Method for testing critical temperature difference corresponding to rapid change of residual strength of ceramic matrix composite under high-precision measurement of thermal shock

Abstract

The application provides a test method for high-precision measurement of a critical temperature difference corresponding to a quick change of a thermal shock residual strength of a ceramic matrix composite material, and solves the problems that the existing method for measuring the critical thermal shock temperature difference of an ultrahigh-temperature ceramic material is relatively complex and cannot be quickly determined. The application determines the critical thermal shock temperature difference through observation of a boiling state closely related to ceramic quenching heat transfer, and the critical thermal shock temperature difference can be determined through only one test, so that the time required for determining the critical thermal shock temperature difference is greatly shortened.

Description

Technical Field

[0001] This invention belongs to the field of thermodynamic property testing technology for brittle materials such as ceramics, and specifically relates to a high-precision method for measuring the critical temperature difference corresponding to rapid changes in the residual strength of ceramic matrix composites under thermal shock. Background Technology

[0002] As the cornerstone of aerospace development, spacecraft face severe challenges to their thermal protection materials and structures when exposed to complex and extreme aerospace environments. This places stringent requirements on the temperature limits and durability of oxidation-resistant materials, as well as their lightweight and toughening properties under high-temperature oxidation and complex load conditions. Materials need to possess excellent high-temperature resistance, thermal shock resistance, oxidation resistance, and ablation resistance, while also maintaining their physical and chemical stability even in the presence of both high temperatures and chemical atmospheres. Currently, the solution to this problem is the application of ultra-high temperature ablation-resistant structural materials, such as ultra-high temperature ceramics (UHTCs) and oxide ceramics. UHTCs possess high strength, excellent thermal shock resistance, oxidation resistance, and ablation resistance at high temperatures, and have experienced rapid development and widespread application over the past 20 years, making them a promising candidate for key thermal structural components such as the nose cone, wing leading edge, and rocket engine nozzle of hypersonic vehicles. However, the inherent brittleness of UHTCs severely impacts the reliability of structural components under actual service conditions, and their poor thermal shock resistance remains a key factor limiting their engineering applications.

[0003] When an object is subjected to thermal shock, the resulting thermal stress is mainly related to the thermal conductivity of the object itself and the surrounding medium. In actual service, both the material's inherent properties and the influence of the service environment on its properties must be considered. Currently, the evaluation of the thermal shock resistance of ultra-high temperature ceramic materials mainly uses the water quenching method. The general understanding of thermal shock failure of ultra-high temperature ceramic materials is that when the material undergoes rapid cooling due to quenching, the surface of the specimen will be subjected to thermal stress caused by the transient non-uniform distribution of the internal temperature field. When the thermal stress experienced by the specimen surface exceeds the inherent strength of the material, the thermal stress will induce microcracks. After the microcracks appear, they will rapidly propagate until the material undergoes overall failure, i.e., thermal shock fracture. The temperature difference between the material and the quenching agent at the time of thermal shock fracture is called the critical thermal shock temperature difference.

[0004] Currently, in water-quenching experiments, ultra-high temperature ceramic materials generally fail at relatively low thermal shock temperatures. Taking the most widely studied ZrB2-based ultra-high temperature ceramic as an example, when the thermal shock temperature difference reaches above 400℃, cracks are found on its surface along the thickness direction, leading to a significant reduction in residual strength. This temperature is far below the melting point of the ceramic itself. Researchers conduct water-quenching thermal shock tests on structural materials used in extreme environments. Multiple sets of water-quenched samples at different temperature differences are subjected to three-point bending tests to obtain the critical thermal shock temperature difference at which the residual strength of the sample decreases significantly, thus determining the material's thermal shock resistance. However, this testing method requires multiple sets of water-quenching tests and strength tests to roughly determine the critical thermal shock temperature difference. The measurement accuracy depends on the number of tests, and this method also suffers from disadvantages such as expensive raw materials, long testing time, and high testing costs. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies, such as the need for multiple quenching tests and three-point bending strength tests to measure the critical thermal shock temperature difference of ultra-high temperature ceramic materials, which cannot quickly determine the critical thermal shock temperature difference; the need for multiple water quenching tests and strength tests to roughly determine the critical thermal shock temperature difference; and the fact that existing test methods are also subject to the drawbacks of expensive raw materials, long testing time, and high testing costs. This invention provides a high-precision test method for measuring the critical temperature difference corresponding to the rapid change of the residual strength of ceramic matrix composites under thermal shock.

[0006] To achieve the above objectives, the technical solution provided by this invention is:

[0007] A high-precision method for measuring the critical temperature difference corresponding to rapid changes in the residual strength of ceramic matrix composites under thermal shock, characterized by comprising the following steps:

[0008] Step 1: Obtain data on the change of the center temperature of the ceramic sample over time and the evolution characteristics of the surface vapor film through water quenching test; Step 2: Based on the data on the change of the center temperature over time obtained in Step 1, calculate the data on the change of the average boundary heat flux over time and the data on the change of the average surface temperature over time, so as to further obtain the curve of the change of the average boundary heat flux over the surface temperature.

[0009] Step 3: Based on the surface vapor film evolution characteristics obtained in Step 1, find the LFP point on the boundary heat flow versus surface temperature curve. The wall temperature of the ceramic sample corresponding to the LFP point is the critical thermal shock temperature difference for rapid change of residual strength.

[0010] Furthermore, step 1 includes the following sub-steps:

[0011] Step 1.1: On the cylindrical ceramic sample, a center hole is machined by wire cutting; the thermocouple is placed inside the hole of the cylindrical ceramic sample, with the top of the thermocouple probe positioned in the middle of the hole; then, the gap between the thermocouple probe and the hole is filled with thermally conductive adhesive; the thermocouple data transmission terminal is set on the electric actuator and connected to the computer.

[0012] Step 1.2: Place the cylindrical ceramic sample into the muffle furnace, with a water tank placed below it. Adjust the orientation of the cylindrical ceramic sample to ensure that after it descends to 60mm below the water surface in the water tank, the groove containing the thermocouple is located behind the camera lens. After the cylindrical ceramic sample is heated to the set temperature range, quickly send it into the water tank below the muffle furnace, stopping when the descent depth is 1.5 to 2 times the height of the cylindrical ceramic sample.

[0013] Step 1.3: A high-speed camera captures the ceramic quenching process, and a data acquisition instrument records the temperature change at the center of the sample. When the temperature drops to the set shooting temperature, the acquisition instrument sends a pulse signal to trigger the high-speed camera to capture the film evolution process during the quenching and cooling process, obtain the film evolution law, and collect the sample center temperature change data over time fed back by the thermocouple.

[0014] Furthermore, in step 1.2, since the groove where the thermocouple is located is behind the back of the imaging lens, aluminum foil can be used to cover the upper opening of the muffle furnace cavity to minimize heat loss caused by natural convection.

[0015] Furthermore, in step 1.2, the temperature is set to 500℃~900℃.

[0016] Furthermore, in step 1.3, the vapor film evolution law requires going through four boiling states: film boiling, transitional boiling, nucleus boiling, and natural convection.

[0017] Furthermore, in step 2, the temperature-time relationship at the center position of the ceramic wall of the cylindrical ceramic sample is measured by thermocouple, and then the average surface temperature Ts and the average boundary heat flux qIIs of the cylindrical ceramic sample during the quenching process are calculated using MATLAB.

[0018] Furthermore, in step 2, the calculation formula is as follows:

[0019]

[0020]

[0021] In the formula T Srod (t) represents the temperature of the cylinder surface at time t, T C(t) represents the temperature at the center position at time t, R is the radius of the cylinder, α is the thermal diffusivity of the cylindrical ceramic sample, and q' S ' rod (t) represents the heat flow at the boundary of the cylinder at time t.

[0022] Beneficial effects

[0023] The advantages of this invention are: it solves the problem that existing methods for measuring the critical thermal shock difference of ultra-high temperature ceramic materials are complex and cannot be determined quickly. This invention utilizes observations of the boiling state, which is closely related to heat transfer during ceramic quenching, to determine the critical thermal shock difference in a single experiment, significantly reducing the time required to determine the critical thermal shock difference. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the quenching test platform.

[0025] Figure 2 This is a schematic diagram of the placement of the thermocouple in the ceramic sample of Example 1.

[0026] Figure 3 This is the change in the center temperature of the ceramic measured in Example 1 over time.

[0027] Figure 4 Example 1 demonstrates the changes in heat flow-temperature and residual strength of ceramic surface and compares the critical thermal shock temperature difference.

[0028] Figure 5 This is an example of the evolution of film boiling from film boiling to transitional boiling.

[0029] Figure 6 It is a surface vapor film evolution characteristic diagram.

[0030] Figure 7 This is a diagram illustrating the four boiling stages.

[0031] In the figure: 1-Thermocouple; 2-Sample; 3-High-speed camera; 4-Muffle furnace; 5-Water tank; 6-Processing computer; 7-Probe position Z=20mm Detailed Implementation

[0032] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0033] The theory of boiling in a pool indicates that the quenching of ultra-high temperature ceramic materials generally goes through four stages: film boiling, transition boiling, nucleation boiling, and natural convection.

[0034] This invention investigates the correlation between the critical thermal shock temperature difference for rapid changes in residual strength and the boiling state of the ceramic surface. It finds that when the wall boiling mode is film boiling, the solid wall is enveloped by a vapor film and remains in a low surface heat flux state for a long period. The extremely low heat flux density at the solid-liquid interface results in a small internal temperature gradient. The temperature point controlling the onset of film boiling is the Leidenfrost characteristic (LFP). Before the wall temperature drops to the LFP, most of the quenching stage is in the film boiling stage. After the LFP, the ceramic quenching enters the transition boiling stage. The phase change heat of water causes random bubble generation at the interface, significantly increasing the surface heat flux, which in turn leads to a large temperature and stress gradient inside the solid.

[0035] Meanwhile, taking ZrB2-based ceramics, which are the most widely studied, as an example, water quenching tests revealed a certain correlation between the heat transfer mode on the high-temperature ceramic surface and the residual strength of the ceramic. The temperature corresponding to the LFP point on the heat flow curve is close to the critical thermal shock temperature difference of this type of ultra-high-temperature ceramic material. This is consistent with the surface heat transfer control mechanism of other types of ceramic materials besides ZrB2-based ceramics.

[0036] Therefore, by accurately measuring the critical thermal shock temperature difference of ultra-high temperature ceramic materials corresponding to the LFP point, the critical temperature difference corresponding to the rapid change of residual thermal shock strength can be quickly obtained.

[0037] See Figures 1-7 The present invention includes the following steps:

[0038] Step 1: Obtain the changes in the center temperature of the ceramic sample over time and the evolution characteristics of the surface vapor film through water quenching test.

[0039] Step 2: Input the data on the center temperature variation with temperature into the heat flux calculation program to perform boundary heat flux calculation. The heat flux calculation uses the inverse problem formula for heat conduction with unknown boundaries and neglecting radial heat conduction, taking the first three terms of the series expansion to calculate the surface temperature T. Srod and boundary heat flow q' S ' rod :

[0040]

[0041]

[0042] In the formula T Srod (t) represents the temperature of the cylinder surface at time t, T C (t) represents the temperature at the center at time t, R is the radius of the cylinder, which is taken as 0.005m in this embodiment; α is the thermal diffusivity of the ZrB2-based ceramic, which is taken as 0.22×10 in this embodiment. -4 ,q' S ' rod(t) represents the heat flow at the boundary of the cylinder at time t.

[0043] Step 3: By analyzing the characteristics of surface vapor film evolution, find the LFP point on the heat flow curve at the corresponding time. The wall temperature of the ceramic sample corresponding to this point is the critical thermal shock temperature difference required in this patent method.

[0044] The technical solution of the present invention will be further explained and illustrated below with specific examples:

[0045] The components that need to be prepared before the experiment are: 40×φ10mm 3 Cylindrical ZrB2-based ceramic samples; thermocouple model TJ120-CAXL-032G-6-CC-XCIB, which has a maximum temperature resistance of 1300℃ in a non-corrosive gas environment; OMEGABOND400 model with a thermal conductivity of 11 W / (m 2 • K) Thermally conductive adhesive with a maximum temperature resistance of 1000℃, quenching and cooling boiling test bench and high-speed camera.

[0046] See Figure 1 and Figure 2 The sample should be processed to cut a 1mm diameter hole at its center for placing the thermocouple. The thermocouple should be placed inside the hole in the center of the cylindrical ZrB2-based ceramic sample, with the top of the thermocouple probe positioned in the middle of the hole. (See attached image.) Figure 2 At Z = 20mm, Z represents the height of the cylindrical sample. Then, thermally conductive adhesive is used to fill the gap between the thermocouple probe and the hole. The thermocouple data transmission terminal is set on the electric actuator and connected to the computer.

[0047] The ceramic was placed inside the muffle furnace on the quenching and cooling boiling test platform. The ceramic's orientation was adjusted to ensure that the groove containing the ceramic thermocouple was behind the imaging lens after descent. Then, aluminum foil was used to cover the upper opening of the muffle furnace cavity to minimize heat loss due to natural convection. After the ceramic was heated to 700℃, it was rapidly fed into the water tank below the muffle furnace at 250mm / s using an electric actuator. The descent was stopped when the ceramic reached a depth of 40mm below the water surface.

[0048] See Figure 3 The process of ceramic quenching is captured by a high-speed camera. The data acquisition instrument records the temperature change at the center of the sample. When the temperature drops to the set shooting temperature, the acquisition instrument sends a 5V pulse signal to trigger the high-speed camera to shoot at a rate of 2000fps, recording the vapor film evolution process of the quenching cooling process and collecting data fed back by thermocouples.

[0049] Four classic boiling states were observed during ceramic quenching: film boiling, transition boiling, nucleus boiling, and natural convection boiling; see [link to relevant documentation]. Figure 7From left to right, they represent film boiling, transition boiling, nucleus boiling, and natural convection, with each stage described below:

[0050] Film boiling: The sample surface is completely covered by a vapor film.

[0051] Transitional boiling: The vapor film first breaks down from the bottom, splitting into several larger bubbles that cover the sample surface. Nucleus boiling: The large bubbles on the surface further split, and a large number of bubbles form from the sample wall and move violently.

[0052] Natural convection: Only a few small bubbles adhere to the sample surface, the bubbles hardly move, and they slowly disappear over time.

[0053] The temperature-time relationship at the center of the ceramic wall was measured using thermocouples. The temperature and time data were compiled into an Excel file named "temperature.xlsx", with temperature as the first column and time as the second column. This file was placed in the working directory of the MATLAB program. After running the MATLAB program, the average surface temperature-time data Ts and average surface heat flux-temperature data qIIs of the cylindrical ZrB2-based ceramic during the quenching process were obtained.

[0054] The LFP point is located on the heat flow-temperature curve of the ceramic surface. The LFP point is the starting point where the heat flow rapidly rises to a plateau as the temperature decreases. The wall temperature of 423℃ corresponding to this point is the critical thermal shock temperature difference for rapid change in residual strength. (See [reference needed]). Figure 4 The occurrence time of LFP can also be determined by observing the vapor film evolution, thus obtaining the corresponding temperature of LFP. Specifically, when the ceramic transitions from film boiling to transition boiling and the surface vapor film ruptures, see [reference needed]. Figure 5 .

[0055] The critical thermal shock temperature difference obtained in this example is similar to the critical thermal shock temperature difference obtained from multiple water quenching tests and strength tests of the same material. (See attached figure.) Figure 4 Therefore, the critical thermal shock temperature difference obtained by the method of this patent can be considered reliable.

[0056] It should be understood that although this specification is described according to various embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

[0057] The detailed descriptions listed above are merely specific illustrations of feasible embodiments of the present invention, and are not intended to limit the scope of protection of the present invention. All equivalent embodiments or modifications made without departing from the spirit of the present invention should be included within the scope of protection of the present invention.

[0058] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the scope of the technology disclosed in the present invention, and such modifications or substitutions should all be covered within the scope of protection of the present invention.

Claims

1. A high-precision method for measuring the critical temperature difference corresponding to rapid changes in the residual strength of ceramic matrix composites under thermal shock, characterized in that, Includes the following steps: Step 1: Obtain data on the change of the center temperature of the ceramic sample over time and the evolution of the surface vapor film through water quenching tests. Including step 1.3: A high-speed camera captures the ceramic quenching process, and a data acquisition instrument records the temperature change at the center of the sample; when the temperature drops to the set shooting temperature, the acquisition instrument sends a pulse signal to trigger the high-speed camera to capture the film evolution process of the quenching cooling process, obtain the film evolution law, and collect the sample center temperature change data over time fed back by the thermocouple. The evolution of the vapor film involves four boiling states: film boiling, transitional boiling, nucleus boiling, and natural convection. Step 2: Based on the center temperature change data over time obtained in Step 1, calculate the average boundary heat flux change data over time and the average surface temperature change data over time, thereby obtaining the curve of average boundary heat flux change with surface temperature. Step 3: Based on the surface vapor film evolution law obtained in Step 1, find the LFP point on the boundary heat flow versus surface temperature curve. The wall temperature of the ceramic sample corresponding to the LFP point is the critical thermal shock temperature difference for rapid change of residual strength.

2. The high-precision method for measuring the critical temperature difference corresponding to rapid changes in the residual strength of ceramic matrix composites under thermal shock as described in claim 1, characterized in that, Step 1 includes the following sub-steps: Step 1.1: On the cylindrical ceramic sample, a center hole is machined by wire cutting; the thermocouple is placed inside the hole of the cylindrical ceramic sample, with the top of the thermocouple probe positioned in the middle of the hole; then, the gap between the thermocouple probe and the hole is filled with thermally conductive adhesive; the thermocouple data transmission terminal is set on the electric actuator and connected to the computer. Step 1.2: Place the cylindrical ceramic sample into the muffle furnace, with a water tank placed below it. Adjust the orientation of the cylindrical ceramic sample to ensure that after it descends to 60mm below the water surface in the water tank, the groove containing the thermocouple is located behind the camera lens. After the cylindrical ceramic sample is heated to the set temperature range, quickly send it into the water tank below the muffle furnace, stopping when the descent depth is 1.5 to 2 times the height of the cylindrical ceramic sample.

3. The high-precision method for measuring the critical temperature difference corresponding to rapid changes in the residual strength of ceramic matrix composites under thermal shock as described in claim 2, characterized in that, In step 1.2, the groove where the thermocouple is located is behind the back of the imaging lens. Aluminum foil can be used to cover the upper opening of the muffle furnace cavity to minimize heat loss caused by natural convection.

4. The high-precision method for measuring the critical temperature difference corresponding to rapid changes in the residual strength of ceramic matrix composites under thermal shock as described in claim 2, characterized in that, In step 1.2, the temperature is set to 500℃~900℃.

5. The high-precision method for measuring the critical temperature difference corresponding to rapid changes in the residual strength of ceramic matrix composites under thermal shock as described in claim 1, characterized in that, In step 2, the temperature-time relationship at the center position of the ceramic wall of the cylindrical ceramic sample is measured by thermocouple. Then, the average surface temperature Ts and the average boundary heat flux qIIs of the cylindrical ceramic sample during the quenching process are calculated using MATLAB.

6. The high-precision method for measuring the critical temperature difference corresponding to rapid changes in the residual strength of ceramic matrix composites under thermal shock as described in claim 1, characterized in that, In step 2, the calculation formula is as follows: In the formula T Srod (t) represents the temperature of the cylinder surface at time t, T C (t) represents the temperature at the center position at time t, R is the radius of the cylinder, and α is the thermal diffusivity of the cylindrical ceramic sample. This represents the heat flow at the boundary of the cylinder at time t.

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