Methods Of Surface Metallization Of Diamond

In the last article, we talked about the reason and principle of diamond surface metallization. In this period, we will talk about the common methods of diamond surface metallization at home and abroad, mainly including chemical plating, salt bath method, vacuum vapor plating, powder covered sintering

 

1. Chemical plating plus electroplating
This method specifically refers to first depositing metals such as Ni, Co, Cu or Ni-W, Co-W and other composite metals on the diamond surface through oxidation-reduction reaction to make the diamond arrive at the store, and then electroplating it to obtain the required coating. The chemical plating and electroplating products of diamond particles are usually only used for resin-bonded tools, which can increase the surface roughness of diamond particles, improve the physical holding force of the bond on the particles, and at the same time, the coating can make up for the internal defects of diamond particles, improve their mechanical properties and thermal stability.
It is worth noting that electroless plating or plating of Ni, Co and Cu on diamond surface is not applicable to metal bond tools. Even products coated with Ni-W and Co-W alloys cannot be used for metal bond tools, because Ni and Co are graphitized elements and cannot play a metallurgical binding role.

2. Salt bath method
Salt bath plating is to add metal powder such as Ti or Cr into the chloride, and then put the diamond particles into the mixed solution, and heat the solution to 850 ℃~1100 ℃. After salt bath treatment for 1~2h, the corresponding carbide coating is formed on the diamond surface. The main advantage of this method is that the coating is firmly combined with diamond; The disadvantage is that the coating temperature is high, the process of separating diamond from salt bath after plating is complex, and the coating cost is high.

3. Vacuum plating
The diamond surface used for metal-bonded diamond tools, such as saw blades and geological bits, should be coated with strong carbide forming elements, such as Ti, cr, Mo, w, V, etc. These metals are generally deposited on the diamond surface by vacuum plating, but this method requires vacuum equipment, and the process is complex. At present, the commonly used vacuum plating technologies at home and abroad include the following:
(1) Vacuum Physical Vapor Deposition (PVD)
Vacuum physical vapor deposition refers to the use of vacuum evaporation coater, magnetron sputtering coater or ion beam coater and other equipment to vacuum the target metal into atoms, molecules or ions and directly deposit them on the surface of the plating piece. According to the different gasification methods during coating, the method can be divided into vacuum evaporation plating, vacuum sputtering plating and vacuum ion plating.
However, this method has some disadvantages, such as low single coating amount, uneven coating, easy to miss plating, physical adhesion between coating and diamond, no chemical metallurgy combination, etc; In order to obtain the best coating structure, it is difficult to realize industrial application because of repeated plating.
(2) Vacuum chemical vapor deposition (CVD)
Chemical vapor deposition (CVP) is the formation of coatings by chemical reaction of gaseous substances on solid surfaces under certain pressure, temperature and time conditions. CVD generally refers to the introduction of gaseous compounds (such as halides, etc.) of the plated metal into the reaction chamber of the plated part, and the thermal decomposition or chemical synthesis of the contact with the workpiece to form the coating.
However, the reaction temperature of this method is generally as high as 900~1200 ℃, which is easy to damage the diamond; Like PVD, the reactive gas phase is difficult to penetrate into the accumulated particles, the amount of single coating is low, and the cost is high.

4. Powder covered sintering
The use of contact reaction between metal powder and diamond at high temperature to form carbide or metal layer on the surface of diamond is called powder covered sintering method or solid powder contact reaction method. The formation of the coating is actually that the metal oxides in the powder (such as WO3, WO2, MoO3, MoO2) are volatile at high temperature and react with carbon atoms on the diamond surface to form carbides.
Although this method can increase the amount of single coating, because the coating temperature is generally higher than 850 ℃, the diamond will be oxidized and the compressive strength will be reduced. The actual application results show that the improvement of diamond tool performance is not obvious.