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Ti(C, N) multilayer PVD coatings
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International Journal of Refractory Metals & Hard Materials 26 (2008) 456–460 www.elsevier.com/locate/IJRMHM
Microstructure and mechanical properties of Ti(C, N) and TiN/Ti(C, N) multilayer PVD coatings Li Chen a
a,b,*
, S.Q. Wang
a,b
, S.Z. Zhou a, Jia Li b, Y.Z. Zhang
a,b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China b Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., Zhuzhou 412007, China Received 2 August 2007; accepted 22 October 2007
Abstract Magnetron sputtered Ti(C, N) and TiN/Ti(C, N) multilayer coatings are deposited onto cemented carbide substrates at 350 C. The crystal structure and microstructure of the deposited coatings are characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), nanoindentation and scratch test. XRD examination indicates that both coatings are of fcc structure. In accordance with SEM observation, both coatings are columnar crystallites structures and the TiN/Ti(C, N) multilayer coating forms the modulation structure with alternating TiN and Ti(C, N) layers. As for TiN/Ti(C, N) multilayer coating, the hardness and adhesion with substrate are improved due to interface effect between TiN and Ti(C, N) layers. Ti(C, N) and TiN/Ti(C, N) multilayer coatings exhibit approximately identical performance and different failure forms in machining carbon steel. But the performance of TiN/Ti(C, N) multilayer coating inserts in machining stainless steel is superior to Ti(C, N) coating inserts due to the better thermal stability of TiN/Ti(C, N) multilayer coating. 2007 Elsevier Ltd. All rights reserved. Keywords: Gradient Ti(C, N) coating; TiN/Ti(C, N) multilayer coating; Nano-hardness; Adhesion; Cutting tests
1. Introduction It is largely accepted that the performance of tools and components in a tribological, corrosive or mechanical loaded environment is mainly determined by the properties of the near surface coating. TiN coating have been successfully employed as wear resistant coatings in many fields for their high hardness, low wear coefficient and other good properties. In order to enhance its hardness and other properties, Al, C and Si were added into it to form (Ti, Al)N, Ti(C, N) and Ti–Si–N composite coatings [1–3]. The tri-component Ti(C, N) coating, combining the advan-
* Corresponding author. Address: State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China. Tel.: +86 733 2889446; fax: +86 731 2887888. E-mail address: [email protected] (L. Chen).
0263-4368/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2007.10.003
tages of the high hardness of TiC, the high ductility of TiN, and high adhesion strength, possesses much better mechanical properties than single-phase TiC or TiN [4,5]. However, the mechanical properties of Ti(C, N) coating decrease rapid with the temperature increasing. The oxidation resistance temperature of Ti(C, N) coating, which is very important property for coating applied in cutting tools is only 400 C, lower than TiN coating (600 C). The search for further improvements in the properties of coating has resulted in the development of multilayer coating consisting of Ti(C, N) and TiN layers [6–9]. The advantages of multilayer have been discussed in [10]. In the present paper, the microstructures and the mechanical properties of Ti(C, N) and TiN/Ti(C, N) multilayer coatings have been evaluated and compared. The aim of this research is to work out TiN/Ti(C, N) multilayer which combines the properties of TiN and Ti(C, N) coating.
L. Chen et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 456–460
2. Experimental 2.1. Coating deposition The cemented carbide (WC–6 wt.%Co) TNMG120408 inserts were manufactured by conventional powder metallurgical technique. After milled and dried, the powders were pressed to TNMG120408 inserts. The pressed compacts were sintered in vacuum. Ti(C, N) and TiN/Ti(C, N) multilayer coatings with a nominal thickness of about 3 lm were deposited onto the mirror-polished substrates at 350 C by means of magnetron sputtering technique. The sputter target was pure titanium (99.5%). High purity argon (99.99%) was the sputtering gas, while nitrogen and ethyne (purity of 99.99%) were used as reactive gas. The base pressure in the chamber was less than 0.5 mPa, and the working pressure consisting of Ar, N2 and C2H2 was set at 580 mPa during the deposition process. Before deposition, the target was cleaned by Ar glow discharge and then, a thin pure Ti layer was first deposited in order to improve the coating adhesion to the substrate. In the process of the Ti(C, N) coating deposition, the reactive gas is the mixture gas of N2 and C2H2 with a gas-flow rate of 200 sccm for N2 and 20 sccm for C2H2. During the TiN/ Ti(C, N) multilayer coating deposition process, C2H2, the reactive gas, was also admitted discontinuously into the vacuum chamber with reactive gas N2. During TiN layer deposition of TiN/Ti(C, N) multilayer coating, the reactive gas is the N2 with a gas-flow rate of 250 sccm, and during Ti(C, N) layer deposition of TiN/Ti(C, N) multilayer coating, the reactive gas is the mixture gas of N2 and C2H2 with a gas-flow rate of 200 sccm for N2 and 20 sccm for C2H2. This process follows each other in rotation for 11 times. 2.2. Microstructure, nano-hardness and adhesion of coatings The phase identification for the coatings was performed using XRD (Bruker D8, Germany). The diffraction experiments were performed on a 2H/X diffractometer at a small angle of incidence of the primary beam (c = 3) and with a nearly parallel diffraction beam. The microstructure of the as-deposited coatings was observed by means of SEM (LEO1525, Germany) instruments with operating volts of 20 kV. A two-step penetration method with nanoindentation was used to measure the nano-hardness of the coatings with a computer-controlled nanoindentation tester (Fischerscope H100VP, Germany) using a Vickers indenter and continuously applied load. A maximum load with holding time of 10 s was selected to measure the nano-hardnesses of the films. According to the experimental results based on the large-load (30 mN) penetration test, a smaller penetration load of 5 mN was chosen to measure the mechanical properties of the coatings. Such a low load could ensure that the deformation under the indentation tip is controlled within the films and the substrate effect can be avoided. The Vickers hardness HV is computed from the load/
457
unload displacement curves by adopting Oliver and Pharr formula [11]. In this step, as many as 20 indentations were made on each specimen. The scratch tests were performed using a scratch tester (MS-T3000 scratcher, China) with a Rockwell C diamond indenter with a tip radius of 200 lm. A scratch was made on the surface of the coating by progressively increasing the load from 0 N. A crack was initiated along the scratch channel at a certain load, and was followed by a complete delamination leading to failure of the coating. Coating and substrate convert and transmit sound emission signals with continuous fluctuation when the load reaches its critical load (Lc). The value of the first Lc at which sound emission peak fluctuates is for evaluating the adhesion of the coating. 2.3. Cutting test One method for evaluating the performance of coatings inserts is the measurement of flank wear. In the present test, the resistance to abrasion was compared by continuous turning of cabon steel containing 0.45%C and stainless steel (1Cr18Ni9Ti) with TNMG120408 style insert, the cutting conditions were a cutting speed (vc) of 160 m/s, a depth of cut (ap) of 0.2 mm and a feed rate (f) of 0.2 mm/r. Flank wear lands were measured using a microscope in interval 2 min and the inserts were deemed to have failed when the wear lands exceeded 0.3 mm. 3. Results and discussion Fig. 1 shows the corresponding XRD diffraction patterns. The TiN coating XRD diffraction pattern comes from [1]. Three coatings are polycrystalline exhibiting diffraction peaks, which can be indexed based on those of fcc TiN with the structure of NaCl. Compared with Ti(C, N) coating, the peak position of TiN/Ti(C, N) multilayer coating is shifted to the higher angles owing to smaller lattice constant arising from low carbon content. There is obvious difference for the crystal orientation of both coatings. Table 1 shows texture
Fig. 1. XRD patterns of TiN, Ti(C, N) and TiN/Ti(C, N) multilayer coatings.
458
L. Chen et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 456–460
Table 1 Crystal orientation and lattice parameter of three coatings Coating technology
Texture coefficient (1 1 1)
(2 0 0)
(2 2 0)
TiN Ti(C, N) TiN/Ti(C, N)
1.3118 0.8095 1.861
0.2556 0.2700 0.455
1.4581 1.9450 0.702
coefficient of three coatings. The formula calculating texture coefficient can be seen in [10]. The Ti(C, N) coating exhibits (2 2 0) preferred orientation. For TiN/Ti(C, N) mul-
tilayer coating, the (2 2 0) preferred orientation weakens and (1 1 1) preferred orientation strengthens. Fig. 2 shows SEM images of fractured cross-sections of Ti(C, N) and TiN/Ti(C, N) multilayer coatings. Both coatings exhibit dense columnar structures with most of grains extending from the interface to the surface. Fig. 2b shows the modulation structure has been formed in the TiN/ Ti(C, N) multilayer coating, which is composed of alternating TiN and Ti(C, N) layers. The thickness ratio of dark Ti(C, N) layer and bright TiCN layer are 100 nm and 180 nm, respectively. The TiN/Ti(C, N) multilayer coating
Fig. 2. SEM images of cross-sections of Ti(C, N) and TiN/Ti(C, N) multiplayer coatings.
L. Chen et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 456–460
is still columnar grain structure though the coating growth periodically is interrupted by TiN and Ti(C, N) layers. The measured nano-hardness values for Ti(C, N) and TiN/Ti(C, N) multilayer coatings are 34.6 Gpa and 32.2 Gpa, respectively. Compared with TiN coating (22.3 Gpa) [12], the nano-hardness value for Ti(C, N) and TiN/Ti(C, N) multilayer coatings is higher. That is due to solid solution hardening from the partial replacement of the nitrogen atoms in the TiN lattice. Besides solid solution hardening, the interfaces between TiN and Ti(C, N) layers play a key role in hardness enhancement of TiN/Ti(C, N) multilayer coating. To provide an estimation of the associated with multilayer effects, the rule of mixture was applied to TiN/Ti(C, N) multilayer coating [13]. H composite ¼ ½tTiðC;NÞ =ttotal H TiðC;NÞ þ ½tTiN =ttotal H TiN
459
ing. The sharp interface between the coating and the substrate may be eliminated by introducing the concept of gradient and multilayer. For TiN/Ti(C, N) multilayer coating, interface in PVD TiN/Ti(C, N) multilayer coating are sites for energy dissipation. So the interface can relax the stress of coating. Moreover, the interface between TiN and Ti(C, N) layers would absorb the energy of microcracks propagation and prevent them during coating deformation. It is possible that the direction of microcrack propagation changes and is along the interface when the microcarck propagates to the interface. It is known that stainless steels have high work hardening even at low deformations rates and low thermal conductivity. These two characteristics make stainless steels more difficult to machine than carbon steels. The high toughness and high ductility of stainless steel lead to the
where (H) is hardness and (t) is thickness of the layers. The calculated hardness value is then 28.5 GPa, which is 3.7 GPa lower than the measured value. This hardness enhancement is due to the high number of interfaces (dislocation blocking strain effects) contribution between TiN and Ti(C, N) layers. Fig. 3 demonstrates sound emission peaks against the applied load according to the scratch test of the coatings. In general, the adhesion between the PVD coatings and substrate is physical adhesion because of low deposition temperature. The adhesion values of Ti(C, N) and TiN/ Ti(C, N) multilayer coatings are 53 N and 85 N, respectively. The adhesion value for TiN/Ti(C, N) multilayer coating is increased remarkably in comparison of Ti(C, N) coating. As far as Ti(C, N) coatings, the microstructures and properties of substrate and coatings have huge difference. During deformation, the stress will concentrate in the boundary between substrate and coatings. It results in the coating cracking and the adhesion strength decreas-
Fig. 3. Sound emission peaks against the applied load according to the scratch test of Ti(C, N) and TiN/Ti(C, N) multilayer coatings.
Fig. 4. The progress of flank wear of coated inserts in continuous turning carbon steel and stainless steel.
460
L. Chen et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 456–460
formation of long continuous chips and work-harden of the workpiece material to the cutting tool surface. Additionally stainless steels have much lower thermal conductivity as compared to structural carbon steels; this inflicts high thermal impact within the chip-tool contact zone. So more heat is generated at the cutting point in machining stainless steel. Fig. 4 shows maximum flank wear as a function of time at a cutting speed of 160 m/s in turning carbon steel and stainless steel. The flank wear lands of both coating inserts are approximately identical. But the inserts in machining stainless steel show obvious difference. The performance of TiN/Ti(C, N) multilayer coating inserts is superior to Ti(C, N) coating inserts due to the better oxidation resistance of TiN/Ti(C, N) multilayer coating. Moreover it must be noted that the failure form of both coating inserts in machining carbon steel is different. According to observation, the failure form of TiN/Ti(C, N) multilayer coating is abrasion. And the failure form of Ti(C, N) coating is mainly that the coating is flaked from substrate owing to the worse adhesion between Ti(C, N) coating and substrate in comparison with that of TiN/ Ti(C, N) multilayer coating. 4. Conclusions • Ti(C, N) and TiN/Ti(C, N) multilayer coatings have fcc structure. The TiN coating is bell mouth columnar structures. The Ti(C, N) coating exhibits (2 2 0) preferred orientation and TiN/Ti(C, N) multilayer coating exhibits (1 1 1) preferred orientation. Both coatings exhibit dense columnar structures with most of grains extending from the interface to the surface. The TiN/Ti(C, N) multilayer coating forms the modulation structure, which is composed of alternating TiN and Ti(C, N) layers. The TiN/Ti(C, N) multilayer coating is still columnar grain structure though the coating growth periodically interrupted by TiN and Ti(C, N) layers. • The hardness of TiN/Ti(C, N) multilayer coating is increased due to interface hardening. The adhesion value of TiN/Ti(C, N) multilayer coating is better than that of Ti(C, N) coating because the interfaces between TiN and Ti(C, N) layer relax the stress during coating deformation.
• Ti(C, N) and TiN/Ti(C, N) multilayer coating exhibits approximately identical performance but different failure forms in machining carbon steel. But the performance of TiN/Ti(C, N) multilayer coating inserts in machining stainless steel is superior to Ti(C, N) coating inserts due to the better oxidation resistance and adhesion of TiN/Ti(C, N) multilayer coating.
References [1] Chen L, Du Y, Wang SQ, Xu HH. Microstructure and mechanical properties of magnetron sputtered (Ti, Al)N coatings with fcc structure. J Ceram Soc Jpn 2006;114:1081–4. [2] Jindal PC, Santhanam AT, Schleinkofer U, Shuster AF. Performance of PVD TiN, TiCN, and TiAlN coated cemented carbide tools in turning. Int J Refract Met Hard Mater 1999;17:163–70. [3] Veprek S, Argon AS. Mechanical properties of superhard nanocomposites. Surf Coat Technol 2001;146–147:175–82. [4] Yang YS, Lee SC, Tsao C-YA. Properties of graded TiCxNy coatings deposited by a low-temperature HCD ion coating technique. Surf Coat Technol 2001;14:178–87. [5] Bull SJ, Bhat DG, Staia MH. Properties and performance of commercial TiCN coatings. Part 1: coating architecture and hardness modeling. Surf Coat Technol 2003;163–164:499–506. [6] Qiao XL, Hou YH, Wu YP, Chen J. Study on functionally gradient coatings of Ti–Al–N. Surf Coat Technol 2000;131:462–4. [7] Konyashin I. A technique for fabrication of coated TiCN-based cermets with functionally graded structure. Int J Refract Met Hard Mater 2001;19:523–6. [8] Bemporad E, Pecchio C, De Rossi S. Characterization and hardness modelling of alternate TiN/TiCN multilayer cathodic arc PVD coating on tool steel. Surf Coat Technol 2001;146– 147:363–70. [9] Suresha SJ, Bhide R, Jayaram V, Biswas SK. Processing, microstructure and hardness of TiN/(Ti, Al)N multilayer coatings. Mater Sci Eng A 2006;429:252–60. [10] Chen L, Du Y, Yin F, Li J. Mechanical properties of (Ti, Al)N monolayer and TiN/(Ti, Al)N multilayer coatings. Int J Refract Met Hard Mater 2007;25:72–6. [11] Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiment. J Mater Res 1992;7:1564–83. [12] Chen L, Wang SQ, Yin F, Li J. Research on mirostructure and mechanical properties of TiN coating. Cemented Carbide 2006;23:41–3. [13] Santana AE, Karimi A. Microstructure and mechanical behavior of TiAlCrN multilayer thin films. Surf Coat Technol 2004;177– 178:334–40.
International Journal of Refractory Metals & Hard Materials 26 (2008) 456–460 www.elsevier.com/locate/IJRMHM
Microstructure and mechanical properties of Ti(C, N) and TiN/Ti(C, N) multilayer PVD coatings Li Chen a
a,b,*
, S.Q. Wang
a,b
, S.Z. Zhou a, Jia Li b, Y.Z. Zhang
a,b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China b Zhuzhou Cemented Carbide Cutting Tools Co., Ltd., Zhuzhou 412007, China Received 2 August 2007; accepted 22 October 2007
Abstract Magnetron sputtered Ti(C, N) and TiN/Ti(C, N) multilayer coatings are deposited onto cemented carbide substrates at 350 C. The crystal structure and microstructure of the deposited coatings are characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), nanoindentation and scratch test. XRD examination indicates that both coatings are of fcc structure. In accordance with SEM observation, both coatings are columnar crystallites structures and the TiN/Ti(C, N) multilayer coating forms the modulation structure with alternating TiN and Ti(C, N) layers. As for TiN/Ti(C, N) multilayer coating, the hardness and adhesion with substrate are improved due to interface effect between TiN and Ti(C, N) layers. Ti(C, N) and TiN/Ti(C, N) multilayer coatings exhibit approximately identical performance and different failure forms in machining carbon steel. But the performance of TiN/Ti(C, N) multilayer coating inserts in machining stainless steel is superior to Ti(C, N) coating inserts due to the better thermal stability of TiN/Ti(C, N) multilayer coating. 2007 Elsevier Ltd. All rights reserved. Keywords: Gradient Ti(C, N) coating; TiN/Ti(C, N) multilayer coating; Nano-hardness; Adhesion; Cutting tests
1. Introduction It is largely accepted that the performance of tools and components in a tribological, corrosive or mechanical loaded environment is mainly determined by the properties of the near surface coating. TiN coating have been successfully employed as wear resistant coatings in many fields for their high hardness, low wear coefficient and other good properties. In order to enhance its hardness and other properties, Al, C and Si were added into it to form (Ti, Al)N, Ti(C, N) and Ti–Si–N composite coatings [1–3]. The tri-component Ti(C, N) coating, combining the advan-
* Corresponding author. Address: State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China. Tel.: +86 733 2889446; fax: +86 731 2887888. E-mail address: [email protected] (L. Chen).
0263-4368/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2007.10.003
tages of the high hardness of TiC, the high ductility of TiN, and high adhesion strength, possesses much better mechanical properties than single-phase TiC or TiN [4,5]. However, the mechanical properties of Ti(C, N) coating decrease rapid with the temperature increasing. The oxidation resistance temperature of Ti(C, N) coating, which is very important property for coating applied in cutting tools is only 400 C, lower than TiN coating (600 C). The search for further improvements in the properties of coating has resulted in the development of multilayer coating consisting of Ti(C, N) and TiN layers [6–9]. The advantages of multilayer have been discussed in [10]. In the present paper, the microstructures and the mechanical properties of Ti(C, N) and TiN/Ti(C, N) multilayer coatings have been evaluated and compared. The aim of this research is to work out TiN/Ti(C, N) multilayer which combines the properties of TiN and Ti(C, N) coating.
L. Chen et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 456–460
2. Experimental 2.1. Coating deposition The cemented carbide (WC–6 wt.%Co) TNMG120408 inserts were manufactured by conventional powder metallurgical technique. After milled and dried, the powders were pressed to TNMG120408 inserts. The pressed compacts were sintered in vacuum. Ti(C, N) and TiN/Ti(C, N) multilayer coatings with a nominal thickness of about 3 lm were deposited onto the mirror-polished substrates at 350 C by means of magnetron sputtering technique. The sputter target was pure titanium (99.5%). High purity argon (99.99%) was the sputtering gas, while nitrogen and ethyne (purity of 99.99%) were used as reactive gas. The base pressure in the chamber was less than 0.5 mPa, and the working pressure consisting of Ar, N2 and C2H2 was set at 580 mPa during the deposition process. Before deposition, the target was cleaned by Ar glow discharge and then, a thin pure Ti layer was first deposited in order to improve the coating adhesion to the substrate. In the process of the Ti(C, N) coating deposition, the reactive gas is the mixture gas of N2 and C2H2 with a gas-flow rate of 200 sccm for N2 and 20 sccm for C2H2. During the TiN/ Ti(C, N) multilayer coating deposition process, C2H2, the reactive gas, was also admitted discontinuously into the vacuum chamber with reactive gas N2. During TiN layer deposition of TiN/Ti(C, N) multilayer coating, the reactive gas is the N2 with a gas-flow rate of 250 sccm, and during Ti(C, N) layer deposition of TiN/Ti(C, N) multilayer coating, the reactive gas is the mixture gas of N2 and C2H2 with a gas-flow rate of 200 sccm for N2 and 20 sccm for C2H2. This process follows each other in rotation for 11 times. 2.2. Microstructure, nano-hardness and adhesion of coatings The phase identification for the coatings was performed using XRD (Bruker D8, Germany). The diffraction experiments were performed on a 2H/X diffractometer at a small angle of incidence of the primary beam (c = 3) and with a nearly parallel diffraction beam. The microstructure of the as-deposited coatings was observed by means of SEM (LEO1525, Germany) instruments with operating volts of 20 kV. A two-step penetration method with nanoindentation was used to measure the nano-hardness of the coatings with a computer-controlled nanoindentation tester (Fischerscope H100VP, Germany) using a Vickers indenter and continuously applied load. A maximum load with holding time of 10 s was selected to measure the nano-hardnesses of the films. According to the experimental results based on the large-load (30 mN) penetration test, a smaller penetration load of 5 mN was chosen to measure the mechanical properties of the coatings. Such a low load could ensure that the deformation under the indentation tip is controlled within the films and the substrate effect can be avoided. The Vickers hardness HV is computed from the load/
457
unload displacement curves by adopting Oliver and Pharr formula [11]. In this step, as many as 20 indentations were made on each specimen. The scratch tests were performed using a scratch tester (MS-T3000 scratcher, China) with a Rockwell C diamond indenter with a tip radius of 200 lm. A scratch was made on the surface of the coating by progressively increasing the load from 0 N. A crack was initiated along the scratch channel at a certain load, and was followed by a complete delamination leading to failure of the coating. Coating and substrate convert and transmit sound emission signals with continuous fluctuation when the load reaches its critical load (Lc). The value of the first Lc at which sound emission peak fluctuates is for evaluating the adhesion of the coating. 2.3. Cutting test One method for evaluating the performance of coatings inserts is the measurement of flank wear. In the present test, the resistance to abrasion was compared by continuous turning of cabon steel containing 0.45%C and stainless steel (1Cr18Ni9Ti) with TNMG120408 style insert, the cutting conditions were a cutting speed (vc) of 160 m/s, a depth of cut (ap) of 0.2 mm and a feed rate (f) of 0.2 mm/r. Flank wear lands were measured using a microscope in interval 2 min and the inserts were deemed to have failed when the wear lands exceeded 0.3 mm. 3. Results and discussion Fig. 1 shows the corresponding XRD diffraction patterns. The TiN coating XRD diffraction pattern comes from [1]. Three coatings are polycrystalline exhibiting diffraction peaks, which can be indexed based on those of fcc TiN with the structure of NaCl. Compared with Ti(C, N) coating, the peak position of TiN/Ti(C, N) multilayer coating is shifted to the higher angles owing to smaller lattice constant arising from low carbon content. There is obvious difference for the crystal orientation of both coatings. Table 1 shows texture
Fig. 1. XRD patterns of TiN, Ti(C, N) and TiN/Ti(C, N) multilayer coatings.
458
L. Chen et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 456–460
Table 1 Crystal orientation and lattice parameter of three coatings Coating technology
Texture coefficient (1 1 1)
(2 0 0)
(2 2 0)
TiN Ti(C, N) TiN/Ti(C, N)
1.3118 0.8095 1.861
0.2556 0.2700 0.455
1.4581 1.9450 0.702
coefficient of three coatings. The formula calculating texture coefficient can be seen in [10]. The Ti(C, N) coating exhibits (2 2 0) preferred orientation. For TiN/Ti(C, N) mul-
tilayer coating, the (2 2 0) preferred orientation weakens and (1 1 1) preferred orientation strengthens. Fig. 2 shows SEM images of fractured cross-sections of Ti(C, N) and TiN/Ti(C, N) multilayer coatings. Both coatings exhibit dense columnar structures with most of grains extending from the interface to the surface. Fig. 2b shows the modulation structure has been formed in the TiN/ Ti(C, N) multilayer coating, which is composed of alternating TiN and Ti(C, N) layers. The thickness ratio of dark Ti(C, N) layer and bright TiCN layer are 100 nm and 180 nm, respectively. The TiN/Ti(C, N) multilayer coating
Fig. 2. SEM images of cross-sections of Ti(C, N) and TiN/Ti(C, N) multiplayer coatings.
L. Chen et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 456–460
is still columnar grain structure though the coating growth periodically is interrupted by TiN and Ti(C, N) layers. The measured nano-hardness values for Ti(C, N) and TiN/Ti(C, N) multilayer coatings are 34.6 Gpa and 32.2 Gpa, respectively. Compared with TiN coating (22.3 Gpa) [12], the nano-hardness value for Ti(C, N) and TiN/Ti(C, N) multilayer coatings is higher. That is due to solid solution hardening from the partial replacement of the nitrogen atoms in the TiN lattice. Besides solid solution hardening, the interfaces between TiN and Ti(C, N) layers play a key role in hardness enhancement of TiN/Ti(C, N) multilayer coating. To provide an estimation of the associated with multilayer effects, the rule of mixture was applied to TiN/Ti(C, N) multilayer coating [13]. H composite ¼ ½tTiðC;NÞ =ttotal H TiðC;NÞ þ ½tTiN =ttotal H TiN
459
ing. The sharp interface between the coating and the substrate may be eliminated by introducing the concept of gradient and multilayer. For TiN/Ti(C, N) multilayer coating, interface in PVD TiN/Ti(C, N) multilayer coating are sites for energy dissipation. So the interface can relax the stress of coating. Moreover, the interface between TiN and Ti(C, N) layers would absorb the energy of microcracks propagation and prevent them during coating deformation. It is possible that the direction of microcrack propagation changes and is along the interface when the microcarck propagates to the interface. It is known that stainless steels have high work hardening even at low deformations rates and low thermal conductivity. These two characteristics make stainless steels more difficult to machine than carbon steels. The high toughness and high ductility of stainless steel lead to the
where (H) is hardness and (t) is thickness of the layers. The calculated hardness value is then 28.5 GPa, which is 3.7 GPa lower than the measured value. This hardness enhancement is due to the high number of interfaces (dislocation blocking strain effects) contribution between TiN and Ti(C, N) layers. Fig. 3 demonstrates sound emission peaks against the applied load according to the scratch test of the coatings. In general, the adhesion between the PVD coatings and substrate is physical adhesion because of low deposition temperature. The adhesion values of Ti(C, N) and TiN/ Ti(C, N) multilayer coatings are 53 N and 85 N, respectively. The adhesion value for TiN/Ti(C, N) multilayer coating is increased remarkably in comparison of Ti(C, N) coating. As far as Ti(C, N) coatings, the microstructures and properties of substrate and coatings have huge difference. During deformation, the stress will concentrate in the boundary between substrate and coatings. It results in the coating cracking and the adhesion strength decreas-
Fig. 3. Sound emission peaks against the applied load according to the scratch test of Ti(C, N) and TiN/Ti(C, N) multilayer coatings.
Fig. 4. The progress of flank wear of coated inserts in continuous turning carbon steel and stainless steel.
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L. Chen et al. / International Journal of Refractory Metals & Hard Materials 26 (2008) 456–460
formation of long continuous chips and work-harden of the workpiece material to the cutting tool surface. Additionally stainless steels have much lower thermal conductivity as compared to structural carbon steels; this inflicts high thermal impact within the chip-tool contact zone. So more heat is generated at the cutting point in machining stainless steel. Fig. 4 shows maximum flank wear as a function of time at a cutting speed of 160 m/s in turning carbon steel and stainless steel. The flank wear lands of both coating inserts are approximately identical. But the inserts in machining stainless steel show obvious difference. The performance of TiN/Ti(C, N) multilayer coating inserts is superior to Ti(C, N) coating inserts due to the better oxidation resistance of TiN/Ti(C, N) multilayer coating. Moreover it must be noted that the failure form of both coating inserts in machining carbon steel is different. According to observation, the failure form of TiN/Ti(C, N) multilayer coating is abrasion. And the failure form of Ti(C, N) coating is mainly that the coating is flaked from substrate owing to the worse adhesion between Ti(C, N) coating and substrate in comparison with that of TiN/ Ti(C, N) multilayer coating. 4. Conclusions • Ti(C, N) and TiN/Ti(C, N) multilayer coatings have fcc structure. The TiN coating is bell mouth columnar structures. The Ti(C, N) coating exhibits (2 2 0) preferred orientation and TiN/Ti(C, N) multilayer coating exhibits (1 1 1) preferred orientation. Both coatings exhibit dense columnar structures with most of grains extending from the interface to the surface. The TiN/Ti(C, N) multilayer coating forms the modulation structure, which is composed of alternating TiN and Ti(C, N) layers. The TiN/Ti(C, N) multilayer coating is still columnar grain structure though the coating growth periodically interrupted by TiN and Ti(C, N) layers. • The hardness of TiN/Ti(C, N) multilayer coating is increased due to interface hardening. The adhesion value of TiN/Ti(C, N) multilayer coating is better than that of Ti(C, N) coating because the interfaces between TiN and Ti(C, N) layer relax the stress during coating deformation.
• Ti(C, N) and TiN/Ti(C, N) multilayer coating exhibits approximately identical performance but different failure forms in machining carbon steel. But the performance of TiN/Ti(C, N) multilayer coating inserts in machining stainless steel is superior to Ti(C, N) coating inserts due to the better oxidation resistance and adhesion of TiN/Ti(C, N) multilayer coating.
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