Tool with a molybdenum sulfide coating and method for producing the same

[0002] This application is a national stage of PCT/DE99/03781 filed Nov. 30, 1999 and based upon German national application 199 09 372.5 of Mar. 3, 1999 under the International Convention.

[0003] The invention relates to a tool, especially a cutting insert, for the machining of metallic workpieces, composed of a hard metal base body, a cermet base body, a ceramic base body or a steel base body and at least one layer deposited thereon, in which the sole layer or in the case of a plurality of layers, the outermost layer or the layer lying beneath the outermost layer is constituted predominantly of molybdenum sulfide.

[0004] The invention also relates to a method of producing the molybdenum sulfide layer by means of CVD (chemical vapor deposition).

[0005] The coating of substrate bodies of hard metal, a cermet, a ceramic, or steel with hard materials or hard material mixtures or ceramic materials is known from the state of the art. Especially, carbides, nitrides, carbonitrides, oxycarbonitrides and/or borides of at least one of the elements of groups IVa to VIa of the Periodic Table are used, such as, for example, TiC, TiN, Ti(C,N) or also ceramic materials like Al₂O₃or ZrO₂. With large cutting speeds and/or large chip cross sections, high temperatures arise at the cutting edge of the machining tool especially in the case of materials which are difficult to machine, thereby giving rise to increased wear which can lead in part also to edge breakage. To minimize the temperatures which arise at the cutting edge of the tool during machining, usually so-called cooling lubricants have been used which contain either toxic and environmentally polluting substances and which must be disposed of at relatively high cost.

[0006] A further possibility, which can reduce the use of coolant lubricants drastically, is the principle of so-called minimum quantity lubrication, whereby very small amounts of coolant lubricants are supplied with point precision through fine passages to the location between the tool cutter and the workpiece. Apart from a high cost for carrying out the method and for the apparatus involved, this approach does not eliminate the waste treatment problem but only minimizes it.

[0007] On the other hand, there are so-called solid dry lubricant films, for example, molybdenum disulfide, known in the art. For example DE-A- 24 15 255 proposes to improve the adhesion of a layer which is preferably based upon MoS₂or WS₂and which is applied by so-called sputtering with a thickness of 1 to 2 μm and in which the application of a dry lubricant layer based also upon MoS₂or WS₂can be effected in a conventional manner by a polishing application. In the East German patent document (DD) 202 898 it has been proposed to apply layers or the like of molybdenum disulfide by sputtering to cutting tools, stamping tools, drawing tools and machining tools, whereby the layer thickness lies in the nm range. The layer which is applied by sputtering to a substrate body provided with a hard material layer should have a hexagonal lattice structure. EP 0 534 905 A2 also proposes the PVD (plasma vapor deposition) coating of machining tools, inter alia with molybdenum disulfide.

[0008] It has, however, been found that MoS₂layers produced by cathodic atomization (sputtering) adhere poorly to the substrate body or to the outer hard material layer applied to the substrate body or have a predominantly unsatisfactory orientation of the MoS₂crystallites. To obtain the lowest friction coefficients, the hexagonal components of the MoS₂layer should be so oriented that the hexagonal base planes lie parallel to the surface. Thereby the oxidation resistance of the layer is also improved.

[0009] It is thus proposed in DE 35 16 933 A1 that the ratio of the water vapor partial pressure to the coating rate be adjusted to be smaller than 10⁻⁷mbar/1.6 mgm⁻²s⁻¹which, in practice, can only be achieved with considerable technological cost.

[0010] Apart from the requirement which is customary for the PVD process for purity, i.e. that the gas atmosphere not contain foreign substances, there is a further drawback with the PVD process in that a directed particle stream is provided from the target source to the substrate whereby the substrate must be rotated about three axes to produce a uniform coating.

[0011] Furthermore WO 97/04142 describes a process for uniform coating of MoS₂and a metal or a metal compound like titanium or titanium nitride through the use of two targets by means of a sputtering process.

[0012] While MoS2 is relatively soft and is not especially wear-resistant, the formation of an MoS₂coating has a significant effect since by the use of a single MoS₂layer or an outermost MoS²layer at the contact location of the tool with the workpiece, the MoS₂may be quickly abraded but the friction force because of the penetration of the MoS₂particles from the edge inwardly (entrainment) is still noticeably reduced for a period of time.

[0013] It is the object of the present invention to provide a single layer or an outermost layer or layer combination which has a dense lattice structure and a higher hardness of greater mechanical stability against wear. It is also an object to provide a method of producing such a layer.

[0014] These objects are achieved by a tool in which the molybdenum sulfide layer contains a mixture of the sulfide phases MoS₂and MoS₂S₃with a substantially random orientation of the phase crystals. The layered lattice structure is comprised of randomly oriented MoS₂grains which partly have a platelet-shaped configuration. The intervening spaces are filled with the Mo₂S₃phase. With these features a surprisingly improved wear resistance of the layer is achieved on machining.

[0015] Thus still further phases of molybdenum sulfur compounds and/or chlorine can be contained in the molybdenum sulfide layer but should not exceed 6 atomic %.

[0016] A single molybdenum sulfide layer or the outermost molybdenum sulfide layer can also be fabricated so as to be of a multilayer type, whereby the respective undermost layer or an underlying layer with a thickness respectively not exceeding 200 nm has a hexagonal lattice structure with a preferred orientation of the hexagonal planes parallel to the tool surface. The remaining layers however are of isotropic orientation. This applies especially for a substrate body which is coated with only a single molybdenum sulfide layer (and no other layer). The layer with the hexagonal MoS₂crystals is thereby bounded directly by the substrate and has a thickness of 100 nm to 200 nm.

[0017] According to a further feature of the invention, the Mo₂S₃component in the molybdenum layer does not remain continuously homogeneous but with increasing distance from the substrate body surface is present in different proportions, preferably varying periodically from 5 to 30 volume %.

[0018] According to a further feature of the invention, the molybdenum sulfide layer contains successive individual layers of different compositions of which one type comprises the mixture of MoS₂and Mo₂S₃sulfide phases and the other type consists substantially of pure MoS₂. Each of the MoS₂layers has a maximum thickness of 200 nm. The thicknesses of layers of the mentioned phase mixture (MoS₂/Mo₂S₃) respectively lie between 200 nm and 400 nm.

[0019] In a further embodiment of the tool of the invention there are still further metallic phases and/or hard material phases in the molybdenum sulfide layer under consideration and whose volume proportion however is preferably not in excess of 10 volume %. The metals in question can be especially molybdenum and the materials especially molybdenum carbide, titanium carbonitride or chromium carbide. Such metal phases or hard material phases serve to increase the stability of this layer which however is in substance the molybdenum sulfide of the mixture described at the outset. The volume proportion of MoS₂in this layer lies between 30 and 98 volume %, preferably between 40 to 95 volume %. The layer thickness lies between 0.5 to 8 μm, preferably between 2 and 4 μm.

[0020] Optionally the single or outermost layer of molybdenum sulfide can have still a further metallic layer of a thickness ≦200 nm applied thereto. Preferably metals of groups IVa to VIa of the Periodic Table are used. Alternatively these thin outer layers of a maximum of 200 nm in thickness can also be composed of a hard material, preferably a carbide, a nitride or a carbonitride of one of the aforementioned elements.

[0021] The molybdenum sulfide layer is applied by means of a CVD process, preferably using the subsequently described process technique.

[0022] For producing the mentioned coating, a CVD process is used in which the gas mixture is composed of molybdenum chloride, hydrogen sulfide, a noble gas and hydrogen and with a coating temperature lying between 500° C. and 900° C., preferably between 600° C. and 850° C. The pressure amounts to 0.1 to 50 kPa, preferably between 0.1 and 1 kPa. The molar ratio between hydrogen sulfide and molybdenum chloride lies between 0.1 and 4, preferably between 0.1 and 2. The proportion of hydrogen and argon together makes up between 80 to 99 volume %, preferably between 90 to 98 volume %.

[0023] Preferably MoCl₅is used although optionally MoCl₄or mixtures of MoCl₅and MoCl₄can be used.

[0024] The maintenance of the above-mentioned parameters is significant for the production of the layer consisting essentially of molybdenum sulfide. With greater than the aforedescribed molar ratios, layers are obtained which consist exclusively or predominantly of hexagonal MoS₂. MoCl₅can decompose at temperatures above 200° into MoCl₄and Cl so that the molybdenum sulfide can be formed both from MoCl₅and also from MoCl₄.

[0025] The mixed layer, which contains hexagonal MoS₂and Mo₂S₃can on the one hand be produced as a coating with a constant Mo₂S₃proportion and random orientation of the MoS₂crystallites. On the other hand the method of the invention can also produce multilayer coatings which can have alternating layers of MoS₂/Mo₂S₃phase mixtures and layers of oriented MoS₂. In the last-mentioned case, the MoS₂layers however should not be thicker than 200 nm since otherwise pores will develop in the molybdenum sulfide layer whereby the density thereof will diminish. In a special embodiment, the molar ratio H₂S/MoCl₅during the coating alternates for equally long time periods in which one of the selected mole ratios is greater than the other by a factor of 7 to 10 so that with a greater mole ratio, pure MoS₂is deposited and with the smaller mole ratio, a phase mixture of MoS2 and Mo₂S₃is deposited. The duration over which the substantially pure MoS₂is deposited is that which will provide a maximum permissible layer thickness of 200 nm. By comparison, the duration in which the mole ratio is set to deposit the phase mixture MoS₂/Mo₂S₃can be two or three times as great. Preferably 8 to 12 layers are deposited of which substantially half are composed of the mentioned phase mixture and the other half are of MoS₂.

[0026] Alternatively here too, the molar.ratio H₂S/MoCl₅can be so controlled that during deposition the Mo₂S₃phase proportion periodically varies.

[0027] The molybdenum sulfide layer is composed in essence of hexagonal MoS₂and Mo₂S₃although it can however contain still further phases which can be formed from the gas mixture of MoCl₅, H₂S, H₂and argon. In this manner, small proportions of rhombo-hedral MoS₂, MoS₂Cl₂and MoCl₃can arise which, however, have no noticeable effect on the layer characteristics.

[0028] The layer consisting essentially of molybdenum sulfide can also be formed in a multilayer manner whereby the individual layers containing essentially MoS₂and Mo₂S₃can be separated by intermediate layers of a metal or a metallic hard material. The intermediate layers are however always thinner than 0.3 μm. As materials for such intermediate layers, molybdenum or molybdenum carbide are to be considered, these enhancing mechanical stability of the molybdenum sulfide layer according to the invention. To deposit molybdenum or molybdenum carbide, the gas mixture composition in the CVD process is so altered that the feed of H₂S is interrupted and replaced by CH₄. By rapid switching of the gas it is possible to obtain quasicontinuous layers which aside from MoS₂and Mo₂S₃also contain other phases. With this feature, the stability of the dry lubricant layer can also be improved. In practice one balances which components of the stability enhancing nonsulphidic impurities are present or intermediate layers are used. The lowest friction coefficients are obtained with layers which are comprised of hexagonal MoS₂but can have the stability by the additional Mo₂S₃phase and further by nonsulphidic impurities. The choice of the respective proportions of stabilizing-enhancing compounds and/or metals of hard materials depends upon the kind of workpiece material to be machined and the cutting forces arising during machining. The invention is described in further detail using concrete examples:

[0029] A CVD apparatus with a heatable quartz glass tube is used as the reaction chamber. The MoCl₅gas is generated by evaporation of MoCl₅salt at 110° C. in an argon flow and fed to the reaction chamber heated to 650° C. In a second test, the other gases are fed. The gas mixture is comprised of 0.4 volume % H₂S, 2.2 volume % MoCl₅, 14.6 volume % H₂and 82.8 volume % Ar. The molar ratio H₂S/MoCl₅amounts to 0.18. During the coating a pressure of 500 Pa is maintained in the reaction chamber. In the chamber, there are to be found uncoated lathe cutting insert plates and cutting insert plates coated with titanium nitride/titanium carbonitride/titanium nitride and thin silicon disks for analytic tests. After the coating duration of 150 minutes, layers of about 3 to 4 μm thickness are formed with a very dense lattice structure. By phase analysis by means of X-ray diffraction and electron diffraction, hexagonal MoS₂and MO₂S₃are found. The ratio in terms of quantity of the two phases can be regarded based upon the visibility of the two phases and is relatively coarsely defined. The volume proportion of the hexagonal MoS₂phase amounted in the present case to about 60%. The relatively thick layer is adhesive and scratch resistant. The Vickers hardness amounted to 400 HVO₂. In tribological tests a coefficient of friction of 0.11 was found (against a steel ball).

[0030] In another test with the above-described apparatus, two gas mixtures are used alternately in which the molar ratio H₂S/MoCl₅is varied by a factor of 10. At the beginning of the test, a gas mixture of 0.2 volume % H₂S, 0.9 volume % MoCl₅, 14.8 volume % H₂and 84.1 volume % Ar is supplied. The deposition temperature is 670° C. and the pressure 500 Pa. Under these conditions, a phase m mixture of hexagonal MoS₂and MO₂S₃is deposited. After 5 minutes, the H₂S gas flow is increased by about ten-fold while all other gases are held constant. The new gas mixture of 1.8 volume % H₂S, 0.9 volume % MoCl₅, 14.6 volume % H₂and 82.7 volume % Ar is fed to the reactor for a duration of two minutes. Under these conditions essentially pure MoS₂is deposited. The alternation between both gas mixtures is effected 10 times. With the switchover times, the total coating interval was about 80 minutes. A dense layer with a thickness of 3.2 μm was obtained. In an X-ray diffractogram it can be seen that the intensity of the (0.02) reflection of the hexagonal MoS₂is significantly greater than in the first embodiment. This means that more of the hexagonal MoS₂is significantly greater than in the first embodiment. This means that more of the hexagonal MoS₂oriented parallel to the surface normal is present. The bases for observation are the textures of the single phase MoS₂layers. The Vickers hardness of the layer amounted to 380 HVO2 and the friction coefficient to 0.08.

[0031] A temperature of 800° C. and a pressure of 500 Pa is established and the gas mixture of 0.4 volume % H₂S, 2.2 volume % MoCl₅, 54.2 volume % H_{2 g}and 43.2 volume % Ar is supplied. After five minutes the H₂S is shut off and all remaining parameters are not changed. From the MoCl₅, H₂and Ar a thin layer of molybdenum metal is formed over five minutes. After 110 minutes, an about 4 μm thick layer with the phases MoS₂and Mo₂S₃is formed which has an enhanced molybdenum content and a dens lattice structure. The hardness value of the layer amounted to 520 HVO₂and the friction coefficient to 0.14.

[0032] Tests were carried out in Example 3 but with methane in the second step instead of H₂S: 6.9 volume % CH₄, 1.8 volume % MoCl₅, 55.3 volume % H₂and 36 volume % Ar. This variant gave rise to the formation of thin intermediate layers of Mo₂C. The hardness value of the layer amounted to 530 KVO2 and the friction coefficient to 0.18.

[0033] The dry lubricant layers were tested using commercial lathe cutting plates of the form CNMG120412 which were constituted of a hard metal alloy of 6% Co, 8% (Ti,Ta,NB)C and 86% WC. The plates were provided with about 12 μm thick coating of the three layers TiN, Ti(C,N) and TiN which were applied in a CVD production apparatus at about 1000° C. Such plates were additionally provided with coatings according to the invention in the above-described test apparatus. In these examples layers with approximately the same thickness (3 μm) of the outermost MoS₂/Mo₂S₃were used as outermost layers. With these coating plates, a shaft of stainless steel was turned in smooth sections until the cutting corner became unusable because of wear or breakage. The turning tests utilized dry cutting with cutting speeds of 200 m/min, cutting depths of 2.5 mm and a feed of 0.25 mm per revolution. The following results were obtained:

[0034] Cutting body CNMG120412,

[0035] Coated with TiN/Ti (C,N)TiN

[0036] These results show that all variants of the coating of the invention increase the use time in dry machining.