Stepwise hydrogen-free carbon-based hard material layer coated on a substrate

Abstract

1. A method for producing a hard coating on a substrate, the hard coating comprising a hydrogen-free amorphous carbon coating, the amorphous carbon coating being deposited on the substrate using cathodic arc discharge deposition; applying a bias voltage to the substrate having an absolute value greater than 0 V, preferably greater than 10 V and less than 1000 V; increasing the absolute value of the bias voltage during the coating process to obtain a first structure and a second structure and a gradient between the first structure and the second structure along the coating thickness, the first structure and the second structure comprising sp2 and sp3 carbon bonds but differing in their relative concentrations; applying at least one coating pause during the coating process; and reducing the substrate temperature during the coating pause.

Description

Background Art

[0001] Carbon-based hard coatings, especially tetrahedral amorphous carbon coatings (ta-C), are used in industrial applications due to their excellent tribological properties. In particular, hydrogen-free amorphous carbon layers can have a layer hardness of 40 GPa or more and an E-modulus of 300 GPa or more. The typical coating thickness of ta-C coatings can vary between 0.2 μm and 30 μm, especially between 0.7 and 2 μm. These coatings are defined according to VDI 2840 (Carbon layers - Basics, Types and Properties of Coatings) or ISO 20523 (Carbon-based Films - Classification and Designation).

[0002] To produce these special coatings, a high-energy physical vapor deposition (PVD) process is required. The commonly used PVD method is the cathodic arc evaporation method. In this process, an energy of about 100 eV per atom can be generated and then injected into the growing coating. To form a ta-C structure, the substrate temperature needs to be below 165 °C, which is a low temperature compared to the high energy of the colliding particles. If the substrate temperature is high, even at a relatively high rate, in addition to the desirable sp3 bonds (diamond bonds), undesirable sp2 bonds (graphite bonds) are formed.

[0003] (Fundamental Problem) The main problem with cathodic arc evaporation is that the formation of macro-particles incorporated as droplets in the layer cannot be avoided. These droplets, especially when using the coating in tribological systems such as bearings, especially bearing piston pins / connecting rod eyes, must be removed in post-treatment. Post-treatment can include sanding or grinding to smooth the surface and avoid wear due to the presence of droplets. Since Ta-C coatings are very hard, these finishing methods are time-consuming and costly.

[0004] There are methods and devices that can be installed in a coating machine to reduce the generation of droplets during the coating process, known as arc filtering. However, introducing this filter causes the deposition rate to drop extremely, which has an adverse effect on the coating cost.

[0005] (Object of the Invention) The object of the present invention is to provide a method for manufacturing a hard coating on a substrate, preferably at a high deposition rate, where the hard coating preferably contains only a few droplets. Another object of the present invention is to provide a workpiece having a hard coating deposited thereon.

[0006] This problem of the present invention is solved by the method according to claim 1 and the workpiece according to claim 14.

[0007] A Ta-C coating having a hardness of more than about 40 GPa and a high E-modulus of more than about 300 GPa. The substrate has a typical hardness of about 6 GPa and an E-modulus of about 200 GPa. As a result, the substrate and the hard coating can exhibit different behaviors under load. As another result, due to the differences in elastic and mechanical properties between the substrate and the hard coating, deformation and subsequent crack formation may start, and ultimately coating peeling may occur. This is sometimes also called the "eggshell" effect, representing an effect where a hard shell wraps around a soft core. To compensate for the mismatch in elastic and mechanical properties, an intermediate adhesion layer can be deposited in front of the hard coating layer to reduce the risk of coating peeling during operation.

[0008] In a tribological structure where two moving bodies are in contact, when the main body and the counter body have the same level of hardness, a wear effect starts to occur. The surface of both friction partners becomes smooth due to their mutual interaction. Also, when only one of the main bodies is coated with ta-C, the wear effect occurs only on the uncoated main body. As a result, after the surface material is lost due to wear, a very smooth surface can be generated.

[0009] The production of a hydrogen-free carbon-based coating having different hardnesses at different locations of the coating over the coating thickness can have several technical and economic advantages.

[0010] If the hardness at the interface between the substrate and the coating is low, the stress due to the mismatch in the E-modulus and hardness between the hard coating and the substrate can be reduced, which is advantageous for crack formation in the coating or peeling of the coating. In some cases, it may also eliminate the need for an intermediate adhesion layer to compensate for different properties between the coating and the substrate.

[0011] Furthermore, towards the outer surface of the coating, a higher hardness is preferred, whereby the wear resistance and the desired tribological properties can be ensured during operation. However, on the other hand, it may also be advantageous to lower the hardness of the surface of the coating in order to smooth the surface and reduce the cost and labor of the post-treatment process necessary to remove droplets.

[0012] In other special applications of tribological systems, it is considered desirable to have a coating that does not have a hard top layer and, conversely, acts like a wear-resistant coating, where one body is rubbed against the other. In that case, while the two bodies are in mechanical contact, the surface of the coated body becomes smoother, and at the same time, the two bodies are kept as close as possible without losing their wear resistance.

[0013] The object of the present invention is to gradually produce different hardnesses and variations in the properties of the aforementioned hydrogen-free carbon-based coating along the coating thickness by applying and varying different coating parameters during the execution of one specific process. This can be done mainly by changing the bias voltage (usually having a negative value) on the substrate, preferably by varying the deposition rate through the arc current itself, and by controlling the coating temperature (i.e., the surface temperature) set through the transfer of energy from the coating material onto the substrate.

[0014] In particular, by increasing the absolute value of the bias voltage applied during the coating process and / or increasing the substrate temperature, a hydrogen-free carbon-based coating can be changed from amorphous carbon mainly having a lower hardness sp2 bond closer to the substrate, preferably anywhere in the middle of the coating, to a tetrahedral amorphous carbon (ta-C) coating having mostly higher hardness sp3 bonds, and preferably by decreasing the absolute value of the applied bias voltage and / or preferably by reducing the substrate temperature, a coating mainly having sp3 bonds can be changed back to a coating in which sp2 bonds having a lower hardness are dominant on the outer surface of the coating. As a result, the hardness of the portion closer to the substrate and the outer surface of the coating will be lower, and the hardness will be higher anywhere between the two regions.

[0015] In a normal coating process where the above-described coating parameters (bias voltage, substrate temperature) are not controlled, the high energy of the process causes the substrate temperature (i.e., surface temperature) to continuously rise, which can have different effects on the coating properties. For example, when the temperature of the substrate exceeds a specific temperature, sp2 bonding is prioritized and the coating loses its ta-C structure. This means that in the same coating process, whenever this specific temperature is reached during the coating process, it is no longer possible to produce a coating on the outer surface that mainly has sp2 bonding and a ta-C structure. Therefore, in order to obtain the desired coating transition, it is necessary to control both the bias voltage and the substrate temperature during the coating process. Using the cathode arc evaporation technique, sp2 bonding begins to predominantly exist during coating at temperatures above about 165°C. In the process of the present invention, the substrate temperature and the structure of the coating can be controlled by applying a negative bias voltage to the substrate whose absolute value can increase at a predetermined rate and stopping at least one coating for at least 1 minute, preferably several minutes, during the coating process in order to cool the substrate during the stop. When the substrate reaches a lower temperature than the predetermined temperature, the coating process can be restarted. The coating can be stopped as many times as necessary in order to keep the substrate temperature, i.e., the surface temperature, low and keep it in the region where sp3 bonding is predominantly generated in the carbon-based coating. By this temperature and process control, it is possible to set a specific region where either a ta-C structure with sp3 bonding or an amorphous carbon structure with sp2 bonding can be obtained along the thickness of the coating.

[0016] In a preferred embodiment, the increase in the absolute value of the bias voltage can be set at a rate defined by a ratio (ΔU / Δs), which is the ratio of the absolute value of the voltage difference to the unit time. In another way, since the deposition rate of the carbon-based coating depends on the coating parameters, the ratio can be defined by a ratio (ΔU / Δd), which is the ratio between the absolute value of the voltage difference and the difference in coating thickness with reference to the total coating thickness. For example, the increase in the absolute value of the bias voltage on the substrate from 0 to 200 V for a coating thickness of 1000 nm can be defined by a ratio ΔU / Δd of 0.2 V / nm. In a preferred embodiment, the increase in the absolute value of the bias voltage during the coating process of the carbon-based coating is greater than 0 V and less than 1000 V, preferably greater than 10 V and less than 1000 V, typically 10 V to 200 V. The increase ratio of the bias voltage with respect to the coating thickness can be set to 0.02 V / nm to 0.5 V / nm, which would correspond to a linear increase in the absolute value of the bias voltage of 100 V for a coating thickness of 5000 nm and 1000 V for a coating thickness of 2000 nm.

[0017] In a special embodiment, instead of linearly increasing the absolute value of the bias voltage during the coating process, the preferred method would be to set different values for the increase ratio of the absolute value of the bias voltage during the coating process. In this way, for example, the region along the coating thickness where a preferred ta-C would be generated could be extended. This effect can be combined with a coating stop applied during the coating process to maintain or lower the specific range of temperatures at which ta-C with sp3 bonds is generated.

[0018] Other improvements to the hard coating system can be achieved by using a pretreatment of the substrate surface before the deposition of the carbon-based coating, such as metal ion etching (MIE), or by adding a very thin adhesion layer such as a Cr-based layer that can be generated using residual Cr from a previous metal ion etching process.

[0019] Another advantage of the present invention is that, as described above, it is possible to post-treat the surface of a carbon-based coating that would have a lower hardness than the core of the coating. Surface smoothing and droplet removal can include sanding, grinding, and in particular band finishing, which will be described in more detail below.

[0020] Band finishing is a process in which a part to be processed, such as a piston pin, is mounted on a rotatable spindle. Thereafter, another part called a "finisher" is pressed against the workpiece via a belt that slowly advances at a defined air pressure. A tape on the belt with a Shore hardness of about A65° or about A85°, preferably about A65° - A85°, rotates with the belt and rubs against the workpiece. The selection of the tape material (type of grains, size of grains, weave) is essential for the finishing result. In particular, diamond microfinishing tape materials having a grain size of about 9 μm to about 30 μm are used depending on the required finish and the finishing time of the surface. Preferably, a diamond tape with a grain size of about 9 μm is used together with a belt having a Shore hardness of about A65° to obtain the best finishing result. In addition to feeding, the finisher can also oscillate left and right along the axis of the rotating workpiece to perform a so-called cross finish. To suppress overheating of the tape during the grinding process, a cooling lubricant is often used, but it is not necessarily required. Typical roughness values of the unfiltered bias slump layer before (while coated) and after finishing can typically be reduced from Rz = 1.5 to 0.5 μm. The target and optimum roughness values shown in parentheses were found to be Spk < 0.4 μm (0.3 μm), Rpk < 0.5 μm (0.3 μm), Rpkx < 0.2 μm (0.1 μm), RfpH5n(F) < 0.6 μm (< 0.3 μm), GKV < 6 μm (3 μm). More detailed values are shown in FIG. 5.

[0021] Roughness measurement is important for coatings produced by cathodic arc discharge where droplets are present within the coating, especially when roughness measurement needs to be considered. Optical methods, such as those developed by Confovis in combination with specially designed algorithms, are used to count droplets on the surface and evaluate the effectiveness of the finishing method.

[0022] The process steps for explaining the production of a tetrahedral carbon layer (ta-C) on a steel substrate using cathodic arc evaporation are as follows:

[0023] Preheating The substrate is heated to approximately 150 °C in a vacuum chamber using an integrated radiant heater. The substrate is rotated with various degrees of freedom, either one rotation, two rotations, or three rotations, for an optimal heat distribution on the surface.

[0024] Etching In the second step, the substrate is etched with argon ions. The etching is carried out by the so-called "Advanced Energy Glow Discharge" (AEGD) technique. In the coating chamber, a titanium target intended for this purpose is operated behind a shutter (shutter at chamber potential) by an arc with a target current of 100 A. The resulting titanium ions (Ti+) are captured by the shutter. At a positive potential, the electrons generated by the ionization of titanium are conducted through the rod anode. At this stage, the argon flow is pressure-controlled and supplied to the coating chamber at approximately 1 x 10 -2 mbar. The ionized argon (Ar + ) is directed towards the substrate via a negative bias voltage of -200 V. Next, the surface is etched by ion bombardment.

[0025] Chromium-MIE (Chromium-Metal-Ion Etching) 1.3 x 10 -3Under a high vacuum of mbar and an argon flow of approximately 178 sccm, the chromium target was ignited with a trigger wire and then operated at a target current of 80 A. As a result, an arc was generated and could move on the target using the magnetic field on the target surface. When a negative bias voltage of -800 V was applied to the substrate, chromium ions (Cr + ) were strongly accelerated from the target to the substrate. Due to the high impact of these ions on the surface, the surface oxide could be removed, and at the same time, the ions could penetrate the substrate material and generate chromium ion implantation. Also, a very thin chromium layer of 20 - 100 nm could be formed by the chromium remaining on the surface. Chromium-Metal Ion Etching (Cr-MIE) ensures a smooth transition at the interface between the substrate and the layer material. Furthermore, the chromium implantation functions as a kind of anchor for the substrate and ensures good adhesion of the layer. Since Cr-MIE is a high-energy process step, the substrate can become quite hot. Therefore, in order to keep the substrate temperature below the annealing temperature of the substrate material, some stops need to be applied.

[0026] Cooling period Before applying the carbon-based coating, the substrate is cooled to approximately 100 °C to improve the adhesion between the thin chromium layer and the carbon layer. During the cooling stage, the substrate is rotated within the vacuum chamber. The cooling time can vary depending on the mass and volume of the substrate.

[0027] Carbon transition layer (C-intermediate layer) 1x10 -3 ~2x10 -3 mbar, preferably approximately 1.7x10 -3 mbar process pressure and a carbon doping is performed with an argon flow of 50 - 80 sccm, preferably approximately 64 sccm. Argon is introduced directly in front of the carbon target by a gas shower. The carbon target is ignited with a trigger wire and operates at a target current of 40 - 55 A, preferably approximately 45 A. The resulting carbon ions (C +) is accelerated onto the substrate at a bias voltage of -500 to -300 V, preferably -500 V. Thereby, carbon is implanted into the thin chromium layer. Thereafter, the absolute value of the negative bias voltage can be decreased from -500 V to -150 V.

[0028] Carbon-based coating process (ta-C coating) Carbon-based coatings, especially ta-C coatings, are produced at a process pressure of 1x10 -3 ~2x10 -3 mbar, preferably about 1.3x10 -3 mbar and an argon flow of 80 to 100 sccm, preferably about 90 sccm. In this case, the carbon target is operated at a lower target current of 25 to 35 A. The absolute value of the negative bias voltage is increased at a predetermined rate (ramp) over the entire coating time of 0 to -200 V. The target current of the carbon target is kept constant. Applying a bias voltage ramp has the advantage that a less hard ta-C layer can be formed at the start of the coating. Thereafter, when the bias voltage is increased, a harder layer is formed in the range of 40 to 50 GPa. When the bias voltage is about -160 V and the substrate temperature is close to 160 °C, the proportion of sp2 bonds increases while the proportion of sp3 bonds decreases. As a result, a graphite-like top layer is brought about, which has a significantly lower hardness than the core of the coating of about 15 to 30 GPa. The low-hardness top layer has the great advantage that the finishing of the layer can be made easier.

[0029] Further advantages, effects and details of the present invention are described in this specification and the accompanying drawings. It is understood that the features described above and the features further described below can be used not only in the combinations shown in each case, but also in other combinations or in isolation without departing from the scope of the present invention.

[0030] The present invention is schematically shown in the drawings based on embodiments and will be further described with reference to the drawings.

Brief Description of the Drawings

[0031]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

[0032] Figure 1 shows a carbon-based coating constructed from left to right, indicating different coating structures along the coating thickness. The temperature of the substrate is shown in red, and the bias voltage is shown in blue. The coating can be divided into three ranges: A, B, and C. Region A near the substrate corresponds to a fine structure with a coating thickness of 200 - 1000 nm, preferably about 700 nm, where the absolute value of the bias voltage rises from 10 V to 100 V and the substrate temperature rises from about 140 °C to 180 °C. The stop of the coating between regions A and B induces a decrease in the substrate temperature of about 20 °C, and then when the bias current increases from 100 V to 150 V, the temperature increases from about 160 °C to 190 °C. The coating in region B is glassy and denser (ta-C), with a coating thickness in the range of 200 - 500 nm, preferably about 400 nm. Region C shows a more porous and rough coating, where the absolute value of the bias voltage rises from 150 V to 200 V and the substrate temperature there rises from 190 °C to 220 °C. In this region of the coating, sp2 bonds are dominant, the coating has a reduced hardness, and the coating thickness is in the range of 200 to 500 nm, preferably about 400 nm.

[0033] Figure 2 shows the same cross-section as in Figure 1 without a legend, with enhanced contrast to show the differences in the coating structure along the coating thickness.

[0034] Figure 3 is a diagram showing the coating described in the present invention with the absolute value of the bias voltage illustrated. (1) is the substrate, (2) is the Cr layer, (3) is the C intermediate layer, (4) is ta-C at a low bias voltage (0 - 20 V), (5) is ta-C at an intermediate bias voltage (20 - 160 V), and (6) is the ta-C top layer at a bias voltage (160 - 200 V).

[0035] Figure 4 shows some related process data such as the cathode arc current, the absolute value of the bias voltage, the gas flow, and the substrate temperature. The values in the legend represent full scale at 100. The X-axis represents time in minutes. The coating process of this example is represented by the dark blue line which is the input of the cathode arc current, lasts for a little over 200 minutes, interrupts the process at about 100 minutes to apply a coating stop for about 20 minutes, and then the coating process continues until the desired coating thickness is achieved. During the entire 200-minute coating time including the coating stop, the absolute value of the bias voltage represented by the green line increases from 10V to 200V. During the break, there is no coating process and the bias voltage remains constant without affecting the substrate. The substrate temperature drops from 175°C to 140°C, becoming a temperature suitable for forming more sp3 bonds for a longer time. As a result, it can be confirmed as a positive effect of the coating stop that the area where the ta-C structure exists on the coating becomes wider. Different data and ramps such as the rising rate of the absolute value of the bias voltage, the coating time, the coating stop time, and the number of coating stops are shown merely by way of example. For example, different increasing rates of the absolute value of the bias voltage can be selected during each interruption of the coating, or the coating can be interrupted multiple times, or the coating stop can last for a time less than 20 minutes or more than 20 minutes but not shorter than 1 minute.

[0036] Figure 5 shows typical roughness values of the unfiltered bias ramp layer before (as coated) and after finishing, which can typically be reduced from Rz = 1.5 to 0.5 μm. The target and optimum roughness values shown in parentheses were found to be Spk < 0.4 μm (0.3 μm), Rpk < 0.5 μm (0.3 μm), Rpkx < 0.2 μm (0.1 μm), RfpH5n(F) < 0.6 μm (< 0.3 μm), GKV < 6 μm (3 μm).

Claims

Claim 1 A method for manufacturing a hard coating on a substrate, wherein the hard coating comprises a hydrogen-free amorphous carbon coating, the hydrogen-free amorphous carbon coating is deposited on the substrate using a cathodic arc deposition method, a bias voltage having an absolute value greater than 0 V and less than 1000 V is applied to the substrate, and during the hydrogen-free amorphous carbon coating process, the absolute value of the bias voltage is increased to obtain a first structure and a second structure and a gradient between the first structure and the second structure along the coating thickness, the first structure and the second structure include sp2 and sp3 carbon bonds, but their relative concentrations are different, at least one coating pause is applied during the hydrogen-free amorphous carbon coating process, the substrate temperature is decreased during the coating pause, the hydrogen-free amorphous carbon coating is divided into three regions A, B, and C, the regions A, B, and C each have the first structure and the second structure, the region A is close to the substrate, the region B is applied on the region A, the region C is applied on the region B, the SP3 bond is higher in the region B than in both the region A and the region C, and the hardness of the region B is higher than the hardness of both the region A and the region C. Claim 2 The method according to claim 1, wherein the coating pause lasts for at least 1 minute. Claim 3 The method according to claim 1 or 2, wherein the increase in the absolute value of the bias voltage is set at a rate defined by a ratio ΔU / Δs, which is the ratio of the absolute value of the voltage difference per unit time. Claim 4 The method according to claim 1 or 2, wherein the increase in the absolute value of the bias voltage is set at a rate defined by a ratio ΔU / Δd, which is the ratio between the absolute value of the voltage difference and the difference in coating thickness. Claim 5 The method according to claim 4, wherein the ratio is between about 0.02 V / nm and about 0.5 V / nm. Claim 6 The method according to any one of claims 1 to 5, wherein the absolute value of the bias voltage increases linearly. Claim 7 The method according to any one of claims 1 to 5, characterized in that the absolute value of the bias voltage increases stepwise during the hard coating process.

8. The method according to any one of claims 1 to 7, characterized in that the substrate is preheated to about 150 °C in a vacuum chamber before the hard coating is applied to the substrate.

9. The method according to any one of claims 1 to 8, characterized in that the substrate is pretreated, in particular by metal ion etching (MIE) and / or by the addition of a very thin adhesion layer of a Cr-based layer, before the hard coating is applied to the substrate.

10. The method according to claim 8, characterized in that the substrate has a temperature of about 100 °C before the hard coating is applied to the substrate, and in particular that the substrate is cooled to about 100 °C after the preheating according to claim 8 in order to ensure good adhesion before the hard coating is applied to the substrate.

11. The carbon transfer layer (C-intermediate layer) is coated at a process pressure of 1x10 －３ to 2x10 －３ mbar and carbon doping is carried out with an argon flow of 50 to 80 sccm. The argon is introduced directly in front of the carbon target by means of a gas shower. The carbon target is ignited by a trigger wire and operated with a target current between 40 and 55 A. The resulting carbon ions (C ＋ ) are then accelerated on the substrate by a bias voltage of -500 V to -300 V, whereby carbon is implanted into the thin chromium layer. The absolute value of the negative bias voltage then decreases to -300 V to -150 V. The method according to any one of claims 1 to 10.

12. The hard coating (ta-C coating) is coated at a process pressure of 1x10 - ３ to 2x10 －３ mbar and an argon flow of 80 to 100 sccm. The carbon target is operated at a low target current of about 25 to about 35 A. The absolute value of the negative bias voltage increases at a rate of about 0 to about -200 V (ramp) over the entire coating time. The target current of the carbon target is kept constant. By applying the ramp of the bias voltage at the start of the hard coating, the ta-C layer having a lower hardness is generated. When the subsequent bias voltage is increased, a harder layer in the range of 40 to 50 GPa is formed. At a bias voltage of about -160 V and a substrate temperature close to 160 °C, the ratio of sp2 bonds increases while the ratio of sp3 bonds decreases. As a result, a graphite-like top layer having a hardness significantly lower than that of the core of the hard coating of about 15 to 30 GPa is obtained. The method according to claim 11, characterized in that.

13. The method according to any one of claims 1 to 12, characterized in that the surface of the carbon-based coating is post-processed by sanding, grinding, and / or band finishing.

14. A workpiece comprising a substrate and a hard coating applied to the substrate, wherein the hard coating comprises a hydrogen-free amorphous carbon coating, the hydrogen-free amorphous carbon coating is deposited on the substrate, the hard coating obtains a first structure and a second structure and a gradient between the first structure and the second structure along the coating thickness, the first structure and the second structure comprise sp2 and sp3 carbon bonds, but their relative concentrations are different, the hydrogen-free amorphous carbon coating is divided into three regions A, B, and C, the regions A, B, and C each have the first structure and the second structure, the region A is close to the substrate, the region B is applied on the region A, the region C is applied on the region B, the region B has a higher SP3 bond than both the region A and the region C, and the hardness of the region B is higher than the hardness of both the region A and the region C.

15. The surface roughness of the workpiece is Rz = 0.5 μm to 1.5 μm, and in particular, the target value of the roughness is Spk < 0.4 μm, and / or in particular Rp < 0.5 μm, and / or in particular Rpkx < 0.2 μm, and / or in particular RfpH5n(F) < 0.6 μm, and / or in particular GKV < 6 μm. The workpiece according to claim 14, characterized in that.

16. The region A corresponds to a microstructure having a thickness of 200 to 1000 nm, the region B is glassy and has a higher density than the region A, the region B has a thickness of 200 to 500 nm, the region C shows a more porous and rough coating compared to the region B, and the thickness of the region C is 200 to 500 nm. The workpiece according to claim 14 or 15, characterized in that.

17. A Cr layer is coated on the base material, a C intermediate layer is coated on the Cr layer, and the hard coating is coated on the C intermediate layer. The workpiece according to any one of claims 14 to 16, characterized in that.

Images