Effects of functionally graded TiN layer and deposition temperature on the structure and surface properties of TiCN coating deposited on plasma nitrided H13 steel by PACVD method

Accepted Manuscript Effects of functionally graded TiN layer and deposition temperature on the structure and surface properties of TiCN coating deposited on plasma nitrided H13 steel by PACVD method Elyad Damerchi, Amir Abdollah-zadeh, Reza Poursalehi, Mahtab Salari Mehr PII:

S0925-8388(18)33321-8

DOI:

10.1016/j.jallcom.2018.09.083

Reference:

JALCOM 47502

To appear in:

Journal of Alloys and Compounds

Received Date: 17 June 2018 Revised Date:

5 September 2018

Accepted Date: 8 September 2018

Please cite this article as: E. Damerchi, A. Abdollah-zadeh, R. Poursalehi, M.S. Mehr, Effects of functionally graded TiN layer and deposition temperature on the structure and surface properties of TiCN coating deposited on plasma nitrided H13 steel by PACVD method, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.09.083. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Effects of functionally graded TiN layer and deposition temperature on the structure and surface properties of TiCN coating deposited on plasma nitrided H13 steel by PACVD method

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Elyad Damerchia, Amir Abdollah-zadeha∗∗, Reza Poursalehia, Mahtab Salari Mehrb a

Department of Materials Eng., Tarbiat Modares University, P.O. Box 14115-143, Tehran, Iran

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Faculty of Materials Eng., Sahand University of Technology, P.O. Box 51335-1996, Tabriz, Iran

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Abstract

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The poor adhesion of nitride coatings to steel substrates is one of the main challenges for industrial applications. In this study, plasma nitriding process and TiN functional intermediate layer were used in order to increase the adhesion of TiCN coating to the hot worked steel H13 substrate and also to improve its mechanical properties. The functionally graded nanostructured TiCN coating was deposited using pulsed-DC plasma-assisted chemical vapor deposition (PACVD) method. The coatings microstructural and mechanical properties were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), optical microscopy (OM), Rockwell-C indentation, micro hardness and ball-on-disk tests. The results showed that peaks of TiN and TiCN phases existed in the XRD pattern and the size of their crystallites is about 7 nm. Carbon and nitrogen contents in the TiCN coating gradually increased from the substrate to its surface. The functional coating prepared at 475 ˚C had higher adhesion to the substrate in comparison with those deposited at 450 and 500 ˚C. The high adhesion led to reduction of radial and peripheral cracks within the coating. Decrease in the friction coefficient and lost volume during ball-on-disk tests indicated that the wear resistance of the functional coating deposited at 475 ˚C was 78% higher than that of the other coatings.

1. Introduction

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Keywords: TiCN, PACVD, Functional graded coating, Surface properties.

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Carbide and nitride coatings are widely used to increase the hardness and thermal stability of cutting tools [1]. TiN and TiC have been used to produce hard and wear resistant coatings. Although being harder, TiC coatings have a lower heat transfer coefficient than TiN coatings [25]. TiCN coating has a good chemical stability in corrosive environments, low friction coefficient, high hardness and also appropriate toughness [5-7]. These special properties are resulted by the presence of various percentages of the covalent, metal and ionic bonds which make TiCN coating a promising candidate to be widely used in cutting and punching tools [811]. TiCN coatings have been developed in recent years in a variety of methods including CVD [12-13], magnetron sputtering [14], DC sputtering [15-16] and large area filtered arc deposition (LAFAD) [17]. In PVD process, coatings grow directionally, so one of the limitations of the this ∗

Corresponding author: [email protected] Tel/Fax:+982188005040

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method, is that obtaining coatings with homogeneous structure is dependent on the rotation system.” [6,11]. In addition, the deposition temperature of the coatings in this process is relatively lower than CVD method [1,18,19]. In CVD method, the process temperature is about 900 ˚C, which is above the tempering temperature of steel at 600 ˚C and causes the substrate strength to decrease after the process [18]. These problems have led the researchers to use plasma during CVD process. Plasma-assisted chemical vapor deposition (PACVD) is one of the most applicable methods for modifying the surfaces of industrial molds and tools. In this process, temperature decreases due to the production of active ions and radicals in plasma. The other advantages of this method are its ability to produce coatings with uniform composition and also completely covering the surface of components with complex geometries [19-21]. Recently, binary and ternary coatings, such as TiN, TiC, TiAlN, TiBN, TiSiN, and TiCN, have been successfully produced by PACVD method [19,24]. In recent years, TiN and TiCN nanostructured coatings with improved hardness, toughness and coefficient of friction have been developed by an effective control over deposition conditions [16-25]. Due to the non-conformance of the elasticity modulus and thermal expansion coefficient in the TiCN coating and the steel substrate, thermal stress is induced after deposition operations that might result in a reduced adhesion in the coating. It is possible to improve the adhesion of the TiN coating using plasma nitriding process [20]. During plasma nitriding, the top section of the steel substrate is functionally hardened due to nitrogen diffusion. This causes the modulus of elasticity and thermal expansion coefficient of TiN layer and the substrate to be similar [20,21,26]. In many studies, hardness in TiC1-xNx coatings reaches its maximum when the x value is about 0.4 or 0.5. The hardness of coatings will increase by forming TiC that is harder than TiN [19,27]. Since TiN and TiC are relatively miscible, with respect to other features of these two phases, the coating architecture can be changed by controlling the chemical composition. Unique properties may be achieved by depositing different layers of TiN and TiC phases, or producing a functional and gradual combination from substrate towards the surface of the coating, that are entirely different from typical monolayer or multilayer coatings [20,21,28]. For instance, formation of multilayer nanostructures can significantly improve the mechanical properties of the coating due to the reduced grain size, high density of phase boundaries, residual stresses and distortion of the lattice [29-30]. If the number of layers increases extensively, the mechanical properties will decrease due to the reduction of the quality of the interface between the layers caused by the time limit for gas change during deposition [31-32]. In functionally graded coatings, the structure and composition of the coating can be gradually changed from substrate towards the coating surface. This improves the mechanical properties of the coating because there is no clear boundary between the coating phases and also there is no destructive inter-phase effect on these types of coatings. The main challenge in hard coatings such as TiCN is securing sufficient adhesion and abrasion resistance [33-35]. PVD method has been suggested and used for functional grading of TiCN coatings for adhesion improvement [36-37]. In previous studies, there is no report about the deposition of functional graded TiCN coatings by PACVD method. The main target of the present work is to use PACVD for this purpose, specifically, to produce a functional TiCN nanostructured coating with a functional TiN interlayer on hotworked AISI H13 steel (DIN: 1.2344). First, plasma nitriding is performed on the substrate of the H13 steel prior to deposition, and in the next step, the functionally graded and nanostructured coating of the TiCN with an intermediate TiN layer is established gradually by the PACVD method. Then the effects of functional TiN interlayer and deposition temperature on the structure and surface properties of functional graded TiCN coatings are investigated.

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2. Materials and methods 2.1. Synthesis of coatings

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Disc shaped pieces of 20 mm in diameter and 5 mm in thickness were made from hot-worked H13 steel according to the AISI standard. Table 1 shows the chemical composition of the samples. The specimens were austenitized for 45 minutes at 1050 ˚C in inert gas atmosphere furnace (PT-1400A) and then quenched in oil to ambient temperature. All of the samples were preheated first at 620 ˚C for 2.5 h and then at 550 ˚C for 2.5 h, respectively, in order to increase toughness and remove the residual stresses [38]. Finally, samples were polished using a suspension containing 0.3 µm Al2O3 particles. Samples were cleaned by acetone in an ultrasonic bath. Before each of the plasma-nitriding and deposition processes, surfaces of the steel samples were bombarded using Ar ions to remove probable oxides. For this purpose, Ar and H2 were entered into the chamber by the amount of 75 sccm for each gas. This operation continued until the temperature of the chamber reached to 500 ˚C. In order to increase the adhesion of the coatings, in the next step all samples were subjected to plasma-nitriding for 5 hours. Pulsed-DC PACVD system (Plasma Fanavar Amin (PFA) Co), with an 800W Pulsed-DC power supply was used to perform plasma-nitriding and deposition processes. In next step, the plasma nitrided samples were extracted from chamber and in the final step, the samples were deposited in three different deposition temperatures according to the conditions mentioned in Table 2 to obtain nanostructured and functionally graded coatings. In order to deposition of coatings in each temperature, in the first step; by entering of Ti atoms into the chamber, a thin layer of Ti atoms was deposited on the substrates. It caused the increasing of coating adhesion to the metallic substrates. In the second step; N2 gas was entered into the chamber by the rate of 2 sccm per 90 seconds, until it became constant in the content of 40 sccm. In this step, TiN intermediate functionally graded layer was deposited. In the third step; methane gas was entered into the chamber by the rate of 0.5 sccm in per 90 seconds until its content became constant in 10 sccm. Finally TiCN functionally graded coatings were deposited on the TiN intermediate functionally graded layer. Due to the gradually increasing of the gases precursors into the chamber and formation of functionally graded coating, the interface between the layers have been omitted. This can cause an improvement in mechanical properties of coating. The possible reactions inside the chamber are schematically shown in Fig. 1.

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2.2. Characterizations

2.2.1. Structural, chemical and surface characterizations X-ray diffraction (XRD) with EQuniox 3000 was used to identify the crystalline structures of the plasma-nitrided samples. The XRD patterns obtained at an angle of radiation between 10-100 degrees with a Cu-Kα radiation source (λ=1.54 Å). Low-angle X-ray diffraction with Pw1730 was used with a 0.5˚ radius of radiation to detect the phases in TiCN coatings. Scanning angle was from 10 to 80˚ with a step size of 0.04˚. Morphology and microstructure of the surface and the cross-section of the coatings were investigated using a scanning electron microscope (SEM) (EDS-ZEISS-EV018).

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2.2.2. Mechanical properties The hardness of coatings and substrate was determined by measuring the indentation under a 10 g load with a Micromet1 Buehler LTD.Lakebluff device. Also the hardness of coatings were measured by nano indentation technique (Hysitron Inc., USA, with a Berkovich tip). The indentation rate was 0.1 Nm/s. To prevent the substrate effect, the indentation load was set to 0.6 mN in order to confine the indent depth to less than 10% of coatings thickness. A Surtronic 25 models roughness measuring device was used to measure the wear profile. The wear profile was studied to evaluate the width and depth of the wearing and to calculate the wear volume. The VDI3198 standard was used for measurement of the coating adhesion to the substrate. This test was repeated for six times on each sample. In this standard, the adhesion quality of coatings was examined by applying a load of 150 Kg with Rockwell C indentation on coating and comparing the resulting states with the standard forms, and measuring the length of the peripheral and radial cracks and the number of cracks. The abrasion behavior of nanocrystalline TiCN coatings was studied using a conventional ball-on-disk method; an AISI 52100 steel ball, 5 mm in diameter, was used to examine the wear mechanisms. The abrasion test was carried out under a force of 2 N, wear rate of 20 cm/s, without lubricant at the temperature of 25 ˚C. The radius of abrasion on samples was 7 mm and the sliding distance was 1000 m. Width and depth values of the wear profile were measured accurately using a roughness meter, SEM and optical microscope. These data were used to calculate the lost volume of coating.

3. Results and discussion 3.1. Plasma nitriding

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In order to further increase the hardness of the substrate close to that of the coating, plasma nitriding was performed on heat treated samples. Fig. 2 shows the optical microscope and SEM images of the cross-sectional area of the plasma-nitrided samples prior to deposition in the conventional method. Fig. 2(a) shows the white layer formed during the plasma nitriding process and it was omitted by polishing before deposition of coatings which can be observed in Fig. 2(b). Presence of nitrogen within the structure of underlying steel substrate is obvious in Fig. 2. This is confirmed by the dark areas emerged after etching (Fig. 2(d)). It can be seen that the thickness of the nitriding layer is about 100 µm. Fig. 3 compares the XRD patterns of the steel substrate and plasma nitrided substrates. Due to presence of nitrogen in the steel structure, the peaks of the pattern shifted toward the lower angles, indicating an increase in the distance between atomic planes in steel lattice. Three diffraction peaks at 2
=44.35˚, 64.52˚ and 81.67˚ could be attributed to (110), (200) and (211) planes of H13 steel and plasma nitrided substrate. With performing plasma nitriding process, the hardness of substrate increases from 286 to 701 HV. Fig. 4 shows the results of the microhardness test on the cross-section of plasma nitrided steel before removing the white layer. It may be observed that the hardness from the surface decreases gradually towards the depth as the nitrogen content decreases in the steel substrate structure and ultimately equals to 701 HV. This can be attributed to the heat treatment of the steel substrate. Considering that hardness value is higher than 701 HV in thicknesses above 100 µm, one may deduce that thickness of the diffusive film is at least 100 µm. This is in well agreement with SEM micrographs of the N diffusive region (Fig. 2).

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3.2. Structural properties

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Fig. 5 shows the XRD patterns for functionally graded coatings at 450, 475 and 500 ˚C. Both TiN and TiCN coatings have FCC structures. Given that carbon is a solid solution in TiCN, and also its atomic radius (RC=0.070 nm) is greater than that of the atmospheric nitrogen (RN=0.065 nm), the displacement of the TiCN diffraction pattern to the left (to the lower 2θ) is not unexpected in the TiN diffraction pattern. In other words, presence of carbon in the structure of TiN, increases the distance between the crystalline planes and reduces the angle of diffraction according to the Bragg equation. Similar to TiN coatings, TiCN nanostructured coating has an FCC lattice in which the slip system is on the (111) plane as it is the densest crystalline plane. Based on the Harris equation [39], the small crystalline texture coefficient on (111) plane suggests the exit of a crystalline plane from one slipping plane to another. By changing the preference plane, the accumulation of dislocations leads to increase in the hardness of coatings. Therefore, the crystalline texture coefficient may somehow indicate a relative increase in strength of material [39]. (1)

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In Eq. 1, I (111), I (200) and I (220), express the intensity of the peaks in the XRD pattern on (111), (200) and (220) planes, respectively. The crystalline texture coefficients on the (111) plane for TiCN coating are 0.804, 0.472 and 0.681 at three different deposition temperatures of 450, 475, 500 ˚C, respectively. The crystalline texture coefficient in the (111) plane is the lowest for the coating produced at 475 ˚C. According to the Harris Equation, it can be concluded that the coating produced at 475 ˚C had the highest hardness. The GIXRD patterns of TiCN nanostructured coatings at various deposition temperatures are shown in Fig. 6. In TiCN coatings, the crystalline reference (200) plane has the highest intensity at all three deposition temperatures. This represents formation of the most of the crystalline structure of the coating on the (200) plane. The crystallite size in TiCN coatings was calculated using Debye-Scherrer equation (Eq. 2) based on the peak intensity of the (200) plane: (2)

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where D is the crystallite size, k is shape factor, λ=1.54 Å is the wave lengths of the X-ray source, β is the full width at half maximum and
is the peak position. The calculated results show that the crystallite size of the coating increases from 5 to 7 nm, with increasing the deposition temperature from 450 to 500 ˚C. As shown in Fig. 6, it can be seen that the temperature increase causes a change in the intensity of the corresponding peaks, so that the peak intensity ratio of the (111) plane to the (200) plane is dropped. It seems that by increasing the deposition temperature, the nucleation and growth of (200) plane is increased more than the other plane. As the deposition temperature rises from 450 to 500 ˚C, the distance between the (200) plane decreases from 2.13 to 2.11 Å, hence the lattice parameter is reduced which, in turn, it causes a compressive stress in the coating. The compressive stress in the coating increased with increase in temperature so that the highest stress value was observed at 500 ˚C.

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3.3. Compositional analysis

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The chemical composition changes in functionally graded coatings were investigated at various temperatures of 450, 475 and 500 ˚C and the results are reported in Figs 7 to 9. Deposition process begins with gradual increase in Ti content followed by addition of N and C up to given contents. According to the conditions applied during the deposition process, it is expected that very thin layers of Ti atoms formed on the substrate surface, and then gradually adding nitrogen to the chamber forms a functional layer of TiN. Formation of the functional TiCN layer continues with gradual increase in the carbon content once nitrogen reached a given content. Chemical composition would no longer alter when the functional layer with certain composition deposited. The described process is clearly seen in the EDS results. The thickness of the coating formed at each temperature is measurable using SEM micrographs and EDS diagrams and the results are consistent with each other. An elemental analysis of the crosssectional area of the coatings was conducted which indicates the proper distribution of the elements in the coating and the arrangement of elements from the substrate surface to the coating surface illustrating the functionality of the coating. Gradual variations of the elements in functionally graded TiCN coatings are seen in Figs. 7 to 9. In the surface of substrate, Ti content was higher than those of other elements and the nitrogen increased further to form a TiN layer, in the end, with the gradual increase of carbon, the functional and final TiCN layer is formed. In Figs. 7 to 9, it is possible to use the horizontal axis indicating the distance from the surface of the coating and to obtain the thickness of the coating in light of the beginning of the chemical composition changes. According to the results obtained from the EDS diagrams, the thickness of the coating decreased from 2.0 to 1.3 µm with temperature rise from 450 to 500 ˚C, respectively. Probably, the reason for the reduction of thickness with increasing temperature is the reduction of the time of atoms presence at the surface when the coating is formed. As the temperature in the environment increases, the distance traveled by the atoms in the plasma decreases on the surface and the possibility of presence of one or more atoms in the proper place on the surface decreases. Also, with increasing temperature, kinetic energy of the atoms is increased and the time of the presence of the atoms on the surface decreases, which ultimately results in a coating with a decreased thickness [40]. Fig. 10 shows the SEM micrographs of functionally graded TiCN coatings produced at various temperatures of 450, 475, and 500 ˚C. The thickness of the coatings was measured with SEM micrographs and this was confirmed in the analysis of the chemical composition of the coating by start and finish of the chemical changes in the composition. According to the results of SEM micrographs, the thickness of the coatings at 450, 475 and 500 ˚C was 2.0, 1.8 and 1.3 µm, respectively. Given that the deposition process and the ratio of the inlet gases were the same at each temperature, it can be concluded that the changes in the coating thickness are due to changes in the amount of active carbon particles in the deposition environment. Methane decomposition decreases with decreasing temperature and its decomposition dependence on temperature is greater than other gases. However, carbon content in the structure did not change with a rise in temperature from 475 to 500 ˚C. Also, the carbon content in the structure increased by 8% with increase in temperature from 450 to 475 ˚C leading to an 8% reduction of titanium content in the coating structure. Considering the TiCN coating as a solid solution of TiN and TiC compounds, the increase in carbon content in the coating structure reflects the effect of temperature on the TiC phase formation. The results of the EDS

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analysis are shown in Table 3 which demonstrates that, the content of amorphous carbon increases in the coating structure with increasing coating temperature. Due to the fact that the titanium and nitrogen elements are stable during the last 3 hours of the deposition process, the rate of methane gas decomposition in the reaction chamber increases and the carbon increases in the plasma with the increase in the coating temperature [41] and finally, it increases the carbon content of the coating structure. The carbon atoms within the coating structure, which are not replaced by nitrogen atoms, are released in the form of amorphous carbon in the structure of coatings. According to the results of EDS, it can be observed that amorphous carbon in the structure significantly increased with increasing temperature from 450 to 500 ˚C. To calculate the amount of amorphous carbon, atomic percent of titanium, carbon and nitrogen elements have been used. Nitrogen atoms are supposed to diffuse into the structure and fill the interstitial positions. Otherwise, they would be pumped out by a vacuum pump and replaced by the existing carbon atoms in the plasma. Now, if carbon atoms exceed the available positions, they would form an amorphous structure [42]. Despite the arguments, it is possible to calculate the amount of amorphous carbon in the structure by increasing the atomic percentages of carbon and nitrogen and calculating its difference with atomic percentage of titanium. 3.4 Tribological and mechanical properties

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Fig. 11 shows the morphology of all the coatings. As it may be seen, the surface is without any porosity in all conditions. In all of the coatings, there are remarkable areas on the surface that were formed as islands. The type of layering and growth mechanism strongly depends on the interaction between the coating and substrate atoms. It is anticipated that the nucleation of functionally graded TiCN nanostructured coating is an islet or can be described according to the Volmer-Weber theory [43]. In this type of nucleation, the film begins to grow as an island from the substrate, and the islands grow on the substrate to reach each other and form a layer of coating. Similarly, layers form together and increase the thickness of the coating [43]. With respect to the values obtained from the average size of the islands, the size of the islands decreases with increasing temperature. As the temperature of the deposition process increased from 450 to 500 ˚C, the size of the islands decreased from 493 to 321 nm, respectively. Probably the increase in temperature has increased nucleation centers in the structure and reduced the final dimensions of the islands in the coating. The surface of all specimens has a hill-shaped morphology. Increasing temperature followed by a decrease in crystallite size, causes the height and width of these hills to decrease and ultimately affect the roughness and coefficient of friction of the coating. In the present study, two steps were taken to strengthen the adhesion of the TiCN coating to the substrate. First, the plasma nitrided process was applied to the substrate. Hardness of the substrate is relatively improved due to formation of CrN and VN in the steel substrate [19]. Second, applying functional graded TiN interlayer between substrate and TiCN functional graded coating. The resulting TiN coating reduces the hardness and thermal expansion coefficient differences between the TiCN coating and the substrate and thus improves its adhesion properties. Exposing structure to methane and, subsequently, producing TiCN coatings, as reported before [44], improves the adhesion of the coating. Therefore the coating deposited at 475 ˚C, has the maximized coating adhesion. Measuring radial cracks and peripheral cracks can be useful in comparing the adhesion of coatings. Accordingly, of the fewer radial cracks and the longer their lengths are, the more the adhesion rate decreases. There exists a direct relationship

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between the distance of radical cracks with one another and improvement in coating adhesion and as the distance between radial cracks increases, the adhesion of the coating to the substrate improves. Also, the number of peripheral cracks is important in determining the adhesion of the coating and as the number of peripheral cracks decreases, coating adhesion would improve [45]. Optical microscope images of the Rockwell C indentation are shown in Fig. 12. According to Fig. 12(b), which is related to the coated sample at 475 ˚C, it can be assumed that due to the absence of peripheral cracks and radial cracks of very low lengths, the functional coating adhesion at this temperature reached the maximum value. Regarding 11 radial cracks with an average length of 130 µm in a coating produced at 450 ˚C, compared to 11 radial cracks with an average length of 124 µm in a coating produced at 500 ˚C, it can be concluded that the functional coatings deposited at 450 ˚C and 500 ˚C are not significantly different in terms of number and dimension of radial cracks. Due to the reduction in the number of peripheral cracks by increasing the temperature and bonding of peripheral cracks in the functional coating deposited at 450 ˚C, it can be concluded that the adhesion of the coating increased with temperature. According to the results of the EDS test, it seems by increasing the temperature and passing through the 475 ˚C, the amount of amorphous carbon in the coatings also increased and formed an inappropriate structure with a low structural conformance with the substrate. Since the presence of excess carbon is likely to give rise to the amorphous carbon phase and this phase does not match the substrate structure, the resulting coating is easily removed in case of plastic deformation of the substrate surface. Fig. 13 shows the micro hardness of the H13 steel, heat treated H13 steel, plasma nitrided H13 steel and functionally graded TiCN nanostructured coating. With heat treatment and plasma nitrided processes the hardness of steel substrates have been increased. Fig. 14 shows the nano indentation results of the functionally graded TiCN nanostructured coating at different temperatures. With increasing temperature from 450 to 475 ˚C and increase of carbon in the structure within this temperature range, the hardness of the coating increased [46]. Also, according to the results of XRD analysis, the functional coating produced at 475 ˚C has the lowest crystalline texture coefficient. According to the results of the structural analysis section, it can be concluded that the coating hardness at 475 ˚C reached its maximum. It seems that compressive stresses in the coating is sufficient to produce coherence and hardness in the coating and according to the micro hardness and adhesion results, the compressive stress at 475 ˚C reached optimum levels. Unlike the previous state, the hardness dropped by increasing the temperature from 475 to 500 ˚C. According to the results of the EDS analysis, the amount of amorphous carbon increased and the presence of amorphous carbon in the structure reduced the stiffness. Also, using the results of the structural analysis, it can be concluded that by increasing the temperature the compressive stress exceeds its optimal level in the coating and reduces the hardness of the coating. Fig. 15 shows optical microscope images of the ball on disk test on functionally graded coatings produced at 450, 475, and 500 ˚C. Regarding the wear center, in Fig. 15(a) it can be seen that the areas of the surface are separated as a part of the wear center, which occurred due to the increase in cracks and separation in these areas. Fig. 15(b) shows the optical microscope images of the abrasion zone from the functional coating produced at 475 ˚C. It can be seen that the abrasion area has a lower width than the functional coating produced at 500 ˚C. This indicates a low wear rate in this coating and it can also be concluded that in the center of the wear zone, after a distance of 1000 m, the coating is not completely destroyed. The debris removed from the coating damaged the coating by micro cracks produced by shear stress

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between the steel ball and the surface. Probably, the presence of debris of coating, which are in the form of hard particles between the steel ball and the coating, makes it possible to polish the lateral areas on the abrasion path. No micro cracks could be observed in the images of the functionally graded coating produced at 475 ˚C. Due to the hardness and high adhesion of the functional coating produced at this temperature, the particles are separated very thinly from the coating under the shear stress. Regarding the deformed region around the wear zone, in a coating produced at a temperature of 500 ˚C, it can be seen that by scraping the steel ball and the surface of the coating and deepening the wear depth, a greater amount of steel balls surface get involved with coating and makes the surface more exposed under shear stress. Fig. 16 shows the friction coefficient based on the distance traveled for functional graded coatings produced at 450, 475, and 500 ˚C. By considering the trend of friction coefficient changes, it can be concluded that all coatings had a friction coefficient of 0.15 at the beginning. This is due to the low level of roughness in this kind of production method. As the wear process continues, the coefficient of friction for functional coatings remains constant at 400 m. The fixed amount of friction coefficient in these coatings indicates that these coatings are very stable and hardly abrasive. For functional coatings deposited at 450 and 500 ˚C, it seems that the coating after this range begins to be destroyed, and with the onset of the destruction of the coating, the surface roughness increases. The coefficient of friction increases as the surface roughness increases. The roughness of the surface will increase until all the pieces of the coating are detached and reach the flat substrate surface or the lower layer. In this range, the reduction of the coefficient of friction will occur due to the decrease of surface roughness. This process will cause a rise and fall in the amount of friction coefficient when the coating is lost. On the other hand, since some steel balls are always on the outer surfaces of the wear section, and as the wear depth increases, lateral areas of the steel ball will also engage with the surface and the process of increasing the coefficient of friction will gradually occur. This trend is shown in Fig. 16. Also, increasing and decreasing the coefficient of friction in the functional coating deposited at 475 ˚C in the 400-meters range is not due to the loss of the coating, because after reducing the friction coefficient in the range of 400 m, the friction coefficient does not increase again, and fixed at its lower limit, and the friction coefficient does not change dramatically until the end of 1000 m. According to the results, it can be stated that during the 1000-meters period, the coating produced at 475 ˚C was not completely destroyed. It is likely that the increase in the friction coefficient in the range of 400-meters is due to the particles separated from the steel ball, which, by placing between the surface of the ball and the coating, increase the coefficient of friction [47]. According to the results obtained from the wear test for the coatings, it can be seen that the functional coating deposited at 475 ˚C is more resistant to wear than the functional coatings at 450 and 500 ˚C. According to the results obtained in the above sections, the functional coating produced at 475 ˚C has a high adhesion and hardness compared with the other coatings and, consequently, has a higher abrasion resistance. The wear profiles of functional coatings at 450, 475, and 500 ˚C are shown in Fig. 17. Considering the thickness of the coating deposited at 450 ˚C and matching the abrasion depth at this temperature, it can be concluded the coating has been completely destroyed after a distance of 1000 m. This is evident from the friction coefficient of the coating produced at 450 ˚C (Fig. 16(a)). The friction coefficient at the end of 1000 m is close to the friction coefficient of the substrate. According to Fig. 17, it can be seen that the wear depth in the functional coating produced at 475 ˚C is less than the depth of abrasion in functional coatings produced at 450 and 500 ˚C. Probably due to the high hardness and adhesion in the functional coating produced at 475

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C as compared to functional coatings produced at 450 and 500 ˚C, the wear rate in this coating is decreased. Depending on the depth of the wear profile and the thickness of functionally graded coating produced at 475 ˚C, it can be concluded that, after a distance of 1000 m, the coating was not completely destroyed. These results are consistent with the results of optical microscope images and friction coefficients. Using the wear depth profiles for functional coatings and calculating the area of the wear in each of the coatings, the total volume lost in the coatings can be obtained. The calculated total wear volumes are reported in Table 4. They match the results of the wear profile for the coatings. It can be concluded that the amount of material lost in the functional coating produced at 475 ˚C is lower than that of the functional coating produced at 450 and 500 ˚ C.

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4. Conclusions

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In this study, TiCN functionally graded nanostructured coatings were deposited on a nitrided H13 steel substrate by pulsed-DC PACVD method. Coatings were produced using H2, N2, Ar, CH4 and TiCl4 at 450, 475, and 500 ˚C. The results are as follows: 1) By performing PN process and nitrogen penetration in the steel structure, the hardness increased from 701 to 1300 HV. 2) The lowest crystalline texture coefficient was related to the coating formed at 475 ˚C with a value of 0.472 that led to increase in hardness of coating (about 30GPa). Amorphous carbon in coatings increased about 1% by increasing the temperature from 450 to 500 ˚C. Increasing the amount of amorphous carbon in the coatings prevented the growth of islands on the surface of the coating and the final size of the islands decreased. The hardness of coating deposited at 500 ˚ C, decreased by increasing the amount of amorphous carbon by 0.58% and increasing the crystalline texture coefficient about 0.209. 3) The size of the islands formed on the surfaces of the coatings decreased from 493 to 321 nm with increasing the temperature from 450 to 500 ˚C. Increasing the temperature reduced the roughness of the surface by reducing the nucleation centers in the structures and the final dimensions of the crystallites. 4) By increasing the temperature from 450 to 500 ˚C, the thickness of the coatings reduces from 2.0 to 1.3 µm. 5) Plasma nitriding process and forming a functional intermediate layer and a functional coating increased the adhesion of the coating to the substrate. Maximum adhesion obtained at 475 ˚C. 6) The lowest coefficient friction and maximum wear resistance of the coatings was related to the coating produced at 475 ˚C. The wear resistance in the coating produced at this temperature improved by 78%. References

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Table 1 Chemical composition of AISI H13 steel (in wt%) C Si Mn Ni Cr Mo 0.367 0.902 0.407 4.883 1.372

V 0.863

Fe remain

Frequency (kHz)

Time (min)

temperature (C˚)

pressure (Torr)

Ar flow rate (sccm)

H2 flow rate (sccm)

N2 flow rate (sccm)

C flow rate (sccm)

TiCl4 flow rate (sccm)

Plasma-nitriding

10

300

500

1.1

75

75

40

-

-

TiCN functional graded coating

10

180

450-475-500

2.1

100

200

0-40

0-10

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Table 2 Plasma-nitriding and deposition conditions

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Table 3 The EDS test results of the chemical composition of the functional TiCN coating at 450, 475 ˚C and 500 ˚C Temperature Ti N2 C O2 Cl amorphous TiCxN1-x (˚C) (at%) (at%) (at%) (at%) (at%) carbon (at%) 51.7 17.3 25.9 2.8 2.3 0 TiC0.64N0.36 450 45.1 12.3 33.4 7.1 2.1 0.57 TiC0.73N0.27 475 44.8 11.9 34.0 7.7 1.6 1.15 TiC0.72N0.28 500

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Table 4 The total volume lost due to abrasion for functional graded coatings produced at 450, 475 ˚C and 500 ˚C Coating temperature (˚C) 450 475 500 7.6×106 7.4×106 9.4×106 Total wear volume (um3)

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Fig. 1. A schematic of chamber and deposition process of functionally graded TiCN coatings.

Fig. 2. Micrograph of the cross-sectional area of the plasma nitrided samples (a) with white layer, (b) and (c) scanning electron microscope after removing white layer, (d) optical microscope after removing white layer.

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Fig. 3. Comparison of XRD patterns for steel and plasma nitrided substrates.

Fig. 4. The hardness profile of the cross-section of the plasma nitrided steel.

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Fig. 5. The XRD patterns of functionally graded coatings at 450, 475 and 500 ˚C.

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Fig. 6. The GIXRD patterns of TiCN nanostructured coatings at various deposition temperatures.

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Fig. 7. The EDS graph from cross-sectional area of in the coating deposited at 450 ˚C.

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Fig. 8. The EDS graph from cross-sectional area of in the coating deposited at 475 ˚C.

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Fig. 9. The EDS graph from cross-sectional area of in the coating deposited at 500 ˚C.

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Fig. 10. Cross-section image of the functional graded TiCN coatings produced at (a) 450 ˚C, (b) 475 ˚C and (c) 500 ˚C.

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Fig. 11. SEM micrographs of functionally graded TiCN coatings produced at (a) 450 ˚C, (b) 475 ˚C and (c) 500 ˚C.

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Fig. 12. Optical microscope images of the Rockwell C indentations on functional graded TiCN coating produced at (a) 450 ˚C, (b) 475 ˚C and (c) 500 ˚C.

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Fig. 13. Micro hardness of the H13 steel, heat treated H13 steel, plasma nitrided H13 steel and functionally graded TiCN nanostructured coatings.

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Fig. 14. The hardness of the functionally graded TiCN nanostructured coatings at different temperatures.

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Fig. 15. Optical microscopic image of wear path for functionally graded coatings at (a) 450 ˚C, (b) 475 ˚C and (c) 500 ˚C.

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Fig. 16. Friction coefficient of coatings produced at (a) 450 ˚C, (b) 475 ˚C and (c) 500 ˚C.

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Fig. 17. Wear profiles of functional graded coatings produced at (a) 450 ˚C, (b) 475 ˚C and (c) 500 ˚C.

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Highlights •

A functionally graded TiN layer was used for enhancing the adhesion of TiCN coating on

The hardness of coating deposited at 500 ˚C, decreases through increasing the amount of amorphous carbon by 0.58%.

•

By increasing the deposition temperature from 450 to 500 ˚C, the thickness of the TiCN coating reduces from 2.0 to 1.3 µm.

Plasma nitriding process and using a functional interlayer increases the adhesion of the TiCN coating to steel.

The lowest coefficient friction and maximum wear resistance of the TiCN coatings was

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related to the coating produced at 475 ˚C.

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hot worked steel H13.