A plasma nitride-oxide composite treatment process method for a hydraulic cylinder piston rod

Abstract

This invention relates to the field of plasma surface treatment technology and discloses a plasma nitrogen oxidation composite treatment process for hydraulic cylinder piston rods. The process includes: low-temperature pulsed nitrogen-containing plasma treatment to form a non-porous metastable nitrogen solid solution substrate, during which the pulse timing is clamped to control the process temperature using an endogenous closed-loop rule; subsequently, a dense oxide film is grown in situ on this substrate. This invention reliably constructs a non-porous metastable nitrogen solid solution substrate by precisely controlling the low-temperature process window and suppressing parasitic heating through an endogenous thermal management closed loop, thus avoiding brittle phase precipitation defects. The in-situ grown oxide film is dense and firmly bonded, improving the corrosion resistance and service reliability of the piston rod.

Description

Technical Field

[0001] This invention relates to a plasma nitrogen oxidation composite treatment process for hydraulic cylinder piston rods, belonging to the field of plasma surface treatment technology. Background Technology

[0002] Currently, plasma nitriding, in particular, is a common technique used in the industrial field to improve the surface properties of key steel components such as hydraulic cylinder piston rods, aiming to increase the surface hardness and wear resistance of the workpiece. However, in the service environment, hydraulic cylinder piston rods, in addition to high wear resistance, must also withstand the corrosion challenges posed by media such as moisture in hydraulic oil. Existing plasma nitriding processes, in pursuit of high hardness, typically operate in the medium-to-high temperature range to form thermodynamically stable iron-nitrogen compound phases. However, the formation of these brittle compound phases, in terms of physical mechanisms, is accompanied by phase transformation stress and volume changes, which can induce micropores and microcracks in the nitrided layer. When attempting to further improve corrosion resistance by performing oxidation treatment on this already defective nitrided layer, these pores become the source of failure. The oxidizing atmosphere will penetrate along the channels, resulting in a mixed layer that is no longer a dense surface passivation film, but rather a loosely structured, porous layer. This loose structure is difficult to provide effective corrosion protection, easily leading to pitting corrosion and component failure.

[0003] Although existing technologies such as laser cladding and spraying have been extensively researched, optimizing the physical morphology and chemical composition of the coating powder is a crucial hardware foundation for improving coating performance in actual production. However, simply relying on the mixing of powder components often fails to fundamentally address the software aspects of the bonding between the cladding layer and the substrate, namely, defects in process control. For example, Chinese invention patent CN110904361B discloses a method for preparing nickel-based alloy composite powder and cladding coating for plasma spraying. This patent claims to reduce the cracking sensitivity of the cladding coating and improve its bonding strength and hardness by subjecting four powders—nickel-based alloy, cerium dioxide, aluminum oxide, and silicon carbide—to radio frequency induction plasma spheroidization treatment to obtain near-spherical composite powder. It further claims this reduces the cracking sensitivity of the cladding coating and improves its bonding strength and hardness. However, the experimental data disclosed in the document shows that the residual stress of the cladding layer prepared from the unspheroidized powder is as high as -315MPa, and the density is poor with a small number of cracks. This indirectly reveals that even if high-quality powder components are used, if the morphology and subsequent spraying process, especially the control of supersonic plasma spraying, cannot achieve the optimal hydrodynamic state, the inconsistent melting state and speed of the sprayed particles when flying in the torch and impacting the substrate will still lead to large residual tensile stress during the coating formation process, which in turn will cause microcracks or reduce the density. This indicates that while pursuing the optimization of powder morphology, the existing technology still faces key technical bottlenecks in how to systematically control the coating structure and internal stress through the synergistic control of powder preparation and spraying process.

[0004] Therefore, the technical problem to be solved by this invention is how to construct a plasma composite treatment process that can impart high hardness to the workpiece while avoiding the formation of brittle compound phases from a mechanistic perspective, and instead construct a high-hardness and dense metastable substrate, and grow a dense passivation oxide film in situ on this substrate. Summary of the Invention

[0005] This invention provides a plasma nitrogen oxidation composite treatment process for hydraulic cylinder piston rods. Its main purpose is to solve the problem in the prior art where the high hardness formation mechanism of the nitrided layer is constrained by the non-porosity of the substrate, which leads to the inability of the subsequent oxide film to be dense.

[0006] To achieve the above objectives, the present invention provides a plasma nitrogen oxidation composite treatment process for hydraulic cylinder piston rods, the method comprising the following steps performed sequentially in the same vacuum chamber:

[0007] Step 101, the metastable nitrogen solid solution substrate construction step, involves applying nitrogen-containing plasma treatment in a pulsed manner under the condition that the workpiece's process temperature is controlled below the boundary of the cryogenic process window (below the nucleation temperature of the iron-nitrogen brittle compound phase). Step 101 also includes an endogenous thermal management closed-loop rule for maintaining the cryogenic process window. This rule is defined as: real-time monitoring of the workpiece's process temperature; and dynamically adjusting the timing parameters of the pulsed method in a closed loop based on the workpiece's process temperature. This adjustment follows the following thermal clamping logic: when the workpiece's process temperature approaches or exceeds the preset upper limit of the cryogenic process window due to the parasitic heating effect of the plasma treatment, the control system forcibly extends the pulse interval time of the pulsed method to instantaneously reduce the average power of the plasma treatment, thereby actively counteracting the parasitic heating effect and ensuring that the process temperature is clamped within the cryogenic process window.

[0008] Step 102, dense oxide film in situ growth step: Subsequently, without breaking the vacuum, a dense passivation oxide film is grown in situ on the nitrogen supersaturated metastable solid solution substrate formed in step 101, which is substantially free of brittle compound phase precipitation and has no pores.

[0009] Preferably, in step 101, while applying pulsed nitrogen-containing plasma treatment to the workpiece, an additional high-frequency electric field is applied; the high-frequency electric field runs continuously as a background field to homogenize the energy distribution of the plasma around the complex geometry of the piston rod and suppress local abnormal discharge and local overheating caused by the complex geometry.

[0010] Preferably, step 101 further includes a closed-loop rule for adaptively determining the endpoint of the step. This rule is defined as follows: real-time monitoring of the plasma electrical characteristics during the application of nitrogen-containing plasma in a pulsed manner; the plasma electrical characteristics are parameters characterizing the growth state of the nitrogen-supersaturated metastable solid solution substrate, calculated based on the real-time acquired pulse voltage-current waveform; and adaptively determining the endpoint of step 101 based on the drift of the plasma electrical characteristics over time; when the drift reaches a preset threshold, the system determines that the nitrogen-supersaturated metastable solid solution substrate has reached a preset state and automatically terminates step 101.

[0011] Preferably, the plasma electrical characteristic is an electrical characteristic parameter characterizing the total capacitance of the plasma-workpiece system. ,in For process time; adaptively determine the endpoint of step 101, specifically following these rules: continuously calculate electrical characteristic parameters. rate of change over time When the rate of change over time The first time it falls below a preset steady-state determination threshold When the time comes, step 101 is terminated.

[0012] Preferably, an intermediate atmosphere replacement step is inserted between steps 101 and 102. The intermediate atmosphere replacement step includes: after stopping the nitrogen-containing plasma treatment in step 101 and before starting the in-situ growth step of the dense passivated oxide film in step 102, an oxidizing atmosphere is introduced into the vacuum chamber in a non-plasma state; and the chemical adsorption of the oxidizing atmosphere is used to replace and remove the residual reducing atmosphere previously adsorbed on the inner wall of the vacuum chamber and the surface of the workpiece, so as to avoid an uncontrolled reaction between the residual reducing atmosphere and the oxidizing atmosphere when step 102 is started.

[0013] Preferably, in step 101, the nitrogen-containing plasma treatment further includes introducing a high-mass inert gas; and step 101 is performed at a relatively high background pressure; to establish a dynamic balance between physical sputtering and collision redeposition on the piston rod surface. Collision redeposition refers to the return of sputtered atoms to the piston rod surface after colliding with the high-mass inert gas in the background pressure, thereby accelerating the net growth rate of the nitrogen-supersaturated metastable solid solution substrate while maintaining the low-temperature process window.

[0014] Preferably, the low-temperature process window is controlled at 350°C. Up to 400 Between; in step 101, nitrogen-containing plasma treatment is applied in a pulsed manner, specifically including applying a high-voltage negative pulse bias to the workpiece using plasma immersion ion implantation, and the atmosphere of the nitrogen-containing plasma treatment includes nitrogen and hydrogen.

[0015] Preferably, in step 102, the in-situ growth of a dense passivation oxide film specifically includes: switching to an oxidizing atmosphere containing oxygen and water vapor, and applying low-energy radio frequency plasma treatment or DC plasma treatment to grow a dense passivation oxide film composed of magnetite.

[0016] Preferably, the timing parameters of the pulse mode are dynamically adjusted in a closed loop, specifically including: keeping the pulse on-time of the pulse mode unchanged or reducing the pulse on-time; and coordinating with forcibly extending the pulse interval time of the pulse mode to reduce the duty cycle of the pulse mode.

[0017] Preferably, an additional high-frequency electric field is applied, specifically including: applying an additional radio frequency electric field with a frequency of 13.56MHz in the vacuum chamber.

[0018] Compared with the prior art, the beneficial effects of the present invention are:

[0019] 1. In the first step, the workpiece temperature is controlled below the specific process window for the nucleation of the brittle iron-nitrogen compound phase, and nitrogen-containing plasma treatment is applied, so that nitrogen atoms form a supersaturated, substantially non-porous metastable solid solution substrate on the piston rod surface without inducing a brittle phase transition. Subsequently, without disrupting the vacuum, the process switches to the second step of oxidizing plasma treatment. At this time, the oxidation process is carried out in situ on the non-porous metastable substrate, and the resulting passivation oxide film is therefore highly dense and tightly bonded to the substrate. This process sequence avoids the physical limitations of traditional nitriding processes, where the precipitation of brittle phases leads to micropores in the substrate, resulting in a loose oxide layer in the subsequent process.

[0020] 2. During the metastable solid solution substrate construction step, an additional high-frequency electric field is applied in conjunction with the existing high-voltage negative pulse bias. This high-frequency electric field alters the discharge state of the plasma in complex geometric features of the workpiece, such as grooves or edge regions, effectively suppressing localized abnormal discharges and instantaneous overheating that may be caused by electric field concentration or the hollow cathode effect. This multi-field synergistic control method ensures that the low-temperature process window relied upon by the main process is consistently maintained across the entire surface of the workpiece, including irregular morphological regions. Furthermore, during the metastable solid solution substrate construction step, the electrical characteristics of the plasma processing are monitored in real time. For example, pulse voltage and current waveforms; due to the growth of the metastable solid solution layer, its electrical properties differ from those of the base metal, causing continuous changes in the electrical parameters of the entire plasma-workpiece system. These changes are reflected in the corresponding drift of electrical characteristics. This method utilizes this physical correlation, using the monitored electrical characteristic drift as the basis for judging the growth state of the solid solution layer, and adaptively determining the endpoint of this step accordingly. This approach eliminates the dependence on fixed process time, enabling the process to actively adapt to the differences in growth rate caused by small fluctuations in the composition of different batches of raw materials such as steel, ensuring the consistency of the final metastable substrate.

[0021] 3. An intermediate atmosphere replacement step is set between the metastable solid solution substrate construction step and the dense oxide film in-situ growth step. In this step, after stopping the preceding nitrogen-containing reducing atmosphere and before starting the subsequent oxidizing plasma treatment, an oxidizing atmosphere is introduced into the vacuum chamber in a non-plasma state. The chemical adsorption properties of the oxidizing atmosphere are used to replace and remove the residual reducing atmosphere, such as hydrogen, previously adsorbed on the inner wall of the chamber and the surface of the workpiece. This transition step avoids the uncontrolled and violent reaction between the residual reducing gas and the oxidizing atmosphere at the moment of subsequent oxidizing plasma start-up, and eliminates the possible sudden changes in process pressure and temperature, providing a clean and stable process environment for the stable and controllable growth of the dense oxide film. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the process steps and closed-loop control logic of the present invention;

[0023] Figure 2 This is a comparison diagram of the pitting potential performance between the sample group and the control group of this invention;

[0024] Figure 3 This is a schematic diagram illustrating the system configuration and control relationship of the process of the present invention. Detailed Implementation

[0025] The specific embodiments of the present invention will be described in detail below. It should be understood that the specific embodiments described herein are only for illustration and explanation of the present invention and are not intended to limit the scope of protection of the present invention.

[0026] This invention discloses a plasma nitriding composite treatment process for hydraulic cylinder piston rods. The process is executed sequentially within the same vacuum chamber, decoupled into two stages. First, step 101, a metastable nitrogen solid solution substrate construction step, is performed to build a substrate that is substantially free of brittle compound phase precipitation and is non-porous. Then, without disrupting the vacuum, step 102, a dense oxide film in-situ growth step, is performed on this substrate to form a composite protective layer on the workpiece surface. In an initial state definition procedure, the hydraulic cylinder piston rod workpiece to be treated is installed in a vacuum chamber. This chamber, as the enabling environment, needs to be equipped with a vacuum pump set, a mass flow controller for precise control of various process atmospheres, a workpiece heating system, a workpiece temperature monitoring system such as thermocouples, and a power supply system for applying nitrogen-containing plasma treatment, for example, a plasma immersion ion implantation power supply capable of outputting a high-voltage negative pulse bias. During step 101, considering the traditional high-temperature nitriding process (typically higher than...), ... This process precipitates brittle iron-nitrogen compounds, inducing microporosity and microcracks in the nitrided layer. This method employs a different approach from traditional high-temperature nitriding processes. Specifically, the workpiece is first heated using a workpiece heating system, and its process temperature is controlled within a low-temperature process window below the nucleation temperature of the brittle iron-nitrogen compounds. A specific low-temperature process window is set at... to Between these temperature boundary conditions, a nitrogen-containing atmosphere containing nitrogen and hydrogen is introduced into the vacuum chamber, and the power system is started. A high-voltage negative pulse bias is applied to the workpiece using plasma immersion ion implantation. Since the process temperature is actively suppressed below the thermodynamic nucleation threshold of the brittle phase, high-flux nitrogen ions are supersaturated and dissolved into the lattice of the workpiece surface in a non-equilibrium state implantation manner, forming a nitrogen supersaturated metastable solid solution substrate without inducing phase transformation precipitation that leads to porosity. The essentially non-porous structural characteristics of this substrate are the physical prerequisite for the in-situ growth of a dense oxide film in the subsequent step 102.

[0027] Engineering calibration and maintenance of process window boundary conditions: The nitrogen-containing plasma treatment applied in step 101 using a pulsed method has its upper limit of process temperature set below the nucleation temperature of the iron-nitrogen brittle compound phase. This boundary condition is specified as follows: to The cryogenic process window, which is determined based on the piston rod body material (e.g., Thermodynamic analysis and non-equilibrium experimental results of the steel were used to ensure that nitrogen atoms could exist in a supersaturated solid solution form under the action of high energy flow injected by plasma, and that the nucleation rate of the iron-nitrogen brittle compound phase was lower than that of the steel. The engineering tolerance threshold is determined by the intrinsic thermal management closed-loop rule. To maintain this low-temperature process window in actual operation, the rule dynamically adjusts the pulse timing by monitoring the process temperature of the workpiece in real time. When the process temperature of the workpiece exceeds the preset upper limit (e.g., ...), the pulse timing will be adjusted accordingly. When this occurs, the control system follows thermal clamping logic, forcibly extending the pulse interval time of the pulse mode while maintaining or reducing the pulse on-time. A specific engineering implementation involves extending the pulse interval time by... With real-time measured temperature overshoot Establish a feedback relationship, for example, using a proportional-integral (PI) relationship. ) control algorithm, where The PI control algorithm used in the endogenous heat management closed-loop rule has a proportional coefficient as its control parameter. With integral coefficient The engineering tuning can be determined through the following procedure: First, the integral coefficients are... Set the initial value to zero, and set the scaling factor. Set the value to a small value, initiate the nitrogen-containing plasma treatment in step 101, and monitor the process temperature response of the workpiece; subsequently, gradually increase the value. The value is taken until the process temperature is observed. It can respond quickly to parasitic heating perturbations, but has not yet shown a response around a preset upper limit (e.g.) The continuous oscillation of ); here Based on the initial value, gradually increase... The value of is determined until the system can eliminate the process temperature. The steady-state error between the preset upper limit and the pulse interval time is ensured. The adjustment process was smooth, and the group and The value is then determined as the control parameter used for the hot clamping logic; by... The increment and the current pulse interval time Perform superposition and force a new pulse timing output. This instantly reduces the average power of the injection, clamping the process temperature at [a certain level]. Within the upper limit.

[0028] To ensure the cryogenic process window is maintained during plasma treatment, step 101 also includes an endogenous thermal management closed-loop rule for maintaining the cryogenic process window; this rule aims to actively counteract the parasitic heating effect caused by high-voltage pulse injection; the rule is defined as: real-time monitoring of the workpiece's process temperature; and setting a preset upper limit within the cryogenic process window (e.g., When the workpiece's process temperature approaches or exceeds the preset upper limit due to parasitic heating, the control system follows a thermal clamping logic to dynamically and in a closed loop adjust the timing parameters of the pulse mode. A specific adjustment method is to forcibly extend the pulse interval time of the pulse mode while keeping the pulse on-time constant or decreasing it. This measure, by instantaneously reducing the duty cycle of the pulse mode, i.e., reducing the average injected power, actively counteracts parasitic heating, ensuring that the process temperature is clamped within the low-temperature process window. After step 101 is completed, i.e., after constructing a non-porous nitrogen-supersaturated metastable solid solution substrate, the method proceeds sequentially to step... Step 102, namely the in-situ growth step of a dense oxide film; an operating procedure is that this switching process does not disrupt the vacuum in order to maintain the cleanliness and activity of the workpiece surface; in specific implementation, the nitrogen-containing atmosphere supply and high-voltage negative pulse bias of step 101 are stopped, and the atmosphere of the vacuum chamber is switched to an oxidizing atmosphere containing oxygen and water vapor; subsequently, a low-energy plasma treatment, such as an RF plasma treatment or a DC plasma treatment, is applied to grow a dense passivation oxide film in situ on the non-porous substrate; since the substrate itself does not have microporous defects, the oxidation process can be carried out uniformly and in a controlled manner on the surface, and the final oxide film is preferably made of magnetite ( It has a dense structure that is firmly bonded to the substrate.

[0029] Electrical characteristic quantification and endpoint determination procedure for metastable substrate growth state: The termination adaptive determination closed-loop rule in step 101 is achieved by real-time monitoring of plasma electrical characteristics, which are electrical characteristic parameters. Characterizing the total capacitance of the plasma-workpiece system, the growth of a nitrogen-supersaturated metastable solid solution substrate, due to the difference in electrical properties between it and the metallic matrix, results in a continuous change in the total capacitance of the system. This is relevant in engineering implementation. The acquisition procedure is as follows: Real-time acquisition of the voltage-current waveform of the high-voltage negative pulse bias voltage during plasma immersion ion implantation. and And for each pulse cycle, calculate the equivalent total capacitance. The quantification parameter, when the substrate growth rate tends to saturate, this electrical characteristic parameter rate of change over time The rate of change will slow down significantly; therefore, the system continuously calculates this rate of change over time and compares it with a preset steady-state threshold. The steady-state determination threshold is compared. The determination was based on statistical analysis of historical data from multiple batches of standard workpieces during the stable growth stage, taking their time-varying rate. The quantile points serve as engineering boundaries to ensure that the substrate layer meets the preset minimum thickness requirement, when the time change rate... First time below At that time, the control system automatically determines that the nitrogen supersaturated metastable solid solution substrate has reached the preset state and automatically terminates the execution of step 101.

[0030] In a preferred embodiment, to address the issue of localized abnormal discharge and overheating that may be caused by complex geometric features such as grooves and chamfers on the piston rod workpiece during plasma treatment, a high-frequency electric field is additionally applied during step 101, while applying pulsed nitrogen-containing plasma treatment to the workpiece. This high-frequency electric field operates continuously as a background field, and can be specifically configured to have a frequency of [frequency value missing]. The radio frequency electric field generates a uniform background plasma to homogenize the energy distribution of the plasma around the complex geometry of the piston rod and suppress partial discharge and overheating caused by electric field concentration or hollow cathode effect, thereby ensuring that the low-temperature process window is consistently maintained across the entire surface of the workpiece, including irregular morphology areas. In another preferred embodiment, to achieve adaptive closed-loop control of the process endpoint of step 101 and eliminate growth rate fluctuations caused by batch differences in the workpiece, the method may also include a closed-loop rule for adaptively determining the endpoint of the step. The physical basis of this rule is that the increase in the thickness of the grown nitrogen supersaturated metastable solid solution substrate (whose electrical properties differ from those of the metal substrate) will cause a continuous change in the total capacitance of the entire plasma-workpiece system. In specific implementation, during the execution of step 101, the system monitors the plasma electrical characteristics during the plasma processing in real time. For example, by acquiring voltage-current waveforms in a pulsed manner in real time, an electrical characteristic parameter characterizing the total capacitance of the system is calculated. ,in This is the process time; subsequently, the control system continuously calculates the rate of change of this electrical characteristic parameter over time, i.e. And a steady-state determination threshold is preset. This threshold corresponds to a state where the basal growth rate slows significantly or approaches saturation; when the rate of change over time... The first time it falls below the preset steady-state determination threshold When the nitrogen supersaturated metastable solid solution substrate reaches the preset state, the system determines that the execution of step 101 has been terminated.

[0031] In another preferred embodiment, to eliminate the chemical interference of the reducing atmosphere (such as hydrogen) used in step 101 on the oxidation process in step 102, an intermediate atmosphere replacement step can be inserted between steps 101 and 102. A specific procedure for this step is as follows: after stopping the nitrogen-containing plasma treatment in step 101 and before starting the oxidizing plasma treatment in step 102, an oxidizing atmosphere (e.g., oxygen) is introduced into the vacuum chamber in a non-plasma state. Taking advantage of the stronger chemical adsorption capacity of this oxidizing atmosphere (such as oxygen) compared to the residual reducing atmosphere (such as hydrogen), through competitive... The adsorption mechanism replaces and removes the residual reducing atmosphere previously adsorbed on the inner wall of the vacuum chamber and the surface of the workpiece. This procedure avoids uncontrolled reactions between two atmospheres with opposite chemical properties at the moment of plasma initiation in step 102, providing a clean and stable process environment for the stable and controllable growth of the dense oxide film. In another preferred embodiment, to increase the net growth rate of the nitrogen-supersaturated metastable solid solution substrate while maintaining a low-temperature process window, the nitrogen-containing plasma treatment in step 101 may also include the introduction of a high-mass inert gas, such as argon; and step 101 is performed at a relatively high background pressure (e.g., higher than...). The mechanism of this parameter configuration is to establish a dynamic balance between physical sputtering and collision redeposition on the piston rod surface. Surface atoms (such as iron and nitrogen atoms) sputtered by high-energy ions collide with massive inert gases (such as argon) in a high background pressure as they fly off the surface, causing their energy to decrease and return to the piston rod surface. This collision redeposition effect macroscopically offsets the net loss caused by sputtering and effectively superimposes a physical thin film deposition growth mode, thereby accelerating the net growth rate of the nitrogen-supersaturated metastable solid solution substrate without relying on high-temperature diffusion.

[0032] Example 1: This example aims to illustrate how this method, facing the inherent parasitic heating challenge of high-power plasma processing, resolves the contradiction between temperature control and processing efficiency through a synergistic internal mechanism. In an industrial-grade plasma immersion ion implantation vacuum chamber for mass processing of hydraulic cylinder piston rods, this batch of workpieces has a large total heat capacity, and the process objective is to... Up to 400 Within the low-temperature process window, step 101 is performed to construct a nitrogen-supersaturated metastable solid solution substrate. To improve processing efficiency, the process is set with high-energy pulse injection parameters. This inevitably produces a significant parasitic heating effect on the workpiece after process startup, causing the actual process temperature of the workpiece to rapidly overshoot and exceed 400°C. The trend towards higher limits, and traditional external heater control systems, due to their large thermal inertia, are unable to respond instantaneously to this rapid internal heat accumulation.

[0033] When the batch of workpieces was heated to 380 When the nitrogen-containing plasma treatment in step 101 is applied in a pulsed manner, the endogenous heat management closed-loop rule is activated; the temperature monitoring system monitors in real time that the process temperature rapidly approaches the parasitic heating effect. When the preset upper limit is reached, the control system immediately executes thermal clamping logic, dynamically and in a closed loop adjusting the timing parameters of the pulse mode without interrupting plasma processing. Specifically, this involves forcibly extending the pulse interval time of the pulse mode, for example, from... Extended to Meanwhile, the pulse on-time is kept constant or reduced; this instantaneous extension of the pulse interval time reduces the duty cycle of the pulse mode on a microsecond scale, thereby instantaneously reducing the average power of the plasma treatment injected into the workpiece, thus achieving active cancellation of parasitic heating effects; under the dynamic adjustment of this closed-loop rule, the process temperature of the workpiece is stably clamped at 350°C throughout the entire execution of step 101. Up to 400 Within the low-temperature process window, this inhibits the nucleation of the iron-nitrogen brittle compound phase; after the process is completed, the obtained nitrogen supersaturated metastable solid solution substrate is tested and confirmed to be a structure that is substantially free of brittle compound phase precipitation and has no pores, and a dense passivation oxide film is formed on this substrate through in-situ growth in step 102.

[0034] Example 2: This example aims to verify, through comparative experiments, the effectiveness of using a low-temperature process window below the nucleation temperature of the iron-nitrogen brittle compound phase in step 101 for constructing a non-porous substrate and ultimately obtaining a highly corrosion-resistant composite layer. Multiple sets of 42CrMo steel hydraulic cylinder piston rods with identical dimensions and material grades were selected for the experiment. After standard pretreatment cleaning, they were placed in the same vacuum chamber equipped with plasma immersion ion implantation, workpiece heating and temperature monitoring systems, and precise atmosphere control. The temperature control accuracy of this chamber was no less than [a certain value missing]. The present invention sets up one sample group and three control groups, namely control group A, control group B and control group C; wherein, the present invention sample group and control groups A and B all perform step 101, the metastable nitrogen solid solution substrate construction step, and step 102, the dense oxide film in-situ growth step; control group C only performs step 102; in step 101 of the present invention sample group, the process temperature is clamped at the specified internal heat management closed-loop rule. The temperature is located at to Within the low-temperature process window; in step 101 of control group A, the process temperature was set and maintained at... The temperature is outside the low-temperature process window; step 101 of control group B is consistent with the process parameters of the sample group of the present invention, but it does not perform the subsequent step 102; control group C does not perform step 101, but directly performs step 102 on its original substrate; after each sample group completes all its process steps, the surface modified layer structure of each sample group is characterized first; the cross-sectional morphology is observed by scanning electron microscopy (SEM), and the results are shown in the substrate structure column of Table 1; the sample group of the present invention and control group B both formed a uniform and dense nitrogen supersaturated metastable solid solution substrate, and no pores or brittle phase precipitation was observed; control group A formed a nitrided layer containing micropores and composed of brittle compound phases; control group C is the original substrate; subsequently, the corrosion resistance of each sample group is tested; an electrochemical workstation is used, in Potentiodynamic polarization tests were performed in the solution to measure the corrosion current density. ) and pitting potential ( The experimental data are summarized in Table 1.

[0035] Table 1: Comparison of the structure and corrosion performance of sample groups treated with different processes.

[0036]

[0037] Refer to Table 1, the corrosion current density of the sample group of the present invention ( Compared to control group A () The pitting potential was reduced by about 50 times, and its pitting potential ( ) was higher than the control group A ( This comparison shows that using 500 The porous, brittle phase substrate formed by the process (control group A) exhibits lower corrosion resistance than that produced by process 390, even after subsequent oxidation. The oxide film formed on a non-porous metastable substrate by the low-temperature process (sample group of this invention) provides a basis for the selection of the boundary of the low-temperature process window; in addition, the corrosion performance of the sample group of this invention is also better than that of control group B, which only performed step 101. for ) and control group C that only performed step 102 for These two comparisons demonstrate that neither the low-temperature nitriding step nor the oxidation step alone can provide the same level of corrosion protection. The performance obtained by the sample group of this invention is the result of the combined effect of the non-porous metastable substrate constructed in step 101 and the dense oxide film grown in situ in step 102.

[0038] Example 3: This example combines Figures 1 to 3 This document describes a plasma nitrogen oxidation composite treatment process for a hydraulic cylinder piston rod, as follows: Figure 1As shown, the process begins with the original workpiece: a hydraulic cylinder piston rod. It first proceeds to step 101: construction of a metastable nitrogen solid solution substrate. The goal of this step is to form a non-porous, high-hardness metastable substrate. During step 101, it is regulated by an endogenous thermal management closed-loop rule, which dynamically adjusts the pulse timing by monitoring the process temperature in real time. Simultaneously, this step is also assisted by an additional applied high-frequency electric field, which homogenizes the energy distribution and suppresses abnormal discharge. The end of step 101 is controlled by a termination adaptive judgment closed-loop rule, which determines the substrate growth state by monitoring the plasma electrical characteristics. After the termination of step 101, the process enters an intermediate atmosphere replacement step, the goal of which is to remove residual reducing atmosphere and provide a clean environment. Then, it proceeds to step 102: in-situ growth of a dense oxide film. The goal of this step is to grow a dense oxide film in situ on the non-porous substrate. Finally, the process produces a high-performance composite protective layer piston rod, which possesses high corrosion resistance and high hardness.

[0039] like Figure 2 As shown in the figure, the Y-axis represents the pitting potential (mV vs SCE), and the X-axis represents different sample groups. The legend indicates that the bars represent the pitting potential. According to the data shown, the pitting potential of the sample group of this invention is 350 mV vs SCE, the pitting potential of control group A is 50 mV vs SCE, the pitting potential of control group B is 100 mV vs SCE, and the pitting potential of control group C is 80 mV vs SCE. Figure 3 As shown, the core of the system is a vacuum chamber, which houses the workpiece hydraulic cylinder piston rod, high-frequency electric field electrodes, and a workpiece heating system, all connected to a vacuum pump group. The entire system is regulated by a process control system that executes closed-loop rules and timing control. Specifically, the high-voltage negative pulse bias of the power supply system is connected to the workpiece, feeding back its plasma electrical characteristics to the process control system. The high-frequency electric field of the power supply system is connected to the high-frequency electric field electrodes and controlled by the process control system. The workpiece heating system is connected to the workpiece heating system and controlled by the process control system. The workpiece temperature monitoring system feeds back the temperature signal to the process control system. The process control system also controls a mass flow controller, which supplies gas to the vacuum chamber from an oxidizing atmosphere or a nitrogen-containing atmosphere from the process gas source.

[0040] Example 4: This comparative experiment aims to verify that when performing step 101, the process temperature is below the lower limit of the low-temperature process window (350). The influence of the process temperature on the final composite layer performance; except for the process temperature setting, the workpiece material, pretreatment, vacuum chamber equipment, nitrogen-containing atmosphere (nitrogen and hydrogen) used in this comparative experiment, plasma immersion ion implantation parameters (pulse voltage, frequency, and on-time) used in step 101, and the oxidation atmosphere (oxygen and water vapor) and low-energy plasma treatment method used in step 102 are all consistent with the sample group of the present invention in Example 2; in this comparative experiment, 42CrMo steel piston rod workpieces from the same batch as in Example 2 were selected, and the process temperature was set and maintained at 300°C when performing step 101. This temperature is below the lower limit of the cryogenic process window, 350°C. After performing plasma immersion ion implantation treatment for the same duration as the sample group of the present invention at this temperature, without breaking the vacuum, the same step 102 as the sample group of the present invention is performed, namely the in-situ growth step of dense oxide film. After the process is completed, the surface modified layer of this comparative experimental sample group is characterized and its performance is tested. The results are compared with the sample group of the present invention in Example 2 and summarized in Table 2. Table 2 additionally includes the Vickers hardness (HV0.1) test results, which are used to characterize the hardness of the surface layer.

[0041] Table 2: Comparison of structure and performance of sample groups treated with different steps of process 101 at different temperatures.

[0042]

[0043] Cross-section observation using a scanning electron microscope revealed that although the substrate formed by this comparative experimental sample did not exhibit obvious pores, its thickness was lower than that of the sample from this invention, and its microhardness test result (820 HV 0.1) was also lower than that of the sample from this invention (1250 HV 0.1). This indicates that at 300... Under these conditions, the nitrogen solid solution diffusion rate was low, failing to form a metastable solid solution substrate of sufficient thickness and nitrogen concentration within the set process time; correspondingly, regarding corrosion performance, see Table 2, the corrosion current density of this comparative test sample group ( The pitting potential is approximately 6.5 times that of the sample group of this invention. It is also lower than the sample group of the present invention ( The experimental results show that if the process temperature in step 101 is lower than 350°C... Because a metastable solid solution substrate that meets the requirements could not be formed, even if the same oxidation steps were subsequently performed, the hardness and corrosion resistance of the final composite layer were inferior to those obtained in the initial oxidation process. to Sample groups processed within the window.

[0044] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention.

[0045] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A plasma nitrogen oxidation composite treatment process for hydraulic cylinder piston rods, characterized in that, The method includes the following steps performed sequentially in the same vacuum chamber: Step 101, Metastable nitrogen solid solution substrate construction step, in which nitrogen-containing plasma treatment is applied in a pulse manner under the boundary conditions of the low temperature process window where the process temperature of the workpiece is controlled below the nucleation temperature of the iron-nitrogen brittle compound phase. Furthermore, step 101 also includes an endogenous thermal management closed-loop rule for maintaining the cryogenic process window. This rule is defined as: real-time monitoring of the workpiece's process temperature; and dynamically adjusting the timing parameters of the pulse mode in a closed loop based on the workpiece's process temperature. This adjustment follows the following thermal clamping logic: when the workpiece's process temperature exceeds the preset upper limit of the cryogenic process window due to the parasitic heating effect of plasma treatment, the control system forcibly extends the pulse interval time of the pulse mode to instantaneously reduce the average power of plasma treatment, thereby actively counteracting the parasitic heating effect and ensuring that the process temperature is clamped within the cryogenic process window. Step 102, dense oxide film in-situ growth step: Subsequently, without breaking the vacuum, a dense passivation oxide film is grown in situ on the nitrogen supersaturated metastable solid solution substrate formed in step 101, which is free of brittle compound phase precipitation and has no pores. Step 101 further includes a closed-loop rule for adaptively determining the endpoint of this step. This rule is defined as follows: real-time monitoring of the plasma electrical characteristics during the application of nitrogen-containing plasma in a pulsed manner; the plasma electrical characteristics are parameters characterizing the growth state of the nitrogen-supersaturated metastable solid solution substrate, calculated based on the real-time acquired pulse voltage-current waveform; and adaptively determining the endpoint of step 101 based on the drift of the plasma electrical characteristics over time; when the drift reaches a preset threshold, the system determines that the nitrogen-supersaturated metastable solid solution substrate has reached a preset state and automatically terminates step 101. The plasma electrical characteristic is an electrical characteristic parameter that characterizes the total capacitance of the plasma-workpiece system. ,in For process time; adaptively determine the endpoint of step 101, specifically following these rules: continuously calculate electrical characteristic parameters. rate of change over time When the rate of change over time The first time it falls below a preset steady-state determination threshold When the time comes, step 101 is terminated.

2. The plasma nitrogen oxidation composite treatment process for a hydraulic cylinder piston rod according to claim 1, characterized in that, In step 101, while applying pulsed nitrogen-containing plasma treatment to the workpiece, an additional high-frequency electric field is applied; the high-frequency electric field continues to operate as a background field to homogenize the energy distribution of the plasma around the complex geometry of the piston rod.

3. The plasma nitrogen oxidation composite treatment process for a hydraulic cylinder piston rod according to claim 1, characterized in that, Between steps 101 and 102, an intermediate atmosphere replacement step is also inserted. The intermediate atmosphere replacement step includes: after stopping the nitrogen-containing plasma treatment in step 101 and before starting the in-situ growth step of the dense passivated oxide film in step 102, an oxidizing atmosphere is introduced into the vacuum chamber in a non-plasma state; and the chemical adsorption of the oxidizing atmosphere is used to replace and remove the residual reducing atmosphere previously adsorbed on the inner wall of the vacuum chamber and the surface of the workpiece, so as to avoid an uncontrolled reaction between the residual reducing atmosphere and the oxidizing atmosphere when step 102 is started.

4. The plasma nitrogen oxidation composite treatment process for a hydraulic cylinder piston rod according to claim 1, characterized in that, In step 101, the nitrogen-containing plasma treatment also includes the introduction of a massive inert gas; and step 101 is performed at a relatively high background pressure; to establish a dynamic balance between physical sputtering and collision redeposition on the piston rod surface, whereby the sputtered atoms collide with the massive inert gas in the background pressure and return to the piston rod surface, thereby accelerating the net growth rate of the nitrogen-supersaturated metastable solid solution substrate while maintaining the low-temperature process window.

5. The plasma nitrogen oxidation composite treatment process for a hydraulic cylinder piston rod according to claim 1, characterized in that, The cryogenic process window is controlled at 350°C. Up to 400 Between; in step 101, nitrogen-containing plasma treatment is applied in a pulsed manner, specifically including applying a high-voltage negative pulse bias to the workpiece using plasma immersion ion implantation, and the atmosphere of the nitrogen-containing plasma treatment includes nitrogen and hydrogen.

6. The plasma nitrogen oxidation composite treatment process for a hydraulic cylinder piston rod according to claim 1, characterized in that, In step 102, a dense passivation oxide film is grown in situ, which specifically includes: switching to an oxidizing atmosphere containing oxygen and water vapor, and applying low-energy radio frequency plasma treatment or DC plasma treatment to grow a dense passivation oxide film composed of magnetite.

7. The plasma nitrogen oxidation composite treatment process for a hydraulic cylinder piston rod according to claim 1, characterized in that, The timing parameters of the pulse mode are dynamically adjusted in a closed loop, specifically including: keeping the pulse on-time of the pulse mode unchanged or reducing the pulse on-time; and coordinating with the forced extension of the pulse interval time of the pulse mode to reduce the duty cycle of the pulse mode.

8. The plasma nitrogen oxidation composite treatment process for a hydraulic cylinder piston rod according to claim 2, characterized in that, An additional high-frequency electric field is applied, specifically: an additional radio frequency electric field with a frequency of 13.56 MHz is applied in the vacuum chamber.

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