Wear performance of AISI 4140 low-alloy steel PVD coated with TiN

ABSTRACT This study studied the tribological properties of PVD AISI 4140 low – alloy TiN coated and reported. The morphologies before testing are assessed with a laser confocal microscope and XRD. Three tests are conducted per sample: the rotating, scratch, and reciprocation tests. The contour of counter bodies and wear scares are studied and reported. The wear scares are also further studied using the SEM-EDS chemical analysis to study the amount of wear that has occurred. The results showed a change in roughness and waviness after coating. In the reciprocating tests showed a high degree of spalling on the TiN sample is observed with a higher wear volume compared to the substrate. However, a lower coefficient of friction is observed. This is similar to the rest of the tests.


Introduction
Developing new technologies in various industries has brought about demands for versatile materials.For engineers and researchers to accurately design systems and machinery, it is very critical for them to understand the behaviour of new and existing materials.It is essential in predicting mechanical systems' life cycle and energy consumption.Friction and other facts have been shown to contribute to more than 20% of factors of energy losses in mechanical systems annually [1][2][3].To overcome these challenges, researchers have explored and developed surface modification methods such as coating.A coating method that has existed for over a hundred years is Titanium Nitride (TiN) physical vapour deposition (PVD) coating.The advantages of this method include its low-cost maintenance, low environmental impact, functionality over various applications, and many more [4].In a recent tribology study, monolayer titanium-based coating has shown excellent resistance to wear when coated on alloy steel, with a 71,73 % reduction in the coefficient of friction [5].It is, therefore, essential to study the reaction of these systems to various surfaces; such studies are generally referred to as tribological research.This area offers great and interesting research opportunities [6].The use of nitrides and carbides has been shown to improve the tool wear and efficiency during CONTACT Rabelani Dennis Murwamadala murward@unisa.co.zaUniversity of South Africa, Department of Mechanical Engineering, Roodepoort, SouthAfrica machining processes.Badiger et al. [5] proposed novel monolayer aluminium and iron-based coatings on a WC-CO substrate for machining applications.A reduction of 22% and 18% in surface roughness and cutting forces, respectively, was reported compared to a WC-CO tool.While the tool wear was reduced by 3,63 and 1,63 times the uncoated tool using the aluminium and iron-based coatings in that order.
EN 19 (AISI 4140) typical applications include gears, shafts, load-bearing tie rods, and other applications requiring high tensile strength and hardness.It has recently gained popularity in the oil and gas industry [7].And quite often, these applications involve surface-to-surface contact with dynamic or static loading.Therefore, selecting the most effective materials efficiently and effectively for designing mechanical systems is critical.This has led to the development of various surface enhancement methods adding increased research opportunities in investigating tribological and other properties caused by these enhancements.
To further enhance the tribological properties of this alloy, it is commonly coated with TiN using physical vapour deposition (PVD).The PVD technique works with different topological modifications, such as ion plating, evaporation, and sputtering.The main focus of these coating methods is initiated by vaporising the coating material and then allowing it to condense on the substrate's surface [8,9].The performance of the coating greatly depends on the adhesion strength of the coating to the substrate [10].The adhesion properties are affected by the type of substrate and surface morphology, thermal expansion coefficient, hardness, and crystal lattice of the substrate and coating [11][12][13].
In this study, the adhesion and wear properties of AISI 4140 coated with TiN are experimentally studied.The instrumentation used for the study allows for a detailed topological study of both the sample's wear track and topological wear on the testing counter body.Based on the literature survey conducted, to our knowledge, there are currently no studies sharing such details.The presented results are useful for the design of both tooling and mechanical parts.

Sample preparation
or this work, two round 25 Ø and 25 mm high AISI 4140 steel samples were precision ground to obtain an almost mirror-shine finish and cleaned.The chemical composition of AISI 4140 is shown in Table 1 [14].To assess the effect of the coating on the steel, one sample is PVD coated with TiN and one is left uncoated, as illustrated in Figure 1.The layer has an average thickness of 5 µm as shown in Figure 2, and the average surface roughness and topology is 0.91 µm.The coating was done at a commercial facility at TD Coating Centre (Pty) Ltd Johannesburg, South Africa.Two tests were conducted, the reciprocating, rotating, and scratch test, using the Rtec MFT-5000.The testing facility allows for a live test recording and direct measurement of the wear tracks and testing probe or ball.

Morphological characterisation of coating and uncoated surfaces
The surface topology of the samples was measured and characterised with a laser confocal microscope, Carl Zeiss LSM 900, XRD analysis of the coated and uncoated surfaces, and  the EDS chemical composition of the test track or scratch with the SEM.Therefore, the samples are evaluated before and after the tests.
The effect of surface roughness on wear is an important factor to take into account when studying the wear rate of a material [15].For that reason, a study of the measured surface topologies was conducted.This is useful information for researchers developing models for wear rate predictions, of machine elements, machining processes, and other applications [16][17][18].It is recommended to use noncontact methods for topography measurements; noncontact methods are both non-invasive and convenient, with the capability to measure a variety of materials with high accuracy and reliability [19].In this study, the measurements are taken using laser confocal microscopy.The measured data is processed and analysed based on ISO 25,178.
From the surface measurements of the base material before coating and after coating there is a clear change in the surface roughness and waviness as shown by Figures 3 and 4.There is more than 10% change in the surface roughness and waviness.Tables 1 and 2 summarises the measured surface properties, roughness, and waviness, respectively, of the base and coating.     of FCC [20], as shown in Figure 6.There is a slight shift in the peaks and this is because of the processing parameters used during the PVD coating.PVD TiN coatings have been shown previously exhibit special magnitudes of orientation.To determine these the Harris texture index, equation 1 is used to illustrate this [21].The texture coefficients are shown in Figure 5.
There is a direct relation between the grain size and the hardness of metallic coatings, to analyse this relationship the Williamson-Hall (W-H) analysis method is used [22,23].The general approach is to determine the size of the average nanocrystalline from the Debye-Scherrer formula in equation 2.Where K is the shape factor (0,9), λis the wavelength and β hkl is the instrument broadening.The estimated strain is also given by the following equation 3: The sum of equation 2 and 3 is assumed to contribute to the line broadening and that each equation is independent to each other with a Cauchy profile, giving equation 4 [24].
The results of the W-H analysis are shown in Figure 7.Where the average crystallite size was determined by the intercept of the fitted line.The W-H analysis indicates a decrease in crystallite size after coating [10].This is caused by oxygen contamination that occurred on the grain boundaries causing low merging earlier than anticipated [25].
The pin-on-disc rotatory test based on ASTM G99 was followed to test the coating wear rate.To measure the reciprocation wear test standard ASTM G133 was followed.Table 3, Table 4 summarises the test parameters used for the study.

Reciprocating test
Shown in Figure 8 are the results of the coefficient of friction of the coated samples and substrate tested over 5 hours using the Al 2 O 3 ball.The slight sudden friction coefficient fluctuations are attributed to the plastic deformation and particles fall off the surface.The friction coefficient of the substrate is significantly higher than the coated sample.This is due to the significant decrease in the surface roughness observed in Figure 4 and Tables 1 and 2 after coating [26].This is also evident in the micrographs of the scratched surfaces and the counter bodies in Figure 9.  N on the substrate material, at an 8 mm radius with a 6 mm diameter WC ball was applied.The TiN coated sample exhibited a lower friction coefficient at 0.55 while the substrate is as high as 0.7.The surface of the counter bodies and wear track were also measured for this test, and this is shown in Figure 10, where (a) is the wear track of the TiN coated sample (b) the counter body used and (c) the wear of the substrate lastly the counter body in (d).In (a) the TiN coating showed a great degree of spalling and plastic deformation and in (b) the counter body was worn greatly leaving a plateau on the contact surface.This is opposed to what was observed for the substrate in (c) and (d).As expected, the size of the spalling debris of the TiN coating is larger than those of the substrate, while the degree of plastic deformation in the substrate track is higher than that of the TiN coated sample.

Scratch test
The scratch tests were conducted based on ISO 20,507; Figure 11 shows the scratch test wear results of the TiN coated sample and Figure 12 the micrographs of the test per failure mode.Three tests were conducted and further summarised in Figure 13.The average critical load LC1 is the first cracking of the coating at 4.42 N, partial delamination occurs at LC2 at an average critical load of 6.58 and lastly full delamination is recorded at LC3 which was an average critical load of 9.22 N as shown in Figure 12(a) to (b).The average wear results of the coating are shown in Figure 14.
Figure 15 shows the scratch wear test results of the substrate.The three tests are summarised in Figure 16; an average critical load of 4,98 N at the start of the scratch and an average final critical load of 10.48 was observed.These are slightly higher than those of the TiN coated sample.However, their maximum depth for the TiN coated sample is lower than that of the substrate.Therefore, there was good adhesion between the substrate and TiN and this aided in improving its COF.
Further analysis of the wear scares was conducted on the SEM using EDS, and results are shown in Figure 17.This was done to chemically analyse the amount of material removed during the TiN coating tests.Based on the micrographs, it is clear that some of the coatings remained intact, or the adhesion seemed better than in other areas of the substrate.On the EDS analysis of the tests, a great amount of Nitrogen was observed still intact in the wear scares of all tests.Some amount of fretting wear is observed in all the tests, this is, however, more prominent in the rotating test.

Conclusion
The present study performed the scratch, rotational, and reciprocation wear tests on AISI 4140 low -alloy steel PVD coated with TiN.A 10% change after coating in morphology was reported, a clear indication of the effect of surface roughness on wear resistance.Concerning the tribological tests, the following conclusions can be made: • In the reciprocating wear test, slight fluctuations in friction coefficient were observed, with those of the untreated sample being higher than those of the coated sample, showing an improvement in wear due to coating.Due to this result, the counter body used on the substrate showed signs of plastic deformation, while that used for the coated sample showed no signs of degradation or deformation.• A similar observation with the rotating wear test was reported.However, the counter body used on the coated sample was deformed or almost flattened with a high degree of spalling along the wear track.While the substrate showed a higher degree of plastic deformation in the wear, the track of the counter body remained intact.• During the scratch test, an average cracking load of 4.42 N on the coated sample was observed, followed by an average partial delamination load of 6.58 N and full delamination at 9.22 N. The average critical load of 4.98 N was recorded for the substrate.
TiN coating application on AISI 4140 increased its wear performance, showing excellent adhesion properties.Future work should look into the optimisation of preprocessing parameters and post processing procedures; and their effects on the tribological properties of TiN coated AISI 4140.

Figure 2 .
Figure 2. (a) SEM micrograph of cross-section showing coating thickness, (b) -(c) EDS analysis of coating and substrate respectively and (d) chemical spectrum of cross-section.

Figure 4 .
Figure 4. Roughness measurements of base material before coating.

Figure 6 .
Figure 6.X-ray diffraction patterns of AISI 4140 and TiN coating.

Figure 7 .
Figure 7. Williamson-Hall analysis of the TiN coated sample and substrate.

Figure 8 .
Figure 8. Reciprocation coefficient of friction over wear time of uncoated and TiN coated.

Figure 9 (
c) shows the wear scare cross-sectional volume of the TiN coated surface.The measured depth was close to 2 µm.In comparison, that of the substrate shown in Figure9(a) was close to 1 µm with more spalling than the TiN coated surface indicating plastic deformation, showing significant improvement to wear resistance.Figure9(d) shows the measured roughness and surface contour of the counter body used to test the TiN coated surface; from the measured roughness there is a clear deformation of the counter body due to the surface's resistance to wear, compared to Figure9(b) where there was almost no deformation on the surface of the counter body.

Figure 10 (Figure 10 .
Figure 10 (a) and (b) show the friction coefficient results of the rotating test on the substrate and coated material.A constant normal force of 5 N on the coated sample and 8

Figure 12 .
Figure 12.Scratch wear results of TiN.

Figure 14 .
Figure 14.Summary of scratch wear results of TiN.

Figure 16 .
Figure 16.Summary of scratch wear results of EN19.

Figure 17 .
Figure 17.SEM EDS chemical analysis of the TiN coated wear tracks.

Table 4 .
Summary of test parameters.