Patent Publication Number: US-9850557-B2

Title: Cermet and cutting tool

Description:
TECHNICAL FIELD 
     The present invention relates to a cermet that contains hard phase particles containing at least Ti and a binding phase containing at least one of Ni and Co and to a cutting tool containing the cermet. 
     BACKGROUND ART 
     Hard materials called cermets have been utilized in main bodies (substrates) of cutting tools. Cermets are sintered bodies in which hard phase particles are bonded together with an iron group metal binding phase, and are hard materials in which a Ti compound, such as titanium carbide (TiC), titanium nitride (TiN), or titanium carbonitride (TiCN), is used as hard phase particles. As compared with cemented carbide in which tungsten carbide (WC) is used in main hard phase particles, cermets have advantages, such as [1] a reduction in the amount of scarce resource W used, [2] high wear resistance, [3] a finely machined surface in steel cutting, and [4] light weight. On the other hand, cermets have problems in that they have lower strength and toughness than cemented carbide, are susceptible to thermal shock, and therefore have limited processing applications. 
     Hard phase particles in some cermets have a cored structure composed of a core and a peripheral portion around the core. The core is rich in TiC or TiCN, and the peripheral portion is rich in a Ti composite compound that contains Ti and another metal (such as periodic table IV, V, and/or VI group element(s)). The peripheral portion improves wettability between the hard phase particles and a binding phase, imparts good sinterability to the cermets, and thereby contributes to improved strength and toughness of the cermets. Attempts have been made to further improve the strength and toughness of cermets, for example, by controlling the composition of such a cored structure (see, for example, Patent Literature 1 to Patent Literature 4). 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. 06-172913 
     PTL 2: Japanese Unexamined Patent Application Publication No. 2007-111786 
     PTL 3: Japanese Unexamined Patent Application Publication No. 2009-19276 
     PTL 4: Japanese Unexamined Patent Application Publication No. 2010-31308 
     SUMMARY OF INVENTION 
     Technical Problem 
     Even though some existing cermets have improved strength and toughness, they might not have sufficient strength and toughness for certain applications. For example, in the case of cutting under severe conditions, such as interrupted cutting at a high cutting speed of 100 m/min or more or interrupted cutting at a high speed and at a high feed rate, cutting tools containing existing cermets sometimes have insufficient fracture resistance. Thus, there is a demand for cermets having sufficient fracture resistance. 
     In view of the situations described above, it is an object of the present invention to provide a cermet that can constitute cutting tools having high fracture resistance and a method for producing the cermet. 
     It is another object of the present invention to provide a cutting tool having high fracture resistance. 
     Solution to Problem 
     The present inventors studied the causes of fractures of existing cermets. As a result, it was found that one of the causes of fractures of existing cermets is accumulations of heat easily built up in a cutting edge and its vicinity, which often results in face wear (crater wear), heat check, and fractures resulting therefrom. The reason that heat tends to accumulate in a cutting edge of an existing cermet and its vicinity during cutting is probably that heat of the cutting edge cannot dissipate through the interior of the cutting tool. Thus, the present inventors studied the thermal properties of cermets and found that a Ti composite compound in a peripheral portion of hard phase particles has a solid solution structure, and therefore the peripheral portion has lower thermal conductivity than the core composed of TiC or TiN. Although the peripheral portion contributes to improved sinterability of cermets, it was found that an excessive peripheral portion in a cermet significantly decreases the thermal conductivity of the cermet, reduces the heat resistance of the cermet, and tends to cause the accumulation of heat in the cutting edge and its vicinity. 
     The present inventors also found in the study that the average particle size of hard phase particles in cermets has an influence on fracture resistance. More specifically, it was found that an excessively small average particle size of hard phase particles is partly responsible for low toughness of the cermet and consequently low fracture resistance of the cermet. On the basis of these findings, a cermet according to one aspect of the present invention is defined as described below. 
     A cermet according to one aspect of the present invention is a cermet that contains hard phase particles containing Ti and a binding phase containing at least one of Ni and Co, and 70% or more (by number) of the hard phase particles have a cored structure containing a core and a peripheral portion around the core. The core of the hard phase particles having the cored structure is composed mainly of at least one of Ti carbide, Ti nitride, and Ti carbonitride. The peripheral portion of the hard phase particles having the cored structure is composed mainly of a Ti composite compound containing Ti and at least one selected from W, Mo, Ta, Nb, and Cr. In a cermet according to one aspect of the present invention, the core has an average particle size α, the peripheral portion has an average particle size β, and α and β satisfy 1.1≦β/α≦1.7. The hard phase particles in the cermet have an average particle size of more than 1.0 μm. 
     Advantageous Effects of Invention 
     A cermet according to the present invention has high fracture resistance. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a scanning electron microscope photograph of a cermet according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Description of Embodiments of Present Invention 
     First, the embodiments of the present invention will be described below. 
     &lt;1&gt; A cermet according to an embodiment of the present invention is a cermet that contains hard phase particles containing Ti and a binding phase containing at least one of Ni and Co, and 70% or more (by number) of the hard phase particles have a cored structure containing a core and a peripheral portion around the core. The core of the hard phase particles having the cored structure is composed mainly of at least one of Ti carbide, Ti nitride, and Ti carbonitride. The peripheral portion of the hard phase particles having the cored structure is composed mainly of a Ti composite compound containing Ti and at least one selected from W, Mo, Ta, Nb, and Cr. The core has an average particle size α, the peripheral portion has an average particle size (that is, the hard phase particles having the cored structure have an average particle size) β, and α and β satisfy 1.1≦β/α≦1.7. The hard phase particles in the cermet have an average particle size of more than 1.0 μm. 
     The hard phase particles having the cored structure that satisfy the formula have a thin peripheral portion having low thermal conductivity and have high thermal conductivity. Thus, a cermet containing hard phase particles having such a cored structure has higher thermal conductivity than existing cermets, retains less heat, suffers less thermal damage, and therefore has high fracture resistance. In particular, in the case that 70% or more of the hard phase particles of the cermet are hard phase particles having the cored structure that satisfies the formula, the cermet has higher toughness and consequently higher fracture resistance when the average particle size of all the hard phase particles is more than 1.0 μm than when the average particle size is 1 μm or less. 
     This is probably because the propagation of cracks, if present at all, in the cermet is suppressed when the average particle size is more than a certain level. 
     The present inventors also found in the study that if hard phase particles have substantially the same average particle size, hard phase particles that do not satisfy the formula tends to have lower hardness than hard phase particles that satisfy the formula. This is probably because the peripheral portion has lower hardness than the core. More specifically, the hard phase particles that do not satisfy the formula have a thick peripheral portion having low hardness and tend to have low hardness. On the other hand, as described above, in the cermet that satisfies the formula, the hard phase particles have a thin peripheral portion, and the core having higher hardness than the peripheral portion is predominant. Thus, if hard phase particles have substantially the same average particle size, hard phase particles that satisfy the formula have higher hardness than hard phase particles that do not satisfy the formula. Consequently, the cermet that satisfies the formula is expected to have high wear resistance. 
     &lt;&lt;Hard Phase Particles&gt;&gt; 
     The hard phase particles having the cored structure constitute 70% or more of all the hard phase particles. Hard phase particles having no cored structure are hard phase particles having almost no peripheral portion, that is, Ti carbide particles, Ti nitride particles, or Ti carbonitride particles. The hard phase particles having the cored structure preferably constitute 90% or more of all the hard phase particles in order to maintain the sinterability of the cermet. 
     The core of the hard phase particles having the cored structure is composed mainly of at least one of Ti carbide, Ti nitride, and Ti carbonitride. That is, the core is substantially composed of the Ti compound. Thus, the Ti content of the core is 50% or more by mass. 
     The peripheral portion of the hard phase particles having the cored structure is composed mainly of a Ti composite compound (=a compound containing Ti and at least one selected from W, Mo, Ta, Nb, and Cr). That is, the peripheral portion is substantially composed of the Ti composite compound. Thus, the W, Mo, Ta, Nb, and Cr content of the peripheral portion is 50% or more by mass. 
     The average particle size α (μm) of the core and the average particle size β (μm) of the peripheral portion in the present specification are average values of the Feret&#39;s diameter in the horizontal direction and the Feret&#39;s diameter in the vertical direction in a cross section image in the image analysis of a cross section of the cermet. More specifically, the Feret&#39;s diameter in the horizontal direction and the Feret&#39;s diameter in the vertical direction are measured in at least 200 hard phase particles having the cored structure in the cross section image. The average values of the Feret&#39;s diameters of the hard phase particles are summed up, and the total is divided by the number of measured particles. When β/α calculated in this manner ranges from 1.1 to 1.7, the peripheral portion has a sufficient thickness to improve wettability between the hard phase particles and the binding phase but is not so thick as to greatly decrease the thermal conductivity of the hard phase particles. β/α preferably ranges from 1.3 to 1.5. The average particle size β of the peripheral portion is identical with the average particle size of the hard phase particles having the cored structure. 
     When the average particle size of all the hard phase particles including the hard phase particles having the cored structure is more than 1.0 μm, the cermet can have high toughness and consequently high fracture resistance. The average particle size is preferably 1.1 μm or more, more preferably 1.4 μm or more. The average particle size of all the hard phase particles can be determined in a cross section image in which the number of all the hard phase particles is 200 or more. The number of all the hard phase particles is the total of the number of the hard phase particles having the cored structure and the number of hard phase particles having no cored structure in the cross section image. The particle size of each of the hard phase particles having the cored structure and the hard phase particles having no cored structure is an average value of the Feret&#39;s diameter in the horizontal direction and the Feret&#39;s diameter in the vertical direction. The average particle size of the hard phase particles can be calculated by summing up the particle sizes of all the hard phase particles and dividing the total by the number of measured particles. 
     &lt;&lt;Binding Phase&gt;&gt; 
     The binding phase contains at least one of Ni and Co and combines the hard phase particles. The binding phase is substantially composed of at least one of Ni and Co and may contain a component of the hard phase particles (Ti, W, Mo, Cr, C, and/or N) and inevitable components. 
     &lt;&lt;Thermal Conductivity of Cermet&gt;&gt; 
     A cermet according to an embodiment of the present invention has higher thermal conductivity than before due to an improvement in the thermal conductivity of the hard phase particles. A cermet preferably has a thermal conductivity of 20 W/m·K or more. 
     &lt;2&gt; A cermet according to an embodiment of the present invention contains hard phase particles having an average particle size of 5.0 μm or less. 
     When the average particle size of all the hard phase particles including the hard phase particles having the cored structure is 5.0 μm or less, the cermet is expected to have high fracture resistance, and wear on the cermet resulting from insufficient hardness is expected to be suppressed. The average particle size of all the hard phase particles is preferably 3.0 μm or less, more preferably 2.0 μm or less, because this is expected to further suppress wear resulting from insufficient hardness while high fracture resistance is maintained. 
     &lt;3&gt; A cermet according to an embodiment of the present invention has a Ti content in the range of 50% to 70% by mass, a W, Mo, Ta, Nb, and Cr content in the range of 15% to 30% by mass, and a Co and Ni content in the range of 15% to 20% by mass. 
     A cermet containing the predetermined amounts of the elements has a good balance of the binding phase and the core and peripheral portion of the hard phase particles having the cored structure and has high toughness and adhesion resistance. For example, when the W, Mo, Ta, Nb, and Cr content of the Ti composite compound in the peripheral portion is 15% or more by mass, the cermet has improved sinterability due to a sufficient absolute amount of the peripheral portion in the cermet. Thus, the cermet tends to have improved toughness. When the W, Mo, Ta, Nb, and Cr content is 30% or less by mass, this can suppress the increase in the number of hard phase particles having no cored structure and containing these elements (for example, WC) in the cermet and suppress the decrease in the adhesion resistance of the cermet. 
     &lt;4&gt; A cutting tool according to an embodiment of the present invention is a cutting tool that contains a cermet according to an embodiment of the present invention as a substrate. 
     A cermet according to an embodiment of the present invention has particularly high fracture resistance. Thus, such a cermet is suitable for substrates of cutting tools for use in cutting that particularly requires fracture resistance, such as high speed cutting or interrupted cutting. 
     A cermet according to an embodiment of the present invention has high wear resistance as well as high fracture resistance and is therefore suitable for substrates of cutting tools. The cutting tools may be of any type, for example, indexable inserts, drills, or reamers. 
     &lt;5&gt; In a cutting tool according to an embodiment of the present invention, at least part of a surface of the substrate is covered with a hard film. 
     The hard film preferably covers a portion of the substrate that is to become a cutting edge and a vicinity of the portion or may cover the entire surface of the substrate. The formation of the hard film on the substrate can improve wear resistance while the toughness of the substrate is maintained. The formation of the hard film on the substrate can increase the chipping resistance of the cutting edge of the substrate and improve the machined surface state of workpieces. 
     The hard film may be monolayer or multilayer and preferably has a thickness in the range of 1 to 20 μm in total. 
     The composition of the hard film may be a carbide, nitride, oxide, or boride of one or more elements selected from periodic table IV, V, and VI metals, aluminum (Al), and silicon (Si), or a solid solution thereof, for example, Ti(C, N), Al 2 O 3 , (Ti, Al)N, TiN, TiC, or (Al, Cr)N. Cubic boron nitride (cBN) and diamond-like carbon are also suitable for the composition of the hard film. The hard film can be formed by a gas phase method, such as a chemical vapor deposition (CVD) method or a physical vapor deposition (PVD) method. 
     Details of Embodiments of Present Invention 
     A cermet according to an embodiment of the present invention will be described below. The present invention is defined by the appended claims rather than by these embodiments. All modifications that fall within the scope of the claims and the equivalents thereof are intended to be embraced by the claims. 
     &lt;Method for Producing Cermet&gt; 
     For example, a cermet according to an embodiment of the present invention can be produced by a production method that includes a preparing step, a mixing step, a shaping step, and a sintering step, as described below.
         Preparing step: Preparing of a first hard phase raw powder containing at least one of Ti carbide, Ti nitride, and Ti carbonitride, a second hard phase raw powder containing at least one selected from W, Mo, Ta, Nb, and Cr, and a binding phase raw powder containing at least one of Co and Ni. The first hard phase raw powder has an average particle size of more than 1.0 μm.   Mixing step: Mixing of the first hard phase raw powder, the second hard phase raw powder, and the binding phase raw powder in an attritor. In the mixing step, the attritor has a peripheral speed in the range of 100 to 400 m/min, and the mixing time ranges from 0.1 to 5 hours.   Shaping step: Shaping of mixed raw materials prepared in the mixing step.   Sintering step: Sintering of a shaped body produced in the shaping step.       

     One of the characteristics of the production method is the mixing of the raw powders in the attritor at the predetermined peripheral speed for the short time, and another one of the characteristics is that the first hard phase raw powder has an average particle size of more than 1.0 μm. This allows the peripheral portion around the core in the hard phase particles having the cored structure to have an appropriate state and can make the average particle size of all the hard phase particles to be more than 1.0 μm. More specifically, [1] the peripheral portion can have a sufficient thickness to improve wettability between the hard phase particles and the binding phase but is not so thick as to greatly decrease the thermal conductivity of the hard phase particles having the cored structure, and [2] all the hard phase particles can have particle sizes that result in high toughness (more than 1.0 μm). 
     &lt;&lt;Preparing Step&gt;&gt; 
     In the preparing step of the production method, the first hard phase raw powder, the second hard phase raw powder, and the binding phase raw powder are prepared. The blend ratio of the raw powders is appropriately selected in accordance with the desired characteristics of the cermet. Typically, the mass ratio of the first hard phase raw powder to the second hard phase raw powder preferably ranges from 50:30 to 70:20, and the mass ratio of the hard phase raw powder to the binding phase raw powder preferably ranges from 80:20 to 90:10. 
     The average particle size of the first hard phase raw powder can be more than 1.0 μm and 5.0 μm or less and may range from 1.2 to 1.8 μm or 1.4 to 1.6 μm. The average particle size of the second hard phase raw powder preferably ranges from 0.5 to 3.0 μm and may be 2.0 μm or less or 1.0 μm or less. The average particle size of the binding phase raw powder preferably ranges from 0.5 to 3.0 μm and may be 2.0 μm or less or 1.0 μm or less. Unlike the average particle size of the hard phase particles in the cermet, the average particle sizes of the raw powders are determined by the Fisher method. The particles of the raw powders are pulverized and deformed through the mixing step and the shaping step, as described below. 
     &lt;&lt;Mixing Step&gt;&gt; 
     In the mixing step of the production method, the first hard phase raw powder, the second hard phase raw powder, and the binding phase raw powder are mixed in the attritor. If necessary, a forming aid (for example, paraffin) may be added to the mixture. 
     The attritor is a mixer that includes a rotating shaft and a plurality of stirring rods protruding circumferentially from the rotating shaft. The peripheral speed (rotation speed) of the attritor ranges from 100 to 400 m/min, and the mixing time ranges from 0.1 hours (=6 minutes) to 5 hours. When the peripheral speed and the mixing time are not less than the lower limits of the specified ranges, the raw powders are sufficiently mixed, the accumulation of the binding phase or the formation of an aggregation phase in the cermet can be suppressed, and hard phase particles having the cored structure can constitute 70% or more of the cermet. When the peripheral speed and the mixing time are not more than the upper limits of the specified ranges, this can prevent the peripheral portion of the hard phase particles having the cored structure in the cermet from becoming excessively thick. The preferred conditions for mixing in the attritor include a peripheral speed in the range of 100 to 250 m/min and a mixing time in the range of 0.1 to 1.5 hours. This is because [1] the raw powders are not excessively pulverized, and it is anticipated that a cermet that contains hard phase particles having an average particle size of more than 1.0 μm can be easily produced, and [2] the thermal conductivity and toughness can be increased. The mixing in the attritor may be performed with cemented carbide ball media or without media. 
     &lt;&lt;Shaping Step&gt;&gt; 
     In the shaping step of the production method, the mixed powders (the first hard phase raw powder+the second hard phase raw powder+the binding phase raw powder+an optional forming aid) are charged and pressed in a mold. The pressing pressure preferably depends on the composition of the raw powders and preferably ranges from approximately 50 to 250 MPa, more preferably 90 to 110 MPa. 
     &lt;&lt;Sintering Step&gt;&gt; 
     In the sintering step of the production method, sintering is preferably performed stepwise. For example, sintering has a forming aid removal period, a first heating period, a second heating period, a holding period, and a cooling period. The forming aid removal period refers to a period during which the temperature is increased to the volatilization temperature of the forming aid, for example, 350° C. to 500° C. During the next first heating period, the shaped body is heated to a temperature in the range of approximately 1200° C. to 1300° C. under vacuum. During the next second heating period, the shaped body is heated to a temperature in the range of approximately 1300° C. to 1600° C. in a nitrogen atmosphere at a pressure in the range of 0.4 to 3.3 kPa. During the holding period, the shaped body is held at the final temperature of the second heating period for 1 to 2 hours. During the cooling period, the shaped body is cooled to room temperature in a nitrogen atmosphere. 
     TEST EXAMPLES 
     Test Example 1 
     A cutting tool containing a cermet was practically produced, and the composition and structure of the cermet and the cutting performance of the cutting tool were examined. 
     &lt;&lt;Production of Samples 1 to 7&gt;&gt; 
     A sample was produced by a sequence of preparing step→mixing step→shaping step→sintering step. These steps will be described in detail below. Among these steps, each of the preparing step and the mixing step is one of features. 
     [Preparing Step] 
     A TiCN powder and a TiC powder were prepared as first hard phase raw powders. A WC powder, a Mo 2 C powder, a NbC powder, a TaC powder, and a Cr 3 C 2  powder were prepared as second hard phase raw powders. A Co powder and a Ni powder were prepared as binding phase raw powders. The first hard phase raw powder, the second hard phase raw powder, and the binding phase raw powder were mixed at a mass ratio listed in Table I. The average particle size of each powder is as follows: TiCN: 1.2 μm, TiC: 1.2 μm, WC: 1.2 μm, Mo 2 C: 1.2 μm, NbC: 1.0 μm, TaC: 1.0 μm, Cr 3 C 2 : 1.4 μm, Co: 1.4 μm, Ni: 2.6 μm. These average particle sizes were measured by the Fisher method. 
     [Mixing Step] 
     The raw powders blended at a mass ratio listed in Table I, a solvent ethanol, and a forming aid paraffin were mixed in an attritor to prepare a mixed raw material slurry. The paraffin constituted 2% by mass of the slurry. The conditions for mixing in the attritor included a peripheral speed of 250 m/min for 1.5 hours. The solvent was volatilized from the raw powder slurry to produce a mixed powder. 
     [Shaping Step] 
     The mixed powder was charged in a mold and was pressed at a pressure of 98 MPa. The shaped body had the SNG432 shape according to the ISO standard. 
     [Sintering Step] 
     The shaped body having the SNG432 shape was sintered. More specifically, the shaped body was first heated to 370° C. to remove the forming aid paraffin. The shaped body was then heated to 1200° C. under vacuum. The shaped body was then heated to 1520° C. in a nitrogen atmosphere at 3.3 kPa and was held at 1520° C. for 1 hour. The shaped body was then cooled to 1150° C. under vacuum and was then cooled to room temperature in a nitrogen atmosphere under pressure, thus forming a sintered body (cermet). 
     &lt;&lt;Production of Samples 21 to 29&gt;&gt; 
     (Samples 21 to 28) 
     The procedure for producing samples 21 to 28 is the same as the procedure for producing the samples 1 to 7 except the following points.
         The average particle size of TiCN prepared as the first hard phase raw powder is 0.7 μm.   The ratio of raw powders (the ratio is listed in Table I).
 
(Sample 29)
       

     The procedure for producing a sample 29 is also the same as the procedure for producing the samples 1 to 7 except the following points.
         The average particle size of TiCN prepared as the first hard phase raw powder is 1.0 μm.   The particle size distribution width of the TiCN is wider than that of TiCN in the other samples.   The ratio of raw powders (the ratio is listed in Table I)   The raw powders were mixed in the attritor at a peripheral speed of 200 m/min for a mixing time of 15 hours.
 
&lt;&lt;Measurement of Characteristics of Samples&gt;&gt;
       

     The structure, composition, thermal conductivity, toughness, and hardness of the cermets of the samples 1 to 7 and 21 to 29 were measured. Table I lists β/α of the structure (the definition of β/α is described below), the average particle size of the hard phase particles, thermal conductivity, toughness, and hardness, as well as the raw powder ratio. 
     &lt;&lt;Measurement of Structure and Composition of Hard Phase Particles&gt;&gt; 
     A cross section of a cermet of each sample was examined with a scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM-EDX) apparatus. Observation of SEM photographs taken with the SEM-EDX apparatus showed that 70% or more of the hard phase particles in the visual field in all the samples had a cored structure that included a core and a peripheral portion around the core.  FIG. 1  shows a SEM photograph of the cermet of the sample 1 as a representative. The black portions in the FIGURE represent the cores of the hard phase particles having the cored structure. The gray portions represent the peripheral portions of the hard phase particles having the cored structure. The white portions represent binding phases. Particles having a black portion or a gray portion alone are hard phase particles having no cored structure. 
     The EDX measurement showed that the core of each hard phase particle having the cored structure was substantially composed of Ti carbonitride (and TiC in the samples 5 and 25), and the Ti content of the core was 50% or more by mass. The EDX measurement showed that the peripheral portion of each hard phase particle having the cored structure was composed of a solid solution of a carbonitride containing Ti (a Ti composite compound), and the W, Mo, Ta, Nb, and Cr content of the peripheral portion was 50% or more by mass. 
     The element contents of the cermet are identical with the element contents of the mixed raw materials. Thus, the Ti content of each sample ranges from 50% to 70% by mass, the W, Mo, Ta, Nb, and Cr content ranges from 15% to 35% by mass, and the Co and Ni content ranges from 15% to 20% by mass. 
     The average particle size α (μm) of the core and the average particle size β (μm) of the peripheral portion in each sample were measured in SEM images (×10000) with an image analyzing apparatus Mac-VIEW (manufactured by Mountech Co., Ltd.) (the average particle size of the peripheral portion is identical with the average particle size of hard phase particles having the cored structure). The average particle size of the hard phase particles having the cored structure was determined by measuring the Feret&#39;s diameter in the horizontal direction and the Feret&#39;s diameter in the vertical direction in 200 or more hard phase particles having the cored structure in each sample, calculating the respective average values, summing up the average values of the hard phase particles having the cored structure, and dividing the total by the number of measured particles. β/α, which is an indicator of the thinness of the peripheral portion in the hard phase particles, was then calculated. A large β/α indicates a relatively thick peripheral portion, and a small β/α indicates a relatively thin peripheral portion. 
     The core and the peripheral portion of the hard phase particles having the cored structure were distinguished by low-cut treatment in which the autoanalysis conditions of image analysis software were set as described below. Values in a low-cut color region indicate that the objective color is close to white or black. A smaller value indicates that the objective color is closer to black. 
     A portion having a value smaller than the low-cut specified value (a portion closer to black) is recognized as a particle.
         Detection mode: color difference, margin of error: 32, scan density: 7, detection accuracy: 0.7   Low-cut specified value in measurement of core: 50 to 100   Low-cut specified value in measurement of peripheral portion: 150 to 200       

     The difference between the low-cut specified values of the core and the peripheral portion of the hard phase particles having the cored structure is fixed at 100. 
     The average particle size of hard phase particles (hard phase particle size in each table) was determined from the number of all the hard phase particles (200 or more) in the SEM image and the particle size of each hard phase particle. The particle size of each hard phase particle was determined with the image analyzing apparatus under the conditions described above. 
     &lt;&lt;Measurement of Thermal Conductivity&gt;&gt; 
     The thermal conductivity (W/m·K) of each sample was calculated by specific heat×thermal diffusivity×density. The specific heat and thermal diffusivity were measured by a laser flash method with TC-7000 manufactured by ULVAC-RIKO, Inc. The density was measured by an Archimedes&#39; principle. The thermal conductivity can be calculated using the equation: heat penetration rate=(thermal conductivity×density×specific heat) 1/2 . The heat penetration rate can be measured with a commercially available thermal microscope. The specific heat can be measured by differential scanning calorimetry (DSC). 
     &lt;&lt;Measurement of Toughness and Hardness&gt;&gt; 
     The toughness (MPa·m 1/2 ) and hardness (GPa) were determined according to JIS R1607 and JIS Z2244, respectively. 
     &lt;&lt;Summary of Measurement Results&gt;&gt; 
     The results in Table I show that the samples 1 to 28, in which the raw powder mixing time was 5 hours or less, tended to have higher thermal conductivity, toughness, and hardness than the sample 29, in which the raw powder mixing time was more than 10 hours. The reason for higher thermal conductivity is probably that the hard phase particles in the samples 1 to 28 had β/α in the range of 1.1 to 1.7, and the hard phase particles in the sample 29 had β/α of more than 2.0 (the peripheral portion of the hard phase particles in the samples 1 to 28 had a smaller thickness than that in the sample 29). The reason that the samples 1 to 7, 21, and 22 and the samples 24 to 28 tended to have higher toughness than the sample 29 is probably that although TiCN used in the sample 29 had a large average particle size, the TiCN had a wide particle size distribution width, and therefore the cermet had a nonuniform structure. The samples 23 and 24, which had an average particle size of not more than one-third the average particle size of the sample 29, had substantially the same toughness as the sample 29. The reason that the samples 1 to 28 had higher hardness than the sample 29 is probably that in the samples 1 to 28, as compared with the sample 29, [1] the core having higher hardness than the peripheral portion is predominant, and [2] the hard phase particles have a small average particle size. 
     The results in Table I show that the sample 1 had higher toughness than the sample 21, in which the average particle size of the TiCN powder was different from that of the sample 1, but the raw powders, the composition, and the production method were the same as those of the sample 1. Like the sample 1 and the sample 21, the comparison of the samples 2 to 7 and the corresponding samples 22 to 27 showed the same tendencies. Thus, when the hard phase particles have a particle size of more than 1.0 μm, the cermet is expected to have high fracture resistance. On the other hand, the samples 21 to 28 tended to have higher hardness than the samples 1 to 7. This is probably because the hard phase particles in the samples 21 to 28 had small particle sizes (1.0 μm or less). 
                                             TABLE I                              Hard                               phase                   particle   Thermal       Sample   Percentage of raw powder (mass %)       size   conductivity   Toughness   Hardness                                                                         No.   TiCN   TiC   WC   Mo 2 C   NbC   TaC   Cr 3 C 2     Co   Ni   β/α   (μm)   (W/m · K)   (MPa · m 1/2 )   (GPa)                                                                                 1   64.1   0.0   19.2   0.0   0.0   0.0   0.0   16.7   0.0   1.4   1.1   25   7.5   14.8       2   64.1   0.0   19.2   0.0   0.0   0.0   0.0   8.4   8.3   1.5   1.1   24   7.0   14.0       3   59.1   0.0   19.2   0.0   0.0   0.0   5.0   16.7   0.0   1.6   1.1   21   7.0   14.7       4   56.7   0.0   19.2   7.4   0.0   0.0   0.0   16.7   0.0   1.6   1.3   22   7.0   14.5       5   60.7   6.2   16.8   0.0   0.0   0.0   0.0   8.2   8.1   1.5   1.5   22   7.1   14.1       6   61.5   0.0   19.2   0.0   2.2   0.0   0.0   8.6   8.5   1.4   1.2   23   7.3   14.3       7   61.5   0.0   19.2   0.0   0.0   2.2   0.0   8.6   8.5   1.4   1.3   23   7.4   14.2       21   64.1   0.0   19.2   0.0   0.0   0.0   0.0   16.7   0.0   1.4   0.7   25   6.6   14.4       22   64.1   0.0   19.2   0.0   0.0   0.0   0.0   8.4   8.3   1.5   0.6   22   5.7   15.2       23   59.1   0.0   19.2   0.0   0.0   0.0   5.0   16.7   0.0   1.5   0.6   22   4.4   15.6       24   56.7   0.0   19.2   7.4   0.0   0.0   0.0   16.7   0.0   1.6   0.6   22   4.8   15.7       25   60.7   6.2   16.8   0.0   0.0   0.0   0.0   8.2   8.1   1.5   0.9   22   6.2   14.7       26   61.5   0.0   19.2   0.0   2.2   0.0   0.0   8.6   8.5   1.6   0.9   21   6.1   14.2       27   61.5   0.0   19.2   0.0   0.0   2.2   0.0   8.6   8.5   1.6   1.0   21   6.2   14.3       28   59.3   0.0   24.0   0.0   0.0   0.0   0.0   16.7   0.0   1.3   0.8   27   6.8   14.0       29   66.9   0.0   16.8   0.0   0.0   0.0   0.0   8.2   8.1   3.1   2.2   14   4.8   12.5                    
&lt;&lt;Cutting Test&gt;&gt;
 
     Cutting tools were then produced with part of the samples and were subjected to a cutting test. The cutting test is a fatigue toughness test. The fatigue toughness test relates to the number of collisions that causes a fracture of a cutting edge of a tip, that is, the life of the tip. 
     The cermets of the samples 1, 6, 21, and 29 were subjected to grinding (flat grinding) and then to cutting edge processing to produce a tip. The tip was fixed to an edge of a bit to produce a cutting tool. The cutting performance of the cutting tool was examined in turning under the conditions listed in Table II. Table III shows the results and the conditions of each sample listed in Table I. 
     
       
         
           
               
               
             
               
                 TABLE II 
               
               
                   
               
               
                   
                 Fatigue toughness test 
               
               
                   
               
             
            
               
                 Workpiece 
                 S35C-flute material (Number of flutes: 4) 
               
               
                 Cutting speed Vc (m/min) 
                 350 
               
               
                 Feed per revolution f 
                 0.25 
               
               
                 (mm/rev) 
                   
               
               
                 Depth of cut ap (mm) 
                 1.5 
               
               
                 Cutting environment 
                 WET 
               
               
                 Evaluation method 
                 Number of collisions that causes fracture 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE III 
               
               
                   
               
               
                   
                   
                 Hard phase 
                 Thermal 
                   
                   
               
               
                 Sample  
                   
                 particle size 
                 conductivity 
                 Toughness 
                 Number of 
               
               
                 No. 
                 β/α 
                 (μm) 
                 (W/m · K) 
                 (MPa · m 1/2 ) 
                 collisions 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 1 
                 1.4 
                 1.1 
                 25 
                 7.5 
                 8637 
               
               
                 6 
                 1.4 
                 1.2 
                 23 
                 7.3 
                 8444 
               
               
                 21 
                 1.4 
                 0.7 
                 25 
                 6.6 
                 4512 
               
               
                 29 
                 3.1 
                 2.2 
                 14 
                 4.8 
                 2355 
               
               
                   
               
            
           
         
       
     
     Table III shows that the cutting tools produced from the samples 1, 6, and 21, which had a thinner peripheral portion of the hard phase particles than the sample 29, had high fracture resistance even in cutting by which the interrupted cutting edge was heated to high temperatures (cutting speed=100 m/min or more). The reasons that the cutting tools produced from the samples 1, 6, and 21 had higher fracture resistance than the sample 29 are probably that the peripheral portion having low thermal conductivity was smaller and that the hard phase particles had high thermal conductivity. It is surmised that high thermal conductivity of the hard phase particles allows heat on the cutting edge generated by cutting to be easily dissipated and thereby reduces heat accumulation in the cutting edge and its vicinity. 
     The samples 1 and 6, in which the hard phase particles had an average particle size of more than 1.0 μm, had higher fracture resistance than the sample 21, in which the hard phase particles had an average particle size of 1.0 μm or less. This is probably because the hard phase particles having a greater average particle size suppressed cracking between the binding phase and the hard phase, thus resulting in high toughness. It was proved from the sample 29 that even when the hard phase particles had a large average particle size of more than 2.0 μm, β/α of more than 2.0 resulted in low fracture resistance. This is probably because the thick peripheral portion resulted in low toughness and thermal conductivity, as described above. 
     Test Example 2 
     In Test Example 2, the effects of the mixing step on the structure of a cermet and cutting performance were examined. 
     First, cutting tools containing the cermets (the samples 8 to 10 and 30) were produced under the same conditions as for the sample 1 in Test Example 1 (the mixing ratio of the raw materials was also the same as in the sample 1) except the peripheral speed and mixing time of the attritor in the mixing step. The mixing conditions for the samples 8 to 10 and 30 were described below.
         Sample8: Peripheral speed of attritor=100 m/min, mixing time=0.1 hours   Sample 9: Peripheral speed of attritor=250 m/min, mixing time=5.0 hours   Sample 10: Peripheral speed of attritor=400 m/min, mixing time=5.0 hours   Sample 30: Peripheral speed of attritor=250 m/min, mixing time=15.0 hours       

     The “average particle size of hard phase particles”, “β/α”, “thermal conductivity”, “toughness”, and “hardness” of each sample were then measured in the same manner as Test Example 1. Table IV shows the results. Table IV also shows the results of the sample 1 of Test Example 1. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE IV 
               
               
                   
               
               
                   
                 Peripheral 
                 Mixing 
                 Hard phase 
                   
                 Thermal 
                   
                   
               
               
                 Sample 
                 speed 
                 time 
                 particle size 
                   
                 conductivity 
                 Toughness 
                 Hardness 
               
               
                 No. 
                 (m/min) 
                 (h) 
                 (μm) 
                 β/α 
                 (W/m · K) 
                 (MPa · m 1/2 ) 
                 (GPa) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 1 
                 250 
                 1.5 
                 1.1 
                 1.4 
                 25 
                 7.5 
                 14.8 
               
               
                 8 
                 100 
                 0.1 
                 1.3 
                 1.2 
                 23 
                 7.7 
                 14.3 
               
               
                 9 
                 250 
                 5.0 
                 1.1 
                 1.6 
                 22 
                 6.9 
                 14.0 
               
               
                 10 
                 400 
                 5.0 
                 1.1 
                 1.7 
                 20 
                 6.8 
                 13.9 
               
               
                 30 
                 250 
                 15.0 
                 0.9 
                 1.9 
                 18 
                 6.3 
                 14.2 
               
               
                   
               
            
           
         
       
     
     Table IV shows that β/α tends to be increased by increasing the peripheral speed of the attritor or the mixing time. In particular, it was found that when the peripheral speed of the attritor ranged from approximately 100 to 250 m/min, and the mixing time ranged from approximately 0.1 to 5 hours, particularly approximately 0.1 to 1.5 hours, cutting tools (cermets) could have high toughness and high fracture resistance due to high thermal conductivity, which contributes to improved adhesion resistance. It was also found that although the hard phase particles had a large average particle size, the cutting tools (cermets) thus produced also had certain hardness. The reason that the sample 30 had substantially the same hardness as the other samples is probably that the hard phase particles had a smallest average particle size among the samples. 
     INDUSTRIAL APPLICABILITY 
     A cermet according to the present invention can be suitably utilized as a substrate of cutting tools. In particular, a cermet according to the present invention can be suitably utilized as a substrate of cutting tools that require fracture resistance.