PVD coated cutting tool and method of its production

Details PVD coated cutting tool and method of its production PVD coated cutting tool and method of its production

2001-11-13

2003-04-29

McDonald, Rodney G. (Department: 1753)

Chemistry: electrical and wave energy

Processes and products

Coating, forming or etching by sputtering

C204S192150, C204S192120, C204S298080, C204S192220

Reexamination Certificate

active

06554971

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a cutting tools for metal machining comprising a body with, at least on the functional parts of the surface thereof, a hard and wear resistant refractory coating. The coating is adherently bonded to the body and covers all functional parts of the tool. The coating is composed of one or more layers of refractory compounds of which at least one layer is fine-crystalline aluminum spinel deposited by Physical Vapor Deposition (PVD) and the non spinel layer(s), if any at all, are metal nitrides and/or carbides with the metal elements chosen from Ti, Nb, Hf, V, Ta, Mo, Zr, Cr, W and Al.
It is well-known that for, e.g., cemented carbide cutting tools used in metal machining, the wear resistance of the tool edge can be increased considerably by applying thin, hard surface layers of metal oxides, carbides or nitrides with the metal either selected from the transition metals from the groups IV, V and VI of the Periodic Table or from silicon, boron and aluminum. The coating thickness usually varies between 1 and 15 um and the most widespread techniques for depositing such coatings are PVD and CVD (Chemical Vapor Deposition). It is also well-known that further improvements of the performance of a cutting tool can be achieved by applying a pure ceramic layer such as Al
2
O
3
on top of layers of metal carbides and nitrides (U.S. Pat. Nos. 5,674,564 and 5,487,625).
Cemented carbide cutting tools coated with alumina layers have been commercially available for over two decades. The CVD technique usually employed involves the deposition of material from a reactive gas atmosphere on a substrate surface held at elevated temperatures. Al
2
O
3
crystallizes into several different phase such as &agr; (alpha), &kgr; (kappa) and &khgr; (chi), called the “&agr;-series ” with hcp (hexagonal close packing) stacking of the oxygen atoms, and into &ggr; (gamma), &thgr; (theta), &eegr; (eta) and &dgr; (delta), called the “&ggr;-series” with fcc (face centered cubic) stacking of the oxygen atoms. The most often occurring Al
2
O
3
-phases in CVD coatings deposited on cemented carbides at conventional CVD temperatures, 1000°-1050° C., are the stable alpha and the metastable kappa phases, however, occasionally the metastable theta phase has also been observed.
The CVD Al
2
O
3
-coatings of the &agr;-, &kgr;- and/or &thgr;-phase are fully crystalline with a grain size in the range of 0.5-5 &mgr;m and having well-faceted grain structures.
The inherently high deposition temperature of about 1000° C. renders the total stress in CVD Al
2
O
3
-coatings on cemented carbide substrates to be tensile, hence the total stress is dominated by thermal stresses caused by the difference in thermal expansion coefficients between the substrate and the coating and less by intrinsic stresses which have their origin from the deposition process itself and are of compressive nature. The tensile stresses may exceed the rupture limit of Al
2
O
3
and cause the coating to crack extensively and thus degrade the performance of the cutting edge in, e.g., wet machining where the corrosive chemicals in the coolant fluid may exploit the cracks in the coating as diffusion paths.
Other than Al
2
O
3
other oxides, mixtures or combinations of oxides or compounds of the spinel type have been proposed as hard coatings deposited by CVD (GB 1,408,294). They have not found a practical acceptance.
Generally CVD-coated tools perform very well when machining various steels and cast irons under dry or wet cutting conditions. However, there exists a number of cutting operations or machining conditions when PVD-coated tools are more suitable, e.g., in drilling, parting and threading and other operations where sharp cutting edges are required. Such cutting operations are often referred to as the “PVD coated tool application area”.
Plasma assisted CVD, PACVD, makes it possible to deposit coatings at lower substrate temperatures as compared to thermal CVD temperatures and thus avoid the dominance of the thermal stresses. Thin Al
2
O
3
PACVD films, free of cracks, have been deposited on cemented carbides at substrate temperatures of 450-700° C. (U.S. Pat. Nos. 5,516,588 and 5,587,233). The PACVD process for depositing Al
2
O
3
includes the reaction between an Al-halogenide, e.g., AlCl
3
, and oxygen donor, e.g., CO
2
, and because of the incompleteness of this chemical reaction, chlorine is trapped in the Al
2
O
3
-coating and its content could be as large as 3.5%. Furthermore, these PACVD Al
2
O
3
-coatings are generally composed of, besides the crystalline alpha- and/or gamma-Al
2
O
3
-phase, a substantial amount of amorphous alumina which, in combination with the high content of halogen impurities, degrades both the chemical and mechanical properties of said coating, hence making the coating material non-optimized as a tool material.
The field of the present invention relates particularly to the art of PVD Al
2
O
3
coated cutting tools or tools of similar hard materials such as cermets, ceramics and high speed steel or the superhard materials such as cubic boron nitride or diamond.
There exist several PVD techniques capable of producing refractory thin films on cutting tools and the most established methods are iron plating, DC- and RF-magnetron sputtering, arc discharge evaporation, IBAD (Ion Beam Assisted Deposition) and Activated Reactive Evaporation (ARE). Each method has its own merits and the intrinsic properties of the produced coatings such as microstructure/grainsize, hardness, state of stress, intrinsic cohesion to the underlying substrate may vary depending on the particular PVD method chosen. Early attempts to PVD deposit Al
2
O
3
at typical PVD temperatures, 400-500° C., resulted in amorphous alumina layers which did not offer any notable improvement in wear resistance when applied on cutting tools. PVD deposition by HF diode- or magnetron sputtering resulted in crystalline &agr;-Al
2
O
3
only when the substrate temperature was kept as high as 1000° C. (Thornton and Chin, Ceramic Bulletin, 56(1977)504). Likewise, applying the ARE method for depositing Al
2
O
3
, only resulted in fully dense and hard Al
2
O
3
-coatings at substrate temperatures around 1000° C. (Bunshah and Schramm, Thin Solid Films, 40(1977)211).
With the invention of the Pulsed Magnetron Sputtering especially in the mode of bipolar pulsed DMS technique (Dual Magnetron Sputtering) which is disclosed in DD 252 205 and U.S. Pat. No. 5,698,314, a wide range of opportunities opened up for the deposition of insulating layers such as Al
2
O
3
and other oxides and, furthermore, the method has made it possible to deposit crystalline Al
2
O
3
-layers at substrate temperatures in the range of 500 to 800° C. In the bipolar dual magnetron system, the two magnetrons alternately act as an anode and a cathode and, hence, preserve a metallic anode over long process times. At high enough frequencies, possible electrical charging on the insulating layers will be suppressed and the otherwise troublesome phenomenon of “arcing” will be limited. Hence, according to U.S. Pat. No. 5,698,314, the DMS sputtering technique is capable of depositing and producing high-quality, well-adherent, crystalline &agr;-Al
2
O
3
thin films at substrate temperatures less than 800° C. The “&agr;-Al
2
O
3
layers” with a typical size of the &agr;-grains varying between 0.2 and 2 &mgr;m, may partially also contain the gamma (&ggr;) phase from the “&ggr; series” of the Al
2
O
3
-polymorphs. The size of the &ggr;-grains in the coating is much smaller than the size of the &agr;-grains. The &ggr;-Al
2
O
3
grain size typically varies between 0.05 and 0.1 &mgr;m. In the Al
2
O
3
-layers where both modifications of &ggr; and &agr;-phase were found, the &ggr;-Al
2
O
3
-phase showed a preferred growth orientation with a (440)-texture. When compared to prior art plasma assisted deposition techniques such as PACVD as described in U.S. Pat. No. 5,587,233, the novel, pulsed DMS sputtering deposition method has the decisive, important advantage that no impurities su

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