Self-lubricating coating for inner surface of air cylinder and preparation method of self-lubricating coating

Abstract

The invention provides a self-lubricating coating on the inner surface of an air cylinder and a preparation method thereof. An adopted magnetron sputtering device comprises a mounting bracket; the rotary driving part is arranged on the mounting bracket; one side of a driving shaft of the rotary driving part is connected with one end of the tubular base material; the target material is of a cylindrical hollow structure, and one end is connected with the driving shaft; the magnet is cylindrical and is arranged in the target material, and one end of the magnet is connected with the driving shaft; the chamber is connected with the other end of the tubular base material; the preparation method comprises the steps that the two ends of the tubular base material are connected with the rotary driving part and the cavity correspondingly; cleaning the inner surface of the tubular base material; sputtering a Cr binding force layer on the inner surface of the cleaned tubular base material; sputtering a CrN transition layer on the Cr binding force layer; and sputtering a ta-C coating on the CrN transition layer. According to the method, the cylindrical hollow target material is adopted, the magnetron sputtering process is regulated and controlled, the coating uniformity and the binding force strength during preparation of the magnetron sputtering coating on the inner wall of the tubular base material are improved, and the method can be used for preparing the coating on the inner surface of the tubular base material with the large length-diameter ratio.

Description

Technical Field

[0001] The present invention belongs to the technical field of coating preparation, and particularly relates to a self-lubricating coating for the inner surface of a cylinder and a preparation method thereof. Background Art

[0002] Liquid-driven piston hydrogen compressors generally have the disadvantages of short maintenance cycles, high maintenance and component replacement costs, and large hydrogen leakage. The main reason is that the piston and the inner wall of the cylinder cannot be lubricated with oil. The piston reciprocates under high temperature and high pressure, which easily scratches the inner surface of the cylinder, and the cylinder fails and needs to be replaced.

[0003] Preparing a nano-coating with high hardness and high wear resistance on the inner surface of the cylinder can improve the self-lubricating performance of the inner surface of the cylinder and reduce the friction coefficient. Magnetron sputtering technology is a physical vapor deposition technology that uses a magnetic field to enhance the sputtering rate and deposition rate, and is an effective method for preparing nano-layers. Its working principle is as follows: An appropriate amount of Ar is filled in a highly vacuum environment, and a DC voltage is applied between the cathode (target) and the anode (substrate). A magnetron-type abnormal glow discharge is generated in the vacuum chamber. Under the action of the electric field (E), electrons collide with argon atoms during the flight to the substrate, causing Ar to ionize to produce argon ions and electrons. The incident ions bombard the target under the action of E, causing neutral atoms or molecules on the surface of the target to obtain sufficient kinetic energy to break away from the surface of the target and deposit on the surface of the substrate to form a coating. The generated secondary electrons are affected by the action of E and the magnetic field (B), generating an E×B drift, and their movement trajectory is approximately a cycloid. If it is a toroidal magnetic field, the electrons move in a circular motion on the target surface in an approximate cycloid form. Their movement paths are not only very long, but also confined in the plasma region near the target surface, where a large number of argon ions are ionized to bombard the target, thus achieving a high deposition rate. As the number of collisions increases, the energy of the secondary electrons is exhausted, and they gradually move away from the target surface and finally deposit on the substrate under the action of E. Since the energy of this electron is very low, the energy transferred to the substrate is very small, resulting in a low temperature rise of the substrate. These jointly contribute to the unique technical advantages of magnetron sputtering.

[0004] The Chinese patent document with the publication number CN107955938B discloses an engine cylinder liner inner cavity vacuum reactive magnetron sputtering coating device and method, which discloses a device and method for magnetron sputtering coating in the inner cavity of a cylinder liner. However, in this patent and the existing technologies, planar targets are mostly used in magnetron sputtering instruments for coating. The sputtering effect of planar target materials can meet the coating requirements of general planar substrates. However, for the inner wall of a cylinder, such target materials can only sputter the two ends of the cylinder, and the sputtering amount for the middle part of the inner wall of the cylinder is insufficient, resulting in uneven sputtering effect for the entire cylinder. Especially for cylinders with a large length-to-diameter ratio and relatively small diameter, the sputtering effect is extremely poor. Therefore, a method suitable for magnetron sputtering coating on the inner surface of a cylinder is needed to improve the uniformity and bonding strength of the coating on the inner surface of the cylinder. Summary of the Invention

[0005] The technical problem solved by the present invention is to provide a self-lubricating coating for the inner surface of a cylinder and a preparation method thereof. A cylindrical hollow target is used, and the process in the magnetron sputtering is regulated, improving the coating uniformity and bonding strength during the preparation of the magnetron sputtering coating on the inner wall of a tubular substrate, and it can be used for the preparation of the inner surface coating of a tubular substrate with a large length-to-diameter ratio.

[0006] To solve the above problems, one aspect of the present invention provides a method for preparing a self-lubricating coating for the inner surface of a cylinder, using a magnetron sputtering device to prepare a coating on the inner surface of a tubular substrate. The magnetron sputtering device includes: An installation bracket; A rotation driving part, arranged on the installation bracket; on one side of the driving shaft of the rotation driving part, there is a first connection part for connecting the rotation driving part with one end of the tubular substrate. A target, the target is a cylindrical hollow structure; one end of the target is connected to the driving shaft of the rotation driving part. A magnet, the magnet is cylindrical and is arranged in the target, and one end of the magnet is connected to the driving shaft of the rotation driving part. A chamber, on the chamber there is a second connection part for connecting the chamber with the other end of the tubular substrate; the chamber is connected with a vacuum pumping device and a gas supply device, and there is a through hole on the chamber for evacuating or supplying gas to the inside of the tubular substrate through the through hole. The preparation method includes the following steps: S1. Connect the two ends of the tubular substrate with the rotation driving part and the chamber respectively. S2. Clean the inner surface of the tubular substrate by glow discharge. S3. Sputter a Cr bonding layer on the inner surface of the cleaned tubular substrate. S4. Sputter a CrN transition layer on the Cr adhesion layer; S5. Sputter a ta-C coating on the CrN transition layer.

[0007] Preferably, sputtering the Cr adhesion layer on the inner surface of the cleaned tubular substrate specifically includes the following steps: using a target made of Cr as the target, introducing Ar into the tubular substrate, turning on the pulsed DC power supply and the bias voltage, and sputtering the Cr adhesion layer on the inner surface of the tubular substrate; the Ar flow rate is 30 - 50 sccm, the sputtering gas pressure is 0.1 - 1 Pa, the sputtering power is 250 - 350 W, the bias voltage is 25 - 35 V, the sputtering time is 10 - 60 min, and the radius of the outer wall of the target is 0.3 - 30 cm smaller than the radius of the inner wall of the tubular substrate.

[0008] Preferably, when sputtering the Cr adhesion layer on the inner surface of the cleaned tubular substrate, the Ar flow rate is 35 - 45 sccm, the sputtering gas pressure is 0.3 - 0.7 Pa, the sputtering power is 280 - 320 W, the bias voltage is 28 - 32 V, the sputtering time is 20 - 40 min, and the radius of the outer wall of the target is 1.5 - 25 cm smaller than the radius of the inner wall of the tubular substrate.

[0009] Preferably, sputtering the CrN transition layer on the Cr adhesion layer specifically includes the following steps: using a target made of Cr as the target, introducing Ar and N2 into the tubular substrate, turning on the pulsed DC power supply and the bias voltage, and sputtering the CrN transition layer on the inner surface of the tubular substrate; the Ar flow rate is 30 - 50 sccm, the N2 flow rate is 30 - 50 sccm, the sputtering gas pressure is 0.1 - 1 Pa, the sputtering power is 250 - 350 W, the bias voltage is 25 - 35 V, the sputtering time is 60 - 120 min, and the radius of the outer wall of the target is 0.3 - 30 cm smaller than the radius of the inner wall of the tubular substrate.

[0010] Preferably, when sputtering the CrN transition layer on the Cr adhesion layer, the Ar flow rate is 35 - 45 sccm, the N2 flow rate is 35 - 45 sccm, the sputtering gas pressure is 0.3 - 0.7 Pa, the sputtering power is 280 - 320 W, the bias voltage is 28 - 32 V, the sputtering time is 80 - 100 min, and the radius of the outer wall of the target is 1.5 - 25 cm smaller than the radius of the inner wall of the tubular substrate.

[0011] Preferably, sputtering the ta-C coating on the CrN transition layer specifically includes the following steps: using a graphite target, introducing Ar into the tubular substrate, turning on the pulsed DC power supply and the bias voltage, and sputtering the ta-C coating on the inner surface of the tubular substrate; the Ar flow rate is 30-50 sccm, the sputtering gas pressure is 0.1-1 Pa, the sputtering power is 250-1000 W, the bias voltage is 25-35 V, the sputtering time is 180-240 min, and the radius of the outer wall of the target is 0.3-30 cm smaller than the radius of the inner wall of the tubular substrate.

[0012] Preferably, when sputtering the ta-C coating on the CrN transition layer, the Ar flow rate is 35-45 sccm, the sputtering gas pressure is 0.3-0.7 Pa, the sputtering power is 350-650 W, the bias voltage is 28-32 V, the sputtering time is 200-220 min, and the radius of the outer wall of the target is 1.5-25 cm smaller than the radius of the inner wall of the tubular substrate.

[0013] Preferably, cleaning the inner surface of the tubular substrate by glow discharge specifically includes the following steps: Introducing Ar into the tubular substrate, turning on the pulsed DC power supply and the bias voltage, turning on the glow discharge, and cleaning the inner surface of the tubular substrate; the Ar flow rate is 50-150 sccm, the gas pressure is 0.1-20 Pa, the sputtering power is 200-400 W, the bias voltage is 200-300 V, the duty cycle is 70%-90%, and the cleaning time is 10-60 min.

[0014] Preferably, before cleaning the inner surface of the tubular substrate by glow discharge, the space formed by the tubular substrate and the chamber is evacuated until the air pressure reaches below 9.9×10 -5 Pa; then the space formed by the tubular substrate and the chamber is inflated until the air pressure is 0.6-1 Pa.

[0015] Another aspect of the present invention provides a coating prepared by the above preparation method.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The preparation method of the self-lubricating coating on the inner surface of the cylinder of the present invention adopts an improved magnetron sputtering device, adjusts the planar target to a cylindrical hollow target, when preparing the coating by magnetron sputtering, both ends of the tubular substrate are connected to the rotation driving part and the chamber through the first connecting part and the second connecting part respectively, the tubular substrate is sleeved outside the cylindrical hollow target, a closed space is formed inside the rotation driving part, the tubular substrate and the chamber, and the inner cavity of the tubular substrate is directly used as the vacuum chamber for magnetron sputtering.

[0017] The preparation method of the self-lubricating coating on the inner surface of the cylinder of the present invention is as follows: 1. A cylindrical hollow target is used, and the hollow target is inserted into the tubular substrate to be processed for sputtering. Compared with the sputtering of the flat target set at both ends of the tubular substrate, the uniform deposition of the coating on the inner surface of the tubular substrate can be achieved, and the uniform deposition of the nano-coating at all places on the inner surface of the tubular substrate can be ensured. 2. The two ends of the tubular substrate to be processed are directly connected to the rotating drive unit and the chamber to form a closed space, and the interior of the tubular substrate is directly used as a vacuum chamber. Compared with the existing magnetron sputtering device, there is no need to set a vacuum chamber outside the workpiece to be processed, which greatly saves space and improves the convenience of installation and disassembly of the tubular substrate.

[0018] In view of the difference in the spatial position relationship between the cylindrical hollow target and the planar target and the substrate, and the difference in the position form of the electric field and the magnetic field, the present invention further regulates the process in the magnetron sputtering process, and is used for a new magnetron sputtering device using a cylindrical hollow target, thereby improving the coating uniformity and bonding strength when the cylindrical hollow target is used to prepare a coating on the inner wall of a tubular substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 is a cross-sectional view of a target material and a workpiece in a magnetron sputtering device according to an embodiment of the present invention; Figure 2 is a schematic structural diagram of a magnetron sputtering device according to an embodiment of the present invention; Figure 3 is a schematic structural diagram of an annular fixing seat in a magnetron sputtering device according to an embodiment of the present invention; Figure 4 is a schematic structural diagram of a target material fixing bracket in a magnetron sputtering device according to an embodiment of the present invention; Figure 5 It is a flow chart of a method for preparing a self-lubricating coating on the inner surface of a cylinder according to an embodiment of the present invention.

[0020] Among them: 1-installing bracket; 2-rotating drive part; 3-first connecting part; 31-first flange; 4-cylinder; 5-target; 6-magnet; 61-sub-magnet; 7-chamber; 8-second connecting part; 81-annular fixing seat; 82-second sealing ring; 9-liner; 11-liquid outlet pipe; 12-longitudinal moving mechanism; 13-connecting shaft; 14-target fixing bracket; 15-support seat. DETAILED DESCRIPTION

[0021] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solution of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] One aspect of the present invention provides a method for preparing a self-lubricating coating on the inner surface of a cylinder. A coating is prepared on the inner surface of a tubular substrate by using a magnetron sputtering device, as Figure 1 , Figure 2 shown. The magnetron sputtering device includes: An installation bracket 1; A rotation driving part 2, which is arranged on the installation bracket 1; a first connecting part 3 is arranged on one side of the driving shaft of the rotation driving part 2, and the first connecting part 3 is used to connect the rotation driving part 2 with one end of the tubular substrate 4; A target 5, the target 5 is a cylindrical hollow structure; one end of the target 5 is connected to the driving shaft of the rotation driving part 2; A magnet 6, the magnet 6 is cylindrical and is arranged in the target 5, and one end of the magnet 6 is connected to the driving shaft of the rotation driving part 2; A chamber 7, a second connecting part 8 is arranged on the chamber 7, and the second connecting part 8 is used to connect the chamber 7 with the other end of the tubular substrate 4; a vacuum pumping device and a gas supply device are connected to the chamber 7, and a through hole is arranged on the chamber for evacuating or supplying gas to the inside of the tubular substrate through the through hole; As Figure 5 shown, the preparation method includes the following steps: S1. Connect the two ends of the tubular substrate to the rotation driving part and the chamber respectively; S2. Clean the inner surface of the tubular substrate by glow discharge; S3. Sputter a Cr bonding layer on the inner surface of the cleaned tubular substrate; S4. Sputter a CrN transition layer on the Cr bonding layer; S5. Sputter a ta-C coating on the CrN transition layer.

[0023] Currently, for magnetron sputtering coating on the inner wall of tubular substrates such as cylinders, planar targets are used. The planar targets are placed at one end or both ends of the tubular substrate for magnetron sputtering. This method can only sputter the coating on the inner surface of the end part of the substrate. For the middle part of the substrate, there are problems of insufficient sputtering amount and poor uniformity. The method for preparing a self-lubricating coating on the inner surface of a cylinder according to the embodiment of the present invention uses an improved magnetron sputtering device. The planar target is adjusted to a cylindrical hollow target. When preparing the coating by magnetron sputtering, the two ends of the tubular substrate are respectively connected to the rotation driving part and the chamber through the first connecting part and the second connecting part. The tubular substrate is sleeved outside the cylindrical hollow target. A closed space is formed inside the rotation driving part, the tubular substrate, and the chamber. The inner cavity of the tubular substrate directly serves as the vacuum chamber for magnetron sputtering.

[0024] The method for preparing the self-lubricating coating on the inner surface of the cylinder according to the embodiment of the present invention is as follows: 1. A cylindrical hollow target is used, and the hollow target is inserted into the tubular substrate to be processed for sputtering. Compared with the sputtering of the flat target set at both ends of the tubular substrate, the uniform deposition of the coating on the inner surface of the tubular substrate can be achieved, and the uniform deposition of the nano-coating at all locations on the inner surface of the tubular substrate can be ensured. 2. The two ends of the tubular substrate to be processed are directly connected to the rotating drive unit and the chamber to form a closed space, and the interior of the tubular substrate is directly used as a vacuum chamber. Compared with the existing magnetron sputtering device, there is no need to set a vacuum chamber outside the workpiece to be processed, which greatly saves space and improves the convenience of installation and disassembly of the tubular substrate.

[0025] Preferably, the two ends of the target 5 are respectively connected with a liquid inlet pipe and a liquid outlet pipe, and a coolant is provided inside the target 5; the magnet 6 is immersed in the coolant inside the target 5. In the preparation method of the embodiment of the present invention, in the magnetron sputtering device, the magnet is directly immersed in the coolant inside the target, and there is no need to set cooling pipes inside the tubular substrate, inside the target, and outside the magnet, which greatly saves space. Therefore, in the magnetron sputtering device, a hollow target with a thinner tube diameter can be used, so it is suitable for coating the inner wall of a thin tube with a large aspect ratio, and is suitable for a tubular substrate with a minimum radius of 15 mm.

[0026] The rotary drive mechanism may adopt a motor, and the output shaft of the motor is connected with the magnet and the target material.

[0027] In some embodiments, the magnetron sputtering device further includes a liner 9, which is disposed between the target material 5 and the magnet 6; the liquid inlet pipe and the liquid outlet pipe are respectively connected to both ends of the liner 9, and the coolant is disposed inside the liner 9. The liner can protect and support the target material.

[0028] In some embodiments, the liquid inlet pipe 10 is connected to the bottom end of the liner 9; the liquid outlet pipe 11 is connected to the top end of the liner 9. The cooling liquid enters the liner from the bottom end, and the cooling effect is higher.

[0029] In some embodiments, the magnet 6 is composed of a plurality of cylindrical sub-magnets 61; the plurality of sub-magnets 61 are connected end to end in sequence by magnetic attraction. The length of the target material and the number of magnets can be adjusted according to the length of the tubular substrate to be deposited, so as to be suitable for coating tubular substrates of different lengths.

[0030] In some embodiments, the magnetron sputtering device further comprises: The longitudinal movement mechanism 12 is provided on the mounting bracket 1, and the rotary drive unit 2 is provided on the moving end of the longitudinal movement mechanism 12. The longitudinal movement mechanism 12 is used to drive the rotary drive unit to move up and down. By adjusting the moving end of the longitudinal movement mechanism 12, the bottom of the rotary drive unit can be adjusted to adapt to tubular substrates to be processed with different lengths. First, move the moving end of the longitudinal movement mechanism upward, place the tubular substrate at the corresponding position in the chamber, and then adjust the moving end to move downward, so that the first connecting portion on the rotary drive unit presses the tubular substrate against the second connecting portion in the chamber. The two ends of the tubular substrate are tightened by the first connecting portion and the second connecting portion, and a closed space is formed by the tubular substrate, the rotary drive unit, and the chamber.

[0031] Specifically, the longitudinal movement mechanism can adopt a longitudinal movement motor, or can also be realized by combining a lead screw-nut pair with a guide rail.

[0032] In some embodiments, the first connecting portion 3 includes a first flange 31, a first annular groove, and a first sealing ring; the first flange 31 is provided on the rotary drive unit 2, the first annular groove is provided on the end face of the first flange 31, and the first sealing ring is provided in the first annular groove. Through the tightening action of the longitudinal movement motor on the tubular substrate, the first sealing ring is extruded, thereby realizing the seal between the outer shell of the rotary drive motor and the tubular substrate.

[0033] As Figure 3 shown, in some embodiments, the second connecting portion 8 includes an annular fixing seat 81 and a second sealing ring 82; the annular fixing seat 81 is detachably connected to the outer wall of the chamber 7 through a fastener, the second sealing ring 82 is provided between the outer wall of the chamber 7 and the annular fixing seat 81, and the middle of the annular fixing seat 81 is suitable for fixing the end of the tubular substrate. The end position of the tubular substrate is fixed by the annular fixing seat, and then the tubular substrate is tightened against the outer wall of the chamber by the longitudinal movement motor, and the seal between the tubular substrate and the chamber is ensured through the action of the sealing ring.

[0034] In some embodiments, the magnetron sputtering device further includes a connecting shaft 13, and the connecting shaft 13 is detachably provided between the rotary drive unit 2 and the tubular substrate; the first connecting portion 3 is provided between the rotary drive unit 2 and the connecting shaft 13; a third sealing ring is further provided between the connecting shaft 13 and the tubular substrate. The connecting shaft can extend the length of the tubular substrate and expand the applicable range of the device. By further providing a detachable connecting shaft, when the length of the tubular substrate is sufficient, the connecting shaft does not need to be added. When the length of the tubular substrate is too small, the connecting shaft can be added to extend the length of the tubular substrate, ensure the tightening action on both ends of the tubular substrate, and improve the sealing performance.

[0035] As Figure 4As shown, in some embodiments, a target fixing bracket is provided in the chamber 7. A tapered groove is provided on the target fixing bracket 14, and the end of the target 5 is rotatably disposed in the tapered groove. The target fixing bracket can fix the end of the target, so that it rotates around a stable rotation axis during rotation without deviation.

[0036] In some embodiments, the magnetron sputtering device further includes a support base 15; the chamber 7 is disposed in the support base 15, the open end at the top of the chamber 7 is connected to the support base 15, the end of the tubular substrate is located above the support base 15, and the second connecting portion is used to connect the tubular substrate and the support base. The support base can play a better supporting role for the bottom end of the tubular substrate.

[0037] When using a planar target to prepare a coating on the inner surface of a tubular substrate by magnetron sputtering technology, the target is located at both ends of the tubular substrate, and the target and the substrate are in a "face-to-face" spatial relationship. In the magnetron sputtering device of the present invention, the cylindrical hollow target is located inside the tubular substrate, and the substrate and the target are in a "surrounding" spatial relationship. The tubular substrate wraps the tubular target, and moreover, the positions and forms of their electric and magnetic fields are also different. Based on the above spatial position differences and electric and magnetic field differences, the process of planar target magnetron sputtering coating cannot be applied to the above new magnetron sputtering device. Therefore, the present invention further regulates the process during magnetron sputtering to improve the coating uniformity and bonding strength when the new cylindrical hollow target prepares a coating on the inner wall of the tubular substrate.

[0038] Gas flow rate, target-substrate distance, sputtering power, sputtering gas pressure, sputtering time, etc. will all significantly affect the quality of the coating. Gas flow rate affects the energy of sputtered ions. Ion energy affects the migration and diffusion abilities of ions when they reach the substrate, thereby changing the densification of the coating. Appropriate energy sputtering can sweep away stray atoms on the coating surface, improving smoothness and uniformity. Excessive gas flow rate can lead to an increase in the density of various defects inside the coating, making the coating more prone to cracking and reducing the coating adhesion; too low gas flow rate will result in a decrease in coating uniformity. The target-substrate distance also significantly affects the quality of the coating. If the target-substrate distance is too large, the number of collisions of sputtered atoms before reaching the substrate is too many, resulting in excessive energy loss, which is not conducive to the nucleation and growth of the coating, and the sputtering rate of the coating will decrease; if the target-substrate distance is too small, the coating will be bombarded by charged particles, causing a large number of defects. Sputtering power can affect the sputtering rate. Too high sputtering power, although it can increase the energy of sputtered particles, but too fast sputtering rate affects the coating quality, and too high sputtering power may also damage the target; too low sputtering power, the energy of sputtered atoms when reaching the substrate is also low, resulting in insufficient coating densification. Increasing the sputtering gas pressure can increase the gas ionization rate, but if the sputtering gas pressure is too high, the mean free path decreases, resulting in too many collisions of sputtered atoms before reaching the substrate, losing a lot of energy, and the migration ability is limited after reaching the substrate, and the crystallization quality deteriorates; if the sputtering gas pressure is too low, the mean free path decreases, and gas ionization becomes difficult, making it difficult to trigger the sputtering glow effect. Sputtering time directly affects the thickness of the coating, and an appropriate sputtering time is set according to the coating requirements.

[0039] In some embodiments, sputtering a Cr adhesion layer on the inner surface of the cleaned tubular substrate specifically includes the following steps: using a Cr target, introducing Ar into the tubular substrate, turning on the pulsed DC power supply and the bias voltage, and sputtering the Cr adhesion layer on the inner surface of the tubular substrate; the Ar flow rate is 30-50 sccm, the sputtering gas pressure is 0.1-1 Pa, the sputtering power is 250-350 W, the bias voltage is 25-35 V, the sputtering time is 10-60 min, and the radius of the outer wall of the target is 0.3-30 cm smaller than the radius of the inner wall of the tubular substrate. Using the above process parameters, the prepared coating has good uniformity and strong adhesion.

[0040] Preferably, when sputtering a Cr adhesion layer on the inner surface of the cleaned tubular substrate, the Ar flow rate is 35-45 sccm, the sputtering gas pressure is 0.3-0.7 Pa, the sputtering power is 280-320 W, the bias voltage is 28-32 V, the sputtering time is 20-40 min, and the radius of the outer wall of the target is 1.5-25 cm smaller than the radius of the inner wall of the tubular substrate. Using the above process parameters, the prepared coating can have better uniformity and stronger adhesion.

[0041] Most preferably, when sputtering the Cr adhesion layer on the inner surface of the cleaned tubular substrate, the Ar flow rate is 40 sccm, the sputtering gas pressure is 0.37 - 0.67 Pa, the sputtering power is 300 W, the bias voltage is 30 V, the sputtering time is 30 min, and the radius of the outer wall of the target is 15 - 20 cm smaller than the radius of the inner wall of the tubular substrate. By using the above process parameters, the prepared coating has the best uniformity and the strongest adhesion force.

[0042] In some embodiments, sputtering the CrN transition layer on the Cr adhesion layer specifically includes the following steps: using a target made of Cr as the target, introducing Ar and N2 into the tubular substrate, turning on the pulsed DC power supply and the bias voltage, and sputtering the CrN transition layer on the inner surface of the tubular substrate; the Ar flow rate is 30 - 50 sccm, the N2 flow rate is 30 - 50 sccm, the sputtering gas pressure is 0.1 - 1 Pa, the sputtering power is 250 - 350 W, the bias voltage is 25 - 35 V, the sputtering time is 60 - 120 min, and the radius of the outer wall of the target is 0.3 - 30 cm smaller than the radius of the inner wall of the tubular substrate. By using the above process parameters, the prepared coating has good uniformity and strong adhesion force.

[0043] Preferably, when sputtering the CrN transition layer on the Cr adhesion layer, the Ar flow rate is 35 - 45 sccm, the N2 flow rate is 35 - 45 sccm, the sputtering gas pressure is 0.3 - 0.7 Pa, the sputtering power is 280 - 320 W, the bias voltage is 28 - 32 V, the sputtering time is 80 - 100 min, and the radius of the outer wall of the target is 1.5 - 25 cm smaller than the radius of the inner wall of the tubular substrate. By using the above process parameters, the prepared coating has better uniformity and stronger adhesion force.

[0044] Most preferably, when sputtering the CrN transition layer on the Cr adhesion layer, the Ar flow rate is 40 sccm, the N2 flow rate is 40 sccm, the sputtering gas pressure is 0.37 - 0.67 Pa, the sputtering power is 300 W, the bias voltage is 30 V, the sputtering time is 90 min, and the radius of the outer wall of the target is 15 - 20 cm smaller than the radius of the inner wall of the tubular substrate. By using the above process parameters, the prepared coating has the best uniformity and the strongest adhesion force.

[0045] In some embodiments, sputtering a ta-C coating on the CrN transition layer specifically includes the following steps: using a graphite target, introducing Ar into the tubular substrate, turning on the pulsed DC power supply and the bias voltage, and sputtering the ta-C coating on the inner surface of the tubular substrate; the Ar flow rate is 30 - 50 sccm, the sputtering gas pressure is 0.1 - 1 Pa, the sputtering power is 250 - 1000 W, the bias voltage is 25 - 35 V, the sputtering time is 180 - 240 min, and the radius of the outer wall of the target is 0.3 - 30 cm smaller than the radius of the inner wall of the tubular substrate. With the above process parameters, the prepared coating has good uniformity and strong adhesion.

[0046] Preferably, when sputtering the ta-C coating on the CrN transition layer, the Ar flow rate is 35 - 45 sccm, the sputtering gas pressure is 0.3 - 0.7 Pa, the sputtering power is 350 - 650 W, the bias voltage is 28 - 32 V, the sputtering time is 200 - 220 min, and the radius of the outer wall of the target is 1.5 - 25 cm smaller than the radius of the inner wall of the tubular substrate. With the above process parameters, the prepared coating has better uniformity and stronger adhesion.

[0047] Most preferably, when sputtering the ta-C coating on the CrN transition layer, the Ar flow rate is 40 sccm, the sputtering gas pressure is 0.37 - 0.67 Pa, the sputtering power is 500 W, the bias voltage is 30 V, the sputtering time is 210 min, and the radius of the outer wall of the target is 15 - 20 cm smaller than the radius of the inner wall of the tubular substrate. With the above process parameters, the prepared coating has the best uniformity and the strongest adhesion.

[0048] Preferably, the cleaning of the inner surface of the tubular substrate by glow discharge specifically includes the following steps: Introducing Ar into the tubular substrate, turning on the pulsed DC power supply and the bias voltage, turning on the glow discharge, and cleaning the inner surface of the tubular substrate; the Ar flow rate is 50 - 150 sccm, the gas pressure is 0.1 - 20 Pa, the sputtering power is 200 - 400 W, the bias voltage is 200 - 300 V, the duty cycle is 70% - 90%, and the cleaning time is 10 - 60 min.

[0049] Further preferably, when cleaning the inner surface of the tubular substrate by glow discharge, the Ar flow rate is 80 sccm, the gas pressure is 10 Pa, the sputtering power is 300 W, the bias voltage is 250 V, the duty cycle is 80%, and the cleaning time is 30 min.

[0050] Preferably, before cleaning the inner surface of the tubular substrate by glow discharge, the space formed by the tubular substrate and the chamber is evacuated until the gas pressure reaches 9.9×10 -5Below Pa; then inflate the space formed by the tubular substrate and the chamber until the air pressure reaches 0.6 - 1 Pa.

[0051] Another aspect of the embodiments of the present invention provides a coating prepared by the above - mentioned preparation method.

[0052] Example 1 For the preparation method of the self - lubricating coating on the inner surface of the cylinder in this example, use the aforementioned magnetron sputtering device to prepare the coating on the inner surface of the cylinder. The inner radius of the cylinder wall is 21 cm, and the following steps are included: S1. Turn on the main switch, turn on the water cooler, and press the start button; place the cylinder to be coated on the chamber, and connect the two ends to the rotary drive part and the chamber respectively; close the air release valve and close the chamber door.

[0053] S2. Turn on the mechanical pump, open the bypass valve, and pump the air pressure inside the chamber and the cylinder to below 8 Pa; close the bypass valve, open the fore - stage valve, turn on the molecular pump until it reaches 3000 r / min, open the gate valve, and pump the air pressure inside the chamber and the cylinder to 9.9×10 -5 Pa; open the inflation valve, and fill in Ar. The Ar flow rate is 80 sccm, and it lasts for 2 min. Adjust the gate valve to the 32nd position to keep the air pressure at about 0.8 Pa; Clean the inner surface of the cylinder through glow discharge: introduce Ar into the cylinder, turn on the pulsed DC power supply and the bias voltage, turn on the glow discharge, and clean the inner surface of the cylinder; the Ar flow rate is 80 sccm, the air pressure is 10 Pa, the sputtering power is 300 W, the bias voltage is 250 V, the duty cycle is 80%, and the cleaning time is 30 min.

[0054] S3. Sputter a Cr adhesion layer on the inner surface of the cleaned cylinder: use a Cr - material target. The outer radius of the target is 3 cm. Introduce Ar into the cylinder, turn on the pulsed DC power supply and the bias voltage, and sputter the Cr adhesion layer on the inner surface of the cylinder; the Ar flow rate is 40 sccm, the sputtering air pressure is controlled at 0.37 - 0.67 Pa, the sputtering power is 300 W, the bias voltage is 30 V, and the sputtering time is 30 min.

[0055] S4. Sputter a CrN transition layer on the Cr adhesion layer: use a Cr - material target. The outer radius of the target is 3 cm. Introduce Ar and N2 into the cylinder, keep the pulsed DC power supply and the bias voltage on, and sputter the CrN transition layer on the inner surface of the cylinder; the Ar flow rate is 40 sccm, the N2 flow rate is 40 sccm, the sputtering air pressure is 0.37 - 0.67 Pa, the sputtering power is 300 W, the bias voltage is 30 V, and the sputtering time is 90 min.

[0056] S5. Sputter the ta-C coating on the CrN transition layer: Replace the target with a graphite target with an outer wall radius of 3 cm. Introduce Ar into the cylinder. Keep the pulsed DC power supply and the bias voltage on, and sputter the ta-C coating on the inner surface of the cylinder; the Ar flow rate is 40 sccm, the sputtering gas pressure is 0.37 - 0.67 Pa, the sputtering power is 500 W, the bias voltage is 30 V, and the sputtering time is 210 min.

[0057] S6. Turn off the bias voltage, turn off the pulsed DC power supply, adjust the gate valve to 100, cool for 40 min, turn the gate valve to 0 position, turn the molecular pump to standby, turn off the fore pump, turn off the mechanical pump, release the air, and take out the cylinder.

[0058] Example 2 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 for the remaining steps and parameters, except that the inner wall radius of the cylinder is 21 cm. In step S3, when sputtering the Cr bonding layer on the inner surface of the cylinder: The outer wall radius of the target is 1 cm, the Ar flow rate is 38 sccm, the sputtering gas pressure is controlled at 0.37 - 0.67 Pa, the sputtering power is 300 W, the bias voltage is 30 V, and the sputtering time is 35 min; In steps S4 and S5, the outer wall radius of the target is 3 cm for both.

[0059] Example 3 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 for the remaining steps and parameters, except that the inner wall radius of the cylinder is 21 cm. In step S3, when sputtering the Cr bonding layer on the inner surface of the cylinder: The outer wall radius of the target is 6 cm, the Ar flow rate is 42 sccm, the sputtering gas pressure is controlled at 0.37 - 0.67 Pa, the sputtering power is 300 W, the bias voltage is 30 V, and the sputtering time is 25 min.

[0060] In steps S4 and S5, the outer wall radius of the target is 3 cm for both.

[0061] Example 4 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 for the remaining steps and parameters, except that the inner wall radius of the cylinder is 26 cm. In step S3, when sputtering the Cr bonding layer on the inner surface of the cylinder: The outer wall radius of the target is 1 cm, the Ar flow rate is 35 sccm, the sputtering gas pressure is controlled at 0.3 - 0.6 Pa, the sputtering power is 320 W, the bias voltage is 28 V, and the sputtering time is 20 min.

[0062] In steps S4 and S5, the outer wall radius of the target is 8 cm for both.

[0063] Example 5 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 for the remaining steps and parameters, except that the inner wall radius of the cylinder is 26 cm. In step S3, when sputtering the Cr bonding layer on the inner surface of the cylinder: The outer wall radius of the target is 24.5 cm, the Ar flow rate is 45 sccm, the sputtering gas pressure is controlled at 0.4 - 0.7 Pa, the sputtering power is 280 W, the bias voltage is 32 V, and the sputtering time is 40 min.

[0064] In steps S4 and S5, the outer wall radius of the target is 8 cm for both.

[0065] Example 6 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 for the remaining steps and parameters, except that the inner wall radius of the cylinder is 31 cm. In step S3, when sputtering the Cr bonding layer on the inner surface of the cylinder: The outer wall radius of the target is 1 cm, the Ar flow rate is 30 sccm, the sputtering gas pressure is controlled at 0.1 - 0.4 Pa, the sputtering power is 350 W, the bias voltage is 25 V, and the sputtering time is 10 min.

[0066] In steps S4 and S5, the outer wall radius of the target is 13 cm for both.

[0067] Example 7 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 for the remaining steps and parameters, except that the inner wall radius of the cylinder is 31 cm. In step S3, when sputtering the Cr bonding layer on the inner surface of the cylinder: The outer wall radius of the target is 30.7 cm, the Ar flow rate is 50 sccm, the sputtering gas pressure is controlled at 0.7 - 1 Pa, the sputtering power is 250 W, the bias voltage is 35 V, and the sputtering time is 60 min.

[0068] In steps S4 and S5, the outer wall radius of the target is 13 cm for both.

[0069] Example 8 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 for the remaining steps and parameters, except that the inner wall radius of the cylinder is 21 cm. In step S4, when sputtering the CrN transition layer on the Cr bonding layer: The outer wall radius of the target is 1 cm, the Ar flow rate is 38 sccm, the N2 flow rate is 38 sccm, the sputtering gas pressure is 0.37 - 0.67 Pa, the sputtering power is 300 W, the bias voltage is 30 V, and the sputtering time is 85 min.

[0070] In steps S3 and S5, the outer wall radius of the target is 3 cm for both.

[0071] Example 9 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 in the remaining steps and parameters. The difference is that the inner wall radius of the cylinder is 21 cm. In step S4, when sputtering the CrN transition layer on the Cr bonding layer: The outer wall radius of the target is 6 cm, the Ar flow rate is 42 sccm, the N2 flow rate is 42 sccm, the sputtering gas pressure is 0.37 - 0.67 Pa, the sputtering power is 300 W, the bias voltage is 30 V, and the sputtering time is 95 min.

[0072] In steps S3 and S5, the outer wall radius of the target is 3 cm.

[0073] Example 10 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 in the remaining steps and parameters. The difference is that the inner wall radius of the cylinder is 26 cm. In step S4, when sputtering the CrN transition layer on the Cr bonding layer: The outer wall radius of the target is 1 cm, the Ar flow rate is 35 sccm, the N2 flow rate is 35 sccm, the sputtering gas pressure is 0.3 - 0.6 Pa, the sputtering power is 320 W, the bias voltage is 28 V, and the sputtering time is 80 min.

[0074] In steps S3 and S5, the outer wall radius of the target is 8 cm.

[0075] Example 11 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 in the remaining steps and parameters. The difference is that the inner wall radius of the cylinder is 26 cm. In step S4, when sputtering the CrN transition layer on the Cr bonding layer: The outer wall radius of the target is 24.5 cm, the Ar flow rate is 45 sccm, the N2 flow rate is 45 sccm, the sputtering gas pressure is 0.4 - 0.7 Pa, the sputtering power is 280 W, the bias voltage is 32 V, and the sputtering time is 100 min.

[0076] In steps S3 and S5, the outer wall radius of the target is 8 cm.

[0077] Example 12 The preparation method of the self-lubricating coating on the inner surface of the cylinder in this example is the same as that in Example 1 in the remaining steps and parameters. The difference is that the inner wall radius of the cylinder is 31 cm. In step S4, when sputtering the CrN transition layer on the Cr bonding layer: The outer wall radius of the target is 1 cm, the Ar flow rate is 30 sccm, the N2 flow rate is 30 sccm, the sputtering gas pressure is 0.1 - 0.4 Pa, the sputtering power is 350 W, the bias voltage is 25 V, and the sputtering time is 60 min.

[0078] In steps S3 and S5, the outer wall radius of the target is 13 cm for both.

[0079] Example 13 For the preparation method of the self-lubricating coating on the inner surface of the cylinder in this example, the remaining steps and parameters are the same as those in Example 1, except that the inner wall radius of the cylinder is 31 cm. In step S4, when sputtering the CrN transition layer on the Cr bonding layer: The outer wall radius of the target is 30.7 cm, the Ar flow rate is 50 sccm, the N2 flow rate is 50 sccm, the sputtering gas pressure is 0.7 - 1 Pa, the sputtering power is 250 W, the bias voltage is 35 V, and the sputtering time is 120 min.

[0080] In steps S3 and S5, the outer wall radius of the target is 13 cm for both.

[0081] Example 14 For the preparation method of the self-lubricating coating on the inner surface of the cylinder in this example, the remaining steps and parameters are the same as those in Example 1, except that the inner wall radius of the cylinder is 21 cm. In step S5, when sputtering the ta-C coating on the CrN transition layer: The outer wall radius of the target is 1 cm, the Ar flow rate is 38 sccm, the sputtering gas pressure is 0.37 - 0.67 Pa, the sputtering power is 500 W, the bias voltage is 30 V, and the sputtering time is 215 min.

[0082] In steps S3 and S4, the outer wall radius of the target is 3 cm for both.

[0083] Example 15 For the preparation method of the self-lubricating coating on the inner surface of the cylinder in this example, the remaining steps and parameters are the same as those in Example 1, except that the inner wall radius of the cylinder is 21 cm. In step S5, when sputtering the ta-C coating on the CrN transition layer: The outer wall radius of the target is 6 cm, the Ar flow rate is 42 sccm, the sputtering gas pressure is 0.37 - 0.67 Pa, the sputtering power is 500 W, the bias voltage is 30 V, and the sputtering time is 205 min.

[0084] In steps S3 and S4, the outer wall radius of the target is 3 cm for both.

[0085] Example 16 For the preparation method of the self-lubricating coating on the inner surface of the cylinder in this example, the remaining steps and parameters are the same as those in Example 1, except that the inner wall radius of the cylinder is 26 cm. In step S5, when sputtering the ta-C coating on the CrN transition layer: The outer wall radius of the target is 1 cm, the Ar flow rate is 35 sccm, the sputtering gas pressure is 0.3 - 0.6 Pa, the sputtering power is 650 W, the bias voltage is 28 V, and the sputtering time is 200 min.

[0086] In steps S3 and S4, the outer wall radius of the target is 8 cm for both.

[0087] Example 17 For the preparation method of the self-lubricating coating on the inner surface of the cylinder in this example, the remaining steps and parameters are the same as those in Example 1, except that the inner wall radius of the cylinder is 26 cm. In step S5, when sputtering the ta-C coating on the CrN transition layer: The outer wall radius of the target is 24.5 cm, the Ar flow rate is 45 sccm, the sputtering gas pressure is 0.4 - 0.7 Pa, the sputtering power is 350 W, the bias voltage is 32 V, and the sputtering time is 220 min.

[0088] In steps S3 and S4, the outer wall radius of the target is 8 cm for both.

[0089] Example 18 For the preparation method of the self-lubricating coating on the inner surface of the cylinder in this example, the remaining steps and parameters are the same as those in Example 1, except that the inner wall radius of the cylinder is 31 cm. In step S5, when sputtering the ta-C coating on the CrN transition layer: The outer wall radius of the target is 1 cm, the Ar flow rate is 30 sccm, the sputtering gas pressure is 0.1 - 0.4 Pa, the sputtering power is 1000 W, the bias voltage is 25 V, and the sputtering time is 180 min.

[0090] In steps S3 and S4, the outer wall radius of the target is 13 cm for both.

[0091] Example 19 For the preparation method of the self-lubricating coating on the inner surface of the cylinder in this example, the remaining steps and parameters are the same as those in Example 1, except that the inner wall radius of the cylinder is 31 cm. In step S5, when sputtering the ta-C coating on the CrN transition layer: The outer wall radius of the target is 30.7 cm, the Ar flow rate is 50 sccm, the sputtering gas pressure is 0.7 - 1 Pa, the sputtering power is 250 W, the bias voltage is 35 V, and the sputtering time is 240 min.

[0092] In steps S3 and S4, the outer wall radius of the target is 13 cm for both.

[0093] Comparative Example 1 For the preparation method of the self-lubricating coating on the inner surface of the cylinder in this comparative example, a conventional magnetron sputtering device is used, planar targets are respectively arranged at both ends of the cylinder, and magnetron sputtering is carried out using the process parameters applicable to planar targets.

[0094] For the coatings prepared in the above examples and comparative examples, the step experiment was used to measure the coating distribution uniformity, and the scratch experiment was used to measure the coating adhesion strength. The test method of the step experiment was as follows: at the inner walls near both ends and the midpoint of the cylinder, 5 points were taken circumferentially at each location to measure the coating thickness, and the ratio of the average thickness difference to the average coating thickness was calculated. The scratch experiment method was as follows: a pressure of 5 - 30 N was applied, and the diamond indenter was pressed downward on the coating, and the critical pressure at which the coating peeled off was determined through multiple experiments. The test results are shown in Table 1 below.

[0095] As can be seen from the data in Table 1, for the coatings obtained by the preparation methods of the examples of the present invention, the distribution uniformity and adhesion are significantly higher than those obtained by the conventional planar target magnetron sputtering method.

[0096] Among them, Examples 1 - 5, 8 - 11, 14 - 17 have better coating uniformity and adhesion compared to other examples, and are preferred embodiments. Examples 1 - 3, 8, 9, 14, 15 are further preferred embodiments.

[0097] Table 1

[0098] Obviously, the above examples are merely illustrations for clear explanation and are not limitations on the embodiments. For those of ordinary skill in the art, other different forms of changes or variations can be made based on the above description. It is not necessary and impossible to list all the embodiments here. And the obvious changes or variations derived therefrom are still within the protection scope of the present invention.

Claims

1. A method for preparing a self-lubricating coating on the inner surface of a cylinder, characterized in that: A coating is prepared on the inner surface of a tubular substrate by using a magnetron sputtering device, wherein the magnetron sputtering device comprises: Mounting bracket; A rotary drive unit is disposed on the mounting bracket; a first connecting portion is disposed on one side of the rotary drive unit located on the driving shaft, and the first connecting portion is used to connect the rotary drive unit to one end of the tubular substrate; A target material, wherein the target material is a cylindrical hollow structure; one end of the target material is connected to the driving shaft of the rotating driving unit; A magnet, the magnet is cylindrical and is disposed in the target material, and one end of the magnet is connected to the driving shaft of the rotating driving unit; A chamber, wherein the chamber is provided with a second connection portion, and the second connection portion is used to connect the chamber with the other end of the tubular substrate; the chamber is connected with a vacuum pumping device and a gas supply device, and the chamber is provided with a through hole, which is used to vacuum or supply gas to the inside of the tubular substrate through the through hole; The preparation method comprises the following steps: S1. connecting the two ends of the tubular substrate to the rotation drive unit and the chamber respectively; S2. Cleaning the inner surface of the tubular substrate by glow discharge; S3. Sputtering a Cr bonding layer on the inner surface of the cleaned tubular substrate; S4. sputtering a CrN transition layer on the Cr bonding layer; S5. Sputtering a ta-C coating on the CrN transition layer.

2. The preparation method according to claim 1, characterized in that: The method of sputtering a Cr bonding layer on the inner surface of a cleaned tubular substrate specifically comprises the following steps: the target material is a Cr target material, Ar is introduced into the tubular substrate, a pulsed DC power supply and a bias voltage are turned on, and the Cr bonding layer is sputtered on the inner surface of the tubular substrate; the Ar flow rate is 30-50sccm, the sputtering gas pressure is 0.1-1Pa, the sputtering power is 250-350W, the bias voltage is 25-35V, the sputtering time is 10-60min, and the radius of the outer wall of the target material is 0.3-30cm smaller than the radius of the inner wall of the tubular substrate.

3. The preparation method according to claim 2, characterized in that: When sputtering the Cr bonding layer on the inner surface of the cleaned tubular substrate, the Ar flow rate is 35-45 sccm, the sputtering gas pressure is 0.3-0.7 Pa, the sputtering power is 280-320 W, the bias voltage is 28-32 V, the sputtering time is 20-40 min, and the radius of the outer wall of the target material is 1.5-25 cm smaller than the radius of the inner wall of the tubular substrate.

4. The preparation method according to claim 1, characterized in that: The sputtering of the CrN transition layer on the Cr bonding layer specifically includes the following steps: the target material uses a Cr target material, Ar and N2 are introduced into the tubular substrate, a pulsed DC power supply and a bias voltage are turned on, and the CrN transition layer is sputtered on the inner surface of the tubular substrate; the Ar flow rate is 30-50sccm, the N2 flow rate is 30-50sccm, the sputtering gas pressure is 0.1-1Pa, the sputtering power is 250-350W, the bias voltage is 25-35V, the sputtering time is 60-120min, and the radius of the outer wall of the target material is 0.3-30cm smaller than the radius of the inner wall of the tubular substrate.

5. The preparation method according to claim 4, characterized in that: When sputtering the CrN transition layer on the Cr bonding layer, the Ar flow rate is 35~45sccm, the N2 flow rate is 35~45sccm, the sputtering gas pressure is 0.3~0.7Pa, the sputtering power is 280~320W, the bias voltage is 28~32V, the sputtering time is 80~100min, and the radius of the outer wall of the target material is 1.5~25cm smaller than the radius of the inner wall of the tubular substrate.

6. The preparation method according to claim 1, characterized in that: The sputtering of the ta-C coating on the CrN transition layer specifically includes the following steps: the target material uses a graphite target material, Ar is introduced into the tubular substrate, a pulsed DC power supply and a bias voltage are turned on, and the ta-C coating is sputtered on the inner surface of the tubular substrate; the Ar flow rate is 30-50sccm, the sputtering gas pressure is 0.1-1Pa, the sputtering power is 250-1000W, the bias voltage is 25-35V, the sputtering time is 180-240min, and the radius of the outer wall of the target material is 0.3-30cm smaller than the radius of the inner wall of the tubular substrate.

7. The preparation method according to claim 6, characterized in that: When the ta-C coating is sputtered on the CrN transition layer, the Ar flow rate is 35-45 sccm, the sputtering gas pressure is 0.3-0.7 Pa, the sputtering power is 350-650 W, the bias voltage is 28-32 V, the sputtering time is 200-220 min, and the radius of the outer wall of the target material is 1.5-25 cm smaller than the radius of the inner wall of the tubular substrate.

8. The preparation method according to claim 1, characterized in that: The step of cleaning the inner surface of the tubular substrate by glow discharge specifically comprises the following steps: Ar is introduced into the tubular substrate, a pulsed DC power supply and a bias are turned on, glow discharge is turned on, and the inner surface of the tubular substrate is cleaned; the Ar flow rate is 50-150 sccm, the gas pressure is 0.1-20 Pa, the sputtering power is 200-400 W, the bias is 200-300 V, the duty cycle is 70%-90%, and the cleaning time is 10-60 min.

9. The preparation method according to claim 1, characterized in that: The method further includes evacuating the space formed by the tubular substrate and the chamber until the pressure reaches 9.9×10 -5 Pa; then the space formed by the tubular substrate and the chamber is inflated to a pressure of 0.6~1Pa.

10. A coating prepared by the preparation method according to any one of claims 1 to 9.

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