Microstructural evolution in AISn-based gas atomised powder and thermally sprayed coatings

This thesis reports on the microstructure of AI-Sn based powders and the development of AI-Sn based coatings for automotive shell bearing applications deposited using the high velocity oxy-liquid fuel (HVOLF) thermal spray technique. The microstructure of the coating and its associated physical and chemical properties, such as microhardness and corrosion resistance, are investigated as a function of the HVOLF thermal spraying parameters. In particular, a detailed microstructural understanding of the thermal sprayed coatings is developed to explain the coating properties. Two alloy systems, AI-12wt. %Sn-1 wt. %Cu and AI-20wt. %Sn-3wt. %Si have been investigated in detail using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. The high resolution transmission electron microscope (HRTEM), electron energy loss spectroscopy (EELS) and energy filtered TEM have also been used to examine nanoscale precipitates as supplementary methods. The statistical image analysis of fine scale particulate dispersions has also been used to study the second phase evolution with annealing. The microstructure of the large gas atomised powder particles used in the HVOLF thermal spray process comprise dendritic Al and interdendritic Sn, whilst the small powder particles exhibit fine scale Sn particles distributed within an Al matrix. The as-sprayed coatings comprise a mixture of melted and partially melted splats due to the full and partial melting of the deposited powder. Nanoscale Sn particles distributed in the Al matrix are present in fully melted regions, whilst micron / sub-micron Sn particle distributions and Sn-particle free Al regions delineate partially melted regions. Cu remains in solid solution within the Al matrix of the AI-12wt. %Sn-1 wt. %Cu as-sprayed coatings, whilst Si formed nanoscale particles in the AI-20wt.%Sn-3wt.%Si as-sprayed coatings. The critical cooling rate to form the metastable liquid phase separationh within AI- 12wt%Sn alloys is put forward according to calculation. If the cooling rate is lower than the critical cooling rate, dendritic Al and interdendritic Sn are formed, thereby explaining the structure of large gas atomised powder particles. If the cooling rate is higher than the critical cooling rate, a liquid phase separation reaction occurs to form fine scale Sn dispersion. The calculated critical AI-12wt.%Sn powder diameter for liquid phase separation is close to the experimentally observed AI-12wt. %Sn-l wt. %Cu powder diameter. The discrepancy between experiment result and theoretical calculation is attributed to the additional element Cu promoting the liquid phase separation. The nano and sub-micron scale Sn distribution in small gas atomised powder particles and the as-sprayed coatings is attributed to the cooling rate being higher than the critical cooling rate. The dendritic structure of the large AI-Sn-Cu gas atomised powder is due to the cooling rate being lower than the critical value. Heat treatments are applied to the as-sprayed coatings to alter the mechanical and chemical properties, such as, microhardness and corrosion resistance, of the bearing material coatings. Annealing causes the nanoscale and sub-micron Sn particles to coarsen within both AI-12wt.%Sn-1 wt.%Cu and AI-20wt.%Sn-wt.3%Si coatings according to the analysis of SEM and TEM images. The Sn particles coarsen greatly within the AI- 12wt.%Sn-1wt.%Cu coatings annealed at 300°C for 5 hours, as compared with coatings annealed for 1 hour. The Ɵ’-phase (CuAl2) also precipitates in the AI-12wt.%Sn- 1 wt.%Cu coatings after annealing at 300°C. Annealing also causes fine scale Si particles to coarsen greatly in the Si containing alloy. The microhardness decreases in the annealed coatings for both alloys and is attributed to a coarsening of Sn particles and the release of residual strain within the as-sprayed coatings. As compared with the as-sprayed coatings and the coatings annealed at 300°C for 1 hour, the corrosion rate in 0.1M NaCI solution of Al-12wt.%Sn-1wt.%Cu coatings annealed at 450°C for 1 hour is very greatly reduced. However, an annealing temperature of ~450°C is not appropriate for these coatings because of the introduction of interlayer cracks and a coating / substrate reaction which might degrade the mechanical properties of the bearing

Characterisation of dispersions within annealed HVOLF thermally sprayed AlSnCu coatings

High velocity oxy-liquid fuel (HVOLF) AlSnCu coatings are characterised following annealing for up to 5 hours at 300°C. A combination of statistical analysis of BSE images and TEM observations demonstrate the decrease in the number of sub-micron and nanoscale Sn particles with annealing, commensurate with a decrease in the coating microhardness. TEM evidence further suggests the coarsening of nanoscale Sn through a mechanism of a liquid phase migration within the Al matrix. EELS and EFTEM additionally allow the identification of the precipitation of theta'

Microstructural evolution in AISn-based gas atomised powder and thermally sprayed coatings

This thesis reports on the microstructure of AI-Sn based powders and the development of AI-Sn based coatings for automotive shell bearing applications deposited using the high velocity oxy-liquid fuel (HVOLF) thermal spray technique. The microstructure of the coating and its associated physical and chemical properties, such as microhardness and corrosion resistance, are investigated as a function of the HVOLF thermal spraying parameters. In particular, a detailed microstructural understanding of the thermal sprayed coatings is developed to explain the coating properties. Two alloy systems, AI-12wt. %Sn-1 wt. %Cu and AI-20wt. %Sn-3wt. %Si have been investigated in detail using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. The high resolution transmission electron microscope (HRTEM), electron energy loss spectroscopy (EELS) and energy filtered TEM have also been used to examine nanoscale precipitates as supplementary methods. The statistical image analysis of fine scale particulate dispersions has also been used to study the second phase evolution with annealing. The microstructure of the large gas atomised powder particles used in the HVOLF thermal spray process comprise dendritic Al and interdendritic Sn, whilst the small powder particles exhibit fine scale Sn particles distributed within an Al matrix. The as-sprayed coatings comprise a mixture of melted and partially melted splats due to the full and partial melting of the deposited powder. Nanoscale Sn particles distributed in the Al matrix are present in fully melted regions, whilst micron / sub-micron Sn particle distributions and Sn-particle free Al regions delineate partially melted regions. Cu remains in solid solution within the Al matrix of the AI-12wt. %Sn-1 wt. %Cu as-sprayed coatings, whilst Si formed nanoscale particles in the AI-20wt.%Sn-3wt.%Si as-sprayed coatings. The critical cooling rate to form the metastable liquid phase separationh within AI- 12wt%Sn alloys is put forward according to calculation. If the cooling rate is lower than the critical cooling rate, dendritic Al and interdendritic Sn are formed, thereby explaining the structure of large gas atomised powder particles. If the cooling rate is higher than the critical cooling rate, a liquid phase separation reaction occurs to form fine scale Sn dispersion. The calculated critical AI-12wt.%Sn powder diameter for liquid phase separation is close to the experimentally observed AI-12wt. %Sn-l wt. %Cu powder diameter. The discrepancy between experiment result and theoretical calculation is attributed to the additional element Cu promoting the liquid phase separation. The nano and sub-micron scale Sn distribution in small gas atomised powder particles and the as-sprayed coatings is attributed to the cooling rate being higher than the critical cooling rate. The dendritic structure of the large AI-Sn-Cu gas atomised powder is due to the cooling rate being lower than the critical value. Heat treatments are applied to the as-sprayed coatings to alter the mechanical and chemical properties, such as, microhardness and corrosion resistance, of the bearing material coatings. Annealing causes the nanoscale and sub-micron Sn particles to coarsen within both AI-12wt.%Sn-1 wt.%Cu and AI-20wt.%Sn-wt.3%Si coatings according to the analysis of SEM and TEM images. The Sn particles coarsen greatly within the AI- 12wt.%Sn-1wt.%Cu coatings annealed at 300°C for 5 hours, as compared with coatings annealed for 1 hour. The Ɵ’-phase (CuAl2) also precipitates in the AI-12wt.%Sn- 1 wt.%Cu coatings after annealing at 300°C. Annealing also causes fine scale Si particles to coarsen greatly in the Si containing alloy. The microhardness decreases in the annealed coatings for both alloys and is attributed to a coarsening of Sn particles and the release of residual strain within the as-sprayed coatings. As compared with the as-sprayed coatings and the coatings annealed at 300°C for 1 hour, the corrosion rate in 0.1M NaCI solution of Al-12wt.%Sn-1wt.%Cu coatings annealed at 450°C for 1 hour is very greatly reduced. However, an annealing temperature of ~450°C is not appropriate for these coatings because of the introduction of interlayer cracks and a coating / substrate reaction which might degrade the mechanical properties of the bearing

Interface engineering of graphene for universal applications as both anode and cathode in organic photovoltaics

The high transparency of graphene, together with its good electrical conductivity and mechanical robustness, enable its use as transparent electrodes in optoelectronic devices such as solar cells. While initial demonstrations of graphene-based organic photovoltaics (OPV) have been promising, realization of scalable technologies remains challenging due to their performance and, critically, poor device reproducibility and yield. In this work, we demonstrate by engineering the interface between graphene and organic layers, device performance and yield become close to devices using indium tin oxide. Our study confirms that the key issue leading to the poor performance or irreproducibility in graphene-based OPV originates from the graphene interface, and can be addressed by a simple interface modification method introduced in this work. We also show similar approach allows graphene to be used as cathode in inverted OPV geometry, thereby demonstrating the universal application of graphene as transparent conductors for both the anode and cathode.open11

High Density Ti6Al4V via Slim Processing: Microstructure and Mechanical Properties

This paper investigates a density improvement method for Ti6Al4V alloy processed by the selective laser melting method. A modified inert gas inlet baffle has been employed to develop improved mechanical properties for these materials. Comparisons of the top surface and cross-section porosities of solid blocks processed by the original and modified gas inlet baffles indicate that the modified baffle greatly increases the properties of the processing blocks. Results showed that the porosity of the Ti6Al4V alloy was lower than 0.1% by area. The microstructure of the SLM Ti6Al4V alloy exhibited martensitic α' phase. The UTS tensile strength was 920-960MPa and the elongation at the fracture was 3-5%. The fracture surfaces of the tensile samples demonstrated a mixture of ductile and brittle fracture.Mechanical Engineerin

Downregulating Notch counteracts KrasG12D-induced ERK activation and oxidative phosphorylation in myeloproliferative neoplasm.

The Notch signaling pathway contributes to the pathogenesis of a wide spectrum of human cancers, including hematopoietic malignancies. Its functions are highly dependent on the specific cellular context. Gain-of-function NOTCH1 mutations are prevalent in human T-cell leukemia, while loss of Notch signaling is reported in myeloid leukemias. Here, we report a novel oncogenic function of Notch signaling in oncogenic Kras-induced myeloproliferative neoplasm (MPN). We find that downregulation of Notch signaling in hematopoietic cells via DNMAML expression or Pofut1 deletion significantly blocks MPN development in KrasG12D mice in a cell-autonomous manner. Further mechanistic studies indicate that inhibition of Notch signaling upregulates Dusp1, a dual phosphatase that inactivates p-ERK, and downregulates cytokine-evoked ERK activation in KrasG12D cells. Moreover, mitochondrial metabolism is greatly enhanced in KrasG12D cells but significantly reprogrammed by DNMAML close to that in control cells. Consequently, cell proliferation and expanded myeloid compartment in KrasG12D mice are significantly reduced. Consistent with these findings, combined inhibition of the MEK/ERK pathway and mitochondrial oxidative phosphorylation effectively inhibited the growth of human and mouse leukemia cells in vitro. Our study provides a strong rational to target both ERK signaling and aberrant metabolism in oncogenic Ras-driven myeloid leukemia

Delivery of the Sox9 gene promotes chondrogenic differentiation of human umbilical cord blood-derived mesenchymal stem cells in an in vitro model

SRY-related high-mobility-group box 9 (Sox9) gene is a cartilage-specific transcription factor that plays essential roles in chondrocyte differentiation and cartilage formation. The aim of this study was to investigate the feasibility of genetic delivery of Sox9 to enhance chondrogenic differentiation of human umbilical cord blood-derived mesenchymal stem cells (hUC-MSCs). After they were isolated from human umbilical cord blood within 24 h after delivery of neonates, hUC-MSCs were untreated or transfected with a human Sox9-expressing plasmid or an empty vector. The cells were assessed for morphology and chondrogenic differentiation. The isolated cells with a fibroblast-like morphology in monolayer culture were positive for the MSC markers CD44, CD105, CD73, and CD90, but negative for the differentiation markers CD34, CD45, CD19, CD14, or major histocompatibility complex class II. Sox9 overexpression induced accumulation of sulfated proteoglycans, without altering the cellular morphology. Immunocytochemistry demonstrated that genetic delivery of Sox9 markedly enhanced the expression of aggrecan and type II collagen in hUC-MSCs compared with empty vector-transfected counterparts. Reverse transcription-polymerase chain reaction analysis further confirmed the elevation of aggrecan and type II collagen at the mRNA level in Sox9-transfected cells. Taken together, short-term Sox9 overexpression facilitates chondrogenesis of hUC-MSCs and may thus have potential implications in cartilage tissue engineering.publishedVersio

Return of 4U~1730--22 after 49 years silence: the peculiar burst properties of the 2021/2022 outbursts observed by Insight-HXMT

After in quiescence for 49 years, 4U~1730--22 became active and had two outbursts in 2021 \& 2022; ten thermonuclear X-ray bursts were detected with Insight-HXMT. Among them, the faintest burst showed a double-peaked profile, placing the source as the 5th accreting neutron star (NS) exhibiting double/triple-peaked type-I X-ray bursts; the other bursts showed photospheric radius expansion (PRE). The properties of double-peaked non-PRE burst indicate that it could be related to a stalled burning front. For the five bright PRE bursts, apart from the emission from the neutron star (NS) surface, we find the residuals both in the soft ($$10 keV) X-ray band. Time-resolved spectroscopy reveals that the excess can be attributed to an enhanced pre-burst/persistent emission or the Comptonization of the burst emission by the corona/boundary-layer. We find, the burst emission shows a rise until the photosphere touches down to the NS surface rather than the theoretical predicted constant Eddington luminosity. The shortage of the burst emission in the early rising phase is beyond the occlusion by the disk. We speculate that the findings above correspond to that the obscured part (not only the lower part) of the NS surface is exposed to the line of sight due to the evaporation of the obscured material by the burst emission, or the burst emission is anisotropic ($\xi>1$) in the burst early phase. In addition, based on the average flux of PRE bursts at their touch-down time, we derive a distance estimation as 10.4 kpc.Comment: arXiv admin note: substantial text overlap with arXiv:2208.13556; text overlap with arXiv:2208.1212