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Influence of Ti–6Al–4 V and Al 2017 substrate morphology on Ni–Al coating adhesion — Impacts of laser treatments
Surface & Coatings Technology 205 (2011) 2702–2708
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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
Influence of Ti–6Al–4 V and Al 2017 substrate morphology on Ni–Al coating adhesion — Impacts of laser treatments Y. Danlos ⁎, S. Costil, H. Liao, C. Coddet LERMPS, Université de Technologie de Belfort-Montbéliard, 90010 Belfort Cedex, France
a r t i c l e
i n f o
Article history: Received 26 May 2010 Accepted in revised form 31 August 2010 Available online 8 September 2010 Keywords: Thermal spraying Laser treatment Interfacial indentation Toughness
a b s t r a c t Surface morphology, chemical composition and surface state are important parameters for the formation of thermally sprayed coatings. More precisely the temperature, wettability, geometry and degree of the surface contamination play an important role in the coating particularly from the adhesion point of view. Several surface preparations (degreasing, high pressure water jet, grit blasting…) before thermal spraying are often used in order to obtain deposits with a good interfacial adherence. In this paper we report on an original laser surface treatment which uses two Nd-YAG laser beams in conjunction with a thermal spray gun in order to modify the substrate surface prior to thermal spraying in order to enhance the adhesion at the coating substrate interface. A first Nd-YAG laser is used to heat the surface and a second to clean it. The system is used to coat two different materials (titanium Ti–6Al–4 V and aluminium 2017 alloy) with a Ni–Al alloy. The relation between the surface modifications induced by these treatments and the adhesion of the coatings on the substrates are studied. © 2010 Published by Elsevier B.V.
1. Introduction Thermal spraying is a well established means to form coatings for surface engineering. A heat source is used to melt and accelerate metallic or non-metallic powders and spray them onto a substrate to elaborate a coating. It seems to be a simple process but there are numerous interdependencies between all parameters. A great number of interactive parameters have to be controlled in order to obtain good coatings and promote substrate adhesion. In fact, the coating adhesion on the substrate, or on the previously deposited layer, is fundamental to promote a good cohesion of the materials and a successful application. To create good conditions to anchore coatings on substrate, appropriate surface preparations are required [1]. Previous studies have shown that in order to obtain good adhesion between the substrate and the coating it is necessary to spray particles on surfaces which have specific chemical, physical and mechanical characteristics [2]. The surfaces have to be cleaned to eliminate the adsorbed molecules which can be evaporated by the contact with the molten sprayed particles and, as a result, generate splashing [3]. The substrate wettability has also to be as high as possible to improve the splat spreading [4]. A clean surface is an important condition for the contact quality which depends on the droplet wetting and desorption of the pollutants adsorbed on the surface or on the underlying layer [5]. In most cases to remove grease, oil or dust from the
⁎ Corresponding author. Tel.: + 33 3 84 58 31 64. E-mail address: [email protected] (Y. Danlos). 0257-8972/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2010.08.147
surface, chemical degreasing is conventionally used. These processes improve the chemical behaviour of the surface to have a good spreading of the particles. But to create the appropriate conditions to anchore coatings on substrate, further surface preparations are required. In general grit blasting is used to obtain efficient surface cleaning and texturing to enhance mechanical adhesion of the coating to the substrate. The creation of asperities on the surface by grit blasting increases the surface roughness and thus the particle anchoring on the surface. However these processes present disadvantages: the elimination of the chemical wastes from the cleaning surface requires precautions for environmental considerations (operators, recycling) and therefore due to grit blasting fatigue property decreases: microcracks on substrate surface or grit inclusions which are harmful for the adhesion [6]. Furthermore the chemical and physical modifications required in order to obtain adequate adhesion are generally induced by different processes constituting successive steps and that are not accomplished during thermal spraying. Within this objective, other cleaning processes have also been developed to eliminate those defects. For example, low pressure plasma processes can be used for surface cleaning. It is possible to remove contaminants by simple plasma sputtering particle bombardment in plasmas of noble gases under low pressure environment. Degreasing oil-contaminated metal sheet and removing weakly bonded oxides were performed by plasma cleaning [7]. Highpressure water jet can be used too in surface preparation before thermal spraying but this jet operation imposes high tensile bond strength [8]. The surface morphology resulting from this process was compared to conventional grit blasting but contrary to this
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conventional process, no grit inclusions occur. Some others, based on jet processes like grit blasting and high-pressure water jet create surface modifications but often cause damage and generate compressive strength on the substrate. Then, new developments like, dry-ice blasting process can remove adhering layers without surface modification. This mechanism is based on induced kinetic and sublimation energy. It could completely remove oil and oxide layer from the surface. The contaminants which have been removed are the only residues of the jet operation [9]. But all these processes cannot be applied simultaneously with thermal spraying. Moreover the treated surfaces could not be precisely controlled by all the technological processes. Laser processes can go beyond this problem as the laser treated area is defined precisely by the laser beam. In order to improve the surface cleanliness, a method based on laser ablation process has been developed. Known as PROTAL® process (French acronym for “PROjection Thermique Assistée par Laser”), this treatment [10] combines two effects of surface treatments associating the photonic ablation and thermal spraying [11]. To ablate surface contaminants and improve the substrate wettability a nanosecond pulsed laser is used [12]. Then, decontamination and thermal spray processes allow to treat the surfaces in only one step contrary to the conventional processes for which usual steps are decoupled. To decrease the particle splash and enhance the adhesion between impacting particles and substrate, the decontamination of the surface is necessary [3]. Nevertheless, considering the material affinity between coating and substrate, it has been established that the substrate temperature is also an important parameter [13]. Increasing the surface temperature changes the splat morphology: on substrate at room temperature, the splats present a splash shape, whereas on heated surfaces they have morphologies similar to the disk which results in a better adhesion of the coating [14]. A transition temperature which depends on both substrate/powder materials has been identified [15]. The splat morphology modification occurs beyond this transition temperature. Several explanations were proposed. Heating the surface clears volatile contaminants adsorbed on the surface, improving the contact between particles and substrate. It was suggested that nanoscale thick oxide layer improved the wettability and contact between the splat and the substrate [16]. The formation of nanoscale peaks [17] was observed on substrate which can be explained by increasing thermal contact between impinging particles and substrate. Thus, this study presents an original solution to prepare surfaces before thermal spraying. This process has been developed for surface cleaning in order to associate chemical and physical surface preparation. It consists in heating the surface by a laser treatment and then initiating surface contaminant desorption by using an ablation laser. The combination of the two techniques: laser preheating and laser cleaning has been already used and encouraging results were obtained on titanium Ti–6Al–4 V alloy [18] and on aluminium 2017 [19]. The aim of this study is to observe the surface modifications induced by laser treatments and to compare of surface adherence between laser surface preparation and conventional preparation (grit blasting). 2. Experimental procedures Cleaning and heating laser treatments are combined before thermal spraying in order to create suitable conditions to anchor coatings. Laser spots are juxtaposed to the particle jet on the substrate in order to limit the recontamination. Coatings are elaborated by the repeated pass of the substrate in front of the plasma torch.
Substrate Plasma
Heating Ablation laser Fig. 1. Schema of the experimental device for laser treatments and thermal spraying.
create the appropriate conditions before thermal spraying in order to promote a maximal adhesion at the coating substrate interface. The surface is heated by a first millisecond laser and then a nanosecond ablation laser is used to clean the surface. A previous study [18] proved that laser cleaning enhances the circular shaped splat but it has also been demonstrated that heating substrates improves the splat spreading. So, associating the two processes would be interesting and would lead to a better adhesion and cohesion of plasma-sprayed coatings. The ablation and preheating lasers were synchronized and their spots were superposed. The delay between the heating and ablation was chosen in order to benefit from the maximum surface temperature while the ablation laser pulse strikes the substrate. Using a laser for the preheating certifies the control of the heating zone and avoids surface distortion or soot deposition which occurs with conventional heating processes (flame, arc wire). 2.2. Material characteristics 2.2.1. Substrates characteristics In this study aluminium alloy 2017 (A2017) and titanium alloy Ti– 6Al–4 V (TA6V) were chosen as substrates and their characteristics are presented in Table 1. Titanium alloy and aluminium alloy substrates are widely used in thermal spraying. TA6V which combines corrosion resistance, biocompatibility and mechanical strength, is used in many industrial fields like aerospace engineering or biomedical applications. Aluminium alloys are as well used in a wide range of applications from electric conductors to automotive field generally in order to lighten mechanical components. However the fatigue behaviour, wear resistance and surface hardness are worse than standard steels which require protective coatings. The investigated materials were chosen because these two materials have different thermal characteristics which lead to different answers faced to laser radiation and thermal phenomena initiated by the impact of the
Table 1 Thermophysical characteristics of materials.
Density(ρ)[kg m−3] Melting temperature (Tfus)[K] Thermal conductivity (k) [W m−1 K−1] Mass heat capacity (Cp)[J kg−1 K−1] Absorptivity at 1064 nm (25 °C) [%]
A2017
TA6V
Ref.
2796 783 140 982 5
4470 1878 7 563 45
[20]
Percentage composition by mass [%] A2017
2.1. Experimental configurations The configuration used in this study (Fig. 1) consists in the superposition and synchronization of two lasers which clean and
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TA6V
Al
Cr
94
0,1
Cu
Mg
Mn
Fe
Ti
Zn
4
0,6
0,7
0,7
0,15
0,25
Al
Ti
V
6
90
4
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melting particles. Aluminium 2017 alloy has a mass heat capacity and a thermal conductivity higher than titanium alloy Ti–6Al–4 V so A2017 evacuates the thermal energy with more efficiency than TA6V. However even if TA6V has a better absorptivity than A2017 (respectively 45 and 5%) the energy (Qmelt) required to melt TA6V is higher than the one to melt A2017. Considering the physical characteristics and applying the following expression, it is possible to evaluate this difference by the relationship: Qmelt = ρCpðT melt −Tatm Þ
ð1Þ
where ρ is the density, Cp the mass heat capacity, Tmelt the melting temperature and Tatm the atmospheric temperature. The simple calculation shows that the energy required to melt A2017 (Qmelt = 1528 J cm−3) is less than half of the energy required to melt TA6V (Qmelt = 4047 J cm−3). Aluminium alloy is therefore more sensitive than TA6V to temperature increase. The substrates were mechanically polished to a mirror surface in order to have the same surface morphologies as these observed with the scanning electron microscope before laser treatments. The procedure consisted in applying successive grades of silicon carbide papers and a final polishing cloth with a 0.05 μm SiC particle suspension. 2.2.2. Powder characteristics The powder sprayed on the substrates was common Ni–Al powder (95–5%, AMDRY 956, Sulzer-Metco) and the particle size range was from 45 μm to 90 μm. 2.2.3. Surface treatment processes For the local surface cleaning, a Quantel1 Q-switched Nd-YAG laser (wavelength 1064 nm, pulse duration 10 ns) was used. The laser spot has a rectangular shape with a homogeneous intensity distribution. The beam profile is generated by crystals which induce phase delay of the wavefront. The angle of incidence of the ablation beam relative to the substrate was equal to 30°. In this study, the dimensions of the laser spot (7 ⁎ 4 mm2) and the power parameters have been chosen in order to irradiate the surfaces with an energy density of 1.7 and 2.3 J cm−2. The energy densities of the ablation laser were chosen for an efficient surface cleaning and to observe the responses of the surfaces irradiated with different energy densities. The pulse repetition rate (60 Hz) and the scanning speed of the spot were also chosen to keep the kinetic thermal spray parameters and to create areas treated by a single shot ablation laser. The heating treatment was accomplished by a Nd-YAG laser designed by the Cheval company.2 It is a pulsed millisecond laser with a wavelength of 1064 nm. The laser spot was circular with a Gaussian energy distribution. The beam is focused on the surface by optical lens and the angle of incidence was equal to 40°. To impose both laser treatments simultaneously, the frequency was the same as the ablation laser (60 Hz). The pulse duration and energy density were respectively fixed at 2 ms and 24.8 J cm−2. The energy density and pulse duration had been chosen because previous studies which were carried out on the spreading behaviour of particles show that circular splats could be obtained under these conditions [21]. The laser spot diameter was equal to 10 mm. The spot scanning speed combined with these parameters create areas treated by 4 pulses of the preheating laser. Fig. 2.a shows the superposition of the ablation laser and the heating laser impacts represented by a rectangle and a circle respectively. The temperatures reached by the laser treated surfaces were observed with a thermal camera (Flir system — Thermovision 320 M). This camera can work at a frequency of 900 Hz. Every 1.1 ms, a thermal image of the substrate is taken 1 Quantel, 17 av. de l'Atlantique - Z.A. Courtaboeuf - B.P. 23 91941 Les Ulis Cedex, France. 2 Cheval, 5 rue de la Louvière - Z.I. de la Louvière - 25 480 PIREY, France.
which can permit to observe and measure the surface temperature during the treatment of the laser heating. So the observed temperatures during laser heating were equal to 85 °C and 118 °C for the aluminium alloy and titanium alloy respectively. The ablation laser pulses strike the substrate which has been heated at these temperatures. Fig. 2.b shows the surface temperature measured by the thermal camera and illustrates the principle of synchronization. To compare the laser treatments with conventional grit blasting processes, the surfaces were degreased and sandblasted with 200– 300 μm SiO2 particles to reach a surface roughness equivalent to 3– 4 μm. 2.3. Plasma spray parameters An Atmospheric Plasma Spray (APS) system with a F4 gun manufactured by Sultzer-Metco was used to elaborate coatings. Nickel–aluminium powder was sprayed onto the substrate with the parameters detailed in Table 2. Coatings were elaborated by successive passes of the samples in front of the plasma torch and twenty-five passes were executed to reach 300 μm thick coatings. The coating thickness was chosen in order to fulfil the conditions required for the characterization tests. 2.4. Observation and characterization instrumentations Surface observations were carried out using Scanning Electron Microscopy (SEM). A roughness meter (Surtronic 3P designed by Taylor Hobson) was used to characterize the surface roughness (Ra). Roughness analyses were carried out on 10 profile measurements which have been taken in different directions to ensure the surface representativeness. The adhesion between the coating and the substrate was characterized by interfacial indentation [22]. Vickers indentations are carried out on cross-section sample and cracks are then initiated and propagated along the interface (Fig. 3). The lengths of the cracks (which are measured with optical microscope observations) are related to the interface toughness and an analytical model defines the toughness representing the coating adhesion. Choulier [23] and Lesage et al. [24] determine a (Pc, ac) couple defining the visible interfacial crack initiation; where P is the applied load and a is the crack length which corresponds to the addition of the half diagonal of the indent and the length of the crack generated from the end of the indentation. The authors propose to define the apparent interface toughness Kca [25] by the relation: Kca = 0:015
1=2 E 3=2 H ac I Pc
ð2Þ
where (E/H)I is defined by: 1=2 E = H I
1=2 1=2 E E H S H C + 1= 2 HS HR 1+ 1+ HC HC
ð3Þ
where E is the Young modulus, H is the hardness and the subscripts, S, C and I stand respectively for the substrate, the coating and the interface. This relation considers the hardness and Young's modulus of substrate and coating therefore the interface toughness Kca is not linked to the couple substrate/coating when comparing titanium and aluminium alloy substrate. It is necessary to know the Young's modulus of the coating but the use of common mechanical methods (such as tensile testing) is not possible. As a result an indentation technique is used to determine the Young's modulus of the coatings and the substrates (which are compared to the referenced Young's modulus in order to validate the
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-a-
2705
-b-
Fig. 2. (a) Schema of the laser spot superposition; (b) Thermal observation of a TA6V surface heated by 4 pulses with an energy density of 24.8 J cm−2.
substrate values). The method is based on the measurement of the elastic recovery of a Knoop indentation (Marshall et al. [26]). The ratio Hk/E (Hk is Knoop's hardness) is related to the lengths of the indentation by the following expression: brec b H = −α K a arec E
ð4Þ
where a and b are respectively the long and short diagonals of the Knoop indentation which are fixed and the ratio b/a is equal to 0.14. α is a numerical constant and it has been adjusted by the comparison between the Young's modulus obtained with the indentation method and those found in the literature. α was chosen equal to 0.45. arec and brec are respectively the long and short length of the Knoop indentation. Interface toughness was obtained with an average of 15 indentations for each sample. Crack lengths were determined with an optical microscope. All of the results are presented with a standard deviation σ. 3. Results and discussion Two different materials have been chosen for this study, each material has been exposed to similar surface laser treatments and then SEM observations and surface characterization have been carried out. A 300 μm thick Ni/Al coating has been realized on treated substrates and the interface toughness has been evaluated by interfacial indentation. 3.1. Surface observations Fig. 4 shows SEM observations of A2017 and TA6V surfaces treated by ablation laser and simultaneously by the preheating laser (Figs. 4-1
Table 2 Spraying conditions. Powder Primary gas flow rate [Nl min−1] Arc current [A] Powder feed rate [g min−1] Carrier gas flow rate [Nl min−1] Spray distance [mm] Injection angle [°] Injection distance from the plasma jet axis [mm] Sample scrolling [mm s−1] Vertical step [mm]
Ni–Al (95–5%) Ar H2
50 8 600 2.4 2 160 75 6 400 6
and 2-.c,d) in comparison with the reference and sandblasted surfaces (Figs. 4-1 and 2-a and b). The A2017 surface reference (Fig. 4-1.a) shows light precipitates, they correspond to transient compounds which constitute the reinforcement phase present in the alloy whereas TA6V reference surface shows no particularity. Inversely, the surface topographies observed after laser irradiation are very different according to the treated material. The morphologies are strongly different from the conventional sand-blasted aspect but differ from the aluminium to the titanium surface. Fig.4 -1 and 4.b show eroded faces for the A2017 and TA6V surfaces. The repeated and high velocity impact of SiO2 particles created surfaces that were considerably rougher than the polished surfaces (Fig.4 -1 and a). A2017 surfaces treated by the ablation and preheating laser simultaneously show nonhomogeneous morphology (Fig. 4-1.c and d) with a great number of craters. The higher the ablation laser energy density, the higher the number of craters. The preheating laser heats the surface which increases the surface absorptivity and when the ablation laser pulse irradiates the substrate more energy is absorbed by the surface. The increase of the ablation laser energy density leads to the formation of a great number of craters because the ablation laser energy is not concentrated only on few surface defects but also on less sensitive and deeper imperfections of the crystallographic structure [27]. Considering now the TA6V sample, a plane morphology (Fig. 4-2.c and d) can be observed on the surfaces treated by the ablation and preheating laser simultaneously. The surfaces treated by the highest energy density of the ablation laser show few craters but a smooth aspect can be detected. Craters created by the ablation laser action show no splash. The crater is surrounded by a ring of matter which has a molten appearance (zoom in Fig. 4-2.d) as the global surface which can be linked to the low thermal conductivity of the material compared to the A2017 and then limits the heat dissipation in the material volume. Unlike A2017 surfaces which were treated by the same processes, TA6V surfaces irradiated by lasers do not present a chaotic aspect. The craters formed on TA6V surfaces are different compared to those observed on A2017. No matter ejection and no splashing occurs after the photonic interaction. The physical characteristics and composition of the two alloys can explain these evolutions. As demonstrated previously, the composition variations or inclusions are responsible mainly for the surface heterogeneities and crater formation [27]. Because the composition of A2017 is more complex than that of TA6V (Tables 1 and 2), A2017 presents more potential inclusion areas. Moreover the energy required to melt TA6V is higher than that to melt A2017 but due to the thermal diffusivity of each material the evacuation of the heat is different. In the case of A2017, the thermal diffusivity is high enough (5.1.10−5 m2 s−1) to induce a rapid dissipation
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1.a
1.b
1.c
1.d
2.a
2.b
2.c
2.d
Fig. 3. SEM observations of A2017(1) or TA6V (2) surfaces (a) reference;( b) sandblasted;(c) preheated by laser with an energy density of 24.8 J cm−2 (85 °C for A2017 and 118 °C for TA6V) and then ablated by laser with an energy density of 1.7 J cm−2 or (d) with an energy density of 2.3 J cm−2.
of the thermal energy into the volume of matter. Considering now the TA6V, the thermal diffusivity is low (2.78.10−6 m2 s−1) so the thermal energy is concentrated on the surface melting the superficial layer. Therefore the nature and composition of the substrate have an important influence on the surface evolutions. It is interesting to compare the A2017 surface observed on Fig. 5 which was treated by the ablation laser only with an energy density of 1.7 J cm−2 and the surface observed on Figs. 4–1.c which was irradiated by ablation laser with the same energy density but at a higher temperature. The density of craters increases on the surface heated before the ablation. Indeed the temperature dependence of absorption of a metallic surface is described by the Drude free electron model or Hagen– Rubens relation: the absorptivity is proportional to the resistivity of the metal which increases when the temperature rises [30]. In metals, the energy absorption operates through successive collisions between electrons and photons. Through electrons and collisions (intraband transitions), the incident energy flux is transmitted to the lattice. The electron movement in the electrical field depends on the amplitude of this electrical field but as well on the density of free electrons. Electron collisions with phonon and free electron density increase with the temperature so the absorption is improved with the heating of the surface. Multiphoton absorption can also occur [31]. This phenomenon occurs if two photons (in the case of two-photon absorption) reach the atom and generate a transition. The atom treats the two
Fig. 4. SEM observation of A2017 surface ablated by laser with an energy density of 1.7 J cm−2.
photons as one photon with half wavelength. The absorption of the atom is then higher. With the temperature increase the energy levels of the atoms are widened and shifted, the probability of multiphoton absorption is thereby increased. The ablation laser removes oxides and surface pollutants. Indeed Christoulis et al. [32] have shown by TEM observations that oxide layer is wiped out during laser cleaning process. Laser treatment creates also surface modification by the creation of craters which are generated by the vaporization of surface or crystallographic defects. The increase of the surface temperature improves the absorption that enhances the action of the ablation laser. Then the heating of the surface improves the ablation laser effect and creates significant surface modifications which can be considered as surface texturing. 3.2. Coating adherence To compare and evaluate the surface pretreatment, Ni/Al coatings were realized on a surface treated by laser and a surface prepared by conventional process (degreasing and grit blasting). Fig. 6 shows in the cross section Ni–Al coatings elaborated on A2017 sandblasted (Fig. 6.a) and irradiated (Fig. 6.b). As conventionally observed, the sandblasted interface shows high roughness. The molten particles penetrate in the asperities which improves the adhesion and limits the crack propagation. However some defects can be observed, the coating is not totally in contact with the substrate. Grain inclusions and holes at the interface exist. Inversely the laser irradiated interface (Fig. 6.b) is smoother and presents no important modifications. The irradiated interface is fine and smooth with a good contact between the coating and the substrate. Then, interfacial indentations were carried out and interfacial toughness was calculated from the crack lengths generated at the interfaces (Fig. 7-1). The interfacial toughness is equal to 2 MPa m1/2 (σ = 0.1 MPa m1/2) for the coating made on sandblasted surface whereas it is equal to 1.61 MPa m1/2 (σ = 0.08 MPa m1/2) and 0.81 MPa m1/2 (σ = 0.07 MPa m1/2) for those realized on surface heated by laser with an energy density of 24.8 J cm-2 and then ablated by the ablation laser with an energy density of 2.3 J cm−2 and 1.7 J cm−2 respectively. For the lowest energy density of the ablation laser, the adhesion is half of the one observed on surfaces irradiated at 2.3 J cm−2. The difference of adhesion seems to be directly connected to
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Fig. 5. Cross section of Ni–Al coatings formed on A2017 surface which was sandblasted (a) and preheated by laser with an energy density of 24.8 J cm−2 and then ablated by laser with an energy density of 2.3 J cm−2(b).
the surface roughness (Fig. 7-2) as expected by the SEM observations. The roughness is the highest for sandblasted surface (Ra = 6.74 μm σ = 0.26 μm) and so is the adhesion. The roughness is the lowest for the surfaces which were preheated (by laser with an energy density of 24.8 J cm−2 ) and then ablated (with an energy density of 1.7 J cm−2 (Ra = 0.33 μm σ = 0.04 μm) and the adhesion follows this evolution. All these adhesion variations seem to be in relation with the roughness of the surfaces. The interfacial toughness obtained with laser treatments is quite similar to the one obtained with sandblasting if the difference of roughness is considered. The roughness of the sandblasted surfaces is twenty times higher than that of irradiated surface while the difference between the interfacial toughness is only equal to 0.39 MPa m1/2. The physical anchoring of the coating is therefore not the only origin of the adhesion phenomenon, chemical affinity seems to play also a role in interfacial bonding. It has been proved that the growth of an oxide layer could improve the surface wettability [28] and enhance the coating adhesion [29]. So Energy Dispersive X-ray spectroscopy has been realized on irradiated surfaces to detect oxygen which can reveal the growth of an oxide layer during laser treatments. Nevertheless no oxygen has been detected and further analysis with more precise instrumentations such as Secondary Ion Mass Spectrometry has to be carried out to detect chemical alteration initiated by the laser treatments. Considering now the TA6V substrates covered by Ni–Al coatings (Fig. 8): the interfacial toughness is equal to 3.4 MPa m 1/2 (σ = 0.19 MPa m1/2) for the coating made on sandblasted surface whereas it is equal to 2.03 MPa m1/2 (σ = 0.2 MPa m1/2) and 1.81 MPa m1/2 (σ = 0.27 MPa m1/2) for those realized on irradiated surfaces. The difference of adhesion between sandblasted and laser irradiated surfaces can be related to the surface roughness too (Fig. 8-2). The highest roughness was measured for sandblasted surfaces (Ra = 5.79 μm, σ = 0.11 μm). The lowest value was noted for the preheated surfaces (by laser with an energy density of 24.8 J cm−2 ) while intermediate values were obtained after laser ablation. In these conditions, the surface morphology is relatively similar to the reference sample which was polished (Ra = 0.13 μm σ = 0.03 μm for the laser treated against Ra = 0.12 μm σ = 0.03 μm for the polished sample). Then, from a general point of view, the interfacial toughness is higher on TA6V sandblasted surfaces than on A1027 (respectively 3.2 and 2 MPa m1/2) while the roughness is similar. On TA6V substrate which was irradiated by laser, the adhesion decreases more roughly in comparison with the toughness difference observed for A2107 substrates. For the same laser treatment, the roughness of TA6V irradiated surfaces is less important than for the A2107 surfaces. The absence of craters on TA6V substrates could explain this difference. For the irradiated surfaces, it is mainly the chemical interactions which are responsible for adhesion phenomenon. As demonstrated with TA6V substrate, differences can be measured after laser treatment whereas no roughness evolution was detected. More significantly than for A2017, the physical anchoring of the coating is not the
only element responsible for the adhesion phenomenon, chemical anchoring plays also a role in interfacial bonding between substrates and coatings. A natural oxide layer exists on polished TA6V surface. Sittig et al. [33] found that the thickness of this oxide layer is equal to 4 or 6 nm. Moreover it has been also observed that this oxide layer grows when the TA6V surface is irradiated by ablation laser with an energy density superior to 2 J cm− 2 [34]. SIMS analyses revealed that the oxide layer can reach a thickness of 10 nm and as established by V. Barnier [35] such oxide layer can grow on aluminium surface treated by nanosecond laser too. Then the improvement of the coating adhesion observed on laser treated aluminium surfaces can be explained by the presence of a thin oxide layer [29]. The enhancement of surface wettability induced by the development of an oxide layer [36] could explain a better adhesion observed on irradiated surfaces.
4. Conclusion An analysis of interfacial toughness related to surface modifications on polished substrates (aluminium 2017 and titanium Ti–6Al– 4 V alloy) induced by laser irradiations was carried out. Topographic modifications were investigated by SEM observations and roughness measurements. The surface modifications induced by laser treatments were linked to the variations of interfacial toughness. The adhesion was evaluated thanks to interfacial indentation and allows the comparison between conventional surface treatment (sandblasting) and laser preparation. Even if the adhesion level is often higher for sandblasted substrates, the influence of the surface roughness was proposed to explain the difference in interfacial bonding. However this study highlights the chemical interactions which play also a significant role in the adhesion. A mixture between mechanical anchorage and chemical affinity seems mostly favourable for the interfacial behaviour. The chemical modifications induced by laser treatments is a complex phenomenon and has to be more investigated to determine the part of physical and chemical anchoring which occurs in the adhesion mechanism. The influence of the surface pretreatment on the residual stresses induced inside the materials has to be investigated too.
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[10] C. Coddet, G. Montavon, S. Costil, O. Freneaux, F. Rigolet, G. Barbezar, F. Folio, A. Diard, P. Wasen, J. Therm. Spray Technol. 8 (1999) 235. [11] H. Li, S. Costil, H. Liao, C.-J. Li, M. Planche, C. Coddet, Surf. Coat. Technol. 200 (2006) 5435. [12] W.M. Healy, J.G. Hartley, S.I. Abdel-Khalik, Int. J. Heat Mass Transfer 44 (2001) 235. [13] Y. Tanaka, M. Fukumoto, Surf. Coat. Technol. 120–121 (1999) 124. [14] A. Abedini, A. Pourmousa, S. Chandra, J. Mostaghimi, Surf. Coat. Technol. 201 (2006) 3350. [15] M. Fukumoto, S. Katoh, I. Okane, Proceedings of the 14th International Thermal Spray Conference, in: A. Ohmori (Ed.), High Temp. Soc. of Japan, Osaka, Japan, Vol. 1, 1995, p. 353. [16] V. Pershin, M. Lufitha, S. Chandra, J. Mostaghimi, J. Therm. Spray Technol. 12 (2003) 370. [17] J. Cedelle, M. Vardelle, P. Fauchais, Surf. Coat. Technol. 201 (2006) 1373. [18] S. Costil, H. Liao, O. Chrétien, A. Loredo, A. Gammoudi, M. Verdier, C. Coddet, Lasers in Eng. 15 (2005) 1. [19] Y. Danlos, S. Costil, H. Liao, C. Coddet, Surf. Coat. Technolo. 202 (2008) 4531. [20] Alloy finder, 3rd editionASM International, Metals Park, Ohio 44073, U.S.A, 2001. [21] Y. Danlos, S. Costil, S. Deng, H. Liao, Proceedings of International Thermal Spray Conference, ASM International, Las Vegas, USA, 2009, 1056. [22] G. Marot, Ph. Démarécaux, J. Lesage, M. Hadad, St. Siegmann, M.H. Staia, Surf. Coat. Technol. 202 (2008) 4411. [23] D.Choulier, Contribution à l'étude de l'adhérence de revêtements projetés à la torche à plasma: Modélisation et utilisation d'un test d'indentation à l'interface, Thèse de doctorat, Université de Technologie de Compiègne, 1989. [24] P. Démarécaux, D. Chicot, J. Lesage, J. Mater. Sci. Lett. 15 (1996) 1377.
[25] D. Chicot, P. Démarécaux, J. Lesage, Thin Solid Films 283 (1996) 151. [26] D.B. Marshall, T. Noma, A.G. Evans, Commun. Am. Chem. Soc. C-175 (1982). [27] H. Li, S. Costil, V. Barnier, R. Oltra, O. Heintz, C. Coddet, Surf. Coat. Technol. 201 (2006) 1383. [28] A. Feng, B.J. McCoy, Z.A. Munir, D. Cagliostro, Mater. Sci. Eng. A. 242 (1998) 50. [29] V. Barnier, 2006, Etude de l'oxydation de l'aluminium induite par traitement laser Nd:YAG (1054 nm, 10 ns): conséquences sur les propriétés d'adhérence d'un revêtement d'alumine élaboré par projection thermique, PhD thesis, Université de Bourgogne, France (in French). [30] A.M. Deus, R. Villar, Laser Processing : Surface Treatment and Film Deposition, in: J. Mazumder, et al., (Eds.), Kluwer Academic Publishers, Netherlands, 1996, p. 195. [31] K. Blidegn, F.O. Olsen, XI International Symposium on Gas Flow and Chemical Lasers and High-Power Laser Conference, Vol.3092, ed. SPIE, Edinburgh, United Kingdom, (2005), 615. [32] D.K. Christoulis, S. Guetta, V. Guipont, M.H. Berger, M. Jeandin, M. Boustie, S. Costil, Y. Ichikawa, K. Ogawa, E. Irrissou, J-G Legoux, C. Moreau., submitted for publication to JTST, (2009). [33] C. Sittig, M. Textor, N.D. Spencer, M. Wieland, P.H. Vallotton, J. Mat, Sci.: Mat Med. 10 (1999) 35. [34] H. Li, (2006), Rôle d'une ablation laser in situ sur les propriétés mécaniques d'un revêtement élaboré par projection thermique, PhD thesis, Université de Technologie de Belfort Montbéliard, France (in French). [35] V. Barnier, O. Heintz, D.E. Roberts, R. Oltra, S. Costil, 11TH European Conference of Applications of Surface and Interface Analysis, ed. John Wiley & sons ltd, Vienna Austria (2005), 406. [36] A. Feng, B.J. McCoy, Z.A. Munir, Z. Cagliostro, Mat. Sci. Eng. A242 (1998) 50.
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
Influence of Ti–6Al–4 V and Al 2017 substrate morphology on Ni–Al coating adhesion — Impacts of laser treatments Y. Danlos ⁎, S. Costil, H. Liao, C. Coddet LERMPS, Université de Technologie de Belfort-Montbéliard, 90010 Belfort Cedex, France
a r t i c l e
i n f o
Article history: Received 26 May 2010 Accepted in revised form 31 August 2010 Available online 8 September 2010 Keywords: Thermal spraying Laser treatment Interfacial indentation Toughness
a b s t r a c t Surface morphology, chemical composition and surface state are important parameters for the formation of thermally sprayed coatings. More precisely the temperature, wettability, geometry and degree of the surface contamination play an important role in the coating particularly from the adhesion point of view. Several surface preparations (degreasing, high pressure water jet, grit blasting…) before thermal spraying are often used in order to obtain deposits with a good interfacial adherence. In this paper we report on an original laser surface treatment which uses two Nd-YAG laser beams in conjunction with a thermal spray gun in order to modify the substrate surface prior to thermal spraying in order to enhance the adhesion at the coating substrate interface. A first Nd-YAG laser is used to heat the surface and a second to clean it. The system is used to coat two different materials (titanium Ti–6Al–4 V and aluminium 2017 alloy) with a Ni–Al alloy. The relation between the surface modifications induced by these treatments and the adhesion of the coatings on the substrates are studied. © 2010 Published by Elsevier B.V.
1. Introduction Thermal spraying is a well established means to form coatings for surface engineering. A heat source is used to melt and accelerate metallic or non-metallic powders and spray them onto a substrate to elaborate a coating. It seems to be a simple process but there are numerous interdependencies between all parameters. A great number of interactive parameters have to be controlled in order to obtain good coatings and promote substrate adhesion. In fact, the coating adhesion on the substrate, or on the previously deposited layer, is fundamental to promote a good cohesion of the materials and a successful application. To create good conditions to anchore coatings on substrate, appropriate surface preparations are required [1]. Previous studies have shown that in order to obtain good adhesion between the substrate and the coating it is necessary to spray particles on surfaces which have specific chemical, physical and mechanical characteristics [2]. The surfaces have to be cleaned to eliminate the adsorbed molecules which can be evaporated by the contact with the molten sprayed particles and, as a result, generate splashing [3]. The substrate wettability has also to be as high as possible to improve the splat spreading [4]. A clean surface is an important condition for the contact quality which depends on the droplet wetting and desorption of the pollutants adsorbed on the surface or on the underlying layer [5]. In most cases to remove grease, oil or dust from the
⁎ Corresponding author. Tel.: + 33 3 84 58 31 64. E-mail address: [email protected] (Y. Danlos). 0257-8972/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2010.08.147
surface, chemical degreasing is conventionally used. These processes improve the chemical behaviour of the surface to have a good spreading of the particles. But to create the appropriate conditions to anchore coatings on substrate, further surface preparations are required. In general grit blasting is used to obtain efficient surface cleaning and texturing to enhance mechanical adhesion of the coating to the substrate. The creation of asperities on the surface by grit blasting increases the surface roughness and thus the particle anchoring on the surface. However these processes present disadvantages: the elimination of the chemical wastes from the cleaning surface requires precautions for environmental considerations (operators, recycling) and therefore due to grit blasting fatigue property decreases: microcracks on substrate surface or grit inclusions which are harmful for the adhesion [6]. Furthermore the chemical and physical modifications required in order to obtain adequate adhesion are generally induced by different processes constituting successive steps and that are not accomplished during thermal spraying. Within this objective, other cleaning processes have also been developed to eliminate those defects. For example, low pressure plasma processes can be used for surface cleaning. It is possible to remove contaminants by simple plasma sputtering particle bombardment in plasmas of noble gases under low pressure environment. Degreasing oil-contaminated metal sheet and removing weakly bonded oxides were performed by plasma cleaning [7]. Highpressure water jet can be used too in surface preparation before thermal spraying but this jet operation imposes high tensile bond strength [8]. The surface morphology resulting from this process was compared to conventional grit blasting but contrary to this
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conventional process, no grit inclusions occur. Some others, based on jet processes like grit blasting and high-pressure water jet create surface modifications but often cause damage and generate compressive strength on the substrate. Then, new developments like, dry-ice blasting process can remove adhering layers without surface modification. This mechanism is based on induced kinetic and sublimation energy. It could completely remove oil and oxide layer from the surface. The contaminants which have been removed are the only residues of the jet operation [9]. But all these processes cannot be applied simultaneously with thermal spraying. Moreover the treated surfaces could not be precisely controlled by all the technological processes. Laser processes can go beyond this problem as the laser treated area is defined precisely by the laser beam. In order to improve the surface cleanliness, a method based on laser ablation process has been developed. Known as PROTAL® process (French acronym for “PROjection Thermique Assistée par Laser”), this treatment [10] combines two effects of surface treatments associating the photonic ablation and thermal spraying [11]. To ablate surface contaminants and improve the substrate wettability a nanosecond pulsed laser is used [12]. Then, decontamination and thermal spray processes allow to treat the surfaces in only one step contrary to the conventional processes for which usual steps are decoupled. To decrease the particle splash and enhance the adhesion between impacting particles and substrate, the decontamination of the surface is necessary [3]. Nevertheless, considering the material affinity between coating and substrate, it has been established that the substrate temperature is also an important parameter [13]. Increasing the surface temperature changes the splat morphology: on substrate at room temperature, the splats present a splash shape, whereas on heated surfaces they have morphologies similar to the disk which results in a better adhesion of the coating [14]. A transition temperature which depends on both substrate/powder materials has been identified [15]. The splat morphology modification occurs beyond this transition temperature. Several explanations were proposed. Heating the surface clears volatile contaminants adsorbed on the surface, improving the contact between particles and substrate. It was suggested that nanoscale thick oxide layer improved the wettability and contact between the splat and the substrate [16]. The formation of nanoscale peaks [17] was observed on substrate which can be explained by increasing thermal contact between impinging particles and substrate. Thus, this study presents an original solution to prepare surfaces before thermal spraying. This process has been developed for surface cleaning in order to associate chemical and physical surface preparation. It consists in heating the surface by a laser treatment and then initiating surface contaminant desorption by using an ablation laser. The combination of the two techniques: laser preheating and laser cleaning has been already used and encouraging results were obtained on titanium Ti–6Al–4 V alloy [18] and on aluminium 2017 [19]. The aim of this study is to observe the surface modifications induced by laser treatments and to compare of surface adherence between laser surface preparation and conventional preparation (grit blasting). 2. Experimental procedures Cleaning and heating laser treatments are combined before thermal spraying in order to create suitable conditions to anchor coatings. Laser spots are juxtaposed to the particle jet on the substrate in order to limit the recontamination. Coatings are elaborated by the repeated pass of the substrate in front of the plasma torch.
Substrate Plasma
Heating Ablation laser Fig. 1. Schema of the experimental device for laser treatments and thermal spraying.
create the appropriate conditions before thermal spraying in order to promote a maximal adhesion at the coating substrate interface. The surface is heated by a first millisecond laser and then a nanosecond ablation laser is used to clean the surface. A previous study [18] proved that laser cleaning enhances the circular shaped splat but it has also been demonstrated that heating substrates improves the splat spreading. So, associating the two processes would be interesting and would lead to a better adhesion and cohesion of plasma-sprayed coatings. The ablation and preheating lasers were synchronized and their spots were superposed. The delay between the heating and ablation was chosen in order to benefit from the maximum surface temperature while the ablation laser pulse strikes the substrate. Using a laser for the preheating certifies the control of the heating zone and avoids surface distortion or soot deposition which occurs with conventional heating processes (flame, arc wire). 2.2. Material characteristics 2.2.1. Substrates characteristics In this study aluminium alloy 2017 (A2017) and titanium alloy Ti– 6Al–4 V (TA6V) were chosen as substrates and their characteristics are presented in Table 1. Titanium alloy and aluminium alloy substrates are widely used in thermal spraying. TA6V which combines corrosion resistance, biocompatibility and mechanical strength, is used in many industrial fields like aerospace engineering or biomedical applications. Aluminium alloys are as well used in a wide range of applications from electric conductors to automotive field generally in order to lighten mechanical components. However the fatigue behaviour, wear resistance and surface hardness are worse than standard steels which require protective coatings. The investigated materials were chosen because these two materials have different thermal characteristics which lead to different answers faced to laser radiation and thermal phenomena initiated by the impact of the
Table 1 Thermophysical characteristics of materials.
Density(ρ)[kg m−3] Melting temperature (Tfus)[K] Thermal conductivity (k) [W m−1 K−1] Mass heat capacity (Cp)[J kg−1 K−1] Absorptivity at 1064 nm (25 °C) [%]
A2017
TA6V
Ref.
2796 783 140 982 5
4470 1878 7 563 45
[20]
Percentage composition by mass [%] A2017
2.1. Experimental configurations The configuration used in this study (Fig. 1) consists in the superposition and synchronization of two lasers which clean and
2703
TA6V
Al
Cr
94
0,1
Cu
Mg
Mn
Fe
Ti
Zn
4
0,6
0,7
0,7
0,15
0,25
Al
Ti
V
6
90
4
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melting particles. Aluminium 2017 alloy has a mass heat capacity and a thermal conductivity higher than titanium alloy Ti–6Al–4 V so A2017 evacuates the thermal energy with more efficiency than TA6V. However even if TA6V has a better absorptivity than A2017 (respectively 45 and 5%) the energy (Qmelt) required to melt TA6V is higher than the one to melt A2017. Considering the physical characteristics and applying the following expression, it is possible to evaluate this difference by the relationship: Qmelt = ρCpðT melt −Tatm Þ
ð1Þ
where ρ is the density, Cp the mass heat capacity, Tmelt the melting temperature and Tatm the atmospheric temperature. The simple calculation shows that the energy required to melt A2017 (Qmelt = 1528 J cm−3) is less than half of the energy required to melt TA6V (Qmelt = 4047 J cm−3). Aluminium alloy is therefore more sensitive than TA6V to temperature increase. The substrates were mechanically polished to a mirror surface in order to have the same surface morphologies as these observed with the scanning electron microscope before laser treatments. The procedure consisted in applying successive grades of silicon carbide papers and a final polishing cloth with a 0.05 μm SiC particle suspension. 2.2.2. Powder characteristics The powder sprayed on the substrates was common Ni–Al powder (95–5%, AMDRY 956, Sulzer-Metco) and the particle size range was from 45 μm to 90 μm. 2.2.3. Surface treatment processes For the local surface cleaning, a Quantel1 Q-switched Nd-YAG laser (wavelength 1064 nm, pulse duration 10 ns) was used. The laser spot has a rectangular shape with a homogeneous intensity distribution. The beam profile is generated by crystals which induce phase delay of the wavefront. The angle of incidence of the ablation beam relative to the substrate was equal to 30°. In this study, the dimensions of the laser spot (7 ⁎ 4 mm2) and the power parameters have been chosen in order to irradiate the surfaces with an energy density of 1.7 and 2.3 J cm−2. The energy densities of the ablation laser were chosen for an efficient surface cleaning and to observe the responses of the surfaces irradiated with different energy densities. The pulse repetition rate (60 Hz) and the scanning speed of the spot were also chosen to keep the kinetic thermal spray parameters and to create areas treated by a single shot ablation laser. The heating treatment was accomplished by a Nd-YAG laser designed by the Cheval company.2 It is a pulsed millisecond laser with a wavelength of 1064 nm. The laser spot was circular with a Gaussian energy distribution. The beam is focused on the surface by optical lens and the angle of incidence was equal to 40°. To impose both laser treatments simultaneously, the frequency was the same as the ablation laser (60 Hz). The pulse duration and energy density were respectively fixed at 2 ms and 24.8 J cm−2. The energy density and pulse duration had been chosen because previous studies which were carried out on the spreading behaviour of particles show that circular splats could be obtained under these conditions [21]. The laser spot diameter was equal to 10 mm. The spot scanning speed combined with these parameters create areas treated by 4 pulses of the preheating laser. Fig. 2.a shows the superposition of the ablation laser and the heating laser impacts represented by a rectangle and a circle respectively. The temperatures reached by the laser treated surfaces were observed with a thermal camera (Flir system — Thermovision 320 M). This camera can work at a frequency of 900 Hz. Every 1.1 ms, a thermal image of the substrate is taken 1 Quantel, 17 av. de l'Atlantique - Z.A. Courtaboeuf - B.P. 23 91941 Les Ulis Cedex, France. 2 Cheval, 5 rue de la Louvière - Z.I. de la Louvière - 25 480 PIREY, France.
which can permit to observe and measure the surface temperature during the treatment of the laser heating. So the observed temperatures during laser heating were equal to 85 °C and 118 °C for the aluminium alloy and titanium alloy respectively. The ablation laser pulses strike the substrate which has been heated at these temperatures. Fig. 2.b shows the surface temperature measured by the thermal camera and illustrates the principle of synchronization. To compare the laser treatments with conventional grit blasting processes, the surfaces were degreased and sandblasted with 200– 300 μm SiO2 particles to reach a surface roughness equivalent to 3– 4 μm. 2.3. Plasma spray parameters An Atmospheric Plasma Spray (APS) system with a F4 gun manufactured by Sultzer-Metco was used to elaborate coatings. Nickel–aluminium powder was sprayed onto the substrate with the parameters detailed in Table 2. Coatings were elaborated by successive passes of the samples in front of the plasma torch and twenty-five passes were executed to reach 300 μm thick coatings. The coating thickness was chosen in order to fulfil the conditions required for the characterization tests. 2.4. Observation and characterization instrumentations Surface observations were carried out using Scanning Electron Microscopy (SEM). A roughness meter (Surtronic 3P designed by Taylor Hobson) was used to characterize the surface roughness (Ra). Roughness analyses were carried out on 10 profile measurements which have been taken in different directions to ensure the surface representativeness. The adhesion between the coating and the substrate was characterized by interfacial indentation [22]. Vickers indentations are carried out on cross-section sample and cracks are then initiated and propagated along the interface (Fig. 3). The lengths of the cracks (which are measured with optical microscope observations) are related to the interface toughness and an analytical model defines the toughness representing the coating adhesion. Choulier [23] and Lesage et al. [24] determine a (Pc, ac) couple defining the visible interfacial crack initiation; where P is the applied load and a is the crack length which corresponds to the addition of the half diagonal of the indent and the length of the crack generated from the end of the indentation. The authors propose to define the apparent interface toughness Kca [25] by the relation: Kca = 0:015
1=2 E 3=2 H ac I Pc
ð2Þ
where (E/H)I is defined by: 1=2 E = H I
1=2 1=2 E E H S H C + 1= 2 HS HR 1+ 1+ HC HC
ð3Þ
where E is the Young modulus, H is the hardness and the subscripts, S, C and I stand respectively for the substrate, the coating and the interface. This relation considers the hardness and Young's modulus of substrate and coating therefore the interface toughness Kca is not linked to the couple substrate/coating when comparing titanium and aluminium alloy substrate. It is necessary to know the Young's modulus of the coating but the use of common mechanical methods (such as tensile testing) is not possible. As a result an indentation technique is used to determine the Young's modulus of the coatings and the substrates (which are compared to the referenced Young's modulus in order to validate the
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-a-
2705
-b-
Fig. 2. (a) Schema of the laser spot superposition; (b) Thermal observation of a TA6V surface heated by 4 pulses with an energy density of 24.8 J cm−2.
substrate values). The method is based on the measurement of the elastic recovery of a Knoop indentation (Marshall et al. [26]). The ratio Hk/E (Hk is Knoop's hardness) is related to the lengths of the indentation by the following expression: brec b H = −α K a arec E
ð4Þ
where a and b are respectively the long and short diagonals of the Knoop indentation which are fixed and the ratio b/a is equal to 0.14. α is a numerical constant and it has been adjusted by the comparison between the Young's modulus obtained with the indentation method and those found in the literature. α was chosen equal to 0.45. arec and brec are respectively the long and short length of the Knoop indentation. Interface toughness was obtained with an average of 15 indentations for each sample. Crack lengths were determined with an optical microscope. All of the results are presented with a standard deviation σ. 3. Results and discussion Two different materials have been chosen for this study, each material has been exposed to similar surface laser treatments and then SEM observations and surface characterization have been carried out. A 300 μm thick Ni/Al coating has been realized on treated substrates and the interface toughness has been evaluated by interfacial indentation. 3.1. Surface observations Fig. 4 shows SEM observations of A2017 and TA6V surfaces treated by ablation laser and simultaneously by the preheating laser (Figs. 4-1
Table 2 Spraying conditions. Powder Primary gas flow rate [Nl min−1] Arc current [A] Powder feed rate [g min−1] Carrier gas flow rate [Nl min−1] Spray distance [mm] Injection angle [°] Injection distance from the plasma jet axis [mm] Sample scrolling [mm s−1] Vertical step [mm]
Ni–Al (95–5%) Ar H2
50 8 600 2.4 2 160 75 6 400 6
and 2-.c,d) in comparison with the reference and sandblasted surfaces (Figs. 4-1 and 2-a and b). The A2017 surface reference (Fig. 4-1.a) shows light precipitates, they correspond to transient compounds which constitute the reinforcement phase present in the alloy whereas TA6V reference surface shows no particularity. Inversely, the surface topographies observed after laser irradiation are very different according to the treated material. The morphologies are strongly different from the conventional sand-blasted aspect but differ from the aluminium to the titanium surface. Fig.4 -1 and 4.b show eroded faces for the A2017 and TA6V surfaces. The repeated and high velocity impact of SiO2 particles created surfaces that were considerably rougher than the polished surfaces (Fig.4 -1 and a). A2017 surfaces treated by the ablation and preheating laser simultaneously show nonhomogeneous morphology (Fig. 4-1.c and d) with a great number of craters. The higher the ablation laser energy density, the higher the number of craters. The preheating laser heats the surface which increases the surface absorptivity and when the ablation laser pulse irradiates the substrate more energy is absorbed by the surface. The increase of the ablation laser energy density leads to the formation of a great number of craters because the ablation laser energy is not concentrated only on few surface defects but also on less sensitive and deeper imperfections of the crystallographic structure [27]. Considering now the TA6V sample, a plane morphology (Fig. 4-2.c and d) can be observed on the surfaces treated by the ablation and preheating laser simultaneously. The surfaces treated by the highest energy density of the ablation laser show few craters but a smooth aspect can be detected. Craters created by the ablation laser action show no splash. The crater is surrounded by a ring of matter which has a molten appearance (zoom in Fig. 4-2.d) as the global surface which can be linked to the low thermal conductivity of the material compared to the A2017 and then limits the heat dissipation in the material volume. Unlike A2017 surfaces which were treated by the same processes, TA6V surfaces irradiated by lasers do not present a chaotic aspect. The craters formed on TA6V surfaces are different compared to those observed on A2017. No matter ejection and no splashing occurs after the photonic interaction. The physical characteristics and composition of the two alloys can explain these evolutions. As demonstrated previously, the composition variations or inclusions are responsible mainly for the surface heterogeneities and crater formation [27]. Because the composition of A2017 is more complex than that of TA6V (Tables 1 and 2), A2017 presents more potential inclusion areas. Moreover the energy required to melt TA6V is higher than that to melt A2017 but due to the thermal diffusivity of each material the evacuation of the heat is different. In the case of A2017, the thermal diffusivity is high enough (5.1.10−5 m2 s−1) to induce a rapid dissipation
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1.a
1.b
1.c
1.d
2.a
2.b
2.c
2.d
Fig. 3. SEM observations of A2017(1) or TA6V (2) surfaces (a) reference;( b) sandblasted;(c) preheated by laser with an energy density of 24.8 J cm−2 (85 °C for A2017 and 118 °C for TA6V) and then ablated by laser with an energy density of 1.7 J cm−2 or (d) with an energy density of 2.3 J cm−2.
of the thermal energy into the volume of matter. Considering now the TA6V, the thermal diffusivity is low (2.78.10−6 m2 s−1) so the thermal energy is concentrated on the surface melting the superficial layer. Therefore the nature and composition of the substrate have an important influence on the surface evolutions. It is interesting to compare the A2017 surface observed on Fig. 5 which was treated by the ablation laser only with an energy density of 1.7 J cm−2 and the surface observed on Figs. 4–1.c which was irradiated by ablation laser with the same energy density but at a higher temperature. The density of craters increases on the surface heated before the ablation. Indeed the temperature dependence of absorption of a metallic surface is described by the Drude free electron model or Hagen– Rubens relation: the absorptivity is proportional to the resistivity of the metal which increases when the temperature rises [30]. In metals, the energy absorption operates through successive collisions between electrons and photons. Through electrons and collisions (intraband transitions), the incident energy flux is transmitted to the lattice. The electron movement in the electrical field depends on the amplitude of this electrical field but as well on the density of free electrons. Electron collisions with phonon and free electron density increase with the temperature so the absorption is improved with the heating of the surface. Multiphoton absorption can also occur [31]. This phenomenon occurs if two photons (in the case of two-photon absorption) reach the atom and generate a transition. The atom treats the two
Fig. 4. SEM observation of A2017 surface ablated by laser with an energy density of 1.7 J cm−2.
photons as one photon with half wavelength. The absorption of the atom is then higher. With the temperature increase the energy levels of the atoms are widened and shifted, the probability of multiphoton absorption is thereby increased. The ablation laser removes oxides and surface pollutants. Indeed Christoulis et al. [32] have shown by TEM observations that oxide layer is wiped out during laser cleaning process. Laser treatment creates also surface modification by the creation of craters which are generated by the vaporization of surface or crystallographic defects. The increase of the surface temperature improves the absorption that enhances the action of the ablation laser. Then the heating of the surface improves the ablation laser effect and creates significant surface modifications which can be considered as surface texturing. 3.2. Coating adherence To compare and evaluate the surface pretreatment, Ni/Al coatings were realized on a surface treated by laser and a surface prepared by conventional process (degreasing and grit blasting). Fig. 6 shows in the cross section Ni–Al coatings elaborated on A2017 sandblasted (Fig. 6.a) and irradiated (Fig. 6.b). As conventionally observed, the sandblasted interface shows high roughness. The molten particles penetrate in the asperities which improves the adhesion and limits the crack propagation. However some defects can be observed, the coating is not totally in contact with the substrate. Grain inclusions and holes at the interface exist. Inversely the laser irradiated interface (Fig. 6.b) is smoother and presents no important modifications. The irradiated interface is fine and smooth with a good contact between the coating and the substrate. Then, interfacial indentations were carried out and interfacial toughness was calculated from the crack lengths generated at the interfaces (Fig. 7-1). The interfacial toughness is equal to 2 MPa m1/2 (σ = 0.1 MPa m1/2) for the coating made on sandblasted surface whereas it is equal to 1.61 MPa m1/2 (σ = 0.08 MPa m1/2) and 0.81 MPa m1/2 (σ = 0.07 MPa m1/2) for those realized on surface heated by laser with an energy density of 24.8 J cm-2 and then ablated by the ablation laser with an energy density of 2.3 J cm−2 and 1.7 J cm−2 respectively. For the lowest energy density of the ablation laser, the adhesion is half of the one observed on surfaces irradiated at 2.3 J cm−2. The difference of adhesion seems to be directly connected to
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Fig. 5. Cross section of Ni–Al coatings formed on A2017 surface which was sandblasted (a) and preheated by laser with an energy density of 24.8 J cm−2 and then ablated by laser with an energy density of 2.3 J cm−2(b).
the surface roughness (Fig. 7-2) as expected by the SEM observations. The roughness is the highest for sandblasted surface (Ra = 6.74 μm σ = 0.26 μm) and so is the adhesion. The roughness is the lowest for the surfaces which were preheated (by laser with an energy density of 24.8 J cm−2 ) and then ablated (with an energy density of 1.7 J cm−2 (Ra = 0.33 μm σ = 0.04 μm) and the adhesion follows this evolution. All these adhesion variations seem to be in relation with the roughness of the surfaces. The interfacial toughness obtained with laser treatments is quite similar to the one obtained with sandblasting if the difference of roughness is considered. The roughness of the sandblasted surfaces is twenty times higher than that of irradiated surface while the difference between the interfacial toughness is only equal to 0.39 MPa m1/2. The physical anchoring of the coating is therefore not the only origin of the adhesion phenomenon, chemical affinity seems to play also a role in interfacial bonding. It has been proved that the growth of an oxide layer could improve the surface wettability [28] and enhance the coating adhesion [29]. So Energy Dispersive X-ray spectroscopy has been realized on irradiated surfaces to detect oxygen which can reveal the growth of an oxide layer during laser treatments. Nevertheless no oxygen has been detected and further analysis with more precise instrumentations such as Secondary Ion Mass Spectrometry has to be carried out to detect chemical alteration initiated by the laser treatments. Considering now the TA6V substrates covered by Ni–Al coatings (Fig. 8): the interfacial toughness is equal to 3.4 MPa m 1/2 (σ = 0.19 MPa m1/2) for the coating made on sandblasted surface whereas it is equal to 2.03 MPa m1/2 (σ = 0.2 MPa m1/2) and 1.81 MPa m1/2 (σ = 0.27 MPa m1/2) for those realized on irradiated surfaces. The difference of adhesion between sandblasted and laser irradiated surfaces can be related to the surface roughness too (Fig. 8-2). The highest roughness was measured for sandblasted surfaces (Ra = 5.79 μm, σ = 0.11 μm). The lowest value was noted for the preheated surfaces (by laser with an energy density of 24.8 J cm−2 ) while intermediate values were obtained after laser ablation. In these conditions, the surface morphology is relatively similar to the reference sample which was polished (Ra = 0.13 μm σ = 0.03 μm for the laser treated against Ra = 0.12 μm σ = 0.03 μm for the polished sample). Then, from a general point of view, the interfacial toughness is higher on TA6V sandblasted surfaces than on A1027 (respectively 3.2 and 2 MPa m1/2) while the roughness is similar. On TA6V substrate which was irradiated by laser, the adhesion decreases more roughly in comparison with the toughness difference observed for A2107 substrates. For the same laser treatment, the roughness of TA6V irradiated surfaces is less important than for the A2107 surfaces. The absence of craters on TA6V substrates could explain this difference. For the irradiated surfaces, it is mainly the chemical interactions which are responsible for adhesion phenomenon. As demonstrated with TA6V substrate, differences can be measured after laser treatment whereas no roughness evolution was detected. More significantly than for A2017, the physical anchoring of the coating is not the
only element responsible for the adhesion phenomenon, chemical anchoring plays also a role in interfacial bonding between substrates and coatings. A natural oxide layer exists on polished TA6V surface. Sittig et al. [33] found that the thickness of this oxide layer is equal to 4 or 6 nm. Moreover it has been also observed that this oxide layer grows when the TA6V surface is irradiated by ablation laser with an energy density superior to 2 J cm− 2 [34]. SIMS analyses revealed that the oxide layer can reach a thickness of 10 nm and as established by V. Barnier [35] such oxide layer can grow on aluminium surface treated by nanosecond laser too. Then the improvement of the coating adhesion observed on laser treated aluminium surfaces can be explained by the presence of a thin oxide layer [29]. The enhancement of surface wettability induced by the development of an oxide layer [36] could explain a better adhesion observed on irradiated surfaces.
4. Conclusion An analysis of interfacial toughness related to surface modifications on polished substrates (aluminium 2017 and titanium Ti–6Al– 4 V alloy) induced by laser irradiations was carried out. Topographic modifications were investigated by SEM observations and roughness measurements. The surface modifications induced by laser treatments were linked to the variations of interfacial toughness. The adhesion was evaluated thanks to interfacial indentation and allows the comparison between conventional surface treatment (sandblasting) and laser preparation. Even if the adhesion level is often higher for sandblasted substrates, the influence of the surface roughness was proposed to explain the difference in interfacial bonding. However this study highlights the chemical interactions which play also a significant role in the adhesion. A mixture between mechanical anchorage and chemical affinity seems mostly favourable for the interfacial behaviour. The chemical modifications induced by laser treatments is a complex phenomenon and has to be more investigated to determine the part of physical and chemical anchoring which occurs in the adhesion mechanism. The influence of the surface pretreatment on the residual stresses induced inside the materials has to be investigated too.
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