Availableonlineatwww.sciencedirect.cor ° Science Direct CERAMICS INTERNATIONAL ELSEVIER Ceramics International 34(2008)197-203 www.elsevier.com/locate/ceramint Potential of Sic multilayer ceramics for high temperature applications in oxidising environment Matteo pavese Paolo Fino a. Alberto ortona Claudio badini a Politecnico di Torino, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, corso Duca degli Abruzzi 24, 10129 Torino, Italy University of Applied Sciences(SUPSI). The iCIMSI Research Institute, Galleria 2, CH 6928 Manno, Switzerlan Received 25 November 2005: received in revised form 6 January 2006: accepted 28 September 2006 Available online 2 November 2006 Abstract Multilayered ceramics seem very promising for applications at very high temperatures in an oxidising environment. Actually, lower cost and better oxidation resistance than many conventional ceramic composites The multilayered SiC oxidation and shock resistance has been investigated on tubular specimens processed by tape casting and pressureless sintering. Microstructure, oxidation and mechanical behaviour were investigated by micro-XRD, SEM, TGA-DTA-MS, indentation and radial compressive tests The mechanical characterization showed that weak interfacial bonds are present between the layers. Together with the residual stresses left after the preparation phase, they caused crack deflection and improved toughness with respect to traditional ceramics. These mechanisms persisted even after long-term oxidation at 1600C or repeated thermal shock tests. The strength was found to depend on the thickness of the single Sic layer, however it was only slightly affected by thermal treatments Keywords: A. Tape casting: D. SiC: Multilayers; Thermal shock resistance 1. Introduction Multilayered ceramics can be obtained by several methods: tape casting, slip casting, rolling, extrusion, followed by Monolithic ceramics show a catastrophic fracture behaviour sintering or hot pressing. The most used method is however tape under applied stress due to the lack of energy absorbing casting [3-9] that consists in casting a thick film of a slurry mechanisms in the failure process. Several tough composites containing the ceramic powders on a polymeric support. Then and multilayer ceramics were, however, developed in recent the tape is dried and the organic substances are removed by years. The key factor improving the toughness of these slow heating. Finally, the sintering can be realised without materials is the presence of weak interfaces between fibres and pressure or by hot pressing. In any case, these materials result he composite matrix or between the ceramic layers. These cheaper than fibre reinforced composites. interfaces allow for energy dissipation before fracture through Several multilayer ceramics have been investigated in the mechanisms of crack deflection, crack bridging, fibre pull out past [4-22]; the most studied materials have been alumina or and interface delamination alumina-zirconia [4-6, 10-11, silicon nitride [12, 13], silicon On fibre-based composites, debonding and pull out are carbide [14-19], even if other composites have been tested frequently achieved by putting a thin interphase layer on the [7, 20-22 fibre surface. For instance, interphases, generally less than In the case of multilayers, two methods have been used to I um thick, made of carbon or boron nitride, were successfully enhance toughness over conventional ceramics, namely the used in SiC/SiCr composites [1, 2] introduction of weak interfaces or the presence of residual stresses In the first case porous interlayers can be used Email adres: matteo, paese polito it (M. pavese,, [7, 11, 12, 14, 18, 19, 23], where the porosity is given by layers not wholly sintered, generally of a different material with 2-8842/$32.00 2006 Elsevier Ltd and Techna Group S.r.L. All rights reserved 10.1016 1-ceramint.2006.09008
Potential of SiC multilayer ceramics for high temperature applications in oxidising environment Matteo Pavese a, *, Paolo Fino a , Alberto Ortona b , Claudio Badini a aPolitecnico di Torino, Dipartimento di Scienza dei Materiali e Ingegneria Chimica, corso Duca degli Abruzzi 24, 10129 Torino, Italy b University of Applied Sciences (SUPSI), The iCIMSI Research Institute, Galleria 2, CH 6928 Manno, Switzerland Received 25 November 2005; received in revised form 6 January 2006; accepted 28 September 2006 Available online 2 November 2006 Abstract Multilayered ceramics seem very promising for applications at very high temperatures in an oxidising environment. Actually, they present lower cost and better oxidation resistance than many conventional ceramic composites. The multilayered SiC oxidation and shock resistance has been investigated on tubular specimens processed by tape casting and pressureless sintering. Microstructure, oxidation and mechanical behaviour were investigated by micro-XRD, SEM, TGA–DTA-MS, indentation and radial compressive tests. The mechanical characterization showed that weak interfacial bonds are present between the layers. Together with the residual stresses left after the preparation phase, they caused crack deflection and improved toughness with respect to traditional ceramics. These mechanisms persisted even after long-term oxidation at 1600 8C or repeated thermal shock tests. The strength was found to depend on the thickness of the single SiC layer, however it was only slightly affected by thermal treatments. # 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Tape casting; D. SiC; Multilayers; Thermal shock resistance 1. Introduction Monolithic ceramics show a catastrophic fracture behaviour under applied stress due to the lack of energy absorbing mechanisms in the failure process. Several tough composites and multilayer ceramics were, however, developed in recent years. The key factor improving the toughness of these materials is the presence of weak interfaces between fibres and the composite matrix or between the ceramic layers. These interfaces allow for energy dissipation before fracture through mechanisms of crack deflection, crack bridging, fibre pull out and interface delamination. On fibre-based composites, debonding and pull out are frequently achieved by putting a thin interphase layer on the fibre surface. For instance, interphases, generally less than 1 mm thick, made of carbon or boron nitride, were successfully used in SiC/SiCf composites [1,2]. Multilayered ceramics can be obtained by several methods: tape casting, slip casting, rolling, extrusion, followed by sintering or hot pressing. The most used method is however tape casting [3–9] that consists in casting a thick film of a slurry containing the ceramic powders on a polymeric support. Then the tape is dried and the organic substances are removed by slow heating. Finally, the sintering can be realised without pressure or by hot pressing. In any case, these materials result cheaper than fibre reinforced composites. Several multilayer ceramics have been investigated in the past [4–22]; the most studied materials have been alumina or alumina–zirconia [4–6,10–11], silicon nitride [12,13], silicon carbide [14–19], even if other composites have been tested [7,20–22]. In the case of multilayers, two methods have been used to enhance toughness over conventional ceramics, namely the introduction of weak interfaces or the presence of residual stresses. In the first case porous interlayers can be used [7,11,12,14,18,19,23], where the porosity is given by layers not wholly sintered, generally of a different material with www.elsevier.com/locate/ceramint Ceramics International 34 (2008) 197–203 * Corresponding author. Tel.: +39 011 564 4708; fax: +39 011 564 4699. E-mail address: matteo.pavese@polito.it (M. Pavese). 0272-8842/$32.00 # 2006 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2006.09.008
M. Pavese et al. /Ceramics International 34(2008)197-203 respect to the main multilayer component [7, 11, 12, 14]or by the addition of pore forming agents in specific layers [18, 19].When a crack approaches a sufficiently weak interface, it deviates, noving along the interface itself; in this way the propagation of cracks from a layer to another is more difficult, and the fracture energy increases. A significant debonding of the interface between layers can be achieved with this method. If the interface is strong, on the contrary, the crack passes over the interface like in a monolithic material, without significant toughening effect The other possibility to increase the toughness of multilayers is to exploit the residual stresses either at the surface or at the nterface between layers of different composition [10, 21, 22, 24- 28. The residual stress is present due to differential sintering or to the difference in thermal expansion coefficient of the components of the composite, and can be tailored in order to optimize the resistance of the material to the growing of a crack Fig. 1. Tape casting apparatus Regarding the effect of oxidation on SiC, the phenomenon is well-known in the literature [29-39]. The oxidation reaction can The layer thickness was controlled by the height of the blade bring to SiO2 and CO/CO,(passive oxidation), with an increase (from 0. 4 to 0.8 mm) and by the speed of advancement of the in weight and the passivation of the surface, or to SiO and co Mylar support (100 mm/min), obtaining layers of different (active oxidation), both gaseous. In the latter case there is no thickness. The organic solvents were then slowly removed by passivating layer forming on the surface of the SiC and there is controlled evaporation in air at ambient temperature. The Sic continual loss of material. The active oxidation however is green tape was carefully detached from the plastic support possible only at low oxygen pressures or very high temperature. and wrapped on a mandrel to obtain tubular specimens(Figs. 2 Depending on the literature source of data used, the temperature and 3) for the transition to active oxidation at 200 mbar of oxygen Due to the presence of the Mylar film, the surface roughness partial pressure is from 1650 to 1950C[29-33. In the case of of the two tape sides(Fig. 4)are rather different. During tape resence of water vapour the reactions are different, and an wrapping, there will always be a rough surface in contact with a oxidation with continual removal of the oxidised layeris possible smooth one, that could be the cause of tape delamination during 34-38. In this case it is the Sio2 formed from the oxidation of fracture. Tubular specimens were submitted to a debinding Sic that reacts with treatment, carried out by slow heating up to 500C under an This paper deals with the processing of a multilayered Sic argon atmosphere. The final pressureless sintering step was ramic fabricated by tape casting and sintering without performed at 2180C under argon pressure. The effect of layer thickness and thermal cycling on The bulk green density, after the debinding treatment, was Sic multilayer oxidation resistance at high temperature was 1.51 g/cm, 48% of the theoretical value; after sintering an investigated by thermo-analytical techniques as well as by apparent density of 3.13 g/cm was measured with comparing microstructure and mechanical behaviour before hydrostatic balance on a single layer, suggesting an almost and after long-term oxidation treatments. complete densification of the ceramic tape; on a multilayer a lower value was measured (2.89 g/em, around 91% of 2. Materials and methods theoretical density), due to the presence of non-accessible porosity between the layers. A severe shrinkage(about 20%o) 2.1. Sample processing Multilayered SiC tubular specimens were fabricated by FN S P.A. Nuove Tecnologie e Servizi Avanzati(Boscomarengo, Italy)[9]. The processing method involved several steps: slurry preparation, tape casting, solvent evaporation, specimen orming, debinding and sintering. The slurry was obtained by dispersing a-SiC powder(Starck UF-10, 15 m/g, with a mean particle size of 0.7 um) in a mixture of ethanol, butanol and tetrachloroethylene; then polyvinyl butyral and polyethy lene glycol were added, respectively, as binder and plasticizer. Boron and carbon(about 2 wt %)were also added in order to aid the final sintering treatment. Thin sheets were produced by casting the slurry on a moving Mylar support(the tape casting apparatus is presented in Fig. 1) Fig. 2. Detachment of Sic green tape from Mylar support
respect to the main multilayer component [7,11,12,14] or by the addition of pore forming agents in specific layers[18,19]. When a crack approaches a sufficiently weak interface, it deviates, moving along the interface itself; in this way the propagation of cracks from a layer to another is more difficult, and the fracture energy increases. A significant debonding of the interface between layers can be achieved with this method. If the interface is strong, on the contrary, the crack passes over the interface like in a monolithic material, without significant toughening effect. The other possibility to increase the toughness of multilayers is to exploit the residual stresses either at the surface or at the interface between layers of different composition [10,21,22,24– 28]. The residual stress is present due to differential sintering or to the difference in thermal expansion coefficient of the components of the composite, and can be tailored in order to optimize the resistance of the material to the growing of a crack. Regarding the effect of oxidation on SiC, the phenomenon is well-known in the literature [29–39]. The oxidation reaction can bring to SiO2 and CO/CO2 (passive oxidation), with an increase in weight and the passivation of the surface, or to SiO and CO (active oxidation), both gaseous. In the latter case there is no passivating layer forming on the surface of the SiC and there is continual loss of material. The active oxidation however is possible only at low oxygen pressures or very high temperature. Depending on the literature source of data used, the temperature for the transition to active oxidation at 200 mbar of oxygen partial pressure is from 1650 to 1950 8C [29–33]. In the case of presence of water vapour the reactions are different, and an oxidation with continual removal of the oxidised layer is possible [34–38]. In this case it is the SiO2 formed from the oxidation of SiC that reacts with the water bringing to volatile species. This paper deals with the processing of a multilayered SiC ceramic fabricated by tape casting and sintering without pressure. The effect of layer thickness and thermal cycling on SiC multilayer oxidation resistance at high temperature was investigated by thermo-analytical techniques as well as by comparing microstructure and mechanical behaviour before and after long-term oxidation treatments. 2. Materials and methods 2.1. Sample processing Multilayered SiC tubular specimens were fabricated by F.N. S.p.A. Nuove Tecnologie e Servizi Avanzati (Boscomarengo, Italy) [9]. The processing method involved several steps: slurry preparation, tape casting, solvent evaporation, specimen forming, debinding and sintering. The slurry was obtained by dispersing a-SiC powder (Starck UF-10, 15 m2 /g, with a mean particle size of 0.7 mm) in a mixture of ethanol, butanol and tetrachloroethylene; then polyvinyl butyral and polyethylene glycol were added, respectively, as binder and plasticizer. Boron and carbon (about 2 wt.%) were also added, in order to aid the final sintering treatment. Thin sheets were produced by casting the slurry on a moving Mylar support (the tape casting apparatus is presented in Fig. 1). The layer thickness was controlled by the height of the blade (from 0.4 to 0.8 mm) and by the speed of advancement of the Mylar support (100 mm/min), obtaining layers of different thickness. The organic solvents were then slowly removed by controlled evaporation in air at ambient temperature. The SiC green tape was carefully detached from the plastic support and wrapped on a mandrel to obtain tubular specimens (Figs. 2 and 3). Due to the presence of the Mylar film, the surface roughness of the two tape sides (Fig. 4) are rather different. During tape wrapping, there will always be a rough surface in contact with a smooth one, that could be the cause of tape delamination during fracture. Tubular specimens were submitted to a debinding treatment, carried out by slow heating up to 500 8C under an argon atmosphere. The final pressureless sintering step was performed at 2180 8C under argon. The bulk green density, after the debinding treatment, was 1.51 g/cm3 , 48% of the theoretical value; after sintering an apparent density of 3.13 g/cm3 was measured with an hydrostatic balance on a single layer, suggesting an almost complete densification of the ceramic tape; on a multilayer a lower value was measured (2.89 g/cm3 , around 91% of theoretical density), due to the presence of non-accessible porosity between the layers. A severe shrinkage (about 20%) 198 M. Pavese et al. / Ceramics International 34 (2008) 197–203 Fig. 1. Tape casting apparatus. Fig. 2. Detachment of SiC green tape from Mylar support
M. Pavese et al. /Ceramics International 34(2008)197-203 pmnmpnnmnTmu Fig. 5. Microstructure of sintered SiC multilayer; are visible the extemal Fig. 3. Tubular SiC specimen after sintering. surface of the multilayer (on the bottom) and an interface between layers (on the top) (a) 422), was used for the thermal ceramic material(100-200 mg in of t Sintered multilayer up to 1500C at a constant rate of 20C/min in flowing chromatographic air (50 ml/min). The gases generated during w快 he thermal analysis were sent to the mass spectrometer that measured the content in the gaseous flow of various species, in particular carbon and silicon oxides Long-term oxidation treatments were also carried out in calm air at high temperatures. Buckles 10 mm long produced Fig4. Roughness profile of fired SiC monolayer:(a)smooth face of green foil by sintered tapes of several thicknesses(the tape thicknesse (lower-side, in contact with Mylar film)and(b)rough face of green foil(upper- referred to the blade height, were 0.4, 0.6 and 0.8 mm)were side) kept at 1600C for 100 h. After the treatment, both the specimen microstructure and the mechanical strength were occurred during sintering; ceramic tubes, 100 mm long with an investigated external diameter of 34 mm were obtained. The final thickness Thermal shock tests were performed by realising the of the tube wall, made of the lay up of several SiC layers, was following thermal cycle: the buckles were inserted in an oven ranging between 0.6 and 1.0 mm(depending on the green tape kept at 1070C, left there 20 min and then extracted and let thickness). Buckles(rings) 10 mm long were machined from cool in calm air. After 20 min they were re-inserted in the oven the tubes by using diamond tools and so on Microstructure, mechanical strength, oxidation and thermal shock resistance of the multilayered material were investigated. 2.3. Material characterization A typical microstructure of the sintered silicon carbide ceramic multilayer, observed at the optical microscope, is presented in The microstructure of the ceramic samples was studied by Fig. 5 microscopy and micro-X-ray diffraction. A Philips 515 scanning electron microscope equipped wit 2.2. Oxidation and thermal shock tests dispersive spectrometer(PV9900) and a Rigaku D/MAX Rapid microdiffractometer were used. The XRD patterns of The oxidation conditions were always chosen in order to both buckle surface and buckle core were compared before and have a passive oxidation on SiC, with the formation of a SiO2 after the oxidising treatments scale. For this reason in all cases air at atmospheric pr The mechanical strength of the buckles was investigated by was used, in order to have an oxygen partial pressure around radial compression tests, carried out according to the Iso 2739 200 mbar [40] specification. This test is not specifically designed for Firstly, the material oxidation resistance was studied by ceramic buckles, so that no absolute values for toughness or thermal gravimetric analysis(TGA). A Mettler-Toledo TGa strength can be extracted from buckles compression tests; analyser, equipped with a mass spectrometer(Balzers Quadstar nevertheless it can be used to compare the mechanical
occurred during sintering; ceramic tubes, 100 mm long with an external diameter of 34 mm were obtained. The final thickness of the tube wall, made of the lay up of several SiC layers, was ranging between 0.6 and 1.0 mm (depending on the green tape thickness). Buckles (rings) 10 mm long were machined from the tubes by using diamond tools. Microstructure, mechanical strength, oxidation and thermal shock resistance of the multilayered material were investigated. A typical microstructure of the sintered silicon carbide ceramic multilayer, observed at the optical microscope, is presented in Fig. 5. 2.2. Oxidation and thermal shock tests The oxidation conditions were always chosen in order to have a passive oxidation on SiC, with the formation of a SiO2 scale. For this reason in all cases air at atmospheric pressure was used, in order to have an oxygen partial pressure around 200 mbar. Firstly, the material oxidation resistance was studied by thermal gravimetric analysis (TGA). A Mettler-Toledo TGA analyser, equipped with a mass spectrometer (Balzers Quadstar 422), was used for the thermal stability tests. Samples of ceramic material (100–200 mg in weight, obtained by cutting sectors of the multilayered SiC rings) were treated in the TGA up to 1500 8C at a constant rate of 20 8C/min in flowing chromatographic air (50 ml/min). The gases generated during the thermal analysis were sent to the mass spectrometer that measured the content in the gaseous flow of various species, in particular carbon and silicon oxides. Long-term oxidation treatments were also carried out in calm air at high temperatures. Buckles 10 mm long produced by sintered tapes of several thicknesses (the tape thicknesses, referred to the blade height, were 0.4, 0.6 and 0.8 mm) were kept at 1600 8C for 100 h. After the treatment, both the specimen microstructure and the mechanical strength were investigated. Thermal shock tests were performed by realising the following thermal cycle: the buckles were inserted in an oven kept at 1070 8C, left there 20 min and then extracted and let cool in calm air. After 20 min they were re-inserted in the oven and so on. 2.3. Material characterization The microstructure of the ceramic samples was studied by microscopy and micro-X-ray diffraction. A Philips 515 scanning electron microscope equipped with an energy dispersive spectrometer (PV9900) and a Rigaku D/MAX Rapid microdiffractometer were used. The XRD patterns of both buckle surface and buckle core were compared before and after the oxidising treatments. The mechanical strength of the buckles was investigated by radial compression tests, carried out according to the ISO 2739 [40] specification. This test is not specifically designed for ceramic buckles, so that no absolute values for toughness or strength can be extracted from buckles compression tests; nevertheless it can be used to compare the mechanical M. Pavese et al. / Ceramics International 34 (2008) 197–203 199 Fig. 3. Tubular SiC specimen after sintering. Fig. 4. Roughness profile of fired SiC monolayer: (a) smooth face of green foil (lower-side, in contact with Mylar film) and (b) rough face of green foil (upperside). Fig. 5. Microstructure of sintered SiC multilayer; are visible the external surface of the multilayer (on the bottom) and an interface between layers (on the top)
M. Pavese et al. /Ceramics International 34 (2008)197-26 properties of the materials under investigations [39), before and after oxidation and thermal shock tests At least three samples for each kind of specimen examined and the corresponding compression test results were =1400 ion tests were performed by pressing specimens between two flat plates at ambient temperature, 5 1000 using a Sintech 10D equipment. These experiments, performed at a constant displacement rate of 0.5 mm/min, made it possible to obtain the stress/displacement curve as well as to calculate the radial compression strength [39] The fracture surfaces were examined by SEM. Indentation JWLAIAI tests were carried out in order to investigate crack propagation 2 theta [degreel inside the multilayer ceramic. Vickers indentations were performed on buckle sections taken both in the parallel and Fig. 7. XRD patterns of the buckle surface, as-prepared (a) and after heat transversal direction with respect to the buckle axis. The crack treatment in air at 1600C for 4 h(b). propagation and the residual stress effect on the crack length were studied by both SEM and optical microscopy. Residual CO2). Moreover, weight loss and CO2 emission are maximum stresses were measured by microdiffraction using the DRASt next to 900C, which is a temperature corresponding to rapid (Debye Ring Analysis for STress measurement)method [41]on air combustion of carbon. the section of the SiC multilayers The thermal characterization has been followed by XRD analysis(Fig. 7). Pattern of the as-processed material shows 3. Results and discussion reflexes belonging to carbon and Sic. treatment in an oxidative environment only SiC and 3.1. Characterization of as-processed composite materials presence is observed. However, it is chiefly the buckle surface which undergoes this reaction, since in the core of the TGA analyses of as-prepared and heat treated multilayer are multilayer no oxidation is present; this is confirmed by SEM resented in Fig. 6. The curves show that the ceramic material observations on oxidised samples(Fig 8), where is seen the undergoes a significant progressive loss of weight during heating passivating layer due to oxidation of silicon carbide air in the temperature range between 700 and 1100 The mechanical behaviour of multilayers was analysed or a). This weight loss can not be ascribed to silicon carbide materials with different layer thickness. The stress-displace oxidation, since the passive oxidation mechanism involves a ment curves have the particular trend shown in Fig 9. The weight gain, and the conditions of oxidation are very far from stress/displacement curve rises up to a maximum, occurring at those typical of active oxidation. The fact that after 4 h at 1600C the failure of any SiC layer, then the stress abruptly falls, but no weight loss is observed (curve b) suggests that the without resulting in the specimen breaking. The multilayer phenomenon is due to the oxidation of carbon residues contained ceramic can still sustain stresses after the onset of fracture. The the material(carbon was added to the slurry in order to help SiC layers not yet damaged support a further stress increase, as sintering and an additional amount of carbon also forms owing to confirmed by the progressive decreasing of the the pyrolysis of plasticizers and binders) modulus of the structure. The delamination phenomena This interpretation is confirmed by mass spectrometry, since for significant sample deformation before the final breaking only mass-to-charge ratios linked to carbon are present(C, Co, The delamination mechanism, which provides a toughening b after heat treatment temperature [C Fig. 6. TGA analysis of Sic multilayer, as-prepared(a)and after heat treatment in air at 1600C for 4 h(b)
properties of the materials under investigations [39], before and after oxidation and thermal shock tests. At least three samples for each kind of specimens were examined and the corresponding compression test results were averaged. Compression tests were performed by pressing the specimens between two flat plates at ambient temperature, using a Sintech 10D equipment. These experiments, performed at a constant displacement rate of 0.5 mm/min, made it possible to obtain the stress/displacement curve as well as to calculate the radial compression strength [39]. The fracture surfaces were examined by SEM. Indentation tests were carried out in order to investigate crack propagation inside the multilayer ceramic. Vickers indentations were performed on buckle sections taken both in the parallel and transversal direction with respect to the buckle axis. The crack propagation and the residual stress effect on the crack length were studied by both SEM and optical microscopy. Residual stresses were measured by microdiffraction using the DRAST (Debye Ring Analysis for STress measurement) method [41] on the section of the SiC multilayers. 3. Results and discussion 3.1. Characterization of as-processed composite materials TGA analyses of as-prepared and heat treated multilayer are presented in Fig. 6. The curves show that the ceramic material undergoes a significant progressive loss of weight during heating in air in the temperature range between 700 and 1100 8C (curve a). This weight loss can not be ascribed to silicon carbide oxidation, since the passive oxidation mechanism involves a weight gain, and the conditions of oxidation are very far from those typical of active oxidation. The fact that after 4 h at 1600 8C no weight loss is observed (curve b) suggests that the phenomenon is due to the oxidation of carbon residues contained in the material (carbon was added to the slurry in order to help sintering and an additional amount of carbon also forms owing to the pyrolysis of plasticizers and binders). This interpretation is confirmed by mass spectrometry, since only mass-to-charge ratios linked to carbon are present (C, CO, CO2). Moreover, weight loss and CO2 emission are maximum next to 900 8C, which is a temperature corresponding to rapid air combustion of carbon. The thermal characterization has been followed by XRD analysis (Fig. 7). Pattern of the as-processed material shows reflexes belonging to carbon and SiC, while after thermal treatment in an oxidative environment only SiC and silica presence is observed. However, it is chiefly the buckle surface which undergoes this reaction, since in the core of the multilayer no oxidation is present; this is confirmed by SEM observations on oxidised samples (Fig. 8), where is seen the passivating layer due to oxidation of silicon carbide. The mechanical behaviour of multilayers was analysed on materials with different layer thickness. The stress–displacement curves have the particular trend shown in Fig. 9. The stress/displacement curve rises up to a maximum, occurring at the failure of any SiC layer, then the stress abruptly falls, but without resulting in the specimen breaking. The multilayer ceramic can still sustain stresses after the onset of fracture. The SiC layers not yet damaged support a further stress increase, as confirmed by the progressive decreasing of the Young’s modulus of the structure. The delamination phenomena allow for significant sample deformation before the final breaking. The delamination mechanism, which provides a toughening 200 M. Pavese et al. / Ceramics International 34 (2008) 197–203 Fig. 6. TGA analysis of SiC multilayer, as-prepared (a) and after heat treatment in air at 1600 8C for 4 h (b). Fig. 7. XRD patterns of the buckle surface, as-prepared (a) and after heat treatment in air at 1600 8C for 4 h (b)
M. Pavese et al. /Ceramics International 34(2008)197-203 100um Fig 8. Silica layer on the surface of the oxidised samples. 1 mm300kU 685E1 1449/01 DIPSMI C Fig. 10. Fracture surface of as-prepared SiC buckle. higher number of interfaces and a better residual strength distribution. The best materials are the one with 0. 6 mm layers since 0.4 mm layers are so thin that are more prone to suffer damage during the preparation phase. On the contrary the oxidation has a more marked effect on mechanical properties of thinner layers, even if the overall mechanical strength after b oxidation remains significantly greater for thinner layers than for thicker ones Crack deflection is another interesting issue that helps to xplain the mechanical behaviour of multilayers. Vickers indentation tests were performed on the polished section of the multilayers, and a typical result is presented in Fig. 11: the Fig.9. Stress/displacement curves of Sic multilayer buckles as-prepared (a) radial cracks formed during the indentation are very short and after heat treatment in air at 1600C for 100h(b) while tangential ones are free to move along the layer; the crack deflection is also visible for radial cracks. This suggests the presence of residual stresses. A confirmation of their presence effect, is most evident when the fracture surfaces are examined was given by performing residual stress measurements by (Fig.10) microdiffraction XRd with DRaSt method It is evident from Fig 9 that the oxidation treatment(100 h at stresses from 600 to 1200 MPa were meas the 00C)does not appreciably change the failure mode of multilayer section. multilayers These results corroborate the expected fracture mechanism The layer thickness on the other side has a rather marked the cracks cannot easily propagate from one layer to another, so influence on the mechanical properties of such materials. that each layer fails singularly and, rather than sudden fracture, Table 1 shows the radial compression strength of samples a structured curve is observed obtained from layers of different thickness(tape thickness, as Thermal shock tests were carried out on samples with measured from the blade height during tape casting, 0.4, 0.6 and 0.6 mm thick layers. The system described in Section 2.2 was used. and the results are shown in Table 2. These results shows that thinner layers give better No significant differences were observed after 10 or 50 mechanical properties to the multilayers, probably due to a thermal shock cycles, thus suggesting that thermal shock from Table I Buckles compression tests results: compression strength for different layer thickness samples 0.4 mm tape thickness 0.6 mm tape thickness 0.8 mm tape thickness As-prepared Heat treated Heat treated Heat treated Compression strength(MPa 235 184 356 210 4
effect, is most evident when the fracture surfaces are examined (Fig. 10). It is evident from Fig. 9 that the oxidation treatment (100 h at 1600 8C) does not appreciably change the failure mode of multilayers. The layer thickness on the other side has a rather marked influence on the mechanical properties of such materials. Table 1 shows the radial compression strength of samples obtained from layers of different thickness (tape thickness, as measured from the blade height during tape casting, 0.4, 0.6 and 0.8 mm). These results shows that thinner layers give better mechanical properties to the multilayers, probably due to a higher number of interfaces and a better residual strength distribution. The best materials are the one with 0.6 mm layers, since 0.4 mm layers are so thin that are more prone to suffer damage during the preparation phase. On the contrary the oxidation has a more marked effect on mechanical properties of thinner layers, even if the overall mechanical strength after oxidation remains significantly greater for thinner layers than for thicker ones. Crack deflection is another interesting issue that helps to explain the mechanical behaviour of multilayers. Vickers indentation tests were performed on the polished section of the multilayers, and a typical result is presented in Fig. 11: the radial cracks formed during the indentation are very short, while tangential ones are free to move along the layer; the crack deflection is also visible for radial cracks. This suggests the presence of residual stresses. A confirmation of their presence was given by performing residual stress measurements by microdiffraction XRD with DRAST method. Compressive stresses from 600 to 1200 MPa were measured on the multilayer section. These results corroborate the expected fracture mechanism: the cracks cannot easily propagate from one layer to another, so that each layer fails singularly and, rather than sudden fracture, a structured curve is observed. Thermal shock tests were carried out on samples with 0.6 mm thick layers. The system described in Section 2.2 was used, and the results are shown in Table 2. No significant differences were observed after 10 or 50 thermal shock cycles, thus suggesting that thermal shock from M. Pavese et al. / Ceramics International 34 (2008) 197–203 201 Fig. 8. Silica layer on the surface of the oxidised samples. Fig. 9. Stress/displacement curves of SiC multilayer buckles as-prepared (a) and after heat treatment in air at 1600 8C for 100 h (b). Fig. 10. Fracture surface of as-prepared SiC buckle. Table 1 Buckles compression tests results: compression strength for different layer thickness samples 0.4 mm tape thickness 0.6 mm tape thickness 0.8 mm tape thickness As-prepared Heat treated As-prepared Heat treated As-prepared Heat treated Compression strength (MPa) 235 184 356 210 143 123
202 M. Pavese et al. /Ceramics International 34 (2008)197-26 4. Conclusions Multilayered ceramics can be a suitable and low expensive way to obtain components apt for working at high temperature Tubular components of silicon carbide with a multilay structure were produced by tape casting and sintering without pressure. Compression tests indicated the presence of delamination phenomena that increase the multilayer toughness over that of a conventional ceramic. The layer thickness influences both the material strength and the oxidation resistance. Buckles containing thinner layers show an increased strength, even if their strength slightly decreases after oxidation 30 um at 1600C. Indentation and microdiffraction tests showed that residual stresses control the crack path. During long-term oxidation a continuous silica coating which acts as a barrier for Fig, lL. Indentation cracks on an as- prepared sample showing cracks moving a further oxygen penetration, forms. In spite of the oxidative preferentially in tangential direction and radially deflected reactions, fracture behaviour of the multilayer ceramic wa found unchanged, even after oxidation treatments carried out in very severe conditions (100 h at 1600C). Thermal shock tests showed that no loss in mechanical strength was observed and that the mechanism of crack deflection is still working even Buckles compression tests after thermal shock cycles after 50 cycles from 1070C No thermal cycling 10 cycles 50 cycles Compression strength(MPa) 287 Acknowledgement Part of this work has been performed within the framework 1070C does not have a profound effect on the mechanical of the Integrated European Project"ExtreMat"(contract NMP properties of such composites CT-2004-500253)with financial support by the Europear Indentation tests were carried out on cycled samples in order Community. It only reflects the view of the authors and the to verify if the crack deflection mechanism was yet active. A European Community is not liable for any use of the typical result is presented in Fig. 12, where it is possible to see information contained therein that the cracks travel only in the tangential direction while are stopped in the radial one. This confirms that residual stress is References still present even after 50 thermal shock cycles, and guide the []O. Dugne, S. Prouhet, A Guette, R. Naslain, R. Fourmeaux, Y. Khin,J. Sevely, J. P. Rocher, J. Cotteret, Interface characterization by TEM, AES and SIMS in tough SiC (ex-PCS)fibre-SiC(CVI) matrix composites with a BN interphase, J. Mater. Sci. 28(1993)3409-3422. [2] O. Lujt, Identification of carbon sublayer in a Hi-Nicalon/BN/SiC com- J. Mater.Sci.Lett.18(1999)1825-1828. B3] R.E. Mistler, E.R. Twiname, Tape Casting. Theory and Practice, The American Ceramic Society. USA, 2000 [4] P. Boch, T. Chartier, M. Huttepain, Tape casting of Al]O3/ZrO2 laminated mposites,. Am. Ceram Soc. 69(1986)C191-C192. [5]A I.Y. Tok, F.T.C. Boey, K.A. Khor, Tape casting of high .Eur. Ceram.Soc.2002000)1691-1697 [7 G.J. Zhang, X.M. Yue, T. Watanabe, AlO,TiC/(MoSig + Mo2Bs)multi- layer composite prepared by tape casting, J. Eur. Ceram. Soc. 19(1999) 2111-2116. [8] M.P. Albano, L B. Garrido, Influence of the slip composition on the properties of tape-cast alumina substrates, Ceram. Int. 31(2005)57-66. 50 Am. [9] C. Badini, P. Fino, A. Ortona, C. Amelio, G. Pasquale, Processing of multilayered SiC ceramic by tape casting and sintering, in: Proceedings of the EuroMat 2001, 2001, published on CD-ROM, ISBN: 88-85298-39-7 [10] M. Jimenez-Melendo, C. Clauss, A Dominguez-Rodriguez, Microstruc- Fig. 12. Indentation cracks on a thermally cycled sample, showing long ture and high-temperature mechanical behaviour of alumina/alumina- tangential cracks deflected toward the external surface Radial cracks are absent yttria-stabilized tetragonal zirconia multilayer composites, J. Am. Ceram. or very short Soe.80(1997)2126-2130
1070 8C does not have a profound effect on the mechanical properties of such composites. Indentation tests were carried out on cycled samples in order to verify if the crack deflection mechanism was yet active. A typical result is presented in Fig. 12, where it is possible to see that the cracks travel only in the tangential direction while are stopped in the radial one. This confirms that residual stress is still present even after 50 thermal shock cycles, and guide the crack path. 4. Conclusions Multilayered ceramics can be a suitable and low expensive way to obtain components apt for working at high temperature. Tubular components of silicon carbide with a multilayer structure were produced by tape casting and sintering without pressure. Compression tests indicated the presence of delamination phenomena that increase the multilayer toughness over that of a conventional ceramic. The layer thickness influences both the material strength and the oxidation resistance. Buckles containing thinner layers show an increased strength, even if their strength slightly decreases after oxidation at 1600 8C. Indentation and microdiffraction tests showed that residual stresses control the crack path. During long-term oxidation a continuous silica coating, which acts as a barrier for a further oxygen penetration, forms. In spite of the oxidative reactions, fracture behaviour of the multilayer ceramic was found unchanged, even after oxidation treatments carried out in very severe conditions (100 h at 1600 8C). Thermal shock tests showed that no loss in mechanical strength was observed and that the mechanism of crack deflection is still working even after 50 cycles from 1070 8C. Acknowledgements Part of this work has been performed within the framework of the Integrated European Project ‘‘ExtreMat’’ (contract NMPCT-2004-500253) with financial support by the European Community. It only reflects the view of the authors and the European Community is not liable for any use of the information contained therein. References [1] O. Dugne, S. Prouhet, A. Guette, R. Naslain, R. Fourmeaux, Y. Khin, J. Sevely, J.P. Rocher, J. Cotteret, Interface characterization by TEM, AES and SIMS in tough SiC (ex-PCS) fibre-SiC (CVI) matrix composites with a BN interphase, J. Mater. Sci. 28 (1993) 3409–3422. [2] O. Lujt, Identification of carbon sublayer in a Hi-Nicalon/BN/SiC composite, J. Mater. Sci. Lett. 18 (1999) 1825–1828. [3] R.E. Mistler, E.R. Twiname, Tape Casting. Theory and Practice, The American Ceramic Society, USA, 2000. [4] P. Boch, T. Chartier, M. Huttepain, Tape casting of Al2O3/ZrO2 laminated composites, J. Am. Ceram. Soc. 69 (1986) C191–C192. [5] A.I.Y. Tok, F.T.C. Boey, K.A. Khor, Tape casting of high dielectric ceramic composite substrates for microelectronic application, J. Mater. Proc. Technol. 89/90 (1999) 508–512. [6] Z. Yuping, J. Dongliang, P. Greil, Tape casting of aqueous Al2O3 slurries, J. Eur. Ceram. Soc. 20 (2000) 1691–1697. [7] G.J. Zhang, X.M. Yue, T. Watanabe, Al2O3/TiC/(MoSi2 + Mo2B5) multilayer composite prepared by tape casting, J. Eur. Ceram. Soc. 19 (1999) 2111–2116. [8] M.P. Albano, L.B. Garrido, Influence of the slip composition on the properties of tape-cast alumina substrates, Ceram. Int. 31 (2005) 57–66. [9] C. Badini, P. Fino, A. Ortona, C. Amelio, G. Pasquale, Processing of multilayered SiC ceramic by tape casting and sintering, in: Proceedings of the EuroMat 2001, 2001, published on CD-ROM, ISBN: 88-85298-39-7. [10] M. Jime`nez-Melendo, C. Clauss, A. Domı`nguez-Rodrı`guez, Microstructure and high-temperature mechanical behaviour of alumina/alumina– yttria-stabilized tetragonal zirconia multilayer composites, J. Am. Ceram. Soc. 80 (1997) 2126–2130. 202 M. Pavese et al. / Ceramics International 34 (2008) 197–203 Fig. 11. Indentation cracks on an as-prepared sample, showing cracks moving preferentially in tangential direction and radially deflected. Table 2 Buckles compression tests after thermal shock cycles No thermal cycling 10 cycles 50 cycles Compression strength (MPa) 287 311 269 Fig. 12. Indentation cracks on a thermally cycled sample, showing long tangential cracks deflected toward the external surface. Radial cracks are absent or very short
M. Pavese et al. /Ceramics International 34(2008)197-203 [11] J. She, T. Inoue, K. Ueno, Multilayer AlO/SiC ceramic with [27] G. Blugan, R. Dobedoe, M. Lugovy, S. Koebel, J. Kuebler, Si3N4-TiN improved mechanical behaviour, J. Eur. Ceram. Soc. 20(2000)1771 based micro-laminates with rising R-curve behaviour, Compos Part B 1775 Eng37(2006459-465 [12] H. Liu, S M. Hsu, multilayer silicon nitride/boron [28] V.M. Sglavo, M. Bertoldi, Design and production of ceramic laminates nitride ceramics. J m 79(1996)2452-24 high mechanical reliability, Compos Part B: Eng 37(2006)481-489 [13] Y Shigeaki, M.E. Toriyama, S Kanzaki, Processing [29] M.J.H. Balat, Determination of the active-to-passive transition in the f a novel multilayered silicon nitride, J. Am. Ceram. Soc. 79(1996) nidation of silicon carbide in standard and microwave-excited air. J 197-2200 Eur. Ceram. Soc. 16(1996)55-62. [14] WJ. Clegg, The fabrication and failure of laminar ceramic composites, [30] K.G. Nickel, The role of condensed silicon monoxide in the active-to- Acta Metall. Mater. 40(1992)3085-3093 [15] R. Huang, H. Gu, J. Zhang, D. Jiang, Effect of Y2Ox-AlO3 ratio on inter- massive oxidation transition of silicon carbide, J. Eur. Ceram. Soc. 9(1992) granular phases and films in tape-casting a-SiC with high toughness, Acta [31] E.A. Gulbransen, S.A. Jansson, The high-temperature oxidation, reduction Mater.53(2005)2521-2529 and volatilization reactions of silicon and silicon carbide Oxid. Met. 4 [16] J.X. Zhang, D L. Jiang, Sh.Y. Qin, Zh. R. Huang, Fracture behavior of (1972)181-201 SiC composites, Ceram. Int. 30(2004)697-709 [32] W.L. Vaughn, H.G. Maahs, Active-to-passive transition in the oxidation of [17]J R. Huang, H. Gu, D. Jiang, Q. Lin, Z Huang, High toughness silicon carbide and silicon nitride in air, J. Am. Ceram. Soc. 73(1990) d SiC ceramics from aqueous tape casting, Scr. Mater. 52(2005) [33] H.E. Kim, AJ. Moorehead, Effect of active oxidation on the flexural [18] C. Reynaud, F. Thevenot, T. Chartier, J. -L. Besson, Mechanical propertie trength of a-SiC, J. Am. Ceram Soc. 73(1990)1868-1872 and mechanical behaviour of Sic dense-porous laminates, J. Eur. Ceram. [34] H.E. Eaton, G D. Linsey, Accelerated oxidation of SiC CMCs by water Soc.2502005)589-59 vapor and protection via environmental barrier coating approach, J.Eur. [19] D. Leguillon, S. Tariolle, E. Martin, T. Chartier, J. L. Besson, Prediction of Ceran.Soc.22(2002)2741-2747 crack deflection in porous/dense ceramic laminates, J. Eur. Ceram Soc. 26 [35] E.J. Opila, R.E. Hann Jr, Paralinear oxidation of CVD SiC in water vapor, (2006)343-349 J.Am. Ceran.Soc.80(1997)197-205 [20] T. Watanabe, G. Zhang, X.M. Yue, Y. Zeng K. Shobu, N. Bahlawane, [36] E.J. Opila, D.S. Fox, N.S. Jacobson, Mass spectrometric identification of Multilayer composites in Al2O3/MoSi] system, Mater. Chem. Phys. 67 Si-O-OHg) species from the reaction of silica with water vapor at (2001)256-26 atmospheric pressure, J. Am. Ceram Soc. 80(1997)1009-101 [21] S.Y. Park, B. Saryhan, H. Schneider, Mullite/zirconia laminate composites [371 E.J. Opila, J.L. Smialek, R.C. Robinson, D.S. Fox, N.S. Jacobson, SiC for high temperature application, J. Eur. Ceram Soc. 20(2000)2463- recession caused by SiO2 scale volatility under combustion conditions. Il. Thermodynamics and gaseous diffusion model, J. Am. Ceram. Soc. 82 [22] S. Qin, D. Jiang, J. Zhang, J. Qin, Design, fabrication and properties of (1999)1826-183 layered SicTiC ceramic with graded thermal residual stress, J. Eur. [38] R.C. Robinson, J. L. Smialek, SiC recession caused by SiO, scale volatility Ceran.Soc.23(2003)1491-1497 under combustion conditions. I mental results and empirical 3] C.A. Folson, F.w. Zok, F.E. Lange, Flexural properties of brittle model, J. Am. Ceram Soc. 82(1999)1817-1825 multilayer materials. I. Modelling. J. Am. Ceram. Soc. 77(1994)689-[39] C. Badini, P. Fino, A. Ortona, C. Amelio, High temperature oxidation of ultilayered SiC processed by tape casting and sintering, J. Eur. Ceram. 24]O. Prakash, P. Sarkar, P.S. Nicholson, Crack defection in ceramic/ceramic Soc.22(2002)2071-2079 laminates with strong interfaces, J. Am. Ceram. Soc. 78(1995)1125- [40 Standard UNI EN ISo 2739: 2000, Bussole metalliche sinterizzate Determinazione della za allo schiacciamento radiale (update of 5] R. Lakshminarayanan, D.K. Shetty, R.A. Cutler, Toughening of layered ISo 2739: 1973. Sintered metal bushes. Determination of radial crushing ceramic composites with residual surface compression, J. Am. Ceram. Soc.79(196)7987. 141 M. Geli, E. B R. Roberti. L.E. 0, X-ray diffraction Debye 26 D.J. Green, PZ. Cai, L. Messing, Residual stresses in alumina-zircc Ring Analysis for STress measurement(DRAST): a new method to aminates, J. Eur. Ceram. Soc. 19(1999)2511-2517 evaluate residual stresses, Acta Mater. 52(2004)583-589
[11] J. She, T. Inoue, K. Ueno, Multilayer Al2O3/SiC ceramic with improved mechanical behaviour, J. Eur. Ceram. Soc. 20 (2000) 1771– 1775. [12] H. Liu, S.M. Hsu, Fracture behaviour of multilayer silicon nitride/boron nitride ceramics, J. Am. Ceram. Soc. 79 (1996) 2452–2457. [13] Y. Shigegaki, M.E. Brito, K. Hirao, M. Toriyama, S. Kanzaki, Processing of a novel multilayered silicon nitride, J. Am. Ceram. Soc. 79 (1996) 2197–2200. [14] W.J. Clegg, The fabrication and failure of laminar ceramic composites, Acta Metall. Mater. 40 (1992) 3085–3093. [15] R. Huang, H. Gu, J. Zhang, D. Jiang, Effect of Y2O3–Al2O3 ratio on intergranular phases and films in tape-casting a-SiC with high toughness, Acta Mater. 53 (2005) 2521–2529. [16] J.X. Zhang, D.L. Jiang, Sh.Y. Qin, Zh.R. Huang, Fracture behavior of laminated SiC composites, Ceram. Int. 30 (2004) 697–709. [17] J. Zhang, R. Huang, H. Gu, D. Jiang, Q. Lin, Z. Huang, High toughness in laminated SiC ceramics from aqueous tape casting, Scr. Mater. 52 (2005) 381–385. [18] C. Reynaud, F. The´venot, T. Chartier, J.-L. Besson, Mechanical properties and mechanical behaviour of SiC dense-porous laminates, J. Eur. Ceram. Soc. 25 (2005) 589–597. [19] D. Leguillon, S. Tariolle, E. Martin, T. Chartier, J.L. Besson, Prediction of crack deflection in porous/dense ceramic laminates, J. Eur. Ceram. Soc. 26 (2006) 343–349. [20] T. Watanabe, G.J. Zhang, X.M. Yue, Y. Zeng, K. Shobu, N. Bahlawane, Multilayer composites in Al2O3/MoSi2 system, Mater. Chem. Phys. 67 (2001) 256–262. [21] S.Y. Park, B. Saryhan, H. Schneider, Mullite/zirconia laminate composites for high temperature application, J. Eur. Ceram. Soc. 20 (2000) 2463– 2468. [22] S. Qin, D. Jiang, J. Zhang, J. Qin, Design, fabrication and properties of layered SiC/TiC ceramic with graded thermal residual stress, J. Eur. Ceram. Soc. 23 (2003) 1491–1497. [23] C.A. Folson, F.W. Zok, F.F. Lange, Flexural properties of brittle multilayer materials. I. Modelling, J. Am. Ceram. Soc. 77 (1994) 689– 696. [24] O. Prakash, P. Sarkar, P.S. Nicholson, Crack deflection in ceramic/ceramic laminates with strong interfaces, J. Am. Ceram. Soc. 78 (1995) 1125– 1127. [25] R. Lakshminarayanan, D.K. Shetty, R.A. Cutler, Toughening of layered ceramic composites with residual surface compression, J. Am. Ceram. Soc. 79 (1996) 79–87. [26] D.J. Green, P.Z. Cai, L. Messing, Residual stresses in alumina-zirconia laminates, J. Eur. Ceram. Soc. 19 (1999) 2511–2517. [27] G. Blugan, R. Dobedoe, M. Lugovy, S. Koebel, J. Kuebler, Si3N4–TiN based micro-laminates with rising R-curve behaviour, Compos Part B: Eng 37 (2006) 459–465. [28] V.M. Sglavo, M. Bertoldi, Design and production of ceramic laminates with high mechanical reliability, Compos Part B: Eng 37 (2006) 481–489. [29] M.J.H. Balat, Determination of the active-to-passive transition in the oxidation of silicon carbide in standard and microwave-excited air, J. Eur. Ceram. Soc. 16 (1996) 55–62. [30] K.G. Nickel, The role of condensed silicon monoxide in the active-topassive oxidation transition of silicon carbide, J. Eur. Ceram. Soc. 9 (1992) 3–8. [31] E.A. Gulbransen, S.A. Jansson, The high-temperature oxidation, reduction and volatilization reactions of silicon and silicon carbide, Oxid. Met. 4 (1972) 181–201. [32] W.L. Vaughn, H.G. Maahs, Active-to-passive transition in the oxidation of silicon carbide and silicon nitride in air, J. Am. Ceram. Soc. 73 (1990) 1540–1543. [33] H.E. Kim, A.J. Moorehead, Effect of active oxidation on the flexural strength of a-SiC, J. Am. Ceram. Soc. 73 (1990) 1868–1872. [34] H.E. Eaton, G.D. Linsey, Accelerated oxidation of SiC CMC’s by water vapor and protection via environmental barrier coating approach, J. Eur. Ceram. Soc. 22 (2002) 2741–2747. [35] E.J. Opila, R.E. Hann Jr., Paralinear oxidation of CVD SiC in water vapor, J. Am. Ceram. Soc. 80 (1997) 197–205. [36] E.J. Opila, D.S. Fox, N.S. Jacobson, Mass spectrometric identification of Si–O–OH(g) species from the reaction of silica with water vapor at atmospheric pressure, J. Am. Ceram. Soc. 80 (1997) 1009–1012. [37] E.J. Opila, J.L. Smialek, R.C. Robinson, D.S. Fox, N.S. Jacobson, SiC recession caused by SiO2 scale volatility under combustion conditions. II. Thermodynamics and gaseous diffusion model, J. Am. Ceram. Soc. 82 (1999) 1826–1834. [38] R.C. Robinson, J.L. Smialek, SiC recession caused by SiO2 scale volatility under combustion conditions. I. Experimental results and empirical model, J. Am. Ceram. Soc. 82 (1999) 1817–1825. [39] C. Badini, P. Fino, A. Ortona, C. Amelio, High temperature oxidation of multilayered SiC processed by tape casting and sintering, J. Eur. Ceram. Soc. 22 (2002) 2071–2079. [40] Standard UNI EN ISO 2739:2000, Bussole metalliche sinterizzate – Determinazione della resistenza allo schiacciamento radiale (update of ISO 2739:1973, Sintered metal bushes. Determination of radial crushing strength). [41] M. Gelfi, E. Bontempi, R. Roberti, L.E. Depero, X-ray diffraction Debye Ring Analysis for STress measurement (DRAST): a new method to evaluate residual stresses, Acta Mater. 52 (2004) 583–589. M. Pavese et al. / Ceramics International 34 (2008) 197–203 203
