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MICROANALYTICAL INVESTIGATIONS OF
REINFORCING COMPONENTS AND COMPOSITES WITH BRITTLE MATRIX
B. Meier, R. Hamminger, G. Grathwohl
To cite this version:
B. Meier, R. Hamminger, G. Grathwohl. MICROANALYTICAL INVESTIGATIONS OF REIN-
FORCING COMPONENTS AND COMPOSITES WITH BRITTLE MATRIX. Journal de Physique
Colloques, 1990, 51 (C1), pp.C1-885-C1-889. �10.1051/jphyscol:19901139�. �jpa-00230050�
COLLOQUE D E PHYSIQUE
Colloque C l , supplbment au n o l , Tome 5 1 , j a n v i e r 1990
MICROANALYTICAL INVESTIGATIONS OF REINFORCING COMPONENTS AND COMPOSITES WITH BRITTLE MATRIX
B . MEIER, R . HAMMINGER* and G . GRATHWOHL
I n s t i t u t fiir Werkstoffkunde 1 1 , U n i v e r s i t d t Karlsruhe ( T H ) ,
! c a i s e r s t r a s s e 1 2 , 0-7500 Karlsruhe, F . R . G . HOECHST A G , 0-6230 Frankfurt 8 0 , F . R . G .
U D e sd r i e s de micro-anelyses ont e t 6 e f f e c t k sur des Cehantillans de f i b r e cerarnique (Sic-fibre/Nicalon, C - f i b r e revetue de Tinx), Sic-lamelles/Pmerican Matrix, a i n s i quc sur &S ceramiques r e n f o r c k s par ces fibres (RBSN, vitro-c6remique en a l u n i n o s i l i c a t e de mgresiun e t des verres en s i l i c a t e de zircon. Ces examens ont et6 e f f e c t d s h L'aidede~lamicroscopie en L m i & r e p o l a r i s k , d i f f r a c t i o n de rayons X, nicroscopie 6lectronique B batayage avec analyse dispersive d ' h r g i e , spectroscopie pour analyse chimiqw, spectroscopie 6 l e c t r o n i q w Auger h haute r6solution. rnicroscopie Clectronique h transmission e t h balayage a i n s i q u e d i f f e r e n r e s mgthodes c h i r n i q Les r 6 s u l t a t s des enalyseschimiquessont danCs pour Les c v e n t s de r e n f o r t dans Leurs 6tats i n i t i a u x , a i n s i q u ' u n conperaison avec leurs interfaces avec La m t r i c e dens les mat6riaux carposite.
Abstract. Microanelytical investigations have been mede on senples o f ceramic f i b r e s (Sic-fibrelNlULON, C-fibre coated u i t h TiNx), Sic-platelets/AWERICAN MATRIX as well as on reinforcedceramics (RBSN matrix
.
nagnesiuna l u n i n o s i l i c a t e glass-ceramic and zirconia s i l i c a t e glass). Polarizing microscopy, X-ray d i f f r a c t i o n , scanning electron microscopy u i t h energy dispersive analysis, electron spectroscopy f o r chemical analysis, high resolution Auger electron spectroscopy, scanning transmission electron microscopy and several chemical methods uere enployed for these exminations. The r e i n f o r c i n g components are analysed i n t h e i r i n i t i a l s t a t e and a carparison i s given u i t h respect t o the interface between reinforcing carponent and matrix i n the conposites.
Advanced ceramics improved considerably, e.g. in strength and other mechanical properties, due to more qualified processing procedures during recent years. However, the inherent brittleness is still a main problem also for advanced ceramics and glasses. Therefore, the addition of reinforcing components such as long ceramic fibres or platelets into these brittle matrices is a promising way to overcome this problcm. The combination of two brittle materials can lead to a "tough" composite material. The increased toughness is due to several reinforcing mechanisms such as crack deviation, formation of multiple cracks or pull-out. It is a well known fact that these processes are concentrated at the interface between the reinforcing component and the matrix. Since both, the chemical state as well as the bonding strength of these boundaries are decisive factors for the efficiency of the reinforcing mechanisms, it is necessary to clear up the conditions of the interfaces. This is also important with respect to the optimization of the processing of the composite materials.
Main questions in this context are related to the chemical compatibility of the reinforcing component and the matrix. On the other hand the influence of impurities on both the optimized densification process and the properties of the composite has to be regarded. In order to characterize any chemical changes at the interfaces and to provide the basis for an optimized processing it is important to analyze first the reinforcing components in their initial states.
The investigations have been made by polarizing microscopy, X-ray diffraction, scanning electron microscopy with energy dispersive analysis (SEM/EDX), electron spectroscopy for chemical analysis (ESCA), high resolution Auger electron spectroscopy (HRAES), scanning transmission electron microscopy (STEM) and several chemical methods. By the combination of these methods the chemical state of three different reinforcing components (Sic-fibre/NICALON, C-fibre coated with TiN ) and SIC-platelets/AMERICAN MATRIX could be achieved. Furthermore, three composites were examine& Sic-platelets reinforced reaction bonded silicon nitride (RBSN), Sic-fibre/NICALON reinforced magnesium aluminosilicate glass-ceramic and C-fibre coated with TiNx reinforced zirconiasilicate glass. Taking into account that all these reinforcing components and composites are quite heterogenous in their physical and chemical properties (e.g. electrical conductivity) the use of the various analytical techniques is necessary.
The Sic-platelets have been supplied by AMERICAN MATRIX (USA). Three samples with different fractions of medium size of the platelets have been investigated: 140-70 pm, 70-45 pm and one sample with all particle sizes. The Sic-platelets reinforced reaction bonded silicon nltride (RBSN) composite materials have been produced at the HOECHST AG (Frankfurt, FRG). The nitridation of samples has partly been performed at AB Glas und Keramik (TU Hamburg-Harburg, FRG). The final density of RBSNISiC composites containing 10 wt-% Sic-platelets was about 80% of the theoretical value / l / .
The Sic-fibres have been manufactured by NIPPON CARBON (Japan). Their trade-name is NICALON. The diameter distribution is extended from 6 to 25 pm with a maximum f r e q u e n c e a t 14 pm. The processing of the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901139
Cl-886 COLLOQUE DE PHYSIQUE
composite using this reinforcing component has been performed at the Institut fiir Nichtmetallische Werkstc (TU Berlin, FRG). The matrix is based on a magnesium aluminosilicate glass-ceramic with additives of T i ,
Ba. The characteristic steps of the processing are: burning off sizing-slurry technique-fibre winding- pressing-partially crystallization of the matrix /2/.
Different types of C-fibres have been coated with TINx at the Institut fiir Chemische Technik (Univers Karlsruhe, FRG) by means of CVD technique. The coated C-fibres were incorporated into a zirconiasilic glass matrix which was prepared making use of a sol-gel-technique. Finally, the densification of the prepre;
provided by hot pressing /3/, both being performed at the Institut fiir Chemische Technik (Univers Karlsruhe, FRG).
Optical microscopy was done using a Zeiss polarizing microscope. Polished sectiohs of about 20 Prn in thickr were prepared f o r this investigation. Phase composition was studied using a Siemens D 500 diffractometer u Cu K, irradiation. The samples were pulverized to particle sizes smaller than 20 pm to reduce any prefer orientations and particle size effects. Auger electron spectroscopy was carried out with a Perkin Elmer (I 600) instrument. Typical working conditions for Auger electron exitation were: primary electron beam curr of ca. 20nA and electron energy of 10 keV. Depth profiles were obtainable by sputtering with an Ar ion be of 6 keV primary energy impinging at an angle of 45' perpendicular to the sample surface. A scann electron microscope (Jeol, Type JSM 840) equipped with an energy dispersive analyser (Kevex Quant Detector) was employed for these examinations. The ESCA investigations were carried out using a Per Elmer (ESCA 5400) instrument. Mg K, radiation was used with a typical beam diameter of about Imm.
STEM-observations have been performed by a 300keV Philips CM30 microscope.
1II.Results and Discussion
Sic-fibre /magnesium aluminosilicate glass-ceramic matrix
The chemical composition of the fibres has been reported earlier /4/ where it was shown that the fibres h:
reached a higher purity concerning impurity elements such as Fe, Cr, Ni, etc. in comparison to earlier analy:
In order to provide a better handling, the fibre bundels have been coated with polyvinyl acetate by 1 manufacturer. This layer is normally removed by burning off in air. Using HRAES, depth profiles on 1
cylindrical surface have been measured as well as the in situ fractured surface of the fibres have be examined /5/. Obviously, the thermal decomposition of the sizing leads to a thin C-rich layer with 0 on I outermost cylindrical surface (about 20-30nm). In addition, the cylindrical surface of the desized fibres ha been examined using ESCA (Fig. l).
98 m ~a, 1111 Im Im IM 1s 106 107 ~ o s
8 1 n d i n g Energy [ e V )
Fig.1: ESCA investigation of the desized Sic-fibres/Nicalon.
It follows from these results that the Si on the fibre surface is in an oxidized state, but no indication for : silicon oxycarbide phase was found in this layer. In order to understand the interface fibre-matrix in 11 composite Sic-fibre/magnesium aluminosilicate glass-ceramic the samples have been in situ fractured in tl HRAES stage (Fig.2). It can be seen that a porous region exists between fibre and matrix. Considering t1 Auger images it becomes clear that this region is enriched with Ti,C,O. The C Auger image shows also an (
rich layer on the cylindrical fibre surface. During fracture the fibre is detached between the C-rich layer a1 the porous Ti-enriched regions. Such C enrichments at interfaces are typical for Sic-fibres/Nicalon composites e.g. /5,6,7/. In comparison with the initial state of the fibre it became evident that the thickness I this layer increased during the sintering process. The question concerning the formation of the Ti enrichc porous region can not be answered so far. Possibly. Ti is active as a foaming agent as it is known fro Mn,Co.Cr /8/. Thus, titaniumoxide will react with the C at the interface from which CO gas is forme
Furthermore, no diffusion of matrix elements into the fibre was detected with HRAES as well as with SEM+EDX. The latter technique gave evidence that the wettability between matrix and fibre is not favorable.
Fig.2: HRAES investigation of the composite Sic-fibre/magnesium aluminosilicate glass-ceramic with additves of Ba and Ti. a,b: secondary electron images; c-f: Auger images of Si(c), C(d), Ti(e), O(f).
C-fibre coated with TiN,/zirconiasilicate matrix -
The coated fibre was chosen to protect the fibre against oxidation in the glass matrix. It has been shown that the coating consists of three regions, the C-fibre, followed by a Ti-C-N region and on top of this TiN, / S / . According to the HRAES analysis the coating occurs continuously, i.e. there are no variations in chemical composition during deposition. The TiN, coating was quantified as TiNo.g-O- depending on the quality of the sample., This was confirmed by X-ray powder diffraction. Furthermore, the
RUES
investigation reveals that a diffusion of C into the coating occurs.The investigation of the composite has been carried out using HRAES and STEM. In Fig.3 the HRAES analysis is presented.COLLOQUE DE PHYSIQUE
Fig.3: HRAES investigation of the in situ fractured composite C-fibre coated with TiNx/zirconiasi matrix. =secondary electron image; b,c,d: point analysis; e:depth profile at point 2.
The analysis at point 2 shows that the coating was found to be stable during processing. Furthermore, no F N -layer was detected, because C was observed all over the coating as it can be seen at the depth p ( ~ i g 5 d ) . A diffusion of C has been taken place during processing. Regarding thc analysis at po (cylindrical fibre surface) and point 3 (fibre imprint in the matrix) a high concentration of C is mea (Fig.3a). From this it may be concluded that during fracture process the fibre is detached from the mat]
its outermost regions. The matrix/coating bond seems to be stronger than the fibre/coating bond.
behaviour can be explained regarding the microstructure of the incorporated fibres. It is a well known that these fibres have a skin/core heterogeneity; the skin having larger, better orientated crystailites tha core / 9 / . In the skin region, the layer planes are essentially parallel to the surface. Hence, the fibre posse anisotropy of the properties e.g. the thermal expansion coefficient, which is larger in the lateral directiol also larger compared to the zirconiasilicate matrix or fibre. Possibly. during cooling of the composite the shrinks more than the matrix. For that reason, the interface fibre/coating especially the outermost regio the C-fibre represents a weak spot in the composite material.
Sic-wlatelets/RBSN matrix
The results of microanalytical investigations of three fractions of Sic-platelets were reported earlier /10/.
chemical analyses shows a lot of typical impurity elements as well as doping additives of S i c cers (Si,C,Fe,Mg,Al,Ti,Cr,Ni,Zr.B). The predominating phases are the 6H and the 4H polytypes of a Observations with the polarizing microscope gave evidence that the large platelets are not always single.'cr:
but intergrowths of some SIC crystals. Furthermore, a significant difference in habit and particle shape ci seen, in particular the platelets are not always flat but also spherical, conical, needle-like, etc. in shapt. I microanalytical techniques the local distribution of the impurity elements have been examined. Most of elements were found to be present on the outermost surfaces (e.g. B or AI) of the platelets or as inclu (e.g. Fe, Ni, Si). Using these platelets composites with RBSN matrix have been prepared. The optimizatic the nitridation process was indispensable to reach the stability region for the platelets during formation o RBSN matrix. No reaction between platelet and matrix have been observed using polarizing microscopy.
platelets have been randomly distributed. Using SEM/EDX a third phase has been identified as a Fe-MI phase which refers to the contamination of both the Sic-platelets as well as the Si-powders. It is a well kr fact that such phases are found to be present in RBSN matrices / l 1,12/. The presence of Fe during R formation favours the nitridation process /13/. In contrast to the former two composites HRAES was suitable for analyzing the interfaces between platelets and matrix because of the extremly poor elecl conductivity of the RBSN matrix. This difference is especially pronounced due to the very small size oi platelets in contrast to long fibres in other matrices for which the earthing to the sample holder is much b
to be realized. Therefore, the characteristics of the interface have been investigated using STEM (Fig.4).
Fig.4: STEM .investigation of the composite of Sic-platelet/RBSN.
In Fig.4a the well-defined interfaces between Sic-platelet and RBSN are illustrated. Moreover, the elements which have been found on the outermost surface of the platelets in their initial state could not be detected anymore at the interface. These elements have been presumably distributed in the matrix. Furthermore, the bright field images gave evidence that the surfaces of the platelets show very low (a) as well as high (b) concentrations of dislocations. Such dislocations have been observed to be generated during 0- to rw-Sic transformation /14/. It is supposed that these dislocations are presented in their initial state; due to its moderate temperature the nitridation process is considered to be inactive in phase transformation induced generation of dislocations.
Finally, it is a known fact from literature that platelets are as effective as whiskers by increasing the fracture energy of composites due to interactions with growing cracks e.g./15/. Thus, the influence of the microstructure of the composite material and especially the role of the interfaces platelet/matrix on the properties of the composite are subjects of high interests to be examined in the future.
Acknowlednemen~.The authors would like to thank Prof.Brlickner (TU Berlin,FRG) and Prof.Fitzer (Universitat Karlsruhe, FRG) for supplying samples. The ESCA investigations were performed by Dr.Roll (Perkin Elmer, Vaterstetten, FRG). We also thank Mr.Nold (Kernforschungszentrurn Karlsruhe, FRG) for valuables discussion and help with the experimental work. The investigations have partly been supported by the Federal Minister of Research and Technology under code number 03M10051CI. The authors are responsible for the content of this publication.
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