use of small scale test specimens offers benefits when handling irradiated reactor core graphite. The results are discussed with respect to the potential benefits, difficulties and
value of small scale mechanical tests for this particular application.

Films and coatings have become widely used in structures and components to protect the underlying... more Films and coatings have become widely used in structures and components to protect the underlying material from mechanical degradation, corrosion, oxidation and high
temperatures or improve surface properties. Residual stress generated in these multi-layer systems is one of the main causes of coating delamination and eventual failure. Systematic
measurement and monitoring of the residual stresses are a vital basis for integrity evaluation and remaining life prediction. Raman spectroscopy has been recognised as one of
the most important approaches to measure the stress in films and coatings. This review considers the measurement of stresses in films and coatings using Raman spectroscopy. It
addresses the following questions: what is Raman spectroscopy, why is stress important for films and coatings, how is strain/stress derived from Raman spectra and what confidence do we have in this technique and the limitations. To elucidate specific issues related to the application of the Raman technique for stress measurement, despite the wide
range of coatings available, important films and coating are chosen as representative examples.

Materials Science & Engineering A, 2014

Residual stresses within thermal barrier coated (TBC) systems are predicted using finite element ... more Residual stresses within thermal barrier coated (TBC) systems are predicted using finite element models and are compared against experimental measurements taken using Raman and photo-stimulated luminescence piezo-spectroscopic methods. Two types of specimens were considered: flat specimens without TBC, and aerofoil shaped specimens with TBC. Comparisons between model predictions and experimentally measured stresses and also the relationship between substrate curvature and residual stresses on subsequent spallation of the aerofoil specimen were made. For the flat specimen, predicted and measured residual stresses are of the same order of magnitude. Both studies have indicated that larger compressive residual stresses arise from higher substrate curvatures. The results improve the understanding of initiation and propagation of TBC cracks which occurs preferentially at locations with high substrate convex curvature.

Materials at High Temperatures

Key Engineering Materials

Key Engineering Materials

Materials Science and Technology

Measurement Science and Technology

The impact of seeding of the diamond growth on the microstructural properties of GaN-on-diamond w... more The impact of seeding of the diamond growth on the microstructural properties of GaN-on-diamond wafers was studied using in situ focused ion beam cross-sectioning and scanning electron microscopy imaging. Microstructural studies revealed that the seeding conditions are a critical parameter to obtain an optimal material, allowing the manufacture of GaN-on-diamond wafers with no microscopic defects and with structural stability under thermal annealing at 825 ⁰C. The use of the right seeding conditions also results in homogeneous thermal properties across four inch GaN-on-diamond wafers, which is of critical importance for their use for ultra-high power microwave electronic devices.

The mechanical and thermo-mechanical integrity of GaN-on-diamond wafers used for ultra-high power... more The mechanical and thermo-mechanical integrity of GaN-on-diamond wafers used for ultra-high power microwave electronic devices was studied using a micro-pillar based in situ mechanical testing approach combined with an optical investigation of the stress and heat transfer across interfaces. We find the GaN/diamond interface to be thermo-mechanically stable, illustrating the potential for this new material for reliable GaN electronic devices.

Neutron diffraction and synchrotron X-Ray diffraction and imaging have been applied to study, in ... more Neutron diffraction and synchrotron X-Ray diffraction and imaging have been applied to study, in situ, the mechanical response to tensile and bending loading of polygranular Gilsocarbon nuclear grade near-isotropic graphite (grade IM1-24). Digital image correlation of X-ray radiographs and digital volume correlation of tomographs allow measurement of bulk elastic moduli and examination of the heterogeneity of deformation in the microstructure. Both the neutron and X-ray studies show the application of tensile strain reduces the bulk elastic modulus. A permanent set is observed to develop with applied tensile strain. The elastic strains within the graphite crystals were measured by diffraction; a cross-correlation analysis method has been applied for greater speed, robustness and improved precision in the measurement of the change in basal plane separation distance. In compression, a linear relation is observed between the elastic strains in the graphite crystals and the applied strain. In tension, this relationship is non-linear. The results are discussed with respect to the distribution of elastic and inelastic strain within the graphite microstructure. It is deduced that the significant residual elastic strains in the as- manufactured graphite are relaxed by microcracking as tensile strain is applied.

In the present work, the microstructure and mechanical properties of Gilsocarbon graphite have be... more In the present work, the microstructure and mechanical properties of Gilsocarbon graphite have been characterized over a range of length-scales. Optical imaging, combined with 3D X-ray computed tomography and 3D high-resolution tomography based on focus ion beam milling has been adopted for microstructural characterization. A range of small-scale mechanical testing approaches are applied including an in situ micro-cantilever technique based in a Dualbeam workstation. It was found that pores ranging in size from nanometers to tens of micrometers in diameter are present which modify the deformation and fracture characteristics of the material. This multi-scale mechanical testing approach revealed the significant change of mechanical properties, for example flexural strength, of this graphite over the length-scale from a micrometer to tens of centimeters. Such differences emphasize why input parameters to numerical models have to be undertaken at the appropriate length-scale to allow predictions of the deformation, fracture and the stochastic features of the strength of the graphite with the required confidence. Finally, the results from a multi-scale model demonstrated that these data derived from the micro-scale tests can be extrapolated, with high confidence, to large components with realistic dimensions.

It has been demonstrated that the initiation and growth of localised, heterogeneously-distributed... more It has been demonstrated that the initiation and growth of localised, heterogeneously-distributed process zones is associated with the non-linear stress-strain response of graphites used to moderate UK gas-cooled civil nuclear reactors. These graphites, such as Gilsocarbon graphite, have heterogeneous complex polygranular microstructures which contain pores and flaws arising from their fabrication. The macroscopic properties of such nuclear graphites are dictated by their microstructure. Due to the presence of pores and
aggregates, the lattice strain is not expected to change 1:1 with the externally bulk strain applied to macroscale specimen. Deformation of the material containing pores and flaws causes localisation of strains and, hence, initiation of inelastic damage. The length-scale at which the localised damage develops during
loading can be characterised by the lattice strain in a bulk volume of material. Therefore, in situ neutron diffraction on a Gilsocarbon graphite bend geometry test specimen has been undertaken at the ENGIN-X, ISIS facility. It was found that lattice strain changes linearly with applied bulk strain but with reduced magnitude. The results are discussed with respect to the evolution of characteristic process zones, as deformation is increased, and the associated of microcracking.

The evolution of the deformation and fracture of a Gilsocarbon nuclear grade graphite has been in... more The evolution of the deformation and fracture of a Gilsocarbon nuclear grade graphite has been investigated in situ at three length-scales (nano-, micro- and macro-scale): (i) Neutron diffraction was used to quantify the lattice spacing changes with externally applied bulk strain. It was found that the lattice strain changes linearly with applied bulk strain but the magnitude is approximately one order of magnitude less. This relationship deviated from linearity when the applied deformation exceeded about two-thirds of the peak strain. (ii) Micro-scale tests that are capable of isolating a selected volume of material, typically 2 μm x 2 μm x10 μm, were used to observe the propagation of damage at this length-scale. These results are typical of the damage and micro-scale fracture that occurs within an individual process zone. (iii) In situ tests under a scanning electron microscope traced the fracture characteristics of macro-cracking. The results obtained from these approaches are discussed with respect to the characteristics of the elastic and inelastic strain accumulation within individual process zones and the formation of macro-cracks in this quasi-brittle nuclear graphite.

The effect of stress state on the flexural strength of a quasi-brittle material is explored using... more The effect of stress state on the flexural strength of a quasi-brittle material is explored using reactor core
Gilsocarbon graphite. The test geometry adopted has a ‘five-point’ bending configuration, i.e. a cruciformshaped
specimen that creates a tensile biaxial stress on the surface. This allows the effect of changing the
biaxial ratio on the load-displacement and fracture characteristics to be considered. An acoustic emission
(AE) technique has been applied to monitor and identify the occurrence of acoustic events and their locations
in specimens loaded either monotonically to failure or via several progressively increasing load-unloading
cycles. It was found that the fracture path changes with biaxial ratio. AE events occurred from low load in
all loading modes, and increased progressively with the increase of applied load. The total number of AE
events and the sum of the cascade energy from the AE measurements were similar for specimens fractured
at the same load under a particular loading condition.

Procedia Materials Science, 2014

A set of Ni-based single crystal superalloy aero-blade sections with electron beam physical vapou... more A set of Ni-based single crystal superalloy aero-blade sections with electron beam physical vapour deposited thermal barrier coating (EBPVD-TBC) were used to investigate the role of substrate curvature on the microstructure and the residual stresses in the coating system. The residual stresses generated in the thermal barrier coating (TBC) and the thermally grown oxide (TGO) have been evaluated non-destructively by Raman spectroscopy (RS) and photo-stimulated luminescence piezo-spectroscopy (PLPS), respectively. In addition, residual stresses were measured in free-standing individual columns extracted from the TBCs for further understanding of the large Raman shift towards the tension side in the ceramic layer. For the TGO, three areas with varied substrate curvature were mapped by PLPS. These residual stresses are analysed as a function of the thermal exposure and the substrate curvature. Discussion focuses on the implications on how to correctly evaluate the residual stresses in EBPVD-TBCs based on Raman shifts and subsequently the role of substrate curvature on the degradation of the microstructure and evolution of residual stresses in this type of TBCs.

A platinum-aluminide environmental coating has been subjected to simulated operating thermal cycl... more A platinum-aluminide environmental coating has been subjected to simulated operating thermal cycles normally encountered in land-based gas turbines. The oxidation was undertaken in a mixture of gas with 6 vol.% steam. The specimens were subject to long term cycles (1000 hrs) at temperatures 850°C, 900°C, 950°C and 1050°C for up to ten cycles. Growth of the thermally grown oxides with thermal exposure is explored, and the fracture and delamination modes of the protective oxide layer were characterised by scanning electron imaging combined with focus ion beam milling. Two representative failure modes of the thermally grown oxide are proposed and discussed.

range of small scale, from centimetre scale to micrometre scale, mechanical testing techniques ha... more range of small scale, from centimetre scale to micrometre scale, mechanical testing techniques have been used to measure the properties of nuclear reactor core graphite, including Pile Grade A (PGA) graphite and Gilsocarbon graphite. These testing methods include four-point bending (centimetre scale), diametral compression (millimetre scale), micro-scale cantilever bending (micrometre scale) and nano-indentation (micrometre scale). These methods provide both a measure of mechanical properties including elastic modulus and fracture strength and detailed information concerning the deformation and fracture
mechanisms. For each test, an example using a particular specimen geometry is given and discussed with respect to the particular mechanical property evaluated and compared with macro-scale data. Nano-indentation was carried out as a conventional approach to validate and assist the understanding of the mechanical properties obtained via other small scale tests. The use of small scale test specimens offers benefits when handling irradiated reactor core graphite. The results are discussed with respect to the potential benefits, difficulties and
value of small scale mechanical tests for this particular application.

Films and coatings have become widely used in structures and components to protect the underlying... more Films and coatings have become widely used in structures and components to protect the underlying material from mechanical degradation, corrosion, oxidation and high
temperatures or improve surface properties. Residual stress generated in these multi-layer systems is one of the main causes of coating delamination and eventual failure. Systematic
measurement and monitoring of the residual stresses are a vital basis for integrity evaluation and remaining life prediction. Raman spectroscopy has been recognised as one of
the most important approaches to measure the stress in films and coatings. This review considers the measurement of stresses in films and coatings using Raman spectroscopy. It
addresses the following questions: what is Raman spectroscopy, why is stress important for films and coatings, how is strain/stress derived from Raman spectra and what confidence do we have in this technique and the limitations. To elucidate specific issues related to the application of the Raman technique for stress measurement, despite the wide
range of coatings available, important films and coating are chosen as representative examples.

Materials Science & Engineering A, 2014

Residual stresses within thermal barrier coated (TBC) systems are predicted using finite element ... more Residual stresses within thermal barrier coated (TBC) systems are predicted using finite element models and are compared against experimental measurements taken using Raman and photo-stimulated luminescence piezo-spectroscopic methods. Two types of specimens were considered: flat specimens without TBC, and aerofoil shaped specimens with TBC. Comparisons between model predictions and experimentally measured stresses and also the relationship between substrate curvature and residual stresses on subsequent spallation of the aerofoil specimen were made. For the flat specimen, predicted and measured residual stresses are of the same order of magnitude. Both studies have indicated that larger compressive residual stresses arise from higher substrate curvatures. The results improve the understanding of initiation and propagation of TBC cracks which occurs preferentially at locations with high substrate convex curvature.

Materials at High Temperatures

Key Engineering Materials

Key Engineering Materials

Materials Science and Technology

Measurement Science and Technology

12th International Conference on Fracture and Damage Mechanics,17-19 September 2013 Alghero, Sardinia, Italy, Sep 17, 2013

The role of stress state on the fracture properties of a quasi-brittle material are explored usin... more The role of stress state on the fracture properties of a quasi-brittle material are explored using reactor core Gilsocarbon graphite. Cruciform specimens have been prepared and tested by a novel biaxial loading method. Pre-slots of 10 mm width and up to a quarter of the depth of the specimen have been manufactured by electric discharge machining. These were machined at the centre of the specimen between two pre-exist vertical through-thickness holes to guide crack propagation upon loading. A loading jig has been designed and built that allows a range of biaxial stresses to be applied. This loading jig allows variation of the length of the loading arm to achieve a change of biaxility at the specimen surface. Clip gauges are adopted for the measure of the strain on the surface and the opening of the crack. Load-displacement and fracture properties of the graphite for different loading conditions have been evaluated. The results are discussed by considering the correlation between the fracture strength and mode and the influence of biaxial loading