Patent Application
20090075114
This application claims priority to United Kingdom Applications Nos. 0713671.6, filed Jul. 13, 2007 and 0800022.6, filed Jan. 2, 2008, the disclosures of which are herein incorporated by reference.
The present invention relates to a method for the manufacture of a hard material coating on a substrate consisting of a metal, TiAl or Ni based alloys or a ceramic material which is, for example, electrically conductive, for example a substrate in the form of a coated tool, a machine component or a turbine blade and to a coated substrate per se.
There are a large number of proposals for the coating of machine tools with hard materials which make it possible to prolong the working life of the tools and/or to increase the cutting speed and the quality of the cut that is possible on a large variety of metal work pieces including relatively hard metals, tool steels, die materials, aerospace materials, steels of diverse kinds, titanium alloys, forgings and castings. Similarly there is a large number of proposals for coating automotive or aerospace components or components in energy systems in order to provide protection against environmental attack at high temperatures.
By way of example European patent 1260603 B1 describes and claims coatings deposited by a PVD (Physical Vapor Deposition) process, consisting of the nitrides TiN, ZrN, TiAlN, TiZrN, TiWN, TiNbN, TiTaN, TiBN or the carbonitrides TiCN, ZrCN, TiAlCN, TiZrCN, TiVCN, TiNbCN, TiTaCN, TiBCN. Moreover, the European patent EP 1260603 B1 discloses that the coating can contain from 0.1 to 5 at % of the rare earth elements Sc, Y, La, Ci.
In addition, the said European patent also discloses coatings consisting of fine (nanoscale) multilayer films having a periodicity from 1 to 10 nm selected from the group comprising TiN/TiAlN, TiN/VN, TiN/NbN, TiN/TaN, TiN/ZrN, TiAlN/CrN, TiAlN/ZrN, TiAlN/VN, CrN/NbN, CrN/TaN, CrN/TiN, Cr/C, Ti/Ci, Zr/C, V/C, Nb/C and Ta/C.
This listing will be understood by the person skilled in the art to mean that the coating consists of alternate layers of the two materials separated by the oblique line in the above material pairings. Again, one of the individual layers listed for these layer pairings can contain 0.1 to 5 at % of the rare earths Sc, Y, La and Ci.
Further particulars of some of the above mentioned coatings can be found in the literature, for example in the publications by Lembke et al in “Significance of Y and Cr in TiAlN hard coatings for dry high speed cutting”, Surface Engineering 17(2) pages 153-158 (2001) in which the coating material Cr has been implanted in the interface using an arc discharge technique and in the paper by P. Eh. Hovsepian, C. Reinhard and A. P. Ehiasarian entitled CrALYN/CrN superlattice coatings deposited by the combined high power impulse magnetron sputtering/unbalanced magnetron sputtering technique in Surface and Coatings Technology 2001 (2006) pages 4105-4110. This reference describes, amongst other things, the pretreatment of the metal substrate in the sense of a cleaning/etching treatment by bombarding the substrate with Cr ions generated by the so-called HIPIMS technique (High Power Impulse Magnetron Sputtering). This technique is also described and claimed in the aforementioned European patent. More specifically, the European patent describes a PVD method for the coating of substrates wherein the coating takes place by means of cathode sputtering after a pretreatment. The substrate is pretreated in the vapour of a pulsed cathode sputtering process assisted by a magnetic field. A field arrangement in the manner of that of a magnetron cathode is used for the magnetic field assistance during the pretreatment and has a strength in the horizontal component in front of the target of 100 to 1500 Gauss. Moreover, the power density of the pulse discharge in the pretreatment is more than 1000 W/cm−2. The power density preferably lies in the range from 2000 to 3000 W/cm−2. The pulse duration lies between 10 and 1000 μs and the pulse interval amounts to between 0.2 ms and 1000 s. The pulse duration is preferably 50 μs and the pulse interval 20 ms. The average pulsed discharge current density is expediently smaller than 10 A/cm−2 and the local maximum pulsed discharge current density typically lies below 100 A/cm−2. The pulses used typically have a peak voltage in the range from 0.5 to 2.5 kV and the pretreatment is usually done in an argon atmosphere in the pressure range from 10−5 to 10−1 bar and especially 10−3 mbar. The negative bias voltages applied to the substrates to achieve etching effects and ion implantation during pretreatment are generally between 0.5 and 1.5 kV. The paper specifically discloses the pretreatment/ion etching/ion implantation using Cr ions, but also discloses that the pretreatment can take place with targets of Cr, V, Ti, Zr, Mo, W, Nb or Ta in a non-reactive atmosphere such as Ne, Ar, Kr or Xe.
Hitherto, the pretreatment, etching and ion implantation of metallic substrates has been carried out by the so-called ABS process (which is a trademark of Hauzer Techno Coating BV) and is described in European patent 0459137 B1. That patent discloses the use of an arc discharge to generate ions for the pretreatment of the substrate and the subsequent use of a cathode sputtering process to deposit coatings on the pretreated etched substrate. The joint use of an arc vaporization process and a magnetron sputtering process in one and the same chamber, which is covered by European patent 0403552 B1, makes it particularly convenient to carry out the ABS process which has been extensively used.
With some materials which are used for the generation of coatings and which contain low melting point elements problems can however arise in the arc process due to the formation of small droplets which are deposited on the substrate during the pretreatment process and which disturb the otherwise smooth coating that would be obtained during the subsequent magnetron sputtering process. It has fortuitously been found that such problems with droplet formation do not occur when the HIPIMS technique is used for the pretreatment of the substrate surface.
As described in the above reference paper by P. Eh. Hovsepian et al in Surface and Coatings Technology 201 CrAlYN/CrN nanoscale multi-layer/superlattice coatings, especially those deposited after pretreatment of the substrate surface with a highly ionized Cr+ and Ar+ plasma generated from a HIPIMS discharge leads to a substrate surface which is clean, free of surface oxides, sharp and free of a macroparticle interface. This provides good conditions for local epitaxial growth over large surface areas, thus resulting in excellent coating adhesion.
Moreover, the last named paper explains that the presence of the yttrium in the superlattice coating leads to a significant reduction in the oxidation rate and suppresses the diffusion of substrate elements, such as Fe, into the coating. It is noted that the intention is not to avoid an oxide coating at all, since tools having an exposed surface layer containing Al2O3, which is a very hard material, can be extremely beneficial in use, in particular when the tools are exposed to very high thermal loads such as occurs at high cutting speeds, and in particular high cutting speeds during dry cutting.
In many high temperature applications the interdiffusion of substrate and coating elements is very common. If a nitride based coating such as TiN, CrN or TiAlN is used, then, at long term exposure at high temperatures, nitrogen very often diffuses into the substrate forming a nitride compound or compounds with the substrate elements. This usually is detrimental to the mechanical properties of the coated substrate leading to reduction of the fatigue strength of the base material in particular.
The object underlying the present invention is to provide a method of preparing a coated substrate and a coated substrate which have even better properties than the coatings described above, but which can nevertheless be used with a large variety of known coatings.
Thus, the method and the coated substrate in accordance with the invention should provide improved cutting properties, improved resistance to wear, improved protection against environmental attack at high temperatures and improved protection against diffusion of elements from the substrate into the applied coating and vice versa, particularly for cutting tools which are used at high temperatures and/or at high cutting speeds and/or in a dry or wet state and for components of automotive engines, aircraft gas turbines or stationary gas turbines exposed to high temperatures.
In order to satisfy these objects there is provided, in accordance with the present invention, a method for the manufacture of a hard material coating on a substrate consisting of a metal or of a ceramic material, e.g. a coated tool for use in a machine tool, or components exposed to high temperature wherein, prior to the deposition of the hard protective material coating, the substrate is pretreated by simultaneous bombardment with ions of more than one element but always including at least one rare earth element thereby resulting in implantation of said ions into said substrate, where the concentration of the implanted elements into said substrate does not exceed 50 at %, preferably does not exceed 20 at % and in particular does not exceed 10 at %.
Furthermore, there is provided a coated substrate made by the method. That is to say a coated substrate comprising a metal or metal alloy such as a high speed steel, a TiAl based alloy or an Ni based alloy or an electrically conductive ceramic material, the substrate having an interfacial layer to the coating produced by simultaneous bombardment with ions of more than one element but always including at least one rare earth element thereby resulting in implantation of said ions into said metal or metal alloy or in the electrically conductive ceramic material, where the concentration of the implanted elements into said substrate does not exceed 50 at %.
It has namely been surprisingly found that the pretreatment, i.e. etching of a metal or electrically conductive ceramic substrate surface using metal ions of rare earth elements, such as Y, La, Ci or Sc, with resulting implantation of rare earth element ions into the substrate surface, surprisingly results in a much higher oxidation resistance for the coating. This is believed to be due to the high affinity of the rare earth elements to the oxygen, as well as to the strong adhesion of the stable oxide that may form to the substrate and to the segregation by thermal displacement of the implanted rare earth material at the substrate material grain boundaries which then block the paths for diffusion of oxygen into this material at elevated temperatures.
The simultaneous implantation of ions of at least one rare earth material together with other ions such as Cr, Ti and/or Al into a substrate during an etching pretreatment of the substrate prior to the deposition of other layer systems thereon is not known from any prior art of which the applicants are aware and has proved to be very advantageous, particularly when the ion implantation/etching is carried out using the HIPIMS technique. Rare earth elements tend to have a large atomic radius and high concentrations thereof lead to the generation of extremely high compressive stresses which are disadvantageous. If implanted in large quantities in the coating interface these high stress values will compromise the adhesion of the coating. The use of the HIPIMS technique allows the generation of an ion flux of more than one type of material, as it is a sputtering process, and a low concentration of rare earth elements in the interface together with other elements can be achieved. The other elements are preferably those or at least some of those used in the actual coating system. It has been found that such co-implantation allows particularly beneficial results to be achieved and allows the above recited objects and advantages to be satisfied and achieved.
Although these effects can be attributed to some degree to rare earth elements incorporated into the coating itself, the concept that the rare earth elements could be implanted during pretreatment into the coating substrate interface either alone or in combination with another material is a completely novel idea and has proved to be very beneficial because the coating substrate interface is a very important and vulnerable zone in a coating-substrate system. The finding that the oxidation resistance of this zone can be further enhanced by the implantation of rare earth elements into the substrate material during the etching stage of the coating production process and that this leads to a further overall improvement of the protective functions of the coatings at high temperatures must be regarded as very surprising.
At this stage it will be noted that prejudices exist against the use of rare earth materials in arc PVD processes because they tend to lead to unstable discharges when present in the evaporation targets. It seems that rare earth elements tend to react with the oxygen from the residual gas in the vacuum chamber and form oxide films on the target surface which can cause arc spot retention problems and can indeed also lead to complete extinction of the arc discharge. The evaporation targets which have hitherto been used containing rare earth elements are usually multi-component alloys of titanium, chromium and aluminum. The presence of elements with a low melting point, such as Al, in the target presents the drawback of generation of large amount of droplet phase and therefore coating defects in arc evaporation process
It is particularly preferred, but not essential, for the pretreatment (etching/ion bombardment) to be carried out using the HIPIMS process because it has been found that this permits the generation of ions of the rare earth elements in droplet-free manner regardless of the precise composition of the target material. The droplet-free ion flux can be accelerated and implanted in the substrate by the application of high-bias voltages to the substrate resulting in the production of extremely sharp and defect-free interfaces. The interfaces that are modified in composition, in this particular case enriched by one or more rare earth elements, such as Y, Lanthanum, or cesium—optionally in combination with other elements such as Cr—provide much higher oxidation resistance to the coating.
Preferred embodiments of the method and of the coated substrate are set forth in the subordinate claims.
The invention will now be explained in more detail with reference to the accompanying drawings in which are shown:
Within 5 nm of the interface, the concentration of the rare earth element should preferably be in the range from 0.1 at % to 10 at %, preferably 0.1 to 1.0% and the concentration of the other ions, i.e. of Cr, or Cr and Al, or Ti or Ti and Al, is generally higher but the total concentration of the implanted ions should not exceed 50 at %, and should preferably not exceed 10 at 5 and especially not exceed 10 at %. The depth of the implanted zone can be as large as 100 nm.
There is a base layer 18 of TiAlCrN on the substrate 10 at the interface 16 and, on top of this base layer 18, there is a layer 20 of TiAlCrYN with an yttrium content of 1 at %.
The base layer 18 and the layer 20 are deposited using the apparatus, targets and parameters described in the above mentioned article by Lembke et al, Surface Engineering 2001, Vol 17, no. 2 pages 153-158. The yttrium ions 12 can be generated by an arc process from a target consisting of CrY, or of CrAlY, or of TiY or of TiAlY (for example) and this means that there are also some atoms of Cr, Cr and Al, Ti and Ti and Al incorporated during the surface cleaning and etching step to a depth of several nanometers in the surface layer of the substrate 10 adjacent the interface 16, which are not, however, shown here. Alternatively, the yttrium can be supplied from a heated vapor source with the plasma in the vicinity of the substrate being generated in the PVD apparatus by any known plasma generating process. A relatively high substrate bias of typically 1200 V can be used to ensure ion bombardment of the substrate with the yttrium ions.
Instead of using the cemented carbide material as the substrate, it is also possible to use TiAl or Ni based alloys or a steel, such as a tool steel or any other suitable material for a high speed machining tool, or stainless steel e.g. suitable for machine components.
It has been found that the incorporation of the rare earth element yttrium together with ions of Cr, or Cr and Al, or Ti or Ti and Al to a depth of several nanometers into the substrate 10 immediately adjacent the interface 16 has a beneficial effect on the properties of the tool as described above and also provides better protection of TiAl or Ni based alloys against environmental attack.
A difference in this example is that the yttrium ions are generated in this embodiment by use of the HIPIMS technique from a target containing yttrium. More specifically the target can be CrY, CrAlY, TiY or TiAlY or any of the foregoing materials in which another rare earth elements is substituted for some or all of the yttrium.
On top of the substrate there is in this case a superlattice structure comprising alternating layers 40 and 42 of CrAlYN/CrN deposited using the apparatus and the process, parameters and targets described by P. Eh. Hovsepian et al in Surface and Coatings Technology 201 2006 4105-4100, i.e. using the HIPIMS process. This superlattice structure is terminated by a further layer 44 consisting, in this example, of a graded layer 44 in which a layer of Al2O3 and Cr2O3 of about 350 nm thickness gradually merges (i.e. with graded composition) into a layer of Al2O3 of about 175 nm thickness.
Again, the conditions for the deposition of the layer system 40, 42, 44 are the same as recited above in the article in Surface and Coating Technology 201 and will not therefore be repeated here.
On top of the substrate 10″ there is provided, above the interface 16, a hard material coating consisting of a Cr layer which has been deposited by an arc process in which some of the Cr ions are implanted into the surface region of the substrate 10″ together with the yttrium ions. Above the Cr layer 50 there is then a base layer 52 of TiAlN followed by a layer 54 of TiAlYN, a CrN layer and finally an Al2O3 top oxide layer. As is usual, the nitrogen in the layers 52, 54 and 56 is supplied by a nitrogen atmosphere in the PVD coating chamber whereas the other layers are produced from targets of the corresponding materials, i.e. from a Cr target, from a TiAlY target, and from a TiAl target. The aluminium oxide top layer can either be produced from an Al2O3 target or simply from an Al target, which results in an Al2O3 oxide layer in a reactive process.
Finally,
The dots 12 again represent yttrium atoms and ions of other elements, for example as recited above incorporated in the surface layer of the substrate by ion bombardment during the surface cleaning and etching step. Again the concentration of the yttrium ions decreases from the interface 16 with progressive depth into the substrate. The concentration of yttrium is the same as was suggested above in connection with
The yttrium ions in this embodiment are generated in the PVD apparatus by any known plasma generating process including the HIPIMS technique from a target containing yttrium.
In fact, the layers 18 and 20 are also the same as the layers 18 and 20 in the embodiment of
In all the examples given, other rare earth metals can be substituted for the yttrium, for example Sc La or Ce.
Moreover, the coatings shown in the examples of
The incorporation of ions of one or more rare earth elements into the surface layer of the substrate especially together with ions of other elements is so effective that it is not always necessary to incorporate one or more rare earth elements into the actual hard material coating, although this is frequently necessary or helpful in order to obtain the best possible results.
It should also be noted that any of the layers described can be deposited with graded transitions to the next adjacent layer by suitable control of the PVD process and the reactive gases supplied to the vacuum treatment chamber.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0713671.6 | Jul 2007 | GB | national |
| 0800022.6 | Jan 2008 | GB | national |
