Coated components used in industry are often exposed to repetitive dynamic impact load. The dynamic impact test is a suitable method for the study of thin protective coatings under such conditions. Aim of this paper is to describe the method of dynamic impact testing and the novel concepts of evaluation of the impact test results, such as the impact resistance and the impact deformation rate. All of the presented results were obtained by testing two W-B-C coatings with different C/W ratio. Different impact test results are discussed with respect to the coatings microstructure, the chemical and phase composition, and the mechanical properties. It is shown that coating adhesion to the HSS substrate played a crucial role in the coatings’ impact lifetime

Buršíková, Vilma

Daniel, Josef

Grossman, Jan

Mirzaei, Saeed

Souček, Pavel

Vašina, Petr

Zábranský, Lukáš

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/APP.2020.27.0037Acta Polytechnica 27(0):37–41, 2020 © Czech Technical University in Prague, 2020available online at http://ojs.cvut.cz/ojs/index.php/appDYNAMIC IMPACT WEAR AND IMPACT RESISTANCE OFW-B-C COATINGSJosef Daniela,∗, Jan Grossman a, Vilma Buršíková b,Lukáš Zábranský b, Pavel Součekb, Saeed Mirzaeib, Petr Vašinaba Institute of Scientific Instruments of the Czech Academy of Sciences, Královopolská 147, Brno 612 64, CzechRepublicb Masaryk University, Faculty of Science, Department of Physical Electronics, Kotlářská 2, Brno 611 37, CzechRepublic∗ corresponding author: jdaniel@isibrno.czAbstract. Coated components used in industry are often exposed to repetitive dynamic impact load.The dynamic impact test is a suitable method for the study of thin protective coatings under suchconditions. Aim of this paper is to describe the method of dynamic impact testing and the novel conceptsof evaluation of the impact test results, such as the impact resistance and the impact deformation rate.All of the presented results were obtained by testing two W-B-C coatings with different C/W ratio.Different impact test results are discussed with respect to the coatings microstructure, the chemicaland phase composition, and the mechanical properties. It is shown that coating adhesion to the HSSsubstrate played a crucial role in the coatings’ impact lifetime.Keywords: Boron carbide, fracture resistance, impact resistance, impact wear, W-B-C.1. IntroductionIndustrially used components coated with protectivecoatings are often exposed to the external load. Insome cases, this load may take the form of repeateddynamic impact. For the investigation of the life-time of the coated components under this form ofload, Knotek et al. developed the method of dy-namic impact test in the early 90s [1]. A tungstencarbide ball repeatedly impacted the surface of thespecimen. The load force of the ball was well-defined,and the impact frequency was set to a constant value.Coatings’ lifetime was evaluated as a maximum num-ber of impacts before coating failure. Knotek alsoshowed that dynamic impact test makes a study ofdynamic wear and fatigue behaviour of the wholecoating/substrate system possible [1]. In the pastdecades, the dynamic impact testing was investigatedby different researchers. Bouzakis et. al. developedFEM simulation of the elastoplastic deformations ofmaterial under dynamic impact load [2] or the inclineddynamic impact test - testing of the coated specimensunder various angles of inclination [3]. Engel et. al.proposed a three-stage model of the dependence of theimpact crater volume on the total number of impactsbased on the coating deformation [4]. Voevodin et. al.investigated the impact lifetime of various Ti-basedcoatings and discussed it with respect to their chemi-cal composition and hardness [5]. Heinke et. al. usedthe dynamic impact test as a supplementary methodto the evaluation of coatings adhesion [6]. Sobotaet. al. investigated the behaviour of DLC coatingsunder dynamic load in various air humidity [7]. In thepast years, the dynamic impact testing also expandedto the field of the thick HVOF sprayed coatings [8].The aim of this paper is to present some crucial re-sults of dynamic impact testing, important for theinvestigation of the behavior of the coating under dy-namic impact load and for development of the newtypes of the protective coatings. Novel parameters ofimpact resistance and impact deformation rate, suit-able for better understanding of the behaviour of thecoating under repeated impact load, were discussed.Presented results were obtained from two W-B-C coat-ings with different chemical composition, structure,and mechanical properties.2. Experimental2.1. Dynamic impact testerThe coatings were tested by a dynamic impact testerdeveloped at the Institute of Scientific Instruments ofthe Czech Academy of Sciences in Brno. The impacthammer with a lift set at a constant value of 2.4mm was electromagnetically driven. Attached to theimpact hammer was a ball-shaped cemented tungstencarbide impact indenter with a diameter of 5 mm. Theindenter ball was rotated to an unworn contact sidebefore every test. The frequency of impact testingwas set to 8 Hz, and the duration of the contact ofthe ball indenter with the surface was 1.5 ms. Theimpact testing was carried out with impact loads of200 N, 400 N and 600 N. Due to its constructionthe impact tester is uniquely suitable also for theinvestigation of the behaviour of the samples undera low number of impacts (≤ 100). Therefore, thenumber of impacts was in the range from 1 to 500000. Every test was repeated three times. The depth,37J. Daniel, J. Grossman, V. Buršíková et al. Acta PolytechnicaC/W W (at. %) C (at. %) B (at. %) H (GPa) E (GPa) H/E H3/E2 (GPa) thick.(µm)0.11 63 7 30 28.6 ± 1.5 500 ± 12 0.057 0.094 1.70.8 40 32 28 23.3 ± 1.0 400 ± 12 0.058 0.079 1.3Table 1. Chemical composition and mechanical properties of the W-B-C coatings.Figure 1. Loading curves of the W-B-C coatings. Dotted lines were added as a visual guide.the radius and the surface morphology of the impactcraters were measured by Talystep profilometer andLext OLS 3100 confocal microscope.2.2. W-B-C coatings preparationTwo W-B-C coatings with different C/W ratioswere deposited on HSS substrate by magnetron co-sputtering of W, C and B4C targets in Ar atmosphere.For the detailed description of the coatings’ deposition,see Ref. [9].3. Results3.1. W-B-C coatings composition,structure and mechanicalpropertiesThe chemical composition and the mechanical proper-ties of the presented W-B-C coatings are summarizedin Table 1. Both coatings exhibited an amorphousmicrostructure. The coating with a higher amountof carbon (C/W = 0.80) exhibited a higher amountof C-C bonds and B-C bonds and a lower amount ofW-B bonds. The detailed description of the chemicalcomposition and the microstructure analyses, the eval-uation of the mechanical properties and the discussionof the relationship between the microstructure andthe mechanical properties can be found in Ref. [9].3.2. Results of impact testingOne of the first results of the dynamic impact test isa loading curve - the dependence of the impact cratervolume on the total number of impacts. The loadingcurves of the studied W-B-C coatings are depictedin Figure 1. The amount of energy supplied fromthe tester to the tested specimen increased with theincreasing number of impacts. This energy caused thedeformation of the specimen and thus the formation ofthe impact crater. As the number of impacts increasedfurther, the supplied energy was dissipated in the formof a continuously increasing internal stress. Afterreaching some critical number of impacts nC , thetested coating/substrate system was unable to receivemore energy from the tester, and the coating failed.The depth and the radius and thus the volume of theimpact crater then rapidly increased. These differentbehaviour modes of the substrate/coating system canbe seen on the loading curve as different slopes andthus it is often possible to determine the nC andtherefore the lifetime of the coating under dynamicimpact load.The impact craters in the W-B-C coatings formedafter 10 000 impacts are depicted in Figure 2. Thecoating with the C/W ratio of 0.11 exhibited impactcraters with a higher diameter than the coating withthe C/W ratio of 0.80. Moreover, in case of the load of400 N, this coating exhibited an almost uncovered sub-strate indicating the first stage of the coating failure.The total coating failure and the uncovered substratein case of the load of 600 N for the coating with theC/W ratio of 0.11 corresponded to a rapid increase ofthe slope of the loading curve of this coating depictedin Figure 1. While the coating with the low C/W ratioexhibited pronounced chipping cracks in the case ofthe loads of 400 N and 600 N, the coating with thehigher C/W ratio only exhibited small concenteredcracks even in case of the highest load of 600 N. Thedetailed view on the edges of impact craters createdwith load of 400 N is depicted in Figure 3.Another parameter describing the behaviour of acoated specimen under the dynamic load is the impactresistance. The impact resistance is defined as follows.The volume of the impact crater created by N1 impactsis denoted as V1 and the volume of the impact cratercreated by N2, impacts is denoted as V2. An incrementof the impact crater volume ∆V = V2 − V1. This38vol. 27/2020 Dynamic impact wear and impact resistance of W-B-C coatingsFigure 2. Comparison of impact craters created by 10 000 impacts.Figure 3. Detail view on the edges of impact craters created with load of 400 N and 10 000 impacts.increment depends on the number of impacts N1 andN2 [4]. Assuming that the increment of ∆V is inverselyrelated to the impact resistance of a coating, theimpact resistance can be calculated from the volumeof impact craters at a given number of impacts. Thisparameter with the unit of[n/mm3]describes howmany impacts (denoted as n ) are required to createa crater with the volume of 1mm3.The impact resistance of the W-B-C coatings isshown in Figure 4. At first, the impact resistanceincreased with the increasing crater volume due to theelastoplastic deformation of coatings. As the numberof impacts increased further, the energy from thetester was dissipated in the form of internal stress andnot in the form of the crater deformation. The impactresistance thus reached the highest values. Then, asthe coating failed, the volume of the impact craterand also the increment of the volume ∆V rapidlyincreased, and the impact resistance correspondinglyrapidly decreased. Therefore, the critical number ofimpacts nc can also be evaluated using the impactresistance.The third way of evaluation of the critical numberof impacts is from the determination of the deforma-tion rate. Assuming that contact time between theimpactor ball and the specimen is the same for everyimpact, it is possible to calculate the deformationrate as the impact crater volume divided by the totalcontact time. The deformation rate of the W-B-Ccoatings is depicted in a log-log scale in Figure 5.The deformation rate linearly decreased as the num-ber of impacts increased. After reaching the criticalnumber of impacts, the impact crater volume rapidlyincreased and thus deformation rate curve deviatedfrom linearity. The final critical number of impactswas evaluated from the combination of the previouslydescribed methods - from the loading curves, theoptical investigation of impact craters (uncovered sub-strate as evidence of the coating failure), the impactresistance and the deformation rates. Figure 6 showsthe Woehler-like dependence of the critical numberof impacts on the load. The coating with the C/Wratio of 0.80 exhibited higher lifetime under dynamicimpact at all of the applied loads.4. DiscussionIn general, the behaviour of coatings under impactload is closely related to the coating’s toughness [2,5]. Both W-B-C coatings exhibited an amorphousstructure and the same H/E ratio that is related tothe fracture resistance [9]. The coating with the C/Wratio of 0.11 exhibited higher hardness and elastic39J. Daniel, J. Grossman, V. Buršíková et al. Acta PolytechnicaFigure 4. Impact resistance of the W-B-C coatings. Dotted lines were added as a visual guide.Figure 5. Deformation rate of the W-B-C coatings.Figure 6. Woehler-like curves of the W-B-C coatings.Dotted lines were added as a visual guide.modulus and therefore, a higher H3/E2 ratio thatis related to resistance to plastic deformation andthe coating toughness [10]. However, better impactresistance was observed in the case of the coatingwith the C/W ratio of 0.80. The main reason wasdifferent adhesion of the coatings to the HSS substrate.As was shown in Figure 3, the impact crater on thecoating with the C/W ratio of 0.11 created after 10000 impacts with the load of 400 N exhibited crackingon its edge, while the impact crater on the coatingwith the C/W ratio of 0.80 created after the samenumber of impacts at the same load exhibited anuninterrupted coated edge. Thus, the adhesion ofthe coating with the C/W ratio of 0.80 was higherthan the adhesion of the coating with the C/W ratioof 0.11. As the H/E ratio was the same for bothcoatings, the adhesion of the studied coatings was themain parameters leading to better impact resistance.5. ConclusionsTwo W-B-C coatings with 30 at. % of boron anddifferent C/W ratios were deposited on HSS substratesand analyzed using the dynamic impact test. Theimpact loads of 200 N, 400 N and 600 N were used.The critical number of impacts determining coatingfailure was estimated using the loading curves, opticalinvestigations and two novel parameters – impactresistance and deformation rate. The higher adhesionand of the coating to the HSS substrate led to thehigher lifetime of the coating under repeated impactload.AcknowledgementsThis work was performed under the support of the projectsLO1212 and LO1411 funded by Ministry of Education,Youth, and Sports of the Czech Republic, and projectGA19-03899S funded by the Czech Science Foundation.References[1] O. Knotek. A new technique for testing the impactload of thin films: the coating impact test. Surface andCoatings Technology 54-55:102–107, 1992.doi:10.1016/0257-8972(92)90147-3.[2] K.-D. Bouzakis, N. Vidakis, T. Leyendecker, et al.Determination of the fatigue properties of multilayerPVD coatings on various substrates, based on the impacttest and its FEM simulation. Thin Solid Films 308-309:315–322, 1997. doi:10.1016/s0040-6090(97)00561-0.[3] K.-D. Bouzakis, A. Asimakopoulos, N. Michailidis,et al. The inclined impact test, an efficient method tocharacterize coatings’ cohesion and adhesion properties.Thin Solid Films 469-470:254 – 262, 2004. Proceedingsof the 31st International Conference on MetallurgicalCoatings and Thin Films, doi:10.1016/j.tsf.2004.08.093.[4] P. Engel. Impact wear of multiplated electricalcontacts. Wear 181-183:730–742, 1995.doi:10.1016/0043-1648(94)07105-5.[5] A. Voevodin, R. Bantle, A. Matthews. Dynamicimpact wear of TiCxNy and ti-DLC composite coatings.Wear 185(1-2):151–157, 1995.doi:10.1016/0043-1648(95)06603-9.40vol. 27/2020 Dynamic impact wear and impact resistance of W-B-C coatings[6] W. Heinke, A. Leyland, A. Matthews, et al. Evaluationof PVD nitride coatings, using impact, scratch androckwell-c adhesion tests. Thin Solid Films 270(1-2):431–438, 1995. doi:10.1016/0040-6090(95)06934-8.[7] J. Sobota, J. Grossman, V. Bursikova, et al.Evaluation of hardness, tribological behaviour andimpact load of carbon-based hard composite coatingsexposed to the influence of humidity. Diamond andRelated Materials 20(4):596–599, 2011.doi:10.1016/j.diamond.2011.01.011.[8] K. Bobzin, L. Zhao, M. Öte, et al. Impact wear of anHVOF-sprayed cr 3 c 2 -NiCr coating. InternationalJournal of Refractory Metals and Hard Materials70:191–196, 2018. doi:10.1016/j.ijrmhm.2017.10.011.[9] M. Alishahi, S. Mirzaei, P. Souček, et al. Evolution ofstructure and mechanical properties of hard yet fractureresistant w-b-c coatings with varying c/w ratio. Surfaceand Coatings Technology 340:103–111, 2018.doi:10.1016/j.surfcoat.2018.02.054.[10] J. Musil. Hard and superhard nanocompositecoatings. Surface and Coatings Technology 125(1-3):322–330, 2000. doi:10.1016/s0257-8972(99)00586-1.41

DYNAMIC IMPACT WEAR AND IMPACT RESISTANCE OF W-B-C COATINGS

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DYNAMIC IMPACT WEAR AND IMPACT RESISTANCE OF W-B-C COATINGS

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