12 ways to test the bond strength of a plating
The bonding force of a coating is the bonding strength of the coating to the base metal or intermediate coating, i.e. the force required to strip the coating from the base metal or intermediate coating per unit surface area. Most of the reasons for poor bonding are due to poor preplating treatment. CB/T 5270- -200X "Test Methods for Adhesion Strength of Overlayers (Electrodeposited and Chemically Deposited Layers) on Metal Substrates" provides the test methods. There are many methods for assessing the adhesion of coatings to the base metal, but most of them are qualitative, while quantitative testing methods are only used in experimental studies due to many difficulties. Qualitative measurement methods are usually used for workshop inspection. It is based on the difference between the physical-mechanical properties of the plated metal and the base metal. This is when the specimen is subjected to uneven deformation, thermal stress or the direct action of external forces to check whether the plating is poorly bonded. The specific method can be selected according to the type of plating and plated parts.
1. Bending test
Bending test is in the role of external forces to bend or bend the specimen, due to the plating and the base metal (or intermediate plating) to different degrees of force, the two generated between the force when the force is greater than its combined strength, the plating is from the base (or intermediate plating) on the peeling. Any signs of peeling, cracking or flaking are considered to be poor bonding. This method is suitable for testing the bonding strength of thin parts such as wire and springs. Bending tests are usually the following: (1) the specimen along the axis of the diameter equal to the thickness of the specimen, repeatedly bend 180 °, until the specimen breaks, the plating does not peel, does not fall off as qualified. (2) will be equal to the diameter of the sample along the axis of the sample thickness, bending 180 °, and then magnified four times to check the bending part, the coating does not peel, do not fall off as qualified. (3) the specimen will be fixed in a vice, repeatedly bending the specimen. Until the substrate is broken, the plating does not peel, do not fall off, or magnify four times to check, the plating and the substrate do not separate are qualified. (4) the diameter of the wire below 1mm, it will be wound in the diameter of the wire 3 times the diameter of the shaft; diameter of the wire above 1mm, wound in the same diameter as the wire on the metal shaft are wound into 10 ~ 15 close to the coil, the plating does not peel, does not fall off as qualified.
2. file scratch test file method is to clamp the plated parts in a vice, with a coarse tooth flat-file its sawing surface, the direction of filing is from the base metal to the plating, runt knife and the surface of the plating about 45% angle. A well-bonded plating should not peel during the test. This method is not suitable for very thin coatings and soft coatings such as zinc and cadmium. The scratch test is carried out by drawing two parallel lines 2mm apart with a hard scribe with a 30° sharp edge. Sufficient pressure should be applied to the line so that the knife cuts through the plating to the base metal in one pass. If any part of the plating between the two scribed lines breaks away from the base metal, the bond is considered poor. Another scribing method for this test is to scribe a square grid with a side length of 1mm and observe whether the plating in the grid peels off from the base metal.
3. Thermal shock test (ASTM B571)
The bonding of many coatings can be determined by heating the specimen under test at a certain temperature and then cooling it abruptly, based on the difference in deformation between the coefficient of thermal expansion of the plated metal and the base metal (or intermediate plating). The specimen is heated in the furnace to the temperature specified in Table 10-1-1, with a temperature error of ±10°C, for a period of 0.5h ~ 1h, and then cooled in room temperature water to check whether the plating is blistering or peeling.
The heating test temperature of the cadmium layer of zinc plating is ( 190 soil 10) ℃ must be noted: easy oxidation of the metal should be in an inert atmosphere or reducing atmosphere heating. If the plated parts with solder to do thermal shock test, the melting point of the solder is lower than the above-specified temperature. The heating temperature is allowed to be reduced accordingly, but this should be stated in the assessment results. This method is only effective if there is a significant difference in the coefficient of expansion of the plating and the metal matrix.
4. Tape traction test
The tape traction test uses pressure-sensitive adhesive tape (transparent tape or tape with a specific adhesive layer) to check whether the coating is peeled off from the base metal surface under a steady force. The tape test is mainly used where it is not suitable to test the bonding of a coating by other means that would damage or deform the substrate, such as the measurement of the bonding of a coating on a plastic surface or a printing plate surface.
5. Rubbing and polishing test
This method can be used for fairly thin coatings. The basic principle is that when a local area of the plated part is rubbed and polished, there is both friction and heat generated. This can cause surface hardening and heat generation. For thin plating, in areas where the adhesion strength is poor under these conditions, the overlay layer will blister and separate from the substrate.
Operation method: If the shape and size of the plated parts allow, in an area of less than 6cm2 plating surface, with a diameter of 6cm top processing into a smooth hemispherical steel bar for polishing tools, friction 15s, the pressure applied should be sufficient to polish the plating in each stroke, but not to cut the plating. If the bond is not good, the plating will blister and continue to rub, the blister will grow until it breaks until the plating is peeled from the substrate. The test piece can also be placed in a tumbler or vibratory polisher with a 3mm diameter steel ball inside and a soap and water solution as a lubricant for the friction polishing test. Blistering occurs when the adhesion strength of the cover layer is very poor. However, this test method is not suitable for thicker coatings.
6. Shot blasting test
The basic principle is to use gravity or compressed airflow to make iron or steel shot fall on the surface of the specimen, due to the hammering effect to deform the coating. If the coating is not well bonded to the substrate, the coating will blister. One method of testing is to use a 150mm long tube with an internal diameter of 19mm as a reservoir for the iron or steel shot (approx. 0.75mm diameter) and connect a nozzle to the device with compressed air at a pressure of 0.07MPa to 0.21MPa and a distance of 3mm to 12mm between the nozzle and the specimen. device to evaluate the adhesion strength of a silver cover layer of 100μm-600μm thickness on a steel substrate. The blasting equipment uses an ordinary compressed air steel shot blasting machine. The average diameter of the shot is φ0. 4mm and the hardness of the shot is not less than HV350. The size is determined by selection and meets the requirements given in Table 10-1 - 2.
The size of the shot is checked at least once a week by taking 100g of a shot from the nozzle and screening it. Prior to blasting, all specimens should be held at (190 ±5) °C for 2h to relieve stress. Protect all surfaces not to be shot-peened. Measure the thickness of the silver plating by a non-destructive method (e.g. magnetic method). Any specimens with a silver plating thickness less than 100μm or greater than 600μm and a difference between the maximum and minimum thickness greater than 125μm should be discarded. Mark the maximum thickness of the blastable specimens and place them in groups with a maximum thickness difference of 125μm or less between the groups. When blasting silver-plated surfaces, the minimum blast intensity required is related to the maximum thickness of the plating measured as shown in Figure 10-1-1.
Before processing each group of specimens, the blast strength must be adjusted by testing on a standard specimen. This is achieved by machining a standard specimen from a 1.6mm thick piece of carbon steel: length (76 +0.2) mm, width (19 +0.1) mm, thickness (1. 30 +0.02) mm, with a hardness range of HV400 to HV500. As shown in Figure 10-1-2, the specimen is fastened in a jig and shot-peened to the exposed surface. After blasting, the specimen is removed from the fixture and the curvature of the blasted surface is measured using a depth gauge. For the measurement, the specimen was supported by four 5mm diameter balls forming a 32mm x 16mm rectangle. On the specimen, the arc height at the centre of the specimen is measured with the depth gauge along a straight line symmetrical to the centre of the specimen, over a length of 32mm. The arc height is measured to an accuracy of 25μm and should not exceed 38μm when measured in accordance with the above regulations. If the arc height does not meet the requirements, the blasting conditions can be adjusted. The arc height can be adjusted to obtain the required arc height. If the silver plating is not well bonded, it will stretch or deform and blister.
7. Tensile peel test
(1) Welded tensile peel test. A 75mm x 10mm x 0.5mm piece of tinned mild steel or tinned brass is bent at a right angle of 10mm from one end and the shorter side is welded to the plated surface of the specimen. A tensile force perpendicular to the weld surface is applied to the long side. If the adhesion strength of the cover layer is less than the strength of the weld joint, the cover layer will separate from the substrate; if the adhesion strength of the cover layer is greater than the strength of the weld joint, a fracture will occur at the weld or within the cover layer. The test method is shown in Figure 10-1-3.
Disadvantages of this method: The temperature of the weld joint during the welding process may change the adhesion strength of the coating. A cured synthetic resin adhesive with sufficient tensile strength can therefore be used instead of welding for the peel test. This test is suitable for the inspection of coatings less than 125μm thick. (2) Adhesive tape tensile peel test. A fibre adhesive tape (adhesive tape adhesion strength value is approximately 8N per 25mm width) is adhered to the plating and rolled on top with a rubber roller of a certain quality to remove the air bubbles within the adhesive surface. After an interval of 10s, the tape is peeled off with a pulling force perpendicular to the plating, if the plating is not peeled off it means that the bonding strength is good. This test is suitable for testing 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. Saw, chisel test
Grinding, sawing, chisel test is with the plating of the specimen or parts are used to grind hacksaw or chisel for mechanical impact, to observe whether the plating and the substrate off, skinning and other phenomena. Grinding is done with an abrasive wheel, grinding the edges of the plated part in the direction from the substrate to the coating. If the adhesion strength is poor, the covering layer will be pierced away from the substrate. A hacksaw can also be used instead of a grinding wheel, but care must be taken that the force exerted on the hacksaw is in a direction that tries to separate the overcoat from the substrate. The grind and saw test are particularly effective on the harder metal coatings of nickel and chromium. The chisel test is suitable for thick overlays (>125μm). One method is to place a sharp chisel on the back of the plated protrusion and give it a sharp hammer blow. If the bonding strength is good, the plating will not separate from the substrate even though the plating may break or chisel through. Another method is combined with the "saw test". The test is carried out by sawing a specimen perpendicular to the covering layer. If the bonding strength is poor, the covering layer will flake off; if the covering layer does not flake off at the fracture, a sharp chisel is used to pry up the coating as much as possible at the edge of the fracture, and if the coating can be peeled off for a considerable period, the bonding strength of the coating is poor. The chisel edge should be sharpened before each test. For thinner coatings, a knife can be used instead of a chisel. A hammer can also be used to give a gentle tap. The chisel test is not suitable for soft metal coverings such as zinc and cadmium.
9. Wrap test
The test consists of winding the specimen (usually a strip or wire plated part). Bending rate. The uniformity of the bending action and the diameter of the round bar used to wrap the specimen. Any signs of peeling, cracking, flaking or flaking during the test are considered to be a sign of poor coating adhesion. When the specimen is bent, the covering layer can be on the inside or on the outside of the specimen. Generally, only the outer side of the specimen needs to be checked to determine how strong the coating is. In some cases, however, an examination of the inside of the specimen may provide a more complete picture.
The deep lead test is often used to check the adhesion strength of thin sheet metal-plated parts, and the common methods used are the "Erikson cup test" and the "Romanov flange cap test". This is done by stamping the overlay and base metal into cups and flanged caps using a punch of some kind. In the Eriksen cupping test, a suitable hydraulic device is used to press a 20 mm diameter spherical punch into the specimen at a speed of 0.2 mm/s to 6 mm/s to the required depth, and a poorly adhered covering layer will peel or flake off after only a few millimetres of deformation. When the adhesion strength is good, the cover layer does not peel even if the punch penetrates the base metal. The Romanov flange cap test set consists of a common pressure tester with an adjustable die for stamping the flange cap. The diameter of the flange is 63.5 mm and the diameter of the cap is 38 mm. the specimen is normally tested until the cap breaks. The undamaged portion after the deep drawing will show how deep drawing affects the structure of the covering. In all cases, the test results must be handled with care, as the test procedure involves the ductility of both the overlay and the base metal. These methods are particularly suitable for harder coatings (e.g. nickel and chromium plating).
11. Cathodic testing
The coated specimen is placed in a solution as a cathode, on which only hydrogen precipitates. When energised, the pressure generated will cause the coating to blister as the precipitated hydrogen diffuses through certain coverings and accumulates in any discontinuities between the coverings and the base metal. The test is to place the specimen in a 5% sodium hydroxide (d=1.054g/mL) solution at 90°C and treat it with a current density of 10A/dm2 for 2min. Many small bubbles will form where the adhesion strength is poor. If the coating is not blistered after 15 min of treatment, the adhesion strength can be considered good. Alternatively, a 5% (mass fraction) sulphuric acid solution at 60°C with a current density of 10A/dm2 can be used for electrolytic treatment. A poorly adhered layer will blister within 5 min to 15 min. This method is limited to coatings where the hydrogen precipitation on the cathode can penetrate. This test method is effective when nickel or nickel + chromium coatings have poor adhesion. It is not suitable for lead, cadmium, zinc, tin or copper coatings.
12. Tensile test
The plated specimen is subjected to tensile stress on a tensile testing machine until it breaks, and the bond between the plated layer and the substrate is observed at the fracture. Specimen specifications. The test bar should be plated under exactly the same conditions as the part and then tested for bond strength, preferably with the same material and heat treatment process as the plated part. This test is suitable for thicker plated parts.
