AN IN-SITU METHOD FOR SYNTHESIZING NI-W-WC COMPOSITE COATING

Abstract

The present invention provides an in-situ method for synthesizing a Ni—W—WC composite coating, which includes the following steps: immersing a carbon steel substrate to be coated in an electroplating solution and electroplating, to obtain a Ni—W—C alloy coating on the surface of the carbon steel substrate; and then subjecting the alloy coating to high temperature heat treatment to obtain the Ni—W—WC composite coating. The electroplating solution comprises the following components: a nickel salt, a tungstate, citric acid, a citrate, a recarburizer, and a wetting agent. The present invention shows merits of simple operation, high current efficiency, simple electroplating process, and is clean and causes no pollution, thus meeting the requirements of environment protection.

Claims

1. An in-situ method for synthesizing a Ni—W—WC composite coating, comprising steps of: immersing a substrate to be coated in an electroplating solution and electroplating, to obtain a Ni—W—C alloy coating on the surface of the substrate; and then subjecting the alloy coating to heat treatment to obtain the Ni—W—WC composite coating; wherein the electroplating solution comprises: a nickel salt, a tungstate, citric acid, a citrate, a recarburizer, and a wetting agent.

2. The in-situ synthesis method according to claim 1, wherein the nickel salt is selected from the group consisting of nickel sulfate, nickel sulfonate, nickel chloride and any combination thereof.

3. The in-situ synthesis method according to claim 1, wherein the tungstate is sodium tungstate.

4. The in-situ synthesis method according to claim 1, wherein the recarburizer is selected from the group consisting of 2-(4-pyridyl)ethanesulfonic acid, 2-pyridinesulfonic acid, 3-pyridinesulfonic acid, pyridinium propanesulfonate and any combination thereof.

5. The in-situ synthesis method according to claim 1, wherein the concentration range of each component in the electroplating solution is: nickel salt 20-70 g/L, tungstate 30-85 g/L, citric acid 7-35 g/L, citrate 10-70 g/L, recarburizer 1-14 g/L, and wetting agent 0.5-9.5 mL/L.

6. The in-situ synthesis method according to claim 1, wherein the electroplating solution is prepared by steps of: mixing the nickel salt, the tungstate, citric acid, and the citrate into water to form a uniform solution; adding the recarburizer, and the wetting agent to the above solution; and then adjusting the solution with a pH of 7.5-7.8 to obtain the electroplating solution.

7. The in-situ synthesis method according to claim 1, wherein during electroplating, the cathode current density is 2-5 A/dm.sup.2.

8. The in-situ synthesis method according to claim 1, wherein the C content in the Ni—W—C alloy coating is 7-12 wt. %, and the W content is 35-45 wt. %.

9. The in-situ synthesis method according to claim 1, wherein a temperature for heat treatment of the Ni—W—C alloy coating is 700-1000° C.

10. The in-situ synthesis method according to claim 1, wherein the Ni—W—WC composite coating has a thickness of 10-20 μm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] To make the disclosure of the present invention more comprehensible, the present invention will be further described in detail by way of specific embodiments of the present invention with reference the accompanying drawings, in which:

[0023] FIG. 1 shows the morphology and element composition of a coating obtained in Example 2 and in Comparative Example 2 of the present invention, in which (a) shows a coating obtained in an electroplating solution containing no recarburizer, and (b) shows a coating obtained in an electroplating solution containing a recarburizer.

[0024] FIG. 2 shows a XPS survey spectrum of a coating obtained Example 2 of the present invention.

[0025] FIG. 3 shows a high-resolution analysis spectrum of C is in a coating obtained in Example 2 of the present invention.

[0026] FIG. 4 is an SEM image showing the morphology of a coating after heat treatment obtained in Example 2 of the present invention.

[0027] FIG. 5 shows an XRD pattern of the coating obtained after heat treatment in Example 2 of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The present invention will be further described below with reference to the accompanying drawings and specific examples, so that those skilled in the art can better understand and implement the present invention; however, the present invention is not limited thereto.

Example 1

[0029] An in-situ method for synthesizing a Ni—W—WC composite coating is provided. The process was specifically as follows.

[0030] (1) Preparation of electroplating solution: 300 mL of deionized water was added to a cleaned 1# beaker, and heated to 65° C.; and then an amount of solid nickel sulfate was added, and stirred until it was completely dissolved. Another 300 mL of deionized water was heated to 75° C. in a 2# beaker, and then sodium citrate, citric acid, ammonium citrate and sodium tungstate were added sequentially, and stirred until they were completely dissolved. The liquid in the 1# beaker was slowly added to the 2# beaker; and then a recarburizer and a wetting agent were added, and stirred until they were completely dissolved. Finally, deionized water was added to make up the solution in the 2# beaker to a constant volume, and the solution was adjusted to a pH of 7.5 with aqueous ammonia. The composition of the electroplating solution is shown in Table 1:

TABLE-US-00001 TABLE 1 Composition of electroplating solution in Example 1 Ingredient Concentration NiSO.sub.4•6H.sub.2O 20 g/L Na.sub.2WO.sub.4•2H.sub.2O 30 g/L Na.sub.3C.sub.6H.sub.5O.sub.7•2H.sub.2O 10 g/L C.sub.6H.sub.8O.sub.7 7 g/L C.sub.6H.sub.17N.sub.3O.sub.7 1 g/L 2-(4-pyridyl)ethanesulfonic acid 1 g/L XP-80 0.5 mL/L

[0031] (2) Pretreatment of carbon steel substrate: The surface (10×10 cm.sup.2) of a carbon steel substrate was polished with 180#-240# carborundum, washed with deionized water, degreased with 350 mL of a degreasing reagent (a mixed solution containing hydrogen sodium oxide of 8 g/L, sodium silicate of 2 g/L, sodium carbonate of 1.5 g/L, and sodium citrate of 0.5 g/L) at 65° C., and washed with water, then with 10% H.sub.2SO.sub.4 for 10 min, and then with water until neutral.

[0032] (3) Low-current electrolysis: An appropriate amount of electroplating solution with composition shown in Table 1 was added to an electroplating bath. Low-current electrolysis was carried out using a constant voltage DC power supply with a corrugated iron plate as the cathode and stainless steel as the anode to allow the complexing agent and additives in the freshly prepared electroplating solution to reach an optimum state. During electrolysis, the temperature was controlled at 65° C., the cathode current density was 1 A/dm.sup.2, and the time was 10 h. (the freshly prepared electroplating solution was pretreated by low-current electrolysis, during which the complexing agent and additives could reach an optimum state).

[0033] (4) Preparation of Ni—W—C alloy coating by electrodeposition: The substrate obtained in step (2) was electroplated in the electroplating solution treated in step (3). The specific electroplating conditions were: an anode of an iridium-tantalum alloy, a cathode of a pretreated carbon steel substrate, an electroplating solution temperature of 65° C., a cathode current density of 2 A/dm.sup.2, and an electroplating time of 60 min. A coating with a thickness of 10 μm was obtained, where the weight ratio of each element in the coating was about Ni:W:C=50:40:10.

[0034] (5) In-situ synthesis of Ni—W—WC composite coating: The coating obtained in step (4) was heated to 700° C. at a ramping rate of 5° C./min and held for 3 h under an argon atmosphere, cooled down to about 200° C. with the furnace, removed from the furnace and air cooled.

Example 2

[0035] An in-situ method for synthesizing a Ni—W—WC composite coating is provided. The process was specifically as follows.

[0036] (1) Preparation of electroplating solution: 300 mL of deionized water was added to a cleaned 1# beaker, and heated to 65° C.; and then an amount of solid nickel sulfate was added, and stirred until they were completely dissolved. Another 300 mL of deionized water was heated to 75° C. in a 2# beaker, and then sodium citrate, citric acid, ammonium citrate and sodium tungstate were added sequentially, and stirred until they were completely dissolved. The liquid in the 1# beaker was slowly added to the 2# beaker; and then a recarburizer and a wetting agent were added, and stirred until they were completely dissolved. Finally, deionized water was added to make up the solution in the 2# beaker to a constant volume, and the solution was adjusted to pH 7.6 with aqueous ammonia. The composition of the electroplating solution is shown in Table 2:

TABLE-US-00002 TABLE 2 Composition of electroplating solution in Example 2 Ingredient Content NiSO.sub.4•6H.sub.2O 40 g/L Na.sub.2WO.sub.4•2H.sub.2O 50 g/L Na.sub.3C.sub.6H.sub.5O.sub.7•2H.sub.2O 45 g/L C.sub.6H.sub.8O.sub.7 20 g/L C.sub.6H.sub.17N.sub.3O.sub.7 5 g/L 3-pyridinesulfonic acid 6 g/L X-114 6 mL/L

[0037] (2) Pretreatment of substrate: The surface (10×10 cm.sup.2) of a carbon steel substrate was polished with 180#-240# carborundum, washed with deionized water, degreased with 350 mL of a degreasing reagent (a mixed solution containing hydrogen sodium oxide of 8 g/L, sodium silicate of 2 g/L, sodium carbonate of 1.5 g/L, and sodium citrate of 0.5 g/L) at 65° C., and washed with water, then with 10% H.sub.2SO.sub.4 for 10 min, and finally with water until neutral.

[0038] (3) Low-current electrolysis: An appropriate amount of electroplating solution having a composition as shown in Table 2 was added to an electroplating bath. Low-current electrolysis was carried out using a constant voltage DC power supply with a corrugated iron plate as the cathode and stainless steel as the anode. to allow the complexing agent and additives in the freshly prepared electroplating solution to reach an optimum state. During electrolysis, the temperature was controlled at 67° C., the cathode current density was 1 A/dm.sup.2, and the electrolytic treatment was continued for 12 h.

[0039] (4) Preparation of Ni—W—C alloy coating by electrodeposition: The substrate obtained in step (2) was electroplated in the electroplating solution treated in step (3). The specific electroplating conditions were: an anode of an iridium-tantalum alloy, a cathode of a pretreated carbon steel substrate, an electroplating solution temperature of 67° C., a cathode current density of 3 A/dm.sup.2, and an electroplating time of 50 min. A coating with a thickness of 15 μm was obtained, where the weight ratio of each element in the coating was about Ni:W:C=46.3:41.7:12.

[0040] (5) In-situ synthesis of Ni—W—WC composite coating: The coating obtained in step (4) was heated to 800° C. at a ramping rate of 5° C./min and held for 2 h under an argon atmosphere, cooled down to about 200° C. with the furnace, removed from the furnace and air cooled.

Example 3

[0041] An in-situ method for synthesizing a Ni—W—WC composite coating is provided. The process was specifically as follows.

[0042] (1) Preparation of electroplating solution: 300 mL of deionized water was added to a cleaned 1# beaker, and heated to 65° C.; and then an amount of solid nickel sulfate was added, and stirred until they were completely dissolved. Another 300 mL of deionized water was heated to 75° C. in a 2# beaker, and then sodium citrate, citric acid, ammonium citrate and sodium tungstate were added sequentially, and stirred until they were completely dissolved. The liquid in the 1# beaker was slowly added to the 2# beaker; and then a recarburizer and a wetting agent were added, and stirred until they were completely dissolved. Finally, deionized water was added to make up the solution in the 2# beaker to a constant volume, and the solution was adjusted to pH 7.7 with aqueous ammonia. The composition of the electroplating solution is shown in Table 3:

TABLE-US-00003 TABLE 3 Composition of electroplating solution in Example 3 Ingredient Content NiSO.sub.4•6H.sub.2O 50 g/L Na.sub.2WO.sub.4•2H.sub.2O 65 g/L Na.sub.3C.sub.6H.sub.5O.sub.7•2H.sub.2O 60 g/L C.sub.6H.sub.8O.sub.7 20 g/L C.sub.6H.sub.17N.sub.3O.sub.7 9 g/L 2-pyridinesulfonic acid 9 g/L XP-90 8 mL/L

[0043] (2) Pretreatment of carbon steel substrate: The surface (10×10 cm.sup.2) of a carbon steel substrate was polished with 180#-240# carborundum, washed with deionized water, degreased with 350 mL of a degreasing reagent (a mixed solution containing hydrogen sodium oxide of 8 g/L, sodium silicate of 2 g/L, sodium carbonate of 1.5 g/L, and sodium citrate of 0.5 g/L) at 65° C., and washed with water, then with 10% H.sub.2SO.sub.4 for 10 min, and then with water until neutral.

[0044] (3) Low-current electrolysis: An appropriate amount of an electroplating solution having a composition as shown in Table 3 was added to an electroplating bath. Low-current electrolysis was carried out using a constant voltage DC power supply with a corrugated iron plate as the cathode and stainless steel as the anode. to allow the complexing agent and additives in the freshly prepared electroplating solution to reach an optimum state. During electrolysis, the temperature was controlled at 68° C., the cathode current density was 1 A/dm.sup.2, and the electrolytic treatment was continued for 10 h.

[0045] (4) Preparation of Ni—W—C alloy coating by electrodeposition: The substrate obtained in step (2) was electroplated in the electroplating solution treated in step (3). The specific electroplating conditions were: an anode of an iridium-tantalum alloy, a cathode of a pretreated carbon steel substrate, an electroplating solution temperature of 68° C., a cathode current density of 4 A/dm.sup.2, and an electroplating time of 45 min. A coating with a thickness of 17 μm was obtained, where the weight ratio of each element in the coating was about Ni:W:C=48.4:42.4:9.2.

[0046] (5) In-situ synthesis of Ni—W—WC composite coating: The coating obtained in step (4) was heated to 900° C. at a ramping rate of 5° C./min and held for 2 h under an argon atmosphere, cooled down to about 200° C. with the furnace, removed from the furnace and air cooled.

Example 4

[0047] An in-situ method for synthesizing a Ni—W—WC composite coating is provided. The process was specifically as follows.

[0048] (1) Preparation of electroplating solution: 300 mL of deionized water was added to a cleaned 1# beaker, and heated to 65° C.; and then an amount of solid nickel sulfate was added, and stirred until they were completely dissolved. Another 300 mL of deionized water was heated to 75° C. in a 2# beaker, and then sodium citrate, citric acid, ammonium citrate and sodium tungstate were added sequentially, and stirred until they were completely dissolved. The liquid in the 1# beaker was slowly added to the 2# beaker; and then a recarburizer and a wetting agent were added, and stirred until they were completely dissolved. Finally, deionized water was added to make up the solution in the 2# beaker to a constant volume, and the solution was adjusted to pH 7.8 with aqueous ammonia. The composition of the electroplating solution is shown in Table 4:

TABLE-US-00004 TABLE 4 Composition of electroplating solution in Example 4 Ingredient Content NiSO.sub.4•6H.sub.2O 70 g/L Na.sub.2WO.sub.4•2H.sub.2O 85 g/L Na.sub.3C.sub.6H.sub.5O.sub.7•2H.sub.2O 70 g/L C.sub.6H.sub.8O.sub.7 35 g/L C.sub.6H.sub.17N.sub.3O.sub.7 13 g/L Pyridinium propanesulfonate 14 g/L XP-70 9.5 mL/L

[0049] (2) Pretreatment of carbon steel substrate: The surface (10×10 cm.sup.2) of a carbon steel substrate was polished with 180#-240# carborundum, washed with deionized water, degreased with 350 mL of a degreasing reagent (a mixed solution containing hydrogen sodium oxide of 8 g/L, sodium silicate of 2 g/L, sodium carbonate of 1.5 g/L, and sodium citrate of 0.5 g/L) at 65° C., and washed with water, then with 10% H.sub.2SO.sub.4 for 10 min, and then with water until neutral.

[0050] (3) Low-current electrolysis: An appropriate amount of an electroplating solution having a composition as shown in Table 4 was added to an electroplating bath. Low-current electrolysis was carried out using a constant voltage DC power supply with a corrugated iron plate as the cathode and stainless steel as the anode. to allow the complexing agent and additives in the freshly prepared electroplating solution to reach an optimum state. During electrolysis, the temperature was controlled at 70° C., the cathode current density was 1.2 A/dm.sup.2, and the electrolytic treatment was continued for 12 h.

[0051] (4) Preparation of Ni—W—C alloy coating by electrodeposition: The substrate obtained in step (2) was electroplated in the electroplating solution treated in step (3). The specific electroplating conditions were: an anode of a carbon steel anode material, an electroplating solution temperature of 70° C., a cathode current density of 5 A/dm.sup.2, and an electroplating time of 40 min. A coating with a thickness of 20 μm was obtained, where the weight ratio of each element in the coating was about Ni:W:C=44.5:45:10.5.

[0052] (5) In-situ synthesis of Ni—W—WC composite coating: The coating obtained in step (4) was heated to 900° C. at a ramping rate of 5° C./min and held for 2 h under an argon atmosphere, cooled down to about 200° C. with the furnace, removed from the furnace and air cooled.

Comparative Example 1 (without Recarburizer)

[0053] An in-situ method for synthesizing a Ni—W—WC composite coating is provided. The process was specifically as follows.

[0054] (1) Preparation of electroplating solution: 300 mL of deionized water was added to a cleaned 1# beaker, and heated to 65° C.; and then an amount of solid nickel sulfate was added, and stirred until they were completely dissolved. Another 300 mL of deionized water was heated to 75° C. in a 2# beaker, and then sodium citrate, citric acid, ammonium citrate and sodium tungstate were added sequentially, and stirred until they were completely dissolved. The liquid in the 1# beaker was slowly added to the 2# beaker; and then a wetting agent were added, and stirred until it was completely dissolved. Finally, deionized water was added to make up the solution in the 2# beaker to a constant volume, and the solution was adjusted to pH 7.6 with aqueous ammonia. The composition of the electroplating solution is shown in Table 5:

TABLE-US-00005 TABLE 5 Composition of electroplating solution in Comparative Example 1 Ingredient Content NiSO.sub.4•6H.sub.2O 40 g/L Na.sub.2WO.sub.4•2H.sub.2O 50 g/L Na.sub.3C.sub.6H.sub.5O.sub.7•2H.sub.2O 45 g/L C.sub.6H.sub.8O.sub.7 20 g/L C.sub.6H.sub.17N.sub.3O.sub.7 5 g/L X-114 6 mL/L

[0055] (2) Pretreatment of substrate: The surface (10×10 cm.sup.2) of a carbon steel substrate was polished with 180#-240# carborundum, washed with deionized water, degreased with 350 mL of a degreasing reagent (a mixed solution containing hydrogen sodium oxide of 8 g/L, sodium silicate of 2 g/L, sodium carbonate of 1.5 g/L, and sodium citrate of 0.5 g/L) at 65° C., and washed with water, then with 10% H.sub.2SO.sub.4 for 10 min, and then with water until neutral.

[0056] (3) Low-current electrolysis: An appropriate amount of an electroplating solution having a composition as shown in Table 5 was added to an electroplating bath. Low-current electrolysis was carried out using a constant voltage DC power supply with a corrugated iron plate as the cathode and stainless steel as the anode. to allow the complexing agent and additives in the freshly prepared electroplating solution to reach an optimum state. During electrolysis, the temperature was controlled at 67° C., the current density in the cathode was 1 A/dm.sup.2, and the electrolytic treatment was continued for 12 h.

[0057] (4) Preparation of Ni—W—C alloy coating by electrodeposition: The substrate obtained in step (2) was electroplated in the electroplating solution treated in step (3). The specific electroplating conditions were: an anode of an iridium-tantalum alloy, a cathode of a pretreated carbon steel substrate, an electroplating solution temperature of 67° C., a cathode current density of 3 A/dm.sup.2, and an electroplating time of 50 min. A coating with a thickness of 15 μm was obtained, where the weight ratio of each element in the coating was about Ni:W:C=56.8:40.5:2.7.

[0058] (5) In-situ synthesis of Ni—W—WC composite coating: The coating obtained in step (4) was heated to 800° C. at a ramping rate of 5° C./min and held for 2 h under an argon atmosphere, cooled down to about 200° C. with the furnace, removed from the furnace and air cooled.

Comparative Example 2 (Electroplating at Low Current Density)

[0059] An in-situ method for synthesizing a Ni—W—WC composite coating is provided. The process was specifically as follows.

[0060] (1) Preparation of electroplating solution: 300 mL of deionized water was added to a cleaned 1# beaker, and heated to 65° C.; and then an amount of solid nickel sulfate was added, and stirred until they were completely dissolved. Another 300 mL of deionized water was heated to 75° C. in a 2# beaker, and then sodium citrate, citric acid, ammonium citrate and sodium tungstate were added sequentially, and stirred until they were completely dissolved. The liquid in the 1# beaker was slowly added to the 2# beaker; and then a recarburizer and a wetting agent were added, and stirred until they were completely dissolved. Finally, deionized water was added to make up the solution in the 2# beaker to a constant volume, and the solution was adjusted to pH 7.6 with aqueous ammonia. The composition of the electroplating solution is shown in Table 2.

[0061] (2) Pretreatment of substrate: The surface (10×10 cm.sup.2) of a carbon steel substrate was polished with 180#-240# carborundum, washed with deionized water, degreased with 350 mL of a degreasing reagent (a mixed solution containing hydrogen sodium oxide of 8 g/L, sodium silicate of 2 g/L, sodium carbonate of 1.5 g/L, and sodium citrate of 0.5 g/L) at 65° C., and washed with water, then with 10% H.sub.2SO.sub.4 for 10 min, and then with water until neutral.

[0062] (3) Low-current electrolysis: An appropriate amount of an electroplating solution having a composition as shown in Table 2 was added to an electroplating bath. Low-current electrolysis was carried out using a constant voltage DC power supply with a corrugated iron plate as the cathode and stainless steel as the anode. to allow the complexing agent and additives in the freshly prepared electroplating solution to reach an optimum state. During electrolysis, the temperature was controlled at 67° C., the cathode current density was 1 A/dm.sup.2, and the electrolytic treatment was continued for 12 h.

[0063] (4) Preparation of Ni—W—C alloy coating by electrodeposition: The substrate obtained in step (2) was electroplated in the electroplating solution treated in step (3). The specific electroplating conditions were: an anode of an iridium-tantalum alloy, a cathode of a pretreated carbon steel substrate, an electroplating solution temperature of 67° C., a cathode current density of 1.5 A/dm.sup.2, and an electroplating time of 60 min. A coating with a thickness of 15 μm was obtained, where the weight ratio of each element in the coating was about Ni:W:C=58.6:35:6.4.

[0064] (5) In-situ synthesis of Ni—W—WC composite coating: The coating obtained in step (4) was heated to 800° C. at a ramping rate of 5° C./min and held for 2 h under an argon atmosphere, cooled down to about 200° C. with the furnace, removed from the furnace and air cooled.

Comparative Example 3 (Heat Treatment at 600° C.)

[0065] An in-situ method for synthesizing a Ni—W—WC composite coating is provided. The process was specifically as follows.

[0066] (1) Preparation of electroplating solution: 300 mL of deionized water was added to a cleaned 1# beaker, and heated to 65° C.; and then an amount of solid nickel sulfate was added, and stirred until they were completely dissolved. Another 300 mL of deionized water was heated to 75° C. in a 2# beaker, and then sodium citrate, citric acid, ammonium citrate and sodium tungstate were added sequentially, and stirred until they were completely dissolved. The liquid in the 1# beaker was slowly added to the 2# beaker; and then a recarburizer and a wetting agent were added, and stirred until they were completely dissolved. Finally, deionized water was added to make up the solution in the 2# beaker to a constant volume, and the solution was adjusted to pH 7.6 with aqueous ammonia. The composition of the electroplating solution is shown in Table 2.

[0067] (2) Pretreatment of substrate: The surface (10×10 cm.sup.2) of a carbon steel substrate was polished with 180#-240# carborundum, washed with deionized water, degreased with 350 mL of a degreasing reagent (a mixed solution containing hydrogen sodium oxide of 8 g/L, sodium silicate of 2 g/L, sodium carbonate of 1.5 g/L, and sodium citrate of 0.5 g/L) at 65° C., and washed with water, then with 10% H.sub.2SO.sub.4 for 10 min, and then with water until neutral.

[0068] (3) Low-current electrolysis: An appropriate amount of an electroplating solution having a composition as shown in Table 2 was added to an electroplating bath. Low-current electrolysis was carried out using a constant voltage DC power supply with a corrugated iron plate as the cathode and stainless steel as the anode. to allow the complexing agent and additives in the freshly prepared electroplating solution to reach an optimum state. During electrolysis, the temperature was controlled at 67° C., the current density in the cathode was 1 A/dm.sup.2, and the electrolytic treatment was continued for 12 h.

[0069] (4) Preparation of Ni—W—C alloy coating by electrodeposition: The substrate obtained in step (2) was electroplated in the electroplating solution treated in step (3). The specific electroplating conditions were: an anode of an iridium-tantalum alloy, a cathode of a pretreated carbon steel substrate, an electroplating solution temperature of 67° C., a cathode current density of 3 A/dm.sup.2, and an electroplating time of 50 min. A coating with a thickness of 15 μm was obtained, where the weight ratio of each element in the coating was about Ni:W:C=46.3:41.7:12.

[0070] (5) In-situ synthesis of Ni—W—WC composite coating: The coating obtained in step (4) was heated to 600° C. at a ramping rate of 5° C./min and held for 2 h under an argon atmosphere, cooled down to about 200° C. with the furnace, removed from the furnace and air cooled.

Comparative Example 4 (Heat Treatment at 1000° C.)

[0071] An in-situ method for synthesizing a Ni—W—WC composite coating is provided. The process was specifically as follows.

[0072] (1) Preparation of electroplating solution: 300 mL of deionized water was added to a cleaned 1# beaker, and heated to 65° C.; and then an amount of solid nickel sulfate was added, and stirred until they were completely dissolved. Another 300 mL of deionized water was heated to 75° C. in a 2# beaker, and then sodium citrate, citric acid, ammonium citrate and sodium tungstate were added sequentially, and stirred until they were completely dissolved. The liquid in the 1# beaker was slowly added to the 2# beaker; and then a recarburizer and a wetting agent were added, and stirred until they were completely dissolved. Finally, deionized water was added to make up the solution in the 2# beaker to a constant volume, and the solution was adjusted to pH 7.6 with aqueous ammonia. The composition of the electroplating solution is shown in Table 2.

[0073] (2) Pretreatment of substrate: The surface (10×10 cm.sup.2) of a carbon steel substrate was polished with 180#-240# carborundum, washed with deionized water, degreased with 350 mL of a degreasing reagent (a mixed solution containing hydrogen sodium oxide of 8 g/L, sodium silicate of 2 g/L, sodium carbonate of 1.5 g/L, and sodium citrate of 0.5 g/L) at 65° C., and washed with water, then with 10% H.sub.2SO.sub.4 for 10 min, and then with water until neutral.

[0074] (3) Low-current electrolysis: An appropriate amount of an electroplating solution having a composition as shown in Table 2 was added to an electroplating bath. Low-current electrolysis was carried out using a constant voltage DC power supply with a corrugated iron plate as the cathode and stainless steel as the anode. to allow the complexing agent and additives in the freshly prepared electroplating solution to reach an optimum state. During electrolysis, the temperature was controlled at 67° C., the cathode current density was 1 A/dm.sup.2, and the electrolytic treatment was continued for 12 h.

[0075] (4) Preparation of Ni—W—C alloy coating by electrodeposition: The substrate obtained in step (2) was electroplated in the electroplating solution treated in step (3). The specific electroplating conditions were: an anode of an iridium-tantalum alloy, a cathode of a pretreated carbon steel substrate, an electroplating solution temperature of 67° C., a cathode current density of 3 A/dm.sup.2, and an electroplating time of 50 min. A coating with a thickness of 15 μm was obtained, where the weight ratio of each element in the coating was about Ni:W:C=46.3:41.7:12.

[0076] (5) In-situ synthesis of Ni—W—WC composite coating: The coating obtained in step (4) was heated to 1000° C. at a ramping rate of 5° C./min and held for 2 h under an argon atmosphere, cooled down to about 200° C. with the furnace, removed from the furnace and air cooled.

Test Examples

[0077] The Ni—W—WC composite coatings synthesized in situ were tested and characterized as follows.

[0078] 1. Coating Characterization

[0079] The coating obtained in Example 2 was characterized. The results are shown in FIGS. 1-4.

[0080] FIG. 1 shows the surface morphology and the element distribution of the coating. It can be seen that the addition of the recarburizer has no obvious influence on the surface morphology and W content in the coating. but significantly changes the C content in the coating. In a coating without the recarburizer, the W content is 40.5 wt. %, and the C content is 2.7 wt. %; while the W content is 41.7 wt. %, and the C content is 12 wt. % in a coating with the addition of the recarburizer.

[0081] FIG. 2 shows that the carbon element in the obtained coating mainly exists in the form of C—C, indicating that there are a large amount of organic matters entrapped in the coating.

[0082] FIG. 3 shows the surface morphology of the Ni—W—C alloy coating after heat treatment at 800° C., in which some uniformly dispersed polygonal particles are in-situ generated tungsten carbide particles.

[0083] FIG. 4 shows the XRD analysis of the coating after heat treatment, in which peaks marked with triangle are the peaks of tungsten carbide. By analyzing the peak intensity, the content of tungsten carbide in the obtained coating is 3-7 wt. %; and the grain size of tungsten carbide calculated by Scherrer's formula is about 30-35 nm, which is consistent with the size of WC particles observed from the SEM image, suggesting that the Ni—W—WC composite coating is prepared in situ.

[0084] 2. Hardness Test

[0085] The coating obtained in Examples 1-4 and Comparative Examples 1-4 was tested for hardness. The test results are shown in Table 6.

TABLE-US-00006 TABLE 6 Heat treatment Heat Average recarburizer is Current density/ temperature/ treatment hardness/ No. added or not A .Math. dm.sup.−2 ° C. time/h WC/wt. % Hv Remark Example 1 Yes 2 700 3 3.42 1569 Example 2 Yes 3 800 2 5.43 1635 Example 3 Yes 4 900 2 4.73 1803 Example 4 Yes 5 900 2 6.81 1756 Comparative Not 3 800 2 — 873 Example 1 Comparative Yes 1.5 800 2 — 739 Example 2 Comparative Yes 3 600 2 — 921 Example 3 Comparative Yes 3 1000 2 5.12 — Uneven Example 4 hardness * * The distribution of elements in the coating after heat treatment at 1000° C. is uneven, and the hardness of each region varies greatly, suggesting that the heat treatment temperature is too high.

[0086] As can be seen from Table 6, the Ni—W—WC composite coating can be synthesized in situ by adding a recarburizer and heat treatment at a high temperature (>700° C.); and in contrast, no WC evolution is observed in the coating without the recarburizer, or with the recarburizer but deposited at low current density or with low heat treatment temperature (<600° C.), indicating that whether to add a recarburizer to the electroplating solution, as well as the current density and heat treatment temperature are essential conditions for the preparation process of synthesizing a Ni—W—WC composite coating in situ. Considering the influence of the electrodeposition conditions and the heat treatment temperature on the final performances, and for the sake of reduced energy consumption, and less operation time, in a preferred preparation process for in-situ synthesis of Ni—W—WC composite coating in the present invention, the electroplating solution has a composition of NiSO.sub.4.Math.6H.sub.2O 40 g/L, Na.sub.2WO.sub.4.Math.2H.sub.2O 50 g/L, Na.sub.3C.sub.6H.sub.5O.sub.7.Math.2H.sub.2O 45 g/L, C.sub.6H.sub.8O.sub.7 20 g/L, C.sub.6H.sub.17N.sub.3O.sub.7 5 g/L, 3-pyridinesulfonic acid 6 g/L, and X-114 6 mL/L; the electrodeposition are carried out at 3 A/cm.sup.2 and for 50 min; and the heat treatment temperature of the coating is 800° C., and the treatment time is 2 h.

[0087] Apparently, the above-described embodiments are merely examples provided for clarity of description, and are not intended to limit the implementations of the present invention. Other variations or changes can be made by those skilled in the art based on the above description. The embodiments are not exhaustive herein. Obvious variations or changes derived therefrom also fall within the protection scope of the present invention.