A.H. Dent

University of Nottingham, Nottigham, England, United Kingdom

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Publications (6)6.76 Total impact

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    ABSTRACT: High velocity oxy-fuel (HVOF) thermal spraying was used to deposit coatings, approximately 200 μm thick, of a commercially available Ni-Cr based alloy containing boron and carbon [Ni-23.5Cr-3.8Cu-0.8Fe-5.9Mo-3.4W-2.8B-4.3C (at.%)]. Powder and coating microstructures were investigated by a combination of X-ray diffraction, scanning electron and transmission electron microscopy. The coatings had layered morphologies due to the deposition and solidification of successive molten or semi-molten splats. The splat microstructures consisted of a Ni-rich metallic matrix containing a small fraction of M23C6 particles ∼50 nm in size. The Ni-rich matrix had a predominantly crystalline structure with only a small fraction of amorphous phase regions. Cr2O3 and NiCr2O4 oxide phases occurred in the form of either intersplat lamellae or globules, with Cr2O3 being the predominant oxide. Coating microhardness values were found to be ∼6.0 GPa which is significantly higher than that of B- and C-free nickel-based alloys similarly deposited.
    No preview · Article · May 2001 · Surface and Coatings Technology
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    ABSTRACT: High velocity oxy-fuel (HVOF) thermal spraying was used to deposit Ni–Cr–Mo–B alloy coatings, approximately 200 μm thick, onto mild steel substrates. Gas atomised powders of three different compositions were used in the experiments. Powder and coating microstructures were investigated by a combination of X-ray diffraction, scanning electron and transmission electron microscopy and differential thermal analysis (DTA). The coatings had layered morphologies due to the deposition and solidification of successive molten or semi-molten splats. The splat microstructures consisted of a Ni-rich metallic matrix containing a small fraction of M3B2 particles 10–30 nm in size and with a tetragonal crystal structure. The Ni-rich matrix comprised both crystalline and amorphous regions. The former was predominantly nanocrystalline with a grain size of ca. 50 nm and DTA showed that the crystallization temperature of the amorphous phase varied from 800 to 860 K depending on the alloy constitution. Cr2O3 and NiCr2O4 oxide phases occurred in the form of either intersplat lamellae or globules. The thin lamellar oxides exhibited the α-Cr2O3 crystal structure whereas globular oxides, up to 1 μm in size, were found with both α-Cr2O3 and NiCr2O4 structures.
    Full-text · Article · May 2000 · Materials Science and Engineering A
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    ABSTRACT: The corrosion characteristics of two 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 0.5 M H2SO4. The deposits were also microstructurally characterized using x-ray diffraction (XRD), scanning electron microscopy (SEM) (utilizing both secondary electron and backscattered electron modes), and transmission electron microscopy (TEM). Results from the microstructural examination of the two alloys have revealed a predominantly amorphous/nanocrystalline face centered cubic (fcc) matrix containing submicron boride precipitates as well as regions of martensitically transformed laths. Apparent recrystallization of the amorphous matrix has also been observed in the form of cellular crystals with a fcc structure. The oxide stringers observed at splat boundaries were found to be columnar grained α-Cr2O3, though regions of the spinel oxide NiCr2O4 with a globular morphology were also observed. The coatings of the two alloys exhibited comparable resistance to corrosion in 0.5 M H2SO4, as revealed by potentiodynamic tests. They both had rest potentials approximately equal to −300 mV saturated calomel electrode (SCE) and passive region current densities of ∼1 mA/cm2. Microstructural examination of samples tested potentiostatically revealed the prevalence of degradation at splat boundaries, especially those where significant oxidation of the deposit occurred.
    No preview · Article · Aug 1999 · Journal of Thermal Spray Technology
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    ABSTRACT: Three Ni-Cr based powders (Metco 700, Armacor C and PSI-1) have been thermally sprayed using the HVOF process. The resultant deposits were then characterized using SEM, TEM and XRD. Microhardness surveys were carried out on the coatings together with abrasive wear tests. Coatings 250 μm thick, were produced with low oxides and porosity levels. The most consistent coatings were prepared from powder sources with a minimum particle size of 25 μm. Coating microstructures were either nano-crystalline or mixtures of amorphous and microcrystalline phases. Boron additions to the powder encouraged the formation of amorphous coatings and higher microhardness values i.e. >675 Hv. The formation of boride precipitates in the coatings does not appear to raise their hardness as the precipitates were either of larger diameter >500 nm or present in low volume fractions. Limited amounts of oxides were produced in the coatings, e.g. as α-Cr2O3 or spinel oxide NiCr2O4 particularly at intersplat boundaries. Abrasive wear resistance of the PSI-1 coating was superior to the other two coatings because it contains more amorphous material. The combination of wear and corrosion resistance associated with these coatings may prove to be attractive in many applications where both these properties are needed, carbides and borides.
    No preview · Article · Mar 1999 · Transactions of the Institute of Metal Finishing
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    ABSTRACT: The corrosion characteristics of two bespoke Ni-Cr-Mo-B alloy powders sprayed by the high velocity oxy-fiiel (HVOF) process have been studied using potentiodynamic and potentiostatic corrosion analysis in 0.5M H 2SO4. 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 sub-micron 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-Cr2O 3, though regions of the spinel oxide NiCr2O4 with a globular morphology were also observed. The coatings of the two alloys exhibited comparable resistance to corrosion in 0.5M H2SO 4, as revealed by potentiodynamic tests. They both had rest potentials approximately equal to -300mV(SCE) and passive region current densities of ca. 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.
    No preview · Article · Jan 1998

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