Erosion protection of carbon—epoxy composites by plasma-sprayed coatings

Surface and Coatings Technology, 49 (1991) 482—488

482

Erosion protection of carbon—epoxy composites by plasma-sprayed coatings F. Alonso, I. Fagoaga and P. Oregui INASMET, Camino de Portuetxe 12. 20009 San Sebastian (Spain)

Abstract This paper deals with the production of plasma-sprayed erosion-resistant coatings on carbon-fibre—epoxy composites, and the study of their erosion behaviour. The heat sensitivity of the composite substrate requires a specific spraying procedure in order to avoid its degradation. In addition, several bonding layers were studied to allow spraying of the protective coatings. Two different functional coatings were sprayed onto an aluminium—glass bonding layer, a WC— l2Co cermet and an Al,0 1 ceramic oxide. The microstructure and properties of these coatings were studied and their erosion behaviour determined experimentally in an erosion-testing device.

1. Introduction Carbon-fibre—epoxy composites are being increasingly used in industrial components, replacing metals because of their better mechanical properties—density ratios. However, these materials present some drawbacks, particularly related to their surface properties, such as tribological behaviour and low wear resistance. Another considerable problem which carbon— epoxy composites suffer~is the lack of thermal and electrical conductivity, excluding some applications. Many of these characteristics could be improved by coating the composites using plasma spraying, as is often done on metals and alloys. Obviously, the standard procedures for the production of sprayed coatings on metallic components are not adequate for carbonfibre—epoxy composites because of the heat sensitivity and degradation of polymeric substrates. If special precautions are not taken during spraying, excessive heating can often deteriorate the composite. In order to avoid this overheating, several approaches have been proposed. Ducos [1] used intensive cooling with liquid gas jets in a controlled atmosphere chamber; other authors, such as Reardon [2], Thorpe [3] and Leclerq [4], developed special bond coatings for these types of substrates and for plastics. As a general trend, it is clear that special control of spraying parameters (distance, surface-torch velocity, angle,.. .) and the use of intensive cooling systems during spraying are required. Other authors developed coating production methods in a completely different way. First, the plasma-sprayed coatings are produced on appropriate substrates, then the coatings are impregnated with epoxy resin and the composite is laid on the coating. Finally, the substrate is removed to obtain the composite—coating system.

This technique is very useful for the production of coatings on complex shapes, for example, internal coatings in tubes [5]. Taking into consideration the industrial needs for production methods as simple and inexpensive as possible, this work has been focused on developing procedures for direct spraying of protective coatings against erosion onto carbon-fibre—epoxy composites and, therefore, on characterizing their wear—erosion behaviour. To obtain these coatings, several bonding layers were tested to select the most suitable for our application. Then, functional coatings of WC— 1 2Co and Al2 03 were sprayed and their microstructure and erosion behaviour studied. All the coatings were produced by commercial atmospheric plasma spraying equipment, with a compressed-air cooling system.

2. Experimental details Eight different possible bonding coats were sprayed and tested for bond strength. Materials and powder characteristics are given in Table 1. Small carbon-fibre— epoxy plates 25 x 25 x 2 mm were used as substrate specimens, being slightly grit blasted (SABLUX device, 8 mm diameter nozzle, 3 bar pressure, 30_600 angle) with white corundum (size FEPA 46) to roughen the surface before spraying. The plasma spraying equipment was a commercial Plasma-Technik M- 1000 system with an F-4 torch. Five air jets were used to control the substrate temperature during spraying, two corresponding to gun-attached nozzles (Silvent 511 BSP) and three to stationary nozzles (Silvent 210 BSP), all of them working at a pressure of 6 bar. After several optimization trials, one set of spraying parameters was selected

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TABLE I. Materials and sprayed powders

energy. The measured particle feed rate was 2.1 g s

for

Coating

Composition

Si02 and 3.2 g s~ for the corundum abrasive. Five specimens were tested under each set of conditions; each

2” 3” 4” 5” 6” 7 8 9 10

Al(45 jim)—10 wt.%glass( 50 jim) blend Al(45 jim) —20 wt.%glass( 50 jim) blend Al(45 jim) —35 wt.%glass( 50 pm) blend Al(45 jim)— 10 wt.%glass( 100 jim) blend Al(45 pm)—20 wt.%glass( 100 jim) blend Al(45 jim) —35 wt.%glass( 100 jim) blend AMDRY 2010 AISi(l50jim)—40 polyester(125pm) METCO 625 stainless-steel—plastic Plasmatex 1003 Al201(25 ±5 jim) Amperit 519.3 WC— l2Co(45 ±4.5 jim)

—‘

specimen was cleaned and weighed before and after testing. The surface aspect of the eroded coatings was analysed by SEM after each exposure. 3. Results and discussion 3.1. Microstructure of coatings The cross-section microstructures of the coatings, shown in Fig. I, consist of a first layer of aluminium —

glass, and an upper functional coating of ceramic oxide or metal carbide presenting the usual lamellar structure of the sprayed coatings with some porosity. All attempts to spray functional coatings directly onto epoxy composites under atmospheric conditions failed because of the high temperature flux involved in the spraying of these materials, which produced systematic substrate degradation. This forced the introduction of a first layer that could be sprayed with low energy. By means of this step, substrate overheating was avoided and a barrier was created as protection from the heat flux related to the spraying of ceramics or carbides. Although low melting metals (aluminium, zinc,...) appear as candidate materials, they suffer from excessive thermal expansion mismatch with the substrate, which leads to spalling or rimming during the spraying of the second layer. To overcome this problem, glass particles were mixed with aluminium, yielding sprayed coatings with a composite structure resulting from aluminium (clear area) and fused glass (dark area) lamellar splats (Figs. 1(A), 1(C)). The glass particles in the aluminium matrix reduce the coefficient of thermal expansion and the heat conduction of the coating, making possible the spray deposition of ceramic or carbide layers.

“Plasmatex 1310 aluminium (45 jim) with glass FilliteTM; commercial composition 27%—33% Al,03, 55%—65% SiO,, less than 4% Fe203 density less than 0.7 g cm ~, coefficient of thermal expansion (CTE), 8 x lO_6 K thermal conductivity 0.09Wm~ K~.

and the coatings produced (the plasma spraying parameters included in Table 2). All coatings were metallographically prepared and their microstructures evaluated. The bond strengths of these coatings were measured according to ASTM C-633 standard, using an INSTRON 6025 universal testing machine. Several analytical techniques, for example X-ray diffraction (Siemens D-500) and scanning electron microscopy (SEM) (JEOL 8600), were used in this work. To determine the erosion behaviour, flat specimens were coated with a bond coat (Al— 10 glass) and an A1203 (—25 + 5 jim) or WC—l2Co (—45 + 5 jim) functional coating. They were exposed, at room temperature, to an abrasive stream of particles from an erosion testing device. Angular Si02 (—595 + 350 j.tm) and corundum (—450 + 300 jim) were used as abrasive particles, with three different impact angles (30°,60°and 90°)and two gas pressures (0.20 and 0.25 MPa), to evaluate the effect of abrasive media, impact angle and particle kinetic

TABLE 2. Plasma spray parameters (Plasma-Technik M-l000/F-4 system) Parameter

Ar (standard I mm ‘) H2 (standard I min~) He (standard I min’) Current (A) Power (kW) Carrier gas (standard I mm ~) Powder feed (gmin~) Injector diameter (mm) Injector distance (mm)/angle (deg) Spray distance (mm) Traverse speed (mm s~) Rotational speed (rev min~)” “Cylinder diameter, 250 mm.

Aluminium—glass (1—6)

Al—Si— polyester (7)

40

70

10

8

Stainless steel—plastic 8 50

WC—l2Co 10 47

—

—

150 720 50 4.5 35 1.8 6/75 170 50 85

—

—

—

500 32 4 8 2 6/+15 170 50 50

450 28.5 4 10 2 6/+15 170 50 50

650 27 4 25 1.8 6/90 170 50 50

A1203 10 41

14 —

600 41 3.4 27 1.8 6/90 170 50 50

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C Fig. I. Micrographs of some plasma sprayed coatings on carbon-fibre—epoxy composites: (A) Al I0wt.-oglass/Al.O~:(B) AISi—polyester/ Al,O~—TiO,: (C) Al— lOwt.%glass/WC—Co; (D) stainless-steel—plastic/Al,0 3 —TiO.,.

20

l~10GIass(-50~tm) 2.AI-20GI~s(-5Ojsm)

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them have a better affinity to polymeric substrates than the aluminium—glass powders, and the bonding to carbon—epoxy composites was improved (Fig. 2). Their microstructure, as shown in Fig. 1(B), consists

5

6

7

8

Coating composition Fig. 2. Adherence strength of the plasma-sprayed coatings on carbon-fibre—epoxy substrates,

Using this approach, other candidate materials were tested for this first coating. Specifically, two commercial powders were employed, an abradable A1Si— polyester and a stainless-steel—plastic powder. Both of

of a mixture of metallic and polymeric spread partides. Although these coatings were sprayed onto the composites with optimum results, they failed when the second layer was sprayed over them, particularly if this coating was A1203 or WC—Co. The polymeric component of these coatings was damaged by the heat and impingement of the melted particles owing to the high energy required to spray Al2 03 and WC—Co materials. In Fig. 1(D) the microstructure of a damaged coating with voids as a result of plastic losses is shown. However, when lower melting point materials, for example metals or even Al2 03 —Ti02 were used, some of these coatings could support the spraying procedure. For example, Fig. 1(B) shows an alternative way of producing Al203 or WC—Co coatings involving the use of a double intermediate coating of metal—polymer and Al2 03 —40TiO2 with the upper functional coating on the top [6].

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Functional coatings of ceramic oxides or carbides were sprayed over these first aluminium—glass layers without apparent substrate degradation. To do this it was necessary to modify the standard spraying parameters, introducing longer spraying distances (170 mm) and intensive air cooling. This change probably has a remarkable influence on the microstructure and mechanical properties of the deposited coatings. For example, the microstructure, as is usual in sprayed coatings, is made of overlapped splats but in this case leads to an increased degree of porosity. From X-ray studies, the structure of Al203 coatings mainly consists of the expected y-Al203, a metastable high temperature phase resulting from the high solidification rate experienced by the sprayed particles, with some c~-Al203remaining from the initial powder. The WC—Co coating shows basically the hexagonal WC phase, and small peaks of W2C, WC1 and Co—W—C phases produced during the spraying (Fig. 3). 3.2. Mechanical properties The measured adherence strength of the bond coats is shown in Fig. 2. The aluminium—glass system presents the highest strength (11.2 MPa) when a mixture of aluminium with 10 wt.% glass of the larger size (100 jim) is used. As a general trend, independently of glass percentage, glass particles of larger sizes gave better adherence results. The A1Si—polyester and the

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Fig. 4. Micrograph of indentation showing development of cracks.

stainless-steel—plastic powders reached higher values (15.4 and 16.3 MPa) than the aluminium—glass system, probably owing to some chemical bonding between polymeric substrate and coating. The cohesiveness of the functional coatings was checked by means of hardness indentations. Figure 4 shows an Al2 03 coating with a network of cracks between lamellar boundaries originating at the indentation, indicating the low cohesive strength of the splats. This fact is related to the spraying conditions used to produce the coatings. The long spraying distance means that the particles have lower velocities and lower ternperatures at the moment of impact with the substrate, affecting the cohesive strength as the mechanical interfitting of the splats is reduced. 3.3. Erosion behaviour The literature on erosion testing shows that the removal of material resulting from the impact of hard particles varies exponentially with the kinetic energy of these abrasive particles, and the mechanism of this removal depends basically on the nature of the eroded .

surface. Brittle materials, such as ceramics, develop extensive microcracking during the impingement, and a fracture mechanism is finally responsible for erosion. Metals and ductile materials suffer from a micromachining process at the impact of the abrasives, with ploughing and cutting, together with a combination of forging and extrusion of distressed platelets [7]. In fact, the principal factor controlling the erosion rate appears

J~~JJJL,Jto be the impingement angle. For brittle behaviour the

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Fig. 3. X-Ray diffraction patterns of (A) Al201 and (B) WC—Co coatings,

erosion rate increases continuously with the angle to a maximum at 90°,while the ductile mode is characterized by the maximum erosion rate at angles between 15° and 30° [8]. When the eroded surface is a sprayed coating, most authors report a general brittle tendency

486

F. Alonso ci

a!. / Protection of carbon—epo vi’

omposites

250 00 200

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WC-Co/ O.2MPa A1203 / 0.2MPa WC-Co/ 0.25MPa

_____________________________________________

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_____

Impact angle 00 (a)

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Impact angle 00 (b) Fig. 5. Comparative erosion rates of Al,0 3 and WC—Co sprayed coatings: (a) effect of pressure or kinetic energy of the abrasive; (b) effect of the type of abrasive particle.

for carbides and ceramics, but the erosion mechanism is affected by other factors such as porosity or cohesive strength [91. The experimental results are summarized in Fig. 5. It should be noted that the alumina coatings seem to show a brittle behaviour, with the maximum erosion rate at a 90°angle of impingement. At low gas pressure, with abrasive particles of low kinetic energy, the Al2 03 coatings show a lower erosion rate, practically constant between 60°and 90°.This could suggest that the mechanism of debris production at low velocities of impingement consists of a separation between splats at their boundaries, rather than their fracture. Figure 6(b) corresponds to an erosion test at 60° and shows the microsection of an eroded alumina surface. It is possible to recognize the presence of cracks at the splat boundaries which might be responsible for the erosion process. When fracture and chipping become the first concern, as occurs at higher pressures, a typical brittle erosion process takes place. The surface appearance of the alumina coating after the erosion test at 90° angle of

(b) Fig. 6. Cracked structure of the upper section of the coatings after erosion tests: (a) WC—l2Co and (b) Al,O~. -

impingement, is shown in Fig. 7(B). It clearly illustrates the predominant brittle fracture mechanism. The erosion rates of A1203 coatings depend on the size and nature of the erosive particles (Fig. 5(b)). Different removal rates are obtained employing corundum or Si02 particles, which increase when particles of bigger size are used. The Si02 particles show a greater erosion rate, mainly owing to their larger size. However, when using corundum abrasives the erosion rate increases continuously with the angle of impingement, which is more typical of brittle behaviour, and it is related to the higher hardness and density values of these abrasives. The WC Co coatings have a distinctly different appearance after erosive impingement. In fact these coatings are composed of hard carbide particles in a ductile —

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corundum abrasive particles produce the same tendencies on the WC—Co coatings as those discussed above on the alumina coatings, increasing the removal rate with the impingement angle in a brittle manner. As a general trend, it is recognized that the erosion performance of thermally sprayed coatings depends on microstructural features such as porosity or unmelted particles present in the coating, but it also depends on the mechanical characteristics, particularly on the cohesive strength between the sprayed splats [10, 111. Obviously, the spraying parameters used do not permit optimum coating qualities and they are responsible for the porosity and low cohesiveness in the coating. It seems necessary to seek for further improvements of both factors using additional methods, such as impregnation with resins or plating techniques.

~_A

4. Conclusions (1) The spraying of aluminium—glass mixtures onto carbon-fibre—epoxy composites allows the subsequent direct spraying of Al2 03 and WC—Co coatings with no

~

Fig. 7. Surface of the plasma-sprayed coatings after the erosion tests: (A) WC—l2Co, (B) A1203.

degradation of the composite substrate. (2) The erosion tests performed on A1203 and WC— Co coatings deposited onto composites show a basically brittle behaviour. However, the clear differences observed between both materials suggest that, while Al2 03 coatings perform in a typical brittle way, WC—Co coatings present some degree of ductility. This is especially clear for low impact energies, but becomes hidden by a brittle mechanism as the impact energies increase. (3) The erosion performance of sprayed coatings depends on their microstructure and mechanical properties, which in turn depend on the spraying conditions. As the carbon—epoxy characteristics limit the spraying

parameters, further studies on additional techniques metallic matrix of cobalt, which produces tougher be- (impregnation, plating,...) for improving the quality haviour. Figure 7(A) shows the eroded surface, with of the coating should be carried out. scars that suggest some degree of ductility. The erosion rates at different angles of impingement is given in Fig. 5(a). It seems that at low pressures the erosion process Acknowledgment meets the characteristics of ductile behaviour, with the maximum removal at 30°, but with relatively high The authors gratefully acknowledge the support for values at 60° and 90°. When the gas pressure is in- this work from the Basque Government. creased, the velocity of the erosive particles is incremented, and more brittle behaviour is observed. The removal rate increases slowly with the angle of impact, References reaching the maximum at 90°. Examinations of the cross-section of the bottom layer after testing (Fig. I M. P. Ducos, Thermal Spraying, ITSC ‘89, Vol. 1, The Welding 6(a)), revealed cracks and fracture processes that Inst., Abington, 1989, p. 66. confirm the cited brittle behaviour. The influence of the 2 J. D. Reardon, Thermal Spray Coatings, New Materials, Processes and Applications, American Society for Metals, Metals Park, OH, type of impacting particles appears to be important. 1989 p 27. The higher values of erosion rate were reached with 3 M. L. Thorpe, Advances in Coating Technology, American Society silica particles owing to their bigger sizes. However, the for Metals, Metals Park, OH, 1988, p. 93.

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4 G. Leclerq, U.S. Patent 3.892,883. 1975. 5 E. Lugscheider, Proc. 1st Plasma-Technik-Symp., 1988 Vol. 1, Plasma-Technik AG, Wohlen, p. 23. 6 F. Alonso, I. Fagoaga and P. Oregui, ASM Heat Treatment and Surface Engineering Conf., 1991, to be published. 7 M. L. Taylor, J. G. Murphy and H. W. King, Advances in Coaling Technology, American Society for Metals, Metals Park, OH, 1988, p. 143.

of carbon —epoxy composites 8 J. J. Wert, D. M. Baker and T. M. McKechnie, Surf Coat. Technol., 33 (1987) 245—265. 9 H. E. Eaton and R. C. Novak, Surf Coat. Technol., 30 (1987) 41 —50. 10 J. A. Sue and R. C. Tucker, Surf Coat. Technol., 32 (1987) 237—248. II A. V. Levy, Surf Coat. Technol., 36 (1988) 387—406.