The choice of thickness of diamond coating is not "the thicker the better".
The quality of diamond coating is more important than the thickness for the improvement of wear resistance.
After experimenting and validation of various cutting tool materials, the service life of CVD diamond coated tools 8 ~ l0 times that of carbide, CVD diamond coated tools allow the cutting speed 2 ~ 3 times higher than the carbide cutting tools and cutting edge is sharp and good consistency and low friction factor, can be in high speed and lower feed of thin-walled graphite mould high speed, high precision machining. Chemical Vapor Deposition (CVD) diamond coating has many excellent properties close to natural diamond, such as high hardness, high thermal conductivity, low friction and low expansion coefficient. It has become the surface coating of cutting tools when processing brittle materials such as graphite and carbon fiber composite materials. It shows excellent performance in improving tool life and machining quality.
According to the grain size, CVD diamond coatings can be divided into micron diamond coatings and nano diamond coatings.
The grain size of micron diamond coating is usually between 500 nm and several microns. It is formed by competitive growth of diamond grains growing in columnar shape. It has excellent characteristics such as high hardness, good surface wear resistance, etc., which can significantly reduce tool wear rate and improve tool life. However, the surface of micron diamond coating is relatively rough (Ra value is usually hundreds of nanometers), and the coating toughness and anti-crack growth ability are poor. The surface cracks are easy to expand along the grain boundary and cause the coating to fall off. As a result, the coating has fallen off before the tool surface reaches the standard of abrasive dull. Nano-diamond coating is composed of nano-sized diamond grains distributed in clusters. The grain size is usually less than 100 nm and the surface is smooth (Ra value is less than 100 nm). In the cutting process, the friction can be significantly reduced, so as to reduce the cutting force and cutting heat and improve the tool life. Nano-diamond coating has good crack propagation resistance. However, compared with micron diamond, its bonding strength with matrix is relatively low, and abnormal failure such as coating peeling is easy to occur in the machining process.
The choice and application of diamond coating tools are mainly based on the properties of workpiece materials and processing quality requirements.
For EDM graphite die processing, the essence of milling process is similar to the "grinding" process, at this time, the number of "micro edge" on the surface of the tool is large and wear-resistant, and the micron diamond coating with large grain should be selected. For the mould used in ceramic processing, the surface finish is required to be high, and nano diamond coating should be selected in the finishing stage. In the processing of aluminum alloy, the severity of "sticking knife" phenomenon is higher than the tool wear, and the requirements for the tool wear resistance are relatively low, so the nano diamond coating with low friction coefficient should be selected. For carbon fiber composites, micro/nano composite diamond coating should be used because the fiber material itself has strong abrasion performance, which requires the tool to have good wear resistance and reduce the cutting force and cutting heat in the machining process.
