Sulfur Containing Alpha-Alumina Coated Cutting Tool

FIELD OF THE INVENTION

The present invention relates to a cutting tool insert consisting of a substrate of cemented carbide, cermet, ceramics, steel or a superhard material such as cubic boron nitride (CBN) and a hard coating consisting of one or more refractory layers of which at least one layer is an 01-AI2O3 layer containing sulphur and having a specified growth orientation defined by the tex- ture coefficient, and a method of manufacturing the cutting tool insert.

BACKGROUND OF THE INVENTION The early approaches to deposit Al ₂ 0 ₃ on a substrate surface in a CVD process based on a AICI3/CO2/H2 reaction gas mixture had a very low deposition rates in the order of about 0.2 μιτι/h on flat surfaces. Besides the fact that it was not possible to control the phase content, this early AI2O3 deposition process also suffered from a pronounced dog-bone-effect, i.e. the deposition rate was higher on the edges than on the flat surfaces of the substrate. US 4,619,866 by Smith and Lindstom discloses that dopants, such as H ₂ S, could be used as a catalyst both to enhance the overall deposition rate but also to suppress the dog-bone effect. With the introduction of H ₂ S as a catalyst for the 01-AI2O3 deposition process the deposition rate increased by a factor of about five coinciding with a more or less complete elimination of the dog-bone effect as compared to the process without any H ₂ S present

Several attempts have been made to deposit industrial alpha and gamma alumina coatings onto cutting tools by CVD or PVD using sulphur containing dopants. In EP-A-0 045 291 the addition of 0.02-0.3 vol-% of sulphur, selenium or tellurium containing gas, preferably H ₂ S, to the deposition gas in the CVD process has been found to increase the growth rate and to im- prove the uniformity of alumina coatings. EP-A-1 788 124 describes the deposition of alpha alumina coatings having a defined crystal grain boundary orientation wherein the deposition of the alumina is performed by CVD adding from 0.25-0.6 vol-% of H ₂ S to the deposition gas. EP-A-1 683 893 describes the deposition of alpha alumina coatings having a defined amount of ∑3 type grain boundary length wherein the deposition of the alumina is performed by CVD adding from 1.5-5 vol-% HCI and from 0.05-0.2 vol-% of H ₂ S to the deposition gas. The prior art literature does not describe the actual sulphur content in the alpha alumina coatings. Since H ₂ S has only been used to enhance the growth rate and prevent the dog-bone effect, there have been no attempts to use higher amounts of sulfur-containing dopants or consider the sulfur content in the 01-AI2O ₃ coatings in general. One reason for this is that, as diclosed in US-A-4, 619,866, the effect of H ₂ S on the growth rate on alumina was found to be at maximum at a H ₂ S concentration of 0.25 to 0.3 vol%. Larger amounts of H ₂ S than about 0.3 vol% were found to result in strongly reduced growth rates"

OBJECT OF THE INVENTION

It is an object of the present invention is to provide a coated cutting tool having an a-AI ₂ 0 ₃ layer that exhibits improved cutting properties, improved chipping resistance and improved crater wear resistance as well as lower friction in contact with the workpiece over the prior-art.

DESCRIPTION OF THE INVENTION

The present invention relates to a cutting tool insert consisting of a substrate of cemented carbide, cermet, ceramics, steel or a superhard material such as cubic boron nitride (CBN) and a coating with a total thickness of 5 to 40 μιη, the coating consisting of one or more refractory layers of which at least one layer is an a-AI ₂ 0 ₃ layer having a thickness of 1 to 25 μιη, wherein the at least one a-AI ₂ 0 ₃ layer having a sulphur content of more than 100 ppm analysed by Secondary Ion Mass Spectroscopy (SIMS) and

the at least one α-ΑΙ ₂ 0 ₃ layer having a texture coefficient TC (0 0 12) > 4 for the (0 0 12) growth direction, the TC (0 0 12) being defined as follows:

7(0 0 12) ^ I (hkl)

TC(0 0 12)

7 ₀ (0 0 12)

(hkl) = measured intensity of the (hkl) reflection

lo (hkl) = standard intensity of the standard powder diffraction data according to JCPDF- card no. 42-1468 n = number of reflections used in the calculation, whereby the (hkl) reflections used are: (012), (104), (1 10), (1 13), (1 16), (300) and (0 0 12).

It has surprisingly been found that improved cutting properties, improved chipping resistance and improved crater wear resistance of the cutting tool insert as well as lower friction in contact with the workpiece can be achieved if the 01-AI2O3 layer has a high sulphur content of more than 100 ppm analysed by Secondary Ion Mass Spectroscopy (SIMS) and a texture coefficient TC (0 0 12) > 4 for the (0 0 12) growth direction. In a preferred embodiment of the present invention the at least one 01-AI2O3 layer of the cutting tool insert has a sulphur content of more than 120 ppm, preferably more than 150 ppm analysed by SIMS. It has been found that the cutting properties of the inventive cutting tool can be further improved by a higher sulphur content. However, the sulphur content of the 01-AI2O3 layer should not exceed 2000 ppm, since a larger sulphur content may impair the properties of the cutting tool, such as grain boundary strength, and, in addition, cause porosity.

In another preferred embodiment of the cutting tool insert of the present invention the coating comprises, in addition to the at least one 01-AI2O3 layer, one or more refractory layers consisting of carbide, nitride, carbonitride, oxycarbonitride or borocarbonitride of one or more of Ti, Zr, V and Hf, or combinations thereof deposited using CVD or MT-CVD, having a thickness of from 0.5 to 20 μιη, preferably from 1 to 10 μιη. Preferably, the coating comprises a first layer adjacent to the substrate body of CVD deposited Ti(C,N), TIN, TiC or HfN, or MT-CVD deposited Ti(C,N), Zr(C,N), Ti(B,C,N), or combinations thereof. Most preferably, the first layer is if Ti(C,N).

In yet another preferred embodiment of the cutting tool insert of the present invention a) the uppermost layer of the coating is the α-ΑΙ ₂ 0 ₃ layer or

b) the uppermost layer of the coating is a layer of carbide, nitride, carbonitride or oxycarbni- tride of one or more of Ti, Zr, V and Hf, or combinations thereof (herein called Ti top coating), having a thickness of from 0.5 to 3 μιη, preferably 0.5 to 1.5 μιη, being deposited atop of the 01-AI2O3 layer or

c) surface areas of the cutting tool insert, preferably the rake face of the cutting tool insert, comprise the 01-AI2O3 layer a) as the uppermost layer whereas the remaining surface areas of the cutting tool insert comprise as the uppermost layer a layer b) of carbide, nitride, carboni- tride or oxycarbnitride of one or more of Ti, Zr, V and Hf, or combinations thereof, having a thickness of from 0.5 to 3 μιη, preferably 0.5 to 1 .5 μιη, being deposited atop of the α-ΑΙ ₂ 0 ₃ layer.

The Ti top coating layer atop the α-ΑΙ ₂ 0 ₃ layer can be provided as a wear indicator or as a layer of other functions. Embodiments, where only parts of the surface areas of the cutting tool insert, preferably the rake face of the cutting tool insert, comprise the 01-AI2O ₃ layer as the uppermost layer whereas the remaining surface areas are covered with the Ti top coating as the outermost layer, can be produced by removing the deposited Ti top coating by way of blasting or any other well known method.

In another preferred embodiment of the cutting tool insert of the present invention the substrate consists of cemented carbide, preferably of cemented carbide consisting of 4 to 12 wt-% Co, optionally 0.3-10 wt-% cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table, preferably Ti, Nb, Ta or combinations thereof, and balance WC.

For steel machining applications the cemented carbide substrate preferably contains 7.0 to 9,0 wt-% cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table, preferably Ti, Nb and Ta, and for cast iron machining applications the cemented carbide substrate preferably contains 0.3 to 3,0 wt-% cubic carbides of the metals from groups IVb, Vb and VIb of the periodic table, preferably Ti, Nb and Ta.

In another preferred embodiment of the cutting tool insert of the present invention the sub- strate consists of cemented carbide comprising a binder phase enriched surface zone having a thickness of 5 to 30 μιη, preferably 10 to 25 μιη, from the substrate surface, the binder phase enriched surface zone having a Co content that is at least 1.5 times higher than in the core of the substrate and having a content of cubic carbides that is less than 0.5 times the content of cubic carbides in the core of the substrate. The thickness of the 01-AI2O3 layer in this embodiment is preferably about 4 to 12 μιτι, most preferably 4 to 8 μιη.

Preferably, the binder phase enriched surface zone of the cemented carbide body is essentially free from cubic carbides. The binder enriched surface zone enhances toughness of the substrate and widens the application range of the tool. Subtrates having a binder enriched surface zone are particularly preferred for cutting tool inserts for metal cutting operations in steel, whereas cutting tool inserts for metal cutting operations in cast iron are preferably produced without binder enriched surface zone.

In another preferred embodiment of the cutting tool insert of the present invention the at least one 01-AI2O ₃ layer has a texture coefficient TC (0 0 12) > 5, more preferably a texture coefficient TC (0 0 12) > 6 for the (0 0 12) growth direction.

The present invention further provides a method of manufacturing a cutting tool insert as defined herein wherein said at least one 01-AI2O ₃ layer is deposited by chemical vapour deposi- tion (CVD) the reaction gas of the CVD process comprising H ₂ , CO2, AICI ₃ and X, with X being H ₂ S, SO2, SF ₆ , or combinations thereof, and optional additions of N ₂ and Ar, wherein the X is present in the reaction gas mixture in an amount of at least 1 ,0 vol-% of the total volume of gases in the CVD reaction chamber and wherein the volume ratio of CO2 and X in the CVD reaction chamber lies within the range of 1≤ CO2/ X≤ 7 during deposition of the at least one 01-AI2O3 layer.

It has surprisingly been found that the inventive 01-AI2O ₃ coating can be controlled by particular deposition conditions. The inventive kind of high sulphur content and the texture coefficient TC(0 0 12) >4 of the 01-AI2O ₃ coating can be achieved by the control of the volume portion of the sulfur containing dopant X in the reaction gas mixture of the total volume of gases in the CVD reaction chamber in an amount of at least 1 ,0 vol-%, preferably at least 1.2 vol%, and, at the same time, by control of the volume ratio of CO2 and X in the CVD deposition reaction. Cutting tests and friction tests have clearly confirmed the beneficial effects of high sulfur content in the 01-AI2O ₃ layer.

If the amount X is less than 1 ,0 vol-% of the total volume of gases in the CVD reaction chamber the sulphur content and the texture coefficient TC(0 0 12) that can be achieved in the a- AI2O ₃ coating will not ne sufficiently high. It has been found that the introduction of a high amount of sulphur containing dopant X alone will not lead to a high sulphur content and the desired texture coefficient TC(0 0 12) in the coating. The inventors have found that the ratio of sulfur containing dopant X to CO2 during CVD strongly affects the sulfur content and the texture coefficient TC(0 0 12) in the deposited 01-AI2O ₃ layer. Studies by the inventors have confirmed that deposition of 01-AI2O ₃ with a high sulfur content and the texture coefficient TC(0 0 12) is difficult if too high CO2/X ratios during deposition are used. It was surprising that the control of the CO2/X ratio in the CVD deposition process of 01-AI2O ₃ is the most important factor to obtain a high sulfur content and the desired texture coefficient TC(0 0 12) in the 01-AI2O ₃ layer and, surprisingly and most importantly, that certain ratios resulted exclusively in high amounts of sulfur with good reproducibly. Thus, the present invention provides for a new a method to control the sulfur content and the texture coefficient TC(0 0 12) of α-ΑΙ ₂ 0 ₃ deposited by CVD.

In a preferred embodiment of the method of the present invention the volume proportion of the component X or the combination of components X is present in the reaction gas mixture during deposition of the at least one 01-AI2O3 layer in an amount of at least 1.2 vol-%, preferably at least 1.5 vol-% of the total volume of gases in the CVD reaction chamber. In another embodiment the volume proportion of the component X or the combination of components X lies within the range of 2.0 to 3.0 vol-%.

It has surprisingly been found that the sulphur content and the texture coefficient TC(0 0 12) of the 01-AI2O3 layer can be further improved by higher X content in the reaction gas mixture during deposition of the 01-AI2O3 layer resulting in improved cutting properties, improved chipping resistance and improved crater wear resistance of the cutting tool insert. However, a too high content of X, for example above 5.0 vol-%, should be avoided due to the danger of handling the sulphur sources. For example, the preferred sulphur source, H ₂ S, is a flammable and ex- tremely hazardous gas.

In another preferred embodiment of the method of the present invention the volume ratio of CO2 and X in the CVD reaction chamber lies within the range of 1≤ CO2/ X≤ 6 during deposition of the at least one 01-AI2O ₃ layer. When the deposition is carried out within this range of CO2/X, both a sufficient amount of sulphur of >100 ppm in the alumina layer together with a strong preferred growth of alumina along the (0 0 12) direction, resulting in a relatively high texture coefficient TC(0 0 12) for the 01-AI2O ₃ layer, can be obtained.

In yet another preferred embodiment of the method of the present invention the volume ratio of CO2 / AICI ₃ in the CVD reaction chamber is equal or smaller than 1 .5 and/or the volume ratio of AICI ₃ / HCI in the CVD reaction chamber is equal or smaller than 1 , during deposition of the at least one 01-AI2O ₃ layer. If the ratio of CO2/AICI ₃ is too high (>1.5) and/or if the ratio of AICI ₃ / HCI is too high (>1 .0), corresponding to too low amounts of HCI, this will enhance growth along the (0 1 2) direction and, consequently, will lead to a lower TC(0 0 12) in the re- suiting alumina coating. The CVD process of the present invention during deposition of the at least one 01-AI2O3 layer is suitably conducted at a temperature in the range of 850 to 1050 °C, preferably 980 to 1050 °C, most preferably 1000 to 1020 °C. If the temperature of the CVD process is too low, the growth rate would be too low, and f the temperature of the CVD process is too high, gas-phase nu- cleation and non-uniform growth will occur.

The reaction gas pressure range where the CVD process of the present invention is conducted during deposition of the at least one 01-AI2O3 layer is preferably from 50 to 120 mbar, more preferably from 50 to 100 mbar.

In yet another preferred embodiment of the method of the present invention the component X in the CVD process is H ₂ S or SO2 or a combination of H ₂ S and SO2, whereby, if the component X in the CVD process is a combination of H ₂ S and SO2, the volume proportion of SO2 does not exceed 20% of the volume amount of H ₂ S. If too much SO2 is used the coating uni- formity can be reduced due to the so-called dog-bone effect.

In yet another preferred embodiment of the method of the present invention the reaction gas of the CVD process comprises additions of N ₂ and/or Ar in a volume amount in the range of 4 to 20 vol%, preferably 10-15 vol%, of the total volume of gases in the CVD reaction chamber.

As will be shown in the examples below, the coatings of the invention exhibit an excellent chipping resistance in a high-speed intermittent cutting and enhanced crater wear resistance in continuous turning over the prior-art coatings.

METHODS

Secondary Ion Mass Spectroscopy (SIMS) The measurement of sulphur in the alumina coatings has been done by Secondary Ion Mass Spectroscopy (SIMS) on a Cameca ims3f spectrometer. The quantitative determination of the sulphur concentration in a sample was done relative to the known aluminum concentration in the sample. For the determination of the sensitivity (relative ion yield) for sulphur relative to aluminum the reference glas SRM 610 of the National Institute of Standards and Technology (NIST) was used. The sulphur concentration in SRM 610 is 575 μg/g, and the relative accuracy of the measurements is about ± 20%. The sample surface was sputtered with negative oxygen ions having an energy of 14.5 keV. The primary ion current was about 30 nA, and the diameter of the focussed primary ion beam at the sample surface was about 30-40 μιη. The generated positive secondary ions were ac- celerated to an energy of 4.5 keV and measured with a mass spectrometer at a mass resolution of m/Arr^^QQ using a secondary ion multiplier in counting modus (for ³² S) and with a Faraday cup (for ²⁷ AI), respectively. The starting energy of the detected secondary ions was 55 ± 20 eV to lower molecular interferences and increase the measurement accuracy (energy filtering). As the measurement results the average of six equal measurement cycles was cal- culated. The integration times per cycle were 25 sec for ³² S and 3 sec for ²⁷ AI, respectively. For each sample the measurements have been repeated 5 times.

TC(0 0 12) X-ray diffraction measurements X-ray diffraction measurements were done on a diffrakto meter XRD3003PTS of GE Sensing and Inspection Technologies using Cu K _a -radiation. The X-ray tube was run at 40 kV and 40 imA focussed to a point. A parallel beam optic using a polycapillary collimating lens with a measuring aperture of fixed size was used on the primary side whereby the irradiated area of the sample was selected to avoid a spill over of the X-ray beam over the coated face of the sample. On the secondary side a Soller slit with a divergence of 0,4° and a 0,25 mm thick Ni K _p filter were used. Θ-2Θ scans within the angle range of 20° < 2Θ < 100° with increments of 0,25° have been conducted. The measurements were done on a flat face of the coated insert, preferably on the flank face. The measurements were done directly on the alumina layer as the outermost layer. Any layer present in the coating above the alumina layer to be measured, if any, is removed by a method that does not substantially influence the XRD measurement results, e. g. etching. For the calculation of the texture coefficient TC(0 0 12) peak height intensities were used. Background subtraction and a parabolic peakfit with 5 measuring points were applied to the XRD raw data. No further corrections such as K _a2 stripping or thin film correction were made.

CVD coatings

All CVD coatings were prepared in a radial flow reactor, type Bernex BPX 325S. EXAMPLES

Example 1 - a-A O^ coatings Cemented carbide substrates for cutting inserts with a composition of 6.0 wt% Co and balance WC (hardness about 1600 HV) were coated with a Ti(C,N) layer by applying MT-CVD using 0.6 vol% CHsCN, 3.8 vol% TiCI ₄ , 20 vol% N ₂ and balance H ₂ . The thickness of the Ti(C,N) MT- CVD layer was about 5μιη. Onto this Ti(C,N) layer of separate substrate samples different layers consisting of about 8 μιη (X-AI2O ₃ were deposited. The coating parameters are given in Table 1 , and the texture coefficients, TC(0 0 12), measured by X-ray diffraction, and the sulphur concentrations in the 01-AI2O ₃ coatings, measured by SIMS, are given in Table 2. The deposition of α-ΑΙ ₂ 0 ₃ was started by depositing a 0.05 μιη to about 1 μιτι, preferably 0.5 μιτι to about 1 μιτι, thick bonding layer on top of the MTCVD layer from the system H ₂ -N ₂ -CO- TiCU-AIC at a pressure of 50 to 100 mbar. For the preparation of the bonding layer the MTCVD layer was treated with a gas mixture of 3 vol% TiCI ₄ , 0.5 vol% AICI ₃ , 4.5 vol% CO, 30 vol% N ₂ and balance H ₂ for about 30 min at a temperature of about 1000°C. The deposition was followed by a purge of 10 min using H ₂ before starting the next step. a-AI ₂ 0 ₃ was nucleated on the (Ti,AI)(C,N,0) bonding layer by treating said layer with a gas mixture of 4 vol% C0 ₂ , 9 vol% CO, 25 vol% N ₂ , balance H ₂ for 5-10 min at a temperature from about 750 to 1050 °C, preferably at about 980 to 1020 °C and most preferably at 1000 to 1020 °C (P= 80 to 100 mbar). The oxidation was followed by a purge of 10 min using Ar.

The alumina deposition was started with by introducing a gas mixture of AICI ₃ , C0 ₂ , Ar ₂ , N ₂ HCI and H ₂ , in the volume amounts as indicated in table 1 , without precursor X for about 10 min at a temperature of about 1000 °C. These precursors were shunted in simultanously except HCI. HCI flow was shunted into the reactor 2 min after the start (8 min before X was introduced). Table 1 - a-A Os coatings

X = invention

Table 2 - g-A Os coatings

X = invention The inserts with coatings 4a and 4b were tested for friction coefficient using the pin-on-disc method. The coating 4a showed a friction coefficient of 0.56, whereas the coatings 4b and 12 showed a lower friction coefficient of 0.42 and 0.39, respectively. Thus, a high sulphur content in the alpha alumina coatings has been identified to be friction reducing.

Example 2 - Edge Toughness Tests

The samples 1 a to 4b of Example 1 were tested with respect to edge toughness (chipping resistance) in longitudinal turning of cast iron (GG25) using the following cutting parameters: Work piece: GG25; cylindrical bar

Insert type: SNUN

Cutting speed v _c =400 m/min

Feed (f) = 0,4 mm/rev

Depth of cut: a _p =2,0 mm

Remarks: dry turning

The inserts were inspected after 2 and 4 minutes of cutting. As shown in Table 3, compared to the coating of the prior art, the edge toughness of the samples was considerably enhanced when the coating was produced according to this invention.

Table 3 - Edge Toughness

X = invention

Example 3 - Turning Tests

The samples 6, 8, 10 and 12 of Example 1 were tested in carbon steel (C45) without coolant using the following cutting parameters: Work piece: C45

Insert type: WNMG080412-NM4

Cutting speed: v _c =280 m/min

Feed (f) = 0,32 mm/rev

Depth of cut: a _p =2,5 mm

Remarks: dry turning

The end of tool life criterion was flank wear > 0.3 mm. Three edges of each variant were tested.

Table 4 - Turning Test results

X = invention