Method for producing a component from a composite Al2O3/titanium aluminide material

The invention relates to a process for producing a component from an Al.sub.2 O.sub.3 /titanium aluminide composite material. To produce the component, a shaped body is pressed from a starting mix of titanium, in particular as an oxide, carbon and/or its precursors, fillers and binders. At a conversion temperature, the shaped body is subjected to a heat treatment in order to form a pressure-stable sacrificial body. In the process, the filler and, if appropriate, the binder is/are also removed by thermal means. The sacrificial body is filled with aluminum and/or an aluminum alloy under pressure. The filling takes place at a filling temperature which is above the conversion temperature. Then, the temperature is reduced to a transformation temperature, the materials of the filled sacrificial body and the aluminum reacting through a solid-state reaction, below the filling temperature, to form an Al.sub.2 O.sub.3 /titanium aluminide composite body.

BACKGROUND OF THE INVENTION
 This invention relates to a process for producing a component from an
 Al.sub.2 O.sub.3 /titanium aluminide composite material in which a shaped
 body is pressed from a starting mix containing titanium, in particular as
 an oxide, the shaped body is subjected to a heat treatment at a conversion
 temperature to produce a sacrificial body, the sacrificial body is filled
 with softened or liquid aluminum and/or an aluminum alloy under pressure
 at a filling temperature, and starting materials of the sacrificial body
 are reacted with the filled aluminum to form an Al.sub.2 O.sub.3 /titanium
 aluminide composite material as is known from DE 196 05 858 A1.
 DE 196 05 858 A1 has disclosed a process for producing a component from an
 Al.sub.2 O.sub.3 /titanium aluminide composite material. The ceramic/metal
 composite material combines the properties of the ceramic and the metallic
 phases and has a high strength and a high fracture toughness. In the
 process on which the invention is based, a starting mix is formed
 containing, inter alia, titanium in the form of an oxide compound. The
 titanium oxide can be reduced by means of aluminum so as to simultaneously
 form aluminide and Al.sub.2 O.sub.3. One titanium oxide of the starting
 mix which may be mentioned is TiO.sub.2. A shaped body which is close to
 its final shape is pressed from the starting mix. The shaped body is
 converted, by means of a heat treatment at a conversion temperature, into
 a sacrificial body, which is then infiltrated with liquid aluminum. Before
 being filled with aluminum, the sacrificial body is sintered under
 pressure. After sintering, the temperature of the sacrificial body is set
 to a filling temperature which is above the melting temperature of
 aluminum and/or an aluminum alloy (referred to below, including the
 claims, as aluminum for simplification purposes). Furthermore, the filling
 temperature is below a reaction temperature at which a so-called SHS
 reaction takes place between the aluminum and at least one of the starting
 materials. An SHS reaction (self-propagating high-temperature synthesis)
 is a reaction which above its reaction temperature takes place very
 quickly, is highly exothermic and is at least almost uncontrollable. At
 the filling temperature, the sacrificial body is filled with aluminum
 under pressure. After filling, the filled sacrificial body is heated
 beyond the filling temperature to a transformation temperature which is
 above the filling temperature, at which an exchange reaction takes place
 between the aluminum and the constituents of the sacrificial body, so as
 to form an Al.sub.2 O.sub.3 /titanium aluminide composite material.
 However, the sacrificial body, as is evident from the examples given in DE
 196 05 858 A1, is only transformed into the Al.sub.2 O.sub.3 /titanium
 aluminide composite material in certain areas. Furthermore, it can also be
 seen from DE 196 05 858 A1 that a sacrificial body containing TiO.sub.2
 can only be completely filled with aluminum in some instances.
 Furthermore, a sacrificial body of this nature can also only be completely
 provided with a titanium aluminide phase in exceptional cases, resulting
 in a high reject rate.
 DE-P 19710671.4, which is not a prior publication, discloses a process for
 producing a component from a metal/ceramic composite material, in which a
 sacrificial body comprising ceramic precursor materials is filled with
 thermally softened metal--in particular aluminum--and/or with a metal
 alloy. The filling temperature is below a reaction temperature, at which
 reaction temperature an exchange reaction between a metal of the ceramic
 precursor material and a metal of the filling metal takes place. After the
 sacrificial body has been filled as completely as possible, the filled
 sacrificial body is heated to the transformation temperature or above, as
 a result of which the exchange reaction just mentioned then takes place.
 This exchange reaction produces a component made from the metal/ceramic
 composite material which comprises a ceramic phase and a metallic phase
 with an intermetallic bond between the metal of the ceramic and the metal
 of the filling metal. As a result of the sacrificial body being filled
 with a metal which has been softened by heating at below a reaction
 temperature at which an exchange reaction takes place between the filling
 metal and the material of the sacrificial body, the ceramic matrix is
 retained during filling and also during the subsequent exchange reaction
 between the introduced metal and the material of the sacrificial body.
 Ideally, the pores of the sacrificial body are completely filled, so that
 when the substances in question are used in stoichiometric amounts, the
 component has reacted fully all through and is free of cracks and
 channels. Preferably, the filling metal is aluminum and the metal of the
 ceramic is titanium, so that after the preferred exchange reaction the
 ceramic phase comprises TiB.sub.x and/or TiC.sub.y and/or TiCN and
 Al.sub.2 O.sub.3, and the intermetallic compound of the metallic phase is
 a high-temperature-resistant titanium aluminide, in particular TiAl. The
 material properties of this metal/ceramic composite material are good.
 Thus, for example, a metal/ceramic composite material which is produced
 using aluminum as filling metal and Ti as metal of the ceramic sacrificial
 body has a density of 3.4 g/cm.sup.3 ; this density is slightly higher
 than that of the MMCs (metal matrix composites) but is only 42% of the
 density of comparable cast iron. Particularly in the preferred embodiment,
 in which the high-temperature-resistant compound is in the form of the
 intermetallic compound TiAl, the use range of the component extends to at
 least 800.degree. C., significantly above the values for grey cast iron.
 The metal/ceramic composite material produced is used, in particular, to
 manufacture friction rings for the frictional surfaces of disc brakes.
 These friction rings are subsequently fixed by means of mechanical joining
 techniques, such as screws, etc., to the hub of the brake disc.
 However, before the sacrificial body is filled with the metal or the alloy,
 the starting materials of the sacrificial body have to be heated, a first
 exchange reaction taking place between the precursor materials, in which
 reaction high-grade, expensive precursor materials form from the exchange
 materials. After filling with the metal, the ceramic phase and the
 metallic phase are formed from these expensive precursor materials and the
 metal, an exchange reaction again being used to form these phases, in this
 case between the precursor material and the filling metal.
 A further process likewise describes the infiltration of a ceramic
 sacrificial body with aluminum (U.S. Pat. No. 4,988,645). In this process,
 the ceramic body is produced using an SHS reaction (SHS reaction:
 self-propagating high-temperature synthesis, meaning the ignition of a
 reactive mixture with the reaction propagating itself and providing the
 desired ceramic matrix as reaction products).
 However, some components produced in this way have unacceptable levels of
 porosity, and consequently the reject rate is high. In particular, the
 filling with sacrificial bodies containing TiO.sub.2 as precursor material
 of the sacrificial body is very poor.
 In general terms, all the above methods have a high energy requirement,
 which is attributable, inter alia, to the various thermal processes, such
 as sintering, first exchange reaction, filling and subsequent second
 exchange reaction at temperatures which are higher than the filling
 temperature. This energy requirement makes the processes expensive.
 WO 84/02927 has disclosed a process for producing fiber-reinforced die-cast
 parts containing aluminum using the so-called squeeze-casting process. In
 the process, firstly a porous green body is pressed from a starting mix
 containing, inter alia, fibers, and this green body is then filled with
 aluminum. To stabilize the porous green body and to maintain the
 orientation of the fibers arranged in the green body, a binder is added to
 the starting mix and is removed by thermal means during filling of the
 green body. Due to the presence of the pores and the strength of the
 binder, the green body does not undergo any deformation, or at most only
 negligible deformation. In this case, there is no chemical reaction
 between the filling aluminum and the starting materials of the green body,
 and consequently the influence of such a reaction on the structure and
 form of the subsequent die-cast component is not known.
 SUMMARY OF THE INVENTION
 The object of the invention is to further develop the known process in such
 a way that the production of components from a metal/ceramic composite
 material is simpler, quicker and, in particular, less expensive and more
 energy-efficient, and in such a way that the volume of the composite body
 can be provided with titanium aluminide reliably and to the greatest
 possible extent.
 With the sacrificial body on which the invention is based, this object is
 achieved by:
 1. adding carbon and/or its precursors (hereafter referred to as a "carbon
 source"), fillers and binders to the starting mix, wherein the binder
 joins the individual constituents of the starting mix to one another, at
 least in certain areas, in a pressure-stabilizing manner;
 2. selecting the conversion temperature to be less than or equal to the
 filling temperature;
 3. selecting a filler and preferably also a binder with a decomposition
 temperature which is less than or equal to the filling temperature;
 4. directly filling the pressure-stabilized sacrificial body with aluminum;
 5. removing the filler and, if appropriate, the binder, preferably by
 thermal means, before or during the filling with aluminum;
 6. wherein the filling temperature is above the melting temperature of the
 aluminum but below a reaction temperature at which an exchange reaction
 takes place between the aluminum and the oxide of titanium;
 7. after the sacrificial body has been filled with the aluminum, cooling
 the filled sacrificial body to a transformation temperature which is below
 the melting temperature of the aluminum, and;
 8. reacting the starting materials of the sacrificial body and the aluminum
 with one another in a solid-state reaction at the transformation
 temperature to form the Al.sub.2 O.sub.3 /titanium aluminide composite
 material.
 By using a pressure-stable sacrificial body which preferably contains
 reduced titanium oxide TiO.sub.x, where x =1, 1.5, 1.67 or, in particular,
 TiO.sub.2 which can be reduced by carbon, and which is preferably formed
 and/or machined close to its final form, it is even possible for the
 molten Al to be infiltrated spontaneously and therefore in particular to
 be very well pressure-infiltrated.
 The reaction between the aluminum and the materials of the sacrificial body
 to form an Al.sub.2 O.sub.3 /titanium aluminide composite material from
 the starting materials may in particular be performed in a single heating
 operation.
 The transformation temperature is preferably below the filling temperature,
 preferably below the melting temperature of the aluminum, and particularly
 preferably below 400.degree. C. In this way, the energy consumption
 required, and also the production time required is reduced.
 To fill the sacrificial body with aluminum or with an aluminum alloy, the
 sacrificial body is heated. It is therefore appropriate, in order to
 produce the sacrificial body, to use inter alia TiO.sub.2 and C, since
 under certain circumstances the reduced titanium oxide TiO.sub.x (TiO,
 Ti.sub.2 O.sub.3 and/or Ti.sub.3 O.sub.5) inter alia may be formed from
 TiO.sub.2 and C when heated.
 Surprisingly, however, during the pressure infiltration of the sacrificial
 body with aluminum, there is as yet no exchange reaction forming Al.sub.2
 O.sub.3 /titanium aluminide composite material. As mentioned above, the
 formation of the Al.sub.2 O.sub.3 /titanium aluminide composite material
 only takes place through a solid-state reaction, the process temperature
 of which is below the melting temperature of the aluminum.
 DESCRIPTION OF THE PREFERRED EMBODIMENTS
 In further suitable embodiments of the invention:
 for filling, the aluminum and the mould and/or the sacrificial body are
 heated to the filling temperature;
 the sacrificial body is filled with the aluminum in the unsintered state;
 the sacrificial body is produced close to its final shape;
 the sacrificial body is machined to close to its final shape;
 the oxide of titanium is a member selected from the group consisting of
 TiO, Ti.sub.2 O.sub.3, Ti.sub.3 O.sub.5, and TiO.sub.2, preferably
 TiO.sub.2 ;
 the TiO.sub.2 is reduced by the carbon, the filler and/or the binder is
 removed thermally, and during the thermal removal of the filler and/or the
 binder, the carbon is formed as an end product and remains in the
 sacrificial body;
 the filler is vaporized below the filling temperature and/or is converted
 into carbon;
 the binder is vaporized below the filling temperature and/or is converted
 into carbon;
 the filler is an organic material, preferably thermoplastic or
 thermosetting material, and particularly preferably starch and/or flour
 and/or a cellulose derivative, in particular cellulose acetate;
 the ingredients of the starting mix are homogeneously dispersed;
 1-3% by weight of binder is added to the starting mix;
 the filler is a powder with a preferred grain size of between 10 .mu.m and
 100 .mu.m, preferably approximately 20 .mu.m;
 the binder is polyvinyl alcohol and/or polyethylene glycol, preferably in
 an aqueous solution;
 nonvolatile additives, in particular TiC and/or SiC and/or BaC and/or
 TiB.sub.2, are added to the starting mix at the filling temperature;
 the Al.sub.2 O.sub.3 of the ceramic phase is bonded together in all three
 spatial directions;
 fibers, in particular of mineral and/or ceramic materials are added to the
 starting mix;
 the aluminum is introduced with only a slight excess pressure;
 the process of the invention is used for producing frictional surfaces of
 tribological systems and/or engine components and/or vehicle components
 and/or brake discs and/or frictional surfaces for brake discs.
 Otherwise, the invention is explained in more detail with reference to a
 number of examples described below.
 A pulverulent ceramic starting mix containing carbon and TiO.sub.2 and a
 binder and a filler is mixed and then pressed.
 By means of a low-temperature heat treatment in vacuo or under protective
 gas, a porous, unsintered, pressure-stable ceramic sacrificial body is
 formed. The preferred protective gas is nitrogen or CO.sub.2. The process
 is carried out between 350.degree. C. and 700.degree. C., preferably at
 400.degree. C. The filler and, if appropriate, the binder, is burnt off in
 vacuo or under protective gas.
 Expediently, thermogravimetric analysis (TG) is carried out at the same
 time, serving to prove that the binder and, if appropriate, the filler
 has/have been completely removed.
 The controlled addition of the fillers and the binder makes it possible to
 establish an accurately defined porosity, pore structure and strength,
 thus allowing pressure infiltration of the sacrificial body with aluminum.
 One of the advantages of the invention is that throughout the entire
 production of a component from a metal/ceramic composite material of this
 nature, i.e. starting from the production of the sacrificial body, through
 the filling of the sacrificial body with aluminum, to the formation of the
 composite material by the exchange reaction, there is no need for
 temperature steps carried out at over 800.degree. C., in particular over
 700.degree. C. On the other hand, production takes place within a short
 time, in particular the filling by pressure casting.
 Furthermore, the aluminum is converted into a high-temperature-resistant
 titanium aluminide. In addition, very favorable raw materials are used;
 the material price is currently approximately 4 DM per kg.
 To produce the starting mix, firstly in particular titanium dioxide and
 graphite are mixed in a defined stoichiometric ratio with respect to one
 another. Then, 1-3% by weight of binder, preferably polyvinyl alcohol
 (PVA) and/or polyethylene glycol (PEG), in an aqueous solution is added to
 the homogeneous mix, followed by kneading. After the binder, a
 water-soluble organic filler in powder or fiber form, preferably a
 cellulose derivative, in particular cellulose acetate, is added to the
 mixture, likewise followed by kneading.
 The filler, which is preferably added in powder form, in particular has a
 mean grain size of between 10 .mu.m and 100 .mu.m preferably of 20 .mu.m.
 The mixture is either dried or left in the moist state (residual moisture
 approx. 10-20% H.sub.2 O), and uniaxially pressed, in particular at 300
 bar. The uniaxial pressing operation is optionally followed by a further
 cold isostatic pressing operation.
 The sacrificial body, which has preferably been pressed close to its final
 shape, is machined to its final dimensions and is placed in a die-casting
 die for subsequent filling of the sacrificial body with liquid aluminum
 during the further production of the component.
 The strength, the modulus of elasticity, the porosity and the pore
 structure of the sacrificial body are of importance for filling with
 aluminum in the die-casting process.
 These properties can be influenced by the selection of binder, of fillers,
 of the quantity of fillers and the pressing pressure. Furthermore, the
 particle sizes of the ceramic powder (TiO.sub.2 etc.) and of the fillers
 also play a role.
 The relationships between the influencing parameters and target parameters
 are shown qualitatively in Table 1 below.

EXAMPLES
 A number of examples of starting mixes for sacrificial bodies are given
 below in Examples 1 to 7.
 Example 1
 3 mol TiO.sub.2 (mean grain diameter d.sub.50 =0.3 .mu.m) undergo
 preliminary mixing with one mole C (d.sub.50 =0.05 .mu.m) for approx. 10
 min in a kneader (e.g. produced by Eirich). 3% by weight of polyethylene
 glycol (in 20% aqueous solution) is added to this mixture, followed by
 kneading. Then, 10% by weight of cellulose acetate (CA) (d.sub.50 =20
 .mu.m) is added to the moist mix, followed by mixing in the kneader. The
 powder is uniaxially pressed at 30 MPa. This is followed by cold isostatic
 pressing at a pressure of 200 MPa. The sacrificial body is heated for 1
 hour at 700.degree. C. under nitrogen (holding time at 350.degree. C.,
 heating rate 1 K/min), during which time all the organic additives burn
 off without leaving a residue. The sacrificial body has a compressive
 strength of 7 MPa and a porosity of 49%. The pore diameters have a bimodal
 distribution, with one maximum at 0.1 .mu.m and one maximum at 20 .mu.m.
 Example 2
 Same as Example 1, except that the molar ratio between TiO.sub.2 and C is
 3/2. In this case, isostatic further pressing at 300 MPa is required.
 Example 3
 Same as Example 1, except that the amount of cellulose acetate is 20% by
 weight.
 Example 4
 Same as Example 1, except that 10% by weight of water is added to the mix
 of TiO.sub.2 /C/PEG/CA before the uniaxial pressing.
 Example 5
 Same as Example 1, except that 1% by weight of methylcellulose is added to
 the mix of TiO.sub.2 /C/PEG/CA before the uniaxial pressing.
 Example 6
 Same as Example 1, except that short constantan wire fibers or C fibers are
 added to the mix of TiO.sub.2 /C/PEG/CA. This increases the elongation at
 break.
 Example 7
 Same as Example 1, except that the grain size of the TiO.sub.2 has a mean
 diameter of 15 .mu.m. This reduces the porosity to 47%. The compressive
 strength increases to 7.5 MPa.
 The sacrificial bodies are provided for subsequent pressurized filling with
 aluminum. After filling, they are subjected to a heat treatment at below
 the melting point of the aluminum, resulting in a component comprising
 composite material which contains in particular homogeneously distributed
 TiC, Al.sub.2 O.sub.3, and Al.sub.3 Ti.
 It should be pointed out here in particular that a solid-state reaction
 takes place during the subsequent heat treatment to produce the composite
 material. Therefore, this reaction can take place at below the melting
 point of the aluminum. The preferably homogeneous composite material is
 high-temperature-resistant and wear-resistant.
 The process according to the invention and therefore also the starting mix
 according to the invention or the sacrificial body according to the
 invention are suitable in particular for the production of frictional
 surfaces of tribological systems or of engine components and/or of vehicle
 components and/or of brake discs and/or of frictional surfaces for brake
 discs. Tribological systems are to be understood as meaning, in addition
 to brake discs, also structural components in jet engines and motors, in
 particular sliding-contact bearings and cutting materials.