J O U R N A L O F M A T E R I A L S S C I E N C E 3 9 (2 0 0 4 ) 4063 – 4066 L E T T E R S

Electrodeposition and mechanical properties

of Ni–La2O3 nanocomposites

YU-JUN XUE ∗, DI ZHU ∗, FEI ZHAODepartment of Mechanical Engineering, Nanjing University of Aeronaustics & Astronaustics,Nanjing 210016, People’s Republic of ChinaE-mail: yjxue2000@yahoo.com.cnE-mail: dzhu@nuaa.edu.cn

Electrodeposition is one method to produce metalmatrix composite coatings. Metal matrix nanocom-posites can be formed by the addition of nano-sizedparticles to a host matrix such as Ni. The nanoparticlesused is reported in the literature [1–7] include SiC,Al2O3, ZrO2, TiO2, PSZ, and YSZ etc. The codepo-sition of the nanoparticles with metal phases to formnanocomposites can bring a remarkable improvementon mechanical and physical properties of the coatings.Meanwhile, the morphology, structure, and propertiesof the composite electrodeposits are affected by severalinterrelated parameters: composition of the electrolyticbath and particles concentration, presence of additives,temperature, pH, agitation, and applied current density[2–5, 8].

Rare earth oxides have been widely used in elec-tronics, materials and chemical engineering, becauseof their special characteristics [9–11]. It was foundthat electrodeposited Ni–La2O3 composite films at hightemperatures had the slowest rate and the best resistanceto thermal cycling compared to those of bare Ni [12].The reason was that La2O3 particles in the nanome-ter size play a major role inhibiting grain-boundarydiffusion. However, the mechanical properties of theNi-La2O3 nanocomposites have not received muchattention.

In this work, the behaviors of composite electro-plating of nanometric La2O3 particles with Ni to formnanocomposites were studied. The effect of the concen-tration of La2O3 in the plating bath and plating param-eters such as current density, temperature, and stirringrate on the amount of La2O3 codeposited in the compos-ites were evaluated. Microhardness of the Ni–La2O3nanocomposites as a function of the weight fraction ofLa2O3 particles in the deposits was also investigated.

Electrodeposition of Ni–La2O3 nanocompositeswere deposited in a nickel sulfamate bath con-sisting of [Ni(NH2SO3)2·4H2O] (400 g/l), NH4Cl(15 g/l), H3BO3 (30 g/l), sodium dodecylsulfate[CH3(CH2)11OSO3Na] (0.1 g/l), and La2O3 particlesfrom 5 to 30 g/l. The La2O3 powder (purity >99.99%)has an average 60-nm particle size. Each plating bathwas mixed by magnetic stirring for 2 h, and subse-quently by ultrasonic agitation for 30 min just beforeelectroplating. Codeposition was carried out in 500-ml

∗Authors to whom all correspondence should be addressed.

beaker for 2 h; a steel plate as cathode with dimen-sions of 20 × 50 × 1 mm was used, and a nickel platewas used as anode. The electrolyte was stirred by amagnetic stirrer during electrodeposition. The temper-ature of the electrolyte was maintained at a predeter-mined value by an automatic controller. The operat-ing conditions for plating were current density rang-ing from 2 to 8 A/dm2, bath temperature from 35 to60 ◦C, and stirring rate from 400 to 1200 rpm. Becauseof La2O3 particles dissolve in the bath, the pH of theelectrolyte increases with the increase of concentra-tion of La2O3 particles in the electrolyte, as listed inTable 1.

The surface morphology and the components ofthe deposits were studied by using a field emissionscanning electron microscope (FESEM, LEO-1530VP)with energy dispersive analyzer system (EDX). Theweight fraction of lanthana was determined by us-ing the lanthanaum to oxygen ratio 2:3 determinedby the chemical formula La2O3. Vickers microhard-ness tests were conducted on an HXS-1000A mi-crohardness tester using a load of 50 g, applied for10 s. Each hardness value was an average of fivemeasurements.

Fig. 1 represents that the wt% of the La2O3 in thecomposite varies with different contents (5–30 g/l) ofthe La2O3 particles in the electrolyte at current densities2 A/dm2, 50 ◦C, and 800 rpm. It is seen that the wt% ofLa2O3 in the deposit rapidly increases with the increaseof the La2O3 content in the bath. The greatest particleincorporation occurs at the highest La2O3 addition of30 g/l. If La2O3 content is too low (15 g/l), the numberof nickel ions adsorbed on La2O3 particles is small,leading to the small amount of codeposited particles.

The effect of the current density on the wt% of La2O3in the composite at pH 5.3, 50 ◦C, and 800 rpm and at30 g/l La2O3 in the electrolyte is shown in Fig. 2. Itis seen that the wt% of La2O3 in the deposit decreaseswith the increase of the current density. As the currentdensity increases, the movement of nickel ions frombulk solution to the cathode surface was faster than thatof La2O3 particles by the mechanical agitation. So, thelower wt% of the codeposited La2O3 was caused at highcurrent densities. As a result, the optimum current den-sity is a relatively low current density, which allowed

0022–2461 C© 2004 Kluwer Academic Publishers 4063

T ABL E I The pH value varied with the content of La2O3 particles inthe electrolyte

La2O3 particles content pH value

0 4.15 4.4

10 4.615 4.920 5.130 5.3

Figure 1 Effect of the La2O3 content in the electrolyte on the wt% ofLa2O3 in the composite.

Figure 2 Effect of the current density on the wt% of La2O3 in the com-posite.

the maximum wt% of the codeposited La2O3. In thisinvestigation, it could be concluded that the optimumcurrent density was 2 A/dm2.

Fig. 3 represents the effect of temperature on the wt%of La2O3 in the composite at 30 g/l La2O3, current den-sities 2 A/dm2, pH 5.3, and 800 rpm. It is clear thatthe temperature of the electrolyte markedly affects theweight fraction of La2O3 in the composite. The em-bedded La2O3 content increases from 2.0 wt% at 35 ◦Cto 3.1 wt% at 50 ◦C. With the further increase of thetemperature value, the wt% of La2O3 in the compositedecreases gradually.

The stirring rate plays an important role in the platingprocess for the transportation of La2O3 particles to the

Figure 3 Effect of the temperature on the wt% of La2O3 in the compos-ite.

Figure 4 Effect of the stirring rate on the wt% of La2O3 in the composite.

cathode for the codeposition. Fig. 4 shows the effectof stirring rate on the wt% of La2O3 in the compositeat pH 5.3, 50 ◦C, current densities 2 A/dm2, and 30 g/lLa2O3 in the electrolyte. It is seen that the amount ofLa2O3 in the composite increases with increasing stir-ring rate up to 800 rpm. The highest wt% of La2O3in the composite is obtained at 800 rpm. The wt% ofLa2O3 in the composite decreases beyond 800 rpm.It is suggested that, under strong stirring conditions,the La2O3 particles absorbed on the cathode are partlytaken away, which resulted in a decreased amount ofthe La2O3 codeposition.

The SEM surface morphologies of electrode-posited pure Ni and Ni-La2O3 nanocomposites isrepresented in Fig. 5. It shows that the compositedeposits register similar nodular surface structure.Compared to pure Ni deposit, the nanocompositecoatings had smaller nodular microstructure. It seemsthat the increase of the wt% La2O3 codepositedlead to the generation of surfaces with more smoothand microcrystalline structures. The EDX resultsshow a good uniformity of dispersed phase, con-firming the parameters operated in this study issuitable.

The hardness of the electrodeposited compositesdepends on many factors. Among them, the mostimportant is the composition of the electrolyte. The

4064

Figure 5 SEM surface micrographs of the electrodeposited pure Ni and Ni-La2O3 nanocomposites obtained at 2 A/dm2, 50 ◦C, and 800 rpm:(a) pure Ni, (b) 1.1 wt% La2O3, (c) 1.5 wt% La2O3, (d) 1.7 wt% La2O3, (e) 2.8 wt% La2O3, and (f) 3.1 wt% La2O3.

relationship between the weight fraction of La2O3 in thenanocomposite and michrohardness is shown in Fig. 6.It appears that higher weight fraction of codeposited-La2O3 provides better microhardness. Gyftou et al.[2] reported that embedding of nanoparticles perturbsthe crystal growth of Ni, inducing a reduction in thecrystal size, gives deposits with significantly increasedhardness values. Wang et al. [3] also reported that theexistence of nano-sized ceramic particle, as the second-phase, reduces the grain size of Ni-matrix. Accord-ingly, the higher microhardness value of the Ni-La2O3nanocomposites may be also due to the decrease of thegrain size of Ni-matrix of the composites, which is fa-vored by the nano-sized La2O3 particles. Future studies

are also aimed at establishing the presence of nanomet-ric La2O3 particles in the composites and investigatingthe microstructure of the coatings by using transmis-sion electron microscopy (TEM) and X-ray diffractionmethods.

In summary, Ni-La2O3 nanocomposites were pre-pared using composite electroplating, which embed-ded nano-sized La2O3 particles as the reinforcingphase. The maximum wt% of La2O3 in the compos-ites was obtained at 30 g/l La2O3 in the electrolyte,50 ◦C, 800 rpm, and current densities 2 A/dm2. Themicrohardness of the Ni-La2O3 nanocomposites in-creased with the weight fraction of codeposited-La2O3nanoparticles.

4065

Figure 6 Variation of microhardness with weight fraction of La2O3 par-ticles in the nanocomposite.

AcknowledgmentsThe present work was supported by China Post-doctoral Science Foundation (2002032192), andby National Nature Science Foundation of China(50075040).

References1. L . B E N E A, P . L . B O N O R A, A. B O R E L L O, S .

M A R T E L L I , F . W E N G R E R, P . P O N T H I A U X and J .G A L L A N D , Solid State Ionics 151 (2002) 89.

2. P . G Y F T O U, M. S T R O U M B O U L I , E . A . P A V L A T O U

and N. S P Y R E L L I S , Trans. IMF 80 (2002) 88.3. S . -C . W A N G and W.-C . J . W E I , Mater. Chem. Phys. 78 (2003)

574.4. A . B . V I D R I N E and E . J . P O D L A H A , J. Appl. Electrochem.

31 (2001) 461.5. T . D E G U C H I , K . I M A I , H . M A T S U I , M. I W A S A K I , H .

T A D A and S . I T O , J. Mater. Sci. 36 (2001) 4723.6. J . L I , J . J I A N G, H. H E and Y. S U N , J. Mater. Sci. Lett. 21

(2002) 939.7. S . T . A R U N A and K. S . R A J A M , Scripta Materialia 48 (2003)

507.8. S . K . K I M and H. J . Y O O , Surf. Coat. Technol. 108–109 (1998)

564.9. B . A . L A T E L L A, A. J . A T A N A C I O and T I A N S H U N L I U ,

J. Mater. Sci. Lett. 21 (2002) 879.10. Z H A N G T I A N S H U, P E T E R H I N G, H A I T A O H U A N G and

J . K I N E R , J. Mater. Sci. 37 (2002) 997.11. D I A N Y I N G C H E N, B A O L I N Z H A N G, J I A N G G U A N G

X U, H A N R U I Z H U A N G and W E N L A N L I , J. Mater. Sci. Lett.22 (2003) 163.

12. X . P E N G, T . L I and W. W U , Oxid. Met. 51 (1999) 314.

Received 28 Octoberand accepted 24 November 2003

4066