A Tribological Study of CrN and TiBN Hard Coatings Deposited on Cobalt Alloys Employed in the Food Industry

Abstract

In this work, a comparative study of the tribological performance of two hard coatings, CrN/TiBN, was conducted for research purposes and industrial applications in food products, particularly for food packaging into cans using the double hermetic sealing process. CrN and TiBN coatings were successfully deposited on a base-cobalt metal substrate of a CoCrW commercial alloy using physical vapor deposition by arc evaporation (AEPVD) technology to improve the tribological properties of the commercial alloy, including wear and corrosion resistance, lower coefficient of friction, and overall durability. This research focuses on conducting scratch and abrasion wear resistance tests in dry conditions; specifically, it pursues to evaluate the wear corrosion properties, known as tribocorrosion performance, on CrN/TiBN hard coatings. The experimental results show that the CrN coating (2.9 μm) is slightly thicker than the TiBN coating (2.7 μm), with a 47 N critical load. It also shows a lower coefficient of friction (CoF) in a dry environment, while the TiBN coating showed total detachment and a high coefficient of friction in a dry environment condition. Tribocorrosion testing in brine aqueous solution indicated that CrN coating shows a high friction coefficient with a higher open circuit potential value (E_corr), and TiBN shows the lowest corrosion potential (E_corr) and the lowest friction coefficient. This suggests that CrN could provide better corrosion protection for commercial cobalt alloys and improve tool performance during the food canning process in brine environments.

1. Introduction

In certain manufacturing industries, metallic parts, tools, mechanical components, and machinery are subjected to the simultaneous action of wear and corrosion damage. Stemp et al. established that the combined effects of wear and corrosion are more damaging than the sum of their separated effects [1]. In the case of the food industry, specifically in equipment for canned pickled food, degradation on the surface of metal roller tools results from the combination of mechanical wear in the sheet-roller contact zone and wear accelerated by corrosion due to the presence of brine at 60 °C [2,3].

During the double hermetic sealing process used to close food cans, a steel blade with a flange in a hook shape is fastened with a chuck, which riddles it into the body of the can full of pickled food; the whole set is in a spinning plate or turntable, as shown in Figure 1a. Next, a mandrel and the spinning plate turn, pressing both hooks and folding them so they get anchored, completing the first closing operation. Immediately, a second spinning roller makes the final shaping, creating the hermetical sealing shown in Figure 1b, completing the second closing operation [4,5]. Since the canning process is performed continuously in a wet environment containing acid products of the brine, tools are subjected to superficial degradation exposure for the action of the tribocorrosion process.

Total integrity on the surface of metal rollers ensures hermetical, homogeneous, and safe sanitary seals during all times of the packaging operation process. For this reason, a suitable selection of materials for these tools is essential to minimize tribocorrosion damage during food canning, where resistance to corrosion deterioration, high hardness value, and excellent wear resistance are the main required properties for this type of industrial procedure. In this sense, steel series 300 and 400, exclusively austenitic and duplex stainless steel, are commonly applied for infrastructure and construction of parts and utensils for the food industry due to their high corrosion resistance. However, these alloys have poor mechanical properties compared with other alloy steel.

In recent years, base cobalt alloys have been widely used in various biomedical, automotive, and food applications, particularly for their high wear and corrosion resistance compared to commercial stainless steel. In these alloys, cobalt (Co) forms a tough matrix, with Cr and W dissolved as carbide compounds that provide additional strength and corrosion resistance [6,7,8]. To further improve these alloys’ mechanical and anticorrosive properties, some researchers have proposed using hard PVD coatings [9,10,11,12,13,14,15,16,17,18,19,20]. Among various alternatives of coating methods, some hard coatings (usually TiBN, CrN, and TiN) can be deposited under favorable conditions on the metal surface as substrate by the Cathodic Arc Physical Vapor Deposition (PVD) process, which has a broad range of applications due to its high density, excellent uniformity, and good adherence, including the strict control of the coating thickness. PVD coatings offer good wear resistance, high hardness, and excellent anticorrosive properties, making this technology attractive and suitable for improving the durability of tools/components and industrial service performance during manufacturing operations.

These hard coatings, like CrN (Chromium Nitride), have a high corrosion resistance because they form a tribo-film of CrO₂ (Chromium Dioxide) during sliding contact, particularly in humid environments in the presence of oxygen in the air that protects the metal surface against chemical reactions, and which continuously reduces the friction forces during mechanical wear stages by acting as self-lubricant, thus giving excellent wear properties (tribological performance) [21,22,23]. Furthermore, TiBN coatings typically exhibit a crystalline structure comprising a combination of hardened titanium nitride (TiN) and titanium diboride (TiB₂) phases, contributing to their exceptional mechanical behavior, particularly in wet conditions. On the other hand, Wagner et al., in their research, suggest that when boride oxides are formed from stacked layers of TiBN in the presence of humidity, they generate a thin lubricant layer that reduces direct contact between the coating and the counter-body of the tool, diminishing friction forces, and the wear resistance of the surface metal increases [19]. Furthermore, TiBN coatings are often used for extremely hard applications and in operations requiring outstanding wear resistance, like cutting tools, forming tools, aerospace components, automotive parts, and even certain medical devices; however, the presence of boron at the surface promotes a homogeneous appearance with a low number of imperfections as pores, micropores, and micro-cracks, thereby avoiding sites where corrosion spots could begin [11,19,20,21,22,23,24]. In summary, the assistance of two advanced hard coatings of TiBN and CrN offers significant benefits in optimizing the industrial performance and durability of tools as well as machinery components. The selection between these two coatings depends on the specific requirements of the industrial application; for example, TiBN is promising due to its extreme hardness and low friction coefficient, while CrN is promising due to its considerable corrosion resistance in humid environments and cost-effectiveness. By considering some of these advantages, manufacturing industries can choose the most suitable hard coating to meet their goals or use both (CrN/TiBN) for better results, as demonstrated in the present research paper.

Consequently, this work aimed to compare CrN and TiBN as a hard coating deposited over a base-cobalt-commercial alloy (CoCrW) for the double hermetic sealing process to close food cans, using arc cathodic PVD technology to reinforce the tribological behavior of withstanding the alloy. Then, a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS) techniques were used to examine the surface morphology, microstructure features, and elemental composition of the deposited coatings, whereas crystallized phases on surface coatings were identified using the X-ray diffraction technique (XRD). The adherence performance of the coatings to the substrate was evaluated using superficial scratch tests. Additionally, the tribological behavior was studied using disk tests under two different conditions: (a) a dry condition and (b) immersed in a commercial chili–brine aqueous solution to simulate the working conditions during the closing of cans; finally, the open circuit potential (OCP) was measured to determine the chemical potential of reactions occurring on the metal surface.

2. Materials and Methods

The material used as substrate was a commercial Co-Cr-W alloy due to its excellent mechanical properties, corrosion, and wear resistance. Disc-shaped specimens of this alloy (38 mm diameter and 6 mm thickness) were prepared using a standard metallography technique, with their surfaces successively grounded using SiC sandpaper of progressively increasing grit sizes from 120, 240, 320, 500, 1000, 1200, to 2400 and then polished with diamond paste of 5 µm and 0.5 µm grades to produce a polished surface with a mirror-like finish. Subsequently, they were subjected to ultrasonic cleaning with acetone. Probes were studied under three different conditions: (i) uncoated as a reference, (ii) coated with chromium nitride (CrN), and (iii) coated with Titanium Boron Nitride (TiBN); for the last two conditions, ultrasonic cleaning with ethanol was strictly required prior to the coating deposition process.

CrN and TiBN coatings were deposited using arc evaporation physical vapor deposition (AEPVD) technology at SADOSA S.A. de C.V., which is a Mexican company located in Mexico City. SADOSA specializes in providing surface treatment and coating technology for industrial applications. This research paper does not disclose the operational conditions of the deposits due to the confidentiality of SADOSA’s coating process. These coatings were surface characterized by a Scanning Electron Microscopy (SEM) with a JEOL equipment of JSM IT100 model, operating at an electron acceleration voltage of 10 kV. This analysis also included an elemental composition examination using an energy-dispersive X-ray spectroscopy (EDS) technique. Further, an X-ray diffraction analysis (XRD) was conducted using a Philips PW1710 X-ray diffractometer, with a Cu-Kα radiation of 1.5406 Å (λ wavelength).

The coatings were mechanically scratched on the surface at least three times per condition using a commercial scratch tester (Revetest-Xpress instrument) to measure the adherence strength over the hard coatings; a Rockwell C diamond was used as a counterpart with a sliding constant velocity of 75 mm/min to evaluate the mechanical properties. Tangential and normal loads were registered and analyzed for the entire experimental set; subsequently, the critical load (${\mathrm{L}}_{\mathrm{C}}$) was estimated using optical microscopy. Meanwhile, wear tests were performed in custom-built pin-on-disc equipment with the configuration shown in Figure 2. The coefficient of friction (CoF) was obtained using an alumina ball counterpart (radius of 3.5 mm) with a normal force (F_n) of 7 N and a relative wear distance of 500 m, considering that alumina could represent an extreme operating condition of resistance and hardness, in addition to its chemical stability in the studied application. After, a Veeco Dektak 150 profilometer unit was used to analyze the resultant worn tracks using 2 cursors to measure the depth and width of the track, analyzing from 1 mm on each side of it at a velocity of 0.2 mm/s, with real vertical resolution ≅ 5 nm. In different parts of the crack, 3 measurements were taken to obtain a characteristic average; wear volume (V) was also determined in this test. All the tests were conducted in accordance with ASTM G99, performing three repetitions per each experimental test condition.

Finally, the tribocorrosion tests were performed by coupling the pin-on-disc device to an electrochemical cell and then connected to a Digi-Ivy DY2300 potentiostat and galvanostat workstation, as shown in Figure 2. All tribocorrosion experiments were conducted in OCP condition (open-circuit potential) in a standard electrochemical cell. The cell used a three-electrode set-up, with a saturated solution of potassium chloride (calomel) electrode as reference electrode (RE); a platinum mesh as a counter electrode (CE); and the study samples, including the coatings, in contact with the test solution with an exposed area of 9.7 cm², acting as working electrode (WE), respectively. A commercial chili brine solution (2%–15% NaCl, 2%–4% acetic acid, pH~4) was used as a test electrolyte at a temperature of about 60 ± 2 °C to partially emulate the industrial process of closure of cans filled with brine food. This is taken into consideration as an extremal condition for the performance of food packaging tools, as they are not directly submerged into the brine solution but are exposed to the environment that contains vapors and substances proceeding from the brine. In this sense, tribocorrosion tests were arranged into three stages, similar to those used by Arenas et al. [25]. In the first stage, the samples remain immersed in the electrolyte solution test for one hour to reach the thermodynamic equilibrium stage or the electrical corrosion potential E_corr at OCP condition. In the second stage, the loading force was applied to the immersed sample for one hour using the same parameters as those tested in dry conditions. The coefficient of friction (CoF) values on the surface of the sliding track and the electrical corrosion potential were simultaneously recorded in a PC workstation. In the third stage, the load was withdrawn, and the sample remained immersed in the electrolytic solution test for one hour. During this stage, measurements of the electrical corrosion potential were registered throughout the entire experiment. All tests were performed for four representative samples for each stage.

3. Results and Discussion

3.1. Surface Characterization

Figure 3a–c show SEM images of the uncoated alloy surface and the coating surface of CrN and TiBN deposited on the CoWCr substrate alloy. In Figure 3a, a typical microstructure of cobalt-base alloy (CoWCr) can be observed; the phase in gray color corresponds to a matrix rich in cobalt, and the light particles dispersed uniformly on the matrix are identified as carbides compounds rich in tungsten. The dark phase corresponds to carbides rich in chromium. So, the good distribution, morphology, and size of those second precipitates are responsible for this alloy’s friction performance, wear resistance, and anticorrosive properties. Figure 3b,c show the surface of the hard coatings (CrN and TiBN) after being deposited on the CoWCr substrate, respectively. Defects were observed on the coated surface. For the CrN coating, a high number of defects, such as small droplets or satellites, pores, and micropores distributed homogeneously, are clearly seen.

Regarding the TiBN coating, the number of defects, mainly pores and droplets, is significantly lower, although their size is larger, and their distribution is more heterogeneous. These defects are characteristic of coatings deposited by the arc evaporation PVD process; the high energy of the arc causes the expulsion of droplets from the cathode, besides atoms of the material deposit. The amount, size, and distribution of defects affect the wear and corrosion behavior of coatings: during the wear stage, the metal of tools comes into contact with their surfaces. Droplets formed on their structure are obstacles to the free sliding movement, which leads to more severe corrosion damage in this condition. Additionally, pores, micropores, and cracks form paths through the solution test penetrates until the substrate-coating interface progressively loses its adherence, resulting in coating detachment and possible corrosion damage to the metal substrate [26]. The thickness measured for the coatings using the ball cratering test was 2.9 μm for CrN and 2.7 μm for TiBN.

Figure 4 shows the Co-Cr-W alloy’s X-ray diffraction patterns, the substrate used, and its corresponding deposited hard coatings (CrN and TiBN). The crystallographic structure and the orientation planes of the substrate material and its coatings were determined by indexing the diffraction peaks using standard reference patterns (databases), as shown in

Table 1. Crystallographic information data obtained from XRD patterns of the substrate Co-Cr-W alloy and its coatings, CrN and TiBN.

Alloy	Symbol	Plane	Structure	Compound
Co-Cr-W	●	(100)	hcp *	Co4W2C
	υ	(101)		(Co,W)6C
	★	(111)		(Co,W)6C
	▼	(220)		-
TiBN		(111)	fcc **	Ti4N3B2
	◯	(200)
CrN	■	(111)	fcc **	CrN
	▲	(200)
		(220)
		(311)

* hcp: Hexagonal close-packed unit cell, crystalline structure. ** fcc: Face-centered cubic unit cell, crystalline structure.

. The XRD patterns provide detailed information about the phases, compound composition, some imperfections/defects present in the crystalline structure, and the symmetry and arrangement of atoms. The substrate material Co-Cr-W showed the presence of Co₄W₂C and (CoW)₆C carbide compounds formed at the characteristic crystallographic planes (100), (101), (111), and (220), as calculated by the Miller indices (hkl), suggesting the growth orientation of the crystallites. Furthermore, a crystalline structure appearing as hexagonal compact (hcp) could be identified.

The results demonstrate a structural change in the atomic arrangement on the surface of the metal substrate that occurs when a CrN or TiBN coating is applied using the PVD vapor deposition process. This atomic arrangement can be identified in the XRD patterns, suggesting a crystalline transformation structure from hexagonal compact (hcp) to a face-centered-cubic (fcc), implying the change in the atoms’ position in the structure; this provides several advantages in terms of surface wear performance, which will be discussed later. In this sense, the CrN coating presented a face-centered cubic crystal structure (fcc). The orientation plane preferential of the CrN coating was (111), (200), (220), and (311) (according to chart diffraction ICDD 04-004-6868); the coating is mainly composed of a uniform CrN compound (see Table 1). Meanwhile, the XRD pattern for TiBN coating showed an intense peak at 2-theta (X-axis) of 36.8° fitting the crystallographic plane of (111) and 42.9° for (200) (ICDD PDF#87-3790), indicating the presence of Ti₄N₃B₂ compounds with a fcc crystalline structure. In addition, peaks located at 44.1°/(101) and 46.9°/(111) were also identified, which are related to the formation of (CoW)₆C compounds, which are characterized by a hcp structure. Consequently, these results suggest that the TiBN coating surface is mainly composed of a complex combination of stoichiometric compounds (Ti₄N₃B₂ and (CoW)₆C), making the coating very attractive for the properties expected in this research study [18].

It can be concluded that depositing a hard coating on a metallic substrate (TiBN/Co-Cr-W) produces the formation of a group of stoichiometric compounds on the surface base of cobalt and tungsten carbides (Co-W)₆C with titanium-nitride borides Ti₄N₃B₂; one of them has a fcc crystalline structure, and the other has an hcp structure. This structural combination provides the following surface benefits: ductility and plasticity suitable for mechanical working and formability, toughness and impact strength, corrosion resistance, and phase stability at high temperatures. This is due to the presence of TiBN as a coating on the Co-Cr-W substrate, which increases the ductility and plasticity of the material/tool due to the fcc structure in the carbide compounds on the coating. This structure has more slip systems, specifically on the planes {111} and directions <110>, leading to atomic and dislocation motion along the crystallography planes and directions. This makes the surface absorb and dissipate energy under the impact of the plastic deformation mechanism on the surface under stress, thus improving abrasion wear resistance. As for the hcp structure results, it has fewer slip patterns but provides higher surface hardness and more outstanding durability.

3.2. Scratching Test

The scratch test was performed to initiate the study of the coatings behavior under frictional forces based on the concept of direct visual inspection of the progress of surface failures on the coating caused by critical loads (Lc) produced using a diamond indenter, which scratches the surface of the test sample. The estimation of the critical loads (Lc) was conducted according to the standard procedures of ISO 20502 [27] and ASTM-C1624 [28] and using Equation (1) [29]. Furthermore, the surface of the scratch tracks was examined using optical microscopy for further explanation of the failure mechanism:

$${\mathrm{L}}_{\mathrm{C}}=\left[{\mathrm{L}}_{\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}{\displaystyle \frac{{\mathrm{l}}_{\mathrm{n}}}{{\mathrm{X}}_{\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}}}\right]+{\mathrm{L}}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{r}\mathrm{t}}$$

(1)

where ${\mathrm{L}}_{\mathrm{C}}$ is the critical scratch load for a defined type of damage at N number of sequences, ${\mathrm{L}}_{\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}$ is the rate of force applied in the specific scratch test, ${\mathrm{l}}_{\mathrm{n}}$ is the distance between the start of the scratch track and the start point of the defined type of damage in the scratch track, ${\mathrm{X}}_{\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}}$ is the rate of horizontal displacement in the specific scratch test, and ${\mathrm{L}}_{\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{r}\mathrm{t}}$ is the preload stylus of force established at the start of the scratch test [29].

For both hard coatings, only one critical load was estimated, ${\mathrm{L}}_{\mathrm{C}1}$, respectively. According to the literature, ${\mathrm{L}}_{\mathrm{C}1}$ was defined in this research as the load associated with the complete detachment of the coating from the substrate, representing an adhesive failure. This failure can be related to increased disturbances in the tangential load [29]. Figure 5a,b show the tangential force (F_t) relationship with its corresponding scratch track images. The adhesive critical load was about 47.1 ± 2.9 N for CrN and 40.7 ± 3.4 N for TiBN coating. These results indicate that TiBN shows worse adhesion than CrN, and the critical load is lower for TiBN. Sakoman et al. (2020) reported a critical load of 50 N for TiBN obtained using the plasma-assisted chemical vapor deposition (PACVD) coating method. So, with the AECVD method, the value is 40.7 N more than 47 N CrN with PAPVD. This indicates the process’s influence on the mechanical and tribological properties. The CrN coating presents better adhesion than TiBN, as seen in the spalling for each coating.

3.3. Wear Test

Following to the wear test, it is crucial to measure the wear volume (V) generated on the sliding surface to estimate the wear constant (k), which represents the wear rate (units mm³/Nm). For this purpose, Co-Cr-W alloy samples, both with and without coating, were analyzed using the Veeco Dektak 150 profilometer, and the calculation data were determined by using the following mathematical expression [30]:

$$\mathrm{k}={\displaystyle \frac{\mathrm{V}}{\mathrm{L}{\mathrm{F}}_{\mathrm{n}}}}$$

(2)

where k is the wear rate and quantifies the volume or mass of material removed per unit of time due to the mechanical interaction with another surface under load. V is the wear volume loss over the surface of the sliding tracks, L is the wear distance, and ${\mathrm{F}}_{\mathrm{n}}$ is the normal applied force. The estimated V values were obtained across the wear tracks; Figure 6a–c show the wear track profiles in the three studied conditions. For the Co-Cr-W alloy (the substrate sample), a material detachment from the surface was observed, reaching a depth of about 1.8 μm, corresponding to a wear loss rate of about 18.4 × 10⁻⁴ mm³/Nm. A coating detachment of about 0.5 μm was observed for the CrN-coated sample; therefore, a wear rate loss of about 2.7 × 10⁻⁴ mm³/Nm was calculated. For the TiBN coated sample, complete detachment of the coating was observed, considering a thickness of about 2.7 μm that was measured with the ball cratering test; for this sample, the wear rate loss of material was measured to be more than 19.1 × 10⁻³ mm³/Nm. Finally, better results were observed for the CrN coating, suggesting higher wear resistance among the three study conditions. These results are summarized in

Table 2. Wear results at 100 m relative wear distance.

Sample Condition	Wear Volume × 10−2 (mm3)	Wear Rate × 10−4 (mm3/Nm)	Wear Depth (μm)
Uncoated	644.0 $\pm$ 25.5	18.4 $\pm$ 0.8	2.0 $\pm$ 0.09
CrN coating	95.6 $\pm$ 7.5	2.7 $\pm$ 0.1	0.5 $\pm$ 0.01
TiBN coating	668.5 $\pm$ 11.5	19.1 $\pm$ 1.2	2.7 $\pm$ 0.11

.

Figure 7 shows the coefficient of friction (CoF) values obtained from the pin-on-disc test. The highest CoF is observed for the uncoated substrate material (CoCrW alloy), which reports values that increase from 0.19 to 0.30 as the sliding distance increases. The CoF values obtained for the coated samples are considerably lower compared to the uncoated sample, reaching up to half of this value. Relating to both hard coatings, the CrN coating shows the lowest value even though it increases as the test progresses, reaching a similar value to the average of the sample coated with TiBN. For the TiBN coatings, the CoF value remains practically constant during the middle of the test, after which it decreases slightly.

According to the CoF values as a function of wear distance shown in the graph (Figure 7), the CoF behavior for any of the coatings reached values approximately 50% lower than that observed by the uncoated sample. An increasing trend of about 0.30 was observed in the CoF behavior of the Co-Cr-W alloy. It should also be noted that the coefficient of friction (CoF) tendency for the uncoated sample increases significantly at the beginning of the wear test; this behavior could be associated with the initial breakdown of superficial roughness, followed by a small period of oscillations. However, this behavior can also be associated with the migration of alumina particles from the pin-on-disc equipment and their possible adhesion to the sample surface, which acts as a temporary lubricant in the wear test. After this removal treatment, the CoF shows a continuous increase in its tendency with unstable oscillation that appears to be associated with the abrasive effects caused by the intense removal of alumina material. In the case of the CrN or TiBN coatings, their tendency was to decrease to a CoF value near 0.15. CrN coating behavior shows a continuous increase to values above 0.15. Meanwhile, the TiNB coating presents a stable CoF trend, but it tends to decrease slightly below 0.15 at the end of the wear test. Compared with the literature, their CoF tendency was increased because they were tested under different environments and load parameters [31,32,33].

For each sample, the tested wear zone was subjected to an EDS analysis, resulting in the spectra that are shown in Figure 8. The analytical EDS spectrum corresponding to the uncoated sample is displayed in the inset of Figure 8a, revealing the constituent elements of the metal substrate. Additionally, traces of the aluminum coming from the alumina used in the pin-on-disc equipment were also detected. The EDS analysis of the sample coated with CrN is shown in Figure 8b; an intense peak was observed that probably belongs to the oxygen alongside the constituents on the coating. This response could be related to the chromium oxide film that grows instantaneously on the coating surface. However, this oxygen could also belong to the alumina powder used in the pin-on-disc equipment.

On the other hand, the EDS spectrum in Figure 8c shows the presence of elemental components of the substrate (Co, Cr, and W), as expected for the sample coated with TiBN, since this hard coating was overcome alongside elements such as B, N, and Ti. As mentioned in the research paper of Wagner et al. [19], boron atoms can diffuse inside the crystal lattice of the substrate, leading to the formation of boron compounds at the bottom of the worn paths, which act as effective lubricant particles. Therefore, this is in accordance with the experimental results. A notable decrease in the CoF values was observed during the intermediate stages of the wear test, which is directly correlated with the compositional analysis.

In the meantime, Figure 8 shows the scanning electron microscope (SEM) images used to investigate and understand the wear mechanisms acting on the metal substrate and its hard coatings during the dry wear test. For the uncoated sample (see Figure 8a), a high amount of adhered material is observed on the surface, confirmed by the EDS analysis mentioned above; this material corresponds to the alumina powder coming from the pin-on-disc equipment. Furthermore, cracks of the carbide’s compounds are observed because, during wear testing, carbides are detached from the surface, and they are caught by the pin on the disc and pressed against the surface, fracturing carbides present in it. For the sample coated with CrN (shown in Figure 8b), tracks in the sliding direction with the characteristic oscillation marks of abrasion wear are observed, and between oscillation marks, attached material from the pin is also observed; for this sample, the characteristic structure of the coating is preserved, as shown in Figure 3b where satellites droplets, pores, and micropores are observed. Figure 8c shows a micrograph of the sample coated with TiBN and subjected to wear testing. Two different phases can be recognized; one of them corresponds to the substrate structure, which implies that all the coating was removed in the contact zone during the wear test. The second phase corresponds to the remaining coating; for this coating, tungsten carbides can be seen, and some of them are cracked in the same way as carbides in the uncoated sample.

Tracking the test sample with a microscope could provide a better understanding of the wear mechanism of the uncoated sample, as shown in Figure 8a. This figure reveals deep scratches resulting from the entrapment of removal particles on the surface of the contact zone. It should be considered that cobalt–tungsten-based alloys contain a large number of compounds in the form of carbides. When these alloys are subjected to a mechanical force, the carbides on their surface come into contact with other carbides, acting as an abrasive particle, leading to unexpected cracking and detachment of the coating, scratches, grooves, or another form of surface alteration. This can result in possible structural damage to the metal substrate. In the same Figure 8, it can also be noted that the most worn surface presents micro-cracks caused by the three-body abrasion mechanism (particle involvement, movement and interaction, and wear mechanism) and caused by the interaction of the fragments with the surface, which causes carbides particles to adhere on the surface through chemical bonds causing wear by adhesion. This could be confirmed by the corresponding EDS analyses, where a high amount of Al dispersed on the coating was found.

For the sample coated with CrN, the low CoF values were related to the formation and removal of the chromium oxide film that is formed on the surface of the coating due to the local increase in temperature at the contact zone, which enables this film to act as a self-lubricant. This suggestion was supported by EDS spectra, which revealed a significant oxygen concentration in the coated surface with CrN [34,35,36,37], whereas for the sample coated with TiBN, a slight decrease in the CoF value was observed after 300 m of sliding; this could be associated with the diffusion of boron atoms into the metal substrate, as observed in the SEM analyses in Figure 8c. As indicated by Erdemir et al. and Hernández-Sanchez et al., the humidity of the environment could react with boron, forming a thin film of boric acid (H₃BO₃), which is considered an excellent solid lubricant due to its crystalline structure [38,39].

The chromium nitride (CrN) coating definitely demonstrates high wear resistance, as indicated by tests conducted in dry environments. This suggests that this hard coating is ideal for all applications where the tools are used in wet and aggressive environments. Additionally, the presence of chromium oxide on the surface not only offers protection to the tools from corrosion but also acts as a self-lubricant. This self-lubrication reduces the friction coefficient between the tool and the working material, which contributes to extending the tool’s service life by minimizing wear and enhancing performance.

3.4. Tribocorrosion Test

Tribocorrosion tests were carried out on the surface of the hard coatings (TiBN and CrN) under two different conditions. In Figure 9a,b, the values of the open circuit potential and friction coefficient measured in the tribocorrosion tests are shown for each condition. The open circuit potential (OCP) was estimated, which provides information about the rates of the oxidation and reduction reactions that occur on the material’s surface (referred to as electrochemical behavior) and the potential value that means the flow electrons from a galvanic couple circuit between the worn or unworn surface area of the exposed material intentionally in any corrosive solution. This electrochemical method tries to simulate the real system conditions very closely, where the electrode potential is controlled by the physicochemical properties of the electrolyte and the tribological behavior between the test sample and the tooling counterpart [40]. Figure 9a describes three distinct corrosion stages. The first stage denotes the stabilization of the steady-state period of immersion time in the electrolytic solution, where the electrochemical reactions reach chemical equilibrium at the metal’s surface. The term corrosion potential (E_corr) refers to internal energy, which is the point where the rates of anodic (oxidation) and cathodic (reduction) reactions on a metal surface are equal in a given environment. The second stage corresponds to the period in which the tooling counterpart slides over the entire surface, breaking the most superficial layer (mainly oxide products), causing that potential to become more negative due to forming new active sites for corrosion. In the final stage, a thermochemical equilibrium is reached again on the metal’s surface, corresponding to a new electrochemical equilibrium condition where the load is withdrawn, and the oxide film can grow up inside the wear tracks until the potential reaches similar values to those seen in the first stage.

The coefficient of friction CoF values obtained from the pin-on-disc tests after immersion in brine (sodium chloride aqueous solution) are shown in Figure 9b. A higher value of CoF was observed for the sample coated with CrN, in which CoF trend behavior increases as the test progresses from a value of 0.14 to 0.17. However, the TiB coating exhibits relatively lower values of CoF, ranging from 0.08 to 0.11 during the entire test. This result suggests that TiB coating has a higher wear resistance than CrN coating, particularly in the brine solution.

In the main stages in which corrosion occurs, which was observed in the initial and final stages, the most negative value of E_corr at the open circuit potential (OCP) was obtained using the electrochemical test for the condition corresponding to TiBN coating. This indicates a possible tendency of greater corrosion damage, while less negative potential is observed in the CrN coating condition, which can be attributed to the formation of the Cr₂O₃ passive film. This passive film effectively protects the metal surface against corrosion attacks or other aggressive reactions. During the wear stage (second stage), the most negative potential value is observed in the sample coated with CrN material. This occurs because the protective oxide film (Cr₂O₃) is constantly being detached, forming new compounds that are transformed into nucleation sites on the surface. These sites make the surface more susceptible to wear and corrosion. In contrast, the sample coated with TiBN exhibits a lower negative potential, indicating that TiBN coating is more thermochemically stable in the brine solution, maintaining a considerable resistance to wear and corrosion under these conditions.

For the sample coated with CrN material, at the beginning of the immersion test, there is an immediate drop in the corrosion potential, which continues dropping slightly during the first stage. After the friction test begins, specifically when rubbing initiates, an abrupt drop of around −500 mV is observed. This significant drop value suggests that the initiation of friction intensifies the corrosion process, probably due to the mechanical wear that disrupts the protective oxide layer on the CrN surface, making it more vulnerable to corrosion. On the other hand, during the rubbing stage, the potential remains almost constant throughout the wear test. Subsequently, after the wear test, the potential gradually increases to more negative values than those observed in the first stage. This behavior could be related to the total removal of the protective oxide film during the wear process. Following the removal, a new oxide film grows in some localized sites, and there is also the possibility of oxide formation on the substrate itself. This transition indicates a dynamic process where the surface is continuously undergoing changes, including damage and subsequent attempts at re-passivation.

The sample coated with TiBN exhibits a slight increase in potential value at the beginning of the immersion test, which decreases slightly during the first stage of the test. When rubbing is applied, the potential diminishes, indicating the occurrence of corrosion damage, such as pitting degradation [20]. However, the potential values are greater than those found in other conditions. During the second stage, two distinct slopes are observed; the first is related to corrosion in TiBN coating, while the second is related to coating detachment. In the final stage, when the load is withdrawn, the potential increases, reaching almost a constant value, which is slightly greater than that shown for the uncoated sample. The final potential value is attributed to the coating detachment and penetration of electrolytes through defects such as droplets, pores, micropores, or cracks in the coating. This behavior suggests that while the TiBN coating offers some initial resistance, its protective capability diminishes over time due to mechanical and chemical degradation.

After the tribocorrosion test, surface observation was accomplished using an optical microscope, as shown in Figure 10, which reveals a significant finding for the CrN and TiBN coatings. Figure 10a shows the wear behavior for the sample coated with CrN; total removal of the CrN coating is observed, exposing the underlying substrate. Additionally, there is a noticeable accumulation of significant corrosion products at the interface of the coating and substrate. Furthermore, preferential corrosion sites were identified on the surface of the substrate, indicating localized areas where the substrate is more vulnerable to corrosion attack when the CrN protective coating failed due to mechanical wear. In Figure 10b, the optical microscope image reveals the total detachment of the TiBN coating. However, in this case, corrosion products are not observed in the wear track, as observed for the CrN coating. This absence of corrosion products can be attributed to the presence of boron inside the wear tracks of the substrate, according to findings from SEM analysis conducted in a dried environment. According to the literature, boron reacts with water molecules, forming H₃BO₃ (boric acid), which is a natural lubricant film. This boric acid film provides a protective barrier, preventing corrosion damage to the substrate surface. This protective effect is further supported by the open-circuit potential values, where the TiBN-coated sample exhibited more stable and less negative potentials, indicating enhanced corrosion resistance.

In tests performed in a brine solution, it was observed that TiBN coating effectively protects the substrate CoCrW alloy from both corrosion attack and wear damage, even if the coating is detached suddenly. Industrial applications of this coating have demonstrated that tools treated with TiBN coatings tend to have a more extensive lifetime of service, as shown in Figure 11. This figure shows that in the TiBN coating implemented in a line production for can enclosure, productivity was enhanced in the process, and the lifetime increases even when the coating is entirely detached; this could be explained by the diffusion of boron in the substrate of the tool, which enhances the surface’s resistance to wear. However, this aspect of boron diffusion and its impact on the substrate’s wear resistance requires further study to fully understand the underlying mechanisms and optimize the coating’s performance.

3.5. Comparision of Behavior in Dry and Brine Media

Regarding the two conditions studied in the tribocorrosion test, both conditions exhibited lower CoF (coefficient of friction) values during the wear stage compared to those obtained during the wear test in a dry condition. This decrease in CoF behavior is attributed to the lubricating effect of the brine solution, which decreases the contact between the tool counterpart and the surface of the coating. Figure 12 presents a comparison of the wear rate for the coatings, which represents the impact effect on the corrosion in wear coatings [30]. For the case of CrN coating, the influence of corrosion on wear rate is considerable; although this coating shows the lowest wear rate in a dried environment, the combined effects of corrosion and wear provoke a substantial detachment of the coating material in the brine solution. In contrast, for the TiBN coating, while the effect of corrosion on the wear rate is also noticeable, this coating demonstrates superior resistance to wear in the brine solution compared to CrN. Despite the challenges posed by corrosion, TiBN maintains a higher level of durability, making it more effective in environments such as brine solutions where both wear and corrosion are critical factors.

In Figure 13, wear profiles are shown, with the dark line corresponding to the wear footprint contour in a dry environment and the gray line corresponding to the contour in a brine media. Figure 13a shows the wear profile of the Co-Cr-W alloy coated with CrN. The profile is relatively shallow in a dried environment, indicating minimal wear. However, in the brine environment, the wear profile shows significant wear, extending beyond the coating into the substrate. Given that the coating thickness is 2.9 μm, the depth of the wear path, nearly 6 μm, suggests that the substrate has been considerably affected by wear once the coating was breached. In contrast, for the TiBN-coated alloy shown in Figure 13b, the substrate remains intact in the dry environment, as the coating effectively protects it. However, the wear profile in the brine environment reveals the highest wear loss, including the detachment of the coating and even wear into the substrate. This indicates that while the TiBN coating offers strong protection in dry conditions, the combined effects of wear and corrosion in a brine environment led to more substantial material loss [40].

4. Conclusions

In this research work, the behavior of two hard coatings, CrN and TiBN, deposited using the AEPVD (Arc-Enhanced Physical Vapor Deposition) process, was successfully studied. The hard coatings were then experimentally studied under two different conditions, in a dry environment and an aqueous brine-based solution, to evaluate their performance in terms of wear resistance and possible corrosion protection.

The findings clearly indicate that the thickness of the CrN coating deposited on the substrate’s surface of Co-Cr-W alloy was significantly greater than that observed in the TiBN coating. This difference in thickness affects the coating’s performance, particularly in terms of wear resistance and more extended durability in different environments during service work.

SEM analyses revealed the typical structure formed in these coatings of CrN and TiBN. For example, a large number of crystallographic defects were observed in the coating CrN, including pores, micropores, cracks, and droplets, which were homogenously distributed along the coating surface. In contrast, the TiBN coating exhibited a lower number of these defects despite having a larger size and a more heterogeneous distribution. This difference in defect characteristics could impact the coatings’ performance in terms of wear resistance and durability.

Therefore, based on the results obtained from the mechanical friction wear tests conducted in dry conditions and in the presence of sodium chloride saline solution (brine), which simulate extreme conditions, such as those encountered in the manufacturing process of sealing cans for food processing and their storage, it is concluded that the sealing materials must be enhanced to improve resistance to both frictional wear and corrosion to ensure reliable performance and extended shelf life of the canned products. In this sense, the application of CrN and TiBN hard coatings deposited on the substrate of the Co-Cr-W alloy, which is commonly used in the food industry, was used in this research for the following reasons:

(a) For tools subjected to wear in an environment free of the acid solution, the CrN coating will protect the substrate from wear.

(b) For tools immersed in a saline solution such as brine, free of load, the CrN coating could protect the substrate from corrosion.

(c) The formation of the thin film of boric acid (H₃BO₃), considered a good solid lubricant, contributes to increasing the lifetime of the tool due to the formation of chrome oxide of alloy that acts as a protecting film.

(d) For tools subjected simultaneously to wear and corrosion in a brine solution, as in the enclosure of brine food, the TiBN coating could protect the substrate, increasing the lifetime of the tool.

(e) The TiBN coating was implemented in a line production for cans’ enclosure; productivity was enhanced in the process, and the lifetime increased even when the coating was completely detached.

Although this study allowed us to find an alternative combination of substrate–coating to the steel used in tools to close cans, the characterization of the coating as roughness and mechanical properties must be analyzed in future work, as well as the interaction between the indenter and the coating, especially the effect of its wear on the coated surface, for a better comprehension of the behavior of these coatings deposited over Co-Cr-W alloys and the alloy itself for more applications.