12 ways to test the adhesion of electroplating layers
The bonding force of the coating refers to the bonding strength between the coating and the base metal or the intermediate coating, that is, the force required to peel off the coating per unit surface area from the base metal or the intermediate coating. The adhesion of the coating is not good, most of which is caused by poor pre-plating treatment. In addition, improper composition and process specification of the plating solution or the disparity of thermal expansion coefficient between the base metal and the coating metal has a significant impact on the adhesion of the coating. CB/T 5270--200X "Test method for adhesion strength of coating layer (electrodeposited layer and chemically deposited layer) on the metal substrate" specifies the test method. There are many methods for evaluating the adhesion between coating and base metal, but most of them are qualitative methods, and quantitative testing methods are only used in experimental research due to many difficulties. A qualitative measurement method is usually used for workshop inspection. It is based on the difference in the physical-mechanical properties of the coating metal and the base metal. That is, when the sample is subjected to uneven deformation, thermal stress, or direct external force, the coating is inspected for poor bonding. Phenomenon. The specific method can be selected according to the type of plating and plating.
1. Bending test
The bending test is to bend or bend the sample under the action of external force. Due to the different degrees of force between the coating and the base metal (or intermediate coating), a component force is generated between the two. When the component force is greater than its bonding strength, the coating will be Peeling on the substrate (or intermediate coating). Any evidence of peeling, chipping, or flaking is considered poor bonding. This method is suitable for the coating adhesion test of thin parts, wires, springs, and other products. Bending tests usually have the following types: (1) Bend the sample repeatedly for 180° along the axis whose diameter is equal to the thickness of the sample, until the sample breaks, and the coating does not peel or fall off as qualified. (2) Bend the sample by 180° along the axis whose diameter is equal to the thickness of the sample, and then magnify four times to check the bent part, if the coating does not peel or fall off, it is qualified. (3) Fix the sample in a vise and bend the sample repeatedly until the base is broken, and the coating does not peel or fall off, or when magnified four times, the coating is not separated from the base. (4) For wires with a diameter of 1 mm or less, wind them on a shaft whose diameter is 3 times that of the wire; for wires with a diameter of 1 mm or more, wind them on a metal shaft with the same diameter as the wire into 10 to 15 tight Coils that are close to each other are qualified if the coating does not peel or fall off.
2. File scratch test
The file method is to clamp the plated part on a vise and file the saw section with a coarse-toothed flat file. The direction of the filing is from the base metal to the coating, and the knife and the surface of the coating are about 45%. horn. Coatings with good adhesion should not peel off during the test. This method is not suitable for very thin coatings and soft coatings such as zinc cadmium. The scratch test is to use a hard scribing knife with an edge ground to an acute angle of 30° to draw two parallel lines with a distance of 2mm. When scribing, enough pressure should be applied so that the scribing knife can pierce the coating to reach the base metal at one time. If any part of the coating between the two scribe lines is detached from the base metal, the bond is considered to be poor. Another method of drawing in this test is: to draw a square grid with a side length of 1 mm, and observe whether the coating in the grid is peeled off from the substrate.
3. Thermal shock test (ASTM B571)
The adhesion of many coatings can be determined by heating the test sample at a certain temperature and then cooling it abruptly, which is based on the difference in deformation due to the difference in thermal expansion coefficients of the coating metal and the base metal (or intermediate coating). Put the sample in the furnace and heat it to the temperature specified in Table 10-1-1, the temperature error is ±10℃, the time is generally 0.5h ~ 1h, and then put it in room temperature water to quench, and check whether the coating is foaming, fall off.
The heating test temperature of the galvanized cadmium layer is (190 ± 10) ℃. It must be noted that the metal that is easily oxidized should be heated in an inert atmosphere or a reducing atmosphere. If the plated parts with welded seams are subjected to a thermal shock test when the melting point of the solder is lower than the temperature specified above, it is allowed to reduce the heating temperature accordingly, but it should be stated in the evaluation results. This method is effective only when the expansion coefficients of the coating and the metal substrate are significantly different.
4. Tape pull test
The tape pull test is to use pressure-sensitive tape (scotch tape or tape with a specific adhesive layer) to check whether the coating is peeled off from the base metal surface under a stable force pull. The tape inspection is mainly used for occasions where it is not suitable to test the bonding force of the coating by destroying or deforming the substrate, such as the bonding force measurement of the coating on the surface of the plastic surface or the surface of the printing plate.
5. Friction and polishing test
This method can be used for relatively thin coatings. The basic principle is that when the local area of the plated part is rubbed and polished, there is both the effect of friction and the generation of heat. It may cause surface hardening and heat generation of the coating. For thin coatings, in areas of poor adhesion under these conditions, the cover will blister and separate from the substrate.
Operation method: If the shape and size of the plated parts allow, on the plated surface with an area of less than 6cm2, use a steel bar with a diameter of 6cm and the top processed into a smooth hemisphere as a polishing tool, rub for 15s, and the applied pressure should be One stroke is sufficient to polish the coating, but not strip it. If the bonding force is not good, the coating will foam, continue to rub, and the bubbles will continue to increase to rupture, until the coating is peeled off from the substrate. The test piece can also be placed in a rolling or vibration polishing machine with a steel ball with a diameter of 3 mm inside, and the friction polishing test can be carried out with a soapy water solution as a lubricant. Foaming occurs when the adhesion strength of the cover layer is very poor. However, this experimental method is not suitable for thicker coatings.
6. Shot peening test
The basic principle is that the iron or steel shot falls on the surface of the sample by means of gravity or compressed airflow, and the coating is deformed due to the action of hammering. If the adhesion between the coating and the substrate is not good, the coating will blister. One of the test methods is to use a pipe with a length of 150mm and an inner diameter of 19mm as a reservoir for an iron shot or steel shot (about 0.75mm in diameter) and connect a nozzle to inject a pressure of 0.07MPa~0.21MPa into the device. In compressed air, the distance between the nozzle and the sample is 3mm~12mm. Another method is to use a standard pneumatic device used for shot peening of steel parts to evaluate the adhesion strength of silver coatings with thicknesses ranging from 100 μm to 600 μm on steel substrates. Ordinary compressed air steel shot ejector for shot peening equipment. For the round steel shot with an average diameter of φ0.4mm and hardness not less than HV350, the size shall be determined by screening and shall meet the requirements.
The size of the steel shot should be checked at least once a week by taking 100g of steel shot from the nozzle for screening. Before shot peening, all samples should be kept at (190 ± 5) ℃ for 2h to relieve stress. Protect all surfaces that do not need blasting. Using non-destructive methods (such as magnetic methods), measure the thickness of the silver layer. Any samples with a thickness of the silver-plated layer less than 100 μm or greater than 600 μm and the difference between the maximum and minimum thicknesses greater than 125 μm should be discarded. Mark the maximum thickness of the samples that can be shot-peened, and place them in groups, the maximum thickness difference between each group is 125 μm or less. Figure 10-1-1 shows the relationship between the required minimum spray intensity and the measured maximum thickness of the coating when shot peening the silver-plated surface.
Before processing each set of samples, the shot peening intensity must be adjusted experimentally on standard samples. The method is to use a carbon steel sheet with a thickness of 1.6mm to process a standard sample: length (76 + 0.2) mm, width (19 + 0. 1) mm, thickness (1. 30 + 0. 02) mm, its hardness The range is HV400 to HV500. As shown in Figure 10-1-2, fasten the sample in the fixture and blast the exposed surface. After shot peening, remove the sample from the fixture and measure the curvature of the shot-peened surface with a depth gauge. During measurement, the sample is supported by four balls with a diameter of 5mm, forming a rectangle of 32mmx16mm. On the test piece, along a line symmetrical with the center position of the test piece, within a length of 32mm, measure the arc height of the center of the test piece with a depth gauge. The measurement accuracy of the arc height value is 25 μm. When measured according to the above regulations, the arc height should not exceed 38μm. When the arc height does not meet the requirements. Adjustable shot peening conditions. In order to obtain the required arc height. If the silver plating is poorly bonded, it will stretch or deform, and it will blister.
7. Tensile peel test
(1) Welding tensile puncture test. A 75mmx10mmx0.5mm tin-plated mild steel or tin-plated brass test piece is bent at a right angle at a distance of 10mm from one end, and the plane of the shorter side is welded to the surface of the coating of the sample. The long side is applied perpendicular to the welding surface. pulling force. If the adhesion strength of the covering layer is less than the strength of the welding point, the covering layer will be separated from the substrate; if the adhesion strength of the covering layer is greater than the strength of the welding point, the fracture will occur at the welding point or inside the covering layer.
The disadvantage of this method: The temperature of the solder joint during the soldering process may change the adhesion strength of the coating. Therefore, a cured synthetic resin adhesive with sufficient tensile strength can be used instead of welding to carry out the peel test. This test is suitable for inspecting coatings with a thickness of less than 125 μm. (2) Adhesive tape tensile peel test. Adhere a fiber adhesive tape (the adhesion strength value of the adhesive tape is about 8N per 25mm width) on the coating, and roll it with a rubber roller of a certain quality to remove the air bubbles in the bonding surface. After an interval of 10s, the tape is peeled off with a tensile force perpendicular to the coating. If the coating does not peel off, it means that the bonding strength is good. This test is suitable for checking the adhesion strength of the plating on the conductors and contacts in the printed circuit board. The test area should be at least 30mm2.
8. Grinding. I Saw, a Chisel test
Grinding, sawing and chisel tests are to mechanically impact the coated samples or parts with a grinding hacksaw or chisel, respectively, to observe whether the coating is peeled off from the substrate or peeled. Grinding is to use of a grinding wheel to grind the edge of the plated part, and the direction of grinding is from the substrate to the cover layer. If the adhesion strength is poor, the cover will pierce from the substrate. It is also possible to use a hacksaw instead of the grinding wheel, but pay attention to the direction of the force applied to the hacksaw, and the stress map separates the cover from the substrate. Grinding and sawing tests are particularly effective on harder metal coatings such as nickel and chromium. The chisel test is suitable for thick overlays (greater than 125 μm). One method is to place a sharp chisel on the backside of the protrusion of the coating and give it a strong hammer blow. If the bond strength is good, the coating will not separate from the substrate even though the coating may crack or gouge. Another method is performed in conjunction with the "saw test". During the test, first saw a sample perpendicular to the cover layer. If the adhesion strength is not good, the cover layer will peel off; if the cover layer at the fracture does not peel off, use a sharp chisel to pry up the coating layer at the edge of the fracture as much as possible. Peeling off a considerable section, it shows that the adhesion strength of the coating is poor. Before each test, the chisel edge should be sharpened. Experiment with a knife instead of a chisel for thinner overlays. And can be tapped lightly with a hammer. The chisel test is less suitable for soft metal coatings such as zinc and cadmium.
9. Winding test
The test is to wrap the sample (usually a strip or wire plated piece), and each part of the test can be standardized, including the length and width of the test strip, the bending rate, the uniformity of the bending action, and the size of the round bar used to wrap the sample. diameter. Any signs of peeling, chipping, or flaking during the test are considered poor coating adhesion. When the sample is bent, the covering layer can be on the inside of the sample or on the outside of the sample. Generally, it is only necessary to check the outer side of the sample to judge the coverage strength of the coating. In some cases, however, it may be possible to inspect the inside of the specimen for a more complete judgment.
10. Deep-pull test Deep-pull test is often used to test the adhesion strength of sheet metal-plated parts, and the commonly used methods are "Erickson cupping test" and "Romanov flange cap test". The cover and base metal are punched into cups and flanged caps with some kind of punch. In the Erickson cupping test, a suitable hydraulic device was used to press a spherical punch with a diameter of 20mm into the sample at a speed of 0.2mm/s ~ 6mm/s to the required depth, adhere to Poor-strength coatings can peel or peel off after only a few millimeters of deformation. When the adhesion strength is good, the coating will not peel even if the punch penetrates the base metal. The Romanov flange cap test device consists of a common pressure testing machine and is equipped with a set of adjustable dies for punching flange caps. The diameter of the flange is 63.5mm and the diameter of the cap is 38mm. The samples are generally tested until the cap ruptures. The undamaged portion after deep lead will show how deep lead affects the structure of the overlay. In all cases, the test results must be handled with care because the test procedure involves the ductility of both the cover and the base metal. These methods are particularly suitable for harder coatings such as nickel and chrome.
11. Cathode test
The plated specimen is placed in the solution as the cathode, and only hydrogen is evolved on the cathode. When energized, the resulting pressure will cause the cover to blister as the evolved hydrogen diffuses through some of the covers and accumulates at any discontinuity between the cover and the base metal. The test is to place the sample in a 5% sodium hydroxide solution (d=1.054g/mL) at 90°C and treat it with a current density of 10A/dm2 for 2min. Many small bubbles will be formed in places with poor adhesion strength. . If the coating does not bubble after 15min treatments, it can be considered that the adhesion strength is good. It can also be electrolytically treated with a 5% (mass fraction) sulfuric acid solution at a current density of 10A/dm2 at 60°C. The coating with poor adhesion strength will foam within 5min~15min. The method is limited to use in capping layers permeable to the evolved hydrogen gas on the cathode. For example, when the adhesion strength of nickel or nickel + chromium coating is poor, this test method is more effective. It is not suitable for coatings such as lead, cadmium, zinc, tin, or copper.
12. Tensile test
The electroplating sample is subjected to tensile stress on the tensile testing machine until it breaks, and the bonding of the coating and the substrate at the fracture is observed. If necessary, a knife can be used for peeling inspection. The specifications, dimensions, and other requirements of the sample shall be designed according to the test bar of the tensile test in the mechanical property test. And the heat treatment process is the same as the plated parts. This test is suitable for plated parts with thicker coatings.
