The composite of nitrided steel of H13 and TiN coatings by plasma duplex treatment and the effect of pre-nitriding

The composite of nitrided steel of H13 and TiN coatings by plasma duplex treatment and the effect of pre-nitriding

Surface and Coatings Technology 137 Ž2001. 116᎐121 The composite of nitrided steel of H13 and TiN coatings by plasma duplex treatment and the effect ...

530KB Sizes 0 Downloads 9 Views

Surface and Coatings Technology 137 Ž2001. 116᎐121

The composite of nitrided steel of H13 and TiN coatings by plasma duplex treatment and the effect of pre-nitriding Shengli Ma, Yanhuai Li, Kewei XuU State-Key Laboratory for Mechanical Beha¨ ior of Materials, Xi’an Jiaotong Uni¨ ersity, Xi’an 710049, PR China Received 3 December 1999; received in revised form 5 July 2000; accepted 30 August 2000

Abstract In order to improve the adhesion behavior of TiN coatings deposited on H13 steel, a layered composite structure has been developed by plasma nitriding and plasma-enhanced CVD in the present studies. Effects of the nitriding process on the microstructure, adhesion and microhardness, as well as the residual stress of composite nitrided H13rTiN coatings were investigated. Experimental results showed that the adhesion of TiN coatings to substrate could be remarkably enhanced by an optimized plasma nitriding process conducted at a 25% flow ratio of N2rŽH 2 q N2 . for 1 h. The formation of a new compound layer during a nitriding process at a 50% flow ratio of N2rŽH 2 q N2 . deteriorates the adhesion of TiN coatings due to premature brittle fracture between the coating and the substrate. The surface hardness of nitrided H13rTiN coatings and compressive residual stress in the diffusion interlayer increase with increasing pre-nitriding time, but the adhesion of the coatings decreases to some extent. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Plasma duplex treatment; Plasma-enhanced chemical vapor deposition ŽPECVD. TiN; Nitriding process; H13 steel

1. Introduction The benefits of plasma surface engineering have been increasingly recognized and proven in various industrial branches during the past decades w1,2x, among which the techniques of plasma-enhanced chemical vapor deposition ŽPECVD. are investigated more extensively. For example, hard coatings of TiN w3,4x, TiCN w5,6x and TiC w7x on high speed steels and cemented carbides deposited by PECVD processes have been developed to improve the service life of tools, dies and components working in heavy wear conditions. However, more wear-resistant surface-hardened and strongly adherent coatings are still needed for hotworking die steels w8,9x, such as AISI H11 and H13, U

Corresponding author. Tel.: q86-29-2668914; fax: q86-293237910. E-mail address: [email protected] ŽK. Xu..

since they are subjected to a severe sliding abrasive wear in an elevated-temperature environment. Wear resistance of such coatings produced by conventional techniques are not good enough in some cases w10x, so a duplex plasma treatment process, i.e. a hard coating deposited on a plasma nitrided substrate, has been developed in recent years. Zlatanovic et al. w11x suggested that a composite nitrided steel of AISI M2 or AISI H11rhard coating structure is important for applications in which a more wear-resistant surface is required. They noted that for all composite structures, the adhesion of coatings was enhanced compared to non-nitrided specimens. Furthermore, a duplex treatment combined a diffusion-dominated plasma nitriding of the substrate with a multilayer coating system of titanium compounds TiN᎐ ŽTi,Si.N᎐TiC by plasmaassisted CVD was proposed by Park et al. w12x. This treatment was recognized to apply well to a workpiece of a stellite 6B that works under severe abrasive wear.

0257-8972r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 1 0 7 3 - 2

S. Ma et al. r Surface and Coatings Technology 137 (2001) 116᎐121

Huang et al. w13x found that the hardness of TiN coating on a nitrided AISI M2 was increased due to a better load carrying capacity, but the adhesion between the coating and the substrate was decreased with increasing flow ratio of N2rŽH 2 q N2 . in nitriding. They suggested that the wear resistance is predominantly affected by the adhesion of coating instead of the load-carrying capacity of the modified substrate. Sirvio et al. w14x reported a similar result for AISI M2 substrates. It is evident, without doubt, that the properties of the nitrided steelrthin hard coatings are dependent on the substrate materials as well as the processing used. For the hot-working die steel AISI H13, experiments on such a duplex treatment are limited. In the present study, the effect of nitriding on the microstructure of the H13 substrate and on the subsequent mechanical properties of composite nitridedrTiN coating structure is explored.

2. Experimental AISI H13 substrate in the form of ␾30 = 10 mm was quenched and tempered to a hardness of 43 " 2 HRC, and ground to a surface roughness of R a s 0.2 ␮m. Prior to plasma treatment, the samples were degreased, dried and sputter etched. The plasma nitriding parameters used are shown in Table 1. TiN hard coatings were deposited on nitrided and non-nitrided samples by pulsed DC plasma-enhanced chemical vapor deposition. According to our previous investigations, the processing parameters have a great effect on the plasma characteristics such as plasma density and electron temperature, for instance, the plasma density decreases with increasing TiCl 4 flow. Therefore, the standard processing parameters are used and listed in Table 2. The plasma nitriding and PECVD TiN deposition were conducted in the same industrial-set plant, as shown schematically in Fig. 1. The cylindrical vacuum chamber, which is 450 mm in diameter and 650 mm in height, can be heated with an auxiliary heating system, the temperature of which is controlled by a thermocouple. The substrate is put directly on the charging plate, which is also used as the cathode of the system. The surrounding wall of the chamber is used as the anode

117

Table 2 PECVD TiN process conditions Pulsed voltage Pulse-on time Pulse-off time Temperature Pressure N2 H2 Ar TiCl4 Žcarrier H2 . Deposition time

650 V 25 ␮s 25 ␮s 520⬚C 350 Pa 400 mlrmin 700 mlrmin 50 mlrmin 110 mlrmin 2h

of the system and the earth potential. The pulsed power supply is able to produce voltage up to 1200 V and a frequency up to 33 kHz. The flow of the different gases is measured and controlled by mass-flow controllers. TiCl 4 is transported to the chamber by the amount of carrier gas ŽH 2 . flowing through the TiCl 4 tank, with temperature kept constant at 40⬚C. During the production of composite nitrided H13r TiN coatings, the flow ratio of N2rŽH 2 q N2 . and the treatment time during nitriding process were varied, while PECVD TiN deposition conditions were kept constant. The thickness of the coatings was approximately 2 ; 3 ␮m, which was measured by SEM. Optical microscopy was used for the microstructure analysis on the metallurgical cross-sections, and the diffusion layer was examined after the samples were polished and etched in 3% HNO3ralcohol solution. A Dmax-IIIA diffractometer using CuK ␣ radiation was used for Xray diffraction ŽXRD. analysis, and residual stress in the nitriding diffusion structure was measured with the sin 2 ␺ method. Vickers microhardness was measured using a load of 0.5 N and an average of three readings was taken. The adhesion of the TiN coatings was characterized by an indentation adhesion test using a modified Rockwell hardness tester, on which the indentation force is applied with a continuous load. The adhesion was denoted by the critical load corresponding to the initiation of the coating spallation, which was

Table 1 Plasma nitriding parameters Pulsed voltage Pulse-on time Pulse-off time Temperature Pressure N2rŽH2 q N2 . flow ratio Nitriding time

650 V 25 ␮s 25 ␮s 520⬚C 500᎐600 Pa 25, 50% 0.5᎐4.0 h

Fig. 1. Schematic diagram of PECVD systems.

S. Ma et al. r Surface and Coatings Technology 137 (2001) 116᎐121

118

monitored by acoustic emission. The validity of the method is illustrated elsewhere w15x. The brittleness of nitrided layer was assessed by the indentation test, on which micro-cracks around the indentation are usually evident when the nitrided layer is brittle w16x.

3. Results and discussion 3.1. Effect of N2 r (H2 q N2 ) flow ratio for nitriding Fig. 2a indicates that the higher N2rŽH 2 q N2 . flow ratio Ž50%. resulted in the formation of a new compound Žthe white layer., and the composite structure was composed of substrate-diffusion layerrcompound layerrTiN coating. However, at the lower N2rŽH 2 q N2 . flow ratio Ž25%., no compound was observed in the interface region; the composite structure consisted only of a substrate-diffusion layerrTiN duplex system as indicated in Fig. 2b. This result suggested that the nitrogen atoms did not react with iron, instead, it acted as an interstitial atom in the ␣-Fe lattice and completely dissolved as a diffusion layer w13x. The same result can be found from the XRD patterns of nitrided specimens at the two N2rŽH 2 q N2 . ratios ŽFig. 3.. This demonstrates that the compound is ␥⬘ŽFe 4 N. at higher N2rŽH 2 q N2 . ratio Ž50%.. The difference in XRD patterns between substrate H13 and nitrided H13 at the lower N2rŽH 2 q N2 . ratio Ž25%. is only evident in the peak shifting and broadening, which may be caused by an increase in the lattice constant due to nitrogen atoms dissolved in the ␣-Fe lattice w14x. The surface hardness measurements are shown in Fig. 4. With 0.5-N indentation, the indentation depth was approximately 1.58 ␮m for coatings on the nonnitrided substrate, and 1.45 and 1.37 ␮m for coatings on nitrided samples at the 25 and 50% ratios of N2rŽH 2 q N2 ., respectively. It can be seen that the

Fig. 2. Optical microscopy of cross-section of composite nitridedrTiN coatings at two nitriding N2rŽH 2 q N2 . ratios: Ža. 50; and Žb. 25%.

surface hardness of the composite nitridedrTiN coatings on H13 is higher than that of TiN coatings on the non-nitrided H13 substrate. For the higher N2rŽH 2 q N2 . ratios the surface hardness obviously increased, and this may be due to the strong support of the compound interlayer. Fig. 5 shows that the adhesion of TiN coatings on nitrided H13 is higher than that on non-nitrided H13 substrate. Surface hardening by plasma nitriding can provide a heavier load-carrying capacity for TiN coated components; this may be one of the reasons for the

Fig. 3. XRD patterns of nitrided H13 at two N2 rŽH 2 q N2 . flow ratios: Ža. 25; Žb. 50%; and Žc. substrate.

S. Ma et al. r Surface and Coatings Technology 137 (2001) 116᎐121

119

Fig. 4. Surface hardness of TiN coatingsr nitrided H13 substrate at two N2 rŽH 2 q N2 . ratios: Ža. 25; and Žb. 50%.

improvement of the adhesion on nitrided H13. However, the indentation adhesion appeared to decrease with increasing N2rŽH 2 q N2 . ratio, as indicated in the figure. In order to explore the reasons, the indented surface morphologies of nitrided H13rTiN coatings were examined. Fig. 6 is an example. It shows that the surface of the coatings on the nitrided sample at the lower N2rŽH 2 q N2 . ratio Ž25%. was not damaged with the pressing force of 1000 N; but at the higher N2rŽH 2 q N2 . ratio Ž50%., a pressing force 500 N exposed a few micro-cracks. In a previous study by Xie w16x, high brittleness was suggested on the indentation morphology of a nitrided sample with micro-cracks. The present study demonstrates that such a brittle performance is a result of the formation of the new compound inter-layer. It is well known that the nitriding layers Žboth diffusion and compound layers. provide a high load-carrying capacity and are useful in sustaining a higher critical load. However, if the inter-layer is subjected to a high brittleness, then coating spallation during the indentation process may result, essentially

Fig. 5. Indentation adhesion of TiN coatings Ž2 ; 3 ␮m. on nonnitrided and nitrided H13 at two N2rŽH 2 q N2 . ratios: Ža. 25; and Žb. 50%.

Fig. 6. Indentation morphology of nitrided samples at two nitriding N2rŽH 2 q N2 . ratios: Ža. 25%, pressing force 1000 N; and Žb. 50%, pressing force 500 N.

from the rupture of the compound layer. The failure mode is illustrated schematically in Fig. 7. When a high indentation force is applied to the coated surface, the brittle fracture within the compound inter-layer would

Fig. 7. Illustration of the failure mode for coating spallation of nitrided H13rTiN structure with a compound interlayer under indentation.

120

S. Ma et al. r Surface and Coatings Technology 137 (2001) 116᎐121

Fig. 9. XRD patterns of TiN-coated samples after nitridation wN2rŽH 2 q N2 . ratio 25%x for different times: Ža. 0.5; Žb. 1.0; Žc. 1.5; and Žd. 4.0 h.

Fig. 8. Nitrided structure of H13 wN2rŽH 2 q N2 . ratio 25%x with nitriding time of: Ža. 0.5; Žb. 1.5; and Žc. 4.0 h.

The surface hardness of nitrided H13rTiN coatings increases with increasing pre-nitriding time ŽFig. 10.. The adhesion of TiN coatings deposited on non-nitrided and nitrided H13 steel with different nitriding times is shown in Fig. 11. It is noted that the adhesion initially increases with nitriding time, but decreases when the nitriding duration increases further. Since the PECVD process was kept constant, the change in adhesion can hardly be attributed to the TiN coatings. The depth of the diffusion layer and the load-carrying capacity increased with nitriding time, which may be useful for stronger adhesion. Nevertheless, the adhesion will be reduced if the premature rupture in the diffusion interlayer takes place for some reason. The effect of nitriding time on residual stress in the diffusion inter-layer is shown in Fig. 12. It indicates that the stress decreased slightly when a short nitriding time was used Ž- 1 h., but showed a pronounced increase with long nitriding times. The stress enhancement due to prolonged plasma nitriding time is responsible for the decrease in TiN adhesion w13x. Although a

happen first, and then propagate outwards to the coating. It follows that a pure diffusion inter-layer without the compound produced by pre-nitriding, with a lower flow ratio, is better for improvement of the adhesion. 3.2. Effect of nitriding time Fig. 8 shows the cross-section of H13rTiN coatings with different nitriding times under constant N2rŽH 2 q N2 . ratio Ž25%.. No compound inter-layer can be observed, which is consistent with the XRD analysis ŽFig. 9., The depth of the diffusion layer increased with the treatment time. The depths are 13, 58 and 133 ␮m for nitriding times of 0.5, 1.5 and 4.0 h, respectively, as indicated in Fig. 8.

Fig. 10. Surface hardness of the pre-nitrided wN2rŽH 2 q N2 . ratio 25%x and TiN-coated Ž2 ␮m. H13 steel with different nitriding times.

S. Ma et al. r Surface and Coatings Technology 137 (2001) 116᎐121

Fig. 11. Indentation adhesion of TiN coatings on non-nitrided and nitrided wN2rŽH 2 q N2 . ratio 25%x H13 steel with different nitriding times.

121

for 1 h. The formation of a new compound layer during a nitriding process at a 50% flow ratio of N2rŽH 2 q N2 . deteriorates the adhesion of TiN coatings, due to its premature brittle fracture between the coating and the substrate. The surface hardness of nitrided H13rTiN coatings increases with increasing pre-nitriding time at a 25% flow ratio of N2rŽH 2 q N2 .; this may be due to the fact that prolonged nitriding time produces a deep diffusion interlayer and consequently provides a good load-carrying capacity for TiN coatings. But the adhesion of TiN coatings decreases with pre-nitriding time; this may result from an accumulation of compressive residual stresses in the diffusion interlayer, which probably causes premature fracture of the interlayer under indentation.

Acknowledgements long nitriding time provides a good load-carrying capacity, an accumulation of higher residual stress in the inter-layer is harmful to the adhesion between nitrided H13 and the TiN coating. Therefore, the nitriding parameters must be carefully optimized so that the industrial application of plasma duplex treatment for improving wear-resistance of H13 is possible.

The authors are grateful for the financial support of the Hi-Tech. Program of China under contract No. 7150080060 and the Doctorate Foundation of Xi’an Jiaotong University ŽDFXJU1999-7. and National Nature Science Foundation of China Ž59921010.. References

4. Conclusions The adhesion of TiN coating to hot-working die steel H13 can be improved by an optimized plasma nitriding process conducted at a 25% flow ratio of N2rŽH 2 q N2 .

Fig. 12. Residual stress in nitriding diffusion inter-layer wN2rŽH 2 q N2 . ratio 25%x with different nitriding times.

w1x R. Sunchentrunk, G. Staudigl, D. Jonke, H.J. Fuesser, Surf. Coat. Technol. 97 Ž1997. 1᎐9. w2x P. Yan, P. Hui, W. Zhu, H. Tan, Surf. Coat. Technol. 102 Ž1998. 175᎐181. w3x K.S. Mogensen, N.B. Thomsen, S.S Eskildsen, C. Mathiasen, J. Bottiger, Surf. Coat. Technol. 99 Ž1998. 140᎐146. ¨ w4x K.T. Rie, A. Gebauer, J. Woehle, Surf. Coat. Technol. 60 Ž1993. 385᎐388. w5x N.J. Archer, Thin Solid Films 80 Ž1981. 221᎐225. w6x K.T. Rie, A. Gebauer, Mater. Sci. Eng. A139 Ž1991. 61᎐66. w7x U. Konig, R. Tabersky, H. Berg, Surf. Coat. Technol. 50 Ž1991. 57᎐62. w8x S.Y. Lee, J.W. Chung, K.B. Kim, J.G. Han, S.S. Kim, Surf. Coat. Technol. 86r87 Ž1996. 325᎐331. w9x M. Zlatanovic, Surf. Coat. Technol. 48 Ž1991. 19᎐24. w10x M. Zlatanovic, T. Gredic, A. Kunosic, N. Backovic, N. Whittle, Surf. Coat. Technol. 63 Ž1994. 35᎐41. w11x M. Zlatanovic, T. Gredic, N. Popovic, Z. Bogdanov, Vacuum 44 Ž1993. 83᎐88. w12x J.-R. Park, Y.K. Song, K.T. Rie, A. Gebauer, Surf. Coat. Technol. 98 Ž1998. 1329᎐1335. w13x H.H. Huang, J.L. He, M.H. Hon, Surf. Coat. Technol. 64 Ž1994. 41᎐46. w14x E.H. Sirvio, M. Sulonen, H. Sundquist, Thin Solid Films 96 Ž1982. 93᎐101. w15x B. Tang, Doctorate thesis, Xi’an Jiaotong University, China, 1998, p. 12 Žin Chinese.. w16x F. Xie, Doctorate thesis, Xi’an Jiaotong University, China, 1997, p. 12 Žin Chinese..