Damage mechanisms and cracking behavior of thermal sprayedWC–CoCr coating under scratch testing

Arash Ghabchi a,b,n, Sanjay Sampath a, Kenneth Holmberg b, Tommi Varis b

a Technical Research Center of Finland (VTT), Espoo, Finlandb Center for Thermal Spray Research, Stony Brook University, Stony Brook, NY USA

a r t i c l e i n f o

Article history:Received 19 November 2013Received in revised form23 February 2014Accepted 24 February 2014Available online 3 March 2014

Keywords:Thermal spray coatingScratch testDamage mechanismSliding wear

a b s t r a c t

Evaluation of wear mechanisms of thick thermal sprayed cermet coatings is a challenging endeavor giventhe numerous process-induced structural and chemical changes as well as presence of residual stresses.In an effort to understand the damage processes under contact load and their sensitivity to the processinduced microstructural attributes, controlled scratch testing was used. Detailed assessment of theresultant damage zone provided repeatable cracking patterns that are categorized as (i) Localizedcollapsing of material, (ii) angular cracks, (iii) primary semi-circular and developed semi-circular cracksand (iv) splat delamination. A correlation was established by linking observed damage mechanisms tothe process induced microstructural descriptions including role of spray particle conditions and residualstresses. Quantitative correlations between delamination load for cracking and the process inducedvariable including particle properties as described by the non-dimensional melting index concept as wellas residual stresses were established. Melting index captures the combined effect of particles' thermaland kinetic history and thus coating porosity and the process induced decarburization. The resultshighlight the critical role of coating density and stress evolution during the coating formation. Theresearch points to scratch testing as a powerful evaluation method to characterize contact response ofthick thermal spray cermet coatings including operative mechanisms.

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

The term “thermal spray” (TS) is referred to an array of coatingprocesses used to apply metallic, ceramic, composite and poly-meric coatings on different substrate materials. Thermal sprayingconsists of introducing the raw materials in form of wire, rod orpowder to the energy source to heat up the materials. Fully orpartially-molten particles are accelerated towards the substrate bygases and projected on the substrate. Upon impact, particles areflattened and solidified to form a disk shape structure called“splat”. Continuous layer of coating is formed by overlapping andadhering splats. Due to the nature of deposition process (indivi-dual particles), deposits will exhibit a lamellar structure. The finalstructure of TS coatings comprises of splat boundaries, pores andcracks. The complex defect-driven microstructure of TS coatingsmakes the understanding of mechanical properties and perfor-mance of coatings an intricate task.

Among the coating materials which have been deposited by TStechniques, composite ceramic-metallic (cermet) coatings (mainlycarbide-based ceramic particles within the metallic binder) havebeen used extensively as wear resistant coatings [1,2]. It is shownthat abrasive and erosive wear performance of materials includingcermet coatings depend on material resistance to penetration ofparticles and its resistance to removal or displacement of materialfrom the surface due to fracture and/or plastic deformation. Thus,the abrasive wear-controlling parameters of material can bedefined by elasto-plastic and fracture characteristics of the mate-rial. In fact these characteristics are the main motivations toemploy cermet coatings in wear resistance applications as theyexhibit a combination of hardness and toughness. The hardness isprovided by hard, brittle dispersed ceramic particles and tough-ness is provided by ductile metallic binder [3–5].

The application of cermet coatings via TS is a well-establishedtechnology dating back to 1960s. The design of these cermetcoatings (WC–Co and CrC–NiCr) evolved from similar work inthe cutting tool industry where liquid phase sintered cermets arethe work horse of tools [5]. However, even advanced TS processsuch as high velocity oxy-fuel combustion spray (HVOF) inducessubstantial chemical and structural changes to the consolidatedmaterial and there by introducing new mechanisms into the

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http://dx.doi.org/10.1016/j.wear.2014.02.0170043-1648 & 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Current address: The Boeing Company, Boeing Researchand Technology, Thermal Spray Advance Research Team, 7701 14th Ave. South,Seattle, WA 98108 USA. Tel.: þ1 206 662 1883; fax: þ1 206 662 6297.

E-mail address: Arash.Ghabchi@Boeing.com (A. Ghabchi).

Wear 313 (2014) 97–105

contact damage situations. Although there is substantial literatureinto the process-microstructure-wear performance of these TScermet coatings, majority of them explore the dependence ofprocessing on wear resistance, rather than examining in detail theunderlying contact mechanics and its relations to the microstruc-ture [6–8]. Much of the published work is based on dynamic wearprocesses (sliding, abrasive or erosive wear) and its correlation toprocess induced microstructural attributes [9–11]. A limited num-bers of publications have looked at the wear mechanisms of thesecoatings [12–14]. Evaluation of wear performance along with thedescription of associated mechanisms is a challenging endeavorfor thick films and overlay coatings, as most of the wear testingmethods are based on measurement of either volume loss or massloss of tested materials that provide quantitative rather thanmechanistic insights into the wear of material. Besides, providingmechanistic insights into wear requires careful and extensive postfacto inspection on traces of wear event. Other studies have lookedat static indentation response of these materials but withouttranslation of the contact bodies [15]. The instrumented indenta-tion technique is one method for controlled loading of materialwith known geometry. The instrumented indentation techniqueprovides practical information regarding elasto-plastic propertiesof materials that can be used in material design [16,17]. None-theless, this technique is primarily a static test and does notcharacterize the interaction of material surface with movingindenter tip. Thus, indentation measured hardness or elasticmodulus as individual parameters are unable to fully describethe wear performance of material [18].

Controlled scratch testing extends the concept of instrumentedindentation by allowing simultaneous load application and slidingin lateral direction in respect to contacting material surfaces.A careful assessment of the damage zone resulting from the testprovides significant information on both the load responses anddynamic sliding effects. Thus, there is an opportunity to elucidatethe dynamic contact problem of these anisotropic coatings usingcontrolled scratch experiments, which is the focus of this study. Ingeneral, scratch testing consists of moving a stylus on the surface ofa material with an increasing normal load [19]. The failures that takeplace in scratch testing usually are complex and associated withmultiple mechanisms that are activated simultaneously. Neverthe-less, the test allows for constructing failure mode maps. An exampleof such a map has been demonstrated by Bull for scratch testing ofthin films. Failure mode map shows different failure modes takingplace during the scratch testing [20]. Of particular interest in scratchtesting is the identification of “critical load” (Lc). Critical load isdefined as the load at which a particular failure mechanism isobserved and repeated frequently at loads higher than critical load.For example Laukkanen et al. used the concept of critical load forintroducing first visible crack on the material surface under scratchtesting to extract the fracture toughness of coated surfaces [21].Scratch testing is now being used pervasively for thin hard filmsapplied in contact damage situations.

There has been some limited works exploring the fractureresponse of thermal sprayed ceramic coatings under scratchtesting. Arata et al. [22] used the onset of changes in slope offriction-normal load to determine the spallation critical load. Xieand Hawthorne [23] investigated the damage mechanisms ofAl2O3, Al2O3–TiO2 and Cr2O3 ceramic TS coatings. They concludedthat predominant damage mechanism is micro-fracture initiatedfrom pre-existing cracks in the microstructure. Vencl et al. usedthe scratch testing on cross section of thick ceramic coating toquantify the adhesion/cohesion of coatings [24]. Ghabchi et al.investigated the failure mode of laser cladded WC–NiCrBSi coat-ings under scratch testing [25].

In current study, we seek to extend the applicability of scratchtesting to TS coatings, in particular hard cermets applied via high

velocity oxy-fuel processes. Spray coatings, even dense HVOFcoatings are more complex compared to thin films due to theirmulti-scale and anisotropic microstructure, considerable hetero-geneity due to splat based assemblage and the presence of processinduced residual stresses. These coatings are also considerablythicker and as such the mechanisms described for thin films arenot readily translatable. Nevertheless, there is an opportunity toextend the knowledge of scratch testing to get a first orderassessment of operative failure mechanisms under sliding contact.To accomplish this, scratch testing was conducted on the topsurface of HVOF sprayed WC-14(CoCr) coatings. The goal was tonot only assess the response to controlled mechanical loading butalso as a method to discriminate differences among the micro-structure. This initial study will provide a frame work to exploitscratch testing concept both as a coating design tool and as a meanto examine process-structure-wear performance relations.

2. Fundamental considerations in scratch testing of thermalsprayed cermet coatings

2.1. Contact condition

When the maximum penetration depth of a spherical indenterdoes not exceed 10% of coating thickness, the plastic as well aselastic field beneath the contact will be limited to the body ofcoating. (Typical TS cermet coatings thickness is 200 μm aftergrinding). Therefore, unlike the thin films some of the interfacial(coating/substrate) failure mechanisms will not be present [26].However, there are still opportunities for other interfacial failuresarising from the lamellar structure and splat–splat interfaces. Inaddition, there is also a possibility of through thickness stressgradient due to layered assemblage of the coating, although itseffects are small for shallow indentation depths compared tocoating thickness.

In response of material to increasing sliding loading (encoun-tered in scratch testing) three mechanisms can be considered:plowing, friction and fracture of material. These steps are shownschematically in Fig. 1 for typical elasto-plastic materials and are

Fig. 1. Three mechanisms, plowing, friction and material fracture in response ofmaterial to increasing sliding load.

A. Ghabchi et al. / Wear 313 (2014) 97–10598

explained as follows: For each of these scenarios, the implicationof the phenomena on layered TS coatings is also considered.

1. Material plowing includes the elasto-plastic deformation ofmaterial under and in front side of the moving tip. Behindthe moving tip only plastic deformation is left which isobserved as scratch groove. In the case of layered thermalsprayed coatings, a combination of contributions will affect thematerial plowing phenomena. They may include compression–consolidation of porosity, interlayer sliding and plastic defor-mation of the splat material itself. There is also significantevidence of non-linear deformation behavior of spray coatings[27,28].

2. Friction is considered to have two contributing components:plowing and adhesion. The material deformation results in theplowing component of friction. Interaction between the mate-rial and the moving tip results in adhesive friction. Theadhesive friction force causes pushing the top layer of materialin front of moving tip and pulling the top layer of materialbehind the moving tip. These two actions, pushing the materialin front and pulling it behind the moving tip, causes formationof pile up and sink-in of material in front and behind of movingtip, respectively (these friction forces acting on the materialsurface in front and behind the moving tip are shown by arrowsin Fig. 1). In the case of thermal spray coatings, it has beennoted that the process of plowing occurs much earlier in thetime scale of interaction due to rapid separation of the inter-splat regions along with their participation in the plowingprocessing which is an indication of low strength and lowtoughness of splat interfaces [3,29].

3. Fracture can be caused by the formation of stresses in front,beneath and behind the moving tip. Behind the moving tip,cracks usually are formed in highest tensile stress regions.

The fundamental concept presented here is similar to priorwork on thin films but modified to fit the thick composite coatingconcepts such as WC-based thermal sprayed coatings [30].

2.2. The stresses around the moving tip:

The origins of stresses around the moving tip in thick coatingsare schematically shown in Fig. 2a and can be considered asfollows:

1-Friction in front side of sliding tip causes compressive stressesand in trail of the sliding tip generates tensional stresses. If thepile-up occurs in front of the moving tip due to friction thentensile stresses will be generated on material surface in front ofthe moving tip.

2-Tip geometry has an effect on the stress state on the materialsurface and beneath the indenter (e.g. sharper indenter generateshigher stresses than a blunt indenter).

3-Intrinsic residual stress is the result of coating process inducedresidual stresses which in the case of HVOF coatings can be eitherin tension or compression depending on the magnitude of thecomponent stresses. Typically, in HVOF sprayed carbide cermets,there are components of tensile quenching stresses, impactinduced peening stresses (compressive) and thermal mismatchstresses associated with cooling from the deposition temperature.Depending on coefficient of thermal expansion the differencebetween substrate and coating, thermal stress can be tensile orcompressive. The superposition of these events leads to a netresidual stress in the coating. The formation of intrinsic residualstresses during the thermal spraying can be evaluated by a non-contact curvature measurement technique. The formation of thesestresses and their significance explained by Kuroda et al. [31,32]and further developed by Sampath and Matejicek [33–35].

The intrinsic stresses will interact with stresses that are resultof plastic deformation introduced to the material surface.

2.3. Groove formation during the scratch testing

In scratch testing the indenter tip first acts as a pure indenta-tion to apply the preloading with no lateral movement. At thisstage the material beneath the indenter deforms elastically and/orplastically. Depending on material's work hardening characteris-tics, sink-in or pile-up of material around the indent imprint willbe observed [36]. With moving the tip and increasing the load agroove with increasing depth will be formed. In front side ofmoving tip compressive stresses are generated due to frictionforce. The friction force between the front side of the tip andmaterial surface pushes the material upward to form a pile-up.At top of the highest pile-up point, tensile stresses will begenerated. At the trail of the tip friction force causes the materialto sink in and generates a tensile stress behind the tip. The grooveformation mechanisms in thick thermal sprayed composite coat-ings are shown in Fig. 2a and b in macro- and micro scales,respectively. All the features in Fig. 2b are shown in relativesize scale.

3. Experimental techniques

3.1. Materials and coating deposition

A commercial WC-14(CoCr) powder (Durmat) was employed asstarting material to deposit coatings with different processingconditions. Table 1 shows the powder characteristics that wereused in this study. The coatings were deposited with a HighVelocity Oxygen Fuel (HVOF) Diamond Jet-2600 (Sulzer Metco)system using hydrogen and nitrogen as fuel gas and carrier gas,respectively. Different spraying parameters were used to depositcoatings with different microstructures. Coatings were depositedat standing off distance of 250 mm. Table 2 shows the processparameters used to deposit different coatings. Hydrogen and

Fig. 2. The origin of stresses around the moving tip and groove formationmechanism in a) macro and b) micro scales.

A. Ghabchi et al. / Wear 313 (2014) 97–105 99

oxygen flows were adjusted to obtain processes with differentfuel/oxygen ratios. Coatings were deposited on steel 1010 sub-strates that were grit-blasted with 590–710 mm alumina particlesat 4.5 bar pressure followed by ultrasonic cleaning in the acetone.Cooling air was applied to keep the substrate temperature at�150 1C during spraying.

3.2. Characterization techniques

In-flight particles temperature and velocity were measured usingthe SprayWatch diagnostics device. Coatings cross sections and scratchgrooves were studied by scanning electron microscope (SEM) andoptical multi-focus microscope. In situ Coating Property (ICP) sensor[32] was used to obtain the stresses during and after the coatingdeposition process. A bonded-specimen sample was prepared follow-ing the steps outlined in Choi et al. [37]. In this technique coating/substrate specimen is cut, and opposing cross sections and topsurfaces are polished. Cross sections are glued and pressed together,and scratch test is carried out on the bonded interface (on top surface).After de-bonding of sample, cross section and top surface of thesamples were examined under SEM. Coatings were evaluated by x-raydiffraction with a Phillips PW3710 with Mo–Kα radiation for detectionof different phases and the W2C/WC intensity peak ratio was used asan indication of the level of decarburization. Porosity values wereevaluated using 10 SEM images from cross section of each sampletaken at 500� magnification. Vickers indentation hardness andelastic modulus of coating cross section were measured using instru-mented indentation (Zwick/Roell) employing 300 g load. Opticalmicroscope was used for measuring the location of specific damagethat is related to normal load in which the damage has occurred.This load was used as “critical load”.

3.3. Scratch testing

Scratch testing was done using a conical diamond tip with200 mm tip radius on the polished top surface of samples. The testwas carried out applying 5 N preload, increasing to 100 N. Averagescratch length was measured to be �9 mm in all cases. For eachsample three scratches were carried out. The scratch test proce-dure is described in more detail in the European Standard prEN1071-3 [38]. Scratch test device was developed at VTT, TechnicalResearch Center of Finland.

4. Results and discussion

The results are presented in three sections: The first sectionpresents a detailed analysis of the scratch test observations for aprototypical HVOF WC–CoCr system from all of the samplesexamined in this study. These mechanisms are outlined throughthe interaction between scratch load and damage observationsresulting in a damage mechanism map. The origin of thesedamages and relevance to the unique thermal spray microstruc-ture are also discussed. The critical normal loads in which thespecific damages were observed are shown in Table 3. In secondsection influence of the process induced microstructural changeson the critical onset of damage (as outlined in the earlier section)is investigated. Although the overall mechanisms for these mate-rials and microstructures are similar, correlations are observedamong processing, microstructure and damage. Finally, the role ofthe process induced residual stresses on the damage evolutionduring scratch is quantified. Such a combined study not onlyallows for understanding the fundamental damage mechanisms inan important class of TS wear resistant coatings but provides aframework for coating design and process control for enhanceddamage tolerance.

4.1. Macro and micro-damage mechanisms in thermal sprayedcermets subjected to controlled scratch tests

Three scratch tests were conducted on each sample. Evolutionof friction coefficient under sliding condition at different normalloads is recorded (Fig. 3). Each curve in Fig. 3 is an average of three

Table 1Characteristics of powder material used in this study.

Powders Apparentdensity(g/cm3)

WC grainsize (μm)

Sizedistributiond10%–d90%

Chemicalcomposition, wt%

C Co Cr W

Conventional 4.91 1–3 24–45 5.34 9.68 3.79 Bal

Table 2Spraying parameters used for deposition of WC-CoCr coatings.

Sample code Spraying parameters(slpm) – Standard liter perminute

NormalizedF/O (γ*)

N2 H2 O2 Air

C1 14 565 225 350 0.95C2 14 605 246 350 0.95C3 14 635 215 350 1.1C4 14 665 230 350 1.1C5 14 660 192 350 1.25C6 14 708 210 350 1.25

Table 3Measured critical loads.

Sample code Critical loads (N)

Angular crack Semi-circular cracks Splat delamination

C1 9.571.2 14.274 27.372.7C2 8.371.6 12.472.4 22.774.8C3 1373 20.372.8 3073C4 11.971.1 14.271.7 28.474C5 10.571.8 16.573.5 3070.7C6 11.472.8 15.271.4 3271.2

Fig. 3. Coefficient of friction versus normal load for different samples.

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friction curves for each sample. Except for sample C3 the frictioncoefficient across different normal loads for all the samples are inthe same range. The generated scratch grooves were studied bymulti-focus optical and scanning electron microscopes to examinevarious failure mechanisms and to develop a failure map. Fivegeneral observations were realized and the related failuremechanisms are discussed in the following sections. Thesemechanisms are based on the fundamental considerations laidout in Section 2. The results are interpreted in the framework of afailure mode map (Fig. 4) with concomitant microscopic imagesdisplaying the observed failure events at each load. In general,formation of each damage mechanism depends on coatingmaterials characteristics and applied maximum load.

(i) Localized collapsing of material: In the low loads of 5–11 N, insome regions where there is higher concentration of closelyspaced open surface porosities, sliding tip suddenly pene-trates into the material. Fine-structured crack networks thatconnect the surface pores were observed in that region. Thepresence of surface open porosities lowers the local loadcarrying capacity of the material. This failure mechanism isreferred to as local collapsing of material (Fig. 4a). Surfacepores might be formed during the deposition of coating orduring the surface preparation and polishing procedure. Atintermediate loads, 15 N–20 N, sudden changes in groovewidth (localized collapsing of material) were also observedthat were not associated with surface open porosities. Thisphenomenon can be attributed to the formation of subsurfacecracks or collapsing of subsurface pores along the scratchpath. The latter collapsing mechanism was examined usingbonded interface-specimen samples[37]. Fig. 4b shows thecross section and top surface of bonded-specimen sample thatconfirms the cracking and pore collapsing beneath the scratchgroove.

(ii) Angular cracks: Angular cracks were observed at the edge ofscratch grooves. These cracks are formed due to combinationof moving action (providing tensile stresses parallel to thescratch groove behind the moving tip) and bending action(providing tensional stress perpendicular to scratch groove)around the moving tip at the edge of groove. The

superimposition of two tensile stresses will dictate theangularity of these types of cracks. Formation of these stressesis shown schematically in Fig. 2a. The angular cracks at thescratch groove are shown in Fig. 4c. At lower loads the angularcracks are finer but at higher loads they become more visible(Fig. 4e). Similar angular cracks have been reported in scratchtesting of thin coatings [20,30]. Angular cracks were analyzedusing SEM. Fig. 4d shows the angular crack morphology andcrack path. Angular cracks tend to go through the carbideparticles and the binder (preferably a straight path). Dottedline in Fig. 4d indicates the approximate groove edge.

(iii) Primary semi-circular and developed semi-circular cracks: Inabsence of friction, maximum tension regions in trail ofmoving diamond stylus will occur below the material surface.However, with increasing friction these maximum tensileregions tend to appear at the contact surface. The cracksmight follow the maximum tensile stress contours on thesurface of material that is developed in trail of moving tip. Atlower loads this kind of cracks are not developed fully and aremainly limited to the scratch groove. By progressing thescratch test (increase in normal load), semi-circular cracksare developed and are expanded outside of scratch groove. Insome areas both angular and developed semi-circular crackscan be observed alongside the scratch groove (Fig. 4e). Also,similar semi-circular cracks have been reported in scratchtesting of thin coatings [20,30]. The main difference betweensemi-circular cracks observed in thin coatings and TS thickcoatings is that in thin coatings crack penetration depth canreach to the substrate/coating interface but in thick coatings itwill be limited to the coating depending on the maximumnormal load. To further study the semi-circular crack pathbeneath the coating surface, bonded-specimen sample's crosssection and top surface were examined using SEM (Fig. 4f).The cracks beneath the surface tend to propagate in adirection almost parallel to the substrate and do not propa-gate all the way through the coating thickness.

(iv) Splat delamination: By further progression of scratch test, loadshigher than 20 N, in which the semi-circular cracks are welldeveloped they reach each other at the splat boundaries andcoating surface. Such interactions between cracks and splat

Fig. 4. (a) Localized collapsing of material under moving stylus due to presence of surface open porosities, (b)SEM image of cross-section and top view of collapsed area(bonded-specimen), (c) Angular cracks observed at the scratch groove edge, (d)SEM image of angular crack, (e)semi-circular cracks, (f) SEM image of cross-section and topview of semi-circular crack, and (g) delamination (moving direction from right to left).

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boundary cause detachment of one or group of splats. Thisexplanation is more plausible by considering the splat bound-aries as weak regions in the coatings due to the nature of TSdeposition. It is also hypothesized that splat delaminationmight occur in front of moving tip due to the formation oftensile stress at highest point of pile up (Fig. 2a). Note thatthis splat delamination mechanism is not same as outlinedfor thin coatings. In thin coatings, one would consider thedetachment or delamination of whole coating from thesubstrate. In case of thick TS coatings delamination is con-sidered as full or partial detachment of one or more splatsfrom the underlying splat (Fig. 4g).

TS coatings are not monolithic structures. Their intrinsic andextrinsic characteristics are strongly influenced by the process.Thus, interpretation of the mechanisms as outlined and explainedin Fig. 4 needs to be put in context of process-microstructures-properties variations. The scratch testing method and damagemechanisms associated with it is sought to be a technique todiscriminate between different microstructures. To further exam-ine this hypothesis in following section, a correlation between theprocess, microstructure, properties, and stresses was establishedand this correlation was extended to understand the potentialeffects of microstructure and stresses on critical loads in which aparticular damage mechanism was observed.

4.2. Establishing a correlation between process variables – in-flightparticles state and microstructure

Characteristics of in-flight particles (mainly thermal and kineticenergy of in-flight particles) in thermal spray are the crucialparameters that significantly affect the final coating. The thermaland kinetic energies of inflight particles are not possible to bemeasured directly. However, temperature and velocity are twocharacteristics of inflight particles that are possible to be measuredby process diagnostic tools. Inflight particles temperature andvelocity are highly affected by the process parameters. Thetemperature and velocity of inflight particles for samples C1 toC6 are shown in Fig. 5. The fuel/oxygen ratio normalized withstoichiometric ratio for hydrogen combustion has been shown onthe same graph (γ*). However, these two parameters (temperatureand velocity) do not represent the real thermal and kinetic historyof particles at the time of their impingement onto the substrate[39]. Descriptive deficiency of temperature and velocity for

ceramic materials were addressed by Vaidya et al. [40,41] usingan index defining the fraction of molten content of in-flightparticles. This index simply is an indication of molten content ofin-flight particles. Higher melting index values represent in-flightparticles with higher molten contents. The melting index (M.I.) inits simple form represents a normalization of the measuredparticle temperature with reported size and dwell time and isdefined as follows:

M:I:¼ TΔtf lyD

In which “T” is the in-flight particle temperature, flight time isΔtf ly ¼ 2s=v, “s” is standing off distance and “v” is particle velocityand “D” is the particle diameter. Further modified version ofM.I. equation developed by Zhang et al. [42]:

M:I:¼ A24kρhfG

11þ4=Bi

ðTs�TmÞΔtf lyD2

In which “A” is the ratio of ðTf �TmÞ=ðTs�TmÞ, “Tf” is the flametemperature, “Ts” is the particle surface temperature measured bydiagnostics and “Tm” is the particle melting point. In this estima-tion flame temperature and particle surface temperature havebeen considered the same. “K” is the thermal conductivity, “ρ” isthe density, “hfg” is the enthalpy of fusion, “Bi” is the Biot numberand “D” is the particle diameter.

The melting state represents the thermal history of in-flightparticles. As the particle size in this study was not available forevery single in-flight particle due to the employed diagnostictechnique, the overall average size of particles was used incalculation of melting index. Fig. 6 shows the correlation ofmelting index with microstructural features, W2C/WC and poros-ity of deposited coating. By increasing the melting index, porosityvalues decrease with concomitant increase in W2C/WC ratioindicative of greater decomposition. Higher melting content ofinflight particle (high melting index) enhances the kinetics ofdecarburization and formation of W2C phase. The same analogycan be used to explain the improved density (lower porosity level).Higher molten content of particle makes the flow and flattening ofsplats easier which results in improved density and less porosity.Fig. 7 shows the back scattered electron microscope images of themicrostructure of two coatings with lowest and highest meltingindex in low and high magnification. Low magnification images(Figs. 7a and b) show a distinct difference between levels ofporosity. Coating with low melting index (M.I.¼0.36) showshigher porosity compared to the coating with higher meltingindex (M.I.¼0.81). High magnification microstructural images

Fig. 5. In-flight particles temperature and velocity at different normalized fuel/oxygen ratio for samples C1 to C6. Fig. 6. Correlation of melting index with W2C/WC and porosity.

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(Figs. 7c and 7d) show the effect of melting index on dissolution ofcarbides and homogenization of binder content. In coating withlower melting index (Fig. 7c), fine carbides are visible, coarsercarbides present with sharp edges and in the binder, localizedcobalt or chromium rich areas are detected. This microstructure,Fig. 7c, is very similar to the microstructure of original startingpowder which indicates minimum heat input to the microstruc-ture. On the other hand, the microstructure of coating with highmelting index (Fig. 7d) shows disappearance of fine carbides, edgerounding of coarse carbides and more homogenized binder. Dis-appearance of fine carbide and edge rounding effects are indica-tions of carbide dissolution/decarburization which potentially leadto the formation of W2C phase. The porosities and different phaseswithin the structure have significant contribution to mechanicalproperties of thermal spray coatings. To capture this attributeFig. 8 shows the correlation between the porosity, indentationelastic modulus and hardness of coatings. Less porous coatingresults in high elastic modulus and high hardness. High hardnessvalues can also be contributed to the presence of higher amount ofW2C phase within the structure.

4.3. Influence of process-microstructure-properties-stressesattributes on scratch behavior: integration of process map anddamage mechanisms

In current study evolution of stresses as well as final residualstresses were evaluated using the non-contact curvature measure-ment technique [33]. The stress evolution during the coatingdeposition, referred to as evolving stress [35], is an indication ofthe rate of stress build up during the coating deposition process.The evolving stress was calculated based on curvature-time databy using the slope of curvature graph. One example of curvatureand temperature monitoring data and representation of evolvingstresses is shown in Fig. 9. Depending on the melting state ofimpinging particles on the substrate the evolving stress can bedominated by quenching tensile stress that is arising from parti-cles with higher melting state or dominated by peening compres-sive stress that is arising from particles with lower melting state.Evolving stress is the net stress from quenching and peeningeffects. Fig. 10 shows the relation between melting index and the

Fig. 7. Low magnification (a), (b) and high magnification (c) ,(d) back scattered electron microscope images of coatings with low and high melting index.

Fig. 8. Correlation between porosity and hardness-elastic modulus of coatings. Fig. 9. An example of curvature/temperature versus time graph, obtained usingcurvature measurement during the deposition process.

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evolving stress in current experiments. Higher melting statepotentially results in the dominant stress formation mechanismthat is based on quenching and solidification of particles. Thus,particles with higher melting state result in coating with highertensile evolving stresses. Lower melting state means presence ofparticles with higher un-molten content. Un-molten particlescontribute more to the peening process which results in lesstensile stresses [31,32]. Evolving stress has implication in evalua-tion of microstructure and cohesive quality of splats. High evolvingstress is a direct result of improved cohesive bonding of splats.Fig. 11 shows the evolving stress versus delamination critical loadin conjunction with the elastic modulus of coatings. Higherevolving stress means enhanced cohesion between splats whichtranslates to higher loads required to delaminate the splats.The improved splat cohesion (higher critical load) is observedfor coatings with high evolving stress and high elastic modulus.

5. Conclusions

A damage mechanism map was developed for thermal sprayedWC–CoCr coatings subjected to scratch testing. Different crackingpatterns were observed due to the surface scratch loading;(i) Localized collapsing of material, (ii) angular cracks, (iii) primary

semi-circular and developed semi-circular cracks and (iv) splatdelamination. Crack types (i) and (iv) are specifically observedin thick thermally sprayed composite coatings while crack types(ii) and (iii) are generic and similar to the ones observed for PVDand CVD thin coatings.

Since the characteristics of thermal sprayed structures isstrongly sensitive to the process parameters and the microstruc-ture, their effects on scratch response were investigated. Theprocessing effects were characterized through monitoring of in-flight particles states during thermal spraying and associatedcorrelations with the microstructure. The non-dimensional melt-ing index concept was used to describe the thermal and kinetichistory of the WC–CoCr particles enabling assessment of bothporosity and process induced decarburization. Particle with highermelting index had conflicting effects on coating the microstructureby reducing porosity but increasing decomposition to form largercontent of W2C. Scratch testing pointed to distinctions in themicrostructure-contact wear response providing both qualitativeand quantitative interpretations. The results pointed to critical roleof stresses and modulus on resistance to scratch. Coatings withhigher evolving stress and modulus resulting from improvedmelting (quantified through melting index parameter) displayshigher critical load for delamination. The methodology provides astrategy for using the in situ process monitoring as a tool fordescribing coating behavior under contact load situations.

Obtaining quantitative values such as different critical loadscan have implications in evaluation of performance related proper-ties of the coating. For instance the critical load for splat delami-nation showed a direct correlationwith evolving stress which is anindication of splat cohesion strength. The critical load for theformation of angular or semi-circular cracks can be used fordetermination of fracture toughness. However, further studiesare necessary to correlate each critical load with developedstresses and mechanical properties of coating. Providing such in-depth knowledge enables the scratch testing to be used asperformance and reliability evaluation tool for thick coatings.

Acknowledgment

AG would like to thank Kati Rissa and Essi Sarlin from TampereUniversity of Technology, Finland for providing SEM images ofcoatings. This study was carried out in the MATERA projectMOTRICOT (model based tribologically optimized thick multi-material coated surfaces). SS acknowledges the support of NSFGOALI program under award CMMI and CMII 1030592. Thefinancial support of Consortium for Thermal Spray Technology(Stony Brook University), Tekes (the Finnish Funding Agency forTechnology and Innovation, The institute for promotion of innova-tion by science and technology of Flanders (IWT), the participatingindustrial companies Metso and Ruukki and VTT TechnicalResearch Center of Finland are gratefully acknowledged.

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