GB1585608A - Method for checking mechanical properties of articles of superhard materials - Google Patents

Description

(54) METHOD FOR CHECKING MECHANICAL PROPERTIES OF ARTICLES OF SUPERHARD MATERIALS (71) We, INSTITUT FIZIKI VYSOKIKH DAVLENY AKADEMII NAUK SSSR, of Akademgorodok, Podolsky raion, Moskovskaya oblast, Union of Soviet Socialist Republics, a corporation organised and existing under the laws of the Union of Soviet Socialist Republics, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:: This invention relates to a method for checking mechanical properties of articles of superhard materials, such as fine crystalline powders of synthetic diamonds, polycrystalline formations of synthetic diamonds of the carbonado and balas type, cubic and wurtzite-like boron nitride, and various composite (compact) products on the basis of powders of natural and synthetic diamond and compact modifications of boron nitride.

The invention will find use in various industries associated with the manufacture of the above-mentioned superhard materials, as well as in the manufacture of tools from such materials or by using such materials, in particular in making monocrystal cutting tools, cutters, drills for machining hard and brittle materials such as glass, ceramics, high silicon aluminium alloys, metallic and cermet hard alloys and high grade construction steel, bits for cutting or drilling rocks, dies for drawing wire, as well as for developing new types of articles used in various constructions, e.g. movable components of precision instruments and vital components of high-pressure equipment.

The basic requirement imposed on articles of superhard materials used in industry are high performance characterizing the mechanical properties of these materials, in particular hardness, modulus of elasticity, compressive and/or bending strength, durability, abrasive and cutting capacity, as well as economical effectiveness.

The present invention provides a method for checking mechanical properties of articles of superhard materials, comprising sampling one article of a group being checked as a reference article, reducing the reference article to powder to relax microstresses built-up during the formation of the superhard material and the manufacture of the article therefrom while retaining the original degree of dispersion crystallites, obtaining X-ray patterns for at least five articles from the group being checked and for the powder of the reference article, measuring the values of broadening of interference bands on the resultant X-ray patterns, determining the values of microstress induced broadening of X-ray bands of the sampled articles by comparing the X-ray patterns for the said articles with the X-ray pattern for the powder, determining one of the characteristic parameters selected from compressive or bending strength, abrasiveness and cutting life of the five articles, establishing a functional relationship between the values of microstress induced broadening of X-ray bands for the five articles and the said selected one of the characteristic parameters thereof, subsequently obtaining X-ray patterns for the remaining articles of the group being controlled, and converting the values of microstress to values of the said selected characteristic parameter by induced broadening of X-ray bands for these articles to values of the said selected characteristic parameter by means of the said established functional relationship to evaluate the mechanical properties of the articles of the group.

The present invention makes it possible to check mechanical properties such as cutting life, abrasiveness, compressive and bending strength of articles of superhard materials without their destruction. In addition, the method according to the invention is rapid and simple in implementation since it eliminates the labour consuming machining of each article being checked. The invention makes it possible to avoid human factors in evaluating the quality, that is mechanical properties, ofarticlesofsuperhard materials.

In considering the present invention, it is first necessary to contemplate the use of a relationship of mechanical properties of articles of superhard materials versus fine structure characteristics of such materials in the articles, such as the value of microstresses and sizes of crystallites. In accordance with the method of the invention, the fraction of broadening of X-ray band of the article being checked associated with the influence of microstresses in the articles being checked is experimentally detected so as to enable an experimental quantitative evaluation of the value of microstresses in this article being checked. The present invention involves the use of a reference article which is employed for a substantially different purpose compared to previously known methods which also required the use of reference articles.

In metals and alloys it is not possible to compare the stressed sampled being checked with a "relaxed" reference sample for quantitative evaluation of the fraction of broadening associated with the influence of microstresses since, upon removal of microstresses in such materials during annealing, the size of crystallites is enlarged as a result of recrystallization.

Therefore, relaxed samples of metals and alloys after the annealing cannot be used as reference samples, and the fraction of broadening of band associated with the influence of microstresses is determined by using either approximation of shape of the band with various functions or by complicated and cumbersome methods of harmonic analysis of shape of X-ray band or moments of second order.

In accordance with the present invention, in order to obtain a reference article, one of the articles being checked is reduced to powder by breaking the structure of the article of superhard material. As the article is reduced to powder, microstressed built-up during the formation of the structure of the material and the manufacture of the article therefrom are completely relaxed, while the size of crystallites (blocks) remains unchanged. The width of X-ray band of the powder is substantially smaller than that of X-patterns of articles being checked and is only related to the effect of degree of dispersion of blocks. Therefore, the resultant powder is used as a relaxed reference sample.Thus, for quantitative evaluation of the fraction of broadening associated with the influence of microstresses, the width of X-ray bands of the articles being checked and of the relaxed reference article is measured, and the resultant values are compared.

Therefore, the experimental method for analysis of microstresses in articles of superhard materials according to the invention, as compared to previously known methods for measuring microstresses elaborated as applied to metals and alloys, permits sufficiently rapid, simple and reliable evaluation of the fraction of microstress induced broadening of X-ray bands of articles, thereby simplifying and accelerating the non-destructive quality of control of articles of superhard materials, while improving its accuracy.

The method according to the invention is carried out in two stages, as follows.

The first stage concerns the establishment of a functional relationship between the values of microstress iriduced broadening of X-ray bands for articles being checked and their mechanical properties. For that purpose, one article of a group of n articles of superhard materials, such as tools of polycrystalline formations of synthetic diamond of the carbon ado and balas or elbor type or samples of a diamond-metal composite, is sampled as a reference article. This reference article is reduced to powder by mechanically or chemically breaking the structure of the article to relax microstresses built-up during the formation of the superhard material and the manufacture of articles therefrom while retaining the original degree of dispersion crystallites.The destruction is for example effected mechanically by grinding and chemically removing metal catalyst inclusions or binder by dissolving in concentrated acids. Then X-ray patterns of at least five, but preferably not more than ten, articles arbitrarily sampled from the group being checked. and of one reference article, are obtained.

The number of 5-10 articles is chosen because such a plurality of articles is necessary and sufficient for plotting graphs and establishing analytical relationships between the physical and mechanical properties of the articles.

X-ray patterns are obtained by the method of back-reflection photography with rotation of film and stationary sample using any X-ray apparatus designed for structural analysis such as an X-ray diffractometer. Broadening of interference bands in the X-ray patterns of samples and powder from the reference article (if recorded on film) is determined from microphotometry curves at one half of the peak of the intensity curve and by the integral intensity method. If an X-ray diffractometer is used. the intermediate microphotometry operation may be dispensed with. The following formulae are used for calculations: (at)2 = p2 + b2, wherein A H is broadening of X-ray band, rad..

ss is physically induced broadening of X-ray band of the sample being checked, rad., b is a value depending on experimental conditions of photographing, and p2 = pb2 + ssd, wherein pb iS a fraction of physically induced broadening related to degree of dispersion of crystallites (blocks), and pd = Ad/dtg H depends on non-uniformity of the lattice pitch Ad/d (microstresses).

Using the above formulae and comparing the value of broadening of X-ray bands for the articles being checked and for the powder of the reference article, the value of microstress induced broadening of X-ray bands for the samples articles being checked is determined.

The mechanical properties of articles of superhard materials are characterized by abrasiveness, cutting life, and compressive or bending strength. One of these characteristic parameters is determined by any appropriate known method for 5-10 sampled articles from the group being checked.

A functional relationship is graphically and/or analytically established to relate the amounts of microstress induced broadening of X-ray bands for the sampled articles being checked to one of characteristic mechanical properties such as the cutting life of articles of superhard materials, that isy = f(x), wherein y is one of the characteristic parameters of the mechanical properties of the article being checked, and x is the microstress induced broadening of X-ray bands for the article being checked. The resultant correlation is provided by the fact that the physical characteristics of the fine structure of superhard material, i.e. microstresses value and crystallite size, experimentally determined from the broadening of X-ray bands are averaged over substantial volumes of the article.

The second stage of the method deals with non-destructive checking of the mechanical properties of articles of superhard materials.

X-ray patterns are obtained by the above-described method for each article of the group being checked, the broadening of X-ray bands of articles is measured, and. by comparing the X-ray patterns for the articles with the earlier obtained X-ray pattern for the powder. the values of microstress induced broadening of X-ray bands of these articles are determined.

Then, by converting each resultant value of microstress induced broadening of X-ray bands to values of the selected characteristic parameter by means of the functional relationship y = f(x). the mechanical properties (quality) of the articles of the group are evaluated.

When operating on a commercial scale, after a certain range of articles are tested. there will be no need to establish the relationship between the value of microstress induced broadening for the articles being checked and one of the characteristic parameters of the mechanical properties thereof for each group being checked. It is sufficient to establish this relationship once for a given type of article of a given superhard material. and non-destructive control of the mechanical properties may then be made by the production flow method on the basis of microstress induced broadening of X-ray bands for articles being manufactured.

The method for checking mechanical properties of articles of superhard materials is not labour intensive and does not require special equipment. When using a standard X-ray diffractometer in combination with a computer. the quality of an article may be checked within several minutes without destruction. Using the graph of the established raltionship y = f(x). wherein y is one of the characteristic parameters of the mechanical properties of the article being checked and xis the microstress induced broadening of X-ray bands of the article being checked. not only may rejection of articles exhibiting low mechanical properties be effected. but a preliminary quantitative evaluation of the mechanical characteristic of the article being controlled and hence correlation with the values of microstress induced broadening of X-ray bands of the article may be made.

The invention will be further described with reference to the following illustrative Examples.

Example I Checking cutting life of tools of polycrystalline formations of synthetic diamond of the carbonado type.

One article of a group being checked consisting of 100 tools of polycrystalline formations of diamond of the carbonado type was sampled as a reference article and reduced to powder ot relaxed diamond by mechanically and chemically braking the structure of the article. Then X-ray patterns of any ten tools of the group being checked and of the diamond powder were obtained. The X-ray patterns were obtained using an X-ray apparatus having a copper anode tube by the back-reflection photography method on a flat film. The X-ray patterns recorded reflections from the system of diamond planes (331). the Bragg-Wulff reflection angle 0 being approximately 70". The width of X-ray band B was determined geometrically at one half of the peak of the intensity curve from microphotometry curves obtained from a microphotometer.Photometry was made by using automatic recording on a photographic plate.

For evaluation of microstress induced broadening of the ten sampled tools. the method based on comparison of X-ray patterns of tools and diamond powder obtained from the tool after its destruction was used. Since the size of crystals of polycrystalline concretion does not increase upon destruction, a change in the width of X-ray band of the powder means that there is a fraction associated with the influence of microstresses in the broadening of the tool band. The actual width of X-ray band for the powder depends on the small size of crystallites (blocks) and is used as a reference for evaluation of microstress induced broadening of bands for the tools of the carbonado type polycrystalline formations.

The values pd of microstress induced broadening were calculated by using a computer on the basis of starting data on the width of X-ray band for the tools B and for the reference article B' in mm.

The values of the above-mentioned parameters are given in Table 1 below.

Table I Nos of B mm of tool B' mm of ssd.103rad. 7 mien tools tool powder of tool of tool 1 1.18 0.99 7.1 26 2 1.20 0.99 7.2 28 3 1.32 0.99 8.2 35 4 1.29 0.99 8.0 40 5 1.34 0.99 8.4 44 6 1.39 0.99 8.8 50 7 1.37 0.99 8.6 52 8 1.48 0.99 9.7 65 9 1.46 0.99 9.5 70 10 1.62 0.99 11.1 75 Mechanical tests of the tools were made by turning tungsten-cobalt hard alloy on a lathe at cutting speeds of 12-15 m/min. Geometrical parameters of the cutting portion of the tools and turning performance were as follows: (p = 400. ç, = 20 . y = -3-5 . a = 10 . R = 0.3-0.4 mm.

S = 0.024 mm/rev.. t = 0.2 mm.

wherein ç is cutting edge angle.

ç, is end cutting edge angle.

a is rake angle.

y is relief angle.

R is nose radius.

S is straight-in feed of tool slide per one revolution of the spindle. and t is cutting depth.

The degree of wear of the tool referred to the flank h3 = 0.3-0.4 mm may be used as the criterion of the tool life.

The values of the tool life "obtained are givan in Table 1.

Using mathematical statistics. the results were processed in a computer. It was established that the relationship y = f(x). wherein x is the amount of microstress induced broadening for tools. pd. 103 is in rad.. andy is tool life T in minutes. was described by a straight line equation V = ax + b.

The above-mentioned back-reflection photography method was then used to obtain X-ray patterns for the remaining 89 tools of the group being checked on a flat film. the broadening of X-ray bands was measured. and the X-ray patterns of the 89 tools were compared with the X-ray pattern of the powder to determine the amounts of microstress induced broadening thereof by means of computer.

Using the graph of the established relationship. low grade tools were rejected on the basis of pd values obtained for 89 tools Example 2 Checking life ofelbor-P in cutting.

One article was sampled as a reference article from a group of 100 tools of elbor-P being checked. and this tool was reduced to power of relaxed cubic boron nitride by mechanicallv and chemically breaking the structure of the article while retaining the original degree of dispersion of crystallite (blocks). Then X-ray patterns of an) five tools from the group being checked and of the cubic boron nitride powder were obtained. X-ray patterns and values of microstress induced broadening of X-ray bands for the five sampled articles were obtained as described in Example 1. but the photographs were taken using cobalt radiation. and reflections from the svstem of planes of cubic boron nitride (400) were recorded on X-ray patterns.

the Bragg-Wulff angle being approximately 8' Table 2 Nos of B mm of B' mm of Pd. 103 rad. 7 min of tools tool powder of tool tool 1 5.15 3.35 54.0 16 2 5.66 3.35 60.2 19 3 5.75 3.35 61.3 22 4 5.98 3.35 64.1 24 5 6.23 3.35 67.1 27 Mechanical tests of the five tools were made on lathes in turning high-grade structural steel under "severe" machining conditions: cutting speed V = 75-90 m/min, S = 0.09 mm/rev, t = 0.2 mm. The geometrical parameters of the cutting portion of the tool corresponded to the following characteristics: cp = 40 ,ç, =200,0 = 10-12", = -3-5 ,R = 0.3-0.6mm.

The degree of wear of the tool referred to the flank h3 = 0.4-0.5 mm was used as the criterion of the tool life.

The values of the above-mentioned parameters of microstress induced broadening of X-ray bands and life of tools of elbor-P are given in Table 2.

By using mathematical statistics, the results were processed in a computer. The relation shipy = frx) was described by straight line equation y = ax + b.

The above-mentioned method was used to obtain X-ray patterns for the remaining 94 tools of the group being checked, and the X-ray patterns of the 94 tools were compared with the X-ray pattern of cubic boron nitride obtained earlier to determine the values of microstress induced broadening of X-ray bands for the tools by means of a computer.

Using the graph of the established relationship, tools with a short cutting life were rejected on the basis of ssd values for the 94 tools.

Example 3 Checking abrasiveness ofsamples of polycrystalline formations of the balas type synthetic diamonds.

One sample was selected as a reference sample from a group of 100 samples of polycrystalline balas type synthetic diamonds being checked, and this sample was reduced to powder of relaxed diamond by mechanically and chemically breaking the structure of the sample while retaining the original degree of dispersion of crystallites. Then X-ray patterns of five samples of the group being checked and of the diamond powder were obtained. X-ray patterns and values of microstress induced broadening of X-ray bands of the five samples selected from the group were obtained as described in Example 1.

Mechanical tests of the five samples were made on diamond grinding wheels. The abrasiveness was determined as the ratio of weight of the abrasive wheel material removed in roughing by a sample wheel of polycrystal of balas type synthetic diamond to the loss of weight of the samples.

The results were treated and the relationship y =f(x). wherein y is the value of microstress induced broadening of X-ray bands of the sample being checked and x is the abrasiveness of the sample being checked in relative units. was established as described in Example 1.

The above method was used to obtain X-ray patterns for the remaining 94 samples of the group being checked.and the X-ray patterns of the 94 samples of polycrystals of the balas type diamond were compared with the earlier obtained X-ray pattern of the diamond powder to determine. by means of a computer, the values of microstress induced broadening of X-ray bands of these samples.

Using the graph of the established relationship. the samples of polycrystalline balas type synthetic diamonds having low abrasiveness were rejected on the basis of ssd values for the 94 samples.

Example 4 Checking compressive strength ofsamples ofdiamond-metal composite.

One sample was taken as a reference sample from a group of 100 samples of diamondmetal composite being controlled. and this sample was reduced to powder of relaxed diamond by mechanically and chemically breaking the structure of the sample while retaining the original degree of dispersion of crystallite. Then X-ray patterns of ten samples of the group being checked and of the diamond powder were obtained. X-ray patterns and values of microstress induced broadening of X-ray bands of the ten selected samples were obtained as described in Example I.

Mechanical tests of the ten samples were made on a press with an axial force of 800 tons.

Compressive strength was determined by crushing cylindrical samples of 12 mm diameter and 5 mm height. The strength P was evaluated by the formula P=F S.

wherein F is the force at which the sample is destructed. and S is the cross-sectional area of the cylindrical sample.

The results were treated and the relationship y = f(x). wherein y is the value of microstress induced broadening of X-ray bands for the sample being checked. pud.103 is in rad.. and x is the axial compresive strength of the sample pe in kgf/mm2. was established as described in Example 1.

The above-mentioned method was used to obtain X-ray patterns for the remaining 89 samples of the group being checked. and the X-ray patterns of the 89 samples of diamondmetal composite were compared with the earlier obtained X-ray pattern of the diamond powder to determine. by means of a computer. the values of microstress induced broadening of X-ray bands for these samples.

Using the graph of the established relationship. samples of diamond-metal composite having a low compressive strength were rejected on the basis of ssd values for the 89 samples.

WHAT WE CLAIM IS: 1. A method for checking mechanical properties of articles of superhard materials.

comprising sampling one article of a group being checked as a reference article. reducing the reference article to powder to relax microstresses built-up during the formation of the superhard material and the manufacture of the article therefrom while retaining the original degree of dispersion crystallites. obtaining X-ray patterns for at least five articles from the group being checked and for the powder of the reference article. measuring the values of broadening of interference bands on the resultant X-ray patterns. determining the values of microstress induced broadening of X-ray bands of the sampled articles by comparing the X-ray patterns for the said articles with the X-ray pattern for the powder. determining one of the characteristic parameters selected from compressive or bending strength. abrasiveness and cutting life of the five articles. establishing a functional relationship between the values of microstress induced broadening of X-ray bands for the five articles and the said selected one of the characteristic parameters thereof. subsequently obtaining X-ray patterns for the remaining articles of the group being controlled. and converting the values of microstress induced broadening of X-ray bands for these articles to values of the said selected characteristic parameter by means of the said established functional relationship to evaluate the mechanical properties of the articles of the group.

2. A method for checking mechanical properties of articles of superhard materials substantially as herein described in any of the foregoing Examples.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (2)

**WARNING** start of CLMS field may overlap end of DESC **. P=F S. wherein F is the force at which the sample is destructed. and S is the cross-sectional area of the cylindrical sample. The results were treated and the relationship y = f(x). wherein y is the value of microstress induced broadening of X-ray bands for the sample being checked. pud.103 is in rad.. and x is the axial compresive strength of the sample pe in kgf/mm2. was established as described in Example 1. The above-mentioned method was used to obtain X-ray patterns for the remaining 89 samples of the group being checked. and the X-ray patterns of the 89 samples of diamondmetal composite were compared with the earlier obtained X-ray pattern of the diamond powder to determine. by means of a computer. the values of microstress induced broadening of X-ray bands for these samples. Using the graph of the established relationship. samples of diamond-metal composite having a low compressive strength were rejected on the basis of ssd values for the 89 samples. WHAT WE CLAIM IS:

1. A method for checking mechanical properties of articles of superhard materials.

comprising sampling one article of a group being checked as a reference article. reducing the reference article to powder to relax microstresses built-up during the formation of the superhard material and the manufacture of the article therefrom while retaining the original degree of dispersion crystallites. obtaining X-ray patterns for at least five articles from the group being checked and for the powder of the reference article. measuring the values of broadening of interference bands on the resultant X-ray patterns. determining the values of microstress induced broadening of X-ray bands of the sampled articles by comparing the X-ray patterns for the said articles with the X-ray pattern for the powder. determining one of the characteristic parameters selected from compressive or bending strength. abrasiveness and cutting life of the five articles. establishing a functional relationship between the values of microstress induced broadening of X-ray bands for the five articles and the said selected one of the characteristic parameters thereof. subsequently obtaining X-ray patterns for the remaining articles of the group being controlled. and converting the values of microstress induced broadening of X-ray bands for these articles to values of the said selected characteristic parameter by means of the said established functional relationship to evaluate the mechanical properties of the articles of the group.

2. A method for checking mechanical properties of articles of superhard materials substantially as herein described in any of the foregoing Examples.