Title Goes Here

Establishment of Interrelation between Mechanical and Tribological Properties of Plasma Nitrided and TiCrN Coated YXR-7 Tool Steel

Sunil Kumar1, a), Saikat Ranjan Maity2, b) and Lokeswar Patnaik3, c)

1,2,3Department of Mechanical Engineering, National Institute of Technology Silchar, Assam-788010, India

a)Corresponding author: sunilnits18@gmail.comb)saikat.jumtech@gmail.com

c)lokeswar.nits@gmail.com

Abstract. This study aims to establish the interrelation between mechanical and tribological properties of nitrided and coated tool steel. TiCrN coating was deposited on to the nitrided YXR-7 tool steel. Nanoindentation and scratch test were conducted on tempered, nitrided and coated tool steel. Hardness of tempered tool steel was enhanced by 20.31% and 167.48% after nitriding and TiCrN coating. There may be possible reasons for enhancement of hardness, one of them is that the coating crystalline comprises lower grain size (0.0144 µm) than the tempered (2.61 µm) and nitrided (1.89 µm) crystalline whereas, another may be due to elemental diffusion (Fe, Cr, W) from substrate to coating. Strain hardening index (n) was calculated for tempered, nitrided and coated tool steel and the obtained values are 0.1, 0.2 and 0.3 respectively. Higher value for coating indicates lower pile-up during nanoindentation. Coated surface depicts lower depth and narrow scratch track than the nitrided surface due to high scratch hardness. Scratch frictional coefficient was also reduced after coating and a direct relation was found between nanoindentation and scratch test results.

Keywords: YXR-7 tool steel; TiCrN; Coating; Nanoindentation; Nanoscratch

INTRODUCTION

Tool steel has wide application in tooling and manufacturing industries due to high hardness, excellent wear and high temperature resistance. These properties can be enhanced using further surface modification such as nitriding and deposition of ceramic coating (TiN, TiAlN, TiCrN, AlCrN etc). The excellent wear properties of TiCrN coating enhances its intensive use for cutting tools and surface modification of die casting die, moulds, forging die and sheet metal dies to produce high quality of components [1]. TiCrN coating can be deposited by many methods such as cathodic arc deposition, reactive magnetron sputtering and ion deposition method. Out of these methods, magnetron sputtering is very common in industries.

In a study done by Thampi et al[2], they have deposited the TiCrN thin film using magnetron sputtering for cutting tool application and observed reduction in build-up edge formation on cutting edge and increase in tool life. Prabhakaran et al. [3] have deposited TiCrN composite coating on mild steel and as a result of which excellent corrosion resistance of mild steel upon coating was reported. Huang et al. [4] have investigated the micro structural properties of TiCrN and they reported that it has excellent mechanical properties under different application.

Based on the previous literature available, this experimental study was designed which focuses on the deposition of TiCrN coating onto the nitrided tool steel. Characterization of tempered, nitrided and coated surface was analyzed using X-ray crystallography (XRD). It illustrates the consequences of particle-size on hardness of tool steel. Nanoindentation was performed to estimate hardness and modulus of elasticity, based on the values of these properties strain hardening index (n) was computed. Later on, value of strain hardening index was correlated with formation of pile-up during indentation. Finally, nanoscratch test was conducted to achieve the tribological properties such as scratch hardness, frictional coefficient and specific wear rate. Finally, the result of nanoindentation and nanoscratch was correlated using bar diagram.

Materials and methodsSample Fabrication

For this work, YXR-7 tool steel was selected to perform vacuum hardening and plasma nitriding. Elemental composition of this is C-0.8%, Cr-4.7%, Mo-5.5%, W-1.3% and V-1.3% by weight. Hardening and tempering was performed in the same chamber to achieve 62-64 HRC hardness. Furthermore, nitriding was performed using plasma nitriding process. During plasma nitriding NH3 gas was supplied to the vacuum chamber. Pressure of this chamber was maintained at ~900 Pa and the temperature at~500oC. Substrate was connected with negative (-ve) terminal whereas, thermal chamber of machine was connected to positive (+ve) terminal to generate suitable potential difference, so that NH3 gas breaks to generate active atomic plasma of N. Later, this active atomic plasma was deposited upon the steel surface.

Additionally, TiCrN coating was deposited onto the hardened and nitrided tool steel. Surface of tool steel was cleaned using manual grinding and polishing before deposition. The deposition chamber was maintained at 3×10-3 Pa vacuum pressure with approx. 1000 V of negative bias of the substrate. Substrate surface was again cleaned using glow discharge for approx. 4 min. At the time of deposition, 70A/25V of I/V was maintained for Cr target whereas, for Ti target it was maintained at60A/20V. Deposition pressure was approx. 0.7Pa and N2 gas flow was 241 sccm. Substrate was rotated with approx. 4 rotations/rev for 40 min to ensure uniform thickness (approx 3.5 µm) of the coating.

Characterization of Surface Structure

The surface structure of tempered, nitrided and TiCrN coated surface was scanned using X-ray crystallography (PAN alytical B.V.). Grain size of these surfaces was calculated using Scherrer expression (Eq. 1) with the help of FWHM of Bragg peak [5].

(1)

In the above expression, t expresses the grain size (in µm), λ expresses X-ray wavelength (1.54Å) and Bragg peak angle is expressed by θ whereas, broadening factor is expressed by B and the value of B is calculated using expression [5], here Bm can be obtained from FWHM and = 0.15o.

Characterization of Nanoindentation

Nanoindentation was performed using Triboindenter, manufactured by Hystron (model: TI 950) and it comprises 100 nm Berkovich shaped indenter. Maximum load (6000 µN) was applied during indentation with loading/unloading rate 600 µN/s. Hardness and modulus of elasticity was obtained by Oliver and Pharr method using expression H= Pmax/A(hc) and 1/Er={(1-v2)/E}+{(1-)/Ei} respectively. Here, maximum applied load expresses by Pmax, A express the contact area, hc expresses contact depth, reduced Young’s modulus expressed by Er (it can be obtained from machine data), E express modulus of elasticity v and vi express the Poisson’s ratio for substrate and indenter material whereas, Ei expresses the modulus of elasticity of indenter material. In addition to this, strain hardening index (n) was calculated for all three zone using Eq. (4) with the help of Eq. (2) and Eq. (3) [6].

(2)

Here, , E express modulus of elasticity and express yield strength of the tempered, nitrided and coated surface, whereas express yield strength of these surface at 0.29 plastic strain.

(3)

(4)

Characterization of Nanoscratch

Nanoscratch was also performed onto the same machine (Triboindenter). A fixed load (8000 µN) was applied onto the indenter. The scratch track length was set to 10 µm with 0.25 µm/s speed of indenter. Scanning probe microscopy (SPM) was used to analyze the scratch behavior. Fraction of lateral force to normal force gives frictional coefficient during scratch test and the scratch hardness (Hs) was determined using Hs= 2.31(PN/w2), here PN express the maximum applied load onto the indenter, w express the scratch track width whereas, specific wear rate (K) was determined using K=V/(W×L). Here, specific wear rate expressed by K (in m2N-1), applied load is expressed by W (in N) and total distance is expressed by L (in m). V expresses the volume of total wear for the scratch (in m3) and this volume was determined using with the help of depth and length of scratch track (in nm) [6].

Results and discussionsCharacterization of Surface Structure

Coarse and fine carbides were formed after heat treatment and tempering of YXR-7 tool steel. These carbide phases coexist with austenitic, ferritic and martensitic structure of the tool steel. The phases such as M7C3, Feα, Feγ and Feδ were observed at 40o, 45o, 75o and 83o diffraction angle respectively (Fig. 1 (a)).As soon as, nitriding was done using plasma nitrided process, these peaks disappear because during nitriding process iron (Fe) at surface of steel reacts with nitrogen (N) and form different structures. These structures are FeN, Fe2N, Fe3N and Fe4N which observed at 42o, 38o, 30o and 47o respectively. One phase of carbide was observed at 34o diffraction angle (Fig. 1 (b)).

Upon deposition of TiCrN coating, two phases were observed i.e TiN and CrN. These crystals have similar crystalline at same diffraction angle. These peaks were consisting of (111, 200 and 220) planes at 36.4o, 43.5o and 64o diffraction angle respectively (Fig. 1 (c)). The average grain size was calculated using Eq. 1, and they are 0.0144 µm, 1.89 µm and 2.61 µm for coated, nitrided and tempered surface respectively. Many researchers have observed the grain size in similar range [7-9].

FIGURE 1. XRD spectra of (a) tempered YXR-7 tool steel, (b) nitrided surface and (c) TiCrN coating

Evaluation of Nanoindentation

In Fig. 2(a), it can be seen that mechanical properties such as hardness, modulus of elastic and residual strength are distributed across the coated zone, nitrided zone and substrate zone. Fig. 2(a), is showing plot for load-vs-displacement of different zones onto the specimen. Average hardness of the coated zone was found around 20.81±2.68 GPa and modulus of elasticity was 330.17±8.74 Gpa. But, when the indenter moves towards nitrided zone the value of hardness decreases and it was recorded as 9.36±1.82 GPa with modulus of elasticity 243.17±9.87 GPa and finally, when indenter reached to the tempered substrate zone hardness value was on its lowest value i.e 7.78±1.14 GPa and modulus of elasticity232.67±7.47 GPa. It can also be validated by the residual depth (hr) and maximum penetration depth (hmax) of the different zone. Lower value of hr indicates higher hardness [10]. One of the reason for enhancement in hardness of coated zone is its crystalline size i.e 0.0144 µm which was lower than the nitrided zone (1.89 µm) and tempered substrate zone (2.61 µm) [hardness related to crystalline size reference]. The higher value of hardness for coated zone is also validated by perceiving the indentation size and formation of pile-up around it.

Furthermore, residual stress distribution for different zones is shown in Fig. 2(b). Residual stress for coated zone varies within the range from0.32- 4.65 GPa from surface to nitrided zone which is tensile in nature and it varies from 0.0004 to 0.013 GPa in the nitrided zone which is compressive in nature due to presence of diffusion zone, whereas presence of carbides in substrate zone brings the stress level in equilibrium state.

From Fig. 3 it can be said that depth of indentation and pile-up formation for TiCrN coated surface is lower than the nitrided and tempered substrate zone. Many researchers have also reported similar results and they concluded that it is directly influenced by the value of strain hardening index and ratio of plastic work done [11-13].

The calculated value of strain hardening index (n) for coated zone is 0.3 which is higher than the nitrided zone (0.2) and tempered substrate zone (0.1). It validates the lower pile-up for coated surface (Fig. 3).

FIGURE 2.Variation in (a) hardness and modulus of elasticity, (b) residual stress corresponding to distance from the surface

FIGURE 3. Indent image and depth profile for (a-a’) tempered YXR-7 tool steel, (b-b’) nitrided surface and (c-c’) TiCrN coating

Additionally, value for the ration of plastic work is also calculated using expression (WP/WT= h*). In this expression, plastic work is expressed by Wp and total work done is expressed by WT. The value of this ratio for coated surface is slightly lower than 0.5 i.e 0.47 and for nitrided surface it is 0.68 which is higher than 0.5. It also validates the coated surface has low pile-up formation than nitrided surface. When the ratio of plastic work done of a material is higher than 0.5 then the material is considered as ductile material which reveals severe pile-up formation due to large deformation [14].

Evaluation of Nanoindentation

Scratch track of coated surface is narrower than the nitrided surface as it is indicated in Fig. 4(a-b). It exhibits that the scratch hardness of coated surface is higher than nitrided surface. The value of scratch hardness was obtained 24.67 Gpa for coated and 12.83 GPa for nitrided surface. On comparison of both the hardness, it can be said that indentation hardness is lower than the scratch hardness. It is due to the presence of friction force during the scratch test which restrains the indenter to slide forward and penetrate to maximum depth. Similar results have been obtained by Shokrieh et al. [15]. Enhancement in the scratch hardness value for nitrided surface might be due to deposition of coating which comprises lower size of crystalline which is hard in nature. The enhancement of scratch hardness decreases the frictional coefficient.

FIGURE 4. 3D SPM image of scratch track for (a) nitrided surface and (b) TiCrN coating, (c) frictional coefficient for both the surfaces

Behavior of frictional coefficient for coated and nitrided surface can be seen in Fig. 4(b) which exhibits lower variation for coated surface, whereas, nitrided surface exhibits higher variation and lager projections in the friction curve. The average value for friction coefficient was recorded as 0.33 for coated surface and 0.47 for nitrided surface. The increment in frictional coefficient is due to adhesion of substrate material to the indenter surface during ploughing. In general, ductile material shows the ploughing mechanism which leads to increment in frictional coefficient [16-17]. Frictional coefficient was reduced by 29.79 % after deposition of coating which also validates that the coated surface having higher value of hardness. Similar trends in the result have been reported earlier [18-20].

Frictional coefficient of the surface directly influences the specific wear rate of the same surface. Specific wear rate of coated and nitrided surface were calculated and the values obtained were 2.97×10-12 m2N-1 and 5.63×10-12 m2N-1 for coated and nitrided surface respectively. The reduction in specific wear rate was 47.25% after deposition of coating which shows superb wear resistance of the coated surface during the scratch test and it also validates high hardness and lower frictional coefficient of coated surface.

Evaluation of Nanoindentation

The present study also elucidates the interrelationship between mechanical and tribological properties as shown in Fig. 5. Many researcher’s claims that the materials with high hardness and modulus of elasticity are having lower frictional coefficient and specific wear rate and some them also focuses on the relationship between n value and wear resistance and they reported that the material with larger value of n leads to excellent wear resistance [13, 21-23]. Similar results have been obtained in the present study as shown in the Fig. 5. It exhibits coated surface having higher hardness, modulus of elasticity and value strain hardening as well as it also has higher scratch hardness and lower frictional coefficient and specific wear rate.

The relationship between these properties is also established by developing the analytical model. It has been developed by Chen et al [13] in the form of expression (Eq. 5).

(5)

From the above expression lateral force () during scratch test was recorded and it was interrelated with hardness (H), reduced modulus of elasticity (Er) and shear strength (). The value of for nitrided and coated surface was calculated by using Eq. 5 and the values are 72.76 GPa and 101.1GPa respectively. The increase in shear strength upon coating deposition indicates higher lateral force which enhances the scratch resistance of the surface.

FIGURE 5. 3D bar chart for the particulars of (a) nanoindentation and (b) nanoscratch

Conclusion

The present study has been performed on tempered, plasma nitrided and TiCrN coated YXR-7 tool steel to investigate its mechanical and tribological properties. Finally, these properties are interrelated and based upon the above results, few conclusions have been noted below:

Hardness and modulus of elasticity of nitrided surface was improved by 167.48% and 20.31% respectively. It enhances the tribological properties like scratch hardness, frictional coefficient and specific wear resistance.

The value of strain hardening index (n) for coated surface was 0.3 which indicates lower pile-up formation and it has also been validated by the value of the ratio of plastic work done.

A tribological property such as scratch hardness enhanced by 92.28%, frictional coefficient and specific wear rate reduced by 25% and 47.25% respectively.

Higher shear strength value of coated surface also validates higher lateral force and scratch hardness during scratch test.

Acknowledgments

Authors would like to acknowledge National Institute of Technology Silchar and Indian Institute of Technology Kharagpur to conduct experimental work such as XRD and Nanoindentation as well as Nanoscratch respectively.

References

F. Zhang, S.Yan, F. Yin, and J. He, Vacuum 157, 115-123 (2018).

VV AnushaThampi, AviBendavid, and B. Subramanian, Cer. Inte. 42, 9940-9948 (2016).

V. Prabakaran, and K. Chandrasekaran, J. of Bio-and Tribo-Corr. 2, 25 (2016).

M. Huang, Z. Chen, Li. Y. Wang, and Y. Wang, Surf. Engg. 32, 284-288 (2016).

A. Chatterjee, N. Kumar, J.R. Abelson, P. Bellon, and A.A Polycarpou, Wear 265, 921-929 (2008).

L. Patnaik, S.R. Maity, and S. Kumar, Cera. Inter. 46, 22805-22818 (2020).

S. Kumar, Maity, S.R. and L. Patnaik, Cera. Inte. 46, 17280-17294 (2020).

S. Kumar, Maity, S.R. and L. Patnaik, Mate. Tod.: Proce. 26, 685-688 (2020).

F. F. Xia, C. Liu, F. Wang, M. H. Wu, J. D. Wang, H. L. Fu, and J. X. Wang, J.ofallo. and comp. 490, 431-435 (2010).

Q. Wang, F. Zhou, and J. Yan, Surf. and Coat. Tech. 285, 203-213 (2016).

A.E. Giannakopoulos and S. Suresh, Scr. Mater. 40, 1191-1198(1999).

J.D. Gale, and A. Achuthan, J. of mat.scie. 49, 5066-5075 (2014).

Y. Chen, S.R.Bakshi, and A.Agarwal, Surf. and Coat. Tech. 204, 2709-2715 (2010).

H.E.Schaefer, R.Würschum, R.Birringer and H. Gleiter, Phy. Rev. B, 38, 9545 (1988).

M.M.Shokrieh, M.R.Hosseinkhani, M.R.Naimi-Jamal and H.J.P.T.Tourani, Poly. Test. 32, 45-51 (2013).

S.R. Bakshi, D. Lahiri, R.R. Patel, A. Agarwal, Thin Solid Films 518, 1703-1711 (2010).

S.K. Ghosh, P.K. Limaye, B.P. Swain, N.L. Soni, R.G. Agarwal, Surf. Coat. Technol. 201, 4609-4618 (2007).

A. Ramalho, J.C. Miranda, Wear 259, 828-834 (2005).

M. Palaniappa, S.K. Seshadri, Mater. Sci. Eng. A 460, 638-644 (2007).

P. Makkar, R.C. Agarwala, V. Agarwala, Adv. Powder Tech. 25, 1653-1660 (2014).

A. Leyland, A. Matthews, Wear 242, 1-11 (2000).

A. Leyland, A. Matthews, Surf. Coat. Tech. 177, 317-324 (2004).

J.L. Bucaille, C. Gauthier, E. Felder, Wear 260, 803-814 (2006).