This paper presents the development process of a new coating method for micro pencil grinding tools (MPGTs). MPGTs, applied for microgrinding, consist of a base body, abrasives, and a metallic bond. The manufacturing process of these microtools presents two challenges. The first being in finding a method to embed the abrasives with a uniform grit distribution and the second finding the correct parameters, required for a bond with adequate grit retention forces. In this research, an electroless plating process is presented. Both the abrasive grit distribution method and the plating parameters will be presented in this paper.
Processing microstructures in hard and brittle materials has become an extensively researched topic . Besides older technologies like laser, lithographie, galvanoformung, abformung (LIGA), or etching, new technologies such as micro-abrasive blasting or micro-ultrasonic machining are researched .
One of these new technologies is microgrinding using MPGTs. As this process requires no mask and is capable to machine a large variety of materials, including hard and brittle materials , it allows flexibility when machining parts with varying structures or prototypes. Moreover, the process offers a better structure quality than most of the other processes and is the more economical solution, due to its simple and much cheaper manufacturing setup .
Micro pencil grinding tools have been manufactured via sintering , chemical vapor deposition (CVD) , or electroplating . Electroplated MPGTs are cheaper to manufacture, allow for smaller diameters than those manufactured in a sintering oven, and can provide a larger chip space than CVD-coated MPGTs . Electroplating is a process that uses electric current to catalytically reduce the metal ions in a plating solution to form a metal coating on an electrode . The electrode is the grinding tool substrate in this case.
A similar process that provides the same advantages as electroplating is the electroless plating process. This process is purely chemical and requires no electric energy for the plating process . The catalytic reduction of the metal ions results from a reducing agent contained in the plating solution itself. Electroless plating promises a harder bond and is not geometry dependent , which allows for a more uniform coating to grow on the substrate . However, electroplating has the advantage that it requires a much lower operating temperature .
Very little research has been conducted on manufacturing MPGTs with the electroless plating method. This paper discusses the manufacturing process of MPGTs made of cemented carbide with an average active diameter of 50 μm. The resulting MPGT will have 1–2 μm abrasive diamond grits embedded in a nickel coating onto the active surface. The composition and the coating parameters needed to manufacture the tools displayed in this paper will be presented.
The Chemical Reaction
The electroless plating method employs a catalytic reducing agent to reduce the metal ions in the plating solution, allowing metal deposition onto the substrate . The following chemical reactions occur when mixing the metal salt with a hypophosphite reducing agent :
The most common reducing agent used in the electroless plating industry is sodium hypophosphite (NaH2PO2) . The metallic salt used in this composition is nickel sulfate. Another important component is a complexing agent, in this case sodium acetate. This additive plays three major roles :
it helps maintain a stable pH value,
it helps prevent precipitation of metal salts, and
it reduces the concentration of free metal ions in the solution.
The electroless plating solution may decompose spontaneously during the metal deposition process . A stabilizer agent like thiourea or maleic acid is added to the plating solution to prevent an early bath decomposition . Above a critical concentration, stabilizers inhibit the nickel deposition completely .
The lattice structure of the nickel deposit is mainly affected by its phosphorus content. A phosphorus content of less than 7% weight percentage (wt. %) results in a crystalline Ni–P structure. The Ni–P deposit has an amorphous structure at phosphorus concentrations above 7 wt. %. The phosphorus content is influenced by the reducing agent, the stabilizer, the pH value, and the process temperature . In general, substrates coated with a plating solution using thiourea as stabilizers show a phosphorus content of less than 7 wt. % and a crystalline structure . The phosphorus content in a nickel coating determines the coating's physical, mechanical, and chemical properties. Lower contents result in a higher hardness of the coating .
The substrates used are made of tungsten carbide with an 8% cobalt content, a particle size of 0.2 μm, a Vickers hardness of 1920 ± 50 HV30 (ISO 3878), and a bending strength of more than 4800 N/mm2. The active part of the tool, a cylindrical tool tip, has a diameter of 48 μm and is 140 μm long in length. Details on the manufacturing of the substrates can be found in Ref. . The abrasive grits used in this paper are synthetic diamonds manufactured by element six with a grit size of 1–2 μm. The diameter of the finished coated tools is 50–52 μm.
The plating process displayed in Fig. 1 follows the coating process of a substrate, which includes six steps. Those are, in sequence, degreasing, cleaning, nickel strike, cleaning, electroless plating, and grit embedment. Figure 2 shows the experimental setup for the degreasing, nickel strike, and electroless plating processes. The substrates are fixed onto a polytetrafluoroethylene (PTFE) holder, which is connected to an electromotor, enabling rotation speeds as low as 1/3 rpm.
The substrate is moved from one solution bath to the next via a slide bearing. A clamp determines the height of the device, and the area not to be coated on the substrates is covered with an inert shrink tube. Both the degreasing bath and the plating solution are heat regulated to assure optimal conditions. The scale used to measure components used in the solutions is Kern EMB 200-31 that measures weight with an accuracy of 0.001 g.
In the degreasing bath, the substrate is cleaned of organic and manufacturing residues. The substrate is soaked for 3 mins in a 70–80 °C heated solution containing 200 g/l NaOH. Following the degreasing process, the tool is soaked for 10 s in a solution made of distilled water and a few drops of hydrochloric acid to neutralize the remainder of the degreasing solution.
Following the cleaning step, the substrate receives an extremely adhesive and wear resistant nickel coating called nickel strike. A nickel strike coating helps to initiate the autocatalytic chemical reduction process in the electroless plating solution . Without a nickel strike layer, no nickel deposition could be achieved on substrates via electroless plating. This thin nickel coating is electroplated to the substrate at room temperature. To electroplate the tool with a nickel layer, an electric cycle needs to be created between the tool and the nickel ring , as shown in Fig. 3. In this electric cycle, the tool is the cathode and the nickel resource acts as an anode. An electric field is formed between the anode and the cathode, allowing the Ni2+ cations to gravitate toward the substrate. The anode transfers nickel electrons to these cations, and a nickel layer is formed. The components used for the nickel strike solution are 250 g/l nickel chloride and 10 g/l hydrochloric acid. The tool is plated for 60 s with a current density of 7 A/dm2.
After the nickel strike, the substrate is cleaned again, followed by the electroless plating. The electroless plating solution is placed inside a tempered beaker filled with water. A magnetic stirrer is placed in the electroless plating solution to keep the diamond grits in a rotatory motion. Due to the centrifugal forces, diamond grit trajectories tend to be closer to the rim of the container, hence the substrate is placed close to the rim. The substrate is kept in rotatory motion, making sure that a uniform grit distribution is achieved during the process. After the main coating process, the magnetic stirrer is stopped, and the diamond grits fall to the bottom of the bath. This allows the coated diamond grits to be embedded with a thicker nickel layer, controlled by the time the substrate remains in the bath.
There are a number of parameters that influence both the nickel deposition rate and the phosphorous content. Nickel coatings with a high phosphor content result in unwanted lower hardness and lower nickel deposition rates. A high phosphor content is indicated by a dark color.
Increasing the pH value is a way to both increase the deposition rate and decrease the phosphorous content. Increasing the temperature increases both the phosphorous content and the deposition rate, making a constant temperature crucial . Decreasing the complexing agent increases both the phosphorus content and the deposition rate , the same is true when increasing the reducing agent .
Figure 4 depicts a substrate with an unwanted black nickel coating. The substrate was coated with 30 g/l nickel sulfate, 31 g/l sodium hypophosphite, 15 g/l sodium acetate, and 2 mg/l thiourea at a temperature of 85 °C and a pH value of 4.3. The experiment was conducted for 45 mins resulting in a 7 μm thick nickel coating.
In general, the solution is insensitive to smaller variations in every parameter except for the thiourea concentration. The slightest variation in the thiourea concentration can either prohibit nickel deposition or decompose the plating solution too early . The influence of this component is hence analyzed in detail in the “The Influence of Thiourea” section.
In all the succeeding experiments presented, a pH value of 5.2–5.4 and a temperature of 87 ± 1 °C are used together with the plating solution composition listed in Table 1.
The Influence of Thiourea
The deposition rates were measured by eye with an optical microscope. The substrates were measured with a 500 times magnification and an accuracy of 1 μm in diameter, using a scale in the eyepiece. For each sample, a fresh plating solution was prepared to ensure repeatability. The plating was conducted for 15 mins with the composition listed in Table 1.
To determine the influence of thiourea, no diamond was applied in the coating process. This eases the determination of the deposition rates.
The impact of the thiourea concentration on the nickel deposition rate is shown in Fig. 5. A thiourea concentration of 1.1 mg/l was identified to be inhibiting the nickel deposition almost completely. The thin nickel layer grown on the substrate had a blackish color and did not change the surface topography resulting from the machining of the substrate (Fig. 6(a)). Hence, the thickness of the layer was considered to be zero.
Substrates plated with a thiourea concentration of 0.9 mg/l and 0.7 mg/l resulted in samples with nickel deposition rates as low as 5 μm/h and samples with a much higher nickel deposition, making the results irreproducible. The surface shows an almost cauliflowerlike structure with frequent pit formation (Fig. 6(b)). Substrates plated with a thiourea concentration of 0.1 mg/l had a reduced nickel deposition and bad surface quality with a rough inhomogeneous structure (Fig. 6(c)) with some substrates even showing pore formation.
Both the concentrations of 0.3 mg/l and 0.6 mg/l had constantly proven to have a good nickel deposition rate, but did not always produce a smooth surface. The concentrations of 0.4 mg/l and 0.5 mg/l proved to be easier to control and produced more reliable results with smooth surfaces (Fig. 7). For all further experiments, a thiourea concentration of 0.4 mg/l was chosen, as it most consistently delivered the best results.
Coating Parameter Study
With the optimum chemical composition and constrains at hand, the parameters to produce MPGTs will be outlined in this section. Those are the diamond concentration in the bath, process time, and the substrate rotation speed.
It was determined that the diamond concentration should be investigated first, in order to prevent accumulated diamond grits to stack on top of each other and to reduce diamond usage for upcoming investigations. The aim was to achieve a 0.7–0.9 μm thick nickel layer to reach enough chip space for most grits while having enough thickness to hold 2 μm diamonds. In accordance with Fig. 5, a coating time of 150 s was hence chosen. The substrate rotated at 1.5 rpm, while the magnetic stirrer was kept at a low rotation speed, achieving the diamond grits to float without building a large eddy in the center. Diamond concentrations of 1000 mg/l, 500 mg/l, and 150 mg/l were investigated. The MPGTs displayed in Figs. 8–11 have all been cleaned in an isopropanol bath with vibration and cleaning clay. Insufficiently embedded grits get removed during the cleaning process.
Figure 8(a) shows an MPGT coated with a 1000 mg/l diamond concentration. Some of the substrates coated with this concentration resulted in overlaying diamond grits with a majority removed during the cleaning processes of the MPGT. The high accumulation of diamond grits reduces the grit adhesion to the substrate and the available chip space. Moreover, the nickel deposition rate is reduced due to stacking diamond grits preventing the growth of additional nickel. However, MPGTs with better diamond distributions have been produced using this diamond concentration, making the resulting coatings less predictable.
Figure 8(b) depicts an MPGT that was coated with a 500 mg/l diamond concentration. Samples coated with this diamond concentration revealed a rather good diamond distribution.
Figure 8(c) displays an MPGT coated with a 150 mg/l diamond concentration. This MPGT exhibits fewer diamond grits than the one in Fig. 8(b), and more pores indicate that a diamond grit was removed during the cleaning process. The substrate requires more embedding time if this concentration is to be used.
For ongoing experiments, a 500 mg/l diamond concentration was chosen, because it presents a more uniform distribution than samples coated with the other concentrations.
The next parameter to be investigated is the rotation speed of the substrate. The initial substrate rotation speed was kept at a low 1.5 rpm to ensure a good distribution. Since it is unknown how higher rotation speeds will affect the plating process, three additional rotation speeds were investigated in this paper: 9 rpm, 25 rpm, and 46 rpm.
Figures 9 and 10 show the SEM micrographs taken of a sample from each test series. With Fig. 9(a) demonstrating the desired result and Fig. 10, a both nodular and rippling structure, the micrographs present a decline in quality when increasing the rotation speed of the substrate. Both Figs. 9(c) and 10 depict nodular nickel deposits, some of which even contain their own diamond grits. Despite showing early symptoms of the same phenomenon, the surface presented in Fig. 9(b) offers an acceptable quality.
Conclusions and Future Work
The electroless plating method proved to be a reliable method allowing the coating of tools with diamond grits with a high reproducibility. The plating solution is easy to produce provided the correct stabilizer concentration is added. It was determined that a 0.4 mg/l thiourea concentration provides a consistent solution, a high nickel deposition rate, and an optimal surface quality.
To achieve a good diamond distribution for 1–2 μm diamond grits, a diamond concentration of 500 mg/l showed an even diamond distribution. Increasing the rotation speed of the substrate showed a decline in surface quality. A maximum rotation speed of 9 rpm is advised.
In future works, MPGT with different grit protrusions will be produced and applied in a microgrinding process to investigate their performance. In addition, the diameter and grit size of the MPGT will be varied. Furthermore, the influence of the phosphorous content on the binder's hardness needs further investigation.
This research was funded by the German Research Foundation (DFG) within the Collaborative Research Center 926 “Microscale Morphology of Component Surfaces.”
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