Coating

At least one layer in a coating located on a surface of a substrate is a domain structure layer constituted of two or more domains different in composition. The average value of the size of each of first domains, defined as the diameter of a virtual circumcircle in contact with each first domain, is 1 nm to 10 nm. The average value of the nearest neighbor distance of each first domain, defined as the length of the shortest straight line connecting the center of the circumcircle with the center of another circumcircle adjacent to the circumcircle, is 1 nm to 12 nm. 95% or more of the first domains has a size within ±25% of the average value of the size, and 95% or more of the first domains has a nearest neighbor distance within ±25% of the average value of the nearest neighbor distance.

TECHNICAL FIELD

The present disclosure relates to a coating located on a surface of a substrate. The present application claims priority to the International Application PCT/JP2015/070314 filed on Jul. 15, 2015, the entire content of which is incorporated herein by reference.

BACKGROUND ART

In order to improve various characteristics of industrial products such as cutting tools, wear-resistant tools, molds, and electronic components, a coating having characteristic physical properties has conventionally been provided on a surface of such a substrate through vapor deposition.

Vapor deposition includes physical vapor deposition (PVD) or chemical vapor deposition (CVD). In particular, since a coating formed through PVD decreases the likelihood of a deterioration in strength of the substrate, it has widely been used for a cutting tool required to be high in strength, such as drills, end mills, and indexable inserts for milling, wear-resistant tools, and molds. In particular, demands for tools include adaptation to dry cutting without the use of a cutting fluid and higher cutting speed. In order to meet such demands, the coating provided on the surface of the substrate is required to have high hardness and good wear resistance.

For the purpose of improving the hardness and wear resistance of a coating, a coating in which a specific compound is stacked or combined on a scale as fine as a nanometer, thus having nanometer order, has been studied. For example, PTD 1 (Japanese Patent Laying-Open No. 07-003432) discloses a coating in which two or more types of layers each having a thickness on the order of nanometers are alternately stacked, and PTD 2 (Japanese Patent Laying-Open No. 06-279990, PTD 3 (Japanese Patent Laying-Open No. 2001-293601), and PTD 4 (Japanese Patent Laying-Open No. 2002-096205) each disclose a coating having a layer containing fine particles on the order of nanometers.

In such a coating which is a stack or combination on a scale as small as the nanometer order, a hardness of the coating itself tends to be high owing to nanometer size effect. Furthermore, strain energy is likely to be accumulated at an interface between compounds different in composition. Coatings in which strain energy is accumulated have been known to be high in hardness. That is, according to the conventional art, attempts to improve the hardness of a coating based on the nanometer size effect and accumulation of strain energy and to thereby improve wear resistance of the coating have been made.

CITATION LIST

Patent Document

SUMMARY OF INVENTION

A coating according to one manner of the present invention is a coating located on a surface of a substrate. The coating includes one or more layers, and at least one of the layers is a domain structure layer constituted of two or more domains different in composition. A first domain and a second domain which are two of the two or more domains are composed of at least one element selected from the group consisting of Al, B, Si, and elements of group 4, 5, 6 in the periodic table and at least one element selected from the group consisting of B, O, C, and N. A plurality of first domains are present in the domain structure layer. When a size of each first domain in an in-plane direction of the domain structure layer is defined as a diameter of a virtual circumcircle in contact with each first domain and a nearest neighbor distance of each first domain in the in-plane direction of the domain structure layer is defined as a length of a shortest straight line connecting a center of the circumcircle with a center of another circumcircle adjacent to the circumcircle, an average value of the size of each first domain is not smaller than 1 nm and not greater than 10 nm and an average value of the nearest neighbor distance of each first domain is not smaller than 1 nm and not greater than 12 nm, 95% or more of the first domains has a size in a range of not more than ±25% of the average value of the size, and 95% or more of the first domains has a nearest neighbor distance in a range of not more than ±25% of the average value of the nearest neighbor distance.

DESCRIPTION OF EMBODIMENTS

[Problems to be Solved by the Present Disclosure]

The conventional art still has a room for improvement in such physical properties as hardness and wear resistance. For example, in PTD 1, though a large number of crystal interfaces are present in the thickness direction, there is also a region where no crystal interface is present in the direction perpendicular to the thickness direction (the in-plane direction). That is, crystal interfaces present in the coating in PTD 1 extend only in one direction. Therefore, the degree of accumulation of strain energy is limited.

In PTDs 2 to 4, control of the extent of distribution of fine particles tends to be difficult and hence there may be a portion where fine particles identical in composition are adjacent to each other. In the portion where fine particles identical in composition are adjacent to each other, larger sized particles are apparently present and the nanometer size effect may not be obtained. In addition, since no strain energy is generated at the crystal interface where fine particles identical in composition are adjacent to each other, strain energy accumulated as a whole may be low.

An object of the present disclosure is to provide a coating excellent in hardness and wear resistance.

[Effects of the Present Disclosure]

According to the above, a coating excellent in hardness and wear resistance can be provided.

[Description of Embodiments of the Present Invention]

A manner of carrying out the present invention will initially be listed and described.

[1] A coating according to one manner of the present invention is a coating located on a surface of a substrate. The coating includes one or more layers, and at least one of the layers is a domain structure layer constituted of two or more domains different in composition. A first domain and a second domain which are two of the two or more domains are composed of at least one element selected from the group consisting of Al, B, Si, and elements of group 4, 5, 6 in the periodic table and at least one element selected from the group consisting of B, O, C, and N. A plurality of first domains are present in the domain structure layer. When the size of each first domain in the in-plane direction of the domain structure layer is defined as the diameter of a virtual circumcircle in contact with each first domain and the nearest neighbor distance of each first domain in the in-plane direction of the domain structure layer is defined as the length of the shortest straight line connecting the center of the circumcircle with the center of another circumcircle adjacent to the circumcircle, the average value of the size of each first domain is not smaller than 1 nm and not greater than 10 nm and the average value of the nearest neighbor distance of the first domain is not smaller than 1 nm and not greater than 12 nm, 95% or more of the first domains has a size in the range of not more than ±25% of the average value of the size, and 95% or more of the first domains has a nearest neighbor distance in the range of not more than ±25% of the average value of the nearest neighbor distance.

Here, the expression “different in composition” herein means a concept including naturally a case that constituent elements do not completely match and also a case that elements completely match, although the content thereof is different. Therefore, for example, “AlN” and “TiN” are different in composition from each other, and “Al0.6Ti0.4N” and “Al0.4Ti0.6N” are also different in composition from each other. Strain energy is generated at an interface between such compositions different from each other.

According to the coating, the average value of the size of the first domain is not smaller than 1 nm and not greater than 10 nm, 95% or more of the first domains has a size which scatters not more than 25% from the average value of the size, and 95% or more of the first domains has a nearest neighbor distance which scatters not more than 25% from the average value of the nearest neighbor distance. That is, in the domain structure layer, the first domain is uniform in nanometer size and uniformly distributed. According to the coating, the average value of the nearest neighbor distance of the first domain is not smaller than 1 nm and not greater than 12 nm. That is, the second domain is also of the nanometer order in a region between adjacent first domains.

Therefore, the domain structure layer can exhibit a great nanometer size effect, and high strain energy is accumulated owing to presence of a large number of interfaces between the first domain and the second domain. Therefore, the domain structure layer can be excellent in hardness and wear resistance.

[2] In the coating in [1], the domain structure layer may be constituted of the first domain and the second domain. In this case as well, the effect above can be achieved.

[3] In the coating in [1] and [2], one of the first domain and the second domain may be composed of a nitride containing at least Al but not containing Ti and the other of the first domain and the second domain may be composed of a nitride containing at least Al and Ti. A nitride containing at least Al and Ti is excellent in hardness and oxidation resistance. Though a nitride containing at least Al but not containing Ti is lower in hardness than the nitride containing Al and Ti, it is superior in oxidation resistance because it contains no Ti. Since the domain structure layer constituted of the first domain and the second domain is composed of the nitride containing Al and Ti excellent in hardness and oxidation resistance and the nitride containing Al which is poorer in hardness but better in oxidation resistance, it can have a hardness and oxidation resistance higher than those of a layer composed of any one nitride.

[4] In the coating in [3], one of the first domain and the second domain can be composed of AlN and the other of the first domain and the second domain can be composed of AlxTi1-xN, where the atomic ratio Al/Ti between Al and Ti in AlxTi1-xN is not lower than 1 and not higher than 1.5. In this case, the effect the same as above is obtained and more inexpensive manufacturing can be achieved due to lower material cost.

[5] In the coating in [1] and [2], one of the first domain and the second domain may be composed of a nitride containing at least Al but not containing Ti and the other of the first domain and the second domain may be composed of a nitride containing at least Ti but not containing Al. Such a domain structure layer has composition similar to the nitride containing Al and Ti as the entire domain structure layer. Therefore, according to the coating, a high hardness and high oxidation resistance specific to the nitride containing Al and Ti can be achieved.

[6] In the coating in [5], one of the first domain and the second domain can be composed of AlN and the other of the first domain and the second domain can be composed of TiN. In this case, the effect the same as above is obtained and more inexpensive manufacturing can be achieved due to lower material cost.

[7] In the coating in [1] to [6], preferably, the atomic ratio Al/Ti between Al and Ti in the entire domain structure layer exceeds 1.5. Since the nitride containing Al and Ti tends to be excellent in hardness and oxidation resistance as the Al/Ti ratio in the entire coating is higher, the coating including such a domain structure layer can have a higher hardness and higher oxidation resistance.

[8] In the coating in [1] and [2], preferably, the first domain and the second domain are composed of a nitride containing at least Al and Ti, and the atomic ratio Al/Ti between Al and Ti in one of the first domain and the second domain is not lower than 1 and the atomic ratio Al/Ti between Al and Ti in the other of the first domain and the second domain is lower than 1. Such a domain structure layer has composition similar to the nitride containing Al and Ti as the entire domain structure layer. Therefore, according to the coating, in addition to the effect described above, a high hardness and high oxidation resistance specific to the nitride containing Al and Ti can further be achieved.

[9] In the coating in [1] to [8], the first domain and the second domain are preferably crystalline. In this case, since high strain energy owing to mismatch in crystal lattice (difference in lattice constant) between the first domain and the second domain is generated, the hardness of the coating can further be higher. Being crystalline is a concept encompassing both single crystals and polycrystals.

[10] In the coating in [1] to [9], the first domain and the second domain preferably have crystal structure of a cubic NaCl type. In this case, since the hardness of each of the first domain and the second domain is high, the hardness of the coating can consequently be even higher.

[11] In the coating in [1] to [10], a plurality of second domains are present in the domain structure layer, when the size of each second domain in the in-plane direction of the domain structure layer is defined as the diameter of a virtual circumcircle in contact with each second domain and the nearest neighbor distance of each second domain in the in-plane direction of the domain structure layer is defined as the length of the shortest straight line connecting the center of the circumcircle with the center of another circumcircle adjacent to the circumcircle, the average value of the size of each second domain is not smaller than 1 nm and not greater than 10 nm and the average value of the nearest neighbor distance of each second domain is not smaller than 1 nm and not greater than 12 nm, 95% or more of the second domains has a size in the range of not more than ±25% of the average value of the size, and 95% or more of the second domains has a nearest neighbor distance in the range of not more than ±25% of the average value of the nearest neighbor distance. In this case, a greater effect resulting from the nanometer order is obtained and strain energy is also higher.

[12] In the coating in [1] to [11], the domain structure layer is formed through PVD. Such a domain structure layer is better in hardness and wear resistance as well as in adhesiveness to a substrate than a film manufactured with CVD.

[Details of Embodiments of the Present Invention]

One embodiment of the present invention (hereinafter denoted as the “present embodiment”) will be described hereinafter, however, the present embodiment is not limited thereto.

An expression in a form “A to B” herein means the upper limit and the lower limit of the range (that is, not less than A and not more than B). When a unit for A is not given and a unit is given only for B, the unit for A and the unit for B are the same. When a compound is herein expressed with a chemical formula such as AlN and when the atomic ratio is not particularly limited, all conventionally known atomic ratios are encompassed and a compound is not necessarily limited only to a compound within a stoichiometric range.

FIG. 1is a cross-sectional view showing one example of a coating according to the present embodiment. Referring toFIG. 1, a coating1is provided on a surface of a substrate2. A shape of substrate2is not particularly limited and the material therefor is not particularly limited either. For example, when substrate2is a base for a cutting tool, a material which can withstand cutting resistance such as cemented carbide, steel, cermet, ceramics, and a diamond sintered material can suitably be employed.

Coating1includes one or more layers, and at least one of the layers is a domain structure layer constituted of two or more domains different in composition. The number of layers in coating1is not particularly limited and the position of the domain structure layer is not particularly limited either. Coating1according to the present embodiment has such a construction that an underlying layer3and a domain structure layer4are stacked in this order as seen from substrate2.

Though the thickness of coating1is not particularly limited either, coating1preferably has a thickness from 0.1 to 10 μm, for example, when substrate2is the base for a tool.

FIG. 2is a cross-sectional view schematically showing a construction of any cross-section in the in-plane direction of the domain structure layer by way of example. The in-plane direction of the domain structure layer means the direction orthogonal to the thickness direction of the domain structure layer.

Referring toFIG. 2, domain structure layer4is constituted of a first domain41and a second domain42. First domain41and second domain42should be different in composition from each other. In the present embodiment, a plurality of first domains41are present in domain structure layer4, and second domain42is continuously present to surround each first domain41. That is, domain structure layer4in the present embodiment has what is called a sea-island structure. In the present embodiment, for facilitating understanding, first domain41is shown to have a square shape and second domain42is shown to have a shape surrounding a plurality of squares, however, a shape of first domain41and second domain42is not particularly limited and their shape may vary.

First domain41and second domain42are composed of at least one element selected from the group consisting of Al, B, Si, and elements of group 4 (Ti, Zr, Hf), 5 (V, Nb, Ta), and 6 (Cr, Mo, W) in the periodic table and at least one element selected from the group consisting of B, O, C, and N. When the former group includes B, the latter group does not include B. That is, when B grouped in the former group representing a group of metal elements is included in each composition, B grouped in the latter group representing a group of non-metal elements cannot be included, and when B grouped in the latter group is included, B grouped in the former group cannot be included. First domain41and second domain42may contain an inevitable impurity.

A compound composed as such is excellent in wear resistance, oxidation resistance, and high-temperature stability. Therefore, domain structure layer4having first domain41and second domain42composed as above is suitable for cutting tools, wear-resistant tools, and molds. The composition of each domain can be confirmed by evaluating a cross-section of domain structure layer4with a transmission electron microscope equipped with an energy dispersive X-ray spectrometer or three-dimensional atom probe tomography.

Each of a plurality of first domains41has a size and the nearest neighbor distance. Herein, the size of first domain41is defined as the diameter of a virtual circumcircle in contact with first domain41, and the nearest neighbor distance of first domain41is defined as the shortest straight line between the center of the virtual circumcircle in contact with first domain41and the center of another virtual circumcircle adjacent to the circumcircle. Specifically, each value is determined as follows.

Referring toFIG. 2, initially, any cross-section in the in-plane direction of domain structure layer4is evaluated with a transmission electron microscope or three-dimensional atom probe tomography. Domains different in composition can thus be distinguished from each other. Then, a virtual circumcircle C1is drawn for a plurality of first domains41, constituting the island structure, of the measured domains. The diameter a1of each virtual circumcircle C1is defined as the size of each first domain41. In addition, each straight line d1between the central point of virtual circumcircle C1in one first domain41and the central point of virtual circumcircle C1in each of other plurality of first domains41adjacent to that one first domain41is measured. The shortest of these straight lines d1is defined as the nearest neighbor distance of that one first domain41.

First domain41in the present embodiment is characterized by satisfying (1) to (4) below for the size and the nearest neighbor distance.

(1) The average value of the size of first domain41is from 1 to 10 nm.

(2) The average value of the nearest neighbor distance of first domain41is from 1 to 12 nm.

(3) Ninety-five percent or more of first domains41has a size in the range of not more than ±25% of the average value of the size.

(4) Ninety-five percent or more of first domains41has a nearest neighbor distance in the range of not more than ±25% of the average value of the nearest neighbor distance.

In connection with (1), the “average value of the size of first domain41” means the average value of diameters a1of at least 100 virtual circumcircles C1. Though the size at which a hardness of first domain41is highest owing to the effect resulting from the nanometer order is different depending on composition of first domain41, a sufficiently high hardness can be exhibited so long as the average value of the size of first domains41is at least within the range above.

In connection with (2), the “average value of the nearest neighbor distance of first domain41” means the average value of the nearest neighbor distances of at least 100 first domains41. From the difference between the average value of the size of first domain41and the average value of the nearest neighbor distance of first domain41, it can be seen that the width of second domain42present between first domains41is on the nanometer order. The average value of the nearest neighbor distance is not smaller than the average value of diameter a1.

In connection with (3), for example, when the average value of the size of 100 first domains41is 10 nm, 95 or more first domains41have the size within the range from 7.5 to 12.5 nm. That is, a plurality of first domains41are uniform in size.

In connection with (4), for example, when the nearest neighbor distance of 100 first domains41is 10 nm, 95 or more first domains41have the nearest neighbor distance within the range from 7.5 to 12.5 nm. That is, a plurality of first domains41are uniformly distributed.

Domain structure layer4in the present embodiment can achieve the following effect by satisfying all of (1) to (4) above. Firstly, since first domains41are uniformly distributed, the conventional problem caused by adjacent particles is suppressed and hence lowering in hardness thereby can be suppressed. Since first domains41are uniform in nanometer size and uniformly distributed, a significant nanometer size effect can be achieved and much strain energy can be accumulated. Furthermore, since both of first domain41and second domain42present between first domains41are of the nanometer order, more interfaces are present within domain structure layer4and hence more strain energy resulting from presence of the interfaces can be accumulated. Therefore, coating1having domain structure layer4can exhibit a high hardness and hence high wear resistance.

In contrast, when domain structure layer4does not satisfy (1) above, for example, when the average value of the size of first domains41is smaller than 1 nm, domain structure layer4exhibits a physical property as if it were composed of a single solid solution. Then, the nanometer size effect is not exhibited and the hardness is lower. Alternatively, when the average value of the size of first domains41is greater than 10 nm, the nanometer size effect is significantly lower. The average value of the size of first domains41is preferably from 1 to 5 nm and further preferably from 2 to 5 nm.

When domain structure layer4does not satisfy (2) above, for example, when the average value of the nearest neighbor distance of first domains41is greater than 12 nm, the total number of first domains41distributed in domain structure layer4is small or the size of second domain42present between first domains41is excessively large, and consequently a degree of accumulation of strain energy lowers. The average value of the nearest neighbor distance of first domains41is preferably from 1 to 11 nm and further preferably from 2 to 11 nm.

When domain structure layer4does not satisfy (3) above, for example, when deviation exceeds ±25%, first domain41low in hardness increases and consequently a hardness of domain structure layer4lowers, because improvement in hardness owing to the nanometer size effect attains to a relative maximum value at a specific value (which is different depending on composition). Ninety-five percent or more of first domains41preferably has a size within the range of not more than ±15% from the average value of the size.

When domain structure layer4does not satisfy (4) above, a portion where first domains41are continuous (adjacent) tends to be present. Since strain energy is generated at an interface between compounds different in composition, no strain energy is accumulated in a portion where first domains41are continuous. When apparent first domain41is no longer of the nanometer order due to first domains41being continuous, the effect resulting from the nanometer order is not exhibited. Ninety-five percent or more of first domains41preferably has the nearest neighbor distance within the range of not more than ±15% from the average value of the nearest neighbor distance.

In domain structure layer4in the present embodiment described above, first domain41and second domain42may be crystalline or amorphous. In any of the cases of first domain41and second domain42both being amorphous, first domain41and second domain42being amorphous and crystalline, respectively, and first domain41and second domain42both being crystalline, the nanometer size effect can be achieved in domain structure layer4so long as composition is different therebetween. Since strain energy is accumulated owing to mismatch in crystal lattice, first domain41and second domain42are preferably crystalline.

When first domain41and second domain42are crystalline, the compositions of first domain41and second domain42are suitably combinations of a composition composed of an element having crystal structure other than a cubic NaCl type at room temperature and atmospheric pressure (composition A) and a composition composed of an element having crystal structure of the cubic NaCl type at room temperature and atmospheric pressure (composition B).

In such domain structure layer4, since the domain having composition A is of the nanometer order, the crystal structure of composition A can change to the crystal structure of the cubic NaCl type under the influence by the crystal structure of composition B in contact therewith. With such change in crystal structure, high strain energy is generated in domain structure layer4and hence a hardness of domain structure layer4is further improved. Since the crystal structure of the cubic NaCl type tends to be higher in hardness than other crystal structures, it is excellent also in this regard. The crystal structure of each domain can be confirmed with a nanobeam electron diffraction method using a transmission electron microscope.

In coating1in the present embodiment, domain structure layer4is preferably composed of a nitride containing at least Al and Ti as a whole. Since the nitride containing Al and Ti is excellent in balance among a hardness, oxidation resistance, toughness, and non-reactivity with iron, coating1having domain structure layer4is suitable as a coating provided on a surface of a cutting tool, a wear-resistant tool, and a mold.

Since the nitride containing Al and Ti tends to be better in hardness and oxidation resistance as the Al/Ti ratio is higher, the Al/Ti ratio in the entire domain structure layer4composed of the nitride containing Al and Ti is preferably high. In domain structure layer4, a high Al/Ti ratio can be realized using a combination of compositions of each domain. In particular, according to domain structure layer4in the present embodiment, the Al/Ti ratio in the entire domain structure layer4can also exceed 1.5.

The high Al/Ti ratio as described above is a value which cannot be achieved by a conventional AlTiN solid solution. In the AlTiN solid solution, when the Al/Ti ratio exceeds 1.5, Al can no longer be dissolved in a solid state in the AlTiN solid solution and Al tends to be segregated as AlN (w-AlN) having crystal structure of a hexagonal wurtzite type or amorphous AlN (a-AlN). Since w-AlN and a-AlN have a lower hardness than the AlTiN solid solution, segregation thereof in the AlTiN solid solution leads to a lowering of the hardness of the coating.

In domain structure layer4composed of the nitride containing Al and Ti, for the purpose of improvement in hardness and oxidation resistance, at least one element (except for Ti) selected from the group consisting of B, Si, and elements of group 4, 5, and 6 in the periodic table may be added as an additional element. Such an additional element may be substituted with Al or Ti or may form a solid solution as being introduced in between lattices. The atomic ratio of each additional element to the total amount of metal elements (the total amount of Al, Ti, and the additional element) is preferably not higher than 0.1 in the entire domain structure layer4and preferably not higher than 0.05 in each domain.

In domain structure layer4composed of the nitride containing Al and Ti described above, for example, one of first domain41and second domain42may be composed of a nitride containing Al but not containing Ti and the other may be composed of a nitride containing Al and Ti. In this case, since the high Al/Ti ratio in the entire domain structure layer4can be realized, a high hardness and high oxidation resistance can be achieved. Since first domain41and second domain42are significantly different in composition from each other, strain energy at an interface therebetween can be increased and hence a hardness of domain structure layer4is improved.

When a domain composed of the nitride containing Al and Ti satisfies composition B above and a domain composed of the nitride containing Al but not containing Ti satisfies composition A above, accumulation of strain energy with change in crystal structure of the domain made of composition A can also be achieved. As domain structure layer4satisfying this condition, a case that one of first domain41and second domain42is composed of AlN and the other is composed of AlxTi1-xN can be mentioned. In this case, the Al/Ti ratio in the domain composed of the nitride containing Al and Ti is preferably from 1 to 1.5. When the ratio is lower than 1, the Al/Ti ratio of the entire domain structure layer is unfavorably low, and when the ratio is higher than 1.5, this tends to cause the unfavorable segregation of w-AlN or a-AlN within the domains.

In domain structure layer4composed of the nitride containing Al and Ti, one of first domain41and second domain42may be composed of a nitride containing Al but not containing Ti and the other may be composed of a nitride containing Ti but not containing Al. In this case as well, since domain structure layer4can realize the high Al/Ti ratio in the entire domain structure layer4, a high hardness and high oxidation resistance can be achieved. Since first domain41and second domain42are significantly different in composition from each other, strain energy at an interface therebetween can be increased and hence a hardness of domain structure layer4is improved.

When a domain composed of the nitride containing Ti but not containing Al satisfies composition B above and a domain composed of the nitride containing Al but not containing Ti satisfies composition A above, accumulation of strain energy with change in crystal structure of the domain composed of composition A can also be achieved. As domain structure layer4satisfying this condition, a case that one of first domain41and second domain42is composed of AlN and the other is composed of TiN can be mentioned.

In domain structure layer4composed of the nitride containing Al and Ti, first domain41and second domain42may be composed of a nitride containing at least Al and Ti, the Al/Ti ratio of one of first domain41and second domain42may be not lower than 1, and the Al/Ti ratio of the other may be lower than 1. In this case, as compared with the case described above (a case that one is composed of AlN and the other is composed of TiN), both domains are composed of the nitride containing Al and Ti and hence the entire domain structure layer is also excellent in both characteristics of a hardness and oxidation resistance. In this case, however, from a point of view of prevention of segregation described above, the Al/Ti ratio is preferably not higher than 1.5.

In domain structure layer4composed of the nitride containing Al and Ti, first domain41or second domain42may naturally contain an additional element described above.

In the present embodiment, underlying layer3is preferably a solid solution layer composed of a solid solution. For example, when substrate2is composed of a sintered material composed of several different materials such as cemented carbide, a more homogenous domain structure layer4is obtained by providing a solid solution layer on the surface of the sintered material. This may be because, when domain structure layer4is provided directly on the surface of the sintered material, uniformity of domain structure layer4may be disturbed as affected by the sintered material. The solid solution preferably contains all elements forming domain structure layer4. In this case, adhesiveness between domain structure layer4and the solid solution layer is further improved.

In the present embodiment described above in detail, a case that domain structure layer4is constituted of first domain41and second domain42has been described, however, the construction of domain structure layer4is not limited thereto. For example, the domain structure layer may be constituted of three types of domains different in composition from one another or four types of domains different in composition from one another.

Though the size of first domain41and the nearest neighbor distance of first domain41are determined in any cross-section in the in-plane direction of domain structure layer4in the present embodiment, similar conditions are preferably satisfied also in any cross-section in the direction of thickness of domain structure layer4.

That is, referring toFIG. 3which is a cross-sectional view schematically showing the construction of the domain structure layer in the direction of thickness, initially, any cross-section in the thickness direction of domain structure layer4is evaluated with a transmission electron microscope or three-dimensional atom probe tomography, and a virtual circumcircle C2is drawn for a plurality of measured first domains41. The diameter a2of each virtual circumcircle C2is defined as the size of each first domain41in the direction of thickness of domain structure layer4. In addition, each distance d2between the central point of virtual circumcircle C2in one first domain41and the central point of virtual circumcircle C2in each of other plurality of first domains41adjacent to that one first domain41is measured. The shortest of these straight lines d2is defined as the nearest neighbor distance of that one first domain41in the thickness direction of domain structure layer4.

Then, (1) to (4) above for the size and the nearest neighbor distance in the direction of thickness of domain structure layer4are preferably satisfied. In this case, the effect described above can further be achieved. The size and the nearest neighbor distance in the direction of thickness of domain structure layer4may be the same as or different from the size and the nearest neighbor distance in the in-plane direction of domain structure layer4described above, respectively. With the fabrication method described later, each value in (1) to (4) above to be controlled tends to be smaller in the thickness direction.

Though a case that domain structure layer4has a sea-island structure has been described in the first embodiment, domain structure layer4is not limited to the construction above. In the present embodiment, a case that first domain41and second domain42are equal to each other in size and nearest neighbor distance will be described. In the present embodiment, differences from the first embodiment will be described and similar description will not be repeated.

FIG. 4is a cross-sectional view schematically showing a construction in the in-plane direction of the domain structure layer by way of another example, andFIG. 5is a cross-sectional view schematically showing the construction in the direction of thickness of the domain structure layer by way of another example. In the present embodiment, first domain41satisfies (1) to (4) above and second domain42also satisfies (1) to (4) above.

FIG. 4, however, shows only a virtual circumcircle C3of first domain41in the in-plane direction of domain structure layer4, the diameter a3of virtual circumcircle C3, and each distance d3between the central point of one virtual circumcircle C3and the central point of virtual circumcircle C3in each of a plurality of other first domains41adjacent to that one virtual circumcircle C3. Virtual circumcircles and distances for second domains42are not shown. Similarly,FIG. 5shows only a virtual circumcircle C4of first domain41in the direction of thickness of domain structure layer4, the diameter a4of virtual circumcircle C4, and each distance d4.

According to such domain structure layer4, since both of first domain41and second domain42are of the nanometer order and uniformly distributed, the effect resulting from the nanometer order is higher. Since the interfaces in domain structure layer4(a surface of contact between different compounds) also increase, strain energy in domain structure layer4is also higher. Therefore, coating1including domain structure layer4in the present embodiment is higher in hardness and hence higher in wear resistance.

Domain structure layer4according to the first embodiment and the second embodiment can be fabricated through vapor deposition in which pulsed electric power can be supplied to targets (evaporation sources) serving as source materials for first domain41and second domain42. In particular, since a film manufactured with PVD is denser, higher in hardness, and better in wear resistance and adhesiveness than a film manufactured with CVD, use of PVD is preferred.

Such PVD includes High Power Impulse Magnetron Sputtering (HiPIMS), pulsed magnetron sputtering, pulsed laser ablation, and pulsed vacuum cathodic arc deposition. Among these, HiPIMS is suitable for fabrication of domain structure layer4, because control of an amount of supply of ions and atoms which can be supplied in response to one pulse is easy and a coating having a dense and smooth surface can be fabricated. A method of manufacturing domain structure layer4with HiPIMS will be described with reference toFIG. 6by way of example of the method of manufacturing domain structure layer4.

FIG. 6is a schematic diagram showing a construction of an apparatus used for fabricating a domain structure layer. This apparatus10is an HiPIMS apparatus with which HiPIMS can be performed. Referring toFIG. 6, apparatus10has a vacuum chamber12provided with a gas introduction port11, and in vacuum chamber12, a substrate holder13rotatable in a direction shown with an arrow in the figure with substrate2being fixed thereto and a heater14capable of heating substrate holder13and substrate2fixed to substrate holder13are arranged.

A substrate bias power supply (not shown) for applying a substrate bias voltage is connected to substrate holder13. Direct current (DC), pulse DC, HiPIMS, or high frequency (RF) can be used as a substrate bias power supply. For a cutting tool, in particular for a tool used for intermittent cutting such as a milling tool, compressive residual stress is preferably introduced into domain structure layer4by enhancing ion bombardment by applying a negative bias voltage to substrate2. By coating a substrate with the coating including domain structure layer4in which compressive residual stress has been introduced, chipping of the substrate such as chipping of a cutting edge can be suppressed.

The “compressive residual stress” herein refers to a type of internal stress (strain energy) present in domain structure layer4, and to stress expressed by a “−” (minus) numeric value. Therefore, a concept of high compressive residual stress means a larger absolute value of the numeric value, and a concept of low compressive residual stress means a smaller absolute value of the numeric value.

Compressive residual stress accumulated in domain structure layer4as above is preferably from −0.2 to −4.0 GPa. When a value for compressive residual stress is smaller than this range, toughness of a cutting edge is insufficient and chipping is likely, and when the value for compressive residual stress exceeds this value, compressive residual stress is excessively high and a coating tends to suffer from micro peeling at the cutting edge. A more preferred value for compressive residual stress is from −0.5 to −2.0 GPa. Compressive residual stress can be measured with a sin2ψ method with the use of an X-ray stress measurement apparatus, a method using Raman spectroscopy, or a method using radiant light.

A substrate bias voltage affects compressive residual stress in domain structure layer4, a hardness, and denseness of domain structure layer4, and a value therefor is desirably from −20 to −150 V. When the voltage is lower than this range, wear resistance tends to lower due to lowering in denseness of domain structure layer4, and when the voltage is higher than this range, compressive residual stress is excessively high. Thus, peeling of the coating from the substrate, for example, peeling of the coating at the cutting edge, is likely. A more preferred range of the substrate bias voltage is from −30 to −100 V.

Evaporation sources15aand15bfor supplying ions or atoms to substrate2are arranged in vacuum chamber12. Evaporation sources15aand15bare composed of elements serving as source materials for first domain41and second domain42, respectively. Normally, a metal element of elements forming each domain is supplied from evaporation sources15aand15b, and a non-metal element of elements forming each domain is introduced through gas introduction port11. It is important that evaporation sources15aand15bare arranged such that ions or atoms which have burst therefrom as a result of sputtering reach the same position on substrate holder13.

Power supplies16aand16bare electrically connected to evaporation sources15aand15b, respectively, and a synchronization device17allowing power supplies16aand16bto alternately supply pulsed electric power is electrically connected to power supplies16aand16b. Thus, while power supply16asupplies electric power, pulsed electric power can be supplied to evaporation source15awhereas electric power cannot be supplied to evaporation source15b. While power supply16bsupplies electric power, pulsed electric power can be supplied to evaporation source15bwhereas electric power cannot be supplied to evaporation source15a.

Under the control by synchronization device17, pulsed electric power supplied alternately by power supplies16aand16bsupplies a pulse current in the form of a pulse train including one or more pulses to evaporation sources15aand15b. Thus, ions or atoms intermittently and alternately burst out of evaporation sources15aand15b, and ions or atoms which have burst reach the same position on substrate holder13.

For example, in fabricating domain structure layer4constituted of first domain41composed of AlN and second domain42composed of TiN with the use of HiPIMS apparatus10, HiPIMS apparatus10is operated as follows.

Firstly, a target composed of Al and a target composed of Ti are attached to evaporation sources15aand15b, respectively, and substrate2is fixed to substrate holder13. Then, while vacuum chamber12is evacuated, heater14heats substrate2. Then, while an inert gas such as argon gas and nitrogen gas as reaction gas are introduced through gas introduction port11, electric power is supplied alternately from power supplies16aand16b. Thus, pulsed electric power in the form of a pulse train is supplied to evaporation sources15aand15b, and ions or atoms alternately burst out of evaporation sources15aand15b. Ions or atoms which have burst out of evaporation sources15aand15bare deposited alternately on substrate2.

Various suitable conditions in HiPIMS are different depending on combination of materials forming domains, such as whether a material forming each domain is a system of a solid solution or a non-solid solution in a thermal equilibrium state or whether a material forming another domain two-dimensionally or three-dimensionally grows on a material forming one domain. At least the conditions below are preferably satisfied.

First, in fabrication of domain structure layer4, a temperature Ts(K) of substrate2preferably satisfies a condition of 0.1≤Ts/Tm≤0.5, with respect to a melting point Tm(K) of the domain composition with the lowest melting point, among the domains forming domain structure layer4(in the present embodiment, first domain41and second domain42). When a value for Ts/Tmis smaller than this range, surface diffusion on substrate2, of film forming species which have come to substrate2is insufficient and consequently a layer composed of a single solid solution tends to be obtained. Alternatively, when a value for Ts/Tmis greater than this range, surface diffusion on substrate2, of film forming species which have come to substrate2is excessively active and hence fabrication of a domain of the nanometer order is difficult or a layer composed of a single solid solution tends to be obtained because a condition close to a thermal equilibrium state is established. Alternatively, phase separation takes place in first domain41or second domain42, or the domain structure layer itself takes on a random structure in which phase separation has taken place, and consequently, domain structure layer4having the aimed-at composition may not be obtained.

Secondly, in connection with evaporation sources15aand15b, the number of ions or atoms supplied to substrate2in response to one pulse train preferably corresponds to 0.1 to 15 monolayers. When the number of ions or atoms is smaller than this range, the domain formed with each evaporation source is excessively small and characteristics as the whole domain structure layer4are unfavorably close to characteristics of a layer as composed of a single solid solution. On the other hand, when the number of ions or atoms is greater than this range, a domain is excessively large and it is difficult to exhibit the nanometer size effect. In connection with evaporation sources15aand15b, the number of ions or atoms supplied to substrate2in response to one pulse train more preferably corresponds to 0.1 to 10 monolayers.

Here, “one pulse train” means electric power intermittently supplied to a single evaporation source. Namely, “one pulse train” means a “pulse train implementing one feed of electric power” in pulsed electric power intermittently supplied to each evaporation source. This one pulse train is constituted of one or more pulses. Furthermore, the expression the number of ions or atoms supplied in response to one pulse train corresponding to 0.1 to 15 monolayers means that the thickness achieved in assuming that a compound film formed on a substrate as a result of reaction between a reaction gas and ions or atoms which have burst out of an evaporation source in response to one pulse train (one feed of electric power) two-dimensionally grows to completely cover the substrate corresponds to 0.1 to 15 monolayers.

For example, when domain structure layer4constituted of first domain41composed of AlN and second domain42composed of TiN is fabricated by intermittently supplying a pulse train constituted of 5 pulses to each of an evaporation source composed of Al and an evaporation source composed of Ti, the above-described value for the source material for first domain41can be calculated as follows.

Firstly, an evaporation source composed of Al and a substrate are set in vacuum chamber12, and electric power in the form of 100 continuous pulses is continuously supplied to the evaporation source while a nitrogen gas as a reaction gas is introduced into vacuum chamber12. Then, the thickness of the fabricated coating composed of AlN (an AlN coating) is measured. For example, when the fabricated AlN coating has a thickness of 100 Å, the AlN coating having a thickness of 1 Å (approximately 0.5 monolayer) is fabricated per 1 pulse and the AlN coating having a thickness of 5 Å (approximately 2.5 monolayers) is fabricated per one pulse train. Therefore, in this case, the number of ions or atoms of a source material for first domain41supplied to substrate2in response to one pulse train corresponds to approximately 2.5 monolayers. The value described above can also be calculated for second domain42composed of TiN with the same method.

In particular, by controlling the number of ions or atoms (monolayers) supplied in response to one pulse train, each size of first domain41and second domain42can be controlled. For example, when evaporation source15ais a source material for first domain41and evaporation source15bis a source material for second domain42, the number of ions or atoms (monolayers) supplied from evaporation source15ain response to one pulse train is made smaller than the number of ions or atoms (monolayers) supplied from evaporation source15bin response to one pulse train, so that domain structure layer4as shown inFIGS. 2 and 3can be fabricated. Alternatively, the number of ions or atoms (monolayers) supplied from evaporation source15ain response to one pulse train is made substantially equal to the number of ions or atoms (monolayers) supplied from evaporation source15bin response to one pulse train, so that domain structure layer4as shown inFIGS. 4 and 5can be fabricated.

Though other conditions are not particularly limited in the HiPIMS method above, it is, for example, preferable that the film formation conditions below are satisfied.

Pulse width (pulse time period of a single pulse within a pulse train): 0.01 to 5 ms

Pressure in chamber: 0.1 to 1 Pa

Using the method of manufacturing a coating in the present embodiment, domain structure layer4satisfying (1) to (4) above can be fabricated and hence coating1including domain structure layer4can be manufactured.

Though fabrication of domain structure layer4constituted of two types of domains of first domain41and second domain42has been described above, for example, when domain structure layer4is constituted of three types of domains, three evaporation sources should be employed. Here, arrangement should be such that a pulse train is alternately supplied to any evaporation source and ions or atoms can be supplied to the same area on substrate holder13.

In fabricating a domain structure layer constituted of two types of domains, at least two evaporation sources, at least two pulse power supplies electrically connected to the respective evaporation sources, and one synchronization device electrically connected to the two pulse power supplies are required. In fabricating a domain structure layer constituted of three types of domains, at least three evaporation sources, at least three pulse power supplies electrically connected to the respective evaporation sources, and one synchronization device electrically connected to the three pulse power supplies are required. For the purpose of reducing a time period for film formation, two or more evaporation sources for forming domains may be arranged.

Before fabricating domain structure layer4on substrate2through the HiPIMS method described above, underlying layer3may be provided on substrate2and thereafter domain structure layer4may be fabricated. For example, when substrate2is made of cemented carbide, a solid solution layer composed of a solid solution is preferably fabricated as underlying layer3on a surface of cemented carbide, because domain structure layer4tends to be fabricated to a desired construction on a surface of a solid solution layer made of single composition more uniformly than on a surface of cemented carbide which is a sintered material consisting of several different compositions.

Though composition of the solid solution layer is not particularly limited, a solid solution composed of an element for evaporation sources15aand15bused for forming domain structure layer4and an element forming a reaction gas is preferred. Specifically, when first domain41is composed of AlN and second domain42is composed of TiN, a solid solution layer composed of AlTiN is preferred. Such a solid solution layer can be fabricated by simultaneously supplying pulsed electric power to power supplies16aand16bto thereby simultaneously supply a pulse train to evaporation sources15aand15b. In this case, the solid solution layer and domain structure layer4can exhibit high adhesiveness to each other, and it is not necessary to separately provide an evaporation source for fabricating a solid solution layer. In addition, since a target composed of Al and a target composed of Ti are relatively inexpensive, domain structure layer4can inexpensively be manufactured.

As the coating according to the first embodiment and the second embodiment described above in detail and the coating manufactured with the manufacturing method according to the third embodiment are provided on the surface of the substrate, various physical properties derived from the domain structure layer can be provided to the substrate. For example, the coating having the domain structure layer excellent in hardness and wear resistance as described above can suitably be made use of for a tool or a mold. Among others, the coating having the domain structure layer excellent additionally in oxidation resistance would be useful also for an application to a tool exposed to a particularly severe environment.

EXAMPLES

Though the present invention will be described in further detail with reference to Examples below, the present invention is not limited thereto. In Examples below, an underlying layer and a domain structure layer (which may hereinafter simply also be denoted as a “structure layer” in a sense including both of the domain structure layer in the present embodiment and a structure layer in a Comparative Example) are fabricated on a surface of a substrate and a structure of the domain structure layer and physical properties were investigated.

In Examples in Study 1, a domain structure layer as shown inFIGS. 2 and 3was fabricated.

(Preparation of Substrate and Target)

Initially, for the purpose of checking a structure and a hardness of a coating, a test piece of which surface to be coated was mirror polished (material name: G10E, manufactured by Sumitomo Electric Industries, Ltd.) was prepared (substrate X). For the purpose of investigating the wear resistance of the coating, an insert for milling (model number: SEET13T3AGSN-G, manufactured by Sumitomo Electric Industries, Ltd.) was prepared (substrate Y). Substrate X and substrate Y were cleaned with an alkali cleaning solution.

Prepared substrate X was set to substrate holder13in HiPIMS apparatus10, a target A was set as evaporation source15a, and a target B was set as evaporation source15b. Each target had a diameter of 4 inches. Composition of target A and target B in Examples 1 to 15 is as shown in Table 1.

(Determination of the Number of Ions or Atoms Per One Pulse Train)

In order to determine the number of ions or atoms (monolayers) per 1 pulse train for each target, the following test was conducted for each target. Initially, while substrate X was heated to 450° C., the pressure in vacuum chamber12was lowered to 0.005 Pa. Then, Ar gas was introduced, and the pressure in vacuum chamber12was held at 0.8 Pa, and substrate X was cleaned with the use of an Ar ion source at a substrate bias voltage of −600 V for 30 minutes.

Then, the Ar gas was exhausted from vacuum chamber12, and thereafter each gas was introduced such that a partial pressure in vacuum chamber12was set to Ar:N2=0.4 Pa:0.2 Pa. Then, under film formation conditions below, a coating composed of an element for target A and N was fabricated, and the number of ions or atoms (monolayers) per 1 pulse supplied to substrate X was calculated. Based on this result, the number of pulses included in 1 pulse train for target A was adjusted such that the number of ions or atoms per 1 pulse train attained the value shown in Table 1.

Vacuum chamber12was then opened, substrate X was replaced with new substrate X, the number of ions or atoms (monolayers) per 1 pulse train for target B was calculated through operations the same as described above, and the number of pulses included in 1 pulse train was adjusted based on this result such that the number of ions or atoms per 1 pulse train for target B attained the value shown in Table 1.

Then, vacuum chamber12was again opened, substrate X was replaced with new substrate X, and a pressure in vacuum chamber12was lowered to 0.005 Pa while the substrate was heated to 450° C. Then, Ar gas was introduced, the pressure in vacuum chamber12was held at 0.8 Pa, and substrate X was cleaned with the use of an Ar ion source at a substrate bias voltage of −600 V for 30 minutes.

Then, the Ar gas was exhausted from vacuum chamber12, and thereafter each gas was introduced such that a partial pressure in vacuum chamber12was set to Ar:N2=0.4 Pa:0.2 Pa. Then, under film formation conditions below, electric power was supplied simultaneously to target A and target B (that is, electric power was supplied continuously to both of target A and target B), so that a solid solution layer (a thickness: 0.1 μm) as the underlying layer was formed on the surface of substrate X. This solid solution layer is composed of a solid solution composed of elements for target A and target B and N.

Pulse width of target A: 0.1 ms

Pulse width of target B: 0.1 ms

Pulsed electric power (targets A and B): 60 kW

Following the formation of the solid solution layer, electric power was alternately supplied to target B and target A under film formation conditions below (that is, starting with target B, electric power was intermittently supplied to target A and target B), to thereby form a domain structure layer (thickness: 3 μm) on a surface of the solid solution layer. The temperature of the substrate was maintained at 450° C. This domain structure layer was constituted of a second domain composed of an element for target A and N and a first domain composed of an element for target B and N. Tables 1 and 2 show features of each coating.

Pulse width of target A: 0.1 ms

Pulse width of target B: 0.1 ms

Pulsed electric power (targets A and B): 60 kW

Table 1 shows composition of target A and target B, the number of ions or atoms (monolayers) per 1 pulse train, and composition of the entire structure layer and the Al/Ti ratio in the entire structure layer. Composition and the Al/Ti ratio of the entire structure layer were measured with an X-ray photoelectron spectrometer. The structure layer in each of Examples 1 to 15 is the domain structure layer described above.

Table 2 shows composition and crystal structure of the first domain and the second domain. “c-NaCl” in the field of crystal structure means crystal structure of the cubic NaCl type, and “h-wurtzite” means crystal structure of a hexagonal wurtzite type.

Through the use of 3D atom probe tomography, the structured layer can be confirmed to consist of first domains and second domains whose compositions differ. Furthermore, that the structured layer consists of first domains and second domain whose compositions differ may also be confirmed using the following procedure. Firstly, a sample of the domain structure layer showing the cross-section in the in-plane direction as well as a sample of the domain structure layer showing the cross-section in the thickness direction were prepared. Then, using mechanical and ion milling, a measured portion of each sample was thinned to a thickness (the thickness in the direction normal to the cross section) from 5 to 20 nm. The samples were then imaged using a transmission electron microscope in the HAADF-STEM (High-angle Annular Dark Field Scanning Transmission Electron Microscopy) mode. The differing compositions of the first domains and the second domains could be confirmed by the differing contrast in the HAADF-STEM images.

The crystal structure of each domain could be confirmed using the nanobeam electron diffraction method of the transmission electron microscope. Furthermore, using a high resolution transmission electron microscope to image the above samples with atomic resolution, it could be confirmed that either of the first domain and the second domain had the cubic NaCl crystal structure. In case the first domain consists of the cubic NaCl crystal structure and the second domain consists of the hexagonal wurtzite crystal structure, the electron diffraction patterns of the cubic NaCl crystal structure and of the hexagonal wurtzite crystal structure were seen overlaid in the electron diffraction measurement. Due to this effect, the crystal structures could be confirmed by acquiring dark field images for the diffraction spots of the cubic NaCl crystal structure as well as the diffraction spots of the hexagonal wurtzite crystal structure and comparing the dark field images to the HAADF-STEM images.

The composition of the first domains and the second domains was analyzed using the line scan function of a transmission electron microscope equipped with an energy dispersive X-ray spectrometer.

Table 2 shows the average value of the size in the in-plane direction of the first domain and deviation (scatter) from the average value, the nearest neighbor distance in the in-plane direction of the first domain and deviation (scatter) from the average value, and the average value of the size in the direction of thickness and deviation (scatter) from the average value. For example, though scatter in size in the in-plane direction is denoted as “±24” in Example 1, this means that 95 or more first domains of 100 first domains observed in any cross-section in the in-plane direction have the size within the range of ±24% from the average value of the size of (100) first domains in the in-plane direction. Each size and each scatter were calculated with the use of a transmission electron microscope.

The average size in the in-plane direction of the above mentioned first domains as well as the relative scatter around this average value were evaluated as described below. Firstly, as described above, the samples were thinned to a thickness in the cross-section in the in-plane direction of 10 nm or smaller before being imaged using the HAADF-STEM mode of a transmission electron microscope. Depending on the size of the first domains, the image size was chosen to between 20 nm×20 nm and 50 nm×50 nm. The brightness and contrast were adjusted so as to obtain a clear contrast of the first domains and the second domains. Then, image analysis software (ImageJ) was used to determine the size and the number of first domains in the HAADF-STEM images and based on this histograms were constructed. Here, the border between first domains and second domains was determined visually, thereby also determining the virtual circumcircle of the first domains.

There have been cases when the above histograms have shown a plurality of peaks. The HAADF-STEM image being a transmission image, this is due to the imaging of two or more different first domains overlapping in the thickness direction of the sample. Imaging two or more different first domains overlapping in the thickness direction of the sample will yield a domain size larger than that of a single first domain, resulting in a histogram with a plurality of peaks.

Therefore, in a histogram with a plurality of peaks, the average size and scatter of the first domains must be determined using the smallest peak in order to extract information from single domains only. In case the peak that corresponds to the smallest measured domain size and the peak that corresponds to the second smallest measured domain size overlap, domains with sizes exceeding those in the valley between the two histogram peaks should be ignored when determining the average and scatter values.

For the same reason, when determining the average value of the nearest neighbor distance in the in-plane direction of the first domains as well as the scatter relative to this value, only single first domains should be used. Furthermore, as to the average size and the scatter relative to this value in the thickness direction, HAADF-STEM images of samples including the cross-section in the thickness direction were used to determine the height of the first domains in the thickness direction.

As it may happen that some of the first domains become truncated in the polishing step, when determining the average size of the first domains and the scatter relative to this value as well as the nearest neighbor distance and the scatter relative to this value from HAADF-STEM images, these first domains may appear smaller than their actual size.

Taking the background of the lighter domains to be 0% contrast and the typical contrast of the darker domains to be 100% contrast, if the first domains are lighter, such as in Examples 1-5, 7-11, 14, and 15, domains showing >50% of the contrast should be discarded, and if the first domains are darker, such as in Examples 6, 12, and 13, domains showing <50% of the contrast should be discarded before the image analysis.

In Comparative Examples 1 and 2, a coating was fabricated with a method the same as in Example 1 except that the number of ions or atoms per 1 pulse train was changed as shown in Table 1. In Comparative Example 3, a coating was fabricated with a method the same as in Example 3 except that electric power was simultaneously supplied to target A and target B. Tables 1 and 2 also show various features in Comparative Examples 1 to 3.

An indentation hardness of the structure layer of each obtained coating was measured by using a nanoindenter (“ENT-1100a” manufactured by Elionix Inc.) and pressing an indenter into the structure layer at a load of 1 g in the direction of normal to a surface of the coating. Table 3 shows results of a hardness of each coating in Examples 1 to 15 and Comparative Examples 1 to 3.

A coating according to each of Examples 1 to 15 and Comparative Examples 1 to 3 was fabricated on the surface of substrate Y with the method the same as above. Thus, an insert having a coating formed on the surface of the insert for milling was fabricated. Each obtained insert was subjected to a milling cutting test under conditions below to measure the wear width of a flank face of the insert. Table 3 shows results.

Depth of cut: 2.0 mm

Distance of cutting: 3600 mm

One insert was attached to the cutter.

Table 3 shows an indentation hardness of the structure layer in the field of “hardness” and a wear width of the coating (the flank face) of the insert before and after the milling cutting test in the field of “wear width”. A higher value for the hardness means a higher hardness of the coating, and a smaller wear width means that an amount of wear of the coating is small and wear resistance is high.

Referring to Tables 1 to 3, the structure layer in each of Examples 1 to 15 had the first domain satisfying all of (1) to (4) above. Namely, the structure layer in each of Examples 1 to 15 was a domain structure layer. The coating having this domain structure layer had a very high hardness not lower than 5500 mgf/μm2. Each coating had a wear width not greater than 0.1 mm and had high wear resistance.

In contrast, the structure layer in each of Comparative Examples 1 and 2 did not have the first domain satisfying all of (1) to (4) above. The coating having this structure layer was lower in hardness and wear resistance than the coatings in Examples 1 to 15.

It was considered in this connection that, in Comparative Example 1, the size of the first domain was smaller than (1), and therefore the structure layer had a physical property like a solid solution as a whole and consequently the hardness was substantially as high as that of the conventional AlTiN solid solution. It was considered that, in Comparative Example 2, the size of the first domain was greater than (1), and therefore the nanometer size effect and accumulation of strain energy were not obtained or were insufficient. The second domain having a crystal structure of the hexagonal wurtzite type also seemed to be connected to the inferior characteristics of Comparative Example 2.

In Comparative Example 3, the pulse current was not alternately supplied to targets A and B, instead the pulse current was simultaneously supplied, and hence Comparative Example 3 did not have a structure having a first domain and a second domain, and a coating composed of Al0.6Ti0.4N of the cubic NaCl type was formed. The coating characteristics were equivalent to those of the coating in Comparative Example 1.

In Examples in Study 2, the domain structure layer as shown inFIGS. 4 and 5was fabricated.

[Examples 16 to 29 and Comparative Examples 4 and 5]

A coating was fabricated on the surface of substrate X and substrate Y in Examples 16 to 29 and Comparative Examples 4 and 5 with the method the same as in Examples in Study 1 by forming target A and target B of elements shown in Table 4 and adjusting the number of ions or atoms (monolayers) per 1 pulse train as shown in Table 4. The number of ions or atoms per 1 pulse train for target A and target B was controlled by controlling the number of pulses included in 1 pulse train as a film formation condition. Tables 4 and 5 show features of each coating and Table 6 shows physical properties of each coating. The methods for measuring each property are the same as in Study 1.

Referring to Tables 4 to 6, the structure layer in each of Examples 16 to 29 had the first domain satisfying all of (1) to (4) above. Namely, the structure layer in each of Examples 16 to 29 was the domain structure layer described above. In the structure layer in each of Examples 16 to 29, the first domain and the second domain were equal to each other in size. This is because the number of ions or atoms per 1 pulse train for target B supplied for fabrication of the first domain was close to the number of ions or atoms per 1 pulse train for target A supplied for fabrication of the second domain. It was confirmed with a transmission electron microscope that the structure layer in each of Examples 16 to 29 had a structure in which the first domain and the second domain were alternately stacked as shown inFIGS. 4 and 5.

Referring to Table 6, the coating in each of Examples 16 to 29 had a very high hardness not lower than 5500 mgf/μm2. Each coating had a wear width not greater than 0.1 mm and had high wear resistance.

In contrast, the structure layer in each of Comparative Examples 4 and 5 did not have the first domain satisfying all of (1) to (4) above. The coating having this structure layer was lower in hardness and wear resistance than the coatings in Examples 16 to 29.

It was considered in this connection that, in Comparative Example 4, the size of the first domain was smaller than (1), and therefore the structure layer had a physical property like a solid solution as a whole and consequently the hardness was substantially as high as that of the conventional AlTiN solid solution. It was considered that, in Comparative Example 5, the size of the first domain was greater than (1), and therefore the nanometer size effect and accumulation of strain energy were not obtained or were insufficient. The second domain having crystal structure of the hexagonal wurtzite type seemed to also be relevant to low characteristics of Comparative Example 5.

REFERENCE SIGNS LIST