Mécanismes de formation de dépôts. Lech Pawlowski Professeur à ENS Chimie Lille Tél et fax : Mél.:lech.pawlowski@ensc-lille.

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1 Mécanismes de formation de dépôts Lech Pawlowski Professeur à ENS Chimie Lille Tél et fax : Mél.:lech.pawlowski@ensc-lille.fr

2 Plan du cours Introduction Impact d une particule Théorie de la nucléation d un grain projeté et les cas particuliers Composition de phases et la formation de phases métastables Inhomogénéités dans un dépôt Croissance du dépôt Température du dépôt et contraintes résiduelles Le cours suit deux ouvrages : 1. Lech Pawlowski, Dépôts Physiques, PPUR, Lausanne, Suisse, Lech Pawlowski, The Science and Engineering of Thermal Spray Coatings, Wiley, Chichester, UK, 2008 d où certains diapositifs sont en Anglais et autres en Français

3 Introduction Propriétés des dépôts sont déterminées par leur microstructure Microstructure dépend du: Procédé de projection Matériau projeté Matériau du substrat Procédé influence par: Vitesse des particules à l impact Température des particules à l impact Atmosphère du dépôt Température du dépôt

4 Introduction : caractéristiques de procédés Procédé v p [m/s] T p /T f Atmosphère du dépôt T d [ C] Matériaux typiques de dépôt Matériaux typiques de substrat HVOF ,6-1 Air (moins si le substrat est refroidi) MMC, carbures Aciers D-gun TM ou Detonation gun Air Carbures, oxydes Aciers FS ,7-1 Air Alliages Aciers APS >1 Air Tous Tous AS >1 Air Métaux et alliages VPS >1 Basse pression Suspension/ solution plasma spraying Proche à la vitesse du plasma >1 Air (gas plasma et vapeur d eau ou vapeur d alcool) Alliages et métaux (courte distance de projection) Oxydes de métaux Aciers Alliages et métaux Alliages et métaux V p -vitesse particule à l impact, T p -température de particule, T d -température du dépôt

5 Introduction: matériaux projetés Tous matériaux peuvent être projetés, quelques uns posent plus de problèmes: Matériaux à fusion avec décomposition (YBa 2 Cu 3 O 7-x, HA, ) solution traitement thermiques post dépôt, Matériaux qui modifient leur composition lors de projection (décarburation de WC, perte d oxygène par HA, TiO 2 ) solution poudre composite p.e. WC-Co Matériaux qui subliment à haute température (graphite)- solution poudre enrobée p.e. graphite enrobée avec Ni CaO+TTCP CaO+HA TTCP +liquid a TCP +liquid a -TCP + TTCP a -TCP + TTCP HA + TTCP HA + a-tcp HA + β-tcp a -TCP+ liquid a-tcp+ liquid a-tcp+ DCP β-tcp+ DCP %CaO Diagramme de phase de HA

6 Introduction: substrats et leurs activation Substrats métalliques: nettoyage et activation par: Sablage Jet d eau Pulses laser (Protal TM ); Substrats céramiques: nettoyage Substrats polymères doivent être protégés lors du dépôt: Couches intermédiaires Refroidissement lors de projection Autres (p.e. graphite) : nettoyage et sablage à sable fin

7 Activation par traitement par laser procédé PROTAL PROjection Thermique Assistée par Laser (PROTAL) Préparation de surface par une action de laser pulsé robot laser Torch plasma Principes: 1. Augmentation de l énergie de surface 2. Promotion de liaisons physicochimiques Brevet de C. Coddet et al. : FR

8 Thermal history of individual particle at spraying

9 Impact d une particule Phénomènes à l impact: Déformation de sphère en lamelle (formation de porosité) Solidification et refroidissement (taille des cristaux) Déformation plastique dans la technique de projection à froid (CGSM) Déformation Formes crêpe (a) et fleur (b) Crêpe: particule fondue, faible vitesse (HF) Fleur: particule fondue, grande vitesse (APS, D-gun, VPS, ) Déformation du substrat Fissuration (a) (b) Couronne

10 Impact of a particle h=d p -v p t Undisturbed liquid at initial velocity v p Compressed liquid Solid substrate Impact can be divided in two stages. A compressible one, starting with a contact with substrate and finishing with a formation of a truncated sphere including compressed liquid near to contact circle (a) (b) Morphology of particles of hydroxyapatite plasma sprayed using electric power of 11 kw - pancake (a) et 24 kw flower (b)

11 Impact of a particle The stage is called subcritical flow regime flow. Its duration was estimated on s. Time scale of entire impact process can be estimated as the time necessary for spreading of particle using following equation in which Re number ρl d p v Re = η η l In typical for thermal spray processes lamellae it is easier to estimate surface area S rather than diameter. That is why one tries to find the parameter ξ defined as: Theory of Madejski does not consider compressibility of liquid in splat but include surface tension, viscosity, and solidification. The elements allow splat size to be determined in function of dimensionless parameters relatives to splat formation: Re and We. In general theory solves the equations describing the radial movement of liquid cylinder (equation of energy balance). The resulting from the analysis flattening parameter ξ is depends on Re and We numbers result from supposition that kinetic energy of impacting particle is dissipated by a viscous flow. p t = i 2 d 3v 2 ξ = d We = p p p 0.2 Re S π ρl d p v σ 2 p

12 Impact of a particle The parameter ξ describing the spread of the liquid particles impacting the substrate following different authors No Re We ξ Approach Reference 1. Optiona (Re ) 0.2 Analytical Madejski l (1976) 2. > Re 0.2 Analytical Madejski (1976) 2 ξ 1 ξ + ( ) We Re >100 >100 Analytical Madejski (1976) 4. >100 (We/3) 0.5 Analytical Madejski (1976) 5. Re 0.2 Numerical Trapaga (1991) Re 0.21 Numerical Yoshida et al.(1992) = 1

13 Impact of a small particle Suspension plasma sprayed TiO 2 particles morphologies after impact Big particles at impact : flower Fine TiO 2 particles at impact : pancake 20µm 5µm 5µm

14 Impact of a small particle Why pancake instead of flower in a case of small particles at solution/suspension spraying? Kinetic energy Surface energy Equilibrium of energies at impact Let us split a molten particle in two

15 Impact of a small particle Small molten particles are more difficult to split in more parts i.e. are more difficult to splash Velocity at impact, m/s Velocity enabling separation of a liquid particle in two Titania Alumina Particle initial diameter, µm

16 Cold Gas Spraying Method/Choix de matériaux Choix systématique d un matériau pour la projection avec la projection à froid Chauffage en contact qui résulte de la pression de l onde de l onde de choc Classification de matériaux possible pour projeter par CGSM 1. Matériaux durcissant sous le stress sont difficile à projeter (par exemple: inox 316L, TiAl8V4, ) 2. Matériaux avec une faible mobilité de dislocations (métaux de structure bcc tels que W, Cr, Mo,..) sont difficile à projeter 3. Facile à projeter de métaux de structure hcp (Mg, Zn, ) 4. Facile à projeter de métaux à faible point de fusion ( T f < 1600 C) par exemple Cu (Cu, ) Métaux et alliages projetés : Ta, NiCr, NiCrAlY, CuAl, plus facile avec He J. Vlcek, L. Gimeno, H. Huber, E. Lugscheider. In: Thermal Spray 20032: Advancing the Science & Applying the Technology, eds. C. Moreau, B. Marple, ASM Int., Materials Park, Ohio, USA, 2003, 37-44)

17 Impact of a particle influence of the spray technique Morphology of molybdenum lamellae sprayed with different techniques (after Steffens et al.,1993) Technique Consumable Powder Wire size, diameter µm mm Spray process variables Lamellae form Remarks FS - wire 3.18 working gas flowrate, spray distance ( mm) FS - powder Pancake with corona spray distance ( mm). Pancake with corona Corona wider spread if greater spray distance Form does not depend on varying parameters APS spray distance ( mm). Pancakes, flowers and intermediate forms More flowers by growing spray distance AS 1.6 atomization gas pressure( MPa), spray distance ( mm) Pancakes, Flowers More flowers by growing atomization gas pressure

18 Impact of a particle 2 d In the practice of thermal spraying, it is important to know thickness of p lamella. It can be found, knowing that the volume of lamella is equal to h = that of particle, from the following equation: ξ 2 To take example, let us evaluate thicknesses of lamella of Al 2 O 3 sprayed obtained by plasma spraying in open air and in vacuum). Thus, an equation for Re>100 and We>100 can be applied. Consequently, the flattening parameter ξ and, using thickness of lamella can be found. 3 Thickness of Al 2 O 3 lamellae sprayed using by plasma in: air after an impact of 46 µm particle having velocity of v p =131 m/s (+); vacuum after an impact of 54 µm particle having velocity of v p =358 m/s ( )

19 Factors influencing shape of particle after impact Bad wetting Good wetting α α wetting of substrate by particle material and thermal contact resistance; roughness of substrate; spraying at angles different of normal; oxidation of particle at splashing; particularities of formation splats from submicrometric (less than 1 µm) or nanometric (less than 100 nm) particles

20 Temperature of contact particle/substrate The model proposed by Kudinov (1977) bases onto following simplifications: substrate is semi-infinite body, having before impact initial temperature T 0 ; particle is liquid prior to the impact and has temperature T m ; contact between particle and substrate is ideal; Heat is propagated in one direction, perpendicular to the substrate surface. Contact temperatures T c and solidification time, t s, for different spray techniques. Spray techn ique Process variables Materials Experimental Calculations Reference Parti cle Subst rate T m, K T 0, K h,µm T c, K t 0,µ s APS Ni Fe Kudinov (1977) AS Atomizing Ni Fe Steffens gas et al. pressure= (1991) 0.7 MPa AS Atomizing gas pressure= 0.25 MPa Ni Fe Steffens et al. (1991)

21 Impact of a particle: all possibilites After Doltsisnis, Harding and Marchese, 1998 Case A Case B Case C Case D Previous splat -1 Case E Curling due to temperature differential between top and bottom Previous splat -2 Case F Case G Unmelted particle Case H δ 1 δ 2 Previous splat-1 Unmelted particle Previous splat-2 Case I Unmelted particle T Symmetric about impact point Case J δ Unmelted particle Impact point Peak Case K Partially melted particle Coating Unmelted particle Case L Melted particle Unmelted particle h 1 h 2 h 3 h mean Substrate Coating

22 Impact of particle suspension spraying L. Pawlowski, 2009 Aerodynamic breadown Evaporation of liquid Sintering of some fine solids Melting of fine solids and agglomerates Evaporation from melt Impact

23 Impact of particle solution spraying Saha, Seal, et al., 2009 Aerodynamic break up Heating and precipitation of solution (fragmentation of external shell can be repeated) Heating and melting of solid, evaporation from the melt Impact

24 Impact of particle solutions spraying Spherical particle Cracked shell Saha, Seal, et al., 2009

25 Solidification and nucleation Solidification starts after contact of liquid particle with substrate The heat is evacuated through cool substrate in the direction of bottom part of lamella. Sometimes solidification finishes well after splat deformation (sometimes solidification is faster (solidification mechanism) or, in a very particular case, these processes are simultaneous. Bottom contact surface of lamella serves as heat sink and nucleation of crystals starts from this surface. For first splats striking cold and smooth substrate, nucleation would be heterogeneous and grains grow as columns. Two elements have to be taken into account: Undercooling effect which renders temperature of nucleation lower than melting point. The effect is more pronounced for homogeneous nucleation. For instance, undercooling effect for hydroxyapatite particles solidifying in-flight is in the range of T/T m = Thermal contact resistance between bottom part of splat and substrate would hamper heat evacuation and slow down solidification of lamella. The resistance is different in different spots below lamella. R th is different on interfaces splat/substrate (coating s interface) and splat/splat (inside coating). The front of solidification can be planar, cellular or dendritic depending on cooling rate. At heterogeneous nucleation on coatings interface the cooling rate is the greatest and planar solidification front is the most probable. The front moves rapidly towards the top of lamella with the velocity in the range of a few cm/s to a few m/s. The residual liquid temperature increases due to recalescence effect (latent heat liberation)

26 Solidification and nucleation Substrate Crystal grains TEM de TiO2 sprayed by suspension plasma spraying Crystals in lamella: columns (a), equiaxial (b)

27 Theory of nucleation The nucleation rate, assuming homogenous nucleation, for a spherical nucleus can be given from the following equation In which the critical free energy for nucleation is given by: The difference in Gibbs free energy ΔG v between liquid and solid is a driving force of the phase transformation. It is proportional to the difference between the temperature of the solidification solid and the melting point, i.e. on the undercooling. The difference can be calculated for each solid and liquid phase by taking the thermodynamic data Another important parameter is the interfacial energy between solid and liquid, γ, which depends on crystallographic data of solid crystal, thermodynamic data of fusion, surface tension of liquid phase and on volume change at solidification as

28 Theory of nucleation case of titania and alumina Pawlowski, x T m TiO2 G * α/ G * γ Solidification anatase/ γ alumina Al2O3 Solidification titania Solidification alumina Line of equal energies G * (rutile)/ G * (anatase) Solidification rutile/ α alumina Temperature of solidification, K Calculated ratios of critical free energies for nucleation vs. temperature of solidification of anatase and rutile from molten titania and of γ-al 2 O 3 and α-al 2 O 3 from molten alumina

29 Theory of nucleation suspension thermal spraying The solidification time of the liquid particle arriving onto the substrate grows in linear way with the thickness of the lamella which is, in turn, proportional to the particle size Consequently, the liquid cooling rate is greater for smaller particles than for coarser ones Additional energy coming from the released heat of fusion would even more lower the liquid cooling rate of coarse particles. Consequently, the nucleation of α - Al 2 O 3 when spraying alumina and rutile at spraying of titania is more probable at spraying of coarse particles than at processing of fine ones.

30 Theory of nucleation experimental verification by suspension thermal spraying Coarse powder Coarse powder plasma spray coating counts (a.u.) TiO 2 (rutile) - Ti X O 2X-1 (Magneli-phases) Θ ( ) Coarse powder milled suspension plasma spray coating TiO 2 (rutile) - Ti X O 2X-1 (Magneli-phases) 180 # * # - TiO 2 (rutile) - TiO 2 (anatase) - Ti X O 2X-1 (Magneli-phases) - steel (substrate) counts (a.u.) Θ ( ) counts (a.u.) # 90 * # * * θ (degree) # #

31 Croissance de cristal Nucléation hétérogène Grain en colonnes Taille des grains Technique de projection Méthode d analyse Dépôts Taille de grains [nm] VPS DRX Cu (40 dans la poudre initiale) 20 µm APS TEM Al 2 O APS TEM Ni APS TEM Ni Pore fermée Contact entre lamelles Grains en forme de colonnes Oxyde de chrome par APS,

32 Mécanisme d adhérence The most obvious and most common mechanism of sprayed coatings adhesion is a mechanical anchorage of splats to irregularities of substrate. The irregularities result from sand blasting. The splat on solidification gets attached to substrate by force resulting from liquid shrinkage.

33 Mécanisme d adhérence Splat does not contact substrate in all its bottom area. It is obviously true for flower type lamellae and also for pancake splat with curling The zones in contact are called sometimes welding points or active zones correspond to a small fraction of splat area (20-30 % ) Adhesion of coating improves, if active zone area gets greater. Formation of pancake splat is a way to achieve it. Area of active zones is reduced by: o oxides in metal and alloy coatings; o air or/an working gas dissolved into liquid particles in-flight; o Air or/and working gas filled pores entrapped by arriving particles on substrate Addition of deoxidizing element in initial feedstock may, by reducing oxides content, increase active zones area e.g. Ni+5 wt. % Al. Another way to increase the active zone is deposition in an oxygen reduced atmosphere (inert or low pressure). Vacuum plasma spraying offers possibility to clean substrate by the action of arc.

34 Mécanisme d adhérence Other spray techniques result in substrates always slightly oxidized during the time elapsed between surface roughening and beginning of coating deposition. The oxidized spots may: act as the points of anchorage ; be dissolved in liquid lamella being the same oxide; react with another oxide to form a spinel. Inside the active zones adhesion can be realized by one of following mechanisms: physical interaction ; metallurgical interaction ; other type of interactions such as e.g. epitaxy.

35 Composition de phases Phases métastables/amorphes à cause de solidification et refroidissement rapides α-al 2 O 3 projetée donne γ- Al 2 O 3 en dépôt Modification d une composition chimique Évaporation sélective (p.e. CuO en YBa 2 Cu 3 O 7-x ) - diagramme Oxydation (métaux) ou réduction (Cr 2 O 3 ) - section d un dépôt Réduction de Cr III au Cr 0 25 µm

36 Inhomogénéités dans un dépôt Inhomogénéités intrinsèques au procédé Pores intra et inter-lamellaires Taille et forme de lamelles différentes car T p et v p à l impact différentes: Injection radiale de poudre (APS, VPS) un grand progrès grâce à la technique d avenir: projection plasma à injection axiale (AXIAL III de Mettech, Canada) Pulsation de flamme (APS, AS, VPS, D-gun) Pulsation du débit de poudre

37 Inhomogénéités dans un dépôt Inhomogénéités contrôlés - dépôts composites Whiskers: dia. 0,05-8 µm, longueur 3-80 µm Fibres (fils) embobinés sur substrat Particules d un renforcement Al Al 2 O 3 25 µm

38 Croissance du dépôt Lamelles s empilent Contact en état solide ou liquide? Calculs pour APS et Al 2 O 3: Tâche de projection diamètre 25 mm, aire 5 cm 2 Données et résultats des calculs Grandeur Valeur Lamelle h l [µm] 4 Procédé Calcul ξ 2,9 t sol [µs] 20 Débit de poudre [g/min] Tâche de projection [cm 2 ] Surface couverte par des particules lors de t sol [cm 2 ] ,01 Temps de couvrir de tâche de projection [ms] 10

39 Croissance du dépôt Vitesse linéaire de torche: typiquement mm/s Détermine nombre de particules dans un passage de torche Détermine l épaisseur/passage Important économiquement (temps de projection) Important pour la qualité du dépôt car trop d épaisseur/passage génère contraintes résiduelles Surface du dépôt reste en contact avec atmosphère lors de passage et après dépôt (oxydation)

40 Température du dépôt Flux thermique du jet/flamme vers dépôt Important sous p atm Sous vide le jet s allonge Réduction par air comprimé ou gaz liquéfié (CO 2, Ar) Flux thermique Source Technique de projection Convection Radiation Solidification des particules Refroidissement des lamelles Gaz chaud de la zone extérieure de la flamme Radiation de la zone de haute température de la flamme Libération de chaleur de solidification Libération de chaleur du refroidissement de lamelles de T f au température actuelle du dépôt APS, FS, HVOF APS, VPS Toutes techniques Toutes techniques

41 Température du dépôt thermal fluxes spray techniques Technique APS VPS Reference Marynowski et al.(1965) Powder Lee (1983) Chemical composition TiC Cu Size,µm Thermal fluxes, MW/m 2 Radiative Convective 1.13 Solidification/cooling 1.42 Total

42 Température du dépôt thermal fluxes influence of parameters No. Processing parameters, Convective heat transfer coefficient, h c, W/(m 2 K) Radiative flux,w/m 2 1. Spray distance:100 mm, No cooling, No barrier, 2. Spray distance:150 mm, No cooling, No barrier, 3. Spray distance:60 mm, cooling gas velocity:110 m/s, barrier gas velocity:140 m/s T s T s T s 3470 T s is substrate temperature. Total flux is equal to q c = h c (T g T g ), T g is temperature of gas

43 Contraintes résiduelles Contraintes sont une somme de: Contraintes de trempe, σ tr Contraintes générées lors de passage de torche Contraintes thermiques de refroidissement après dépôt, σ t ; Contraintes de transformation de phase, σ tp ; Contraintes mécaniques de choc des particules (HVOF, CGSM) 10 µm Fissuration dans ZrO 2 +7 % mass. Y 2 O 3, due aux contraintes

44 Contraintes résiduelles Contraintes de trempe, toujours de tension, mesurées par courbature d une plaque σ tr = α d ( T T ) Contraintes thermiques, compression ou tension E ( α α )( ) σ t = d d f s 1 ν d T d d T s E d

45 Contraintes résiduelles Contraintes de transformation de phase, compression ou tension σ tp = ρ 1 E d (1 ) ρ 2 3 ( 2ν 1 ) Contraintes mécaniques, compression dans les procédés HVOF, D-gun, CGSM (projection à froid) d