Application of tungsten-coated tungsten carbide shell-core powder in inhibition of coating cracking and pores

Abstract

The invention belongs to the field of preparation of wear-resistant coatings through laser cladding, and discloses application of tungsten-coated tungsten carbide shell-core powder in inhibition of coating cracking and pores. The invention discloses application of tungsten-coated tungsten carbide shell-core powder in preparation of a tungsten carbide metal ceramic coating through laser cladding to inhibit cracking and pores of the coating. The tungsten-coated tungsten carbide shell-core powder is shell-core structure powder with tungsten carbide as a core and metal tungsten as a shell. According to the invention, it is revealed for the first time that the tungsten-coated tungsten carbide shell-core powder is applied to the laser cladding process, and coating cracking and pores can be inhibited; wC is coated with metal W, so that decomposition of WC in a laser molten pool can be effectively inhibited, formation of brittle carbides is reduced, and then the cracking sensitivity of a laser cladding layer is reduced. Moreover, the tungsten-coated tungsten carbide shell-core powder disclosed by the invention can be particularly prepared by adopting an oxidation-reduction preparation method, and effective and controllable preparation of a W shell layer can be realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of laser cladding preparation of wear-resistant coating, more particularly, relates to the application of tungsten-coated tungsten carbide shell-core powder in inhibiting cracking and porosity of the coating, which can inhibit cracking and porosity sensitivity of laser cladding tungsten carbide cermet coating. BACKGROUND

[0002] Cermet composite material with tungsten carbide (WC) as reinforcing particles has high hardness, excellent wear resistance and excellent erosion resistance, making it an ideal protective coating material for the surface of parts such as drill bits, turbine blades and oil pipelines. The high hardness of WC cermet composite material is due to the WC phase, while the toughness is mainly determined by the binder metal. The binder metal generally includes iron-based, nickel-based, cobalt-based and copper-based alloys. Using surface engineering technology such as laser cladding to deposit a tungsten carbide cermet composite layer on the surface of a common substrate can significantly improve its wear resistance. However, during laser cladding, WC particles will partially melt and decompose in the high-temperature molten pool, releasing carbon atoms, which react with alloying elements in the binder phase to form brittle carbides (intrinsic brittleness), reducing the toughness of the binder phase and increasing the crack sensitivity of the coating. On the other hand, C atoms react with O2 mixed into the molten pool to form CO2, resulting in reaction porosity.

[0003] Preheating the substrate can to some extent inhibit the formation of cracks in the tungsten carbide cermet coating. However, it does not reduce the intrinsic brittleness of the coating. Reducing the content of carbide-forming elements in the binder phase can reduce the precipitation of carbides, but usually at the expense of its corrosion resistance and wear resistance, and is prone to produce a large number of pores.

[0004] The use of shell-core WC powder coated with metal, such as nickel-coated WC and cobalt-coated WC, can to some extent reduce the decomposition of WC without compromising other properties. However, nickel and cobalt will rapidly decompose in the molten pool, which cannot effectively inhibit the decomposition of WC and avoid the formation of brittle carbides. SUMMARY

[0005] In view of the above shortcomings of nickel-coated WC and cobalt-coated WC, the present application first discloses the application of tungsten-coated tungsten carbide shell-core powder in laser cladding process, which can inhibit cracking and porosity of the coating; metal W-coated WC can effectively inhibit the decomposition of WC in the laser molten pool, thereby reducing the formation of brittle carbides and further reducing the cracking sensitivity of the laser cladding layer. Moreover, the tungsten-coated tungsten carbide shell-core powder in the present application can be prepared by an oxidation-reduction method, which can achieve effective and controllable preparation of W shell layer.

[0006] To achieve the above object, according to one aspect of the present application, there is provided an application of tungsten-coated tungsten carbide core-shell powder in laser cladding preparation of tungsten carbide cermet coating to inhibit coating cracking and porosity, characterized in that the tungsten-coated tungsten carbide core-shell powder is a core-shell structure powder with tungsten carbide as core and metallic tungsten as shell.

[0007] As a further preferred embodiment of the present application, the laser cladding preparation of tungsten carbide cermet coating is specifically to place the tungsten-coated tungsten carbide core-shell powder and the binder metal powder in the powder hopper of the powder feeder, and to prepare the tungsten carbide cermet coating on the surface of the substrate by using the laser cladding process. Preferably, in the tungsten-coated tungsten carbide core-shell powder and the binder metal powder, the mass fraction of the tungsten-coated tungsten carbide core-shell powder is not less than 60 wt%.

[0008] As a further preferred embodiment of the present application, the tungsten-coated tungsten carbide core-shell powder is prepared by a preparation method comprising the following steps: (1) Spread the WC powder in a crucible, put it into a heating furnace for oxidation to form tungsten oxide on the surface, and obtain tungsten oxide-coated tungsten carbide core-shell powder; (2) Put the tungsten oxide-coated tungsten carbide core-shell powder into a reduction furnace for heating reduction to completely reduce the tungsten oxide into tungsten, and obtain the tungsten-coated tungsten carbide core-shell powder.

[0009] As a further preferred embodiment of the present application, in step (1), the particle size of the WC powder is 80 mesh to 500 mesh, and the powder layer spread in the crucible is 1-2 mm thick; the oxidation temperature is 510 ℃ to 600 ℃, and the oxidation time is 60 min to 120 min; after the oxidation is completed, the crucible is taken out of the heating furnace for air cooling.

[0010] As a further preferred embodiment of the present application, in step (2), the tungsten oxide-coated tungsten carbide core-shell powder is first spread in a crucible and then put into a reduction furnace; The powder layer spread in the crucible is 1-2 mm thick.

[0011] As a further preferred embodiment of the present application, in step (2), the heating reduction is carried out in a pure hydrogen environment. Preferably, the pure hydrogen environment is that before heating starts, the air in the reduction furnace is first discharged, and then pure hydrogen is continuously introduced at a rate of 1-2 L / min.

[0012] As a further preferred embodiment of the present application, the heating reduction is performed by a two-step reduction method, the reduction temperature of the first step is 650-800 DEG C, and the holding time is 2-3 hours; after the first step reduction, the furnace temperature is raised to 830-900 DEG C for the second step reduction, and the holding time of the second step reduction is 2-3 hours; after the reduction, the furnace is cooled; preferably, the heating rate of the first step reduction and the second step reduction is 2-5 DEG C / min.

[0013] Compared with the prior art, the present application can reduce the cracking sensitivity and inhibit pores of the laser cladding WC cermet coating by using tungsten-coated WC powder.

[0014] The present application uses W-coated WC, which is different from Ni-coated WC and Co-coated WC in that it not only protects WC before entering the molten pool but also avoids the brittleness problem caused by the decomposition of WC after entering the molten pool.

[0015] Specifically, the following beneficial effects can be achieved: 1. By introducing a metal W shell, the present application inhibits the decomposition of WC in the laser cladding molten pool, thereby reducing the generation of carbides and reducing the cracking sensitivity.

[0016] 2. By introducing a metal W shell, the present application inhibits the decomposition of WC in the laser cladding molten pool, thereby avoiding the reaction of C atoms with O2 to form CO2 and inhibiting the generation of pores in the coating.

[0017] 3. By using tungsten-coated tungsten carbide powder in the laser cladding process, the present application can not only inhibit the cracks and pores in the laser cladding tungsten carbide cermet coating and improve its mechanical properties.

[0018] 4. Unlike the prior art of nickel-coated WC and cobalt-coated WC, the coated WC can be prepared by electrodeposition and chemical plating. The tungsten-coated WC powder in the present application is not suitable for electrodeposition and chemical plating. Electrodeposition, chemical plating and vapor deposition are common material surface coating preparation processes. Due to the potential characteristics of W element close to hydrogen, in the electrodeposition process, most of the water molecules in the electrolyte participate in the reaction, and tungsten compounds or ions do not participate in the reaction, resulting in the inability to prepare pure tungsten on the surface of tungsten carbide by electrodeposition. On the other hand, the chemical plating W layer is thin and has poor adhesion, which cannot effectively protect the WC. Chemical vapor deposition or physical vapor deposition can prepare a pure W layer on the surface of WC, but the thickness is at the nanometer level, and there are problems of high technical difficulty, high cost and long preparation period to prepare a micron-level pure tungsten layer. The present application uses WC powder as raw material, first carries out oxidation treatment to obtain tungsten oxide coated tungsten carbide shell-core powder, and then carries out reduction treatment to reduce all the tungsten oxide to tungsten, thereby finally obtaining tungsten coated tungsten carbide shell-core powder. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 is the preparation route of the W-coated WC powder of the present application and its principle of reducing crack sensitivity.

[0020] Figure 2 is the microstructure and flaw detection result of Inconel 625 and pure spherical cast WC laser cladding according to mass ratio (4:6), (3:7) and (2:8) of the first embodiment of the present application; wherein, Figure 2 (a), (a1) and (a2) in (a), (a1) and (a2) are the microstructures of different magnifications of mass ratio (3:7), Figure 2 (a3) in (a3) is the flaw detection result of each ratio.

[0021] Figure 3 is the surface and cross-sectional morphology of the "flower-like" W-WC preparation process of the second embodiment of the present application: wherein, Figure 3 (a) and (a1) in (a) and (a1) are the surface and cross-sectional morphology of the "flower-like" WO3-WC after oxidation, Figure 3 (b), (b1) and (b2) in (b), (b1) and (b2) are the surface and cross-sectional morphology of the "flower-like" WO2-WC after the first reduction, Figure 3 (c), (c1) and (c2) in (c), (c1) and (c2) are the surface and cross-sectional morphology of the "flower-like" W-WC after the second reduction. And, Figure 3 (a1), (b2) and (c2) in (a1), (b2) and (c2) are cross-sectional morphologies, and the others are surface morphologies.

[0022] Figure 4are the microstructure and surface flaw detection results of Inconel 625 and "flower-like" W-WC with different laser powers in the second embodiment of the present application, wherein, Figure 4 (a), (a1) and (a2) in (a) are the microstructure of the cladding layer with a laser power of 1500W, Figure 4 (b) in (a) is the flaw detection result of the cladding layer under different laser powers, Figure 4 (c), (c1) and (c2) in (a) are the damage of the W shell layer after cladding under different laser powers.

[0023] Figure 5 are the surface and cross-sectional morphology of the "spherical" W-WC preparation process in the third embodiment of the present application, wherein, Figure 5 (a) and (a1) in (a) are the surface and cross-sectional morphology of the "spherical" WO3-WC after oxidation, Figure 5 (b) in (a) is the surface morphology of the "spherical" WO2-WC after the first reduction, Figure 5 (c) in (a) is the surface morphology of the "spherical" W-WC after the second reduction. And, Figure 5 (a1) in (a) is the cross-sectional morphology, and the others are the surface morphology.

[0024] Figure 6 are the surface flaw detection and microstructure results of Inconel 625 and "spherical" W-WC with different laser powers in the third embodiment of the present application, wherein, Figure 6 (a) in (a) is the flaw detection result of the cladding layer under different laser powers, Figure 6 (b), (b1) and (b2) in (a) are the damage of the W shell layer after cladding under different laser powers. DETAILED DESCRIPTION

[0025] In order to make the purpose, technical scheme and advantages of the present application clearer and more apparent, the present application will be further described in detail below in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0026] The spherical cast WC powder and Inconel 625 powder used in the following embodiments are commonly used powder materials on the market.

[0027] Hereinafter, from the second embodiment, a certain thickness of metal W shell layer is prepared on the surface of WC powder by oxidation-reduction reaction to form a tungsten-coated tungsten carbide shell-core structure powder. For example, Figure 1As shown, the surface of the WC powder (i.e., pure spherical cast WC powder) is first oxidized to a certain thickness of tungsten oxide (WO3) to prepare WO3-coated WC powder (WO3-WC). Next, the WO3-WC powder is reduced to W-coated WC powder (W-WC). The powder is then used for laser cladding. When the W-WC powder enters the molten pool, the W shell layer first prevents the laser from directly irradiating the WC powder and second prevents the high-temperature molten pool from directly contacting the WC powder, thereby inhibiting the decomposition of the WC in the early stage. During the solidification of the molten pool, the W shell layer is slightly decomposed, forming gaps, allowing the molten pool liquid to contact the WC from the gaps, and a small amount of WC is dissolved, releasing a small amount of C atoms from the gaps to the molten pool. Therefore, the presence of the tungsten shell layer reduces the release of C atoms, thereby inhibiting the generation of brittle carbides in the binder metal and reducing the crack sensitivity of the tungsten carbide cermet coating.

[0028] The following are specific examples: Example One (This example is a comparative example, which uses WC powder, i.e., pure spherical cast WC powder, for laser cladding).

[0029] Spherical Inconel 625 powder of 200-325 mesh and spherical WC powder of 200-325 mesh were mixed in mass ratios of (4:6), (3:7), and (2:8), loaded into a powder feeder, and the powder feeding rate was adjusted to 18 g / min. The laser beam spot was designed as a circle with a diameter of 5 mm, the laser power was set to 1500 W, the scanning speed was 700 mm / min, the powder feeding gas flow was 6 L / min, and the protective gas flow was 12 L / min. Then, the laser cladding process was carried out to prepare Inconel 625 / WC coatings.

[0030] The macro penetrant inspection results of the coatings in each ratio are shown in Figure 2 (a3), where the metallograph of the coating with a mass ratio of (3:7) is shown in Figure 2 (a), (a1), and (a2). The WC powder is largely decomposed in the high-temperature molten pool, and a large amount of released carbon atoms react with the carbide-forming elements in the binder phase. The elemental analysis results of each phase in the microstructure are shown in Table 1; .

[0031] The decomposition of WC generates three types of carbides in the binder phase, as shown in Figure 2 (a2) as point 2 (blocky), point 5 (net-like), and point 6 (radial), with the carbon content ranking as: blocky carbide > radial carbide > net-like carbide. From Figure 2As shown in (a), the crack propagation pathway is: WC → massive carbides → radial carbides → massive carbides → WC. The microstructure of the metal-ceramic cladding layers with mass ratios of (4:6), (3:7), and (2:8) is similar. This indicates that massive and radial carbides significantly reduce the toughness of the binder phase, leading to a large number of cracks in the metal-ceramic cladding layers with mass ratios of (4:6), (3:7), and (2:8), such as... Figure 2 As shown in (a3).

[0032] Example 2.

[0033] This embodiment uses tungsten-coated tungsten carbide powder for laser cladding. The specific steps include: 200-325 mesh WC powder was placed in a crucible and then oxidized in a muffle furnace. The powder layer thickness was 2 mm, the oxidation temperature was 600℃, and the oxidation time was 60 min. Afterward, it was removed from the furnace and air-cooled. The resulting flower-shaped WO3-WC powder had the following morphology: Figure 3 As shown in (a) and (a1) in the table. The spectroscopic elemental analysis results of point a on the surface of WO3-WC powder are shown in Table 2. The results show that the W:O atomic ratio is close to 1:3. WO3 exhibits a "flowering" pattern and has a relatively thick oxide layer.

[0034] Flowering WO3-WC powder was reduced in a hydrogen reduction furnace (the hydrogen atmosphere was maintained at one standard atmosphere during the reduction process, the same below). The powder layer thickness was 2 mm. A two-step reduction method was used: first, the temperature was increased to 800℃ at 5℃ / min and held for 3 hours to obtain "flowering" WO2-WC powder, as shown below. Figure 3 As shown in (b), (b1), and (b2) in Table 2, the fine particles at point b were identified as WO2 by WDS. The reduced powder maintained good structural consistency with the oxidized powder. The temperature was then increased to 900℃ at 5℃ / min and held for 3 hours. This resulted in a "flowering" W-WC powder, as shown in Table 2. Figure 3 As shown in (c), (c1), and (c2), the W layer is thick and consists of relatively coarse W grains. Its elemental composition is determined by the WDS results at points c and d in Table 2. The W shell grains of the "flowering" W-WC powder are coarse and loose, but the W shell is relatively thick. .

[0035] The prepared "flower-like" W-WC powder was mixed with Inconel 625 powder at a mass ratio of 7:3, loaded into a powder feeder, and the powder feeding rate was adjusted to 12 g / min. The laser beam spot size was adjusted to 3 mm, the laser power was set to 1200 W, 1500 W and 1800 W, the scanning speed was 700 mm / min, the powder feeding gas flow was 6 L / min, and the protective gas flow was 12 L / min. Subsequently, the laser cladding process was carried out to prepare the Inconel 625 / W-WC coating. The damage to the W shell layer after cladding at different laser powers is shown in (c), (c1) and (c2) of FIG. 6. Figure 4

[0036] The microstructure of the coating at a laser power of 1500 W is shown in (a), (a1) and (a2) of FIG. 5. It can be seen that the W shell layer still exists around the WC, but the shell layer is partially decomposed in the molten pool, and gaps appear. In the microstructure, the blocky carbides and radial carbides with high carbon content disappear, and are replaced by fine network carbides with low carbon content. The carbon content of the network carbides is lower than that of the network carbides produced by laser cladding using spherical cast WC in Example One, as shown in Table 3. Figure 4 .

[0037] This indicates that the C content in the Inconel 625 / W-WC cladding layer is lower than that in the Inconel 625 / WC cladding layer. The reason is that the W shell layer protects the WC in the molten pool from decomposing in large quantities, thereby inhibiting the release of C elements and reducing the brittleness of the material, so the cladding layer does not crack. The macroscopic morphology is shown in the "1500 W" sample in (b) of FIG. 4. Similarly, when the laser power is 1200 W, the cladding layer obtained is also not cracked, as shown in the "1200 W" sample in (b) of FIG. 4. That is, when the laser power does not exceed 1500 W (for example, the laser power can be 1200 W~1500 W), an uncracked laser cladding layer can be obtained. As shown in (c2) of FIG. 6, when a high laser power of 1800 W is used, the shell layer decomposes intensively, resulting in the formation of blocky carbides with high carbon content, which reduces the toughness of the material, and thus the cladding layer cracks, as shown in the "1800 W" sample in (b) of FIG. 4. This illustrates the role of the complete W shell layer in inhibiting the decomposition of tungsten carbide and the formation of carbides. Although the coating cracks, it has much fewer cracks than the cracks produced by laser cladding using pure spherical cast WC powder at the same content, further indicating the effect of using tungsten-coated tungsten carbide powder for Inconel 625 / W-WC coating preparation to inhibit cracking. Figure 4 Figure 4 Figure 4 Figure 4

[0038] Example Three.​​​​​​

[0039] This embodiment is to use tungsten-coated tungsten carbide powder for laser cladding. Specifically, the following steps are included: After the WC powder of 200-325 mesh is loaded into the crucible and placed in the muffle furnace for oxidation, the powder layer is 1 mm thick, the oxidation temperature is 510℃, the oxidation time is 120 min, and then it is taken out of the furnace and air-cooled. The obtained spherical WO3-WC powder has a high surface sphericity and a thin oxidation layer, as shown in (a) and (a1) of Figure 5

[0040] The spherical WO3-WC powder is placed in a hydrogen reduction furnace for reduction (during the reduction process, the hydrogen atmosphere can be maintained at one standard atmosphere, and the same applies below), the powder layer is 2 mm thick, and a two-step reduction method is used. First, heat to 650℃ at a rate of 5℃ / min and keep for 2h to obtain spherical WO2-WC powder, as shown in (b) of Figure 5 The WO2 is in the form of fine particles loosely attached to the WC core, and the overall structure of the powder remains consistent with the oxidized powder; then heat to 830℃ at a rate of 5℃ / min and keep for 2h. The obtained spherical W-WO3 powder is shown in (c) of Figure 5 The W shell layer of the spherical W-WO3 powder is dense and has fine grains, but the W shell layer is thin.

[0041] The prepared spherical W-WC powder is mixed with Inconel 625 powder at a mass ratio of 7:3, loaded into the powder feeder, and the powder feeding rate is adjusted to 12 g / min. The laser beam spot size is adjusted to 3mm, the laser power is set to 900W, 1200W and 1500W, the scanning speed is 700 mm / min, the powder feeding gas flow is 6 L / min, and the protective gas flow is 12 L / min. Then, the laser cladding process is carried out to prepare Inconel 625 / W-WC coating.

[0042] The flaw detection results of the laser cladding coating under different powers are shown in (a) of Figure 6 All the coatings are not cracked, and it can be seen that a good non-cracking effect can be achieved when the power does not exceed 1500W. The coating cross-section metallographic image is shown in Figure 6 ​As shown in (b), (b1) and (b2) in FIG. 6, the W shell of the spherical W-WC powder is thinner, but the integrity of the shell is better. Therefore, at lower power (900 W and 1200 W), the decomposition of WC is less, no bulk carbide is generated in the molten pool, and the proportion of network carbide is also reduced, which means that the cracking tendency is further reduced. When the power is increased to 1500 W, the thinner W shell is completely decomposed at the late solidification stage of the molten pool, which causes damage to WC, and bulk carbide grows into the molten pool, but since the molten pool temperature has been quickly solidified, the bulk carbide does not grow large, and the toughness of the binder phase is slightly reduced, so the cladding layer does not crack. As can be seen, even if the W shell of the spherical W-WC is thinner, its better compactness and integrity can also have a good crack inhibition effect at lower power (laser power does not exceed 1200 W, such as 900 W-1200 W in the embodiment). At high power (laser power exceeds 1200 W, such as 1500 W in the embodiment), even if the thinner W shell is decomposed, it delays the time when WC contacts the molten liquid phase, thereby reducing the generation of carbide.

[0043] The above embodiments are only examples. For example, the decomposition of the W shell and the types of carbide in the microstructure and the cracking of the cladding layer can be observed according to the actual cladding situation, and the laser cladding process parameters can be optimized.

[0044] Those skilled in the art will readily understand that the above description is only preferred embodiments of the present application and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. The application of tungsten-coated tungsten carbide core powder in inhibiting coating cracking and porosity during laser cladding preparation of tungsten carbide cermet coatings, characterized in that, The tungsten-coated tungsten carbide core-shell powder is a core-shell structure powder with tungsten carbide as the core and metallic tungsten as the shell.

2. The application as described in claim 1, characterized in that, The laser cladding process for preparing tungsten carbide metal ceramic coating involves placing tungsten-coated tungsten carbide core powder and binder metal powder in a powder feeder hopper, and then using a laser cladding process to prepare the tungsten carbide metal ceramic coating on the substrate surface. Preferably, in the tungsten-coated tungsten carbide core powder and the binder metal powder, the mass percentage of the tungsten-coated tungsten carbide core powder is not less than 60 wt%.

3. The application as described in claim 1, characterized in that, The tungsten-coated tungsten carbide core powder is prepared by a method including the following steps: (1) Spread WC powder in a crucible and heat it in a furnace to oxidize it, so that tungsten oxide is formed on its surface, and tungsten carbide core powder coated with tungsten oxide is obtained; (2) The tungsten oxide-coated tungsten carbide shell and core powder is placed in a reduction furnace for heating and reduction, so that all the tungsten oxide is reduced to tungsten, and tungsten-coated tungsten carbide shell and core powder is obtained.

4. The application as described in claim 3, characterized in that, In step (1), the particle size of the WC powder is 80 mesh to 500 mesh, and the thickness of the powder layer spread in the crucible is 1-2 mm; the oxidation temperature is 510 ℃ to 600 ℃, and the oxidation time is 60 min to 120 min; after the oxidation is completed, the crucible is taken out of the heating furnace and air-cooled.

5. The application as described in claim 3, characterized in that, In step (2), the tungsten oxide-coated tungsten carbide core powder is first spread out in a crucible and then placed in a reduction furnace; The thickness of the powder layer spread inside the crucible is 1-2 mm.

6. The application as described in claim 3, characterized in that, In step (2), the heating reduction is carried out in a pure hydrogen environment; Preferably, the pure hydrogen environment is achieved by first purging the air from the reduction furnace before heating begins, and then continuously introducing pure hydrogen at a rate of 1-2 L / min.

7. The application as described in claim 3, characterized in that, The heating reduction adopts a two-step reduction method. The reduction temperature of the first step is 650 ℃~800 ℃, and the holding time is 2 h~3 h. After the first step reduction is completed, the furnace temperature is raised to 830 ℃~900 ℃ for the second step reduction, and the holding time of the second step reduction is 2 h~3 h. After the reduction is completed, the furnace is cooled. Preferably, the heating rate used for both the first step reduction and the second step reduction is 2-5 ℃ / min.

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