MAX-phase coatings produced by thermal spraying

This paper presents a comparative study on the Ti₂AlC coatings produced by different thermal spray methods, as Ti₂AlC is one of the most studied materials from the MAX-phase family.Представлено порівняльне дослідження покриттів Ti₂AlC, отриманих різними методами термічного розпилення, оскільки Ti₂AlC є одним з найбільш вивчених матеріалів з сімейства фаз MAX.Представлено сравнительное исследование покрытий Ti₂AlC, полученных различными методами термического распыления, поскольку Ti₂AlC является одним из наиболее изученных материалов из семейства MAX-фаз

A Synchrotron X-ray diffraction deconvolution method for the measurement of residual stress in thermal barrier coatings as a function of depth

The average residual stress distribution as a function of depth in an air plasma-sprayed yttria stabilized zirconia top coat used in thermal barrier coating (TBC) systems was measured using synchrotron radiation X-ray diffraction in reflection geometry on station I15 at Diamond Light Source, UK, employing a series of incidence angles. The stress values were calculated from data deconvoluted from diffraction patterns collected at increasing depths. The stress was found to be compressive through the thickness of the TBC and a fluctuation in the trend of the stress profile was indicated in some samples. Typically this fluctuation was observed to increase from the surface to the middle of the coating, decrease a little and then increase again towards the interface. The stress at the interface region was observed to be around 300 MPa, which agrees well with the reported values. The trend of the observed residual stress was found to be related to the crack distribution in the samples, in particular a large crack propagating from the middle of the coating. The method shows promise for the development of a nondestructive test for as-manufactured samples

Sliding and abrasive wear behaviour of HVOF- and HVAF-sprayed Cr3C2-NiCr hardmetal coatings

This paper provides a comprehensive characterisation of HVOF- and HVAF-sprayed Cr3C2-25 wt.% NiCr hardmetal coatings. One commercial powder composition with two different particle size distributions was processed using five HVOF and HVAF thermal spray systems. All coatings contain less Cr3C2 than the feedstock powder, possibly due to the rebound of some Cr3C2-rich particles during high-velocity impact onto the substrate. Dry sand-rubber wheel abrasive wear testing causes both grooving and pull-out of splat fragments. Mass losses depend on inter- and intra-lamellar cohesion, being higher (≥70 mg after a wear distance of 5904 m) for the coatings deposited with the coarser feedstock powder or with one type of HVAF torch. Sliding wear at room temperature against alumina involves shallower abrasive grooving, small-scale delamination and carbide pull-outs, and it is controlled by intra-lamellar cohesion. The coatings obtained from the fine feedstock powder exhibit the lowest wear rates (≈5×10-6 mm3/(Nm)). At 400 °C, abrasive grooving dominates the sliding wear behaviour; wear rates increase by one order of magnitude but friction coefficients decrease from ≈0.7 to ≈0.5. The thermal expansion coefficient of the coatings (11.08×10-6 °C-1 in the 30-400 °C range) is sufficiently close to that of the steel substrate (14.23×10-6 °C-1) to avoid macro-cracking

Tribology of HVOF- and HVAF-sprayed WC-10Co4Cr hardmetal coatings: A comparative assessment

This paper provides a comprehensive assessment of the sliding and abrasive wear behaviour of WC-10Co4Cr hardmetal coatings, representative of the existing state-of-the-art. A commercial feedstock powder with two different particle size distributions was sprayed onto carbon steel substrates using two HVOF and two HVAF spray processes. Mild wear rates of <10-7mm3/(Nm) and friction coefficients of 480.5 were obtained for all samples in ball-on-disk sliding wear tests at room temperature against Al2O3 counterparts. WC-10Co4Cr coatings definitely outperform a reference electrolytic hard chromium coating under these test conditions. Their wear mechanisms include extrusion and removal of the binder matrix, with the formation of a wavy surface morphology, and brittle cracking. The balance of such phenomena is closely related to intra-lamellar features, and rather independent of those properties (e.g. indentation fracture toughness, elastic modulus) which mainly reflect large-scale inter-lamellar cohesion, as quantitatively confirmed by a principal component analysis. Intra-lamellar dissolution of WC into the matrix indeed increases the incidence of brittle cracking, resulting in slightly higher wear rates. At 400\ub0C, some of the hardmetal coatings fail because of the superposition between tensile residual stresses and thermal expansion mismatch stresses (due to the difference between the thermal expansion coefficients of the steel substrate and of the hardmetal coating). Those which do not fail, on account of lower residual stresses, exhibit higher wear rates than at room temperature, due to oxidation of the WC grains.The resistance of the coatings against abrasive wear, assessed by dry sand-rubber wheel testing, is related to inter-lamellar cohesion, as proven by a principal component analysis of the collected dataset. Therefore, coatings deposited from coarse feedstock powders suffer higher wear loss than those obtained from fine powders, as brittle inter-lamellar detachment is caused by their weaker interparticle cohesion, witnessed by their systematically lower fracture toughness as well

Oxidation Behavior of HVAF-Sprayed NiCoCrAlY Coating in H-2-H2O Environment

Isothermal oxidation behavior of an HVAF-sprayed NiCoCrAlY coating on AISI 304L was studied in an Ar-10 %H-2-20 %H2O environment at 600 A degrees C. Techniques such as BIB/SEM, EDS, and XRD were used to comprehensively characterize the coating and the coating/substrate interface to investigate the oxidation mechanisms. Results were also compared with those obtained from an uncoated AISI 304L substrate. The alumina-forming NiCoCrAlY coating was found to exhibit superior oxidation behavior due to the formation of a slow-growing and protective Al2O3 scale, while the chromia-forming bare 304L substrate lost its protective capability due to the formation of a duplex [Fe3O4 on (Fe,Cr)(3)O-4 spinel oxide] corrosion product layer

Characterization of cold lap defects in tandem arc MAG welding

The objective of this investigation was to classify and characterize the small lack of fusion defects, called cold lap, located at the weld toe. Since the defects are very small (0.01-1.5 mm) and difficult to detect by NDT methods, a better understanding of the formation mechanism is required to be able to avoid their formation. The investigation consisted of two parts. Firstly, a study was made on the type and frequency of cold laps. Three types were identified, namely "spatter cold lap", and "overlap cold lap" and "spatter-overlap cold lap". No relation between type or frequency of cold laps and the welding parameters could be established. Secondly, the interface between spatter and the base material was investigated using optical and scanning electron microscopy, to better understand the cold lap formation mechanism. Manganese-silicate particles were found in the interface located in such a way that they may assist cold lap formation

Oxidation behaviour of HVAF-sprayed NiCr coating in moisture-laden environment

\ua9 Copyright 2017 by DVS Media GmbH. All rights reserved. Reducing CO 2 emissions from power generation plants is intimately related to enhancing their thermal efficiency, which can be achieved by increasing the temperature/pressure of steam. However, any increase in steam temperature is inevitably accompanied by accelerated oxidation of boiler components. The use of renewable fuels such as biomass increases the problem by introducing a number of corrosive compounds into the boiler environment, resulting in more rapid degradation of components. Although thermal sprayed coatings are techno-commercially attractive solutions for augmenting the durability of degradation-prone boiler components and are already used, further improvements in their performance are continuously sought. High-velocity air fuel (HVAF) coatings are promising in this context. In the present work, isothermal oxidation behavior of a candidate HVAF-sprayed Ni21Cr was studied in N 2 + 5% O 2 + 20% H 2 O at 600\ub0C for 168h. The oxide scale growth mechanisms were studied by BIB/SEM/EDX to evaluate the effectiveness of the coatings. It was found that the water vapor effect is insignificant due to the Cr reservoir in the Ni21Cr coating, which yielded enhanced oxidation protection by forming nano-scale Cr 2 O 3