1 Introduction

An electroplated diamond wheel is a kind of diamond grinding wheel made of diamond wrapped on the base (steel or other materials) by metal electrodeposition method. It is widely used in the fields of mechanical electronics, glass, building materials, petroleum drilling.

With the development of economy and advancement of science and technology, the requirements of electroplated diamond wheels in different industries are basically the same, that is, high efficiency, long life, and high grinding precision. To ensure these features, the coated metal not only needs to have higher hardness and wear resistance but also needs to be evenly distributed in various parts of the matrix to avoid the peeling of the coated layer, which shortens the life of the tool.

The amount of feed is controlled at about 0.3mm in some special industries such as strong grinding of the magnetic material industry. The requirements of the bonding force between the coated metal and steel base are particularly harsh for the dry grinding of large feed quantity in the ceramic industry. In the manufacturing process of electroplated diamond wheels, most of the manufacturers only notice the type, hardness and wear resistance of the coating metal, and often ignore the problem of the bonding strength between the coated metal and the matrix. The plating peeling off is common in the actual use process. This paper analyzes the cause of this problem and discusses the solution.

2 Types of plating peeling off

Owing to the conditions such as grinding force, temperature rise, the impact of the workpiece, etc., it causes the diamond-containing metal plating layer separation from steel base during the use of the electroplated diamond grinding wheels. This is the peeling off of the plating layer. The peeling off of the plating layer is usually partial peeling off. In the actual use, there are 3 types of plating peeling off as follows:

• The plating is peeled off to the surface of the base, that is, the diamond-containing metal plating layer and the diamond-free metal bottom plating layer are simultaneously separated from the steel base.
• The layer is peeled off to the metal bottom plating layer, that is, the diamond-free metal bottom plating layer is not separated from the steel base, but the diamond-containing metal plating layer is peeled off from the metal bottom plating layer.
• Layer separation of metal in diamond-containing metal plating: During the use of diamond-containing metal plating, the plating metal in contact with the workpiece is not normally worn, but is abnormally sliced or powdered off, and diamond is not all fall off, but grains are partially peeled off. If not pay attention to this phenomenon, it causes the product life short and gives the impression of the plating metal with poor holding force or wear resistance.

3 The reasons for the fall off of the electroplated diamond wheel

There are many processes in the manufacturing electroplated diamond wheels. Any step is not carried out properly, the plating will fall off.

1. Effect of pre-plating treatment

The treatment process of the steel base entering the plating bath is called pre-plating treatment. Pre-plating treatment includes mechanical polishing, degreasing, etching and activation steps. The purpose of the pre-plating treatment is to remove burrs, oil stains, oxide film, rust and oxide coating on the surface of the base to expose the metal base for normal growth of the metal lattice, forming the intermolecular bonding force. If the pre-plating treatment is not good, there is a thin oil film and an oxide film on the base surface and the metal lattice of the base metal cannot be sufficiently exposed, which hinders the intermolecular bonding force between the plating metal and the base metal. It is only a mechanical mosaic effect to make the bonding force poor. So, poor pre-plating treatment is the main reason for plating peeling off.

2. Effect of the plating solution

The formula of the plating solution directly affects type, hardness and wear resistance of the plating metal. Different process parameters can control the coarseness of metal crystallization and density of plating metal and stress in the plating. For the production of electroplated diamond wheels, most of them use nickel or nickel-cobalt alloy. If the effect of plating solution impurities is not considered, the factors affecting the peeling of the plating are:

• The internal stress of the plating is generated during the electrodeposition process. The additives in the solution, as well as their decomposition products and hydroxides,  can increase the internal stress. This stress can cause lattice defects due to the effect of some deposition factors during the plating process. In particular, the action of certain metal ions and organic additives can significantly increase the internal stress of the plating. There are macroscopic stress and microscopic stress. The macroscopic stress shows metal slice will be single-sided electroplated, and the slice is affected by the internal stress of the plating to bending. Microscopic stress is expressed by improving the plating hardness. Macroscopic stress can cause pores, cracks, and peeling during the storage of the plating.

The electroplated nickel or nickel-cobalt alloys is composited of different plating solutions with different internal stresses. The higher the chloride content, the bigger the internal stress is. For the main salt is nickel sulfate of the plating solution,

The internal stress of the watt plating solution is smaller than that of other types of plating solutions. It can significantly reduce the macroscopic internal stress of the plating and increase its microscopic internal stress by adding organic brightener or stress reducer. Different process combinations such as current density, PH, and temperature can cause different internal stress in the plating of the same plating solution. Therefore, should strictly control the process range of the plating solution to reduce the effect of internal stress, ensure that the internal stress of the plating is within the range of process requirement.

• The effect of hydrogen evolution is in any plating solution, regardless of PH value, a certain amount of hydrogen ions always exist due to the dissociation of water molecules. Therefore, electroplating is done in acidic, neutral or alkaline electrolyte, and hydrogen is often precipitated on the cathode while being precipitated with the metal when the conditions are appropriate. After the hydrogen ions are reduced at the cathode, one part forms the hydrogen gas to escape, and another part is infiltrated into the base metal and the plating in the state of atomic hydrogen. It makes the lattice distorted, causing large internal stress and the plating deformation. At this time, although the defect was not seen from the appearance, its mechanical performance was unqualified. When the temperature of the medium rises, the adsorbed hydrogen accumulated in the base metal or the plating metal expands to cause bubbling and peeling during the use of the tools.

3. The effect of the plating process

If the composition of the plating solution and other process control effects are excluded, the power failure is an important cause of the plating peeling off during the plating process. The electroplating process of electroplated diamond wheels is quite different from other types of electroplating. The electroplating process of the electroplated diamond wheels includes vacuum plating, sanding, and thickening.

It is possible that the base leaves the plating solution in each process, that is, long or short power failure. For example, observe the quality of the bottom nickel and whether the diamond is evenly distributed on the base after a certain period of vacuum plating. If there are sand-planting and sand-unloading steps in the above process, then sand-unloading sometimes needs to leave the plating bath and in another bath. Observe whether the diamond coverage is in place during the thickening process.

A short time power failure has little effect on the plating. If the power-off time is too long, a thin oxide film will be formed on the surface of the plating metal in an instant, makes the subsequent electrodeposited metal atoms cannot grow along the original metal lattice, affecting the bonding of the two. When the external force of the tool is larger than that of the bonding force between the two layers during use, it is unavoidable for the separation of the layers between the two layers. Therefore, more reasonable technology and the process can reduce the plating peeling off.

     4  Measures to solve the plating peeling off

In view of the above reasons for the peeling of the plating, should take the following measures to solve the problem of peeling off the plating.

• Strengthen the pre-plating treatment, remove the burrs, oil stains, oxide film, rust and

Oxide coating on the surface of the base as much as possible, making the normal growth of the metal lattice of the plating, improving the bonding force between the plating metal and the base metal.

• Optimize the plating solution formulation and electroplating process, take the charging into the bath to avoid the bipolar. Take a short time high current impact vacuum plating for the workpiece with complex shape to reduce the effect of stress and hydrogen evolution in the plating and improve the plating quality.
• Optimize the technology and process and reduce the power-off time during sand unloading, and even unload the sand in the original sand bath, thicken it or sand unload in a spare bath with electricity, improving the bonding force between the diamond grains and the plating. If the power is off during the thickening process, the workpiece should be placed in the electrolyte for cathode reduction when re-thickening. After reduction, it is charged into the bath to ensure the bonding force of the plating.

5 Conclusions

In the production process of the electroplated diamond grinding wheel, in addition to considering the hardness and wear resistance of the plating, pay attention to the internal stress of the plating, qualitatively measure the internal stress of the plating and the effect of various additives on the internal stress when choosing the plating solution.

At the same time, pay full attention to the various steps of pre-plating treatment, production process control specifications in the production to ensure the plating solution is clean and the impurity content is within the process range. Only in this way, high quality and stable quality electroplated diamond wheels are produced.

Only pay attention to the pre-plating treatment and strictly control the production technology, it can effectively control the problem of the peeling of the plating of electroplated diamond wheels.