There are an insufficient holding force and grain peeling off in the electroplated diamond tools, which seriously affect the service life and efficiency of the tools. This paper reviews the methods developed in recent years to improve the performance of electroplated diamond tools. It is summarized into the following 3 types.
- Improve the performance of the coating matrix material and improve the support and bonding effect of the coating on the diamond.
- Eliminate the gap between the diamond and the coating by increasing the contact area of diamond and matrix.
- Treat the surface of diamond grains to form the chemical bond between the diamond and the coating.
This paper describes 3 methods in detail and recommends some potential methods in order to have a comprehensive and systematic understanding of the improved methods of electroplated diamond tools.
With high hardness, high strength, high wear resistance and low coefficient of linear expansion of diamond, diamond tools are used for processing hard and brittle hard-to-machine materials. Electroplated diamond tools are made by the electrodeposition of the metal, bonding the loose diamond grains in the coating to make diamond grains have the cutting ability.
Electroplated diamond tools have a low manufacturing temperature to avoid heat loss of diamond. It has a simple production process, short manufacturing cycle, easy molding, and easy dressing. Therefore, electroplated diamond products have various tools such as diamond grinding wheels, mounted points, needle files, dressing rollers, geological drills, reamers, cutting blades, and etc. They are widely used in the fields of machinery, electronics, construction, drilling, and optical glass.
At present, there are main problems when the diamond tools are made by electroplating at home and abroad. They are that the bonding force between the coating matrix and diamond grains is low, and the diamond grains are easily loosened and fall off when subjected to the force to make short service life. The main cause for these problems is that low manufacturing temperature of electroplated diamond tools makes the surface of diamond grains not easily infiltrated by the general metal, which not obtain a strong chemical bond, but also often produce gaps. In addition, the coating metal of diamond tools is limited due to the effect of plating technology( only limited to nickel, chromium and other metals and their alloys). It is not as wide as the metal type used in hot pressing tools.
In views of the above problems, various measures have been taken to solve the bonding force between diamond and coating metal. This paper comprehensively introduces the methods developed in recent years to improve the performance of electroplated diamond tools and summarizes them, hoping to give a detailed and clear understanding.
1 Improve the matrix material of electroplated diamond tools
The plating in the electroplated diamond tools support and bond to the diamond, and it called the matrix or matrix metal. It determines whether the diamond grains can fully perform the cutting effect. It is usually required that is high hardness, high wear resistance, and high performance. So, first, consider improved diamond tools by improving the performance of the matrix material.
- Alloying of matrix metal
Although the single plating (such as nickel) has high strength, especially toughness, its hardness usually is lower, so, alloy plating is often applied.
1.1.1 Ni-Co binary alloy coating
Cobalt not only improves the strength of nickel metal( compressive strength of nickel-cobalt alloy matrix is 1600MPa), but also improves the heat resistance of the matrix metal. The strength limit of Ni-Co binary alloy matrix metal is 500MPa at 800 °C, and it can also improve the toughness of the matrix metal. So, Ni-Co binary alloy coating is widely used as matrix material. But sometimes the hardness of Ni-Co binary alloy coating is not enough, and the matrix is consumed quickly when processing hard and strong abrasiveness material. Moreover, only the cobalt content reaches about 30% in the Ni-Co coating to ensure high hardness and wear resistance. A large amount of expensive metal cobalt increases the cost.
1.1.2 Ni-Mn binary alloy coating
Metal manganese can improve the hardness, strength and wear resistance of nickel matrix than cobalt. The hardness of the nickel-manganese matrix is about 10HRC than that of nickel-cobalt matrix. The manganese content in the alloy is small but has a great effect on matrix performance. When the nickel-manganese matrix diamond drill bits are drilled in the hard and strong abrasiveness ground, the average life and aging are 55% and 30% than that of the nickel-cobalt matrix diamond drill bits, respectively. At the same time, the nickel-manganese matrix drill bit does not high speed and large pressure, which is good for reducing material consumption and drilling costs. However, Ni-Mn binary alloy coating is more brittle and cracks easily, which makes the working layers crushed.
1.1.3 Ni-Co-Mn ternary alloy coating
The Ni-Co-Mn ternary alloy coating has higher comprehensive performance. That hardness is higher than that of Ni-Co, and that brittleness is lower than that of Ni-Mn, which is in line with the requirements of the electroplated diamond products for the matrix.
The stone tools made of Ni-Co-Mn ternary alloy coating are sharper and more durable than that the Ni-Co binary alloy coating products, especially for hard stones.
The cost of Ni-Co-Mn ternary alloy coating is low due to the saving of a large amount of expensive material cobalt. The mechanical performance of Ni-Co-Mn ternary alloy coatings can be adjusted in the wide range to meet the needs of more applications. However, the composition of the plating solution of Ni-Co-Mn ternary alloy coating is complicated and the stability is not easily controlled.
1.2 Composite of matrix metal
The composite plating is a special plating formed by one or several insoluble solid grains and fibers uniformly interposing into the metal plating by coprecipitation method. Owing to a large number of solid grains uniformly in the composite coating, these hard grains can greatly hinder the slip between the crystal grains and effectively strengthen the metal.
1.2.1 Ni-Co fine-grained diamond composite coating
Add some nano-diamond powder to the plating solution, and the hardness of Ni-Co diamond composite coating is obviously high, and the hardness can reach 601.53 HV, and the friction and wear performance is high. The friction coefficient of the nickel-cobalt alloy coating is about 0.35, and the average life is 0.022km when the friction radius is 14mm.
The friction coefficient of Ni-Co diamond coating containing nanodiamond powder is about 0.3, and the coating life is 0.15km when the friction radius is 14mm. Diamond drill bits made of the Ni-Co-diamond composite coating diamond bit matrix has good resistance, fast drilling, long life and can prevent hole inclination in drilling hard and strong abrasiveness ground. Since the ultrafine diamond powder gets together easily and makes the performance cannot be fully exerted. So, make the measure to disperse the diamond powder, which restricts the use value and application prospect of ultrafine diamond powder.
1.2.2 Ni-Co rare earth element composite coating
Adding a small amount of rare earth compound can improve the performance of the plating solution and the coating with varying degrees. In the electrodeposition process, the cation is mainly adsorbed on the surface of the metal deposit, and the rare earth metal ion expresses strong adsorption on the electrode. The rare earth metal ions are easily adsorbed on the active point of the crystal growth, that is, adsorbed on the growth point of the crystal plane and can effectively restrain the crystal growth. Therefore, after adding the rare earth element in the plating solution, the coating with fine crystal grains can be obtained. The grinding test study of diamond tools for ceramic by bright nickel and adding rare earth element bright nickel using the universal cylindrical grinding machine M1420E is shown. It is found that the addition of rare earth elements improves the grinding ratio of the diamond tools. Bright nickel-plated diamond tools have poor wear resistance, fast matrix consumption, cannot guarantee a high edge of the diamond, diamond fall off quickly. The bright nickel bond with rare earth elements improves the wear resistance of the matrix and wraps the diamond better, and the diamond edge height is high. So, the efficiency of the tools is improved.
1.2.3 Ni-Co-CNT composite coating
Carbon nanotubes(CNTs) have superior strength and toughness and can be used as reinforcements for advanced composite materials to greatly improve the strength and toughness of composite materials. In addition, CNTs feature good chemical stability and low friction coefficient and it is expected to make a new type of composite coating with high wear resistance, wear reduction and corrosion resistance. Observe the morphology of the composite coating, the surface of the matrix is covered by a thick layer of carbon nanotubes. One end of these carbon nanotubes are deeply embedded in the matrix, and the other end is exposed to the outside of the matrix, which obviously protects the matrix.
1.3 Grain refinement of matrix metal
The crystallization process of the coating is controlled by the nucleation rate and grain growth rate. The faster the rate of nucleation, the slower the grain growth rate, and the finer the crystallization, the denser the coating and the better the hardness and toughness. According to the electrochemical theory, the larger the cathode electrochemical polarization overpotential, the easier it is to form a crystal nucleus, and the finer the crystallization, the denser the coating. Therefore, improving the electrochemical polarization overpotential and refining the crystal grains to improve the matrix materials.
1.3.1 Refined additives
After adding the additive to the electrolyte, owing to its adsorption on the electrode surface, the electrochemical polarization is increased, the covered crystal grains stop growing, and new crystal nucleus are generated. The new crystal grains are soon covered again, and a new crystal nucleus center is generated. So, you can get fine crystals. Secondly, the adsorption of the additive on the surface of the crystal can reduce the surface energy of the crystal, thereby reducing the formation of crystallites, which is good for forming a new crystal nucleus. The refined additives are mainly sulfonic acid, sulfonates, disulfonic acid, for example, saccharin sodium, p-toluenesulfonamide, benzenesulphinic acid, naphthalene disulfonate, and etc. It can be found from the morphology of the surface of the coating before and after the addition of the aromatic ketone additive that the crystal grains are large before the addition of the additive, and the crystallinity of the grains is poor, the crystal grains are loose, and the crystal grains are significantly smaller after the addition of the additive, and the crystal is dense.
1.3.2 Ultrasonic method
Ultrasound enables the material to undergo intense mechanical vibrations and also produces unidirectional force. When a certain frequency of ultrasonic passes through the liquid, the small bubbles resonate. In the sparse stage of ultrasound, the small bubbles rapidly expand and become larger. In the thick stage, the small bubbles are suddenly compressed until they collapse. When the small bubbles are suddenly compressed, the surrounding liquid fills the cavity at high speed, and the nearby liquid or solid is subjected to the high pressure of thousands of atmospheres, which is cavitation. Electroplating with high current density under ultrasonic conditions makes the coating more compact, smooth, uniform in thickness, non-porous, well bonded to the base, has high strength and hardness. Nickel plating in an ultrasonic field with a frequency of 16kHz can increase the hardness by 30% to 50%. It can also make the internal stress of the coating lower than that of the coating without ultrasonic plating under appropriate process conditions. The reason is that the cathode polarization increases sharply, causing the hydrogen evolution to increase, the PH value rises, and nickel hydroxide solution appears at the cathode when the cathode current density is high to a certain value. The cavitation of the ultrasonic
plays the refining, dispersion, and stabilization for the sol to avoid the sol condensation and precipitation. Ultrasonic should not be used in the whole process of diamond sanding, and only used in the thickening period. When the diamond abrasive grains are buried in a certain thickness of the coating, ultrasonic is used, and the diamond abrasive grains will not be shaken, which will not affect the number of sanding of diamond tools.
1.3.3 Pulse electroplating and nano matrix material
Pulse electroplating is a new type of plating technology developed in the 1960s. The electrochemical principle is that in one pulse period, when the current is turned on, the electrochemical polarization increases, the metal ions near the cathode region are fully deposited, and the plating crystallization is fine and bright. When the current is turned off,
The discharge ions nearby the cathode region return to the initial concentration and the concentration polarization is eliminated. Therefore, pulse electroplating is a new type of powder application. The relaxation of the current or voltage pulse is used to reduce the concentration polarization of the cathode, thereby allowing higher current density to achieve higher electrode polarization and ultimately refining the grains. At present, electrodeposition technology has become an important preparation method for nanomaterials. These materials have high hardness and good toughness, which used to make electroplated diamond tools, the wear resistance of diamond tools is significantly higher. Nano-nickel diamond tools made by pulse electrodeposition method are tested by wear experiment, it shows that the average life of pulse nano-nickel diamond tool is significantly higher than that of conventional nickel-cobalt electroplated diamond tools, which is about 1.5 times.
2 Improve the contact area between diamond and matrix
2.1 Surface roughened diamond grains used
The roughening method makes some tiny pits and cracks on the diamond surface, increasing the contact surface of diamond and matrix, improving the mechanical integration force of the diamond and metal, enhancing the “mechanical anchor chain” effect. A strong roughening method is to cover the diamond with fluorine-based salt( mainly NaCl+ BaCl2) and a small amount of deoxidizing agent, cover with the ceramic crucible, heat it in the furnace to 1000 ° C ~ 1100 ° C, and then remove the fluorine-based salt by boiling water. Heat the diamond, and the fluorine-based salt is melted to cause graphitization of the diamond corrosion, makes the surface forms rough pits and cracks.
Another weakening method is to etch the diamond in the roughening solution( nitric acid+ sulfuric acid or nitric acid+ hydrogen peroxide) at room temperature or under heat condition with stirring and then clean it by distilled water. The diamond will form some defects( such as pits, cracks) and slight graphitization under the corrosion of strong oxidizing acid.
2.2 Eliminate the gap between the diamond grains and the matrix
Diamond is non-metal, and it has not good affinity with metals, resulting in high interfacial energy between diamond and common metals or alloys, often producing gap and reducing the bonding between diamond grains and the matrix of the coating. In this case, the grain surface modification method, CVD method, ultrasonic method, electroless plating can avoid or compensate for this gap.
2.2.1 Grain surface modification method
Do the oxidation treatment for diamond grains and makes the surface from the hydrophilic chemical group, thereby improving the hydrophilicity of the diamond surface and tightly bonding the diamond grains to the coating. If the hydrophilic groups on the diamond surface are replaced by some more hydrophilic organic genes by chemical means, further high effects can be obtained.
2.2.2 Electroless Plating
Electroless plating is that the oxidation-reduction reaction of the autocatalytic process deposit the metal on the surface of the diamond, forms a film coating with uniform thickness and dense thickness under the non-applied current condition. It has been found that there is no gap between the diamond and the metal matrix from the diamond tools by electroless plating technology. Accompanied by ultrasonic vibration, there will be expansion around the diamond grains, completely eliminating the gap. The diamond grains can be directed to obtain the tools by electroless plating, which can be uniformly suspended in the plating solution. We recommend that first electroplating and sanding for the large-grain diamond, and electroless plating is applied in the thickening process of diamond pre-embedded coating.
3 Improve the chemical bond between the grains and the matrix
The carbon atoms and the metal atoms on the surface form metal/carbon chemical bond by diamond treatment, which can completely solve the problem that the not strong bonding force between the diamond and the matrix in the electroplated diamond tools
3.1 Surface metallization
The surface treatment of the diamond grains by electroless plating can form a firm and tight connection between the diamond and the coating. However, if the diamond surface is completely metalized, the surface of the diamond grains has good electrical conductivity and is not suitable for electroplating to do diamond tools. In the process of burying sand, the plated diamond and steel matrix and the coating form the cathode together, there are many diamond grains bonded to each other forming the blocks. Therefore, a diamond with dispersed conductive dots on the surface is used to make electroplated diamond tools. The method is to control the degree of electroless plating on the diamond surface, strictly control the concentration of the sensitizing solution and the activating solution, and the time of sensitization and activation treatment, making the number of metal dots on the diamond surface keeping within the proper range. Although the number of conductive dots on the diamond surface increases, it can increase the connection point with the plated metal and improves the bonding performance between the coating and the diamond. But
When the metal dots are too thick, it will form the metal thin sheet.
After electroless plating of a diamond, the obvious boundary between the diamond and the nickel-cobalt-based coating disappears, and there are some scattered nickel-cobalt joints grown on the joint surface of the diamond and the coating. The electroplated diamond grinding wheel is made by the activated diamond. The material removal amount is 1.5 times that of the inactivated treatment when grinding Al2O3 ceramic workpiece. However, it may be only the original chemical bond between diamond grains and the coating, and it is not a true chemical bond, and the intermolecular force may account for a large proportion.
3.2 CVD method
The diamond CVD deposition technology is used to dress the diamond tools, which not only allows the newly formed diamond to be deposited in the gaps in the tools but also allows the diamond grains in the tools to be regenerated, further develops the surface, improving the performance of diamond grains. The MPCVD method has been successfully used for gap dressing between diamond grains and the matrix after electroplating. The edge and surface irregularities were found on the surface topography of the electroplated diamond tools with 16 μm diamond grains that were observed under SEM. There were concave surfaces and gaps between diamond grains and the matrix. Put the tools in the MPCVD system, the average size of the diamond grains increases to 25 μm. The gap between the diamond grains and the matrix metal is compensated by SEM observation, and the surface of diamond grains is regular. Dressing electroplated diamond tools has higher cutting force, wear resistance and grains bonding than the undressing electroplated diamond tools. It can form a carbon-metal bond between the diamond and the matrix during the MPCVD process at high temperature, making the strong bond between the diamond grains and the matrix metal.
In the actual applications, should choose the suitable method to solve the problem of insufficient holding force of diamond tools according to the actual situation and processing materials.