GB2336603A - A method and apparatus for plasma boronising - Google Patents

Abstract

The method comprises injecting a boron containing gas into a plasma chamber 10 containing a workpiece, heating the workpiece to a temperature between 650-1050‹C while maintaining the pressure in the range 2-10 mbar. The boron containing gas may be boron trifluoride or boron trichloride, optionally mixed with at least one other gas (e.g. hydrogen and/or argon). Apparatus suitable for performing this method comprise an anode 23 and a cathode (e.g. the workpiece on the workpiece support 28) between which a glow discharge occurs which heats the workpiece. To reduce the heat up period and improve the temperature uniformity of the workpiece, it is surrounded by heating elements 16. The workpiece support 28 and/or the heating elements 16 may be connected to a current lead 12 and 14 which have means (e.g. cap 60, Fig. 2; spacer 104, Fig. 3) for spacing insulation (e.g. 52, 54 and 56; Fig 2 and 98, 100 and 102, Fig. 3) evenly around its conducting part (48, Fig. 2 and 96, Fig. 3).

Description

2336603 IMPROVEMENTS IN AND RELATING TO PLASMA PROCESSING This invention

is concerned with improvements in and relating to plasma processing of metals such as iron, steel, titanium and aluminium. Such plasma processing includes plasma nitriding. plasma nitrocarburising, plasma sulphonitrocarburi sing, plasma carbonitriding, plasma carburising and plasma boronising. This invention is particularly concerned with a method of, and apparatus for, plasma boronising.

Within the periodic classification the elements of the 15 series B, C and N have similar potential for the hardening of iron and steel. There are a number of similarities since they all form intermetallic components such as FeC, FeN and FB, and they can affect the austenite to ferrite transformation kinetic and introduce interstitial hardening effects.

Commercially, carbon and nitrogen are extensively exploited in the thermo chemical hardening of iron and steel. Boron in comparison, although producing higher hardness levels, has only been used as a last resort and where the cost and complexity of existing processes can be justified.

It is an object of the present invention to provide apparatus for plasma processing and a method of, and apparatus for, plasma boronising.

The present invention is a method of plasma boronising, the method comprising injecting a boron containing gas into a plasma chamber containing a workpiece, and heating the workpiece to a temperature in the range 650-1050'C while maintaining the pressure in the chamber in the range 2-10mbar.

Preferably, the temperature is in the range 750-950C.

The pressure may be in the range 4-7mbar.

The boron containing gas may be boron trifluoride.

The boron containing gas may be mixed with at least one other gas.

The present invention is also apparatus for plasma boronising comprising a plasma chamber, means for injecting gas into the chamber, means for maintaining in the chamber a temperature in the range 650-1050C, and means for maintaining the pressure in the chamber in the range 2-10mbar.

The means for maintaining the temperature may comprise a Power source connected with an anode and a cathode to establish a glow discharge between the anode and the cathode.

Preferably, an active screen is provided surrounding the workpiece.

The present invention also provides apparatus for the plasma processing of metals, the apparatus comprising a plasma chamber, a support in the chamber for a workpiece, and a current lead for passing current into the chamber, the current lead including a conductor, insulation around the conductor and means for spacing the insulation evenly around the conductor.

The conductor may be connected to the workpiece support.

The conductor may be connected to a heating screen.

The heating screen may be in the form of a cage.

The heating screen may have heating elements having a U10 shaped cross section.

1 An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Fig.1 is a partially sectioned elevation of apparatus according to the present invention of apparatus for plasma processing, in particular plasma boronising; Fig.2 is a partially sectioned elevation of a part of Fig.1, the main current connector; is a partially sectioned elevation of another part of Fig.1, the current connector for the active screen; Figs.4 and 5 are respectively elevation and plan views of the active screen of Fig.1; Fig.6 is an elevation of a modified heating element of the active screen; Fig.7 is a secti on on the line 7-7 in Fig.6; and Fig.8 is a partially sectioned view of a further part of Fig.1, the thermocouple lead-in.

Fig. 3 Referring now to Fig.1, apparatus according to the present invention for plasma processing, in particular plasma boronising, comprises a plasma chamber 10 having a main current lead 12, a 1 secondary current lead 14, an active screen 16, a thermocouple lead-in 18 and a gas inlet 20 leading to a gas distributer 22 inside the chamber. A gas outlet is also provided, but is not visible in the drawings.

- The chamber 10 is enclosed by a domed shell 23 mounted on a base consisting of members 24 and 26 which, together with the shell 23, are water cooled. The main current lead 12 is mounted in the base member 26, extends axially into the chamber 10 and terminates at a workpiece support table 28. Horizontal annular thermal shields 30 surround the current lead 12 below the table 28 and are mounted on insulators 32 on the base member 24.

The domed shell 23 may be separated from the base to allow workpieces to be placed in and retrieved from the chamber, but in use the chamber is gas tight to permit the low pressures necessary for plasma generation.

The base member 24 also supports the gas inlet 20 with its associated gas distributer 22 and the thermocouple lead-in 18 with its associated thermocouple 34 on the table 28. The gas distributer 22 consists of perforated gas pipes for distributing gas evenly throughout the chamber.

At the top 9f the shell 23 is mounted the secondary current lead 14 which supports, at its bottom end, the active screen 16 and, above the screen 16, three thermal shields 36 which are circular in cross section and from the lead 14 extend out to the wall of the shell 23 and then down to below the level of the workpiece table 28.

Referring now to Fig.2, the main current lead 12 comprises a connector 44 for a supply cable, the connector being screwed into the base 46 of a main shaft or conductor 48 which then extends upwardly to support the workpiece table 28 on a nut 50. The main shaft 48 is electrically insulated by three annular members 52, 54 and 56, the members 52 and 54 insulating the base 46 of the shaft while the third member 56, a ceramic tube, insulates the main length of the shaft 48. At the top of the tube 56 is a cap 60 which is supported on a shoulder 62 on the shaft 48 and embraces the end of the tube 56 to ensure that the tube 56 is evenly spaced about the shaft 48 throughout its length.

At the bottom end of the ceramic tube 56, and above the insulating member 54 is a mounting member 66 which has a shoulder 68 abutting the outer surface of the base member 26 and a threaded boss 70 which extends through a hole in the base member. Above the base member 26 are provided a washer 72 and a nut 74 which is screwed down on the threaded boss 70 to secure the mounting member 66 in position.

The lower periphery of the mounting member 66 is also threaded at 76 and an end cap 78 is there screwed onto the mounting member 66, the cap 78 having an internal flange 80 which, through a ball race 82, maintains the insulating member 52, the base 46 of the shaft and the members 54, 56 and 66 in their assembled positions as illustrated. The ball race isolates the member 52 f rom any torque as the cap is screwed onto the mounting member. Finally an end cover 86 is mounted-on the cap 78 to guard the connector 44.

Annular seals 88 are provided to ensure that there is no gas leakage past the lead 12, and a rubber ring 90 is provided to prevent the appearance of an electrical discharge between the shaft 48 and the mounting member 66.

The secondary current lead 14 shown in Fig.3 is essentially the same as the primary current lead 12, and has a connecter 92 engaging the base 94 of a main shaft 96, three insulating members 98, 100 and 102, a spacer 104 at the lower end of the shaft to maintain the tubular insulator 102 evenly spaced around the shaft 96# a mounting member 106 having an extension 108 for passing through a hole in a boss 110 (Fig.1) at the top of the shell 23, and an end cap 112 screwed onto the mounting member 106. At the bottom of the shaft is an annular recess 114. The bottom of the extension 108 is threaded for cooperation with a nut (not illustrated) securing the lead 14 in the boss 110. Annular seals are again provided to seal against gas leakage, and an end cover, similar to the end cover 86, may be provided.

The active screen 16 shown in Figs. 4 and 5 is in the form of a circular cage surrounding the workpiece table. The screen 16 has at its top a pair of semirings 120 by means of which the screen is secured to the annular recess 114 at the bottom of the secondary current lead. From the semirings 120 heating elements 122 incline down and out to a first support ring 124 and thence down to a second support ring 126 which, as can be seen f rom Fig.1 is at or just below the level of the workpiece table 28.

The heating elements 122 of this embodiment are,-simply flat conductive strips, but they could be in the form shown in Figs.

6 and 7, i.e. of U shaped cross section, which can use the hollow cathode effect and enable temperatures of 900-1000 - C to be achieved easily.

The thermocouple lead-in 18 is shown in Fig.8 and consists of a housing 130 which is externally threaded at 131 to be secured in a hole in the base member 24 by a pair of nuts which are not illustrated in Fig.3. Insulators 132, 134 and 136 are located inside the housing 130 and sealing rings 138 and 140 are located between the insulator 134 and the housing 130 and between the two insulators 134 and 136.

The whole lead-in assembly is secured together by a washer 144 engaged by a nut 146 screwed into the bottom of the housing 130. The thermocouple connecting cable 148 passes through a central passage, and as the nut 146 is tightened the sealing ring 140 is compressed and seals against the periphery of the cable 148 while the other sealing r:Lng 138 is compressed and seals between the housing 130 and the insulator 132. The thermocouple 34 is located in a ceramic sheath in a special recess in a member 152 on the workpiece table 28.

Working gas is supplied to the chamber through the gas inlet and gas distributer from a gas supply unit which ensures that the desired mixture of gases is supplied to the chamber at the desired mass flow rate and pressure. The gas supply unit includes an active digital display for a Barocel gauge head which indicates the treatment pressure in the chamber independently of the type of gas used.

Switches for rotary pump, isolation valve, Pirani gauge and mass flow controllers are mounted on a panel where the pressur e in a mix tank can also be read. The mix tank is supplied with hydrogen, nitrogen and argon through respective mass flow controllers, while a boron containing gas, such as boron trifluoride or boron trichloride is added, through its own mass flow controller, to the mixture coming from the mix tank before 5 the gas inlet 20.

A source of electrical power is connected to establish the shell 23 as the anode and, in normal operation, the workpiece as the cathode so that at sub-atmospheric pressures a glow discharge is established which heats the workpiece the temperature of which is monitored by the thermocouple.

The active screen is used to reduce the heating up period, and thus the productivity, and to improve the temperature uniformity of the workpieces when workpieces with different geometry are being treated, and when the active screen is in use it is connected through the secondary lead directly to the negative terminal of the power supply, the main current lead then being connected to the power supply negative through a resistor In use, the workpiece(s) are first cleaned to be free of any rust,, oxides, paint, wax, grease or other contaminant and are then located on the table 28. The chamber is then closedf evacuated and purged with the working gas before the power supply is switched on. The workpiece load is gradually heated up to the treatment temperature and this temperature is kept constant for the duration of the treatment. During the heating up period the pressure is also gradually increased to the desired value.

When the treatment is finished the power is switched off and the load is allowed to cool down to a temperature at which the chamber can be unloaded.

The working gas is a mixture of a boron containing gas such as boron trifluoride or boron trichloride and at least one other gas such as hydrogen or argon. The process temperature is in the range 650-1050C, preferably in the range 750-950'C, and the process pressure is in the range 2-10mbar, preferably in the range 4-7mbar.

Examnle Treatment parameters:- Plasma boronising of a plain carbon steel sample.

Temp. 900C Pressure 5.2 - 5.9 mbar Duration 2 hours Gas composition: 100% Ar during heating to 375'C with active screen, thereafter 47% H,, + 47% Ar + 6% BF, As a result of this treatment a boronised layer was produced at the surface of the sample for a depth of about 100pm, giving a microhardness in the range 1500 - 1800 HVO.02

Claims (16)

1. A method of plasma boronising, the method comprising injecting a boron containing gas into a plasma chamber containing a workpiece, and heating the workpiece to a temperature in the range 650-1050'C while maintaining the pressure in the chamber in the range 2-10mbar.

2. A method as claimed in claim 1, in which the temperature is in the range 750-9500C. -

3. A method as claimed in claim 1 or claim 2, in which the pressure is in the range 4-7mbar.

4. A method as claimed in any preceding claim, in which the boron containing gas is boron trifluoride.

5. A method as claimed in any preceding claim, in which the boron containing gas may be mixed with at least one other gas.

6. Apparatus for plasma boronising comprising a plasma chamber, means for injecting gas into the chamber, means for maintaining in the chamber a temperature in the range 650-1050'C, and means for maintaining the pressure in the chamber in the range 210mbar.

7. Apparatus as claimed in claim 6, in which the means for maintaining the temperature may comprise a power source connected with an anode and a cathode to establish a glow discharge between the anode and the cathode.

8. Apparatus as claimed in claim 6 or claim 7, in which an active screen is provided surrounding the workpiece.

9. Apparatus for the plasma processing of metals, the apparatus comprising a plasma chamber, a support in the chamber for a workpiece, and a current lead for passing current into the chamber, the current lead including a conductor, insulation around the conductor and means for spacing the insulation evenly 15 around the conductor.

10. Apparatus as claimed in claim 9, in which the conductor is connected to the workpiece support.

11. Apparatus as claimed in claim 8 or claim 9, in which the conductor is connected to a heating screen.

12. Apparatus as claimed in claim 11, in which the heating screen is in the form of a cage.

13. Apparatus as claimed in claim 11 or claim 12, in which The heating screen has heating elements having a U-shaped cross section.

14. A method of plasma boronising substantially as hereinbefore described with reference to the accompanying drawings.

15. Apparatus for plasma boronising substantially as hereinbef ore described with ref erence to and as shown in the accompanying drawings.

16. Apparatus f or the plasma processing of metals substantially as hereinbefore described with reference to and as shown in the 10 accompanying drawings.