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Effect of surface roughness parameters on thermally sprayed PEEK coatings
Surface & Coatings Technology 204 (2010) 3567–3572
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Effect of surface roughness parameters on thermally sprayed PEEK coatings Krishal Patel a, Colin S. Doyle a, Daisuke Yonekura b, Bryony J. James a,⁎ a b
Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand Department of Mechanical Engineering, The University of Tokushima, 2-1 Minami-josanjima-cho, Tokushima 770-8506, Japan
a r t i c l e
i n f o
Article history: Received 17 September 2009 Accepted in revised form 9 April 2010 Available online 18 April 2010 Keywords: Roughness Skewness Thermal spray High velocity oxygen fuel (HVOF) High velocity air fuel (HVAF) Poly ether ether ketone (PEEK)
a b s t r a c t Single splats and coatings of PEEK (polyether ether ketone) were produced using high velocity oxygen fuel (HVOF) and assisted combustion high velocity air fuel (AC-HVAF) thermal spray techniques respectively to investigate the effect of varying surface roughness and chemistry of the ANSI304 stainless steel substrates had on the formation of PEEK coatings. As received degreased, etched and steel grit blasted surface treatments were used. XPS analysis showed that the etched substrate had the most chemically clean surface confirmed by contact angle measurement which showed that the etched substrate had the lowest contact angle indicating good wetting properties followed by the degreased and the steel grit blasted substrates. Roughness and skewness measurements of the treatment substrates showed the steel grit blasted surface to have the highest roughness value followed by the etched and degreased substrates whose roughness values were an order of magnitude lower. Skewness was positive on the etched substrate, slightly negative on the steel grit and negative on the degreased substrate. PEEK single splat image analysis showed that splats per mm2 were highest on the steel grit blasted substrate followed by degreased and etched respectively. Scratch testing of the PEEK coatings showed coating failure on the steel grit substrate to be cohesive, on the degreased substrate, adhesive and on the etched substrate a combination of both. From these results it was concluded that a positive skewness ANSI304 stainless steel surface can be produced using an etching process, and for surfaces with suitable topography mechanical interlocking may play a more significant role than surface chemistry for room temperature substrates. HVOF and AC-HVAF thermal spraying of PEEK coatings onto stainless steel surfaces and for ANSI304 stainless steel surfaces with comparable roughness and PEEK coatings on surfaces with positive skewness show better coating adhesion than similar surfaces with negative skewness. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polyether ether ketone (PEEK) is a semi-crystalline engineering thermoplastic which possesses excellent chemical resistance, along with good thermal stability, mechanical properties, wear and reduced frictional properties [1–3]. PEEK has high glass transition and melting temperatures (Tg = 143 °C, Tm = 343 °C) [4] which make it an excellent choice for many engineering applications requiring good chemical resistance at high temperatures. This makes PEEK an ideal candidate for coating applications. The high chemical resistance of PEEK towards solvent attack means that solution based coating methods are neither efficient nor environmentally friendly methods of coating deposition (PEEK is soluble in media such as diphenyl sulphone, high boiling point esters, benzophenone, and 1-chloronaphthalene at temperatures above Tm) [5]. Thermal spray is a group of coating methods which uses thermal energy to heat the coating material to a molten or semi-molten state.
⁎ Corresponding author. E-mail address: [email protected] (B.J. James). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.04.026
The heated particles are accelerated towards a substrate by process gas or atomisation jets. Upon impact the softened particles plastically deform, producing thin disc-like platelets, termed “splats”. A thermal spray coating is made my subsequent particles impacting onto the already deposited splats, building up a thick interlocking lamellar structure. The solvent-less nature of thermal spray [6] makes it an ideal coating process for PEEK coatings [7]. The quality of a thermally sprayed coating depends on a number of factors, including the surface preparation of the substrate. The substrate parameters can be divided into two broad classes, representing the chemistry and the topology of the surface. The substrate parameters which have been identified as being of key importance in producing good coatings are the cleanliness (reflecting the surface chemistry) and the roughness (reflecting the topology) of the surface [8]. Cleanliness of the surface promotes better wetting of the coating media to the substrate and roughness provides sites for the mechanical interlocking [9] of the coating media. The wetting properties of a surface can be determined by using the sessile drop contact angle method [10]. There are a number of parameters which can be used as a measure of the roughness of a surface, the most common of which is the profile average or centre line roughness parameter Ra [11]. Ra alone is not
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adequate in assessing the surface topography of a substrate as different surface profiles can exhibit the same or similar Ra values [12,13]. If the varying heights along a profile of a (rough) surface are considered as a distribution, it is possible to gauge whether a profile is skewed towards broad peaks and spike-like valleys, or towards broad valleys and spike-like peaks. The parameter used to give this information is skewness or Rsq [12–14]. The skewness parameter can also be used to describe a surface profile along with Ra. The additional information derived from including the skewness along with the roughness is shown in Fig. 1, which shows a schematic of three surfaces with equal roughness (Ra), but with varying skewness. In this work ANSI stainless steel 304 has been subjected to three different substrate pre-treatments to investigate the effect of varying surface roughness and chemistry on the formation of PEEK coatings. Coatings were investigated as single splats and as coatings formed by the build-up of multiple sprayed layers of PEEK using high velocity thermal spray techniques. The three surface treatments were: as received degreased; chemically etched; and steel grit blasted. Grit or bead blasting is a common surface preparation technique used widely in industry [15,16] for the cleaning and roughening of surfaces prior to coating application. The surface chemistry of the ANSI304 stainless steel substrates was characterised prior to surface treatment and thermal spraying using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM), atomic force microscopy (AFM) and laser profilometry were be used to characterise the surface topology. Image analysis of the PEEK single splats and scratch testing of the coatings were done to determine the effect of the surface pretreatments on the PEEK single splats and coatings.
Etched substrates were produced by treatment with an etchant solution comprising of (by volume) 3 parts water, 2 parts concentrated hydrochloric acid, and 1 part concentrated nitric acid. The degreased stainless steel coupons were immersed in the etching solution for 5 min. After which the substrates were washed under water and dried. The steel grit (SAE-G 50) media of mesh size 50 (290 µm) was used in a SBC 220 vertical sand blast cabinet with nominal air pressure of 60 psi (410 kPa) to prepare the grit blasted coupons. 2.2. Single splat and coating formation The PEEK powder was manufactured by VICTREX (Victrex PEEK®, Lancashire, UK) and was of grade 150 PF. This powder has a nominal size of 45 μm. Spray conditions and techniques were selected on the basis of an optimisation process detailed in [17]. Single splats of PEEK were produced using a Stallite Jet Kote II HVOF thermal spray system using a Miller thermal rotary powder feeder with gun parameters of 150 mm nozzle length, 280 mm spray distance and transverse speed of 300 mm s− 1. Coatings (with thicknesses in the order of 50 μm) were produced using the UniqueCoat Technologies AC-HVAF thermal spray system with a Browning Mark XV powder feeder. A 150 mm nozzle was used on the spray gun which operated at a distance of 300 mm from the substrate. Coatings were produced in three passes with a gun transverse speed of 250 mm s− 1. The three substrates were mounted in the spray booth together and were subsequently sprayed at the same time allowing for the direct comparison of the three surface treatments. 2.3. Substrate characterisation
2. Experimental 2.1. Substrate preparation Single splat experiments were conducted on 20 mm square coupons while coatings were produced on 100 mm square coupons. All coupons were degreased by submersing the stainless steel coupons in iso-propyl alcohol for approximately 5 min while gently rubbing the surface of the samples with solvent proof gloves.
Fig. 1. Illustration of three surfaces with equal roughness (Ra) parameter. (a) Positive skewness, (b) zero skewness, and (c) negative skewness.
A Kratos Axis Ultra DLD XPS system was used to determine the composition and chemical state of the elements present on the substrate surface prior to thermal spraying. This system uses monochromatized Aluminium Kα X-rays (1486.69 eV) along with a hybrid electrostatic and magnetic lensing system to create an analysis area of 300 × 700 µm. Elemental survey scans were run on all the samples with pass energy of 160 eV, step size of 1 eV and sweep time of 180 s. Chemical state scans of the iron, chromium, oxygen and carbon regions were conducted with pass energy of 20 eV, step size of 0.1 eV and sweep time of 180 s. Quantification of the survey scans and fitting of the chemical state scans were performed using the CasaXPS package [18]. The binding energy scale was corrected using the signal from adventitious carbon (≡285.0 eV) as an internal standard. Contact angle measurements were collected on the three treated substrates using the KSV instruments CAM101 contact angle goniometer with water as the probe liquid to determine the effect of the difference in surface chemistry. Four contact angle measurements were taken at random positions on each substrate. Contact angle measurements were taken 1 min after the probe liquid made contact with the substrate surface in order for equilibrium to be reached. An average of the four contact angles was calculated. For the degreased and etched substrates a Digital Instruments multimode AFM in contact mode was employed to obtain surface roughness values, 3D surface plots and line profiles of the surface. Roughness values were obtained at four separate random spots on each sample over an area of 500 µm2 and averaged. For the steel grit blasted substrate, which was too rough for AFM measurement, the roughness profiles were obtained using the SmartScope ZIP 250 laser profilometer (Rakon Ltd., Auckland, NZ). Four measurements at random positions were taken then averaged. Backscattered Secondary Electron (BSE) SEM images were taken using a FEI Quanta Environmental SEM with an accelerating voltage of 20 kV. Image analysis was performed on BSE SEM images using ImageJ [19], an open source software package that can be used to quantify
K. Patel et al. / Surface & Coatings Technology 204 (2010) 3567–3572
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Table 1 Substrate surface elemental composition. Surface treatment
%C
%O
%Fe
%Cr
%N
%Ca
%F
%Si
%Na
Degreased Etched Steel grit
58.1 60.6 50.3
32.1 33.6 42.4
2.4 0.6 3.3
1.9 2.5 0.4
4.4 2.7 0.3
0.7 – 0.7
0.2 – –
– – 2.6
0.3 – –
electron microscopy images. The image analysis software allowed the determination of the number density of splats (splats mm− 2) and the average circularity of splats. The circularity is a shape factor which relates the measured area of an object to the area of a circle with the same perimeter. The circularity thus gives a measure of how well formed a splat is. A uniform disc-like splat will have a circularity approaching unity, whereas fingered or splashy splats will have lower circularity indicating their more jagged perimeter [19,20]. Scratch testing was done using a CSEM Revetest scratch tester with a spherical diamond indenter with 50 µm radius tip. A linearly increasing load (0 to 40 N) was applied till coating failure. The point of coating failure was determined to be where the first sign of coating delamination from the substrate was observed.
Fig. 2. Graph of contact angle, splat density (left axis) and circularity (right axis) for the three surface treatments.
3. Results and discussion However the two surfaces differ in that the degreased surface had a negative skewness and the etched surface a positive skewness. The skewness differences can be seen in the AFM line profiles of each surface which are shown in Figs. 3 and 4. Fig. 3 shows the degreased substrate surface and a line scan showing the surface topography. From the line scan it can be seen that there are board peaks and sharp valleys indicative of a negative skewness. The line scan of the etched surface in Fig. 4 shows that there are more peaks than valleys. The peaks are also sharp. This give the etched substrate surface a positive skewness. The highest roughness was on the steel grit blasted surface, which was an order of magnitude higher than on the degreased or etched substrate surfaces. This is due to the mechanical abrasion from the irregular shaped steel grit media and is an expected result. The skewness of the surface is negative which indicates that there are more valleys than peaks or that the peaks are broader than the valleys of the surface.
3.1. X-ray photoelectron spectroscopy (XPS) Table 1 shows the elemental composition of the three substrate surfaces as determined by survey XPS scans. The etched substrate seems to be the most “chemically” clean surface with contaminants of calcium, fluorine, silicon and sodium not present in contrast to the as treated and mechanically abraded substrate surfaces. Silicon contamination on the steel grit blasted substrate is most likely due to contamination from residual silica in the blasting cabinet. The results of the chemical state XPS scans are summarised in Table 2. It is again evident that the etched substrate surface is chemically cleaner than the other two substrate surfaces, as the percentage of carbon as C–H, C–O, C O along with oxygen as C–O, C O and H2O/OH are lowest on the etched substrate surface. 3.2. Contact angle measurements Contact angle measurements showed that the etched surface had the most favourable wetting properties followed by the degreased then steel grit blasted substrate as in Fig. 2.
3.4. ImageJ analysis of single splat images Fig. 2 shows the average number density of splats (splats mm− 2) determined from automated splat counting routines by ImageJ on SEM images of single splats on each of the substrates. The steel grit blasted substrate had the highest splat coverage followed by the degreased surface and etched surface. The steel grit blasted surface was an order of magnitude rougher than the degreased and etched surfaces and therefore the highest splat coverage on this surface is not unexpected. However if the surface roughness was the sole predictor of splat adhesion, it would be expected that the etched surface, being the next roughest surface should have the next highest splat coverage. However this is not the case, with the etched surface having the lowest splat coverage. Other than the surface chemistry of the two substrates (the etched substrate being cleaner than the degreased and
3.3. Roughness measurements The roughness and skewness values determined by AFM and laser profilometry for the three substrate treatments are shown in Table 3. Each value is an average of measurements from 4 randomly selected areas, 500 μm × 500 μm, with each roughness measurement being based on 256 lines within each sample and extracted using the Digital Instruments software that is bundled with the AFM control software. Analysis of the AFM line scans of the degreased and etched surface revealed that the etched substrate surface has a higher roughness value but of the same order of magnitude as the degreased surface. Table 2 Substrate surface element chemical states. %C
Degreased Etched Steel grit
%O
%Fe
%Cr
285 ± 0.5 (C,C–OR)
286 ± 0.5 (C–H)
288 ± 0.5 (C–O, C O)
293 ± 0.5 (organic)
530 ± 0.5 (FeOx)
531 ± 0.5 (CrOx)
533 ± 0.5 (C–O, C O, H2O/OH)
710 ± 0.5 (FeOx)
707 ± 0.5 (Fe)
576 ± 0.5 (CrOx)
574 ± 0.5 (Cr)
42.3 56.7 33.6
38 32.5 41.9
17.9 10.8 24.5
1.8
27 25.3 17.6
41.4 64 23.2
31.6 10.8 38.1
90.2 70.9 91.6
9.8 29.1 8.4
95.6 93.9 93.7
4.4 6.1 6.3
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Table 3 Substrate roughness and skewness values.
Degreased Etched Steel grit
Ra
Rsk
Method
140 nm 360 nm 4.6 µm
− 1.1 0.6 − 0.3
AFM AFM Laser profilometry
comparable contact angles) the only property of the two substrates which differed significantly was the skewness parameter. Therefore it can be suggested that surface topology may be more significant than the surface chemistry for the adhesion of PEEK splats onto stainless 304, and that for surfaces with a similar roughness, a positive skewness value leads to a lower coverage of splats than a surface with a negative skewness value. Fig. 3 also shows the average circularity of splats deposited on the three substrates. The splats deposited on the etched substrate have the highest average circularity, followed by the degreased and then steel grit blasted substrate. This follows the order of chemically clean substrates. However if surface chemistry is not as significant in governing PEEK splats as suggested then possibly the positive skewness value of the etched substrate could be promoting the more circular or disc shaped splats. It has been shown by Fukumoto et al. [13] that on surfaces of the same (order of magnitude) roughness the circularity of Cu splats on ANSI304 stainless steel was increased by changing from negative to positive skewness.
Both Fukumoto et al. and Cadelle et al. [21,22] have shown that increasing the substrate temperature of ANSI304 stainless steel leads to a change in the skewness from negative to positive. Heating the substrate also leads to the evaporation of absorbates from the surface which also leads to a chemically cleaner surface. The circularity of splats in this situation also increases as there are more disc type splats than splashy type splats. Cadelle et al. [22] suggest that a surface exhibiting positive skewness increases the wetting and therefore contacts with the substrate of the coating liquid. Both situations lead to more circular splats on surfaces with a positive skewness parameter. 3.5. Scratch testing Table 4 shows the typical average load (N) range at which the PEEK coatings failed during scratch testing. Fig. 5 shows SEM images of a typical scratch test performed on ACHVAF sprayed PEEK coatings on a degreased substrate. The scratch path shows a clean substrate with minimal PEEK residue, indicating adhesive failure of the coating occurred. The coating folded over indicating poor bonding of the coating to the surface and reinforcing the adhesive mode of failure of the coating. Fig. 6 shows the scratch track on the PEEK coating formed on the etched substrate. On this substrate the coating exhibits features typical of both adhesive and cohesive modes of failure as PEEK is visible on the scratch track. The scratch track shows regions of both
Fig. 3. AFM surface scan and surface profile of degreased substrate.
Fig. 4. AFM surface scan and surface profile of etched substrate.
K. Patel et al. / Surface & Coatings Technology 204 (2010) 3567–3572
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Table 4 Scratch test results. Substrate
Coating failure load (N) range
Degreased Etched Steel Grit
6.1–9.0 9.1–12.0 12.1–15.0
coating fold-over and pile-up, which are indicated in Fig. 6. Pile-up of the coating can be taken as indicative of the cohesive failure of the coating. Both the fold-over and pile-up of the coating are illustrated by a simple schematic in Fig. 7. Fig. 8 shows the scratch track on the PEEK coating formed on the steel grit blasted substrate. This coating shows only cohesive coating failure as significant amounts of PEEK are still present on the substrate surface. The results of the scratch testing of coatings formed on the three treated surfaces indicate that the coatings on the steel grit blasted surfaces showed the greatest adhesion to the stainless steel surfaces, with the etched surface having the poorest adhesion. The applied load at failure, shown in Table 4, was also highest for the steel grit blasted substrate.
3.6. Roughness, skewness, and mechanical interlocking It can be concluded that the higher surface roughness of the steel grit blasted substrate promotes better mechanical interlocking of the PEEK to the substrate ensuring a better adhesive strength than the PEEK coatings on the degreased and etched surfaces (Table 4). The improved mechanical interlocking of the coating with the grit blasted
Fig. 7. Schematic showing cohesive (pile-up) and adhesive (fold-over) failure modes of PEEK coatings on ANSI304 stainless steel.
surface resulted in cohesive coating failure as opposed to adhesive failure which was observed on both the degreased and etched surfaces. The degreased substrate was the least rough and the most negatively skewed. The poor performance of the coating on this substrate can be attributed to the surface not providing many locations for mechanical interlocking of the PEEK coating. The etched substrate shows coating failure in-between that of the degreased and steel grit blasted values as shown in Table 4. The roughness value of the etched surface is higher than the roughness value on the degreased surface however both are in the nanometer range, as opposed to the much greater roughness of the steel grit blasted substrate. The small difference in the roughness between the etched and the degreased substrate cannot be the only factor in the improved performance of the coatings on etched
Fig. 5. SEM images of scratch test carried out of PEEK coating on degreased substrate.
Fig. 6. SEM images of scratch test carried out of PEEK coating on etched substrate.
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Fig. 8. SEM images of scratch test carried out of PEEK coating on steel grit blasted substrate.
substrates. This is because PEEK splats typically have dimensions on the order of microns, and small changes in the roughness on the nanometer scale would not be expected to account for the difference in the failure loads observed for coatings on the degreased and etched substrates. An increase in available surface area for adhesion or more mechanical interlocking sites for the PEEK to adhere would explain this behaviour. Comparison of the line profiles of the etched and degreased surface, in Figs. 3 and 4 shows that the profile of the etched surface does provide a better surface for mechanical interlocking, which would explain the increase in coating adhesion. This increase is due to the positive skewness parameter of the etched substrate, indicating that the surface has a profile which favours greater mechanical interlocking. Fig. 9 shows a very simple schematic showing surfaces with highly idealised negative and positive skewness, and how this may affect coating adhesion. A (PEEK) splat impacting onto a negatively skewed surface would not flow into the sharp valleys as much as a splat would flow into broad valleys due to the sharp peaks of a surface with a positive skewness value. This gives more available surface area for increased adhesion, as well as mechanical interlocking between the coating and the substrate surface. This hypothesis works well with the results of the PEEK coatings in this work and also with the work by Cadelle et al. [22] on the impingement of free falling droplets.
skewness (Rsk) show better coating adhesion than similar surfaces with negative skewness. Preparing surfaces with suitable topography (that promotes mechanical interlocking) plays a more important role than the chemistry of the substrate surface when spraying onto substrates that have no preheating. Acknowledgements This work was supported by a New Economy Research Fund grant (NERF) from the Foundation for Research, Science, and Technology (FRST, NZ), contract number UOAX0410. We would like to thank Professor Richard Knight and Mr Dustin Doss of the Drexel University (Philadelphia, PA, USA) thermal spray group for their valuable advice and the use of their Jet Kote II HVOF thermal spray system. We would like to thank Celia Fonseca and Shufen Li of Rakon Ltd., Auckland, New Zealand for their help with laser profilometry measurements. References [1] [2] [3] [4] [5] [6]
4. Conclusions Formation of surface roughness on ANSI304 stainless steel with a positively skewed distribution can be produced using an etching process. The skewness of roughness as well as its magnitude must be considered when considering the influence of roughness parameters of adhesion of thermal spray coatings, especially those with gross splat flow or deformation during coating formation. A positively skewed roughness is desirable to promote mechanical interlocking. For ANSI304 stainless steel surfaces with comparable roughness (Ra), the skewness of the roughness distribution affects the adhesion of the coating formed. PEEK coatings on surfaces with positive
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Fig. 9. Simplistic schematic showing possible splat interaction with negative and positive skewness value surfaces.
T. Palathai, J. Tharajak, N. Sombatsompop, Mater. Sci. Eng. 485 (2008) 66. V. Hnatowicz, et al., Nucl. Instrum. Methods Phys. Res., Sect. B 266 (2) (2008) 283. P.J. Rae, E.N. Brown, E.B. Orler, Polymer 48 (2) (2007) 598. VICTREX, PEEK: Properties Guide, Lancashire, 2002. D. Parker, et al., Polymers, High-temperature, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 2002. M. Ivosevic, R.A. Cairncross, R. Knight, Int. J. Heat Mass Transfer 49 (19–20) (2006) 3285. E. Petrovicova, L.S. Schadler, Int. Mater. Rev. 47 (4) (2002) 169. J. Varacalle, J. Dominic, et al., J. Therm. Spray Tech. 15 (3) (2006) 348. A.V. Pocius, Adhesion and Adhesives Technology, Hanser Publications, USA, 1997. L. Ponsonnet, et al., Mater. Sci. Eng., C 23 (4) (2003) 551. T.R. Thomas, Precis. Eng. 3 (2) (1981) 97. T.R. Thomas, Rough Surfaces, 2nd ed., Imperial College Press, London, 1999. M. Fukumoto, H. Nagai, T. Yasui, J. Therm. Spray Tech. 15 (4) (2006) 759. K.J. Stout, T.G. King, D.J. Whitehouse, Wear 43 (1) (1977) 99. M. Ivosevic, et al., J. Therm. Spray Tech. 15 (2006) 725. M. Bahbou, P. Nylén, J. Wigren, J. Therm. Spray Tech. 13 (4) (2004) 508. Patel, K High Velocity Thermal Spraying of PEEK onto Stainless Steel, 2009, PhD thesis, University of Auckland, New Zealand. CasaXPS, Processing Software for XPS, AES, AIMS and More, 2009 (cited 2009; Available from: www.casaxps.com). W. Rasband, ImageJ, National Institutes of Health, USA, 1997–2006. J.J. Friel, Measurements, Practical Guide to Image Analysis, ASM International, Materials Park, OH, 2000, p. 101. M. Fukumoto, E. Nishioka, T. Matsubara, Surf. Coat. Technol. 120–121 (1999) 9131. J. Cadelle, A. Vardele, P. Fauchais, Surf. Coat. Technol. 201 (2006) 1373.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Effect of surface roughness parameters on thermally sprayed PEEK coatings Krishal Patel a, Colin S. Doyle a, Daisuke Yonekura b, Bryony J. James a,⁎ a b
Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland 1142, New Zealand Department of Mechanical Engineering, The University of Tokushima, 2-1 Minami-josanjima-cho, Tokushima 770-8506, Japan
a r t i c l e
i n f o
Article history: Received 17 September 2009 Accepted in revised form 9 April 2010 Available online 18 April 2010 Keywords: Roughness Skewness Thermal spray High velocity oxygen fuel (HVOF) High velocity air fuel (HVAF) Poly ether ether ketone (PEEK)
a b s t r a c t Single splats and coatings of PEEK (polyether ether ketone) were produced using high velocity oxygen fuel (HVOF) and assisted combustion high velocity air fuel (AC-HVAF) thermal spray techniques respectively to investigate the effect of varying surface roughness and chemistry of the ANSI304 stainless steel substrates had on the formation of PEEK coatings. As received degreased, etched and steel grit blasted surface treatments were used. XPS analysis showed that the etched substrate had the most chemically clean surface confirmed by contact angle measurement which showed that the etched substrate had the lowest contact angle indicating good wetting properties followed by the degreased and the steel grit blasted substrates. Roughness and skewness measurements of the treatment substrates showed the steel grit blasted surface to have the highest roughness value followed by the etched and degreased substrates whose roughness values were an order of magnitude lower. Skewness was positive on the etched substrate, slightly negative on the steel grit and negative on the degreased substrate. PEEK single splat image analysis showed that splats per mm2 were highest on the steel grit blasted substrate followed by degreased and etched respectively. Scratch testing of the PEEK coatings showed coating failure on the steel grit substrate to be cohesive, on the degreased substrate, adhesive and on the etched substrate a combination of both. From these results it was concluded that a positive skewness ANSI304 stainless steel surface can be produced using an etching process, and for surfaces with suitable topography mechanical interlocking may play a more significant role than surface chemistry for room temperature substrates. HVOF and AC-HVAF thermal spraying of PEEK coatings onto stainless steel surfaces and for ANSI304 stainless steel surfaces with comparable roughness and PEEK coatings on surfaces with positive skewness show better coating adhesion than similar surfaces with negative skewness. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Polyether ether ketone (PEEK) is a semi-crystalline engineering thermoplastic which possesses excellent chemical resistance, along with good thermal stability, mechanical properties, wear and reduced frictional properties [1–3]. PEEK has high glass transition and melting temperatures (Tg = 143 °C, Tm = 343 °C) [4] which make it an excellent choice for many engineering applications requiring good chemical resistance at high temperatures. This makes PEEK an ideal candidate for coating applications. The high chemical resistance of PEEK towards solvent attack means that solution based coating methods are neither efficient nor environmentally friendly methods of coating deposition (PEEK is soluble in media such as diphenyl sulphone, high boiling point esters, benzophenone, and 1-chloronaphthalene at temperatures above Tm) [5]. Thermal spray is a group of coating methods which uses thermal energy to heat the coating material to a molten or semi-molten state.
⁎ Corresponding author. E-mail address: [email protected] (B.J. James). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.04.026
The heated particles are accelerated towards a substrate by process gas or atomisation jets. Upon impact the softened particles plastically deform, producing thin disc-like platelets, termed “splats”. A thermal spray coating is made my subsequent particles impacting onto the already deposited splats, building up a thick interlocking lamellar structure. The solvent-less nature of thermal spray [6] makes it an ideal coating process for PEEK coatings [7]. The quality of a thermally sprayed coating depends on a number of factors, including the surface preparation of the substrate. The substrate parameters can be divided into two broad classes, representing the chemistry and the topology of the surface. The substrate parameters which have been identified as being of key importance in producing good coatings are the cleanliness (reflecting the surface chemistry) and the roughness (reflecting the topology) of the surface [8]. Cleanliness of the surface promotes better wetting of the coating media to the substrate and roughness provides sites for the mechanical interlocking [9] of the coating media. The wetting properties of a surface can be determined by using the sessile drop contact angle method [10]. There are a number of parameters which can be used as a measure of the roughness of a surface, the most common of which is the profile average or centre line roughness parameter Ra [11]. Ra alone is not
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adequate in assessing the surface topography of a substrate as different surface profiles can exhibit the same or similar Ra values [12,13]. If the varying heights along a profile of a (rough) surface are considered as a distribution, it is possible to gauge whether a profile is skewed towards broad peaks and spike-like valleys, or towards broad valleys and spike-like peaks. The parameter used to give this information is skewness or Rsq [12–14]. The skewness parameter can also be used to describe a surface profile along with Ra. The additional information derived from including the skewness along with the roughness is shown in Fig. 1, which shows a schematic of three surfaces with equal roughness (Ra), but with varying skewness. In this work ANSI stainless steel 304 has been subjected to three different substrate pre-treatments to investigate the effect of varying surface roughness and chemistry on the formation of PEEK coatings. Coatings were investigated as single splats and as coatings formed by the build-up of multiple sprayed layers of PEEK using high velocity thermal spray techniques. The three surface treatments were: as received degreased; chemically etched; and steel grit blasted. Grit or bead blasting is a common surface preparation technique used widely in industry [15,16] for the cleaning and roughening of surfaces prior to coating application. The surface chemistry of the ANSI304 stainless steel substrates was characterised prior to surface treatment and thermal spraying using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Scanning electron microscopy (SEM), atomic force microscopy (AFM) and laser profilometry were be used to characterise the surface topology. Image analysis of the PEEK single splats and scratch testing of the coatings were done to determine the effect of the surface pretreatments on the PEEK single splats and coatings.
Etched substrates were produced by treatment with an etchant solution comprising of (by volume) 3 parts water, 2 parts concentrated hydrochloric acid, and 1 part concentrated nitric acid. The degreased stainless steel coupons were immersed in the etching solution for 5 min. After which the substrates were washed under water and dried. The steel grit (SAE-G 50) media of mesh size 50 (290 µm) was used in a SBC 220 vertical sand blast cabinet with nominal air pressure of 60 psi (410 kPa) to prepare the grit blasted coupons. 2.2. Single splat and coating formation The PEEK powder was manufactured by VICTREX (Victrex PEEK®, Lancashire, UK) and was of grade 150 PF. This powder has a nominal size of 45 μm. Spray conditions and techniques were selected on the basis of an optimisation process detailed in [17]. Single splats of PEEK were produced using a Stallite Jet Kote II HVOF thermal spray system using a Miller thermal rotary powder feeder with gun parameters of 150 mm nozzle length, 280 mm spray distance and transverse speed of 300 mm s− 1. Coatings (with thicknesses in the order of 50 μm) were produced using the UniqueCoat Technologies AC-HVAF thermal spray system with a Browning Mark XV powder feeder. A 150 mm nozzle was used on the spray gun which operated at a distance of 300 mm from the substrate. Coatings were produced in three passes with a gun transverse speed of 250 mm s− 1. The three substrates were mounted in the spray booth together and were subsequently sprayed at the same time allowing for the direct comparison of the three surface treatments. 2.3. Substrate characterisation
2. Experimental 2.1. Substrate preparation Single splat experiments were conducted on 20 mm square coupons while coatings were produced on 100 mm square coupons. All coupons were degreased by submersing the stainless steel coupons in iso-propyl alcohol for approximately 5 min while gently rubbing the surface of the samples with solvent proof gloves.
Fig. 1. Illustration of three surfaces with equal roughness (Ra) parameter. (a) Positive skewness, (b) zero skewness, and (c) negative skewness.
A Kratos Axis Ultra DLD XPS system was used to determine the composition and chemical state of the elements present on the substrate surface prior to thermal spraying. This system uses monochromatized Aluminium Kα X-rays (1486.69 eV) along with a hybrid electrostatic and magnetic lensing system to create an analysis area of 300 × 700 µm. Elemental survey scans were run on all the samples with pass energy of 160 eV, step size of 1 eV and sweep time of 180 s. Chemical state scans of the iron, chromium, oxygen and carbon regions were conducted with pass energy of 20 eV, step size of 0.1 eV and sweep time of 180 s. Quantification of the survey scans and fitting of the chemical state scans were performed using the CasaXPS package [18]. The binding energy scale was corrected using the signal from adventitious carbon (≡285.0 eV) as an internal standard. Contact angle measurements were collected on the three treated substrates using the KSV instruments CAM101 contact angle goniometer with water as the probe liquid to determine the effect of the difference in surface chemistry. Four contact angle measurements were taken at random positions on each substrate. Contact angle measurements were taken 1 min after the probe liquid made contact with the substrate surface in order for equilibrium to be reached. An average of the four contact angles was calculated. For the degreased and etched substrates a Digital Instruments multimode AFM in contact mode was employed to obtain surface roughness values, 3D surface plots and line profiles of the surface. Roughness values were obtained at four separate random spots on each sample over an area of 500 µm2 and averaged. For the steel grit blasted substrate, which was too rough for AFM measurement, the roughness profiles were obtained using the SmartScope ZIP 250 laser profilometer (Rakon Ltd., Auckland, NZ). Four measurements at random positions were taken then averaged. Backscattered Secondary Electron (BSE) SEM images were taken using a FEI Quanta Environmental SEM with an accelerating voltage of 20 kV. Image analysis was performed on BSE SEM images using ImageJ [19], an open source software package that can be used to quantify
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Table 1 Substrate surface elemental composition. Surface treatment
%C
%O
%Fe
%Cr
%N
%Ca
%F
%Si
%Na
Degreased Etched Steel grit
58.1 60.6 50.3
32.1 33.6 42.4
2.4 0.6 3.3
1.9 2.5 0.4
4.4 2.7 0.3
0.7 – 0.7
0.2 – –
– – 2.6
0.3 – –
electron microscopy images. The image analysis software allowed the determination of the number density of splats (splats mm− 2) and the average circularity of splats. The circularity is a shape factor which relates the measured area of an object to the area of a circle with the same perimeter. The circularity thus gives a measure of how well formed a splat is. A uniform disc-like splat will have a circularity approaching unity, whereas fingered or splashy splats will have lower circularity indicating their more jagged perimeter [19,20]. Scratch testing was done using a CSEM Revetest scratch tester with a spherical diamond indenter with 50 µm radius tip. A linearly increasing load (0 to 40 N) was applied till coating failure. The point of coating failure was determined to be where the first sign of coating delamination from the substrate was observed.
Fig. 2. Graph of contact angle, splat density (left axis) and circularity (right axis) for the three surface treatments.
3. Results and discussion However the two surfaces differ in that the degreased surface had a negative skewness and the etched surface a positive skewness. The skewness differences can be seen in the AFM line profiles of each surface which are shown in Figs. 3 and 4. Fig. 3 shows the degreased substrate surface and a line scan showing the surface topography. From the line scan it can be seen that there are board peaks and sharp valleys indicative of a negative skewness. The line scan of the etched surface in Fig. 4 shows that there are more peaks than valleys. The peaks are also sharp. This give the etched substrate surface a positive skewness. The highest roughness was on the steel grit blasted surface, which was an order of magnitude higher than on the degreased or etched substrate surfaces. This is due to the mechanical abrasion from the irregular shaped steel grit media and is an expected result. The skewness of the surface is negative which indicates that there are more valleys than peaks or that the peaks are broader than the valleys of the surface.
3.1. X-ray photoelectron spectroscopy (XPS) Table 1 shows the elemental composition of the three substrate surfaces as determined by survey XPS scans. The etched substrate seems to be the most “chemically” clean surface with contaminants of calcium, fluorine, silicon and sodium not present in contrast to the as treated and mechanically abraded substrate surfaces. Silicon contamination on the steel grit blasted substrate is most likely due to contamination from residual silica in the blasting cabinet. The results of the chemical state XPS scans are summarised in Table 2. It is again evident that the etched substrate surface is chemically cleaner than the other two substrate surfaces, as the percentage of carbon as C–H, C–O, C O along with oxygen as C–O, C O and H2O/OH are lowest on the etched substrate surface. 3.2. Contact angle measurements Contact angle measurements showed that the etched surface had the most favourable wetting properties followed by the degreased then steel grit blasted substrate as in Fig. 2.
3.4. ImageJ analysis of single splat images Fig. 2 shows the average number density of splats (splats mm− 2) determined from automated splat counting routines by ImageJ on SEM images of single splats on each of the substrates. The steel grit blasted substrate had the highest splat coverage followed by the degreased surface and etched surface. The steel grit blasted surface was an order of magnitude rougher than the degreased and etched surfaces and therefore the highest splat coverage on this surface is not unexpected. However if the surface roughness was the sole predictor of splat adhesion, it would be expected that the etched surface, being the next roughest surface should have the next highest splat coverage. However this is not the case, with the etched surface having the lowest splat coverage. Other than the surface chemistry of the two substrates (the etched substrate being cleaner than the degreased and
3.3. Roughness measurements The roughness and skewness values determined by AFM and laser profilometry for the three substrate treatments are shown in Table 3. Each value is an average of measurements from 4 randomly selected areas, 500 μm × 500 μm, with each roughness measurement being based on 256 lines within each sample and extracted using the Digital Instruments software that is bundled with the AFM control software. Analysis of the AFM line scans of the degreased and etched surface revealed that the etched substrate surface has a higher roughness value but of the same order of magnitude as the degreased surface. Table 2 Substrate surface element chemical states. %C
Degreased Etched Steel grit
%O
%Fe
%Cr
285 ± 0.5 (C,C–OR)
286 ± 0.5 (C–H)
288 ± 0.5 (C–O, C O)
293 ± 0.5 (organic)
530 ± 0.5 (FeOx)
531 ± 0.5 (CrOx)
533 ± 0.5 (C–O, C O, H2O/OH)
710 ± 0.5 (FeOx)
707 ± 0.5 (Fe)
576 ± 0.5 (CrOx)
574 ± 0.5 (Cr)
42.3 56.7 33.6
38 32.5 41.9
17.9 10.8 24.5
1.8
27 25.3 17.6
41.4 64 23.2
31.6 10.8 38.1
90.2 70.9 91.6
9.8 29.1 8.4
95.6 93.9 93.7
4.4 6.1 6.3
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Table 3 Substrate roughness and skewness values.
Degreased Etched Steel grit
Ra
Rsk
Method
140 nm 360 nm 4.6 µm
− 1.1 0.6 − 0.3
AFM AFM Laser profilometry
comparable contact angles) the only property of the two substrates which differed significantly was the skewness parameter. Therefore it can be suggested that surface topology may be more significant than the surface chemistry for the adhesion of PEEK splats onto stainless 304, and that for surfaces with a similar roughness, a positive skewness value leads to a lower coverage of splats than a surface with a negative skewness value. Fig. 3 also shows the average circularity of splats deposited on the three substrates. The splats deposited on the etched substrate have the highest average circularity, followed by the degreased and then steel grit blasted substrate. This follows the order of chemically clean substrates. However if surface chemistry is not as significant in governing PEEK splats as suggested then possibly the positive skewness value of the etched substrate could be promoting the more circular or disc shaped splats. It has been shown by Fukumoto et al. [13] that on surfaces of the same (order of magnitude) roughness the circularity of Cu splats on ANSI304 stainless steel was increased by changing from negative to positive skewness.
Both Fukumoto et al. and Cadelle et al. [21,22] have shown that increasing the substrate temperature of ANSI304 stainless steel leads to a change in the skewness from negative to positive. Heating the substrate also leads to the evaporation of absorbates from the surface which also leads to a chemically cleaner surface. The circularity of splats in this situation also increases as there are more disc type splats than splashy type splats. Cadelle et al. [22] suggest that a surface exhibiting positive skewness increases the wetting and therefore contacts with the substrate of the coating liquid. Both situations lead to more circular splats on surfaces with a positive skewness parameter. 3.5. Scratch testing Table 4 shows the typical average load (N) range at which the PEEK coatings failed during scratch testing. Fig. 5 shows SEM images of a typical scratch test performed on ACHVAF sprayed PEEK coatings on a degreased substrate. The scratch path shows a clean substrate with minimal PEEK residue, indicating adhesive failure of the coating occurred. The coating folded over indicating poor bonding of the coating to the surface and reinforcing the adhesive mode of failure of the coating. Fig. 6 shows the scratch track on the PEEK coating formed on the etched substrate. On this substrate the coating exhibits features typical of both adhesive and cohesive modes of failure as PEEK is visible on the scratch track. The scratch track shows regions of both
Fig. 3. AFM surface scan and surface profile of degreased substrate.
Fig. 4. AFM surface scan and surface profile of etched substrate.
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Table 4 Scratch test results. Substrate
Coating failure load (N) range
Degreased Etched Steel Grit
6.1–9.0 9.1–12.0 12.1–15.0
coating fold-over and pile-up, which are indicated in Fig. 6. Pile-up of the coating can be taken as indicative of the cohesive failure of the coating. Both the fold-over and pile-up of the coating are illustrated by a simple schematic in Fig. 7. Fig. 8 shows the scratch track on the PEEK coating formed on the steel grit blasted substrate. This coating shows only cohesive coating failure as significant amounts of PEEK are still present on the substrate surface. The results of the scratch testing of coatings formed on the three treated surfaces indicate that the coatings on the steel grit blasted surfaces showed the greatest adhesion to the stainless steel surfaces, with the etched surface having the poorest adhesion. The applied load at failure, shown in Table 4, was also highest for the steel grit blasted substrate.
3.6. Roughness, skewness, and mechanical interlocking It can be concluded that the higher surface roughness of the steel grit blasted substrate promotes better mechanical interlocking of the PEEK to the substrate ensuring a better adhesive strength than the PEEK coatings on the degreased and etched surfaces (Table 4). The improved mechanical interlocking of the coating with the grit blasted
Fig. 7. Schematic showing cohesive (pile-up) and adhesive (fold-over) failure modes of PEEK coatings on ANSI304 stainless steel.
surface resulted in cohesive coating failure as opposed to adhesive failure which was observed on both the degreased and etched surfaces. The degreased substrate was the least rough and the most negatively skewed. The poor performance of the coating on this substrate can be attributed to the surface not providing many locations for mechanical interlocking of the PEEK coating. The etched substrate shows coating failure in-between that of the degreased and steel grit blasted values as shown in Table 4. The roughness value of the etched surface is higher than the roughness value on the degreased surface however both are in the nanometer range, as opposed to the much greater roughness of the steel grit blasted substrate. The small difference in the roughness between the etched and the degreased substrate cannot be the only factor in the improved performance of the coatings on etched
Fig. 5. SEM images of scratch test carried out of PEEK coating on degreased substrate.
Fig. 6. SEM images of scratch test carried out of PEEK coating on etched substrate.
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Fig. 8. SEM images of scratch test carried out of PEEK coating on steel grit blasted substrate.
substrates. This is because PEEK splats typically have dimensions on the order of microns, and small changes in the roughness on the nanometer scale would not be expected to account for the difference in the failure loads observed for coatings on the degreased and etched substrates. An increase in available surface area for adhesion or more mechanical interlocking sites for the PEEK to adhere would explain this behaviour. Comparison of the line profiles of the etched and degreased surface, in Figs. 3 and 4 shows that the profile of the etched surface does provide a better surface for mechanical interlocking, which would explain the increase in coating adhesion. This increase is due to the positive skewness parameter of the etched substrate, indicating that the surface has a profile which favours greater mechanical interlocking. Fig. 9 shows a very simple schematic showing surfaces with highly idealised negative and positive skewness, and how this may affect coating adhesion. A (PEEK) splat impacting onto a negatively skewed surface would not flow into the sharp valleys as much as a splat would flow into broad valleys due to the sharp peaks of a surface with a positive skewness value. This gives more available surface area for increased adhesion, as well as mechanical interlocking between the coating and the substrate surface. This hypothesis works well with the results of the PEEK coatings in this work and also with the work by Cadelle et al. [22] on the impingement of free falling droplets.
skewness (Rsk) show better coating adhesion than similar surfaces with negative skewness. Preparing surfaces with suitable topography (that promotes mechanical interlocking) plays a more important role than the chemistry of the substrate surface when spraying onto substrates that have no preheating. Acknowledgements This work was supported by a New Economy Research Fund grant (NERF) from the Foundation for Research, Science, and Technology (FRST, NZ), contract number UOAX0410. We would like to thank Professor Richard Knight and Mr Dustin Doss of the Drexel University (Philadelphia, PA, USA) thermal spray group for their valuable advice and the use of their Jet Kote II HVOF thermal spray system. We would like to thank Celia Fonseca and Shufen Li of Rakon Ltd., Auckland, New Zealand for their help with laser profilometry measurements. References [1] [2] [3] [4] [5] [6]
4. Conclusions Formation of surface roughness on ANSI304 stainless steel with a positively skewed distribution can be produced using an etching process. The skewness of roughness as well as its magnitude must be considered when considering the influence of roughness parameters of adhesion of thermal spray coatings, especially those with gross splat flow or deformation during coating formation. A positively skewed roughness is desirable to promote mechanical interlocking. For ANSI304 stainless steel surfaces with comparable roughness (Ra), the skewness of the roughness distribution affects the adhesion of the coating formed. PEEK coatings on surfaces with positive
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Fig. 9. Simplistic schematic showing possible splat interaction with negative and positive skewness value surfaces.
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