HK1111742A - Method and system for coating internal surfaces using reverse-flow cycling and other techniques - Google Patents

Description

Method and system for coating internal surfaces using reverse flow circulation and other techniques

Technical Field

The present invention relates generally to chemical vapor deposition methods and systems, and more particularly, but not by way of limitation, to methods and systems for coating the interior surfaces of components, such as delivery tubes.

Background

Much effort has been made to improve the corrosion resistance of specialty metal alloys, such as Stainless Steel (SS), by precisely determining the chemical element content levels (e.g., 16-18% Cr in 316L stainless steel) and reducing the impurity levels remaining after melting and refining (e.g., less than 0.03% S and C in 316L stainless steel). This requires specialized steel smelting processes such as Vacuum Oxygen Decarburization (VOD), vacuum electromagnetic induction melting (VIM), and Vacuum Arc Remelting (VAR), which significantly increase the cost. Another consideration of low impurity steels is that their machinability, hardness and other related properties are negatively affected. Often expensive post-machining operations such as polishing and electropolishing must be performed to meet the hardness and surface roughness requirements specified by some organizations, particularly the International society for Semiconductor device Materials (SEMI). one solution to these problems is to coat the lower-grade base material with a high-quality coating having the desired mechanical, electrical, or optical properties, such as high hardness, corrosion resistance, wear resistance, or low friction.

Waste pipes, not only in the semiconductor industry, but also in the chemical processing industry, are often manufactured from other expensive specialty Alloys such as Hastelloy and Inconel (both of which are federally registered trademarks of Huntington Alloys Corporation). These alloys exhibit high temperature strength and corrosion resistance. Also, if a suitable surface coating is applied to the interior surfaces that are exposed to the corrosive environment, a less expensive base material can be used.

Prior art coating methods include Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), plasma spray, electroplating, and sol-gel. Of these methods, CVD and PVD produce films of the highest quality in terms of purity, bond strength, uniformity and other properties. Both techniques require a dedicated vacuum chamber, making it difficult to coat already fully assembled parts with it. In the case of pipes, valves, pumps or pipes for conveying corrosive materials (such as in the oil/petrochemical industry), it is necessary to coat the inner surfaces that come into contact with the corrosive materials. For very low pressure technologies such as PVD (which are at pressures below or near the molecular flow region), coating the interior surface is limited to large diameter and short length (i.e., small aspect ratio) tubes. Also, CVD techniques are limited to such applications because chemical reactions require the supply of heat, which can damage heat sensitive substrates. Plasma enhanced cvd (pecvd) can be used to reduce the temperature required for the reaction, but it is difficult to maintain a uniform plasma state within the tube and to prevent depletion of the source gases as the gases flow out of the tube.

Plasma Immersion Ion Implantation and Deposition (PIIID) techniques have been shown to be useful for coating the outer surfaces of complex shapes. PIIID is performed by applying a negative bias to the workpiece, which pulls positive ions toward the workpiece if the plasma sheath is compatible therewith. The properties of the coating film, such as adhesion strength and film density, can also be improved by bombarding the workpiece with ions.

In prior art PVD and CVD chambers, the chamber is sized to distribute pressure evenly throughout the chamber with little variation. However, when the workpiece is a chamber, one cannot control the shape of the workpiece. It is therefore necessary to design a process that can coat workpieces having a high aspect ratio (length/diameter) for which the pressure from the gas inlet to the gas outlet is significantly reduced. The method of the invention enables such workpieces to be coated with good uniformity.

Methods of coating the interior surface of a pipe have been described in which the source material to be applied is inserted into the pipe and then applied to the pipe by sputtering or arc coating. For example, one method described in U.S. patent 5026466 (Wesemeyer et al) is to insert a cathode into a tube and apply a cathode material to the interior of the tube via an electric arc. U.S. patent 4407712 (issued to Henshaw et al) describes inserting a hollow cathode containing a high temperature vaporized metal into a tube and coating the source material onto the interior surface of the tube by cathodic arc removal from the hollow cathode. This arrangement has several disadvantages, including: limited to large diameter tubes (since the hollow cathode tube with heat shield and cooling tube must be inserted into the tube to be coated); complicated means are required to move the anode and the hollow cathode in the duct; cathodic arc can produce macroparticles. U.S. patent 4714589 (issued to Auwerda et al) describes a method of coating the interior of a pipe by a plasma activated gas mixture deposition process, but the method is limited to electrically insulated pipes and coatings and involves a complex system of moving a microwave source along the outside of the pipe. A less complex solution is being sought.

Summary of The Invention

The method of the present invention utilizes the workpiece itself as a deposition chamber and utilizes the reverse flow of coating material in the workpiece to coat the interior surfaces of a pipe, tube or other workpiece. The method comprises the following steps: applying a negative bias between the workpiece itself and the anode; flowing a gas containing a surface modifying material through the workpiece in a first direction; reducing the pressure in the workpiece; establishing a hollow cathode effect in the workpiece; modifying an inner surface of the workpiece by applying the surface modifying material to the surface; the gas is then counter-flowed across the workpiece in the next application step.

In certain embodiments, the present methods and systems for performing the same are used to provide reverse cycle coating of a workpiece having more than two openings. On the other hand, if the cycle is performed by a device inserted into the workpiece, the reverse cycle coating method can also be applied to workpieces having an opening.

The source gas may be activated by heating or plasma methods or a combination of both to coat the workpiece surface. When using the heating method, the workpiece can be put into a heating furnace or wrapped in a heat insulation blanket with a heating coil without using the heating furnace. Heating techniques can only be used for substrates that are not sensitive to heat. For heat sensitive substrates, a certain amount of plasma activation must be used to lower the required activation temperature.

The invention can be used not only to form internal surface coatings but also to modify surface or sub-surface properties, such as nitriding of steel or argon sputter cleaning of surfaces. The present techniques may be used not only in chemical vapor deposition processes (e.g., chemical reactions of precursor gases or ionized gases on a surface), but also in techniques classified as physical vapor deposition (i.e., physical reactions rather than chemical reactions to form coatings or subsurface modifications by bombarding a workpiece with ions), or combinations of these techniques. The invention is described only in terms of using plasma enhanced techniques due to its broader application and greater complexity, but the invention is also applicable to simpler thermal deposition or surface treatment methods.

The method of the invention provides for the uniform coating of such workpieces by repeatedly and rapidly reversing the pressure drop from the inlet to the outlet over a coating length at least twice the length of the pipe to be coated as compared to the prior art.

The method preferably includes adjusting one or more of: a gas supply, a vacuum supply to the workpiece, a pressure in the workpiece, and a bias applied to the workpiece to maintain a hollow cathode effect during the altering of the surface. This control may be automatic, and may be repeated if desired. The pressure can cause the electron mean free path to be slightly smaller than the diameter of the tube, causing the electrons to oscillate back and forth in the tube, creating multiple ionizing collisions and a stronger plasma. The improvement over the prior art PECVD method in which the plasma is generated from outside the workpiece is that the ionization disappears as the gas flows through the tube, so that less film deposition occurs at the workpiece exit. If the pressure drop across the tube becomes too great, the hollow cathode effect, which depends on pressure, the plasma intensity and the resulting film thickness and mass will vary along the length of the tube. In contrast, the present invention achieves a more uniform ionized plasma along the length of the workpiece due to the counter-current flow of the gas streams, thus providing a more uniform deposition. Deposition uniformity can be improved by controlling the pressure drop across the workpiece and implementing a reverse-cycle coating process to provide a uniform plasma.

The method can coat the inner surface of a delivery pipe, a pipeline, a valve, an air pump or a workpiece with more complex geometric shapes. While the openings may be referred to as "inlets" or "outlets," their roles are reversed when the direction of flow is reversed. This flow cycling can significantly reduce the likelihood of the coating thickness gradually decreasing from one end to the other due to the gradual decrease in coating density in the plasma as coating is drawn from the plasma onto the inner surface of the workpiece. The improvement in coating thickness from one end to the other is also due to the following: the gas reservoirs provide fresh reactant gas from openings in each workpiece, which can flow or diffuse into the tube as the gas is consumed or depleted during the coating process.

According to one embodiment of a system for carrying out the present invention, the workpiece is connected to a biasing system such that the workpiece functions as a cathode. The first opening of the workpiece is first connected to a gas source as an inlet, while the second opening of the workpiece is connected to a vacuum source as an outlet. The system is then operated to flow coating material through the workpiece from the inlet to the outlet, thus performing a first round of coating. The workpiece is left in place and the airflow direction is reversed for a second coating pass. If the particular application of the invention is to further improve the coating uniformity by cycling, the cycle can be repeated. In some embodiments, the biasing system is also cycled (changed direction).

The applied bias voltage is preferentially a negative pulsed DC voltage applied to the workpiece relative to the electrode and includes a duty cycle having an "on" phase during which a negative voltage is applied to the conductive workpiece to attract ions to the inner surface and an "off" phase during which gas is at least partially replenished.

When the workpiece includes at least two openings, an anode can be coupled to each opening, the anode being physically and electrically isolated from the workpiece by a retractable seal. Likewise, a gas reservoir is coupled to each opening, and the pressure of the gas at each opening can be controlled by flowing into the reservoir from a gas source and out of the gas reservoir into a delivery tube or a suction pump. Thus, the pressure gradient across the delivery tube can be precisely controlled.

In another embodiment of the invention, a device is inserted into the workpiece for performing the reverse-cycle coating process. The device includes at least one aperture to allow gas to flow into or out of the device. In one cycle, gas flows out of the insertion device, through the electrically conductive workpiece, and out of an opening in the workpiece. This embodiment is particularly suitable for coating the inner surface of a workpiece having one opening. The gas flow can be reversed and then through the workpiece to the apparatus. The device may include a tube having a length that is adjustable and a plurality of holes, the number of holes also being varied when adjusting the length. This adjustability allows the apparatus to be used to effectively coat workpieces of different sizes.

Another embodiment of the present invention provides a method of modifying an interior surface of a workpiece having an interior (segment), the method comprising: sealing the interior of the component from the outside atmosphere; providing an anode; providing an inlet for gas into the interior of the workpiece and an outlet for gas from the interior; reducing the pressure of the interior and applying a negative bias between the workpiece and the anode to establish a hollow cathode effect within the interior; introducing a gas containing a surface modifying material into the interior; modifying the surface of the interior of the workpiece by chemical vapor deposition; the gas flow between the inlet and the outlet is reversed in the next surface modification step.

The inlet and outlet may conveniently be provided at respective ends of the length of the delivery tube.

The method may optionally include a pre-cleaning step comprising: introducing a sputtering gas into the interior, reducing the pressure in the interior, and applying a negative DC pulsed voltage between the workpiece and the anode. Conveniently, the sputtering gas may be argon.

One of the best options is an injection step in which the inner surface is injected with an adhesive material and then the surface is modified.

The inner surface is preferably modified with an adhesive material that is capable of forming a chemical bond with the substrate and also with a coating deposited on top of the adhesive layer.

The injection is preferably carried out by applying a bias voltage such that the coating penetrates below the substrate surface to form a bond between the coating and the substrate.

The most preferred surface modifying materials are selected from: metal, ceramic, carbon such as diamond. The most preferred body is acetylene. Alternatively, the gas is selected from: acetylene, methane and toluene, or mixtures thereof. In one arrangement, the gas comprises a hydrocarbon material containing from 1 to 8 carbon atoms.

The method may benefit from: adding hydrogen to the modified gas; and/or introducing a dopant into the modifying gas. It is desirable to incorporate the dopant as a silicon-containing molecule which may be tetramethylsilane, hexamethyldisiloxane, trimethylsilane, or mixtures thereof.

The method preferably includes adding a dopant comprising a metal selected from the group consisting of: titanium, chromium, zirconium, tantalum or tungsten or mixtures thereof.

The method preferably includes the step of adjusting the surface treatment by varying one or more of the bias voltage, gas flow and vacuum pressure during the treatment. And may include the step of changing the composition of the gas during the process.

Brief description of the drawings

Fig. 1 is a functional diagram of a coating apparatus according to an embodiment of the present invention.

Fig. 2 is a functional diagram of a coating apparatus according to a second embodiment of the present invention.

Fig. 3 is a functional diagram of a coating apparatus according to a third embodiment of the present invention.

Fig. 4 is a bottom view of an anode of a third embodiment of the present invention.

Fig. 5 is a functional diagram of a coating apparatus according to a fourth embodiment of the present invention.

Fig. 6 is a bottom view of an anode structure of a fourth embodiment of the present invention.

FIG. 7 is a flow chart of steps in implementing the present invention.

Detailed Description

Referring to FIG. 1, a conductive delivery tube or "workpiece" 10 is connected to a pulsed DC power supply 12, to which a pulsed negative bias is applied. With the negative bias: (a) generating a plasma between a cathode and an anode; (b) drawing an ionized reactant gas to the surface to be coated; (c) ion bombardment of the coating film to improve film properties (e.g., density and stress level); and (d) controlling uniformity by adjusting duty cycle to allow for replenishment of source gas and depletion of surface charges generated by the coating process during "off' periods in the cycle. Here, the workpiece 10 acts as a cathode, while the anodes 18 and 20 are connected to the anode side of a pulsed DC power supply. Gas reservoirs 23 and 25 are connected to each end of the workpiece. In this embodiment, they are electrically isolated from the workpiece and grounded. In other embodiments, they may be biased as cathodes or floating with grounded anodes. Pressure sensors 58 and 60 are mounted on each gas tank so that the pressure drop across the delivery pipe can be monitored and controlled. The anodes are mounted adjacent the workpiece openings 14 and 16 and are physically and electrically isolated from the conductive workpiece and other functional subsystems by insulators 22, 24, 26, 28, 30, 32. The gas supply subsystem 34 and the pump subsystem 44 are coupled to the gas reservoirs and the workpiece openings 14 and 16 via flow control valves 46, 48, 50, 52, 54.

In fig. 1, the workpiece 10 is shown as a single block, but may also be an assembly of pipes or segments. The assembly preferably has completed all welding and assembly steps and should be tested for leaks prior to the coating process described below. The workpiece may be a conductive conduit that is connected to a system that includes the gas supply subsystem 34 and the pumping subsystem 44. Readily available non-toxic carbon-containing gases (such as methane or acetylene) are provided from the first gas supply vessel 36. The gas can be used to form a diamond-like carbon (DLC) coating on the inside of the workpiece. The second gas supply vessel 38 provides argon or other sputtering gas to plasma "pre-clean" the tube surfaces and mix the argon and carbon-containing gases.

The flow control valves 46, 48, 54 are "opened" (flow control valves 50, 52 are still "closed") and the pump subsystem 44 pumps gas from opening 14 through the workpiece 10 to opening 16 for a first coating cycle. The gas is regulated to a given flow rate by adjustable flow control valves 52 and 54, and the gas flow rates are also independently controlled by mass flow controllers 39 and 40. These valves are used to regulate the flow and pressure of gas through the workpiece. At the completion of the first coating cycle, flow control valves 48 and 54 are closed and flow control valves 50 and 52 are opened to allow source gas to flow through the workpiece from gas supply subsystem 34 through opening 16 to opening 14 for a second coating cycle.

The pressure controller 56 receives information from the optical probe 58 and the langmuir probe 60, which are positioned as follows: the line of sight of the optical probe enters the plasma, while the langmuir probe is in contact with the plasma. These 2 probes are capable of detecting plasma density and generating information indicative of the intensity level. The pressure controller may use this information to determine the appropriate settings for flow control valves 52 and 54. This arrangement should establish a condition in the workpiece 10 such that the electron mean free path is slightly shorter than the inner diameter of the workpiece, creating electron oscillation and enhancing ionizing collisions by the "hollow cathode" effect. Thus, a stronger plasma is generated in the workpiece. Since the electron mean free path increases with decreasing pressure, the pressure necessarily decreases with increasing diameter of the transport conduit. For example, an 1/4 inch (6.35 cm) diameter gas line may produce a hollow cathode plasma at a pressure of about 200 mTorr, while a 4 inch (101.6 cm) pump line may produce a plasma at about 12 mTorr. These are approximate values showing a general trend of lower pressure for larger diameters, but these values, which can vary significantly in pressure range, still maintain the hollow cathode plasma.

Pressure controller 56 may also be used to monitor the pressure drop across the piping, which may be adjusted using suction pump throttle valves 52 and 54 or fast reactant flow control valves 48 and 50. As previously mentioned, for small diameter (3.8 cm) and long (61 cm) tubes, it is necessary to prevent too much pressure drop and flow rate to ensure that a uniform high density hollow cathode effect plasma is maintained along the length of the tube in the pulsed DC power "on" state. On the other hand, during the "off" cycle of a DC burst pulsed plasma burst, when the conduit needs to be refilled with reactant gas quickly, the pressure head and flow rate may be increased.

It is also necessary to change the duty cycle in different bursts (pulsed plasma). For example, a deposition burst is performed with 100kHz and 55% duty cycle "on" (i.e., 4.5 microseconds off and 5.5 microseconds on). The 4.5 microseconds is not sufficient to replenish the reactant gas for the entire length of the long small diameter tube, so the deposition burst should be performed for about 10 microseconds. A longer burst at a 100% "off" duty cycle follows to allow the entire pipeline to be replenished with gas. As the diameter becomes smaller and longer, the "off" cycle should increase for a 3.8 cm diameter and 91 cm long pipe, which is only feasible for about 2 seconds.

The degree of ionization or plasma intensity is critical to the effectiveness of the PIIID technique because only the accelerated ionized gas can pass through the plasma sheath into the workpiece 10. The hollow cathode effect provides a plasma that is stronger than existing DC or RF plasmas. This intensity increase can be achieved without other complex structures for generating strong plasmas, such as electromagnetic or microwave plasma sources. This method also does not require isolated heating of the workpiece 10. Optical and langmuir probes 58 and 60 are placed at the anode end connection points to monitor when the intense hollow cathode is properly created.

As shown, the computer software control 66 is connected to the gas supply subsystem 34 and the pressure controller 54. In addition, computer software control can generate and transmit control signals to the DC pulse power subsystem 12 via the interface cable 64 to control various operations.

When considering the flow rate and pressure required to pass through a workpiece with a high aspect ratio (length to diameter), the Poiseuille equation can be applied if the inner cross-section is a long circular tube with laminar flow:

in this equation, V is the volumetric flow rate, r is the channel radius, Δ P is the average pressure, 1 is the channel length, and h is the viscosity. In this equation, r increases by a power of 4, resulting in a significant decrease in V. For example, a 3.8 cm diameter pipe will have a 16-fold reduction in flow compared to a pipe of the same diameter and length of 7.6 cm, all other factors being equal. Δ P-VR where R is the flow resistance, R-8 η l/π R⁴As R increases, the pressure gradient Δ P must be increased to maintain the same flow rate.

The effect of increasing the length is the same as decreasing the diameter, but to a lesser extent. For example, a 3.8 cm tube of 78.7 cm diameter, flowing argon of viscosity 0.02cP, had a pressure drop across the tube of 5.3Pa (40 mTorr) and a flow rate of 176 cm 3/sec, while a 7.6 cm diameter tube of the same length and pressure drop had a gas flow rate of 2811 cm 3/sec. A3.8 cm pipe requires a pressure drop of 85Pa (640 mTorr) to achieve the same volumetric flow rate (2811 cm 3/sec). If we assume that the plasma plume flows and divides V by the cross-sectional area to calculate the residence time, the residence time t for a 3.8 cm x 78.7 cm tube (Δ P5.3 Pa) is 5 seconds, while for a 7.6 cm diameter tube t 1.3 seconds under the same conditions. Smaller tubes give a residence time of 1.3 seconds and ap must be increased to 21Pa (159 mtorr), which has a negative impact on plasma uniformity. These residence times give a rough indication of the time required to refill the tube with fresh reaction gas. For small diameter tubes, the plasma off-time can be increased to refill the tube with gas, or the pressure gradient can be increased to reduce the residence time, bearing in mind that too large a pressure drop negatively affects plasma uniformity. A combination of increased plasma "off" time and increased pressure drop can also be achieved, but care should be taken not to use too great a pressure drop to adversely affect plasma uniformity.

Lowering V, increasing the pressure gradient, while increasing the aspect ratio (length/diameter) will have a significant effect on the uniformity of deposition across the length of the workpiece. Since deposition rate is proportional to pressure, and since the pressure at the inlet is higher than at the outlet, uniformity will gradually deteriorate as the length/diameter ratio increases. Thus, to obtain good coating uniformity, a low cross-line pressure drop Δ Ρ is required. On the other hand, if Δ Ρ and flow rate V become too low, the reactant gas will be depleted before reaching the end of the tube. The invention provides a method for coating such a workpiece with good uniformity: by repeatedly and rapidly reversing the pressure drop from the inlet to the outlet, the coated length of the pipe is doubled at least by a factor of two compared to the prior art. In addition, a method is provided for independently and precisely controlling the pressure drop across the conduit to achieve maximum uniformity and replenish reactant gases from all openings of the conduit that react with the inner surface of the conduit.

In a preferred embodiment, valves 50 and 52 are closed as described in the first coating cycle. However, these valves are adjustable and may be set to a partially open condition, but less open than valves 48 and 54. The degree to which valves 50 and 52 are closed is determined by the desired cross-line pressure. To minimize the cross-over gas pressure difference, the pump speed on the outlet side may be slightly higher than on the inlet side, and the same gas flow rate requires that throttle valve 52 be closed more than throttle valve 54. Or the outlet side gas flow rate is slightly lower than the inlet side, the same pump pumping rate, requires mass flow control valve 50 to close more than mass flow control valve 48. Thus, the hollow cathode plasma can be precisely controlled to ensure uniformity of the hollow cathode plasma across the entire workpiece 10. The degree of opening or closing of valves 48-54 is reversed during the second coating cycle.

In some applications of the present invention, the first and second coating cycles are repeated to provide a more uniform coating along the entire length of the inner surface of the workpiece. This "plasma cycling" technique is advantageous because the high pressure end or gas flow inlet (14 for the first cycle and 16 for the second cycle) of the workpiece 10 is deposited at a higher rate and with a thicker coating than at the low pressure end or gas flow outlet. By reversing the direction of the gas flow and the pressure gradient across the workpiece, uniform coating of the inner surface of the tube can be achieved.

Another embodiment of the invention is shown in figure 2. FIG. 2 depicts a method of coating an interior surface of a workpiece with an opening. In this case, the cylinder 68 with an opening 70 has a short section of sacrificial tubing 72 attached to it. The purpose of the sacrificial tube is to ensure that the plasma is ignited before the gas actually enters the workpiece. This ensures a full hollow cathode plasma density at the workpiece entrance. An anode 76 is connected to the sacrificial tube but is electrically isolated therefrom by the insulator 74. A gas injector 78 with a series of holes 80 along its length is inserted in the cylinder opening 70.

As shown in fig. 2, the DC pulse power source 12 is connected to the cylinder 68 (workpiece) and the gas injector 78 through a DC cable. The cylinder and gas injector are negatively biased with respect to the anode 76. Methods of connecting a DC cable to a gas injector are known in the art. The method of applying a bias voltage to the injector 78 as a cathode will enable the use of a large diameter cylindrical hollow cathode plasma for the pressure boosting operation, since some electrons will oscillate between the cylinder wall and the injector, effectively reducing the spacing of the hollow plasma from the diameter to the cylinder radius. In another embodiment, the syringe may be allowed to electrically float for a smaller diameter barrel. I.e. it is biased neither as cathode nor as anode.

In another embodiment, the length of the gas injector may be adjusted to accommodate workpieces of different lengths and diameters. In this embodiment, the gas injector is patterned to have a plurality of holes 80 along its length. As the gas injector is extended and retracted, the number of holes exposed along its length increases and decreases, respectively. However, a single orifice gas injector may also be used in the practice of the present invention.

Referring to FIG. 1, flow control valves 46-54 connect gas supply subsystem 34 and pump subsystem 44 to cylinder 68 and gas injector 78. These flow control valves are configured to perform the first and second coating cycles as previously described. In this embodiment, the first cycle is performed by flowing gas into the cylinder through the gas injector, drawing it into the cylinder, and flowing it out through the opening 70. The airflow is then reversed to begin a second coating cycle.

Another embodiment of the invention is shown in fig. 3. The process is modified here to coat a section of pipe with a very large length to diameter aspect ratio, for example greater than 50: 1. In this example, the "plasma cycling" process would produce a coating with poor uniformity due to the loss of plasma density as ions are pumped out to coat very long pipes. That is, even though "plasma circulation" has its advantages, the thickness of the coating in the central region of a pipe or pipe system having such an aspect ratio is much lower than in the end regions. More typically, however, the embodiment of fig. 3 may be used where lengths of pipe that have been coated with a "plasma-cycling" process to have a uniform coating are welded together to form an extremely long pipe. Corrosion resistant coatings are still needed for the weld and the area around the weld where the coating is damaged by the welding operation.

Referring to fig. 3 and 4, the anodes 18 and 20 are mounted on an insulating drum 82. A retractable vacuum seal 84 surrounds the anode. The vacuum seal, when extended, physically and electrically isolates the anode from the tube. Anodes are inserted into both ends of the pipe 10 (workpiece) and at or near the weld 86 to be coated. Flexible gas supply lines and a suction pump line 88 are connected to the anode in a known manner. The gas supply line and the pump line are electrically isolated from the anode by an insulator 89, see fig. 4.

The vacuum seal 84 is extended and a negative bias is applied to the tube 10 using a DC pulsed power supply, which acts as a cathode. Even if the entire tube 10 is biased as a cathode, plasma is only generated inside the tube between the anodes 18 and 20, since only this portion of the interior of the tube is at a low pressure and meets the spacing and pressure requirements for initiating the generation of the plasma. Thus coating only the inner surface of the pipe section containing the low pressure region.

As previously described and shown in fig. 1, the first and second coating cycles are performed using flow control valves 46-54, gas supply subsystem 34, and pump subsystem 44. After the coating process is complete, the interior of the pipe section is vented to atmospheric pressure. The retractable vacuum seal 84 is retracted and the anodes 18 and 20 are moved with the flexible line 88 to the next weld or pipe section to be coated.

In another embodiment, see fig. 5 and 6, the electrode structure 90 is mounted on the insulated drum 82 and has been inserted into the workpiece 10. The electrode structure includes an RF electrode 92 and a DC anode 94. Flexible gas supply lines and vacuum supply lines 88 are also connected to the electrode structure. The RF electrode and the DC anode are isolated from each other by a circular insulator 96, which can be seen more clearly in fig. 6. A retractable vacuum seal 84 surrounds the anode structure as described in the previous embodiments.

In this embodiment, an RF power supply 98 is connected to the RF electrode, see FIG. 5. The DC anode 94 is connected to the pulsed DC power supply 12. This arrangement can be controlled and controlled by varying the power, bias amplitude and frequency of the RF power supply to generate a plasma in the tube 10. In addition, by varying the power and bias of the DC pulsed power supply, various coating properties can be individually tailored to any plasma tuning. The first and second coating cycles and the movement of the anode structure are performed as described in the previous embodiments.

One embodiment of the process of the present invention is described with reference to fig. 1 and 7. At step 100, the workpiece is mounted on other components of the piping system such that heating of the workpiece is not required after the internal coating process is completed. Thus, the interior of the workpiece can be coated after all welding steps associated with the workpiece have been completed. As previously mentioned, although the workpiece is shown as a complete pipe, the workpiece may be an assembly of pipes or components.

At step 102, a preclean is performed. The pre-clean may introduce a sputtering gas, such as argon, from the first gas supply vessel 36. Argon is the preferred sputtering gas because it is an inert gas (group 8 of the periodic table) with atoms enclosed in a shell that transfers energy well to contaminating atoms that are bonded to the substrate, either "knocking them out" or removing them from the surface via sputtering. Can be reduced to 1 × 10 by the air pump^-3Torr or preferably below 1 x 10^-4After the cleaning, pre-cleaning is started. When a negative DC pulse is applied by the power supply 20, contaminants on the inner surface of the workpiece are removed by the sputtering gas.

An optional carbon implantation step 104 may be employed in certain applications. Carbon implantation can form a subsurface carbon layer in the workpiece material (e.g., stainless steel). This layer improves adhesion to DLC and other materials. The carbon implantation should be performed at a higher magnitude bias than other steps of the coating process. Suitable bias voltages are those in excess of 5 kV. For small diameter tubes, care must be taken in this step so that the size of the plasma sheath does not become larger than the radius of the workpiece.

One preferred and alternative method of improving adhesion to steel substrates relies on the use of chemical bonding, the material used in the method being such as to form a chemical bond with the workpiece to be coated and the resulting coating itself. While many different materials are known to those skilled in the art, it will be appreciated that when coating steel components with a carbon-containing coating, one can use a silicon-containing bond coat because it can form a strong bond with the iron in the workpiece and the carbon in the coating without the need for the high bias required for implantation. Such high bias voltages can cause arc damage and require more expensive power supplies. A more detailed description of suitable materials will be provided herein below.

After the optional injection step 104, at least one precursor gas is introduced in step 106. Acceptable precursor gases include methane, acetylene or toluene. The DC pulse voltage is reduced in this step to provide thin film deposition, rather than implantation. The application of the DC pulse voltage is shown in step 108 of fig. 7. During the coating step, argon may be mixed with the carbon-containing precursor gas, as shown in step 110.

At step 112, the coating parameters in the coating process may be dynamically adjusted. The computer software control 66 and pressure controller 56 may use the probe to provide information to maintain the various parameters within their tolerances. Thus, the factors that determine the pressure within the workpiece can be adjusted as desired, or the amplitude and duty cycle of the pulsed bias can be adjusted if desired.

After the first coating cycle is completed, the airflow is reversed in step 114. At this step, the flow control valves 46-54 are reset for a second cycle. The process flow step 106 and 114 may be repeated to ensure uniform coating of the inner surface of the workpiece of different lengths and diameters.

In principle, any metal, ceramic, or DLC coating (such as TiN, CrN, etc.) having the desired hardness and corrosion resistance properties can be used. However, the coatings used in the art employ non-toxic gases. DLC precursor gases containing 1 to 4 carbon atoms, such as methane, toluene, are used in the preferred embodiment, or acetylene is used as the source gas in the preferred embodiment. DLC has been shown to provide a hard, corrosion resistant and low friction coating. By adjusting the bonding hybridization ratio of sp3 (diamond), sp2 (graphite), and sp1 (linear) in the coating film, various properties of the coating film can be adjusted. Since hydrogen can be incorporated into the membrane using a hydrocarbon precursor gas, the hydrogen content also affects the membrane performance. For example, a high sp3 content film is obtained with a high hydrogen content, resulting in a soft polymer-like film, since the sp3 bonding comes from C-H4 type bonds, rather than hard diamond C-C4 type bonds. The performance of DLC films is controlled by the energy per carbon atom.

The highest sp3 bonding hybridization (carbon bond form of tetrahedral diamond as opposed to graphite (sp2) or sp1 form) is available with ion energies between 40eV and 100eV, above or below which the diamond content decreases. The mechanism of how to form a DLC film from a gas precursor is briefly described below. A molecular precursor (e.g. acetylene) is ionized with "hot" high energy electrons, which have energies higher than the ionization energy of the gas precursor (> 10eV), which comes from the high energy tail of the electron profile, with most electrons having "cold" or "medium" energies, following a maxwell-boltzmann profile. Of course, the gas contains not only ions, but also energetic molecules (which have been excited without losing electrons), radicals, and neutral molecules. The plasma body is a quasi-neutral body (equal number of electrons and ions) and the electric field across which the plasma is formed falls at the cathode. This is due to the very high velocity of electrons compared to ions, so that when a negative DC pulse is applied, the electrons quickly leave the cathode, leaving a sheath of positively charged ions around the cathode. The ions are then accelerated through the sheath and hit the substrate, where intramolecular bonds break and separate into individual atoms, which may also be heated to implant or knock out (sputter) depending on the ion energy. Ion bombardment can also result in the sputtering of electrons. Further ionization can be caused by accelerated electrons going back through the plasma sheath field becoming "hot" into the bulk of the plasma, which results in self-sustaining of the plasma.

The ion energy can be controlled with a bias voltage and can be reduced if the pressure is too high (i.e., if the mean free path of the ions becomes smaller than the plasma sheath width, which would cause collisions and energy losses). Power also affects ion energy, as an increase in power increases plasma density (number of ions or electrons per cubic centimeter), which reduces sheath size and reduces the chance of collisions. With larger molecules, the energy per carbon atom is also reduced and shared when collisions with the surface cause the molecule to break (e.g., acetylene requires about 2 times the bias voltage of methane to achieve the same energy of carbon ions). If a hard film (about 20GPa) is desired at a medium bias voltage (about-1000V), acetylene is the best precursor gas, and the film contains 67% sp3 (as measured by Raman spectroscopy) and 25% hydrogen at a pressure of about 100 mTorr.

Typically, the highest sp3 ratio (mostly diamond-like) is obtained from methane, but it also produces lower deposition rates, higher compressive stresses, and film thickness limited to about 5000 Å compared to higher carbon molecules. The addition of certain dopants (e.g., silicon or silicon oxide) to DLC improves thermal stability and can reduce compressive stress. Molecules containing a large number of carbon atoms, such as toluene, will produce higher deposition rates, but will yield softer films. Hydrogen containing DLC is an amorphous film called a-C: H with very short range bonding magnitudes. The added dopant is capable of forming a "nano-composite" film with a dual bonding matrix, e.g., amorphous carbon: a mixture of hydrogen (a-C: H) and amorphous carbon silicon oxygen (a-C: H: SiO), orThe a-C: H and a-C: H: SiO can be deposited in different thin layers respectively. Silicon or silicon-containing dopants are in many cases preferred because silicon also forms sp3 type bonds (i.e. the sp3 content is still high), and readily forms FeSi type bonds (forming heat-19 kcal/mol, while Fe forms) in combination with steel₃C forms heat of + 5.7 kcal/mole), provides a strong adhesive layer, and also reduces compressive stress due to its larger atoms and longer bonds. Stress and adhesion are critical to the formation of thick films because weak adhesion is likely to be overcome with less film stress, while strong adhesion is overcome with very high film stress. There can be two main sources of film stress; intrinsic stress, resulting from the way the film is formed (tensile stress can be caused by pores in the film due to lack of atomic surface movement; compressive stress due to close packing of atoms by high energy ion bombardment as in DLC), or extrinsic stress due to mismatch in the coefficient of thermal expansion between the substrate and the film, for example, when the contact temperature is cycled. Can be prepared by using organic precursor gas (such as hexamethyldisiloxane (C)₆H₁₈Si₂O) or tetramethylsilane (Si (CH)₃)₄) To introduce silicon dopants.

Some substrates cannot form strong silicon bonds (e.g., high nickel alloys), in which case a metal (e.g., Ti, W, or Cr) that can form carbon bonds (TiC forms-110 kcal/mol) may be used. One example of a hydrocarbon precursor gas that may be used to incorporate titanium into the film is tetrakis (dimethylamino) titanium (TDMAT), depending on the process conditions (e.g., temperature and TDMAT/C)₂H₂Relative proportions) TDMAT will form DLC containing varying amounts of carbon, titanium, hydrogen, and nitrogen doping. Metal dopants can also reduce the resistivity of the film. These metal dopants can also be used to increase the ductility of the film, which will improve the toughness of the film/substrate combination when exposed to shock particles (erosion). This technique also makes it possible to form pure metal, metal nitride or metal oxide films by driving off the carbon with heat (either exogenous or ion bombardment generated).

The higher the sp3 content of the DLC film, the closer the film properties are to diamond, resulting in a high hardness, high wear resistance, low coefficient of friction, and high corrosion resistance film. The properties of the film can also be adjusted by specifically determining the sp3 content of the film, for example, different films can be made with a hardness of from 6Gpa to 30 Gpa. This can be accomplished by adding dopants, or adjusting process parameters such as bias, pressure, or power. The amorphous properties of DLC films are critical for corrosion resistance because, in contrast to polycrystalline films, DLC films do not have (crystalline) grain boundaries that can act as diffusion paths for corrosive species to the substrate, and are also insulators, thus cutting off the current through the film, which is necessary to produce corrosion. The thicker film also has improved corrosion resistance due to the longer diffusion path to the substrate.

The choice of precursor gas not only determines the desired film properties, but also the following issues involved: health and safety (flammability, toxicity, etc.) associated with the precursor gas, ease of delivery of the precursor gas (e.g., delivery of a gas is much easier than a solid), cost, and availability. As previously mentioned, the volume of the precursor gas and the relative amounts of carbon and hydrogen therein (high C/H yields a harder film), as well as the energy required for ionization, are also critical.

Thus, the properties of the film can be specifically determined by the choice of precursor gases, or layered films can be deposited. For example, if a particular process (e.g., very rough welding) requires a thick deposit coating, the above-described process location can be improved by: (1) depositing a strong adhesion layer with tetramethylsilane containing silicon; (2) depositing a thick, low stress layer with a mixture of acetylene and tetramethylsilane; (3) with pure C₂H₂A thin (due to higher stress) but hard upper layer is deposited. If only a low coefficient of friction film is desired, a thin adhesive layer may be applied followed by a thin layer of C₂H₂And (4) a covering layer. The balance between the desired mechanical, electrical or optical properties of the film, and the deposition rate and stress of a given precursor gas and the bonding hybridization (sp3 vs. sp2) can be optimized for a given application.

The process can be scaled up for larger diameter pipes by increasing the amount of air flow in proportion to the pipe surface area. But the bias voltage must be slightly increased to compensate for the increase in electron travel length.

The method has the advantages that: the benefits of ion bombardment of the aforementioned PIID process can be exploited to improve film adhesion and density. This is achieved in the best embodiment by applying a negative pulsed DC bias to the workpiece (relative to the anode). Since the DLC coating is an insulator, the use of a short pulse (1-20 milliseconds) prevents the creation of excessive positive charges on the coating. This charge is compensated when the off-cycle plasma sheath collapses. In one preferred approach, a small (about 100-500V) positive pulse is used to rapidly dissipate the positive charge generated on the resistive film, which can be done with a bipolar pulse generator. The workpiece or coating surface may be bombarded with energetic cations generated by a hollow cathode within the workpiece. Coating complex shaped workpieces (i.e. pipe threads) requires a Hollow Cathode Effect (HCE). Because the "hot" energetic electrons are "captured" by the cathode or caused to oscillate between relatively negative electric fields on the cathode. These "hot" electrons cause increased ionization of the gas molecules (see section 54), which in turn reduces the thickness of the plasma sheath to pull the ions into the (surface) shape of the substrate, resulting in a very conformal film. The energy of the ions can be controlled by the magnitude and pressure of the applied voltage (higher pressure, stronger collisions, lower energy for a given voltage).

Another advantage is that a multi-step process can be used to tailor the quality of the film deposited on the inner surface of the workpiece. The first step of the process may also be to pre-clean the surface of the workpiece by introducing a sputtering gas (e.g., argon) and then lowering the pump pressure to 1X 10-3 Torr or preferably l X10-4 Torr. When a negative DC pulse is applied, contaminants on the inner surface of the workpiece are removed by the sputtering gas. A second step may then be performed to form a subsurface carbon layer on the steel using carbon implantation. This carbon layer can improve adhesion of the DLC. This can be done by increasing the magnitude of the bias voltage above 5 kV. For small diameter tubing care was taken not to allow the size of the plasma sheath to exceed the radius of the tubing. The formula for calculating the minimum radius at which the cylinder does not overlap the plasma sheath is as follows:

where V is the voltage amplitude and n is the plasma density.

After this injection step, a DLC deposition step is performed with the above-described methane, acetylene or toluene precursor gas. This processing step reduces the DC pulse voltage (e.g., 100V-10kV) to provide film deposition rather than implantation. In these coating steps, argon may be mixed with a carbon-containing precursor gas to provide ion bombardment to increase film density. Argon also produces the so-called "Penning" effect "which ionizes neutral gas particles by the energy transferred by the metastable argon atoms, thereby increasing the percent ionization and plasma density. The anode can also be purged with argon, since argon is a non-reactive gas, which helps to keep the coating built up by the anode clean, which may lead to so-called anode disappearance. Uniformity can also be controlled by duty cycle of the DC pulse, when the pulse is "off, source gas and make-up flow through the tubing. One skilled in the art will recognize that uniformity may also be controlled by the selection of gas flow rates and pump speeds.

It will be appreciated that the pre-cleaning step may be omitted where the surface is sufficiently clean to accept the coating process without further cleaning. It is also recognized that the addition of the second gas at step 110 is optional and may be omitted in some deposition processes. It is further recognized that in some cases, dynamic adjustment of the coating parameters of the deposition step 112 may not be required. In any process, one or all of these steps may be omitted. It will also be appreciated that in some instances, if the deposition level of the material applied in a step is acceptable, it may not be necessary to reverse the gas flow direction to complete the deposition step, nor to repeat the process.

The claims (modification according to treaty clause 19)

Statement of amendment according to PCT treaty clause 19

In accordance with PCT treaty item 19, the applicant filed modifications to the claims of this application in light of the delivery notice of international search reports and the written comments of the international search institution.

Claims 1 and 33 have been modified to show more clearly that electrical connections are made to the workpiece by applying a negative bias. Thus, the methods of claims 1 and 33 modify the characteristics of the interior surface of the workpiece in a combination of discrete steps that include the features of gas counter-flow, providing electrical connections, and allowing the workpiece to function as a cathode. The combination of these features distinguishes the claimed invention from the prior art in addressing the way in which the inner surface of the workpiece is altered.

Independent claim 15 has been modified to incorporate the features of the above claim 19. The system (apparatus) of claim 15 is now said to include a cyclic control of the flow control means for controlling the gas flow in the first direction during the first duty cycle and the gas flow in the second direction during the second duty cycle. The original claim 19 is deleted.

While independent claim 1 is generic to the various embodiments illustrated in the present application, independent claim 25 is specific to the embodiment of fig. 2.

Furthermore, the claims 51 are modified to eliminate ambiguity as to their application to other claims.

1. A method for modifying an interior surface of a workpiece, comprising:

applying a negative bias between the workpiece itself and the anode such that the workpiece acts as a cathode by being electrically connected to the workpiece;

passing a gas containing a surface modifying material through the workpiece in a first direction;

reducing the pressure within the workpiece;

establishing a hollow cathode effect within the workpiece;

modifying the inner surface of the workpiece by applying the surface modifying material to the inner surface of the workpiece;

the gas flow through the workpiece is reversed in the next application step.

2. The method of claim 1, wherein the method comprises: controlling pressure within the workpiece during application of the surface modifying material to the surface.

3. The method of claim 1 or 2, comprising providing a source of the gas to an inlet and an outlet of the workpiece.

4. A method according to any of claims 1-3, wherein the pressure is reduced by applying a vacuum pump.

5. The method of any of claims 1-4, wherein the bias voltage is applied between the workpiece and an electrode at an opening in the workpiece.

6. The method of any of claims 1-4, wherein the bias voltage is applied between the workpiece and the electrodes at the first and second openings in the workpiece.

7. The method of any one of claims 1 to 6, wherein the method comprises adjusting one or more of: a supply of gas, a supply of vacuum to the workpiece, a pressure within the workpiece, and a bias voltage applied to the workpiece to maintain a hollow cathode effect during modification of the surface.

8. The method of any one of claims 1 to 7, comprising repeating the surface modification step.

9. A method according to any of claims 1-8, characterized in that the method comprises controlling the process under automatic control.

10. The method of any of claims 1-9, wherein the bias voltage is a negative pulsed DC voltage applied to the workpiece relative to the electrode and comprises a duty cycle comprising "on" and "off phases, wherein a negative voltage is applied to the conductive workpiece during the" on "phase to attract ions to the inner surface and the gas is at least partially replenished during the" off "phase.

11. The method of any of claim 10, wherein the method comprises: reducing the pressure gradient within the workpiece, thereby reducing the amount of gas flow through the workpiece in the "on" state;

increasing the pressure gradient within the workpiece, thereby increasing the amount of airflow through the workpiece in the "off" state.

12. The method of claim 10 or 11, wherein the method comprises: applying a reverse voltage sufficient to deplete the surface charge generated by the coating on the interior surface during the "off" phase.

13. The method of any of claims 1-12, comprising: the gas is provided as a hydrocarbon gas having the surface-modifying material formed therein as diamond-like carbon.

14. The method of any of claims 1-13, comprising the step of thermally activating the gas by heating the workpiece.

15. A system for coating an interior surface of an electrically conductive workpiece having an interior space, comprising:

an anode;

an electrical bias system connected to the electrically conductive workpiece and the anode;

a vacuum source coupled to an interior of the conductive workpiece;

a gas source coupled to the interior of the electrically conductive workpiece for introducing a coating material-containing gas;

a flow control system for causing the gas to flow in a first direction through the workpiece during a first operating cycle and in a second, opposite direction through the workpiece during a second operating cycle; and

a loop control for controlling the flow control system.

16. The system of claim 15, comprising a gas reservoir having an inlet coupled to the gas source, an outlet coupled to the vacuum source, and a workpiece connection opening connected to the workpiece, the flow control system further configured to control a pressure at the inlet and the outlet of the gas reservoir.

17. The system of claim 16, comprising a control system for controlling the biasing system, the vacuum source, and the gas source.

18. The system of claim 16, comprising a control system for controlling the bias system, the vacuum source, and the gas source to generate a plasma within the workpiece and establish a hollow cathode effect therein.

19. The system of claim 15, wherein the cycling control is an adjustable cycling control for varying the cycle.

20. The system of any of claims 15-19, wherein the biasing system comprises a negatively pulsed DC voltage source and comprises a duty cycle having an "on" phase in which a negative voltage is applied to the conductive workpiece and an "off" phase in which a lower or no voltage is applied.

21. The system of any of claims 15-20, further comprising a pressure controller.

22. The system of claim 20, further comprising a pressure controller for reducing a pressure gradient in the workpiece during the "on" phase and increasing the pressure gradient during the "off" phase.

23. The system of any of claims 15-22, comprising a voltage inverter.

24. A method for coating an interior surface of an electrically conductive workpiece having at least one opening, comprising:

providing an electrode within the workpiece;

connecting a voltage bias system between the electrode and the workpiece such that the workpiece is negatively biased with respect to the electrode;

coupling a vacuum source to at least one opening of the electrically conductive workpiece;

inserting a device into the at least one opening of the electrically conductive workpiece, the device having at least one hole for allowing gas to flow into and out of the device;

flowing a gas through the electrically conductive workpiece in a direction from the device to at least one opening of the electrically conductive workpiece to effect a first coating cycle; and

the conductive workpiece is left in place and a gas is flowed counter-currently through the conductive workpiece, from the at least one opening of the conductive workpiece to the device, to effect a second coating cycle.

25. The method of claim 24, further comprising generating a plasma inside the electrically conductive workpiece, the plasma having an intensity that is adjustable by varying the biasing system.

26. The method of claim 24 or claim 25, further comprising repeating the first and second coating cycles.

27. The method of any one of claims 24 to 26 including providing a length adjustable conduit having a plurality of said holes along said adjustable length.

28. The method of claim 27, further comprising varying the adjustable length based on a length and a diameter of the workpiece.

29. A method as claimed in claim 27 or claim 28, comprising varying the number of outlets along the length of the conduit.

30. A method according to any of claims 27-29, characterized in that the device is inserted into a conductive workpiece having only one opening.

31. The method of any of claims 27-30, further comprising connecting the inserted device to the biasing system to act as a cathode.

32. A method for modifying an interior surface of a workpiece having an interior space, comprising:

sealing the interior of the assembly from the external environment;

providing an anode;

a gas inlet provided to the interior and a gas outlet from the interior;

reducing the pressure within the interior and applying a negative bias to establish a hollow cathode effect within the interior by electrically connecting to the workpiece and the anode;

introducing a gas containing a surface modifying material into the interior;

modifying an interior surface of the workpiece by chemical vapor deposition;

the gas flow between the inlet and the outlet is reversed in the next surface modification step.

33. The method of claim 32, wherein the inlet and outlet are located at respective ends of a length of tubing.

34. The method of claim 31 or 32, further comprising a pre-cleaning step, wherein the inner surface is pre-cleaned by introducing a sputtering gas into the interior, reducing the pressure in the interior, and applying a negative DC pulsed voltage between the workpiece and the anode.

35. The method of claim 34, wherein the sputtering gas is argon.

36. The method of any of claims 32-34, further comprising an injection step, wherein the inner surface is injected with an adhesive material prior to modification of the surface.

37. The method of any of claims 32-36, wherein the inner surface is modified with an adhesive material that forms a chemical bond with the substrate and also with a coating deposited on top of the adhesive layer.

38. The method of claim 36, wherein the injecting is performed by applying a bias voltage to cause the coating to penetrate below the surface of the substrate to form a bond between the coating and the substrate.

39. The method of any of claims 32-38, wherein the surface modifying material is selected from the group consisting of metals, ceramics, and diamond-like carbon.

40. The method of any one of claims 32-39, wherein the gas is acetylene.

41. A method according to any one of claims 32 to 40, wherein the gas is selected from acetylene, methane and toluene or mixtures thereof.

42. The method of any of claims 32-37, wherein the gas comprises a hydrocarbon material comprising 1-8 carbon atoms.

43. The method of any of claims 32-42, further comprising adding hydrogen to the modifying gas.

44. The method of any of claims 32-43, further comprising introducing a dopant into the modifying gas.

45. The method of claim 44, wherein the dopant is introduced as a silicon-containing molecule.

46. The method of claim 44, wherein the introduced dopant is tetramethylsilane, hexamethyldisiloxane, trimethylsilane, or a mixture thereof.

47. The method of any of claims 32-43, further comprising adding a metal-containing dopant.

48. The method of claim 47, wherein the metal dopant is selected from titanium, chromium, zirconium, tantalum, or tungsten, or mixtures thereof.

49. The method of any of claims 32-48, further comprising adjusting the surface treatment by varying one or more of: bias voltage, gas flow, and vacuum pressure during processing.

50. The method of any of claims 32-49, further comprising the step of changing the composition of the gas during the treatment.

51. The method of any of claims 32-39, wherein the precursor contains 1 to 4 carbon atoms.

Claims (52)

1. A method for modifying an interior surface of a workpiece, comprising:

applying a negative bias between the workpiece itself and the anode;

flowing a gas containing a surface modifying material through the workpiece in a first direction;

reducing the pressure within the workpiece;

establishing a hollow cathode effect within the workpiece;

modifying the surface of the workpiece by applying the surface modifying material to the inner surface of the workpiece;

the gas is flowed through the workpiece in the reverse direction in the next application step.

2. The method of claim 1, wherein the method comprises: controlling pressure within the workpiece during application of the surface modifying material to the surface.

3. The method of claim 1 or 2, wherein the method comprises: a source of the gas is provided to an inlet and an outlet of the workpiece.

4. A method according to any of claims 1-3, characterized in that the pressure is reduced by applying a vacuum suction pump.

5. The method of any of claims 1-4, wherein the bias voltage is applied between the workpiece and an electrode at an opening in the workpiece.

6. The method of any of claims 1-4, wherein the bias voltage is applied between the workpiece and the electrodes at the first and second openings in the workpiece.

7. The method of any one of claims 1 to 6, wherein the method comprises adjusting one or more of: a supply of gas, a supply of vacuum to the workpiece, a pressure within the workpiece, and a bias voltage applied to the workpiece to maintain a hollow cathode effect during modification of the surface.

8. The method of any one of claims 1 to 7, comprising repeating the surface modification step.

9. A method according to any of claims 1-8, characterized in that the method comprises controlling the process under automatic control.

10. The method of any of claims 1-9, wherein the bias voltage is a negative pulsed DC voltage applied to the workpiece relative to the electrode and comprises a duty cycle comprising "on" and "off phases, wherein a negative voltage is applied to the conductive workpiece during the" on "phase to attract ions to the inner surface and the gas is at least partially replenished during the" off "phase.

11. The method of any of claim 10, wherein the method comprises: reducing the pressure gradient within the workpiece, thereby reducing the amount of gas flow through the workpiece in the "on" state;

increasing the pressure gradient within the workpiece, thereby increasing the amount of airflow through the workpiece in the "off" state.

12. The method of claim 10 or 11, wherein the method comprises: applying a reverse voltage sufficient to deplete the surface charge generated by the coating on the interior surface during the "off" phase.

13. The method of any of claims 1-12, comprising: the gas is provided as a hydrocarbon gas having the surface-modifying material formed therein as diamond-like carbon.

14. The method of any of claims 1-13, comprising the step of thermally activating the gas by heating the workpiece.

15. A system for coating an interior surface of an electrically conductive workpiece having an interior space, comprising:

an anode;

an electrical bias system connected to the electrically conductive workpiece and the anode;

a vacuum source coupled to an interior of the conductive workpiece;

a gas source coupled to the interior of the electrically conductive workpiece for introducing a coating material-containing gas;

a flow control system for causing said gas to flow in a first direction through said workpiece during a first operating cycle and in a second, opposite direction through said workpiece during a second operating cycle.

16. The system of claim 15, comprising a gas reservoir having an inlet coupled to the gas source, an outlet coupled to the vacuum source, and a workpiece connection opening connected to the workpiece, the flow control system further configured to control a pressure at the inlet and the outlet of the gas reservoir.

17. The system of claim 16, comprising a control system for controlling the biasing system, the vacuum source, and the gas source.

18. The system of claim 16, comprising a control system for controlling the bias system, the vacuum source, and the gas source to generate a plasma within the workpiece and establish a hollow cathode effect therein.

19. A system according to any of claims 15-18, comprising a loop control for controlling the flow control means.

20. The system of claim 19, wherein the cycling control is an adjustable cycling control for varying the cycle.

21. The system of any of claims 15-20, wherein the biasing system comprises a negatively pulsed DC voltage source and comprises a duty cycle having an "on" phase in which a negative voltage is applied to the conductive workpiece and an "off" phase in which a lower or no voltage is applied.

22. The system of any of claims 15-21, further comprising a pressure controller.

23. The system of claim 21, further comprising a pressure controller for reducing a pressure gradient in the workpiece during the "on" phase and increasing the pressure gradient during the "off" phase.

24. The system of any of claims 15-23, comprising a voltage inverter.

25. A method for coating an interior surface of an electrically conductive workpiece having at least one opening, comprising:

providing an electrode within the workpiece;

connecting a voltage bias system between the electrode and the workpiece such that the workpiece is negatively biased with respect to the electrode;

coupling a vacuum source to at least one opening of the electrically conductive workpiece;

inserting a device into the at least one opening of the electrically conductive workpiece, the device having at least one hole for allowing gas to flow into and out of the device;

flowing a gas through the electrically conductive workpiece in a direction from the device to at least one opening of the electrically conductive workpiece to effect a first coating cycle; and

the conductive workpiece is left in place and a gas is flowed counter-currently through the conductive workpiece, from the at least one opening of the conductive workpiece to the device, to effect a second coating cycle.

26. The method of claim 25, further comprising generating a plasma inside the conductive workpiece, the plasma having an intensity that is adjustable by varying the bias system.

27. The method of claim 25 or claim 26, further comprising repeating the first and second coating cycles.

28. The method of any one of claims 25 to 27 including providing a length adjustable conduit having a plurality of said holes along said adjustable length.

29. The method of claim 28, further comprising varying the adjustable length based on a length and a diameter of the workpiece.

30. A method as claimed in claim 28 or claim 29, comprising varying the number of outlets along the length of the conduit.

31. A method according to any of claims 28-30, characterized in that the device is inserted into a conductive workpiece having only one opening.

32. The method of any of claims 28-31, further comprising connecting the inserted device to the biasing system to act as a cathode.

33. A method for modifying an interior surface of a workpiece having an interior space, comprising:

sealing the interior of the assembly from the external environment;

providing an anode;

a gas inlet provided to the interior and a gas outlet from the interior;

reducing the pressure in the interior and applying a negative bias between the workpiece and the anode to establish a hollow cathode effect within the interior;

introducing a gas containing a surface modifying material into the interior;

modifying an interior surface of the workpiece by chemical vapor deposition;

the gas flow between the inlet and the outlet is reversed in the next surface modification step.

34. The method of claim 33, wherein the inlet and outlet are located at respective ends of a length of tubing.

35. The method of claim 32 or 33, further comprising a pre-cleaning step, wherein the inner surface is pre-cleaned by introducing a sputtering gas into the interior, reducing the pressure in the interior, and applying a negative DC pulsed voltage between the workpiece and the anode.

36. The method of claim 35, wherein the sputtering gas is argon.

37. The method of any of claims 33-35, further comprising an injection step, wherein the inner surface is injected with an adhesive material prior to modification of the surface.

38. The method of any of claims 33-37, wherein the inner surface is modified with an adhesive material that forms a chemical bond with the substrate and also with a coating deposited on top of the adhesive layer.

39. The method of claim 37, wherein the injecting is performed by applying a bias to cause the coating to penetrate below the surface of the substrate to form a bond between the coating and the substrate.

40. The method of any one of claims 33-39, wherein the surface modifying material is selected from the group consisting of metals, ceramics, and diamond-like carbon.

41. The method of any one of claims 33-40, wherein the gas is acetylene.

42. The method of any one of claims 33 to 41, wherein the gas is selected from acetylene, methane and toluene or mixtures thereof.

43. The method of any of claims 33-38, wherein the gas comprises a hydrocarbon material comprising 1-8 carbon atoms.

44. The method of any of claims 33-43, further comprising adding hydrogen to the modifying gas.

45. The method of any of claims 33-44, further comprising introducing a dopant into the modifying gas.

46. The method of claim 45, wherein the dopant is introduced as a silicon-containing molecule.

47. The method of claim 45, wherein the introduced dopant is tetramethylsilane, hexamethyldisiloxane, trimethylsilane, or a mixture thereof.

48. The method of any of claims 33-44, further comprising adding a metal-containing dopant.

49. The method of claim 48, wherein the metal dopant is selected from titanium, chromium, zirconium, tantalum, or tungsten, or mixtures thereof.

50. The method of any of claims 33-49, further comprising adjusting the surface treatment by varying one or more of: bias voltage, gas flow, and vacuum pressure during processing.

51. The method of any preceding claim, further comprising the step of varying the composition of the gas during the treatment.

52. The method of any of claims 33-40, wherein the precursor contains 1-4 carbon atoms.