CVD-coated titanium based carbonitride cutting toll insert

Details CVD-coated titanium based carbonitride cutting toll insert CVD-coated titanium based carbonitride cutting toll insert

1998-02-09

1999-12-28

Turner, Archene

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Web or sheet containing structurally defined element or...

Physical dimension specified

428469, 428472, 428698, 428701, 428702, 428704, 51298, 51307, 51309, C23C 1630, B23B 2714

Patent

active

060079096

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

The present invention relates to a cutting tool insert of a carbonitride alloy with titanium as main component and containing tungsten and cobalt useful for machining, e.g., turning, milling and drilling, of metal and alloys. The insert is provided with at least one wear resistant layer free from cooling cracks, which in combination with a moderate compressive stress, gives the tool insert improved properties compared to prior art tools in several cutting tool applications.
WC-Co based alloys (cemented carbide) coated with one or more layers of a wear resistant material, e.g., TiC, Ti(C,N), TiN and Al.sub.2 O.sub.3, are the dominating type of materials used for cutting tool inserts. The coatings are most often produced by employing chemical vapor deposition (CVD) techniques at relatively high deposition temperatures (700-1100.degree. C.). One weakness of such CVD-coatings in combination with WC-Co alloys is that a network of cooling cracks are formed in the coating during cooling down the CVD-load after the coating run. The cracks are caused by the mismatch in thermal expansion between the WC-Co based alloy and the coating materials. The WC-Co alloy has a thermal expansion coefficient, .alpha., in the approximate range 4.6-6.7.multidot.10.sup.-6 .degree. C..sup.-1, while typical values for the coating materials are .alpha..sub.TiC .apprxeq.7.6, .alpha..sub.TiN .apprxeq.8.0, .alpha..sub.Ti(C,N) .apprxeq.7.8 and .alpha..sub..alpha.-Al.sbsb.2.sub.O.sbsb.3 .apprxeq.7.8.multidot.10.sup.-6 .degree. C..sup.-1. This means in all cases that the coating will contract more than the WC-Co alloy upon cooling to room temperature. This contraction leads to tensile stresses in the coating which in part are relaxed by the formation of the cooling cracks.
Cooling cracks may be detrimental to the performance of the cutting tool in certain machining applications for at least three reasons: perpendicular to the cutting edge) and edge fracture. stable than the coating, is exposed through the cracks to attack by cutting fluids, work piece material and the surrounding atmosphere. operation, thus enlarging the initial cracks.
In addition, the residual tensile stresses in the coating may lead to spalling of the coating when used in a cutting operation.
CVD-coatings on inserts of WC-Co alloys result in a reduction in transverse rupture strength (TRS) of the cutting insert which negatively influences the toughness properties of the insert. It is thought that cooling cracks and tensile stresses in the coating are of importance for this reduction.
The problem of crack formation can to a certain extent be solved by employing low temperature coating processes such as physical vapour deposition (PVD), plasma assisted CVD or similar techniques. However, coatings produced by these techniques generally have inferior wear properties, lower adhesion and lower cohesiveness. Furthermore, although these techniques may be used to deposit TiC, Ti(C,N) or TiN coatings, so far it is not possible to deposit high quality Al.sub.2 O.sub.3 -coatings with good crystallinity. In the Swedish patent application 9304283-6 a method of producing essentially crack free coatings is disclosed. However, these coatings always have a specific 114-textured .alpha.-Al.sub.2 O.sub.3 layer with a certain grain size and grain shape (platelet type grains). These coatings on ordinary WC-Co alloys always possess tensile stresses.
It is generally known that a tensile residual stress in a coating can be reduced by a mechanical treatment of the coating, e.g., by shoot peening the coating with small steel balls or similar particles. The tensile stresses are released by inducing defects in the coating or by generating further cracks (see U.S. Pat. No. 123,934). Additional cracks are not desirable for conditions mentioned above and the positive effect of the induced defects will in many cases be lost during the cutting operation when the tool insert tip may reach very high temperatures (up to 1000.degree. C.).
In U.S. Pat. No. 5,395,680 a method to obt

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