Process For Increasing The Resistance To Wear Of The Surface Of Hard Metal Cemented Carbide Parts Subject To Wear

Abstract

A process for increasing the resistance to wear of the surface of hard metal parts subject to wear, such as the cutting blade of cutting tools, by coating the surface of the hard cemented carbide articles with a layer of refractory oxide such as aluminium oxide, ziroconium oxide or stabilized zirconium oxide in a thickness of up to 20 microns.

Description

FIELD OF THE INVENTION

This invention relates to a process for increasing the resistance to wear of cutting tools or other cemented carbide hard metal parts subject to wear and to the cutting tools obtained by this process. This invention relates to a process for increasing the wear resistance of cemented carbide articles subject to wear, for instance cutting tools, and to the cemented carbide articles, in particular cutting tools, obtained by this process.

BACKGROUND OF THE INVENTION

As is known parts made of "hard metal" otherwise known as cemented carbides consist of a mixture of at least one metal serving as a binder and at least one metal carbide of great hardness. The carbide may be particularly tungsten, titanium, tantalum or niobium carbide or a mixed carbide of tantalum and niobium. The binder metal may be, for example, cobalt, iron or nickel. The surface of such cemented carbide articles is very hard and resistant to abrasion, more than that of common metals and alloys, particularly steel. Therefore, such parts can be used for many applications in which the surface of the parts must have a great hardness and resistance to abrasion, particularly for producing cutting tools, which cannot be reground, such as those used for machining hard metals such as steel, in drawing dies, etc.

Obviously, it is very desirable to further increase the resistance to wear of the surface of or articles. In particular, in the case of cutting blades such an increase of wear resistance would increase the useful life of the cutting blades for a given cutting speed and would permit to increase the cutting speed for a given useful life or would even permit to increase simultaneously the cutting speed and the useful life.

A process for increasing the resistance to wear of hard metal blades for cutting tools is already known. This process consists in providing the surface of the blades with a coating having a resistance to wear higher than that of the original surface of the blade, this coating being formed of at least one carbide selected from the same constituents which form the hard metal itself, particularly titanium carbide TiC.

THE INVENTION

It is therefore the object of the present invention to provide a process for increasing the resistance to wear of the surface of hard cemented carbides metal parts, particularly of cutting blades for cutting tools which cannot be reground, which process provides a resistance to wear higher than that which can be obtained by the known process described above.

The process according to the invention comprises coating at least a portion of the surface of the cemented carbide article with a layer of a refractory oxide selected from the group including aluminium oxide, zirconium oxide and stabilized zirconium oxide in a thickness of up to 20 microns.

The stabilized zirconium oxide used may be, for example, zirconium oxide stabilized by 10 mole percent of magnesium oxide or 5 mole percent of calcium oxide or at least one rare earth oxide in an appropriate ratio. The refractory oxides mentioned above may be used either alone or in the form of a mixture of these oxides.

To increase the resistance to wear and consequently the useful life of a hard metal part by means of a coating of refractory oxide is quite an unexpected result. In fact, although it is well known that refractory oxides, particularly aluminium oxide and stabilized zirconium oxide, are very hard, it is equally well known that these oxides are more brittle than "hard metal" cemented carbide at least when these oxides are in the form of relatively large bodies of a size of at least some millimetres. Consequently, by coating the surface of a hard metal part with a layer of refractory oxide normally there was not to be expected an adhesion of the coating on the surface of the cemented carbide part which would be sufficient to result in a durable improvement of the resistance to wear of such surface.

The advantageous and unexpected result obtained is probably due to the careful choice of the thickness of the layer of refractory oxide. In fact, preferably the thickness of the refractory oxide layer must be in the range between 0.1 and 10 microns to obtain the greatest increase in the resistance to wear. When the thickness of the refractory oxide layer is less than 0.1 microns it wears off rapidly and when the thickness is greater than about 10 microns its toughness decreases.

For depositing the refractory oxide layer on the surface of the hard metal part any appropriate known method may be used which permits to obtain a compact, coherent and homogeneous adhering coating of a thickness which is substantially uniform at least over the portions of the surface to be coated in which the resistance to wear is to be increased. For example, particles of a refractory oxide powder which at least in part are in a liquid state may be cast on to the surface to be coated by some known appropriate means, for example, a plasma torch. To obtain a well adhering coating, the deposition of the coating layer may advantageously be effected by a treatment at a high temperature and/or by subjecting the surface of the hard metal part, after the application of the coating layer, to a further thermal treatment at a high temperature for increasing the adhesion of the refractory oxide layer on said surface by diffusion with substitution of atoms. The deposition of the coating layer may also be effected by electrophoresis with a subsequent thermal treatment, at a high temperature, of the surface of the coating layer. In any case the termal treatment is advantageously carried out at a temperature between 700.degree. and 1200.degree.C. for a duration of at least half an hour. The thermal treatment may also be carried out for a duration of more than half an hour at a temperature of about 700.degree.C. The oxide layer is preferably deposited from a gaseous state, particularly by evaporation and condensation under vacuum, cathodic spraying and deposition by chemical reaction in the gaseous phase, this method being usually referred to as "chemical vapour deposition" or C.V.D. This latter method is particularly employed in the preferred form of putting the invention into practice and it permits to obtain the deposition of a refractory oxide coating layer which to a large extent possesses the above-mentioned desired properties.

DETAILED DESCRIPTION

Among the various chemical reactions which may be used for depositing the refractory oxide coating layer preferably the reaction of a volatile halide, particularly a chloride, of the metal corresponding to the oxide, with water or with a mixture of carbon dioxide and hydrogen is chosen.

Thus, for example, for depositing an aluminium oxide coating layer one of the following two reactions may be used:

2 AlCl.sub.3 + 3 H.sub.2 O .fwdarw. Al.sub.2 O.sub.3 + 6 HCL

or

2 AlCl.sub.3 + 3 CO.sub.2 + 3 H.sub.2 .fwdarw. Al.sub.2 O.sub.3 + 3 CO + 6 HCl

For depositing a coating layer of stabilized zirconium oxide (also called stabilized zirconium), on the one hand, one of the following two reactions:

ZrCl.sub.4 + 2 H.sub.2 0 .fwdarw. ZrO.sub.2 + 4 HCl

or

ZrCl.sub.4 + 2 CO.sub.2 + 2 H.sub.2 .fwdarw. ZrO.sub.2 + 2 CO + 4 HCl

and, on the other hand, one of the following two reactions for the formation of a stabilized ZrO.sub.2 oxide may be used, which is effected simultaneously with the corresponding reaction for the formation of zirconium oxide (merely by way of example, the two reactions are indicated hereafter for the case of the stabilizing oxide component being formed by magnesium oxide):

MgCl.sub.2 + H.sub.2 0 .fwdarw. MgO + 2 HCl

or

MgCl.sub.2 + CO.sub.2 + 2 H.sub.2 .fwdarw. MgO + CO + 2 HCl.

In this case it is sufficient to select the proportion of zirconium chloride and of the element corresponding to the stabilizing oxide (here magnesium chloride) to obtain the desired proportion of stabilizing oxide (for example, 10 mole percent in the case of magnesium oxide) in the stabilized zirconium oxide.

As to the temperature and pressure conditions which permit the deposition of the refractory oxide coating layer, they must be selected according to the nature of the chemical compounds used as starting compounds. This selection can be made by one skilled in the art as it is evident from the abundant literature which has already been published on the conditions which are suitable for the deposition of various refractory oxides by chemical reaction in the gaseous phase (cf. for example, the book "Vapor deposition" by C. F. Powell, J. H. Oxley and J. M. Blocher, published by John Wiley and Sons Inc., New York, London, Sidney).

For example, for depositing aluminium oxide by reaction of aluminium chloride with water preferably the following conditions are chosen:

Temperature of the surface of the part to be coated with the aluminium layer: 600.degree. to 1200.degree.C.

Overall pressure of the gaseous phase: 1 to 760 torr (preferably between 30 and 80 torr).

For depositing aluminium oxide by reaction of aluminium chloride with carbon dioxide and hydrogen, the following conditions are preferably chosen:

Temperature of the surface of the part to be coated: 700.degree. to 1200.degree.C. (preferably between 900.degree. and 1150.degree.C).

Overall pressure of the gaseous phase: 1 to 760 torr (preferably between 10 and 125 torr).

For depositing stabilized or unstabilized zirconium oxide the conditions are similar and may be selected, for example, by taking into account the indications given at page 400 of the above-mentioned book.

A zirconium oxide coating layer may also be produced by oxidizing, for example, with oxygen, carbon dioxide or other similar oxygenated compounds, a layer of zirconium carbide or nitride deposited on a substrate by chemical reaction in the gaseous phase.

For depositing refractory oxides by chemical reaction in the gaseous phase any device may be used which is suitable for the starting compounds as well as the dimensions and number of articles to be coated. Such devices are known per se and many forms of construction and variations thereof have been described in the relevant technical literature.

A preferred embodiment of the invention will now be described by way of example and with reference to the accompanying drawing which schematically shows a device for depositing an aluminium oxide coating layer on the surface of a cemented carbide article by chemical reaction in the gaseous phase according to one of the reactions indicated above, and in which:

FIG. 1 is a schematic overall view of the device, and

FIG. 2 is a sectional view, on a larger scale, showing the portion of the device with the part to be coated (the reaction chamber).

The device shown in FIG. 1 comprises a reaction chamber 1 of quartz, provided with a movable support bar 2, likewise of quartz, shiftably mounted in a gasket 3 which is cooled by cold water. A coiled copper pipe 4, which is cooled by a flow of water and connected to a high frequency electric current generator, permits the cemented carbide article 5 to be heated by induction, the article 5 being placed on the support bar 2 and being the part to be coated, in the illustrated embodiment, with aluminium oxide.

The reaction chamber 1 is supplied through a conduit 6 with a mixture of hydrogen and aluminium chloride from a device 7 for producing aluminium chloride in the gaseous phase and for mixing this gas with hydrogen at a variable ratio. The walls of the conduit 6 are kept at 200.degree.C. by heating means not shown in the drawing. A further conduit 8 supplies the reaction chamber 1 with carbon dioxide or with a mixture of hydrogen and water vapour, depending on the type of reaction selected for depositing the aluminium. One or the other of these mixtures is supplied by a device 9 for mixing the gas.

The devices 7 and 9 are provided with means for purging and rinsing by an inert gas such as argon which is supplied by an outside storage container not shown in the drawing. A pumping unit 11 is connected to the reaction chamber 1 through a conduit 10 and permits to establish in the reaction chamber a pressure which can be adjusted according to the requirements of the process, this pressure being between 1 and 760 torr.

The manner in which the article 5 to be coated is arranged on the support bar 2 is shown in greater detail in FIG. 2. In the illustrated embodiment, a removable support 12 formed by an aluminium oxide plate is interposed between the article 5 and the support bar 2. FIG. 2 also shows the device for mixing the gas flows supplied to the reaction chamber 1 through the conduits 6 and 8. This device substantially comprises a bell-mouthed tube 13 having a smaller diameter than that of the conduit 6. A thermocouple 14, not shown in FIG. 1, permits to measure the temperature of the part 5.

A device similar to that which has been described above can be used for depositing a zirconium oxide coating layer or a stabilized zirconium oxide coating layer or a layer consisting of a mixture of at least two of the above-mentioned oxides. For this purpose it is only necessary to replace the device 7 for producing gaseous aluminium chloride by a device for producing the volatile zirconium compound or the mixture of volatile zirconium and the element corresponding to the oxide stabilizing the zirconium oxide, or by a device for producing a mixture of volatile compounds of aluminium, zirconium and, if desired, a stabilizing compound.

Some practical Examples for carrying out the process of the present invention will now be described in greater detail.

EXAMPLE 1

An aluminium oxide coating layer having a thickness of 5 microns was deposited on a cutting blade of a hard cemented carbide metal cutting tool by using said first mentioned reaction (reaction of aluminium chloride with water vapour).

The reaction conditions were as follows:

Time of treatment 5 hrs. Temperature 1000.degree.C. Overall pressure of the gaseous phase 5 torr Feed rate of the gaseous hydrogen mixture (carrier gas) (amount reduced to 20.degree.C. and 760 torr) 400 cm.sup.3 /min. Aluminium chloride (AlCl.sub.3) 10 mg/min. Water vapour 4 mg/min.

It was found that the major portion of the coating layer was formed by alpha alumina.

The composition of the hard metal cemented carbide of the cutting tool was as follows (in percent by weight):

Cobalt 9.5 Titanium carbide 11.9 Tantalum carbide 6 Niobium carbide 4 Tungsten carbide 68.6

EXAMPLE 2

An aluminium oxide coating layer having a thickness of 1 micron was deposited on a cutting blade for a cutting tool of hard metal of the same composition as that described in Example 1 by using said second reaction indicated above (reaction of aluminium chloride with carbon dioxide and hydrogen) under the following reaction conditions:

Time of depositing operation 7 minutes Temperature 1000.degree.C. Overall pressure of the gaseous phase 50 torr Feed rate of the gaseous mixture (amounts reduced to 20.degree.C. and at a pressure of 760 torr): Hydrogen 200 cm.sup.3 /min. Carbon dioxide 200 cm.sup.3 /min. Aluminium chloride (AlCl.sub.3) 10 mg/min

It was found that the coating layer was formed by alpha alumina.

EXAMPLE 3

The process as described in Example 2 was repeated, but with a time of 30 minutes for the depositing operation. Apart from this, all the reaction conditions were the same as described in Example 2. In this manner an alpha, alumina coating layer having a thickness of 6 microns is deposited on the hard metal cemented carbide cutting tool.

EXAMPLE 4

The process as described in Example 2 was repeated, but after the depositing operation the cutting blade was kept at 1000.degree.C. for 30 minutes under a hydrogen atmosphere.

Comparative cutting tests were carried out on a lathe with samples of the cemented carbide cutting tools coated with alumina as described in Example 2 and 3 and with uncoated hard metal cemented carbide cutting tools and hard metal cemented carbide cutting tools coated with a surface layer of titanium carbide. The samples consisted of hard steel of the following composition (in percent by weight):

Carbon 0.96 Silicon 0.27 Manganese 0.25 Phosphorus 0.019 Sulphur 0.015 Chrominium 0.15 Iron the remainder

These comparative tests have shown that the resistance to wear of the cutting blades produced by the process of the present invention is considerably improved with respect to the resistance to wear of the cutting blades produced by conventional methods. The results of the comparative tests were as follows:

Series No. 1 ______________________________________ Testing operation Turning Test material Steel of the composition as indicated above Cutting conditions Speed 140 m/min. Feed 0,40 mm/revolution Cutting depth 2,0 mm Different types of test material Life of cutting blade in minutes Hard metal cemented carbide of standard ISO P30 3.7 Hard metal cemented carbide of standard ISO P10 13.0 Hard metal cemented carbide of standard ISO P 30 coated with a layer of TiC having a thickness in the order of 5 microns 21.7 Hard metal cemented carbide of the standard ISO P30 coated with alpha alumina according to Example 3 43.1 ______________________________________

The cemented carbide of the standard ISO P10 has the following composition (in % by weight):

Cobalt 9.5 Titanium carbide 19 Tantalum carbide 12.2 Niobium carbide 3.8 Tungsten carbide 55.5

Series No. 2 ______________________________________ Testing operation Turning Test material Steel Cutting conditions Speed 160 m/min. Feed 0.30 mm/revolution Cutting depth 2.0 mm Different types of test material Life of cutting blade in minutes Cemented carbide of the standard ISO P30 3.0 Cemented carbide of the standard ISO P10 10.0 Cemented carbide of the standard ISO P30 coated with TiC 19.2 Cemented carbide of the standard ISO P30 coated with alpha alumina according to Example 2 14.5 Cemented carbide of standard ISO P30 coated with alpha alumina according to Example 3 35.4 Cemented carbide of the standard ISO P30 coated with alpha aluminium according to Example 4 25.0 ______________________________________

Although some preferred Examples for carrying out the process of the invention and a preferred embodiment of the apparatus for carrying out the process have been described herein in detail it is to be understood that the invention is not limited to these precise Examples and to this embodiment of the apparatus and that numerous changes and modification may be made therein without departing from the scope of the invention.

Claims

What we claim is:

1. A cemented carbide tool provided with a surface coating produced by coating at least a portion of the surface of the cemented carbide with a layer 0.1 to 20 microns thick of refractory oxide selected from the group including aluminium oxide, zirconium oxide and stabilized zirconium oxide.

2. A cemented carbide tool as claimed in claim 1, wherein said surface coating has a thickness of between 0.1 and 10 microns.

3. A cemented carbide tool as claimed in claim 1 wherein said cemented carbide is composed of a carbide of a metal selected from the group consisting of tungsten, titanium, tantalum and niobium, or a mixed carbide of tantalum and niobium; and a binder metal selected from the group consisting of cobalt, iron, and nickel.