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Vacuum 75 (2004) 339–345

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doi:10.1016/j.vac

Kinetics of titanium nitride coatings deposited bythermo-reactive deposition technique

Ugur Sen*

Department of Metal Education, Technical Education Faculty, Sakarya University, Esentepe Campus, 54187 Sakarya, Turkey

Received 31 January 2004; received in revised form 7 April 2004; accepted 22 April 2004

Abstract

In this study, the growth kinetics of titanium nitride layer deposited on pre-nitrided AISI 1020 steel samples by

thermo-reactive diffusion (TRD) techniques in a solid medium was reported. Steel was at first tufftrided and then

titanium nitride coating treatment was performed in a powder mixture consisting of ferro-titanium, ammonium

chloride and alumina at 1173, 1223 and 1273K for 1–4 h. Titanium nitride layer thickness on the titanium nitride coated

AISI 1020 steel ranged from 5.5 to 19.2 mm depending on treatment time and temperature. Layer growth kinetics was

analyzed by measuring the depth of titanium nitride layer as a function of time and temperature. The kinetics equation

of the reaction has also been determined with Arhenius equation K ¼ Koexpð�Q=ðRTÞ: The result showed that the

diffusion coefficient (K) of the process increased with treatment temperature. Activation energy (Q) for TRD process

was calculated as 187.09 kJ/mol. The diffusion coefficients (K) changed between 6.637� 10�11 and 2.097� 10�10 cm2/s

depending on the process temperature.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Titanium nitride; Kinetic; TRD; Diffusion; Microstructure

1. Introduction

Hard coating with nitride, carbide or carboni-tride of transition metals is a common method ofimproving the wear resistance of ferrous materials.This can also be achieved by vapor depositionprocesses such as physical vapor deposition (PVD)and chemical vapor deposition (CVD), both ofwhich have their respective advantages. CVDusually involves high processing temperatures

2-574-7993; fax: +90-264-346-0262.

ss: ugursen@sakarya.edu.tr (U. Sen).

front matter r 2004 Elsevier Ltd. All rights reserv

uum.2004.04.003

(700–1200�C) in order to achieve deposition ofthe coating material. However, this high tempera-ture can lead to the heavy distortion of the treatedpart. PVD process, which is usually performed at200–500�C, well below the tempering range of toolsteels, requires expensive and complicated equip-ment. Due to the limited amount of diffusion thatoccurs during PVD process, adhesion strength ofthe coating layer is weaker than that of thermo-reactive diffusion (TRD) treatments [1].

Besides CVD and PVD techniques to depositTiN coating, TRD technique can also be utilizedfor titanium nitride coating on pre-nitrided iron

ed.

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Table 1

Technological parameters of titanium nitride coating process

Deposition parameters Value

Tufftride temperature (K) 853

Tufftride time (min) 120

Salt bath Commercial

Titanium nitride coating

temperature(K)

1173, 1223 and 1273

Titanium nitride deposition time (min) 60, 120, 180 and 240

Pack content (ferro-titanium/

ammonium chloride/alumina, by

weight)

2/1/2

Cooling time (min) 60

U. Sen / Vacuum 75 (2004) 339–345340

base alloys. Some surface treatment techniques canbe combined or performed one after another in theindustry. Boro-vanadizing, boro-chromizing [2],nitro-vanadizing [3] and nitro-titanizing are someof such treatments. There is a great deal of studyabout duplex treatment, which involves nitridingand titanium nitride coating successively [4–10].However, there is a lack of study on titaniumnitride coating by TRD on pre-nitrided steels inthe available literature [11].

The main objective of this study was to producetitanium nitride thin layer on the steel substrate bya two-step treatment. The first step is a tufftridingtreatment for producing iron nitride phases andsolid solution of nitrogen on the substrate. Thesecond step is a titanizing treatment for producingtitanium nitride phases on the pre-nitrided steel.This technique was named as nitro-titanizingtreatment in this study. Tufftriding process is asoft salt bath nitriding process for iron-basedmaterial [12]. Unlike conventional diffusion meth-ods whereby specific elements (carbon and nitro-gen) in a treating agent diffuse into the substratefor hardening, the TRD method results in anintentional build up of a coating layer on thesubstrate [13–15].

2. Experimental procedure

AISI 1020 steel consisting of 0.227% C, 0.2%Si, 0.64% Mn, 0.024% P, 0.031% S, 0.029% Cr,0.021% Mo, 0.027% Ni and 0.001%V wastufftrided in the salt bath. Then titanium nitridecoating was performed on the tufftrided sample byTRD process. The TRD process was performedutilizing a pack box containing ferro-titanium,ammonium chloride and alumina powders, in thehigh temperature tube furnace cleaned by vacuumand then argon gases. Values of the major processparameters are presented in Table 1.

The commercial salt bath and commercialpowders were used for tufftriding treatment andtitanium nitride coating treatment. The substratewas AISI 1020 plain carbon steel in disk shape(diameter 20� 5mm, ground with 1200 grid emerypaper). The preparation of the substrates for theprocess involved cleaning ultrasonically with

acetone and ethyl alcohol. Firstly, the steel samplewas tufftrided in the salt bath at 853K for 2 h.Nitrided samples were cleaned in boiling waterfrom the salt remainder coming from tufftridingtreatment and samples were then cleaned ultra-sonically with acetone and ethyl alcohol followedby grinding with 1200 grid emery paper for a shortperiod of time (2–3 s). Finally, titanium nitridecoating was performed by TRD process, theparameters of which are given in Table 1.

The equipment used in analysing is as follows:

* The morphology of titanium nitride coatinglayer was investigated by B O71 optical micro-scopy and scanning electron microscopy-backscattered electron image (SEM-BEI). The depthof coating layer was measured by an opticalmicrometer attached to the optical microscope.

* Microhardness measurements were performedunder the load of 25 g.

* The chemical analysis of boride layer andtitanium nitride coating was determined by PhilipsX-ray diffractometer (with CuKa radiation).

* Contour diagrams treatment time and tempera-ture for titanium nitride coating layer wereconstructed by using SigmaPlot 7.0 software.

3. Result and discussion

3.1. Properties of titanium nitride layer

Both optical and SEM cross-sectional examina-tions of titanium nitride coated AISI 1020 steel

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revealed that titanium nitride layer formed on thesubstrate has a denticular morphology, as shownin Figs. 1–2. Titanium nitride layer is dense,compact and porosity free. On the cross-sectionof titanium nitride coated AISI 1020 steel surfaces,there were three distinct regions at higher magni-fications. These are (i) a surface layer consisting ofTiN and Ti2N type coating layer the phases ofwhich were confirmed by X-ray diffraction analy-sis, as shown in Fig. 3. Some earlier studies showedthat CVD [16,17] and other TiN coating techni-ques [18] caused the formation of TiN and Ti2Nphases in the titanium nitride layer which is also

Fig. 1. Optical microstructure of titanium nitride coated AISI

1020 steel at 900�C for 2 h.

Fig. 2. SEM-BEI of titanium nitride coated AISI 1020 steel at

900�C for 2 h.

the use in this present study (Fig. 3). (ii) Transitionzone consisting of nitrogen which came from pre-nitriding treatment, as shown in Figs. 1–2, (whitezone) beneath titanium nitride layer and (iii) steelmatrix which is not affected by titanium andnitrogen.

The Vickers microhardness of nitrided steelsurfaces and the titanium nitride layer weremeasured to be 520756 and 14507223HV,respectively. Microhardness testing is an indenta-tion method for measuring the hardness of amaterial on a microscopic scale. A precisiondiamond indenter is impressed into the materialat loads from 10 to 1000 gf. The impression length,measured microscopically, and the test load areused to calculate a hardness value. The hardnessvalues of some surface treatments are given inTable 2. The hardness of titanium nitride layerprovides an extremely hard surface compared withthe traditional chromium plating, nitriding, car-burizing and carbo-nitriding [10]. It is believedthat the high hardness of titanium nitride layerformed on the pre-nitrided steel is due to theformation of non-oxide ceramic coating layerconsisting of TiN and Ti2N phases. These com-pounds make the surface of the titanium nitridecoated steels resist against chemical effects, ad-hesive, abrasive and erosive wear [20].

3.2. Titanium nitride layer growth kinetics

Depending on the treatment time and tempera-ture, the thickness of titanium nitride layerchanged between 5.5 and 19.2 mm. Assuming that:(i) the rate of titanium nitride layer growth iscontrolled by titanium diffusion rate in the TiNand Ti2N sub-layers by nitrogen diffusion ratefrom pre-nitrided steel sample, (ii) titanium nitridelayer growth occurs perpendicular to the pre-nitrided substrate as a consequence of titaniumdiffusion steel sample. Titanium nitride layerthickness varies parabolic law with time accordingto the following equation (see Fig. 4):

d2 ¼ Kt; ð1Þ

where d is the titanium nitride layer thickness(cm), t is the treatment time (s) and K is thediffusion coefficient (cm2/s). Titanium diffusion in

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Fig. 3. X-ray diffraction pattern of titanium nitride layer formed on the steel substrate at 1273K for 2 h.

Fig. 4. Titanium nitride layer thickness formed on steel as a

function of treatment time and temperature.

Table 2

Hardness of some surface treatments

Materials Hardness (HV)

Boronized steel 1600–1900 [2]

Nitrided steel 650–1700 [2]

Carburized low alloy steel 650–950 [2]

Hard chromium coating 1000–1200 [2]

Plasma nitrided AISI H13 steel 1387[8]

PVD TiN coating 2498–2509 [8]

CVD TiN coating 1320–1750 [15]

HCD–IP TiN coating 1490–3360 [19]

Nitro-titanizing (present study) 14507223

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the titanium nitride layer is the primary factoraffecting the coating layer thickness. As shown inFig. 4, the higher the treatment temperature, thelonger the treatment time and the thicker thetitanium nitride layer became. Furthermore, acontour diagram derived from Fig. 4 by means ofSiqmaPlot.7 software is to show the processparameters (treatment time and temperature) forpre-determined layer thickness for industrial ap-plications. This diagram shows that the treatmenttemperature and time for a pre-determined layerthickness can be clearly predicted to that of

experimental study (Fig. 5). The plot of the squareof the layer thickness (d2) versus treatment time (t)was shown to be linear in Fig. 6. The diffusioncoefficient values (growth rate constant), K ; of thetitanium nitride layers were calculated from theslopes of the plots. Diffusion coefficient ordiffusivity (K) is a temperature dependent coeffi-cient related to the rate of which atoms, ions orother species diffuse. The diffusion coefficient

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Fig. 5. Contour diagram of titanium nitride layer thickness

(mm) of coated AISI 1020 steel.

Fig. 6. Square of the layer thickness, d; versus titanium nitride

coating time, t:

Fig. 7. LnK versus 1=T for titanium nitride coated AISI 1020

steel.

U. Sen / Vacuum 75 (2004) 339–345 343

depends on temperature, the composition andmicrostructure of the host material and alsoconcentration of diffusing species [21]. The rela-tionship between the diffusion coefficients, K ;activation energy, Q; and the process temperaturein Kelvin, T ; can be expressed using Arheniusequation [22]:

K ¼ Ko exp �Q

RT

� �; ð2Þ

where Ko is the frequency factor (pre-exponentialconstant) and R is the gas constant. Eq. (3) wasexpressed from the natural logarithm of the Eq. (2)as follows:

ln K ¼ ln Ko �Q

RT: ð3Þ

The graph of ln K versus reciprocal treatmenttemperature is thus shown to be linear in Fig. 7.The activation energy, Q; was calculated by theslope of the plot (ln K � T�1) in Fig. 7.

The results showed that K increased withincreasing treatment temperature. Activation en-ergy (Q) for titanium nitride coating was deter-mined as 181.57 kJ/mol. Activation energy is anenergy required to cause a particular reaction tooccur. In diffusion, the activation energy is relatedto the energy required to move an atom from onelattice site to another. The atom is originally in alow-energy, relatively stable location. In order tomove to a new location the atom must pass overan energy barrier. The energy barrier is theactivation energy. Heat supplies the atom withthe energy needed to exceed this barrier [21]. Thediffusion coefficient (K) ranged from 6.637� 10�11

to 2.097� 10�10 cm2/s. The derived formulas fromthe slopes of the plots in graph showing therelation between d2 and t are as follows:

K ¼ 944 849� 10�3eð�21 850=TÞ; ð4Þ

where K is the diffusion coefficient and T is thetemperature (K). The practical formula, for

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Table 3

Kinetic data for some surface treatments

Treatment Diffusion coefficient (m2/s) Activation energy (kJ/mol)

Carburized steels 1.6� 10�10–5� 10�10 [23] 122.68 [23]

Nitrogen diffusion (in a iron at 575�C) 1.2� 10�10 [24,25] 76.2 [24,25]

Boronizing 3� 10�9 [2] 218.4 [2]

LPCVD TiN coating — 61–78.9 [26]

CVD TiN coating — 96.3 [15]

Conventional CVD TiN coating — 24–55 [27]

Laser CVD TiN coating — 46.8 [27]

LCVD TiN coating (on Ti-6Al-4V substrate) — 65.2 [27]

Nitro-titanizing (present study) 6.64� 10�15–2.1� 10�14 187.09

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calculating the layer thickness (mm) for can bederived from Eq. (1) and Eq. (4) as follows:

d ¼ 58 322� 104ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiteð�21 850=TÞ

p: ð5Þ

The titanium nitride layer thickness obtainedfrom the experimental study is very close to thatderived from Eq. (5). Comparing both results givesthe reliable results for practical applications. Somediffusion data for traditional surface treatmentand titanium nitride coatings are given in Table 3.

4. Conclusion

1. Titanium nitride coating can be formed on thepre-nitrided steel sample by TRD method.

2. The coating layer has denticular, dense andporosity-free morphology.

3. The longer the treatment time, the higher thetreatment temperature and the thicker thetitanium nitride layer became.

4. Titanium nitride layer thickness on the titaniumnitride coated AISI 1020 steel ranged from 5.5to 19.2 mm depending on treatment time andtemperature.

5. The Vickers microhardness of nitrided steelsurfaces and the titanium nitride layer weremeasured to be 520756 and 14507223HV,respectively.

6. The activation energy of the titanium nitrideformation in TRD method is 187.09 kJ/mol.

7. An equation was established for calculatingdiffusion coating layer (Eq. (5)).

Acknowledgements

Author of this work thanks Dr. Sakip koksal,Adem Coskun (B.Sc.) and Bilal Inan (B.Sc.) fromTechnical Education Faculty of Sakarya Univer-sity for laboratory testing.

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