A coating for the surface of a cathode of a CT cathode tube and a method for producing the same

Abstract

The application discloses a CT cathode tube cathode surface coating and a preparation method thereof. The coating is prepared by improving a magnetron sputtering process, using iridium (Ir) and osmium (Os) as metal targets, and depositing a metal coating with a thickness of 10-20 microns on a molybdenum base. The prepared metal coating has a bonding force to the base which can exceed 100 N, and the coating is dense and has good uniformity, and can be used as a cathode for exciting electrons on the surface of a cathode tube in a CT X-ray tube.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal surface treatment, and relates to surface modification treatment of a CT cathode tube, in particular to a cathode surface coating of a CT cathode tube and a preparation method thereof. BACKGROUND

[0002] As an important component of a CT system, an X-ray tube mainly comprises a cathode, an anode, a vacuum chamber, a window and a power supply. The cathode is mainly used for generating a sufficient number and energy of electrons and bombarding an anode target to generate X-rays, and is one of core components of the X-ray tube, and the performance of the cathode directly affects the emission intensity of X-rays and the overall service life of the component. In order to obtain high electron yield and rectification, the X-ray tube of the existing CT system generally adopts a cathode tube built-in heating mode, and the working mode of generating hot electrons from the surface of the tube. However, when the X-ray tube is in a working state, the cathode material needs to face a high-temperature and vacuum environment, and the service environment is extremely harsh. Therefore, the cathode tube needs to have the characteristics of high high-temperature strength, good thermal shock resistance, excellent heat conduction performance, low electron work function, large and stable current density and low thermal evaporation speed.

[0003] Early cathode tube materials are generally made of refractory metals, such as W, Mo, Re, Nb, Ir and Os. However, the use of refractory metals to manufacture the cathode tube has the problem of low application value. First, the preparation and mechanical processing of the refractory metal are difficult, for example, W, Mo, Ir and Os have the problems of high melting point and high brittleness; second, some metals have stable emission but low emission efficiency, such as W; third, some refractory metals have large electron work function, which makes the working temperature high, such as Ir, Os and Re; and finally, the connection technology of the tube body and the heating system of different materials has high technical requirements, and the conventional welding technology is difficult to achieve and has high cost. Therefore, the thin film cathode derived from the pure metal cathode with high emission efficiency gradually replaces the pure metal cathode to become the main material of the cathode tube. At the same time, the preparation and forming processing problems of the cathode tube material are also well solved.

[0004] There are many common coating preparation technologies, such as electroplating, chemical plating, chemical vapor deposition (CVD), magnetron sputtering and laser cladding. Among them, magnetron sputtering is the most widely used thin film preparation method in industry, and its principle is to use high-energy particles to bombard the surface of a solid target material to make the atoms on the surface of the target material escape and form a thin film on the surface of the sample. The advantages are: (1) the selected sputtering target material is almost not limited, whether it is a metal target material or a ceramic target material, a corresponding thin film can be obtained by sputtering; (2) the thin film obtained by sputtering has high purity, good density and good uniformity; (3) the magnetron sputtering process has good repeatability, and the same thin film structure can be repeatedly prepared.

[0005] Therefore, in view of the problems of great processing difficulty, low emission efficiency, high working temperature, great difficulty in hetero-connection, and high cost of the tube body cathode of the CT cathode tube, a refractory coating meeting actual application is prepared on a refractory metal substrate by using a magnetron sputtering deposition process, so as to obtain a thin film type hot cathode tube with high emission efficiency and high stability, which has important scientific significance and engineering application value. SUMMARY

[0006] The purpose of the present application is to provide a CT cathode tube cathode surface coating and a preparation method thereof, which can effectively solve the problems of low electron emission efficiency, great processing difficulty, and great difficulty in tube body connection of pure refractory metal materials for CT cathode tubes.

[0007] To achieve the above purpose, the preparation method of the CT cathode tube cathode surface coating provided by the present application comprises the following steps:

[0008] (1) Pretreatment

[0009] The molybdenum substrate is sequentially polished and cleaned, and then dried for standby use;

[0010] (2) Bias reverse sputtering cleaning

[0011] The dried molybdenum substrate in step (1) is placed in a vacuum furnace cavity of a magnetron sputtering device, and vacuumized to not more than 2x10 -4 Pa, and the molybdenum substrate is cleaned by bias reverse sputtering under an argon atmosphere;

[0012] (3) Target pre-sputtering

[0013] The metal target is pre-sputtered under an argon atmosphere; the metal target is iridium or osmium;

[0014] (4) Depositing a metal coating

[0015] The metal target is sputtered under the argon atmosphere in step (3) at a sputtering gas pressure of 0.6-1.0 Pa, an argon flow rate of 60-100 sccm, a sputtering power of 200-300 W, a target-substrate distance of 6-7 cm, and a deposition temperature of 200-400℃, until the metal coating deposited on the surface of the molybdenum substrate reaches a set thickness;

[0016] (5) Stress relief and deformation correction post-treatment

[0017] After the deposition of the metal coating on the surface of the molybdenum substrate is completed, the vacuum is extracted to not more than 4x10 -4After the deposition temperature, standing for 5-8h, then furnace cooling to below 100℃, and then closing the vacuum system and standing for at least 12h, the stress and deformation of the metal coating on the surface of the molybdenum substrate can be removed, and the metal coating on the surface of the CT cathode tube is obtained.

[0018] In step (1) of the above method for preparing the cathode surface coating of the CT cathode tube, the molybdenum substrate is pretreated by suitable processing equipment and processing means, so as to have better adhesion of the thick metal coating. Therefore, under the premise of achieving the purpose, the conventional pretreatment method in the art can be used to remove the oxide and adsorbed substances on the surface of the molybdenum substrate. In the present application, the surface of the molybdenum substrate is preferably mechanically polished, and then ultrasonic cleaned with acetone and anhydrous ethanol for 10-20min, respectively, to remove the grease and impurities such as adsorbed substances on the surface of the substrate, and then dried for use. Further, the mechanical polishing method can be used in the conventional manner in the art. In the present application, the molybdenum substrate is preferably polished with water sandpaper from coarse to fine (such as 240#, 600#, 1000#, 1500# and 3000# water sandpaper), and finally polished on a metallographic polishing machine using polishing paste.

[0019] In step (2) of the above method for preparing the cathode surface coating of the CT cathode tube, the oxide and / or adsorbed impurities on the surface of the target material can be further removed by bias sputter etching, such as removing the oxide layer, machining burrs, oil stains and dirt on the surface of the workpiece (molybdenum substrate). The bias sputter etching can use the conventional bias sputter etching parameters in the art. In the present application, the bias sputter etching conditions are as follows: when the vacuum in the vacuum furnace chamber is not greater than 2x10 -4 After Pa, Ar gas is introduced to make the vacuum degree reach 2-4Pa, and then the molybdenum substrate is subjected to bias sputter etching under the conditions of a bias voltage of -600 to -800V and a target-substrate distance of 6-7cm, and the cleaning time is 10-20min.

[0020] In step (3) of the above method for preparing the cathode surface coating of the CT cathode tube, the oxide and / or adsorbed impurities on the surface of the target material are removed by pre-sputtering, so as to clean the surface of the target material. The pre-sputtering conditions of the target material in the present application are not particularly limited, and the conventional pre-sputtering parameters can be used. The pre-sputtering conditions are as follows: the argon gas flow is adjusted to 60-100sccm to make the pre-sputtering pressure reach 0.6-1.0Pa, and then the metal target is turned on after the shutter is closed under the conditions of a pre-sputtering pressure of 0.6-1.0Pa and a pre-sputtering power of 100-200W, and the metal target is subjected to pre-sputter etching, and the cleaning time is 10-20min.

[0021] In the preparation method of the cathode surface coating of the CT cathode tube, in step (4), after the target material pre-sputtering in step (3) is completed, the Ar atmosphere is maintained, the sputtering parameters are adjusted to the predetermined parameters, that is, the sputtering gas pressure is 0.6-1.0 Pa, the argon flow rate is 60-100 sccm, the sputtering power is 200-300 W, the deposition temperature is 200-400 DEG C, the target-substrate distance is 6-7 cm, and the target head baffle is opened to deposit the metal coating; in order to avoid the formation of interface defects in the coating, which leads to the decrease of the electron emission performance of the coating, the one-time deposition method of the metal coating is adopted in the present application, and the deposition is not continuously segmented until the required thickness is reached.

[0022] In the preparation method of the cathode surface coating of the CT cathode tube, the purpose of step (5) is to perform stress relief treatment on the metal coating deposited on the molybdenum substrate, so as to avoid the stress peeling and surface oxidation of the coating caused by external pressure and temperature change. In this step, the standing time after the vacuum system is closed is preferably 12-24 h. The magnetron sputtering has a certain stress, and the heating is beneficial to eliminating the stress in the coating, preventing the peeling of the coating, and making the coating and the substrate more closely combined. The lower the temperature during deposition, the longer the heat preservation standing time.

[0023] In the preparation method of the cathode surface coating of the CT cathode tube, in the prior art, the magnetron sputtering is used to prepare the refractory metal coating, in order to inhibit the surface roughness caused by the grain growth and the decrease or peeling of the coating caused by the stress concentration, the intermediate pause homogeneous multilayer deposition process is usually adopted. However, for the coating applied in the present application, the multilayer deposition process will form a large number of interlayer interfaces in the coating, and the grain refinement will produce a large number of grain boundaries, and these defects will cause difficulties in the surface escape of electrons, the transportation of electrons in the coating, and the heat transfer. Therefore, based on the similar thermal expansion coefficients of Mo, Os and Ir, the one-time deposition coating under the heating condition is adopted, and the long heat preservation standing and furnace standing treatment are performed, so as to inhibit the formation of defects in the coating and eliminate the residual stress in the coating as much as possible, so as to realize the dual purposes of residual stress elimination and defect formation inhibition of the coating.

[0024] The present application also provides a CT cathode tube cathode surface coating prepared by the above method, and the thickness of the metal coating is greater than 10 microns, and preferably the total thickness of the metal coating is 10-20 microns.

[0025] Compared with the prior art, the CT cathode tube cathode surface coating and the preparation method thereof provided by the present application have the following beneficial effects:

[0026] (1) In the magnetron sputtering, only argon is used as the working gas, and the metal coating with high bonding force, uniform thickness and dense surface is prepared on the molybdenum substrate by one-time deposition under the conditions of heating deposition, heat preservation standing and furnace standing.

[0027] (2) The application adopts mature magnetron sputtering technology, which has better adhesion, purity and thickness than traditional wet coating method, and overcomes the disadvantages of traditional wet coating method, such as heavy ions, waste liquid and waste residue, which are easy to pollute the environment, and has the advantages of simple operation, low pollution, low cost, environmental friendliness and good process repeatability, and has good application prospect and is worth popularization and application.

[0028] (3) The application can accurately control the sputtering power of the target material, and the thickness of the coating can be adjusted and controlled, and has more advantages than other preparation technologies of physical vapor deposition technology, such as more dense and uniform coating prepared by multi-arc ion plating, and higher coating / substrate adhesion than coating prepared by evaporation plating. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 The physical map of the Ir coating prepared in Example 1 of the application is shown in the figure;

[0030] Figure 2 The surface SEM map of the Ir coating prepared in Example 1 of the application is shown in the figure: (a) surface morphology, (b) cross-sectional morphology;

[0031] Figure 3 The acoustic emission signal map of the Ir coating prepared in Example 1 of the application under the dynamic load of the scratch tester is shown in the figure;

[0032] Figure 4 The physical map of the Os coating prepared in Example 2 of the application is shown in the figure;

[0033] Figure 5 The surface SEM map of the Os coating prepared in Example 2 of the application is shown in the figure: (a) surface morphology, (b) cross-sectional morphology;

[0034] Figure 6 The acoustic emission signal map of the Os coating prepared in Example 2 of the application under the dynamic load of the scratch tester is shown in the figure. DETAILED DESCRIPTION

[0035] The technical solutions of the embodiments of the application will be described clearly and completely in combination with the drawings. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor belong to the application.

[0036] In the following examples, the magnetron sputtering equipment adopts a radio frequency reaction magnetron sputtering coating equipment; the purity of the molybdenum substrate used is 99.99%; and the purity of the working gas Ar is 99.999%. The metal target used is an iridium (Ir) target with a purity of 99.99%. The metal target used is an osmium (Os) target with a purity of 99.99%.

[0037] Example 1

[0038] The CT cathode tube cathode surface coating provided by the embodiment is prepared according to the following steps:

[0039] (1) Pretreatment

[0040] The molybdenum substrate of the cathode tube cylindrical surface is polished from coarse to fine by using 240#, 600#, 1000#, 1500# and 3000# water sandpaper, and finally polished on a metallographic polishing machine using polishing paste. After polishing and smoothing, the molybdenum substrate is ultrasonically cleaned with acetone and anhydrous ethanol for 15 minutes, respectively, and then dried for standby after cleaning.

[0041] (2) Bias sputter cleaning

[0042] The cathode tube treated in step (1) is placed in the vacuum furnace cavity of the magnetron sputtering equipment. First, low vacuum is pumped by a mechanical pump, and then high vacuum is pumped by a molecular pump to reach a base vacuum of 2×10 -4 Pa. Argon gas is introduced as the working gas. The molybdenum substrate is subjected to bias sputter cleaning at a bias voltage of -600 V, a sputter pressure of 4 Pa, and a target-substrate distance of 6 cm. The cleaning time is 20 minutes.

[0043] (3) Target pre-sputtering

[0044] After the surface of the molybdenum substrate is cleaned by reverse sputtering, the argon gas flow is adjusted to 60 sccm to reach a pre-sputtering pressure of 0.6 Pa. After the Ir target is rapidly ignited by a radio frequency power source, the shutter is closed. The Ir target material is pre-sputtered at a pre-sputtering pressure of 0.6 Pa and a pre-sputtering power of 100 W. The pre-sputtering time is 10 minutes.

[0045] (4) Depositing Ir coating

[0046] Under the argon atmosphere of step (3), the argon flow is adjusted to 100 sccm. The Ir target material is sputtered at a sputtering pressure of 1.0 Pa, a sputtering power of 200 W, a target-substrate distance of 6 cm, and a deposition temperature of 200°C. The sputtering deposition time is 3 hours.

[0047] (5) Stress relief and deformation correction treatment

[0048] After the deposition of the Ir coating on the surface of the molybdenum substrate is completed, the gas supply is stopped, and the vacuum is pumped to 4×10 -4 Pa. Then, the sample is placed at the deposition temperature for 5 hours, and then cooled to below 100°C in the furnace. The vacuum system is then turned off and the sample is left to stand for 12 hours. This completes the stress relief and deformation correction treatment of the metal coating on the surface of the molybdenum substrate, and the Ir coating on the surface of the CT cathode tube cathode is obtained. The deposited thickness of the Ir coating is about 12.1 μm.

[0049] Example 2

[0050] The CT cathode tube cathode surface coating provided by the embodiment is prepared according to the following steps:

[0051] (1) Pretreatment

[0052] The molybdenum substrate of the cathode tube cylindrical surface is polished from coarse to fine by using 240#, 600#, 1000#, 1500# and 3000# water sandpaper, and finally polished on a metallographic polishing machine using polishing paste. After polishing and smoothing, the molybdenum substrate is ultrasonically cleaned with acetone and anhydrous ethanol for 15 minutes, respectively, and then dried for standby after cleaning.

[0053] (2) Bias sputter cleaning

[0054] The cathode tube treated in step (1) is placed in the vacuum furnace cavity of the magnetron sputtering equipment. First, low vacuum is pumped by a mechanical pump, and then high vacuum is pumped by a molecular pump to reach a base vacuum of 2×10 -4 Pa. Argon gas is introduced as the working gas. The molybdenum substrate is subjected to bias sputter cleaning at a bias voltage of -800 V, a sputter pressure of 3 Pa, and a target-substrate distance of 7 cm. The cleaning time is 10 minutes.

[0055] (3) Target pre-sputtering

[0056] After the surface of the molybdenum substrate is cleaned by reverse sputtering, the argon gas flow is adjusted to 100 sccm to make the pre-sputtering pressure reach 1.0 Pa. After the Os target is rapidly ignited by the radio frequency power source, the shutter is closed. The Os target material is pre-sputtered at a pre-sputtering pressure of 1.0 Pa and a pre-sputtering power of 200 W. The pre-sputtering time is 20 minutes.

[0057] (4) Depositing Os coating

[0058] Under the argon atmosphere of step (3), the argon flow is adjusted to 100 sccm. The Os target material is sputtered at a sputtering pressure of 1.0 Pa, a sputtering power of 300 W, a target-substrate distance of 7 cm, and a deposition temperature of 400℃. The total sputtering deposition time is 2 hours.

[0059] (5) Stress relief and deformation correction treatment

[0060] After the deposition of the Os coating on the surface of the molybdenum substrate is completed, the gas supply is stopped, and the vacuum is pumped to 4×10 -4 Pa. Then, the sample is placed at the deposition temperature for 5 hours, and then cooled to below 100℃ in the furnace. The vacuum system is then turned off and the sample is left to stand for 24 hours. This completes the stress relief and deformation correction treatment of the metal coating on the surface of the molybdenum substrate, and the Os coating on the surface of the CT cathode tube is obtained. The deposited thickness of the Os coating is about 20.1 μm.

[0061] The morphology and performance of the metal coating for the surface of the CT cathode tube prepared in Examples 1-2 were analyzed as follows.

[0062] (I) Morphology analysis

[0063] The actual photographs of the Ir and Os coatings deposited on the molybdenum substrate prepared in Examples 1-2 are shown in Figure 1 and Figure 4 respectively. The surface micro-morphology of the Ir and Os coatings deposited on the molybdenum substrate prepared in Examples 1-2 was analyzed by scanning electron microscopy, as shown in Figure 2 and Figure 5 respectively. As can be seen from the figures, the Ir and Os coatings prepared in Examples 1-2 respectively have a dense and continuous surface, and no obvious defects are observed in a large range.

[0064] (II) Performance analysis

[0065] The adhesion strength of the coating was estimated using an automatic scratch tester, and the scratch test was performed under linear dynamic increasing load, while the scratch morphology was observed by scanning electron microscopy. The adhesion test results of the Ir and Os coatings prepared in Examples 1-2 are shown in Figure 3 and Figure 6 respectively. No acoustic emission signal was observed for the Ir and Os coatings at 0-100 N, indicating that the coating and the substrate are tightly combined, and the surface adhesion strength of the coating and the substrate exceeds 100 N, indicating that the coating and the substrate have excellent adhesion performance, which is crucial for the coating to meet the actual engineering application.

Claims

1. A method of preparing a cathode surface coating for a CT cathode tube, characterized by: The method comprises the following steps: (1) Pretreatment Polish and clean the molybdenum substrate in sequence, and dry for standby; (2) Bias reverse sputtering cleaning The dried molybdenum substrate of step (1) is placed in a vacuum furnace chamber of a magnetron sputtering apparatus, and vacuumed to not more than 2 x 10 - 4 Pa, the molybdenum substrate is cleaned by bias sputter etching in an argon atmosphere; (3) Target pre-sputtering Pre-sputter the metal target in argon atmosphere; the metal target is iridium or osmium; (4) Depositing metal coating Keep the argon atmosphere in step (3), and sputter the metal target under the conditions of sputtering gas pressure of 0.6-1.0 Pa, argon flow rate of 60-100 sccm, sputtering power of 200-300 W, target-substrate distance of 6-7 cm, and deposition temperature of 200-400 ℃, until the metal coating deposited on the surface of the molybdenum substrate reaches the set thickness; (5) Stress removal and deformation correction post-treatment After the surface of the molybdenum substrate is coated with the metal coating, the vacuum is extracted to not more than 4 x 10 -4 After the surface of the molybdenum substrate is coated with the metal coating, the vacuum is extracted to not more than 4 x 10 -4 After the surface of the molybdenum substrate is coated with the metal coating, the vacuum is extracted to not more than 4 x 10 -4 After the surface of the molybdenum substrate is coated with the metal coating, the vacuum is extracted to not more than 4 x 10 -4 After the surface of the molybdenum substrate is coated with the metal coating, the vacuum is extracted to not more than 4 x 10 -4 After the surface of the molybdenum substrate is coated with the metal coating, the vacuum is extracted to not more than 4 x 10 < 2. The method of claim 1, wherein: In step (1), mechanically polish the surface of the molybdenum substrate, and clean the polished surface with acetone and alcohol for 10-20 min in sequence.

3. The method of claim 1, wherein the coating is applied by a method selected from the group consisting of: sputtering, evaporation, and chemical vapor deposition. In step (2), the bias sputter cleaning conditions are: when the vacuum in the vacuum furnace chamber is not more than 2x10 -4 After the pressure reaches 2-4 Pa, the molybdenum substrate is subjected to bias sputter cleaning under the conditions of a bias of -600 to -800 V and a target-substrate distance of 6-7 cm for 10-20 min.

4. The method of claim 1, wherein: In step (3), the pre-sputtering conditions are as follows: adjust the argon flow rate to 60-100 sccm, so that the pre-sputtering gas pressure is stabilized to 0.6-1.0 Pa, then under the conditions of pre-sputtering gas pressure of 0.6-1.0 Pa and pre-sputtering power of 100-200 W, turn on the metal target and close the shutter, and pre-sputter the metal target for 10-20 min.

5. The CT cathode tube cathode surface coating prepared by the method of any one of claims 1-4.

6. The CT cathode tube cathode surface coating of claim 5, wherein, The thickness of the metal coating is greater than 10 μm.

7. The metal coating for a CT cathode tube surface according to claim 6, characterized in that The thickness of the metal coating is 10-20 μm.

Images