# Patent

## Title
Plasma boriding method

## Abstract

The present invention relates to a method of preparing wear-resistant metallic surfaces.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority to U.S. provisional patent application No. 60/720,251, filed Sep. 22, 2005, the entirety of which is hereby incorporated herein by reference.


FIELD OF THE INVENTION
The present invention relates to a method of preparing wear-resistant metallic surfaces.
BACKGROUND OF THE INVENTION
Boriding is known to increase wear-resistance in metallic surfaces. Various methods of boronizing metallic surfaces are known. Such methods produce a boron layer on a metal surface. Typically, these methods utilize reactive boron species which diffuse into the metal surface. Such reactive boron species include gaseous diborane and boron trihalides, including BCl3 and BF3.
One method for boriding metallic surfaces is the “pack” method. In this methods, the boron source is in the form of a solid powder, paste, or in granules. The metal surface is packed with the solid boron source and then heated to release and transfer the boron species into the metal surface. This method has many disadvantages including the need for using a large excess of the boron source resulting in the disposal of excessive toxic waste.
Another method for boriding metallic surfaces utilizes a plasma charge to assist in the transfer of boron to the metal surface. Typically, plasma boronization methods utilize diborane, BCl3, or BF3 where the plasma charge is applied to the gaseous boron-containing reagent to release reactive boron species. See U.S. Pat. No. 6,306,225 and U.S. Pat. No. 6,783,794, for example. However, these methods utilize corrosive and highly toxic gases and are thus difficult to utilize on an industrial scale.
Plasma boriding processes have several advantages, including speed and localized heating of the substrate. This prevents the bulk metal in the borided piece from annealing, obviating additional heat treatments to restore the original microstructure and crystal structure. As a result, it is desirable to have plasma boriding processes that retain the advantages of plasma treatment while reducing the hazards and costs connected with noxious chemicals.


DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS
The present invention provides a method for boriding a metal surface. According to methods of the present invention, KBX4, wherein X is a halogen, is provided as a boron source. Use of KBX4 is advantageous in that it is a solid substance which is readily available and easily handled. In certain embodiments, KBX4 is provided in solid form in the presence of a metal surface to be borided. Heat is applied such that the KBX4 releases BX3 gas to which a plasma charge is applied. Without wishing to be bound by any particular theory, it is believed that the plasma charge results in the formation of one or more active boron species which diffuse into the metal surface. As used herein, the term “activated boron species” refers to any one or more of the boron species created from applying the plasma charge to the gas resulting from heating KBX4. In certain embodiments, the one or more activated boron species include, but are not limited to, B+, BX+, BX2+, and BX3+.
As used herein, the terms “boriding” and “boronizing” are used interchangeably and refer to the process of incorporating a boron layer on a metal surface.
As used herein, the term “plasma” refer to an ionized gas and the term “plasma charge” refers to an electric current applied to a gas to form a plasma. In certain embodiments, a plasma of the present invention comprises one or more activated boron species including, but not limited to, B+, BX+, BX2+, and BX3+, wherein each X is a halogen.
As used herein, the term “glow discharge” refers to a type of plasma formed by passing a current at 100 V to several kV through a gas. In some embodiments, the gas is argon or another noble gas.
In certain embodiments, each X is chlorine and the KBX4 is KBCl4.
In other embodiments, each X is fluorine and the KBX4 is KBF4.
In certain embodiments, the present invention provides a method for boriding a metal surface, comprising the steps of:

    (a) providing KBX4, wherein each X is halogen;
    (b) heating the KBX4 at a temperature sufficient to release BX3; and
    (c) applying a plasma charge to the BX3 to create one or more activated boron species for diffusing into the metal surface.


In other embodiments, the present invention provides a method for boriding a metal surface, comprising the steps of:

    (a) providing KBX4, wherein each X is halogen, in the presence of the metal surface;
    (b) heating the KBX4 at a temperature sufficient to release BX3; and
    (c) applying a plasma charge to the BX3 to create one or more activated boron species for diffusing into the metal surface.


In certain embodiments, the metal surface to be boronized is an iron-containing metal. Iron-containing metals are well known to one of ordinary skill in the art and include steels, high iron chromes, and titanium alloys. In certain embodiments, the iron-containing metal is a stainless steel or 4140 steel. In other embodiments, the stainless steel is selected from 304, 316, 316L steel. According to one embodiment, the iron-containing metal is a steel selected from 301, 301L, A710, 1080, or 8620. In other embodiments, the metal surface to be boronized is titanium or a titanium-containing metal. Such titanium-containing metals include titanium alloys.
In other embodiments, the KBX4 is provided in solid form in a chamber containing the metal surface to be borided. The KBX4 is heated to release BX3. A plasma charge is applied at the opposite side of the chamber to create a plasma comprising one or more activated boron species. The temperature at which the KBX4 is heated is sufficient to release BX3 therefrom. In certain embodiments, the KBX4 is heated at a temperature of 700 to 900° C.
The amount of KBX4 utilized in methods of the present invention is provided in an amount sufficient to maintain a pressure of about 10 to about 1500 Pascals within the reaction chamber. In certain embodiments, the pressure is from about 50 to about 1000 Pascals. In other embodiments, the pressure is from about 100 to about 750 Pascals. One of ordinary skill in the art will appreciate that the thermodecomposition of KBX4 to BX3 results in an increase of pressure within the reaction chamber. Without wishing to be bound by any particular theory, it is believed that the number of moles of BX3 gas created may be calculated by measuring the increase of pressure.
In certain embodiments, hydrogen gas is introduced into the chamber with the KBX4 and BX3 resulting from the thermodecomposition thereof. Without wishing to be bound by any particular theory, it is believed that elemental hydrogen facilitates the decomposition of BX3 into the one or more activated boron species upon treatment with the plasma charge. In certain embodiments, hydrogen gas is introduced in an amount that is equal to or in molar excess as compared to the amount of BX3 liberated.
In some embodiments, the BX3 and optional hydrogen gases are carried into a plasma by a stream of an inert gas, for example, argon. The plasma allows quicker diffusion of reactive elements and higher velocity impact of reactive boron species against the metal surface being treated. In certain embodiments, the plasma is a glow plasma. The substrate may be any material that is suitable for use with plasma treatment methods, for example, steels or titanium alloys. The KBX4 may be decomposed in a separate decomposition chamber connected to the plasma chamber, or both the decomposition and the plasma treatment may occur in separate areas of a single reaction vessel.
As described herein, methods of the present invention include the step of applying a plasma charge to create one or more activated boron species. In certain embodiments, the plasma charge is a pulsed plasma charge. In other embodiments, the plasma charge is applied wherein the voltage is regulated from between about 0 to about 800 V. In still other embodiments, the amperage is about 200 A max.
Other embodiments of the invention will be apparent to those skilled in the art from a consideration of the specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.
EXAMPLES
A steel part is placed into a reaction chamber along with 50 g KBF4 in a boron nitride crucible. The reaction chamber is evacuated to 0.01 Pa. The crucible is heated to 900° C. resulting in decomposition of KBF4 to BF3. A 10% H2/Ar2 gas mixture is added to the reaction chamber to a pressure of 500 Pa. An electrical discharge is applied at 600 V and 150 Amps. The reaction is continued for about 3 hours or until desired boron penetration is accomplished.

## Claims

1. A method for boriding a metal surface, comprising:
heating KBX4, wherein each X is a halogen, at a temperature sufficient to release BX3;
applying a plasma charge to the BX3 to create a plasma comprising one or more activated boron species; and
diffusing the plasma onto the metal surface, wherein:
the KBX4 is in the presence of the metal surface in a single reaction vessel such that both thermal decomposition of the KBX4 and plasma treatment of the metal surface occur in separate areas of the reaction vessel; or
thermal decomposition of the KBX4 occurs in a separate decomposition chamber connected to a reaction vessel containing the metal surface for plasma treatment of the metal surface.

2. The method according to claim 1, wherein the metal surface is titanium or a titanium-containing metal.

3. The method according to claim 1, wherein the KBX4 is heated at a temperature of 700 to 900° C.

4. The method according to claim 1, wherein the one or more activated boron are selected from B+, BX+, BX2+, or BX3+.

5. The method according to claim 4, wherein the plasma charge is a glow plasma.

6. The method according to claim 1, wherein the metal surface is an iron-containing metal surface.

7. The method according to claim 6, wherein the metal surface comprises a steel, an iron chromium alloy, or a titanium alloy.

8. The method according to claim 1, further comprising introducing hydrogen gas.

9. The method according to claim 8, wherein the hydrogen gas is introduced in a stream of argon.

10. A method of plasma boriding, comprising
thermally decomposing KBX4, wherein each X is a halogen, to produce KX and BX3;
directing said BX3 into a plasma formed by an inert gas, wherein the composition and plasma formation conditions are selected such that the BX3 is decomposed into BX2+ and X−; and
allowing said BX2+ to react with a metal, wherein:
the KBX4 is in the presence of the metal in a single reaction vessel such that both thermal decomposition of the KBX4 and plasma treatment of the metal occur in separate areas of the reaction vessel; or
thermal decomposition of the KBX4 occurs in a separate decomposition chamber connected to a reaction vessel containing the metal for plasma treatment of the metal.

11. The method according to claim 10, wherein X is fluorine.

12. The method according to claim 10, wherein X is chlorine.

13. The method according to claim 10, wherein X is bromine.

14. The method according to claim 10, further introducing hydrogen gas.

15. The method according to claim 14, wherein the hydrogen gas is introduced in a stream of argon.