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High temperature oxidation behavior of HVOF-sprayed unreinforced and reinforced molybdenum disilicide powders
Surface and Coatings Technology 146 – 147 (2001) 19–26
High temperature oxidation behavior of HVOF-sprayed unreinforced and reinforced molybdenum disilicide powders a ¨ Guido Reisela,*, Bernhard Wielagea, Siegfried Steinhauser , Ingrid Morgenthalb, Roland Schollb a
Institute of Composite Materials and Surface Technology, Technical University Chemnitz, Erfenschlager Straße 73, 09107 Chemnitz, Germany b Fraunhofer-Institute for Applied Materials Research (IFAM), Division for Powder Metallurgy and Composite Materials in Dresden, Winterbergstraße 28, 01277 Dresden, Germany
Abstract Intermetallics like silicides are useful for protective coatings against high-temperature corrosion. Especially molybdenum disilicide which has a great potential as protective coating e.g. in aircraft engines and gas turbines in the temperature range between 1400 and 18008C due to its high melting point and its low brittle-ductile transition temperature of approximately 800–11008C. Four types of coatings were produced by high velocity oxyfuel spraying (HVOF): unreinforced MoSi2 with low porosity, unreinforced MoSi2 with high porosity, with silicon carbide reinforced MoSi2 and with alumina reinforced MoSi2. The coatings as sprayed were characterized by XRD, SEM and EDX. Microhardness and porosity were measured. The oxidation behavior of the coatings was determined at 500, 1000 and 15008C. The influence of the heating rate was investigated during oxidation tests at 10008C. The tests at 5008C showed that the pesting depends on the porosity of the coating. SiC as reinforcing phase seems to accelerate pesting, while alumina reduces this reaction. Unreinforced MoSi2 coatings form a protective SiO2 layer on the surface with a thickness below 10 mm during oxidation at 15008C. The layer seems to be glassy with cristobalite inclusions. The microstructure of the coating changes to a high crystalline two-phase system of a-MoSi2 and hexagonal Mo5Si3. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: High temperature; HVOF-sprayed; Molybdenum disilicide; Powders
1. Introduction Monolithic molybdenum disilicide (MoSi2) is a possible structural material for high-temperature applications like furnace components or aircraft turbine engines due to its high melting point of 20308C combined with its moderate density (6.25 g cmy3) which is much lower than the density of other high temperature structural materials like nickel based alloys, tungsten or niobium. Furthermore, molybdenum disilicide shows an excellent high temperature oxidation resistance, a low coefficient of thermal expansion, a good thermal as well as a good electrical conductivity. Also the brittle-ductile transition temperature of approximately 800–11008C is relatively * Corresponding author. Tel.: q49-371-531-5391; fax: q49-371531-6179. E-mail address: [email protected] (G. Reisel).
low, so that MoSi2 shows at temperatures above 12008C, an acceptable deformation behavior caused by its high toughness w1–4x. Today the main application of molybdenum disilicide is as heating elements for electric heated furnaces w5,6x Unfortunately, there are some disadvantages of monolithic molybdenum disilicide like the low fracture toughness especially at room temperature and the low creep resistance at temperatures above the brittle-ductile transition which hinder its wide use today. For some possible applications, it is necessary to reinforce MoSi2 with materials like silicon carbide or alumina, e.g. MoSi2 is chemically compatible with SiC up to highest temperatures w7–9x. Another great problem is the socalled pesting of MoSi2 at temperatures between 300 and 6008C, which can lead to the disintegration of the bulk material into powder. In the case of monolithic
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 6 4 - 0
G. Reisel et al. / Surface and Coatings Technology 146 – 147 (2001) 19–26
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Table 1 Experimental design of oxidation tests Route
Starting temp. (8C)
Heating rate (Kymin)
Working temp. (8C)
Holding time at working temp. (h)
1 2
20 20
500 1000
100 1
3 4
20 20
50 1, 5, 10, 20, 30, 40, 50 50 50
1000 1500
60 28
MoSi2, high porosity is named as the main reason for the pesting w10,11x. MoSi2 can be used as coating onto various substrates, e.g. as a protective layer against corrosion. The coatings are produced normally by plasma spraying. In the case of vacuum plasma spraying (VPS), the coatings exhibit a low porosity as well as low oxygen content, but no protective silicon oxide layer is formed on the coatings surface. This layer is known as one possible method to avoid pesting in the case of molybdenum disilicide produced as bulk material. If atmospheric plasma spraying (APS) is used the coatings show a relatively high porosity and a high oxygen content. In both technologies after spraying Mo5Si3 was found w12–15x. High velocity oxyfuel spraying (HVOF) could be an alternative thermal spraying method to produce molybdenum disilicide coatings. In the present paper, the oxidation behavior of HVOF sprayed unreinforced and with silicon carbide or alumina reinforced MoSi2 is examined at different temperatures. For evaluating the coatings resistance against pesting a temperature of 5008C was chosen. Furthermore, tests were taken out at 1000 and 15008C. 2. Experimental details 2.1. Coatings processing The processing routes of the unreinforced molybdenum disilicide powder as well as with 20 vol.% silicon carbide reinforced MoSi2 by high energy milling followed by heat treatment and gentle milling were described in previous works w16,17x. With 15 vol.% alumina reinforced MoSi2 powder was also produced by high energy milling in a similar way. The powders were sprayed by high velocity oxyfuel spraying (HVOF) onto a polished carbon steel substrate. Caused by the smooth surface and the different thermal coefficients of expansion the coatings did not adhere on the substrate, so that pure coatings without a substrate could be produced for the following oxidation examinations. To ascertain the influence of the coatings porosity on the oxidation behavior in the case of unreinforced MoSi2 a coating with a low and a coating with a high porosity were
sprayed. Furthermore, one silicon carbide and one alumina-reinforced coating were investigated. The HVOF system used was the Diamond Jet from Sulzer Metco, USA. 2.2. Thermogravimetric measurements (TG) For the oxidation tests the simultaneous thermogravimetric equipment STA409C, Netzsch, Germany, was used. For accelerated oxidation compressed air with a rate of 100 ccmymin flowed through the furnace. Four types of tests with different temperature-time-regimes which are given in Table 1 were carried out. The temperature of 5008C was chosen to investigate the oxidation behavior of the coatings at low temperatures. 15008C were necessary to examine the high temperature oxidation while 10008C is an often used working temperature in the case of nickel-based materials which are possible candidates to be substituted by molybdenum disilicide. Also, the influence of the heating rate on the oxidation is evaluated by thermogravimetry. 2.3. Scanning electron microscopy (SEM) Scanning electron microscopy was used to examine the coatings as fractured and as polished cross-sections with the aim to characterize their microstructure. The samples were investigated in the scanning electron microscope JSM-840, Jeol, Japan, as well as in a microscope of the type 1455 VP, LEO, Germany. 2.4. Energy dispersive X-ray analysis (EDX) Phase compositions of the coatings are executed at polished cross-sections by the energy dispersive X-ray analysis system EDISON from Getac, Germany. It was used to identify phases inside the coatings. 2.5. X-Ray diffraction analysis (XRD) To determine the microstructure and the phase composition of the powders and coatings surfaces XRD was taken out by an equipment of the type D5000, Siemens, Germany. Cu-radiation was used, the analyzed range of
G. Reisel et al. / Surface and Coatings Technology 146 – 147 (2001) 19–26
21
Fig. 1. SEM investigations of the coatings as sprayed w(a) MoSi2 LP with low porosity; (b) MoSi2 HP with high porosity; (c) MoSi2-SiC coating; (d) MoSi2-Al2O3 coatingx.
the diffraction angle 2u was between 108 and 1008 by a step width of 0.028. 2.6. Porosity The porosity of the coatings before oxidation test (‘as sprayed’) was measured by an image analysis system on polished cross-sections. The system used was Quantimet 570C, Leica, Germany. The determination works due to the principle of gray value analysis. For each sample 10 measurements were carried out. 2.7. Vickers microhardness Microhardness measurements were made on polished cross-sections of the coatings. An instrument of the type Micromet1, Buehler Ltd., USA, with a diamond of Vickers geometry was used. The load of 0.5 N was applied 15 s on the sample and the Vickers microhardness HV0.05 determined. Ten tests were taken out on each coating.
3. Results and discussion 3.1. Characterization of the powders and the coatings before oxidation XRD of the unreinforced MoSi2 powder showed that only tetragonal MoSi2 could be found. The average diameter of the powder was 6.25 mm and the oxygen content 1.42% (determined by hot gas extrusion). The MoSi2-20 vol.% SiC powder showed as the main phase MoSi2 (tetragonal). Additional phases were SiC and tetragonal Mo5Si3 with a very small amount. The average diameter of the SiC particles was 11 mm and of the whole powder 10.25 mm, 0.82% oxygen was measured. The MoSi2-15 vol.% Al2O3 powder was composed of tetragonal MoSi2 as main phase, Mo5Si3 and alumina as corundum. The diffraction pattern of this powder is shown in Fig. 4. The average diameter of the reinforcing alumina particles was 1 mm and of the composite powder 3.46 mm. SEM pictures of the four examined coatings assprayed are given in Fig. 1, while Table 2 shows some
Table 2 Coating properties ‘as sprayed’ (LP: Low Porosity, HP: High Porosity) Coating
Phase composition
Porosity w%x
Microhardness HV0.05
MoSi2 LP MoSi2 HP MoSi2-SiC MoSi2-Al2O3
a-MoSi2, b-MoSi2, Mo5Si3 (tetr.), Mo a-MoSi2, b-MoSi2, Mo5Si3 (hex.) a-MoSi2, b-MoSi2, Mo5Si3 (tetr.), SiC a-MoSi2, b-MoSi2, Mo5Si3 (tetr. And hex.), Mo, Al2O3
2.05 20.60 4.34 6.01
1074 825 804 813
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G. Reisel et al. / Surface and Coatings Technology 146 – 147 (2001) 19–26
Fig. 2. TG curves of various coatings during oxidation at 5008C.
characteristics of these coatings. The MoSi2 coating with low porosity (MoSi2 LP) is shown in Fig. 1a, while in Fig. 1b the MoSi2 coating with high porosity (MoSi2 HP) is visible. The higher porosity level of the right coating is clearly detectable. Also it is recognizable, that one phase which is detected with EDX as MoSi2 has the greatest part in the microstructure in both MoSi2 coatings. MoSi2 in its tetragonal (a-) modification with a C11b structure was also detected as main phase by XRD, but another MoSi2 phase was found too. It is the high temperature modification (b-modification) of molybdenum disilicide with a hexagonal lattice and a C40 structure. Its equilibrium is above 19008C. Caused by the rapid cooling of the layer during and after spraying the hexagonal phase exists in the as-sprayed coatings. This phase was also detected in vacuum plasma sprayed MoSi2-coatings w18x. In comparison with MoSi2 HP (its XRD-pattern is shown in Fig. 9) the MoSi2 coating with low porosity exhibits much more MoSi2 in its b-modification, little molybdenum and little Mo5Si3 with a tetragonal lattice, while in MoSi2 HP only little Mo5Si3 with a hexagonal lattice and no molybdenum was found. A possible reason for this phenomenon could be the different spraying parameters, so that the coating with higher porosity cooled down slower and more MoSi2 of the b-modification was converted into the a-modification while caused by these different temperatures during spraying the silicon loss was higher in the case of MoSi2 with low porosity so that molybdenum could be found. The picture of the MoSi2-SiC coating (Fig. 1c) shows clearly the silicon carbide particles, which are embedded in the matrix without great pores on the boundary. The coating is dense and only one phase is recognizable in the matrix, which is determined to MoSi2 by EDX. XRD confirmed this result. The main phase in the coating is a-MoSi2. Only very small amounts of b-
MoSi2 and tetragonal Mo5Si3 were found. The silicon carbide content was measured with image analysis and below 2%. Because of the small grain size, no Al2O3-particles are visible in the SEM picture of the MoSi2-Al2O3 coating (Fig. 1d). With EDX measurement on the whole coating area of the polished cross-section the alumina content was determined to be approximately 3%. In the picture a mixture of much phases is visible. These phases could be identified by XRD to a- and b-MoSi2, hexagonal and tetragonal Mo5Si3, Al2O3 (corundum) and molybdenum (Fig. 4). With EDX also an eutectic was detectable with approximately 55 at.% Si and 45 at.% Mo (mixture of MoSi2 and Mo5Si3). Like the high temperature modification of MoSi2 the eutectic which is stable at 19008C exists after spraying caused by the rapid cooling of the coating. 3.2. Oxidation behavior at 5008C The oxidation behavior at 5008C was tested on all four coatings. The measured thermogravimetric curves are shown in Fig. 2. The MoSi2 coating with low porosity as well as with alumina reinforced MoSi2 coating show a relatively small mass growth and their visible appearance did not change. The mass of the MoSi2 HP and the MoSi2-SiC coatings grew more than 60%. These two coatings disintegrated into a green powder without any rests of the original coating and their volumes extended extremely. This phenomenon is called pest oxidation or pesting and is known for monolithic MoSi2. Monolithic MoSi2 is prone to oxidation at temperatures between 400 and 6008C, because no protective film of SiO2 is formed on the surface of the material in this temperature range. Oxidation proceeds through pores, cracks and grain boundaries. Two reactions are
G. Reisel et al. / Surface and Coatings Technology 146 – 147 (2001) 19–26
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Fig. 3. XRD pattern of MoSi2 LP coating (1) and MoSi2 HP coating (2) after oxidation at 5008C for 100 h ws, a-MoSi2; q, MoO3; d, SiO2 (coesite); n, SiO2 (cristobalite)x.
thermodynamically possible w19,21x 2MoSi2 (s)q7O2™2MoO3 (s)q4SiO2 (s)
(1)
5MoSi2 (s)q7O2™Mo5Si3 (s)q7SiO2 (s)
(2)
but Eq. (1) is favored and results in pesting. The pesting leads to the simultaneous formation of mainly crystalline MoO3-whiskers or platelets and amorphous SiO2-clusters as well as residual MoSi2 crystals in the material. Because of the volume expansion during the formation especially of the voluminous MoO3-whiskers, great internal stresses lead to the disintegration of the material into powder. The disintegration reaches a maximum at 5008C. A porosity lower than 4% combined with low Mo5Si3- and low SiO2-contents avoids pesting in the case of sintered MoSi2 w10,19–21x. Also in the case of the examined unreinforced coatings the porosity seems to play an important role in avoiding pesting. To check the transferability of the reactions during pesting to coatings, XRD was taken out of the samples. Fig. 3 compares the XRD patterns of the both unreinforced coatings. The differences are clear. In the case of the coating with high porosity only MoO3 and a small amount of SiO2 in form of Coesite are detectable, but no MoSi2 was found. The detection of Coesite shows that not all silicon oxide develops in an amorphous form. The coating with low porosity consists of MoSi2 in its a-modification, MoO3 and SiO2 in form of Coesite and Cristobalite. To find out the reaction during the pest oxidation, a MoSi2 HP coating was tested for 20 h. TG curve showed that the disintegration into powder was started, but not complete. XRD detected a-MoSi2, SiO2 in the form of Coesite and MoO3. No Mo5Si3 or b-MoSi2 was found. It can be concluded that b-MoSi2 changed into a-MoSi2. This reaction is also described by Westermann who used a heat treatment for this
conversion w18x. The lack of Mo5Si3 proves the correctness of the reaction described by Eq. (1). Reinforcing MoSi2 with silicon carbide seems to accelerate the pest oxidation, because despite the lower porosity compared to the MoSi2 HP coating the disintegration occurred in a similar time. Alumina as reinforcing material seems to be suitable for avoiding pest oxidation. With a porosity of 7%, which is higher than the porosity of SiC reinforced coating, no disintegration started during the test. Fig. 4 shows the XRD patterns for the MoSi2-Al2O3 powder as well as for the coatings before and after oxidation. After oxidation the phase composition is similar to the oxidized MoSi2 LP sample. 3.3. Influence of the heating rate on the oxidation behavior To discover the influence of the time the coatings stay in the temperature range between 400 and 6008C on the oxidation behavior the heating rate was varied between 1 and 50 Kymin. The holding time at 10008C was 1 h. There is no influence of the heating rate detectable on phase composition and mass change in the case of MoSi2 LP and MoSi2-Al2O3, that means for the coatings which are not prone to pest oxidation. Both coatings that disintegrated during the oxidation test at 5008C show another behavior. For heating rates below 20 Ky min a dependency of the mass change is recognizable. If the heating rate is very low, there is a mass growth up to a temperature of approximately 7008C. At higher temperatures the mass of the coating is reduced. If the heating rate is higher this effect is less. For heating rates equal or higher than 20 Kymin the mass growth and the following loss are independent of the heating rate. The mass growth can be correlated with the formation
24
G. Reisel et al. / Surface and Coatings Technology 146 – 147 (2001) 19–26
Fig. 4. XRD pattern of MoSi2-Al2O3 powder (1), MoSi2-Al2O3 coating as sprayed (2) and MoSi2-Al2O3 coating after oxidation at 5008C for 100 h (3) ws, a-MoSi2; q, MoO3; d, SiO2 (coesite); n, SiO2 (cristobalite); m, Mo; q, Mo5Si3; h, b-MoSi2; j, Al2O3x.
of amorphous SiO2 and MoO3, which becomes volatile 8008C and evaporates, so that now a mass loss is caused. Fig. 5 shows the MoSi2 HP coating after oxidation. The heating rate was 1 Kymin. There is no visible difference in the microstructure to the as-sprayed sample. XRD shows no differences too. 3.4. Oxidation behavior at 10008C To evaluate the effect of a working temperature in the range of the brittle-ductile-transition an oxidation test at 10008C was taken out for 60 h. Fig. 6 presents the results of the thermogravimetric measurements for the MoSi2 LP, the MoSi2 HP and the MoSi2-SiC coatings. In the MoSi2 HP and the MoSi2-SiC samples a mass growth followed by a mass loss was recognizable during heating. This process is mentioned above. After heating all coatings lost mass between 0.5 and 1.0%.
Fig. 5. Coating MoSi2 HP after oxidation at 10008Cy1 h with a heating rate of 1 Kymin (SEM).
After these events, no more relevant reactions were measured. XRD investigations after the oxidation test showed that the coatings are composed of the three phases a-MoSi2, tetragonal Mo5Si3 and molybdenum. In the MoSi2 HP silicon oxide was found too. SEM of this coating confirmed the silicon oxide content. On the coating surface the formation of a SiO2 layer started. The hexagonal phases of both MoSi2 and Mo5Si3 were converted into the tetragonal lattice structure. A possible oxidation reaction for Mo5Si3 is given by Akinc et al. w22x and can be expressed as: Mo5Si3q3 O2 ™5 Moq3 SiO2
(3)
This reaction is caused by the lower free energy of formation of amorphous silicon oxide compared with molybdenum oxide and explains the detection of molybdenum by XRD.
Fig. 6. TG curves of various coatings during oxidation at 10008C w(1) MoSi2 LP, (2) MoSi2 HP and (3) MoSi2-SiCx.
G. Reisel et al. / Surface and Coatings Technology 146 – 147 (2001) 19–26
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Fig. 7. TG curve of MoSi2 HP coating during oxidation at 15008C.
3.5. Oxidation behavior at 15008C For the applicability of MoSi2 as a protective coating in the temperature range from 12008C to 18008C it is necessary to evaluate the coatings oxidation behavior at this temperatures. Unreinforced MoSi2 coatings were examined at 15008C. The coating with low porosity shows similar results like the now presented MoSi2 HP coating, whose TG curve is shown in Fig. 7. The coatings mass increases in a parabolic curve. After 28 h the mass growth was below 0.6% and the slope of the curve is very low. It can be expected that the mass growth will end below 0.6%. The microstructure as well as the fractured crosssection of the oxidized coating are shown in Fig. 8. In both SEM pictures a layer with a thickness below 10 mm is visible on the coatings surface. The microstructure of the coating shows two phases, which are separated by sharp boundaries. EDX analysis showed that the layer consists of SiO2, the light phase of Mo5Si3 and the darker phase of MoSi2. The SiO2 layer seems to be glassy, but a XRD investigation, which is presented in Fig. 9, proves the existence of crystalline SiO2 in the modification of cristobalite. The two other
phases could be identified as a-MoSi2 and hexagonal Mo5Si3. Furthermore, a small amount of MoO2 was detected by XRD. The observed SiO2 layer thickness corresponds to the investigations of Becker et al. w23x, who examined the high temperature oxidation behavior of MoSi2 samples produced by hot isostatic pressing. They found after 826 h oxidation at 14008C a SiO2 layer with a thickness below 10 mm. Inside this layer they detected some small inclusions, but it was not possible to determine their nature. They suspected that the crystals were probably crystalline SiO2 embedded in an amorphous SiO2 matrix. This theory would explain the results of XRD and the more amorphous appearance of the SiO2 layer in Fig. 8. 4. Conclusions
1. Pesting of the MoSi2 coatings depends on the porosity of the coating. Silicon carbide as reinforcing phase seems to accelerate pesting, while alumina seems to reduce this reaction.
Fig. 8. Coating MoSi2 HP after oxidation at 15008Cy28 h: (a) polished cross-section; (b) fractured cross-section (SEM).
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G. Reisel et al. / Surface and Coatings Technology 146 – 147 (2001) 19–26
Fig. 9. XRD pattern of MoSi2 HP coating as sprayed (1) and MoSi2 HP coating after oxidation at 15008C for 28 h (2) ws, a-MoSi2; q, bMoSi2; n, hex. Mo5Si3; h, SiO2 (cristobalite); d, MoO2x.
2. The heating rate has no significant influence on the short-term oxidation behavior of unreinforced and reinforced MoSi2 coatings at 10008C. 3. All tested coatings showed no significant damage after 60 h oxidation at 10008C. Inside the coatings molybdenum was found which suggests oxidation of Mo5Si3. 4. Unreinforced MoSi2 coatings form a protective SiO2 layer on the surface with a thickness below 10 mm during oxidation at 15008C. The layer seems to be glassy with cristobalite inclusions. The microstructure of the coating changes to a high crystalline twophase system of a-MoSi2 and hexagonal Mo5Si3. References w1x Y.L. Jeng, E.J. Lavernia, J. Mater. Sci 29 (1994) 2557–2571. w2x C. Friedrich, G. Berg, E. Broszeit, C. Berger, Mater.-Wiss. U. Werkstofftechnik 28 (1997) 59–76. w3x J.J. Petrovic, A.K. Vasudevan, Mater. Sci. Eng. A261 (1999) 1–5. w4x A. Newman, T. Jewett, S. Sampath, C. Berndt, H. Herman, J. Mater. Res. 13 (1998) 2662–2671. w5x S.D. Conzone, D.P. Butt, A.H. Bartlett, J. Mater. Sci. 32 (1997) 3369–3374. w6x G.S. Dhupia, H. Kelichhaus, M. Konig, ¨ Ker. Zeitschrift 42 (1990) 824–829. w7x K.S. Kumar, G. Bao, Comp. Sci. Tech. 52 (1994) 127–150. w8x P.J. Meschter, D.S. Schwartz, JOM 41 (11) (1989) 52–55. w9x E.L. Courtright, Mater. Sci. Eng. A261 (1999) 53–63.
w10x J. Schlichting, High Temp. High Press. 10 (1978) 241–269. w11x R. Mitra, V.V. Rama-Rao, Y.R. Mahajan, Mater. Sci. Tech. 13 (1997) 415–419. w12x K.H. Baik, P.S. Grant, A.R. Bhatti, R.A. Shatwell, in: C.C. Berndt (Ed.), Thermal Spray: Surface Engineering via Applied Research, 1st ITSC, ASM Int, 2000, pp. 821–827. w13x A. Newman, S. Sampath, H. Herman, Mater. Sci. Eng. A261 (1999) 252. w14x K.J. Hollis, A. Bartlett, R.G. Castro, R.A. Neiser, in: C.C. Berndt (Ed.), Thermal Spray: Practical Solutions for Engineering Problems, 9th NTSC, ASM Int, 1996, pp. 429–437. w15x Jansen, F., Neue Aspekte zur Formkorperherstellung ¨ aus MoSi2 durch klassisches Sintern und zur Schichtentwicklung durch Plasmaspritzen, Thesis, RWTH Aachen, 1997 w16x B. Wielage, Reisel G., S. Steinhauser, I. Morgenthal, R. Scholl, in: C.C. Berndt (Ed.), Thermal Spray: Surface Engineering via Applied Research, 1st ITSC, 2000, ASM Int., 2000, pp. 865– 869. w17x Reisel, G., B. Wielage, S. Steinhauser, I. Morgenthal, R. Scholl, in Proceedings. of the Materials Week 2000, publisher. in Internet (http:y ywww.materialsweek.orgyproceedingsy index.htm) w18x U.Westermann, Refraktarmetall-Silizide ¨ als Struktur- und Bes¨ hohe Temperaturen, Thesis, RWTH chichtungswerkstoffe fur Aachen, 1992 w19x T.C. Chou, T.G. Nieh, J. Mater. Res. 8 (1993) 214–226. w20x J. Chen, C. Li, Z. Fu, X. Tu, M. Sundberg, R. Pompe, Mater. Sci. Eng. A261 (1999) 239–244. w21x Z. Yao, J. Stiglich, T.S. Sudarshan, J. Mater. Eng. Perf. 8 (1999) 291–304. w22x M. Akinc, M.K. Meyer, M.J. Kramer, A.J. Thom, J.J. Huebsch, B. Cook, Mater. Sci. Eng. A261 (1999) 16–23. w23x S. Becker, A. Rahmel, M. Schutze, ¨ Solid State Ionics 53–56 (1992) 280–289.
High temperature oxidation behavior of HVOF-sprayed unreinforced and reinforced molybdenum disilicide powders a ¨ Guido Reisela,*, Bernhard Wielagea, Siegfried Steinhauser , Ingrid Morgenthalb, Roland Schollb a
Institute of Composite Materials and Surface Technology, Technical University Chemnitz, Erfenschlager Straße 73, 09107 Chemnitz, Germany b Fraunhofer-Institute for Applied Materials Research (IFAM), Division for Powder Metallurgy and Composite Materials in Dresden, Winterbergstraße 28, 01277 Dresden, Germany
Abstract Intermetallics like silicides are useful for protective coatings against high-temperature corrosion. Especially molybdenum disilicide which has a great potential as protective coating e.g. in aircraft engines and gas turbines in the temperature range between 1400 and 18008C due to its high melting point and its low brittle-ductile transition temperature of approximately 800–11008C. Four types of coatings were produced by high velocity oxyfuel spraying (HVOF): unreinforced MoSi2 with low porosity, unreinforced MoSi2 with high porosity, with silicon carbide reinforced MoSi2 and with alumina reinforced MoSi2. The coatings as sprayed were characterized by XRD, SEM and EDX. Microhardness and porosity were measured. The oxidation behavior of the coatings was determined at 500, 1000 and 15008C. The influence of the heating rate was investigated during oxidation tests at 10008C. The tests at 5008C showed that the pesting depends on the porosity of the coating. SiC as reinforcing phase seems to accelerate pesting, while alumina reduces this reaction. Unreinforced MoSi2 coatings form a protective SiO2 layer on the surface with a thickness below 10 mm during oxidation at 15008C. The layer seems to be glassy with cristobalite inclusions. The microstructure of the coating changes to a high crystalline two-phase system of a-MoSi2 and hexagonal Mo5Si3. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: High temperature; HVOF-sprayed; Molybdenum disilicide; Powders
1. Introduction Monolithic molybdenum disilicide (MoSi2) is a possible structural material for high-temperature applications like furnace components or aircraft turbine engines due to its high melting point of 20308C combined with its moderate density (6.25 g cmy3) which is much lower than the density of other high temperature structural materials like nickel based alloys, tungsten or niobium. Furthermore, molybdenum disilicide shows an excellent high temperature oxidation resistance, a low coefficient of thermal expansion, a good thermal as well as a good electrical conductivity. Also the brittle-ductile transition temperature of approximately 800–11008C is relatively * Corresponding author. Tel.: q49-371-531-5391; fax: q49-371531-6179. E-mail address: [email protected] (G. Reisel).
low, so that MoSi2 shows at temperatures above 12008C, an acceptable deformation behavior caused by its high toughness w1–4x. Today the main application of molybdenum disilicide is as heating elements for electric heated furnaces w5,6x Unfortunately, there are some disadvantages of monolithic molybdenum disilicide like the low fracture toughness especially at room temperature and the low creep resistance at temperatures above the brittle-ductile transition which hinder its wide use today. For some possible applications, it is necessary to reinforce MoSi2 with materials like silicon carbide or alumina, e.g. MoSi2 is chemically compatible with SiC up to highest temperatures w7–9x. Another great problem is the socalled pesting of MoSi2 at temperatures between 300 and 6008C, which can lead to the disintegration of the bulk material into powder. In the case of monolithic
0257-8972/01/$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 1 . 0 1 3 6 4 - 0
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Table 1 Experimental design of oxidation tests Route
Starting temp. (8C)
Heating rate (Kymin)
Working temp. (8C)
Holding time at working temp. (h)
1 2
20 20
500 1000
100 1
3 4
20 20
50 1, 5, 10, 20, 30, 40, 50 50 50
1000 1500
60 28
MoSi2, high porosity is named as the main reason for the pesting w10,11x. MoSi2 can be used as coating onto various substrates, e.g. as a protective layer against corrosion. The coatings are produced normally by plasma spraying. In the case of vacuum plasma spraying (VPS), the coatings exhibit a low porosity as well as low oxygen content, but no protective silicon oxide layer is formed on the coatings surface. This layer is known as one possible method to avoid pesting in the case of molybdenum disilicide produced as bulk material. If atmospheric plasma spraying (APS) is used the coatings show a relatively high porosity and a high oxygen content. In both technologies after spraying Mo5Si3 was found w12–15x. High velocity oxyfuel spraying (HVOF) could be an alternative thermal spraying method to produce molybdenum disilicide coatings. In the present paper, the oxidation behavior of HVOF sprayed unreinforced and with silicon carbide or alumina reinforced MoSi2 is examined at different temperatures. For evaluating the coatings resistance against pesting a temperature of 5008C was chosen. Furthermore, tests were taken out at 1000 and 15008C. 2. Experimental details 2.1. Coatings processing The processing routes of the unreinforced molybdenum disilicide powder as well as with 20 vol.% silicon carbide reinforced MoSi2 by high energy milling followed by heat treatment and gentle milling were described in previous works w16,17x. With 15 vol.% alumina reinforced MoSi2 powder was also produced by high energy milling in a similar way. The powders were sprayed by high velocity oxyfuel spraying (HVOF) onto a polished carbon steel substrate. Caused by the smooth surface and the different thermal coefficients of expansion the coatings did not adhere on the substrate, so that pure coatings without a substrate could be produced for the following oxidation examinations. To ascertain the influence of the coatings porosity on the oxidation behavior in the case of unreinforced MoSi2 a coating with a low and a coating with a high porosity were
sprayed. Furthermore, one silicon carbide and one alumina-reinforced coating were investigated. The HVOF system used was the Diamond Jet from Sulzer Metco, USA. 2.2. Thermogravimetric measurements (TG) For the oxidation tests the simultaneous thermogravimetric equipment STA409C, Netzsch, Germany, was used. For accelerated oxidation compressed air with a rate of 100 ccmymin flowed through the furnace. Four types of tests with different temperature-time-regimes which are given in Table 1 were carried out. The temperature of 5008C was chosen to investigate the oxidation behavior of the coatings at low temperatures. 15008C were necessary to examine the high temperature oxidation while 10008C is an often used working temperature in the case of nickel-based materials which are possible candidates to be substituted by molybdenum disilicide. Also, the influence of the heating rate on the oxidation is evaluated by thermogravimetry. 2.3. Scanning electron microscopy (SEM) Scanning electron microscopy was used to examine the coatings as fractured and as polished cross-sections with the aim to characterize their microstructure. The samples were investigated in the scanning electron microscope JSM-840, Jeol, Japan, as well as in a microscope of the type 1455 VP, LEO, Germany. 2.4. Energy dispersive X-ray analysis (EDX) Phase compositions of the coatings are executed at polished cross-sections by the energy dispersive X-ray analysis system EDISON from Getac, Germany. It was used to identify phases inside the coatings. 2.5. X-Ray diffraction analysis (XRD) To determine the microstructure and the phase composition of the powders and coatings surfaces XRD was taken out by an equipment of the type D5000, Siemens, Germany. Cu-radiation was used, the analyzed range of
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Fig. 1. SEM investigations of the coatings as sprayed w(a) MoSi2 LP with low porosity; (b) MoSi2 HP with high porosity; (c) MoSi2-SiC coating; (d) MoSi2-Al2O3 coatingx.
the diffraction angle 2u was between 108 and 1008 by a step width of 0.028. 2.6. Porosity The porosity of the coatings before oxidation test (‘as sprayed’) was measured by an image analysis system on polished cross-sections. The system used was Quantimet 570C, Leica, Germany. The determination works due to the principle of gray value analysis. For each sample 10 measurements were carried out. 2.7. Vickers microhardness Microhardness measurements were made on polished cross-sections of the coatings. An instrument of the type Micromet1, Buehler Ltd., USA, with a diamond of Vickers geometry was used. The load of 0.5 N was applied 15 s on the sample and the Vickers microhardness HV0.05 determined. Ten tests were taken out on each coating.
3. Results and discussion 3.1. Characterization of the powders and the coatings before oxidation XRD of the unreinforced MoSi2 powder showed that only tetragonal MoSi2 could be found. The average diameter of the powder was 6.25 mm and the oxygen content 1.42% (determined by hot gas extrusion). The MoSi2-20 vol.% SiC powder showed as the main phase MoSi2 (tetragonal). Additional phases were SiC and tetragonal Mo5Si3 with a very small amount. The average diameter of the SiC particles was 11 mm and of the whole powder 10.25 mm, 0.82% oxygen was measured. The MoSi2-15 vol.% Al2O3 powder was composed of tetragonal MoSi2 as main phase, Mo5Si3 and alumina as corundum. The diffraction pattern of this powder is shown in Fig. 4. The average diameter of the reinforcing alumina particles was 1 mm and of the composite powder 3.46 mm. SEM pictures of the four examined coatings assprayed are given in Fig. 1, while Table 2 shows some
Table 2 Coating properties ‘as sprayed’ (LP: Low Porosity, HP: High Porosity) Coating
Phase composition
Porosity w%x
Microhardness HV0.05
MoSi2 LP MoSi2 HP MoSi2-SiC MoSi2-Al2O3
a-MoSi2, b-MoSi2, Mo5Si3 (tetr.), Mo a-MoSi2, b-MoSi2, Mo5Si3 (hex.) a-MoSi2, b-MoSi2, Mo5Si3 (tetr.), SiC a-MoSi2, b-MoSi2, Mo5Si3 (tetr. And hex.), Mo, Al2O3
2.05 20.60 4.34 6.01
1074 825 804 813
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Fig. 2. TG curves of various coatings during oxidation at 5008C.
characteristics of these coatings. The MoSi2 coating with low porosity (MoSi2 LP) is shown in Fig. 1a, while in Fig. 1b the MoSi2 coating with high porosity (MoSi2 HP) is visible. The higher porosity level of the right coating is clearly detectable. Also it is recognizable, that one phase which is detected with EDX as MoSi2 has the greatest part in the microstructure in both MoSi2 coatings. MoSi2 in its tetragonal (a-) modification with a C11b structure was also detected as main phase by XRD, but another MoSi2 phase was found too. It is the high temperature modification (b-modification) of molybdenum disilicide with a hexagonal lattice and a C40 structure. Its equilibrium is above 19008C. Caused by the rapid cooling of the layer during and after spraying the hexagonal phase exists in the as-sprayed coatings. This phase was also detected in vacuum plasma sprayed MoSi2-coatings w18x. In comparison with MoSi2 HP (its XRD-pattern is shown in Fig. 9) the MoSi2 coating with low porosity exhibits much more MoSi2 in its b-modification, little molybdenum and little Mo5Si3 with a tetragonal lattice, while in MoSi2 HP only little Mo5Si3 with a hexagonal lattice and no molybdenum was found. A possible reason for this phenomenon could be the different spraying parameters, so that the coating with higher porosity cooled down slower and more MoSi2 of the b-modification was converted into the a-modification while caused by these different temperatures during spraying the silicon loss was higher in the case of MoSi2 with low porosity so that molybdenum could be found. The picture of the MoSi2-SiC coating (Fig. 1c) shows clearly the silicon carbide particles, which are embedded in the matrix without great pores on the boundary. The coating is dense and only one phase is recognizable in the matrix, which is determined to MoSi2 by EDX. XRD confirmed this result. The main phase in the coating is a-MoSi2. Only very small amounts of b-
MoSi2 and tetragonal Mo5Si3 were found. The silicon carbide content was measured with image analysis and below 2%. Because of the small grain size, no Al2O3-particles are visible in the SEM picture of the MoSi2-Al2O3 coating (Fig. 1d). With EDX measurement on the whole coating area of the polished cross-section the alumina content was determined to be approximately 3%. In the picture a mixture of much phases is visible. These phases could be identified by XRD to a- and b-MoSi2, hexagonal and tetragonal Mo5Si3, Al2O3 (corundum) and molybdenum (Fig. 4). With EDX also an eutectic was detectable with approximately 55 at.% Si and 45 at.% Mo (mixture of MoSi2 and Mo5Si3). Like the high temperature modification of MoSi2 the eutectic which is stable at 19008C exists after spraying caused by the rapid cooling of the coating. 3.2. Oxidation behavior at 5008C The oxidation behavior at 5008C was tested on all four coatings. The measured thermogravimetric curves are shown in Fig. 2. The MoSi2 coating with low porosity as well as with alumina reinforced MoSi2 coating show a relatively small mass growth and their visible appearance did not change. The mass of the MoSi2 HP and the MoSi2-SiC coatings grew more than 60%. These two coatings disintegrated into a green powder without any rests of the original coating and their volumes extended extremely. This phenomenon is called pest oxidation or pesting and is known for monolithic MoSi2. Monolithic MoSi2 is prone to oxidation at temperatures between 400 and 6008C, because no protective film of SiO2 is formed on the surface of the material in this temperature range. Oxidation proceeds through pores, cracks and grain boundaries. Two reactions are
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Fig. 3. XRD pattern of MoSi2 LP coating (1) and MoSi2 HP coating (2) after oxidation at 5008C for 100 h ws, a-MoSi2; q, MoO3; d, SiO2 (coesite); n, SiO2 (cristobalite)x.
thermodynamically possible w19,21x 2MoSi2 (s)q7O2™2MoO3 (s)q4SiO2 (s)
(1)
5MoSi2 (s)q7O2™Mo5Si3 (s)q7SiO2 (s)
(2)
but Eq. (1) is favored and results in pesting. The pesting leads to the simultaneous formation of mainly crystalline MoO3-whiskers or platelets and amorphous SiO2-clusters as well as residual MoSi2 crystals in the material. Because of the volume expansion during the formation especially of the voluminous MoO3-whiskers, great internal stresses lead to the disintegration of the material into powder. The disintegration reaches a maximum at 5008C. A porosity lower than 4% combined with low Mo5Si3- and low SiO2-contents avoids pesting in the case of sintered MoSi2 w10,19–21x. Also in the case of the examined unreinforced coatings the porosity seems to play an important role in avoiding pesting. To check the transferability of the reactions during pesting to coatings, XRD was taken out of the samples. Fig. 3 compares the XRD patterns of the both unreinforced coatings. The differences are clear. In the case of the coating with high porosity only MoO3 and a small amount of SiO2 in form of Coesite are detectable, but no MoSi2 was found. The detection of Coesite shows that not all silicon oxide develops in an amorphous form. The coating with low porosity consists of MoSi2 in its a-modification, MoO3 and SiO2 in form of Coesite and Cristobalite. To find out the reaction during the pest oxidation, a MoSi2 HP coating was tested for 20 h. TG curve showed that the disintegration into powder was started, but not complete. XRD detected a-MoSi2, SiO2 in the form of Coesite and MoO3. No Mo5Si3 or b-MoSi2 was found. It can be concluded that b-MoSi2 changed into a-MoSi2. This reaction is also described by Westermann who used a heat treatment for this
conversion w18x. The lack of Mo5Si3 proves the correctness of the reaction described by Eq. (1). Reinforcing MoSi2 with silicon carbide seems to accelerate the pest oxidation, because despite the lower porosity compared to the MoSi2 HP coating the disintegration occurred in a similar time. Alumina as reinforcing material seems to be suitable for avoiding pest oxidation. With a porosity of 7%, which is higher than the porosity of SiC reinforced coating, no disintegration started during the test. Fig. 4 shows the XRD patterns for the MoSi2-Al2O3 powder as well as for the coatings before and after oxidation. After oxidation the phase composition is similar to the oxidized MoSi2 LP sample. 3.3. Influence of the heating rate on the oxidation behavior To discover the influence of the time the coatings stay in the temperature range between 400 and 6008C on the oxidation behavior the heating rate was varied between 1 and 50 Kymin. The holding time at 10008C was 1 h. There is no influence of the heating rate detectable on phase composition and mass change in the case of MoSi2 LP and MoSi2-Al2O3, that means for the coatings which are not prone to pest oxidation. Both coatings that disintegrated during the oxidation test at 5008C show another behavior. For heating rates below 20 Ky min a dependency of the mass change is recognizable. If the heating rate is very low, there is a mass growth up to a temperature of approximately 7008C. At higher temperatures the mass of the coating is reduced. If the heating rate is higher this effect is less. For heating rates equal or higher than 20 Kymin the mass growth and the following loss are independent of the heating rate. The mass growth can be correlated with the formation
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Fig. 4. XRD pattern of MoSi2-Al2O3 powder (1), MoSi2-Al2O3 coating as sprayed (2) and MoSi2-Al2O3 coating after oxidation at 5008C for 100 h (3) ws, a-MoSi2; q, MoO3; d, SiO2 (coesite); n, SiO2 (cristobalite); m, Mo; q, Mo5Si3; h, b-MoSi2; j, Al2O3x.
of amorphous SiO2 and MoO3, which becomes volatile 8008C and evaporates, so that now a mass loss is caused. Fig. 5 shows the MoSi2 HP coating after oxidation. The heating rate was 1 Kymin. There is no visible difference in the microstructure to the as-sprayed sample. XRD shows no differences too. 3.4. Oxidation behavior at 10008C To evaluate the effect of a working temperature in the range of the brittle-ductile-transition an oxidation test at 10008C was taken out for 60 h. Fig. 6 presents the results of the thermogravimetric measurements for the MoSi2 LP, the MoSi2 HP and the MoSi2-SiC coatings. In the MoSi2 HP and the MoSi2-SiC samples a mass growth followed by a mass loss was recognizable during heating. This process is mentioned above. After heating all coatings lost mass between 0.5 and 1.0%.
Fig. 5. Coating MoSi2 HP after oxidation at 10008Cy1 h with a heating rate of 1 Kymin (SEM).
After these events, no more relevant reactions were measured. XRD investigations after the oxidation test showed that the coatings are composed of the three phases a-MoSi2, tetragonal Mo5Si3 and molybdenum. In the MoSi2 HP silicon oxide was found too. SEM of this coating confirmed the silicon oxide content. On the coating surface the formation of a SiO2 layer started. The hexagonal phases of both MoSi2 and Mo5Si3 were converted into the tetragonal lattice structure. A possible oxidation reaction for Mo5Si3 is given by Akinc et al. w22x and can be expressed as: Mo5Si3q3 O2 ™5 Moq3 SiO2
(3)
This reaction is caused by the lower free energy of formation of amorphous silicon oxide compared with molybdenum oxide and explains the detection of molybdenum by XRD.
Fig. 6. TG curves of various coatings during oxidation at 10008C w(1) MoSi2 LP, (2) MoSi2 HP and (3) MoSi2-SiCx.
G. Reisel et al. / Surface and Coatings Technology 146 – 147 (2001) 19–26
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Fig. 7. TG curve of MoSi2 HP coating during oxidation at 15008C.
3.5. Oxidation behavior at 15008C For the applicability of MoSi2 as a protective coating in the temperature range from 12008C to 18008C it is necessary to evaluate the coatings oxidation behavior at this temperatures. Unreinforced MoSi2 coatings were examined at 15008C. The coating with low porosity shows similar results like the now presented MoSi2 HP coating, whose TG curve is shown in Fig. 7. The coatings mass increases in a parabolic curve. After 28 h the mass growth was below 0.6% and the slope of the curve is very low. It can be expected that the mass growth will end below 0.6%. The microstructure as well as the fractured crosssection of the oxidized coating are shown in Fig. 8. In both SEM pictures a layer with a thickness below 10 mm is visible on the coatings surface. The microstructure of the coating shows two phases, which are separated by sharp boundaries. EDX analysis showed that the layer consists of SiO2, the light phase of Mo5Si3 and the darker phase of MoSi2. The SiO2 layer seems to be glassy, but a XRD investigation, which is presented in Fig. 9, proves the existence of crystalline SiO2 in the modification of cristobalite. The two other
phases could be identified as a-MoSi2 and hexagonal Mo5Si3. Furthermore, a small amount of MoO2 was detected by XRD. The observed SiO2 layer thickness corresponds to the investigations of Becker et al. w23x, who examined the high temperature oxidation behavior of MoSi2 samples produced by hot isostatic pressing. They found after 826 h oxidation at 14008C a SiO2 layer with a thickness below 10 mm. Inside this layer they detected some small inclusions, but it was not possible to determine their nature. They suspected that the crystals were probably crystalline SiO2 embedded in an amorphous SiO2 matrix. This theory would explain the results of XRD and the more amorphous appearance of the SiO2 layer in Fig. 8. 4. Conclusions
1. Pesting of the MoSi2 coatings depends on the porosity of the coating. Silicon carbide as reinforcing phase seems to accelerate pesting, while alumina seems to reduce this reaction.
Fig. 8. Coating MoSi2 HP after oxidation at 15008Cy28 h: (a) polished cross-section; (b) fractured cross-section (SEM).
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G. Reisel et al. / Surface and Coatings Technology 146 – 147 (2001) 19–26
Fig. 9. XRD pattern of MoSi2 HP coating as sprayed (1) and MoSi2 HP coating after oxidation at 15008C for 28 h (2) ws, a-MoSi2; q, bMoSi2; n, hex. Mo5Si3; h, SiO2 (cristobalite); d, MoO2x.
2. The heating rate has no significant influence on the short-term oxidation behavior of unreinforced and reinforced MoSi2 coatings at 10008C. 3. All tested coatings showed no significant damage after 60 h oxidation at 10008C. Inside the coatings molybdenum was found which suggests oxidation of Mo5Si3. 4. Unreinforced MoSi2 coatings form a protective SiO2 layer on the surface with a thickness below 10 mm during oxidation at 15008C. The layer seems to be glassy with cristobalite inclusions. The microstructure of the coating changes to a high crystalline twophase system of a-MoSi2 and hexagonal Mo5Si3. References w1x Y.L. Jeng, E.J. Lavernia, J. Mater. Sci 29 (1994) 2557–2571. w2x C. Friedrich, G. Berg, E. Broszeit, C. Berger, Mater.-Wiss. U. Werkstofftechnik 28 (1997) 59–76. w3x J.J. Petrovic, A.K. Vasudevan, Mater. Sci. Eng. A261 (1999) 1–5. w4x A. Newman, T. Jewett, S. Sampath, C. Berndt, H. Herman, J. Mater. Res. 13 (1998) 2662–2671. w5x S.D. Conzone, D.P. Butt, A.H. Bartlett, J. Mater. Sci. 32 (1997) 3369–3374. w6x G.S. Dhupia, H. Kelichhaus, M. Konig, ¨ Ker. Zeitschrift 42 (1990) 824–829. w7x K.S. Kumar, G. Bao, Comp. Sci. Tech. 52 (1994) 127–150. w8x P.J. Meschter, D.S. Schwartz, JOM 41 (11) (1989) 52–55. w9x E.L. Courtright, Mater. Sci. Eng. A261 (1999) 53–63.
w10x J. Schlichting, High Temp. High Press. 10 (1978) 241–269. w11x R. Mitra, V.V. Rama-Rao, Y.R. Mahajan, Mater. Sci. Tech. 13 (1997) 415–419. w12x K.H. Baik, P.S. Grant, A.R. Bhatti, R.A. Shatwell, in: C.C. Berndt (Ed.), Thermal Spray: Surface Engineering via Applied Research, 1st ITSC, ASM Int, 2000, pp. 821–827. w13x A. Newman, S. Sampath, H. Herman, Mater. Sci. Eng. A261 (1999) 252. w14x K.J. Hollis, A. Bartlett, R.G. Castro, R.A. Neiser, in: C.C. Berndt (Ed.), Thermal Spray: Practical Solutions for Engineering Problems, 9th NTSC, ASM Int, 1996, pp. 429–437. w15x Jansen, F., Neue Aspekte zur Formkorperherstellung ¨ aus MoSi2 durch klassisches Sintern und zur Schichtentwicklung durch Plasmaspritzen, Thesis, RWTH Aachen, 1997 w16x B. Wielage, Reisel G., S. Steinhauser, I. Morgenthal, R. Scholl, in: C.C. Berndt (Ed.), Thermal Spray: Surface Engineering via Applied Research, 1st ITSC, 2000, ASM Int., 2000, pp. 865– 869. w17x Reisel, G., B. Wielage, S. Steinhauser, I. Morgenthal, R. Scholl, in Proceedings. of the Materials Week 2000, publisher. in Internet (http:y ywww.materialsweek.orgyproceedingsy index.htm) w18x U.Westermann, Refraktarmetall-Silizide ¨ als Struktur- und Bes¨ hohe Temperaturen, Thesis, RWTH chichtungswerkstoffe fur Aachen, 1992 w19x T.C. Chou, T.G. Nieh, J. Mater. Res. 8 (1993) 214–226. w20x J. Chen, C. Li, Z. Fu, X. Tu, M. Sundberg, R. Pompe, Mater. Sci. Eng. A261 (1999) 239–244. w21x Z. Yao, J. Stiglich, T.S. Sudarshan, J. Mater. Eng. Perf. 8 (1999) 291–304. w22x M. Akinc, M.K. Meyer, M.J. Kramer, A.J. Thom, J.J. Huebsch, B. Cook, Mater. Sci. Eng. A261 (1999) 16–23. w23x S. Becker, A. Rahmel, M. Schutze, ¨ Solid State Ionics 53–56 (1992) 280–289.