The aim of the study is to extend the NbN coating on MS with Nb interlayer to explore the benefits of hard nitride coatings on low-cost structural material and to compare the coating with NbN monolithic coating on SS. NbN on MS and SS was deposited by reactive d.c. magnetron sputtering at various N2/Ar flow ratios and substrate bias. Deposition rate decreased from 20 to 10 nm/min (without biasing) and from 16 to 8 nm/min (−50 V biasing) when
Thin films of binary, ternary, and multicomponent nitride coatings, multilayers, duplex, and nanocrystalline coatings are widely deposited by magnetron sputtering [
Mild steel (MS) is widely used as a structural material due to its low cost. Properties of mild steel such as hardness, wear resistance, and corrosion resistance are not adequate. Application of hard nitride coatings on mild steel can modify the surface of mild steel necessary for practical applications. Hard nitride coatings deposited by physical vapor deposition (PVD) techniques exhibit high hardness, good wear resistance, and excellent chemical inertness. However, corrosion behavior of these coatings is often insufficient because of the presence of pin-hole porosity and microcracks inherent in PVD coatings. The corrosive media can attack the steel through these pores and microcracks. Various interlayers have been used to address the problem [
The aim of the present study is to propose a prospective coating combination on mild steel for versatile application. In the present study, NbN coatings have been extended on to MS substrate with niobium (Nb) interlayer deposited by sputtering and the results have been compared with the monolithic NbN coating on SS. For this NbN, coatings were deposited on MS and stainless steel (SS) substrates. Coatings were studied for their thickness, structure, hardness, and adhesion. Process parameters were optimized, and then, NbN coating with Nb interlayer was deposited on to MS substrate. These duplex coatings were studied for the improvement with respect to adhesion by scratch tests, surface hardness by Knoop microindentation, and corrosion performance by potentiodynamic polarization technique. Open circuit potentials were also measured.
NbN films were deposited using reactive DC magnetron sputtering on SS-, MS-, and Nb-coated MS substrates. An Nb (99.99% purity) metallic target, 160 mm diameter and 4 mm thick, was mechanically clamped to a planar sputter source mounted horizontally on the base of the chamber evacuated to a base pressure of
Deposition parameters for Nb and NbN coatings.
Parameter | Value |
---|---|
Base pressure | |
Operating pressure | |
Argon gas flow rate | 20 sccm |
Nitrogen gas flow rate | 0–14 sccm |
Substrate biasing | 0 to −150 V |
Target current NbN/Nb | 0.25/0.3 Ampere |
Target-substrate distance |
As used cleaning cycle.
Weight gain of the samples was recorded and thickness of the coatings was calculated using bulk density values. Actual coating thicknesses were studied by microabrasion using Calotest technique. In Calotest, an AISI 52100 hardened chrome steel ball (hardness = 65 HRC) is rotated against the coated specimen using the diamond particles suspended in a fluid. Roughness of the bare and coated samples was measured by a contact type diamond stylus profilometer. The phase structure of the films was investigated by X-ray diffraction (XRD) with CuK
Electrochemical evaluation of the coated samples was carried out using the standard potentiodynamic measurement technique with a computer controlled Santronic Electrochemical Analyzer. Tests were carried out using a three-electrode cell. Coated samples were soldered (with indium) to a copper wire coated with enamel. The samples were masked by Shailmask 800 lacquer (proprietary) to get the 1 cm2 surface area exposed. All potentials were measured with respect to a saturated calomel electrode (SCE). The auxiliary or counter electrode was platinum. The anodic and cathodic electrochemical polarization curves of all the samples were obtained in N2 de-aerated 1N H2SO4 electrolyte at room temperature. Open circuit potentials (OCPs) were measured in deaerated 1 N H2SO4 solution for 2 hrs. Before potentiodynamic measurements, the samples were allowed to reach equilibrium potential (
A variation of 5%–10% in the thickness of the coatings was found between the calculated values (weight gain method) and the actual values (Calotest technique). This was due to the density of the coatings being lower than the bulk values. In the deposition using biasing, there was a continuous ion bombardment at the substrate, which reduced the effective deposition rate. Therefore, more time was required to get the same coating thickness for coatings deposited at higher bias voltages. NbN coating thicknesses of 1.8
Deposition rate of Nb-N coatings as a function of N2/Ar flow ratio has been plotted in Figure
Deposition rate of Nb-N coatings versus
Figure
Deposition rate of Nb-N coating versus substrate biasing (
X-ray diffraction patterns of Nb-N films deposited on SS at various N2/Ar flow ratios are shown in Figure
X-ray diffraction patterns of Nb-N Coatings on SS deposited at 5%–70% of
Figure
X-ray diffraction patterns of Nb/MS and NbN/Nb/MS (Nb and NbN coatings were deposited in pure Ar and
Knoop microhardness values for Nb-N coatings on SS, taken at a load of 25 gf, have been plotted as a function of N2/Ar flow ratio in Figure
Surface hardness (at 25 gf) of Nb-N coatings deposited on SS substrate at various
To get the true hardness of the NbN coating, a coating thickness of about 4
Grain size also influences the hardness of the materials. Hardness changes as per the Hall-Petch relationship. Grain size has been found to increase with the increase in the partial pressure of nitrogen [
Roughness of the coated samples was found to replicate the values of the polished surface (0.06–0.08
Figure
Surface hardness (at 25 gf) of Nb-N coatings on SS (deposited at
The development of the morphology and microstructure of sputtered films bombarded by energetic ions during their growth is described by the Thornton diagram [
Hardness is found correlated directly to the compressive stresses also. Researchers have reported similar enhancement of the hardness and compressive stress for a variety of hard coatings deposited by magnetron sputtering [
The surface hardness of MS substrate, Nb, NbN, and duplex coating of NbN with Nb interlayer on MS are given in Table
Surface hardness of MS, and Nb, NbN, and duplex coatings on MS.
Coating | Hardness (HK25) |
---|---|
Substrate (MS) | 198 |
Nb | 434 |
NbN | 1084 |
Nb + NbN | 1436 |
Friction force and depth of indentation for all the scratch tests were recorded online along with the indenter movement to confirm the critical loads for cracks, chipping, delamination, coating failure, or other such observations. Tests performed at different loading rates were observed to give almost similar results, and variation in loading rate had little impact; therefore, loading rate for scratch tests was kept at 30 N/min.
Several types of observations were revealed as the scratch progressed, such as upper mono layers removal, pile-up on the sides, visibility of small cracks to long wide cracks within the coatings, pores, chipping, and partial or complete delamination of the coating. Figure
Scratch test for NbN coating (deposited at N2/Ar flow = 20%) on MS at (a) 9 N load, showing chipping and cracks and (b) 28 N load, showing nearly complete delamination.
Figure
Scratch test for NbN coating (deposited at N2/Ar flow = 20%) on SS at (a) 1 N, showing start of the scratch, (b) 12 N, revealing segregation, cracks and pores, (c) 22 N displaying chipping, cracks, pores, pile-up, and (d) 32 N, showing chipping, cracks, pores, pile-up, and delamination.
Scratch test for NbN coating (deposited at N2/Ar flow = 20%) on MS with Nb interlayer at (a) 1 N, showing start of the scratch, (b) 12 N, revealing segregation, cracks, and pores, (c) 22 N, displaying cracks, pores, and pile-up, and (d) 32 N, showing chipping, pile-up, and partial delamination.
Two critical loads, Lc1 and Lc2, have been defined for the failure of the coatings. Lc1, the first critical load, corresponds to initial cohesive failure of the coating such as appearance of first cracks within the coating. Lc2, the second critical load, corresponds to initial adhesive failure of the coating, that is, first observation of adhesive failure such as chipping, partial delamination, pores, or some such phenomena, where substrate beneath coating gets exposed. Lc1 and Lc2 for coating on MS samples were observed to be between 6–8 N and 9–12 N loads, respectively. For coatings deposited on SS substrates Lc1 varied between 7–15 N and Lc2 between 12–25 N. NbN coatings on SS showed better performance than NbN coatings on MS. However, for Nb interlayered NbN coatings on MS, the critical loads improved; Lc1 was found to be between 8–14 N and Lc2 between 10–24 N, showing almost equal or sometimes slightly better performance than NbN coatings on SS.
Coefficient of friction (
Load (N) | Coefficient of friction ( | ||
MS | SS | Nb/MS | |
20 | 0.23 | 0.20 | 0.18 |
30 | 0.28 | 0.25 | 0.24 |
40 | 0.35 | 0.30 | 0.30 |
60 | 0.45 | 0.40 | 0.38 |
Depth of penetration increased with the increase in applied load. At 30 N load, on an average, MS samples had 20–30
The effect of loading rate was studied for NbN coatings on MS and SS substrates. The effect of loading rate on Lc1 and Lc2 was found to have little impact. The Lc1 value was found to shift from 7 N at 10 N/min to 7.5 N at 30 N/min and further to 8.2 N at 50 N/min. Similarly, Lc2 value shifted from 8.2 N to 9 N and further to 9.5 N with similar increase in applied loading rates.
For SS samples, the Lc1 changed from 15 to 17 N and Lc2 changed from 22 N to 24 N when the applied loading rate was increased successively from 20 N/min to 80 N/min.
Effect of biasing was studied on NbN coated SS samples. Increase in biasing voltage from zero to −75 V (in a step of 25 V), keeping other factors constant, led to successive increase in the values of Lc1 and Lc2. At −75 V, biasing the Lc1 and Lc2 values was found to be 15.6 N and 26 N, respectively. However, at −100 V, the coating became brittle, and Lc1 and Lc2 values dropped drastically to 7 and 11 N, respectively. The values at various substrate biasing are shown in Table
Effect of biasing on critical loads during scratch tests for NbN coatings on SS.
Biasing (−V) | Lc1 (N) | Lc2 (N) |
---|---|---|
0 | 6.5 | 10.5 |
25 | 10.5 | 20 |
50 | 12.0 | 24 |
75 | 15.6 | 26 |
100 | 7.0 | 11 |
Nb-N coatings on SS samples deposited at N2/Ar flow ratio ranging from 10% to 70%, (keeping the biasing voltage constant at −50 V) were tested for scratch adhesion. Results with respect to critical loads are shown in Table
Effect of N2 flow on critical loads during scratch tests for NbN coatings on SS.
Lc1 (N) | Lc2 (N) | |
---|---|---|
10 | 11 | 18 |
20 | 12 | 24 |
30 | 11 | 25 |
40 | 10 | 20 |
50 | 8 | 20 |
60 | 8 | 18 |
70 | 7 | 14 |
It is difficult to deposit the hard coatings by physical vapor deposition (PVD) techniques without any micro porosity. Thus, when a PVD coated sample is exposed to the corrosive environment, the electrochemical behavior of the coated sample is the combined behavior of the coating and the substrate. The polarization curve of such a specimen may be considered as a combination of two curves—one representing the base material and the other the coating.
Figure
A high
Coating | ||
---|---|---|
Substrate | −496.4 | 1440.3 |
NbN | −412.1 | 150.2 |
Nb | −469.3 | 121.8 |
Nb + NbN | −396.2 | 14.8 |
Nb interlayer was found to improve the corrosion resistance of NbN coated MS substrates effectively. For Nb interlayered NbN coating,
Potentiodynamic curves for MS, NbN/MS and NbN/Nb/MS.
NbN coatings were deposited on MS and SS substrates by reactive DC magnetron sputtering.
Deposition rate decreased from 20 to 10 nm/min (without biasing) and from 16 to 8nm/min (with biasing at −50 V) with the increase in N2 flow from 0% to 70%. Deposition rate decreased from 15.9 to 6 nm/min when the bias voltage was increased from zero to −150 V successively. Coatings showed presence of hexagonal
Surface hardness on SS reached a maximum of 2040 HK25 at a N2/Ar flow ratio of 20% and then decreased slowly with the further increase in N2/Ar flow ratio. Surface hardness increased from 1084 HK25 to 1618 HK25 when incorporated with Nb interlayer. Critical loads for cohesive (Lc1) and adhesive (Lc2) failures during scratch test for coatings on MS samples were observed to be between 6–8 N and 9–12 N loads, respectively; while for coatings on SS substrates, the values were between 7–15 N and 12–25 N, respectively. For duplex coating on MS, the values were between 8–14 and 10–24 N, respectively. Coefficient of friction (
Open circuit potential for NbN coated MS sample decreased from an initial value of −252 mV to −478 mV in 30 min and to −498 mV in 40 min after which it stabilized. The equilibrium value was found to be quite similar to MS substrate, thus indicating the presence of pores in the coating, resulting in corrosion taking place beneath coating. For duplex coating, OCP shifted to −154 mV in the beginning and stabilized to −443 mV after 40 min. With duplex coating the corrosion resistance increased remarkably.