FR2576813A1 - Abrasive product, which can be brazed, containing an ultra-hard product, and process for manufacturing such a product - Google Patents

Abstract

The product comprises an active part consisting of a sintered compact 10, containing more than 60 % by volume of particles of ultra-hard product, integral with a support consisting of sintered grains of at least one refractory and hard compound. It further comprises, between the active part and the support, at least one diffusion barrier containing a portion of between 10 and 50 % of the grains of ultra-hard product contained in the active part, sintered with particles of a refractory and hard compound, and of a catalyst binder, the abrasive product constituting a one-piece unit. The product can be brazed or indexed on a cutting or drilling tool.

Description

Brazable abrasive product containing an ultra-hard product and process for manufacturing such a product
The invention relates to brazable abrasive products but which can be used on tools where they are mechanically fixed, of the type comprising an active part constituted by a sintered compact containing more than 60X. by volume of particles of ultra-hard product, integral with a support consisting of sintered grains of at least one hard and refractory compound.

 The term ultra-hard product is used to designate "the few products which have an exceptionally high hardness, ie essentially diamond and boron nitride crystallized in the cubic system, often denoted by the abbreviation" CBN ". A" compact "is a sintered product constituted by a set of grains or particles of different dimensions linked together by bridges, created by diffusion of matter in a plastic state. This sintering in plastic phase is not obtained, with diamond or CBN grains, only at pressures and temperatures of the order of magnitude of the pressures and temperatures used for the synthesis of these same grains. These pressures and temperatures can be reduced by adding to the starting material conversion catalysts, which are now well known. In order to obtain a diamond or CBN compact on an industrial scale, a sintering binder having such a catalytic action is used which dissolves each grain of surface. in the pressure and temperature conditions necessary for sintering in the plastic phase. The most commonly used binder for diamonds is cobalt, which has excellent catalytic action. The most frequently used sintering binder for CBN is aluminum, which is a good catalyst for converting hexagonal boron nitride to CBN. Very recently. the use of a mixture of silicon and an aluminum filler material, with a content not exceeding 507. by volume. appeared to be much more advantageous.

 It has been known for a long time that it is not possible to directly braze diamond or CBN compacts. so that these compacts can only be mounted on tools by indexing, that is to say by clamping the edge of the compact.

 To overcome this brazing difficulty, it has been proposed (US Pat. No. 2,476,699) to produce diamond tools in which the diamond crystals were linked to a sintered carbide support.

 As early as 1966, the company MEGADIAMOND also proposed to replace the diamond monocrystals previously used. a product called "polycrystalline diamond", manufactured by subjecting a mass of diamond particles of small particle size and binder to conditions of temperature and pressure close to those necessary for the manufacture of diamond. A product is thus obtained which does not have a preferential cleavage plane, and which is much less fragile than natural diamond and is more suitable for industrial applications. such as cutting very hard products.

 More recently (US Pat. No. 3,745,623) has been proposed a product with a diamond working end for the direct machining of metals with diamond. the active part of which contains more than 707. by volume of diamond and is linked to a carbide support of volume considerably greater than that of the active part, the interface between the active part and the support being constituted by mechanical connection of the carbide and diamond.

 If the presence of the support has the advantage of making the product brazable, it has in return a certain number of drawbacks.

 First of all, there is, during the manufacture by simultaneous sintering of the grains of ultra-hard product and of carbide, diffusion of the sintering binder which is added to the carbide to guarantee the perfect cohesion of the grains of carbide between them. In the case, for example, where the active part contains diamond, cobalt will generally be used as a binder. During sintering, there is diffusion of cobalt from the carbide to the diamond and depletion of cobalt from the carbide near the carbide interface. diamond and correspondingly a peak of cobalt concentration at the interface. This depleted sernent weakens the product and makes it sensitive to mechanical shock, which, among other consequences, led initially to consider that this abrasive product was only suitable for cutting metals, where resistance to mechanical shock is less essential than resistance to abrasion and thermal shock.

 It should be noted in passing that, whatever the binder used for sintering the grains of ultra-hard product, the diffusion of cobalt always takes place in the direction of enriching the active part to the detriment of the support, so that the problem remains in all cases.

 A second defect is the appearance of voltages at the interface of the active part and of the support, generated by the differences in retraction between the two constituents during cooling after pure sintering. This phenomenon is very sensitive during sintering of diamond or CBN on a carbide support due to the large volume variations due to the shrinkage of the part to be densified, that is to say of the part containing the diamond grains. or CBN. The remaining stresses in tension cause the product to be sensitized to stresses of all kinds, whether mechanical or thermal. The active part containing the ultra-hard product is under extension stress while the carbide support is under compression. The stresses are such that they often give the interface a homogeneous and no longer flat form.

 impo-rtance of these constraints is such that it was initially thought quite naturally that it was not possible to give sufficient cohesion to the abrasive product unless the carbide support is solid, the active part being on the contrary thin.

The above defects could have been partly ruled out and we currently know of composite abrasive products, sold under the brand "MEGAPAX, in which the compact is produced from a powder already containing the catalyst binder involved in its sintering. For this, the mass giving rise to the compact can consist of diamond grains mixed with a fine powder of the binder,
The diffusion of the binder then takes place very quickly within the compact by a process very different from the infiltration phenomenon, requiring a passage of binder in the liquid phase from the carbide to the diamond. It is then possible to maintain the product only for a short time at the required sintering temperatures and to limit migration. We arrived by this process and by increasing the cobalt content of the carbide grains near the interface, to produce inserts for tools for cutting metals, wood, plastics, etc. intended to be brazed on a tool and in which 1 thickness of support does not exceed the thickness of the active part despite the fact that the small particle size of the grains of ultra-hard product, desirable to guarantee a good surface condition, is unfavorable from the point of view of constraints.

 It has also been possible to produce pins, the active part of which contains diamond grains of large particle size. sufficiently resistant to mechanical shock and wear so that it can be mounted on drill bits.

 However, this is only an imperfect solution because the constraints arising from the difference in the thermal expansion coefficients remain at the interface between the support and the active part. Furthermore. supersaturation with cobalt at the interface leads to embrittlement and a reduction in resistance to thermal shocks.

 The invention aims to provide an abrasive product of the type defined above that meets the requirements of the practice better than those previously known. in particular in that the internal stresses are reduced and in that the support retains, after sintering. the mechanical characteristics it would have in the absence of the active part.

 To this end, the invention provides in particular an abrasive product of the type defined above, further comprising, between the active part and the support. at least one diffusion barrier containing a proportion of between 10 and 50Z of grains of the ultra-hard product contained in the active part, sintered with particles of a hard and refractory compound and of a catalyst binder, the abrasive product constituting a monobloc assembly. The diffusion barrier will often contain the same refractory compound as the support, alone or with another.

 It is practically necessary that the concentration of the diffusion barrier in ultra-hard product does not exceed 50Z in volume to avoid the phenomena of migration of binder and the appearance of the constraints mentioned above: this concentration will usually be between 10 and 50Z in volume.

 The thickness of the diffusion barrier should be at least 100 µm. As a general rule, there is no advantage in giving it a thickness exceeding 1 mm. In practice, a thickness of 250 μm will often give good results, in particular in the manufacture of cutting inserts the active part of which has a thickness of approximately 0.5 mm and the carbide support a thickness of approximately 0.5 mm. When the particle size of the ultra-hard product in the active layer must be low, it will generally be preferable to give a higher particle size to the ultra-hard product in the diffusion barrier, in order to reduce shrinkage during sintering.

 In the case of pins intended for drilling tools, the particle size of the ultra-hard product in the active layer is generally high, for example 115 μm on average, and the particle size can be the same in the active part and in the barrier broadcast.

 It is possible to provide several successive diffusion barriers, having carbide contents which increase from the active part towards the support. which further reduces the constraints.

 Among the hard and refractory compounds which can be used, mention may be made of titanium carbides. tantalum, zirconium and especially tungsten. The binders can in particular be nickel, iron and especially cobalt in the case of diamonds, aluminum and aluminum-silicon in the case of CBN. The addition, in the latter case, to the diffusion barrier, of titanium nitride or of another compound preventing the migration of the cobalt binder from carbide to the compact, at a content not exceeding 10 × by volume, is also favorable. . Incidentally, it can be noted that the effect sought by this addition is to avoid infiltration, which nevertheless constitutes a conventional technique for adding diamond sintering binder to traditional solutions.

According to another aspect of the invention, the latter provides a brazable abrasive product comprising
- a compact of sintered PCD or CBN particles and constituting a continuous matrix, with a binder belonging to catalysts for converting graphite into diamond or hexagonal boron nitride into
CON, which may also contain a hard and refractory ceramic phase in the dispersed state in the matrix, such as borides, carbides and nitrides of silicon, titanium, zirconium, tantalum and tungsten, alumina, zirconia and aluminum-silicon oxynitride,
a support consisting of sintered hard and refractory carbide and a binder such as nickel or cobalt, characterized in that at least one diffusion barrier linked to the compact and to the support consists of PCD or
CBN with a content of between 10 and 50Z by volume, imbricated with a hard and refractory carbide, the content of binder, of a type also belonging to the conversion catalysts, exhibiting no irregularities at the interfaces.

The invention also aims to provide a method of manufacturing an abrasive product of the type defined above, according to which: a layer is placed in a protective metal cell intended to give rise to an active part consisting of a mixture of ultra-hard product and binder with an ultra-hard product content greater than 60Z, then at least one diffusion barrier containing ultra-hard product at a volume concentration of less than 507, a binder, and a powder pre-sintered with at least one hard and refractory compound, the cell is closed using a support for said hard and refractory compound in cemented form, and the assembly thus constituted is brought to a temperature and a pressure causing the sintering plastic phase of the ultra-hard product and the hard and refractory compound,
The term "cemented" designates a product comprising the hard and refractory compound in the form of grains connected by the binder which ensures the bonding by a process of melting and then solidification on the grains, obtained at a temperature significantly below the temperature of Diamond or CON transformation, without growth of the grains of ultra-hard products and formation of bridges connecting them to ensure a coherent matrix.

It is only during sintering that there will be bonding of the grains in the compact to form a hard and very coherent support, the carbide remaining in the cemented state me after application of the sintering pressures and temperatures of the diamond or CBN.

The manufacturing can be carried out in a press of the type called "belt" described in the patent
US-A-2,941,248. However, it is preferable to use a press comprising six anvils delimiting a cubic pressure space, of a type also known, which has the advantage of guaranteeing a much more homogeneous pressure distribution within the mass to be sintered.

In practice, sintering can be obtained by keeping the cell and the support punctured for a short time under conditions of temperature and pressure which cause plastic flow and compacification of the ultra-hard product, while remaining at - above the stability limit of the ultra-hard product. In the case of diamonds and CON, it will generally suffice, when the ultra-hard product already contains catalyst binder, to subject the cell and the support to a pressure of at least 55 kbar and to a temperature of at least minus 1300-C for a period of at least one minute. The delay will be shorter the higher the sintering temperature and vice versa. In practice, the usable temperature range is 1000 to 1800 ° C and corresponds to a pressure range of 35 to 65 kbar. it is obviously necessary to place itself above the curve of limit of stability of the ultra-hard product used and to maintain the conditions of temperature and pressure for a sufficient time so that there is fusion of the whole of the binder and plastic deformation and compacification of the grains of ultra-hard product. The nature of the catalyst binder exerts an important action on these latter phenomena by a surface activation effect. It is obviously desirable to keep the duration of maintenance in high pressure and high temperature as low as possible and, in practice, a time is very quickly reached beyond which an additional extension has little influence on the density of the compact and the growth of the grains of ultra-hard product, When one is placed near the upper limit of the conditions of temperature and pressure mentioned above, the duration of maintenance under the sintering conditions can generally be reduced to a value barely greater than 2 min,
Microscopic examination of the product obtained shows that the bonds which exist in the abrasive product obtained between the diffusion barrier on the one hand, and the support on the other hand, are very different in nature from that of a product devoid of diffusion barrier. The diamond grains constituting the diffusion barrier are connected to each other and to the diamond grains of the active part by bridges characteristic of sintering in the plastic phase. The tungsten carbide grains of the diffusion barrier are connected to each other and with the tungsten carbide grains which are tenuous from the support by cementation. there is therefore a real interpenetration of the diamond layer in the tungsten carbide support via the diffusion barrier when the PCD concentration of the latter is less than 507 .. The cobalt flux of the carbide support is then regulated by the diffusion barrier which acts as a buffer rattle and thus avoids depletion or over-concentration of binder which is harmful to the mechanical properties of the compact.

 We will now give, purely by way of illustration, a few specific examples of implementation of the invention.

The description refers to the accompanying drawings, in which
- Figure 1 is a schematic sectional view of an abrasive product constituting an exemplary embodiment of the invention;
- Figure 2 is a curve representative of the variation of the tungsten and cobalt content along an abrasive product in which the support is cemented tungsten carbide containing cobalt and the active part a compact polycrystalline diamond containing cobalt
- Figure 3 is a photomicrograph showing the separation layer of a product according to the invention and the adjacent areas of the support and the compact
- Figure 4 is a curve representative of the variation in the tungsten and cobalt content along an abrasive product according to the invention.

EXAMPLE 1 Abrasive Product with Diamond Active Part and
a double diffusion barrier
First of all four powders are prepared having the following compositions
Powder 1: Mixture of polycrystalline diamond (PCD) in the form of grains having a particle size distribution of semi-logarithmic type with an average size of 8 μm and 137. (by weight of the mixture) of -cobalt having a fine particle size, typically of around 10 µm.

Powder 2: Diamond mixture in the form of grains having an average diameter of 20 μm and 13Z (by weight of the mixture) of fine cobalt powder.

 Powder: Addition to powder 2 of 50 '(by volume of the mixture) of grains of tungsten carbide presintered with ITX of cobalt.

Powder 4: Incorporation into powder 2 of 75Z (by volume of the mixture) of grains of tungsten carbide pre-sintered at 177 '. cobalt.

The powders 1, 3 and 4 are placed in an oven under a hydrogen atmosphere at 800 "C for 1 hour,
Then equal volumes of the powders 1 3 and 4 are superimposed in a cell made of zirconium and molybdenum (nickel, tantalum, tungsten and stainless steel - which may also be suitable). The cell is closed using a cemented tungsten carbide support to form a sealed assembly. The cell is housed in a plastic pressure transmission medium which can be any of the products known for this purpose, such as sodium chloride. The assembly is placed in a resistance heating envelope, in graphite or metal. Then this assembly is inserted in the chamber of a press capable of providing hyperpressures and provided with heating means by passage of an electric current in the envelope. after having surrounded it with a product such as pyrophillite which transmits the pressures and forms a joint. The press is advantageously of the type called "cubic", all the walls of which consist of anvils associated with jacks (US Patent 3,913,280). We put under a pressure of 55 kbars. Then apply the heating. We arrive, about ten seconds, at a temperature of 1300-C which is maintained for 2 minutes - The heating is switched off. Then, after partial cooling, the press is opened. The abrasive product is recovered by destroying the casing and the cell by chemical attack, projection or attack with a grinding wheel. The carbide support is ground and possibly. the abrasive product is shaped by laser cutting, electron beam or EDM.

We thus obtain a product of the kind shown in
Figure 1, comprising a compact 10, a diffusion barrier in two zones 12 and 14 of increasing carbide content, and finally a carbide support 16.

Such a product was cut in two halves and measurements of tungsten and cobalt concentration were carried out by X-ray analysis with a scanning electron microscope. The results are given in FIG. 4 which shows that the diffusion of cobalt at the interface is perfectly controlled, which makes the internal stresses disappear without affecting the resistance to wear whereas, in the case of a product previously known without diffusion barrier, there is a supersaturation of cobalt at the interface (Figure 2). The cobalt content is substantially constant in the sample
Example 2: Abrasive product with active part, diamond and
a single diffusion barrier
A volume of the powder 1 mentioned in Example 1 is placed in a zirconium and molybdenum cell. It is surmounted by a volume of half the powder 2.

A tungsten carbide support is placed on the whole to obtain a tight assembly. The whole is integrated into a high pressure cell, which in turn is placed in an envelope. The assembly is subjected to a pressure of 55 kbars and a temperature of 13-00 "C for 2 minutes.

 Observation of the interface using a scanning electron microscope (Figure 3) makes it possible to observe the continuity between the compact and the support via the diffusion barrier. The plastic phase sintering makes it possible to obtain a homogeneous and compact layer, the shrinkage coefficient of which is intermediate between those of the compact and of the support, hence a very considerable reduction in residual stresses.

Example 3: Active Dartie Abrasive Product Containing
CBN
First, two powders are prepared having the following compositions
Powder 1: Mixture of CBN in the form of grains having an average diameter of the order of 8 μm and a distribution of the semi-logarithmic and aluminum-silicon type, at a content of 17 × by volume of the mixture obtained.

Powder 2: Mixture of equal volumes of powder 1 and grains of tungsten carbide pre-sintered with 17g of cobalt.

The powders are treated and loaded as in Example 2. The cell is subjected to a pressure of 55 kbar and a temperature of 1400 μl for 2 minutes.
The product obtained is still almost free from constraints.

Claims (8)

 1. Brazable abrasive product comprising an active part consisting of a sintered compact (10) containing more than 60X by volume of particles of ultra-hard product, secured to a support consisting of sintered grains of at least one hard and refractory compound, characterized in that it further comprises, between the active part and the support, at least one diffusion barrier containing a proportion of between 10 and 50X. grains of the ultra-hard product contained in the active part, sintered with particles of a hard and refractory compound and of a catalyst binder, the abrasive product constituting a one-piece assembly.  2. Brazable abrasive product comprising  - a compact of sintered PCD or CBN particles and constituting a continuous matrix, with a binder belonging to the catalysts for converting graphite into diamond or hexagonal boron nitride into CBN, which may also contain a hard ceramic phase and refractory to l dispersed state in the matrix, such as borides, carbides and nitrides of silicon, titanium, zirconium, tantalum and tungsten, alumina, zirconia and aluminum-silicon oxynitride,  - a support consisting of sintered hard and refractory carbide and a binder such as nickel or cobalt,  characterized in that at least one diffusion barrier linked to the compact and to the support consists of PCD or CBN at a content of between 10 and 507 by volume, nested with a hard and refractory carbide, the content of binder also belonging to the catalysts conversion with no irregularities at the interfaces.  3. Product according to claim 1 or 2, characterized in that the thickness of the diffusion barrier is between 100 µm and 1 mm.  4. Product according to claim 1, 2 or 3, ca acterized in that the diffusion barrier contains titanium nitride.  5. A method of manufacturing an abrasive product according to claim 1 or 2, characterized in that one places, in a protective metal cell, a layer intended to give rise to an active part and constituted by a mixture powders of ultra-hard product and binder with an ultra-hard product content greater than 607, then at least one diffusion barrier containing ultra-hard product at a volume concentration of less than 507, a binder, and a pre-sintered powder of at least one hard and refractory compound, the cell is closed using a support for said hard and refractory compound in cemented form or a mixture of powders, and the assembly thus constituted is worn, for at least one minute, at a temperature and a pressure causing the plastic phase sintering of the ultra-hard product and of the hard and refractory compound.  6. Method according to claim 5, characterized in that the initial content of binder is higher in the diffusion barrier than in the active part  7. Method according to claim 5 or 6, charac terized in that the cell is subjected to a pressure of at least 55 kbar and at a temperature of at least 130O'C for a period of at least 2 minutes ,  8. Method according to claim 5, characterized in that the diffusion barrier is constituted by a mixture of powder of the type intended to give rise to the active part and of a mixture of pre-sintered carbide powder containing binder in proportions such as the diamond content is less than 507.