Studies of nanocrystalline Pd alloy films coated by electroless deposition 

G.K. Strukova, G.V. Strukov, I.E. Batov, M.K. Sakharov, E.A.Kudrenko and A.A. Mazilkin Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka,

Russia E-mail:strukova@issp.ac.ru

The microstructures of thin coating films of pure palladium and palladium alloys deposited from organic electrolytes onto different metallic substrates by electroless plating method have been investigated. The coatings are dense, pore-free 0.005-1µm thick films with high adhesive strength to the substrate surface. X-ray spectral analysis, X-ray phase analysis, transmission and scanning electron microscopy were used to determine the composition and structure of alloy coatings of binary systems: Pd-Au, Pd-Ag, Pd-Ni, Pd-Pb, and ternary system Pd-Au-Ni. The coatings of Pd-Au, Pd-Ag, and Pd-Ni have a solid solution structure, whereas Pd-Pb is intermetallic compound. It has been found that the deposited films consist of nanocrystalline grains with sizes in the range of 11-35 nm. Scanning and transmission electron microscopy investigations reveal the existence of clusters formed by nanocrystalline grains. The origin for the formation of nanocrystalline structures of coating films is discussed.

Main resultsMain results

20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0

Inte

nsity (

arb

.un

its)

2 ( o )

Ag

Cu

311 222

311

311Ag

311Cu

200Cu

200Ag

(100)

(110) <111>

<100>

<311>

222Cu

222Ag

111Ag

111Cu

220200111

200 220111

(110)

Composition from EDX-dates of the Pd-Ag coatings on copper substrate. at.%Pd 100 77 72 60 51 42 36 24 4 2Ag 0 23 28 40 49 58 64 76 96 98

Composition of the Pd-Ni coatings on copper substrate (at.%) Pd 100 91 79 53 35 31 19 0Ni 0 9 21 47 65 69 81 100

Scanning electron micrograph of cross section of the Pd-Au coating on the copper substrate. The film thickness is 200 nm.

Ms-metal of substrate, Mc-metal of coating, RX- halogenide, L,L1-organic ligande(aprotonic dipolar solvent or its derivatives), (L)n (RX)m and (L1)n (RX)m –molecularassociated compound

X-ray diffraction pattern of Ag film on Cu X-ray diffraction pattern of Ag film on Cu single crystal substratesingle crystal substrate

• A novel method has been developed for thin film plating of palladium and its alloys onto metal substrates, i.e. immersion plating from organic solutions on the base of metal chlorides, ammonium chloride, and aprotonic dipolar solvents. The distinctive feature of the method is a possibility to obtain immersion coatings of alloys of different metals due to the ability of the aprotonic dipolar solvent to form coordination compounds with transition metals. Coatings of two-component (Pd-Ag, Pd-Au, Pd-Ni, Pd-Pb) and three-component (Pd-Ni-Au) alloys have been produced.

• The method of X-ray diffraction has been used to find that the films of Pd-Ag, Pd-Au, and Pd-Ni have a solid solution structure. The Pd-Pb coating film contains the single phase of thePd 3Pb intermetallide. The Pd3Pb coating on a copper substrate was obtained by immersion plating for the first time.

• The peaks broadening on the diffraction patterns, the SEM photographs of the films surfaces, and the TEM photographs show that all the coatings obtained by the immersion plating from the organic solutions are nanocrystalline ones. The grain size, derived from the peaks broadening on the diffraction patterns, is 11-35 nm.

•At different growth stages of the immersion coatings of fcc metals the SEM photographs demonstrate spheroidal particles which cover the whole of the substrate surface, then over this film there grow spheroidal clusters 200 -300 nm in size which consist of nanocrystalline grains.

Schematic representation of organic solution – metal substrate interaction

Sample No.

Coating Substrate

Lattice parameter, Ǻ

∆ Grain

size, nm

Coating Substrate

1 Pd Cu 3.89 3.6148 +0.2752 35

2 Pd Ni 3.89 3.538 +0.3520 16

3 Ni Cu 3.538 3.6148 -0.0768 20

4 Ni Cu/Zn 3.538 3.697 -0.1590 13

1 Pd Cu 352 Ag Cu (single

crystal)23 (100)

18 (111)

12 (311)

3 Pd/Ag-77/23

Pd/Ag-72/28

Cu 21

16

4 Pd/Ni-79/21 Cu 11

5 Pd/Au-17/83 Cu 35

6 Pd/Au/Ni-19/64/17

Cu 15

7 Pd3Pb Cu 258 Ni Cu 209 Ni Cu-Zn 13

10 Pd Ni 16

11 Au Ni 20

Grain size in the various coating films (film thickness: 0.1-0.35 µm)

Grain size and lattice parameters of some coatings and substrates

As a result, atoms of the substrate metal are replaced by a noble metal which incorporates into the crystal lattice of the substrate forming a coating strongly bound with the substrate. An epitaxial coating film was obtained on treatment under the analogic conditions of a copper single crystal, i,e. the film grows oriented

The coating films of pure palladium and palladium alloys were deposited using a novel electroless plating method [G.K. Strukova, G.V. Strukov, et al.,Thin metal coatings on metal substrates obtained by electroless deposition from nonaqueous solutions, Metal Finishing, 102 (2004) Issue 3, 20-25.] In accordance with the method, metallic substrates were exposed to the solution of metal halogenides and ammonium chloride in an aprotonic-dipolar organic solvent at a fixed temperature. The aprotonic-dipolar solvent posseses the required combination of donor and ionizing capabilities and forms coordination compounds with many transition metals. Substrates (foils of copper, brass, nickel, and permalloy) were prepared for plating by standard procedures (cleaning, activation, rinsing). The composition and thickness of coatings on the same substrate were controlled by the variation of the components of the organic solution, temperature (from 50 to 90оC), and duration of the substrate exposure to the organic solution (from 0.5 to 30 min.). A distinguishing feature of the method is the transfer of the reaction between a metallic substrate and a plating solution to a non-aqueous media. This leads to a significant change in the structure of coatings obtained.

Serious advantage of an offered method is the reception of the coatings from metal alloys.

It allows to supply functional properties of thin metal coatings as the alloys have, as a rule, properties, interesting to practice. The examples of the metal coatings received by electroless plating from organic solutions are given in the tables.

X-ray diffraction patterns of Pd, Pd/Ag-77/23 at.%, and Pd/Ag-72/28 at.% films on copper substrates.The films form solid solutions in the process of growing. The peak positions shift regularly with a change in Ag- content of Pd-Ag alloy. Broadening of peaks results from nanocrystalline structure of the plated film.

The SEM-photographs of the Pd-Ag coatings on the copper substrate obtained for the exposure time 30 sec, 2 and 5 min, are demonstrated the immersion coating growth stages. It is seen that each cluster in turn consists of nanocrystallites.

SEM-photo of Ni film deposited on copper. Nickel nanocrystallites are seen clearly.

All the reflexes correspond to a coating of pure nickel. Pd-Ni films form solid solutions. The grain sizes determined from the peak broadenings were 20 and 11 nm for Ni and Pd/Ni-79/21 аt.% alloy films on a copper substrate, respectively.

The deposition of pure nickel (without phosphorus or boron) onto a copper substrate by electroless “immersion” method was not previously known. In this work, nickel films on copper and brass substrates were successfully deposited. The thickness of the plated Ni films was 0.1-0.3 µm.

The SEM-photo and the diffraction patterns of binary Pd-Au and ternary Pd/Au/Ni alloys plated on copper substrates.

The dark-field TEM micrograph of a 50 nm thick Pd/Ag-72/28 at.% film and the corresponding electron diffraction pattern. It can be seen that the film consists of equiaxial grains with a mean grain size about 19 nm, which is in accordance with X-ray estimations.

The SEM-photo and the diffraction pattern of the Pd/Pb- 75/25 at.% alloy film on a copper substrate. The film consists of clusters of Pd3Pb nanocrystallites.The reflexions in the diffraction pattern coincide with those calculated for the Cu substrate and the intermetallic compound Pd3Pb.

Pd-Ag alloy films were grown on a copper substrate by electroless plating over a wide range of compositions. Using electroless plating, we also succeeded in depositing Pd-Ni films on copper substrates in a wide range of compositions. A distinguishing feature of Pd-Pb coatings is that the alloy compositions of the coatings are substantially the same despite the different coating conditions. Together with the observation that pure Pb does not form, under the same conditions, an immersion plating on a copper substrate these facts point to the formation of an immersion plating from the intermetallide. The diffraction pattern confirmed that the coating consisted one phase of the intermetallide Pd3Pb.

Also Pd-Hg and Pd-Bi, as Pd-Pb coatings show substantially the same alloy compositions despite the different coating conditions. This clearly demonstrates the formation of intermetallic compounds.