HARD MATERIAL COATING WITH YTTRIUM AND METHOD FOR ITS DEPOSITION

Abstract: Ternary hard material layers are described to which a small proportion of yttrium is added to increase the resistance to wear at elevated temperatures.

Description: HARD MATERIAL COATING WITH YTTRIUM AND METHOD FOR ITS DEPOSITION The invention relates to PVD hard coating materials for engi¬ neering components used in a hot environment, especially for tools for the coolant-free and lubricant-free machining of materials. The coolant-free and lubricant-free machining of materials, in particular of metals like die steels or aluminium alloys, is increasingly gaining in significance, in particular in the automotive industry. TiN and TiCN layers deposited by the known PVD and CVD meth¬ ods cannot satisfy this object satisfactorily because both begin to oxidize at operating temperatures beyond 400°C and thus considerable wear arises, in particular at the cutting edges. This is because the oxides which are formed are very brittle and spall off from the TiN or TiCN coating due to their low bond strength and different thermal expansion. This leads to a continuous and considerable reduction of the layer thickness in the edge region during cutting operations at elevated temperatures. Analogous effects can also arise with forming tools. It has been found that TiAlN applied by the PVD process as a hard material coating is much better suited to resisting the oxidation processes. It has been reported that the tempera¬ ture at which oxidation sets in can be increased to 700 to 800°C when the hard material layer consists, for example, of 50 at % Ti and of 50 at % Al . ( .-D. Mύnz, J. Vac. Sci. Tech- nol. , A4 (6) (1986) 2117) . Even higher Al contents have been reported, i.e. Al contents of up to 70 at % of the respective metal content. It is known from the field of turbine blade coating that the addition of yttrium increases the oxidation resistance of superalloys such as CoCrAlY or of thermally insulating layers of zirco¬ nium oxide. In the hard material coating field a layer of TiYN has admit¬ tedly been reported with a Y-content of more than 10 at %; however, this layer proved to be unsuitable for industrial use, mainly because of its brittleness. (J.R. Roos, J.P.Celis, E. Vancoille, H. Veltrop, S. Boelens, F. Jungblut, J. Ebberink, H. Homberg, Thin Solid Films, 193/194 (1990) 1542) . An object of the invention is to thermally stabilize the in¬ terface between the hard material coating and the respective substrate and to further reduce the onset of oxidation. For this purpose the invention proposes the addition of small quantities of yttrium to binary, ternary or quaternary TiAl- based multi-component layers. Preferably the yttrium is un¬ evenly distributed with respect to the growth direction of the coating. That means that the distribution is not uniform perpendicular to the substrate surface. In particular, pure yttrium or alloys containing yttrium and scandium, are added to todays well known TiAlN. In accordance with the invention yttrium is preferably added in the concentration range between 0.1 to 4.0 at % in ternary TiAlN alloys or in TiAlN/CrN, TiAlN/ZrN, TiAlN/TiN, TiAlN/MoN and TiAlN/WN multi-layer coatings. It will be understood that the percentage value of yttrium is quoted as a percentage of the metallic constituents only. Preferably a Y-content of only 1.5 to 2.0 at % of the Ti and Al constituents is recom¬ mended and used. The coating of the tools is preferably produced by sputtering (unbalanced magnetron UB ) ( .-D. Munz, Surf. Coat. Technol. , 48(1991) 81) , cathodic arc evaporation (e.g. steered arc) or with combination methods as cathodic arc evaporation/ sputtering (W.-D. Mϋnz, D. Schulze, F.J. . Hauzer, Surf. Coat .Technol . , 50 (1992) 169) or sputtering/low voltage elec¬ tron beam evaporation (anodic arc evaporation) or combined low voltage evaporation/cathodic arc evaporation. The uneven or non-uniform distribution of yttrium in the growth direction of the hard coating is achieved by the spe¬ cial choice of the deposition conditions. It has to be stated that the lateral distribution of the yttrium parallel to the substrate surface has to be as uniform as possible. The un¬ even distribution in growth direction is obtained by using a multitarget unbalanced magnetron sputtering machine. Fig. 1 shows the cross-section of an industrial four target PVD coater (detailed description: W.-D. Mϋnz, D. Schulze, F.J.M. Hauzer, Surf. Coat. Technol., 50 (1992) 169) . Four cathodes are mounted vertically into the doors of the vacuum chamber. The cathodes are of linear design. Four rectangular targets with a typical dimension of 60 cm x 20 cm (target thickness: 1-2 cm) are mounted on the four cathodes. The substrates to be coated are mounted on a rotating turntable as shown in Fig. 1. They are subjected to a three-fold planetary rotation as outlined by the arrows in Fig. 1. During coating they are passing all four cathodes collecting vaporised atoms sput¬ tered from the cathodes. If a uniform distribution of an al¬ loy material is required all four targets should be mounted with identical sputtering materials. In case of the TiAlYN deposition all four cathodes should be equipped with targets of the same composition. It is a preferred feature of the invention to avoid yttrium being incorporated over the entire hard material layer thick¬ ness with high uniformity as would be the case if the yttrium would be distributed in equal concentrations over all targets in a multi-target deposition system involved in the deposi¬ tion process. Thus, according to the invention the yttrium is unevenly dis¬ tributed over the entire hard material layer thickness and this special distribution can be obtained in a multi-target deposition system in which the yttrium is not alloyed into all targets and preferably only into a single target. Advantageous embodiments of the invention are described in the subclaims and will be explained in the following descrip¬ tion of examples in connection with Figures 1 to 8. When using multi-target deposition systems with the yttrium being distributed in equal concentrations over all targets involved in the deposition process of TiAlN, it can be shown that yttrium is incorporated over the entire hard material layer with high uniformity. Whereas this may influence the formation of dense oxide lay¬ ers on top of the hard nitride coating positively, e.g. on TiAlN, it must be stated that there is destabilisation of the interface between the nitride coating and the substrate tak¬ ing place with increasing temperatures. It was only possible to prevent this destabilisation by the described uneven dis¬ tribution of the Y in the growth direction of the coating. In detail the following target assembly has been used. To carry out the combined steered arc/unbalanced magnetron depo¬ sition process, one cathode was equipped with a Cr target. The Cr target was used to perform the metal-ion-etch process in a steered arc discharge to establish in vacuo a clean sub¬ strate surface. Cr was chosen as target material to reduce the generation of droplets which deposit on the substrate surface and cause growth defects there in the deposited TiAlN (W.-D. Mϋnz, I.v. Smith, L.A. Donohue, V.S. Brooks, German Patent Application 195 47 305.1) or TiAlYN coating. Two tar¬ gets consisted of 50 at % Ti and 50 at % Al . The Y-containing target with typcially 48 at % Ti, 48 at % Al and 4 at % Y was mounted between the two TiAl targets. Fig. 1 describes the actual target assembly. A schematic process sequence is outlined in Fig. 2. It can be seen there also that the deposition of the hard coating with the unbal¬ anced magnetron starts initially without using the TiAlY tar¬ get. It has been found that the deposition of a pure TiAlN base layer enhances the adhesion of a TiAlN based quaternary coating like TiAlNbN or as in the case described here with TiAlYN. It has also to be mentioned that the Cr cathode was used during the deposition of TiAlN and TiAlYN in the unbal¬ anced magnetron mode on a very low power level. Whereas the TiAl and TiAlY targets were operated with 8 to 10 kW, the Cr target was powered with 0.2 to 1 kW, typically with 0.5 kW. Initially, it was thought to prevent cross-contamination from the TiAl targets. However, it has been found out surprisingly that a low chromium content of the coating led to an increase of the oxidation resistance when compared to a pure TiAlN coating. A detailed set of coating parameters is given in Ta¬ ble 1. Process Steps Units Parameter Range Recommended Conditions Substrate Heat-up °C 250 - 550 450 Temperature Metal Ion Etch Target Material at % 100 Cr or 100 Cr at % 100 Mo Pressure Argon mbar 0 - 5.10 "3 0.3 - 0.6 x 10 "3 Current A 50 - 200 100 Substrate Bias V -1800 to -1500 -1200 Temperature °C 350 - 550 420 - 450 Duration min 1-20 10 Deposition Base aver (UBM) Pressure total mbar 2-10 x 10° 3.15 x 10 "3 (Ar + N 2 ) Pressure Ar mbar 1.5 - 9 x 10° 3.25 x 10 "3 Number UBM cathodes 2 2 Power per TiAl kW 6 - 12 8 cathode Power Cr Cathode kW 0 - 1.5 0.5 Bias Voltage V -50 to -100 -75 Bias Current Density mA/cm 2 2 - 5 3 Temperature β C 350 - 550 420 - 450 Thickness μm 0.1 - 2 0.2 Deposition of TiAlYN Pressure total (Ar+N 2 ) mbar 2-10 x 10 '3 3.5 x 10 "3 Pressure Ar mbar 1.5 - 9 x 10° 3.25 X 10 "3 Power TiAl (2x) kW 6 - 12 8 Power TiAlY (lx) kW 6 - 12 8 Power Cr kW 0.2 - 1.5 0.5 Bias Voltage V -50 to -100 -75 Bias Current Density mA/cm 3 2 - 5 3 Temperature °C 350 - 550 420 - 450 Thickness μm 2 - 6 3 - 5 Coolincr down of Substrate Temperature °C 150 - 300 200+ Table 1: Important Process Parameters for a Four Cathode Combined Arc/ Unbalanced Magnetron PVD Coating Machine with a Typical Target Size of 60 cm x 20 cm When using the "preferred deposition conditions" as outlined in Table 1 the following film composition of the metallic constituents of the hard material coating has been found by using WDX (Wavelength Dispersive X-Ray Analysis) : 40 at % Ti 56 at % Al 2 at % Y 2 at % Cr The stabilisation of the interface between the hard coating and a steel substrate by the uneven incorporation of Y is shown in Figs. 3 to 5 using SNMS (Secondary Neutral Mass Spectroscopy) analyses. Fig. 3 shows the completely uniform distribution of the coat¬ ing constituents Al(4), Ti(6) , Cr(7), N(3) over the complete thickness range. The intensity of the metal concentration is given in non-calibrated counts. In this coating no Y was in¬ corporated. Therefore Y is only identified as a low concen¬ trated random element. The sample has been heat treated for one hour in air at 800°C. Despite this heat treatment a sharp decay of the Fe signal (8) is observed at the interface indi¬ cating that no significant diffusion of Fe into the hard coating has taken place. Only a rather thin oxide layer is formed on top of the TiAlN coating as indicated by the sharp increase of the oxygen signal (1) and the parallel decrease of the N signal (3) . However, if one increases the heat treatment temperature to 900°C a significant diffusion of Fe (6) into the coating is observed (Fig. 4) . The formation of a surface oxide layer is intensified. Concluding from the shape of signal 4 (Al) , 5 (Ti) , 1(0) and 3 (N) an almost N-free ox¬ ide is formed with an Al rich top layer and a Ti rich oxide between the Al-oxide and the TiAlN film. The formation of this N-free sandwich oxide on top of a TiAlN coating has been confirmed by earlier work (D. Mclntyre, J.E. Greene, G. Hakansson, J.-E. Sundgren, W.-D. Mϋnz, J. Appl . Phy. 67 (1990) 1542) . Extrapolation from the observed data (point "A" in Fig. 4) allow the estimation of an oxide thickness of 0.8 μm. The uneven incorporation of Y into the coating completely changes the results gained by SNMS analyses (Fig. 5) . Similar to the case at 800°C a steep decay of the Fe signal /6 is ob¬ served at the interface thus demonstrating clearly that the Fe diffusion is completely suppressed. The Y itself seem to be distributed uniformly between the top oxide and the inter¬ face. SNMS does not permit the resolution of the layered and therefore uneven incorporation of Y into the coating. Because of the rotation of the substrate holders the Y concentration may be regarded as a layered concentration, e.g. varying ap¬ proximately sinusoidally in the growth direction. SNMS shows only that the Al rich oxide is more or less Y-free and that there exists some indication that Y is incorporated into the Ti oxide portion. It can also be seen that the thickness of the sandwich oxide is reduced by the incorporation of Y to an approximate thickness of 0.65 μm. The positive influence of low concentrated Y but also of Cr onto the oxidation resistance respectively onto the formation of a passivating oxide coating on top of the hard nitride coating has been established by TG (Thermo Gravimetric) analyses, too. Fig. 6 shows isotherms recorded at a tempera¬ ture of 900°C and over a reaction time in air up to 10 hours. It can be clearly seen that 2 at % Cr already reduce the weight gain due to oxidation during the reaction of TiAlN with hot air. However, the most pronounced improvement was found when Y was added in the "uneven" mode into the coating. For comparison reasons the oxidation behaviour of TiN is given in Fig. 6 also. One recognises a steep increase in weight gain due to the spontaneous formation of Ti0 2 at 900°C. The coating is completely oxidised already after 1 hour. Therefore no further weight gain is observed due to the fact that the stainless steel substrate material oxidises only marginally in this temperature range. The discrete concentration of Y into one target led to a sub¬ stantial influence of the growth mechanism of the coating. Coatings without Y showed in X-ray analyses with pronounced <111> preferred orientation. Fig. 7a exhibits an XRD diagram of a Ti 042 Al 9 sa N coating. The XRD diagram of TiAlN with con¬ tinuous Y distribution is very similar although the peaks are somewhat broader, indicating higher internal stresses and smaller grain size of the polycrystalline coating. Incorporating Y from one single target completely changes the XRD spectrum. The <111> preferred orientation is converted into a <200> preferred orientation with much less sharp peaks as shown in Fig. 7b. Both the broader peaks and the <200> preferred orientation indicate smaller grain sizes and con¬ tinuous re-nucleation of the growing film. It has been found that the above described drastic thermal stabilisation of the interface can - for example - be achieved under target arrangement conditions as described in Fig. 1. It is also important to note that linear cathodes al¬ low a very uniform distribution of the Y content parallel to the substrate surface and across the full height of the cath¬ ode length as compared to cathode arrangements with a series of small circular cathodes as very often used in cathodic arc evaporation systems. The assumption of a continuous re-nucleation and grain re¬ finement caused by the uneven distribution of Y in the growth direction of the coating can be strengthened by cross-section TEM (Transmission Electron Microscopy) analyses. Figs. 8a and 8b show computer processed TEM images which show in Fig. 8a the typical columnar growth of TiAlN coatings as reported in earlier work (G. Hakansson, J.-E. Sundgren, D. Mclntyre, E. Greene, W.-D. Mϋnz, Thin Solid Films, 153 (1987) 55) . Fig. 8b however demonstrates a very fine grained film growth with clear indications of regular columnar growth only in the direct vicinity of the interface. In this section of the coating the base layer was deposited as outlined in Fig. 2. Fig. 8b supports also the understanding of reduced oxidation of Y containing TiAlN coating due to the obviously prolonged diffusion paths for oxygen along the reduced grain sizes. It is also common knowledge that fine grained hard coatings are subjected to high internal stresses. The TEM results there¬ fore justify the introduction of the Y-free base layer since via this graded interface the influence of the internal stresses onto the adhesion of the coatings should be reduced. The higher internal stresses of the fine grained TiAlYN may be quantified also by an enhanced hardness HK 2700 in com¬ parison to the Y-free coating with typically HK 2400. Experiments showed that the achievements generated by the de¬ scribed incorporation of Y can be improved further when Yt¬ trium is alloyed with Scandium. A typical composition of 50 at % Y and 50 at % Sc has been found to have a significant effect. Furthermore, it has been shown that the hard material layer should itself be very smooth (W.-D. Mϋnz, I.V. Smith, L.A. Donohue, J.S. Brooks, Patent Application 195 47 305.1) in or¬ der to bring about the advantageous influence of the yttrium. With a surface roughness in the range Ra = 0.150 to 0.3 μm, produced for example by droplet formation during cathodic arc discharge evaporation, the improvement of the cutting behav¬ iour of twist drills in uncooled operation is rather limited. With a roughness of Ra < 0.050 μm one could, however, find substantial improvements of the service life. The following tests were made for twist drills: tool twist drills 8 mm diameter blind holes 27 mm in diameter material machined GG25 (test iron) cutting speed 60 m/min feed 0.2 mm/revolution The drilling tests brought the following results drill uncoated 17 holes drill TiAlN (Ra = 0.15 - 0.20 μm) 100 holes drill TiAlN (Ra < 0.05 μm) 250 holes drill TiAlYN (Ra < 0.05 μm) 600 1,100 holes References W.-D. Mϋnz J. Vac. Sci. Technol. A4 (6) (1986) 2117 J.R. Roos, J.P. Celis, E. Vancoille, H. Veltrop, S. Boelens, F. Jungblut, J. Ebberink, H. Homberg Thin Solid Films, 193/194 (1990) 1542 W.-D. Mϋnz Surf. Coat. Technol. 48 (1991) 81 W.-D. Mϋnz, D. Schulze, F.J.M Hauzer Surf. Coat. Technol., 50 (1992) 169 W.-D. Mϋnz Werkstoffe und Korrosion, 41 (1990) 753 W.-D. Mϋnz, I.V. Smith, L.A. Donohue, J.S. Brooks German Patent Application 195 47 305.1 W.-D. Mϋnz. T.T. Trink, T. Hurkmans D 44 05 477 Al D.McIntyre, J.E. Greene, G. Hakansson, J.-E. Sundgren, W.-D. Mϋnz J. Appl. Phys. 67(1990) 1542 G. Hakansson, J.-E. Sundgren, D. Mclntyre, J.E. Greene, W.-D. Mϋnz Thin Solid Films, 153 (1987) 55

Claims: Patent Claims 1. Hard material coatings manufactured by means of cathodic arc evaporation or sputtering or combination processes of sputtering/cathodic arc evaporation or sputtering/low voltage electron beam evaporation or low voltage evapo- ration/cathodic arc evaporation and consisting substan¬ tially of a binary, ternary or quaternary hard material layer comprising nitride or carbonitride, characterised in that the layer contains about 0.1 to 4 at % yttrium. 2. Hard material coating in accordance with claim 1, char¬ acterised in that the layer consists substantially of the ternary hard material layer TiAlN with an Al-content of 10 to 70 at % and in that 0.1 to 4 at % yttrium is added to this layer. 3. Hard material coating in accordance with claim 2, char¬ acterised in that the layer consists of a polycrystal- line fee structure (Bl-NaCl-type) with a <200> preferred orientation. 4. Hard material coating in accordance with claim 2 or claim 3, characterised in that the intensities of the major XRD peaks from the pattern (111) , (220) , (311) are smaller than that of the (200) peak intensity by at least 70 %. 5. Hard material coating in accordance with any of the claims 2, 3 or 4, characterised in that the full width at half maximum (FWHM) of the (200) XRD peak is larger than 1° of 2θ. 6. Hard material coating in accordance with claim 2 or claim 3, characterised in that the yttrium is unevenly distributed in the growth direction of the coating. 7. Hard material coating in accordance with any one of the preceding claims, characterised in that the yttrium con¬ tent amounts to approximately 1.5 to 2.0 at %. 8. Hard material coating in accordance with any one of the preceding claims, characterised in that the yttrium is replaced by an alloy of yttrium and scandium. 9. Hard material coating in accordance with any one of the preceding claims, characterised in that the scandium content amounts to 50 at %. 10. Hard material coating in accordance with any one of the preceding claims, characterised in that the base layer adjacent the substrate contains substantially no yt¬ trium. 11. Hard material coating in accordance with any one of the preceding claims, characterised in that the coating con¬ sists of a multi-layer superlattice of TiAlN/TiN, TiAlN/ZrN, TiAlN/CrN, TiAlN/MoN, TiAlN/WN with an Y con¬ tent of 0.1 to 4 at %. 12. Hard material coating in accordance with any one of teh preceding claims, characterised in that the coating con¬ sists of a multi-layer superlattice of TiAlN/TiN, TiAlN/ZrN, TiAlN/CrN, TiAlN/MoN, TiAlN/WN, whereby the Y is incorporated in the TiAlN layers of the superlattice coating. 13. Hard material coating in accordance with one of the pre¬ ceding claims, characterised in that 1 to 10 at %, pref¬ erably 2 at % of the total metal content consist of Cr. 14. Hard material coating in accordance with claim 13, characterised in that Cr is used in place of yttrium. 15. Hard material coating in accordance with one of the pre¬ ceding claims, characterised in that an additional car¬ bon content from 30 to 70 at % is present in the layer. 16. Hard material coating in accordance with claim 15, characterised in that the carbon is exclusively intro¬ duced into the outer layer zone and the layer thickness of the outer layer zone amounts to 0.5 to 1 μm. 17. Hard material coating in accordance with one of the pre¬ ceding claims, characterised in that the total layer thickness of the hard PVD coating amounts to approxi¬ mately 1.5 to 5 μm independently of the composition of the layer. 18. Hard material coating in accordance with one of the pre¬ ceding claims, characterised in that the surface rough¬ ness of the hard PVD layer on a substrate polished to Ra 0.010 μm amounts preferably to Ra 0.050 μm and smaller values. 19. Hard material coating in accordance with one of the pre¬ ceding claims, characterised in that the substrate mate¬ rial consists of high-speed steel (HSS) or hard metal (cemented carbide) and the parts to be coated are espe¬ cially formed by twist drills, milling cutters, reamers, exchangeable cutting tips or forming tools such as stamping punches or forging dies. 20. PVD-method for coating substrates with a hard material coating in accordance with one of the preceding claims, characterised by the use of multi-target deposition sys¬ tems in which at least one of the cathodes is not equipped with a Y containing target and the substrates to be coated are mounted on rotatable substrate fixtur- ing systems so that the substrates are intermittently exposed to the Y containing target . 21. Method according to claim 20, wherein the coating is de¬ posited in a four target combined arc/unbalanced magne¬ tron PVD deposition machine having four cathodes, espe¬ cially four vertically arranged linear cathodes, only one of which is equipped with a Y containing target and the substrates to be coated are mounted on a rotatable substrate fixturing system allowing at least twofold substrate rotations. 22. Method according to claim 20 or 21, characterised in that one of the cathodes being used as a cathodic arc etching cathode, is especially equipped with a Cr tar¬ get, whereas the other three cathode positions are fur¬ nished with TiAl targets and only one of these three cathodes is equipped with a Y containing target . 23. Method in accordance with claim 22, characterised in that Cr is mixed into at least one of the TiAl targets of a multi-target PVD coating machine, in particular an arc coating machine, and is optionally used in place of yttrium. 24. Method in accordance with any one of the claims 19 to 23, characterised in that the cathode behind the target containing yttrium is first switched on after the depo¬ sition of a base layer on the substrate containing sub¬ stantially no yttrium, i.e. only yttrium present as an impurity rather than as an intentional constituent of the layer.

Inventors: BROOKS JOHN STUART, DONOHUE LEE ADRIAN, MUENZ WOLF-DIETER, SMITH IAN, BROOKS, JOHN, STUART, DONOHUE, LEE, ADRIAN, MUENZ, WOLF-DIETER, SMITH, IAN
Applicants: BROOKS JOHN STUART, DONOHUE LEE ADRIAN, HAUZER IND BV, MUENZ WOLF DIETER, SMITH IAN, BROOKS, JOHN, STUART, DONOHUE, LEE, ADRIAN, HAUZER INDUSTRIES BV, MUENZ, WOLF-DIETER, SMITH, IAN
Application year: 1997
Main classification: C23C14
IPC codes: C23C14