Structure and impedance of TiN coated 304L SS substrates

In the present work, the effect of the deposition time on the structure and electrical impedance of reactive DC magnetron sputtered TiN coatings was studied. The structure of the coatings and the substrates was studied using X-ray diffraction (XRD). The electrical impedance and high frequency stability of the specimens was experimentally measured and the results were indicative of whether the application of the coatings as protective layers between the electrical contact pads of electrical contact systems is feasible.


Introduction
Most commonly, the preferred materials used for stationary electrical contacts for both high and low current applications are copper and aluminum due to their excellent thermal and electrical properties, however, in some rare cases due to the adverse environment the electrical contact has to be comprised of high corrosion resistance materials such as stainless steel.The 304L stainless steel is predominantly used for aerospace, automotive, household, and corrosive resistant applications due to its excellent mechanical and corrosion resistance properties [1].However, the electrical properties of this type of steel are vastly inferior compared to aluminum and copper due to its' significantly higher electrical resistivity, which could result in great power losses and heating of the electrical contacts, particularly when a high current application is employed.In addition, in case the entire electrical system is not made of stainless steel a need for a junction of dissimilar metals is imminent.This could lead to the formation of galvanic corrosion and increase of the electrical contact resistance of the electrical system [2].
In order to improve the electrical properties of the 304L SS and minimize the possibility of galvanic corrosion a protective layer between the electrical contact pads can be utilized.TiN is a highly popular material used in mechanical engineering as a protective coating due to its' excellent mechanical properties, high melting temperature, superb tribological properties, and more [3].Furthermore, TiN characterizes with good electrical conductivity due to its' unique electron configuration [4] and excellent chemical stability which remains intact even in high temperature conditions up to 500 °C [5].Due to these reasons, this material is a likely candidate to be employed in electrical contact systems in the form of a thin film formed between the electrical contact pads of both similar and dissimilar metals.The different properties of the coatings imminently should have a result on their electrical impedance and high frequency electrical stability, which are currently much less studied.the investigation of the impedance of DC magnetron sputtered TiN and VN coatings deposited on aluminum electrodes showed that the last exhibited improved mechanical and electrical properties as a result of the applied coatings.Only a slight deviation of the expected electrical impedance values was noticed in the high-frequency range as a function of the thickness of the coatings [6].Stainless steel has a very limited application in the electrical engineering industry, however, this could be potentially improved by applying similar conductive coatings on the surface of the electrodes.Considering this, a very limited amount of research has been conducted regarding the usability of transition-metal nitride coatings on the surface of stainless steel for electrical engineering purposes.Furthermore, the current investigation provides opportunity for the examination of the influence of the thickness of the applied films on the resultant electrical impedance and high-frequency stability.This is why in this work, the effect of TiN coatings of different thicknesses on the electrical impedance of 304L SS substrates was studied, and the possibility of improving the electrical properties of the substrate was examined.The results were discussed concerning the potential applicability of such coatings in the electrical industry.

Experimental setup
The formation of the coatings is carried out using reactive DC magnetron sputtering.Most of the technological conditions of layer deposition are summarized in table 1.The coatings are applied on 304L substrates with a diameter of 16 mm and a thickness of 3 mm.The gases used for the experiments are Ar and N 2 both with a 99.9999% purity.The duration of the magnetron sputtering process was varied between 40 minutes and 80 minutes.The samples corresponding to those deposition times were denoted as suggested in table 1, namely "TiN-40" and "TiN-80".The working pressure was 1.2x10 -1 Pa.The pressure ratio in the Ar:N 2 system was 23:1.The phase structure of the coatings and the substrates was studied using X-ray diffraction (XRD) with a CuKα wavelength of 1.54 Å in a standard range of 30 to 80 degrees of the 2θ scale.A symmetrical Bragg-Brentano mode was used for all experiments.The phase composition of the samples was determined using the crystallographic database compiled by the International Centre for Diffraction Data (ICDD).The data sheets used to determine the composition of the samples were PDF #330397, and PDF #381420.
The impedance of the pure substrate and the TiN coated substrates was studied using the parallel method with two electrodes, which consists of a sandwich structure of two parallel electrodes and the sample positioned in between them, forming a plane-parallel electrical contact.The measurements were carried out using an Agilent 4294A impedance analyzer combined with an Agilent 16451B attachment.An applied contact force of 5 N was used in all cases.In order to study the influence of theskin effect on the impedance of the samples a frequency sweep in the range of 40 Hz to 1 MHz was employed.

Results and discussion
Figure 1 shows the XRD patterns of the considered specimens.The substrates characterize with a double phase structure comprised of αFe and γFe, which is a typical configuration for austenitic steels [7].The (111), (200), and (220) crystallographic planes were detected corresponding to the γFe phase, and the (220) plane corresponding to the αFe phase was also detected, where the main phase is the gamma one.The diffraction maxima corresponding to the TiN coating deposited for a time of 40 minutes belong to the {111} and {100} families of crystallographic planes.Regarding the coating deposited for a time of 80 minutes a single peak was observed corresponding to the {100} family of crystallographic planes.Following the previous observations and to confirm the change in the texture of the samples the pole density of the observed diffraction maxima was calculated in agreement with the equation given in [8].The performed calculations are presented in figure 2 (a), and figure 2 (b), and confirm that in the case of the TiN coating applied to the TiN-40 sample peaks corresponding to the {111} and {100} family of crystallographic planes were observed and their partial ratio in the studied area of the sample was nearly 1:1.In the case of the coating formed using a deposition time of 80 minutes of the contribution of {100} family of crystallographic planes is 100 %.This means that in that case the preferred crystallographic orientation of the TiN coating in the studied are is directed towards the {100} family of planes.
Figure 3 shows the results of the performed impedance experiments.The bulk data represents the impedance of the entire probe as supposed to the equivalent resistance of the separate components of the circuit.Most of the observed impedance is due to the electrical contact resistance formed in the zone of contact between the sample and the two electrodes of the analyzer.The impedance of the pure substrate at 40 Hz was 1.58 Ω and at the end of the chart it reaches 1.71 Ω.The impedance of the TiN-40 and the TiN-80 samples at 40 Hz was 1.24 Ω and 1.18 Ω, respectively, and increase to 1.35 Ω and 1.39 Ω. Evidently, the impedance values rise in cases influenced by the skin effect.The skin effect depth of the pure substrate was calculated using equation (1).
The skin effect depth δ (m) is directly correlated to the specific electrical resistivity of the material ρ (Ωm), the frequency f (Hz), the specific material permeability µr (H/m) and the permeability constant µ0 (H/m).The results of the calculations are given in figure 4. Up to a frequency of 770 Hz the skin effect depth is 16 mm which is equal to the diameter of the studied samples.This indicates that frequencies below this value have no effect on their impedance.The skin effect depth decreases exponentially with the increase of the frequency.At 1 MHz the depth of the usable area of the studied conductor is just 0.44 mm.When comparing the impedance data in both cases the substrate has a higher impedance compared to the studied samples.This is attributed to the much lower specific electrical resistivity of TiN compared to steel.Furthermore, the applied coatings unify the surface of the sample, eliminating the presence of potential micro-deformations formed during the machining process.The coating applied for a time of 80 minutes has a slightly lower impedance compared to the one deposited for 40 minutes.This may be attributed to a number of factors.The most likely one is the change in the preferred crystallographic orientation of the thicker coating directed towards a single family of planes.This improves the mobility of electrons through the material and thus reducing the electrical resistance of the coatings ever so slightly.Improved electrical resistivity with the change in the texture of TiN thin films from micro-volumes belonging to the {111} family of planes which are parallel to the surface towards the {100} deposited using magnetron sputtering was also noticed by Ponon et al. [9].They also theorize that this change in the orientation of the films is predominantly caused by the increase of the partial pressure.At about 770 kHz a more rapid increase of the impedance of the TiN-80 sample was observed.Beyond this frequency, the impedance of this sample increases beyond the impedance of the TiN-40 sample.This phenomena is could be attributed to imperfections in the sample.The increase of the deposition time leads a worsening of the adhesion between the surface of the substrate and the deposited layer.This leads to the formation of a "flaky" coating with poor adhesion at the edge of the samples.With the increase of the skin effect the electrons are directed closer towards the edge of the samples where the concentration of defects and the adhesion are increasingly worse, which leads to the increase of the impedance values.The crystal structure of the substrate did not undergo significant changes during the deposition process, which means that the presented data is primarily influenced by the coating deposition procedure and the structure of the coatings.

Conclusions
The present study proves on a basic scale the concept of using TiN as a protective coating in the electrical engineering field, particularly in the form of conductive layers that improve the conductivity of stainless steel contact pads.Since these coatings also possess high corrosion resistance they can be applied even in high corrosive environments where improved reliability is required.The deposited coatings can be easily applied to stationary electrical contacts where no commutation takes place.Since many processes such as electrical, thermal, mechanical, corrosive and more affect commutating electrical devices, the usability of such coatings in the electrical contact systems of such devices is yet to be definitively determined, and further experiments need to be performed.

Figure 1 .
Figure 1.Diffraction patterns of the samples.Figure 2. Texture of the coatings of the (a) TiN-40, and (b) TiN-80 samples.

Figure 2 .
Figure 1.Diffraction patterns of the samples.Figure 2. Texture of the coatings of the (a) TiN-40, and (b) TiN-80 samples.

Table 1 .
Parameters used during the reactive DC magnetron sputtering and thickness of the film.