Oxide coated cutting tool

#The present invention relates to a coated cutting tool for chipforming machining. The coating includes at least one alumina (Al2O3) layer characterized by fine, equiaxed grains.

#In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention.

#Cemented carbide cutting tools coated with various types of Al2O3 layers using Chemical Vapour Deposition (CVD), e.g., pure κ-Al2O3, mixtures of κ- and α-Al2O3 coarse grained α-Al2O3 and fine grained textured α-Al2O3 have been commercially available for years generally in multilayer combinations with other metal carbide and/or nitride layers, the metal being selected from transition metals of the IVB, VB and VIB groups of the Periodic Table.

#FIGS. 1a-c show Scanning Electron Microscope (SEM) micrographs of the present invention. FIG. 1a shows an SEM micrograph of an Al2O3 layer according to the present invention at a high magnification in top view projection, FIG. 1b shows an SEM micrograph of a cross sectioned sample according to the present invention at a high magnification, and FIG. 1c shows an SEM micrograph of a polished cross-section of an Al2O3 layer with Si-rich layers, A, according to the present invention in so called backscatter mode at high magnification.

#FIGS. 2a-b show SEM micrographs of a prior art κ-Al2O3 layer. FIG. 2a shows an SEM micrograph at a high magnification in top view projection and FIG. 2b shows an SEM micrograph in so called backscatter mode at high magnification of a polished cross-section.

#FIGS. 3a-b show SEM micrographs of a prior art multilayer κ-Al2O3+TiN coating. FIG. 3a shows an SEM micrograph at a high magnification in top view projection and FIG. 3b shows an SEM micrograph in so called backscatter mode at high magnification of a polished cross-section.

#Surprisingly, it has been found that a non-columinar, fine grained, equiaxed κ-Al2O3 layer can be deposited by repeatedly introducing a controlled amount of a silicon halide, preferably SiCl4, during an Al2O3-deposition process

#The duration of the treatment periods as well as the silicon halide concentration are important parameters which must be optimised in order to obtain the desired result. If the silicon halide concentration is too low or/and treatment time is too short, the renucleation of the Al2O3 layer will not be sufficiently dense to cover a sufficient portion of the whole coating surface. If, on the other hand, the silicon halide concentration is too high and/or the treatment time is too long, the cohesion between the grains will be too weak resulting in a low quality coating.

#The method of the present invention thus relates to the coating of a body with a κ-Al2O3 layer during which the body is brought in contact with a hydrogen carrier gas containing one or more halides of aluminium, preferably AlCl3, and a hydrolysing and/or oxidising agent, preferably CO2, at temperature of the body between 800 and 1050° C. During the Al2O3 growth, a silicon halide, preferably SiCl4, is added to the reaction mixture at intervals of 1-5 minutes in a concentration of 20-50% of the AlCl3 flow. This procedure is repeatedly carried out in order to obtain a fine grained κ-Al2O3 layer structure with the desired grain size.