Deposition of carbide and nitride based composite coating by Atmospheric Plasma Spraying

Z. Károly1, B. Cecília1, I. Mohai1, I. Sajó1, L. Boros1, J. Szépvölgyi1,2

1Institute of Materials and Environmental Chemistry, Chemical Research Center, Budapest, Hungary

2 Research Institute of Chemical and Process Engineering, University of Pannonia, Veszprém, Hungary

Abstract: SiC and Si3N4 composite coatings have been prepared by atmospheric plasma spraying (APS). The powder mixtures used for spraying composed of various oxides capable of melting beside SiC and Si3N4. Our finding was that the composite particles best for spray-ing could be made by a consecutive milling and sintering processes. Using the as-prepared powders we could prepare composite coatings, in which the particles could be prevented from oxidation and decomposition, as well. Keywords: ceramic composites, APS, silicon-nitride, silicon-carbide, SIALON

1. Introduction

SiC and Si3N4 ceramics have long been known about their excellent wear and corrosion resistance and attractive prop-erties such as high thermal conductivity, thermal shock re-sistance, strength, and hardness, which are retained even at higher temperatures [1]. Due to the abovementioned proper-ties they are frequently used as structural materials in bulk form. There is also a great demand to use them as coatings. Thermal spraying of these non-oxide ceramics, however, is prohibited as with rising temperature they decompose above 1800 °C without a liquid phase being formed [2,3]. To overcome this problem specially prepared particles were used for spraying that comprised of oxide com-pounds in some proportion that are routinely used at liq-uid phase sintering to promote the development of a liq-uid phase. As a result, a composite coating structure could be built, in which the discrete particles of non-oxide ceram-ics are being embedded in a ceramic matrix.

In this work we made attempts to prepare such composite coatings and to develop a method for making powder mix-tures suitable for spraying. We investigated two kinds of composite systems: one is a SiC-based composite with a mullite (3Al2O3x2SiO2) matrix. In the other system Si3N4 particles were transformed into SIALON and dispersed in a ceramic matrix. 2. Experimental

The starting materials for atmospheric plasma spraying were SiC and Si3N4-containing powder mixtures prepared by various methods.

The SiC containing powder was made by planetary milling of the fine powder of commercial SiC and mullite powder in a 70 to 30 weight ratio into spherical agglom-erates.

The Si3N4 containing powders were prepared in a con-secutive milling and sintering processes to reach an ap-propriate particle size and spherical grains. In addition to Si3N4, other ceramic materials including Al2O3 (12%, ALCOA A16), Y2O3 (4%, HC Starck) and AlN (15%, HC

Starck) were also used. In this way two batches of pow-ders of similar chemical composition but different mor-phology (marked as A and B) were prepared. The differ-ence in powder A and B is that at powder A the second sintering step was omitted. The aim of the second sinter-ing step is to transform the particles of powder A into harder, more favorable ones in terms of feeding by creat-ing a glassy phase on the surface. Before spraying the powders were sifted to a powder fraction of 50-125 µm.

The as-prepared composite powders were deposited into a heat resistant steel sheet. The metal sheet was pre-viously coated with metallic bond coat in around 100 µm thickness to improve adhesion. The bond powder was a NiCoCrAlY alloy composed of 360 µm size particles (Fig. 1). The metal sheet was preheated to 250-300 °C just be-fore plasma spraying.

Fig. 1 SEM image of bond powder

For APS we used a commercial plasma spray gun (Metco 9MB). The main operating conditions are summarized in Table 1. Applied power of the plasma arc during spraying was around 40 kW in each tests.

The particle size distribution of the starting powders was analyzed by laser diffraction method. Morphology of the powder and structure of the coatings was character-ized by SEM, XRD, GDO-ES techniques. Adhesion was also tested by a standard tension test.

Table 1. Operating conditions of sprayed powders I.

Powders Gases

( l·min-1)

Spray distance

[mm]

Bond coating Plasma: Ar(42) H2 (5.2) Carrier: Ar (8)

100

Mullit - SiC Plasma: Ar (40) H2 (6) Carrier: Ar (12)

70

Powder A and B Plasma: Ar (38) H2 (13) Carrier: Ar (7)

65

3. Results

Mullite-SiC nanocomposite During plasma spraying of carbide containing powder

mixture we face two risks. One is the oxidation of SiC particles on getting into contact with air at elevated tem-perature. The other one is the possible decomposition of SiC at the high temperature of the plasma flame. The in-jected powder, however, should be subjected to high temperature as the mullite, which is the matrix forming component of the composite ceramic coating melts only above 1850 °C [4]. Comparing the XRD diffractograms of the starting powder mixture and the coated substrate no considerable changes occurred. Only the characteristic peaks of mullite, alumina and SiC can be revealed on the X-ray diffractogram of the coating (Fig. 2). It means that both the oxidation and the decomposition could be pre-vented as biggest part of the plasma energy was used to melting of the mullite particles.

Fig. 2 XRD diffractogram of SiC-based coating

SEM micrograph of the cross section of the coating is shown in Fig. 2. The thickness of the coating can be esti-mated to be about 200 µm. To determine the thickness more precisely, we have made chemical analysis on the cross section of the coated substrate by different methods. EDS analyses were carried out at the spots marked with

whole numbers in Fig. 3. Evaluating the relative ratio of the chemical elements we can roughly determine the in-terface of the substrate/bond and the bond/ceramic coat-ing, which are situated at points 3 and 2, respectively. Continuous chemical analyses in respect to distance from the front of the coating could be made by GD-OES analyses. The resulted curves of the constituent chemical elements are illustrated in Fig. 4. The quickly decreasing Si line suggests that the ceramic coating was actually only 50 µm thick, while the much thicker bond layer comes to end after 140 µm. It is followed by a 40 µm thick zone in which the Ni content is decreasing with increasing Fe content. This zone was probably formed during spraying due to diffusion at the elevated temperature of the sub-strate. As oxygen could not be detected in the bond coat, oxidation of this metallic layer could be avoided. The obvious contradiction of the results of the presented two methods is possibly caused by the not uniform thickness of the coating due to manual spraying.

Fig. 3 SEM image of the cross section of SiC-based coating

Fig. 4 Concentration of the elements along the cross section of the SiC-based composite coating by GD-OES

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lit

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ullit

mul

lit

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ullit

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ullit

+AL2

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C

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Si3N4-based nanocomposites SEM micrographs of powders A and B (Fig. 5 a, b)

show that the applied procedure resulted in homogenous and large particles. At higher magnification it can be ob-served that the powder mixture sintered twice (B) is composed of hardly round particles having several edges in contrast to powder A, which was not subjected to a second sintering step. Additional work will be required before understanding of this unexpected result. According to laser diffraction size analysis, both powders (i.e. A and B) are composed of particles with size between 50-110 µm. The efficiency of the second sintering step was veri-fied as particles of powder B did not broke apart on the effect of ultrasonic agitation in contrast to powder A. Even so, powder A did not brake up during feeding, what’s more it could be fed much better than powder B.

XRD diffractograms suggest that particles are princi-pally composed of crystalline phases, including yttrium aluminum oxide (40%), corundum and β-SIALON (20-20%). Considerable oxidation of the powders could be prevented in spite of the high temperature of the fur-nace.

a

b Fig. 5 SEM images of powder A (a) and B (b)

Coatings made by plasma spraying The thickest coating was prepared using powder A (300

µm), whilst using powder B the obtained thickness was only ~100 µm. During spraying of powder B the feeding was not uniform probably due to the aforementioned un-favorable morphology, which eventually resulted in sev-eral larger non-melted agglomerates embedded in the coating.. Seemingly perfect uniform coating formed using powder A. Peaks on the XRD diffractograms (Fig. 6) cor-respond to the crystalline phases of β-SIALON, corun-dum, YAG and yttrium-oxide in an estimated amount of 30, 30, 10 and 3 wt%, respectively. In addition, a consid-erable amount of glassy phase (30%) was also present. Comparing this results with the phase composition of the starting material (Fig. 7) it can be concluded that mostly the YAG phase was vitrified while corundum and the β-SIALON retained its crystalline characteristic.

Adhesion was tested by tensile adhesion test according to standard MSZ EN ISO 4624:2003. In this test the coated sample is glued to an uncoated counterpart and pulled with increasing force in an universal testing ma-chine. In the test the two parts separated from each other at force of 3 MPa. As separation took place mostly inside the glue the real adhesion is probably higher.

Fig. 6 XRD peaks of Si3N4-based coating

Fig. 7 XRD peaks of powder mixture used for spraying

4. Summary SiC and Si3N4 containing ceramic composite

powders were investigated for plasma spraying. The powder mixture prepared from a few micrometer-sized particles by attrition and consecutive sintering could be easily fed into the plasma flame. The oxides mixed into the composite powders successfully prevented the carbide and nitride particles both from oxidation and from de-composition, as well. Uniform thickness could be achieved by an automatic spraying process.

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Microstructures, Chapman & Hall (1985) [2] S. Thiele et al, J. Thermal Spray Techn., 2, 11 (2002) [3] Hyun-Ki Kang, Suk Bong Kang, Mat. Sci & Eng A

428 (2006) [4] L. Li, Z. J. Tang, W. Y. Sun, and P. L. Wang, J. Ma-

ter. Sci. Technol. (Shenyang, People's Repub. China), 15, [5], 439-443 (1999)