4,260 research outputs found
Tribological properties of HVOF as-sprayed and heat treated Co–Mo–Cr–Si coatings
No full textHVOF-sprayed Co–28%Mo–17%Cr–3% Si alloy tribological performance was tested in the as-sprayed condition and after thermal treatments at 200, 400, 600°C for 1 h. As-sprayed coating possesses low hardness, undergoes adhesive wear against 100Cr6steel and displays an high-friction coefficient causing relevant thermal effects. The 600°C-heat treatment increases microhardness, thus preventing adhesive wear and reducing friction
Heat Treatment Effects on the Tribological Performance of HVOF-Sprayed Co-Mo-Cr-Si Coatings
No full textThe tribological behavior of high-velocity oxyfuel sprayed Co-28%Mo-17%Cr-3%Si coatings, both as sprayed and after heat treatments at 200, 400, and 600 °C for 1 h, has been studied. The as-sprayed coating contains oxide stringers and is mostly amorphous. It has low hardness (∼6.7 GPa) and toughness and undergoes adhesive wear against 100Cr6 steel. The friction coefficient increases up to ∼0.9, so the flash temperature reaches a critical oxidation value; then, friction decreases and increases again. This phenomenon occurs periodically. Much adhesive wear occurs in the first stage. Abrasive wear prevails against alumina pin: the coating wear rate is lower because it possesses good plasticity. Thermal effects still occur. The 600 °C treatment causes formation of submicrometric crystals. Hardness increases (∼8.8 GPa), adhesive wear is prevented, the friction coefficient has no peaks. Against the alumina pin, wear rates remain similar to the as-sprayed case. Nevertheless, the friction coefficient has no peaks and its final value is lowered (from 0.84 to0.75)
Heat treatment effects on the corrosion resistance of some HVOF-sprayed metal alloy coatings
No full textThe present study evaluates the effects of a 600 °C, 1 h heat treatment on the corrosion resistance of threeHigh Velocity Oxygen Fuel (HVOF) flame-sprayed alloy coatings: a Co–28Mo–17Cr–3Si (similar to Tribaloy-800) coating, a Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe (Diamalloy-4006) coating and a Ni–32Mo–16Cr–3Si–2Co (similar to Tribaloy-700) coating. Electrochemical polarization tests and free corrosion tests wereperformed in 0.1 M HCl aqueous solution. The corrodkote test (ASTM B380-97R02) was also performed, toevaluate the coatings qualitatively. The heat treatment improves the corrosion resistance of the Co–28Mo–17Cr–3Si coating and of the Ni–20Cr–10W–9Mo–4Cu–1C–1B–1Fe coating by enhancing their passivationability. The precipitation of sub-micron sized secondary phases after the treatment may produce galvanicmicrocells at intralamellar scale, but the beneficial contribution provided by the healing of the very small but dangerous interlamellar defects (normally present in thermal spray coatings but not detectable usingordinary scanning electron microscopy) prevails. The effect on Ni–32Mo–16Cr–3Si–2Co coatings is moreambiguous: its sensitivity to crevice corrosion is worsened by the heat treatment
Ti6Al4V-ELI Alloy Manufactured via Laser Powder-Bed Fusion and Heat-Treated below and above the β-Transus: Effects of Sample Thickness and Sandblasting Post-Process
No full textTi6Al4V-ELI is the most-used lightweight alloy in the aerospace industrial sector thanks to its high mechanical strength and corrosion resistance. The present paper aims, firstly, to evaluate the effects induced by different heat treatments, which were performed above and below the β-transus temperature on Ti6Al4V-ELI samples manufactured via Laser Powder-Bed Fusion in different orientations (XZ, XY, Z and 45°). The first set of tensile samples and bars were heat-treated at 1050 °C × 1 h, while the second and third set were heat-treated at 704 °C × 120′ following the AMS2801 standard specification, and at 740 °C × 130′. These heat treatments were chosen to improve the as-built mechanical properties according to the ASTM F3001 and also ASTM F2924-14 standard specifications. Optical and SEM measurements reveal primary, secondary and tertiary α-laths below the β-transus, while above this temperature, the microstructure varies in relation to the sample’s thickness. Secondly, this work analyzed the results obtained after a sandblasting process, which was performed on half of all the available heat-treated tensile samples, through XRD and Vickers microhardness measurements. XRD analysis also highlighted the presence of α2-Ti3Al and TiAl3 precipitates and the microstructural change in terms of the α-phase
Defect-Correlated Vickers Microhardness of Al-Si-Mg Alloy Manufactured by Laser Powder Bed Fusion with Post-process Heat Treatments
No full textHVOF-sprayed WC-CoCr coatings on Al alloy: Effect of the coating thickness on the tribological properties
No full textThe microstructure, the micromechanical properties, the wear behaviour and the impact resistance of WC-CoCr cermet coatings, deposited onto an aluminium alloy substrate by High Velocity Oxygen-Fuel (HVOF) flame-spraying, were examined as a function of the coating thickness, which was varied between 50 mu m and 150 mu m by performing different numbers of scans of the HVOF torch in front of the substrate. The coatings became denser and significantly harder as the number of torch scans increased: the analysis of single WC-CoCr splats by combined SEM and Focused Ion Beam (FIB) techniques enabled the interpretation of the mechanisms underlying this phenomenon. In accordance to such densification, the sliding wear resistance increased with the number of torch scans, as abrasive grooving and brittle failure mechanisms were progressively suppressed. The resistance to cyclic impact was also enhanced. In comparison to anodised films, the WC-CoCr coatings appeared much more resistant against wear and cyclic impact; specifically, three torch scans seem enough to produce a coating having suitable characteristics. (C) 2009 Elsevier B.V. All rights reserved
Mechanical and tribological properties of electrolytic hard chrome and HVOF-sprayed coatings
No full textThe microstructure, mechanical properties (Vickers microhardness, fracture toughness, elastic modulus) and tribological behaviour (abrasion and unlubricated sliding wear resistance) of various kinds of electrolitic hard chrome (EHC) coatings and of metallic and cermet HVOF-sprayed coatings (WC-17Co, WC-10Co-4Cr, Co-28Mo-17Cr-3Si) have been studied. EHC coatings are very tough, in fact no cracks could be produced with 10 N Vickers microindentations. However, their superficial morphology and Vickers microhardness are significantly affected by the substrate preparation technique, the deposition process characteristics, and by post-deposition treatments. This causes the coatings to show different behaviors in different wear conditions; in particular, harder coatings are more abrasion-resistant, but undergo a more severe mass loss under unlubricated sliding conditions. HVOF-sprayed cermet coatings are harder but less tough than EHC ones. Therefore, they undergo a comparable or even higher mass loss when subjected to three-body abrasion conditions. However, their two-body sliding resistance definitely overcomes that of EHC coatings, because they form a tough and uniform surface film protecting them from further damage
Variation of splat shape with processing conditions in plasma sprayed alumina coatings
No full textThis paper is on various splat shapes obtained using three alumina based powders sprayed on various
substrates. The parameters considered were substrate preheating temperature, nozzle diameter, and
secondary and primary gas flow rates. The splat shape was found to be strongly dependent on spraying
conditions. The substrate preheating temperature determined the degree of substrate wetting by the
splat. A change in either nozzle diameter or primary gas flow rate brought about a change in the particle
momentum and subsequently, a change in splat shape. The splat shape differed widely on an as – sprayed
bond coat as compared to a polished one, owing to splat confinement by surface asperities. Sub-microscale
surface roughness of polished substrate surfaces showed an increase with the preheating temperature
and this in turn, resulted in better substrate wetting by the splats
Diamond-Like Carbon (DLC) and AlCrN films onto Ti-6Al-4V substrates by Laser-Powder Bed Fusion (L-PBF): Effect of substrate heat treatment and surface finish
No full textThis paper focuses on the performance of thin-film coatings onto additively manufactured Ti-6Al-4V. Specifically, because metal parts obtained by laser-powder bed fusion (L-PBF) often require post-process heat treatments and surface finishing to meet end-user specifications, we studied how the resulting changes to mechanical strength and surface roughness affect the performance of films deposited by physical or plasma-enhanced chemical vapor deposition (PVD, PE-CVD). L-PBF Ti-6Al-4V substrates were heat-treated either below or above the β-transus and finished by grinding with different grit sizes, and then were coated with PVD AlCrN or a PE-CVD DLC-based film.
Scratch adhesion on harder surfaces treated below the β-transus was higher with both coating types, whilst the substrate finishing had a negligible effect. Conversely, in ball-on-disc sliding wear tests, substrate roughness had a dominant effect: rough surfaces always resulted in earlier cracking and delamination of the coatings. Substrate hardness had a minor effect only with the AlCrN film. Moreover, the DLC-based films, because of the low-friction conditions they establish through a graphitized tribofilm as well as their higher H/E ratio, survived severe contact conditions better than the stiffer AlCrN. The results were interpreted in the light of the plasticity indices of the coated systems and their tribochemical interactions
Interesting aspects of indentation and scratch methods for characterization of thermally-sprayed coatings
No full textIn order to improve the knowledge on the use and significance of instrumented indentation and scratchtesting on thermally-sprayed materials, a wide range of tests was performed on thermally-sprayed ceramic,cermet and metal coatings. A scale-dependent behavior of hardness was observed as a function ofindentation depth for all coatings: at low penetration depths, the hardness value depends on theintralamellar material properties, whereas at larger depths it reflects the long-range cohesive strength of thecoating. In all cases, hardness becomes independent of the indentation depth above a threshold value of~2000 nm. The elastic modulus is also scale-dependent, but it never stabilizes to a depth-independent value,probably on account of crack opening/closing mechanisms. Scratch test on the cross-section has been deeplyinvestigated and identified as a comparative method to quantify the cohesion of the coatings
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