CN107794485A - A kind of preparation technology of metal ceramic powder used for hot spraying - Google Patents

Abstract

The invention discloses a preparation process of cermet powder for thermal spraying, which comprises the following steps: (1) mixing and ball-milling metal powder and ceramic powder to obtain slurry A; (2) spraying and granulating slurry A to obtain Powder B; (3) Classify powder B to obtain powder C with more than two grades of different particle sizes; (4) Sinter powders with different particle sizes at different temperatures, and crush and classify them after sintering to obtain powders of different sizes. Powder D with packing density; (5) Mix powders D with different bulk densities to obtain cermet powder for spraying. The invention improves the utilization rate of the cermet powder, improves the deposition rate of thermal spraying, improves the density of the coating, and obtains a better comprehensive effect.

Description

A preparation process of cermet powder for thermal spraying

technical field

The invention relates to a preparation process of cermet powder for thermal spraying, which belongs to the field of thermal spraying.

Background technique

The cermet powders currently used for thermal spraying, especially supersonic flame spraying, include WC-Ni, WC-Co, WC-CoCr, WC-Cr ₃ C ₂ -Ni, Cr ₃ C ₂ -NiCr and other cermet powder materials. The material can be used to prepare corresponding composite coatings, and is widely used in industrial production where anti-corrosion and wear-resisting are required. At present, most cermet powders are prepared by the agglomeration method, and the process generally consists of material mixing and ball milling, spray granulation, sintering, crushing and sieving and classification.

In the above process, spray granulation is to agglomerate fine particles of metal and ceramic particles into larger spherical particles, which is convenient for smooth powder delivery during spraying. The purpose of sintering is to remove the organic binder in the spherical particles, and it can also improve the mechanical strength of the spherical powder particles so that they are not easily broken during the spraying and powder feeding process.

The inventors found that the current cermet powders obtained by spray granulation are all sintered at the same temperature, and the sintered materials are then crushed, screened and air-flow classified to obtain thermal spray powders with uniform bulk density.

When the spray-granulated cermet powder is sintered at a lower temperature, the bulk density of the obtained powder is also low, and the larger-sized powder particles in this powder can be fully melted or softened in the spray flame (obtained The coating is relatively dense), but the small-sized powder particles are prone to over-melting, and the over-melting particles often adhere to the inner wall of the gun barrel, causing gun blocking and large droplets (even if the lower limit of the powder is increased to 20 microns). This large droplet phenomenon cannot be avoided), which eventually leads to the instability of the spraying process, and the corresponding coating has pits on the surface after grinding, which leads to a low pass rate.

When the spray-granulated cermet powder is sintered at high temperature, the obtained powder particles have a high density (high powder bulk density). During the spraying process, the small-sized dense particles in this powder are heated and melted or softened sufficiently, but the large-sized dense particles are not heated sufficiently in the spray flame, and when the particles hit the substrate, their cores are not fully melted or The softened part is prone to rebound, resulting in a lower deposition rate of the powder.

When the spray-granulated cermet powder is sintered at a moderate temperature, the bulk density of the obtained powder is also moderate, and the powder deposition rate and coating density are moderate. For this kind of powder with moderate bulk density, the powder with fine particles still has a greater tendency to over-melt during the spraying process. Therefore, in order to reduce this over-melt phenomenon, it is necessary to increase the lower limit size of the powder particle size distribution to 15 Micron or even 20 microns, resulting in greatly reduced utilization of spray granulation powder. In addition, in order to prepare a high-density coating by spraying this medium bulk density cermet powder, high spray parameters must be selected (increased fuel and oxygen consumption during spraying), which in turn will lead to powder deposition. rate is greatly reduced.

It can be seen that it is difficult to obtain satisfactory results by using cermet powders with uniform bulk density to prepare corresponding coatings, whether it is low, medium bulk density or high bulk density. Therefore, it is urgent to optimize the design and preparation process of traditional cermet powders to obtain cermet powders with stable spraying process, high powder deposition rate and high coating density.

Contents of the invention

The invention intends to solve the problems of how to improve the utilization rate of the spray granulated cermet powder, maintain the stability of the spraying process, and that the deposition rate of the powder spraying and the density of the coating can not be achieved at the same time.

The technical solution of the present invention is to provide a preparation process of cermet powder for thermal spraying, comprising the following steps:

(1) Metal powder and ceramic powder are mixed and ball milled to obtain slurry A;

(2) Carry out spray granulation to slurry A, obtain powder B;

(3) Classifying powder B to obtain powder C with different particle sizes in more than two stages (respectively powder C1, C2, ... Cn);

(4) Powders C (powders C1, C2, ... Cn) with different particle sizes are sintered at different temperatures respectively, and after the sintering is completed, they are crushed and classified respectively to obtain powders D with different bulk densities (respectively powders D1, D2,...Dn);

(5) Mix powders D with different bulk densities to obtain cermet powder for spraying.

Preferably, in the powder C, the smallest particle size is a micron, and the largest particle size is b micron; wherein 4≤a≤8; 50≤b≤60.

Preferably, in step (3), powder B is divided into n grades, according to particle size from small to large, respectively powder C1, C2, ..., Cn, sintering temperatures are T1, T2, ..., Tn, sintering temperature The unit is °C; wherein, T1>T2>...>Tn, n is a natural number greater than 1.

Preferably, the powder B is divided into two grades to obtain a powder C1 with a particle size not exceeding c microns and a powder C2 with a particle size larger than c microns, wherein 25≤c≤35.

Preferably, the sintering temperature of powder C1 is 1170-2000°C; the sintering temperature of powder C2 is 1160-1190°C.

Preferably, the sintering temperature of the powder C1 is T1°C, the sintering temperature of the powder C2 is T2°C, and T2+40°C≥T1≥T2+10°C.

Preferably, the powder B is divided into three grades, which are respectively powders C1, C2 and C3, and their particle sizes are respectively d, e and f microns; wherein, d<e<f, a≤d≤a+14 microns; b- 14 microns ≤ f ≤ b, the particle size distribution e of powder C2 is between d and f.

Preferably, the sintering temperature of powder C1 is T1°C, the sintering temperature of powder C2 is T2°C, the sintering temperature of powder C3 is T3°C, T2+30°C≥T1≥T2+8°C; T3+30°C≥T2≥T3 +8°C. Preferably, T2+20°C≥T1≥T2+10°C; T3+20°C≥T2≥T3+10°C.

Preferably, in the powder D, the smallest particle size is a micron, and the largest particle size is b micron.

Preferably, the metal powder is one or more of Ni, Co, Cr, Fe, Al and Mo; the ceramic powder is WC, Cr ₃ C ₂ , TiC, TiB ₂ , WB and Mo ₂ B one and many.

Specifically, the steps that preparation technique of the present invention comprises have:

(1) A certain proportion of metal powder (one or more of Ni, Cr, Co, Fe and Mo) and ceramic powder (WC, Cr ₃ C ₂ , TiC, WB, TiB ₂ , Mo ₂ B one or more) and cemented carbide balls, deionized water, and polyvinyl alcohol were put into a ball mill for ball milling for 10-40 hours to obtain a cermet mixed slurry (when the total amount of cermet powder was set as 100 parts, hard The ratio of high-quality alloy balls is 700 parts, the ratio of deionized water is 20-25 parts, and the ratio of polyvinyl alcohol is 2 parts);

(2) importing the above slurry into a centrifugal spray tower for spray granulation to obtain a spherical cermet composite powder;

(3) The above-mentioned granulated particles are screened and air-flow classified to obtain two or more spherical cermet powders with different particle size distributions; (the particle size distribution is divided into two sections: 5-30 microns and 30-30 microns) 53 microns, or a particle size distribution of 3 segments 5-20 microns, 20-38 microns and 38-53 microns, or more segments);

(4) Sintering the above-mentioned powders with different particle size distributions in reducing atmosphere furnaces and vacuum furnaces at different temperatures, wherein the fine-grained powders are sintered at high temperatures to obtain high-density fine-grained powders (high bulk density); Medium-sized powders are sintered at medium temperatures to obtain medium-density powders; coarse-grained powders are sintered at lower temperatures to obtain low-density coarse-grained powders (low bulk density);

(5) Crushing the powder obtained by the above sintering, and re-sieving and airflow classification to remove particles in each powder whose particle size is smaller than the lower limit size of the powder particle size distribution due to the crushing process.

(6) Remove powders smaller than the lower limit of the particle size distribution of the powder and then perform mechanical mixing to obtain a new type of cermet powder (the particle size distribution range of the mixed powder is 5-53 microns, and the powder with a smaller size has a higher bulk density and a smaller size. Larger powders have lower bulk densities).

The present invention will be further explained below, wherein the codes of materials are represented by labels such as P1, P2..., etc. in the text, which are only used to facilitate distinction and have no definite meaning.

For example, select one or more metal elements in Ni, Cr, Co, Fe and Mo (the metal accounts for 7%-40% of the total mass percentage of powder P1) metal powder and WC, Cr ₃ C ₂ , TiC, WB, One or more ceramic powders of TiB ₂ and Mo ₂ B (ceramics account for 60%-93% of the total mass of the powder P1) are combined to obtain the mixed powder P1. P1 is ball milled and mixed with 20-25% deionized water and 2% polyvinyl alcohol accounting for the total mass of P1 respectively to obtain slurry P2 formed by uniformly mixing and ball milling the above components. The slurry P2 is introduced into a centrifugal spray tower for spray drying and granulation to obtain a cermet powder P3 with a particle size distribution in a certain range, and the cermet powder P3 obtained by spray drying is subjected to sieving and air classification to obtain fine particles with different particle size distributions Powder P4, medium particle size powder P5 and coarse particle size powder P6.

Put the classified powders P4, P5 and P6 into graphite boats or ceramic boats respectively, and put them into an atmosphere furnace or a vacuum furnace with a reducing atmosphere for sintering. Among them, the fine particle powder P4 is sintered and crushed at a higher temperature to obtain a high-density powder P7 (high bulk density of the powder); the medium-sized powder P5 is sintered and crushed at a moderate sintering temperature to obtain a medium density The powder P8 (the bulk density of the powder is moderate); the coarse powder P6 is sintered and crushed at a lower temperature to obtain the powder P9 with low density (the bulk density of the powder is low).

Particles smaller than the lower limit of the respective powder size distributions in the P7, P8 and P9 powders due to crushing were removed by sub-sieving and airflow classification to obtain powders P10, P11 and P12 respectively.

The classified powders P10, P11 and P12 were mechanically mixed to obtain mixed metal-ceramic powder P13 with different bulk densities (the larger the particle size of the powder, the lower the bulk density).

Among the P13 powders with different bulk densities obtained by the above process, the small-sized powder has a high density. Although the mass is small during the powder feeding process, due to its high density, under the action of the powder feeding carrier gas, it can It is sent into the center of the flame flow; and, due to the high density of the fine particle powder, it is not easy to over-melt in the barrel of the spray gun, which greatly reduces the tendency of the over-melt powder to adhere to the inner wall of the barrel. The density of medium-sized and large-sized powder particles decreases in turn, on the one hand, they can be fully heated to a molten or softened state in the flame flow, and on the other hand, they can also spread better when they hit the substrate Performance, improve the deposition rate of the powder and the density of the corresponding coating. In this way, the bulk density of the particles in different ranges of particle size distribution in the powder P13 is also different, which enables the powder to basically reach a more consistent and fully melted or softened state during the spraying process. This not only basically avoids the phenomenon that the fine powder adheres to the inner wall of the gun barrel due to over-melting, but also avoids the serious rebound of the powder due to the insufficient degree of melting and softening of the core of the large particle powder, so that the deposition rate of the powder and the The coating density is higher. In addition, the bulk density of the fine powder is greatly improved, and the probability of over-melting during the spraying process is also greatly reduced. Therefore, the lower limit size of the particle size distribution of the powder can also be reduced from the original 15-20 microns to 5 microns. Finally, the utilization rate of the cermet powder obtained by spray granulation is also greatly improved.

The beneficial effect of the invention is that the utilization rate of the cermet powder is improved, the deposition rate of thermal spraying is improved, the density of the coating is increased, and a better comprehensive effect is obtained.

Description of drawings

Fig. 1 represents the preparation process flowchart of a kind of cermet powder in the prior art;

Fig. 2 is the preparation process flowchart of a kind of cermet powder among the present invention;

Figure 3 is the WC-12Co powder sintered at 1195 ° C to obtain the interface of the powder and the cross-sectional photo of the WC-12Co coating prepared from the powder;

Figure 4 is the WC-12Co powder sintered at 1185 ° C to obtain the powder interface and the WC-12Co coating section photo prepared from the powder;

Figure 5 is a photo of the interface of the powder obtained by sintering WC-12Co powder at 1175 °C and the WC-12Co coating prepared from the powder;

Figure 6 shows WC-12Co powder mechanically mixed with high bulk density powder (sintered at 1195°C) with a particle size distribution of 5-25 microns and low bulk density powder (sintered at 1180°C) with a particle size distribution of 25-53 microns Cross-sectional photographs and cross-sectional photographs of WC-12Co coatings prepared from this powder.

Detailed ways

The present invention will be further described below in conjunction with embodiment.

Comparative example 1

(1) 88wt.% WC (particle size is ~ 2.5 microns) and 12wt.% Co powder (particle size is 1-3 microns) are added to the ball mill barrel respectively, and the powder raw material (WC Powder + Co powder, the same below) with a total mass of 23wt.% deionized water and 2wt.% polyvinyl alcohol, added WC-Co cemented carbide balls at a ball-to-material ratio of 6:1, and ball milled for 30 hours to obtain slurry P1.

(2) The slurry P1 is gradually introduced into the top container of the spray drying tower, and spray-dried to obtain spherical WC-12Co powder P2 with a particle size distribution within a certain range.

(3) P2 powder was sintered at 1195°C to obtain high-density P3 powder. The sintering furnace used can be a furnace with a reducing ( _H2 or _NH3 decomposition gas) atmosphere, or a vacuum furnace, which is kept at the set sintering temperature for 2 hours.

(5) The P3 powder obtained by sintering is crushed, and the obtained WC-12Co powder is respectively P4.

(6) The particles smaller than 15 microns and larger than 53 microns in the crushed powder of P4 were removed by sub-sieving and air-flow classification, and the obtained WC-12Co powder was coded as P5.

(8) Use kerosene as fuel to spray P5 powder, under standard parameters (kerosene flow rate: 22.6L/h, oxygen flow rate: 56.6m ³ /h, powder feeding rate 75g/min, spraying distance: 380mm), the obtained WC-12Co coating has a porosity of 1.7% (tested by the gray scale method); the powder spraying process is stable, but the inner wall of the gun barrel is worn more seriously, and the powder deposition rate is 35.2% (corresponding powder and coating section See Figure 3 for photos).

Comparative example 2

(1) Add 88wt.% WC (particle size ~2.5 microns) and 12wt.% Co powder (1-3 microns particle size) to the ball milling barrel respectively, and add 88 wt.% of the total mass of powder raw materials to the barrel 23wt.% of deionized water and 2wt.% of polyvinyl alcohol were added into WC-Co cemented carbide balls according to the ball-to-material ratio of 6:1, and ball milled for 30 hours to obtain slurry P1.

(2) The slurry P1 is gradually introduced into the top container of the spray drying tower, and spray-dried to obtain spherical WC-12Co powder P2 with a particle size distribution within a certain range.

(3) P2 powder was sintered at 1185°C to obtain P3 powder with moderate density. The sintering furnace used can be a furnace with a reducing ( _H2 or _NH3 decomposition gas) atmosphere, or a vacuum furnace, which is kept at the set sintering temperature for 2 hours.

(5) The P3 powder obtained by sintering is crushed, and the obtained WC-12Co powder is respectively P4.

(6) The particles smaller than 15 microns and larger than 53 microns in the crushed powder of P4 were removed by sub-sieving and air-flow classification, and the obtained WC-12Co powder was coded as P5.

(8) Use kerosene as fuel to spray P5 powder, under standard parameters (kerosene flow rate: 22.6L/h, oxygen flow rate: 56.6m ³ /h, powder feeding rate 75g/min, spraying distance: 380mm), the porosity of the obtained WC-12Co coating is 0.7% (tested by the gray scale method); the powder spraying process is relatively stable, and the powder deposition rate is 40.4% (see Figure 4 for the corresponding powder and coating section photos).

Comparative example 3

(1) Add 88wt.% WC (particle size ~2.5 microns) and 12wt.% Co powder (1-3 microns particle size) to the ball milling barrel respectively, and add 88 wt.% of the total mass of powder raw materials to the barrel 23wt.% of deionized water and 2wt.% of polyvinyl alcohol were added into WC-Co cemented carbide balls according to the ball-to-material ratio of 6:1, and ball milled for 30 hours to obtain slurry P1.

(2) The slurry P1 is gradually introduced into the top container of the spray drying tower, and spray-dried to obtain spherical WC-12Co powder P2 with a particle size distribution within a certain range.

(3) P2 powder was sintered at 1170°C to obtain low-density P3 powder. The sintering furnace used can be a furnace with a reducing ( _H2 or _NH3 decomposition gas) atmosphere, or a vacuum furnace, which is kept at the set sintering temperature for 2 hours.

(5) The P3 powder obtained by sintering is crushed, and the obtained WC-12Co powder is respectively P4.

(6) The particles smaller than 15 microns and larger than 53 microns in the crushed powder of P4 were removed by sub-sieving and air-flow classification, and the obtained WC-12Co powder was coded as P5.

(8) Use kerosene as fuel to spray P5 powder, under standard parameters (kerosene flow rate: 22.6L/h, oxygen flow rate: 56.6m ³ /h, powder feeding rate 75g/min, spraying distance: 380mm), the obtained WC-12Co coating has a porosity of 0.3% (tested by the gray scale method); but the powder spraying process is unstable, large droplets are produced, and the surface of the coating is pitted after grinding, and the coloring flaw detection is red Seriously, the coating is unqualified.

Example 1

(1) Add 88wt.% WC (particle size ~2.5 microns) and 12wt.% Co powder (1-3 microns particle size) to the ball milling barrel respectively, and add 88 wt.% of the total mass of powder raw materials to the barrel 23wt.% of deionized water and 2wt.% of polyvinyl alcohol were added into WC-Co cemented carbide balls according to the ball-to-material ratio of 6:1, and ball milled for 30 hours to obtain slurry P1.

(2) The slurry P1 is gradually introduced into the top container of the spray drying tower, and subjected to centrifugal spray drying to obtain spherical WC-12Co powder P2 with different particle sizes.

(3) The P2 powder is classified into two kinds of WC-12Co powders P3 and P4 with a particle size of 5-25 μm and 25-53 μm by sieving and airflow classification.

(4) P3 powder was sintered at 1195°C to obtain high-density P5 powder, and P4 powder was sintered at 1175°C to obtain low-density P6 powder. The sintering furnace used can be a furnace with a reducing (H2 or _NH3 decomposition gas) atmosphere, or a vacuum furnace, which is kept at the set sintering temperature for 2 hours.

(5) The P5 and P6 powders obtained by sintering were crushed, and the obtained WC-12Co powders were P7 and P8, respectively.

(6) Using the method of sub-sieving and airflow classification again, the particles less than 5 microns in the crushed powder of P7 are removed, and the obtained WC-12Co powder is numbered as P9, and the particles less than 25 microns in the crushed powder of P8 are removed to obtain The WC-12Co powder number is P10.

(7) The two powders of P19 and P10 were uniformly mixed by mechanical mixing to obtain WC-12Co powder P11 with different densities.

(8) Spray P11 powder with a supersonic flame spray gun fueled by kerosene, under standard parameters (kerosene flow rate: 22.6L/h, oxygen flow rate: 56.6m ³ /h, powder feeding rate 75g/min, spraying distance: 380mm) to obtain a porosity of 0.31%; the powder spraying process is stable (there is no phenomenon that the overmelted powder adheres to the inner wall of the gun barrel, and the wear rate of the inner wall of the gun barrel is also low); in addition, under the same spraying parameters, the The deposition rate of the new WC-12Co powder is 46.6% (see Figure 6 for the corresponding powder and coating section photos).

Example 2

(1) Add 88wt.% WC (particle size ~2.5 microns) and 12wt.% Co powder (1-3 microns particle size) to the ball milling barrel respectively, and add 88 wt.% of the total mass of powder raw materials to the barrel 23wt.% of deionized water and 2wt.% of polyvinyl alcohol were added into WC-Co cemented carbide balls according to the ball-to-material ratio of 6:1, and ball milled for 30 hours to obtain slurry P1.

(2) The slurry P1 is gradually introduced into the top container of the spray drying tower, and subjected to centrifugal spray drying to obtain spherical WC-12Co powder P2 with different particle sizes.

(3) The P2 powder is classified into three kinds of WC-12Co powders P3, P4 and P5 with particle sizes of 5-20 microns, 20-38 microns and 38-53 microns by sieving and airflow classification.

(4) P3 powder was sintered at 1195°C to obtain high-density P6 powder, and P4 powder was sintered at 1185°C to obtain medium-density P7 powder. P5 powder was sintered at 1175°C to obtain P8 powder with low density. The sintering furnace used can be a furnace with a reducing (H2 or _NH3 decomposition gas) atmosphere, or a vacuum furnace, which is kept at the set sintering temperature for 2 hours.

(5) The P6, P7 and P8 powders obtained by sintering were crushed, and the obtained WC-12Co powders were P9, P10 and P11, respectively.

(6) Using the method of sub-sieving and airflow classification again, remove the particles less than 5 microns in the crushed powder of P9, the WC-12Co powder obtained is numbered P12, remove the particles less than 20 microns in the crushed powder of P10, and obtain The number of the WC-12Co powder obtained is P13, and the particles smaller than 38 microns in the crushed powder of P11 are removed, and the obtained WC-12Co powder is numbered P14.

(7) The three powders of P12, P13 and P14 were uniformly mixed by mechanical mixing to obtain WC-12Co powder P15 with different densities.

(8) Use kerosene as fuel to spray P15 powder, under standard parameters (kerosene flow rate: 22.6L/h, oxygen flow rate: 56.6m ³ /h, powder feeding rate 75g/min, spraying distance: 380mm) to obtain a porosity of 0.27%; the powder spraying process is stable (there is no phenomenon that the overmelted powder adheres to the inner wall of the gun barrel, and the wear rate of the inner wall of the gun barrel is also low); in addition, under the same spraying parameters, the The deposition rate of the new WC-12Co powder is 49.6%.

Claims (10)

1. A preparation process for thermal spraying cermet powder, comprising the following steps: (1) Metal powder and ceramic powder are mixed and ball milled to obtain slurry A; (2) Carry out spray granulation to slurry A, obtain powder B; It is characterized in that it also includes: (3) Classifying powder B to obtain powder C with different particle sizes in more than two stages; (4) Sintering powders with different particle sizes at different temperatures, and then crushing and grading after sintering to obtain powders D with different bulk densities; (5) Mix powders D with different bulk densities to obtain cermet powder for spraying. 2. The preparation process according to claim 1, characterized in that, in the powder C, the smallest particle size is a micron, and the largest particle size is b micron; wherein 4≤a≤8; 50≤b≤60. 3. The preparation process as claimed in claim 1 or 2, characterized in that, in step (3), powder B is divided into n grades to obtain powder C, from small to large in particle size, powder C is respectively powder C1, C2, ..., Cn, the sintering temperatures are T1, T2, ..., Tn respectively, and the units of the sintering temperatures are °C; wherein, T1>T2>...>Tn, n is a natural number greater than 1. 4. The preparation process according to claim 3, wherein the powder B is divided into two stages to obtain powder C1 with a particle size not exceeding c microns and powder C2 with a particle size greater than c microns, wherein 25≤c≤35. 5. The preparation process according to claim 4, wherein the sintering temperature of powder C1 is 1170-1200°C; the sintering temperature of powder C2 is 1160-1190°C. 6. The preparation process according to claim 4, wherein the sintering temperature of the powder C1 is T1°C, the sintering temperature of the powder C2 is T2°C, and T2+40°C≥T1≥T2+10°C. 7. The preparation process as claimed in claim 3, wherein powder B is divided into three grades, namely powder C1, C2 and C3, and its particle size is respectively d, e and f microns; wherein, d<e< f; a≤d≤a+14 microns; b-14 microns≤f≤b. 8. The preparation process according to claim 7, wherein the sintering temperature of powder C1 is T1°C, the sintering temperature of powder C2 is T2°C, the sintering temperature of powder C3 is T3°C, T2+30°C≥T1≥ T2+8℃; T3+30℃≥T2≥T3+8℃. 9. The preparation process as claimed in claim 2, characterized in that, in the powder D, the particle size is a micron at the smallest and a micron at the largest. 10. The preparation process according to claim 1, wherein the metal powder is one or more of Ni, Co, Cr, Fe, Al and Mo; the ceramic powder is WC, Cr ₃ C _2. One or more of TiC, TiB ₂ , WB and Mo ₂ B.