Received 20 October 2006, in final form 24 November 2006Published 19 January 2007Online at stacks.iop.org/JPhysD/40/769
AbstractThe room temperature nitriding of titanium is accomplished by utilizingnitrogen ion beams delivered by a 2.3 kJ plasma focus discharge. Titaniumsamples are exposed to ions at different axial positions (3, 5, 7 and 9 cmfrom the focus) in order to correlate their surface properties with ion beamparameters such as energy, number density, current density and energy flux(energy deliverance per unit time per unit volume). A BPX65 photodiodedetector is employed to measure the ion beam parameters by using the timeof flight technique. X-ray diffraction analysis as well as field emissionscanning electron microscopy along with the energy dispersive x-rayspectroscopy is carried out to explore the structural, morphological andcompositional profiles of the treated samples. The results demonstrate theformation of nanocrystalline TiN thin film with surface features stronglydependent on ion beam energy flux. A Vickers microindentationmeasurement reveals that the surface hardness is improved 4–5 times fortypical nitrided samples.
(Some figures in this article are in colour only in the electronic version)
1. Introduction
The dense plasma focus (DPF) is a pulsating discharge thatmakes efficient use of a self-generated magnetic field to forma very high density (1025–1026 m−3) and high temperature(∼1–2 keV) short duration (∼10−7 s) plasma column [1]. Itis a potential source of relativistic electrons, ions, neutronsand x-rays over a broad energy spectrum [2–6]. The DPFdevices operating from a few kJ to MJ energy emit ionsof characteristic energy from hundreds of keV to tens ofMeV [7, 8]. Recently, the ion beams emitted from DPFdevices have been successfully employed in plasma assistedsurface modifications including ion implantation, thermalsurface treatment, phase changes of thin film and thin filmdeposition [9–12]. Plasma focus based deposition of TiNthin films on titanium significantly improves its surfaceproperties such as wear and corrosion resistance and hardness,making it attractive in many industrial applications includingaeronautical, biomedical and nuclear fields. Moreover, TiN
thin films find prospective applications in the machine industryas hard and wear resistant coatings on the mechanical parts ofmachine tools [13]. The TiN thin film deposition is executedby employing various techniques such as chemical vapourdeposition [14], sputtering deposition [15], arc evaporation[16] and pulsed laser deposition [17]. But the plasma focusassisted deposition of TiN layer possesses superior qualitiesincluding good adhesion to the substrate owing to the broadspectrum of ion energy and growth in room temperatureenvironment. The active species including ions and radicalsof focused plasma assist the growth of films composed ofnanoparticles with features quite different from those ofthe bulk substrate [18]. Therefore, ion–surface interactionsdemand a careful investigation of the ion beam parameters inorder to understand the mechanisms of their production and theassociated ion induced surface changes [19–25]. Temporalinvestigations of ions streams emitted from the DPF havebeen reported by employing various diagnostics such as themagnetic energy analyzer, photoconductive GaAs detector,
Thomson parabola spectrometer and biased ion collector(BIC) [3, 26–29]. A photodiode detector having nanosecondresponse can be advantageous to characterize the ion beamwith better temporal resolution.
This paper reports the plasma focus assisted depositionof TiN thin film on titanium at different axial positions. Themicrostructural, morphological and compositional profiles ofthe film are correlated with the ion beam parameters evaluatedby using a BPX65 diode in the ambient gas pressure of thenormal device operation. Though this ambient gas pressuremay cause the energy loss of the ions, the major idea is toinvestigate the ion streams by placing the detectors in thesample treatment environment and to analyse the behaviourof the ion-assisted deposition of TiN film on titanium interms of ion parameters. Various studies on the measurementof ion beam parameters have been conducted to exploredifferent mechanisms leading to the emission and accelerationof charged species [5,19], but very little is reported on the directcorrelation of ion beam parameters with thin film propertiesincluding surface morphology, elemental composition anddistribution, microstructure and microhardness. The BPX65photodiode is employed for the first time to characterize the ionbeams and the results are compared with the existing scalinglaw, for the present DPF device, which reasonably agree withthe other investigations of low energy devices. The growth andthe quality of the film are discussed in terms of an accumulativeion beam parameter, that is energy flux (energy deliverance perunit time per unit volume).
2. Experimental
A Mather type DPF device [30] powered by a single Maxwell32 µF, 15 kV capacitor is used as a source of energetic nitrogenions for the treatment of titanium samples and is shown infigure 1. The electrode system comprises a copper rod of152 mm length and 18 mm diameter serving as the anodesurrounded by six copper rods each having a diameter of 9 mm,serving as the cathode. A Pyrex glass insulator sleeve ofexternal diameter 24.5 mm, wall thickness 2 mm and effectivelength 25 mm is fitted in between the electrodes to support thelow inductance breakdown. The tip of the anode is engraveddown to a depth of 20 mm in order to reduce the impuritiesintroduced from the anode tip [2]. The discharge chamberis evacuated down to 10−2 mbar by a rotary vane pump andfilled with N2 at an optimum pressure of 1.25 mbar. A resistordivider is used as a high voltage probe to monitor the focusingof plasma and a Rogowski coil to record the current waveform.A peak discharge current of 190–200 kA is recorded by usinga TDS 500 MHz Digital Phosphor Oscilloscope. The totalparasitic inductance of the system is about 80 nH. Whenthe capacitor voltage is applied across the electrodes, gasbreakdown occurs. The resultant current sheath is acceleratedaxially up the chamber in the shape of a parabolic by the
−→J ×−→
B
force and undergoes radial collapse on reaching the top of theanode. Thus, an extremely hot/dense pinched plasma columnis formed with the ion density 1025–1026 m−3, which is thendisrupted as a result of instabilities producing energetic ionsmoving towards the top of the chamber owing to the intenseelectric fields [31]. These energetic ions are characterized
Figure 1. Schematic diagram of the DPF device used for theexperiment.
- 45 V
0.47 µ
56 Ω
50 Ω Coaxial cable
BPX 65 Photodiode
To
Oscilloscope
10 k
Ω
F
Figure 2. Schematic arrangement of a BPX65 photodiode withbiasing circuit used in the experiment.
by employing a BPX65 diode detector and then used for thenitriding of titanium.
Figure 2 depicts the schematic of the biasing network forthe BPX65 diode detector. The protective glass of the diodeis removed enabling the ions and soft x-rays photons to reachits active area. The diode has a response time of 12 ns with anactive layer of thickness 10 µm. A 600 µm diameter aperturewith an area of 0.28 mm2 limits the ion beam flux striking thedetection area of the diode. The ion induced electrical signalis coupled to the oscilloscope via a 50 coaxial transmissionline.
The TOF technique [32] is employed for the evaluationof ion parameters. The nitrogen ion velocity is estimatedby taking the ratio of the distance to the flight time of ionsfrom the source to the detector. The flight time of the ions isevaluated by correlating the ion beam pulse with the x-ray pulseemitted due to the intense electric fields at the time of maximumcompression during the pinch phase. The ion velocity v, thus
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Nitriding of titanium by using an ion beam
estimated, is used in the calculation of the energy of the ionsreaching the detector at different instants of time by using theexpression
E = 1
2mv2, (1)
where m is the atomic mass of nitrogen ions.The ion number density Nd having velocity v and charge
q can be written as
Nd = V
RqAv, (2)
where V is the maximum voltage of the ion pulse developedacross the resistor R and A is the area of the entrance window.The ion beam current density J is also estimated from theBPX65 diode signals using the simple expression
J = I
A, (3)
where I = dQ
dtis the ion current and Q = ∫
VR
dt is the chargecollected at the collector.
To check the efficacy of the results evaluated by thediode detector, the ion beam parameters are also evaluated byemploying the spectral law [24]
dN
dE= αE−k, (4)
where N is the number of ions having the energy E of afew keV to several MeV with the exponent k in the range2–5 and the proportionality constant α = 1.12 × 1019 a.u..Different values of k have been tried for the estimation of theion number density in the probable energy range. For k = 5,the numerical results reasonably agree with the experiment.Thus, equation (4) yields the number of ions as
N(E) = α
4[E−4
1 − E−42 ], (5)
where E1 and E2 are the low and high energy limitscorresponding to an energy interval. The mean energy perion E is calculated by the equation as
E = 1
N
∫ N(E2)
N(E1)
E(N) dN. (6)
The energy flux (energy deliverance per unit volume per unittime) delivered to the sample by the ions with a certain energyinterval is calculated [33] using the equation
Q = NdE
τ= NE
τhσl
, (7)
where Nd = Nhσl
is the ion number density, N is the number
of ions, E is the mean energy per ion, τ represents the timeof interactions between the ions of this energy interval and thetarget, h is the mean layer thickness, which is estimated withthe help of the ion penetration range using the SRIM code [34],and σl is the cross-section of the conic beam at a distance l fromthe focus given as σl = π(l tan 20 )2.
These characteristic ions are then utilized for the surfacenitriding of commercially available pure titanium. Titanium
samples of 9 × 9 × 4 mm3 dimensions are mechanicallypolished, cleaned with acetone and an ultrasonic water bath.The samples are then placed above the anode tip at differentaxial positions (3, 5, 7, and 9 cm) with the help of an axiallymoveable holder and are irradiated for 30 focus shots afterproper focusing.
Sufficient energy deliverance by the most energetic ionsapproaching the titanium substrate first, increases its surfacetemperature causing local melting and evaporation. As aresult, the evaporated substrate material may chemically reactwith the subsequent ions to form a TiN compound, whichis deposited again on the surface of the titanium [35]. Thisinitiates the nucleation (crystal growth) of a compound layeron the surface of the substrate due to sublimation, encounteringvarious mechanisms.
The crystallographic structures of the deposited film havebeen investigated to explore the ion induced changes onthe substrate by employing an X’Pert PRO MPD θ–θ x-raydiffractometer operated at a voltage of 40 kV and a current of40 mA using a Cu-Kα (λ = 1.544 Å) radiation source. Surfacemorphology, elemental composition and elemental distributionin the deposited film have been demonstrated with the help ofa field emission JSM-6700F SEM provided with the OxfordInstr. UK EDS attachment in order to reveal the qualitative,quantitative and spatial profiles of the film. Microindentationmeasurements were performed to test the mechanical strengthof the nitrided titanium using a Wilson Wolpert 401MVAVickers microhardness tester.
3. Results and discussion
Figure 3 shows a typical ion beam signal along with the highvoltage probe signal recorded with the BPX65 diode detectorplaced at the axial distance of 7 cm. The first peak in theBPX65 diode signal corresponds to the x-rays pulse owing tothe high sensitivity of the diode for x-rays, while the secondone is the desired pulse of the ion beam generated at the timeof x-rays emission. Such ion beam pulses have already beenreported in the literature [8, 18]. It is a well-established factthat the acceleration of the ion beam and relativistic electronstakes place at the moment of onset of the x-ray emission.Thus, the flight time of the ion beam for a fixed distanceof the diode detector from the ion source is determined bymeasuring the time delay between a point on the x-ray signaland the respective point on the ion beam signal. The ion beamenergy estimated from equation (1) for this typical ion pulseis distributed from 30 to 558 keV. The ion number density isestimated from equation (2) and is found to be distributedfrom 3.2 × 1013 to 7.3 × 1013 cm−3 corresponding to theion energies 558–30 keV. At each axial position, ion beamsignals with five shots are recorded to estimate the averagevalues of ion beam parameters and the standard deviation ofdata.
Figure 4 plots the ion energy estimated by the diode as afunction of the ion evolution time. Since the ion accelerationtakes place at the time of maximum radial compression, theenergy gained by the multi-charged ion beam may depend onthe charge state of the ions. After the compression phase ofpinched plasma, the most energetic ions approach the detector
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M Hassan et al
Figure 3. Typical ion beam signal recorded by the BPX65photodiode.
40 60 80 100 120 140
100
200
300
400
500
600 Ion energy
Ion
en
erg
y (k
eV)
Time evolution (nsec)
Figure 4. Ion energy (keV) as a function of time evolution (ns)estimated with the BPX65 photodiode.
in about 30–45 ns, whereas the probable pulse duration is foundto be ∼140 ns.
The distribution of ion number density as a function of ionenergy estimated from the typical signal of the BPX65 diodeis presented in figure 5. It is obvious from the energy spectrumthat the ion energy ranges from 30 to 550 keV, and the largestcontribution to the spectrum is made by the 30–200 keV ions.
Figure 6 presents the axial variation of the ion energycorresponding to the maximum ion number density evaluatedby the BPX65 diode detector. The maximum value of ionnumber density estimated by the diode varies from 10.1×1013
to 4.3 × 1013 cm−3 corresponding to the probable ion energyranging from 82 to 57 keV when the detector is displaced from3 to 9 cm axial position. The probable energy of the ion beamdecreases more slowly as compared with the correspondingnumber density when measured along the axis.
Figure 7 presents the ion current density estimated bythe BPX65 diode placed at different axial positions. The ion
100 200 300 400 500 6002
3
4
5
6
7
8
Ion
num
ber
dens
ity
(x10
13cm
-3)
Ion number density
Ion energy (keV)
Figure 5. Distribution of ion number density (cm −3) as a functionof ion energy (keV) estimated with the BPX65 photodiode.
3 4 5 6 7 8 930
45
60
75
90
105
Axial position (cm)
Ion energy
Max
. io
n n
umbe
r d
ensi
ty (1
013cm
-3)
Ion
en
erg
y (k
eV)
3
4
5
6
7
8
9
10
11
Max. ion number density
Figure 6. Ion energy (keV) and the corresponding maximum ionnumber density (cm −3) as a function of axial positions (cm)evaluated with the BPX65 photodiode.
current density decreases with the increase in axial distanceowing to the lateral spreading of the ion beam due to scatteringas well as the energy attenuation by the ambient gas pressure(1.25 mbar) of the normal device operation. The ion currentdensity decreases from 1349 to 1059 A cm−2 as the diode ismoved from 3 to 9 cm axial position.
To test the validity of BPX65 diode measurements, theion parameters are evaluated by using the scaling law, whichis discussed as follows: for the probable energy interval of30–558 keV, the number of ions N(E) evaluated by usingequation (5) is 3.45 × 1012. From equation (6), the meanenergy per ion E turns out to be 40 keV for the aboveenergy interval. At the axial distance of l = 7 cm, thebeam cross-section is σl = 20.38 cm2. Substituting h =0.66 µm for titanium (using the SRIM code), the ion numberdensity Nd is also evaluated, which is found to be 2.56 ×1015 cm−3 corresponding to the above energy interval. Usingequation (7), the energy flux Q delivered to the titanium
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Nitriding of titanium by using an ion beam
3 4 5 6 7 8 91000
1100
1200
1300
1400 Ion current density
Ion
curr
ent
dens
ity
(A c
m-2
)
Axial position (cm)
Figure 7. The ion current density (A cm−2) as a function of axialpositions (cm) recorded with the BPX65 photodiode.
sample is estimated as follows: substituting the mean energyE = 40 keV, τ = 140 ns, the energy flux delivered to thetitanium is 7.32 × 1014 keV cm−3 ns−1. The diode basedmeasurements of ion beam parameters are consistent with theother investigations of low energy DPF devices [8, 27, 29, 33]and fit well into the existing scaling law.
The x-ray diffraction (XRD) spectra presented in figure 8demonstrate the axial dependence of the ion induced structuralchanges on the titanium surface exposed for 30 shots. Thecharacteristic peaks of TiN in the XRD spectra confirm thedeposition of TiN film on the sample surfaces treated at 5, 7 and9 cm axial positions. At these axial positions, the diffractionpeak of Ti(0 0 2) plane reflection belonging to the substratedisappears along with the appearance of new peaks at 2θ valuesof 36.91 and 43.04 corresponding to TiN(1 1 1) and Ti(2 0 0)plane reflections, respectively. A significant broadening ofall the diffraction peaks of Ti reveals the degradation ofcrystallinity of the substrate surface due to ion bombardment.The relative intensity of the peaks of TiN is maximum forthe sample treated at 7 cm axial position indicating the higherrelative proportion of the compound layer. The maximumion number density reaching the sample is 5.9 × 1013 cm−3,having the probable energy of 64 keV as estimated from thephotodiode measurements. The ion current density estimatedat this axial position is 1142 A cm−2. The energy flux of theion beam at this position is 2.69 × 1013 keV cm−3 ns−1.
The high energy high density ion beams of very short(∼140 ns) pulse duration cause melting and subsequent rapidcooling of the substrate surface. Eventually, the original phasesof the substrate recrystallize with the development of a newphase at the sample surface during the molten state [36], whichis more likely to occur due to the DPF ion induced stress effects.
The crystalline size of the TiN film deposited at 7 cmaxial position is measured from the diffraction peak broadeningusing the Scherrer’s formula [37]:
grain size = Kλ
β cos θ,
where K = 0.99 is the numerical constant, λ = 1.54 Å isthe wavelength of x-ray source, β is the broadening (rad) of
the diffraction peak and θ is the Bragg’s angle. For the typicaldiffraction peak of TiN at the 2θ value of 36.91 correspondingto the (1 1 1) plane reflection having β = 0.0017 (rad), theestimated crystalline size is found to be 90 ± 5 nm, which isof the order of nanoscale features obvious from the scanningelectron microscopy (SEM) images.
Table 1 presents the XRD data showing the relativeintensities and broadening (FWHM) of the typical peaks ofTiN corresponding to (1 1 1) and (2 0 0) plane reflections,respectively, when the samples are exposed at different axialpositions. An increase in the broadening of diffraction peaksat higher axial positions depicts the deposition of a TiN filmwith smaller grain sizes. The crystalline size is dependent onthe ion beam energy flux, and hence the resultant substratetemperature. Usually, smaller grain sizes are fostered at lowersubstrate temperatures appearing at distant axial positions.
Figure 9 displays the SEM micrographs of the titaniumsample surfaces treated at the axial positions of 3 cm,5 cm, 7 cm and 9 cm, respectively. The micrograph of thesample treated at 3 cm (figure 9(a)) exhibits non-uniformsurface features. A significant radiation damage (surfaceswelling) appearing on the substrate surface without anymicrocrystalline pattern can be attributed to the higher energyflux of 5.91 × 1013 keV cm−3 ns−1 of the ion beam at this axialdistance. However, the subsequent ions having lower energyflux find insufficient time to react with the vaporized substratematerial to form TiN. At 5 cm axial position (figure 9(b)), thesurface roughness is decreased due to a comparatively low ionenergy flux of 3.94 × 1013 keV cm−3 ns−1. Noticeably, it ishere that the onset of the coagulation of nanocrystalline surfacefeatures occurs. The energy flux of the ion beam may ablateand vaporize the substrate, which in turn gets sufficient time toreact with the subsequent ions, thus forming the TiN layer. Acareful observation reveals the beginning of pattern formationof the TiN compound layer on the sample surface, which isalso supported by the XRD spectrum at 5 cm. At 7 cm axial
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M Hassan et al
Table 1. XRD data showing the relative intensity (%) and the broadening (2θ0) of the typical peaks of TiN film deposited on titanium atdifferent axial positions.
3 cm No peak No peak No peak No peak5 cm 6.72 0.1424 No peak No peak7 cm 52.92 0.1968 26.94 0.47239 cm 17.61 0.2362 6.33 0.6298
Figure 9. Typical SEM micrographs of TiN films deposited at different axial positions: (a) 3 cm, ×40 000, (b) 5 cm, ×40 000, (c) 7 cm,×10 000 and (d) 9 cm, ×20 000.
distance (figure 9(c)), the muscular coagulation of the surfacefeatures occurs giving rise to the TiN film petal-like surfacemorphology. The TiN film is thick and uniform with respectto the appearance of the surface features. The rich coagulationpattern demonstrates the deposition of TiN layer with a distinctnanocrystalline structure, which is also recognized by themaximum relative intensity of the diffraction peaks in the XRDspectrum at 7 cm. This axial position bombarded with the ionenergy flux of 2.69 × 1013 keV cm−3 ns−1 is found to be theoptimum one offering the best quality of the TiN film depositedwith the present device. A smoother but microporous filmsurface is grown at 9 cm axial distance (figure 9(d)) treated withthe ion energy flux of 1.75 × 1013 keV cm−3 ns−1. The petal-like features exist but their boundaries are not well distinctive.The porous nature of the film is owing to the large spread ofthe ion beam at this axial position. Such randomly scatteredmicropores of varying sizes may be attributed to the anisotropicbehaviour of the ion beam flux.
Figure 10 demonstrates the surface morphology of theTiN film with different magnifications, deposited at the axialposition of 7 cm. The film has petal-like microcrystallinefeatures, which are spatially uniform and rich (figure 10(b)).These multidimensional petals of about 1 µm diameter arecomposed of nanocrystalline tubular structures having very
little size dispersion. These tubular structures thermallycoagulate due to successive focus shots forming petal-likefeatures, which are linked together in a network forming aTiN layer. The nanostructure pattern of the ion-bombardedtitanium substrate is developed from nucleates of a few nmsize. These nucleates are grown by the ion induced collisioncascades extending upto a few tens of nm range for ionenergies of ∼a few tens of keV to MeV. A sufficient amountof energy is transferred to the substrate during ion-surfaceinteractions, via electronic excitations. Eventually collisioncascades occur within the substrate leading to the developmentof unusual defects and nanostructures with uniqueproperties [38].
The kinetics of nucleation may be understood on amicroscopic scale as follows: by increasing the number offocus shots, the film growth proceeds through nucleation andgrowth stages, with the monolayer thick islands nucleatedfirst. A ledge, or step on the substrate surface captures thearriving atoms or ions within a certain zone on either side of thestep. Individual steps on the surface grow with the successivefocus shots filling the voids and channels in between, andeventually the growth decelerates. In general, rough surfacesgrow faster than smooth surfaces, so that the final growth formconsists entirely of slow growing faces. At low substrate
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Nitriding of titanium by using an ion beam
Figure 10. Typical SEM micrographs of (a) titanium substrate at x1000 and TiN film deposited at 7 cm at (b) ×10 000, (c) ×20 000 and(d) ×50 000 magnifications.
Table 2. EDS data showing the nitrogen and titanium concentration(at. % and wt. %) in nitrided titanium at different axial positions.
Axial position Nitrogen content Nitrogen content(at.%) (wt%)
Z = 3 cm 23.07 9.76 ± 3.42Z = 5 cm 18.88 8.11 ± 2.83Z = 7 cm 35.45 13.84 ± 3.97Z = 9 cm 1.98 0.59 ± 0.20
temperature, the surfaces are almost smooth [39]. As thetemperature is raised, the surface becomes rougher. Thisis also obvious from the rough surface features appearingin the samples treated with the ions of higher energy fluxcausing a sufficient temperature rise at 3 and 5 cm axialpositions.
The energy dispersive x-ray spectroscopy (EDS) datashowing the nitrogen concentration on titanium surface treatedat various axial positions is presented in table 2. Themaximum nitrogen content is estimated to be 35.45 at.% and13.84 ± 3.97 wt% in the surface of the TiN film deposited at7 cm axial position. At this axial position, the energy flux isenough to deposit a TiN compound layer with the maximumrelative proportion on the substrate. This is quite consistentwith the XRD results providing maximum relative intensityof the diffraction peaks corresponding to the TiN film at thisposition, and hence the maximum relative proportion of thecompound layer. At lower axial distances, the energy flux is toohigh causing surface damage due to sputtering, thus providinga small fraction of the compound. Whereas at the extremeposition (9 cm), the energy flux is rendered insufficient due toattenuation, thus providing a less compositional profile of theTiN film.
Figure 11 presents the SEM image along with EDSmaps of the TiN film deposited at 7 cm axial position. The
EDS maps exhibit the distribution of elements present in thedeposited layer using the imaging energy filters. Figure 11(a)shows the SEM image used for the selection of mappedarea, figure 11(b) demonstrates the TiKα map providing theelemental distribution of titanium and figure 11(c) shows theNKα map presenting the elemental distribution of nitrogen onthe sample surface. The intensity of the TiKα and NKα in themaps is a qualitative measure of the elemental concentration.A homogeneous spatial distribution of the NKα intensity overthe sample surface indicates the uniform deposition of the TiNfilm.
Figure 12 shows the surface hardness (MPa) as a functionof the indentation depth (µm) with different test loads (10, 25,50, 100, 200, 300 and 500 g) on the sample surfaces treatedat various axial positions. The maximum surface hardnessis found for the TiN film deposited at 7 cm axial position.The typical microhardness of the TiN layer is 7650 ± 10 MPacorresponding to the indentation depth of 1 µm. A steepfall of the microhardness values in the near surface region ofthe sample treated at 7 cm suggests a concentration gradienttowards the bulk. However, the improved microhardness uptoa few µm depth in the surfaces of all the treated samplesreports a stress induced hardening by the collision cascadesof energetic ions. The increase in the microhardness valuesis attributed to the incorporation of nitrogen in the TiN thinfilm [40], which in turn improves the fatigue strength ofthe material making it attractive for industrial applications.However, the increase of the ion dose per shot (flux) from acertain optimum value makes the surface rough and limits itstribological applications. Thus a more influential parameteris the energy flux that accommodates both the energy and theflux. Apart from the energy flux, operating voltage, fillingpressure, shot to shot reproducibility and the repetition rate ofthe shots may influence the deposition characteristics of thefilm at that particular axial position.
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M Hassan et al
Figure 11. Typical SEM micrograph with its EDS maps showingtitanium and nitrogen elemental distribution on TiN thin filmdeposited at 7 cm.
4. Conclusion
The DPF ions irradiated titanium samples are analysed inorder to correlate the surface properties with the ion beamparameters. Prior to the exposure of samples, nitrogen ions
02
46
810
1214
01000
2000
3000
4000
5000
6000
7000
Z=9cmZ=7cmZ=5cm
Z=3cmUntreated
Mic
roha
rdne
ss(M
Pa)
Indentationdepth (µm
)
Figure 12. Variation of microhardness (MPa) as a function ofindentation depth (µm) with different test loads (10, 25, 50, 100,200, 300 and 500 g) on sample surfaces treated at different axialpositions.
are characterized for parameters such as energy, numberdensity, energy distribution and current density, by employinga BPX65 photodiode. The measurements are consistent withthe previous investigations of low energy DPF devices and fitwell into the existing scaling law.
The XRD analysis of the nitrided samples shows thedeposition of a nanocrystalline TiN thin film at different axialpositions, with the maximum relative proportion at the axialposition of 7 cm. The grain size of the TiN film is estimated tobe about 90 nm while the compound layer thickness is about0.66 µm. The SEM results exhibit that the surface roughness ofthe TiN thin film is strongly influenced by the ion beam energyflux and decreases with the increase in the axial distance of thesamples. At 3 cm, the surface roughness is maximum withoutformation of the compound layer. The roughness is reducedat 5 cm along with the development of the nanocrystalline TiNfilm having petal-like features, which appears prominentlyat 7 cm. This is consistent with the XRD spectra and theEDS compositional analysis showing the maximum relativeproportion of the TiN compound at 7 cm axial position. TheEDS maps of the film reveal a spatially homogeneous surfaceprofile with respect to the nitrogen elemental distribution atthis axial position. The film surface becomes porous at 9 cmaxial position owing to the reduced energy flux of the ion beam.Thus, 7 cm is the optimum axial position for the deposition ofquality film on titanium with the present DPF device operatedin the nitrogen discharge, where the energy flux of the ionbeam is 2.69 × 1013 keV cm−3 ns−1. The probable energy ofthe ions reaching this position is 64 keV with the maximum ionnumber density of 5.9 × 1013 cm−3. The corresponding ioncurrent density is 1142 A cm−2. The surface microhardnessis also maximum at this axial position with the typical valueof 7650 ± 10 MPa corresponding to the indentation depth of1 µm. Based on these investigations, the DPF device is foundto be an effective ion accelerator that can remove the native
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Nitriding of titanium by using an ion beam
oxide layer and activate the nitrogen to form a TiN compoundat room temperature environment.
Acknowledgments
The authors would like to acknowledge the support providedby the HEC Research project entitled ‘Nitriding of Materialsin Plasma Environment’ at the GC University, Lahore. One ofthe authors (MH) is grateful to the NESCOM for providing thefinancial support to carry out material analysis at the NanyangTechnological University, Singapore.
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