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TiN and CrN PVD coatings on electroless nickel-coated steel substrates
474
Surface and Coatings Technologi,
(i()
(1993) 474—479
TiN and CrN PVD coatings on electroless nickel-coated steel substrates A. Leyland, M. Bin-Sudin, A. S. James, M. R. Kalantary, P. B. Wells and A. Matthews Research Centre in Surffice Engineering, Unjiersitv of hull, Hull, Hu,nherside, h!U6 7RX (UK
J. Housden and B. Garside Teciac Ltd., Stow Cum Quv, Cambridge. CB5 9AB (UK)
Abstract Plasma-assisted (PA) PVD ceramic coatings such as TiN have so far achieved only very limited use on cheap low-alloy steels. owing to problems relating to both corrosion resistance and the need for load support from the underlying material. Here we report tests to assess the wear and corrosion performance of TiN and CrN PAPVD coatings on phosphorus-doped electroless nickel (ENiP)-coated steels. It is shown that this route offers a potentially cost-effective means of utilising PAPVD ceramic films on lower grade steels. In particular, CrN/ENiP on A151 304 stainless steel is shown to exhibit a promising combination of wct abrasion resistance with good corrosion properties.
I. Introduction PAPVD TiN coatings are now widely used on HSS cutting tools to extend life and improve productivity and product quality [I]. On forming tools, they also find an increasing number of applications, although here too, only expensive alloy steels (e.g. hot working steels or HSS) are generally suitable. The market for TiN is thus constrained by factors such as the need for a high level of substrate load support, a limiting maximum hard-coating thickness (due to internal stresses) and consequentially poor corrosion resistance from the coating/substrate pair. If the hard-coating treatment could be cost-effectively applied to low-carbon steels (and other cheaper substrates) the potential range of applications would be considerably widened. In a recent survey [2], it was shown that, in the UK, the TiN PVD market is less than one-fiftieth of either the electroplating or thermal-spraying market by value, and probably less than one-thousandth by volume (worldwide, these figures are likely to be similar). There is a clear need, from the diverse viewpoints of both total system performance and increased environmental awareness, to try to widen the applicability of the PAPVD processes by permitting their use on cheaper low-grade steels. Cost will obviously be one major factor in the equation, but the potential for enhanced or unique capabilities from the optimal substrate/coating combination also needs to be considered. Wear-resistant coatings presently used for improving the performance of cheap substrate materials provide many advantages, but there are also major disadvan-
0257—8972/93/56.00
tages. Electroplated chromium, for instance, can be deposited with high thickness and (with a nickel interlayer) good corrosion properties, but the environmental drawbacks of these types of process (for example, in by-product disposal) are now an increasingly sensitive issue. Electroless nickel (particularly when incorporating phosphorus, which enables it to he heat-treated to high hardness) is an alternative to hard chrome with good coating integrity and less environmental impact. However, it is more expensive to deposit and process, particularly at higher thicknesses (above 50 jim). Also. its corrosion performance may not necessarily be satisfactory in certain applications. The thickness limitations of PAPVD TiN (due to coating internal stresses) restrict its corrosion performance (accelerated pitting via through-coating porosity being the main problem [3—5]). There is evidence to suggest that CrN can be deposited by PAPVD techniques onto steel substrate materials with lower internal compressive stresses, and thus to greater thickness (i.e. 10 jim or more [6]). The increased thickness should reduce through-coating porosity, whilst also improving resistance to abrasive wear by hard particles. Perhaps more importantly, however, its electrochemical compatibility with an electroless nickel interlayer should be better than that of the TiN/Ni system (it is known, for example, that Cr/Ni-layered electroplate can provide excellent corrosion resistance—superior to that which is available from either chrome electroplate or electroless nickel [7]). The improved wear properties of CrN over Cr electroplate reduces the thickness of coating required.
1993
Elsevier Sequoia. All rights reservcd
A. Leyland et al.
/
TiN and CrN PVD coatings on electroless Ni-coated steels
particularly if additional load support is available from a precipitation-hardened electroless nickel layer. For the latter, typically layers 20—30 jim thick are adequate for most of the established applications. It is therefore apparent that there could be an overall materials (and possibly cost) saving in moving from thick electro-(or electroless) plate to a “duplex” (i.e. two processes combined) system such as TiN or CrN above a comparatively thin ENiP layer. Thus, for a wide variety of reasons, the combination of a hard PAPVD coating with ENiP offers many attractions. An additional processing aspect which we examine here is the hardening of ENiP by heat treatment (which is traditionally carried out in a separate process stage at a temperature of 400—500 °C).This is the typical PAPVD processing range and there is therefore the potential to harden the ENiP durii~g hard coating deposition, giving improved load support to the PVD treatment whilst eliminating a treatment stage (with associated cost benefits). 2. Experimental details 2,1. Deposition details The substrate materials employed were mild steel and AISI 304 stainless steel. These were polished to 0.05 jim roughness, then wet-blasted in a 10 jim alumina slurry to provide a controlled roughened surface with an Ra value of 0.5 jim. This was intended to provide similar pre- and post-coated roughnesses, ensuring repeatability in corrosion and wear tests. The electroless nickel—phosphorus coatings were deposited using a standard autocatalytic technique, with a commercial Lea Ronal solution (NPA 8009). A coating of 16 jim nominal thickness was produced, with an asreceived hardness of about 500 Hk (200 g). The phosphorus content was approximately 10 at.%. The PVD coatings were deposited by Tecvac Ltd., using a commercial thermionic triode-plasma-assisted electron beam (EB) PYD system. The main deposition parameters are presented in Table 1. In the case of the duplex ENiP/hard-coating combinations the PVD-coating cycle was intended to promote precipitation hardening of the ENiP layer. As mentioned previously, this hardening would be produced commercially by a sepaRa
TABLE 1. PVD coating condi~ons
TiN
CrN
Deposition temp ( C) Deposition time (mm) Evaporation rate (g min~) Coating thickness (urn)
400—480 90 0.3 1.75
330—470 60 0.86 1.25
475
rate heat-treatment cycle at a similar temperature (about 400—500 °C)under vacuum or inert-gas-furnace conditions, to give a layer hardness of up to 1000 Hk. To ensure direct comparability, the “ENiP-only” coatings used in the present work were subjected to a plasma heat-treatment cycle in an argon d.c. triode plasma, to simulate the hardening effects of the PVD process on the “duplex” samples.
2.2. Testing Thickness was measured using a ball-crater technique, hardness using a Leitz “Miniload” tester with a Knoop dtamond, and adhesion was assessed using a VTT Tech scratch tester with acoustic emission and tangential force monitoring. For abrasive wear tests an ASTM-type rubber-wheel wet abrasion tester was used, shown schematically in Fig. 1 [8]. The normal force on the sample was 10 N and the rotational speed was 200 rev min The abrasive grit was 55 jim alumina. The weight loss of the sample was measured every 200 revolutions, up to a total of 1000. Corrosion studies were carried out using a Schlumberger test facility. Polarisation measurements were obtained at a sweep rate of 100 mY min’ using a 200 cm3 solution of 0.5 M H2S04 electrolyte, which was ~.
replaced after each sweep run. This solution was sparged with air for 10 mm prior to corrosion measurements being made. A potential sweep range of —1.0 to +2.5 V was employed, the reference being a standard calomel electrode (SCE). The exposed area the work2, the auxiliary (orof“counter”)ing electrode was 0.28 cm electrode was platinum. Salt-water immersion tests have also been carried out on mild steel in combination with ENiP, TiN and duplex ENiP/TiN to provide a visual indication of relative corrosion resistance.
Rubber
Sample
Weights
Abrasive slurry ____________________________________ Fig. 1. Schematic representation of rubber-wheel abrasive wear test.
476
A. Leyland et al.
/
TiN and CrN PVD coatings on electroless Ni-coated steels
3. Results and discussion 3.1. Hardness Owing to the roughened nature of the surfaces, it was difficult to measure hardness values at low loads, since the small indent lengths were not reliably discernable. Thus, a 200 g load was employed to give a “composite” hardness figure for the layered surface. These results are summarised in Table 2. As one would expect, the stainless steel substrate was significantly harder than the mild steel, and this is reflected, to some extent, in the composite hardness levels recorded. With the two-layer system, values over 1000 Hk are achievable, equivalent to or greater than that of a fully heat-treated high-speed steel or of chromium electroplate. It can be observed that the ENiP coatings alone (which were subjected to the plasma heat-treatment cycle) gave a hardness of about 750 Hk at the 200 g load employed. This would suggest that the true hardness of the treated ENiP was in fact slightly higher (taking account of substrate hardness contribution effects) and roughly comparable to what might be provided by a conventional heat-treatment stage. The PVD coating stage could, therefore, be expected to give the desirable additional benefit of an integral hardening mechanism, effectively making the combined ENiP/PYD coating procedure cheaper than might be inferred from the sum cost of the two individual treatments. It is interesting to note that despite the CrN coating being thinner than the TiN, the “composite” hardness (in combination with ENiP) is superior (Table 1). We attribute this to the CrN having a higher hardness (although it would also appear to be more brittle, as evidenced by the poor composite hardness on, for instance, mild steel alone). The superior load support of the ENiP (and to some extent the underlying stainless steel), together with the chemical compatability of the Ni/Cr(N) combination (when compared with Ni/Ti(N)), probably combine to alleviate the brittle characteristics of the CrN, whilst promoting the benefits of its superior hardness and (as is demonstrated in the present work) abrasion resistance. 3.2. Adhesion Owing to the rough nature of the surface, scratch-test acoustic emission was not a reliable indicator of debond-
ing; similarly, significant step changes in tangential friction force were not detected. It was necessary, therefore, to utilise visual observation to detect debonding. Since the CrN was similar in colour to the ENiP (and the substrates), adhesion changes were most easily assessed for the TiN-coated samples. Typical critical load values obtained for TiN are presented in Table 3. It is well known that the scratch test is a comparative technique which should be used on similar coatings with similar thickness and substrate hardness. However, it is significant that in these trials it was shown that the composite duplex coating system can achieve critical loads greater than 30 N and even (in the case of TiN + ENiP on 304 stainless steel) greater than 50 N. This is a respectable figure for TiN on, for instance, hardened high-speed steel and further confirms the considerable benefits this duplex system might provide. 3.3. Abrasive wear resistance Figures 2 and 3 show the abrasive wheel test results. Figure 2 shows the results for mild steel substrate material. As expected, the uncoated performance was poor. Perhaps less expected are the results of either PVD coating alone on mild steel. Both performed badly, owing to poor substrate load support. The performance of the ENiP alone is initially surprising, apparently being better than that of the duplex treatments during the early stages of the test (with minimal weight loss). The wear mechanism of the ENiP is, however, characterised by initial “smearing” of the (rough, nodular) surface, followed by eventual large-scale removal of the coating (presumably by fatigue fracture) after a certain incubaTABLE 3. Scratch test critical load (L~,(values for TiN
_____________________________________________________ Substratecoating
~
MS/TIN MSENiPTiN SS/TiN SS/ENiP/TiN
9.3 ±1.0 34.3 + 1.0 197 + 35 53.7+2.1
_______________________ ~
0.03
0
0.025
(N)
____________ -
002
TABLE 2. Knoop “composite” hardness values(Hk 200g) Mild steel substrate
0015
Stainless-steel substrate
MS 160.3 ±7.4 SS 425.0±3.0 MS/TiN 426.8 ±81.2 SS/TiN 903.0 ±103.6 MS/CrN 411.0±12.8 SS/CrN 932.5± 146.8 MS/ENiP 743.0+61.4 SS/ENiP 767.5±63.2 MS/ENiP/TiN 810.0 ±50.2 SS/ENIP/TiN 1022.0 ±111.7 MS ENiP CrN 10120+720 SS ENiP CrN 13080±3015 ______________________________________________
001
~ 0
-
_______________________________________ 0
200
400
600
800
1000
Revolutions Fig. 2. Abrasive wear test data for mild steel substrates. • MS; ~. MS ENiP • MS TiN • MS UrN V MS ENiP TiN MS ENiPCrN.
A. Leyland et a!.
/
TiN and CrN PVD coatings on electroless Ni-coated steels
TABLE 4. Corrosion potential and current density of coating combinations
0.03 0) ~
0025
j :~ E
477
0.02
__________________________________
0
200
400
600
800
1000
Substrate/coating
Eoorr/mV
MS/ENiP/TiN
—675
MS/ENiP/CrN
—635
0.55
SS 55/TiN SS/ENiP 55/ENiP/TiN SS/CrN SS/ENiP/CrN
—825 —802
80.00
—635 —598 —841 —598
17.00 0.11 1.40 0.45
Icorr/uA
cm —2
Revolutions Fig. 3. Abrasive wear test data for stainless steel substrates. +, SS; LI, SS/ENiP; A, SS/TiN; 0, 55/UrN; ~, SS/ENiP/TiN; ~, SS/ENiP/CrN.
tion period. This incubation period is particularly marked for the mild steel substrate (Fig. 2). The PVD coatings suppressed this mechanism, with obviously beneficial results over the long term (Figs. 2 and 3). Considering Fig. 3 in more detail, it is apparent that the stainless-steel substrate alone exhibits a higher wear rate than its mild steel counterpart in this test; the same trend can be observed with the SS/CrN (thin) combination. The effect may be due to oxide debris from the stainless-steel surface accelerating wear by “gouging” the underlying material. This can clearly be seen by comparing MS/TiN(CrN) with SS/TiN(CrN) after 1000 wheel revolutions (Figs. 4 and 5), where the weight losses are virtually the same for each pair. Figures 4 and 5 both illustrate further the benefits which the greater load support of the stainless-steel substrate provides (particularly for the hard coating alone). In the case of CrN, the poor contrast between coating(s) and substrate(s) to some extent conceals the wear scars produced. However, Fig. 5 appears to confirm visually the indication of Figs. 2 and 3 that the ENiP/CrN-coating combination can provide benefits over ENiP/TiN. In the case of mild steel substrates, the ENiP/CrN combination is slightly superior to its TiN equivalent; with the stainless steel, the wear rates are virtually identical (this occurs despite the CrN coating being thinner), 3.4. Corrosion resistance The corrosion potentials and current densities measured for each substrate/coating combination are presented in Table 4. A high (i.e., less negative) corrosion potential (Ecorr) and a low corrosion current density (‘corr) are considered indicative of good corrosion resistance. Although these values give useful pointers about corrosion performance, a summary of the trends observed is given in Table 5. (This takes into account other factors such as corrosion current variations across the full voltage sweep and preliminary data on porosity, obtained from linear polarisation resistance (LPR) measurements around Ecorr.)
0.58
TABLE 5. Summary of corrosion data Mild steel substrate MS/CrN MS/TiN
Stainless steel substrate
I I
Improved corrosion
Worse than substrate alone
SS/CrN SS/TiN
MS only MS/ENiP/TiN
Better than
SS only SS/ENiP/TiN
MS/ENiP/CrN MS/ENiP
substrate alone
SS/ENiP SS/ENiP/CrN
In general, the TiN- and CrN-coated samples mimicked the current density profiles of the untreated substrates. Similarly, the duplex treatments (to a lesser extent) mimicked the corrosion behaviour of the ENiP coating. This confirms that [3, 6] through-coating porosity is an important factor in determining the corrosion performance of thin PVD coatings. The ENiP and (particularly) the duplex treatments exhibited a much higher Ecorr and lower ‘corr than either the untreated substrates or the PYD coatings alone. Although the porosity of the CrN coating might be expected to be high (LPR measurements confirmed this) compared with the thicker TiN, this did not appear to have significant adverse effects on the ‘corr values, particularly in the case of the duplex treatment combinations. The CrN-based duplex treatments tended to exhibit a larger increase in IcOrr above Ecorr than did TiN (which might be considered indicative of poor corrosion resistance). However, this rise in current appeared, in practice, to be characterised by a build-up of surface corrosion products on the CrN rather than the catastrophic pitting corrosion (and coating removal) commonly observed with TiN (and other Ti-based PYD coatings [3, 4]). It is important to note that electron-beam-evaporated
478
A. Leyland et a!.
MS/TiN
MS/ENiP/TiN
MS/TiN
MS’ENiP’TiN
TiN and (‘r\’ PVD coatings on e!e Iroless Aj-c sued steels
SSITiN
SS/ENIP/TjN
SS TiN
SS’ENjP ‘fiN
Hg. 4. Abrasise s~earscars for ‘I iN and [NiP/TiN tions (top) and 1000 revolutions (bottom).
after 2)uO resolu—
PVD hard coatings almost invariably incorporate a metallic interlayer at the substrate interface. This effect is often used to benefit adhesion and mechanical properties. However, it will also strongly influence the corrosion properties of the composite via through-coating porosity. The pitting corrosion failure of the (Ti/)TiN is most probably due to galvanically induced dissolution of the Ti-interlayer in contact with the substrate (Fe, or Fe/Cr) or ENiP underlayer [3]. With the (Cr/)CrN, this effect would appear (in the case of duplex ENiP/CrN at least) to be reduced or eliminated. The processes occurring are, however, obviously extremely complicated and, in practice, are likely to vary from application to application. The rather idealised corrosion tests reported here tend to suggest that the ENiP/TiN coating combination gives marginally less benefit than ENiP/CrN (or even ENiP alone). However, it should be remembered that the differences are small in comparison with the untreated substrates and thin PYD-only coatings. The apparent benefits in wear resistance should also be taken into account. Other simple tests carried out using aqueous salt solution (5% NaC1) rather than an acid environment (in which mild steel—in combination with ENiP, TiN
MS/CrN
SS’CrN
MS/ENiP/CrN
SS/ENi P~CrN
MS/CrN
MS’ENiP’CrN
SS/CrN
SS”ENiP CrN
FigS. Abrasise ~~earscars for UrN and [NiP UrN after 2)10 resolutions (top) and 1000 re’.olutions (bottom).
and duplex ENiP/TiN—was immersed for I h, then left exposed in the laboratory for 48 h) showed the substantial benefits which a duplex layer of this type can provide (Fig. 6). As suggested here, however, one might still expect (from both basic electrochemical data, and practical examples of Ni/Cr use) that the Ni/Cr(N) galvanic coupling would, overall, give a corrosion performance superior to that of the Ni/Ti(N) couple [7].
4. Conclusion This preliminary investigation confirms that significant performance benefits may be provided by duplex coating systems involving PAPVD ceramic films and electroless coatings on (relatively) cheap steel substrates. In particular, a combination of AISI 304 stainless steel + ENiP + CrN shows excellent promise as a system for certain wet abrasive wear applications (although all duplex treatments gave some degree of benefit in both wear and corrosion). It appears likely that treatment costs will not be prohibitively high when compared with traditional electroless and electroplating techniques. A lower overall coating
A. Leyland et a!.
/
TiN and CrN PVD coatings on electroless Ni-coated steels
479
Acknowledgments The financial support of the UK Science and Engineering Research Council is gratefully acknowledged. The advice and help of our colleagues in the Research Centre in Surface Engineering is greatly appreciated.
D
Fig. 6. Mild steel test plates immersed in 5% NaCI aqueous solution and left to dry at normal room temperature and humidity: (A) mild steel only (B) MS/TiN (C) MS/ENiP (D) MS/ENiP/TiN.
thickness (and the proven viability of an integral hardening of the electroless layer during PVD treatment) could provide equivalent (in the case of corrosion) or superior (in the case of abrasive wear) performance, with benefits in materials costs and reduced environmental impact.
References 1 A. Matthews, Proc. A Cutting Edge for the 90s Conf., Sheffield, 1990, The Institute of Metals, 1991. 2 A. Matthews, R. J. Artley, P. S. Holiday and P. R. Stevenson, The U.K. Coatings Industry in 2005, The University of Hull, 1992. 3 M. J. Park, A. Leyland and A. Matthews, Surf Coat. Technol., 43/44 (1990) 481. 4 J. Aromaa, H. Ronkainen, A. Mahiout, 5.-P. Hannula, A. Leyland, A. Matthews, B. Matthes and E. Broszeit, Mater. Sd. Eng. A, 140 (1991) 722. 5 H. A. Jehn and M. E. Baumgartner, Surf. Coat. Technol., 54/55 6 7 8
(1992) 108. A. Leyland and A. Matthews, unpublished work. D. R. GabeInstitute (ed.), Coatings for Protection: A London, Guide for1983. Production Engineers, of Production Engineers, G. A. Saltzmann, in R. G. Bayer (ed), Selection & Use of Wear Tests for Coatings, ASTM 5TP769, American Society for Testing and Materials, Philadelphia, PA, 1982, p. 71.
Surface and Coatings Technologi,
(i()
(1993) 474—479
TiN and CrN PVD coatings on electroless nickel-coated steel substrates A. Leyland, M. Bin-Sudin, A. S. James, M. R. Kalantary, P. B. Wells and A. Matthews Research Centre in Surffice Engineering, Unjiersitv of hull, Hull, Hu,nherside, h!U6 7RX (UK
J. Housden and B. Garside Teciac Ltd., Stow Cum Quv, Cambridge. CB5 9AB (UK)
Abstract Plasma-assisted (PA) PVD ceramic coatings such as TiN have so far achieved only very limited use on cheap low-alloy steels. owing to problems relating to both corrosion resistance and the need for load support from the underlying material. Here we report tests to assess the wear and corrosion performance of TiN and CrN PAPVD coatings on phosphorus-doped electroless nickel (ENiP)-coated steels. It is shown that this route offers a potentially cost-effective means of utilising PAPVD ceramic films on lower grade steels. In particular, CrN/ENiP on A151 304 stainless steel is shown to exhibit a promising combination of wct abrasion resistance with good corrosion properties.
I. Introduction PAPVD TiN coatings are now widely used on HSS cutting tools to extend life and improve productivity and product quality [I]. On forming tools, they also find an increasing number of applications, although here too, only expensive alloy steels (e.g. hot working steels or HSS) are generally suitable. The market for TiN is thus constrained by factors such as the need for a high level of substrate load support, a limiting maximum hard-coating thickness (due to internal stresses) and consequentially poor corrosion resistance from the coating/substrate pair. If the hard-coating treatment could be cost-effectively applied to low-carbon steels (and other cheaper substrates) the potential range of applications would be considerably widened. In a recent survey [2], it was shown that, in the UK, the TiN PVD market is less than one-fiftieth of either the electroplating or thermal-spraying market by value, and probably less than one-thousandth by volume (worldwide, these figures are likely to be similar). There is a clear need, from the diverse viewpoints of both total system performance and increased environmental awareness, to try to widen the applicability of the PAPVD processes by permitting their use on cheaper low-grade steels. Cost will obviously be one major factor in the equation, but the potential for enhanced or unique capabilities from the optimal substrate/coating combination also needs to be considered. Wear-resistant coatings presently used for improving the performance of cheap substrate materials provide many advantages, but there are also major disadvan-
0257—8972/93/56.00
tages. Electroplated chromium, for instance, can be deposited with high thickness and (with a nickel interlayer) good corrosion properties, but the environmental drawbacks of these types of process (for example, in by-product disposal) are now an increasingly sensitive issue. Electroless nickel (particularly when incorporating phosphorus, which enables it to he heat-treated to high hardness) is an alternative to hard chrome with good coating integrity and less environmental impact. However, it is more expensive to deposit and process, particularly at higher thicknesses (above 50 jim). Also. its corrosion performance may not necessarily be satisfactory in certain applications. The thickness limitations of PAPVD TiN (due to coating internal stresses) restrict its corrosion performance (accelerated pitting via through-coating porosity being the main problem [3—5]). There is evidence to suggest that CrN can be deposited by PAPVD techniques onto steel substrate materials with lower internal compressive stresses, and thus to greater thickness (i.e. 10 jim or more [6]). The increased thickness should reduce through-coating porosity, whilst also improving resistance to abrasive wear by hard particles. Perhaps more importantly, however, its electrochemical compatibility with an electroless nickel interlayer should be better than that of the TiN/Ni system (it is known, for example, that Cr/Ni-layered electroplate can provide excellent corrosion resistance—superior to that which is available from either chrome electroplate or electroless nickel [7]). The improved wear properties of CrN over Cr electroplate reduces the thickness of coating required.
1993
Elsevier Sequoia. All rights reservcd
A. Leyland et al.
/
TiN and CrN PVD coatings on electroless Ni-coated steels
particularly if additional load support is available from a precipitation-hardened electroless nickel layer. For the latter, typically layers 20—30 jim thick are adequate for most of the established applications. It is therefore apparent that there could be an overall materials (and possibly cost) saving in moving from thick electro-(or electroless) plate to a “duplex” (i.e. two processes combined) system such as TiN or CrN above a comparatively thin ENiP layer. Thus, for a wide variety of reasons, the combination of a hard PAPVD coating with ENiP offers many attractions. An additional processing aspect which we examine here is the hardening of ENiP by heat treatment (which is traditionally carried out in a separate process stage at a temperature of 400—500 °C).This is the typical PAPVD processing range and there is therefore the potential to harden the ENiP durii~g hard coating deposition, giving improved load support to the PVD treatment whilst eliminating a treatment stage (with associated cost benefits). 2. Experimental details 2,1. Deposition details The substrate materials employed were mild steel and AISI 304 stainless steel. These were polished to 0.05 jim roughness, then wet-blasted in a 10 jim alumina slurry to provide a controlled roughened surface with an Ra value of 0.5 jim. This was intended to provide similar pre- and post-coated roughnesses, ensuring repeatability in corrosion and wear tests. The electroless nickel—phosphorus coatings were deposited using a standard autocatalytic technique, with a commercial Lea Ronal solution (NPA 8009). A coating of 16 jim nominal thickness was produced, with an asreceived hardness of about 500 Hk (200 g). The phosphorus content was approximately 10 at.%. The PVD coatings were deposited by Tecvac Ltd., using a commercial thermionic triode-plasma-assisted electron beam (EB) PYD system. The main deposition parameters are presented in Table 1. In the case of the duplex ENiP/hard-coating combinations the PVD-coating cycle was intended to promote precipitation hardening of the ENiP layer. As mentioned previously, this hardening would be produced commercially by a sepaRa
TABLE 1. PVD coating condi~ons
TiN
CrN
Deposition temp ( C) Deposition time (mm) Evaporation rate (g min~) Coating thickness (urn)
400—480 90 0.3 1.75
330—470 60 0.86 1.25
475
rate heat-treatment cycle at a similar temperature (about 400—500 °C)under vacuum or inert-gas-furnace conditions, to give a layer hardness of up to 1000 Hk. To ensure direct comparability, the “ENiP-only” coatings used in the present work were subjected to a plasma heat-treatment cycle in an argon d.c. triode plasma, to simulate the hardening effects of the PVD process on the “duplex” samples.
2.2. Testing Thickness was measured using a ball-crater technique, hardness using a Leitz “Miniload” tester with a Knoop dtamond, and adhesion was assessed using a VTT Tech scratch tester with acoustic emission and tangential force monitoring. For abrasive wear tests an ASTM-type rubber-wheel wet abrasion tester was used, shown schematically in Fig. 1 [8]. The normal force on the sample was 10 N and the rotational speed was 200 rev min The abrasive grit was 55 jim alumina. The weight loss of the sample was measured every 200 revolutions, up to a total of 1000. Corrosion studies were carried out using a Schlumberger test facility. Polarisation measurements were obtained at a sweep rate of 100 mY min’ using a 200 cm3 solution of 0.5 M H2S04 electrolyte, which was ~.
replaced after each sweep run. This solution was sparged with air for 10 mm prior to corrosion measurements being made. A potential sweep range of —1.0 to +2.5 V was employed, the reference being a standard calomel electrode (SCE). The exposed area the work2, the auxiliary (orof“counter”)ing electrode was 0.28 cm electrode was platinum. Salt-water immersion tests have also been carried out on mild steel in combination with ENiP, TiN and duplex ENiP/TiN to provide a visual indication of relative corrosion resistance.
Rubber
Sample
Weights
Abrasive slurry ____________________________________ Fig. 1. Schematic representation of rubber-wheel abrasive wear test.
476
A. Leyland et al.
/
TiN and CrN PVD coatings on electroless Ni-coated steels
3. Results and discussion 3.1. Hardness Owing to the roughened nature of the surfaces, it was difficult to measure hardness values at low loads, since the small indent lengths were not reliably discernable. Thus, a 200 g load was employed to give a “composite” hardness figure for the layered surface. These results are summarised in Table 2. As one would expect, the stainless steel substrate was significantly harder than the mild steel, and this is reflected, to some extent, in the composite hardness levels recorded. With the two-layer system, values over 1000 Hk are achievable, equivalent to or greater than that of a fully heat-treated high-speed steel or of chromium electroplate. It can be observed that the ENiP coatings alone (which were subjected to the plasma heat-treatment cycle) gave a hardness of about 750 Hk at the 200 g load employed. This would suggest that the true hardness of the treated ENiP was in fact slightly higher (taking account of substrate hardness contribution effects) and roughly comparable to what might be provided by a conventional heat-treatment stage. The PVD coating stage could, therefore, be expected to give the desirable additional benefit of an integral hardening mechanism, effectively making the combined ENiP/PYD coating procedure cheaper than might be inferred from the sum cost of the two individual treatments. It is interesting to note that despite the CrN coating being thinner than the TiN, the “composite” hardness (in combination with ENiP) is superior (Table 1). We attribute this to the CrN having a higher hardness (although it would also appear to be more brittle, as evidenced by the poor composite hardness on, for instance, mild steel alone). The superior load support of the ENiP (and to some extent the underlying stainless steel), together with the chemical compatability of the Ni/Cr(N) combination (when compared with Ni/Ti(N)), probably combine to alleviate the brittle characteristics of the CrN, whilst promoting the benefits of its superior hardness and (as is demonstrated in the present work) abrasion resistance. 3.2. Adhesion Owing to the rough nature of the surface, scratch-test acoustic emission was not a reliable indicator of debond-
ing; similarly, significant step changes in tangential friction force were not detected. It was necessary, therefore, to utilise visual observation to detect debonding. Since the CrN was similar in colour to the ENiP (and the substrates), adhesion changes were most easily assessed for the TiN-coated samples. Typical critical load values obtained for TiN are presented in Table 3. It is well known that the scratch test is a comparative technique which should be used on similar coatings with similar thickness and substrate hardness. However, it is significant that in these trials it was shown that the composite duplex coating system can achieve critical loads greater than 30 N and even (in the case of TiN + ENiP on 304 stainless steel) greater than 50 N. This is a respectable figure for TiN on, for instance, hardened high-speed steel and further confirms the considerable benefits this duplex system might provide. 3.3. Abrasive wear resistance Figures 2 and 3 show the abrasive wheel test results. Figure 2 shows the results for mild steel substrate material. As expected, the uncoated performance was poor. Perhaps less expected are the results of either PVD coating alone on mild steel. Both performed badly, owing to poor substrate load support. The performance of the ENiP alone is initially surprising, apparently being better than that of the duplex treatments during the early stages of the test (with minimal weight loss). The wear mechanism of the ENiP is, however, characterised by initial “smearing” of the (rough, nodular) surface, followed by eventual large-scale removal of the coating (presumably by fatigue fracture) after a certain incubaTABLE 3. Scratch test critical load (L~,(values for TiN
_____________________________________________________ Substratecoating
~
MS/TIN MSENiPTiN SS/TiN SS/ENiP/TiN
9.3 ±1.0 34.3 + 1.0 197 + 35 53.7+2.1
_______________________ ~
0.03
0
0.025
(N)
____________ -
002
TABLE 2. Knoop “composite” hardness values(Hk 200g) Mild steel substrate
0015
Stainless-steel substrate
MS 160.3 ±7.4 SS 425.0±3.0 MS/TiN 426.8 ±81.2 SS/TiN 903.0 ±103.6 MS/CrN 411.0±12.8 SS/CrN 932.5± 146.8 MS/ENiP 743.0+61.4 SS/ENiP 767.5±63.2 MS/ENiP/TiN 810.0 ±50.2 SS/ENIP/TiN 1022.0 ±111.7 MS ENiP CrN 10120+720 SS ENiP CrN 13080±3015 ______________________________________________
001
~ 0
-
_______________________________________ 0
200
400
600
800
1000
Revolutions Fig. 2. Abrasive wear test data for mild steel substrates. • MS; ~. MS ENiP • MS TiN • MS UrN V MS ENiP TiN MS ENiPCrN.
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TiN and CrN PVD coatings on electroless Ni-coated steels
TABLE 4. Corrosion potential and current density of coating combinations
0.03 0) ~
0025
j :~ E
477
0.02
__________________________________
0
200
400
600
800
1000
Substrate/coating
Eoorr/mV
MS/ENiP/TiN
—675
MS/ENiP/CrN
—635
0.55
SS 55/TiN SS/ENiP 55/ENiP/TiN SS/CrN SS/ENiP/CrN
—825 —802
80.00
—635 —598 —841 —598
17.00 0.11 1.40 0.45
Icorr/uA
cm —2
Revolutions Fig. 3. Abrasive wear test data for stainless steel substrates. +, SS; LI, SS/ENiP; A, SS/TiN; 0, 55/UrN; ~, SS/ENiP/TiN; ~, SS/ENiP/CrN.
tion period. This incubation period is particularly marked for the mild steel substrate (Fig. 2). The PVD coatings suppressed this mechanism, with obviously beneficial results over the long term (Figs. 2 and 3). Considering Fig. 3 in more detail, it is apparent that the stainless-steel substrate alone exhibits a higher wear rate than its mild steel counterpart in this test; the same trend can be observed with the SS/CrN (thin) combination. The effect may be due to oxide debris from the stainless-steel surface accelerating wear by “gouging” the underlying material. This can clearly be seen by comparing MS/TiN(CrN) with SS/TiN(CrN) after 1000 wheel revolutions (Figs. 4 and 5), where the weight losses are virtually the same for each pair. Figures 4 and 5 both illustrate further the benefits which the greater load support of the stainless-steel substrate provides (particularly for the hard coating alone). In the case of CrN, the poor contrast between coating(s) and substrate(s) to some extent conceals the wear scars produced. However, Fig. 5 appears to confirm visually the indication of Figs. 2 and 3 that the ENiP/CrN-coating combination can provide benefits over ENiP/TiN. In the case of mild steel substrates, the ENiP/CrN combination is slightly superior to its TiN equivalent; with the stainless steel, the wear rates are virtually identical (this occurs despite the CrN coating being thinner), 3.4. Corrosion resistance The corrosion potentials and current densities measured for each substrate/coating combination are presented in Table 4. A high (i.e., less negative) corrosion potential (Ecorr) and a low corrosion current density (‘corr) are considered indicative of good corrosion resistance. Although these values give useful pointers about corrosion performance, a summary of the trends observed is given in Table 5. (This takes into account other factors such as corrosion current variations across the full voltage sweep and preliminary data on porosity, obtained from linear polarisation resistance (LPR) measurements around Ecorr.)
0.58
TABLE 5. Summary of corrosion data Mild steel substrate MS/CrN MS/TiN
Stainless steel substrate
I I
Improved corrosion
Worse than substrate alone
SS/CrN SS/TiN
MS only MS/ENiP/TiN
Better than
SS only SS/ENiP/TiN
MS/ENiP/CrN MS/ENiP
substrate alone
SS/ENiP SS/ENiP/CrN
In general, the TiN- and CrN-coated samples mimicked the current density profiles of the untreated substrates. Similarly, the duplex treatments (to a lesser extent) mimicked the corrosion behaviour of the ENiP coating. This confirms that [3, 6] through-coating porosity is an important factor in determining the corrosion performance of thin PVD coatings. The ENiP and (particularly) the duplex treatments exhibited a much higher Ecorr and lower ‘corr than either the untreated substrates or the PYD coatings alone. Although the porosity of the CrN coating might be expected to be high (LPR measurements confirmed this) compared with the thicker TiN, this did not appear to have significant adverse effects on the ‘corr values, particularly in the case of the duplex treatment combinations. The CrN-based duplex treatments tended to exhibit a larger increase in IcOrr above Ecorr than did TiN (which might be considered indicative of poor corrosion resistance). However, this rise in current appeared, in practice, to be characterised by a build-up of surface corrosion products on the CrN rather than the catastrophic pitting corrosion (and coating removal) commonly observed with TiN (and other Ti-based PYD coatings [3, 4]). It is important to note that electron-beam-evaporated
478
A. Leyland et a!.
MS/TiN
MS/ENiP/TiN
MS/TiN
MS’ENiP’TiN
TiN and (‘r\’ PVD coatings on e!e Iroless Aj-c sued steels
SSITiN
SS/ENIP/TjN
SS TiN
SS’ENjP ‘fiN
Hg. 4. Abrasise s~earscars for ‘I iN and [NiP/TiN tions (top) and 1000 revolutions (bottom).
after 2)uO resolu—
PVD hard coatings almost invariably incorporate a metallic interlayer at the substrate interface. This effect is often used to benefit adhesion and mechanical properties. However, it will also strongly influence the corrosion properties of the composite via through-coating porosity. The pitting corrosion failure of the (Ti/)TiN is most probably due to galvanically induced dissolution of the Ti-interlayer in contact with the substrate (Fe, or Fe/Cr) or ENiP underlayer [3]. With the (Cr/)CrN, this effect would appear (in the case of duplex ENiP/CrN at least) to be reduced or eliminated. The processes occurring are, however, obviously extremely complicated and, in practice, are likely to vary from application to application. The rather idealised corrosion tests reported here tend to suggest that the ENiP/TiN coating combination gives marginally less benefit than ENiP/CrN (or even ENiP alone). However, it should be remembered that the differences are small in comparison with the untreated substrates and thin PYD-only coatings. The apparent benefits in wear resistance should also be taken into account. Other simple tests carried out using aqueous salt solution (5% NaC1) rather than an acid environment (in which mild steel—in combination with ENiP, TiN
MS/CrN
SS’CrN
MS/ENiP/CrN
SS/ENi P~CrN
MS/CrN
MS’ENiP’CrN
SS/CrN
SS”ENiP CrN
FigS. Abrasise ~~earscars for UrN and [NiP UrN after 2)10 resolutions (top) and 1000 re’.olutions (bottom).
and duplex ENiP/TiN—was immersed for I h, then left exposed in the laboratory for 48 h) showed the substantial benefits which a duplex layer of this type can provide (Fig. 6). As suggested here, however, one might still expect (from both basic electrochemical data, and practical examples of Ni/Cr use) that the Ni/Cr(N) galvanic coupling would, overall, give a corrosion performance superior to that of the Ni/Ti(N) couple [7].
4. Conclusion This preliminary investigation confirms that significant performance benefits may be provided by duplex coating systems involving PAPVD ceramic films and electroless coatings on (relatively) cheap steel substrates. In particular, a combination of AISI 304 stainless steel + ENiP + CrN shows excellent promise as a system for certain wet abrasive wear applications (although all duplex treatments gave some degree of benefit in both wear and corrosion). It appears likely that treatment costs will not be prohibitively high when compared with traditional electroless and electroplating techniques. A lower overall coating
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TiN and CrN PVD coatings on electroless Ni-coated steels
479
Acknowledgments The financial support of the UK Science and Engineering Research Council is gratefully acknowledged. The advice and help of our colleagues in the Research Centre in Surface Engineering is greatly appreciated.
D
Fig. 6. Mild steel test plates immersed in 5% NaCI aqueous solution and left to dry at normal room temperature and humidity: (A) mild steel only (B) MS/TiN (C) MS/ENiP (D) MS/ENiP/TiN.
thickness (and the proven viability of an integral hardening of the electroless layer during PVD treatment) could provide equivalent (in the case of corrosion) or superior (in the case of abrasive wear) performance, with benefits in materials costs and reduced environmental impact.
References 1 A. Matthews, Proc. A Cutting Edge for the 90s Conf., Sheffield, 1990, The Institute of Metals, 1991. 2 A. Matthews, R. J. Artley, P. S. Holiday and P. R. Stevenson, The U.K. Coatings Industry in 2005, The University of Hull, 1992. 3 M. J. Park, A. Leyland and A. Matthews, Surf Coat. Technol., 43/44 (1990) 481. 4 J. Aromaa, H. Ronkainen, A. Mahiout, 5.-P. Hannula, A. Leyland, A. Matthews, B. Matthes and E. Broszeit, Mater. Sd. Eng. A, 140 (1991) 722. 5 H. A. Jehn and M. E. Baumgartner, Surf. Coat. Technol., 54/55 6 7 8
(1992) 108. A. Leyland and A. Matthews, unpublished work. D. R. GabeInstitute (ed.), Coatings for Protection: A London, Guide for1983. Production Engineers, of Production Engineers, G. A. Saltzmann, in R. G. Bayer (ed), Selection & Use of Wear Tests for Coatings, ASTM 5TP769, American Society for Testing and Materials, Philadelphia, PA, 1982, p. 71.