It is well known that for e. g. cemented carbide cut- ting tools used in metal machining, the wear resistance of the tool edge considerably can be increased by apply- ing thin, hard surface layers of metal oxides, carbides or nitrides with the metal either selected from the transition metals from the groups IV, V and VI of the Periodic Table or from silicon, boron and aluminium. The coating thickness usually varies between 1 and 15 pm and the most widespread techniques for depositing such coat- ings are PVD and CVD (Chemical Vapor Deposition). It is also known that further improvements of the performance of a cutting tool can be achieved by applying a pure ce- ramic layer such as A1203 on top of layers of metal car- bides and nitrides (U. S. 5,674,564. U. S. 5,487,625).

Cemented carbide cutting tools coated with alumina layers have been commercially available for over two decades. The CVD technique usually employed involves the deposition of material from a reactive gas atmosphere on a substrate surface held at elevated temperatures. A1203 crystallizes into several different phases such as a (alpha), K (kappa) and x (chi) called the"a-series"with

hcp (hexagonal close packing) stacking of the oxygen atoms, and into y (gamma), 0 (theta), (eta) and 6 (delta) called the"y-series"with fcc (face centered cubic) stacking of the oxygen atoms. The most often occurring A1203-phases in CVD coatings deposited on cemented car- bides at conventional CVD temperatures, 1000°-1050°C, are the stable alpha and the metastable kappa phases, however, occasionally the metastable theta phase has also been observed.

The CVD A1203-coatings of the a-, K-and/or 6-phase are fully crystalline with a grain size in the range 0.5-5 J. m and having well-facetted grain structures.

The inherently high deposition temperature of about 1000 °C renders the total stress in CVD A1203-coatings on cemented carbide substrates to be tensile, hence the total stress is dominated by thermal stresses caused by the difference in thermal expansion coefficients between the substrate and the coating and less by intrinsic stresses which have their origin from the deposition process itself and are of compressive nature. The ten- sile stresses may exceed the rupture limit of A1203 and cause the coating to crack extensively and thus degrade the performance of the cutting edge in e. g. wet machin- ing where the corrosive chemicals in the coolant fluid may exploit the cracks in the coating as diffusion paths.

Generally CVD-coated tools perform very well when machining various steels and cast irons under dry or wet cutting conditions. However, there exists a number of cutting operations or machining conditions when PVD- coated tools are more suitable e. g. in drilling, parting and threading and other operations where sharp cutting edges are required. Such cutting operations are often referred to as the"PVD coated tool application area".

Plasma assisted CVD technique, PACVD, makes it pos- sible to deposit coatings at lower substrate tempera-

tures as compared to thermal CVD temperatures and thus avoid the dominance of the thermal stresses. Thin A1203 PACVD films, free of cracks, have been deposited on ce- mented carbides at substrate temperatures 450-700 °C (DE 41 10 005; DE 41 10 006; DE 42 09 975). The PACVD pro- cess for depositing A1203 includes the reaction between an Al-halogenide, e. g. AlC13, and an oxygen donor, e. g.

CO2, and because of the Al incompleteness of this chemi- cal reaction, chlorine is to a large extent trapped in the A1203-coating and its content could be as large as 3.5 %. Furthermore, these PACVD A1203-coatings are ge- nerally composed of, besides the crystalline alfa- and/or gamma-Al203-phase, a substantial amount of amor- phous alumina which in combination with the high content of halogen impurities, degrades both the chemical and mechanical properties of said coating, hence making the coating material non-optimised as a tool material, The field of the present invention relates particu- larly to the art of PVD A1203 coated carbide cutting tools or tools of similar hard materials such as cer- mets, ceramics and high-speed steel.

There exist several PVD techniques capable of pro- ducing refractory thin films on cutting tools and the most established methods are ion plating, DC-and RF- magnetron sputtering, arc discharge evaporation, IBAD (Ion Beam Assisted Deposition) and Activated Reactive Evaporation (ARE). Each method has its own merits and the intrinsic properties of the produced coatings such as microstructure/grain size, hardness, state of stress, intrinsic cohesion and adhesion to the underlying sub- strate may vary depending on the particular PVD method chosen. Early attempts to PVD deposit A1203 at typical PVD temperatures, 400-500 °C, resulted in amorphous alu- mina layers which did not offer any notable improvement in wear resistance when applied on cutting tools. PVD deposition by HF diode-or magnetron sputtering resulted

in crystalline a-A1203 only when the substrate tempera- ture was kept as high as 1000 °C (Thornton and Chin, Ce- ramic Bulletin, 56 (1977) 504). Likewise, applying the ARE method for depositing A1203, only resulted in fully dense and hard A1203-coatings at substrate temperatures around 1000 °C (Bunshah and Schramm, Thin Solid Films, 40 (1977) 211).

With the invention of the bipolar pulsed DMS tech- nique (Dual Magnetron Sputtering) which is disclosed in DD 252 205 and DE 195 18 779, a wide range of opportuni- ties opened up for the deposition of insulating layers such as A1203 and, furthermore, the method has made it possible to deposit crystalline A1203-layers at sub- strate temperatures in the range 500 to 800 °C. In the bipolar dual magnetron system, the two magnetrons alter- nately act as an anode and a cathode and, hence, pre- serve a metallic anode over long process times. At high enough frequencies, possible electron charging on the insulating layers will be suppressed and the otherwise troublesome phenomenon of"arcing"will be limited.

Hence, according to DE 195 18 779, the DMS sputtering technique is capable of depositing and producing high- quality, well-adherent, crystalline a-A1203 thin films at substrate temperatures less than 800 °C. The"a- A1203-layers", with a typical size of the a-grains vary- ing between 0.2 and 2 Am, may partially also contain the gamma (y) phase from the"y-series"of the A1203-poly- morphs. The size of the y-grains in the coating is much smaller than the size of the a-grains. The y-A1203 grain size typically varies between 0.05 and 0.1 jj. m. In the A1203-layers where both modifications of y and a-phase were found, the y-Al203-phase showed a preferred growth orientation with a (440)-texture. When compared to prior art plasma assisted deposition techniques such as PACVD as described in DE 49 09 975, the novel, pulsed DMS sputtering deposition method has the decisive, important

advantage that no impurities such as halogen atoms, e. g. chlorine, are incorporated in the A1203-coating.

Fig. 1 is an EDS-analysis of an A1203-layer depo- sited by PACVD (with AlCl3 as a precursor) containing Cl-impurities.

Fig. 2 is an EDS-analysis of a y-Al203-layer accor- ding to the invention.

Fig. 3 shows an X-ray diffraction pattern from an A1203-layer containing y-phase.

Fig. 4 shows an X-ray diffraction pattern from an A1203-layer also containing 0-phase.

Fig. 5 shows an electron diffraction pattern from an A1203-layer deposited at a substrate temperature of 650 °C.

According to the present invention there is provided a cutting tool for metal machining such as turning (threading and parting), milling and drilling comprising a body of a hard alloy of cemented carbide, cermet, ce- ramics or high speed steel onto which a hard and wear resistant refractory coating is deposited by the DMS PVD method at substrate temperatures of 450 to 700 °C, pre- ferably 550 to 650 °C, depending on the particular mate- rial of the tool body, and that said wear resistant coating is composed of one or more layers of refractory compounds of which at least one layer, preferably the outermost layer, consists of A1203 and that the inner- most layer (s), if any at all, between the tool body and the A1203-layer, is composed of metal nitrides and/or carbides with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al. In contrast to the state of the art, the A1203-layers consist of high-qua- lity, dense, fine-grained crystalline y-Al203 with a grain size less than 0.1 pm. Furthermore, the y-Al203- layers are virtually free of cracks and halogen impuri- ties. The latter property is illustrated in Fig, 1 which is an EDS-analysis of an A1203-layer deposited by PACVD

(with AlCl3 as a precursor) containing Cl-impurities and in Fig. 2 which is an EDS-analysis of a y-Al203-layer according to the invention. In the latter A1203-layer no detectable impurities are present.

The y-Al203-layers according to the invention fur- ther give the cutting edges of the tool an extremely smooth surface finish which, compared to prior art a- A1203 coated tools, results in an improved surface fi- nish also of the workpiece being machined. The very smooth surface finish can be attributed to the very fine crystallinity of the coating. The"y-A1203"-layers may also partially contain other phases from the"y-series" like 0,8 and. Identification of the y-and/or 6-phases in the A1203-layers according to the invention can pre- ferably be made by X-ray diffraction. Reflexes from the (400) and (440) planes of the y-A1203-layers occurring at the 20-angles 45.8° and 66.8° when using CuKa radia- tion, unequivocally identifies the y-phase (Fig. 3).

Weaker reflexes from the (222), (200) and (311) planes of the y-phase can occasionally be identified. When the 0-phase is present in the A1203-layers according to the invention, said phase is identified by the reflexes from the (200,20-2) planes (Fig. 4).

A second identification method for the A1203-phases is based on electron diffraction in a Transmission Elec- tron Microscope (TEM). A diffraction pattern from an A1203-layer deposited at a substrate temperature of 650 °C is shown in Fig. 5. The pattern shows rings from a polycrystalline phase with grains considerably smaller than the diameter of the electron beam and, furthermore, the intensity of the rings and the distances between the rings again unequivocally identifies the y-phase of A1203.

The fine-grained, crystalline y-Al203 according to the invention is strongly textured in the [440]-direc- tion, A Texture Coefficient, TC, can be defined as.

where I (hkl) = measured intensity of the (hkl) reflection Io (hkl) = standard intensity from the ASTM standard powder pattern diffraction data n = number of reflections used in the calculation (hkl) reflections used are: (111), (311), (222), (400) and (440) and whenever the TC (hkl) > 1, there is a texture in the [hkl]-direction. The larger the value of TC (hkl), the more pronounced is the texture. According to the present invention, the TC for the set of (440) crystal planes is greater than 1.5.

When the very fine-grained y-A1203 coated cemented carbide cutting tools according to the invention are used in the machining of steel or cast iron, several im- portant improvements compared to prior art have been ob- served which will be demonstrated in the forthcoming examples. Surprisingly, the PVD y-A1203 without contain- ing any portion of the coarser and thermodynamically stable a-A1203-phase, shows in certain metal machining operations, a wear resistance which is equal to the wear resistance found in coarser CVD a-A1203-coatings depo- sited at temperatures around 1000 °C. Furthermore, the fine-grained PVD y-A1203-coatings show a wear resistance considerably better than prior art PVD-coatings. These observations open up the possibility to considerably im- prove the cutting performance and prolong the tool lives of coated PVD tools. The low deposition temperature will also make it possible to deposit PVD y-A1203-coatings on high-speed steel tools.

A further improvement in cutting performance can be anticipated if the edges of the y-Al203 coated cutting tools according to the invention are treated by a gentle

wet-blasting process or by edge brushing with brushes based on e. g. SiC as disclosed in the Swedish patent application 9402534-4.

The total coating thickness according to the present invention varies between 0.5 and 20 m, preferably bet- ween 1 and 15 m with the thickness of the non-Al203- layer (s) varying between 0.1 and 10 J. m, preferably bet- ween 0.5 and 5 pm. The fine-grained y-A1203-coating can also be deposited directly onto the cutting tool sub- strate of cemented carbide, cermet, ceramics or high speed steel and the thickness of said y-Al203 varies then between 0.5 and 15 pm preferably between 1 and 10 pm. Likewise can further coatings of metal nitrides and/or carbides with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al be deposited on top of the A1203-layer.

The y-A1203-layer according to the invention is de- posited by a bipolar dual magnetron sputtering technique at substrate temperatures of 450-700 °C, preferably 550- 650 °C, using aluminium targets, a gas mixture of Ar and 02 and a process pressure in the range 1-5 pbar. The substrate may be floating or pulsed biased, the exact conditions depending to a certain extent on the design of the equipment being used.

It is within the purview of the skilled artisan to determine whether the requisite grain size and phase compositions have been obtained and to modify the depo- sition conditions in accordance with the present speci- fication, if desired, to affect the nanostructure of the A1203-layer within the frame of the invention.

The layer (s) described in the present invention, comprising metal nitrides and/or carbides and/or car- bonitrides and with the metal elements selected from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al can be deposited by PVD-technique, CVD-and/or MTCVD-technique (Medium Tem- perature Chemical Vapor Deposition).

The superiority of the fine-grained y-Al203 PVD-lay- ers according to the present invention, compared to prior art PVD-coatings is demonstrated in Examples 1,2 <BR> <BR> <BR> and 5. Examples 3,4 and 6 demonstrate the surprisingly<BR> <BR> <BR> <BR> <BR> good wear resistance properties of the fine-grained y-<BR> <BR> <BR> <BR> <BR> <BR> A1203-layers compared to traditionally CVD-deposited single phase K-A1203 and single phase a-A1203-layers.

Example 1 A) Commercially available cemented carbide threading inserts of style R166.0G-16MM01-150 having a composition of 10 w% Co and balance WC, coated with an approximately 2 pm TiN-layer by an ion plating tech- nique.

B) TiN coated tools from A) were coated with a 1 Fm fine-grained y-A1203-layer in a separate experiment with the pulsed magnetron sputtering technique. The deposition temperature was 650 °C and the process pres- sure was 1 pbar.

C) Cemented carbide threading inserts of style R166.0G-16MM011-150 having a composition of 10 w% Co and <BR> <BR> <BR> balance WC, coated with an approximately 3 Am TiN-layer by an ion plating technique.

Coated tool inserts from B) and C) were then tested in a threading operation at a customers site in the pro- duction of engine oil plugs of cast iron (SS0125; 180- 240 HB). The thread of the plug being produced was of size M36 x 1.5.

Cutting data: Speed: 154 m/min 5 passages per thread

The results below are expressed as the number of machined plugs per cutting edge.

C) prior art 300 plugs Large crater wear, cutting edge is worn out B) invention >500 plugs No detectable wear on the cutting edge. The edge can produce more plugs From the above results it is obvious that the alu- mina coated insert according to the invention is supe- rior with respect to cutting performance.

Samole 2 D) Commercial PVD-TiN coated cemented carbide drilling inserts of style LCMX 040308-53 with a coating thickness of approximately 3 Fm having a cemented car- bide composition of 10 w% Co and balance WC.

E) TiN coated tools from D), coated with a 1 pm fine-grained y-A1203-layer in a separate experiment with the pulsed magnetron sputtering technique. The deposi- tion temperature was 650 °C and the process pressure was 1 pbar.

The alumina coating from E) appeared transparent and very smooth. SEM-studies of a fracture cross section of the alumina coating showed a very fine-grained struc- ture. An XRD-investigation identified the alumina phase as pure y-Al203.

Coated tool inserts from D) and E) were then tested in a drilling operation in a workpiece material of a low alloyed, non-hardened steel (SS 2541).

Cutting data: Speed: 150 m/min Feed: 0.12 mm/rev Hole diameter: 25 mm Hole depth: 46 mm Coolant being used

Both flank and crater wear were developed on the cutting edges. The extent of the flank wear determined the life time of the cutting tool. The results below ex- press the number of holes being drilled per cutting edge.

D) prior art 150 holes Flank wear 0.15 mm 200 holes Flank wear 0.22 mm, cutting edge damaged E) invention 150 holes Flank wear 0,07 mm 200 holes Flank wear 0.09 mm 250 holes Flank wear 0.10 mm cutting edge slightly damaged From the above results it is obvious that the alu- mina coated inserts according to the invention are able to drill more holes than the prior art inserts.

Fxam F) Cemented carbide inserts of style CNMA 120412- KR having a composition of 6 w% Co and balance WC, coated with a first layer of 8 m TiCN and thereafter with a top layer of 4.7 pm a-A1203. Both the TiCN and the A1203-layer were deposited by conventional CVD-tech- nique. The A1203-layer had an average grain size of 1.2 ) J. m.

G) Cemented carbide inserts of the same style and composition as in F), first coated with an approximately 3.6 m TiCN-layer by conventional CVD-technique and thereafter coated with a 2.3 m fine-grained y-Al203- layer in a separate experiment with the pulsed magnetron sputtering technique. The deposition temperature was 650 °C and the process pressure was 1 pbar.

Coated inserts from F) and G) were then tested in a continuous turning operation in a ball bearing steel (Ovako 825). The crater wear of the cutting edges was measured.

Cutting data: Speed: 210 m/min Feed: 0.25 mm/rev Depth of cut: 2.0 mm Coolant being used The cutting operation was periodically interrupted in order to measure the crater wear of the cutting edges. The crater wear was measured in an optical micro- scope. The machining time until the A1203-layer was worn through, was registered (i. e. when the inner TiCN-coat- ing just becoming visible). In order to define a figure of merit for the intrinsic wear resistance of the A1203- layers, the thickness (m) of the A1203-layer was di- vided by the above defined machining time (min). The re- sults below express the wear rate figure of merit.

F) prior art a-A1203-layers 0.5 m/min C) invention 0.5 m/min From the above results it is obvious that the wear resistance of the fine-grained y-A1203-layer surpris- ingly is as good as the wear resistance of the coarser- grained a-A1203-layer deposited by CVD technique.

Rwamnle a H) Cemented carbide inserts of style CNMA 120412- KR having a composition of 6 w% Co and balance WC, coated with a first layer of 6 pm TiCN and thereafter with a top layer of 1.1 pm K-Al203. Both the TiCN and the A1203-layer were deposited by conventional CVD tech- nique. The A1203-layer had an average grain size of 1 Am.

I) Cemented carbide inserts of the same style and composition as in H), coated with an approximately 2.5 pm TiN-layer by an ion plating technique.

J) TiN coated tools from I), coated with a 1.2 4m fine-grained y-Al203-layer in a separate experiment with the pulsed magnetron sputtering technique. The deposi-

tion temperature was 600 °C and the process pressure was 1 J. bar.

K) TiN coated tools from I), coated with a 1.7 pm fine-grained y-Al203-layer in a separate experiment with the pulsed magnetron sputtering technique. The deposi- tion temperature was 730 °C and the process pressure was 1 J. bar.

Coated inserts from H), J) and K), were then tested in a continuous turning operation in a ball bearing steel (Ovako 825). The crater wear of the cutting edges was measured.

Speed 250 m/min Feed 0.25 mm/rev Depth of cut 2.0 mm Coolant being used The cutting operation was periodically interrupted in order to measure the crater wear of the cutting edges. The crater wear was measured in an optical micro- scope. The machining time until the A1203-layer was worn through, was registered (i. e. when the inner TiN or TiCN-coating just becoming visible). In order to define a figure of merit for the intrinsic wear-resistance of the A1203-layers, the thickness (pm) of the A1203-layer was divided by the above defined machining time (min).

The results below express the wear rate figure of merit.

H) prior art K-A1203-layers 0.44 m/min J) invention TiN + y-A1203 0.40 pm/min K) invention TiN + y-A1203 0.46 pm/min From the above results it is obvious that the wear resistance of the fine-grained y-Al203-layer surpris- ingly is as good as the wear resistance of the coarser- grained K-A1203-layer deposited by CVD technique.

Example 5 Coated cutting inserts from I), J) and K) in Example 4 were tested under the same cutting conditions and cut-

ting data as in Example 4. The machining time until a predetermined crater wear had developed on the rake face of the inserts was registered. The results below express said machining time until the predetermined crater wear.

I) prior art TiN 4 min J) invention TiN + y-Al203 9 min K) invention TiN + y-Al203 9.7 min From the above results it is obvious that a top coating of the fine-grained y-Al203-layer on PVD TiN considerably improves the crater wear resistance of the cutting tool.

Example L) Cemented carbide inserts of style CNMA 120412- KR having a composition of 6 w% Co and balance WC, coated with a first layer of 6 Rm TiCN and thereafter with a top layer of 4.8 pm a-Al203. Both the TiCN and the A1203-layer were deposited by conventional CVD-tech- nique. The A1203-layer had an average grain size of 1 pm.

M) Cemented carbide inserts of the same style and composition as in L), first coated with an approximately 5 pm TiAlN-layer and thereafter, without vacuum inter- ruption, coated with a 4.4 pm fine-grained y-Al203- layer, both layers deposited with the pulsed magnetron sputtering technique. The deposition temperature was 600 °C and the process pressure was 1 pbar.

Coated inserts from L) and M) were then tested in a continuous turning operation in a low alloyed, non-hard- ened steel (SS2541). The crater wear of the cutting edges was measured.

Speed: 250 m/min Feed: 0.25 mm/rev Depth of cut: 2.0 mm Coolant being used

The cutting operation was periodically interrupted in order to measure the crater wear of the cutting edges. The crater wear was measured in an optical micro- scope. The machining time until the A1203-layer was worn through, was registered (i. e. when the inner TiCN or TiAlN-coating just becoming visible). In order to define a figure of merit for the intrinsic wear resistance of the A1203-layers, the thickness (m) of the A1203-layer was divided by the above defined machining time (min).

The results below express the wear rate figure of merit.

L) prior art a-Al203-layers 0.69 m/min M) invention 0.73 m/min From the above results it is obvious that the wear resistance of the fine-grained y-Al203-layer surpris- ingly is as good as the wear resistance of the coarser- grained a-Al203-layer deposited by CVD technique.