Compared to Al-based materials or low-alloyed and unalloyed steels, Ti-based alloys have a significantly lower thermal conductivity. In addition, the tensile strength of Ti-based materials is significantly higher. Due to the low thermal conductivity of λ=4 to 1 W/mK, heat dissipation plays an important role during the chip generation process in the machining. For example, when machining Ti-based materials, about 30% more energy goes into the tool in comparison to when machining CK45 with otherwise equal process parameters. This results in an increased thermal load on the cutting tool and thus promotes tool wear. Conversely, the technological parameters for machining Ti-based materials are reduced so that the productivity and thus the efficiency are significantly lower when machining Ti-based materials than when machining other materials.
Ni-based materials such as Inconel have a high thermal stability and are therefore frequently encountered, particularly in turbine construction. In addition, the material has sufficient strength even at extremely high temperatures.
Due to the generally high requirements of the chip-removing machining process when machining Ti- and Ni-based materials, the technological parameters are comparatively low and thus the productivity and efficiency are also low.
In addition, the resulting high thermal load in the chip-removing machining process results in comb-like edge crack formation and/or crater wear, which further promotes the abrasive wear mechanism on the active surfaces of machining tools. Adhesive wear and plastic deformation of the cut material are also observed, depending on the material.
In addition, current testing shows that purely AlCrN-based layer systems and purely AlTiN-based layer systems are subject to wear mechanisms that are similar to those of the surfaces of uncoated substrates and as a result, neither of the two layer systems achieves a significant advantage.
According to the prior art, some oxynitride-based hard material layers are nevertheless considered to be well suited for chip-removing machining of hard-to-machine materials.
The patent JP2012192513A, for example, discloses a coated cutting tool for chip-removing machining whose coating enables a higher performance in the wet chip-removing machining of hard-to-machine materials such as titanium-based alloys. The coating is composed of an inner layer and an outer layer; the inner layer is an oxynitride layer composed of titanium and aluminum with a layer thickness of between 0.5 and 3.4 μm and the outer layer is a nitride layer composed of titanium and aluminum with a layer thickness of between 0.8 and 4.0 μm. The oxynitride layer has micropores that are distributed across the layer thickness and have a diameter of between 0.1 and 1.5 μm. In addition, the composition of such an oxide layer corresponds to the following equation in atomic percent: (Ti1-xAlx)N1-yOy, where x is between 0.4 and 0.75 and y is between 0.1 and 0.4.
The patent JP2009167498A also relates to oxynitride edge layers. In this case, the oxynitride edge layers are produced by means of the anodic oxidation of the substrate, with layer thicknesses of 5 to 30 μm. This also reduces the risk of layer spalling due to excessive internal compressive stresses. The composition of such layers is defined in atomic percent by the following equation: (Me1-aXa)α(N1-x-yCxOy), where Me is one or more elements selected from among the groups 4a, 5a, and 6a of elements in the periodic system, X is one or more elements selected from among the group Al, Si, B, and S, where 0.10≦a≦0.65, 0≦x≦10, 0≦y≦10, and 0.85≦α≦1.25. In addition, edge layers of this kind should have a face-centered cubic structure.
The prior art does not, however, disclose methods as to how PVD oxynitride hard material layers with a predetermined thermal conductivity can be manufactured.
The object of the present invention is to provide a method for manufacturing oxynitride hard material layers that have a predetermined thermal conductivity. Another object of the present invention is to provide a hard material layer system that includes such oxynitride hard material layers. Preferably, these oxynitride hard material layers should in particular have a higher wear resistance at high temperatures so that these hard material layers can be well suited particularly for the chip-removing machining of hard-to-machine materials.
In particular, the aim is to provide a method that makes it possible to produce a coating with a high thermal stability, reduced thermal conductivity, increased anisotropy of the thermal conductivity, and consequently an extended service life, thus making it possible to increase productivity in the chip-removing machining of hard-to-machine materials.
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