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Effect of ultra-fine TiC0.5N0.5 on the microstructure and properties of gradient cemented carbide
Journal of Materials Processing Technology 209 (2009) 5293–5299
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Effect of ultra-fine TiC0.5 N0.5 on the microstructure and properties of gradient cemented carbide Ji Xiong a , Zhixing Guo a,∗ , Mei Yang b , Sujian Xiong a , Jianzhong Chen a , Yuemei Wu a , Bin Wen c , Ding Cao a a b c
School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, PR China College of Materials and Chemistry and Chemical Engineering, Chengdu University of Technology, Chengdu 610059, PR China Baker Hughes Incorporated, United States
a r t i c l e
i n f o
Article history: Received 25 October 2008 Received in revised form 24 March 2009 Accepted 28 March 2009 Keywords: Gradient cemented carbide Ultra-fine TiC0.5 N0.5 Microstructure and properties
a b s t r a c t To increase the performance of cemented carbide, it was common to coat the wear surfaces with thin layers of hard materials. Cemented carbides with surface zone depleted of hard cubic phase and enriched in ductile binder phase were commonly used as the substrates. At the present, the so-called “gradient cemented carbides” were usually prepared by adding TiN or medium-sized TiCN, and a two-step sintering process including pre-sintering in nitrogen and gradient sintering in vacuum was usually adopted. In this paper, gradient cemented carbide based on WC–5.19 wt.%Ti–9.2 wt.%Co was prepared by simple vacuum sintering with the addition of 0.13 m ultra-fine TiC0.5 N0.5 , compared with medium-sized TiN or TiCN. The result showed that gradient cemented carbide prepared with 0.13 m ultra-fine TiC0.5 N0.5 had better properties. With the increase of ultra-fine TiC0.5 N0.5 content, the thickness of gradient surface zone decreased. TiCN coatings were deposited on gradient substrate and conventional substrate by moderate temperature chemical vapor deposition (MTCVD), respectively. Gradient substrate resulted in better adhesion and cutting performance. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Cemented carbides were commonly used as cutting tools for metals. To increase the performance of the tools further, it was common to coat the wear surfaces with a thin layer of hard materials such as TiC, TiN and Al2 O3 , or multi-layers of such hard materials (Ekroth et al., 2004). The coated layers were usually grown by chemical vapor deposition (CVD) at temperatures around 1000 ◦ C. Cracks would unavoidably form in the coatings due to differences in thermal expansion coefficients between coatings and substrates during cooling after the coating process. When the inserts were used for machining, these cracks might propagate into the substrates and cause failures. Therefore, it was of interest to prepare gradient substrates with tough surface zones, which were enriched in tough binder phase, and depleted of hard and brittle carbides or cubic carbo-nitrides. It was reported that surface gradient layers formed when titanium and nitrogen-containing cemented carbides were sintered under denitride conditions (Suzuki et al., 1981; Schwarzkopf et al.,
∗ Corresponding author at: School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, PR China. Tel.: +86 13688313720; fax: +86 28 85990773. E-mail address: [email protected] (Z. Guo). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.03.018
1988; Gustafson and Ostlund, 1994). Many researchers reported that the preparation process was usually divided into two steps (Ekroth et al., 2004; Chen et al., 2000; Zhang et al., 2005; Frykholm et al., 2001, 2002a,b; Frykholm and Andren, 2001). The fist step, including de-waxing and deoxidization, was performed in a controlled atmosphere at about 50 mbar to hinder outward N diffusion. The gas was introduced when the temperature reached 1350 ◦ C. After reaching 1390 ◦ C and a 15 min isothermal holding time, the samples followed furnace cooling. At the second heat treatment, the samples were kept at 1450 ◦ C for 2 h in a nitrogen-free atmosphere, consisting mainly of Ar and CO, in order to develop a controlled gradient surface zone. During sintering, there existed a gradient in nitrogen activity in the materials, leading to an outward diffusion of N. Due to the thermodynamic coupling between N and Ti, the outward diffusion of N would lead to an inward diffusion of Ti, and a surface zone depleted of cubic carbides was formed. At the present, nitrogen was usually introduced by the addition of micro-sized TiCN (Zhang et al., 2005) or TiN (Chen et al., 2000). However, TiN decomposed during sintering and pores tended to form, and the two-step sintering process was a little complicated. Therefore, in the paper, N was introduced by adding 0.13 m ultra-fine TiC0.5 N0.5 , and simple vacuum sintering was adopted to prepare the WC–5.19 wt.%Ti–9.2 wt.%Co cemented carbide with a surface zone depleted of hard cubic carbo-nitride phase and enriched in ductile binder phase. The effect of ultra-
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Table 1 Characteristics of the raw materials. Raw materials
F.S.S.S. (m)
Total C/N content (wt.%)
Manufacturer
WC (W,Ti)C Co TiN Conventional TiC0.5 N0.5 Ultra-fine TiC0.5 N0.5
5.65 1.2 1.3 3 1.2 0.13
5.98/– 13.08/– –/– –/22.62 9.85/15 9.85/15
Zigong Cemented Carbide Corp., LTD. of China Changsha Wing High High-tech New Material Corp., LTD. of China Nanjing Hanrui Cobalt Corp., LTD. of China Changsha Wing High High-tech New Material Corp., LTD. of China Changsha Wing High High-tech New Material Corp., LTD. of China Changsha Wing High High-tech New Material Corp., LTD. of China
Table 2 Powder mixture of all the cemented carbides. Cemented carbide
Powder
1 2 3 4 5 6
Co
0.13 m TiC0.5 N0.5
9.2 9.2 9.2 9.2 9.2 9.2
3.9
1.2 m TiC0.5 N0.5 3.9
1.99 3.0 5.0
fine TiC0.5 N0.5 addition level on the gradient layer thickness and properties was studied. TiCN coatings were deposited on gradient and conventional cemented carbides, and adhesion strength and cutting performance of both coated inserts were investigated. 2. Experimental procedures Table 1 showed the characteristics of raw materials. The cemented carbides studied in the paper were based on mixtures of various raw powders according to Table 2. Table 3 showed the chemical compositions of all the cemented carbides. The cemented carbides were prepared according to normal powder metallurgical procedures. The powders were ball milled in alcohol in stainless steel lined mills for 36 h, using WC–8 wt.%Co with a diameter of 10 mm as milling bodies, the milling speed were 56 rpm, after milling the pulp was dried and 1.1 wt.% synthetic rubber was added as a pressing aid. Rectangular specimens of 6.5 mm × 5.25 mm × 20 mm in dimension were pressed for transverse rupture strength test, and ISO standard type WNMG080408-ZM inserts were pressed for cutting performance test. All the pressed pieces (green bodies) were sintered in vacuum furnace of pilot scale at 1440 ◦ C for 1 h. TiCN films were deposited on all the substrates by MTCVD in a CTI-C280 industrial reactor developed by Chengdu Tool Research Institute of China. The coating temperature was 750–900 ◦ C and controlled by Type K thermocouples. Acetonitrile (CH3 CN) was used as an organic C/N source. The gas phase was a mixture of CH3 CN, TiCl4 and H2 with a volume ratio of 0.01/0.02/1. The total flow rate was 2000 cm3 /min and the pressure during deposition was 5–20 kPa. The deposition time was 4 h, thus ensuring a film thickness of 6–10 m. Coated WNMG080408-ZM inserts were used for continuous turning tests. The inserts were clamped to a tool holder. Workpiece was a cylinTable 3 Chemical compositions of all the cemented carbides. Cemented carbide
1 2 3 4 5 6
3 m TiN
Chemical compositions (wt.%) Co
Ti
C
N
W
9.2 9.2 9.2 9.2 9.2 9.2
5.19 5.19 5.19 5.19 5.19 5.19
5.96 5.96 5.90 6.05 5.85 6.35
0.45 0.45 0.45 0.34 0.57 –
Balance Balance Balance Balance Balance Balance
(W,Ti)C
WC
5.3 5.3 9.13 7.08 3.15 12.98
81.6 81.6 78.68 80.72 82.65 77.82
der 1Cr18Ni9Ti steel. The cutting tests were conducted using a C630 turning lathe (Shenyang Jichuang Yichang, China). Flank wear was measured by 15J toolmaker’s microscope every 3 min of cutting. Tool life was evaluated according to a critical flank wear of 0.3 mm. S300-N SEM (Hitachi, Japan) was used for microstructure observation. To perform scanning electron microscopy (SEM) observation, the prepared gradient cemented carbide inserts were cut perpendicular to the original surface in order to make a crosssection visible. Due to two-dimensional diffusion near corners of the insert, there was dependence between the zone thickness and the distance to nearest corner. In a metal atom controlled diffusion process, the surface zone would be thinner near corners. To make sure that there were no effects from two-dimensional diffusion, the cutting was performed as far as possible from the corners. Transverse rupture strength test was performed using WE-100B universal material testing machine (Changchun testing machine plant, China), hardness was measured using ARK-600 Rockwell hardness tester (AKashi, Japan), coercive force was measured by KOERZIMAT1.095 instrument (FOERSTER IMADEN, Germany), and Archimedes method was used for density test. Scratch tests were performed with a conical diamond scriber (120◦ cone angle and 0.2 mm tip radius). Each coated sample was tested by applying loads of 60 N at the diamond indenter. The indenter was replaced with a new one after one set of measurements. The substitution of a new indenter was necessary because of the modification of the tip geometry, which was controlled by performing Rockwell A (HRA, 60 kg) hardness measurements on a standard cemented carbide sample with certified hardness.
3. Results and discussions 3.1. Different N introduction methods Fig. 1 showed the SEM images of 3 m TiN, 1.2 m TiC0.5 N0.5 and 0.13 m TiC0.5 N0.5 , indicating that 0.13 m TiC0.5 N0.5 powder was well dispersed with finer particle size than 3 m TiN and 1.2 m TiC0.5 N0.5 . The above three powders were added into the cemented carbides to introduce N into the gradient cemented carbides. Fig. 2 showed SEM images of a cross-section of gradient cemented carbide 1, 2 and 3, respectively. During vacuum sintering of N-containing cemented carbide (including TiN and TiC0.5 N0.5 ), there was a gradient of N activity from the bulk to the surface of the material, and the decrease of N activity would result in the outward dif-
J. Xiong et al. / Journal of Materials Processing Technology 209 (2009) 5293–5299
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Fig. 1. SEM image of 0.13 m TiC0.5 N0.5 , 3 m TiN and 1.2 m TiC0.5 N0.5 powder. (a) 0.13 m TiC0.5 N0.5 , (b) 3 m TiN and (c) 1.2 m TiC0.5 N0.5 .
Fig. 2. SEM image of cemented carbide prepared with 0.13 m TiC0.5 N0.5 , 1.2 m TiC0.5 N0.5 and 3 m TiN, and gradient layers could be seen in all of them. (a) With 0.13 m TiC0.5 N0.5 , (b) with 1.2 m TiC0.5 N0.5 and (c) with 3 m TiN.
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Fig. 3. SEM image and Ti distribution of gradient cemented with different ultra-fine TiC0.5 N0.5 content. There was a decrease of gradient zone thickness with increase of ultra-fine TiC0.5 N0.5 addition level. (a) 3.0 wt.% TiC0.5 N0.5 , (b) 3.9 wt.% TiC0.5 N0.5 and (c) 5.0 wt.% TiC0.5 N0.5 .
J. Xiong et al. / Journal of Materials Processing Technology 209 (2009) 5293–5299
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Table 4 Properties and low magnification microstructure of cemented carbide 1, 2 and 3. Properties and low magnification microstructure
Average WC grain size (m) Low magnification microstructure Pores (25–75 m) Pores (75–125 m) Pores (125–150 m) Transverse rupture strength (MPa) Hardness (HRA)
Cemented carbide 1
2
3
3 A00B00C00 – – – 1831 90.8
3.2 A00B02C00 1 – – 1782 89.3
4.5 A00B04C00 2 1 – 1765 89.2
fusion of N content (Frykholm and Andren, 2001). The outward diffusion of nitrogen created an inward diffusion of titanium simultaneously (Ekroth et al., 2004). Therefore, a surface zone depleted of cubic phase could be seen in the prepared cemented carbides with the addition of TiN, middle particle sized TiC0.5 N0.5 and ultra-fine TiC0.5 N0.5 . The properties and low magnification microstructure of the above three cemented carbides were shown in Table 4. Since TiN was less stable than TiC0.5 N0.5 at high temperature, it tended to be decomposed. N2 might be formed (Yang et al., 2003), which would result in pores. Therefore, gradient cemented carbide 3 had the lowest transverse rupture strength due to higher porosity than 1 and 2. After the addition of 0.13 m ultra-fine TiC0.5 N0.5 powder, the grain size of gradient cemented carbide 1 was refined. According to Hall–Petch relation, the hardness and transverse rupture strength of the cemented carbides were enhanced due to the decrease of grain size. Therefore, specimen 1 showed the best properties. 3.2. Effect of ultra-fine TiC0.5 N0.5 content on the microstructure and properties of gradient cemented carbide Fig. 3 showed the SEM images and surface Ti distribution of gradient cemented carbides 4, 1 and 5. Surface zones depleted of Ti content were formed in all the three cemented carbide with different ultra-fine TiC0.5 N0.5 content. It was obvious that different addition level of ultra-fine TiC0.5 N0.5 resulted in different gradient layer thickness, and the gradient zone thickness decreased with increase of ultra-fine TiC0.5 N0.5 addition level. When the above three cemented carbides were sintered in vacuum, N activity inside the alloy was higher than the surface. As nitrogen diffused out from the alloy, the decrease of nitrogen activity in the surface zone leaded to a local activity decrease of elements with a high affinity for nitrogen, such as Ti, and provoked their diffusion in the opposite direction. Therefore, the gradient surface zone was formed as a sequence of thermodynamic coupling of the outward nitrogen dif-
Table 5 Properties of the gradient cemented carbide 1, 4 and 5 of different addition level of ultra-fine TiC0.5 N0.5 , and conventional cemented carbide 6. Properties
Ultra-fine TiC0.5 N0.5 content (wt.%) Transverse rupture strength (MPa) Hardness (HRA) Coercive force (kA/m) Thickness of gradient zone (m)
Cemented carbide 4
1
5
6
3.0 1737 90.7 9.9 40
3.9 1831 90.8 11.1 34
5.0 1820 90.9 11.5 32
– 1699 90.6 9.7 –
fusion and the inward titanium diffusion. According to Frykholm et al. (2002b), the outward diffusion of N was not driven by the gradient in N concentration, but by the gradient in N activity. Therefore, a stronger gradient could be achieved, not only by increasing the content of N, but also by changing the bulk content of other elements in the substrate. With the increase of bulk C content, the gradient in N activity would be stronger, and the driving force for gradient surface zone formation increased. It can be concluded that one way to increase the N activity was to increase the C activity. From Table 3, it could be seen that the bulk carbon content decreased with the increase of ultra-fine TiC0.5 N0.5 addition as for gradient cemented carbides 4, 1 and 5. The decrease of C activity resulted in the decrease of driving force for gradient surface zone formation. Therefore, in some way, it was the decrease of C/N ratio that accounted for the decrease of gradient surface zone thickness. Table 5 showed the properties of gradient cemented carbide 1, 4 and 5 of different addition level of ultra-fine TiC0.5 N0.5 . The transverse rupture strength increased firstly due to the strengthen effect of the ultra-fine powder, and reached the maximum value of 1831 MPa when 3.9 wt.% ultra-fine TiC0.5 N0.5 was added. Since TiCN was a brittle phase in essence, excessive addition of ultrafine TiC0.5 N0.5 would result in the decrease of transverse rupture strength, and 5.0 wt.% might be too much for the alloy. Coercive
Fig. 4. SEM image of coated cemented carbide with gradient and conventional substrate. Columnar structures could be seen in both coatings, and gradient substrate resulted in tidy and compact bonding. (a) With conventional substrate 6 and (b) with gradient substrate 1.
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Fig. 5. SEM image of the scratch of coated cemented carbide with gradient and conventional substrate. Gradient substrate showed better adhesion strength. (a) With conventional substrate 6 and (b) with gradient substrate 1.
force was an indirect measure of the hard phase grain size and the distribution of the binder phase. With the increase of ultra-fine TiC0.5 N0.5 content, coercive force increased due to the fining of hard phase grains and more homogeneous distribution of Co. The hardness also increased due to the introduction of TiC0.5 N0.5 hard phase (Yang et al., 2003). Generally, the properties of the three cemented carbides were quite similar, and 3.9 wt.% TiC0.5 N0.5 was the relatively optimal addition level in view of the hardness and transverse rupture strength. 3.3. Adhesion test and cutting performance Gradient cemented carbide 1 and conventional cemented carbide 6 were coated with TiCN layers by MTCVD, and SEM image of cross-section of both coated cemented carbides were shown in Fig. 4. It indicated that MTCVD coating layers grown on gradient cemented carbide 1 showed dense and well-developed columnar structures, and the bonding between coatings and gradient substrate was more tidy and compact than conventional cemented carbide 6. In order to evaluate the adhesion level between the coatings and substrates, scratch tests were performed by applying load of 60 N at the diamond indenter. Fig. 5 showed the surface images of coated alloys after scratch test. With the same load, delamination of the coatings on gradient cemented carbide 1 was less serious than that on conventional cemented carbide 6. Therefore, gradient cemented carbide resulted in better adhesion strength. Turning tests for both coated cemented carbide were carried out with a cutting speed Vc = 100 m/min, a feed rate f = 0.24 mm/r, a radial cutting depth ap = 2 mm. Fig. 6 showed the flank wear curves of both coated cemented carbide inserts as a function of cutting time. It could be seen that both coated inserts underwent a rapid wear. It took 9 and 12 min for coated cemented
carbide 1 and 6 to exceed the critical flank wear value of 0.3 mm, respectively. The above results confirmed the superiority of gradient cemented carbide when used as the substrate for coating inserts. 4. Conclusions In the paper, WC–6.25 wt.%TiC–9.2 wt.%Co cemented carbide with a surface zone depleted of hard cubic carbo-nitride phase and enriched in ductile binder phase was prepared with 0.13 m ultra-fine TiC0.5 N0.5 by simple vacuum sintering. Equal amount of nitrogen was introduced into the cemented carbides by the addition of 0.13 m ultra-fine TiC0.5 N0.5 , 1.2 m TiCN and 3 m TiN, respectively. Gradient layers could be achieved by all the additions. However, gradient cemented carbide prepared with ultra-fine TiC0.5 N0.5 showed lower porosity and higher hardness of 90.8 HRA and transverse rupture strength of 1831 MPa than that made with 1.2 m TiCN and 3 m TiN. Effect of ultra-fine TiC0.5 N0.5 addition level on the microstructure and properties of the gradient cemented carbide was also investigated. The thickness of gradient layers decreased from 40 to 32 m with the addition of ultra-fine TiC0.5 N0.5 increased from 3.0 to 5.0 wt.%. 3.9 wt.% TiC0.5 N0.5 was the optimal addition level in view of hardness and transverse rupture strength. Finally, MTCVD coatings were deposited on WC–5.19 wt.%Ti– 9.2 wt.%Co substrates with and without gradient layers. Gradient cemented carbide substrate resulted in better bonding strength and cutting performance. Acknowledgements The research is partially supported by Sichuan Province Key Technology R&D Program of China (No. 2008GZ0179). The authors are grateful to Chengdu Mingwu Science and Technology Corp., LTD. of China for the use of facilities. References
Fig. 6. Flank wear curves of both coated cemented carbide inserts as a function of cutting time. Coating insert with gradient substrate showed less flank wear.
Chen, Limin, Lengauer, Walter, Dreyer, Klaus, 2000. Advances in modern nitrogencontaining hard metals and cermets. Int. J. Refract. Met. Hard Mater. 18, 153– 161. Ekroth, M., Frykholm, R., Lindholm, M., Andren, H.-O., Agren, J., 2004. Gradient Zones in WC–Ti(C,N)–Co-based cemented carbides: experimental study and computer simulations. Acta Mater. 48, 2177–2185. Frykholm, R., Andren, H.-O., 2001. Development of the microstructure during gradient sintering of a cemented carbide. Mater. Chem. Phys. 67, 203–208. Frykholm, R., Ekroth, M., Jansson, B., Andren, H.-O., Agren, J., 2001. Effect of cubic phase composition on gradient zone formation in cemented carbides. Int. J. Refract. Met. Hard Mater. 19, 527–538.
J. Xiong et al. / Journal of Materials Processing Technology 209 (2009) 5293–5299 Frykholm, R., Jansson, B., Andren, H.-O., 2002a. The influence of carbon content on formation of carbo-nitride free surface layers in cemented carbides. Int. J. Refract. Met. Hard Mater. 20, 345–353. Frykholm, R., Jansson, B., Andren, H.-O., 2002b. Methods for atom probe analysis of microgradients in functionally graded cemented carbides. Micron 33, 639– 646. Gustafson, P., Ostlund, A., 1994. Binder-phase enrichment by dissolution of cubic carbides. Int. J. Refract. Met. Hard Mater. 12, 129–136. Schwarzkopf, M., Exner, E., Fischmeister, H.F., Schintlmeister, W., 1988. Kinetics of compositional modification of (W,Ti)C–WC–Co alloy surfaces. Mater. Sci. Eng., 225–231, A105/106.
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Suzuki, H., Hayashi, K., Taniguchi, Y., 1981. The beta-free layer formed near the surface of vacuum-sintered WC-beta-Co alloys containing nitrogen. Trans. Jpn. Inst. Met. 22, 758–764. Yang, G.G., Wang, H.B., Liu, Y., 2003. Fabrication methods and formation mechanism of surface gradient zone of cemented carbides. Powder Metall. Technol. 21, 295–299. Zhang, W.Z., Liu, Y., He, Y.H., Wang, H.B., 2005. Effect of Ti(C, N) content on the gradient structure and properties of cemented carbides. Rare Met. Cemented Carbides 33, 28–30.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Effect of ultra-fine TiC0.5 N0.5 on the microstructure and properties of gradient cemented carbide Ji Xiong a , Zhixing Guo a,∗ , Mei Yang b , Sujian Xiong a , Jianzhong Chen a , Yuemei Wu a , Bin Wen c , Ding Cao a a b c
School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, PR China College of Materials and Chemistry and Chemical Engineering, Chengdu University of Technology, Chengdu 610059, PR China Baker Hughes Incorporated, United States
a r t i c l e
i n f o
Article history: Received 25 October 2008 Received in revised form 24 March 2009 Accepted 28 March 2009 Keywords: Gradient cemented carbide Ultra-fine TiC0.5 N0.5 Microstructure and properties
a b s t r a c t To increase the performance of cemented carbide, it was common to coat the wear surfaces with thin layers of hard materials. Cemented carbides with surface zone depleted of hard cubic phase and enriched in ductile binder phase were commonly used as the substrates. At the present, the so-called “gradient cemented carbides” were usually prepared by adding TiN or medium-sized TiCN, and a two-step sintering process including pre-sintering in nitrogen and gradient sintering in vacuum was usually adopted. In this paper, gradient cemented carbide based on WC–5.19 wt.%Ti–9.2 wt.%Co was prepared by simple vacuum sintering with the addition of 0.13 m ultra-fine TiC0.5 N0.5 , compared with medium-sized TiN or TiCN. The result showed that gradient cemented carbide prepared with 0.13 m ultra-fine TiC0.5 N0.5 had better properties. With the increase of ultra-fine TiC0.5 N0.5 content, the thickness of gradient surface zone decreased. TiCN coatings were deposited on gradient substrate and conventional substrate by moderate temperature chemical vapor deposition (MTCVD), respectively. Gradient substrate resulted in better adhesion and cutting performance. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Cemented carbides were commonly used as cutting tools for metals. To increase the performance of the tools further, it was common to coat the wear surfaces with a thin layer of hard materials such as TiC, TiN and Al2 O3 , or multi-layers of such hard materials (Ekroth et al., 2004). The coated layers were usually grown by chemical vapor deposition (CVD) at temperatures around 1000 ◦ C. Cracks would unavoidably form in the coatings due to differences in thermal expansion coefficients between coatings and substrates during cooling after the coating process. When the inserts were used for machining, these cracks might propagate into the substrates and cause failures. Therefore, it was of interest to prepare gradient substrates with tough surface zones, which were enriched in tough binder phase, and depleted of hard and brittle carbides or cubic carbo-nitrides. It was reported that surface gradient layers formed when titanium and nitrogen-containing cemented carbides were sintered under denitride conditions (Suzuki et al., 1981; Schwarzkopf et al.,
∗ Corresponding author at: School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, PR China. Tel.: +86 13688313720; fax: +86 28 85990773. E-mail address: [email protected] (Z. Guo). 0924-0136/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2009.03.018
1988; Gustafson and Ostlund, 1994). Many researchers reported that the preparation process was usually divided into two steps (Ekroth et al., 2004; Chen et al., 2000; Zhang et al., 2005; Frykholm et al., 2001, 2002a,b; Frykholm and Andren, 2001). The fist step, including de-waxing and deoxidization, was performed in a controlled atmosphere at about 50 mbar to hinder outward N diffusion. The gas was introduced when the temperature reached 1350 ◦ C. After reaching 1390 ◦ C and a 15 min isothermal holding time, the samples followed furnace cooling. At the second heat treatment, the samples were kept at 1450 ◦ C for 2 h in a nitrogen-free atmosphere, consisting mainly of Ar and CO, in order to develop a controlled gradient surface zone. During sintering, there existed a gradient in nitrogen activity in the materials, leading to an outward diffusion of N. Due to the thermodynamic coupling between N and Ti, the outward diffusion of N would lead to an inward diffusion of Ti, and a surface zone depleted of cubic carbides was formed. At the present, nitrogen was usually introduced by the addition of micro-sized TiCN (Zhang et al., 2005) or TiN (Chen et al., 2000). However, TiN decomposed during sintering and pores tended to form, and the two-step sintering process was a little complicated. Therefore, in the paper, N was introduced by adding 0.13 m ultra-fine TiC0.5 N0.5 , and simple vacuum sintering was adopted to prepare the WC–5.19 wt.%Ti–9.2 wt.%Co cemented carbide with a surface zone depleted of hard cubic carbo-nitride phase and enriched in ductile binder phase. The effect of ultra-
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Table 1 Characteristics of the raw materials. Raw materials
F.S.S.S. (m)
Total C/N content (wt.%)
Manufacturer
WC (W,Ti)C Co TiN Conventional TiC0.5 N0.5 Ultra-fine TiC0.5 N0.5
5.65 1.2 1.3 3 1.2 0.13
5.98/– 13.08/– –/– –/22.62 9.85/15 9.85/15
Zigong Cemented Carbide Corp., LTD. of China Changsha Wing High High-tech New Material Corp., LTD. of China Nanjing Hanrui Cobalt Corp., LTD. of China Changsha Wing High High-tech New Material Corp., LTD. of China Changsha Wing High High-tech New Material Corp., LTD. of China Changsha Wing High High-tech New Material Corp., LTD. of China
Table 2 Powder mixture of all the cemented carbides. Cemented carbide
Powder
1 2 3 4 5 6
Co
0.13 m TiC0.5 N0.5
9.2 9.2 9.2 9.2 9.2 9.2
3.9
1.2 m TiC0.5 N0.5 3.9
1.99 3.0 5.0
fine TiC0.5 N0.5 addition level on the gradient layer thickness and properties was studied. TiCN coatings were deposited on gradient and conventional cemented carbides, and adhesion strength and cutting performance of both coated inserts were investigated. 2. Experimental procedures Table 1 showed the characteristics of raw materials. The cemented carbides studied in the paper were based on mixtures of various raw powders according to Table 2. Table 3 showed the chemical compositions of all the cemented carbides. The cemented carbides were prepared according to normal powder metallurgical procedures. The powders were ball milled in alcohol in stainless steel lined mills for 36 h, using WC–8 wt.%Co with a diameter of 10 mm as milling bodies, the milling speed were 56 rpm, after milling the pulp was dried and 1.1 wt.% synthetic rubber was added as a pressing aid. Rectangular specimens of 6.5 mm × 5.25 mm × 20 mm in dimension were pressed for transverse rupture strength test, and ISO standard type WNMG080408-ZM inserts were pressed for cutting performance test. All the pressed pieces (green bodies) were sintered in vacuum furnace of pilot scale at 1440 ◦ C for 1 h. TiCN films were deposited on all the substrates by MTCVD in a CTI-C280 industrial reactor developed by Chengdu Tool Research Institute of China. The coating temperature was 750–900 ◦ C and controlled by Type K thermocouples. Acetonitrile (CH3 CN) was used as an organic C/N source. The gas phase was a mixture of CH3 CN, TiCl4 and H2 with a volume ratio of 0.01/0.02/1. The total flow rate was 2000 cm3 /min and the pressure during deposition was 5–20 kPa. The deposition time was 4 h, thus ensuring a film thickness of 6–10 m. Coated WNMG080408-ZM inserts were used for continuous turning tests. The inserts were clamped to a tool holder. Workpiece was a cylinTable 3 Chemical compositions of all the cemented carbides. Cemented carbide
1 2 3 4 5 6
3 m TiN
Chemical compositions (wt.%) Co
Ti
C
N
W
9.2 9.2 9.2 9.2 9.2 9.2
5.19 5.19 5.19 5.19 5.19 5.19
5.96 5.96 5.90 6.05 5.85 6.35
0.45 0.45 0.45 0.34 0.57 –
Balance Balance Balance Balance Balance Balance
(W,Ti)C
WC
5.3 5.3 9.13 7.08 3.15 12.98
81.6 81.6 78.68 80.72 82.65 77.82
der 1Cr18Ni9Ti steel. The cutting tests were conducted using a C630 turning lathe (Shenyang Jichuang Yichang, China). Flank wear was measured by 15J toolmaker’s microscope every 3 min of cutting. Tool life was evaluated according to a critical flank wear of 0.3 mm. S300-N SEM (Hitachi, Japan) was used for microstructure observation. To perform scanning electron microscopy (SEM) observation, the prepared gradient cemented carbide inserts were cut perpendicular to the original surface in order to make a crosssection visible. Due to two-dimensional diffusion near corners of the insert, there was dependence between the zone thickness and the distance to nearest corner. In a metal atom controlled diffusion process, the surface zone would be thinner near corners. To make sure that there were no effects from two-dimensional diffusion, the cutting was performed as far as possible from the corners. Transverse rupture strength test was performed using WE-100B universal material testing machine (Changchun testing machine plant, China), hardness was measured using ARK-600 Rockwell hardness tester (AKashi, Japan), coercive force was measured by KOERZIMAT1.095 instrument (FOERSTER IMADEN, Germany), and Archimedes method was used for density test. Scratch tests were performed with a conical diamond scriber (120◦ cone angle and 0.2 mm tip radius). Each coated sample was tested by applying loads of 60 N at the diamond indenter. The indenter was replaced with a new one after one set of measurements. The substitution of a new indenter was necessary because of the modification of the tip geometry, which was controlled by performing Rockwell A (HRA, 60 kg) hardness measurements on a standard cemented carbide sample with certified hardness.
3. Results and discussions 3.1. Different N introduction methods Fig. 1 showed the SEM images of 3 m TiN, 1.2 m TiC0.5 N0.5 and 0.13 m TiC0.5 N0.5 , indicating that 0.13 m TiC0.5 N0.5 powder was well dispersed with finer particle size than 3 m TiN and 1.2 m TiC0.5 N0.5 . The above three powders were added into the cemented carbides to introduce N into the gradient cemented carbides. Fig. 2 showed SEM images of a cross-section of gradient cemented carbide 1, 2 and 3, respectively. During vacuum sintering of N-containing cemented carbide (including TiN and TiC0.5 N0.5 ), there was a gradient of N activity from the bulk to the surface of the material, and the decrease of N activity would result in the outward dif-
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Fig. 1. SEM image of 0.13 m TiC0.5 N0.5 , 3 m TiN and 1.2 m TiC0.5 N0.5 powder. (a) 0.13 m TiC0.5 N0.5 , (b) 3 m TiN and (c) 1.2 m TiC0.5 N0.5 .
Fig. 2. SEM image of cemented carbide prepared with 0.13 m TiC0.5 N0.5 , 1.2 m TiC0.5 N0.5 and 3 m TiN, and gradient layers could be seen in all of them. (a) With 0.13 m TiC0.5 N0.5 , (b) with 1.2 m TiC0.5 N0.5 and (c) with 3 m TiN.
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Fig. 3. SEM image and Ti distribution of gradient cemented with different ultra-fine TiC0.5 N0.5 content. There was a decrease of gradient zone thickness with increase of ultra-fine TiC0.5 N0.5 addition level. (a) 3.0 wt.% TiC0.5 N0.5 , (b) 3.9 wt.% TiC0.5 N0.5 and (c) 5.0 wt.% TiC0.5 N0.5 .
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Table 4 Properties and low magnification microstructure of cemented carbide 1, 2 and 3. Properties and low magnification microstructure
Average WC grain size (m) Low magnification microstructure Pores (25–75 m) Pores (75–125 m) Pores (125–150 m) Transverse rupture strength (MPa) Hardness (HRA)
Cemented carbide 1
2
3
3 A00B00C00 – – – 1831 90.8
3.2 A00B02C00 1 – – 1782 89.3
4.5 A00B04C00 2 1 – 1765 89.2
fusion of N content (Frykholm and Andren, 2001). The outward diffusion of nitrogen created an inward diffusion of titanium simultaneously (Ekroth et al., 2004). Therefore, a surface zone depleted of cubic phase could be seen in the prepared cemented carbides with the addition of TiN, middle particle sized TiC0.5 N0.5 and ultra-fine TiC0.5 N0.5 . The properties and low magnification microstructure of the above three cemented carbides were shown in Table 4. Since TiN was less stable than TiC0.5 N0.5 at high temperature, it tended to be decomposed. N2 might be formed (Yang et al., 2003), which would result in pores. Therefore, gradient cemented carbide 3 had the lowest transverse rupture strength due to higher porosity than 1 and 2. After the addition of 0.13 m ultra-fine TiC0.5 N0.5 powder, the grain size of gradient cemented carbide 1 was refined. According to Hall–Petch relation, the hardness and transverse rupture strength of the cemented carbides were enhanced due to the decrease of grain size. Therefore, specimen 1 showed the best properties. 3.2. Effect of ultra-fine TiC0.5 N0.5 content on the microstructure and properties of gradient cemented carbide Fig. 3 showed the SEM images and surface Ti distribution of gradient cemented carbides 4, 1 and 5. Surface zones depleted of Ti content were formed in all the three cemented carbide with different ultra-fine TiC0.5 N0.5 content. It was obvious that different addition level of ultra-fine TiC0.5 N0.5 resulted in different gradient layer thickness, and the gradient zone thickness decreased with increase of ultra-fine TiC0.5 N0.5 addition level. When the above three cemented carbides were sintered in vacuum, N activity inside the alloy was higher than the surface. As nitrogen diffused out from the alloy, the decrease of nitrogen activity in the surface zone leaded to a local activity decrease of elements with a high affinity for nitrogen, such as Ti, and provoked their diffusion in the opposite direction. Therefore, the gradient surface zone was formed as a sequence of thermodynamic coupling of the outward nitrogen dif-
Table 5 Properties of the gradient cemented carbide 1, 4 and 5 of different addition level of ultra-fine TiC0.5 N0.5 , and conventional cemented carbide 6. Properties
Ultra-fine TiC0.5 N0.5 content (wt.%) Transverse rupture strength (MPa) Hardness (HRA) Coercive force (kA/m) Thickness of gradient zone (m)
Cemented carbide 4
1
5
6
3.0 1737 90.7 9.9 40
3.9 1831 90.8 11.1 34
5.0 1820 90.9 11.5 32
– 1699 90.6 9.7 –
fusion and the inward titanium diffusion. According to Frykholm et al. (2002b), the outward diffusion of N was not driven by the gradient in N concentration, but by the gradient in N activity. Therefore, a stronger gradient could be achieved, not only by increasing the content of N, but also by changing the bulk content of other elements in the substrate. With the increase of bulk C content, the gradient in N activity would be stronger, and the driving force for gradient surface zone formation increased. It can be concluded that one way to increase the N activity was to increase the C activity. From Table 3, it could be seen that the bulk carbon content decreased with the increase of ultra-fine TiC0.5 N0.5 addition as for gradient cemented carbides 4, 1 and 5. The decrease of C activity resulted in the decrease of driving force for gradient surface zone formation. Therefore, in some way, it was the decrease of C/N ratio that accounted for the decrease of gradient surface zone thickness. Table 5 showed the properties of gradient cemented carbide 1, 4 and 5 of different addition level of ultra-fine TiC0.5 N0.5 . The transverse rupture strength increased firstly due to the strengthen effect of the ultra-fine powder, and reached the maximum value of 1831 MPa when 3.9 wt.% ultra-fine TiC0.5 N0.5 was added. Since TiCN was a brittle phase in essence, excessive addition of ultrafine TiC0.5 N0.5 would result in the decrease of transverse rupture strength, and 5.0 wt.% might be too much for the alloy. Coercive
Fig. 4. SEM image of coated cemented carbide with gradient and conventional substrate. Columnar structures could be seen in both coatings, and gradient substrate resulted in tidy and compact bonding. (a) With conventional substrate 6 and (b) with gradient substrate 1.
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Fig. 5. SEM image of the scratch of coated cemented carbide with gradient and conventional substrate. Gradient substrate showed better adhesion strength. (a) With conventional substrate 6 and (b) with gradient substrate 1.
force was an indirect measure of the hard phase grain size and the distribution of the binder phase. With the increase of ultra-fine TiC0.5 N0.5 content, coercive force increased due to the fining of hard phase grains and more homogeneous distribution of Co. The hardness also increased due to the introduction of TiC0.5 N0.5 hard phase (Yang et al., 2003). Generally, the properties of the three cemented carbides were quite similar, and 3.9 wt.% TiC0.5 N0.5 was the relatively optimal addition level in view of the hardness and transverse rupture strength. 3.3. Adhesion test and cutting performance Gradient cemented carbide 1 and conventional cemented carbide 6 were coated with TiCN layers by MTCVD, and SEM image of cross-section of both coated cemented carbides were shown in Fig. 4. It indicated that MTCVD coating layers grown on gradient cemented carbide 1 showed dense and well-developed columnar structures, and the bonding between coatings and gradient substrate was more tidy and compact than conventional cemented carbide 6. In order to evaluate the adhesion level between the coatings and substrates, scratch tests were performed by applying load of 60 N at the diamond indenter. Fig. 5 showed the surface images of coated alloys after scratch test. With the same load, delamination of the coatings on gradient cemented carbide 1 was less serious than that on conventional cemented carbide 6. Therefore, gradient cemented carbide resulted in better adhesion strength. Turning tests for both coated cemented carbide were carried out with a cutting speed Vc = 100 m/min, a feed rate f = 0.24 mm/r, a radial cutting depth ap = 2 mm. Fig. 6 showed the flank wear curves of both coated cemented carbide inserts as a function of cutting time. It could be seen that both coated inserts underwent a rapid wear. It took 9 and 12 min for coated cemented
carbide 1 and 6 to exceed the critical flank wear value of 0.3 mm, respectively. The above results confirmed the superiority of gradient cemented carbide when used as the substrate for coating inserts. 4. Conclusions In the paper, WC–6.25 wt.%TiC–9.2 wt.%Co cemented carbide with a surface zone depleted of hard cubic carbo-nitride phase and enriched in ductile binder phase was prepared with 0.13 m ultra-fine TiC0.5 N0.5 by simple vacuum sintering. Equal amount of nitrogen was introduced into the cemented carbides by the addition of 0.13 m ultra-fine TiC0.5 N0.5 , 1.2 m TiCN and 3 m TiN, respectively. Gradient layers could be achieved by all the additions. However, gradient cemented carbide prepared with ultra-fine TiC0.5 N0.5 showed lower porosity and higher hardness of 90.8 HRA and transverse rupture strength of 1831 MPa than that made with 1.2 m TiCN and 3 m TiN. Effect of ultra-fine TiC0.5 N0.5 addition level on the microstructure and properties of the gradient cemented carbide was also investigated. The thickness of gradient layers decreased from 40 to 32 m with the addition of ultra-fine TiC0.5 N0.5 increased from 3.0 to 5.0 wt.%. 3.9 wt.% TiC0.5 N0.5 was the optimal addition level in view of hardness and transverse rupture strength. Finally, MTCVD coatings were deposited on WC–5.19 wt.%Ti– 9.2 wt.%Co substrates with and without gradient layers. Gradient cemented carbide substrate resulted in better bonding strength and cutting performance. Acknowledgements The research is partially supported by Sichuan Province Key Technology R&D Program of China (No. 2008GZ0179). The authors are grateful to Chengdu Mingwu Science and Technology Corp., LTD. of China for the use of facilities. References
Fig. 6. Flank wear curves of both coated cemented carbide inserts as a function of cutting time. Coating insert with gradient substrate showed less flank wear.
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