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A.H. Dent
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Proceedings Papers
ITSC2000, Thermal Spray 2000: Proceedings from the International Thermal Spray Conference, 495-500, May 8–11, 2000,
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BaTiO3 has been successfully sprayed by HVOF to produce dense 25-150 µm thick deposits for use as dielectric and capacitive layers within prototype multilayer conformal electronics. Parameter optimization has been shown to play a critical role in the effective spraying of these materials as thin structurally homogeneous deposits. The affect of standoff distance and combustion chamber size on the phase structure of the coatings have been studied and related to the dielectric properties of the layer. The proportion of crystalline to amorphous phase was found to be critically dependent upon the degree of melting of the particles in the flame and the rate of cooling of the deposits. The crystalline/amorphous ratio is directly related to the dielectric properties of the layer with greater crystallinity giving higher values of dielectric constant. Microcracks and splat/splat interfaces are also believed to adversely affect the dielectric properties. The maximum dielectric constant (K) values achieved using the HVOF method for deposition have been in the range 70-115.
Proceedings Papers
ITSC1998, Thermal Spray 1998: Proceedings from the International Thermal Spray Conference, 83-88, May 25–29, 1998,
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Dilute aluminium alloys with additions of tin and indium when deposited by thermal spraying no longer behave as barrier coatings but demonstrate sacrificial corrosion properties when they exist on corrodible substrates. The degree to which the sacrificial attack occurs depends upon the spraying conditions and the tin or indium contents of the coating. The form in which the tin and/or indium exists in these coatings has not been specified but both elements are known to be sparingly soluble in aluminium. A series of experiments have been carried out using Al-12wt%Sn alloy powder as a feedstock for high velocity oxy-fuel (HVOF) spraying on to a steel substrate. The as-sprayed coatings were highly reactive in distilled water and dissolved in a few minutes. Heat-treatment of the coatings at 450°C for increasing amounts of time up to 20 hours reduced the reactivity to water but did not influence the corrosion rate in 0.1M NaCl solution. SEM/TEM observations on the coating provided evidence of the coarsening of tin particles from 15nm (as sprayed) to 0.5-2µm (as heat-treated). A second alloy with a copper addition i.e. Al-12wt%Sn-1wt%Cu was also sprayed to form coatings. The copper addition prevented reaction in water but did not influence the high corrosion rate of the as-sprayed coating in 0.1M NaCl. Heat treatment at 450°C reduced the corrosion rate and allowed passive films to form over limited ranges of electrode potential. The size and distribution of the tin phase was different in the copper containing coatings and this influenced the corrosion rate.
Proceedings Papers
Microstructure and Corrosion Behaviour of HVOF Sprayed Nickel-Based Amorphous/Nanocrystalline Alloys
ITSC1998, Thermal Spray 1998: Proceedings from the International Thermal Spray Conference, 665-670, May 25–29, 1998,
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The corrosion characteristics of two bespoke Ni-Cr-Mo-B alloy powders sprayed by the high velocity oxy-fuel (HVOF) process have been studied using potentiodynamic and potentiostatic corrosion analysis in 0.5M H2SO4. The deposits have also been microstructurally characterised using X-ray diffraction (XRD), scanning electron microscopy (utilising both secondary electron (SE) and backscattered electron (BE) modes), and transmission electron microscopy (TEM). Results from the microstructural examination of the two alloys have revealed a predominantly amorphous/nanocrystalline (fcc) matrix containing submicron boride precipitates as well as regions of martensitically transformed laths. Apparent recrystallisation of the amorphous matrix has also been observed in the form of cellular crystals with an fcc structure. The oxide stringers observed at splat boundaries were found to be columnar grained α-Cr 2 O 3 , though regions of the spinel oxide NiCr 2 O 4 with a globular morphology were also observed. The coatings of the two alloys exhibited comparable resistance to corrosion in 0.5M H 2 SO 4 , as revealed by potentiodynamic tests. They both had rest potentials approximately equal to -300mV(SCE) and passive region current densities of around 1mAcm-2. Microstructural examination of samples tested potentiostatically revealed the prevalence of degradation at splat boundaries, especially those where significant oxidation of the deposit had occurred.
Proceedings Papers
ITSC1997, Thermal Spray 1997: Proceedings from the United Thermal Spray Conference, 917-923, September 15–18, 1997,
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Two boron-rich Fe/Cr based gas atomised powders (Armacor M and Armacor C) have been thermally sprayed using the HVOF process and the resultant deposits subsequently characterised, using X-ray diffraction, scanning electron microscopy (SEM), plan view transmission electron microscopy (TEM), and microhardness measurements. The wear and corrosion characteristics of the two alloy coatings have also been investigated by three body abrasive wear (utilising cross-sectional TEM to examine the worn surfaces) and potentiodynamic corrosion testing respectively. Results from microstructural analysis of the as-sprayed deposits revealed the presence of small chromium-iron boride precipitates within a predominantly amorphous matrix in the Fe-based Armacor M coating. The Fe-Cr-based Armacor C coating, however, consisted mainly of regions of nano- and microcrystalline material interspersed with chromium boride precipitation. Iron-chromium oxides have been observed within both of the alloy coatings studied. Both of these alloys exhibit good abrasive wear resistance when compared with other metallic based HVOF sprayed coatings. Both Armacor M and Armacor C also exhibit extensive passivation on exposure to an acidic solution. The wear and corrosion test results are related to the microstructural observations.