Patent Application
20220136096
A broad range of industrial processes take place under severe conditions e.g. high temperature. Demanding efficiency improvements scale up this severity drastically. One approach to protect industrial tools in such operational conditions is to separate the tools from the harsh environments with implementation of thin coatings over the tools surface. Among coatings methods physical vapor deposition (PVD) technique has gained immense attention both in academia and industry and has validated immense successes.
Nonetheless, still today high temperatures can affect the coating applied on a tool surface as well as the tool surface covered with the coating (i.e. the substrate surface covered with the coating).
A well-known example is coated cemented carbide cutting tools. During use of tools, especially in case of tools made of or comprising cemented carbide, it can happen that the exposure of the tool surfaces to high temperatures results in out-diffusion of cobalt into the coating. Moreover, already during deposition, if this proceeds at high temperatures and intense plasma treatment, the out-diffusion of cobalt to the substrate surface may occur.
Since, exposure of the substrate, e.g. the cutting edge of a cutting tool, to high temperature in certain applications, e.g. in cutting Ti-containing material, will be inevitable despite of the use of coatings, there is a need of coatings that can help to avoid damages of the substrate surface in operations involving exposition to high temperatures.
In the context of the present invention the term “high temperatures” is used in particular for referring to temperatures in range between 600 and 1200° C.
The main objective of the present invention is to provide a coating that helps to suppress the diffusion of elements from the substrate surface to the coating, when the coated substrate surface is subjected to high temperatures. This problem can occur normally:
The objective of the present invention is attained by providing a coated substrate comprising a substrate 1 comprising cobalt (Co) and a coating 2 deposited on a surface of the substrate, the coating comprising at least one boron-comprising layer 20, which further comprises aluminum and in which boron is present as boride, wherein the at least one boron-comprising layer 20 is deposited directly on the surface of the Co-comprising substrate 1 (see
The inventors observed that the boride comprised in the boron-comprising layer 20 has surprisingly the property of suppressing diffusion of cobalt from the Co-comprising substrate 1 to the coating 2.
This surprisingly effect is especially useful for avoiding damage of substrate and damage of coating in cases in which tools made of Co-comprising materials are used in machining operations, e.g. when cemented carbide tools are used.
The inventors observed that the mechanism of suppressing the diffusion process works in the interface between substrate surface and coating.
The inventors observed a formation of a Co-containing layer 21 (see
Thus, it was possible to suppress diffusion of cobalt from a Co-comprising workpiece to the coating, because of the formation of intermetallic phases of Al—Co and B—Co in the interface between the substrate 1 and the coating 2.
In addition, the inventors observed a further beneficial effect, when the inventive coated substrates where exposed to high temperatures in an atmosphere comprising oxygen. This further effect was a formation of an aluminum oxide comprising layer 22 (Al—O comprising layer 22) atop the boron-containing layer 20 (see
Borides and their superior properties are known. The present invention allows using boride coatings (which are very promising coating materials) in applications involving exposure to high temperatures, inclusive in atmospheres comprising oxygen. It is possible because the inventive coated tool results in higher thermal stability and simultaneously suppress Co diffusion by forming intermetallic compounds at interface of coating and substrate.
For example, in case of using a boron containing layer 20 composed of AlWB2, a layer 21 comprising following kind of intermetallic phases of Al—Co and/or B—Co (e.g. AlCo, Al2Co5, Co2B and/or Co3B) can be formed. These intermetallic phases in the interface between substrate and layer 20, allowing the rest of the coating (e.g. layer 20) remains intact.
Additionally, one major week point of boride coatings is the poor oxidation resistance of them.
Therefore, by having Al-boride coatings, e.g. AlWB2, protective Al—O layer can be formed and suppress the inwards diffusion of O2 from atmosphere to the coating.
In this manner, it was further possible to attain that coating material by itself realizes also in the contact with the workpiece reduced diffusion, especially if the workpiece contains fast diffusing elements like cobalt.
The inventive coating is particularly beneficial for protecting tools having substrates comprising cobalt.
Therefore, according to a preferred embodiment of the present invention, the inventive coating is deposited comprising a boride comprising layer 20 having the property of suppressing diffusion (hereafter in the context of the present invention also called diffusion barrier layer) of cobalt, wherein the diffusion barrier layer is deposited directly on the substrate.
The present invention will be explained in more detail with the help of some examples and figures.
Figures caption:
The following description and examples should not be understood as a limitation of the invention.
The depicted out-diffusion in this annealing experiment illustrates the diffusion processes which may proceed during deposition as well as during utilization of the coated substrate as explained above. The diffusion of Co alters the chemical composition of the coatings and its properties thereof. As a result, Co out-diffusion can lead to a weak coating-tool interface and influencing coating properties resulting in a reduction of lifetime of the coated tool.
As mentioned above, diffusion processes may occur or be initiated during the deposition process and during operation of the coated substrate. During deposition, prevalent substrate temperatures in PVD processes are between 300° C. and 600° C., i.e. below the temperature range of 600° C. and 1200° C. which is defined as diffusion relevant. However, the deposition process is characterized not only by the condensation of neutral elements at the surface but also by the simultaneously bombardment with energetic ions. This will result in much higher “temperatures” in the uppermost surface of the substrate and increases the mobility of Co atoms for diffusion. In the other case, during operation of the coated substrate, the surface of the coated tool in contact with the work piece (e.g. the cutting edge of a cutting tool) can be become very hot, and my exceed 1000° C., e.g. for machining of Ti-Alloys. Under these conditions, the diffusion of Co is also initiated.
A first objective of the present invention is to provide a coating which suppresses the diffusion of Co at high temperatures from the substrate material into the coating.
Another objective of the present invention is to provide a coating with superior mechanical properties with respect to hardness and Youngs modulus at room temperature as well as at high temperatures.
Binary and ternary Al—B compounds show very attractive properties such as high hardness in excess of 50 GPa which make them promising candidates for wear resistant coating to protect cutting tools and components in harsh environment.
The composition of of Al—B-based coatings by PVD is more or less determined by the composition of the target utilized for sputtering or cathodic arc evaporation.
According to a preferred embodiment of the present invention the coating or at least part of the coating is produced by using PVD (physical vapor deposition) sputtering techniques, preferably non-reactive PVD sputtering techniques, i.e. only a non-reactive gas, e.g. argon is used as process gas.
According to a further preferred embodiment of the present invention the coating (for producing the inventive coated tool) the coating or at least part of the coating is deposited by using PVD (physical vapor deposition) cathodic arc evaporation techniques, preferably non-reactive PVD cathodic arc evaporation techniques, i.e. no process gas or only a non-reactive gas, e.g. argon is used as process gas.
Depending on the parameters in these processes (target composition and fabrication, evaporation process, reactive and noble gas background atmosphere, substrate bias etc.), the Al—B-based coatings may contain, in addition to the elemental constituents, different Al—B phases according to the corresponding phase diagram.
An important contribution of the present invention is furthermore the prevention of oxygen diffusion towards the substrate/coating interface and towards the substrate which initiates the outward diffusion of Co through the coating. Because the Al—B-based coatings inherently contain Al which reacts with oxygen, it forms at the surface of the coating as well as in the regions where oxygen can diffuse to a Al—O which acts as diffusion barrier.
On the other hand, Co diffusion is reduced also in regions where this mechanism is not present due to the lack of oxygen. Here the high temperature initiates the formation of intermetallic phases of Al—Co and B—Co which reduce the diffusion of elemental Co.
The present invention relates concretely to a coating system comprising at least one diffusion barrier layer, wherein the at least one diffusion transition layer is:
In the transition region between the cobalt containing substrate and the boride containing layer, a gradient layer containing Al and W can be added for improved adhesion and suppressing cobalt out-diffusion to the substrate surface during the deposition process.
The inventive coating can be deposited exhibiting a monolithic or layered coating architecture comprising one or more of the above described diffusion barrier layers.
The inventive coating can be used for coating cemented carbide tools in a range of applications where the coated tool is exposed to high temperatures, such as cutting of Ti—Al compounds.
The inventive coating can be used for coating cemented carbide tools in a range of applications where the coated tool is exposed to high temperatures, such applications preferably are cutting of Ni-based, Co-based or Ni—Co-based superalloys.
The presence of the Al in these layers is preferred because Al can be important for attaining suppression of Co out-diffusion into the coating. The presence of borides compounds in these layers is preferred because boride compounds can be decisive for providing superior properties such as superior hardness, in particular a hardness of over 35 GPa, which is a very important property in different operations, e.g. in different cutting tool applications.
Prevented Co out-diffusion into the coating ensures the unique properties of these inventive coatings to remain intact during applications at high temperatures.
The binary and ternary borides in the context of the present invention can be synthesized for example by non-reactive physical vapor deposition (PVD) sputtering technique where merely Ar is used as the process gas. Binary boride targets are sputtered in an Ar atmosphere for synthesis of binary borides and simultaneous sputter of two binary boride targets, e.g. WB2 and AlB2, synthesizes the ternary boride over a broad range of chemical compositions, in this case WAlB2. In such a combinatorial synthesis of the later one, chemical composition of the synthesized coating can be tuned based on the power applied to the each binary boride target and the change in sputtering rate thereof. Required process gas pressure range for attaining a stable process is from 0.3 to 0.9 Pa. Adjusting process gas pressure in the above-mentioned range allows ensuring process stability. Substrate temperature ranges from 400 to 600° C. ensure the required mobility when low bias voltage values from floating to 50 V are used. In this setting, favored crystal structures can be obtained and at the same time the residual stress level of the boride coatings does not produce coating delamination.
For deposition of an intermediate/gradient layer between the substrate surface and the borides, preferably Al—W targets with more than 20 at. % W are utilized.
The above given example describes the deposition by sputtering. Of course, it is also possible to deposit Al—B-based coating by cathodic arc evaporation. In this process, targets Al—B or Al—B—W containing targets are utilized as cathodes.
The most important aspects of the invention can be summarized as following:
The present invention is furthermore related to the use of a coated tool according to any of the above-mentioned embodiments or combinations thereof, wherein the coated tool or the coated surface is exposed to temperatures in a range between approximately 600 and 1200° C.
The present invention is furthermore related to the use of a coated tool according to any of the above-mentioned embodiments or combinations thereof, wherein the substrate of the coated tool is cemented carbide and above the diffusion barrier layer a diamond layer is deposited. In such a case, it is not necessary that the cemented carbide substrate undergoes a pretreatment for removing cobalt before deposition of the diamond layer.
The present invention is furthermore related to a method for coating a coated tool according to any of the previous claims 1 to 7, characterized in that the coating is sputtered using PVD, in particular non-reactive PVS (physical vapor deposition).
All the above-mentioned embodiments of coated tools according to the present invention could be used for machining of super alloy materials.
The diffusion boride layers in the context of the present invention could be used as diffusion barrier layers for stopping diffusion of other elements different from cobalt and nickel.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/053345 | 2/10/2020 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62802838 | Feb 2019 | US |
