Damage tolerant functionally graded WC–Co/Stainless Steel HVOF coatings

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    Functionally graded materialsHigh Velocity Oxygen-Fuel (HVOF) sprayingWear resistanceImpact testingResidual stress

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    found by scanning electron microscopy and d

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    prole [1], designed in order to improve certain characteristics numerous synergistic factors leading to such cracking (including the

    Surface & Coatings Technology 205 (2010) 21972208

    Contents lists available at ScienceDirect

    Surface & Coatin

    l s(usually mechanical), over monolithic materials [25]. FGM manu-facturing is non-trivial, but is well-suited for thermal spray (TS)deposition methods, largely due to the ability of TS to process dis-similar materials concurrently, in sequential layers of differentcomposition [6]. Layer-by-layer compositional gradients can indeedbe easily manufactured by simultaneously feeding different powdersin the hot ame of the spraying torch [79], and gradually varying theratio of the two or more powders [1016]. This gradation can beobtained by the simultaneous control of the feed rate in various

    mismatch in thermo-mechanical properties, but also the oxidation ofthe metallic bond coat material) [14]. Few studies have, up to now,addressed the subject of thermally-sprayed FGMs for wear andcorrosion protection [1722]. As traditional single-layer coatings forwear protection generally exhibit much higher hardness and elasticmodulus than the substrate (the typical combination being a cermet,like WCCo or WCCoCr, deposited onto steel or light alloy sub-strates), the stress distribution produced under localised contactconditions concentrates inside the coating and along the coating/powder feeders, by strategic injection of theplume, or by previously blending the powdproportions, and periodically replacing thesented here).

    Corresponding author.E-mail address: alfredo.valarezo@gmail.com (A. Vala

    0257-8972/$ see front matter 2010 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2010.08.148icrostructure of whichding to a predetermined

    (MCrAlY) / top coat (Y-PSZ) interface. As a result of those investiga-tions, very complex architectures have been proposed to address thecomposite materials, the composition and mare not spatially homogeneous but vary accor1. Introduction

    Functionally graded materials (FGcurvature measurement technique reveals that during the deposition of the FGC, compressive stresses exist inthe lower, metallic layers owing to peening effect of successive impact, and these gradually evolve to hightensile, in the top layers. Tensile stresses appear to be due to quenching alone; thermal stresses are lowbecause of the gradation. All of this is benecial for the deposition of a thick coating.The FGC structure shows the ability to reduce cracking with increased compliance in the top layer duringstatic and dynamic normal contact loading, while retaining excellent sliding wear resistance (ball-on-disktests). Results are discussed in comparison to the behavior and properties of coatings of similar individualcompositions and thicknesses, as well as a thick monolithic WC12Co sprayed coating. Further improvementsin the processing are proposed to enhance the adhesion strength and avoid coating delamination under highload contact-fatigue conditions.

    2010 Elsevier B.V. All rights reserved.

    re a special category of

    The application of the FGM concept to TS has been extensivelystudied for thermal barrier coating (TBC) applications [1416], in anattempt to suppress the generation of cracks along the bond coatvarious powders in theer materials in differentfeedstock (as it is pre-

    substrate interfa[2325]. Anti-coorder to minimihowever, duringsively large residmacrocracks or pre-examinationaddition, advancrezo).

    l rights reserved.mposition and hardness through the coating thickness isepth-sensing microindentation, respectively. The in-situKeywords:Damage tolerant coatingsincrease in compliance with depth, and 2) an increase in fracture resistance by containment, arrest anddeection of cracks. A smooth gradation in the co12Co (top layer) depositedDamage tolerant functionally graded WC

    Alfredo Valarezo a,, Giovanni Bolelli b, Wanhuk B. CLuca Lusvarghi b, Roberto Rosa b

    a Center for Thermal Spray Research, Materials Science and Engineering Department, 130b Department of Materials and Environmental Engineering, University of Modena and Reg

    a b s t r a c ta r t i c l e i n f o

    Article history:Received 23 April 2010Accepted in revised form 31 August 2010Available online 8 September 2010

    In this paper, effective damoxygen fuel (HVOF) sprayichange in composition from

    j ourna l homepage: www.eo/Stainless Steel HVOF coatings

    i a, Sanjay Sampath a, Valeria Cannillo b,

    vy Engineering Bldg., Stony Brook University, Stony Brook, NY, 11794-2275, USAmilia, Via Vignolese 905, 41125 Modena (MO), Italy

    tolerance of a functionally graded coating (FGC) deposited by high velocitys observed. The thick FGC (1.2 mm) consists of 6 layers with a stepwise0 vol.% ductile AISI316 stainless steel (bottom layer) to 100 vol.% hard WCo an AISI316 stainless steel substrate. Damage tolerance is observed via 1) an

    gs Technology

    ev ie r.com/ locate /sur fcoatce, where cracking and delamination become likelyrrosion coatings should be as thick as possible, inse or eliminate the interconnected porosity [26,27];the deposition of thick single-layer coatings, exces-ual stress is often built up [28,29] producing verticalremature delamination. These issues have led to theof thermal sprayed FGMs as potential solutions. Ines in process science, such as the in situmeasurement

  • of stress states, have enabled new insights into materials andprocesses providing new opportunities of layered and gradedmaterials design.

    In this paper, the approach of damage tolerance, which has beenextensively used in the design of brittle components that withstandload without failure despite the presence of defects (e.g. cracks), isapplied to TS coatings for wear resistance. Given that TS coating layersare built by individual molten particles, the presence of defects (e.g.intersplat interfaces and pores) and, potentially, of cracksmakes themsingular systems that can prematurely fail at the weakest regions via

    Cross-sectional samples of all of the coatings were prepared bycutting, hot-mounting in phenolic resin, grinding with SiC papers (upto 2500 mesh) and polishing with diamond slurry (up to 0.5 m). Thepolished sections were observed by scanning electron microscopy(SEM, Quanta-200 and XL30, FEI, Eindhoven, The Netherlands) andthe composition of the various layers was assessed by image analysis(NIH ImageJ version 1.37): at least 5 micrographs at 400 magni-cation were employed for each layer.

    The residual stress build-up during the deposition of the FGC andof each single-layer coating was monitored by the in situ coating

    2198 A. Valarezo et al. / Surface & Coatings Technology 205 (2010) 21972208macro- or micro-cracking under applications of contact loads in wearsituations. Thus, damage tolerance applied to TS coatings, in thisresearch, considers the combination of process and microstructuraldesign concepts by grading structures to achieve enhanced perfor-mance. Here, i) crack inhibition is proposed to be achieved in hardmetal-based cermet coatings (e.g. usually embrittled WCCo, CrCNiCr due to decarburization) by exploiting the process conditions inorder to induce compressive residual stress by peening in the bottomlayers, and ii) crack arrest and deection is achieved by addition of atough metallic phase.

    2. Experimental procedures

    2.1. Coating deposition

    The WC12 wt.%Co material (hereafter WCCo) used in theseexperiments, with particle size 62+42 m, was an experimentalbatch of ne carbide size produced by Osram Sylvania [30] withdesignation SX-432. The stainless steel 316 powder (hereafter SS316)was produced by ANVAL, particle size 46+24 m.

    The WCCo/SS316 functionally graded coating (FGC) was depos-ited onto grit-blasted AISI316 stainless steel plates of228.625.41.6 mm size, using a Sulzer-Metco DJ 2700 torch withpropylene as fuel. The WCCo and SS316 powders were mixed invarious proportions to produce a FGC. Starting from a 100 vol.% steellayer, the amount ofWCCowas increased by 20 vol.% until a top layerconsisting of pure WCCo was deposited, thus obtaining a 6-layersystem of about 0.2 mm per layer. The parameters employed for thedeposition of each layer are listed in Table 1. For comparativepurposes, monolithic single-layer coatings corresponding to eachlayer of the FGC (and having identical thickness) were also depositedonto the stainless steel plates, using the same deposition parameterslisted in Table 1. They will be hereafter referred to as single-layercoatings. Additionally, a thick (0.6 mm) pure WCCo layer wasproduced, in order to compare the thick FGC to a homogeneouscoating of similar thickness. This layer could not be grown thickerthan 0.6 mm without debonding and severely distorting thesubstrate.

    2.2. Microstructural and micromechanical characterization

    X-ray diffractometry (X'Pert PRO, Panalytical, Almelo, TheNetherlands) was performed on the top layer of the FGC and on thesingle-layer coatings, using CuK radiation in the 30b2b90range.

    Table 1Parameters employed in the HVOF-deposition of the various layers of the FGC.

    Nominal powder blendcomposition (vol.%)

    Layer thickness(mm)

    Numberof passes



    100% SS316L 0.188 7 67 25180%SS316L-20%WCCo 0.184 8 78 25160%SS316L-40%WCCo 0.265 14 78 26640%SS316L-60%WCCo 0.164 9 78 26620%SS316L-80%WCCo 0.189 13 78 280100% WCCo 0.313 20 78 280property (ICP) sensor [3133]. ICP uses a laser displacement sensor tomeasure continuously the stress-induced curvature of a thin platesubjected to layer-by-la