Interesting aspects of indentation and scratch methods for characterization of thermally-sprayed coatings

In 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

Depth-sensing indentation for assessing the mechanical properties ofcold-sprayed Ta

The stress\u2013strain behaviour of cold-sprayed Ta coatings, deposited onto Al and steel substrates, was studiedby depth-sensing spherical and sharp (Berkovich) indentation testing, and was compared to that of anannealed Ta sheet. The mechanical properties of the coatings, free of any scale-dependence, are insensitive tothe presence of a lamellar structure, indicating strong, tight bonding between cold-sprayed Ta particles.Accordingly, the coatings are isotropic. The stress\u2013strain curves reveal that the coating material experiencedsome degree of work-hardening during cold-spraying; indeed, the sprayed particles were plasticallydeformed at impact. This was confirmed by line profile analysis on XRD patterns. The consistency between theresults from different indentation testing techniques provides a means to cross-validate the entire analysis

Wear and corrosion behaviour of HVOF WC-CoCr / CVD DLC hybrid coating systems deposited onto aluminum substrate

Hybrid coating systems consisting of a HVOF-sprayed WC–CoCr interlayer (75 μm or 125 μm thick) and a thinDLC-based top layer (obtained by electron cyclotron wave resonance–chemical vapour deposition: ECWRCVD)were deposited onto AA6082-T6 substrates; their tribological and anti-corrosion performances wereevaluated and compared to those of the DLC film deposited directly onto the Al alloy substrate withoutinterlayer.In scratch adhesion testing, the interlayer delays the onset of cracking and spallation of the film, because itprovides better mechanical support than the bare Al substrate. As soon as cracking begins, however, completedelamination of the films deposited on cermet interlayers occurs, revealing limited interface adhesion.Accordingly, in ball-on-disk wear tests, the films deposited on cermet interlayers can stand more severecontact conditions without cracking, but, as the contact pressure increases, most of the film delaminates.If no cracking occurs, the film is very effective at producing low wear and low friction, but it is not as effectiveat preventing penetration of corrosive agents, as shown by corrosion tests in aqueous environments, becauseit contains some small defects

Tribological and mechanical performance evaluation of metal prosthesis components manufactured via metal injection molding

The increasing number of total joint replacements, in particular for the knee joint, has a growing impact on the healthcare system costs. New cost-saving manufacturing technologies are being explored nowadays. Metal injection molding(MIM) has already demonstrated its suitability for the production of CoCrMo alloy tibial trays, with a significant reduction in production costs, by holding both corrosion resistance and biocompatibility. In this work, mechanical and tribological properties were evaluated on tibial trays obtained via MIM and conventional investment casting. Surface hardness and wear properties were evaluated through Vickers hardness, scratch and pin on disk tests. The MIM and cast finished tibial trays were then subjected to a fatigue test campaign in order to obtain their fatigue load limit at 5 millions cycles following ISO 14879-1 directions. CoCrMo cast alloy exhibited 514HVhardness compared to 335 HV of MIM alloy, furthermore it developed narrower scratches with a higher tendency towards microploughing than microcutting, in comparison to MIM CoCrMo. The observed fatigue limits were (1,766 +/- 52) N for cast tibial trays and (1,625 +/- 44) N for MIM ones. Fracture morphologies pointed out to a more brittle behavior of MIM microstructure. These aspects were attributed to the absence of a fine toughening and surface hardening carbide dispersion in MIM grains. Nevertheless, MIM tibial trays exhibited a fatigue limit far beyond the 900 N of maximum load prescribed by ISO and ASTM standards for the clinical application of these devices

Micromechanical properties and sliding wear behaviour of HVOF-sprayed Fe-based alloy coatings

The tribological performance of two Fe–Cr–Ni–Si–B–C (Colferoloy) alloy coatings manufactured by HVOFthermal spraying was studied by rubber-wheel dry particle abrasion test and ball-on-disk sliding wear tests. The results were compared to those obtained on Ni–Cr–Fe–Si–B–C and Cr3C2–NiCr layers (also manufactured by HVOF-spraying), hard chromium electroplating and bulk tool steel. At room temperature, the sliding wear loss of the Colferoloy coatings against alumina counterpart,caused by a mix of mild abrasion, delamination and tribo-oxidation, was larger than that of Cr3C2–NiCrand tool steel but lower than that of Ni–Cr–Fe–Si–B–C and hard chromium plating. At 400 ◦C and 700 ◦C,Colferoloy coatings mainly suffered abrasive grooving: they were still superior to Ni–Cr–Fe–Si–B–C butinferior to Cr3C2–NiCr. Against steel, Colferoloy coatings, with limited delamination and negligible wearloss, were comparable to Cr3C2–NiCr and superior to Ni–Cr–Fe–Si–B–C, tool steel and electrolytic hardchromium, although they inflicted quite significant wear to the steel counterbody. Colferoloy coatings were therefore validated as alternatives to Ni-based alloys and electroplatedchromium under sliding wear conditions, but appeared unsuitable for particle abrasion resistance. The different sliding wear behaviours of HVOF-sprayed coatings could be explained by coupling micro and nano-hardness to scratch testing, which reflected cracking resistance and plastic deformability

Wear behaviour of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings produced using micron- and nano-sized powder suspensions

Three Al2O3-based suspensions were prepared using two nano-sized Al2O3 powders (having analogous size distribution and chemical composition but different agglomeration behaviour) and a micron-sized one. High velocity suspension flame sprayed (HVSFS) coatings were produced using these suspensions as feedstock and adopting two different sets of deposition parameters.The coatings produced by the micrometric powder suspension are denser and more wear resistant (as revealed by ball-on-disk testing) than those deposited using the two nanopowder suspensions, irrespective of the deposition parameters. The tribological behaviour of all coatings is dictated by the formation and subsequent removal of surface tribofilms: these tribofilms are more stable in the coatings obtained by the micrometric powder suspension, although they exhibit the largest tensile residual stress (as determined by X-ray diffraction and hole drilling techniques).Even though the as-deposited coatings are quite smooth (Ra around 1.3–2 μm), polishing to Ra around 0.1 μm further improves the wear resistance in all cases, as it delays the onset of tribofilm delamination phenomena

Microstructure and in vitro behaviour of 45S5 bioglass coatings deposited by high velocity suspension \ufb02ame spraying (HVSFS)

The high-velocity suspension \ufb02ame spraying technique (HVSFS) was employed in order to deposit 45S5 bioactive glass coatings onto titanium substrates, using a suspension of micron-sized glass powders dispersed in a water ? isopropanol mixture as feedstock. By modifying the process parameters, \ufb01ve coatings with different thick- ness and porosity were obtained. The coatings were entirely glassy but exhibited a through-thickness micro- structural gradient, as the deposition mechanisms of the glass droplets changed at every torch cycle because of the increase in the system temperature during spraying. After soaking in simulated body \ufb02uid, all of the coatings were soon covered by a layer of hydroxyapatite; furthermore, the coatings exhibited no cytotoxicity and human osteosar- coma cells could adhere and proliferate well onto their surfaces. HVSFS-deposited 45S5 bioglass coatings are therefore highly bioactive and have potentials as replace- ment of conventional hydroxyapatite in order to favour osseointegration of dental and prosthetic implants