Claims:1. A coating solution comprising:
nickel (Ni) ion source at concentration ranging from about 30 g to about 50 g per litre of the coating solution;
reducing agent at concentration ranging from about 0.88g to about 88 g per litre of the coating solution;
complexing agent at concentration ranging from about 1g to about 150 g per litre of the coating solution;
and
optionally phosphorous (P) ion source and stabilizing agent;
and wherein said coating solution has pH of about 6 to about 7.5.

2. A method for obtaining the coating solution as claimed in claim 1, said method comprising acts of:
adding nickel ion source, reducing agent, complexing agent, optionally along with phosphorous ion source and mixing in water at temperature ranging from about 70°C to about 95°C to obtain coating solution; and
maintaining pH of the coating solution between 6 and 7.5 and optionally adding stabilizing agent to obtain the final coating solution.

3. The coating solution as claimed in claim 1 or the method as claimed in claim 2, wherein the Ni ion source is a nickel salt selected from group comprising nickel chloride, nickel sulfate, nickel formate, and nickel hypophosphite, or combinations thereof, preferably nickel sulfate.

4. The coating solution as claimed in claim 1 or the method as claimed in claim 2, wherein the P ion source is selected from hypophosphorous acid and hypophosphite; wherein the hypophosphite is selected from sodium hypophosphite and calcium hypophosphite, preferably sodium hypophosphite; and wherein the P ion source is present at concentration ranging from about 20 g to about 30 g per litre of the coating solution.

5. The coating solution as claimed in claim 1 or the method as claimed in claim 2, wherein the reducing agent is selected from group comprising hypophosphorous acid, hypophosphite, borohydride, dimethylamine borane, trimethylamine borane, hydrazine, thiosulfate and ascorbate, or combinations thereof; wherein the borohydride is sodium borate, the hypophosphite is selected from sodium hypophosphite and calcium hypophosphite, the thiosulfate is sodium thiosulfate and the ascorbate is sodium ascorbate; and wherein the reducing agent is preferably sodium hypophosphite.

6. The coating solution as claimed in claim 1 or the method as claimed in claim 2, wherein the complexing agent is selected from a group comprising ammonia and organic complex-forming agents containing functional groups selected from primary amino, secondary amino, tertiary amino, imino, carboxy and hydroxy, or combinations thereof.

7. The coating solution or the method as claimed in claim 6, wherein the complexing agent is selected from group comprising ethylenediamine, diethylene triamine, triethylene tetramine, triethylenetriamine, sodium citrate, sodium pyrophosphate, organic acid, water soluble salt of organic acid, and amino acid, or combinations thereof; wherein the water soluble salt of organic acid is sodium succinate; and wherein the organic acid is selected from group comprising malic acid, succinic acid, lactic acid, oxalic acid, citric acid, tartaric acid, ethylene diamine tetra acetic acid, glycolic acid, or combinations thereof, preferably glycolic acid.

8. The coating solution as claimed in claim 1 or the method as claimed in claim 2, wherein the stabilizing agent is selected from a group comprising ammonium acetate, thiourea, ammonium succinate and lead ion (Pb2+), or combinations thereof, preferably thiourea; and wherein the stabilizing agent is present at concentration ranging from about 0.05 mg to about 10 mg per litre of the coating solution, preferably at concentration ranging from about 0.5 ppm to about 1.5 ppm.

9. The coating solution as claimed in claim 1 or the method as claimed in claim 2, wherein the pH of the coating solution is maintained between 6 and 7.5 by adding pH stabilizing agent; wherein the pH stabilizing agent is alkali, selected from sodium hydroxide and potassium hydroxide; and wherein the pH stabilizing agent is present at concentration ranging from about 5 weight% to about 15 weight%.

10. A metallic glass coating comprising Nickel and Phosphorous, wherein the coating has thickness ranging from about 0.4 microns to about 1.0 microns.

11. The metallic glass coating as claimed in claim 10, wherein the coating comprises Ni at concentration ranging from about 88 % w/w to about 96 % w/w and P at concentration ranging from about 4 % w/w to about 12 % w/w; and wherein the coating has thickness ranging from about 0.4 microns to about 0.6 microns.

12. An article comprising the metallic glass coating as claimed in claim 10.

13. The article as claimed in claim 12, wherein the article is coated with the metallic glass coating on atleast one surface.

14. A method of coating article with the metallic glass coating as claimed in claim 10, said method comprising acts of:
contacting at least one surface of article with the coating solution as claimed in claim 1 and heating to a temperature ranging from about 70°C to about 95°C for time period ranging from about 0.5 minutes to about 1 minute, to obtain the coated article.

15. The method as claimed in claim 14, wherein the contacting is carried out by immersing the article in the coating solution.

16. The method as claimed in claim 14, wherein prior to contacting the article with the coating solution, the article is subjected to cleaning.

17. The method as claimed in claim 16, wherein the cleaning is carried out by treating the article with alkali at temperature ranging from about 55°C to about 65°C for time period of about 2 minutes to about 3 minutes.

18. The article as claimed in claim 12 or the method as claimed in claim 14, wherein the coating provides low co-efficient of friction, low surface roughness, high hardness, corrosion resistance and wear resistance to the article.

19. The article or the method as claimed in claim 18, wherein the co-efficient of friction ranges from about 0.3 to about 0.1, the surface roughness ranges from about 0.4 micron to about 0.2 micron; and the hardness ranges from about 65Rc to about 67Rc.

20. The article as claimed in claim 12 or the method as claimed in claim 14, wherein the article is composed of steel and ceramic, or a combination thereof.

21. The article or the method as claimed in claim 20, wherein the article is selected from group comprising bearing, piston, piston ring, piston shaft, crank shaft and valve.

22. The article or the method as claimed in claim 21, wherein the article is a bearing, preferably a ball-bearing.
, Description:TECHNICAL FIELD
The present disclosure relates to a metallic glass coating exhibiting high corrosion resistance and wear resistance. The coating of the present disclosure comprises Nickel and Phosphorous and is deposited on article(s) from a coating solution comprising Nickel and Phosphorous ions, metal complexing agent(s), reducing agent(s) and optional stabilizing agent(s). The present disclosure also relates to article(s) deposited with the metallic glass coating.

BACKGROUND OF THE DISCLOSURE
Electroless metal deposition solutions ("electroless plating solutions") deposit metal over a catalytically active surface by chemical reduction in the absence of an external electric circuit. Nickel containing solutions are prevalent in a wide range of industrial coating applications. A number of variants of Nickel based coatings such as Nickel-boron, nickel-cobalt and nickel-phosphorus coatings are recognized in the art for their corrosion resistance, hardness and associated wear-resistance. A typical electroless nickel plating solution generally comprises a water-soluble nickel salt, a water-soluble alloying salt (if an alloy is present), a reducing agent, and a chelating or complexing agent. Additives may also be added in relatively low concentrations to enhance various characteristics of the solution of plated article. Despite the number of inventions made in the domain, there is need to increase the stability of electroless nickel plating solutions. Additives, when added to the coating solutions, adversely affect Ni-coating formation at times. A few additives may even result in discoloration of the deposits. Hence, there is need of a coating solution that can be deposited electrolessly on metal surface such as bearings without causing any adverse effect on the metal surface. In general, the presently available Ni-P coating solutions function at acidic or alkaline pH and are crystalline in nature. Further, the presently available coatings have a high thickness of approximately 10-100 microns, for exhibiting corrosion resistance and wear resistance. Thus, there is a need for Ni-P coatings which are amorphous, function at neutral pH and which are ideal for applications requiring thickness of less than one micron, while still being corrosion and wear resistant.

STATEMENT OF THE DISCLOSURE
Accordingly, the present disclosure relates to coating solution comprising: nickel (Ni) ion source at concentration ranging from about 30 g to about 50 g per litre of the coating solution, reducing agent at concentration ranging from about 0.88g to about 88 g per litre of the coating solution, complexing agent at concentration ranging from about 1g to about 150 g per litre of the coating solution, and optionally phosphorous (P) ion source and stabilizing agent, and wherein said coating solution has pH of about 6 to about 7.5; a method for obtaining the coating solution as above, said method comprising acts of: adding nickel ion source, reducing agent, complexing agent, optionally alongwith phosphorous ion source and mixing in water at temperature ranging from about 70°C to about 95°C to obtain coating solution, and maintaining pH of the coating solution between 6 and 7.5 and optionally adding stabilizing agent to obtain the final coating solution; a metallic glass coating comprising Nickel and Phosphorous, wherein the coating has thickness ranging from about 0.4 microns to about 1.0 microns; an article comprising the metallic glass coating as above; and a method of coating article with the metallic glass coating as above, said method comprising acts of: contacting at least one surface of article with the coating solution as above and heating to a temperature ranging from about 70°C to about 95°C for time period ranging from about 0.5 minutes to about 1 minute, to obtain the coated article.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
In order that the disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with a detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure where:

Figure 1 depicts the different parts of bearing used for coating development, according to the present disclosure.

Figure 2 is a pictorial view depicting the process of coating, according to the present disclosure.

Figure 3 is a pictorial view depicting the different coated parts of the bearing.

Figures 4(a) and 4(b) depict the depth image of the Ni-P coating deposited on the bearing surface by electroless process, according to the present disclosure.

Figure 5 depicts the XRD peaks of the coated steel substrate of bearing.

Figure 6 is a pictorial view depicting oil and Ni-P coated bearings after salt spray exposure for about 96 hours.

DETAILED DESCRIPTION OF THE DISCLOSURE
The present disclosure relates to a coating solution comprising:
nickel (Ni) ion source at concentration ranging from about 30 g to about 50 g per litre of the coating solution;
reducing agent at concentration ranging from about 0.88g to about 88 g per litre of the coating solution;
complexing agent at concentration ranging from about 1g to about 150 g per litre of the coating solution; and
optionally phosphorous (P) ion source and stabilizing agent;
and wherein said coating solution has pH of about 6 to about 7.5.

The present disclosure also relates to a method for obtaining the coating solution as above, said method comprising acts of:
adding nickel ion source, reducing agent, complexing agent, optionally along with phosphorous ion source and mixing in water at temperature ranging from about 70°C to about 95°C to obtain coating solution; and
maintaining pH of the coating solution between 6 and 7.5 and optionally adding stabilizing agent to obtain the final coating solution.
In an embodiment of the present disclosure, the Ni ion source is a nickel salt selected from group comprising nickel chloride, nickel sulfate, nickel formate, and nickel hypophosphite, or combinations thereof, preferably nickel sulfate.

In another embodiment of the present disclosure, the P ion source is selected from hypophosphorous acid and hypophosphite; wherein the hypophosphite is selected from sodium hypophosphite and calcium hypophosphite, preferably sodium hypophosphite; and wherein the P ion source is present at concentration ranging from about 20 g to about 30 g per litre of the coating solution.

In yet another embodiment of the present disclosure, the reducing agent is selected from group comprising hypophosphorous acid, hypophosphite, borohydride, dimethylamine borane, trimethylamine borane, hydrazine, thiosulfate and ascorbate, or combinations thereof; wherein the borohydride is sodium borate, the hypophosphite is selected from sodium hypophosphite and calcium hypophosphite, the thiosulfate is sodium thiosulfate and the ascorbate is sodium ascorbate; and wherein the reducing agent is preferably sodium hypophosphite.

In yet another embodiment of the present disclosure, the complexing agent is selected from a group comprising ammonia and organic complex-forming agents containing functional groups selected from primary amino, secondary amino, tertiary amino, imino, carboxy and hydroxy, or combinations thereof.

In yet another embodiment of the present disclosure, the complexing agent is selected from group comprising ethylenediamine, diethylene triamine, triethylene tetramine, triethylenetriamine, sodium citrate, sodium pyrophosphate, organic acid, water soluble salt of organic acid, and amino acid, or combinations thereof; wherein the water soluble salt of organic acid is sodium succinate; and wherein the organic acid is selected from group comprising malic acid, succinic acid, lactic acid, oxalic acid, citric acid, tartaric acid, ethylene diamine tetra acetic acid, glycolic acid, or combinations thereof, preferably glycolic acid.

In yet another embodiment of the present disclosure, the stabilizing agent is selected from a group comprising ammonium acetate, thiourea, ammonium succinate and lead ion (Pb2+), or combinations thereof, preferably thiourea; and wherein the stabilizing agent is present at concentration ranging from about 0.05 mg to about 10 mg per litre of the coating solution, preferably at concentration ranging from about 0.5 ppm to about 1.5 ppm.

In still another embodiment of the present disclosure, the pH of the coating solution is maintained between 6 and 7.5 by adding pH stabilizing agent; wherein the pH stabilizing agent is alkali, selected from sodium hydroxide and potassium hydroxide; and wherein the pH stabilizing agent is present at concentration ranging from about 5 weight% to about 15 weight%.

The present disclosure also relates to a metallic glass coating comprising Nickel and Phosphorous, wherein the coating has thickness ranging from about 0.4 microns to about 1.0 microns.

In an embodiment of the present disclosure, the coating comprises Ni at concentration ranging from about 88 % w/w to about 96 % w/w and P at concentration ranging from about 4 % w/w to about 12 % w/w; and wherein the coating has thickness ranging from about 0.4 microns to about 0.6 microns.

The present disclosure also relates to an article comprising the metallic glass coating as above.

In an embodiment of the present disclosure, the article is coated with the metallic glass coating on at least one surface.

The present disclosure further relates to a method of coating article with the metallic glass coating as above, said method comprising acts of:
contacting at least one surface of article with the coating solution as above and heating to a temperature ranging from about 70°C to about 95°C for time period ranging from about 0.5 minutes to about 1 minute, to obtain the coated article.

In an embodiment of the present disclosure, the contacting is carried out by immersing the article in the coating solution.

In another embodiment of the present disclosure, prior to contacting the article with the coating solution, the article is subjected to cleaning.

In yet another embodiment of the present disclosure, the cleaning is carried out by treating the article with alkali at temperature ranging from about 55°C to about 65°C for time period of about 2 minutes to about 3 minutes.

In yet another embodiment of the present disclosure, the coating provides low co-efficient of friction, low surface roughness, high hardness, corrosion resistance and wear resistance to the article.

In yet another embodiment of the present disclosure, the co-efficient of friction ranges from about 0.3 to about 0.1, the surface roughness ranges from about 0.4 micron to about 0.2 micron; and the hardness ranges from about 65Rc to about 67Rc.

In yet another embodiment of the present disclosure, the article is composed of steel and ceramic, or combination thereof.

In yet another embodiment of the present disclosure, the article is selected from group comprising bearing, piston, piston ring, piston shaft, crank shaft and valve.

In still another embodiment of the present disclosure, the article is a bearing, preferably a ball-bearing.

The present disclosure relates to a metallic glass coating, particularly, a Nickel-Phosphorus (Ni-P) composite coating for deposition/coating of surface of articles/substrates. The metallic glass coating deposited on the article surface exhibits low coefficient of friction, low surface roughness, high hardness, corrosion resistance and wear resistance. Particularly, the coating provides caustic and chloride resistance to the coated article.

In an embodiment of the present disclosure, the metallic glass coating is amorphous in nature and comprises about 88 weight% to about 96 weight% Nickel and about 4 weight% to about 12 weight% Phosphorus. Nickel is the metallic component in the coating and provides barrier protection, while Phosphorous helps in corrosion resistance and for lubrication. Phosphorous also reduces co-efficient of friction.

Deposition of the metallic glass coating on articles/substrates is accomplished by contacting said articles/substrates with a coating solution/coating bath. The metallic glass coating of the present disclosure has a thickness ranging from about 0.4 microns to about 0.1 microns, preferably ranging from about 0.4 microns to about 0.6 microns.

The present disclosure also relates to coating solution/coating bath for providing the metallic glass coating on an article/substrate.

In an embodiment of the present disclosure, the coating solution comprises Nickel ions, Phosphorous ions, reducing agent, metal complexing agent and optionally stabilizing agent in effective amounts and is maintained at a neutral pH of between 6 to 7.5, preferably 7.

In an embodiment of the present disclosure, the neutral pH of the coating solution is maintained by means of an alkali, including but not limited to, sodium hydroxide and potassium hydroxide. Thus, in an embodiment, the coating solution also comprises a pH stabilizing agent such as alkali, such as but not limiting to sodium hydroxide and potassium hydroxide, at concentration ranging from about 5 weight % to about 15 weight %.

In an embodiment of the present disclosure, the neutral pH of the coating solution nullifies hydrogen embrittlement problem, which commonly occurs due to the evolution of hydrogen gas at acidic pH conditions. This is an advantage of maintaining the coating solution of the present disclosure at neutral pH conditions. Further, the neutral pH condition also ensures the right amount of phosphorous in the coating, which in turn results in the amorphous nature of the coating.

In another embodiment of the present disclosure, nickel ions in the coating solution are present in the form of a water-soluble nickel salt selected from the group comprising nickel chloride, nickel sulfate, nickel formate, and nickel hypophosphite, or combinations thereof. In a preferable, non-limiting embodiment, the nickel salt is nickel sulfate. Further, the nickel salt is present at a concentration ranging from about 30 g to about 50 g per litre of the coating solution.

In another embodiment of the present disclosure, source of phosphorus ions in the coating solution is selected from hypophosphorous acid and hypophosphite such as, but not limited to sodium hypophosphite and calcium hypophosphite, or combinations thereof. Further, the phosphorus ions are present at a concentration ranging from about 20g to about 30g per litre of the coating solution.

In another embodiment of the present disclosure, the reducing agent is selected from the group comprising hypophosphorous acid, hypophosphites such as but not limited to sodium hypophosphite and calcium hypophosphite, borohydrides such as but not limited to sodium borate, dimethylamine borane, trimethylamine borane, hydrazine, thiosulfates such as but not limited to sodium thiosulfate and ascorbates such as but not limited to sodium ascorbate or combinations thereof. In a preferred non-limiting embodiment, the reducing agent is sodium hypophosphite. Further, the reducing agent is present at a concentration ranging from about 0.88g to 88 g per litre of the coating solution.

In certain non-limiting embodiments of the present disclosure, the reducing agent also functions as a source of phosphorous ions in the coating solution. For example, reducing agents such as hypophosphorous acid and hypophosphites also provide phosphorus ions in the coating solution, thereby eliminating the need for addition of separate phosphorous ion source in the coating solution.

In another embodiment of the present disclosure, the stabilizing agent is selected from the group comprising ammonium acetate, thiourea, ammonium succinate and lead ions (Pb2+), or combinations thereof. In a preferred non-limiting embodiment, the stabilizing agent is thiourea. Further, the stabilizing agent is present at a concentration ranging from about 0.05 mg to about 10 mg per litre of the coating solution, preferably from about 0.5 ppm to about 1.5 ppm. The concentration of the stabilizing agent should be optimum. While the presence of stabilizing agent up to certain ppm, hastens the deposition reaction of the coating solution, higher concentrations would result in the stabilizing agent acting negatively and even stopping the deposition of Nickel completely.

In another embodiment of the present disclosure, the metal ion complexing agent is selected from the group comprising ammonia and organic complex-forming agents comprising functional groups such as, but not limited to, primary amino, secondary amino, tertiary amino, imino, carboxy and hydroxy.

Particularly, the complexing agent is selected from the group comprising ethylenediamine, diethylene triamine, triethylene tetramine, triethylenetriamine, sodium citrate, sodium pyrophosphate, organic acids, water soluble salts of organic acids, and amino acids. The organic acids are selected from the group comprising malic acid, succinic acid, lactic acid, oxalic acid, citric acid, tartaric acid, ethylene diamine tetra acetic acid and glycolic acid. In a preferred, non-limiting embodiment, the complexing agent is glycolic acid.

In a non-limiting embodiment of the present disclosure, the water soluble salt of organic acid is sodium succinate and the amino acid may be any amino acid.

In a particular non-limiting embodiment of the present disclosure, the amino acid includes but is not limited to glutamic acid, leucine and alanine.
In another embodiment, the complexing agent is present at a concentration sufficient to inhibit the precipitation of Ni ions from the coating solution, i.e., to ensure limited availability of Ni ions in the solution for controlled reduction. The concentration of metal complexing agent ranges from about 1g to about 150 g per litre of the coating solution.

Although generally, longer O-OH chain is responsible for formation of nickel ion complex, it is observed in the present disclosure that compounds with shorter O-OH chain, such as glycolic acid, also function at neutral pH to enable speedy coating process, with less phosphorus in the coating, which is responsible for the amorphous nature of the coating.

In an embodiment of the present disclosure, the coating solution is prepared by mixing the Ni ion source and P ion source, and adding the reducing agent, complexing agent, and optionally stabilizing agent, at concentrations hereinbefore disclosed. The remainder of the solution is made up with water. The solution is heated to a temperature of about 70°C to about 95°C and the pH of the solution is maintained between 6 and 7.5.

In embodiments where the reducing agent also functions as the P ion source, there is no requirement of addition of a separate P ion source.

In an embodiment of the present disclosure, the pH of the coating solution is maintained by means of a pH stabilizing agent such as an alkali, particularly, sodium hydroxide or potassium hydroxide. In a more preferred embodiment, the pH of the coating solution is maintained by adding diluted sodium hydroxide solution or diluted potassium hydroxide solution.
The present disclosure also relates to a method of coating an article/substrate with metallic glass coating comprising Nickel and Phosphorous, which includes acts of cleaning the article/substrate and contacting the required surface(s) of the cleaned article/substrate with the coating solution, in order to obtain the coated article.
In an embodiment, the article/substrate is contacted with the coating solution by means of dipping/immersing in the coating solution. Further, the coating of the article/substrate takes place by electroless process.
The cleaning of the article/substrate is carried out by alkali cleaning process to remove oil and dirt from the surface, followed by rinsing in water. The cleaned article/substrate is then immersed in coating bath having temperature ranging from about 70°C to about 95°C to initiate the coating process. The process is continued until deposition of the coating has progressed to the desired thickness of about 0.4 microns to about 1 micron, preferably about 0.4 microns to about 0.6 microns which takes about 0.5 minutes to about 1 minute. The coating thickness is directly proportional to coating time. After electroless coating, the article/substrate is rinsed in tap water and subsequently dried in open atmosphere.
Deposition rate is sensitive to concentration of the different metallic and non-metallic ingredients in the solution, pH and temperature of the solution. Further, temperature of the coating bath plays a significant role in optimum deposition. The temperature of bath is preferably maintained at temperature ranging from about 80°C to about 90°C for optimum deposition. At temperatures above 950 C, Nickel gets dissolved out from the surface.
The method of the present disclosure enables uniform coating on article surface and also enables coating on one or more surfaces of same or different shapes of articles simultaneously.

In an embodiment of the present disclosure, the article/substrate may be any article/substrate having surfaces composed of steel and ceramic, or a combination thereof. Further, the article/substrate may be selected from group comprising bearings, piston, piston ring, piston shaft, crank shaft and valve.

In a preferred non-limiting embodiment of the present disclosure, the article is a bearing, particularly, a ball-bearing. In a preferred, non-limiting embodiment of the present disclosure, the article is a ball-bearing made of steel. In another preferred non-limiting embodiment of the present disclosure, the article is inner and/or outer races of ball bearing.

The present disclosure also relates to article/substrate coated on at least a portion of its surface with a hard, wear and corrosion resistant metallic glass coating comprising nickel and phosphorus.

In an embodiment of the present disclosure, the metallic glass coating comprises about 88 weight% to about 96 weight % Nickel and about 4 weight% to about 12 weight% Phosphorus, and is amorphous in nature. Additionally, the coating has a thickness of about 0.4 microns to about 1 micron, preferably from about 0.4 microns to about 0.6 microns.

In an embodiment of the present disclosure, the article/substrate may be any article/substrate having surfaces composed of steel and ceramic, or a combination thereof. Further, the article/substrate may be selected from group comprising bearings, piston, piston ring, piston shaft, crank shaft and valve.

In a preferred non-limiting embodiment of the present disclosure, the article is a bearing, particularly, a ball-bearing. In a preferred, non-limiting embodiment of the present disclosure, the article is a ball-bearing made of steel. In another preferred non-limiting embodiment of the present disclosure, the article is inner and/or outer races of ball bearing.

The present disclosure further relates to use of the metallic glass coating of the present disclosure for providing low surface roughness, low coefficient of friction, high hardness, corrosion resistance and wear resistance to articles.

In an embodiment, the metallic glass coating of the present disclosure provides surface roughness ranging from about 0.4 microns to about 0.2 microns, coefficient of friction ranging from about 0.3 to about 0.1 and high hardness ranging from about 65 Rc to about 67Rc.

In an embodiment, the present disclosure relates to use of the metallic glass coating for providing caustic and chloride resistance to articles.

Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in the art based upon the description provided herein. The embodiments herein provide various features and advantageous details thereof in the description. Descriptions of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the invention herein provides for examples illustrating the above described embodiments, and in order to illustrate the embodiments of the present invention, certain aspects have been employed. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein.

EXAMPLES
WORKING EXAMPLE 1- Preparation of Coating Solution
About 40 g of nickel sulfate is first taken in a borosil beaker and about 20 g of sodium hypophosphite is added to the beaker and finally about 25 ml of glycolic acid is added before addition of about 800 ml of distilled water to mix all the components in the water medium. After addition of distilled water, the beaker is put on a stirrer to get proper mixing of all chemicals added and temperature is maintained at about 85°C. The pH of the solution is adjusted between 6 and 7.5 by addition of about 10 ml of diluted sodium hydroxide solution after ensuring complete mixing of all the components to obtain about 1 litre of the coating solution.

COMPARATIVE EXAMPLE 1
About 30 g of nickel chloride and about 10g of nickel sulfate are first taken in a borosil beaker and about 20 g of sodium hypophosphite is added to the beaker and finally about 20 g of sodium citrate is added before addition of about 800 ml of distilled water to mix all the components in the water medium. After addition of distilled water, the beaker is put on a stirrer to get proper mixing of all chemicals added and temperature is maintained at about 85°C, to obtain about 1 litre of the coating solution. The pH of the solution is found to be between 3 and 5.

COMPARATIVE EXAMPLE 2
About 5 g of nickel chloride and about 35 g of nickel sulfate is first taken in a borosil beaker and about 20 ml of orthophosphorous acid is added to the beaker and finally about 20 g of sodium citrate is added before addition of about 800 ml of distilled water to mix all the components in the water medium. After addition of distilled water, the beaker is put on a stirrer to get proper mixing of all chemicals added and temperature is maintained at about 85°C, to obtain about 1 litre of the coating solution. The pH of the solution is found to be between 3 and 5.

EXAMPLE 2-Coating of ball-bearings with Ni-P coating of the present disclosure
The different parts of bearing used for coating development is shown in Fig. 1.

Initially, process parameters are optimized to remove oil and dirt from the steel surface of the bearings. Henkel make alkaline powder is used for the cleaning purpose. Distilled water is used to make the solution. Specifically, about 2.5g of Ridoline is mixed in 100ml of distilled water to form the cleaning solution. Alkali cleaning process parameters are as mentioned in Table 1 below:
Alkali chemicals Alkali make Weight % Temperature Dipping time
Ridoline Henkel 2.5 55-650 C 2-3 minutes
TABLE 1: Alkali Cleaning Parameters
After alkali cleaning, rinsing is done thoroughly in tap water before dipping the ball bearings in the electroless coating solution prepared as per Working Example 1. Figure 2 shows the pictorial view of the coating process. The ball bearings are immersed in the coating bath having temperature ranging from about 70°C to about 95°C to initiate the coating process.

Table 2 below shows the temperature and time period for which the coating solution is immersed in the coating solution:

Sl. No. Temperature Time period
1 85°C 0.5 minutes
2 85°C 0.75 minutes
3 85°C 1 minute
TABLE 2: Temperature and time period for coating

Cathodic and anodic reactions occur simultaneously on the steel substrate of the ball bearing as mentioned in equations (i & ii).

After electroless coating, the samples are rinsed in tap water and subsequently dried in open atmosphere. The surfaces of different coated parts of the bearing appear brighter as shown in Fig. 3.

The different parts are cut into small pieces for coating development and subsequent characterization. The depth image of the coating and composition of it throughout the depth is shown in Fig. 4(a) and Table 3 below, respectively.
pts Ni P Fe
1 91.59 8.41
2 91.46 8.54
3 92.03 7.97
4 93.84 6.16
5 96.42 3.58
6 96.75 3.25
7 97.8

TABLE 3: Composition of Coating through the depth of the bearing

As can be observed, concentration of Ni decreases from steel surface to outer coating layer whereas concentration of P increases from steel surface to outer coating layer. This type of coating attribute is an added advantage from point of application aspect. The drop of Ni deposition rate with lapse of time can be explained by the increase in H+ ions in the solution. The reduction current for Ni2+ is dropped as the reduction current for H+ is increased.

Further the SEM depth image of the coating formed under the process parameters of Table 2, Sl. No. 2 as shown in Fig. 4(b) shows the coating having thickness of about 1 micron.

Additionally, the coating is found to contain about 88 weight% to about 96 weight % Nickel and about 4 weight% to about 12 weight% Phosphorus.

Further, XRD analysis of the coated steel substrate of bearing formed under the process parameters of Table 2, Sl. No. 2 is carried out and as can be observed from Figure 5, the coating on the steel substrate is found to be amorphous in nature.
Similarly, bearings are immersed in the coating solutions prepared in Comparative Examples 1 and 2, and the temperatures and time periods maintained for the coating process is outlined in Table 4 below:

Sl. No. Comparative Example Temperature Time period
1 1 85°C 10 minutes
2 2 85°C 15 minutes
TABLE 4: Temperature and time period for coating

Upon XRD analysis of the coated steel substrate of bearings prepared by immersing in the coating baths of Comparative Examples 1 and 2, it is found that the coating is crystalline in nature.

EXAMPLE 3- Testing Surface Roughness, Co-Efficient of Friction and Hardness of Coated Surface
Surface roughness and co-efficient of friction are measured using Olympus Laser Confocal Microscopy (OLS) and IMADA respectively, for the coated bearings prepared as per Example 2, Table 2, Sl. No. 2 and compared with uncoated samples. Results are tabulated in Table 5 below:
Material Surface roughness Co-efficient of friction
Bare steel 0.6 µm 0.5
Coated steel 0.27 µm 0.18

TABLE 5: Comparison of Surface Roughness and Co-efficient of Friction of coated and uncoated bearings
Surface roughness must be in the range of about 0.4 microns to about 0.2 microns. Further, the co-efficient of friction must be in the range of about 0.3 to about 0.1.

From the data in Table 5, it is evident that after coating, surface roughness and co-efficient of friction drop down significantly, and are within the acceptable range.

Further, bearings are immersed in coating solution prepared as per Working Example 1 at temperature of about 85°C and at different time periods of 0.5 minutes, 0.75 minutes and 1 minute, and the surface roughness, co-efficient of friction and hardness of the resultant coated article is calculated for each time period. Further, the experiment is repeated thrice for each time period and the values are provided in Table 6 below:

Time Period Surface Roughness (µm) Coefficient of Friction Hardness (Rc)
Trial 1 Trial 2 Trial 3 Trial 1 Trial 2 Trial 3 Trial 1 Trial 2 Trial 3
0.5 minutes 0.40 0.43 0.36 0.30 0.31 0.28 66 65 65
0.75 minutes 0.32 0.28 0.27 0.18 0.19 0.21 66 65 66
1 minute 0.22 0.20 0.21 0.10 0.11 0.12 67 67 66
TABLE 6: Surface Roughness, Coefficient of Friction and Hardness of Coated Bearing Formed at Different Time Periods

From the above table, it is clear that the coated bearings of the present disclosure exhibit surface roughness, coefficient of friction and hardness, well within the acceptable range.

EXAMPLE 4-Testing Corrosion Resistance of the Coated Bearings
Material loss of the coated bearings prepared as per Example 2, Table 2, Sl. Nos. 1, 2 and 3 on subjecting to chloride attack is determined and compared with the material loss of uncoated bearings. The data is tabulated in Table 7 below:
Material Material loss
Bare steel 20.8 mpy
Coated steel (Example 2, Table 2, Sl. No. 2) 5.1 mpy
Coated steel (Example 2, Table 2, Sl. No. 1) 8 mpy
Coated steel (Example 2, Table 2, Sl. No. 3) 5 mpy

Table 7: Material loss of Coated and Uncoated Bearings

As can be observed, after coating, material loss against aggressive corrosive environment drops down significantly.

Further, the coated bearing of the present disclosure as well as conventional oil coated bearing are exposed to salt spray for about 96 hours, using standard protocol ASTM B117. As can be observed from Figure 6, the coated bearing of the present disclosure shows much better resistance against chloride attack compared to conventional oil coated bearing, since no red rust is observed up to 200 hours in case of the coated bearing of the present disclosure.

Table 8 below shows the salt spray test (SST) life of the various bearings prepared using different coating baths and at different coating conditions:

Bearing SST Life
Working Example 1, Table 2, Sl. No. 1 100 hours
Working Example 1, Table 2, Sl. No. 2 150 hours
Working Example 1, Table 2, Sl. No. 3 200 hours
Comparative Example 1, Table 4, Sl. No. 1 100 hours
Comparative Example 2, Table 4, Sl. No. 2 100 hours
TABLE 8: SST Life of Coated Bearings

From the above data, it is evident that the Ni-P coated bearing of the present disclosure shows significant improvement in corrosion resistance against chloride attack. Particularly, even short coating time is able to provide coated bearings showing high corrosion resistance, while bearings subjected to longer duration of coating in other coating solutions show comparatively less corrosion resistance.

EXAMPLE 5- Rig Test Performance for Coated Bearing
Rig test ensures more than 50% improvement in fatigue life of coated bearing under cyclic load condition. Fatigue life of bench mark carbonitride treated bearing is maximum of 800 hours. The Ni-P coated bearing of the present disclosure has crossed 1250 hours without any trouble, thus showing that the coating of the present disclosure exhibit excellent wear resistance capability.

Table 9 below shows the rig test life of the bearings prepared using the coating bath of the present disclosure:

Bearing Rig Test Life
Working Example 1, Table 2, Sl. No. 1 1000 hours
Working Example 1, Table 2, Sl. No. 2 1300 hours
Working Example 1, Table 2, Sl. No. 3 1200 hours
TABLE 9: Rig Test Life of Coated Bearings

Additional embodiments and features of the present invention will be apparent to one of ordinary skill in art based on the description provided herein. The embodiments herein provide various features and advantageous details thereof in the description. Descriptions of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments herein.

The foregoing description of the specific embodiments fully reveals the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications of such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments in this invention have been described in terms of preferred embodiments, those of skill in the art will recognize that the embodiments herein can be practiced with modifications within the spirit and scope of the embodiments described herein.

Throughout the specification, the word “comprise”, or variations such as “comprises” or “comprising” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

Any discussion of documents, acts, materials, devices, articles and the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art and that the scope of the present invention is accordingly limited by nothing other than the appended claims.