Surface and Coatings Technology 169–170(2003) 438–442

0257-8972/03/$ - see front matter� 2003 Elsevier Science B.V. All rights reserved.PII: S0257-8972Ž03.00185-3

The effect of voltage on microstructure of iron by nitrogen plasmaimmersion ion implantation

Yue Sun*, Jixin Chen, Xinxin Ma, Xingui Liu

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China

Abstract

The influence of the applied voltage on the microstructure of nitrogen implanted iron by plasma immersion ion implantation at360 8C was studied. Glancing X-ray diffraction shows that the amount of nitride decreases with increase of the applied voltage.In the surface layer of the iron samples,´ and Fe N nitrides are formed at the surface, andg9 nitrides are formed in the sub-2

layer of the surface. There is only a small fraction of´ nitride at the surface for the applied voltage up to 45 kV, which may bethe result of so-called ‘outer diffusion’ of nitrogen near the surface. The high-energy ion-bombardment might cause a high densityof defects at the sample surface, and thus the defects promote the nitrogen diffusion. As a result, the formation of ‘highconcentration’ nitride(such as´ phase) might be restrained. Even though a nitrided layer with the thickness of 150mm isobtained by our process owing to the high dose rate. The observation of scanning electron microscopy(SEM) shows that thethickness of the nitrided layer is not homogenous at the applied voltage less than 15 kV, which may result from the preferentialdiffusion of nitrogen along the grain boundaries. It is also found that long needle-likeg9 nitride appears as a parallel distributionin the diffusion zone. The surface SEM morphology at different voltages shows that the formation of nitrides in different zone isnot homogenous.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: Nitrogen; Plasma immersion ion implantation; Iron; X-ray diffraction; Structure

1. Introduction

Plasma immersion ion implantation(PIII) is a methodthat modifies the surface properties of the materials withthree-dimensional shapes so as to improve the wear andcorrosion resistancew1–4x. Nowadays, PIII are generallyperformed at elevated temperatures due to significantheating effect caused by ion impinging, which results indiffusion of the implanted species well beyond theimplantation range and increases the thickness of mod-ified layers. Up to now, the influences of the criticalprocess parameters such as applied pulse voltage andsample temperature on microstructure of implanted layerby PIII at elevated temperatures have not fully beenstudied w5–8x. Usually, increasing the applied voltage,the substrate temperature increases, and the ion energyalso increases. Both of the temperature and ion energy

*School of Materials Science and Engineering, Harbin Institute ofTechnology, P.O. Box 433, Harbin 150001, PR China. Tel.:q86-451-6413935; fax:q86-451-6413922.

E-mail address: hgdcysy@public.hr.hl.cn(Y. Sun).

will affect the microstructure. It is difficult to distinguishthe effects of the two factors.

In the present study, we concentrate on the influenceof the pulse voltage on the microstructure of ironsubjected to nitrogen PIII treatment. In order to avoidthe interference of other factors, the implantation doseand temperature were maintained constant on the whole.

2. Experimental details

The material used was iron(99.9%). Disk-tape spec-imens in diameter of 30 mm and in thickness of 5 mmwere cut from a bar. Before PIII treatment, the specimenswere surface ground, followed by electrolytic polishing.

PIII treatments were carried out in a DLZ-01 systemmade by Harbin Institute of Technology. The chamberwas filled with argon at the pressure of 0.5 Pa after itwas pumped down to base pressure below 5=10 Pa,y3

and the specimens were heated up to 3608C underargon bombardment environment at the working voltageto clean the surface. After reaching the treatment tem-perature, nitrogen was filled into the chamber to the

439Y. Sun et al. / Surface and Coatings Technology 169 –170 (2003) 438–442

Fig. 1. GXRD spectra of iron surface layer implanted with nitrogen ions at 15 kV, 3608C.

intended working pressure of 0.5 Pa. The plasma wasgenerated by 700 W power source with a radio frequen-cy of 13.56 MHz.

The treatment temperature of 3608C was maintainedby an adjustable thermal-resistant blocks under theworking stage, and the temperature was in situ detectedby a Thermalert GP infrared detector. The PIII treat-�

ments were carried out for 4 h with the high pulsevoltages of 15, 25, 35 and 45 kV, the average currentof 20, 17, 24, 15 mA, respectively, and the implanteddose were 2.9=10 , 2.5=10 , 3.5=10 , 2.2=1019 19 19 19

cm , respectively.y2

Glancing angle X-ray diffraction with Cu Ka radia-tion was used to determine the phase changes thatoccurred in surface layer. The microstructure in themodified layers were investigated using cross-sectionalphotographs by a scanning electron microscopy(SEM).

3. Results and discussion

3.1. Structural changes in the surface layer

Glancing X-ray diffraction(GXRD) pattern of mod-ified iron sample implanted at 15 kV at a low incidentangle of 18 showed that there is a nitride layer, whichmainly contains´ (Fe N), Fe N nitrides and some2–3 2

fraction of Fe O as shown in Fig. 1. At the incident3 4

angle larger than 38, a-Fe peaks can also be detected inthe pattern, and the peak intensity ofg9 (Fe N) nitride4

increases, which suggests thatg9 nitride mainly existsin the sub-surface at the iron substrate.

The modified sample of iron shows a difference inmicrostructure at different voltages. Fig. 2a is the GXRDpatterns of nitrided layer for the different applied volt-ages from 15 to 45 kV at the incident angle of 18. Itcan be seen that the nitrides mainly exist in the surfacelayer at the applied voltage less than 35 kV. It might becaused by the intensive ‘outer diffusion’ of nitrogen intothe implanted layerw9x. Therefore, the formation of‘high concentration’ nitride(such as phase) is possiblyrestrained. The high-energy nitrogen ion bombardmentinduced high density of defects, and the higher thevoltage, the higher the density of the defects, whichmaybe the answer to the result of phase composition inthe surface.

When the incident angle is 78, corresponding to thedeepest detect thickness in the XRD experimentation,the peaks ofa-Fe and g9 nitrides appear for all ofapplied voltages as shown in Fig. 2b. It indicates thatthe sub-surface mainly consist of these two phases.

3.2. Cross-sectional morphology of the modified surfacelayer

Fig. 3 shows the SEM photography representing themicrostructures of the nitrided layers at different implan-tation voltages. It is found that the microstructure ofnitrided layers is similar at the treatment temperature of360 8C. The thickness of nitrided layer decreases withvoltage increasing. The depth of nitrided layer contain-ing nitrides is more than 150mm in Fig. 3a, and it isonly approximately 50mm in Fig. 3d. The phase andFe N nitrides are difficult to be recognized from the2

440 Y. Sun et al. / Surface and Coatings Technology 169 –170 (2003) 438–442

Fig. 2. GXRD spectra of iron surface layer implanted with nitrogen ions at 15, 25, 35, 45 kV, glancing angle at(a) 18 and(b) 78.

SEM photography, but long needle-likeg9 nitrides canbe seen in the surface layer as shown in Fig. 3. Inaddition, the thickness of the nitrided layer is nothomogenous at the applied voltage of 15 kV, which mayresult from the preferential diffusion of nitrogen alongthe grain boundaries in the iron substrate.

In Fig. 3c and d, we can also see thatg9 nitridesprecipitated underside of surface nitrided layer in paralleland some fine nucleus ofg9 nitrides remained in exis-tence. It shows thatg9 nitrides formed nucleus prefer-entially in the high nitrogen concentration zone, such asexisted´ andg9 nitrides.

3.3. The morphology of the implanted surface

The surface morphology of nitrogen implanted surfaceis obviously different at different applied voltages. Evenat the same applied voltage, the surface morphology isnot homogenous on the modified surface as shown inFig. 4. The selective sputtering may occur at designatedcrystal surfaces with high index numbers having a lowbinding energy. Above results also corresponded to theexistence of nitrides in the near surface region of treateda-Fe. From the results of GXRD and SEM photographswe found that the nitrides can be formed at compara-

441Y. Sun et al. / Surface and Coatings Technology 169 –170 (2003) 438–442

Fig. 3. The microstructure of the nitrided layers at different implantation voltages:(a) 15 kV, (b) 25 kV, (c) 35 kV and(d) 45 kV.

Fig. 4. The surface morphology of implanted surface at different implantation voltages:(a) 15 kV, (b) 45 kV.

tively low applied voltage of 15 kV under the experi-mental conditions. The results of Fig. 4 strongly suggestthat there is a different sputtering rate between differentstructures.

4. Conclusions

The results of GXRD showed that the Fe N,´-Fe N2 2–3

andg9-Fe N were mainly formed in surface layer mod-4

ified by nitrogen PIII at the elevated temperatures of360 8C, and the sub-surface was a diffusion zoneconsisted ofg9 and solid solution of nitrogen ina-Fe.

The SEM analysis showed that the thickness of themodified layer decreased with pulse voltage enhancingat a certain temperature such as 3608C. The thicknessof nitrided layer can be greater than 150mm. In addition,at lower voltage of 15 kV, the thickness of the nitridedlayer is not homogenous, which may result from the

442 Y. Sun et al. / Surface and Coatings Technology 169 –170 (2003) 438–442

preferential diffusion of nitrogen along the grain bound-aries. It is also found that long needle-likeg9 nitridewith a parallel distribution in the diffusion zone.

Acknowledgments

The authors would like to express their gratitude forthe support of this work by Prof. Weidong Fei and DrWeili Li of Harbin Institute of Technology, PR China.

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