Surface treatment: Abstract: This article in view of the electroplated diamond tools in use in the process of coating dropped three common types, discusses the shedding of coating influencing factor, put forward to solve the shedding of coating basic countermeasures.
Key words: electroplating diamond tools; plating; plating processing; internal stress
1 foreword
Electroplated diamond tools by means of metal electrodeposition method, so that the diamond are firmly metal matrix is wrapped in a metal ( steel or other material ) production into a diamond tool, it is widely used in electronic machinery, glass, building materials, oil drilling and other industries. With the development of economy, the progress of science and technology, different sectors of the electroplated diamond tools demand basically is the same, namely, high efficiency, long service life, high grinding precision. To ensure that these characteristics, metal coating not only have high hardness, wear resistance, but also in the matrix to be evenly distributed to each part, make the tool life shortening plating peeling off. In some special industries, such as magnetic materials industry grinding, feed is controlled around 0.3mm; ceramic industry large amount of feed of dry grinding of metal coating and steel substrate, the bonding force is particularly harsh. In the production of electroplated diamond tools in the process, most manufacturers do pay attention only to the plating metal type, hardness, wear resistance, but often ignore metal coating combined with matrix problems. In using a process actually, plating peeling off phenomenon it is often seen. This paper makes a analysis of the causes of, and solutions are discussed. [1, 2, 3, 4]
2 types of plating peeling off
Electroplated diamond tools in use process, as a result of the use of conditions such as grinding force, temperature rise, the impact of workpiece and other reasons, may cause containing diamond metal coating and steel substrate separation phenomenon, this is the film to fall off. Plating peeling off are generally local shedding, one-time whole coating peeling phenomenon rare. In actual use, coating dropped generally has three kinds as follows:
( 1) to the surface of the substrate plating peeling off: diamond containing metal coating and diamond containing metal bottom coating and steel substrate separation.
( 2) falling to the bottom layer of metal plating: i.e. excluding diamond metal bottom coating and steel without matrix separation, only diamond containing metal and bottom plating stripping.
( 3) containing diamond metal plating coating metal in the lamellar separation: diamond containing metal coating in use process, in contact with the workpiece portions of the metal coating is not normal wear, but abnormal plots or powder form off, diamond is not all fall off, but local granular exfoliation. This phenomenon is not easy to notice, the consequences are products of short service life, often give a person a kind of coating metal holding force or wear resistance is poor. Exclusion of thick coating of char and coating metal poor wear resistance and other factors, tools used in the normal course of diamond particles, visual performance as the tool surface has a continuous piece of large holes, such coatings should be off.
The 3 coating shedding reason
Electroplated diamond tools in the manufacturing process involves many processes, any procedure are not sufficient, will cause the film to fall off.
Effect of 3.1 treatment before plating
Steel substrate in the plating tank before entering the process called treatment before plating. Before plating processing include: mechanical polishing, degreasing, pickling and activating step. Treatment before plating are designed to remove the burrs on the substrate surface, oil, oxidation, rust and oxide skin, to expose the base metal so that the metal lattice normal growth, formation of intermolecular binding force. If the treatment before plating is not good, the surface of the substrate and a thin oil film and film, the matrix metal lattice cannot fully exposed, it will impede the metal coating and the base metal form intermolecular binding forces, merely mechanical mosaic effect, poor binding force.
Therefore, before plating plating peeling off bad is caused by the main reason.
The influence of 3.2 plating solution
Plating solution formula directly influence the coating metal type, hardness, wear resistance, with different process parameters can also control the plating metal crystal thickness, density and coating internal stress. For the production of electroplated diamond tools, the vast majority of nickel or nickel cobalt alloy, without considering the influence of impurities in plating solution, plating peeling off of the influence factors:
( 1) the effects of internal stress on internal stress in the electrodeposition process, additives in the electrolyte and its decomposition products and hydroxide will increase the internal stress. The stress is in the electroplating process plating deposition by some factors, caused by lattice defects. Especially some metal ions and organic additives, can significantly increase the coating internal stress. The internal stress of the coating layer has the macroscopic stress and microscopic stress two. Macroscopic stress will be reflected in a metal sheet for a single sheet by electroplating, plating internal stress impact bending. Microscopic stress is mainly by improving the hardness of coating performance.
Macroscopic stress can cause the coating in storage, generated in the using process of bubble, crack, fall off.
In electroplated nickel or nickel cobalt alloy, the different composition of plating solution, stress difference, chloride content higher, more internal stress. For as the main salt of nickel sulfate bath, W type plating internal stress were less than other types of plating solution. By adding organic brightener or stress elimination agent, can obviously reduce the coating macro internal stress increases its microscopic stress. The different combination of process, such as current density, pH value, temperature, can make the same plating coating with different stress. Therefore, to reduce the effect of internal stress of plating process must be strictly controlled, so as to ensure the coating internal stress in the process of the range of requirements.
( 2) hydrogen effect in any plating solution pH value, regardless of how, as the water molecular dissociation, always there is a certain amount of hydrogen ion. Therefore, in appropriate circumstances, whether in acidic, neutral or alkaline electrolyte on the cathode and the plating, metal is deposited at the same time, often with hydrogen evolution. Hydrogen ions in the cathode reduction, formed a part of hydrogen evolved, in part to atomic hydrogen state into the base metal and coating. The lattice distortion, caused great internal stress of plating, also make significant deformation. At this time, although not be seen from the outer appearance defect, but its mechanical properties are not qualified. Tool during use, when the surrounding medium temperature increases, assembled in the base metal or metal coating within the adsorbed hydrogen swell and the coating bubble generation, fall off phenomenon.
Electroplating nickel, cathodic current efficiency is 95%, only 5% for hydrogen evolution reaction. But if the temperature is too high, the low pH value, each incorrectly can all make hydrogen aggravate. Therefore, how to control the plating of the hydrogen evolution reaction to control the internal stress of the coating layer is a problem that is worth to discuss.
A 3.3 electroplating process effect
Excluding electroplating bath composition and other process control effects, the electroplating process failure is caused by plating peeling off an important reason.
Electroplated diamond tools electroplating production process and other types of plating with a larger difference. Electroplated diamond tools electroplating process includes empty plating ( backing ), sand, the thickening process. In every process in both the existence of the substrate leaving the bath, or long or short outages may. For example, empty plating after a certain period of time, the need to observe the bottom nickel and the quality of diamond on the substrate is uniformly distributed; the process if the sand and sand and sand unloading steps, sand unloading sometimes need to leave the bath, in another trough; thickening processes observed in diamond cover is in place. Short time power, on the coating effect, if the power-off time, coating metal surface then in a moment to form a layer of compact oxide film, so that the electrodeposition of metal atoms along the original metal lattice growth, influence the binding force.
Tool during use, the external force is greater than the two interlayer bond strength, the two layer between the layer and the layer separation is inevitable. Therefore, a more reasonable technological process, also can reduce plating peeling off phenomenon.
4 measures to solve plating peeling off
In view of the above coating shedding reason, the author thinks we can take the following measures to solve the problem of plating peeling off:
( 1) strengthen treatment before plating, as far as possible the complete removal of the burrs on the substrate surface, oil, oxidation, rust and oxide skin, make coating metal lattice normal growth, improve the metal coating and the base metal binding force between the.
( 2) optimizing the formulation of bath solution and electroplating process, taken charged into the groove, preventing bipolar phenomenon, for complex shape workpiece using the short time high current impulse air to reduce the internal stress of plating, plating and hydrogen evolution effect, improving the quality of the coating.
( 3 ) optimization of process, process, reduce the sand unloading when power-off time, even with constant electric in the original sand tank sand unloading, thickening or in a standby tank charged sand unloading, in order to improve the diamond particles and the coating binding force between the. If the thickening process in the event of power failure phenomenon, to thicken, the workpiece should be put into the electrolyte for the cathodic reduction, reduction of charged into the groove after plating to ensure coating binding force.
5 Conclusion
In the production of electroplated diamond tools in the selected process, a composition of the plating solution formula, besides considering the coating hardness, resistance to wear, still should pay full attention to the internal stress of the coating layer, qualitative measurement of coating internal stress and various additives on the effects of internal stress. At the same time in the production process should pay full attention to pre-treatment steps, the production process control standard, ensure the bath clean, impurity content in technology area, the only way to ensure production of quality, quality and stability of the electroplated diamond tools.
Reference.
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[2 ] Guo Hetong, Chen Jianxun, Liu Shulan. Electroplating technology [ M]. Tianjin Science and technology press.
[3 ] of Shanghai light industry college. Principles and technology of electroplating [ M]. Shanghai science and technology press.
Wang Qinsheng [ 4]. Superhard materials electroplating technology [ M]. China Abrasives Industry company.