Multiple-Approach Evaluation of WSPCoatings Adhesion/Cohesion Strength

Radek Musalek, Vaclav Pejchal, Monika Vilemova, and Jirı Matejıcek

(Submitted June 26, 2012; in revised form October 11, 2012)

Adhesion/cohesion testing represents one of the most common methods for benchmarking and opti-mization of thermal spray coatings. However, owing to the inhomogeneous coating microstructure, suchtesting may be quite troublesome. In this study, adhesion/cohesion strength of representative metallicand ceramic coatings deposited by water-stabilized plasma (WSP) spraying was evaluated by four dif-ferent methods: tensile adhesion test, pin test, tubular coating tensile test, and shear test. Combination ofvarious methods enabled the evaluation of the coating adhesion/cohesion strength under differentloading conditions. Limitations and benefits of each method for testing of WSP coatings are demon-strated. Dominating failure micromechanisms were determined by supplementary fractographic analysis.

Keywords adhesion testing, adhesive strength, alumina,cohesion, stainless steel, water-stabilized plasma

1. Introduction

Satisfactory adhesion/cohesion strength of thermalspray coatings is essential for their application (Ref 1).However, it is not easy to evaluate, as the coatings may beexposed to different modes of loading (e.g., in-plane, out-of-plane, tension, compression, bending, or shear) in dif-ferent types of environments (including elevated or cryo-genic temperatures, thermal shocks, corrosive andabrasive environments, etc.) and their combinations. As itis not feasible to test coatings for any possible type ofloading during in-service conditions, laboratory tests mustbe used. However, such testing may be insufficient andeven misleading, as coatings may show different behaviorsunder different conditions because of their anisotropiclamellar microstructures and inhomogeneities (such aspores, cracks, unmelted or resolidified particles, oxides

and impurities, etc.). Moreover, the family of thermalspray coatings is very wide, ranging from brittle ceramicsto ductile metals with different types of microstructures.None of the existing methods is therefore universal(Marot in Ref 2 reported that more than 80 adhesion testmethods existed in 2006). The most common adhesion/cohesion testing methods are based on evaluation of themacroscopic coating strength, which is in principle a crit-ical load normalized by the specimen geometry. Suchtesting methods are represented by the widely appliedtensile adhesion test (TAT) or its modifications (Ref 3-8),a shear test (Ref 9-11), a tubular coating tensile (TCT) test(Ref 12, 13), or a pin test (Ref 11, 14), and many others.Another approach is represented by testing of the coatingadhesion/cohesion on the microscopic level using inden-tation testing (Ref 15, 16) or scratch testing (Ref 17, 18).

For some of the previously mentioned tests, there existmethods based on fracture mechanics which may accountfor stress concentration and provide an alternative evalu-ation of the results in terms of the coating fracturetoughness or interfacial toughness (Ref 2, 8, 14-16, 19-23).

It has to be pointed out that each method has someconsiderable limitations such as the strength of the glueand its penetration into the coating (TAT test—Ref 3, 4,11, and 21), need of additional sample grinding/polishing(e.g., shear test—Ref 9 and 11), requirement of specialequipment for testing and specimen geometry (e.g., sheartest and pin test—Ref 2, 9-11, and 14), knowledge of thecoating mechanical properties (indentation tests—Ref 2,15, 16, and 20), significant stress concentration (e.g., TCTtest—Ref 12 and 13), etc. It may be therefore beneficial tocombine several methods (Ref 2, 12, 18-22).

In this article, a combination of four different types ofstraightforward adhesion/cohesion tests (TAT test, sim-plified pin-test, TCT test and shear test) based on evalu-ation of macroscopic coating strength was selected inorder to observe coatings failure under different loadingconditions, to attempt to overcome limitations of indi-vidual tests, and to evaluate their validity for routine testing

This article is an invited paper selected from presentations at the2012 International Thermal Spray Conference and has beenexpanded from the original presentation. It is simultaneouslypublished in Thermal Spray 2012: Proceedings of theInternational Thermal Spray Conference, Air, Land, Water, andthe Human Body: Thermal Spray Science and Applications,Houston, Texas, USA, May 21-24, 2012, Basil R. Marple, ArvindAgarwal, Laura Filofteia-Toma, Margaret M. Hyland, Yuk-ChiuLau, Chang-Jiu Li, Rogerio S. Lima, and Andre McDonald, Ed.,ASM International, Materials Park, OH, 2012.

Radek Musalek, Monika Vilemova and Jirı Matejıcek, Depart-ment of Materials Engineering, Institute of Plasma Physics AS CR,v.v.i, Za Slovankou 3, 182 00 Praha 8, Czech Republic; andVaclav Pejchal, Department of Materials, Faculty of NuclearSciences and Physical Engineering, Czech Technical Universityin Prague, Trojanova 13, 120 00 Praha 2, Czech Republic. Contacte-mail: musalek@ipp.cas.cz.

JTTEE5 22:221–232

DOI: 10.1007/s11666-012-9850-2

1059-9630/$19.00 � ASM International

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of plasma sprayed coatings. Samples were deposited usingthe water-stabilized plasma (WSP) process developed atour institute, for which a high enthalpy resulting in highachievable feed rate and coating thickness is characteristic(Ref 1). However, it is assumed that such testing may bealso applicable for other types of thermal spray coatings.

2. Methods

2.1 Sample Preparation

For all tests, two types of coatings were deposited usinga water-stabilized plasma system (WSP� 500, Institute ofPlasma Physics AS CR, CZ). Al2O3 and AISI 410 stainlesssteel were selected as representative ceramic and metal-lic coatings to compare the results of the tests. The sub-strates were machined from a structural steel (EuropeanS235JRC grade) to dimensions prescribed for each test (seebelow). The substrates were grit-blasted with alumina gritimmediately before the coating deposition (Ra = 7.7 lm)and cleaned with acetone. In order to observe the influenceof the coating thickness on the test results (e.g., expectedcoating reinforcement due to the glue penetration orresidual stress profile), samples with thicknesses rangingfrom 0.1 to 1.4 mm were prepared during one spraying runby stepwise removal of the samples after a different numberof passes. For the spraying conditions, see Table 1. Coatingthickness of each sample was measured after mechanicaltesting on polished cross sections using a calibrated lightmicroscope.

Typical coating microstructures are illustrated in theFig. 1. For the Al2O3, numerous vertical intrasplat cracksas well as intersplat boundaries and pores were typical.For the AISI 410 coating, intersplat pores as well as someoxidation verified by EDX mapping (darker phase inFig. 1b) of the splats, especially at the splat boundaries,were observed. Interfaces between splats also had ‘‘wave-like’’ character, which indicates a possibility of a goodmutual mechanical interlocking of the splats especiallyduring the in-plane loading.

2.2 Mechanical Testing

2.2.1 TAT Test. Testing was carried out in accordancewith the EN 582 standard (Ref 4). Heat-curing adhesiveE1100S (DFD Instruments, UK) was used to bond thecoated substrate and the counterpart cylinder. Nominal(apparent) coating tensile adhesion/cohesion strength wasevaluated from the maximum load Fmax as follows:

RTAT ¼ Fmax=S0 ðEq 1Þ

where S0 is the initial area of the critical cross section.

2.2.2 Pin Test. The specimens for the pin test methodconsist of a cylindrical pin and coaxial ring (see Fig. 2)machined from the same steel to prevent coating damagebecause of the mismatch in thermal expansion duringspraying. Before the coating deposition, cleaned ring andpin may move freely and are aligned so that their top flat(grit-blasted) surfaces are in the same plane. The typical gapbetween the pin and the ring was approximately 40 lm.Subsequently, coating is deposited onto the surface. Coat-ing stiffness then prevents mutual movement of the pin andthe ring. The pin test is then based in pulling-off the cylin-drical pin from the coaxial ring and evaluation of the loadneeded to achieve a macroscopic coating failure. Two dif-ferent pin diameters (12 and 25 mm) were used to evaluatethe influence of the specimen dimensions on the test results.

During the test, the pull-off force against displacementis recorded until a failure of the coating occurs. Typically,three modes of failure occur (see Fig. 3):

� Mode I (tensile mode)—typical for thick coatings.Adhesion and/or cohesion failure occurs in the planeparallel to the coating-substrate interface.

� Mode II (shear mode)—typical for thin coatings.Failure develops within the coating perpendicularly toits plane, above the pin circumference.

� Mixed mode—typically a combination of initial failurein Mode I and subsequently in Mode II.

Table 1 Spraying conditions

Coating Al2O3 AISI 410

Powder size, lm 30-80 140-180Feeding distance, mm 88 115Spraying distance, mm 400 450Feed rate, kg/h 10.1 23.4

Fig. 1 Typical microstructures of the coatings. SEM-BSE-COMPO

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From the maximum load Fmax achieved during the test,the maximum nominal normal stress rN and the maximumnominal shear stress rS applied on the coating arecalculated:

rN ¼ Fmax= pR2� �

ðEq 2Þ

rS ¼ Fmax= 2pRtð Þ ðEq 3Þ

where R is the radius of the pin, and t is the coatingthickness. If Mode I or Mode II failure occurs, then rN orrS is denoted as critical, respectively. Please note that theratio

rS=rN ¼ R=2t ðEq 4Þ

is dependent only on the pin radius and coating thickness.2.2.3 TCT Test. The test was carried out in analogy to

the method described in Ref 12 and 13. Coatings weredeposited on the grit-blasted curved surfaces of two joinedcylinders with diameter and height of 25 mm which werecoaxially fixed together during spraying by a screw. Thescrew was removed before the testing so that the cylinderswere bonded together only by the deposited coating. Theaxial load needed to separate the cylinders is then ameasure of the in-plane coating cohesion strength. TCTstrength was evaluated from the maximum load Fmax asfollows:

RTCT ¼ Fmax=S0 ðEq 5Þ

where S0 is the area of the critical cross section of thecoating.

In all three above-mentioned tests, samples were loa-ded with a constant load rate of 0.5 mm/min in a fullyarticulating fixture to avoid any undesirable bendingmoment. TAT specimens were tested on an adapted

universal testing machine Inspekt 100 (Hegewald &Peschke, D). The pin test and TCT test were carried outusing an Instron 1362 upgraded with Instron 8800 serieselectronics and adapted for each test.

2.2.4 Shear Test. The test was carried out in accor-dance with the EN 15340 standard (Ref 9). Coatings weredeposited on the base of 10 9 5 9 30 mm prisms. Sheardistance (distance of the edge of the shear plate from thecoating-substrate interface at the beginning of the test)was adjusted with a micrometer screw to the value of50 ± 20 lm as recommended by the EN 15340 standard.Interchangeable carbide shear plates SPEW 1204ADEN:8230 (Pramet Tools, CZ) were used to ensure a sharpshear edge for each sample. Edges were observed aftertesting in the SEM microscope, and no significant damageto the shear edges was observed. Loading was carried outusing an Instron 1362 machine with 8800 series electronicsand shear plate displacement rate of 3 mm/min. Shearresistance of the coatings was then calculated from themaximum load achieved during the test as follows:

RST ¼ Fmax=S0 ðEq 6Þ

where S0 is area of the prism base (50 mm2). According tothe shape of the load-displacement curves and crack path,mode of failure of the samples was evaluated according tothe EN 15340 standard.

2.3 Fractographic Analysis

The fracture surfaces of the broken specimens wereobserved in an EVO� MA 15 (Carl Zeiss SMT, D) scan-ning electron microscope equipped with an EDX detectorXFlash� 5010 (Bruker, D) to identify the typical failuremicromechanisms.

3. Results and Discussion

3.1 TAT Test

For both ceramic and metallic coatings, the load-dis-placement curves were linear until sudden failure. Nosigns of a decrease in the coating stiffness, which wouldindicate gradual coating failure, were observed.

Results of the TAT test are plotted in Fig. 4. For bothcoatings, values of the nominal coating strength convergedto the strength of the glue (~70 MPa) with decreasingcoating thickness. However, the overall behavior of theFig. 2 Scheme of the pin test. Adapted from Ref 11

Fig. 3 Typical failure modes observed during pin test. Crack path highlighted in black. (a) Mode I, (b) Mode II, and (c) mixed mode

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coatings was significantly different. For the Al2O3 coating,apparent tensile adhesion/cohesion strength (as evaluatedfrom Eq 1) significantly decreased with increasing coatingthickness to approximately 18 MPa whereas for the AISI410 coating, the coating strength was in the range of 61-69 MPa with only a slight decrease with increasing coatingthickness.

Significant differences were also observed in the frac-ture modes of both coatings. For the thinnest coatings fromboth sets, a significant amount of glue failure (>30%) wasobserved. As the coating thickness increased, the amountof glue on the fracture surface decreased. The results forthe samples which showed significant presence of the glueon the fracture surface were invalidated (see Fig. 4). Forthicknesses above 0.5 mm, no glue failure was apparentduring visual inspection with the unaided eye or stereo-microscope (see Fig. 5), so that those results were desig-nated according to EN 582 standard as valid. However,

detailed SEM observation of the fracture surfaces revealedsome ability of the glue to locally penetrate into the coatingmicrostructure, so that a careful detailed investigation ofthe fracture surfaces is recommended (see further).

For the Al2O3 coating, cohesion failure was typical.The failure propagated within the coating parallel to thecoating-substrate interface. It may be concluded that thebonding strength of the coating to the substrate (adhesion)is higher than mutual bonding of individual coating layers(cohesion). In general, regions of two types of residualthicknesses of the coating could be observed on the frac-ture surface—thin, consisting of only several residuallayers, and thick, where the crack propagated closer to theoriginal coating free surface (see Fig. 5). For the coatingswith thickness smaller than 0.5 mm, a thin residual layeron the coating fracture surface (outside the glue fracture)was characteristic. For thicker coatings, the presence ofboth thin and thick residual layers was typical. Penetrationof the glue into the Al2O3 coating microstructure wasconfirmed by EDX mapping of carbon which was the mainconstituent of the used glue (see Fig. 6). Carbon-rich areaswere observed throughout the whole fracture surface ofAl2O3 coatings with thickness smaller than 0.5 mm. Forthe coatings with thickness of about 1 mm, glue penetra-tion was observed only in the distance up to approximately2 mm from the edge of the sample, forming a rim of areinforced thicker residual coating (see Fig. 5) and leavingthe center of the specimen free of contamination. The rimrepresents only a small fraction of the fracture surface,and so its influence on the resulting adhesion/cohesionstrength may be neglected. Formation of the reinforcedrim on the sample edge is the result of the preferentialorientation of the horizontal intrasplat pores (see Fig. 1)which are wider, more numerous, and therefore easier topenetrate in the in-plane direction, resulting in a differentmaximum glue penetration depth in the in-plane and out-of-plane directions (about 2 and 0.5 mm, respectively).Typical failure mechanisms were splat decohesion andsplat cracking.

Fig. 4 Tensile adhesion strength vs. coating thickness. Sampleswith significant amount of glue on the fracture surface (observedwith unaided eye) were invalidated (hollow symbols)

Fig. 5 Representative fracture surfaces of the samples after TAT. Al2O3 with initial thickness of t = 0.94 mm. Cohesion failure (Bright:thick residual coating, dark: thin residual coating). AISI 410 sample with initial coating thickness of t = 0.53 mm. Combined adhesion/cohesion failure

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For the AISI 410 coating, a change in the macroscopicappearance of the fracture surface with increasing coatingthickness was also observed. For the coatings with thick-ness smaller than 0.5 mm, a significant amount of the gluefailure was observed, whereas for the thicknesses greaterthan 0.5 mm, the glue failure was not apparent with theunaided eye or at low magnifications available in theoptical microscope (see Fig. 5), so that the results couldbe, according to the EN 582 standard, designated as valid.SEM observation revealed that the coatings failed in themixed cohesion/adhesion mode near the coating-substrateinterface. Islands of the cohesive failure could be easilyidentified within the adhesion failure area by EDX map-ping of chromium, which is present in the AISI 410 steelbut not in the used substrate. Glue penetration could notbe unambiguously confirmed nor denied by EDX mappingas a little carbon is present in both feedstock and thesubstrate. However, some areas of locally higher carboncontent were identified on the coating fracture surfaces,and a small degree of the glue penetration and coatingreinforcement cannot be therefore excluded, even thoughit was, according to the obtained results, not as significantas in the case of the Al2O3 coating. Typical micromecha-nisms of the AISI 410 failure were adhesion failure, splatdecohesion, and splat cracking. Plastic deformation ofthe individual splats near the interlocking sites was alsoobserved, which led to their permanent deforma-tion—typically stretching of the splats in the loadingdirection.

In general, it may be concluded from the above that theglue penetration into the coating microstructure repre-sents a significant limit of the TAT method as it cansignificantly compromise the results of the test and itsinterpretation especially for the coatings with some degree

of open porosity. Moreover, the glue penetration is noteasy to detect with the unaided eye or optical microscopy(especially if the glue is transparent).

It has to be pointed out that the presented TAT resultsobtained for coatings with thickness greater than 0.5 mmcould be, according to the EN 582 standard, claimedwithout hesitation as valid, as no significant glue failurewas observed on the fracture surfaces during visualinspection. However, such condition of the macroscopicabsence of the glue on the fracture surface may not besufficient and detailed inspection (preferably using SEM)is highly recommended to validate the test results.

From a practical point of view, it may be expected thatthere exists some critical coating thickness, beyond whichthe glue penetration and its influence on the TAT resultsmay be neglected as the failure takes place predominantlydeeper in the coating not reinforced by the glue. For thetested Al2O3 coatings, such critical thickness was experi-mentally determined to be approximately 0.5 mm, beyondwhich no significant glue penetration was observed on thefracture surface. For the AISI 410 coating, critical thick-ness could not be so easily identified as it was not possibleto confirm or exclude the presence of glue within thecoating and on the fracture surface. However, from thecomparison with the results obtained for the other types oftests (see further), it may be concluded that testing ofAISI 410 coatings with a thickness greater than 0.5 mmalso provided representative results. However, generalestimation of the value of such critical thickness may bequite problematic because the glue penetration abilitymay be different for different coatings and spraying pro-cesses as it is given by the character of the coating openporosity (namely, density and size of pores), wettingbetween the glue and the coating and glue fluidity, etc.

Fig. 6 Glue contamination in the glue-reinforced area (rim) near the edge of the fracture surface of Al2O3 from Fig. 4 as observed inSEM by EDX mapping of carbon (left) and backscattered electrons COMPO mode (SEM-BSE-COMPO) (right)

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The situation is even more complicated as there are manydifferent glue types available in the market. Any recom-mendation on the minimum coating thickness (e.g.,0.38 mm provided by ASTM C633 standard) shouldbe therefore taken only as informative and evaluatedindividually for the investigated coatings to provide arepresentative value of the coating tensile adhesionstrength. Another problem of the TAT method may bethat samples with sufficiently high thickness may not beavailable or representative for the coating application.However, even if the test results were influenced by theglue penetration, the method may be used for identifica-tion of the dominant failure micromechanisms, as it wasalways possible to find areas not influenced by the glue.

3.2 Pin Test

During loading of the pin test specimens, load-dis-placement curves were linear until the maximum load wasreached. If the specimens failed in Modes I or II, then asudden decrease of the load was observed. If the speci-mens failed in the mixed mode, then the decrease of theload was gradual, which corresponded to the ongoingfailure propagation.

The mode of coating failure was determined by theratio of the coating thickness to the pin diameter (seeFig. 7). Based on the fractographic analysis (see Fig. 8)and direct observation of the coating surface with themicroscope during loading, the following model of thecoating failure was established. The coating failure initi-ated near the edge of the pin. If the coating thickness wassmall, then sudden through-thickness ‘‘shear’’ failure(Mode II) occurred. For the thicker coatings which couldresist a through-thickness shearing, the coating failure alsostarted near the edge of the pin but formed a circumfer-ential crack which propagated toward the pin center. Forthe thickest coatings, failure propagated in the planeparallel to the coating-substrate interface (Mode I) until asudden detachment of the coating from the pin. For thecoatings with medium thickness, i.e., for the 12-mm pinswith coating thickness of about 350 lm and 25-mm pinswith 500-650-lm coating thickness, mixed failure modewas observed: at some crack length, and owing to theinevitable denting of the coating during loading, thecoating stiffness was insufficient to support the coatingagainst shear, and the transition to a sudden through-thickness coating failure was observed resulting in theformation of ‘‘step’’ on the fracture surface (see Fig. 8).

In the case of Al2O3 coatings, some substrate revelationwas observed on the edge of the pin, but the dominantfailure was due to the cohesion failure. In the case ofthe AISI 410 coating, islands of cohesion failure wereobserved in the adhesion failure area (see Fig. 8).

The maximum load achieved during the test correlatedwell with the coating thickness (see Fig. 9) and the pindiameter, which demonstrates that the load needed torupture the coating was a function only of the coatingthickness and that the friction between the ring and the pinduring the test was negligible. Using Eq 2 and 3, maximumnominal normal and shear stresses were calculated.

The results are shown in Fig. 10. The ratio of the shear tonormal stress as a function of the coating thickness isplotted in Fig. 11. From the figures, several findings may bepointed out. If the nominal shear to normal stress ratio washigher than ~12, then failure Mode II occurred, and thecorresponding critical shear stress was in the range of

Fig. 7 Representative specimens (12- and 25-mm diameter)after the test (rings and pins): Al2O3 (left) and AISI 410 (right).Numbers indicate average coating set thickness in lm and failuremode

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about 2-33 MPa for Al2O3 and 25-35 MPa for AISI 410coatings. Rather high scatter, especially for Al2O3, wascaused by a relatively high scatter in the measured maxi-mum loads combined with low sample thickness.

If the ratio was smaller than ~7, then failure Mode Iwas dominant with average critical normal stress of about1-2.6 MPa and 6.7-13.8 MPa for Al2O3 and AISI 410coatings, respectively. From the Fig. 10, dependence ofmaximum nominal normal stress on the coating thicknessmay be observed which is probably caused by a higher out-of-plane stiffness of the thicker coatings and resistance ofthe coating to denting during pulling of the pin. In otherwords, the thicker the coating is, the smaller is theimportance of an additional bending momentum whichleads to a higher maximum nominal normal stress. It maybe therefore recommended that coatings with comparablethickness should be benchmarked. Please note that forsamples failed in Mode I, maximum achieved nominalshear stress is comparable regardless of the coatingthickness, as a higher maximum load is compensated by ahigher coating thickness.

The results confirm significant anisotropy in the coatingstrength, as for an isotropic material, a transitional valueof the ratio rS/rN should be close to ‘‘1’’ instead of theobserved range of 7-12, as the values of the shear andnormal stresses should be comparable for such material.

Fig. 8 Fracture surface of the coatings which failed in the mixedmode. Edge of the pin. SEM-BSE-COMPO

Fig. 9 Dependence of maximum load on the coating thicknessand specimen diameter in pin test

Fig. 10 Relation between maximum nominal shear and normalstresses. Numbers indicate average coating thickness set in lm. Ifseveral samples were available, then error bars are displayed. Forthe failure Mode I, normal stress was critical; for the Mode II,shear stress was critical

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Moreover, the coating strength was higher for AISI 410for both through-thickness and in-plane failure propaga-tion modes, which indicates more effective bonding of thesplats than for Al2O3.

Results obtained for both coatings also indicate some‘‘size effect’’ as the critical nominal normal stress for thespecimens with 12-mm diameter pin was almost twice thatof the critical stress of specimens with a pin with 25-mmdiameter and comparable coating thickness. It may betherefore concluded that only results obtained for thesame pin diameter should be compared, and a pin diam-eter of 12 mm is suitable for benchmarking. This is ananalogy to TAT, where a specimen diameter of 25 mm (or1 in.) was widely adapted as standard.

3.3 TCT Test

Results of the TCT test are illustrated in Fig. 12. Duringtesting, load-displacement curves were linear up to a sud-den coating failure in the whole critical cross section. Forthe Al2O3 coating, the coating strength was in the range of8-18 MPa and for the AISI 410 coating in the range of98-133 MPa, with very small sensitivity to the coatingthickness for both coatings. No signs of the substratesbonding due to the coating deposition (i.e., neithermechanical joining due to the grit-blasting nor metallur-gical welding due to the surface exposure to high temper-atures), which could increase the maximum load obtainedfrom the test, were observed during SEM inspection of thetested samples.

The typical coating failure was due to the splat crackingand splat decohesion. In the case of the AISI 410 coating,signs of plastic deformation of the splats were alsoobserved (see Fig. 13).

3.4 Shear Test

Results of the shear test are plotted in Fig. 14. For bothtypes of coatings, a sudden drop of the loading force frommaximum to practically zero was observed, which corre-sponded to the total delamination of the coating at themaximum load. The Al2O3 coating showed much smallershear resistance (in the range of critical shear stress

Fig. 11 Ratio between nominal shear and normal stresses as afunction of the coating thickness for all tested samples. Note thatdata points lie on the hyperbolic curves (given by Eq 4)

Fig. 12 TCT strength as a function of the coating thickness

Fig. 13 Fracture surface of the coatings after TCT test. SEM-BSE-COMPO

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3.6-7.4 MPa) when compared with the AISI 410 coatingwith the critical shear stress in the range of 38.9-54.0 MPa.

Fractographic analysis of failed specimens (see Fig. 15)revealed that the Al2O3 coating failed in Mode 3b, asdefined by EN 15340 standard, as cohesive failure withinthe coating, parallel with the coating-substrate interface,was observed. The AISI 410 coating failed in Mode 1,where the delamination propagated along the substrate-coating interface, and even if the fracture started at somedistance from the interface, the crack path immediatelyinclined to the interface. Dominant adhesive failure withsome presence of cohesive failure islands was observed.

3.5 Comparison of the Tests

All four tests presented in this article proved to beapplicable for the testing of WSP coatings and demon-strated their ability to evaluate the apparent coatingstrength in different loading modes, even though each testbore some considerable limitations (see Table 2). For theTAT test, glue penetration is the most problematic issue,which could be partially suppressed by increasing thecoating thickness and completely avoided in the otherperformed tests. The Pin test enabled the evaluation of thecoating strength anisotropy but turned out to be quitelaborious (especially when concerning sample preparationbefore the coating deposition) and is therefore not con-venient for large-scale testing of many samples. The TCTtest is relatively simple, but a proper alignment of thesamples (so that the substrate cylinders are placed duringassembly coaxially) turned out to be quite laborious.Moreover, only in-plane cohesion strength may be exam-ined in this test, but this also may be beneficial, if only thisfeature is the subject of investigation. Besides the TATtest, the shear test has been already standardized andprovided straightforward results. The main limitations ofthis test are demands on the sample geometry and posi-tioning of the shear blade with respect to the roughcoating-substrate interface, which complicates testing ofcoatings with small thicknesses.

Coating strengths, as evaluated from the tests, are listedin Table 2 and plotted in Fig. 16. For easier comparison,

values were interpolated for the coating thicknesses of0.5 and 1 mm from the measured data. Note that for thepin test, only failure Mode I was taken into account as itwas representative for the considered coating thicknesses.

It is obvious that the apparent coating strength isstrongly dependent on the testing method but much lessdependent on the coating thickness, which demonstrates asatisfactory validity of the tests as no significant gradientsof the coating properties with increasing distance from thecoating-substrate interface are expected. Moreover, withdecreasing coating thickness, relative uncertainty of thecoating thickness measurement increases, which maycompromise the precision of the results for thin coatings(thickness of tens to several hundreds of microns). Slightvariations of the test results with coating thickness mayalso occur because of the effects connected with coatingbuild-up (e.g., residual stresses, local thermal history of thematerial, etc.) which were not in the scope of this study.Some decrease in the coating adhesion/cohesion strengthalso could be explained by so the called ‘‘free-edge effect’’described by Greving et al. in Ref 24. Only in the case ofthe TAT test was a strong dependence on the coatingthickness observed for Al2O3, which was explained by thecoating reinforcement due to the glue penetration.

In all tests, the AISI 410 coating consistently showedsignificantly higher adhesion/cohesion strength than the

Fig. 14 Coating shear strength vs. coating thickness

Fig. 15 Fracture surface of the coatings after shear test. High-lighted areas denote islands of cohesive failure in AISI 410coating. SEM-BSE-COMPO

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Al2O3 coating. The differences of the strength valuesobtained from different tests can be attributed to thesensitivity of the coatings to different loading modesbecause of their anisotropic microstructure and characterof the stress concentration in the critical areas in differenttests, which leads to differences in the coating failureresistance in the in-plane and out-of-plane direction. Thetype of test should therefore be carefully selected withrespect to the intended coating application or, if required,their combination should be considered.

Coating strengths obtained from the TCT test were sig-nificantly higher than those obtained with other methods,especially for AISI 410 coating. In the TCT test, the appliedload in the critical cross section of the sample is transmittedonly by the coating, and the coating adhesion across thecoating-substrate interface plays a negligible role (if thecoating adhesion is sufficient, which was this case). There-fore, only in-plane cohesion strength of the coating can beevaluated in this test. A high value of the cohesion strengthof the AISI 410 coating, as evaluated from the TCT test, istherefore not surprising because all other tests showed thatthis coating has cohesion strength even higher than adhe-sion. Therefore, if just the cohesion is evaluated in the TCTtest, then the absolute strength values must be relativelyhigh. Authors in Ref 12 recommend further multiplicationof the TCT strength with a factor of 1.5-1.7 because of thestress concentration to estimate ultimate coating strength; itwas omitted for this study, as it would not change the mutualrelation of the TCT results for both coatings.

Obtained results confirm that there exists an urgentneed of the thermal spray community to carry out furtherdevelopment of the testing methods suitable for coatingadhesion/cohesion strength evaluation (especially for themodes of loading relevant for in-service conditions) andtheir standardization.

3.6 Discussion of Coating Failure Mechanisms

The main mechanisms of coatings failure were the lossof adhesion to the substrate, the mutual splat decohesion,T

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and the splat cracking observed also in Ref 25 and 26.High adhesion and especially cohesion strength of theAISI 410 coating was caused by an effective mutualbonding of the splats due to the wave-like character of thecoating microstructure, which provided a high density ofsites available for mechanical anchoring (interlocking).Moreover, even though numerous oxides were observed atthe splat boundaries, formation of some local metallicbonding may be anticipated, which should significantlyreinforce the AISI 410 coating as metallic bonds usuallyprovide a high strength of the interface (Ref 23). Thebonding of the AISI 410 coating to the underlaying sub-strate was less effective; therefore, loss of coating adhesionto the substrate was a typical failure mechanism for thiscoating. However, the presence of numerous islands ofcohesion failure on the fracture surface indicates that thelocal cohesion strength of the AISI 410 coating was notsubstantially higher than its adhesion to the substrate.Finally, stress localization in the metallic coating duringloading could be to some extent compensated by plasticdeformation of the metallic splats, which could furtherincrease the apparent coating strength. Plastic deforma-tion occurred locally in the areas of increased bonding ofthe splats and extensively took place during the abruptrelease of the accumulated strain energy at the terminalstage of the loading, and so it is not surprising that it didnot deteriorate linearity of the load-displacement curvesobtained from the tests.

On the other hand, such microstructural yielding wasnot possible for the Al2O3 coating, which, together withthe presence of numerous intrasplat cracks and intersplatvoids, resulted in significantly lower mutual bonding of thesplats and therefore overall lower coating performance interms of adhesion/cohesion. However, fractographicanalysis consistently showed that the adhesion of theAl2O3 coating to the substrate is relatively high (higherthan the coating cohesion), which is caused by the effec-tive mechanical anchorage of the first layer of the splats onthe rough grit-blasted substrate surface.

4. Summary and Conclusions

Combination of four different methods (TAT test, Pintest, TCT test, and Shear test) for evaluation of coatingadhesion/cohesion strength was demonstrated. As wasshown, none of the testing methods is universal as each ofthe used methods has certain limitations and can evenprovide different strength values depending on the loadingtype and direction, loaded volume, etc. Therefore, onlyvalues obtained from one type of test, preferably havingcomparable sample geometry and coating thickness, andrelevant for the coating application should be comparedor used for benchmarking. Combination of the tests isbeneficial as it provides a deeper insight into the coatingsperformance and the coating failure process especiallywhen supplemented with detailed fractographic analysis.The tensile test of adhesion, pin test, and shear test con-sistently showed that the adhesion of the Al2O3 coating to

the substrate is higher than its cohesion. On the contrary,for the AISI 410 coating, adhesion to the substrate waslower than its cohesion. The pin test also revealed signif-icant anisotropy of the coatings strength, showing that theresistance of both coatings to the through-thickness shearfailure was significantly higher than resistance to the fail-ure in tensile loading when loaded in the out-of-planedirection. In-plane coating’s cohesion strength was suc-cessfully evaluated by the TCT test. Establishment of thecombined adhesion/cohesion testing will allow furtheroptimization of the coating deposition parameters for thein-service relevant types of loading.

Acknowledgments

This study was supported by the Grant No. P108/12/P552 (Czech Science Foundation).

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