- Email: [email protected]
(Cr5Si3–CrSi) metal silicide alloy
Materials Science and Engineering A 452–453 (2007) 746–750
High-temperature wear behaviors of a laser melted Cuss/(Cr5Si3–CrSi) metal silicide alloy Y.X. Yin a,b , H.M. Wang a,∗ a
Laboratory of Laser Materials Processing and Manufacturing, School of Materials Science and Engineering, Beihang University, 37 Xueyuan Road, Beijing 100083, China b Division of Mineral Resources, Metallurgy and Materials, General Research Institute for Nonferrous Metals, 2 Xinjiekouwai Road, Beijing 100088, China Received 12 October 2006; received in revised form 25 October 2006; accepted 28 October 2006
Abstract Wear-resistant Cu-based solid solution-toughened Cr5 Si3 –CrSi metal silicide alloy is fabricated by the laser melting process using Cr–Cu–Si elemental powder blends. High-temperature wear resistance of the laser melted Cuss /(Cr5 Si3 –CrSi) metal silicide alloy is evaluated on a selfdesigned pin-on-disc high-temperature sliding wear tester in open air. The alloy has excellent wear resistance under high-temperature sliding wear test conditions due to its unique microstructural characteristics and tribological behaviors of the phase constituents. With the increasing wear test temperature, the wear mass loss of the alloy decreases slightly. © 2006 Elsevier B.V. All rights reserved. Keywords: Transition metal silicides; Intermetallics; Wear; Laser processing
1. Introduction In metallurgical, chemical, petrochemical and marine industries, many mechanical moving components working under heavy load, high speed tribological environment, such as sliding bearing, mechanical seals in valves and pumps, are required to have excellent wear resistance, high-temperature mechanical properties, high hardness and good tribological compatibility. Traditional metal components and lubricants cannot withstand the high temperatures. Ceramics could be used as replacement materials because of their good high-temperature properties, such as thermal stability, low thermal conductivity, light weight, etc. However, their friction and wear properties are too poor to sustain sliding [1]. Recently, transition metal silicide alloys have attracted increasing attentions as promising alternative high-temperature structural materials due to their outstanding combination of high melting point, low density, excellent high-temperature oxidation and creep resistance, etc. [2–7]. However, it is also preliminarily demonstrated that many of these ordered transition metal silicides alloys, such as Ti5 Si3 , Cr5 Si3 ,
∗
Corresponding author. Tel.: +86 10 8231 7102; fax: +86 10 8231 7102. E-mail address: [email protected] (H.M. Wang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.143
W5 Si3 , Mo5 Si3 , etc., having the topologically closed packed (TCP) W5 Si3 type phase crystal structure, exhibit excellent tribological properties because of their inherent high hardness, covalent-dominated strong atomic bonds and are expected to be a new class of abrasive and adhesion wear-resistant materials for those tribological moving components working under high-temperature or aggressive service environments [8–11]. Unfortunately, serious room-temperature brittleness and poor processing capability are the major drawbacks preventing them from industrial application as either wear-resistant bulk or coating materials. Addition of one or more alloying elements especially by introducing a ductile second metallic phase is one of the most effective ways to improve the toughness of the silicide alloys. Moreover, in this way, it is very effective to control the tribological properties of the metal silicides alloys by adjusting the balance between hardness and toughness, which the wear behavior of the metal silicide alloys is directly proportion to [12–16]. The copper-based solid solution (hereafter referred as Cuss ), well famous for its high thermal, electrical conductivity, low coefficient of friction and excellent tribological compatibility and ductility, is therefore expected to be an ideal phase to toughen the wear-resistant metal silicide alloys [17–20]. In our previous study, a group of Cuss -toughened Cr5 Si3 –CrSi metal silicide alloys are designed and fabricated, and the room temperature dry
Y.X. Yin, H.M. Wang / Materials Science and Engineering A 452–453 (2007) 746–750
747
sliding wear behaviors of the alloys are systematically investigated. Results indicated that the Cuss /(Cr5 Si3 –CrSi) metal silicide alloys exhibit reasonable toughness, excellent wear resistance and low coefficient of friction under room temperature dry sliding wear test conditions [21,22]. As a wear-resistant material working under high-temperature or aggressive service environments, a basic understanding of its high-temperature wear behavior is necessary from both academic and application points of view. In this paper, the high-temperature wear behaviors of the Cuss -toughened Cr5 Si3 –CrSi metal silicide alloy is evaluated on a self-designed pin-on-disc high-temperature sliding wear tester. 2. Experimental procedures Commercially pure chromium, silicon and copper elemental powders with an average particle size ranging from 45 to 125 m were selected as the precursor materials. The chemical composition (at.%) of the designed experimental Cr–Si–Cu alloy is 11% Cu–48% Cr–41% Si. The Cr–Cu–Si metal silicide alloy was fabricated by the melting process in a newly patented water cooled copper-mold laser melting furnace, using a high power laser beam delivered from an 8 kW continuous wave CO2 laser. Details of the laser melting procedures were reported elsewhere [23,24]. The laser melting parameters were as follows: laser beam output power, 3.5 kW; beam diameter, 14 mm; laser melting time, 15–25 s. Shot-shaped ingots with an ingot diameter of approximately 15 mm and an ingot height of approximately 18 mm are produced. Metallographic samples were prepared using conventional mechanical polishing procedures and were etched in HNO3 –20 vol.% CH3 COOH–20 vol.% H2 O. The microstructure analysis was performed in an Olypus BX51M optical microscope (OM) equipped with a Sisc IAS6.0 image analyzing software and KYKY-2800B scanning electron microscopes (SEM). Powder X-ray diffraction (XRD) was conducted using the Rigaku D/max 2200 pc automatic X-ray diffractometer with Cu target K␣ radiation. Chemical compositions of the phase constituents were analyzed by energy dispersive X-ray analysis (EDS) using Noran Ventage DSI spectrometer. Volume fractions of the Cr5 Si3 , CrSi dendrite and the Cuss of the Cuss /(Cr5 Si3 –CrSi) alloys were measured by quantitative metallographic analysis method on high-contrast optical photographs (200× magnifications) using a commercial contrast-based image analyzing software. Hardness was measured using an Everone MH-6 semi-automatic Vickers micro-hardness tester with a test load of 500 g and a load-dwell time of 15 s. High-temperature sliding wear tests were carried out on a self-designed pin-on-disc high-temperature sliding wear tester in open air. As illustrated in Fig. 1, two square pin-like specimens, 6 mm × 6 mm × 6 mm in size, slide on the top surface of a rotating disc made of the hot-rolled and solid-solutionstrengthened nickel-based superalloy GH5K with a chemical composition (wt.%) of 29% Cr–1.1% Cu–0.2% Fe–0.1% Si–0.08%–Ce and Ni as the balance. The test parameters were as follows: load 98, test temperatures of 400, 500 and 600 ◦ C, sliding velocity of 0.1 m/s and a total sliding distance of 180 m. A hot-rolled and solution-treated austenitic stainless steel AISI321
Fig. 1. Schematic illustration of high-temperature pin-on-disc sliding wear tester.
was selected as the reference material. The wear mass loss was measured using an electronic balance (Sartorius BS110) with an accuracy of 0.1 mg. Worn surfaces and subsurface during the wear testing process were examined by SEM to assist in the analysis of wear mechanisms. 3. Results Results of X-ray diffraction analysis, as shown in Fig. 2, and EDS indicate that the main constitution phases of the alloy are Cr5 Si3 , CrSi and Cuss [21,22]. It is well known that the metal silicide Cr5 Si3 has a high melting point (1680 ◦ C) and the large negative free energy of formation in the Cr–Si–Cu ternary alloy system. As a result, Cr5 Si3 preferably precipitates from the laser melt pool as the primary phase during the solidification process. Following the precipitation of Cr5 Si3 from the melt pool, the
Fig. 2. XRD diffraction patterns of the laser melted Cuss /(Cr5 Si3 –CrSi) metal silicide alloy.
748
Y.X. Yin, H.M. Wang / Materials Science and Engineering A 452–453 (2007) 746–750
Fig. 3. SEM micrograph showing the typical microstructure of the Cuss /(Cr5 Si3 –CrSi) alloy.
peritectic reaction L + Cr5 Si3 → CrSi occurs. However, the peritectic reaction cannot proceed completely and the final peritectic products are Cr5 Si3 and CrSi. Accompany the precipitation of the phase Cr5 Si3 and CrSi, the residual liquid is highly enriched in Cu, and finally the copper-based solid solution is formed in the interdendritic regions, as shown in Fig. 3. EDS result
Fig. 4. Variation of wear mass loss vs. temperature for the stainless steel AISI321 and the Cuss /(Cr5 Si3 –CrSi) alloy under the high-temperature wear conditions.
(73.42% Cu–25.47% Si–1.11% Cr, at.%) indicates that the Cuss is highly supersaturated with Si and Cr. Supersaturated Cr and Si in Cuss can further improve the toughness of Cuss , thus, the Cuss can make a more effective contribution to firmly support the Cr5 Si3 and CrSi. Volume fraction of the Cr5 Si3 , CrSi and Cuss in the laser melted Cuss -toughened Cr5 Si3 –CrSi alloy is
Fig. 5. SEM micrograph showing the worn surface morphology of (a) the austenitic stainless steel AISI 321, (b and c) the Cuss /(Cr5 Si3 –CrSi) alloy and (d) the worn subsurface morphologies of the Cuss /(Cr5 Si3 –CrSi) alloy after high-temperature sliding wear test at 400 ◦ C.
Y.X. Yin, H.M. Wang / Materials Science and Engineering A 452–453 (2007) 746–750
approximately 60%, 22% and 18%, respectively. Because of the high volume fraction and uniform distribution of the hard metal silicides Cr5 Si3 and CrSi, the alloy exhibits a high hardness of approximately HV1000. Results of high-temperature sliding wear tests, as shown in Fig. 4, indicate that the laser melted Cuss /(Cr5 Si3 –CrSi) metal silicide alloy has excellent high-temperature wear resistance. The wear mass loss of the alloy is much lower than that of the reference material austenitic stainless steel AISI 321. Meanwhile, it is very interesting to note that wear mass loss of the alloy decreases slowly while that of the AISI 321 increases drastically with the increasing wear test temperature. SEM micrographs of worn surface of the laser melted Cuss /(Cr5 Si3 –CrSi) metal silicide alloy and the reference material AISI 321 are shown in Fig. 5. The worn surface of the austenitic stainless steel AISI 321 is very rough with deep plowing grooves, severe metallic adhesion and plastic deformation features, as indicated in Fig. 5a. On the contrary, the worn surface of the Cuss /(Cr5 Si3 –CrSi) alloy is so smooth that neither abrasion nor adhesion characteristics are observed, as illustrated in Fig. 5b. Only a few wear debris powder aggregations can be observed on the worn surface, as shown in Fig. 5c. EDS analysis indicates that chemical compositions (at.%) of
Fig. 6. SEM micrographs showing the worn surface morphologies for the Cuss /(Cr5 Si3 –CrSi) alloy after high-temperature sliding wear test at (a) 500 ◦ C and (b) 600 ◦ C.
749
these powder aggregations is approximately 54.78% Ni–27.79% Cr–10.58% O–3.22% Fe–1.71% Si–1.08%Cu–0.83% Al. The powder aggregations are highly enriched in Ni, Cr and O, containing a little Fe, Si, Cu and Al and thus mainly transferred from the couple materials superalloy GH5K. The wear debris powders are actually complex oxides. These oxides can make a positive effect on the sliding wear performance of the friction couple by acting as the high-temperature self-lubricant. To assist the analysis of the wear mechanisms, subsurface microstructure of the Cuss /(Cr5 Si3 –CrSi) alloy is investigated. As shown in Fig. 5d, no noticeable evidence of plastic deformation or selective wear occurs. Similar characteristics are also observed on the worn surface of the Cuss /(Cr5 Si3 –CrSi) alloy after high-temperature sliding wear test at 500 and 600 ◦ C, as indicated in Fig. 6. The worn surfaces of the alloy are both very smooth and similar to a polished and etched metallographic section on which the primary metal silicide phases are clearly visible. 4. Discussion The excellent wear resistance of the Cuss /(Cr5 Si3 –CrSi) metal silicide alloy under high-temperature sliding wear test conditions is attributed to its unique microstructural characteristics and tribological behaviors of the phase constituents. First, there is high volume fraction of high hardness Cr5 Si3 and CrSi phases dispersed uniformly in the alloy. When sliding with the counterpart superalloy GH5K, the contacting asperities of the mating counterpart GH5K can hardly press into the alloy to generate micro-cutting and can only slightly scratch the alloy through the mechanism of “soft abrasion”. These provide the alloy with good high-temperature abrasive wear resistance. Simultaneously, because the volume fraction of the Cr5 Si3 and CrSi is high, it can also prevent the softer toughened phase Cuss from wear by the sliding counterpart asperities. Second, as the metal silicide Cr5 Si3 and CrSi are of unique covalent-dominant strong atomic bonds, it is very hard for them to generate plastic deformation, adhesion and materials transfer as well as coldweld to the metallic asperities on the actual contacting surface of the slide-coupling counterpart superalloy GH5K. The sliding counterpart GH5K can only scratch the alloy very slowly through the above-mentioned “soft abrasion” method, thus the alloy has excellent high-temperature adhesive wear resistance. Third, the highly ductile Cuss has a positive effect on improving the toughness of the Cr5 Si3 and CrSi, and provides powerful support for the Cr5 Si3 and CrSi during the wear process. Meanwhile, it can also effectively release the stress concentration of the alloy under the high-temperature conditions. Finally, it is well known that oxides can effectively reduce metal wear loss rates [25]. So, the oxide debris particles, which are mostly transferred from the counterpart superalloy, are helpful to improve the wear performance of the friction couple by acting as the high-temperature self-lubricant, and further decrease the alloy’s wear loss. In addition, the hardness and the strength of the counterpart GH5K are all decreasing while the Cuss /(Cr5 Si3 –CrSi) alloy still keeps high hardness under high-temperature environments, the asperities of the counterpart superalloy can even harder scratch the surface of
750
Y.X. Yin, H.M. Wang / Materials Science and Engineering A 452–453 (2007) 746–750
the alloy. Therefore, wear mass loss of the Cuss /(Cr5 Si3 –CrSi) alloy decreases slightly with the increasing test temperature. 5. Conclusions The laser melted Cuss /(Cr5 Si3 –CrSi) metal silicide alloy exhibits excellent wear properties under high-temperature sliding wear conditions due to the high hardness and covalentdominant strong atomic bonds of the chromium silicide Cr5 Si3 and CrSi, and the excellent ductility of the toughening Cu-base solid solution. The wear mass loss of the alloy decreases slightly with the increasing wear test temperature. The wear mechanism is mostly dominated by the “soft abrasion”. Acknowledgements The work was supported by National Natural Science Foundation of China (Grant no. 50331010). References [1] K.J. Wahl, L.E. Seitzman, R.N. Bolster, I.L. Singer, M.B. Peterson, Surf. Coat. Technol. 89 (1997) 245–251. [2] S.V. Meschel, O.J. Kleppa, J. Alloys Compd. 267 (1998) 128–135. [3] J.H. Ma, Y.L. Gu, L. Shi, L.Y. Chen, Z.H. Yang, Y.T. Qian, J. Alloys Compd. 375 (2004) 249–252.
[4] A. Tomasi, R. Ceccato, M. Nazmy, S. Gialanella, Mater. Sci. Eng. A 239–240 (1997) 877–881. [5] E. Strom, S. Eriksson, H. Rundlof, J. Zhang, Acta Mater. 53 (2005) 357–365. [6] P. Jehanno, M. Heilmaier, H. Kestler, Intermetallics 12 (2004) 1005– 1009. [7] H. Bei, E.P. George, G.M. Pharr, Scripta Mater. 51 (2004) 875–879. [8] J.R. Jokisaari, S. Bhaduri, S.B. Bhaduri, Mater. Sci. Eng. A 323 (2002) 478–483. [9] H.M. Wang, Y.F. Liu, Mater. Sci. Eng. A 338 (2002) 126–132. [10] X.B. Liu, H.M. Wang, Surf. Coat. Technol. 200 (2006) 4462–4470. [11] L.X. Cai, H.M. Wang, Appl. Surf. Sci. 235 (2004) 501–506. [12] W.Y. Kim, H. Tanaka, A. Kasama, S. Hanada, Intermetallics 9 (2001) 827–834. [13] H. Bei, E.P. George, E.A. Kenik, G.M. Pharr, Acta Mater. 51 (2003) 6241–6252. [14] J.J. Petrovic, Intermetallics 8 (2000) 1175–1182. [15] Y. Liu, H.M. Wang, Mater. Sci. Eng. A 396 (2005) 240–250. [16] X.D. Lu, H.M. Wang, Surf. Coat. Technol. 200 (2005) 2380–2385. [17] N. Zarrinfar, A.R. Kennedy, P.H. Shipway, Scripta Mater. 50 (2004) 949–952. [18] Z.Y. Shi, D.Q. Wang, Appl. Surf. Sci. 167 (2000) 107–112. [19] S.C. Tjong, K.C. Lau, Mater. Sci. Eng. A 282 (2000) 183–186. [20] J.P. Tu, W. Rong, S.Y. Guo, Y.Z. Yang, Wear 255 (2003) 832–835. [21] Y.X. Yin, H.M. Wang, J. Mater. Res. 20 (2005) 1122–1130. [22] Y.X. Yin, H.M. Wang, J. Alloys Compd. 420 (2006) 218–224. [23] H.M. Wang, D.Y. Luan, L.Y. Zhang, Scripta Mater. 48 (2003) 1179–1184. [24] H.M. Wang, D.Y. Luan, L.Y. Cai, Metall. Mater. Trans. A 34 (2003) 2005–2015. [25] F.H. Stott, Tribol. Int. 35 (2002) 489–495.
High-temperature wear behaviors of a laser melted Cuss/(Cr5Si3–CrSi) metal silicide alloy Y.X. Yin a,b , H.M. Wang a,∗ a
Laboratory of Laser Materials Processing and Manufacturing, School of Materials Science and Engineering, Beihang University, 37 Xueyuan Road, Beijing 100083, China b Division of Mineral Resources, Metallurgy and Materials, General Research Institute for Nonferrous Metals, 2 Xinjiekouwai Road, Beijing 100088, China Received 12 October 2006; received in revised form 25 October 2006; accepted 28 October 2006
Abstract Wear-resistant Cu-based solid solution-toughened Cr5 Si3 –CrSi metal silicide alloy is fabricated by the laser melting process using Cr–Cu–Si elemental powder blends. High-temperature wear resistance of the laser melted Cuss /(Cr5 Si3 –CrSi) metal silicide alloy is evaluated on a selfdesigned pin-on-disc high-temperature sliding wear tester in open air. The alloy has excellent wear resistance under high-temperature sliding wear test conditions due to its unique microstructural characteristics and tribological behaviors of the phase constituents. With the increasing wear test temperature, the wear mass loss of the alloy decreases slightly. © 2006 Elsevier B.V. All rights reserved. Keywords: Transition metal silicides; Intermetallics; Wear; Laser processing
1. Introduction In metallurgical, chemical, petrochemical and marine industries, many mechanical moving components working under heavy load, high speed tribological environment, such as sliding bearing, mechanical seals in valves and pumps, are required to have excellent wear resistance, high-temperature mechanical properties, high hardness and good tribological compatibility. Traditional metal components and lubricants cannot withstand the high temperatures. Ceramics could be used as replacement materials because of their good high-temperature properties, such as thermal stability, low thermal conductivity, light weight, etc. However, their friction and wear properties are too poor to sustain sliding [1]. Recently, transition metal silicide alloys have attracted increasing attentions as promising alternative high-temperature structural materials due to their outstanding combination of high melting point, low density, excellent high-temperature oxidation and creep resistance, etc. [2–7]. However, it is also preliminarily demonstrated that many of these ordered transition metal silicides alloys, such as Ti5 Si3 , Cr5 Si3 ,
∗
Corresponding author. Tel.: +86 10 8231 7102; fax: +86 10 8231 7102. E-mail address: [email protected] (H.M. Wang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.10.143
W5 Si3 , Mo5 Si3 , etc., having the topologically closed packed (TCP) W5 Si3 type phase crystal structure, exhibit excellent tribological properties because of their inherent high hardness, covalent-dominated strong atomic bonds and are expected to be a new class of abrasive and adhesion wear-resistant materials for those tribological moving components working under high-temperature or aggressive service environments [8–11]. Unfortunately, serious room-temperature brittleness and poor processing capability are the major drawbacks preventing them from industrial application as either wear-resistant bulk or coating materials. Addition of one or more alloying elements especially by introducing a ductile second metallic phase is one of the most effective ways to improve the toughness of the silicide alloys. Moreover, in this way, it is very effective to control the tribological properties of the metal silicides alloys by adjusting the balance between hardness and toughness, which the wear behavior of the metal silicide alloys is directly proportion to [12–16]. The copper-based solid solution (hereafter referred as Cuss ), well famous for its high thermal, electrical conductivity, low coefficient of friction and excellent tribological compatibility and ductility, is therefore expected to be an ideal phase to toughen the wear-resistant metal silicide alloys [17–20]. In our previous study, a group of Cuss -toughened Cr5 Si3 –CrSi metal silicide alloys are designed and fabricated, and the room temperature dry
Y.X. Yin, H.M. Wang / Materials Science and Engineering A 452–453 (2007) 746–750
747
sliding wear behaviors of the alloys are systematically investigated. Results indicated that the Cuss /(Cr5 Si3 –CrSi) metal silicide alloys exhibit reasonable toughness, excellent wear resistance and low coefficient of friction under room temperature dry sliding wear test conditions [21,22]. As a wear-resistant material working under high-temperature or aggressive service environments, a basic understanding of its high-temperature wear behavior is necessary from both academic and application points of view. In this paper, the high-temperature wear behaviors of the Cuss -toughened Cr5 Si3 –CrSi metal silicide alloy is evaluated on a self-designed pin-on-disc high-temperature sliding wear tester. 2. Experimental procedures Commercially pure chromium, silicon and copper elemental powders with an average particle size ranging from 45 to 125 m were selected as the precursor materials. The chemical composition (at.%) of the designed experimental Cr–Si–Cu alloy is 11% Cu–48% Cr–41% Si. The Cr–Cu–Si metal silicide alloy was fabricated by the melting process in a newly patented water cooled copper-mold laser melting furnace, using a high power laser beam delivered from an 8 kW continuous wave CO2 laser. Details of the laser melting procedures were reported elsewhere [23,24]. The laser melting parameters were as follows: laser beam output power, 3.5 kW; beam diameter, 14 mm; laser melting time, 15–25 s. Shot-shaped ingots with an ingot diameter of approximately 15 mm and an ingot height of approximately 18 mm are produced. Metallographic samples were prepared using conventional mechanical polishing procedures and were etched in HNO3 –20 vol.% CH3 COOH–20 vol.% H2 O. The microstructure analysis was performed in an Olypus BX51M optical microscope (OM) equipped with a Sisc IAS6.0 image analyzing software and KYKY-2800B scanning electron microscopes (SEM). Powder X-ray diffraction (XRD) was conducted using the Rigaku D/max 2200 pc automatic X-ray diffractometer with Cu target K␣ radiation. Chemical compositions of the phase constituents were analyzed by energy dispersive X-ray analysis (EDS) using Noran Ventage DSI spectrometer. Volume fractions of the Cr5 Si3 , CrSi dendrite and the Cuss of the Cuss /(Cr5 Si3 –CrSi) alloys were measured by quantitative metallographic analysis method on high-contrast optical photographs (200× magnifications) using a commercial contrast-based image analyzing software. Hardness was measured using an Everone MH-6 semi-automatic Vickers micro-hardness tester with a test load of 500 g and a load-dwell time of 15 s. High-temperature sliding wear tests were carried out on a self-designed pin-on-disc high-temperature sliding wear tester in open air. As illustrated in Fig. 1, two square pin-like specimens, 6 mm × 6 mm × 6 mm in size, slide on the top surface of a rotating disc made of the hot-rolled and solid-solutionstrengthened nickel-based superalloy GH5K with a chemical composition (wt.%) of 29% Cr–1.1% Cu–0.2% Fe–0.1% Si–0.08%–Ce and Ni as the balance. The test parameters were as follows: load 98, test temperatures of 400, 500 and 600 ◦ C, sliding velocity of 0.1 m/s and a total sliding distance of 180 m. A hot-rolled and solution-treated austenitic stainless steel AISI321
Fig. 1. Schematic illustration of high-temperature pin-on-disc sliding wear tester.
was selected as the reference material. The wear mass loss was measured using an electronic balance (Sartorius BS110) with an accuracy of 0.1 mg. Worn surfaces and subsurface during the wear testing process were examined by SEM to assist in the analysis of wear mechanisms. 3. Results Results of X-ray diffraction analysis, as shown in Fig. 2, and EDS indicate that the main constitution phases of the alloy are Cr5 Si3 , CrSi and Cuss [21,22]. It is well known that the metal silicide Cr5 Si3 has a high melting point (1680 ◦ C) and the large negative free energy of formation in the Cr–Si–Cu ternary alloy system. As a result, Cr5 Si3 preferably precipitates from the laser melt pool as the primary phase during the solidification process. Following the precipitation of Cr5 Si3 from the melt pool, the
Fig. 2. XRD diffraction patterns of the laser melted Cuss /(Cr5 Si3 –CrSi) metal silicide alloy.
748
Y.X. Yin, H.M. Wang / Materials Science and Engineering A 452–453 (2007) 746–750
Fig. 3. SEM micrograph showing the typical microstructure of the Cuss /(Cr5 Si3 –CrSi) alloy.
peritectic reaction L + Cr5 Si3 → CrSi occurs. However, the peritectic reaction cannot proceed completely and the final peritectic products are Cr5 Si3 and CrSi. Accompany the precipitation of the phase Cr5 Si3 and CrSi, the residual liquid is highly enriched in Cu, and finally the copper-based solid solution is formed in the interdendritic regions, as shown in Fig. 3. EDS result
Fig. 4. Variation of wear mass loss vs. temperature for the stainless steel AISI321 and the Cuss /(Cr5 Si3 –CrSi) alloy under the high-temperature wear conditions.
(73.42% Cu–25.47% Si–1.11% Cr, at.%) indicates that the Cuss is highly supersaturated with Si and Cr. Supersaturated Cr and Si in Cuss can further improve the toughness of Cuss , thus, the Cuss can make a more effective contribution to firmly support the Cr5 Si3 and CrSi. Volume fraction of the Cr5 Si3 , CrSi and Cuss in the laser melted Cuss -toughened Cr5 Si3 –CrSi alloy is
Fig. 5. SEM micrograph showing the worn surface morphology of (a) the austenitic stainless steel AISI 321, (b and c) the Cuss /(Cr5 Si3 –CrSi) alloy and (d) the worn subsurface morphologies of the Cuss /(Cr5 Si3 –CrSi) alloy after high-temperature sliding wear test at 400 ◦ C.
Y.X. Yin, H.M. Wang / Materials Science and Engineering A 452–453 (2007) 746–750
approximately 60%, 22% and 18%, respectively. Because of the high volume fraction and uniform distribution of the hard metal silicides Cr5 Si3 and CrSi, the alloy exhibits a high hardness of approximately HV1000. Results of high-temperature sliding wear tests, as shown in Fig. 4, indicate that the laser melted Cuss /(Cr5 Si3 –CrSi) metal silicide alloy has excellent high-temperature wear resistance. The wear mass loss of the alloy is much lower than that of the reference material austenitic stainless steel AISI 321. Meanwhile, it is very interesting to note that wear mass loss of the alloy decreases slowly while that of the AISI 321 increases drastically with the increasing wear test temperature. SEM micrographs of worn surface of the laser melted Cuss /(Cr5 Si3 –CrSi) metal silicide alloy and the reference material AISI 321 are shown in Fig. 5. The worn surface of the austenitic stainless steel AISI 321 is very rough with deep plowing grooves, severe metallic adhesion and plastic deformation features, as indicated in Fig. 5a. On the contrary, the worn surface of the Cuss /(Cr5 Si3 –CrSi) alloy is so smooth that neither abrasion nor adhesion characteristics are observed, as illustrated in Fig. 5b. Only a few wear debris powder aggregations can be observed on the worn surface, as shown in Fig. 5c. EDS analysis indicates that chemical compositions (at.%) of
Fig. 6. SEM micrographs showing the worn surface morphologies for the Cuss /(Cr5 Si3 –CrSi) alloy after high-temperature sliding wear test at (a) 500 ◦ C and (b) 600 ◦ C.
749
these powder aggregations is approximately 54.78% Ni–27.79% Cr–10.58% O–3.22% Fe–1.71% Si–1.08%Cu–0.83% Al. The powder aggregations are highly enriched in Ni, Cr and O, containing a little Fe, Si, Cu and Al and thus mainly transferred from the couple materials superalloy GH5K. The wear debris powders are actually complex oxides. These oxides can make a positive effect on the sliding wear performance of the friction couple by acting as the high-temperature self-lubricant. To assist the analysis of the wear mechanisms, subsurface microstructure of the Cuss /(Cr5 Si3 –CrSi) alloy is investigated. As shown in Fig. 5d, no noticeable evidence of plastic deformation or selective wear occurs. Similar characteristics are also observed on the worn surface of the Cuss /(Cr5 Si3 –CrSi) alloy after high-temperature sliding wear test at 500 and 600 ◦ C, as indicated in Fig. 6. The worn surfaces of the alloy are both very smooth and similar to a polished and etched metallographic section on which the primary metal silicide phases are clearly visible. 4. Discussion The excellent wear resistance of the Cuss /(Cr5 Si3 –CrSi) metal silicide alloy under high-temperature sliding wear test conditions is attributed to its unique microstructural characteristics and tribological behaviors of the phase constituents. First, there is high volume fraction of high hardness Cr5 Si3 and CrSi phases dispersed uniformly in the alloy. When sliding with the counterpart superalloy GH5K, the contacting asperities of the mating counterpart GH5K can hardly press into the alloy to generate micro-cutting and can only slightly scratch the alloy through the mechanism of “soft abrasion”. These provide the alloy with good high-temperature abrasive wear resistance. Simultaneously, because the volume fraction of the Cr5 Si3 and CrSi is high, it can also prevent the softer toughened phase Cuss from wear by the sliding counterpart asperities. Second, as the metal silicide Cr5 Si3 and CrSi are of unique covalent-dominant strong atomic bonds, it is very hard for them to generate plastic deformation, adhesion and materials transfer as well as coldweld to the metallic asperities on the actual contacting surface of the slide-coupling counterpart superalloy GH5K. The sliding counterpart GH5K can only scratch the alloy very slowly through the above-mentioned “soft abrasion” method, thus the alloy has excellent high-temperature adhesive wear resistance. Third, the highly ductile Cuss has a positive effect on improving the toughness of the Cr5 Si3 and CrSi, and provides powerful support for the Cr5 Si3 and CrSi during the wear process. Meanwhile, it can also effectively release the stress concentration of the alloy under the high-temperature conditions. Finally, it is well known that oxides can effectively reduce metal wear loss rates [25]. So, the oxide debris particles, which are mostly transferred from the counterpart superalloy, are helpful to improve the wear performance of the friction couple by acting as the high-temperature self-lubricant, and further decrease the alloy’s wear loss. In addition, the hardness and the strength of the counterpart GH5K are all decreasing while the Cuss /(Cr5 Si3 –CrSi) alloy still keeps high hardness under high-temperature environments, the asperities of the counterpart superalloy can even harder scratch the surface of
750
Y.X. Yin, H.M. Wang / Materials Science and Engineering A 452–453 (2007) 746–750
the alloy. Therefore, wear mass loss of the Cuss /(Cr5 Si3 –CrSi) alloy decreases slightly with the increasing test temperature. 5. Conclusions The laser melted Cuss /(Cr5 Si3 –CrSi) metal silicide alloy exhibits excellent wear properties under high-temperature sliding wear conditions due to the high hardness and covalentdominant strong atomic bonds of the chromium silicide Cr5 Si3 and CrSi, and the excellent ductility of the toughening Cu-base solid solution. The wear mass loss of the alloy decreases slightly with the increasing wear test temperature. The wear mechanism is mostly dominated by the “soft abrasion”. Acknowledgements The work was supported by National Natural Science Foundation of China (Grant no. 50331010). References [1] K.J. Wahl, L.E. Seitzman, R.N. Bolster, I.L. Singer, M.B. Peterson, Surf. Coat. Technol. 89 (1997) 245–251. [2] S.V. Meschel, O.J. Kleppa, J. Alloys Compd. 267 (1998) 128–135. [3] J.H. Ma, Y.L. Gu, L. Shi, L.Y. Chen, Z.H. Yang, Y.T. Qian, J. Alloys Compd. 375 (2004) 249–252.
[4] A. Tomasi, R. Ceccato, M. Nazmy, S. Gialanella, Mater. Sci. Eng. A 239–240 (1997) 877–881. [5] E. Strom, S. Eriksson, H. Rundlof, J. Zhang, Acta Mater. 53 (2005) 357–365. [6] P. Jehanno, M. Heilmaier, H. Kestler, Intermetallics 12 (2004) 1005– 1009. [7] H. Bei, E.P. George, G.M. Pharr, Scripta Mater. 51 (2004) 875–879. [8] J.R. Jokisaari, S. Bhaduri, S.B. Bhaduri, Mater. Sci. Eng. A 323 (2002) 478–483. [9] H.M. Wang, Y.F. Liu, Mater. Sci. Eng. A 338 (2002) 126–132. [10] X.B. Liu, H.M. Wang, Surf. Coat. Technol. 200 (2006) 4462–4470. [11] L.X. Cai, H.M. Wang, Appl. Surf. Sci. 235 (2004) 501–506. [12] W.Y. Kim, H. Tanaka, A. Kasama, S. Hanada, Intermetallics 9 (2001) 827–834. [13] H. Bei, E.P. George, E.A. Kenik, G.M. Pharr, Acta Mater. 51 (2003) 6241–6252. [14] J.J. Petrovic, Intermetallics 8 (2000) 1175–1182. [15] Y. Liu, H.M. Wang, Mater. Sci. Eng. A 396 (2005) 240–250. [16] X.D. Lu, H.M. Wang, Surf. Coat. Technol. 200 (2005) 2380–2385. [17] N. Zarrinfar, A.R. Kennedy, P.H. Shipway, Scripta Mater. 50 (2004) 949–952. [18] Z.Y. Shi, D.Q. Wang, Appl. Surf. Sci. 167 (2000) 107–112. [19] S.C. Tjong, K.C. Lau, Mater. Sci. Eng. A 282 (2000) 183–186. [20] J.P. Tu, W. Rong, S.Y. Guo, Y.Z. Yang, Wear 255 (2003) 832–835. [21] Y.X. Yin, H.M. Wang, J. Mater. Res. 20 (2005) 1122–1130. [22] Y.X. Yin, H.M. Wang, J. Alloys Compd. 420 (2006) 218–224. [23] H.M. Wang, D.Y. Luan, L.Y. Zhang, Scripta Mater. 48 (2003) 1179–1184. [24] H.M. Wang, D.Y. Luan, L.Y. Cai, Metall. Mater. Trans. A 34 (2003) 2005–2015. [25] F.H. Stott, Tribol. Int. 35 (2002) 489–495.