Cutting tool for chip forming machining of metals

The present invention relates to a coated cutting tool, suitable for chip forming machining of metals, such as an insert for use in holding systems for turning, milling or drilling, or a solid tool, such as a drill or an endmill, having improved resistance to corrosion caused by cooling liquid, and a method of making thereof.

Modern high productivity chip forming machining of metals requires reliable tools with high wear resistance, good toughness properties and excellent resistance to plastic deformation. The cutting tools generally comprise a hard body of, e.g., cemented carbide and a wear resistant coating of single layer or multilayer type, most commonly comprising wear layers of transition metal carbides, nitrides, borides and carbonitrides, and layers of oxides of, e.g., Al and Zr. For the deposition of the different layers onto the body, Chemical Vapour Deposition (CVD) , Physical Vapour Deposition (PVD) or similar coating techniques are used. It is well known that the elevated temperatures used in CVD in combination with the difference in coefficient of thermal expansion between the body material and the coating often cause the formation of cracks in the coating when the coated tool is cooled down from process temperature to room temperature. These narrow cracks often extend essentially perpendicularly from the coating surface all the way through the coating and penetrate into the body as is disclosed in, e.g., US Patent No. 5,123,934, and as a result have a very high depth to width ratio. However, a coated tool with such cracks present is susceptible to at least two harmful mechanisms affecting the performance of the tool. For certain cutting operations machining must be performed under wet conditions, during which cooling liquid flows over the work piece and the cutting tool. Through the cracks, cooling liquid may instantly reach layers underneath the outer layer, generally being less resistant to oxidation than the outer layer, as well as reach the body beneath the coating, starting a corrosion process. Independently on whether machining is performed under wet conditions or not,

the cracks in the coating constitute defect points where contaminants can penetrate the coating, causing an oxidation of the body and coating material. Beside the chemical effect, contaminants being squeezed into the cracks may widen them by means of shear physical force, leading to even further degradation of the coating.

U.S. Pat. No. 5,250,367 and U.S. Pat. No. 5,364,209 discloses coated cutting tools comprising a coating including at least one CVD layer and at least one PVD layer, wherein the PVD layer is in a state of residual compressive stress.

It is an object of present invention to solve the problem of cutting tool corrosion and oxidation, as well as widening of coating cracks, caused by cooling liquid and other contaminants.

This object is solved by providing a coated cutting tool and a method of making thereof, wherein the coating deposition process is performed under certain conditions as to obtain a sealed coating.

Fig. 1 is a schematic drawing of an exemplary coating according to the invention.

Fig. 2 is a Scanning Electron Microscope (SEM) image of a polished surface of an exemplary coating according to the invention .

Fig. 3 is a SEM image of a polished cross section of an exemplary coating according to the invention.

Fig. 4 is a Scanning Electron Microscope (SEM) image of a surface of a coating according to prior art.

Fig. 5 is a Scanning Electron Microscope (SEM) image of a polished cross section of a coating according to prior art.

It has surprisingly been found that by performing a deposition process under certain conditions it is possible to fill substantial parts of the cooling cracks, despite their high depth to width ratio, creating an effective seal of the coating

layer, thus preventing cooling liquid and other contaminants from penetrating the coating.

According to the present invention there is provided a method of making a coated cutting tool comprising the steps of providing a hard body, such as a cemented carbide or ceramic body, preferably a cemented carbide body, performing a first coating deposition process, to obtain a first coating, comprising depositing, by CVD deposition techniques known in the art, one or more layers of refractory material (s) of which at least one preferably has a Coefficient of Thermal expansion (CTE) being at least 1.5*10 ^~6 K ^"1 larger than that of the body, lowering the temperature of the cutting tool to at least a temperature where cooling cracks form in the coating, preferably to a temperature at least 150 ⁰ C, more preferably at least 200 ⁰ C, lower than the highest deposition temperature used during the deposition of the first coating, or, alternatively, preferably lowering the temperature to 750 ⁰ C or below, more preferably 730 ⁰ C or below, most preferably 700 ⁰ C or below, additionally performing a crack sealing step, whereby substantial parts of the cooling cracks are filled with a crack filling material, being the same as or different from the material (s) in the first coating, using a deposition technique having sufficiently high step coverage, such as Chemical Vapour deposition (CVD) , Plasma Activated CVD (PACVD) or Atomic Layer Deposition (ALD) , at a process temperature which may be higher, lower or equal to the temperature in the preceding step, preferably at a process temperature of 100 to 750 ⁰ C, optionally performing a second coating process comprising depositing further layers, such as lubricating, colouring and/or wear detection layers, and optionally performing a post-coating treatment, such as brushing or blasting, using either wet or dry process conditions, for the purpose of, e.g., smoothing the outer most layer surface and/or removing one or several of the outer layers at least along the edge line.

When using CVD or PACVD for the crack sealing step it is preferred that the lowering of the temperature to obtain cooling cracks in the coating is not lower than 500 ⁰ C, preferably not lower than 400 ⁰ C, below the highest deposition temperature used during the deposition of the first coating, or, alternatively, preferably not below 500 ⁰ C, more preferably not below 600 ⁰ C. In this case it is also preferred that the crack sealing step is preformed at a temperature between 250 ⁰ C, preferably 500 ⁰ C, most preferably 600 ⁰ C, and 750 ⁰ C. More specifically the first coating process may comprise depositing by using CVD or Plasma Activated CVD (PACVD)

- a first, 0.1 to 3 μm, preferably 0.3 to 2 μm, most preferably 0.5 to 1.5 μm, thick wear resistant layer sequence comprising one or several individual layers, the first layer being a transition metal compound being a carbide, nitride, oxide, carbonitride or carbooxynitride, preferably one of TiC, TiN, Ti(C,N), ZrN, HfN, most preferably TiN, at a temperature of about 850 to 1000 ⁰ C,

- a second, 0.5 to 30 μm, preferably 3 to 20 μm, thick layer sequence comprising one or more layers of a transition metal compound being a nitride, carbide or carbonitride, preferably TiN, TiC, Ti(C,N), Zr (C, N), most preferably Ti (C, N) or Zr (C, N) with a columnar grain structure, and possibly a Ti (C, N, O) layer having a plate like structure, at a temperature of about 800 to 1050 ⁰ C, and

- a third, 0.5 to 25 μm, preferably 2 to 19 μm, most preferably 3 to 15 μm, thick layer sequence comprising at least one oxide layer, preferably Al ₂ O ₃ , ZrO ₂ , Ti ₂ O ₃ or HfO ₂ , most preferably Al ₂ O ₃ and ZrO ₂ , at a temperature of about 900 to 1050 ⁰ C, to a total thickness of the first coating of preferably > 3.5 μm, more preferably > 5 μm, most preferably > 7 μm.

It has been found that the temperature where cooling cracks form in the coating varies strongly between different combinations of coatings and bodies, from temperatures close to the deposition temperature of the coating down to temperatures near room temperature. For body-coating combinations for which

the difference in CTE between the body and the coating material is at least 1.5*10 ^~6 K ^"1 , a temperature difference between the highest deposition temperature of the first coating and the crack sealing step of at least 150 ⁰ C has been found to be sufficient in order for the cracks to form and to be sufficiently filled. Cooling to a temperature below 750 ⁰ C has been found sufficiently low to form cooling cracks for the combinations of body and CVD coating frequently used for cutting tools. It is however within the purview of the skilled artisan to determine the temperature at which the cooling cracks form for a chosen body-coating combination.

It has been found advantageous performing the crack sealing step using a crack filling material of a metal compound being a carbide, nitride, oxide or boride or combinations thereof, such as TiN, TiC, Ti (C, N), Al ₂ O ₃ , ZrN, ZrO ₂ , Zr (C, N) or Ti (C, N), preferably Al ₂ O ₃ and ZrO ₂ when a highly oxidation resistant coating is required, or alternatively, preferably TiN, Zr (C, N) or Ti (C, N) when demands on oxidation resistance are not so high. The technique used for the deposition must have such characteristics that the gaseous molecules can penetrate the narrow cooling cracks forming a coating inside the cooling cracks. Deposition techniques having sufficient step coverage are CVD, PACVD, Atomic Layer Deposition (ALD) , also known as Atomic Layer Epitaxy (ALE) or Atomic Layer CVD, or similar techniques and process conditions adjusted so that a very low deposition rate of preferably less than 0.5 μm/h, more preferably 0.4 μm/h, is used ensuring that the process gases are given sufficient time to diffuse deep into the cracks thus avoiding the formation of a "bridge" on top of the crack. The deposition temperature when using CVD or PACVD is preferably 250 to 750 ⁰ C, depending on the deposition chemistry. When the crack sealing step is performed using ALD, the deposition temperature is preferably 100 to 500 ⁰ C. In order to allow a sufficient amount of filling of the cracks, the crack sealing step is performed until the crack filling material forms a layer having a thickness of preferably 0.1 to 1.5 μm, more preferably 0.1 to 1.2

μm, most preferably 0.1 to 0.9 μm on the surface of the first coating.

It is within the scope of the invention that the crack sealing step can be performed in the same equipment as the first coating process. In this case it is advantageous if the deposition of the first coating, the crack forming step and the crack filling step are run in sequence, wherein the process temperature is at least 100 ⁰ C during the whole of this sequence. Alternatively, the crack sealing step can be performed in a separate reaction chamber optimized for deposition at low temperatures .

For the determination of the degree of crack-filling that has been reached a sample preparation technique has been developed in which approximately 1 μm of the thickness of the first coating is removed by a top polishing operation after which the sample is investigated by means of image analysis of SEM micrographs for the determination of the lateral degree of crack- f i l l ing .

In order to obtain an improvement in performance, at least 50% of the area of the cracks in the surface of a top polished cutting tool, prepared according to the above, thus at a depth of 1 μm of the first coating, should be filled by the crack filling material. However, to obtain a more substantial improvement at least 70% of the crack area of the cracks in the surface should be filled.

As a complement to a top polished sample, a polished cross section of the sample is also investigated by means of SEM. The cooling cracks should preferably be filled to at least 50% of the depth of the crack, or at least on an average 2 μm of the first coating thickness if the total thickness of the first coating exceeds 4 μm, in order for a substantial improvement in performance to be obtained. Fig. 1 is a schematic illustration of the invention showing a body and a coating wherein the cooling cracks in the coating have been at least partially filled during a crack sealing operation, wherein

A - body,

B - first layer,

C - second layer,

D - third layer, and E - partially filled cracks.

In one embodiment the method comprises depositing onto a cemented carbide substrate a first coating comprising a first, inner, layer of TiN with a thickness of 0.1 to 3 μm, preferably of 0.3 to 2 μm, most preferably of 0.5 to 1.5 μm, using CVD at a process pressure of 120-200 mbar and a process temperature of 910-950 ⁰ C, a second layer of ZrC _x N _y or TiC _x N _y , preferably TiC _x N _y , where x>0 , y≥O , preferably x and y are both 0 . 45- 0 . 55 , and x+y=l , using MTCVD at 50 - 60 mbar and 850 - 900 ⁰ C, with columnar structure, with a thickness of 0.5 to 30 μm, preferably 3 to 20 μm, most preferably 3 to 16 μm, a third layer of TiC _x N _y O _z where x>0, z>0, y≥O and x+y+z=l, using CVD at 50-60 mbar and 950-1050 ⁰ C, having a plate-like structure and a thickness of 0.1 to 3 μm, preferably 0.3 to 2 μm, most preferably 0.5 to 1.6 μm, a fourth layer of alpha or kappa Al ₂ O ₃ , preferably alpha Al ₂ O ₃ , with a thickness 0.5 to 25 μm, preferably of 2 to 19 μm, most preferably of 3 to 15 μm, using CVD at 50-60 mbar and 950- 1050 ⁰ C, where after the process temperature is lowered to 750 ⁰ C or lower to ensure the formation of cooling cracks, performing a crack sealing step using the CVD technique depositing ZrO ₂ as a crack filling material at a process temperature of 250-750 ⁰ C and a process pressure of about 50-60 mbar to a layer thickness on the surface of the first coating of 0.1-1.5 μm, preferably 0.1 to 1.2 μm, most preferably 0.1 to 0.9 μm, whereby substantial parts of the coating cooling cracks are filled with ZrO ₂ , and optionally performing a CVD deposition process, at a process temperature of 900-1020 ⁰ C, to deposit one or several outer layers, to a thickness of 0.3 to 3 μm, preferably 0.5 to 2.5 μm,

most preferably 0.8 to 1.5 μm, comprising ,e.g., an outermost TiN layer giving the cutting tool a golden surface colour.

According to the present invention there is also provided a coated cutting tool comprising a hard body, such as a cemented carbide or ceramic body, preferably a cemented carbide body, and a first coating deposited by CVD techniques known in the art, comprising one or more layers of refractory material (s) of which at least one preferably has a Coefficient of Thermal expansion (CTE) being at least 1.5*10 ^~6 K ^"1 larger than that of the body, wherein said coating has cooling cracks and wherein substantial parts of the cooling cracks are filled with a crack filling material, being the same or different from the material (s) in the first coating, preferably using CVD, PACVD or ALD. Optionally the coating comprises additional, outer, layers, such as lubricating, colouring and/or wear detection layers, and may as a further option have a smooth outermost layer surface and/or one or several of the outer layers removed, at least along the edge line, as a result of a post-coating treatment, such as brushing or blasting.

More specifically the first coating may comprise

- a first, 0.1 to 3 μm, preferably 0.3 to 2 μm, most preferably 0.5 to 1.5 μm, thick wear resistant layer sequence comprising one or several individual layers, the first layer being a transition metal compound being a carbide, nitride, oxide, carbonitride or carbooxynitride, preferably one of TiC, TiN, Ti(C,N), ZrN, HfN, most preferably TiN, - a second, 0.5 to 30 μm, preferably 3 to 20 μm, thick layer sequence comprising one or more layers of a transition metal compound being a nitride, carbide or carbonitride, preferably TiN, TiC, Ti(C,N), Zr (C, N), most preferably Ti (C, N) or Zr (C, N) with a columnar grain structure, and a Ti (C, N, O) layer, and - a third, 0.5 to 25 μm, preferably 2 to 19 μm, most preferably 3 to 15 μm, thick layer sequence comprising at least one oxide layer, preferably Al ₂ O ₃ , ZrO ₂ , Ti ₂ O ₃ or HfO ₂ , most preferably Al ₂ O ₃ and ZrO ₂ ,

wherein a total thickness of the first coating is preferably > 3.5 μm, more preferably > 5 μm, most preferably > 7 μm.

It is advantageous if the cooling cracks are at least partly filled with a crack filling material of a metal compound being a carbide, nitride, or oxide or combinations thereof, such as TiN, TiC, Ti(C,N), Al ₂ O ₃ , ZrN, ZrO ₂ , Zr (C, N) or Ti (C, N), preferably Al ₂ O ₃ and ZrO ₂ when a highly oxidation resistant coating is required, or alternatively, preferably TiN, Zr (C, N) or Ti (C, N) when demands on oxidation resistance are not so high.

In one embodiment of the invention the cutting tool comprises a cemented carbide body and a first coating comprising a first, inner, layer of TiN with a thickness of 0.1 to 3 μm, preferably of 0.3 to 2 μm, most preferably of 0.5 to 1.5 μm, a second layer of ZrC _x N _y or TiC _x N _y , preferably TiC _x N _y , where x>0, y≥O, preferably x and y are both 0.45-0.55, and x+y=l, preferably produced by MTCVD, with columnar structure, with a thickness of 0.5 to 30 μm, preferably 3 to 20 μm, most preferably 3 to 16 μm, a third layer of TiC _x N _y O _z where x≥O, z>0, y≥O and x+y+z=l, having a plate-like structure and a thickness of 0.1 to 3 μm, preferably 0.3 to 2 μm, most preferably 0.5 to 1.6 μm, a fourth layer of alpha or kappa Al ₂ O ₃ , preferably alpha Al ₂ O ₃ , with a thickness of 0.5 to 25 μm, preferably of 2 to 19 μm, most preferably of 3 to 15 μm, wherein substantial parts of the cooling cracks of the first coating are filled with ZrO ₂ as a crack filling material such that the ZrO ₂ layer thickness on the coating surface is 0.1 to 1.5 μm, preferably 0.1 to 1.2 μm, most preferably 0.1 to 0.9 μm. Optionally the coating is provided with an outermost TiN layer with a thickness of 0.3 to 3 μm, preferably 0.5 to 2.5 μm, most preferably 0.8 to 1.5 μm, deposited to give the cutting tool a golden surface colour.

Example 1 (prior art)

Cemented carbide cutting inserts with the composition 6.2 wt-% Co, and balance WC (CTE = about 6*10 ^~6 K ^"1 ) were coated with

a 0.5 μm thick layer of TiN using conventional CVD-technique at 930 °C followed by a 7 μm TiC _x N _y layer employing the MTCVD- technique using TiCl ₄ , H ₂ , N ₂ , HCL and CH ₃ CN as process gases at a temperature of 885 °C . In subsequent process steps during the same coating cycle, a layer of TiC _x O _z about 0.5 μm thick was deposited at 1000 °C using TiCl ₄ , CO and H2, and then the reactor was flushed with a mixture of 2 % CO ₂ , 5 % HCl and 93 % H ₂ for 2 min before a 7 μm thick layer of CC-Al ₂ O ₃ was deposited. The process conditions during the deposition steps were as shown in Table 1.

The inserts were then allowed to cool where after they were examined using Scanning Electron Microscopy (SEM) in SEI mode. The investigation showed extensive cracking of the coating as shown in Fig. 4.

Table 1.

Example 2 (invention)

Inserts from Example 1 were heated to 750 °C and subject to a 90 min ZrO ₂ deposition process using the parameters shown in Table 2 during which a 0.6 μm thick layer was formed.

The inserts were then allowed to cool to room temperature where after a sample preparation was performed where about 1 μm

of the Al ₂ O ₃ layer was removed by means of a polishing operation. The inserts were then examined using SEM in backscatter mode. The investigation showed that more than 95% of the cracks had been at least partially filled during the ZrO ₂ deposition process, as shown in Fig. 2. An examination of a polished cross section of the specimen showed that the topmost about 5 μm of the cracks were filled with ZrO ₂ as shown in Fig. 3.

Table 2.

Example 3 (invention)

Inserts from Example 1 were heated to 700 °C and subject to a 90 min. Al ₂ O ₃ deposition process using the parameters shown in Table 3 during which a 0.5 μm thick layer was formed.

Table 3.

The inserts were then allowed to cool to room temperature where after a sample preparation and examination according to Example 2 was performed. The investigation showed that more than

85% of the cracks had been filled to an average of about 4 μm during the Al ₂ O ₃ deposition process.

Example 4 (invention)

Cemented carbide inserts with the composition 6.2 wt-% Co and balance WC were coated using the deposition steps as in Table 1. The inserts were allowed to cool to 700 °C, allowing cooling cracks to form, and was subject to a 90 min. ZrO ₂ deposition process using the parameters shown in Table 2 during which a 0.6 μm thick layer was formed.

The inserts were then allowed to cool to room temperature where after a sample preparation and examination according to Example 2 was performed. The investigation showed that more than 90% of the cracks had been filled to an average of about 4 μm during the ZrO ₂ deposition process.

Example 5 (invention)

Cemented carbide inserts with the composition 6.2 wt-% Co and balance WC was coated using the deposition steps as in Table 1. In subsequent steps the inserts were allowed to cool to 700 °C, allowing cooling cracks to form, and was subject to a ZrO ₂ deposition process using the parameters shown in Table 2 during which a 0.5 μm thick layer was formed. In subsequent steps the inserts were coated with a 1.3 μm thick TiN layer using the parameters shown in Table 4, giving the inserts a golden lustre. Finally the inserts were subjected to a blasting operation removing the TiN and exposing the OC-Al ₂ O ₃ layer on the rake face of the inserts, but leaving the TiN layer on the flank faces.

Table 4.

Example 6 (prior art)

Cemented carbide inserts with the composition 6.2 wt-% Co and balance WC was coated using the deposition steps as in Table 1. In subsequent steps the inserts were allowed to cool to room temperature, allowing cooling cracks to form, and the inserts were transferred to a PVD reaction chamber. The inserts were then coated with a 2 μm thick TiN layer using arc evaporation with the parameters shown in Table 5. The inserts were allowed to cool to room temperature after which a sample preparation according to Example 2 was performed. The investigation showed that no filling of the cracks was obtained, as shown in Fig. 5.

Table 5.

Time Temperature Arc Bias Pressure

Layer Target [min] [ ⁰ C] current [V] [μbar] [A]

TiN Ti 130 450 180 -100 30 N ₂

Example 7

Inserts from Examples 1 to 6 were blasted using a mixture of Al ₂ O ₃ -grits and water and a pressure of about 2.2 bar exposing the CC-Al ₂ O ₃ layer on the rake face of the inserts. The flank faces were shielded of, leaving them unaffected by the blasting operation .

The inserts were then tested in a milling test in cast iron with respect to flank wear and partial destruction of the edge line caused by propagation of thermal cracks.

Cutting data:

Work piece material: SS0125

Cutting speed: 300 m/min

Feed/tooth: 0.15 mm

Depth of cut A _e /A _p : 3/50 mm

Note: Wet milling

Tool life criteria: flank wear >= 0.35 mm

Table 6 Results, average of tests on 5 individual inserts

Insert Tool life (%)

Example 1 (prior art) 100

Example 2 (invention) 120

Example 3 (invention) 115

Example 4 (invention) 118

Example 5 (invention) 121

Example 6 (prior art) 104

The results clearly show that the tool life of the insert with the high degree of crack filling is between 15 and 20% longer than that without any filling of the cracks. They also show that the influence of the PVD top layer is very limited and probably only within the experimental variations.