Process for producing shrinkage-matched ceramic composites

Details Process for producing shrinkage-matched ceramic composites Process for producing shrinkage-matched ceramic composites

1997-09-30

2003-06-10

Walker, W. L. (Department: 1723)

Plastic and nonmetallic article shaping or treating: processes

Outside of mold sintering or vitrifying of shaped inorganic...

Producing microporous article

C264S113000, C264SDIG004, C210S490000, C210S500250, C210S500260

Reexamination Certificate

active

06576182

ABSTRACT:

BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention relates to a process for producing shrinkage-matched composites which comprise at least one ceramic component and the composites obtainable by this process.
DESCRIPTION OF THE BACKGROUND
The production of materials-based components frequently includes the problem of combining various materials (e.g. ceramic and metal or ceramic I and ceramic II) with one another to form one unit. If such composites are produced by powder-metallurgical methods, there is frequently the additional requirement of first pairing unlike materials in the unfired state in order to then sinter them together in a co-firing to produce the actual product.
However, the production of unlike material pairings via sintering processes presents the fundamental problem that each component has specific material properties such as modulus of elasticity, coefficient of thermal expansion and material transport properties (sinter activity). The differences in the sinter activity lead to different shrinkages during the sintering process, so that distortion or complete destruction of the component can occur already during the densification phase. Furthermore, intact components frequently have very high residual stresses which are attributable, on the one hand, to the different shrinkages on sintering and, on the other hand, to the differences in the coefficients of thermal expansion of the materials involved. There have therefore been many attempts to master the problems of the different shrinkages on sintering and different coefficients of thermal expansion by materials-related measures. Among the most widely employed techniques is the use of low-temperature-sintering glass phases by means of which, in combination with appropriate starting powders, sintering temperature and shrinkage on sintering are adjusted. However, for many applications glass phases cannot be used since they adversely affect the material properties of one or more components or the required properties are not achieved at all in the system. In these cases, attempts are made to realize the different material pairings by means of the grain size dependence of the sinter activity of powders. This method of matching the shrinkage on sintering can be employed for many material combinations, but it is associated with considerable technical complication and financial cost. In complicated processes, the grain size distributions of the components have to be matched to one another or powders of appropriate fineness have to be produced first. All techniques have in common the fact that not only selected raw materials but also organic additives are required. These additives, on the one hand, are to disperse the powders homogeneously and in a deagglomerated state in a dispersion medium. On the other hand, they assume the function of processing aids by means of which the rheological properties or the processability of the ceramic compositions are matched to the requirements of the respective shaping process. Depending on the shaping process, the proportion of organics can be up to 50% by volume and has to be removed before sintering, sometimes by complicated and time-consuming processes. This step becomes particularly difficult when different materials such as ceramic/metal are joined to form one part, since the removal of the organic processing aids then has to be additionally carried out under inert conditions. In order to overcome these difficulties and limitations, it would be extremely worthwhile to develop techniques, processes or materials which permit very substantial matching of the shrinkages on sintering when using unlike materials and at the same time make do without addition of organic processing aids or make do with greatly reduced proportions of these. In the ideal case, the function of the organic processing aids should be taken over by inorganic materials which are converted into the ceramic material during sintering.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a process for producing composites which comprise at least one ceramic component, which process enables the shrinkage behaviour of this or these ceramic component(s) on sintering to be matched to that of the other ceramic and/or non-ceramic components.
It has surprisingly been found that this object is achieved by using nanosize ceramic powder as starting material (or a constituent thereof) for the ceramic component(s) whose shrinkage behaviour on sintering is to be matched to the other component(s). This process provides completely new possibilities in pairing materials and optionally in densifying them in a co-firing.
The present invention accordingly provides a process for producing composites which comprise at least one shrinkage-matched ceramic component, which process is characterized in that the starting material for the ceramic component(s) whose shrinkage behaviour on sintering is to be matched to the remaining component(s) is selected such that the ceramic-forming constituent of the same consists essentially of:
(a) at least one ceramic powder (i) comprising particles having a size of up to 500 nm;
(b) at least one ceramic powder (i) as defined in (a) in admixture with at least one powder (ii) comprising at least one sintering-inhibiting substance having a particle size equal to or smaller than that of the powder (i); or
(c) at least one ceramic powder (i) as defined in (a) in admixture with at least one ceramic powder (iii) having a particle size above that of the powder (i) used and up to 500 &mgr;m.
DETAILED DESCRIPTION OF THE INVENTION
The ceramic powder (i) preferably comprises particles having a size of up to 300 nm and in particular up to 200 nm. Although there is no critical lower limit for the particle size, for reasons of being able to make the powder it is usually about 1 nm. Furthermore, the ceramic powder (i), like the other ceramic powders used, can be used in pretreated form; this pretreatment can include, in particular, a surface modification of the powder particles with short-chain (preferably bifunctional) organic or organometallic compounds, as described, for example, in DE-A-4212633. The purpose of this pretreatment can be, for example, to adjust the rheology of the mix and/or (particularly in the case of nanosize powders) the solids content.
The particles of the ceramic powders used in the present invention can have various shapes, for example spherical, tabular, fibre-shaped, etc. The term particle size as used herein refers in each case to the longest dimension of these particles, which corresponds, for example, to the diameter in the case of spherical particles. Furthermore, for example, agglomerates can first be produced from these powders and then be subjected to thermal post-treatment in order to adjust the sinter activity.
The ceramic materials used in the present invention are preferably derived from metal (mixed) oxides and carbides, nitrides, borides, silicides and carbonitrides of metals and nonmetals. Examples are (optionally hydrated) Al
2
O
3
, partially and fully stabilized ZrO
2
, mullite, cordierite, perovskites, spinels, e.g. BaTiO
3
, PZT, PLZT, etc., and also SiC, Si
3
N
4
, B
4
C, BN, MoSi
2
, TiB
2
, TiN, TiC and Ti(C,N). Of course, it is also possible to use mixtures of oxides or mixtures of non-oxides and mixtures of oxides and non-oxides. Particularly preferred ceramic starting materials are (&agr;- or &ggr;-)Al
2
O
3
and ZrO
2
(in unstabilized, partially stabilized or fully stabilized form).
The above alternative (b) of the process of the invention is of particular interest when composites are to be produced from a (ceramic) powder which sinters relatively sluggishly and a powder (i) (e.g. in the production of filters). In this case, the shrinkage behaviour of the (very sinter active) powder (i) has to be matched to that of the coarser powder. Sintering-inhibiting secondary phases in the form of the powder (ii) are used for this purpose. The particular materials used as powder (ii) depend on the nature of the powder (i

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