EP3808864A1

EP3808864A1 - Premix alloy powders for diamond tools

Info

Publication number: EP3808864A1
Application number: EP19203219.1A
Authority: EP
Inventors: Matteo Zanon
Original assignee: Ecka Granules Germany GmbH
Current assignee: Ecka Granules Germany GmbH
Priority date: 2019-10-15
Filing date: 2019-10-15
Publication date: 2021-04-21
Anticipated expiration: 2039-10-15
Also published as: ES2919199T3; EP3808864B1

Abstract

The present invention provides a powder composition comprising a mixed powder of the formula (1)

Fe _a Co _b Cu _c Sn _d Ni _e B _f Cr _g C _h (1)

Wherein, based on (a+b+c+d+e+f+g+h) = 100 (wt.-%), a-h indicate the content of the individual elements (wt.-%):
a
is 0-85.0
b
is 0-85.0
with (a+b) being 40.0-85.0
c
is 10.0-50.0
d
is 0.1-17.5
with (c+d) being 10.1-55.0 [100^∗ d/(c+d)] being 1.0-35.0
e
is 0.01-15.0
f
is 0.02-1.0
g
is 0.10-4.0
h
is 0.01-1.0
wherein the content of the composition of formula (1) is ≥ 98 wt.-% of the entire composition. Also provided are compositions comprising the above powder composition and other material suitable for preparing a matrix (bond) for diamonds in a diamond cutting tool; a composite material made of diamond and the present compositions and a process for making these composite materials, and further the use of a powder of a XCrBC-alloy powder (X = Fe and/or Co) as a diamond binding and/or reinforcing agent.

Description

State of the Art

Since the invention of synthetic diamonds in the 1950s, diamond tools industry has kept on expanding into the most diverse areas; besides stone cutting, sintered diamond tools are used for concrete, road repair, glass (e.g. for mobile phone screens) and ceramics. The cutting elements are composite materials where diamonds are embedded into a metallic matrix, the so-called "bond". Its role is twofold: to hold the diamond as long as possible, and to wear at a rate compatible with the material being cut. Metal powder is mixed with diamonds (typically 5-10 vol-%), granulated and then cold pressed. The composite is then consolidated close to full density via hot pressing (HP) or free sintering (FS) processes.
These two processes pose different requirements on matrix powder, with FS requiring more reactive materials than HP. The final sintered density must be higher than 95 % of the theoretical maximum density, since otherwise diamond retention and thus tool life would be compromised.
Matrix formulations have historically been based on cobalt, thanks to its excellent diamond retention, ease of processing by hot pressing and adjustable wear rate (by adding bronze / tungsten carbide). In the 1990s, chemically precipitated alloys were developed, based on Fe-Cu-Co system, as a response to instability in Co price and supply (J. Konstanty, Powder Metallurgy 2013, 56(3), pp. 184-188). Such products have gained a significant market share in Europe. Other powder producers offer chemical, mechanically alloyed or also premixed products, based on the Fe-Cu system.
Chemically precipitated/hydrometallurgical pre-alloys feature many technical advantages. Their fine granulometry (D₅₀ = 5-12 µm) implies high reactivity during sintering, high mechanical properties via grain refinement, virtually no segregation, strong diamond bonding via metallurgical interaction. Their main drawback, however, is the intrinsically high cost, due to batch processing of metallic salts followed by rinsing, drying and annealing under hydrogen (see e.g. US 2005/0106057 A1 ). This process also generates considerable amounts of contaminated water, which needs to be processed before being re-utilized or disposed. Another disadvantage of such powders is their low compressibility, which leads to sinter-to-green linear shrinkage rates after free sintering on the order of 15-20%. This significantly compromises the achievable dimensional precision, limiting their possible applications via this high throughput consolidation technique.
Several known powders also include cobalt in their formulation at variable content. The high and unstable price of cobalt is problematic, and the unsafe supply due to ever increasing demand from Li-ion batteries and non-conflict minerals regulation, as well as the tightening of related environmental/safety regulation, are strong incentives to find ways how to reduce or even fully substitute Co in either premixed and pre-alloyed powders.
Co-free powders already exist since several years, but their performance as compared to Co-containing alternatives is inferior, and thus it was not possible to displace the Co-based alternatives.
Mechanically alloyed products have also found a stable niche in this sector and are also the subject of recent R&D efforts (see, e.g., J. Konstanty et al., Arch. Metall. Mater. 2015, Vol. 60, pp. 634-637). The deformation energy stored in the milled powders activates their sintering (see e.g. US 2005/0106057 A1 ), besides making them homogeneous and thus non-segregating. Main drawbacks are once again cost and low compressibility.
Premixed powders on the one hand are intrinsically cheaper and can preserve the high compressibility of their constituents. On the other hand, their performance is generally inferior to hydrometallurgical and mechanically alloyed products, mainly due to coarser grain size of their constituents and lack of stored mechanical energy. An example is disclosed in EP-A-2 082 072 directly comparing chemically equivalent hydrometallurgical and premix powders. After 1 h at 950°C, the Fe-Co-Cu-P premix reaches only 89,1% density, while the hydrometallurgical goes up to 97,2%. On the Asian market such products are reported to be gaining ground thanks to their attractive cost (see, e.g., J. Borowiecka-Jamrozek et al., Arch. Metall. Mater. 2017, Vol. 62(3), pp. 1713-1720; J. Borowiecka-Jamrozek et al., Adv. Mat. Res. 2014, Vol. 1052, pp. 520-523). These are heterogeneous mixes of atomized, electrolytic, carbonyl Fe-Cu powders and even bronze flakes, that can reach moderate properties under hot pressing. Being too coarse, however, they are not suitable for free sintering. Other commercial products are known which share a basic concept of the invention, i.e. mixing commonly available, relatively cheap raw materials to create a value-added product, but their properties are not satisfactory due to an unbalanced formulation and a lack of innovative ingredients.
Hence, there exists a still unmet need in the market for products which show a performance comparable to hydrometallurgical powders but are available at substantially lower cost, which are further environmentally friendly (i.e. low on Co and/or Ni and without waste water generation), and which have high compressibility and thus lead to relatively low shrinkage in free sintering processes, e.g. at temperatures around 900-950°C.

Description of the Figures

Fig. 1: shows the relationship between green density and required isotropic shrinkage for achieving a specified sintered density for prior art pre-alloyed powder and the inventive powder, respectively.
Fig. 2: shows SEM photographs of diamonds on fracture surfaces obtained in Example 1 with Mix 1 and Mix 2 at different sintering temperatures (910°C, 930°C and 950°C).
Fig. 3: shows SEM photographs of diamonds on fracture surfaces obtained in Comparative Example 1 with Mix 3 (invention) and Mix 4 (comparison) at a sintering temperature of 930°C.

Summary of the Invention

The present invention addresses all the requirements herein described, fulfilling them via the introduction of an alloy system not considered in the present field before, namely FeCrBC, and further by choosing and balancing other base constituents. In this way the present invention has been achieved.
Thus, the present invention provides a composition which is a powder composition comprising a mixed powder of the formula (1)

Fe _a Co _b Cu _c Sn _d Ni _e B _f Cr _g C _h (1)

wherein, based on (a+b+c+d+e+f+g+h) = 100 (wt.-%), a-h indicate the content of the individual elements (wt.-%):

a: is 0-85.0
b: is 0-85.0
with (a+b) being 40.0-85.0
c: is 10.0-50.0
d: is 0.1-17.5
with (c+d) being 10.1-55.0
[100*d/(c+d)] being 1.0-35.0
e: is 0.01-15.0
f: is 0.02-1.0
g: is 0.10-4.0
h: is 0.01-1.0

Also, the present invention provides a powder composition comprising, based on the entire composition, ≥ 50 wt. and preferably ≥ 60 wt.-% of the composition of any of claims 1-6, and at least one other material suitable for preparing a matrix (bond) for diamonds in a diamond cutting tool.
Even further, the present invention provides a process for producing a composite material comprising the steps of

(i) mixing a composition of any of claims 1-7 with diamond,
(ii) granulating the obtained mixture,
(iii) cold pressing the granulated mixture, and
(iv) densifying the material obtained in step (iii).

Yet further, the present invention provides a composite material made of diamond and a composition of the present invention, and a diamond tool comprising this composite material.
In addition, the present invention provides the use of a powder of a XCrBC-alloy powder, wherein X is Fe, Co or Fe and Co, as a diamond binding and/or reinforcing agent.
Preferred embodiments of the invention are as defined in the appended dependent claims and/or in the following detailed description.

Detailed Description of the Invention

The present invention provides a composition which is a powder composition comprising a mixed powder of the formula (1)

Fe _a Co _b Cu _c Sn _d Ni _e B _f Cr _g C _h (1)
The sum of the weight percentages of the constituents a-h equals 100 %, excluding impurities and oxygen. Thus, (a+b+c+d+e+f+g+h) = 100. Said mixture may then be diluted with other typical materials known to those skilled in the art, such as but not limited to tungsten powder, tungsten carbide (WC), CuAg brazing alloys, bronze powders, Ni-based Ni-Cr-Si-B alloys, NiP, FeP, etc. in order to alter its wear resistance or to confer special properties such as "self-brazing" behavior. In this case, defining j as the total weight percentage of the inventive mixture and k as the total weight percentage of the remainder constituents, any composition for which [100*j/(j+k)] ≥ 50 is considered to fall within the scope of the invention.
In other words, in one embodiment a powder composition of the invention comprises, based on the entire composition, ≥ 50 wt. and preferably ≥ 60 wt.-% of the composition of the above formula (1) together with at least one other material suitable for preparing a matrix (bond) for diamonds in a diamond cutting tool.
As a general feature of the premix route in comparison to pre-alloyed products, being the single unalloyed or low-alloyed powders relatively soft and coarse, it guarantees a good compressibility of the green parts. This results in a relatively low shrinkage rate after free sintering, because they start from a density closer to the final one. The final density for most applications needs to be at least around 96-97 % relative to theoretical full density.
The inventive mix has Fe and Cu as main constituents. Given their low mutual solubility, they form two distinct phases in the final microstructure, one Fe-rich and the other one Cu-rich. Fe has been used since decades as a lower cost, lower performance substitute for cobalt; it provides the primary diamond bonding, both mechanical as well as metallurgical, being carbon (C), the essential constituent of diamond, highly soluble into Fe. In this invention then Fe could be at least partially replaced by cobalt, without contradicting its spirit; in one preferred embodiment however, it is essentially Co-free.
In the present composition iron (Fe) is present in the composition of formula (1) in an amount corresponding to a value of a of 0-85.0, preferably 20.0-80.0, more preferably 30.0-75.0, even more preferably 40.0-70.0, and most preferably 55.0-65.0.
Also, the content of cobalt (Co), expressed in terms of b in formula (1), is 0-85.0, preferably ≤ 50.0, more preferably ≤ 30.0, even more preferably ≤ 15.0, yet more preferably ≤ 10.0, and especially may Co be absent (b = 0).
The total content of Fe and Co in the powder of formula (1), expressed as the sum (a+b), is 40.0-85.0, preferably 45.0-80.0, more preferably 50.0-75.0, even more preferably 52.0-70.0, and yet more preferably 55.0-65.0.
The ratio of Fe and Co in the powder of formula (1), expressed as the relationship [100*b/(a+b)], is 0-90.0, preferably 0-40.0, more preferably 0-20.0, and even more preferably 0-10.0.
Iron can be introduced under the form of water atomized Fe powder, electrolytic Fe powder, Fe powder made from Fe carbonyl, "sponge" iron made by e.g. direct ore reduction (Höganäs process) or any other market-available powder, in a single form or a combination thereof. In a preferred embodiment of the invention, Fe is introduced as a combination of water-atomized annealed powder (PS₉₅ = 106 µm, and preferably PS₉₅ = 75 µm) and annealed carbonyl iron (PS₉₅ = 20 µm, and preferably PS₉₅ = 15 µm).
The term "PS₉₅ = XX µm" used above here and in the following means a particle size distribution of the powder according to which about ≤ 5 wt.-% of the powder, after sieving analysis according to DIN ISO 3310-1 and -2, remain on top of the sieve having a mesh size of XX µm. Thus, the PS₉₅ value indicates that about ≥ 95 wt.-% of the powder particles have a size of less than XX µm.
The solubility of carbon into copper (Cu) is negligible. The primary role of Cu in the present powders is to increase the sintering activity of the matrix, by rendering possible its sintering under 1,000°C. Copper is introduced under the form of water atomized, air atomized, gas atomized, electrolytic, oxide-reduced or any other market-available powder in a single form or a combination thereof. In a preferred embodiment of the invention, Cu is introduced as electrolytic powder with grain size of PS₉₅ = 106 µm, and more preferably PS₉₅ = 63 µm.
In the present composition Cu is present in the composition of formula (1) in an amount corresponding to a value of c of 10.0-50.0, preferably 15.0-45.0, more preferably 20.0-40.0, and even more preferably 25.0-35.0.
Tin (Sn) is added both to reinforce copper phase as well as to lower sintering temperature, given its strong effect on melting point of bronze (Cu-Sn) alloy. Total content of bronze constituents (c+d) determines the sintering reactivity of the matrix; if too low it will undermine its densification, if too high will make the bond too weak and soft.
Therefore, the content of Sn in the present composition, expressed in terms of d in formula (1), is 0.1-17.5, preferably 0.5-15.0, more preferably 1.0-10.0, and yet more preferably 1.5-7.0.
The proportion of Sn in the bronze phase, in other words the ratio d/(c+d), can be adjusted and optimized according to sintering temperature. A too low Sn content will however be ineffective, a too high content will generate an excess of brittle delta phase.
Thus, the total content of Cu and Sn in the powder of formula (1), expressed as the sum (c+d), is 10.1-55.0, preferably 15-50.0, more preferably 25-45, and even more preferably 30-40.
The ratio of Cu and Sn in the powder of formula (1), expressed as the relationship [100*d/(c+d)], is 1.0-35.0, preferably 5.0-25.0, more preferably 6.0-18.0, and even more preferably 7.0-12.0.
Tin (Sn) can be introduced as either a CuSn- or FeSn-based alloy produced via water, air or gas atomization, diffusion-bonding or any other market-available product of this kind, as Sn-based alloy or elemental Sn powder manufactured via air, gas atomization or other customary technologies, or as a combination of both. In the preferred embodiment, Sn is added as elemental air atomized powder, with grain size of PS₉₅ = 63 µm, and more preferably PS₉₅ = 25 µm, as determined according to DIN ISO 3310-1 and -2.
Nickel (Ni) is used to adjust the overall hardness level of the bond, given its solid solution strengthening in Fe and Cu and the well-known spinodal precipitation hardening in Cu-Sn-Ni system. For soft stones, an essentially Ni-free product within the scope of the invention may still be a viable solution; for harder, more difficult to saw materials a stronger bond is required, and Ni addition is a very effective way to accommodate this demand, up to a saturation point. Its content "e" shall thus be 0,01 ≤ e ≤ 15. Nickel can be added either as elemental Ni in the form of water, air or gas atomized, electrolytic, carbonyl or any other market-available powder, or as Ni-based, FeNi-based, CoNi-based or CuNi-based alloy. In the preferred embodiment, Ni is mainly added as elemental Ni powder made from Ni carbonyl, with grain size of PS₉₅ =63 µm, and more preferably PS₉₅ =40 µm, as determined according to DIN ISO 3310-1 and -2.
The content of Ni in the present composition, expressed in terms of e in formula (1), is 0.01-15.0, preferably 0.1-12.0, more preferably 1.0-9.0, even more preferably 1.5-7.5, and yet more preferably 2.0-6.5.
Boron (B) and chromium (Cr) are notorious strong carbide formers and have already been included in formulations for diamond tool bonds to improve diamond retention by carbide formation on its surface, which serve as interface between the metallic bond and the matrix. They are typically added as fine elemental powders or sometimes as tool steels, also together with other carbide formers as Mn, Mo, W, V (see, e.g., J. Konstanty, Powder Metallurgy diamond tools, Elsevier 2005, p. 55; L. Duan, Metals 2018, pp. 4-5). They suffer however from several limitations.
The availability of such elements in the bond is affected by their local discontinuous distribution, the extremely limited redistribution due to their atomic size and the insufficient diffusion time at processing temperature, their sensitivity to oxidation. The latter is especially critical during free sintering, where no help from external pressure is available as in hot pressing and so only a very reactive powder can be successfully consolidated above the required 95% relative density, more preferably at least 97% ("near full density").
A key finding of the inventive mix is to have found a way to successfully incorporate such carbide formers while improving at the same time diamond bonding and overall mechanical strength, without substantially affecting the free sintering capability. This was achieved by introducing B and Cr by means of a FeCrBC alloy. This alloy system has been known for many years in the hardfacing industry (see e.g. A.A. Sorour, PhD thesis, Dept. of Mining and Materials Engineering, McGill University Montreal, April 2014), where such alloys are deposited by PTA, HVOF, MIG welding, Plasma Spray, etc. as anti-wear coatings. Microstructure is composed of a Fe-based matrix (microhardness ≈ 600 HV) with a dispersion of lamellar chromium-iron borides (Fe,Cr)₂B (microhardness ≈ 2.400 HV).
Fe-B system forms a eutectic at 4 wt.-% B with melting temperature of 1,174°C, down from 1,538°C for pure iron. For pure B, melting point is 2,092°C. B has thus a double role of hardener and melting agent. In the context of the invention, this renders the powder much more sinter-active than elemental B, and by diluting it in Fe also less oxidation-sensitive. Other typical additions are Si and Mn (Fe matrix hardening), Mo and V (carbide formers), Ni (better corrosion resistance thanks to austenitization on Fe-matrix).
Carbon is introduced to further reinforce the matrix, form carbides but even more importantly in this context to further lower the melting point and to protect from oxidation via the well-known C-O Boudouard reaction. The usage of pre-alloyed FeCrBC-based pre-alloys as strengthening and diamond binding agent for sintered diamond tools is not to be found anywhere in scientific or technical literature, and it is thus held as inventive in itself. Such FeCrBC-based powders are typically manufactured via atomization methods, either water, air, gas or a combination thereof. In the preferred embodiment, FeCrBC is added as gas-atomized powder with grain size of PS₉₅ = 75 µm, and more preferably of PS₉₅ = 45 µm, as determined according to DIN ISO 3310-1 and -2.
Typical but not exclusive compositions in the hardfacing industry are Fe, 10-35% Cr, 3-5% B, 0,5-2,5% C, 0-20% Ni, 0-5% Si, 0-5% Mn, plus other possible additions of Mo, V, W, Nb, N. Total boron and chromium content in the inventive mix, introduced via FeCrBC alloy, shall be balanced in order to avoid on one side ineffectiveness, and on the other side excessive embrittlement and oxidation sensitivity.
Therefore, the content of B in the present composition, expressed in terms of f in formula (1), is 0.02-1.0, preferably 0.05-0.70, more preferably 0.07-0.50, and even more preferably 0.10-0.30.
The content of Cr in the present composition, expressed in terms of g in formula (1), is 0.10-4.0, preferably 0.20-3.0, more preferably 0.30-2.5, even more preferably 0.35-1.5, and yet more preferably 0.40-0.90.
C is introduced into the inventive mix at least in conjunction with FeCrBC alloy powder. In addition to this, it was found to be advantageous, under some embodiments of the invention, to add it also as fine graphite powder. Its function is to harden the base Fe phase by interstitial solid solution and Fe-Fe₃C pearlite formation and to protect diamonds from oxidation/graphitization by acting as "sacrificial anode". The general usage of graphite in diamond tools matrix has been already studied, so it does not constitute per se an inventive feature, nor an essential one.
In the present invention, the content of C in the present composition, expressed in terms of h in formula (1), is 0.01-1.0, preferably 0.05-0.8, more preferably 0.08-0.6, still more preferably 0.10-0.50, and even more preferably 0.12-0.40.
The above described constituents shall be mixed according to usual procedures familiar to those skilled in the art, to create a homogeneous, agglomerate-free dispersion. The precise mixing method is not seen as a critical aspect of the invention; it may involve, without being limited to, double-cone mixer, rotating cylinder, "turbula mixer" or any other device, with or without mixing aids. Ball milling may also be employed in this phase. The mixture may then be admixed with an organic binder and other pressing aids to be granulated, or directly mixed with the quantity and grade of diamonds according to required application; this step may also occur at the same time the single constituents are brought together. The bond - diamond mix is then cold pressed, typically under 200-300 MPa pressure. If the hot-pressing route is selected, the bond - diamond mix may be directly fed to the graphite die.
In general, in the production of a composite material made of diamond and a composition of the present invention, the present metal powder composition is mixed with diamonds (typically 5-10 vol-%), granulated and then cold pressed. The composite is then consolidated close to full density via hot pressing or free sintering processes.
In hot pressing (HP), a graphite die is resistance-heated thus allowing the simultaneous application of heat and pressure. Typical conditions are temperatures of 650-900°C, preferably 700-850°C, and more preferably 750-800°C, and pressures of 20-45 MPa, preferably 25-40 MPa, and more preferably 30-35 MPa. The pressing may be conducted for e.g. 1-10 minutes, such as 2-8 or 3-5 minutes in air, inert gas or under vacuum. For example, hot pressing at 780-850°C and 35 MPa for 3 minutes under air/N₂/vacuum is a common process in the industry.
Free sintering (FS) is the "standard" sintering process on a belt furnace. Typical conditions are 850-1,000°C, preferably 900-950°C, and more preferably 910-930°C for 30-120 minutes, typically 45-90 minutes, such as 60 minutes, under N₂ + H₂. This process has become widespread for wire beads thanks to higher throughput. Such beads have also to be brazed to a steel sleeve in a second step or by using self-brazing matrixes.
The bond - diamond mix is then consolidated via hot-pressing, free sintering, hot isostatic pressing (HIP) or any of the conventional techniques known to those skilled in the art. No complicated extra steps nor special equipment, atmospheres, etc. are required. Under free sintering consolidation the inventive product is particularly attractive, because of relatively low linear shrinkage required to reach near full density, in comparison to traditional pre-alloys. This descends directly from its better compressibility, which means it already starts from a higher density (around 77 % of theoretical value, while for pre-alloys around 60% is typical; see, e.g., J. M. Sanchez, Powder Metallurgy Powder Metallurgy 2014, Vol. 56, p. 362-373) and thus less shrinkage is required to reach the same final value. The chart shown in Figure 1 illustrates this relationship. This allows for a very significant improvement in dimensional precision of the sintered components and thus reduced scrap rate.

Examples

Example 1 (Ex-1)

Water atomized iron (Fe_WA) powder of PS₉₅ = 75 µm, annealed carbonyl iron (Fe_CO) powder of PS₉₅ = 15 µm, electrolytic copper powder of PS₉₅ = 63 µm, air atomized tin powder of PS₉₅ = 20 µm, natural graphite of PS₉₅ = 15 µm, carbonyl nickel of PS₉₅ = 40 µm, gas atomized FeCr_14,5Ni_6,2B_3,4Si_2,8C_2,1Mn_0,3 -50 µm are mixed together in the proportions shown in Table 1 for 30 minutes in a rotating can under air. Can filling factor is 33%. A homogeneous mixture of the constituents is thus obtained, total weight 200 g for each mixture. Table 1: Composition of inventive mixtures 1' and 2'

Content (%)

Mix Fe_WA Fe_CO C Cu Sn Ni FeCrBC

1' 27,90 31,47 0,03 32,36 2,81 2,30 3,13

2' 25,46 28,71 0,15 31,23 2,72 5,70 6,04
The mixtures of are then admixed with 0,6% of solid lubricant (Acrawax) and further homogenized for 15 minutes. The lubricant is only added to facilitate the subsequent pressing operation, for the testing of the mixture. These new mixtures will be henceforth referred to as "Mix 1" and "Mix 2".
100 g of Mix 1 and Mix 2 are each mixed with 2,79% of synthetic diamonds type "SASG80_40/50" (of Shannon Abrasives). This ratio corresponds to a 6,27% volume concentration, i. e. C25 (1,1 carats/cm³). Mixing time is 15 minutes. The obtained diamond-containing mixtures are referred to as "Mix 1D" and "Mix 2D".
Tensile bars according to ISO norm 2740 and weighing around 14 g each are compacted for Mix 1 and Mix 2 at 200, 300 or 400 MPa. Density is then measured via geometric method, by measuring bar dimensions with micrometer. Percentage density is calculated based on theoretical values, respectively 8,19 g/cm³ form Mix 1 and 8,20 g/cm³ for Mix 2 and subtracting the 0,60% lubricant contribution to weight. Table 2 presents the results of compressibility curves; it can be readily seen that the inventive products can be compacted to a relatively high density around 76% of theoretical values, even reaching 80% and above with 300 MPa. Table 2: Compressibility curve of inventive mixtures

P [MPa] Green density [g/cm³] Green density [%]

200 6,29 76,3

Mix 1 300 6,69 81,1

400 6,93 84,1

Mix 2 200 6,24 75,6

300 6,57 79,6

400 6,84 82,9
For Mix 1, Mix 2, Mix 1D and Mix 2D TRS bars are also compacted at 200 MPa in a die with cavity measuring 35,08 x 7,13 mm. Weight of the single bars is around 8 g each.
Sintering is performed in a static tubular furnace according to the parameters described in Table 3, for test pieces compacted at 200 MPa. T_max is the sintering i.e. dwell temperature and, in this case, was set to 890°C, 910°C, 930°C or 950°C. Test and TRS bars are sintered together under such conditions on steel trays, then after Step 3 the trays are manually moved to the forward zone of the furnace and left to cool naturally for 60 minutes (Step 4), before being taken out and spontaneously further cooled to room temperature. Atmosphere is 40% H₂ and 60% N₂, with total flow of 370 l/h. Table 3: Description of sintering cycle for Example 1

Step

1 2 3 4

Temperature [°C] 400 400 → T_max T_max T_max → 300°C

Dwelling time [min] 15 * 60 60

Heating rate [°C/h] * 150 * *
The sintered components are subjected to the following investigations. Dimensional change ΔL% is evaluated by taking the length of sintered tensile bars and expressing it as percentage variation to die cavity length of 89,40 mm. Density is assessed using water displacement method (Archimedes method) for all sintered components. Hardness is measured on the surface of tensile bars after polishing with #320 sand paper, according to ISO 6506 using a Brinell 2,5 mm spherical indenter and 187,5 kg load. Tensile properties (tensile strength Rm, 0,20% yield strength Rp_0,2, plastic elongation at fracture A%) are assessed on a Zwick machine according to DIN EN ISO 10002/2000, with 10 mm/min crosshead speed. Bending strength is measured for sintered TRS bars according to DIN EN ISO 7438/2000, under 3-points bending with support span of 25 mm and 10 mm/min crosshead speed. The ratio of bending strength between diamond-containing and diamond-free mixes, identified as Mix nD and Mix n (with n = 1, 2, ...), is called "holding force coefficient" %σ_R: $% σ_{R} = 100 \times \frac{σ_{R, Mix nD}}{σ_{R, Mix n}}$
where σ_{R,Mix nD} and σ_{R,Mix n} are respectively the averaged bending strengths of Mix nD and Mix n. Since the diamond acts as a stress concentrator and a discontinuity in the material, it invariably leads to a lower bending strength, particularly in case of discontinuous interface with metal matrix, brittle phase formation, local development of gases from reduction reactions, etc. The closer to 100 then %σ_R is, the stronger the metallurgical bonding between diamond and matrix, at least in general terms (see, e.g., Xiaojun Zhao et al. Metals 2018, pp. 4-5). Further investigation to study diamond - matrix interaction is then carried out using scanning electronic microscope on fracture surface of diamond-containing mix, i.e. Mix 1D and Mix 2D.

Table 4 shows the results for the inventive mixtures Mix 1 and Mix 2. A density above 96% of theoretical can be reached within a wide processing window above 900°C, particularly between 910°C and 930°C which are typical processing temperatures for the free sintering of diamond tools. Tensile properties are on par or even superior with what reported in scientific literature for pre-alloyed grades (see, e.g., A. M. Mancisidor et al., "Effect of the sintering atmosphere on the densification, mechanical properties and diamond stability of prealloyed diamond impregnated composites obtained by free-sintering", International Powder Metallurgy Congress and Exhibition, Euro PM 2013). The different hardness levels of Mix 1 and Mix 2 make them suitable for different applications, according to hardness and abrasivity of material to be cut.

Table 4: Results after sintering for tensile bars

Mix	TS*	Sinter density		ΔL%	Rm	Rp0,2	A%	H.*
Mix	[°C]	[g/cm³]	[%]	[%]	[MPa]	[MPa]	[%]	H.*
1	890	7,66	93,5	-5,5	505	430	3,1	172
	910	8,03	98,1	-7,1	620	500	5,0	215
	930	8,03	98,0	-7,0	660	530	4,4	219
	950	8,01	97,8	-7,2	635	555	2,2	221
2	890	7,50	91,5	-5,1	485	425	0,7	197
	910	7,94	96,8	-6,6	750	710	0,4	298
	930	7,95	97,0	-6,7	770	755	0,2	304
	950	7,90	96,3	-6,5	725	700	0,2	300

*: TS = Sintering Temperature, H = Brinell Hardness [HB_2,5/187,5]

Results on test bars confirm the densification behavior obtained on tensile bars, with minimal loss due to the introduction of diamonds. Bending strength with and without diamonds are also on par with pre-alloyed grades as reported in scientific literature (see, e.g., A. M. Mancisidor et al., "Effect of the sintering atmosphere on the densification, mechanical properties and diamond stability of prealloyed diamond impregnated composites obtained by free-sintering", International Powder Metallurgy Congress and Exhibition, Euro PM 2013), as well as the holding force coefficient for the respective hardness levels.

Table 5: Results after sintering for TRS bars

		Without Diamonds			With Diamonds
Mix	T sinter	Sinter density		σR	Sinter density		σR	%σR
Mix	[°C]	[g/cm³]	[%]	[MPa]	[g/cm³]	[%]	[Mpa]	[%]
1	890	7,65	93,4	1144	7,24	91,6	829	72,5
	910	8,04	98,2	1364	7,73	97,8	1016	74,5
	930	8,03	98,1	1328	7,70	97,5	999	75,2
	950	8,01	97,8	1320	7,68	97,2	909	68,9
2	890	7,49	91,4	1010	7,19	90,9	719	71,2
	910	7,95	96,9	1383	7,61	96,2	830	60,0
	930	7,97	97,2	1384	7,65	96,8	872	63,0
	950	7,93	96,7	1298	7,61	96,2	884	68,1

SEM analysis of diamonds on fracture surface are shown in Fig. 2. The SEM photographs confirm a high compatibility with the metal bond. Edges remain sharp, without any sign of degradation up to 930°C. The fractured spots observable on 930°C pictures are an indication of strong matrix - diamond interfacial bonding, with local preferential rupture path going through the stone. Some graphitization starts to occur for Mix 1D at 950°C, while Mix 2D can still well preserve the stones even at this relatively high temperature.

Comparative Example 1 (CE-1)

Water atomized iron powder (Fe_WA) of PS₉₅ = 75 µm, annealed carbonyl iron (Fe_CO) powder of PS₉₅ = 15 µm, electrolytic copper powder of PS₉₅ = 63 µm, air atomized tin powder of PS₉₅ = 20 µm, natural graphite of PS₉₅ = 15 µm, carbonyl nickel of PS₉₅ = 40 µm, gas atomized FeCr_14,5Ni_6,2B_3,4Si_2,8C_2,1Mn_0,3 of PS₉₅ = 50 µm and tool steel powder H13 (FeCr₅Mo_1,5Si₁V₁C_0,4Mn_0,3) of PS₉₅ = 63 µm are mixed together in the proportions shown in Table 6 for 30 minutes in a rotating can under air. Can filling factor is around 33 %. A homogeneous mixture of the constituents is thus obtained, total weight 200 g for each mixture. It shall be noticed that Mixture 4' is not according to the invention, belonging H13 steel not to FeCrBC family. Table 6: Composition of mixtures 3' (invention) and 4' (comp.)

Content in %

Mix Fe_WA Fe_CO C Cu Sn Ni FeCrBC H13

3' 22,71 28,41 0,14 34,96 3,04 5,00 5,70 0,00

4' 25,58 25,58 0,14 34,96 3,04 5,00 0,00 5,70
The mixtures are then admixed with 0,6 % of solid lubricant (Acrawax) and further homogenized for 15 minutes. The lubricant is only added to facilitate the subsequent pressing operation, for the testing of the mixture. These new mixtures will be henceforth referred to as "Mix 3" and "Mix 4".
100 g of Mix 3 and Mix 4 are each mixed with 2,79 % of synthetic diamonds type "SASG80_40/50" (of Shannon Abrasives). This ratio corresponds to a 6,27% volume concentration, i. e. C25 (1,1 carats/cm³). Mixing time is 15 minutes. These obtained diamond-containing mixtures are referred to as "Mix 3D" and "Mix 4D". Tensile and TRS bars are pressed at 200 MPa, as per procedures already described in Ex-1. Sintering is performed in a static tubular furnace according to the parameters described in Table 7. Test and TRS bars are sintered together under such conditions on steel trays, then after Step 3 the trays are manually moved to the forward zone of the furnace and left to cool naturally for 60 minutes (Step 4), before being taken out and spontaneously further cooled to room temperature. Atmosphere is 20% H₂ and 80% N₂, with total flow of 290 l/hour. Table 7: Sintering cycle for Comparative Example 1

Step 1 Step 2 Step 3 Step 4

Temperature [°C] 400 400 → Tmax 930 Tmax → 300

Dwelling time [min] 15 * 60 60

Heating rate [°C/h] * 300 * *
The sintered components are then subjected to the same tests described in Example 1. Tables 8 and 9 show the results. Although a very good densification can be achieved with H13 addition, mechanical strength and hardness are inferior. Yield strength is 50% improved by FeCrBC addition, which directly translates into better mechanical diamond retention and thus superior cutting properties. Table 8: sintering at 930 °C for Mix 3 (inv.) and Mix 4 (comp.)

Mix Sinter density ΔL% Rm Rp0,2 A% Hardness*

[g/cm3] [%] [%] [MPa] [MPa] [%]

3 7,87 95,9 -6,48 705 652 0,6 268

4 8,08 98,3 -7,43 614 441 1,9 246

*: Hardness = Brinell Hardness [HB_2,5/185,5]

Mechanical properties with diamonds are also improved, although the difference is less marked. The SEM investigation as shown in Fig. 3 however revealed a marked difference in the diamond-matrix interaction. Inventive Mix 3 preserves the stone cutting edges, with fracture happening by pull-out and partially through the stone itself, a sign of strong interface. Non-inventive Matrix 4 achieve similar holding force coefficient by a much deeper alteration of the stones, due to extensive surface interactions triggered by carbide-forming elements such as Cr and V. The fracture pathway runs mainly along the carbides-based interface, which is a sign of its brittleness.

Table 9: Sintering at 930°C for Mix 3 (inv.) and Mix 4 (comp.)

Mix	Without diamonds			With Diamonds
	Sinter density		σR	Sinter density		σR	%σR
	[g/cm3]	[%]	[MPa]	[g/cm3]	[%]	[Mpa]	[%]
3	7,69	96,3	1189	7,73	96,8	821	69
4	7,91	98,8	1115	7,91	98,8	778	70

Comparative Example 2 (CE-2)

Water atomized iron (Fe_WA) powder of PS₉₅ = 75 µm, annealed carbonyl iron (Fe_CO) powder of PS₉₅ = 15 µm, electrolytic copper powder of PS₉₅ = 63 µm, air atomized tin powder of PS₉₅ = 20 µm, carbon in form of natural graphite of PS₉₅ = 15 µm, atomized FeCr₁₃V₆C_3,7 of PS₉₅ = 63 µm or -30 µm are mixed together in the proportions shown in Table 9 for 30 minutes in a rotating can under air. Can filling factor is around 33 %. A homogeneous mixture of the constituents is thus obtained, total weight 200 g for each mixture. It shall be noticed that Mixture 5' and 6' are not according to the invention, belonging to FeCr₁₃V₆C_3,7 steels and not to the FeCrBC family. Table 10: Composition of comparative mixtures 5' and 6'

Content (%)

Mix Fe_WA Fe_CO C Cu Sn Ni FeCrVC 63 µm FeCrVC 30 µm

5' 23,93 23,93 0,15 36,80 3,20 - 12,0 0,00

6' 23,93 23,93 0,15 36,80 3,20 - 0,00 12,0
The mixtures are then admixed with 0,6 % of solid lubricant (Acrawax) and further homogenized for 15 minutes. The lubricant is only added to facilitate the subsequent pressing operation, for the testing of the mixture. These new mixtures will be henceforth referred to as "Mix 5" and "Mix 6". Tensile bars are pressed at 200 MPa, as per procedures already described in Ex-1. Sintering is performed in a static tubular furnace according to the same parameters described in CE-1. The sintered components are then subjected to the same investigations described in CE-1, with results detailed in Table 11. The addition of the non-inventive FeCrVC alloy yields very negative sintering behavior, with sinter density below 90%. As a consequence, all mechanical properties drop to an unacceptable extent. This is to be ascribed to the oxidation sensitivity typical of high Cr alloys not part of the invention, which transfers to the overall matrix and compromises its densification. Table 11: Sintering at 930°C of comparative tensile bars.

Mix Sinter density ΔL% Rm Rp0,2 A% Hardness*

[g/cm³] [%] [%] [MPa] [MPa] [%]

5 7,20 88,0 -3,54 183 NA <0,1 78

6 7,34 89,7 -3,59 211 NA <0,1 78

*: Hardness = Brinell Hardness [HB_2,5/187,5]
The invention described in the present application in detail above includes, among others, the following embodiments:

(1) A composition which is a powder composition comprising a mixed powder of the formula (1)

Fe _a Co _b Cu _c Sn _d Ni _e B _f Cr _g C _h (1)

Wherein, based on (a+b+c+d+e+f+g+h) = 100 (wt.-%), a-h indicate the content of the individual elements (wt.-%):

a

is 0-85.0

b

is 0-85.0
with (a+b) being 40.0-85.0

c

is 10.0-50.0

d

is 0.1-17.5 with (c+d) being 10.1-55.0
[100^∗ d/(c+d)] being 1.0-35.0

e

is 0.01-15.0

f

is 0.02-1.0

g

is 0.10-4.0

h

is 0.01-1.0

wherein the content of the composition of formula (1) is ≥ 98 wt.-% of the entire composition.
(2) The composition of embodiment (1), which
contains, based on the entire composition, ≥ 99 wt.-% of the composition of formula (1), and
preferably consists of the mixed powder of the formula (1) and unavoidable impurities.
(3) The composition of embodiment (1) or (2), wherein the mixed powder of formula (1) includes powder of a XCrBC-alloy which
- comprises X (wherein X is Fe, Co, or Fe and Co), Cr, B and C; and
- preferably is a XCrBC-alloy consisting of
  - 5.0-41.0 wt.-%, preferably 10.0-35.0 wt.-%, of Cr,
  - 1.0-6.0 wt.-%, preferably 3.0-5.0 wt.-%, of B,
  - 0.3-3.5 wt.-%, preferably 1.0-3.0 wt.-%, of C,
  - 0-20.0 wt.-% of Ni,
  - 0-5.0 wt.-% of Si,
  - 0-5.0 wt.-% of Mn,
  - 0-20.0 wt.-%, preferably 0-15.0 wt.-% and more preferably 0-10.0 wt.-%, of a total of other elements, and
  - the remainder being Fe, Co or a mixture of both, and unavoidable impurities.
(4) The composition of embodiment (3), wherein the other elements are selected from Mo, V, W, Nb and N.
(5) The composition of embodiment (3) or (4), wherein in the mixed powder of formula (1) the entire content of B and Cr is present in the XCrBC-alloy powder.
(6) The composition of any of embodiments (3)-(5), wherein in the mixed powder of formula (1) X is Fe.
(7) The composition of any of embodiments (1)-(6), wherein the mixed powder of the formula (1) includes natural graphite.
(8) The composition of any of embodiments (1)-(7), wherein in formula (1) a is 20-80, preferably 30-75, more preferably 40-70, and even more preferably 55-65.
(9) The composition of any of embodiments (1)-(8), wherein in formula (1) b is ≤ 50, preferably ≤ 30, more preferably ≤ 15, even more preferably ≤ 10, and yet more preferably b = 0.
(10) The composition of any of embodiments (1)-(9), wherein in formula (1) (a+b) is 45-80, preferably 50-75, more preferably 52-70, and even more preferably 55-65.
(11) The composition of any of embodiments (1)-(10), wherein in formula (1) [100*b/(a+b)] is 0-90, preferably 0-40, more preferably 0-20, and even more preferably 0-10.
(12) The composition of any of embodiments (1)-(11), wherein in formula (1) c is 15-45, preferably 20-40, and more preferably 25-35.
(13) The composition of any of embodiments (1)-(12), wherein in formula (1) d is 0.5-15.0, preferably 1.0-10.0, and more preferably 1.5-7.0.
(14) The composition of any of embodiments (1)-(13), wherein in formula (1) (c+d) is 15-50, preferably 25 - 45, and even more preferably 30 - 40.
(15) The composition of any of embodiments (1)-(14), wherein in formula (1) [100*d/(c+d)] is 5.0-25.0, preferably 6.0-18.0, and more preferably 7.0-12.0.
(16) The composition of any of embodiments (1)-(15), wherein in formula (1) e is 0.1-12.0, preferably 1.0-9.0, more preferably 1.5-7.5, and even more preferably 2.0-6.5.
(17) The composition of any of embodiments (1)-(16), wherein in formula (1) f is 0.05-0.70, preferably 0.07-0.50, and more preferably 0.10-0.30.
(18) The composition of any of embodiments (1)-(17), wherein in formula (1) g is 0.20-3.0, preferably 0.30-2.5, more preferably 0.35-1.5, and even more preferably 0.40-0.90.
(19) The composition of any of embodiments (1)-(18), wherein in formula (1) h is 0.05-0.8, preferably 0.08-0.6, more preferably 0.10-0.50, and even more preferably 0.12-0.40.
(20) The composition of any of embodiments (1)-(19), wherein the powder has a particle size (D₅₀) of 5-50 µm, preferably 10-40 µm and more preferably 15-25 µm.
(21) A powder composition comprising, based on the entire composition, ≥ 50 wt. and preferably ≥ 60 wt.-% of the composition of any of embodiments (1)-(20), and at least one other material suitable for preparing a matrix (bond) for diamonds in a diamond cutting tool.
(22) The powder composition of embodiment (21), wherein the other material is at least one of tungsten powder, tungsten carbide (WC), CuAg brazing alloys, bronze powders, Ni-based Ni-Cr-Si-B alloys, NiP and FeP.
(23) A process for producing a composite material comprising the steps of
1. (i) mixing a composition of any of embodiments (1)-(22) with diamond,
2. (ii) granulating the obtained mixture,
3. (iii) cold pressing the granulated mixture, and
4. (iv) densifying the material obtained in step (iii).
(24) The process of embodiment (23), wherein the amount of diamond mixed in step (i) is 0.5-10 parts by weight (pbw), preferably 1.0-8.0 pbw, and more preferably 2.0-6.0 pbw per 100 pbw of the composition of any of embodiments (1)-(20).
(25) The process of embodiment (23) or (24), wherein step (vi) is performed by
- free sintering, preferably at a temperature of 850-1,000°C, preferably 900-950°C, and more preferably 910-930°C; or
- hot pressing, preferably at a temperature of 650-900°C, preferably 700-850°C, and more preferably 750-800°C.
(26) The process of any of embodiments (23)-(25), wherein step (vi) is performed by hot pressing at a pressure of 20-45 MPa, preferably 25-40 MPa, and more preferably 30-35 MPa.
(27) Composite material made of diamond and a composition of any of embodiments (1)-(22).
(28) The composite material of embodiment (27) which is obtainable by the process of any of embodiments 23-26.
(29) Diamond tool, preferably a diamond cutting tool, comprising the composite material of embodiment (27) or (28) .
(30) The use of a powder of a XCrBC-alloy powder, wherein X is Fe, Co or a mixture of both, and preferably X is Fe, as a diamond binding and/or reinforcing agent.

Claims

A composition which is a powder composition comprising a mixed powder of the formula (1)

Fe _a Co _b Cu _c Sn _d Ni _e B _f Cr _g C _h (1)

Wherein, based on (a+b+c+d+e+f+g+h) = 100 (wt.-%), a-h indicate the content of the individual elements (wt.-%):
a is 0-85.0

b is 0-85.0
with (a+b) being 40.0-85.0

c is 10.0-50.0

d is 0.1-17.5
with (c+d) being 10.1-55.0
[100*d/(c+d)] being 1.0-35.0

e is 0.01-15.0

f is 0.02-1.0

g is 0.10-4.0

h is 0.01-1.0
wherein the content of the composition of formula (1) is ≥ 98 wt.-% of the entire composition.
The composition of claim 1, wherein the mixed powder of formula (1) includes powder of a XCrBC-alloy which
- comprises X (wherein X is Fe, Co, or Fe and Co), Cr, B and C; and

- preferably is a XCrBC-alloy consisting of 5.0-41.0 wt.-%, preferably 10.0-35.0 wt.-%, of Cr, 1.0-6.0 wt.-%, preferably 3.0-5.0 wt.-%, of B, 0.3-3.5 wt.-%, preferably 1,0-3,0 wt.-%, of C, 0-20.0 wt.-% of Ni, 0-5.0 wt.-% of Si, 0-5.0 wt.-% of Mn, 0-20.0 wt.-%, preferably 0-15.0 wt.-% and more preferably 0-10.0 wt.-%, of a total of other, and
the remainder being Fe, Co or a mixture of both, and unavoidable impurities.
The composition of claim 1 or 2, wherein in the mixed powder of formula (1) the entire content of B and Cr is present in the XCrBC-alloy powder.
The composition of any of claims 1-3, wherein X in the XCrBC-alloy is Fe.
The composition of any of claims 1-4, wherein in formula (1) b is ≤ 50.0, preferably ≤ 30.0, more preferably ≤ 15.0, even more preferably ≤ 10.0, and yet more preferably b = 0.
A powder composition comprising, based on the entire composition, ≥ 50 wt. and preferably ≥ 60 wt.-% of the composition of any of claims 1-5, and at least one other material suitable for preparing a matrix (bond) for diamonds in a diamond cutting tool.
The powder composition of claim 6, wherein the other material is at least one of tungsten powder, tungsten carbide (WC), CuAg brazing alloys, bronze powders, Ni-based Ni-Cr-Si-B alloys, NiP and FeP.
A process for producing a composite material comprising the steps of
(i) mixing a composition of any of claims 1-7 with diamond,

(ii) granulating the obtained mixture,

(iii) cold pressing the granulated mixture, and

(iv) densifying the material obtained in step (iii).
The process of claim 8, wherein the amount of diamond mixed in step (i) is 0.5-10 parts by weight (pbw), preferably 1.0-8.0 pbw, and more preferably 2.0-6.0 pbw per 100 pbw of the composition of any of claims 1-19.
The process of claim 8 or 9, wherein step (vi) is performed by
- free sintering, preferably at a temperature of 850-1,000°C, preferably 900-950°C, and more preferably 910-930°C; or

- hot pressing, preferably at a temperature of 650-900°C, preferably 700-850°C, and more preferably 750-800°C.
Composite material made of diamond and a composition of any of claims 1-7.
Diamond tool, preferably a diamond cutting tool, comprising the composite material of claim 11.
The use of a powder of a XCrBC-alloy powder, wherein X is Fe, Co or a mixture of both, and preferably X is Fe, as a diamond binding and/or reinforcing agent.