Cemented carbide, coated articles having the cemented carbide as base, in particular coated hard tools

Abstract

 Novel cemented carbide having improved hardness and corrosion resistance comprising 3 to 25 % by weight of the sum of Co and Ni, 10 to 30 % by weight of chromium in the term of chromium carbide with respect to the sum of Co and Ni and the balance being tungsten carbide and inevitable impurities. The cemented carbide is advantageously used in cutting tools for hard-to-cut materials and in various fields which require improved hardness and corrosion resistance such as structural parts, machine parts and ornaments. On a base made of the cemented carbide, the cutting tool has a surface layer including at least one layer made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides of at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si.

Description

[0001] The present invention relates to a novel cemented carbide and coated cemented carbide having the cemented carbide as a base, and coated hard tools prepared therefrom.

[0002] The cemented carbide and coated cemented carbide according to the invention are advantageously used to prepare machining tools having improved hardness and corrosion resistance, and can be used in a variety applications which require improved hardness and corrosion resistance such as structural parts, machine parts and ornaments or decorations.

[0003] The coated hard tools having a base of the cemented carbide according to the present invention show very high resistance to wear welding and can be used in cutting work in such materials that are difficult to be cut.

[0004] Improvement in hardness and strength are uninterruptedly requested in cemented carbide for machining tools, since materials to be worked become harder and cutting speed become faster.

[0005] For example, in cutting tools, cemented carbide is required to resist so-called interface-wear caused by contact with a surface (so called black skin) of material to be cut, in addition to resistance to flank-wear. High resistance to crater-wear which is often observed at a cutting face is also required. Recently, resistance to welding of material to be cut onto cutting tools is also required. Such welding is often observed in cutting of materials that are difficult to be cut (hereinafter, hard-to-cut materials) such as Ni alloy, Ti alloy and steels of high hardness and forms a build-up edge which causes chipping of tools, resulting in shortening of tool life.

[0006] JP-A-61-12847 discloses to add V (vanadium) and Cr (chromium) to limit growth of grains of WC (tungsten carbide) so as to improve the corrosion resistance.

[0007] JP-A-4-31012 and "Powders and powder metallurgy" 31, 1984, p56 describes such a fact that addition of Cr₃C₂ (chromium carbide) improve the corrosion resistance.

[0008] In practice, in cemented carbide for cutting tools, fine particles of WC is used to improve the hardness and Cr₃C₂ is added to limit growth of grains and to resist interface-wear and crater-wear so as to improve the corrosion resistance.

[0009] However, decrement in resistance to propagation of crack is often observed in cemented carbide produced with fine particles. In fact, greater energy is necessary to propagation crack in the coarse particles and hence the coarse particles contribute to resist to propagation of crack.

[0010] Still more, fine particles are easily fall off than coarse particles because gripping force of hard particles in bonding phase of cemented carbide used as cutting tool is lost or lowered at high cutting temperatures, resulting in rapid abrasion wearing.

[0011] In this type cemented carbide, higher content of inevitable impurities cause formation of fragile phase in alloy when Cr₃C₂ is added, resulting in that the resistance to crack propagation is lost and hence the strength is lost.

[0012] On the other hand, attempt has been made to deposit a surface layer such as TiN on a surface of cemented carbide to improve its wear resistance. However, the base material of cemented carbide suffers from severe wear so that satisfactory effect or advantage is not yet obtained.

[0013] An object of the present invention is to solve the above-mentioned problems and to provide a novel cemented carbide possessing desired characteristics for cutting tools and improved in wear resistance.

[0014] Another object of the present invention is to provide a coated cemented carbide comprising the cemented carbide as a base and a surface layer and possessing high resistance to corrosion and welding, which is advantageously used in hard cutting tools for hard-to-cut materials.

[0015] The present invention provides a cemented carbide comprising 3 to 25 % by weight of the sum of Co and Ni, 10 to 30 % by weight of chromium in the term of chromium carbide (Cr₃C₂ ) with respect to the sum of Co and Ni and the balance being tungsten carbide (WC) and inevitable impurities.

[0016] The present invention also provide a coated cemented carbide, in particular coated hard tool, comprising a base of the cemented carbide and a surface layer deposited on the base, the surface layer including at least one layer made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides of at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si.

[0017] In a preferred embodiment of cemented carbide according to the present invention, a proportion of Ni ranges from 0.4 to 80 % by weight to the sum of Co and Ni.

[0018] In another preferred embodiment of cemented carbide according to the present invention, an average particle size of tungsten carbide ranges from 0.3 to 5 µm.

[0019] In still another preferred embodiment of cemented carbide according to the present invention, tungsten carbide is composed of fine particles having an average particle size of 0.3 to 1.1 µm and coarse particles having an average particle size of 1.2 to 5 µm, and the ratio of said coarse particles to the total amount of said tungsten carbide being 0.1 to 0.9.

[0020] The cemented carbide according to the present invention is characterized in increased proportion of Cr₃C₂ (chromium carbide) comparing to the conventional cemented carbides.

[0021] In fact, the cemented carbide according to the present invention comprising 3 to 25 % by weight of Co + Ni, 10 to 30 % by weight of chromium in the term of chromium carbide (Cr₃C₂ ) with respect to the sum of Co and Ni and the balance being tungsten carbide (WC) and inevitable impurities, assure extremely longer life time in cutting tools comparing to known types cemented carbides such as WC/Co, WC/trace of Cr₃C₂-Co and WC/trace of Cr₃C₂/Co/trace of Ni type in cutting of Ni alloys such as inconel and nimonic.

[0022] This characteristic is far more improved when the cemented carbide contains 0.4 to 80 % by weight of Ni to the sum of Co and Ni.

[0023] Chromium can be introduced in a form of chromium carbide or of elemental metal chromium or other chromium compound as far as the proportion of chromium expressed in term of chromium carbide falls in the defined range.

1) Amount of Co and Ni

[0024] If the contents of Co and Ni is not higher than 3 % by weight, the toughness is undesirably lowers. On the other hand, if the contents of Co and Ni exceeds 25 % by weight, the resistance to plastic deformation and wearing become disadvantageously low. Therefore, the contents of Co and Ni should be in the range of 3 to 25 % by weight.

2) Amount of Cr

[0025] If a proportion of chromium in term of chromium carbide (Cr₃C₂) with respect to the amount of Co and Ni is not higher than 10 % by weight, resistance to oxidation and interface-wearing become poor. On the other hand, if it exceeds 30 % by weight, fragile phase is generated resulting in sharp drop in toughness. Therefore, the weight ratio of Cr₃C₂ to the sum of Co and Ni is preferably in a range of 10 to 30 % by weight.

3) Amount of Ni

[0026] If the proportion of Ni is not higher than 0.4 % by weight to the sum of Co and Ni, desired resistance to wearing and interface wearing can not be obtained. On the other hand, if the proportion of Ni exceeds 80 % by weight, sintering become insufficient resulting in lowering of toughness or sintering must be effected at higher temperature which deteriorate wearing resistance.

4) Average particle size of tungsten carbide

[0027] If the average particle size of tungsten carbide is not higher than 0.3 µm, sufficient sintering can not be effected and hence the strength become poor. If the average particle size becomes larger than 5 µm, wearing resistance become insufficient.

5) Ratio of coarse particles to the total tungsten carbide

[0028] If the ratio of course particles to the total amount of tungsten carbide is not higher than 0.1, the strength become insufficient and if it exceeds 0.9, the wear-resistance becomes insufficient.

[0029] The coated hard tool according to the present invention comprises a base made of the cemented carbide according to the present invention and a surface layer deposited on a surface of the cemented carbide base. The surface layer includes at least one layer made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides of at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si.

[0030] The coated hard tools having the surface layer possess very longer tool life comparing to known coated hard tools produced by the conventional technique.

[0031] In the preferred embodiment of the coated hard tool according to the present invention, the surface layer includes a multi-layered structure consisting of at least two unit layers superimposed alternately, each unit layer being made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides of at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si, each unit layer having a thickness of 0.2 to 100 nm, and the multi-layered structure having a total thickness of 0.5 to 10 µm.

[0032] If the thickness of each unit layer is not higher than 0.2 nm, adjacent layers are intermixed each other and hence advantages of the multi-layered structure can not be obtained. Similarly, if the thickness of each unit layer exceeds 100 nm, interaction of adjacent unit layers can not be expected so that advantage of the multi-layered structure is not obtained.

[0033] Advantages of coating are not obtained if the total thickness of the multi-layered structure is not higher than 0.5 µm. On the other hand, if the total thickness exceeds 10 µm, chipping increases disadvantageously.

[0034] Addition of at least one element selected from a group comprising Ge, Sn and Pb to at least one unit layer makes tool life longer.

[0035] In another embodiment of the coated hard tools according to the present invention, the surface layer comprises the multi-layered structure and mono-layer structure having no layered structure, two structures being superimposed alternately at least for five (5) times, said mono-layer structure being made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides comprising at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si and having a thickness of 100 to 5000 nm, and the surface layer having a total thickness of 0.5 to 10 µm.

[0036] Thanks to this structure, stress in the coated surface layer decrease and chipping of the coated surface layer decrease, which assure very long tool life. In this embodiment, advantage of stress relaxation can not be expected if the multi-layered structure and mono-layer structure are not superimposed alternately for more than 5 times (more than 10 structures).

[0037] The coated hard tool according to the invention has preferably a bottom layer consisting of TiN and having a thickness of 0.02 to 2 µm. If this bottom layer has not a thickness higher than 0.2 µm, no effect as coating layer is obtained. On the contrary, if the thickness exceed 2 µm, wear-resistance is spoiled disadvantageously.

[0038] The coated hard tool according to the invention has preferably an outermost layer having a surface roughness "Ra" in accordance with JIS B 0601 is 0.18 µm. This permits tips to flow smoothly and prevent welding of chip so that a work to be cut can be maintained in very good condition on its surface and cutting edge chipping and breaking from the welded portions can be prevented advantageously, resulting in longer tool life. If the surface roughness "Ra" exceed 0.18 µm, trouble of welding increase.

[0039] Now, the present invention will be explained with reference to Examples. The following disclosure is merely for explanation of the present invention but no way limits the scope of the present invention.

Example 1

[0040] Following components were mixed in a wet form to prepare material powder No. 1:

	% by weight
WC powder having average particle size of 2 µm:	28
WC powder having average particle size of 0.7 µm:	65.7
Cr3C2 powder having average particle size of 2 µm	0.8
Ni powder having average particle size of 1.5 µm	0.5
Co powder having average particle size of 1.5 µm	5

[0041] The material power No. 1 thus prepared was compacted and then sintered in vacuum under 10^-2 Torr at a temperature of 1400 °C to obtain sample No. 1. In this sample No. 1, a proportion of Cr₃C₂ to the sum of Co and Ni is 14.5 % by weight, a proportion of Ni to the sum of Co and Ni is 9.1 % by weight. An average particle size of total WC is 1 µm and a ratio of coarse WC powder having average particle size of 2 µm to the total WC is 0.3.

[0042] Following components were mixed in a wet form to prepare material powder No. 2.

	% by weight
WC powder having average particle size of 1 µm:	89
Cr3C2 powder having average particle size of 2 µm	2
Ni powder having average particle size of 1.5 µm	3
Co powder having average particle size of 1.5 µm	6

[0043] The material power No. 2 thus prepared was compacted and then sintered in vacuum under 10^-2 Torr at a temperature of 1,400 °C to obtain sample No. 2. In this sample No. 2, a proportion of Cr₃C₂ to the sum of Co and Ni is 22.2 % by weight, a proportion of Ni to the sum of Co and Ni is 33.3 % by weight.

[0044] For comparison, material powder No. 3 whose composition is outside the present invention was also prepared. The material powder No. 3 was prepared from the same formulation as sample No. 1 but the content of Cr₃C₂ powder was changed to 0.3 % by weight and the content of Co powder was changed to 5.5 % by weight.

[0045] The material powder No. 3 was compacted and sintered under the same conditions as sample No. 1 to obtain sample No. 3. In this sample No. 3, a proportion of Cr₃C₂ to the sum of Co and Ni is 5 % by weight and a proportion of Ni to the sum of Co and Ni is 8.3 % by weight.

[0046] Table 1 shows the results of mechanical properties and corrosion resistance of samples No. 1 to 3.

[0047] In Table 1, corrosion loss was determined after samples were placed in 36 % HCl solution at 50 °C for 8 hours. Oxidation gain was determined after the samples were left in atmospheric environment at 1,000 °C for 30 minutes.

Table 1

	Hardness (Hv)	Deflective strength (kg/mm2)	Corrosion loss (g/m2·hr)	Oxdation gain (mg/mm2·hr)
Sample 1	1820	235	2.13	0.02
Sample 2	1730	210	1.97	0.01
Sample 3*	1780	285	10.5	1.57

(* : Comparative Example)

[0048] Table 1 reveals such facts that the samples No. 1 and 2 according to the present invention possess slightly lower deflective strength due to higher Cr₃C₂ contents but highly improved corrosion loss and oxidation gain comparing to sample No. 3 of comparative example.

[0049] In the present invention, sample No. 2 having higher content of Cr₃C₂ shows better corrosion resistance than sample No. 1.

Example 2

[0050] Cutting tools were manufactured from the samples No. 1 to 3 prepared in Example 1. The performances of tools were evaluated by actual cutting work effected under the conditions shown in Table 2.

Table 2

	Cutting condition 1	Cutting condition 2	Cutting condition 3
Shape of tool	CNMG432	CNMG432	CNMG432
Material to be cut	SUS304 round rod	SCM435 (Hs=250) round rod having longitudinal four grooves	Incone 1718 round rod
Cutting speed	120 m/min.	200 m/min.	50 m/min.
Feed	0.2 mm/revolution	0.28 mm/revolution	0.2 mm/revolution
Depth	1.5 mm	1.0 mm	1.5 mm
Cutting oil	water soluble	-	water soluble
Cutting time	15 minutes	5 minutes	20 minutes
Evaluation	Maximum width of wear (mm)	Number of broken cutting edges out of 20	Average width of wear (mm)

[0051] Performance of tools was evaluated by the maximum width of wear in the condition 1, by the number of broken cutting edges out of 20 cutting edges in the condition 2 and by the average width of wear in the condition 3 respectively. The result are summarized in Table 3.

Table 3

Sample No.	Note	Cutting condition 1	Cutting condition 2	Cutting condition 3
1	Example	0.08	4	0.12
2	Example	0.25	7	0.26
3	Comparative Example	1.24	20	more than 1 mm in 10 min.

[0052] Table 3 reveals that cutting tools made from cemented carbide according to the present invention possess very high performances in the resistance to wear and breakage.

[0053] Sample No. 1 shows better characteristics than sample No. 2 in cutting properties.

Example 3

[0054] Samples No. 4 to 14 having composition shown in Table 4 were prepared. Samples No. 12 to 14 were comparative examples.

[0055] Table 5 summarizes the results of characteristics of samples tested.

[0056] In this table, oxidation gain was evaluated by the same method as Example 1 and cutting condition 3 was the same as Example 2.

Table 5

Sample No.	Hardness (Hv)	Deflective strength (kg/mm2)	Oxidation gain	Cutting condition 3
4	1580	270	0.02	0.17
5	1840	210	0.02	0.38
6	1510	255	0.01	0.56
7	1670	240	0.02	0.24
8	1490	220	0.01	0.41
9	1600	270	0.02	0.37
10	1280	305	0.01	0.87
11	1360	280	0.02	0.74
12	1760	145	0.01	*1
13	1510	210	9.44	**2
14	1300	290	11.44	***3

*1: broken in 2 minutes

**2: cutting surface was waved in 5 minutes so that cutting was stopped (interface-wear : more than 2 mm)

***3: more than 1 mm in one minute.

Example 4

[0057] Coated hard tools were manufactured by depositing following coatings A to N on cemented carbides of Examples 1 to 3 and then performance of the tools thus obtained were evaluated.

Sample

[0058] 

A :

sample coated with 2 µm of ZrN

B :

sample coated with 10 µm of HfN

C :

sample coated with 8 µm of VCN

D :

sample coated with 10 µm of TiAIN

E :

sample coated with 2 µm of BN

F :

sample coated with 5 µm of Al₂O₃ containing 1 % Si

G :

sample coated with a surface layer having the total thickness of 3.5 µm and comprising 2,500 TiN layers each having a thickness of 0.2 nm and 2,500 AIN layers each having a thickness of 0.5 nm superimposed alternately.

H=

sample coated with a surface layer having the total thickness of 3.5 µm and comprising 250 TiN layers each having a thickness of 2 nm and 250 AIN layers each having a thickness of 5 nm superimposed alternately.

I=

sample coated with a surface layer having the total thickness of 9 µm and comprising 25 Al₂O₃ layers each having a thickness of 80 nm and 25 HfC layers each having a thickness of 100 nm superimposed alternately.

J=

sample coated with a surface layer having the total thickness of 3.5 µm and comprising 250 TiN layers each having a thickness of 2 nm and 250 AIN (containing 1% Si) layers each having a thickness of 5 nm superimposed alternately.

K=

sample coated with a surface layer having the total thickness of 2.05 µm and comprising five layers of multi-layer structure and five layers of mono-layer structure, the multi-layer structure comprising 15 TiN layers each having a thickness of 2 nm and 15 AlN₄ layers each having a thickness of 5 nm while the mono-layer structure is made of TiCN layer having a thickness of 0.2 µm.

L=

sample coated with a surface layer having the total thickness of 3.52 µm and comprising a TiN layer of 0.02 µm thickness at an interface with a base material and a layer of multi-layer structure comprising 250 TiN layers each having a thickness of 2 nm and 250 AIN layers each having a thickness of 5 nm superimposed alternately.

M=

sample coated with a surface layer having the total thickness of 3.51 µm and comprising a TiN layer of 0.01 µm thickness at an interface with a base material and a layer of multi-layer structure comprising 250 TiN layers each having a thickness of 2 nm and 250 AIN layers each having a thickness of 5 nm superimposed alternately.

N=

sample coated with a surface layer having the total thickness of 14.5 µm and comprising a TiN layer of 11 µm thickness at an interface with a base material and a layer of multi-layer structure comprising 250 TiN layers each having a thickness of 2 nm and 250 AIN layers each having a thickness of 5 nm superimposed alternately.

[0059] Cutting performance of samples were evaluated under the condition 3 of Example 2.

[0060] Table 6 to Table 8 summarize structures of surface layers used.

[0061] Table 9 is a list showing relation between sample numbers and the surface layer and base material sample Nos.

[0062] Table 10 shows the results obtained.

Table 6

Surface layer list (1)
Surface layer No.	Component	Thickness (µm)	Doped Element
A	ZrN	2
B	HfN	10
C	VCN	8
D	TiAIN	10
E	BN	2
F	Al2O3	5	Si 1%

Table 8

Surface layer list (3)
Surface layer	Multi-layer structure			TiN layer at Interface thickness (µm)
	Unit layer 1 (thickness µm)	Unit layer 2 (thickness µm)	Number of layers
L	TiN (2)	AIN (5)	500	0.02
M	TiN (2)	AIN (5)	500	0.01
N	Al2O3 ( 2)	HfC ( 5)	500	11

Table 9

List of sample No.
Surface layer	Base material sample No.
	1	2	3
A	A-1	A-2	A-3
B	B-1	B-2	B-3
C	C-1	C-2	C-3
D	D-1	D-2	D-3
E	E-1	E-2	E-3
F	F-1	F-2	F-3
G	G-1	G-2	G-3
H	H-1	H-2	H-3
I	I-1	I-2	I-3
J	J-1	J-2	J-3
K	K-1	K-2	K-3
L	L-1	L-2	L-3
M	M-1	M-2	M-3
N	N-1	N-2	N-3

Table 10

Surface layer	Sample No.	Wear4	Sample No.	Wear4	Sample No.	Wear4
A	A-1	0.069	A-2	0.169	A-3	more than 1 mm in 10 min.
B	B-1	0.072	B-2	0.173	B-3	more than 1 mm in 10 min.
C	C-1	0.076	C-2	0.188	C-3	more than 1 mm in 10 min.
D	D-1	0.068	D-2	0.17	D-3	more than 1 mm in 10 min.
E	E-1	0.078	E-2	0.175	E-3	more than 1 mm in 10 min.
F	F-1	0.075	F-2	0.174	F-3	more than 1 mm in 10 min.
G	G-1	0.049	G-2	0.148	G-3	more than 1 mm in 10 min.
H	H-1	0.045	H-2	0.145	H-3	more than 1 mm in 10 min.
I	I-1	0.055	I-2	0.15	I-3	more than 1 mm in 10 min.
J	J-1	0.039	J-2	0.136	J-3	more than 1 mm in 10 min.
K	K-1	0.033	K-2	0.126	K-3	more than 1 mm in 15 min.
L	L-1	0.042	L-2	0.138	L-3	more than 1 mm in 12 min.
M1	M-1	0.12	M-2	0.26	M-3	more than 1 mm in 10 min.
N2	N-1	0.01	N-2	0.24	N-3	more than 1 mm in 10 min.
Control3	1	0.12	2	0.26	3	more than 1 mm in 10 min.
M1 : M-1, M-2 are outside the scope of invention
N2 : N-1, N-2 are outside the scope of invention
Control3: sample having no surface layer
Wear4: mm

Example 5

[0063] In order to validate the effect of surface roughness, the sample No. 1 of Example 1 was coated with a variety of TiAIN layers each having a surface roughness (Ra) shown in Table 11 and the same cutting test as Example 4 was effected. The results are shown in Table 11.

Table 11

Sample No.	Ra (mm)	wea (mm)	welding
5-1	0.10	0.068	no deposition
5-2	0.19	0.090	observed
5-3	0.25	1.200	severe welding

[0064] This result shows that risk of welding is reduced when the surface roughness become smaller, so that better cutting performance is obtained.

[0065] As explained above, the cemented carbide according to the present invention possess excellent corrosion resistance. Therefore, its use is not limited to cutting tools but it can be used, as highly reliable cemented carbide material, in a variety of applications where good corrosion resistance is required, in particular in the field of cutting work of hard-to-cut materials such as high hardness steel, Ni based alloy, Co based alloy and Ti based alloy for hot rolling rolls, watch frames, sleeves and mechanical seal for sea water pump, high pressure valve seat and ball which require high corrosion resistance and hard ornaments or decorations.

Claims

1. Cemented carbide comprising 3 to 25 % by weight of the sum of Co and Ni, 10 to 30 % by weight of chromium in the term of chromium carbide with respect to the sum of Co and Ni and the balance being tungsten carbide and inevitable impurities.

2. The cemented carbide set forth in claim 1 wherein a proportion of Ni is 0.4 to 80 % by weight to the sum of Co and Ni.

3. The cemented carbide set forth in claim 1 or 2 wherein an average particle size of said tungsten carbide is 0.3 to 5 µm.

4. The cemented carbide set forth in any one of claims 1 to 3 wherein said tungsten carbide is composed of fine particles having an average particle size of 0.3 to 1.1 µm and coarse particles having an average particle size of 1.2 to 5 µm, and the ratio of said coarse particles to the total amount of said tungsten carbide being 0.1 to 0.9.

5. A coated cemented carbide comprising a base of said cemented carbide set forth in any one of claim 1 to 4 and a surface layer deposited on said base.

6. The coated cemented carbide set forth in claim 5 wherein said surface layer includes at least one layer made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides of at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si.

7. The coated cemented carbide set forth in claim 5 or 6 wherein said surface layer includes a multi-layered structure consisting of at least two unit layers superimposed alternately, each unit layer being made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides of at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si, each unit layer having a thickness of 0.2 to 100 nm, and said multi-layered structure having a total thickness of 0.5 to 10 µm.

8. The coated cemented carbide set forth in claim 7 wherein that at least one of unit layers contains at least one elements selected from a group comprising Ge, Sn and Pb.

9. The coated cemented carbide set forth in claim 7 or 8 wherein said surface layer comprises said multi-layered structure and mono-layer structure having no layered structure, two structures being superimposed alternately at least for five times, said mono-layer structure being made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides comprising at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si and having a thickness of 100 to 5000 nm, and said surface layer having a total thickness of 0.5 to 10 µm.

10. A coated hard tool comprising a base made of said cemented carbide set forth in any one of claims 1 to 4 and a surface layer deposited on a surface of said cemented carbide base, said surface layer including at least one layer made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides of at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si.

11. The coated hard tool set forth in claim 10 wherein said surface layer includes a multi-layered structure consisting of at least two unit layers superimposed alternately, each unit layer being made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides of at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si, each unit layer having a thickness of 0.2 to 100 nm, and said multi-layered structure having a total thickness of 0.5 to 10 µm.

12. The coated hard tool set forth in claim 11 wherein that at least one of unit layers contains at least one elements selected from a group comprising Ge, Sn and Pb.

13. The coated hard tool set forth in claim 11 or 12 wherein said surface layer comprises said multi-layered structure and mono-layer structure having no layered structure, said multi-layered structure and said mono-layer structure being superimposed alternately at least for five times, said mono-layer structure being made of at least one compound selected from a group comprising nitrides, carbides, carbonitrides and oxides comprising at least one element selected from a group comprising IVa elements, Va elements, Al, B and Si and having a thickness of 100 to 5000 nm, and said surface layer having a total thickness of 0.5 to 10 µm.

14. The coated hard tool set forth in any one of claims 10 to 13 wherein said surface layer includes a bottom layer made of TiN having a thickness of 0.02 to 2 µm.

15. The coated hard tool set forth in any one of claims 10 to 14 wherein said surface layer has an outer most layer whose surface roughness Ra is smaller than 0.18 µm.

16. Use of said coated cemented carbide set forth in any one of claim 5 to 9 in hard tools.