METHOD FOR DEPOSITING FINE-GRAINED ALUMINA COATINGS ON CUTTING TOOLS

Abstract: The present invention relates to a method for depositing refractory alumina (Al2O3) thin layers on cutting tools made of cemented carbide, cermet, ceramics or high speed steel. The invented method is a Plasma Activated Chemical Vapor Deposition (PACVD) process in which the plasma is produced by applying a bipolar pulsed DC voltage across two electrodes to which the tool substrates to be coated are fixtured and electrically connected. In contrast to prior art methods, built-up electrical charge on non-conducting surfaces is suppressed and hence, no arcing occurs on said surfaces. This will permit stable, long-term processing. With the invented method, high-quality coatings consisting of either single phase gamma -Al2O3 or of a mixture of gamma - and alpha -Al2O3 phases can be deposited on cutting tools at deposition temperatures as low as 500 DEG C. When coated cemented carbide tools according to the invention are used in the machining of steel or cast iron, several important improvements to prior art produced Al2O3- coated tools have been observed.

Description: Method for depositing fine-grained alumina coa ings on cutti g tool p, The present invention relates to a method for depo- siting fine-grained, crystalline Al 2 0 3 (alumina) coatings on cutting tools having a body of cemented carbide, cermet, ceramics or high speed steel by means of a Plasma Activated Chemical Vapour Deposition (PACVD) process with the chemical reactants A1C1 3 , 0 2 , H 2 and Ar. The plasma is produced by applying a bipolar pulsed DC voltage on two electrodes or two sets of electrodes. It is possible, with the invented method, to deposit at high deposition rate, smooth, high-quality coatings consisting of either single phase γ-Al 2 0 3 or of a mixture of γ- and OC.- I2O3 phases, said coatings having a good wear resistance when applied on cutting tools. With the invented method, crystalline Al 2 0 3 coatings can be obtained at deposition temperatures as low as 500 °C . It is well known that for cemented carbide cutting tools used in metal machining, the wear resistance of the tool considerably can be increased by applying 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 μm and the most widespread techniques for depositing such coatings are PVD (Physical Vapour Deposition) and CVD (Chemical Vapour Deposition) . 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 of A1C1 3 , C0 2 and H 2 , on a substrate surface held at elevated temperatures around 1000 °C. Al 2 0 3 crystallizes into several different phases such as (alfa), κ(kappa) and χ(chi) called the "α-series" with hep (hexagonal close packing) stacking of the oxygen atoms, and into γ (gamma), θ(theta), η (eta) and δ (delta) called the "γ-series" with fee (face centered cubic) stacking of the oxygen atoms . The most often occurring Al 2 0 3 -phases in coatings deposited with CVD methods on cemented carbides at temperatures 1000-1050 °C, are the stable α- and the metastable κ-phases, however, occasionally the metastable θ-phase has also been observed. These A1 2 0 3 coatings of the α- , K- and/or θ-phase are fully crystalline with a grain size in the range 0.5-5 μm and the coatings have a well -faceted grain structure. The inherently high deposition temperature of about 1000 °C renders the total stress in CVD A1 2 0 3 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 cemented carbide substrate and the coating. The tensile stresses may exceed the rupture limit of Al 2 0 3 and cause the coating to crack extensively and a network of cooling cracks will be generated over the entire Al 2 0 3 layer. Alternative deposition methods for the production of refractory coatings such as alumina, are desirable to find and particularly, methods capable of operating at lower substrate temperatures, hence not only allowing more temperature sensitive tool substrates to be coated, such as high speed steel, but also eliminating cooling cracks caused by the thermal stresses in the coating. A refractory coating deposited at lower temperatures would also result in a finer grain structure and possibly, a higher hardness of the coating. Potential low temperature deposition technologies for the production of refractory coatings such as Tie, TiN and Al 2 0 3 on cutting tools are PVD (Physical Vapour Deposition) and PACVD (Plasma Activated CVD) . However, certain problems will arise when employing these plasma based techniques for the deposition of highly insulating layers such as Al 2 0 3 . The alumina layer grows not only on the substrates but equally on all surfaces in the vici- nity of the plasma as well as on the cathodes/electrodes. Furthermore, these insulating layers will become charged which may cause electrical breakdown and arcing. This letter phenomenon naturally effects both the growth rate and the quality of the coating in a detrimental way. One solution to the above problems has been the invention of the bipolar pulsed DMS technique (Dual Magnetron Sputtering) which is disclosed in DD 252 205 and US 5,698,314. In the bipolar dual magnetron system, the two magnetrons alternately act as an anode and as a cathode and hence, preserve the magnetron targets in a metallic state over long process times. At high enough frequencies, possible surface charging on the insulating layers will be suppressed and the otherwise troublesome phenomenon of arcing will be limited. According to US 5,698,314 the DMS sputtering technique is capable of depositing and producing high-quality, well-adherent , crystalline α- Al 2 0 3 thin films at substrate temperatures less than 800 °C. The PVD techniques in general have, due to the low process pressure, the disadvantage of being so called "line of sight" methods, i.e. only surfaces facing the ion source will be coated. This disadvantage can to a certain extent be compensated for by rotating the substrates during the deposition. A prior art Plasma Assisted CVD method for the deposition of Al 2 0 3 layers of the α- and/or γ-Al 2 0 3 polymorphs at substrate temperatures between 450 and 700 °C is disclosed in US 5,516,588 and 5,587,233. This PACVD process includes the reaction between an Al-halogenide, A1C1 3 , and C0 2 , H 2 and Ar in a plasma generated by applying a unipolar pulsed DC voltage on the substrate body connected as a cathode which means that the substrate is constantly held at a negative potential. A disadvantage with the DC voltage generation of plasmas, including the unipolar pulsed DC voltage technique, is that the surface charging on the non-conducting layers cannot totally be suppressed. Specifically, the charging is most severe on sharp corners and along edges of the substrates resulting in a significant decrease in the layer thickness and also in the quality of the coating. In more general terms, the fact that the insulating alumina layer grows not only on the substrates but equally on all surfaces in the vicinity the plasma as well as on the electrodes, will negatively influence the stability of the plasma and the entire deposition process may eventually end in the extinction of the discharge. Still another factor which effects the growth rate of the coating is that the deposition process will be interrupted every time the unipolar pulsed DC voltage is at zero potential. In US 5,093,151, the unipolar pulsed DC voltage being used to produce the plasma, is deliberately not allowed to attain zero potential between the pulses but is held at a residual potential which is always larger than the lowest ionisation potential of any of the element in the reaction mixture H, H 2 , Ar, O, 0 2 and A1C1 3 . The ratio of the residual voltage and the maximum voltage of the pulse is said to be 0.02-0.5. By not allowing the voltage to attain zero potential may have a preferable effect on the deposition rate but simulta- neously resulting in a more severe charge built-up on non-conducting surfaces. It is the object of the present invention to avoid or alleviate the problems of prior art PACVD techniques and according to the present invention, there is now pro- vided a method for depositing hard and dense, fine- grained crystalline coatings of α-Al 2 0 3 and/or γ-Al 2 0 3 at substrate temperatures between 500° and 800° C. Coatings produced by the invented method are transparent and have a very smooth surface finish. Said coatings are further characterized by a high hardness larger than or equal to 19 GPa and a grain size estimated between 5 and 200 nm. The invented method is based on a plasma activation of a reaction mixture of the chemical reactants AlCl 3 , H 2 and Ar together with an oxygen-donor 0 2 , C0 2 , CO and N 2 0. Preferably the oxygen-donor is 0 2 . The plasma is produced by applying a bipolar pulsed DC voltage across two electrodes or two sets of electrodes to which the substrate bodies to be coated are fixtured and electrically connected. Alternatively, the reactor wall may serve as an electrode. The two electrodes or the two sets of electrodes alternately act as anode (s) and cathode (s). The alternation of the voltage pulse applied to the electrodes between positive and negative potential has several advantages. Firstly, the undesirable built-up elec- trical charge on non-conducting surfaces during the negative pulse period will be discharged during the positive pulse period and, by selecting a high enough frequency, > 5 kHz, of the bipolar pulsed DC voltage, arcing can be eliminated. This will permit stable, long-term proces- sing. Secondly, if no pause time between the positive and the negative pulses is allowed for, the plasma will constantly be activated resulting in a higher deposition rate compared to prior art unipolar pulsed DC voltage techniques. A further advantage of the invented method is that the growth rate of the coating is practically constant on all surfaces alike, edges, corners and flat surfaces of the body subjected to coating. The invented method, bipolar pulsed DC voltage PACVD, can also successfully be used for the deposition of non- insulating coatings such as Tie, TiN, TiCN and TiAlN, or other car- bides and/or nitrides with the metal element chosen from Nb, Hf, V, Ta, Mo, Zr, Cr and W. Fig. 1 shows a micrograph of the top surface and another micrograph of a polished cross section of an α- Al 2 0 3 coating deposited on a cemented carbide cutting tool with the bipolar pulsed DC voltage PACVD technique. Fig. 2 shows an X-ray Diffractogram (XRD) of the same α- Al 2 0 3 coating as in Fig. 1. The α-phase of alumina is unequivocally defined by the reflexes from the (012) , (104), (110), (113), (024) and (116) planes. The coating equipment used for the invented process is depicted in its simplest form in Fig. 3 with a reactor tube (1) , a resistance heated furnace (2) , electrodes (3) , a gas distributor (4) , shield (5) and power supply (6) . The substrates to be coated can be placed on any of the two electrodes or on both electrodes. For practical purposes the substrates are placed on the lower electrode in this particular electrode arrangement. Fig. 4 shows a different coating system with a reac- tor retort (1), a resistance heated furnace (2), electrodes (3) , gas inlet tube (4) , shield (5) , power supply (6) and tool substrates to be coated (7) . This coating system may be suitable for the mass production of e.g. alumina coated cemented carbide cutting tools. In this system all the electrodes in the reactor serve as substrate tables/holders and the reactants are fed into the reactor through a centre pipe. Alternatively, the reactor wall serves as an electrode. In order to attain optimum coating quality and growth rate when applying the bipolar pulsed DC voltage technique, the frequency, pulse amplitudes, positive and negative pulse on/off times can be varied which is illustrated in Fig. 5. A frequency in the range 5-100 kHz can be used, preferably 8-20 kHz. The pulse amplitudes can vary between 300 and 1000 Volts, preferably between 600 and 900 Volts, and the negative and positive pulse on times as well as the negative and positive pulse off times (zero potential) within the period T of the pulsed DC voltage can also be varied in order to modify the coating properties and the stability of the process. Following the definitions of the parameters P + , P., t + , t_, A + and A_ below: Pulse on time positive=P + Pulse on time negative=P_ Pulse off time positive=t + Pulse off time negative=t_ Positive pulse amplitude=A + Negative pulse amplitude=A_ where P_ > P + > 0.1P_, preferably 0.5P_ > P + > 0.1P_, and where also P_ > 0.1T. The negative and positive pulse off times should be set to larger or equal to zero i.e. t_ > 0 and t + 0. The amplitude A + is essentially of equal magnitude as amplitude A ^ . The following gas composition, process pressure and substrate temperature may be used: Possible range Preferred range A Al1CCl1 3 , 0 0..11--22 %% 0.2-0.4 % o 2 0 . 1 - 3 . 9 0.1-2.0 % H 2 25 - 95 % 70-80 % Ar 5 - 75 % . 20-30 % Process p prreessssuurree:: 0 0..0055--11 kkPa 0.1-0.4 kPa Substrate temperature : 500 - 800 600-700 °C It is within the purview of the skilled artisan to determine whether the requisite grain size and phase com- positions have been obtained and to modify the deposition conditions in accordance with the present specification, if desired, to effect the structure of the Al 2 0 3 coating within the frame of the invention. The benefits of the invented method are demonstrated by Examples 1-3. F.yample 1 A) 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 μm TiCN and thereafter with a top layer of 1.5 μm κ-Al 2 0 3 . Both the TiCN and the Al 2 0 3 layer were deposited by conventional CVD technique. The Al 2 0 3 layer had an average grain size of 1 μm. B) Cemented carbide inserts of the same style and composition as in A, first coated with an approximately 2.5 μm TiN layer by an ion plating technique. C) Inserts from B, coated with 5.3 μm fine-grained α-Al 2 0 3 layer in a separate experiment with the bipolar pulsed DC voltage PACVD technique under the conditions given below: D) Inserts from B, coated with 5.9 μm fine-grained γ-Al 2 0 3 layer in a separate experiment with the bipolar pulsed DC voltage PACVD technique under the conditions given below: C D AlClj 0.4 % 0.2 % o 2 0.4 % 0.3 % H 2 76 % 76 % Ar 24 %. 24 % Pressure : 0.2 kPa 0.2 kPa Temperature : 700 °C 700 °C Frequency: 16.6 kHz 16.6 kHz Pulse on time positive (P + ) =Pulse on time negative (P.) Pulse off time positive (t + ) =0 Pulse off time negative (t_)=0 X-ray diffraction (XRD) analysis when using Cu κα radiation, chlorine concentration analysis and hardness measurements HV(0.02) gave the following results: D XRD α-Al 2 0 3 (Fig.2) γ-Al 2 0 3 (Fig.6) HV(0.02) 21 GPa 21 GPa Appearance half transparent transparent Cl content 0.5 at % 0.5 at % Coated inserts from A, C and D were then tested in a continuous turning operation in a ball bearing steel (Ovako 825) under the following conditions: 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 microscope. The machining time until the A1 2 0 3 layer was worn through, was registered (i.e. when the inner TiCN coating just becoming visible) . In order to define a figure of merit for the intrinsic wear resistance of the Al 2 0 3 layers, the thickness (μm) of the Al 2 0 3 layer was divided by the above defined machining time (min) . The results below express the wear rate figure of merit . A) prior art layers 1.0 μm/min C) invention α-Al 2 0 3 0.76 μm/min D) invention γ-Al 2 0 3 0.92 μm/min From the above results it is obvious that the wear resistance of the fine-grained γ-Al 2 0 3 and the α-Al 2 0 3 layer produced with the invented method is surprisingly at least as good as the wear resistance of the coarser grained κ-Al 2 0 3 layer deposited by CVD technique. The following two examples demonstrate further pos- sibilities of the invented method to obtain Al 2 0 3 coatings with slightly different grain morphology and XRD- pattern. Example 2 E) Cemented carbide inserts of style CNMA 120412 -KR having a composition of 6 w% Co and balance WC, coated with a first layer of approximately 2.5 μm TiN by an ion plating technique and thereafter coated with a 3 μm fine- grained Al 2 0 3 layer in a separate experiment with the bipolar pulsed DC voltage PACVD technique under the following deposition conditions: E A1C1 3 0.2 0 2 0.4 H 2 76 i Ar 24 % Pressure: 0.2 kPa Temperature: 550 °C Frequency: 16.6 kHz Pulse on time positive (P + ) =Pulse on time negative (PJ Pulse off time positive (t + ) =0 Pulse off time negative (t_)=0 XRD analysis (Fig. 7) showed a broad peak at the 2θ- angle 66.8° corresponding to the (440) planes of γ-Al 2 0 3 . The broad XRD peak indicates a very fine-grained γ-Al 2 0 3 . The microhardness HV(0.02) of the coating was determined to 19 GPa. A chlorine concentration of 1.5 at % was determined. The coating appeared fully transparent. F.xampl e 3 F) Cemented carbide inserts of style CN.MA 120412- KR having a composition of 6 w% Co and balance WC, coated with a first layer of approximately 2.5 μm TiN layer by an ion plating technique and thereafter coated with a 2.5 μm fine-grained Al 2 0 3 layer in a separate experiment with the bipolar pulsed DC voltage PACVD technique under the following deposition conditions: F AlCl j 0.2 % 0 2 0.4 % H 2 76 % Ar 24 % Pressure: 0.2 kPa Temperature: 650°C Frequency: 8.3 kHz Pulse on time positive (P + ) =Pulse on time negative (P.) Pulse off time positive (t + ) =0 Pulse off time negative (t.) =75 μs XRD analysis (Fig. 8) showed a distinct peak at the 2θ-angle 66.8° corresponding to the (440) planes of γ- Al 2 0 3 . The microhardness HV(0.02) was determined to 22 GPa.

Claims: Cl a ims 1. A Plasma Activated CVD method for the coating of cutting tools based on sintered cemented carbide or cermet, ceramics or high speed steel, with a fine-grained (5-200 nm) coating of α-Al 2 0 3 and/or γ-Al 2 0 3 with a hardness larger than or equal to 19 GPa, and said method is charact eri zed in that the plasma is produced by a bipolar pulsed DC voltage applied between two electrodes or two sets of electrodes to which the substrate bodies to be coated are fixtured and electrically connected, and said electrodes or set of electrodes alternately act as anode (s) and cathode (s), that the chemical reactants are AlCl 3 , H 2 and Ar together with an oxygen- donor 0 2 , C0 2 , CO or N 2 0, that the pulse frequency is set between 5 and 100 kHz, preferably between 8 and 20 kHz, that the pulse amplitudes are set in the range 300 to 1000 Volts, preferably in the range 600 to 900 Volts and that the substrate temperature is in the range 500° to 800° C, preferably 550° to 650° C, depending on the mate- rial of the tool body being coated. 2. A method according to claim 1 charac t eri zed in that the oxygen-donor is 0 2 . 3. A method according to claim 2 charac teri z ed in that the chemical reactants are set to in vol-% : A1C1 3 : 0.1-2.0, preferably 0.2-0.4, 0 2 : 0.1-3.9, preferably 0.1-2, H 2 : 25-95, preferably 70-80 and Ar 5-75, preferably 20-30. 4. A method according to claim 3 charac teri zed in that the total process pressure is set in the range 0.05 to 1 kPa, preferably in the range 0.1 to 0.4 kPa. 5. A method according claim 4 charac t eri z ed in that the pulse on time negative (P.) is larger or equal to the pulse on time positive (P + ) , and the latter is larger or equal to 10% of the pulse on time negative, i.e. P_ > P + > 0.1P_, and further the pulse on time negative is larger or equal to 10% of the period T, i.e. P_ > 0.1T, and that the negative and positive pulse off times are set to be larger or equal to zero, i.e. t_ > 0 and t + > 0. 6. A method according to claim 5 characteri zed in that the amplitudes of the positive (A + ) and negative (AJ pulses of the bipolar pulsed DC voltage are of equal magnitude.

Inventors: ENDLER INGOLF, LEONHARDT ALBRECHT, LJUNGBERG BJOERN, SJOESTRAND MATS, TAESCHNER CHRISTINE, ENDLER, INGOLF, LEONHARDT, ALBRECHT, LJUNGBERG, BJOERN, SJOESTRAND, MATS, TAESCHNER, CHRISTINE
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 1998
Main classification: C23C16
IPC codes: B23B27, B23P15, C23C16