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Plasma nitrided and TiCN coated AISI H13 steel by pulsed dc PECVD and its application for hot-working dies
Surface & Coatings Technology 191 (2005) 201 – 205 www.elsevier.com/locate/surfcoat
Plasma nitrided and TiCN coated AISI H13 steel by pulsed dc PECVD and its application for hot-working dies Shengli Ma a,b,*, Kewei Xu a, Wanqi Jie b a b
State-Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, 710049, PR China State-Key Laboratory for Solidification Processing, Northwestern Polytechnic University, Xi’an, 710072, PR China Received 17 October 2003; accepted in revised form 26 March 2004 Available online 2 June 2004
Abstract Pulsed direct current (dc) plasma-enhanced chemical-vapor deposition (PECVD) holds great advantages such as plasma permeation and deposition in the inner wall of holes and narrow cracks usually encountered in complex components. In this work, plasma nitriding and TiCN coatings on H13 steel have been developed using pulsed dc PECVD. The composite layer has been characterized with respect to its microhardness, adhesion, and microstructure. The results indicate that with the assistance of plasma nitriding, the composite layer improves the surface microhardness and interfacial adhesion between the coating and substrate. The application of this duplex-plasma processing on a hot-working die made of AISI H13 steel provides a significant increase in lifetime for the steel. This is a more-successful processing technique than conventional techniques, which usually do not improve the surface properties of dies, with complex shapes and demanding performance requirement conditions. However, extra hardening of this composite layer is not always advantageous. Microhardness (related to layer’s strength) and adhesion, as well as the toughness of the coating system, should all be considered together. D 2004 Elsevier B.V. All rights reserved. Keywords: Composite layer; Plasma nitriding; Pulsed dc PECVD; Hot-working die; TiCN
1. Introduction Pulsed direct current (dc) plasma-enhanced chemicalvapor deposition (PECVD) is one of the most promising methods for surface modification of dies and tools used in various industries. Until now, binary-coating and ternarycoating systems such as TiN, TiC, TiAlN, TiBN, TiSiN, and TiCN have been made by PECVD and successfully applied [1 – 6]. However, surface-hardened and strongly adherent coatings with improved wear-resistance, especially the coatings used at elevated temperatures, are still needed in some cases. For example, hot-working dies made of AISI H13 have to sustain an impacting load at a high-temperature environment. In recent years [7,8], we and other researchers suggested a duplex-plasma process which in-situ produces a TiN hard coating deposited on a plasma-nitrided substrate. It is found that such an optimized composite layer makes a
* Corresponding author. State-Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, 710049, PR China. Tel.: +86-29-2668395; fax: +86-29-2663453. E-mail address: [email protected] (S. Ma). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.03.048
significant improvement to the mechanical behavior of single TiN coatings. Ternary coating systems including TiCN and TiAlN, however, have better mechanical properties than simple TiN coatings; however, these coatings are not clearly understood yet. For a deeper understanding and extensive exploration of the plasma-duplex processing suggested in our earlier publication [7], we decided to examine another hard-coating TiCN system. Using the PECVD technique, we examined the TiCN system’s application for a hotworking die made of H13 steel. The main results are presented in this article.
2. Experimental The AISI H13 substrate, in the bar of 30 mm (diameter) 8 mm (height), was quenched and tempered to the hardness of 43 F 2 HRC, then ground to the surface roughness of Ra = 0.2 Am. Prior to plasma treatment, the samples were degreased, dried, and sputter etched. Table 1 shows plasma-nitriding parameters used. TiCN hard coatings were deposited on nitrided and non-nitrided
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Table 1 Plasma-nitriding parameters Pulsed voltage Pulse-on time Pulse-off time Temperature Pressure N2/(H2 + N2) flow ratio Nitriding time
650 V 25 As 25 As 520 jC 600 Pa 25% 0.5 – 30 h
samples by PECVD in order to compare the samples with each other. The standard processing parameters were used and listed in Table 2. The plasma-nitriding and PECVD TiCN deposition were conducted in situ in the industrial-scale plant shown schematically in Fig. 1. More details about the PECVD system can be found in our earlier reference [9]. The cylindrical vacuum chamber, which was 450 mm in diameter and 650 mm in height, was heated with an auxiliary heating system, the temperature of which was controlled by a thermocouple. The substrate was loaded directly onto the charging plate, which was also used as the cathode of the system. The surrounding wall of the chamber was used as an anode of the system and the earth potential. The pulsed power supply was able to produce voltages up to 1200 V and frequencies up to 33 kHz. The flow of the different gases was measured and controlled by massflow controllers. TiCl4 was led into the chamber by a flow of carrier gas (H2) flowing through the TiCl4 tank. The temperature was kept constant at 40 jC. During the production of the composite layer of nitrided H13/TiCN, the flow ratio of X = CH4/(CH4 + N2)% was changed, while other PECVD TiCN deposition conditions were kept constant. The thickness of the coatings was about 3 – 4 Am, which was measured under scanningelectron micrograph (SEM). A Regaku Dmax-IIIA diffractometer using Cu-Ka radiation was used for X-ray diffraction (XRD) analysis. Vickers microhardness was measured using a load of 0.25 N and an average of three readings was taken. The adhesion of the TiCN coatings was characterized by an indentation – adhesion test using a modified Rockwell hardness tester on which the indentaTable 2 PECVD TiCN coatings processing condition Pulsed voltage Pulse-on time Pulse-off time Temperature Pressure X=[CH4/(CH4 + N2)]% CH4 + N2 H2 Ar TiCl4 (carrier H2) Deposition time
650 V 25 As 25 As 520 jC 200 Pa 0 – 100% 150 ml/min 700 ml/min 50 ml/min 40 ml/min 6h
Fig. 1. Schematic drawing of PECVD systems.
tion force was applied on the samples with a continuous load. The adhesion was denoted by the critical load corresponding to the initiation of the coating spallation, which was monitored by acoustic emission. To some extent, the validity of this method was illustrated elsewhere [10]. The application of this improved plasma-duplex treatment processing was finally evaluated in practice using an actual hot-working die (with dimensions 0f 240 120 100 mm) made of H13 steel.
3. Results and discussion 3.1. Characterization of microstructure of TiCN coatings on plasma-nitrided H13 steel Fig. 2 shows clearly that the TiCN coatings on H13 steel are free of columnar structure, become denser, and have a finer microstructure than TiN coatings. This is a contribution to the microhardness, and therefore better wear-resistance and better corrosion resistance of H13 steel, which generally shows a poor corrosion property as a die-steel. Improvement to structure is probably due to the addition of carbon. Fig. 3 shows the XRD patterns of TiCN coatings with different carbon contents, we can see well that there is a shift in the lattice constants when the higher carbon included in TiCN coatings. However, it is apparent that the ternary TiCN coating system is always a single TiN-based continuous transition coating in which carbon atoms are simply dissolved in the lattice structure of TiN. With the higher ratio of X = CH4/(CH4 + N2)%, the ternary TiCN coating system can be produced which has superior properties such as the microhardness and adhesions seen in our next section. In our previous investigation for plasma nitriding of H13 steel at lower N2/(H2 + N2) flow ratio (25%), the nitrided compound
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Fig. 2. Cross-sectional SEM morphology of (a) TiCN(X = 38%) and (b) TiN coatings.
was observed near the interface region. The composite structure in that case consisted only of a substrate/nitrided diffusion layer/coating system. This result suggests that the nitrogen did not react with iron at a low-nitrogen
potential; instead, the nitrogen acted as an interstitial atom in the a-Fe lattice and was completely dissolved as a diffusion layer. However, the higher N2/(H2 + N2) flow ratio (50%) resulted in the formation of a new compound phase (usually called the white layer) and the composite structure was composed of substrate/diffusion layer/compound layer/coatings. This composite layer will decrease the adhesion of the coating due to premature brittle fracture between the coating and the substrate, as pointed out in our earlier publication [7]. 3.2. Microhardness and adhesion of TiCN coatings on plasma-nitrided H13 steel
Fig. 3. X-ray diffraction patterns of TiCN coatings deposited with different gas ratios of X = CH4/(CH4 + N2)%.
Fig. 4 shows that TiCN coatings with the thickness of 3 Am have higher microhardness than TiN coatings. This would be partially due to the enhancement of the TiCN phase, which has a higher microhardness than TiN coatings (see Fig. 4) and therefore is certainly useful to improve the wear resistance of ternary TiCN hard coatings. The microhardness of the composite layer of nitrided H13 steel can be adjusted through the plasma pre-nitriding process. Fig. 5 clearly shows that a plasma-nitrided layer can provide a high load-carrying capacity and is useful to sustain high, critical loading. This appears to be due to the
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Fig. 4. Surface microhardness of TiCN coatings deposited on 30-h prenitriding H13 steel vs. different gas ratios of X = CH4/(CH4 + N2)%.
increased thickness of the nitrided layer with the increase of plasma-nitriding time. Fig. 6 shows that the adhesion of the TiCN coatings is improved significantly as compared with TiN coatings deposited on nitrided or non-nitrided H13 substrate. We suggest that this is due to the matching of the lattice structure of the TiCN coatings with the nitrided layer, which produces a moderate transition of lattice structure and mechanical properties. In addition, the high microhardness, which represents a high load-carrying capacity, will contribute to the increase of adhesion. When this composite layer system is used, the system usually exhibits better properties than TiN coatings on nitrided steel, but the extra microhardness value of the composite layer is not always an advantage as seen in its application in the next section. 3.3. Application of plasma in-situ duplex processing for hotworking die made of H13 steel Hot-working dies still represent challenges for the improvement of these dies’ surface properties. We chose
Fig. 5. Surface microhardness of the pre-nitrided and TiCN coated (X = 38% and 3 Am thickness) H13 steel with different nitriding times.
Fig. 6. Effect of 30-h pre-nitriding on interfacial adhesion between coating and a H13 substrate, A is TiN coating on a non-nitrided substrate, B is TiN coating on a nitrided substrate, and C is TiCN coating (X = 38%) on nitrided substrate.
a hot-working die made of H13 steel in engine-blade production as an example. The challenges include the ability to combine high microhardness, high fatigue wearresistance, high toughness, and also high corrosion-resistance in working conditions of engine-blade production. For example, temperatures in the range of 800– 1000 jC are possible and also dramatic temperature changes. The first problem using hot-working die made of H13 steel is its relatively soft surface of substrate which is needed for its high toughness. In order to improve the surface microhardness and wear resistance of the dies, while retaining the high toughness of the substrate bulk materials of H13, normal gas nitriding and/or plasma nitriding, as well as carbon –nitrogen processing have been popular solutions. Hard coatings deposited on the surface of hotworking dies by different methods have been developed in recent years. The plasma-duplex treatment, which consists of in-situ plasma nitriding followed by hard coating, has been demonstrated to be a promising method for this type of die. In the present study, after optimized processing, the hot-working die made of H13 steel treated by plasma nitriding and then coated with a TiCN layer was given industry testing in the blade production. The service life is increased by a factor of 3, compared to a plasma-nitrided die, and also an increase by a factor of 2 compared to a die treated with a composite layer of a TiN/nitrided substrate. However, it will not be good for the hot-working die if the microhardness of composite coatings is higher (which is always the result from the high content of the TiCN phase) because this high microhardness is characterized by its brittleness and poor adhesion between coating and the nitrided H13 substrate as demonstrated in our earlier study [7]. The effort to get higher microhardness may be counterproductive for a die that sustains a heavy-load impacting in service. Other properties such as toughness of coating and interfacial adhesion must be all considered together. Different dies
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interfacial adhesion between coating and substrate. This plasma-duplex treatment processing for hot-working dies made of H13 steel used in engine-blade production was successful in improving lifetime over conventional techniques, the highest microhardness of the hot-working die obtained by using high CH4 concentrations were not as good as these with 38%CH4, due to brittleness and poor adhesion between coating and the nitrided-H13 substrate.
Acknowledgements
Fig. 7. Test results for differently treated hot-working dies made of H13 for the production of a blade. A shows the quenched state, B shows the nitrided state, C shows TiN coating on nitrided substrate, D and E are TiCN(X = 38%) coating on nitrided substrate, and F is the TiCN(X = 86%) coating on nitrided substrate. The nitriding time in all cases is the same 30 h, except for D which was 22 h.
and tools should have specifically optimized microstructures and corresponding properties of coatings. From a practical point of view, this plasma-duplex treatment is a new and successful technique for a hot-working die made of H13 steel (Fig. 7).
4. Conclusions Our results show that the composite layer of nitridedH13 steel and TiCN coatings prepared by pulsed dc PECVD is useful technique for surface modification of a hot-working die made of H13 steel. The composite layer consists of a continuous transition coatings in this TiCN ternary system deposited on a nitrided-H13 steel substrate. With the assistance of plasma nitriding, the composite layer improves the surface microhardness and
Thanks to the National High-Tech program of China (No. 2001AA883010) and National Natural Science Foundation of China (No. 50271053, No. 50371067) for financial support. This work has been also supported in part by the European Community’s fifth framework program (GRD-2001-40419).
References [1] S.L. Ma, Y.H. Li, J.M. Nan, K.W. Xu, Trans. Nonferr. Met. Soc. China 10 (4) (2000) 489. [2] D. Heim, R. Hochreiter, Surf. Coat. Technol. 98 (1998) 1553. [3] S. Veprek, A. Niederhofer, K. Moto, T. Bolom, Surf. Coat. Technol. 133 – 134 (2000) 152. [4] B. Matthes, E. Broszeit, K.H. Kloos, Surf. Coat. Technol. 57 (1993) 97. [5] B. Selbach, K. Schmidt, M. Wang, Thin Solid Films 188 (1990) 267. [6] P. Hardt, M. Eckel, M. Schmidt, H. Wulff, Surf. Coat. Technol. 20 – 121 (1999) 238. [7] S.L. Ma, Y.H. Li, K.W. Xu, Surf. Coat. Technol. 137 (2001) 116. [8] S.Y. Lee, J.W. Chung, K.B. Kim, J.G. Han, S.S. Kim, Surf. Coat. Technol. 86 – 87 (1996) 325. [9] S.L. Ma, Y.H. Li, K.W. Xu, Surf. Coat. Technol. 131 (2000) 131. [10] S.L. Ma, Doctoral thesis, Xi’an Jiaotong University, Xi’an, China, 2001.
Plasma nitrided and TiCN coated AISI H13 steel by pulsed dc PECVD and its application for hot-working dies Shengli Ma a,b,*, Kewei Xu a, Wanqi Jie b a b
State-Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, 710049, PR China State-Key Laboratory for Solidification Processing, Northwestern Polytechnic University, Xi’an, 710072, PR China Received 17 October 2003; accepted in revised form 26 March 2004 Available online 2 June 2004
Abstract Pulsed direct current (dc) plasma-enhanced chemical-vapor deposition (PECVD) holds great advantages such as plasma permeation and deposition in the inner wall of holes and narrow cracks usually encountered in complex components. In this work, plasma nitriding and TiCN coatings on H13 steel have been developed using pulsed dc PECVD. The composite layer has been characterized with respect to its microhardness, adhesion, and microstructure. The results indicate that with the assistance of plasma nitriding, the composite layer improves the surface microhardness and interfacial adhesion between the coating and substrate. The application of this duplex-plasma processing on a hot-working die made of AISI H13 steel provides a significant increase in lifetime for the steel. This is a more-successful processing technique than conventional techniques, which usually do not improve the surface properties of dies, with complex shapes and demanding performance requirement conditions. However, extra hardening of this composite layer is not always advantageous. Microhardness (related to layer’s strength) and adhesion, as well as the toughness of the coating system, should all be considered together. D 2004 Elsevier B.V. All rights reserved. Keywords: Composite layer; Plasma nitriding; Pulsed dc PECVD; Hot-working die; TiCN
1. Introduction Pulsed direct current (dc) plasma-enhanced chemicalvapor deposition (PECVD) is one of the most promising methods for surface modification of dies and tools used in various industries. Until now, binary-coating and ternarycoating systems such as TiN, TiC, TiAlN, TiBN, TiSiN, and TiCN have been made by PECVD and successfully applied [1 – 6]. However, surface-hardened and strongly adherent coatings with improved wear-resistance, especially the coatings used at elevated temperatures, are still needed in some cases. For example, hot-working dies made of AISI H13 have to sustain an impacting load at a high-temperature environment. In recent years [7,8], we and other researchers suggested a duplex-plasma process which in-situ produces a TiN hard coating deposited on a plasma-nitrided substrate. It is found that such an optimized composite layer makes a
* Corresponding author. State-Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, 710049, PR China. Tel.: +86-29-2668395; fax: +86-29-2663453. E-mail address: [email protected] (S. Ma). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.03.048
significant improvement to the mechanical behavior of single TiN coatings. Ternary coating systems including TiCN and TiAlN, however, have better mechanical properties than simple TiN coatings; however, these coatings are not clearly understood yet. For a deeper understanding and extensive exploration of the plasma-duplex processing suggested in our earlier publication [7], we decided to examine another hard-coating TiCN system. Using the PECVD technique, we examined the TiCN system’s application for a hotworking die made of H13 steel. The main results are presented in this article.
2. Experimental The AISI H13 substrate, in the bar of 30 mm (diameter) 8 mm (height), was quenched and tempered to the hardness of 43 F 2 HRC, then ground to the surface roughness of Ra = 0.2 Am. Prior to plasma treatment, the samples were degreased, dried, and sputter etched. Table 1 shows plasma-nitriding parameters used. TiCN hard coatings were deposited on nitrided and non-nitrided
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Table 1 Plasma-nitriding parameters Pulsed voltage Pulse-on time Pulse-off time Temperature Pressure N2/(H2 + N2) flow ratio Nitriding time
650 V 25 As 25 As 520 jC 600 Pa 25% 0.5 – 30 h
samples by PECVD in order to compare the samples with each other. The standard processing parameters were used and listed in Table 2. The plasma-nitriding and PECVD TiCN deposition were conducted in situ in the industrial-scale plant shown schematically in Fig. 1. More details about the PECVD system can be found in our earlier reference [9]. The cylindrical vacuum chamber, which was 450 mm in diameter and 650 mm in height, was heated with an auxiliary heating system, the temperature of which was controlled by a thermocouple. The substrate was loaded directly onto the charging plate, which was also used as the cathode of the system. The surrounding wall of the chamber was used as an anode of the system and the earth potential. The pulsed power supply was able to produce voltages up to 1200 V and frequencies up to 33 kHz. The flow of the different gases was measured and controlled by massflow controllers. TiCl4 was led into the chamber by a flow of carrier gas (H2) flowing through the TiCl4 tank. The temperature was kept constant at 40 jC. During the production of the composite layer of nitrided H13/TiCN, the flow ratio of X = CH4/(CH4 + N2)% was changed, while other PECVD TiCN deposition conditions were kept constant. The thickness of the coatings was about 3 – 4 Am, which was measured under scanningelectron micrograph (SEM). A Regaku Dmax-IIIA diffractometer using Cu-Ka radiation was used for X-ray diffraction (XRD) analysis. Vickers microhardness was measured using a load of 0.25 N and an average of three readings was taken. The adhesion of the TiCN coatings was characterized by an indentation – adhesion test using a modified Rockwell hardness tester on which the indentaTable 2 PECVD TiCN coatings processing condition Pulsed voltage Pulse-on time Pulse-off time Temperature Pressure X=[CH4/(CH4 + N2)]% CH4 + N2 H2 Ar TiCl4 (carrier H2) Deposition time
650 V 25 As 25 As 520 jC 200 Pa 0 – 100% 150 ml/min 700 ml/min 50 ml/min 40 ml/min 6h
Fig. 1. Schematic drawing of PECVD systems.
tion force was applied on the samples with a continuous load. The adhesion was denoted by the critical load corresponding to the initiation of the coating spallation, which was monitored by acoustic emission. To some extent, the validity of this method was illustrated elsewhere [10]. The application of this improved plasma-duplex treatment processing was finally evaluated in practice using an actual hot-working die (with dimensions 0f 240 120 100 mm) made of H13 steel.
3. Results and discussion 3.1. Characterization of microstructure of TiCN coatings on plasma-nitrided H13 steel Fig. 2 shows clearly that the TiCN coatings on H13 steel are free of columnar structure, become denser, and have a finer microstructure than TiN coatings. This is a contribution to the microhardness, and therefore better wear-resistance and better corrosion resistance of H13 steel, which generally shows a poor corrosion property as a die-steel. Improvement to structure is probably due to the addition of carbon. Fig. 3 shows the XRD patterns of TiCN coatings with different carbon contents, we can see well that there is a shift in the lattice constants when the higher carbon included in TiCN coatings. However, it is apparent that the ternary TiCN coating system is always a single TiN-based continuous transition coating in which carbon atoms are simply dissolved in the lattice structure of TiN. With the higher ratio of X = CH4/(CH4 + N2)%, the ternary TiCN coating system can be produced which has superior properties such as the microhardness and adhesions seen in our next section. In our previous investigation for plasma nitriding of H13 steel at lower N2/(H2 + N2) flow ratio (25%), the nitrided compound
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Fig. 2. Cross-sectional SEM morphology of (a) TiCN(X = 38%) and (b) TiN coatings.
was observed near the interface region. The composite structure in that case consisted only of a substrate/nitrided diffusion layer/coating system. This result suggests that the nitrogen did not react with iron at a low-nitrogen
potential; instead, the nitrogen acted as an interstitial atom in the a-Fe lattice and was completely dissolved as a diffusion layer. However, the higher N2/(H2 + N2) flow ratio (50%) resulted in the formation of a new compound phase (usually called the white layer) and the composite structure was composed of substrate/diffusion layer/compound layer/coatings. This composite layer will decrease the adhesion of the coating due to premature brittle fracture between the coating and the substrate, as pointed out in our earlier publication [7]. 3.2. Microhardness and adhesion of TiCN coatings on plasma-nitrided H13 steel
Fig. 3. X-ray diffraction patterns of TiCN coatings deposited with different gas ratios of X = CH4/(CH4 + N2)%.
Fig. 4 shows that TiCN coatings with the thickness of 3 Am have higher microhardness than TiN coatings. This would be partially due to the enhancement of the TiCN phase, which has a higher microhardness than TiN coatings (see Fig. 4) and therefore is certainly useful to improve the wear resistance of ternary TiCN hard coatings. The microhardness of the composite layer of nitrided H13 steel can be adjusted through the plasma pre-nitriding process. Fig. 5 clearly shows that a plasma-nitrided layer can provide a high load-carrying capacity and is useful to sustain high, critical loading. This appears to be due to the
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Fig. 4. Surface microhardness of TiCN coatings deposited on 30-h prenitriding H13 steel vs. different gas ratios of X = CH4/(CH4 + N2)%.
increased thickness of the nitrided layer with the increase of plasma-nitriding time. Fig. 6 shows that the adhesion of the TiCN coatings is improved significantly as compared with TiN coatings deposited on nitrided or non-nitrided H13 substrate. We suggest that this is due to the matching of the lattice structure of the TiCN coatings with the nitrided layer, which produces a moderate transition of lattice structure and mechanical properties. In addition, the high microhardness, which represents a high load-carrying capacity, will contribute to the increase of adhesion. When this composite layer system is used, the system usually exhibits better properties than TiN coatings on nitrided steel, but the extra microhardness value of the composite layer is not always an advantage as seen in its application in the next section. 3.3. Application of plasma in-situ duplex processing for hotworking die made of H13 steel Hot-working dies still represent challenges for the improvement of these dies’ surface properties. We chose
Fig. 5. Surface microhardness of the pre-nitrided and TiCN coated (X = 38% and 3 Am thickness) H13 steel with different nitriding times.
Fig. 6. Effect of 30-h pre-nitriding on interfacial adhesion between coating and a H13 substrate, A is TiN coating on a non-nitrided substrate, B is TiN coating on a nitrided substrate, and C is TiCN coating (X = 38%) on nitrided substrate.
a hot-working die made of H13 steel in engine-blade production as an example. The challenges include the ability to combine high microhardness, high fatigue wearresistance, high toughness, and also high corrosion-resistance in working conditions of engine-blade production. For example, temperatures in the range of 800– 1000 jC are possible and also dramatic temperature changes. The first problem using hot-working die made of H13 steel is its relatively soft surface of substrate which is needed for its high toughness. In order to improve the surface microhardness and wear resistance of the dies, while retaining the high toughness of the substrate bulk materials of H13, normal gas nitriding and/or plasma nitriding, as well as carbon –nitrogen processing have been popular solutions. Hard coatings deposited on the surface of hotworking dies by different methods have been developed in recent years. The plasma-duplex treatment, which consists of in-situ plasma nitriding followed by hard coating, has been demonstrated to be a promising method for this type of die. In the present study, after optimized processing, the hot-working die made of H13 steel treated by plasma nitriding and then coated with a TiCN layer was given industry testing in the blade production. The service life is increased by a factor of 3, compared to a plasma-nitrided die, and also an increase by a factor of 2 compared to a die treated with a composite layer of a TiN/nitrided substrate. However, it will not be good for the hot-working die if the microhardness of composite coatings is higher (which is always the result from the high content of the TiCN phase) because this high microhardness is characterized by its brittleness and poor adhesion between coating and the nitrided H13 substrate as demonstrated in our earlier study [7]. The effort to get higher microhardness may be counterproductive for a die that sustains a heavy-load impacting in service. Other properties such as toughness of coating and interfacial adhesion must be all considered together. Different dies
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interfacial adhesion between coating and substrate. This plasma-duplex treatment processing for hot-working dies made of H13 steel used in engine-blade production was successful in improving lifetime over conventional techniques, the highest microhardness of the hot-working die obtained by using high CH4 concentrations were not as good as these with 38%CH4, due to brittleness and poor adhesion between coating and the nitrided-H13 substrate.
Acknowledgements
Fig. 7. Test results for differently treated hot-working dies made of H13 for the production of a blade. A shows the quenched state, B shows the nitrided state, C shows TiN coating on nitrided substrate, D and E are TiCN(X = 38%) coating on nitrided substrate, and F is the TiCN(X = 86%) coating on nitrided substrate. The nitriding time in all cases is the same 30 h, except for D which was 22 h.
and tools should have specifically optimized microstructures and corresponding properties of coatings. From a practical point of view, this plasma-duplex treatment is a new and successful technique for a hot-working die made of H13 steel (Fig. 7).
4. Conclusions Our results show that the composite layer of nitridedH13 steel and TiCN coatings prepared by pulsed dc PECVD is useful technique for surface modification of a hot-working die made of H13 steel. The composite layer consists of a continuous transition coatings in this TiCN ternary system deposited on a nitrided-H13 steel substrate. With the assistance of plasma nitriding, the composite layer improves the surface microhardness and
Thanks to the National High-Tech program of China (No. 2001AA883010) and National Natural Science Foundation of China (No. 50271053, No. 50371067) for financial support. This work has been also supported in part by the European Community’s fifth framework program (GRD-2001-40419).
References [1] S.L. Ma, Y.H. Li, J.M. Nan, K.W. Xu, Trans. Nonferr. Met. Soc. China 10 (4) (2000) 489. [2] D. Heim, R. Hochreiter, Surf. Coat. Technol. 98 (1998) 1553. [3] S. Veprek, A. Niederhofer, K. Moto, T. Bolom, Surf. Coat. Technol. 133 – 134 (2000) 152. [4] B. Matthes, E. Broszeit, K.H. Kloos, Surf. Coat. Technol. 57 (1993) 97. [5] B. Selbach, K. Schmidt, M. Wang, Thin Solid Films 188 (1990) 267. [6] P. Hardt, M. Eckel, M. Schmidt, H. Wulff, Surf. Coat. Technol. 20 – 121 (1999) 238. [7] S.L. Ma, Y.H. Li, K.W. Xu, Surf. Coat. Technol. 137 (2001) 116. [8] S.Y. Lee, J.W. Chung, K.B. Kim, J.G. Han, S.S. Kim, Surf. Coat. Technol. 86 – 87 (1996) 325. [9] S.L. Ma, Y.H. Li, K.W. Xu, Surf. Coat. Technol. 131 (2000) 131. [10] S.L. Ma, Doctoral thesis, Xi’an Jiaotong University, Xi’an, China, 2001.