Diamond powder chemical nickel plating

With the transformation of manufacturing industry to high-end, the rapid development of clean energy, and the development of semiconductor and photovoltaic industries, the demand for diamond tools with high efficiency and high precision processing capabilities is growing. However, artificial diamond powder, as the most important raw material for diamond tools, has problems such as weak holding force with the matrix and easy premature carbonization, resulting in a short tool life. To solve these problems, the industry generally adopts the technical means of coating metal materials on the surface of diamond powder to improve its surface properties, enhance durability, and thus improve the overall quality of the tool.

 

Diamond powder surface is coated with metal materials to improve its surface properties, enhance durability, and thus improve the overall quality of the tool. Diamond powder chemical plating is a commonly used method.

 

At present, there are many methods for diamond powder surface coating, including chemical plating, electroplating, magnetron sputtering plating, vacuum micro-evaporation plating, thermal explosion reaction, etc. Among them, chemical plating and electroplating have become the two most commonly used technical means in the industry due to their mature process, uniform coating, precise control of coating composition and thickness, and customized coating.

 

 

1. Chemical plating

Diamond powder chemical plating is to put the treated diamond powder into the chemical plating solution, and the metal ions in the plating solution are catalytically reduced and deposited on the diamond surface through the action of the reducing agent in the chemical plating solution to form a dense metal coating. At present, the most widely used diamond chemical plating is chemical nickel-phosphorus (Ni-P) binary alloy, usually called chemical nickel plating.

 

01 Chemical nickel plating solution composition

The composition of the chemical plating solution has a decisive influence on the smooth progress of its chemical reaction, stability and coating quality. It usually contains multiple components such as main salt, reducing agent, complexing agent, buffer, stabilizer, accelerator, surfactant, etc. The proportion of each component needs to be carefully adjusted to achieve the best coating effect.

 

• 1. Main salt: usually nickel sulfate, nickel chloride, nickel aminosulfonate, nickel carbonate, etc., its main function is to provide nickel source.
• 2. Reducing agent: mainly provides atomic hydrogen, reduces Ni2+ in the plating solution to Ni and deposits it on the surface of diamond particles. It is the main component in the plating solution. In industry, sodium hypophosphite with strong reducing ability, low cost and good plating solution stability is mainly used as a reducing agent. This reduction system can achieve chemical plating at low and high temperatures.
• 3. Complexing agent: can precipitate the plating solution, enhance the stability of the plating solution, extend the service life of the plating solution, increase the deposition rate of nickel, and improve the quality of the plating. Generally, organic acids such as succinic acid, citric acid, lactic acid and their salts are used.
• 4. Other ingredients: Stabilizers can inhibit the decomposition of the plating solution, but because they will affect the occurrence of chemical plating reactions, they need to be used in moderation; buffers can generate H+ during the chemical nickel plating reaction to ensure the continued stability of the pH value; surfactants can reduce the porosity of the coating.

 

 

 

02 Chemical nickel plating process

Chemical plating of sodium hypophosphite system requires that the substrate must have a certain catalytic activity, while the diamond surface itself does not have catalytic active centers. Therefore, diamond micropowder needs to be pretreated before chemical plating. The traditional pretreatment methods for chemical plating are degreasing, roughening, sensitization, and activation.

(1) Degreasing and roughening: Degreasing is mainly to remove grease, stains and other organic pollutants on the surface of diamond micropowder to ensure the tight fit and good performance of the subsequent coating. Roughening can form some tiny pits and cracks on the diamond surface, increase the surface roughness of the diamond, which is conducive to the adsorption of metal ions at this location, facilitating the subsequent chemical plating and electroplating, and can also form steps on the diamond surface, providing favorable conditions for the growth of chemically plated or electroplated metal deposition layers.

 

Usually, the degreasing step usually uses alkaline solutions such as NaOH as the degreasing solution, and the roughening step uses acidic solutions such as nitric acid as the roughening solution to etch the diamond surface. In addition, both of these steps should be used in conjunction with an ultrasonic cleaning machine, which is beneficial to improving the efficiency of degreasing and roughening of diamond micropowder, saving time in the degreasing and roughening process, and ensuring the effect of degreasing and roughening.

 

(2) Sensitization and activation: The sensitization and activation process is the most critical step in the entire chemical plating process, which is directly related to whether chemical plating can be carried out. Among them, sensitization is to adsorb easily oxidizable substances on the surface of diamond micropowder that does not have self-catalytic ability. And activation is to adsorb metal ions (such as metal palladium) that have catalytic activity for the oxidation of hypophosphorous acid and the reduction of nickel particles on the surface of diamond micropowder particles to accelerate the deposition rate of the coating on the surface of diamond micropowder.

 

Generally speaking, if the sensitization and activation treatment time is too short, the metal palladium particles on the diamond surface will be relatively small, the adsorption force on the coating will be insufficient, and the formed coating will easily fall off or it will be difficult to form a complete coating. If the treatment time is too long, it will cause waste of palladium particles. Therefore, the optimal time for sensitization and activation treatment is 20 to 30 minutes.

 

(3) Chemical nickel plating: The chemical nickel plating process is not only affected by the composition of the plating solution, but also by the temperature and pH value of the plating solution. Traditional high-temperature chemical nickel plating generally has a temperature of 80-85°C. Above 85°C, the plating solution is prone to decomposition. When the temperature is below 85°C, the higher the temperature, the faster the reaction rate. In terms of pH value, the deposition rate of the coating increases with the increase of pH value, but the increase of pH value will also cause the formation of nickel salt precipitates to inhibit the chemical reaction rate. Therefore, in the process of chemical nickel plating, it is necessary to optimize the composition and ratio of the chemical plating solution and the chemical plating process conditions, and control the method of improving the deposition rate, density, corrosion resistance and density of the chemical plating layer, so as to plate diamond micropowder that meets the needs of industrial development.

 

 

In addition, a single plating may not achieve the ideal coating thickness, and defects such as bubbles and pinholes may occur. Therefore, multiple plating can be adopted to improve the coating quality and increase the dispersibility of the plated diamond powder.

 

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