Surface and Coatings Technology 83 (1996) 307-312
Wear and fretting wear behaviour of ion-implanted Zircaloy-4 Jeon G. Han a, Jae S. Lee a, Byung H. Choi b, W. Kim b, Guoyi Tang b a Department
of Metallwgical Engineeying, Sung Kyun Kwan University, 300 Chunchun-dong, Jangan-Ku, Suwon, South Korea b Korea Atomic Energy Research institute, P.O. Box 7, Daedeok Science Town, Taejeon, South Korea
Abstract Zircaloy-4 wasimplantedwith nitrogen at 120keV to various total ion dosesbetween1 x 1O1’and 1 x lOi ionscmm2at various temperaturesin the range 310-660“C. The implanted surfaceswere analysedby Auger electron spectroscopy(AES) and X-ray diffraction (XRD). Wear and frettin g wear tests were performed at various loads under unlubricated conditions and water immersionrespectively. Nitrogen implantation produced zirconium nitride and oxide which enhancedthe surfacehardnessup to 1800HK (0.1 N) for a total ion doseof 1 x lOI* ionscm-’ at 660“C (300 HK (0.1 N) for unimplantedspecimen).The ball-on-discwear resistancewas improved for nitrogen-implantedspecimensabove 500“C. In addition, oxide layer and ZrN formation by nitrogen implantation in an oxygen atmospheregreatly enhancedthe wear resistanceof Zircaloy-4. The fretting wear resistancewasenhancedby nitrogen implantation. Successful improvement of the fretting wear resistancewas obtainedfor the specimenimplantedat 550“C to a total doseof 8 x 101’ionscm-‘. Keywords:
Fretting wear; Ion implantation; Zircaloy-4; Zirconium nitride
1. Introduction Nitrogen implantation is a precise hardening process for the improvement of the tribological properties of various ferrous alloys and other materials [l-5]. The hardening caused by nitrogen implantation is due to the formation of fine nitride precipitates, the generation of irradiation damage and dislocation pinning by nitrogen, etc. The transition metals of group 4A, such as Ti and Zr, produce nitrides such as T&N, TiN, ZrN, etc. on nitrogen implantation [ 6,7]. In this study, nitrogen implantation was applied to
Zircaloy-4, which is used to fabricate fuel cladding tubes in pressurized light water nuclear power reactors, in an attempt to improve the wear and fretting wear resistance. The formation behaviour of ZrN and ZrO, was studied at various total ion doses and elevated temperatures. The wear and fretting wear behavioural changes caused Table 1 Implantation
by nitrogen implantation were evaluated in conjunction with the structural modifications associated with the implantation conditions. 2. Experimental
Zircaloy-4 plate and tube were implanted with nitrogen ions at various total ion doses and temperatures at 120 keV. Tables 1 and 2 list the implantation conditions for plate and tube specimens respectively. In addition, some of the plate specimenswere implanted with nitrogen at O2 partial pressures of 2.0 x 10m3 Pa and 2.8 x 10e3 Pa to promote the formation of a ZrO, layer as well as ZrN precipitates. The composition profile and compound formation in the implanted surface region were analysed by Auger electron spectroscopy (AES) and X-ray diffraction (XRD). Ball-on-disc wear tests were carried out using
conditions of plate specimens
Processing temperature (“C) Current density (PA cm-*)
Ion dose (ions cmwZ) 1 x lOl7
3 x 101’
5 x 1O1’ 400 55.56
0257-8972/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved
6 x 1Ol7
1 x 101s
J. G. Han et al.JSurface and Coatings Technology83 (1996) 307-312
Table 2 Implantation conditions of tube specimens Conditions
Processing temperature (“C) Current density (PA cm-‘)
Ion dose (ions cmm2) 1 x 1017
3 x 1017
AISI52100 and A&O, balls against plate specimensat various loads in the range 0.1-1.0 N under unlubricated conditions. Fretting wear tests were performed using a laboratory designed fretting wear tester (Fig. 1). Two tube specimenswere cross contacted and moved each other with a vertical vibration of 10 Hz between -t 15 pm with a reciprocating translational movement of 1 mm under water immersion at 20 “C. The applied load and translational cycle ranges for fretting wear tests were l-10 N and 9000-360000 cyclesrespectively. 3. Results and discussion
Figs. 2(a) and 2(b) illustrate the AES depth profiles of nitrogen-implanted Zircaloy-4 at 671 and 660 “C. The nitrogen concentration increaseswith the total ion dose and penetratesover 0.5 ltrn through radiation-enhanced diffusion at elevated temperature. Oxygen and, especially, carbon contamination is observedin the implanted region. The contamination is attributed to the inherent strong gettering effect of oxygen by Zircaloy-4 and the
@ Load cell
@ Upper specimen holder @IMicro-moving table @ Lower specimen holder @ Dove table @ L. M. guide
@ Water bath
0 Actuator Amp.
0 Frequency counter
@ILoad cell controller
8 IWounting block
@ Load cell holder bracket
Fig. 1. Schematic diagram of fretting wear tester.
5 x 10” 400 64.82
8 x 10” '
back streaming of carbonaceousgas from the diffusion pump oil of the vacuum pumping systemduring the ion implantation process. In contrast, nitrogen implantation under an oxygen partial pressureinduces significant oxygen penetration at high concentration (over 70 at.%) to a thickness of approximately 0.7 ltm, i.e. beyond the nitrogen profile range (see Fig. 2(c)). This indicates that oxygen gas is adsorbedto the Zircaloy-4 surfaceby a strong gettering effect and diffusesdeeply into Zircaloy-4 by radiationenhanced diffusion together with nitrogen ions during nitrogen implantation. Thereforeit is suggestedthat the formation of a dense oxide layer of a strong oxygen gettering material can be promoted by ion irradiation at elevatedtemperature. Fig. 3 illustrates typical XRD patterns of nitrogenimplanted Zircaloy-4 for various process temperatures and total ion doses. Zr02 is produced in implanted specimensup to 500 “C, while ZrN formation is observed for the implanted specimenat 620 “C. However,nitrogen implantation at 660 “C produces ZrN at all total ion dosesof 1 x 1017to 1 x 1OL8ions crnm2.The orientations of ZrN producing nitrogen implantation into Zircaloy-4 appear to be (111) and (200). The increasein the ZrN peak intensity with increasing total ion dose indicates the enhancementof ZrN precipitation. Zr02 formation is also highly promoted by nitrogen implantation in an oxygenatmosphereasshownin Fig. 3(f ). The promotion of ZrO, formation is consistent with the AES data analysisin Fig. 2. It can be concludedfrom theseresults that ZrN formation can be obtained by nitrogen implantation into Zircaloy-4 at high temperatures(above approximately 620 “C) with a high total dose. In addition, ZrO, layer formation can be promoted by nitrogen implantation in an oxygen atmosphere at elevated temperatures. Figs. 4 and 5 illustrate the hardness change as a function of the implantation temperature and total ion dose.The hardnessgradually increaseswith temperature up to 500 “C and then increasesstrongly to 1500 HK (from 300 HK for unimplanted Zircaloy-4) at 620°C as shown in Fig. 4. A significant increase in hardness is observedfor implanted specimenswith increasingtotal ion doseat 660 “C (Fig. 5). The significant improvement in hardnessabove 620 “C strongly supports the conclusion that ZrN formation plays an important role in the
J. G. Han et al./Surface and Coatitlgs Technology83 (1996) 307-312
Sputtering time (min.) 100 80
Sputtering time (min.)
Fig. 2. AES depth profiles of nitrogen-implanted Zircaloy-4 (sputtering rate, 150 A min-I): (a) 6 x 1Ol7ions cm-‘, 671 “C; (b) 1 x 101*ions cm-‘, 660 “C; (c) 3 x 1OL7ions cmm2,480 “C, 0, atmosphere.
J. G. Han et al.lS’wface and Coatings Technology83 (1996) 307-312
/+Harrlness+Ratiocfi~ (d) Fig. 5. Knoop microhardness (0.1 N) and ratio of hardness increase for unimplanted and N+- implanted Zircaloy-4 at various ion doses (120 keV; processing temperature, 660 “C). (e)
,,li,J,/I/ li I..,:,,” /
Fig. 3. XRD patterns of unimplanted and N’-implanted Zircaloy-4 for various implantation conditions (ion energy, 120keV; 0, ZrN (111); I, ZrN (200); A, ZrO, (111); 0, Zr): (a) unimplanted; (b) 3 x 1Ol7ions cm-‘, 500 “C; (c) 3 x 1Ol7ions cmY2, 620 “C; (d) 6 x 1017 ions cme2, 660 “C; (e) 1 x lOis ions cmd2, 660°C; (f) 5 x 1O1’ ions cmv2, 320 “C, O2 atmosphere.
hardening of nitrogen-implanted Zircaloy-4 relative to other hardening effectsexpected for an ion-implanted surface,such as irradiation hardening and dislocation pinning by nitrogen. ZrO, layer formation by nitrogen implantation in an oxygen atmospherealso leads to an increasedhardnessup to 1300 IX (0.1 N). Surface hardening by nitrogen implantation significantly reducesthe wear of Zircaloy-4 as shown in Fig. 6. The enhancement in the wear resistance is clearly observedfor nitrogen-implantedspecimenswith increasing total ion dosesabove 500 “C. The friction coefficient appears to increase during initial wear testing against an AM52100 ball, while an opposite effectis observed against an A1,03 ball. It is found from an analysis of the wear track and ball contact area that, for AIS152100, -adhesivewear initially occurs due to its low hardness
1600 , 1400 1200 * 4 IOOO-
400 500 270 Processingtemp. (T)
1--c Hardness +
Ratio of increase /
Fig. 4. Knoop microhardness (0.1 N) and ratio of hardness increase for unimplanted and N+- implanted Zircaloy-4 at various processing temperatures (120 keV; 3 x 10” ions cm-‘).
Fig. 6. Variation of the weight loss against AISI52100 and A&O, balls for unimplanted and N*-implanted Zircaloy-4 for various ion doses at 660 “C (wear testing conditions: 0.5 N; 0.4 m s-‘; 1000 m).
J. G. Han et aLlSurface and Coatings Technology83 (1996) 307-312
(550 HK) compared with the hardened layer of the implanted specimen, followed by a mixture of abrasive wear and oxidation as shown in Fig. 7. The friction coefficient is strongly reduced between Zircaloy-4 nitrogen implanted in an oxygen atmosphere and the A&O, ball (Fig. 8), indicating a significantly improved wear resistance. This implies that ZrO, layer formation is effective in reducing the friction coefficient and wear against an oxide counterpart. Fretting wear between Zircaloy-4 tubes under water immersion is significant under accelerated fretting wear conditions. The rate of increase in the crater depth due to fretting wear (reciprocating translational movement
of 1 mm with vertical vibration of + 15 pm at 10 Hz) is approximately 20 A per cycle under a load of 10 N. Nitrogen implantation at various total ion doses and temperatures slightly improves the fretting wear resistance as illustrated in Figs. 9 and 10. A successful improvement is found for the specimen implanted to a total dose of 8 x 1Ol7ions cmm2 at 550 “C. The rate of inc;ease in the crater depth is reduced to approximately 12 A per cycle through surface hardening by nitrogen implantation as shown in Fig. 11. The features of the fretting wear craters for unimplanted and nitrogenimplanted Zircaloy-4 tube surfaces are illustrated in Fig. 12. It is suggestedfrom the results that the formation
Fig. 7. Optical micrographs of wear scar for unimplanted and Ni-implanted Zircaloy-4 (wear testing conditions: 0.5 N; 0.4 m s-i; 1000 m; processing temperature, 660 “C): (a) substrate against AISI52100 steel ball; (b) 6 x 1Ol7ions cmeZ against AISI52100 steel ball; (c) substrate against Al,O, ball; (d) 6 x lOi ions cm-’ against A&O, ball.
I f loo0 1500 Sliding time (sec.)
Fig. 8. Variation of the dynamic friction coefficient against an Al,O, ball for unimplanted and N’-implanted processing temperature, 480 “C) in an oxygen atmosphere (wear testing conditions: 0.5 N; 0.4 m s-l; 1000 m).
(3 x lOI7 ions cm-z;
J. G. Han et ailSurface and Coatings Technology 83 (1996) 307-312
Fig. 9. Variation of crater depth for unimplanted and N*-implanted Zircaloy-4 for various ion doses after fretting wear tests (fretting wear test conditions: 20 “C; 1 N; 9000 cycles; in water bath; vertical, 10 Hz, & 15 pm; horizontal, 10 Hz, 1 mm).
Fretting cycle --t
Unimplanted. --C N implanted.
Fig. Il. Variation of crater depth for unimplanted and Nt-implanted Zircaloy-4 (8 x 1017ions cmq2) for various cycles after fretting wear tests (fretting wear test conditions: 20 “C; 1 N, in water bath; vertical, 10 Hz, + 15 pm; horizontal, 10 Hz, 1 mm).
Fig. 10. Variation of crater depth for unimplanted and N+-implanted Zircaloy-4 for various processing temperatures after fretting wear tests (fretting wear test conditions: 20 “C; in water bath; vertical, 10 Hz, rir:15 pm; horizontal, 10 Hz, 1 mm).
of densenitrides and oxidesby implantation successfully enhancesthe fretting wear resistance.
Fig. 12. Optical micrographs of crater scar for unimplanted (a) and N*- implanted (b) Zircaloy-4 (120 keV, 3 x 10” ions cms2, 515 “G) in fretting wear tests (fretting wear test conditions: 20 “G; 3 N, 18000 cycles; in water bath; vertical, 10 Hz, &15 pm; horizontal, 10 Hz, 1 mm).
sphere at elevatedtemperature also greatly reducesthe friction coefficient against an A&O3 ball, thereby improving the wear resistance.The fretting wear resistance is also improved by nitrogen implantation to a total dose of 8 x 1017ions cmV2at 550 “C!.
The wear and fretting wear behaviour, together with the structural changes, were studied for nitrogenimplanted Zircaloy-4 at various total ion doses and elevated processing temperatures.ZrN precipitates of (111) and (200) orientations were produced for nitrogen implantation above 620 “C. Nitrogen implantation in an oxygen atmospherepromotes ZrO, layer formation to a thickness of 0.7 pm. The formation of ZrN and ZrO, significantly enhances the surfacehardnessby a factor of 5.5,correspondingto 1780 HK (0.1 N), and thereby improves the wear and fretting wear resistance.The wear resistanceis enhanced by as much as fourfold for nitrogen- implanted Zircaloy-4 at 620°C to a total ion dose of 1 x 101* ions cm- 2. Nitrogen implantation in an oxygen atmo-
This work was supported by the Korea Atomic Energy ResearchInstitute. References   [S] 
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