Wear-resistant hard alloy and preparation method thereof

Abstract

The invention relates to the field of hard alloy materials, in particular to a wear-resistant hard alloy and a preparation method thereof.The wear-resistant hard alloy is composed of a hard alloy matrix, a transition layer and a wear-resistant coating, the transition layer is made of rare earth-iron binary alloy and LaMO3, and M is a transition metal element. And the thermal stress distribution among the layers is more uniform, and the stress concentration phenomenon is reduced, so that the bonding strength between the wear-resistant coating and the hard alloy matrix is improved, and the service life is prolonged.

Description

Technical Field

[0001] The present invention relates to the field of cemented carbide materials, in particular to a wear-resistant cemented carbide and a preparation method thereof. Background Art

[0002] With the rapid development of dry high-performance cutting technology, tools need to withstand higher temperatures and faster cutting speeds during the machining process. Especially when cutting difficult-to-machine materials such as high-temperature alloys, violent chemical reactions will occur at the cutting edge of the tool, resulting in severe wear on the workpiece surface and shortened tool life.

[0003] Attaching a wear-resistant coating on the surface of cemented carbide to form a surface coated cemented carbide can improve the wear resistance of the tool to a certain extent and increase its service life. TiN coating has been widely used in the field of tool coating due to its high wear resistance and chemical stability. However, it is mainly used under conditions of lower temperatures and simple working conditions. When the cutting temperature exceeds 500°C, the TiN coating will produce severe oxidation wear, and eventually cause the coating to fall off. TiAIN coating is obtained by doping Al elements. It is a metastable stable solution formed by Al atoms replacing some Ti atoms in TiN, and a dense aluminum oxide protective film can be generated on the surface of the TiAIN coated tool, so that it still maintains good cutting performance under working conditions above 800°C.

[0004] However, whether it is TiN coating or TiAIN coating, the thermal expansion coefficient between them and cemented carbide is significantly different, resulting in insufficient interface bonding strength. The coating is easily peeled off from the substrate during high-speed cutting. How to improve the bonding strength between the coating and the substrate has become a research hotspot for technicians in this field. Summary of the Invention

[0005] Purpose of the invention: In order to solve the above technical problems, the present invention provides a wear-resistant cemented carbide and a preparation method thereof.

[0006] The technical solutions adopted are as follows: A wear-resistant cemented carbide, consisting of a cemented carbide substrate, a transition layer and a wear-resistant coating: The transition layer is made of rare earth-iron binary alloy and LaMO3, where M is a transition metal element.

[0007] Furthermore, the mass ratio of the rare earth-iron binary alloy to the rare earth oxide is 1-5:1-5.

[0008] Furthermore, the rare earth-iron binary alloy is TbFe2 and / or DyFe2.

[0009] Furthermore, M is Ni 3+ 、Co 3+ or Cr 3+ .

[0010] Furthermore, the wear-resistant coating is a TiAlN coating.

[0011] Furthermore, the TiAlN coating is deposited from a titanium aluminum alloy target in a mixed atmosphere of nitrogen and silane.

[0012] Furthermore, the silane is a product obtained by replacing one or more carbon atoms in methane, ethane, propane, butane, pentane or hexane with silicon atoms.

[0013] Furthermore, the silane is n-butylsilane (CAS NO: 1600-29-9).

[0014] The present invention also provides a method for preparing wear-resistant cemented carbide: The cemented carbide substrate is ground, polished, cleaned and dried, and the rare earth-iron binary alloy and LaMO3 are evenly mixed to form a transition layer on the surface of the cemented carbide substrate using laser cladding technology. Subsequently, the TiAlN coating is deposited on the transition layer in a mixed atmosphere of nitrogen and silane.

[0015] Furthermore, the process parameters of laser cladding are as follows: The laser power is 3-5kW, the scanning speed is 200-400mm / min, the spot diameter is 1-5mm, argon protection is used, and the argon flow rate is 1-10L / min.

[0016] Furthermore, the flow ratio of nitrogen to silane is 40-50:1.

[0017] It has the following beneficial effects: The difference in thermal expansion coefficient between cemented carbide and wear-resistant coating is a key factor affecting the interfacial bonding performance and coating service life. During the cooling process after deposition or high-temperature service, the wear-resistant coating shrinks more, resulting in tensile and compressive residual stresses at the interface. This stress can cause coating peeling and shorten service life. By adding a transition layer, a gradient structure of thermal expansion coefficient can be formed, making the thermal stress distribution between the layers more uniform, thereby reducing stress concentration, improving the bonding strength between the wear-resistant coating and the cemented carbide substrate, and extending service life.

[0018] The present invention provides a wear-resistant cemented carbide. The high-temperature stability and oxygen vacancy characteristics of LaMO3 can promote chemical bonding and offset the internal stress of the coating through compressive stress. The transition layer made by combining with a rare earth-iron binary alloy with a low thermal expansion coefficient can effectively alleviate the difference in thermal expansion coefficient between the wear-resistant coating and the cemented carbide substrate, thereby improving the bonding strength of the interface between the two. In addition, the rare earth-iron binary alloy promotes interfacial wettability and enhances physical bonding through high surface energy. At the same time, the rare earth element has high activity and can form a strong chemical bond with the cemented carbide substrate. At the same time, it undergoes a partial diffusion reaction with the elements in the wear-resistant coating, thereby improving the interface bonding performance.

[0019] When silane is introduced during TiAlN coating deposition, the silicon generated by the decomposition of silane forms amorphous Si3N4 within the TiAlN coating. This not only increases the grain boundaries between different phases but also provides internal resistance within the coating, promoting grain isolation, preventing dislocation formation, and reducing inter-grain motion, thereby increasing the coating's hardness. During high-speed friction, the amorphous Si3N4 reacts with moisture in the air to form SiO2·nH2O, a lubricating compound that also reduces the coating's friction coefficient and improves wear resistance. The carbon generated during the decomposition of silane replaces some nitrogen atoms in the TiAlN unit cell, forming a TiAlCN solid solution, which increases the coating's hardness. Furthermore, the lubricating effect of carbon also helps reduce the coating's friction coefficient and extend its service life. Of course, the above information is a reasonable speculation based on performance test results and existing relevant research. It may be imperfect or even oversights, and further research is needed. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a cross-sectional morphology of the wear-resistant cemented carbide prepared in Example 1. DETAILED DESCRIPTION

[0021] Unless otherwise specified, the following examples and comparative examples were conducted in parallel, using the same processing steps and parameters.

[0022] Example 1: This embodiment provides a wear-resistant cemented carbide, which is composed of a cemented carbide substrate, a transition layer and a wear-resistant coating. Figure 1 : The transition layer is made of DyFe2 alloy and LaNiO3 with a mass ratio of 1:1, and the wear-resistant coating is a TiAlN coating.

[0023] The preparation method of the above-mentioned wear-resistant cemented carbide is as follows: A cemented carbide substrate with a Co content of 12 wt.% and a WC grain size of 1.6 μm was used as the substrate. The cemented carbide substrate was ground, polished, cleaned with alcohol, and then dried with hot air. DyFe2 alloy and LaNiO3 with a mass ratio of 1:1 were evenly mixed and then laser cladding technology was used to form a transition layer with a thickness of 1.5±0.1 μm on the surface of the cemented carbide substrate. The laser cladding process parameters were as follows: laser power 4 kW, scanning speed 300 mm / min, spot diameter 3 mm, argon protection, and argon flow rate of 5 L / min. Subsequently, the cemented carbide substrate was placed in the chamber of a cathodic arc evaporation equipment. Ti33Al67 alloy was used as the target material, and nitrogen and n-butylsilane with a flow ratio of 45:1 were used as the working gas. The TiAlN coating was deposited for 240 min at a target current of 180 A, a bias voltage of -40 V, a pressure of 3.2 Pa, and a deposition temperature of 700°C.

[0024] Example 2: This embodiment provides a wear-resistant cemented carbide, which is composed of a cemented carbide substrate, a transition layer and a wear-resistant coating: The transition layer is made of DyFe2 alloy and LaNiO3 with a mass ratio of 1:1, and the wear-resistant coating is a TiAlN coating.

[0025] The preparation method of the above-mentioned wear-resistant cemented carbide is as follows: A cemented carbide with a Co content of 12 wt.% and a WC grain size of 1.6 μm was used as the substrate. The cemented carbide substrate was ground, polished, and cleaned with alcohol, then dried with hot air. DyFe2 alloy and LaNiO3 with a mass ratio of 1:1 were evenly mixed and then laser cladding technology was used to form a transition layer with a thickness of 1.5±0.1 μm on the surface of the cemented carbide substrate. The laser cladding process parameters were as follows: laser power 4 kW, scanning speed 300 mm / min, spot diameter 3 mm, argon protection, and argon flow rate of 5 L / min. Subsequently, the cemented carbide substrate was placed in the chamber of a cathodic arc evaporation equipment. Ti33Al67 alloy was used as the target material and nitrogen was used as the working gas. The TiAlN coating was deposited for 240 min at a target current of 180 A, a bias voltage of -40 V, a pressure of 3.2 Pa, and a deposition temperature of 700°C.

[0026] The difference between Example 2 and Example 1 is that n-butylsilane was not introduced during the deposition of the TiAlN coating, which resulted in a decrease in the performance of the wear-resistant cemented carbide. This may be because n-butylsilane was introduced during the deposition of the TiAlN coating. The silicon generated by the decomposition of n-butylsilane forms amorphous Si3N4 in the TiAlN coating, which not only increases the grain boundaries between different phases but also provides internal resistance in the coating, promotes the isolation of grains, prevents dislocation formation, and reduces inter-grain motion, thereby improving the hardness of the coating. During high-speed friction, the amorphous Si3N4 reacts with moisture in the air to generate SiO2·nH2O with a lubricating effect, which can also reduce the friction coefficient of the coating and improve wear resistance. The carbon element during the decomposition of silane replaces some nitrogen atoms in the TiAlN unit cell to form a TiAlCN solid solution, which improves the hardness of the coating. The lubricating effect of carbon also helps to reduce the friction coefficient of the coating and extend its service life.

[0027] Example 3: This embodiment provides a wear-resistant cemented carbide, which is composed of a cemented carbide substrate, a transition layer and a wear-resistant coating: The transition layer is made of DyFe2 alloy and LaNiO3 with a mass ratio of 5:1, and the wear-resistant coating is TiAlN coating.

[0028] The preparation method of the above-mentioned wear-resistant cemented carbide is as follows: A cemented carbide substrate with a Co content of 12 wt.% and a WC grain size of 1.6 μm was used as the substrate. The cemented carbide substrate was ground, polished, cleaned with alcohol, and then dried with hot air. DyFe2 alloy and LaNiO3 with a mass ratio of 5:1 were evenly mixed and then laser cladding technology was used to form a transition layer with a thickness of 1.5±0.1 μm on the surface of the cemented carbide substrate. The laser cladding process parameters were as follows: laser power 5 kW, scanning speed 400 mm / min, spot diameter 5 mm, argon protection, and argon flow rate of 10 L / min. Subsequently, the cemented carbide substrate was placed in the chamber of a cathodic arc evaporation equipment. Ti33Al67 alloy was used as the target material, and nitrogen and n-butylsilane with a flow ratio of 50:1 were used as the working gas. The TiAlN coating was deposited for 240 min at a target current of 180 A, a bias voltage of -40 V, a pressure of 3.2 Pa, and a deposition temperature of 700°C.

[0029] The difference between Example 3 and Example 1 is that the mass ratio of DyFe2 alloy and LaNiO3 is different. It is possible that changing the ratio of the two will affect the construction of the thermal expansion gradient system of the wear-resistant cemented carbide, thereby affecting the performance of the wear-resistant cemented carbide.

[0030] Example 4: This embodiment provides a wear-resistant cemented carbide, which is composed of a cemented carbide substrate, a transition layer and a wear-resistant coating: The transition layer is made of DyFe2 alloy and LaNiO3 with a mass ratio of 1:5, and the wear-resistant coating is a TiAlN coating.

[0031] The preparation method of the above-mentioned wear-resistant cemented carbide is as follows: A cemented carbide substrate with a Co content of 12 wt.% and a WC grain size of 1.6 μm was used as the substrate. The cemented carbide substrate was ground, polished, cleaned with alcohol, and then dried with hot air. DyFe2 alloy and LaNiO3 with a mass ratio of 1:5 were evenly mixed and then laser cladding technology was used to form a transition layer with a thickness of 1.5±0.1 μm on the surface of the cemented carbide substrate. The laser cladding process parameters were as follows: laser power 3 kW, scanning speed 200 mm / min, spot diameter 1 mm, argon protection, and argon flow rate of 1 L / min. Subsequently, the cemented carbide substrate was placed in the chamber of a cathodic arc evaporation equipment. Ti33Al67 alloy was used as the target material, and nitrogen and n-butylsilane with a flow ratio of 40:1 were used as the working gas. The TiAlN coating was deposited for 240 min at a target current of 180 A, a bias voltage of -40 V, a pressure of 3.2 Pa, and a deposition temperature of 700°C.

[0032] The difference between Example 4 and Example 1 is that the mass ratio of DyFe2 alloy and LaNiO3 is different. It is possible that changing the ratio of the two will affect the construction of the thermal expansion gradient system of the wear-resistant cemented carbide, thereby affecting the performance of the wear-resistant cemented carbide.

[0033] Example 5: This embodiment provides a wear-resistant cemented carbide, which is composed of a cemented carbide substrate, a transition layer and a wear-resistant coating: The transition layer is made of TbFe2 alloy and LaNiO3 with a mass ratio of 1:1, and the wear-resistant coating is a TiAlN coating.

[0034] The preparation method of the above-mentioned wear-resistant cemented carbide is as follows: A cemented carbide substrate with a Co content of 12 wt.% and a WC grain size of 1.6 μm was used as the substrate. The cemented carbide substrate was ground, polished, cleaned with alcohol, and then dried with hot air. TbFe2 alloy and LaNiO3 with a mass ratio of 1:1 were evenly mixed and then laser cladding technology was used to form a transition layer with a thickness of 1.5±0.1 μm on the surface of the cemented carbide substrate. The laser cladding process parameters were as follows: laser power 4 kW, scanning speed 300 mm / min, spot diameter 3 mm, argon protection, and argon flow rate of 5 L / min. Subsequently, the cemented carbide substrate was placed in the chamber of a cathodic arc evaporation equipment. Ti33Al67 alloy was used as the target material, and nitrogen and n-butylsilane with a flow ratio of 45:1 were used as the working gas. The TiAlN coating was deposited for 240 min at a target current of 180 A, a bias voltage of -40 V, a pressure of 3.2 Pa, and a deposition temperature of 700°C.

[0035] The difference between Example 5 and Example 1 is that TbFe2 alloy is used instead of DyFe2 alloy, which may affect the construction of the thermal expansion gradient system of the wear-resistant cemented carbide, thereby affecting the performance of the wear-resistant cemented carbide. The activity difference of different rare earth elements may also lead to changes in the performance of the wear-resistant cemented carbide.

[0036] Example 6: This embodiment provides a wear-resistant cemented carbide, which is composed of a cemented carbide substrate, a transition layer and a wear-resistant coating: The transition layer is made of DyFe2 alloy and LaCoO3 with a mass ratio of 1:1, and the wear-resistant coating is a TiAlN coating.

[0037] The preparation method of the above-mentioned wear-resistant cemented carbide is as follows: A cemented carbide substrate with a Co content of 12 wt.% and a WC grain size of 1.6 μm was used as the substrate. The cemented carbide substrate was ground, polished, cleaned with alcohol, and then dried with hot air. DyFe2 alloy and LaCoO3 with a mass ratio of 1:1 were evenly mixed and then laser cladding technology was used to form a transition layer with a thickness of 1.5±0.1 μm on the surface of the cemented carbide substrate. The laser cladding process parameters were as follows: laser power 4 kW, scanning speed 300 mm / min, spot diameter 3 mm, argon protection, and argon flow rate of 5 L / min. Subsequently, the cemented carbide substrate was placed in the chamber of a cathodic arc evaporation equipment. Ti33Al67 alloy was used as the target material, and nitrogen and n-butylsilane with a flow ratio of 45:1 were used as the working gas. The TiAlN coating was deposited for 240 min at a target current of 180 A, a bias voltage of -40 V, a pressure of 3.2 Pa, and a deposition temperature of 700°C.

[0038] The difference between Example 6 and Example 1 is that LaCoO3 is used instead of LaNiO3, which may affect the construction of the thermal expansion gradient system of the wear-resistant cemented carbide, thereby affecting the performance of the wear-resistant cemented carbide. The activity difference of different transition metal elements may also lead to changes in the performance of the wear-resistant cemented carbide.

[0039] Example 7: This embodiment provides a wear-resistant cemented carbide, which is composed of a cemented carbide substrate, a transition layer and a wear-resistant coating: The transition layer is made of DyFe2 alloy and LaCrO3 with a mass ratio of 1:1, and the wear-resistant coating is a TiAlN coating.

[0040] The preparation method of the above-mentioned wear-resistant cemented carbide is as follows: A cemented carbide substrate with a Co content of 12 wt.% and a WC grain size of 1.6 μm was used as the substrate. The cemented carbide substrate was ground, polished, cleaned with alcohol, and then dried with hot air. DyFe2 alloy and LaCrO3 with a mass ratio of 1:1 were evenly mixed and then laser cladding technology was used to form a transition layer with a thickness of 1.5±0.1 μm on the surface of the cemented carbide substrate. The laser cladding process parameters were as follows: laser power 4 kW, scanning speed 300 mm / min, spot diameter 3 mm, argon protection, and argon flow rate of 5 L / min. Subsequently, the cemented carbide substrate was placed in the chamber of a cathodic arc evaporation equipment. Ti33Al67 alloy was used as the target material, and nitrogen and n-butylsilane with a flow ratio of 45:1 were used as the working gas. The TiAlN coating was deposited for 240 min at a target current of 180 A, a bias voltage of -40 V, a pressure of 3.2 Pa, and a deposition temperature of 700°C.

[0041] The difference between Example 7 and Example 1 is that LaCrO3 is used instead of LaNiO3, which may affect the construction of the thermal expansion gradient system of the wear-resistant cemented carbide, thereby affecting the performance of the wear-resistant cemented carbide. The activity difference of different transition metal elements may also lead to changes in the performance of the wear-resistant cemented carbide.

[0042] Comparative Example 1: It is basically the same as Example 1, except that it does not contain a transition layer; This comparative example provides a wear-resistant cemented carbide, which consists of a cemented carbide substrate and a wear-resistant coating: Wherein, the wear-resistant coating is a TiAlN coating.

[0043] The preparation method of the above-mentioned wear-resistant cemented carbide is as follows: A cemented carbide substrate with a Co content of 12 wt.% and a WC grain size of 1.6 μm was used as the substrate. After grinding, polishing, and alcohol cleaning, the cemented carbide substrate was hot-air dried. The substrate was placed in the chamber of a cathodic arc evaporation device. A Ti33Al67 alloy target was used, and nitrogen and n-butylsilane were used as the working gases at a flow ratio of 45:1. The TiAlN coating was deposited for 240 minutes at a target current of 180 A, a bias voltage of -40 V, a pressure of 3.2 Pa, and a deposition temperature of 700°C. Since there is no transition layer, the difference in thermal expansion coefficient between cemented carbide and wear-resistant coating affects the interface bonding performance and service life of the coating. During the cooling process after deposition of the wear-resistant coating or high-temperature service, the wear-resistant coating shrinks more, resulting in tensile residual stress and compressive residual stress at the interface, which leads to the deterioration of the performance of the wear-resistant cemented carbide.

[0044] Comparative Example 2: It is basically the same as Example 1, except that the transition layer does not contain LaNiO 3； This comparative example provides a wear-resistant cemented carbide, which consists of a cemented carbide substrate, a transition layer and a wear-resistant coating: The transition layer is made of DyFe2 alloy, and the wear-resistant coating is TiAlN coating.

[0045] The preparation method of the above-mentioned wear-resistant cemented carbide is as follows: A cemented carbide substrate with a Co content of 12 wt.% and a WC grain size of 1.6 μm was used as the substrate. The cemented carbide substrate was ground, polished, cleaned with alcohol, and then hot-air dried. DyFe2 alloy was laser clad on the surface of the cemented carbide substrate to form a transition layer with a thickness of 1.5±0.1 μm. The laser cladding process parameters were as follows: laser power 4 kW, scanning speed 300 mm / min, spot diameter 3 mm, argon protection, and argon flow rate of 5 L / min. The cemented carbide substrate was then placed in the chamber of a cathodic arc evaporation device. Ti33Al67 alloy was used as the target material, and nitrogen and n-butylsilane with a flow ratio of 45:1 were used as the working gas. The TiAlN coating was deposited for 240 min at a target current of 180 A, a bias voltage of -40 V, a pressure of 3.2 Pa, and a deposition temperature of 700°C. Since the transition layer does not contain LaNiO3, a thermal expansion gradient system cannot be constructed, resulting in a decrease in the performance of the wear-resistant cemented carbide.

[0046] Comparative Example 3: The method is basically the same as Example 1, except that the DyFe2 alloy is replaced by Fe-36Ni Invar alloy; Although Fe-36Ni Invar alloy also has a low thermal expansion coefficient and can be used with LaNiO3 to construct a thermal expansion gradient system, it lacks highly active rare earth elements, which results in a decrease in the performance of wear-resistant cemented carbide.

[0047] Performance testing: The wear-resistant cemented carbides prepared in Examples 1-7 of the present invention and Comparative Examples 1-3 were used as samples for performance testing.

[0048] The bond strength between the coating and the carbide substrate was measured using a WS-500 automatic scratch tester. A diamond stylus applied a uniform normal pressure of 0-140 N to the coating surface. The large fluctuations and sharp increases in the friction coefficient and acoustic signal during the scratch test correlate with the exposure of the substrate after coating failure. Therefore, the acoustic signal, friction coefficient, and normal load can be combined to determine the bond strength between the coating and the substrate.

[0049] The sliding friction test of the sample coating was carried out at room temperature. The friction and wear test was carried out on an Rtec MFT-5000 friction and wear tester using GCr15 grinding balls with a load of 5N, a wear radius of 3mm, a rotation speed of 180r / min, and a wear time of 30min.

[0050] The test results are shown in Table 1 below:

[0051] It can be seen from Table 1 above that the wear-resistant cemented carbide prepared by the method of the present invention has excellent wear resistance and hardness, and the hardness of the TiAlN coating and the bonding strength with the cemented carbide substrate are high.

[0052] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A wear-resistant hard alloy, characterized in that: It consists of cemented carbide substrate, transition layer and wear-resistant coating: The transition layer is made of rare earth-iron binary alloy and LaMO3, where M is a transition metal element.

2. The wear-resistant hard alloy according to claim 1, characterized in that: The mass ratio of the rare earth-iron binary alloy to the rare earth oxide is 1-5:1-5.

3. The wear-resistant hard alloy according to claim 1, characterized in that: The rare earth-iron binary alloy is TbFe2 and / or DyFe2.

4. The wear-resistant hard alloy according to claim 1, wherein M is Ni 3+ 、Co 3+ or Cr 3+ .

5. The wear-resistant hard alloy according to claim 1, wherein: The wear-resistant coating is a TiAlN coating.

6. The wear-resistant hard alloy according to claim 5, characterized in that: The TiAlN coating is obtained by depositing a titanium-aluminum alloy target in a mixed atmosphere of nitrogen and silane.

7. The wear-resistant hard alloy according to claim 6, characterized in that: The silane is a product obtained by replacing one or more carbon atoms in methane, ethane, propane, butane, pentane or hexane with silicon atoms.

8. A method for preparing a wear-resistant cemented carbide according to any one of claims 5 to 7, characterized in that: The cemented carbide substrate is ground, polished, cleaned and dried, and the rare earth-iron binary alloy and LaMO3 are evenly mixed to form a transition layer on the surface of the cemented carbide substrate using laser cladding technology. Subsequently, the TiAlN coating is deposited on the transition layer in a mixed atmosphere of nitrogen and silane.

9. The method for preparing the wear-resistant cemented carbide according to claim 8, wherein: The process parameters of laser cladding are as follows: The laser power is 3-5kW, the scanning speed is 200-400mm / min, the spot diameter is 1-5mm, argon protection is used, and the argon flow rate is 1-10L / min.

10. The method for preparing wear-resistant cemented carbide according to claim 8, wherein: The flow ratio of nitrogen to silane is 40-50:1.

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