DE102007045636A1 - Method for determining the thermal shock robustness and material strength of brittle-failing materials - Google Patents

Abstract

The invention relates to a method for determining the thermal shock robustness and the material strength of brittle failing, in particular ceramic materials, comprising the following steps: (a) homogeneously heating at least one material sample to a desired temperature, (b) inducing a local thermal shock by cooling with a Form and volume of fluid, wherein the fluid has a Weber number of We> = 130, (c) testing the material sample for cracking, (d) optionally steps a) to c) are repeated with the material sample, wherein in step a) a higher target temperature level is set. The invention also relates to a device for carrying out the method.

Description

The The present invention relates to a method for determining the Thermal shock robustness and the material strength of brittle-failing Materials, in particular of ceramic materials, as well as a Apparatus for carrying out this method.

One continuous development process leads to more and more demanding and smarter technologies. With the same dynamics increase also the demands made on new and modern materials become. Especially higher material strengths for Material saving constructions, higher security, the Thermal shock behavior and a longer life play in technical applications and in the selection of those suitable materials a central role.

ceramic Materials have already proven themselves in a variety of applications, where high hardness, high wear resistance, high Corrosion resistance good high temperature stability needed with a low specific weight become. The properties of ceramic materials are not only by the chemical composition, but also relevant determined by the respective structure and microstructure. The targeted setting of certain microstructures, the like called structure design, allows the influence of mechanical and physical characteristics. New ceramic Materials must meet the respective requirements and technical Problems suffice. Are the characteristics of the material known, taking into account the thermal, mechanical and chemical stresses, if necessary, before production of a component materials that are particularly suitable Have properties. Likewise ceramic materials that are unsuitable for are a particular application profile. An essential point of view when using ceramic materials is their brittleness and the thermal shock sensitivity. Under thermal shock understands the fast, shocking change in temperature a material or workpiece. This leads to mechanical Tensions between the outer and inner part of the material, as the heat on the surface dissipate faster than inside. Exceed the resulting voltages have a critical value, it comes to Damage, such as cracking, of the material. For example, it may in the exhaust system of a motor vehicle by Water for a local thermal shock on the ceramic sensor element which leads to cracking and consequently to a component failure can lead. For use over a long distance Temperature ranges and with rapid temperature changes must the temperature change or thermal shock resistance Therefore, be assured of materials and components made from them.

State of the art

to Determining the mechanical strength of ceramic materials is It is known, so-called four-point bending tests or a double-ring bending test apply. For thermal shock testing already exist Method for component testing, such. B. Wasserabschreckversuche. However, all these experiments and methods do not provide material characteristics.

The EP 0 992 784 B1 describes an apparatus and method for measuring the strength and thermal shock behavior of material samples. In this method, a locally narrowly limited area of a material sample is shock-heated, for example with a lamp or a laser. Using a pyrometer and a thermal camera, a local temperature profile is recorded and its chronological progression is monitored. The mechanical processes in the sample are observed via a microscope and a video camera. The heat energy input is increased until, due to the thermal stresses, a break occurs in the material sample. Knowing the temperature distribution at break time and a variety of material characteristics, such as the modulus of elasticity, the Poisson number and the coefficient of thermal expansion, the stress ratios at break time and thus the strength of the material sample can be calculated.

Disclosure of the invention

• (a) homogenes Aufbeizen mindestens einer Materialprobe auf eine Solltemperatur,
• (b) induzieren eines lokalen Thermoschocks durch Abkühlen mit einem in Form und Volumen definierten Fluid, wobei das Fluid eine Weberzahl von W_e > 130 aufweist,
• (c) Prüfung der Materialprobe auf eine Rissbildung,
• (d) optional werden die Schritte a) bis c) mit der Materialprobe wiederholt wobei in Schritt a) einem höheres Soll-Temperaturniveau eingestellt wird.

According to the invention, a method is proposed for measuring the thermal shock robustness and the material strength of brittle-failing, in particular ceramic materials, comprising the following steps

• (a) homogeneous pickling of at least one material sample to a desired temperature,
• (b) inducing a local thermal shock by cooling with a fluid defined in shape and volume, the fluid having a Weber number of W _e > 130,
• (c) testing the material sample for cracking,
• (D) optionally, the steps a) to c) are repeated with the material sample wherein in step a) a higher target temperature level is set.

The measuring method according to the invention makes it possible to characterize brittle-failing materials, in particular ceramic materials, with regard to thermal shock robustness and the mechanical, temperature-dependent material strength. Likewise, other brittle-failing materials, such as glasses or metallic materials prepared by powder technology, can be characterized according to the invention. Thermal shock robustness is also called thermal shock resistance. The material strength and the thermal shock resistance are determined here as material characteristics. In contrast to the known methods, the evaluation of the measurements obtained for the thermal shock resistance can advantageously be carried out without the knowledge of further material parameters, such as modulus of elasticity, Poisson's coefficient or thermal expansion coefficient. The measurement can also be performed as a material test. Advantageously, in the run-up to the possibly very complex and costly production of components, an examination of the mechanical properties of the materials, in particular the thermal shock robustness, can take place. Thus, it can be judged whether the respective material is fundamentally suitable for a particular application due to its identified thermal shock robustness and material strength.

ρ:

Dichte des Fluid [kg/m³]

ν:

Geschwindigkeit des Fluid [m/s]

L:

Charakteristische Länge/Durchmesser des Fluid [m] und

σ:

Oberflächenspannung [N/m] bedeutet.

The fluid used according to the invention has a Weber number of W _e ≥ 130. The Weber number is a measure of the drop deformation and is represented by the following formula:

ρ:

Density of the fluid [kg / m ³ ]

ν:

Velocity of the fluid [m / s]

L:

Characteristic length / diameter of the fluid [m] and

σ:

Surface tension [N / m] means.

In In a preferred embodiment of the method, the material sample have a maximum thickness of 0.5 mm, preferably of at most 0.3 mm. The induced cooling by the fluid can in this case especially advantageously over the entire sample thickness and so on particularly favorable tension conditions in the Be provoked sample.

In Another preferred variant of the method provides that in step b) the diameter d1 of the induced local cooling through the fluid to the material sample diameter D1 a ratio of at least 1: 4. That is, preferred according to the invention the diameter of the material sample (D1) is at least 4 times larger can be as the diameter of the induced local cooling through the fluid (d1). This will be for the inventive Material test achieved particularly favorable voltage conditions.

According to the invention various material sample forms, in particular ceramic film samples, be used, which have a plane stress state, which means that until reaching the breaking point essentially no Denting takes place. Particularly preferred may be circular Washers are used.

In In a preferred embodiment, in step a) the material sample contactless, particularly preferably by a high-power lamp, be heated to the set temperature. This has on the one hand Advantage that the material sample no further mechanical stress is suspended. On the other hand, the heating process can be particularly homogeneous and done quickly. In addition, advantageously a variety of samples are heated simultaneously.

Around a quick yet accurate determination of the material characteristics To achieve the respective material sample / n, according to the invention, the Temperatursteigerungsinkrement in step d) ≤ 10 ° C, preferably ≤ 5 ° C to get voted. There may also be smaller temperature increase increments be selected to further increase the accuracy. However, this may increase the duration of the procedure. It can according to the invention but also first for a rough determination also a bigger one Temperature increment can be selected. In the temperature range in which then a cracking is detected, can then below performs a fine determination with smaller temperature increments which then improves the accuracy of the characteristic determination.

In a further preferred embodiment of the invention Process, the fluid may be a liquid, preferably a be aqueous medium. Advantageously, can Such fluids are handled well and usually safely. By aqueous media are meant media that at least 80 wt .-% consist of water. For example, the water can Substances or substance mixtures are added, the surface-active Have properties. For example, surfactants can be added to the water or surfactant mixtures which have the wetting properties improve the water. The fluid can thus be much more controlled to hit the material sample. Preference is given to the substances or Substance mixtures in quantities of 5-10% by weight of the water added. Aqueous media have beyond the advantage that they can be used again and also environmentally friendly in disposal and also inexpensive are.

According to the invention, the test for crack formation in step c) in the process, visually, preferably with a fluorescent liquid under UV light occurred. The test can be carried out automatically or semi-automatically directly by an operator or even with known means and methods. For example, an automatic camera can record the course of the test continuously or at selectable time intervals and subsequently evaluate it. Other methods are based, for example, on the acoustic detection as in the ultrasonic resonance or on X-rays.

In a further preferred embodiment of the invention, the At least partially automated measuring method according to the invention be performed. This allows a high and fast throughput of material samples. Also a statistical hedge the measured material parameters for thermal shock robustness and material strength can thus be done much faster.

According to the invention furthermore an apparatus for carrying out the above described method and its various embodiments for determining thermal shock robustness and material strength provided by brittle-failing materials, these being at least one sample tray for material samples, one agent for non-contact homogeneous heating of the material sample / s, a means for non-contact temperature measurement, a Dosing device for applying a defined in shape and volume of fluid on a material sample, a device for crack detection and a Evaluation has.

Especially Preferably, the device is semi-automated or fully automatic operated, so until the completion of the test advantageously no intervention by an operator is necessary. The exam is finished when all the samples used are cracking show or the maximum temperature of the invention Device for the test is achieved.

When Means for non-contact homogeneous heating of the material sample / s For example, it is possible to use a high-power lamp. advantageously, The heating process can thus be particularly gentle on the Samples and done very fast. As a means of contactless Temperature measurement, for example, a pyrometer can be used. The metered application of a defined in shape and volume of fluid on a material sample can in the inventive Device can be realized by a corresponding dropper.

The Evaluation unit of the device according to the invention can be a computerized control and a corresponding Software program included. In the control can all required parameters such as the sample thickness, the Fluid form and the temperature increments to be observed and stored and all required for evaluation Data is collected. After the examiner can then the evaluation advantageously be done automatically.

Especially Preferably, the device may comprise a sample tray which to record at least 30 material samples. This can be used for a high and fast throughput of samples. on the other hand Advantageously, the results of the material characteristic characterization can be statistical be secured.

In a particularly preferred embodiment of the inventive method can be positioned on a sample round plate of a device according to the invention for testing a plurality of samples, in particular ceramic sample sheets. The samples may preferably be configured identically in shape and size. The samples may have, for example, a maximum thickness of 0.3 mm and a minimum of 4 times the diameter. The maximum diameter should not exceed 16 mm. This procedure allows a high and fast throughput of material samples as well as a statistical validation of the measured material characteristics. Each sample can be heated without contact by means of a high-power lamp to a predetermined target temperature homogeneous. Advantageously, the heating of the entire sample volume used by the high-power lamp can be done simultaneously, which in turn promotes rapid implementation of the method. A temperature control can also be done without contact, for example via a pyrometer. After reaching the desired temperature, the samples can be left for a remaining time at this temperature, so that a homogeneous temperature control of the entire sample / s is particularly ensured. The retention time may preferably be for a period of 5 seconds. to 3 min., more preferably for a duration of 10 sec. to 60 sec., to be selected. After this holding time, a fluid defined in shape and volume is brought to the center of the sample (s), thus inducing a local thermal shock. A liquid preferably an aqueous fluid can be applied, for example, by a dropper as a metering device. According to the invention, a ratio of the diameter of the centrally initiated cooling by the fluid (d1) to the sample diameter (D1) of 1: 4 is particularly preferred. The fluid has a Weber number W _e ≥ 130. After carrying out the thermal shock test of all samples on the sample tray, a visual check for cracking of the samples may preferably be carried out. For better visualization of the crack picture In addition, a fluorescent liquid can advantageously be applied to the material samples and the test can be carried out under UV light. The cracking test may preferably be automated by known methods. For the samples in which no cracking is determined, then automatically set a next higher temperature level and the inventive method with steps a) to c) are repeated.

drawings

The Invention will be described below by way of example with reference to a Embodiment in conjunction with the drawing closer illustrated without being limited thereto.

In this shows:

1 a schematic sectional view of a material sample according to the invention.

1 shows a schematic sectional view of a material sample according to the invention 1 from a brittle-failing material. The material sample 1 is preferably made of a ceramic material. The material sample 1 may have at least a diameter D1 which is four times as large as the diameter d1 of the region of local cooling 3 by a fluid defined in shape and volume 2 is effected. The fluid 2 according to the invention has a Weber number of W _e ≥ 130 and induced in the material sample 1 a thermal shock.

The material strengths were according to the standard DIN EN 843-1 certainly.

According to the invention Thus, a method and an apparatus for carrying out This method provides, with a simple way Characterization of brittle-failing materials, in particular of ceramic materials, in terms of thermal shock robustness and the mechanical, temperature-dependent material strength is possible. The material strength and thermal shock resistance can be determined here as material characteristics. In contrast to the previously known methods, the characterization advantageously without the knowledge of further material parameters respectively.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• EP 0992784 B [0005]

Cited non-patent literature

• - DIN EN 843-1 [0027]

Claims (10)

Method for determining the thermal shock robustness and the material strength of brittle-failing, in particular ceramic materials, characterized by the following steps: (a) homogeneously heating at least one material sample to a desired temperature, (b) inducing a local thermal shock by cooling with a fluid defined in shape and volume, wherein the fluid has a Weber number of W _e ≥ 130, (c) testing the material sample for cracking, (d) optionally repeating steps a) to c) with the material sample, setting a higher desired temperature level in step a) , Method according to claim 1, characterized that the material sample has a maximum thickness of 0.5 mm, preferably of a maximum of 0.3 mm. Method according to one of claims 1 to 2 characterized in that in step b) the diameter d1 the induced local cooling by the fluid to the material sample diameter D1 has a ratio of 1: 4. Method according to one of claims 1 to 3 characterized in that in step a) the material sample contactless, preferably by a high-power lamp, to the target temperature is heated. Method according to one of the preceding claims 1 to 4, characterized in that the Temperatursteigerungsinkrement in step d) ≤ 10 ° C, preferably ≤ 5 ° C is selected. Method according to one of the preceding claims 1 to 5, characterized in that the fluid is a liquid, preferably an aqueous medium. Method according to one of the preceding claims 1 to 6, characterized in that the steps a) to d) at least partially automated. Apparatus for carrying out the method according to one of claims 1 to 7 for determining the thermal shock robustness and the material strength of brittle-failing materials, at least containing a sample tray for material samples, a means for non-contact heating of the material sample / s, a means for non-contact temperature measurement, a Dosing device for applying a defined in shape and volume Fluids on a material sample, a device for crack detection and an evaluation unit. Device according to claim 8, characterized that the sample tray can hold at least 30 material samples. Device according to claim 8 or 9, characterized that the material test and / or the evaluation unit for the measurements are controlled by a software program.