Patent Publication Number: US-2022235487-A1

Title: Quantitative textured polycrystalline coatings

Description:
RELATED APPLICATION DATA 
     The present application claims priority pursuant to Article 8 of the Patent Cooperation Treaty to U.S. Provisional Patent Application Ser. No. 62/850,953 filed May 21, 2019, which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present invention relates to refractory coatings and, in particular, to quantitative textured polycrystalline layers of refractory coatings. 
     BACKGROUND 
     Cutting tools, including cemented carbide cutting tools, have been used in both coated and uncoated conditions for machining various metals and alloys. In order to increase cutting tool wear resistance, performance and lifetime, one or more layers of refractory material have been applied to cutting tool surfaces. TiC, TiCN, TiN and/or Al 2 O 3 , for example, have been applied to cemented carbide substrates by chemical vapor deposition (CVD) and by physical vapor deposition (PVD). While effective in inhibiting wear and extending tool lifetime in a variety of applications, refractory coatings based on single or multi-layer constructions of the foregoing refractory materials have increasingly reached their performance limits, thereby calling for the development of new coating architectures for cutting tools, including coatings having various textures and grain orientations for enhanced performance. 
     However, significant issues exist with proper characterization of coating architectures and related properties. Current analytical methods are qualitative and subjective in nature, wherein coating characterization is largely dependent on specific data sets selected by the lab. Texture coefficient of a polycrystalline layer measured by the Harris equation is one of many examples where a coating property can vary significantly based on user input. The Harris equation is 
     
       
         
           
             
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     where I(hkl) is the measured intensity of the (hkl) reflection, I 0 (hkl) is the powder diffraction intensity according to the PDF reference card from the International Center for Diffraction Data (ICDD) and N is the number of reflections used in the calculation. 
     Based on this equation and associated underlying assumptions, the texture coefficient is the average intensity ratio difference from the experimental versus the reference. It is noted that the texture evaluation is arbitrary due to the subjective selection of references and numbers of reflection peaks. Different references will lead to different reflection peak intensity, and thus the deviation of final calculation of texture coefficient. Another concern is that there are many peaks in a polycrystalline layer such as alumina. Therefore, the operator is required to select the number of reflection peaks for the texture analysis. Some operators select 6 peaks, some operators select 8 peaks and some operators select even more peaks for the calculation. Due to the subjective selection of these numerous input parameters, the calculated texture coefficient value is only of relative meaning and is not fully quantitative. The texture coefficient for a polycrystalline layer can vary significantly based on the input data, especially when characterizing the “maximum” texture coefficient which is based on the number of reflections used in the calculation (N). For example, the maximum texture coefficient will be 5 if the operator selects 5 reflections; it will be 8 if the operator selects 8 reflections and so forth from the Harris equation. Accordingly, texture coefficient based on the Harris formula can only draw qualitative, not fully quantitative, meaningful conclusions regarding coating architecture and related coating performance. Such limitations also preclude accurate description of crystal evolution in polycrystalline coatings. 
     SUMMARY 
     In view of the foregoing, quantitative methods of polycrystalline coating development and characterization are needed for understanding coating architecture and coating performance properties. In one aspect, methods of making coated articles are described herein. A method, in some embodiments, comprises providing a substrate, and depositing a coating by CVD and/or PVD over a surface of the substrate, the coating comprising at least one polycrystalline layer, wherein one or more CVD and/or PVD conditions are selected to induce one or more properties of the polycrystalline layer. The presence of the one or more properties in the polycrystalline layer is quantified by two-dimensional (2D) X-ray diffraction analysis. In some embodiments, the method further comprises quantifying the presence of the one or more properties of the polycrystalline layer by electron backscatter diffraction (EBSD). As described further herein, one or more of the CVD or PVD parameters or conditions can be adjusted in response to quantification of the one or more properties by the 2D X-ray analysis. Properties of the polycrystalline layer induced by the deposition conditions and quantified by the 2D X-ray analysis are not limited and may be selected according to various considerations, including compositional identity of the polycrystalline layer and desired properties of the polycrystalline layer. In some embodiments, for example, the one or more properties include grain orientation in the polycrystalline layer and/or residual stress of the polycrystalline layer. 
     In another aspect, coated articles are provided. As described further herein, properties of the coated articles can be induced by the associated CVD or PVD parameters and quantified by 2D X-ray diffraction. A coated article, for example, comprises a substrate and a coating comprising a layer of alumina deposited by CVD over the substrate, wherein less than 50 volume percent of the grains of the alumina layer exhibit random orientation. In some embodiments, greater than 30 volume percent of the grains of the alumina layer exhibit fiber orientation. Additionally, grains of the alumina layer can exhibit one or more preferred orientations, including the (006) growth (e.g. 0001 or basal plane) direction. In some embodiments, for example, 15-50 volume percent of the grains in the alumina layer exhibit (006) texture. 
     In addition to grain orientation properties, the alumina layer can exhibit various levels of residual stress depending on CVD parameters or conditions, the residual stress quantified by 2D X-ray diffraction. In some embodiments, the alumina layer exhibits a residual stress of 0.2 GPa to −3 GPa. As is known to the skilled artisan, a positive value for residual stress indicates tensile stress whereas a negative value indicates compressive stress. 
     These and other embodiments are further described in the following detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  provides a cross-sectional scanning electron micrograph (SEM) of a coated substrate and schematics of each coated substrate detailed in the Example herein. 
         FIG. 2  illustrates residual stress evolution in an Al 2 O 3  layer as a function of layer thickness, according to some embodiments. 
         FIG. 3  illustrates grain orientation/texture evolution in an Al 2 O 3  layer as a function of layer thickness, according to some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments described herein can be understood more readily by reference to the following detailed description and examples and their previous and following descriptions. Elements, apparatus and methods described herein, however, are not limited to the specific embodiments presented in the detailed description and examples. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations will be readily apparent to those of skill in the art without departing from the spirit and scope of the invention. 
     In one aspect, methods of making coated articles are described herein. A method, in some embodiments, comprises providing a substrate, and depositing a coating by CVD and/or PVD over a surface of the substrate, the coating comprising at least one polycrystalline layer, wherein one or more CVD and/or PVD conditions are selected to induce one or more properties of the polycrystalline layer. The presence of the one or more properties in the polycrystalline layer is quantified by the 2D X-ray diffraction analysis. In some embodiments, the method further comprises quantifying the presence of the one or more properties of the polycrystalline layer by EBSD. 
     As described further herein, one or more of the CVD or PVD parameters or conditions can be adjusted in response to quantification of the one or more properties by the 2D X-ray analysis. In some embodiments, for example, the one or more properties of the polycrystalline layer are not detected by the 2D X-ray analysis or are not present in the polycrystalline layer at the desired level. CVD or PVD parameters may be adjusted until the one or more properties are achieved in the polycrystalline layer at the desired level. Adjustment of CVD or PVD parameters to induce the desired properties in the polycrystalline may require an iterative process wherein samples of the polycrystalline layer are deposited and subsequently analyzed by the 2D X-ray analysis. This iterative process can yield information regarding property evolution in the polycrystalline layer relative to various deposition parameters. In this way, a library can be developed for deposition conditions to induce desired coating architectures. 
     Properties of the polycrystalline layer induced by the deposition conditions and quantified by the 2D X-ray analysis are not limited and may be selected according to various considerations including compositional identity of the polycrystalline layer and desired properties of the polycrystalline layer. In some embodiments, for example, the one or more properties include grain orientation in the polycrystalline layer and/or residual stress of the polycrystalline layer. Moreover, one or more CVD parameters that may be varied or adjusted to induce or achieve a desired property in the polycrystalline layer can include reactant partial pressures, gas flow rates into and out of the CVD reactor, CVD reactor temperature, deposition/growth rate of the polycrystalline and/or deposition time of the polycrystalline layer. One or more PVD parameters that may be varied can include cathode composition, substrate bias voltage, cathode arrangement relative to the substrate, cathode and/or substrate rotation, substrate temperature, deposition/growth rate of the polycrystalline and/or deposition time of the polycrystalline layer. 
     In some embodiments, for example, one or more deposition conditions are selected to control random grain orientation of the polycrystalline layer. Less than 50 volume percent of the grains of the polycrystalline layer can exhibit random orientation, in some embodiments. Deposition conditions can also control fiber orientation of the grains in the polycrystalline layer. Fiber orientation, as used herein, refers to grains having one crystallographic axis parallel to the substrate normal. In some embodiments, greater than 30 volume percent of the grains, such as 30-90 volume percent, exhibit fiber orientation. Further, deposition conditions can also control one or more preferred grain orientations in the polycrystalline layer, including the (006) orientation. In some embodiments, for example, 10-95 volume percent or 15-50 volume percent of the grains in the alumina layer exhibit (006) texture. 
     In addition to controlling grain orientation as quantified by the 2D X-ray analysis, deposition parameters can also control residual stress of the polycrystalline layer. In some −3 GPa. 
     Polycrystalline layers of methods and compositions described herein can have any desired composition. In some embodiments, the polycrystalline layers are formed of refractory ceramic materials including metal oxides, nitrides, carbides, carbonitrides, or oxycarbonitrides. The polycrystalline layer can comprise one or more metallic elements selected from the group consisting of aluminum and metallic elements of Groups IVB, VB and VIB of the Periodic Table and one or more non-metallic elements selected from the group consisting of non-metallic elements of Groups IIIA, IVA, VA and VIA of the Periodic Table. In some embodiments, for example, the polycrystalline layer comprises alumina, such as alpha-alumina, kappa-alumina or mixtures thereof. Additionally, coatings described herein can comprise a single polycrystalline layer or multiple polycrystalline layers. Layer thickness of a single polycrystalline layer can generally range from 0.5 μm to 20 μm. 
     Coated articles according to methods described herein can comprise any substrate not inconsistent with the objectives of the present invention. For example, a substrate can be a cutting tool or tooling used in wear applications. Cutting tools include, but are not limited to, indexable cutting inserts, end mills or drills. Indexable cutting inserts can have any desired ANSI standard geometry for milling or turning applications. Substrates of coated articles described herein can be formed of cemented carbide, carbide, ceramic, cermet, steel or other alloy. A cemented carbide substrate, in some embodiments, comprises tungsten carbide (WC). WC can be present in a cutting tool substrate in an amount of at least about 80 weight percent or in an amount of at least about 85 weight percent. Additionally, metallic binder of cemented carbide can comprise cobalt or cobalt alloy. Cobalt, for example, can be present in a cemented carbide substrate in an amount ranging from 1 weight percent to 15 weight percent. In some embodiments, cobalt is present in a cemented carbide substrate in an amount ranging from 5-12 weight percent or from 6-10 weight percent. Further, a cemented carbide substrate may exhibit a zone of binder enrichment beginning at and extending inwardly from the surface of the substrate. 
     Cemented carbide substrates can also comprise one or more additives such as, for example, one or more of the following elements and/or their compounds: titanium, niobium, vanadium, tantalum, chromium, zirconium and/or hafnium. In some embodiments, titanium, niobium, vanadium, tantalum, chromium, zirconium and/or hafnium form solid solution carbides with WC of the substrate. In such embodiments, the substrate can comprise one or more solid solution carbides in an amount ranging from 0.1-5 weight percent. Additionally, a cemented carbide substrate can comprise nitrogen. 
     These and other embodiments are further illustrated in the following non-limiting examples. 
     EXAMPLE 1—Control of Residual Stress and Grain Orientation via 2D-XRD 
     Four cemented carbide substrates of the same grade were coated provided CVD coatings according to Table I. 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 Coated cemented carbide substrates 
               
            
           
           
               
               
               
            
               
                   
                 Coating Sample 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Coating Structure 
                 M8A2 
                 M8A4 
                 M8A6 
                 M8A12 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 TiN 
                 (μm) 
                 0.3 
                 0.7 
                 0.2 
                 0.3 
               
               
                 MT-TiCN 
                 (μm) 
                 8.6 
                 8.3 
                 8.5 
                 7.8 
               
               
                 HT-TiCN 
                 (μm) 
                 1.6 
                 1.2 
                 1.1 
                 1.2 
               
               
                 Al 2 O 3   
                 (μm) 
                 1.7 
                 3.7 
                 5.8 
                 12.1 
               
               
                 Total 
                 (μm) 
                 12.3 
                 13.9 
                 15.6 
                 21.5 
               
               
                   
               
            
           
         
       
     
     As provided in Table I, only the thickness of the Al 2 O 3  layer was varied, with the remaining layers exhibiting substantially the similar thicknesses across the samples under the same deposition conditions.  FIG. 1  provides a cross-sectional scanning electron micrograph of a coated substrate and schematics of each coated substrate according to Table I. Each coating was deposited according to the parameters of Tables II and III with the principal difference being the deposition time of the Al 2 O 3  layer to achieve thickness values listed in Table I. Deposition times for the Al 2 O 3  layer ranged from 1.8 to 10 hours at a rate of 1-1.5 μm/hr. For enhanced adhesion, a bonding layer including HT-TiCN and TiOCN is adjacent to the Al 2 O 3  layer. 
     
       
         
           
               
             
               
                 TABLE II 
               
             
            
               
                   
               
               
                 CVD Deposition of Coating Layers 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 H 2   
                 N 2   
                 TiCl 4   
                 CH 3 CN 
                 CH 4   
                 AlCl 3   
                 CO 2   
                 CO 
                 HCl 
                 H 2 S 
               
               
                 Process Step 
                 vol. % 
                 vol. % 
                 vol. % 
                 vol. % 
                 vol. % 
                 vol. % 
                 vol. % 
                 vol. % 
                 vol. % 
                 vol. % 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 TiN 
                 Bal. 
                 18.40 
                 0.95 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 MT-TiCN 
                 Bal. 
                 27.8 
                 1.31 
                 0.001 
                 — 
                 — 
                 — 
                 — 
                 1.40 
                 — 
               
               
                 HT-TiCN 
                 Bal. 
                 16.69 
                 0.76 
                 — 
                 3.70 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                 TiOCN 
                 Bal. 
                 17.50 
                 1.08 
                 — 
                 2.52 
                 — 
                 — 
                 1.10 
                 1.10 
                 — 
               
               
                 Al 2 O 3   
                 Bal. 
                 — 
                 — 
                 — 
                 — 
                 4.84 
                 2.42 
                 — 
                 3.00 
                 0.10 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE III 
               
             
            
               
                   
               
               
                 CVD Deposition Steps 
               
            
           
           
               
               
               
               
            
               
                 Process Step 
                 Temperature ° C. 
                 Pressure mbar 
                 Time min. 
               
               
                   
               
               
                 TiN 
                 850-960  
                 60-90  
                 10-90  
               
               
                 MT-TiCN 
                 900-940  
                 70-100 
                 50-400 
               
               
                 HT-TiCN 
                 900-1050 
                 60-150 
                 30-200 
               
               
                 TiOCN 
                 950-1050 
                 200-500  
                 30-70  
               
               
                 Al 2 O 3   
                 950-1050 
                 50-120 
                 10-750 
               
               
                   
               
            
           
         
       
     
     Data collection for the Al 2 O 3  layer residual stress and grain orientation/texture was performed on a Bruker D8 diffractometer with the general area detector diffraction system HI-STAR GADDS and a rotating anode, operating at 40 kV and 20 mA. The core of the GADDS is the 2D area detector that is a photon counter over a large area. With the sample mounted on an χ and Φ eucentric cradle so that the film is normal to the Φ axis. The cradle rotated about a vertical axis denoted as ω which changes the orientation of the χ axis in the horizontal plane. Basic advantage is speed and more diffraction information. At the same time, the 2D may provide the better accuracy due to less defocus in the collection process. Pole figure measurement on materials like thin polycrystalline films or multilayers may require long measurement times and a high directional resolution in the pole figures due to the low diffracted intensities and expected sharp textures. The most efficient diffraction diagrams were determined with area detectors (XRD 2 ). X-ray pole figures were acquired utilizing filtered copper optics. In order to characterize the crystallographic texture, three incomplete (0-90° ) pole figures {001}, {100}, and {110} were measured. The macrotexture orientation distribution functions (ODFs) were calculated and visualized using integration software MulTEX. 
     The component description enabled an efficient concentration of grain orientation/texture information. A solution for the ODFs can be obtained which takes into account only the smallest necessary number of components. Estimates may be appropriate if the quality or quantity of measured data does not allow a precise calculation. The approach to determining texture components requires that the number of available pole figures and the measured ranges are large enough that any crystal orientation can be determined unambiguously. The least necessary amount of texture information in experimental pole figures only depends on crystal symmetry and the type of Miller indices of the diffracting lattice planes. 
     The volume fractions of the corresponding grain orientation/texture components were calculated using a spread of 15° from their ideal orientation. The overall intensity of the textures was characterized by the respective texture index T which was calculated as: T= [f(g)] 2 dg where f(g) is the orientation density function and g denotes the orientation defined by the three Euler angles g=(φ1, Φ,φ2). Spherical texture component (peak component) is the volume fraction, which does not depend on the rotation axis turning during the X-ray diffraction collection. 
     For residual stress analysis, Pearson VII mode was employed for peak evaluation in the two-dimensional XRD, and stress tensors were derived based on a triaxial stress model. Diffraction rings (024), (116), (214) and (300) used for the residual stress calculations. 
       FIG. 2  illustrates residual stress evolution in the Al 2 O 3  layer with increasing thickness for the layer. According to this quantitative analysis by 2D XRD, residual stress in the Al 2 O 3  layer can be controlled or set to the desired level based on control of layer thickness.  FIG. 3  illustrates results of the grain orientation/texture analysis conducted on the Al 2 O 3  layers of the coating samples via 2D X-ray diffraction. As set forth in  FIG. 3 , random orientation of the Al 2 O 3  grains was reduced with increasing layer thickness. Additionally, fiber orientation of the Al 2 O 3  grains increased with layer thickness. In view of these results, Al 2 O 3  grain orientation/texture can be controlled with increasing Al 2 O 3  layer thickness according the deposition parameters in Table II and III above. 
     Quantification of residual stress and grain orientation/texture of Al 2 O 3  layers of the present example via 2D X-ray diffraction establishes the ability to control coating architectures via selection of appropriate coating depositions parameters. Notably, the quantitative nature of the 2D X-ray diffraction analysis enables reproducibility of the coating architectures and removes the subjective nature of current qualitative characterizations methods, such as the Harris formula. With the methods and compositions described herein, polycrystalline layer properties and associated performance can be elucidated and tailored for a number of coating applications, including high performance tooling applications involving cutting inserts and various rotary cutting tools. 
     Various embodiments of the invention have been described in fulfillment of the various objects of the invention. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the invention.