Title:
Process for boriding refractory metals and their alloys
United States Patent 3870569
Abstract:
Refractory metals are borided in a vacuum using amorphous boron that has been previously annealed in a vacuum under oxygen free conditions.


Inventors:
KRZYMINSKI HARALD
Application Number:
05/360190
Publication Date:
03/11/1975
Filing Date:
05/14/1973
Assignee:
Deutsche, Gold-und Silber-scheideanstalt Vormals Roessler (Frankfurt, DT)
Primary Class:
International Classes:
C23C8/68; (IPC1-7): C23F7/00
Field of Search:
148/6 117
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US Patent References:
Primary Examiner:
Rosdol, Leon D.
Assistant Examiner:
Wolfe Jr., Charles R.
Attorney, Agent or Firm:
Cushman, Darby & Cushman
Claims:
What is claimed is

1. A process for boriding a refractory metal consisting essentially of annealing amorphous boron powder in an oxygen-free vacuum and then packing the refractory metal in the oxygen-free amorphous boron powder and boriding the metal in a vacuum at an elevated temperature.

2. A process according to claim 1 wherein the refractory metal is a member of the group consisting of titanium, zirconium and hafnium.

3. A process according to claim 1 wherein the annealed boron is subjected to an atmosphere of a noble gas prior to the boriding step.

4. A process according to claim 1, wherein the refractory metal is titanium.

5. A process according to claim 1, wherein the boriding is carried out at 800° to 1300°C.

6. A process according to claim 5 wherein the boriding is carried out at 10-1 to 10- 5 Torr.

7. A process according to claim 6 wherein the refractory metal is titanium, hafnium, tantalum, niobium, zirconium, molybdenum, vanadium, tungsten or an alloy of such metals.

8. A process according to claim 7 wherein the boriding is carried out at 10-3 Torr.

9. A process according to claim 7 wherein the metal is titanium, tantalum or niobium and the temperature is 850° to 1000°C.

10. A process according to claim 7 wherein the metal is hafnium or zirconium and the temperature is 100° to 1300°C.

11. A process according to claim 1 wherein the oxygen free annealing is carried out at 1000°C for 1 to 3 hours with a vacuum of 10-4 to 10-5 Torr.

12. A process according to claim 1 consisting of (1) annealing amorphous boron powder in an oxygen-free vacuum, (2) subjecting the annealed boron to an atmosphere of noble gas, (3) packing the refractory metal in the oxygen-free noble gas laden amorphous boron powder, and (4) boriding the metal in a vacuum at an elevated temperature.

Description:
The purpose of the invention is the development of a process in which boride layers are produced by the diffusion of boron into refractory metals and alloys of refractory metals. Such layers are characterized by an extremely high hardness and impart an extraordinarily high wear resistance to parts which can consist, for example, of titanium, tantalum, niobium, hafnium, zirconium molybdenum, tungsten or vanadium.

The improvement produced by the boriding is above all of significance for metals which are inclined to wear and seizing. This is true, for example, of titanium and all titanium alloys. The alloys of titanium, because of their high strength and their low weights, in many respects are ideal work materials. However, they can be added only to a very limited extent for construction parts which both must have strenth and be able to transmit motion because of a decided inclination to seizing. The boride layers, furthermore, increase the compressive strength and the bending strength, the oxidation resistance and the corrosion resistance. Finally, it is known that the boride layers of various refractory metals is characterized by a small wettability and a high resistance to molten metals.

For the production of boride layers on non-ferrous metals there have been described in the past various procedures. Thus, there have been tried the boriding of metals in borax containing and fluoride salt melts, which, partly, are carried out with the help of electrolysis. In such cases it has turned out that only extremely pure and nearly oxygen free salt melts work somewhat satisfactorily. Also, such expensive precautions as the melting of the salts in a vacuum, the carrying out of protracted electrolytic purification and the operation of the salt bath under protective gas do not always guarantee that adhesive layers are formed and that the surfaces of the parts are not attacked. The boriding of non-ferrous metals in salt melts, therefore, places requirements almost impossible to meet in practical use.

Furthermore, there has been investigated the treatment of titanium and other refractory metals in boron or boron carbide containing powder which recently have found use in the boriding of iron materials. The result of these experiments is that the previously available solid boriding agents are usable only in exceptional cases for the treatment of non-ferrous metals. Such exceptions are nickel and molybdenum which can be borided in boron carbide containing powders or granulates without special precautions. For most non-ferrous metals this simple procedure is eliminated, however, because the air oxygen, absorptively bound in solid boriding agents, oxidizes the surface of the metal and besides causes embrittlement by concentration. Thus it has been found in the treatment of titanium and titanium alloys that the bad effect of oxygen even then cannot be avoided if the customary boriding agents contain specific getter materials and besides are used under protective gas.

By far more positive boriding experiments have been carried out with amorphous boron which has been previously annealed in the temperature region of 1000°C for several hours with the introduction of the purest, oxygen free argon. With the pretreated boron there can be produced mixtures with extenders and activating additives which are used later in the true boriding process again with the introduction of the purest argon. According to these processes it was possible for the first time to build faultless boride layers on samples of titanium and titanium alloys.

To be sure the process has two decisive disadvantages. In the first place, the high consumption of the purest argon makes the process expensive and makes its carrying out difficult. Experiments to replace the purest argon by cheap gases were unsuccessful. The use of hydrogen, for example, likewise with the elimination of injurious air oxygen, leads to a hydrogen embrittlement of several of the metals, especially of titanium.

In the second place there occurs a renewed oxygen take up of the annealed boron in producing of the final mixture and in longer storing which has a result faulty layers and oxidation damage to the treated materials.

The recited disadvantages are removed by the process of the invention in which both the oxygen free annealing of the boriding agent and also the true boriding process occur under a vacuum. Desirably the vacuum is such that the pressure is not over 10-1 Torr.

In a surprising manner it has been ascertained that the boriding process, contrary to known consideration, runs extraordinarily effectively in the vacuum. Since the diffusion of the boron into the material also takes place by way of the gaseous phase when using solid boriding agent it was originally assumed that using a vacuum the boron distributing gas would be so extensively diluted that the boriding activity would be reduced. In fact, however, titanium and other refractory metals can be more effectively borided under a vacuum than by any other known process, by proceeding according to the invention.

The process of the invention is characterized by the following particulars.

A vessel of heat resistant material, best made of ceramic oxide, is filled with amorphous boron and slowly heated under a vacuum of 10-4 to 10-5 Torr to 1000°C. It is held at this temperature for 1 to 3 hours. (The temperature can range from 850° to 1300°C, in this pretreatment the time of pretreatment can be widely varied). Then the furnace chamber is flooded with the purest argon or one of the other pure noble gases, e.g. helium or neon, and cooled to room temperature. After this treatment not only is the amorphous boron oxygen free but instead of being laden with air oxygen is loaded with noble gas. The adsorptive bond of the noble gas to the amorphous boron is very stabile so that the furnace chamber can be opened without an immediate reloading with air oxygen occurring.

After the oxygen free annealing the true boriding process takes place. The part to be treated is packed in the pretreated boron, whereupon the furnace chamber is again evacuated and is brought to the treatment temperature provided for. Since the evacuation occurs before the high heating the powder packing is again completely oxygen free before reaching the critical temperatures, that is also if in bringing the parts into the powder packing larger amounts of oxygen are taken up adsorptively. The boriding process preferably does not occur under high vacuum but preferably at a pressure of 10-3. Torr.

At this pressure there is attained a definite optimum in regard to thickness of the layer, freedom from pores, surface roughness and adhesive strength. The boriding process can be carried out at 10-1 to 10-5 Torr, preferably at 10-3 Torr. x)

The treatment temperature, according to the working material borided and the layer thickness strived for, can be from 850° to 1300°C. Titanium, tantalum and niobium are preferably borided in the lower temperature range, thus between 850° and 1000°C. Hafnium and zirconium, because of the slower speed of growth of their layers, are more suitably treated above 1000°C, the time of treatment, according to the thickness of the layer sought and the material, varies between 2 and 12 hours. In special cases, however, it can also be extended to 24 hours or more. After expiration of the fixed time of treatment the furnace chamber is again flooded with argon (or other noble gas) and the parts after cooling to room temperature can be taken out from the powder packing.

The next boriding treatment can take place in the same powder packing without the need for renewed oxygen free annealing. The use of pure, amorphous boron has the advantage that this substance is reusable as often as desired. The consumption is extraordinarily small and even after 50 fold use no chemical change of the boron occurs.

Additionally, the amorphous boron has no tendency to sinter together but remains loose and powdery. The packing in and taking out of the parts from the powder, therefore, creates no difficulties and is, if the parts are not too bulky, able to be carried out without a recharging of the boron powder. The practically unlimited reusability of the amorphous boron and extaordinarily small consumption of noble gas makes the process very thrifty.

The solid boriding agents developed for iron materials as a rule contain diluents and activating additives. As diluents there can be used aluminum oxide, magnesium oxide, silicon carbide and graphite. For an activation there can be used chlorides, fluorides and bromides. Such diluents and activating additives can also on principle be employed for the boriding of refractory metals. Their use is recommended, for example, in combination with the already mentioned processes which provide a boriding of titanium work materials under purest argon. Finally, this can still be considered to cheapen the boriding agents by replacing the expensive, amorphous boron by cheaper boron compounds, as for example, boron carbide or ferroboron.

All of these modes of action are also fundamentally usable according to the process of the invention. However, among the substantial advantages of the process of the invention there belongs the fact that these procedures can be relinquished, namely for the following reasons:

First, because of the multiple reuse of the amorphous boron under vacuum, the use of cheap diluents and other cheaper boron donors is without substantial utility, especially such additives which as a rule reduce the boriding activity.

Second, the process of the invention makes available a high effectiveness even without the addition of known activators. Since, besides, the activators are consumed in the course of time they limit the reusability of the boriding agent and are therefore rather of disadvantage than of advantage.

The effectiveness of the process of the invention is explained in the following examples.

Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1

Samples of pure titanium and the titanium alloy Ti A1 6 V4 were heated for 5 hours at 1000°C, and at a pressure of 10-3 Torr in amorphous, oxygen free annealed boron. In both workpieces there were produced closed, completely pore free boride layers which were toothed in characteristic manner with the base material. The compact part of the layer had in the case of pure titanium, a thickness of 10 μm and in the case of the alloy Ti A1 6 V4 a thickness of 8 μm. If the treatment temperature is increased to 1200°C, then, after the same time of treatment the thickness of the pure titanium is 35 μm and of the alloy Ti A1 6 V4 is 30 μm.

By increasing the time of treatment to 12 hours the thickness of the layer of both workpieces can be raised to above 50 μm. Also under these conditions the layers are built up completely fault free.

The compact portion of the layer consists of the titanium-boride Ti B2. The dentrites extending deeper into the base material consist predominantly of the boron poor compound Ti B. Both compounds are extremely hard. Their Vicker's hardness is between 3500 and 3800 kp/mm2.

EXAMPLE 2

Samples of niobium and tantalum were treated for 5 hours at 900°C, and a pressure of 10-3 Torr in amorphous, oxygen free annealed boron. In both workpieces the layers were built up without flaw and their thickness was between 12 and 18 μm. If the temperature of holding the treatment for 5 hours is increased to 1000°C there the thickness of the layer grows to 45 to 50 μm. The thicker layers are still free of pores but are inclined, however, to the formation of cracks which signifies an increased brittleness of the thicker layers. The layer produced from tantalum consists of the compound Ta B2 and the layer produced from niobium consists of the compound Nb B2. The Vicker's hardness of both layers is in the range of 3800 to 4200 kp/mm2.

EXAMPLE 3

Samples of hafnium and the zirconium alloy, zircaloy, which contains 1.5% of tin were borided for 5 hours at 1100°C under a pressure of 10-3 Torr. In both cases the layer was toothed with the base material and faultlessly formed. In the Zircaloy there was measured a thickness of the layer of 10 μm and in the hafnium a thickness of 15 μm.

If the treatment temperature for 5 hours is increased to 1200°C, there the layer strength of the Zircaloy increases to 18 μm and of the hafnium to nearly 50 μm. the layers formed from the compounds Zr B2 and Hf B2 attained Vicker's hardness values of 3400 kp/mm2 (Zr B2) and 4100 kp/mm2 (Hf B2).