NiCrBSi-ZrB2 METAL CERAMIC POWDER, COMPOSITE COATING FOR HIGH TEMPERATURE PROTECTION, AND PREPARATION METHOD THEREFOR

Abstract

The metal ceramic powder with a particle size of 15-45 .Math.m and suitable for thermal spraying is prepared through a combination of mechanical ball milling, spray granulation, and vacuum sintering. The metal ceramic powder is sprayed on a surface of a steel substrate adopting the high velocity oxygen fuel (HVOF) technology with oxygen-propane as fuel and taking oxygen as a combustion improver, propane as fuel, nitrogen as powder feeding carrier gas, and air as a cooling medium to prepare and form the NiCrBSi—ZrB2 composite coating. The present disclosure solves the problem that ZrB.sub.2 ceramic is difficult to compact during sintering and improves powder bonding strength and fluidity. The preparation method is simple, has advantages of high coating deposition efficiency and convenient equipment operation, and is cost-effective. The preparation method can improve thermal corrosion resistance and high-temperature wear resistance of a surface of boiler, and prolonging lifetime of the boiler.

Claims

1. A method for preparing a NiCrBSi—ZrB.sub.2 metal ceramic powder for high temperature protection, comprising the following steps: a, mechanical ball milling: adding NiCrBSi powder, ZrB.sub.2 powder, alcohol and zirconia grinding balls to a ball milling tank in proportion, and conducting ball milling at 300-350 r/min for 30-40 h to obtain a mixed powder solution containing alcohol; b, drying of the mixed powder solution: heating the mixed powder solution containing alcohol in a constant temperature blast drying oven at 50° C. for 12 h to obtain a dried mixed powder; c, preparation of slurry: adding a binder polyvinyl alcohol (PVA), a defoamer n-octanol, and deionized water into the dried mixed powder, and stirring and standing to obtain water-based composite slurry; d, spray granulation: continuously stirring the water-based composite slurry, and sending the water-based composite slurry to a high-speed centrifugal spray dryer through a constant-flow pump for atomization to form spherical powder particles, wherein the high-speed centrifugal spray dryer has an inlet temperature of 200-240° C., an outlet temperature of 100-130° C., and an atomizing rotary frequency of 36 Hz, and the constant-flow pump works at 26 r/min; e, vacuum sintering: conducting vacuum sintering on the spherical powder particles using a vacuum sintering furnace to obtain a metal ceramic powder; and f, sieving and grading: conducting sieving and grading on metal ceramic powder by a vibrating sieve and an ultrasonic vibrator to obtain the NiCrBSi—ZrB.sub.2 metal ceramic powder, wherein a pulse frequency of the ultrasonic vibrator is 2-3 Hz.

2. The method according to claim 1, wherein in step a, the NiCrBSi powder has a particle size of 20-50 .Math.m, and comprises the following elements in mass percentage: 0.3%-1.0% of C, 8%-18% of Cr, 2.5%-5.5% of Si, 1.8%-4.5% of B, 65%-85% of Ni, and less than 5% of Fe; and the ZrB.sub.2 powder has a particle size of 1-3 .Math.m and a purity greater than or equal to 99.85%.

3. The method according to claim 1, wherein in step a, the NiCrBSi powder and the ZrB.sub.2 powder have a mass ratio of (6-8):(4-2), 55.5 mL of the alcohol is added into every 100 g of the NiCrBSi and ZrB2 powders, and the zirconia grinding balls with diameters of 15 mm, 13 mm, 11 mm, 10 mm, and 6 mm are mixed with a quantity ratio of 1:3:3:2:1, and the grinding balls and the NiCrBSi and ZrB.sub.2 powders have a mass ratio of 2:1.

4. The method according to claim 1, wherein in step c, a percentage of the binder PVA to a total mass of the NiCrBSi and ZrB.sub.2 powders is 3%-3.5%, a percentage of the defoamer n-octanol to the total mass of the NiCrBSi and ZrB.sub.2 powders is 0.4%-0.5%, and the deionized water is added with an amount enabling a solid content of the NiCrBSi and ZrB.sub.2 powders in the slurry to reach 40%.

5. The method according to claim 1, wherein in step e, the vacuum sintering comprises: heating the spherical powder particles for 40 min to rise a temperature from a room temperature to 300° C. and keeping 300° C. for 30 min, heating the spherical powder particles for 80 min to rise the temperature from 300° C. to 900-1,100° C. and keeping 900-1,100° C. for 6 h, and stopping heating the spherical powder particles and cooling to the room temperature.

6. An NiCrBSi—ZrB.sub.2 metal ceramic powder for high temperature protection prepared by the method according to claim 1, wherein the NiCrB Si—ZrB.sub.2 metal ceramic powder has a particle size of 15-45 .Math.m, an apparent density of 1.51-2.13 g/cm.sup.2, and fluidity of 69.8-98.3 s/50 g.

7. A method for preparing a NiCrBSi—ZrB.sub.2 composite coating for high temperature protection, comprising the following steps: S1: preparing the NiCrBSi—ZrB.sub.2 metal ceramic powder for high temperature protection according to claim 6; S2: conducting oil removal and purification on a steel substrate surface of a boiler, conducting sand blasting on the surface, and preheating the substrate; and S3: spraying the metal ceramic powder obtained in step S1 on the steel substrate surface of the boiler by using a high velocity oxygen fuel (HVOF) technology with oxygen-propane as fuel and taking oxygen as a combustion improver, propane as fuel, nitrogen as powder feeding carrier gas, and air as a cooling medium to prepare and form the NiCrBSi—ZrB.sub.2 composite coating.

8. The method according to claim 7, wherein in step S2, the sand blasting is conducted with a sand blasting material of brown corundum sand with a particle size of 25 meshes at a pressure of 3-5 MPa; and after the sand blasting, surface roughness of the substrate reaches 2.5-3 .Math.m, and a preheating temperature of the substrate reaches 80-120° C.

9. The method according to claim 7, wherein in step S3, the propane has a flow rate of 60-70 L/min, the oxygen has a flow rate of 230-250 L/min, the air has a flow rate of 320-350 L/min, 230-250 mm of the spraying is conducted with a step of 3 mm at 800 mm/s, a powder feeding voltage is 5-5.5 V, and a powder feeding rate is 50-60 g/min.

10. An NiCrBSi—ZrB.sub.2 composite coating for high temperature protection prepared by the method according to claim 7, wherein the coating has a thickness of 200-300 .Math.m and hardness of 700-1,000 HV, bonding strength between the coating and the substrate exceeds 75 MPa, and a porosity of the coating reaches 0.4%-0.5%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 shows a preparation process of NiCrBSi—ZrB.sub.2 metal ceramic powder water-based composite slurry in the present disclosure;

[0032] FIG. 2 shows surface morphology and energy dispersive spectroscopy (EDS) results of a NiCrBSi—ZrB.sub.2 powder in Examples 1, 2 and 3 of the present disclosure: (a) NiCrBSi-20ZrB.sub.2, (b) NiCrBSi-30ZrB.sub.2, and (c) NiCrBSi-40ZrB.sub.2;

[0033] FIG. 3 shows a macroscopic picture of apparent density and fluidity of the NiCrBSi-ZrB.sub.2 powder measured with a Hall flowmeter;

[0034] FIG. 4 shows sectional morphology and EDS results of a NiCrBSi-ZrB.sub.2 composite coating in Examples 1, 2 and 3 of the present disclosure: (a) NiCrBSi-20ZrB.sub.2, (b) NiCrBSi-30ZrB.sub.2, and (c) NiCrBSi-40ZrB.sub.2;

[0035] FIG. 5 shows an X-ray diffraction (XRD) pattern of a surface of the NiCrBSi—ZrB.sub.2 composite coating in a sprayed state and after thermal corrosion in Examples 1, 2 and 3 of the present disclosure: (a) and (d) NiCrBSi-20ZrB.sub.2, (b) and (e) NiCrBSi-30ZrB.sub.2, and (c) and (f) NiCrBSi-40ZrB.sub.2;

[0036] FIG. 6 is a comparison diagram of thermal corrosion weight gain and a thermal corrosion kinetic constant between the NiCrBSi—ZrB.sub.2 composite coating and an Ni60-40TiB.sub.2 coating in Examples 1, 2 and 3 of the present disclosure; and

[0037] FIG. 7 is a comparison diagram of a high-temperature wear volume and wear rate between the NiCrBSi—ZrB.sub.2 composite coating and a NiCrBSi coating in Examples 1, 2 and 3 of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The above and other technical features and advantages of the present disclosure will be described below in more details in connection with the accompanying drawings.

Example 1

[0039] NiCrBSi powder with a mass percentage of 80% and ZrB.sub.2 powder with a mass percentage of 20% were taken, mixed and added into a ball milling tank. Zirconia grinding balls with a mass twice that of the powder were added into the ball milling tank. The zirconia grinding balls with diameters of 15 mm, 13 mm, 11 mm, 10 mm, and 6 mm are mixed with a quantity ratio of 1:3:3:2: 1. Alcohol was added into the ball milling tank at a ratio of adding 55.5 mL of the alcohol into every 100 g of the NiCrBSi and ZrB.sub.2 powders. Ball milling was conducted at 320 r/min for 40 h.

[0040] After the ball milling, the mixed powder solution containing alcohol was placed in a constant temperature blast drying oven, a heating temperature was set at 50° C., and heat preservation was conducted for 12 h. After the powder was dried, a binder PVA with an amount being 3.5% of a total mass of the powder, a defoamer n-octanol with an amount being 0.5% of the total mass of the powder, and deionized water were added, so as to enable a solid content of the powder to reach 40%. The prepared water-based composite slurry was constantly stirred, such that the powder particles and the binder were evenly dispersed in the slurry.

[0041] The water-based composite slurry was constantly stirred, and sent to a high-speed centrifugal spray dryer through a constant-flow pump for atomization to form spherical powder particles. The high-speed centrifugal spray dryer was set at an inlet temperature of 240° C., an outlet temperature of 100° C., and an atomizing rotary frequency of 36 Hz, and the constant-flow pump worked at 26 r/min.

[0042] The metal ceramic powder after spray granulation was collected and placed in an alumina crucible for vacuum sintering. A vacuum sintering program was set. The powder was heated for 40 min to rise a temperature from a room temperature to 300° C. and the temperature was kept for 30 min, and then heated for 80 min to rise the temperature from 300° C. to 1,000° C. and the temperature was kept for 6 h, and the powder was stopped heating and cooled to the room temperature with the furnace.

[0043] The metal ceramic powder obtained by vacuum sintering was sieved and graded. The powder was sieved with metal screens with mesh numbers of 15 .Math.m and 45 .Math.m. An ultrasonic vibrator was loaded at the edge of the screen frame to assist powder sieving when a screen with a small mesh number was used. A pulse frequency of the ultrasonic vibrator is selected as 3 Hz. The NiCrBSi—ZrB.sub.2 metal ceramic powder with a particle size distribution of 15-45 .Math.m was obtained by sieving with screens with different mesh numbers.

[0044] Oil removal and purification were conducted on a steel substrate surface of a boiler with removal powder, alcohol and acetone. Sand blasting was conducted on the surface subjected to oil removal. The sand blasting was conducted with a sand blasting material of brown corundum sand with a particle size of 25 meshes (Al.sub.2O.sub.3). An air valve was adjusted to make the sand blasting pressure reach 3 MPa. After the sand blasting, the surface roughness Ra of the substrate reached 2.5 .Math.m. The power supply, gas path switch and cooling water switch of the spraying equipment were turned on. The oxygen was used as a combustion improver, propane as fuel, nitrogen as powder feeding carrier gas, and air as a cooling medium. A sample was fixed on the workbench. The operation program of the mechanical arm was modified, such that the spraying distance reach 250 mm, and the spraying was conducted at 800 mm/s with a step of 3 mm. The propane, oxygen and air flow valves were opened. The flow rate of the propane was adjusted to 65 L/min, the flow rate of the oxygen to 240 L/min, and the flow rate of the air to 350 L/min. The propane flame flow was ignited to preheat the substrate surface, such that the surface temperature reached 80-120° C. The powder feeder switch was turned on. The powder feeding voltage was adjusted to 5 V. The powder feeding rate was maintained at 50 g/min. After every 5 times of spraying, the coating surface was purged and cooled with an air gun, and the coating thickness was measured with a spiral micrometer. When the coating temperature was reduced to about 80° C., the spraying equipment was started to continue spraying, and the operations were repeated to finally make the coating thickness reach about 250 .Math.m.

Example 2

[0045] NiCrBSi powder with a mass percentage of 70% and ZrB2 powder with a mass percentage of 30% were taken, mixed and added into a ball milling tank. Zirconia grinding balls with a mass twice that of the powder were added into the ball milling tank. The zirconia grinding balls with diameters of 15 mm, 13 mm, 11 mm, 10 mm, and 6 mm were selected and were compounded at a ratio of 1:3:3:2:1. Alcohol was added into the ball milling tank at a ratio of adding 55.5 mL of the alcohol into every 100 g of the NiCrBSi and ZrB.sub.2 powders. Ball milling was conducted at 300 r/min for 30 h.

[0046] After the ball milling, the mixed powder solution containing alcohol was placed in a constant temperature blast drying oven, a heating temperature was set at 50° C., and heat preservation was conducted for 12 h. After the powder was dried, a binder PVA with an amount being 3% of a total mass of the powder, a defoamer n-octanol with an amount being 0.4% of the total mass of the powder, and deionized water were added, so as to enable a solid content of the powder to reach 40%. The prepared water-based composite slurry was constantly stirred, such that the powder particles and the binder were evenly dispersed in the slurry.

[0047] The water-based composite slurry was constantly stirred, and sent to a high-speed centrifugal spray dryer through a constant-flow pump for atomization to form spherical powder particles. The high-speed centrifugal spray dryer was set at an inlet temperature of 240° C., an outlet temperature of 110° C., and an atomizing rotary frequency of 36 Hz, and the constant-flow pump worked at 26 r/min.

[0048] The metal ceramic powder after spray granulation was collected and placed in an alumina crucible for vacuum sintering. A vacuum sintering program was set. The powder was heated for 40 min to rise the temperature from a room temperature to 300° C. and the temperature was kept for 30 min, and then heated for 80 min to rise the temperature from 300° C. to 900° C. and the temperature was kept for 6 h, and the powder was stopped heating and cooled to the room temperature with the furnace.

[0049] The metal ceramic powder obtained by vacuum sintering was sieved and graded. The powder was sieved with metal screens with mesh numbers of 15 .Math.m and 45 .Math.m. An ultrasonic vibrator was loaded at the edge of the screen frame to assist powder sieving when a screen with a small mesh number was used. A pulse frequency of the ultrasonic vibrator is selected as 2.5 Hz. The NiCrBSi—ZrB.sub.2 metal ceramic powder with a particle size distribution of 15-45 .Math.m was obtained by sieving with screens with different mesh numbers.

[0050] Oil removal and purification were conducted on a steel substrate surface of a boiler with removal powder, alcohol and acetone. Sand blasting was conducted on the surface subjected to oil removal. The sand blasting was conducted with a sand blasting material of brown corundum sand with a particle size of 25 meshes (Al.sub.2O.sub.3). An air valve was adjusted to make the sand blasting pressure reach 3 MPa. After the sand blasting, the surface roughness Ra of the substrate reached 2.5 .Math.m. The power supply, gas path switch and cooling water switch of the spraying equipment were turned on. The oxygen was used as a combustion improver, propane as fuel, nitrogen as powder feeding carrier gas, and air as a cooling medium. A sample was fixed on the workbench. The operation program of the mechanical arm was modified, such that the spraying distance reach 230 mm, and the spraying was conducted at 800 mm/s with a step of 3 mm. The propane, oxygen and air flow valves were opened. The flow rate of the propane was adjusted to 60 L/min, the flow rate of the oxygen to 230 L/min, and the flow rate of the air to 320 L/min. The propane flame flow was ignited to preheat the substrate surface, such that the surface temperature reached 80-120° C. The powder feeder switch was turned on. The powder feeding voltage was adjusted to 5 V. The powder feeding rate was maintained at 50 g/min. After every 5 times of spraying, the coating surface was purged and cooled with an air gun, and the coating thickness was measured with a spiral micrometer. When the coating temperature was reduced to about 80° C., the spraying equipment was started to continue spraying, and the operations were repeated to finally make the coating thickness reach about 250 .Math.m.

Example 3

[0051] NiCrBSi powder with a mass percentage of 60% and ZrB.sub.2 powder with a mass percentage of 40% were taken, mixed and added into a ball milling tank. Zirconia grinding balls with a mass twice that of the powder were added into the ball milling tank. The zirconia grinding balls with diameters of 15 mm, 13 mm, 11 mm, 10 mm, and 6 mm were selected and were compounded at a ratio of 1:3:3:2:1. Alcohol was added into the ball milling tank at a ratio of adding 55.5 mL of the alcohol into every 100 g of the NiCrBSi and ZrB.sub.2 powders. Ball milling was conducted at 320 r/min for 40 h.

[0052] After the ball milling, the mixed powder solution containing alcohol was placed in a constant temperature blast drying oven, a heating temperature was set at 50° C., and heat preservation was conducted for 12 h. After the powder was dried, a binder PVA with an amount being 3.5% of a total mass of the powder, a defoamer n-octanol with an amount being 0.5% of the total mass of the powder, and deionized water were added, so as to enable a solid content of the powder to reach 40%. The prepared water-based composite slurry was constantly stirred, such that the powder particles and the binder were evenly dispersed in the slurry.

[0053] The water-based composite slurry was constantly stirred, and sent to a high-speed centrifugal spray dryer through a constant-flow pump for atomization to form spherical powder particles. The high-speed centrifugal spray dryer was set at an inlet temperature of 240° C., an outlet temperature of 100° C., and an atomizing rotary frequency of 36 Hz, and the constant-flow pump worked at 26 r/min.

[0054] The metal ceramic powder after spray granulation was collected and placed in an alumina crucible for vacuum sintering. A vacuum sintering program was set. The powder was heated for 40 min to rise the temperature from a room temperature to 300° C. and the temperature was kept for 30 min, and then heated for 80 min to rise the temperature from 300° C. to 1,000° C. and the temperature was kept for 6 h, and the powder was stopped heating and cooled to the room temperature with the furnace.

[0055] The metal ceramic powder obtained by vacuum sintering was sieved and graded. The powder was sieved with metal screens with mesh numbers of 15 .Math.m and 45 .Math.m. An ultrasonic vibrator was loaded at the edge of the screen frame to assist powder sieving when a screen with a small mesh number was used. A pulse frequency of the ultrasonic vibrator is selected as 3 Hz. The NiCrBSi—ZrB.sub.2 metal ceramic powder with a particle size distribution of 15-45 .Math.m was obtained by sieving with screens with different mesh numbers.

[0056] Oil removal and purification were conducted on a steel substrate surface of a boiler with removal powder, alcohol and acetone. Sand blasting was conducted on the surface subjected to oil removal. The sand blasting was conducted with a sand blasting material of brown corundum sand with a particle size of 25 meshes (Al.sub.2O.sub.3). An air valve was adjusted to make the sand blasting pressure reach 3 MPa. After the sand blasting, the surface roughness Ra of the substrate reached 2.5 .Math.m. The power supply, gas path switch and cooling water switch of the spraying equipment were turned on. The oxygen was used as a combustion improver, propane as fuel, nitrogen as powder feeding carrier gas, and air as a cooling medium. A sample was fixed on the workbench. The operation program of the mechanical arm was modified, such that the spraying distance reach 250 mm, and the spraying was conducted at 800 mm/s with a step of 3 mm. The propane, oxygen and air flow valves were opened. The flow rate of the propane was adjusted to 70 L/min, the flow rate of the oxygen to 250 L/min, and the flow rate of the air to 350 L/min. The propane flame flow was ignited to preheat the substrate surface, such that the surface temperature reached 80-120° C. The powder feeder switch was turned on. The powder feeding voltage was adjusted to 5 V. The powder feeding rate was maintained at 50 g/min. After every 5 times of spraying, the coating surface was purged and cooled with an air gun, and the coating thickness was measured with a spiral micrometer. When the coating temperature was reduced to about 80° C., the spraying equipment was started to continue spraying, and the operations were repeated to finally make the coating thickness reach about 250 .Math.m.

I. Performance Test of NiCrBSi-ZrB.SUB.2 Metal Ceramic Powder Prepared in Examples 1 to 3 of the Present Disclosure

[0057] The fluidity and apparent density of the powder were measured with a Hall flowmeter. Each powder sample was tested three times, and the average value was taken as the fluidity index and apparent density of the powder sample. The test results are shown in Table 1.

TABLE-US-00001 Fluidity and apparent density of NiCrBSi—ZrB.sub.2 metal ceramic powder prepared in Examples 1 to 3 Example 1 Example 2 Example 3 Fluidity (s/50 g) 98.35 80.42 67.24 Apparent density (g/cm.sup.3) 1.51 1.63 1.59

II. Performance Test of NiCrBSi-ZrB.SUB.2 Composite Coating Prepared in Examples 1 to 3 of the Present Disclosure

[0058] The microhardness of the coating was tested with a Vickers hardness tester. The load was 300 gf. The loading time was 5 s. 10 points were tested for each coating. The average value was taken as the microhardness of the coating. The test results are shown in Table 2.

TABLE-US-00002 Microhardness of NiCrBSi—ZrB.sub.2 composite coating prepared in Examples 1 to 3 Example 1 Example 2 Example 3 Microhardness (HV.sub.300) 784 815 1006

[0059] The KCl molten salt corrosion resistance of the NiCrBSi—ZrB.sub.2 composite coating was tested in a tubular furnace. Three groups were tested for each coating. The experiment was carried out at 700° C. for 100 h. The samples were taken out and weighed every 10 h. The thermal corrosion weight gain of the coating was recorded, and the average thermal corrosion kinetic constant of the coating was calculated. The test results are shown in FIG. 6. Compared with the Ni60-40TiB.sub.2 coating, the NiCrBSi-40ZrB.sub.2 coating prepared in Example 3 formed the SiO.sub.2 and m-ZrO.sub.2 phases during thermal corrosion, and the m-ZrO.sub.2 did not undergo an obvious conversion to t-ZrO.sub.2. The continuous and compact oxide film on the coating surface could effectively prevent the corrosion diffusion of chloride molten salt into the coating, such that the coating had more excellent thermal corrosion resistance.

[0060] The high-temperature wear resistance of the NiCrBSi—ZrB.sub.2 composite coating was tested in a HT-1000 high-temperature friction and wear testing machine. The wear load was 10 N, the wear temperature was 700° C., the frequency was 5.7 Hz, the friction radius was 3.5 mm, and the wear time was 60 min. The grinding balls adopted Al.sub.2O.sub.3 ceramic balls with a diameter of 5 mm, and the wear volume of the coating was calculated with a KLA-P7 probe profiler. The test results are shown in FIG. 7. The test results show that compared with the NiCrBSi coating, the NiCrBSi-40ZrB.sub.2 coating prepared in Example 4 has high hardness and optimal high-temperature wear resistance because it contains many ZrB.sub.2 hard phases.

[0061] In summary, the NiCrBSi—ZrB.sub.2 composite coating for high temperature protection prepared by the present disclosure meets the requirements of high-temperature corrosion resistance and high-temperature wear resistance on the steel substrate surface of energy and chemical equipment, and the optimal powder and coating preparation technology can be obtained by improving the method and process flow. The metal ceramic powder which is prepared by means of a combination of mechanical ball milling, spray granulation, and vacuum sintering, has a mass ratio of NiCrBSi powder to ZrB.sub.2 powder of 6:4, and is obtained after vacuum sintering at 1,000° C. has optimal fluidity and apparent density. By optimizing the spraying process parameters, optimal spraying process parameters are obtained: a propane flow rate of 70 L/min, an oxygen flow rate of 250 L/min, an air flow rate of 350 L/min, a spraying distance of 250 mm, a spraying step of 3 mm, a spraying speed of 800 mm/s, a powder feeding voltage of 5 V, and a powder feeding rate of 50 g/min. The composite coating prepared under these parameters has optimal high-temperature corrosion resistance due to the presence of the SiO.sub.2 and m-ZrO.sub.2 phases, and the composite coating has optimal high-temperature wear resistance due to the presence of many ZrB.sub.2 phases.

[0062] The above described are merely preferred examples of the present disclosure, and are merely illustrative rather than restrictive. It is to be understood that many alterations, modifications or even equivalent replacements can be made within the spirit and scope defined by the claims of the present disclosure, and should fall within the protection scope of the present disclosure.