What is the coating thickness test?

Date：2021-08-03 16:20:00Views：1502

catalogue

summary	significance	application area
Reference standard	type	method
principle	Sample requirements	evaluation criterion

Overview of coating / coating thickness test

Plating is to coat a layer of plastic on the metal surface of some articles for good-looking or storage, or a thin layer of metal, or to imitate a precious metal, a thin layer of this precious metal is plated on the surface of ordinary metal. Coating thickness test the thickness test of metal and oxide coatings on the surface of materials. The surface covering layer is to make the material surface have certain strengthening, protection (such as anti-oxidation and anti-corrosion) decoration or special functions (such as conductivity, insulation, invisibility, etc.), and form one or more layers of covering layers with a certain thickness and different from the base material on the surface of metal or non-metal substrate by physical or chemical methods, so as to meet the requirements of product design and use.

The covering layer includes coating, non-metallic coating, coating, chemically generated film, etc. the covering layer is applied to the material surface to realize the special functions and properties of the material.

Significance of thickness test of coated / coated products

In industrial production, if the thickness of the covering layer is too thin, it will be difficult to give full play to the special functions and properties of the material. If it is too thick, it will cause economic waste. Moreover, if the thickness of the covering layer is uneven or does not meet the specified requirements, it will have an adverse impact on its mechanical and physical properties. Therefore, the thickness uniformity of material surface coating is one of the most important product quality indexes.

The detection of material surface coating has become the most important process for material processing industry and users to detect product quality. It is also a necessary means for products to reach high-quality standards.

Application field of coating / coating thickness test

Electronics, automobile, aviation, materials, metals, ceramics and other manufacturing fields.

Reference standards for coating / coating thickness measurement

GB / T 6462-2005 measurement of thickness of metal and oxide coatings microscope method

Metallic coatings -- Measurement of coating thickness -- X-ray spectrometric method

ASTM b487-1985 (2007) test method for determination of metal and oxide layer by cross-sectional microscopic observation

ASTM b748-1990 (2010) test method for measuring metal coating thickness by measuring cross section with scanning electron microscope

Type of coating / coating thickness test

Protective coatings commonly used in industry include metal coatings and non-metal coatings:

1. Thickness of metal coating

The thickness and uniformity of metal coating is an important quality mark of coating, which affects the reliability and service life of products to a great extent. Detecting the thickness and uniformity of the metal coating on the material surface is helpful to monitor the product quality, improve the process and improve the benefit.

Objective: the thickness and uniformity of metal coating is an important quality mark of coating, which affects the reliability and service life of products to a great extent. Detecting the thickness and uniformity of the metal coating on the material surface is helpful to monitor the product quality, improve the process and improve the benefit.

Scope of application: it is mainly used for the protection of small and precision instrument parts and the repair of anti-wear shafts, as well as the beautiful appearance, decoration or conductivity, anti-corrosion performance of materials, etc.

2. Thickness of non-metallic coating

The surface of metal parts and equipment is covered with corrosion-resistant non-metallic protective layer, which is one of the important means of metal corrosion prevention. The purpose is to replace the metal / environment interface with the metal / non-metallic layer / environment interface, and use the corrosion resistance, impermeability and corrosion inhibition properties of non-metallic materials to protect metals from environmental erosion, so as to prolong their service life.

Objective: the surface of metal parts and equipment covered with corrosion-resistant non-metallic protective layer is one of the important means of metal corrosion prevention. The purpose is to replace the metal / environment interface with the metal / non-metallic layer / environment interface, and use the corrosion resistance, impermeability and corrosion inhibition properties of non-metallic materials to protect metals from environmental erosion, so as to prolong their service life. For example, the coating can be coated on the object surface by different construction processes, and a solid film with certain strength and continuity can be formed by using its own adhesion and curing ability, that is, the coating, also known as paint film or film. Many coatings are inert to acid, alkali, salt and other corrosive media, isolating the object from the corrosive environment, thus playing an anti-corrosion role.

Scope of application: according to the characteristics of metal parts and equipment and different corrosion environments, different types and thicknesses of corrosion-resistant non-metallic protective layers can be covered to protect the metal surface. This method has the characteristics of good protection effect, wide applicability, flexible use, convenient and simple construction, and is widely used in the anti-corrosion protection of metal components in various industries, such as automobile, bicycle, machinery manufacturing and other fields.

The test steps for the thickness of metal coating and non-metal coating are the same: sampling → cleaning → vacuum inlay → grinding → polishing → micro etching (if necessary) → gold spraying (if necessary) → observation.

Test method for coating thickness

The general methods to detect the coating thickness are: 1. Metallographic method; 2. Coulometric method; 3. X-ray method

Scope of application

Metallographic method:

It is a method to measure the local thickness of metal coating and oxide film by detecting the cross section with metallographic microscope. Generally, the thickness measurement needs to be greater than 1um to ensure that the measurement result is within the error range; The greater the thickness, the smaller the error.

Coulometric method:

It is suitable for measuring the thickness of single-layer and multi-layer metal coatings. Anodic dissolution coulometry includes measuring the thickness of multi-layer systems, such as Cu / Ni / Cr, alloy coatings and alloying diffusion layers. It can measure not only the coating thickness of plane samples, but also the coating thickness of cylinders and wires, especially suitable for measuring the metal and its potential difference of multi-layer nickel coating. The types of measured coatings are Au, Ag, Zn, Cu, Ni, DNI and Cr.

X-ray method:

It is suitable for measuring the thickness of metal coating that needs to be analyzed in electroplating, electronic circuit board and other industries. It includes the thickness of metal elements such as gold (AU), silver (Ag), tin (SN), copper (Cu), nickel (Ni), chromium (CR).

This measurement method can simultaneously measure the three-layer overburden system or the thickness and composition of the three-layer components.

Principle of coating thickness measurement

Metallographic method:

Based on the principle of metallographic microscope, the coating thickness is enlarged for accurate observation and measurement.

Coulometric method:

The covering layer with accurately limited area is dissolved by appropriate electrolyte anode. The sharp change of electrolytic cell voltage shows that the covering layer is completely dissolved in essence. The thickness of the covering layer is calculated through the power consumption. Due to different anodic dissolution methods, the power consumption of the measured coating thickness is also different. When dissolved with constant current density, it can be calculated from the beginning of the test to the end of the test; When dissolved with non constant current density, the cumulative power consumption is calculated by the cumulative power consumption, and the cumulative power consumption is displayed by the electricity meter.

X-ray method:

X-ray spectroscopy is based on the interaction between a beam of intense and narrow polychromatic X-rays and the substrate and the coating. This interaction produces secondary radiation with discrete wavelength and energy, which has the characteristics of constituting covering layer and matrix elements. There is a certain relationship between the mass per unit area of the overburden (if the density is known, it is the linear thickness of the overburden) and the secondary radiation intensity. The relationship is first determined by the overburden correction standard block correction with known mass per unit area. If the density of the covering material is known and the actual density is given, such a standard block can give the linear thickness of the covering.

Coating thickness test sample requirements

Metallographic method:

Since the thickness of samples measured by metallographic method is local thickness, for some samples with inconsistent thickness, the customer needs to specify specific parts. If there are no special requirements, we will take a more uniform part for measurement.

Coulometric method:

At present, we can only measure the plating thickness of the plane, and the sample needs at least one 5 mm2 plane.

X-ray method:

Its area shall be at least greater than 0.05 × 0.25mm

Standard for quality evaluation of Electrodeposited Coatings

The quality inspection of electrodeposited coating includes the appearance, thickness, adhesion with base metal, ductility, microhardness, brittleness, corrosion resistance, wear resistance, weldability, etc. Although the specific quality inspection contents vary from part to part and coating, the appearance, thickness, corrosion resistance and adhesion with the base metal of the coating are the contents that must be inspected for all coatings. Each country has its own national standards and unified international standards for the quality inspection methods and evaluation of Electrodeposited coatings; Different enterprises also formulate corresponding enterprise standards.

1) Appearance of coating

The appearance of the coating is one of the items that must be checked for any part and any kind of plating. Generally, the appearance of the coating is directly observed with the naked eye under natural light. The content includes the macro uniformity, color, brightness, crystallization and macro adhesion of the forging layer. Generally speaking, in addition to its unique color and luster, the coating shall also be uniform, meticulous and have good adhesion. Pinholes, stripes, blisters, peeling, burrs, nodules, pockmarks, charring, cracking, peeling, falling off, abnormal color or missing plating are not allowed. However, for hanging plated parts, slight defects at the hook are generally allowed.

2) Thickness of coating

To ensure the service performance of parts, the coating on the surface of parts must reach a certain thickness. The commonly used coating thickness measurement methods include destructive and non-destructive. Destructive thickness measurement methods include anodic dissolution coulometry, metallographic method, dissolution weighing method, liquid flow method, drop method, etc; Non destructive thickness measurement methods include mechanical gauge method, magnetic method, eddy current method β X-ray backscattering, X-ray spectroscopy, etc. When using these methods to measure the thickness of the coating, please refer to the corresponding national standards.

3) Corrosion resistance of coating

There are two main test methods to evaluate the corrosion resistance of coatings: natural environment test and artificial accelerated corrosion test. The former includes field test under service environment and atmospheric exposure test under different climatic conditions. This kind of method can truly evaluate the corrosion resistance of coating, but the disadvantage is that the cycle is too long; The latter includes neutral salt spray test (NSS), copper acetate accelerated test (CASS), corrosion paste test, electrolytic corrosion test, industrial gas corrosion test, damp heat test, etc. All these corrosion resistance tests have corresponding national standards, which specify the test conditions and evaluation methods.

4) Adhesion of coating

There are many methods to test the bonding force (also known as bonding strength) between electrodeposited coating and base metal, but they are qualitative tests. The common test methods of coating adhesion include friction polishing test, peeling test, file test, cross marking test, bending test, thermal vibration test, deep drawing test, etc. Different methods are applicable to different coatings and have different evaluation standards. Please refer to the corresponding national standards for specific use.