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EXCHANGE OF EXPERIENCE AND INFORMATION METALLIZATION FIGURE IN PLASMA AND DETONATION SPRAY COATING E. A. Astakhov and S. Yu. Sharivker Investigation of the processes of high-temperature spray deposition of protective coatings by means of a plasma jet and devices utilizing detonation-wave energy is an important task of present-day materials science [11. The available literature data on the distribution of sprayed material over the spray target (metalliza- tion figure) are very fragmentary and incomplete [2, 3]. Yet, this characteristic is important both for un- derstanding the physical processes taking place during the application of a coating and for practical pur- poses, namely, choice of a spray pitch and deposition of a uniformly thick coating. In plasma sprayeoating, particle scattering in the stream obeys Gauss' law of normal distribution (Fig. 1). In the figure, the axis of ordinates represents the relative intensity of spraying, I, calculated from the formula: h I -- hmax, (1) where h is the height of a layer deposited with both the nozzle and the substrate stationary, at a distance r (in ram) from the nozzle axis and the center of the spray target, and hma x is the height of the layer depos- ited by the same technique in the center of the spray target (in ram). Thus, Fig. 1 depicts the spray target in a transverse cross section (seen from above, the target is circular). It has been proposed [4] that the h = f(r) curve in such instances eould be described by the equa- tion: r ~ l h = hma x eQ", (2) where p is the radius of scattering, mm. The value of p can be determined, assuming normal distribution, as the distance from the spray target axis to the point where the relative intensity of spraying is 0.368. /o 12 8 4 o !0 ~6 363 Distance from nozzle axis, mm Fig. 1. Distribution of NbC powder in plasma spray deposition. Are cur- rent 290 A, spray range 75 ram. Using the concept of "radius of scattering" simplifies procedure in describing the character of a sprayed material. Moreover, the magnitude p is a characteristic of this techno- logical process. Thus, it has been demonstrated [4] that, with increasing pitch between parallel sprayed bands, the resulting nonuniformity of sprayed-layer thickness is determined by the radius of scattering (Table 1). The value of pitch recommended for attaining a uniform thickness is 1.2-1.3 p in manual spray- ing and 1.4 p with automatic travel of the nozzle or the sprayed part. The resulting coating-thickness variation does not ex- ceed 3%. The authors investigated the influence of technological parameters on the radius of scattering in the plasma spraying of zirconium and niobium carbides. The spraying technique, starting materials, and equipment used in this investigation were described earlier [5]. Figure 2 shows the results ob- tained. The increase in the radius of scattering with rise in arc-current intensity may be attributed to improved spray- Institute of Materials Science, Academy of Sciences of the Ukrainian SSR. Translated from Porosh- kovaya Metallurgiya, No, i0 (82),pp. 105-109, October 1969. Original article submitted April 1, 1968. 859

Metallization figure in plasma and detonation spray coating

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Page 1: Metallization figure in plasma and detonation spray coating

E X C H A N G E O F E X P E R I E N C E A N D I N F O R M A T I O N

METALLIZATION FIGURE IN PLASMA

AND DETONATION SPRAY COATING

E. A. Astakhov and S. Yu. Sharivker

I n v e s t i g a t i o n of the p r o c e s s e s of h i g h - t e m p e r a t u r e s p r a y d e p o s i t i o n of p r o t e c t i v e c o a t i n g s by m e a n s of a p l a s m a j e t and d e v i c e s u t i l i z i n g d e t o n a t i o n - w a v e e n e r g y is an i m p o r t a n t t a s k of p r e s e n t - d a y m a t e r i a l s

s c i e n c e [11.

The a v a i l a b l e l i t e r a t u r e d a t a on the d i s t r i b u t i o n of s p r a y e d m a t e r i a l o v e r the s p r a y t a r g e t ( m e t a l l i z a - t ion f igure) a r e v e r y f r a g m e n t a r y and i n c o m p l e t e [2, 3]. Yet, th i s c h a r a c t e r i s t i c i s i m p o r t a n t both fo r un- d e r s t a n d i n g the p h y s i c a l p r o c e s s e s t ak ing p l a c e d u r i n g the a p p l i c a t i o n of a c o a t i n g and fo r p r a c t i c a l p u r - p o s e s , n a m e l y , c h o i c e of a s p r a y p i t c h and d e p o s i t i o n of a u n i f o r m l y th ick coa t ing . In p l a s m a s p r a y e o a t i n g , p a r t i c l e s c a t t e r i n g in the s t r e a m o b e y s G a u s s ' law of n o r m a l d i s t r i b u t i o n (F ig . 1). In the f i gu re , the ax i s of o r d i n a t e s r e p r e s e n t s the r e l a t i v e i n t e n s i t y of s p r a y i n g , I, c a l c u l a t e d f r o m the f o r m u l a :

h I -- hmax, (1)

w h e r e h i s the he igh t of a l a y e r d e p o s i t e d with both the nozz le and the s u b s t r a t e s t a t i o n a r y , a t a d i s t a n c e r (in ram) f r o m the n o z z l e ax i s and the c e n t e r of the s p r a y t a r g e t , and h m a x is the he igh t of the l a y e r d e p o s - i ted by the s a m e t echn ique in the c e n t e r of the s p r a y t a r g e t (in ram).

Thus , F ig . 1 d e p i c t s the s p r a y t a r g e t in a t r a n s v e r s e c r o s s s e c t i o n ( s een f r o m above , the t a r g e t is c i r c u l a r ) . It ha s b e e n p r o p o s e d [4] tha t the h = f ( r ) c u r v e in such i n s t a n c e s eou ld be d e s c r i b e d by the e q u a - t ion:

r ~ l

h = hma x �9 e Q", (2)

w h e r e p is the r a d i u s of s c a t t e r i n g , m m . The va lue of p can be d e t e r m i n e d , a s s u m i n g n o r m a l d i s t r i b u t i o n , a s the d i s t a n c e f r o m the s p r a y t a r g e t ax i s to the po in t w h e r e the r e l a t i v e i n t e n s i t y of s p r a y i n g is 0.368.

/o

12 8 4 o

!0

~6

363

Distance from nozzle axis, mm

F ig . 1. D i s t r i b u t i o n of NbC p o w d e r in p l a s m a s p r a y d e p o s i t i o n . A r e c u r - r e n t 290 A, s p r a y r a n g e 75 ram.

Using the concept of "radius of scattering" simplifies procedure in describing the character of a sprayed material. Moreover, the magnitude p is a characteristic of this techno- logical process. Thus, it has been demonstrated [4] that, with increasing pitch between parallel sprayed bands, the resulting nonuniformity of sprayed-layer thickness is determined by the radius of scattering (Table 1). The value of pitch recommended fo r a t t a i n i n g a u n i f o r m t h i c k n e s s i s 1 .2 -1 .3 p in manua l s p r a y - ing and 1.4 p with a u t o m a t i c t r a v e l of the n o z z l e o r the s p r a y e d p a r t . The r e s u l t i n g c o a t i n g - t h i c k n e s s v a r i a t i o n does not e x - ceed 3%.

The authors investigated the influence of technological parameters on the radius of scattering in the plasma spraying of zirconium and niobium carbides. The spraying technique, starting materials, and equipment used in this investigation were described earlier [5]. Figure 2 shows the results ob- tained. The increase in the radius of scattering with rise in arc-current intensity may be attributed to improved spray-

Institute of Materials Science, Academy of Sciences of the Ukrainian SSR. Translated from Porosh- kovaya Metallurgiya, No, i0 (82),pp. 105-109, October 1969. Original article submitted April 1, 1968.

859

Page 2: Metallization figure in plasma and detonation spray coating

T A B L E 1. N o n u n i f o r m i t y of T h i c k n e s s of S p r a y e d L a y e r . R a t i o of G r e a t e s t to L e a s t T h i c k n e s s

Pitch between ~parallel spray bands, mm

0,8 1,2 1,6 2,0 2,4

Thickness nbauniformity of sprayed layer, %

0,00 0,42 8,13

29,00 52,91

p a r t i c l e hea t i ng c ond i t i ons p r e v a i l i n g in p l a s m a j e t s of g r e a t e r power . Th i s i n c r e a s e s the n u m b e r of p a r t i c l e s on the j e t p e r i p h e r y whose t e m p e r a t u r e is su f f i c i en t fo r d e p o s i t f o r m a t i o n . Th i s e x p l a n a t i o n i s c o n f i r m e d by the w e l l - k n o w n fac t that the c o e f f i c i e n t of p o w d e r u t i l i z a t i o n in s p r a y i n g i n c r e a s e s with r i s e in p l a s m a - j e t p o w e r [6, 7].

The g rowth of p with i n c r e a s i n g r a n g e is c l e a r l y l i n k e d with the w i d e n - ing of the s p r a y - p a r t i c l e cone a s i t t r a v e l s away f r o m the nozz l e .

I t i s i n t e r e s t i n g to note that , unde r the s a m e p r o c e s s cond i t i ons , the r a d i u s of s c a t t e r i n g fo r z i r c o n i u m and n iob ium c a r b i d e s is v i r t u a l l y i d e n t i - ca l (F ig . 2), which is due to the s i m i l a r i t y of the t h e r m o p h y s i c a l c h a r a c t e r i s - t i c s of t h e s e m a t e r i a l s .

When s p r a y i n g i s p e r f o r m e d with a s y s t e m having a s m a l l e r d i s t a n c e be tween the ca thode and the p o w d e r - a d m i s s i o n poin t , the va lue of p s h a r p l y i n c r e a s e s (F ig . 2a). T h i s , too, m a y be a t t r i b u t e d to the fac t tha t i n t r o d u c i n g the p a r t i c l e s into a h o t t e r zone of the p l a s m a j e t i m p r o v e s t h e i r h e a t i n g c o n d i t i o n s .

In the de tona t ion d e p o s i t i o n of c o a t i n g s , c u r v e s of s p r a y i n g i n t e n s i t y d i s t r i b u t i o n fo r the m a t e r i a l in the t a r g e t a r e a a s s u m e a d i f f e r e n t shape (F ig . 3). T h e s e c u r v e s can be d e s c r i b e d by the equa t ion :

a (3) h = hma x a + r " '

w h e r e h is a d i m e n s i o n l e s s c o e f f i c i e n t and a is a c o e f f i c i e n t e x p r e s s e d in m m n.

The c o e f f i c i e n t s a and n can be d e t e r m i n e d by a r b i t r a r i l y d iv id ing the d i s t r i b u t i o n c u r v e into two zones

(Fig. 3) ;

Zone I, in which spray-coating intensity (I) shows practically no change with increasing distance from the target axis. Allowing for measurement error, this zone may be taken to end at a point with the abscissa ref , where the coating thickness differs by no more than 10% from the thickness at the center (ref is the eff- ective radius of spraying);

Zone If, in which the intensity of spraying sharply falls off with increasing distance from the target axis. The coordinates bounding this zone of the curve are ref and r i - a point where the coating thickness

is 10% of hma x (r t is the radius of spraying).

Taking into account these conditions and performing suitable algebraic transformations, we can find the values of a and n in a general form:

a ----- O. 11 rT; (4)

015 " 1I

9

7 o

.~ 5

g 3

I o-Ire io-N C 1,

I

5

4

--2.9101 n ~

logr I --logref

/

7 / / ~ I I

/ yC__:ef_

o 100 200 300 400 50 100 t50 Spray-target radius, mm ,4; 12 tO 8 5 4 2 0 4 5 0 10 12 N a Arc current, A b Distance from barrel axis, mm

F i g . 2. I n f luence of a r c c u r r e n t (a) and s p r a y r a n g e (b) on r a d i u s of s c a t t e r i n g in p l a s m a s p r a y - ing. S p r a y i n g with p l a s m a t r o n with d i s t a n c e f r o m ca thode to p o w d e r - a d m i s s i o n po in t of: 1) 11

m m ; 2) 27 m m .

F i g . 3. D i s t r i b u t i o n of s p r a y e d p o w d e r in de tona t ion s p r a y coa t ing . S p r a y i n g of Ni, r a n g e 90 m m ,

b a r r e l d i a m e t e r : 1) 21.5: 2) 16; 3) 12 ram.

(5)

860

Page 3: Metallization figure in plasma and detonation spray coating

3O

E 25 ~A

22

y /8 )

co 14

2 J

f

,30 50 ,90 I20 Spray range, mm

a

"Z,6 ~5

2~

?3

?2

~8 ,50

l

60 90 fgO I50 Spray range, mm

b

Fig. 4. Influence of range in detonation spray coat- ing on spray-target radius (a) and index n (b). Spray depositionofNi, barrel bore: 1) 21.5; 2) 16; 3) 12 ram.

a2 I .~- a~ ~f O,,5---~n~ I = ' v I,# 18 2z

Barrel bore, mm 24

Fig. 5. Influence of barrel bore on ratio of effective radius of spraying to barrel radius. Detonation spray deposition of Ni, spray range 60-150 mm.

As with plasma spray coating, a study was

made of the influence of process parameters in the deposition of nickel coatings by the detonation pro-

cess on the formation of the metallization figure. Spray coating was performed on a laboratory deto- nation apparatus; the apparatus operates on the principle of utilization of detonation energy in an oxygen-acetylene mixture present in a barrel with one end closed. The volume ratio of the gases was chosen so that their combustion products should form a neutral atmosphere. The particle size of the nickel powder was 10-20 p. Particles of this size were fully melted during the process, as a re- sult of which they formed a nonporous coating strongly adhering to the substrate. The weight of a powder batch was from 70 to I00 rag. Substrates were provided by polished brass plates.

In the investigation, two process parameters- barrel bore and distance from the barrel orifice to the substrate to be coated-were varied. The bar- rel bore was changed in the range 12-21.5 ram. At small diameters, heat losses are substantial, mak- ing it difficult to detonate a gas mixture in a tube [8]. Under such conditions, the thickness of the coating applied in a single discharge decreases, with increasing diameter, from 12 to 5 #, while at the same time the size of the spray-coated surface increases (see Fig. 3). The total number of dis- charges for different barrel bores was selected so as to obtain coatings 100 p thick.

The distance from the barrel muzzle to the substrate has a considerable effect on deposit quality, because it determines the residence time of the powder particles in the combustion products after their emission from the barrel and their impact velocity at the surface of the part being coated. The choice of a distance depends largely on the specific gravity of the material being sprayed, its melting point, and the substrate material. In our experiments, the spray range was varied from 60 to 150 ram.

Deposit thickness was measured on microsections under a microscope at a magnification of 485 dia- meters, using an MOV-I measuring attachment. Owing to some chipping on the outer surface of deposits, the scatter of measured values was 10%.

Figures 4 and 5 show the results obtained in this study. As can be seen from the data, with increasing spraying range, the target diameter (2ri) increases, while the effective spray-target diameter (2ref) re- mains essentially unchanged. Thus, the growth of the spray target in this instance is due to an increase of zone If, which leads to a reduction of the index n (Fig. 4b). With increasing barrel bore, both the target diameter (Fig. 4a) and the index n increase. The effective spray-target diameter also increases with in- creasing bore. It is interesting, however, that an increase is exhibited not only by its absolute value, but also by the ratio of the effective diameter to the barrel bore (Fig. 5).

The relationships established can be explained as follows. Decreasing the barrel bore increases the velocity of sprayed particles, produces turbulence in the boundary layer, and, consequently, intensifies the influence exerted by the jet drag against the external medium on the characier of spraying. This leads to a relative reduction Of the jet portion in which the material being sprayed is almost uniformly distributed and, consequently, tea less rapid drop in intensity in zone II of Fig. 3 (the index n decreases).

When the spray range is extended from 60 to 150 ram, the character of distribution in the jet portion which is not affected by interaction with the external medium remains practically unchanged (constant ref). This is due to the fact that the velocity of combustion products in detonation is, according to our measure- ments, 2-4 M and does not change in the range indicated. On the other hand, the drag of the boundary jet

861

Page 4: Metallization figure in plasma and detonation spray coating

regions becomes m o r e pronounced as the s p r a y range is extended. As a resul t , zone II becomes f la t ter (the index n diminishes) . It will be seen f rom Fig. 3 that, to produce a l aye r of uniform thickness in detonation sp ray coating, the sp r ay t a rge t d i sp lacement should exceed r e f by i/2 (r 1 - r e f ).

The authors wish to exp re s s thei r grat i tude to Academic ian G. V. Samsonov* for constant in teres t shown in this work,and to I. A. Rozenfelfd for a s s i s t ance in the mathemat ica l p roces s ing of data.

1.

2.

3.

4.

5.

6.

7.

8.

LITERATURE CITED

A. A. Appen, Heat Res i s t an t Inorganic Coatings [in Russian], Izd-vo Khimiya, Moscow-Len ingrad (1967). Yu. A. Pirogov, An Inves t iga t ion of the Forma t ion P r o c e s s e s and P r o p e r t i e s of Oxidation Res i s tan t Coatings [in Russian], Candidatefs Disser ta t ion , KhPI (1966). G. V. Bobrou and V . I . P r ivezen t sev , Svarochn. Pro izv . , No. 6 (1967). V. Nawara, Schweiss technik (DDR), 1..~3, 9 (1963). S. Yu. Shar ivker , V. G. ZilTberberg, and M. I. Olievskii , Poroshkovaya Met., No. 1, 38 (1969). D. R. Mash, N. E. Weare , and D. Z. Walker , J. Metals , 12, 7 (1961). F. I. Kitaev and A. G. Tsidulko, Poroshkovaya Met., No. 6 (1968). Ya. B. ZelVdovich and A. S. Kompanee ts , Theory of Detonation [in Russian], Moscow (1965).

* Corresponding Member , Academy of Sciences of the Ukrainian SSR.

862