7,419 research outputs found
High temperature indentation of WC/Co hardmetals
No full textWC/Co is the material of choice for most high wear applications such as hot forming operations, drilling, cutting, machining and wire drawing due to its superior wear properties. In the evaluation of its material properties to withstand wear, particularly that of its hardness, most of the available information is based on room temperature investigations. However, when WC/Co is used in the actual applications, it also experiences very high contact temperatures and this localized heating has a detrimental effect on the WC/Co material causing microstructural changes on the surface and sub-micron surface that affect its wear resistance properties. Creep also plays an important role in the damage mechanisms at elevated temperatures especially in prolonged contact to heat such as those used in hot metal forming operations. However, the current information on these high temperature properties is not complete and has contradictory results. Moreover, creep studies of WC/Co have been done at temperatures >800°C and information below this temperature is also lacking.Therefore, the primary goal of this thesis is to obtain a first time in-depth understanding of its high temperature property–composition–microstructure interaction that affects its wear properties from room temperature up to 800°C using comprehensive scratch tests, high temperature indentation hardness tests and indentation creep tests. These tests are supplemented with advanced 3D imaging, microstructural and composition analysis using SEM, EBSD, FIB, EDX, SIMS and laser confocal microscopy. Moreover, due to the lack of a commercially available high temperature microindentation test system, this thesis also aims to design, build and commission a high temperature and high vacuum microindentation test system to carry out these investigations.The results of the scratch tests showed new information on the damage mechanisms, particularly on the formation of a tribo-layer on the scratched surfaces and the evolution of surface damages incurred on the indenter tip which is found to affect the coefficient of friction. These provide baseline information on the damage mechanisms occurring at room temperature and were instrumental in the development of the methodology used in the high temperature indentation tests.The results of the high temperature indentation tests showed that the behaviour of the WC/Co hardmetals were categorised according to two temperature regimes. In the low temperature regime (20°C?500°C), the controlling factor is dominated by the WC grain size and that the damage mechanisms are characterised by plastic deformation of the WC grains via slip, intragranular fracture of the WC grains within the surface of the indent and intergranular fracture along the edges of the indent. In the high temperature regime (>500°C), the controlling factor was dominated by the Co binder and the damage mechanism showed first time observation of severe plastic flow accompanied by shape change on the WC grains. A tribo-layer was also found to exist on the room temperature indents but not on the high temperature indents. In addition, pitting of the indenter tip occurred at test temperatures >700°C and its contribution to the measured hardness were determined.These results provide significant information on the best combination of WC grain size and Co content that are suitable for applications requiring high hardness retention and creep resistance. In addition, the thorough investigation of the damage mechanisms ensuing at these temperatures is beneficial in the design of WC/Co with better wear properties at elevated temperatures
Consolidation of WC-Co nanocomposites synthesised by mechanical alloying
Full text linkA thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of PhilosophyThe influence of mechanical alloying (MA) milling time, temperature, sintering method and microstructure on the mechanical properties of a tungsten carbide-cobalt (WC-Co) hardmetal, based on 10wt% Co, has been established. The effects of high-energy milling for 30, 60, 180 and 300 min and the interrelation between milling time and powder properties, and the resultant effects on the mechanical properties of the consolidated WC-10Co material, has been obtained for a horizontally designed ball mill. Nanostructured WC-10Co powder was synthesised after 60 min cyclic milling at room temperature with an average WC domain size of 21 nm. In direct comparison, a WC-10Co composition MA at -30°C for 60 min produced an average WC domain size of 26 nm with a higher lattice strain. WC domain size showed a slight increase with milling time, measured at 27 nm after 300 min ball milling. Extended ball milling (300 min) reduced the mean particle size from 0.148 μm for 60 min milling to 0.117 μm. Thermal analysis showed that the onset temperature of the WC-Co eutectic was related to particle size with increased milling time reducing the onset temperature from 1344°C after 60 min milling to 1312°C after 300 min milling. Onset temperature was further reduced by the addition of vanadium carbide (VC), reducing the onset temperature to 1283°C after 300 min milling. Powder contamination increased with increased milling time with Fe content measured at ~ 3wt% after 300 min ball milling. Milling at -30°C reduced Fe contamination to an almost undetectable level. Increased ball milling time resulted in decreased levels of green density with the powders milled for 30 and 300 min achieving 62.5% and 59.5% TD, respectively. Relative density increased for the powder milled at -30°C compared to the RT milled powder due to its flattened, slightly rounded morphology. A large difference in VC starting particle size compared to WC and Co led to non-uniform dispersion of the inhibitor during milling. Densification and hardness reached optimum levels for the 60 min milled powder for both pressureless sintering and sinter-HIP. Both properties decreased with increased milling time, regardless of the sintering method. Low temperature milling resulted in a higher hardness value of 1390 HV30 compared to 1326 HV30 for the 60 min, RT milled material after pressureless sintering. Densification levels of the doped materials were restricted to < 90% TD for both sintering methods due to inhomogeneity in the microstructures. Palmqvist fracture toughness (WK) of the RT milled powders increased with increased milling time and increasing WC grain size for both sintering methods. WK reached 11.6 MN.m3/2 with 300 min milling after pressureless sintering but reached 16.1 MN.m32 for the same material after sinter-HIP due to the effect of mean WC grain size and binder phase mean free path. The -30°C milled powder exhibited higher fracture toughness for both sintering methods than the 60 min, RT milled material. Spark plasma sintering (SPS) showed that the onset of densification was dependent upon particle size with the powder from 300 min milling showing an onset temperature of ~ 800°C compared to ~ 1000°C for the 60 min milled powder. The low temperature milled powder showed an onset temperature of ~ 980°C, which suggested that low temperature milling provided enhanced densification kinetics
Investigation into laser re-melting of inconel 625 HVOF coating blended with WC
Full text linkHigh velocity oxy-fuel (HVOF) spraying of Diamalloy 1005 powders mixed with WC particles onto steel (304) is considered and laser re-melting of the resulting coatings is examined. Laser re-melting process is modeled to determine the melt layer thickness while temperature increase is formulated using the Fourier heating law. The morphological and metallurgical analyses prior and post laser re-melting process are carried out using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). X-ray diffraction (XRD) technique is used to determine the residual stress developed in the coating while the analytical formulation is adopted to predict the residual stress levels at the coating base material interface. The indentation tests are carried out to determine the Young’s modulus and fracture toughness of the coating prior to laser re-melting. Corrosion resistance of coating is measured using potentiodynamic polarization technique prior and post laser treatment process. The predictions of the melt layer thickness are in good agreement with experimental results. The presence of WC particles modifies temperature rise and its gradient in the coating while affecting the Young’s modulus, residual stress levels, and fracture toughness of the coating. The differences in the thermal properties of Inconel 625 powders and WC particles result in formation of small size cellular structure through polyphase solidification. WC dissolution in the central region of the large polycrystalline cells is observed due to the loss of carbon through carbonic gas formation. The results of corrosion tests prevail that significant improvement of corrosion resistance can be achieved after laser treatment process
Testing for Candidate gene linkage disequilibrium using a dense array of single nucleotide polymorphisms in case-parents studies
No full textSearching for disease-susceptibility loci by testing for Hardy-Weinberg disequilibrium in a gene bank of affected individuals
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New fluorene-acceptor random copolymers: Towards pure white light emission from a single polymer
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