cohesion of alumina coatings

,WH/A¢ a 4;TA/TN6S

ilNOLO / ELSEVIER

Surface and Coatings Technology81 (1996) 275 286

Influence of substrate roughness and temperature on the adhesion/ cohesion of alumina coatings M. Mellali, P. Fauchais, A. Grimaud Laboratoire Mat~riaux C~ramiques et Traitements de Surface, Equipe "Plasma, Laser, Mat&iaux", URA CNRS 320, Facultk des Sciences, University of Limoges, 123, Av. Albert Thomas, 87060 Limoges Cedex, France Received 8 March 1995; accepted in final form 16 June 1995

Abstract This article is devoted to the study of the adhesion/cohesion (A/C) of alumina coatings sprayed on four different substrates: aluminium alloy, titanium alloy, cast iron and mild steel. Fused and crushed alumina particles (-45 + 22 gin) were sprayed with a custom-made torch with a nozzle of internal diameter 7 mm working with an arc current of 600 A and a plasma gas mixture of 45 slm Ar, 15 slm H2, which resulted in a voltage of 64 V and a thermal efficiency of 52%. The particles were injected internally and were almost fully molten (less than 3% ~-phase in the collected powders after their passage in the plasma jet). The influence of substrate roughness and temperature before (preheating), during and after spraying was systematically studied. R, values were 6 8, 10-13 and 14-21 gm and the substrates were preheated to temperatures Tp of 170, 200, 300, 350, 380 and 500 °C. The most important results were the following: • for cold substrates A/C increased with Ra up to a maximum of 20 MPa • for substrates whose expansion mismatch with alumina was less than 4.10 -6 K, the highest A/C values (50-60 MPa) were obtained with Tp between 350 and 500 °C provided the substrates were not oxidized • at these temperatures the highest values of A/C were obtained with the lowest R, • when particle velocity and surface temperature upon impact decreased, for example by load effect, A/C values decreased too

Keywords: Coating adhesion/cohesion; Heat flux; Preheating; Plasma spraying; Surface roughness; Surface temperature control

1. Introduction Plasma-sprayed coatings consist of a highly anisotropic layered structure with individual splats oriented parallel to the substrate surface [ 1]. Among the different properties of coatings, adhesion is one of the most important but unfortunately it is still poorly understood. This is due to: • the difficulties in comparing experimental results. Firstly, very often the spraying conditions are not all well defined (torch working parameters, powder injection, substrate and coating temperature control, substrate roughness, relative movements torch to substrate etc.). Secondly, A/C values have been obtained by using different methods, as summarized recently by Lin and Berndt [2]. The most widely used are the tensile adhesion tests ASTM-C633 or DIN-50160. They are simple but do not promote any understanding of coating performance, for which other methods based on fracture mechanisms have been developed. However, the results obtained with 0257-8972/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI 0257-8972 ( 95 ) 02540-5

these different methods for the same spraying conditions are very different, even when comparing those obtained with two normalized tensile tests (ASTM and DIN). the lack of reliable models. Zaat [ 3 ] proposes three mechanisms: - chemical adhesion, for example with the formation of MoFe when spraying Mo particles on an iron substrate. This implies both the possible chemical reaction and the melting of the substrate by the impinging particle [4]. However, once the first layer is sprayed, the adhesion between the splats (coating cohesion) is no longer governed by the same mechanism. - diffusion: this mechanism is promoted if the substrate is kept at high temperature during spraying, which is the case for alloys or metals sprayed under soft vacuum conditions on a metallic substrate, as confirmed for example by the measurements of Steffens et al. [ 5 ] and Itoh et al. [6]. - t h e mechanical anchorage phenomenon which,

276

M. Mellaliet al./Surface and Coatings Technology81 (1996) 275-286

according to Zaat, is the only possible one for ceramic materials. Beside these mechanisms residual stresses may enhance or decrease the adhesion of the coating [7] especially for thick ones. When looking at the literature related to the A/C of oxide coatings on metallic substrates, different spraying parameters influencing this property can be identified, but with no clear correlations, often due to incomplete information about the spraying conditions: • All authors agree that the spraying angle between the torch axis and the substrate plays a great role, the best A/C values being obtained for 90 ° and starting to decrease for values lower than 60 ° [,8-10]. • For cold substrates all authors also agree that the A/C increases with the substrate roughness up to a certain limit related to the particle size [,8,10-12]. Similar results were obtained with W C - C o particles sprayed by H V O F [13]. • When preheating the surface up to a certain limit, which depends on the residual stresses generated upon cooling (stresses due to expansion coefficient mismatch between substrate and coating), the A/C of ceramic coatings for a given surface roughness, increases with preheating temperature [-14-19]. • Other parameters have also been mentioned as acting on the A/C of ceramic coatings, but sometimes with no clear correlation because very often spraying parameters, such as substrate temperature, were not specified. The A/C of coating is influenced by the particle velocity upon impact, a higher velocity resulting in a higher A/C [20,21]. • There are also variations in coating properties across a spray pattern [22], relative velocity torch to substrate [23], residual stresses close to the c o a t i n ~ substrate interface [24], and the molten state of the particles upon impact. In addition, the A/C diminishes when particle size increases for the same spaying conditions [,20,21]. Very recently, the study of A1203 and ZrO2 splat formation [25 29] has brought a new insight into the A/C of coatings: • When sprayed on a cold (T<100°C) smooth (Ra~0.05 pm) stainless steel substrate, alumina or zirconia splats are extensively fingered (shape factor (SF) below 0.6) with poor contacts with the substrate. Their cooling rate on stainless steel substrates is below 10 v K s 1 as measured by fast pyrometry [-25-28]. • When sprayed on the same hot substrates (T> 200°C), the splats, all over the spray cone [29], have almost a perfect lenticular shape (SF >0.9) with a mean size 50% higher than that of those collected on a cold substrate (no material splashing). They are microcracked all over their surface (very good contact, inducing the relaxation of the quenching stresses all over the surface). Their cooling rate on stainless steel

is between 5 x 1 0 7 and 8 x 108 K s -1 [-27,28] corresponding to an almost perfect contact between the splat and the substrate. This cooling rate increases with the impact velocity as correspondingly the splat thickness decreases. • When sprayed on rough surfaces the same trends are observed, the only difference being an increase in the splat thickness with an increase of roughness, resulting in a decrease of the cooling rate by more than one order of magnitude. If no more lenticular shapes are observed on hot substrates, the microcrack network also covers all the splat surface and the mean diameter on hot substrates is substantially higher than that obtained on the cold ones. This behaviour is probably due to better wettability of the impacting droplet at the end of its flattening on the substrate (true substrate or previously deposited layers) which improves, at least on the microscopic scale, the A/C of the splat. In order to check the influence of the different spraying parameters of alumina particles on the A/C of the resulting coatings, experiments have been performed, where the spraying parameters were controlled as accurately as possible with a special focus on the substrate temperature and roughness. Thus in the following experimental work, the influence of the substrate roughness with and without preheating, that of the preheating temperature for a given roughness and that of roughness for a rather high (350-500 °C) preheating temperature will be presented successively.

2. Experiment procedure 2.1. Plasma spraying conditions

In order to achieve good temperature control of the substrate and coating, disc-shaped samples (4 = 25 ram, e = 4 ram) were disposed in eight holes on a stainless steel cylindrical holder (SCH), 110 mm in diameter and 3 mm thick. The samples were held by three screws. The torch, whose axis was orthogonal to that of the SCH, was translated parallel to the SCH at a constant velocity (Fig. 1). Spraying was performed with a custom-built torch with a cylindrical anode nozzle whose internal diameter (i.d.) was 7 mm. The powder was injected inside the nozzle with an injector 1.8 mm in i.d., disposed orthogonally to the torch axis 3 mm upstream of the torch nozzle exit, in order to achieve good melting of the particles [30]. Fused and crushed (F.C.) alumina particles with a size distribution ( - 45 + 22 gm) were sprayed with the following working parameters of the torch: arc current I = 600 A, voltage V= 64 V, argon flow rate 45 slm, hydrogen flow rate 15 slm, thermal efficiency of torch Pth = 52%. In order to limit the load effect, the powder flow rate

M. Mellali et al./Surface and Coatings Technology 81 (1996) 275-286

Computer

g

4~ t_ .,t Translation

•

•

i

i

Scanner

Fig. 1. Sketch of the spraying device with cooling air jets. mpo w a s

1 kg h - 1 . To check the influence of the particle molten state upon impact, bigger F.C. alumina particles ( - 9 5 + 45 gin) where also sprayed with the same spraying conditions. The influence of the particle molten state was also studied by increasing the powder flow rate to 3 kg h -1 (load effect). The spraying distance was chosen in order to obtain a good molten state of the particles together with easy control of substrate and coating temperatures. According to the measurements of Vardelle et al. [30], for standoff distances (d) greater than 80 mm the surface temperature of the particles starts to decrease very slowly, the decrease becoming larger for distances greater than 120 mm. The heat flux imposed on the substrate and substrate holder had a roughly Gaussian shape: = ~bo exp

-

(1)

where ~bo and rg are respectively the maximum heat flux and the radius of the gaussian) [31,32]. The value of ~bo decreases drastically with d, as shown in Fig. 2 for two arc currents. That is why finally d = 100 mm was chosen as the best compromise. o was The powder argon carrier gas flow rate meg adjusted (meg=6.5 o slm) in such a way that the mean trajectory of the particles made an angle of 3.5 ° with the torch axis [18,30]. It is worth noting that such an ~o (MW/m 2)

-x-600A - A-400A

3 0

i

50

I

70

,

I

90

,

injection condition corresponds to the highest deposition efficiency (pd=62%). Pa was measured by spraying on an aluminium cylinder (4--60 ram, e = 4 mm, l = 150 mm) of known weight for a given time. Weighting the cylinder and deposit allowed Pa to be determined. Such a deposition efficiency was obtained by adjusting the rotation velocity of the substrate holder and the translation velocity of the torch in such a way that the sprayed beads' overlap was 50%; this value seems to give the best spraying conditions, especially for lowering the residual stresses within the coatings [33]. For our substrate spraying conditions, it corresponds to a rotation of the substrate holder of 160 r.p.m, and a translation velocity of the torch of 16 m m s 1. These conditions were slightly modified when the powder flow rate was 3 kg h 1 in order to account for the change in bead dimensions.

2.2. Substrate and coating temperature monitoring As shown in Fig. 1, the substrates and coatings were cooled during spraying by air jets in one of two ways: • One blown orthogonally (air barrier called B) to the plasma jet 20 mm in front of the substrates, 12 mm below the torch axis. According to the measurements of Monnerie et al. [31] it is possible, with an air flow rate of 40 scmh, to reduce the plasma heat flux to the substrate by a ratio of 6. However, such a device reduces the impact velocity of ( - 4 5 + 2 2 gm) F.C. alumina particles by 12-20% and their surface temperature by 10-15% [-30]. It also eliminates most of the small unmolten particles which did not penetrate the plasma jet but were sucked downstream by the engulfment process induced by the plasma jet [34]. • One or four of them, called 1R or 4R, blown at the cylinder surface opposite to the plasma jet, each one being disposed orthogonally to the cylinder surface 5 mm from it. These air jets were achieved with machined slots 28 mm in length and 1 mm in width. The surface temperature of coatings and substrates was continuously measured by IR monochromatic pyrometry (2 = 5.2 4-0.3 ~tm, 10 Hz). The emissivities of the substrates and coatings with different roughnesses were measured by heating them in a furnace and comparing their emitted flux for a given temperature to that of a black body (isoflux method [-35]). In the following the roughness will be defined by the R,:

lfo

R~=~

× i

110

,

I

t30

,

I

150

277

lyldx

(2)

,

170

d (mm)

Fig. 2. Evolution vs. stand-off distance d of the maximum heat flux ~o (supposed to be gaussian) imposed by the plasma jet to a cold flat substrate.

where l is the length of the analysed cross-section and the integral represents the area of the peaks and undercuts along I. For example, Fig. 3(a) represents the emissivity of the alumina coatings, which is almost independent of temperature and surface roughness, and

M. Mellali et al./SurJktce and Coatings Technology 81 (1996) 275 286

278

~(~.,~T)

1 0.8 0.6 0.4

•

Ra=fpm

A

Ra= 13pm

0.2 0 100

(a)

200

300

400

500

600

700

Ts (°C) ~(7.,txT)

1

#

Ra = 13 -16 pm

0.8 0.6 t? 0.4

l* Ra ~ 1 tJm

2.3. Substrate preparation and spraying conditions

0.2 0

(b)

powder flow rate of 3 kg h - l ) . It is worth noting that, with no preheating, when spraying a coating thickness of 300-350 pm the final surface temperature of the coating without preheating was close to that obtained with preheating (see Fig. 4). Cooling was achieved by switching off simultaneously the plasma jet, the cooling air jets and the powder feeder and letting the coating and substrate cool down slowly in air with the substrate holder still rotating. It is worth noticing that cooling is faster with no preheating due to the lower substrate temperature. The problem related to these temperature measurements is the low response time of the pyrometer (10 Hz) which damps the temperature fluctuations due to the substrate rotation and torch translation.

0

I

]

p

I

I

I

100

200

300

400

500

600

Ts(°C)

700

Fig. 3. Evolution for different values of R a of the emissivity ez (2 = 5.2 ~tm) vs. temperature. (a) Alumina coatings; (b) cast iron (FT25).

Fig. 3(b) that of cast iron (FT25) for Ra respectively of 13-16 ~tm and 1 ~tm. It is worth noting that for T> 450 °C oxidation of the surface drastically changes the emissivity. At 500 °C measurements were performed with clean substrates. Their heating took about 200 s and the emission coefficient kept increasing up to 600 s. The precision of the temperature measurements was estimated to be 4% over 180 °C. Below 150 °C the precision was poor (15%), the pyrometer being influenced by the light reflected from the plasma. Fig. 4 represents typical heating curves with and without preheating. In the first case the substrate was preheated for 150 s with the plasma jet working in the spraying conditions (stand-off distance of 100 mm) up to a temperature of 460 °C. After substrate preheating the powder was injected, resulting in a temperature increase of 80-100 °C with a powder flow rate of 1 kg h -1 and 160-180 °C with a

The substrate roughness was achieved by using three white alumina grit sizes: 0.5, 1 and 1.4 mm in mean diameter. The blasting distance was 100 mm, the blasting angle 90 ° and the blasting pressure 0.3 MPa. A pressure blasting machine was used with a B 4 C nozzle 8 mm in internal diameter. Such blasting conditions [-18,36] resulted in an almost linear variation of the substrate roughness with the grit size, the obtained roughness decreasing when the substrate Young's modulus increased. The grit blasting time was limited to 3 s. With such conditions the grit wear was a minimum. In order to account for this grit wear (for example 15 wt.% wear during one passage (3 s) of the 1.4 mm grit) all blastings were performed with "new" grit. Four substrates were used, which means that the linear expansion coefficients below 600 °C were the following: • aluminium alloy AU4G (c~=23 x 1 0 - 6 K -1) • cast iron FT25 ( e = 1 0 -5 K -1) • mild steel 34CD4 (c~= 12 x 10 - 6 K - 1 ) • aluminium-titanium alloy Ti6A14V (TA6V) (~= 5×10

6K-l)

In the same temperature range, for alumina coatings the expansion coefficient parallel to the substrate is ~ = 8 X 10 . 6 K - 1 .

The sand blasting resulted in the Ra (gm) summarized in Table 1. The values given account for the dispersion, which increases with grit size [-18,36].

Ts(°C) 600 500

400 S'\

Table 1 Substrate roughness according to grit size

300 2O0

Substrate nature

100

Grit size

tA~J 0

-150

0

i

n

~

I

150

300

450

600

Time (sec) Fig. 4. Substrate and coating surface temperature evolution with (P)

and without (N) preheating.

FT25, 34CD4 AU4G TA6V

0.5

1.0

1.4

6-8 7 9 6-8

10 13 13-17 12 15

14 21 18 25 16-25

M. Mellali et al./Surface and Coatings Technology 81 (1996) 275 286

279

Table 2 Cooling conditions during spraying Conditions

I

II

III

IV

V

Used cooling devices With (P), without (NP) preheating Ti (°C)

B+ R NP

B+ R P

R NP

R P

4R P

<50 250 _+ 20 --

170_+20 250 _+ 20 30

150___40b 550 _+ 40 --

380-+ 15 550 ± 40 70

300_+ 15 380 _+ 30 130

Zf (°c)a A T (°C)a

a Measured with 1 kg h 1 powder flow rate b The higher value of Ti compared to case (I) is due to the fact that the powder injection takes a few seconds after opening the valve, thus allowing the substrate which is heated directly by the plasma jet (no more air barrier) to heat up

Three preheating (P) temperatures were achieved using respectively: • the barrier (31 32 scmh)+one cooling rear slot (48 scmh) noted in the following (B + R) • four rear slots (120 scmh) (4R) • one rear slot (48 scmh) (R) Two conditions of no preheating (NP) were associated respectively with B + R and 1R. Table 2 summarizes all these conditions with the corresponding substrate temperatures: Ti when starting spraying; Tf, the final temperature of the coating when spraying is stopped; and AT, the temperature variation measured with the pyrometer focused just over the spray cone (i.e just after powder deposition) and just before it (i.e. just before the next layer deposition). AT measurement was only performed for preheated substrates. It is clear from the A T values that the barrier is an efficient means of reducing the temperature fluctuations due to the relative movement of the torch to the substrate, but unfortunately, as already emphasized, it cools and slows down the particles before impact. If the four rear slots allow the substrate to be kept at a preheating temperature of 300 °C without the barrier, the temperature variation between the successive passes is large and probably larger than that shown in Table 2 due to the low response time of the pyrometer. These cooling conditions result in different values of the coating roughnesses. For example, Table 3 summarizes the values of the surface roughness (Ra) obtained for 380 _+ 30 lam thick coatings sprayed onto a cast iron substrate with an Ra of 14-21 gm. From this table, it is clear that the coating reduces the initial substrate roughness. The use of a barrier, eliminating the unmolten small particles, drastically reduces the coating roughness. Table 3 Surface roughness of 500 g m alumina coatings on FT25 substrate (R,=14 21gm) Conditions Ra(gm)

I 5.7_+0.4

II 5.4_+0.4

III 10.7_+0.8

IV 11.1_+0.9

V 10.6_+0.8

There is no clear effect of the preheating on the surface roughness of the coatings.

2.4. Coating and surface characterization The surface roughness R~ was measured with a perthometer. Each result is the mean value of 20 measurements performed along two orthogonal directions. The phase analysis was performed by XRD with the Cu e-line and using an automatized code (Diffract: Eva or Fit, Search). The coating porosity was measured either by mercury porosimetry or by image analysis. In this last case the same preparation procedure of the samples was followed. Vacuum impregnation of the coatings (Mecapex ss resin), cutting of the coating with a diamond disk, polishing of the coatings with paper disks covered with SiC of different grades: successively 300, 500, 1000, and 1200 for 5 rain each with a load of 2 units of the Struers Pedemax machine and finally polished with diamond impregnated cloths with diamonds whose sizes were successively 6, 3 and 1 lam. The adhesion/cohesion of the coatings was measured according to the DIN 50160 test and six measurements were carried out for each spraying condition, allowing the mean value and its standard deviation to be determined. The Vickers hardness was measured with a 5 N load applied for 10 s on coating cross-sections. Twelve measurements were performed for each sample.

3. Results

3.1. Influence of substrate roughness (classical plasma spraying) The two alumina powders were sprayed on FT25 substrates with the three grit blasting conditions. The substrates were not preheated and substrate and coating were cooled during spraying by one rear slot. The stationary temperature achieved for a coating

M. Mellali et al./Surface and Coatings Technology 81 (1996) 275-286

280

30

o (MPa)

25 2O 15 -£

10 5

Z

0 8

12

20

16

Roughness Ra (pm)

Fig. 5. Evolution of the coating A/C vs. the substrate roughness for two alumina particle sizes. I 1 : - 4 5 + 2 2 ~tm; A: - 9 0 + 4 5 ~tm.

450-500 lain thick was 500 °C. Fig. 5 shows the results obtained. As predicted by most authors [10 12] the A/C increases with roughness at least for the well-molten particles ( - 4 5 + 2 2 ~tm). Coatings with big particles which were partially molten exhibit lower values of A/C. However the decrease in A/C for the highest roughness has no clear explanation except maybe that the partially molten particles flatten the substrate peaks. In all cases the rupture was purely adhesive. According to these results, in the following study of the influence of the substrate temperature we chose to use only ( - 4 5 + 2 2 gm) particles and to work with the highest roughness.

3.2. Influence of substrate temperature First we tested whether the substrate temperature was influencing the thickness of the coatings. The results are presented in Fig. 6. The values are not drastically different and they are within the error bars. However, as could be expected without preheating, the coating thickness is a little bit smaller with the barrier than without it. Of course, when preheating to 500 °C the adhesion of the splats is better and their mean diameter is larger 1-21,26 29]; thus the coating is slightly thicker than that obtained with the same cooling condition but without preheating. It is only slightly Coating thickness (~Jm) 800 700 600 500 400 300 200 100 0

,t

.5&¢ ~ _ .a522~~" ,,S~" ~

+ P+R

.~,~.~_~S'~ ~'~

o B+R

thicker because, without preheating, as soon as the coating thickness is more than 80 gin, the particles impact on alumina layers, whose temperature is higher than 250 °C, resulting in well-adhering splats. The starting powder is pure e-phase, as shown by the XRD pattern in Fig. 7(a). The spectra of the coatings are all normalized relative to the most intense line (004) of the ?,-alumina phase obtained in spraying condition (I) (Table 2). The substrate cooling conditions during spraying do not seem to play a role in the e-phase formation, as shown by Figs. 7(b) (f). In all cases the e-phase percentage of the coatings is less than 3%. Comparison of the spectra shows that the highest is the substrate temperature and the lowest are the lines of the planes (004) and (400) corresponding to the ),-alumina phase. This might be due to the increase in the microcracking of the splats due to the relaxation of the quenching stresses with the excellent contact between the splat and the layers already deposited [-18,28,29] (Fig. 8(a) represents an almost perfect lenticular-shaped splat obtained on a steel substrate at 300 °C). The e peak (311) showing up in condition (IV) may be due to nucleation in the e-phase at high temperature [37]. As will be shown later, the best adhesion was obtained with the highest surface temperature with preheating of the substrate to 380 °C (condition IV). However, as shown by recent work on splat formation [ 18,28,29] performed at our laboratory, if for substrate temperatures over 200 °C perfect lenticular-shaped splats are obtained on a smooth surface, when the substrate surface is oxidized, the splats again become extensively fingered, like those obtained below 150 °C (see Fig. 8(b), which represents a splat obtained at 75 °C on a steel substrate). As, for example, for FT25 substrates, the colour of the surface changes for heating times longer than 130 s, the A/C of the coatings sprayed on FT25 decreases for heating times longer than 100 s, as summarized in Table 4. Thus in the following, the preheating of the substrates was performed in 90 s by approaching the torch to 80mm before withdrawing it to 100ram to start spraying. Tables 5-7 summarize the results obtained respectively for AU4G, 34CD4 and FT25 substrates. For the A/C results (A) means that the rupture was purely adhesive, ( ~ C ) that it occurred very close to the substrate or partially at the interface and partially near it, and (glue) that it occurred in the glue. When considering the results obtained with AU4G

~.~ i

i

i

20

40

60

80

Number of torch passages

Fig. 6. Evolution of coating thickness vs. the number of torch passages without preheating (cooling achieved with B + R and R) and with preheating (cooling achieved with R) noted P + R.

Table 4 Evolution of the A/C of the coatings sprayed on FT25 (R. = 1 4 2 1 gm) in condition (IV) with different preheating times Preheating time (s) A/C (MPa)

90 574- 1

150 38 _+ 2

300 22 + 4

M. Mellali et al./Surface and Coatings Technology 81 (1996) 275 286

100

281

Height (%)

Height (%)

loo r

(3.

Ot 75 50

50

25

25

;

i L_

40

55

0 25

10 (a)

10o

,/

ol 70

85

2 0 ( °)

10

25

40

Height (%)

100

75

70

85

55

7O

85

70

85

Height (%)

7

75

50

55

20 (°)

(b)

50

7 25

25

0

0 25

lO

40

55

7O

85

100

10

25

40

20 (o)

(d)

20 (°)

(c) Height (%)

100

Height (%) y

75

75

50

50

3'

25 0

0 10

25

40

(e)

55

70

85

2o (o)

0

(f)

25

40

55 2 0 (°)

Fig. 7. XRD pattern of the starting F.C. alumina powder ( - 4 5 +22 gm) (I.a) and of the coatings sprayed with the different cooling conditions described in Table 2 (I.b, II.c, II.d, IV.e, V.f).

(Table 5), it is clear that, with the important expansion coefficient mismatch, the preheating temperatures of 300 and 380 °C are far too high. Assuming that the coating Young's modulus is 120 GPa [38], the coating and substrate temperature is uniform upon cooling. Using the calculation method of Hsueh et al. [39] and neglecting the tensile quenching stresses, which according to Kuroda et al. [40] are less than 20 MPa, at 400 °C the compressive residual stresses are about 350 MPa (compared with less than 50 MPa for FT25 and 34CD4). Table 5 Adhesion/cohesion, mean pore size and porosity of alumina coatings (500 gm thick) sprayed on AU4G I

A/C (MPa)

II

10.6 _+ 2 11.4+4 (A)

(A) Mean pore size (gm) Porosity (%) mercury

2.28 7

2.18 6

III

IV

V

No

Detached during cooling

Partial adhesion

That is why conditions (IV) and (V) result in completely or partially detached coatings during cooling. The preheating to 170 °C is not sufficient to improve the A/C of the coatings. With condition (III) when the temperature gradient within the coating is important (~ 1600 K mm -1) the coatings are not detached at the end of cooling, but they can be detached very easily (o < 1-2 MPa). The A/C results are much better with 34CD4 and FT25 (Tables 6 and 7). With preheating, the highest A/C values are obtained for the highest surface temperature. According to the splat formation studies [18,21,28,29], as soon as a substrate or previously deposited layers have a temperature higher than 250-300 °C, their adhesion is quite good. Thus the A/C with a preheating temperature of 300 °C (condition V) should be close to that obtained at 380 °C (condition IV). However, as shown in Table 2, for condition (V) the four cooling slots induce a large temperature variation between two successive passes (a pass being the layer deposited during the translation of the torch in

282

M. Mellali et al./Surface and Coatings Technology 81 (1996) 275-286

(a)

(b) Fig. 8. Alumina splats collected on smooth steel substrates (Ra= 0.5 gm): (a) preheated to 300 °C in less than 100 s; (b) cold (<75 ~'C).

front of the coating), resulting in higher residual stresses between them, and coating cross-sections show cracks parallel to the substrate between each pass and a lower

A/C value. The good A/C of the coatings at high temperature (conditions IV) seems to be due firstly to the excellent adhesion between the successive splats and secondly to the growth of the columnar structure all across one pass constituted of 6-7 layered splats [29]. The development of the cracks between the successive passes with condition (V) is underlined by the highest porosity of the coatings sprayed with condition (V). The hardness of these coatings is also the smallest, while that of the coatings sprayed at high temperature (condition IV) is the highest. With this condition there are lowtemperature fluctuations (Table 2) and the A/C between the successive passes is much better. It is interesting to note the difference in porosity values measured by mercury porosimetry (MP) and by image analysis (IA) (Table 7). In the case of IA the preparation of the samples (Section 2.4) induced new cracks and partially molten particles pulling out, especially when the A/C of the coatings was not too good, that is, when starting with a rather cold substrate and letting a temperature gradient develop within the coating as in cases (I) and (III). Preheating to about 170 °C slightly improved the coating A/C (compare the results of conditions (I) and (II)). The A/C of case (III) (no preheating, cooling by one rear slot) is slightly better than that of case (I), where the barrier cooled and slowed down the particles before impact. Moreover, in case (III) the surface temperature of the first layers was rapidly higher than that obtained in case (I). However, in case (III), as shown by image analysis, the cohesion of this coating does not seem to be as good as that obtained in condition (II): probably for coating thicknesses over 100 lam the temperature gradients induced too drastic residual stresses within the coating. Finally,

Table 6 Adhesion/cohesion, microhardness, mean pore size and porosity of alumina coatings (500 gm thick) sprayed on FT25

A/C (MPa) Vickers microhardness (5 N load) Mean pore size (gm) Porosity (%) mercury

I

II

III

IV

V

17.2 _+ 3 (A) 1200 ___96 2.44 7

22.6 _+ 2 (A) 1258 _ 100 2.70 6

23.1 _+4 (A) 1175 + 94 2.04 5

> 40 (glue) 1301 + 100 2.44 4-5

32.5 _ 3 (A-C) 1197 + 95 2.5 7-8

Table 7 Adhesion/cohesion, microhardness, porosity and mean pore size of alumina coatings (500 gm thick) sprayed on 34CD4

A/C (MPa) Vickers microhardness (5 N load) Porosity % image analysis Mean pore size (gm) Porosity (%) mercury

I

II

III

IV

V

14 + 2 (A) 1122 _+ 100 11.1 2.64 7

16.7 _+ 3 (A) 1198 _+ 96 7.5 2.34 6

23.7 _+2 (A) 1156 _+90 9.8 2.67 5

> 36 (glue) 1263 + 100 4.9 2.47 4-5

> 31.5 (glue) 1073 + 86 -7 8

M. Mellali et al./Surface and Coatings Technology 81 (1996) 275-286

283

Table 8 Spraying conditions, A/C and microhardness values for 500 g m thick alumina coating ( - 4 5 + 22 g m FC alumina powder) Spraying condition Cooling mode Air flow rate (scmh) Preheating temperature Ti (°C) Powder flow rate (kg h -1) Tf (°C) A/C on FT25

VI R 47 350 _+ 15 1 550 + 40 > 40 (Glue) > 36 (Glue) 1200-1400

A/C on 34CD4 Microhardness (HV5)

VII R 47 350 ___15 3 630 _+ 50 ~28.5 (C) a 26.46 (C) a 1200 1300

VIII 4R 120 350 _+ 15 1 550 _+ 40 ~32.5 (Glue) > 31.5 (Glue) 1197-1300

IX 4R 120 350 _+ 15 3 630 + 50 ~29.3 (C) a > 29.2 (Glue) b

a (C): rupture occurred within coating b Difficult to measure with an extremely porous coating

in case IV, in which the A/C is good, the porosities measured by IA and MP are about the same.

3.3. Load effect The load effect, occurring when the powder flow rate was increased to 3 kg h - l , resulted in a cooling of the plasma jet and a reduction in velocity and temperature of the particles in flight (surface temperature decrease of 300 K and velocity of 30 m s -1) [30,41]. Table8 summarizes the spraying conditions and the A/C values obtained with FT25 and 34CD4 preheated substrates. Compared with the coating sprayed with 1 kg h-1 (see Fig. 7) the a-phase content of the coating (,~7% compared with 3%) is slightly higher (Fig. 9), confirming the cooling of the particles in flight. When adjusting the rotation and translation velocities to achieve a bead 100 Height (%) 75 50

7

25 0 10

I

I

25

40

(a) 100

55

70

85

20(o) Height (%) 7

7

75 50

Y

overlapping of 50% the thickness of the beads and passes is also almost tripled. The cooling of the particles, as well as probably the higher temperature gradients within the passes and coatings, reduces the A/C values of the coatings obtained at 3 kg h - i . However, it is worth noting that the cooling with four rear slots seems to have less effect on the A/C values compared with those measured with the coatings sprayed at 1 kg h-l). This might be due to the higher surface temperature inducing a better contact between the splats and a growth of the columnar structure within one thicker pass, and also to the smaller temperature fluctuations (A T= 110 °C) due to the higher heat capacity of the thicker passes.

3.4. Influence of substrate roughness for a given preheating temperature According to the excellent results obtained for the splats and coatings at preheating temperatures over 200 °C, the effect of the substrate roughness was tested with a powder feed rate of 1 kg h-1 in the conditions defined in Table 9 for two substrates: FT25 and TA6V (in which oxidation kinetic is slower than that of FT25). It is worth noting that, compared with the previous conditions, to achieve preheating temperatures between 300 and 500 °C only one rear slot was used, where the air flow rate was varied. The A/C values obtained with the two substrates are summarized respectively in Tables 10 and 11, and they were obtained with a new glue exhibiting much better adhesion. From these tables, it is clear that the results obtained Table 9 Spraying conditions

10

(b)

25

40

55

70

85

20 (°)

Fig. 9. X R D pattern of the coatings sprayed with 3 kg h 1 in conditions (VII) and (IX).

Conditions Preheating Cooling mode T~ (°C)

III No R 150-1-40

X Yes B+ R 200___25

XI Yes R 300__+15

XII Yes R 500_+35

M. Mellali et aL /Surface and Coatings Technology 81 (1996) 275-286

284

Table 10 Adhesion/cohesion values for 500 gm thick A120 a coatings sprayed on FT25. (A): 100% adhesive, (A)* > 8 0 % adhesive, (A--C)**~70% cohesive, (C): 100% cohesive

o (MPa)

50 40

Conditions

R a (gm) 6-8

III X XI

15_+ 1 (A) 35+1 (A)* 58 _+ 3 (C)

i

60

(C)

30 10 13 18_+2 (A) 26_+4 (A) 43 _+4 (A-C)**

XII

15 21 23__.4 (A) 27_+1 (A) 57 _+ 1 (A C)** 52 _+ 1

20

(A)

10 0 300

500

(A)

I

I

I

700

900

1100

1300

Coating thickness (pm)

Fig. 10. Adhesion/cohesion evolution with the coating thickness sprayed in condition (IV) on FT25 substrate with R a = 14-21 gm. (A): adhesion; (C): cohesion.

(~C)** Table 11 Adhesion/cohesion values for 500 gm thick AI203 coatings sprayed on TA6V. (A): 100% adhesive, (A)*>80% adhesive, (C): 100% cohesive Conditions

R~ (gm) 6-8

III XI XII

10 13

15 21

23 _ 2

27 ± 1

(A)

(A)

60_+3

50_+2

57_+ 1

(c)

(A)*

(a)*

--

61 _+ 2

(c) with cold substrates and no temperature contro ! (except to limit it to 550°C) are confirmed: the adhesion increases with substrate roughness (see the results of conditions (III) in Tables 10 and 11). With cast iron preheated to 300 °C (condition (XI) the A/C value is increased and it is very interesting to see that the highest value is obtained with the smallest R, (6-8 gin), where the rupture is purely cohesive. According to the work of Richard et al. [24], this could be due to the fact that when the roughness decreases, the residual stresses near the interface decrease also, but it has to be confirmed by systematic measurements with the incremental hole drilling method. This tendency is confirmed when preheating to 500°C (see condition (X I) in Table 10). However, we have no clear explanation for the smallest value obtained either with FT25 o r TA6V with the intermediate roughness (R = 10-13 p.m) in conditions (X) and (XI). The oxidation effect can be seen with FT25 when it is preheated to 500 °C (condition (XII) in Table 10). It is not the same for TA6V, where A/C values increase with temperature and become purely cohesive at 500 °C (condition (XIII) in Table 11). The oxidation problem with FT25 is also probably responsible for the decrease in A/C with thick coatings in condition (IV) with R , = 14-21 gm, as shown in Fig. 10. As soon as the thickness is over 450gm (which

corresponds to about 200 s spraying) the A/C values are reduced by a factor of 2 and become purely adhesive instead of cohesive for 450 gm thick coatings. Due to the porosity of the coatings (Table 5), parts of the surface of the coated substrate become oxidized as checked after detaching the coatings.

3.5. Influence of substrate and coating cooling When, instead of letting the substrate and coating slowly cool down on the rotating substrate after spraying (Fig. 4: about 800 s from 580 °C to 100 °C), the cooling air jets were kept blowing after the plasma was switched off (about 150 s from 580°C to 100°C), the coating A/C was drastically reduced: from A/C>41 MPa in conditions (IV) to A/C=22.3 MPa.

4. Conclusions

When studying alumina and zirconia splats on smooth or rough surfaces either of metal or of plasma-sprayed alumina or zirconia coatings, it has been recently demonstrated that as soon as the surface temperature is over 200°C, the contact between the splats and the underneath surface is much better than at lower temperature, provided the surface is not oxidized [-26-29]. The aim of this article was to check the influence of substrate temperature and roughness on the adhesion/cohesion (A/C) of the resulting coatings, measured by the test DIN 50160. For that, spraying conditions were chosen in order to obtain well-molten alumina fused and crushed particles ( - 4 5 + 2 2 gin) [30] and only the substrate roughness and its temperature varied. Three different roughnesses were studied with four different substrates: aluminium alloy, titanium alloy, cast iron and mild steel. The substrates were either preheated to 170, 200, 300, 350, 380 and 500 °C before spraying or not preheated at all with starting temperatures of 50 or 150 °C. In order to limit oxidation, the preheating time with the plasma

~/L Mellali et al./Surface and Coatings Technology 81 (1996) 275-286

torch (Ar-H2 25 vol.% plasma forming gas) was performed for 90 s maximum. Temperature control was achieved with different air jets and the surface temperatures of the substrate and coating were measured by IR monochromatic pyrometry, after calibration of the emission coefficients of the different surfaces studied. The main results are the following: • When starting with a cold substrate (< 100 °C) the adhesion/cohesion (A/C) increases almost linearly with the substrate roughness. • The adhesion/cohesion is best when substrate temperature is between 300 and 500 °C, at least for substrates whose expansion coefficient is not too far from that of alumina (Ac~<4 x 10 - 6 K - l ) . • The cooling of the substrate and coating after spraying has to be as slow as possible to avoid high temperature gradients. • When particle surface temperature and velocity in flight decrease (load effect or use of an air barrier to reduce the heat flux from the plasma jet) the A/C diminishes. • The adhesion decreases if the preheated substrates are oxidized; at the limit, it tends to the values obtained with cold substrates. • At high substrate temperature (preheating to 350 500°C) the A/C is the highest for the smallest value of the studied Ra. According to the literature this could be due to a reduction in the stresses close to substrate coating interface. • When the expansion mismatch is too high (aluminium alloy), the compressive residual stress generated upon cooling do not allow the coating to adhere to the substrate if the preheating temperature is higher than 250 °C. Of course these results are preliminary ones and they should be completed by residual stresses distribution measurements, accounting for the partial relaxation, by the preheating, of the compressive residual stresses generated by grit blasting.

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