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NiSi metal silicide coatings
Acta Materialia 52 (2004) 5419–5426 www.actamat-journals.com
High-temperature phase stability and tribological properties of laser clad Mo2Ni3Si/NiSi metal silicide coatings X.D. Lu, H.M. Wang
*
Laboratory of Laser Materials Processing and Surface Engineering, School of Materials Science and Engineering, Beihang University (formerly Beijing University of Aeronautics and Astronautics), 37 Xue Yuan Road, Beijing 100083, PR China Received 11 February 2004; accepted 6 August 2004
Abstract Mo2Ni3Si/NiSi wear-resistant metal silicide composite coatings consisting of Mo2Ni3Si primary dendrite and interdendritic Mo2Ni3Si/NiSi eutectic were fabricated on substrate of an austenitic stainless steel AISI321 by laser cladding using Ni–Mo–Si elemental powder blends. The high-temperature structural stability of the coating was evaluated by aging at 800 C for 1–50 h. High-temperature sliding wear resistance of the as-laser clad and aged coatings was evaluated at 600 C. Results indicate that the Mo2Ni3Si/NiSi metal silicides coating has excellent high temperature phase stability. No phase transformation except the dissolution of the eutectic Mo2Ni3Si and the corresponding growth of the Mo2Ni3Si primary dendrite and no elemental diffusion from the coating into the substrate were detected after aging the coating at 800 C for 50 h. Aging of the coating at 800 C leads to gradual dissolution of the interdendritic eutectic Mo2Ni3Si and subsequent formation of a dual-phase structure with equiaxed Mo2Ni3Si primary grains distributed in the NiSi single-phase matrix. Because of the strong covalent-dominated atomic bonds and high volume fraction of the ternary metal silicide Mo2Ni3Si, both the original and the aged Mo2Ni3Si/NiSi coating has excellent wear resistance under pin-on-disc high-temperature sliding wear test conditions, although hardness of the aged coating is slightly lower than that of the as-clad coating. 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Laser cladding; Metal silicide; Phase stability; Wear resistance; Coating
1. Introduction Transition metal silicides such as MoSi2, Ni3Si, Cr3Si etc., have many attractive properties for structural applications in the aerospace and energy production industries [1–3]. Their low density, good mechanical and oxidation properties up to temperatures of 1200 C make these materials excellent candidates for high temperature structural applications [4,5]. It is also preliminarily [6–8] demonstrated that many of these * Corresponding author. Tel.: +86 10 82317102; fax: +86 10 82317105. E-mail addresses: [email protected], wanghuaming@buaa. edu.cn (H.M. Wang).
transition metal silicide-based materials exhibit excellent tribological properties and are likely a new class of abrasion and adhesion wear-resistant materials for tribological components working under high-temperature or aggressive service conditions. Serious room-temperature brittleness is the main obstacle preventing the materials from industrial applications [2,9–11]. Introduction of a second ductile phase and production of a multi-phase structure are recognized as two of the most efficient means to improve the toughness of intermetallic alloys [12–17]. Compared to the binary metal molybdenum silicides, such as MoSi2 and Mo5Si3, the ternary metal silicide Mo2Ni3Si with the hP12 Laves crystal structure is anticipated to possess better toughness while still keeping the inherent high hardness and strong atomic bonds
1359-6454/$30.00 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2004.08.006
X.D. Lu, H.M. Wang / Acta Materialia 52 (2004) 5419–5426
and therefore, is expected to be an ideal wear resistant reinforcement. In our previous study, the Mo2Ni3Si/ NiSi multi-phase metal silicides coatings were designed and fabricated by the laser cladding process and exhibited excellent wear resistance under both room and high-temperature sliding wear test conditions [16,17]. The long-term high-temperature phase stability is of critical importance for a coating working in high-temperature environments especially for a laser clad coating with a rapidly solidified non-equilibrium microstructure because unfavorable phase transformation (e.g. formation of a harmful phase, etc.) and serious elemental diffusion from the coating into the substrate during long-term elevated-temperature exposure are two of the most important reasons leading to degradation of a high-temperature coating [18–21]. In this paper, the long-term high-temperature phase stability of a laser clad Mo2Ni3Si/NiSi metal silicide coating was studied by aging the coating at 800 C for 1–50 h. Aging behaviors and the effect of age-treatment on wear resistance of the coating were investigated as functions of aging time under pin-on-disc high-temperature sliding wear test conditions.
2. Experimental procedures A solution-treated commercial austenitic stainless steel AISI321 was selected as the substrate material. Commercial pure elemental powder blends (particle size 70–140 lm) in nominal chemical composition (wt%) of 35Ni–39Mo–26Si was utilized as the laser cladding precursor materials. The powders are preplaced on surface of the austenitic stainless steel specimens, 50 mm · 20 mm · 10 mm in size, prior to laser cladding with a powder-bed thickness of approximately 1 mm. Laser cladding was conducted on a 5 kW transverse-flow continuous-wave CO2 laser materials processing system equipped with a 4-axis computer numerical controlled (CNC) laser materials processing machine under ambient atmospheric environment. The optimized laser cladding parameters are as follows: laser power 1.8 kW, laser beam diameter 3 mm and beam traverse speed 200 mm/min. Six overlap tracks were clad side by side with a overlap ratio of approximately 25% in order to clad the whole surface of 50 mm · 20 mm on the substrate specimen. Average thickness of laser clad coatings is approximately 0.8 mm. Metallographic and wear test specimens were prepared by electric discharging machining followed by mechanical milling and grinding. Metallographic sections were prepared using standard mechanical polishing procedures and were etched in HF:HNO3:H2O water solution in volume ratio of 1:6:7. Microstructure was characterized using a Nephot optical microscope (OM) and a KYKY-2800 and a Hitachi S-3500N scan-
ning electron microscopes (SEM). Phase constituents were identified by X-ray diffraction (XRD) using a Rigaku D/max 2200 X-ray diffractometer with Cu Ka radiation at a scanning rate of 5/min. Chemical compositions of the phases were analyzed by a Oxford Inca energy dispersive spectroscope (EDS). Hardness profiles along the depth direction of the laser clad coatings were measured using a MH-6 semi-automatic Vickers microhardness tester with a test load of 1.96 N and a dwell time of 15 s. The eutectic and liquidus temperatures were measured using a Netzsch Sta 449C differential scanning calorimetry (DSC) with a heating rate of 20 C/min and an Ar flow rate of 20 ml/min. Results of DSC (as shown in Fig. 1) indicated that the eutectic temperature of the coating is 815 C (dH = 9.076 J/g) and the liquidus is 1110 C (dH = 33.47 J/g). The aging temperature was selected at 800 C which is slightly lower than the eutectic temperature of the coating. High-temperature phase stability of the coating was evaluated by aging the coating at 800 C for 1–50 h. High-temperature wear resistance of the original (as-laser clad) and aged coatings was evaluated on a HTW-II type pin-on-disc high-temperature sliding wear tester (Fig. 2) at 600 C, where two laser clad specimens, 6 mm · 6 mm · 6 mm in size, slide under a total test load of 98 N on a rotating disc made of a solid-solution strengthened nickel-base superalloy Nimonic 75 (500 HV at room-temperature). The relative sliding speed is 0.066 m/s and the total wear sliding distance is 238 m. The solution-treated austenitic stainless steel AISI321 was selected as the reference material for all the wear tests. Wear mass loss was measured using a Sartorius BS 110 precision electronic balance in accuracy of 0.1 mg. Wear mechanisms of the laser clad Mo2Ni3Si/NiSi metal silicide coating were discussed based on SEM observations of the worn surface morphologies and subsurface microstructure.
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Temperature (˚˚C) Fig. 1. DSC curve of the laser clad Mo2Ni3Si/NiSi metal silicide composite coating.
X.D. Lu, H.M. Wang / Acta Materialia 52 (2004) 5419–5426
Fig. 2. Schematic illustration of the high-temperature sliding wear tester [13].
3. Results The original laser clad Mo–Ni–Si intermetallic coatings have a fine and uniform dendritic structure and are metallurgically bonded to the substrate, as shown in Figs. 3 and 4. Results of XRD (as indicated in Fig. 5(a)) and EDS analysis indicate that the primary dendrites are the Mo2Ni3Si ternary metal silicide which has the hP12 topologically closed packed (TCP) Laves crystal structure [22] and the interdendritic lamellar eutectic structure is the Mo2Ni3Si/NiSi, as shown in Fig. 4(a). Volume fraction of the Mo2Ni3Si primary dendrite is approximately 45%. As shown clearly in Fig. 5(b), aging the coating at 800 C for 50 h produces no any changes on phase constituents of the coating, indicating that the laser clad Mo2Ni3Si/NiSi metal silicide coatings have excellent high-temperature phase stability. However, hardness
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of the aged coating was slightly decreased by a magnitude of approximately HV100, as shown in Fig. 6. Results of line-scan EDS analysis indicate that the no any elemental inter-diffusions between the coating and the substrate were detected after aging treatment at 800 C for 50 h as shown in Fig. 7. As indicated in Fig. 8 and more clearly in Fig. 9, drastic changes of microstructure during ageing process occurred in the first 10 h. The relative volume fraction of interdendritic eutectic decreases drastically with the increasing aging time (as indicated in Fig. 8) and rapid dissolution and coarsening of the interdendritic fine lamellar Mo2Ni3Si/NiSi eutectic structure and notable breaking or granulation of the Mo2Ni3Si primary dendrites occurred (as shown in Fig. 9(a) and (b)) during the first approximately 10 h. During the subsequent aging process, the microstructure changes are quite mild with the increasing aging time. The residual interdendritic eutectic thick plates are completely dissolved and the Mo2Ni3Si primary dendrites are further granulated during the subsequent aging process, leading to the formation of a
Fig. 3. OM micrograph showing the overview transverse cross-section of the laser clad Mo2Ni3Si/NiSi metal silicide composite coating.
Fig. 4. SEM micrographs showing microstructure of the laser clad Mo2Ni3Si/NiSi metal silicide composite coating, (a) in the central coating and (b) in the coating/substrate transition zone.
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Distance from coating surface (µm) Fig. 6. Hardness profiles of (a) the original and (b) the aged (800 C/ 50 h) Mo2Ni3Si/NiSi coatings.
Relative volume fraction of eutectics (%)
Fig. 5. XRD patterns of (a) the original and (b) the aged (800 C/50 h) Mo2Ni3Si/NiSi coatings.
100 80 60 40 20 0 0
10
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Ageing time (h) Fig. 8. Relative volume fraction of the interdendritic eutectic as a function of aging time.
Fig. 7. EDS line-scan analysis showing the major elements distributions of (a) the as-laser clad and (b) aged Mo2Ni3Si/NiSi coating at 800 C for 50 h.
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Fig. 9. SEM micrographs showing the typical microstructure of the Mo2Ni3Si/NiSi coating after aging treatment at 800 C for (a) 1 h, (b) 10 h, (c) 30 h and (d) 50 h.
Wear mass loss (mg)
30 25
as-clad coating aged coating AISI321
25.6
20 15 10 5
4.5
5.8
0 Fig. 10. Wear mass loss of the as-laser clad and aged (800 C/50 h) Mo2Ni3Si/NiSi metal silicide coatings in comparison to the reference material AISI321 under sliding wear test conditions at 600 C with a test load of 98N and a sliding speed of 0.066 m/s.
dual-phase intermetallic structure consisting of coarse granular Mo2Ni3Si grains evenly distributed in the NiSi matrix, as shown in Fig. 9(d). Volume fraction of the granular Mo2Ni3Si phase in the aged Mo2Ni3Si/NiSi coating measured by contrast-based quantative metallographic method is approximately 62%. As indicated in Fig. 10, aging the laser clad Mo2Ni3Si/NiSi intermetallic coating at 800 C for 50 h leads to
no significant decrease of the wear resistance. Because of the complete dissolution of interdendritic eutectic structure and the granulation and coarsening of the Mo2Ni3Si primary dendrites, which leads to slight hardness reduction of aged coating, wear mass loss of the aged coating is slightly increased. In comparison to the reference material of AISI321, however, both the as-laser clad and the aged Mo2Ni3Si/NiSi metal silicide coatings have excellent tribological properties under sliding wear test conditions at 600 C.
4. Discussion Because both phase constituent of the laser clad Mo2Ni3Si/NiSi intermetallic coating i.e., the ternary metal silicide Mo2Ni3Si and binary metal silicide NiSi, are intermetallic compounds with strong covalentdominant atomic bonds and high thermodynamic stability, no any phase transformation, phase decomposition and elemental inter-diffusion between the coating and the substrate occurred during the longterm high-temperature aging process at so high a temperature just only approximately 15 C below its eutectic temperature. This illustrated that the Mo2Ni3Si/NiSi metal silicide coatings have excellent high-temperature structural stability, which is helpful for the
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coating to be applied as high-temperature tribological coatings at service temperatures near to their eutectic temperatures. However, the selected aging temperature of 800 C is so high which is only approximately 15 C below its eutectic temperature, significant atomic diffusion is able to occure and the kinetic condition is favorable for dissolution of the fine lamellar eutectic Mo2Ni3Si phase and breaking and granulation and coarsening of the Mo2Ni3Si primary dendrites, leading to the formation of a dual-phase intermetallic structure consisting of the coarse granular Mo2Ni3Si grains and the NiSi single-phase matrix, as indicated in Fig. 9(d). The dissolution of the fine lamellar eutectic Mo2Ni3Si and the breaking, granulation, coarsening of the Mo2Ni3Si primary phase are the natural spontaneous Ostwald ripening process at high-temperature exposure conditions leading to reduction of the interfacial energy of the coating by greatly reducing the Mo2Ni3Si/NiSi interface areas. Under high-temperature sliding wear test conditions coupling with the nickel-base superalloy rotating disc counterpart, the sliding surface of the austenitic stainless steel is easily plastically deformed, ploughed or transferred, leading to detachment of vary large chips and adhesive wear debris. The surface of the austenitic stainless steel suffers severe abrasive and adhesive wear [17]. On the contrary, the worn surface of the Mo2Ni3Si/NiSi
metal silicide coatings is extremely smooth, without any cutting or plowing grooves and plastic deformation features observable, as shown in Figs. 11(a) and 12(a). Because of the high hardness and its abnormal hardness-temperature relationship, it is difficult for the Mo2Ni3Si to be plastically deformed or ploughed during the high-temperature sliding wear process, imparting the as-laser clad coating with excellent abrasive wear resistance. Moreover, the unique atomic bonding characteristics of Mo2Ni3Si with strong covalent bonds provide Mo2Ni3Si with excellent resistance to metallic adhesion when coating with a sliding counterpart. Meanwhile, the interdendritic Mo2Ni3Si/NiSi eutectics played a positive role in resisting wear attacks via the mechanism of providing the wear-resistant Mo2Ni3Si primary dendrite with powerful support. Moreover, it is interesting that significant dissolution of interdendritic Mo2Ni3Si/NiSi eutectic structure also occurred during the high-temperature sliding wear process at 600 C for the as-laser clad Mo2Ni3Si/NiSi coatings, as show in Fig. 11(b) and more clearly Fig. 12(a). Worn surface morphologies of the as-laser clad (as indicated in Fig. 11(a)) and the aged (see Fig. 11(c)) Mo2Ni3Si/NiSi intermetallic coatings at low magnifications are very similar and are very resemble to a polish-and-etched metallographic section, on which there are nearly no any cutting or plowing grooves and no
Fig. 11. SEM micrographs showing worn surface morphologies of: (a), (b) the as-laser clad and (c), (d) the aged (800 C/50 h) Mo2Ni3Si/NiSi metal silicide coatings after sliding wear test at 600 C for 1 h.
X.D. Lu, H.M. Wang / Acta Materialia 52 (2004) 5419–5426
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Fig. 12. SEM micrographs showing the subsurface microstructure of (a) the as-laser clad and (b) the aged (800 C/50 h) Mo2Ni3Si/NiSi metal silicide coatings after sliding wear test at 600 C for 1 h.
any wear features in terms of metallic adhesion and material-transfer observable. This illustrates that both the as-laser clad and the aged coatings have excellent capability to resist abrasive and adhesive wear attacks during high temperature metallic sliding wear process because microstructural phase constituents of both the original and the aged coatings are the same, i.e., the Mo2Ni3Si and NiSi. However, close examinations of the worn surface under higher magnifications (as indicated in Fig. 11(b) and (d)) indicated that the wear behaviors and wear mechanisms are with some notable difference between the as-laser clad and the aged coatings. For the as-laser clad coating, the Mo2Ni3Si primary dendrites are firmly hold by the matrix (i.e., the Mo2Ni3Si/NiSi eutectic) and no any microcracks or fragmentation were observed on the slightly protruded Mo2Ni3Si dendrites on the worn surface, as indicated in Fig. 11(b) and in the subsurface, as shown in Fig. 12(a). Whereas the coarse granular or spherical Mo2Ni3Si grains of the aged coatings with a dual-phase microstructure are over-protruded out of the worn surface because the relatively weaker and softer NiSi matrix was preferentially worn out, making the hard and wear-resistant Mo2Ni3Si grains loose adequate support. Under subsequent repeated high contact-stress sliding actions of the coupling counterpart, mass cracks are generated and propagated in the over-protruded Mo2Ni3Si grains, leading to some fragmentation, brittle fracturing of the hard and wear resistant Mo2Ni3Si grains, as shown on the worn surface (Fig. 11(d)) and in the subsurface (Fig. 12(b)). Therefore, complete dissolution of the interdendritic eutectic Mo2Ni3Si during aging treatment is harmful to the tribological properties of the laser clad Mo2Ni3Si/NiSi intermetallic coatings especially under high-load sliding wear service conditions. Consequently, the wear mass loss of the aged coatings is higher than the original laser clad Mo2Ni3Si/NiSi metal silicide coatings.
5. Conclusions (1) The Mo2Ni3Si/NiSi metal silicides coating has excellent high temperature phase stability. No phase transformation except the dissolution of the eutectic Mo2Ni3Si and the corresponding growth of the Mo2Ni3Si primary dendrite and no any elemental inter-diffusion between the coating and the substrate were detected after aging the coating at 800 C for 50 h. The laser clad Mo2Ni3Si/NiSi metal silicide coating has a dual-phase structure with equiaxed Mo2Ni3Si primary grains distributed in the NiSi single-phase matrix after aging at 800 C for 50 h. (2) Both the as-laser clad and aged Mo2Ni3Si/NiSi metal silicide coatings have high hardness and excellent wear resistance under high-temperature sliding wear test conditions. The hardness and wear resistance of the aged coatings are slightly decreased because of the complete dissolution of interdendritic Mo2Ni3Si/NiSi eutectic and the spheroidization and coarsening of the Mo2Ni3Si primary dendrite.
Acknowledgements The research was supported by National High-Tech R&D Program (Contract Nos. 2002AA331030 and 2003AA305750), National Natural Science Foundation of China (Grant No. 50071004), Beijing Municipal Natural Science Foundation (Grant No. 2022012), Science Funds Office of AVIC (No. 02H510011), Trans-Century Training Program for Outstanding Talents of Ministry of Education of China and Basic Scientific Research Program of the Commission of Science, Technology and Industries for National Defense of China. The authors are grateful to Mr. Lingyun Zhang and Rongli
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Yu for their assistance during laser cladding and metallographic experiments.
References [1] Shah DM, Berczik D, Anton DL, Hecht R. Mater Sci Eng A 1992;A155:45. [2] Petrovic JJ, Vasudevan AK. Mater Sci Eng A 1999;A261:1. [3] Vasudevan AK. Mater Sci Eng A 1992;A155:1. [4] Maloney MJ, Hecht RJ. Mater Sci Eng A 1992;A155:19. [5] Kircher TA, Courtright EL. Mater Sci Eng A 1992;A155:67. [6] Wang HM, Wang CM, Cai LX. Surf Coat Technol 2003;168:202. [7] Duan G, Wang HM. Scripta Mater 2002;46:107. [8] Wang HM, Duan G. Mater Sci Eng A 2002;A336:117. [9] Sharif AA, Misra A, Petrovic JJ, Mitchell TE. Intermetallics 2001;9:869. [10] Kajuch J, Rigney JD, Lewandowski JJ. Mater Sci Eng A 1991;A155:59.
[11] Liu CT, Stringer J, Mundy JN, Horton LL, Angelini P. Intermetallics 1997;5:579. [12] Gil I, Munoz-Morris MA, Morris DG. Intermetallics 2001;9: 373. [13] Wang HM, Luan DY, Cai LX. Metall Mater Trans 2003;34A:2005. [14] Wang HM, Cao F, Cai LX, Tang HB, Yu RL, Zhang LY. Acta Mater 2003;51(20):6319. [15] Wang HM, Duan G, Zhang LY, Xi WJ. Proc SPIE 2002;4915:162. [16] Lu XD, Wang HM. J Alloys Comp 2003;359:287. [17] Lu XD, Wang HM. Appl Surf Sci 2003;214:190. [18] Lin KL, Chao WH, Wu CD. Surf Coat Technol 1997;89:279. [19] Kim HJ, Walter ME. Mater Sci Eng A 2003;A360:7. [20] Xiang ZD, Rose SR, Datta PK. Mater Sci Eng A 2003;A356: 181. [21] Li ZH, Liu XP, Zhao JX, Gao Y, Xu Z. Surf Coat Technol 2000;131:579. [22] Villars P, Prince A, Okamoto H. Handbook of ternary alloy phase diagrams New York: ASM International; 1994. p. 12540.
High-temperature phase stability and tribological properties of laser clad Mo2Ni3Si/NiSi metal silicide coatings X.D. Lu, H.M. Wang
*
Laboratory of Laser Materials Processing and Surface Engineering, School of Materials Science and Engineering, Beihang University (formerly Beijing University of Aeronautics and Astronautics), 37 Xue Yuan Road, Beijing 100083, PR China Received 11 February 2004; accepted 6 August 2004
Abstract Mo2Ni3Si/NiSi wear-resistant metal silicide composite coatings consisting of Mo2Ni3Si primary dendrite and interdendritic Mo2Ni3Si/NiSi eutectic were fabricated on substrate of an austenitic stainless steel AISI321 by laser cladding using Ni–Mo–Si elemental powder blends. The high-temperature structural stability of the coating was evaluated by aging at 800 C for 1–50 h. High-temperature sliding wear resistance of the as-laser clad and aged coatings was evaluated at 600 C. Results indicate that the Mo2Ni3Si/NiSi metal silicides coating has excellent high temperature phase stability. No phase transformation except the dissolution of the eutectic Mo2Ni3Si and the corresponding growth of the Mo2Ni3Si primary dendrite and no elemental diffusion from the coating into the substrate were detected after aging the coating at 800 C for 50 h. Aging of the coating at 800 C leads to gradual dissolution of the interdendritic eutectic Mo2Ni3Si and subsequent formation of a dual-phase structure with equiaxed Mo2Ni3Si primary grains distributed in the NiSi single-phase matrix. Because of the strong covalent-dominated atomic bonds and high volume fraction of the ternary metal silicide Mo2Ni3Si, both the original and the aged Mo2Ni3Si/NiSi coating has excellent wear resistance under pin-on-disc high-temperature sliding wear test conditions, although hardness of the aged coating is slightly lower than that of the as-clad coating. 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Laser cladding; Metal silicide; Phase stability; Wear resistance; Coating
1. Introduction Transition metal silicides such as MoSi2, Ni3Si, Cr3Si etc., have many attractive properties for structural applications in the aerospace and energy production industries [1–3]. Their low density, good mechanical and oxidation properties up to temperatures of 1200 C make these materials excellent candidates for high temperature structural applications [4,5]. It is also preliminarily [6–8] demonstrated that many of these * Corresponding author. Tel.: +86 10 82317102; fax: +86 10 82317105. E-mail addresses: [email protected], wanghuaming@buaa. edu.cn (H.M. Wang).
transition metal silicide-based materials exhibit excellent tribological properties and are likely a new class of abrasion and adhesion wear-resistant materials for tribological components working under high-temperature or aggressive service conditions. Serious room-temperature brittleness is the main obstacle preventing the materials from industrial applications [2,9–11]. Introduction of a second ductile phase and production of a multi-phase structure are recognized as two of the most efficient means to improve the toughness of intermetallic alloys [12–17]. Compared to the binary metal molybdenum silicides, such as MoSi2 and Mo5Si3, the ternary metal silicide Mo2Ni3Si with the hP12 Laves crystal structure is anticipated to possess better toughness while still keeping the inherent high hardness and strong atomic bonds
1359-6454/$30.00 2004 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2004.08.006
X.D. Lu, H.M. Wang / Acta Materialia 52 (2004) 5419–5426
and therefore, is expected to be an ideal wear resistant reinforcement. In our previous study, the Mo2Ni3Si/ NiSi multi-phase metal silicides coatings were designed and fabricated by the laser cladding process and exhibited excellent wear resistance under both room and high-temperature sliding wear test conditions [16,17]. The long-term high-temperature phase stability is of critical importance for a coating working in high-temperature environments especially for a laser clad coating with a rapidly solidified non-equilibrium microstructure because unfavorable phase transformation (e.g. formation of a harmful phase, etc.) and serious elemental diffusion from the coating into the substrate during long-term elevated-temperature exposure are two of the most important reasons leading to degradation of a high-temperature coating [18–21]. In this paper, the long-term high-temperature phase stability of a laser clad Mo2Ni3Si/NiSi metal silicide coating was studied by aging the coating at 800 C for 1–50 h. Aging behaviors and the effect of age-treatment on wear resistance of the coating were investigated as functions of aging time under pin-on-disc high-temperature sliding wear test conditions.
2. Experimental procedures A solution-treated commercial austenitic stainless steel AISI321 was selected as the substrate material. Commercial pure elemental powder blends (particle size 70–140 lm) in nominal chemical composition (wt%) of 35Ni–39Mo–26Si was utilized as the laser cladding precursor materials. The powders are preplaced on surface of the austenitic stainless steel specimens, 50 mm · 20 mm · 10 mm in size, prior to laser cladding with a powder-bed thickness of approximately 1 mm. Laser cladding was conducted on a 5 kW transverse-flow continuous-wave CO2 laser materials processing system equipped with a 4-axis computer numerical controlled (CNC) laser materials processing machine under ambient atmospheric environment. The optimized laser cladding parameters are as follows: laser power 1.8 kW, laser beam diameter 3 mm and beam traverse speed 200 mm/min. Six overlap tracks were clad side by side with a overlap ratio of approximately 25% in order to clad the whole surface of 50 mm · 20 mm on the substrate specimen. Average thickness of laser clad coatings is approximately 0.8 mm. Metallographic and wear test specimens were prepared by electric discharging machining followed by mechanical milling and grinding. Metallographic sections were prepared using standard mechanical polishing procedures and were etched in HF:HNO3:H2O water solution in volume ratio of 1:6:7. Microstructure was characterized using a Nephot optical microscope (OM) and a KYKY-2800 and a Hitachi S-3500N scan-
ning electron microscopes (SEM). Phase constituents were identified by X-ray diffraction (XRD) using a Rigaku D/max 2200 X-ray diffractometer with Cu Ka radiation at a scanning rate of 5/min. Chemical compositions of the phases were analyzed by a Oxford Inca energy dispersive spectroscope (EDS). Hardness profiles along the depth direction of the laser clad coatings were measured using a MH-6 semi-automatic Vickers microhardness tester with a test load of 1.96 N and a dwell time of 15 s. The eutectic and liquidus temperatures were measured using a Netzsch Sta 449C differential scanning calorimetry (DSC) with a heating rate of 20 C/min and an Ar flow rate of 20 ml/min. Results of DSC (as shown in Fig. 1) indicated that the eutectic temperature of the coating is 815 C (dH = 9.076 J/g) and the liquidus is 1110 C (dH = 33.47 J/g). The aging temperature was selected at 800 C which is slightly lower than the eutectic temperature of the coating. High-temperature phase stability of the coating was evaluated by aging the coating at 800 C for 1–50 h. High-temperature wear resistance of the original (as-laser clad) and aged coatings was evaluated on a HTW-II type pin-on-disc high-temperature sliding wear tester (Fig. 2) at 600 C, where two laser clad specimens, 6 mm · 6 mm · 6 mm in size, slide under a total test load of 98 N on a rotating disc made of a solid-solution strengthened nickel-base superalloy Nimonic 75 (500 HV at room-temperature). The relative sliding speed is 0.066 m/s and the total wear sliding distance is 238 m. The solution-treated austenitic stainless steel AISI321 was selected as the reference material for all the wear tests. Wear mass loss was measured using a Sartorius BS 110 precision electronic balance in accuracy of 0.1 mg. Wear mechanisms of the laser clad Mo2Ni3Si/NiSi metal silicide coating were discussed based on SEM observations of the worn surface morphologies and subsurface microstructure.
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Heat Flow (W/g)
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1.0
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0.0 400
600
800
1000
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Temperature (˚˚C) Fig. 1. DSC curve of the laser clad Mo2Ni3Si/NiSi metal silicide composite coating.
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Fig. 2. Schematic illustration of the high-temperature sliding wear tester [13].
3. Results The original laser clad Mo–Ni–Si intermetallic coatings have a fine and uniform dendritic structure and are metallurgically bonded to the substrate, as shown in Figs. 3 and 4. Results of XRD (as indicated in Fig. 5(a)) and EDS analysis indicate that the primary dendrites are the Mo2Ni3Si ternary metal silicide which has the hP12 topologically closed packed (TCP) Laves crystal structure [22] and the interdendritic lamellar eutectic structure is the Mo2Ni3Si/NiSi, as shown in Fig. 4(a). Volume fraction of the Mo2Ni3Si primary dendrite is approximately 45%. As shown clearly in Fig. 5(b), aging the coating at 800 C for 50 h produces no any changes on phase constituents of the coating, indicating that the laser clad Mo2Ni3Si/NiSi metal silicide coatings have excellent high-temperature phase stability. However, hardness
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of the aged coating was slightly decreased by a magnitude of approximately HV100, as shown in Fig. 6. Results of line-scan EDS analysis indicate that the no any elemental inter-diffusions between the coating and the substrate were detected after aging treatment at 800 C for 50 h as shown in Fig. 7. As indicated in Fig. 8 and more clearly in Fig. 9, drastic changes of microstructure during ageing process occurred in the first 10 h. The relative volume fraction of interdendritic eutectic decreases drastically with the increasing aging time (as indicated in Fig. 8) and rapid dissolution and coarsening of the interdendritic fine lamellar Mo2Ni3Si/NiSi eutectic structure and notable breaking or granulation of the Mo2Ni3Si primary dendrites occurred (as shown in Fig. 9(a) and (b)) during the first approximately 10 h. During the subsequent aging process, the microstructure changes are quite mild with the increasing aging time. The residual interdendritic eutectic thick plates are completely dissolved and the Mo2Ni3Si primary dendrites are further granulated during the subsequent aging process, leading to the formation of a
Fig. 3. OM micrograph showing the overview transverse cross-section of the laser clad Mo2Ni3Si/NiSi metal silicide composite coating.
Fig. 4. SEM micrographs showing microstructure of the laser clad Mo2Ni3Si/NiSi metal silicide composite coating, (a) in the central coating and (b) in the coating/substrate transition zone.
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5000
0 20
40
60
80
0 20
40
60
A(312)
1000
A(220)
A(110) A(103)
2000
A(206)
B(214)(124)
A--Mo2Ni3Si B--NiSi
B(115)
3000
100
2theta (deg)
(a)
4000
B(400) B(303)
500
Intensity (cps)
1000
A(206)
1500
A(200)
A(313)
2000
B(410) A(220)
2500
B(122) B(400)
3000
A--Mo2Ni3Si B--NiSi B(311)(131)
A(110) A(200) A(103) A(201)B(115) B(213)(300) B(214)(124)
Intensity (cps)
3500
80
100
2theta (deg)
(b)
Microhardness (Hv)
1000 800 600 as-clad treatment aged treatment
400
Laser clad coating Substrate 200 0
100
200
300
400
500
600
700
Distance from coating surface (µm) Fig. 6. Hardness profiles of (a) the original and (b) the aged (800 C/ 50 h) Mo2Ni3Si/NiSi coatings.
Relative volume fraction of eutectics (%)
Fig. 5. XRD patterns of (a) the original and (b) the aged (800 C/50 h) Mo2Ni3Si/NiSi coatings.
100 80 60 40 20 0 0
10
20
30
40
50
Ageing time (h) Fig. 8. Relative volume fraction of the interdendritic eutectic as a function of aging time.
Fig. 7. EDS line-scan analysis showing the major elements distributions of (a) the as-laser clad and (b) aged Mo2Ni3Si/NiSi coating at 800 C for 50 h.
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Fig. 9. SEM micrographs showing the typical microstructure of the Mo2Ni3Si/NiSi coating after aging treatment at 800 C for (a) 1 h, (b) 10 h, (c) 30 h and (d) 50 h.
Wear mass loss (mg)
30 25
as-clad coating aged coating AISI321
25.6
20 15 10 5
4.5
5.8
0 Fig. 10. Wear mass loss of the as-laser clad and aged (800 C/50 h) Mo2Ni3Si/NiSi metal silicide coatings in comparison to the reference material AISI321 under sliding wear test conditions at 600 C with a test load of 98N and a sliding speed of 0.066 m/s.
dual-phase intermetallic structure consisting of coarse granular Mo2Ni3Si grains evenly distributed in the NiSi matrix, as shown in Fig. 9(d). Volume fraction of the granular Mo2Ni3Si phase in the aged Mo2Ni3Si/NiSi coating measured by contrast-based quantative metallographic method is approximately 62%. As indicated in Fig. 10, aging the laser clad Mo2Ni3Si/NiSi intermetallic coating at 800 C for 50 h leads to
no significant decrease of the wear resistance. Because of the complete dissolution of interdendritic eutectic structure and the granulation and coarsening of the Mo2Ni3Si primary dendrites, which leads to slight hardness reduction of aged coating, wear mass loss of the aged coating is slightly increased. In comparison to the reference material of AISI321, however, both the as-laser clad and the aged Mo2Ni3Si/NiSi metal silicide coatings have excellent tribological properties under sliding wear test conditions at 600 C.
4. Discussion Because both phase constituent of the laser clad Mo2Ni3Si/NiSi intermetallic coating i.e., the ternary metal silicide Mo2Ni3Si and binary metal silicide NiSi, are intermetallic compounds with strong covalentdominant atomic bonds and high thermodynamic stability, no any phase transformation, phase decomposition and elemental inter-diffusion between the coating and the substrate occurred during the longterm high-temperature aging process at so high a temperature just only approximately 15 C below its eutectic temperature. This illustrated that the Mo2Ni3Si/NiSi metal silicide coatings have excellent high-temperature structural stability, which is helpful for the
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coating to be applied as high-temperature tribological coatings at service temperatures near to their eutectic temperatures. However, the selected aging temperature of 800 C is so high which is only approximately 15 C below its eutectic temperature, significant atomic diffusion is able to occure and the kinetic condition is favorable for dissolution of the fine lamellar eutectic Mo2Ni3Si phase and breaking and granulation and coarsening of the Mo2Ni3Si primary dendrites, leading to the formation of a dual-phase intermetallic structure consisting of the coarse granular Mo2Ni3Si grains and the NiSi single-phase matrix, as indicated in Fig. 9(d). The dissolution of the fine lamellar eutectic Mo2Ni3Si and the breaking, granulation, coarsening of the Mo2Ni3Si primary phase are the natural spontaneous Ostwald ripening process at high-temperature exposure conditions leading to reduction of the interfacial energy of the coating by greatly reducing the Mo2Ni3Si/NiSi interface areas. Under high-temperature sliding wear test conditions coupling with the nickel-base superalloy rotating disc counterpart, the sliding surface of the austenitic stainless steel is easily plastically deformed, ploughed or transferred, leading to detachment of vary large chips and adhesive wear debris. The surface of the austenitic stainless steel suffers severe abrasive and adhesive wear [17]. On the contrary, the worn surface of the Mo2Ni3Si/NiSi
metal silicide coatings is extremely smooth, without any cutting or plowing grooves and plastic deformation features observable, as shown in Figs. 11(a) and 12(a). Because of the high hardness and its abnormal hardness-temperature relationship, it is difficult for the Mo2Ni3Si to be plastically deformed or ploughed during the high-temperature sliding wear process, imparting the as-laser clad coating with excellent abrasive wear resistance. Moreover, the unique atomic bonding characteristics of Mo2Ni3Si with strong covalent bonds provide Mo2Ni3Si with excellent resistance to metallic adhesion when coating with a sliding counterpart. Meanwhile, the interdendritic Mo2Ni3Si/NiSi eutectics played a positive role in resisting wear attacks via the mechanism of providing the wear-resistant Mo2Ni3Si primary dendrite with powerful support. Moreover, it is interesting that significant dissolution of interdendritic Mo2Ni3Si/NiSi eutectic structure also occurred during the high-temperature sliding wear process at 600 C for the as-laser clad Mo2Ni3Si/NiSi coatings, as show in Fig. 11(b) and more clearly Fig. 12(a). Worn surface morphologies of the as-laser clad (as indicated in Fig. 11(a)) and the aged (see Fig. 11(c)) Mo2Ni3Si/NiSi intermetallic coatings at low magnifications are very similar and are very resemble to a polish-and-etched metallographic section, on which there are nearly no any cutting or plowing grooves and no
Fig. 11. SEM micrographs showing worn surface morphologies of: (a), (b) the as-laser clad and (c), (d) the aged (800 C/50 h) Mo2Ni3Si/NiSi metal silicide coatings after sliding wear test at 600 C for 1 h.
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Fig. 12. SEM micrographs showing the subsurface microstructure of (a) the as-laser clad and (b) the aged (800 C/50 h) Mo2Ni3Si/NiSi metal silicide coatings after sliding wear test at 600 C for 1 h.
any wear features in terms of metallic adhesion and material-transfer observable. This illustrates that both the as-laser clad and the aged coatings have excellent capability to resist abrasive and adhesive wear attacks during high temperature metallic sliding wear process because microstructural phase constituents of both the original and the aged coatings are the same, i.e., the Mo2Ni3Si and NiSi. However, close examinations of the worn surface under higher magnifications (as indicated in Fig. 11(b) and (d)) indicated that the wear behaviors and wear mechanisms are with some notable difference between the as-laser clad and the aged coatings. For the as-laser clad coating, the Mo2Ni3Si primary dendrites are firmly hold by the matrix (i.e., the Mo2Ni3Si/NiSi eutectic) and no any microcracks or fragmentation were observed on the slightly protruded Mo2Ni3Si dendrites on the worn surface, as indicated in Fig. 11(b) and in the subsurface, as shown in Fig. 12(a). Whereas the coarse granular or spherical Mo2Ni3Si grains of the aged coatings with a dual-phase microstructure are over-protruded out of the worn surface because the relatively weaker and softer NiSi matrix was preferentially worn out, making the hard and wear-resistant Mo2Ni3Si grains loose adequate support. Under subsequent repeated high contact-stress sliding actions of the coupling counterpart, mass cracks are generated and propagated in the over-protruded Mo2Ni3Si grains, leading to some fragmentation, brittle fracturing of the hard and wear resistant Mo2Ni3Si grains, as shown on the worn surface (Fig. 11(d)) and in the subsurface (Fig. 12(b)). Therefore, complete dissolution of the interdendritic eutectic Mo2Ni3Si during aging treatment is harmful to the tribological properties of the laser clad Mo2Ni3Si/NiSi intermetallic coatings especially under high-load sliding wear service conditions. Consequently, the wear mass loss of the aged coatings is higher than the original laser clad Mo2Ni3Si/NiSi metal silicide coatings.
5. Conclusions (1) The Mo2Ni3Si/NiSi metal silicides coating has excellent high temperature phase stability. No phase transformation except the dissolution of the eutectic Mo2Ni3Si and the corresponding growth of the Mo2Ni3Si primary dendrite and no any elemental inter-diffusion between the coating and the substrate were detected after aging the coating at 800 C for 50 h. The laser clad Mo2Ni3Si/NiSi metal silicide coating has a dual-phase structure with equiaxed Mo2Ni3Si primary grains distributed in the NiSi single-phase matrix after aging at 800 C for 50 h. (2) Both the as-laser clad and aged Mo2Ni3Si/NiSi metal silicide coatings have high hardness and excellent wear resistance under high-temperature sliding wear test conditions. The hardness and wear resistance of the aged coatings are slightly decreased because of the complete dissolution of interdendritic Mo2Ni3Si/NiSi eutectic and the spheroidization and coarsening of the Mo2Ni3Si primary dendrite.
Acknowledgements The research was supported by National High-Tech R&D Program (Contract Nos. 2002AA331030 and 2003AA305750), National Natural Science Foundation of China (Grant No. 50071004), Beijing Municipal Natural Science Foundation (Grant No. 2022012), Science Funds Office of AVIC (No. 02H510011), Trans-Century Training Program for Outstanding Talents of Ministry of Education of China and Basic Scientific Research Program of the Commission of Science, Technology and Industries for National Defense of China. The authors are grateful to Mr. Lingyun Zhang and Rongli
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Yu for their assistance during laser cladding and metallographic experiments.
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