Abstract: The bond strength of binder and diamond grits in metal-based diamond tools, the state of diamond cutting edge, and the performance and life of diamond tools have important influences. The bond strength between the bond and the diamond grits in the metal-based diamond tool has an important influence on the state of the diamond cutting edge, and the performance and the life of the diamond tool. In this paper, the action mechanism and interface characteristics of carbide-forming elements and rare earth elements in multi-layer and single-layer metal-based diamond tools on the interface between bond and diamond are reviewed.
With the rapid development of the modern manufacturing industry, especially the development of electronic information, aerospace, new materials and other industries, a large number of high-performance diamond tools are needed for cutting, grinding and polishing semiconductors, functional ceramics, high temperature alloys, magnetic materials, optical crystals and other advanced hard and brittle materials. At present, diamond tools can be mainly divided into three types: metal-based, resin-based and ceramic-based according to the properties of the binder.
Due to the excellent matching of metal strength, hardness and toughness, metal-bond diamond tools have become widely used tools for processing hard and brittle materials. Since the interface bonding state between diamond powder and metal bond directly affects the bonding strength of diamond particles, the height of the cutting edge, and the processing efficiency, precision, and life of the tool, the research on the interface between diamond and metal bond is of great importance. This paper discusses the structure of the interface between industrial diamond powder and bond in multilayer and single-layer metal-based diamond tools, respectively.
1. The interface between diamond and binder in multilayer diamond tools
Multilayer diamond tools are usually densified by sintering synthetic diamond grits and metal binder mixture powder, so they are also called sintered diamond tools. At present, the binder of sintered diamond tool is mainly Co-based, Cu-based and Fe-based. Co is the best binder to the bond diamonds at present. The cu-based binder is usually based on Cu-S alloy, which is widely used as a binder due to its low melting point and good sintering performance. The appearance of the Fe-based bond is a new bond proposed to reduce the cost of Co and Cu. Because Fe and diamond powder can react to form carbides, Fe-based bond has strong bond strength to diamond. However, due to the strong etching effect of Fe on the surface of diamond particles, the strength of diamond itself will be reduced. In addition, Fe powder is easy to oxidize, when the sintering temperature is high, not easy to edge, and needs to add a certain amount of other alloy elements, such as Co, Ni and so on. Therefore, Fe – based binders are not widely used.
In order to improve the bonding state between the metal bond and the diamond, on the one hand, the diamond can be coated or directly add a small amount of other elements (such as carbide forming elements, rare earth elements, etc.)， on the other hand, On the other hand, a layer of carbides forming elements with affinity to diamond is coated on the diamond surface to make diamond have the characteristics of metal.
The layer of carbide-forming elements that have an affinity for diamond gives the diamond its metallic character.
1.1 Interface effects of carbide-forming elements in binders
Adding carbide forming elements such as Ti, Cr, V, Mo, W and Zr into the bond of sintered diamond tools can obviously reduce the wettability Angle of the bond on the diamond. When Ti, Cr, V and other elements are added into Cu, the wettability Angle of the diamond decreases from 145° to 50° or even 0°. It not only improves the bonding strength of the binder to diamond grains but also improves the hardness and bending strength of the matrix.
1.2 Interaction of rare earth elements in binders at interfaces
Rare earth elements can be called industrial vitamins and can be added in small amounts to purify the grain boundary impurities of most materials, thus strengthening them. For sintered diamond tools, there are pollution from oxygen in the air and the surrounding environment in the process of gold treatment. Therefore, the main role of rare earth element is to purify and strengthen the grain boundary of the bond agent, and also to improve the bond state between the bond agent and diamond polishing powder.
1.3 The role of carbide-forming elements on the interface of diamond surface coating
Another form of addition of carbide-forming elements to sintered diamond tools is to pre-coat the diamond surface. Coating metal or alloy on the diamond grits surface can further improve the wettability of the bond to diamond, especially coating carbide-forming elements such as Ti, Cr, V, W, Mo, etc.
2. The interface between diamond and binder in single-layer diamond tools
Single-layer diamond tools have the characteristics of high cutting edge and do not need to be trimmed, and their sharpness and cutting efficiency are much better than multi-layer diamond tools, so they have been widely used in the field of processing hard and brittle materials. At present, the manufacturing methods of single-layer diamond tools mainly include electroplating method and brazing method.
2.1 Interface of electroplated monolayer diamond tools
Electroplating diamond tools is based on the principle of electroplating Ni deposition on the steel substrate and diamond surface, and the diamond is wrapped, so the Ni coating and diamond grit is just a simple mechanical inlay, some diamond and Ni coating gap will appear. Therefore, the interface bonding force between electroplated diamond and binder is very weak. Of course, other methods can also be used to improve the combination of electroplated diamond and Ni coating, such as the use of laser surface treatment, so that the local coating melting can also form a certain metallurgical combination with diamond.
(SEM photo of the bonding state of electroplated metal and diamond)
2.2 Brazing interface of monolayer diamond tool
The brazing single-layer diamond tool is to melt the solder alloy at high temperature, fill the gap between diamond and steel matrix by capillary action, and pile up the diamond root to a certain height, the edge height is very high, can reach more than 70% of diamond abrasive diameter. The commonly used filler metals for brazing diamond mainly include Ag-Cu, Cu-Sn and Ni-Cr alloys. The interface bonding state and micro-structure characteristics of the bond and diamond in metal bond diamond tools have important effects on the bond strength, edge performance and life of the diamond, so it is very important to study the interface between the bond and diamond.
At present, the research on the interaction mechanism of carbide-forming elements, rare earths in the binder and diamond at the interface is not enough, especially the lack of in-depth microstructure analysis. Therefore, it is necessary to use various advanced analysis methods, such as transmission electron microscopy, Raman spectroscopy, X-ray diffraction, Auger energy spectroscopy to analyze and test the microstructure, microdomain composition, phase structure. It provides a strong scientific basis for improving the composition and preparation method of the binder and improving the final processing performance and life of the Metal bond diamond tool.