Application analysis of coated cemented carbide in machining tools

Since the advent of coated cemented carbide, it has been widely used in machining tools, significantly improving the efficiency of metal cutting. About 70% of cemented carbide tools are currently surface treated. In recent years, with the continuous improvement of metal cutting processing requirements, tool coating technology has also continuously achieved new developments. At present, the commonly used coating methods are chemical vapor deposition (CVD), physical vapor deposition (PVD), physical chemical vapor deposition (PCVD), etc.; coating materials mainly include TiC, TiN, TiCN, Al2O3, etc.; The layer approach has evolved from a single coating to a composite coating. At the same time, in order to carry out in-depth research on coating properties and coating process, the related coating analysis and inspection technology has also been continuously improved. This paper introduces the metallographic analysis method of cemented carbide coating.

1. Preparation of coated metallographic samples
Carbide coatings are hard, brittle, and thin, and are typically only a few microns to a dozen microns thick. If the sample is slightly inadvertent, the surface coating of the sample may collapse or chamfer, so care should be taken to protect the coating during handling. In the process of sample preparation, in order to ensure that the coating and the cemented carbide substrate are located on the same plane of view as the metallographic microscope, the coated sample surface should be in the same plane as the cemented carbide based system sample. Through repeated sample preparation tests, it was found that the following sample preparation methods can obtain better sample preparation results: firstly, the sample is coarsely and finely ground on a machine tool with a diamond grinding wheel, and then the sample pair is set in sulfur, using a polishing machine and The diamond paste is finely polished. When grinding the sample, a resin-bonded bowl-shaped diamond grinding wheel (particle size in the range of 320#, M20, M14) can be used. To reflect the true thickness of the coating, the surface of the coated sample and the surface of the coating should be polished. Keep vertical; according to the pre-order grinding surface quality, the sample can be ground and polished with diamond abrasive paste with particle size M5 or M1. Figures 1 and 2 show the morphology of a single coating and a composite coating prepared by the above method, respectively.

2. Observation and analysis of coating morphology
The prepared coated metallographic sample was subjected to high-magnification observation (1000-1500 times) under a metallographic microscope. It can be observed in the microscope field of view that the sample section is composed of a surface coating, a transition zone and a cemented carbide substrate.
Metallographic analysis of the transition layer (intermediate layer) When the prepared sample is subjected to high-magnification observation, it is sometimes found that there is a continuous band-shaped (or intermittent) white bright color narrow region between the coating layer and the substrate. After being eroded with red blood salt and sodium hydroxide solution, the color transition of the region is from orange to dark brown to black, which is one of the typical characteristics of h phase WxCoxC. Usually, the industrial alloy is deficient in carbon, and the h1 phase appears. It has a great influence on the performance of coated alloys (currently there are different views on the role of h2 phase, limited by space, this article does not discuss). In summary, metallographic analysis of the transition layer is an essential step in the observation of the sample.
Appearance and Morphology Observation of the Coating Microstructure Since the coating is a very thin single or multi-layer film, it is necessary to be particularly careful when revealing its microstructure. For coatings of different materials, different reagents are required for erosion display.
Development of Titanium Carbide Coating After polishing the titanium carbide coating sample with 10% K3Fe(CN)6+10% NaOH aqueous solution for 20-30 seconds, the microstructure of the titanium carbide coating can be performed under a metallographic microscope. High magnification observation.
The titanium nitride and titanium carbonitride coatings can be used for the titanium nitride and titanium carbonitride (or titanium carbide) coating samples, and can be dripped with a mixed solution of 10 ml of nitric acid + 10 ml of hydrofluoric acid + 10 ml of water. It appears after ~30 seconds. The appearance of the composite coating For the composite coating, the microstructure should be visualized by the segmental erosion method according to the specific coating type, and then the morphology observation under the metallographic microscope.

3. Observation of coating defects
When there are some problems with the surface coating process, various defects occur on the surface of the coating.

4. Coating and transition layer thickness measurement
Tests have shown that the thickness of the coating (surface layer) and the transition layer (intermediate layer) have a great influence on the performance of the cemented carbide coated insert. Therefore, measuring the thickness of each layer of the sample is important for analyzing the performance of the coating.

Coating thickness refers to the distance from the surface of the coating to the interface between the coating and the transition layer (or matrix). The thickness of the transition layer (also referred to as the decarburization layer) refers to the distance from the interface between the coating layer and the transition layer to the interface between the transition layer and the substrate. The thickness of the coating is typically from about a few microns to about 20 microns. For composite coatings, the thickness of each layer needs to be measured separately. The conventional method of measuring the thickness of a coating is to measure using an eyepiece micrometer of a metallographic microscope, but the measurement accuracy of the method is poor and the operation is cumbersome. At present, image analysis systems (such as the products of Sichuan University Zhisheng Company) can be used for measurement. The method is convenient to operate and has high measurement accuracy. In addition, the system has the function of multiple photographing and synthesizing images, which can solve the problem of small difference of focus on the coating sample surface.