Schönau am Königssee, Germany
Schönau am Königssee, Germany

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Ntsoane T.P.,NECSA Ltd | Ntsoane T.P.,University of Pretoria | Topic M.,IThemba LABS National Research Foundation | Harting M.,University of Cape Town | And 2 more authors.
Surface and Coatings Technology | Year: 2016

Hydroxyapatite coatings (HAp, Ca10(PO4)6OH2) were deposited by air plasma spraying onto Ti6Al4V substrates and investigated to determine the depth-dependent behaviour of phase composition, crystallinity, and residual stress using diffractometric techniques. Through-thickness characterisation was carried out by conventional X-ray and synchrotron radiations in reflection and transmission geometries. Results showed HAp together with its thermal decomposition products, tetracalcium phosphate (TTCP), tricalcium phosphate (TCP) and calcium oxide to be present throughout the coating thickness. Quantitative phase identification employing Rietveld refinement showed HAp and TTCP to be the two major phases, with the former decreasing with depth whilst the latter increases. The largest changes were observed adjacent to the coating-substrate interface region. Crystallinity investigation showed a similar trend, revealing a more crystalline near-surface region and increasing amorphisation toward the coating-substrate interface. Residual stress investigation revealed the normal components σ11 and σ33 to be tensile and compressive, respectively. The stresses relax and increase to respective minimum and maximum within the first 145 μm. With further penetration depth, both normal stress components became tensile. © 2016 Elsevier B.V.


Heimann R.B.,Am Stadtpark 2A | Chirikure S.,University of Cape Town | Killick D.,University of Arizona
European Journal of Mineralogy | Year: 2010

Vitreous slag remains of prehistoric tin smelting activities (1650-1850 CE) excavated at Rooiberg, Limpopo Province, South Africa were analyzed by wavelength-dispersive X-ray fluorescence spectrometry (WD-XRF), energy-dispersive X-ray spectroscopy (EDX), and electron-microprobe analysis (EPMA). The slags were found to contain high concentrations of tin oxide suggesting a low level of metal recovery that resulted in substantial tin losses. In addition, skeletal cassiterite, and complex spinels, as well as tin prills ranging in size from a few to tens of micrometres were observed. The contribution discusses the role tin(II) and (IV) oxides are thought to play during formation of slag as well as the crystal chemistry of spinels precipitated during cooling. © 2010 E. Schweizerbart'sche Verlagsbuchhandlung.


Heimann R.B.,Am Stadtpark 2A | Kleiman J.I.,Integrity Testing Laboratory Inc. | Litovsky E.,Integrity Testing Laboratory Inc. | Marx S.,GfE Fremat GmbH | And 5 more authors.
Surface and Coatings Technology | Year: 2014

Alumina-aluminum MMC coatings were deposited by cold gas dynamic spraying onto aluminum alloy (Al 7570) substrates. The microstructure, phase composition, thermal conductivity, microhardness, and thermal optical properties (solar absorptance αs and thermal emittance ε) of four types of coating were determined on as-sprayed samples and samples subjected to atomic oxygen (AO) plasma, intended to simulate conditions in outer space environment. In addition, time-of-flight (ToF) secondary ion mass spectrometry (SIMS) was applied to determine the responses of the coatings to oxidation by AO plasma. The results indicate that the very low thermal conductivity (λ), low solar absorptance (αs), comparatively high infrared emittance (ε) and oxidation stability of the investigated coatings may provide adequate protection as environmental control coatings of space-bound materials against deleterious effects of space environment within the Low Earth Orbit (LEO) and beyond. © 2014 Elsevier B.V.


Litovsky E.,Integrity Testing Laboratory Inc. | Kleiman J.I.,Integrity Testing Laboratory Inc. | Shagalov M.,Integrity Testing Laboratory Inc. | Heimann R.B.,Am Stadtpark 2A
Surface and Coatings Technology | Year: 2014

A novel method has been developed to measure the apparent thermal conductivity of thin coatings based on measuring the heat flux through the coatings as well as the external heat transfer coefficient of an uncoated sample using a modified slug calorimeter configuration. The method was tested on cold gas dynamically sprayed alumina-reinforced aluminum coatings deposited on Al 7075 alloy substrates, and verified by a standard steady-state method. The very low apparent thermal conductivities measured between - 150. °C and +. 200. °C have been attributed to the presence of numerous splat boundaries, pores and microcracks the width of which changes in response to temperature gradients and hence differential thermal expansion coefficients. © 2014 Elsevier B.V.


Heimann R.B.,Am Stadtpark 2A
Journal of Thermal Spray Technology | Year: 2011

Surface roughness strongly controls essential properties of thermally sprayed wear- and corrosion-resistant coatings including their mechanical adhesion to the substrate, tribological performance, efficient retention of lubricating materials, and also the presence of sufficient carrying surface able to support the wear couple along the line of contact expressed by the Abbott-Firestone curve. The determination of the surface fractal geometry may yield useful information on the topography of plasma-sprayed coatings beyond that provided by a single roughness parameter such as R a or R z. The fractal geometry of atmospheric plasma-sprayed chromium oxide coatings, deposited according to two different statistical experimental design protocols, was assessed through determination of the Hurst exponent H of fractal Brownian motion (fBM), as well as the area-scaled fractal complexity (ASFC) obtained by triangular tessellation ("patchwork" method). Attempts were made to correlate fractality with coating adhesion strength. © 2011 ASM International.


Heimann R.B.,Am Stadtpark 2A
Surface and Coatings Technology | Year: 2016

Low-temperature coating deposition is carried out at or near ambient temperature temperatures, much below the incongruent melting point of hydroxyapatite. However, frequently post-depositional heat treatment must be applied to either crystallise amorphous calcium phosphate (ACP), transform (dehydrated/dehydroxylated) precursor phases such as octacalcium phosphate to hydroxyapatite, and/or to remove organic compounds used in coating preparation, for example during sol–gel, dip coating, electrochemical, and electrophoretic deposition. The contributions to this Special Issue deal predominately with low-temperature deposition of hydroxyapatite coatings applied by electrochemical deposition (ECD), plasma electrolytic oxidation (PEO), electrophoretic deposition (EPD), and ion-beam assisted deposition (IBAD) to the surfaces of Ti6Al4V, cp-Ti, CoCrMo, Mg, as well as Zr and PEEK substrates. © 2015 Elsevier B.V.


Heimann R.B.,Am Stadtpark 2A
Surface and Coatings Technology | Year: 2010

Highly accelerated powder particles obtained by low-frequency pulsed detonation spraying (LFPDS), high-frequency pulsed detonation spraying (HFPDS), and high-current pulsed plasma spraying (HCPPS) were applied with the aim to deposit dense protective silicon nitride coatings that adhere well to steel and nickel superalloy surfaces. The silicon nitride-based feedstock powders comprise α-silicon nitride with sintering additives such as alumina, yttria and minor amounts of aluminum nitride and/or silica and magnesia. Coatings were characterized by testing their critical wear path length, tensile adhesion strength, microindentation hardness, salt-spray corrosion and thermoshock resistances, and friction coefficient (pin-on-disk). © 2010 Elsevier B.V.


Heimann R.B.,Am Stadtpark 2A
American Mineralogist | Year: 2015

In modern orthopedics, plasma-sprayed hydroxylapatite coatings are applied routinely to metallic parts of hip and knee prostheses, as well as to dental root implants to render them osseoconductive, that is, able to assist the body in creating new bone by ingrowth of bone cells, blood capillaries, and soft tissue. In this work, hydroxylapatite coatings were deposited by atmospheric plasma spraying on titanium alloy substrates, and characterized by synchrotron radiation X-ray diffraction and solid-state nuclear magnetic resonance spectroscopy. The deposition parameters were varied using a statistical design of experiments methodology. Depending on the degree of heat input and heat transfer rates, (1) dehydroxylated hydroxylapatite phases such as oxy- and oxyhydroxylapatite, (2) thermal decomposition phases (tri- and tetracalcium phosphates, CaO), and (3) amorphous calcium phosphate were formed. Implications of this research are that oxyapatite appears to be unstable at ambient conditions, and that proper selection of intrinsic plasma-spray parameters is key to the chemical stability and mechanical performance of the coating. © Walter de Gruyter Berlin/Boston 2015.

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