DE69205983T2 - COATING OF SUBSTRATES. - Google Patents

Description

The invention relates to methods for coating substrates and in particular to methods for coating substrates by means of electroless coating.

In coating processes, a substrate, which may be a metallic alloy, composite or non-metallic substrate, is immersed in a coating solution to provide the substrate with a desired coating. The coating may be obtained by applying an electric current (electrolysis) or without such a current (electroless). In electroless coating, the substrate is placed in a container of a suitable coating solution, which in turn is placed in a thermostatically controlled water bath. The solution is heated to a temperature and for a time sufficient to achieve a desired coating thickness.

In such coating, for a given coating solution, the deposition rate and grain structure of the coating are controlled by adjusting the current (electrolytic plating) or by the electrolyte temperature (electroless plating). However, such increases in current/temperature can have disadvantages in terms of reducing the lifetime of the coating solution.

According to the invention, there is provided a method of depositing a coating on a substrate comprising placing a substrate in a coating solution for coating the substrate and then heating the coating solution in a chamber by a microwave source during the coating process to produce a required coating.

Preferably, the method also comprises heating the coating solution by the microwave source prior to inserting the substrate.

It has been found that improvements in precipitation rate and grain structure can be achieved by using a microwave source for heating.

Preferably, the method also includes adjusting the power output of the microwave source to control the temperature of the coating solution.

The method may also include controlling the temperature to a constant temperature throughout the coating process. Alternatively, the method may include pulsing the power to the microwave source. The method may also include heat treating the coated substrate.

Preferably, the coating process is an electroless coating process.

The following is a detailed description of some embodiments of the invention by way of example, with reference to the accompanying drawings and electron micrographs, in which:

Figure 1 shows a schematic view of an apparatus for coating a metallic substrate by an electroless process and it includes a microwave energy source,

Figure 2a shows two micrographs of the crystal structure of a copper substrate with a nickel-phosphorus coating from a first coating solution applied by a known method (left) and by a first, exemplary method according to the invention (right),

Figure 2b shows two scanning electron micrographs of the crystal structure of a soft, unalloyed steel with a nickel-phosphorus coating from a first coating solution, applied by a known method (left) and by a first exemplary method according to the invention (right),

Figure 2c shows two micrographs of the crystal structure of a stainless steel substrate with a nickel-phosphorus coating from a first coating solution applied by a known method (left) and by a first exemplary method according to the invention (right),

Figure 3a shows two scanning electron micrographs of the crystal structure of a copper substrate after treatment in a second coating solution by a known method (left) and by a second, exemplary method according to the invention (right),

Figure 3b shows two scanning electron micrographs of the crystal structure of a soft, unalloyed steel substrate after treatment in a second coating solution by a known method (left) and by a second, exemplary method according to the invention (right),

Figure 3c shows two scanning electron micrographs of the crystal structure of a stainless steel substrate after treatment in a second coating solution by a known method (left) and by a second, exemplary method according to the invention (right),

Figure 4 shows six scanning electron micrographs at a magnification of x2000 showing the surface topography and arranged in three vertically arranged pairs, each pair showing the topography of a copper substrate, a soft, unalloyed steel substrate and a stainless steel substrate, each coated with a nickel-phosphorus coating from a first coating solution applied by a known method (upper micrographs) and a third exemplary method according to the invention (lower micrographs),

Figure 5 shows two scanning electron micrographs at a magnification of x2000 of the surface topography of a soft, unalloyed steel substrate with a nickel-phosphorus coating formed from a second coating solution applied by a known method (top) and a fourth exemplary method according to the invention (bottom).

As shown in Figure 1, the electroless coating device comprises a domestic microwave oven 10 having a maximum power output of 500W from a microwave source 12. The oven is equipped with an internal turntable 11 and a microwave stirrer to ensure even distribution of energy within the chamber. The oven 10 is modified to provide continuous irradiation at variable power using a variac or autotransformer 13.

An accurate indication of the temperature of the coating solution can be easily obtained using a miniature gas thermometer. This is connected via a capillary 14 to a pressure transducer 15 which is attached to the outside of the microwave cavity. The output from the transducer 15 is connected to a digital voltmeter 16. In this way, using a gas thermometer of known dimensions as described in the article by G. Bond, RB Moyes, SD Pollington and DA Whan, Meas. Sci. Technol. 2 (1991), 571-572, and with air as the working gas, calibration of the thermometer can be achieved by reference to fixed points or by simultaneously immersing the gas thermometer and a thermocouple. into a heated liquid without the presence of microwave energy. During the course of the experiment, the thermometer is placed in the bath and by monitoring the output from the pressure sensor 15, a constant temperature can be maintained.

A 500 milliliter beaker 17 is used to hold coating solutions and substrates as described above.

Tests were conducted using three different substrates and two different coating solutions.

The substrates had the following compositions and dimensions: Substrate No. Material (cm) Width (cm) Length (cm) Thickness Copper (Goodfellow, 99.9% purity, semi-hard) Soft, unalloyed steel (M-Steel) Stainless steel (S-Steel)

The two coating solution compositions used were as follows:

COATING SOLUTION 1

Nickel sulfate 30g/l

Sodium hypophosphite 20g/l

Lactic acid 25g/l

Propionic acid 5g/l

Lead as lead nitrate 4mg/l

pH4.5

COATING SOLUTION 2

one for

OMI proprietary solution Enplate NI 425A, 425B (sold by OMI-IMASA Marketing (Europe) Ltd)

pH (at 90ºC) 5

Before coating, each substrate was pre-treated as follows. First, the substrate was soaked or soaked in Enbond 808 (trademark) at 80ºC for three minutes and then rinsed. The substrate was then soaked in Enbond 808 (trademark) and subjected to anodic electrolytic cleaning at 80ºC for three minutes under a current density of 6 A/dm2 and then rinsed. It was then immersed in tin chloride (SnCl₂), rinsed and immersed in palladium chloride (PdCl₂) and rinsed.

The substrates were then treated using two different methods. In the first method (Method 1), the coating solution (either Coating Solution 1 or Coating Solution 2) was placed in a beaker and the beaker was placed in a thermostatically controlled water bath. The bath was maintained at a temperature of 80ºC (Coating Solution 1) or 85ºC (Coating Solution 2) and the deposition rate of the coating was measured after a total treatment time of 2 hours.

In the second method (Method 2), the appropriate coating solution was placed in the beaker 17 containing the substrate and the beaker was placed on the platen 11 of the microwave chamber 10 described above with reference to Figure 1. The coating solution can be heated using the microwave source prior to insertion of the substrate. The microwave energy source was turned on and the required temperature (80°C for coating solution 1, 85°C for coating solution 2) was maintained using the Variac 13. Again, the coating rate was measured after a total treatment time of 2 hours. It was found that the use of the microwave energy source eliminated the need for agitation of the coating solution. In addition, it was found that the fact that the chamber is closed allows for the containment of any debris that may be generated during the process.

The results of these coating processes are given in the attached Table 1.

Results with coating solution 1

It will be seen that for all substrates the coating rate was greater with Process 2 (the process according to the invention) than with Process 1 (the prior art process). For copper, the ratio of nickel to phosphorus in the coating was essentially unchanged by the use of Process 2. As can be seen in Figures 2a, 2b and 2c, the morphology of the nickel-phosphorus coating on the copper and soft carbon steel and stainless steel substrates coated with Process 1 is comparable to those coated with Process 2. These figures also indicate that different substrates can provide different coating structures for both processes. Nevertheless, the surface coating structure of the coatings of the Process 2 are superior to those of Process 1. The surface topography of the coating on the stainless steel substrate for the two processes (Examples 9 and 10) tend to differ slightly, which can be taken as further evidence that the surface texture is dependent on the substrate type.

Results with coating solution 2

It will be seen that with coating solution 2 the plating rate on mild steel was greater with method 2 than with method 1 (Examples 7 and 8). The coating was unsatisfactory on both copper and stainless steel with either method.

Figures 3a, 3b and 3c show the surface topography of the substrates after treatment in a coating solution 2. This confirms that the copper substrate was not coated by either the process 1 or the process 2. This could be a consequence of the fact that the coating solution 2 is a proprietary solution intended to be used on substrates other than copper, according to the manufacturer's instructions.

Figure 3a confirms that the coating obtained on a stainless steel substrate by method 1 was patchy and that no deposition occurred with method 2. Again, this may be because coating solution 2 is the one designed for substrates other than stainless steel.

Substrates according to Examples 1 and 2 were then taken and heated in a muffle furnace at 400ºC for 80 minutes. Knoop microhardness measurements were made on the examples before and after heat treatment using a Leitz (trademark) mini-load Knoop microhardness tester.

The results were as follows: Example No. heat treated Average₆(HK₆) ΔHK no yes

It will be seen that even without heat treatment, substrates coated according to Process 2 are generally harder than substrates treated by Process 1. After heat treatment, the hardness of the substrates treated by Process 2 remains superior to the hardness of the substrates treated by Process 1. The significance of microhardness, ΔHK, indicates that the variations in the values do not overlap, meaning that consistent improvements in hardness can be expected with Process 2.

Figure 4 shows the surface topography of the substrates of Examples 1, 2, 5, 6, 9 and 10 after being heat treated at 400°C for 80 minutes. Figure 4 shows substrates of Examples 7 and 8 heat treated at 400°C for 80 minutes. This clearly shows that the structures become smoother after heat treatment. The hardness of the deposits is enhanced because smoothness is directly related to grain size, i.e., the smaller the grain size, the higher the hardness values. In addition, it can be seen that the coated substrates of Process 2 have smoother coatings than those of Process 1, which accounts for the improved hardness of the coated substrates according to Process 2.

It will therefore be seen that the use of a microwave energy source to heat the coating solution produces increased coating rates compared to a water bath method. Consequently, the use of a microwave source may enable satisfactory coating rates to be achieved at lower temperatures than with a water bath. In addition, the use of a microwave energy source may enable improvements in grain structure (and hardness) to be achieved at lower temperatures than with a water bath. This may extend the life of the coating solution and avoid instability in the coating solution.

Although the structures described above are metals or metal alloys, the substrates could be of any other suitable material.

The use of microwave heating can also provide improvements in grain structure and deposition rates when used with electrolytic plating and other anodic electrolytic processes.

It will be appreciated that the plating solution used could be any suitable plating solution, such as a plating solution for copper plating a substrate. TABLE 1 Example No. Substrate No. Electrolyte No. Process No. Plating Rate (m/hr) Nickel % Phosphorus % No Deposit (Spotted)

Claims (9)

1. A method of depositing a coating on a substrate, comprising placing a substrate in a coating solution (17) to coat the substrate and then heating the coating solution in a chamber by a microwave source (12) during the coating process to produce a required coating. 2. The method of claim 1 and further comprising heating the coating solution by the microwave source (12) prior to inserting the substrate. 3. The method of claim 2 and further comprising adjusting the power output of the microwave source (12) to control the temperature of the coating solution. 4. The method of claim 3, further comprising controlling the temperature to a constant temperature during the coating process. 5. The method of claim 3, further comprising pulsing the power of the microwave source. 6. A method according to any one of claims 1 to 5 and further comprising heat treating the coated substrate. 7. Method according to one of claims 1 to 6, wherein the coating process is an electroless coating process. 8. The method according to any one of claims 1 to 7, wherein the substrate is a metallic substrate. 9. A coated substrate produced by the process of any of claims 1 to 8.