Electroplated composite coatings Ni-B
Benigna Szeptycka Instytut Mechaniki Precyzyjnej, ul. Duchnicka 3, 01-796 Warszawa
Annals 2 No. 4, 2002 pages 248-252
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abstract Electroplating of Ni-B coatings may be an alternative to conventional technology of chromium plating based on the use of concentrated solutions of high toxic chromic acid. The processes of electrodeposition of Ni-B coatings with the low content of nickel ion bath and the amorphous boron have been developed. The main components of the bath are nickel sulphate 130 g/dm3, nickel chloride 70 g/dm3, boric acid 45 g/dm3 and the saccharin 3 g/dm3 or WRN 20 ml/dm3. The electrodeposition was carried out with mechanical stirring at ik = 4 A/dm2, t = 318 K, pH = 3.8. Six surface - active compounds were used to preparation of the boron disperse in the nickel bath. The boron content in the coatings was examined gravimetrically. Brightness was measured with a Johansonn 8510-1 glossmeter. IS-meter was used to the measurement of the internal stress. The microhardness of the deposited layers was measured using Vickers’ method and a Hanemann microhardness tester at a load of 0.01 and 0.05 kg. In order to determine wear resistance the nickel foils was tested using a technique based on a measuring system comprising a flat surface and ball. The layers were subjected to wear by dry slide friction using a 30 mm diameter ball at an angle of 35o, duration of friction 5000 turns. On the basis of the wear traces and measurement of their diameter, the depth of the wear was calculated, which was a measure of wear resistance. The boron content in the composite coatings Ni-B plating in the presence of the saccharin was from 1 to 9.97% at. (Tab. 1) while in the presence WRN it was from 3.89 to 12.22% at. (Tab. 2). Brightness those coatings was from 18 to 53% (Tabs 1, 2). The boron content in the coating change with the boron content varies in the bath (Fig. 1) as much as 30% at. The internal stress of the composite coatings Ni-B were compressive from 40 to 80 MPa for the thickness as much as 30 μm (Fig. 2). The least internal stress has coatings plating in the presence of saccharin and a cationic high-fluoride surfactant. The variation in the microhardness of Ni-B deposits plated with the investigated surface - active compounds is shown in Tables 3 and 4. Under the conditions used in the present investigation the microhardness of the deposit is in the range of 256÷527 μHV0.01. The microhardness of nickel coating is 358 μHV0.01 for deposit plated in present of the saccharin and 346 μHV0.01 for deposit plated in present of WRN. It can be seen from Tables 1 and 2 that the change of the brightener and the surface-active compound has an influence on the boron contents and from Tables 3 and 4 that has an influence on the microhardness. However, the highest microhardness is not related with the highest boron contents. Probably, superimposing the effects for the strengthening from the fine crystal grains onto that from the boron disperse and the high density dislocations can describe the variation of the microhardness of Ni-B deposit with the change of the bath composition. The results obtained from wear tests are summarized in Tables 3 and 4. Data regarding the nickel deposits are also included for comparison. A relative increase of 69% for the sliding wear resistance of the composite coating plated with saccharin and WFK in the bath compared with the nickel coating have been determined (Tab. 3, sample 1). The wear behaviour of the specimens plated with WRN was differentiating. The sample 6 (Tab. 4) has the least wear resistance and sample 1 has a relative 44% greater wear resistance compared with the nickel coating plated with WRN. But the sample 1 plated with saccharin and WFK has a relative increase of 66% for the wear test compared with the nickel plated with WRN and was the best specimen from all examined.