Short range structure of amorphous electrodeposited Cr80C10H10 and Cr73C12H15 alloys by means of X-ray and neutron diffraction

JOURNA

Journal of Non-Crystalhne Sohds 156-158 (1993) 181-184 North-Holland

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Short range structure of amorphous electrodeposited Cr80C10Ha0 and Cr73C12H15alloys by means of X-ray and neutron diffraction Mi. Nuding, P. L a m p a r t e r and S. Steeb Max Planek Instttut fitr Metallforschung, Instttut fiir Werkstoffwtssenschaft, Seestr 92, IV-7000 Stuttgart, Germany

The atomic structure of amorphous electrodeposlted Cr80CloHxo and Cr73C12H15 alloys was investigated by wide angle X-ray and neutron scattering The total structure factors and pair correlation functions were determined By combination of X-ray and neutron &ffractlon data, partial coordination numbers and atomic distances were obtained. Further, using a fast X-ray diffractometer equipped with a position sensitive detector, the m sltu crystallization behavior of these alloys was studied

1. Introduction Chromium plating is an important process to furnish metallic surfaces with corrosion resistant as well as decorative and hard coatings. The electrolytes most commonly used are those developed by Sargent [1] and Fink [2]. If formic acid is added to the plating bath, amorphous, bright chromium layers can be obtained [3]. They have the advantage that the defect concentration is greatly reduced compared with crystalline coatings and, in addition, the annealing at 500°C causes a large increase of their hardness. To date, X-ray diffraction, differential scanning calorimetry, hardness and crystallization measurements have been performed [3,4].

the following reagent grade chemicals, dissolved in deionized water: chromium oxide (Cr(VI)O 3) 100 g/l; sulfuric acid (H2SO 4 95%) - 2.83 ml/1; and formic acid (HCOOH 88%) - 20 ml/1. The cathode used was a 75 txm thick copper foil. The anode used was a lead foil of purity 99.95%. The plating was carried out for 30 min with a constant current density of 350 m A / c m z at plating temperatures of T = 15, 25 and 50°C.

2.2. Dtffraction expertments X-ray diffraction experiments were performed in transmission mode using Mo K a radiation. Neutron diffraction experiments were performed at the Institute Laue-Langevin, Grenoble. The two-axis diffractometer, D20, was used, operating at a neutron wavelength of 0.879 A.

2. Experimental

2.1. Sample preparation

3. Results

Amorphous Cr80CloHlo and Cr73C12H15 alloys were prepared from plating baths containing

3.1. X-ray dtffraction

Correspondence to Dr M. Nudlng, Max Planck Institut fur Metallforschung, Instltut fiir Werkstoffwlssenschaft, Seestr 92, W-7000 Stuttgart, Germany. Tel +49-711 2095 384 Telefax: +49-711 226 5722 E-mall. nuding @ vaxww2.mpl-stuttgart.mpg de

The evaluation of the structure factors from the measured intensities was done according to ref. [5]. Figure 1 shows the total structure factors according to Faber-Ziman of three CrCH alloys prepared at different electrolyte temperatures. In

0022-3093/93/$06.00 © 1993 - Elsevier Soence Pubhshers B.V All rights reserved

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Mr. N u d m g et al / Short range structure

table 1, some characteristic features of the total structure factors obtained from X-ray diffraction are listed. Although the chemical analysis of samples prepared at plating temperatures T = 15 and 25°C, respectively, resulted in the same chemical composition, Cr80C10H10 , there are differences observable in the shape of the total structure factor. The total structure factor of a ternary Crs0C10Hl0 alloy can be divided into six partial structure factors:

6 5

~

~ 4 3 2

T = 25°C

1

S t ( Q ) = 0.931Scrc, + 0.001Scc + 0.000SHH + 0.058S¢r C + 0.010Scr H + 0.000ScH,

(1) where the coefficients Wu are given for Q = 0. The Wu < 10 -3 are set to zero. From eq. (1), one clearly sees that the total X-ray structure factor is mainly determined by the C r - C r atomic pair correlations. Figure 2 shows the total X-ray pair correlation functions. Characteristic data are listed in table 1. While the position of the first maximum in the pair correlation function remains unchanged at o R = 2.64 A, as the plating temperature increases, the peak halfwidth decreases from 0.40 .~ to 0.38 ,~ and the peak height increases from 6.94 .~-2 to 7.37 .~-2. Thus, the nearest neighbour correla-

0 0

2

4

6

8

10

12

tion within the first coordination shell sharpens up while the correlations at larger distances are damped out.

3.2. Neutron diffraction Neutron diffraction was done on the alloy Cr80C10H10 prepared at an electrolyte temperature T = 25°C. Figure 3 shows the total neutron structure factor. Compared with X-ray diffrac-

X-ray diffraction

Neutron diffraction

CrsoC 1oH 10

CrsoC toll 10

Cr73C12H 15

T (°C):

15

25

50

25

Qt ( ' ~ - 1): S(Q1)" AQ 1 (,~- t):

3.00 3.97 0.416

3.00 3.87 0.456

2 97 3.70 0 487

3.05 4.79 0.436

R 1 (A) G ( R t ) (,~-2) AR t (,~): Nt:

2.64 6 94 0.40 13.47

2.64 7.00 0 40 13.32

2.64 7 37 0.38 13.30

2 66 6.65 0.39 -

CrsoC 1oH 10

RH_Cr (,~):

-

1.73

-

1 73

R c _ c r (,~):

-

2.12

-

2.12

-

2.62 5.4 7.6 10.9

-

2.62

-

54 7.6 10.9

r (ilk):

ZH_Cr" Zc_cr: Zcr_cr.

-

16

Fig. 1. Total X-ray structure factors, M o K a .

Table 1 Structural data for different CrCH alloys

Rcr_C

14

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Short range structure

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T = 15°C

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Fig. 2 Total X-ray pmr correlation functions

Fig 4 Total neutron pair correlation function for Cr8oCloHlo.

tion, the first maximum of St(Q) is shifted to a higher Q value Q1 = 3.05 ,~-1 and the halfwidth decreases to a value of 0.44 ,~-1. The total neutron structure factor of Cr8oC10Hlo can be divided into six partial structure factors:

comes in with a negative sign becomes important in data analysis if neutron diffraction data are combined with X-ray diffraction data. Figure 4 shows the total pair correlation function for the CraoCt0Hto alloy. By contrast with the X-ray pair correlation function of C r 8 0 C 1 0 H 1 0 , the neutron pair correlation function shows a negative peak at R = 1.72 ,~ going deep under the straight line -4"rrpoR, indicating a C r - H pair correlation. The Cr-C pair correlation occurs at R = 2.14 ,~ as a little tip at the left hand side of the dominant Cr-Cro pair correlation peak situated at R =

S t ( Q ) = 0.827Scrcr + 0.043Scc + 0.014Srn J + 0.378Scr c - 0.213Scr H - 0.049SCH.

(2) The most contributing partial structure factor to S t is Scrcr. The fact that the C r - H correlation

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T = 25°C

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Q [.~ ~] Fig. 3 Total neutron structure factor for Crs0CtoH10; A = 0.879 ,~

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Fig. 5. Difference function G~(R)- 0.8883G~'(R)

20

184

Mr. Nudmg et aL / Short range structure

4. Discussion. Combination of X-ray and neutron diffraction data In order to further evaluate information concerning the short range order of the alloy Crs0C10Ha0, it is quite favorable to combine neutron with X-ray diffraction data in such a way that the C r - C r correlation, which is the predominant correlation in both spectra, disappears. This effect can be achieved by taking the difference

600

T[°C]

G t ( R ) - 0.8883Gt ( R )

28 [degree]

= 0.000Gcrcr + 0.043Gcc + 0.014GHH + 0.327Gcr c - 0.222Gcr H - 0.049GcH.

(3)

The difference spectrum is mainly determined by the C r - C and C r - H pair correlations which contribute to the difference spectrum with opposite signs. Figure 5 shows the difference spectrum which consists of a deep negative peak at R = 1.72 /k belonging to the C r - H pair correlation. The C r - C pair correlation now comes out as a separate peak at R = 2.14 P,. Atomic distances, R,j, as well as partial coordination numbers, Z,j, were determined from the difference spectrum g t ( R ) - 0.8883gtX(R), where g(R) = 1 + G(R)/4"rrpoR.

5. Crystallization experiments Isochronous crystallization experiments in vacuum were performed on the alloy Crs0C10H10. The sample was heated from T = 20 to 600°C with a heating rate of 2°C/min while measuring diffraction angles between 20 = 5 ° and 20 = 70 ° with a detector speed of 10°/min. Figure 6 shows the diffraction patterns as obtained during the i s o c h r o n o u s crystallization e x p e r i m e n t on CrsoC10H m. The results are summarized as follows. The alloy shows a primary crystallization behavior. The first chromium peaks appear at a temperature of T = 384°C. Above T = 505°C, additional peaks are observed, marked with an ar-

70

Fig 6 Diffraction patterns of an lsochronous crystalhzanon experiment on Crs0CmH w; Mo Ka. row, caused by the formation of the carbide C r T C 3.

6. Conclusion The addition of formic acid to conventionally used hard chromium electroplating baths resulted in the formation of bright, amorphous CrsoCloHlo and Cr73C12H15 alloys. The chemical composition was altered by varying the electrolyte temperature. The short range structure of these alloys was studied by X-ray diffraction. For the Cr80CloHlo alloy, combination of X-ray and neutron data yielded the atomic distances and coordination numbers of the C r - C and the C r - H pair correlations. Isochronous in situ crystallization experiments performed on the alloy Crs0C10H~0 showed a primary crystallization behavior.

References [1] G J Sargent, Trans Am. Electrochem. Soc 37 (1920) 479. [2] C G. Flnk, US Patent no 1,581,188 (1926). [3] S Hoshlno, H.A. Lamnen and G.B. Hoflund, J Electrochem. Soc 133 (1986) 681. [4] R.Y. Tsal and S T. Wu, J. Electrochem Soc. 136 (1989) 1341 [5] P. Lamparter, A Habenschuss and A H Narten, J NonCryst Sohds 86 (1986) 109.