Nuclear Instruments and Methods in Physics Research B 307 (2013) 536–540

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Nuclear Instruments and Methods in Physics Research B

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Surface damage of Ti3SiC2 by MeV iodine bombardment

0168-583X/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.nimb.2013.03.021

⇑ Corresponding author. Tel.: +86 21 6564 2292.E-mail address: lqshi@fudan.edu.cn (L. Shi).

Chaozhuo Liu a,b, Liqun Shi b,⇑, Qiang Qi b, D.J. O’Connor c, B.V. King c, E.H. Kisi d, X.B. Qing e, B.Y. Wang e

a College of Science, China University of Petroleum, Qingdao 266580, PR Chinab Institute of Modern Physics, Fudan University, Shanghai 200433, PR Chinac School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, NSW 2308, Australiad School of Engineering, University of Newcastle, Callaghan, NSW 2308, Australiae Institute of High Energy Physics, CAS, Beijing 100049, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 September 2012Received in revised form 12 March 2013Accepted 13 March 2013Available online 23 March 2013

Keywords:Titanium silicon carbideMAX phaseIon irradiationSurface damage

The damage produced by a 2 MeV iodine ion beam in MAX-phase Ti3SiC2 polycrystalline targets at roomtemperature has been studied at doses up to 3 � 1015 ions cm�2. The evolution of microstructure andinduced defects of the irradiated sample with different doses was surveyed by combining grazing inci-dent X-ray diffraction (GIXRD) using synchrotron radiation and variable energy positron beam analysis(PBA). With increasing irradiation dose, the crystallinity degrades gradually and leads to a combinationof damaged Ti3SiC2 in combination with the precipitation of a TiCx phase. For high dose irradiation, anano-dispersed TiCx phase becomes the dominant component. The PBA measurements indicate the for-mation of a new large vacancy-type defect that could be a cluster or void. The combination of GIXRD andPBA demonstrates that the damage of the MAX phase is more serious in the first 10 nm surface layer thanthat in the deeper layers closer to the final resting position of the projectile in the solid. The possible dam-age mechanisms have been discussed.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

A class of nano laminated ternary carbides or nitrides, whichcombine the superior properties of both metals and ceramics [1],have shown potential applications in many fields and drawnincreasing research concern. One group in particular called MAX-phases has a generic chemical formula as Mn+1AXn, where M isan early transition metal, A is an element from the IIIA or IVAgroups, X is carbon or nitrogen, and n = 1, 2 or 3. One of the moststudied components of this class, the typical MAX-phase Ti3SiC2

has been revealed the outstanding characteristics, which includethe good ductility and machinability even at room temperature,the thermal stability at high temperature, the high thermal andelectrical conductivity, the low thermal expansion coefficient,and the resistance to chemical attack and thermal shock [2–6]. Re-cent researches on microstructure change under ion irradiation atroom temperature and high temperature has revealed its damagetolerance [7–9]. These properties suggest that MAX-phase Ti3SiC2

is a promising candidate structural material for the future nuclearreactors [10,11].

Until now, there have only been limited studies of the defectstructure of Ti3SiC2 surface under irradiation. Our previous work[9] on ion irradiation showed a significant damage to the surface

and formation of decomposed phase TiC in damage region by thegrazing incident X-ray diffraction (GIXRD). This work has been ex-tended to study in detail the nature of the damage and its depthdistribution induced by various doses of iodine ion irradiation.The defect structure of the irradiated samples were characterizedby GIXRD and the variable energy positron beam analysis (PBA),while the topography changes due to damage were studied usingthe scanning electron microscopy (SEM) and atomic force micros-copy (AFM).

2. Experimental

The powder of Ti, SiC and C with the stoichiometric proportion3:1:1 was cold-pressed into a cylindrical ingot with subsequentfour-hour sintering at 1600 �C in argon gas. The additive Al wasused to suppress the formation of TiCx (x � 0.67) and help to gainthe MAX-phase Ti3SiC2 at a relatively low temperature. The bulkmaterial was cut into 15 mm � 12 mm slices and polished usingfine metallographic abrasive paper with Al2O3 suspensions of a sizedown to 5 m, and then cleaned by rinsing in ultrasonic baths ofacetone and ethanol. The specimen was annealed at 800 �C for1 h in a vacuum of 2 � 10�5 Pa to release the residual strain beforethe irradiation.

The specimen was irradiated with 2 MeV iodine ion deliveredby a NEC 9SDH-2 tandem accelerator at Institute of Modern Phys-ics of Fudan University. The incidence angle of the beam to sample

C. Liu et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 536–540 537

normal was set at 45� to create more damages in the near surfacelayer. The irradiation flux was 1.2 � 1011 ions cm�2 s�1. The irradi-ation experiments were carried out for four fluences: 6 � 1013,3 � 1014, 1 � 1015 and 3�1015 ions cm�2. The ion beam heating ofthe sample during irradiation may increase the sample tempera-ture to as much as 200 �C.

GIXRD measurements were performed at the BL14B1 beamlinestation of Shanghai Synchrotron Radiation Facility (SSRF). A Huber5021 diffractometer equipped with NaI scintillation detector wasused for data acquisition. The X-ray wavelength was 1.2398 Åwhile the spot size of X-ray beam was approximately 0.5 mm.The angle of incidence was set at the angles above the critical angleto get a stronger signal from the near surface region.

PBA experiments were carried out using 0.1–30 keV positronbeams at the Institute of High Energy Physics. The Doppler broad-ened spectra of annihilation radiation were recorded by a HPGedetector. The traditional S-parameter and W-parameter wereadopted to quantify the widening or sharpening of the 511 keVpeak. The S-parameter was defined as the ratio of the gamma-raycounts in the central part of the peak (510.24–511.76 keV) to thatin the total peak (504.2–517.8 keV). Meanwhile, W-parameter wasdefined as the ratio of the summed counts in the ranges of 504.2–508.4 keV–513.6–517.8 keV to the total counts of the peak.

To eliminate the possible annealing effect during the polishingprocedure, one sample that did not undergo the complete polish-ing was used for the surface observations. Its surface topographybefore and after the large dose of irradiation was observed bySEM (Philips XL30 FEG) with 20 keV electron beam in combinationwith AFM (Omicron Multi Probe S) in the tapping operation mode.

3. Results and discussion

3.1. Phase analysis by GIXRD

GIXRD scans (Fig. 1) were performed at 0.6� incidence to gainpredominantly the properties of the first 200 nm. The scan of virginsample reveals a polycrystalline MAX phase of Ti3SiC2 with a smallamount of Al2O3 impurity, which formed in the sample during thereactive sintering.

With increasing irradiation dose up to 3 � 1014 ions cm�2, thediffraction peaks related to MAX phase broaden in shape and re-duce in intensity. Previous research [9] has reported that thebroadening of MAX-phase peaks was caused by an increase of lat-tice microstrain. Further irradiation continued the broadening ofthe MAX phase peaks and some of them become invisible that

27 28 29 30 31 32 33 34 35 36 37

(200

)

Inte

nsity

(a.u

.)

2θ (°)

Ti3SiC

2Al

2O

3TiC

x

(111

)

6×1013 cm-2

3×1014 cm-2

1×1015 cm-2

3×1015 cm-2

Virgin

(104

)

(101

)

(102

)

(103

)

(008

)

(105

)

Fig. 1. GIXRD scans at 0.6� incidence for samples irradiated with a range of doses.

means a distortion of the MAX phase lattice. The significant shiftof (008) peak indicates continuous expansion of the unit cell alongthe c axis due to irradiation. The irradiation-induced damagecauses a continuous but anisotropic expansion along the differentcrystallographic directions of the hexagonal lattice. Meanwhile,the GIXRD background signal gets significant, revealing a high levelof disorder in the crystal structure. The crystal lattice has been dis-torted by the damage defects, and the crystalline quality decreasesaccordingly.

Irradiation with the dose of 6 � 1013 ionscm�2 has induced theemergence of a peak at 33.60�, which is close to the (105) peak at34.00� and therefore contributes to a marked broadening of (105)peak. This emergent peak becomes more prominent when the doseincreases up to 3 � 1014 ionscm�2 and can be distinguished fromthe (105) peak. This new peak has been identified as TiCx (x is nearto 1) from the decomposition of Ti3SiC2 [9]. Relative to the tightbonding between the Ti and C atoms in the MAX-phase lattice,the Si atoms has weaker bonding to adjacent Ti layer, and are easilyknocked from their lattice. So more of them escape from the dis-placement cascade region. This process drives the transition frompure MAX phase to TiCx in the cascade region.

With increasing dose up to 1 � 1015 ionscm�2, the strength ofTiCx (200) peak reduced and a broad low peak formed at 29.06�corresponding to TiCx (111). The wide peak of TiCx phase indicatesthe TiCx presents as the type of nano-sized grain or microcrystal.Finally, a nano-dispersed damage phase TiCx and a damagedMAX phase coexist in the bulk.

The GIXRD scans for different angles of incidence to the sampleirradiated with a dose of 3 � 1015 ions cm�2 is presented in Fig. 2.The incidence angle can be related to the characteristic penetrationdepth and therefore the sampling depth from which most of the X-rays are scattered (Fig. 3). When the incidence angle is smaller thanthe critical angle (0.208�), X-rays will be reflected from the surfacelayer and will yield the surface structural information to about10 nm beneath the surface. In case of 0.1� and 0.2�, there are onlythree peaks of TiC (111), (200) and Ti3SiC2 (104), which are muchlower than that for the higher angles (deeper layers). With increas-ing incidence angle, greater depths are probed and the other peaksof MAX phase emerge and become intense, as also does the dam-age phase TiCx. All these indicate the level of damage near the sur-face is different from that in the bulk.

To evaluate the damage level of the MAX phase, the GIXRD pat-terns in Fig. 2 were decomposed to calculate the peak area ratio ofTiCx (111)–Ti3SiC2 (104). The areal ratio of TiCx-to-Ti3SiC2 peakand corresponding detection depth of X-ray with different inci-dence angle were shown in Fig. 3. Comparing with the MAX-phase

27 28 29 30 31 32 33 34 352θ (°)

(105)

(200

)

(111

)(104

)

(110

)

(113

)

Ti3SiC2

Al2O3

TiCx

Inte

nsity

(a.u

.)

0.4°

0.1°0.2°

0.6°

0.8°

1.5°

3.0°

(008) (104)(101)

(102

)

(103

)

Fig. 2. GIXRD scans at different incidence angles for the irradiated sample with adose of 3 � 1015 ions cm�2.

0.0 0.5 1.0 1.5 2.0 2.5 3.00

1

2

3

4

5

6

Diff

ract

ion

ratio

of

TiC

x-to-

Ti3S

iC2

Grazing incidence angle (°)

Crit

ical

ang

le

0

200

400

600

800

1000

X-ra

y de

tect

ion

dept

h (n

m)

Fig. 3. The areal ratio of TiCx (111) peak to Ti3SiC2 (104) peak (left axis) as afunction of depth which can be related to the X-ray characteristic detection depth(right axis) as a function of the incidence angle in GIXRD.

538 C. Liu et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 536–540

component in the material, TiCx component is dominant in thedepth range of 10 nm. However, it should be mentioned here thatthis is a approximate estimation for relative TiC level near the sur-face because the diffraction peaks below critical angle is attributedto surface structural information. As the detection depth increases,the ratio decreases but still remain comparable in the depth up to900 nm, demonstrating that there is significant damage through-out the range of ion projectile.

3.2. Defect characterization by PBA

The S-parameter as a function of positron energy for the sam-ples with different irradiation fluences is shown in Fig. 4. At the lo-cal open-volume defects, the positron will have a lower probabilityto interact with the core electrons that possess the higher kineticenergy and a higher probability to annihilate with the outer-shellelectrons that have lower kinetic energy. Positrons tend to diffuseto a vacancy, void, bubble, and interface or surface if possible; thenthey will be more likely to annihilate with a low kinetic energyelectron resulting in a small Doppler broadening of the resultantgamma rays, and hence will be characterized by a higher S-param-eter. As positrons diffuse through a solid, it is not possible to assigna particular depth to positron energy. For a 2 keV positron beam,the signal is dominated by the structure of 10 nm thickness; whilethe beam up to 10 keV corresponds predominantly to the depth ofabout 200 nm. The low S-parameter for the virgin sample over thefull range of positron energies reveals a uniform material with few-er defects in the bulk.

0 2 4 6 8 10 12 14 16 18 200.45

0.46

0.47

0.48

0.49

0.50

0.51

600nm

200nm

S-p

aram

eter

3×1015 cm-2

1×1015 cm-2

3×1014 cm-2

6×1013 cm-2

Virgin

Positron energy (keV)

10nm

Fig. 4. The S-parameter versus the positron implantation energy for the irradiatedsamples with the denoted dose.

At the lowest irradiation fluence, i.e. 6 � 1013 ions cm�2, com-paring to the deep position in the bulk, a significantly larger S-parameter in the depth range of 10–600 nm is obtained showingthat vacancy defects have been created by the irradiation. The low-er S-parameter in the range 0–2 keV reflects the fact that initiallythe damage by the ion beam is localized at the greater depths nearwhere the projectile comes to rest. The irradiation with a dose of3 � 1014 ions cm�2 has caused a reduction in S-parameter value,which still maintains a uniform increase to the un-irradiated sam-ple. Some recombination of vacancies and interstitials or disloca-tion formation may have happened due to the continuousirradiation effects. The decrease in S-parameter at 1–2 keV indi-cates a lower density of vacancies near the surface compared tothat in the bulk for the lower damage cases.

Further increasing the irradiation dose continues to develop theS-parameter value in the bulk, while little change is observed inthe near-surface zone. The diffusion of vacancies into the bulk is ob-served. When the dose increases to the highest, i.e. 3 � 1015 -ions cm�2, the S-parameter increases dramatically in the range ofpositron energy 0–2 keV over that in the bulk, which illustrates thatunder the conditions of high damage rates at the end of the projectilerange, defects in significant quantities move to the surface layer.

The relationship between the S-parameter and W-parameter ofpositron annihilation spectra for the irradiated samples is shown infig. 5. Similar plots have been used to discriminate the type of de-fects in SiC [12] and SiGe [13]. Here two different behaviours of the(S,W) characteristics are observed. When the irradiation fluence isless than 3 � 1015 ions cm�2, all the (S,W) values are aligned on astraight line, indicating that the positron-sensitive defects are va-cancy-type with tiny size. However, at the highest fluence3 � 1015 ions cm�2, almost all (S,W) values are aligned on anotherstraight line with larger slope, except those at the high positionenergies (>16 keV) responsible for the zone deeper than the dam-age range, This suggests that a new type of defect has evolved, suchas void or nanocluster, whose size is larger than the defects createdat lower dose. As the S-parameter in the near surface at this dose ismuch larger than that in the bulk, it is believed that more abundantlarger-sized defects are produced near the surface.

3.3. Surface observation by SEM and AFM

The SEM images of the sample before and after the irradiationto the dose 3 � 1015 ionscm�2 (Fig. 6) reveal that the effect of irra-diation is to produce more small particles and some degradation ofthe previously clean crystallite structure. The damage cascade

0.05 0.06 0.07 0.080.45

0.46

0.47

0.48

0.49

0.50 3×1015 cm-2

1×1015 cm-2

3×1014 cm-2

6×1013 cm-2

Virgin

W-parameter

S-pa

ram

eter

Surface

Bulk

Fig. 5. The relationship of S-parameter and W-parameter of positron annihilationspectra for the irradiated samples with the denoted dose.

Fig. 6. SEM images of the sample (a) before and (b) after the irradiation to the dose3 � 1015 ions cm�2.

Fig. 7. AFM topographies of the sample (a) before and (b) after the irradiation to thedose 3 � 1015 ions cm�2.

C. Liu et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 536–540 539

quenching and defect accumulation can lead to such macroscopicchanges. These effects can be achieved by either collision processesor ionizing irradiation [14] as in the case of electron excitation byhigh-energy ions. It can also be found from Fig. 6 that the irradiat-tion causes microcracks, compared with unirradiated samplesurface.

The roughening of the surface of the individual crystals is alsoevident in the comparison of the virgin and irradiated surfacesmeasured by AFM (Fig. 7). The individual grains of the virgin sam-ple have a uniform and smooth surface, while the irradiated sam-ple shows evidence of serious surface roughness. As the estimatedthickness of the sputtered layer during this irradiation is about2 nm, which is much less than the changes observed in the surfacetopology, there must be processes in play which cause serious dif-fusion of damage to the surface which leads to the enhanced sur-face damage.

3.4. Further discussion

The distribution of iodine ions and irradiation induced displace-ment damage were simulated by the SRIM code [15] with themode ‘‘Detailed Calculation with Full Damage Cascades’’ to helpillustrate the range and damage outcomes. The energetic ion pene-trating into the target transfers its energy to target atoms and elec-trons before coming to rest in the subsurface region. For 2 MeV Iion irradiation at 45� incidence, the ion and damage-inducedvacancies form a distribution which ranges to 600 nm with a meanrange of about 310 nm.

The atoms struck directly by the iodine projectile are called pri-mary knock-on atoms (PKA), and if these particles gain with suffi-cient energy, they will permanently displace the atoms from theirlattice sites and produce point defects such as vacancy and intersti-tial. This collision cascade process creates numerous point defectsdistributed over a range of depths. Taking the displacement energy

of Ti, Si, C atom as 25, 15, 28 eV respectively, the computer simu-lation SRIM predicts that one iodine projectile can create 470 PKAand 16,200 vacancies. The ratio of created vacancies of Ti, Si and Cis about 1.73: 1: 0.78 (7980: 4610: 3610 in Fig. 8). Comparing tothe ideal ratio 3: 1: 2 of the MAX phase, Si atoms has a higher prob-ability to be displaced from their lattice position (as it has a smallerdisplacement energy) to form interstitial atoms and move out thedisplacement cascade region, and the ratio of Ti and C atoms re-main in the cascade region and it enhances the probability of form-ing the TiCx phase with x = 0.67–1. The out-diffusion Si atoms islikely to aggregate into clusters or form complexes with other de-fects, while the remaining atoms in the cascade region can berecrystallized into TiCx phase due to the thermal spike effects. Eventhough the Si atom has a larger sputtering coefficient compared toTi and C atoms, the influence of preferential sputtering is negligibleas a dose of 3 � 1015 ionscm�2 leads to the sputtering of only a2 nm surface layer.

Taking the density of Ti3SiC2 to be 8.303 � 1022 atoms cm�3, themean range of the damage distribution for 2 MeV Iodine incidentat 45� to the normal is 600 nm and for the irradiation dose3 � 1014 ions cm�2 it is estimated that the peak in the damage dis-tribution is approximately 1 dpa (displacement per atom). Thepoint defects will lead to microstress of the crystallites as observedexperimentally by the broadening of the diffraction peaks.

At 10 dpa damage level, i.e. the irradiation dose of 3 � 1015 -ions cm�2, the characteristic peaks of the MAX phase are retained inthe GIXRD patterns at incidence angles of 0.6� and 1.5�, revealing thatthe MAX-phase Ti3SiC2 has good radiation tolerance and some crystal

0 100 200 300 400 500 6000

10

20

30

40

50

60

70 Displacement Ti Vacancy Si Vacancy C Vacancy

Total number per I ionDisplacements: 16670PKA: 470 Vacancies: 16200Ti Vacancies: 7980Si Vacancies: 4610C Vacancies: 3610

Primary Knock-on Atom(PKA)

Dis

tribu

tion(

nm-1)

Target Depth (nm)

0

1

2

3

4

5

6

7

Distribution of I ion ( 10

-3nm-1)

I ion

Fig. 8. Distribution of damage and ion range given by SRIM code.

540 C. Liu et al. / Nuclear Instruments and Methods in Physics Research B 307 (2013) 536–540

structure remains despite this high level of damage. The abundantmobile vacancies induced by the displacement cascade will diffuseto trapping sites and form large vacancy and interstitial clusters.

A detailed analysis of the GIXRD and PBA results reveals that athigh irradiation dose levels, the damage formed is higher closer tothe surface than is predicted by the collision cascade programSRIM. This is unlikely to be caused by a gross error in the simula-tion but more likely to a higher than expected mobility for thevacancies and interstitials created by the collision cascade.

4. Conclusion

The ion beam induced damage of MAX-phase Ti3SiC2 producedby 2 MeV iodine irradiation has been investigated by GIXRD andPBA. Despite high irradiation doses leading to a predicted 10 dpa,the material still shows good radiation tolerance.

Under irradiation, the induced point defects will initially pro-duce microstress in the crystallites. At intermediate doses, the

damage will evolve and lead to the precipitation of the decom-posed phase TiCx. Further irradiation will lead to high levels of de-fects, which have been found to migrate towards the surfaceresulting in a highly damaged and amorphous surface layer ofabout 10 nm.

Acknowledgment

The authors will acknowledge the staff on the beamlineBL14B1 in SSRF. This work gained a support of the Australia/Chi-na International Linkage program (No. CH080126) of the Depart-ment of Industry, Innovation and Science Research, Australia.Our work was also supported by the National Nature ScienceFoundation of China (Grant No. 91126019) and the FundamentalResearch Funds for the Central Universities of China (Grant No.12CX04085A).

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