Reasons for plating peeling off of electroplated diamond tools

1、Preface

Electroplated diamond tools refers to the method of metal electrodeposition so that the diamond is firmly wrapped by the carcass metal in the substrate (steel or other materials) made of a diamond tool which is widely used in machinery and electronics, glass, building materials, oil drilling and other industries. With the economic development of science and technology progress of different industries on the electroplated diamond tool requirements are basically the same, namely, high efficiency, long life, high precision grinding. To ensure that these characteristics of the plated metal should not only have a high hardness, wear resistance and the requirements of the various parts of the matrix to be uniformly distributed to avoid plating off to shorten the life of the tool. In some special industries, such as magnetic materials industry, the amount of powerful grinding feed is controlled at about 0.3mm; ceramic industry, the amount of large feed dry grinding, etc. on the plated metal and steel substrate bonding force requirements are particularly harsh. In the production process of electroplated diamond tools, most manufacturers only pay attention to the type of plated metal, hardness, wear resistance and often ignore the problem of the bond between the plated metal and the substrate. In the actual use of the phenomenon of plating off is not uncommon. This paper analyzes the causes of this problem and discusses the solution measures.

2、Types of plating shedding

Electroplated diamond tools in the use of the process due to the use of conditions such as the size of the grinding force, temperature rise, the impact of the workpiece and other reasons will cause containing diamond metal plating and steel substrate separation of the phenomenon that is plating off. This is the phenomenon of plating shedding. Plating shedding is generally localized, and the phenomenon of one-time total stripping of the plating is rare. In the actual use of the plating off in the process of the following three kinds of situations:

(1) The plating layer falls off to the surface of the substrate: that is, the diamond-containing metal plating layer and the non-diamond-containing metal bottom plating layer are separated from the steel substrate at the same time.

(2) The layer falls off to the metal undercoat: that is, the metal undercoat without diamond is not separated from the steel substrate, but the metal undercoat with diamond is peeled off from the metal undercoat.

(3) Diamond-containing metal plating layer in the coating metal layer separation: diamond-containing metal plating layer in the use of the process of contact with the workpiece part of the coating metal is not the normal wear and tear, but non-normal into a piece or powder fall off the diamond is not all off but localized granular fall off. This phenomenon is not easy to pay attention to the consequences of the product life is shorter often give a kind of plating metal holding power or wear resistance is not good false impression. Excluding thickening when the plating burnt and plated metal wear resistance and other factors in the normal use of tools in the process of diamond particles fall off intuitively manifested on the surface of the tool there are continuous pieces of larger holes should be this kind of plating fall off.

3、The reason of plating off

Electroplated diamond tools in the manufacturing process involves a number of processes, any one of which is not carried out sufficiently will cause the plating layer to fall off.

3.1 Influence of pre-plating treatment

Steel substrate into the plating tank before the treatment process is called pre-plating treatment. Pre-plating treatment includes: mechanical polishing, degreasing, etching and activation steps. The purpose of pre-plating treatment is to remove burrs on the surface of the substrate, oil, oxide film, rust and oxidized skin in order to expose the substrate metal so that the normal growth of the metal lattice to form an intermolecular bond. If the plating pretreatment is not good substrate surface has a very thin oil film and oxide film base metal metal lattice can not be fully exposed will prevent the plating metal and the formation of intermolecular bonding of the base metal is only a mechanical mosaic effect of poor bonding.

Therefore, poor pre-plating treatment is the main cause of plating off.

3.2 Influence of plating solution

The formulation of the plating solution directly affects the type of plated metal, hardness, wear resistance with different process parameters can also control the thickness of the plated metal crystals, densities, and the size of the plated layer of internal stress. For the production of electroplated diamond tools, the vast majority of nickel or nickel - cobalt alloy if you do not take into account the influence of impurities in the plating solution, the plating layer off the factors are:

(1) the influence of internal stress plating internal stress is produced in the electrodeposition process in the solution of additives and their decomposition products and hydroxides will increase the internal stress. This stress is in the electroplating process plating layer by the influence of some deposition factors caused by lattice defects. In particular, the role of certain metal ions and organic additives can significantly increase the internal stress of the plating layer. Plating internal stress has two types of macro stress and micro stress. Macroscopic stress is manifested in a metal sheet for one-sided plating sheet plated by the plating of internal stress and produce bending. Microscopic stress is mainly manifested by increasing the hardness of the plating layer.

Macroscopic stress can cause plating in the storage, use of the process of bubbles, cracking, peeling and other phenomena.

For electroplating nickel or nickel - cobalt alloy for different composition of the plating solution internal stress varies greatly, the higher the chloride content of the internal stress is greater. For the main salt for the nickel sulfate plating solution, the internal stress of the watt class plating solution is smaller than other classes of plating solution. The addition of organic brighteners or stress relieving agents can significantly reduce the macroscopic internal stress and increase the microscopic internal stress of the plated layer. Different process combinations such as current density, PH value, temperature can make the same kind of plating solution plating layer with different internal stress. Therefore, to reduce the impact of internal stress must strictly control the process range of the plating solution so as to ensure that the internal stress of the plated layer in the process requirements of the range.

(2) The effect of hydrogen precipitation in any plating solution regardless of its PH value due to the dissociation of water molecules will always exist a certain amount of hydrogen ions. Therefore, under appropriate conditions, whether in acidic, neutral or alkaline electrolyte for plating in the cathode and metal precipitation at the same time there is often hydrogen precipitation. Hydrogen ions in the cathode after the reduction of part of the formation of hydrogen gas escapes part of the state of the atomic hydrogen penetration into the base metal and plating layer. This distorts the crystal lattice and causes a great deal of internal stress and significant deformation of the plated layer. At this time, although from the appearance of the defects can not be seen, but its mechanical properties are unqualified. When the temperature of the surrounding medium rises during the use of the tool, the adsorbed hydrogen gathered in the base metal or the plated metal will expand and cause the plated layer to produce blistering and peeling phenomenon.

The cathodic current efficiency of nickel plating is 95%, and only 5% is hydrogen precipitation reaction. However, if the temperature is too high and the PH value is too low, the hydrogen precipitation will be aggravated if the components are inappropriate. Therefore, how to control the hydrogenation reaction during plating in order to control the internal stress of the plated layer is a problem worth exploring.

3.3 Influence of plating process

If we exclude the composition of the plating solution and other process control aspects of the impact of plating process power failure is an important cause of plating off.

Electroplated diamond tools plating production process and other types of plating has a big difference. The plating process of electroplated diamond tools includes empty plating (priming), sanding and thickening process. In each process there is a substrate to leave the plating solution that is, or long or short of the possibility of power failure. For example, empty plating for a certain period of time after the need to observe the quality of the bottom nickel and diamond in the substrate is uniformly distributed; the above process if the sand on the sand planting and unloading sand steps unloading sand sometimes need to leave the plating tank in another tank; thickening process to observe the diamond coverage in place and so on. Short power failure on the plating is not much if the power failure time is too long the surface of the plated metal will be generated in a dense layer of oxidized film in an instant so that the subsequent electrodeposition of metal atoms can not be along the original growth of the metal lattice to affect the combination of the two.

Tools in the use of the process of external forces greater than the bonding force between the two layers when the layer between the two layers of the layer and layer separation is inevitable. Therefore, the use of more reasonable technology, process can also reduce the phenomenon of plating off.

4、Measures to solve the plating off

In view of the above reasons for plating off the author believes that the following measures can be taken to solve the problem of plating off:

(1) Strengthen the pre-plating treatment as far as possible to completely remove the substrate surface burrs, oil, oxide film, rust and oxidized skin to promote the normal growth of the plated metal lattice to improve the bonding force between the plated metal and the substrate metal.

(2) Optimize the formulation of the plating solution and plating process, take charged into the tank to prevent the phenomenon of bipolar for the complex shape of the workpiece using a short period of high-current impact plating in order to reduce the internal stress of the plating layer and precipitation of hydrogen phenomenon to improve the quality of the plating layer.

(3) Optimize the technology, process to reduce the power-off time when unloading sand or even without power in the original sand tank unloading sand, thickening, or in a spare tank charged unloading sand in order to improve the bonding force between diamond particles and plating layer. If in the process of thickening in the event of power failure phenomenon to re-thickening the workpiece should be put into the electrolyte for cathodic reduction reduction after charged into the tank plating to ensure that the bonding force of the plating layer.

5、Conclusion

In the production process of electroplated diamond tools in the selected composition of a plating solution formula in addition to consider the hardness of the plated metal, abrasion resistance, but also should pay full attention to the plating layer stress qualitative measurement of plating layer stress and various additives on the impact of internal stress. At the same time in the production process should pay full attention to the plating pre-treatment of each step of the production process control specifications to ensure that the plating solution is clean, impurity content in the process within the scope of the only way to ensure that the production of high-quality, stable quality of electroplated diamond tools.