Dry vacuum pump and manufacturing method thereof

Abstract

A dry vacuum pump has a stator (2) and two rotors (5) housed in at least one compression chamber (3) of the stator (2), the pair of rotors (5) configured to rotate synchronously in opposite directions to drive a gas to be pumped between an intake port and an outlet port of the vacuum pump, the rotor (5) and the compression chamber (3) of the stator (2) are coated with a nickel-phosphorus coating (11) containing 9% to 14% phosphorus and having a thickness of more than 20 μm, the nickel-phosphorus coating (11) being 700H V The alloy has been subjected to a hardening heat treatment comprising heating at a treatment temperature of greater than 250°C for a treatment period of greater than 1 hour so as to have a hardness of greater than 1000 ppm.

Description

[Technical field]

[0001] The present invention relates to a dry vacuum pump, such as a vacuum pump of the "Roots" type, "claw" type, or spiral type, or screw type, or based on another similar principle. The present invention also relates to a method for manufacturing such a vacuum pump. [Background technology]

[0002] Dry vacuum pumps can be used to evacuate corrosive gases and aggressive particulates, such as halogenated gases and abrasive particles, especially from reaction by-products of certain manufacturing processes. Corrosion layers can form on the surfaces of vacuum pump components, which can reduce the functional clearance between the rotor and stator and alter the performance of the vacuum pump. Typically, a nickel coating, or a Teflon type polymer coating, is used to protect cast iron from corrosive attack. Summary of the Invention [Problem to be solved by the invention]

[0003] However, these solutions are not really satisfactory: in particular, the inherent ductility of these coatings means that at the slightest impact or contact, they will plastically deform, leading to the build-up of blob-like material between components, which can entail the risk of the pump seizing. Another disadvantage of this type of coating is that although it improves the cast iron's resistance to corrosive gases, it does not necessarily protect the vacuum pump from wear.

[0004] Another solution is to reduce the temperature of the vacuum pump in order to lower the temperature of the pumped gas, thus reducing the thermal activation of the corrosion rate. However, lowering the gas temperature promotes condensation or solidification, especially of precursors, carrier gases, or other reaction by-products, and may increase the formation of deposits, especially of polymer, metal, or oxide type deposits, which may also entail the risk of vacuum pump seizure. It is also known to use nickel-reinforced cast irons of the Ni-resist type. These cast irons have the advantage of being much more resistant to corrosion and oxidation than conventional cast irons. However, this material cannot easily be substituted for conventional cast irons for the manufacture of vacuum pump parts, due to its difficulty in machining and its high cost.

[0005] One of the aims of the present invention is to at least partially remedy the aforementioned drawbacks by proposing a vacuum pump that is resistant, in particular, to corrosive gases and abrasive powders and is not excessively expensive. [Means for solving the problem]

[0006] To this end, the subject of the present invention is a dry vacuum pump having a stator and two rotors housed in at least one compression chamber of the stator, the pair of rotors being adapted to rotate synchronously in opposite directions so as to drive a gas to be pumped between an inlet and an outlet of the vacuum pump, the pair comprising: The rotor and the compression chamber of the stator are coated with a nickel-phosphorus coating containing 9% to 14% phosphorus and having a thickness of more than 20 μm; The nickel-phosphorus coating has a durability of 700H V The hardening heat treatment includes a step in which the treatment temperature exceeds 250°C and the treatment period exceeds 1 hour so as to have a hardness of at least 100%.

[0007] The hardening heat treatment is carried out to precipitate and crystallize compounds in the nickel-phosphorus coating in order to increase the hardness of the nickel-phosphorus coating. Hardening the coating by heat treatment makes it more brittle due to the appearance of micro-cracks in its microstructure, which can flake off and disperse as dust if there is mechanical contact between the rotor and stator, or between a pair of rotors. The coating does not deform into an expanding mass as in the case of prior art coatings, but instead falls off in the form of fine particles which can be easily and gradually pumped out without interfering with the rotation of the vacuum pump, thus avoiding seizure of the vacuum pump.

[0008] In addition, the nickel-phosphorus coating makes it possible to avoid the formation of a corrosion layer in the vacuum pump, and thus, by heat treatment to harden the coating, it is possible to improve the resistance of the vacuum pump to corrosive gases and wear. Increasing the regulated temperature of the vacuum pump's stator body also makes it possible to avoid condensation and solidification of reaction by-products and thus avoid the formation of deposits of condensable material that may cause the vacuum pump to seize.

[0009] The dry vacuum pump may also have one or more of the features described below, considered alone or in combination. The treatment period is, for example, more than 8 hours. A treatment period of more than 8 hours can make the microstructure of the coating uniform. Such a treatment period can also limit the internal stress of the coating and make the coating more durable. In addition, a treatment period of more than 8 hours can degas the hydrogen gas trapped in the coating during the deposition stage of the coating.

[0010] The treatment period is, for example, less than 15 hours. If the treatment period exceeds 15 hours, there is a risk that the desired hardening quality will not be obtained. Hardness is 800H V ~1000H V It is best to place it somewhere between . The treatment temperature may be less than 350°C. The nickel-phosphorus coating may contain 10% to 13% phosphorus. The nickel-phosphorus coating has a thickness of 60 μm or less, such as, for example, 25 μm±5 μm.

[0011] The vacuum pump may, for example, have at least two pump stages each having a compression chamber, the compression chambers of successive pump stages being connected in series by at least one interstage channel provided in the body of the stator and also having a nickel-phosphorus coating. More specifically, a nickel-phosphorus coating, for example, covers all walls of the vacuum pump likely to come into contact with the gas being pumped. The stator body and the rotor body are manufactured, for example, from cast iron or steel. The vacuum pump may be configured to rotate at speeds greater than 40 Hz.

[0012] Another subject of the invention is a method for manufacturing a dry vacuum pump, characterized in that it comprises the following steps: Depositing a nickel-phosphorus coating containing 9% to 14% phosphorus and having a thickness of more than 20 μm on the inner wall of the stator and the wall of the rotor; The nickel-phosphorus coating of the stator and rotor was V Hardness exceeding 800H V ~1000H V and heating at a treatment temperature of greater than 250° C. for a treatment period of greater than 1 hour so as to have a hardness between 0.01 and 0.15.

[0013] The manufacturing method may have one or more of the features described below, taken alone or in any combination. The treatment period is, for example, greater than 8 hours and / or less than 15 hours. The hardening heat treatment may include at least one ramp step, where the temperature set point is increased from ambient temperature to the treatment temperature at a ramp rate of 1°C / min to 3°C / min. These ramp rates provide an acceptable compromise between a relatively short treatment period for an industrial process and a rate slow enough to avoid the development of excessive forces at the interface between the nickel-phosphorus coating and the stator wall or the interface between the nickel-phosphorus coating and the rotor wall, which have slightly different thermal expansion coefficients.

[0014] The nickel-phosphorus coating is deposited on the inner walls of the stator and the walls of the rotor using, for example, a technique of dipping the stator body and the rotor body. Other characteristics and advantages of the invention will become apparent from the following description, given by way of non-limiting example with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a highly schematic diagram of elements of a dry vacuum pump, with only three-quarters of the stator of the first pump stage being shown. [Diagram 2] FIG. 2 is a highly schematic diagram of a cross-section of a pump stage of the vacuum pump of FIG. 1; [Diagram 3] 1 is a graph showing an example of a temperature setpoint profile for a curing heat treatment using temperature (° C.) on the Y-axis as a function of time (in hours) on the X-axis. [Figure 4a] 1 is a scanning micrograph of a nickel-phosphorus coating that has been subjected to a hardening heat treatment. [Figure 4b] FIG. 4b is an enlarged photograph showing a detail of FIG. 4a. [Figure 5a] FIG. 1 shows a sample of a prior art coating having grooves formed therein. [Figure 5b] FIG. 5b shows a sample of a hardening heat treated nickel-phosphorus coating that had grooves formed therein similar to those formed in the coating of FIG. 5a. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] In the following drawings, identical elements are given the same reference numbers. The following embodiments are some examples. Although the following description refers to one or more embodiments, this does not necessarily mean that each reference relates to the same embodiment or that a feature applies only to a single embodiment. Individual features of various embodiments may also be combined or exchanged to provide other embodiments. In the following, for ease of understanding, only the elements necessary for the operation of the pump are shown.

[0017] The invention applies to any type of dry vacuum pump 1 with one or more stages, be it a "Roots" type vacuum pump, a double claw or "claw" type vacuum pump, a spiral or screw type vacuum pump or based on another similar principle. These dry vacuum pumps are used in particular in certain manufacturing methods for integrated circuits, photovoltaic solar cells, flat panel displays, light emitting diodes, etc. These manufacturing methods include evacuating the corrosive reactive gases from a process chamber of the manufacturing method before releasing the treated gases into the atmosphere, with a vacuum pump having an inlet connected to the process chamber and an outlet connected to a gas treatment device.

[0018] FIG. 1 illustrates an exemplary embodiment of a dry vacuum pump 1, such as a roughing vacuum pump 1, configured to deliver gas pumped at atmospheric pressure. The vacuum pump 1 comprises a stator 2 (or pump body) which comprises at least one pump stage 1a-1e. The vacuum pump 1 has at least two pump stages 1a-1e, for example, arranged in series between an inlet 4 and an outlet of the vacuum pump 1, through which the pumped gas can circulate (the direction of circulation of the pumped gas is indicated by the arrow G in FIG. 1). The pump stage 1a in communication with the inlet 4 of the vacuum pump 1 is the stage with the lowest pressure, and the pump stage 1e in communication with the outlet is the stage with the highest pressure.

[0019] In this embodiment, the vacuum pump 1 has five pump stages 1a-1e, each of which defines a compression chamber 3 in a stator 2 that houses two rotors 5 of the vacuum pump 1, each of which has an inlet 6 and an outlet 7 (FIG. 2). The compression chambers 3 of successive pump stages 1a-1e are connected in series one after the other by at least one interstage channel 8 connecting the respective reservoirs from the outlet 7 of the previous pump stage to the inlet 6 of the next pump stage. The interstage channels 8 are provided, for example, in the body 9 of the stator 2, for example adjacent to the compression chambers 3. For example, there are two interstage channels 8 per pump stage, which are connected in parallel between the outlets 7 and the inlets 6 and are located on either side of the compression chambers 3.

[0020] The pair of rotors 5 have lobes with identical profiles, for example of the "Roots", "claw" or screw type, or based on another similar positive displacement vacuum pump principle. The pair of rotors 5 are configured to rotate synchronously in opposite directions in pump stages 1a-1e (FIG. 2). During rotation, gas drawn through inlet 6 is trapped in the volume created by the pair of rotors 5 and compression chamber 3 of the stator 2 of the pump stage, then compressed by the pair of rotors 5 and driven towards the next stage.

[0021] The pair of rotors 5 are rotated by, for example, one motor of the vacuum pump 1 disposed at one end. The vacuum pump 1 is particularly configured to rotate at a speed exceeding 40 Hz, such as between 50 Hz and 150 Hz. The vacuum pump 1 is referred to as "dry" because, during operation, the pair of rotors 5 rotate inside the stator 2 without any mechanical contact between them or with the stator 2, thereby allowing for the absence of oil in the compression chamber 3.

[0022] The body 9 of the stator 2 and the body 10 of the rotor 5 are made, for example, from cast iron or steel. They are made, for example, from spheroidal graphite cast iron, such as ductile iron, also called SG cast iron. During the manufacturing process of the vacuum pump 1 , a nickel-phosphorus coating 11 is deposited on the inner wall of the body 9 of the stator 2 and on the wall of the body 10 of the rotor 5 .

[0023] A nickel-phosphorus coating 11 is, for example, deposited on all walls of the vacuum pump 1 likely to come into contact with the gas being pumped, in particular the inner walls of the compression chambers 3 and the walls of the inter-stage channels 8 provided in the body 9 of the stator 2. A coating 11 of nickel-phosphorus is deposited, for example, using an immersion technique on the body 9 of the stator 2 and the body 10 of the rotor 5 . The nickel-phosphorus coating 11 contains 9% to 14% phosphorus by weight, such as 10% to 13% phosphorus, and has a thickness e of greater than 20 μm.

[0024] Next, the nickel-phosphorus coating 11 of the stator 2 and rotor 5 is heated for 700H V Hardness exceeding 800H (Vickers hardness at a load of 0.1 kGf) V ~1000H V In order to give the substrate a hardness of 100 mm or more, the substrate is heat-treated in a heating step 102 until the treatment temperature T exceeds 250° C. and the treatment duration D exceeds 1 hour. This hardening heat treatment is carried out to precipitate and crystallize the compounds of the nickel-phosphorus coating 11, increasing its hardness. This hardening heat treatment must be carried out on the nickel-phosphorus coating 11 of the stator 2 and on the nickel-phosphorus coating 11 of the rotor 5 to benefit from an improvement in the coefficient of friction between the two.

[0025] The thickness e is, for example, 60 μm or less, such as 25 μm±5 μm (FIG. 4a). A larger thickness e increases the cost and deposition time of the nickel-phosphorus coating 11. The processing temperature T of the heating step 102 is, for example, less than 350°C, for example, 300°C±20°C. The treatment duration D of the heating step 102 is, for example, greater than 8 hours and, for example, less than 15 hours.

[0026] A treatment period D of more than 8 hours makes it possible to homogenize the microstructure of the coating 11. This treatment period D also makes it possible to limit the internal stresses of the coating 11 and therefore to make it more robust. Furthermore, a treatment period D of more than 8 hours makes it possible to degas the hydrogen gas trapped in the coating 11 during the deposition phase of the coating 11. In contrast, a treatment period D of more than 15 hours runs the risk of not achieving the desired curing quality.

[0027] A phosphorus percentage between 9% and 14% is referred to as "high phosphorus" in contrast to "low phosphorus", which contains 1% to 3% phosphorus by weight, and "medium phosphorus", which contains 6% to 8% phosphorus. This high percentage of phosphorus allows the hardening heat treatment to produce the desired hardness behavior: the hardness of the "high phosphorus" nickel-phosphorus coating 11 increases and then plateaus at a substantially high level, whereas for "low phosphorus" type coatings, the hardness increases more rapidly but then tends to decrease with treatment time.

[0028] The hardening heat treatment is carried out, for example, in an industrial furnace. The curing heat treatment may include at least one temperature ramp step 101 where the temperature set point is ramped from ambient temperature to the heat treatment temperature, for example at a ramp rate of 1° C. / min to 3° C. / min.

[0029] These temperature rise rates provide an acceptable compromise between relatively short processing times in industrial processes and speeds slow enough to avoid the development of excessive forces at the interfaces located between the nickel-phosphorus coating 11 and the inner wall of the stator 2 or between the nickel-phosphorus coating 11 and the body 10 of the rotor 5, in particular because of the slightly different thermal expansion coefficients of the two.

[0030] FIG. 3 shows an example of a profile of temperature settings during the hardening heat treatment. Although the process temperature of the heating step 102 effectively obtained in an industrial furnace is relatively stable, this temperature is relatively variable during the temperature ramp-up and ramp-down steps as well as during transition phases, especially during the level stabilization phase, due in particular to the relatively high inertia of the furnace.

[0031] This temperature setpoint profile includes a first temperature ramp step 101 of 2 hours during which the temperature setpoint is ramped from ambient temperature to the process temperature. The curing heat treatment then includes the actual heating step 102 during which the treatment temperature is maintained at or above 250° C., in this example 300° C., for at least 1 hour, such as at least 8 hours, in this example 12 hours. Finally, the curing heat treatment includes a 2 hour temperature ramp step 103 during which the temperature set point is ramped down from 300°C to 200°C. Heating is then stopped to allow the stator 2 and rotor 5 to cool to ambient temperature.

[0032] Hardening of the coating 11 by heat treatment makes the coating 11 more brittle due to the appearance of microcracks in the microstructure of the coating 11 (FIGS. 4a and 4b). In the event of mechanical contact between the rotor 5 and the stator 2 or between a pair of rotors 5, the coating 11 will flake off and disperse as dust. This is shown in Figure 5b, which shows a sample of the nickel-phosphorus coating that had been subjected to a hardening heat treatment and had grooves formed in it. The edges of the grooves have spalled off and dispersed. The coating without a curing heat treatment, shown in Figure 5a, shows that the coating does not transform into an inflatable body.

[0033] Thus, any particulates that may be generated during use as a result of contact between the rotor 5 and the stator 2, or between a pair of rotors 5, are easily and progressively evacuated by pumping, without impeding the rotation of the vacuum pump 1. Thus, seizure can be avoided. Furthermore, the nickel-phosphorus coating 11 makes it possible to avoid the formation of a corrosion layer in the vacuum pump 1. A heat treatment to harden the coating 11 thus makes it possible to improve the resistance of the vacuum pump 1 to corrosive gases and to wear.

[0034] It also makes it possible to increase the regulated temperature of the body 9 of the stator 2 of the vacuum pump 1 in order to avoid condensation and solidification of the reaction by-products and thus to avoid the formation of deposits of condensable substances which could cause the vacuum pump 1 to burn out. The hardened nickel-phosphorus coating 11 therefore makes it possible to reduce the risk of seizing of the vacuum pump 1 . [Explanation of symbols]

[0035] 1. Vacuum pump 1a-1e Pump stage 2 Stator 3 Compression chamber 4 Intake port 5 Rotor 6 Interstage Channel Entrance 7. Exit of interstage channel 8 Inter-Stage Channels 9 Stator body 10 Rotor body D. Processing Period e-coating thickness G Circulation direction of compressed gas T processing temperature

Claims

1. A dry vacuum pump (1) comprising a stator (2) and two rotors (5) housed in at least one compression chamber (3) of the stator (2), the pair of rotors (5) being configured to rotate synchronously in opposite directions to drive gas to be pumped between an intake port (4) and an exhaust port of the vacuum pump (1), The vacuum pump (1) has at least two pump stages (1a-1e), each of which has one of the compression chambers (3), and the compression chambers (3) of successive pump stages (1a-1e) are connected in series by at least one inter-stage channel (8) provided in the body (9) of the stator (2); The dry vacuum pump is one in which, during operation, the pair of rotors rotate inside the stator without mechanical contact between them or with the stator, thereby eliminating the need for oil in the compression chamber. the rotor (5), the compression chambers (3) of the stator (2) and the inter-stage channel (8) are coated with a nickel-phosphorus coating (11) containing 10% to 13% phosphorus and having a thickness (e) of 20 μm to 60 μm; The nickel-phosphorus coating (11) has a tough, brittle, uniform microstructure having a hardness of 700 HV or more after undergoing a hardening heat treatment.

2. A vacuum pump (1) according to claim 1, A dry vacuum pump, characterized in that the nickel-phosphorus coating (11) does not contain hydrogen gas.

3. A vacuum pump (1) according to any one of claims 1 to 2, The dry vacuum pump has a hardness of 800 HV to 1000 HV.

4. A vacuum pump (1) according to any one of claims 1 to 3, A dry vacuum pump, characterized in that the nickel-phosphorus coating (11) covers all walls of the vacuum pump (1) likely to come into contact with the gas being pumped.

5. A vacuum pump (1) according to any one of claims 1 to 4, A dry vacuum pump, characterised in that the body (9) of the stator (2) and the body (10) of the rotor (5) are made of cast iron or steel.

6. A method for manufacturing a dry vacuum pump (1), comprising the steps of: The dry vacuum pump includes a stator (2) and a pair of rotors (5) housed in at least one compression chamber (3) of the stator (2), the pair of rotors (5) being configured to rotate synchronously in opposite directions to drive gas to be pumped between an intake port (4) and a discharge port; The vacuum pump (1) has at least two pump stages (1a-1e), each of which has one of the compression chambers (3), and the compression chambers (3) of successive pump stages (1a-1e) are connected in series by at least one inter-stage channel (8) provided in the body (9) of the stator (2); The dry vacuum pump is one in which, during operation, the pair of rotors rotate inside the stator without mechanical contact between them or with the stator, thereby eliminating the need for oil in the compression chamber. depositing a nickel-phosphorus coating (11) containing 10% to 13% phosphorus and having a thickness (e) of 20 μm to 60 μm on the inner wall of the stator (2) and on the rotor (5); a hardening heat treatment step (102) of heating the nickel-phosphorus coating (11) of the stator (2), the rotor (5) and the inter-stage channel (8) to a treatment temperature (T) greater than 250° C. and less than 350° C. for a treatment duration (D) of between 8 hours and 15 hours; The method for manufacturing a dry vacuum pump is characterized in that the nickel-phosphorus coating (11) is formed as a tough, brittle, uniform microstructure having a hardness of 700 HV or more.

7. A method for manufacturing a vacuum pump (1) according to claim 6, comprising the steps of: The method for manufacturing a dry vacuum pump, characterized in that the heat treatment comprises at least one temperature ramping step (101), during which the temperature set point is ramped from ambient temperature to the treatment temperature (T) at a ramp rate of 1° C. / min to 3° C. / min.

8. A method for manufacturing a vacuum pump (1) according to any one of claims 6 to 7, comprising the steps of: A method for manufacturing a dry vacuum pump, characterized in that said nickel-phosphorus coating (11) is deposited on the body (9) of said stator (2) and on the body (10) of said rotor (5) using an immersion technique.

9. A method for manufacturing a vacuum pump (1) according to any one of claims 6 to 8, comprising the steps of: The method for producing a dry vacuum pump, characterized in that the treatment period (D) degasses hydrogen gas trapped in the coating during the deposition stage of the coating.

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