JPH1171197A - Surface-reinforced diamond and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a diamond crystal increased in compression crushing strength by heterogeneously introducing a dopant element into the diamond crystal so that the concentration of the dopant element impurity increases between the center and surface of the diamond crystal or so that the concentration gradient of a positive dopant element exists in the diamond crystal. SOLUTION: This method for producing the diamond crystal improved in the compression crushing strength comprises a process for producing a diamond crystal having three-dimensional small surfaces and having a diameter of maximally about 2 cm, and a process for introducing a dopant element impurity selected from the group consisting of B, N, H, Li, Ni, Co, Na, K, Al, P, O and their mixtures into the outermost surfaces of the diamond crystal in an amount of about 10-10,000 ppm and in a depth of about 0.25-50 μm from the outermost surfaces to expand the diamond lattices with the introduced dopant element and generate a compression stress of maximally about 5,000 mega pascal in the tangent direction on at least one small surface of the crystal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to diamond compositions and products manufactured at elevated temperatures and pressures and exhibiting an increase in compressive fracture strength (CFS). More particularly, the present invention relates to increasing the fracture strength and performance by heterogeneously introducing certain elements towards and / or near the outer surface of the diamond crystal. The present invention also relates to a method for producing diamond crystals surface-enriched and strengthened with elements that increase the compressive fracture strength.

[0002]

Background of the Invention Diamond crystals are commonly used for the manufacture of diamond tools such as grinding wheels, dressing or reshaping tools for grinding wheels, and saw blades. In order to function effectively in the above applications, it is desirable that the diamond crystal has the highest strength including the compressive fracture strength. Compressive fracture strength is a fundamental property for diamond crystals, which is often correlated with the performance of diamond in such applications. In particular, the crystal fracture strength as measured by a roll crusher correlates with the performance of the diamond in applications such as quarrying.

[0003] Compressive fracture strength can be measured by compressive fracture of a population of diamond grains before and after treatment. In the roll crusher method,
An apparatus is used which comprises a pair of hard reversing rollers and means for measuring at the moment of the grain breakage the compressive force exerted by the rollers on the grains passing between them. That is, by measuring the movement of one roller using a suitable transducer (eg, a linear differential transformer), a voltage signal proportional to the wobble of the roller and thus to the compressive force applied to the diamond grains is generated. Generated.

[0004] Diamond is a brittle solid that breaks down by cracking. Its elastic constant (the Young's modulus of diamond) is high, 1.143 × 10 ¹² gigapascal. The ductile-brittle transition temperature of diamond is about 1150 ° C
It is. For such applications, diamond is used in brittle fracture areas from room temperature to about 1000 ° C.

[0005] Impurity atoms dissolved in diamond crystals (eg, boron, nitrogen and hydrogen) create stress due to the significant lattice expansion they cause. For example,
Dissolved nitrogen, boron and hydrogen atoms expand the surrounding diamond lattice by 40%, 33.7% and 31%, respectively. If the distribution of dissolved nitrogen, boron or hydrogen is uniform in the diamond crystal, the lattice will only expand uniformly and no long-range stress will occur. However, non-uniform distribution of dissolved nitrogen, boron or hydrogen will result in non-uniform strain in the diamond. Such non-uniform strain results in large long-range stresses.

At present, the impurity concentration in a synthetic diamond crystal decreases as the radius of the diamond crystal increases. There are several reasons for this. For example, in a high-temperature and high-pressure method (HTHP method) used for synthesizing a diamond crystal, the nitrogen concentration in the melt decreases with time because the growing diamond absorbs nitrogen from the melt. (A melt is a molten metal catalyst / solvent that serves to transport carbon to diamond crystals grown from a graphite source under high temperature and pressure.) Another reason is that the growth rate of diamond crystals is Decrease with increasing, and impurity incorporation decreases with decreasing growth rate. Furthermore, the impurity concentration in the diamond crystal may depend on the growth sector.
By growth sector is meant the crystallographic direction in which growth is (or is occurring), and the area in the crystal where growth has occurred along the same direction.

[0007] Such an impurity concentration (ie, a negative concentration gradient) that decreases with increasing radius results in tangential tensile stress on the diamond surface. Since diamond is a brittle solid, such tangential tensile stress reduces its compressive fracture strength. As technology advances, the next generation of diamonds will require higher strength. For this reason, there is a demand for a diamond crystal having an increased compressive fracture strength. There is also a need for a method for producing such diamond crystals.

[0008]

SUMMARY OF THE INVENTION In accordance with the above needs, the present invention provides a method for compressing a diamond crystal in which a tangential compressive stress is generated on the surface of the diamond crystal by increasing the concentration of dopant element impurities between the center and the surface of the diamond crystal. Provided is an element-introduced diamond crystal having an increased fracture strength. The present invention also provides an element having increased compressive fracture strength, characterized in that a tangential compressive stress is generated on the surface of a diamond crystal by the presence of a positive dopant element concentration gradient between the center and the surface of the diamond crystal. A diamond crystal comprising the introduced diamond crystal is also provided. Illustrative examples of dopant elements include, but are not limited to, boron, nitrogen, hydrogen, lithium, nickel, cobalt, sodium, potassium, aluminum, phosphorus, and oxygen. In this specification, a dopant element may be called an impurity atom, an impurity element, or an impurity concentration. Other examples of impurities are interstitial atoms (including interstitial carbon atoms) and vacancies in the crystal structure.

According to another aspect of the present invention, there is provided a product (eg, a saw, a grinding wheel and a stone cutting tool) comprising the diamond crystal of the present invention. According to still another aspect of the present invention, there is provided a method for producing a diamond crystal having an increased compressive fracture strength. In order to create a non-uniform impurity concentration or distribution, different amounts of impurities are introduced into different crystal growth sectors or the growth conditions are temporarily changed to create a radial impurity concentration gradient. I just need to do it. Since the distribution of impurities in the diamond is controlled by the growth conditions, the diamond of the present invention can be enhanced by using different growth conditions.

One object of the present invention is to increase the compressive fracture strength of synthetic diamond, such as diamond grit used in tools. It is another object of the present invention to increase the compressive fracture strength of synthetic diamond by increasing the concentration of impurity atoms in or near the surface of the diamond.

Yet another object of the present invention is to provide a method for increasing the compressive fracture strength of synthetic diamond crystals. Other objects of the present invention will become apparent from reading the following description with reference to the accompanying drawings.

[0012]

DETAILED DESCRIPTION OF THE INVENTION The positive radial concentration gradient (ie, increasing concentration with increasing radius) of the impurity element causing the expansion of the diamond lattice creates a tangential compressive stress on the diamond surface, thereby strengthening the tempered glass. It has been found that the diamond is strengthened in the same way as. Ideal examples of such impurities or dopant elements include boron, nitrogen, hydrogen, lithium, nickel, cobalt, sodium, potassium, aluminum, phosphorus and oxygen. In addition, interstitial atoms (including interstitial carbon atoms) and vacancies in the crystal can also be regarded as impurities.
Boron, hydrogen and nitrogen can be easily introduced into the diamond during growth. Boron has the additional advantage of improving oxidation resistance. In the present invention, nitrogen can be used at a local concentration of about 0 to 1000 ppm, hydrogen can be used at a concentration of about 0 to 1000 ppm, and boron, nitrogen and hydrogen can be used at a concentration of about 0 to 10000 ppm. Mixtures can be used.

The natural isotope abundance diamond is 2
Species carbon isotopes, ie 98.9%¹²C and 1.1
%of¹³Consists of C. More than 1.1%¹³C isotope
Impure impurities in natural isotope abundance diamonds
Can be considered a thing.¹³C isotope is diamond case
Causes child to contract. Therefore, in the diamond crystal ¹³
Creating a negative concentration gradient of C isotope
The tangential compression stress on the surface causes the diamond
Is strengthened.

The diamond crystals used in the present invention are synthetic diamonds, which have a pressure in excess of about 45 kbar and a temperature of 1200 ° C. in the PT region of carbon such that the diamond forms a thermodynamically favorable phase. It is customarily produced by a high-temperature and high-pressure method comprising a combination of a temperature higher than Thereby, various diamond crystals can be manufactured. The present invention also provides a method for producing diamond by chemical vapor deposition (CV).
D method) can also be used.

It should be understood that the method of the present invention is also suitable for use with a variety of raw diamond crystals (hereinafter sometimes referred to as "abrasive particles"). Commercially available diamond abrasive particles of various strengths and toughness are available. One factor that can be used to control the properties of diamond abrasive particles is to vary the growth rate during manufacture. Perfect diamond crystals tend to show more regular cleavage and wear. Crystal defects, twinning and the like may occur, but these may result in changes in the properties of the diamond crystal.

The raw diamond crystal preferably has an uncoated surface. The method of the present invention is used to increase the compressive fracture strength of diamond prior to attachment to a tool. For example, the diamond crystal of the present invention is at least nominally a single crystal,
It may have one or more twin planes. It is a three-dimensional (3-D) faceted diamond crystal. The diamond crystals of the present invention have a diameter of up to about 2 centimeters (2 cm). Such diamond crystals have an impurity concentration that increases with increasing radius, thereby creating a tangential compressive stress on the surface of the crystal. According to one embodiment of the present invention, the impurity concentration exhibits a maximum at the surface of the crystal, decreases with decreasing radius, and is at least about 5 mm from the surface of the diamond crystal.
The local minimum is reached at a position below the micrometer. 0
The impurity concentration in the outermost layer portion of ５０50 micrometers is in the range of about 10 to 10000 ppm. The present invention also provides a three-dimensional diamond in which one component (ie, the tangential component) that constitutes the stress state of some facets of the crystal is a compressive stress in the range of about 10 to 5000 megapascals (MPa). Including crystals. The tangential compressive surface stress is up to 5000 MPa. The surface stress in the radial direction is always 0 MPa. Also, the tangential compressive stress applied to the diamond crystal of the present invention can be superimposed on the existing tangential tensile stress applied to the crystal. The range of tangential compressive stress superimposed on existing tangential tensile stress in diamond crystals is about 10
５5000 MPa.

The invention will now be demonstrated using boron, nitrogen and hydrogen. Due to the extremely high binding energy and atomic density of diamond, boron, nitrogen and hydrogen can dissolve to a significant extent in diamond.
However, even in the case of boron, nitrogen and hydrogen, the solubility is limited to less than a few hundred ppm for nitrogen and to less than 1% for boron. Despite the limited solubility in diamond, dissolved nitrogen, boron or hydrogen can generate stress. The reason is that they cause a large lattice expansion. If the distribution of nitrogen, boron or hydrogen is not uniform, uneven strain will occur in the diamond. These non-uniform strains result in large long-range stresses. The approximate value of the magnitude of the stress can be obtained by multiplying the Young's modulus of the diamond (10 ¹² Pascal) by the strain generated by the impurity.
Regarding nitrogen in diamond, the maximum strain occurs at the maximum solubility of nitrogen in diamond, which is about 4.5 × 10 ⁻⁴ . Such an amount of strain produces a stress of the order of 5 × 10 ⁸ Pascal. This is an experimental diamond crush strength of 0.2 → 2 ×
It is comparable to 10 ¹⁰ Pascal. That is, the stress caused by the uneven distribution of nitrogen in diamond can be as high as 2.5-25% of the crush strength of diamond.

[0018] The non-uniform distribution of nitrogen or boron in diamond can result from two different sources. The first cause of the uneven distribution of nitrogen or boron is:
Distribution coefficient of boron and nitrogen depending on the crystal growth surface of diamond (ratio of solubility of impurities in liquid phase and solid phase)
Is to change. For example, nitrogen uptake is (1
11) Most often in the growth sector, slightly less in the (100) growth sector, and even less in the (113) and (110) growth sectors. Normally, the concentration of nitrogen reaches several hundred ppm. If a cross-section is made through the center of a typical synthetic diamond crystal, such a cross-section will typically show (111) and (100) growth sectors. The large lattice expansion caused by nitrogen in (111) growth sectors is incompatible with the smaller lattice expansion in (100) growth sectors with low nitrogen concentrations. These incompatible strains create tensile stress in the (100) growth sector and compressive stress in the (111) growth sector.
Tensile stresses in the (100) growth sector can weaken diamond crystals during mechanical grinding and sawing applications.

Boron also has a distribution coefficient that depends on the crystal growth surface of diamond. In the high boron concentration regions of interest, gem diamonds and CVD
In any case of diamond, boron is (111)
Priority in the growth sector. Boron expands the diamond lattice by as much as 33.7%, and its solubility in diamond can reach as high as 0.9%. Therefore, the incorporation of boron also causes stress in the diamond crystal as in the case of nitrogen. For boron in diamond, the largest strain occurs at the maximum solubility of boron in diamond, which is of the order of 3 × 10 ⁻³ . Such a strain amount is 3.4 ×
Stresses of the order of 10 ⁹ Pascals can be produced, which is the experimental diamond crush strength of 0.2 → 2 × 10 ¹⁰
It is remarkable compared to Pascal. That is, the stress caused by the uneven distribution of boron in diamond can be as high as 17-170% of the crush strength of diamond.

A second cause of the non-uniform distribution of boron or nitrogen in the diamond crystal is a change in growth cell conditions that occurs during crystal growth. Temporary changes in pressure, temperature, impurity concentration in the melt, and / or crystal growth rate may change the amount of boron or nitrogen incorporated into the crystal as a function of time. The non-uniformity resulting from such causes creates a radial impurity concentration gradient in the crystal. The stresses resulting from such radial concentration gradients suggest that diamond crystals can be strengthened by appropriately changing the growth cell conditions during crystal growth.

In order to obtain an analytical answer, an equiaxed diamond crystal having a faceted shape similar to a soccer ball will be approximated by a diamond sphere. In fact, this is a reasonably good approximation to the finest diamond grit with a shape close to a sphere. Furthermore, let us consider in detail only the case where the impurity concentration gradient is exclusively a function of the radial distance R from the center of the diamond crystal.

The mechanical balance of any small spherical element requires a balance between the differential radial forces on the opposing faces of the element. If the angle formed by the spherical element with respect to the two tangential directions is φ, the radial stress is σ _r , the tangential stress is σ _t , and the radius of the spherical element is R, then to balance the radial force, An expression is required.

[0023]

(Equation 1)

When dR and dφ approach zero, the above equation reduces to equation (2).

[0025]

(Equation 2)

Strain in diamond is caused by stress as well as by the presence of impurities that cause the diamond lattice to expand or contract. The strain ε _i caused by impurities is given by the following equation.

[0027]

(Equation 3)

Where α is a constant depending on the impurity, and C is the atomic concentration of the impurity in the lattice. For nitrogen, boron and hydrogen in diamond, α is 0.4, 0.337 and 0.31, respectively. The stress that develops in diamond is related to strain by Hooke's law.

[0029]

(Equation 4)

Where ν is the Poisson's ratio, E is the Young's modulus of the diamond, ε _r is the radial strain, and ε _t is the tangential strain. Impurity strain ε in equations 4a and 4b
_i is subtracted from the respective elastic strain epsilon _r and epsilon _t. This is because impurity strain is not maintained by elastic stress, but rather is created by a change in lattice size caused by an oversized or undersized impurity atom. The generally accepted Poisson's ratio value for diamond is 0.07. The Young's modulus of the diamond averaged in the orientation direction is 1.143 × 10 ¹² N / m ² .

Equations 4a and 4b can be solved for radial stress σ _r and tangential stress σ _t .

[0032]

(Equation 5)

Radial strain ε _r and tangential stress ε _t
Can be represented by a radial displacement u.

[0034]

(Equation 6)

Substituting equations 6a and 6b into equations 5a and 5b and then into equation 2 gives

[0036]

(Equation 7)

Or

[0038]

(Equation 8)

Which is the Laplace equation in spherical coordinates. The displacement u (R) is obtained by integrating the equation (7b).

[0040]

(Equation 9)

Where A and B are integration constants and C (r)
Is the concentration of nitrogen or boron in diamond as a function of radius. For a solid diamond sphere, due to symmetry, the displacement u → 0 when R → 0, and therefore B = 0
Becomes Rearranging Equation 8 into Equations 6a, 6b, 5a and 5b yields the radial and tangential stresses in diamond.

[0042]

(Equation 10)

The radial stress σ _{r at} the external free surface (R = S) of diamond must be zero (R =
Σ _r = 0 at S). Therefore, the constant A is obtained as follows from Expression 9a.

[0044]

[Equation 11]

Where S is the radius of the diamond. If the constant A is introduced into the expressions 9a and 9b, the following expression is obtained.

[0046]

(Equation 12)

Equation 10a gives an interesting physical interpretation.
That is, the radial stress is obtained by calculating the average impurity concentration in the entire diamond (the first integral in the expression 10a) and the average impurity concentration in the diamond portion up to the radius R to be considered (the second integral in the expression 10a). It is proportional to the difference. When the average impurity concentration up to the radius R is higher than the average impurity concentration in the entire diamond, the radial stress becomes a compressive stress with respect to the impurity that expands the lattice.

The three cases demonstrated here are the case where the impurity concentration increases along the radial direction, the case where the impurity concentration decreases along the radial direction, and the case where a thin shell of impurities exists. These different impurity distributions create different stress states in the diamond crystal and may strengthen the crystal. The following considerations relate to the stresses that occur in diamond crystals having a particular impurity distribution along the radial direction.

The first case to be considered is when the concentration of the impurity increases linearly with radius, ie, in the range from r = 0 to r = S, C (r) = _Cor / S This is the case. Substitution into equations 10a and 10b yields:

[0050]

(Equation 13)

The following is the case when α is positive, ie when the impurities cause the diamond lattice to expand. Radial stress is tensile (ie, positive), which has a maximum at the center of the diamond,
And it decreases linearly until it becomes zero on the surface of the diamond. The tangential stress also has a maximum at the center of the diamond, where it is the tensile stress. However, at a radius equal to 2/3 of the total radius of the diamond, the tangential stress is zero. Thereafter, the tangential stress becomes a compressive stress (ie, negative) and reaches maximum compression at the diamond surface. Such tangential compressive stresses on the surface of the diamond make the diamond less susceptible to fracture and cracking in tension, thus strengthening the diamond.

If the impurities cause the diamond lattice to shrink (α <0), the signs of the stresses are just reversed. That is, the radial stress is a compressive stress, which has a maximum absolute value at the center of the diamond and decreases linearly to zero at the diamond surface. Tangential stress is also a compressive stress and has a maximum absolute value at the center of the crystal. Of the full radius of the diamond
At a radius equal to 2/3, the tangential stress changes from compressive to tensile, and reaches a maximum at the diamond surface. Such tangential tensile stresses on the surface of the diamond can weaken the diamond by making it more susceptible to fracture in tension and cracking due to small scratches.

Further, since the shear yield strength rapidly decreases with an increase in temperature, the shear stress may cause diamond breakage, especially at high temperatures. Maximum shear stress σ _s in diamond with radial concentration gradient
Is 1/2 of the difference between the radial stress σ _r and the tangential stress σ _t
Given by

[0054]

[Equation 14]

It should be noted that the shear stress increases with radius and reaches a maximum at the surface of the diamond.
When the shear stress exceeds the shear yield stress K of the diamond as shown in FIG. 3, the diamond breaks, but the shear yield stress depends on the crystal direction, crystal plane and temperature. FIG. 5 plots the concentration of the impurity and the radial, tangential and shear stress versus radius for the case where an impurity such as boron or nitrogen expands the diamond lattice.

The second case to consider (demonstrating the prior art) is that if the concentration of the impurity decreases linearly with radius, that is, C (r) in the range from r = 0 to r = S ) = C _o (1−r / S). Substitution into equations 10a and 10b yields:

[0057]

(Equation 15)

The absolute value of the stress does not change as compared with the case described above, and only the signs thereof change. Concentration formula C (r) = C
The first term in _o (1-r / S) (i.e., C _o ) represents a uniform concentration term, so it does not create stress. -C _o r / S section left is Apart from code,
The same density distribution as in the case described above is shown. In the normal case for diamonds (ie, when α is positive),
Impurities cause the expansion of the diamond lattice. Radial stress is a compressive stress (ie, negative), which exhibits a maximum absolute value at the center of the diamond and decreases linearly to zero at the surface of the diamond. Tangential stress also has a maximum absolute value at the center of the diamond, where it is the compressive stress. However, at a radius equal to 2/3 of the total radius of the diamond, the tangential stress is zero. Thereafter, the tangential stress becomes tensile (ie, positive) and reaches a maximum at the diamond surface. Such tangential tensile stresses on the surface of the diamond predispose to fracture and cracking of the diamond in tension, thus weakening the diamond.

If the impurities cause the diamond lattice to shrink (α <0), the signs of the stresses are just reversed. That is, the radial stress is a tensile stress, which has a maximum at the center of the diamond and decreases linearly to zero at the surface of the diamond. Tangential stress is also tensile stress and has a maximum at the center of the crystal. At a radius equal to 2/3 of the total radius of the diamond, the tangential stress changes from tensile to compressive, and reaches a maximum absolute value at the surface of the diamond. Such tangential compressive stresses on the diamond surface may strengthen the diamond by making it less prone to fracture in tension and cracks due to surface defects and scratches.

When the expansion due to impurities is changed to a contraction, the signs of the radial stress and the tangential stress are only reversed, so that the shear stress derived from the difference between the radial stress and the tangential stress is as described above. Same as case. FIG. 4 (prior art), which shows the case of a concentration gradient decreasing along the radial direction, shows the concentration of the impurities and the radial, tangential and shear stresses for the case where impurities such as boron or nitrogen expand the diamond lattice. Stress is plotted against radius.

A third case to consider is when the impurities are present in a thin shell of thickness x adjacent to the outer surface of the diamond. Such cases may be caused by boron, nitrogen or hydrogen diffusing into the thin shell of the diamond crystal as a result of exposure of the diamond to boron, nitrogen or hydrogen at elevated temperatures. Similarly, when diamond is exposed to a high energy plasma containing hydrogen, nitrogen or boron, such impurities may be implanted in a thin shell adjacent to the outer surface of the diamond.

The distribution of such impurities is mathematically described as follows. That is, assuming that the thickness of the outer shell is x, when 0 <R <(S−x), C = 0 and (S−
x) C = _{Co when} <R <S. Equations 10a and 1
0b, the thin shell condition (ie, x
The following equation is obtained for ≪S).

[0063]

(Equation 16)

If α is positive, that is, if the impurities cause the diamond lattice to expand, the radial stress in the inner area of the diamond without impurities is a constant small tensile stress. Radial stress decreases rapidly as it traverses the outer shell of the impurity and reaches zero at the diamond surface, as expected. In the inner area of the diamond, which does not contain impurities, the tangential stress is also a low tensile stress. However, the tangential stress changes to a large compressive stress on the surface of the diamond. Such a state of compressive stress on the diamond surface makes the diamond less susceptible to fracture and cracking in tension, thereby strengthening the diamond. FIG. 6 shows an impurity concentration distribution and a stress distribution with respect to a diamond crystal having a thin outer shell on the outer surface.

If the impurities cause the diamond lattice to shrink (α <0), the signs of the stresses are just reversed. That is, in a diamond containing a uniform level of impurities, if the impurities are lost from the thin outer shell adjacent to the outer surface (eg, due to the diffused release of impurities after crystal growth), the absolute value of the resulting stress is given by Equations 14a through 14a. 1
Same as that represented by 4d, but the signs of all stresses are reversed.

The shear stress σ _s created by the thin shell of impurities is given by the difference between radial and tangential stress in diamond.

[0067]

[Equation 17]

In the region containing no impurities, the shear stress is zero due to the existence of a pure hydrostatic tension state. The shear stress increases as it passes through the thin spherical shell of impurities and reaches a maximum at the diamond surface. The rate of linear crystal growth typically decreases during the growth process, and impurities in the growth medium are typically incorporated into the diamond throughout the operation. In one method of the present invention using a high-temperature, high-pressure method to produce diamond crystals, a source of powdered boron [eg, boron carbide (B
₄ C), boron nitride (BN) or elemental boron] is encapsulated by carbon, and is added to the growth cell. Encapsulation of the powdered boron source is achieved by CVD by thermal decomposition of methane, coating and drying with colloidal graphite, sputtering, and the like. In the initial stage of diamond growth under high temperature and pressure, boron cannot be used for growing diamond crystals. As the growth progresses, the carbon coating dissolves in the metal solvent / catalyst,
Finally, the boron source is exposed. At this point, the boron will begin to dissolve in the solvent / catalyst and will be incorporated into the outer portion of the diamond crystal. The boron level and thickness of the boron-introduced layer can be controlled by varying the concentration and size of the boron-containing particles in the growth medium and the thickness of the carbon coating.

Another method is to produce a crystal with a positive radial nitrogen concentration gradient by using a nitrogen source (eg, Fe _x N) encapsulated with carbon. If desired, the use of getters in the growth medium, the use of low porosity cores, or the elimination of air from the growth cell immediately prior to growth can also reduce the nitrogen level in the center of the crystal.

Another alternative in accordance with the present invention is to form a boron or nitrogen rich layer on lightly doped or undoped diamond crystals grown in a conventional manner. This can be achieved by using a non-impurity-introduced crystal as a seed crystal in a second high-temperature, high-pressure operation in which the growth medium contains a sufficient amount of boron, nitrogen or both. An impurity-introduced epitaxial layer may be formed thereon. In this alternative method, a second growth operation is required. However, the number of diamond crystals contained in the growth cell can be higher than usual, since only a thin layer of about 0.5 to about 50 micrometers in thickness is required.

Still another method according to the present invention comprises
This is to diffuse nitrogen or hydrogen into the diamond crystal. Other impurities (eg, oxygen, lithium, sodium, phosphorus, aluminum, nickel, cobalt or mixtures thereof) can also be diffused.
The impurity concentration is about 0.25 to 50 from the diamond surface.
About 10 to 10 at a depth in the range of micrometers
It is sufficient if it is 000 ppm. The preferred depth is about 0.3.
25 to 10 micrometers. In order to diffuse the impurities, exposure to a plasma containing hydrogen and / or a gas containing nitrogen and boron may be performed.
The diffusion distance of vacancies in diamond is 1100 ° C.
Reported about 100 microns in one week annealing time. Boron or nitrogen diffusion is slower, but still fast enough to achieve strengthening of the diamond crystal when viewed on a suitable time scale.
The presence of hydrogen in the gas phase may help stabilize the diamond crystals, thereby allowing for long-term processing at temperatures above 1000 ° C. without significant graphitization.

Another method for diffusing atoms into diamond is at about 700 ° -1400 ° C. for about 1 hour.
Exposing the diamond to a gas containing impurity atoms (e.g., hydrogen, boron or nitrogen) for a period of time from 0 seconds to about 100 hours. Yet another method for diffusing impurity atoms to the diamond surface is to coat the surface of the diamond crystal with a solid or liquid film containing hydrogen, boron or nitrogen and then diffuse the impurity atoms to the outer surface of the diamond. The coated crystals are heat treated at a temperature of about 700-1400 ° C. for a sufficient time. Such time is about 1
It can range from 0 seconds to about 100 hours.

In yet another method, undoped or lightly doped crystals are placed in a chemical vapor deposition (CVD) apparatus, and boron or nitrogen is added to the usual raw reactants (ie, hydrogen, methane and oxygen). The impurity-introduced epitaxial layer can be grown while adding the contained gas. When grown at a support temperature approximately equal to or less than 740 ° C., the doped diamond layer is subjected to an additional intrinsic compressive stress, thus achieving the objectives of the present invention without doping. it can. The thickness of the impurity-introduced diamond layer is about 0.25 to 50 micrometers, and the concentration of the introduced impurity in the outer layer is higher than about 10 to 10000 ppm, which is the impurity concentration on the outermost surface of the underlying diamond crystal. .

One method for generating stress in a diamond crystal depending on the crystal growth conditions is to vary the pressure and temperature in the cell over time. With proper control, such fluctuations can create a positive radial impurity concentration gradient in the diamond crystal. For impurities such as boron, nitrogen and hydrogen that expand the diamond lattice, a positive radial concentration gradient creates a compressive stress state on the surface of the diamond, which strengthens the diamond and makes it less prone to cracking in tension. . Increasing the diamond crystal growth rate as a function of radius (time) can also increase the uptake or concentration of impurities in the diamond. Furthermore, the impurity concentration can be increased with respect to the radius by increasing the impurity concentration in the melt with time.

Yet another method for obtaining CFS-enhanced diamond crystals in accordance with the present invention is the high temperature high pressure method (H
This is to expand the vicinity of the outer surface of the diamond crystal grown by the THP method). For this purpose, radiation damage may be caused by ion or electron impact.
Such radiation damage causes expansion of the diamond lattice by creating interstitial atoms and vacancies. In this case, the interstitial atoms may consist of carbon atoms or impurity atoms or a combination thereof. Such a thin shell of lattice expanding impurities creates a compressive stress state on the surface of the diamond crystal, thereby making cracking less likely. Unlike the strengthening achieved by a radial concentration gradient, the protective effect of such a thin compressed shell is lost when the thin shell is punctured by wear of the crystals during use.
Three-dimensional crystals that have undergone ion implantation or radiation damage to their surface may contain impurity atoms, interstitial atoms, and vacancies at concentrations in the range of about 1 to 10,000 ppm.

Other methods for introducing impurities into the outermost surface of the diamond crystal include a method of introducing impurities into layers by hydrothermal growth, electrochemical growth, liquid-phase solid source growth, or molten salt growth. . Table 1 below demonstrates the invention and shows the effect of nitrogen, boron and hydrogen impurity concentrations on the compressive fracture strength (CFS) on the outer surface of the diamond crystal.

[0077]

[Table 1]Table 1 Non-uniform impurity concentration near the surface Increases the strength of diamond crystals Maximum tangential direction Maximum tangential direction Increase in strength Increase in strength Impurity Maximum concentration Stress (gradient) Stress (thin layer) Rate (gradient) Rate (thin layer)  [ppm] [Mechanical skull] [Mechanical skull]     Nitrogen 1,000 123 492 7.7% 30.8% Boron 10,000 1,040 4,150 65% 259% Hydrogen 1,000 95 381 5.9% 23.8% 40/45 MBS Diamond strength-1.6 gigapaths
Cal (10⁹Pascal) E₀= 1.143 × 10¹²Pascal α_N= 0.4 α_B= 0.337 α_H= 0.31 ν = 0.07 1−ν = 0.93 E₀/(1.ν)=1.23×10¹²Pascal E₀/4(1-ν)=3.07×10¹²Pascal The following example is intended to further illustrate the invention.
You.Example 1 Starting materials are prepared by high-temperature and high-pressure methods known to those skilled in the art.
Manufactured, commercially available with a particle size of 45/50 mesh
GE Superabrasives
s) MBS970 crystal (General Electric
Product). Nitrogen in raw crystals
Distribution produces a tangential tensile stress of about 50 MPa on the surface.
, Thereby weakening the crystal. Untreated crystal pressure
The crimp strength (CFS) is measured by a roll crusher
It was 63.5 pounds (lbs).

The concentration of nitrogen (N) on the (100) crystal plane was measured by secondary ion mass spectrometry (SIMS) and found to be 4.2 ppm (not an absolute value).
Further, the concentration of nitrogen (N) on the (111)
It was 29.5 ppm (not an absolute value) as measured by IMS. The SIMS signal was converted to concentration using an ion-implanted diamond standard sample. However, ion implantation can cause grating damage and affect sensitivity. Combustion analysis of whole crystal (LE
When the nitrogen concentration obtained by CO) and SIMS data are compared, the concentration measured by SIMS is 2 to 2.
It turns out that it is four times too low.

Next, microwave plasma chemical vapor deposition (C
VD) A diamond crystal was placed on a support in the apparatus. The processing gas is hydrogen (H ₂ ) of 1,000 sccm,
Consisted 5sccm methane and 10sccm of nitrogen (N _2). The pressure was about 70 torr, the substrate temperature was about 700-750 ° C, and the microwave power was about 3 kilowatts. In this way, the diamond crystal was coated for 3 hours. After the growth of the film was interrupted and the crystals were rolled, the film was repeatedly grown for another 3 hours to obtain a uniform film.

The thickness of the nitrogen-introduced CVD diamond layer was about 5 micrometers. The compression fracture strength (CFS) of the coated crystals was measured by a roll crusher apparatus to be 65.9 pounds, indicating a 3.8% increase. The nitrogen concentration in the diamond layer on the (100) plane of the coated crystal was measured by SIMS and found to be 62.5 ppm (not an absolute value). The nitrogen (N) concentration on the (111) plane of the coated crystal was measured by SIMS.
It was 5 ppm.

As predicted from the theoretical model of the present invention using the average surface concentration measured by SIMS, such coatings produce a tangential compressive stress of 26.6 MPa, and the resulting increase in fracture strength is 1. 7%. However, if the surface nitrogen concentration measured by SIMS is corrected to 2 to 4 times as described above and considered, the predicted increase in fracture strength is 3.4 to 6.8%.
This is in good agreement with the measured value of 3.8%.

Note that the obtained strengthening effect is slightly lower than the calculated value due to pocking, surface roughness (see FIGS. 7 and 8), and unevenness of the coating layer (see FIG. 9). Example 2 The starting material was a commercially available GE Super Abrasives MBS970 crystal with a particle size of 45/50 mesh (a product of General Electric Company) manufactured by a high-temperature high-pressure method known to those skilled in the art. . The distribution of nitrogen in the untreated crystals is approximately 50
A tangential tensile stress of MPa is created, thereby weakening the crystal. When the compressive fracture strength (CFS) of the untreated crystal was measured by a roll crusher, it was 7
0.6 pounds.

Next, microwave plasma chemical vapor deposition (C
VD) A diamond crystal was placed on a support in the apparatus. The processing gas consisted of 900 sccm of hydrogen (H ₂ ). Pressure is about 100 Torr, support temperature is about 700-
900 ° C., and the microwave power was about 3 kilowatts. Thus, the diamond crystal was exposed to the hydrogen plasma for 2 hours. After the hydrogen plasma operation was interrupted and the crystal was rolled, the crystal was uniformly exposed to the hydrogen plasma by repeating the hydrogen plasma operation twice for an additional two hours.

The compression crushing strength (CFS) of the coated crystal was measured using a roll crusher apparatus and found to be 75.7.
Pound, which represents a 7.2% increase. Report on the uptake of hydrogen into diamond by hydrogen plasma treatment [MI Landstrass
(MI Landstrass) et al. [Appl. Phys. Lett. 55, 97
5 (1989)] and a paper by T. Maki et al. [Jpn. J.
Appl. Phys. 31, L1446 (1992)] and G Popo Beach
(G. Popovici) et al. [J. Appl. Phys. 77, 5103 (199
5)]], the surface hydrogen concentration is estimated to reach as high as 1,000 ppm. According to the theoretical model of the present invention,
The surprising result is that the hydrogen atoms introduced by diffusion introduce tangential compressive stresses as high as 380 MPa and the resulting increase in fracture strength reaches 24%. The observed increase in fracture strength is well within this range.

[Brief description of the drawings]

FIG. 1 is a graph plotting growth rate versus radius of a diamond crystal for the prior art.

FIG. 2 is a graph plotting nitrogen impurity levels (in ppm) against the radius of a diamond crystal, also for the prior art.

FIG. 3 is also a graph plotting the plastic yield stress (solid line) of diamond versus temperature for the prior art. In addition, the equilibrium line (dashed line) between diamond and graphite is also shown.

FIG. 4 is a graph showing the radial, tangential and shear stresses in diamond having a negative radial linear impurity concentration gradient, also in the prior art.

FIG. 5 is a graph illustrating radial, tangential and shear stresses in diamond having a positive radial linear impurity concentration gradient in accordance with the present invention.

FIG. 6 is a graph showing radial, tangential and shear stresses in diamond having a thin shell of impurities adjacent to the outer surface of the diamond in accordance with the present invention.

FIG. 7 is a micrograph of a crystal structure of a (111) crystal plane on a diamond crystal of the present invention having an impurity-introduced coating on the surface of the diamond.

FIG. 8 is a micrograph of a crystal structure of a (100) crystal plane on a diamond crystal of the present invention having an impurity-introduced coating on the surface of the diamond.

FIG. 9 is a micrograph of a crystal structure of a cross section in the thickness direction of the diamond of the present invention.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Thomas Richard Anthony, Inventor United States, New York, Schenectady, Lynwood Drive, No. 2142 (72) Inventor Clifford Lawrence Spiro United States, New York, Niska Yuna, Troy − Schenecta Day Road, No. 2801

Claims (13)

[Claims] 1. A diamond material having a positive dopant element concentration gradient near the surface of the diamond crystal, wherein said dopant element causes expansion of the diamond lattice and which results in tangential compressive stress on the surface of said diamond crystal. A diamond crystal comprising an element-introduced diamond crystal exhibiting an increase in compressive fracture strength. 2. The diamond crystal according to claim 1, wherein said dopant element is selected from the group consisting of boron, nitrogen, hydrogen, lithium, nickel, cobalt, sodium, potassium, aluminum, phosphorus, oxygen, and mixtures thereof. 3. The diamond crystal according to claim 1, wherein impurities including interstitial atoms, interstitial carbon atoms, and vacancies are present in the crystal. 4. The method according to claim 1, wherein the concentration of the dopant element in the diamond crystal is about 10 to 10,000 ppm in an outermost layer of 0 to 50 micrometers of the diamond crystal.
The diamond crystal according to claim 1, which is within the range of: 5. An impurity having a diameter of up to 2 centimeters and having a higher concentration towards or near the outermost surface of the crystal than the center of the crystal, wherein the impurity concentration is A three-dimensional faceted diamond crystal characterized in that it increases as a function of radius in the crystal and has a tangential compressive stress of up to 5000 megapascals in one facet of the crystal. 6. A diamond film having a thickness of about 0.25 to 50 μm on the outer surface of said diamond crystal, wherein impurities in said coating are made of a film of an element-introduced diamond crystal. 6. The three-dimensional faceted diamond crystal according to claim 5, wherein the diamond crystal is present at a concentration higher than the impurity concentration of about 10 to 10000 ppm on the outer surface. 7. The three-dimensional faceted diamond crystal according to claim 5, having a surface that has been subjected to ion implantation or radiation treatment so that the impurity concentration is in the range of about 1 to 10,000 ppm. 8. The three-dimensional faceted diamond of claim 5, wherein a tangential compressive stress in the range of about 10 to 5000 megapascals is superimposed on an existing tangential tensile stress in the diamond crystal. crystal. 9. A diameter of up to about 2 centimeters containing impurities that enhance the outer surface at a concentration within the range of about 10 to 10,000 ppm at a depth of about 0.25 to 50 micrometers from the outer surface of the crystal. A method for producing a diamond crystal having a tangential compressive stress of up to about 5000 megapascals on one facet, comprising a step of growing a three-dimensional diamond crystal having the following by a high-temperature and high-pressure method. 10. A process for producing a three-dimensional faceted diamond crystal having a diameter of up to about 2 centimeters, wherein about 10 to 10
Impurities in an amount of 10000 ppm from the outermost surface to about 0.
By introducing to a depth of 25 to 50 micrometers, a maximum of about 500
Generating a tangential compressive stress of up to 0 megapascals. 11. Radiation damage due to ion or electron bombardment creates interstitial atoms, interstitial carbon atoms and vacancies, resulting in expansion of the outermost surface of the diamond.
A method for producing a diamond having improved compressive fracture strength according to claim 10. 12. The diamond layer into which impurities are introduced is formed on the outermost surface by growing the diamond crystal by a high-temperature and high-pressure method and then growing the diamond crystal again by a high-temperature and high-pressure method.
A method for producing diamond having improved compressive fracture strength as described above. 13. The method according to claim 10, wherein the impurities are introduced in a layer form by hydrothermal growth, electrochemical growth, liquid-phase solid source growth or molten salt growth.