ELECTRODEPOSITION AND CHARACTERIZATION OF Ni-Co-P-BN DISPERSION COATINGS

Transcription

1 ELECTRODEPOSITION AND CHARACTERIZATION OF Ni-Co-P-BN DISPERSION COATINGS N.Furukawa, K. Uchiyama, I. Nakayama and T. Hayashi (Dept, of Applied Chem., Univ. of Osaka Prefecture, Sakai-shi, Osaka 593, Japan) 5 2&-@ ABSTRACT Ni-Co-P-BN dispersion coatings were electroplated from sulfamate bath using hexagonal BN particles as a dispersoid. The Ni-Co-P-BN dispersion coatings containing wt% Co, 6-8 wt% P and 3-4 wt% BN plated at 5-10 A/dm2 had amorphous structure, and they were found to have better lubricating and wear characteristics at high temperatures. INTRODUCTION Functional plating such as alloys or dispersion coatings have been widely used in metal finishing industry. In general, dispersion coatings are deposited from plating baths containing suspended particles such as metal oxides, carbides, metals or organic materials,(l-3) Codeposited particles in the dispersion coatings have the remarkable influence on the properties of the deposits depending on their type, size, contents and also mode of the distribution of the particles. Metal matrix also play an important role in the dispersion coatings. Therefore, it is very important to study the metal deposition parameters in the preparation of dispersion coatings and also to control the codeposition process of the particles in the metal matrix.(4-9) Ni-P alloy deposits containing PTFE particles are the typical dispersion coatings which have the nobel lubricating properties. ( ) Hardness of Ni-P-PTFE dispersion coating was found to increase upon the heat treatment of the coating at 400 C to give Hv. The Ni-P alloy deposits containing 7-10 wt% P have been found to have the amorphous structure and the hardness of the deposits were increased by the heat treatment. The hardness of Ni-Co-P alloy deposits was retained to considerablly greater level compared with Ni-P alloy coating. Ni-Co-P-Si3N4 dispersion coatings have been found to have the excellent wear properties.(l2) Hexagonal boron nitride(bn) is an inert and heat-resistant material and has the lubricating property when used at higher temperatures. In the present study, electroplating of Ni-Co-P-BN dispersion coatings has been carried out to provide the lubricating, wearand corrosion-resistant coatings having high hardness. 1

2 EXPERIMENTAL Electroplating bath used in this study is shown in Table 1. Fine powders of hexagonal BN (Products of Showa Denko Co. Ltd.) were used as a dispersoid. The well-washed particles were intoduced in the form of a slurry into the plating bath and kept in suspension by using an impeller agitation. To obtain the uniform codeposition of the particles, the special wall made of acrylic resin plate was used in the plating bath. Ni-S and Co-S alloy plates were used as a separate anode. Electrodeposition was carried out at 60 C and ph in 2000 ml cell. Incorporated particles in the dispersion coatings were determined by gravimetric analysis. The content of Co in the alloy deposits and also in the plating bath was analyzed by the atomic absorption spectroscopy. The content of P in the alloy coatings was analyzed by the conventional spectroscopic method using phosphorous-molybdenum complex solution. The cathode potential in the electrodeposition of Ni-Co-P alloy and Ni-Co-P-BN dispersion coatings was measured by the galvanostatic method at 60 C. The hardness of the deposited Ni-Co-P and Ni-Co-P-BN coatings was measured by the Vickers microhardness tester with a load of 100 and 200 g. The tribological characteristics of the electrodeposited coatings were evaluated by means of the coupled contact for the plated coatings. The electroplated steel plate with a constant load (1.0 kg) were placed on the rotating cylinder plated with Ni-Co-P or Ni-Co-P-BN and the wear resistance was measured by the weight loss of the plated article. The extent of damage of the deposits was cheked by the wear track. RESULTS AND DISCUSSION 1) Electrodeposition of Ni-Co-P and Ni-Co-P-BN coatings The contents of Ni, Co and P in the Ni-Co-P alloy coating are shown in Fig. 1 as a function of the cathode potential in the course of electrodeposition of alloys. The content of Co in the alloy deposits increased as the cathode potentials were shifted from -0.5 V to V (vs. Ag/AgCl electrode). The Co content in the deposit gave the maximum value( 17 wt%) at V, when the concentration of *. ~ Co2+ in tne bath is 20 gjdm3. nowever, Co content decreased with further increase of the cathode overpotential. These results can be explained by the fact that the mass transfer process in the electrodeposition of Co is in the more negative potential region than V. The Ni content in the alloy deposits seemed to decrease with an increase in the cathode potential and these phenomena seemed to be contrary to the characteristics of the Co deposition. The contnt of P in the alloy deposits decreased when the cathode potential was shifte from -0.5 to -1.0 V

3 The cathode current efficiency for the Ni-Co-P alloy deposition increased with an increase of the cathode overpotential shown in Fig. 2. ( Current efficiency ca. 30% at V and ca. 80% at -0.9 V) The effect of the cathode current density on the BN content in the Ni-Co-P-BN dispersion coatings are shown in Fig. 3. The less pronounced influence of the current density on the BN content in the dispersion coating was obtained in the current density range between 3.0 to 8.0 A/dm2. The content of BN in the Ni-Co-P-BN coatings was found to increase when the concentration of BN in the bath was increased. The distribution of the BN particles in the coatings is quite uniform throughout the deposits as shown in Fig. 4. 2) Properties of electrodeposited coatings The hardness of various electrodeposits, Ni-Co-P and Ni-Co-P-BN, as deposited and also after heat treatment ( 600 C, 1 hr in Ar atmosphere) is shown in Fig. 5. The hardness of the Ni-Co-P alloy deposits was found to increase after heat treatment. These results can be explained by the precipitation of finely dispersed nickel and cobalt phosphides. The hardness of the Ni-Co-BN and Ni-Co-P-BN dispersion coatings decreased slightly compared with those of each alloy matrix. These phenomena seemed to be attributable to the presence of softer BN particles in the deposits. Tribological tests were carried out for the various type of the coatings by using a wear test apparatus. In this test, electroplated cylindrical specimen were made contact against the loaded electrodeposited plate at 400 rpm. The Ni-Co-P-BN dispersion coatings were found to give the best results in the abrasion wear test as shown in Fig. 6. Comparing the data obtained from the different type of the electrodeposits, incorporation of P and BN particle in the deposits was found to be essential to improve the wear resistance of the dispersion coatings. The BN particles in the deposits seemed to act as a solid lubricant at high temperatures. CONCLUSION Ni-Co-P and Ni-Co-P-BN coatings were electrodeposited from svllfamate bath. Hexagonal EN particles was used as a dispersoid in the preparation of the Ni-Co-P-BN dispersion coatings. The Ni-Co-P-BN dispersion coatings containing wt% Co, 6-8 wt% P and 3-4 wt% BN had amorphous structure and found to have the excellent lubricating and wear characteristics at high temperatures. Incorporation of P and BN in the dispersion coatings was found to improve the wear resistance of the deposits. The BN particles in the deposits seemed to act as a solid lubricant at higher temperatures

4 REFERENCES 1) W. Metzger, P. Ott, Metalloberflache, 31, 494 (1977) 2) N. Feldstein, T, Lancsek, D. Lindsay, L. Salorno, Metal Finishing, 81 (8), 35 (1983) 3) M, Izzard, J, K. Dennis, Trans. Inst. Metal Finishing, 65, 85 (1987) 4) v. P. Greco, Plating and Surface Finishing, 76(10), 68 (1989) 5) J. P, Celis, J. R, ROOS, A. Piekarska, Trans. Inst. Metal Finishing, 68, 124 (1990) 6) K. Mertz, A. Gemmler, W. Metzger, Galvanotechnik, 83, 2295 (1992) 7) B. J. Hwang, C. S. Hwang, J. Electrochem SOC., 140, 979 (1993) 8) 0. Berkh, S. Eskin, J. Zahavi, Plating and Surface Finishing, 81 (3), 62 (1994) 9) P. R. Webb, N. L. Robertson, J. Electrochem. SOC., 141, 669 (1994) 10)s. S. Tulsi, Trans. Inst. Metal Finishing, 61, 147 (1983) 11)P. R. Ebdon, ibid., 65, 80 (1987) 12)N. Furukawa, S. Noya, T. Hayashi, Proc. 77th AESF Annual Technical Conference (1990)

5 Table 1 Bath composition Ni (NH,S03 1, 4H,O 350 gdf3 35,7 gd~n-~ 6 5 sd~n-~) H3p03 NiBr, H3BO3 EN particle 0-20 gdm-3 5 gdm- 30 gdm gdm- 100pm Fig. 4 Photomicrograph of the surface of Ni-Co-P-BN dispersion coating

6 Ni 70 co t *O 15 A P 10 5 A 0 I I I I I U ,8 019 Potential -E / V vsl Ag/AgCI Figl 1 Relationship between cathode potential and Ni, Co and P content in deposit

7 loo I I- F I I 1 I I I Potential -E / V vs, Ag/AgCl Relationship between cathode potential and cathode current efficiency

8 2, gdm U n O,! 3OgdmIi 20gdm I I I 1 I 1 I I I 3 2 E 10-2 Current density i Adm Fig13 Effect of current density on BN content in the Ni-Co-BN coatings

9 Ni -Co HT Ni-Co-P HT Ni-Co-BN Ni -Co-P-BN I HT I I I a I Hardness (Hv) Fig. 5 Hardness of electrodeposits HT;Heat treated at 600 C(lh, Ar)

10 Ni-Co Ni-Co-P Ni-Co-BN HT Ni-Co-P-BN Fig.6 I I Wear loss (mg/mm 2 ) Results of wear testing for electrodeposits HT;Heat treated at 400 C(lh,Ar)