Coating processes – Measuring – testing – or indicating
Reexamination Certificate
2000-06-15
2004-08-31
Padgett, Marianne (Department: 1762)
Coating processes
Measuring, testing, or indicating
C427S569000, C427S573000, C427S576000
Reexamination Certificate
active
06783794
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to methods and arrangements for producing a boride layer on a surface by plasma boronizing in which a gas mixture containing a boron-releasing gas is supplied to a reactor in which a glow discharge is generated.
The boronizing process, which belongs to the group of thermochemical methods of treatment, makes it possible to produce wear-resistant surface layers, preferably on metal components, which provide excellent protection against high abrasive and adhesive wear stresses. Until now, industrial boronizing processes have frequently used solid boron-releasing media in the form of, for example, powders or pastes. However, such processes have a number of drawbacks which limit the production of borides to certain applications for which no alternative treatments that would provide a comparable wear resistance exist. These drawbacks include, for example, the high cost of manual handling of the materials used. In this regard, the component to be boronized must be packed in boron-releasing powder or the boronizing paste must be spread on the component, and the residual boronizing agent must be removed after completion of the boronizing. For ecological reasons, all residues of boronizing agent must be disposed of at suitable waste-disposal dumps. Frequently, the prior-art methods cannot be adequately controlled or cannot be controlled at all and automation of such processes is impossible.
For this reason, various methods have been developed for producing a boride layer on a surface by plasma boronizing in which a gas mixture containing a boron-releasing gas is supplied to a reactor and a glow discharge is generated within the reactor. Such a process is described, for example, in German Offenlegungsschrift No. 196 02 639. That publication discusses the problem of plasma boronizing of metal surfaces, for example, in which the resulting layers have a substantial porosity. Such porosity has a negative effect on the wear resistance of the boronized surface. Moreover, the plasma boronizing method as described in the aforementioned publication cannot be developed for industrial series applications.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method and arrangement for plasma boronizing which overcomes disadvantages of the prior art.
Another object of the invention is to provide a plasma boronizing method which produces substantially pore-free boronized surfaces in a reliable manner and is therefore suitable for industrial series applications.
These and other objects of the invention are attained by providing a boronizing method in which a gas mixture containing a boron-releasing gas is supplied to a reactor in which the gas releases boron in a glow discharge plasma and a glow discharge parameter is controlled to maintain the amount of at least one glow discharge product of the boron-releasing gas, or the relation of that amount to the amount of another glow discharge product, within selected maximum and minimum limits, or in which the glow discharge pulse ratio is maintained above a selected level, or the glow discharge pulse duration is maintained below a maximum value.
The invention further provides an arrangement for producing a boride layer on a surface by plasma boronizing including a reactor having a treatment chamber to which a gas mixture containing a boron-releasing gas is supplied and in which a glow discharge is generated by a DC voltage having a variable pulse width or pulse pause. This arrangement is suitable for carrying out the method of the invention in accordance with the abovementioned parameters, and will be described in detail hereinafter.
First, various alternatives for carrying out the method according to the invention will be described in greater detail. It has now been established by extensive testing that it is important in plasma boronizing to properly select the production parameters of the plasma generated in the treatment chamber of the reactor. It has surprisingly been found that these parameters can be selected advantageously in such a way that an increased proportion of excited boron particles is produced in the plasma. If the plasma contains a relatively large amount of excited boron, boron layers of low porosity will be produced. This fact was demonstrated by optical emission spectroscopy and plasma analysis during development work leading to the method of the invention. If, on the other hand, the plasma contains a high concentration of excited BC
1
particles, highly porous layers are produced which should be avoided for the abovementioned reasons. During the studies which resulted in the invention, the inventors established that various parameters with respect to both plasma generation and the individual components contained in the gas mixture supplies to the reactor can have an effect on the desired concentration of excited boron particles. It is important that, in order to obtain the desired boride layer of low porosity, certain threshold values of excited boron be attained in the plasma.
In accordance with one embodiment of the plasma boronizing method of the invention, a glow discharge is preferably generated with a pulsed DC voltage. In this connection, it was found surprisingly that control of the pulse-duty factor, which is defined as the ratio of the duration of the voltage pulse to the subsequent pulse pause before the next voltage pulse, facilitates the desired production of an increased concentration of excited boron particles and thus facilitates optimum control of the plasma generation method. According to one variation of the method of the invention, the pulse-duty factor should be greater than 1.1:1, preferably in the range from 1.25:1 to 5:1, and desirably in the range from 1.5:1 to 3.5:1. Furthermore, the pulse period, which is the sum of the durations of the voltage pulse and pulse pause, is preferably less than about 230 &mgr;s, but ≧50 &mgr;s.
Desirably, the pulse period is less than about 230 &mgr;s and more than 50 &mgr;s, for example, about 210 &mgr;s. According to one embodiment of the method of the invention, the DC voltage used for the pulsed current to produce the glow discharge is preferably in the range between about 500 volts and about 1000 volts, desirably in the range between about 600 volts and about 900 volts, and more desirably in the range between about 650 volts and about 800 volts. It was further found that, when working with higher voltage, the use of a longer pulse pause is advantageous. However, a good result is also achieved when applying lower voltage, preferably within the abovementioned voltage ranges, but even in this case, the composition of the individual components of the gas mixture supplied to the reactor can exert an influence on the resulting coating characteristics.
In the method of the invention, it is preferable to use, as a first component of the gas mixture supplied to the reactor, a boron-releasing gas in the form of a boron halide, for example, boron trichloride or boron trifluoride. Preferably used as second component of the gas mixture is hydrogen gas, and optionally, as third component of the gas mixture, a noble gas such as argon. It has been found that, when using argon as a third component in the method of the invention, good boride layers can be obtained even when applying relatively low voltages.
The concentration of boron trihalide as a boron-releasing gas in the gas mixture generally has an effect on the boride layer produced by the method. In general, the boron trihalide content should not be too low and, as rule, should not be less than 1% by volume, since otherwise a suitable boride layer may not be obtained. In accordance with one embodiment of the method of the invention, the boron trihalide content is preferably in the range of about 2% by volume to 50% by volume. It should be noted, however, that excessively high boron trihalide contents cause a relatively high boron trihalide loss. The lost boron trihalide is contained in the waste gas from the reactor and results in high cost f
Biemer Swen
Cabeo Emilio Rodriguez
Laudien Günter
Rie Kyong-Tschong
Baker & Botts L.L.P.
Padgett Marianne
Volkswagen AG
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