Deposition of carbide and nitride based composite coating by ...

Deposition of carbide and nitride based composite coating by AtmosphericPlasma SprayingZ. Károly 1 , B. Cecília 1 , I. Mohai 1 , I. Sajó 1 , L. Boros 1 , J. Szépvölgyi 1,21 Institute of Materials and Environmental Chemistry, Chemical Research Center, Budapest, Hungary2 Research Institute of Chemical and Process Engineering, University of Pannonia, Veszprém, HungaryAbstract: SiC and Si 3 N 4 composite coatings have been prepared by atmospheric plasmaspraying (APS). The powder mixtures used for spraying composed of various oxides capableof melting beside SiC and Si 3 N 4 . Our finding was that the composite particles best for sprayingcould be made by a consecutive milling and sintering processes. Using the as-preparedpowders we could prepare composite coatings, in which the particles could be prevented fromoxidation and decomposition, as well.Keywords: ceramic composites, APS, silicon-nitride, silicon-carbide, SIALON1. IntroductionSiC and Si3N4 ceramics have long been known about theirexcellent wear and corrosion resistance and attractive propertiessuch as high thermal conductivity, thermal shock resistance,strength, and hardness, which are retained even athigher temperatures [1]. Due to the abovementioned propertiesthey are frequently used as structural materials in bulkform. There is also a great demand to use them as coatings.Thermal spraying of these non-oxide ceramics, however, isprohibited as with rising temperature they decomposeabove 1800 °C without a liquid phase being formed [2,3].To overcome this problem specially prepared particleswere used for spraying that comprised of oxide compoundsin some proportion that are routinely used at liquidphase sintering to promote the development of a liquidphase. As a result, a composite coating structure couldbe built, in which the discrete particles of non-oxide ceramicsare being embedded in a ceramic matrix.In this work we made attempts to prepare such compositecoatings and to develop a method for making powder mixturessuitable for spraying. We investigated two kinds ofcomposite systems: one is a SiC-based composite with amullite (3Al 2 O 3 x2SiO 2 ) matrix. In the other system Si 3 N 4particles were transformed into SIALON and dispersed in aceramic matrix.2. ExperimentalThe starting materials for atmospheric plasma sprayingwere SiC and Si 3 N 4 -containing powder mixtures preparedby various methods.The SiC containing powder was made by planetarymilling of the fine powder of commercial SiC and mullitepowder in a 70 to 30 weight ratio into spherical agglomerates.The Si 3 N 4 containing powders were prepared in a consecutivemilling and sintering processes to reach an appropriateparticle size and spherical grains. In addition toSi 3 N 4 , other ceramic materials including Al 2 O 3 (12%,ALCOA A16), Y 2 O 3 (4%, HC Starck) and AlN (15%, HCStarck) were also used. In this way two batches of powdersof similar chemical composition but different morphology(marked as A and B) were prepared. The differencein powder A and B is that at powder A the secondsintering step was omitted. The aim of the second sinteringstep is to transform the particles of powder A intoharder, more favorable ones in terms of feeding by creatinga glassy phase on the surface. Before spraying thepowders were sifted to a powder fraction of 50-125 µm.The as-prepared composite powders were depositedinto a heat resistant steel sheet. The metal sheet was previouslycoated with metallic bond coat in around 100 µmthickness to improve adhesion. The bond powder was aNiCoCrAlY alloy composed of 360 µm size particles (Fig.1). The metal sheet was preheated to 250-300 °C just beforeplasma spraying.Fig. 1 SEM image of bond powderFor APS we used a commercial plasma spray gun (Metco9MB). The main operating conditions are summarized inTable 1. Applied power of the plasma arc during sprayingwas around 40 kW in each tests.The particle size distribution of the starting powderswas analyzed by laser diffraction method. Morphology ofthe powder and structure of the coatings was characterizedby SEM, XRD, GDO-ES techniques. Adhesion wasalso tested by a standard tension test.

Table 1. Operating conditions of sprayed powders I.PowdersBond coatingMullit - SiCPowder A and BGases( l·min -1 )Plasma: Ar(42)H 2 (5.2)Carrier: Ar (8)Plasma: Ar (40)H 2 (6)Carrier: Ar (12)Plasma: Ar (38)H 2 (13)Carrier: Ar (7)Spraydistance[mm]1003. ResultsMullite-SiC nanocompositeDuring plasma spraying of carbide containing powdermixture we face two risks. One is the oxidation of SiCparticles on getting into contact with air at elevated temperature.The other one is the possible decomposition ofSiC at the high temperature of the plasma flame. The injectedpowder, however, should be subjected to hightemperature as the mullite, which is the matrix formingcomponent of the composite ceramic coating melts onlyabove 1850 °C [4]. Comparing the XRD diffractogramsof the starting powder mixture and the coated substrate noconsiderable changes occurred. Only the characteristicpeaks of mullite, alumina and SiC can be revealed on theX-ray diffractogram of the coating (Fig. 2). It means thatboth the oxidation and the decomposition could be preventedas biggest part of the plasma energy was used tomelting of the mullite particles.7065whole numbers in Fig. 3. Evaluating the relative ratio ofthe chemical elements we can roughly determine the interfaceof the substrate/bond and the bond/ceramic coating,which are situated at points 3 and 2, respectively.Continuous chemical analyses in respect to distance fromthe front of the coating could be made by GD-OESanalyses. The resulted curves of the constituent chemicalelements are illustrated in Fig. 4. The quickly decreasingSi line suggests that the ceramic coating was actually only50 µm thick, while the much thicker bond layer comes toend after 140 µm. It is followed by a 40 µm thick zone inwhich the Ni content is decreasing with increasing Fecontent. This zone was probably formed during sprayingdue to diffusion at the elevated temperature of the substrate.As oxygen could not be detected in the bond coat,oxidation of this metallic layer could be avoided. Theobvious contradiction of the results of the presented twomethods is possibly caused by the not uniform thicknessof the coating due to manual spraying.mullitmullitmullit+SiCSiCFig. 3 SEM image of the cross section of SiC-basedcoatingSiC+ mullitmullitmullitmullitmullitmullitMullit+AL2O3mullitmullitmullitmullitSiCmullitAL3NiAl3NiFig. 2 XRD diffractogram of SiC-based coatingSEM micrograph of the cross section of the coating isshown in Fig. 2. The thickness of the coating can be estimatedto be about 200 µm. To determine the thicknessmore precisely, we have made chemical analysis on thecross section of the coated substrate by different methods.EDS analyses were carried out at the spots marked withFig. 4 Concentration of the elements along the crosssection of the SiC-based composite coating by GD-OES

Si 3 N 4 -based nanocompositesSEM micrographs of powders A and B (Fig. 5 a, b)show that the applied procedure resulted in homogenousand large particles. At higher magnification it can be observedthat the powder mixture sintered twice (B) iscomposed of hardly round particles having several edgesin contrast to powder A, which was not subjected to asecond sintering step. Additional work will be requiredbefore understanding of this unexpected result. Accordingto laser diffraction size analysis, both powders (i.e. A andB) are composed of particles with size between 50-110µm. The efficiency of the second sintering step was verifiedas particles of powder B did not broke apart on theeffect 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 principallycomposed of crystalline phases, including yttriumaluminum oxide (40%), corundum and β-SIALON(20-20%). Considerable oxidation of the powders couldbe prevented in spite of the high temperature of the furnace.Coatings made by plasma sprayingThe thickest coating was prepared using powder A (300µm), whilst using powder B the obtained thickness wasonly ~100 µm. During spraying of powder B the feedingwas not uniform probably due to the aforementioned unfavorablemorphology, which eventually resulted in severallarger non-melted agglomerates embedded in thecoating.. Seemingly perfect uniform coating formed usingpowder A. Peaks on the XRD diffractograms (Fig. 6) correspondto the crystalline phases of β-SIALON, corundum,YAG and yttrium-oxide in an estimated amount of30, 30, 10 and 3 wt%, respectively. In addition, a considerableamount of glassy phase (30%) was also present.Comparing this results with the phase composition of thestarting material (Fig. 7) it can be concluded that mostlythe YAG phase was vitrified while corundum and theβ-SIALON retained its crystalline characteristic.Adhesion was tested by tensile adhesion test accordingto standard MSZ EN ISO 4624:2003. In this test thecoated sample is glued to an uncoated counterpart andpulled with increasing force in an universal testing machine.In the test the two parts separated from each otherat force of 3 MPa. As separation took place mostly insidethe glue the real adhesion is probably higher.aFig. 6 XRD peaks of Si 3 N 4 -based coatingbFig. 5 SEM images of powder A (a) and B (b)Fig. 7 XRD peaks of powder mixture used for spraying

4. SummarySiC and Si 3 N 4 containing ceramic compositepowders were investigated for plasma spraying. Thepowder mixture prepared from a few micrometer-sizedparticles by attrition and consecutive sintering could beeasily fed into the plasma flame. The oxides mixed intothe composite powders successfully prevented the carbideand nitride particles both from oxidation and from decomposition,as well. Uniform thickness could beachieved by an automatic spraying process.References[1] William E. Lee, W. Mark Rainforth, CeramicMicrostructures, 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 A428 (2006)[4] L. Li, Z. J. Tang, W. Y. Sun, and P. L. Wang, J. Mater.Sci. Technol. (Shenyang, People's Repub. China),15, [5], 439-443 (1999)