Process and apparatus for the surface treatment of workpieces by glow discharge

For the glow discharge between a receptacle and a workpiece, the glow discharge path is connected in series with a first switch. A capacitor circuit is charged via the glow discharge path and two diodes. By closing a switch, the previously positive pole of the capacitor circuit is set to the zero potential, whereby the potential of the other pole is shifted negative. By closing a switch, the capacitor circuit is discharged through the glow discharge path while the first switch is closed. Upon the discharging of the capacitor circuit, the supply voltage continues to be present at the glow discharge path for the remaining pulse time. In the initial range, the voltage pulses have a pulse peak for ignition followed by a maintenance range, the height of which corresponds to the supply voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for surface treatment of workpieces by 
glow discharge by providing voltage pulses for igniting and maintaining a 
glow discharge between a workpiece disposed in a receptacle and a 
counterelectrode. 
2. Prior Art 
It has been known to treat workpiece surfaces by glow discharge, thus 
exposing, for instance, metallic workpieces to surface transformation by 
nitration, etc. It is also possible to use glow discharge for coating, 
tempering, annealing, etc. Usually, the workpiece is used as the cathode 
and the walls of a vacuum vessel surrounding the workpiece as a 
counterelectrode, these electrodes being connected to a voltage source of 
some 100 to 1000 volts during the glow discharge, which extends over the 
total workpiece surface to be treated. The so-called active species which 
result in the desired surface modification are created thereby. The glow 
discharge is normally fed by direct current. 
On the one hand, the voltage for glow discharge must be high enough to 
cover the total workpiece surface to be treated (complete glowing), while 
it must not be so high as to cause an arc discharge, as this would involve 
the risk of damage to the workpiece. If direct current voltage is 
continuously supplied for the glow discharge, it will be difficult to 
maintain the glow conditions. As a rule, use is made of a rapid 
disconnecting device which, in case of an arc discharge, will cause a 
short circuit to protect the workpiece accordingly. If direct current is 
used to feed the discharge, the temperature existing at the workpiece is 
particularly dependent upon voltage, pressure and gas type. This is a 
basic disadvantage because pressure and temperature, above all, should be 
decoupled for reasons relating to the process technique. On the other 
hand, workpiece temperature has an influence on the transition from glow 
discharge to arc discharge. 
It has been known to feed the glow discharge with square-topped pulses 
having a variable pulse duty factor in order to decouple the process 
parameters and avoid the risk of arc discharges. If the pulse intervals 
are too long, reignition problems will occur, the pulse voltage sufficient 
to maintain an ignited glow discharge being sometimes not capable of 
causing the repeated ignition. Moreover, in the known processes, switching 
losses are high because the connecting and disconnecting operations are 
taking place during current flow. Therefore, the pulse processes have not 
been successful to date. 
SUMMARY OF THE INVENTION 
It is the object of the invention to provide a process of the type 
mentioned above by which arc discharges and workpiece damage are avoided, 
on the one hand, while reignition problems do not occur on the other hand. 
At the same time, the process is well controllable with a high degree of 
efficiency. 
To solve the problems referred to, the invention provides that, in the 
initial range, each voltage pulse has a pulse peak for igniting the glow 
discharge while subsequently the voltage pulse has a range of amplitudes 
suitable for maintaining the glow discharge. 
The initial range of each pulse, according to the process of the invention, 
displays a pulse peak that is higher than the ignition voltage of the glow 
discharge. After having been ignited with this short-time pulse peak, the 
glow discharge is maintained during the voltage pulse by a voltage having 
a lower amplitude. Arc discharges are avoided by said lower voltage being 
beneath the ignition voltage. The process of the invention permits the 
realization of varying pulse ratios, it being possible to select very long 
pulse intervals without reignition problems. Thus, suitable pulse voltages 
and pulse duty ratios can be selected for each type of treatment and 
workpiece temperature. The switching is mostly currentless during the on 
and off operation. 
According to a preferred embodiment of the invention, the voltage pulses 
contain a constant low amplitude section prior to the pulse peak, the 
voltage pulses not starting directly with the increased pulse peak. The 
increased pulse peak is generated after a certain delay after the charging 
current has decayed in preparation therefor. By this means, switching 
losses are reduced to quite a small amount. 
The pulse duty ratio of voltage pulses to pulse intervals is preferably 
very small and on the order of 1:10 to 1:500. 
An apparatus for performing the process of the invention is characterized 
in that the glow discharge path between a supply voltage is provided to 
the workpiece and counterelectrode in series with a pulsecontrolled first 
switch, that a capacitor circuit chargeable via the glow discharge path is 
placed so that one pole is at a fixed reference potential when the first 
switch is closed and its other pole takes a potential situated outside the 
range of the supply voltage, and that the capacitor circuit is 
dischargeable via the glow discharge path through a second switch when the 
second switch is closed. As a result, after termination of a voltage 
pulse, the capacitor circuit is charged via the glow discharge path. 
Current is taken from the capacitor circuit through the first switch so 
that the losses caused by disconnection are very low. 
When the first switch is closed, a change of potential occurs at the 
capacitor circuit in that one pole of the capacitor is connected to one 
pole of the supply voltage, the capacitor voltage being added to the 
supply voltage. 
The capacitor circuit is subsequently discharged by closing the second 
switch at a certain time interval after closing of the first switch, the 
workpiece being temporarily exposed to a voltage higher than the supply 
voltage by the amount of the charging voltage of the capacitor circuit. 
Upon discharge of the capacitor circuit, the second switch can be opened 
again while the glow discharge is maintained by the supply voltage applied 
to the glow discharge path. The voltage pulse is terminated by opening the 
first switch. 
In a very simple design, the capacitor circuit consists of one capacitor or 
of a plurality of capacitors connected in parallel. Upon closing the first 
switch, this capacitor circuit is charged to the full supply voltage. In 
other words, the amplitude of the pulse peak is nearly twice as high as 
the supply voltage. However, such a high pulse peak is undesirable in some 
cases. To generate lower pulse peaks, a preferred further embodiment of 
the invention comprises a capacitor circuit having a plurality of 
capacitors which are so coupled via diodes that they are connected in 
series during charging and in parallel upon the closing of the first and 
second switches when they are discharged through the glow discharge path. 
If the individual capacitors are similar to one another, each capacitor of 
the capacitor circuit is charged to the value U.sub.V /N, where U.sub.V is 
the supply voltage and N is the number of capacitors. The pulse peak above 
the supply voltage is smaller by a factor 1/N than the supply voltage. It 
is possible accordingly to influence the size of the pulse peak by the 
number of capacitors. 
In case of short pulse intervals, the capacitor circuit is not fully 
charged via the glow discharge path until the next pulse is ignited. 
Therefore, the amplitude of the pulse peak of the next pulse is 
approximately proportional to the time of the pulse interval. This is a 
desirable effect because with voltage pulses of a short-pulse interval, 
peak pulses required for the ignition of the glow discharge are less than 
those required with long pulse intervals. 
With the method of the invention, novel plasma processes are feasible 
which, however, call for treating vessels other than those used 
previously. In fact, the conventional receptacles cannot be insulated 
thermally, or can be so-insulated only to a negligible extent. This is 
because, for physical reasons, a minimum power input, subject to the type 
of gas and pressure, is required per workpiece area for the glow coating 
necessary for the treatment. In some plasma processes, condensates 
occurring at the cold surfaces may inhibit the provided treatment. 
According to a preferred embodiment of the invention, the receptacle 
consists of a closed vessel provided with a heating means and surrounded 
at variable radial distance by a heat-insulating jacket so that the space 
between the vessel and the jacket can be traversed by a cooling means. By 
conveniently selecting the flow of the cooling means and the heating, the 
wall of the vessel may be kept at the temperature required for the 
process. Preferably, the jacket is divided into several segments quickly 
removable so that, upon termination of the process, a rapid cooling of the 
workpiece is possible. 
Some embodiments of the invention will be explained hereunder with 
reference to the drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS 
According to FIG. 1, a receptacle R is provided in which the workpiece W to 
be treated is connected to an electrode 10 while the receptacle R itself 
forms the counterelectrode, the space between the workpiece W and the 
receptacle R forming the glow discharge path 11. 
The counterelectrode is connected to the positive pole of the supply 
voltage U.sub.V and to one end of a resistor R1, the other end of which is 
connected to the electrode 10 which is connected to the anode of a diode 
D1 whose cathode is connected via a switch S and a current sensor 12 to 
the negative pole of the supply voltage U.sub.V. 
From the connecting point A between the cathode of the diode D1 and the 
switch S2, a series circuit comprising a capacitor C1 and a diode D2 
extends to the negative pole of the supply voltage U.sub.V. The connecting 
point between the capacitor C1 and the anode of the diode D2 is designated 
as point B. 
From point B, a conduit extends via the second switch S2 and a resistor R2 
to the electrode 10 or to point C. 
To simplify the illustration, switches S1 and S2 are of the mechanical 
type. However, it is more convenient to use electronic switches, for 
example, transistors or thyristors. 
Switch S1 is controlled by a timed pulse generator 13 while switch S2 is 
controlled by a control circuit 14 responsive to the current of the 
current sensor 12, to the voltage at C1, and the time. 
FIG. 2 illustrates the voltages U.sub.A, U.sub.B and U.sub.C at points A, B 
and C of the circuit of FIG. 1. In addition, voltage U.sub.G realized at 
the glow discharge path 11 is shown. Upon connection of the supply voltage 
U.sub.V, the capacitor C1 is charged through resistor R1, diode D1 and 
diode D2. As a result, voltages U.sub.A and U.sub.C are developed at 
points A and C, respectively. In FIG. 2, the minus potential of the supply 
voltage U.sub.V is considered as a zero potential. The lower plate of 
capacitor C1 being by nearly the factor U.sub.V more negative than the 
upper plate, a potential more negative than the zero potential by this 
factor is formed at point B. At this moment (S1 on), the voltage U.sub.A 
goes to zero. 
The control circuit 14 contains a delay circuit which, after some short 
time when the current through the current sensor 11 has exceeded a 
threshold value, becomes responsive and closes switch S2 (S2 on). As a 
result, the capacitor C1 is discharged via the closed switch S2, the 
resistor R2 and the glow discharge path 11. Moreover, in accordance with 
an e-function, the voltage U.sub.B is slowly reduced to zero, while 
voltage U.sub.C is suddenly negative to subsequently slowly rise to zero. 
After a predetermined time period, the control circuit 14 again opens 
switch S2 (S2 off), thus placing point C at zero potential so that the 
supply voltage U.sub.V is again present at the glow discharge path 11. 
If the switch S1 is then opened again by the pulse generator 13 (S1 off), 
the capacitor C1 is recharged via the glow discharge path 11, the 
parallel-connected resistor R1 and the diode D1. However, the resistance 
of the glow discharge path is considerably less than the value of resistor 
R1 thus permitting a charging that is quicker than the initial charging 
upon the connection of the supply voltage U.sub.V. 
The curve of the voltage pulse 15 occurring at the glow discharge path 11 
is shown in the last line of FIG. 2. First, the voltage pulse 15 displays 
a stepped range 16 in which the value of the supply voltage U.sub.V is 
reached. The time T.sub.V of range 16 is predetermined by the control 
device 14. Range 16 is followed by the pulse peak 17 having a peak value 
corresponding to the ignition voltage U.sub.Z nearly twice as high as the 
supply voltage U.sub.V. 
From the short time pulse peak 17, the voltage drops according to an 
e-function to the range 18 which maintains the glow discharge. The range 
18 corresponds to the value of U.sub.V. The voltage pulse is terminated 
with the opening of switch S1. 
The embodiment of FIG. 3 is different only from that of FIG. 1 in that the 
sole capacitor C1 is replaced by the capacitor circuit 19 consisting of 
two capacitors C1.sub.N and C2.sub.N which are coupled by diodes, one pole 
of capacitor C1.sub.N being connected to point A, the other pole being 
connected to the anode of diode D5.sub.N. The series circuit of the 
capacitor C1.sub.N and the diode D5.sub.N are bridged by diode D3.sub.N, 
the anode of which is connected to the cathode of diode D5.sub.N and the 
cathode of which is connected to point A. The cathode of diode D5.sub.N is 
connected to one pole of capacitor C2.sub.N whose other pole is joined to 
the anode of diode D2. The anode of diode D2 is connected to the anode of 
a diode D4.sub.N whose cathode is connected to the anode of diode D5.sub.N 
and to switch S2. 
Prior to the closing of switch S1, the capacitors C1.sub.N and C2.sub.N 
form a series connection, both capacitors being equal to one another. Each 
is charged to half the supply voltage U.sub.V /2 by the diodes D5.sub.N 
and D2 operated in forward direction. Upon the closing of the switches S1 
and S2, the diodes D5.sub.N and D2 are operated in blocking direction, 
while diodes D3.sub.N and D4.sub.N are operated in a forward direction. 
Accordingly, the capacitors C1.sub.N and C2.sub.N are operated in a 
parallel circuit so as to be discharged. As a result, the voltage peak 17 
shown in FIG. 3 is only half as high (above the voltage level U.sub.V) as 
that in the embodiment of FIG. 1. The circuit of FIG. 3 is selected if the 
desired value of the voltage peak 17 is to be lower. In place of two 
capacitors C1.sub.N and C2.sub.N use can be made also of three or more 
capacitors which are connected in series for charging and in parallel for 
discharging. If so, the height of the voltage peak would be reduced even 
more than that. The treating vessel shown in FIGS. 4 and 5 is used in 
association with the circuit of FIG. 1 or FIG. 3. It comprises a tubular 
receptacle R for arranging the workpiece W in contact with line 10. 
Receptacle R, having a thermally insulated bottom 20, is closed with a 
cover 21, also thermally insulated. A heating coil 22 is wound about the 
receptacle R so as to obtain by externally applied heat a specific 
temperature at the workpiece W, the energy required for this effect being 
applied by the heating coil 22, i.e., by external separate heating. 
The receptable R is enclosed at a variable radial distance designated "A" 
by a heat-insulating jacket 23 which is open at the top. A cooling means, 
e.g., air, is absorbed downwardly through the annular space 24 between the 
receptacle R and the jacket 23. The distance between the insulating jacket 
and the container wall is variable so that it can be adjusted to obtain 
the desired annular gap width. During heating, the insulation adjoins the 
container. With forced cooling, the cooling medium is absorbed at high 
speed through the annular gap, the absorption being achieved by an annular 
channel 25 arranged at the lower end of the treating vessel and 
communicating with a blower 26 that causes a downward cooling air flow 
(arrow 27) in the annular space 24. Due to said cooling air flow, a 
convection flow (arrow 28) produced inside the receptacle by heat exchange 
through the wall, is also directed downwardly. The gas contained inside 
the receptacle R flows downwardly along the outer wall and upwardly along 
the workpiece which is uniformly exposed to thermal action. 
To permit a quick cooling of the workpiece W upon termination of the glow 
discharge treatment, the annular jacket 23 is divided into several 
segments 231, 232, 233 between which the sealing elements 29 are disposed. 
Said segments may be moved or swivelled individually in radial direction 
to the outside so that the receptacle R will be freely exposed to quickly 
cool under the influence of outside air or by blower cooling.