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Improvement in the properties of plasma-sprayed chromium carbide coatings using nickel-clad powders
Surface and Coatings Technology 130 Ž2000. 15᎐19
Improvement in the properties of plasma-sprayed chromium carbide coatings using nickel-clad powders Jianfeng LiU , Chuanxian Ding Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China
Abstract In this paper, electroless plating was used to prepare Ni-clad Cr3 C2 powders with different nickel contents, and plasma spraying was carried out to deposit the corresponding coatings. The deposited coatings were then compared with coatings sprayed using mechanically blended Cr3 C2 ᎐NiCr powders in terms of microstructure, phase composition, mechanical and tribological properties. The results obtained indicated that the elemental nickel in the coatings using the Ni-clad powders was better distributed than that using the mechanically blended Cr3 C2 ᎐NiCr powders; and the free carbon content was also lower in the former coatings than in the latter coatings. Comparing properties of the different coatings with each other, it was found that the Ni-clad Cr3 C2 powders obviously improved not only the elastic modulus, fracture toughness and wear resistance of chromium carbide coatings, but also the Weibull modulus of the microhardness and elastic modulus. The above results may be ascribed to an increase in the real contact and in the bonding strength between the lamellae of the coatings. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Plasma spraying; Chromium carbide; Coating; Mechanical property; Wear resistance
1. Introduction Thermal sprayed chromium carbide coatings are widely used in high temperature wear-resistant and corrosion-resistant applications in aerospace, power engineering, steel and automation industries w1᎐3x. In order to offset the brittleness of chromium carbide, a metal binder phase of Ž75᎐80 wt.%. Ni᎐ Ž20᎐25 wt.%. Cr is usually used in the starting powders of the coatings. The reason why the NiCr alloy is used lies in that the coatings are developed from sintered Cr3 C2 ᎐NiCr, in which pure nickel is not in thermodynamic equilibrium with Cr3 C2 and chromium, or its carbides will be dissolved into the binders during sintering w4x. However, whatever the effects of pure nickel used in starting powders on the properties of chromium carbide coatings, there is no report on it.
U
Corresponding author. Fax: q86-21-62513093. E-mail address: [email protected] ŽJ. Li..
Although clad chromium carbide powders are commercially available, for example Sulzer Metco Diamalloy 3007, the NiCr alloy is always used as cladding layer in the powders. Here, electroless plating was used, choosing hydrazine hydrate as a reductant to successfully prepare pure Ni-clad Cr3 C2 powders with different nickel contents. In the present paper, an improvement in the mechanical and tribological properties of plasma-sprayed chromium carbide coatings using the Ni-clad powders was reported.
2. Experimental details Different nickel contents of the Ni-clad Cr3 C2 powders were prepared by changing the chemical composition of the electroless plating solution. The commercial Sulzer Metco 82VF-NS powder, with 7 wt.% NiCr alloy, was chosen as the primary Cr3 C2 powder. Table 1 lists the chemical compositions of electroless Ni plating solutions and their operating
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 7 8 - 2
J. Li, C. Ding r Surface and Coatings Technology 130 (2000) 15᎐19
16
Table 1 The chemical compositions of electroless Ni plating solutions for cladding Cr3 C 2 powders and corresponding operating conditions Powder code
CRC15NI
CRC20NI
CRC25NI
NiSO4 ⭈ 6H 2 O Žg. N2 H 4 ⭈ H 2 O Žml. C 4 H 4 C 6 Na 2 ⭈ 2H 2 O Žg. C 10 H 14 N2 Na 2 O 8 ⭈ 2H 2 O Žg. PbAC Žmg. pH value a Temperature Ž⬚C. Volume of solution Žl. Weighty of primary powder Žg. Plating time Žmin.
40 260 40 20 1.2 13᎐13.5 92᎐95 2 50 60
60 390 60 30 1.8 13᎐13.5 92᎐95 2 50 60
80 520 80 40 2.4 13᎐13.5 92᎐95 2 50 60
a
Fig. 1. SEM micrograph of cross-section of CRC20NI powder.
Adjusting the pH value of the solutions with NaOH.
conditions. In Table 1, the numbers in powder codes stand for nickel content by weight, including the nickel from the NiCr alloy of the primary powder. Choosing hydrazine hydrate as the reductant, the plated layer consisted of pure Ni. Plasma spraying was carried out using an atmospheric plasma spraying system consisting of a Sulzer Metco F4-MB plasma spray gun mounted on an ABB S3 robot. Coating samples, with thicknesses of approximately 1 mm, were applied onto 1Cr18Ni9Ti stainless steel substrates, forming the specific dimensions for mechanical and tribological properties testing. The plasma spraying parameters are listed in Table 2. Except for containing a 25-wt.% NiCr alloy, the other aspects of the commercial Sulzer Metco 81VF-NS powder are almost the same as 82VF-NS. The porosities of the coatings were measured by the microscopic count method with a magnification of 1000. Their microhardness and elastic modulus were measured using Knoop indentation techniques w5x. Their fracture toughness was determined using the Vickers indentation technique w6x. An Akashi Avk-A indenter ŽAkashi Seisakusho Ltd., Japan. was used to measure the indentation parameters. The applied indenter load was 294 N, and the specimen dimensions were 20 = 10 = 3 mm, with a coating thickness of approximately 1 mm. The indentation parameters of radial crack length, 2c, and impression diagonal, 2a, were measured by optical microscopy immediately after releasing the load.
Their tribological properties were tested with a blockon-ring arrangement and operating conditions of dry friction, a load of 200 N, and a sliding velocity of 0.42 mrs. The experimental procedures and statistical analyses on the porosity, mechanical and tribological properties have been described in detail elsewhere w5᎐8x. The morphologies of the powders, coatings and worn tracks were observed employing an EPMA-8705QHII scanning electron microscope ŽSEM.. The phases of the Ni-clad Cr3 C2 powders and the as-sprayed coatings were determined by JAPAN-RICOH X-ray diffraction ŽXRD..
Fig. 2. XRD spectrum of CRC20NI powder.
Table 2 The plasma spraying parameters Powder
Current ŽA.
Voltage ŽV.
Ar ŽSPLM.
H2 ŽSPLM.
Powder feed rate Žgrmin.
Spraying distance Žmm.
Gun translation speed Žmrmin.
82VF-NS 81VF-NS CRC15NI CRC20NI CRC25NI
600 600 600 600 600
58 50 50 50 50
55 65 65 65 65
12 3 3 3 3
40 40 40 40 40
130 130 130 130 130
75 75 75 75 75
J. Li, C. Ding r Surface and Coatings Technology 130 (2000) 15᎐19
17
Fig. 3. SEM micrographs of cross-sections of Ža. 81VF-NS and Žb. CRC25NI coatings.
3. Results and discussion 3.1. Characterization of Ni-clad Cr3 C2 powders As an example of the Ni-clad Cr3 C2 powders, Fig. 1 and Fig. 2 show SEM micrographs of a cross-section of the CRC20NI powder, and an XRD spectrum of the powder. The SEM micrograph shows that Ni was plated onto the surfaces of Cr3 C2 particles as a dense clad layer, uniform in thickness. XRD results reveal that the electroless plated Ni layer consisted of face-centeredcubic tiny grains. Fig. 4. XRD spectra of Ža. 81VF-NS and Žb. CRC20NI coatings.
3.2. Microstructures of coatings Fig. 3 shows the SEM micrographs of cross-sections of coatings sprayed using 81VF-NS and CRC25NI powders. Table 3 gives the results of statistical analyses of porosity data sets, with a reading of 20, for coatings sprayed using different powders. Fig. 3 and Table 3 indicate that the metal binders were better distributed and porosities were obviously lower for the coatings sprayed using Ni-clad Cr3 C2 powders than those using mechanically blended Cr3 C2 ᎐NiCr powders. Comparing the XRD spectrum of the CRC20NI with that of the 81VF-NS coating ŽFig. 4., it can be seen that the diffraction peaks were widened for both of the two kinds of coatings; however, the Ni-clad Cr3 C2 powders apparently reduced the content of free carbon in coatTable 3 The means, their 90% confidence intervals and Weibull parameters of porosity data sets for the different coatings Sample code
Mean
Var. Coe.
y 90
q 90
x0
m
82VF-NS 81VF-NS CRC15NI CRC20NI CRC25NI
5.4 4.9 2.8 2.9 3.8
0.23 0.19 0.21 0.24 0.26
4.8 4.4 2.6 2.6 3.4
5.9 5.3 3.0 3.2 4.2
5.8 5.2 2.9 3.2 4.0
4.2 5.4 6.2 4.1 4.6
ing. In addition, it can also be seen that the Ni content in the CRC20NI coating was slightly lower than that in the 81VF-NS coating. 3.3. Mechanical properties of coatings Tables 4᎐6 list the results of statistical analyses of Knoop hardness, elastic modulus and fracture toughness for different coatings. For the Knoop hardness and elastic modulus, 20 indentations were measured, and for the fracture toughness, 10 indentations were measured for every specimen, respectively. Although the Ni-clad Cr3 C2 powders had no significant effect on the means of the microhardness of the coatings, they improved not only the elastic modulus and fracture toughness of chromium carbide coatings, but also the Weibull modulus of the microhardness and elastic modulus. As for the decrease in the Weibull modulus of fracture toughness of coatings sprayed using the Ni-clad Cr3 C2 powders, compared with those using the mechanically blended Cr3 C2-NiCr powders, it may be related to wider confidence intervals of the Weibull modulus, as shown in Table 6. The wider confidence intervals resulted from the fact that the measurement reading of 10 is too small w8x.
J. Li, C. Ding r Surface and Coatings Technology 130 (2000) 15᎐19
18
Table 4 The means, their 90% confidence intervals and Weibull modulus of Knoop hardness data sets for the different coatings Code
Testing load Mean Var. Coe. y 90 q 90 m Žgf.
82VF-NS
1000 300 81VF-NS 1000 CRC15NI 300 CRC20NI 300 CRC25NI 300
643.7 711.8 587.2 742.2 731.4 676.7
0.04 0.09 0.06 0.11 0.09 0.08
633.6 686.5 574.4 711.3 712.7 657.6
651.1 732.3 598.8 765.0 753.5 694.5
27.9 12.1 19.9 12.3 16.0 18.2
Table 6 The means, their 90% confidence intervals and Weibull modulus of fracture toughness data sets measured at the load of 294 N for the different coatings Code
Mean Var. Coe. y 90 q 90 my 90 m
mq 90
82VF-NS 81VF-NS CRC15NI CRC20NI CRC25NI
6.1 6.2 8.4 8.7 9.8
29.2 43.7 20.4 21.3 16.1
0.06 0.04 0.08 0.07 0.09
5.9 6.1 7.7 8.1 9.0
6.3 6.4 8.9 9.3 10.6
11.8 17.6 8.2 8.6 6.5
21.5 32.2 15.0 15.7 11.8
3.4. Tribological properties Fig. 5 and Fig. 6 give the friction and wear coefficients of different coatings mated to the 82VF-NS coating as sliding pairs at room temperature. From the figures, it can be seen that the friction coefficients of the coatings sprayed using Ni-clad Cr3 C2 powders mated to the 82VF-NS coating were slightly higher than those using mechanically blended Cr3 C2 ᎐NiCr powders mated to the 82VF-NS coating. However, the coatings sprayed using the three kinds of Ni-clad Cr3 C2 powders had far lower wear coefficients than those using the mechanically blended Cr3 C2-NiCr powders. The SEM micrographs of the worn tracks of the 81VFNS and CRC20NI coatings, mated to the 82VF-NS coating, show that their wear mechanisms are both interlamellar cracking and the flaking-off of lamellae ŽFig. 7., which results from intense adhesion between the contact surface and the adhesive force acting as strain᎐stress, vertical to the interlamellae w8x. Therefore, the wear coefficients of different friction pairs demonstrated that the bonding strength between the lamellae of coatings sprayed using the three kinds of Ni-clad Cr3 C2 powders was higher than those sprayed using the mechanically blended Cr3 C2 ᎐NiCr powders.
Fig. 5. Friction coefficients of different coatings mated to 82VF-NS coating as sliding pairs.
3.5. Discussion The above results displayed that the pure Ni-clad Cr3 C2 powders had notably repressed the decarbonization and improved the mechanical properties and wear Table 5 The means, their 90% confidence intervals and Weibull modulus of elastic modulus data sets for the different coatings Code
82VF-NS
Testing load Žgf.
1000 300 81VF-NS 1000 CRC15NI 300 CRC20NI 300 CRC25NI 300
Mean
Var. Coe.
y 90
q 90
m
85.8 120.0 106.7 169.6 273.3 266.1
0.22 0.29 0.38 0.35 0.26 0.20
78.4 102.9 88.6 140.1 238.7 226.8
90.6 137.0 114.8 199.0 307.9 305.4
8.7 3.3 4.5 4.2 5.0 5.8
Fig. 6. Wear coefficients of different coatings mated to 82VF-NS coating as sliding pairs.
resistance of the chromium carbide coating compared with mechanically blended Cr3 C2 ᎐NiCr powders. The former was related to the fact that the Ni-clad Cr3 C2 powders increased in the contact between Ni and Cr3 C2 . Because nickel has lower solidifying point than chromium carbides, the molten nickel covering the would-be solidifying chromium carbides reduced the cooling and solidifying velocity of the carbides, which resulted in a decrease in the decarbonization of the
J. Li, C. Ding r Surface and Coatings Technology 130 (2000) 15᎐19
19
Fig. 7. SEM micrographs of worn tracks: Ža. 81VF-NS mated to Žb. 82VF-NS and Žc. CRC20NI mated to Žd. 82VF-NS coatings.
carbides. The latter resulted from increase of the real contact and the bonding strength between lamellae of the coatings. It reveals that the dissolution of nickel to chromium or its carbides had taken place before the solidifying of the coatings, which had no apparently destructive effect on the microstructures and properties of the coatings.
4. Conclusion Electroless plating was used to successfully prepare pure Ni-clad Cr3 C2 powders with different nickel contents, and plasma spraying was carried out to deposit the corresponding coatings. It was found that elemental nickel in the coatings using the Ni-clad powders was better distributed than in those using the mechanically blended Cr3 C2 ᎐NiCr powders, and the free carbon content was also lower in the former coatings than in the latter coatings. The mechanical properties and wear-resistant performance of the plasma-sprayed chromium carbide coatings using the Ni-clad Cr3 C2 were outstandingly improved compared with the coat-
ings using the mechanically blended Cr3 C2 ᎐NiCr powders. The results obtained may be ascribed to an increase in the real contact and in the bonding strength between the lamellae of the coatings. References w1x G. Barbezat, A.R. Nicoll, A. Sickinger, Wear 529 Ž1993. 162᎐164. w2x J. Takeuchi, Y. Murata, Y. Harada, T. Tomita, S. Nakahama, T. Go, Thermal Spray: Meeting the Challenges of the 21st Century, in: C. Coddet ŽEd.., ASM international, Materials Park, OH 44073-002, USA, 1998, p. 1425. w3x D.-Y. Kim, M.-S. Han, J.-G. Youn, Thermal Spray: Practical Solutions for Engineering Problems, in: C.C. Berndt ŽEd.., ASM international, Materials Park, Ohio, USA, 1996, p. 123. w4x L.-M. Berger, W. Hermel, P. Vuoristo, T. Mantyla, ¨ ¨ W. Lengauer, P. Ettmayer, ibid, p. 89. w5x S.H. Leigh, C.K. Lin, C.C. Berndt, J. Am. Ceram. Soc. 80 Ž8. Ž1997. 2093. w6x G.K. Beshish, C.W. Florey, F.J. Worzala, J. Therm. Spray Technol. 2 Ž1. Ž1993. 36. w7x J.F. Li, C.X. Ding, J.Q. Huang, P.Y. Zhang, Wear 211 Ž1997. 177. w8x J.F. Li. Dissertation for Ph. D. of Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1999.
Improvement in the properties of plasma-sprayed chromium carbide coatings using nickel-clad powders Jianfeng LiU , Chuanxian Ding Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China
Abstract In this paper, electroless plating was used to prepare Ni-clad Cr3 C2 powders with different nickel contents, and plasma spraying was carried out to deposit the corresponding coatings. The deposited coatings were then compared with coatings sprayed using mechanically blended Cr3 C2 ᎐NiCr powders in terms of microstructure, phase composition, mechanical and tribological properties. The results obtained indicated that the elemental nickel in the coatings using the Ni-clad powders was better distributed than that using the mechanically blended Cr3 C2 ᎐NiCr powders; and the free carbon content was also lower in the former coatings than in the latter coatings. Comparing properties of the different coatings with each other, it was found that the Ni-clad Cr3 C2 powders obviously improved not only the elastic modulus, fracture toughness and wear resistance of chromium carbide coatings, but also the Weibull modulus of the microhardness and elastic modulus. The above results may be ascribed to an increase in the real contact and in the bonding strength between the lamellae of the coatings. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Plasma spraying; Chromium carbide; Coating; Mechanical property; Wear resistance
1. Introduction Thermal sprayed chromium carbide coatings are widely used in high temperature wear-resistant and corrosion-resistant applications in aerospace, power engineering, steel and automation industries w1᎐3x. In order to offset the brittleness of chromium carbide, a metal binder phase of Ž75᎐80 wt.%. Ni᎐ Ž20᎐25 wt.%. Cr is usually used in the starting powders of the coatings. The reason why the NiCr alloy is used lies in that the coatings are developed from sintered Cr3 C2 ᎐NiCr, in which pure nickel is not in thermodynamic equilibrium with Cr3 C2 and chromium, or its carbides will be dissolved into the binders during sintering w4x. However, whatever the effects of pure nickel used in starting powders on the properties of chromium carbide coatings, there is no report on it.
U
Corresponding author. Fax: q86-21-62513093. E-mail address: [email protected] ŽJ. Li..
Although clad chromium carbide powders are commercially available, for example Sulzer Metco Diamalloy 3007, the NiCr alloy is always used as cladding layer in the powders. Here, electroless plating was used, choosing hydrazine hydrate as a reductant to successfully prepare pure Ni-clad Cr3 C2 powders with different nickel contents. In the present paper, an improvement in the mechanical and tribological properties of plasma-sprayed chromium carbide coatings using the Ni-clad powders was reported.
2. Experimental details Different nickel contents of the Ni-clad Cr3 C2 powders were prepared by changing the chemical composition of the electroless plating solution. The commercial Sulzer Metco 82VF-NS powder, with 7 wt.% NiCr alloy, was chosen as the primary Cr3 C2 powder. Table 1 lists the chemical compositions of electroless Ni plating solutions and their operating
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 6 7 8 - 2
J. Li, C. Ding r Surface and Coatings Technology 130 (2000) 15᎐19
16
Table 1 The chemical compositions of electroless Ni plating solutions for cladding Cr3 C 2 powders and corresponding operating conditions Powder code
CRC15NI
CRC20NI
CRC25NI
NiSO4 ⭈ 6H 2 O Žg. N2 H 4 ⭈ H 2 O Žml. C 4 H 4 C 6 Na 2 ⭈ 2H 2 O Žg. C 10 H 14 N2 Na 2 O 8 ⭈ 2H 2 O Žg. PbAC Žmg. pH value a Temperature Ž⬚C. Volume of solution Žl. Weighty of primary powder Žg. Plating time Žmin.
40 260 40 20 1.2 13᎐13.5 92᎐95 2 50 60
60 390 60 30 1.8 13᎐13.5 92᎐95 2 50 60
80 520 80 40 2.4 13᎐13.5 92᎐95 2 50 60
a
Fig. 1. SEM micrograph of cross-section of CRC20NI powder.
Adjusting the pH value of the solutions with NaOH.
conditions. In Table 1, the numbers in powder codes stand for nickel content by weight, including the nickel from the NiCr alloy of the primary powder. Choosing hydrazine hydrate as the reductant, the plated layer consisted of pure Ni. Plasma spraying was carried out using an atmospheric plasma spraying system consisting of a Sulzer Metco F4-MB plasma spray gun mounted on an ABB S3 robot. Coating samples, with thicknesses of approximately 1 mm, were applied onto 1Cr18Ni9Ti stainless steel substrates, forming the specific dimensions for mechanical and tribological properties testing. The plasma spraying parameters are listed in Table 2. Except for containing a 25-wt.% NiCr alloy, the other aspects of the commercial Sulzer Metco 81VF-NS powder are almost the same as 82VF-NS. The porosities of the coatings were measured by the microscopic count method with a magnification of 1000. Their microhardness and elastic modulus were measured using Knoop indentation techniques w5x. Their fracture toughness was determined using the Vickers indentation technique w6x. An Akashi Avk-A indenter ŽAkashi Seisakusho Ltd., Japan. was used to measure the indentation parameters. The applied indenter load was 294 N, and the specimen dimensions were 20 = 10 = 3 mm, with a coating thickness of approximately 1 mm. The indentation parameters of radial crack length, 2c, and impression diagonal, 2a, were measured by optical microscopy immediately after releasing the load.
Their tribological properties were tested with a blockon-ring arrangement and operating conditions of dry friction, a load of 200 N, and a sliding velocity of 0.42 mrs. The experimental procedures and statistical analyses on the porosity, mechanical and tribological properties have been described in detail elsewhere w5᎐8x. The morphologies of the powders, coatings and worn tracks were observed employing an EPMA-8705QHII scanning electron microscope ŽSEM.. The phases of the Ni-clad Cr3 C2 powders and the as-sprayed coatings were determined by JAPAN-RICOH X-ray diffraction ŽXRD..
Fig. 2. XRD spectrum of CRC20NI powder.
Table 2 The plasma spraying parameters Powder
Current ŽA.
Voltage ŽV.
Ar ŽSPLM.
H2 ŽSPLM.
Powder feed rate Žgrmin.
Spraying distance Žmm.
Gun translation speed Žmrmin.
82VF-NS 81VF-NS CRC15NI CRC20NI CRC25NI
600 600 600 600 600
58 50 50 50 50
55 65 65 65 65
12 3 3 3 3
40 40 40 40 40
130 130 130 130 130
75 75 75 75 75
J. Li, C. Ding r Surface and Coatings Technology 130 (2000) 15᎐19
17
Fig. 3. SEM micrographs of cross-sections of Ža. 81VF-NS and Žb. CRC25NI coatings.
3. Results and discussion 3.1. Characterization of Ni-clad Cr3 C2 powders As an example of the Ni-clad Cr3 C2 powders, Fig. 1 and Fig. 2 show SEM micrographs of a cross-section of the CRC20NI powder, and an XRD spectrum of the powder. The SEM micrograph shows that Ni was plated onto the surfaces of Cr3 C2 particles as a dense clad layer, uniform in thickness. XRD results reveal that the electroless plated Ni layer consisted of face-centeredcubic tiny grains. Fig. 4. XRD spectra of Ža. 81VF-NS and Žb. CRC20NI coatings.
3.2. Microstructures of coatings Fig. 3 shows the SEM micrographs of cross-sections of coatings sprayed using 81VF-NS and CRC25NI powders. Table 3 gives the results of statistical analyses of porosity data sets, with a reading of 20, for coatings sprayed using different powders. Fig. 3 and Table 3 indicate that the metal binders were better distributed and porosities were obviously lower for the coatings sprayed using Ni-clad Cr3 C2 powders than those using mechanically blended Cr3 C2 ᎐NiCr powders. Comparing the XRD spectrum of the CRC20NI with that of the 81VF-NS coating ŽFig. 4., it can be seen that the diffraction peaks were widened for both of the two kinds of coatings; however, the Ni-clad Cr3 C2 powders apparently reduced the content of free carbon in coatTable 3 The means, their 90% confidence intervals and Weibull parameters of porosity data sets for the different coatings Sample code
Mean
Var. Coe.
y 90
q 90
x0
m
82VF-NS 81VF-NS CRC15NI CRC20NI CRC25NI
5.4 4.9 2.8 2.9 3.8
0.23 0.19 0.21 0.24 0.26
4.8 4.4 2.6 2.6 3.4
5.9 5.3 3.0 3.2 4.2
5.8 5.2 2.9 3.2 4.0
4.2 5.4 6.2 4.1 4.6
ing. In addition, it can also be seen that the Ni content in the CRC20NI coating was slightly lower than that in the 81VF-NS coating. 3.3. Mechanical properties of coatings Tables 4᎐6 list the results of statistical analyses of Knoop hardness, elastic modulus and fracture toughness for different coatings. For the Knoop hardness and elastic modulus, 20 indentations were measured, and for the fracture toughness, 10 indentations were measured for every specimen, respectively. Although the Ni-clad Cr3 C2 powders had no significant effect on the means of the microhardness of the coatings, they improved not only the elastic modulus and fracture toughness of chromium carbide coatings, but also the Weibull modulus of the microhardness and elastic modulus. As for the decrease in the Weibull modulus of fracture toughness of coatings sprayed using the Ni-clad Cr3 C2 powders, compared with those using the mechanically blended Cr3 C2-NiCr powders, it may be related to wider confidence intervals of the Weibull modulus, as shown in Table 6. The wider confidence intervals resulted from the fact that the measurement reading of 10 is too small w8x.
J. Li, C. Ding r Surface and Coatings Technology 130 (2000) 15᎐19
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Table 4 The means, their 90% confidence intervals and Weibull modulus of Knoop hardness data sets for the different coatings Code
Testing load Mean Var. Coe. y 90 q 90 m Žgf.
82VF-NS
1000 300 81VF-NS 1000 CRC15NI 300 CRC20NI 300 CRC25NI 300
643.7 711.8 587.2 742.2 731.4 676.7
0.04 0.09 0.06 0.11 0.09 0.08
633.6 686.5 574.4 711.3 712.7 657.6
651.1 732.3 598.8 765.0 753.5 694.5
27.9 12.1 19.9 12.3 16.0 18.2
Table 6 The means, their 90% confidence intervals and Weibull modulus of fracture toughness data sets measured at the load of 294 N for the different coatings Code
Mean Var. Coe. y 90 q 90 my 90 m
mq 90
82VF-NS 81VF-NS CRC15NI CRC20NI CRC25NI
6.1 6.2 8.4 8.7 9.8
29.2 43.7 20.4 21.3 16.1
0.06 0.04 0.08 0.07 0.09
5.9 6.1 7.7 8.1 9.0
6.3 6.4 8.9 9.3 10.6
11.8 17.6 8.2 8.6 6.5
21.5 32.2 15.0 15.7 11.8
3.4. Tribological properties Fig. 5 and Fig. 6 give the friction and wear coefficients of different coatings mated to the 82VF-NS coating as sliding pairs at room temperature. From the figures, it can be seen that the friction coefficients of the coatings sprayed using Ni-clad Cr3 C2 powders mated to the 82VF-NS coating were slightly higher than those using mechanically blended Cr3 C2 ᎐NiCr powders mated to the 82VF-NS coating. However, the coatings sprayed using the three kinds of Ni-clad Cr3 C2 powders had far lower wear coefficients than those using the mechanically blended Cr3 C2-NiCr powders. The SEM micrographs of the worn tracks of the 81VFNS and CRC20NI coatings, mated to the 82VF-NS coating, show that their wear mechanisms are both interlamellar cracking and the flaking-off of lamellae ŽFig. 7., which results from intense adhesion between the contact surface and the adhesive force acting as strain᎐stress, vertical to the interlamellae w8x. Therefore, the wear coefficients of different friction pairs demonstrated that the bonding strength between the lamellae of coatings sprayed using the three kinds of Ni-clad Cr3 C2 powders was higher than those sprayed using the mechanically blended Cr3 C2 ᎐NiCr powders.
Fig. 5. Friction coefficients of different coatings mated to 82VF-NS coating as sliding pairs.
3.5. Discussion The above results displayed that the pure Ni-clad Cr3 C2 powders had notably repressed the decarbonization and improved the mechanical properties and wear Table 5 The means, their 90% confidence intervals and Weibull modulus of elastic modulus data sets for the different coatings Code
82VF-NS
Testing load Žgf.
1000 300 81VF-NS 1000 CRC15NI 300 CRC20NI 300 CRC25NI 300
Mean
Var. Coe.
y 90
q 90
m
85.8 120.0 106.7 169.6 273.3 266.1
0.22 0.29 0.38 0.35 0.26 0.20
78.4 102.9 88.6 140.1 238.7 226.8
90.6 137.0 114.8 199.0 307.9 305.4
8.7 3.3 4.5 4.2 5.0 5.8
Fig. 6. Wear coefficients of different coatings mated to 82VF-NS coating as sliding pairs.
resistance of the chromium carbide coating compared with mechanically blended Cr3 C2 ᎐NiCr powders. The former was related to the fact that the Ni-clad Cr3 C2 powders increased in the contact between Ni and Cr3 C2 . Because nickel has lower solidifying point than chromium carbides, the molten nickel covering the would-be solidifying chromium carbides reduced the cooling and solidifying velocity of the carbides, which resulted in a decrease in the decarbonization of the
J. Li, C. Ding r Surface and Coatings Technology 130 (2000) 15᎐19
19
Fig. 7. SEM micrographs of worn tracks: Ža. 81VF-NS mated to Žb. 82VF-NS and Žc. CRC20NI mated to Žd. 82VF-NS coatings.
carbides. The latter resulted from increase of the real contact and the bonding strength between lamellae of the coatings. It reveals that the dissolution of nickel to chromium or its carbides had taken place before the solidifying of the coatings, which had no apparently destructive effect on the microstructures and properties of the coatings.
4. Conclusion Electroless plating was used to successfully prepare pure Ni-clad Cr3 C2 powders with different nickel contents, and plasma spraying was carried out to deposit the corresponding coatings. It was found that elemental nickel in the coatings using the Ni-clad powders was better distributed than in those using the mechanically blended Cr3 C2 ᎐NiCr powders, and the free carbon content was also lower in the former coatings than in the latter coatings. The mechanical properties and wear-resistant performance of the plasma-sprayed chromium carbide coatings using the Ni-clad Cr3 C2 were outstandingly improved compared with the coat-
ings using the mechanically blended Cr3 C2 ᎐NiCr powders. The results obtained may be ascribed to an increase in the real contact and in the bonding strength between the lamellae of the coatings. References w1x G. Barbezat, A.R. Nicoll, A. Sickinger, Wear 529 Ž1993. 162᎐164. w2x J. Takeuchi, Y. Murata, Y. Harada, T. Tomita, S. Nakahama, T. Go, Thermal Spray: Meeting the Challenges of the 21st Century, in: C. Coddet ŽEd.., ASM international, Materials Park, OH 44073-002, USA, 1998, p. 1425. w3x D.-Y. Kim, M.-S. Han, J.-G. Youn, Thermal Spray: Practical Solutions for Engineering Problems, in: C.C. Berndt ŽEd.., ASM international, Materials Park, Ohio, USA, 1996, p. 123. w4x L.-M. Berger, W. Hermel, P. Vuoristo, T. Mantyla, ¨ ¨ W. Lengauer, P. Ettmayer, ibid, p. 89. w5x S.H. Leigh, C.K. Lin, C.C. Berndt, J. Am. Ceram. Soc. 80 Ž8. Ž1997. 2093. w6x G.K. Beshish, C.W. Florey, F.J. Worzala, J. Therm. Spray Technol. 2 Ž1. Ž1993. 36. w7x J.F. Li, C.X. Ding, J.Q. Huang, P.Y. Zhang, Wear 211 Ž1997. 177. w8x J.F. Li. Dissertation for Ph. D. of Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1999.