Patent Application
20080090099
The following will explain this invention with the aid of examples and with reference to the exemplary figures which illustrate the following:
The example per test #1), described below in more detail, covers a complete coating cycle according to the invention, employing a weak, essentially vertical magnetic field in the area of the target surface.
The workpieces were placed in appropriately provided double- or triple-rotatable holders, the holders were positioned in the vacuum processing chamber, whereupon the vacuum chamber was pumped down to a pressure of about 10−4 mbar.
For generating the process temperature, supported by radiation heaters, a low voltage arc (LVA) plasma was ignited between a baffle-separated cathode chamber housing a hot cathode and the anodic workpieces in an argon-hydrogen atmosphere.
The following heating parameters were selected:
Those skilled in the art will be familiar with possible alternatives. As a matter of preference the substrate was connected as the anode for the low voltage arc and also preferably pulsed in unipolar or bipolar fashion.
As the next procedural step the etching was initiated by activating the low voltage arc between the filament and the auxiliary anode. Here as well, a DC-, pulsed DC- or AC-operated MF or RF power supply can be connected between the workpieces and frame ground. By preference, however, a negative bias voltage was applied to the workpieces.
The following etching parameters were selected:
The next procedural step consisted in the coating of the substrate with an AlCrO layer and a TiAlN interface layer. If higher ionization is needed, all coating processes can be assisted by means of the low voltage arc plasma.
For the deposition of the TiAlN interface layer the following parameters were selected:
For the transition of about 15 min to the actual functional layer the AlCr arc sources were switched on with a DC source current of 200 A, with the positive pole of the DC source connected to the annular anode of the source and to frame ground. During that phase a DC substrate bias of −40 V was applied to the substrate. 5 minutes after activation of the AlCr (50/50) targets the oxygen inflow was started and was then ramped up within 10 min from 50 to 1000 sccm. At the same time the TiAl (50/50) targets were turned off and the N2 was reduced back to approx. 100 sccm. Just before the introduction of oxygen the substrate bias was switched from DC to bipolar pulses and increased to U=−60 V. That completed the interface layer and the transition to the functional layer. The targets were powder-metallurgically produced targets. Alternatively, melt-metallurgical targets may be used as well. To reduce the spattering rate, monophase targets as described in DE 19522331 may be used.
The coating of the substrate with the actual functional layer took place in pure oxygen. Since aluminum oxide constitutes an insulating layer, either a pulsed or an AC bias supply was used.
The key functional-layer parameters were selected as follows:
The process described yielded well-bonded, hard layers. Comparison tests of the coating on lathe-work and milling tools revealed a product life significantly improved over traditional TiAlN coatings, although the surface roughness was clearly higher than the roughness values of optimized pure TiAlN coatings.
The test examples #2 to #22 shown in Table 1 refer to simple layer systems according to the invention, each consisting of a double oxide layer of the (Al1-xCrx)2O3 type produced at a coating temperature of between 450 and 600° C. The remaining parameters were identical to the parameters described above for producing the functional layer. The stoichiometric component of the layer composition was measured by Rutherford backscattering spectrometry (RBS). The largest deviation from the target alloy composition shown in column 2 was encountered in tests #10 to #12, with a deviation of 3.5 percentage points at a 70/30 Al/Cr ratio. The metal components of the layer are scaled to the total metal content of the oxide. In terms of the stoichiometry of the oxygen, however, there were somewhat greater deviations of up to over 8%. All layers nevertheless exhibited a clearly corundum-like lattice structure. Preferably, therefore, layers produced according to the invention should have an oxygen-related stoichiometry shortage of 0 to 10% since even with an oxygen deficit of as much as 15% the desired lattice structure will be obtained.
The crystal structure of the layers is compact-grained, in large measure with an average crystallite size of less than 0.2 μm. Only in cases of large chromium concentrations and at coating temperatures of 650° C. were crystallite sizes found to be between 0.1 and 0.2 μm.
For the tests #2 to #22,
Parallel tests were performed on mixed-crystal layers 2 μm thick, employing the methods described above (Stoney's bending strip method and bending disk method). The layer stress values measured ranged from stress-free to minor compressive and tensile stress values less than or equal to 0.5 GPa. However, thicker PVD coatings can still be deposited with layers exhibiting a somewhat higher layer stress of about 0.8 GPa. Another possibility consists in a sequence of thin layers (≦1 μm) deposited with alternating tensile and compressive stress, constituting a multilayer system.
As shown in Table 2, test #2, the temperature and oxidation resistance of the corundum structure of the deposited (Al1-xCrx)2O3 layers was tested by heating coated carbide metal test objects with an elevated Co content to a temperature of 1000° and, respectively, 1100° C. over a period of 50 minutes, then holding them there for 30 minutes and finally cooling them to 300° C. over a time span of 50 minutes. Once cooled to room temperature, the lattice constants were reevaluated. According to the phase diagram [W. Sitte, Mater. Sci. Monogr., 28A, React. Solids 451-456, 1985] referred to in Phase Equilibria Diagrams Volume XII Oxides published by the American Ceramic Society, there is a miscibility gap in the range between about 5 and 70% aluminum, i.e. (Al0.05-0.7Crx0.95-0.30)2O3 for temperatures up to about 1150° C., which would predict a segregation of the (Al1-xCrx)2O3 mixed crystal into Al2O3 and Cr2O3 and an (Al1-xCrx)2O3 mixed crystal of some other composition. From that diagram it is also evident that with the process according to this invention it is possible to shift the thermodynamic formation temperature for (Al1-xCrx)2O3 mixed-crystal layers from 1200° C. to between 450° and 600° C. Surprisingly it was also found that the mixed-crystal layers produced by this inventive method experience only minimal changes in their lattice constants as a result of the glow process and that there is no segregation into their binary components. The maximum deviation, shown in
The hardness values of the (Al0.5Cr0.5)2O3 layers were determined to be about 2000 HV50. Measurements performed on other multi-oxides such as (Al0.5Ti0.3Cr0.2)2O3, or (Al0.6Ti0.4)2O3, (V0.5Cr0.5)2O3, (Al0.2Cr0.8)2O3, on their part yielded values between 1200 and 2500 HV.
Tables 3 to 6 list additional multilayer implementations of the layer system according to the invention. Process parameters for producing AlCrO and AlCrON mixed-crystal layers on a 4-source coating system (RCS) are shown in Table 7 while corresponding process parameters for producing individual strata for various support layers are shown in Table 8.
The tests #23 to #60 in Tables 3 and 4 refer to layer systems in which the oxidic mixed-crystal layer is of a corundum structure throughout and is mostly formed as a monolayer. Only in tests #25, #29 and #31 the mixed-crystal layer is formed from two consecutive individual strata of different chemical compositions. In test #29 the only difference between the mixed-crystal layers is their respective Al/Cr ratio.
The tests #61 to #107 in Tables 5 and 7 refer to layer systems in which the mixed-crystal layer is composed of 5 to as many as 100 very thin strata measuring between 50 nm and 1 μm. In these cases, there may be alternating oxidic mixed-crystal layers of a corundum structure with different chemical compositions and corresponding mixed-crystal layers with different layer systems.
In comparison tests on various turning and milling tools, the layers used in tests #23, #24 and #61 to #82 proved clearly superior in turning and milling applications over conventional layer systems such as TiAlN, TiN/TiAlN and AlCrN. Even when compared to CVD coatings, tool product-life improvements were achieved in the milling arena and in some lathe applications.
Although, as stated above, analyses and tests have already been conducted on a substantial number of layer systems, those skilled in the art will use conventional measures, where necessary, to adapt certain characteristics of the inventive layer system to specific requirements. For example, one may consider adding further constituent elements to individual or all layers of the system but in particular to the mixed-crystal layer. Elements known to improve for instance the heat resistance at least of nitridic layers include Zr, Y, La or Ce.
Coil current of the source magnetic system 0.1 to 2 A
