ABSTRACT One of the major problems encountered in hard anodizing cast aluminium alloys is the ina... more ABSTRACT One of the major problems encountered in hard anodizing cast aluminium alloys is the inability to grow a sufficiently thick and homogeneous anodic oxide layer. This work analyzes the possibility of using a semisolid metal forming technology (SSM), like Thixocasting, to solve this surface treatment problem. When compared to conventional castings, Thixocasting reduces porosity and amount of trapped gas, which allow cast components to be heat treated without blistering and the chemical reactivity of the surface also changes. The aim of this work is to study the heat treatment influence on the anodization of A357 aluminium alloy produced by a Thixocasting process. SEM was used to examine the microstructural features found on, within and under the anodized layer. Experiments using a tribometer (ball-on-disc configuration) have been performed in order to evaluate the friction and wear properties of the different layers. The anodic oxide layer formed on the as-cast and T5 substrates had a similar appearance. However, the anodic oxide layer formed over the T6 substrate exhibited a more homogeneous thickness and spheroidized silicon particles became embedded in the anodic oxide layer. Furthermore, the T6 anodized samples showed a slight reduction in friction coefficient and specific wear rate than the as-cast anodized substrate.
One of the most important uses of HVOF thermal sprayed CrC-NiCr coatings is for wear resistance. ... more One of the most important uses of HVOF thermal sprayed CrC-NiCr coatings is for wear resistance. In this work Cr3C275-NiCr25 coatings were obtained by high-velocity oxy-fuel system from three agglomerated feedstock powders with various powder size distributions (- 30, -10 and –5 µm). The powders were agglomerated, sintered, plasmafused and crushed, in order to increase their density. The coating microstructures were characterised by SEM microscopy. Differences in coating roughness have been determined by profilometry. The ultra-microindentation technique was applied to measure the hardness and the elasto-plastic properties of the coatings. Experiments using a tribometer (pin on disc configuration) under lubricated and dry conditions have been performed in order to evaluate the friction and wear properties of the different coatings. It was found that the coatings obtained with the lowest feedstock powder size presented the best sliding wear resistance under all the conditions. This fact could be explained in terms of differences of cohesion between the carbide particles and the binder phase. The lower feedstock powder presents a lower carbide particle size that involves a better distribution and cohesion of the chromium carbide with the NiCr binder phase. This fact leads a minor production of “third body” hard particles in the wear tests that influenced quite considerably in the final wear rate of the studied coatings.
ABSTRACT In this work the affects of laser characteristics on surface morphology, microstructure,... more ABSTRACT In this work the affects of laser characteristics on surface morphology, microstructure, microhardness and wear rates of high velocity oxygen fuel (HVOF) WC‐CoCr coatings were investigated. Continuous and pulsed laser treatments were applied on the HVOF coatings using a CO2 laser of 10.6 μm wavelength. Continuous laser treatment significantly improved homogeneity of the coatings. Pulsed laser treatment produced significantly higher discontinuities compared to continuous mode. Microhardness increased by 45% for continuous mode laser modified WC‐CoCr coatings. However, wear resistance deteriorated, with a notable 40% increase in wear rate. The variations in coating properties were attributed to the re‐melting and rapid quenching of the CoCr bonding matrix allowing for WC redistribution.
Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion a... more Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work the oxidation behavior of conventional and nanostructured HVOF NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Free-standing bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at a 1000°C for different time periods in order to form the thermally grown oxide (TGO) layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared to that of the conventional one. This behavior is a result of the formation of a continuous Al2O3 layer on the top surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating.
ABSTRACT One of the major problems encountered in hard anodizing cast aluminium alloys is the ina... more ABSTRACT One of the major problems encountered in hard anodizing cast aluminium alloys is the inability to grow a sufficiently thick and homogeneous anodic oxide layer. This work analyzes the possibility of using a semisolid metal forming technology (SSM), like Thixocasting, to solve this surface treatment problem. When compared to conventional castings, Thixocasting reduces porosity and amount of trapped gas, which allow cast components to be heat treated without blistering and the chemical reactivity of the surface also changes. The aim of this work is to study the heat treatment influence on the anodization of A357 aluminium alloy produced by a Thixocasting process. SEM was used to examine the microstructural features found on, within and under the anodized layer. Experiments using a tribometer (ball-on-disc configuration) have been performed in order to evaluate the friction and wear properties of the different layers. The anodic oxide layer formed on the as-cast and T5 substrates had a similar appearance. However, the anodic oxide layer formed over the T6 substrate exhibited a more homogeneous thickness and spheroidized silicon particles became embedded in the anodic oxide layer. Furthermore, the T6 anodized samples showed a slight reduction in friction coefficient and specific wear rate than the as-cast anodized substrate.
One of the most important uses of HVOF thermal sprayed CrC-NiCr coatings is for wear resistance. ... more One of the most important uses of HVOF thermal sprayed CrC-NiCr coatings is for wear resistance. In this work Cr3C275-NiCr25 coatings were obtained by high-velocity oxy-fuel system from three agglomerated feedstock powders with various powder size distributions (- 30, -10 and –5 µm). The powders were agglomerated, sintered, plasmafused and crushed, in order to increase their density. The coating microstructures were characterised by SEM microscopy. Differences in coating roughness have been determined by profilometry. The ultra-microindentation technique was applied to measure the hardness and the elasto-plastic properties of the coatings. Experiments using a tribometer (pin on disc configuration) under lubricated and dry conditions have been performed in order to evaluate the friction and wear properties of the different coatings. It was found that the coatings obtained with the lowest feedstock powder size presented the best sliding wear resistance under all the conditions. This fact could be explained in terms of differences of cohesion between the carbide particles and the binder phase. The lower feedstock powder presents a lower carbide particle size that involves a better distribution and cohesion of the chromium carbide with the NiCr binder phase. This fact leads a minor production of “third body” hard particles in the wear tests that influenced quite considerably in the final wear rate of the studied coatings.
ABSTRACT In this work the affects of laser characteristics on surface morphology, microstructure,... more ABSTRACT In this work the affects of laser characteristics on surface morphology, microstructure, microhardness and wear rates of high velocity oxygen fuel (HVOF) WC‐CoCr coatings were investigated. Continuous and pulsed laser treatments were applied on the HVOF coatings using a CO2 laser of 10.6 μm wavelength. Continuous laser treatment significantly improved homogeneity of the coatings. Pulsed laser treatment produced significantly higher discontinuities compared to continuous mode. Microhardness increased by 45% for continuous mode laser modified WC‐CoCr coatings. However, wear resistance deteriorated, with a notable 40% increase in wear rate. The variations in coating properties were attributed to the re‐melting and rapid quenching of the CoCr bonding matrix allowing for WC redistribution.
Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion a... more Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion and oxidation. They consist of a metal bond coat (MCrAlY, M = Ni, Co) and a ceramic top layer (ZrO2/Y2O3). In this work the oxidation behavior of conventional and nanostructured HVOF NiCrAlY coatings has been compared. Commercially available NiCrAlY powder was mechanically cryomilled and HVOF sprayed on a nickel alloy foil to form a nanocrystalline coating. Free-standing bodies of conventional and nanostructured HVOF NiCrAlY coatings were oxidized at a 1000°C for different time periods in order to form the thermally grown oxide (TGO) layer. The experiments show an improvement in oxidation resistance in the nanostructured coating when compared to that of the conventional one. This behavior is a result of the formation of a continuous Al2O3 layer on the top surface of the nanostructured HVOF NiCrAlY coating. This layer protects the coating from further oxidation and avoids the formation of mixed oxide protrusions present in the conventional coating.
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