Coated body with nanocrystalline CVD coating for enhanced edge toughness and reduced friction

[0002] Coated bodies for use in the cutting of metal are well-known. Typically, the bodies are made of a cemented carbide, cermet or ceramic and the coatings are one or more of a Group VIB metal carbide, nitride, oxide or mixtures thereof. For example, bodies of a cemented carbide coated with layers of TiC, Al₂O₃and TiN are widely used. There are many variations in layer composition and thickness. The layers are applied by various methods such as CVD (chemical vapor deposition), both conducted at normal temperatures of from about 900 to 1250° C. and medium temperature chemical vapor deposition (MTCVD) conducted at temperatures of from about 700 to 900° C. and PVD (physical vapor deposition).

[0003] CVD TiC coatings are usually composed of equiaxed grains with the grain size being from about 0.5 to 1.0 microns. CVD TiN as well as MTCVD Ti(C,N) coatings are composed of columnar grains with the length of the grains approaching the coating layer thickness. The morphology of CVD coatings can be slightly modified by process adjustments. The MTCVD coatings are, however, very difficult to modify by conventional process adjustments.

[0004] The hardness of polycrystalline materials in general (including coating layers as well) obey the Hall-Petch equation: H=H°+C/d where H is the hardness of a polycrystalline material, H° is the hardness of a single crystal, C is a material constant and d is the grain size. As may be seen from this equation, the hardness of a material can be increased by decreasing the grain size. Nonetheless, conventional CVD and MTCVD coatings have grain sizes of at least 0.5 microns and above. MTCVD coatings are particularly characterized by the presence of large columnar grains with the length of the crystals approaching the thickness of the coating layer.

[0005] The use of a dopant such as a tetravalent titanium, hafnium and/or zirconium compound in the formation of an Al₂O₃layer to promote the formation of a particular phase is shown in U.S. Reissue Pat. No. 31,526. Also, the use of a dopant selected from the group consisting of sulfur, selenium, tellerium, phosphorous, arsenic, antimony, bismuth and mixtures thereof to increase the growth rate of Al₂O₃applied by CVD as well as to promote even layers of the coating is disclosed in U.S. Pat. No. 4,619,886.

[0006] CO₂has been used as part of the coating process as well. In particular, it has been used in oxidizing processes where it reacts with H₂to form H₂O, the oxidizing gas. See, for example, U.S. Pat. No. 5,827,570.

[0007] It is an object of this invention to avoid or alleviate the problems of the prior art.

[0008] It is further an object of this invention to provide a coating layer of significantly smaller grain size and concomitant hardness.

[0009] In one aspect of the invention there is provided a coated body having as a coating layer, a layer of Ti(C,N,O) having a grain size of 25 nm or less.

[0010] In another aspect of the invention there is provided a method of forming a coated body of Ti(C,N,O) comprising contacting a body with a gas containing titanium halide, a nitrogen compound, a carbon compound, a reducing agent and a dopant addition of CO and/or CO₂sufficient to form the Ti(C,N,O) in a size less than 25 mm.

[0011] It has now been found that the grain size of coatings applied by MTCVD can be refined to much smaller grain size levels and to an equiaxed shape by adding small amounts of a dopant of CO or CO₂or mixtures thereof, preferably CO, to the coating gas during the MTCVD process. In order to obtain a grain size of the resulting coating in the order of 25 nm or less, preferably 10 nm or less, the amount of CO in the MTCVD gaseous mixture should be from about 5 to 10%, preferably from about 7 to 9%, of the total gaseous mixture. When CO₂is used, it should be present in an amount of from about 0.5 to 1.0%, preferably 0.4 to 0.6% of the total gaseous mixture. The CO and/or CO ₂dopant can be added at anytime throughout the reaction, continuously or in the interrupted mode. When CO₂and/or CO/CO₂mixtures are used, care should be taken within the skill of the artisan to avoid the formation of Magnelli phases.

[0012] While the dopant addition can be made to the reactant gas admixture used for various coating layers, it has found particular utility in the formation of a Ti(C,N,O) layer which would have been a Ti(C,N) layer in the absence of the dopant. In the Ti(C,N,O) layer, the ratio of constituents generally has been as follows: O/Ti from 0.10 to 0.40, preferably 0.20 to 0.30, C/Ti from about 0.40 to 0.60, preferably 0.50 to 0.60, and N/Ti from about 0.15 to 0.35, preferably 0.20 to 0.30. While a Ti (C,O,N) layer is preferred, the method of the present invention can be applied to form a Ti(C,O) layer which would have been a TiC layer in the absence of the dopant.

[0013] The nanocrystalline layer may be applied as the outermost layer or as an inner layer. As will be shown below, the nanocrystalline coatings are harder but exhibit at higher temperatures (at higher cutting speeds) grain boundary sliding leading to plastic deformation. Due to the extremely fine grain size of this coating, the surface smoothness is increased and friction coefficient is reduced. Consequently, nanocrystalline coatings obviously are acting as friction reducing/lubricating layers and should be consequently deposited atop of the existing coating structure. However, new coatings of MTCVD/CVD with alternating nanocrystalline layers (doping switched ON/OFF during MTCVD/CVD process, nanolayered structures of MTCVD/nanocrystalline layers are possible) should exhibit outstanding/new properties. The nanocrystalline layers could also be used in combination with other coating materials like alumina (kappa or alpha), or other oxides or TiN forming a nanolayered structure being composed of layers of MTCVD and nanograined coatings. Very thin nanocrystalline layers inserted in the MTCVD coatings can be used to control the grain size of the MTCVD coating when a coating composed of mainly Ti(C,N) is preferred. When used as the outermost layer, the nanocrystalline layer may be applied onto an Al₂O₃layer, which itself can be applied onto one or more other layers such as, for example, TiC. The Al₂O₃layer can be an alpha phase, a kappa phase or a mixture of alpha and kappa phase Al₂O₃. The nanocrystalline layer may also be applied onto a TiN layer.

[0014] Similarly, when the nanocrystalline layer is applied as an inner layer, there may be other layers such as Al₂O₃, TiC, Ti(C,N), TiN or the like applied atop the nanocrystalline layer.

[0015] These various other inner and/or outer layers may be applied by CVD, MTCVD or PVD.

[0016] By equiaxed, it is meant that the grains have essentially the same dimension in all directions.

[0017] The invention is additionally illustrated in connection with the following Examples which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the Examples.

[0018] In this case CO doping was applied. The following five experimental coatings (referred to as coatings 1, 2, 3, 4 and 5) were produced at a pressure of 65 mbar according to the process data given in Table 1.

[0019] The coatings 1-5 were investigated using Transmission electron microscopy (TEM) in order to elucidate the effect of CO doping on the grain size. It appeared clear that the microstructure of the MTCVD Ti(C,N) coating being composed of large columnar crystals can be strongly refined by CO doping. The structure became nanocrystalline at a CO doping level of about 8%.

[0020] Coatings 1, 3, 4 and 5 were studied by using XRD . The grain refinement is clearly manifested as line broadening. In Table 2, data for line broadening together with observed grain sizes are presented.

[0021] Full Width at Half Maximum of the reference is B₀

[0022] Full Width at Half Maximum of the reference of the nanograined coating is B_n(n=4.0, 6.0, 8.0)

[0023] Line broadening is B_n/B₀.

[0024] Line broadening is defined as absolute values (°2θ) and as relative values. The line broadening should be-from 0.30 to 0.60°2θ, preferably from 0.33 to 0.4°2θ, and from 2.0 to 4.0, preferably from 2.2 to 2.7 (relative value, reference MTCVD Ti(C,N)).

[0025] The two definitions for line broadening are in this case characterized by the fact that the coating is under slight tensile stresses. The line broadening is consequently solely due to grain size alone and is not due to compressive stresses together with a small grain size as is the case in PVD coatings.

[0026] Hardness

[0027] Hardness of the coatings 1, 3, 4 and 5 were measured by using nanoindentation technique. The results are presented in Table 3.

[0028] Coating Chemistry (incorporation of oxygen)

[0029] The experiments have shown that by doping, considerable amounts of oxygen can be incorporated into the coatings, Table 4. It is clear that the carbon content in the coating is in principle, unaffected by the increase in CO doping. The nitrogen level decreases, while there is a drastic increase in the oxygen content. However, titanium oxides (Magnelli phases) were not found. The stoichiometry of the coating layers increases from 0.88 to 1.03.

[0030] Friction

[0031] The friction coefficients between steel (SS1672) and the experimental coatings were measured using a pin-on disk method. Reduced friction could be confirmed, Table 5.

[0032] Single layers of coatings 1, 3, 4 and 5 were deposited on turning (SNUN120408) and milling SEKN1203 AFN) inserts. All the coating layers had the same thickness of 6 μm. As is clear from Table 6, the nanocrystalline (coating 5) single layers exhibited good wear properties at lower cutting speed in turning. At the highest cutting speed, the coating failed due to plastic deformation, however, with clearly reduced crater, flank wear and chipping as compared to the non-doped coating layer (coating 1). In milling (in this case at medium speed) the nanocrystalline coating (coating 5) exhibited a clearly increased life time and enhanced edge strength (chipping resistance), Table 7.

[0033] From the results obtained, it is clear that a combination of nanocrystalline coating layer atop a layer of Ti(C,N), TiN, TiC, Al₂O₃or a combination of these, obviously results in enhanced wear properties, especially when turning at higher speed and chipping resistance in milling are concerned.

[0034] The nanograined coating layer was applied atop a MTCVD Ti(C,N) layer and atop at Ti(C,N)-Al₂O₃multicoating layer. The coating combinations were the following, Table 8. The coatings 1-4 were deposited on turning (SNUN120408) and milling (SEKN1203 AFN) inserts.

[0035] The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention.