Hard coating coated parts

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Reexamination Certificate

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C428S336000, C428S325000, C428S697000, C428S699000

Reexamination Certificate




This invention is about wear-resistant parts having improved solid-state friction properties or lubrication capability, a higher wear resistance, an improved oxidation behavior and a superior adhesion property between the coating and the substrate.
In the fields of cutting tools, molds and mechanical components, it is popular to coat various hard coatings or layers in order to have superior wear resistance, oxidation behavior and friction properties. The typical coatings TiN and TiCN have a good wear resistance, but still they have problems in fitting the need for a sufficient oxidation resistance. Furthermore, TiAlN-based coatings proposed by published Japanese patent Hei 7-310173 have good wear resistance and oxidation behavior but the friction property is still insufficient.
CrN-, CrCN-based coatings have good friction properties, but they have a lower coating hardness and lower wear resistance.
As shown above, conventional coatings show inferiority in wear resistance, oxidation behavior or friction properties and still have some problems in various applications. In order to provide better friction properties, it was proposed to coat MoS-based thin films on the surface of hard coatings. However both the wear resistance and often also the adhesion of these types of thin films are poor; thus they do not provide sufficient improvement.
In general, the conventional coatings do not have the maximum adhesion force between the substrate and the coating; often a partial peeling-off of the coating takes place in heavy-duty cutting and therefore a stable cutting cannot be realized.
Because such conventional coatings still have some problems with top layers other than MoS-based layers, Japanese Laid-Open patent Hei 11-156992 proposed to coat a CrN-based coating on the surface of a TiAlN-based layer. However this coating type does not show a sufficient wear resistance, due to the limitation of the entire coating thickness.
The present invention aims at a superior adhesion between the coating and the substrate without a reduction of the wear resistance, of the oxidation resistance or of the friction properties, but improving every one of the coating properties to achieve a stable behavior under heavy-duty cutting, resulting in a long lifetime.
In order to solve the above-described optimization problem of the coating properties, in the present invention Ti, Al, Cr, N are necessary elements and, by adding oxygen to these elements, special optimized hard coatings can be created.
In addition, by optimizing the growth rate of the coating it is possible to deposit coatings characterized by a superior adhesion.
In the coating composition, Ti and Al contribute as wear-resistant elements, while Cr helps to improve the friction properties. By adding oxygen to the TiAlCrN coating, a further improvement, both of the oxidization-resistance and of the friction properties, is created.
In order to improve both the adhesion and the wear resistance, an optimization of the growth rate is carried out that should enable a continuous growth between the crystal lattice of WC grain of cemented carbide substrates and the hard coating, resulting in an at least partial epitaxial growth. The results are hard coatings with superior adhesion to the substrate, and therefore a longer cutting life under heavy-duty cutting is achievable.
First, studies of properties of hard coatings with different chemical composition showed the following effect: the oxidation behavior is improved when Cr is added to TiAlN. In case of TiAlN, it is well known that due to higher temperature load under oxygen attack Al diffuses to the surface of the layer and creates an aluminum oxide layer, resulting in the effect that the oxygen penetration from outside is suppressed and therefore an improvement of the oxidation behavior can be achieved. However, in this case, especially if a thermal and/or mechanical shock is acting on the cutting tool, then the aluminum oxide layer can easily flake off, because a very porous titanium oxide layer is generated underneath the aluminum oxide layer during oxidation. The same phenomena are occurring in the application of the hard coatings to different cutting tools or other tools like dies or molds or others.
It was proved that porous Ti oxide created underneath Al oxide turns from titanium oxide, in case of an AlTiN coating, into a TiCr oxide by adding Cr into the hard layer. It was observed that the TiCr oxide forms very dense layers. This dense oxide layer supports the aluminum oxide created on the top of the hard coating, resulting in a sufficient adhesion combined with an improved oxidation behavior.
The second effect of the Cr addition is the improvement of the friction properties.
The friction values of TiAlN hard layers against steel are in the range of 0.7-0.8.
The friction values were measured to be in the range of 0.3-0.6 if Cr is added. This is a significant improvement of the friction behavior.
This friction coefficient depends on the volume of Cr addition.
However, if the volume of Cr addition is too high, then the coating hardness decreases, resulting in inferior wear resistance, and therefore it is better to establish an upper limit on the volume of Cr addition.
It is confirmed that the Cr addition can improve the friction properties and oxidation behavior of the TiAlN-based coatings, but it has been found that the Cr addition is not enough for the optimization of the hard coating properties. Thus further improvement is recognized when oxygen is added.
The effect of oxygen addition results in a drastic improvement of oxidation behavior as well as in friction properties.
It is considered that the reason for the improvement of the oxidation behavior is that the oxygen addition inside the coating results in the effect that the grain structure becomes finer, the coating itself becomes more dense and the grain boundaries become more dense, resulting in the effect that the speed of oxygen diffusion of the oxygen from the hard layer surface inside the layer is reduced.
The improvement of the friction properties has not yet been analyzed in detail, but it is considered that affinity with steel becomes lower by adding oxygen.
The second effect of oxygen addition is that wear resistance is improved by an improvement of the adhesion of the hard coating due to lowering of the residual compressive stress in coating.
The adhesion of the hard coating is very important especially in heavy-duty cutting or in the field of forging dies. There is a trend of increasing wear caused by small peeling-off of layers, and if a large peeling-off takes place then the end of the lifetime is suddenly reached.
For demonstration of the improved adhesion, selected measurements using a standard scratch test unit are given now. The maximum loads of AlTiCrN-based coatings are in the range of 60 N-80 N, while it is improved to more than 100 N by adding oxygen.
If the volume of oxygen addition increases, the wear resistance will be improved due to the above-mentioned improvements of the oxidation behavior, friction properties and adhesion of the hard coating. However, the coating hardness is decreased if the oxygen content becomes too high, resulting in an inferior abrasive wear resistance. Accordingly, it is important and desirable to make multi-layer coatings consisting of single layers with optimized elements that contribute to the improvement both of the oxidation behavior and the friction properties and of single layers with optimized elements that contribute to an improvement of the abrasive wear resistance. The advantages of the above two kinds of single layers are then combined in the multi-layer coating. It is possible to adjust the friction properties and wear resistance not only by the variation of the oxygen volume in multi-layer coatings but also by the optimization of the Cr content.
Because extremely high mechanical impact forces are acting in the heavy-duty cutting field, it is not sufficient to optimize the chemical composition of the hard coatings regarding the friction properties, th


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