erosion behavior of uncoated waspaloy and waspaloy coated wi

8/8/2019 Erosion Behavior of Uncoated Waspaloy and Waspaloy Coated Wi

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ELSEVIER Surface and Coatings Technology 94-95 (1997) 64-69

Erosion behavior of uncoated Waspaloy and Waspaloycoated with titanium carbideVesselin Shanov, Widen Tabakoff*, J.A. Gunaraj

Depn rfmm f of Aerospace Engineering rind Engineering Mechunics, University of Cirrrinnati. Cincinnnfi, OH 4.5221, USA

AbstractTurbines and engines operating in a particulate flow environment experience erosion and performance deterioration. The industrialapproach for decreasing the erosion o f machine components is to apply wear resistance coatings. This paper describes an experimentalinvestigation performed to compare the behavior of uncoated and coated Waspaloy eroded in a media composed of chromite particles. Thespecimens were coated with titanium carbide (Tic) by a chemical vapor deposition (CVD) technique. The experimental program covered atemperature range From ambient tempcmture to 538OC, the particle velocities ranging from 180 m s- to 305 m s-, and impingement anglesvarying from 10 to 90. The faci lit y used for this work was a custom-made, high temperature erosion wind tunnel. The eroded surfacemorphology was examined by scanning electron microscopy (SEM). The results obtained depict the influence of the temperature. velocityand the impingement angle on the erosion rate. In addition to this, further data show the variation of the coating erosion rate with thequantity of chromite powder used. The erosion rate behavior of the TiC coating with respect to the impingement angles reveals a brittlecharacteristic trend. The uncoated superalloy behaves as a ductile material because the maximum erosion is between 30 and 4.5. It is

found that the sample temperature has a significant eff ect on the mat&al erosion rate. The erosion resistance of the CVD coating increasesat elevated temperatures, whereas that of uncoated Waspaloy decreases. The results indicate that the erosion rate for both uncoated andcoated samples is proportional to the particlc impact veloc ity to the power n. This investigation showed that the tested CVD titaniumcarbide coating provides ver y good erosion protection for Waspaloy in a particulate flow environment t elevatedemperatures. 0 1997Elsevier Science S.A.Ke~n,ords: Waspaloy; Chromite particles; Titanium carbide coating; Chemical vapor deposition (CVD); Impingement angle; Erosion rateat high temperatures

1. IntroductionThe ingestion of particle matter such as sand and dustwears the system componentsexposed to particulate flow.An important research ask s the development of better andmore durable high temperatureprotective coatings n orderto increase the lifetime of systems which are exposed toaggressive erosion-corrosion environment. Ceramic coat-

ings such as refractory metal carbides,nitrides, and oxideshave been investigated very intensively becauseof theirhigh resistance o erosion and corrosion. A comprehensivesource of information is reported in Ref. [l]. Titanium car-bide (Tic), titanium nitride (TiN), and alumina (AlzO$, arewidely used as wear resistancecoatings for cemented car-bide cutting tools such as inserts, drill bits, and saws 121.The concept of a composite machine part, combining sur-* Corresponding author.

face layers of these coating materials and a bulk substrate,being ough enough o stopcracks generatedat the surface sattractive in the designof somemachine componentsoper-ating in a particulate flow environment. Erosion studiesofceramic coatings applied on stainlesssteel (SS) or super-alloy-basedsubstrates y plasmaspraying, sputtering, deto-nation gun spraying (D-gun), and electro-spark detonationare well described n many articles [3-71.The erosion resistanceof ceramic coatings is stronglydependenton the coating processand on the substratemate-rial [3,4]. Two basic coating techniqueshave been devel-oped over the years: chemical vapor deposition (CVD) andphysical vapor deposition (PVD). Some PVD techniques,notably sputtering and ion plating, can produce carbides

and nitrides of the same structure quality as a CVD deposi-tion. CVD, however, currently remains he best coating fordeposition of thin ceramic, although competition from thePVD techniques ately has become more intense. Our pre-

02.57~8972f97i917.00 0 1997 Elsevier Science S.A. All rights reservedPII s0257-8972(97)00477-5


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vious work demonstrated the excellent protection that CVDcoatings provide for superalloy substrates in particu late flowenvironment [8,9].

The objective of the presented study was to investigatethe effect of the erosion parameters on the wear behavior ofWaspaloy, coated with CVD titanium carbide. For compar-ison, erosion data of uncoated Waspaloy were also col-lected. This nickel-based alloy is heat resistant and revealshigh strength. Waspaloy is frequently used for manufactur-ing jet engine buckets, disks and high temperature bolts[lo]. The effect of temperature on the erosion mechanismof Waspaloy at constant particle velocity of 50 m s-l andimpingement angle of 30 is discussed in Ref. [I 11. To thebest of our knowledge, there is no information in the litera-ture regarding erosion study of CVD coated Waspaloy.

2. Experimental details2.1. Experimental set-up

A reactor hot wall was used for CVD deposition of theTiC coating [7]. The charge is placed in a vacuum bellwhich is heated up by an electr ical furnace. The reactoroperated at a temperature of 1000C and total pressure of40 Torr (5333 Pa) [12,13]. The following gas mixture wasintroduced into the reactor for TiC deposition: TiCId-H2-CH+ Waspaloy was used as a substrate material on whichthe CVD coating was grown at a deposition rate of about 1Pd.The high temperature erosion test facil ity was designed toprovide erosion data in the range of operat ing temperaturesexperienced in compressors and turbines. In addition to thehigh temperatures, the facil ity simulates al l the erosion para-meters determined to be important from an aerodynamicpoin t of view. These parameters include particle velocity,angle of impact, particle size, particle concentration andsample size [14].2.2. Test conditiom and mnterials

The particle velocity, particle impingement angle, parti-cle characteristics, and material sample temperaturestrongly influence the erosion rate. These parameters werevaried in the present study for the tested TiC coatingsapplied on Waspaloy. This metal alloy selected for thetest program is commonly used for turbine components.The composition of Waspaloy was as follows: Waspaloy:M 59.0, Cr 19.5, Co 13.5, MO 4.2, T i 3.0, Mn 0.7, Fe 0.2, Al1.2, c 0*07.

The partic le velocity was controlled by changing the tun-nel air pressure. The partic le impingement angle was set byrotating the sample relative to the flow stream direct ion. Thesample temperature was varied by heat ing the flow streamwhich heats the sample to the desired temperature. Since thesolid particles in the steam turbines are mainly boiler scales,

chromite powder was used in the present investigation. Thiserodent material is usually considered to be solid solutionsof various spinels. Chromite used for the erosion tests wasreported in our previous publications [3,7,15]. The erodentpowder was sifted and the fraction below 75 pm was usedfor the experiments. This is a standard requirement for test-ing erosion resistant surfaces for coatings used on steamturbines.

Flat rectangular coupons were machined from Waspaloyplate . The specimens were 26 mm long, 3 mm thick and 13mm wide. The sample surfaces exposed to the environmen-tal flow were coated with a 15-pm thick titanium carbidefilm. The impact velocities changed from 180 to 305 m s attemperatures from ambient to 538C. Test data were accu-mulated by setting the particle impingement angle at 20, 30,50, 70 and 90. The erosion tests were conducted to obtaintwo types of erosion data, namely the erosion rates and thecumulative erosion mass loss for the CVD coated anduncoated Waspaloy. The erosion rate is defined as theratio between the change of the sample mass and the massof the impacting particles. The erosion rate tests were car-ried out in one cycle using a certain amount of particlesimpacting the sample surface. The wear did not exceed75% of the total coating mass. The cumulative erosionmass loss tests on the other hand were conducted in multip lecycles. In each cycle, the specimen was impacted by a pre-weighed increment of the partic le mass. After each particlemass increment had impacted the sample, its surface wascleaned, the specimen was weighed, and the change in thespecimen mass was recorded. The erosion rate for eachsuccessive particle increment was determined from theobtained experimental data. Erosion duplicate tests wererun and only the mean values are presented. The uncertaintyof the obtained erosion rates was within 7%.

3. Erosion test results and discussionThe TiC coating on Waspaloy prepared by a CVD process

exhibits fine grained structure which was observed by scan-ning electron microscopy (SEM) (Fig. 1).3.1. Impact angle eflect on the erosion rnte

The variation in the erosion rate as a function of thepartic le impact angle for CVD coated and uncoated Waspa-loy is shown in Fig. 2. On inspection of this plot , it can beseen that the erosion rate of the TiC coating increases withincrease in the impingement angle. The plot also shows thatthe maximum erosion rate for the TiC coating correspondsto a particle impact ang le of 90. Titanium carbide coatingexhibits a brittle erosion behavior when exposed to parti-culate flows. The same erosion pattern was observed pre-viously for TiC coating, on the Ni-based superalloys MAR246 [S] and INCO 718 [9]. During the exposure of thesample to the oncoming particu late flow the erosion rate


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66 I? Shanov et al. / Sq%ce cmd Conrings Technology 94-95 (1997) 64-69

Fig. 1. Scanning electron micrograph af TIC coating on Waspaloy beforeerosion: plan view.of the TiC coating was largely due to chipping. On furtherobservation by SEM (Fig. 3), the eroded surface displayedno cracks or plastic deformation in the coating. This beha-vior, we believe, is affected by the fine grained structure ofthe coating and its good adhesion to the substrate. Similarconclusions were made by Levy et al. concerning the lowerosion wear of CVD silicon carbide coating [ 121.Variation of the erosion rate with the impact angle foruncoated Waspaloy is also shown in Fig. 2. The erosionrates of these specimens passes through a maximum closeto 45 impact angle. This pattern indicates the ductile natureof the substrate. The chromite particles, striking theuncoated specimen, cut metal chips along the surfaceand cause plastic deformation. This is shown in Fig. 3 fora Waspaloy specimen at an impingement angle of 90. The

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Rg. 2. Erosion rate variation of uncoated and coated Waspaloy withimpingement angle: T= 538C, 16 = 305 m s-] , 20 g chromite mass.

Fig. 3. Scanning electron micrograph of TIC coating on Waspaloy aftererosion at 90 impingement angle: T = 538C, VP = 305 m s-, 20 g chro-mite mass.material removal mechanism is based on flaking andploughing. Similar surface morphology was observed byChinnadurai and Bahadur [ 111 or Waspaloy when impactedwith Sic particles. For uncoated Waspaloy, we obtained anerosion rate of one order of magnitude higher than that ofthe coated alloy at the same conditions. It can hence beconcluded that the CVD titanium carbide coating providesvery good erosion protection for the Waspaloy in particulateflow environment.3.2. Temnpemture ffect

The variation in the TiC coated and uncoated Waspaloyerosion rate with respect to the temperature of the particu-late flow at an impact angle of 90 is presented in Fig. 5. Itwas noticed that the erosion rate of the uncoated substratesfirs t decreased to 300C and then increased as the tempera-ture was increased to 538C. According to Chinnadurai and

Fig. 4. Scanning electron micrograph of uncoated Waspaloy after erosionat 90 impingement angle: T = 538C, tJP 305 m s-l, 20 g chromite mass.


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V. Shnnov et nl. / Surjluce and Coarings Technology 94-95 (1997) 64-69 67

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Fig. 5. Variation of the uncoated and coated Waspaloy erosion rate withtemperature at 90 impingement angle: V, = 180 m SK,20 g chromiteparticles.Bahadur ill], the hardness of uncoated Waspaloy decreaseswith an increase in temperature and this causes an increasein the erosion rate.As seen from Fig. 5, the erosion rate of coated specimensdecreases with rise in temperature. It is clear from theseresults that the TIC coating better protects the superalloyat elevated temperatures. Our previous study of CVD TiCcoatings on INCO 718 and stainless steel 410 showed simi-lar behavior [9]. SE&I observations of the eroded coating onWaspaloy at ambient temperature and at 538C reveal moredestruction by the impacted particles on the surface mor-phology at the lower temperature. The high temperatureerosion structure of the coating shown in Fig. 6 becomesinterrupted at ambient temperature by small craters (Fig. 7).

Fig. 6. Scanning electron micrograph of TiC coating on Waspaloy aftererosion at 90 impingement angle: T = 53PC, VP = 180 m P, 20 g chro-mite mass.

Fig. 7. Scanning electron micrograph of TiC coating on Waspaloy aftererosion at 90 impingement angle: T= 25C, VP = 180 m s-, 20 g chro-mite mass.The effect of the temperature on the materials physicalproperties such as strength and hardness is of substantialimportance for further interpretation of the obtained results[11,16-18-j.3.3. Particle velocity effect

The effect of the particle impact velocity on the erosionrate of TIC coated and uncoated Waspaloy is presented in

100 400VELOCIN (m/s)

Fig. 8. Variation of the uncoated and coated Waspaloy erosion rate withvelocity at 90 impingement angle: T = 538C, 20 g chromite particles.


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68 V. Shrorov et ui. / Surjircr and Courings Technology 94-95 (1997) 64-69

0 25 50 75 100 125 150MASS OF PARTIC LES (g)

Fig. 9. Titanium carbide coating weight loss variation with particle mass at90 impingem ent angle: T = 538C, 16 = 30.5 m s-, chromite particles.

Fig. 8. The experimental data show that the partic le velocityhas a significant influence on the wear of both coating andsubstrate material. The logarithmic plots prepared fromexperimental data reveal linear behavior of the erosionrate with the velocity. The velocity exponents II obtainedfrom the power low curve fitt ing are 2.51 for coated Was-paloy and 2.3 1 for uncoated substrare. The TIC exponent t7is close to that reported for the same coating on stainlesssteel-S5 3 10 substrate 191.3.4. Pnrticlc rims effu oil the erosion rate

The cumulative mass erosion test result5 for the TiC coat-ing on Waspaloy at 90 impingement angle are shown inFigs. 9 and 10. The non-linear rrlationship between theweight loss and the mass of impacting particles (Fig. 9)indicates that there is a continuous change in the erosionrate. This is displayed in Fig. 10, nrhich presents the erosionrate variation with the cumulative mass of particles. Theerosion rate of the TiC coating on Waspaloy decreases ini ti-ally due to the removal of a very thin top layer which hasbeen exposed to the coating process and to the atmosphericenvironment. This thin top layer can be removed via polish-ing. However, after a partic le dose of 10 g, the erosion rateremains steady with further increase of the impacting ero-dent mass. After a partic le dose of 40 g, the erosion rateincreases rapidly and reaches a constant value of about 2mg/g. The erosion rate of the uncoated Waspaloy is around3.4 mglg and it was not reached after particle dose of 183 g.The latter indicates existence of an intermediate layerbetween the coating and the substrate probably formed dur-

ing the exposure of Waspaloy to the high temperature coat-ing process. The erosion process and the surface coated lifedepends on the particles concentration, size, hardness andmany other factors. A further composition and microstruc-ture study of the coating/substrate interface is required andsuch an investigation is in progress.

4. Summary and conclusionsA comparative study was conducted to investigate the

erosion behavior of uncoated Waspaloy and Waspaloycoated with CVD titanium carbide. The experimental resultscharacterized the tested coating subjected to high tempera-ture erosion by chromite particles. The effect of the impactangle, particle velocity and temperature on the erosion rateof the studied materials has been experimentally investi-gated. It was found that the tested coating on Waspaloybehaves as a brittle substance and its erosion rate decreasesat high temperatures. The uncoated Ni superalloy exhibitsduct ile behavior and wears more at elevated temperatures.The data obtained for the TiC coating showed one order ofmagnitude less erosion rate compared to the uncoated meta lsubstrate. It was established that the erosion rates of bothuncoated and coated Waspaloy are proportional to the par-ticle velocity to the power 12,where the exponent I? is 2.51for the coated metal and 2.31 for the uncoated substrate.This study demonstrated that the CVD titanium carbidecoating provides very good erosion protection for Waspaloywhen exposed to particulate flow at high temperatures.

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Fig. 10. Variation bf the titanium carbide coating erosion rate with particlemass at 90 impingemen t angle: T= 538C, V, = 305 m s-l, chromiteparticles.


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V. Shanoi et al. / Swfuce and Coatings Technolog) 94-95 (1997) 64-69 69

AcknowledgementsThis research was sponsored by NationaI Science Foun-

dation, Washington, DC, Grant INT-9204963.-Dr. V, Sha-nov would like to thank the Fulbright Foreign ScholarshipBoard for financia l support. The authors thank Dr. J. Lianfor performing the SEM work.

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Coating Prodrtcrioiz, High Temperature Properties and Applicatio ns,Longman Scient ific and Technical, UK, 1989, p. 349.:2] P.A. Dearney and E.M. Rent, Met&- Technology, February, Vol. 9.The Metal Society, 1989.[3] J. Qureshi and W. Tabakoff, Su$ Gout. Techrrol., 33 (1988) 433.[4] W. Tabakoff. Srtr$ Coat. Technol., 39/40 (1989) 97.[5] P.J. Burnet and D.S. Rickerby, J. Muter. Sci., 23 (1988) 2429.161 B. Jonsson, L. Akre, S. Johansson and S. Hogmark, Thin Solid Films,137 j19S6j 65.171 P. Walsh and W. Tabakoff, Adv. Beam Turbine Tec h&. PoiverGeneration, 10 (1990) 1.

[S] W. Tabakoff and V. Shanov, Sut$ Conr. Technol., 76/77 (1995)15.[9] V. Shanov and W. Tabakoff, Eruion Resisrance OJ Coatirzgs /orMeral Prorecrion at E!evared Tem peratures, presented at the Inter-national Conference on Metallurgical Coatings and Thin Films, April22-26, 1996, San Diego, CA.[lo] J.P. Frick, Wooidmans Engecring Alloys, 7th edn., ASM InternationalMaterials Park, Ohio, 1990, p. 1318.[l l] S. Chinnadurai and S. Baludur, I\eu, 186/187 (1995) 299.[12] V. Shanov, W. Tabakoff and M. Metwally, Sur[i: Coat. Technoi., 54/55 (1992) 25.[ 131 V. Shanov, W. Tabakoff and A. Hamed, Sur$ Coat. Techn ol., 68/69(1994) 92.[14] W. Tabakoff and and T. Wakeman, Test Facility for Marerial Ero-sion ar High Temperarure, ASME Special Publication, 664 (1979) p.123.1151 W. Tabakoff, M. Metwally and A. Hamed, J. Eng. Gas TurbirzePobcer, ZZ7(Ij (1995) 146.[16] A. Levy . D. Boone, A. Dav is and E. Scholz, in J.E. Field and N.S.Corney (eds.), Proc. 6th Itzi. Conf: on Erosion by Liquid and SolidImpacr. Cavendish Laboratog, Cambridge, 1983 p. 46.[17] N. Gat and W. Tabakoff, American Society for Testing and Materi-als, 1. Tes?iizg Evaluation, 8j4) (1980) 177.[18] T. Wakeman and W. Tabakoff, J. Aircraff, J6(12) (1978) 828.

erosion behavior of uncoated waspaloy and waspaloy coated wi

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