Thin film performance from hybrid PVD-powder coating process

Surface & Coatings Technology 236 (2013) 388–393

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Thin film performance from hybrid PVD-powder coating process David T. Gawne a,⁎, Yuqing Bao a, Jiming Gao a, Cristina Zubizarreta b, Josu Goikoetxea b, Javier Barriga b a b

Department of Engineering and Design, London South Bank University, London SE1 0AA, United Kingdom Surface Physics and Technology, IK4-TEKNIKER, c/Iñaki Goenaga, 5, 20600 Eibar, Guipuzcoa, Spain

a r t i c l e

i n f o

Article history: Received 21 July 2013 Accepted in revised form 10 October 2013 Available online 18 October 2013 Keywords: PVD films Polymer powder coatings Residual thermal stress Stress analysis Hybrid PVD powder coating process

a b s t r a c t A new in-line reciprocating PVD prototype machine is described that enables a significant increase in throughput and product flexibility over conventional PVD machines. The research shows that conventional cathodic arc evaporation can be used in tandem with powder coating to produce planarized PVD films. However, poor adhesion was obtained for arc-evaporated zirconium films deposited on epoxy powder coatings in the prototype machine. Subsequent stress analysis showed that the epoxy undercoat substantially raised the residual stress in the zirconium and that the inadequate adhesion in the prototype machine was due to its rapid deposition rate. The analysis also predicted that inserting a material of an intermediate expansion coefficient between the film and epoxy undercoat should reduce the residual stress. Experimental trials confirmed that a copper interlayer raised the adhesion strength substantially and successful deposition could be achieved with the prototype machine. The zirconium films were dense and adherent with planarized, high gloss surfaces. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Physical vapour deposition (PVD) has for many years been considered as a potential alternative to the traditional coating technologies, especially electroplating and other liquid-deposition processes. In practice, however, this potential has not been realized owing mainly to economic and technical reasons. The economics are hindered by the high capital cost of vacuum equipment, the high operating costs due to pump-down, heating/cooling cycles and slow deposition rate. This makes it difficult for PVD to compete economically with the low-cost traditional metal-finishing industries. The technical obstacles are that PVD coatings are very thin compared with traditional coatings (e.g. 0.3 μm compared with 30 μm), PVD coatings replicate the substrate surface without any smoothing or levelling, and PVD coatings usually provide little corrosion protection owing to their thinness, columnar structure, occluded droplets and dust particles together with the fact that the coatings are often more chemically noble than the substrate. Nevertheless, PVD is a clean and dry coating technology without the serious environmental problems of electroplating, anodizing and galvanizing. For example, the hexavalent chromic acid baths used in chromium plating are carcinogenic and cause health risks to workers in production and environmental problems in toxic waste disposal [1]. Besides the latter process benefits, PVD has several end-product advantages. It has the ability to provide not only metallic but also metal and ceramic alloy coatings with an almost unlimited range of chemical compositions and consequent control of product properties. ⁎ Corresponding author. E-mail address: [email protected] (D.T. Gawne). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.10.019

PVD chromium nitride, for instance, exhibits superior wear resistance and is a potential alternative to chromium plating in some applications [2]. Other product performance attributes of PVD include high wear resistance, biocompatibility, high gloss and a range of attractive colours. However, the thinness of PVD coatings is a serious disadvantage as it brings poor corrosion protection and an inability for levelling or planarizing the substrate surface. As a result, a rough metal substrate will remain rough after coating and will not provide a glossy PVD surface, unless it is given an expensive polishing treatment before deposition. A potential solution to the above corrosion and levelling problems is to use a polymer powder coating as an undercoat or primer between the PVD coating and the metal substrate. The powder coating is much thicker than the PVD film (70 μm compared with 0.3 μm), nonconducting and dense. Consequently, this hybrid technology should, in principle, provide a corrosion-resistant barrier layer as well as a smooth, glossy coating and the avoidance of the need for polishing. The polymer chosen was an epoxy as its ability for crosslinking suppresses permeability while enhancing strength. The deposition technique used was electrostatic spraying owing to its widespread usage, although other possibilities are fluidized bed dipping, liquid painting and thermal spraying [3,4]. The selection of a PVD technique is much more challenging. It is linked to the wider issue of developing a more cost-effective PVD process able to compete economically with the existing traditional processes by increasing the throughput of coated products. One approach is to reduce the cost per item of coating by building larger PVD machines. There is another approach and the one used in the current project as part of a wider programme of research. This is to

D.T. Gawne et al. / Surface & Coatings Technology 236 (2013) 388–393

2. Experimental details 2.1. PVD equipment A new design has been developed and a prototype machine constructed (IK4-Tekniker, Eibar, Spain). This in-line design consists of a number of modules in a row, each of which is devoted to a specific task. In this configuration, as opposed to that in a conventional batch process, the module devoted to the highest value operation (coating deposition) is never idling as there are other modules or chambers to carry out the less costly processes, such as vacuum, heating and venting. Conventional PVD is divided into four stages: vacuum generation, heating, coating and venting (a cleaning stage may also be included, generally by a glow discharge of the substrate before coating). Each of these stages may be carried out in a separate chamber. However, the new design [11] consists of a central chamber in which coating takes place and two side chambers for loading/unloading of the central chamber and where vacuum generation, venting and heating of the substrates are carried out. This enables the coating chamber to be always under vacuum and loaded with substrates to coat. After a high vacuum has been reached, the heating is carried out with rapid infrared heaters and then a linear-motion system transfers the samples to the coating chamber. The coating chamber of the prototype is 300 mm in height and 200 mm in diameter; more detailed dimensions are given in Reference [11]. There are two principal advantages of the new design. The first is improved throughput because the coating chamber is lock-loaded and so the vacuum is never broken. This avoids the need for vacuum generation, which is the most time-consuming part of the PVD process, and leads to much shorter process times than those in conventional industrial PVD machines. Typical times for the prototype with a substrate temperature of 150 °C are: vacuum generation of 60 s, heating of 40 s, load/unload of 5 s, coating of 90 s and venting of 10 s. However, the prototype is designed to treat many charges sequentially and vacuuming is only needed for the first charge. Once the prototype machine is running under these conditions, the total process time for each charge is close to the coating time of 90 s. The total process time for a conventional industrial PVD machine is typically 70 min. The latter machine has dimensions of 900mm in diameter and 1200mm in height, and taking sample holders and tooling into account, this machine has a coating volume of 36 times that of the prototype. Expressing throughput as the coating volume treated per unit time, the throughput of the prototype is 30% higher than that of the conventional machine. The second advantage of the new design is that its relatively small size, besides providing reduced equipment costs, enables increased product flexibility in that short runs of various coating types and parts can be readily undertaken.

2.2. Film deposition Zirconium films were deposited on epoxy-coated aluminium substrates by cathodic arc evaporation using the PVD prototype equipment described above. The deposition was performed with a constant cathode current in the range of 80–120 A, argon flow at a constant rate of 150 sccm and a pressure in the vacuum chamber of 6 × 10−3 mbar. The substrates were biased at a voltage of −30 V. The total deposition time was 90 s for a film thickness of 0.15 μm, which corresponds to a deposition rate of 100 nm/min. 2.3. Epoxy coatings A phenolic epoxy system (Vacuprime 301, DuPont Powder Coatings Ltd., Gateshead, UK) developed for higher temperature resistance was used in the investigation. Fig. 1 shows the differential scanning calorimetry (DSC) curves of the powder and the subsequently fully cured coating. The large exothermic peak in Fig. 1 centred around 200 °C is due to the curing process of the epoxy. The crosslinking reaction during curing is needed to strengthen the epoxy before film deposition. Fig. 1 indicates that curing is essentially complete at 232 °C and so this is used as the curing temperature. Fig. 1 also suggests that no substantial degradation of the epoxy takes place up to 300 °C. The powder was deposited on aluminium substrates by electrostatic spraying and subsequently cured in a stoving oven at 232 °C for 30 min. The thickness of the epoxy coatings was between 50 and 70 μm. The substrates used were aluminium (grade 6063) flat plates of dimensions 100 mm × 50 mm × 3 mm. 2.4. Film property evaluation The surface roughness of the coatings was characterized using a Talysurf profilometer and the film structure by optical and electron microscopy. The adhesion of the polymer undercoat to metal substrates was assessed by the pull-off tensile test with an AVERY universal testing machine (Birmingham, England), which conforms to ASTM D4541 and BS 3900-E10. The crosshead speed used in the tensile tests was 5 mm/min. A test dolly was bonded to the coating using an adhesive and the tensile force required to detach the coating from substrate was measured. The adhesion between the PVD film and the undercoat was assessed by the cross-cut test, which was performed using a multi-blade cutting tool in accordance with ENISO2409:2007. The test measures the percentage of the film detached as a result of the cross-cutting and tape stripping. The adhesion of the coatings is presented as a six-step classification (0–5) in accordance with that defined in ISO 2409:2007:

0.4 0

Heat flow ( W/g)

construct a reciprocating in-line machine (see Section 2.1) rather than a batch technique as described above. A further important way to maximize productivity and offset the high capital cost of PVD machines is to use a coating process with a high deposition rate. In this project, cathodic arc-evaporation was employed owing to its rapid deposition (40 nm/min in conventional processes). In this technique, an electric arc is used to vaporize material from the cathode target, which then condenses on the substrate to form a thin film. A difficulty with arc evaporation is its tendency to eject molten droplets from the target and additionally, there is little information concerning arc-evaporation deposition on epoxies. This paper investigates the possibility of depositing an arcevaporated film on an epoxy undercoat on an aluminium substrate using the PVD prototype. The film material selected was zirconium owing to its wide range of potential applications, including medical [5,6], magnetic [7,8] and decorative [9,10].

389

Un-cured

-0.4 -0.8

Fully-cured -1.2 -1.6 -2 20

60

100

140

180

220

260

300

Temperature (oC) Fig. 1. DSC curves of uncured epoxy powder and cured epoxy coating.

390

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Step 0 gives no detectable detachment of the film and the highest adhesion performance; Step 1 gives less than 5% by area detached, Step 2 gives 5–15% detached, Step 3 gives 15–35%, Step 4 gives 35– 65% and Step 5 more than 65% film detachment with the lowest adhesion performance.

the substrate. The density and the width of network boundaries were found to be related to the PVD conditions used. For example, the width of the buckled film bands was observed to increase with increased deposition time. This behaviour suggests that residual stress plays an important role.

3. Results and discussion

3.3. Residual stress analysis

3.1. Adhesion of epoxy coating to aluminium substrate

The zirconium PVD films produced by cathodic arc evaporation using the prototype machine showed poor adhesion to the epoxy undercoat. Various deposition conditions were tried but not enough adhesion was achieved. In order to provide more information on possible failure mechanisms, the as-deposited zirconium films on the epoxy undercoat were examined microscopically. The top surface of the film showed a surface network (Fig. 2a), while the cross section (Fig. 2b) revealed that the network boundaries were where the film had locally lifted away from

There are three main contributions to residual stress in PVD films [5,6,12–14]: (i) intrinsic stress; (ii) extrinsic stress and (iii) thermal stress. Intrinsic stress is formed by the atom species impacting the substrate or growing surface layer in the chamber during deposition. It can be compressive or tensile depending upon the PVD conditions and the film structure. SEM examination of the Zr-based films on epoxy undercoat (Fig. 3) reveals that the film consists of densely packed columnar grain structures with diameters of the order of 50 nm. This type of structure tends to result in a compressive stress in the film [5,6]. Extrinsic stress is due to various molecules, particularly gaseous polar species, penetrating open voids or pores, adsorbing on pore walls and creating residual stress. However, film detachment was observed during cooling under low pressure in the chamber and so extrinsic stress is unlikely to be a major contributor. Thermal stress is due to differential expansion: the mismatch of thermal expansion between the film and the substrate upon cooling from the deposition temperature to ambient. Epoxy has a thermal expansion coefficient of 100 × 10−6 K−1, which is much higher than that of zirconium (5.9 × 10−6 K−1). This difference is expected to have

Fig. 2. SEM micrograph showing the Zr film deposited by cathodic arc evaporation on an epoxy undercoat: (a) top surface; (b) cross section.

Fig. 3. SEM micrograph of through-thickness cross section of coatings on aluminium substrate: (a) Zr–epoxy and (b) Zr–Cu–epoxy.

The average value of pull-off strength recorded for the epoxy coatings to the aluminium substrate was 8 MPa. However, the fracture was observed within the adhesive used, which indicates that the true coating-substrate adhesive strength would be greater than 8 MPa. This indicates strong epoxy coating-substrate adhesion. 3.2. Adhesion of PVD film to epoxy coating

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substantial effect on the level of thermal stress. It is significant that zirconium films can be easily deposited on bare aluminium substrate with good adhesion. The fact that the film buckling bands formed on the epoxy undercoat and not on the bare metal substrate suggests that thermal stress has a dominant effect on the failure of the films on the epoxy undercoat. Some degree of intrinsic stress may be present but the observed influence of dissimilar materials and cooling implies that thermal stress is the controlling mechanism. A quantitative stress analysis was therefore undertaken to estimate thermal stress in order to clarify the understanding the observed behaviour. It is assumed that the zirconium film, epoxy interlayer and aluminium substrate have homogenous structures and properties, and undergo uniform biaxial thermal strain in response to changes in temperature (the thermal strain along the thickness direction is neglected). Due to the differences in thermal expansion coefficients, the strain in the epoxy will be restricted by the zirconium film and the aluminium substrate, as they both have smaller free expansions than epoxy. During cooling from the deposition temperature to room temperature, therefore, the epoxy will be under a biaxial tensile force while the zirconium film and aluminium substrate will be under biaxial compressive forces. As there are no external forces acting upon the cross-section of the combined multilayer system, the stress in each layer can be determined by solving the following equations from equilibrium conditions [15,16]: Force equilibrium :

n X

Fi ¼ 0

ð1Þ

i

Bending moment ðMÞ equilibrium :

n X i

Mi ¼

n 1X Ft 2 i i i

ð2Þ

where F is the internal force that arises owing to the restrained thermal strain, M is the moment of resistance and t the thickness. The total strain ε in each layer equals the sum of the direct strain caused by direct force Fi, the bending strain due the bending moment and a thermal strain due to the temperature T variation. The strain at the interface of adjacent layers is therefore: εi ¼ αi ΔTi þ

  Fi 1 νi 2 þ ðti þ δÞ: − Ei ti b a R

ð3Þ

These must be compatible at each adjacent interface: εi ¼ εiþ1 :

ð4Þ

The stress in the film, epoxy interlayer and aluminium substrate can be calculated: σi ¼

Fi Ei  y bti R i

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Table 1 Room temperature properties used in the stress analysis [18–20]. Properties

Zirconium

Copper

Epoxy

Aluminium

Density (kg m−3) Linear coefficient of thermal expansion (10−6 K−1) Young's modulus (GPa) Poisson's ratio

4510 5.9

8900 17

1200 100

2700 23.6

99 0.34

115 0.33

2.41 0.34

69 0.33

3.4. Failure mechanisms The above equations were applied to calculate residual stress as a function of the deposition temperature during PVD using the material properties in Table 1. The results are given in Fig. 4 for two coating systems: (i) a 0.15 μm zirconium film on a bare aluminium substrate of thickness 3 mm; (ii) the same 0.15 μm zirconium film but now on an undercoat or primer of 60 μm-thick epoxy, which is itself on the 3mm-thick aluminium substrate. The results show that for both coating systems, compressive stresses are developed in the zirconium film. However, the introduction of the epoxy undercoat produces a largescale increase in residual stress. At a deposition temperature of 200 °C, for example the stress in the zirconium film on bare aluminium is 250 MPa and introducing an epoxy undercoat increases this up to 403 MPa. The residual stress model in Section 3.3 was also applied to show how the stress varies through the thickness of the coating system. Fig. 5 reveals sharp discontinuities in stress at the interfaces. For the Zr–Al system the stress is +0.05 MPa tensile in the aluminium and −257 MPa compressive in the zirconium. In the presence of epoxy, however, the stress differential is much larger: from +39 MPa tensile in the epoxy to −403 MPa compressive in the zirconium film. This major increase in stress is expected to contribute to the film failure. Close examination of Fig. 3 shows separations at the interface of the order of at least 100 nm and under the prevailing high levels of compressive stress, these separations are expected to initiate the buckling [17] as observed in Fig. 2. It is pointed out that the driving force for buckling, eventual spallation and adhesive failure is provided by the residual stress and particularly the stress discontinuity at the interface. The origin of the high levels of residual stress is thermal expansion mismatch, particularly between the metallic zirconium and the polymeric epoxy. The stress analysis indicates that one way to reduce this is to insert a material with an intermediate expansion coefficient between the zirconium film and the epoxy undercoat. Copper has a thermal expansion coefficient of 17 × 10−6 K−1, which is higher than that of zirconium (5.9 × 10−6 K−1) but lower than that of epoxy (100 × 10−6 K−1). The

ð5Þ

where R is the radius of curvature of the neutral axis in the combined multilayer system. E is the elastic modulus of the material, υ is the Poisson ratio, δ is the distance between epoxyaluminium interface to the neutral axis of the cross-sections, ti the thickness of individual layers, a the length of the sample (100 mm), b the width of the sample (50 mm) and yi is the distance from the neutral axis. The position of the neutral axis is determined by considering that the resultant normal force acting on the cross section is zero and using the equation below: Z n X Ei ydA ¼ 0: i

i

ð6Þ

Fig. 4. Calculated stresses in Zr films in two coating systems under various deposition temperatures. Zr–Al: a 0.15 μm Zr film on a 3 mm aluminium substrate. Zr–EP–Al: a 0.15 μm zirconium film on epoxy (60 μm) coated aluminium (3 mm) substrate. EP refers to epoxy.

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adhesion performance. The SEM micrograph in Fig. 3 shows a cross section of the two-layered film on the epoxy undercoat. The use of a copper interlayer between the zirconium and epoxy has, therefore, successfully overcome the cracking and adhesion problem. Fig. 7 shows the stress distribution through the thickness of the Zr– Cu–epoxy–Al system for various deposition temperatures. The deposition temperature in the prototype could not be measured in the experimental trials, although it was expected to be higher than that in the conventional machine as the substrate was closer to the evaporator. Moreover, the

Fig. 5. Calculated stress distribution through the thickness of Zr–Al and Zr–epoxy–Al at 200 °C deposition temperature. EP refers to epoxy.

calculation was performed for a Zr–Cu–epoxy–Al system using the stress model and a thickness of the Cu film of 0.1 μm. The calculated results are presented in Fig. 6. Although the difference in stress between the zirconium and epoxy is the same, the stress differentials across the interfaces are now much less. In particular, the stress differential across the weaker metal–epoxy (zirconium–epoxy) interface is approximately half the previous value: 226 MPa compared with 442 MPa. Experimental trials were then undertaken to deposit a Zr–Cu– epoxy–Al system by arc-evaporation using the prototype machine. The deposition was carried out and a 0.1μm-thick copper was deposited as an interlayer. The films were assessed in terms of adhesion and microstructure. The cross-cutting test on the zirconium surface on the Zr–Cu–epoxy–Al system gave a Step 0 grade indicating the highest

Fig. 6. Calculated stress distribution through the thickness in the Zr–EP–Al and Zr–Cu–EP– Al systems under 200 °C deposition temperature. EP refers to epoxy.

140oC 200 oC 240 oC 300 oC

Fig. 7. Calculated stress distribution across the thickness of the Zr–Cu–EP–Al system under various deposition temperatures. EP refers to epoxy.

Fig. 8. SEM micrographs of: (a) surface of as-received aluminium plate before coating, (b) surface of epoxy coating on aluminium and (c) surface of zirconium–copper thin film on epoxy undercoat (at high magnification).

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deposition rate in the prototype machine was also higher than that in the conventional machine (100 nm/min and 40 nm/min respectively), which will substantially raise the rate of heat input into the underlying epoxy/ substrate and increase the deposition temperature. The results in Fig. 7 imply that the increased deposition temperature due to the higher deposition rate in the prototype machine will raise the residual stress and this is expected to contribute to the poor adhesion of the coated materials. The results also indicate, however, that the use of a copper interlayer substantially reduces the residual stress and provides good adhesion.

undercoat would reduce the stress in the film and thereby improve its adhesion. Experimental trials showed that a copper interlayer reduced the stress differentials across the interfaces and raised the adhesion strength substantially. The results indicate that successful deposition on epoxy could be achieved using the prototype machine by the use of a copper interlayer. • The zirconium films produced were dense and adherent, and also exhibited a glossy appearance due to levelling by the epoxy interlayer.

3.5. Surface finish of PVD films

Acknowledgement

Fig. 8 gives scanning electron micrographs showing (a): the surface of the as-received aluminium plate before coating, (b) the surface of the same plate coated with epoxy and (c) then with the PVD zirconium film. The surface roughness in terms of Ra values was respectively: 0.34 μm, 0.08 μm and 0.06 μm. The undercoat has a thickness of 50 μm and effectively covers the topography of the aluminium substrate to give a smooth, well-levelled surface. The PVD zirconium film replicates the topography of the epoxy undercoat to produce a smooth, glossy surface. These results show that the topography of the PVD films is governed by that of the epoxy undercoat and demonstrate that a powder coating primer can enable a smooth PVD film on a rough substrate.

The authors wish to thank the European Commission (Sixth Framework Programme COLL-CT-2006-030409 Flexicoat) for financial support of this work.

4. Conclusions • A new in-line reciprocating PVD machine is described that enables a significant increase in throughput and product flexibility over conventional PVD machines. The research has shown that the machine can be used in tandem with a powder coating process to produce dense, adherent PVD films with fine surface finishes. • The deposition of arc-evaporated zirconium films on epoxy powder coatings using the prototype machine resulted in poor adhesion as a result of its high deposition rate and thermal expansion mismatch. • Quantitative stress analysis predicted the generation of high compressive stresses in the PVD film, which were responsible for the adhesion failure. The level of the compressive stress rose with increasing deposition temperature and time. • The stress analysis predicted that applying an interlayer material of an intermediate expansion coefficient between the film and epoxy

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