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Heat treatments for electroless nickel–boron plating on aluminium alloys
Surface and Coatings Technology 160 (2002) 239–248
Heat treatments for electroless nickel–boron plating on aluminium alloys F. Delaunois*, P. Lienard ´ Faculte´ Polytechnique de Mons, Service de Metallurgie, Rue du Joncquois, 53 B-7000 Mons, Belgium Received 4 March 2002; accepted in revised form 24 June 2002
Abstract Electroless nickel–boron baths reduced with sodium borohydride can be stabilized with various agents such as thallium nitrate or lead tungstate with no fundamental modification of deposition rates and stability. To improve the mechanical properties of electroless nickel–boron deposits, various heat treatments are applied. At low temperatures, no fundamental changes in the deposit structure are observed, only an improvement of adhesion on aluminium substrate. The values of the Knoop microhardness obtained on these heat-treated deposits are near 600 hk100. At higher temperatures, structural changes take place and the nickel–boron deposits crystallize. The microhardness rises until 1050 hk50 for heat treatments at 350 8C for 4 h. A diffusion layer between the electroless nickel deposit and the aluminium substrate appears at high heat treatment temperatures. The results of scratch tests show that the Ni–B deposits, with or without heat treatments, have good tribological properties under external solicitations. They are hard, wear and abrasion resistant, and also have good adhesion to the aluminium substrate. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electroless plating; Nickel; Boron; Aluminium; Heat treatments
1. Introduction Borohydride electroless nickel coatings find extensive applications in aerospace, automotive, chemical and electrical industries specially because of their solderability, high hardness, and wear and abrasion resistances. Similarly to nickel–phosphorus, nickel–boron deposit characteristics change with boron content w1x. The amount of boron contained in borohydride electroless nickel coatings ranges between 1 and 10% weight w2x. The structure of the deposits is a mixture of microcrystalline nickel and amorphous Ni–B phases in the asdeposited condition. The quantity of amorphous phase increases with boron content w3–6x. Gawrilov and Baudrand give a comparison of physical, mechanical and tribological properties of a borohydride-reduced electroless nickel (5% B weight) and a hypophosphite one (10.5% P weight) w6,7x. The nickel–boron density is very similar to the nickel–phosphorous one of equal *Corresponding author. Tel.: q32-65-374541; fax: q32-65374600. E-mail address: [email protected] (F. Delaunois).
alloy weight content and does not change with heat treatment. The melting point of a 5% B coating is relatively high (1080 8C for Ni–B in comparison with 890 8C for Ni–P). In the as-deposited condition, its electrical resistivity is similar to that of Ni–P alloys (89 mV) and Ni–B coatings are very weakly ferromagnetic. Strength and ductility of the Ni–5% B coatings are only approximately one-fifth of that of high phosphorus content deposits. The principal advantage of electroless Ni–B is its high hardness also in the as-deposited condition, and its superior wear resistance. After the bath, the Ni–B deposit microhardness is generally near 650–750 hv100. Electroless Ni–B coatings are also naturally lubricious w6,7x. To enhance the mechanical and tribological properties of the Ni–B deposits, appropriate heat treatments are performed, in agreement with the substrate. They are modifying deposit structure allowing the precipitation of Ni–B phases according to Ni–B phase diagram. The literature is rich concerning heat treatments explanations for electroless Ni deposits on steel w2,5–8x and more reserved on aluminium substrates w8x. Two different
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 4 1 5 - 2
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Table 1 Chemical analysis of the aluminium alloys used (%) Al alloy
Fe
Si
Mg
Mn
Cu
Cr
Ti
Zn
Be
Sb
B
1050 AS7G06
0.199 0.09
0.049 6.85
0.000 0.51
0.000 -0.01
0.000 -0.002
0.001 –
0.000 0.11
0.016 -0.006
0.001 –
– -0.02
0.002 –
types of heat treatment of electroless Ni–B deposits should be distinguished w8x: the heat treatment at temperatures approximately 200 8C and the heat treatment at temperatures above 280 8C. For the first one, a heat treatment of 1 h 30 min at 190–210 8C improves the deposit adhesion on aluminium substrates but does not change the deposit hardness. Moreover, a heat treatment at 250–260 8C significantly enhances the Ni–B deposit hardness due to the formation of metastable Ni2B and Ni3B crystallized particles for treatment above 250 8C. Above 370–380 8C, the deposit is completely crystallized. Heat treatments carried out at temperatures from approximately 280 to approximately 600 8C (generally for deposits on steel substrates) improve the wear resistance and other tribological characteristics of Ni–B deposits. The final structure of the Ni–B heat-treated deposit is made of Ni–B intermetallic compounds (principally Ni3B and metastable Ni2B) and Ni fcc (f10% depending of the initial amount of B). Maximum hardness is generally found for heat treatment at 400 8C for 1 h (hardness values up to 1200 hv100 reached after 1– 2 h). At 500 8C, a decrease in hardness is observed with time treatment after 15 min (maximum hardness f10 min f1200 hv100) due to the growth of nickel borides. Moreover, for heat treatments at low temperatures approximately 200–300 8C for extended periods, a remarkably high hardness of the Ni–B deposits can be reached, up to 1800–2000 hv100 after 40 weeks, due to a finer nickel boride dispersion than at higher temperatures and with the formation of iron borides when steel substrates are used w6,7x. This paper is the continuation of a first publication: Delaunois et al. w11x. 2. Electroless nickel baths composition Electroless nickel solutions contain a reducing agent to supply electrons for the reduction of nickel. The most powerful reducing agent available for electroless nickel plating is the borohydride ion and sodium borohydride is preferred w7x. To help control reduction, stabilizers are necessary. They control the reduction reaction so that the deposition occurs at a predictable rate and only on the substrate to be plated. They are adsorbed on many colloidal particles in the solution and prevent the reduction of nickel on their surface. w7x Many stabilizers can then be used with good efficiency w18x. The first one is thallium nitrate
w9,17,18x. But due to its toxicity and due to environmental problems, it can be replaced by lead tungstate w10,18x without any problem of bath stability or deposition rate. Some stabilizers can be co-deposited in the Ni–B coating as it is the case for thallium nitrate w8x and can lead to the formation of a ternary alloy such as Ni–B–Tl e.g. w18x. They can also have beneficial effects to brighten the deposit w7x. For thallium nitrate, the stabilizer co-deposition is very important and the global deposition reactions w2,7,8,19x are given in the Eq. (1). McComas w9x has found that the deposits obtained contain between 0.5 and 10% weight of boron and 1–8% weight of metallic thallium. When lead tungstate is used as stabilizer, McComas w10x has found that the deposits obtained contain no lead and between 2.5 and 10% weight of boron. Global deposition reactions for electroless Ni borohydride-reduced with thallium nitrate stabilizer (Eq. (1)): y y 2Ni2qqBHy 4 q4OH ™2NiqBO2 q2H2O ≠ q y y q2H2 4Tl qBH4 q4OH ™4TlqBO2y y y q2H2Oq2H≠ 2 2BH4 q2H2O™2Bq2OH ≠ q5H2
(1)
3. Experimental procedures 3.1. Electroless nickel–boron plating on aluminium substrates The choice of an aluminium substrate is directly linked to the practical use of the covered specimen and to later heat treatments applied. Due to many applications of electroless nickel deposition on complex geometry pieces, it is important to study the behaviour of a cast aluminium alloy. For these reasons, two aluminium alloys are chosen: an extruded 1050 as reference and a cast AS7G06. The chemical composition of these alloys is given in Table 1. The classical heat treatment of asmade AS7G06 aluminium alloy is annealing at 540 8C during 8 h, water quenching and tempering at 180 8C during 4 h. It produces aluminium alloy hardening with precipitation of Mg2Si particles. The general process for electroless Ni–B deposition on these aluminium substrates is the same as in the previous paper w11x: 1. Mechanical polishing of the aluminium substrate,
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Table 2 Optimal composition of Ni–B baths stabilized by thallium nitrate or lead tungstate Bath 1 (gyl)
Bath 2 (gyl)
Hexahydrated nickel chloride (NiCl2Ø6H2O) Then Ni Sodium hydroxide (NaOH)
20 4.94 90
24
Complexing agent
Ethylene diamine (NH2–CH2–CH2–NH2)
90 (100 mlyl)
59 (60 mlyl)
Stabilizer
Thallium nitrate Tlq (TlNO3) Lead tungstate Pb2q (PbWO4) Borohydrure de sodium (NaBH4)
Ni–B solutions Metallic ions Alkalinity reserve
Reducing agent
2. Acid degreasing and deoxidising of the aluminium substrate in a bath containing fluorides, 3. Double zincating conversion to improve electroless deposit adhesion, 4. Intermediate deposition of an electroless 3–4 mm thickness Ni–P layer to protect the aluminium substrates from corrosion by high alkaline Ni borohydride-reduced solution, 5. Electroless nickel–boron deposition. The specimens are properly rinsed with demineralised water between each step. The specimen dimensions are approximately 10 mm thick for a diameter of approximately 25 mm (f17.7 cm2 area). The standard compositions of the electroless Ni–B baths stabilized with thallium nitrate (bath 1) or with lead tungstate (bath 2) are given in Table 2. A Pyrex cell of 1 l contains the Ni–B solution thermostated with an oil bath at a stable temperature near the optimal temperature (95"1 8C). The solution is stirred by rotation of the specimen to be coated with a mechanical stirring rod at a speed of 40 rpm. Evaporation from the solution is condensed and directed back to the solution. The immersion time in the baths is by default of 1 h. Heat treatments under a constant heating rate of 20 8Cymin are performed on some specimen in an air controlled oven with 95% Ar and 5% H2 to avoid chemical surface modifications. Specimens are heated up to temperatures ranging from 160 to 450 8C during 1–4 h. 3.2. Analysis of electroless nickel–boron deposits The thickness and the quality of the deposits are estimated using optical microscopy and SEM analysis. The use of an energy selection X-ray spectrometer allows the identification of the different chemical elements present in the Ni–B deposit, such as Ni, Tl, Pb, P and Al (map scanning). Boron, which is a very light element, cannot be detected due to matrix effects. This analysis (linescan) is performed on a transverse layer of deposit from its external surface to the substrate. The
39
0.11 1.05
0.02 0.48
average chemical quantitative analysis of the Ni–B deposits is obtained by dissolving in regal water, and the solution obtained is analysed by inductively coupled plasma to determine the average contents of Ni, B, Tl, Pb, W, P and Zn in the deposits. To increase the microhardness measuring precision, a Knoop diamond penetrator is used, which gives a diamond-shaped imprint with one diagonal longer than the other. The microhardnesses are made on the cross section of the sample with a LECO M-400-A hardness tester. X-rays diffraction (XRD) patterns of the samples in as-deposited condition and after heat treatments are recorded at room temperature, using a Siemens D500 X-rays u–2u ˚ and no Cu Ka apparatus applying Cu Ka (15 406 A) ˚ radiation. (15 406 A) To estimate the tribological behaviour of the deposits under external solicitations, a scratch test is used. This is a method which gives a good idea of the ‘practical adhesion’ of the deposit on the substrate, taking into account the complexity of the effects of the material microstructure, the external loading and the environmental aspects w12x. During the scratch test, the specimen is moved with a constant scratching speed whilst increasing the indenting load. Scratches are generated on the sample using a Rockwell C diamond stylus (1208 cone, 200-mm radius spherical tip). The minimum load at which the adhesive failure occurs has been defined as the critical load LC, which can be used to accurately characterize the adhesive strength of the depositysubstrate system w13,14x. With modern scratch testing instruments, this critical load can be determined by acoustic emission, optical microscopy, variation in penetration depth or variation in the tangential frictional force between the tip and the sample w14x. In its current form, the scratch test can also be used to generate the same damage mechanisms as are observed in abrasive wear but cannot be simply used to make a quantitative prediction of wear rate w15x. In the present study, to determine the critical loads, the continuously increasing normal load mode is selected with a table speed of approximately 5.28 mmymin, a loading rate of approx-
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Table 3 Average chemical quantitative analysis of the Ni–B deposits after the bath Elements
Ni B Zn P Tl Pb W
Average content (% weight) TlNO3 as stabilizer
PBWO4 as stabilizer
88.78 3.99 0.02 0.31 6.90 – –
92.46 5.75 0.03 0.49 – 1.25 0.01
imately 200 Nymin and an initial load of 0.1 N. A minimum of four tests is made for each after the bath and heat-treated specimen to give an average value of LC. The instrument used was a Revetest CSEM Scratch Tester. 4. Results and discussion The main purpose of this paper is to study the heat treatments applied on Ni–B deposits and their influences on the deposits properties. 4.1. Thallium nitrate as stabilizer In the previous paper, it was shown that the deposition rate of the Ni–B bath stabilized with thallium nitrate was approximately 25–30 mmyh. The average chemical quantitative analysis of the deposit after the bath is given in Table 3. The Ni–B deposits contain approximately 4% weight of B and are intermediate boron content coatings. With approximately 7% weight of Tl co-deposited, it is clear that the deposits obtained with these Ni–B baths compositions do not give a Ni–B binary structure but give a ternary one that is Ni–B–Tl, and that Tl content will influence the Ni–B deposit properties. Many studies have confirmed that as-deposited Ni–B coatings consist of extremely fine crystallites or else they are amorphous in terms of X-rays structure w3,8x. Fig. 1 shows the diffraction pattern of a Ni–B– Tl deposit in the as-deposited condition. The presence of a mixture of one diffuse amorphous peak and sharp crystallineymicrocrystalline nickel peaks can be noticed, as it was the case for the work of Staia and Puchi w16x
Fig. 1. XRD pattern of an electroless Ni–B–Tl coating on aluminium substrate in as-deposited condition.
for electroless Ni–P. The total XRD profile can be separated in the crystalline and amorphous one to take into account both structures w3,16x. The Ni–B–Tl deposit is almost amorphous with an extremely fine Ni crystallized structure near the nanometer. In terms of Knoop microhardness, it was previously shown that under a 100-gf load, the obtained value approaches 630 hk100 (Table 4), which is an exceptional microhardness value after the bath. Heat treatments favour diffusions at the interfaces Ni–B–TlyNi–P and Ni–PyAl. In fact, a SEM mapping realized on a heat-treated deposit at 400 8C for 1 h shows a light diffusion of Ni from the deposit to the Al substrate (Fig. 2). On the other hand, the thallium distribution is not affected by this heat treatment. Fig. 3 shows XRD data after heat treatment at 400 8C for 1 h. The Ni–B–Tl deposit is totally crystallized and contains the following phases: mainly Ni fcc, nickel borides in the Ni3B form and metallic Tl. The metastable Ni2B phase has nearly disappeared. Appropriate heat treatments applied to the Ni–B–Tl deposits allow very high hardnesses to be reached w2,7,8x. For example, a heat
Table 4 Knoop microhardness of a Ni–B–Tl deposit (f31 mm) without bath replenishment, with or without heat treatment
Ni–B–Tl deposit after 1 h (;31 mm) without replenishment Without heat treatment Ni–B–Tl deposit after 1 h (;31 mm) without replenishment With heat treatment (400 8C—1 h)
hk200
hk100
hk50
hk25
535"1
625"15
793"40 1133"10
Failures
Failures
722"15 892"30 Failures?
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Fig. 2. Map scanning of an electroless Ni–B–Tl deposit on aluminium substrate (cross-section) after heat treatment at 400 8C for 1 hyelements distribution (Al, P, Tl, Ni): diffusion layer visualization.
treatment at 400 8C for 1 h hardens the deposit to values near 1130 hk25 (Table 4) due to the maximal precipitation of nickel borides. However, these heat-treated deposits become slightly fragile and microhardness measurements with higher loads lead to deposit failures. Consequently, these heat treatments must be adapted to obtain the best compromise between hardness and wear resistance without brittleness. 4.2. Lead tungstate as stabilizer The deposition rate of the Ni–B bath stabilized with lead tungstate is approximately 20–25 mmyh. The asdeposit Ni–B–(Pb) structure is the same as for the Ni– B–Tl one: a ‘cauliflower’-like structure (Fig. 4) that gives to the deposit the naturally property of autolubricious and increases the wear resistance by reducing the surface contact. Moreover, the Ni–B–(Pb) ‘cauliflower’-like structure is finer and contains less open
porosity than the Ni–B–Tl one. The Ni–B–(Pb) deposit present is perfectly uniform and adherent on the Al substrate (Fig. 5). The open porosity is only present on the surface of the deposit and does not decrease the corrosion resistance of the sample. It is also possible to deposit thick Ni–B layers. For example, a thick deposit can be obtained after immersion of 1 h in six consecutive baths. The deposit thickness reaches in this case 130 mm. The average chemical quantitative analysis of the deposit after the bath is given in Table 3. The Ni–B deposits contain approximately 6% weight of B and are intermediate boron content coatings, such as the Ni–B– Tl ones. The lead content does not exceed 1.5% weight. It is uniformly distributed in the Ni–B deposit and its concentration is near the SEM limit detection. For this reason, the Ni–B deposit obtained from baths stabilized with lead tungstate is a binary one and not a ternary one as for the Ni–B–Tl deposits obtained from baths stabilized with thallium nitrate. Fig. 6 shows that the
244
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Fig. 3. XRD pattern of an electroless Ni–B–Tl deposit on aluminium substrate after heat treatment at 400 8C for 1 h.
Ni–B–(Pb) structure of our deposit is extremely finely crystallized with a Ni crystal structure near the nanometer, which is almost amorphous, as it is the case for as-deposited Ni–B–Tl layer. The total XRD profile can also be separated in the crystalline and amorphous profiles. In term of Knoop microhardness, the value obtained on an as-deposited Ni–B–(Pb) layer reaches approximately 600 hk100 (Table 5) under a 100-gf load. After heat treatments, a diffusion layer appears at the interface Ni–PyAl substrate. Practically invisible after heat treatment at 180 8C for 4 h (Fig. 7), it becomes thicker after heat treatment at 450 8C for 1 h (Fig. 9). The Ni diffusion from the deposit to the Al substrate is
then obvious (Figs. 8 and 10) and the defined compounds formed are a mixture of Ni2Al3 and NiAl3 compounds in accordance with the Ni–Al phase diagram. Fig. 11 shows XRD data after heat treatment at 180 8C for 4 h. The Ni–B–(Pb) deposit begins to
Fig. 4. SEM micrograph of an electroless Ni–B–(Pb) deposit on aluminium substrate.
Fig. 5. Optical micrograph of an electroless Ni–B–(Pb) deposit (;23 mm) on AS7G06 aluminium substrate.
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Fig. 8. Linescan (qualitative chemical analysis) of an electroless Ni– B–(Pb) deposit with heat treatment at 180 8C for 4 h.
Fig. 6. XRD patterns for an electroless Ni–B–(Pb) as-deposited layer on aluminium substrate.
crystallize and the metastable Ni2B phase appears. It is totally crystallized for heat treatment at 400 8C for 1 h and contains the following phases: mainly Ni3B, metastable Ni2B and Ni fcc (Fig. 12). Table 5 shows the Knoop microhardness evolution with temperature and heat treatments time. The Knoop microhardness of the Ni–B–(Pb) deposits increases with the increase of the heat treatment temperature and with the heat treatment time. This is due to the decrease of the amorphous structure of the heat-treated deposits with benefit to the crystallized one. A treatment at 350 8C for 4 h allows approximately 1050 hk50 to be reached due to the fine crystallization of the nickel borides and the total disappearance of the amorphous phase. The optimal heat
Fig. 7. Electroless Ni–B–(Pb) deposit on aluminium substrate after heat treatment at 180 8C for 4 h.
treatment applied on AS7G06 substrate, that is 180 8C for 4 h, hardens the Ni–B–(Pb) deposit to values near 750 hk50. Moreover, these heat-treated Ni–B–(Pb) deposits are less fragile than the same heat-treated Ni– B–Tl ones. Microhardnesses measurements with loads until 50-gf can be realized without apparition of cracks for heat treatments reaching 450 8C for 1 h. 4.3. Scratch tests The amount of information given by the results obtained after the scratch tests on our samples needs a detailed analyse. In fact, the scratches have various morphologies depending on the mechanical behaviour of the deposit during the tests. For these reasons, we only give in this paper the tribological behaviour of the deposits under external solicitations for two chosen
Fig. 9. Electroless Ni–B–(Pb) deposit on aluminium substrate after heat treatment at 450 8C for 1 h.
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Table 5 Knoop microhardness of Ni–B–(Pb) deposits without bath replenishment, with or without heat treatment
As-deposit 180 8C—1 200 8C—1 250 8C—1 260 8C—1 300 8C—1 350 8C—1 400 8C—1 450 8C—1
h h h h h h h h
hk200
hk100
hk50
hk25
516"5 554"15 547"10 513"20 531"5 – – – –
599"10 649"10 637"25 617"15 613"15 721"50 756"50 – –
668"15 718"20 712"50 699"10 722"10 865"50 982"30 988"40 1021"30
726"25 736"20 810"60 751"40 766"30 1070"70 1053"40 1152"50 1114"30
specimens. The complete interpretation of the whole scratch test results will be the object of a later publication. However, we can claim from these scratch tests that the as-deposited Ni–B–Tl and Ni–B–(Pb) layers have good adhesion on the Al substrate (1050 and AS7G06) and are not fragile. The observation of the scratches shows that the delamination damage induced by the indenter is relatively plastic, without any cracks. Heat treatments at high temperatures (e.g. at 400 8C for 1 h) modify the deposit behaviour under solicitation but do not weaken them and no cracks in these heat-treated Ni–B deposits are observed. Moreover, the ‘practical adhesion’ of these deposits onto the substrate is still acceptable. The two chosen specimens for this paper are: an asdeposited Ni–B–(Pb) layer on AS7G06 substrate and a Ni–B–(Pb) deposit on an AS7G06 substrate and heattreated at 400 8C for 1 h. Figs. 13 and 14 show the micrographs of the scratches. These two scratches are very different. In fact, the deposit behaviour with the scratch test are different and the method of deposit delamination too. The critical charges LC found for these
Fig. 10. Linescan (qualitative chemical analysis) of an electroless Ni– B–(Pb) deposit on aluminium substrate after heat treatment at 450 8C for 1 h.
As-deposit 160 8C—4 180 8C—4 200 8C—4 220 8C—4 240 8C—4 260 8C—4 300 8C—4 350 8C—4
h h h h h h h h
hk200
hk100
hk50
Hk25
516"5 522"5 563"15 556"20 532"10 554"5 575"15 – –
599"10 627"20 667"10 655"15 663"10 655"10 712"10 746"20 –
668"15 712"30 757"10 725"20 725"20 732"30 814"35 911"30 1068"35
726"25 782"25 793"15 810"30 822"10 937"50 845"60 1036"60 1181"50
two specimens are approximately 82 and 102 N respectively for the as-deposited Ni–B–(Pb) and for the Ni– B–(Pb) deposit heat treated at 400 8C for 1 h. These values show that the 400 8C—1 h heat treatment enhances the resistance of the Ni–B–(Pb) depositAS7G06 substrate system under external mechanical solicitations and also enhances the ‘practical adhesion’ of the deposit on the substrate. Moreover, the scratch tests made on deposits with the two different substrates Al 1050 and AS7G06 also give different scratches. This point will be treated in a later publication. 5. Conclusions The use of electroless Ni–B baths reduced with sodium borohydride and stabilized with thallium nitrate or lead tungstate allowed homogeneous thin or thick Ni–B deposits on aluminium 1050 or AS7G06 substrate
Fig. 11. XRD patterns for an electroless Ni–B–(Pb) deposit on aluminium substrate after heat treatment at 180 8C for 4 h.
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Fig. 12. XRD patterns for an electroless Ni–B–(Pb) deposit on aluminium substrate after heat treatment at 400 8C for 1 h.
to be realized. Due to health reasons and environmental problems, lead tungstate is preferred. The quality of the as-deposit Ni–B layers are excellent in both cases: they are amorphous, hard, adherent to the substrate and have
very good tribological characteristics under mechanical solicitations. After heat treatment at high temperature, the microhardness of these deposits increases highly (f1050 hk50 for Ni–B–(Pb) at 400 8C during 1 h). The deposits then crystallized in various phases: mainly Ni3B, metastable Ni2B and Ni fcc depending on the temperature and the duration of the treatment. Their tribological characteristics under mechanical solicitations change but remain very good: good adhesion on the substrate and no cracking in the heat-treated deposit. Ni–B–(Pb) deposits seem to have a better comportment in both as-plated and heat-treated stages. Acknowledgments
Fig. 13. Optical micrograph of an electroless Ni–B–(Pb) deposit on AS7G06 aluminium substrate without heat treatment: scratch obtained after scratch test (ls3.9 mm, 40=).
The authors wish to thank the Materials Science’s Service (Materia Nova) of the Faculty of Engineering of Mons (FPMs) (Belgium) and the INISMa (Mons, Belgium) for their technical support, respectively for the XRD apparatus and for the scratch tester. References
Fig. 14. Optical micrograph of an electroless Ni–B–(Pb) deposit on AS7G06 aluminium substrate after heat treatment at 400 8C for 1 h: scratch obtained after scratch test (ls3.9 mm, 40=).
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Heat treatments for electroless nickel–boron plating on aluminium alloys F. Delaunois*, P. Lienard ´ Faculte´ Polytechnique de Mons, Service de Metallurgie, Rue du Joncquois, 53 B-7000 Mons, Belgium Received 4 March 2002; accepted in revised form 24 June 2002
Abstract Electroless nickel–boron baths reduced with sodium borohydride can be stabilized with various agents such as thallium nitrate or lead tungstate with no fundamental modification of deposition rates and stability. To improve the mechanical properties of electroless nickel–boron deposits, various heat treatments are applied. At low temperatures, no fundamental changes in the deposit structure are observed, only an improvement of adhesion on aluminium substrate. The values of the Knoop microhardness obtained on these heat-treated deposits are near 600 hk100. At higher temperatures, structural changes take place and the nickel–boron deposits crystallize. The microhardness rises until 1050 hk50 for heat treatments at 350 8C for 4 h. A diffusion layer between the electroless nickel deposit and the aluminium substrate appears at high heat treatment temperatures. The results of scratch tests show that the Ni–B deposits, with or without heat treatments, have good tribological properties under external solicitations. They are hard, wear and abrasion resistant, and also have good adhesion to the aluminium substrate. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Electroless plating; Nickel; Boron; Aluminium; Heat treatments
1. Introduction Borohydride electroless nickel coatings find extensive applications in aerospace, automotive, chemical and electrical industries specially because of their solderability, high hardness, and wear and abrasion resistances. Similarly to nickel–phosphorus, nickel–boron deposit characteristics change with boron content w1x. The amount of boron contained in borohydride electroless nickel coatings ranges between 1 and 10% weight w2x. The structure of the deposits is a mixture of microcrystalline nickel and amorphous Ni–B phases in the asdeposited condition. The quantity of amorphous phase increases with boron content w3–6x. Gawrilov and Baudrand give a comparison of physical, mechanical and tribological properties of a borohydride-reduced electroless nickel (5% B weight) and a hypophosphite one (10.5% P weight) w6,7x. The nickel–boron density is very similar to the nickel–phosphorous one of equal *Corresponding author. Tel.: q32-65-374541; fax: q32-65374600. E-mail address: [email protected] (F. Delaunois).
alloy weight content and does not change with heat treatment. The melting point of a 5% B coating is relatively high (1080 8C for Ni–B in comparison with 890 8C for Ni–P). In the as-deposited condition, its electrical resistivity is similar to that of Ni–P alloys (89 mV) and Ni–B coatings are very weakly ferromagnetic. Strength and ductility of the Ni–5% B coatings are only approximately one-fifth of that of high phosphorus content deposits. The principal advantage of electroless Ni–B is its high hardness also in the as-deposited condition, and its superior wear resistance. After the bath, the Ni–B deposit microhardness is generally near 650–750 hv100. Electroless Ni–B coatings are also naturally lubricious w6,7x. To enhance the mechanical and tribological properties of the Ni–B deposits, appropriate heat treatments are performed, in agreement with the substrate. They are modifying deposit structure allowing the precipitation of Ni–B phases according to Ni–B phase diagram. The literature is rich concerning heat treatments explanations for electroless Ni deposits on steel w2,5–8x and more reserved on aluminium substrates w8x. Two different
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 4 1 5 - 2
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Table 1 Chemical analysis of the aluminium alloys used (%) Al alloy
Fe
Si
Mg
Mn
Cu
Cr
Ti
Zn
Be
Sb
B
1050 AS7G06
0.199 0.09
0.049 6.85
0.000 0.51
0.000 -0.01
0.000 -0.002
0.001 –
0.000 0.11
0.016 -0.006
0.001 –
– -0.02
0.002 –
types of heat treatment of electroless Ni–B deposits should be distinguished w8x: the heat treatment at temperatures approximately 200 8C and the heat treatment at temperatures above 280 8C. For the first one, a heat treatment of 1 h 30 min at 190–210 8C improves the deposit adhesion on aluminium substrates but does not change the deposit hardness. Moreover, a heat treatment at 250–260 8C significantly enhances the Ni–B deposit hardness due to the formation of metastable Ni2B and Ni3B crystallized particles for treatment above 250 8C. Above 370–380 8C, the deposit is completely crystallized. Heat treatments carried out at temperatures from approximately 280 to approximately 600 8C (generally for deposits on steel substrates) improve the wear resistance and other tribological characteristics of Ni–B deposits. The final structure of the Ni–B heat-treated deposit is made of Ni–B intermetallic compounds (principally Ni3B and metastable Ni2B) and Ni fcc (f10% depending of the initial amount of B). Maximum hardness is generally found for heat treatment at 400 8C for 1 h (hardness values up to 1200 hv100 reached after 1– 2 h). At 500 8C, a decrease in hardness is observed with time treatment after 15 min (maximum hardness f10 min f1200 hv100) due to the growth of nickel borides. Moreover, for heat treatments at low temperatures approximately 200–300 8C for extended periods, a remarkably high hardness of the Ni–B deposits can be reached, up to 1800–2000 hv100 after 40 weeks, due to a finer nickel boride dispersion than at higher temperatures and with the formation of iron borides when steel substrates are used w6,7x. This paper is the continuation of a first publication: Delaunois et al. w11x. 2. Electroless nickel baths composition Electroless nickel solutions contain a reducing agent to supply electrons for the reduction of nickel. The most powerful reducing agent available for electroless nickel plating is the borohydride ion and sodium borohydride is preferred w7x. To help control reduction, stabilizers are necessary. They control the reduction reaction so that the deposition occurs at a predictable rate and only on the substrate to be plated. They are adsorbed on many colloidal particles in the solution and prevent the reduction of nickel on their surface. w7x Many stabilizers can then be used with good efficiency w18x. The first one is thallium nitrate
w9,17,18x. But due to its toxicity and due to environmental problems, it can be replaced by lead tungstate w10,18x without any problem of bath stability or deposition rate. Some stabilizers can be co-deposited in the Ni–B coating as it is the case for thallium nitrate w8x and can lead to the formation of a ternary alloy such as Ni–B–Tl e.g. w18x. They can also have beneficial effects to brighten the deposit w7x. For thallium nitrate, the stabilizer co-deposition is very important and the global deposition reactions w2,7,8,19x are given in the Eq. (1). McComas w9x has found that the deposits obtained contain between 0.5 and 10% weight of boron and 1–8% weight of metallic thallium. When lead tungstate is used as stabilizer, McComas w10x has found that the deposits obtained contain no lead and between 2.5 and 10% weight of boron. Global deposition reactions for electroless Ni borohydride-reduced with thallium nitrate stabilizer (Eq. (1)): y y 2Ni2qqBHy 4 q4OH ™2NiqBO2 q2H2O ≠ q y y q2H2 4Tl qBH4 q4OH ™4TlqBO2y y y q2H2Oq2H≠ 2 2BH4 q2H2O™2Bq2OH ≠ q5H2
(1)
3. Experimental procedures 3.1. Electroless nickel–boron plating on aluminium substrates The choice of an aluminium substrate is directly linked to the practical use of the covered specimen and to later heat treatments applied. Due to many applications of electroless nickel deposition on complex geometry pieces, it is important to study the behaviour of a cast aluminium alloy. For these reasons, two aluminium alloys are chosen: an extruded 1050 as reference and a cast AS7G06. The chemical composition of these alloys is given in Table 1. The classical heat treatment of asmade AS7G06 aluminium alloy is annealing at 540 8C during 8 h, water quenching and tempering at 180 8C during 4 h. It produces aluminium alloy hardening with precipitation of Mg2Si particles. The general process for electroless Ni–B deposition on these aluminium substrates is the same as in the previous paper w11x: 1. Mechanical polishing of the aluminium substrate,
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Table 2 Optimal composition of Ni–B baths stabilized by thallium nitrate or lead tungstate Bath 1 (gyl)
Bath 2 (gyl)
Hexahydrated nickel chloride (NiCl2Ø6H2O) Then Ni Sodium hydroxide (NaOH)
20 4.94 90
24
Complexing agent
Ethylene diamine (NH2–CH2–CH2–NH2)
90 (100 mlyl)
59 (60 mlyl)
Stabilizer
Thallium nitrate Tlq (TlNO3) Lead tungstate Pb2q (PbWO4) Borohydrure de sodium (NaBH4)
Ni–B solutions Metallic ions Alkalinity reserve
Reducing agent
2. Acid degreasing and deoxidising of the aluminium substrate in a bath containing fluorides, 3. Double zincating conversion to improve electroless deposit adhesion, 4. Intermediate deposition of an electroless 3–4 mm thickness Ni–P layer to protect the aluminium substrates from corrosion by high alkaline Ni borohydride-reduced solution, 5. Electroless nickel–boron deposition. The specimens are properly rinsed with demineralised water between each step. The specimen dimensions are approximately 10 mm thick for a diameter of approximately 25 mm (f17.7 cm2 area). The standard compositions of the electroless Ni–B baths stabilized with thallium nitrate (bath 1) or with lead tungstate (bath 2) are given in Table 2. A Pyrex cell of 1 l contains the Ni–B solution thermostated with an oil bath at a stable temperature near the optimal temperature (95"1 8C). The solution is stirred by rotation of the specimen to be coated with a mechanical stirring rod at a speed of 40 rpm. Evaporation from the solution is condensed and directed back to the solution. The immersion time in the baths is by default of 1 h. Heat treatments under a constant heating rate of 20 8Cymin are performed on some specimen in an air controlled oven with 95% Ar and 5% H2 to avoid chemical surface modifications. Specimens are heated up to temperatures ranging from 160 to 450 8C during 1–4 h. 3.2. Analysis of electroless nickel–boron deposits The thickness and the quality of the deposits are estimated using optical microscopy and SEM analysis. The use of an energy selection X-ray spectrometer allows the identification of the different chemical elements present in the Ni–B deposit, such as Ni, Tl, Pb, P and Al (map scanning). Boron, which is a very light element, cannot be detected due to matrix effects. This analysis (linescan) is performed on a transverse layer of deposit from its external surface to the substrate. The
39
0.11 1.05
0.02 0.48
average chemical quantitative analysis of the Ni–B deposits is obtained by dissolving in regal water, and the solution obtained is analysed by inductively coupled plasma to determine the average contents of Ni, B, Tl, Pb, W, P and Zn in the deposits. To increase the microhardness measuring precision, a Knoop diamond penetrator is used, which gives a diamond-shaped imprint with one diagonal longer than the other. The microhardnesses are made on the cross section of the sample with a LECO M-400-A hardness tester. X-rays diffraction (XRD) patterns of the samples in as-deposited condition and after heat treatments are recorded at room temperature, using a Siemens D500 X-rays u–2u ˚ and no Cu Ka apparatus applying Cu Ka (15 406 A) ˚ radiation. (15 406 A) To estimate the tribological behaviour of the deposits under external solicitations, a scratch test is used. This is a method which gives a good idea of the ‘practical adhesion’ of the deposit on the substrate, taking into account the complexity of the effects of the material microstructure, the external loading and the environmental aspects w12x. During the scratch test, the specimen is moved with a constant scratching speed whilst increasing the indenting load. Scratches are generated on the sample using a Rockwell C diamond stylus (1208 cone, 200-mm radius spherical tip). The minimum load at which the adhesive failure occurs has been defined as the critical load LC, which can be used to accurately characterize the adhesive strength of the depositysubstrate system w13,14x. With modern scratch testing instruments, this critical load can be determined by acoustic emission, optical microscopy, variation in penetration depth or variation in the tangential frictional force between the tip and the sample w14x. In its current form, the scratch test can also be used to generate the same damage mechanisms as are observed in abrasive wear but cannot be simply used to make a quantitative prediction of wear rate w15x. In the present study, to determine the critical loads, the continuously increasing normal load mode is selected with a table speed of approximately 5.28 mmymin, a loading rate of approx-
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Table 3 Average chemical quantitative analysis of the Ni–B deposits after the bath Elements
Ni B Zn P Tl Pb W
Average content (% weight) TlNO3 as stabilizer
PBWO4 as stabilizer
88.78 3.99 0.02 0.31 6.90 – –
92.46 5.75 0.03 0.49 – 1.25 0.01
imately 200 Nymin and an initial load of 0.1 N. A minimum of four tests is made for each after the bath and heat-treated specimen to give an average value of LC. The instrument used was a Revetest CSEM Scratch Tester. 4. Results and discussion The main purpose of this paper is to study the heat treatments applied on Ni–B deposits and their influences on the deposits properties. 4.1. Thallium nitrate as stabilizer In the previous paper, it was shown that the deposition rate of the Ni–B bath stabilized with thallium nitrate was approximately 25–30 mmyh. The average chemical quantitative analysis of the deposit after the bath is given in Table 3. The Ni–B deposits contain approximately 4% weight of B and are intermediate boron content coatings. With approximately 7% weight of Tl co-deposited, it is clear that the deposits obtained with these Ni–B baths compositions do not give a Ni–B binary structure but give a ternary one that is Ni–B–Tl, and that Tl content will influence the Ni–B deposit properties. Many studies have confirmed that as-deposited Ni–B coatings consist of extremely fine crystallites or else they are amorphous in terms of X-rays structure w3,8x. Fig. 1 shows the diffraction pattern of a Ni–B– Tl deposit in the as-deposited condition. The presence of a mixture of one diffuse amorphous peak and sharp crystallineymicrocrystalline nickel peaks can be noticed, as it was the case for the work of Staia and Puchi w16x
Fig. 1. XRD pattern of an electroless Ni–B–Tl coating on aluminium substrate in as-deposited condition.
for electroless Ni–P. The total XRD profile can be separated in the crystalline and amorphous one to take into account both structures w3,16x. The Ni–B–Tl deposit is almost amorphous with an extremely fine Ni crystallized structure near the nanometer. In terms of Knoop microhardness, it was previously shown that under a 100-gf load, the obtained value approaches 630 hk100 (Table 4), which is an exceptional microhardness value after the bath. Heat treatments favour diffusions at the interfaces Ni–B–TlyNi–P and Ni–PyAl. In fact, a SEM mapping realized on a heat-treated deposit at 400 8C for 1 h shows a light diffusion of Ni from the deposit to the Al substrate (Fig. 2). On the other hand, the thallium distribution is not affected by this heat treatment. Fig. 3 shows XRD data after heat treatment at 400 8C for 1 h. The Ni–B–Tl deposit is totally crystallized and contains the following phases: mainly Ni fcc, nickel borides in the Ni3B form and metallic Tl. The metastable Ni2B phase has nearly disappeared. Appropriate heat treatments applied to the Ni–B–Tl deposits allow very high hardnesses to be reached w2,7,8x. For example, a heat
Table 4 Knoop microhardness of a Ni–B–Tl deposit (f31 mm) without bath replenishment, with or without heat treatment
Ni–B–Tl deposit after 1 h (;31 mm) without replenishment Without heat treatment Ni–B–Tl deposit after 1 h (;31 mm) without replenishment With heat treatment (400 8C—1 h)
hk200
hk100
hk50
hk25
535"1
625"15
793"40 1133"10
Failures
Failures
722"15 892"30 Failures?
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Fig. 2. Map scanning of an electroless Ni–B–Tl deposit on aluminium substrate (cross-section) after heat treatment at 400 8C for 1 hyelements distribution (Al, P, Tl, Ni): diffusion layer visualization.
treatment at 400 8C for 1 h hardens the deposit to values near 1130 hk25 (Table 4) due to the maximal precipitation of nickel borides. However, these heat-treated deposits become slightly fragile and microhardness measurements with higher loads lead to deposit failures. Consequently, these heat treatments must be adapted to obtain the best compromise between hardness and wear resistance without brittleness. 4.2. Lead tungstate as stabilizer The deposition rate of the Ni–B bath stabilized with lead tungstate is approximately 20–25 mmyh. The asdeposit Ni–B–(Pb) structure is the same as for the Ni– B–Tl one: a ‘cauliflower’-like structure (Fig. 4) that gives to the deposit the naturally property of autolubricious and increases the wear resistance by reducing the surface contact. Moreover, the Ni–B–(Pb) ‘cauliflower’-like structure is finer and contains less open
porosity than the Ni–B–Tl one. The Ni–B–(Pb) deposit present is perfectly uniform and adherent on the Al substrate (Fig. 5). The open porosity is only present on the surface of the deposit and does not decrease the corrosion resistance of the sample. It is also possible to deposit thick Ni–B layers. For example, a thick deposit can be obtained after immersion of 1 h in six consecutive baths. The deposit thickness reaches in this case 130 mm. The average chemical quantitative analysis of the deposit after the bath is given in Table 3. The Ni–B deposits contain approximately 6% weight of B and are intermediate boron content coatings, such as the Ni–B– Tl ones. The lead content does not exceed 1.5% weight. It is uniformly distributed in the Ni–B deposit and its concentration is near the SEM limit detection. For this reason, the Ni–B deposit obtained from baths stabilized with lead tungstate is a binary one and not a ternary one as for the Ni–B–Tl deposits obtained from baths stabilized with thallium nitrate. Fig. 6 shows that the
244
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Fig. 3. XRD pattern of an electroless Ni–B–Tl deposit on aluminium substrate after heat treatment at 400 8C for 1 h.
Ni–B–(Pb) structure of our deposit is extremely finely crystallized with a Ni crystal structure near the nanometer, which is almost amorphous, as it is the case for as-deposited Ni–B–Tl layer. The total XRD profile can also be separated in the crystalline and amorphous profiles. In term of Knoop microhardness, the value obtained on an as-deposited Ni–B–(Pb) layer reaches approximately 600 hk100 (Table 5) under a 100-gf load. After heat treatments, a diffusion layer appears at the interface Ni–PyAl substrate. Practically invisible after heat treatment at 180 8C for 4 h (Fig. 7), it becomes thicker after heat treatment at 450 8C for 1 h (Fig. 9). The Ni diffusion from the deposit to the Al substrate is
then obvious (Figs. 8 and 10) and the defined compounds formed are a mixture of Ni2Al3 and NiAl3 compounds in accordance with the Ni–Al phase diagram. Fig. 11 shows XRD data after heat treatment at 180 8C for 4 h. The Ni–B–(Pb) deposit begins to
Fig. 4. SEM micrograph of an electroless Ni–B–(Pb) deposit on aluminium substrate.
Fig. 5. Optical micrograph of an electroless Ni–B–(Pb) deposit (;23 mm) on AS7G06 aluminium substrate.
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Fig. 8. Linescan (qualitative chemical analysis) of an electroless Ni– B–(Pb) deposit with heat treatment at 180 8C for 4 h.
Fig. 6. XRD patterns for an electroless Ni–B–(Pb) as-deposited layer on aluminium substrate.
crystallize and the metastable Ni2B phase appears. It is totally crystallized for heat treatment at 400 8C for 1 h and contains the following phases: mainly Ni3B, metastable Ni2B and Ni fcc (Fig. 12). Table 5 shows the Knoop microhardness evolution with temperature and heat treatments time. The Knoop microhardness of the Ni–B–(Pb) deposits increases with the increase of the heat treatment temperature and with the heat treatment time. This is due to the decrease of the amorphous structure of the heat-treated deposits with benefit to the crystallized one. A treatment at 350 8C for 4 h allows approximately 1050 hk50 to be reached due to the fine crystallization of the nickel borides and the total disappearance of the amorphous phase. The optimal heat
Fig. 7. Electroless Ni–B–(Pb) deposit on aluminium substrate after heat treatment at 180 8C for 4 h.
treatment applied on AS7G06 substrate, that is 180 8C for 4 h, hardens the Ni–B–(Pb) deposit to values near 750 hk50. Moreover, these heat-treated Ni–B–(Pb) deposits are less fragile than the same heat-treated Ni– B–Tl ones. Microhardnesses measurements with loads until 50-gf can be realized without apparition of cracks for heat treatments reaching 450 8C for 1 h. 4.3. Scratch tests The amount of information given by the results obtained after the scratch tests on our samples needs a detailed analyse. In fact, the scratches have various morphologies depending on the mechanical behaviour of the deposit during the tests. For these reasons, we only give in this paper the tribological behaviour of the deposits under external solicitations for two chosen
Fig. 9. Electroless Ni–B–(Pb) deposit on aluminium substrate after heat treatment at 450 8C for 1 h.
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Table 5 Knoop microhardness of Ni–B–(Pb) deposits without bath replenishment, with or without heat treatment
As-deposit 180 8C—1 200 8C—1 250 8C—1 260 8C—1 300 8C—1 350 8C—1 400 8C—1 450 8C—1
h h h h h h h h
hk200
hk100
hk50
hk25
516"5 554"15 547"10 513"20 531"5 – – – –
599"10 649"10 637"25 617"15 613"15 721"50 756"50 – –
668"15 718"20 712"50 699"10 722"10 865"50 982"30 988"40 1021"30
726"25 736"20 810"60 751"40 766"30 1070"70 1053"40 1152"50 1114"30
specimens. The complete interpretation of the whole scratch test results will be the object of a later publication. However, we can claim from these scratch tests that the as-deposited Ni–B–Tl and Ni–B–(Pb) layers have good adhesion on the Al substrate (1050 and AS7G06) and are not fragile. The observation of the scratches shows that the delamination damage induced by the indenter is relatively plastic, without any cracks. Heat treatments at high temperatures (e.g. at 400 8C for 1 h) modify the deposit behaviour under solicitation but do not weaken them and no cracks in these heat-treated Ni–B deposits are observed. Moreover, the ‘practical adhesion’ of these deposits onto the substrate is still acceptable. The two chosen specimens for this paper are: an asdeposited Ni–B–(Pb) layer on AS7G06 substrate and a Ni–B–(Pb) deposit on an AS7G06 substrate and heattreated at 400 8C for 1 h. Figs. 13 and 14 show the micrographs of the scratches. These two scratches are very different. In fact, the deposit behaviour with the scratch test are different and the method of deposit delamination too. The critical charges LC found for these
Fig. 10. Linescan (qualitative chemical analysis) of an electroless Ni– B–(Pb) deposit on aluminium substrate after heat treatment at 450 8C for 1 h.
As-deposit 160 8C—4 180 8C—4 200 8C—4 220 8C—4 240 8C—4 260 8C—4 300 8C—4 350 8C—4
h h h h h h h h
hk200
hk100
hk50
Hk25
516"5 522"5 563"15 556"20 532"10 554"5 575"15 – –
599"10 627"20 667"10 655"15 663"10 655"10 712"10 746"20 –
668"15 712"30 757"10 725"20 725"20 732"30 814"35 911"30 1068"35
726"25 782"25 793"15 810"30 822"10 937"50 845"60 1036"60 1181"50
two specimens are approximately 82 and 102 N respectively for the as-deposited Ni–B–(Pb) and for the Ni– B–(Pb) deposit heat treated at 400 8C for 1 h. These values show that the 400 8C—1 h heat treatment enhances the resistance of the Ni–B–(Pb) depositAS7G06 substrate system under external mechanical solicitations and also enhances the ‘practical adhesion’ of the deposit on the substrate. Moreover, the scratch tests made on deposits with the two different substrates Al 1050 and AS7G06 also give different scratches. This point will be treated in a later publication. 5. Conclusions The use of electroless Ni–B baths reduced with sodium borohydride and stabilized with thallium nitrate or lead tungstate allowed homogeneous thin or thick Ni–B deposits on aluminium 1050 or AS7G06 substrate
Fig. 11. XRD patterns for an electroless Ni–B–(Pb) deposit on aluminium substrate after heat treatment at 180 8C for 4 h.
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Fig. 12. XRD patterns for an electroless Ni–B–(Pb) deposit on aluminium substrate after heat treatment at 400 8C for 1 h.
to be realized. Due to health reasons and environmental problems, lead tungstate is preferred. The quality of the as-deposit Ni–B layers are excellent in both cases: they are amorphous, hard, adherent to the substrate and have
very good tribological characteristics under mechanical solicitations. After heat treatment at high temperature, the microhardness of these deposits increases highly (f1050 hk50 for Ni–B–(Pb) at 400 8C during 1 h). The deposits then crystallized in various phases: mainly Ni3B, metastable Ni2B and Ni fcc depending on the temperature and the duration of the treatment. Their tribological characteristics under mechanical solicitations change but remain very good: good adhesion on the substrate and no cracking in the heat-treated deposit. Ni–B–(Pb) deposits seem to have a better comportment in both as-plated and heat-treated stages. Acknowledgments
Fig. 13. Optical micrograph of an electroless Ni–B–(Pb) deposit on AS7G06 aluminium substrate without heat treatment: scratch obtained after scratch test (ls3.9 mm, 40=).
The authors wish to thank the Materials Science’s Service (Materia Nova) of the Faculty of Engineering of Mons (FPMs) (Belgium) and the INISMa (Mons, Belgium) for their technical support, respectively for the XRD apparatus and for the scratch tester. References
Fig. 14. Optical micrograph of an electroless Ni–B–(Pb) deposit on AS7G06 aluminium substrate after heat treatment at 400 8C for 1 h: scratch obtained after scratch test (ls3.9 mm, 40=).
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