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    • 专利摘要:
    A polymer ceramic precursor comprising a base precursor comprising an Si / C / N derived polymer system with a polysilazane block. It comprises additives comprising an addition precursor comprising an Si / C-derived polymer system with a polycarbosilane block, a crosslinking initiator and a carbon-rich chemical reactive with the basic precursor hydrosilyl, vinyl or allyl. . Process for producing a micro-mechanical component made of such reinforced ceramic material: a-b) Producing said precursor; c) filling a mold with said improved ceramic precursor; d) conducting a cross-linking of said precursor to form a green body without graphitization; e) demolding said green body; f) conducting pyrolysis to obtain said micromechanical component.
    公开号:CH705690B1
    申请号:CH00486/13
    申请日:2011-07-12
    公开日:2016-06-15
    发明作者:Bakumov Vadym；Blugan Gurdial；Kübler Jakob
    申请人:Eta Sa Mft Horlogère Suisse；
    IPC主号:
    专利说明:

    Field of the invention
    [0001] The invention relates to ceramic precursors with a polymer matrix called "PDC" or polymer-derived ceramic. PDCs are a relatively new category of so-called "oxide-free" structural and functional materials, which are gaining increasing interest from research and industry.
    [0002] The tender nature of polymer raw materials provides a unique opportunity for manufacturing various profiles (for example by liquid molding), which are converted into rigid ceramic structures by so-called "curing" followed by a pyrolytic treatment at 600 ° C – 1000 ° C.
    [0003] The shaping of the chemical structure of the starting polymer precursor, at the molecular level, and different pyrolysis conditions, allow adjustment of the final composition, its microstructure and its associated properties.
    This approach has been successfully commercialized for fibers (eg Tyranno Fiber <®>, Nicalon ™) and for ceramic matrix composites. It is commonly extended to the field of functional and protective coatings, porous bodies, filaments, and is conceivable to create a market for the manufacture of miniature components.
    Background of the invention
    [0005] The current technology of microelectromechanical systems, known as MEMS, is mainly based on the use of silicon, whose modest mechanical and tribological properties make it necessary to identify and develop new alternative materials.
    The manufacture of various fine PDC structures based on variants of soft or non-soft lithographic techniques has been known for the past decade.
    [0007] However, in order to open the way to wide use of MEMS PDCs, it would be necessary to validate the state of their mechanical properties and specific to applications, for example tribological.
    [0008] Similar to diamond and DLC, PDCs show a low coefficient of friction and wear under moderate operating conditions, and appear to be promising candidates for MEMS.
    [0009] However, the contact and sliding pressures of small moving components can exceed the maximum acceptable radio pressures and lead to difficult conditions of friction and wear becoming sources of failure.
    [0010] Among the wide range of so-called preceramic polymers, the family of Ceraset <®> polysilazane liquid precursors, which leads to silicon carbonitrides, has attracted the highest attention of the research community on PDCs, as being the material of choice for many works devoted to MEMS PDCs.
    [0011] MEMS based on silicon oxycarbons and glassy carbon have already been presented.
    Surprisingly, polycarbosilane, used industrially for the manufacture of the SiC matrix in brake discs (for example a polymer system derived from Si / C), and which is therefore promising for other friction applications , has not so far raised the attention of the MEMS research community.
    It has been shown, in the few studies on the tribology of PDCs, that, for the Ceraset <®> system of Si / C / N derivative, there is a transition between a moderate diet and a severe diet of wear, when reaching a threshold contact pressure value.
    [0014] Free carbon has been attributed the origin of the interesting friction properties under moderate wear.
    However, the question still arises of determining the optimum composition in Si / C / N as a function of resistance, friction and wear, as well as the conditions and limitations specific to these materials. for industrial applications.
    The main obstacle preventing a large and exhaustive characterization of the intrinsic properties of PDCs is undoubtedly the difficulty of manufacturing monolithic test specimens on a macroscopic scale, difficulty linked to cracking caused by a gas modification and a consequent shrinkage during the pyrolysis.
    [0017] Most test specimens comprising a PDC matrix were produced using fine techniques (or fillers), and / or pressing techniques, which are not suitable for MEMS manufacturing, and the process steps affect the results of these studies.
    [0018] The die-casting method cited by Shah and Raj, and modified by Janakiraman, seems to present a break with this problem. However, its applicability for MEMS remains to be proven.
    [0019] While it seems possible to produce components, these prove to be unsatisfactory with regard to their mechanical strength and their tribological properties.
    We are referring here to the manufacture of components with a thickness of less than 1 mm, in PDC, by a direct casting method which can be used for the production of MEMS and for their characterization with regard to their resistance behavior and to the friction.
    Summary of the invention
    [0021] The object of the present invention is to partially or totally remedy the drawbacks set out above, by providing a new type of precursor, suitable for use in the shaping of reinforced ceramic micromechanical components.
    The invention relates to the use of polymers based on silicon as ceramic sources, said oxide-free, amorphous and obtained by pyrolytic treatment.
    [0023] This invention focuses on the manufacture in finite dimensions, on the mechanical and tribological properties of an Si / C / N derived polymer system.
    Small ceramic test discs and bars, less than 1 mm thick, were manufactured by casting mixtures of polysilazane and / or polycarbosilane precursors in elastomeric counterforms based on polydimethylsiloxane, followed by crosslinking and pyrolysis.
    Additional carbon was introduced at the molecular level, using triphenylvinylsilane as a precursor, its crosslinking with the polymers by hydrosilylation prevents the phase separation of the graphite.
    [0026] The characteristic strength of about 700 MPa, with sliding friction coefficients that are low and stable, compared to similar PDCs, confirms their potential for microelectromechanical system applications.
    On this basis, according to a first variant, the invention relates to a ceramic precursor with a polymer matrix comprising a base precursor comprising a polymer system derived from Si / C / N with at least one polysilazane block, characterized in that it further comprises additives comprising an addition precursor comprising an Si / C derivative polymer system with at least one polycarbosilane block, a crosslinking initiator and at least one carbon-rich chemical substance comprising hydrosilyl and vinyl groups attached to a silicon atom and reacting with the hydrosilyl, vinyl or allyl group of the base precursor so that the manufacture of said precursor benefits from the strength of said Si / C / N derived polymer system, the rigidity of said Si / C derived polymer system, and the reduced coefficient of friction of said at least one carbon-rich chemical.
    In addition, according to a second variant, the invention relates to a ceramic precursor with a polymer matrix comprising a base precursor comprising a polymer system derived from Si / C / N with at least one polyureasilazane block, characterized in that it comprises in addition additives comprising an addition precursor comprising an Si / C derivative polymer system with at least one polycarbosilane block, a crosslinking initiator and at least one carbon-rich chemical substance comprising hydrosilyl and vinyl groups attached to a silicon atom and reacting with the hydrosilyl, vinyl or allyl group of the base precursor so that the manufacture of said precursor benefits from the mechanical strength of said Si / C / N derived polymer system, from the rigidity of said Si / C derived polymer system, and the reduced coefficient of friction of said at least one carbon-rich chemical.
    The peculiarity and the advantage of the present invention come from the fact that all the components are present in the liquid state, at low viscosity, or in the soluble state, which allows the manufacture of various complex structures by many different techniques, including microfabrication. The homogeneous distribution of carbon and silicon carbide components results in a smooth, defect-free surface.
    [0030] According to other characteristics of the invention:The precursor comprises 49-79 wt% of said Si / C / N derivative polymer system, 10-40 wt% of said Si / C derivative polymer system, and comprises 2 to 20 wt% of said at least one rich chemical. in carbon.said Si / C / N derivative polymer system comprises at least one polysilazane block;said Si / C / N derivative polymer system comprises at least one polycarbosilane group;said at least one carbon-rich chemical comprises triphenylvinylsilane.
    [0031] The invention further relates to a process for the production of a micromechanical component made of reinforced ceramic material, characterized in that it comprises the following steps: a – b) producing an improved ceramic polymer precursor; c) filling a mold, according to the shape corresponding to the future micromechanical component, using said improved ceramic polymer precursor; d) crosslinking, at least partially, said precursor, so as to form a green body, also called green body, without formation of graphite; e) unmold the green body; f) carrying out a pyrolysis operation to obtain the reinforced ceramic micromechanical component benefiting from the strength and rigidity of the ceramic matrix, and from the reduced coefficient of friction of the amorphous carbon and / or of the integrated graphene.
    [0032] According to other characteristics of the invention:The process includes a step a): providing an Si / C / N derivative polymer system, an Si / C derivative polymer system and at least one carbon-rich chemical substance,and step b): stirring all the elements to make an improved ceramic polymer precursor.
    The method further comprises, between steps b) and c), step g):mixing a crosslinking initiator to initiate the crosslinking of step d);said precursor comprises from 0.1 to 5% by mass of said crosslinking initiator;said crosslinking initiator comprises 1,1'-azobis (cyclohexanecarbonitrile);the pyrolysis is carried out between 1100 ° C and 1450 ° C in an atmosphere of inert gas at low pressure so as to avoid any cracking;the mold cavity is less than 1 mm deep to avoid swelling by gas evolution.
    Brief description of the drawings
    [0034] FIG. 1 shows load / deflection curves of three-point bending tests of ceramic green body bars (20 x 2.2 x 0.25 mm) (1) from 100% Ceraset <®> PSZ 20 ( mixture C), with an addition of 60% by mass of SMP-10; fig. 2a shows a transparent, molded and crosslinked polysilazane disc, and the resulting ceramics: bar and disc intact on the left, cracked disc on the right; fig. 2b shows the surface of the disk of FIG. 2a after pyrolysis under strong nitrogen circulation, which reveals severe flaking; fig. 2c represents discs convex on the left, or concave on the right, pyrolyzed under a moderate flow of nitrogen (~ 5 cm <3> / min); fig. 3 shows thermogravimetric curves of specimens crosslinked with compositions according to Table 1; fig. 4 shows absorption spectra of sputtered A and B specimens, which reveal a higher level of Si-N bonds in material B and a jump in the low wavelength region of the Si-C band; fig. 5 shows the X-ray diffraction spectrum of ceramic A pyrolyzed at 1370 ° C; fig. 6 represents a Weibull distribution plot for bending tests on 3 points of rectangular bars (20 mm x 1.5 mm x 0.18 mm) of material A; fig. 7 shows the change in the coefficient of friction during continuous sliding on an alumina counter-body, compared to an SiC derived from SMP-10, both under a load of 1 N and a sliding speed of 5 mm / s; fig. 7b represents the evolution of the coefficient of friction during a continuous sliding on an alumina counter-body, and on SiC with 5% additional carbon (F), both under a load of 5 N and a sliding speed of 5 mm / s; fig. 7c represents the evolution of the coefficient of friction during continuous sliding on a counter-body derived from SMP-10 with 10% additional carbon (mixture E), both against A and B under a load of 1 N and a speed of slip of 10 mm / s; fig. 8a represents the traces of wear of sample A after sliding against alumina (1 N, 14 mm / s, 100 m) immediately after the test; fig. 8b shows the traces of wear of sample A after sliding on alumina (1 N, 14 mm / s, 100 m) after removal of debris in an ultrasonic bath; fig. 9a represents a comparison of the wear marks of sample B after sliding on alumina (1 N, 14 mm / s, 100 m); fig. 9b shows a comparison of the wear marks of sample B against SiC derived from SMP-10 (1 N, 5 mm / s); fig. 10a shows EDX spectra taken on a wear trace of sample B from the center ("C" in Fig. 9a); fig. 10b shows EDX spectra taken from a wear trace of sample B from the side sediment heels ("L" in Fig. 9a).
    Detailed description of the embodiments of the invention
    The aim of the invention is to provide a new type of polymer-based ceramic precursor intended for the production of micromechanical components which can withstand high mechanical stresses.
    In particular, the studies are oriented on the application to a mobile component of a timepiece, for which a mechanical strength greater than 500 MPa, a coefficient of friction less than 0.2, and a surface roughness where the ridge-valley distance (Ra) is less than 25 nm.
    [0037] In addition, with regard to the process for producing the micromechanical component, which preferably uses a mold, it is also necessary that the viscosity of the precursor is between 2 and 200 mPa.s and, preferably of order of 50 mPa.s, so as to facilitate filling of the mold, that is to say to prevent the precursor from trapping air in the corners of the mold.
    It was found, during the study, that it was possible to satisfy all of the above conditions, by chemically modifying a precursor by incorporating substances rich in carbon.
    The base precursor preferably contains at least one polysilazane group, such as for example the product Ceraset <®> polysilazane 20 from Kion <®>. Polyureasilazane products, such as Ceraset <®> polyuréasilazane from Kion <®>, can nevertheless be used as an alternative.
    [0040] In fact, the ceramic obtained by pyrolysis of the improved precursor contains amorphous carbon and / or graphenes, which, in particular, quite considerably lower its coefficient of friction.
    It has been found that there is a wide variety of substances rich in carbon, which can be incorporated into the basic precursor, provided that coupling groups, called linkers, which react chemically with hydrosilyl groups, vinyl are incorporated. or allyl, based on polysilazane precursor.
    The most suitable among the characteristic groups, or "moieties", of substances rich in carbon, are the hydrosilyl and vinyl groups attached to the silicon atom. The carbonaceous characteristic groups of said carbon-rich substances are preferably aryl, phenyl or naphthyl groups. However, inclusions are possible with the following commercially available substances, including:
    It has also been found that the inclusions can be facilitated by increasing the concentration of reactive Si-H groups in the mixture of the base precursor and the carbon-rich substance. For example, this can be done by adding at least one polycarbosilane block. In case the carbon rich substance contains triphenylvinylsilane (TVPS), said polycarbosilane is preferably of the SMP-10 type from Starfire <®> Systems.
    [0044] After these inclusions of 2 to 20% of at least one substance rich in carbon, that is to say after chemical improvement of the base precursor, the improved precursor can be used for the production of micromechanical components.
    The invention uses commercially available liquid precursor polymers chosen for this study: Ceraset <®> polysilazane 20 (Clariant Charlotte, NC, USA) and allylhydridopolycarbosilane SMP-10 (Starfire <®> Systems, Malta, NY, USA). Triphenylvinylsilane (TVPS). The additional free carbon source was sourced from ABCR GmbH (Karlsruhe, Germany). To promote crosslinking, a crosslinking initiator can be dispersed in the precursors, in an amount of 0.1 to 5% by mass, preferably from 0.3 to 1% by mass. 1,1'-Azobis (cyclohexanecarbonitrile) was used as initiator for thermal crosslinking of polymer precursors (Sigma-Aldrich, Buchs, Switzerland). Two-component elastomeric polydimethylsiloxane (PDMS) Sylgard <®> -184 (Dow Corning, Midland, USA) was used for the duplication of flexible molds from a photosensitive resin matrix SU-8 (MicroChem Corp., Newton , USA). All chemicals were used as received, without purification or modification.
    The manufacturing method according to the invention uses an SU-8 matrix. Structures with dimensions of 20 mm x 2.1 mm x 0.3 mm (bars) and diameter 20 mm x 0.3 mm (disks) were created on silicon wafers by lithography.
    For the manufacture of the PDMS molds, the two components of Sylgard <®> -184, in a ratio of 1:10 as prescribed by the seller, were mixed, degassed under a vacuum of 2 to 4 mbar for 15 minutes, and poured onto the silicon wafer supporting the complementary SU-8 matrix.
    After manual inversion of the plate for a uniform distribution of the PDMS, the plate was placed in a drying chamber, and the temperature was brought to 140 ° C, with a temperature gradient of 20 to 40 ° C by hour and held for two hours.
    After cooling, the PDMS replica is carefully peeled, cut into circles 50 mm in diameter, and glued on its unstructured side on PTFE discs 7 mm thick, which serve as a rigid support for the flexible PDMS . For the manufacture of thicker (0.6mm) PDC specimens, the corresponding cavities were ground directly into PTFE discs (approximately 0.8mm thick).
    [0050] Table 1. Composition of the mixtures of precursors
    [0051] The precursors were mixed in the proportions of Table 1, using for the purposes of homogeneity and acceleration of the process an ultrasonic micro-stirrer (5 W) for 30-60 seconds (liquid heated to 60 ° C).
    The mixtures were degassed at 0.3 to 0.5 mbar for 20 to 30 minutes with intense stirring (800 to 1100 revolutions per minute) then poured using a micropipette in the PDMS molds lubricated beforehand (with silicone grease).
    The utility of the degassing procedure is dictated by the fact that the two precursor polymers contain volatile oligomers and slowly expel hydrogen and / or ammonia, even in normal storage, causing macroporosity and even swelling of the samples by evolution of gas during heating and evaporation of dissolved volatile bodies.
    Particular attention is paid to the volume injected into the cavities of the mold so as to minimize the meniscus created at the top. For the manufacture of the PDC counter-body for the tribological tests, precursor cups were placed directly on the PTFE support without PDMS.
    It is then also possible, in particular, to form a resin mold by photolithography, a silicon, quartz or corundum mold by dry or wet etching. In addition, the filling is carried out by any process such as injection, molding, spraying or even pressing. The mold is preferably agitated and / or compressed so as to restrict the entrapment of gas in the improved precursor.
    [0056] Due to the large contact angle between the polymer and the PTFE, the drops do not expand and solidify during crosslinking in the form of hemispheres.
    The latter are converted by pyrolysis to ceramic, and are glued to an aluminum pin and fixed on a strain gauge in a tribometer (see following paragraph).
    The braced and filled molds are stacked one on top of the other and transferred to a hermetically sealed Büchi (Büchi AG, Uster, Switzerland) miniclave chamber, which is emptied and subjected to an atmosphere under argon. Then, an argon pressure of 3 bar is applied, and the chamber is heated to a target temperature of 165 to 175 ° C for a period of 4 hours, using a silicone oil bath, and the temperature is maintained at 175 ° C overnight crosslinking to form a green body. More generally, the crosslinking is carried out under the prolonged action of heat (2 to 96 h), preferably 8 to 36 h, at a temperature of 70 to 220 ° C, preferably 140 to 190 ° C. Additionally, UV light exposure can be applied prior to thermal crosslinking.
    After cooling, the solidified samples are mechanically removed from the molds, placed in open or closed SiC cups and subjected to pyrolysis in an alumina tube furnace (Carbolite STF 16/610, Carbolite Ltd., Hope Valley , UK) under a stream of nitrogen (Alphagas1, purity 99.999%) up to the target temperature. The heating and cooling gradients were 0.45 and 3.3 K / min, respectively, the annealing time was 2 hours.
    More generally, the pyrolysis is preferably carried out under a controlled atmosphere such as an inert atmosphere (nitrogen, argon, etc.) or under vacuum, in a temperature range of 1000 to 1450 ° C. The heating gradient is in the range of 0.5 to 100 K / h, preferably 10 to 60 K / h. The maximum temperature holding time is 0 to 96 hours, and preferably 2 to 12 hours.
    The resistance of the rectangular specimens was measured on a Zwick Z005 test machine (Zwick GmbH, Ulm, Germany), in 3-point bending mode, using a reach of 8 mm, a preload force of 0.2 N , and a descent speed of 0.5 mm / min.
    The data were processed according to the Weibull distributions with two parameters using the regression of maximum probability.
    [0063] The fracture toughness was measured according to the sample beam method notched in a V on one side only, on four bars of 0.6 mm of material H (Table 1).
    For this purpose, a notch of depth 0.1 mm was polished in the middle of the bars, using a steel razor blade and diamond polishing paste. The bars were loaded according to the 3-point bending mode (mode I) in a manner similar to the strength tests described above.
    The hardness was measured on a Leitz Durimet microhardness device equipped with a Vickers punch under a load of 500 g.
    An internal built-up tribometer, comprising a linear actuator and a strain gauge measuring the tangential force, was used for the friction measurements.
    The specimen discs were glued to the plate driven by the actuator, the counter-body was loaded with a prescribed mass, fixed to a strain gauge, and brought into contact with the specimen to achieve sliding friction .
    The FF-IR spectra were measured on a Golden Gate ATR. Scanning electron microscopes, using 10 kV acceleration and a secondary electron detector, were purchased from VEGA Tescan (Brno, Czech Republic).
    Manufacture of PDC specimens by direct casting and pyrolysis
    The compositions of the samples used for this study were determined by the need to demonstrate the effects of the precursor on the manufacturing process, the microstructure, the mechanical and tribological properties of the macroscopic PDCs.
    [0070] The manufacture of self-supporting test specimens encountered some difficulties related to the expulsion of volatile bodies, and the removal during the two stages of crosslinking and pyrolysis.
    Among the series of Ceraset <®> products, Polysilazane 20, in comparison with polyureasilazane, has a smaller difference in density between the liquid and solid states. We chose it to minimize shrinkage during crosslinking, and the associated stresses. SMP-10 polycarbosilane has a density of approximately 1.0 g / cm <3>, both in the raw and cured states.
    The problem of gas expulsion and swelling by gas evolution, or bloating, can be overcome by keeping the thickness of the polymer layer less than 0.7 mm, and by implementing degassing and a slow and uniform heating, as described in the section on tests.
    The peculiarity of the present approach is the absence of use of pressurized and hermetically sealed molds, and therefore its suitability for MEMS.
    [0074] Other problems of cracking, or cracking, and / or deformation, appearing during the pyrolysis step, have been described in the literature.
    The study by variation of the experimental parameters (composition, heating and gas flow gradients, geometry and dimensions of the samples, receptacle, etc.) allowed us to establish the origin of the forces acting on a specimen during pyrolysis and leading to non-uniform shrinkage in the specimen, and to discern external and internal factors affecting shape fidelity.
    [0076] The external factors are the pyrolysis conditions, which determine the uniformity of the heat distribution and its transfer to the sample, for example the thermal conductivity of the specimen receptacles, the length of the heating zone, and the gas flow gradient. The flow of inert gas which protects the specimens from oxidation was, in our experimentation, the main source of temperature disturbances, and it must be reduced to the minimum level, while remaining high enough to maintain a slight overpressure at the inside the tube, and prevent any inclusion of air in the heating zone.
    Internal parameters specific to the specimen are the rigidity of the green body, the presence of carbon, and the form factor. Thicker discs have less tendency to warp than thinner ones. We have further established that the degree of deformation is correlated with the composition of the precursor SMP-10, and thus is correlated with the flexibility of green bodies (Fig. 1). In addition, discs and bars of larger diameters and lengths have been deformed more than those with smaller linear dimensions.
    The direction of warping during pyrolysis has been predetermined by the curvature of the surface, which comes from the meniscus obtained at the top of the liquid before solidification. A positive curvature (derived from a convex meniscus), as well as a negative curvature (derived from a concave meniscus) increases in magnitude during pyrolysis, also affecting the opposite, initially flat side. Flat samples (with negligible curvature) exposed to a considerable temperature gradient, for example due to a violent flow of inert gas, suffered extensive cracking.
    Small cracks at the edge of the flat specimens develop, if the latter are exposed to non-optimized pyrolysis conditions. These deformations occurred below 500 ° C, with cracks occurring in the range 540–570 ° C.
    These observations can be explained as follows: the temperature gradient which is created along the sample, and due to external factors of limited thermal conductivity and / or due to cooling by a flow of inert gas, causes various withdrawals and stresses within the sample.
    Larger samples were exposed to greater temperature differences at opposite ends, and exhibited a leverage effect for warping-generating stresses, while their rigidity (Young's modulus E) and thickness samples (internal factors) oppose the deformation. The presence of positive or negative curvature of the surface (with a Laplace pressure exerted respectively towards the outside or the inside) triggers the direction of the forces, then determining the convex or concave shape of the sample, and dissipates the tensions.
    Phase composition
    The carbon species in the PDCs, distributed homogeneously in an amorphous Si / C / N matrix, are considered as lubricating agents. On the other hand, the poor mechanical and wear properties of graphite should restrict its content for structural applications, and make necessary the possibility of a controlled introduction of carbon into the starting materials.
    It is then possible to use, as a carbon source, triphenylvinylsilane (TVPS), which is soluble in ceramic polymers and binds to them during crosslinking by hydrosilylation.
    [0084] Following the binding, the carbon conservation of the TVPS is effected, as indicated by thermal gravimetry (Fig. 3), which shows an improved ceramic yield compared to polysilazane or polycarbosilane in new condition.
    An increased level of polycarbosilane leads to a sample A enriched in SiC; Sample B shows a quantifiable proportion of Si-N bonds. The X-ray diffraction spectrum of FIG. 5 reveals nanocrystals of silicon carbide of the β type and of amorphous carbon.
    The absence of crystalline phases in graphite form in the carbon-enriched material A, treated at temperatures reaching 1370 ° C, is remarkable, because the formation of graphite is generally observed around 1200 ° C.
    Mutine has shown that the chemical structure of amorphous Si / C / O systems depends only on the raw composition, and is developed by redistribution reactions between SiCxO4 – xtetrahedral. Similarly, one can expect redistribution reactions between SiCxN4 – x tetrahedra and the formation of mixed silicon carbonitrides from the characteristic polysilazane and polycarbosilane groups.
    However, the precipitation of silicon carbide (and even for materials with a low polycarbosilane content) suggests that the possibility of a chemical interaction between silazane and carbosilane groups is limited to the pyrolysis phase, and that of SiC and Si / C / N derived from the corresponding precursors exist in separate domains.
    This can be explained by the macromolecular nature of the starting materials, which are presented for the most part as entangled chains even under or after intensive shearing during mixing. In addition, crosslinking restricts chain mobility and the low diffusion coefficients of amorphous Si / C / N / H hybrid materials (duomers) make carbon redistribution or graphitization reactions impossible.
    The pyrolysis conditions also influence the composition, and, while pyrolysis under nitrogen flow (specimen B) results in ceramics based on silicon carbonitrides, the treatment under a stationary atmosphere consisting predominantly of 'hydrocarbons expelled, leads to enriched SiC (specimen A). These variations as a function of the polycarbosilane content determine the composition of the Si / C / N and have a significant effect on the tribology.
    Mechanical properties
    [0091] The difficulties in the direct manufacture of monolithic carbon and silicon-based test samples have limited to date the characterization of their intrinsic mechanical properties.
    A characteristic strength of 110 MPa has been shown for ceramics derived from Ceraset <®> die-cast and polished.
    However, the microstructure, in particular the porosity of PDCs, is strongly linked to the process, and any variation in the preparation steps makes it difficult to predict the mechanical properties.
    With regard to the samples prepared in the present study, the 3-point bending tests (fig. 6) revealed a characteristic resistance of about 700 MPa, which slightly exceeds that obtained for the samples molded under pressure and not polishes prepared by Shah.
    In our experiments, the variations in the composition of SMP-10 in the range of 0 to 50% by mass, and the pyrolysis temperature (1130 to 1370 ° C) did not significantly affect the resistance, and a slight variation in value could mainly be attributed to the statistical nature of the parameter.
    Indeed, the relatively low Weibull modulus, 6.1 in the present work, closely resembles the statistical behavior of glass fibers, while samples die-cast under the reference 23 have revealed resistances in upper (polished) or lower (unpolished) flexion.
    [0097] For such low tenacity materials, the surface finish of the specimens is a key factor, with respect to strength, and the small size of MEMS should make it a preferred niche for the application of PDCs.
    We have also been able to manufacture 0.6 mm thick bars of composition H for SEVNB, while thick samples with other compositions generally suffer from cracking during pyrolysis.
    The material H reveals a tenacity of 1.3 ± 0.2 MPa · m <1/2>.
    [0100] This value roughly corresponds to the assessment by Janakiraman et al. for Si / C / N derived from Ceraset <®> annealed at the same temperature range. To our knowledge, this is the first report on the fracture toughness of monolithic Si / C / N PDCs without filler, measured by the mode-l loading technique by SEVNB.
    Tribological properties
    [0101] The potential of PDCs for tribological applications has been recognized for ten years. In systematic studies on polysilazane-based systems, the increase in performance with increasing annealing temperature was observed, and the nature of the material of the sub-body, alumina or steel, was found to be of no influence. on the friction characteristics. In the present study, PDC counterbodies showed a low coefficient of friction compared to that of alumina, and the difference was even more pronounced under a higher load (Fig. 7a, 7b).
    [0102] This observation can be attributed to the higher carbon content in the contact zone, and / or to a reduced contact radio pressure, due to a low Young's modulus of amorphous PDCs, in comparison with alumina. Indeed, the rate of TVPS derived carbon in the pin lowers the coefficient of friction against material A (fig. 7a and 7c). SiC-enriched sample A performs better than B (fig. 7c), one possible explanation is the thermal conductivity of nanocrystalline SiC which results from faster heat dissipation. One can notice here the absence of a soft graphite phase (Fig. 5) despite the presence of additional carbon source (TVPS), which indicates the efficiency of the crosslinking reactions in obstructed graphitization.
    [0103] Amorphous carbon is an excellent lubricating constituent, which does not suffer from poor mechanical properties like its graphitic counterpart. The measured hardness (Table 1) supports this conclusion. All the materials tested reveal a comparable hardness, which is far below the 15 to 26 GPa obtained by Shah and others, but slightly exceeds the hardness measured under the reference 24. The strong impact of the pre-pyrolytic process, as well as others parameters, on mechanical properties, emerged from Janakiraman's studies, and we believe that the remarkably lower hardness of material H (Table 1) must be attributed to the increased thickness of the test specimens (0.6 mm against 0.2 mm for the rest of the samples).
    [0104] The comparison of the materials pyrolyzed under argon shows that the secondary additives for Ceraset <®>, for example polycarbosilane and / or TVPS, have a slightly positive effect on hardness. We attribute the increased hardness of B relative to A to the partial nitriding of the surface of the parent material. It also contributes to the wide dispersion of measured hardnesses and reduced friction properties.
    [0105] Observation with a scanning microscope of the wear patterns reveals a particular behavior of the debris: unlike the wear pattern of the material A (fig. 8), which is covered, and therefore probably protected, by debris, debris containing aluminum is thrown around the periphery for sample B (Figs. 9 and 10). This has the effect of constantly exposing a fresh surface to frictional stresses, which leads to a higher degree of wear.
    [0106] Thus, according to the invention, it is possible to manufacture small objects in PDC, with a dimension of less than 1 mm, by direct casting, crosslinking, and pyrolysis of polysilazanes and polycarbosilanes. The resulting ceramics show considerable resistance (around 700 MPa) and a low coefficient of friction (less than 0.1) when sliding against PDCs. A decrease in the coefficient of friction is obtained by increasing the carbon and / or the SiC level. The combination of properties is very promising for MEMS applications.
    Example
    [0107] 6 g of Ceraset <®> Polysilazane 20 (Clariant Charlotte, NC, USA), 3 g of allylhydridopolycarbosilane SMP-10 (Starfire <®> Systems, Malta, NY, USA), 1 g of Triphenylvinylsilane (ABCR GmbH, Karlsruhe, Germany) and 0.1 g of 1,1 ́-azobis (cyclohexanecarbonitrile) (Sigma-Aldrich) were mixed in a 25 ml ampoule, using for the purposes of homogenization an ultrasonic micro-shaker for 30-60 seconds ( the liquid is heated up to 60 ° C). Accordingly, the solution includes about 59 wt% Ceraset <®> Polysilazane 20, about 30 wt% SMP-10, about 10 wt% TVPS, and about 1 wt% crosslinking initiator.
    The resulting mixture is evacuated at 0.3 to 0.6 mbar for 20 to 30 minutes, with intensive stirring (800 to 1100 revolutions / min), then it is poured into PTFE molds. The filled molds are transferred to a hermetically sealed chamber. The chamber is heated to a target temperature of 165-175 ° C and held overnight.
    [0109] After cooling, the solidified and hardened samples are mechanically removed from the molds, placed in SiC cups, and pyrolyzed under a flow of nitrogen up to 1350 ° C, with a thermal gradient of 10 Kh <–> <1 > and a 6 hour hold time. After cooling (with a gradient of 300 Kh <–> <1>), the parts showed an average flexural strength of 0.6 GPa, a maximum surface roughness Ra of 20 nm, and a friction coefficient of 0 0.03 to 0.08 against alumina.
    [0110] It is understood that the present invention is not restricted to the example illustrated, but is capable of numerous variations and modifications which will be obvious to those skilled in the art. In particular, other basic precursors and / or other carbon-rich substances and / or other addition components can be used.
    [0111] In addition, one can imagine applications other than a mobile component, in or outside the field of watchmaking, due to the improved mechanical qualities of the ceramic obtained according to the invention.
    [0112] Finally, one can imagine other methods of producing reinforced ceramics, which do not necessarily use a mold.
    权利要求:
    Claims (20)
    [1]
    1. Ceramic precursor with a polymer matrix comprising a base precursor comprising an Si / C / N derivative polymer system formed by at least one polysilazane block, characterized in that it further comprises additives comprising an addition precursor comprising a polymer system Si / C derivative formed by at least one polycarbosilane block, a crosslinking initiator and at least one carbon-rich chemical substance comprising hydrosilyl and vinyl groups attached to a silicon atom and reacting with the hydrosilyl, vinyl or allyl group of the precursor of base so that the manufacture of said precursor benefits from the mechanical strength of said Si / C / N derived polymer system, the rigidity of said Si / C derived polymer system, and the reduced coefficient of friction of said at least one rich chemical substance in carbon.
    [2]
    2. Ceramic precursor with a polymer matrix comprising a base precursor comprising an Si / C / N derivative polymer system formed by at least one polyureasilazane block, characterized in that it further comprises additives comprising an addition precursor comprising a system Si / C derivative polymer formed by at least one polycarbosilane block, a crosslinking initiator and at least one carbon-rich chemical substance comprising hydrosilyl and vinyl groups attached to a silicon atom and reacting with the hydrosilyl, vinyl or allyl group of the precursor base so that the manufacture of said precursor benefits from the mechanical strength of said Si / C / N derived polymer system, the rigidity of said Si / C derived polymer system, and the reduced coefficient of friction of said at least one chemical substance rich in carbon.
    [3]
    3. Precursor according to claim 1 or 2, characterized in that it comprises 49 to 79% by mass of said basic precursor.
    [4]
    4. Precursor according to one of the preceding claims, characterized in that it comprises 10 to 40% by mass of said addition precursor.
    [5]
    5. Precursor according to one of the preceding claims, characterized in that it comprises 2 to 20% by mass of said chemical substance rich in carbon.
    [6]
    6. Precursor according to the preceding claim, characterized in that said chemical substance rich in carbon comprises triphenylvinylsilane.
    [7]
    7. Precursor according to one of the preceding claims, characterized in that it comprises 0.3 to 1% by mass of said crosslinking initiator.
    [8]
    8. Precursor according to the preceding claim, characterized in that said crosslinking initiator comprises 1,1 ́-azobis (cyclohexanecarbonitrile).
    [9]
    9. Process for the production of a ceramic precursor with polymer matrix, characterized in that it comprises the following steps:a) provide a base precursor comprising an Si / C / N derivative polymer system with at least one polysilazane block, an addition precursor comprising an Si / C derivative polymer system with at least one polycarbosilane block, a crosslinking initiator and at least one least one carbon-rich chemical having hydrosilyl and vinyl groups attached to a silicon atom and reacting with the hydrosilyl, vinyl or allyl of the base precursor;b) pre-mix and degas all the elements to make a polymer matrix ceramic precursor.
    [10]
    10. Process for the production of a ceramic precursor with a polymer matrix, characterized in that it comprises the following steps:a) provide a base precursor comprising an Si / C / N derivative polymer system with at least one polyureasilazane block, an addition precursor comprising an Si / C derivative polymer system with at least one polycarbosilane block, a crosslinking initiator and at least one least one carbon-rich chemical having hydrosilyl and vinyl groups attached to a silicon atom and reacting with the hydrosilyl, vinyl or allyl group of the base precursor;b) stirring and degasing all the elements to make a polymer matrix ceramic precursor.
    [11]
    11. The method of claim 9 or 10, characterized in that the ceramic precursor includes 49 to 79% by weight of base precursor.
    [12]
    12. Method according to one of claims 9 to 11, characterized in that the ceramic precursor includes 10 to 40% by mass of addition precursor.
    [13]
    13. Method according to one of claims 9 to 12, characterized in that the ceramic precursor includes 2 to 20% by mass of said chemical substance rich in carbon.
    [14]
    14. Method according to the preceding claim, characterized in that said chemical substance rich in carbon comprises triphenylvinylsilane.
    [15]
    15. Method according to one of claims 9 to 14, characterized in that it includes 0.3 to 1% by weight of crosslinking initiator.
    [16]
    16. Method according to the preceding claim, characterized in that said crosslinking initiator comprises 1,1 ́-azobis (cyclohexanecarbonitrile).
    [17]
    17. Process for the production of a micromechanical component made of a reinforced ceramic material, characterized in that it comprises the following steps:a – b) Producing a polymer matrix ceramic precursor according to one of claims 9 to 16;c) filling a mold with a profile corresponding to the future micromechanical component using said ceramic precursor;d) conducting, at least partially, a crosslinking of said precursor so as to form a compact green body without graphitization;e) demoulding said compact green body;f) carrying out a pyrolysis operation to obtain the reinforced ceramic micromechanical component inheriting strength and rigidity from the ceramic matrix, and reduced coefficient of friction from the integrated amorphous carbon and / or graphene.
    [18]
    18. Method according to the preceding claim, characterized in that the crosslinking is carried out between 140 ° C and 190 ° C under an argon atmosphere with a holding time of between 8 hours and 36 hours.
    [19]
    19. Method according to one of claims 17 or 18, characterized in that the pyrolysis is carried out between 1100 ° C and 1450 ° C with a holding time of between 2 hours and 12 hours in an inert gas atmosphere at low pressure. so as to avoid any cracking.
    [20]
    20. Method according to one of claims 17 to 19, characterized in that the mold cavity is less than 1 mm deep so as to prevent swelling by gas evolution.
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    同族专利:
    公开号 | 公开日
    WO2012025291A2|2012-03-01|
    WO2012025291A3|2012-06-07|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    US5208192A|1990-01-12|1993-05-04|Ethyl Corporation|Preceramic polysilazane compositions|
    DE102004058119A1|2004-12-02|2006-06-08|Daimlerchrysler Ag|Porous SiC bodies with microchannels and process for their preparation|EP2579106A1|2011-10-04|2013-04-10|ETA SA Manufacture Horlogère Suisse|Shaping of an integral transparent clock component|
    CN104725879B|2015-03-19|2017-08-04|山东大学|A kind of organic silicon composite of resistance to ablation and preparation method and application|
    法律状态:
    优先权:
    申请号 | 申请日 | 专利标题
    EP10173841A|EP2423174A1|2010-08-24|2010-08-24|Reinforced ceramic micromechanical component|
    EP10188599|2010-10-22|
    PCT/EP2011/061812|WO2012025291A2|2010-08-24|2011-07-12|MECHANICAL AND TRIBOLOGICAL PROPERTIES OF POLYMER-DERIVED Si/C/N SUBMILLIMETER THICK MINIATURIZED COMPONENTS FABRICATED BY DIRECT CASTING|
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