GB2061324A

GB2061324A - Coated sintered hard metal carbide inserts

Info

Publication number: GB2061324A
Application number: GB8031409A
Authority: GB
Original assignee: Iscar Ltd
Current assignee: Iscar Ltd
Priority date: 1979-10-24
Filing date: 1980-09-29
Publication date: 1981-05-13
Also published as: IT1194949B; FR2467689B1; IT8012691A0; DE3039775A1; IL58548A; ZA806039B; DE3039775C2; IL58548A0; GB2061324B; FR2467689A1

Abstract

Sintered hard metal carbide products, such as disposable cutting inserts are coated with one or more layers of titanium carbide, titanium nitride or titanium carbonitride, with an intermediate layer consisting essentially for aluminium oxide and having a thickness of from 0.1 to 3 microns and interposed between the metal carbide and said coating. In a preferred embodiment said coating further comprises an outermost surface coating layer consisting of aluminium oxide and having a thickness of from 0.4 to 20 microns.

Description

SPECIFICATION Sintered hard metal products having a multi-layer wear-resistant coating This invention relates to sintered hard metal products coated with an improved surface coating and having both increased wear resistance and increased hardness and relatively high strength.

Sintered hard metals, also known as cemented carbides, consist of a mixture of one or more carbides of, mainly, tungsten, titanium, tantalum or niobium and a binder metal which in most cases is cobalt, but may also be nickel or a mixture of cobalt and nickel. Owing to their great surface hardness, wear resistance and strength, these sintered hard metals are extensively used for industrial applications in cutting tools and the like for machining of steel and other metals.

A considerable number of methods have been developed in recent years to enhance the wearresistance and other desirable mechanical properties of hard metal products, such as cutting inserts, thus to lengthen their useful lifetime. Thus is is known to apply to hard metal products very thin surface coatings consisting of carbides, nitrides or carbonitrides of certain metals, especially titanium, and various combinations of such compounds. It is also known that further advantages can be obtained bv using a coating consisting of two or more different layers of the abovementioned carbide and nitride compounds, the layers being applied one above the other.In particular it has been proposed to provide in such a multi-layer coating an outermost layer of titanium nitride, serving as a diffusion barrier layer between the coated hard metal tool and the machined material, over an innermost layer of titanium carbide and a second, intermediate layer of titanium carbonitride.

It is also known to coat sintered metal products with very thin, wear-resistant ceramic coatings consisting principally of aluminium oxide, either as a single layer having a thickness of a few microns, or as an outer coating layer having a thickness of 0.2-20 microns over one or more inner layers of the conventional carbide, nitride or carbonitride coating compounds.

Practically all of the above-described known coated hard metal products possess the drawbacks that the various combinations of coatings and the processes used to apply them, result in the formation of a hard and frangible layer, intermediate to the substrate and the coating, with the consequent reduction in the rupture strength of the coated product. The increased wear resistance of such known coated products has thus been achieved at the cost of a loss in strength of the coated substrate.

Furthermore, each of the various types of the known coated hard metal products is adapted only to a limited range of applications, which compels the user and manufacturer in the field of metal machining to stock a large variety of coated hard metal tools so as to meet the numerous different specifications for the different metal machining tasks at hand.

It is thus the object of this invention to provide a coated sintered hard metal product wherein the coating combines an excellent wear resistance against all wear mechanisms, together with retention of maximal strength of the coated product, so as to adapt it to the widest possible spectrum of different applications.

The above object is achieved in accordance with the invention by providing a sintered hard metal product consisting of a cemented metal carbide substrate and a thin wear-resistant surface coating comprising one or more layers of titanium carbide, titanium nitride or titanium carbonitride and, optionally, an outermost layer of aluminium oxide, characterized in that an intermediate layer consisting essentially of aluminium oxide and having a thickness of from 0.1 to 3 microns is interposed between said substrate and said coating.

In accordance with the invention it has unexpectedly been found that the provision of the abovementioned very thin intermediate layer of aluminium oxide enables the formation thereabove of a number of outer coating layers, by the chemical vapour deposition technique, without any adverse effect on the rupture strength of the coated product. It is assumed that the intermediate aluminium oxide layer serves as a barrier between the substrate and the outer layer(s), in that it prevents the diffusion of carbon atoms from the hard metal substrate into the coating.It is known that, in the absence of such a barrier layer, diffusion of carbon out of the substrate results in the formation, in the surface layer thereof, immediately beiow the coating, of a carbon-deficient layer, consisting mainly of the compound Co3W3C3, which layer is generally referred to as the "71 phase" and is extremely frangible.

The intermediate aluminium oxide layer in accordance with the invention also serves as a barrier against crater breakthrough which is known to occur in wear processes in machining of metals. This effect is due to the fact that the intermediate aluminium oxide layer also prevents diffusion of carbon atoms from the cutting tool into the chips, thus arresting the diffusional wear mechanism. Furthermore, the intermediate aluminium oxide layer enveloping the entire surface of the substrate, underneath all the upper layers, affords good wear-protection after all these upper layers have been worn off. In the hitherto known coated hard metal tools such eventual exposure of the substrate, as a rule, results in an enhanced rate of wear of the substrate itself.However, the presence of the said protective intermediate layer of aluminium oxide in accordance with the invention, decreases the rate of this wear by inhibiting the development of the adhesive and abrasive wear mechanisms and also prevents oxidation of the hard metal substrate.

The intermediate aluminium oxide layer in accordance with the invention, as well as the remaining outer layer (or layers) are applied by the known chemical vapour deposition techniques, such as .described, e.g. in U.S. Patent specification Nos. 3,836,392; 3,914,473; 3,977,061; 3,837,896; 4,035,541 and 4,052,530. In accordance with the present invention the various parameters of this process of chemical vapour deposition are selected so as to allow a very thin intermediate layer of aluminium oxide to build up in a dense and fine crystalline structure and with a thickness of 0.13 microns.These desired properties of the intermediate aluminium oxide layer in accordance with the invention are obtained by deposition from the gaseous phase on the sintered hard metal substrate which is heated in a furnace to a temperature in the range of from 700 to 12000C, preferably from 927 to 11 270C. It has been found in accordance with the invention that the pressure at which the chemical vapour deposition is carried out is quite critical for the quality and structure of the deposited intermediate aluminium oxide layer. The required dense and thin layer can be obtained, at controlled thicknesses, by deposition under reduced pressure and it has been found that the lower the pressure, the slower is the rate of deposition and the denser the structure of the obtained layer.Thus, for the formation of the intermediate aluminium oxide layer in accordance with the invention, the preferred pressure range inside the furnace is from 10 to 100 torr. Against this, pressures of from 20 to 200 torr can be used when depositing the outermost aluminium oxide layer, in accordance with a preferred embodiment of the invention, to be described hereinbelow. It has further been found that if the chemical vapour deposition of the aluminium oxide layer is carried out at pressures higher than about 200 torr, the layers obtained are porous and have a less dense structure. This thin and dense aluminium oxide layer has a further advantage in that it ensures the deposition thereupon of the subsequent outer layers in a similar dense, fine-crystalline structure.The result is that the rate of deposition of the outer layers on top of the intermediate aluminium oxide layer according to the invention, becomes independent of the composition and structure of the substrate.

The above features of the coating of the sintered hard metal products according to the invention make it possible to achieve identical wear resistance properties with hard metal substrates of differing compositions but equal in hardness. Thus, the above described separation of the substrate from the outer coating layers by means of the protective intermediate aluminium oxide layer, enables the coated hard metal product to retain a mechanical strength which is almost independent of the total thickness of the coating up to, say, 30 microns as contrasted to the conventional coating methods (lacking this protective intermediate layer), where an increase in the thickness of the coating invariably results in a decrease of the strength of the coated hard metal product.

In accordance with a preferred embodiment of the invention the wear resistant surface coating comprises, in addition to the intermediate thin aluminium oxide layer, at least two outer layers, of which the outermost one is also composed essentially of aluminium oxide and has a thickness of 0.4-20 microns. The advantage achieved by providing such an outermost aluminium oxide layer are those hitherto reported in respect of known ceramic-coated hard metal products, namely: high wear resistance at high temperatures owing to the stability of the aluminium oxide at such high temperatures, inhibition of oxidation and lesser decrease of hardness with increasing temperatures (so-called "hothardness") as compared to other coating materials.The use of such an outer aluminium oxide coating layer affords protection against abrasive wear even at high temperatures and the structural stability of the aluminium oxide, even at high temperatures, additionally prevents the adhesive wear which is due to removal of material particles from the coating. Furthermore, the presence of an outermost layer of aluminium oxide in the coating, prevents crater formation in tools adapted for machining, because this outer layer serves as a barrier between the tool and the chip, preventing loss by diffusional wear of carbon atoms from the coating and the substrate to the chip, which mechanism usualiy causes accelerated crater wear.

By virtue of all the above, wear protection of the hard metal products by an outer coating layer of aluminium oxide, in accordance with this preferred embodiment of the invention, enables the production of tools capable of withstanding considerably higher machining speeds than hitherto known coated hard metal tools. Furthermore, tools having an outermost aluminium oxide layer in accordance with this embodiment are suitable for machining of such materials (e.g. super alloys) which are known to cause very rapid wear of conventional carbide- or nitride-coated hard metal tools. This is due to the fact that the coefficient of friction of alumina on steel is lower than the coefficients of the metal carbides and nitrides.

As stated above, the wear resistant coating of the sintered hard metal products in accordance with the invention consists of one or more outer layers of titanium carbide, titanium nitride and/or titanium carbonitride superimposed on the intermediate thin aluminium oxide layer and underneath the optional outermost layer of aluminium oxide, if such is present. All these outer layers are also applied, in accordance with the invention, by the known chemical vapour deposition technique mentioned above, and each of these outer layers preferably has a thickness of from 1 to 1 5 microns. The total thickness of the coating including said intermediate aluminium oxide layer which is the essential feature of the present invention, should preferably not exceed about 20 microns.

As regards the specific nature, number and sequence of the aforesaid outer layers, the present invention allows, and is meant to include a considerable number of variations and combinations, each imparting to the coated product a specific set of physical properties and wear-resistance characteristics.

The following embodiments are mentioned by way of example: (1) In accordance with one embodiment of the invention, the outer layers of the coating, above the intermediate thin aluminium oxide layer, consist of a first layer of titanium carbide having a thickness of 1 to 12 microns, and a second, outermost aluminium oxide layer, as described above in connection with the preferred embodiment of the invention. This combination of coating layers on hard metal cutting inserts affords the lowest rate of flank wear in the machining of both steel and cast iron.

(2) Similar advantageous results are obtained, in accordance with another embodiment of the invention, when the titanium carbide layer in the embodiment described under (1) above, is replaced by a titanium carbonitride layer having a thickness of from 1 to 12 microns.

(3) If the titanium carbide layer in the embodiment described under (1) above is replaced by a layer of titanium nitride having a thickness of 1 to 12 microns, there results a coated hard metal product possessing the highest end strength as well as a good resistance to crater formation.

(4) In accordance with yet a further embodiment of the invention, remarkable resistance against both flank wear and crater wear is obtained with a coating comprising, on top of the thin intermediate aluminium oxide layer, a first outer layer of titanium carbide, a second transition layer of titanium carbonitride, a third outer layer of titanium nitride and a fourth, outermost layer of aluminium oxide. The total thickness of all the coating layers in this embodiment is preferably from 2 to 1 5 microns.

(5) A similar embodiment of the invention is one wherein the positions of the titanium carbide and the titanium nitride layers on both sides of the transition layer of titanium carbonitride in the embodiment described under (4) above, is reversed. The total thickness of the coating is again, preferably, from 2 to 1 5 microns. in this case the coated product possesses an improved resistance against flank wear.

It has been found in accordance with the invention that in all cases where the first outer layer, immediately adjacent the thin aluminium oxide layer according to the invention, is composed of titanium nitride, maximal values of end strength are obtained. This may be due to the following reasons: (a) Both the thin intermediate aluminium oxide layer and the adjacently subsequent titanium nitride layer require no carbon for their build-up and, therefore, da not disturb the carbon balance in the hard metal substrate.

(b) The thermal expansion coefficient (at) of titanium nitride (9.4 x 10-6/ K) is close, but somewhat higher than that of aluminium oxide (8.4 x 10-6/ K) which fact prevents the coating frown peeling off the substrate, even when the thickness of the first intermediate aluminium oxide layer reaches the upper limits of the afore-specified range.

The invention will now be illustrated in more detail by the following non-limiting examples. The sintered hard metal substrates used for the coating experiments in Examples 1 to 6 were conventional cutting inserts, according to the international ISO classification as shown in Table I herein, and have the compositions given in that table.

TABLE I

ISO classification code WO % TaC % NbC % TiC % Co % K 20 94 6 K10 91 .7 2.5 5.8 M15 83 6.5 0.5 3 7 P40 77 7A 0.6 4 11 EXAMPLE NO. 1 Sintered hard metal cutting inserts were placed in a furnace provided with a gas inlet and a gas outlet connected to a vacuum pump.The furnace was heated to 1 0270C and fed with a gaseous mixture consisting of 92.5% by volume of H2,3% of AIC13 and 4.5% of CO2, the pressure inside the furnace being maintained at 50 torr. The total feed rate of the gaseous mixture was 40 NL per minute.

This process was continued for 30 minutes, whereafter the gas feed was stopped, the pressure inside the furnace was equated to atmospheric pressure and the furnace was filled with H2. The furnace was then fed with a gaseous mixture consisting of 88% by volume of H2, 5% of TiCI4 and 7% of methane, at a total feed rate of 80 NL per minute. This stage was continued for 1 50 minutes and the furnace was then allowed to reach ambient temperature and pressure.

There were obtained sintered hard metal platelets coated with an inner layer of aluminium oxide having a thickness of 0.3 to 0.4 u and an outer layer of titanium carbide having a thickness of 5 to 6,u.

EXAMPLE NO.2 Sintered hard metal cutting inserts were placed in the furnace described in Example 1 and the furnace was heated to 1 0270C. A gas mixture consisting of 92.5% by volume of H2,3% of AICI3 and 4.5% of CO2 was fed to the furnace at a pressure of 50 torr and a total feed rate of 40 NL per minute, for 15 minutes. The pressure inside the furnace was then equated to atmospheric pressure, the furnace was filled with hydrogen and then fed with a gas mixture consisting of 88% by volume of H, 5% of TiCI4 and 7% of methane, at a total feed rate of 70 NL per minute, for 150 minutes.The feed was then interrupted, the pressure in the furnace reduced to 100 torr by means of the vacuum pump connected thereto, and the furnace fed with a gaseous mixture consisting of 90% by volume of H2,4% of AICI3 and 6% of CO2, at a total feed rate of 70 NL per minute, for 120 minutes. Thereafter the furnace was allowed to reach ambient temperature and pressure.

There were obtained sintered hard metal cutting inserts coated with a first, inner layer of aluminium oxide having a thickness of 0.2 to O.3,u, a second layer of titanium carbide having a thickness of 5 to 6 ,u and a third, outer layer of aluminium oxide having a thickness of 1.5 to 2 .

EXAMPLE NO.3 Sintered hard metal cutting inserts were coated by placing them in the furnace described in Example 1 which was then heated to 10270C. A gaseous mixture consisting of 92.5% by volume of H2, 3% of AICI3 and 4.5% of CO2 was fed into the furnace at a pressure of 50 torr and at a rate of 40 NL per minute, for 15 minutes. The furnace was then allowed to reach atmospheric pressure, filled with hydrogen and thereafter fed with a gas mixture consisting of 70% by volume of H2, 5% of TiCI4 and 25% of N2 at a total feed rate of 100 NL per minute, for 130 minutes. The pressure was then reduced by means of the vacuum system to 100 torr and the furnace fed with a gaseous mixture consisting of 90% by volume of H2,4% of AICI3 and 6% of CO2 at a rate of 70 NL per minute, for 120 minutes.After the furnace had been allowed to reach ambient temperature and pressure, the sintered hard metal cutting inserts were discharged therefrom and found to be coated with a first, inner layer of aluminium oxide having a thickness of 0.2 to 0.3 , a second layer of titanium nitride having a thickness of 5.5 to 6.5 and a third, outer layer of aluminium oxide having a thickness of 2 to 2.5 Su.

EXAMPLE NO.4 Sintered hard metal cutting inserts were coated by a procedure similar to that described in Examples 2 and 3, the temperature of the furnace being maintained thoughout the process at 1 0270C.

In the first stage the furnace was fed with a gaseous mixture consisting of 92.5% by volume of H2,3% of AICI3 and 4.5% of CO2 at a pressure of 50 torr and a total feed rate of 40 NL per minute for 1 5 minutes.

Thereafter the furnace was allowed to reach atmospheric pressure, filled with hydrogen and fed with a mixture of gases consisting of 79% by volume of H2,5% of TiCI4 35% of methane and 12.5% of N2, at a total feed rate of 90 NL per minute, for 135 minutes. Thereafter the pressure within the furnace was reduced to 100 torr and the furnace was fed with a mixture of gases consisting of 90% by volume of H2, 4% of AICI3 and 6% of CO2 for 120 minutes at a total feed rate of 70 NL per minute.

As a result the sintered hard metal cutting inserts were coated with a first, inner layer of aluminium oxide having a thickness of 0.2 to 0.3 , a second layer of titanium carbonitride having a thickness of 5 to 6 y and a third, outer layer of aluminium oxide having a thickness of 1.5 to 2 .

EXAMPLE NO. 5 Sintered hard metal cutting inserts were coated, in the furnace described in Example 1, with a first inner layer of aluminium oxide in the same manner as described in Examples 2 4. Thereafter the furnace was allowed to reach atmospheric pressure, filled with hydrogen and fed with a gaseous mixture consisting of 88% by volume of H2, 5% of TiCI4 and 7% of methane at a total feed rate of 80 NL per minute, for 120 minutes. Thereafter the gas feed was replaced by a different mixture consisting of 79% by volume of H2, 5% by volume of TiCl4, 3.5% of methane and 12,5% of N2 at a total feed rate of 90 NL per minute, for 25 minutes. Next, the gaseous feed mixture was changed again to a mixture consisting of 70% by volume of H2, 5% of TiCl4 and 25% of N2, at a total rate of 100 NL per minute, for 60 minutes.The pressure inside the furnace was then reduced to 100 torr and the furnace fed with yet another gaseous mixture consisting of 90% by volume of H2, 4% of AlCl3 and 6% of CO2 at a total feed rate of 70 NL per minute for 120 minutes. The furnace was then allowed to reach ambient temperature and pressure and the inserts discharged therefrom.

There resulted coated sintered hard metal cutting inserts, the coating consisting of a first, inner layer of aluminium oxide having a thickness of 0.2 to 0.3 , a second layer of titanium carbide having a thickness of 3.5 to 4,u, a third layer of titanium carbonitride having a thickness of 1 to 1.5 , a fourth layer ot titanium nitride having a thickness of 2 to 2.5 y and a fifth, outer layer of aluminium oxide having a thickness of 2 to 2.5 .

EXAMPLE NO.6 Sintered hard metal cutting inserts were placed in a furnace as described in Example 1 and coated with a first layer of aluminium oxide by feeding the furnace with a gas mixture consisting of 92.5% by volume of H2,3% of AICI3 and 4.5% of CO2 at 50 torr and a total feed rate of 40 NL per minute, for 30 minutes. The furnace was then allowed to reach atmospheric pressure, it was filled with hydrogen and then fed with a gas mixture consisting of 70% by volume of H2,5% of TiCI4 and 25% of N2 at a rate of 100 NL per minute, for 60 minutes. The gas mixture feed was then changed to one consisting of 79% by volume of H2, 5% of TiCI4,3.5% of methane and 12.5% of N2, at a feed rate of 90 NL per minute, for 25 minutes.The gas mixture feed was then changed again to one consisting of 88% by volume of H2, 5% of TiCI4 and 7% of methane, at a total feed rate of 80 NL per minute, for 120 minutes. Thereafter the pressure inside the furnace was reduced by means of the vacuum pump to 100 torr and at that pressure the furnace was fed with a gas mixture consisting of 90% by volume of H2,4% of AICI3 and 6% of CO2, at a total feed rate of 70 NL per minute, for 120 minutes. The furnace was then cooled to room temperature and allowed to reach atmospheric pressure.

There were obtained sintered hard metal cutting inserts coated with a first, inner layer of aluminium oxide having a thickness of 0.3 to 0.4 , a second layer of titanium nitride having a thickness of 2 to 2.5,u, a third layer of titanium carbonitride having a thickness of 1 to 1.5 u, a fourth layer of titanium carbide having a thickness of 3.5 to 4 y and a fifth, outer layer of aluminium oxide having a thickness of 1.5 to 2 y.

EXAMPLE NO. 7 The metal cutting performance of coated cutting inserts prepared in accordance with Examples 1-6 was compared with that of conventional, commercially available coated cutting inserts by machining of steels and of cast iron, by means of the following tests: 1. Machining test on steel This test was performed on carbon steel AiSi 1050 at a speed of 230 m per minute, a feed of 0.25 mm per revolution and a depth of cut of 2.5 mm. The cutting inserts were in the form of TNMG 432.

The flank wear VB was measured after 2, 4, 8, 12, 16 and 20 minutes. The rate of wear VB was measured in the range between 2 and 16 minutes of turning.

2. Turning test on grey cast iron This test was performed on cast iron at a speed of 130 m per minute at a feed of 0.25 mm per revolution and at a depth of cut of 2.5 mm. The coated inserts were in the form TNMA 432.

Flank wear VB was measured after 2, 4, 8, 12, 16 and 20 minutes and the rate of wear Ve was measured in the range between 2 and 16 minutes of turning.

The depth of the formed crater Kt was measured after 1 6 minutes of turning.

3. Milling test (cf. "Short Milling Test", A. Ber, S. Kaldor and E.Lenz, CIRP Meeting, India, August 1977) The milling test was performed in order to determine the transverse rupture strength at the corners of the insert after coating. The test was made on AiSi 1060 steel with a milling cutter having a diameter of 100 mm at a speed of 88 m per minute and a depth of cut of 3 mm and increasing feeds of 0.1 mm per tooth. There were measured the breaking feeds Sz at which the coated inserts broke. The inserts tested were the same as in Test 1 above, i.e. of the shape TNMG 432.

The feeds tested were 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1 mm per tooth. Each feed was tested along 700 mm.

The results of the performance tests are shown in Table II.

The results in Table II show as follows: 1. Cutting inserts coated with a single coat of aluminium oxide having a thickness of 5 y have a very low transverse rupture strength.

2. Inserts coated in accordance with the invention with a first inner layer of aluminium oxide having a small thickness and, thereupon, a layer of titanium carbide and an outer layer of aluminium oxide, have a transverse rupture strength 14% higher than similar inserts not comprising the said first inner layer of aluminium oxide. Further, the wear resistance of the inserts according to the invention shows an increase of 43% in machining of cast iron and of 30% in machining of steel.

3. A coating according to the invention consisting of 5 layers in the sequence: aluminium oxide, titanium nitride, titanium carbonitride, titanium carbide and an outer layer of aluminium oxide, on an ISO Ml 5 substrate exhibits the highest transverse rupture strength in the milling test, as well as the best resistance to crater formation in machining of cast iron.

4. Resistance to crater formation and wear resistance in machining of cast iron are the same for the different substrates ISO M15 and ISO K1 0.

5. When cutting inserts coated with a single layer of titanium carbide are compared with inserts coated with a first thin layer of aluminium oxide, in accordance with the invention, topped by an outer layer of titanium carbide, the latter inserts show a 14% increase of transverse rupture strength and a 32% increase of wear resistance in the machining of steel.

TABLE II

OLC' !tEO Depth of m R,ate, of Wear Crater Coating Rate of Wear Feed 5z mm VB it/min Kt tL after ~ Sequence of layer: innerouter YB /min tooth (cast in 16 min' out turning) milling) (cast iron) Commercial ISO MIS 5 Al3O3 16 0.3 5 3 CT 55N TiCN +2.5 Al2O3 12 0.8 8 5 ox E l sC '? tq 'E CO O. eo. s. eg oM c3 O aM t q C .

m O ' - ILO) 4 O'25Al2O3+6TiCN+2Al2O3 9 0.8 5 2 5 1 sC C 6 s 9 0,9 z 5 a " 2 ISO K10 054 A12O3 +5NTiC +2 Al2O3 4' 3 2 lSOK20 0,254AI2O3+5.5TiC+2Al2Q 5 4 Om ISO P40 uncoated 1 A 6 > -o q , O f 0.2 Al2O3 +41LTiC +1TiCN +2TiN + o N Z t < zt 66: z . O ct Z t cD - F oz cy u) Z- ~ Z F U < ar + + + ot S + + ~ t eD N O i ( N N N N n + N . < 3 i- Y iS S tS) F F O '- S O S t U) . N N CM CS CX C 1 N c) co t to o o o o o o o o 3 f o ov 0Or 00) c o , cq cq .Y uz o O o &commat; cL O ILI ID O L

Claims

1. A sintered hard metal product consisting of a cemented metal carbide substrate and a thin wear-resistant surface coating comprising one or more layers of titanium carbide, titanium nitride or titanium carbonitride and, optionally, an outermost layer of aluminium oxide, characterized in that an intermediate layer consisting essentially of aluminium oxide and having a thickness of from 0.1 to 3 microns is interposed between said substrate and said coating.

2. A product according to Claim 1 comprising, in addition to said intermediate layer of aluminium oxide, a single outer layer of titanium carbide, titanium nitride or titanium carbonitride, having a thickness of from 1 to 15 microns.

3. A product according to Claim 1 comprising immediately adjacent said intermediate aluminium oxide layer, a first outer layer and thereover a second outer layer , each of said first and second outer layers composed of a different one of titanium carbide, titanium nitride and titanium carbonitride, the total thickness of said first and second outer layers being from 2 to 1 5 microns.

4. A product according to Claim 1 comprising, immediately adjacent said intermediate aluminium oxide layer, a first outer layer of titanium carbide, a second outer layer of titanium carbonitride and a third outer layer of titanium nitride, the total thickness of said first, second and third outer layers being from 2 to 1 5 microns.

5. A product according to Claim 1 comprising, immediately adjacent said intermediate aluminium oxide layer, a first outer layer of titanium nitride, a second outer layer of titanium carbonitride and a third outer layer of titanium carbide, the total thickness of said first, second and third outer layers being from 2 to 1 5 microns.

6. A product according to any one of Claims 2 to 5 further comprising an outermost surface coating layer consisting essentially of aluminium oxide and having a thickness of from 0.4 to 20 microns.

7. A product according to any one of the preceeding claims, wherein all the surface coating layers have been applied by chemical vapour deposition.

8. A product according to Claim 7 wherein the chemical vapour deposition of all the coating layer has been conducted at a temperature of from 700 to 12O00C.

9. A product according to Claim 8 wherein the chemical vapour deposition of all the coating layers has been conducted at a temperature of from 927 to 1 1270C.

10. A product according to Claims 8 or 9 wherein the intermediate layer of aluminium oxide has been applied at a pressure of from 10 to 100 torr.

11. A product according to Claims 6, 7 and 8 or 9 wherein the outermost aluminium oxide layer has been applied at a pressure of from 20 to 200 torr.

12. A product according to any one of the preceding Claims in which the cemented carbide substrate comprises tungsten carbide and optionally tantalum carbide, niobium carbide and/or titanium carbide, in a cobalt matrix.

1 3. A product according to any one of the preceding Claims in the form of a disposable cutting insert for use in the machining of metals and other materials.

14. A product according to any one of the preceding Claims in which the total thickness of the coating, including the intermediate layer of aluminium oxide, is less than 1 5 microns.

1 5. A coated sintered hard metal product substantially as herein described and exemplified.