WO1993011274A1

WO1993011274A1 - Process for treating sintered tungsten carbide parts and parts obtained such as cutting tools and wear parts

Info

Publication number: WO1993011274A1
Application number: PCT/FR1992/001125
Authority: WO
Inventors: Alain Basse; Sylvain Bintivegna; Pascal Nisio; Bernard Thomassery
Original assignee: Coating Developpement
Priority date: 1991-12-05
Filing date: 1992-12-02
Publication date: 1993-06-10
Also published as: FR2684692B1; IL103946A0; FR2684692A1

Abstract

Process for treating a cobalt sintered tungsten carbide part. The process consists in chemically vapour-depositing (CVD) on the part a crystallized titanium carbide coating, in the presence of boron, so as to obtain a discontinuous underlayer of CoWB crystals under the titanium carbide layer, and then in chemically vapour-depositing an amorphous boron carbide coating, in which the basic carbon content is between 20 % and 35 %. The process of the invention can be used to treat cutting tools or tungsten carbide wear parts in order to improve their mechanical performance and surface qualities.

Description

PROCESS FOR TREATING SINTERED TUNGSTEN CARE PARTS AND PARTS OBTAINED SUCH AS CUTTING TOOLS

OR WEAR PARTS

The invention relates to a method for treating cobalt sintered tungsten carbide parts such as cutting tools or wearing parts. It extends to the parts obtained. A significant proportion of cutting tools

(turning, milling, drilling, reaming, drilling ...) or wearing parts (bearings, wire guides, stretching cores, shaping tools ...) is made of sintered tungsten carbide, the binder of which consists of cobalt (generally in proportion by weight of the order of 5 to 20 '); sometimes this frit contains a minority proportion of other compounds such as mixed carbide of titanium, tantalum and niobium. This type of frit has an excellent hardness / toughness compromise, well suited to severe working conditions.

To further improve the performance of this sintered material or extend its field of application, tool manufacturers have coated it with crystallized layers, generally deposited by CVD, such as TiC, TiN, O, TiCN or carbide layer of crystallized boron. These coatings lead to an improvement in performance, in particular to better resistance to wear and abrasion. However, these performances are still insufficient in certain fields (in particular for the machining of metals other than standard steel: cast iron, titanium, certain stainless steels, etc.). In addition, the crystalline nature of the coating layers leads to a certain number of surface defects, originating from their structure in oriented grains of given morphology. In particular, in the case of very sharp edges, it is not possible to maintain the sharpness of said edges after depositing the coating. In addition, the crystal structure conditions an imperfect surface state with a significant microroughness, which increases the friction of tools or parts in the working phase, which results in a significant increase in temperatures. surface and cutting forces and a reduction in the quality of machining.

Furthermore, in different sectors,

5 amorphous deposits of boron carbide have already been produced, in particular for coating fibers, composite fabrics, carbon ribbons, with the aim of manufacturing composite materials with specific characteristics (patents

US 3-799.830, 4.287.259, 3-967.029, 3.537-877). Attempts have been made to use this type of amorphous deposit on cutting tools in order to improve their performance; however, they have led to failure mainly due to problems of adhesion and brittleness of the coating. Some tests have, in particular, consisted in manufacturing a thick layer of amorphous boron carbide on a substrate, in detaching it from its substrate and in bonding or brazing it on the tool; however, this method leads to very fragile cutting edges, subject to chipping and cracking so that this treatment method is currently not used, and all the more so since its cost is very high.

In addition, an article published in April 1988 describes tests for the deposition of boron carbide and evokes the manufacture of tools coated with a layer of amorphous boron carbide ("Mikael Olsson et al., Mechanical and tribological properties of chemically vapor -deposited boron Carbide Coatings, Mat. Sci. Eng. A 105/106, 1988, p. 453 to 463 "). These deposits made by CVD on tools already coated with titanium carbide sometimes led to crystallized boron carbide, sometimes to amorphous boron carbide; in the case of these amorphous deposits, the authors of this article indicate:

- page 453, col. 1, lines 22-23: "••• the adhesion of the amorphous coating to TiC is very poor.",

- page 453, col. 1, line 27 at col. 2, line 2: "... the amorphous boron carbide gave very poor performance owing to its poor adhesion.",

- page 459, col. 1, lines 4-8: "In the case of the amorphous boron carbide, the hardness could not be established since the indentation induced spallation of the coating even at very low loads. This indieates an extremely poor adhesion of the amorphous boron carbide to TiC ",

- page 459, col. 2, lines 38-41: "... the amorphous coating displays an inferior erosion resistance, even compared with TiC. This is not unexpected, considering the poor adhesion observed earlier.".

This article therefore highlights the adhesion problems of amorphous boron carbide and, despite the improvements that one might expect, concludes that such a layer cannot be used on tools:

- page 461, col. 2, lines 35-37: "The results from the simulated tool wear test do not indicate any beneficiiai effect from boron carbide in the machining of steel." The present invention proposes to solve the abovementioned problems of adhesion in order to allow parts of sintered tungsten carbide to be coated with a surface layer of amorphous boron carbide which is perfectly adherent to the part. The objective of the invention is thus to provide solid parts made of cobalt sintered tungsten carbide, such as cutting tools or wearing parts, which benefit from increased mechanical performance and an improved surface condition compared to to existing parts, making it possible in particular to obtain excellent sharpness of the cutting edges, slower and more regular wear and better machining quality.

To this end, the process according to the invention for treating a piece of cobalt sintered tungsten carbide consists of:

. to carry out a chemical vapor deposition (CVD) of crystallized titanium carbide TiC on the part, said deposition being carried out in the presence of boron so as to obtain a discontinuous sublayer of boride crystals under the layer of titanium carbide TiC mixed cobalt and tungsten CoWB by reaction of boron on tungsten carbide in the vicinity of the joints occupied by cobalt,

. to then carry out a chemical vapor deposition (CVD) of boron carbide, said deposition being carried out so as to obtain on the αe carbide αe titanium layer TiC a layer of amorphous boron carbide a-BC in which the elementary carbon content is between 20% and

35%. The process of the invention provides a coated part in which the layers adhere perfectly and are not subject to any tendency to detachment. It has been observed that, in the case of very high loads, flaking occurs inside the tungsten carbide and not in the coating. The coated part thus fully benefits from the performance conferred by the amorphous structure of the deposit of surface boron carbide. The absence of grains makes it possible to obtain a much finer surface microroughness than in the case of crystallized layers and a greater edge acuity (isotropy of the amorphous deposit). In addition, the mechanical properties are notably improved compared to the crystallized layers of boron carbide due to the absence of grain boundaries (which constitute in the crystalline structures initiators of rupture and privileged paths of diffusion of external elements) .

The treatment method of the invention was designed and developed at the end of a study comprising three stages consisting in understanding and identifying the causes of poor adhesion of previously manufactured amorphous boron carbide coatings, in imagining a process for eliminating these causes, to design and develop practical means of implementation. Firstly, the studies carried out made it possible to highlight that the detachments and flaking of the coating had two causes coming, on the one hand, from the diffusion of carbon from sintered tungsten carbide towards the WC-Co / TiC interface , on the other hand, from the diffusion of cobalt towards this interface, then through the TiC layer, towards the TiC / a-BC interface. The first phenomenon produces at the WC-Co / Tic interface an eta CogWgC phase sublayer which is very fragile. The second phenomenon produced at the TiC / a-BC interface more or less contiguous islets, poorly identified but probably formed of cobalt, cobalt boride and titanium diboride: these islets which are interposed between the TiC layer and the layer amorphous a-BC cause very poor adhesion at this level. The second stage of the study consisted in finding a process to oppose these causes (formation of the eta phase at the sintered material / TiC interface, diffusion of cobalt towards

5 TiC / a-BC interface). This process consists in forming at the sintered material / TiC interface a barrier made up of a compound more stable than the eta phase which will therefore prevent the formation of this phase and also prevent the diffusion of cobalt through the TiC layer. The last step of the study consisted in defining the nature of the barrier to be used and the production method; in the invention, this barrier consists of a crystallized sublayer of mixed cobalt and tungsten boride which meets the abovementioned requirements; it should be noted that this 5 sub-layer is discontinuous and forms, in a network, at the joints between the grains of tungsten carbide: it fulfills its function perfectly since the diffusion of carbon and cobalt takes place through these joints .

In addition, this sublayer whose hardness is 0 intermediate between that of tungsten carbide WC-Co and that of titanium carbide TiC contributes to creating in the part a hardness gradient extremely favorable to the mechanical resistance of the assembly ( better stress distribution). 5 The process of the invention lends itself to an implementation at reduced cost, not increased compared to known methods. Indeed, the boron intended for manufacturing the mixed boride sublayer can very simply be previously placed in the reaction chamber of the deposition reactor (in particular in the form of a deposit of boron carbide on the walls) so that displacement of boron occurs at the start of chemical vapor deposition of the TiC layer. The boron carbide present on the walls of the reactor can come from a previous deposition cycle since each cycle ends with a deposition of a-BC. Thus, the process of the invention simply requires that boron is available in the reaction chamber during the first manufacturing cycle and that the layers of TiC and a-BC are produced successively in the same reactor. Q Preferably CVD repositories to form o the two layers TiC and a-BC are produced successively without intermediate cooling. Tests have shown that this embodiment further increases the adhesion of the a-BC amorphous layer on the TiC crystal layer.

The treatment process according to the invention is advantageously carried out under the following conditions.

CVD deposition of titanium carbide TiC is carried out by reducing a gaseous mixture of titanium tetrachloride TiCl _j , and methane in the presence of hydrogen under the following operating conditions: temperature between 930 ° C and 1050 ° C, absolute pressure included between 10 ^ and 10 ^ Pa, molar flow of the gas mixture between 3.10 "" °. and 6.10 "^ mole.cm ~ 2.s ~ 1, atomic ratio C / Ti of the mixture between 0.5 and 15, ratio atomic H / Ti between 5 and 150.

This deposition of titanium carbide TiC is preferably carried out for a period suitable for obtaining a layer with a thickness of between 0.15 and 5 microns. This thickness is preferable in practice because, below the lower limit, there exists during the second CVD deposition, risks of migration of boron through the layer towards the WC-Co / TiC interface with an excessive richness in boron susceptible lead to the formation of harmful compounds. Above the upper limit, the thickness of the coating is increased without gaining performance, with the related drawbacks (longer process duration, higher internal stresses, loss of acuity on sharp edges).

CVD deposition of amorphous boron carbide a-BC is carried out by reducing a gaseous mixture of boron trichloride BCl and methane in the presence of hydrogen under the following operating conditions in order to lead to the abovementioned amorphous boron carbide a-BC: temperature between 900 ° C and 1050 ° C, absolute pressure between 10 ^ eε 10 ^ Pa, molar flow of the gas mixture between 4.10 ~ 6 and 4.1 ^' 0 ~ ⁱ mole.cm ~ ² .s ~ ¹ , atomic ratio C / B of the mixture between 1 and 6, atomic ratio H / B between _ between 4 and 50. The parameters molar flux and ratio atomic C / B are essential to lead to amorphous carbide; for example, the molar flux could be advantageously adjusted to around 2.2.10 "^ mole.cm ~ 2.s- 1 and the C / B ratio to around 2.

This deposition of amorphous boron carbide a-BC is preferably carried out for a period suitable for obtaining a layer of thickness between 0.5 and 5 microns. This thickness range makes it possible to benefit from the performances already mentioned without additional thickness (seat of high internal stresses which could compromise the stability of the layer).

Furthermore, it has been found that it is desirable for the gas mixture intended for producing the TiC deposit to be brought to the reaction temperature for a period of between 1 and 10 s prior to its contact with the part to be treated, so as to advance the decomposition of the reactive gases, this decomposition having a relatively slow kinetics. On the contrary, it is desirable that the gas mixture intended for the production of the amorphous a-BC deposit be rapidly brought into contact with the part to be treated (in practice, temperature setting time less than 0.2 s) taking into account the very fast kinetics in this case of the reaction of decomposition of the reactive gases. According to another advantageous characteristic of the process, the part to be treated is rectified beforehand on its active faces in order to remove the cobalt crust on the surface and to confine this compound to the joints between grains of tungsten carbide. Thus, the diffusion of cobalt which is located around the grains of tungsten carbide is already limited (while in the absence of rectification it covers the entire surface); This enables a barrier ^"mixed boride thinner for the same effectiveness.

The method of the invention can be applied to treat any type of tungsten carbide sintered with cobalt, in particular tungsten carbide containing between and 20% of cobalt, with where appropriate in a minor proportion of the mixed carbide of titanium, tantalum and niobium. The invention extends, as products new, to parts manufactured by the process of the invention, in particular cutting tools or wearing parts; these parts have a substrate made of tungsten carbide sintered with cobalt 5 and are characterized in that they ^' successively comprise on the surface a discontinuous sublayer of crystals of mixed boride of cobalt and tungsten CoWB, a layer of crystallized titanium carbide TiC and a layer of amorphous boron carbide a-BC in which the elementary carbon content is between 20% and 35%. The parts treated in accordance with the invention are also characterized by a hardness gradient from the core to the surface, the hardness of successive materials increasing from tungsten carbide to amorphous boron carbide. In the preferential conditions for implementing the process which have been defined above, it has been found that a tungsten carbide substrate of Vickers hardness between 1500 and 2200 Hv is obtained, a discontinuous sub-layer of mixed boride of hardness Vickers between 2200 and 2500 Hv, a layer of titanium carbide of hardness - Vickers between 3000 and 4000 Hv and a layer of amorphous boron carbide of hardness between 4500 and 6000 Hv.

In addition, it was found that the discontinuous mixed boride sublayer had a penetration thickness in the tungsten carbide of between a minimum thickness corresponding to the limit of detection of CoWB by X-ray diffraction and a maximum thickness of 5 microns. . It should be noted that the existence of this under-layer which forms a barrier is essential, as indicated above; however, it is advantageous for its thickness to be small (limited to the value of 5 microns mentioned above) because an excess would lead to embrittlement of the part.

The absolute roughness of the surface of the layer of amorphous boron carbide of the parts obtained is less than 1 / 10th of the thickness of said layer. Such roughness is much lower than that of the best crystalline layers of boron carbide obtained (roughness generally greater than 0.3 microns). In addition, a titanium carbide is obtained which is generally crystallized in the cubic system with a preferential orientation (200); comparative tests have shown that this orientation promotes the low roughness of the layer of amorphous boron carbide deposited thereon, probably due to a denser nucleation during CVD deposition of said amorphous layer (compared to a TiC of texture (111)).

The following examples illustrate the implementation of the process of the invention and the performances obtained. Figure 1 is a schematic sectional view on an expanded scale of the coating obtained in the examples.

EXAMPLE 1

Nature of coated parts:

Milling inserts in cobalt sintered tungsten carbide, grade K10 (6.5% Co). Reference plates: "LNE 32302 HBF 5x15x10". Coating:

The wafers are placed in a CVD reactor, the walls of which are covered with amorphous boron carbide. They are heated to 1000 ° C under a reducing atmosphere. The CoWB layer and the TiC layer are obtained under the following conditions:

Gas flow: TiCl4 3.7- 10 ^-7 mole cm ^-2 s ^"1 CH4 2.5- 10 ~ ⁶ mole cm ^-2 s ~ ¹ H2 1.5.10 ~ ⁵ .mole cm ^{" 2} s ^-1 Pressure: 4kPa Temperature: 1000 ° C

Duration: 25 minutes The layer of amorphous boron carbide is obtained in succession by decreasing temperature and pressure.

^'' Gas flow: BC13 1,2.10 ^-6 mole cm ^-2 s ^-1 CH4 2,5.10 ~ ⁶ mole cm ^-2 s ^-1

H2 1,2.10-5 mole cm " ² s ^-1 Pressure: 2 kPa Temperature: 950 ° C Duration: 35 minutes The characteristics of the coating obtained are: CoWB thickness: 1 micron TiC thickness: 1 micron a-BC thickness: 1 micron Roughness: 0.08 micron (Ra)

Milling test:

The removable inserts thus obtained were tested in the milling application of GS (spheroidal graphite) cast iron, with the following conditions: 0 80 mm cutter, 20 inserts per tool,

Feed 0.05 mm per tooth Depth of pass 5 mm Speed 120 m / min Lubrication by soluble oil The increase in service life is 5 to 8 times compared to SNUN 12 04 12 inserts coated with TiC / A1203 / T1 -

EXAMPLE 2:

Nature of coated parts:

Wire guide in cobalt sintered tungsten carbide, 15% CO. Internal diameter 10 mm, height 15 mm. Coating:

The wafers are placed in a CVD reactor, the walls of which are covered with amorphous boron carbide. They are heated to 1000 ° C under a reducing atmosphere. The CoWB layer and the d-e TiC layer are obtained under the following conditions:

Gas flow: TiCl4 3.7.10 ~ ⁷ mole cm ^-2 s ^-1 CH4 2.5- 10 ~ ⁵ mole cm ^"2 s ^-1

H2 1.5-1O ^-5 mole cm ^"2 s ~ ¹ Pressure: 4kPa Temperature: 1000 ° C Duration: 50 minutes The layer of amorphous boron carbide is obtained as a result by decreasing temperature and pressure:

Gas flow: BC13 1.2.10 "^ mole cm" ² s ^-1 CH4 2.5-10 "° mole cm" ² s ^"1 H2 1.2.10 ~ ⁵ mole cm" ² s ^-1 Pressure: 2kPa Temperature: 950 ° C

Duration: 75 minutes The characteristics of the coating obtained are: CoWB thickness: 2 micron

TiC thickness: 2 micron

A-BC thickness: 2 micron

Roughness: 0.16 micron (Ra) Application test: The wire guides thus coated have been tested for the passage of nickel-plated copper wire of diameter: 0.55 mm. Their service life has been multiplied by 10 compared to uncoated wire guides.

EXAMPLE 3:

Nature of coated parts:

Cobalt sintered tungsten carbide cutting inserts, reference: FG 10 792 UT (VALENITE GTE). Realization of the coating: Idem example 1. Test of turning: Material drawn: stainless steel 304 L Feed rate: 0.15 mm / rev Number of pieces machined at 70 m / min without coating: 125

72 m / min with coating: 750 92 m / min with coating: 1855 The coating allowed a gain of 2000% of machined parts for a cutting speed increased by 30% compared to uncoated inserts.

Claims

1 / - P r o c e d e t r a i t e m e n t of a tungsten carbide part cobalt fried as a tool for

5 cutting or wearing part, in order to increase its mechanical performance and its surface qualities, characterized in that it consists:

. carrying out a chemical vapor deposition (CVD) of crystallized titanium carbide TiC on the part, said deposit being carried out in the presence of boron so as to obtain under the layer of titanium carbide TiC a discontinuous sublayer of crystals of mixed cobalt and tungsten boride CoWB by reaction of boron on tungsten carbide in the vicinity of the joints occupied by cobalt, 15. then carrying out a chemical vapor deposition (CVD) of boron carbide, said deposition being carried out so as to obtain on the layer of titanium carbide TiC a layer of amorphous boron carbide a-BC in which the elementary carbon content is between 20% and 20 35%.

2 / - Treatment process according to claim 1, characterized in that CVD deposition of titanium carbide TiC is carried out in a reactor containing boron or a boron compound in its reaction chamber, so as to produce a displacement of boron at the start of chemical vapor deposition to form the CoWB mixed boride sublayer.

3 / - Treatment method according to one of claims 1 or 2, wherein the CVD deposition of 0 titanium carbide TiC consists in reducing a gaseous mixture of titanium tetrachloride TiClj _j and methane in the presence of hydrogen under the conditions following procedures: temperature between 930 ° C and 1050 ° C, absolute pressure between 10 ² and 10 ^ pa, molar flow of the gas mixture 5 between 3.10 "° and 6.10" ^ mole.cm ^-2 .s " ¹ , C / Ti atomic ratio of the mixture between 1 and 15, H / Ti atomic ratio between 5 and 150.

4 / - Treatment process according to claims 2 and 3 taken together, characterized in that 0 is used to carry out the CVD deposition of titanium carbide, a reaction chamber previously coated with boron carbide.

5 / - Treatment method according to one of claims 3 or 4, characterized in that one carries out the CVD deposition of titanium carbide TiC for a period suitable for obtaining a layer of thickness between 0.15 and 5 microns.

6 / - Treatment process according to one of the preceding claims, in which the CVD deposition of boron carbide consists in reducing a gaseous mixture of boron trichloride BCl and methane in the presence of hydrogen, characterized in that this deposition is carried out under the following operating conditions with a view to producing carbide. a-BC aforementioned amorphous boron: temperature between 900 ° C and 1050 ° C, absolute pressure between

10 and 10 ^ pa, molar flow of the gas mixture between

4.10 ~ ^ϋ and 4.10 "^ mole .cm ^{" 2} . s " ¹ , C / B atomic ratio of the mixture between 1 and 6, H / B atomic ratio between 4 and 50.

7 / - Treatment method according to claim 6, characterized in that the deposition is carried out

CVD of amorphous boron carbide for a period suitable for obtaining a layer of thickness between 0.5 and 5 microns.

8 / - Treatment process according to claims 3 and 6 taken together, characterized in that one operates successively CVD deposition of titanium carbide and amorphous boron carbide in the same reactor without intermediate cooling.

9 / - Treatment method according to claim 8, characterized in that:

. to carry out the CVD deposition of titanium carbide TiC, the gaseous mixture is admitted into the reaction chamber so as to be brought to the reaction temperature for a period of between 1 and 10 s prior to its contact with the part to be treated,

. to carry out the CVD deposition of amorphous boron carbide, the gas mixture is admitted into the reaction chamber so as to be brought to the reaction temperature for a period of less than 0.2 s prior to its contact with the part to be treated.

10 / - Treatment method according to one of the preceding claims, in which the part to be treated is previously rectified on its active faces in order to remove the cobalt crust on the surface and to confine this compound to the joints of the tungsten carbide grains .

11 / - Method according to one of claims 1 to 10, applied to the treatment of a massive piece of sintered tungsten carbide, containing between 5 and 20% of cobalt with, where appropriate, in a minor proportion of mixed titanium carbide, tantalum and niobium.

12 / - Part in particular cutting tool or wearing part, made of tungsten carbide sintered with cobalt, characterized in that it successively comprises on the surface a discontinuous sub-layer of crystals of mixed boron of cobalt and tungsten CoWB, a layer of titanium carbide crystallized TiC and a layer of amorphous boron carbide a-BC in which the elementary carbon content is between 20% and 35%, the hardness of the successive materials increasing from tungsten carbide to carbide mixed boron.

13 / - Piece according to claim 12, characterized in that it comprises a tungsten carbide substrate of Vickers hardness between 1500 and 2200 Hv, a discontinuous sub-layer of mixed boride of Vickers hardness between 2200 and 2500 Hv, a layer of titanium carbide of Vickers hardness between 3000 and 4000 Hv and a layer of amorphous boron carbide of hardness between 4500 and 6000 Hv.

T _ part according to one of claims 12 or 13, characterized in that the absolute roughness of the surface of the layer of amorphous boron carbide is less than 1 / 10th of the thickness of said layer.

15 / - Part according to one of claims

12, 13 or 14, in which the titanium carbide is crystallized in the cubic system with a preferential orientation (200). 16 / - Part according to one of claims 12, 13, 14 or 15, in which:

. the discontinuous boride sublayer has a penetration thickness in the tungsten carbide of between a minimum thickness corresponding to the detection limit of CoWB by X-ray diffraction and a maximum thickness of 5 microns,

. the layer of titanium carbide has a thickness of between 0.15 and 5 microns,. the layer of amorphous boron carbide has a thickness of between 0.5 and 5 microns.