Influence of La/Al Doping via Magnetron Sputtering on the Mechanical and Tribological Properties of TiN Coatings

Abstract

Adding alloying elements to binary nitrides enables the design of hard and tough coatings. To improve the mechanical and tribological performances of TiN-based coatings, La atoms were added to TiAlN coatings to form TiAlLaN coatings. Magnetron sputtering was conducted to prepare the TiAlLaN coatings. Thereafter, scanning electron microscopy (SEM), x-ray diffraction (XRD), nano-indentation, and a tribometer were utilized to test their microstructure, phases, and mechanical and tribological performances. Next, this study analyzed how lanthanum affected the microstructure and tribological performances of the TiAlLaN coatings. Incorporating La atoms in TiAlN coatings reduced the crystallite size and enhanced the coating toughness and hardness. The hardness H and elastic modulus E of the TiAlLaN coatings first increased and then decreased with the increase in La. Meanwhile, the coatings had improved wear and friction properties. The increased H/E and H³/E² levels, which have been considered to reflect the hard coating’s toughness, were acquired based on the TiAlLaN coating, possessing enhanced hardness (19.8 GPa). The coefficient of friction and the wear rates of the coatings reduced and then increased with the increase in La. The TiAlLaN coating with 1.4 % of lanthanum had the lowest friction coefficient and wear rate of around 0.383 and 1.59 × 10⁻⁸ mm³/N·m, respectively, corresponding to a higher H/E (~0.086) and H³/E² (~0.147 GPa). Adding an appropriate amount of La can substantially enhance the TiAlN coating’s tribological and mechanical properties. The TiAlLaN coating with remarkable characteristics may be applied to a steel substrate.

1. Introduction

Hard metal nitride coatings have been more widely applied in industry over the last several decades because of their unique mechanical and tribological properties [1,2]. Among them, TiN has been applied to improve the surface properties of steel, carbides, and diverse alloys, as a hard ceramic coating [3,4]. Although TiN coatings possess numerous merits, they have a high friction coefficient when grinding against the workpiece material, and they can overheat and oxidize under high-speed friction or up to 550 °C, resulting in poor wear resistance [5,6]. Due to this disadvantage of TiN coatings, ternary coatings including TiCrN, TiAlN, and TiMoN have been developed [7,8].

TiAlN coatings possess great hardness, favorable oxidation resistance, and abrasive wear resistance up to 800 °C. As a tool-coating material, the coatings have been extensively utilized for improving the lifespan and performance of cutting tools [9]. TiAlN coatings containing 12% Al had the highest hardness (25.1 ± 1.5 GPa), while further adding Al atoms decreased the hardness [6]. The hardness of Ti0.5Al0.5N coatings is 27.1 GPa [10]. TiAlN-based coatings are deposited through reactive cathodic arc evaporation plus molybdenum codeposition from a planar magnetron source; appropriate Mo atoms may reduce the coating friction by introducing components for forming oxides, and these oxides can thus serve as a solid lubricant [8]. More studies have been conducted to analyze the impact of novel dopants, although not Al in a TiN lattice like Nb, V, Si, or Cr [11]. In order to meet the development of modern high-speed dry cutting technology, further improving the hardness of TiAlN thin coatings and reducing their higher friction coefficient is the key to expanding the application of TiAlN coatings [5,6].

Recent research has assessed the alterations resulting from gradually applying a low amount of rare earth elements to coatings [12,13]. It was shown that adding Y into a TiAlCrN coating can increase the interface stability; TiAlCrYN represents the most advanced coating, and it is successfully used in operations that demand dry high-speed cutting [14]. Cathodic arc evaporation was conducted to deposit novel nanocomposite TiSiLaN coatings. The study investigated how diverse lanthanum levels (0–13%) affected nanocomposites with regard to their chemical composition, mechanical performance, and structural evolution. According to the study’s findings, increasing the lanthanum level dramatically reduced the crystallite size from 6.9 nm to 1.7 nm; notably, coatings that contained lanthanum at 0.4 % had the highest hardness (38.7 ± 1 GPa), wear resistance (H/E = 0.095), and plastic deformation resistance (H³/E² = 0.350 GPa) [15]. A microwave plasma chemical vapor deposition system was utilized to deposit La-doped diamond coatings; thereafter, the impacts of adding La on the diamond coating morphology, quality, and microstructure were comprehensively analyzed. Based on the results, an appropriate amount of lanthanum enhanced the (110) orientation grain growth and promoted the sp³ phase fraction within the deposited diamond coatings [16]. Doping with small amounts of Y or La can improve the interface stability or microstructure, while La can also enhance the surface hardness and wear resistance of coating via cathodic arc evaporation. La, as a rare earth element, possesses a high atomic radius (0.1877 nm) and low attraction to outer electrons. It has unique physicochemical properties and is extensively applied for surface engineering. Nevertheless, there are few studies introducing La to TiN via magnetron sputtering. Thus far, coupling the deposition of AL atoms and the doping of La elements to co-modify TiN has been rarely reported.

Chemical vapor deposition (CVD) arc evaporation [17,18] and DC magnetron sputtering systems [19,20] have been used to fabricate TiAlN coatings. The advantages of magnetron sputtering are the low operation temperature, high deposition rate, uniformity of the thin coating, and flexibility of the process parameters (working pressure, substrate temperature, target-to-substrate distance, target power, and deposition time) [21]. In this work, TiN, TiAlN, and TiAlLaN coatings were deposited via magnetron sputtering. The surface morphology, microstructure, and friction coefficient of the coatings were analyzed and tested via SEM, XRD, and a friction tester. The different characteristics and properties of the TiN, TiAlN, and TiAlLaN coatings were analyzed.

2. Experiments

2.1. Sample Synthesis

The magnetron sputtering system was utilized to prepare TiN, TiAlN, and TiAlLaN coatings. Figure 1 shows the schematic of the magnetron sputtering process. The Ti target, the Al target with 99.99% purity, and the La-Ti alloy target at the 1:1 atomic ratio were employed as sputtering targets (Φ50.8 mm × 3 mm). Ar and N2 were used as the working gas and reaction gas, respectively (purity 99.99%). By using a 304 stainless-steel sheet (Φ30 mm × 2 mm) as the substrate, the coating micromorphology was observed, the tribological properties were tested, and a single crystal silicon sheet with a size of 10 mm × 10 mm × 650 μm was utilized to test the coating hardness and elastic modulus. Sandpaper and grinding paste were utilized to grind the 304 stainless-steel substrate prior to polishing. Before being put in a vacuum chamber, the 304 stainless-steel substrate underwent 15 min of cleaning using acetone and ethanol (Shenyang Science Instrument, Shenyang, China). To improve the coating adhesion, we deposited a Ti interlayer onto the substrate for 20 min between the coatings and the stainless-steel substrate. In the process of deposition, both sputtering targets worked simultaneously, the workpiece table rotated at 20 r/min, and the samples were prepared with two sputtering targets in turn. The main process parameters in the deposition process were as follows: the vacuum was 5 × 10⁻⁴ Pa, the flow rate of the Ar working gas was 40 mL/min, the deposition temperature was 200 °C, and the total deposition time of the coating was 120 min. The specific deposition pressure and power of the coatings are shown in

Table 1. Deposition parameters of the TiN, TiAlN, and TiAlLaN coatings.

Sample Code	Coating	Al Target Power/W	Ti Target Power/W	La-Ti Target Power W	N2 Flow VN2/mL·min−1	Ar Flow VAr/mL·min−1
TiN 1	TiN	-	150-RF	-	40	40
TiN 2		-	200-RF	-	40	40
TiN 3		-	250-RF	-	40	40
TiAlN1	TiAlN	120-DC	200-RF	-	40	60
TiAlN2		150-DC	200-RF	-	40	60
TiAlN3		180-DC	200-RF	-	40	60
TiAlLaN1	TiAlLaN	150-DC	-	150-RF	20	60
TiAlLaN2		150-DC	-	200-RF	20	60
TiAlLaN3		150-DC	-	250-RF	20	60

.

2.2. Sample Analysis

A scanning electron microscope (ZEiSS Sigma 500 Carl Zeiss AG, Oberkochen, Germany) was utilized to observe the coating surface and wear morphology, whereas EDS with SEM was conducted to examine the coating composition. X-ray diffraction (Smartlab RIGAKU, Tokyo, Japan, Cu target, K α radiation, scanning range of 10° to 80°, and scanning speed 1°/min) was carried out to analyze the coating phase.

Additionally, the iNano nano-indentation machine(iNano, Milpitas, CA, USA) was employed to analyze the coating hardness and elastic modulus, and the Berkovich indenter was adopted to test the single-point hardness on a single crystal silicon wafer. To avoid errors, this test was carried out in 5 different positions, and the average of the test results was used. The test load and maximal indentation depth were 50 mN and ≤1/10 of the coating thickness, respectively.

The HT-1000 high-temperature friction and wear tester was employed to analyze the tribological properties of the sample coatings under the following conditions: the GCr15 (AISI 52100) steel ball (φ6 mm) was adopted as a grinding part, the loading load was 1 N, the grinding time was 8 min, the friction radius was 2 mm, and the sliding speed was 4.2 m/min. The test condition was an atmospheric environment, and the friction mode was circumferential cyclic sliding friction in a dry friction condition, whereas the critical friction coefficient was 1. The wear profile of the sample coating was measured using a white-light interference three-dimensional profiler, the wear area was acquired through profile integration, the wear volume was obtained by multiplication of the wear area by the total wear mark length, and the wear rate was determined using Formula (1):

$$\mathrm{W}={\displaystyle \frac{\mathrm{V}}{(\mathrm{F}·\mathrm{L})}}$$

(1)

where W stands for the wear rate, m³/(N·m); V represents the worn volume, m³; F indicates the normal load applied, N; L suggests the total friction stroke, m. Additionally, the mean wear rate from 3 friction tests was determined to reduce the error, whereas the wear rate was employed to be the index to determine the coating wear performance.

3. Results and Discussion

3.1. TiN, TiAlN, and TiAlLaN Coating Morphologies

Figure 2 shows the TiN, TiAlN, and TiAlLaN coating surface morphologies. From Figure 2a–c, the coating prepared under the Ti target, with an RF power of 150 W, exhibited an even and smooth deposition, and there were few pores. After increasing the power to 200 W, the coating surface was smooth and dense; as for the TiN coating prepared at 250 W, a few hole defects were observed on the coating surface, due to the higher target RF power [22].

In Figure 2d, the TiAlN was arranged under a DC power of 120 W, and it showed numerous inclusions and pores on its surface. This is probably due to the tiny Al particle entanglement on the coating surface, thus creating voids on the surface at the lower power. The TiAlN coatings arranged under 150 W of DC power had solid agglomerated particles on their surface, but their grain size decreased compared to the TiN, due to the increase in the DC power with the increased aluminum content in the coating. The agglomerated particles are probably associated with the particles’ random and Brownian motion, finally inducing the sticking and collision of particles at a high power [23]. The TiAlN coating prepared under the DC power of 180 W showed a higher grain surface and surface roughness than the TiAlN coatings under a DC power of 150 W; the increasing power induced excessive aluminum content.

The TiAlLaN coatings were finer and more dense than the TiN and TiAlN coatings arranged under the RF power of 200 W. Adding La into the TiAlLaN coating resulted in grain refinement, probably due to the La₂O₃ or LaN phase generation at the boundaries, leading to restrained crystal growth [24]. Numerous materials also show this phenomenon; for instance, ZnAl-LaDHs is inhibited following lanthanum doping, leading to the denser yet finer nanosheet, where La inhibits the growth of the crystals [15]. This suggests that adding La suppresses the TiAlN coating growth and promotes the coating compactness. Typically, adding La affects the coating morphology, which contributes to the square edge crystal growth [16]. In this process, La atoms are a partial solid solution within the TiAlN lattice, whereas there might be more La gathered intergranularly, leading to a reduced grain size and surface segregation. According to the SEM images, there were many solid agglomerated particles on the TiAlLaN coating surface, suggesting the particles’ random and Brownian motion, similar to that observed at the high power.

According to prior research on TiAlN coatings, the surface morphology is important for the coating’s oxidation resistance, and a coating showing the dense packing of fine grains is more resistant to oxidation [25]. The TiAlLaN coating was smoother and denser than the TiAlN coating; therefore, the TiAlLaN coatings developed under an RF power of 200 W might possess superior mechanical properties and wear resistance to the additional two samples tested.

3.2. Structural Characterization Through XRD

Figure 3 shows the XRD patterns for the coatings arranged under different powers. The XRD patterns show the TiN’s, TiAlN’s, and TiAlLaN’s diffraction phase. All the coatings included the crystalline cubic B1 NaCl-type TiN phase; in addition, every pattern showed significant 304 peaks, because of the 304 phase presence within the iron compound, but these are not marked in the figure.

As shown in Figure 3a, the TiN1, TiN2, and TiN3 curves correspond to three samples with a sputtering power of 150 W, 200 W, and 250 W, respectively. With the increase in the sputtering power, the diffraction peak of the (111) plane increased at first, and then decreased. The diffraction peak of the (200) plane was the strongest in the TiN2 curve, and the corresponding sputtering power was 200 W, while in the TiN1 curve corresponding to 150 W, the diffraction peak of the (111) plane tended to disappear. In the TiN3 curve corresponding to 250 W, the intensity of the (111) plane and (200) plane was not high. In general, the applied sputtering power directly influences the energy distribution of particles emitted from the target material. When the power is low, the atomic weight of Ti sputtered is less, which not only affects the binding of the Ti atoms and N atoms but also affects the condensation and binding of the atomic groups on the substrate. Excessive sputtering power will lead to the existence of excess Ti atoms, and the surplus Ti atoms do not produce more TiN. On the contrary, the surplus Ti atoms may dissociate the bound TiN through collision.

In the TiAlN1 curve corresponding to 120 W, the diffraction peak of the (111) plane was stronger; however, with the increase in power, the diffraction peak intensity of the TiN (111) plane decreased and disappeared gradually. In the TiAlN2 curve corresponding to 150 W, the diffraction peak intensity of the (200) plane increased gradually, and the preferred orientation of the crystal plane changed in the process of thin coating deposition. This is due to the fact that the atomic radius RAl < RTi, and the face-centered cubic structure (Ti,Al)N phase is formed by Al atoms replacing part of the Ti atoms in the face-centered cubic structure TiN. With the increase in power, the continuous addition of Al atoms distorts the face-centered cubic lattice. In the TiAlN3 curve, the atoms within the FCC-TiAlN unit cell exhibit varying vibrational amplitudes due to the influence of interatomic forces. Therefore, with the increase in power, there is a higher degree of distortion in the TiAlN phase in the FCC structure, and lattice distortion stress occurs, which changes the trend of the preferential growth of the TiAlN phase along the crystal plane. At the same time, the increase in Al atoms also suppresses the diffusion of the Ti and N atoms. The surface density and flatness are improved, and the surface structure is improved. Similarly, Shyam Bharatkumar Patel [6] observed the XRD patterns of TiAlN coatings at (111), (200), and (220) crystal planes in their experimental work.

Figure 3c displays the XRD patterns for the TiAlLaN coatings of diverse lanthanum concentrations, where Ti atoms are substituted with La and Al. Generally, adding La markedly promotes the (110) orientation grain growth [16]. The preferential orientation slightly altered from (111) to (200), after increasing the RF power from 150 W to 250 W, suggesting internal stress reduction. Additionally, this finding may be interpreted using the broken bond theory, probably associated with atomic mobility [15]. In line with the above theory, we determined the surface energies of orientations as (111) > (200), and the (200) orientation had increased strain energy in comparison with the (111) orientation. Consequently, the addition of La probably increased the atomic mobility of the deposited atoms, thereby leading to the generation of the (200) orientation. Moreover, the TiN (111) position shows a low-angle shift after increasing the RF power from 150 W to 200 W. This is probably due to the larger-sized La atom solid solution within the FCC TiN lattice [15]. According to Lembk et al. [14], Y might be gathered intergranularly, leading to a grain size reduction and persistent renucleation. Therefore, La may limit the crystalline grain growth. On the contrary, La was detected around 44.3° through the XRD experimentation at the RF powers of 150 W and 250 W. Also at a 200 W RF power, faint La peaks around 62.3° were found. The grain size was determined using the Debye–Scherrer formula [26]. Figure 3d displays the determined grain sizes based on peaks (111) and (200). For those deposited coatings, their grain sizes declined with the increase in the RF power, suggesting that La limits the TiAlLaN crystalline grain growth. This observation indicates that with 1.4% of La, the grain sizes of TiAlLaN3 were the smallest, and the coating became dense.

3.3. Elemental Analysis of the TiN, TiAlN, and TiAlLaN Coatings via EDS

Different elemental compositions (Ti, Al, La, and N) as per atomic at% can be observed in

Table 2. Composition of the various elements of the TiN, TiAlN, and TiAlLaN coatings.

Element	TiN1	TiN2	TiN3	TiAlN1	TiAlN2	TiAlN3	TiAlLaN1	TiAlLaN2	TiAlLaN3
Ti (%)	52.5	54.7	59.1	41.6	40.8	36.1	39.2	41.6	44.8
N (%)	47.5	45.3	40.9	43.2	44.9	46.9	43.9	43.6	38.1
Al (%)	-	-	-	15.2	14.3	17.0	14.7	13.4	15.4
La (%)	-	-	-	-	-	-	2.2	1.4	1.7

. Clearly, the Ti level increases, but the N level decreases within the TiN coatings, as the Ti target power increases. Additionally, for the TiAlN coatings arranged under diverse Al target powers, with the increase in the Al target sputtering power, the atomic content of Al increases gradually, while the content of Ti decreases, and the content of N increases. Based on the analysis, at a low Al content, the Al atom number is limited, while Ti preferentially reacts with N₂ to form TiN. With the increase in the Al content, part of the Al atoms replace part of the Ti atoms to form the Al content in the TiAlN coatings. An increase in the LaTi target power decreased the La content in the coatings from 2.2% to 1.4%, while at an Al target power of 250 W, the La content increased to 1.7%. The TiAlLaN coatings show gently increased Ti/La atomic ratios in comparison with the target, probably associated with the atomic mass. Due to the increased La atomic mass in comparison with the Ti atom, the La atom may be backscattered. The La content within the deposited coatings increased from 1.4% to 2.2% because La has an excessively high atomic mass, causing backscattering in deposition; therefore, it is quite difficult to control La in magnetron sputtering.

3.4. Mechanical Property Prediction Based on Nano-Indentation

Figure 4 displays the hardness (H) and elastic modulus (E) for the TiN, TiAlN, and TiAlLaN coatings arranged under diverse target powers. Typically, the maximal H and E levels in the TiN coating are 12.3 and 201.1 GPa at a 200 W Ti target power. With regard to the TiAlN and TiAlLaN coatings, their maximal H and E are 14 and 203.3 and 19.8 and 229.1 GPa, respectively. The H and E levels in the deposited coatings elevate as the target power increases, which is good for those diverse coatings. There are some factors enhancing the coating mechanical properties, including the orientation, internal macro stress, and crystallite size. In this study, the hardness elevates after adding La, which is probably associated with the grain boundary hardening and solid solution strengthening via the Hall–Petch relationship [24]. The crystallite size reduces as the target level increases, as observed in the XRD images [Figure 3d]. TiAlLaN has an increased hardness in comparison with TiN and TiAlN after adding La and Al into the TiN matrix, giving rise to the reduced crystallite size observed in Figure 3d. Notably, the lower La level (~1.4%) positively affected the nanohardness enhancement. In contrast, the higher La level (~2.2%) elevated the grain size to about 12 nm, resulting in reduced nanohardness. This reduction is associated with the grain boundary sliding, after numerous grain boundary defects led to fast stress-induced atom and vacancy diffusion.

The H/E ratio contributes to predicting the wear resistance of hard coatings, whereas the plastic deformation resistance can be determined by adopting the H³/E² parameter [15]. A high H/E level suggests reduced contact pressure because of the load distribution applied over a large area; consequently, an increased H/E ratio can be expected for hard coatings in various tribological practices [13,15]. According to Figure 4b, the maximal H/E and H³/E² levels were around 0.086 and 0.147 GPa, respectively. Consequently, the maximal wear resistance and plastic deformation resistance were obtained in the TiAlLaN coatings with 1.4% of lanthanum, indicating La can improve the mechanical properties.

3.5. Friction and Wear Behaviors

Figure 5 displays the friction coefficient curves for different TiAlN and TiAlLaN coatings. Clearly, the mean friction coefficient for TiAlN1 prepared with a 120 W Al target power is about 0.601; after increasing the Al target power to 150 W, the TiAlN2 sample coating has a very stable friction coefficient (around 0.494). When the power is increased to 180 W in the TiAlN3 coating, the friction coefficient increases (0.663). As the power increases, the friction coefficient decreases at first and then increases. The same change also exists in the TiAlLaN coatings. As the target power of Al increases from 150 W to 250 W, the friction coefficient first shows a decreasing and later an increasing trend. TiAlLaN2 has the lowest average friction coefficient, which is about 0.383, while that of TiAlLaN1 and TiAlLaN3 is about 0.51 and 0.489, respectively. Therefore, TiAlLaN has a lower friction coefficient than that of TiAlN, demonstrating that the friction coefficient of the TiAlLaN2 coating decreases through adding the appropriate amount of La. This occurs since the surface of the TiAlLaN2 coating is smooth and compact, with fewer defects and higher hardness. It is calculated that the wear rates of the TiAlN1, TiAlN2, and TiAlN3 coatings are 6.13, 2.32, and 4.25 × 10⁻⁸ m³/(N·m), respectively, while those of the TiAlLaN1, TiAlLaN2, and TiAlLaN3 coatings are only 5.23, 1.59, and 3.57 × 10⁻⁸ m³/(N·m), respectively, decreasing by about 20% relative to the TiAlN coatings. Consequently, the wear resistance of the TiAlN coating is improved by adding La. It can be seen that the TiAlLaN coating with 1.4 % of lanthanum has the lowest friction coefficient (0.383) and wear rate (1.59 × 10⁻⁸ mm⁻³/N·m), which is associated with the increased H/E and H³/E² levels. This indicates that the addition of La enhances the tribological properties.

Figure 6 exhibits the worn surface morphologies of the coatings. On the worn surface of the TiAlN1 coating, shown in Figure 6a, cracks are observed, and some areas of the coating surface have even accumulated and peeled off the worn surface. The morphology of the wear marks of the TiAlN3 coating shown in Figure 6c is significantly improved; the number of furrows is reduced, the width of the wear marks remains large, and there are obvious marks at the edges. In Figure 6b, the wear mark depth is obviously shallow, the width is also narrow, the surface of wear marks is neat and smooth, showing a few plowing marks, and the whole surface shows slight abrasive wear characteristics, evidencing good wear resistance. This is because the TiAlN2 coating has better compactness and hardness, and the amount of wear debris produced in the friction process is less. Simultaneously, the wear debris size formed by the refined grains in the friction process is smaller, and some of them are rolled into the bottom of the grinding channel under the extrusion of the friction pair, which partially weakens the effect of abrasive wear, conforming to the above conclusion that it has a low and stable friction coefficient. The worn surface of the TiAlLaN coating in Figure 6d,f shows that the width of the wear mark is large, there are some particles adhering around the wear mark, and it has the characteristics of adhesive wear and abrasive wear, which are considered to be due to the uneven size of the crystal particles, the poor plastic deformation resistance, and the low hardness of the coating surface. The wear surface for the TiAlLaN coating shown in Figure 6e shows a smooth wear surface, with no crack caused by friction. This is related to the increased ductile features in the TiAlLaN3 coating containing increased H/E and H³/E² levels. Furthermore, the dense surface is hard, tough, and smooth, corresponding to the decreased wear rate. Adding an appropriate amount of La helps avoid the cracking of the coating and local delamination and improves the tribological performance of the TiAlN coating.

4. Conclusions

The TiAlLaN coatings possessing an FCC structure are deposited through magnetron sputtering, which form dense and void-free microstructures, and adding La gradually increases the (200) texture. In addition, adding La can substantially enhance the TiAlN coating’s tribological and mechanical properties because of the suppression of the grain growth and the reduction in the grain size. Hard and tough coatings were obtained, which showed better mechanical properties. The higher H/E and H³/E² levels are identified as major factors related to the reduced wear rate and friction coefficient of the TiAlLaN coatings. The TiAlLaN coatings that have increased H/E and H³/E² levels exhibit superior scratch resistance and elastic recovery. The TiAlLaN coating with 1.4 % of lanthanum has the lowest friction coefficient (0.383) and wear rate (1.59 × 10⁻⁸ mm⁻³/N·m), associated with the increased H/E and H³/E² levels. The results of this study allow for establishing a rational coating composition for deposition on tools, providing an increase in the machining efficiency of the materials used in engineering.