Study of Surface Modification of Niobium Caused by Nitriding and Cathodic Cage Deposition

In this work, plasma nitriding (PN) and cathode cage plasma deposition (CCPN) treatments were carried out with temperatures of 400 and 450 °C to evaluate the modifications caused on the surface of pure niobium samples. XRD, SEM, Vickers microhardness, and sphere-disk analyses were used to characterize the coatings’ composition, morphology, hardness, and wear resistance. The results showed that the treated samples increased hardness and wear resistance, with the sample submitted to CCPN at 450 °C presenting the best tribological behavior.


Introduction
Brazil has the world's largest deposits of Niobium (Nb), and this metal is strategically important as a target for research to determine different uses and new industrial applications.In recent years, the application of Nb for different uses, such as micro-alloyed steels to promote good weldability and toughness [1][2][3] , superalloys 4 , thin films 5 , medical implants 6 , and superconductors, has steadily increasing 7 .Furthermore, this metal does not oxidize at room temperature, and its properties vary depending on the content of interstitial elements 8 .
Nitriding and plasma deposition has been widely used to form niobium nitrides to improve the tribological behavior of several steels.In addition, these nitrides can act as protective layers against oxidation processes 9 and improve tribological performance 10 .Niobium nitride has excellent physical and chemical properties such as hardness 11 , wear resistance 12 , chemical stability 13 , and superconducting properties 14 high electrical conductivity, and superconducting transition temperature 15,16 .Furthermore, plasma nitriding can form a transition layer produced by nitrogen diffusion on the metal surface.This can reduce tensions resulting from the interface between film and substrate, which results from the contrast between the mechanical properties between the metal and the film deposited on it 17 .
Singh et al. 18 , in 2012, performed the deposition of NbN films to improve the surface hardness of various steel substrates.This work is related to the formation of hard films composed of phases of the Nb x N y type.
Furthermore, the authors varied the nitrogen flow to cause changes in the phases produced in the sputtering process and observed that films composed of Nb 2 N had the lowest hardness values among the films produced 18 .NbN coatings have been an excellent alternative for improving tribological behavior by showing greater wear resistance than commercially used coatings such as TiN, reports Zhitomirsky 19 in 2007.
In this work, plasma nitriding (PN) and cathodic cage plasma deposition (CCPD) will be performed with temperature variation to produce surface modification by inserting nitrogen on the surface and deposition of a hard and wear-resistant Nb x N y composite film featuring better tribological performance and longer functional life of the studied metal.The results obtained should contribute to the discussion about the relevance of using Nb and NbN as functional coatings for application in the metalworking industry.

Materials and Methods
This work used cylindrical samples of pure niobium metal (Nb).Those samples were treated with plasma under four different conditions, and your results were compared with samples without treatment.All samples had their upper surfaces prepared with 400 to 1200 mesh sandpaper and polished with diamond paste until a mirrored and scratchfree appearance was acquired.Before the plasma treatment, the samples were dipped in alcohol, subjected to ultrasound for 10 minutes to eliminate remaining impurities, and dried with airflow at room temperature 20,21 .
*e-mail: maxwellprog@hotmail.com Materials Research The equipment used for plasma treatments consists of a gas supply system, a reactor, a direct current source with a maximum value of 2 A, a vacuum pump, and a digital thermometer.The whole apparatus is better described in previous works 21,22 .The treatments were performed with a pre-sputtering stage lasting one hour, 350 °C, and 1:1 flow (Ar/H 2 ) to remove remaining residues.Soon after, PN or CCPD treatments are effectively started with the parameters shown in Table 1.
A niobium cage of approximately 7.2 cm in diameter, 3.5 cm in height, and 2 mm in thickness was used for the cathodic cage deposition (CCPD) step.An alumina disk was used to keep the samples on fluctuating potential during the depositions.
The phase composition was analyzed using an X-ray diffractogram (XRD) from Shimadzu, model XRD-6000.Cu Kα lines (wavelength: 0.154 nm) were used, operating with 40 kV and 30 mA in the range of 30 to 65 ° in the Bragg-Brentano configuration.The scanning electron microscope FEI COMPANY, model QUANTA FEG 250, was used to evaluate the thickness and morphology of the deposited layers.Five thickness measurements were made along the length of the layers to obtain the average values and their standard deviations.The hardness test was carried out using an INSIZE microdurometer, model ISH-TDV 1000.A Vickers indenter (HV) with a load of 50 gf and duration of 15 seconds was used in the test, and five measurements were taken to obtain the hardness value average.The micro-abrasive wear test was performed with ball-disc-type equipment to assess the wear volume of the samples.The SAE 52100 steel ball has a diameter of 25.4 mm, and the tests were carried out with a normal load of 8 N, a rotation frequency of 40 Hz, for 600 s, without lubricant.Three tests were performed on each sample to obtain measurements of the average diameter of the wear trail.

Results and Discussion
The X-ray diffraction results of pure niobium and samples submitted to PN and CCPD treatments are shown in Figure 1.Despite the conventional nitriding process, samples PN400 and PN450 did not show Nb x N y phases.However, the PN treatments caused diffraction peak displacement due to the insertion of nitrogen in the niobium matrix (Nb -ISCD 76416).According to Kertscher et al. 23 , o the solvus curve for the α-Nb(N) phase, the solubility of nitrogen in niobium bcc ranges from < 3 wt% N at a temperature of ≈ 2350 °C to zero at room temperature 23 .Therefore, the low amount of nitrogen in the niobium matrix caused peak shifts of little significance.On the other hand, the diffractograms of the samples treated by CCPD showed the formation of phases Nb 4 N 5 (ICSD 26251) and Nb 2 N (ICSD 76387) that present high hardness and thermodynamic stability, in addition to improving resistance to wear and corrosion [24][25][26][27] .
Figure 2 shows the surface electronic microscopy of the Base sample (without treatment).The sample preparation process consisting of sanding and polishing can reduce irregularities.Despite this, the presence of remaining surface imperfections is noted.
The electronic micrographs of the treated samples are shown in Figure 3.The surfaces of the PN400 and PN450 samples were more regular than the surface of the Base sample, except for some areas of discontinuity.This can occur due to irregularities of the pre-treatment surface or possible opening of electrical arcs on the sample during nitriding.
Samples submitted to CCPD treatment showed fewer surface defects due to the deposition of Nb x N y films.Figure 3b shows that the film produced by CCPD at 400 °C is very thin, unable to highlight it in the cross-sectional SEM.The surface image of the CCPD400 sample shows the formation of refined grains, which characterizes a low deposition rate under the conditions adopted in the treatment.On the other hand, the CCPD450 sample showed an expressive layer evolution and the formation of larger grains.The increase in temperature caused a higher sputtering rate and more significant deposition of the Nb x N y film on the substrate.
Table 2 presents the layer thickness values and their respective standard deviations.As observed in the electron microscopy images, layers were formed by nitriding.However, these thick layers are constituted by niobium, with a low content of nitrogen diffused in its structure.The low thickness of the layer formed in the CCPD400 sample shows that under this temperature and pressure condition, it was impossible to promote an efficient deposition rate like that observed in the deposition at 450 °C (CCPD450).
Figure 4 shows the surface hardness values of the nitrided and CCPD-treated samples and compares the treated samples with the base sample (Nb).All samples subjected to plasma treatment showed higher surface hardness than the untreated sample.Samples nitrided at 400 °C and 450 °C obtained hardness values of approximately 279 HV and 245 HV, respectively.The CCPD400 sample showed higher hardness due to the formation of niobium nitride phases, as shown in the x-ray diffractogram in  The optical microscopy images of the trail formed in the sphere-disc wear test are shown in Figure 5.The untreated sample showed a more extensive trail with regions marked by adhesive wear resulting from sliding contact with the sphere used in the tribological test.Conversely, the nitrided samples showed a smaller trail size, characterizing less wear.In these samples, wear by scratching was more evident on the surfaces.On the other hand, the samples submitted to the deposition process showed little wear trail.The CCPD400 sample showed less trail and adhesive wear.Samples with niobium nitride thin films were more wear-resistant than nitrided samples.The Vickers hardness results showed low values for the CCPD450 sample to the nitrided samples.However, the image of the wear trail of this sample showed more excellent resistance in the sliding contact.
Figure 6 shows the wear volumes of the samples submitted to the sphere-disk type tribological test.Plasmatreated samples had less wear volume when compared to the untreated sample.Among the treated samples, PN450 showed the worst average performance.On the other hand, the CCPD450 sample had a smaller wear volume, which characterizes greater resistance to wear than the sample with niobium nitride film.The PN400 sample also showed behavior comparable to the deposited samples in the tribological test.As seen in Figure 5b, the region of contact between the surfaces in the tribological test was close to surface deformations that may have influenced the accuracy of the numerical result.Despite this, the result for PN400 is close to the values obtained for PN450, considering the standard deviation.

Conclusions
The results obtained in this work allowed the following conclusions: • The nitriding treatments carried out at 400 °C, and 450 °C showed only Nb phases distorted in the diffractograms by the presence of a small amount of accompaniment in the steel matrix.

•
The samples kept in the CCPD treatment showed the formation of the Nb 4 N 5 and Nb 2 N phases responsible for the hardening of the surface of the samples.

•
In the results of scanning electron microscopy also observed the formation of thick layers, except in the CCPD400 sample, which did not present adequate temperature and gas pressure conditions to form niobium nitride phases.

•
The wear test showed that the treatments improved wear resistance.In addition, nitriding and deposition of niobium nitrides resulted in the best tribological behavior of the studied steel.
Therefore, the appreciated results that formed a layer composed of Nb with diffusion in nitriding and Nb x N y in deposition reduce steel wear in industrial applications.

Figure 1 .
Figure 1.X-ray diffractogram and the 2θ position of the 100% peak.

Figure 2 .
Figure 2. Surface SEM of the Base sample.

Figure 4 .
Figure 4. Microhardness values of treated and untreated niobium samples.

Figure 1 .
Figure 1.Unexpectedly, the surface hardness value of the CCPD450 sample was the lowest among the treated samples.This can be explained by possibly forming a less dense, less resistant film than the layer modified by the deposition treatment at 400 °C.The optical microscopy images of the trail formed in the sphere-disc wear test are shown in Figure5.The untreated sample showed a more extensive trail with regions marked by adhesive wear resulting from sliding contact with the sphere used in the tribological test.Conversely, the nitrided samples showed a smaller trail size, characterizing less wear.In these samples, wear by scratching was more evident on the surfaces.On the other hand, the samples submitted to the deposition process showed little wear trail.The CCPD400 sample showed less trail and adhesive wear.Samples with niobium nitride thin films were more wear-resistant than nitrided samples.The Vickers hardness results showed low values for the CCPD450 sample to the nitrided samples.However, the image of the wear trail of this sample showed more excellent resistance in the sliding contact.Figure6shows the wear volumes of the samples submitted to the sphere-disk type tribological test.Plasmatreated samples had less wear volume when compared to the untreated sample.Among the treated samples, PN450 showed the worst average performance.On the other hand, the CCPD450 sample had a smaller wear volume, which characterizes greater resistance to wear than the

Figure 6 .
Figure 6.Volume of wear of the samples submitted to the spheredisk type tribological test.

Table 2 .
Mean and standard deviation layer thicknesses of samples with conventional nitriding and cathodic cage nitriding.