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Precipitation phenomenon in nanostructured Cr3C2–NiCr coatings
Materials Science and Engineering A301 (2001) 69 – 79 www.elsevier.com/locate/msea
Precipitation phenomenon in nanostructured Cr3C2 –NiCr coatings Jianhong He, Enrique J. Lavernia * Department of Chemical and Biochemical Engineering and Materials Science, Uni6ersity of California, Ir6ine, CA92697 -2575, USA Received 17 December 1999; received in revised form 2 April 2000
Abstract Precipitation in the nanostructured Cr3C2/NiCr coatings was investigated. Ultrafine Cr2O3 particles with an average size of 8.3 nm were observed using transmission electron microscopy in the nanostructured Cr3C2/NiCr coatings exposed to elevated temperatures. In addition to the precipitation of oxide particles, the phase transformations in the original NiCr amorphous phase, which was always observed in the as-sprayed nanostructured Cr3C2/NiCr coatings, were also identified. Internal oxidation was thought to be responsible for the precipitation of the dispersed oxide particles. The results of microhardness and scratch-resistance tests showed that microhardness of the conventional coating slightly increased only with an increase in the exposure temperature, while that of the nanostructured coating increased significantly from 1020 to 1240 HV300. Compared with the conventional Cr3C2/NiCr coatings, the scratch-resistance and coefficient of friction were found to be increased and reduced respectively in the nanostructured coatings. Heat treatment led to further increase in scratch-resistance and further decrease in coefficient of friction of the nanostructured coatings. The increases in microhardness and scratch-resistance and decrease in coefficient of friction of the nanostructured coatings were attributed to a high density of oxide nanoparticles precipitating within the coating as the exposure temperature increased. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Precipitation; Nanostructured coating; Thermal spraying; TEM examination.
1. Introduction At the beginning of this century, Wilm [1] discovered the phenomenon of precipitation hardening in Al alloys. It was postulated that the increase in hardness of Al alloys with time was due to precipitation of a new phase in an initially supersaturated solid solution [2]. Subsequent studies using transmission electron microscopy confirmed the precipitation of very fine particle in aged alloys [3]. Mott and Nabarro [4] and Orowan [5] derived the famous dislocation models of precipitation/dispersion hardening. Today, precipitation behavior is an important sector of research in materials science. A systematic description of mechanics models relating precipitation strengthening is available in a review article by Ardell [6]. In addition to the strengthening caused by precipitation, the behavior of precipitates in individual alloy system has been studied in detail [7–13] because precipitation plays a crucial * Corresponding author. Tel.: +1-949-8248277; fax: + 1-9498242262. E-mail address: [email protected] (E.J. Lavernia).
role on alloy properties. The phenomenon of supersaturation, which results in subsequent precipitation, is commonly observed in materials manufactured by nonequilibrium processes, i.e. rapid-quenching, mechanical alloying/milling and thermal spraying. In related studies, a significant amount of W and C was dissolved into the binder phase of WC–Co coatings [14], and a nonequilibrium microstructure was detected in the assprayed nanostructured Cr3C2 –NiCr coatings synthesized by mechanical milling and high velocity oxygen fuel (HVOF) thermal spraying [15]. Therefore, it is anticipated that precipitation will occur in these coatings under certain conditions. The aim of the present paper is to examine the precipitation in nanostructured Cr3C2 –25(Ni20Cr) coatings and the influence of precipitation on the coating properties.
2. Experimental procedure Pre-alloyed Cr3C2 –NiCr powders (Dialloy 3004 blended Cr3C2-25 (Ni20Cr), produced by Sulzer Metco (US) Inc.) were chosen for this study. The powders
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Table 1 Chemical composition of Cr3C2–NiCr powders (weight percent)
Conventional powder Nanostructured powder
Cr
Ni
C
N
O
70.0 64.3
19.2 18.8
9.83 9.36
0.20 0.51
0.21 1.93
were immersed in Hexane [H3C(CH2)4CH3] and mechanically milled with a modified Szegvari attritor Model B at a rate of 180 rpm for 20 h in a stainless steel tank with stainless steel balls. X-ray diffraction and transmission electron microscope (TEM) observations indicated that the as-milled powder is nanocomposite of NiCr phase and Cr3C2 particles; the average grain size of carbide phase the nanostructured powder was 15 nm [16]. The chemical compositions of the nanostructured and the conventional (as-received) Cr3C2 –NiCr powders are shown in Table 1. These two powders were used as feedstock powder to synthesize nanostructured and conventional Cr3C2 – NiCr coatings, respectively, by high velocity oxygen fuel (HVOF) thermal spraying. To prepare Cr3C2 – NiCr coatings, a Sulzer Metco Diamond Jet HVOF thermal spray facility was used. The main constituents of this facility are described in detail elsewhere [15,17]. The resulting coatings were thermally exposed in air at 473, 673, 873 and 1073 K for 8 h. The cross-section of the coating was examined using a Philips XL 30 FEG SEM. The microhardness was tested on a Buehler Micromet 2004 Microhardness tester using a load of 300 g. Each microhardness value was obtained from an average value of 30 tests. The friction coefficient and scratch-resistance of the coatings were measured by the
Center For Tribology, Inc. (Mountain View, California) using its own CETR Micro-Tribometer. The scratch head was a sapphire ball with a radius of 0.75 mm, and the scratch tests were performed under a scratch rate of 22.7 rpm (535 mm/min) and scratch normal load of 5 N. After removal of the substrate by polishing, TEM specimens were prepared by cutting out a section of the coating and forming 3 mm diameter disks. The disks were dimpled to around 30 mm thick using a dimpler fitted with diamond grinders. The grinding size descended from a 6 mm grade, down to 3 mm, and finally to a 1 mm grade. The final thinning perforation process was performed using an argon ion mill. With the prepared samples, TEM observations were carried out on the Philips CM20 microscope operated at 200 keV.
3. Results and discussion
3.1. Microstructure of the as-sprayed coatings The SEM back-scattered electron images of the assprayed conventional and nanostructured Cr3C2 – 25(Ni20Cr) coatings are shown in Fig. 1. A uniform and dense microstructure is observed in the nanostructured coatings, compared to the conventional Cr3C2 – 25(Ni20Cr) coating which is observed to have an inhomogeneous microstructure. The TEM bright field image of the nanostructured coating, the corresponding dark field image and the diffraction pattern are shown in Fig. 2(a–c), respectively. The average carbide particle size is approximately 24 nm. This indicates that the
Fig. 1. Microstructure of Cr3C2 –25(Ni20Cr) coatings. (a) Conventional coating; (b) Magnification of (a); (c) Nanostructured coating; and (d) Magnification of (c).
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rings are thus indicative of an amorphous phase rather than a false appearance caused by under/over focusing. These elongated amorphous phases, which have dimensions of around 100 nm wide and 1 mm in length, are discontinuously distributed in the nanostructured Cr3C2/NiCr coating. The formation of the amorphous phase is thought to occur as follows; HVOF spraying process yields a maximum temperature of 2400 K in the nanocomposite Cr3C2/NiCr powder causing the NiCr binder matrix powder, with a melting point of 1700 K [19], in the nanocomposite powder to completely melt. Regions of the molten matrix were rendered amorphous during impingement and rapid cooling with the surface, a behavior commonly referred as splat-quenching [20,21]. Fig. 4 shows a SEM image of the surface of the nanostructured Cr3C2/NiCr coating, in which the arrow illustrates the morphology of the solidified droplet.
3.2. Precipitation in the nanostructured Cr3C2 /NiCr coatings
Fig. 2. TEM observation of nanostructured Cr3C2 –25(Ni20Cr) coating. (a) Bright field image; (b) Dark field image; and (c) Corresponding diffraction pattern.
coating has a nano structure. In the nanostructured WC–12%Co coating [17], TEM examination revealed a microstructure consisting of nano sized WC carbide particles distributed in an amorphous matrix phase. In the nanostructured Cr3C2 – NiCr coatings, the diffraction pattern shown in Fig. 2(c), does not clearly reveal the presence of an amorphous matrix phase. Guilemany and Calero [18] also observed amorphous matrix phases in a conventional HVOF thermally sprayed Cr3C2 – NiCr coating. Instead of an amorphous matrix phase, a few discontinuous elongated amorphous phases are observed in the nanostructured Cr3C2 – NiCr coating, shown in Fig. 3(a – d). The inserted diffraction patterns, which were from the elongated phases marked A, show diffuse rings. Many fine diffraction spots are sharply imaged in the diffraction patterns, this indicates that the diffraction patterns are well focused. The diffuse
Fig. 5(a–c) show the TEM bright field image, dark field image and the corresponding diffraction pattern of the nanostructured coating treated at 1073 K. In addition to original carbide particles, some very fine precipitates are observed. The detailed view shown in Fig. 6(a–b) reveals spherically shaped precipitates that are likely to have formed by nucleation and growth in the matrix. The average size of the original carbide particles increases from 24 nm in the as-sprayed nanostructured coating to 39 nm in the nanostructured coating exposed at 1073 K, whereas the precipitates have an average size of 8.3 nm. In addition to the precipitation, structural changes in the original elongated amorphous phases are also observed. In the as-sprayed nanostructured coatings, elongated amorphous phases are often observed, see Fig. 3. As shown in Fig. 7, the morphology and dimensions of the elongated amorphous phases have not changed during heat treatment. However, a few changes can be observed in Fig. 8 consisting of a series of TEM micrographs. Fig. 8(a) shows a TEM bright field image. Arrow M indicates the matrix containing a high density of precipitates and arrow A illustrates an original elongated amorphous phase. The selected area diffraction (SAD) pattern of area A, shown Fig. 8(b), indicates that the original amorphous phase has been crystallized. Very thin and long streaks in the SAD pattern indicate the presence of thin twins. Using a diffraction streak, the dark field image of the original elongated phase was taken and shown in Fig. 8(c), in which very thin thick twins can be seen. Precipitates are not found in this elongated structure. Therefore, the changes in the original elongated amorphous phases during annealing can be summarized as follows: (1) the morphology and dimensions have not been
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altered; (2) the amorphous structure has been crystallized; (3) deformation twins can be observed inside the phase; and (4) no precipitates are found inside. Fig. 8 (d) is a SAD pattern of the matrix M consisting of two sets of diffraction spots from the f.c.c. matrix and the Cr2O3 particles. Arrows P indicate the diffraction spots of Cr2O3 and arrow M indicates diffraction spots of f.c.c. matrix. The dark field image of the precipitates was taken using a P spot and shown in Fig. 8(e), the
spherically shaped precipitates are densely distributed in the matrix. The dark field image of the matrix taken using a M spot is shown in Fig. 8(f). The average grain size of the matrix is approximately 150 nm. Phase transformations in conventional Cr3C2/NiCr coatings at high temperatures have been reported in the literature [22–26]. Lai [25,26] showed that there are structural changes in carbides from Cr3C2 to Cr7C3 to Cr23C6 during exposure to high temperature. The rela-
Fig. 3. Elongated amorphous phase in nanostructured Cr3C2 – 25(Ni20Cr) coating. (a) Bright field image; (b) Dark field image; (c) Bright field image; and (d) Dark field image.
Fig. 4. SEM image of surface of the nanostructured coatings illustrating droplet solidified.
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Fig. 5. TEM observation of the nanostructured coating exposed at 1073 K. (a) Bright field image; (b) Dark field image; and (c) Corresponding diffraction pattern.
Fig. 6. Precipitates in the nanostructured coating. (a) Bright field image; and (b) Dark field image. Arrows O indicates the original carbides, and Arrows P indicates the precipitates.
tive thermodynamic stability of carbides was determined by their standard free energies of formation. The Cr2O3 phase has been detected by X-ray diffraction in the Cr3C2/NiCr coatings exposed to air [22,24], high pressure helium [25,26] and N2 – 3%H2 gas [23] at high temperatures. Fig. 9 through Fig. 11 show the results of X-ray mapping illustrating the distribution of oxygen in the coatings. Oxygen is not observed in the NiCr binder phases, thus the precipitate is not found in the original
elongated amorphous phases. In the as-sprayed coatings, a high oxygen content is observed in the nanostructured coating, while there is a low oxygen content in the conventional coating. In the coatings exposed at 873 K, high oxygen contents are found in both conventional and nanostructured coatings. Therefore, oxidization occurs in the carbide phases of Cr3C2/NiCr coatings exposed to air at high temperatures regardless of original oxygen content in the coatings.
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3.3. Variation of microhardness and scratch-resistance with heat treatment temperature The average microhardness, measured on the crosssection, of both conventional and nanostructured Cr3C2 –25(Ni20Cr) coatings is plotted in Fig. 12. The microhardness of the as-sprayed coatings increases from a value of 846 for the conventional coating to 1020 HV300 for the nanostructured coating. Hence the nanostructured coating exhibits a 20.5% increase in microhardness as compared with the corresponding conventional coating. Microhardness of the conventional coating only increases slightly with increased exposure to all temperature ranges, while that of the nanostructured coating drastically increases from 1020 to 1240 HV300 in the temperature range 700–900 K, and then approaches a constant value. The relationship between scratch depth and time under a normal load of 5 N are shown in Fig. 13. A reading of scratch depth was taken every 0.01 s during the tests that were automatically controlled by a computer, thus a few tens of thousands of data illustrating the relation between scratch depth and time were recorded during a single scratch run. Fig. 13(a) compares scratch depths in the as-sprayed conventional coating and in the as-sprayed nanostructured coating. The as-sprayed conventional coating produces an average scratch depth of approximately 100 mm, whereas a depth of around 50 mm is found in the as-sprayed nanostructured coating. Thus, the nanostructured coating exhibits a scratch resistance that is twice that of the conventional one. Fig. 13(b) reveals the influence of heat treatment on scratch depth of the nanostructured coatings. The scratch depth decreases with increasing heat treatment temperature. No scratch was found in
Fig. 7. TEM image of the original elongated amorphous phase in the nanostructured coating exposed at 1073 K.
the nanostructured coating treated at 873 K under the present experimental conditions. The scratch depth results are supported by the measured microhardness values. Coefficient of friction, under the mode of the ‘ball on disk’ friction, was also obtained from the scratch tests and are listed in Table 2. Compared with the coefficient of ftiction in the as-sprayed conventional coating, a reduced coefficient of friction is observed in the assprayed nanostructured coating. Kear and McCandlish [27] also observed a decreased friction coefficient in a nanostructured WC-23% Co coating. The coefficient of friction is further decreased as heat treatment temperature increases. A number of publications report the application of Cr3C2/NiCr coatings at elevated temperatures [18,22– 24,28–33]. In a related study [22], the Cr3C2/NiCr was heated at 1123 K for 2 h and the results showed that the hardness on cross-sections did not decrease even though Cr2O3was detected by X-ray diffraction. The results published by Fukuda and Kumon [24] showed that heat treatment at 923 K for 1000 h yielded a maximum hardness value of 1100 for D-gun sprayed Cr3C2 –25(Ni20Cr) coatings. In their experiments, 900 Hv for the as-sprayed coatings, 1100 for the coatings heated at 923 K for 1000 h and 1000 for the coatings heated at 1123 K for 1000 h were reported. Cr3C2/NiCr coatings with the highest hardness offered the best abrasive and erosive wear resistance [29]. Therefore, it can be concluded that, on basis of the published studies and the present experimental results, a suitable postsprayed heat treatment is beneficial for properties of both conventional and nanostructured Cr3C2/NiCr coatings. Microhardness of a material depends strongly on its microstructure. Precipitation, depending on size, distribution and behavior of the precipitates formed, usually causes hardening [4,5]. High density of fine, hard and dispersed precipitates in the matrix significantly increases microhardness [6]. The size, distribution and density of precipitates result from the combined effect of nucleation and growth of the precipitates, which are determined by aging processing, mainly aging temperature and time [7–13]. For most types of precipitates, the relationship between the nucleation of precipitates and aging temperature can be represented by a Cshaped curve [34], in other words, an incubation is needed for the nucleation of precipitates and the shortest incubation appears in a specified temperature range. The temperature corresponding to the shortest incubation is usually chosen as the aging temperature. A typical hardness 6s aging time curve in precipitation hardening alloys shows three stages, under-aged, aged and overaged stages [34]. The hardness of the alloy increases sharply in the under-aged stage and reaches its maximum value in the aged stage, then decreases
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Fig. 8. TEM analysis of precipitates, matrix and changes in the original amorphous phases. (a) Bright field image containing matrix, precipitates and the original amorphous phase; (b) SAD pattern of the original amorphous phase, very thin and long streaks illustrating thin twins are observed; (c) Dark field image of the original amorphous phase; (d) SAD pattern containing diffraction spots from both matrix and precipitates; (e) Dark field image of the precipitates; and (f) Dark field image illustrating grains of the matrix.
with increasing aging time in the over-aged stage because of coarsening of precipitates. On the basis of the distribution of oxygen in the coatings, shown in Figs. 9 –11, it is proposed that the observed Cr2O3 particles in the nanostructured coatings evolve as follows. First,
there is a high oxygen potential in the as-sprayed nanostructured coatings, due to the presence of oxygen in the starting powder (1.93 wt%, see Table 1), as well as oxygen entrainment during thermal spraying. Second, oxygen reacts with chromium to form CrO, CrO2,
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CrO3, Cr2O3, and Cr3O4, however, CrO, CrO2, CrO3 and Cr3O4 are metastable and/or present only under a high pressure [35]. Therefore during thermal exposure, Cr2O3 is directly formed from the following chemical reaction:
Cr2C3 + 92O2 Cr2O3 + 3CO2
(1)
The formation of the Cr2O3 phase is supported by the TEM analysis that reveals the presence of diffraction pattern from the hexagonal (a= 0.4954 nm, c=13.584)
Fig. 9. X-ray mapping of the as-sprayed conventional Cr3C2/NiCr coating. (a) SEM back scattered electron image; (b) Corresponding Cr element map; (c) Corresponding Ni element map; and (d) Corresponding O element map.
Fig. 11. X-ray oxygen element mapping result of the coatings exposed at 873 K. (a) SEM back scattered electron image of the conventional coating exposed at 873 K; (b) Corresponding O element map; (c) SEM back scattered electron image of the nanostructured coating exposed at 873 K; (d) Corresponding O element map.
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ternal oxidation was thought to be responsible for the precipitation of the dispersed oxide particles. 2. The phase transformations in the original NiCr amorphous phase were identified in the nanostructured Cr3C2/NiCr coatings exposed to high temperature. The amorphous structure has been crystallized and the deformation twins can be observed inside.
Fig. 12. Variation of microhardness of Cr3C2/NiCr coatings with heat treatment temperature.
Cr2O3 phase. The Cr2O3 phase, with the Gibbs free energy of formation of per mole volume of DG = 251.70 kilocalories [36] is relatively stable. Third, the nucleation and growth of Cr2O3 phase uniformly occurs throughout the coating because most of oxygen is primarily observed in nano-Cr3C2 particles, see Fig. 10 and Fig. 11, that are uniformly distributed in the coating. Therefore, this internal oxidation process leads to the formation of fine (8.3 nm) and dispersed Cr2O3 precipitates in the nanostructured Cr3C2 coatings. During thermal exposure, the crystallization of amorphous phase in the coatings occurs, see Fig. 8. Crystallization of amorphous phase usually causes a decrease in hardness [37], therefore, the change in microstructure of amorphous phase has a negative influence on hardness of the coatings. However, the decrease in hardness caused by crystallization of the amorphous phase is compensated by the increase originated from the precipitation of oxide particles. Accordingly, on the basis of the discussion above, the observed increase in microhardness of Cr3C2/NiCr coatings exposed to high temperatures is attributed to the precipitation of oxides. Particularly in the nanostructured coatings, the high density, nano sized oxide particles lead to significant increases in the microhardness and hence an increase in scratch-resistance.
4. Conclusions The precipitation in the nanostructured Cr3C2/NiCr coatings was investigated using SEM, TEM, microhardness and Micro-Tribometer tests. The main conclusions are briefly summarized as follows: 1. Ultrafine Cr2O3 particles with an average size of 8.3 nm were observed in the nanostructured Cr3C2/ NiCr coatings exposed to elevated temperature. In-
Fig. 13. Scratch-resistance of Cr3C2/NiCr coatings. (a) Scratch depths of the as-sprayed coatings; and (b) Influence of heat treatment temperature on scratch depth.
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Table 2 Coefficients of ffiction of the coatings As-sprayed conventional coating
As-sprayed nanostructured coating
Nanostructured coating treated at 673 K
Nanostructured coating treated at 873 K
0.495
0.216
0.205
0.183
Fig. 10. X-ray mapping of the as-sprayed nanostructured Cr3C2 – 25(Ni20Cr) coating. (a) SEM back scattered electron image; (b) Corresponding Cr element map; (c) Corresponding Ni element map; and (d) Corresponding O element map.
3. Microhardness of the conventional coating increased slightly with temperature, while that of the nanostructured coating drastically increased from 1020–1240 HV300 for the same temperature increases. 4. Compared with the conventional Cr3C2/NiCr coatings, the increased scratch resistance and reduced coefficient of friction were found in the nanostructured coatings. Heat treatment led to further increases in scratch-resistance and further decreases in the coefficient of friction for the nanostructured coatings.
Acknowledgements The authors would like to thank Michael Ice, a formerly graduate student who currently works at Orthodyne Electronics, Irvine, California, for his involving in partial work. The authors also gratefully acknowledge financial support provided by the Office of Naval Research (Grants No.: N00014-94-1-0017, N00014-971-0844, N00014-981-0569 and N00014-00-1-0109).
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Precipitation phenomenon in nanostructured Cr3C2 –NiCr coatings Jianhong He, Enrique J. Lavernia * Department of Chemical and Biochemical Engineering and Materials Science, Uni6ersity of California, Ir6ine, CA92697 -2575, USA Received 17 December 1999; received in revised form 2 April 2000
Abstract Precipitation in the nanostructured Cr3C2/NiCr coatings was investigated. Ultrafine Cr2O3 particles with an average size of 8.3 nm were observed using transmission electron microscopy in the nanostructured Cr3C2/NiCr coatings exposed to elevated temperatures. In addition to the precipitation of oxide particles, the phase transformations in the original NiCr amorphous phase, which was always observed in the as-sprayed nanostructured Cr3C2/NiCr coatings, were also identified. Internal oxidation was thought to be responsible for the precipitation of the dispersed oxide particles. The results of microhardness and scratch-resistance tests showed that microhardness of the conventional coating slightly increased only with an increase in the exposure temperature, while that of the nanostructured coating increased significantly from 1020 to 1240 HV300. Compared with the conventional Cr3C2/NiCr coatings, the scratch-resistance and coefficient of friction were found to be increased and reduced respectively in the nanostructured coatings. Heat treatment led to further increase in scratch-resistance and further decrease in coefficient of friction of the nanostructured coatings. The increases in microhardness and scratch-resistance and decrease in coefficient of friction of the nanostructured coatings were attributed to a high density of oxide nanoparticles precipitating within the coating as the exposure temperature increased. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Precipitation; Nanostructured coating; Thermal spraying; TEM examination.
1. Introduction At the beginning of this century, Wilm [1] discovered the phenomenon of precipitation hardening in Al alloys. It was postulated that the increase in hardness of Al alloys with time was due to precipitation of a new phase in an initially supersaturated solid solution [2]. Subsequent studies using transmission electron microscopy confirmed the precipitation of very fine particle in aged alloys [3]. Mott and Nabarro [4] and Orowan [5] derived the famous dislocation models of precipitation/dispersion hardening. Today, precipitation behavior is an important sector of research in materials science. A systematic description of mechanics models relating precipitation strengthening is available in a review article by Ardell [6]. In addition to the strengthening caused by precipitation, the behavior of precipitates in individual alloy system has been studied in detail [7–13] because precipitation plays a crucial * Corresponding author. Tel.: +1-949-8248277; fax: + 1-9498242262. E-mail address: [email protected] (E.J. Lavernia).
role on alloy properties. The phenomenon of supersaturation, which results in subsequent precipitation, is commonly observed in materials manufactured by nonequilibrium processes, i.e. rapid-quenching, mechanical alloying/milling and thermal spraying. In related studies, a significant amount of W and C was dissolved into the binder phase of WC–Co coatings [14], and a nonequilibrium microstructure was detected in the assprayed nanostructured Cr3C2 –NiCr coatings synthesized by mechanical milling and high velocity oxygen fuel (HVOF) thermal spraying [15]. Therefore, it is anticipated that precipitation will occur in these coatings under certain conditions. The aim of the present paper is to examine the precipitation in nanostructured Cr3C2 –25(Ni20Cr) coatings and the influence of precipitation on the coating properties.
2. Experimental procedure Pre-alloyed Cr3C2 –NiCr powders (Dialloy 3004 blended Cr3C2-25 (Ni20Cr), produced by Sulzer Metco (US) Inc.) were chosen for this study. The powders
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 3 8 3 - 6
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Table 1 Chemical composition of Cr3C2–NiCr powders (weight percent)
Conventional powder Nanostructured powder
Cr
Ni
C
N
O
70.0 64.3
19.2 18.8
9.83 9.36
0.20 0.51
0.21 1.93
were immersed in Hexane [H3C(CH2)4CH3] and mechanically milled with a modified Szegvari attritor Model B at a rate of 180 rpm for 20 h in a stainless steel tank with stainless steel balls. X-ray diffraction and transmission electron microscope (TEM) observations indicated that the as-milled powder is nanocomposite of NiCr phase and Cr3C2 particles; the average grain size of carbide phase the nanostructured powder was 15 nm [16]. The chemical compositions of the nanostructured and the conventional (as-received) Cr3C2 –NiCr powders are shown in Table 1. These two powders were used as feedstock powder to synthesize nanostructured and conventional Cr3C2 – NiCr coatings, respectively, by high velocity oxygen fuel (HVOF) thermal spraying. To prepare Cr3C2 – NiCr coatings, a Sulzer Metco Diamond Jet HVOF thermal spray facility was used. The main constituents of this facility are described in detail elsewhere [15,17]. The resulting coatings were thermally exposed in air at 473, 673, 873 and 1073 K for 8 h. The cross-section of the coating was examined using a Philips XL 30 FEG SEM. The microhardness was tested on a Buehler Micromet 2004 Microhardness tester using a load of 300 g. Each microhardness value was obtained from an average value of 30 tests. The friction coefficient and scratch-resistance of the coatings were measured by the
Center For Tribology, Inc. (Mountain View, California) using its own CETR Micro-Tribometer. The scratch head was a sapphire ball with a radius of 0.75 mm, and the scratch tests were performed under a scratch rate of 22.7 rpm (535 mm/min) and scratch normal load of 5 N. After removal of the substrate by polishing, TEM specimens were prepared by cutting out a section of the coating and forming 3 mm diameter disks. The disks were dimpled to around 30 mm thick using a dimpler fitted with diamond grinders. The grinding size descended from a 6 mm grade, down to 3 mm, and finally to a 1 mm grade. The final thinning perforation process was performed using an argon ion mill. With the prepared samples, TEM observations were carried out on the Philips CM20 microscope operated at 200 keV.
3. Results and discussion
3.1. Microstructure of the as-sprayed coatings The SEM back-scattered electron images of the assprayed conventional and nanostructured Cr3C2 – 25(Ni20Cr) coatings are shown in Fig. 1. A uniform and dense microstructure is observed in the nanostructured coatings, compared to the conventional Cr3C2 – 25(Ni20Cr) coating which is observed to have an inhomogeneous microstructure. The TEM bright field image of the nanostructured coating, the corresponding dark field image and the diffraction pattern are shown in Fig. 2(a–c), respectively. The average carbide particle size is approximately 24 nm. This indicates that the
Fig. 1. Microstructure of Cr3C2 –25(Ni20Cr) coatings. (a) Conventional coating; (b) Magnification of (a); (c) Nanostructured coating; and (d) Magnification of (c).
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rings are thus indicative of an amorphous phase rather than a false appearance caused by under/over focusing. These elongated amorphous phases, which have dimensions of around 100 nm wide and 1 mm in length, are discontinuously distributed in the nanostructured Cr3C2/NiCr coating. The formation of the amorphous phase is thought to occur as follows; HVOF spraying process yields a maximum temperature of 2400 K in the nanocomposite Cr3C2/NiCr powder causing the NiCr binder matrix powder, with a melting point of 1700 K [19], in the nanocomposite powder to completely melt. Regions of the molten matrix were rendered amorphous during impingement and rapid cooling with the surface, a behavior commonly referred as splat-quenching [20,21]. Fig. 4 shows a SEM image of the surface of the nanostructured Cr3C2/NiCr coating, in which the arrow illustrates the morphology of the solidified droplet.
3.2. Precipitation in the nanostructured Cr3C2 /NiCr coatings
Fig. 2. TEM observation of nanostructured Cr3C2 –25(Ni20Cr) coating. (a) Bright field image; (b) Dark field image; and (c) Corresponding diffraction pattern.
coating has a nano structure. In the nanostructured WC–12%Co coating [17], TEM examination revealed a microstructure consisting of nano sized WC carbide particles distributed in an amorphous matrix phase. In the nanostructured Cr3C2 – NiCr coatings, the diffraction pattern shown in Fig. 2(c), does not clearly reveal the presence of an amorphous matrix phase. Guilemany and Calero [18] also observed amorphous matrix phases in a conventional HVOF thermally sprayed Cr3C2 – NiCr coating. Instead of an amorphous matrix phase, a few discontinuous elongated amorphous phases are observed in the nanostructured Cr3C2 – NiCr coating, shown in Fig. 3(a – d). The inserted diffraction patterns, which were from the elongated phases marked A, show diffuse rings. Many fine diffraction spots are sharply imaged in the diffraction patterns, this indicates that the diffraction patterns are well focused. The diffuse
Fig. 5(a–c) show the TEM bright field image, dark field image and the corresponding diffraction pattern of the nanostructured coating treated at 1073 K. In addition to original carbide particles, some very fine precipitates are observed. The detailed view shown in Fig. 6(a–b) reveals spherically shaped precipitates that are likely to have formed by nucleation and growth in the matrix. The average size of the original carbide particles increases from 24 nm in the as-sprayed nanostructured coating to 39 nm in the nanostructured coating exposed at 1073 K, whereas the precipitates have an average size of 8.3 nm. In addition to the precipitation, structural changes in the original elongated amorphous phases are also observed. In the as-sprayed nanostructured coatings, elongated amorphous phases are often observed, see Fig. 3. As shown in Fig. 7, the morphology and dimensions of the elongated amorphous phases have not changed during heat treatment. However, a few changes can be observed in Fig. 8 consisting of a series of TEM micrographs. Fig. 8(a) shows a TEM bright field image. Arrow M indicates the matrix containing a high density of precipitates and arrow A illustrates an original elongated amorphous phase. The selected area diffraction (SAD) pattern of area A, shown Fig. 8(b), indicates that the original amorphous phase has been crystallized. Very thin and long streaks in the SAD pattern indicate the presence of thin twins. Using a diffraction streak, the dark field image of the original elongated phase was taken and shown in Fig. 8(c), in which very thin thick twins can be seen. Precipitates are not found in this elongated structure. Therefore, the changes in the original elongated amorphous phases during annealing can be summarized as follows: (1) the morphology and dimensions have not been
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altered; (2) the amorphous structure has been crystallized; (3) deformation twins can be observed inside the phase; and (4) no precipitates are found inside. Fig. 8 (d) is a SAD pattern of the matrix M consisting of two sets of diffraction spots from the f.c.c. matrix and the Cr2O3 particles. Arrows P indicate the diffraction spots of Cr2O3 and arrow M indicates diffraction spots of f.c.c. matrix. The dark field image of the precipitates was taken using a P spot and shown in Fig. 8(e), the
spherically shaped precipitates are densely distributed in the matrix. The dark field image of the matrix taken using a M spot is shown in Fig. 8(f). The average grain size of the matrix is approximately 150 nm. Phase transformations in conventional Cr3C2/NiCr coatings at high temperatures have been reported in the literature [22–26]. Lai [25,26] showed that there are structural changes in carbides from Cr3C2 to Cr7C3 to Cr23C6 during exposure to high temperature. The rela-
Fig. 3. Elongated amorphous phase in nanostructured Cr3C2 – 25(Ni20Cr) coating. (a) Bright field image; (b) Dark field image; (c) Bright field image; and (d) Dark field image.
Fig. 4. SEM image of surface of the nanostructured coatings illustrating droplet solidified.
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Fig. 5. TEM observation of the nanostructured coating exposed at 1073 K. (a) Bright field image; (b) Dark field image; and (c) Corresponding diffraction pattern.
Fig. 6. Precipitates in the nanostructured coating. (a) Bright field image; and (b) Dark field image. Arrows O indicates the original carbides, and Arrows P indicates the precipitates.
tive thermodynamic stability of carbides was determined by their standard free energies of formation. The Cr2O3 phase has been detected by X-ray diffraction in the Cr3C2/NiCr coatings exposed to air [22,24], high pressure helium [25,26] and N2 – 3%H2 gas [23] at high temperatures. Fig. 9 through Fig. 11 show the results of X-ray mapping illustrating the distribution of oxygen in the coatings. Oxygen is not observed in the NiCr binder phases, thus the precipitate is not found in the original
elongated amorphous phases. In the as-sprayed coatings, a high oxygen content is observed in the nanostructured coating, while there is a low oxygen content in the conventional coating. In the coatings exposed at 873 K, high oxygen contents are found in both conventional and nanostructured coatings. Therefore, oxidization occurs in the carbide phases of Cr3C2/NiCr coatings exposed to air at high temperatures regardless of original oxygen content in the coatings.
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3.3. Variation of microhardness and scratch-resistance with heat treatment temperature The average microhardness, measured on the crosssection, of both conventional and nanostructured Cr3C2 –25(Ni20Cr) coatings is plotted in Fig. 12. The microhardness of the as-sprayed coatings increases from a value of 846 for the conventional coating to 1020 HV300 for the nanostructured coating. Hence the nanostructured coating exhibits a 20.5% increase in microhardness as compared with the corresponding conventional coating. Microhardness of the conventional coating only increases slightly with increased exposure to all temperature ranges, while that of the nanostructured coating drastically increases from 1020 to 1240 HV300 in the temperature range 700–900 K, and then approaches a constant value. The relationship between scratch depth and time under a normal load of 5 N are shown in Fig. 13. A reading of scratch depth was taken every 0.01 s during the tests that were automatically controlled by a computer, thus a few tens of thousands of data illustrating the relation between scratch depth and time were recorded during a single scratch run. Fig. 13(a) compares scratch depths in the as-sprayed conventional coating and in the as-sprayed nanostructured coating. The as-sprayed conventional coating produces an average scratch depth of approximately 100 mm, whereas a depth of around 50 mm is found in the as-sprayed nanostructured coating. Thus, the nanostructured coating exhibits a scratch resistance that is twice that of the conventional one. Fig. 13(b) reveals the influence of heat treatment on scratch depth of the nanostructured coatings. The scratch depth decreases with increasing heat treatment temperature. No scratch was found in
Fig. 7. TEM image of the original elongated amorphous phase in the nanostructured coating exposed at 1073 K.
the nanostructured coating treated at 873 K under the present experimental conditions. The scratch depth results are supported by the measured microhardness values. Coefficient of friction, under the mode of the ‘ball on disk’ friction, was also obtained from the scratch tests and are listed in Table 2. Compared with the coefficient of ftiction in the as-sprayed conventional coating, a reduced coefficient of friction is observed in the assprayed nanostructured coating. Kear and McCandlish [27] also observed a decreased friction coefficient in a nanostructured WC-23% Co coating. The coefficient of friction is further decreased as heat treatment temperature increases. A number of publications report the application of Cr3C2/NiCr coatings at elevated temperatures [18,22– 24,28–33]. In a related study [22], the Cr3C2/NiCr was heated at 1123 K for 2 h and the results showed that the hardness on cross-sections did not decrease even though Cr2O3was detected by X-ray diffraction. The results published by Fukuda and Kumon [24] showed that heat treatment at 923 K for 1000 h yielded a maximum hardness value of 1100 for D-gun sprayed Cr3C2 –25(Ni20Cr) coatings. In their experiments, 900 Hv for the as-sprayed coatings, 1100 for the coatings heated at 923 K for 1000 h and 1000 for the coatings heated at 1123 K for 1000 h were reported. Cr3C2/NiCr coatings with the highest hardness offered the best abrasive and erosive wear resistance [29]. Therefore, it can be concluded that, on basis of the published studies and the present experimental results, a suitable postsprayed heat treatment is beneficial for properties of both conventional and nanostructured Cr3C2/NiCr coatings. Microhardness of a material depends strongly on its microstructure. Precipitation, depending on size, distribution and behavior of the precipitates formed, usually causes hardening [4,5]. High density of fine, hard and dispersed precipitates in the matrix significantly increases microhardness [6]. The size, distribution and density of precipitates result from the combined effect of nucleation and growth of the precipitates, which are determined by aging processing, mainly aging temperature and time [7–13]. For most types of precipitates, the relationship between the nucleation of precipitates and aging temperature can be represented by a Cshaped curve [34], in other words, an incubation is needed for the nucleation of precipitates and the shortest incubation appears in a specified temperature range. The temperature corresponding to the shortest incubation is usually chosen as the aging temperature. A typical hardness 6s aging time curve in precipitation hardening alloys shows three stages, under-aged, aged and overaged stages [34]. The hardness of the alloy increases sharply in the under-aged stage and reaches its maximum value in the aged stage, then decreases
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Fig. 8. TEM analysis of precipitates, matrix and changes in the original amorphous phases. (a) Bright field image containing matrix, precipitates and the original amorphous phase; (b) SAD pattern of the original amorphous phase, very thin and long streaks illustrating thin twins are observed; (c) Dark field image of the original amorphous phase; (d) SAD pattern containing diffraction spots from both matrix and precipitates; (e) Dark field image of the precipitates; and (f) Dark field image illustrating grains of the matrix.
with increasing aging time in the over-aged stage because of coarsening of precipitates. On the basis of the distribution of oxygen in the coatings, shown in Figs. 9 –11, it is proposed that the observed Cr2O3 particles in the nanostructured coatings evolve as follows. First,
there is a high oxygen potential in the as-sprayed nanostructured coatings, due to the presence of oxygen in the starting powder (1.93 wt%, see Table 1), as well as oxygen entrainment during thermal spraying. Second, oxygen reacts with chromium to form CrO, CrO2,
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CrO3, Cr2O3, and Cr3O4, however, CrO, CrO2, CrO3 and Cr3O4 are metastable and/or present only under a high pressure [35]. Therefore during thermal exposure, Cr2O3 is directly formed from the following chemical reaction:
Cr2C3 + 92O2 Cr2O3 + 3CO2
(1)
The formation of the Cr2O3 phase is supported by the TEM analysis that reveals the presence of diffraction pattern from the hexagonal (a= 0.4954 nm, c=13.584)
Fig. 9. X-ray mapping of the as-sprayed conventional Cr3C2/NiCr coating. (a) SEM back scattered electron image; (b) Corresponding Cr element map; (c) Corresponding Ni element map; and (d) Corresponding O element map.
Fig. 11. X-ray oxygen element mapping result of the coatings exposed at 873 K. (a) SEM back scattered electron image of the conventional coating exposed at 873 K; (b) Corresponding O element map; (c) SEM back scattered electron image of the nanostructured coating exposed at 873 K; (d) Corresponding O element map.
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ternal oxidation was thought to be responsible for the precipitation of the dispersed oxide particles. 2. The phase transformations in the original NiCr amorphous phase were identified in the nanostructured Cr3C2/NiCr coatings exposed to high temperature. The amorphous structure has been crystallized and the deformation twins can be observed inside.
Fig. 12. Variation of microhardness of Cr3C2/NiCr coatings with heat treatment temperature.
Cr2O3 phase. The Cr2O3 phase, with the Gibbs free energy of formation of per mole volume of DG = 251.70 kilocalories [36] is relatively stable. Third, the nucleation and growth of Cr2O3 phase uniformly occurs throughout the coating because most of oxygen is primarily observed in nano-Cr3C2 particles, see Fig. 10 and Fig. 11, that are uniformly distributed in the coating. Therefore, this internal oxidation process leads to the formation of fine (8.3 nm) and dispersed Cr2O3 precipitates in the nanostructured Cr3C2 coatings. During thermal exposure, the crystallization of amorphous phase in the coatings occurs, see Fig. 8. Crystallization of amorphous phase usually causes a decrease in hardness [37], therefore, the change in microstructure of amorphous phase has a negative influence on hardness of the coatings. However, the decrease in hardness caused by crystallization of the amorphous phase is compensated by the increase originated from the precipitation of oxide particles. Accordingly, on the basis of the discussion above, the observed increase in microhardness of Cr3C2/NiCr coatings exposed to high temperatures is attributed to the precipitation of oxides. Particularly in the nanostructured coatings, the high density, nano sized oxide particles lead to significant increases in the microhardness and hence an increase in scratch-resistance.
4. Conclusions The precipitation in the nanostructured Cr3C2/NiCr coatings was investigated using SEM, TEM, microhardness and Micro-Tribometer tests. The main conclusions are briefly summarized as follows: 1. Ultrafine Cr2O3 particles with an average size of 8.3 nm were observed in the nanostructured Cr3C2/ NiCr coatings exposed to elevated temperature. In-
Fig. 13. Scratch-resistance of Cr3C2/NiCr coatings. (a) Scratch depths of the as-sprayed coatings; and (b) Influence of heat treatment temperature on scratch depth.
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Table 2 Coefficients of ffiction of the coatings As-sprayed conventional coating
As-sprayed nanostructured coating
Nanostructured coating treated at 673 K
Nanostructured coating treated at 873 K
0.495
0.216
0.205
0.183
Fig. 10. X-ray mapping of the as-sprayed nanostructured Cr3C2 – 25(Ni20Cr) coating. (a) SEM back scattered electron image; (b) Corresponding Cr element map; (c) Corresponding Ni element map; and (d) Corresponding O element map.
3. Microhardness of the conventional coating increased slightly with temperature, while that of the nanostructured coating drastically increased from 1020–1240 HV300 for the same temperature increases. 4. Compared with the conventional Cr3C2/NiCr coatings, the increased scratch resistance and reduced coefficient of friction were found in the nanostructured coatings. Heat treatment led to further increases in scratch-resistance and further decreases in the coefficient of friction for the nanostructured coatings.
Acknowledgements The authors would like to thank Michael Ice, a formerly graduate student who currently works at Orthodyne Electronics, Irvine, California, for his involving in partial work. The authors also gratefully acknowledge financial support provided by the Office of Naval Research (Grants No.: N00014-94-1-0017, N00014-971-0844, N00014-981-0569 and N00014-00-1-0109).
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