Thermally-sprayed alumina based materials, e.g., alumina-titania (Al 2 O 3-TiO 2), are commonly a... more Thermally-sprayed alumina based materials, e.g., alumina-titania (Al 2 O 3-TiO 2), are commonly applied as wear resistant coatings in industrial applications. Properties of the coatings depend on the spray process, powder morphology, and chemical composition of the powder. In this study, wear resistant coatings from Al 2 O 3 and Al 2 O 3-13TiO 2 powders were sprayed with plasma and high-velocity oxygen-fuel (HVOF) spray processes. Both, fused and crushed, and agglomerated and sintered Al 2 O 3-13TiO 2 powders were studied and compared to pure Al 2 O 3. The coatings were tested for abrasion, erosion, and cavitation resistances in order to study the effect of the coating structure on the wear behavior. Improved coating properties were achieved when agglomerated and sintered nanostructured Al 2 O 3-13TiO 2 powder was used in plasma spraying. Coatings with the highest wear resistance in all tests were produced by HVOF spraying from fused and crushed powders.
ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviou... more ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviour of WC-10Co4Cr hardmetal coatings, representative of the existing state-of-the-art. A commercial feedstock powder with two different particle size distributions was sprayed onto carbon steel substrates using two HVOF and two HVAF spray processes.
This study deals with the corrosion behaviour of high-pressure cold-sprayed Ta coatings, compared... more This study deals with the corrosion behaviour of high-pressure cold-sprayed Ta coatings, compared with Ta bulk material and inert-atmosphere plasma-sprayed Ta coatings. Electrochemical polarization and electrochemical impedance spectroscopy measurements are carried out in 1M KOH solution; immersion tests are performed in 1wt.% HF solution, and the resistance to high-temperature oxidation and molten salt attack is evaluated by hot corrosion tests.
ABSTRACT Thermally sprayed iron-based coatings are being widely studied as alternative solution t... more ABSTRACT Thermally sprayed iron-based coatings are being widely studied as alternative solution to conventional hardmetal (cermet) and Ni-based coatings for wear and corrosion applications in order to reduce costs, limit environmental impact and enhance the health safety. The aim of the present work is to study the cavitation erosion behaviour in distilled water and the corrosion properties in acidic solution of four high-velocity oxy-fuel (HVOF) sprayed Fe-based composite coatings. Fe-Cr-Ni-B-C powder was selected for its good sliding wear properties. In addition, a powder composition with an addition of Mo was studied in order to increase the corrosion resistance whereas additions of 20 wt. % and 40 wt. % WC-12Co as blended powder mixtures were investigated in order to increase wear resistance. Improvement of coating properties was significant with the advanced powder compositions. Dense coating structures with low porosity were detected with microstructural characterization. In addition, good cavitation wear resistance was achieved. The cavitation resistance of customized Fe-based coating with Mo addition was reported to be twice as high as that of conventional Ni-based and WC-CoCr coatings. The corrosion properties of HVOF Fe-based coatings were also evaluated by studying electrochemical behaviour in order to analyse their potential to use as corrosion barrier coatings.
ASME 2011 9th International Conference on Fuel Cell Science, Engineering and Technology, 2011
ABSTRACT Protective coatings are used on ferritic stainless steel interconnects of solid oxide fu... more ABSTRACT Protective coatings are used on ferritic stainless steel interconnects of solid oxide fuel cells (SOFCs) to prevent the oxidation and evaporation of volatile chromium compounds. Oxide scale is formed of chromium oxide (Cr2 O3 ) which tends to react with the oxygen and water, forming chromium trioxide (CrO3 ) and chromium hydroxides (Cr2 (OH)2 ). These compounds will migrate to the triple phase barrier (TPB) of a cathode and reduce back to Cr2 O3 . This reaction pathway is a notable reason for the degradation phenomena of the cell. Plasma spraying (PS) and suspension plasma spraying (SPS) were studied as possible manufacturing processes for thin Mn-Co-(Fe) spinel coatings. Powder for PS was manufactured by using a solid state reaction method from carbonates and oxide to form a MnCo1.8 Fe0.2 O4 and powder for SPS by co-precipitation process from nitrates to form a MnCo2 O4 spinel structure. Using PS, coatings with thin and relatively dense structures were obtained. The composition of the coatings was homogeneous although, the decomposition of the spinel structure was noticed. The crystal structures of the PS coatings were partially restored by a separate annealing process. The spray parameters had a strong influence on the coating structure and the composition when SPS was used. The most homogenous coating structure was formed when low energy spraying parameters were used, whereas high energy parameters formed a columnar structure with larger cobalt rich areas. The decomposed spinel structure of the SPS coatings were fully restored by the annealing treatment. In SPS, more process optimization is needed to improve the coating quality and especially denseness.
ABSTRACT The current study is focused on deposition of different carbide based powders as WC/10Co... more ABSTRACT The current study is focused on deposition of different carbide based powders as WC/10Co/4Cr, Cr3C2/25NiCr, CrC/37WC/18M using High Velocity Oxygen Fuel (HVOF) spraying technique onto the surface of low carbon steel substrates. The obtained coatings were characterized by means of X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) to study their phases and morphology. Hardness and porosity values of the coatings were also measured. The sliding wear behavior was evaluated using the pin-on-disk method and the corrosion resistance was determined by open cell potential measurements and salt spray tests. Comparative results of the investigated samples showed that the WC/10Co/4Cr coating had the best wear properties and the Cr3C2/25NiCr coating had the best corrosion resistance.
ABSTRACT Cold spraying is a coating deposition method for the production of high-quality metallic... more ABSTRACT Cold spraying is a coating deposition method for the production of high-quality metallic coatings. Copper, for instance, is widely used as a coating material due to its suitability for cold spray process. Cold-sprayed copper coatings are known to have interesting technical properties such as high electrical and thermal conductivity. Low-pressure cold spraying (LPCS) is a cold spray technique of preparing relatively dense and pure copper coatings. The deposition behaviour of low-pressure cold-sprayed spherical and dendritic copper particles impacting the substrate was evaluated in this study. The effect of powder type was clearly detected in the observations of single spray beads. Higher deposition efficiency and denser coatings were prepared by using the spherical copper powder.
ABSTRACT Cold spraying enables high quality Cu coatings to be deposited for applications where hi... more ABSTRACT Cold spraying enables high quality Cu coatings to be deposited for applications where high electrical and/or thermal conductivity is needed. Fully dense Cu coatings can provide an effective corrosion barrier in specific environments. The structure of cold-sprayed Cu coatings is characterized by high deformation which imparts excellent properties. Coating properties depend on powder, the cold spray process and post treatments. First of all, powder characteristics have a strong influence on the formation of pure coatings. Secondly, cold spraying provides dense, adherent, and conductive coatings by using HPCS and LPCS. Furthermore, an addition of Al2O3 particles to the Cu powder in LPCS process significantly improves coating properties. Also, heat treatments improve electrical conductivity. This study summarizes optimal characteristics of Cu powder optimized for cold spraying, achieving high coating quality and compares properties of HPCS Cu, LPCS Cu and Cu+Al2O3 coatings prepared from the same batch of OFHC Cu powder.
ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviou... more ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviour of WC-10Co4Cr hardmetal coatings, representative of the existing state-of-the-art. A commercial feedstock powder with two different particle size distributions was sprayed onto carbon steel substrates using two HVOF and two HVAF spray processes.
ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviou... more ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviour of WC-10Co4Cr hardmetal coatings, representative of the existing state-of-the-art. A commercial feedstock powder with two different particle size distributions was sprayed onto carbon steel substrates using two HVOF and two HVAF spray processes.
ABSTRACT Fe-based coatings are promising alternatives to Ni-based ones, because of lower cost and... more ABSTRACT Fe-based coatings are promising alternatives to Ni-based ones, because of lower cost and lower toxicity. Following a previous research, where the sliding wear resistance of HVOF-sprayed Fe–Cr–Ni–Si–B–C alloy coatings was found to compare favorably with that of a Ni–Cr–B–Si–C alloy and of electroplated chromium, the present study investigates the wear resistance of Fe–Cr–Ni–Si–B–C + WC–Co composite coatings. The Fe-alloy feedstock powder was therefore blended with 0, 20 and 40 wt.% of a WC–12 wt.% Co powder and sprayed by HVOF and HVAF processes. HVAF-sprayed coatings exhibit less structural alteration than HVOF-sprayed ones, which results in lower intrinsic nanohardness of both Fe-alloy and WC–Co splats; however, HVOF- and HVAF-sprayed coatings exhibit similar Vickers microhardness. Somewhat poorer interlamellar bonding in HVAF-sprayed coatings results in a greater tendency to microcracking during dry sliding wear testing at room temperature; however, dry sliding wear rates of HVOF- and HVAF-sprayed samples never differ significantly. The reinforcing effect of WC–Co decreases the wear rate of composite coatings (≈ 10− 6 mm3/(Nm)) by more than order of magnitude, compared to unreinforced ones (≈ 1–2 ∗ 10− 5 mm3/(Nm)). As the test temperature is increased to 400 °C and 700 °C, the dry sliding wear rates of all samples increase (up to 10− 4 mm3/(Nm) or greater). The greatest changes are observed when the WC–Co content is larger, as it suffers from oxidation and thermal alteration more than the Fe-alloy matrix. The abrasive wear resistance of the Fe-based coatings, evaluated by rubber-wheel testing, is also significantly improved by the addition of WC–Co.
ABSTRACT Thermally sprayed iron-based coatings are being widely studied as alternative solution t... more ABSTRACT Thermally sprayed iron-based coatings are being widely studied as alternative solution to conventional hardmetal (cermet) and Ni-based coatings for wear and corrosion applications in order to reduce costs, limit environmental impact and enhance the health safety. The aim of the present work is to study the cavitation erosion behaviour in distilled water and the corrosion properties in acidic solution of four high-velocity oxy-fuel (HVOF) sprayed Fe-based composite coatings. Fe-Cr-Ni-B-C powder was selected for its good sliding wear properties. In addition, a powder composition with an addition of Mo was studied in order to increase the corrosion resistance whereas additions of 20 wt. % and 40 wt. % WC-12Co as blended powder mixtures were investigated in order to increase wear resistance. Improvement of coating properties was significant with the advanced powder compositions. Dense coating structures with low porosity were detected with microstructural characterization. In addition, good cavitation wear resistance was achieved. The cavitation resistance of customized Fe-based coating with Mo addition was reported to be twice as high as that of conventional Ni-based and WC-CoCr coatings. The corrosion properties of HVOF Fe-based coatings were also evaluated by studying electrochemical behaviour in order to analyse their potential to use as corrosion barrier coatings.
ABSTRACT Cold spraying has shown its potential to produce metallic and composite coatings with hi... more ABSTRACT Cold spraying has shown its potential to produce metallic and composite coatings with high quality and performance. For instance, the impermeability of the coatings is the criterion for the corrosion resistance and thus, fully dense coatings can act as real corrosion barrier coatings. Our previous study has demonstrated the good corrosion properties of high-pressure cold-sprayed (HPCS) Ni and NiCu coatings whereas the present study focuses on the analysis of structural characteristics behind the dense coatings. Microstructures of as-sprayed and heat-treated HPCS Ni, Ni20Cu and Ni20Cu + Al2O3 coatings have been evaluated with FESEM from top-view direction. This revealed clearly particle deformation and particle boundaries whereas particle bonding was evaluated with cavitation–erosion test. Density was proven with Corrodkote test and impermeable HPCS NiCu + Al2O3 coating structures were achieved. Furthermore, cavitation–erosion resistance of NiCu coating was improved by heat treatment.
Thermally-sprayed alumina based materials, e.g., alumina-titania (Al 2 O 3-TiO 2), are commonly a... more Thermally-sprayed alumina based materials, e.g., alumina-titania (Al 2 O 3-TiO 2), are commonly applied as wear resistant coatings in industrial applications. Properties of the coatings depend on the spray process, powder morphology, and chemical composition of the powder. In this study, wear resistant coatings from Al 2 O 3 and Al 2 O 3-13TiO 2 powders were sprayed with plasma and high-velocity oxygen-fuel (HVOF) spray processes. Both, fused and crushed, and agglomerated and sintered Al 2 O 3-13TiO 2 powders were studied and compared to pure Al 2 O 3. The coatings were tested for abrasion, erosion, and cavitation resistances in order to study the effect of the coating structure on the wear behavior. Improved coating properties were achieved when agglomerated and sintered nanostructured Al 2 O 3-13TiO 2 powder was used in plasma spraying. Coatings with the highest wear resistance in all tests were produced by HVOF spraying from fused and crushed powders.
ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviou... more ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviour of WC-10Co4Cr hardmetal coatings, representative of the existing state-of-the-art. A commercial feedstock powder with two different particle size distributions was sprayed onto carbon steel substrates using two HVOF and two HVAF spray processes.
This study deals with the corrosion behaviour of high-pressure cold-sprayed Ta coatings, compared... more This study deals with the corrosion behaviour of high-pressure cold-sprayed Ta coatings, compared with Ta bulk material and inert-atmosphere plasma-sprayed Ta coatings. Electrochemical polarization and electrochemical impedance spectroscopy measurements are carried out in 1M KOH solution; immersion tests are performed in 1wt.% HF solution, and the resistance to high-temperature oxidation and molten salt attack is evaluated by hot corrosion tests.
ABSTRACT Thermally sprayed iron-based coatings are being widely studied as alternative solution t... more ABSTRACT Thermally sprayed iron-based coatings are being widely studied as alternative solution to conventional hardmetal (cermet) and Ni-based coatings for wear and corrosion applications in order to reduce costs, limit environmental impact and enhance the health safety. The aim of the present work is to study the cavitation erosion behaviour in distilled water and the corrosion properties in acidic solution of four high-velocity oxy-fuel (HVOF) sprayed Fe-based composite coatings. Fe-Cr-Ni-B-C powder was selected for its good sliding wear properties. In addition, a powder composition with an addition of Mo was studied in order to increase the corrosion resistance whereas additions of 20 wt. % and 40 wt. % WC-12Co as blended powder mixtures were investigated in order to increase wear resistance. Improvement of coating properties was significant with the advanced powder compositions. Dense coating structures with low porosity were detected with microstructural characterization. In addition, good cavitation wear resistance was achieved. The cavitation resistance of customized Fe-based coating with Mo addition was reported to be twice as high as that of conventional Ni-based and WC-CoCr coatings. The corrosion properties of HVOF Fe-based coatings were also evaluated by studying electrochemical behaviour in order to analyse their potential to use as corrosion barrier coatings.
ASME 2011 9th International Conference on Fuel Cell Science, Engineering and Technology, 2011
ABSTRACT Protective coatings are used on ferritic stainless steel interconnects of solid oxide fu... more ABSTRACT Protective coatings are used on ferritic stainless steel interconnects of solid oxide fuel cells (SOFCs) to prevent the oxidation and evaporation of volatile chromium compounds. Oxide scale is formed of chromium oxide (Cr2 O3 ) which tends to react with the oxygen and water, forming chromium trioxide (CrO3 ) and chromium hydroxides (Cr2 (OH)2 ). These compounds will migrate to the triple phase barrier (TPB) of a cathode and reduce back to Cr2 O3 . This reaction pathway is a notable reason for the degradation phenomena of the cell. Plasma spraying (PS) and suspension plasma spraying (SPS) were studied as possible manufacturing processes for thin Mn-Co-(Fe) spinel coatings. Powder for PS was manufactured by using a solid state reaction method from carbonates and oxide to form a MnCo1.8 Fe0.2 O4 and powder for SPS by co-precipitation process from nitrates to form a MnCo2 O4 spinel structure. Using PS, coatings with thin and relatively dense structures were obtained. The composition of the coatings was homogeneous although, the decomposition of the spinel structure was noticed. The crystal structures of the PS coatings were partially restored by a separate annealing process. The spray parameters had a strong influence on the coating structure and the composition when SPS was used. The most homogenous coating structure was formed when low energy spraying parameters were used, whereas high energy parameters formed a columnar structure with larger cobalt rich areas. The decomposed spinel structure of the SPS coatings were fully restored by the annealing treatment. In SPS, more process optimization is needed to improve the coating quality and especially denseness.
ABSTRACT The current study is focused on deposition of different carbide based powders as WC/10Co... more ABSTRACT The current study is focused on deposition of different carbide based powders as WC/10Co/4Cr, Cr3C2/25NiCr, CrC/37WC/18M using High Velocity Oxygen Fuel (HVOF) spraying technique onto the surface of low carbon steel substrates. The obtained coatings were characterized by means of X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) to study their phases and morphology. Hardness and porosity values of the coatings were also measured. The sliding wear behavior was evaluated using the pin-on-disk method and the corrosion resistance was determined by open cell potential measurements and salt spray tests. Comparative results of the investigated samples showed that the WC/10Co/4Cr coating had the best wear properties and the Cr3C2/25NiCr coating had the best corrosion resistance.
ABSTRACT Cold spraying is a coating deposition method for the production of high-quality metallic... more ABSTRACT Cold spraying is a coating deposition method for the production of high-quality metallic coatings. Copper, for instance, is widely used as a coating material due to its suitability for cold spray process. Cold-sprayed copper coatings are known to have interesting technical properties such as high electrical and thermal conductivity. Low-pressure cold spraying (LPCS) is a cold spray technique of preparing relatively dense and pure copper coatings. The deposition behaviour of low-pressure cold-sprayed spherical and dendritic copper particles impacting the substrate was evaluated in this study. The effect of powder type was clearly detected in the observations of single spray beads. Higher deposition efficiency and denser coatings were prepared by using the spherical copper powder.
ABSTRACT Cold spraying enables high quality Cu coatings to be deposited for applications where hi... more ABSTRACT Cold spraying enables high quality Cu coatings to be deposited for applications where high electrical and/or thermal conductivity is needed. Fully dense Cu coatings can provide an effective corrosion barrier in specific environments. The structure of cold-sprayed Cu coatings is characterized by high deformation which imparts excellent properties. Coating properties depend on powder, the cold spray process and post treatments. First of all, powder characteristics have a strong influence on the formation of pure coatings. Secondly, cold spraying provides dense, adherent, and conductive coatings by using HPCS and LPCS. Furthermore, an addition of Al2O3 particles to the Cu powder in LPCS process significantly improves coating properties. Also, heat treatments improve electrical conductivity. This study summarizes optimal characteristics of Cu powder optimized for cold spraying, achieving high coating quality and compares properties of HPCS Cu, LPCS Cu and Cu+Al2O3 coatings prepared from the same batch of OFHC Cu powder.
ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviou... more ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviour of WC-10Co4Cr hardmetal coatings, representative of the existing state-of-the-art. A commercial feedstock powder with two different particle size distributions was sprayed onto carbon steel substrates using two HVOF and two HVAF spray processes.
ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviou... more ABSTRACT This paper provides a comprehensive assessment of the sliding and abrasive wear behaviour of WC-10Co4Cr hardmetal coatings, representative of the existing state-of-the-art. A commercial feedstock powder with two different particle size distributions was sprayed onto carbon steel substrates using two HVOF and two HVAF spray processes.
ABSTRACT Fe-based coatings are promising alternatives to Ni-based ones, because of lower cost and... more ABSTRACT Fe-based coatings are promising alternatives to Ni-based ones, because of lower cost and lower toxicity. Following a previous research, where the sliding wear resistance of HVOF-sprayed Fe–Cr–Ni–Si–B–C alloy coatings was found to compare favorably with that of a Ni–Cr–B–Si–C alloy and of electroplated chromium, the present study investigates the wear resistance of Fe–Cr–Ni–Si–B–C + WC–Co composite coatings. The Fe-alloy feedstock powder was therefore blended with 0, 20 and 40 wt.% of a WC–12 wt.% Co powder and sprayed by HVOF and HVAF processes. HVAF-sprayed coatings exhibit less structural alteration than HVOF-sprayed ones, which results in lower intrinsic nanohardness of both Fe-alloy and WC–Co splats; however, HVOF- and HVAF-sprayed coatings exhibit similar Vickers microhardness. Somewhat poorer interlamellar bonding in HVAF-sprayed coatings results in a greater tendency to microcracking during dry sliding wear testing at room temperature; however, dry sliding wear rates of HVOF- and HVAF-sprayed samples never differ significantly. The reinforcing effect of WC–Co decreases the wear rate of composite coatings (≈ 10− 6 mm3/(Nm)) by more than order of magnitude, compared to unreinforced ones (≈ 1–2 ∗ 10− 5 mm3/(Nm)). As the test temperature is increased to 400 °C and 700 °C, the dry sliding wear rates of all samples increase (up to 10− 4 mm3/(Nm) or greater). The greatest changes are observed when the WC–Co content is larger, as it suffers from oxidation and thermal alteration more than the Fe-alloy matrix. The abrasive wear resistance of the Fe-based coatings, evaluated by rubber-wheel testing, is also significantly improved by the addition of WC–Co.
ABSTRACT Thermally sprayed iron-based coatings are being widely studied as alternative solution t... more ABSTRACT Thermally sprayed iron-based coatings are being widely studied as alternative solution to conventional hardmetal (cermet) and Ni-based coatings for wear and corrosion applications in order to reduce costs, limit environmental impact and enhance the health safety. The aim of the present work is to study the cavitation erosion behaviour in distilled water and the corrosion properties in acidic solution of four high-velocity oxy-fuel (HVOF) sprayed Fe-based composite coatings. Fe-Cr-Ni-B-C powder was selected for its good sliding wear properties. In addition, a powder composition with an addition of Mo was studied in order to increase the corrosion resistance whereas additions of 20 wt. % and 40 wt. % WC-12Co as blended powder mixtures were investigated in order to increase wear resistance. Improvement of coating properties was significant with the advanced powder compositions. Dense coating structures with low porosity were detected with microstructural characterization. In addition, good cavitation wear resistance was achieved. The cavitation resistance of customized Fe-based coating with Mo addition was reported to be twice as high as that of conventional Ni-based and WC-CoCr coatings. The corrosion properties of HVOF Fe-based coatings were also evaluated by studying electrochemical behaviour in order to analyse their potential to use as corrosion barrier coatings.
ABSTRACT Cold spraying has shown its potential to produce metallic and composite coatings with hi... more ABSTRACT Cold spraying has shown its potential to produce metallic and composite coatings with high quality and performance. For instance, the impermeability of the coatings is the criterion for the corrosion resistance and thus, fully dense coatings can act as real corrosion barrier coatings. Our previous study has demonstrated the good corrosion properties of high-pressure cold-sprayed (HPCS) Ni and NiCu coatings whereas the present study focuses on the analysis of structural characteristics behind the dense coatings. Microstructures of as-sprayed and heat-treated HPCS Ni, Ni20Cu and Ni20Cu + Al2O3 coatings have been evaluated with FESEM from top-view direction. This revealed clearly particle deformation and particle boundaries whereas particle bonding was evaluated with cavitation–erosion test. Density was proven with Corrodkote test and impermeable HPCS NiCu + Al2O3 coating structures were achieved. Furthermore, cavitation–erosion resistance of NiCu coating was improved by heat treatment.
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Papers by Heli Koivuluoto