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Pretoria, South Africa

Venter A.M.,Necsa Ltd | Venter A.M.,University of Witwatersrand | Oladijo O.P.,University of Witwatersrand | Luzin V.,Australian Nuclear Science and Technology Organisation | And 2 more authors.
Thin Solid Films | Year: 2013

Integral to the performance of high-velocity oxygen-fuel (HVOF) coatings is the thermo-mechanical interaction associated with the thermal misfit, or differences in thermal expansion coefficients (CTEs), between coating and substrate. This investigation reports results on the microstructures, chemical phase content, coating-substrate misfit residual stress, and wear resistance. For this purpose a systematic characterization of WC-Co sprayed coatings on a number of substrates covering a range of CTE values were pursued for both the as-coated and heat-treated conditions. The neutron diffraction technique in conjunction with sub-millimeter sized gauge volumes enabled depth-resolved studies of the stress in the coatings and substrates by paying special attention to the determination of the stress contribution attributed by the final spray process. In the as-coated condition the stress values in the coatings were compressive for CTEs larger than that of WC-Co and tensile for CTE lower than WC-Co. Wear resistance increased for increased compressive stress and macrohardness. In the heat-treated condition, this trend became enhanced due to increased compressive stress in the coatings. © 2013 Elsevier B.V. Source

Jun T.-S.,University of Oxford | Venter A.M.,Necsa Ltd | Korsunsky A.M.,University of Oxford
Experimental Mechanics | Year: 2011

This paper deals with the analysis of elastic strain and eigenstrain in non-uniformly shaped shot-peened 17-4PH stainless steel samples. Based on residual strain measurements by synchrotron X-ray diffraction, the finite element (FE) models are established for the inverse problem of eigenstrain analysis in slice conical sample. The eigenstrains obtained in the slice are then implemented into the FE model of the solid conical sample. It is found that the dependence of elastic strain distributions on the peening intensity and sample shape/thickness could be elucidated via the understanding of underlying permanent strain, or eigenstrain. The effect of the peening process is therefore best described in terms of the induced eigenstrain. The proposed framework is useful for the predictive modelling of residual stresses in non-uniformly shaped shot-peened materials, in that it allows efficient reconstruction of complete residual stress states. In addition, it provides an excellent basis for developing predictive tools for in service performance and design optimisation. © 2010 Society for Experimental Mechanics. Source

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. Source

Ntsoane T.P.,Necsa Ltd | Topic M.,IThemba Laboratory | Bucher R.,IThemba Laboratory
Powder Diffraction | Year: 2011

Coatings of plasma sprayed hydroxyapatite (HAp), incubated in simulated body fluid for periods varying from 1 to 56 days, were characterized using conventional laboratory X rays. Quantitative phase analysis, employing TOPAS software, showed an opposite trend in the two main phases of the coating, viz., HAp and tetracalcium phosphate (TTCP). The former increased within the first 7 days of incubation whilst the latter decreased during the same period; both phases stabilized with further incubation. The crystallinity of the coatings exhibited a trend similar to that of HAp i.e., an increase in the early stages of incubation stabilization with further incubation. Results of residual stress determined with Bruker's D8 Discover and analyzed with LEPTOS software, showed both the normal stress tensor components, σ11 and σ22, to be tensile, relaxing significantly in the early stages of incubation before stabilizing with further incubation. © 2011 International Centre for Diffraction Data. Source

Oladijo O.P.,University of Witwatersrand | Sacks N.,University of Witwatersrand | Cornish L.A.,University of Witwatersrand | Venter A.M.,Necsa Ltd
International Journal of Refractory Metals and Hard Materials | Year: 2012

WC-17 wt.% Co coatings were deposited using high velocity oxy-fuel (HVOF) spraying onto four different substrate materials, namely aluminium, brass, 304 stainless steel and super-invar. These substrates have different coefficients of thermal expansion which have been shown to influence the final coating microstructural properties. The abrasive wear properties of the coatings were characterised using an ASTM-G65 three body abrasive wear machine with silica sand as the abrasive. The highest mass loss was recorded for the coating on the aluminium substrate whilst the coated 304 stainless steel showed the lowest mass loss. The coatings on brass and super invar experienced similar mass losses. SEM studies of the worn surfaces showed preferential removal of the Co binder phase as well as cracking and rounding of the carbide grains. The differences in wear behaviour may be attributed to the presence of residual stresses where the highest compressive residual stress led to the highest wear rate. The coatings deposited onto brass showed compressive stresses whilst those deposited onto super-invar had tensile stresses, yet these two coatings had similar wear rates. Thus further study is required to provide conclusive evidence of the role of residual stresses on the abrasion resistance of these coatings. © 2012 Elsevier Ltd. All rights reserved. Source

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