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, Al2O3 and 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 1250xc2x0 C. and medium temperature chemical vapor deposition (MTCVD) conducted at temperatures of from about 700 to 900xc2x0 C., and PVD (physical vapor deposition).
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.
The hardness of polycrystalline materials in general (including coating layers as well) obey the Hall-Petch equation: H=Hxc2x0+C/d where H is the hardness of a polycrystalline material, Hxc2x0 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.
The use of a dopant such as a tetravalent titanium, hafnium and/or zirconium compound in the formation of an Al2O3 layer 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 Al2O3 applied by CVD as well as to promote even layers of the coating is disclosed in U.S. Pat. No. 4,619,886.
CO2 has been used as part of the coating process as well. In particular, it has been used in oxidizing processes where it reacts with H2 to form H2O, the oxidizing gas. See, for example, U.S. Pat. No. 5,827,570.
It is an object of this invention to avoid or alleviate the problems of the prior art.
It is further an object of this invention to provide a coating layer of significantly smaller grain size and concomitant hardness.
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.
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 CO2 sufficient to form the Ti(C,N,O) in a size less than 25 mm.
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 CO2 or 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 CO2 is 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 CO2 dopant can be added at anytime throughout the reaction, continuously or in the interrupted mode. When CO2 and/or CO/CO2 mixtures are used, care should be taken within the skill of the artisan to avoid the formation of Magnelli phases.
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.
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 Al2O3 layer, which itself can be applied onto one or more other layers such as, for example, TiC. The Al2O3 layer 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.
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.
These various other inner and/or outer layers may be applied by CVD, MTCVD or PVD.
By equiaxed, it is meant that the grains have essentially the same dimension in all directions.