MICROSTRUCTURAL CHANGES OF THERMALLY SPRAYED COATINGS AFTER HIGH TEMPERATURE CYCLING

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1 , Ržnv pd Radhštěm, Česká Republika MICROSTRUCTURAL CHANGES OF THERMALLY SPRAYED COATINGS AFTER HIGH TEMPERATURE CYCLING Rstislav MEDLÍN a, Olga BLÁHOVÁ a, František ZAHÁLKA b, Jan ŘÍHA a a New Technlgies - Research Centre - University f West Bhemia, Plzeň, Univerzitní 22, Plzeň, Czech Republic, medlin@ntc.zcu.cz b ŠKODA VÝZKUM s.r.., Tylva 1/57, 316 Plzeň, Czech Republic, frantisek.zahalka@skdavyzkum.cz Abstract The technlgy f thermally sprayed catings by the HVOF prcess is an auspicius technlgy f creating mre than 5 μm thick catings. Using this fleible, versatile and ecnmical technlgy enables t adapt surface prperties f parts t their wrking cnditins. It leads t etensin f lifetime and increasing f reliability and safety f cated parts. In dependence f cating materials, they can be used fr many applicatins. The grup f wear resistant catings with favurable frictin prperties can be used in pwer industry fr increasing the lifetime and reliability f rtatin parts f turbines. In that case the resistance f catings t changes in temperature is ne f the necessary cnditins. The paper is fcused n micrstructural changes in the fllwing thermally sprayed catings: Cr 3 C 2-35NiCr, NiCrMWFe, CMCrSi, Cr 3 C 2 -CNiCrAlY and WC-CrC-Ni, which are initiated by electrically induced thermal cycles. The catings were eamined by X-ray diffractin, SEM imaging and micrhardness testing. Keywrds: Thermal Spraying, Catings, HVOF, SEM, X-Ray diffractin, EDS. 1. INTRODUCTION HVOF thermally sprayed catings are used fr their lw prsity, high bnd strength and lw residual stress. Thanks t the very high kinetic energy f particles striking the substrate surface, they d nt require the particles t be fully melted t frm high quality catings. This is certainly an advantage fr the carbide cermet-type catings and in this area the HVOF prcess really ecels. HVOF catings are typically used as wear and crrsin resistant layers. With the usage f these catings in a wide range f applicatins their high-temperature behaviur becmes significant as well as the high temperature behaviur f ther materials, like WC-C described in [1] and as a cmple WC-(W,Cr)-Ni materials characteristics described in [2]. The evaluated materials belng t tw grups f catings: Cr 3 C 2-35NiC, Cr 3 C 2 -CNiCrAlY, WC-CrC-Ni are cermet catings, characterized by a beneficial cmbinatin f carbides with high hardness and a matri, respnsible fr tughness f the material. High wear resistance f the catings is the result f bth cntributins. Their superir triblgical prperties are well knwn [3]. The NiCrMWFe and CMCrSi are allys, widely used as materials fr triblgical applicatins. The main aim f the eperiment is t prve the stability f micrstructure and prperties f the cating in the cnditins f thermal cycling ccurring in turbine parts. 2. EXPERIMENTAL MATERIALS Cmmercial feedstck pwders were used fr depsitin f the catings. The catings were sprayed nt a grit-blasted steel substrate using the parameters, which were previusly ptimized and are currently used in ŠKODA VÝZKUM s.r.. in the JP 5 (TAFA, USA) spraying equipment.

2 , Ržnv pd Radhštěm, Česká Republika Cating Feedstck Pwder Granularity CMCrSi M μm NiCrMWFe M μm WC-2%CrC-7%Ni μm Cr 3 C 2-25%CNiCrAlY μm Cr 3 C 2-35%NiCr μm Table 1. Materials f HVOF thermally sprayed catings The CMCrSi material cntains 28% M, 8.5% Cr and 2.5% Si and the balance f C. It is a variatin f the material dented as Tribaly 4. It is wear-resistant, crrsin resistant up t 81 C and has lw frictin cefficient. The NiCrMWFe material cntains 15.5% Cr, 15.5% M, 4% W, 3% Fe and Ni balance. It ffers idatin and high-temperature crrsin resistance. The cermet WC-2%CrC-7%Ni is - in cnfrntatin with ther WC-based materials - idatin and crrsin resistant up t 75 C. The cermet Cr 3 C 2-25%CNiCrAlY ehibits higher idatin resistance - up t 1, C - than Cr 3 C 2 -NiCr. It is used mainly fr turbine and furnace cmpnents. The cermet Cr 3 C 2-35%NiCr shws lwer hardness but higher tughness than Cr 3 C 2-25%NiCr. The cating is suitable fr high temperature applicatin up t 87 C fr its idatin and crrsin resistance. As a highly wear-resistant material it is als used fr prtectin against abrasin and ersin wear f cmpnents. It als ehibits gd frictin behaviur thanks t frmatin f a Cr 2 O 3 ide film n the surface. The film has lw frictin cefficient and gd adhesin t the parent material. It is used fr valve stems, turbine cmpnents, hydraulic pistns and ther prducts. 2.1 High Temperature Cycling Using the SMITWELD TTU 22 machine in labratries f ŠKODA VÝZKUM s.r.., all samples were heated by electrical current. Electric heating affects nly a shrt line between the pints f cntact. Heating f material was very quick; arund 15 C/s. The heat stresses acting n peripheral parts f samples were small. The temperature changed linearly withut pauses during testing within the limits f 2 and 66 C at the rate f 15 C/s. One temperature cycle tk 1 minute and fr each material 5 temperature cycles were applied. Fig. 1 SMITWELD TTU 22 therm-mechanical cycling simulatr with samples. 3. MEASURED DATA AND DISCUSSION Samples were eamined by the electrn micrscpe FEI SEM Quanta 2 and by the X-Ray Diffractmeter Bruker AXS D8 with the area detectr HiStar X-Ray Diffractin In all samples high amrphus backgrund was registered. Mst amrphus were the samples f CrC- 35NiCr and CrC-CNiCrAlY catings. The phases detected in the sample CrC-35NiCr were Ni and Cr 3 C 2,

3 , Ržnv pd Radhštěm, Česká Republika withut changes in high temperature samples. In the sample WC-CrC-Ni there are WC, W 2 C, C, NiCO 3 and a minrity CrO 2 phase. In high temperature sample the CrO 2 phase disappeared and Cr 2 O 3 and WO 3 phases ccurred. We discuss that material in mre detail later. The sample CMCrSi cntains C 3 M 2 Si, Cr and M, with M 5 Si 3 phase int high temperature sample (see Fig. 4). CrC-CNiCrAlY sample cntains nt clearly recgnized Cr and Cr 2 O 3 phase int mstly amrphus backgrund withut change nt high temperature and sample NiCrMWFe embdy FeNi and M phase with high temperature change int CrFeNi (ferrite) and minrity M 4 O 11 and NiO. Fig. 2 shws an X-Ray diffractin pattern f WC-CrC-Ni material. It cntains curves fr the basic catings material at the bttm, tgether with high temperature diffractgrams frm the peripheral area (middle) and the central heating line (abve). In Fig. 3 there are parts f this diffractin pattern shwn in detail and aligned t clearly display the increasing intensity f diffractins frm idic phases. CMCrSi cating diffractin patterns are in Fig. 4 with M 5 Si 3 phase shwn at high temperature. The appearance f a mstly amrphus sample is shwn in Fig. 5 (CrC-35NiCr) WC W 2 C Cr 2 O 3 WO 3 Δ C (+3 cps) Δ Δ Δ 2 (+15 cps) P M Fig.2 Diffractin patterns f WC-CrC-Ni samples 9-6 (riginal cating), M (peripheral part f a heated sample) and P (central part f heated sample) WO WC P 3 Cr M 2 O W 2 C Cr P 2 2 O M WO Fig.3 Details f diffractin patterns mutually aligned. Increasing WO 3 lines () n the left and right and Cr 2 O 3 lines (X) in the middle and n the right.

4 , Ržnv pd Radhštěm, Česká Republika C 3 M 2 Si Cr M M 5 Si 3 (+2 cps) (+1 cps) P M Fig. 4 Diffractin patterns f CMCrSi samples, 9-57 (riginal cating), M (peripheral part f a heated sample) and P (central part f heated sample) with M 5 Si 3 phase Ni Cr 3 C 2 (+2 cps) 2 (+1 cps) P M Fig. 5 Diffractin patterns with a mstly amrphus-type respnse frm the CrC-35NiCr samples Micrstructure Micrgraphs were taken by the electrn micrscpe FEI Quanta 2 n a transversal sectin. On Fig. 6 and Fig. 7 there are micrstructures f CrC-35NiCr and WC-CrC-Ni catings in the middle f the area affected by temperature cycling (left) and in the regin withut temperature changes (right). There is n clear difference in micrstructure between the basic and high temperature cycling catings. Neither are there any cracks r evidence f catings' damage. Changes detected by X-Ray diffractin are t marginal as well.

5 , Ržnv pd Radhštěm, Česká Republika Fig. 6 Micrstructures f CrC-CNiCrAlY cating upn high temperature cycling (left) and withut its effects (right). Fig. 7 Micrstructures f WC-CrC-Ni cating upn high temperature cycling (left) and withut its effects (right) Vlume Changes f Oygen in the Micrstructure All catings were analyzed with EPMA (Electrn Prbe Micr Analysis) frm EDAX in the electrn micrscpe FEI Quanta 2. Oygen present in catings mstly shares energy f its characteristic emissin with L series f the characteristic emissin frm heavier elements which makes it harder t detect. Tab. 2 shws results f element measurements frm an area f 2525 μm. There is tendency t indicate higher ygen cntent in catings epsed t high temperature cycling, ecluding CMCrSi and CrC-35NiCr samples.

6 , Ržnv pd Radhštěm, Česká Republika Cating CMCrSi NiCrMWFe WC-2%CrC-7%Ni Cr 3 C 2-25%CNiCrAlY Cr 3 C 2-35%NiCr Heated by HT cycling Oygen cntent [wt %] HT.67 N HT.71 HT 2.94 N HT 2.74 HT 2.53 N HT 1.75 HT 3.66 N HT 2.93 HT N HT Tab. 2. Oygen cntent in catings measured by EDS 3.4 Hardness HV.3 All samples were measured at the minimum f 1 pints per sample fr HV.3 hardness. Tab. 3 cntains the measured results. There is an eplicit tendency t ehibit higher hardness after high temperature cycling, NiCrMWFe and CrC-35NiCr catings, which are withut margin change. 4. CONCLUSION Cating HV.3 withut HT cycling HV.3 with HT cycling CMCrSi 458 ± ±44 NiCrMWFe 38 ± ±29 WC-2%CrC-7%Ni 596 ±84 76 ±98 Cr 3 C 2-25%CNiCrAlY 549 ± ±51 Cr 3 C 2-35%NiCr 492 ± ±6 Tab. 3. HV.3 hardness f catings Catings after 5 thermal cycles up t 66 C still shw nly very small changes. In the present samples wasn t bserve any cracks r ther types f damage. Materials ehibited signs f very weak idatin prcess with frmatin f hard and mre fragile ide phases with measurable impact n hardness. Hwever, idatin prcesses in these high temperature cating materials frm a part f their temperature resistance mechanism. Yet, in sme samples were nt strng enugh t be detected. Only marginal changes prve the temperature resistance f eamined materials. ACKNOWLEDGMENTS This wrk was supprted by prject MPO ČR FT - TA 5 / 72. REFERENCES [1] - Š.Hudkvá, F. Zahálka, M. Kašparvá, Triblgical behavir f thermally sprayed catings at elevated temperatures, In: Prc. f Internatinal Thermal Spray Cnference 28, 2-4 June, Maastricht, The Netherlands, ISBN: , pp [2] - L.-M. Berger, S. Saar,N. Tbias, K. Michaela, F. Zahalka, Micrstructure and Prperties f HVOF- Sprayed WC-(W,Cr)2 C-Ni Catings, Jurnal f Thermal Spray Technlgy, 28,17 (3) , ISSN: [3] - Hudkvá, Š., Zábranská, I. & Zahálka, F., Vliv přítmnsti maziva na třecí charakteristiky HVOF stříkaných pvlaků, aplikvatelných ve spalvacích mtrech. Prceedings f Metal 29:18th Internatinal cnference n Metallurgy and materials, Ostrava : Tanger Ltd., 29 ISBN ; ISBN (CD-ROM)