The Wear Resistance of Cr-C-Al2O3 Composite Deposits Prepared on a Cu Substrate using Cr3+-based Plating Baths

Cr-C-Al 2 O 3 deposits with different Al 2 O 3 concentrations were successfully prepared on a Cu substrate using Cr 3+ -based electroplating baths. The microstructures of the Cr-C-Al 2 O 3 deposits were examined using optical, scanning and transmission electron microscopes. The hardness values, the corrosion and wear resistance of the Cr-C and Cr-C-Al 2 O 3 deposited specimens were evaluated. Based on the experimental results, the hardness values of the Cr-C-Al 2 O 3 deposits increased with increasing Al 2 O 3 concentration in the electroplating bath. According to our microstructure study, Al 2 O 3 nanoparticles are uniformly distributed within the Cr-C deposits after electroplating in a Cr 3+ -based plating bath. The wear resistance of the Cr-C-deposited specimens could be noticeably improved by adding Al2O3 nanoparticles to the deposit. The Cr-C-Al 2 O 3 deposited specimens, which were prepared in a plating bath with an Al 2 O 3 concentration of 50 gL -1 , had a relatively high wear resistance compared to the other specimens.


Introduction
Due to the material's lustre appearance, as well as superior corrosion and wear resistance, Cr electroplating has been widely used in industrial applications for almost a century.Conventional Cr electroplating is performed in an electroplating bath containing highly toxic Cr +6 ions [1].In February of 2003, the Restriction of Hazardous Substances Directive (RoHS) was announced by the European Union.This directive restricts the use of six toxic substances, including Cr +6 ions, in the manufacturing process for electrical and electronic products.Therefore, the development of alternative coatings has become an important topic in recent years [2,3].Trivalent Cr electroplating, among all of the alternative options, is considered to have a great potential in replacing conventional hexavalent Cr electroplating for its relatively low toxicity and high current efficiency [4][5][6].
In our previous works, we proposed that the hardness value of an as-plated Cr-C deposit increased from ca. 780 Hv to ca. 1600 Hv after annealing at 600 °C for 1 h [7] or to ca. 1600 Hv through reduction flame heating for 1 s [8].However, the cracks in the Cr-C deposits severely widened after annealing, reducing the corrosion protection of the coating.
Hence, increasing the mechanical properties of Cr-C deposits without weaken the corrosion resistance is important.Composite electroplating may be a useful method for strengthening as-plated Cr-C deposits [9,10].In general, ceramic particles, such as SiC or Al 2 O 3 , are often used as hardening phases to form composite coatings via electroplating [11,12].The aim of this study is to fabricate Cr-C-Al 2 O 3 composite deposits using Cr 3+ -based plating baths with different Al 2 O 3 concentrations.The microstructures, hardness values, and wear resistances of the resulting Cr-C-Al 2 O 3 deposits will be investigated and discussed.

Experimental Procedure
A commercially pure Cu disc with a diameter of 20 mm and a thickness of 2 mm was used as the substrate for Cr-C-Al 2 O 3 electroplating.Before electroplating, the Cu substrate was mechanically polished with 600-grid emery paper, ultrasonically cleaned in an alcohol bath, and dried with an air blaster.The trivalent Cr plating bath was composed of 0.8 M CrCl 3 •6H 2 O as the main metal salt, urea as a complex agent and a small amount of buffer salts to maintain a pH value of 1.1 [7].Al 2 O 3 particles with an average size of 100 nm were added to the plating bath at concentrations of 0, 50, 100 and 150 gL -1 to produce the Cr-C and Cr-C-Al 2 O 3 deposits on the Cu substrate.A deposit with a thickness of ca.50 ȝm was prepared with an electroplating current density of 35 Adm í2 for 1500 s.The bath temperature was kept at 30 ± 1 °C during electroplating.To increase its circulation, the plating bath was stirred with a magnetic stirrer during electroplating.
After electroplating, the surface and cross-sectional morphologies were studied with a scanning electron microscope (SEM; Hitachi S-3000N) and an optical microscope (OM; Olympus BH2-HLSH).The hardness values of the Cr-C and Cr-C-Al 2 O 3 deposits were measured using a micro-hardness tester (Matsuzawa Digital, Model MXT-Į7e) with a load of 25 g.The mean hardness and standard deviation of a Cr-C or Cr-C-Al 2 O 3 deposit were evaluated by measuring at five arbitrary positions that were approximately in the centre of its cross section mounted in epoxy.The microstructures of the as-plated Cr-C and Cr-C-Al 2 O 3 deposits were characterised using optical, scanning and transmission electron microscopes (TEM; Philips Tecnai F30).
The wear resistance of the Cr-C and Cr-C-Al 2 O 3 deposited specimens was evaluated using a ball-on-plate wear tester, in which a 6 mm counterpart ball made of steel with a hardness value of ca.450 Hv was used.During the wear-resistance test, a constant load of 10 N was applied normally to the Cr-C or Cr-C/Al 2 O 3 deposited specimens under an unlubricated condition at 25 °C.The wear-resistance test was conducted with a circular track with a diameter of 3 mm, a frequency of 10 Hz, and a total ground distance of 50 m.The weight-difference value of the Cr-C or Cr-C-Al 2 O 3 deposited specimens was measured before and after the wear-resistance test using a scale with a precision of 0.01 mg.Optical microscopes was used to examine the surface morphologies of the Cr-C and Cr-C-Al 2 O 3 deposited specimen before and after the wear tests.According to our previous study [8], the hardness of the as-plated Cr-C deposits can be significantly increased to 1600 Hv after flame heating for 1 s.The wear resistance of the as-plated Cr-C deposited specimen could also be obviously improved through flame heating.However, the cracks in the flame-heated Cr-C deposits became wider and longer, leading to decrease in their corrosion resistance.In this study, we confirmed that the crack density could be significantly reduced and the crack width could be narrowed in the Cr-C deposits in the presence of Al 2 O 3 nanoparticles.Moreover, the as-plated Cr-C-Al 2 O 3 deposits have a relatively high hardness value, above 790 Hv, which is higher than that of fully quench-hardened steels that are used as tool and cutting materials.

3.3Wear Resistance
The weight-difference values of the Cr-C and Cr-C-Al 2 O 3 deposited specimens, before and after the wear-resistance test, are shown in Fig. 4. Clearly, a weight loss of 0.43 mg was detected from the Cr-C deposited specimen after the wear-resistance test.However, weight gains were found from the worn Cr-C-Al 2 O 3 deposited specimens prepared from the plating baths with 50 and 100 gL -1 of Al 2 O 3 .A slight weight loss of 0.02 mg was detected from the Cr-C-Al 2 O 3 deposited specimens prepared in the electroplating bath with 150 gL -1 of Al 2 O 3 .This indicates that the addition of Al 2 O 3 nanoparticles to the Cr-C deposits noticeably increased their wear resistance. of the surface of the as-plated Cr-C deposits was covered, whereas the other half did not markedly change, revealing a typical nodular surface.However, the nodular surface of the worn Cr-C-Al 2 O 3 deposited specimens was no longer observed; the surface had a circular scratched appearance.Because weight gains could be detected from the Cr-C-Al 2 O 3 deposited specimen prepared in the plating baths with 50 and 100 gL -1 of Al 2 O 3 , it can be expected that the circular scratched marks were possibly ground and cold-welded by the steel counterpart.The wear resistance of the as-plated and anneal-hardened Cr-C deposited specimens was evaluated in our previous study [13].We found that the wear resistance of the as-plated Cr-C deposited specimens could be significantly improved after anneal-hardening.A slight weight loss was detected from the anneal-hardened Cr-C deposited specimens after the wear-resistance test.In this study, a cold-welded layer, smeared from the steel counterpart, was detected on the surface of the Cr-C-Al 2 O 3 deposited specimens, leading to an increase in their weight.Although the hardness of the anneal-hardened Cr-C deposits is much higher than that of the Cr-C-Al 2 O 3 deposits, a slight weight loss was detected from the anneal-hardened Cr-C deposited specimens.Because Al 2 O 3 particles are widely used as an abrasive substance for grinding, the steel counterpart could be abraded by the Cr-C-Al 2 O 3 deposits during the wear-resistance test and cold-welded on the deposited specimens.

Figs. 1
Figs.1 (a, b) show the SEM micrographs of the Cr-C and Cr-C-Al 2 O 3 deposit surfaces and their chemical composition analyses, which were performed using an energy-dispersive x-ray spectrometer (EDS).It can be observed that the Al 2 O 3 nanoparticles, shown in a bright colour, were uniformly distributed on the deposit surface (see Fig.1(b)).The results

Fig. 1 Fig. 2
Fig. 1 Surface morphologies and EDS-analysis of (a) Cr-C deposits and (b) Cr-C-Al 2 O 3 deposits prepared in an electroplating bath with 50 gL -1 Al 2 O 3 .

Fig. 3
Fig. 3 shows the hardness and standard deviation values of the Cr-C-Al 2 O 3 deposits with different Al 2 O 3 concentrations.The hardness of the as-plated Cr-C deposits is 683 Hv, whereas the values of 791, 814 and 852 Hv were detected for the Cr-C-Al 2 O 3 deposits prepared in the baths with 50, 100 and 150 gL -1 of Al 2 O 3 , respectively.That is, the hardness values of Cr-C-Al 2 O 3 deposits increased with an increasing Al 2 O 3 concentration in the Cr 3+ -based plating bath.According to our previous study[8], the hardness of the as-plated Cr-C deposits can be significantly increased to 1600 Hv after flame heating for 1 s.The wear resistance of the as-plated Cr-C deposited specimen could also be obviously improved through flame heating.However, the cracks in the flame-heated Cr-C deposits became wider and longer, leading to decrease in their corrosion resistance.In this study, we confirmed that the crack density could be significantly reduced and the crack width could be narrowed in the Cr-C deposits in the presence of Al 2 O 3 nanoparticles.Moreover, the as-plated Cr-C-Al 2 O 3 deposits have a relatively high hardness value, above 790 Hv, which is higher than that of fully quench-hardened steels that are used as tool and cutting materials.

Fig. 3
Fig. 3 Hardness values and standard deviations of as-plated Cr-C deposits and Cr-C-Al 2 O 3 deposits prepared from electroplating baths with varying concentrations of Al 2 O 3 .

Fig. 4
Fig. 4 Weight-difference values of as-plated Cr-C and Cr-C-Al 2 O 3 deposited specimens after the wear-resistance test.The surface morphologies of the Cr-C and Cr-C-Al 2 O 3 deposited specimens after the wear-resistance test are shown in Figs.5(a)-(d).As shown in Fig. 5(a), approximately half

Fig. 5
Fig. 5 Surface morphologies of (a) Cr-C deposited specimens and Cr-C-Al 2 O 3 deposited specimens prepared in electroplating baths with (b) 50, (c) 100, and (d) 150 gL -1 Al 2 O 3 , after the wear-resistance test.The cross-sectional morphologies of the Cr-C and Cr-C-Al 2 O 3 deposited specimens after the wear-resistance tests are shown in Figs.6(a) and (b), in which the specified specimens have either an inferior or superior wear resistance, respectively.The surface profile of the Cr-C deposited specimens levelled off significantly after the wear resistance test; however, the worn Cr-C-Al 2 O 3 deposited specimens did not significantly alter their surface profiles.These findings suggest that the addition of Al 2 O 3 nanoparticles within the Cr-C deposits could increase their wear resistance.As shown in Fig. 6(b), a cold-welded layer smeared from the steel counterpart can be found on the surface of the Cr-C-Al 2 O 3 deposits after the wear-resistance test.This result could explain the weight gain that was detected in the Cr-C-Al 2 O 3 deposited specimens after the wear-resistance test.As shown in Fig. 5(d), some shallow holes were observed in the Cr-C-Al 2 O 3 deposits that were prepared in the electroplating bath containing 150 gL -1 Al 2 O 3 after the wear resistance test.This result implies that some fragments of the Cr-C-Al 2 O 3 deposits were peeled off during the wear-resistance test, resulting in a slight weight loss, though a smeared steel counterpart covered its surface after the wear-resistance test.

Fig. 6
Fig. 6 Cross-sectional morphologies of (a) Cr-C deposited specimens and (b) Cr-C-Al 2 O 3 deposited specimens prepared in an electroplating bath with 50 gL -1 Al 2 O 3 after the wear-resistance test.
In this study, Cr-C-Al 2 O 3 deposits with different Al 2 O 3 concentrations could be successfully prepared in Cr 3+ -based electroplating baths.After electroplating, the corrosion and wear resistance of the Cr-C and Cr-C-Al 2 O 3 deposited specimens were investigated.The hardness values of the Cr-C-Al 2 O 3 deposits increased with increasing concentrations of Al 2 O 3 nanoparticles in the Cr 3+ -based electroplating baths.The wear resistance of the Cr-C deposited specimens can be markedly increased via co-deposition with Al 2 O 3 nanoparticles.Through-deposit cracks in the Cr-C deposits were reduced by adding Al 2 O 3 nanoparticles to the deposits.The Cr-C-Al 2 O 3 deposited specimens prepared in the electroplating bath with 50 gL -1 Al 2 O 3 had relatively high wear resistances compared to the other specimens.