US 3717496 A
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United States Patent US. Cl. 117-49 8 Claims ABSTRACT OF THE DISCLOSURE Machine parts made of hard metal comprising at least one metal carbide and a metal binder may be provided with an improved wearand abrasion-resistance by forming a nitride or carbonitride surface layer thereon of one or more metals of Groups IVa to Vla of the Periodic Table. In particular embodiments of the invention a surface layer of titanium nitride or carbonitride is formed. The invention also provides a method of producing such surface layers which includes the step of heating the machine parts in the presence of a thermally-decomposable metal halide and a purified gas mixture of nitrogen, hydrogen and possibly methane.
This invention relates to the provision of an abrasionand wear-resistant coating on hard metal parts used for machining and plastic forming, to protect said parts from abrasion and wear.
The cutting surface of metal tools which are used for machining materials which form long and short chips are always subject to some degree of abrasion which shortens the life of the tool. A known technique for preventing this is to sinter coating layers of titanium-carbide cobalt hard metal on cutting tools made of tungstencarbide cobalt hard metal. This is intended to preserve the toughness of the base material and also to make use of the high resistance to abrasion of titanium-carbide containing metals. The differential coefficients of thermal expansion and the varying sintering stresses between the coating layer and the base material nevertheless cause cracks to occur, and for this reason such composite hard metal surface layers have not been a success.
A recent development in protecting the surface of hard metal cutting tools is to deposit a hard titanium carbide coating layer less than 10 m. thick by vaporisation. Such a layer provides some improvement in the abrasive wearing properties of the tool.
It is the object of the present invention to provide a coating that will provide further improvement in the protection against abrasion and wear than coatings hitherto used.
It has been discovered that coating layers of metal nitrides or of metal carbonitrides of metals as hereinafter described have better abrasion resistance than conventional titanium carbide coating layers on hard metal parts comprising at least one metal carbide and a metal binder, e.g. iron, cobalt and/or nickel. Although metal nitrides and metal carbonitrides are not as hard as metal carbides their resistance to abrasive wear is higher. Among other factors this may be due to the nitrided hard metals being less liable to weld and stick to metal chips and their oxide skins than metal carbides.
The invention provides a machining part made from a hard metal comprising at least one metal carbide and a metal binder, having a surface layer consisting of a nitride or carbonitride of one or more metals of Groups IVa to VIa of the Periodic Table of Elements.
3,717,496 Patented Feb. 20, 1973 Coating layers of metal carbides, nitrides, borides and silicides on parts made of metal or metal alloys are as such well known in the art. For instance, German patent specification No. 1,056,449 relates to a method of producing coatings of metal carbide, particularly titanium carbide, on the surface of metal and non-metal parts, such as steel cutting and drawing tools, by vaporising and depositing the carbide from the gas phase. The result is a hard and wear resistant surface on these tools.
German patent specification 1,056,450 describes coating layers of metal carbides, nitrides, borides and silicides on objects made of tool steel or cast iron.
German patent specification No. 1,065,442 describes the provision of workpieces and tools made of steel with coatings of titanium carbide and titanium nitride.
However, the specifications hereinbefore mentioned do not disclose that coatings of metal nitrides or carbonitrides on hard metal tools according to the invention have a particularly favourable effect on the abrasion resistance of the tool. Hitherto it has been generally accepted that coating layers of metal carbides have the best qualities regarding wear and abrasion resistance, as hereinbefore mentioned, it transpires that this is not so.
In a preferred embodiment of the invention the coating layer consists of titanium nitride or titanium carbonitride. In a coating of titanium carbonitride, it is preferred that from 10% to 20% titanium carbide is present in the crystalline carbonitride solid solution. The thickness of the coating on the hard metal parts may be between 1 and 50 ,um.
In a preferred method of producing the proposed coating layer the nitride or carbonitride is deposited on the surfaces of hard metal parts by thermal decomposition of a halide of a metal of Groups IVa to VIa of the Periodic Table of Elements in the presence of a prepurified gas mixture of nitrogen, hydrogen and possibly a gaseous. hydrocarbon for example methane, in a furnace in which the hard metal parts that are to be coated are heated to a temperature between 800 and 1150 C.
The method according to the invention of producing a coating layer of metal nitride or carbonitride on hard metal parts preferably includes the pretreatment step of embedding the hard metal parts in a powder mixture of metal nitride or carbonitride to remove the sinter skin from the hard metal parts. This will ensure that the coating layer is uniformly thick on every surface of the parts during treatment in the furnace. A preferred procedure comprises initially moving the hard metal parts in the embedding powder mixture, for instance by rotation. This has the effect of rubbing off the sinter skin from the hard metal parts, as this might inhibit the formation of the coating layer.
Two examples of the production of a titanium nitride and a titanium carbonitride coating on hard metal parts according to the invention are provided:
EXAMPLE 1 A stream of high temperature-dried hydrogen and nitrogen was charged with titanium tetrachloride vapour by passing the gases through a vessel filled with liquid titanium tetrachloride. The previously purified gas mixture was conducted over plates of hard metal consisting of 94% by weight of tungsten carbide and 6% of molybdenum in a furnace wherein the plates were heated to between 800 and 1150". The gas mixture reacted at the surface of the white hot hard metal plates and formed a golden yellow layer of titanium nitride. By this procedure the adhesion of the titanium nitride layer was unexpectedly particularly good if the sinter skin was first ground or rubbed off from all faces and if the duration of the nitriding treatment was from 30 to 60 minutes. This led to the formation of a layer that was between and 50 pm. thick.
EXAMPLE 2 For the production of a titanium carbonitride coating, the procedure of Example 1 was repeated except that the gas mixture was obtained by combining dry methane in aliquot proportions with a gas mixture of hydrogen, nitrogen and titanium tetrachloride. A coating layer containing 10% to of titanium carbide in the crystalline solid carbonitride solution was obtained.
With increasing content of titanium carbide the colour of the layer changes from golden yellow to a reddish gold, the latter colour being attained when the composition is approximately 4TiN/1TiC.
When used in machining operations, hard metal parts provided with golden yellow titanium nitride or red golden titanium carbonitride coating layers according to the invention may have a working life several times longer than that of uncoated plates, and from 50 to 100% longer with a correspondingly better abrasion resistance, than hard metal parts coated with titanium carbide. Parts treated according to the invention also have the technical advantage that the coating temperatures may be between 100 and 300 C. lower than when applying coatings of metal carbides, so that the hard metal machining parts will not undergo undesirable structural change and will not sufler an excessive decarburisation at the surface, i.e. the formation of an -phase intermediate layer.
The application of nitrides of other transition metals of Groups IVa to Va and Cr N, is a little more difficult because the chlorides of the said metals are all solids at room temperature and they therefore have to be melted and further heated before being exposed to the nitrogen and become entrained in the vapour phase. However in such circumstances a proportion of the solid chlorides can be dissolved in liquid titanium tetrachloride and gas mixtures can thus be obtained that will form coatings of crystalline solid nitride solutions when nitrogen or hydrogen is passed through. In the simultaneous presence of methane, multi-component carbonitride layers are formed.
Suitable furnaces for performing the nitriding process are horizontal, stationary, ceramic tube kilns or also ceramic rotary kilns. In the latter type of furnace it is particularly important that the parts should be embedded in the above described manner in a powder mixture that will ensure that corners and edges of the plates are gently treated, apart from a gentle abrading eifect, i.e., removal of sinter skin. Other suitable furnaces are of the kind used for chromising steel and substantially have the form of a retort.
Hard metal parts which may be treated according to the invention include machine tools, or parts of machine tools for example cutting inserts, die inserts and drawing dies.
What is claimed is:
1. A machining part made from a hard metal com prising a major proportion of at least one metal carbide and a metal binder, having a surface layer consisting of a nitride of one or more metals of Groups IVa to VIa of the Periodic Table of Elements or of a carbonitride of one or more metals of Groups IVa to VIa of the Periodic Table of Elements.
2. A machine part according to claim 1, in which the said surface layer is selected from the class consisting of titanium nitride and titanium carbonitride.
3. A machine part according to claim 2, in which the said surface layer is crystalline carbonitride solid solution containing from 10% to 20% of titanium carbide.
4. A machine part according to claim 1, in which the said surface layer is between 1 and m. thick.
5. A machine part according to claim 1, in which the machine part comprises tungsten carbide and a metal binder selected from the class conisting of iron, cobalt and nickel.
6. A method of producing a surface layer on a machine part made from a major proportion of at least one metal carbide and a metal binder, comprising removing the outer skin of the surface of said part by embedding the part in a powder mixture of metal nitride or carbonitride and heated therein in a furnace, depositing a nitride or carbonitride surface layer on the said part at a temperature between 800 and 1150 C. by the thermal decomposition of a halide of metals of Groups IVa to VIa of the Periodic Table of Elements in the presence of a purified gas mixture of nitrogen, hydrogen, and in the case of carbonitrides, at least one gaseous hydrocarbon.
7. A method as in claim 6 wherein the gaseous hydrocarbon is a methane.
8. A method as in claim 6 in which the part is moved in the embedding power mixture during heating.
References Cited UNITED STATES PATENTS 2,884,894 5/1959 Ruppert et al. 117-106 R 2,972,556 2/1961 Vrahiotes et al. 117-106 R FOREIGN PATENTS 731,990 6/1955 Great Britain 117-106 R 699,790 12/ 1964 Canada 117-106 C 1,041,320 10/1958 Germany 117-106 R 1,069,448 11/1959 Germany 117-106 R 1,817,339 8/1969 Germany 117-106 C 1,092,271 11/1960 Germany 117-106 R OTHER REFERENCES Translation of Czch Pat. No. 95,792, published June WILLIAM D. MARTIN, Primary Examiner B. D. PIANALTO, Assistant Examiner US. Cl. X.R. 117-106 C, 127