Patent Abstract:
Provided is a coated cutting tool having a coating and a substrate. The coating includes at least one chemical vapor deposited (CVD) layer and at least one physical vapor deposited (PVD) layer. The outermost layer of the coating is a PVD layer, containing residual compressive stresses. The substrate is a composite having hard refractory grains bonded together by a binder material.

Full Description:
This is a continuation of copending application Ser. No. 07/925,890, filed on Aug. 6, 1992, now abandoned, which is a continuation of Ser. No. 07/583,889, filed on Sep. 17, 1990, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of coated cutting tools. 
     In the past, cemented carbide cutting tools have been used in both a coated and an uncoated condition to machine metals and alloys. The application of a coating having one or more layers of a refractory material to a tungsten carbide-cobalt cemented carbide substrate has been used to improve the wear resistance and application range of cemented carbide cutting tools. In the past, refractory coatings, such as TiC, TiCN, TiN, and Al 2  O 3 , have been applied by CVD (chemical vapor deposition) techniques. In addition, TiN coatings have been applied by PVD (physical vapor deposition) techniques. Such CVD coatings deposited on cemented carbide substrates are characterized by thermal cracks, and residual tensile stresses. PVD TiN coatings are characterized by a dense, pore free structure without thermal cracks, and may have residual compressive stresses. The application of CVD coatings to cemented carbide substrates results in a reduction in the transverse rupture strength of the insert and, therefore, produces a greater susceptibility to chipping and breakage during use. 
     SUMMARY OF THE INVENTION 
     Applicants have now discovered that the transverse rupture strength of CVD coated cemented carbide may be improved by applying a PVD layer to the CVD coated substrate. In order to provide the aforementioned improvements, it is believed that the PVD coating must contain residual compressive stresses. 
     Therefore, in accordance with the present invention, improved cutting tools are provided having a substrate coated with one or more CVD layers and one or more PVD layers. The outermost layer of the CVD and PVD layers preferably is a PVD layer having residual compressive stresses. The substrate is a composite material having hard refractory grains (e.g., WC) bonded together by a binder material, preferably, metallic (e.g., Co). 
     These and other aspects of the present invention will become more apparent upon review of the detailed specification in conjunction with the FIGURE which is briefly described below: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE shows an isometric view of an embodiment of a cutting tool in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, the FIGURE shows a preferred embodiment of an indexable cutting insert 10 having cutting edges 12 at the junction of rake face 14 with flank faces 16. The cutting insert 10 shown in the FIGURE is an SPGN-433 (ANSI B212.4-1986) which may be in a sharp, or preferably, honed cutting edge condition. 
     In a preferred embodiment, the substrate is a WC based cemented carbide substrate containing at least 70 weight percent WC, and more preferably, at least 80 weight percent WC. The binder is preferably cobalt or a cobalt alloy and, preferably, has a bulk concentration of 5 to 15 weight percent. More preferably, the bulk cobalt content is about 7 to about 13 weight percent, and most preferably, about 8 to about 12 weight percent. The substrate may also preferably contain solid solution carbide forming elements such as Ti, Hf, Zr, Nb, Ta, V, preferably selected from Ti, Nb and Ta alone or in combination with each other. These elements preferably may be added to the mix as an element, alloy, carbide, nitride or carbonitride. Preferably, the concentration of these elements is as follows: Ta 0-12 w/o; Ti 0-10 w/o and Nb 0-6 w/o. More preferably, the sum of Ta plus Nb is between about 3 to about 7 w/o and the titanium content is between about 0.5 to 10 w/o. Most preferably, the titanium content is about 1.5 to 4.0 w/o. 
     These elements form solid solution carbides with the WC in the substrate. Chromium, preferably as Cr 3  C 2 , may also be added in small amounts. 
     Bonded on to the substrate is a hard refractory coating having at least two layers, a CVD layer and a PVD layer, with the last PVD layer preferably being outside the last CVD layer. Applicants have found that, when the PVD layer also has a compressive residual stress, that the transverse rupture strength of the CVD coated article is improved. 
     In a preferred embodiment, the inner CVD layer is preferably a hard nitride, such as the nitrides of Ti, Hf, Zr, their alloys with each other and their alloys with other elements, and more preferably Ti, Zr or Hf nitride. Nitrides are preferred over refractory carbides or carbonitrides for the inner layer adjacent to the substrate in order to minimize the formation of eta phase (Co 3  W 3  C and/or Co 6  W 6  C) at the cemented tungsten carbide based substrate peripheral boundary. Eta phase is a brittle phase which may make the cutting edge more susceptible to chipping. A second layer in the coating is preferably a CVD hard carbonitride, or carbide, such as Ti, Zr or Hf carbonitride or carbide, because of the higher hardness and abrasion resistance of the carbides and carbonitrides of Ti, Zr and Hf compared to their nitrides. Carbonitrides of the alloys of Ti, Hf and Zr with each other and their alloys with other elements may also be utilized. The CVD second layer and any additional CVD layers may include, for example, alumina, either as a single layer or multiple layers separated from each other by a layer of another refractory material, such as a nitride, carbonitride or carbide of Ti, Hf or Zr, preferably, Ti. 
     Where it is desired to have an Al 2  O 3  CVD layer next to the outer PVD layer, it is preferred, in order to improve the bond strength between the Al 2  O 3  and the PVD layer that a CVD nitride, carbonitride or carbide layer of Ti, Hf or Zr is deposited between the alumina CVD layer and the PVD layer. 
     The final and outermost layer of the coating is preferably a hard refractory PVD layer, such as a nitride, carbonitride of Ti, Zr or Hf, their alloys with each other, and their alloys with other elements. This outermost layer is characterized by residual compressive stresses. 
     The present invention is further illustrated by the following examples which are provided purely for illustration and not limitation. 
     The effect of coatings on the transverse rupture strength of cemented carbide substrate was demonstrated by applying the following coatings on a conventional WC-6 weight percent cobalt-0.5 weight percent Cr 3  C 2  cemented carbide substrate. 
     Coatings 
     1. CVD {TiN (1 μm)/TiCN (3 μm)/TiN (4 μm)} 
     2. CVD {TiN (1 μm)/TiCN (3 μm)}/PVD TiN (4 μm) 
     3. PVD TiN (8 μm) 
     The CVD coatings were applied by conventional CVD techniques at about 950 to 1000 cegrees Centigrade. The PVD TiN coating was applied with a Balzers BAI-830 (Balzers AG, Liechtenstein) ion plating PVD coating unit at about 500 degrees Centigrade using a titanium source and a nitrogen atmosphere (see U.S. Pat. No. 4,448,802). As is common practice during the initial stages of coating a very thin layer (detectable by high resolution TEM) of titanium was deposited on the CVD TiCN layer to provide improved adhesion between it and the PVD-TiN layer. The PVD layer produced by this technique has compressive residual stresses. 
     The transverse rupture strength of transverse rupture bars coated as described above and in an uncoated condition were measured using the ASTM standard B406 transverse rupture test procedure. The transverse rupture strength data are given below in Table I. 
     
                       TABLE I______________________________________          Average Standard          TRS (Ksi)                  Deviation (Ksi)______________________________________Uncoated         559       27Coating 1 (CVD)  193       12Coating 2 (CVD + PVD)            281       11Coating 3 (PVD)  566       58______________________________________ 
    
     As shown by the examples, the application of CVD layers to a cemented carbide substrate results in a drastic decrease in transverse rupture strength. This reduction is believed to be due to: (a) the high temperature annealing effect of the CVD deposition temperature (950-1000 degrees Centigrade) which reduces beneficial compressive residual stresses in the surface due to grinding of the cemented carbide substrate; and (b) the presence of thermal cracks and residual tensile stresses in the CVD layers caused by thermal expansion mismatch between the coating and the cemented carbide substrate. 
     The deposition of PVD TiN over the CVD {TiN/TiCN} partially restores the loss in TRS observed with CVD coating alone. This improvement (about 45 percent) in TRS is believed to be due to the presence of compressive residual stresses and the absence of thermal cracks within the outer PVD TiN layer. 
     It, therefore, follows that a coating of solely PVD should not degrade transverse rupture properties. This is shown by the results of the samples coated with coating 3. 
     It is the applicants&#39; belief that, in view of the above results, combinations of CVD and PVD coatings may be applied to a variety of cemented carbide substrates to obtain improvements in the overall properties and performance of the material. It is contemplated that this invention may be used to its best advantage where the cobalt content of the substrate is 7 to 13 weight percent and, more preferably, 8 to 12 weight percent. Specific examples of substrates where the present invention may be applied are as follows: 
     Substrate 1 11.5 w/o Co, 2.5 w/o Ta (Nb)C, 86 w/o WC - 89.8 R A , Hc 160 Oe, A and/or B porosity 
     Substrate 2 6.0 w/o Co, 0.5 w/o Cr 3  C 2 , 93.5 w/o WC - 93 R A , Hc 300 Oe, A and/or B porosity (used in Table I examples) 
     Substrate 3 9.75 w/o Co, 90.25 w/o WC - 91 R A , Hc 200 Oe, A and/or B porosity 
     Substrate 4 10 w/o Co, 90 w/o WC - 89 R A , Hc 120 Oe, A and/or B porosity 
     Substrate 5 10.5 w/o Co, 10 w/o Ta(Nb)C, 7 w/o TiC, 72.5 w/o WC - 91.4 R A , Hc 180 Oe, A and/or B porosity 
     Substrate 6 8.5 w/o Co, 11 w/o Ta(Nb)C, 7.5 w/o TiC, 73 w/o WC - 91.2 R A , Hc 140 Oe, A and/or B porosity 
     CVD and PVD coated binder enriched substrates and other embodiments of the present invention are described in copending application U.S. Ser. No. 07/583,544 entitled &#34;Binder Enriched CVD and PVD Coated Cutting Tool,&#34; filed on the same date as the present application, now U.S. Pat. No. 5,250,367. This copending application describes the application of these cutting inserts to milling of steel and shows the superiority of binder enriched grades over nonenriched grades in the severe milling applications described therein. The present applicants believe that, for less severe milling applications and turning applications, the application of CVD+PVD coatings to nonenriched substrates as described in the present application should be sufficient to provide improved chipping resistance. 
     All applications, patents and other documents referred to herein are hereby incorporated by reference. 
     Other embodiments of the invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.

Technology Classification (CPC): 2