WO1996036961A2 - Selected ceramic mouldings and process for selecting faultless ceramic mouldings - Google Patents

Abstract

The description relates to a process and device for producing focussed underwater shock waves by multiple simultaneous underwater spark discharge and focussing by means of confocal ellipsoids, in which a maximum pressure the GPa range is attained in the commun focus, and its use as a proof test for ceramic mouldings. The ceramic mouldings subjects to the proof test are initially faultless.

Description

Selected ceramic molded parts and process for the selection of faultless ceramic molded parts

Ceramic molded parts have a number of properties, such as temperature resistance, hardness, low density, low coefficient of thermal expansion, low thermal conductivity and corrosion resistance, etc., which they have for a large number of applications in the high temperature range, under temperature changes, even in a corrosive environment, etc predestine. The high brittleness of molded ceramic parts is a disadvantage. Due to the high brittleness, even the smallest material defects, especially in the case of moving ceramic components, e.g. as components of internal combustion engines, for their destruction.

Due to the manufacturing process for ceramic components, starting from possibly granulated powders, shaping by dry pressing or injection molding, heating or burning out the shaping aid and

Sintering and, if necessary, reworking the surface, material defects in the finished molded part cannot be reliably avoided.

For critical areas of application, it is therefore necessary to reject faulty components after manufacture.

For this purpose, methods of acoustic sound analysis, ultrasound analysis as well as mechanical and thermal shock tests were developed, which provide non-destructive information about any defects in the component. Although a large part of the defective molded parts can be sorted out in this way, the security of the identification of a defective component is not sufficient for many desirable areas of application. So the broad fails

Use of a ceramic valve stem in the motor vehicle engine on the still inadequate freedom from defects of such molded parts. For the valve stem in the motor vehicle engine, it must be demanded that for every million such components there is less than one defective one. There is therefore a need for a "proof test" that allows defective molded parts to be recognized and sorted out with almost absolute certainty. In comparison to this, the improvement of the manufacturing process for ceramic molded parts to avoid defects is only a measure that improves the economic efficiency and with which the basic usability of such components cannot be guaranteed. According to the invention, it is now proposed to apply a focused underwater shock wave to ceramic molded parts after the sintering, in such a way that defective molded parts are destroyed and sorted out by the compressive and tensile stresses generated thereby.

The technique of focused underwater shock waves has been developed especially for the area of kidney stone destruction and has been widely used. One possibility for generating focused underwater shock waves is to bring about a plasma explosion in the focal point of a rotation ellipsoid by means of a discharging spark gap, the pressure wave of the plasma explosion taking the form of a

Shock wave propagates, is reflected on the walls of the ellipsoid and is focused in the other focal point. The spark discharge energies permitted for kidney stone destruction are, however, in the range of only up to a maximum of 100 joules, whereas the spark discharge energies required for the proof set of ceramic components lie in the 4 m range of a few kilojoules. A

However, increasing the spark discharge energy in the required range is not easily possible. On the one hand, an increase in the discharge energy leads to an increased erosion of the electrodes of the spark gap, furthermore, increased requirements must be placed on the impedance-free electrical supply of the spark gap in order to ensure a sufficient energy density

(short discharge time). An increase in the spark discharge energy accordingly requires an extension of the distance between the electrodes, so that the discharge is triggered at a higher voltage. However, an increase in the distance between the electrodes of the spark gap leads to a broadening of the focus, so that the energy density in the focus does not increase accordingly. A focusing of the shock wave suitable for the proof test of ceramic components is still achieved with underwater spark gaps, with which an energy of 400 to 800 joules is released. With such discharge energies, the half-value surface of the focus lies in the range from 3 to 5 cm ² . The half-value surface of the focus is to be understood as the closed area (approximately an ellipsoid) within which the pressure of the focused shock wave is greater than half the maximum pressure.

According to the invention, it is therefore proposed to carry out several simultaneous spark discharges in the one focus of each hollow ellipsoid, the several ellipsoids being arranged such that they have a common second one Have focus. The present invention accordingly relates to a method for generating a focused underwater shock wave by means of underwater spark discharge in a focus of a hollow ellipsoid, characterized in that the total spark discharge energy is released by at least two simultaneous spark discharges in one focus each of a focusing ellipsoid, the at least two Ellipsoids have a common second focus.

Since the focused shock waves meet in the common focus of the ellipsoids, it is possible to increase the energy shock density in the focus without widening the focus.

At least three underwater spark discharges are preferably triggered simultaneously. The number of spark discharges to be triggered at the same time can only be increased by the geometry, i.e. limited by the expansion of the focusing ellipsoids. According to the invention, it is not excluded to focus up to 12 shock waves in one focus. The achievable according to the invention

Maximum pressure in focus can be between 0.5 and 2.5 GPa.

The present invention also relates to a device for generating a focused underwater shock wave with high energy density, comprising at least two confocal partial ellipsoids, in the other focus of which a spark gap is provided, and means for generating simultaneous spark discharges in the

Partial ellipsoids.

Part ellipsoids according to the invention are part-hollow rotational ellipsoids, the spark gap being in the focus within the part ellipsoid and the second focus outside the part ellipsoid. At least the partial ellipsoid should extend to a plane perpendicular to the major axis of the ellipsoid, which is twice the distance from the point of penetration of the major axis through the ellipsoid, like the focus. Due to the propagation behavior of the shock wave generated by the spark gap arranged parallel to and in the major axis of the ellipsoid, the energy bundling cannot generally be improved if the partial ellipsoid is larger than 1/2 ellipsoid.

Spark discharges are usually generated by a capacitor bank using a capacitor charging unit at voltages of 10 is charged up to 50 kilovolts, and the discharge is ignited via a switching spark gap by means of a trigger generator, which generates a voltage peak in the switching spark gap.

The means for generating simultaneous spark discharges can consist in the fact that the several spark gaps are connected in series.

Another means of generating simultaneous spark discharges is to trigger the trigger generators for the spark discharges at the same time.

Devices according to the invention are preferred which release a total energy of 1 to 5 kilojoules by means of underwater spark discharges, the half-value surface of the focus being less than 5 cm ² , particularly preferably less than 3 cmr. This corresponds approximately to a focus diameter of 7 to 12 mm.

The half-value surface is the ellipsoidal surface enclosing the location of the pressure surge maximum, on which the mean pressure is half the pressure maximum.

The present invention also relates to a method for producing fault-free ceramic components by sintering green bodies, which is characterized in that each component is not subjected to a focused underwater shock wave which was generated by means of underwater spark discharge with an energy of 1 to 5 kilojoules before the sintering was carried out and wherein the half-value surface of the focus is less than 5 cm ² , defective components being eliminated by destruction.

The invention further relates to a ceramic molded part selected by exposure to at least one focused underwater shock wave, the underwater shock waves using an underwater spark discharge of an energy of

1 to 5 kilojoules was generated and the half-value surface of the focus is less than 5 cm ² .

The ceramic moldings are preferably after sintering and before

Machining by mechanical removal subjected to the proof test so that the complex mechanical post-processing for the defective molded parts remains. Furthermore, it is advantageous to carry out any notches, engravings or other mechanical machining operations that can be mechanically narrowed only after the proof test, since such cross-sectional constrictions can lead to excessive stress from the proof test. The ceramic components preferably have dimensions in at least one dimension of 5 to 30 mm. Shaped parts that have larger dimensions in at least two dimensions must be subjected to the focused shock wave several times at different points, since otherwise their freedom from errors cannot be guaranteed with certainty. In larger molded parts, the shock wave propagates with weakening, so that points further away from the impinging focus are no longer subjected to sufficient energy.

Partial ellipsoids are preferably used, the large elipsoid axis being between 15 and 25 cm and the axis ratio being 1.1 to 1.5, preferably up to 1.3.

The spark discharge energy in the range from 1 to 5 kJ is selected in

Dependence on the ceramic material and the shape and size of the molded part.

Suitable ceramic materials are Si ₃ N ₄ , A1N, BN, SiC, B ₄ C, the transition metal carbides, nitrides, silicides, borides, the oxides Al ₂ O ₃ , ZrO ₂ , SiO ₂ , La-O ₃ , Rare earth oxides, Cr ₂ O ₃ , TiO ₂ , titanates, ferrites and any mixtures of these

Connections or mixed phases from these compounds, especially if the corresponding moldings were produced by sintering. Silicon nitride, aluminum oxide, zirconium oxide, silicon carbide and mixtures thereof are particularly preferred.

The molded parts selected according to the invention can initially be characterized as 100% error-free.

The invention is explained below with reference to the accompanying figures.

1 shows a device according to the invention with which three underwater spark discharges are focused in a common focus.

FIG. 2 shows a horizontal section AA through the device according to FIG. 1. Fig. 3 shows the principle of electrical spark discharge for the simultaneous triggering of two spark discharges.

4 shows an alternative embodiment of the electrical circuit for simultaneously igniting two spark discharges.

FIG. 5 exemplifies the proof test for a valve stem made of silicon nitride.

6 shows an example of an embodiment of the device according to the invention with 6 confocal spark discharges.

The device according to FIG. 1 consists of an upwardly open water tank 1 with a water surface 2 indicated, which in a corner has three circular openings 3 arranged at right angles to one another with respect to its longitudinal axis a and to which semi-ellipsoids 4 which are open from the outside of the container are flanged . In the focus F of each half ellipsoid, a spark gap (not shown) is arranged through an insertion opening provided on the back of the ellipsoid. The second focus of each of the ellipsoids 4 is the common focus F _c . The ceramic molding to be tested is arranged in the focus with a surface such that the focused shock waves triggered at the same time can enter the molding as evenly as possible.

Fig. 2 shows a horizontal section AA through Fig. 1, which contains the two horizontal axes of the ellipsoids. The spark gap 6 is additionally indicated in these ellipsoids.

Fig. 3 shows in principle the switching arrangement for generating the spark discharge. For this purpose, a capacitor battery 10 is provided, which is charged by means of a capacitor charging unit via the charging resistor 12, for example to a voltage of 40 kV. To trigger the spark discharge, the trigger generator 13 generates a high-voltage pulse in the switching spark gap by means of an electrode 15, by means of which the switching spark gap is ionized, so that the total resistance in the circuit comprising the capacitor bank 10, the switching spark gap 14 and the two spark gaps 6 is reduced in this way that the spark gaps ignite. The fact that the two spark gaps 6 are connected in series ensures that they ignite at the same time. 4, the two spark gaps 6a and 6b have separate identical supply and switching units as shown in FIG. 3. For the simultaneous ignition of the spark discharge, the two trigger generators 13 are connected via a synchronization line 16.

5 shows as a ceramic component a valve stem 20 with the valve plate 21 and the integrally formed axis 22. The valve stem is arranged in the device according to FIG. 1 with suitable holding devices such that the common focus F _{c is} in the region of the transition from the Valve plate to the valve axis is located. The three arrows I, II and III indicate the relative arrangement of the large ellipse axes from FIG. 1.

FIG. 6 shows a device according to the invention with 6 spark gaps 6 arranged in a circular symmetry of the plane and the corresponding partial ellipsoids 4, which focus on the common focus F _c . A cylindrical shaped part 3 with an axis perpendicular to the plane of the drawing is shown as an example.

Claims

claims 1. Device for generating a focused underwater shock wave of high energy density, containing at least two confocal partial ellipsoids, in the other focus of which a spark gap is provided, and means for generating simultaneous spark discharges in the partial ellipsoids. 2. A method for generating a focused underwater shock wave by means of underwater spark discharge in a focus of a partial ellipsoid, characterized in that the spark discharge energy is released by at least two simultaneous spark discharges in one focus each of a partial ellipsoid and the partial ellipsoids have a common second focus. 3. Process for the production of defect-free ceramic components by sintering shaped green bodies, characterized in that each component is not subjected to a focused underwater shock wave, which was generated by means of underwater spark discharge, of an energy of 1 to 5 kJ, and the half-value surface of the Focus is less than 5 cm ² , defective components are sorted out by destruction. 4. Ceramic molded part selected by exposure to a focused underwater shock wave, the underwater shock wave using Underwater spark discharge with an energy of 1 to 5 kJ was generated and the half-value surface of the focus is less than 5 cm ² .