Oxide ceramic fibre reinforced material and the use thereof

Translation of description of equivalent WO 03045874 A2

p0001Oxide ceramic fiber composites and their use

p0002Field of the Invention The invention relates to oxide-ceramic fiber composites, as are, for example, according to the invention in systems for energy conversion using.

p0003State of the art in the interest in future considerably to enhancing energy yields are observed> 1400 ° C increased efforts to provide ceramic materials for energy production facilities at temperatures. Congruent estimates are assuming that for load-bearing components under such extreme conditions

p0004- Both sufficient damage tolerance (ie thermal shock stability and high room-temperature fracture toughness of actually brittle ceramics) as well as for demanding creep stability of the components will be realized only by means of a sit-Faserkompo- Designes,

p0005- The problem of thermodynamic Hocfttemperafur stability in oxidizing atmospheres only with per se oxidation resistant oxide ceramics is controlled permanently.

p0006Similarly differentiated claims related. Damage tolerance, temperature and corrosion stability must meet high performance brake discs. At present, the mechanical and thermophysical requirements are best fulfilled by composites made of long carbon fibers with reaction bonded SiC matrix, the corrosion resistance, however, is limited, just because the corrosion along the non-oxide fibers progresses to the inside of the discs. It has been observed that the use of short carbon fibers, although the corrosion stability is improved, on the other hand, the level of mechanical properties is limited (Ga R. Dow, pp 15-29 in. Ceram Eng Be Proc Vol 21/3,.... The Am. Ceram. Soc, Wester- ville / OH, 2000). Pure oxide composites could therefore provide a favorable mechanical recycling alternative here. Given the complex mechanical requirements is obvious with the above estimates for all such applications that the mechanical properties of the fiber / matrix bonding plays a crucial role in the realization of such oxide composites. However, It is also evident, which is why it has been impossible to develop a concept that the demands for thermal shock stability and to creep is alike:

p0007- The requirement for transmission of the creep resistance of a (preferably multi-dimensional) fiber structure to the component as a whole leads, at least with a view to long-term use> 10.000 hours for obvious demand for a fixed fiber / matrix bond (IW Donald among others, J. Mater . Be 11 (1976) 5, 949-972). AG Evans and others, S. 929-955 in Creep and Fracture of Engineering Materials, London, 1987). Although one could hope to achieve through a multi-dimensional fiber design a certain macroscopic dimensional stability of the parts even without a fixed fiber / matrix bonding solely by a high creep resistance of the fiber arrangement is, however, atmospheres and high pressure gradients under conditions of flowing in turbines very given complex stresses (including with respect to erosion load) to 1400 ° C hardly which can withstand a longer period composites generally weak interfaces at temperatures.

p0008- In contrast, the demand for a thermoschockrestistenten for repetitive strain ceramic appears currently only be achieved with a composite design, the consumption of (brittle) fracture energy (dissipation of energy) after stretching and partial breakdown of fibers by means of a considerable shear deformation along the fiber / matrix interfaces implemented (crack deflection, "pull-out" - or "debonding' effects, made possible by weak fiber / matrix bond).

p0009In implementation of this understanding a variety of proposals have been made which include all, without exception, the formation of weak fiber / matrix interfaces (interfaces). This is done partly by fiber coating and partly by a corresponding design of the matrix.

p0010To describe Pejryd others (EP 639 165 A1, US 5,567,518), "a ceramic composite, particularly for use at Temperatures above 1400 ° C" with a stack of oxide Fiber / oxide matrix and a per claim such specific material selection and possibly greater coating thickness of the fibers> 2 microns can be achieved that explicitly weak interfaces ( "weak bond Nable to debonding"). As in most of the known from the literature suggestions is however also the real utility for the declared application does not disclose: the "proof" of power consumption at break (in the form of non-linear stress-strain effects) occurs in the examples by performing tests at room temperature, especially for the real thermal shock behavior and creep stability no conclusions permits from the interventions mentioned as a target beyond 1400 ° C.

p0011Saruhan-Brings others (EP 890 559 A1) describes a "process for coating oxidic fibrous materials for the production failure tolerant hochtemperaturfe- art oxidation resistant composites"; as an example, a La-aluminate coating on polycrystalline AI₂θ₃Fiber connected to a mullite matrix. Here too, no high-temperature tests or their results are disclosed. The same lack adhered to the Patent of Long and others (US 5,856,252), where under "Damage tolerant ceramic matrix composities by a precursor infiltration" a claim an "all oxide ceramic composite" of uncoated AI₂θ₃Fibers and porous mullite matrix describes disclosed without detail high temperature performance; Main content of US 5,856,252 is the description of oxide composites with schadenstolerantem behavior, achieved by "delamination ... with extensive regions of cracking normal to the rupture plane".

p0012A similar approach find at Dariol among others in a "Procede d^'elaboration d'un materiau d'inerphase, materiau obtenu, procede de traitment d^'un fiber ceramides que de renfort avec ce materiau et materiau thermoplastic structural incluant de telles fibers "(FR 27 78 655 A1), where the formation of a micro-porous fiber / matrix interfacial area by adding a Porositätsbildners (eg carbon, the least is partially oxidized) occurs. at a micro-structurally similar conclusion reached by Lundberg and others (EP 946 458 A1), which for oxide fiber / oxide matrix composites, intended specifically for use in oxidizing atmospheres> 1400 ° C, microporous fiber / matrix -Grenzflächen by dipping the fibers in slurry with carbon and Zr0₂ produce. It overlooked the suggestions that the creep rates increase by about 4 orders of magnitude just Candidate oxides between 1300 ° C and 1700 ° C (many non-oxide ceramics are kriechresistenter, yet oxidative onsanfällig) so that sufficient mechanical long-term stability ( > 10,000 h) with polycrystalline fibers in polycrystalline matrices quite impossible to realize is n.

p0013The own invention therefore relies exclusively on composites of oxide-ceramic materials with at least regionally monocrystalline fibers whose mean coherence length of the single crystal regions at least 150 .mu.m, preferably> 400 microns especially preferably,> 1 mm. Amazingly, can be found in the above publications only in EP 639 165 A1, an embodiment corresponding to this requirement inasmuch as thick single crystal sapphire fibers (Saphi- kon, USA; fiber thickness> 100 microns) with Zr0₂ coated and AI in a₂θ3 matrix embedded. However, in EP 639 165 are missing any detail really interesting as a target behavior under temperature stress. Even when using or in-situ production of monocrystalline fibers weak interfaces is the concept held, inter alia, with the consequence that the fibers decrease in the range of low temperatures, the strengths rather than improved (AA Kolchin Others Composite Sci. Technol. 61 (2001) 8, 1079-1082).

p0014The previously known proposed solutions is thus common that even in case of target definition for use at temperatures> 1400 ° C the property development is oriented shock stress exclusively on relevant only at lower temperatures thermal, and that even this is no disclosure of the material properties. All known developments aiming as target microstructural design for "versagenstolerantem" behavior ( "damage tolerant"), which is realized by weak fiber / matrix interfaces. Inexplicably, the attainment of this objective is not detected in any case, by appropriate thermal shock tests in technically relevant temperature ranges, but all mentioned embodiments limited to pure room temperature fracture experiments. If these "weak" fiber / matrix bond is generated by bindungsschwächende fiber coatings with suitable materials or by artificially created for micro-pores, so will continue to overlook the fact that such micro-pores in the desired operating range of temperature unstable at least in the long-term use be and that the bindungsschwächende effect of a fiber coating at higher temperatures, such as at 1400 ° C very different showing than in exclusively studied the room temperature range. and of course, "weak" fiber / matrix interfaces can not just at> 1400 ° meet C dominant requirement for creep stability.

p0015Thus offer the known proposals for oxide-ceramic fiber composites for use at temperatures> 1400 ° C for either

p0016(1) associated with the heating / cooling thermal shock stresses nor for

p0017(2) the serious problem at such high temperatures the creep a workable solution or just a suggestion.

p0018therefore of the Invention The present invention sets for its object a solution of the two previously incompatible demands for (1) improved Sprödbruchverhal- th to secure the thermal shock stability and damage tolerance in the temperature range <1000 ° C, and (2) after mechanical high-temperature stability ensuring the long-term dimensional stability (creep resistance) of components of such oxide ceramic composites a fundamentally new concept.

p0019The object is achieved by the specified in the claims the invention. Further developments are subject of the dependent claims.

p0020The oxide-ceramic fiber composites according to the invention, the fibers with at least regionally-crystal structure include (average length of the single-crystal "coherent" areas of the fiber at least 150 .mu.m, preferably> 400 .mu.m, more preferably> 1 mm), characterized in that the fiber / matrix -Grenzflächen at high temperature stress along the fiber axis in a mesoscopic length ranges on the average fiber Coherence length tight binding show, said binding NEN when stressed species above room temperature, which are accompanied by the spread of macro cracks (fracture, thermal shock), only microscopic, locally small on portions than said coherence length and with no more than 200 microns limited Delaminatio- (debonding) and having such a quasi-macroscopic separation of the fibers at break (pull-out) also prevented as extensive creep of the interface at high temperatures.

p0021The notion of a along the fiber axis on the fiber coherence length mesosko- pisch strength fiber / matrix interface means according to the invention that under thermal momechanischer load to no continuous tearing of the matrix of the fiber ( "pull-out" in the axial direction) with a is the average coherence length of the fiber, and with no more than 200 microns in excess of plan dimension. thus, while "delamination ... with extensive ... cracking" (US 5,856,252) are avoided here occur in microscopic areas which are smaller than the average coherence length of the fibers and with no more than 200 microns, locally-limited separations quite on. These microscopic localized delamination can, however, due to the axial direction of the fibers in mesoscopic solid interfacial bonding not lead to longer pull-out effects of the fibers in the fracture surface; the potential to fracture surfaces observed pull-out lengths remain small compared with the specified minimum coherence length of 150 microns, and in no case exceed an average value of 200 microns.

p0022This illustration also shows that a general classification of the fiber / matrix interfaces as "fixed" or makes little sense "weak" because of different temperature ranges and without specification of the considered length dimensions. Thus, the above laid called mesoscopic fiber / matrix bond could considering its brittle in the temperature range of material behavior (<1000 ° C) microscopic delamination one hand, as mentioned relatively weak in this temperature range are compared with the possibly stronger, creep ermöglichendem character same interface at high temperatures above 1000 ° C; On the other hand, this fiber / matrix interface but <1000 ° C is in the range undoubtedly relatively "solid" compared to other interfaces that, as described by the prior art, at break to peel lengths of multiples chen of fiber diameter lead. Surprisingly, associated with microscopic local fiber separation energy dissipation inventive composites is sufficient that Sprödbruchvorgänge as strength and thermal shock resistance in the temperature range determine <1000 ° C, to affect significantly. The hitherto insurmountable contradiction between the demands on the control of the mechanical behavior in this low temperature range and to form stability (creep) and microstructure cohesion (eg facing erosion stability) at very high temperatures is the inventive design for pure-oxide Faververbund materials first solved.

p0023The choice of material and the production technology are compared to the inventive basic design of the composite concept of secondary importance. The use of known as creep-resistant materials, such as mullite or Y Al garnet for matrix and fibers generally offers advantages. Also additional measures to control structure under long-term high temperature stresses can be useful (as, inter alia, the use of special allocations from duplex or generally polyphase matrix materials in order to prevent or limit grain growth). The particular choice of the basic material for matrix and fibers and / or additives but will always be determined by the respective application and subject under the inventive composite designs described herein no restrictions. The material selection of a possible fiber coating as part of the disclosed herein design concept can be carried out within a wide range, because the material of the fiber / matrix interface defines only first prerequisites for the fulfillment of the feature of the new composite designs: the real development of an invention mesoscopic strength fiber / matrix interface, which allows for loading conditions above room temperature, which are accompanied by the spread of macro cracks (fracture, thermal shock), microscopic localized delaminations (debonding) takes place, only by vote of material selection with another, for se known process steps (such as the generation of an optimized degree of sintering of the matrix by varying the sintering temperature). If piece Plated fibers are used, the thickness of the coating must be maintained at <2 microns, in order to realize the inventive mesoscopically firm bond.

p0024Of course it is also immaterial whether the composite is prepared using a prefabricated single-crystalline oxide-ceramic fiber with the aforementioned coherence length or of polycrystalline fibers, fabrics or other fibrous precursors which (produced by impregnation, for example) only in the course of sintering of the composite in situ in monocrystalline state described in this invention have been transferred.

p0025The present invention can be used in a special way for thermal- highly loaded and / or long-term-highly loaded components or systems. They are also advantageously used for components which are used in particular under changing thermal and / or long-term loads. these composites are advantageously used in particular for components and / or systems for energy conversion or for high performance brake discs.

p0026BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the invention is illustrated in an exemplary embodiment.

p0027Sapphire fibers with about 160 microns in diameter (Advanced Crystal Products, USA) by sol / gel method with 0.15 to 1.2 microns thick SrO_*6Al₂0₃Layers provided and then together with a corundum powder of high purity (> 99.99% Al₂0₃) And 0.2 micron average particle size (TM-DAR, Boehringer Ingelheim Chemicals, Japan) compacted in a hot press to form flat sheets of about 5 mm thick. Comparatively also the characteristics of fiber-free sample were examined. The influence of the fiber thickness is not shown here, but generally thin fibers are to be used advantageously.

p0028For the exemplary experiments, a relatively large mutual fiber distance of about 1 was - 1.5 mm set, the dimension of the sample was 3.6 x 6.8 x 60 mm³, The fibers were aligned parallel to the longitudinal axis of the flexural rods and thus oriented perpendicularly to the resultant (macroscopic) fracture surfaces. the grain size of the matrix of the dense oxide-ceramic fiber composites produced was changed by different hot-pressing temperatures; such an adjustment of the grain size of the matrix is, inter alia, important for the mechanical stability at very high temperature: 1330 ° C / 2 h - 0.6 microns,

p00291550 ° C / 2 h - 6.0 microns,

p00301800 ° C / 2 h - 15.5 microns.

p0031All the mechanical testing in the range of room temperature to 1400 ° C were carried out in 3-point bending with a loading rate of 0.5 mm / min; in preliminary tests between 0.1 and 1 mm / min, no influence of the loading rate, the packing density of the fiber or an additional hot isostatic densification of the composites had been found previously.

p0032The investigation results of the fracture surfaces and the mechanical data show the character of the invention the oxide-ceramic composites prepared by way of example here.

p0033The figures show an example of fracture surfaces Connected whose sapphire fibers with 0.6 and 1.2 microns thick Sr aluminate layers were provided:

p0034Fig. 1 No longer fiber detachment from the matrix at break at room temperature, the fibers (coating thickness 1.2 micron) break in nearly the same plane as the surrounding matrix (minimum pull-out lengths <100 microns).

p0035Fig.2a / b generated the higher magnification at room temperature

p0036Fracture surface shows that the strength-increasing effect of the coated fibers (coating thickness 1.2 .mu.m) in the temperature range brittle material behavior (see table below.) Accompanied by (i) a local, microscopic coexistence of (i) after breaking even festversinterten regions of the fiber / matrix interface (which prolonged shedding fibers from the matrix prevented) and (ii) microscopic localized cracking (energiedissipative affect flussung of brittle fracture with effect as in the sense of increased strength or improved thermal shock resistance).

p0037Fig. 3 brittle Also in the upper temperature range material behavior occurs no significant pull-out effect on (here eg 900 ° C; coating thickness 1, 2 microns).

p0038Fig. 4a / b fracture surface 1200 ° C (similarly at 1400 ° C): Even at very high temperature (in the example with 0.6 dictated ker Sr aluminate coating) a mesoscopic for fiber coatings which lead to the invention, oxide-ceramic composites strength fiber / matrix bond realized that. the macroscopic fracture without long fiber detachment from the matrix survives (similar result with 1, 2 micron thick fiber coating)

p0039Mechanical tests were exploring the behavior of the oxide-ceramic fiber composite materials under temperature exposure.

p0040(Which is for example in thermal shock in the temperature range between 20 and 1000 ° C addressed) The desired positive effect of fibers with coating and with it, coordinated sintering shows the following table with respect to the brittle fracture behavior; the data relating to the structure with 0.6 micron average particle size matrix:

p0041<img id="imgf000011_0001" he="68" wi="157" file="imgf000011_0001.tif" img-format="tif" img-content="table" orientation="portrait" inline="yes" /> At very high temperature contrast brittle fracture behavior and thermal shock resistance play a subordinate role, is important above all a strong fiber / matrix bond as a condition for a sufficient long-term dimensional stability of the component. During 1200 -. 1400 ° C, the microscopic strength of the interfaces and the absence of pull-out mechanisms with Figure 4a / b is occupied table below shows for the example of the composite materials with a matrix grain size of 6 microns, the constancy of the macroscopic strength between 1200 to at least 1400 ° C, regardless of the thickness of the fiber coating in the range investigated (similar data for matrix with 14.5 micron grain size):

p0042<img id="imgf000012_0001" he="61" wi="157" file="imgf000012_0001.tif" img-format="tif" img-content="table" orientation="portrait" inline="yes" />