The invention concerns a coated cutting tool. More specifically, the invention pertains to a coated cutting tool wherein the coating scheme for the cutting tool includes a layer of a Group IVB metal-aluminum alloy applied by physical vapor deposition (PVD) and a layer of alumina applied by PVD to the layer of the Group IVB-aluminum alloy.
Alumina as a bulk material exhibits high oxidation resistance, high chemical stability, high hardness and good wear resistance. Alumina thus has desirable properties for use in material removal, e.g., metal removal, applications. Notwithstanding these desirable properties, the use of alumina cutting tools has certain drawbacks due to the low toughness and poor formability of alumina. To improve the cutting performance, some manufacturers have included additives such as chromium oxide, magnesium oxide, titanium oxide, nickel oxide and refractory metal carbides to the alumina-based cutting tool. The presence of these additives has resulted in alumina cutting tools with improved performance properties.
Another way to overcome the disadvantages of alumina and yet still achieve the beneficial properties thereof is to coat tougher and more readily formed cutting tool substrates with an alumina coating via physical vapor deposition (PVD) techniques. The end result of this process is an alumina-coated cutting tool.
While alumina-coated cutting tools achieve certain performance levels, there have been drawbacks with these tools. In this regard, the following documents concern the application of an amorphous alumina coating to a substrate.
The article by Knotek et al. entitled "Sputtered hard materials based on titanium and aluminum for wear protection", International Pulvermet. Tagung., DDR, Dresden (1985), pp. 181-196, mentions the PVD (magnetron sputtering) application of separate coatings of alumina, TiN and TiAlN. The article states that these coatings have equivalent, or in the case of TiAlN better, performance than CVD coatings.
The Sumomogi et al article entitled "Adhesion Evaluation of RF-Sputtered Aluminum Oxide and Titanium Carbide Thick Films Grown on Carbide Tools", Thin Solid Films, 79 (1981), pp. 91-100, discusses the PVD application of alumina coatings on cemented carbide tools (ISO P20).
The Shinzato et al. article entitled "Internal Stress in Sputter-Deposited Al.sub.2 O.sub.3 Films", Thin Solid Films, 97 (1982), pp. 333-337, mentions the application of an alumina coating to cemented carbide (ISO P20), high speed steel, and Corning Pyrex glass substrates via either a conventional r.f. apparatus or a planar magnetron r.f. apparatus.
The Kazama et al. article entitled "Alumina Films Prepared by Ion Plating" alumina on a WC cutting tool!, mentions the application of an alumina coating via ion plating to a WC substrate to from an alpha-alumina coating.
Japanese Patent Application S54-2982 to Murayama et al. addresses the use of the ion plating method to apply an alumina (or alumina and TiC) coating to a substrate. Practical Example II concerns the alumina coating on a cutting tool (ISO P30 alloy).
One drawback of alumina coated cutting tools pertains to the lack of adequate adhesion between the substrate and the alumina coating or layer. One way to improve the adhesion of the alumina coating is to provide an intermediate layer or layers. The following documents discuss the use of an intermediate layer.
In Ramos et al., "Adhesion improvement of RF-sputtered alumina coatings as determined by the scratch test", Jour. Adhesion Sci. Technol., Vol. 7, No. 8, pp. 801-811 (1993), the authors looked at the use of a titanium or a titanium nitride intermediate layer between a high speed steel substrate (AISI M2) and an alumina coating applied via RF-sputtering. The overall conclusion was that the presence of a titanium or titanium nitride intermediate layer improves the adhesion of the alumina coating to the high speed steel substrate.
Japanese Patent Publication No. 57-120667 to Doi et al. mentions in the context of a cutting tool the PVD application of an intermediate layer of alumina over a base layer of TiC. The outer layer is TiN. This patent also mentions a six-layer coating scheme including alumina as the outer layer with TiC, alumina, TiCO as the intermediate layers using a plasma CVD device. Example No. 3 of this patent comprises an eight layer scheme in which the outer layer is alumina and the intermediate layers include TiC, alumina, TiCO and TiCN.
Japanese Patent Application No. H4-308075 to Matsuda et al. discusses the application by ion plating of an alumina coating to a substrate. The alumina may be applied directly to the surface of the substrate. The alumina may also be applied to an intermediate layer previously formed on the substrate. The intermediate layer may be titanium, titanium carbide or titanium nitride.